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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "Switch 3" | |
132 | .TH Switch 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | Switch \- A switch statement for Perl | |
135 | .SH "VERSION" | |
136 | .IX Header "VERSION" | |
137 | This document describes version 2.10 of Switch, | |
138 | released Dec 29, 2003. | |
139 | .SH "SYNOPSIS" | |
140 | .IX Header "SYNOPSIS" | |
141 | .Vb 1 | |
142 | \& use Switch; | |
143 | .Ve | |
144 | .PP | |
145 | .Vb 1 | |
146 | \& switch ($val) { | |
147 | .Ve | |
148 | .PP | |
149 | .Vb 11 | |
150 | \& case 1 { print "number 1" } | |
151 | \& case "a" { print "string a" } | |
152 | \& case [1..10,42] { print "number in list" } | |
153 | \& case (@array) { print "number in list" } | |
154 | \& case /\ew+/ { print "pattern" } | |
155 | \& case qr/\ew+/ { print "pattern" } | |
156 | \& case (%hash) { print "entry in hash" } | |
157 | \& case (\e%hash) { print "entry in hash" } | |
158 | \& case (\e&sub) { print "arg to subroutine" } | |
159 | \& else { print "previous case not true" } | |
160 | \& } | |
161 | .Ve | |
162 | .SH "BACKGROUND" | |
163 | .IX Header "BACKGROUND" | |
164 | [Skip ahead to \*(L"\s-1DESCRIPTION\s0\*(R" if you don't care about the whys | |
165 | and wherefores of this control structure] | |
166 | .PP | |
167 | In seeking to devise a \*(L"Swiss Army\*(R" case mechanism suitable for Perl, | |
168 | it is useful to generalize this notion of distributed conditional | |
169 | testing as far as possible. Specifically, the concept of \*(L"matching\*(R" | |
170 | between the switch value and the various case values need not be | |
171 | restricted to numeric (or string or referential) equality, as it is in other | |
172 | languages. Indeed, as Table 1 illustrates, Perl | |
173 | offers at least eighteen different ways in which two values could | |
174 | generate a match. | |
175 | .PP | |
176 | .Vb 1 | |
177 | \& Table 1: Matching a switch value ($s) with a case value ($c) | |
178 | .Ve | |
179 | .PP | |
180 | .Vb 3 | |
181 | \& Switch Case Type of Match Implied Matching Code | |
182 | \& Value Value | |
183 | \& ====== ===== ===================== ============= | |
184 | .Ve | |
185 | .PP | |
186 | .Vb 2 | |
187 | \& number same numeric or referential match if $s == $c; | |
188 | \& or ref equality | |
189 | .Ve | |
190 | .PP | |
191 | .Vb 3 | |
192 | \& object method result of method call match if $s->$c(); | |
193 | \& ref name match if defined $s->$c(); | |
194 | \& or ref | |
195 | .Ve | |
196 | .PP | |
197 | .Vb 3 | |
198 | \& other other string equality match if $s eq $c; | |
199 | \& non-ref non-ref | |
200 | \& scalar scalar | |
201 | .Ve | |
202 | .PP | |
203 | .Vb 1 | |
204 | \& string regexp pattern match match if $s =~ /$c/; | |
205 | .Ve | |
206 | .PP | |
207 | .Vb 3 | |
208 | \& array scalar array entry existence match if 0<=$c && $c<@$s; | |
209 | \& ref array entry definition match if defined $s->[$c]; | |
210 | \& array entry truth match if $s->[$c]; | |
211 | .Ve | |
212 | .PP | |
213 | .Vb 5 | |
214 | \& array array array intersection match if intersects(@$s, @$c); | |
215 | \& ref ref (apply this table to | |
216 | \& all pairs of elements | |
217 | \& $s->[$i] and | |
218 | \& $c->[$j]) | |
219 | .Ve | |
220 | .PP | |
221 | .Vb 2 | |
222 | \& array regexp array grep match if grep /$c/, @$s; | |
223 | \& ref | |
224 | .Ve | |
225 | .PP | |
226 | .Vb 3 | |
227 | \& hash scalar hash entry existence match if exists $s->{$c}; | |
228 | \& ref hash entry definition match if defined $s->{$c}; | |
229 | \& hash entry truth match if $s->{$c}; | |
230 | .Ve | |
231 | .PP | |
232 | .Vb 2 | |
233 | \& hash regexp hash grep match if grep /$c/, keys %$s; | |
234 | \& ref | |
235 | .Ve | |
236 | .PP | |
237 | .Vb 2 | |
238 | \& sub scalar return value defn match if defined $s->($c); | |
239 | \& ref return value truth match if $s->($c); | |
240 | .Ve | |
241 | .PP | |
242 | .Vb 2 | |
243 | \& sub array return value defn match if defined $s->(@$c); | |
244 | \& ref ref return value truth match if $s->(@$c); | |
245 | .Ve | |
246 | .PP | |
247 | In reality, Table 1 covers 31 alternatives, because only the equality and | |
248 | intersection tests are commutative; in all other cases, the roles of | |
249 | the \f(CW$s\fR and \f(CW$c\fR variables could be reversed to produce a | |
250 | different test. For example, instead of testing a single hash for | |
251 | the existence of a series of keys (\f(CW\*(C`match if exists $s\->{$c}\*(C'\fR), | |
252 | one could test for the existence of a single key in a series of hashes | |
253 | (\f(CW\*(C`match if exists $c\->{$s}\*(C'\fR). | |
254 | .PP | |
255 | As perltodo observes, a Perl case mechanism must support all these | |
256 | \&\*(L"ways to do it\*(R". | |
257 | .SH "DESCRIPTION" | |
258 | .IX Header "DESCRIPTION" | |
259 | The Switch.pm module implements a generalized case mechanism that covers | |
260 | the numerous possible combinations of switch and case values described above. | |
261 | .PP | |
262 | The module augments the standard Perl syntax with two new control | |
263 | statements: \f(CW\*(C`switch\*(C'\fR and \f(CW\*(C`case\*(C'\fR. The \f(CW\*(C`switch\*(C'\fR statement takes a | |
264 | single scalar argument of any type, specified in parentheses. | |
265 | \&\f(CW\*(C`switch\*(C'\fR stores this value as the | |
266 | current switch value in a (localized) control variable. | |
267 | The value is followed by a block which may contain one or more | |
268 | Perl statements (including the \f(CW\*(C`case\*(C'\fR statement described below). | |
269 | The block is unconditionally executed once the switch value has | |
270 | been cached. | |
271 | .PP | |
272 | A \f(CW\*(C`case\*(C'\fR statement takes a single scalar argument (in mandatory | |
273 | parentheses if it's a variable; otherwise the parens are optional) and | |
274 | selects the appropriate type of matching between that argument and the | |
275 | current switch value. The type of matching used is determined by the | |
276 | respective types of the switch value and the \f(CW\*(C`case\*(C'\fR argument, as | |
277 | specified in Table 1. If the match is successful, the mandatory | |
278 | block associated with the \f(CW\*(C`case\*(C'\fR statement is executed. | |
279 | .PP | |
280 | In most other respects, the \f(CW\*(C`case\*(C'\fR statement is semantically identical | |
281 | to an \f(CW\*(C`if\*(C'\fR statement. For example, it can be followed by an \f(CW\*(C`else\*(C'\fR | |
282 | clause, and can be used as a postfix statement qualifier. | |
283 | .PP | |
284 | However, when a \f(CW\*(C`case\*(C'\fR block has been executed control is automatically | |
285 | transferred to the statement after the immediately enclosing \f(CW\*(C`switch\*(C'\fR | |
286 | block, rather than to the next statement within the block. In other | |
287 | words, the success of any \f(CW\*(C`case\*(C'\fR statement prevents other cases in the | |
288 | same scope from executing. But see \*(L"Allowing fall\-through\*(R" below. | |
289 | .PP | |
290 | Together these two new statements provide a fully generalized case | |
291 | mechanism: | |
292 | .PP | |
293 | .Vb 1 | |
294 | \& use Switch; | |
295 | .Ve | |
296 | .PP | |
297 | .Vb 1 | |
298 | \& # AND LATER... | |
299 | .Ve | |
300 | .PP | |
301 | .Vb 1 | |
302 | \& %special = ( woohoo => 1, d'oh => 1 ); | |
303 | .Ve | |
304 | .PP | |
305 | .Vb 2 | |
306 | \& while (<>) { | |
307 | \& switch ($_) { | |
308 | .Ve | |
309 | .PP | |
310 | .Vb 3 | |
311 | \& case (%special) { print "homer\en"; } # if $special{$_} | |
312 | \& case /a-z/i { print "alpha\en"; } # if $_ =~ /a-z/i | |
313 | \& case [1..9] { print "small num\en"; } # if $_ in [1..9] | |
314 | .Ve | |
315 | .PP | |
316 | .Vb 3 | |
317 | \& case { $_[0] >= 10 } { # if $_ >= 10 | |
318 | \& my $age = <>; | |
319 | \& switch (sub{ $_[0] < $age } ) { | |
320 | .Ve | |
321 | .PP | |
322 | .Vb 5 | |
323 | \& case 20 { print "teens\en"; } # if 20 < $age | |
324 | \& case 30 { print "twenties\en"; } # if 30 < $age | |
325 | \& else { print "history\en"; } | |
326 | \& } | |
327 | \& } | |
328 | .Ve | |
329 | .PP | |
330 | .Vb 2 | |
331 | \& print "must be punctuation\en" case /\eW/; # if $_ ~= /\eW/ | |
332 | \& } | |
333 | .Ve | |
334 | .PP | |
335 | Note that \f(CW\*(C`switch\*(C'\fRes can be nested within \f(CW\*(C`case\*(C'\fR (or any other) blocks, | |
336 | and a series of \f(CW\*(C`case\*(C'\fR statements can try different types of matches | |
337 | \&\*(-- hash membership, pattern match, array intersection, simple equality, | |
338 | etc. \*(-- against the same switch value. | |
339 | .PP | |
340 | The use of intersection tests against an array reference is particularly | |
341 | useful for aggregating integral cases: | |
342 | .PP | |
343 | .Vb 8 | |
344 | \& sub classify_digit | |
345 | \& { | |
346 | \& switch ($_[0]) { case 0 { return 'zero' } | |
347 | \& case [2,4,6,8] { return 'even' } | |
348 | \& case [1,3,4,7,9] { return 'odd' } | |
349 | \& case /[A-F]/i { return 'hex' } | |
350 | \& } | |
351 | \& } | |
352 | .Ve | |
353 | .Sh "Allowing fall-through" | |
354 | .IX Subsection "Allowing fall-through" | |
355 | Fall-though (trying another case after one has already succeeded) | |
356 | is usually a Bad Idea in a switch statement. However, this | |
357 | is Perl, not a police state, so there \fIis\fR a way to do it, if you must. | |
358 | .PP | |
359 | If a \f(CW\*(C`case\*(C'\fR block executes an untargeted \f(CW\*(C`next\*(C'\fR, control is | |
360 | immediately transferred to the statement \fIafter\fR the \f(CW\*(C`case\*(C'\fR statement | |
361 | (i.e. usually another case), rather than out of the surrounding | |
362 | \&\f(CW\*(C`switch\*(C'\fR block. | |
363 | .PP | |
364 | For example: | |
365 | .PP | |
366 | .Vb 7 | |
367 | \& switch ($val) { | |
368 | \& case 1 { handle_num_1(); next } # and try next case... | |
369 | \& case "1" { handle_str_1(); next } # and try next case... | |
370 | \& case [0..9] { handle_num_any(); } # and we're done | |
371 | \& case /\ed/ { handle_dig_any(); next } # and try next case... | |
372 | \& case /.*/ { handle_str_any(); next } # and try next case... | |
373 | \& } | |
374 | .Ve | |
375 | .PP | |
376 | If \f(CW$val\fR held the number \f(CW1\fR, the above \f(CW\*(C`switch\*(C'\fR block would call the | |
377 | first three \f(CW\*(C`handle_...\*(C'\fR subroutines, jumping to the next case test | |
378 | each time it encountered a \f(CW\*(C`next\*(C'\fR. After the thrid \f(CW\*(C`case\*(C'\fR block | |
379 | was executed, control would jump to the end of the enclosing | |
380 | \&\f(CW\*(C`switch\*(C'\fR block. | |
381 | .PP | |
382 | On the other hand, if \f(CW$val\fR held \f(CW10\fR, then only the last two \f(CW\*(C`handle_...\*(C'\fR | |
383 | subroutines would be called. | |
384 | .PP | |
385 | Note that this mechanism allows the notion of \fIconditional fall-through\fR. | |
386 | For example: | |
387 | .PP | |
388 | .Vb 4 | |
389 | \& switch ($val) { | |
390 | \& case [0..9] { handle_num_any(); next if $val < 7; } | |
391 | \& case /\ed/ { handle_dig_any(); } | |
392 | \& } | |
393 | .Ve | |
394 | .PP | |
395 | If an untargeted \f(CW\*(C`last\*(C'\fR statement is executed in a case block, this | |
396 | immediately transfers control out of the enclosing \f(CW\*(C`switch\*(C'\fR block | |
397 | (in other words, there is an implicit \f(CW\*(C`last\*(C'\fR at the end of each | |
398 | normal \f(CW\*(C`case\*(C'\fR block). Thus the previous example could also have been | |
399 | written: | |
400 | .PP | |
401 | .Vb 4 | |
402 | \& switch ($val) { | |
403 | \& case [0..9] { handle_num_any(); last if $val >= 7; next; } | |
404 | \& case /\ed/ { handle_dig_any(); } | |
405 | \& } | |
406 | .Ve | |
407 | .Sh "Automating fall-through" | |
408 | .IX Subsection "Automating fall-through" | |
409 | In situations where case fall-through should be the norm, rather than an | |
410 | exception, an endless succession of terminal \f(CW\*(C`next\*(C'\fRs is tedious and ugly. | |
411 | Hence, it is possible to reverse the default behaviour by specifying | |
412 | the string \*(L"fallthrough\*(R" when importing the module. For example, the | |
413 | following code is equivalent to the first example in \*(L"Allowing fall\-through\*(R": | |
414 | .PP | |
415 | .Vb 1 | |
416 | \& use Switch 'fallthrough'; | |
417 | .Ve | |
418 | .PP | |
419 | .Vb 7 | |
420 | \& switch ($val) { | |
421 | \& case 1 { handle_num_1(); } | |
422 | \& case "1" { handle_str_1(); } | |
423 | \& case [0..9] { handle_num_any(); last } | |
424 | \& case /\ed/ { handle_dig_any(); } | |
425 | \& case /.*/ { handle_str_any(); } | |
426 | \& } | |
427 | .Ve | |
428 | .PP | |
429 | Note the explicit use of a \f(CW\*(C`last\*(C'\fR to preserve the non-fall-through | |
430 | behaviour of the third case. | |
431 | .Sh "Alternative syntax" | |
432 | .IX Subsection "Alternative syntax" | |
433 | Perl 6 will provide a built-in switch statement with essentially the | |
434 | same semantics as those offered by Switch.pm, but with a different | |
435 | pair of keywords. In Perl 6 \f(CW\*(C`switch\*(C'\fR will be spelled \f(CW\*(C`given\*(C'\fR, and | |
436 | \&\f(CW\*(C`case\*(C'\fR will be pronounced \f(CW\*(C`when\*(C'\fR. In addition, the \f(CW\*(C`when\*(C'\fR statement | |
437 | will not require switch or case values to be parenthesized. | |
438 | .PP | |
439 | This future syntax is also (largely) available via the Switch.pm module, by | |
440 | importing it with the argument \f(CW"Perl6"\fR. For example: | |
441 | .PP | |
442 | .Vb 1 | |
443 | \& use Switch 'Perl6'; | |
444 | .Ve | |
445 | .PP | |
446 | .Vb 8 | |
447 | \& given ($val) { | |
448 | \& when 1 { handle_num_1(); } | |
449 | \& when ($str1) { handle_str_1(); } | |
450 | \& when [0..9] { handle_num_any(); last } | |
451 | \& when /\ed/ { handle_dig_any(); } | |
452 | \& when /.*/ { handle_str_any(); } | |
453 | \& default { handle anything else; } | |
454 | \& } | |
455 | .Ve | |
456 | .PP | |
457 | Note that scalars still need to be parenthesized, since they would be | |
458 | ambiguous in Perl 5. | |
459 | .PP | |
460 | Note too that you can mix and match both syntaxes by importing the module | |
461 | with: | |
462 | .PP | |
463 | .Vb 1 | |
464 | \& use Switch 'Perl5', 'Perl6'; | |
465 | .Ve | |
466 | .Sh "Higher-order Operations" | |
467 | .IX Subsection "Higher-order Operations" | |
468 | One situation in which \f(CW\*(C`switch\*(C'\fR and \f(CW\*(C`case\*(C'\fR do not provide a good | |
469 | substitute for a cascaded \f(CW\*(C`if\*(C'\fR, is where a switch value needs to | |
470 | be tested against a series of conditions. For example: | |
471 | .PP | |
472 | .Vb 2 | |
473 | \& sub beverage { | |
474 | \& switch (shift) { | |
475 | .Ve | |
476 | .PP | |
477 | .Vb 9 | |
478 | \& case sub { $_[0] < 10 } { return 'milk' } | |
479 | \& case sub { $_[0] < 20 } { return 'coke' } | |
480 | \& case sub { $_[0] < 30 } { return 'beer' } | |
481 | \& case sub { $_[0] < 40 } { return 'wine' } | |
482 | \& case sub { $_[0] < 50 } { return 'malt' } | |
483 | \& case sub { $_[0] < 60 } { return 'Moet' } | |
484 | \& else { return 'milk' } | |
485 | \& } | |
486 | \& } | |
487 | .Ve | |
488 | .PP | |
489 | The need to specify each condition as a subroutine block is tiresome. To | |
490 | overcome this, when importing Switch.pm, a special \*(L"placeholder\*(R" | |
491 | subroutine named \f(CW\*(C`_\|_\*(C'\fR [sic] may also be imported. This subroutine | |
492 | converts (almost) any expression in which it appears to a reference to a | |
493 | higher-order function. That is, the expression: | |
494 | .PP | |
495 | .Vb 1 | |
496 | \& use Switch '__'; | |
497 | .Ve | |
498 | .PP | |
499 | .Vb 1 | |
500 | \& __ < 2 + __ | |
501 | .Ve | |
502 | .PP | |
503 | is equivalent to: | |
504 | .PP | |
505 | .Vb 1 | |
506 | \& sub { $_[0] < 2 + $_[1] } | |
507 | .Ve | |
508 | .PP | |
509 | With \f(CW\*(C`_\|_\*(C'\fR, the previous ugly case statements can be rewritten: | |
510 | .PP | |
511 | .Vb 7 | |
512 | \& case __ < 10 { return 'milk' } | |
513 | \& case __ < 20 { return 'coke' } | |
514 | \& case __ < 30 { return 'beer' } | |
515 | \& case __ < 40 { return 'wine' } | |
516 | \& case __ < 50 { return 'malt' } | |
517 | \& case __ < 60 { return 'Moet' } | |
518 | \& else { return 'milk' } | |
519 | .Ve | |
520 | .PP | |
521 | The \f(CW\*(C`_\|_\*(C'\fR subroutine makes extensive use of operator overloading to | |
522 | perform its magic. All operations involving _\|_ are overloaded to | |
523 | produce an anonymous subroutine that implements a lazy version | |
524 | of the original operation. | |
525 | .PP | |
526 | The only problem is that operator overloading does not allow the | |
527 | boolean operators \f(CW\*(C`&&\*(C'\fR and \f(CW\*(C`||\*(C'\fR to be overloaded. So a case statement | |
528 | like this: | |
529 | .PP | |
530 | .Vb 1 | |
531 | \& case 0 <= __ && __ < 10 { return 'digit' } | |
532 | .Ve | |
533 | .PP | |
534 | doesn't act as expected, because when it is | |
535 | executed, it constructs two higher order subroutines | |
536 | and then treats the two resulting references as arguments to \f(CW\*(C`&&\*(C'\fR: | |
537 | .PP | |
538 | .Vb 1 | |
539 | \& sub { 0 <= $_[0] } && sub { $_[0] < 10 } | |
540 | .Ve | |
541 | .PP | |
542 | This boolean expression is inevitably true, since both references are | |
543 | non\-false. Fortunately, the overloaded \f(CW'bool'\fR operator catches this | |
544 | situation and flags it as a error. | |
545 | .SH "DEPENDENCIES" | |
546 | .IX Header "DEPENDENCIES" | |
547 | The module is implemented using Filter::Util::Call and Text::Balanced | |
548 | and requires both these modules to be installed. | |
549 | .SH "AUTHOR" | |
550 | .IX Header "AUTHOR" | |
551 | Damian Conway (damian@conway.org). The maintainer of this module is now Rafael | |
552 | Garcia-Suarez (rgarciasuarez@free.fr). | |
553 | .SH "BUGS" | |
554 | .IX Header "BUGS" | |
555 | There are undoubtedly serious bugs lurking somewhere in code this funky :\-) | |
556 | Bug reports and other feedback are most welcome. | |
557 | .SH "LIMITATIONS" | |
558 | .IX Header "LIMITATIONS" | |
559 | Due to the heuristic nature of Switch.pm's source parsing, the presence | |
560 | of regexes specified with raw \f(CW\*(C`?...?\*(C'\fR delimiters may cause mysterious | |
561 | errors. The workaround is to use \f(CW\*(C`m?...?\*(C'\fR instead. | |
562 | .PP | |
563 | Due to the way source filters work in Perl, you can't use Switch inside | |
564 | an string \f(CW\*(C`eval\*(C'\fR. | |
565 | .PP | |
566 | If your source file is longer then 1 million characters and you have a | |
567 | switch statement that crosses the 1 million (or 2 million, etc.) | |
568 | character boundary you will get mysterious errors. The workaround is to | |
569 | use smaller source files. | |
570 | .SH "COPYRIGHT" | |
571 | .IX Header "COPYRIGHT" | |
572 | .Vb 3 | |
573 | \& Copyright (c) 1997-2003, Damian Conway. All Rights Reserved. | |
574 | \& This module is free software. It may be used, redistributed | |
575 | \& and/or modified under the same terms as Perl itself. | |
576 | .Ve |