OS/2 Professional/OS2PRO194.ISO/os2/prgramer/unix/info/elisp.i05

GNU Info File | 1993-06-14 | 50.7 KB | 1,283 lines

This is Info file elisp, produced by Makeinfo-1.47 from the input file
elisp.texi.

   This file documents GNU Emacs Lisp.

   This is edition 1.03 of the GNU Emacs Lisp Reference Manual, for
Emacs Version 18.

   Published by the Free Software Foundation, 675 Massachusetts Avenue,
Cambridge, MA 02139 USA

   Copyright (C) 1990 Free Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Foundation.


File: elisp,  Node: Symbol Components,  Next: Definitions,  Prev: Symbols,  Up: Symbols

Symbol Components
=================

   Each symbol has four components (or "cells"), each of which
references another object:

Print name
     The "print name cell" holds a string which names the symbol for
     reading and printing.  See `symbol-name' in *Note Creating
     Symbols::.

Value
     The "value cell" holds the current value of the symbol as a
     variable.  When a symbol is used as a form, the value of the form
     is the contents of the symbol's value cell.  See `symbol-value' in
     *Note Accessing Variables::.

Function
     The "function cell" holds the function definition of the symbol.
     When a symbol is used as a function, its function definition is
     used in its place.  This cell is also used by the editor command
     loop to record keymaps and keyboard macros.  Because each symbol
     has separate value and function cells, variables and function
     names do not conflict.  See `symbol-function' in *Note Function
     Cells::.

Property list
     The "property list cell" holds the property list of the symbol.
     See `symbol-plist' in *Note Property Lists::.

   The print name cell always holds a string, and cannot be changed.
The other three cells can be set individually to any specified Lisp
object.

   The print name cell holds the string that is the name of the symbol.
Since symbols are represented textually by their names, it is important
not to have two symbols with the same name.  The Lisp reader ensures
this: every time it reads a symbol, it looks for an existing symbol with
the specified name before it creates a new one.  (In GNU Emacs Lisp,
this is done with a hashing algorithm that uses an obarray; see *Note
Creating Symbols::.)

   In normal usage, the function cell usually contains a function or
macro, as that is what the Lisp interpreter expects to see there (*note
Evaluation::.).  Keyboard macros (*note Keyboard Macros::.), keymaps
(*note Keymaps::.) and autoload objects (*note Autoloading::.) are also
sometimes stored in the function cell of symbols.  We often refer to
"the function `foo'" when we really mean the function stored in the
function cell of the symbol `foo'.  The distinction will be made only
when necessary.

   Similarly, the property list cell normally holds a correctly
formatted property list (*note Property Lists::.), as a number of
functions will expect to see a property list there.

   The function cell or the value cell may be "void", which means that
the cell does not reference any object.  (This is not the same thing as
holding the symbol `void', nor the same as holding the symbol `nil'.)
Examining the value of a cell which is void results in an error, such
as `Symbol's value as variable is void'.

   The four functions `symbol-name', `symbol-value', `symbol-plist',
and `symbol-function' return the contents of the four cells.  Here as
an example we show the contents of the four cells of the symbol
`buffer-file-name':

     (symbol-name 'buffer-file-name)
          => "buffer-file-name"
     (symbol-value 'buffer-file-name)
          => "/gnu/elisp/symbols.texi"
     (symbol-plist 'buffer-file-name)
          => (variable-documentation 29529)
     (symbol-function 'buffer-file-name)
          => #<subr buffer-file-name>

Because this symbol is the variable which holds the name of the file
being visited in the current buffer, the value cell contents we see are
the name of the source file of this chapter of the Emacs Lisp Manual.
The property list cell contains the list `(variable-documentation
29529)' which tells the documentation functions where to find
documentation about `buffer-file-name' in the `DOC' file. (29529 is the
offset from the beginning of the `DOC' file where the documentation for
the function begins.)  The function cell contains the function for
returning the name of the file.  Since `buffer-file-name' is a
primitive function, its function definition has no read syntax and
prints in hash notation (*note Primitive Function Type::.).  A function
definition written in Lisp will have a lambda expression (or byte-code)
in this cell.


File: elisp,  Node: Definitions,  Next: Creating Symbols,  Prev: Symbol Components,  Up: Symbols

Defining Symbols
================

   A "definition" in Lisp is a special form that announces your
intention to use a certain symbol in a particular way.  In Emacs Lisp,
you can define a symbol as a variable, or define it as a function (or
macro), or both independently.

   A definition construct typically specifies a value or meaning for the
symbol for one kind of use, plus documentation for its meaning when used
in this way.  Thus, when you define a symbol as a variable, you can
supply an initial value for the variable, plus documentation for the
variable.

   `defvar' and `defconst' are definitions that establish a symbol as a
global variable.  They are documented in detail in *Note Defining
Variables::.

   `defun' defines a symbol as a function, creating a lambda expression
and storing it in the function cell of the symbol.  This lambda
expression thus becomes the function definition of the symbol. (The
term "function definition", meaning the contents of the function cell,
is derived from the idea that `defun' gives the symbol its definition
as a function.)  *Note Functions::.

   `defmacro' defines a symbol as a macro.  It creates a macro object
and stores it in the function cell of the symbol.  Note that a given
symbol can be a macro or a function, but not both at once, because both
macro and function definitions are kept in the function cell, and that
cell can hold only one Lisp object at any given time. *Note Macros::.

   In GNU Emacs Lisp, a definition is not required in order to use a
symbol as a variable or function.  Thus, you can make a symbol a global
variable with `setq', whether you define it first or not.  The real
purpose of definitions is to guide programmers and programming tools.
They inform programmers who read the code that certain symbols are
*intended* to be used as variables, or as functions.  In addition,
utilities such as `etags' and `make-docfile' can recognize definitions,
and add the appropriate information to tag tables and the
`emacs/etc/DOC-VERSION' file. *Note Accessing Documentation::.


File: elisp,  Node: Creating Symbols,  Next: Property Lists,  Prev: Definitions,  Up: Symbols

Creating and Interning Symbols
==============================

   To understand how symbols are created in GNU Emacs Lisp, it is
necessary to know how Lisp reads them.  It is essential to ensure that
every time Lisp reads the same set of characters, it finds the same
symbol. Failure to do so would be disastrous.

   When the Lisp reader encounters a symbol, it reads all the characters
of the name.  Then it "hashes" those characters to find an index in a
table called an "obarray".  Hashing is an efficient method of looking
something up.  For example, instead of searching a telephone book cover
to cover when looking up Jan Jones, you start with the J's and go from
there.  That is a simple version of hashing.  Each element of the
obarray is a "bucket" which holds all the symbols with a given hash
code; to look for a given name, it is sufficient to look through all
the symbols in the bucket for that name's hash code.

   If a symbol with the desired name is found, then it is used.  If no
such symbol is found, then a new symbol is created and added to the
obarray bucket.  Adding a symbol to an obarray is called "interning"
it, and the symbol is then called an "interned symbol".  In Emacs Lisp,
a symbol may be interned in only one obarray.

     Common Lisp note: in Common Lisp, a symbol may be interned in
     several obarrays at once.

   If a symbol is not in the obarray, then there is no way for Lisp to
find it when its name is read.  Such a symbol is called an "uninterned
symbol" relative to the obarray.  An uninterned symbol has all the
other characteristics of symbols.  It is possible, though uncommon, for
two different symbols to have the same name in different obarrays; they
are not `eq' or `equal'.

   In Emacs Lisp, an obarray is represented as a vector.  Each element
of the vector is a bucket; its value is either an interned symbol whose
name hashes to that bucket, or 0 if the bucket is empty.  Each interned
symbol has an internal link (invisible to the user) to the next symbol
in the bucket.  Because these links are invisible, there is no way to
scan the symbols in an obarray except using `mapatoms' (below). The
order of symbols in a bucket is not significant.

   In an empty obarray, every element is 0, and you can create an
obarray with `(make-vector LENGTH 0)'.  Prime numbers as lengths tend
to result in good hashing; lengths one less than a power of two are also
good.

   Most of the functions below take a name and sometimes an obarray as
arguments.  A `wrong-type-argument' error is signaled if the name is
not a string, or if the obarray is not a vector.

 -- Function: symbol-name SYMBOL
     This function returns the string that is SYMBOL's name.  For
     example:

          (symbol-name 'foo)
               => "foo"

     Changing the string by substituting characters, etc, will change
     the name of the symbol, but will fail to update the obarray, so
     don't do it!

 -- Function: make-symbol NAME
     This function returns a newly-allocated uninterned symbol whose
     name is NAME (which must be a string).  Its value and function
     definition are void, and its property list is `nil'.  In the
     example below, the value of `sym' is not `eq' to `foo' because it
     is a distinct uninterned symbol whose name is also `foo'.

          (setq sym (make-symbol "foo"))
               => foo
          (eq sym 'foo)
               => nil

 -- Function: intern NAME &optional OBARRAY
     This function returns the interned symbol whose name is NAME.  If
     there is no such symbol in the obarray, a new one is created,
     added to the obarray, and returned.  If OBARRAY is supplied, it
     specifies the obarray to use; otherwise, the value of the global
     variable `obarray' is used.

          (setq sym (intern "foo"))
               => foo
          (eq sym 'foo)
               => t

 -- Function: intern-soft NAME &optional OBARRAY
     This function returns the symbol whose name is NAME, or `nil' if a
     symbol with that name is not found in the obarray.  Therefore, you
     can use `intern-soft' to test whether a symbol with a given name is
     interned.  If OBARRAY is supplied, it specifies the obarray to
     use; otherwise the value of the global variable `obarray' is used.

          (intern-soft "frazzle")                ; No such symbol exists.
               => nil
          (make-symbol "frazzle")                ; Create an uninterned one.
               => frazzle
          (intern-soft "frazzle")                ; That one cannot be found.
               => nil
          (setq sym (intern "frazzle"))          ; Create an interned one.
               => frazzle
          (intern-soft "frazzle")                ; That one can be found!
               => frazzle
          (eq sym 'frazzle)                      ; And it is the same one.
               => t

 -- Variable: obarray
     This variable is the standard obarray for use by `intern' and
     `read'.

 -- Function: mapatoms FUNCTION &optional OBARRAY
     This function applies FUNCTION to every symbol in OBARRAY. It
     returns `nil'.  If OBARRAY is not supplied, it defaults to the
     value of `obarray', the standard obarray for ordinary symbols.

          (setq count 0)
               => 0
          (defun count-syms (s)
            (setq count (1+ count)))
               => count-syms
          (mapatoms 'count-syms)
               => nil
          count
               => 1871

     See `documentation' in *Note Accessing Documentation::, for another
     example using `mapatoms'.


File: elisp,  Node: Property Lists,  Prev: Creating Symbols,  Up: Symbols

Property Lists
==============

   A "property list" ("plist" for short) is a list of paired elements
stored in the property list cell of a symbol.  Each of the pairs
associates a property name (usually a symbol) with a property or value.
 Property lists are generally used to record information about a
symbol, such as how to compile it, the name of the file where it was
defined, or perhaps even the grammatical class of the symbol
(representing a word) in a language understanding system.

   The property names and property values may be any Lisp objects, but
the names are usually symbols.  They are compared using `eq'.  Here is
an example of a property list, found on the symbol `progn' when the
compiler is loaded:

     (lisp-indent-hook 0 byte-compile byte-compile-progn)

Here `lisp-indent-hook' and `byte-compile' are property names, and the
other two elements are the corresponding values.

   Association lists (*note Association Lists::.) are very similar to
property lists.  In contrast to association lists, the order of the
pairs in the property list is not significant since the property names
must be distinct.

   Property lists are better than association lists when it is necessary
to attach information to various Lisp function names or variables.  If
all the pairs are recorded in one association list, it will be necessary
to search that entire list each time a function or variable is to be
operated on.  By contrast, if the information is recorded in the
property lists of the function names or variables themselves, each
search will scan only the length of one property list, which is usually
short.  For this reason, the documentation for a variable is recorded in
a property named `variable-documentation'.  The byte compiler likewise
uses properties to record those functions needing special treatment.

   However, association lists have their own advantages.  Depending on
your application, it may be faster to add an association to the front of
an association list than to update a property.  All properties for a
symbol are stored in the same property list, so there is a possibility
of a conflict between different uses of a property name.  (For this
reason, it is a good idea to use property names that are probably
unique, such as by including the name of the library in the property
name.)  An association list may be used like a stack where associations
are pushed on the front of the list and later discarded; this is not
possible with a property list.

 -- Function: symbol-plist SYMBOL
     This function returns the property list of SYMBOL.

 -- Function: setplist SYMBOL PLIST
     This function sets SYMBOL's property list to PLIST. Normally,
     PLIST should be a well-formed property list, but this is not
     enforced.

          (setplist 'foo '(a 1 b (2 3) c nil))
               => (a 1 b (2 3) c nil)
          (symbol-plist 'foo)
               => (a 1 b (2 3) c nil)

     For symbols in special obarrays, which are not used for ordinary
     purposes, it may make sense to use the property list cell in a
     nonstandard fashion; in fact, the abbrev mechanism does so (*note
     Abbrevs::.).

 -- Function: get SYMBOL PROPERTY
     This function finds the value of the property named PROPERTY in
     SYMBOL's property list.  If there is no such property, `nil' is
     returned.  Thus, there is no distinction between a value of `nil'
     and the absence of the property.

     The name PROPERTY is compared with the existing property names
     using `eq', so any object is a legitimate property.

     See `put' for an example.

 -- Function: put SYMBOL PROPERTY VALUE
     This function puts VALUE onto SYMBOL's property list under the
     property name PROPERTY, replacing any previous value.

          (put 'fly 'verb 'transitive)
               =>'transitive
          (put 'fly 'noun '(a buzzing little bug))
               => (a buzzing little bug)
          (get 'fly 'verb)
               => transitive
          (symbol-plist 'fly)
               => (verb transitive noun (a buzzing little bug))


File: elisp,  Node: Evaluation,  Next: Control Structures,  Prev: Symbols,  Up: Top

Evaluation
**********

   The "evaluation" of expressions in Emacs Lisp is performed by the
"Lisp interpreter"--a program that receives a Lisp object as input and
computes its "value as an expression".  The value is computed in a
fashion that depends on the data type of the object, following rules
described in this chapter.  The interpreter runs automatically to
evaluate portions of your program, but can also be called explicitly
via the Lisp primitive function `eval'.

* Menu:

* Intro Eval::  Evaluation in the scheme of things.
* Eval::        How to invoke the Lisp interpreter explicitly.
* Forms::       How various sorts of objects are evaluated.
* Quoting::     Avoiding evaluation (to put constants in the program).


File: elisp,  Node: Intro Eval,  Next: Eval,  Prev: Evaluation,  Up: Evaluation

Introduction to Evaluation
==========================

   The Lisp interpreter, or evaluator, is the program which computes
the value of an expression which is given to it.  When a function
written in Lisp is called, the evaluator computes the value of the
function by evaluating the expressions in the function body.  Thus,
running any Lisp program really means running the Lisp interpreter.

   How the evaluator handles an object depends primarily on the data
type of the object.

   A Lisp object which is intended for evaluation is called an
"expression" or a "form".  The fact that expressions are data objects
and not merely text is one of the fundamental differences between
Lisp-like languages and typical programming languages.  Any object can
be evaluated, but in practice only numbers, symbols, lists and strings
are evaluated very often.

   It is very common to read a Lisp expression and then evaluate the
expression, but reading and evaluation are separate activities, and
either can be performed alone.  Reading per se does not evaluate
anything; it converts the printed representation of a Lisp object to the
object itself.  It is up to the caller of `read' whether this object is
a form to be evaluated, or serves some entirely different purpose.
*Note Input Functions::.

   Do not confuse evaluation with command key interpretation.  The
editor command loop translates keyboard input into a command (an
interactively callable function) using the current keymaps, and then
uses `call-interactively' to invoke the command.  The execution of the
command itself involves evaluation if the command is written in Lisp,
but that is not a part of command key interpretation itself. *Note
Command Loop::.

   Evaluation is a recursive process.  That is, evaluation of a form may
cause `eval' to be called again in order to evaluate parts of the form.
 For example, evaluation of a function call first evaluates each
argument of the function call, and then evaluates each form in the
function body.  Consider evaluation of the form `(car x)': the subform
`x' must first be evaluated recursively, so that its value can be
passed as an argument to the function `car'.

   The evaluation of forms takes place in a context called the
"environment", which consists of the current values and bindings of all
Lisp variables.  Whenever the form refers to a variable without
creating a new binding for it, the value of the current binding is used.
*Note Variables::.

   Evaluation of a form may create new environments for recursive
evaluation by binding variables (*note Local Variables::.).  These
environments are temporary and will be gone by the time evaluation of
the form is complete.  The form may also make changes that persist;
these changes are called "side-effects".  An example of a form that
produces side-effects is `(setq foo 1)'.

   Finally, evaluation of one particular function call, `byte-code',
invokes the "byte-code interpreter" on its arguments.  Although the
byte-code interpreter is not the same as the Lisp interpreter, it uses
the same environment as the Lisp interpreter, and may on occasion invoke
the Lisp interpreter.  (*Note Byte Compilation::.)

   The details of what evaluation means for each kind of form are
described below (*note Forms::.).


File: elisp,  Node: Eval,  Next: Forms,  Prev: Intro Eval,  Up: Evaluation

Eval
====

   Most often, forms are evaluated automatically, by virtue of their
occurrence in a program being run.  On rare occasions, you may need to
write code that evaluates a form that is computed at run time, such as
when the form is read from text being edited or found on a property
list.  On these occasions, use the `eval' function.

   The functions and variables described in this section evaluate
forms, specify limits to the evaluation process, or record recently
returned values.  Evaluation is also performed by `load' (*note
Loading::.).

 -- Function: eval FORM
     This is the basic function for performing evaluation.  It evaluates
     FORM in the current environment and returns the result.  How the
     evaluation proceeds depends on the type of the object (*note
     Forms::.).

     Since `eval' is a function, the argument expression that appears
     in a call to `eval' is evaluated twice: once as preparation before
     `eval' is called, and again by the `eval' function itself. Here is
     an example:

          (setq foo 'bar)
               => bar
          (setq bar 'baz)
               => baz
          ;; `eval' is called on the form `bar', which is the value of `foo'
          (eval foo)
               => baz

     The number of currently active calls to `eval' is limited to
     `max-lisp-eval-depth'.

 -- Command: eval-current-buffer &optional STREAM
     This function evaluates the forms in the current buffer.  It reads
     forms from the buffer and calls `eval' on them until the end of the
     buffer is reached, or until an error is signaled and not handled.

     If STREAM is supplied, the variable `standard-output' is bound to
     STREAM during the evaluation (*note Output Functions::.).

     `eval-current-buffer' always returns `nil'.

 -- Command: eval-region START END &optional STREAM
     This function evaluates the forms in the current buffer in the
     region defined by the positions START and END.  It reads forms from
     the region and calls `eval' on them until the end of the region is
     reached, or until an error is signaled and not handled.

     If STREAM is supplied, `standard-output' is bound to it for the
     duration of the command.

     `eval-region' always returns `nil'.

 -- Variable: max-lisp-eval-depth
     This variable defines the maximum depth allowed in calls to
     `eval', `apply', and `funcall' before an error is signaled (with
     error message `"Lisp nesting exceeds max-lisp-eval-depth"').
     `eval' is called recursively to evaluate the arguments of Lisp
     function calls and to evaluate bodies of functions.

     This limit, with the associated error when it is exceeded, is one
     way that Lisp avoids infinite recursion on an ill-defined function.

     The default value of this variable is 200.  If you set it to a
     value less than 100, Lisp will reset it to 100 if the given value
     is reached.

 -- Variable: values
     The value of this variable is a list of values returned by all
     expressions which were read from buffers (including the
     minibuffer), evaluated, and printed.  The elements are in order,
     most recent first.

          (setq x 1)
               => 1
          (list 'A (1+ 2) auto-save-default)
               => (A 3 t)
          values
               => ((A 3 t) 1 ...)

     This variable is useful for referring back to values of forms
     recently evaluated.  It is generally a bad idea to print the value
     of `values' itself, since this may be very long.  Instead, examine
     particular elements, like this:

          ;; Refer to the most recent evaluation result.
          (nth 0 values)
               => (A 3 t)
          ;; That put a new element on, so all elements move back one.
          (nth 1 values)
               => (A 3 t)
          ;; This gets the element that was next-to-last before this example.
          (nth 3 values)
               => 1


File: elisp,  Node: Forms,  Next: Quoting,  Prev: Eval,  Up: Evaluation

Kinds of Forms
==============

   A Lisp object that is intended to be evaluated is called a "form".
How Emacs evaluates a form depends on its data type.  Emacs has three
different kinds of form that are evaluated differently: symbols, lists,
and "all other types".  All three kinds are described in this section,
starting with "all other types" which are self-evaluating forms.

* Menu:

* Self-Evaluating Forms::   Forms that evaluate to themselves.
* Symbol Forms::            Symbols evaluate as variables.
* Classifying Lists::       How to distinguish various sorts of list forms.
* Function Forms::          Forms that call functions.
* Macro Forms::             Forms that call macros.
* Special Forms::           "Special forms" are idiosyncratic primitives,
                              most of them extremely important.
* Autoloading::             Functions set up to load files
                              containing their real definitions.


File: elisp,  Node: Self-Evaluating Forms,  Next: Symbol Forms,  Prev: Forms,  Up: Forms

Self-Evaluating Forms
---------------------

   A "self-evaluating form" is any form that is not a list or symbol.
Self-evaluating forms evaluate to themselves: the result of evaluation
is the same object that was evaluated.  Thus, the number 25 evaluates to
25, and the string `"foo"' evaluates to the string `"foo"'. Likewise,
evaluation of a vector does not cause evaluation of the elements of the
vector--it returns the same vector with its contents unchanged.

     '123               ; An object, shown without evaluation.
          => 123
     123                ; Evaluated as usual---result is the same.
          => 123
     (eval '123)        ; Evaluated ``by hand''---result is the same.
          => 123
     (eval (eval '123)) ; Evaluating twice changes nothing.
          => 123

   It is common to write numbers, characters, strings, and even vectors
in Lisp code, taking advantage of the fact that they self-evaluate.
However, it is quite unusual to do this for types that lack a read
syntax, because it is inconvenient and not very useful; however, it is
possible to put them inside Lisp programs when they are constructed
from subexpressions rather than read.  Here is an example:

     ;; Build such an expression.
     (setq buffer (list 'print (current-buffer)))
          => (print #<buffer eval.texi>)
     ;; Evaluate it.
     (eval buffer)
          -| #<buffer eval.texi>
          => #<buffer eval.texi>


File: elisp,  Node: Symbol Forms,  Next: Classifying Lists,  Prev: Self-Evaluating Forms,  Up: Forms

Symbol Forms
------------

   When a symbol is evaluated, it is treated as a variable.  The result
is the variable's value, if it has one.  If it has none (if its value
cell is void), an error is signaled.  For more information on the use of
variables, see *Note Variables::.

   In the following example, the value of a symbol is set with `setq'.
When the symbol is later evaluated, that value is returned.

     (setq a 123)
          => 123
     (eval 'a)
          => 123
     a
          => 123

   The symbols `nil' and `t' are treated specially, so that the value
of `nil' is always `nil', and the value of `t' is always `t'.  Thus,
these two symbols act like self-evaluating forms, even though `eval'
treats them like any other symbol.


File: elisp,  Node: Classifying Lists,  Next: Function Forms,  Prev: Symbol Forms,  Up: Forms

Classification of List Forms
----------------------------

   A form that is a nonempty list is either a function call, a macro
call, or a special form, according to its first element.  These three
kinds of forms are evaluated in different ways, described below.  The
rest of the list consists of "arguments" for the function, macro or
special form.

   The first step in evaluating a nonempty list is to examine its first
element.  This element alone determines what kind of form the list is
and how the rest of the list is to be processed.  The first element is
*not* evaluated, as it would be in some Lisp dialects including Scheme.

   If the first element of the list is a symbol, as it most commonly is,
then the symbol's function cell is examined, and its contents are used
instead of the original symbol.  If the contents are another symbol,
this process, called "symbol function indirection", is repeated until a
non-symbol is obtained.

   One possible consequence of this process is an infinite loop, in the
event that a symbol's function cell refers to the same symbol.  Or a
symbol may have a void function cell, causing a `void-function' error.
But if neither of these things happens, we eventually obtain a
non-symbol, which ought to be a function or other suitable object.

   More precisely, we should now have a Lisp function (a lambda
expression), a primitive function, a Lisp macro, a special form, or an
autoload object.  Each of these types is a case described in one of the
following sections.  If the object is not one of these types, the error
`invalid-function' is signaled.

   The following example illustrates the symbol indirection process.  We
use `fset' to set the function cell of a symbol and `symbol-function'
to get the function cell contents (*note Function Cells::.).
Specifically, we store the symbol `car' into the function cell of
`first', and the symbol `first' into the function cell of `erste'.

     ;; Build this function cell linkage:
     ;;    -------------       -----        -------        -------
     ;;   | #<subr car> | <-- | car |  <-- | first |  <-- | erste |
     ;;    -------------       -----        -------        -------

     (symbol-function 'car)
          => #<subr car>
     (fset 'first 'car)
          => car
     (fset 'erste 'first)
          => first
     (erste '(1 2 3))           ; Call the function referenced by `erste'.
          => 1

   By contrast, the following example calls a function without any
symbol function indirection, because the first element is an anonymous
Lisp function, not a symbol.

     ((lambda (arg) (erste arg))
      '(1 2 3))
          => 1

After that function is called, its body is evaluated; this does involve
symbol function indirection when calling `erste'.


File: elisp,  Node: Function Forms,  Next: Macro Forms,  Prev: Classifying Lists,  Up: Forms

Evaluation of Function Forms
----------------------------

   If the first element of a list being evaluated is a Lisp function
object or primitive function object, then that list is a "function
call".  For example, here is a call to the function `+':

     (+ 1 x)

   When a function call is evaluated, the first step is to evaluate the
remaining elements of the list in the order they appear.  The results
are the actual argument values, one argument from each element.  Then
the function is called with this list of arguments, effectively using
the function `apply' (*note Calling Functions::.).  If the function is
written in Lisp, the arguments are used to bind the argument variables
of the function (*note Lambda Expressions::.); then the forms in the
function body are evaluated in order, and the result of the last one is
used as the value of the function call.


File: elisp,  Node: Macro Forms,  Next: Special Forms,  Prev: Function Forms,  Up: Forms

Lisp Macro Evaluation
---------------------

   If the first element of a list being evaluated is a macro object,
then the list is a "macro call".  When a macro call is evaluated, the
elements of the rest of the list are *not* initially evaluated.
Instead, these elements themselves are used as the arguments of the
macro.  The macro definition computes a replacement form, called the
"expansion" of the macro, which is evaluated in place of the original
form.  The expansion may be any sort of form: a self-evaluating
constant, a symbol or a list.  If the expansion is itself a macro call,
this process of expansion repeats until some other sort of form results.

   Normally, the argument expressions are not evaluated as part of
computing the macro expansion, but instead appear as part of the
expansion, so they are evaluated when the expansion is evaluated.

   For example, given a macro defined as follows:

     (defmacro cadr (x)
       (list 'car (list 'cdr x)))

an expression such as `(cadr (assq 'handler list))' is a macro call,
and its expansion is:

     (car (cdr (assq 'handler list)))

Note that the argument `(assq 'handler list)' appears in the expansion.

   *Note Macros::, for a complete description of Emacs Lisp macros.


File: elisp,  Node: Special Forms,  Next: Autoloading,  Prev: Macro Forms,  Up: Forms

Special Forms
-------------

   A "special form" is a primitive function specially marked so that
its arguments are not all evaluated.  Special forms define control
structures or perform variable bindings--things which functions cannot
do.

   Each special form has its own rules for which arguments are evaluated
and which are used without evaluation.  Whether a particular argument is
evaluated may depend on the results of evaluating other arguments.

   Here is a list, in alphabetical order, of all of the special forms in
Emacs Lisp with a reference to where each is described.

`and'
     *note Combining Conditions::.

`catch'
     *note Catch and Throw::.

`cond'
     *note Conditionals::.

`condition-case'
     *note Errors::.

`defconst'
     *note Defining Variables::.

`defmacro'
     *note Defining Macros::.

`defun'
     *note Defining Functions::.

`defvar'
     *note Defining Variables::.

`function'
     *note Anonymous Functions::.

`if'
     *note Conditionals::.

`interactive'
     *note Interactive Call::.

`let'
     *note Local Variables::.

`let*'
     *note Local Variables::.

`or'
     *note Combining Conditions::.

`prog1'
     *note Sequencing::.

`prog2'
     *note Sequencing::.

`progn'
     *note Sequencing::.

`quote'
     *note Quoting::.

`save-excursion'
     *note Excursions::.

`save-restriction'
     *note Narrowing::.

`save-window-excursion'
     *note Window Configurations::.

`setq'
     *note Setting Variables::.

`setq-default'
     *note Creating Buffer-Local::.

`unwind-protect'
     *note Nonlocal Exits::.

`while'
     *note Iteration::.

`with-output-to-temp-buffer'
     *note Temporary Displays::.

     Common Lisp note: here are some comparisons of special forms in
     GNU Emacs Lisp and Common Lisp.  `setq', `if', and `catch' are
     special forms in both Emacs Lisp and Common Lisp. `defun' is a
     special form in Emacs Lisp, but a macro in Common Lisp.
     `save-excursion' is a special form in Emacs Lisp, but doesn't
     exist in Common Lisp.  `throw' is a special form in Common Lisp
     (because it must be able to throw multiple values), but it is a
     function in Emacs Lisp (which doesn't have multiple values).


File: elisp,  Node: Autoloading,  Prev: Special Forms,  Up: Forms

Autoloading
-----------

   The "autoload" feature allows you to call a function or macro whose
function definition has not yet been loaded into Emacs.  When an
autoload object appears as a symbol's function definition and that
symbol is used as a function, Emacs will automatically install the real
definition (plus other associated code) and then call that definition.
(*Note Autoload::.)


File: elisp,  Node: Quoting,  Prev: Forms,  Up: Evaluation

Quoting
=======

   The special form `quote' returns its single argument "unchanged".

 -- Special Form: quote OBJECT
     This special form returns OBJECT, without evaluating it.  This
     allows symbols and lists, which would normally be evaluated, to be
     included literally in a program.  (It is not necessary to quote
     numbers, strings, and vectors since they are self-evaluating.)  Use
     `function' instead of `quote' when quoting lambda expressions
     (*note Anonymous Functions::.).

     Because `quote' is used so often in programs, a convenient read
     syntax is defined for it.  An apostrophe character (`'') followed
     by a Lisp object (in read syntax) expands to a list whose first
     element is `quote', and whose second element is the object.  Thus,
     the read syntax `'x' is an abbreviation for `(quote x)'.

     Here are some examples of expressions that use `quote':

          (quote (+ 1 2))
               => (+ 1 2)
          (quote foo)
               => foo
          'foo
               => foo
          ''foo
               => (quote foo)
          '(quote foo)
               => (quote foo)
          ['foo]
               => [(quote foo)]


File: elisp,  Node: Control Structures,  Next: Variables,  Prev: Evaluation,  Up: Top

Control Structures
******************

   A Lisp program consists of expressions or "forms" (*note Forms::.).
We control the order of execution of the forms by enclosing them in
"control structures".  Control structures are special forms which
control when, whether, or how many times to execute the forms they
contain.

   The simplest control structure is sequential execution: first form
A, then form B, and so on.  This is what happens when you write several
forms in succession in the body of a function, or at top level in a
file of Lisp code--the forms are executed in the order they are
written.  We call this "textual order".  For example, if a function
body consists of two forms A and B, evaluation of the function
evaluates first A and then B, and the function's value is the value of
B.

   Naturally, Emacs Lisp has many kinds of control structures, including
other varieties of sequencing, function calls, conditionals, iteration,
and (controlled) jumps.  The built-in control structures are special
forms since their subforms are not necessarily evaluated.  You can use
macros to define your own control structure constructs (*note
Macros::.).

* Menu:

* Sequencing::             Evaluation in textual order.
* Conditionals::           `if', `cond'.
* Combining Conditions::   `and', `or', `not'.
* Iteration::              `while' loops.
* Nonlocal Exits::         Jumping out of a sequence.


File: elisp,  Node: Sequencing,  Next: Conditionals,  Prev: Control Structures,  Up: Control Structures

Sequencing
==========

   Evaluating forms in the order they are written is the most common
control structure.  Sometimes this happens automatically, such as in a
function body.  Elsewhere you must use a control structure construct to
do this: `progn', the simplest control construct of Lisp.

   A `progn' special form looks like this:

     (progn A B C ...)

and it says to execute the forms A, B, C and so on, in that order.
These forms are called the body of the `progn' form. The value of the
last form in the body becomes the value of the entire `progn'.

   When Lisp was young, `progn' was the only way to execute two or more
forms in succession and use the value of the last of them.  But
programmers found they often needed to use a `progn' in the body of a
function, where (at that time) only one form was allowed.  So the body
of a function was made into an "implicit `progn'": several forms are
allowed just as in the body of an actual `progn'.  Many other control
structures likewise contain an implicit `progn'.  As a result, `progn'
is not used as often as it used to be.  It is needed now most often
inside of an `unwind-protect', `and', or `or'.

 -- Special Form: progn FORMS...
     This special form evaluates all of the FORMS, in textual order,
     returning the result of the final form.

          (progn (print "The first form")
                 (print "The second form")
                 (print "The third form"))
               -| "The first form"
               -| "The second form"
               -| "The third form"
          => "The third form"

   Two other control constructs likewise evaluate a series of forms but
return a different value:

 -- Special Form: prog1 FORM1 FORMS...
     This special form evaluates FORM1 and all of the FORMS, in textual
     order, returning the result of FORM1.

          (prog1 (print "The first form")
                 (print "The second form")
                 (print "The third form"))
               -| "The first form"
               -| "The second form"
               -| "The third form"
          => "The first form"

     Here is a way to remove the first element from a list in the
     variable `x', then return the value of that former element:

          (prog1 (car x) (setq x (cdr x)))

 -- Special Form: prog2 FORM1 FORM2 FORMS...
     This special form evaluates FORM1, FORM2, and all of the following
     FORMS, in textual order, returning the result of FORM2.

          (prog2 (print "The first form")
                 (print "The second form")
                 (print "The third form"))
               -| "The first form"
               -| "The second form"
               -| "The third form"
          => "The second form"


File: elisp,  Node: Conditionals,  Next: Combining Conditions,  Prev: Sequencing,  Up: Control Structures

Conditionals
============

   Conditional control structures choose among alternatives.  Emacs Lisp
has two conditional forms: `if', which is much the same as in other
languages, and `cond', which is a generalized case statement.

 -- Special Form: if CONDITION THEN-FORM ELSE-FORMS...
     `if' chooses between the THEN-FORM and the ELSE-FORMS based on the
     value of CONDITION.  If the evaluated CONDITION is non-`nil',
     THEN-FORM is evaluated and the result returned. Otherwise, the
     ELSE-FORMS are evaluated in textual order, and the value of the
     last one is returned.  (The ELSE part of `if' is an example of an
     implicit `progn'.  *Note Sequencing::.)

     If CONDITION has the value `nil', and no ELSE-FORMS are given,
     `if' returns `nil'.

     `if' is a special form because the branch which is not selected is
     never evaluated--it is ignored.  Thus, in the example below,
     `true' is not printed because `print' is never called.

          (if nil
              (print 'true)
            'very-false)
          => very-false

 -- Special Form: cond CLAUSE...
     `cond' chooses among an arbitrary number of alternatives.  Each
     CLAUSE in the `cond' must be a list.  The CAR of this list is the
     CONDITION; the remaining elements, if any, the BODY-FORMS.  Thus,
     a clause looks like this:

          (CONDITION BODY-FORMS...)

     `cond' tries the clauses in textual order, by evaluating the
     CONDITION of each clause.  If the value of CONDITION is non-`nil',
     the BODY-FORMS are evaluated, and the value of the last of
     BODY-FORMS becomes the value of the `cond'.  The remaining clauses
     are ignored.

     If the value of CONDITION is `nil', the clause "fails", so the
     `cond' moves on to the following clause, trying its CONDITION.

     If every CONDITION evaluates to `nil', so that every clause fails,
     `cond' returns `nil'.

     A clause may also look like this:

          (CONDITION)

     Then, if CONDITION is non-`nil' when tested, the value of
     CONDITION becomes the value of the `cond' form.

     The following example has four clauses, which test for the cases
     where the value of `x' is a number, string, buffer and symbol,
     respectively:

          (cond ((numberp x) x)
                 ((stringp x) x)
                 ((bufferp x)
                  (setq temporary-hack x) ; multiple body-forms
                  (buffer-name x))        ; in one clause
                 ((symbolp x) (symbol-value x)))

     Often we want the last clause to be executed whenever none of the
     previous clauses was successful.  To do this, we use `t' as the
     CONDITION of the last clause, like this: `(t BODY-FORMS)'.  The
     form `t' evaluates to `t', which is never `nil', so this clause
     never fails, provided the `cond' gets to it at all.

     For example,

          (cond ((eq a 1) 'foo)
                (t "default"))
          => "default"

     This expression is a `cond' which returns `foo' if the value of
     `a' is 1, and returns the string `"default"' otherwise.

   Both `cond' and `if' can usually be written in terms of the other.
Therefore, the choice between them is a matter of taste and style.  For
example:

     (if A B C)
     ==
     (cond (A B) (t C))


File: elisp,  Node: Combining Conditions,  Next: Iteration,  Prev: Conditionals,  Up: Control Structures

Constructs for Combining Conditions
===================================

   This section describes three constructs that are often used together
with `if' and `cond' to express complicated conditions.  The constructs
`and' and `or' can also be used individually as kinds of multiple
conditional constructs.

 -- Function: not CONDITION
     This function tests for the falsehood of CONDITION.  It returns
     `t' if CONDITION is `nil', and `nil' otherwise. The function `not'
     is identical to `null', and we recommend using `null' if you are
     testing for an empty list.

 -- Special Form: and CONDITIONS...
     The `and' special form tests whether all the CONDITIONS are true.
     It works by evaluating the CONDITIONS one by one in the order
     written.

     If any of the CONDITIONS evaluates to `nil', then the result of
     the `and' must be `nil' regardless of the remaining CONDITIONS; so
     the remaining CONDITIONS are ignored and the `and' returns right
     away.

     If all the CONDITIONS turn out non-`nil', then the value of the
     last of them becomes the value of the `and' form.

     Here is an example.  The first condition returns the integer 1,
     which is not `nil'.  Similarly, the second condition returns the
     integer 2, which is not `nil'.  The third condition is `nil', so
     the remaining condition is never evaluated.

          (and (print 1) (print 2) nil (print 3))
               -| 1
               -| 2
          => nil

     Here is a more realistic example of using `and':

          (if (and (consp foo) (eq (car foo) 'x))
              (message "foo is a list starting with x"))

     Note that `(car foo)' is not executed if `(consp foo)' returns
     `nil', thus avoiding an error.

     `and' can be expressed in terms of either `if' or `cond'. For
     example:

          (and ARG1 ARG2 ARG3)
          ==
          (if ARG1 (if ARG2 ARG3))
          ==
          (cond (ARG1 (cond (ARG2 ARG3))))

 -- Special Form: or CONDITIONS...
     The `or' special form tests whether at least one of the CONDITIONS
     is true.  It works by evaluating all the CONDITIONS one by one in
     the order written.

     If any of the CONDITIONS evaluates to a non-`nil' value, then the
     result of the `or' must be non-`nil'; so the remaining CONDITIONS
     are ignored and the `or' returns right away.  The value it returns
     is the non-`nil' value of the condition just evaluated.

     If all the CONDITIONS turn out `nil', then the `or' expression
     returns `nil'.

     For example, this expression tests whether `x' is either 0 or
     `nil':

          (or (eq x nil) (= x 0))

     Like the `and' construct, `or' can be written in terms of `cond'.
     For example:

          (or ARG1 ARG2 ARG3)
          ==
          (cond (ARG1)
                (ARG2)
                (ARG3))

     You could almost write `or' in terms of `if', but not quite:

          (if ARG1 ARG1
            (if ARG2 ARG2
              ARG3))

     This is not completely equivalent because it can evaluate ARG1 or
     ARG2 twice.  By contrast, `(or ARG1 ARG2 ARG3)' never evaluates
     any argument more than once.


File: elisp,  Node: Iteration,  Next: Nonlocal Exits,  Prev: Combining Conditions,  Up: Control Structures

Iteration
=========

   Iteration means executing part of a program repetitively.  For
example, you might want to repeat some expressions once for each
element of a list, or once for each integer from 0 to N.  You can do
this in Emacs Lisp with the special form `while':

 -- Special Form: while CONDITION FORMS...
     `while' first evaluates CONDITION.  If the result is non-`nil', it
     evaluates FORMS in textual order.  Then it reevaluates CONDITION,
     and if the result is non-`nil', it evaluates FORMS again.  This
     process repeats until CONDITION evaluates to `nil'.

     There is no limit on the number of iterations that may occur.  The
     loop will continue until either CONDITION evaluates to `nil' or
     until an error or `throw' jumps out of it (*note Nonlocal
     Exits::.).

     The value of a `while' form is always `nil'.

          (setq num 0)
               => 0
          (while (< num 4)
            (princ (format "Iteration %d." num))
            (setq num (1+ num)))
               -| Iteration 0.
               -| Iteration 1.
               -| Iteration 2.
               -| Iteration 3.
               => nil

     If you would like to execute something on each iteration before the
     end-test, put it together with the end-test in a `progn' as the
     first argument of `while', as shown here:

          (while (progn
                   (forward-line 1)
                   (not (looking-at "^$"))))

     This moves forward one line and continues moving by lines until an
     empty line is reached.