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Text File | 1998-10-28 | 50.8 KB | 1,453 lines

oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) NNNNAAAAMMMMEEEE overload - Package for overloading perl operations SSSSYYYYNNNNOOOOPPPPSSSSIIIISSSS package SomeThing; use overload '+' => \&myadd, '-' => \&mysub; # etc ... package main; $a = new SomeThing 57; $b=5+$a; ... if (overload::Overloaded $b) {...} ... $strval = overload::StrVal $b; CCCCAAAAVVVVEEEEAAAATTTT SSSSCCCCRRRRIIIIPPPPTTTTOOOORRRR Overloading of operators is a subject not to be taken lightly. Neither its precise implementation, syntax, nor semantics are 100% endorsed by Larry Wall. So any of these may be changed at some point in the future. DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN DDDDeeeeccccllllaaaarrrraaaattttiiiioooonnnn ooooffff oooovvvveeeerrrrllllooooaaaaddddeeeedddd ffffuuuunnnnccccttttiiiioooonnnnssss The compilation directive package Number; use overload "+" => \&add, "*=" => "muas"; declares function _N_u_m_b_e_r::_a_d_d() for addition, and method _m_u_a_s() in the "class" Number (or one of its base classes) for the assignment form *= of multiplication. Arguments of this directive come in (key, value) pairs. Legal values are values legal inside a &{ ... } call, so the name of a subroutine, a reference to a subroutine, or an anonymous subroutine will all work. Note that values specified as strings are interpreted as methods, not subroutines. Legal keys are listed below. The subroutine add will be called to execute $a+$b if $a is a reference to an object blessed into the package Number, or if $a is not an object from a package with defined mathemagic addition, but $b is a reference to a Number. It Page 1 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) can also be called in other situations, like $a+=7, or $a++. See the section on _M_A_G_I_C _A_U_T_O_G_E_N_E_R_A_T_I_O_N. (Mathemagical methods refer to methods triggered by an overloaded mathematical operator.) Since overloading respects inheritance via the @ISA hierarchy, the above declaration would also trigger overloading of + and *= in all the packages which inherit from Number. CCCCaaaalllllllliiiinnnngggg CCCCoooonnnnvvvveeeennnnttttiiiioooonnnnssss ffffoooorrrr BBBBiiiinnnnaaaarrrryyyy OOOOppppeeeerrrraaaattttiiiioooonnnnssss The functions specified in the use overload ... directive are called with three (in one particular case with four, see the section on _L_a_s_t _R_e_s_o_r_t) arguments. If the corresponding operation is binary, then the first two arguments are the two arguments of the operation. However, due to general object calling conventions, the first argument should always be an object in the package, so in the situation of 7+$a, the order of the arguments is interchanged. It probably does not matter when implementing the addition method, but whether the arguments are reversed is vital to the subtraction method. The method can query this information by examining the third argument, which can take three different values: FALSE the order of arguments is as in the current operation. TRUE the arguments are reversed. undef the current operation is an assignment variant (as in $a+=7), but the usual function is called instead. This additional information can be used to generate some optimizations. Compare the section on _C_a_l_l_i_n_g _C_o_n_v_e_n_t_i_o_n_s _f_o_r _M_u_t_a_t_o_r_s. CCCCaaaalllllllliiiinnnngggg CCCCoooonnnnvvvveeeennnnttttiiiioooonnnnssss ffffoooorrrr UUUUnnnnaaaarrrryyyy OOOOppppeeeerrrraaaattttiiiioooonnnnssss Unary operation are considered binary operations with the second argument being undef. Thus the functions that overloads {"++"} is called with arguments ($a,undef,'') when $a++ is executed. CCCCaaaalllllllliiiinnnngggg CCCCoooonnnnvvvveeeennnnttttiiiioooonnnnssss ffffoooorrrr MMMMuuuuttttaaaattttoooorrrrssss Two types of mutators have different calling conventions: ++ and -- The routines which implement these operators are expected to actually _m_u_t_a_t_e their arguments. So, assuming that $obj is a reference to a number, Page 2 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n} is an appropriate implementation of overloaded ++. Note that sub incr { ++$ {$_[0]} ; shift } is OK if used with preincrement and with postincrement. (In the case of postincrement a copying will be performed, see the section on _C_o_p_y _C_o_n_s_t_r_u_c_t_o_r.) x= and other assignment versions There is nothing special about these methods. They may change the value of their arguments, and may leave it as is. The result is going to be assigned to the value in the left-hand-side if different from this value. This allows for the same method to be used as averloaded += and +. Note that this is _a_l_l_o_w_e_d, but not recommended, since by the semantic of the section on _F_a_l_l_b_a_c_k Perl will call the method for + anyway, if += is not overloaded. WWWWaaaarrrrnnnniiiinnnngggg.... Due to the presense of assignment versions of operations, routines which may be called in assignment context may create self-referencial structures. Currently Perl will not free self-referential structures until cycles are explicitly broken. You may get problems when traversing your structures too. Say, use overload '+' => sub { bless [ \$_[0], \$_[1] ] }; is asking for trouble, since for code $obj += $foo the subroutine is called as $obj = add($obj, $foo, undef), or $obj = [\$obj, \$foo]. If using such a subroutine is an important optimization, one can overload += explicitly by a non-"optimized" version, or switch to non-optimized version if not defined $_[2] (see the section on _C_a_l_l_i_n_g _C_o_n_v_e_n_t_i_o_n_s _f_o_r _B_i_n_a_r_y _O_p_e_r_a_t_i_o_n_s). Even if no _e_x_p_l_i_c_i_t assignment-variants of operators are present in the script, they may be generated by the optimizer. Say, ",$obj," or ',' . $obj . ',' may be both optimized to my $tmp = ',' . $obj; $tmp .= ','; Page 3 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) OOOOvvvveeeerrrrllllooooaaaaddddaaaabbbblllleeee OOOOppppeeeerrrraaaattttiiiioooonnnnssss The following symbols can be specified in use overload directive: +o _A_r_i_t_h_m_e_t_i_c _o_p_e_r_a_t_i_o_n_s "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=", "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=", For these operations a substituted non-assignment variant can be called if the assignment variant is not available. Methods for operations "+", "-", "+=", and "-=" can be called to automatically generate increment and decrement methods. The operation "-" can be used to autogenerate missing methods for unary minus or abs. See the section on _M_A_G_I_C _A_U_T_O_G_E_N_E_R_A_T_I_O_N, the section on _C_a_l_l_i_n_g _C_o_n_v_e_n_t_i_o_n_s _f_o_r _M_u_t_a_t_o_r_s and the section on _C_a_l_l_i_n_g _C_o_n_v_e_n_t_i_o_n_s _f_o_r _B_i_n_a_r_y _O_p_e_r_a_t_i_o_n_s) for details of these substitutions. +o _C_o_m_p_a_r_i_s_o_n _o_p_e_r_a_t_i_o_n_s "<", "<=", ">", ">=", "==", "!=", "<=>", "lt", "le", "gt", "ge", "eq", "ne", "cmp", If the corresponding "spaceship" variant is available, it can be used to substitute for the missing operation. During sorting arrays, cmp is used to compare values subject to use overload. +o _B_i_t _o_p_e_r_a_t_i_o_n_s "&", "^", "|", "neg", "!", "~", "neg" stands for unary minus. If the method for neg is not specified, it can be autogenerated using the method for subtraction. If the method for "!" is not specified, it can be autogenerated using the methods for "bool", or "\"\"", or "0+". +o _I_n_c_r_e_m_e_n_t _a_n_d _d_e_c_r_e_m_e_n_t "++", "--", If undefined, addition and subtraction methods can be used instead. These operations are called both in prefix and postfix form. +o _T_r_a_n_s_c_e_n_d_e_n_t_a_l _f_u_n_c_t_i_o_n_s Page 4 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", If abs is unavailable, it can be autogenerated using methods for "<" or "<=>" combined with either unary minus or subtraction. +o _B_o_o_l_e_a_n, _s_t_r_i_n_g _a_n_d _n_u_m_e_r_i_c _c_o_n_v_e_r_s_i_o_n "bool", "\"\"", "0+", If one or two of these operations are unavailable, the remaining ones can be used instead. bool is used in the flow control operators (like while) and for the ternary "?:" operation. These functions can return any arbitrary Perl value. If the corresponding operation for this value is overloaded too, that operation will be called again with this value. +o _S_p_e_c_i_a_l "nomethod", "fallback", "=", see the section on _S_P_E_C_I_A_L _S_Y_M_B_O_L_S _F_O_R _u_s_e _o_v_e_r_l_o_a_d. See the section on _F_a_l_l_b_a_c_k for an explanation of when a missing method can be autogenerated. A computer-readable form of the above table is available in the hash %overload::ops, with values being space-separated lists of names: with_assign => '+ - * / % ** << >> x .', assign => '+= -= *= /= %= **= <<= >>= x= .=', str_comparison => '< <= > >= == !=', '3way_comparison'=> '<=> cmp', num_comparison => 'lt le gt ge eq ne', binary => '& | ^', unary => 'neg ! ~', mutators => '++ --', func => 'atan2 cos sin exp abs log sqrt', conversion => 'bool "" 0+', special => 'nomethod fallback =' IIIInnnnhhhheeeerrrriiiittttaaaannnncccceeee aaaannnndddd oooovvvveeeerrrrllllooooaaaaddddiiiinnnngggg Inheritance interacts with overloading in two ways. Strings as values of use overload directive If value in use overload key => value; Page 5 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) is a string, it is interpreted as a method name. Overloading of an operation is inherited by derived classes Any class derived from an overloaded class is also overloaded. The set of overloaded methods is the union of overloaded methods of all the ancestors. If some method is overloaded in several ancestor, then which description will be used is decided by the usual inheritance rules: If A inherits from B and C (in this order), B overloads + with \&D::plus_sub, and C overloads + by "plus_meth", then the subroutine D::plus_sub will be called to implement operation + for an object in package A. Note that since the value of the fallback key is not a subroutine, its inheritance is not governed by the above rules. In the current implementation, the value of fallback in the first overloaded ancestor is used, but this is accidental and subject to change. SSSSPPPPEEEECCCCIIIIAAAALLLL SSSSYYYYMMMMBBBBOOOOLLLLSSSS FFFFOOOORRRR uuuusssseeee oooovvvveeeerrrrllllooooaaaadddd Three keys are recognized by Perl that are not covered by the above description. LLLLaaaasssstttt RRRReeeessssoooorrrrtttt "nomethod" should be followed by a reference to a function of four parameters. If defined, it is called when the overloading mechanism cannot find a method for some operation. The first three arguments of this function coincide with the arguments for the corresponding method if it were found, the fourth argument is the symbol corresponding to the missing method. If several methods are tried, the last one is used. Say, 1-$a can be equivalent to &nomethodMethod($a,1,1,"-") if the pair "nomethod" => "nomethodMethod" was specified in the use overload directive. If some operation cannot be resolved, and there is no function assigned to "nomethod", then an exception will be raised via _d_i_e()-- unless "fallback" was specified as a key in use overload directive. FFFFaaaallllllllbbbbaaaacccckkkk The key "fallback" governs what to do if a method for a particular operation is not found. Three different cases are possible depending on the value of "fallback": Page 6 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) +o undef Perl tries to use a substituted method (see the section on _M_A_G_I_C _A_U_T_O_G_E_N_E_R_A_T_I_O_N). If this fails, it then tries to calls "nomethod" value; if missing, an exception will be raised. +o TRUE The same as for the undef value, but no exception is raised. Instead, it silently reverts to what it would have done were there no use overload present. +o defined, but FALSE No autogeneration is tried. Perl tries to call "nomethod" value, and if this is missing, raises an exception. NNNNooootttteeee.... "fallback" inheritance via @ISA is not carved in stone yet, see the section on _I_n_h_e_r_i_t_a_n_c_e _a_n_d _o_v_e_r_l_o_a_d_i_n_g. CCCCooooppppyyyy CCCCoooonnnnssssttttrrrruuuuccccttttoooorrrr The value for "=" is a reference to a function with three arguments, i.e., it looks like the other values in use overload. However, it does not overload the Perl assignment operator. This would go against Camel hair. This operation is called in the situations when a mutator is applied to a reference that shares its object with some other reference, such as $a=$b; ++$a; To make this change $a and not change $b, a copy of $$a is made, and $a is assigned a reference to this new object. This operation is done during execution of the ++$a, and not during the assignment, (so before the increment $$a coincides with $$b). This is only done if ++ is expressed via a method for '++' or '+=' (or nomethod). Note that if this operation is expressed via '+' a nonmutator, i.e., as in $a=$b; $a=$a+1; then $a does not reference a new copy of $$a, since $$a does not appear as lvalue when the above code is executed. If the copy constructor is required during the execution of some mutator, but a method for '=' was not specified, it can be autogenerated as a string copy if the object is a plain scalar. Page 7 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) EEEExxxxaaaammmmpppplllleeee The actually executed code for $a=$b; Something else which does not modify $a or $b.... ++$a; may be $a=$b; Something else which does not modify $a or $b.... $a = $a->clone(undef,""); $a->incr(undef,""); if $b was mathemagical, and '++' was overloaded with \&incr, '=' was overloaded with \&clone. Same behaviour is triggered by $b = $a++, which is consider a synonim for $b = $a; ++$a. MMMMAAAAGGGGIIIICCCC AAAAUUUUTTTTOOOOGGGGEEEENNNNEEEERRRRAAAATTTTIIIIOOOONNNN If a method for an operation is not found, and the value for "fallback" is TRUE or undefined, Perl tries to autogenerate a substitute method for the missing operation based on the defined operations. Autogenerated method substitutions are possible for the following operations: _A_s_s_i_g_n_m_e_n_t _f_o_r_m_s _o_f _a_r_i_t_h_m_e_t_i_c _o_p_e_r_a_t_i_o_n_s $a+=$b can use the method for "+" if the method for "+=" is not defined. _C_o_n_v_e_r_s_i_o_n _o_p_e_r_a_t_i_o_n_s String, numeric, and boolean conversion are calculated in terms of one another if not all of them are defined. _I_n_c_r_e_m_e_n_t _a_n_d _d_e_c_r_e_m_e_n_t The ++$a operation can be expressed in terms of $a+=1 or $a+1, and $a-- in terms of $a-=1 and $a-1. abs($a) can be expressed in terms of $a<0 and -$a (or 0-$a). _U_n_a_r_y _m_i_n_u_s can be expressed in terms of subtraction. _N_e_g_a_t_i_o_n ! and not can be expressed in terms of boolean conversion, or string or numerical conversion. _C_o_n_c_a_t_e_n_a_t_i_o_n can be expressed in terms of string conversion. Page 8 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) _C_o_m_p_a_r_i_s_o_n _o_p_e_r_a_t_i_o_n_s can be expressed in terms of its "spaceship" counterpart: either <=> or cmp: <, >, <=, >=, ==, != in terms of <=> lt, gt, le, ge, eq, ne in terms of cmp _C_o_p_y _o_p_e_r_a_t_o_r can be expressed in terms of an assignment to the dereferenced value, if this value is a scalar and not a reference. LLLLoooossssiiiinnnngggg oooovvvveeeerrrrllllooooaaaaddddiiiinnnngggg The restriction for the comparison operation is that even if, for example, `cmp' should return a blessed reference, the autogenerated `lt' function will produce only a standard logical value based on the numerical value of the result of `cmp'. In particular, a working numeric conversion is needed in this case (possibly expressed in terms of other conversions). Similarly, .= and x= operators lose their mathemagical properties if the string conversion substitution is applied. When you _c_h_o_p() a mathemagical object it is promoted to a string and its mathemagical properties are lost. The same can happen with other operations as well. RRRRuuuunnnn----ttttiiiimmmmeeee OOOOvvvveeeerrrrllllooooaaaaddddiiiinnnngggg Since all use directives are executed at compile-time, the only way to change overloading during run-time is to eval 'use overload "+" => \&addmethod'; You can also use eval 'no overload "+", "--", "<="'; though the use of these constructs during run-time is questionable. PPPPuuuubbbblllliiiicccc ffffuuuunnnnccccttttiiiioooonnnnssss Package overload.pm provides the following public functions: overload::StrVal(arg) Gives string value of arg as in absence of stringify overloading. overload::Overloaded(arg) Returns true if arg is subject to overloading of some operations. Page 9 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) overload::Method(obj,op) Returns undef or a reference to the method that implements op. OOOOvvvveeeerrrrllllooooaaaaddddiiiinnnngggg ccccoooonnnnssssttttaaaannnnttttssss For some application Perl parser mangles constants too much. It is possible to hook into this process via _o_v_e_r_l_o_a_d::_c_o_n_s_t_a_n_t() and _o_v_e_r_l_o_a_d::_r_e_m_o_v_e__c_o_n_s_t_a_n_t() functions. These functions take a hash as an argument. The recognized keys of this hash are integer to overload integer constants, float to overload floating point constants, binary to overload octal and hexadecimal constants, q to overload q-quoted strings, constant pieces of qq- and qx-quoted strings and here-documents, qr to overload constant pieces of regular expressions. The corresponding values are references to functions which take three arguments: the first one is the _i_n_i_t_i_a_l string form of the constant, the second one is how Perl interprets this constant, the third one is how the constant is used. Note that the initial string form does not contain string delimiters, and has backslashes in backslash-delimiter combinations stripped (thus the value of delimiter is not relevant for processing of this string). The return value of this function is how this constant is going to be interpreted by Perl. The third argument is undefined unless for overloaded q- and qr- constants, it is q in single-quote context (comes from strings, regular expressions, and single-quote HERE documents), it is tr for arguments of tr/y operators, it is s for right-hand side of s-operator, and it is qq otherwise. Since an expression "ab$cd,," is just a shortcut for 'ab' . $cd . ',,', it is expected that overloaded constant strings are equipped with reasonable overloaded catenation operator, otherwise absurd results will result. Similarly, negative numbers are considered as negations of positive constants. Note that it is probably meaningless to call the functions _o_v_e_r_l_o_a_d::_c_o_n_s_t_a_n_t() and _o_v_e_r_l_o_a_d::_r_e_m_o_v_e__c_o_n_s_t_a_n_t() from anywhere but _i_m_p_o_r_t() and _u_n_i_m_p_o_r_t() methods. From these methods they may be called as Page 10 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) sub import { shift; return unless @_; die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant'; overload::constant integer => sub {Math::BigInt->new(shift)}; } BBBBUUUUGGGGSSSS Currently overloaded-ness of constants does not propagate into eval '...'. IIIIMMMMPPPPLLLLEEEEMMMMEEEENNNNTTTTAAAATTTTIIIIOOOONNNN What follows is subject to change RSN. The table of methods for all operations is cached in magic for the symbol table hash for the package. The cache is invalidated during processing of use overload, no overload, new function definitions, and changes in @ISA. However, this invalidation remains unprocessed until the next blessing into the package. Hence if you want to change overloading structure dynamically, you'll need an additional (fake) blessing to update the table. (Every SVish thing has a magic queue, and magic is an entry in that queue. This is how a single variable may participate in multiple forms of magic simultaneously. For instance, environment variables regularly have two forms at once: their %ENV magic and their taint magic. However, the magic which implements overloading is applied to the stashes, which are rarely used directly, thus should not slow down Perl.) If an object belongs to a package using overload, it carries a special flag. Thus the only speed penalty during arithmetic operations without overloading is the checking of this flag. In fact, if use overload is not present, there is almost no overhead for overloadable operations, so most programs should not suffer measurable performance penalties. A considerable effort was made to minimize the overhead when overload is used in some package, but the arguments in question do not belong to packages using overload. When in doubt, test your speed with use overload and without it. So far there have been no reports of substantial speed degradation if Perl is compiled with optimization turned on. There is no size penalty for data if overload is not used. The only size penalty if overload is used in some package is that _a_l_l the packages acquire a magic during the next blessing into the package. This magic is three-words-long for packages without overloading, and carries the cache tabel if the package is overloaded. Page 11 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) Copying ($a=$b) is shallow; however, a one-level-deep copying is carried out before any operation that can imply an assignment to the object $a (or $b) refers to, like $a++. You can override this behavior by defining your own copy constructor (see the section on _C_o_p_y _C_o_n_s_t_r_u_c_t_o_r). It is expected that arguments to methods that are not explicitly supposed to be changed are constant (but this is not enforced). MMMMeeeettttaaaapppphhhhoooorrrr ccccllllaaaasssshhhh One may wonder why the semantic of overloaded = is so counterintuive. If it _l_o_o_k_s counterintuive to you, you are subject to a metaphor clash. Here is a Perl object metaphor: _o_b_j_e_c_t _i_s _a _r_e_f_e_r_e_n_c_e _t_o _b_l_e_s_s_e_d _d_a_t_a and an arithmetic metaphor: _o_b_j_e_c_t _i_s _a _t_h_i_n_g _b_y _i_t_s_e_l_f. The _m_a_i_n problem of overloading = is the fact that these metaphors imply different actions on the assignment $a = $b if $a and $b are objects. Perl-think implies that $a becomes a reference to whatever $b was referencing. Arithmetic-think implies that the value of "object" $a is changed to become the value of the object $b, preserving the fact that $a and $b are separate entities. The difference is not relevant in the absence of mutators. After a Perl-way assignment an operation which mutates the data referenced by $a would change the data referenced by $b too. Effectively, after $a = $b values of $a and $b become _i_n_d_i_s_t_i_n_g_u_i_s_h_a_b_l_e. On the other hand, anyone who has used algebraic notation knows the expressive power of the arithmetic metaphor. Overloading works hard to enable this metaphor while preserving the Perlian way as far as possible. Since it is not not possible to freely mix two contradicting metaphors, overloading allows the arithmetic way to write things _a_s _f_a_r _a_s _a_l_l _t_h_e _m_u_t_a_t_o_r_s _a_r_e _c_a_l_l_e_d _v_i_a _o_v_e_r_l_o_a_d_e_d _a_c_c_e_s_s _o_n_l_y. The way it is done is described in the section on _C_o_p_y _C_o_n_s_t_r_u_c_t_o_r. If some mutator methods are directly applied to the overloaded values, one may need to _e_x_p_l_i_c_i_t_l_y _u_n_l_i_n_k other values which references the same value: Page 12 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) $a = new Data 23; ... $b = $a; # $b is "linked" to $a ... $a = $a->clone; # Unlink $b from $a $a->increment_by(4); Note that overloaded access makes this transparent: $a = new Data 23; $b = $a; # $b is "linked" to $a $a += 4; # would unlink $b automagically However, it would not make $a = new Data 23; $a = 4; # Now $a is a plain 4, not 'Data' preserve "objectness" of $a. But Perl _h_a_s a way to make assignments to an object do whatever you want. It is just not the overload, but _t_i_e()ing interface (see the tie entry in the _p_e_r_l_f_u_n_c manpage). Adding a _F_E_T_C_H() method which returns the object itself, and _S_T_O_R_E() method which changes the value of the object, one can reproduce the arithmetic metaphor in its completeness, at least for variables which were _t_i_e()d from the start. (Note that a workaround for a bug may be needed, see the section on _B_U_G_S.) CCCCooooooookkkkbbbbooooooookkkk Please add examples to what follows! TTTTwwwwoooo----ffffaaaacccceeee ssssccccaaaallllaaaarrrrssss Put this in _t_w_o__f_a_c_e._p_m in your Perl library directory: package two_face; # Scalars with separate string and # numeric values. sub new { my $p = shift; bless [@_], $p } use overload '""' => \&str, '0+' => \&num, fallback => 1; sub num {shift->[1]} sub str {shift->[0]} Use it as follows: require two_face; my $seven = new two_face ("vii", 7); printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1; print "seven contains `i'\n" if $seven =~ /i/; (The second line creates a scalar which has both a string Page 13 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) value, and a numeric value.) This prints: seven=vii, seven=7, eight=8 seven contains `i' SSSSyyyymmmmbbbboooolllliiiicccc ccccaaaallllccccuuuullllaaaattttoooorrrr Put this in _s_y_m_b_o_l_i_c._p_m in your Perl library directory: package symbolic; # Primitive symbolic calculator use overload nomethod => \&wrap; sub new { shift; bless ['n', @_] } sub wrap { my ($obj, $other, $inv, $meth) = @_; ($obj, $other) = ($other, $obj) if $inv; bless [$meth, $obj, $other]; } This module is very unusual as overloaded modules go: it does not provide any usual overloaded operators, instead it provides the the section on _L_a_s_t _R_e_s_o_r_t operator nomethod. In this example the corresponding subroutine returns an object which encupsulates operations done over the objects: new symbolic 3 contains ['n', 3], 2 + new symbolic 3 contains ['+', 2, ['n', 3]]. Here is an example of the script which "calculates" the side of circumscribed octagon using the above package: require symbolic; my $iter = 1; # 2**($iter+2) = 8 my $side = new symbolic 1; my $cnt = $iter; while ($cnt--) { $side = (sqrt(1 + $side**2) - 1)/$side; } print "OK\n"; The value of $side is ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]], undef], 1], ['n', 1]] Note that while we obtained this value using a nice little script, there is no simple way to _u_s_e this value. In fact this value may be inspected in debugger (see the _p_e_r_l_d_e_b_u_g manpage), but ony if bareStringify OOOOption is set, and not via p command. Page 14 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) If one attempts to print this value, then the overloaded operator "" will be called, which will call nomethod operator. The result of this operator will be stringified again, but this result is again of type symbolic, which will lead to an infinite loop. Add a pretty-printer method to the module _s_y_m_b_o_l_i_c._p_m: sub pretty { my ($meth, $a, $b) = @{+shift}; $a = 'u' unless defined $a; $b = 'u' unless defined $b; $a = $a->pretty if ref $a; $b = $b->pretty if ref $b; "[$meth $a $b]"; } Now one can finish the script by print "side = ", $side->pretty, "\n"; The method pretty is doing object-to-string conversion, so it is natural to overload the operator "" using this method. However, inside such a method it is not necessary to pretty-print the _c_o_m_p_o_n_e_n_t_s $a and $b of an object. In the above subroutine "[$meth $a $b]" is a catenation of some strings and components $a and $b. If these components use overloading, the catenation operator will look for an overloaded operator ., if not present, it will look for an overloaded operator "". Thus it is enough to use use overload nomethod => \&wrap, '""' => \&str; sub str { my ($meth, $a, $b) = @{+shift}; $a = 'u' unless defined $a; $b = 'u' unless defined $b; "[$meth $a $b]"; } Now one can change the last line of the script to print "side = $side\n"; which outputs side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]] and one can inspect the value in debugger using all the possible methods. Something is is still amiss: consider the loop variable $cnt of the script. It was a number, not an object. We cannot Page 15 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) make this value of type symbolic, since then the loop will not terminate. Indeed, to terminate the cycle, the $cnt should become false. However, the operator bool for checking falsity is overloaded (this time via overloaded ""), and returns a long string, thus any object of type symbolic is true. To overcome this, we need a way to compare an object to 0. In fact, it is easier to write a numeric conversion routine. Here is the text of _s_y_m_b_o_l_i_c._p_m with such a routine added (and slightly modifed _s_t_r()): package symbolic; # Primitive symbolic calculator use overload nomethod => \&wrap, '""' => \&str, '0+' => \# sub new { shift; bless ['n', @_] } sub wrap { my ($obj, $other, $inv, $meth) = @_; ($obj, $other) = ($other, $obj) if $inv; bless [$meth, $obj, $other]; } sub str { my ($meth, $a, $b) = @{+shift}; $a = 'u' unless defined $a; if (defined $b) { "[$meth $a $b]"; } else { "[$meth $a]"; } } my %subr = ( n => sub {$_[0]}, sqrt => sub {sqrt $_[0]}, '-' => sub {shift() - shift()}, '+' => sub {shift() + shift()}, '/' => sub {shift() / shift()}, '*' => sub {shift() * shift()}, '**' => sub {shift() ** shift()}, ); sub num { my ($meth, $a, $b) = @{+shift}; my $subr = $subr{$meth} or die "Do not know how to ($meth) in symbolic"; $a = $a->num if ref $a eq __PACKAGE__; $b = $b->num if ref $b eq __PACKAGE__; $subr->($a,$b); } All the work of numeric conversion is done in %subr and _n_u_m(). Of course, %subr is not complete, it contains only operators used in teh example below. Here is the extra- Page 16 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) credit question: why do we need an explicit recursion in _n_u_m()? (Answer is at the end of this section.) Use this module like this: require symbolic; my $iter = new symbolic 2; # 16-gon my $side = new symbolic 1; my $cnt = $iter; while ($cnt) { $cnt = $cnt - 1; # Mutator `--' not implemented $side = (sqrt(1 + $side**2) - 1)/$side; } printf "%s=%f\n", $side, $side; printf "pi=%f\n", $side*(2**($iter+2)); It prints (without so many line breaks) [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]] 2]]] 1] [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912 pi=3.182598 The above module is very primitive. It does not implement mutator methods (++, -= and so on), does not do deep copying (not required without mutators!), and implements only those arithmetic operations which are used in the example. To implement most arithmetic operattions is easy, one should just use the tables of operations, and change the code which fills %subr to my %subr = ( 'n' => sub {$_[0]} ); foreach my $op (split " ", $overload::ops{with_assign}) { $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; } my @bins = qw(binary 3way_comparison num_comparison str_comparison); foreach my $op (split " ", "@overload::ops{ @bins }") { $subr{$op} = eval "sub {shift() $op shift()}"; } foreach my $op (split " ", "@overload::ops{qw(unary func)}") { print "defining `$op'\n"; $subr{$op} = eval "sub {$op shift()}"; } Due to the section on _C_a_l_l_i_n_g _C_o_n_v_e_n_t_i_o_n_s _f_o_r _M_u_t_a_t_o_r_s, we do not need anything special to make += and friends work, except filling += entry of %subr, and defining a copy constructor (needed since Perl has no way to know that the implementation of '+=' does not mutate the argument, compare the section on _C_o_p_y _C_o_n_s_t_r_u_c_t_o_r). Page 17 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) To implement a copy constructor, add '=' = \&cpy> to use overload line, and code (this code assumes that mutators change things one level deep only, so recursive copying is not needed): sub cpy { my $self = shift; bless [@$self], ref $self; } To make ++ and -- work, we need to implement actual mutators, either directly, or in nomethod. We continue to do things inside nomethod, thus add if ($meth eq '++' or $meth eq '--') { @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference return $obj; } after the first line of _w_r_a_p(). This is not a most effective implementation, one may consider sub inc { $_[0] = bless ['++', shift, 1]; } instead. As a final remark, note that one can fill %subr by my %subr = ( 'n' => sub {$_[0]} ); foreach my $op (split " ", $overload::ops{with_assign}) { $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; } my @bins = qw(binary 3way_comparison num_comparison str_comparison); foreach my $op (split " ", "@overload::ops{ @bins }") { $subr{$op} = eval "sub {shift() $op shift()}"; } foreach my $op (split " ", "@overload::ops{qw(unary func)}") { $subr{$op} = eval "sub {$op shift()}"; } $subr{'++'} = $subr{'+'}; $subr{'--'} = $subr{'-'}; This finishes implementation of a primitive symbolic calculator in 50 lines of Perl code. Since the numeric values of subexpressions are not cached, the calculator is very slow. Here is the answer for the exercise: In the case of _s_t_r(), we need no explicit recursion since the overloaded .-operator will fall back to an existing overloaded operator "". Overloaded arithmetic operators _d_o _n_o_t fall back to numeric conversion if fallback is not explicitly requested. Page 18 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) Thus without an explicit recursion _n_u_m() would convert ['+', $a, $b] to $a + $b, which would just rebuild the argument of _n_u_m(). If you wonder why defaults for conversion are different for _s_t_r() and _n_u_m(), note how easy it was to write the symbolic calculator. This simplicity is due to an appropriate choice of defaults. One extra note: due to teh explicit recursion _n_u_m() is more fragile than _s_y_m(): we need to explicitly check for the type of $a and $b. If componets $a and $b happen to be of some related type, this may lead to problems. _R_e_a_l_l_y symbolic calculator One may wonder why we call the above calculator symbolic. The reason is that the actual calculation of the value of expression is postponed until the value is _u_s_e_d. To see it in action, add a method sub STORE { my $obj = shift; $#$obj = 1; @$obj->[0,1] = ('=', shift); } to the package symbolic. After this change one can do my $a = new symbolic 3; my $b = new symbolic 4; my $c = sqrt($a**2 + $b**2); and the numeric value of $c becomes 5. However, after calling $a->STORE(12); $b->STORE(5); the numeric value of $c becomes 13. There is no doubt now that the module symbolic provides a _s_y_m_b_o_l_i_c calculator indeed. To hide the rough edges under the hood, provide a _t_i_e()d interface to the package symbolic (compare with the section on _M_e_t_a_p_h_o_r _c_l_a_s_h). Add methods sub TIESCALAR { my $pack = shift; $pack->new(@_) } sub FETCH { shift } sub nop { } # Around a bug (the bug is described in the section on _B_U_G_S). One can use this new interface as Page 19 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) tie $a, 'symbolic', 3; tie $b, 'symbolic', 4; $a->nop; $b->nop; # Around a bug my $c = sqrt($a**2 + $b**2); Now numeric value of $c is 5. After $a = 12; $b = 5 the numeric value of $c becomes 13. To insulate the user of the module add a method sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; } Now my ($a, $b); symbolic->vars($a, $b); my $c = sqrt($a**2 + $b**2); $a = 3; $b = 4; printf "c5 %s=%f\n", $c, $c; $a = 12; $b = 5; printf "c13 %s=%f\n", $c, $c; shows that the numeric value of $c follows changes to the values of $a and $b. AAAAUUUUTTTTHHHHOOOORRRR Ilya Zakharevich <_i_l_y_a@_m_a_t_h._m_p_s._o_h_i_o-_s_t_a_t_e._e_d_u>. DDDDIIIIAAAAGGGGNNNNOOOOSSSSTTTTIIIICCCCSSSS When Perl is run with the ----DDDDoooo switch or its equivalent, overloading induces diagnostic messages. Using the m command of Perl debugger (see the _p_e_r_l_d_e_b_u_g manpage) one can deduce which operations are overloaded (and which ancestor triggers this overloading). Say, if eq is overloaded, then the method (eq is shown by debugger. The method () corresponds to the fallback key (in fact a presence of this method shows that this package has overloading enabled, and it is what is used by the Overloaded function of module overload). BBBBUUUUGGGGSSSS Because it is used for overloading, the per-package hash %OVERLOAD now has a special meaning in Perl. The symbol table is filled with names looking like line-noise. For the purpose of inheritance every overloaded package behaves as if fallback is present (possibly undefined). This may create interesting effects if some package is not overloaded, but inherits from two overloaded packages. Page 20 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) Relation between overloading and _t_i_e()ing is broken. Overloading is triggered or not basing on the _p_r_e_v_i_o_u_s class of _t_i_e()d value. This happens because the presence of overloading is checked too early, before any _t_i_e()d access is attempted. If the _F_E_T_C_H()ed class of the _t_i_e()d value does not change, a simple workaround is to access the value immediately after _t_i_e()ing, so that after this call the _p_r_e_v_i_o_u_s class coincides with the current one. NNNNeeeeeeeeddddeeeedddd:::: a way to fix this without a speed penalty. Barewords are not covered by overloaded string constants. This document is confusing. There are grammos and misleading language used in places. It would seem a total rewrite is needed. Page 21 (printed 10/23/98) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) 1111////AAAAuuuugggg////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) oooovvvveeeerrrrllllooooaaaadddd((((3333)))) Page 22 (printed 10/23/98)