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