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Fix misleading example in perlretut.pod
[perl5.git] / lib / overload.pm
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1package overload;
2
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3$overload::hint_bits = 0x20000;
4
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5sub nil {}
6
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7sub OVERLOAD {
8 $package = shift;
9 my %arg = @_;
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10 my ($sub, $fb);
11 $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching.
12 *{$package . "::()"} = \&nil; # Make it findable via fetchmethod.
4633a7c4 13 for (keys %arg) {
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14 if ($_ eq 'fallback') {
15 $fb = $arg{$_};
16 } else {
17 $sub = $arg{$_};
18 if (not ref $sub and $sub !~ /::/) {
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19 $ {$package . "::(" . $_} = $sub;
20 $sub = \&nil;
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21 }
22 #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n";
23 *{$package . "::(" . $_} = \&{ $sub };
24 }
4633a7c4 25 }
a6006777 26 ${$package . "::()"} = $fb; # Make it findable too (fallback only).
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27}
28
29sub import {
30 $package = (caller())[0];
31 # *{$package . "::OVERLOAD"} = \&OVERLOAD;
32 shift;
33 $package->overload::OVERLOAD(@_);
34}
35
36sub unimport {
37 $package = (caller())[0];
a6006777 38 ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table
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39 shift;
40 for (@_) {
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41 if ($_ eq 'fallback') {
42 undef $ {$package . "::()"};
43 } else {
44 delete $ {$package . "::"}{"(" . $_};
45 }
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46 }
47}
48
49sub Overloaded {
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50 my $package = shift;
51 $package = ref $package if ref $package;
52 $package->can('()');
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53}
54
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55sub ov_method {
56 my $globref = shift;
57 return undef unless $globref;
58 my $sub = \&{*$globref};
59 return $sub if $sub ne \&nil;
60 return shift->can($ {*$globref});
61}
62
4633a7c4 63sub OverloadedStringify {
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64 my $package = shift;
65 $package = ref $package if ref $package;
44a8e56a 66 #$package->can('(""')
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67 ov_method mycan($package, '(""'), $package
68 or ov_method mycan($package, '(0+'), $package
69 or ov_method mycan($package, '(bool'), $package
70 or ov_method mycan($package, '(nomethod'), $package;
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71}
72
73sub Method {
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74 my $package = shift;
75 $package = ref $package if ref $package;
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76 #my $meth = $package->can('(' . shift);
77 ov_method mycan($package, '(' . shift), $package;
78 #return $meth if $meth ne \&nil;
79 #return $ {*{$meth}};
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80}
81
82sub AddrRef {
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83 my $package = ref $_[0];
84 return "$_[0]" unless $package;
85 bless $_[0], overload::Fake; # Non-overloaded package
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86 my $str = "$_[0]";
87 bless $_[0], $package; # Back
a6006777 88 $package . substr $str, index $str, '=';
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89}
90
91sub StrVal {
f6b3007c 92 (OverloadedStringify($_[0]) or ref($_[0]) eq 'Regexp') ?
a6006777 93 (AddrRef(shift)) :
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94 "$_[0]";
95}
96
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97sub mycan { # Real can would leave stubs.
98 my ($package, $meth) = @_;
99 return \*{$package . "::$meth"} if defined &{$package . "::$meth"};
100 my $p;
101 foreach $p (@{$package . "::ISA"}) {
102 my $out = mycan($p, $meth);
103 return $out if $out;
104 }
105 return undef;
106}
107
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108%constants = (
109 'integer' => 0x1000,
110 'float' => 0x2000,
111 'binary' => 0x4000,
112 'q' => 0x8000,
113 'qr' => 0x10000,
114 );
115
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116%ops = ( with_assign => "+ - * / % ** << >> x .",
117 assign => "+= -= *= /= %= **= <<= >>= x= .=",
2877bd81 118 num_comparison => "< <= > >= == !=",
ee239bfe 119 '3way_comparison'=> "<=> cmp",
2877bd81 120 str_comparison => "lt le gt ge eq ne",
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121 binary => "& | ^",
122 unary => "neg ! ~",
123 mutators => '++ --',
124 func => "atan2 cos sin exp abs log sqrt",
125 conversion => 'bool "" 0+',
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126 iterators => '<>',
127 dereferencing => '${} @{} %{} &{} *{}',
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128 special => 'nomethod fallback =');
129
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130sub constant {
131 # Arguments: what, sub
132 while (@_) {
133 $^H{$_[0]} = $_[1];
d5448623 134 $^H |= $constants{$_[0]} | $overload::hint_bits;
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135 shift, shift;
136 }
137}
138
139sub remove_constant {
140 # Arguments: what, sub
141 while (@_) {
142 delete $^H{$_[0]};
143 $^H &= ~ $constants{$_[0]};
144 shift, shift;
145 }
146}
147
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1481;
149
150__END__
151
152=head1 NAME
153
cb1a09d0 154overload - Package for overloading perl operations
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155
156=head1 SYNOPSIS
157
158 package SomeThing;
159
160 use overload
161 '+' => \&myadd,
162 '-' => \&mysub;
163 # etc
164 ...
165
166 package main;
167 $a = new SomeThing 57;
168 $b=5+$a;
169 ...
170 if (overload::Overloaded $b) {...}
171 ...
172 $strval = overload::StrVal $b;
173
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174=head1 DESCRIPTION
175
176=head2 Declaration of overloaded functions
177
178The compilation directive
179
180 package Number;
181 use overload
182 "+" => \&add,
183 "*=" => "muas";
184
185declares function Number::add() for addition, and method muas() in
186the "class" C<Number> (or one of its base classes)
187for the assignment form C<*=> of multiplication.
188
189Arguments of this directive come in (key, value) pairs. Legal values
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190are values legal inside a C<&{ ... }> call, so the name of a
191subroutine, a reference to a subroutine, or an anonymous subroutine
192will all work. Note that values specified as strings are
193interpreted as methods, not subroutines. Legal keys are listed below.
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194
195The subroutine C<add> will be called to execute C<$a+$b> if $a
196is a reference to an object blessed into the package C<Number>, or if $a is
197not an object from a package with defined mathemagic addition, but $b is a
198reference to a C<Number>. It can also be called in other situations, like
199C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical
200methods refer to methods triggered by an overloaded mathematical
201operator.)
202
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203Since overloading respects inheritance via the @ISA hierarchy, the
204above declaration would also trigger overloading of C<+> and C<*=> in
205all the packages which inherit from C<Number>.
e7ea3e70 206
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207=head2 Calling Conventions for Binary Operations
208
209The functions specified in the C<use overload ...> directive are called
210with three (in one particular case with four, see L<Last Resort>)
211arguments. If the corresponding operation is binary, then the first
212two arguments are the two arguments of the operation. However, due to
213general object calling conventions, the first argument should always be
214an object in the package, so in the situation of C<7+$a>, the
215order of the arguments is interchanged. It probably does not matter
216when implementing the addition method, but whether the arguments
217are reversed is vital to the subtraction method. The method can
218query this information by examining the third argument, which can take
219three different values:
220
221=over 7
222
223=item FALSE
224
225the order of arguments is as in the current operation.
226
227=item TRUE
228
229the arguments are reversed.
230
231=item C<undef>
232
233the current operation is an assignment variant (as in
234C<$a+=7>), but the usual function is called instead. This additional
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235information can be used to generate some optimizations. Compare
236L<Calling Conventions for Mutators>.
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237
238=back
239
240=head2 Calling Conventions for Unary Operations
241
242Unary operation are considered binary operations with the second
243argument being C<undef>. Thus the functions that overloads C<{"++"}>
244is called with arguments C<($a,undef,'')> when $a++ is executed.
245
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246=head2 Calling Conventions for Mutators
247
248Two types of mutators have different calling conventions:
249
250=over
251
252=item C<++> and C<-->
253
254The routines which implement these operators are expected to actually
255I<mutate> their arguments. So, assuming that $obj is a reference to a
256number,
257
258 sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n}
259
260is an appropriate implementation of overloaded C<++>. Note that
261
262 sub incr { ++$ {$_[0]} ; shift }
263
264is OK if used with preincrement and with postincrement. (In the case
265of postincrement a copying will be performed, see L<Copy Constructor>.)
266
267=item C<x=> and other assignment versions
268
269There is nothing special about these methods. They may change the
270value of their arguments, and may leave it as is. The result is going
271to be assigned to the value in the left-hand-side if different from
272this value.
273
f610777f 274This allows for the same method to be used as overloaded C<+=> and
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275C<+>. Note that this is I<allowed>, but not recommended, since by the
276semantic of L<"Fallback"> Perl will call the method for C<+> anyway,
277if C<+=> is not overloaded.
278
279=back
280
281B<Warning.> Due to the presense of assignment versions of operations,
282routines which may be called in assignment context may create
f610777f 283self-referential structures. Currently Perl will not free self-referential
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284structures until cycles are C<explicitly> broken. You may get problems
285when traversing your structures too.
286
287Say,
288
289 use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
290
291is asking for trouble, since for code C<$obj += $foo> the subroutine
292is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
293\$foo]>. If using such a subroutine is an important optimization, one
294can overload C<+=> explicitly by a non-"optimized" version, or switch
295to non-optimized version if C<not defined $_[2]> (see
296L<Calling Conventions for Binary Operations>).
297
298Even if no I<explicit> assignment-variants of operators are present in
299the script, they may be generated by the optimizer. Say, C<",$obj,"> or
300C<',' . $obj . ','> may be both optimized to
301
302 my $tmp = ',' . $obj; $tmp .= ',';
303
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304=head2 Overloadable Operations
305
ee239bfe 306The following symbols can be specified in C<use overload> directive:
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307
308=over 5
309
310=item * I<Arithmetic operations>
311
312 "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=",
313 "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
314
315For these operations a substituted non-assignment variant can be called if
316the assignment variant is not available. Methods for operations "C<+>",
317"C<->", "C<+=>", and "C<-=>" can be called to automatically generate
318increment and decrement methods. The operation "C<->" can be used to
319autogenerate missing methods for unary minus or C<abs>.
320
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321See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
322L<"Calling Conventions for Binary Operations">) for details of these
323substitutions.
324
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325=item * I<Comparison operations>
326
327 "<", "<=", ">", ">=", "==", "!=", "<=>",
328 "lt", "le", "gt", "ge", "eq", "ne", "cmp",
329
330If the corresponding "spaceship" variant is available, it can be
331used to substitute for the missing operation. During C<sort>ing
332arrays, C<cmp> is used to compare values subject to C<use overload>.
333
334=item * I<Bit operations>
335
336 "&", "^", "|", "neg", "!", "~",
337
338"C<neg>" stands for unary minus. If the method for C<neg> is not
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339specified, it can be autogenerated using the method for
340subtraction. If the method for "C<!>" is not specified, it can be
341autogenerated using the methods for "C<bool>", or "C<\"\">", or "C<0+>".
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342
343=item * I<Increment and decrement>
344
345 "++", "--",
346
347If undefined, addition and subtraction methods can be
348used instead. These operations are called both in prefix and
349postfix form.
350
351=item * I<Transcendental functions>
352
353 "atan2", "cos", "sin", "exp", "abs", "log", "sqrt",
354
355If C<abs> is unavailable, it can be autogenerated using methods
1fef88e7 356for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
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357
358=item * I<Boolean, string and numeric conversion>
359
360 "bool", "\"\"", "0+",
361
f5284f61 362If one or two of these operations are not overloaded, the remaining ones can
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363be used instead. C<bool> is used in the flow control operators
364(like C<while>) and for the ternary "C<?:>" operation. These functions can
365return any arbitrary Perl value. If the corresponding operation for this value
366is overloaded too, that operation will be called again with this value.
367
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368=item * I<Iteration>
369
370 "<>"
371
372If not overloaded, the argument will be converted to a filehandle or
373glob (which may require a stringification). The same overloading
374happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and
375I<globbing> syntax C<E<lt>${var}E<gt>>.
376
377=item * I<Dereferencing>
378
379 '${}', '@{}', '%{}', '&{}', '*{}'.
380
381If not overloaded, the argument will be dereferenced I<as is>, thus
382should be of correct type. These functions should return a reference
383of correct type, or another object with overloaded dereferencing.
384
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385=item * I<Special>
386
387 "nomethod", "fallback", "=",
388
389see L<SPECIAL SYMBOLS FOR C<use overload>>.
390
391=back
392
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393See L<"Fallback"> for an explanation of when a missing method can be
394autogenerated.
395
396A computer-readable form of the above table is available in the hash
397%overload::ops, with values being space-separated lists of names:
398
399 with_assign => '+ - * / % ** << >> x .',
400 assign => '+= -= *= /= %= **= <<= >>= x= .=',
2877bd81 401 num_comparison => '< <= > >= == !=',
ee239bfe 402 '3way_comparison'=> '<=> cmp',
2877bd81 403 str_comparison => 'lt le gt ge eq ne',
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404 binary => '& | ^',
405 unary => 'neg ! ~',
406 mutators => '++ --',
407 func => 'atan2 cos sin exp abs log sqrt',
408 conversion => 'bool "" 0+',
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409 iterators => '<>',
410 dereferencing => '${} @{} %{} &{} *{}',
ee239bfe 411 special => 'nomethod fallback ='
4633a7c4 412
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413=head2 Inheritance and overloading
414
774d564b 415Inheritance interacts with overloading in two ways.
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416
417=over
418
419=item Strings as values of C<use overload> directive
420
774d564b 421If C<value> in
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422
423 use overload key => value;
424
774d564b 425is a string, it is interpreted as a method name.
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426
427=item Overloading of an operation is inherited by derived classes
428
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429Any class derived from an overloaded class is also overloaded. The
430set of overloaded methods is the union of overloaded methods of all
431the ancestors. If some method is overloaded in several ancestor, then
e7ea3e70 432which description will be used is decided by the usual inheritance
774d564b 433rules:
e7ea3e70 434
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435If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads
436C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">,
437then the subroutine C<D::plus_sub> will be called to implement
438operation C<+> for an object in package C<A>.
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439
440=back
441
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442Note that since the value of the C<fallback> key is not a subroutine,
443its inheritance is not governed by the above rules. In the current
444implementation, the value of C<fallback> in the first overloaded
445ancestor is used, but this is accidental and subject to change.
e7ea3e70 446
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447=head1 SPECIAL SYMBOLS FOR C<use overload>
448
449Three keys are recognized by Perl that are not covered by the above
450description.
451
774d564b 452=head2 Last Resort
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453
454C<"nomethod"> should be followed by a reference to a function of four
455parameters. If defined, it is called when the overloading mechanism
456cannot find a method for some operation. The first three arguments of
457this function coincide with the arguments for the corresponding method if
458it were found, the fourth argument is the symbol
459corresponding to the missing method. If several methods are tried,
460the last one is used. Say, C<1-$a> can be equivalent to
461
462 &nomethodMethod($a,1,1,"-")
463
464if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
465C<use overload> directive.
466
467If some operation cannot be resolved, and there is no function
468assigned to C<"nomethod">, then an exception will be raised via die()--
469unless C<"fallback"> was specified as a key in C<use overload> directive.
470
471=head2 Fallback
472
473The key C<"fallback"> governs what to do if a method for a particular
474operation is not found. Three different cases are possible depending on
475the value of C<"fallback">:
476
477=over 16
478
479=item * C<undef>
480
481Perl tries to use a
482substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it
483then tries to calls C<"nomethod"> value; if missing, an exception
484will be raised.
485
486=item * TRUE
487
488The same as for the C<undef> value, but no exception is raised. Instead,
489it silently reverts to what it would have done were there no C<use overload>
490present.
491
492=item * defined, but FALSE
493
494No autogeneration is tried. Perl tries to call
495C<"nomethod"> value, and if this is missing, raises an exception.
496
497=back
498
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499B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone
500yet, see L<"Inheritance and overloading">.
501
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502=head2 Copy Constructor
503
504The value for C<"="> is a reference to a function with three
505arguments, i.e., it looks like the other values in C<use
506overload>. However, it does not overload the Perl assignment
507operator. This would go against Camel hair.
508
509This operation is called in the situations when a mutator is applied
510to a reference that shares its object with some other reference, such
511as
512
513 $a=$b;
ee239bfe 514 ++$a;
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515
516To make this change $a and not change $b, a copy of C<$$a> is made,
517and $a is assigned a reference to this new object. This operation is
ee239bfe 518done during execution of the C<++$a>, and not during the assignment,
4633a7c4 519(so before the increment C<$$a> coincides with C<$$b>). This is only
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520done if C<++> is expressed via a method for C<'++'> or C<'+='> (or
521C<nomethod>). Note that if this operation is expressed via C<'+'>
522a nonmutator, i.e., as in
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523
524 $a=$b;
525 $a=$a+1;
526
527then C<$a> does not reference a new copy of C<$$a>, since $$a does not
528appear as lvalue when the above code is executed.
529
530If the copy constructor is required during the execution of some mutator,
531but a method for C<'='> was not specified, it can be autogenerated as a
532string copy if the object is a plain scalar.
533
534=over 5
535
536=item B<Example>
537
538The actually executed code for
539
540 $a=$b;
541 Something else which does not modify $a or $b....
542 ++$a;
543
544may be
545
546 $a=$b;
547 Something else which does not modify $a or $b....
548 $a = $a->clone(undef,"");
549 $a->incr(undef,"");
550
551if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>,
552C<'='> was overloaded with C<\&clone>.
553
554=back
555
f610777f 556Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for
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557C<$b = $a; ++$a>.
558
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559=head1 MAGIC AUTOGENERATION
560
561If a method for an operation is not found, and the value for C<"fallback"> is
562TRUE or undefined, Perl tries to autogenerate a substitute method for
563the missing operation based on the defined operations. Autogenerated method
564substitutions are possible for the following operations:
565
566=over 16
567
568=item I<Assignment forms of arithmetic operations>
569
570C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
571is not defined.
572
573=item I<Conversion operations>
574
575String, numeric, and boolean conversion are calculated in terms of one
576another if not all of them are defined.
577
578=item I<Increment and decrement>
579
580The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>,
581and C<$a--> in terms of C<$a-=1> and C<$a-1>.
582
583=item C<abs($a)>
584
585can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>).
586
587=item I<Unary minus>
588
589can be expressed in terms of subtraction.
590
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591=item I<Negation>
592
593C<!> and C<not> can be expressed in terms of boolean conversion, or
594string or numerical conversion.
595
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596=item I<Concatenation>
597
598can be expressed in terms of string conversion.
599
600=item I<Comparison operations>
601
602can be expressed in terms of its "spaceship" counterpart: either
603C<E<lt>=E<gt>> or C<cmp>:
1fef88e7 604
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605 <, >, <=, >=, ==, != in terms of <=>
606 lt, gt, le, ge, eq, ne in terms of cmp
607
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608=item I<Iterator>
609
610 <> in terms of builtin operations
611
612=item I<Dereferencing>
613
614 ${} @{} %{} &{} *{} in terms of builtin operations
615
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616=item I<Copy operator>
617
618can be expressed in terms of an assignment to the dereferenced value, if this
619value is a scalar and not a reference.
620
621=back
622
ee239bfe 623=head1 Losing overloading
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624
625The restriction for the comparison operation is that even if, for example,
626`C<cmp>' should return a blessed reference, the autogenerated `C<lt>'
627function will produce only a standard logical value based on the
628numerical value of the result of `C<cmp>'. In particular, a working
629numeric conversion is needed in this case (possibly expressed in terms of
630other conversions).
631
632Similarly, C<.=> and C<x=> operators lose their mathemagical properties
633if the string conversion substitution is applied.
634
635When you chop() a mathemagical object it is promoted to a string and its
636mathemagical properties are lost. The same can happen with other
637operations as well.
638
639=head1 Run-time Overloading
640
641Since all C<use> directives are executed at compile-time, the only way to
642change overloading during run-time is to
643
644 eval 'use overload "+" => \&addmethod';
645
646You can also use
647
648 eval 'no overload "+", "--", "<="';
649
650though the use of these constructs during run-time is questionable.
651
652=head1 Public functions
653
654Package C<overload.pm> provides the following public functions:
655
656=over 5
657
658=item overload::StrVal(arg)
659
660Gives string value of C<arg> as in absence of stringify overloading.
661
662=item overload::Overloaded(arg)
663
664Returns true if C<arg> is subject to overloading of some operations.
665
666=item overload::Method(obj,op)
667
668Returns C<undef> or a reference to the method that implements C<op>.
669
670=back
671
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672=head1 Overloading constants
673
674For some application Perl parser mangles constants too much. It is possible
675to hook into this process via overload::constant() and overload::remove_constant()
676functions.
677
678These functions take a hash as an argument. The recognized keys of this hash
679are
680
681=over 8
682
683=item integer
684
685to overload integer constants,
686
687=item float
688
689to overload floating point constants,
690
691=item binary
692
693to overload octal and hexadecimal constants,
694
695=item q
696
697to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted
698strings and here-documents,
699
700=item qr
701
702to overload constant pieces of regular expressions.
703
704=back
705
706The corresponding values are references to functions which take three arguments:
707the first one is the I<initial> string form of the constant, the second one
708is how Perl interprets this constant, the third one is how the constant is used.
709Note that the initial string form does not
710contain string delimiters, and has backslashes in backslash-delimiter
711combinations stripped (thus the value of delimiter is not relevant for
712processing of this string). The return value of this function is how this
713constant is going to be interpreted by Perl. The third argument is undefined
714unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
715context (comes from strings, regular expressions, and single-quote HERE
716documents), it is C<tr> for arguments of C<tr>/C<y> operators,
717it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
718
719Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
720it is expected that overloaded constant strings are equipped with reasonable
721overloaded catenation operator, otherwise absurd results will result.
722Similarly, negative numbers are considered as negations of positive constants.
723
724Note that it is probably meaningless to call the functions overload::constant()
725and overload::remove_constant() from anywhere but import() and unimport() methods.
726From these methods they may be called as
727
728 sub import {
729 shift;
730 return unless @_;
731 die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
732 overload::constant integer => sub {Math::BigInt->new(shift)};
733 }
734
735B<BUGS> Currently overloaded-ness of constants does not propagate
736into C<eval '...'>.
737
4633a7c4
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738=head1 IMPLEMENTATION
739
740What follows is subject to change RSN.
741
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742The table of methods for all operations is cached in magic for the
743symbol table hash for the package. The cache is invalidated during
744processing of C<use overload>, C<no overload>, new function
745definitions, and changes in @ISA. However, this invalidation remains
746unprocessed until the next C<bless>ing into the package. Hence if you
747want to change overloading structure dynamically, you'll need an
748additional (fake) C<bless>ing to update the table.
749
750(Every SVish thing has a magic queue, and magic is an entry in that
751queue. This is how a single variable may participate in multiple
752forms of magic simultaneously. For instance, environment variables
753regularly have two forms at once: their %ENV magic and their taint
754magic. However, the magic which implements overloading is applied to
755the stashes, which are rarely used directly, thus should not slow down
756Perl.)
4633a7c4
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757
758If an object belongs to a package using overload, it carries a special
759flag. Thus the only speed penalty during arithmetic operations without
760overloading is the checking of this flag.
761
774d564b
PP
762In fact, if C<use overload> is not present, there is almost no overhead
763for overloadable operations, so most programs should not suffer
764measurable performance penalties. A considerable effort was made to
765minimize the overhead when overload is used in some package, but the
766arguments in question do not belong to packages using overload. When
767in doubt, test your speed with C<use overload> and without it. So far
768there have been no reports of substantial speed degradation if Perl is
769compiled with optimization turned on.
4633a7c4 770
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771There is no size penalty for data if overload is not used. The only
772size penalty if overload is used in some package is that I<all> the
773packages acquire a magic during the next C<bless>ing into the
774package. This magic is three-words-long for packages without
f610777f 775overloading, and carries the cache table if the package is overloaded.
4633a7c4
LW
776
777Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
778carried out before any operation that can imply an assignment to the
779object $a (or $b) refers to, like C<$a++>. You can override this
780behavior by defining your own copy constructor (see L<"Copy Constructor">).
781
782It is expected that arguments to methods that are not explicitly supposed
783to be changed are constant (but this is not enforced).
784
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785=head1 Metaphor clash
786
f610777f
A
787One may wonder why the semantic of overloaded C<=> is so counter intuitive.
788If it I<looks> counter intuitive to you, you are subject to a metaphor
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789clash.
790
791Here is a Perl object metaphor:
792
793I< object is a reference to blessed data>
794
795and an arithmetic metaphor:
796
797I< object is a thing by itself>.
798
799The I<main> problem of overloading C<=> is the fact that these metaphors
800imply different actions on the assignment C<$a = $b> if $a and $b are
801objects. Perl-think implies that $a becomes a reference to whatever
802$b was referencing. Arithmetic-think implies that the value of "object"
803$a is changed to become the value of the object $b, preserving the fact
804that $a and $b are separate entities.
805
806The difference is not relevant in the absence of mutators. After
807a Perl-way assignment an operation which mutates the data referenced by $a
808would change the data referenced by $b too. Effectively, after
809C<$a = $b> values of $a and $b become I<indistinguishable>.
810
811On the other hand, anyone who has used algebraic notation knows the
812expressive power of the arithmetic metaphor. Overloading works hard
813to enable this metaphor while preserving the Perlian way as far as
814possible. Since it is not not possible to freely mix two contradicting
815metaphors, overloading allows the arithmetic way to write things I<as
816far as all the mutators are called via overloaded access only>. The
817way it is done is described in L<Copy Constructor>.
818
819If some mutator methods are directly applied to the overloaded values,
820one may need to I<explicitly unlink> other values which references the
821same value:
822
823 $a = new Data 23;
824 ...
825 $b = $a; # $b is "linked" to $a
826 ...
827 $a = $a->clone; # Unlink $b from $a
828 $a->increment_by(4);
829
830Note that overloaded access makes this transparent:
831
832 $a = new Data 23;
833 $b = $a; # $b is "linked" to $a
834 $a += 4; # would unlink $b automagically
835
836However, it would not make
837
838 $a = new Data 23;
839 $a = 4; # Now $a is a plain 4, not 'Data'
840
841preserve "objectness" of $a. But Perl I<has> a way to make assignments
842to an object do whatever you want. It is just not the overload, but
843tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
844which returns the object itself, and STORE() method which changes the
845value of the object, one can reproduce the arithmetic metaphor in its
846completeness, at least for variables which were tie()d from the start.
847
848(Note that a workaround for a bug may be needed, see L<"BUGS">.)
849
850=head1 Cookbook
851
852Please add examples to what follows!
853
854=head2 Two-face scalars
855
856Put this in F<two_face.pm> in your Perl library directory:
857
858 package two_face; # Scalars with separate string and
859 # numeric values.
860 sub new { my $p = shift; bless [@_], $p }
861 use overload '""' => \&str, '0+' => \&num, fallback => 1;
862 sub num {shift->[1]}
863 sub str {shift->[0]}
864
865Use it as follows:
866
867 require two_face;
868 my $seven = new two_face ("vii", 7);
869 printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1;
870 print "seven contains `i'\n" if $seven =~ /i/;
871
872(The second line creates a scalar which has both a string value, and a
873numeric value.) This prints:
874
875 seven=vii, seven=7, eight=8
876 seven contains `i'
877
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878=head2 Two-face references
879
880Suppose you want to create an object which is accessible as both an
8db13b63
DC
881array reference and a hash reference, similar to the
882L<pseudo-hash|perlref/"Pseudo-hashes: Using an array as a hash">
883builtin Perl type. Let's make it better than a pseudo-hash by
884allowing index 0 to be treated as a normal element.
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IZ
885
886 package two_refs;
887 use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
888 sub new {
889 my $p = shift;
890 bless \ [@_], $p;
891 }
892 sub gethash {
893 my %h;
894 my $self = shift;
895 tie %h, ref $self, $self;
896 \%h;
897 }
898
899 sub TIEHASH { my $p = shift; bless \ shift, $p }
900 my %fields;
901 my $i = 0;
902 $fields{$_} = $i++ foreach qw{zero one two three};
903 sub STORE {
904 my $self = ${shift()};
905 my $key = $fields{shift()};
906 defined $key or die "Out of band access";
907 $$self->[$key] = shift;
908 }
909 sub FETCH {
910 my $self = ${shift()};
911 my $key = $fields{shift()};
912 defined $key or die "Out of band access";
913 $$self->[$key];
914 }
915
916Now one can access an object using both the array and hash syntax:
917
918 my $bar = new two_refs 3,4,5,6;
919 $bar->[2] = 11;
920 $bar->{two} == 11 or die 'bad hash fetch';
921
922Note several important features of this example. First of all, the
923I<actual> type of $bar is a scalar reference, and we do not overload
924the scalar dereference. Thus we can get the I<actual> non-overloaded
925contents of $bar by just using C<$$bar> (what we do in functions which
926overload dereference). Similarly, the object returned by the
927TIEHASH() method is a scalar reference.
928
929Second, we create a new tied hash each time the hash syntax is used.
930This allows us not to worry about a possibility of a reference loop,
931would would lead to a memory leak.
932
933Both these problems can be cured. Say, if we want to overload hash
934dereference on a reference to an object which is I<implemented> as a
935hash itself, the only problem one has to circumvent is how to access
936this I<actual> hash (as opposed to the I<virtual> exhibited by
937overloaded dereference operator). Here is one possible fetching routine:
938
939 sub access_hash {
940 my ($self, $key) = (shift, shift);
941 my $class = ref $self;
942 bless $self, 'overload::dummy'; # Disable overloading of %{}
943 my $out = $self->{$key};
944 bless $self, $class; # Restore overloading
945 $out;
946 }
947
948To move creation of the tied hash on each access, one may an extra
949level of indirection which allows a non-circular structure of references:
950
951 package two_refs1;
952 use overload '%{}' => sub { ${shift()}->[1] },
953 '@{}' => sub { ${shift()}->[0] };
954 sub new {
955 my $p = shift;
956 my $a = [@_];
957 my %h;
958 tie %h, $p, $a;
959 bless \ [$a, \%h], $p;
960 }
961 sub gethash {
962 my %h;
963 my $self = shift;
964 tie %h, ref $self, $self;
965 \%h;
966 }
967
968 sub TIEHASH { my $p = shift; bless \ shift, $p }
969 my %fields;
970 my $i = 0;
971 $fields{$_} = $i++ foreach qw{zero one two three};
972 sub STORE {
973 my $a = ${shift()};
974 my $key = $fields{shift()};
975 defined $key or die "Out of band access";
976 $a->[$key] = shift;
977 }
978 sub FETCH {
979 my $a = ${shift()};
980 my $key = $fields{shift()};
981 defined $key or die "Out of band access";
982 $a->[$key];
983 }
984
985Now if $baz is overloaded like this, then C<$bar> is a reference to a
986reference to the intermediate array, which keeps a reference to an
987actual array, and the access hash. The tie()ing object for the access
988hash is also a reference to a reference to the actual array, so
989
990=over
991
992=item *
993
994There are no loops of references.
995
996=item *
997
998Both "objects" which are blessed into the class C<two_refs1> are
999references to a reference to an array, thus references to a I<scalar>.
1000Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no
1001overloaded operations.
1002
1003=back
1004
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1005=head2 Symbolic calculator
1006
1007Put this in F<symbolic.pm> in your Perl library directory:
1008
1009 package symbolic; # Primitive symbolic calculator
1010 use overload nomethod => \&wrap;
1011
1012 sub new { shift; bless ['n', @_] }
1013 sub wrap {
1014 my ($obj, $other, $inv, $meth) = @_;
1015 ($obj, $other) = ($other, $obj) if $inv;
1016 bless [$meth, $obj, $other];
1017 }
1018
1019This module is very unusual as overloaded modules go: it does not
1020provide any usual overloaded operators, instead it provides the L<Last
1021Resort> operator C<nomethod>. In this example the corresponding
f610777f 1022subroutine returns an object which encapsulates operations done over
ee239bfe
IZ
1023the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new
1024symbolic 3> contains C<['+', 2, ['n', 3]]>.
1025
1026Here is an example of the script which "calculates" the side of
1027circumscribed octagon using the above package:
1028
1029 require symbolic;
1030 my $iter = 1; # 2**($iter+2) = 8
1031 my $side = new symbolic 1;
1032 my $cnt = $iter;
3cb6de81 1033
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IZ
1034 while ($cnt--) {
1035 $side = (sqrt(1 + $side**2) - 1)/$side;
1036 }
1037 print "OK\n";
1038
1039The value of $side is
1040
1041 ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]],
1042 undef], 1], ['n', 1]]
1043
1044Note that while we obtained this value using a nice little script,
1045there is no simple way to I<use> this value. In fact this value may
1046be inspected in debugger (see L<perldebug>), but ony if
1047C<bareStringify> B<O>ption is set, and not via C<p> command.
1048
1049If one attempts to print this value, then the overloaded operator
1050C<""> will be called, which will call C<nomethod> operator. The
1051result of this operator will be stringified again, but this result is
1052again of type C<symbolic>, which will lead to an infinite loop.
1053
1054Add a pretty-printer method to the module F<symbolic.pm>:
1055
1056 sub pretty {
1057 my ($meth, $a, $b) = @{+shift};
1058 $a = 'u' unless defined $a;
1059 $b = 'u' unless defined $b;
1060 $a = $a->pretty if ref $a;
1061 $b = $b->pretty if ref $b;
1062 "[$meth $a $b]";
1063 }
1064
1065Now one can finish the script by
1066
1067 print "side = ", $side->pretty, "\n";
1068
1069The method C<pretty> is doing object-to-string conversion, so it
1070is natural to overload the operator C<""> using this method. However,
1071inside such a method it is not necessary to pretty-print the
1072I<components> $a and $b of an object. In the above subroutine
1073C<"[$meth $a $b]"> is a catenation of some strings and components $a
1074and $b. If these components use overloading, the catenation operator
1075will look for an overloaded operator C<.>, if not present, it will
1076look for an overloaded operator C<"">. Thus it is enough to use
1077
1078 use overload nomethod => \&wrap, '""' => \&str;
1079 sub str {
1080 my ($meth, $a, $b) = @{+shift};
1081 $a = 'u' unless defined $a;
1082 $b = 'u' unless defined $b;
1083 "[$meth $a $b]";
1084 }
1085
1086Now one can change the last line of the script to
1087
1088 print "side = $side\n";
1089
1090which outputs
1091
1092 side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
1093
1094and one can inspect the value in debugger using all the possible
1095methods.
1096
1097Something is is still amiss: consider the loop variable $cnt of the
1098script. It was a number, not an object. We cannot make this value of
1099type C<symbolic>, since then the loop will not terminate.
1100
1101Indeed, to terminate the cycle, the $cnt should become false.
1102However, the operator C<bool> for checking falsity is overloaded (this
1103time via overloaded C<"">), and returns a long string, thus any object
1104of type C<symbolic> is true. To overcome this, we need a way to
1105compare an object to 0. In fact, it is easier to write a numeric
1106conversion routine.
1107
1108Here is the text of F<symbolic.pm> with such a routine added (and
f610777f 1109slightly modified str()):
ee239bfe
IZ
1110
1111 package symbolic; # Primitive symbolic calculator
1112 use overload
1113 nomethod => \&wrap, '""' => \&str, '0+' => \&num;
1114
1115 sub new { shift; bless ['n', @_] }
1116 sub wrap {
1117 my ($obj, $other, $inv, $meth) = @_;
1118 ($obj, $other) = ($other, $obj) if $inv;
1119 bless [$meth, $obj, $other];
1120 }
1121 sub str {
1122 my ($meth, $a, $b) = @{+shift};
1123 $a = 'u' unless defined $a;
1124 if (defined $b) {
1125 "[$meth $a $b]";
1126 } else {
1127 "[$meth $a]";
1128 }
1129 }
1130 my %subr = ( n => sub {$_[0]},
1131 sqrt => sub {sqrt $_[0]},
1132 '-' => sub {shift() - shift()},
1133 '+' => sub {shift() + shift()},
1134 '/' => sub {shift() / shift()},
1135 '*' => sub {shift() * shift()},
1136 '**' => sub {shift() ** shift()},
1137 );
1138 sub num {
1139 my ($meth, $a, $b) = @{+shift};
1140 my $subr = $subr{$meth}
1141 or die "Do not know how to ($meth) in symbolic";
1142 $a = $a->num if ref $a eq __PACKAGE__;
1143 $b = $b->num if ref $b eq __PACKAGE__;
1144 $subr->($a,$b);
1145 }
1146
1147All the work of numeric conversion is done in %subr and num(). Of
f610777f 1148course, %subr is not complete, it contains only operators used in the
ee239bfe
IZ
1149example below. Here is the extra-credit question: why do we need an
1150explicit recursion in num()? (Answer is at the end of this section.)
1151
1152Use this module like this:
1153
1154 require symbolic;
1155 my $iter = new symbolic 2; # 16-gon
1156 my $side = new symbolic 1;
1157 my $cnt = $iter;
3cb6de81 1158
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IZ
1159 while ($cnt) {
1160 $cnt = $cnt - 1; # Mutator `--' not implemented
1161 $side = (sqrt(1 + $side**2) - 1)/$side;
1162 }
1163 printf "%s=%f\n", $side, $side;
1164 printf "pi=%f\n", $side*(2**($iter+2));
1165
1166It prints (without so many line breaks)
1167
1168 [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1]
1169 [n 1]] 2]]] 1]
1170 [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912
1171 pi=3.182598
1172
1173The above module is very primitive. It does not implement
1174mutator methods (C<++>, C<-=> and so on), does not do deep copying
1175(not required without mutators!), and implements only those arithmetic
1176operations which are used in the example.
1177
f610777f 1178To implement most arithmetic operations is easy, one should just use
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IZ
1179the tables of operations, and change the code which fills %subr to
1180
1181 my %subr = ( 'n' => sub {$_[0]} );
1182 foreach my $op (split " ", $overload::ops{with_assign}) {
1183 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1184 }
1185 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1186 foreach my $op (split " ", "@overload::ops{ @bins }") {
1187 $subr{$op} = eval "sub {shift() $op shift()}";
1188 }
1189 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1190 print "defining `$op'\n";
1191 $subr{$op} = eval "sub {$op shift()}";
1192 }
1193
1194Due to L<Calling Conventions for Mutators>, we do not need anything
1195special to make C<+=> and friends work, except filling C<+=> entry of
1196%subr, and defining a copy constructor (needed since Perl has no
1197way to know that the implementation of C<'+='> does not mutate
1198the argument, compare L<Copy Constructor>).
1199
1200To implement a copy constructor, add C<'=' => \&cpy> to C<use overload>
1201line, and code (this code assumes that mutators change things one level
1202deep only, so recursive copying is not needed):
1203
1204 sub cpy {
1205 my $self = shift;
1206 bless [@$self], ref $self;
1207 }
1208
1209To make C<++> and C<--> work, we need to implement actual mutators,
1210either directly, or in C<nomethod>. We continue to do things inside
1211C<nomethod>, thus add
1212
1213 if ($meth eq '++' or $meth eq '--') {
1214 @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference
1215 return $obj;
1216 }
1217
1218after the first line of wrap(). This is not a most effective
1219implementation, one may consider
1220
1221 sub inc { $_[0] = bless ['++', shift, 1]; }
1222
1223instead.
1224
1225As a final remark, note that one can fill %subr by
1226
1227 my %subr = ( 'n' => sub {$_[0]} );
1228 foreach my $op (split " ", $overload::ops{with_assign}) {
1229 $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}";
1230 }
1231 my @bins = qw(binary 3way_comparison num_comparison str_comparison);
1232 foreach my $op (split " ", "@overload::ops{ @bins }") {
1233 $subr{$op} = eval "sub {shift() $op shift()}";
1234 }
1235 foreach my $op (split " ", "@overload::ops{qw(unary func)}") {
1236 $subr{$op} = eval "sub {$op shift()}";
1237 }
1238 $subr{'++'} = $subr{'+'};
1239 $subr{'--'} = $subr{'-'};
1240
1241This finishes implementation of a primitive symbolic calculator in
124250 lines of Perl code. Since the numeric values of subexpressions
1243are not cached, the calculator is very slow.
1244
1245Here is the answer for the exercise: In the case of str(), we need no
1246explicit recursion since the overloaded C<.>-operator will fall back
1247to an existing overloaded operator C<"">. Overloaded arithmetic
1248operators I<do not> fall back to numeric conversion if C<fallback> is
1249not explicitly requested. Thus without an explicit recursion num()
1250would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild
1251the argument of num().
1252
1253If you wonder why defaults for conversion are different for str() and
1254num(), note how easy it was to write the symbolic calculator. This
1255simplicity is due to an appropriate choice of defaults. One extra
f610777f
A
1256note: due to the explicit recursion num() is more fragile than sym():
1257we need to explicitly check for the type of $a and $b. If components
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IZ
1258$a and $b happen to be of some related type, this may lead to problems.
1259
1260=head2 I<Really> symbolic calculator
1261
1262One may wonder why we call the above calculator symbolic. The reason
1263is that the actual calculation of the value of expression is postponed
1264until the value is I<used>.
1265
1266To see it in action, add a method
1267
1268 sub STORE {
1269 my $obj = shift;
1270 $#$obj = 1;
1271 @$obj->[0,1] = ('=', shift);
1272 }
1273
1274to the package C<symbolic>. After this change one can do
1275
1276 my $a = new symbolic 3;
1277 my $b = new symbolic 4;
1278 my $c = sqrt($a**2 + $b**2);
1279
1280and the numeric value of $c becomes 5. However, after calling
1281
1282 $a->STORE(12); $b->STORE(5);
1283
1284the numeric value of $c becomes 13. There is no doubt now that the module
1285symbolic provides a I<symbolic> calculator indeed.
1286
1287To hide the rough edges under the hood, provide a tie()d interface to the
1288package C<symbolic> (compare with L<Metaphor clash>). Add methods
1289
1290 sub TIESCALAR { my $pack = shift; $pack->new(@_) }
1291 sub FETCH { shift }
1292 sub nop { } # Around a bug
1293
1294(the bug is described in L<"BUGS">). One can use this new interface as
1295
1296 tie $a, 'symbolic', 3;
1297 tie $b, 'symbolic', 4;
1298 $a->nop; $b->nop; # Around a bug
1299
1300 my $c = sqrt($a**2 + $b**2);
1301
1302Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value
1303of $c becomes 13. To insulate the user of the module add a method
1304
1305 sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; }
1306
1307Now
1308
1309 my ($a, $b);
1310 symbolic->vars($a, $b);
1311 my $c = sqrt($a**2 + $b**2);
1312
1313 $a = 3; $b = 4;
1314 printf "c5 %s=%f\n", $c, $c;
1315
1316 $a = 12; $b = 5;
1317 printf "c13 %s=%f\n", $c, $c;
1318
1319shows that the numeric value of $c follows changes to the values of $a
1320and $b.
1321
4633a7c4
LW
1322=head1 AUTHOR
1323
1fef88e7 1324Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>.
4633a7c4
LW
1325
1326=head1 DIAGNOSTICS
1327
1328When Perl is run with the B<-Do> switch or its equivalent, overloading
1329induces diagnostic messages.
1330
e7ea3e70
IZ
1331Using the C<m> command of Perl debugger (see L<perldebug>) one can
1332deduce which operations are overloaded (and which ancestor triggers
1333this overloading). Say, if C<eq> is overloaded, then the method C<(eq>
1334is shown by debugger. The method C<()> corresponds to the C<fallback>
1335key (in fact a presence of this method shows that this package has
1336overloading enabled, and it is what is used by the C<Overloaded>
ee239bfe 1337function of module C<overload>).
e7ea3e70 1338
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1339=head1 BUGS
1340
aa689395
PP
1341Because it is used for overloading, the per-package hash %OVERLOAD now
1342has a special meaning in Perl. The symbol table is filled with names
1343looking like line-noise.
4633a7c4 1344
a6006777
PP
1345For the purpose of inheritance every overloaded package behaves as if
1346C<fallback> is present (possibly undefined). This may create
1347interesting effects if some package is not overloaded, but inherits
1348from two overloaded packages.
4633a7c4 1349
ee239bfe
IZ
1350Relation between overloading and tie()ing is broken. Overloading is
1351triggered or not basing on the I<previous> class of tie()d value.
1352
1353This happens because the presence of overloading is checked too early,
1354before any tie()d access is attempted. If the FETCH()ed class of the
1355tie()d value does not change, a simple workaround is to access the value
1356immediately after tie()ing, so that after this call the I<previous> class
1357coincides with the current one.
1358
1359B<Needed:> a way to fix this without a speed penalty.
1360
b3ac6de7
IZ
1361Barewords are not covered by overloaded string constants.
1362
ee239bfe
IZ
1363This document is confusing. There are grammos and misleading language
1364used in places. It would seem a total rewrite is needed.
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1365
1366=cut
1367