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 { |
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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 $str, '='; |
4633a7c4 |
87 | } |
88 | |
89 | sub StrVal { |
a6006777 |
90 | (OverloadedStringify($_[0])) ? |
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 $out; |
102 | } |
103 | return undef; |
104 | } |
105 | |
b3ac6de7 |
106 | %constants = ( |
107 | 'integer' => 0x1000, |
108 | 'float' => 0x2000, |
109 | 'binary' => 0x4000, |
110 | 'q' => 0x8000, |
111 | 'qr' => 0x10000, |
112 | ); |
113 | |
ee239bfe |
114 | %ops = ( with_assign => "+ - * / % ** << >> x .", |
115 | assign => "+= -= *= /= %= **= <<= >>= x= .=", |
116 | str_comparison => "< <= > >= == !=", |
117 | '3way_comparison'=> "<=> cmp", |
118 | num_comparison => "lt le gt ge eq ne", |
119 | binary => "& | ^", |
120 | unary => "neg ! ~", |
121 | mutators => '++ --', |
122 | func => "atan2 cos sin exp abs log sqrt", |
123 | conversion => 'bool "" 0+', |
f5284f61 |
124 | iterators => '<>', |
125 | dereferencing => '${} @{} %{} &{} *{}', |
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126 | special => 'nomethod fallback ='); |
127 | |
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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 | |
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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 $b; |
171 | |
172 | =head1 CAVEAT SCRIPTOR |
173 | |
174 | Overloading of operators is a subject not to be taken lightly. |
175 | Neither its precise implementation, syntax, nor semantics are |
176 | 100% endorsed by Larry Wall. So any of these may be changed |
177 | at some point in the future. |
178 | |
179 | =head1 DESCRIPTION |
180 | |
181 | =head2 Declaration of overloaded functions |
182 | |
183 | The compilation directive |
184 | |
185 | package Number; |
186 | use overload |
187 | "+" => \&add, |
188 | "*=" => "muas"; |
189 | |
190 | declares function Number::add() for addition, and method muas() in |
191 | the "class" C<Number> (or one of its base classes) |
192 | for the assignment form C<*=> of multiplication. |
193 | |
194 | Arguments of this directive come in (key, value) pairs. Legal values |
e7ea3e70 |
195 | are values legal inside a C<&{ ... }> call, so the name of a |
196 | subroutine, a reference to a subroutine, or an anonymous subroutine |
197 | will all work. Note that values specified as strings are |
198 | interpreted as methods, not subroutines. Legal keys are listed below. |
4633a7c4 |
199 | |
200 | The subroutine C<add> will be called to execute C<$a+$b> if $a |
201 | is a reference to an object blessed into the package C<Number>, or if $a is |
202 | not an object from a package with defined mathemagic addition, but $b is a |
203 | reference to a C<Number>. It can also be called in other situations, like |
204 | C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical |
205 | methods refer to methods triggered by an overloaded mathematical |
206 | operator.) |
207 | |
774d564b |
208 | Since overloading respects inheritance via the @ISA hierarchy, the |
209 | above declaration would also trigger overloading of C<+> and C<*=> in |
210 | all the packages which inherit from C<Number>. |
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211 | |
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212 | =head2 Calling Conventions for Binary Operations |
213 | |
214 | The functions specified in the C<use overload ...> directive are called |
215 | with three (in one particular case with four, see L<Last Resort>) |
216 | arguments. If the corresponding operation is binary, then the first |
217 | two arguments are the two arguments of the operation. However, due to |
218 | general object calling conventions, the first argument should always be |
219 | an object in the package, so in the situation of C<7+$a>, the |
220 | order of the arguments is interchanged. It probably does not matter |
221 | when implementing the addition method, but whether the arguments |
222 | are reversed is vital to the subtraction method. The method can |
223 | query this information by examining the third argument, which can take |
224 | three different values: |
225 | |
226 | =over 7 |
227 | |
228 | =item FALSE |
229 | |
230 | the order of arguments is as in the current operation. |
231 | |
232 | =item TRUE |
233 | |
234 | the arguments are reversed. |
235 | |
236 | =item C<undef> |
237 | |
238 | the current operation is an assignment variant (as in |
239 | C<$a+=7>), but the usual function is called instead. This additional |
ee239bfe |
240 | information can be used to generate some optimizations. Compare |
241 | L<Calling Conventions for Mutators>. |
4633a7c4 |
242 | |
243 | =back |
244 | |
245 | =head2 Calling Conventions for Unary Operations |
246 | |
247 | Unary operation are considered binary operations with the second |
248 | argument being C<undef>. Thus the functions that overloads C<{"++"}> |
249 | is called with arguments C<($a,undef,'')> when $a++ is executed. |
250 | |
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251 | =head2 Calling Conventions for Mutators |
252 | |
253 | Two types of mutators have different calling conventions: |
254 | |
255 | =over |
256 | |
257 | =item C<++> and C<--> |
258 | |
259 | The routines which implement these operators are expected to actually |
260 | I<mutate> their arguments. So, assuming that $obj is a reference to a |
261 | number, |
262 | |
263 | sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n} |
264 | |
265 | is an appropriate implementation of overloaded C<++>. Note that |
266 | |
267 | sub incr { ++$ {$_[0]} ; shift } |
268 | |
269 | is OK if used with preincrement and with postincrement. (In the case |
270 | of postincrement a copying will be performed, see L<Copy Constructor>.) |
271 | |
272 | =item C<x=> and other assignment versions |
273 | |
274 | There is nothing special about these methods. They may change the |
275 | value of their arguments, and may leave it as is. The result is going |
276 | to be assigned to the value in the left-hand-side if different from |
277 | this value. |
278 | |
f610777f |
279 | This allows for the same method to be used as overloaded C<+=> and |
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280 | C<+>. Note that this is I<allowed>, but not recommended, since by the |
281 | semantic of L<"Fallback"> Perl will call the method for C<+> anyway, |
282 | if C<+=> is not overloaded. |
283 | |
284 | =back |
285 | |
286 | B<Warning.> Due to the presense of assignment versions of operations, |
287 | routines which may be called in assignment context may create |
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288 | self-referential structures. Currently Perl will not free self-referential |
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289 | structures until cycles are C<explicitly> broken. You may get problems |
290 | when traversing your structures too. |
291 | |
292 | Say, |
293 | |
294 | use overload '+' => sub { bless [ \$_[0], \$_[1] ] }; |
295 | |
296 | is asking for trouble, since for code C<$obj += $foo> the subroutine |
297 | is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj, |
298 | \$foo]>. If using such a subroutine is an important optimization, one |
299 | can overload C<+=> explicitly by a non-"optimized" version, or switch |
300 | to non-optimized version if C<not defined $_[2]> (see |
301 | L<Calling Conventions for Binary Operations>). |
302 | |
303 | Even if no I<explicit> assignment-variants of operators are present in |
304 | the script, they may be generated by the optimizer. Say, C<",$obj,"> or |
305 | C<',' . $obj . ','> may be both optimized to |
306 | |
307 | my $tmp = ',' . $obj; $tmp .= ','; |
308 | |
4633a7c4 |
309 | =head2 Overloadable Operations |
310 | |
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311 | The following symbols can be specified in C<use overload> directive: |
4633a7c4 |
312 | |
313 | =over 5 |
314 | |
315 | =item * I<Arithmetic operations> |
316 | |
317 | "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=", |
318 | "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=", |
319 | |
320 | For these operations a substituted non-assignment variant can be called if |
321 | the assignment variant is not available. Methods for operations "C<+>", |
322 | "C<->", "C<+=>", and "C<-=>" can be called to automatically generate |
323 | increment and decrement methods. The operation "C<->" can be used to |
324 | autogenerate missing methods for unary minus or C<abs>. |
325 | |
ee239bfe |
326 | See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and |
327 | L<"Calling Conventions for Binary Operations">) for details of these |
328 | substitutions. |
329 | |
4633a7c4 |
330 | =item * I<Comparison operations> |
331 | |
332 | "<", "<=", ">", ">=", "==", "!=", "<=>", |
333 | "lt", "le", "gt", "ge", "eq", "ne", "cmp", |
334 | |
335 | If the corresponding "spaceship" variant is available, it can be |
336 | used to substitute for the missing operation. During C<sort>ing |
337 | arrays, C<cmp> is used to compare values subject to C<use overload>. |
338 | |
339 | =item * I<Bit operations> |
340 | |
341 | "&", "^", "|", "neg", "!", "~", |
342 | |
343 | "C<neg>" stands for unary minus. If the method for C<neg> is not |
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344 | specified, it can be autogenerated using the method for |
345 | subtraction. If the method for "C<!>" is not specified, it can be |
346 | autogenerated using the methods for "C<bool>", or "C<\"\">", or "C<0+>". |
4633a7c4 |
347 | |
348 | =item * I<Increment and decrement> |
349 | |
350 | "++", "--", |
351 | |
352 | If undefined, addition and subtraction methods can be |
353 | used instead. These operations are called both in prefix and |
354 | postfix form. |
355 | |
356 | =item * I<Transcendental functions> |
357 | |
358 | "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", |
359 | |
360 | If C<abs> is unavailable, it can be autogenerated using methods |
1fef88e7 |
361 | for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction. |
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362 | |
363 | =item * I<Boolean, string and numeric conversion> |
364 | |
365 | "bool", "\"\"", "0+", |
366 | |
f5284f61 |
367 | If one or two of these operations are not overloaded, the remaining ones can |
4633a7c4 |
368 | be used instead. C<bool> is used in the flow control operators |
369 | (like C<while>) and for the ternary "C<?:>" operation. These functions can |
370 | return any arbitrary Perl value. If the corresponding operation for this value |
371 | is overloaded too, that operation will be called again with this value. |
372 | |
f5284f61 |
373 | =item * I<Iteration> |
374 | |
375 | "<>" |
376 | |
377 | If not overloaded, the argument will be converted to a filehandle or |
378 | glob (which may require a stringification). The same overloading |
379 | happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and |
380 | I<globbing> syntax C<E<lt>${var}E<gt>>. |
381 | |
382 | =item * I<Dereferencing> |
383 | |
384 | '${}', '@{}', '%{}', '&{}', '*{}'. |
385 | |
386 | If not overloaded, the argument will be dereferenced I<as is>, thus |
387 | should be of correct type. These functions should return a reference |
388 | of correct type, or another object with overloaded dereferencing. |
389 | |
4633a7c4 |
390 | =item * I<Special> |
391 | |
392 | "nomethod", "fallback", "=", |
393 | |
394 | see L<SPECIAL SYMBOLS FOR C<use overload>>. |
395 | |
396 | =back |
397 | |
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398 | See L<"Fallback"> for an explanation of when a missing method can be |
399 | autogenerated. |
400 | |
401 | A computer-readable form of the above table is available in the hash |
402 | %overload::ops, with values being space-separated lists of names: |
403 | |
404 | with_assign => '+ - * / % ** << >> x .', |
405 | assign => '+= -= *= /= %= **= <<= >>= x= .=', |
406 | str_comparison => '< <= > >= == !=', |
407 | '3way_comparison'=> '<=> cmp', |
408 | num_comparison => 'lt le gt ge eq ne', |
409 | binary => '& | ^', |
410 | unary => 'neg ! ~', |
411 | mutators => '++ --', |
412 | func => 'atan2 cos sin exp abs log sqrt', |
413 | conversion => 'bool "" 0+', |
f5284f61 |
414 | iterators => '<>', |
415 | dereferencing => '${} @{} %{} &{} *{}', |
ee239bfe |
416 | special => 'nomethod fallback =' |
4633a7c4 |
417 | |
e7ea3e70 |
418 | =head2 Inheritance and overloading |
419 | |
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420 | Inheritance interacts with overloading in two ways. |
e7ea3e70 |
421 | |
422 | =over |
423 | |
424 | =item Strings as values of C<use overload> directive |
425 | |
774d564b |
426 | If C<value> in |
e7ea3e70 |
427 | |
428 | use overload key => value; |
429 | |
774d564b |
430 | is a string, it is interpreted as a method name. |
e7ea3e70 |
431 | |
432 | =item Overloading of an operation is inherited by derived classes |
433 | |
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434 | Any class derived from an overloaded class is also overloaded. The |
435 | set of overloaded methods is the union of overloaded methods of all |
436 | the ancestors. If some method is overloaded in several ancestor, then |
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437 | which description will be used is decided by the usual inheritance |
774d564b |
438 | rules: |
e7ea3e70 |
439 | |
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440 | If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads |
441 | C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">, |
442 | then the subroutine C<D::plus_sub> will be called to implement |
443 | operation C<+> for an object in package C<A>. |
e7ea3e70 |
444 | |
445 | =back |
446 | |
774d564b |
447 | Note that since the value of the C<fallback> key is not a subroutine, |
448 | its inheritance is not governed by the above rules. In the current |
449 | implementation, the value of C<fallback> in the first overloaded |
450 | ancestor is used, but this is accidental and subject to change. |
e7ea3e70 |
451 | |
4633a7c4 |
452 | =head1 SPECIAL SYMBOLS FOR C<use overload> |
453 | |
454 | Three keys are recognized by Perl that are not covered by the above |
455 | description. |
456 | |
774d564b |
457 | =head2 Last Resort |
4633a7c4 |
458 | |
459 | C<"nomethod"> should be followed by a reference to a function of four |
460 | parameters. If defined, it is called when the overloading mechanism |
461 | cannot find a method for some operation. The first three arguments of |
462 | this function coincide with the arguments for the corresponding method if |
463 | it were found, the fourth argument is the symbol |
464 | corresponding to the missing method. If several methods are tried, |
465 | the last one is used. Say, C<1-$a> can be equivalent to |
466 | |
467 | &nomethodMethod($a,1,1,"-") |
468 | |
469 | if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the |
470 | C<use overload> directive. |
471 | |
472 | If some operation cannot be resolved, and there is no function |
473 | assigned to C<"nomethod">, then an exception will be raised via die()-- |
474 | unless C<"fallback"> was specified as a key in C<use overload> directive. |
475 | |
476 | =head2 Fallback |
477 | |
478 | The key C<"fallback"> governs what to do if a method for a particular |
479 | operation is not found. Three different cases are possible depending on |
480 | the value of C<"fallback">: |
481 | |
482 | =over 16 |
483 | |
484 | =item * C<undef> |
485 | |
486 | Perl tries to use a |
487 | substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it |
488 | then tries to calls C<"nomethod"> value; if missing, an exception |
489 | will be raised. |
490 | |
491 | =item * TRUE |
492 | |
493 | The same as for the C<undef> value, but no exception is raised. Instead, |
494 | it silently reverts to what it would have done were there no C<use overload> |
495 | present. |
496 | |
497 | =item * defined, but FALSE |
498 | |
499 | No autogeneration is tried. Perl tries to call |
500 | C<"nomethod"> value, and if this is missing, raises an exception. |
501 | |
502 | =back |
503 | |
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504 | B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone |
505 | yet, see L<"Inheritance and overloading">. |
506 | |
4633a7c4 |
507 | =head2 Copy Constructor |
508 | |
509 | The value for C<"="> is a reference to a function with three |
510 | arguments, i.e., it looks like the other values in C<use |
511 | overload>. However, it does not overload the Perl assignment |
512 | operator. This would go against Camel hair. |
513 | |
514 | This operation is called in the situations when a mutator is applied |
515 | to a reference that shares its object with some other reference, such |
516 | as |
517 | |
518 | $a=$b; |
ee239bfe |
519 | ++$a; |
4633a7c4 |
520 | |
521 | To make this change $a and not change $b, a copy of C<$$a> is made, |
522 | and $a is assigned a reference to this new object. This operation is |
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523 | done during execution of the C<++$a>, and not during the assignment, |
4633a7c4 |
524 | (so before the increment C<$$a> coincides with C<$$b>). This is only |
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525 | done if C<++> is expressed via a method for C<'++'> or C<'+='> (or |
526 | C<nomethod>). Note that if this operation is expressed via C<'+'> |
527 | a nonmutator, i.e., as in |
4633a7c4 |
528 | |
529 | $a=$b; |
530 | $a=$a+1; |
531 | |
532 | then C<$a> does not reference a new copy of C<$$a>, since $$a does not |
533 | appear as lvalue when the above code is executed. |
534 | |
535 | If the copy constructor is required during the execution of some mutator, |
536 | but a method for C<'='> was not specified, it can be autogenerated as a |
537 | string copy if the object is a plain scalar. |
538 | |
539 | =over 5 |
540 | |
541 | =item B<Example> |
542 | |
543 | The actually executed code for |
544 | |
545 | $a=$b; |
546 | Something else which does not modify $a or $b.... |
547 | ++$a; |
548 | |
549 | may be |
550 | |
551 | $a=$b; |
552 | Something else which does not modify $a or $b.... |
553 | $a = $a->clone(undef,""); |
554 | $a->incr(undef,""); |
555 | |
556 | if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>, |
557 | C<'='> was overloaded with C<\&clone>. |
558 | |
559 | =back |
560 | |
f610777f |
561 | Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for |
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562 | C<$b = $a; ++$a>. |
563 | |
4633a7c4 |
564 | =head1 MAGIC AUTOGENERATION |
565 | |
566 | If a method for an operation is not found, and the value for C<"fallback"> is |
567 | TRUE or undefined, Perl tries to autogenerate a substitute method for |
568 | the missing operation based on the defined operations. Autogenerated method |
569 | substitutions are possible for the following operations: |
570 | |
571 | =over 16 |
572 | |
573 | =item I<Assignment forms of arithmetic operations> |
574 | |
575 | C<$a+=$b> can use the method for C<"+"> if the method for C<"+="> |
576 | is not defined. |
577 | |
578 | =item I<Conversion operations> |
579 | |
580 | String, numeric, and boolean conversion are calculated in terms of one |
581 | another if not all of them are defined. |
582 | |
583 | =item I<Increment and decrement> |
584 | |
585 | The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>, |
586 | and C<$a--> in terms of C<$a-=1> and C<$a-1>. |
587 | |
588 | =item C<abs($a)> |
589 | |
590 | can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>). |
591 | |
592 | =item I<Unary minus> |
593 | |
594 | can be expressed in terms of subtraction. |
595 | |
3bc6ec80 |
596 | =item I<Negation> |
597 | |
598 | C<!> and C<not> can be expressed in terms of boolean conversion, or |
599 | string or numerical conversion. |
600 | |
4633a7c4 |
601 | =item I<Concatenation> |
602 | |
603 | can be expressed in terms of string conversion. |
604 | |
605 | =item I<Comparison operations> |
606 | |
607 | can be expressed in terms of its "spaceship" counterpart: either |
608 | C<E<lt>=E<gt>> or C<cmp>: |
1fef88e7 |
609 | |
4633a7c4 |
610 | <, >, <=, >=, ==, != in terms of <=> |
611 | lt, gt, le, ge, eq, ne in terms of cmp |
612 | |
f5284f61 |
613 | =item I<Iterator> |
614 | |
615 | <> in terms of builtin operations |
616 | |
617 | =item I<Dereferencing> |
618 | |
619 | ${} @{} %{} &{} *{} in terms of builtin operations |
620 | |
4633a7c4 |
621 | =item I<Copy operator> |
622 | |
623 | can be expressed in terms of an assignment to the dereferenced value, if this |
624 | value is a scalar and not a reference. |
625 | |
626 | =back |
627 | |
ee239bfe |
628 | =head1 Losing overloading |
4633a7c4 |
629 | |
630 | The restriction for the comparison operation is that even if, for example, |
631 | `C<cmp>' should return a blessed reference, the autogenerated `C<lt>' |
632 | function will produce only a standard logical value based on the |
633 | numerical value of the result of `C<cmp>'. In particular, a working |
634 | numeric conversion is needed in this case (possibly expressed in terms of |
635 | other conversions). |
636 | |
637 | Similarly, C<.=> and C<x=> operators lose their mathemagical properties |
638 | if the string conversion substitution is applied. |
639 | |
640 | When you chop() a mathemagical object it is promoted to a string and its |
641 | mathemagical properties are lost. The same can happen with other |
642 | operations as well. |
643 | |
644 | =head1 Run-time Overloading |
645 | |
646 | Since all C<use> directives are executed at compile-time, the only way to |
647 | change overloading during run-time is to |
648 | |
649 | eval 'use overload "+" => \&addmethod'; |
650 | |
651 | You can also use |
652 | |
653 | eval 'no overload "+", "--", "<="'; |
654 | |
655 | though the use of these constructs during run-time is questionable. |
656 | |
657 | =head1 Public functions |
658 | |
659 | Package C<overload.pm> provides the following public functions: |
660 | |
661 | =over 5 |
662 | |
663 | =item overload::StrVal(arg) |
664 | |
665 | Gives string value of C<arg> as in absence of stringify overloading. |
666 | |
667 | =item overload::Overloaded(arg) |
668 | |
669 | Returns true if C<arg> is subject to overloading of some operations. |
670 | |
671 | =item overload::Method(obj,op) |
672 | |
673 | Returns C<undef> or a reference to the method that implements C<op>. |
674 | |
675 | =back |
676 | |
b3ac6de7 |
677 | =head1 Overloading constants |
678 | |
679 | For some application Perl parser mangles constants too much. It is possible |
680 | to hook into this process via overload::constant() and overload::remove_constant() |
681 | functions. |
682 | |
683 | These functions take a hash as an argument. The recognized keys of this hash |
684 | are |
685 | |
686 | =over 8 |
687 | |
688 | =item integer |
689 | |
690 | to overload integer constants, |
691 | |
692 | =item float |
693 | |
694 | to overload floating point constants, |
695 | |
696 | =item binary |
697 | |
698 | to overload octal and hexadecimal constants, |
699 | |
700 | =item q |
701 | |
702 | to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted |
703 | strings and here-documents, |
704 | |
705 | =item qr |
706 | |
707 | to overload constant pieces of regular expressions. |
708 | |
709 | =back |
710 | |
711 | The corresponding values are references to functions which take three arguments: |
712 | the first one is the I<initial> string form of the constant, the second one |
713 | is how Perl interprets this constant, the third one is how the constant is used. |
714 | Note that the initial string form does not |
715 | contain string delimiters, and has backslashes in backslash-delimiter |
716 | combinations stripped (thus the value of delimiter is not relevant for |
717 | processing of this string). The return value of this function is how this |
718 | constant is going to be interpreted by Perl. The third argument is undefined |
719 | unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote |
720 | context (comes from strings, regular expressions, and single-quote HERE |
721 | documents), it is C<tr> for arguments of C<tr>/C<y> operators, |
722 | it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise. |
723 | |
724 | Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>, |
725 | it is expected that overloaded constant strings are equipped with reasonable |
726 | overloaded catenation operator, otherwise absurd results will result. |
727 | Similarly, negative numbers are considered as negations of positive constants. |
728 | |
729 | Note that it is probably meaningless to call the functions overload::constant() |
730 | and overload::remove_constant() from anywhere but import() and unimport() methods. |
731 | From these methods they may be called as |
732 | |
733 | sub import { |
734 | shift; |
735 | return unless @_; |
736 | die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant'; |
737 | overload::constant integer => sub {Math::BigInt->new(shift)}; |
738 | } |
739 | |
740 | B<BUGS> Currently overloaded-ness of constants does not propagate |
741 | into C<eval '...'>. |
742 | |
4633a7c4 |
743 | =head1 IMPLEMENTATION |
744 | |
745 | What follows is subject to change RSN. |
746 | |
e7ea3e70 |
747 | The table of methods for all operations is cached in magic for the |
748 | symbol table hash for the package. The cache is invalidated during |
749 | processing of C<use overload>, C<no overload>, new function |
750 | definitions, and changes in @ISA. However, this invalidation remains |
751 | unprocessed until the next C<bless>ing into the package. Hence if you |
752 | want to change overloading structure dynamically, you'll need an |
753 | additional (fake) C<bless>ing to update the table. |
754 | |
755 | (Every SVish thing has a magic queue, and magic is an entry in that |
756 | queue. This is how a single variable may participate in multiple |
757 | forms of magic simultaneously. For instance, environment variables |
758 | regularly have two forms at once: their %ENV magic and their taint |
759 | magic. However, the magic which implements overloading is applied to |
760 | the stashes, which are rarely used directly, thus should not slow down |
761 | Perl.) |
4633a7c4 |
762 | |
763 | If an object belongs to a package using overload, it carries a special |
764 | flag. Thus the only speed penalty during arithmetic operations without |
765 | overloading is the checking of this flag. |
766 | |
774d564b |
767 | In fact, if C<use overload> is not present, there is almost no overhead |
768 | for overloadable operations, so most programs should not suffer |
769 | measurable performance penalties. A considerable effort was made to |
770 | minimize the overhead when overload is used in some package, but the |
771 | arguments in question do not belong to packages using overload. When |
772 | in doubt, test your speed with C<use overload> and without it. So far |
773 | there have been no reports of substantial speed degradation if Perl is |
774 | compiled with optimization turned on. |
4633a7c4 |
775 | |
e7ea3e70 |
776 | There is no size penalty for data if overload is not used. The only |
777 | size penalty if overload is used in some package is that I<all> the |
778 | packages acquire a magic during the next C<bless>ing into the |
779 | package. This magic is three-words-long for packages without |
f610777f |
780 | overloading, and carries the cache table if the package is overloaded. |
4633a7c4 |
781 | |
782 | Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is |
783 | carried out before any operation that can imply an assignment to the |
784 | object $a (or $b) refers to, like C<$a++>. You can override this |
785 | behavior by defining your own copy constructor (see L<"Copy Constructor">). |
786 | |
787 | It is expected that arguments to methods that are not explicitly supposed |
788 | to be changed are constant (but this is not enforced). |
789 | |
ee239bfe |
790 | =head1 Metaphor clash |
791 | |
f610777f |
792 | One may wonder why the semantic of overloaded C<=> is so counter intuitive. |
793 | If it I<looks> counter intuitive to you, you are subject to a metaphor |
ee239bfe |
794 | clash. |
795 | |
796 | Here is a Perl object metaphor: |
797 | |
798 | I< object is a reference to blessed data> |
799 | |
800 | and an arithmetic metaphor: |
801 | |
802 | I< object is a thing by itself>. |
803 | |
804 | The I<main> problem of overloading C<=> is the fact that these metaphors |
805 | imply different actions on the assignment C<$a = $b> if $a and $b are |
806 | objects. Perl-think implies that $a becomes a reference to whatever |
807 | $b was referencing. Arithmetic-think implies that the value of "object" |
808 | $a is changed to become the value of the object $b, preserving the fact |
809 | that $a and $b are separate entities. |
810 | |
811 | The difference is not relevant in the absence of mutators. After |
812 | a Perl-way assignment an operation which mutates the data referenced by $a |
813 | would change the data referenced by $b too. Effectively, after |
814 | C<$a = $b> values of $a and $b become I<indistinguishable>. |
815 | |
816 | On the other hand, anyone who has used algebraic notation knows the |
817 | expressive power of the arithmetic metaphor. Overloading works hard |
818 | to enable this metaphor while preserving the Perlian way as far as |
819 | possible. Since it is not not possible to freely mix two contradicting |
820 | metaphors, overloading allows the arithmetic way to write things I<as |
821 | far as all the mutators are called via overloaded access only>. The |
822 | way it is done is described in L<Copy Constructor>. |
823 | |
824 | If some mutator methods are directly applied to the overloaded values, |
825 | one may need to I<explicitly unlink> other values which references the |
826 | same value: |
827 | |
828 | $a = new Data 23; |
829 | ... |
830 | $b = $a; # $b is "linked" to $a |
831 | ... |
832 | $a = $a->clone; # Unlink $b from $a |
833 | $a->increment_by(4); |
834 | |
835 | Note that overloaded access makes this transparent: |
836 | |
837 | $a = new Data 23; |
838 | $b = $a; # $b is "linked" to $a |
839 | $a += 4; # would unlink $b automagically |
840 | |
841 | However, it would not make |
842 | |
843 | $a = new Data 23; |
844 | $a = 4; # Now $a is a plain 4, not 'Data' |
845 | |
846 | preserve "objectness" of $a. But Perl I<has> a way to make assignments |
847 | to an object do whatever you want. It is just not the overload, but |
848 | tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method |
849 | which returns the object itself, and STORE() method which changes the |
850 | value of the object, one can reproduce the arithmetic metaphor in its |
851 | completeness, at least for variables which were tie()d from the start. |
852 | |
853 | (Note that a workaround for a bug may be needed, see L<"BUGS">.) |
854 | |
855 | =head1 Cookbook |
856 | |
857 | Please add examples to what follows! |
858 | |
859 | =head2 Two-face scalars |
860 | |
861 | Put this in F<two_face.pm> in your Perl library directory: |
862 | |
863 | package two_face; # Scalars with separate string and |
864 | # numeric values. |
865 | sub new { my $p = shift; bless [@_], $p } |
866 | use overload '""' => \&str, '0+' => \&num, fallback => 1; |
867 | sub num {shift->[1]} |
868 | sub str {shift->[0]} |
869 | |
870 | Use it as follows: |
871 | |
872 | require two_face; |
873 | my $seven = new two_face ("vii", 7); |
874 | printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1; |
875 | print "seven contains `i'\n" if $seven =~ /i/; |
876 | |
877 | (The second line creates a scalar which has both a string value, and a |
878 | numeric value.) This prints: |
879 | |
880 | seven=vii, seven=7, eight=8 |
881 | seven contains `i' |
882 | |
f5284f61 |
883 | =head2 Two-face references |
884 | |
885 | Suppose you want to create an object which is accessible as both an |
886 | array reference, and a hash reference, similar to the builtin |
887 | L<array-accessible-as-a-hash|perlref/"Pseudo-hashes: Using an array as |
888 | a hash"> builtin Perl type. Let us make it better than the builtin |
889 | type, there will be no restriction that you cannot use the index 0 of |
890 | your array. |
891 | |
892 | package two_refs; |
893 | use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} }; |
894 | sub new { |
895 | my $p = shift; |
896 | bless \ [@_], $p; |
897 | } |
898 | sub gethash { |
899 | my %h; |
900 | my $self = shift; |
901 | tie %h, ref $self, $self; |
902 | \%h; |
903 | } |
904 | |
905 | sub TIEHASH { my $p = shift; bless \ shift, $p } |
906 | my %fields; |
907 | my $i = 0; |
908 | $fields{$_} = $i++ foreach qw{zero one two three}; |
909 | sub STORE { |
910 | my $self = ${shift()}; |
911 | my $key = $fields{shift()}; |
912 | defined $key or die "Out of band access"; |
913 | $$self->[$key] = shift; |
914 | } |
915 | sub FETCH { |
916 | my $self = ${shift()}; |
917 | my $key = $fields{shift()}; |
918 | defined $key or die "Out of band access"; |
919 | $$self->[$key]; |
920 | } |
921 | |
922 | Now one can access an object using both the array and hash syntax: |
923 | |
924 | my $bar = new two_refs 3,4,5,6; |
925 | $bar->[2] = 11; |
926 | $bar->{two} == 11 or die 'bad hash fetch'; |
927 | |
928 | Note several important features of this example. First of all, the |
929 | I<actual> type of $bar is a scalar reference, and we do not overload |
930 | the scalar dereference. Thus we can get the I<actual> non-overloaded |
931 | contents of $bar by just using C<$$bar> (what we do in functions which |
932 | overload dereference). Similarly, the object returned by the |
933 | TIEHASH() method is a scalar reference. |
934 | |
935 | Second, we create a new tied hash each time the hash syntax is used. |
936 | This allows us not to worry about a possibility of a reference loop, |
937 | would would lead to a memory leak. |
938 | |
939 | Both these problems can be cured. Say, if we want to overload hash |
940 | dereference on a reference to an object which is I<implemented> as a |
941 | hash itself, the only problem one has to circumvent is how to access |
942 | this I<actual> hash (as opposed to the I<virtual> exhibited by |
943 | overloaded dereference operator). Here is one possible fetching routine: |
944 | |
945 | sub access_hash { |
946 | my ($self, $key) = (shift, shift); |
947 | my $class = ref $self; |
948 | bless $self, 'overload::dummy'; # Disable overloading of %{} |
949 | my $out = $self->{$key}; |
950 | bless $self, $class; # Restore overloading |
951 | $out; |
952 | } |
953 | |
954 | To move creation of the tied hash on each access, one may an extra |
955 | level of indirection which allows a non-circular structure of references: |
956 | |
957 | package two_refs1; |
958 | use overload '%{}' => sub { ${shift()}->[1] }, |
959 | '@{}' => sub { ${shift()}->[0] }; |
960 | sub new { |
961 | my $p = shift; |
962 | my $a = [@_]; |
963 | my %h; |
964 | tie %h, $p, $a; |
965 | bless \ [$a, \%h], $p; |
966 | } |
967 | sub gethash { |
968 | my %h; |
969 | my $self = shift; |
970 | tie %h, ref $self, $self; |
971 | \%h; |
972 | } |
973 | |
974 | sub TIEHASH { my $p = shift; bless \ shift, $p } |
975 | my %fields; |
976 | my $i = 0; |
977 | $fields{$_} = $i++ foreach qw{zero one two three}; |
978 | sub STORE { |
979 | my $a = ${shift()}; |
980 | my $key = $fields{shift()}; |
981 | defined $key or die "Out of band access"; |
982 | $a->[$key] = shift; |
983 | } |
984 | sub FETCH { |
985 | my $a = ${shift()}; |
986 | my $key = $fields{shift()}; |
987 | defined $key or die "Out of band access"; |
988 | $a->[$key]; |
989 | } |
990 | |
991 | Now if $baz is overloaded like this, then C<$bar> is a reference to a |
992 | reference to the intermediate array, which keeps a reference to an |
993 | actual array, and the access hash. The tie()ing object for the access |
994 | hash is also a reference to a reference to the actual array, so |
995 | |
996 | =over |
997 | |
998 | =item * |
999 | |
1000 | There are no loops of references. |
1001 | |
1002 | =item * |
1003 | |
1004 | Both "objects" which are blessed into the class C<two_refs1> are |
1005 | references to a reference to an array, thus references to a I<scalar>. |
1006 | Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no |
1007 | overloaded operations. |
1008 | |
1009 | =back |
1010 | |
ee239bfe |
1011 | =head2 Symbolic calculator |
1012 | |
1013 | Put this in F<symbolic.pm> in your Perl library directory: |
1014 | |
1015 | package symbolic; # Primitive symbolic calculator |
1016 | use overload nomethod => \&wrap; |
1017 | |
1018 | sub new { shift; bless ['n', @_] } |
1019 | sub wrap { |
1020 | my ($obj, $other, $inv, $meth) = @_; |
1021 | ($obj, $other) = ($other, $obj) if $inv; |
1022 | bless [$meth, $obj, $other]; |
1023 | } |
1024 | |
1025 | This module is very unusual as overloaded modules go: it does not |
1026 | provide any usual overloaded operators, instead it provides the L<Last |
1027 | Resort> operator C<nomethod>. In this example the corresponding |
f610777f |
1028 | subroutine returns an object which encapsulates operations done over |
ee239bfe |
1029 | the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new |
1030 | symbolic 3> contains C<['+', 2, ['n', 3]]>. |
1031 | |
1032 | Here is an example of the script which "calculates" the side of |
1033 | circumscribed octagon using the above package: |
1034 | |
1035 | require symbolic; |
1036 | my $iter = 1; # 2**($iter+2) = 8 |
1037 | my $side = new symbolic 1; |
1038 | my $cnt = $iter; |
1039 | |
1040 | while ($cnt--) { |
1041 | $side = (sqrt(1 + $side**2) - 1)/$side; |
1042 | } |
1043 | print "OK\n"; |
1044 | |
1045 | The value of $side is |
1046 | |
1047 | ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]], |
1048 | undef], 1], ['n', 1]] |
1049 | |
1050 | Note that while we obtained this value using a nice little script, |
1051 | there is no simple way to I<use> this value. In fact this value may |
1052 | be inspected in debugger (see L<perldebug>), but ony if |
1053 | C<bareStringify> B<O>ption is set, and not via C<p> command. |
1054 | |
1055 | If one attempts to print this value, then the overloaded operator |
1056 | C<""> will be called, which will call C<nomethod> operator. The |
1057 | result of this operator will be stringified again, but this result is |
1058 | again of type C<symbolic>, which will lead to an infinite loop. |
1059 | |
1060 | Add a pretty-printer method to the module F<symbolic.pm>: |
1061 | |
1062 | sub pretty { |
1063 | my ($meth, $a, $b) = @{+shift}; |
1064 | $a = 'u' unless defined $a; |
1065 | $b = 'u' unless defined $b; |
1066 | $a = $a->pretty if ref $a; |
1067 | $b = $b->pretty if ref $b; |
1068 | "[$meth $a $b]"; |
1069 | } |
1070 | |
1071 | Now one can finish the script by |
1072 | |
1073 | print "side = ", $side->pretty, "\n"; |
1074 | |
1075 | The method C<pretty> is doing object-to-string conversion, so it |
1076 | is natural to overload the operator C<""> using this method. However, |
1077 | inside such a method it is not necessary to pretty-print the |
1078 | I<components> $a and $b of an object. In the above subroutine |
1079 | C<"[$meth $a $b]"> is a catenation of some strings and components $a |
1080 | and $b. If these components use overloading, the catenation operator |
1081 | will look for an overloaded operator C<.>, if not present, it will |
1082 | look for an overloaded operator C<"">. Thus it is enough to use |
1083 | |
1084 | use overload nomethod => \&wrap, '""' => \&str; |
1085 | sub str { |
1086 | my ($meth, $a, $b) = @{+shift}; |
1087 | $a = 'u' unless defined $a; |
1088 | $b = 'u' unless defined $b; |
1089 | "[$meth $a $b]"; |
1090 | } |
1091 | |
1092 | Now one can change the last line of the script to |
1093 | |
1094 | print "side = $side\n"; |
1095 | |
1096 | which outputs |
1097 | |
1098 | side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]] |
1099 | |
1100 | and one can inspect the value in debugger using all the possible |
1101 | methods. |
1102 | |
1103 | Something is is still amiss: consider the loop variable $cnt of the |
1104 | script. It was a number, not an object. We cannot make this value of |
1105 | type C<symbolic>, since then the loop will not terminate. |
1106 | |
1107 | Indeed, to terminate the cycle, the $cnt should become false. |
1108 | However, the operator C<bool> for checking falsity is overloaded (this |
1109 | time via overloaded C<"">), and returns a long string, thus any object |
1110 | of type C<symbolic> is true. To overcome this, we need a way to |
1111 | compare an object to 0. In fact, it is easier to write a numeric |
1112 | conversion routine. |
1113 | |
1114 | Here is the text of F<symbolic.pm> with such a routine added (and |
f610777f |
1115 | slightly modified str()): |
ee239bfe |
1116 | |
1117 | package symbolic; # Primitive symbolic calculator |
1118 | use overload |
1119 | nomethod => \&wrap, '""' => \&str, '0+' => \# |
1120 | |
1121 | sub new { shift; bless ['n', @_] } |
1122 | sub wrap { |
1123 | my ($obj, $other, $inv, $meth) = @_; |
1124 | ($obj, $other) = ($other, $obj) if $inv; |
1125 | bless [$meth, $obj, $other]; |
1126 | } |
1127 | sub str { |
1128 | my ($meth, $a, $b) = @{+shift}; |
1129 | $a = 'u' unless defined $a; |
1130 | if (defined $b) { |
1131 | "[$meth $a $b]"; |
1132 | } else { |
1133 | "[$meth $a]"; |
1134 | } |
1135 | } |
1136 | my %subr = ( n => sub {$_[0]}, |
1137 | sqrt => sub {sqrt $_[0]}, |
1138 | '-' => sub {shift() - shift()}, |
1139 | '+' => sub {shift() + shift()}, |
1140 | '/' => sub {shift() / shift()}, |
1141 | '*' => sub {shift() * shift()}, |
1142 | '**' => sub {shift() ** shift()}, |
1143 | ); |
1144 | sub num { |
1145 | my ($meth, $a, $b) = @{+shift}; |
1146 | my $subr = $subr{$meth} |
1147 | or die "Do not know how to ($meth) in symbolic"; |
1148 | $a = $a->num if ref $a eq __PACKAGE__; |
1149 | $b = $b->num if ref $b eq __PACKAGE__; |
1150 | $subr->($a,$b); |
1151 | } |
1152 | |
1153 | All the work of numeric conversion is done in %subr and num(). Of |
f610777f |
1154 | course, %subr is not complete, it contains only operators used in the |
ee239bfe |
1155 | example below. Here is the extra-credit question: why do we need an |
1156 | explicit recursion in num()? (Answer is at the end of this section.) |
1157 | |
1158 | Use this module like this: |
1159 | |
1160 | require symbolic; |
1161 | my $iter = new symbolic 2; # 16-gon |
1162 | my $side = new symbolic 1; |
1163 | my $cnt = $iter; |
1164 | |
1165 | while ($cnt) { |
1166 | $cnt = $cnt - 1; # Mutator `--' not implemented |
1167 | $side = (sqrt(1 + $side**2) - 1)/$side; |
1168 | } |
1169 | printf "%s=%f\n", $side, $side; |
1170 | printf "pi=%f\n", $side*(2**($iter+2)); |
1171 | |
1172 | It prints (without so many line breaks) |
1173 | |
1174 | [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] |
1175 | [n 1]] 2]]] 1] |
1176 | [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912 |
1177 | pi=3.182598 |
1178 | |
1179 | The above module is very primitive. It does not implement |
1180 | mutator methods (C<++>, C<-=> and so on), does not do deep copying |
1181 | (not required without mutators!), and implements only those arithmetic |
1182 | operations which are used in the example. |
1183 | |
f610777f |
1184 | To implement most arithmetic operations is easy, one should just use |
ee239bfe |
1185 | the tables of operations, and change the code which fills %subr to |
1186 | |
1187 | my %subr = ( 'n' => sub {$_[0]} ); |
1188 | foreach my $op (split " ", $overload::ops{with_assign}) { |
1189 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; |
1190 | } |
1191 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); |
1192 | foreach my $op (split " ", "@overload::ops{ @bins }") { |
1193 | $subr{$op} = eval "sub {shift() $op shift()}"; |
1194 | } |
1195 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { |
1196 | print "defining `$op'\n"; |
1197 | $subr{$op} = eval "sub {$op shift()}"; |
1198 | } |
1199 | |
1200 | Due to L<Calling Conventions for Mutators>, we do not need anything |
1201 | special to make C<+=> and friends work, except filling C<+=> entry of |
1202 | %subr, and defining a copy constructor (needed since Perl has no |
1203 | way to know that the implementation of C<'+='> does not mutate |
1204 | the argument, compare L<Copy Constructor>). |
1205 | |
1206 | To implement a copy constructor, add C<'=' => \&cpy> to C<use overload> |
1207 | line, and code (this code assumes that mutators change things one level |
1208 | deep only, so recursive copying is not needed): |
1209 | |
1210 | sub cpy { |
1211 | my $self = shift; |
1212 | bless [@$self], ref $self; |
1213 | } |
1214 | |
1215 | To make C<++> and C<--> work, we need to implement actual mutators, |
1216 | either directly, or in C<nomethod>. We continue to do things inside |
1217 | C<nomethod>, thus add |
1218 | |
1219 | if ($meth eq '++' or $meth eq '--') { |
1220 | @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference |
1221 | return $obj; |
1222 | } |
1223 | |
1224 | after the first line of wrap(). This is not a most effective |
1225 | implementation, one may consider |
1226 | |
1227 | sub inc { $_[0] = bless ['++', shift, 1]; } |
1228 | |
1229 | instead. |
1230 | |
1231 | As a final remark, note that one can fill %subr by |
1232 | |
1233 | my %subr = ( 'n' => sub {$_[0]} ); |
1234 | foreach my $op (split " ", $overload::ops{with_assign}) { |
1235 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; |
1236 | } |
1237 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); |
1238 | foreach my $op (split " ", "@overload::ops{ @bins }") { |
1239 | $subr{$op} = eval "sub {shift() $op shift()}"; |
1240 | } |
1241 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { |
1242 | $subr{$op} = eval "sub {$op shift()}"; |
1243 | } |
1244 | $subr{'++'} = $subr{'+'}; |
1245 | $subr{'--'} = $subr{'-'}; |
1246 | |
1247 | This finishes implementation of a primitive symbolic calculator in |
1248 | 50 lines of Perl code. Since the numeric values of subexpressions |
1249 | are not cached, the calculator is very slow. |
1250 | |
1251 | Here is the answer for the exercise: In the case of str(), we need no |
1252 | explicit recursion since the overloaded C<.>-operator will fall back |
1253 | to an existing overloaded operator C<"">. Overloaded arithmetic |
1254 | operators I<do not> fall back to numeric conversion if C<fallback> is |
1255 | not explicitly requested. Thus without an explicit recursion num() |
1256 | would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild |
1257 | the argument of num(). |
1258 | |
1259 | If you wonder why defaults for conversion are different for str() and |
1260 | num(), note how easy it was to write the symbolic calculator. This |
1261 | simplicity is due to an appropriate choice of defaults. One extra |
f610777f |
1262 | note: due to the explicit recursion num() is more fragile than sym(): |
1263 | we need to explicitly check for the type of $a and $b. If components |
ee239bfe |
1264 | $a and $b happen to be of some related type, this may lead to problems. |
1265 | |
1266 | =head2 I<Really> symbolic calculator |
1267 | |
1268 | One may wonder why we call the above calculator symbolic. The reason |
1269 | is that the actual calculation of the value of expression is postponed |
1270 | until the value is I<used>. |
1271 | |
1272 | To see it in action, add a method |
1273 | |
1274 | sub STORE { |
1275 | my $obj = shift; |
1276 | $#$obj = 1; |
1277 | @$obj->[0,1] = ('=', shift); |
1278 | } |
1279 | |
1280 | to the package C<symbolic>. After this change one can do |
1281 | |
1282 | my $a = new symbolic 3; |
1283 | my $b = new symbolic 4; |
1284 | my $c = sqrt($a**2 + $b**2); |
1285 | |
1286 | and the numeric value of $c becomes 5. However, after calling |
1287 | |
1288 | $a->STORE(12); $b->STORE(5); |
1289 | |
1290 | the numeric value of $c becomes 13. There is no doubt now that the module |
1291 | symbolic provides a I<symbolic> calculator indeed. |
1292 | |
1293 | To hide the rough edges under the hood, provide a tie()d interface to the |
1294 | package C<symbolic> (compare with L<Metaphor clash>). Add methods |
1295 | |
1296 | sub TIESCALAR { my $pack = shift; $pack->new(@_) } |
1297 | sub FETCH { shift } |
1298 | sub nop { } # Around a bug |
1299 | |
1300 | (the bug is described in L<"BUGS">). One can use this new interface as |
1301 | |
1302 | tie $a, 'symbolic', 3; |
1303 | tie $b, 'symbolic', 4; |
1304 | $a->nop; $b->nop; # Around a bug |
1305 | |
1306 | my $c = sqrt($a**2 + $b**2); |
1307 | |
1308 | Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value |
1309 | of $c becomes 13. To insulate the user of the module add a method |
1310 | |
1311 | sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; } |
1312 | |
1313 | Now |
1314 | |
1315 | my ($a, $b); |
1316 | symbolic->vars($a, $b); |
1317 | my $c = sqrt($a**2 + $b**2); |
1318 | |
1319 | $a = 3; $b = 4; |
1320 | printf "c5 %s=%f\n", $c, $c; |
1321 | |
1322 | $a = 12; $b = 5; |
1323 | printf "c13 %s=%f\n", $c, $c; |
1324 | |
1325 | shows that the numeric value of $c follows changes to the values of $a |
1326 | and $b. |
1327 | |
4633a7c4 |
1328 | =head1 AUTHOR |
1329 | |
1fef88e7 |
1330 | Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>. |
4633a7c4 |
1331 | |
1332 | =head1 DIAGNOSTICS |
1333 | |
1334 | When Perl is run with the B<-Do> switch or its equivalent, overloading |
1335 | induces diagnostic messages. |
1336 | |
e7ea3e70 |
1337 | Using the C<m> command of Perl debugger (see L<perldebug>) one can |
1338 | deduce which operations are overloaded (and which ancestor triggers |
1339 | this overloading). Say, if C<eq> is overloaded, then the method C<(eq> |
1340 | is shown by debugger. The method C<()> corresponds to the C<fallback> |
1341 | key (in fact a presence of this method shows that this package has |
1342 | overloading enabled, and it is what is used by the C<Overloaded> |
ee239bfe |
1343 | function of module C<overload>). |
e7ea3e70 |
1344 | |
4633a7c4 |
1345 | =head1 BUGS |
1346 | |
aa689395 |
1347 | Because it is used for overloading, the per-package hash %OVERLOAD now |
1348 | has a special meaning in Perl. The symbol table is filled with names |
1349 | looking like line-noise. |
4633a7c4 |
1350 | |
a6006777 |
1351 | For the purpose of inheritance every overloaded package behaves as if |
1352 | C<fallback> is present (possibly undefined). This may create |
1353 | interesting effects if some package is not overloaded, but inherits |
1354 | from two overloaded packages. |
4633a7c4 |
1355 | |
ee239bfe |
1356 | Relation between overloading and tie()ing is broken. Overloading is |
1357 | triggered or not basing on the I<previous> class of tie()d value. |
1358 | |
1359 | This happens because the presence of overloading is checked too early, |
1360 | before any tie()d access is attempted. If the FETCH()ed class of the |
1361 | tie()d value does not change, a simple workaround is to access the value |
1362 | immediately after tie()ing, so that after this call the I<previous> class |
1363 | coincides with the current one. |
1364 | |
1365 | B<Needed:> a way to fix this without a speed penalty. |
1366 | |
b3ac6de7 |
1367 | Barewords are not covered by overloaded string constants. |
1368 | |
ee239bfe |
1369 | This document is confusing. There are grammos and misleading language |
1370 | used in places. It would seem a total rewrite is needed. |
4633a7c4 |
1371 | |
1372 | =cut |
1373 | |