Commit | Line | Data |
---|---|---|
3ed9f206 JH |
1 | package Switch; |
2 | ||
3 | use strict; | |
4 | use vars qw($VERSION); | |
5 | use Carp; | |
6 | ||
a1813bef | 7 | $VERSION = '2.03'; |
3ed9f206 JH |
8 | |
9 | ||
10 | # LOAD FILTERING MODULE... | |
11 | use Filter::Util::Call; | |
12 | ||
13 | sub __(); | |
14 | ||
15 | # CATCH ATTEMPTS TO CALL case OUTSIDE THE SCOPE OF ANY switch | |
16 | ||
17 | $::_S_W_I_T_C_H = sub { croak "case statement not in switch block" }; | |
18 | ||
19 | my $offset; | |
20 | my $fallthrough; | |
3ed9f206 JH |
21 | |
22 | sub import | |
23 | { | |
3961318e | 24 | $DB::single = 1; |
3ed9f206 JH |
25 | $fallthrough = grep /\bfallthrough\b/, @_; |
26 | $offset = (caller)[2]+1; | |
a1813bef | 27 | filter_add({}) unless @_>1 && $_[1] eq 'noimport'; |
3ed9f206 JH |
28 | my $pkg = caller; |
29 | no strict 'refs'; | |
30 | for ( qw( on_defined on_exists ) ) | |
31 | { | |
32 | *{"${pkg}::$_"} = \&$_; | |
33 | } | |
34 | *{"${pkg}::__"} = \&__ if grep /__/, @_; | |
35 | 1; | |
36 | } | |
37 | ||
38 | sub unimport | |
39 | { | |
40 | filter_del() | |
41 | } | |
42 | ||
43 | sub filter | |
44 | { | |
45 | my($self) = @_ ; | |
46 | local $Switch::file = (caller)[1]; | |
47 | ||
48 | my $status = 1; | |
49 | $status = filter_read(10_000); | |
50 | return $status if $status<0; | |
51 | $_ = filter_blocks($_,$offset); | |
52 | $_ = "# line $offset\n" . $_ if $offset; undef $offset; | |
53 | # print STDERR $_; | |
54 | return $status; | |
55 | } | |
56 | ||
57 | use Text::Balanced ':ALL'; | |
58 | ||
59 | sub line | |
60 | { | |
61 | my ($pretext,$offset) = @_; | |
62 | ($pretext=~tr/\n/\n/)+$offset, | |
63 | } | |
64 | ||
65 | sub is_block | |
66 | { | |
67 | local $SIG{__WARN__}=sub{die$@}; | |
68 | local $^W=1; | |
69 | my $ishash = defined eval 'my $hr='.$_[0]; | |
70 | undef $@; | |
71 | return !$ishash; | |
72 | } | |
73 | ||
74 | my $casecounter = 1; | |
75 | sub filter_blocks | |
76 | { | |
77 | my ($source, $line) = @_; | |
78 | return $source unless $source =~ /case|switch/; | |
79 | pos $source = 0; | |
80 | my $text = ""; | |
81 | component: while (pos $source < length $source) | |
82 | { | |
3961318e | 83 | if ($source =~ m/(\G\s*use\s+Switch\b)/gc) |
3ed9f206 JH |
84 | { |
85 | $text .= q{use Switch 'noimport'}; | |
86 | next component; | |
87 | } | |
88 | my @pos = Text::Balanced::_match_quotelike(\$source,qr/\s*/,1,1); | |
89 | if (defined $pos[0]) | |
90 | { | |
3961318e | 91 | $text .= " " . substr($source,$pos[2],$pos[18]-$pos[2]); |
3ed9f206 JH |
92 | next component; |
93 | } | |
94 | @pos = Text::Balanced::_match_variable(\$source,qr/\s*/); | |
95 | if (defined $pos[0]) | |
96 | { | |
3961318e | 97 | $text .= " " . substr($source,$pos[0],$pos[4]-$pos[0]); |
3ed9f206 JH |
98 | next component; |
99 | } | |
100 | ||
101 | if ($source =~ m/\G(\n*)(\s*)switch\b(?=\s*[(])/gc) | |
102 | { | |
103 | $text .= $1.$2.'S_W_I_T_C_H: while (1) '; | |
3961318e | 104 | @pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\(/,qr/\)/,qr/[[{(<]/,qr/[]})>]/,undef) |
3ed9f206 JH |
105 | or do { |
106 | die "Bad switch statement (problem in the parentheses?) near $Switch::file line ", line(substr($source,0,pos $source),$line), "\n"; | |
107 | }; | |
108 | my $arg = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); | |
109 | $arg =~ s {^\s*[(]\s*%} { ( \\\%} || | |
110 | $arg =~ s {^\s*[(]\s*m\b} { ( qr} || | |
111 | $arg =~ s {^\s*[(]\s*/} { ( qr/} || | |
112 | $arg =~ s {^\s*[(]\s*qw} { ( \\qw}; | |
113 | @pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef) | |
114 | or do { | |
115 | die "Bad switch statement (problem in the code block?) near $Switch::file line ", line(substr($source,0, pos $source), $line), "\n"; | |
116 | }; | |
117 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); | |
118 | $code =~ s/{/{ local \$::_S_W_I_T_C_H; Switch::switch $arg;/; | |
119 | $text .= $code . 'continue {last}'; | |
120 | next component; | |
121 | } | |
122 | elsif ($source =~ m/\G(\s*)(case\b)(?!\s*=>)/gc) | |
123 | { | |
124 | $text .= $1."if (Switch::case"; | |
125 | if (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)) { | |
126 | my $code = substr($source,$pos[0],$pos[4]-$pos[0]); | |
127 | $text .= " sub" if is_block $code; | |
128 | $text .= " " . filter_blocks($code,line(substr($source,0,$pos[0]),$line)) . ")"; | |
129 | } | |
130 | elsif (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/[[(]/,qr/[])]/,qr/[[({]/,qr/[])}]/,undef)) { | |
131 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); | |
132 | $code =~ s {^\s*[(]\s*%} { ( \\\%} || | |
133 | $code =~ s {^\s*[(]\s*m\b} { ( qr} || | |
134 | $code =~ s {^\s*[(]\s*/} { ( qr/} || | |
135 | $code =~ s {^\s*[(]\s*qw} { ( \\qw}; | |
136 | $text .= " $code)"; | |
137 | } | |
138 | elsif ( @pos = Text::Balanced::_match_quotelike(\$source,qr/\s*/,1,1)) { | |
139 | my $code = substr($source,$pos[2],$pos[18]-$pos[2]); | |
140 | $code = filter_blocks($code,line(substr($source,0,$pos[2]),$line)); | |
141 | $code =~ s {^\s*m} { qr} || | |
142 | $code =~ s {^\s*/} { qr/} || | |
143 | $code =~ s {^\s*qw} { \\qw}; | |
144 | $text .= " $code)"; | |
145 | } | |
146 | elsif ($source =~ m/\G\s*(([^\$\@{])[^\$\@{]*)(?=\s*{)/gc) { | |
147 | my $code = filter_blocks($1,line(substr($source,0,pos $source),$line)); | |
148 | $text .= ' \\' if $2 eq '%'; | |
149 | $text .= " $code)"; | |
150 | } | |
151 | else { | |
152 | die "Bad case statement (invalid case value?) near $Switch::file line ", line(substr($source,0,pos $source), $line), "\n"; | |
153 | } | |
154 | ||
155 | @pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef) | |
156 | or do { | |
157 | if ($source =~ m/\G\s*(?=([};]|\Z))/gc) { | |
158 | $casecounter++; | |
159 | next component; | |
160 | } | |
161 | die "Bad case statement (problem in the code block?) near $Switch::file line ", line(substr($source,0,pos $source),$line), "\n"; | |
162 | }; | |
163 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); | |
164 | $code =~ s/}(?=\s*\Z)/;last S_W_I_T_C_H }/ | |
165 | unless $fallthrough; | |
166 | $text .= "{ while (1) $code continue { goto C_A_S_E_$casecounter } last S_W_I_T_C_H; C_A_S_E_$casecounter: }"; | |
167 | $casecounter++; | |
168 | next component; | |
169 | } | |
170 | ||
171 | $source =~ m/\G(\s*(\w+|#.*\n|\W))/gc; | |
172 | $text .= $1; | |
173 | } | |
174 | $text; | |
175 | } | |
176 | ||
177 | ||
178 | ||
179 | sub in | |
180 | { | |
181 | my ($x,$y) = @_; | |
182 | my @numy; | |
183 | for my $nextx ( @$x ) | |
184 | { | |
a1813bef | 185 | my $numx = ref($nextx) || defined $nextx && (~$nextx&$nextx) eq 0; |
3ed9f206 JH |
186 | for my $j ( 0..$#$y ) |
187 | { | |
188 | my $nexty = $y->[$j]; | |
a1813bef | 189 | push @numy, ref($nexty) || defined $nexty && (~$nexty&$nexty) eq 0 |
3ed9f206 JH |
190 | if @numy <= $j; |
191 | return 1 if $numx && $numy[$j] && $nextx==$nexty | |
192 | || $nextx eq $nexty; | |
193 | ||
194 | } | |
195 | } | |
196 | return ""; | |
197 | } | |
198 | ||
199 | sub on_exists | |
200 | { | |
201 | my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ }; | |
202 | [ keys %$ref ] | |
203 | } | |
204 | ||
205 | sub on_defined | |
206 | { | |
207 | my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ }; | |
208 | [ grep { defined $ref->{$_} } keys %$ref ] | |
209 | } | |
210 | ||
211 | sub switch(;$) | |
212 | { | |
213 | my ($s_val) = @_ ? $_[0] : $_; | |
214 | my $s_ref = ref $s_val; | |
215 | ||
216 | if ($s_ref eq 'CODE') | |
217 | { | |
218 | $::_S_W_I_T_C_H = | |
219 | sub { my $c_val = $_[0]; | |
220 | return $s_val == $c_val if ref $c_val eq 'CODE'; | |
221 | return $s_val->(@$c_val) if ref $c_val eq 'ARRAY'; | |
222 | return $s_val->($c_val); | |
223 | }; | |
224 | } | |
a1813bef | 225 | elsif ($s_ref eq "" && defined $s_val && (~$s_val&$s_val) eq 0) # NUMERIC SCALAR |
3ed9f206 JH |
226 | { |
227 | $::_S_W_I_T_C_H = | |
228 | sub { my $c_val = $_[0]; | |
229 | my $c_ref = ref $c_val; | |
230 | return $s_val == $c_val if $c_ref eq "" | |
a1813bef | 231 | && defined $c_val |
3ed9f206 JH |
232 | && (~$c_val&$c_val) eq 0; |
233 | return $s_val eq $c_val if $c_ref eq ""; | |
234 | return in([$s_val],$c_val) if $c_ref eq 'ARRAY'; | |
235 | return $c_val->($s_val) if $c_ref eq 'CODE'; | |
236 | return $c_val->call($s_val) if $c_ref eq 'Switch'; | |
237 | return scalar $s_val=~/$c_val/ | |
238 | if $c_ref eq 'Regexp'; | |
239 | return scalar $c_val->{$s_val} | |
240 | if $c_ref eq 'HASH'; | |
241 | return; | |
242 | }; | |
243 | } | |
244 | elsif ($s_ref eq "") # STRING SCALAR | |
245 | { | |
246 | $::_S_W_I_T_C_H = | |
247 | sub { my $c_val = $_[0]; | |
248 | my $c_ref = ref $c_val; | |
249 | return $s_val eq $c_val if $c_ref eq ""; | |
250 | return in([$s_val],$c_val) if $c_ref eq 'ARRAY'; | |
251 | return $c_val->($s_val) if $c_ref eq 'CODE'; | |
252 | return $c_val->call($s_val) if $c_ref eq 'Switch'; | |
253 | return scalar $s_val=~/$c_val/ | |
254 | if $c_ref eq 'Regexp'; | |
255 | return scalar $c_val->{$s_val} | |
256 | if $c_ref eq 'HASH'; | |
257 | return; | |
258 | }; | |
259 | } | |
260 | elsif ($s_ref eq 'ARRAY') | |
261 | { | |
262 | $::_S_W_I_T_C_H = | |
263 | sub { my $c_val = $_[0]; | |
264 | my $c_ref = ref $c_val; | |
265 | return in($s_val,[$c_val]) if $c_ref eq ""; | |
266 | return in($s_val,$c_val) if $c_ref eq 'ARRAY'; | |
267 | return $c_val->(@$s_val) if $c_ref eq 'CODE'; | |
268 | return $c_val->call(@$s_val) | |
269 | if $c_ref eq 'Switch'; | |
270 | return scalar grep {$_=~/$c_val/} @$s_val | |
271 | if $c_ref eq 'Regexp'; | |
272 | return scalar grep {$c_val->{$_}} @$s_val | |
273 | if $c_ref eq 'HASH'; | |
274 | return; | |
275 | }; | |
276 | } | |
277 | elsif ($s_ref eq 'Regexp') | |
278 | { | |
279 | $::_S_W_I_T_C_H = | |
280 | sub { my $c_val = $_[0]; | |
281 | my $c_ref = ref $c_val; | |
282 | return $c_val=~/s_val/ if $c_ref eq ""; | |
283 | return scalar grep {$_=~/s_val/} @$c_val | |
284 | if $c_ref eq 'ARRAY'; | |
285 | return $c_val->($s_val) if $c_ref eq 'CODE'; | |
286 | return $c_val->call($s_val) if $c_ref eq 'Switch'; | |
287 | return $s_val eq $c_val if $c_ref eq 'Regexp'; | |
288 | return grep {$_=~/$s_val/ && $c_val->{$_}} keys %$c_val | |
289 | if $c_ref eq 'HASH'; | |
290 | return; | |
291 | }; | |
292 | } | |
293 | elsif ($s_ref eq 'HASH') | |
294 | { | |
295 | $::_S_W_I_T_C_H = | |
296 | sub { my $c_val = $_[0]; | |
297 | my $c_ref = ref $c_val; | |
298 | return $s_val->{$c_val} if $c_ref eq ""; | |
299 | return scalar grep {$s_val->{$_}} @$c_val | |
300 | if $c_ref eq 'ARRAY'; | |
301 | return $c_val->($s_val) if $c_ref eq 'CODE'; | |
302 | return $c_val->call($s_val) if $c_ref eq 'Switch'; | |
303 | return grep {$_=~/$c_val/ && $s_val->{"$_"}} keys %$s_val | |
304 | if $c_ref eq 'Regexp'; | |
305 | return $s_val==$c_val if $c_ref eq 'HASH'; | |
306 | return; | |
307 | }; | |
308 | } | |
309 | elsif ($s_ref eq 'Switch') | |
310 | { | |
311 | $::_S_W_I_T_C_H = | |
312 | sub { my $c_val = $_[0]; | |
313 | return $s_val == $c_val if ref $c_val eq 'Switch'; | |
314 | return $s_val->call(@$c_val) | |
315 | if ref $c_val eq 'ARRAY'; | |
316 | return $s_val->call($c_val); | |
317 | }; | |
318 | } | |
319 | else | |
320 | { | |
321 | croak "Cannot switch on $s_ref"; | |
322 | } | |
323 | return 1; | |
324 | } | |
325 | ||
326 | sub case($) { $::_S_W_I_T_C_H->(@_); } | |
327 | ||
328 | # IMPLEMENT __ | |
329 | ||
330 | my $placeholder = bless { arity=>1, impl=>sub{$_[1+$_[0]]} }; | |
331 | ||
332 | sub __() { $placeholder } | |
333 | ||
334 | sub __arg($) | |
335 | { | |
336 | my $index = $_[0]+1; | |
337 | bless { arity=>0, impl=>sub{$_[$index]} }; | |
338 | } | |
339 | ||
340 | sub hosub(&@) | |
341 | { | |
342 | # WRITE THIS | |
343 | } | |
344 | ||
345 | sub call | |
346 | { | |
347 | my ($self,@args) = @_; | |
348 | return $self->{impl}->(0,@args); | |
349 | } | |
350 | ||
351 | sub meta_bop(&) | |
352 | { | |
353 | my ($op) = @_; | |
354 | sub | |
355 | { | |
356 | my ($left, $right, $reversed) = @_; | |
357 | ($right,$left) = @_ if $reversed; | |
358 | ||
359 | my $rop = ref $right eq 'Switch' | |
360 | ? $right | |
361 | : bless { arity=>0, impl=>sub{$right} }; | |
362 | ||
363 | my $lop = ref $left eq 'Switch' | |
364 | ? $left | |
365 | : bless { arity=>0, impl=>sub{$left} }; | |
366 | ||
367 | my $arity = $lop->{arity} + $rop->{arity}; | |
368 | ||
369 | return bless { | |
370 | arity => $arity, | |
371 | impl => sub { my $start = shift; | |
372 | return $op->($lop->{impl}->($start,@_), | |
373 | $rop->{impl}->($start+$lop->{arity},@_)); | |
374 | } | |
375 | }; | |
376 | }; | |
377 | } | |
378 | ||
379 | sub meta_uop(&) | |
380 | { | |
381 | my ($op) = @_; | |
382 | sub | |
383 | { | |
384 | my ($left) = @_; | |
385 | ||
386 | my $lop = ref $left eq 'Switch' | |
387 | ? $left | |
388 | : bless { arity=>0, impl=>sub{$left} }; | |
389 | ||
390 | my $arity = $lop->{arity}; | |
391 | ||
392 | return bless { | |
393 | arity => $arity, | |
394 | impl => sub { $op->($lop->{impl}->(@_)) } | |
395 | }; | |
396 | }; | |
397 | } | |
398 | ||
399 | ||
400 | use overload | |
401 | "+" => meta_bop {$_[0] + $_[1]}, | |
402 | "-" => meta_bop {$_[0] - $_[1]}, | |
403 | "*" => meta_bop {$_[0] * $_[1]}, | |
404 | "/" => meta_bop {$_[0] / $_[1]}, | |
405 | "%" => meta_bop {$_[0] % $_[1]}, | |
406 | "**" => meta_bop {$_[0] ** $_[1]}, | |
407 | "<<" => meta_bop {$_[0] << $_[1]}, | |
408 | ">>" => meta_bop {$_[0] >> $_[1]}, | |
409 | "x" => meta_bop {$_[0] x $_[1]}, | |
410 | "." => meta_bop {$_[0] . $_[1]}, | |
411 | "<" => meta_bop {$_[0] < $_[1]}, | |
412 | "<=" => meta_bop {$_[0] <= $_[1]}, | |
413 | ">" => meta_bop {$_[0] > $_[1]}, | |
414 | ">=" => meta_bop {$_[0] >= $_[1]}, | |
415 | "==" => meta_bop {$_[0] == $_[1]}, | |
416 | "!=" => meta_bop {$_[0] != $_[1]}, | |
417 | "<=>" => meta_bop {$_[0] <=> $_[1]}, | |
418 | "lt" => meta_bop {$_[0] lt $_[1]}, | |
419 | "le" => meta_bop {$_[0] le $_[1]}, | |
420 | "gt" => meta_bop {$_[0] gt $_[1]}, | |
421 | "ge" => meta_bop {$_[0] ge $_[1]}, | |
422 | "eq" => meta_bop {$_[0] eq $_[1]}, | |
423 | "ne" => meta_bop {$_[0] ne $_[1]}, | |
424 | "cmp" => meta_bop {$_[0] cmp $_[1]}, | |
425 | "\&" => meta_bop {$_[0] & $_[1]}, | |
426 | "^" => meta_bop {$_[0] ^ $_[1]}, | |
427 | "|" => meta_bop {$_[0] | $_[1]}, | |
428 | "atan2" => meta_bop {atan2 $_[0], $_[1]}, | |
429 | ||
430 | "neg" => meta_uop {-$_[0]}, | |
431 | "!" => meta_uop {!$_[0]}, | |
432 | "~" => meta_uop {~$_[0]}, | |
433 | "cos" => meta_uop {cos $_[0]}, | |
434 | "sin" => meta_uop {sin $_[0]}, | |
435 | "exp" => meta_uop {exp $_[0]}, | |
436 | "abs" => meta_uop {abs $_[0]}, | |
437 | "log" => meta_uop {log $_[0]}, | |
438 | "sqrt" => meta_uop {sqrt $_[0]}, | |
439 | "bool" => sub { croak "Can't use && or || in expression containing __" }, | |
440 | ||
441 | # "&()" => sub { $_[0]->{impl} }, | |
442 | ||
443 | # "||" => meta_bop {$_[0] || $_[1]}, | |
444 | # "&&" => meta_bop {$_[0] && $_[1]}, | |
445 | # fallback => 1, | |
446 | ; | |
447 | 1; | |
448 | ||
449 | __END__ | |
450 | ||
451 | ||
452 | =head1 NAME | |
453 | ||
454 | Switch - A switch statement for Perl | |
455 | ||
456 | =head1 VERSION | |
457 | ||
a1813bef JH |
458 | This document describes version 2.03 of Switch, |
459 | released May 15, 2001. | |
3ed9f206 JH |
460 | |
461 | =head1 SYNOPSIS | |
462 | ||
463 | use Switch; | |
464 | ||
465 | switch ($val) { | |
466 | ||
467 | case 1 { print "number 1" } | |
468 | case "a" { print "string a" } | |
469 | case [1..10,42] { print "number in list" } | |
470 | case (@array) { print "number in list" } | |
471 | case /\w+/ { print "pattern" } | |
472 | case qr/\w+/ { print "pattern" } | |
473 | case (%hash) { print "entry in hash" } | |
474 | case (\%hash) { print "entry in hash" } | |
475 | case (\&sub) { print "arg to subroutine" } | |
476 | else { print "previous case not true" } | |
477 | } | |
478 | ||
479 | =head1 BACKGROUND | |
480 | ||
481 | [Skip ahead to L<"DESCRIPTION"> if you don't care about the whys | |
482 | and wherefores of this control structure] | |
483 | ||
484 | In seeking to devise a "Swiss Army" case mechanism suitable for Perl, | |
485 | it is useful to generalize this notion of distributed conditional | |
486 | testing as far as possible. Specifically, the concept of "matching" | |
487 | between the switch value and the various case values need not be | |
488 | restricted to numeric (or string or referential) equality, as it is in other | |
489 | languages. Indeed, as Table 1 illustrates, Perl | |
490 | offers at least eighteen different ways in which two values could | |
491 | generate a match. | |
492 | ||
493 | Table 1: Matching a switch value ($s) with a case value ($c) | |
494 | ||
495 | Switch Case Type of Match Implied Matching Code | |
496 | Value Value | |
497 | ====== ===== ===================== ============= | |
498 | ||
499 | number same numeric or referential match if $s == $c; | |
500 | or ref equality | |
501 | ||
502 | object method result of method call match if $s->$c(); | |
503 | ref name match if defined $s->$c(); | |
504 | or ref | |
505 | ||
506 | other other string equality match if $s eq $c; | |
507 | non-ref non-ref | |
508 | scalar scalar | |
509 | ||
510 | string regexp pattern match match if $s =~ /$c/; | |
511 | ||
512 | array scalar array entry existence match if 0<=$c && $c<@$s; | |
513 | ref array entry definition match if defined $s->[$c]; | |
514 | array entry truth match if $s->[$c]; | |
515 | ||
516 | array array array intersection match if intersects(@$s, @$c); | |
517 | ref ref (apply this table to | |
518 | all pairs of elements | |
519 | $s->[$i] and | |
520 | $c->[$j]) | |
521 | ||
522 | array regexp array grep match if grep /$c/, @$s; | |
523 | ref | |
524 | ||
525 | hash scalar hash entry existence match if exists $s->{$c}; | |
526 | ref hash entry definition match if defined $s->{$c}; | |
527 | hash entry truth match if $s->{$c}; | |
528 | ||
529 | hash regexp hash grep match if grep /$c/, keys %$s; | |
530 | ref | |
531 | ||
532 | sub scalar return value defn match if defined $s->($c); | |
533 | ref return value truth match if $s->($c); | |
534 | ||
535 | sub array return value defn match if defined $s->(@$c); | |
536 | ref ref return value truth match if $s->(@$c); | |
537 | ||
538 | ||
539 | In reality, Table 1 covers 31 alternatives, because only the equality and | |
540 | intersection tests are commutative; in all other cases, the roles of | |
541 | the C<$s> and C<$c> variables could be reversed to produce a | |
542 | different test. For example, instead of testing a single hash for | |
543 | the existence of a series of keys (C<match if exists $s-E<gt>{$c}>), | |
544 | one could test for the existence of a single key in a series of hashes | |
545 | (C<match if exists $c-E<gt>{$s}>). | |
546 | ||
547 | As L<perltodo> observes, a Perl case mechanism must support all these | |
548 | "ways to do it". | |
549 | ||
550 | ||
551 | =head1 DESCRIPTION | |
552 | ||
553 | The Switch.pm module implements a generalized case mechanism that covers | |
554 | the numerous possible combinations of switch and case values described above. | |
555 | ||
556 | The module augments the standard Perl syntax with two new control | |
557 | statements: C<switch> and C<case>. The C<switch> statement takes a | |
558 | single scalar argument of any type, specified in parentheses. | |
559 | C<switch> stores this value as the | |
560 | current switch value in a (localized) control variable. | |
561 | The value is followed by a block which may contain one or more | |
562 | Perl statements (including the C<case> statement described below). | |
563 | The block is unconditionally executed once the switch value has | |
564 | been cached. | |
565 | ||
566 | A C<case> statement takes a single scalar argument (in mandatory | |
567 | parentheses if it's a variable; otherwise the parens are optional) and | |
568 | selects the appropriate type of matching between that argument and the | |
569 | current switch value. The type of matching used is determined by the | |
570 | respective types of the switch value and the C<case> argument, as | |
571 | specified in Table 1. If the match is successful, the mandatory | |
572 | block associated with the C<case> statement is executed. | |
573 | ||
574 | In most other respects, the C<case> statement is semantically identical | |
575 | to an C<if> statement. For example, it can be followed by an C<else> | |
576 | clause, and can be used as a postfix statement qualifier. | |
577 | ||
578 | However, when a C<case> block has been executed control is automatically | |
579 | transferred to the statement after the immediately enclosing C<switch> | |
580 | block, rather than to the next statement within the block. In other | |
581 | words, the success of any C<case> statement prevents other cases in the | |
582 | same scope from executing. But see L<"Allowing fall-through"> below. | |
583 | ||
584 | Together these two new statements provide a fully generalized case | |
585 | mechanism: | |
586 | ||
587 | use Switch; | |
588 | ||
589 | # AND LATER... | |
590 | ||
591 | %special = ( woohoo => 1, d'oh => 1 ); | |
592 | ||
593 | while (<>) { | |
594 | switch ($_) { | |
595 | ||
596 | case %special { print "homer\n"; } # if $special{$_} | |
597 | case /a-z/i { print "alpha\n"; } # if $_ =~ /a-z/i | |
598 | case [1..9] { print "small num\n"; } # if $_ in [1..9] | |
599 | ||
600 | case { $_[0] >= 10 } { # if $_ >= 10 | |
601 | my $age = <>; | |
602 | switch (sub{ $_[0] < $age } ) { | |
603 | ||
604 | case 20 { print "teens\n"; } # if 20 < $age | |
605 | case 30 { print "twenties\n"; } # if 30 < $age | |
606 | else { print "history\n"; } | |
607 | } | |
608 | } | |
609 | ||
610 | print "must be punctuation\n" case /\W/; # if $_ ~= /\W/ | |
611 | } | |
612 | ||
613 | Note that C<switch>es can be nested within C<case> (or any other) blocks, | |
614 | and a series of C<case> statements can try different types of matches | |
615 | -- hash membership, pattern match, array intersection, simple equality, | |
616 | etc. -- against the same switch value. | |
617 | ||
618 | The use of intersection tests against an array reference is particularly | |
619 | useful for aggregating integral cases: | |
620 | ||
621 | sub classify_digit | |
622 | { | |
623 | switch ($_[0]) { case 0 { return 'zero' } | |
624 | case [2,4,6,8] { return 'even' } | |
625 | case [1,3,4,7,9] { return 'odd' } | |
626 | case /[A-F]/i { return 'hex' } | |
627 | } | |
628 | } | |
629 | ||
630 | ||
631 | =head2 Allowing fall-through | |
632 | ||
633 | Fall-though (trying another case after one has already succeeded) | |
634 | is usually a Bad Idea in a switch statement. However, this | |
635 | is Perl, not a police state, so there I<is> a way to do it, if you must. | |
636 | ||
637 | If a C<case> block executes an untargetted C<next>, control is | |
638 | immediately transferred to the statement I<after> the C<case> statement | |
639 | (i.e. usually another case), rather than out of the surrounding | |
640 | C<switch> block. | |
641 | ||
642 | For example: | |
643 | ||
644 | switch ($val) { | |
645 | case 1 { handle_num_1(); next } # and try next case... | |
646 | case "1" { handle_str_1(); next } # and try next case... | |
647 | case [0..9] { handle_num_any(); } # and we're done | |
648 | case /\d/ { handle_dig_any(); next } # and try next case... | |
649 | case /.*/ { handle_str_any(); next } # and try next case... | |
650 | } | |
651 | ||
652 | If $val held the number C<1>, the above C<switch> block would call the | |
653 | first three C<handle_...> subroutines, jumping to the next case test | |
654 | each time it encountered a C<next>. After the thrid C<case> block | |
655 | was executed, control would jump to the end of the enclosing | |
656 | C<switch> block. | |
657 | ||
658 | On the other hand, if $val held C<10>, then only the last two C<handle_...> | |
659 | subroutines would be called. | |
660 | ||
661 | Note that this mechanism allows the notion of I<conditional fall-through>. | |
662 | For example: | |
663 | ||
664 | switch ($val) { | |
665 | case [0..9] { handle_num_any(); next if $val < 7; } | |
666 | case /\d/ { handle_dig_any(); } | |
667 | } | |
668 | ||
669 | If an untargetted C<last> statement is executed in a case block, this | |
670 | immediately transfers control out of the enclosing C<switch> block | |
671 | (in other words, there is an implicit C<last> at the end of each | |
672 | normal C<case> block). Thus the previous example could also have been | |
673 | written: | |
674 | ||
675 | switch ($val) { | |
676 | case [0..9] { handle_num_any(); last if $val >= 7; next; } | |
677 | case /\d/ { handle_dig_any(); } | |
678 | } | |
679 | ||
680 | ||
681 | =head2 Automating fall-through | |
682 | ||
683 | In situations where case fall-through should be the norm, rather than an | |
684 | exception, an endless succession of terminal C<next>s is tedious and ugly. | |
685 | Hence, it is possible to reverse the default behaviour by specifying | |
686 | the string "fallthrough" when importing the module. For example, the | |
687 | following code is equivalent to the first example in L<"Allowing fall-through">: | |
688 | ||
689 | use Switch 'fallthrough'; | |
690 | ||
691 | switch ($val) { | |
692 | case 1 { handle_num_1(); } | |
693 | case "1" { handle_str_1(); } | |
694 | case [0..9] { handle_num_any(); last } | |
695 | case /\d/ { handle_dig_any(); } | |
696 | case /.*/ { handle_str_any(); } | |
697 | } | |
698 | ||
699 | Note the explicit use of a C<last> to preserve the non-fall-through | |
700 | behaviour of the third case. | |
701 | ||
702 | ||
703 | ||
704 | =head2 Higher-order Operations | |
705 | ||
706 | One situation in which C<switch> and C<case> do not provide a good | |
707 | substitute for a cascaded C<if>, is where a switch value needs to | |
708 | be tested against a series of conditions. For example: | |
709 | ||
710 | sub beverage { | |
711 | switch (shift) { | |
712 | ||
713 | case sub { $_[0] < 10 } { return 'milk' } | |
714 | case sub { $_[0] < 20 } { return 'coke' } | |
715 | case sub { $_[0] < 30 } { return 'beer' } | |
716 | case sub { $_[0] < 40 } { return 'wine' } | |
717 | case sub { $_[0] < 50 } { return 'malt' } | |
718 | case sub { $_[0] < 60 } { return 'Moet' } | |
719 | else { return 'milk' } | |
720 | } | |
721 | } | |
722 | ||
723 | The need to specify each condition as a subroutine block is tiresome. To | |
724 | overcome this, when importing Switch.pm, a special "placeholder" | |
725 | subroutine named C<__> [sic] may also be imported. This subroutine | |
726 | converts (almost) any expression in which it appears to a reference to a | |
727 | higher-order function. That is, the expression: | |
728 | ||
729 | use Switch '__'; | |
730 | ||
731 | __ < 2 + __ | |
732 | ||
733 | is equivalent to: | |
734 | ||
735 | sub { $_[0] < 2 + $_[1] } | |
736 | ||
737 | With C<__>, the previous ugly case statements can be rewritten: | |
738 | ||
739 | case __ < 10 { return 'milk' } | |
740 | case __ < 20 { return 'coke' } | |
741 | case __ < 30 { return 'beer' } | |
742 | case __ < 40 { return 'wine' } | |
743 | case __ < 50 { return 'malt' } | |
744 | case __ < 60 { return 'Moet' } | |
745 | else { return 'milk' } | |
746 | ||
747 | The C<__> subroutine makes extensive use of operator overloading to | |
748 | perform its magic. All operations involving __ are overloaded to | |
749 | produce an anonymous subroutine that implements a lazy version | |
750 | of the original operation. | |
751 | ||
752 | The only problem is that operator overloading does not allow the | |
753 | boolean operators C<&&> and C<||> to be overloaded. So a case statement | |
754 | like this: | |
755 | ||
756 | case 0 <= __ && __ < 10 { return 'digit' } | |
757 | ||
758 | doesn't act as expected, because when it is | |
759 | executed, it constructs two higher order subroutines | |
760 | and then treats the two resulting references as arguments to C<&&>: | |
761 | ||
762 | sub { 0 <= $_[0] } && sub { $_[0] < 10 } | |
763 | ||
764 | This boolean expression is inevitably true, since both references are | |
765 | non-false. Fortunately, the overloaded C<'bool'> operator catches this | |
766 | situation and flags it as a error. | |
767 | ||
768 | =head1 DEPENDENCIES | |
769 | ||
770 | The module is implemented using Filter::Util::Call and Text::Balanced | |
771 | and requires both these modules to be installed. | |
772 | ||
773 | =head1 AUTHOR | |
774 | ||
775 | Damian Conway (damian@conway.org) | |
776 | ||
777 | =head1 BUGS | |
778 | ||
779 | There are undoubtedly serious bugs lurking somewhere in code this funky :-) | |
780 | Bug reports and other feedback are most welcome. | |
781 | ||
782 | =head1 COPYRIGHT | |
783 | ||
784 | Copyright (c) 1997-2000, Damian Conway. All Rights Reserved. | |
7a57cd46 JH |
785 | This module is free software; you can redistribute it and/or |
786 | modify it under the same terms as Perl itself. |