Commit | Line | Data |
4633a7c4 |
1 | package overload; |
2 | |
98225a64 |
3 | our $VERSION = '1.04'; |
b75c8c73 |
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; |
b3c0ec7c |
85 | |
86 | require Scalar::Util; |
87 | my $class = Scalar::Util::blessed($_[0]); |
88 | my $class_prefix = defined($class) ? "$class=" : ""; |
89 | my $type = Scalar::Util::reftype($_[0]); |
90 | my $addr = Scalar::Util::refaddr($_[0]); |
91 | return sprintf("$class_prefix$type(0x%x)", $addr); |
4633a7c4 |
92 | } |
93 | |
1b1d102f |
94 | *StrVal = *AddrRef; |
4633a7c4 |
95 | |
44a8e56a |
96 | sub mycan { # Real can would leave stubs. |
97 | my ($package, $meth) = @_; |
98 | return \*{$package . "::$meth"} if defined &{$package . "::$meth"}; |
99 | my $p; |
100 | foreach $p (@{$package . "::ISA"}) { |
101 | my $out = mycan($p, $meth); |
102 | return $out if $out; |
103 | } |
104 | return undef; |
105 | } |
106 | |
b3ac6de7 |
107 | %constants = ( |
9cfe5470 |
108 | 'integer' => 0x1000, # HINT_NEW_INTEGER |
109 | 'float' => 0x2000, # HINT_NEW_FLOAT |
110 | 'binary' => 0x4000, # HINT_NEW_BINARY |
111 | 'q' => 0x8000, # HINT_NEW_STRING |
112 | 'qr' => 0x10000, # HINT_NEW_RE |
b3ac6de7 |
113 | ); |
114 | |
ee239bfe |
115 | %ops = ( with_assign => "+ - * / % ** << >> x .", |
116 | assign => "+= -= *= /= %= **= <<= >>= x= .=", |
2877bd81 |
117 | num_comparison => "< <= > >= == !=", |
ee239bfe |
118 | '3way_comparison'=> "<=> cmp", |
2877bd81 |
119 | str_comparison => "lt le gt ge eq ne", |
ee239bfe |
120 | binary => "& | ^", |
121 | unary => "neg ! ~", |
122 | mutators => '++ --', |
f216259d |
123 | func => "atan2 cos sin exp abs log sqrt int", |
ee239bfe |
124 | conversion => 'bool "" 0+', |
f5284f61 |
125 | iterators => '<>', |
126 | dereferencing => '${} @{} %{} &{} *{}', |
ee239bfe |
127 | special => 'nomethod fallback ='); |
128 | |
6b82e2f5 |
129 | use warnings::register; |
b3ac6de7 |
130 | sub constant { |
131 | # Arguments: what, sub |
132 | while (@_) { |
6b82e2f5 |
133 | if (@_ == 1) { |
4498a751 |
134 | warnings::warnif ("Odd number of arguments for overload::constant"); |
6b82e2f5 |
135 | last; |
136 | } |
137 | elsif (!exists $constants {$_ [0]}) { |
4498a751 |
138 | warnings::warnif ("`$_[0]' is not an overloadable type"); |
6b82e2f5 |
139 | } |
140 | elsif (!ref $_ [1] || "$_[1]" !~ /CODE\(0x[\da-f]+\)$/) { |
141 | # Can't use C<ref $_[1] eq "CODE"> above as code references can be |
142 | # blessed, and C<ref> would return the package the ref is blessed into. |
143 | if (warnings::enabled) { |
6b82e2f5 |
144 | $_ [1] = "undef" unless defined $_ [1]; |
4498a751 |
145 | warnings::warn ("`$_[1]' is not a code reference"); |
6b82e2f5 |
146 | } |
147 | } |
148 | else { |
149 | $^H{$_[0]} = $_[1]; |
f22a2069 |
150 | $^H |= $constants{$_[0]}; |
6b82e2f5 |
151 | } |
b3ac6de7 |
152 | shift, shift; |
153 | } |
154 | } |
155 | |
156 | sub remove_constant { |
157 | # Arguments: what, sub |
158 | while (@_) { |
159 | delete $^H{$_[0]}; |
160 | $^H &= ~ $constants{$_[0]}; |
161 | shift, shift; |
162 | } |
163 | } |
164 | |
4633a7c4 |
165 | 1; |
166 | |
167 | __END__ |
168 | |
b267980d |
169 | =head1 NAME |
4633a7c4 |
170 | |
7adf7a02 |
171 | overload - Package for overloading Perl operations |
4633a7c4 |
172 | |
173 | =head1 SYNOPSIS |
174 | |
175 | package SomeThing; |
176 | |
b267980d |
177 | use overload |
4633a7c4 |
178 | '+' => \&myadd, |
179 | '-' => \&mysub; |
180 | # etc |
181 | ... |
182 | |
183 | package main; |
184 | $a = new SomeThing 57; |
185 | $b=5+$a; |
186 | ... |
187 | if (overload::Overloaded $b) {...} |
188 | ... |
189 | $strval = overload::StrVal $b; |
190 | |
4633a7c4 |
191 | =head1 DESCRIPTION |
192 | |
193 | =head2 Declaration of overloaded functions |
194 | |
195 | The compilation directive |
196 | |
197 | package Number; |
198 | use overload |
b267980d |
199 | "+" => \&add, |
4633a7c4 |
200 | "*=" => "muas"; |
201 | |
202 | declares function Number::add() for addition, and method muas() in |
203 | the "class" C<Number> (or one of its base classes) |
b267980d |
204 | for the assignment form C<*=> of multiplication. |
4633a7c4 |
205 | |
206 | Arguments of this directive come in (key, value) pairs. Legal values |
e7ea3e70 |
207 | are values legal inside a C<&{ ... }> call, so the name of a |
208 | subroutine, a reference to a subroutine, or an anonymous subroutine |
209 | will all work. Note that values specified as strings are |
210 | interpreted as methods, not subroutines. Legal keys are listed below. |
4633a7c4 |
211 | |
212 | The subroutine C<add> will be called to execute C<$a+$b> if $a |
213 | is a reference to an object blessed into the package C<Number>, or if $a is |
214 | not an object from a package with defined mathemagic addition, but $b is a |
215 | reference to a C<Number>. It can also be called in other situations, like |
216 | C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical |
217 | methods refer to methods triggered by an overloaded mathematical |
218 | operator.) |
219 | |
774d564b |
220 | Since overloading respects inheritance via the @ISA hierarchy, the |
221 | above declaration would also trigger overloading of C<+> and C<*=> in |
222 | all the packages which inherit from C<Number>. |
e7ea3e70 |
223 | |
4633a7c4 |
224 | =head2 Calling Conventions for Binary Operations |
225 | |
226 | The functions specified in the C<use overload ...> directive are called |
227 | with three (in one particular case with four, see L<Last Resort>) |
228 | arguments. If the corresponding operation is binary, then the first |
229 | two arguments are the two arguments of the operation. However, due to |
230 | general object calling conventions, the first argument should always be |
231 | an object in the package, so in the situation of C<7+$a>, the |
232 | order of the arguments is interchanged. It probably does not matter |
233 | when implementing the addition method, but whether the arguments |
234 | are reversed is vital to the subtraction method. The method can |
235 | query this information by examining the third argument, which can take |
236 | three different values: |
237 | |
238 | =over 7 |
239 | |
240 | =item FALSE |
241 | |
242 | the order of arguments is as in the current operation. |
243 | |
244 | =item TRUE |
245 | |
246 | the arguments are reversed. |
247 | |
248 | =item C<undef> |
249 | |
250 | the current operation is an assignment variant (as in |
251 | C<$a+=7>), but the usual function is called instead. This additional |
ee239bfe |
252 | information can be used to generate some optimizations. Compare |
253 | L<Calling Conventions for Mutators>. |
4633a7c4 |
254 | |
255 | =back |
256 | |
257 | =head2 Calling Conventions for Unary Operations |
258 | |
259 | Unary operation are considered binary operations with the second |
260 | argument being C<undef>. Thus the functions that overloads C<{"++"}> |
261 | is called with arguments C<($a,undef,'')> when $a++ is executed. |
262 | |
ee239bfe |
263 | =head2 Calling Conventions for Mutators |
264 | |
265 | Two types of mutators have different calling conventions: |
266 | |
88c28ceb |
267 | =over |
ee239bfe |
268 | |
269 | =item C<++> and C<--> |
270 | |
271 | The routines which implement these operators are expected to actually |
272 | I<mutate> their arguments. So, assuming that $obj is a reference to a |
273 | number, |
274 | |
275 | sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n} |
276 | |
277 | is an appropriate implementation of overloaded C<++>. Note that |
278 | |
279 | sub incr { ++$ {$_[0]} ; shift } |
280 | |
281 | is OK if used with preincrement and with postincrement. (In the case |
282 | of postincrement a copying will be performed, see L<Copy Constructor>.) |
283 | |
284 | =item C<x=> and other assignment versions |
285 | |
286 | There is nothing special about these methods. They may change the |
287 | value of their arguments, and may leave it as is. The result is going |
288 | to be assigned to the value in the left-hand-side if different from |
289 | this value. |
290 | |
f610777f |
291 | This allows for the same method to be used as overloaded C<+=> and |
ee239bfe |
292 | C<+>. Note that this is I<allowed>, but not recommended, since by the |
293 | semantic of L<"Fallback"> Perl will call the method for C<+> anyway, |
294 | if C<+=> is not overloaded. |
295 | |
296 | =back |
297 | |
d1be9408 |
298 | B<Warning.> Due to the presence of assignment versions of operations, |
b267980d |
299 | routines which may be called in assignment context may create |
300 | self-referential structures. Currently Perl will not free self-referential |
ee239bfe |
301 | structures until cycles are C<explicitly> broken. You may get problems |
302 | when traversing your structures too. |
303 | |
b267980d |
304 | Say, |
ee239bfe |
305 | |
306 | use overload '+' => sub { bless [ \$_[0], \$_[1] ] }; |
307 | |
308 | is asking for trouble, since for code C<$obj += $foo> the subroutine |
b267980d |
309 | is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj, |
ee239bfe |
310 | \$foo]>. If using such a subroutine is an important optimization, one |
311 | can overload C<+=> explicitly by a non-"optimized" version, or switch |
b267980d |
312 | to non-optimized version if C<not defined $_[2]> (see |
ee239bfe |
313 | L<Calling Conventions for Binary Operations>). |
314 | |
315 | Even if no I<explicit> assignment-variants of operators are present in |
316 | the script, they may be generated by the optimizer. Say, C<",$obj,"> or |
317 | C<',' . $obj . ','> may be both optimized to |
318 | |
319 | my $tmp = ',' . $obj; $tmp .= ','; |
320 | |
4633a7c4 |
321 | =head2 Overloadable Operations |
322 | |
ee239bfe |
323 | The following symbols can be specified in C<use overload> directive: |
4633a7c4 |
324 | |
325 | =over 5 |
326 | |
327 | =item * I<Arithmetic operations> |
328 | |
329 | "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=", |
330 | "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=", |
331 | |
332 | For these operations a substituted non-assignment variant can be called if |
fa8a6580 |
333 | the assignment variant is not available. Methods for operations C<+>, |
334 | C<->, C<+=>, and C<-=> can be called to automatically generate |
335 | increment and decrement methods. The operation C<-> can be used to |
4633a7c4 |
336 | autogenerate missing methods for unary minus or C<abs>. |
337 | |
ee239bfe |
338 | See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and |
339 | L<"Calling Conventions for Binary Operations">) for details of these |
340 | substitutions. |
341 | |
4633a7c4 |
342 | =item * I<Comparison operations> |
343 | |
344 | "<", "<=", ">", ">=", "==", "!=", "<=>", |
345 | "lt", "le", "gt", "ge", "eq", "ne", "cmp", |
346 | |
347 | If the corresponding "spaceship" variant is available, it can be |
348 | used to substitute for the missing operation. During C<sort>ing |
349 | arrays, C<cmp> is used to compare values subject to C<use overload>. |
350 | |
351 | =item * I<Bit operations> |
352 | |
353 | "&", "^", "|", "neg", "!", "~", |
354 | |
fa8a6580 |
355 | C<neg> stands for unary minus. If the method for C<neg> is not |
3bc6ec80 |
356 | specified, it can be autogenerated using the method for |
fa8a6580 |
357 | subtraction. If the method for C<!> is not specified, it can be |
358 | autogenerated using the methods for C<bool>, or C<"">, or C<0+>. |
4633a7c4 |
359 | |
360 | =item * I<Increment and decrement> |
361 | |
362 | "++", "--", |
363 | |
364 | If undefined, addition and subtraction methods can be |
365 | used instead. These operations are called both in prefix and |
366 | postfix form. |
367 | |
368 | =item * I<Transcendental functions> |
369 | |
f216259d |
370 | "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int" |
4633a7c4 |
371 | |
372 | If C<abs> is unavailable, it can be autogenerated using methods |
1fef88e7 |
373 | for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction. |
4633a7c4 |
374 | |
f216259d |
375 | Note that traditionally the Perl function L<int> rounds to 0, thus for |
376 | floating-point-like types one should follow the same semantic. If |
377 | C<int> is unavailable, it can be autogenerated using the overloading of |
378 | C<0+>. |
379 | |
4633a7c4 |
380 | =item * I<Boolean, string and numeric conversion> |
381 | |
fa8a6580 |
382 | 'bool', '""', '0+', |
4633a7c4 |
383 | |
f5284f61 |
384 | If one or two of these operations are not overloaded, the remaining ones can |
4633a7c4 |
385 | be used instead. C<bool> is used in the flow control operators |
fa8a6580 |
386 | (like C<while>) and for the ternary C<?:> operation. These functions can |
4633a7c4 |
387 | return any arbitrary Perl value. If the corresponding operation for this value |
388 | is overloaded too, that operation will be called again with this value. |
389 | |
1554e226 |
390 | As a special case if the overload returns the object itself then it will |
391 | be used directly. An overloaded conversion returning the object is |
392 | probably a bug, because you're likely to get something that looks like |
393 | C<YourPackage=HASH(0x8172b34)>. |
394 | |
f5284f61 |
395 | =item * I<Iteration> |
396 | |
397 | "<>" |
398 | |
399 | If not overloaded, the argument will be converted to a filehandle or |
400 | glob (which may require a stringification). The same overloading |
401 | happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and |
402 | I<globbing> syntax C<E<lt>${var}E<gt>>. |
403 | |
54f8c773 |
404 | B<BUGS> Even in list context, the iterator is currently called only |
405 | once and with scalar context. |
406 | |
f5284f61 |
407 | =item * I<Dereferencing> |
408 | |
409 | '${}', '@{}', '%{}', '&{}', '*{}'. |
410 | |
411 | If not overloaded, the argument will be dereferenced I<as is>, thus |
412 | should be of correct type. These functions should return a reference |
413 | of correct type, or another object with overloaded dereferencing. |
414 | |
b267980d |
415 | As a special case if the overload returns the object itself then it |
416 | will be used directly (provided it is the correct type). |
417 | |
418 | The dereference operators must be specified explicitly they will not be passed to |
419 | "nomethod". |
420 | |
4633a7c4 |
421 | =item * I<Special> |
422 | |
0d863452 |
423 | "nomethod", "fallback", "=", "~~", |
4633a7c4 |
424 | |
425 | see L<SPECIAL SYMBOLS FOR C<use overload>>. |
426 | |
427 | =back |
428 | |
ee239bfe |
429 | See L<"Fallback"> for an explanation of when a missing method can be |
430 | autogenerated. |
431 | |
432 | A computer-readable form of the above table is available in the hash |
433 | %overload::ops, with values being space-separated lists of names: |
434 | |
435 | with_assign => '+ - * / % ** << >> x .', |
436 | assign => '+= -= *= /= %= **= <<= >>= x= .=', |
2877bd81 |
437 | num_comparison => '< <= > >= == !=', |
ee239bfe |
438 | '3way_comparison'=> '<=> cmp', |
2877bd81 |
439 | str_comparison => 'lt le gt ge eq ne', |
ee239bfe |
440 | binary => '& | ^', |
441 | unary => 'neg ! ~', |
442 | mutators => '++ --', |
443 | func => 'atan2 cos sin exp abs log sqrt', |
444 | conversion => 'bool "" 0+', |
f5284f61 |
445 | iterators => '<>', |
446 | dereferencing => '${} @{} %{} &{} *{}', |
ee239bfe |
447 | special => 'nomethod fallback =' |
4633a7c4 |
448 | |
e7ea3e70 |
449 | =head2 Inheritance and overloading |
450 | |
774d564b |
451 | Inheritance interacts with overloading in two ways. |
e7ea3e70 |
452 | |
88c28ceb |
453 | =over |
e7ea3e70 |
454 | |
455 | =item Strings as values of C<use overload> directive |
456 | |
774d564b |
457 | If C<value> in |
e7ea3e70 |
458 | |
459 | use overload key => value; |
460 | |
774d564b |
461 | is a string, it is interpreted as a method name. |
e7ea3e70 |
462 | |
463 | =item Overloading of an operation is inherited by derived classes |
464 | |
774d564b |
465 | Any class derived from an overloaded class is also overloaded. The |
466 | set of overloaded methods is the union of overloaded methods of all |
467 | the ancestors. If some method is overloaded in several ancestor, then |
e7ea3e70 |
468 | which description will be used is decided by the usual inheritance |
774d564b |
469 | rules: |
e7ea3e70 |
470 | |
774d564b |
471 | If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads |
472 | C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">, |
473 | then the subroutine C<D::plus_sub> will be called to implement |
474 | operation C<+> for an object in package C<A>. |
e7ea3e70 |
475 | |
476 | =back |
477 | |
774d564b |
478 | Note that since the value of the C<fallback> key is not a subroutine, |
479 | its inheritance is not governed by the above rules. In the current |
480 | implementation, the value of C<fallback> in the first overloaded |
481 | ancestor is used, but this is accidental and subject to change. |
e7ea3e70 |
482 | |
4633a7c4 |
483 | =head1 SPECIAL SYMBOLS FOR C<use overload> |
484 | |
485 | Three keys are recognized by Perl that are not covered by the above |
486 | description. |
487 | |
774d564b |
488 | =head2 Last Resort |
4633a7c4 |
489 | |
490 | C<"nomethod"> should be followed by a reference to a function of four |
491 | parameters. If defined, it is called when the overloading mechanism |
492 | cannot find a method for some operation. The first three arguments of |
493 | this function coincide with the arguments for the corresponding method if |
494 | it were found, the fourth argument is the symbol |
495 | corresponding to the missing method. If several methods are tried, |
496 | the last one is used. Say, C<1-$a> can be equivalent to |
497 | |
498 | &nomethodMethod($a,1,1,"-") |
499 | |
500 | if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the |
501 | C<use overload> directive. |
502 | |
b267980d |
503 | The C<"nomethod"> mechanism is I<not> used for the dereference operators |
504 | ( ${} @{} %{} &{} *{} ). |
505 | |
506 | |
4633a7c4 |
507 | If some operation cannot be resolved, and there is no function |
508 | assigned to C<"nomethod">, then an exception will be raised via die()-- |
509 | unless C<"fallback"> was specified as a key in C<use overload> directive. |
510 | |
b267980d |
511 | |
512 | =head2 Fallback |
4633a7c4 |
513 | |
514 | The key C<"fallback"> governs what to do if a method for a particular |
515 | operation is not found. Three different cases are possible depending on |
516 | the value of C<"fallback">: |
517 | |
518 | =over 16 |
519 | |
520 | =item * C<undef> |
521 | |
522 | Perl tries to use a |
523 | substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it |
524 | then tries to calls C<"nomethod"> value; if missing, an exception |
525 | will be raised. |
526 | |
527 | =item * TRUE |
528 | |
529 | The same as for the C<undef> value, but no exception is raised. Instead, |
530 | it silently reverts to what it would have done were there no C<use overload> |
531 | present. |
532 | |
533 | =item * defined, but FALSE |
534 | |
535 | No autogeneration is tried. Perl tries to call |
b267980d |
536 | C<"nomethod"> value, and if this is missing, raises an exception. |
4633a7c4 |
537 | |
538 | =back |
539 | |
e7ea3e70 |
540 | B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone |
541 | yet, see L<"Inheritance and overloading">. |
542 | |
5e68dedd |
543 | =head2 Smart Match |
544 | |
545 | The key C<"~~"> allows you to override the smart matching used by |
546 | the switch construct. See L<feature>. |
547 | |
4633a7c4 |
548 | =head2 Copy Constructor |
549 | |
550 | The value for C<"="> is a reference to a function with three |
551 | arguments, i.e., it looks like the other values in C<use |
552 | overload>. However, it does not overload the Perl assignment |
553 | operator. This would go against Camel hair. |
554 | |
555 | This operation is called in the situations when a mutator is applied |
556 | to a reference that shares its object with some other reference, such |
557 | as |
558 | |
b267980d |
559 | $a=$b; |
ee239bfe |
560 | ++$a; |
4633a7c4 |
561 | |
562 | To make this change $a and not change $b, a copy of C<$$a> is made, |
563 | and $a is assigned a reference to this new object. This operation is |
ee239bfe |
564 | done during execution of the C<++$a>, and not during the assignment, |
4633a7c4 |
565 | (so before the increment C<$$a> coincides with C<$$b>). This is only |
ee239bfe |
566 | done if C<++> is expressed via a method for C<'++'> or C<'+='> (or |
567 | C<nomethod>). Note that if this operation is expressed via C<'+'> |
568 | a nonmutator, i.e., as in |
4633a7c4 |
569 | |
b267980d |
570 | $a=$b; |
4633a7c4 |
571 | $a=$a+1; |
572 | |
573 | then C<$a> does not reference a new copy of C<$$a>, since $$a does not |
574 | appear as lvalue when the above code is executed. |
575 | |
576 | If the copy constructor is required during the execution of some mutator, |
577 | but a method for C<'='> was not specified, it can be autogenerated as a |
578 | string copy if the object is a plain scalar. |
579 | |
580 | =over 5 |
581 | |
582 | =item B<Example> |
583 | |
b267980d |
584 | The actually executed code for |
4633a7c4 |
585 | |
b267980d |
586 | $a=$b; |
4633a7c4 |
587 | Something else which does not modify $a or $b.... |
588 | ++$a; |
589 | |
590 | may be |
591 | |
b267980d |
592 | $a=$b; |
4633a7c4 |
593 | Something else which does not modify $a or $b.... |
594 | $a = $a->clone(undef,""); |
595 | $a->incr(undef,""); |
596 | |
597 | if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>, |
598 | C<'='> was overloaded with C<\&clone>. |
599 | |
600 | =back |
601 | |
f610777f |
602 | Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for |
ee239bfe |
603 | C<$b = $a; ++$a>. |
604 | |
4633a7c4 |
605 | =head1 MAGIC AUTOGENERATION |
606 | |
607 | If a method for an operation is not found, and the value for C<"fallback"> is |
608 | TRUE or undefined, Perl tries to autogenerate a substitute method for |
609 | the missing operation based on the defined operations. Autogenerated method |
610 | substitutions are possible for the following operations: |
611 | |
612 | =over 16 |
613 | |
614 | =item I<Assignment forms of arithmetic operations> |
615 | |
616 | C<$a+=$b> can use the method for C<"+"> if the method for C<"+="> |
617 | is not defined. |
618 | |
b267980d |
619 | =item I<Conversion operations> |
4633a7c4 |
620 | |
621 | String, numeric, and boolean conversion are calculated in terms of one |
622 | another if not all of them are defined. |
623 | |
624 | =item I<Increment and decrement> |
625 | |
626 | The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>, |
627 | and C<$a--> in terms of C<$a-=1> and C<$a-1>. |
628 | |
629 | =item C<abs($a)> |
630 | |
631 | can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>). |
632 | |
633 | =item I<Unary minus> |
634 | |
635 | can be expressed in terms of subtraction. |
636 | |
3bc6ec80 |
637 | =item I<Negation> |
638 | |
639 | C<!> and C<not> can be expressed in terms of boolean conversion, or |
640 | string or numerical conversion. |
641 | |
4633a7c4 |
642 | =item I<Concatenation> |
643 | |
644 | can be expressed in terms of string conversion. |
645 | |
b267980d |
646 | =item I<Comparison operations> |
4633a7c4 |
647 | |
648 | can be expressed in terms of its "spaceship" counterpart: either |
649 | C<E<lt>=E<gt>> or C<cmp>: |
1fef88e7 |
650 | |
4633a7c4 |
651 | <, >, <=, >=, ==, != in terms of <=> |
652 | lt, gt, le, ge, eq, ne in terms of cmp |
653 | |
f5284f61 |
654 | =item I<Iterator> |
655 | |
656 | <> in terms of builtin operations |
657 | |
658 | =item I<Dereferencing> |
659 | |
660 | ${} @{} %{} &{} *{} in terms of builtin operations |
661 | |
4633a7c4 |
662 | =item I<Copy operator> |
663 | |
664 | can be expressed in terms of an assignment to the dereferenced value, if this |
665 | value is a scalar and not a reference. |
666 | |
667 | =back |
668 | |
84fc275b |
669 | =head1 Minimal set of overloaded operations |
670 | |
671 | Since some operations can be automatically generated from others, there is |
672 | a minimal set of operations that need to be overloaded in order to have |
299476e0 |
673 | the complete set of overloaded operations at one's disposal. |
674 | Of course, the autogenerated operations may not do exactly what the user |
675 | expects. See L<MAGIC AUTOGENERATION> above. The minimal set is: |
84fc275b |
676 | |
677 | + - * / % ** << >> x |
678 | <=> cmp |
679 | & | ^ ~ |
680 | atan2 cos sin exp log sqrt int |
681 | |
682 | Additionally, you need to define at least one of string, boolean or |
299476e0 |
683 | numeric conversions because any one can be used to emulate the others. |
684 | The string conversion can also be used to emulate concatenation. |
84fc275b |
685 | |
ee239bfe |
686 | =head1 Losing overloading |
4633a7c4 |
687 | |
688 | The restriction for the comparison operation is that even if, for example, |
689 | `C<cmp>' should return a blessed reference, the autogenerated `C<lt>' |
690 | function will produce only a standard logical value based on the |
691 | numerical value of the result of `C<cmp>'. In particular, a working |
692 | numeric conversion is needed in this case (possibly expressed in terms of |
693 | other conversions). |
694 | |
695 | Similarly, C<.=> and C<x=> operators lose their mathemagical properties |
696 | if the string conversion substitution is applied. |
697 | |
698 | When you chop() a mathemagical object it is promoted to a string and its |
699 | mathemagical properties are lost. The same can happen with other |
700 | operations as well. |
701 | |
702 | =head1 Run-time Overloading |
703 | |
704 | Since all C<use> directives are executed at compile-time, the only way to |
705 | change overloading during run-time is to |
706 | |
707 | eval 'use overload "+" => \&addmethod'; |
708 | |
709 | You can also use |
710 | |
711 | eval 'no overload "+", "--", "<="'; |
712 | |
713 | though the use of these constructs during run-time is questionable. |
714 | |
715 | =head1 Public functions |
716 | |
717 | Package C<overload.pm> provides the following public functions: |
718 | |
719 | =over 5 |
720 | |
721 | =item overload::StrVal(arg) |
722 | |
6a0e9e72 |
723 | Gives string value of C<arg> as in absence of stringify overloading. If you |
724 | are using this to get the address of a reference (useful for checking if two |
725 | references point to the same thing) then you may be better off using |
726 | C<Scalar::Util::refaddr()>, which is faster. |
4633a7c4 |
727 | |
728 | =item overload::Overloaded(arg) |
729 | |
730 | Returns true if C<arg> is subject to overloading of some operations. |
731 | |
732 | =item overload::Method(obj,op) |
733 | |
734 | Returns C<undef> or a reference to the method that implements C<op>. |
735 | |
736 | =back |
737 | |
b3ac6de7 |
738 | =head1 Overloading constants |
739 | |
7adf7a02 |
740 | For some applications, the Perl parser mangles constants too much. |
bfce84ec |
741 | It is possible to hook into this process via C<overload::constant()> |
7adf7a02 |
742 | and C<overload::remove_constant()> functions. |
b3ac6de7 |
743 | |
744 | These functions take a hash as an argument. The recognized keys of this hash |
7adf7a02 |
745 | are: |
b3ac6de7 |
746 | |
747 | =over 8 |
748 | |
749 | =item integer |
750 | |
751 | to overload integer constants, |
752 | |
753 | =item float |
754 | |
755 | to overload floating point constants, |
756 | |
757 | =item binary |
758 | |
759 | to overload octal and hexadecimal constants, |
760 | |
761 | =item q |
762 | |
763 | to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted |
764 | strings and here-documents, |
765 | |
766 | =item qr |
767 | |
768 | to overload constant pieces of regular expressions. |
769 | |
770 | =back |
771 | |
772 | The corresponding values are references to functions which take three arguments: |
773 | the first one is the I<initial> string form of the constant, the second one |
b267980d |
774 | is how Perl interprets this constant, the third one is how the constant is used. |
b3ac6de7 |
775 | Note that the initial string form does not |
b267980d |
776 | contain string delimiters, and has backslashes in backslash-delimiter |
b3ac6de7 |
777 | combinations stripped (thus the value of delimiter is not relevant for |
b267980d |
778 | processing of this string). The return value of this function is how this |
b3ac6de7 |
779 | constant is going to be interpreted by Perl. The third argument is undefined |
780 | unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote |
781 | context (comes from strings, regular expressions, and single-quote HERE |
b267980d |
782 | documents), it is C<tr> for arguments of C<tr>/C<y> operators, |
b3ac6de7 |
783 | it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise. |
784 | |
785 | Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>, |
786 | it is expected that overloaded constant strings are equipped with reasonable |
b267980d |
787 | overloaded catenation operator, otherwise absurd results will result. |
b3ac6de7 |
788 | Similarly, negative numbers are considered as negations of positive constants. |
789 | |
790 | Note that it is probably meaningless to call the functions overload::constant() |
791 | and overload::remove_constant() from anywhere but import() and unimport() methods. |
792 | From these methods they may be called as |
793 | |
794 | sub import { |
795 | shift; |
796 | return unless @_; |
797 | die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant'; |
798 | overload::constant integer => sub {Math::BigInt->new(shift)}; |
799 | } |
800 | |
4633a7c4 |
801 | =head1 IMPLEMENTATION |
802 | |
803 | What follows is subject to change RSN. |
804 | |
e7ea3e70 |
805 | The table of methods for all operations is cached in magic for the |
806 | symbol table hash for the package. The cache is invalidated during |
807 | processing of C<use overload>, C<no overload>, new function |
808 | definitions, and changes in @ISA. However, this invalidation remains |
809 | unprocessed until the next C<bless>ing into the package. Hence if you |
810 | want to change overloading structure dynamically, you'll need an |
811 | additional (fake) C<bless>ing to update the table. |
812 | |
813 | (Every SVish thing has a magic queue, and magic is an entry in that |
814 | queue. This is how a single variable may participate in multiple |
815 | forms of magic simultaneously. For instance, environment variables |
816 | regularly have two forms at once: their %ENV magic and their taint |
817 | magic. However, the magic which implements overloading is applied to |
818 | the stashes, which are rarely used directly, thus should not slow down |
819 | Perl.) |
4633a7c4 |
820 | |
821 | If an object belongs to a package using overload, it carries a special |
822 | flag. Thus the only speed penalty during arithmetic operations without |
823 | overloading is the checking of this flag. |
824 | |
774d564b |
825 | In fact, if C<use overload> is not present, there is almost no overhead |
826 | for overloadable operations, so most programs should not suffer |
827 | measurable performance penalties. A considerable effort was made to |
828 | minimize the overhead when overload is used in some package, but the |
829 | arguments in question do not belong to packages using overload. When |
830 | in doubt, test your speed with C<use overload> and without it. So far |
831 | there have been no reports of substantial speed degradation if Perl is |
832 | compiled with optimization turned on. |
4633a7c4 |
833 | |
e7ea3e70 |
834 | There is no size penalty for data if overload is not used. The only |
835 | size penalty if overload is used in some package is that I<all> the |
836 | packages acquire a magic during the next C<bless>ing into the |
837 | package. This magic is three-words-long for packages without |
f610777f |
838 | overloading, and carries the cache table if the package is overloaded. |
4633a7c4 |
839 | |
b267980d |
840 | Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is |
4633a7c4 |
841 | carried out before any operation that can imply an assignment to the |
842 | object $a (or $b) refers to, like C<$a++>. You can override this |
843 | behavior by defining your own copy constructor (see L<"Copy Constructor">). |
844 | |
845 | It is expected that arguments to methods that are not explicitly supposed |
846 | to be changed are constant (but this is not enforced). |
847 | |
ee239bfe |
848 | =head1 Metaphor clash |
849 | |
f610777f |
850 | One may wonder why the semantic of overloaded C<=> is so counter intuitive. |
b267980d |
851 | If it I<looks> counter intuitive to you, you are subject to a metaphor |
852 | clash. |
ee239bfe |
853 | |
854 | Here is a Perl object metaphor: |
855 | |
856 | I< object is a reference to blessed data> |
857 | |
858 | and an arithmetic metaphor: |
859 | |
860 | I< object is a thing by itself>. |
861 | |
862 | The I<main> problem of overloading C<=> is the fact that these metaphors |
863 | imply different actions on the assignment C<$a = $b> if $a and $b are |
864 | objects. Perl-think implies that $a becomes a reference to whatever |
865 | $b was referencing. Arithmetic-think implies that the value of "object" |
866 | $a is changed to become the value of the object $b, preserving the fact |
867 | that $a and $b are separate entities. |
868 | |
869 | The difference is not relevant in the absence of mutators. After |
870 | a Perl-way assignment an operation which mutates the data referenced by $a |
b267980d |
871 | would change the data referenced by $b too. Effectively, after |
ee239bfe |
872 | C<$a = $b> values of $a and $b become I<indistinguishable>. |
873 | |
b267980d |
874 | On the other hand, anyone who has used algebraic notation knows the |
ee239bfe |
875 | expressive power of the arithmetic metaphor. Overloading works hard |
876 | to enable this metaphor while preserving the Perlian way as far as |
d1be9408 |
877 | possible. Since it is not possible to freely mix two contradicting |
ee239bfe |
878 | metaphors, overloading allows the arithmetic way to write things I<as |
879 | far as all the mutators are called via overloaded access only>. The |
880 | way it is done is described in L<Copy Constructor>. |
881 | |
882 | If some mutator methods are directly applied to the overloaded values, |
b267980d |
883 | one may need to I<explicitly unlink> other values which references the |
ee239bfe |
884 | same value: |
885 | |
886 | $a = new Data 23; |
887 | ... |
888 | $b = $a; # $b is "linked" to $a |
889 | ... |
890 | $a = $a->clone; # Unlink $b from $a |
891 | $a->increment_by(4); |
892 | |
893 | Note that overloaded access makes this transparent: |
894 | |
895 | $a = new Data 23; |
896 | $b = $a; # $b is "linked" to $a |
897 | $a += 4; # would unlink $b automagically |
898 | |
899 | However, it would not make |
900 | |
901 | $a = new Data 23; |
902 | $a = 4; # Now $a is a plain 4, not 'Data' |
903 | |
904 | preserve "objectness" of $a. But Perl I<has> a way to make assignments |
905 | to an object do whatever you want. It is just not the overload, but |
906 | tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method |
b267980d |
907 | which returns the object itself, and STORE() method which changes the |
ee239bfe |
908 | value of the object, one can reproduce the arithmetic metaphor in its |
909 | completeness, at least for variables which were tie()d from the start. |
910 | |
911 | (Note that a workaround for a bug may be needed, see L<"BUGS">.) |
912 | |
913 | =head1 Cookbook |
914 | |
915 | Please add examples to what follows! |
916 | |
917 | =head2 Two-face scalars |
918 | |
919 | Put this in F<two_face.pm> in your Perl library directory: |
920 | |
921 | package two_face; # Scalars with separate string and |
922 | # numeric values. |
923 | sub new { my $p = shift; bless [@_], $p } |
924 | use overload '""' => \&str, '0+' => \&num, fallback => 1; |
925 | sub num {shift->[1]} |
926 | sub str {shift->[0]} |
927 | |
928 | Use it as follows: |
929 | |
930 | require two_face; |
931 | my $seven = new two_face ("vii", 7); |
932 | printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1; |
933 | print "seven contains `i'\n" if $seven =~ /i/; |
934 | |
935 | (The second line creates a scalar which has both a string value, and a |
936 | numeric value.) This prints: |
937 | |
938 | seven=vii, seven=7, eight=8 |
939 | seven contains `i' |
940 | |
f5284f61 |
941 | =head2 Two-face references |
942 | |
943 | Suppose you want to create an object which is accessible as both an |
6d822dc4 |
944 | array reference and a hash reference. |
f5284f61 |
945 | |
946 | package two_refs; |
947 | use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} }; |
b267980d |
948 | sub new { |
949 | my $p = shift; |
f5284f61 |
950 | bless \ [@_], $p; |
951 | } |
952 | sub gethash { |
953 | my %h; |
954 | my $self = shift; |
955 | tie %h, ref $self, $self; |
956 | \%h; |
957 | } |
958 | |
959 | sub TIEHASH { my $p = shift; bless \ shift, $p } |
960 | my %fields; |
961 | my $i = 0; |
962 | $fields{$_} = $i++ foreach qw{zero one two three}; |
b267980d |
963 | sub STORE { |
f5284f61 |
964 | my $self = ${shift()}; |
965 | my $key = $fields{shift()}; |
966 | defined $key or die "Out of band access"; |
967 | $$self->[$key] = shift; |
968 | } |
b267980d |
969 | sub FETCH { |
f5284f61 |
970 | my $self = ${shift()}; |
971 | my $key = $fields{shift()}; |
972 | defined $key or die "Out of band access"; |
973 | $$self->[$key]; |
974 | } |
975 | |
976 | Now one can access an object using both the array and hash syntax: |
977 | |
978 | my $bar = new two_refs 3,4,5,6; |
979 | $bar->[2] = 11; |
980 | $bar->{two} == 11 or die 'bad hash fetch'; |
981 | |
982 | Note several important features of this example. First of all, the |
983 | I<actual> type of $bar is a scalar reference, and we do not overload |
984 | the scalar dereference. Thus we can get the I<actual> non-overloaded |
985 | contents of $bar by just using C<$$bar> (what we do in functions which |
986 | overload dereference). Similarly, the object returned by the |
987 | TIEHASH() method is a scalar reference. |
988 | |
989 | Second, we create a new tied hash each time the hash syntax is used. |
990 | This allows us not to worry about a possibility of a reference loop, |
d1be9408 |
991 | which would lead to a memory leak. |
f5284f61 |
992 | |
993 | Both these problems can be cured. Say, if we want to overload hash |
994 | dereference on a reference to an object which is I<implemented> as a |
995 | hash itself, the only problem one has to circumvent is how to access |
1fd16925 |
996 | this I<actual> hash (as opposed to the I<virtual> hash exhibited by the |
f5284f61 |
997 | overloaded dereference operator). Here is one possible fetching routine: |
998 | |
999 | sub access_hash { |
1000 | my ($self, $key) = (shift, shift); |
1001 | my $class = ref $self; |
b267980d |
1002 | bless $self, 'overload::dummy'; # Disable overloading of %{} |
f5284f61 |
1003 | my $out = $self->{$key}; |
1004 | bless $self, $class; # Restore overloading |
1005 | $out; |
1006 | } |
1007 | |
1fd16925 |
1008 | To remove creation of the tied hash on each access, one may an extra |
f5284f61 |
1009 | level of indirection which allows a non-circular structure of references: |
1010 | |
1011 | package two_refs1; |
1012 | use overload '%{}' => sub { ${shift()}->[1] }, |
1013 | '@{}' => sub { ${shift()}->[0] }; |
b267980d |
1014 | sub new { |
1015 | my $p = shift; |
f5284f61 |
1016 | my $a = [@_]; |
1017 | my %h; |
1018 | tie %h, $p, $a; |
1019 | bless \ [$a, \%h], $p; |
1020 | } |
1021 | sub gethash { |
1022 | my %h; |
1023 | my $self = shift; |
1024 | tie %h, ref $self, $self; |
1025 | \%h; |
1026 | } |
1027 | |
1028 | sub TIEHASH { my $p = shift; bless \ shift, $p } |
1029 | my %fields; |
1030 | my $i = 0; |
1031 | $fields{$_} = $i++ foreach qw{zero one two three}; |
b267980d |
1032 | sub STORE { |
f5284f61 |
1033 | my $a = ${shift()}; |
1034 | my $key = $fields{shift()}; |
1035 | defined $key or die "Out of band access"; |
1036 | $a->[$key] = shift; |
1037 | } |
b267980d |
1038 | sub FETCH { |
f5284f61 |
1039 | my $a = ${shift()}; |
1040 | my $key = $fields{shift()}; |
1041 | defined $key or die "Out of band access"; |
1042 | $a->[$key]; |
1043 | } |
1044 | |
1fd16925 |
1045 | Now if $baz is overloaded like this, then C<$baz> is a reference to a |
f5284f61 |
1046 | reference to the intermediate array, which keeps a reference to an |
1047 | actual array, and the access hash. The tie()ing object for the access |
1fd16925 |
1048 | hash is a reference to a reference to the actual array, so |
f5284f61 |
1049 | |
88c28ceb |
1050 | =over |
f5284f61 |
1051 | |
1052 | =item * |
1053 | |
1054 | There are no loops of references. |
1055 | |
1056 | =item * |
1057 | |
1058 | Both "objects" which are blessed into the class C<two_refs1> are |
1059 | references to a reference to an array, thus references to a I<scalar>. |
1060 | Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no |
1061 | overloaded operations. |
1062 | |
1063 | =back |
1064 | |
ee239bfe |
1065 | =head2 Symbolic calculator |
1066 | |
1067 | Put this in F<symbolic.pm> in your Perl library directory: |
1068 | |
1069 | package symbolic; # Primitive symbolic calculator |
1070 | use overload nomethod => \&wrap; |
1071 | |
1072 | sub new { shift; bless ['n', @_] } |
1073 | sub wrap { |
1074 | my ($obj, $other, $inv, $meth) = @_; |
1075 | ($obj, $other) = ($other, $obj) if $inv; |
1076 | bless [$meth, $obj, $other]; |
1077 | } |
1078 | |
1079 | This module is very unusual as overloaded modules go: it does not |
88c28ceb |
1080 | provide any usual overloaded operators, instead it provides the L<Last |
1081 | Resort> operator C<nomethod>. In this example the corresponding |
f610777f |
1082 | subroutine returns an object which encapsulates operations done over |
ee239bfe |
1083 | the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new |
1084 | symbolic 3> contains C<['+', 2, ['n', 3]]>. |
1085 | |
1086 | Here is an example of the script which "calculates" the side of |
1087 | circumscribed octagon using the above package: |
1088 | |
1089 | require symbolic; |
1090 | my $iter = 1; # 2**($iter+2) = 8 |
1091 | my $side = new symbolic 1; |
1092 | my $cnt = $iter; |
3cb6de81 |
1093 | |
ee239bfe |
1094 | while ($cnt--) { |
1095 | $side = (sqrt(1 + $side**2) - 1)/$side; |
1096 | } |
1097 | print "OK\n"; |
1098 | |
1099 | The value of $side is |
1100 | |
1101 | ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]], |
1102 | undef], 1], ['n', 1]] |
1103 | |
1104 | Note that while we obtained this value using a nice little script, |
1105 | there is no simple way to I<use> this value. In fact this value may |
1106 | be inspected in debugger (see L<perldebug>), but ony if |
1107 | C<bareStringify> B<O>ption is set, and not via C<p> command. |
1108 | |
1109 | If one attempts to print this value, then the overloaded operator |
1110 | C<""> will be called, which will call C<nomethod> operator. The |
1111 | result of this operator will be stringified again, but this result is |
1112 | again of type C<symbolic>, which will lead to an infinite loop. |
1113 | |
1114 | Add a pretty-printer method to the module F<symbolic.pm>: |
1115 | |
1116 | sub pretty { |
1117 | my ($meth, $a, $b) = @{+shift}; |
1118 | $a = 'u' unless defined $a; |
1119 | $b = 'u' unless defined $b; |
1120 | $a = $a->pretty if ref $a; |
1121 | $b = $b->pretty if ref $b; |
1122 | "[$meth $a $b]"; |
b267980d |
1123 | } |
ee239bfe |
1124 | |
1125 | Now one can finish the script by |
1126 | |
1127 | print "side = ", $side->pretty, "\n"; |
1128 | |
1129 | The method C<pretty> is doing object-to-string conversion, so it |
1130 | is natural to overload the operator C<""> using this method. However, |
1131 | inside such a method it is not necessary to pretty-print the |
1132 | I<components> $a and $b of an object. In the above subroutine |
1133 | C<"[$meth $a $b]"> is a catenation of some strings and components $a |
1134 | and $b. If these components use overloading, the catenation operator |
1fd16925 |
1135 | will look for an overloaded operator C<.>; if not present, it will |
ee239bfe |
1136 | look for an overloaded operator C<"">. Thus it is enough to use |
1137 | |
1138 | use overload nomethod => \&wrap, '""' => \&str; |
1139 | sub str { |
1140 | my ($meth, $a, $b) = @{+shift}; |
1141 | $a = 'u' unless defined $a; |
1142 | $b = 'u' unless defined $b; |
1143 | "[$meth $a $b]"; |
b267980d |
1144 | } |
ee239bfe |
1145 | |
1146 | Now one can change the last line of the script to |
1147 | |
1148 | print "side = $side\n"; |
1149 | |
1150 | which outputs |
1151 | |
1152 | side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]] |
1153 | |
1154 | and one can inspect the value in debugger using all the possible |
b267980d |
1155 | methods. |
ee239bfe |
1156 | |
d1be9408 |
1157 | Something is still amiss: consider the loop variable $cnt of the |
ee239bfe |
1158 | script. It was a number, not an object. We cannot make this value of |
1159 | type C<symbolic>, since then the loop will not terminate. |
1160 | |
1161 | Indeed, to terminate the cycle, the $cnt should become false. |
1162 | However, the operator C<bool> for checking falsity is overloaded (this |
1163 | time via overloaded C<"">), and returns a long string, thus any object |
1164 | of type C<symbolic> is true. To overcome this, we need a way to |
1165 | compare an object to 0. In fact, it is easier to write a numeric |
1166 | conversion routine. |
1167 | |
1168 | Here is the text of F<symbolic.pm> with such a routine added (and |
f610777f |
1169 | slightly modified str()): |
ee239bfe |
1170 | |
1171 | package symbolic; # Primitive symbolic calculator |
1172 | use overload |
1173 | nomethod => \&wrap, '""' => \&str, '0+' => \# |
1174 | |
1175 | sub new { shift; bless ['n', @_] } |
1176 | sub wrap { |
1177 | my ($obj, $other, $inv, $meth) = @_; |
1178 | ($obj, $other) = ($other, $obj) if $inv; |
1179 | bless [$meth, $obj, $other]; |
1180 | } |
1181 | sub str { |
1182 | my ($meth, $a, $b) = @{+shift}; |
1183 | $a = 'u' unless defined $a; |
1184 | if (defined $b) { |
1185 | "[$meth $a $b]"; |
1186 | } else { |
1187 | "[$meth $a]"; |
1188 | } |
b267980d |
1189 | } |
1190 | my %subr = ( n => sub {$_[0]}, |
1191 | sqrt => sub {sqrt $_[0]}, |
ee239bfe |
1192 | '-' => sub {shift() - shift()}, |
1193 | '+' => sub {shift() + shift()}, |
1194 | '/' => sub {shift() / shift()}, |
1195 | '*' => sub {shift() * shift()}, |
1196 | '**' => sub {shift() ** shift()}, |
1197 | ); |
1198 | sub num { |
1199 | my ($meth, $a, $b) = @{+shift}; |
b267980d |
1200 | my $subr = $subr{$meth} |
ee239bfe |
1201 | or die "Do not know how to ($meth) in symbolic"; |
1202 | $a = $a->num if ref $a eq __PACKAGE__; |
1203 | $b = $b->num if ref $b eq __PACKAGE__; |
1204 | $subr->($a,$b); |
1205 | } |
1206 | |
1207 | All the work of numeric conversion is done in %subr and num(). Of |
f610777f |
1208 | course, %subr is not complete, it contains only operators used in the |
ee239bfe |
1209 | example below. Here is the extra-credit question: why do we need an |
1210 | explicit recursion in num()? (Answer is at the end of this section.) |
1211 | |
1212 | Use this module like this: |
1213 | |
1214 | require symbolic; |
1215 | my $iter = new symbolic 2; # 16-gon |
1216 | my $side = new symbolic 1; |
1217 | my $cnt = $iter; |
3cb6de81 |
1218 | |
ee239bfe |
1219 | while ($cnt) { |
1220 | $cnt = $cnt - 1; # Mutator `--' not implemented |
1221 | $side = (sqrt(1 + $side**2) - 1)/$side; |
1222 | } |
1223 | printf "%s=%f\n", $side, $side; |
1224 | printf "pi=%f\n", $side*(2**($iter+2)); |
1225 | |
1226 | It prints (without so many line breaks) |
1227 | |
1228 | [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] |
1229 | [n 1]] 2]]] 1] |
1230 | [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912 |
1231 | pi=3.182598 |
1232 | |
1233 | The above module is very primitive. It does not implement |
1234 | mutator methods (C<++>, C<-=> and so on), does not do deep copying |
1235 | (not required without mutators!), and implements only those arithmetic |
1236 | operations which are used in the example. |
1237 | |
1fd16925 |
1238 | To implement most arithmetic operations is easy; one should just use |
ee239bfe |
1239 | the tables of operations, and change the code which fills %subr to |
1240 | |
1241 | my %subr = ( 'n' => sub {$_[0]} ); |
1242 | foreach my $op (split " ", $overload::ops{with_assign}) { |
1243 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; |
1244 | } |
1245 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); |
1246 | foreach my $op (split " ", "@overload::ops{ @bins }") { |
1247 | $subr{$op} = eval "sub {shift() $op shift()}"; |
1248 | } |
1249 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { |
1250 | print "defining `$op'\n"; |
1251 | $subr{$op} = eval "sub {$op shift()}"; |
1252 | } |
1253 | |
1254 | Due to L<Calling Conventions for Mutators>, we do not need anything |
1255 | special to make C<+=> and friends work, except filling C<+=> entry of |
1256 | %subr, and defining a copy constructor (needed since Perl has no |
1257 | way to know that the implementation of C<'+='> does not mutate |
1258 | the argument, compare L<Copy Constructor>). |
1259 | |
1fd16925 |
1260 | To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload> |
ee239bfe |
1261 | line, and code (this code assumes that mutators change things one level |
1262 | deep only, so recursive copying is not needed): |
1263 | |
1264 | sub cpy { |
1265 | my $self = shift; |
1266 | bless [@$self], ref $self; |
1267 | } |
1268 | |
b267980d |
1269 | To make C<++> and C<--> work, we need to implement actual mutators, |
ee239bfe |
1270 | either directly, or in C<nomethod>. We continue to do things inside |
1271 | C<nomethod>, thus add |
1272 | |
1273 | if ($meth eq '++' or $meth eq '--') { |
1274 | @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference |
1275 | return $obj; |
1276 | } |
1277 | |
b267980d |
1278 | after the first line of wrap(). This is not a most effective |
ee239bfe |
1279 | implementation, one may consider |
1280 | |
1281 | sub inc { $_[0] = bless ['++', shift, 1]; } |
1282 | |
1283 | instead. |
1284 | |
1285 | As a final remark, note that one can fill %subr by |
1286 | |
1287 | my %subr = ( 'n' => sub {$_[0]} ); |
1288 | foreach my $op (split " ", $overload::ops{with_assign}) { |
1289 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; |
1290 | } |
1291 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); |
1292 | foreach my $op (split " ", "@overload::ops{ @bins }") { |
1293 | $subr{$op} = eval "sub {shift() $op shift()}"; |
1294 | } |
1295 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { |
1296 | $subr{$op} = eval "sub {$op shift()}"; |
1297 | } |
1298 | $subr{'++'} = $subr{'+'}; |
1299 | $subr{'--'} = $subr{'-'}; |
1300 | |
b267980d |
1301 | This finishes implementation of a primitive symbolic calculator in |
1302 | 50 lines of Perl code. Since the numeric values of subexpressions |
ee239bfe |
1303 | are not cached, the calculator is very slow. |
1304 | |
1305 | Here is the answer for the exercise: In the case of str(), we need no |
1306 | explicit recursion since the overloaded C<.>-operator will fall back |
1307 | to an existing overloaded operator C<"">. Overloaded arithmetic |
1308 | operators I<do not> fall back to numeric conversion if C<fallback> is |
1309 | not explicitly requested. Thus without an explicit recursion num() |
1310 | would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild |
1311 | the argument of num(). |
1312 | |
1313 | If you wonder why defaults for conversion are different for str() and |
1314 | num(), note how easy it was to write the symbolic calculator. This |
1315 | simplicity is due to an appropriate choice of defaults. One extra |
f610777f |
1316 | note: due to the explicit recursion num() is more fragile than sym(): |
1317 | we need to explicitly check for the type of $a and $b. If components |
ee239bfe |
1318 | $a and $b happen to be of some related type, this may lead to problems. |
1319 | |
1320 | =head2 I<Really> symbolic calculator |
1321 | |
1322 | One may wonder why we call the above calculator symbolic. The reason |
1323 | is that the actual calculation of the value of expression is postponed |
1324 | until the value is I<used>. |
1325 | |
1326 | To see it in action, add a method |
1327 | |
b267980d |
1328 | sub STORE { |
1329 | my $obj = shift; |
1330 | $#$obj = 1; |
ee239bfe |
1331 | @$obj->[0,1] = ('=', shift); |
1332 | } |
1333 | |
1334 | to the package C<symbolic>. After this change one can do |
1335 | |
1336 | my $a = new symbolic 3; |
1337 | my $b = new symbolic 4; |
1338 | my $c = sqrt($a**2 + $b**2); |
1339 | |
1340 | and the numeric value of $c becomes 5. However, after calling |
1341 | |
1342 | $a->STORE(12); $b->STORE(5); |
1343 | |
1344 | the numeric value of $c becomes 13. There is no doubt now that the module |
1345 | symbolic provides a I<symbolic> calculator indeed. |
1346 | |
1347 | To hide the rough edges under the hood, provide a tie()d interface to the |
1348 | package C<symbolic> (compare with L<Metaphor clash>). Add methods |
1349 | |
1350 | sub TIESCALAR { my $pack = shift; $pack->new(@_) } |
1351 | sub FETCH { shift } |
1352 | sub nop { } # Around a bug |
1353 | |
1354 | (the bug is described in L<"BUGS">). One can use this new interface as |
1355 | |
1356 | tie $a, 'symbolic', 3; |
1357 | tie $b, 'symbolic', 4; |
1358 | $a->nop; $b->nop; # Around a bug |
1359 | |
1360 | my $c = sqrt($a**2 + $b**2); |
1361 | |
1362 | Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value |
1363 | of $c becomes 13. To insulate the user of the module add a method |
1364 | |
1365 | sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; } |
1366 | |
1367 | Now |
1368 | |
1369 | my ($a, $b); |
1370 | symbolic->vars($a, $b); |
1371 | my $c = sqrt($a**2 + $b**2); |
1372 | |
1373 | $a = 3; $b = 4; |
1374 | printf "c5 %s=%f\n", $c, $c; |
1375 | |
1376 | $a = 12; $b = 5; |
1377 | printf "c13 %s=%f\n", $c, $c; |
1378 | |
1379 | shows that the numeric value of $c follows changes to the values of $a |
1380 | and $b. |
1381 | |
4633a7c4 |
1382 | =head1 AUTHOR |
1383 | |
1fef88e7 |
1384 | Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>. |
4633a7c4 |
1385 | |
1386 | =head1 DIAGNOSTICS |
1387 | |
1388 | When Perl is run with the B<-Do> switch or its equivalent, overloading |
1389 | induces diagnostic messages. |
1390 | |
e7ea3e70 |
1391 | Using the C<m> command of Perl debugger (see L<perldebug>) one can |
1392 | deduce which operations are overloaded (and which ancestor triggers |
1393 | this overloading). Say, if C<eq> is overloaded, then the method C<(eq> |
1394 | is shown by debugger. The method C<()> corresponds to the C<fallback> |
1395 | key (in fact a presence of this method shows that this package has |
1396 | overloading enabled, and it is what is used by the C<Overloaded> |
ee239bfe |
1397 | function of module C<overload>). |
e7ea3e70 |
1398 | |
6ad11d81 |
1399 | The module might issue the following warnings: |
6b82e2f5 |
1400 | |
1401 | =over 4 |
1402 | |
1403 | =item Odd number of arguments for overload::constant |
1404 | |
1405 | (W) The call to overload::constant contained an odd number of arguments. |
1406 | The arguments should come in pairs. |
1407 | |
1408 | =item `%s' is not an overloadable type |
1409 | |
1410 | (W) You tried to overload a constant type the overload package is unaware of. |
1411 | |
1412 | =item `%s' is not a code reference |
1413 | |
1414 | (W) The second (fourth, sixth, ...) argument of overload::constant needs |
1415 | to be a code reference. Either an anonymous subroutine, or a reference |
1416 | to a subroutine. |
1417 | |
1418 | =back |
1419 | |
4633a7c4 |
1420 | =head1 BUGS |
1421 | |
aa689395 |
1422 | Because it is used for overloading, the per-package hash %OVERLOAD now |
1423 | has a special meaning in Perl. The symbol table is filled with names |
1424 | looking like line-noise. |
4633a7c4 |
1425 | |
a6006777 |
1426 | For the purpose of inheritance every overloaded package behaves as if |
1427 | C<fallback> is present (possibly undefined). This may create |
1428 | interesting effects if some package is not overloaded, but inherits |
1429 | from two overloaded packages. |
4633a7c4 |
1430 | |
b267980d |
1431 | Relation between overloading and tie()ing is broken. Overloading is |
ee239bfe |
1432 | triggered or not basing on the I<previous> class of tie()d value. |
1433 | |
b267980d |
1434 | This happens because the presence of overloading is checked too early, |
ee239bfe |
1435 | before any tie()d access is attempted. If the FETCH()ed class of the |
b267980d |
1436 | tie()d value does not change, a simple workaround is to access the value |
ee239bfe |
1437 | immediately after tie()ing, so that after this call the I<previous> class |
1438 | coincides with the current one. |
1439 | |
1440 | B<Needed:> a way to fix this without a speed penalty. |
1441 | |
b3ac6de7 |
1442 | Barewords are not covered by overloaded string constants. |
1443 | |
ee239bfe |
1444 | This document is confusing. There are grammos and misleading language |
1445 | used in places. It would seem a total rewrite is needed. |
4633a7c4 |
1446 | |
1447 | =cut |
1448 | |