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Common Lisp the Language, 2nd Edition ------------------------------------------------------------------------------- 28. Common Lisp Object System By Daniel G. Bobrow, Linda G. DeMichiel, Richard P. Gabriel, Sonya E. Keene, Gregor Kiczales, and David A. Moon [change_begin] PREFACE: X3J13 voted in June 1988 (CLOS) to adopt the first two chapters (of three) of the Common Lisp Object System specification as a part of the forthcoming draft Common Lisp standard. This chapter presents the bulk of the first two chapters of the Common Lisp Object System specification; it is substantially identical to these two specification chapters as previously published elsewhere [5,6,7]. I have edited the material only very lightly to conform to the overall style of this book and to save a substantial number of pages by using a typographically condensed presentation. I have inserted a small number of bracketed remarks, identified by the initials GLS. The chapter divisions of the original specification have become section divisions in this chapter; references to the three chapters of the original specification now refer to the three ``parts'' of the specification. (See the Acknowledgments to this second edition for acknowledgments to others who contributed to the Common Lisp Object System specification.) This is not the last word on CLOS; X3J13 may well refine this material further. Keene has written a good tutorial introduction to CLOS [26]. - Guy L. Steele Jr. [change_end] ------------------------------------------------------------------------------- * Programmer Interface Concepts o Error Terminology o Classes + Defining Classes + Creating Instances of Classes + Slots + Accessing Slots o Inheritance + Inheritance of Methods + Inheritance of Slots and Slot Options + Inheritance of Class Options + Examples o Integrating Types and Classes o Determining the Class Precedence List + Topological Sorting + Examples o Generic Functions and Methods + Introduction to Generic Functions + Introduction to Methods + Agreement on Parameter Specializers and Qualifiers + Congruent Lambda-Lists for All Methods of a Generic Function + Keyword Arguments in Generic Functions and Methods o Method Selection and Combination + Determining the Effective Method + Standard Method Combination + Declarative Method Combination + Built-in Method Combination Types o Meta-objects + Metaclasses + Standard Metaclasses + Standard Meta-objects o Object Creation and Initialization + Initialization Arguments + Declaring the Validity of Initialization Arguments + Defaulting of Initialization Arguments + Rules for Initialization Arguments + Shared-Initialize + Initialize-Instance + Definitions of Make-Instance and Initialize-Instance o Redefining Classes + Modifying the Structure of Instances + Initializing Newly Added Local Slots + Customizing Class Redefinition + Extensions o Changing the Class of an Instance + Modifying the Structure of an Instance + Initializing Newly Added Local Slots + Customizing the Change of Class of an Instance o Reinitializing an Instance + Customizing Reinitialization * Functions in the Programmer Interface ------------------------------------------------------------------------------- 28.1. Programmer Interface Concepts [change_begin] The Common Lisp Object System (CLOS) is an object-oriented extension to Common Lisp. It is based on generic functions, multiple inheritance, declarative method combination, and a meta-object protocol. The first two parts of this specification describe the standard Programmer Interface for the Common Lisp Object System. The first part, Programmer Interface Concepts, contains a description of the concepts of the Common Lisp Object System, and the second part, Functions in the Programmer Interface, contains a description of the functions and macros in the Common Lisp Object System Programmer Interface. The third part, The Common Lisp Object System Meta-Object Protocol, explains how the Common Lisp Object System can be customized. [The third part has not yet been approved by X3J13 for inclusion in the forthcoming Common Lisp standard and is not included in this book.-GLS] The fundamental objects of the Common Lisp Object System are classes, instances, generic functions, and methods. A class object determines the structure and behavior of a set of other objects, which are called its instances. Every Common Lisp object is an instance of a class. The class of an object determines the set of operations that can be performed on the object. A generic function is a function whose behavior depends on the classes or identities of the arguments supplied to it. A generic function object contains a set of methods, a lambda-list, a method combination type, and other information. The methods define the class-specific behavior and operations of the generic function; a method is said to specialize a generic function. When invoked, a generic function executes a subset of its methods based on the classes of its arguments. A generic function can be used in the same ways as an ordinary function in Common Lisp; in particular, a generic function can be used as an argument to funcall and apply and can be given a global or a local name. A method is an object that contains a method function, a sequence of parameter specializers that specify when the given method is applicable, and a sequence of qualifiers that is used by the method combination facility to distinguish among methods. Each required formal parameter of each method has an associated parameter specializer, and the method will be invoked only on arguments that satisfy its parameter specializers. The method combination facility controls the selection of methods, the order in which they are run, and the values that are returned by the generic function. The Common Lisp Object System offers a default method combination type and provides a facility for declaring new types of method combination. [change_end] ------------------------------------------------------------------------------- * Error Terminology * Classes o Defining Classes o Creating Instances of Classes o Slots o Accessing Slots * Inheritance o Inheritance of Methods o Inheritance of Slots and Slot Options o Inheritance of Class Options o Examples * Integrating Types and Classes * Determining the Class Precedence List o Topological Sorting o Examples * Generic Functions and Methods o Introduction to Generic Functions o Introduction to Methods o Agreement on Parameter Specializers and Qualifiers o Congruent Lambda-Lists for All Methods of a Generic Function o Keyword Arguments in Generic Functions and Methods * Method Selection and Combination o Determining the Effective Method o Standard Method Combination o Declarative Method Combination o Built-in Method Combination Types * Meta-objects o Metaclasses o Standard Metaclasses o Standard Meta-objects * Object Creation and Initialization o Initialization Arguments o Declaring the Validity of Initialization Arguments o Defaulting of Initialization Arguments o Rules for Initialization Arguments o Shared-Initialize o Initialize-Instance o Definitions of Make-Instance and Initialize-Instance * Redefining Classes o Modifying the Structure of Instances o Initializing Newly Added Local Slots o Customizing Class Redefinition o Extensions * Changing the Class of an Instance o Modifying the Structure of an Instance o Initializing Newly Added Local Slots o Customizing the Change of Class of an Instance * Reinitializing an Instance o Customizing Reinitialization ------------------------------------------------------------------------------- 28.1.1. Error Terminology [change_begin] The terminology used in this chapter to describe erroneous situations differs from the terminology used in the first edition. The new terminology involves situations; a situation is the evaluation of an expression in some specific context. For example, a situation might be the invocation of a function on arguments that fail to satisfy some specified constraints. In the specification of the Common Lisp Object System, the behavior of programs in all situations is described, and the options available to the implementor are defined. No implementation is allowed to extend the syntax or semantics of the Object System except as explicitly defined in the Object System specification. In particular, no implementation is allowed to extend the syntax of the Object System in such a way that ambiguity between the specified syntax of the Object System and those extensions is possible. ``When situation S occurs, an error is signaled.'' This terminology has the following meaning: o If this situation occurs, an error will be signaled in the interpreter and in code compiled under all compiler safety optimization levels. o Valid programs may rely on the fact that an error will be signaled in the interpreter and in code compiled under all compiler safety optimization levels. o Every implementation is required to detect such an error in the interpreter and in code compiled under all compiler safety optimization levels. ``When situation S occurs, an error should be signaled.'' This terminology has the following meaning: o If this situation occurs, an error will be signaled at least in the interpreter and in code compiled under the safest compiler safety optimization level. o Valid programs may not rely on the fact that an error will be signaled. o Every implementation is required to detect such an error at least in the interpreter and in code compiled under the safest compiler safety optimization level. o When an error is not signaled, the results are undefined (see below). ``When situation S occurs, the results are undefined.'' This terminology has the following meaning: o If this situation occurs, the results are unpredictable. The results may range from harmless to fatal. o Implementations are allowed to detect this situation and signal an error, but no implementation is required to detect the situation. o No valid program may depend on the effects of this situation, and all valid programs are required to treat the effects of this situation as unpredictable. ``When situation S occurs, the results are unspecified.'' This terminology has the following meaning: o The effects of this situation are not specified in the Object System, but the effects are harmless. o Implementations are allowed to specify the effects of this situation. o No portable program can depend on the effects of this situation, and all portable programs are required to treat the situation as unpredictable but harmless. ``The Common Lisp Object System may be extended to cover situation S.'' The meaning of this terminology is that an implementation is free to treat situation S in one of three ways: o When situation S occurs, an error is signaled at least in the interpreter and in code compiled under the safest compiler safety optimization level. o When situation S occurs, the results are undefined. o When situation S occurs, the results are defined and specified. In addition, this terminology has the following meaning: o No portable program can depend on the effects of this situation, and all portable programs are required to treat the situation as undefined. ``Implementations are free to extend the syntax S.'' This terminology has the following meaning: o Implementations are allowed to define unambiguous extensions to syntax S. o No portable program can depend on this extension, and all portable programs are required to treat the syntax as meaningless. The Common Lisp Object System specification may disallow certain extensions while allowing others. [change_end] ------------------------------------------------------------------------------- 28.1.2. Classes [change_begin] A class is an object that determines the structure and behavior of a set of other objects, which are called its instances. A class can inherit structure and behavior from other classes. A class whose definition refers to other classes for the purpose of inheriting from them is said to be a subclass of each of those classes. The classes that are designated for purposes of inheritance are said to be superclasses of the inheriting class. A class can have a name. The function class-name takes a class object and returns its name. The name of an anonymous class is nil. A symbol can name a class. The function find-class takes a symbol and returns the class that the symbol names. A class has a proper name if the name is a symbol and if the name of the class names that class. That is, a class C has the proper name S if S = (class-name C) and C = (find-class S). Notice that it is possible for (find-class ) = (find-class ) and . If C = (find-class S), we say that C is the class named S. A class is a direct superclass of a class if explicitly designates as a superclass in its definition. In this case, is a direct subclass of . A class is a superclass of a class if there exists a series of classes such that is a direct superclass of for 1 i < n. In this case, is a subclass of . A class is considered neither a superclass nor a subclass of itself. That is, if is a superclass of , then . The set of classes consisting of some given class C along with all of its superclasses is called ``C and its superclasses.'' Each class has a class precedence list, which is a total ordering on the set of the given class and its superclasses. The total ordering is expressed as a list ordered from most specific to least specific. The class precedence list is used in several ways. In general, more specific classes can shadow, or override, features that would otherwise be inherited from less specific classes. The method selection and combination process uses the class precedence list to order methods from most specific to least specific. When a class is defined, the order in which its direct superclasses are mentioned in the defining form is important. Each class has a local precedence order, which is a list consisting of the class followed by its direct superclasses in the order mentioned in the defining form. A class precedence list is always consistent with the local precedence order of each class in the list. The classes in each local precedence order appear within the class precedence list in the same order. If the local precedence orders are inconsistent with each other, no class precedence list can be constructed, and an error is signaled. The class precedence list and its computation is discussed in section 28.1.5. Classes are organized into a directed acyclic graph. There are two distinguished classes, named t and standard-object. The class named t has no superclasses. It is a superclass of every class except itself. The class named standard-object is an instance of the class standard-class and is a superclass of every class that is an instance of standard-class except itself. There is a mapping from the Common Lisp Object System class space into the Common Lisp type space. Many of the standard Common Lisp types have a corresponding class that has the same name as the type. Some Common Lisp types do not have a corresponding class. The integration of the type and class systems is discussed in section 28.1.4. Classes are represented by objects that are themselves instances of classes. The class of the class of an object is termed the metaclass of that object. When no misinterpretation is possible, the term metaclass will be used to refer to a class that has instances that are themselves classes. The metaclass determines the form of inheritance used by the classes that are its instances and the representation of the instances of those classes. The Common Lisp Object System provides a default metaclass, standard-class, that is appropriate for most programs. The meta-object protocol provides mechanisms for defining and using new metaclasses. Except where otherwise specified, all classes mentioned in this chapter are instances of the class standard-class, all generic functions are instances of the class standard-generic-function, and all methods are instances of the class standard-method. [change_end] ------------------------------------------------------------------------------- * Defining Classes * Creating Instances of Classes * Slots * Accessing Slots ------------------------------------------------------------------------------- 28.1.2.1. Defining Classes [change_begin] The macro defclass is used to define a new named class. The definition of a class includes the following: * The name of the new class. For newly defined classes this is a proper name. * The list of the direct superclasses of the new class. * A set of slot specifiers. Each slot specifier includes the name of the slot and zero or more slot options. A slot option pertains only to a single slot. If a class definition contains two slot specifiers with the same name, an error is signaled. * A set of class options. Each class option pertains to the class as a whole. The slot options and class options of the defclass form provide mechanisms for the following: * Supplying a default initial value form for a given slot. * Requesting that methods for generic functions be automatically generated for reading or writing slots. * Controlling whether a given slot is shared by instances of the class or whether each instance of the class has its own slot. * Supplying a set of initialization arguments and initialization argument defaults to be used in instance creation. * Indicating that the metaclass is to be other than the default. * Indicating the expected type for the value stored in the slot. * Indicating the documentation string for the slot. [change_end] ------------------------------------------------------------------------------- 28.1.2.2. Creating Instances of Classes [change_begin] The generic function make-instance creates and returns a new instance of a class. The Object System provides several mechanisms for specifying how a new instance is to be initialized. For example, it is possible to specify the initial values for slots in newly created instances either by giving arguments to make-instance or by providing default initial values. Further initialization activities can be performed by methods written for generic functions that are part of the initialization protocol. The complete initialization protocol is described in section 28.1.9. [change_end] ------------------------------------------------------------------------------- 28.1.2.3. Slots [change_begin] An object that has standard-class as its metaclass has zero or more named slots. The slots of an object are determined by the class of the object. Each slot can hold one value. The name of a slot is a symbol that is syntactically valid for use as a variable name. When a slot does not have a value, the slot is said to be unbound. When an unbound slot is read, the generic function slot-unbound is invoked. The system-supplied primary method for slot-unbound signals an error. The default initial value form for a slot is defined by the :initform slot option. When the :initform form is used to supply a value, it is evaluated in the lexical environment in which the defclass form was evaluated. The :initform along with the lexical environment in which the defclass form was evaluated is called a captured :initform. See section 28.1.9. A local slot is defined to be a slot that is visible to exactly one instance, namely the one in which the slot is allocated. A shared slot is defined to be a slot that is visible to more than one instance of a given class and its subclasses. A class is said to define a slot with a given name when the defclass form for that class contains a slot specifier with that name. Defining a local slot does not immediately create a slot; it causes a slot to be created each time an instance of the class is created. Defining a shared slot immediately creates a slot. The :allocation slot option to defclass controls the kind of slot that is defined. If the value of the :allocation slot option is :instance, a local slot is created. If the value of :allocation is :class, a shared slot is created. A slot is said to be accessible in an instance of a class if the slot is defined by the class of the instance or is inherited from a superclass of that class. At most one slot of a given name can be accessible in an instance. A shared slot defined by a class is accessible in all instances of that class. A detailed explanation of the inheritance of slots is given in section 28.1.3.2. [change_end] ------------------------------------------------------------------------------- 28.1.2.4. Accessing Slots [change_begin] Slots can be accessed in two ways: by use of the primitive function slot-value and by use of generic functions generated by the defclass form. The function slot-value can be used with any slot name specified in the defclass form to access a specific slot accessible in an instance of the given class. The macro defclass provides syntax for generating methods to read and write slots. If a reader is requested, a method is automatically generated for reading the value of the slot, but no method for storing a value into it is generated. If a writer is requested, a method is automatically generated for storing a value into the slot, but no method for reading its value is generated. If an accessor is requested, a method for reading the value of the slot and a method for storing a value into the slot are automatically generated. Reader and writer methods are implemented using slot-value. When a reader or writer is specified for a slot, the name of the generic function to which the generated method belongs is directly specified. If the name specified for the writer option is the symbol name, the name of the generic function for writing the slot is the symbol name, and the generic function takes two arguments: the new value and the instance, in that order. If the name specified for the accessor option is the symbol name, the name of the generic function for reading the slot is the symbol name, and the name of the generic function for writing the slot is the list (setf name). A generic function created or modified by supplying reader, writer, or accessor slot options can be treated exactly as an ordinary generic function. Note that slot-value can be used to read or write the value of a slot whether or not reader or writer methods exist for that slot. When slot-value is used, no reader or writer methods are invoked. The macro with-slots can be used to establish a lexical environment in which specified slots are lexically available as if they were variables. The macro with-slots invokes the function slot-value to access the specified slots. The macro with-accessors can be used to establish a lexical environment in which specified slots are lexically available through their accessors as if they were variables. The macro with-accessors invokes the appropriate accessors to access the specified slots. Any accessors specified by with-accessors must already have been defined before they are used. [change_end] ------------------------------------------------------------------------------- 28.1.3. Inheritance [change_begin] A class can inherit methods, slots, and some defclass options from its superclasses. The following sections describe the inheritance of methods, the inheritance of slots and slot options, and the inheritance of class options. [change_end] ------------------------------------------------------------------------------- * Inheritance of Methods * Inheritance of Slots and Slot Options * Inheritance of Class Options * Examples ------------------------------------------------------------------------------- 28.1.3.1. Inheritance of Methods [change_begin] A subclass inherits methods in the sense that any method applicable to all instances of a class is also applicable to all instances of any subclass of that class. The inheritance of methods acts the same way regardless of whether the method was created by using one of the method-defining forms or by using one of the defclass options that causes methods to be generated automatically. The inheritance of methods is described in detail in section 28.1.7. [change_end] ------------------------------------------------------------------------------- 28.1.3.2. Inheritance of Slots and Slot Options [change_begin] The set of names of all slots accessible in an instance of a class C is the union of the sets of names of slots defined by C and its superclasses. The structure of an instance is the set of names of local slots in that instance. In the simplest case, only one class among C and its superclasses defines a slot with a given slot name. If a slot is defined by a superclass of C, the slot is said to be inherited. The characteristics of the slot are determined by the slot specifier of the defining class. Consider the defining class for a slot S. If the value of the :allocation slot option is :instance, then S is a local slot and each instance of C has its own slot named S that stores its own value. If the value of the :allocation slot option is :class, then S is a shared slot, the class that defined S stores the value, and all instances of C can access that single slot. If the :allocation slot option is omitted, :instance is used. In general, more than one class among C and its superclasses can define a slot with a given name. In such cases, only one slot with the given name is accessible in an instance of C, and the characteristics of that slot are a combination of the several slot specifiers, computed as follows: * All the slot specifiers for a given slot name are ordered from most specific to least specific, according to the order in C's class precedence list of the classes that define them. All references to the specificity of slot specifiers immediately following refer to this ordering. * The allocation of a slot is controlled by the most specific slot specifier. If the most specific slot specifier does not contain an :allocation slot option, :instance is used. Less specific slot specifiers do not affect the allocation. * The default initial value form for a slot is the value of the :initform slot option in the most specific slot specifier that contains one. If no slot specifier contains an :initform slot option, the slot has no default initial value form. * The contents of a slot will always be of type (and ... ) where are the values of the :type slot options contained in all of the slot specifiers. If no slot specifier contains the :type slot option, the contents of the slot will always be of type t. The result of attempting to store in a slot a value that does not satisfy the type of the slot is undefined. * The set of initialization arguments that initialize a given slot is the union of the initialization arguments declared in the :initarg slot options in all the slot specifiers. * The documentation string for a slot is the value of the :documentation slot option in the most specific slot specifier that contains one. If no slot specifier contains a :documentation slot option, the slot has no documentation string. A consequence of the allocation rule is that a shared slot can be shadowed. For example, if a class defines a slot named S whose value for the :allocation slot option is :class, that slot is accessible in instances of and all of its subclasses. However, if is a subclass of and also defines a slot named S, 's slot is not shared by instances of and its subclasses. When a class defines a shared slot, any subclass of will share this single slot unless the defclass form for specifies a slot of the same name or there is a superclass of that precedes in the class precedence list of that defines a slot of the same name. A consequence of the type rule is that the value of a slot satisfies the type constraint of each slot specifier that contributes to that slot. Because the result of attempting to store in a slot a value that does not satisfy the type constraint for the slot is undefined, the value in a slot might fail to satisfy its type constraint. The :reader, :writer, and :accessor slot options create methods rather than define the characteristics of a slot. Reader and writer methods are inherited in the sense described in section 28.1.3.1. Methods that access slots use only the name of the slot and the type of the slot's value. Suppose a superclass provides a method that expects to access a shared slot of a given name, and a subclass defines a local slot with the same name. If the method provided by the superclass is used on an instance of the subclass, the method accesses the local slot. [change_end] ------------------------------------------------------------------------------- 28.1.3.3. Inheritance of Class Options [change_begin] The :default-initargs class option is inherited. The set of defaulted initialization arguments for a class is the union of the sets of initialization arguments specified in the :default-initargs class options of the class and its superclasses. When more than one default initial value form is supplied for a given initialization argument, the default initial value form that is used is the one supplied by the class that is most specific according to the class precedence list. If a given :default-initargs class option specifies an initialization argument of the same name more than once, an error is signaled. [change_end] ------------------------------------------------------------------------------- 28.1.3.4. Examples [change_begin] (defclass C1 () ((S1 :initform 5.4 :type number) (S2 :allocation :class))) (defclass C2 (C1) ((S1 :initform 5 :type integer) (S2 :allocation :instance) (S3 :accessor C2-S3))) Instances of the class C1 have a local slot named S1, whose default initial value is 5.4 and whose value should always be a number. The class C1 also has a shared slot named S2. There is a local slot named S1 in instances of C2. The default initial value of S1 is 5. The value of S1 will be of type (and integer number). There are also local slots named S2 and S3 in instances of C2. The class C2 has a method for C2-S3 for reading the value of slot S3; there is also a method for (setf C2-S3) that writes the value of S3. [change_end] ------------------------------------------------------------------------------- 28.1.4. Integrating Types and Classes [change_begin] The Common Lisp Object System maps the space of classes into the Common Lisp type space. Every class that has a proper name has a corresponding type with the same name. The proper name of every class is a valid type specifier. In addition, every class object is a valid type specifier. Thus the expression (typep object class) evaluates to true if the class of object is class itself or a subclass of class. The evaluation of the expression (subtypep class1 class2) returns the values t and t if class1 is a subclass of class2 or if they are the same class; otherwise it returns the values nil and t. If I is an instance of some class C named S and C is an instance of standard-class, the evaluation of the expression (type-of I) will return S if S is the proper name of C; if S is not the proper name of C, the expression (type-of I) will return C. Because the names of classes and class objects are type specifiers, they may be used in the special form the and in type declarations. Many but not all of the predefined Common Lisp type specifiers have a corresponding class with the same proper name as the type. These type specifiers are listed in table 28-1. For example, the type array has a corresponding class named array. No type specifier that is a list, such as (vector double-float 100), has a corresponding class. The form deftype does not create any classes. Each class that corresponds to a predefined Common Lisp type specifier can be implemented in one of three ways, at the discretion of each implementation. It can be a standard class (of the kind defined by defclass), a structure class (defined by defstruct), or a built-in class (implemented in a special, non-extensible way). A built-in class is one whose instances have restricted capabilities or special representations. Attempting to use defclass to define subclasses of a built-in class signals an error. Calling make-instance to create an instance of a built-in class signals an error. Calling slot-value on an instance of a built-in class signals an error. Redefining a built-in class or using change-class to change the class of an instance to or from a built-in class signals an error. However, built-in classes can be used as parameter specializers in methods. It is possible to determine whether a class is a built-in class by checking the metaclass. A standard class is an instance of standard-class, a built-in class is an instance of built-in-class, and a structure class is an instance of structure-class. Each structure type created by defstruct without using the :type option has a corresponding class. This class is an instance of structure-class. The :include option of defstruct creates a direct subclass of the class that corresponds to the included structure. The purpose of specifying that many of the standard Common Lisp type specifiers have a corresponding class is to enable users to write methods that discriminate on these types. Method selection requires that a class precedence list can be determined for each class. The hierarchical relationships among the Common Lisp type specifiers are mirrored by relationships among the classes corresponding to those types. The existing type hierarchy is used for determining the class precedence list for each class that corresponds to a predefined Common Lisp type. In some cases, the first edition did not specify a local precedence order for two supertypes of a given type specifier. For example, null is a subtype of both symbol and list, but the first edition did not specify whether symbol is more specific or less specific than list. The CLOS specification defines those relationships for all such classes. Table 28-1 lists the set of classes required by the Object System that correspond to predefined Common Lisp type specifiers. The superclasses of each such class are presented in order from most specific to most general, thereby defining the class precedence list for the class. The local precedence order for each class that corresponds to a Common Lisp type specifier can be derived from this table. Individual implementations may be extended to define other type specifiers to have a corresponding class. Individual implementations can be extended to add other subclass relationships and to add other elements to the class precedence lists in the above table as long as they do not violate the type relationships and disjointness requirements specified in section 2.15. A standard class defined with no direct superclasses is guaranteed to be disjoint from all of the classes in the table, except for the class named t. [At this point the original CLOS report specified that certain Common Lisp types were to appear in table 28-1 if and only if X3J13 voted to make them disjoint from cons, symbol, array, number, and character. X3J13 voted to do so in June 1988 (DATA-TYPES-HIERARCHY-UNDERSPECIFIED) . I have added these types and their class precedence lists to the table; the new types are indicated by asterisks.-GLS] ---------------------------------------------------------------- Table 28-1: Class Precedence Lists for Predefined Types Predefined Class Precedence List Common Lisp Type for Corresponding Class ============================================================== array (array t) bit-vector (bit-vector vector array sequence t) character (character t) complex (complex number t) cons (cons list sequence t) float (float number t) function * (function t) hash-table * (hash-table t) integer (integer rational number t) list (list sequence t) null (null symbol list sequence t) number (number t) package * (package t) pathname * (pathname t) random-state * (random-state t) ratio (ratio rational number t) rational (rational number t) readtable * (readtable t) sequence (sequence t) stream * (stream t) string (string vector array sequence t) symbol (symbol t) t (t) vector (vector array sequence t) ============================================================== [An asterisk indicates a type added to this table as a consequence of a portion of the CLOS specification that was conditional on X3J13 voting to make that type disjoint from certain other built-in types (DATA-TYPES-HIERARCHY-UNDERSPECIFIED).---GLS] ---------------------------------------------------------------- [change_end] ------------------------------------------------------------------------------- 28.1.5. Determining the Class Precedence List [change_begin] The defclass form for a class provides a total ordering on that class and its direct superclasses. This ordering is called the local precedence order. It is an ordered list of the class and its direct superclasses. The class precedence list for a class C is a total ordering on C and its superclasses that is consistent with the local precedence orders for C and its superclasses. A class precedes its direct superclasses, and a direct superclass precedes all other direct superclasses specified to its right in the superclasses list of the defclass form. For every class C, define where are the direct superclasses of C in the order in which they are mentioned in the defclass form. These ordered pairs generate the total ordering on the class C and its direct superclasses. Let be the set of C and its superclasses. Let R be The set R may or may not generate a partial ordering, depending on whether the , , are consistent; it is assumed that they are consistent and that R generates a partial ordering. When the are not consistent, it is said that R is inconsistent. To compute the class precedence list for C, topologically sort the elements of with respect to the partial ordering generated by R. When the topological sort must select a class from a set of two or more classes, none of which are preceded by other classes with respect to R, the class selected is chosen deterministically, as described below. If R is inconsistent, an error is signaled. [change_end] ------------------------------------------------------------------------------- * Topological Sorting * Examples ------------------------------------------------------------------------------- 28.1.5.1. Topological Sorting [change_begin] Topological sorting proceeds by finding a class C in such that no other class precedes that element according to the elements in R. The class C is placed first in the result. Remove C from , and remove all pairs of the form (C, D), , from R. Repeat the process, adding classes with no predecessors to the end of the result. Stop when no element can be found that has no predecessor. If is not empty and the process has stopped, the set R is inconsistent. If every class in the finite set of classes is preceded by another, then R contains a loop. That is, there is a chain of classes such that precedes , 1 <= i < n, and precedes . Sometimes there are several classes from with no predecessors. In this case select the one that has a direct subclass rightmost in the class precedence list computed so far. If there is no such candidate class, R does not generate a partial ordering - the , , are inconsistent. In more precise terms, let , m >= 2, be the classes from with no predecessors. Let , n >= 1, be the class precedence list constructed so far. is the most specific class, and is the least specific. Let 1 <= j <= n be the largest number such that there exists an i where 1 <= i <= m and is a direct superclass of ; is placed next. The effect of this rule for selecting from a set of classes with no predecessors is that classes in a simple superclass chain are adjacent in the class precedence list and that classes in each relatively separated subgraph are adjacent in the class precedence list. For example, let and be subgraphs whose only element in common is the class J. Suppose that no superclass of J appears in either or . Let be the bottom of ; and let be the bottom of . Suppose C is a class whose direct superclasses are and in that order; then the class precedence list for C will start with C and will be followed by all classes in except J. All the classes of will be next. The class J and its superclasses will appear last. [change_end] ------------------------------------------------------------------------------- 28.1.5.2. Examples [change_begin] This example determines a class precedence list for the class pie. The following classes are defined: (defclass pie (apple cinnamon) ()) (defclass apple (fruit) ()) (defclass cinnamon (spice) ()) (defclass fruit (food) ()) (defclass spice (food) ()) (defclass food () ()) The set S = {pie, apple, cinnamon, fruit, spice, food, standard-object, t}. The set R = {(pie, apple), (apple, cinnamon), (cinnamon, standard-object), (apple, fruit), (fruit, standard-object), (cinnamon, spice), (spice, standard-object), (fruit, food), (food, standard-object), (spice, food), (standard-object, t)} [The original CLOS specification [5,6] contained a minor error in this example: the pairs (cinnamon, standard-object), (fruit, standard-object), and (spice, standard-object) were inadvertently omitted from R in the preceding paragraph. It is important to understand that defclass implicitly appends the class standard-object to the list of superclasses when the metaclass is standard-class (the normal situation), in order to insure that standard-object will be a superclass of every instance of standard-class except standard-object itself (see section 28.1.2). is then generated from this augmented list of superclasses; this is where the extra pairs come from. I have corrected the example by adding these pairs as appropriate throughout the example. The final result, the class precedence list for pie, is unchanged.-GLS] The class pie is not preceded by anything, so it comes first; the result so far is (pie). Remove pie from S and pairs mentioning pie from R to get S = {apple, cinnamon, fruit, spice, food, standard-object, t} and R = {(apple, cinnamon), (cinnamon, standard-object), (apple, fruit), (fruit, standard-object), (cinnamon, spice), (spice, standard-object), (fruit, food), (food, standard-object), (spice, food), (standard-object, t)}. The class apple is not preceded by anything, so it is next; the result is (pie apple). Removing apple and the relevant pairs results in S = {cinnamon, fruit, spice, food, standard-object, t} and R = {(cinnamon, standard-object), (fruit, standard-object), (cinnamon, spice), (spice, standard-object), (fruit, food), (food, standard-object), (spice, food), (standard-object, t)}. The classes cinnamon and fruit are not preceded by anything, so the one with a direct subclass rightmost in the class precedence list computed so far goes next. The class apple is a direct subclass of fruit, and the class pie is a direct subclass of cinnamon. Because apple appears to the right of pie in the precedence list, fruit goes next, and the result so far is (pie apple fruit). S = {cinnamon, spice, food, standard-object, t} and R = {(cinnamon, standard-object), (cinnamon, spice), (spice, standard-object), (food, standard-object), (spice, food), (standard-object, t)}. The class cinnamon is next, giving the result so far as (pie apple fruit cinnamon). At this point S = {spice, food, standard-object, t} and R = {(spice, standard-object), (food, standard-object), (spice, food), (standard-object, t)}. The classes spice, food, standard-object, and t are then added in that order, and the final class precedence list for pie is (pie apple fruit cinnamon spice food standard-object t) It is possible to write a set of class definitions that cannot be ordered. For example: (defclass new-class (fruit apple) ()) (defclass apple (fruit) ()) The class fruit must precede apple because the local ordering of superclasses must be preserved. The class apple must precede fruit because a class always precedes its own superclasses. When this situation occurs, an error is signaled when the system tries to compute the class precedence list. The following might appear to be a conflicting set of definitions: (defclass pie (apple cinnamon) ()) (defclass pastry (cinnamon apple) ()) (defclass apple () ()) (defclass cinnamon () ()) The class precedence list for pie is (pie apple cinnamon standard-object t) The class precedence list for pastry is (pastry cinnamon apple standard-object t) It is not a problem for apple to precede cinnamon in the ordering of the superclasses of pie but not in the ordering for pastry. However, it is not possible to build a new class that has both pie and pastry as superclasses. [change_end] ------------------------------------------------------------------------------- 28.1.6. Generic Functions and Methods [change_begin] A generic function is a function whose behavior depends on the classes or identities of the arguments supplied to it. The methods define the class-specific behavior and operations of the generic function. The following sections describe generic functions and methods. [change_end] ------------------------------------------------------------------------------- * Introduction to Generic Functions * Introduction to Methods * Agreement on Parameter Specializers and Qualifiers * Congruent Lambda-Lists for All Methods of a Generic Function * Keyword Arguments in Generic Functions and Methods ------------------------------------------------------------------------------- 28.1.6.1. Introduction to Generic Functions [change_begin] A generic function object contains a set of methods, a lambda-list, a method combination type, and other information. Like an ordinary Lisp function, a generic function takes arguments, performs a series of operations, and perhaps returns useful values. An ordinary function has a single body of code that is always executed when the function is called. A generic function has a set of bodies of code of which a subset is selected for execution. The selected bodies of code and the manner of their combination are determined by the classes or identities of one or more of the arguments to the generic function and by its method combination type. Ordinary functions and generic functions are called with identical function-call syntax. Generic functions are true functions that can be passed as arguments, returned as values, used as the first argument to funcall and apply, and otherwise used in all the ways an ordinary function may be used. A name can be given to an ordinary function in one of two ways: a global name can be given to a function using the defun construct; a local name can be given using the flet or labels special forms. A generic function can be given a global name using the defmethod or defgeneric construct. A generic function can be given a local name using the generic-flet, generic-labels, or with-added-methods special forms. The name of a generic function, like the name of an ordinary function, can be either a symbol or a two-element list whose first element is setf and whose second element is a symbol. This is true for both local and global names. The generic-flet special form creates new local generic functions using the set of methods specified by the method definitions in the generic-flet form. The scoping of generic function names within a generic-flet form is the same as for flet. The generic-labels special form creates a set of new mutually recursive local generic functions using the set of methods specified by the method definitions in the generic-labels form. The scoping of generic function names within a generic-labels form is the same as for labels. The with-added-methods special form creates new local generic functions by adding the set of methods specified by the method definitions with a given name in the with-added-methods form to copies of the methods of the lexically visible generic function of the same name. If there is a lexically visible ordinary function of the same name as one of the specified generic functions, that function becomes the method function of the default method for the new generic function of that name. The generic-function macro creates an anonymous generic function with the set of methods specified by the method definitions that appear in the generic-function form. When a defgeneric form is evaluated, one of three actions is taken: * If a generic function of the given name already exists, the existing generic function object is modified. Methods specified by the current defgeneric form are added, and any methods in the existing generic function that were defined by a previous defgeneric form are removed. Methods added by the current defgeneric form might replace methods defined by defmethod or defclass. No other methods in the generic function are affected or replaced. * If the given name names a non-generic function, a macro, or a special form, an error is signaled. * Otherwise a generic function is created with the methods specified by the method definitions in the defgeneric form. Some forms specify the options of a generic function, such as the type of method combination it uses or its argument precedence order. They will be referred to as ``forms that specify generic function options.'' These forms are defgeneric, generic-function, generic-flet, generic-labels, and with-added-methods. Some forms define methods for a generic function. They will be referred to as ``method-defining forms.'' These forms are defgeneric, defmethod, generic-function, generic-flet, generic-labels, with-added-methods, and defclass. Note that all the method-defining forms except defclass and defmethod are also forms that specify generic function options. [change_end] ------------------------------------------------------------------------------- 28.1.6.2. Introduction to Methods [change_begin] A method object contains a method function, a sequence of parameter specializers that specify when the given method is applicable, a lambda-list, and a sequence of qualifiers that are used by the method combination facility to distinguish among methods. A method object is not a function and cannot be invoked as a function. Various mechanisms in the Object System take a method object and invoke its method function, as is the case when a generic function is invoked. When this occurs it is said that the method is invoked or called. A method-defining form contains the code that is to be run when the arguments to the generic function cause the method that it defines to be invoked. When a method-defining form is evaluated, a method object is created and one of four actions is taken: * If a generic function of the given name already exists and if a method object already exists that agrees with the new one on parameter specializers and qualifiers, the new method object replaces the old one. For a definition of one method agreeing with another on parameter specializers and qualifiers, see section 28.1.6.3. * If a generic function of the given name already exists and if there is no method object that agrees with the new one on parameter specializers and qualifiers, the existing generic function object is modified to contain the new method object. * If the given name names a non-generic function, a macro, or a special form, an error is signaled. * Otherwise a generic function is created with the methods specified by the method-defining form. If the lambda-list of a new method is not congruent with the lambda-list of the generic function, an error is signaled. If a method-defining form that cannot specify generic function options creates a new generic function, a lambda-list for that generic function is derived from the lambda-lists of the methods in the method-defining form in such a way as to be congruent with them. For a discussion of congruence, see section 28.1.6.4. Each method has a specialized lambda-list, which determines when that method can be applied. A specialized lambda-list is like an ordinary lambda-list except that a specialized parameter may occur instead of the name of a required parameter. A specialized parameter is a list (variable-name parameter-specializer-name), where parameter-specializer-name is either a name that names a class or a list (eql form). A parameter specializer name denotes a parameter specializer as follows: * A name that names a class denotes that class. * The list (eql form) denotes the type specifier (eql object), where object is the result of evaluating form. The form form is evaluated in the lexical environment in which the method-defining form is evaluated. Note that form is evaluated only once, at the time the method is defined, not each time the generic function is called. Parameter specializer names are used in macros intended as the user-level interface (defmethod), while parameter specializers are used in the functional interface. [It is very important to understand clearly the distinction made in the preceding paragraph. A parameter specializer name has the form of a type specifier but is semantically quite different from a type specifier: a parameter specializer name of the form (eql form) is not a type specifier, for it contains a form to be evaluated. Type specifiers never contain forms to be evaluated. All parameter specializers (as opposed to parameter specializer names) are valid type specifiers, but not all type specifiers are valid names and treat them as specifications for constructing certain type specifiers (parameter specializers) that may then be used with such functions as find-method.-GLS] Only required parameters may be specialized, and there must be a parameter specializer for each required parameter. For notational simplicity, if some required parameter in a specialized lambda-list in a method-defining form is simply a variable name, its parameter specializer defaults to the class named t. Given a generic function and a set of arguments, an applicable method is a method for that generic function whose parameter specializers are satisfied by their corresponding arguments. The following definition specifies what it means for a method to be applicable and for an argument to satisfy a parameter specializer. Let be the required arguments to a generic function in order. Let be the parameter specializers corresponding to the required parameters of the method M in order. The method M is applicable when each satisfies . If is a class, and if is an instance of a class C, then it is said that satisfies when or when C is a subclass of . If is of the form (eql object), then it is said that satisfies when the function eql applied to and object is true. Because a parameter specializer is a type specifier, the function typep can be used during method selection to determine whether an argument satisfies a parameter specializer. In general a parameter specializer cannot be a type specifier list, such as (vector single-float). The only parameter specializer that can be a list is (eql object). This requires that Common Lisp define the type specifier eql as if the following were evaluated: (deftype eql (object) `(member ,object)) [See section 4.3.-GLS] A method all of whose parameter specializers are the class named t is called a default method; it is always applicable but may be shadowed by a more specific method. Methods can have qualifiers, which give the method combination procedure a way to distinguish among methods. A method that has one or more qualifiers is called a qualified method. A method with no qualifiers is called an unqualified method. A qualifier is any object other than a list, that is, any non-nil atom. The qualifiers defined by standard method combination and by the built-in method combination types are symbols. In this specification, the terms primary method and auxiliary method are used to partition methods within a method combination type according to their intended use. In standard method combination, primary methods are unqualified methods, and auxiliary methods are methods with a single qualifier that is one of :around, :before, or :after. When a method combination type is defined using the short form of define-method-combination, primary methods are methods qualified with the name of the type of method combination, and auxiliary methods have the qualifier :around. Thus the terms primary method and auxiliary method have only a relative definition within a given method combination type. [change_end] ------------------------------------------------------------------------------- 28.1.6.3. Agreement on Parameter Specializers and Qualifiers [change_begin] Two methods are said to agree with each other on parameter specializers and qualifiers if the following conditions hold: * Both methods have the same number of required parameters. Suppose the parameter specializers of the two methods are and . * For each 1 <= i <= n, agrees with . The parameter specializer agrees with if and are the same class or if , , and (eql ). Otherwise and do not agree. * The lists of qualifiers of both methods contain the same non-nil atoms in the same order. That is, the lists are equal. [change_end] ------------------------------------------------------------------------------- 28.1.6.4. Congruent Lambda-Lists for All Methods of a Generic Function [change_begin] These rules define the congruence of a set of lambda-lists, including the lambda-list of each method for a given generic function and the lambda-list specified for the generic function itself, if given. * Each lambda-list must have the same number of required parameters. * Each lambda-list must have the same number of optional parameters. Each method can supply its own default for an optional parameter. * If any lambda-list mentions &rest or &key, each lambda-list must mention one or both of them. * If the generic function lambda-list mentions &key, each method must accept all of the keyword names mentioned after &key, either by accepting them explicitly, by specifying &allow-other-keys, or by specifying &rest but not &key. Each method can accept additional keyword arguments of its own. The checking of the validity of keyword names is done in the generic function, not in each method. A method is invoked as if the keyword argument pair whose keyword is :allow-other-keys and whose value is t were supplied, though no such argument pair will be passed. * The use of &allow-other-keys need not be consistent across lambda-lists. If &allow-other-keys is mentioned in the lambda-list of any applicable method or of the generic function, any keyword arguments may be mentioned in the call to the generic function. * The use of &aux need not be consistent across methods. If a method-defining form that cannot specify generic function options creates a generic function, and if the lambda-list for the method mentions keyword arguments, the lambda-list of the generic function will mention &key (but no keyword arguments). [change_end] ------------------------------------------------------------------------------- 28.1.6.5. Keyword Arguments in Generic Functions and Methods [change_begin] When a generic function or any of its methods mentions &key in a lambda-list, the specific set of keyword arguments accepted by the generic function varies according to the applicable methods. The set of keyword arguments accepted by the generic function for a particular call is the union of the keyword arguments accepted by all applicable methods and the keyword arguments mentioned after &key in the generic function definition, if any. A method that has &rest but not &key does not affect the set of acceptable keyword arguments. If the lambda-list of any applicable method or of the generic function definition contains &allow-other-keys, all keyword arguments are accepted by the generic function. The lambda-list congruence rules require that each method accept all of the keyword arguments mentioned after &key in the generic function definition, by accepting them explicitly, by specifying &allow-other-keys, or by specifying &rest but not &key. Each method can accept additional keyword arguments of its own, in addition to the keyword arguments mentioned in the generic function definition. If a generic function is passed a keyword argument that no applicable method accepts, an error is signaled. For example, suppose there are two methods defined for width as follows: (defmethod width ((c character-class) &key font) ...) (defmethod width ((p picture-class) &key pixel-size) ...) Assume that there are no other methods and no generic function definition for width. The evaluation of the following form will signal an error because the keyword argument :pixel-size is not accepted by the applicable method. (width (make-instance 'character-class :char #\Q) :font 'baskerville :pixel-size 10) The evaluation of the following form will signal an error. (width (make-instance 'picture-class :glyph (glyph #\Q)) :font 'baskerville :pixel-size 10) The evaluation of the following form will not signal an error if the class named character-picture-class is a subclass of both picture-class and character-class. (width (make-instance 'character-picture-class :char #\Q) :font 'baskerville :pixel-size 10) [change_end] ------------------------------------------------------------------------------- 28.1.7. Method Selection and Combination [change_begin] When a generic function is called with particular arguments, it must determine the code to execute. This code is called the effective method for those arguments. The effective method is a combination of the applicable methods in the generic function. A combination of methods is a Lisp expression that contains calls to some or all of the methods. If a generic function is called and no methods apply, the generic function no-applicable-method is invoked. When the effective method has been determined, it is invoked with the same arguments that were passed to the generic function. Whatever values it returns are returned as the values of the generic function. [change_end] ------------------------------------------------------------------------------- * Determining the Effective Method * Standard Method Combination * Declarative Method Combination * Built-in Method Combination Types ------------------------------------------------------------------------------- 28.1.7.1. Determining the Effective Method [change_begin] The effective method for a set of arguments is determined by the following three-step procedure: 1. Select the applicable methods. 2. Sort the applicable methods by precedence order, putting the most specific method first. 3. Apply method combination to the sorted list of applicable methods, producing the effective method. Selecting the Applicable Methods. This step is described in section 28.1.6.2. Sorting the Applicable Methods by Precedence Order. To compare the precedence of two methods, their parameter specializers are examined in order. The default examination order is from left to right, but an alternative order may be specified by the :argument-precedence-order option to defgeneric or to any of the other forms that specify generic function options. The corresponding parameter specializers from each method are compared. When a pair of parameter specializers are equal, the next pair are compared for equality. If all corresponding parameter specializers are equal, the two methods must have different qualifiers; in this case, either method can be selected to precede the other. If some corresponding parameter specializers are not equal, the first pair of parameter specializers that are not equal determines the precedence. If both parameter specializers are classes, the more specific of the two methods is the method whose parameter specializer appears earlier in the class precedence list of the corresponding argument. Because of the way in which the set of applicable methods is chosen, the parameter specializers are guaranteed to be present in the class precedence list of the class of the argument. If just one parameter specializer is (eql object), the method with that parameter specializer precedes the other method. If both parameter specializers are eql forms, the specializers must be the same (otherwise the two methods would not both have been applicable to this argument). The resulting list of applicable methods has the most specific method first and the least specific method last. Applying Method Combination to the Sorted List of Applicable Methods. In the simple case-if standard method combination is used and all applicable methods are primary methods-the effective method is the most specific method. That method can call the next most specific method by using the function call-next-method. The method that call-next-method will call is referred to as the next method. The predicate next-method-p tests whether a next method exists. If call-next-method is called and there is no next most specific method, the generic function no-next-method is invoked. In general, the effective method is some combination of the applicable methods. It is defined by a Lisp form that contains calls to some or all of the applicable methods, returns the value or values that will be returned as the value or values of the generic function, and optionally makes some of the methods accessible by means of call-next-method. This Lisp form is the body of the effective method; it is augmented with an appropriate lambda-list to make it a function. The role of each method in the effective method is determined by its method qualifiers and the specificity of the method. A qualifier serves to mark a method, and the meaning of a qualifier is determined by the way that these marks are used by this step of the procedure. If an applicable method has an unrecognized qualifier, this step signals an error and does not include that method in the effective method. When standard method combination is used together with qualified methods, the effective method is produced as described in section 28.1.7.2. Another type of method combination can be specified by using the :method-combination option of defgeneric or of any of the other forms that specify generic function options. In this way this step of the procedure can be customized. New types of method combination can be defined by using the define-method-combination macro. The meta-object level also offers a mechanism for defining new types of method combination. The generic function compute-effective-method receives as arguments the generic function, the method combination object, and the sorted list of applicable methods. It returns the Lisp form that defines the effective method. A method for compute-effective-method can be defined directly by using defmethod or indirectly by using define-method-combination. A method combination object is an object that encapsulates the method combination type and options specified by the :method-combination option to forms that specify generic function options. ------------------------------------------------------------------------------- Implementation note: In the simplest implementation, the generic function would compute the effective method each time it was called. In practice, this will be too inefficient for some implementations. Instead, these implementations might employ a variety of optimizations of the three-step procedure. Some illustrative examples of such optimizations are the following: * Use a hash table keyed by the class of the arguments to store the effective method. * Compile the effective method and save the resulting compiled function in a table. * Recognize the Lisp form as an instance of a pattern of control structure and substitute a closure that implements that structure. * Examine the parameter specializers of all methods for the generic function and enumerate all possible effective methods. Combine the effective methods, together with code to select from among them, into a single function and compile that function. Call that function whenever the generic function is called. ------------------------------------------------------------------------------- [change_end] ------------------------------------------------------------------------------- 28.1.7.2. Standard Method Combination [change_begin] Standard method combination is supported by the class standard-generic-function. It is used if no other type of method combination is specified or if the built-in method combination type standard is specified. Primary methods define the main action of the effective method, while auxiliary methods modify that action in one of three ways. A primary method has no method qualifiers. An auxiliary method is a method whose method qualifier is :before, :after, or :around. Standard method combination allows no more than one qualifier per method; if a method definition specifies more than one qualifier per method, an error is signaled. * A :before method has the keyword :before as its only qualifier. A :before method specifies code that is to be run before any primary method. * An :after method has the keyword :after as its only qualifier. An :after method specifies code that is to be run after primary methods. * An :around method has the keyword :around as its only qualifier. An :around method specifies code that is to be run instead of other applicable methods but that is able to cause some of them to be run. The semantics of standard method combination are as follows: * If there are any :around methods, the most specific :around method is called. It supplies the value or values of the generic function. * Inside the body of an :around method, call-next-method can be used to call the next method. When the next method returns, the :around method can execute more code, perhaps based on the returned value or values. The generic function no-next-method is invoked if call-next-method is used and there is no applicable method to call. The function next-method-p may be used to determine whether a next method exists. * If an :around method invokes call-next-method, the next most specific :around method is called, if one is applicable. If there are no :around methods or if call-next-method is called by the least specific :around method, the other methods are called as follows: o All the :before methods are called, in most-specific-first order. Their values are ignored. An error is signaled if call-next-method is used in a :before method. o The most specific primary method is called. Inside the body of a primary method, call-next-method may be used to call the next most specific primary method. When that method returns, the previous primary method can execute more code, perhaps based on the returned value or values. The generic function no-next-method is invoked if call-next-method is used and there are no more applicable primary methods. The function next-method-p may be used to determine whether a next method exists. If call-next-method is not used, only the most specific primary method is called. o All the :after methods are called in most-specific-last order. Their values are ignored. An error is signaled if call-next-method is used in an :after method. * If no :around methods were invoked, the most specific primary method supplies the value or values returned by the generic function. The value or values returned by the invocation of call-next-method in the least specific :around method are those returned by the most specific primary method. In standard method combination, if there is an applicable method but no applicable primary method, an error is signaled. The :before methods are run in most-specific-first order and the :after methods are run in least-specific-first order. The design rationale for this difference can be illustrated with an example. Suppose class modifies the behavior of its superclass, , by adding :before and :after methods. Whether the behavior of the class is defined directly by methods on or is inherited from its superclasses does not affect the relative order of invocation of methods on instances of the class . Class 's :before method runs before all of class 's methods. Class 's :after method runs after all of class 's methods. By contrast, all :around methods run before any other methods run. Thus a less specific :around method runs before a more specific primary method. If only primary methods are used and if call-next-method is not used, only the most specific method is invoked; that is, more specific methods shadow more general ones. [change_end] ------------------------------------------------------------------------------- 28.1.7.3. Declarative Method Combination [change_begin] The macro define-method-combination defines new forms of method combination. It provides a mechanism for customizing the production of the effective method. The default procedure for producing an effective method is described in section 28.1.7.2. There are two forms of define-method-combination. The short form is a simple facility; the long form is more powerful and more verbose. The long form resembles defmacro in that the body is an expression that computes a Lisp form; it provides mechanisms for implementing arbitrary control structures within method combination and for arbitrary processing of method qualifiers. The syntax and use of both forms of define-method-combination are explained in section 28.2. [change_end] ------------------------------------------------------------------------------- 28.1.7.4. Built-in Method Combination Types [change_begin] The Common Lisp Object System provides a set of built-in method combination types. To specify that a generic function is to use one of these method combination types, the name of the method combination type is given as the argument to the :method-combination option to defgeneric or to the :method-combination option to any of the other forms that specify generic function options. The names of the built-in method combination types are +, and, append, list, max, min, nconc, or, progn, and standard. The semantics of the standard built-in method combination type were described in section 28.1.7.2. The other built-in method combination types are called simple built-in method combination types. The simple built-in method combination types act as though they were defined by the short form of define-method-combination. They recognize two roles for methods: * An :around method has the keyword symbol :around as its sole qualifier. The meaning of :around methods is the same as in standard method combination. Use of the functions call-next-method and next-method-p is supported in :around methods. * A primary method has the name of the method combination type as its sole qualifier. For example, the built-in method combination type and recognizes methods whose sole qualifier is and; these are primary methods. Use of the functions call-next-method and next-method-p is not supported in primary methods. The semantics of the simple built-in method combination types are as follows: * If there are any :around methods, the most specific :around method is called. It supplies the value or values of the generic function. * Inside the body of an :around method, the function call-next-method can be used to call the next method. The generic function no-next-method is invoked if call-next-method is used and there is no applicable method to call. The function next-method-p may be used to determine whether a next method exists. When the next method returns, the :around method can execute more code, perhaps based on the returned value or values. * If an :around method invokes call-next-method, the next most specific :around method is called, if one is applicable. If there are no :around methods or if call-next-method is called by the least specific :around method, a Lisp form derived from the name of the built-in method combination type and from the list of applicable primary methods is evaluated to produce the value of the generic function. Suppose the name of the method combination type is operator and the call to the generic function is of the form (generic-function ... ) Let be the applicable primary methods in order; then the derived Lisp form is (operator ... ) If the expression is evaluated, the method will be applied to the arguments . For example, if operator is or, the expression is evaluated only if , 1 <= j < i, returned nil. The default order for the primary methods is :most-specific-first. However, the order can be reversed by supplying :most-specific-last as the second argument to the :method-combination option. The simple built-in method combination types require exactly one qualifier per method. An error is signaled if there are applicable methods with no qualifiers or with qualifiers that are not supported by the method combination type. An error is signaled if there are applicable :around methods and no applicable primary methods. [change_end] ------------------------------------------------------------------------------- 28.1.8. Meta-objects [change_begin] The implementation of the Object System manipulates classes, methods, and generic functions. The meta-object protocol specifies a set of generic functions defined by methods on classes; the behavior of those generic functions defines the behavior of the Object System. The instances of the classes on which those methods are defined are called meta-objects. Programming at the meta-object protocol level involves defining new classes of meta-objects along with methods specialized on these classes. [change_end] ------------------------------------------------------------------------------- * Metaclasses * Standard Metaclasses * Standard Meta-objects ------------------------------------------------------------------------------- 28.1.8.1. Metaclasses [change_begin] The metaclass of an object is the class of its class. The metaclass determines the representation of instances of its instances and the forms of inheritance used by its instances for slot descriptions and method inheritance. The metaclass mechanism can be used to provide particular forms of optimization or to tailor the Common Lisp Object System for particular uses. The protocol for defining metaclasses is discussed in the third part of the CLOS specification, The Common Lisp Object System Meta-Object Protocol. [The third part has not yet been approved by X3J13 for inclusion in the forthcoming Common Lisp standard and is not included in this book.-GLS] [change_end] ------------------------------------------------------------------------------- 28.1.8.2. Standard Metaclasses [change_begin] The Common Lisp Object System provides a number of predefined metaclasses. These include the classes standard-class, built-in-class, and structure-class: * The class standard-class is the default class of classes defined by defclass. * The class built-in-class is the class whose instances are classes that have special implementations with restricted capabilities. Any class that corresponds to a standard Common Lisp type might be an instance of built-in-class. The predefined Common Lisp type specifiers that are required to have corresponding classes are listed in table 28-1. It is implementation-dependent whether each of these classes is implemented as a built-in class. * All classes defined by means of defstruct are instances of structure-class. [change_end] ------------------------------------------------------------------------------- 28.1.8.3. Standard Meta-objects [change_begin] The Object System supplies a standard set of meta-objects, called standard meta-objects. These include the class standard-object and instances of the classes standard-method, standard-generic-function, and method-combination. * The class standard-method is the default class of methods that are defined by the forms defmethod, defgeneric, generic-function, generic-flet, generic-labels, and with-added-methods. * The class standard-generic-function is the default class of generic functions defined by the forms defmethod, defgeneric, generic-function, generic-flet, generic-labels, with-added-methods, and defclass. * The class named standard-object is an instance of the class standard-class and is a superclass of every class that is an instance of standard-class except itself. * Every method combination object is an instance of a subclass of the class method-combination. [change_end] ------------------------------------------------------------------------------- 28.1.9. Object Creation and Initialization [change_begin] The generic function make-instance creates and returns a new instance of a class. The first argument is a class or the name of a class, and the remaining arguments form an initialization argument list. The initialization of a new instance consists of several distinct steps, including the following: combining the explicitly supplied initialization arguments with default values for the unsupplied initialization arguments, checking the validity of the initialization arguments, allocating storage for the instance, filling slots with values, and executing user-supplied methods that perform additional initialization. Each step of make-instance is implemented by a generic function to provide a mechanism for customizing that step. In addition, make-instance is itself a generic function and thus also can be customized. The Object System specifies system-supplied primary methods for each step and thus specifies a well-defined standard behavior for the entire initialization process. The standard behavior provides four simple mechanisms for controlling initialization: * Declaring a symbol to be an initialization argument for a slot. An initialization argument is declared by using the :initarg slot option to defclass. This provides a mechanism for supplying a value for a slot in a call to make-instance. * Supplying a default value form for an initialization argument. Default value forms for initialization arguments are defined by using the :default-initargs class option to defclass. If an initialization argument is not explicitly provided as an argument to make-instance, the default value form is evaluated in the lexical environment of the defclass form that defined it, and the resulting value is used as the value of the initialization argument. * Supplying a default initial value form for a slot. A default initial value form for a slot is defined by using the :initform slot option to defclass. If no initialization argument associated with that slot is given as an argument to make-instance or is defaulted by :default-initargs, this default initial value form is evaluated in the lexical environment of the defclass form that defined it, and the resulting value is stored in the slot. The :initform form for a local slot may be used when creating an instance, when updating an instance to conform to a redefined class, or when updating an instance to conform to the definition of a different class. The :initform form for a shared slot may be used when defining or re-defining the class. * Defining methods for initialize-instance and shared-initialize. The slot-filling behavior described above is implemented by a system-supplied primary method for initialize-instance which invokes shared-initialize. The generic function shared-initialize implements the parts of initialization shared by these four situations: when making an instance, when re-initializing an instance, when updating an instance to conform to a redefined class, and when updating an instance to conform to the definition of a different class. The system-supplied primary method for shared-initialize directly implements the slot-filling behavior described above, and initialize-instance simply invokes shared-initialize. [change_end] ------------------------------------------------------------------------------- * Initialization Arguments * Declaring the Validity of Initialization Arguments * Defaulting of Initialization Arguments * Rules for Initialization Arguments * Shared-Initialize * Initialize-Instance * Definitions of Make-Instance and Initialize-Instance ------------------------------------------------------------------------------- 28.1.9.1. Initialization Arguments [change_begin] An initialization argument controls object creation and initialization. It is often convenient to use keyword symbols to name initialization arguments, but the name of an initialization argument can be any symbol, including nil. An initialization argument can be used in two ways: to fill a slot with a value or to provide an argument for an initialization method. A single initialization argument can be used for both purposes. An initialization argument list is a list of alternating initialization argument names and values. Its structure is identical to a property list and also to the portion of an argument list processed for &key parameters. As in those lists, if an initialization argument name appears more than once in an initialization argument list, the leftmost occurrence supplies the value and the remaining occurrences are ignored. The arguments to make-instance (after the first argument) form an initialization argument list. Error checking of initialization argument names is disabled if the keyword argument pair whose keyword is :allow-other-keys and whose value is non-nil appears in the initialization argument list. An initialization argument can be associated with a slot. If the initialization argument has a value in the initialization argument list, the value is stored into the slot of the newly created object, overriding any :initform form associated with the slot. A single initialization argument can initialize more than one slot. An initialization argument that initializes a shared slot stores its value into the shared slot, replacing any previous value. An initialization argument can be associated with a method. When an object is created and a particular initialization argument is supplied, the generic functions initialize-instance, shared-initialize, and allocate-instance are called with that initialization argument's name and value as a keyword argument pair. If a value for the initialization argument is not supplied in the initialization argument list, the method's lambda-list supplies a default value. Initialization arguments are used in four situations: when making an instance, when re-initializing an instance, when updating an instance to conform to a redefined class, and when updating an instance to conform to the definition of a different class. Because initialization arguments are used to control the creation and initialization of an instance of some particular class, we say that an initialization argument is ``an initialization argument for'' that class. [change_end] ------------------------------------------------------------------------------- 28.1.9.2. Declaring the Validity of Initialization Arguments [change_begin] Initialization arguments are checked for validity in each of the four situations that use them. An initialization argument may be valid in one situation and not another. For example, the system-supplied primary method for make-instance defined for the class standard-class checks the validity of its initialization arguments and signals an error if an initialization argument is supplied that is not declared valid in that situation. There are two means of declaring initialization arguments valid. * Initialization arguments that fill slots are declared valid by the :initarg slot option to defclass. The :initarg slot option is inherited from superclasses. Thus the set of valid initialization arguments that fill slots for a class is the union of the initialization arguments that fill slots declared valid by that class and its superclasses. Initialization arguments that fill slots are valid in all four contexts. * Initialization arguments that supply arguments to methods are declared valid by defining those methods. The keyword name of each keyword parameter specified in the method's lambda-list becomes an initialization argument for all classes for which the method is applicable. Thus method inheritance controls the set of valid initialization arguments that supply arguments to methods. The generic functions for which method definitions serve to declare initialization arguments valid are as follows: o Making an instance of a class: allocate-instance, initialize-instance, and shared-initialize. Initialization arguments declared valid by these methods are valid when making an instance of a class. o Re-initializing an instance: the functions reinitialize-instance and shared-initialize. Initialization arguments declared valid by these methods are valid when re-initializing an instance. o Updating an instance to conform to a redefined class: update-instance-for-redefined-class and shared-initialize. Initialization arguments declared valid by these methods are valid when updating an instance to conform to a redefined class. o Updating an instance to conform to the definition of a different class: update-instance-for-different-class and shared-initialize. Initialization arguments declared valid by these methods are valid when updating an instance to conform to the definition of a different class. The set of valid initialization arguments for a class is the set of valid initialization arguments that either fill slots or supply arguments to methods, along with the predefined initialization argument :allow-other-keys. The default value for :allow-other-keys is nil. The meaning of :allow-other-keys is the same here as when it is passed to an ordinary function. [change_end] ------------------------------------------------------------------------------- 28.1.9.3. Defaulting of Initialization Arguments [change_begin] A default value form can be supplied for an initialization argument by using the :default-initargs class option. If an initialization argument is declared valid by some particular class, its default value form might be specified by a different class. In this case :default-initargs is used to supply a default value for an inherited initialization argument. The :default-initargs option is used only to provide default values for initialization arguments; it does not declare a symbol as a valid initialization argument name. Furthermore, the :default-initargs option is used only to provide default values for initialization arguments when making an instance. The argument to the :default-initargs class option is a list of alternating initialization argument names and forms. Each form is the default value form for the corresponding initialization argument. The default value form of an initialization argument is used and evaluated only if that initialization argument does not appear in the arguments to make-instance and is not defaulted by a more specific class. The default value form is evaluated in the lexical environment of the defclass form that supplied it; the result is used as the initialization argument's value. The initialization arguments supplied to make-instance are combined with defaulted initialization arguments to produce a defaulted initialization argument list. A defaulted initialization argument list is a list of alternating initialization argument names and values in which unsupplied initialization arguments are defaulted and in which the explicitly supplied initialization arguments appear earlier in the list than the defaulted initialization arguments. Defaulted initialization arguments are ordered according to the order in the class precedence list of the classes that supplied the default values. There is a distinction between the purposes of the :default-initargs and the :initform options with respect to the initialization of slots. The :default-initargs class option provides a mechanism for the user to give a default value form for an initialization argument without knowing whether the initialization argument initializes a slot or is passed to a method. If that initialization argument is not explicitly supplied in a call to make-instance, the default value form is used, just as if it had been supplied in the call. In contrast, the :initform slot option provides a mechanism for the user to give a default initial value form for a slot. An :initform form is used to initialize a slot only if no initialization argument associated with that slot is given as an argument to make-instance or is defaulted by :default-initargs. The order of evaluation of default value forms for initialization arguments and the order of evaluation of :initform forms are undefined. If the order of evaluation matters, use initialize-instance or shared-initialize methods. [change_end] ------------------------------------------------------------------------------- 28.1.9.4. Rules for Initialization Arguments [change_begin] The :initarg slot option may be specified more than once for a given slot. The following rules specify when initialization arguments may be multiply defined: * A given initialization argument can be used to initialize more than one slot if the same initialization argument name appears in more than one :initarg slot option. * A given initialization argument name can appear in the lambda-list of more than one initialization method. * A given initialization argument name can appear both in an :initarg slot option and in the lambda-list of an initialization method. If two or more initialization arguments that initialize the same slot are given in the arguments to make-instance, the leftmost of these initialization arguments in the initialization argument list supplies the value, even if the initialization arguments have different names. If two or more different initialization arguments that initialize the same slot have default values and none is given explicitly in the arguments to make-instance, the initialization argument that appears in a :default-initargs class option in the most specific of the classes supplies the value. If a single :default-initargs class option specifies two or more initialization arguments that initialize the same slot and none is given explicitly in the arguments to make-instance, the leftmost argument in the :default-initargs class option supplies the value, and the values of the remaining default value forms are ignored. Initialization arguments given explicitly in the arguments to make-instance appear to the left of defaulted initialization arguments. Suppose that the classes and supply the values of defaulted initialization arguments for different slots, and suppose that is more specific than ; then the defaulted initialization argument whose value is supplied by is to the left of the defaulted initialization argument whose value is supplied by in the defaulted initialization argument list. If a single :default-initargs class option supplies the values of initialization arguments for two different slots, the initialization argument whose value is specified farther to the left in the default-initargs class option appears farther to the left in the defaulted initialization argument list. If a slot has both an :initform form and an :initarg slot option, and the initialization argument is defaulted using :default-initargs or is supplied to make-instance, the captured :initform form is neither used nor evaluated. The following is an example of the preceding rules: (defclass q () ((x :initarg a))) (defclass r (q) ((x :initarg b)) (:default-initargs a 1 b 2)) Defaulted Initialization Contents Form Argument List of Slot ======================================================================= (make-instance 'r) (a 1 b 2) 1 (make-instance 'r 'a 3) (a 3 b 2) 3 (make-instance 'r 'b 4) (b 4 a 1) 4 (make-instance 'r 'a 1 'a 2) (a 1 a 2 b 2) 1 ======================================================================= [change_end] ------------------------------------------------------------------------------- 28.1.9.5. Shared-Initialize [change_begin] The generic function shared-initialize is used to fill the slots of an instance using initialization arguments and :initform forms when an instance is created, when an instance is re-initialized, when an instance is updated to conform to a redefined class, and when an instance is updated to conform to a different class. It uses standard method combination. It takes the following arguments: the instance to be initialized, a specification of a set of names of slots accessible in that instance, and any number of initialization arguments. The arguments after the first two must form an initialization argument list. The second argument to shared-initialize may be one of the following: * It can be a list of slot names, which specifies the set of those slot names. * It can be nil, which specifies the empty set of slot names. * It can be the symbol t, which specifies the set of all of the slots. There is a system-supplied primary method for shared-initialize whose first parameter specializer is the class standard-object. This method behaves as follows on each slot, whether shared or local: * If an initialization argument in the initialization argument list specifies a value for that slot, that value is stored into the slot, even if a value has already been stored in the slot before the method is run. The affected slots are independent of which slots are indicated by the second argument to shared-initialize. * Any slots indicated by the second argument that are still unbound at this point are initialized according to their :initform forms. For any such slot that has an :initform form, that form is evaluated in the lexical environment of its defining defclass form and the result is stored into the slot. For example, if a :before method stores a value in the slot, the :initform form will not be used to supply a value for the slot. If the second argument specifies a name that does not correspond to any slots accessible in the instance, the results are unspecified. * The rules mentioned in section 28.1.9.4 are obeyed. The generic function shared-initialize is called by the system-supplied primary methods for the generic functions initialize-instance, reinitialize-instance, update-instance-for-different-class, and update-instance-for-redefined-class. Thus methods can be written for shared-initialize to specify actions that should be taken in all of these contexts. [change_end] ------------------------------------------------------------------------------- 28.1.9.6. Initialize-Instance [change_begin] The generic function initialize-instance is called by make-instance to initialize a newly created instance. It uses standard method combination. Methods for initialize-instance can be defined in order to perform any initialization that cannot be achieved with the simple slot-filling mechanisms. During initialization, initialize-instance is invoked after the following actions have been taken: * The defaulted initialization argument list has been computed by combining the supplied initialization argument list with any default initialization arguments for the class. * The validity of the defaulted initialization argument list has been checked. If any of the initialization arguments has not been declared valid, an error is signaled. * A new instance whose slots are unbound has been created. The generic function initialize-instance is called with the new instance and the defaulted initialization arguments. There is a system-supplied primary method for initialize-instance whose parameter specializer is the class standard-object. This method calls the generic function shared-initialize to fill in the slots according to the initialization arguments and the :initform forms for the slots; the generic function shared-initialize is called with the following arguments: the instance, t, and the defaulted initialization arguments. Note that initialize-instance provides the defaulted initialization argument list in its call to shared-initialize, so the first step performed by the system-supplied primary method for shared-initialize takes into account both the initialization arguments provided in the call to make-instance and the defaulted initialization argument list. Methods for initialize-instance can be defined to specify actions to be taken when an instance is initialized. If only :after methods for initialize-instance are defined, they will be run after the system-supplied primary method for initialization and therefore they will not interfere with the default behavior of initialize-instance. The Object System provides two functions that are useful in the bodies of initialize-instance methods. The function slot-boundp returns a boolean value that indicates whether a specified slot has a value; this provides a mechanism for writing :after methods for initialize-instance that initialize slots only if they have not already been initialized. The function slot-makunbound causes the slot to have no value. [change_end] ------------------------------------------------------------------------------- 28.1.9.7. Definitions of Make-Instance and Initialize-Instance [change_begin] The generic function make-instance behaves as if it were defined as follows, except that certain optimizations are permitted: (defmethod make-instance ((class standard-class) &rest initargs) (setq initargs (default-initargs class initargs)) ... (let ((instance (apply #'allocate-instance class initargs))) (apply #'initialize-instance instance initargs) instance)) (defmethod make-instance ((class-name symbol) &rest initargs) (apply #'make-instance (find-class class-name) initargs)) The elided code in the definition of make-instance checks the supplied initialization arguments to determine whether an initialization argument was supplied that neither filled a slot nor supplied an argument to an applicable method. This check could be implemented using the generic functions class-prototype, compute-applicable-methods, function-keywords, and class-slot-initargs. See the third part of the Common Lisp Object System specification for a description of this initialization argument check. [The third part has not yet been approved by X3J13 for inclusion in the forthcoming Common Lisp standard and is not included in this book.-GLS] The generic function initialize-instance behaves as if it were defined as follows, except that certain optimizations are permitted: (defmethod initialize-instance ((instance standard-object) &rest initargs) (apply #'shared-initialize instance t initargs))) These procedures can be customized at either the Programmer Interface level, the meta-object level, or both. Customizing at the Programmer Interface level includes using the :initform, :initarg, and :default-initargs options to defclass, as well as defining methods for make-instance and initialize-instance. It is also possible to define methods for shared-initialize, which would be invoked by the generic functions reinitialize-instance, update-instance-for-redefined-class, update-instance-for-different-class, and initialize-instance. The meta-object level supports additional customization by allowing methods to be defined on make-instance, default-initargs, and allocate-instance. Parts 2 and 3 of the Common Lisp Object System specification document each of these generic functions and the system-supplied primary methods. [The third part has not yet been approved by X3J13 for inclusion in the forthcoming Common Lisp standard and is not included in this book.-GLS] Implementations are permitted to make certain optimizations to initialize-instance and shared-initialize. The description of shared-initialize in section 28.2 mentions the possible optimizations. Because of optimization, the check for valid initialization arguments might not be implemented using the generic functions class-prototype, compute-applicable-methods, function-keywords, and class-slot-initargs. In addition, methods for the generic function default-initargs and the system-supplied primary methods for allocate-instance, initialize-instance, and shared-initialize might not be called on every call to make-instance or might not receive exactly the arguments that would be expected. [change_end] ------------------------------------------------------------------------------- 28.1.10. Redefining Classes [change_begin] A class that is an instance of standard-class can be redefined if the new class will also be an instance of standard-class. Redefining a class modifies the existing class object to reflect the new class definition; it does not create a new class object for the class. Any method object created by a :reader, :writer, or :accessor option specified by the old defclass form is removed from the corresponding generic function. Methods specified by the new defclass form are added. When the class C is redefined, changes are propagated to its instances and to instances of any of its subclasses. Updating such an instance occurs at an implementation-dependent time, but no later than the next time a slot of that instance is read or written. Updating an instance does not change its identity as defined by the eq function. The updating process may change the slots of that particular instance, but it does not create a new instance. Whether updating an instance consumes storage is implementation-dependent. Note that redefining a class may cause slots to be added or deleted. If a class is redefined in a way that changes the set of local slots accessible in instances, the instances will be updated. It is implementation-dependent whether instances are updated if a class is redefined in a way that does not change the set of local slots accessible in instances. The value of a slot that is specified as shared both in the old class and in the new class is retained. If such a shared slot was unbound in the old class, it will be unbound in the new class. Slots that were local in the old class and that are shared in the new class are initialized. Newly added shared slots are initialized. Each newly added shared slot is set to the result of evaluating the captured :initform form for the slot that was specified in the defclass form for the new class. If there is no :initform form, the slot is unbound. If a class is redefined in such a way that the set of local slots accessible in an instance of the class is changed, a two-step process of updating the instances of the class takes place. The process may be explicitly started by invoking the generic function make-instances-obsolete. This two-step process can happen in other circumstances in some implementations. For example, in some implementations this two-step process will be triggered if the order of slots in storage is changed. The first step modifies the structure of the instance by adding new local slots and discarding local slots that are not defined in the new version of the class. The second step initializes the newly added local slots and performs any other user-defined actions. These steps are further specified in the next two sections. [change_end] ------------------------------------------------------------------------------- * Modifying the Structure of Instances * Initializing Newly Added Local Slots * Customizing Class Redefinition * Extensions ------------------------------------------------------------------------------- 28.1.10.1. Modifying the Structure of Instances [change_begin] The first step modifies the structure of instances of the redefined class to conform to its new class definition. Local slots specified by the new class definition that are not specified as either local or shared by the old class are added, and slots not specified as either local or shared by the new class definition that are specified as local by the old class are discarded. The names of these added and discarded slots are passed as arguments to update-instance-for-redefined-class as described in the next section. The values of local slots specified by both the new and old classes are retained. If such a local slot was unbound, it remains unbound. The value of a slot that is specified as shared in the old class and as local in the new class is retained. If such a shared slot was unbound, the local slot will be unbound. [change_end] ------------------------------------------------------------------------------- 28.1.10.2. Initializing Newly Added Local Slots [change_begin] The second step initializes the newly added local slots and performs any other user-defined actions. This step is implemented by the generic function update-instance-for-redefined-class, which is called after completion of the first step of modifying the structure of the instance. The generic function update-instance-for-redefined-class takes four required arguments: the instance being updated after it has undergone the first step, a list of the names of local slots that were added, a list of the names of local slots that were discarded, and a property list containing the slot names and values of slots that were discarded and had values. Included among the discarded slots are slots that were local in the old class and that are shared in the new class. The generic function update-instance-for-redefined-class also takes any number of initialization arguments. When it is called by the system to update an instance whose class has been redefined, no initialization arguments are provided. There is a system-supplied primary method for the generic function update-instance-for-redefined-class whose parameter specializer for its instance argument is the class standard-object. First this method checks the validity of initialization arguments and signals an error if an initialization argument is supplied that is not declared valid (see section 28.1.9.2.) Then it calls the generic function shared-initialize with the following arguments: the instance, the list of names of the newly added slots, and the initialization arguments it received. [change_end] ------------------------------------------------------------------------------- 28.1.10.3. Customizing Class Redefinition [change_begin] Methods for update-instance-for-redefined-class may be defined to specify actions to be taken when an instance is updated. If only :after methods for update-instance-for-redefined-class are defined, they will be run after the system-supplied primary method for initialization and therefore will not interfere with the default behavior of update-instance-for-redefined-class. Because no initialization arguments are passed to update-instance-for-redefined-class when it is called by the system, the :initform forms for slots that are filled by :before methods for update-instance-for-redefined-class will not be evaluated by shared-initialize. Methods for shared-initialize may be defined to customize class redefinition (see section 28.1.9.5). [change_end] ------------------------------------------------------------------------------- 28.1.10.4. Extensions [change_begin] There are two allowed extensions to class redefinition: * The Object System may be extended to permit the new class to be an instance of a metaclass other than the metaclass of the old class. * The Object System may be extended to support an updating process when either the old or the new class is an instance of a class other than standard-class that is not a built-in class. [change_end] ------------------------------------------------------------------------------- 28.1.11. Changing the Class of an Instance [change_begin] The function change-class can be used to change the class of an instance from its current class, , to a different class, ; it changes the structure of the instance to conform to the definition of the class . Note that changing the class of an instance may cause slots to be added or deleted. When change-class is invoked on an instance, a two-step updating process takes place. The first step modifies the structure of the instance by adding new local slots and discarding local slots that are not specified in the new version of the instance. The second step initializes the newly added local slots and performs any other user-defined actions. These steps are further described in the following two sections. [change_end] ------------------------------------------------------------------------------- * Modifying the Structure of an Instance * Initializing Newly Added Local Slots * Customizing the Change of Class of an Instance ------------------------------------------------------------------------------- 28.1.11.1. Modifying the Structure of an Instance [change_begin] In order to make an instance conform to the class , local slots specified by the class that are not specified by the class are added, and local slots not specified by the class that are specified by the class are discarded. The values of local slots specified by both the class and the class are retained. If such a local slot was unbound, it remains unbound. The values of slots specified as shared in the class and as local in the class are retained. This first step of the update does not affect the values of any shared slots. [change_end] ------------------------------------------------------------------------------- 28.1.11.2. Initializing Newly Added Local Slots [change_begin] The second step of the update initializes the newly added slots and performs any other user-defined actions. This step is implemented by the generic function update-instance-for-different-class. The generic function update-instance-for-different-class is invoked by change-class after the first step of the update has been completed. The generic function update-instance-for-different-class is invoked on two arguments computed by change-class. The first argument passed is a copy of the instance being updated and is an instance of the class ; this copy has dynamic extent within the generic function change-class. The second argument is the instance as updated so far by change-class and is an instance of the class . The generic function update-instance-for-different-class also takes any number of initialization arguments. When it is called by change-class, no initialization arguments are provided. There is a system-supplied primary method for the generic function update-instance-for-different-class that has two parameter specializers, each of which is the class standard-object. First this method checks the validity of initialization arguments and signals an error if an initialization argument is supplied that is not declared valid (see section 28.1.9.2). Then it calls the generic function shared-initialize with the following arguments: the instance, a list of names of the newly added slots, and the initialization arguments it received. [change_end] ------------------------------------------------------------------------------- 28.1.11.3. Customizing the Change of Class of an Instance [change_begin] Methods for update-instance-for-different-class may be defined to specify actions to be taken when an instance is updated. If only :after methods for update-instance-for-different-class are defined, they will be run after the system-supplied primary method for initialization and will not interfere with the default behavior of update-instance-for-different-class. Because no initialization arguments are passed to update-instance-for-different-class when it is called by change-class, the :initform forms for slots that are filled by :before methods for update-instance-for-different-class will not be evaluated by shared-initialize. Methods for shared-initialize may be defined to customize class redefinition (see section 28.1.9.5). [change_end] ------------------------------------------------------------------------------- 28.1.12. Reinitializing an Instance [change_begin] The generic function reinitialize-instance may be used to change the values of slots according to initialization arguments. The process of reinitialization changes the values of some slots and performs any user-defined actions. Reinitialization does not modify the structure of an instance to add or delete slots, and it does not use any :initform forms to initialize slots. The generic function reinitialize-instance may be called directly. It takes one required argument, the instance. It also takes any number of initialization arguments to be used by methods for reinitialize-instance or for shared-initialize. The arguments after the required instance must form an initialization argument list. There is a system-supplied primary method for reinitialize-instance whose parameter specializer is the class standard-object. First this method checks the validity of initialization arguments and signals an error if an initialization argument is supplied that is not declared valid (see section 28.1.9.2). Then it calls the generic function shared-initialize with the following arguments: the instance, nil, and the initialization arguments it received. [change_end] ------------------------------------------------------------------------------- * Customizing Reinitialization ------------------------------------------------------------------------------- 28.1.12.1. Customizing Reinitialization [change_begin] Methods for the generic function reinitialize-instance may be defined to specify actions to be taken when an instance is updated. If only :after methods for reinitialize-instance are defined, they will be run after the system-supplied primary method for initialization and therefore will not interfere with the default behavior of reinitialize-instance. Methods for shared-initialize may be defined to customize class redefinition (see section 28.1.9.5). [change_end] ------------------------------------------------------------------------------- 28.2. Functions in the Programmer Interface [change_begin] This section describes the functions, macros, special forms, and generic functions provided by the Common Lisp Object System Programmer Interface. The Programmer Interface comprises the functions and macros that are sufficient for writing most object-oriented programs. This section is reference material that requires an understanding of the basic concepts of the Common Lisp Object System. The functions are arranged in alphabetical order for convenient reference. The description of each function, macro, special form, and generic function includes its purpose, its syntax, the semantics of its arguments and returned values, and often an example and cross-references to related functions. The syntax description for a function, macro, or special form describes its parameters. The description of a generic function includes descriptions of the methods that are defined on that generic function by the Common Lisp Object System. A method signature is used to describe the parameters and parameter specializers for each method. The following is an example of the format for the syntax description of a generic function with the method signature for one primary method: [Generic function] f x y &optional z &key :k [Primary method] f (x class) (y t) &optional z &key :k This description indicates that the generic function f has two required parameters, x and y. In addition, there is an optional parameter z and a keyword parameter :k. The method signature indicates that this method on the generic function f has two required parameters, x, which must be an instance of the class class, and y, which can be any object. In addition, there is an optional parameter z and a keyword parameter :k. The signature also indicates that this method on f is a primary method and has no qualifiers. The syntax description for a generic function describes the lambda-list of the generic function itself, while the method signatures describe the lambda-lists of the defined methods. The generic functions described in this book are all standard generic functions. They all use standard method combination. Any implementation of the Common Lisp Object System is allowed to provide additional methods on the generic functions described here. It is useful to categorize the functions and macros according to their role in this standard: * Tools used for simple object-oriented programming These tools allow for defining new classes, methods, and generic functions and for making instances. Some tools used within method bodies are also listed here. Some of the macros listed here have a corresponding function that performs the same task at a lower level of abstraction. call-next-method initialize-instance change-class make-instance defclass next-method-p defgeneric slot-boundp defmethod slot-value generic-flet with-accessors generic-function with-added-methods generic-labels with-slots * Functions underlying the commonly used macros add-method reinitialize-instance class-name remove-method compute-applicable-methods shared-initialize ensure-generic-function slot-exists-p find-class slot-makunbound find-method slot-missing function-keywords slot-unbound make-instances-obsolete update-instance-for-different-class no-applicable-method update-instance-for-redefined-class no-next-method * Tools for declarative method combination call-method method-combination-error define-method-combination method-qualifiers invalid-method-error * General Common Lisp support tools class-of print-object documentation symbol-macrolet [Note that describe appeared in this list in the original CLOS proposal [5,7], but X3J13 voted in March 1989 (DESCRIBE-UNDERSPECIFIED) not to make describe a generic function after all (see describe-object).-GLS] [At this point the original CLOS report contained a description of the [[ and ]] notation; that description is omitted here. I have adopted the notation for use throughout this book. It is described in section 1.2.5.-GLS] [Generic function] add-method generic-function method [Primary method] add-method (generic-function standard-generic-function) (method method) The generic function add-method adds a method to a generic function. It destructively modifies the generic function and returns the modified generic function as its result. The generic-function argument is a generic function object. The method argument is a method object. The lambda-list of the method function must be congruent with the lambda-list of the generic function, or an error is signaled. The modified generic function is returned. The result of add-method is eq to the generic-function argument. If the given method agrees with an existing method of the generic function on parameter specializers and qualifiers, the existing method is replaced. See section 28.1.6.3 for a definition of agreement in this context. If the method object is a method object of another generic function, an error is signaled. See section 28.1.6.3 as well as defmethod, defgeneric, find-method, and remove-method. [Macro] call-method method next-method-list The macro call-method is used in method combination. This macro hides the implementation-dependent details of how methods are called. It can be used only within an effective method form, for the name call-method is defined only within the lexical scope of such a form. The macro call-method invokes the specified method, supplying it with arguments and with definitions for call-next-method and for next-method-p. The arguments are the arguments that were supplied to the effective method form containing the invocation of call-method. The definitions of call-next-method and next-method-p rely on the list of method objects given as the second argument to call-method. The call-next-method function available to the method that is the first subform will call the first method in the list that is the second subform. The call-next-method function available in that method, in turn, will call the second method in the list that is the second subform, and so on, until the list of next methods is exhausted. The method argument is a method object; the next-method-list argument is a list of method objects. A list whose first element is the symbol make-method and whose second element is a Lisp form can be used instead of a method object as the first subform of call-method or as an element of the second subform of call-method. Such a list specifies a method object whose method function has a body that is the given form. The result of call-method is the value or values returned by the method invocation. See call-next-method, define-method-combination, and next-method-p. [Function] call-next-method &rest args The function call-next-method can be used within the body of a method defined by a method-defining form to call the next method. The function call-next-method returns the value or values returned by the method it calls. If there is no next method, the generic function no-next-method is called. The type of method combination used determines which methods can invoke call-next-method. The standard method combination type allows call-next-method to be used within primary methods and :around methods. The standard method combination type defines the next method according to the following rules: * If call-next-method is used in an :around method, the next method is the next most specific :around method, if one is applicable. * If there are no :around methods at all or if call-next-method is called by the least specific :around method, other methods are called as follows: o All the :before methods are called, in most-specific-first order. The function call-next-method cannot be used in :before methods. o The most specific primary method is called. Inside the body of a primary method, call-next-method may be used to pass control to the next most specific primary method. The generic function no-next-method is called if call-next-method is used and there are no more primary methods. o All the :after methods are called in most-specific-last order. The function call-next-method cannot be used in :after methods. For further discussion of the use of call-next-method, see sections 28.1.7.2 and 28.1.7.4. When call-next-method is called with no arguments, it passes the current method's original arguments to the next method. Neither argument defaulting, nor using setq, nor rebinding variables with the same names as parameters of the method affects the values call-next-method passes to the method it calls. When call-next-method is called with arguments, the next method is called with those arguments. When providing arguments to call-next-method, the following rule must be satisfied or an error is signaled: The ordered set of methods applicable for a changed set of arguments for call-next-method must be the same as the ordered set of applicable methods for the original arguments to the generic function. Optimizations of the error checking are possible, but they must not change the semantics of call-next-method. If call-next-method is called with arguments but omits optional arguments, the next method called defaults those arguments. The function call-next-method returns the value or values returned by the method it calls. Further computation is possible after call-next-method returns. The definition of the function call-next-method has lexical scope (for it is defined only within the body of a method defined by a method-defining form) and indefinite extent. For generic functions using a type of method combination defined by the short form of define-method-combination, call-next-method can be used in :around methods only. The function next-method-p can be used to test whether or not there is a next method. If call-next-method is used in methods that do not support it, an error is signaled. See sections 28.1.7, 28.1.7.2, and 28.1.7.4 as well as the functions define-method-combination, next-method-p, and no-next-method. [Generic function] change-class instance new-class [Primary method] change-class (instance standard-object) (new-class standard-class) change-class (instance t) (new-class symbol) The generic function change-class changes the class of an instance to a new class. It destructively modifies and returns the instance. If in the old class there is any slot of the same name as a local slot in the new class, the value of that slot is retained. This means that if the slot has a value, the value returned by slot-value after change-class is invoked is eql to the value returned by slot-value before change-class is invoked. Similarly, if the slot was unbound, it remains unbound. The other slots are initialized as described in section 28.1.11. The instance argument is a Lisp object. The new-class argument is a class object or a symbol that names a class. If the second of the preceding methods is selected, that method invokes change-class on instance and (find-class new-class). The modified instance is returned. The result of change-class is eq to the instance argument. Examples: (defclass position () ()) (defclass x-y-position (position) ((x :initform 0 :initarg :x) (y :initform 0 :initarg :y))) (defclass rho-theta-position (position) ((rho :initform 0) (theta :initform 0))) (defmethod update-instance-for-different-class :before ((old x-y-position) (new rho-theta-position) &key) ;; Copy the position information from old to new to make new ;; be a rho-theta-position at the same position as old. (let ((x (slot-value old 'x)) (y (slot-value old 'y))) (setf (slot-value new 'rho) (sqrt (+ (* x x) (* y y))) (slot-value new 'theta) (atan y x)))) ;;; At this point an instance of the class x-y-position can be ;;; changed to be an instance of the class rho-theta-position ;;; using change-class: (setq p1 (make-instance 'x-y-position :x 2 :y 0)) (change-class p1 'rho-theta-position) ;;; The result is that the instance bound to p1 is now ;;; an instance of the class rho-theta-position. ;;; The update-instance-for-different-class method ;;; performed the initialization of the rho and theta ;;; slots based on the values of the x and y slots, ;;; which were maintained by the old instance. After completing all other actions, change-class invokes the generic function update-instance-for-different-class. The generic function update-instance-for-different-class can be used to assign values to slots in the transformed instance. The generic function change-class has several semantic difficulties. First, it performs a destructive operation that can be invoked within a method on an instance that was used to select that method. When multiple methods are involved because methods are being combined, the methods currently executing or about to be executed may no longer be applicable. Second, some implementations might use compiler optimizations of slot access, and when the class of an instance is changed the assumptions the compiler made might be violated. This implies that a programmer must not use change-class inside a method if any methods for that generic function access any slots, or the results are undefined. See section 28.1.11 as well as update-instance-for-different-class. [Generic function] class-name class [Primary method] class-name (class class) The generic function class-name takes a class object and returns its name. The class argument is a class object. The new-value argument is any object. The name of the given class is returned. The name of an anonymous class is nil. If S is a symbol such that S =(class-name C) and C = (find-class S), then S is the proper name of C (see section 28.1.2). See also section 28.1.2 and find-class. [Generic function] (setf class-name) new-value class [Primary method] (setf class-name) new-value (class class) The generic function (setf class-name) takes a class object and sets its name. The class argument is a class object. The new-value argument is any object. [Function] class-of object The function class-of returns the class of which the given object is an instance. The argument to class-of may be any Common Lisp object. The function class-of returns the class of which the argument is an instance. [Function] compute-applicable-methods generic-function function-arguments Given a generic function and a set of arguments, the function compute-applicable-methods returns the set of methods that are applicable for those arguments. The methods are sorted according to precedence order. See section 28.1.7. The generic-function argument must be a generic function object. The function-arguments argument is a list of the arguments to that generic function. The result is a list of the applicable methods in order of precedence. See section 28.1.7. [Macro] defclass class-name ({superclass-name}*) ({slot-specifier}*) [[?class-option]] class-name ::= symbol superclass-name ::= symbol slot-specifier ::= slot-name | (slot-name [[?slot-option]]) slot-name ::= symbol slot-option ::= {:reader reader-function-name}* | {:writer writer-function-name}* | {:accessor reader-function-name}* | {:allocation allocation-type}* | {:initarg initarg-name}* | {:initform form}* | {:type type-specifier}* | {:documentation string}* reader-function-name ::= symbol writer-function-name ::= function-name/ function-name ::= {symbol | (setf symbol)} initarg-name ::= symbol allocation-type ::= :instance | :class class-option ::= (:default-initargs initarg-list) | (:documentation string) | (:metaclass class-name) initarg-list ::= {initarg-name default-initial-value-form}* The macro defclass defines a new named class. It returns the new class object as its result. The syntax of defclass provides options for specifying initialization arguments for slots, for specifying default initialization values for slots, and for requesting that methods on specified generic functions be automatically generated for reading and writing the values of slots. No reader or writer functions are defined by default; their generation must be explicitly requested. Defining a new class also causes a type of the same name to be defined. The predicate (typep object class-name) returns true if the class of the given object is class-name itself or a subclass of the class class-name. A class object can be used as a type specifier. Thus (typep object class) returns true if the class of the object is class itself or a subclass of class. The class-name argument is a non-nil symbol. It becomes the proper name of the new class. If a class with the same proper name already exists and that class is an instance of standard-class, and if the defclass form for the definition of the new class specifies a class of class standard-class, the definition of the existing class is replaced. Each superclass-name argument is a non-nil symbol that specifies a direct superclass of the new class. The new class will inherit slots and methods from each of its direct superclasses, from their direct superclasses, and so on. See section 28.1.3 for a discussion of how slots and methods are inherited. Each slot-specifier argument is the name of the slot or a list consisting of the slot name followed by zero or more slot options. The slot-name argument is a symbol that is syntactically valid for use as a variable name. If there are any duplicate slot names, an error is signaled. The following slot options are available: * The :reader slot option specifies that an unqualified method is to be defined on the generic function named reader-function-name to read the value of the given slot. The reader-function-name argument is a non-nil symbol. The :reader slot option may be specified more than once for a given slot. * The :writer slot option specifies that an unqualified method is to be defined on the generic function named writer-function-name to write the value of the slot. The writer-function-name argument is a function-name. The :writer slot option may be specified more than once for a given slot. * The :accessor slot option specifies that an unqualified method is to be defined on the generic function named reader-function-name to read the value of the given slot and that an unqualified method is to be defined on the generic function named (setf reader-function-name) to be used with setf to modify the value of the slot. The reader-function-name argument is a non-nil symbol. The :accessor slot option may be specified more than once for a given slot. * The :allocation slot option is used to specify where storage is to be allocated for the given slot. Storage for a slot may be located in each instance or in the class object itself, for example. The value of the allocation-type argument can be either the keyword :instance or the keyword :class. The :allocation slot option may be specified at most once for a given slot. If the :allocation slot option is not specified, the effect is the same as specifying :allocation :instance. o If allocation-type is :instance, a local slot of the given name is allocated in each instance of the class. o If allocation-type is :class, a shared slot of the given name is allocated. The value of the slot is shared by all instances of the class. If a class C defines such a shared slot, any subclass C of C will share this single slot unless the defclass form for C specifies a slot of the same name or there is a superclass of C that precedes C in the class precedence list of C and that defines a slot of the same name. * The :initform slot option is used to provide a default initial value form to be used in the initialization of the slot. The :initform slot option may be specified at most once for a given slot. This form is evaluated every time it is used to initialize the slot. The lexical environment in which this form is evaluated is the lexical environment in which the defclass form was evaluated. Note that the lexical environment refers both to variables and to functions. For local slots, the dynamic environment is the dynamic environment in which make-instance was called; for shared slots, the dynamic environment is the dynamic environment in which the defclass form was evaluated. See section 28.1.9. No implementation is permitted to extend the syntax of defclass to allow (slot-name form) as an abbreviation for (slot-name :initform form). * The :initarg slot option declares an initialization argument named initarg-name and specifies that this initialization argument initializes the given slot. If the initialization argument has a value in the call to initialize-instance, the value will be stored into the given slot, and the slot's :initform slot option, if any, is not evaluated. If none of the initialization arguments specified for a given slot has a value, the slot is initialized according to the :initform slot option, if specified. The :initarg slot option can be specified more than once for a given slot. The initarg-name argument can be any symbol. * The :type slot option specifies that the contents of the slot will always be of the specified data type. It effectively declares the result type of the reader generic function when applied to an object of this class. The result of attempting to store in a slot a value that does not satisfy the type of the slot is undefined. The :type slot option may be specified at most once for a given slot. The :type slot option is further discussed in section 28.1.3.2. * The :documentation slot option provides a documentation string for the slot. Each class option is an option that refers to the class as a whole or to all class slots. The following class options are available: * The :default-initargs class option is followed by a list of alternating initialization argument names and default initial value forms. If any of these initialization arguments does not appear in the initialization argument list supplied to make-instance, the corresponding default initial value form is evaluated, and the initialization argument name and the form's value are added to the end of the initialization argument list before the instance is created (see section 28.1.9). The default initial value form is evaluated each time it is used. The lexical environment in which this form is evaluated is the lexical environment in which the defclass form was evaluated. The dynamic environment is the dynamic environment in which make-instance was called. If an initialization argument name appears more than once in a :default-initargs class option, an error is signaled. The :default-initargs class option may be specified at most once. * The :documentation class option causes a documentation string to be attached to the class name. The documentation type for this string is type. The form (documentation class-name 'type) may be used to retrieve the documentation string. The :documentation class option may be specified at most once. * The :metaclass class option is used to specify that instances of the class being defined are to have a different metaclass than the default provided by the system (the class standard-class). The class-name argument is the name of the desired metaclass. The :metaclass class option may be specified at most once. The new class object is returned as the result. If a class with the same proper name already exists and that class is an instance of standard-class, and if the defclass form for the definition of the new class specifies a class of class standard-class, the existing class is redefined, and instances of it (and its subclasses) are updated to the new definition at the time that they are next accessed (see section 28.1.10). Note the following rules of defclass for standard classes: * It is not required that the superclasses of a class be defined before the defclass form for that class is evaluated. * All the superclasses of a class must be defined before an instance of the class can be made. * A class must be defined before it can be used as a parameter specializer in a defmethod form. The Object System may be extended to cover situations where these rules are not obeyed. Some slot options are inherited by a class from its superclasses, and some can be shadowed or altered by providing a local slot description. No class options except :default-initargs are inherited. For a detailed description of how slots and slot options are inherited, see section 28.1.3.2. The options to defclass can be extended. An implementation must signal an error if it observes a class option or a slot option that is not implemented locally. It is valid to specify more than one reader, writer, accessor, or initialization argument for a slot. No other slot option may appear more than once in a single slot description, or an error is signaled. If no reader, writer, or accessor is specified for a slot, the slot can be accessed only by the function slot-value. See sections 28.1.2, 28.1.3, 28.1.10, 28.1.5, 28.1.9 as well as slot-value, make-instance, and initialize-instance. [Macro] defgeneric function-name lambda-list [[?option | {method-description}*]] function-name ::= {symbol | (setf symbol)} lambda-list ::= ({var}* [&optional {var | (var)}*] [&rest var] [&key {keyword-parameter}* [&allow-other-keys]]) keyword-parameter ::= var | ({var | (keyword var)}) option ::= (:argument-precedence-order {parameter-name}+) | (declare {declaration}+) | (:documentation string) | (:method-combination symbol {arg}*) | (:generic-function-class class-name) | (:method-class class-name) method-description ::= (:method {method-qualifier}* specialized-lambda-list [[ {declaration}* | documentation ]] {form}*) method-qualifier ::= non-nil-atom specialized-lambda-list ::= ({var | (var parameter-specializer-name)}* [&optional {var | (var [initform [supplied-p-parameter]])}*] [&rest var] [&key {specialized-keyword-parameter}* [&allow-other-keys]] [&aux {var | (var [initform])}*]) specialized-keyword-parameter ::= var | ({var | (keyword var)} [initform [supplied-p-parameter]]) parameter-specializer-name ::= symbol | (eql eql-specializer-form) The macro defgeneric is used to define a generic function or to specify options and declarations that pertain to a generic function as a whole. If (fboundp function-name) is nil, a new generic function is created. If (fdefinition function-specifier) is a generic function, that generic function is modified. If function-name/ names a non-generic function, a macro, or a special form, an error is signaled. [X3J13 voted in March 1989 (FUNCTION-NAME) to use fdefinition in the previous paragraph, as shown, rather than symbol-function, as it appeared in the original report on CLOS [5,7]. The vote also changed all occurrences of function-specifier in the original report to function-name; this change is reflected here.-GLS] Each method-description defines a method on the generic function. The lambda-list of each method must be congruent with the lambda-list specified by the lambda-list option. If this condition does not hold, an error is signaled. See section 28.1.6.4 for a definition of congruence in this context. The macro defgeneric returns the generic function object as its result. The function-name argument is a non-nil symbol or a list of the form (setf symbol). The lambda-list argument is an ordinary function lambda-list with the following exceptions: * The use of &aux is not allowed. * Optional and keyword arguments may not have default initial value forms nor use supplied-p parameters. The generic function passes to the method all the argument values passed to it, and only those; default values are not supported. Note that optional and keyword arguments in method definitions, however, can have default initial value forms and can use supplied-p parameters. The following options are provided. A given option may occur only once, or an error is signaled. * The :argument-precedence-order option is used to specify the order in which the required arguments in a call to the generic function are tested for specificity when selecting a particular method. Each required argument, as specified in the lambda-list argument, must be included exactly once as a parameter-name so that the full and unambiguous precedence order is supplied. If this condition is not met, an error is signaled. * The declare option is used to specify declarations that pertain to the generic function. The following standard Common Lisp declaration is allowed: o An optimize declaration specifies whether method selection should be optimized for speed or space, but it has no effect on methods. To control how a method is optimized, an optimize declaration must be placed directly in the defmethod form or method description. The optimization qualities speed and space are the only qualities this standard requires, but an implementation can extend the Common Lisp Object System to recognize other qualities. A simple implementation that has only one method selection technique and ignores the optimize declaration is valid. The special, ftype, function, inline, notinline, and declaration declarations are not permitted. Individual implementations can extend the declare option to support additional declarations. If an implementation notices a declaration that it does not support and that has not been proclaimed as a non-standard declaration name in a declaration proclamation, it should issue a warning. * The :documentation argument associates a documentation string with the generic function. The documentation type for this string is function. The form (documentation function-name/ 'function) may be used to retrieve this string. * The :generic-function-class option may be used to specify that the generic function is to have a different class than the default provided by the system (the class standard-generic-function). The class-name argument is the name of a class that can be the class of a generic function. If function-name specifies an existing generic function that has a different value for the :generic-function-class argument and the new generic function class is compatible with the old, change-class is called to change the class of the generic function; otherwise an error is signaled. * The :method-class option is used to specify that all methods on this generic function are to have a different class from the default provided by the system (the class standard-method). The class-name argument is the name of a class that is capable of being the class of a method. * The :method-combination option is followed by a symbol that names a type of method combination. The arguments (if any) that follow that symbol depend on the type of method combination. Note that the standard method combination type does not support any arguments. However, all types of method combination defined by the short form of define-method-combination accept an optional argument named order, defaulting to :most-specific-first, where a value of :most-specific-last reverses the order of the primary methods without affecting the order of the auxiliary methods. The method-description arguments define methods that will be associated with the generic function. The method-qualifier and specialized-lambda-list arguments in a method description are the same as for defmethod. The form arguments specify the method body. The body of the method is enclosed in an implicit block. If function-name is a symbol, this block bears the same name as the generic function. If function-name is a list of the form (setf symbol), the name of the block is symbol. The generic function object is returned as the result. The effect of the defgeneric macro is as if the following three steps were performed: first, methods defined by previous defgeneric forms are removed; second, ensure-generic-function is called; and finally, methods specified by the current defgeneric form are added to the generic function. If no method descriptions are specified and a generic function of the same name does not already exist, a generic function with no methods is created. The lambda-list argument of defgeneric specifies the shape of lambda-lists for the methods on this generic function. All methods on the resulting generic function must have lambda-lists that are congruent with this shape. If a defgeneric form is evaluated and some methods for that generic function have lambda-lists that are not congruent with that given in the defgeneric form, an error is signaled. For further details on method congruence, see section 28.1.6.4. Implementations can extend defgeneric to include other options. It is required that an implementation signal an error if it observes an option that is not implemented locally. See section 28.1.6.4 as well as defmethod, ensure-generic-function, and generic-function. [Macro] define-method-combination name [[?short-form-option]] define-method-combination name lambda-list ({method-group-specifier}*) [(:arguments . lambda-list)] [(:generic-function generic-fn-symbol)] [[{declaration}* | doc-string]] {form}* short-form-option ::= :documentation string | :identity-with-one-argument boolean | :operator operator method-group-specifier ::= (variable {{qualifier-pattern}+ | predicate} [[?long-form-option]]) long-form-option ::= :description format-string | :order order | :required boolean The macro define-method-combination is used to define new types of method combination. There are two forms of define-method-combination. The short form is a simple facility for the cases that are expected to be most commonly needed. The long form is more powerful but more verbose. It resembles defmacro in that the body is an expression, usually using backquote, that computes a Lisp form. Thus arbitrary control structures can be implemented. The long form also allows arbitrary processing of method qualifiers. In both the short and long forms, name is a symbol. By convention, non-keyword, non-nil symbols are usually used. The short-form syntax of define-method-combination is recognized when the second subform is a non-nil symbol or is not present. When the short form is used, name is defined as a type of method combination that produces a Lisp form (operator method-call method-call ... ). The operator is a symbol that can be the name of a function, macro, or special form. The operator can be specified by a keyword option; it defaults to name. Keyword options for the short form are the following: * The :documentation option is used to document the method-combination type. * The :identity-with-one-argument option enables an optimization when boolean is true (the default is false). If there is exactly one applicable method and it is a primary method, that method serves as the effective method and operator is not called. This optimization avoids the need to create a new effective method and avoids the overhead of a function call. This option is designed to be used with operators such as progn, and, +, and max. * The :operator option specifies the name of the operator. The operator argument is a symbol that can be the name of a function, macro, or special form. By convention, name and operator are often the same symbol. This is the default, but it is not required. None of the subforms is evaluated. These types of method combination require exactly one qualifier per method. An error is signaled if there are applicable methods with no qualifiers or with qualifiers that are not supported by the method combination type. A method combination procedure defined in this way recognizes two roles for methods. A method whose one qualifier is the symbol naming this type of method combination is defined to be a primary method. At least one primary method must be applicable or an error is signaled. A method with :around as its one qualifier is an auxiliary method that behaves the same as an :around method in standard method combination. The function call-next-method can be used only in :around methods; it cannot be used in primary methods defined by the short form of the define-method-combination macro. A method combination procedure defined in this way accepts an optional argument named order, which defaults to :most-specific-first. A value of :most-specific-last reverses the order of the primary methods without affecting the order of the auxiliary methods. The short form automatically includes error checking and support for :around methods. For a discussion of built-in method combination types, see section 28.1.7.4. The long-form syntax of define-method-combination is recognized when the second subform is a list. The lambda-list argument is an ordinary lambda-list. It receives any arguments provided after the name of the method combination type in the :method-combination option to defgeneric. A list of method group specifiers follows. Each specifier selects a subset of the applicable methods to play a particular role, either by matching their qualifiers against some patterns or by testing their qualifiers with a predicate. These method group specifiers define all method qualifiers that can be used with this type of method combination. If an applicable method does not fall into any method group, the system signals the error that the method is invalid for the kind of method combination in use. Each method group specifier names a variable. During the execution of the forms in the body of define-method-combination, this variable is bound to a list of the methods in the method group. The methods in this list occur in most-specific-first order. A qualifier pattern is a list or the symbol *. A method matches a qualifier pattern if the method's list of qualifiers is equal to the qualifier pattern (except that the symbol * in a qualifier pattern matches anything). Thus a qualifier pattern can be one of the following: the empty list (), which matches unqualified methods; the symbol *, which matches all methods; a true list, which matches methods with the same number of qualifiers as the length of the list when each qualifier matches the corresponding list element; or a dotted list that ends in the symbol * (the * matches any number of additional qualifiers). Each applicable method is tested against the qualifier patterns and predicates in left-to-right order. As soon as a qualifier pattern matches or a predicate returns true, the method becomes a member of the corresponding method group and no further tests are made. Thus if a method could be a member of more than one method group, it joins only the first such group. If a method group has more than one qualifier pattern, a method need only satisfy one of the qualifier patterns to be a member of the group. The name of a predicate function can appear instead of qualifier patterns in a method group specifier. The predicate is called for each method that has not been assigned to an earlier method group; it is called with one argument, the method's qualifier list. The predicate should return true if the method is to be a member of the method group. A predicate can be distinguished from a qualifier pattern because it is a symbol other than nil or *. If there is an applicable method whose qualifiers are not valid for the method combination type, the function invalid-method-error is called. Method group specifiers can have keyword options following the qualifier patterns or predicate. Keyword options can be distinguished from additional qualifier patterns because they are neither lists nor the symbol *. The keyword options are: * The :description option is used to provide a description of the role of methods in the method group. Programming environment tools use (apply #'format stream format-string (method-qualifiers method)) to print this description, which is expected to be concise. This keyword option allows the description of a method qualifier to be defined in the same module that defines the meaning of the method qualifier. In most cases, format-string will not contain any format directives, but they are available for generality. If :description is not specified, a default description is generated based on the variable name and the qualifier patterns and on whether this method group includes the unqualified methods. The argument format-string is not evaluated. * The :order option specifies the order of methods. The order argument is a form that evaluates to :most-specific-first or :most-specific-last. If it evaluates to any other value, an error is signaled. This keyword option is a convenience and does not add any expressive power. If :order is not specified, it defaults to :most-specific-first. * The :required option specifies whether at least one method in this method group is required. If the boolean argument is non-nil and the method group is empty (that is, no applicable methods match the qualifier patterns or satisfy the predicate), an error is signaled. This keyword option is a convenience and does not add any expressive power. If :required is not specified, it defaults to nil. The boolean argument is not evaluated. The use of method group specifiers provides a convenient syntax to select methods, to divide them among the possible roles, and to perform the necessary error checking. It is possible to perform further filtering of methods in the body forms by using normal list-processing operations and the functions method-qualifiers and invalid-method-error. It is permissible to use setq on the variables named in the method group specifiers and to bind additional variables. It is also possible to bypass the method group specifier mechanism and do everything in the body forms. This is accomplished by writing a single method group with * as its only qualifier pattern; the variable is then bound to a list of all of the applicable methods, in most-specific-first order. The body forms compute and return the Lisp form that specifies how the methods are combined, that is, the effective method. The effective method uses the macro call-method. The definition of this macro has lexical scope and is available only in an effective method form. Given a method object in one of the lists produced by the method group specifiers and a list of next methods, the macro call-method will invoke the method so that call-next-method will have available the next methods. When an effective method has no effect other than to call a single method, some implementations employ an optimization that uses the single method directly as the effective method, thus avoiding the need to create a new effective method. This optimization is active when the effective method form consists entirely of an invocation of the call-method macro whose first subform is a method object and whose second subform is nil. Each define-method-combination body is responsible for stripping off redundant invocations of progn, and, multiple-value-prog1, and the like, if this optimization is desired. The list (:arguments . lambda-list) can appear before any declaration or documentation string. This form is useful when the method combination type performs some specific behavior as part of the combined method and that behavior needs access to the arguments to the generic function. Each parameter variable defined by lambda-list is bound to a form that can be inserted into the effective method. When this form is evaluated during execution of the effective method, its value is the corresponding argument to the generic function. If lambda-list is not congruent to the generic function's lambda-list, additional ignored parameters are automatically inserted until it is congruent. Thus it is permissible for lambda-list to receive fewer arguments than the number that the generic function expects. Erroneous conditions detected by the body should be reported with method-combination-error or invalid-method-error; these functions add any necessary contextual information to the error message and will signal the appropriate error. The body forms are evaluated inside the bindings created by the lambda-list and method group specifiers. Declarations at the head of the body are positioned directly inside bindings created by the lambda-list and outside the bindings of the method group variables. Thus method group variables cannot be declared. Within the body forms, generic-function-symbol is bound to the generic function object. If a doc-string argument is present, it provides the documentation for the method combination type. The functions method-combination-error and invalid-method-error can be called from the body forms or from functions called by the body forms. The actions of these two functions can depend on implementation-dependent dynamic variables automatically bound before the generic function compute-effective-method is called. Note that two methods with identical specializers, but with different qualifiers, are not ordered by the algorithm described in step 2 of the method selection and combination process described in section 28.1.7. Normally the two methods play different roles in the effective method because they have different qualifiers, and no matter how they are ordered in the result of step 2 the effective method is the same. If the two methods play the same role and their order matters, an error is signaled. This happens as part of the qualifier pattern matching in define-method-combination. The value returned by the define-method-combination macro is the new method combination object. Most examples of the long form of define-method-combination also illustrate the use of the related functions that are provided as part of the declarative method combination facility. ;;; Examples of the short form of define-method-combination (define-method-combination and :identity-with-one-argument t) (defmethod func and ((x class1) y) ...) ;;; The equivalent of this example in the long form is: (define-method-combination and (&optional (order ':most-specific-first)) ((around (:around)) (primary (and) :order order :required t)) (let ((form (if (rest primary) `(and ,@(mapcar #'(lambda (method) `(call-method ,method ())) primary)) `(call-method ,(first primary) ())))) (if around `(call-method ,(first around) (,@(rest around) (make-method ,form))) form))) ;;; Examples of the long form of define-method-combination ;;; The default method-combination technique (define-method-combination standard () ((around (:around)) (before (:before)) (primary () :required t) (after (:after))) (flet ((call-methods (methods) (mapcar #'(lambda (method) `(call-method ,method ())) methods))) (let ((form (if (or before after (rest primary)) `(multiple-value-prog1 (progn ,@(call-methods before) (call-method ,(first primary) ,(rest primary))) ,@(call-methods (reverse after))) `(call-method ,(first primary) ())))) (if around `(call-method ,(first around) (,@(rest around) (make-method ,form))) form)))) ;;; A simple way to try several methods until one returns non-nil (define-method-combination or () ((methods (or))) `(or ,@(mapcar #'(lambda (method) `(call-method ,method ())) methods))) ;;; A more complete version of the preceding (define-method-combination or (&optional (order ':most-specific-first)) ((around (:around)) (primary (or))) ;; Process the order argument (case order (:most-specific-first) (:most-specific-last (setq primary (reverse primary))) (otherwise (method-combination-error "~S is an invalid order.~@ :most-specific-first and :most-specific-last ~ are the possible values." order))) ;; Must have a primary method (unless primary (method-combination-error "A primary method is required.")) ;; Construct the form that calls the primary methods (let ((form (if (rest primary) `(or ,@(mapcar #'(lambda (method) `(call-method ,method ())) primary)) `(call-method ,(first primary) ())))) ;; Wrap the around methods around that form (if around `(call-method ,(first around) (,@(rest around) (make-method ,form))) form))) ;;; The same thing, using the :order and :required keyword options (define-method-combination or (&optional (order ':most-specific-first)) ((around (:around)) (primary (or) :order order :required t)) (let ((form (if (rest primary) `(or ,@(mapcar #'(lambda (method) `(call-method ,method ())) primary)) `(call-method ,(first primary) ())))) (if around `(call-method ,(first around) (,@(rest around) (make-method ,form))) form))) ;;; This short-form call is behaviorally identical to the preceding. (define-method-combination or :identity-with-one-argument t) ;;; Order methods by positive integer qualifiers; note that :around ;;; methods are disallowed here in order to keep the example small. (define-method-combination example-method-combination () ((methods positive-integer-qualifier-p)) `(progn ,@(mapcar #'(lambda (method) `(call-method ,method ())) (stable-sort methods #'< :key #'(lambda (method) (first (method-qualifiers method))))))) (defun positive-integer-qualifier-p (method-qualifiers) (and (= (length method-qualifiers) 1) (typep (first method-qualifiers) '(integer 0 *)))) ;;; Example of the use of :arguments (define-method-combination progn-with-lock () ((methods ())) (:arguments object) `(unwind-protect (progn (lock (object-lock ,object)) ,@(mapcar #'(lambda (method) `(call-method ,method ())) methods)) (unlock (object-lock ,object)))) The :method-combination option of defgeneric is used to specify that a generic function should use a particular method combination type. The argument to the :method-combination option is the name of a method combination type. See sections 28.1.7 and 28.1.7.4 as well as call-method, method-qualifiers, method-combination-error, invalid-method-error, and defgeneric. [Macro] defmethod function-name {method-qualifier}* specialized-lambda-list [[ {declaration}* | doc-string]] {form}* function-name ::= {symbol | (setf symbol)} method-qualifier ::= non-nil-atom parameter-specializer-name ::= symbol | (eql eql-specializer-form) The macro defmethod defines a method on a generic function. If (fboundp function-name) is nil, a generic function is created with default values for the argument precedence order (each argument is more specific than the arguments to its right in the argument list), for the generic function class (the class standard-generic-function), for the method class (the class standard-method), and for the method combination type (the standard method combination type). The lambda-list of the generic function is congruent with the lambda-list of the method being defined; if the defmethod form mentions keyword arguments, the lambda-list of the generic function will mention &key (but no keyword arguments). If function-name names a non-generic function, a macro, or a special form, an error is signaled. If a generic function is currently named by function-name, where function-name is a symbol or a list of the form (setf symbol), the lambda-list of the method must be congruent with the lambda-list of the generic function. If this condition does not hold, an error is signaled. See section 28.1.6.4 for a definition of congruence in this context. The function-name argument is a non-nil symbol or a list of the form (setf symbol). It names the generic function on which the method is defined. Each method-qualifier argument is an object that is used by method combination to identify the given method. A method qualifier is a non-nil atom. The method combination type may further restrict what a method qualifier may be. The standard method combination type allows for unqualified methods or methods whose sole qualifier is the keyword :before, the keyword :after, or the keyword :around. A specialized-lambda-list is like an ordinary function lambda-list except that the name of a required parameter can be replaced by a specialized parameter, a list of the form (variable-name parameter-specializer-name). Only required parameters may be specialized. A parameter specializer name is a symbol that names a class or (eql eql-specializer-form). The parameter specializer name (eql eql-specializer-form) indicates that the corresponding argument must be eql to the object that is the value of eql-specializer-form for the method to be applicable. If no parameter specializer name is specified for a given required parameter, the parameter specializer defaults to the class named t. See section 28.1.6.2. The form arguments specify the method body. The body of the method is enclosed in an implicit block. If function-name is a symbol, this block bears the same name as the generic function. If function-name is a list of the form (setf symbol), the name of the block is symbol. The result of defmethod is the method object. The class of the method object that is created is that given by the method class option of the generic function on which the method is defined. If the generic function already has a method that agrees with the method being defined on parameter specializers and qualifiers, defmethod replaces the existing method with the one now being defined. See section 28.1.6.3 for a definition of agreement in this context. The parameter specializers are derived from the parameter specializer names as described in section 28.1.6.2. The expansion of the defmethod macro refers to each specialized parameter (see the ignore declaration specifier), including parameters that have an explicit parameter specializer name of t. This means that a compiler warning does not occur if the body of the method does not refer to a specialized parameter. Note that a parameter that specializes on t is not synonymous with an unspecialized parameter in this context. See sections 28.1.6.2, 28.1.6.4, and 28.1.6.3. [At this point the original CLOS report [5,7] contained a specification for describe as a generic function. This specification is omitted here because X3J13 voted in March 1989 (DESCRIBE-UNDERSPECIFIED) not to make describe a generic function after all (see describe-object).-GLS] [Generic function] documentation x &optional doc-type [Primary method] documentation (method standard-method) &optional doc-type documentation (generic-function standard-generic-function) &optional doc-type documentation (class standard-class) &optional doc-type documentation (method-combination method-combination) &optional doc-type documentation (slot-description standard-slot-description) &optional doc-type documentation (symbol symbol) &optional doc-type documentation (list list) &optional doc-type The ordinary function documentation (see section 25.2) is replaced by a generic function. The generic function documentation returns the documentation string associated with the given object if it is available; otherwise documentation returns nil. The first argument of documentation is a symbol, a function-name list of the form (setf symbol), a method object, a class object, a generic function object, a method combination object, or a slot description object. Whether a second argument should be supplied depends on the type of the first argument. * If the first argument is a method object, a class object, a generic function object, a method combination object, or a slot description object, the second argument must not be supplied, or an error is signaled. * If the first argument is a symbol or a list of the form (setf symbol), the second argument must be supplied. o The forms (documentation symbol 'function) and (documentation '(setf symbol) 'function) return the documentation string of the function, generic function, special form, or macro named by the symbol or list. o The form (documentation symbol 'variable) returns the documentation string of the special variable or constant named by the symbol. o The form (documentation symbol 'structure) returns the documentation string of the defstruct structure named by the symbol. o The form (documentation symbol 'type) returns the documentation string of the class object named by the symbol, if there is such a class. If there is no such class, it returns the documentation string of the type specifier named by the symbol. o The form (documentation symbol 'setf) returns the documentation string of the defsetf or define-setf-method definition associated with the symbol. o The form (documentation symbol 'method-combination) returns the documentation string of the method combination type named by the symbol. An implementation may extend the set of symbols that are acceptable as the second argument. If a symbol is not recognized as an acceptable argument by the implementation, an error must be signaled. The documentation string associated with the given object is returned unless none is available, in which case documentation returns nil. [Generic function] (setf documentation) new-value x &optional doc-type [Primary method] (setf documentation) new-value (method standard-method) &optional doc-type (setf documentation) new-value (generic-function standard-generic-function) &optional doc-type (setf documentation) new-value (class standard-class) &optional doc-type (setf documentation) new-value (method-combination method-combination) &optional doc-type (setf documentation) new-value (slot-description standard-slot-description) &optional doc-type (setf documentation) new-value (symbol symbol) &optional doc-type (setf documentation) new-value (list list) &optional doc-type The generic function (setf documentation) is used to update the documentation. The first argument of (setf documentation) is the new documentation. The second argument of documentation is a symbol, a function-name list of the form (setf symbol), a method object, a class object, a generic function object, a method combination object, or a slot description object. Whether a third argument should be supplied depends on the type of the second argument. See documentation. [Function] ensure-generic-function function-name &key :lambda-list:argument-precedence-order:declare:documentation:generic-function-class:method-combination:method-class:environment function-name ::= {symbol | (setf symbol)} The function ensure-generic-function is used to define a globally named generic function with no methods or to specify or modify options and declarations that pertain to a globally named generic function as a whole. If (fboundp function-name) is nil, a new generic function is created. If (fdefinition function-name) is a non-generic function, a macro, or a special form, an error is signaled. [X3J13 voted in March 1989 (FUNCTION-NAME) to use fdefinition in the previous paragraph, as shown, rather than symbol-function, as it appeared in the original report on CLOS [5,7]. The vote also changed all occurrences of function-specifier in the original report to function-name; this change is reflected here.-GLS] If function-name specifies a generic function that has a different value for any of the following arguments, the generic function is modified to have the new value: :argument-precedence-order, :declare, :documentation, :method-combination. If function-name specifies a generic function that has a different value for the :lambda-list argument, and the new value is congruent with the lambda-lists of all existing methods or there are no methods, the value is changed; otherwise an error is signaled. If function-name specifies a generic function that has a different value for the :generic-function-class argument and if the new generic function class is compatible with the old, change-class is called to change the class of the generic function; otherwise an error is signaled. If function-name specifies a generic function that has a different :method-class value, the value is changed but any existing methods are not changed. The function-name argument is a symbol or a list of the form (setf symbol). The keyword arguments correspond to the option arguments of defgeneric, except that the :method-class and :generic-function-class arguments can be class objects as well as names. The :environment argument is the same as the &environment argument to macro expansion functions. It is typically used to distinguish between compile-time and run-time environments. The :method-combination argument is a method combination object. The generic function object is returned. See defgeneric. [Function] find-class symbol &optional errorp environment The function find-class returns the class object named by the given symbol in the given environment. The first argument to find-class is a symbol. If there is no such class and the errorp argument is not supplied or is non-nil, find-class signals an error. If there is no such class and the errorp argument is nil, find-class returns nil. The default value of errorp is t. The optional environment argument is the same as the &environment argument to macro expansion functions. It is typically used to distinguish between compile-time and run-time environments. The result of find-class is the class object named by the given symbol. The class associated with a particular symbol can be changed by using setf with find-class. The results are undefined if the user attempts to change the class associated with a symbol that is defined as a type specifier in chapter 4. See section 28.1.4. [Generic function] find-method generic-function method-qualifiers specializers &optional errorp [Primary method] find-method (generic-function standard-generic-function) method-qualifiers specializers &optional errorp The generic function find-method takes a generic function and returns the method object that agrees on method qualifiers and parameter specializers with the method-qualifiers and specializers arguments of find-method. See section 28.1.6.3 for a definition of agreement in this context. The generic-function argument is a generic function. The method-qualifiers argument is a list of the method qualifiers for the method. The order of the method qualifiers is significant. The specializers argument is a list of the parameter specializers for the method. It must correspond in length to the number of required arguments of the generic function, or an error is signaled. This means that to obtain the default method on a given generic function, a list whose elements are the class named t must be given. If there is no such method and the errorp argument is not supplied or is non-nil, find-method signals an error. If there is no such method and the errorp argument is nil, find-method returns nil. The default value of errorp is t. The result of find-method is the method object with the given method qualifiers and parameter specializers. See section 28.1.6.3. [Generic function] function-keywords method [Primary method] function-keywords (method standard-method) The generic function function-keywords is used to return the keyword parameter specifiers for a given method. The method argument is a method object. The generic function function-keywords returns two values: a list of the explicitly named keywords and a boolean that states whether &allow-other-keys had been specified in the method definition. [Special Form] generic-flet ({(function-name lambda-list [[?option | {method-description}* ]])}*) {form}* The generic-flet special form is analogous to the flet special form. It produces new generic functions and establishes new lexical function definition bindings. Each generic function is created with the set of methods specified by its method descriptions. The special form generic-flet is used to define generic functions whose names are meaningful only locally and to execute a series of forms with these function definition bindings. Any number of such local generic functions may be defined. The names of functions defined by generic-flet have lexical scope; they retain their local definitions only within the body of the generic-flet. Any references within the body of the generic-flet to functions whose names are the same as those defined within the generic-flet are thus references to the local functions instead of to any global functions of the same names. The scope of these generic function definition bindings, however, includes only the body of generic-flet, not the definitions themselves. Within the method bodies, local function names that match those being defined refer to global functions defined outside the generic-flet. It is thus not possible to define recursive functions with generic-flet. The function-name, lambda-list, option, method-qualifier, and specialized-lambda-list arguments are the same as for defgeneric. A generic-flet local method definition is identical in form to the method definition part of a defmethod. The body of each method is enclosed in an implicit block. If function-name is a symbol, this block bears the same name as the generic function. If function-name is a list of the form (setf symbol), the name of the block is symbol. The result returned by generic-flet is the value or values returned by the last form executed. If no forms are specified, generic-flet returns nil. See generic-labels, defmethod, defgeneric, and generic-function. [Macro] generic-function lambda-list [[?option | {method-description}*]] option ::= (:argument-precedence-order {parameter-name}+) | (declare {declaration}+) | (:documentation string) | (:method-combination symbol {arg}*) | (:generic-function-class class-name) | (:method-class class-name) method-description ::= (:method {method-qualifier}* specialized-lambda-list {declaration | documentation}* {form}*) The generic-function macro creates an anonymous generic function. The generic function is created with the set of methods specified by its method descriptions. The option, method-qualifier, and specialized-lambda-list arguments are the same as for defgeneric. The generic function object is returned as the result. If no method descriptions are specified, an anonymous generic function with no methods is created. See defgeneric, generic-flet, generic-labels, and defmethod. [Special Form] generic-labels ((function-name lambda-list [[?option | {method-description}*]])}*) {form}* The generic-labels special form is analogous to the labels special form. It produces new generic functions and establishes new lexical function definition bindings. Each generic function is created with the set of methods specified by its method descriptions. The special form generic-labels is used to define generic functions whose names are meaningful only locally and to execute a series of forms with these function definition bindings. Any number of such local generic functions may be defined. The names of functions defined by generic-labels have lexical scope; they retain their local definitions only within the body of the generic-labels construct. Any references within the body of the generic-labels construct to functions whose names are the same as those defined within the generic-labels form are thus references to the local functions instead of to any global functions of the same names. The scope of these generic function definition bindings includes the method bodies themselves as well as the body of the generic-labels construct. The function-name, lambda-list, option, method-qualifier, and specialized-lambda-list arguments are the same as for defgeneric. A generic-labels local method definition is identical in form to the method definition part of a defmethod. The body of each method is enclosed in an implicit block. If function-name is a symbol, this block bears the same name as the generic function. If function-name is a list of the form (setf symbol), the name of the block is symbol. The result returned by generic-labels is the value or values returned by the last form executed. If no forms are specified, generic-labels returns nil. See generic-flet, defmethod, defgeneric, generic-function. [Generic function] initialize-instance instance &rest initargs [Primary method] initialize-instance (instance standard-object) &rest initargs The generic function initialize-instance is called by make-instance to initialize a newly created instance. The generic function initialize-instance is called with the new instance and the defaulted initialization arguments. The system-supplied primary method on initialize-instance initializes the slots of the instance with values according to the initialization arguments and the :initform forms of the slots. It does this by calling the generic function shared-initialize with the following arguments: the instance, t (this indicates that all slots for which no initialization arguments are provided should be initialized according to their :initform forms) and the defaulted initialization arguments. The instance argument is the object to be initialized. The initargs argument consists of alternating initialization argument names and values. The modified instance is returned as the result. Programmers can define methods for initialize-instance to specify actions to be taken when an instance is initialized. If only :after methods are defined, they will be run after the system-supplied primary method for initialization and therefore will not interfere with the default behavior of initialize-instance. See sections 28.1.9, 28.1.9.4, and 28.1.9.2 as well as shared-initialize, make-instance, slot-boundp, and slot-makunbound. [Function] invalid-method-error method format-string &rest args The function invalid-method-error is used to signal an error when there is an applicable method whose qualifiers are not valid for the method combination type. The error message is constructed by using a format string and any arguments to it. Because an implementation may need to add additional contextual information to the error message, invalid-method-error should be called only within the dynamic extent of a method combination function. The function invalid-method-error is called automatically when a method fails to satisfy every qualifier pattern and predicate in a define-method-combination form. A method combination function that imposes additional restrictions should call invalid-method-error explicitly if it encounters a method it cannot accept. The method argument is the invalid method object. The format-string argument is a control string that can be given to format, and args are any arguments required by that string. Whether invalid-method-error returns to its caller or exits via throw is implementation-dependent. See define-method-combination. [Generic function] make-instance class &rest initargs [Primary method] make-instance (class standard-class) &rest initargs make-instance (class symbol) &rest initargs The generic function make-instance creates a new instance of the given class. The generic function make-instance may be used as described in section 28.1.9. The class argument is a class object or a symbol that names a class. The remaining arguments form a list of alternating initialization argument names and values. If the second of the preceding methods is selected, that method invokes make-instance on the arguments (find-class class) and initargs. The initialization arguments are checked within make-instance (see section 28.1.9). The new instance is returned. The meta-object protocol can be used to define new methods on make-instance to replace the object-creation protocol. See section 28.1.9 as well as defclass, initialize-instance, and class-of. [Generic function] make-instances-obsolete class [Primary method] make-instances-obsolete (class standard-class) make-instances-obsolete (class symbol) The generic function make-instances-obsolete is invoked automatically by the system when defclass has been used to redefine an existing standard class and the set of local slots accessible in an instance is changed or the order of slots in storage is changed. It can also be explicitly invoked by the user. The function make-instances-obsolete has the effect of initiating the process of updating the instances of the class. During updating, the generic function update-instance-for-redefined-class will be invoked. The class argument is a class object symbol that names the class whose instances are to be made obsolete. If the second of the preceding methods is selected, that method invokes make-instances-obsolete on (find-class class). The modified class is returned. The result of make-instances-obsolete is eq to the class argument supplied to the first of the preceding methods. See section 28.1.10 as well as update-instance-for-redefined-class. [Function] method-combination-error format-string &rest args The function method-combination-error is used to signal an error in method combination. The error message is constructed by using a format string and any arguments to it. Because an implementation may need to add additional contextual information to the error message, method-combination-error should be called only within the dynamic extent of a method combination function. The format-string argument is a control string that can be given to format, and args are any arguments required by that string. Whether method-combination-error returns to its caller or exits via throw is implementation-dependent. See define-method-combination. [Generic function] method-qualifiers method [Primary method] method-qualifiers (method standard-method) The generic function method-qualifiers returns a list of the qualifiers of the given method. The method argument is a method object. A list of the qualifiers of the given method is returned. Example: (setq methods (remove-duplicates methods :from-end t :key #'method-qualifiers :test #'equal)) See define-method-combination. [Function] next-method-p The locally defined function next-method-p can be used within the body of a method defined by a method-defining form to determine whether a next method exists. The function next-method-p takes no arguments. The function next-method-p returns true or false. Like call-next-method, the function next-method-p has lexical scope (for it is defined only within the body of a method defined by a method-defining form) and indefinite extent. See call-next-method. [Generic function] no-applicable-method generic-function &rest function-arguments [Primary method] no-applicable-method (generic-function t) &rest function-arguments The generic function no-applicable-method is called when a generic function of the class standard-generic-function is invoked and no method on that generic function is applicable. The default method signals an error. The generic function no-applicable-method is not intended to be called by programmers. Programmers may write methods for it. The generic-function argument of no-applicable-method is the generic function object on which no applicable method was found. The function-arguments argument is a list of the arguments to that generic function. [Generic function] no-next-method generic-function method &rest args [Primary method] no-next-method (generic-function standard-generic-function) (method standard-method) &rest args The generic function no-next-method is called by call-next-method when there is no next method. The system-supplied method on no-next-method signals an error. The generic function no-next-method is not intended to be called by programmers. Programmers may write methods for it. The generic-function argument is the generic function object to which the method that is the second argument belongs. The method argument is the method that contains the call to call-next-method for which there is no next method. The args argument is a list of the arguments to call-next-method. See call-next-method. [Generic function] print-object object stream [Primary method] print-object (object standard-object) stream The generic function print-object writes the printed representation of an object to a stream. The function print-object is called by the print system; it should not be called by the user. Each implementation must provide a method on the class standard-object and methods on enough other classes so as to ensure that there is always an applicable method. Implementations are free to add methods for other classes. Users can write methods for print-object for their own classes if they do not wish to inherit an implementation-supplied method. The first argument is any Lisp object. The second argument is a stream; it cannot be t or nil. The function print-object returns its first argument, the object. Methods on print-object must obey the print control special variables named *print-xxx* for various xxx. The specific details are the following: * Each method must implement *print-escape*. * The *print-pretty* control variable can be ignored by most methods other than the one for lists. * The *print-circle* control variable is handled by the printer and can be ignored by methods. * The printer takes care of *print-level* automatically, provided that each method handles exactly one level of structure and calls write (or an equivalent function) recursively if there are more structural levels. The printer's decision of whether an object has components (and therefore should not be printed when the printing depth is not less than *print-level*) is implementation-dependent. In some implementations its print-object method is not called; in others the method is called, and the determination that the object has components is based on what it tries to write to the stream. * Methods that produce output of indefinite length must obey *print-length*, but most methods other than the one for lists can ignore it. * The *print-base*, *print-radix*, *print-case*, *print-gensym*, and *print-array* control variables apply to specific types of objects and are handled by the methods for those objects. * X3J13 voted in June 1989 (DATA-IO) to add the following point. All methods for print-object must obey *print-readably*, which takes precedence over all other printer control variables. This includes both user-defined methods and implementation-defined methods. If these rules are not obeyed, the results are undefined. In general, the printer and the print-object methods should not rebind the print control variables as they operate recursively through the structure, but this is implementation-dependent. In some implementations the stream argument passed to a print-object method is not the original stream but is an intermediate stream that implements part of the printer. Methods should therefore not depend on the identity of this stream. All of the existing printing functions (write, prin1, print, princ, pprint, write-to-string, prin1-to-string, princ-to-string, the ~S and ~A format operations, and the ~B, ~D, ~E, ~F, ~G, ~$, ~O, ~R, and ~X format operations when they encounter a non-numeric value) are required to be changed to go through the print-object generic function. Each implementation is required to replace its former implementation of printing with one or more print-object methods. Exactly which classes have methods for print-object is not specified; it would be valid for an implementation to have one default method that is inherited by all system-defined classes. [Generic function] reinitialize-instance instance &rest initargs [Primary method] reinitialize-instance (instance standard-object) &rest initargs The generic function reinitialize-instance can be used to change the values of local slots according to initialization arguments. This generic function is called by the Meta-Object Protocol. It can also be called by users. The system-supplied primary method for reinitialize-instance checks the validity of initialization arguments and signals an error if an initialization argument is supplied that is not declared valid. The method then calls the generic function shared-initialize with the following arguments: the instance, nil (which means no slots should be initialized according to their :initform forms) and the initialization arguments it received. The instance argument is the object to be initialized. The initargs argument consists of alternating initialization argument names and values. The modified instance is returned as the result. Initialization arguments are declared valid by using the :initarg option to defclass, or by defining methods for reinitialize-instance or shared-initialize. The keyword name of each keyword parameter specifier in the lambda-list of any method defined on reinitialize-instance or shared-initialize is declared a valid initialization argument name for all classes for which that method is applicable. See sections 28.1.12, 28.1.9.4, 28.1.9.2 as well as initialize-instance, slot-boundp, update-instance-for-redefined-class, update-instance-for-different-class, slot-makunbound, and shared-initialize. [Generic function] remove-method generic-function method [Primary method] remove-method (generic-function standard-generic-function) method The generic function remove-method removes a method from a generic function. It destructively modifies the specified generic function and returns the modified generic function as its result. The generic-function argument is a generic function object. The method argument is a method object. The function remove-method does not signal an error if the method is not one of the methods on the generic function. The modified generic function is returned. The result of remove-method is eq to the generic-function argument. See find-method. [Generic function] shared-initialize instance slot-names &rest initargs [Primary method] shared-initialize (instance standard-object) slot-names &rest initargs The generic function shared-initialize is used to fill the slots of an instance using initialization arguments and :initform forms. It is called when an instance is created, when an instance is re-initialized, when an instance is updated to conform to a redefined class, and when an instance is updated to conform to a different class. The generic function shared-initialize is called by the system-supplied primary method for initialize-instance, reinitialize-instance, update-instance-for-redefined-class, and update-instance-for-different-class. The generic function shared-initialize takes the following arguments: the instance to be initialized, a specification of a set of names of slots accessible in that instance, and any number of initialization arguments. The arguments after the first two must form an initialization argument list. The system-supplied primary method on shared-initialize initializes the slots with values according to the initialization arguments and specified :initform forms. The second argument indicates which slots should be initialized according to their :initform forms if no initialization arguments are provided for those slots. The system-supplied primary method behaves as follows, regardless of whether the slots are local or shared: * If an initialization argument in the initialization argument list specifies a value for that slot, that value is stored into the slot, even if a value has already been stored in the slot before the method is run. * Any slots indicated by the second argument that are still unbound at this point are initialized according to their :initform forms. For any such slot that has an :initform form, that form is evaluated in the lexical environment of its defining defclass form and the result is stored into the slot. For example, if a :before method stores a value in the slot, the :initform form will not be used to supply a value for the slot. * The rules mentioned in section 28.1.9.4 are obeyed. The instance argument is the object to be initialized. The slot-names argument specifies the slots that are to be initialized according to their :initform forms if no initialization arguments apply. It is supplied in one of three forms as follows: * It can be a list of slot names, which specifies the set of those slot names. * It can be nil, which specifies the empty set of slot names. * It can be the symbol t, which specifies the set of all of the slots. The initargs argument consists of alternating initialization argument names and values. The modified instance is returned as the result. Initialization arguments are declared valid by using the :initarg option to defclass, or by defining methods for shared-initialize. The keyword name of each keyword parameter specifier in the lambda-list of any method defined on shared-initialize is declared a valid initialization argument name for all classes for which that method is applicable. Implementations are permitted to optimize :initform forms that neither produce nor depend on side effects by evaluating these forms and storing them into slots before running any initialize-instance methods, rather than by handling them in the primary initialize-instance method. (This optimization might be implemented by having the allocate-instance method copy a prototype instance.) Implementations are permitted to optimize default initial value forms for initialization arguments associated with slots by not actually creating the complete initialization argument list when the only method that would receive the complete list is the method on standard-object. In this case, default initial value forms can be treated like :initform forms. This optimization has no visible effects other than a performance improvement. See sections 28.1.9, 28.1.9.4, 28.1.9.2 as well as initialize-instance, reinitialize-instance, update-instance-for-redefined-class, update-instance-for-different-class, slot-boundp, and slot-makunbound. [Function] slot-boundp instance slot-name The function slot-boundp tests whether a specific slot in an instance is bound. The arguments are the instance and the name of the slot. The function slot-boundp returns true or false. This function allows for writing :after methods on initialize-instance in order to initialize only those slots that have not already been bound. If no slot of the given name exists in the instance, slot-missing is called as follows: (slot-missing (class-of instance) instance slot-name 'slot-boundp) The function slot-boundp is implemented using slot-boundp-using-class. See slot-missing. [Function] slot-exists-p object slot-name The function slot-exists-p tests whether the specified object has a slot of the given name. The object argument is any object. The slot-name argument is a symbol. The function slot-exists-p returns true or false. The function slot-exists-p is implemented using slot-exists-p-using-class. [Function] slot-makunbound instance slot-name The function slot-makunbound restores a slot in an instance to the unbound state. The arguments to slot-makunbound are the instance and the name of the slot. The instance is returned as the result. If no slot of the given name exists in the instance, slot-missing is called as follows: (slot-missing (class-of instance) instance slot-name 'slot-makunbound) The function slot-makunbound is implemented using slot-makunbound-using-class. See slot-missing. [Generic function] slot-missing class object slot-name operation &optional new-value [Primary method] slot-missing (class t) object slot-name operation &optional new-value The generic function slot-missing is invoked when an attempt is made to access a slot in an object whose metaclass is standard-class and the name of the slot provided is not a name of a slot in that class. The default method signals an error. The generic function slot-missing is not intended to be called by programmers. Programmers may write methods for it. The required arguments to slot-missing are the class of the object that is being accessed, the object, the slot name, and a symbol that indicates the operation that caused slot-missing to be invoked. The optional argument to slot-missing is used when the operation is attempting to set the value of the slot. If a method written for slot-missing returns values, these values get returned as the values of the original function invocation. The generic function slot-missing may be called during evaluation of slot-value, (setf slot-value), slot-boundp, and slot-makunbound. For each of these operations the corresponding symbol for the operation argument is slot-value, setf, slot-boundp, and slot-makunbound, respectively. The set of arguments (including the class of the instance) facilitates defining methods on the metaclass for slot-missing. [Generic function] slot-unbound class instance slot-name [Primary method] slot-unbound (class t) instance slot-name The generic function slot-unbound is called when an unbound slot is read in an instance whose metaclass is standard-class. The default method signals an error. The generic function slot-unbound is not intended to be called by programmers. Programmers may write methods for it. The function slot-unbound is called only by the function slot-value-using-class and thus indirectly by slot-value. The arguments to slot-unbound are the class of the instance whose slot was accessed, the instance itself, and the name of the slot. If a method written for slot-unbound returns values, these values get returned as the values of the original function invocation. An unbound slot may occur if no :initform form was specified for the slot and the slot value has not been set, or if slot-makunbound has been called on the slot. See slot-makunbound. [Function] slot-value object slot-name The function slot-value returns the value contained in the slot slot-name of the given object. If there is no slot with that name, slot-missing is called. If the slot is unbound, slot-unbound is called. The macro setf can be used with slot-value to change the value of a slot. The arguments are the object and the name of the given slot. The result is the value contained in the given slot. If an attempt is made to read a slot and no slot of the given name exists in the instance, slot-missing is called as follows: (slot-missing (class-of instance) instance slot-name 'slot-value) If an attempt is made to write a slot and no slot of the given name exists in the instance, slot-missing is called as follows: (slot-missing (class-of instance) instance slot-name 'setf new-value) The function slot-value is implemented using slot-value-using-class. Implementations may optimize slot-value by compiling it in-line. See slot-missing and slot-unbound. [At this point the original CLOS report [5,7] contained a specification for symbol-macrolet. This specification is omitted here. Instead, a description of symbol-macrolet appears with those of related constructs in chapter 7.-GLS] [Generic function] update-instance-for-different-class previous current &rest initargs [Primary method] update-instance-for-different-class (previous standard-object) (current standard-object) &rest initargs The generic function update-instance-for-different-class is not intended to be called by programmers. Programmers may write methods for it. This function is called only by the function change-class. The system-supplied primary method on update-instance-for-different-class checks the validity of initialization arguments and signals an error if an initialization argument is supplied that is not declared valid. This method then initializes slots with values according to the initialization arguments and initializes the newly added slots with values according to their :initform forms. It does this by calling the generic function shared-initialize with the following arguments: the instance, a list of names of the newly added slots, and the initialization arguments it received. Newly added slots are those local slots for which no slot of the same name exists in the previous class. Methods for update-instance-for-different-class can be defined to specify actions to be taken when an instance is updated. If only :after methods for update-instance-for-different-class are defined, they will be run after the system-supplied primary method for initialization and therefore will not interfere with the default behavior of update-instance-for-different-class. The arguments to update-instance-for-different-class are computed by change-class. When change-class is invoked on an instance, a copy of that instance is made; change-class then destructively alters the original instance. The first argument to update-instance-for-different-class, previous, is that copy; it holds the old slot values temporarily. This argument has dynamic extent within change-class; if it is referenced in any way once update-instance-for-different-class returns, the results are undefined. The second argument to update-instance-for-different-class, current, is the altered original instance. The intended use of previous is to extract old slot values by using slot-value or with-slots or by invoking a reader generic function, or to run other methods that were applicable to instances of the original class. The initargs argument consists of alternating initialization argument names and values. The value returned by update-instance-for-different-class is ignored by change-class. See the example for the function change-class. Initialization arguments are declared valid by using the :initarg option to defclass, or by defining methods for update-instance-for-different-class or shared-initialize. The keyword name of each keyword parameter specifier in the lambda-list of any method defined on update-instance-for-different-class or shared-initialize is declared a valid initialization argument name for all classes for which that method is applicable. Methods on update-instance-for-different-class can be defined to initialize slots differently from change-class. The default behavior of change-class is described in section 28.1.11. See sections 28.1.11, 28.1.9.4, and 28.1.9.2 as well as change-class and shared-initialize. [Generic function] update-instance-for-redefined-class instance added-slots discarded-slots property-list &rest initargs [Primary method] update-instance-for-redefined-class (instance standard-object) added-slots discarded-slots property-list &rest initargs The generic function update-instance-for-redefined-class is not intended to be called by programmers. Programmers may write methods for it. The generic function update-instance-for-redefined-class is called by the mechanism activated by make-instances-obsolete. The system-supplied primary method on update-instance-for-different-class checks the validity of initialization arguments and signals an error if an initialization argument is supplied that is not declared valid. This method then initializes slots with values according to the initialization arguments and initializes the newly added slots with values according to their :initform forms. It does this by calling the generic function shared-initialize with the following arguments: the instance, a list of names of the newly added slots, and the initialization arguments it received. Newly added slots are those local slots for which no slot of the same name exists in the old version of the class. When make-instances-obsolete is invoked or when a class has been redefined and an instance is being updated, a property list is created that captures the slot names and values of all the discarded slots with values in the original instance. The structure of the instance is transformed so that it conforms to the current class definition. The arguments to update-instance-for-redefined-class are this transformed instance, a list of the names of the new slots added to the instance, a list of the names of the old slots discarded from the instance, and the property list containing the slot names and values for slots that were discarded and had values. Included in this list of discarded slots are slots that were local in the old class and are shared in the new class. The initargs argument consists of alternating initialization argument names and values. The value returned by update-instance-for-redefined-class is ignored. Initialization arguments are declared valid by using the :initarg option to defclass or by defining methods for update-instance-for-redefined-class or shared-initialize. The keyword name of each keyword parameter specifier in the lambda-list of any method defined on update-instance-for-redefined-class or shared-initialize is declared a valid initialization argument name for all classes for which that method is applicable. See sections 28.1.10, 28.1.9.4, and 28.1.9.2 as well as shared-initialize and make-instances-obsolete. (defclass position () ()) (defclass x-y-position (position) ((x :initform 0 :accessor position-x) (y :initform 0 :accessor position-y))) ;;; It turns out polar coordinates are used more than Cartesian ;;; coordinates, so the representation is altered and some new ;;; accessor methods are added. (defmethod update-instance-for-redefined-class :before ((pos x-y-position) added deleted plist &key) ;; Transform the x-y coordinates to polar coordinates ;; and store into the new slots. (let ((x (getf plist 'x)) (y (getf plist 'y))) (setf (position-rho pos) (sqrt (+ (* x x) (* y y))) (position-theta pos) (atan y x)))) (defclass x-y-position (position) ((rho :initform 0 :accessor position-rho) (theta :initform 0 :accessor position-theta))) ;;; All instances of the old x-y-position class will be updated ;;; automatically. ;;; The new representation has the look and feel of the old one. (defmethod position-x ((pos x-y-position)) (with-slots (rho theta) pos (* rho (cos theta)))) (defmethod (setf position-x) (new-x (pos x-y-position)) (with-slots (rho theta) pos (let ((y (position-y pos))) (setq rho (sqrt (+ (* new-x new-x) (* y y))) theta (atan y new-x)) new-x))) (defmethod position-y ((pos x-y-position)) (with-slots (rho theta) pos (* rho (sin theta)))) (defmethod (setf position-y) (new-y (pos x-y-position)) (with-slots (rho theta) pos (let ((x (position-x pos))) (setq rho (sqrt (+ (* x x) (* new-y new-y))) theta (atan new-y x)) new-y))) [Macro] with-accessors ({slot-entry}*) instance-form {declaration}* {form}* The macro with-accessors creates a lexical environment in which specified slots are lexically available through their accessors as if they were variables. The macro with-accessors invokes the appropriate accessors to access the specified slots. Both setf and setq can be used to set the value of the slot. The result returned is that obtained by executing the forms specified by the body argument. Example: (with-accessors ((x position-x) (y position-y)) p1 (setq x y)) A with-accessors expression of the form (with-accessors ( ... ) instance ... ) ... ) expands into the equivalent of (let ((in instance)) (symbol-macrolet (( ( in)) ... ( ( in))) ... ) ... ) [X3J13 voted in March 1989 (SYMBOL-MACROLET-SEMANTICS) to modify the definition of symbol-macrolet substantially and also voted (SYMBOL-MACROLET-DECLARE) to allow declarations before the body of symbol-macrolet but with peculiar treatment of special and type declarations. The syntactic changes are reflected in this definition of with-accessors.-GLS] See with-slots and symbol-macrolet. [Special Form] with-added-methods (function-name lambda-list [[?option | {method-description}*]]) {form}* The with-added-methods special form produces new generic functions and establishes new lexical function definition bindings. Each generic function is created by adding the set of methods specified by its method definitions to a copy of the lexically visible generic function of the same name and its methods. If such a generic function does not already exist, a new generic function is created; this generic function has lexical scope. The special form with-added-methods is used to define functions whose names are meaningful only locally and to execute a series of forms with these function definition bindings. The names of functions defined by with-added-methods have lexical scope; they retain their local definitions only within the body of the with-added-methods construct. Any references within the body of the with-added-methods construct to functions whose names are the same as those defined within the with-added-methods form are thus references to the local functions instead of to any global functions of the same names. The scope of these generic function definition bindings includes the method bodies themselves as well as the body of the with-added-methods construct. The function-name, option, method-qualifier, and specialized-lambda-list arguments are the same as for defgeneric. The body of each method is enclosed in an implicit block. If function-name is a symbol, this block bears the same name as the generic function. If function-name is a list of the form (setf symbol), the name of the block is symbol. The result returned by with-added-methods is the value or values of the last form executed. If no forms are specified, with-added-methods returns nil. If a generic function with the given name already exists, the lambda-list specified in the with-added-methods form must be congruent with the lambda-lists of all existing methods on that function as well as with the lambda-lists of all methods defined by the with-added-methods form; otherwise an error is signaled. If function-name specifies an existing generic function that has a different value for any of the following option arguments, the copy of that generic function is modified to have the new value: :argument-precedence-order, declare, :documentation, :generic-function-class, :method-combination. If function-name specifies an existing generic function that has a different value for the :method-class option argument, that value is changed in the copy of that generic function, but any methods copied from the existing generic function are not changed. If a function of the given name already exists, that function is copied into the default method for a generic function of the given name. Note that this behavior differs from that of defgeneric. If a macro or special form of the given name already exists, an error is signaled. If there is no existing generic function, the option arguments have the same default values as the option arguments to defgeneric. See generic-labels, generic-flet, defmethod, defgeneric, and ensure-generic-function. [Macro] with-slots ({slot-entry}*) instance-form {declaration}* {form}* slot-entry ::= slot-name | (variable-name slot-name) The macro with-slots creates a lexical context for referring to specified slots as though they were variables. Within such a context the value of the slot can be specified by using its slot name, as if it were a lexically bound variable. Both setf and setq can be used to set the value of the slot. The macro with-slots translates an appearance of the slot name as a variable into a call to slot-value. The result returned is that obtained by executing the forms specified by the body argument. Example: (with-slots (x y) position-1 (sqrt (+ (* x x) (* y y)))) (with-slots ((x1 x) (y1 y)) position-1 (with-slots ((x2 x) (y2 y)) position-2 (psetf x1 x2 y1 y2)))) (with-slots (x y) position (setq x (1+ x) y (1+ y))) A with-slots expression of the form: (with-slots ( ... ) instance ... ) ... ) expands into the equivalent of (let ((in instance)) (symbol-macrolet ( ... ) ... ) ... ) where is ( (slot-value in ' )) if is a symbol and is ( (slot-value in ' )) if is of the form ( ). [X3J13 voted in March 1989 (SYMBOL-MACROLET-SEMANTICS) to modify the definition of symbol-macrolet substantially and also voted (SYMBOL-MACROLET-DECLARE) to allow declarations before the body of symbol-macrolet but with peculiar treatment of special and type declarations. The syntactic changes are reflected in this definition of with-slots.-GLS] See with-accessors and symbol-macrolet. [change_end]