Commit | Line | Data |
4633a7c4 |
1 | package overload; |
2 | |
d5448623 |
3 | $overload::hint_bits = 0x20000; |
4 | |
a6006777 |
5 | sub nil {} |
6 | |
4633a7c4 |
7 | sub OVERLOAD { |
8 | $package = shift; |
9 | my %arg = @_; |
a6006777 |
10 | my ($sub, $fb); |
11 | $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching. |
12 | *{$package . "::()"} = \&nil; # Make it findable via fetchmethod. |
4633a7c4 |
13 | for (keys %arg) { |
a6006777 |
14 | if ($_ eq 'fallback') { |
15 | $fb = $arg{$_}; |
16 | } else { |
17 | $sub = $arg{$_}; |
18 | if (not ref $sub and $sub !~ /::/) { |
44a8e56a |
19 | $ {$package . "::(" . $_} = $sub; |
20 | $sub = \&nil; |
a6006777 |
21 | } |
22 | #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n"; |
23 | *{$package . "::(" . $_} = \&{ $sub }; |
24 | } |
4633a7c4 |
25 | } |
a6006777 |
26 | ${$package . "::()"} = $fb; # Make it findable too (fallback only). |
4633a7c4 |
27 | } |
28 | |
29 | sub import { |
30 | $package = (caller())[0]; |
31 | # *{$package . "::OVERLOAD"} = \&OVERLOAD; |
32 | shift; |
33 | $package->overload::OVERLOAD(@_); |
34 | } |
35 | |
36 | sub unimport { |
37 | $package = (caller())[0]; |
a6006777 |
38 | ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table |
4633a7c4 |
39 | shift; |
40 | for (@_) { |
a6006777 |
41 | if ($_ eq 'fallback') { |
42 | undef $ {$package . "::()"}; |
43 | } else { |
44 | delete $ {$package . "::"}{"(" . $_}; |
45 | } |
4633a7c4 |
46 | } |
47 | } |
48 | |
49 | sub Overloaded { |
a6006777 |
50 | my $package = shift; |
51 | $package = ref $package if ref $package; |
52 | $package->can('()'); |
4633a7c4 |
53 | } |
54 | |
44a8e56a |
55 | sub ov_method { |
56 | my $globref = shift; |
57 | return undef unless $globref; |
58 | my $sub = \&{*$globref}; |
59 | return $sub if $sub ne \&nil; |
60 | return shift->can($ {*$globref}); |
61 | } |
62 | |
4633a7c4 |
63 | sub OverloadedStringify { |
a6006777 |
64 | my $package = shift; |
65 | $package = ref $package if ref $package; |
44a8e56a |
66 | #$package->can('(""') |
ee239bfe |
67 | ov_method mycan($package, '(""'), $package |
68 | or ov_method mycan($package, '(0+'), $package |
69 | or ov_method mycan($package, '(bool'), $package |
70 | or ov_method mycan($package, '(nomethod'), $package; |
4633a7c4 |
71 | } |
72 | |
73 | sub Method { |
a6006777 |
74 | my $package = shift; |
75 | $package = ref $package if ref $package; |
44a8e56a |
76 | #my $meth = $package->can('(' . shift); |
77 | ov_method mycan($package, '(' . shift), $package; |
78 | #return $meth if $meth ne \&nil; |
79 | #return $ {*{$meth}}; |
4633a7c4 |
80 | } |
81 | |
82 | sub AddrRef { |
a6006777 |
83 | my $package = ref $_[0]; |
84 | return "$_[0]" unless $package; |
85 | bless $_[0], overload::Fake; # Non-overloaded package |
4633a7c4 |
86 | my $str = "$_[0]"; |
87 | bless $_[0], $package; # Back |
a6006777 |
88 | $package . substr $str, index $str, '='; |
4633a7c4 |
89 | } |
90 | |
91 | sub StrVal { |
f6b3007c |
92 | (OverloadedStringify($_[0]) or ref($_[0]) eq 'Regexp') ? |
a6006777 |
93 | (AddrRef(shift)) : |
4633a7c4 |
94 | "$_[0]"; |
95 | } |
96 | |
44a8e56a |
97 | sub mycan { # Real can would leave stubs. |
98 | my ($package, $meth) = @_; |
99 | return \*{$package . "::$meth"} if defined &{$package . "::$meth"}; |
100 | my $p; |
101 | foreach $p (@{$package . "::ISA"}) { |
102 | my $out = mycan($p, $meth); |
103 | return $out if $out; |
104 | } |
105 | return undef; |
106 | } |
107 | |
b3ac6de7 |
108 | %constants = ( |
109 | 'integer' => 0x1000, |
110 | 'float' => 0x2000, |
111 | 'binary' => 0x4000, |
112 | 'q' => 0x8000, |
113 | 'qr' => 0x10000, |
114 | ); |
115 | |
ee239bfe |
116 | %ops = ( with_assign => "+ - * / % ** << >> x .", |
117 | assign => "+= -= *= /= %= **= <<= >>= x= .=", |
2877bd81 |
118 | num_comparison => "< <= > >= == !=", |
ee239bfe |
119 | '3way_comparison'=> "<=> cmp", |
2877bd81 |
120 | str_comparison => "lt le gt ge eq ne", |
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121 | binary => "& | ^", |
122 | unary => "neg ! ~", |
123 | mutators => '++ --', |
124 | func => "atan2 cos sin exp abs log sqrt", |
125 | conversion => 'bool "" 0+', |
f5284f61 |
126 | iterators => '<>', |
127 | dereferencing => '${} @{} %{} &{} *{}', |
ee239bfe |
128 | special => 'nomethod fallback ='); |
129 | |
b3ac6de7 |
130 | sub constant { |
131 | # Arguments: what, sub |
132 | while (@_) { |
133 | $^H{$_[0]} = $_[1]; |
d5448623 |
134 | $^H |= $constants{$_[0]} | $overload::hint_bits; |
b3ac6de7 |
135 | shift, shift; |
136 | } |
137 | } |
138 | |
139 | sub remove_constant { |
140 | # Arguments: what, sub |
141 | while (@_) { |
142 | delete $^H{$_[0]}; |
143 | $^H &= ~ $constants{$_[0]}; |
144 | shift, shift; |
145 | } |
146 | } |
147 | |
4633a7c4 |
148 | 1; |
149 | |
150 | __END__ |
151 | |
152 | =head1 NAME |
153 | |
cb1a09d0 |
154 | overload - Package for overloading perl operations |
4633a7c4 |
155 | |
156 | =head1 SYNOPSIS |
157 | |
158 | package SomeThing; |
159 | |
160 | use overload |
161 | '+' => \&myadd, |
162 | '-' => \&mysub; |
163 | # etc |
164 | ... |
165 | |
166 | package main; |
167 | $a = new SomeThing 57; |
168 | $b=5+$a; |
169 | ... |
170 | if (overload::Overloaded $b) {...} |
171 | ... |
172 | $strval = overload::StrVal $b; |
173 | |
4633a7c4 |
174 | =head1 DESCRIPTION |
175 | |
176 | =head2 Declaration of overloaded functions |
177 | |
178 | The compilation directive |
179 | |
180 | package Number; |
181 | use overload |
182 | "+" => \&add, |
183 | "*=" => "muas"; |
184 | |
185 | declares function Number::add() for addition, and method muas() in |
186 | the "class" C<Number> (or one of its base classes) |
187 | for the assignment form C<*=> of multiplication. |
188 | |
189 | Arguments of this directive come in (key, value) pairs. Legal values |
e7ea3e70 |
190 | are values legal inside a C<&{ ... }> call, so the name of a |
191 | subroutine, a reference to a subroutine, or an anonymous subroutine |
192 | will all work. Note that values specified as strings are |
193 | interpreted as methods, not subroutines. Legal keys are listed below. |
4633a7c4 |
194 | |
195 | The subroutine C<add> will be called to execute C<$a+$b> if $a |
196 | is a reference to an object blessed into the package C<Number>, or if $a is |
197 | not an object from a package with defined mathemagic addition, but $b is a |
198 | reference to a C<Number>. It can also be called in other situations, like |
199 | C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical |
200 | methods refer to methods triggered by an overloaded mathematical |
201 | operator.) |
202 | |
774d564b |
203 | Since overloading respects inheritance via the @ISA hierarchy, the |
204 | above declaration would also trigger overloading of C<+> and C<*=> in |
205 | all the packages which inherit from C<Number>. |
e7ea3e70 |
206 | |
4633a7c4 |
207 | =head2 Calling Conventions for Binary Operations |
208 | |
209 | The functions specified in the C<use overload ...> directive are called |
210 | with three (in one particular case with four, see L<Last Resort>) |
211 | arguments. If the corresponding operation is binary, then the first |
212 | two arguments are the two arguments of the operation. However, due to |
213 | general object calling conventions, the first argument should always be |
214 | an object in the package, so in the situation of C<7+$a>, the |
215 | order of the arguments is interchanged. It probably does not matter |
216 | when implementing the addition method, but whether the arguments |
217 | are reversed is vital to the subtraction method. The method can |
218 | query this information by examining the third argument, which can take |
219 | three different values: |
220 | |
221 | =over 7 |
222 | |
223 | =item FALSE |
224 | |
225 | the order of arguments is as in the current operation. |
226 | |
227 | =item TRUE |
228 | |
229 | the arguments are reversed. |
230 | |
231 | =item C<undef> |
232 | |
233 | the current operation is an assignment variant (as in |
234 | C<$a+=7>), but the usual function is called instead. This additional |
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235 | information can be used to generate some optimizations. Compare |
236 | L<Calling Conventions for Mutators>. |
4633a7c4 |
237 | |
238 | =back |
239 | |
240 | =head2 Calling Conventions for Unary Operations |
241 | |
242 | Unary operation are considered binary operations with the second |
243 | argument being C<undef>. Thus the functions that overloads C<{"++"}> |
244 | is called with arguments C<($a,undef,'')> when $a++ is executed. |
245 | |
ee239bfe |
246 | =head2 Calling Conventions for Mutators |
247 | |
248 | Two types of mutators have different calling conventions: |
249 | |
250 | =over |
251 | |
252 | =item C<++> and C<--> |
253 | |
254 | The routines which implement these operators are expected to actually |
255 | I<mutate> their arguments. So, assuming that $obj is a reference to a |
256 | number, |
257 | |
258 | sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n} |
259 | |
260 | is an appropriate implementation of overloaded C<++>. Note that |
261 | |
262 | sub incr { ++$ {$_[0]} ; shift } |
263 | |
264 | is OK if used with preincrement and with postincrement. (In the case |
265 | of postincrement a copying will be performed, see L<Copy Constructor>.) |
266 | |
267 | =item C<x=> and other assignment versions |
268 | |
269 | There is nothing special about these methods. They may change the |
270 | value of their arguments, and may leave it as is. The result is going |
271 | to be assigned to the value in the left-hand-side if different from |
272 | this value. |
273 | |
f610777f |
274 | This allows for the same method to be used as overloaded C<+=> and |
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275 | C<+>. Note that this is I<allowed>, but not recommended, since by the |
276 | semantic of L<"Fallback"> Perl will call the method for C<+> anyway, |
277 | if C<+=> is not overloaded. |
278 | |
279 | =back |
280 | |
281 | B<Warning.> Due to the presense of assignment versions of operations, |
282 | routines which may be called in assignment context may create |
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283 | self-referential structures. Currently Perl will not free self-referential |
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284 | structures until cycles are C<explicitly> broken. You may get problems |
285 | when traversing your structures too. |
286 | |
287 | Say, |
288 | |
289 | use overload '+' => sub { bless [ \$_[0], \$_[1] ] }; |
290 | |
291 | is asking for trouble, since for code C<$obj += $foo> the subroutine |
292 | is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj, |
293 | \$foo]>. If using such a subroutine is an important optimization, one |
294 | can overload C<+=> explicitly by a non-"optimized" version, or switch |
295 | to non-optimized version if C<not defined $_[2]> (see |
296 | L<Calling Conventions for Binary Operations>). |
297 | |
298 | Even if no I<explicit> assignment-variants of operators are present in |
299 | the script, they may be generated by the optimizer. Say, C<",$obj,"> or |
300 | C<',' . $obj . ','> may be both optimized to |
301 | |
302 | my $tmp = ',' . $obj; $tmp .= ','; |
303 | |
4633a7c4 |
304 | =head2 Overloadable Operations |
305 | |
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306 | The following symbols can be specified in C<use overload> directive: |
4633a7c4 |
307 | |
308 | =over 5 |
309 | |
310 | =item * I<Arithmetic operations> |
311 | |
312 | "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=", |
313 | "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=", |
314 | |
315 | For these operations a substituted non-assignment variant can be called if |
316 | the assignment variant is not available. Methods for operations "C<+>", |
317 | "C<->", "C<+=>", and "C<-=>" can be called to automatically generate |
318 | increment and decrement methods. The operation "C<->" can be used to |
319 | autogenerate missing methods for unary minus or C<abs>. |
320 | |
ee239bfe |
321 | See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and |
322 | L<"Calling Conventions for Binary Operations">) for details of these |
323 | substitutions. |
324 | |
4633a7c4 |
325 | =item * I<Comparison operations> |
326 | |
327 | "<", "<=", ">", ">=", "==", "!=", "<=>", |
328 | "lt", "le", "gt", "ge", "eq", "ne", "cmp", |
329 | |
330 | If the corresponding "spaceship" variant is available, it can be |
331 | used to substitute for the missing operation. During C<sort>ing |
332 | arrays, C<cmp> is used to compare values subject to C<use overload>. |
333 | |
334 | =item * I<Bit operations> |
335 | |
336 | "&", "^", "|", "neg", "!", "~", |
337 | |
338 | "C<neg>" stands for unary minus. If the method for C<neg> is not |
3bc6ec80 |
339 | specified, it can be autogenerated using the method for |
340 | subtraction. If the method for "C<!>" is not specified, it can be |
341 | autogenerated using the methods for "C<bool>", or "C<\"\">", or "C<0+>". |
4633a7c4 |
342 | |
343 | =item * I<Increment and decrement> |
344 | |
345 | "++", "--", |
346 | |
347 | If undefined, addition and subtraction methods can be |
348 | used instead. These operations are called both in prefix and |
349 | postfix form. |
350 | |
351 | =item * I<Transcendental functions> |
352 | |
353 | "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", |
354 | |
355 | If C<abs> is unavailable, it can be autogenerated using methods |
1fef88e7 |
356 | for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction. |
4633a7c4 |
357 | |
358 | =item * I<Boolean, string and numeric conversion> |
359 | |
360 | "bool", "\"\"", "0+", |
361 | |
f5284f61 |
362 | If one or two of these operations are not overloaded, the remaining ones can |
4633a7c4 |
363 | be used instead. C<bool> is used in the flow control operators |
364 | (like C<while>) and for the ternary "C<?:>" operation. These functions can |
365 | return any arbitrary Perl value. If the corresponding operation for this value |
366 | is overloaded too, that operation will be called again with this value. |
367 | |
f5284f61 |
368 | =item * I<Iteration> |
369 | |
370 | "<>" |
371 | |
372 | If not overloaded, the argument will be converted to a filehandle or |
373 | glob (which may require a stringification). The same overloading |
374 | happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and |
375 | I<globbing> syntax C<E<lt>${var}E<gt>>. |
376 | |
377 | =item * I<Dereferencing> |
378 | |
379 | '${}', '@{}', '%{}', '&{}', '*{}'. |
380 | |
381 | If not overloaded, the argument will be dereferenced I<as is>, thus |
382 | should be of correct type. These functions should return a reference |
383 | of correct type, or another object with overloaded dereferencing. |
384 | |
4633a7c4 |
385 | =item * I<Special> |
386 | |
387 | "nomethod", "fallback", "=", |
388 | |
389 | see L<SPECIAL SYMBOLS FOR C<use overload>>. |
390 | |
391 | =back |
392 | |
ee239bfe |
393 | See L<"Fallback"> for an explanation of when a missing method can be |
394 | autogenerated. |
395 | |
396 | A computer-readable form of the above table is available in the hash |
397 | %overload::ops, with values being space-separated lists of names: |
398 | |
399 | with_assign => '+ - * / % ** << >> x .', |
400 | assign => '+= -= *= /= %= **= <<= >>= x= .=', |
2877bd81 |
401 | num_comparison => '< <= > >= == !=', |
ee239bfe |
402 | '3way_comparison'=> '<=> cmp', |
2877bd81 |
403 | str_comparison => 'lt le gt ge eq ne', |
ee239bfe |
404 | binary => '& | ^', |
405 | unary => 'neg ! ~', |
406 | mutators => '++ --', |
407 | func => 'atan2 cos sin exp abs log sqrt', |
408 | conversion => 'bool "" 0+', |
f5284f61 |
409 | iterators => '<>', |
410 | dereferencing => '${} @{} %{} &{} *{}', |
ee239bfe |
411 | special => 'nomethod fallback =' |
4633a7c4 |
412 | |
e7ea3e70 |
413 | =head2 Inheritance and overloading |
414 | |
774d564b |
415 | Inheritance interacts with overloading in two ways. |
e7ea3e70 |
416 | |
417 | =over |
418 | |
419 | =item Strings as values of C<use overload> directive |
420 | |
774d564b |
421 | If C<value> in |
e7ea3e70 |
422 | |
423 | use overload key => value; |
424 | |
774d564b |
425 | is a string, it is interpreted as a method name. |
e7ea3e70 |
426 | |
427 | =item Overloading of an operation is inherited by derived classes |
428 | |
774d564b |
429 | Any class derived from an overloaded class is also overloaded. The |
430 | set of overloaded methods is the union of overloaded methods of all |
431 | the ancestors. If some method is overloaded in several ancestor, then |
e7ea3e70 |
432 | which description will be used is decided by the usual inheritance |
774d564b |
433 | rules: |
e7ea3e70 |
434 | |
774d564b |
435 | If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads |
436 | C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">, |
437 | then the subroutine C<D::plus_sub> will be called to implement |
438 | operation C<+> for an object in package C<A>. |
e7ea3e70 |
439 | |
440 | =back |
441 | |
774d564b |
442 | Note that since the value of the C<fallback> key is not a subroutine, |
443 | its inheritance is not governed by the above rules. In the current |
444 | implementation, the value of C<fallback> in the first overloaded |
445 | ancestor is used, but this is accidental and subject to change. |
e7ea3e70 |
446 | |
4633a7c4 |
447 | =head1 SPECIAL SYMBOLS FOR C<use overload> |
448 | |
449 | Three keys are recognized by Perl that are not covered by the above |
450 | description. |
451 | |
774d564b |
452 | =head2 Last Resort |
4633a7c4 |
453 | |
454 | C<"nomethod"> should be followed by a reference to a function of four |
455 | parameters. If defined, it is called when the overloading mechanism |
456 | cannot find a method for some operation. The first three arguments of |
457 | this function coincide with the arguments for the corresponding method if |
458 | it were found, the fourth argument is the symbol |
459 | corresponding to the missing method. If several methods are tried, |
460 | the last one is used. Say, C<1-$a> can be equivalent to |
461 | |
462 | &nomethodMethod($a,1,1,"-") |
463 | |
464 | if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the |
465 | C<use overload> directive. |
466 | |
467 | If some operation cannot be resolved, and there is no function |
468 | assigned to C<"nomethod">, then an exception will be raised via die()-- |
469 | unless C<"fallback"> was specified as a key in C<use overload> directive. |
470 | |
471 | =head2 Fallback |
472 | |
473 | The key C<"fallback"> governs what to do if a method for a particular |
474 | operation is not found. Three different cases are possible depending on |
475 | the value of C<"fallback">: |
476 | |
477 | =over 16 |
478 | |
479 | =item * C<undef> |
480 | |
481 | Perl tries to use a |
482 | substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it |
483 | then tries to calls C<"nomethod"> value; if missing, an exception |
484 | will be raised. |
485 | |
486 | =item * TRUE |
487 | |
488 | The same as for the C<undef> value, but no exception is raised. Instead, |
489 | it silently reverts to what it would have done were there no C<use overload> |
490 | present. |
491 | |
492 | =item * defined, but FALSE |
493 | |
494 | No autogeneration is tried. Perl tries to call |
495 | C<"nomethod"> value, and if this is missing, raises an exception. |
496 | |
497 | =back |
498 | |
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499 | B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone |
500 | yet, see L<"Inheritance and overloading">. |
501 | |
4633a7c4 |
502 | =head2 Copy Constructor |
503 | |
504 | The value for C<"="> is a reference to a function with three |
505 | arguments, i.e., it looks like the other values in C<use |
506 | overload>. However, it does not overload the Perl assignment |
507 | operator. This would go against Camel hair. |
508 | |
509 | This operation is called in the situations when a mutator is applied |
510 | to a reference that shares its object with some other reference, such |
511 | as |
512 | |
513 | $a=$b; |
ee239bfe |
514 | ++$a; |
4633a7c4 |
515 | |
516 | To make this change $a and not change $b, a copy of C<$$a> is made, |
517 | and $a is assigned a reference to this new object. This operation is |
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518 | done during execution of the C<++$a>, and not during the assignment, |
4633a7c4 |
519 | (so before the increment C<$$a> coincides with C<$$b>). This is only |
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520 | done if C<++> is expressed via a method for C<'++'> or C<'+='> (or |
521 | C<nomethod>). Note that if this operation is expressed via C<'+'> |
522 | a nonmutator, i.e., as in |
4633a7c4 |
523 | |
524 | $a=$b; |
525 | $a=$a+1; |
526 | |
527 | then C<$a> does not reference a new copy of C<$$a>, since $$a does not |
528 | appear as lvalue when the above code is executed. |
529 | |
530 | If the copy constructor is required during the execution of some mutator, |
531 | but a method for C<'='> was not specified, it can be autogenerated as a |
532 | string copy if the object is a plain scalar. |
533 | |
534 | =over 5 |
535 | |
536 | =item B<Example> |
537 | |
538 | The actually executed code for |
539 | |
540 | $a=$b; |
541 | Something else which does not modify $a or $b.... |
542 | ++$a; |
543 | |
544 | may be |
545 | |
546 | $a=$b; |
547 | Something else which does not modify $a or $b.... |
548 | $a = $a->clone(undef,""); |
549 | $a->incr(undef,""); |
550 | |
551 | if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>, |
552 | C<'='> was overloaded with C<\&clone>. |
553 | |
554 | =back |
555 | |
f610777f |
556 | Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for |
ee239bfe |
557 | C<$b = $a; ++$a>. |
558 | |
4633a7c4 |
559 | =head1 MAGIC AUTOGENERATION |
560 | |
561 | If a method for an operation is not found, and the value for C<"fallback"> is |
562 | TRUE or undefined, Perl tries to autogenerate a substitute method for |
563 | the missing operation based on the defined operations. Autogenerated method |
564 | substitutions are possible for the following operations: |
565 | |
566 | =over 16 |
567 | |
568 | =item I<Assignment forms of arithmetic operations> |
569 | |
570 | C<$a+=$b> can use the method for C<"+"> if the method for C<"+="> |
571 | is not defined. |
572 | |
573 | =item I<Conversion operations> |
574 | |
575 | String, numeric, and boolean conversion are calculated in terms of one |
576 | another if not all of them are defined. |
577 | |
578 | =item I<Increment and decrement> |
579 | |
580 | The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>, |
581 | and C<$a--> in terms of C<$a-=1> and C<$a-1>. |
582 | |
583 | =item C<abs($a)> |
584 | |
585 | can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>). |
586 | |
587 | =item I<Unary minus> |
588 | |
589 | can be expressed in terms of subtraction. |
590 | |
3bc6ec80 |
591 | =item I<Negation> |
592 | |
593 | C<!> and C<not> can be expressed in terms of boolean conversion, or |
594 | string or numerical conversion. |
595 | |
4633a7c4 |
596 | =item I<Concatenation> |
597 | |
598 | can be expressed in terms of string conversion. |
599 | |
600 | =item I<Comparison operations> |
601 | |
602 | can be expressed in terms of its "spaceship" counterpart: either |
603 | C<E<lt>=E<gt>> or C<cmp>: |
1fef88e7 |
604 | |
4633a7c4 |
605 | <, >, <=, >=, ==, != in terms of <=> |
606 | lt, gt, le, ge, eq, ne in terms of cmp |
607 | |
f5284f61 |
608 | =item I<Iterator> |
609 | |
610 | <> in terms of builtin operations |
611 | |
612 | =item I<Dereferencing> |
613 | |
614 | ${} @{} %{} &{} *{} in terms of builtin operations |
615 | |
4633a7c4 |
616 | =item I<Copy operator> |
617 | |
618 | can be expressed in terms of an assignment to the dereferenced value, if this |
619 | value is a scalar and not a reference. |
620 | |
621 | =back |
622 | |
ee239bfe |
623 | =head1 Losing overloading |
4633a7c4 |
624 | |
625 | The restriction for the comparison operation is that even if, for example, |
626 | `C<cmp>' should return a blessed reference, the autogenerated `C<lt>' |
627 | function will produce only a standard logical value based on the |
628 | numerical value of the result of `C<cmp>'. In particular, a working |
629 | numeric conversion is needed in this case (possibly expressed in terms of |
630 | other conversions). |
631 | |
632 | Similarly, C<.=> and C<x=> operators lose their mathemagical properties |
633 | if the string conversion substitution is applied. |
634 | |
635 | When you chop() a mathemagical object it is promoted to a string and its |
636 | mathemagical properties are lost. The same can happen with other |
637 | operations as well. |
638 | |
639 | =head1 Run-time Overloading |
640 | |
641 | Since all C<use> directives are executed at compile-time, the only way to |
642 | change overloading during run-time is to |
643 | |
644 | eval 'use overload "+" => \&addmethod'; |
645 | |
646 | You can also use |
647 | |
648 | eval 'no overload "+", "--", "<="'; |
649 | |
650 | though the use of these constructs during run-time is questionable. |
651 | |
652 | =head1 Public functions |
653 | |
654 | Package C<overload.pm> provides the following public functions: |
655 | |
656 | =over 5 |
657 | |
658 | =item overload::StrVal(arg) |
659 | |
660 | Gives string value of C<arg> as in absence of stringify overloading. |
661 | |
662 | =item overload::Overloaded(arg) |
663 | |
664 | Returns true if C<arg> is subject to overloading of some operations. |
665 | |
666 | =item overload::Method(obj,op) |
667 | |
668 | Returns C<undef> or a reference to the method that implements C<op>. |
669 | |
670 | =back |
671 | |
b3ac6de7 |
672 | =head1 Overloading constants |
673 | |
674 | For some application Perl parser mangles constants too much. It is possible |
675 | to hook into this process via overload::constant() and overload::remove_constant() |
676 | functions. |
677 | |
678 | These functions take a hash as an argument. The recognized keys of this hash |
679 | are |
680 | |
681 | =over 8 |
682 | |
683 | =item integer |
684 | |
685 | to overload integer constants, |
686 | |
687 | =item float |
688 | |
689 | to overload floating point constants, |
690 | |
691 | =item binary |
692 | |
693 | to overload octal and hexadecimal constants, |
694 | |
695 | =item q |
696 | |
697 | to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted |
698 | strings and here-documents, |
699 | |
700 | =item qr |
701 | |
702 | to overload constant pieces of regular expressions. |
703 | |
704 | =back |
705 | |
706 | The corresponding values are references to functions which take three arguments: |
707 | the first one is the I<initial> string form of the constant, the second one |
708 | is how Perl interprets this constant, the third one is how the constant is used. |
709 | Note that the initial string form does not |
710 | contain string delimiters, and has backslashes in backslash-delimiter |
711 | combinations stripped (thus the value of delimiter is not relevant for |
712 | processing of this string). The return value of this function is how this |
713 | constant is going to be interpreted by Perl. The third argument is undefined |
714 | unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote |
715 | context (comes from strings, regular expressions, and single-quote HERE |
716 | documents), it is C<tr> for arguments of C<tr>/C<y> operators, |
717 | it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise. |
718 | |
719 | Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>, |
720 | it is expected that overloaded constant strings are equipped with reasonable |
721 | overloaded catenation operator, otherwise absurd results will result. |
722 | Similarly, negative numbers are considered as negations of positive constants. |
723 | |
724 | Note that it is probably meaningless to call the functions overload::constant() |
725 | and overload::remove_constant() from anywhere but import() and unimport() methods. |
726 | From these methods they may be called as |
727 | |
728 | sub import { |
729 | shift; |
730 | return unless @_; |
731 | die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant'; |
732 | overload::constant integer => sub {Math::BigInt->new(shift)}; |
733 | } |
734 | |
735 | B<BUGS> Currently overloaded-ness of constants does not propagate |
736 | into C<eval '...'>. |
737 | |
4633a7c4 |
738 | =head1 IMPLEMENTATION |
739 | |
740 | What follows is subject to change RSN. |
741 | |
e7ea3e70 |
742 | The table of methods for all operations is cached in magic for the |
743 | symbol table hash for the package. The cache is invalidated during |
744 | processing of C<use overload>, C<no overload>, new function |
745 | definitions, and changes in @ISA. However, this invalidation remains |
746 | unprocessed until the next C<bless>ing into the package. Hence if you |
747 | want to change overloading structure dynamically, you'll need an |
748 | additional (fake) C<bless>ing to update the table. |
749 | |
750 | (Every SVish thing has a magic queue, and magic is an entry in that |
751 | queue. This is how a single variable may participate in multiple |
752 | forms of magic simultaneously. For instance, environment variables |
753 | regularly have two forms at once: their %ENV magic and their taint |
754 | magic. However, the magic which implements overloading is applied to |
755 | the stashes, which are rarely used directly, thus should not slow down |
756 | Perl.) |
4633a7c4 |
757 | |
758 | If an object belongs to a package using overload, it carries a special |
759 | flag. Thus the only speed penalty during arithmetic operations without |
760 | overloading is the checking of this flag. |
761 | |
774d564b |
762 | In fact, if C<use overload> is not present, there is almost no overhead |
763 | for overloadable operations, so most programs should not suffer |
764 | measurable performance penalties. A considerable effort was made to |
765 | minimize the overhead when overload is used in some package, but the |
766 | arguments in question do not belong to packages using overload. When |
767 | in doubt, test your speed with C<use overload> and without it. So far |
768 | there have been no reports of substantial speed degradation if Perl is |
769 | compiled with optimization turned on. |
4633a7c4 |
770 | |
e7ea3e70 |
771 | There is no size penalty for data if overload is not used. The only |
772 | size penalty if overload is used in some package is that I<all> the |
773 | packages acquire a magic during the next C<bless>ing into the |
774 | package. This magic is three-words-long for packages without |
f610777f |
775 | overloading, and carries the cache table if the package is overloaded. |
4633a7c4 |
776 | |
777 | Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is |
778 | carried out before any operation that can imply an assignment to the |
779 | object $a (or $b) refers to, like C<$a++>. You can override this |
780 | behavior by defining your own copy constructor (see L<"Copy Constructor">). |
781 | |
782 | It is expected that arguments to methods that are not explicitly supposed |
783 | to be changed are constant (but this is not enforced). |
784 | |
ee239bfe |
785 | =head1 Metaphor clash |
786 | |
f610777f |
787 | One may wonder why the semantic of overloaded C<=> is so counter intuitive. |
788 | If it I<looks> counter intuitive to you, you are subject to a metaphor |
ee239bfe |
789 | clash. |
790 | |
791 | Here is a Perl object metaphor: |
792 | |
793 | I< object is a reference to blessed data> |
794 | |
795 | and an arithmetic metaphor: |
796 | |
797 | I< object is a thing by itself>. |
798 | |
799 | The I<main> problem of overloading C<=> is the fact that these metaphors |
800 | imply different actions on the assignment C<$a = $b> if $a and $b are |
801 | objects. Perl-think implies that $a becomes a reference to whatever |
802 | $b was referencing. Arithmetic-think implies that the value of "object" |
803 | $a is changed to become the value of the object $b, preserving the fact |
804 | that $a and $b are separate entities. |
805 | |
806 | The difference is not relevant in the absence of mutators. After |
807 | a Perl-way assignment an operation which mutates the data referenced by $a |
808 | would change the data referenced by $b too. Effectively, after |
809 | C<$a = $b> values of $a and $b become I<indistinguishable>. |
810 | |
811 | On the other hand, anyone who has used algebraic notation knows the |
812 | expressive power of the arithmetic metaphor. Overloading works hard |
813 | to enable this metaphor while preserving the Perlian way as far as |
814 | possible. Since it is not not possible to freely mix two contradicting |
815 | metaphors, overloading allows the arithmetic way to write things I<as |
816 | far as all the mutators are called via overloaded access only>. The |
817 | way it is done is described in L<Copy Constructor>. |
818 | |
819 | If some mutator methods are directly applied to the overloaded values, |
820 | one may need to I<explicitly unlink> other values which references the |
821 | same value: |
822 | |
823 | $a = new Data 23; |
824 | ... |
825 | $b = $a; # $b is "linked" to $a |
826 | ... |
827 | $a = $a->clone; # Unlink $b from $a |
828 | $a->increment_by(4); |
829 | |
830 | Note that overloaded access makes this transparent: |
831 | |
832 | $a = new Data 23; |
833 | $b = $a; # $b is "linked" to $a |
834 | $a += 4; # would unlink $b automagically |
835 | |
836 | However, it would not make |
837 | |
838 | $a = new Data 23; |
839 | $a = 4; # Now $a is a plain 4, not 'Data' |
840 | |
841 | preserve "objectness" of $a. But Perl I<has> a way to make assignments |
842 | to an object do whatever you want. It is just not the overload, but |
843 | tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method |
844 | which returns the object itself, and STORE() method which changes the |
845 | value of the object, one can reproduce the arithmetic metaphor in its |
846 | completeness, at least for variables which were tie()d from the start. |
847 | |
848 | (Note that a workaround for a bug may be needed, see L<"BUGS">.) |
849 | |
850 | =head1 Cookbook |
851 | |
852 | Please add examples to what follows! |
853 | |
854 | =head2 Two-face scalars |
855 | |
856 | Put this in F<two_face.pm> in your Perl library directory: |
857 | |
858 | package two_face; # Scalars with separate string and |
859 | # numeric values. |
860 | sub new { my $p = shift; bless [@_], $p } |
861 | use overload '""' => \&str, '0+' => \&num, fallback => 1; |
862 | sub num {shift->[1]} |
863 | sub str {shift->[0]} |
864 | |
865 | Use it as follows: |
866 | |
867 | require two_face; |
868 | my $seven = new two_face ("vii", 7); |
869 | printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1; |
870 | print "seven contains `i'\n" if $seven =~ /i/; |
871 | |
872 | (The second line creates a scalar which has both a string value, and a |
873 | numeric value.) This prints: |
874 | |
875 | seven=vii, seven=7, eight=8 |
876 | seven contains `i' |
877 | |
f5284f61 |
878 | =head2 Two-face references |
879 | |
880 | Suppose you want to create an object which is accessible as both an |
881 | array reference, and a hash reference, similar to the builtin |
882 | L<array-accessible-as-a-hash|perlref/"Pseudo-hashes: Using an array as |
883 | a hash"> builtin Perl type. Let us make it better than the builtin |
884 | type, there will be no restriction that you cannot use the index 0 of |
885 | your array. |
886 | |
887 | package two_refs; |
888 | use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} }; |
889 | sub new { |
890 | my $p = shift; |
891 | bless \ [@_], $p; |
892 | } |
893 | sub gethash { |
894 | my %h; |
895 | my $self = shift; |
896 | tie %h, ref $self, $self; |
897 | \%h; |
898 | } |
899 | |
900 | sub TIEHASH { my $p = shift; bless \ shift, $p } |
901 | my %fields; |
902 | my $i = 0; |
903 | $fields{$_} = $i++ foreach qw{zero one two three}; |
904 | sub STORE { |
905 | my $self = ${shift()}; |
906 | my $key = $fields{shift()}; |
907 | defined $key or die "Out of band access"; |
908 | $$self->[$key] = shift; |
909 | } |
910 | sub FETCH { |
911 | my $self = ${shift()}; |
912 | my $key = $fields{shift()}; |
913 | defined $key or die "Out of band access"; |
914 | $$self->[$key]; |
915 | } |
916 | |
917 | Now one can access an object using both the array and hash syntax: |
918 | |
919 | my $bar = new two_refs 3,4,5,6; |
920 | $bar->[2] = 11; |
921 | $bar->{two} == 11 or die 'bad hash fetch'; |
922 | |
923 | Note several important features of this example. First of all, the |
924 | I<actual> type of $bar is a scalar reference, and we do not overload |
925 | the scalar dereference. Thus we can get the I<actual> non-overloaded |
926 | contents of $bar by just using C<$$bar> (what we do in functions which |
927 | overload dereference). Similarly, the object returned by the |
928 | TIEHASH() method is a scalar reference. |
929 | |
930 | Second, we create a new tied hash each time the hash syntax is used. |
931 | This allows us not to worry about a possibility of a reference loop, |
932 | would would lead to a memory leak. |
933 | |
934 | Both these problems can be cured. Say, if we want to overload hash |
935 | dereference on a reference to an object which is I<implemented> as a |
936 | hash itself, the only problem one has to circumvent is how to access |
937 | this I<actual> hash (as opposed to the I<virtual> exhibited by |
938 | overloaded dereference operator). Here is one possible fetching routine: |
939 | |
940 | sub access_hash { |
941 | my ($self, $key) = (shift, shift); |
942 | my $class = ref $self; |
943 | bless $self, 'overload::dummy'; # Disable overloading of %{} |
944 | my $out = $self->{$key}; |
945 | bless $self, $class; # Restore overloading |
946 | $out; |
947 | } |
948 | |
949 | To move creation of the tied hash on each access, one may an extra |
950 | level of indirection which allows a non-circular structure of references: |
951 | |
952 | package two_refs1; |
953 | use overload '%{}' => sub { ${shift()}->[1] }, |
954 | '@{}' => sub { ${shift()}->[0] }; |
955 | sub new { |
956 | my $p = shift; |
957 | my $a = [@_]; |
958 | my %h; |
959 | tie %h, $p, $a; |
960 | bless \ [$a, \%h], $p; |
961 | } |
962 | sub gethash { |
963 | my %h; |
964 | my $self = shift; |
965 | tie %h, ref $self, $self; |
966 | \%h; |
967 | } |
968 | |
969 | sub TIEHASH { my $p = shift; bless \ shift, $p } |
970 | my %fields; |
971 | my $i = 0; |
972 | $fields{$_} = $i++ foreach qw{zero one two three}; |
973 | sub STORE { |
974 | my $a = ${shift()}; |
975 | my $key = $fields{shift()}; |
976 | defined $key or die "Out of band access"; |
977 | $a->[$key] = shift; |
978 | } |
979 | sub FETCH { |
980 | my $a = ${shift()}; |
981 | my $key = $fields{shift()}; |
982 | defined $key or die "Out of band access"; |
983 | $a->[$key]; |
984 | } |
985 | |
986 | Now if $baz is overloaded like this, then C<$bar> is a reference to a |
987 | reference to the intermediate array, which keeps a reference to an |
988 | actual array, and the access hash. The tie()ing object for the access |
989 | hash is also a reference to a reference to the actual array, so |
990 | |
991 | =over |
992 | |
993 | =item * |
994 | |
995 | There are no loops of references. |
996 | |
997 | =item * |
998 | |
999 | Both "objects" which are blessed into the class C<two_refs1> are |
1000 | references to a reference to an array, thus references to a I<scalar>. |
1001 | Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no |
1002 | overloaded operations. |
1003 | |
1004 | =back |
1005 | |
ee239bfe |
1006 | =head2 Symbolic calculator |
1007 | |
1008 | Put this in F<symbolic.pm> in your Perl library directory: |
1009 | |
1010 | package symbolic; # Primitive symbolic calculator |
1011 | use overload nomethod => \&wrap; |
1012 | |
1013 | sub new { shift; bless ['n', @_] } |
1014 | sub wrap { |
1015 | my ($obj, $other, $inv, $meth) = @_; |
1016 | ($obj, $other) = ($other, $obj) if $inv; |
1017 | bless [$meth, $obj, $other]; |
1018 | } |
1019 | |
1020 | This module is very unusual as overloaded modules go: it does not |
1021 | provide any usual overloaded operators, instead it provides the L<Last |
1022 | Resort> operator C<nomethod>. In this example the corresponding |
f610777f |
1023 | subroutine returns an object which encapsulates operations done over |
ee239bfe |
1024 | the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new |
1025 | symbolic 3> contains C<['+', 2, ['n', 3]]>. |
1026 | |
1027 | Here is an example of the script which "calculates" the side of |
1028 | circumscribed octagon using the above package: |
1029 | |
1030 | require symbolic; |
1031 | my $iter = 1; # 2**($iter+2) = 8 |
1032 | my $side = new symbolic 1; |
1033 | my $cnt = $iter; |
3cb6de81 |
1034 | |
ee239bfe |
1035 | while ($cnt--) { |
1036 | $side = (sqrt(1 + $side**2) - 1)/$side; |
1037 | } |
1038 | print "OK\n"; |
1039 | |
1040 | The value of $side is |
1041 | |
1042 | ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]], |
1043 | undef], 1], ['n', 1]] |
1044 | |
1045 | Note that while we obtained this value using a nice little script, |
1046 | there is no simple way to I<use> this value. In fact this value may |
1047 | be inspected in debugger (see L<perldebug>), but ony if |
1048 | C<bareStringify> B<O>ption is set, and not via C<p> command. |
1049 | |
1050 | If one attempts to print this value, then the overloaded operator |
1051 | C<""> will be called, which will call C<nomethod> operator. The |
1052 | result of this operator will be stringified again, but this result is |
1053 | again of type C<symbolic>, which will lead to an infinite loop. |
1054 | |
1055 | Add a pretty-printer method to the module F<symbolic.pm>: |
1056 | |
1057 | sub pretty { |
1058 | my ($meth, $a, $b) = @{+shift}; |
1059 | $a = 'u' unless defined $a; |
1060 | $b = 'u' unless defined $b; |
1061 | $a = $a->pretty if ref $a; |
1062 | $b = $b->pretty if ref $b; |
1063 | "[$meth $a $b]"; |
1064 | } |
1065 | |
1066 | Now one can finish the script by |
1067 | |
1068 | print "side = ", $side->pretty, "\n"; |
1069 | |
1070 | The method C<pretty> is doing object-to-string conversion, so it |
1071 | is natural to overload the operator C<""> using this method. However, |
1072 | inside such a method it is not necessary to pretty-print the |
1073 | I<components> $a and $b of an object. In the above subroutine |
1074 | C<"[$meth $a $b]"> is a catenation of some strings and components $a |
1075 | and $b. If these components use overloading, the catenation operator |
1076 | will look for an overloaded operator C<.>, if not present, it will |
1077 | look for an overloaded operator C<"">. Thus it is enough to use |
1078 | |
1079 | use overload nomethod => \&wrap, '""' => \&str; |
1080 | sub str { |
1081 | my ($meth, $a, $b) = @{+shift}; |
1082 | $a = 'u' unless defined $a; |
1083 | $b = 'u' unless defined $b; |
1084 | "[$meth $a $b]"; |
1085 | } |
1086 | |
1087 | Now one can change the last line of the script to |
1088 | |
1089 | print "side = $side\n"; |
1090 | |
1091 | which outputs |
1092 | |
1093 | side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]] |
1094 | |
1095 | and one can inspect the value in debugger using all the possible |
1096 | methods. |
1097 | |
1098 | Something is is still amiss: consider the loop variable $cnt of the |
1099 | script. It was a number, not an object. We cannot make this value of |
1100 | type C<symbolic>, since then the loop will not terminate. |
1101 | |
1102 | Indeed, to terminate the cycle, the $cnt should become false. |
1103 | However, the operator C<bool> for checking falsity is overloaded (this |
1104 | time via overloaded C<"">), and returns a long string, thus any object |
1105 | of type C<symbolic> is true. To overcome this, we need a way to |
1106 | compare an object to 0. In fact, it is easier to write a numeric |
1107 | conversion routine. |
1108 | |
1109 | Here is the text of F<symbolic.pm> with such a routine added (and |
f610777f |
1110 | slightly modified str()): |
ee239bfe |
1111 | |
1112 | package symbolic; # Primitive symbolic calculator |
1113 | use overload |
1114 | nomethod => \&wrap, '""' => \&str, '0+' => \# |
1115 | |
1116 | sub new { shift; bless ['n', @_] } |
1117 | sub wrap { |
1118 | my ($obj, $other, $inv, $meth) = @_; |
1119 | ($obj, $other) = ($other, $obj) if $inv; |
1120 | bless [$meth, $obj, $other]; |
1121 | } |
1122 | sub str { |
1123 | my ($meth, $a, $b) = @{+shift}; |
1124 | $a = 'u' unless defined $a; |
1125 | if (defined $b) { |
1126 | "[$meth $a $b]"; |
1127 | } else { |
1128 | "[$meth $a]"; |
1129 | } |
1130 | } |
1131 | my %subr = ( n => sub {$_[0]}, |
1132 | sqrt => sub {sqrt $_[0]}, |
1133 | '-' => sub {shift() - shift()}, |
1134 | '+' => sub {shift() + shift()}, |
1135 | '/' => sub {shift() / shift()}, |
1136 | '*' => sub {shift() * shift()}, |
1137 | '**' => sub {shift() ** shift()}, |
1138 | ); |
1139 | sub num { |
1140 | my ($meth, $a, $b) = @{+shift}; |
1141 | my $subr = $subr{$meth} |
1142 | or die "Do not know how to ($meth) in symbolic"; |
1143 | $a = $a->num if ref $a eq __PACKAGE__; |
1144 | $b = $b->num if ref $b eq __PACKAGE__; |
1145 | $subr->($a,$b); |
1146 | } |
1147 | |
1148 | All the work of numeric conversion is done in %subr and num(). Of |
f610777f |
1149 | course, %subr is not complete, it contains only operators used in the |
ee239bfe |
1150 | example below. Here is the extra-credit question: why do we need an |
1151 | explicit recursion in num()? (Answer is at the end of this section.) |
1152 | |
1153 | Use this module like this: |
1154 | |
1155 | require symbolic; |
1156 | my $iter = new symbolic 2; # 16-gon |
1157 | my $side = new symbolic 1; |
1158 | my $cnt = $iter; |
3cb6de81 |
1159 | |
ee239bfe |
1160 | while ($cnt) { |
1161 | $cnt = $cnt - 1; # Mutator `--' not implemented |
1162 | $side = (sqrt(1 + $side**2) - 1)/$side; |
1163 | } |
1164 | printf "%s=%f\n", $side, $side; |
1165 | printf "pi=%f\n", $side*(2**($iter+2)); |
1166 | |
1167 | It prints (without so many line breaks) |
1168 | |
1169 | [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] |
1170 | [n 1]] 2]]] 1] |
1171 | [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912 |
1172 | pi=3.182598 |
1173 | |
1174 | The above module is very primitive. It does not implement |
1175 | mutator methods (C<++>, C<-=> and so on), does not do deep copying |
1176 | (not required without mutators!), and implements only those arithmetic |
1177 | operations which are used in the example. |
1178 | |
f610777f |
1179 | To implement most arithmetic operations is easy, one should just use |
ee239bfe |
1180 | the tables of operations, and change the code which fills %subr to |
1181 | |
1182 | my %subr = ( 'n' => sub {$_[0]} ); |
1183 | foreach my $op (split " ", $overload::ops{with_assign}) { |
1184 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; |
1185 | } |
1186 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); |
1187 | foreach my $op (split " ", "@overload::ops{ @bins }") { |
1188 | $subr{$op} = eval "sub {shift() $op shift()}"; |
1189 | } |
1190 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { |
1191 | print "defining `$op'\n"; |
1192 | $subr{$op} = eval "sub {$op shift()}"; |
1193 | } |
1194 | |
1195 | Due to L<Calling Conventions for Mutators>, we do not need anything |
1196 | special to make C<+=> and friends work, except filling C<+=> entry of |
1197 | %subr, and defining a copy constructor (needed since Perl has no |
1198 | way to know that the implementation of C<'+='> does not mutate |
1199 | the argument, compare L<Copy Constructor>). |
1200 | |
1201 | To implement a copy constructor, add C<'=' => \&cpy> to C<use overload> |
1202 | line, and code (this code assumes that mutators change things one level |
1203 | deep only, so recursive copying is not needed): |
1204 | |
1205 | sub cpy { |
1206 | my $self = shift; |
1207 | bless [@$self], ref $self; |
1208 | } |
1209 | |
1210 | To make C<++> and C<--> work, we need to implement actual mutators, |
1211 | either directly, or in C<nomethod>. We continue to do things inside |
1212 | C<nomethod>, thus add |
1213 | |
1214 | if ($meth eq '++' or $meth eq '--') { |
1215 | @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference |
1216 | return $obj; |
1217 | } |
1218 | |
1219 | after the first line of wrap(). This is not a most effective |
1220 | implementation, one may consider |
1221 | |
1222 | sub inc { $_[0] = bless ['++', shift, 1]; } |
1223 | |
1224 | instead. |
1225 | |
1226 | As a final remark, note that one can fill %subr by |
1227 | |
1228 | my %subr = ( 'n' => sub {$_[0]} ); |
1229 | foreach my $op (split " ", $overload::ops{with_assign}) { |
1230 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; |
1231 | } |
1232 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); |
1233 | foreach my $op (split " ", "@overload::ops{ @bins }") { |
1234 | $subr{$op} = eval "sub {shift() $op shift()}"; |
1235 | } |
1236 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { |
1237 | $subr{$op} = eval "sub {$op shift()}"; |
1238 | } |
1239 | $subr{'++'} = $subr{'+'}; |
1240 | $subr{'--'} = $subr{'-'}; |
1241 | |
1242 | This finishes implementation of a primitive symbolic calculator in |
1243 | 50 lines of Perl code. Since the numeric values of subexpressions |
1244 | are not cached, the calculator is very slow. |
1245 | |
1246 | Here is the answer for the exercise: In the case of str(), we need no |
1247 | explicit recursion since the overloaded C<.>-operator will fall back |
1248 | to an existing overloaded operator C<"">. Overloaded arithmetic |
1249 | operators I<do not> fall back to numeric conversion if C<fallback> is |
1250 | not explicitly requested. Thus without an explicit recursion num() |
1251 | would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild |
1252 | the argument of num(). |
1253 | |
1254 | If you wonder why defaults for conversion are different for str() and |
1255 | num(), note how easy it was to write the symbolic calculator. This |
1256 | simplicity is due to an appropriate choice of defaults. One extra |
f610777f |
1257 | note: due to the explicit recursion num() is more fragile than sym(): |
1258 | we need to explicitly check for the type of $a and $b. If components |
ee239bfe |
1259 | $a and $b happen to be of some related type, this may lead to problems. |
1260 | |
1261 | =head2 I<Really> symbolic calculator |
1262 | |
1263 | One may wonder why we call the above calculator symbolic. The reason |
1264 | is that the actual calculation of the value of expression is postponed |
1265 | until the value is I<used>. |
1266 | |
1267 | To see it in action, add a method |
1268 | |
1269 | sub STORE { |
1270 | my $obj = shift; |
1271 | $#$obj = 1; |
1272 | @$obj->[0,1] = ('=', shift); |
1273 | } |
1274 | |
1275 | to the package C<symbolic>. After this change one can do |
1276 | |
1277 | my $a = new symbolic 3; |
1278 | my $b = new symbolic 4; |
1279 | my $c = sqrt($a**2 + $b**2); |
1280 | |
1281 | and the numeric value of $c becomes 5. However, after calling |
1282 | |
1283 | $a->STORE(12); $b->STORE(5); |
1284 | |
1285 | the numeric value of $c becomes 13. There is no doubt now that the module |
1286 | symbolic provides a I<symbolic> calculator indeed. |
1287 | |
1288 | To hide the rough edges under the hood, provide a tie()d interface to the |
1289 | package C<symbolic> (compare with L<Metaphor clash>). Add methods |
1290 | |
1291 | sub TIESCALAR { my $pack = shift; $pack->new(@_) } |
1292 | sub FETCH { shift } |
1293 | sub nop { } # Around a bug |
1294 | |
1295 | (the bug is described in L<"BUGS">). One can use this new interface as |
1296 | |
1297 | tie $a, 'symbolic', 3; |
1298 | tie $b, 'symbolic', 4; |
1299 | $a->nop; $b->nop; # Around a bug |
1300 | |
1301 | my $c = sqrt($a**2 + $b**2); |
1302 | |
1303 | Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value |
1304 | of $c becomes 13. To insulate the user of the module add a method |
1305 | |
1306 | sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; } |
1307 | |
1308 | Now |
1309 | |
1310 | my ($a, $b); |
1311 | symbolic->vars($a, $b); |
1312 | my $c = sqrt($a**2 + $b**2); |
1313 | |
1314 | $a = 3; $b = 4; |
1315 | printf "c5 %s=%f\n", $c, $c; |
1316 | |
1317 | $a = 12; $b = 5; |
1318 | printf "c13 %s=%f\n", $c, $c; |
1319 | |
1320 | shows that the numeric value of $c follows changes to the values of $a |
1321 | and $b. |
1322 | |
4633a7c4 |
1323 | =head1 AUTHOR |
1324 | |
1fef88e7 |
1325 | Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>. |
4633a7c4 |
1326 | |
1327 | =head1 DIAGNOSTICS |
1328 | |
1329 | When Perl is run with the B<-Do> switch or its equivalent, overloading |
1330 | induces diagnostic messages. |
1331 | |
e7ea3e70 |
1332 | Using the C<m> command of Perl debugger (see L<perldebug>) one can |
1333 | deduce which operations are overloaded (and which ancestor triggers |
1334 | this overloading). Say, if C<eq> is overloaded, then the method C<(eq> |
1335 | is shown by debugger. The method C<()> corresponds to the C<fallback> |
1336 | key (in fact a presence of this method shows that this package has |
1337 | overloading enabled, and it is what is used by the C<Overloaded> |
ee239bfe |
1338 | function of module C<overload>). |
e7ea3e70 |
1339 | |
4633a7c4 |
1340 | =head1 BUGS |
1341 | |
aa689395 |
1342 | Because it is used for overloading, the per-package hash %OVERLOAD now |
1343 | has a special meaning in Perl. The symbol table is filled with names |
1344 | looking like line-noise. |
4633a7c4 |
1345 | |
a6006777 |
1346 | For the purpose of inheritance every overloaded package behaves as if |
1347 | C<fallback> is present (possibly undefined). This may create |
1348 | interesting effects if some package is not overloaded, but inherits |
1349 | from two overloaded packages. |
4633a7c4 |
1350 | |
ee239bfe |
1351 | Relation between overloading and tie()ing is broken. Overloading is |
1352 | triggered or not basing on the I<previous> class of tie()d value. |
1353 | |
1354 | This happens because the presence of overloading is checked too early, |
1355 | before any tie()d access is attempted. If the FETCH()ed class of the |
1356 | tie()d value does not change, a simple workaround is to access the value |
1357 | immediately after tie()ing, so that after this call the I<previous> class |
1358 | coincides with the current one. |
1359 | |
1360 | B<Needed:> a way to fix this without a speed penalty. |
1361 | |
b3ac6de7 |
1362 | Barewords are not covered by overloaded string constants. |
1363 | |
ee239bfe |
1364 | This document is confusing. There are grammos and misleading language |
1365 | used in places. It would seem a total rewrite is needed. |
4633a7c4 |
1366 | |
1367 | =cut |
1368 | |