Commit | Line | Data |
3ed9f206 |
1 | package Switch; |
2 | |
3 | use strict; |
4 | use vars qw($VERSION); |
5 | use Carp; |
6 | |
74a6a946 |
7 | $VERSION = '2.04'; |
3ed9f206 |
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 | |
74a6a946 |
17 | $::_S_W_I_T_C_H = sub { croak "case/when statement not in switch/given block" }; |
3ed9f206 |
18 | |
19 | my $offset; |
20 | my $fallthrough; |
74a6a946 |
21 | my ($Perl5, $Perl6) = (0,0); |
3ed9f206 |
22 | |
23 | sub import |
24 | { |
3961318e |
25 | $DB::single = 1; |
3ed9f206 |
26 | $fallthrough = grep /\bfallthrough\b/, @_; |
27 | $offset = (caller)[2]+1; |
a1813bef |
28 | filter_add({}) unless @_>1 && $_[1] eq 'noimport'; |
3ed9f206 |
29 | my $pkg = caller; |
30 | no strict 'refs'; |
31 | for ( qw( on_defined on_exists ) ) |
32 | { |
33 | *{"${pkg}::$_"} = \&$_; |
34 | } |
35 | *{"${pkg}::__"} = \&__ if grep /__/, @_; |
74a6a946 |
36 | $Perl6 = 1 if grep(/Perl\s*6/i, @_); |
37 | $Perl5 = 1 if grep(/Perl\s*5/i, @_) || !grep(/Perl\s*6/i, @_); |
3ed9f206 |
38 | 1; |
39 | } |
40 | |
41 | sub unimport |
42 | { |
43 | filter_del() |
44 | } |
45 | |
46 | sub filter |
47 | { |
48 | my($self) = @_ ; |
49 | local $Switch::file = (caller)[1]; |
50 | |
51 | my $status = 1; |
52 | $status = filter_read(10_000); |
53 | return $status if $status<0; |
54 | $_ = filter_blocks($_,$offset); |
55 | $_ = "# line $offset\n" . $_ if $offset; undef $offset; |
56 | # print STDERR $_; |
57 | return $status; |
58 | } |
59 | |
60 | use Text::Balanced ':ALL'; |
61 | |
62 | sub line |
63 | { |
64 | my ($pretext,$offset) = @_; |
74a6a946 |
65 | ($pretext=~tr/\n/\n/)+($offset||0); |
3ed9f206 |
66 | } |
67 | |
68 | sub is_block |
69 | { |
70 | local $SIG{__WARN__}=sub{die$@}; |
71 | local $^W=1; |
72 | my $ishash = defined eval 'my $hr='.$_[0]; |
73 | undef $@; |
74 | return !$ishash; |
75 | } |
76 | |
77 | my $casecounter = 1; |
78 | sub filter_blocks |
79 | { |
80 | my ($source, $line) = @_; |
74a6a946 |
81 | return $source unless $Perl5 && $source =~ /case|switch/ |
82 | || $Perl6 && $source =~ /when|given/; |
3ed9f206 |
83 | pos $source = 0; |
84 | my $text = ""; |
85 | component: while (pos $source < length $source) |
86 | { |
3961318e |
87 | if ($source =~ m/(\G\s*use\s+Switch\b)/gc) |
3ed9f206 |
88 | { |
89 | $text .= q{use Switch 'noimport'}; |
90 | next component; |
91 | } |
92 | my @pos = Text::Balanced::_match_quotelike(\$source,qr/\s*/,1,1); |
93 | if (defined $pos[0]) |
94 | { |
3961318e |
95 | $text .= " " . substr($source,$pos[2],$pos[18]-$pos[2]); |
3ed9f206 |
96 | next component; |
97 | } |
98 | @pos = Text::Balanced::_match_variable(\$source,qr/\s*/); |
99 | if (defined $pos[0]) |
100 | { |
3961318e |
101 | $text .= " " . substr($source,$pos[0],$pos[4]-$pos[0]); |
3ed9f206 |
102 | next component; |
103 | } |
104 | |
74a6a946 |
105 | if ($Perl5 && $source =~ m/\G(\n*)(\s*)(switch)\b(?=\s*[(])/gc |
106 | || $Perl6 && $source =~ m/\G(\n*)(\s*)(given)\b(?=\s*[(])/gc) |
3ed9f206 |
107 | { |
74a6a946 |
108 | my $keyword = $3; |
3ed9f206 |
109 | $text .= $1.$2.'S_W_I_T_C_H: while (1) '; |
3961318e |
110 | @pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\(/,qr/\)/,qr/[[{(<]/,qr/[]})>]/,undef) |
3ed9f206 |
111 | or do { |
74a6a946 |
112 | die "Bad $keyword statement (problem in the parentheses?) near $Switch::file line ", line(substr($source,0,pos $source),$line), "\n"; |
3ed9f206 |
113 | }; |
114 | my $arg = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
115 | $arg =~ s {^\s*[(]\s*%} { ( \\\%} || |
116 | $arg =~ s {^\s*[(]\s*m\b} { ( qr} || |
117 | $arg =~ s {^\s*[(]\s*/} { ( qr/} || |
118 | $arg =~ s {^\s*[(]\s*qw} { ( \\qw}; |
119 | @pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef) |
120 | or do { |
74a6a946 |
121 | die "Bad $keyword statement (problem in the code block?) near $Switch::file line ", line(substr($source,0, pos $source), $line), "\n"; |
3ed9f206 |
122 | }; |
123 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
124 | $code =~ s/{/{ local \$::_S_W_I_T_C_H; Switch::switch $arg;/; |
125 | $text .= $code . 'continue {last}'; |
126 | next component; |
127 | } |
74a6a946 |
128 | elsif ($Perl5 && $source =~ m/\G(\s*)(case\b)(?!\s*=>)/gc |
129 | || $Perl6 && $source =~ m/\G(\s*)(when\b)(?!\s*=>)/gc) |
3ed9f206 |
130 | { |
74a6a946 |
131 | my $keyword = $2; |
3ed9f206 |
132 | $text .= $1."if (Switch::case"; |
133 | if (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)) { |
134 | my $code = substr($source,$pos[0],$pos[4]-$pos[0]); |
135 | $text .= " sub" if is_block $code; |
136 | $text .= " " . filter_blocks($code,line(substr($source,0,$pos[0]),$line)) . ")"; |
137 | } |
138 | elsif (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/[[(]/,qr/[])]/,qr/[[({]/,qr/[])}]/,undef)) { |
139 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
140 | $code =~ s {^\s*[(]\s*%} { ( \\\%} || |
141 | $code =~ s {^\s*[(]\s*m\b} { ( qr} || |
142 | $code =~ s {^\s*[(]\s*/} { ( qr/} || |
143 | $code =~ s {^\s*[(]\s*qw} { ( \\qw}; |
144 | $text .= " $code)"; |
145 | } |
74a6a946 |
146 | elsif ($Perl6 && do{@pos = Text::Balanced::_match_variable(\$source,qr/\s*/)}) { |
147 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
148 | $code =~ s {^\s*%} { \%} || |
149 | $code =~ s {^\s*@} { \@}; |
150 | $text .= " $code)"; |
151 | } |
3ed9f206 |
152 | elsif ( @pos = Text::Balanced::_match_quotelike(\$source,qr/\s*/,1,1)) { |
153 | my $code = substr($source,$pos[2],$pos[18]-$pos[2]); |
154 | $code = filter_blocks($code,line(substr($source,0,$pos[2]),$line)); |
155 | $code =~ s {^\s*m} { qr} || |
156 | $code =~ s {^\s*/} { qr/} || |
157 | $code =~ s {^\s*qw} { \\qw}; |
158 | $text .= " $code)"; |
159 | } |
74a6a946 |
160 | elsif ($Perl5 && $source =~ m/\G\s*(([^\$\@{])[^\$\@{]*)(?=\s*{)/gc |
161 | || $Perl6 && $source =~ m/\G\s*([^:;]*)()/gc) { |
3ed9f206 |
162 | my $code = filter_blocks($1,line(substr($source,0,pos $source),$line)); |
163 | $text .= ' \\' if $2 eq '%'; |
164 | $text .= " $code)"; |
165 | } |
166 | else { |
74a6a946 |
167 | die "Bad $keyword statement (invalid $keyword value?) near $Switch::file line ", line(substr($source,0,pos $source), $line), "\n"; |
3ed9f206 |
168 | } |
169 | |
74a6a946 |
170 | die "Missing colon or semi-colon after 'when' value near $Switch::file line ", line(substr($source,0,pos $source), $line), "\n" |
171 | unless !$Perl6 || $source =~ m/\G(\s*)(:|(?=;))/gc; |
172 | |
173 | do{@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)} |
3ed9f206 |
174 | or do { |
175 | if ($source =~ m/\G\s*(?=([};]|\Z))/gc) { |
176 | $casecounter++; |
177 | next component; |
178 | } |
74a6a946 |
179 | die "Bad $keyword statement (problem in the code block?) near $Switch::file line ", line(substr($source,0,pos $source),$line), "\n"; |
3ed9f206 |
180 | }; |
181 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
182 | $code =~ s/}(?=\s*\Z)/;last S_W_I_T_C_H }/ |
183 | unless $fallthrough; |
184 | $text .= "{ while (1) $code continue { goto C_A_S_E_$casecounter } last S_W_I_T_C_H; C_A_S_E_$casecounter: }"; |
185 | $casecounter++; |
186 | next component; |
187 | } |
188 | |
189 | $source =~ m/\G(\s*(\w+|#.*\n|\W))/gc; |
190 | $text .= $1; |
191 | } |
192 | $text; |
193 | } |
194 | |
195 | |
196 | |
197 | sub in |
198 | { |
199 | my ($x,$y) = @_; |
200 | my @numy; |
201 | for my $nextx ( @$x ) |
202 | { |
a1813bef |
203 | my $numx = ref($nextx) || defined $nextx && (~$nextx&$nextx) eq 0; |
3ed9f206 |
204 | for my $j ( 0..$#$y ) |
205 | { |
206 | my $nexty = $y->[$j]; |
a1813bef |
207 | push @numy, ref($nexty) || defined $nexty && (~$nexty&$nexty) eq 0 |
3ed9f206 |
208 | if @numy <= $j; |
209 | return 1 if $numx && $numy[$j] && $nextx==$nexty |
210 | || $nextx eq $nexty; |
211 | |
212 | } |
213 | } |
214 | return ""; |
215 | } |
216 | |
217 | sub on_exists |
218 | { |
219 | my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ }; |
220 | [ keys %$ref ] |
221 | } |
222 | |
223 | sub on_defined |
224 | { |
225 | my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ }; |
226 | [ grep { defined $ref->{$_} } keys %$ref ] |
227 | } |
228 | |
229 | sub switch(;$) |
230 | { |
231 | my ($s_val) = @_ ? $_[0] : $_; |
232 | my $s_ref = ref $s_val; |
233 | |
234 | if ($s_ref eq 'CODE') |
235 | { |
236 | $::_S_W_I_T_C_H = |
237 | sub { my $c_val = $_[0]; |
238 | return $s_val == $c_val if ref $c_val eq 'CODE'; |
239 | return $s_val->(@$c_val) if ref $c_val eq 'ARRAY'; |
240 | return $s_val->($c_val); |
241 | }; |
242 | } |
a1813bef |
243 | elsif ($s_ref eq "" && defined $s_val && (~$s_val&$s_val) eq 0) # NUMERIC SCALAR |
3ed9f206 |
244 | { |
245 | $::_S_W_I_T_C_H = |
246 | sub { my $c_val = $_[0]; |
247 | my $c_ref = ref $c_val; |
248 | return $s_val == $c_val if $c_ref eq "" |
a1813bef |
249 | && defined $c_val |
3ed9f206 |
250 | && (~$c_val&$c_val) eq 0; |
251 | return $s_val eq $c_val if $c_ref eq ""; |
252 | return in([$s_val],$c_val) if $c_ref eq 'ARRAY'; |
253 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
254 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
255 | return scalar $s_val=~/$c_val/ |
256 | if $c_ref eq 'Regexp'; |
257 | return scalar $c_val->{$s_val} |
258 | if $c_ref eq 'HASH'; |
259 | return; |
260 | }; |
261 | } |
262 | elsif ($s_ref eq "") # STRING SCALAR |
263 | { |
264 | $::_S_W_I_T_C_H = |
265 | sub { my $c_val = $_[0]; |
266 | my $c_ref = ref $c_val; |
267 | return $s_val eq $c_val if $c_ref eq ""; |
268 | return in([$s_val],$c_val) if $c_ref eq 'ARRAY'; |
269 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
270 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
271 | return scalar $s_val=~/$c_val/ |
272 | if $c_ref eq 'Regexp'; |
273 | return scalar $c_val->{$s_val} |
274 | if $c_ref eq 'HASH'; |
275 | return; |
276 | }; |
277 | } |
278 | elsif ($s_ref eq 'ARRAY') |
279 | { |
280 | $::_S_W_I_T_C_H = |
281 | sub { my $c_val = $_[0]; |
282 | my $c_ref = ref $c_val; |
283 | return in($s_val,[$c_val]) if $c_ref eq ""; |
284 | return in($s_val,$c_val) if $c_ref eq 'ARRAY'; |
285 | return $c_val->(@$s_val) if $c_ref eq 'CODE'; |
286 | return $c_val->call(@$s_val) |
287 | if $c_ref eq 'Switch'; |
288 | return scalar grep {$_=~/$c_val/} @$s_val |
289 | if $c_ref eq 'Regexp'; |
290 | return scalar grep {$c_val->{$_}} @$s_val |
291 | if $c_ref eq 'HASH'; |
292 | return; |
293 | }; |
294 | } |
295 | elsif ($s_ref eq 'Regexp') |
296 | { |
297 | $::_S_W_I_T_C_H = |
298 | sub { my $c_val = $_[0]; |
299 | my $c_ref = ref $c_val; |
300 | return $c_val=~/s_val/ if $c_ref eq ""; |
301 | return scalar grep {$_=~/s_val/} @$c_val |
302 | if $c_ref eq 'ARRAY'; |
303 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
304 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
305 | return $s_val eq $c_val if $c_ref eq 'Regexp'; |
306 | return grep {$_=~/$s_val/ && $c_val->{$_}} keys %$c_val |
307 | if $c_ref eq 'HASH'; |
308 | return; |
309 | }; |
310 | } |
311 | elsif ($s_ref eq 'HASH') |
312 | { |
313 | $::_S_W_I_T_C_H = |
314 | sub { my $c_val = $_[0]; |
315 | my $c_ref = ref $c_val; |
316 | return $s_val->{$c_val} if $c_ref eq ""; |
317 | return scalar grep {$s_val->{$_}} @$c_val |
318 | if $c_ref eq 'ARRAY'; |
319 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
320 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
321 | return grep {$_=~/$c_val/ && $s_val->{"$_"}} keys %$s_val |
322 | if $c_ref eq 'Regexp'; |
323 | return $s_val==$c_val if $c_ref eq 'HASH'; |
324 | return; |
325 | }; |
326 | } |
327 | elsif ($s_ref eq 'Switch') |
328 | { |
329 | $::_S_W_I_T_C_H = |
330 | sub { my $c_val = $_[0]; |
331 | return $s_val == $c_val if ref $c_val eq 'Switch'; |
332 | return $s_val->call(@$c_val) |
333 | if ref $c_val eq 'ARRAY'; |
334 | return $s_val->call($c_val); |
335 | }; |
336 | } |
337 | else |
338 | { |
339 | croak "Cannot switch on $s_ref"; |
340 | } |
341 | return 1; |
342 | } |
343 | |
344 | sub case($) { $::_S_W_I_T_C_H->(@_); } |
345 | |
346 | # IMPLEMENT __ |
347 | |
348 | my $placeholder = bless { arity=>1, impl=>sub{$_[1+$_[0]]} }; |
349 | |
350 | sub __() { $placeholder } |
351 | |
352 | sub __arg($) |
353 | { |
354 | my $index = $_[0]+1; |
355 | bless { arity=>0, impl=>sub{$_[$index]} }; |
356 | } |
357 | |
358 | sub hosub(&@) |
359 | { |
360 | # WRITE THIS |
361 | } |
362 | |
363 | sub call |
364 | { |
365 | my ($self,@args) = @_; |
366 | return $self->{impl}->(0,@args); |
367 | } |
368 | |
369 | sub meta_bop(&) |
370 | { |
371 | my ($op) = @_; |
372 | sub |
373 | { |
374 | my ($left, $right, $reversed) = @_; |
375 | ($right,$left) = @_ if $reversed; |
376 | |
377 | my $rop = ref $right eq 'Switch' |
378 | ? $right |
379 | : bless { arity=>0, impl=>sub{$right} }; |
380 | |
381 | my $lop = ref $left eq 'Switch' |
382 | ? $left |
383 | : bless { arity=>0, impl=>sub{$left} }; |
384 | |
385 | my $arity = $lop->{arity} + $rop->{arity}; |
386 | |
387 | return bless { |
388 | arity => $arity, |
389 | impl => sub { my $start = shift; |
390 | return $op->($lop->{impl}->($start,@_), |
391 | $rop->{impl}->($start+$lop->{arity},@_)); |
392 | } |
393 | }; |
394 | }; |
395 | } |
396 | |
397 | sub meta_uop(&) |
398 | { |
399 | my ($op) = @_; |
400 | sub |
401 | { |
402 | my ($left) = @_; |
403 | |
404 | my $lop = ref $left eq 'Switch' |
405 | ? $left |
406 | : bless { arity=>0, impl=>sub{$left} }; |
407 | |
408 | my $arity = $lop->{arity}; |
409 | |
410 | return bless { |
411 | arity => $arity, |
412 | impl => sub { $op->($lop->{impl}->(@_)) } |
413 | }; |
414 | }; |
415 | } |
416 | |
417 | |
418 | use overload |
419 | "+" => meta_bop {$_[0] + $_[1]}, |
420 | "-" => meta_bop {$_[0] - $_[1]}, |
421 | "*" => meta_bop {$_[0] * $_[1]}, |
422 | "/" => meta_bop {$_[0] / $_[1]}, |
423 | "%" => meta_bop {$_[0] % $_[1]}, |
424 | "**" => meta_bop {$_[0] ** $_[1]}, |
425 | "<<" => meta_bop {$_[0] << $_[1]}, |
426 | ">>" => meta_bop {$_[0] >> $_[1]}, |
427 | "x" => meta_bop {$_[0] x $_[1]}, |
428 | "." => meta_bop {$_[0] . $_[1]}, |
429 | "<" => meta_bop {$_[0] < $_[1]}, |
430 | "<=" => meta_bop {$_[0] <= $_[1]}, |
431 | ">" => meta_bop {$_[0] > $_[1]}, |
432 | ">=" => meta_bop {$_[0] >= $_[1]}, |
433 | "==" => meta_bop {$_[0] == $_[1]}, |
434 | "!=" => meta_bop {$_[0] != $_[1]}, |
435 | "<=>" => meta_bop {$_[0] <=> $_[1]}, |
436 | "lt" => meta_bop {$_[0] lt $_[1]}, |
437 | "le" => meta_bop {$_[0] le $_[1]}, |
438 | "gt" => meta_bop {$_[0] gt $_[1]}, |
439 | "ge" => meta_bop {$_[0] ge $_[1]}, |
440 | "eq" => meta_bop {$_[0] eq $_[1]}, |
441 | "ne" => meta_bop {$_[0] ne $_[1]}, |
442 | "cmp" => meta_bop {$_[0] cmp $_[1]}, |
443 | "\&" => meta_bop {$_[0] & $_[1]}, |
444 | "^" => meta_bop {$_[0] ^ $_[1]}, |
445 | "|" => meta_bop {$_[0] | $_[1]}, |
446 | "atan2" => meta_bop {atan2 $_[0], $_[1]}, |
447 | |
448 | "neg" => meta_uop {-$_[0]}, |
449 | "!" => meta_uop {!$_[0]}, |
450 | "~" => meta_uop {~$_[0]}, |
451 | "cos" => meta_uop {cos $_[0]}, |
452 | "sin" => meta_uop {sin $_[0]}, |
453 | "exp" => meta_uop {exp $_[0]}, |
454 | "abs" => meta_uop {abs $_[0]}, |
455 | "log" => meta_uop {log $_[0]}, |
456 | "sqrt" => meta_uop {sqrt $_[0]}, |
457 | "bool" => sub { croak "Can't use && or || in expression containing __" }, |
458 | |
459 | # "&()" => sub { $_[0]->{impl} }, |
460 | |
461 | # "||" => meta_bop {$_[0] || $_[1]}, |
462 | # "&&" => meta_bop {$_[0] && $_[1]}, |
463 | # fallback => 1, |
464 | ; |
465 | 1; |
466 | |
467 | __END__ |
468 | |
469 | |
470 | =head1 NAME |
471 | |
472 | Switch - A switch statement for Perl |
473 | |
474 | =head1 VERSION |
475 | |
74a6a946 |
476 | This document describes version 2.04 of Switch, |
477 | released July 30, 2001. |
3ed9f206 |
478 | |
479 | =head1 SYNOPSIS |
480 | |
481 | use Switch; |
482 | |
483 | switch ($val) { |
484 | |
485 | case 1 { print "number 1" } |
486 | case "a" { print "string a" } |
487 | case [1..10,42] { print "number in list" } |
488 | case (@array) { print "number in list" } |
489 | case /\w+/ { print "pattern" } |
490 | case qr/\w+/ { print "pattern" } |
491 | case (%hash) { print "entry in hash" } |
492 | case (\%hash) { print "entry in hash" } |
493 | case (\&sub) { print "arg to subroutine" } |
494 | else { print "previous case not true" } |
495 | } |
496 | |
497 | =head1 BACKGROUND |
498 | |
499 | [Skip ahead to L<"DESCRIPTION"> if you don't care about the whys |
500 | and wherefores of this control structure] |
501 | |
502 | In seeking to devise a "Swiss Army" case mechanism suitable for Perl, |
503 | it is useful to generalize this notion of distributed conditional |
504 | testing as far as possible. Specifically, the concept of "matching" |
505 | between the switch value and the various case values need not be |
506 | restricted to numeric (or string or referential) equality, as it is in other |
507 | languages. Indeed, as Table 1 illustrates, Perl |
508 | offers at least eighteen different ways in which two values could |
509 | generate a match. |
510 | |
511 | Table 1: Matching a switch value ($s) with a case value ($c) |
512 | |
513 | Switch Case Type of Match Implied Matching Code |
514 | Value Value |
515 | ====== ===== ===================== ============= |
516 | |
517 | number same numeric or referential match if $s == $c; |
518 | or ref equality |
519 | |
520 | object method result of method call match if $s->$c(); |
521 | ref name match if defined $s->$c(); |
522 | or ref |
523 | |
524 | other other string equality match if $s eq $c; |
525 | non-ref non-ref |
526 | scalar scalar |
527 | |
528 | string regexp pattern match match if $s =~ /$c/; |
529 | |
530 | array scalar array entry existence match if 0<=$c && $c<@$s; |
531 | ref array entry definition match if defined $s->[$c]; |
532 | array entry truth match if $s->[$c]; |
533 | |
534 | array array array intersection match if intersects(@$s, @$c); |
535 | ref ref (apply this table to |
536 | all pairs of elements |
537 | $s->[$i] and |
538 | $c->[$j]) |
539 | |
540 | array regexp array grep match if grep /$c/, @$s; |
541 | ref |
542 | |
543 | hash scalar hash entry existence match if exists $s->{$c}; |
544 | ref hash entry definition match if defined $s->{$c}; |
545 | hash entry truth match if $s->{$c}; |
546 | |
547 | hash regexp hash grep match if grep /$c/, keys %$s; |
548 | ref |
549 | |
550 | sub scalar return value defn match if defined $s->($c); |
551 | ref return value truth match if $s->($c); |
552 | |
553 | sub array return value defn match if defined $s->(@$c); |
554 | ref ref return value truth match if $s->(@$c); |
555 | |
556 | |
557 | In reality, Table 1 covers 31 alternatives, because only the equality and |
558 | intersection tests are commutative; in all other cases, the roles of |
559 | the C<$s> and C<$c> variables could be reversed to produce a |
560 | different test. For example, instead of testing a single hash for |
561 | the existence of a series of keys (C<match if exists $s-E<gt>{$c}>), |
562 | one could test for the existence of a single key in a series of hashes |
563 | (C<match if exists $c-E<gt>{$s}>). |
564 | |
565 | As L<perltodo> observes, a Perl case mechanism must support all these |
566 | "ways to do it". |
567 | |
568 | |
569 | =head1 DESCRIPTION |
570 | |
571 | The Switch.pm module implements a generalized case mechanism that covers |
572 | the numerous possible combinations of switch and case values described above. |
573 | |
574 | The module augments the standard Perl syntax with two new control |
575 | statements: C<switch> and C<case>. The C<switch> statement takes a |
576 | single scalar argument of any type, specified in parentheses. |
577 | C<switch> stores this value as the |
578 | current switch value in a (localized) control variable. |
579 | The value is followed by a block which may contain one or more |
580 | Perl statements (including the C<case> statement described below). |
581 | The block is unconditionally executed once the switch value has |
582 | been cached. |
583 | |
584 | A C<case> statement takes a single scalar argument (in mandatory |
585 | parentheses if it's a variable; otherwise the parens are optional) and |
586 | selects the appropriate type of matching between that argument and the |
587 | current switch value. The type of matching used is determined by the |
588 | respective types of the switch value and the C<case> argument, as |
589 | specified in Table 1. If the match is successful, the mandatory |
590 | block associated with the C<case> statement is executed. |
591 | |
592 | In most other respects, the C<case> statement is semantically identical |
593 | to an C<if> statement. For example, it can be followed by an C<else> |
594 | clause, and can be used as a postfix statement qualifier. |
595 | |
596 | However, when a C<case> block has been executed control is automatically |
597 | transferred to the statement after the immediately enclosing C<switch> |
598 | block, rather than to the next statement within the block. In other |
599 | words, the success of any C<case> statement prevents other cases in the |
600 | same scope from executing. But see L<"Allowing fall-through"> below. |
601 | |
602 | Together these two new statements provide a fully generalized case |
603 | mechanism: |
604 | |
605 | use Switch; |
606 | |
607 | # AND LATER... |
608 | |
609 | %special = ( woohoo => 1, d'oh => 1 ); |
610 | |
611 | while (<>) { |
612 | switch ($_) { |
613 | |
74a6a946 |
614 | case (%special) { print "homer\n"; } # if $special{$_} |
615 | case /a-z/i { print "alpha\n"; } # if $_ =~ /a-z/i |
616 | case [1..9] { print "small num\n"; } # if $_ in [1..9] |
3ed9f206 |
617 | |
618 | case { $_[0] >= 10 } { # if $_ >= 10 |
619 | my $age = <>; |
620 | switch (sub{ $_[0] < $age } ) { |
621 | |
622 | case 20 { print "teens\n"; } # if 20 < $age |
623 | case 30 { print "twenties\n"; } # if 30 < $age |
624 | else { print "history\n"; } |
625 | } |
626 | } |
627 | |
628 | print "must be punctuation\n" case /\W/; # if $_ ~= /\W/ |
629 | } |
630 | |
631 | Note that C<switch>es can be nested within C<case> (or any other) blocks, |
632 | and a series of C<case> statements can try different types of matches |
633 | -- hash membership, pattern match, array intersection, simple equality, |
634 | etc. -- against the same switch value. |
635 | |
636 | The use of intersection tests against an array reference is particularly |
637 | useful for aggregating integral cases: |
638 | |
639 | sub classify_digit |
640 | { |
641 | switch ($_[0]) { case 0 { return 'zero' } |
642 | case [2,4,6,8] { return 'even' } |
643 | case [1,3,4,7,9] { return 'odd' } |
644 | case /[A-F]/i { return 'hex' } |
645 | } |
646 | } |
647 | |
648 | |
649 | =head2 Allowing fall-through |
650 | |
651 | Fall-though (trying another case after one has already succeeded) |
652 | is usually a Bad Idea in a switch statement. However, this |
653 | is Perl, not a police state, so there I<is> a way to do it, if you must. |
654 | |
655 | If a C<case> block executes an untargetted C<next>, control is |
656 | immediately transferred to the statement I<after> the C<case> statement |
657 | (i.e. usually another case), rather than out of the surrounding |
658 | C<switch> block. |
659 | |
660 | For example: |
661 | |
662 | switch ($val) { |
663 | case 1 { handle_num_1(); next } # and try next case... |
664 | case "1" { handle_str_1(); next } # and try next case... |
665 | case [0..9] { handle_num_any(); } # and we're done |
666 | case /\d/ { handle_dig_any(); next } # and try next case... |
667 | case /.*/ { handle_str_any(); next } # and try next case... |
668 | } |
669 | |
670 | If $val held the number C<1>, the above C<switch> block would call the |
671 | first three C<handle_...> subroutines, jumping to the next case test |
672 | each time it encountered a C<next>. After the thrid C<case> block |
673 | was executed, control would jump to the end of the enclosing |
674 | C<switch> block. |
675 | |
676 | On the other hand, if $val held C<10>, then only the last two C<handle_...> |
677 | subroutines would be called. |
678 | |
679 | Note that this mechanism allows the notion of I<conditional fall-through>. |
680 | For example: |
681 | |
682 | switch ($val) { |
683 | case [0..9] { handle_num_any(); next if $val < 7; } |
684 | case /\d/ { handle_dig_any(); } |
685 | } |
686 | |
687 | If an untargetted C<last> statement is executed in a case block, this |
688 | immediately transfers control out of the enclosing C<switch> block |
689 | (in other words, there is an implicit C<last> at the end of each |
690 | normal C<case> block). Thus the previous example could also have been |
691 | written: |
692 | |
693 | switch ($val) { |
694 | case [0..9] { handle_num_any(); last if $val >= 7; next; } |
695 | case /\d/ { handle_dig_any(); } |
696 | } |
697 | |
698 | |
699 | =head2 Automating fall-through |
700 | |
701 | In situations where case fall-through should be the norm, rather than an |
702 | exception, an endless succession of terminal C<next>s is tedious and ugly. |
703 | Hence, it is possible to reverse the default behaviour by specifying |
704 | the string "fallthrough" when importing the module. For example, the |
705 | following code is equivalent to the first example in L<"Allowing fall-through">: |
706 | |
707 | use Switch 'fallthrough'; |
708 | |
709 | switch ($val) { |
710 | case 1 { handle_num_1(); } |
711 | case "1" { handle_str_1(); } |
712 | case [0..9] { handle_num_any(); last } |
713 | case /\d/ { handle_dig_any(); } |
714 | case /.*/ { handle_str_any(); } |
715 | } |
716 | |
717 | Note the explicit use of a C<last> to preserve the non-fall-through |
718 | behaviour of the third case. |
719 | |
720 | |
721 | |
74a6a946 |
722 | =head2 Alternative syntax |
723 | |
724 | Perl 6 will provide a built-in switch statement with essentially the |
725 | same semantics as those offered by Switch.pm, but with a different |
726 | pair of keywords. In Perl 6 C<switch> with be spelled C<given>, and |
727 | C<case> will be pronounced C<when>. In addition, the C<when> statement |
728 | will use a colon between its case value and its block (removing the |
729 | need to parenthesize variables. |
730 | |
731 | This future syntax is also available via the Switch.pm module, by |
732 | importing it with the argument C<"Perl6">. For example: |
733 | |
734 | use Switch 'Perl6'; |
735 | |
736 | given ($val) { |
737 | when 1 : { handle_num_1(); } |
738 | when $str1 : { handle_str_1(); } |
739 | when [0..9] : { handle_num_any(); last } |
740 | when /\d/ : { handle_dig_any(); } |
741 | when /.*/ : { handle_str_any(); } |
742 | } |
743 | |
744 | Note that you can mix and match both syntaxes by importing the module |
745 | with: |
746 | |
747 | use Switch 'Perl5', 'Perl6'; |
748 | |
749 | |
3ed9f206 |
750 | =head2 Higher-order Operations |
751 | |
752 | One situation in which C<switch> and C<case> do not provide a good |
753 | substitute for a cascaded C<if>, is where a switch value needs to |
754 | be tested against a series of conditions. For example: |
755 | |
756 | sub beverage { |
757 | switch (shift) { |
758 | |
759 | case sub { $_[0] < 10 } { return 'milk' } |
760 | case sub { $_[0] < 20 } { return 'coke' } |
761 | case sub { $_[0] < 30 } { return 'beer' } |
762 | case sub { $_[0] < 40 } { return 'wine' } |
763 | case sub { $_[0] < 50 } { return 'malt' } |
764 | case sub { $_[0] < 60 } { return 'Moet' } |
765 | else { return 'milk' } |
766 | } |
767 | } |
768 | |
769 | The need to specify each condition as a subroutine block is tiresome. To |
770 | overcome this, when importing Switch.pm, a special "placeholder" |
771 | subroutine named C<__> [sic] may also be imported. This subroutine |
772 | converts (almost) any expression in which it appears to a reference to a |
773 | higher-order function. That is, the expression: |
774 | |
775 | use Switch '__'; |
776 | |
777 | __ < 2 + __ |
778 | |
779 | is equivalent to: |
780 | |
781 | sub { $_[0] < 2 + $_[1] } |
782 | |
783 | With C<__>, the previous ugly case statements can be rewritten: |
784 | |
785 | case __ < 10 { return 'milk' } |
786 | case __ < 20 { return 'coke' } |
787 | case __ < 30 { return 'beer' } |
788 | case __ < 40 { return 'wine' } |
789 | case __ < 50 { return 'malt' } |
790 | case __ < 60 { return 'Moet' } |
791 | else { return 'milk' } |
792 | |
793 | The C<__> subroutine makes extensive use of operator overloading to |
794 | perform its magic. All operations involving __ are overloaded to |
795 | produce an anonymous subroutine that implements a lazy version |
796 | of the original operation. |
797 | |
798 | The only problem is that operator overloading does not allow the |
799 | boolean operators C<&&> and C<||> to be overloaded. So a case statement |
800 | like this: |
801 | |
802 | case 0 <= __ && __ < 10 { return 'digit' } |
803 | |
804 | doesn't act as expected, because when it is |
805 | executed, it constructs two higher order subroutines |
806 | and then treats the two resulting references as arguments to C<&&>: |
807 | |
808 | sub { 0 <= $_[0] } && sub { $_[0] < 10 } |
809 | |
810 | This boolean expression is inevitably true, since both references are |
811 | non-false. Fortunately, the overloaded C<'bool'> operator catches this |
812 | situation and flags it as a error. |
813 | |
814 | =head1 DEPENDENCIES |
815 | |
816 | The module is implemented using Filter::Util::Call and Text::Balanced |
817 | and requires both these modules to be installed. |
818 | |
819 | =head1 AUTHOR |
820 | |
821 | Damian Conway (damian@conway.org) |
822 | |
823 | =head1 BUGS |
824 | |
825 | There are undoubtedly serious bugs lurking somewhere in code this funky :-) |
826 | Bug reports and other feedback are most welcome. |
827 | |
828 | =head1 COPYRIGHT |
829 | |
830 | Copyright (c) 1997-2000, Damian Conway. All Rights Reserved. |
7a57cd46 |
831 | This module is free software; you can redistribute it and/or |
832 | modify it under the same terms as Perl itself. |