Commit | Line | Data |
4633a7c4 |
1 | package overload; |
2 | |
a6006777 |
3 | sub nil {} |
4 | |
4633a7c4 |
5 | sub OVERLOAD { |
6 | $package = shift; |
7 | my %arg = @_; |
a6006777 |
8 | my ($sub, $fb); |
9 | $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching. |
10 | *{$package . "::()"} = \&nil; # Make it findable via fetchmethod. |
4633a7c4 |
11 | for (keys %arg) { |
a6006777 |
12 | if ($_ eq 'fallback') { |
13 | $fb = $arg{$_}; |
14 | } else { |
15 | $sub = $arg{$_}; |
16 | if (not ref $sub and $sub !~ /::/) { |
44a8e56a |
17 | $ {$package . "::(" . $_} = $sub; |
18 | $sub = \&nil; |
a6006777 |
19 | } |
20 | #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n"; |
21 | *{$package . "::(" . $_} = \&{ $sub }; |
22 | } |
4633a7c4 |
23 | } |
a6006777 |
24 | ${$package . "::()"} = $fb; # Make it findable too (fallback only). |
4633a7c4 |
25 | } |
26 | |
27 | sub import { |
28 | $package = (caller())[0]; |
29 | # *{$package . "::OVERLOAD"} = \&OVERLOAD; |
30 | shift; |
31 | $package->overload::OVERLOAD(@_); |
32 | } |
33 | |
34 | sub unimport { |
35 | $package = (caller())[0]; |
a6006777 |
36 | ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table |
4633a7c4 |
37 | shift; |
38 | for (@_) { |
a6006777 |
39 | if ($_ eq 'fallback') { |
40 | undef $ {$package . "::()"}; |
41 | } else { |
42 | delete $ {$package . "::"}{"(" . $_}; |
43 | } |
4633a7c4 |
44 | } |
45 | } |
46 | |
47 | sub Overloaded { |
a6006777 |
48 | my $package = shift; |
49 | $package = ref $package if ref $package; |
50 | $package->can('()'); |
4633a7c4 |
51 | } |
52 | |
44a8e56a |
53 | sub ov_method { |
54 | my $globref = shift; |
55 | return undef unless $globref; |
56 | my $sub = \&{*$globref}; |
57 | return $sub if $sub ne \&nil; |
58 | return shift->can($ {*$globref}); |
59 | } |
60 | |
4633a7c4 |
61 | sub OverloadedStringify { |
a6006777 |
62 | my $package = shift; |
63 | $package = ref $package if ref $package; |
44a8e56a |
64 | #$package->can('(""') |
ee239bfe |
65 | ov_method mycan($package, '(""'), $package |
66 | or ov_method mycan($package, '(0+'), $package |
67 | or ov_method mycan($package, '(bool'), $package |
68 | or ov_method mycan($package, '(nomethod'), $package; |
4633a7c4 |
69 | } |
70 | |
71 | sub Method { |
a6006777 |
72 | my $package = shift; |
73 | $package = ref $package if ref $package; |
44a8e56a |
74 | #my $meth = $package->can('(' . shift); |
75 | ov_method mycan($package, '(' . shift), $package; |
76 | #return $meth if $meth ne \&nil; |
77 | #return $ {*{$meth}}; |
4633a7c4 |
78 | } |
79 | |
80 | sub AddrRef { |
a6006777 |
81 | my $package = ref $_[0]; |
82 | return "$_[0]" unless $package; |
83 | bless $_[0], overload::Fake; # Non-overloaded package |
4633a7c4 |
84 | my $str = "$_[0]"; |
85 | bless $_[0], $package; # Back |
a6006777 |
86 | $package . substr $str, index $str, '='; |
4633a7c4 |
87 | } |
88 | |
89 | sub StrVal { |
f6b3007c |
90 | (OverloadedStringify($_[0]) or ref($_[0]) eq 'Regexp') ? |
a6006777 |
91 | (AddrRef(shift)) : |
4633a7c4 |
92 | "$_[0]"; |
93 | } |
94 | |
44a8e56a |
95 | sub mycan { # Real can would leave stubs. |
96 | my ($package, $meth) = @_; |
97 | return \*{$package . "::$meth"} if defined &{$package . "::$meth"}; |
98 | my $p; |
99 | foreach $p (@{$package . "::ISA"}) { |
100 | my $out = mycan($p, $meth); |
101 | return $out if $out; |
102 | } |
103 | return undef; |
104 | } |
105 | |
b3ac6de7 |
106 | %constants = ( |
107 | 'integer' => 0x1000, |
108 | 'float' => 0x2000, |
109 | 'binary' => 0x4000, |
110 | 'q' => 0x8000, |
111 | 'qr' => 0x10000, |
112 | ); |
113 | |
ee239bfe |
114 | %ops = ( with_assign => "+ - * / % ** << >> x .", |
115 | assign => "+= -= *= /= %= **= <<= >>= x= .=", |
2877bd81 |
116 | num_comparison => "< <= > >= == !=", |
ee239bfe |
117 | '3way_comparison'=> "<=> cmp", |
2877bd81 |
118 | str_comparison => "lt le gt ge eq ne", |
ee239bfe |
119 | binary => "& | ^", |
120 | unary => "neg ! ~", |
121 | mutators => '++ --', |
122 | func => "atan2 cos sin exp abs log sqrt", |
123 | conversion => 'bool "" 0+', |
f5284f61 |
124 | iterators => '<>', |
125 | dereferencing => '${} @{} %{} &{} *{}', |
ee239bfe |
126 | special => 'nomethod fallback ='); |
127 | |
b3ac6de7 |
128 | sub constant { |
129 | # Arguments: what, sub |
130 | while (@_) { |
131 | $^H{$_[0]} = $_[1]; |
132 | $^H |= $constants{$_[0]} | 0x20000; |
133 | shift, shift; |
134 | } |
135 | } |
136 | |
137 | sub remove_constant { |
138 | # Arguments: what, sub |
139 | while (@_) { |
140 | delete $^H{$_[0]}; |
141 | $^H &= ~ $constants{$_[0]}; |
142 | shift, shift; |
143 | } |
144 | } |
145 | |
4633a7c4 |
146 | 1; |
147 | |
148 | __END__ |
149 | |
150 | =head1 NAME |
151 | |
cb1a09d0 |
152 | overload - Package for overloading perl operations |
4633a7c4 |
153 | |
154 | =head1 SYNOPSIS |
155 | |
156 | package SomeThing; |
157 | |
158 | use overload |
159 | '+' => \&myadd, |
160 | '-' => \&mysub; |
161 | # etc |
162 | ... |
163 | |
164 | package main; |
165 | $a = new SomeThing 57; |
166 | $b=5+$a; |
167 | ... |
168 | if (overload::Overloaded $b) {...} |
169 | ... |
170 | $strval = overload::StrVal $b; |
171 | |
4633a7c4 |
172 | =head1 DESCRIPTION |
173 | |
174 | =head2 Declaration of overloaded functions |
175 | |
176 | The compilation directive |
177 | |
178 | package Number; |
179 | use overload |
180 | "+" => \&add, |
181 | "*=" => "muas"; |
182 | |
183 | declares function Number::add() for addition, and method muas() in |
184 | the "class" C<Number> (or one of its base classes) |
185 | for the assignment form C<*=> of multiplication. |
186 | |
187 | Arguments of this directive come in (key, value) pairs. Legal values |
e7ea3e70 |
188 | are values legal inside a C<&{ ... }> call, so the name of a |
189 | subroutine, a reference to a subroutine, or an anonymous subroutine |
190 | will all work. Note that values specified as strings are |
191 | interpreted as methods, not subroutines. Legal keys are listed below. |
4633a7c4 |
192 | |
193 | The subroutine C<add> will be called to execute C<$a+$b> if $a |
194 | is a reference to an object blessed into the package C<Number>, or if $a is |
195 | not an object from a package with defined mathemagic addition, but $b is a |
196 | reference to a C<Number>. It can also be called in other situations, like |
197 | C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical |
198 | methods refer to methods triggered by an overloaded mathematical |
199 | operator.) |
200 | |
774d564b |
201 | Since overloading respects inheritance via the @ISA hierarchy, the |
202 | above declaration would also trigger overloading of C<+> and C<*=> in |
203 | all the packages which inherit from C<Number>. |
e7ea3e70 |
204 | |
4633a7c4 |
205 | =head2 Calling Conventions for Binary Operations |
206 | |
207 | The functions specified in the C<use overload ...> directive are called |
208 | with three (in one particular case with four, see L<Last Resort>) |
209 | arguments. If the corresponding operation is binary, then the first |
210 | two arguments are the two arguments of the operation. However, due to |
211 | general object calling conventions, the first argument should always be |
212 | an object in the package, so in the situation of C<7+$a>, the |
213 | order of the arguments is interchanged. It probably does not matter |
214 | when implementing the addition method, but whether the arguments |
215 | are reversed is vital to the subtraction method. The method can |
216 | query this information by examining the third argument, which can take |
217 | three different values: |
218 | |
219 | =over 7 |
220 | |
221 | =item FALSE |
222 | |
223 | the order of arguments is as in the current operation. |
224 | |
225 | =item TRUE |
226 | |
227 | the arguments are reversed. |
228 | |
229 | =item C<undef> |
230 | |
231 | the current operation is an assignment variant (as in |
232 | C<$a+=7>), but the usual function is called instead. This additional |
ee239bfe |
233 | information can be used to generate some optimizations. Compare |
234 | L<Calling Conventions for Mutators>. |
4633a7c4 |
235 | |
236 | =back |
237 | |
238 | =head2 Calling Conventions for Unary Operations |
239 | |
240 | Unary operation are considered binary operations with the second |
241 | argument being C<undef>. Thus the functions that overloads C<{"++"}> |
242 | is called with arguments C<($a,undef,'')> when $a++ is executed. |
243 | |
ee239bfe |
244 | =head2 Calling Conventions for Mutators |
245 | |
246 | Two types of mutators have different calling conventions: |
247 | |
248 | =over |
249 | |
250 | =item C<++> and C<--> |
251 | |
252 | The routines which implement these operators are expected to actually |
253 | I<mutate> their arguments. So, assuming that $obj is a reference to a |
254 | number, |
255 | |
256 | sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n} |
257 | |
258 | is an appropriate implementation of overloaded C<++>. Note that |
259 | |
260 | sub incr { ++$ {$_[0]} ; shift } |
261 | |
262 | is OK if used with preincrement and with postincrement. (In the case |
263 | of postincrement a copying will be performed, see L<Copy Constructor>.) |
264 | |
265 | =item C<x=> and other assignment versions |
266 | |
267 | There is nothing special about these methods. They may change the |
268 | value of their arguments, and may leave it as is. The result is going |
269 | to be assigned to the value in the left-hand-side if different from |
270 | this value. |
271 | |
f610777f |
272 | This allows for the same method to be used as overloaded C<+=> and |
ee239bfe |
273 | C<+>. Note that this is I<allowed>, but not recommended, since by the |
274 | semantic of L<"Fallback"> Perl will call the method for C<+> anyway, |
275 | if C<+=> is not overloaded. |
276 | |
277 | =back |
278 | |
279 | B<Warning.> Due to the presense of assignment versions of operations, |
280 | routines which may be called in assignment context may create |
f610777f |
281 | self-referential structures. Currently Perl will not free self-referential |
ee239bfe |
282 | structures until cycles are C<explicitly> broken. You may get problems |
283 | when traversing your structures too. |
284 | |
285 | Say, |
286 | |
287 | use overload '+' => sub { bless [ \$_[0], \$_[1] ] }; |
288 | |
289 | is asking for trouble, since for code C<$obj += $foo> the subroutine |
290 | is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj, |
291 | \$foo]>. If using such a subroutine is an important optimization, one |
292 | can overload C<+=> explicitly by a non-"optimized" version, or switch |
293 | to non-optimized version if C<not defined $_[2]> (see |
294 | L<Calling Conventions for Binary Operations>). |
295 | |
296 | Even if no I<explicit> assignment-variants of operators are present in |
297 | the script, they may be generated by the optimizer. Say, C<",$obj,"> or |
298 | C<',' . $obj . ','> may be both optimized to |
299 | |
300 | my $tmp = ',' . $obj; $tmp .= ','; |
301 | |
4633a7c4 |
302 | =head2 Overloadable Operations |
303 | |
ee239bfe |
304 | The following symbols can be specified in C<use overload> directive: |
4633a7c4 |
305 | |
306 | =over 5 |
307 | |
308 | =item * I<Arithmetic operations> |
309 | |
310 | "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=", |
311 | "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=", |
312 | |
313 | For these operations a substituted non-assignment variant can be called if |
314 | the assignment variant is not available. Methods for operations "C<+>", |
315 | "C<->", "C<+=>", and "C<-=>" can be called to automatically generate |
316 | increment and decrement methods. The operation "C<->" can be used to |
317 | autogenerate missing methods for unary minus or C<abs>. |
318 | |
ee239bfe |
319 | See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and |
320 | L<"Calling Conventions for Binary Operations">) for details of these |
321 | substitutions. |
322 | |
4633a7c4 |
323 | =item * I<Comparison operations> |
324 | |
325 | "<", "<=", ">", ">=", "==", "!=", "<=>", |
326 | "lt", "le", "gt", "ge", "eq", "ne", "cmp", |
327 | |
328 | If the corresponding "spaceship" variant is available, it can be |
329 | used to substitute for the missing operation. During C<sort>ing |
330 | arrays, C<cmp> is used to compare values subject to C<use overload>. |
331 | |
332 | =item * I<Bit operations> |
333 | |
334 | "&", "^", "|", "neg", "!", "~", |
335 | |
336 | "C<neg>" stands for unary minus. If the method for C<neg> is not |
3bc6ec80 |
337 | specified, it can be autogenerated using the method for |
338 | subtraction. If the method for "C<!>" is not specified, it can be |
339 | autogenerated using the methods for "C<bool>", or "C<\"\">", or "C<0+>". |
4633a7c4 |
340 | |
341 | =item * I<Increment and decrement> |
342 | |
343 | "++", "--", |
344 | |
345 | If undefined, addition and subtraction methods can be |
346 | used instead. These operations are called both in prefix and |
347 | postfix form. |
348 | |
349 | =item * I<Transcendental functions> |
350 | |
351 | "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", |
352 | |
353 | If C<abs> is unavailable, it can be autogenerated using methods |
1fef88e7 |
354 | for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction. |
4633a7c4 |
355 | |
356 | =item * I<Boolean, string and numeric conversion> |
357 | |
358 | "bool", "\"\"", "0+", |
359 | |
f5284f61 |
360 | If one or two of these operations are not overloaded, the remaining ones can |
4633a7c4 |
361 | be used instead. C<bool> is used in the flow control operators |
362 | (like C<while>) and for the ternary "C<?:>" operation. These functions can |
363 | return any arbitrary Perl value. If the corresponding operation for this value |
364 | is overloaded too, that operation will be called again with this value. |
365 | |
f5284f61 |
366 | =item * I<Iteration> |
367 | |
368 | "<>" |
369 | |
370 | If not overloaded, the argument will be converted to a filehandle or |
371 | glob (which may require a stringification). The same overloading |
372 | happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and |
373 | I<globbing> syntax C<E<lt>${var}E<gt>>. |
374 | |
375 | =item * I<Dereferencing> |
376 | |
377 | '${}', '@{}', '%{}', '&{}', '*{}'. |
378 | |
379 | If not overloaded, the argument will be dereferenced I<as is>, thus |
380 | should be of correct type. These functions should return a reference |
381 | of correct type, or another object with overloaded dereferencing. |
382 | |
4633a7c4 |
383 | =item * I<Special> |
384 | |
385 | "nomethod", "fallback", "=", |
386 | |
387 | see L<SPECIAL SYMBOLS FOR C<use overload>>. |
388 | |
389 | =back |
390 | |
ee239bfe |
391 | See L<"Fallback"> for an explanation of when a missing method can be |
392 | autogenerated. |
393 | |
394 | A computer-readable form of the above table is available in the hash |
395 | %overload::ops, with values being space-separated lists of names: |
396 | |
397 | with_assign => '+ - * / % ** << >> x .', |
398 | assign => '+= -= *= /= %= **= <<= >>= x= .=', |
2877bd81 |
399 | num_comparison => '< <= > >= == !=', |
ee239bfe |
400 | '3way_comparison'=> '<=> cmp', |
2877bd81 |
401 | str_comparison => 'lt le gt ge eq ne', |
ee239bfe |
402 | binary => '& | ^', |
403 | unary => 'neg ! ~', |
404 | mutators => '++ --', |
405 | func => 'atan2 cos sin exp abs log sqrt', |
406 | conversion => 'bool "" 0+', |
f5284f61 |
407 | iterators => '<>', |
408 | dereferencing => '${} @{} %{} &{} *{}', |
ee239bfe |
409 | special => 'nomethod fallback =' |
4633a7c4 |
410 | |
e7ea3e70 |
411 | =head2 Inheritance and overloading |
412 | |
774d564b |
413 | Inheritance interacts with overloading in two ways. |
e7ea3e70 |
414 | |
415 | =over |
416 | |
417 | =item Strings as values of C<use overload> directive |
418 | |
774d564b |
419 | If C<value> in |
e7ea3e70 |
420 | |
421 | use overload key => value; |
422 | |
774d564b |
423 | is a string, it is interpreted as a method name. |
e7ea3e70 |
424 | |
425 | =item Overloading of an operation is inherited by derived classes |
426 | |
774d564b |
427 | Any class derived from an overloaded class is also overloaded. The |
428 | set of overloaded methods is the union of overloaded methods of all |
429 | the ancestors. If some method is overloaded in several ancestor, then |
e7ea3e70 |
430 | which description will be used is decided by the usual inheritance |
774d564b |
431 | rules: |
e7ea3e70 |
432 | |
774d564b |
433 | If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads |
434 | C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">, |
435 | then the subroutine C<D::plus_sub> will be called to implement |
436 | operation C<+> for an object in package C<A>. |
e7ea3e70 |
437 | |
438 | =back |
439 | |
774d564b |
440 | Note that since the value of the C<fallback> key is not a subroutine, |
441 | its inheritance is not governed by the above rules. In the current |
442 | implementation, the value of C<fallback> in the first overloaded |
443 | ancestor is used, but this is accidental and subject to change. |
e7ea3e70 |
444 | |
4633a7c4 |
445 | =head1 SPECIAL SYMBOLS FOR C<use overload> |
446 | |
447 | Three keys are recognized by Perl that are not covered by the above |
448 | description. |
449 | |
774d564b |
450 | =head2 Last Resort |
4633a7c4 |
451 | |
452 | C<"nomethod"> should be followed by a reference to a function of four |
453 | parameters. If defined, it is called when the overloading mechanism |
454 | cannot find a method for some operation. The first three arguments of |
455 | this function coincide with the arguments for the corresponding method if |
456 | it were found, the fourth argument is the symbol |
457 | corresponding to the missing method. If several methods are tried, |
458 | the last one is used. Say, C<1-$a> can be equivalent to |
459 | |
460 | &nomethodMethod($a,1,1,"-") |
461 | |
462 | if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the |
463 | C<use overload> directive. |
464 | |
465 | If some operation cannot be resolved, and there is no function |
466 | assigned to C<"nomethod">, then an exception will be raised via die()-- |
467 | unless C<"fallback"> was specified as a key in C<use overload> directive. |
468 | |
469 | =head2 Fallback |
470 | |
471 | The key C<"fallback"> governs what to do if a method for a particular |
472 | operation is not found. Three different cases are possible depending on |
473 | the value of C<"fallback">: |
474 | |
475 | =over 16 |
476 | |
477 | =item * C<undef> |
478 | |
479 | Perl tries to use a |
480 | substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it |
481 | then tries to calls C<"nomethod"> value; if missing, an exception |
482 | will be raised. |
483 | |
484 | =item * TRUE |
485 | |
486 | The same as for the C<undef> value, but no exception is raised. Instead, |
487 | it silently reverts to what it would have done were there no C<use overload> |
488 | present. |
489 | |
490 | =item * defined, but FALSE |
491 | |
492 | No autogeneration is tried. Perl tries to call |
493 | C<"nomethod"> value, and if this is missing, raises an exception. |
494 | |
495 | =back |
496 | |
e7ea3e70 |
497 | B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone |
498 | yet, see L<"Inheritance and overloading">. |
499 | |
4633a7c4 |
500 | =head2 Copy Constructor |
501 | |
502 | The value for C<"="> is a reference to a function with three |
503 | arguments, i.e., it looks like the other values in C<use |
504 | overload>. However, it does not overload the Perl assignment |
505 | operator. This would go against Camel hair. |
506 | |
507 | This operation is called in the situations when a mutator is applied |
508 | to a reference that shares its object with some other reference, such |
509 | as |
510 | |
511 | $a=$b; |
ee239bfe |
512 | ++$a; |
4633a7c4 |
513 | |
514 | To make this change $a and not change $b, a copy of C<$$a> is made, |
515 | and $a is assigned a reference to this new object. This operation is |
ee239bfe |
516 | done during execution of the C<++$a>, and not during the assignment, |
4633a7c4 |
517 | (so before the increment C<$$a> coincides with C<$$b>). This is only |
ee239bfe |
518 | done if C<++> is expressed via a method for C<'++'> or C<'+='> (or |
519 | C<nomethod>). Note that if this operation is expressed via C<'+'> |
520 | a nonmutator, i.e., as in |
4633a7c4 |
521 | |
522 | $a=$b; |
523 | $a=$a+1; |
524 | |
525 | then C<$a> does not reference a new copy of C<$$a>, since $$a does not |
526 | appear as lvalue when the above code is executed. |
527 | |
528 | If the copy constructor is required during the execution of some mutator, |
529 | but a method for C<'='> was not specified, it can be autogenerated as a |
530 | string copy if the object is a plain scalar. |
531 | |
532 | =over 5 |
533 | |
534 | =item B<Example> |
535 | |
536 | The actually executed code for |
537 | |
538 | $a=$b; |
539 | Something else which does not modify $a or $b.... |
540 | ++$a; |
541 | |
542 | may be |
543 | |
544 | $a=$b; |
545 | Something else which does not modify $a or $b.... |
546 | $a = $a->clone(undef,""); |
547 | $a->incr(undef,""); |
548 | |
549 | if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>, |
550 | C<'='> was overloaded with C<\&clone>. |
551 | |
552 | =back |
553 | |
f610777f |
554 | Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for |
ee239bfe |
555 | C<$b = $a; ++$a>. |
556 | |
4633a7c4 |
557 | =head1 MAGIC AUTOGENERATION |
558 | |
559 | If a method for an operation is not found, and the value for C<"fallback"> is |
560 | TRUE or undefined, Perl tries to autogenerate a substitute method for |
561 | the missing operation based on the defined operations. Autogenerated method |
562 | substitutions are possible for the following operations: |
563 | |
564 | =over 16 |
565 | |
566 | =item I<Assignment forms of arithmetic operations> |
567 | |
568 | C<$a+=$b> can use the method for C<"+"> if the method for C<"+="> |
569 | is not defined. |
570 | |
571 | =item I<Conversion operations> |
572 | |
573 | String, numeric, and boolean conversion are calculated in terms of one |
574 | another if not all of them are defined. |
575 | |
576 | =item I<Increment and decrement> |
577 | |
578 | The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>, |
579 | and C<$a--> in terms of C<$a-=1> and C<$a-1>. |
580 | |
581 | =item C<abs($a)> |
582 | |
583 | can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>). |
584 | |
585 | =item I<Unary minus> |
586 | |
587 | can be expressed in terms of subtraction. |
588 | |
3bc6ec80 |
589 | =item I<Negation> |
590 | |
591 | C<!> and C<not> can be expressed in terms of boolean conversion, or |
592 | string or numerical conversion. |
593 | |
4633a7c4 |
594 | =item I<Concatenation> |
595 | |
596 | can be expressed in terms of string conversion. |
597 | |
598 | =item I<Comparison operations> |
599 | |
600 | can be expressed in terms of its "spaceship" counterpart: either |
601 | C<E<lt>=E<gt>> or C<cmp>: |
1fef88e7 |
602 | |
4633a7c4 |
603 | <, >, <=, >=, ==, != in terms of <=> |
604 | lt, gt, le, ge, eq, ne in terms of cmp |
605 | |
f5284f61 |
606 | =item I<Iterator> |
607 | |
608 | <> in terms of builtin operations |
609 | |
610 | =item I<Dereferencing> |
611 | |
612 | ${} @{} %{} &{} *{} in terms of builtin operations |
613 | |
4633a7c4 |
614 | =item I<Copy operator> |
615 | |
616 | can be expressed in terms of an assignment to the dereferenced value, if this |
617 | value is a scalar and not a reference. |
618 | |
619 | =back |
620 | |
ee239bfe |
621 | =head1 Losing overloading |
4633a7c4 |
622 | |
623 | The restriction for the comparison operation is that even if, for example, |
624 | `C<cmp>' should return a blessed reference, the autogenerated `C<lt>' |
625 | function will produce only a standard logical value based on the |
626 | numerical value of the result of `C<cmp>'. In particular, a working |
627 | numeric conversion is needed in this case (possibly expressed in terms of |
628 | other conversions). |
629 | |
630 | Similarly, C<.=> and C<x=> operators lose their mathemagical properties |
631 | if the string conversion substitution is applied. |
632 | |
633 | When you chop() a mathemagical object it is promoted to a string and its |
634 | mathemagical properties are lost. The same can happen with other |
635 | operations as well. |
636 | |
637 | =head1 Run-time Overloading |
638 | |
639 | Since all C<use> directives are executed at compile-time, the only way to |
640 | change overloading during run-time is to |
641 | |
642 | eval 'use overload "+" => \&addmethod'; |
643 | |
644 | You can also use |
645 | |
646 | eval 'no overload "+", "--", "<="'; |
647 | |
648 | though the use of these constructs during run-time is questionable. |
649 | |
650 | =head1 Public functions |
651 | |
652 | Package C<overload.pm> provides the following public functions: |
653 | |
654 | =over 5 |
655 | |
656 | =item overload::StrVal(arg) |
657 | |
658 | Gives string value of C<arg> as in absence of stringify overloading. |
659 | |
660 | =item overload::Overloaded(arg) |
661 | |
662 | Returns true if C<arg> is subject to overloading of some operations. |
663 | |
664 | =item overload::Method(obj,op) |
665 | |
666 | Returns C<undef> or a reference to the method that implements C<op>. |
667 | |
668 | =back |
669 | |
b3ac6de7 |
670 | =head1 Overloading constants |
671 | |
672 | For some application Perl parser mangles constants too much. It is possible |
673 | to hook into this process via overload::constant() and overload::remove_constant() |
674 | functions. |
675 | |
676 | These functions take a hash as an argument. The recognized keys of this hash |
677 | are |
678 | |
679 | =over 8 |
680 | |
681 | =item integer |
682 | |
683 | to overload integer constants, |
684 | |
685 | =item float |
686 | |
687 | to overload floating point constants, |
688 | |
689 | =item binary |
690 | |
691 | to overload octal and hexadecimal constants, |
692 | |
693 | =item q |
694 | |
695 | to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted |
696 | strings and here-documents, |
697 | |
698 | =item qr |
699 | |
700 | to overload constant pieces of regular expressions. |
701 | |
702 | =back |
703 | |
704 | The corresponding values are references to functions which take three arguments: |
705 | the first one is the I<initial> string form of the constant, the second one |
706 | is how Perl interprets this constant, the third one is how the constant is used. |
707 | Note that the initial string form does not |
708 | contain string delimiters, and has backslashes in backslash-delimiter |
709 | combinations stripped (thus the value of delimiter is not relevant for |
710 | processing of this string). The return value of this function is how this |
711 | constant is going to be interpreted by Perl. The third argument is undefined |
712 | unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote |
713 | context (comes from strings, regular expressions, and single-quote HERE |
714 | documents), it is C<tr> for arguments of C<tr>/C<y> operators, |
715 | it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise. |
716 | |
717 | Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>, |
718 | it is expected that overloaded constant strings are equipped with reasonable |
719 | overloaded catenation operator, otherwise absurd results will result. |
720 | Similarly, negative numbers are considered as negations of positive constants. |
721 | |
722 | Note that it is probably meaningless to call the functions overload::constant() |
723 | and overload::remove_constant() from anywhere but import() and unimport() methods. |
724 | From these methods they may be called as |
725 | |
726 | sub import { |
727 | shift; |
728 | return unless @_; |
729 | die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant'; |
730 | overload::constant integer => sub {Math::BigInt->new(shift)}; |
731 | } |
732 | |
733 | B<BUGS> Currently overloaded-ness of constants does not propagate |
734 | into C<eval '...'>. |
735 | |
4633a7c4 |
736 | =head1 IMPLEMENTATION |
737 | |
738 | What follows is subject to change RSN. |
739 | |
e7ea3e70 |
740 | The table of methods for all operations is cached in magic for the |
741 | symbol table hash for the package. The cache is invalidated during |
742 | processing of C<use overload>, C<no overload>, new function |
743 | definitions, and changes in @ISA. However, this invalidation remains |
744 | unprocessed until the next C<bless>ing into the package. Hence if you |
745 | want to change overloading structure dynamically, you'll need an |
746 | additional (fake) C<bless>ing to update the table. |
747 | |
748 | (Every SVish thing has a magic queue, and magic is an entry in that |
749 | queue. This is how a single variable may participate in multiple |
750 | forms of magic simultaneously. For instance, environment variables |
751 | regularly have two forms at once: their %ENV magic and their taint |
752 | magic. However, the magic which implements overloading is applied to |
753 | the stashes, which are rarely used directly, thus should not slow down |
754 | Perl.) |
4633a7c4 |
755 | |
756 | If an object belongs to a package using overload, it carries a special |
757 | flag. Thus the only speed penalty during arithmetic operations without |
758 | overloading is the checking of this flag. |
759 | |
774d564b |
760 | In fact, if C<use overload> is not present, there is almost no overhead |
761 | for overloadable operations, so most programs should not suffer |
762 | measurable performance penalties. A considerable effort was made to |
763 | minimize the overhead when overload is used in some package, but the |
764 | arguments in question do not belong to packages using overload. When |
765 | in doubt, test your speed with C<use overload> and without it. So far |
766 | there have been no reports of substantial speed degradation if Perl is |
767 | compiled with optimization turned on. |
4633a7c4 |
768 | |
e7ea3e70 |
769 | There is no size penalty for data if overload is not used. The only |
770 | size penalty if overload is used in some package is that I<all> the |
771 | packages acquire a magic during the next C<bless>ing into the |
772 | package. This magic is three-words-long for packages without |
f610777f |
773 | overloading, and carries the cache table if the package is overloaded. |
4633a7c4 |
774 | |
775 | Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is |
776 | carried out before any operation that can imply an assignment to the |
777 | object $a (or $b) refers to, like C<$a++>. You can override this |
778 | behavior by defining your own copy constructor (see L<"Copy Constructor">). |
779 | |
780 | It is expected that arguments to methods that are not explicitly supposed |
781 | to be changed are constant (but this is not enforced). |
782 | |
ee239bfe |
783 | =head1 Metaphor clash |
784 | |
f610777f |
785 | One may wonder why the semantic of overloaded C<=> is so counter intuitive. |
786 | If it I<looks> counter intuitive to you, you are subject to a metaphor |
ee239bfe |
787 | clash. |
788 | |
789 | Here is a Perl object metaphor: |
790 | |
791 | I< object is a reference to blessed data> |
792 | |
793 | and an arithmetic metaphor: |
794 | |
795 | I< object is a thing by itself>. |
796 | |
797 | The I<main> problem of overloading C<=> is the fact that these metaphors |
798 | imply different actions on the assignment C<$a = $b> if $a and $b are |
799 | objects. Perl-think implies that $a becomes a reference to whatever |
800 | $b was referencing. Arithmetic-think implies that the value of "object" |
801 | $a is changed to become the value of the object $b, preserving the fact |
802 | that $a and $b are separate entities. |
803 | |
804 | The difference is not relevant in the absence of mutators. After |
805 | a Perl-way assignment an operation which mutates the data referenced by $a |
806 | would change the data referenced by $b too. Effectively, after |
807 | C<$a = $b> values of $a and $b become I<indistinguishable>. |
808 | |
809 | On the other hand, anyone who has used algebraic notation knows the |
810 | expressive power of the arithmetic metaphor. Overloading works hard |
811 | to enable this metaphor while preserving the Perlian way as far as |
812 | possible. Since it is not not possible to freely mix two contradicting |
813 | metaphors, overloading allows the arithmetic way to write things I<as |
814 | far as all the mutators are called via overloaded access only>. The |
815 | way it is done is described in L<Copy Constructor>. |
816 | |
817 | If some mutator methods are directly applied to the overloaded values, |
818 | one may need to I<explicitly unlink> other values which references the |
819 | same value: |
820 | |
821 | $a = new Data 23; |
822 | ... |
823 | $b = $a; # $b is "linked" to $a |
824 | ... |
825 | $a = $a->clone; # Unlink $b from $a |
826 | $a->increment_by(4); |
827 | |
828 | Note that overloaded access makes this transparent: |
829 | |
830 | $a = new Data 23; |
831 | $b = $a; # $b is "linked" to $a |
832 | $a += 4; # would unlink $b automagically |
833 | |
834 | However, it would not make |
835 | |
836 | $a = new Data 23; |
837 | $a = 4; # Now $a is a plain 4, not 'Data' |
838 | |
839 | preserve "objectness" of $a. But Perl I<has> a way to make assignments |
840 | to an object do whatever you want. It is just not the overload, but |
841 | tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method |
842 | which returns the object itself, and STORE() method which changes the |
843 | value of the object, one can reproduce the arithmetic metaphor in its |
844 | completeness, at least for variables which were tie()d from the start. |
845 | |
846 | (Note that a workaround for a bug may be needed, see L<"BUGS">.) |
847 | |
848 | =head1 Cookbook |
849 | |
850 | Please add examples to what follows! |
851 | |
852 | =head2 Two-face scalars |
853 | |
854 | Put this in F<two_face.pm> in your Perl library directory: |
855 | |
856 | package two_face; # Scalars with separate string and |
857 | # numeric values. |
858 | sub new { my $p = shift; bless [@_], $p } |
859 | use overload '""' => \&str, '0+' => \&num, fallback => 1; |
860 | sub num {shift->[1]} |
861 | sub str {shift->[0]} |
862 | |
863 | Use it as follows: |
864 | |
865 | require two_face; |
866 | my $seven = new two_face ("vii", 7); |
867 | printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1; |
868 | print "seven contains `i'\n" if $seven =~ /i/; |
869 | |
870 | (The second line creates a scalar which has both a string value, and a |
871 | numeric value.) This prints: |
872 | |
873 | seven=vii, seven=7, eight=8 |
874 | seven contains `i' |
875 | |
f5284f61 |
876 | =head2 Two-face references |
877 | |
878 | Suppose you want to create an object which is accessible as both an |
879 | array reference, and a hash reference, similar to the builtin |
880 | L<array-accessible-as-a-hash|perlref/"Pseudo-hashes: Using an array as |
881 | a hash"> builtin Perl type. Let us make it better than the builtin |
882 | type, there will be no restriction that you cannot use the index 0 of |
883 | your array. |
884 | |
885 | package two_refs; |
886 | use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} }; |
887 | sub new { |
888 | my $p = shift; |
889 | bless \ [@_], $p; |
890 | } |
891 | sub gethash { |
892 | my %h; |
893 | my $self = shift; |
894 | tie %h, ref $self, $self; |
895 | \%h; |
896 | } |
897 | |
898 | sub TIEHASH { my $p = shift; bless \ shift, $p } |
899 | my %fields; |
900 | my $i = 0; |
901 | $fields{$_} = $i++ foreach qw{zero one two three}; |
902 | sub STORE { |
903 | my $self = ${shift()}; |
904 | my $key = $fields{shift()}; |
905 | defined $key or die "Out of band access"; |
906 | $$self->[$key] = shift; |
907 | } |
908 | sub FETCH { |
909 | my $self = ${shift()}; |
910 | my $key = $fields{shift()}; |
911 | defined $key or die "Out of band access"; |
912 | $$self->[$key]; |
913 | } |
914 | |
915 | Now one can access an object using both the array and hash syntax: |
916 | |
917 | my $bar = new two_refs 3,4,5,6; |
918 | $bar->[2] = 11; |
919 | $bar->{two} == 11 or die 'bad hash fetch'; |
920 | |
921 | Note several important features of this example. First of all, the |
922 | I<actual> type of $bar is a scalar reference, and we do not overload |
923 | the scalar dereference. Thus we can get the I<actual> non-overloaded |
924 | contents of $bar by just using C<$$bar> (what we do in functions which |
925 | overload dereference). Similarly, the object returned by the |
926 | TIEHASH() method is a scalar reference. |
927 | |
928 | Second, we create a new tied hash each time the hash syntax is used. |
929 | This allows us not to worry about a possibility of a reference loop, |
930 | would would lead to a memory leak. |
931 | |
932 | Both these problems can be cured. Say, if we want to overload hash |
933 | dereference on a reference to an object which is I<implemented> as a |
934 | hash itself, the only problem one has to circumvent is how to access |
935 | this I<actual> hash (as opposed to the I<virtual> exhibited by |
936 | overloaded dereference operator). Here is one possible fetching routine: |
937 | |
938 | sub access_hash { |
939 | my ($self, $key) = (shift, shift); |
940 | my $class = ref $self; |
941 | bless $self, 'overload::dummy'; # Disable overloading of %{} |
942 | my $out = $self->{$key}; |
943 | bless $self, $class; # Restore overloading |
944 | $out; |
945 | } |
946 | |
947 | To move creation of the tied hash on each access, one may an extra |
948 | level of indirection which allows a non-circular structure of references: |
949 | |
950 | package two_refs1; |
951 | use overload '%{}' => sub { ${shift()}->[1] }, |
952 | '@{}' => sub { ${shift()}->[0] }; |
953 | sub new { |
954 | my $p = shift; |
955 | my $a = [@_]; |
956 | my %h; |
957 | tie %h, $p, $a; |
958 | bless \ [$a, \%h], $p; |
959 | } |
960 | sub gethash { |
961 | my %h; |
962 | my $self = shift; |
963 | tie %h, ref $self, $self; |
964 | \%h; |
965 | } |
966 | |
967 | sub TIEHASH { my $p = shift; bless \ shift, $p } |
968 | my %fields; |
969 | my $i = 0; |
970 | $fields{$_} = $i++ foreach qw{zero one two three}; |
971 | sub STORE { |
972 | my $a = ${shift()}; |
973 | my $key = $fields{shift()}; |
974 | defined $key or die "Out of band access"; |
975 | $a->[$key] = shift; |
976 | } |
977 | sub FETCH { |
978 | my $a = ${shift()}; |
979 | my $key = $fields{shift()}; |
980 | defined $key or die "Out of band access"; |
981 | $a->[$key]; |
982 | } |
983 | |
984 | Now if $baz is overloaded like this, then C<$bar> is a reference to a |
985 | reference to the intermediate array, which keeps a reference to an |
986 | actual array, and the access hash. The tie()ing object for the access |
987 | hash is also a reference to a reference to the actual array, so |
988 | |
989 | =over |
990 | |
991 | =item * |
992 | |
993 | There are no loops of references. |
994 | |
995 | =item * |
996 | |
997 | Both "objects" which are blessed into the class C<two_refs1> are |
998 | references to a reference to an array, thus references to a I<scalar>. |
999 | Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no |
1000 | overloaded operations. |
1001 | |
1002 | =back |
1003 | |
ee239bfe |
1004 | =head2 Symbolic calculator |
1005 | |
1006 | Put this in F<symbolic.pm> in your Perl library directory: |
1007 | |
1008 | package symbolic; # Primitive symbolic calculator |
1009 | use overload nomethod => \&wrap; |
1010 | |
1011 | sub new { shift; bless ['n', @_] } |
1012 | sub wrap { |
1013 | my ($obj, $other, $inv, $meth) = @_; |
1014 | ($obj, $other) = ($other, $obj) if $inv; |
1015 | bless [$meth, $obj, $other]; |
1016 | } |
1017 | |
1018 | This module is very unusual as overloaded modules go: it does not |
1019 | provide any usual overloaded operators, instead it provides the L<Last |
1020 | Resort> operator C<nomethod>. In this example the corresponding |
f610777f |
1021 | subroutine returns an object which encapsulates operations done over |
ee239bfe |
1022 | the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new |
1023 | symbolic 3> contains C<['+', 2, ['n', 3]]>. |
1024 | |
1025 | Here is an example of the script which "calculates" the side of |
1026 | circumscribed octagon using the above package: |
1027 | |
1028 | require symbolic; |
1029 | my $iter = 1; # 2**($iter+2) = 8 |
1030 | my $side = new symbolic 1; |
1031 | my $cnt = $iter; |
3cb6de81 |
1032 | |
ee239bfe |
1033 | while ($cnt--) { |
1034 | $side = (sqrt(1 + $side**2) - 1)/$side; |
1035 | } |
1036 | print "OK\n"; |
1037 | |
1038 | The value of $side is |
1039 | |
1040 | ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]], |
1041 | undef], 1], ['n', 1]] |
1042 | |
1043 | Note that while we obtained this value using a nice little script, |
1044 | there is no simple way to I<use> this value. In fact this value may |
1045 | be inspected in debugger (see L<perldebug>), but ony if |
1046 | C<bareStringify> B<O>ption is set, and not via C<p> command. |
1047 | |
1048 | If one attempts to print this value, then the overloaded operator |
1049 | C<""> will be called, which will call C<nomethod> operator. The |
1050 | result of this operator will be stringified again, but this result is |
1051 | again of type C<symbolic>, which will lead to an infinite loop. |
1052 | |
1053 | Add a pretty-printer method to the module F<symbolic.pm>: |
1054 | |
1055 | sub pretty { |
1056 | my ($meth, $a, $b) = @{+shift}; |
1057 | $a = 'u' unless defined $a; |
1058 | $b = 'u' unless defined $b; |
1059 | $a = $a->pretty if ref $a; |
1060 | $b = $b->pretty if ref $b; |
1061 | "[$meth $a $b]"; |
1062 | } |
1063 | |
1064 | Now one can finish the script by |
1065 | |
1066 | print "side = ", $side->pretty, "\n"; |
1067 | |
1068 | The method C<pretty> is doing object-to-string conversion, so it |
1069 | is natural to overload the operator C<""> using this method. However, |
1070 | inside such a method it is not necessary to pretty-print the |
1071 | I<components> $a and $b of an object. In the above subroutine |
1072 | C<"[$meth $a $b]"> is a catenation of some strings and components $a |
1073 | and $b. If these components use overloading, the catenation operator |
1074 | will look for an overloaded operator C<.>, if not present, it will |
1075 | look for an overloaded operator C<"">. Thus it is enough to use |
1076 | |
1077 | use overload nomethod => \&wrap, '""' => \&str; |
1078 | sub str { |
1079 | my ($meth, $a, $b) = @{+shift}; |
1080 | $a = 'u' unless defined $a; |
1081 | $b = 'u' unless defined $b; |
1082 | "[$meth $a $b]"; |
1083 | } |
1084 | |
1085 | Now one can change the last line of the script to |
1086 | |
1087 | print "side = $side\n"; |
1088 | |
1089 | which outputs |
1090 | |
1091 | side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]] |
1092 | |
1093 | and one can inspect the value in debugger using all the possible |
1094 | methods. |
1095 | |
1096 | Something is is still amiss: consider the loop variable $cnt of the |
1097 | script. It was a number, not an object. We cannot make this value of |
1098 | type C<symbolic>, since then the loop will not terminate. |
1099 | |
1100 | Indeed, to terminate the cycle, the $cnt should become false. |
1101 | However, the operator C<bool> for checking falsity is overloaded (this |
1102 | time via overloaded C<"">), and returns a long string, thus any object |
1103 | of type C<symbolic> is true. To overcome this, we need a way to |
1104 | compare an object to 0. In fact, it is easier to write a numeric |
1105 | conversion routine. |
1106 | |
1107 | Here is the text of F<symbolic.pm> with such a routine added (and |
f610777f |
1108 | slightly modified str()): |
ee239bfe |
1109 | |
1110 | package symbolic; # Primitive symbolic calculator |
1111 | use overload |
1112 | nomethod => \&wrap, '""' => \&str, '0+' => \# |
1113 | |
1114 | sub new { shift; bless ['n', @_] } |
1115 | sub wrap { |
1116 | my ($obj, $other, $inv, $meth) = @_; |
1117 | ($obj, $other) = ($other, $obj) if $inv; |
1118 | bless [$meth, $obj, $other]; |
1119 | } |
1120 | sub str { |
1121 | my ($meth, $a, $b) = @{+shift}; |
1122 | $a = 'u' unless defined $a; |
1123 | if (defined $b) { |
1124 | "[$meth $a $b]"; |
1125 | } else { |
1126 | "[$meth $a]"; |
1127 | } |
1128 | } |
1129 | my %subr = ( n => sub {$_[0]}, |
1130 | sqrt => sub {sqrt $_[0]}, |
1131 | '-' => sub {shift() - shift()}, |
1132 | '+' => sub {shift() + shift()}, |
1133 | '/' => sub {shift() / shift()}, |
1134 | '*' => sub {shift() * shift()}, |
1135 | '**' => sub {shift() ** shift()}, |
1136 | ); |
1137 | sub num { |
1138 | my ($meth, $a, $b) = @{+shift}; |
1139 | my $subr = $subr{$meth} |
1140 | or die "Do not know how to ($meth) in symbolic"; |
1141 | $a = $a->num if ref $a eq __PACKAGE__; |
1142 | $b = $b->num if ref $b eq __PACKAGE__; |
1143 | $subr->($a,$b); |
1144 | } |
1145 | |
1146 | All the work of numeric conversion is done in %subr and num(). Of |
f610777f |
1147 | course, %subr is not complete, it contains only operators used in the |
ee239bfe |
1148 | example below. Here is the extra-credit question: why do we need an |
1149 | explicit recursion in num()? (Answer is at the end of this section.) |
1150 | |
1151 | Use this module like this: |
1152 | |
1153 | require symbolic; |
1154 | my $iter = new symbolic 2; # 16-gon |
1155 | my $side = new symbolic 1; |
1156 | my $cnt = $iter; |
3cb6de81 |
1157 | |
ee239bfe |
1158 | while ($cnt) { |
1159 | $cnt = $cnt - 1; # Mutator `--' not implemented |
1160 | $side = (sqrt(1 + $side**2) - 1)/$side; |
1161 | } |
1162 | printf "%s=%f\n", $side, $side; |
1163 | printf "pi=%f\n", $side*(2**($iter+2)); |
1164 | |
1165 | It prints (without so many line breaks) |
1166 | |
1167 | [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] |
1168 | [n 1]] 2]]] 1] |
1169 | [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912 |
1170 | pi=3.182598 |
1171 | |
1172 | The above module is very primitive. It does not implement |
1173 | mutator methods (C<++>, C<-=> and so on), does not do deep copying |
1174 | (not required without mutators!), and implements only those arithmetic |
1175 | operations which are used in the example. |
1176 | |
f610777f |
1177 | To implement most arithmetic operations is easy, one should just use |
ee239bfe |
1178 | the tables of operations, and change the code which fills %subr to |
1179 | |
1180 | my %subr = ( 'n' => sub {$_[0]} ); |
1181 | foreach my $op (split " ", $overload::ops{with_assign}) { |
1182 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; |
1183 | } |
1184 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); |
1185 | foreach my $op (split " ", "@overload::ops{ @bins }") { |
1186 | $subr{$op} = eval "sub {shift() $op shift()}"; |
1187 | } |
1188 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { |
1189 | print "defining `$op'\n"; |
1190 | $subr{$op} = eval "sub {$op shift()}"; |
1191 | } |
1192 | |
1193 | Due to L<Calling Conventions for Mutators>, we do not need anything |
1194 | special to make C<+=> and friends work, except filling C<+=> entry of |
1195 | %subr, and defining a copy constructor (needed since Perl has no |
1196 | way to know that the implementation of C<'+='> does not mutate |
1197 | the argument, compare L<Copy Constructor>). |
1198 | |
1199 | To implement a copy constructor, add C<'=' => \&cpy> to C<use overload> |
1200 | line, and code (this code assumes that mutators change things one level |
1201 | deep only, so recursive copying is not needed): |
1202 | |
1203 | sub cpy { |
1204 | my $self = shift; |
1205 | bless [@$self], ref $self; |
1206 | } |
1207 | |
1208 | To make C<++> and C<--> work, we need to implement actual mutators, |
1209 | either directly, or in C<nomethod>. We continue to do things inside |
1210 | C<nomethod>, thus add |
1211 | |
1212 | if ($meth eq '++' or $meth eq '--') { |
1213 | @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference |
1214 | return $obj; |
1215 | } |
1216 | |
1217 | after the first line of wrap(). This is not a most effective |
1218 | implementation, one may consider |
1219 | |
1220 | sub inc { $_[0] = bless ['++', shift, 1]; } |
1221 | |
1222 | instead. |
1223 | |
1224 | As a final remark, note that one can fill %subr by |
1225 | |
1226 | my %subr = ( 'n' => sub {$_[0]} ); |
1227 | foreach my $op (split " ", $overload::ops{with_assign}) { |
1228 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; |
1229 | } |
1230 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); |
1231 | foreach my $op (split " ", "@overload::ops{ @bins }") { |
1232 | $subr{$op} = eval "sub {shift() $op shift()}"; |
1233 | } |
1234 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { |
1235 | $subr{$op} = eval "sub {$op shift()}"; |
1236 | } |
1237 | $subr{'++'} = $subr{'+'}; |
1238 | $subr{'--'} = $subr{'-'}; |
1239 | |
1240 | This finishes implementation of a primitive symbolic calculator in |
1241 | 50 lines of Perl code. Since the numeric values of subexpressions |
1242 | are not cached, the calculator is very slow. |
1243 | |
1244 | Here is the answer for the exercise: In the case of str(), we need no |
1245 | explicit recursion since the overloaded C<.>-operator will fall back |
1246 | to an existing overloaded operator C<"">. Overloaded arithmetic |
1247 | operators I<do not> fall back to numeric conversion if C<fallback> is |
1248 | not explicitly requested. Thus without an explicit recursion num() |
1249 | would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild |
1250 | the argument of num(). |
1251 | |
1252 | If you wonder why defaults for conversion are different for str() and |
1253 | num(), note how easy it was to write the symbolic calculator. This |
1254 | simplicity is due to an appropriate choice of defaults. One extra |
f610777f |
1255 | note: due to the explicit recursion num() is more fragile than sym(): |
1256 | we need to explicitly check for the type of $a and $b. If components |
ee239bfe |
1257 | $a and $b happen to be of some related type, this may lead to problems. |
1258 | |
1259 | =head2 I<Really> symbolic calculator |
1260 | |
1261 | One may wonder why we call the above calculator symbolic. The reason |
1262 | is that the actual calculation of the value of expression is postponed |
1263 | until the value is I<used>. |
1264 | |
1265 | To see it in action, add a method |
1266 | |
1267 | sub STORE { |
1268 | my $obj = shift; |
1269 | $#$obj = 1; |
1270 | @$obj->[0,1] = ('=', shift); |
1271 | } |
1272 | |
1273 | to the package C<symbolic>. After this change one can do |
1274 | |
1275 | my $a = new symbolic 3; |
1276 | my $b = new symbolic 4; |
1277 | my $c = sqrt($a**2 + $b**2); |
1278 | |
1279 | and the numeric value of $c becomes 5. However, after calling |
1280 | |
1281 | $a->STORE(12); $b->STORE(5); |
1282 | |
1283 | the numeric value of $c becomes 13. There is no doubt now that the module |
1284 | symbolic provides a I<symbolic> calculator indeed. |
1285 | |
1286 | To hide the rough edges under the hood, provide a tie()d interface to the |
1287 | package C<symbolic> (compare with L<Metaphor clash>). Add methods |
1288 | |
1289 | sub TIESCALAR { my $pack = shift; $pack->new(@_) } |
1290 | sub FETCH { shift } |
1291 | sub nop { } # Around a bug |
1292 | |
1293 | (the bug is described in L<"BUGS">). One can use this new interface as |
1294 | |
1295 | tie $a, 'symbolic', 3; |
1296 | tie $b, 'symbolic', 4; |
1297 | $a->nop; $b->nop; # Around a bug |
1298 | |
1299 | my $c = sqrt($a**2 + $b**2); |
1300 | |
1301 | Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value |
1302 | of $c becomes 13. To insulate the user of the module add a method |
1303 | |
1304 | sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; } |
1305 | |
1306 | Now |
1307 | |
1308 | my ($a, $b); |
1309 | symbolic->vars($a, $b); |
1310 | my $c = sqrt($a**2 + $b**2); |
1311 | |
1312 | $a = 3; $b = 4; |
1313 | printf "c5 %s=%f\n", $c, $c; |
1314 | |
1315 | $a = 12; $b = 5; |
1316 | printf "c13 %s=%f\n", $c, $c; |
1317 | |
1318 | shows that the numeric value of $c follows changes to the values of $a |
1319 | and $b. |
1320 | |
4633a7c4 |
1321 | =head1 AUTHOR |
1322 | |
1fef88e7 |
1323 | Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>. |
4633a7c4 |
1324 | |
1325 | =head1 DIAGNOSTICS |
1326 | |
1327 | When Perl is run with the B<-Do> switch or its equivalent, overloading |
1328 | induces diagnostic messages. |
1329 | |
e7ea3e70 |
1330 | Using the C<m> command of Perl debugger (see L<perldebug>) one can |
1331 | deduce which operations are overloaded (and which ancestor triggers |
1332 | this overloading). Say, if C<eq> is overloaded, then the method C<(eq> |
1333 | is shown by debugger. The method C<()> corresponds to the C<fallback> |
1334 | key (in fact a presence of this method shows that this package has |
1335 | overloading enabled, and it is what is used by the C<Overloaded> |
ee239bfe |
1336 | function of module C<overload>). |
e7ea3e70 |
1337 | |
4633a7c4 |
1338 | =head1 BUGS |
1339 | |
aa689395 |
1340 | Because it is used for overloading, the per-package hash %OVERLOAD now |
1341 | has a special meaning in Perl. The symbol table is filled with names |
1342 | looking like line-noise. |
4633a7c4 |
1343 | |
a6006777 |
1344 | For the purpose of inheritance every overloaded package behaves as if |
1345 | C<fallback> is present (possibly undefined). This may create |
1346 | interesting effects if some package is not overloaded, but inherits |
1347 | from two overloaded packages. |
4633a7c4 |
1348 | |
ee239bfe |
1349 | Relation between overloading and tie()ing is broken. Overloading is |
1350 | triggered or not basing on the I<previous> class of tie()d value. |
1351 | |
1352 | This happens because the presence of overloading is checked too early, |
1353 | before any tie()d access is attempted. If the FETCH()ed class of the |
1354 | tie()d value does not change, a simple workaround is to access the value |
1355 | immediately after tie()ing, so that after this call the I<previous> class |
1356 | coincides with the current one. |
1357 | |
1358 | B<Needed:> a way to fix this without a speed penalty. |
1359 | |
b3ac6de7 |
1360 | Barewords are not covered by overloaded string constants. |
1361 | |
ee239bfe |
1362 | This document is confusing. There are grammos and misleading language |
1363 | used in places. It would seem a total rewrite is needed. |
4633a7c4 |
1364 | |
1365 | =cut |
1366 | |