Commit | Line | Data |
3ed9f206 |
1 | package Switch; |
2 | |
3 | use strict; |
4 | use vars qw($VERSION); |
5 | use Carp; |
6 | |
218ab4da |
7 | $VERSION = '2.09'; |
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 | { |
25 | $fallthrough = grep /\bfallthrough\b/, @_; |
26 | $offset = (caller)[2]+1; |
a1813bef |
27 | filter_add({}) unless @_>1 && $_[1] eq 'noimport'; |
3ed9f206 |
28 | my $pkg = caller; |
29 | no strict 'refs'; |
30 | for ( qw( on_defined on_exists ) ) |
31 | { |
32 | *{"${pkg}::$_"} = \&$_; |
33 | } |
34 | *{"${pkg}::__"} = \&__ if grep /__/, @_; |
74a6a946 |
35 | $Perl6 = 1 if grep(/Perl\s*6/i, @_); |
36 | $Perl5 = 1 if grep(/Perl\s*5/i, @_) || !grep(/Perl\s*6/i, @_); |
3ed9f206 |
37 | 1; |
38 | } |
39 | |
40 | sub unimport |
41 | { |
42 | filter_del() |
43 | } |
44 | |
45 | sub filter |
46 | { |
47 | my($self) = @_ ; |
48 | local $Switch::file = (caller)[1]; |
49 | |
50 | my $status = 1; |
51 | $status = filter_read(10_000); |
52 | return $status if $status<0; |
53 | $_ = filter_blocks($_,$offset); |
54 | $_ = "# line $offset\n" . $_ if $offset; undef $offset; |
3ed9f206 |
55 | return $status; |
56 | } |
57 | |
58 | use Text::Balanced ':ALL'; |
59 | |
60 | sub line |
61 | { |
62 | my ($pretext,$offset) = @_; |
74a6a946 |
63 | ($pretext=~tr/\n/\n/)+($offset||0); |
3ed9f206 |
64 | } |
65 | |
66 | sub is_block |
67 | { |
68 | local $SIG{__WARN__}=sub{die$@}; |
69 | local $^W=1; |
70 | my $ishash = defined eval 'my $hr='.$_[0]; |
71 | undef $@; |
72 | return !$ishash; |
73 | } |
74 | |
d38ca171 |
75 | |
76 | my $EOP = qr/\n\n|\Z/; |
77 | my $CUT = qr/\n=cut.*$EOP/; |
78 | my $pod_or_DATA = qr/ ^=(?:head[1-4]|item) .*? $CUT |
79 | | ^=pod .*? $CUT |
80 | | ^=for .*? $EOP |
81 | | ^=begin \s* (\S+) .*? \n=end \s* \1 .*? $EOP |
82 | | ^__(DATA|END)__\n.* |
83 | /smx; |
84 | |
3ed9f206 |
85 | my $casecounter = 1; |
86 | sub filter_blocks |
87 | { |
88 | my ($source, $line) = @_; |
74a6a946 |
89 | return $source unless $Perl5 && $source =~ /case|switch/ |
90 | || $Perl6 && $source =~ /when|given/; |
3ed9f206 |
91 | pos $source = 0; |
92 | my $text = ""; |
6596d39b |
93 | $DB::single = 1; |
3ed9f206 |
94 | component: while (pos $source < length $source) |
95 | { |
3961318e |
96 | if ($source =~ m/(\G\s*use\s+Switch\b)/gc) |
3ed9f206 |
97 | { |
98 | $text .= q{use Switch 'noimport'}; |
99 | next component; |
100 | } |
d38ca171 |
101 | my @pos = Text::Balanced::_match_quotelike(\$source,qr/\s*/,1,0); |
3ed9f206 |
102 | if (defined $pos[0]) |
103 | { |
3961318e |
104 | $text .= " " . substr($source,$pos[2],$pos[18]-$pos[2]); |
3ed9f206 |
105 | next component; |
106 | } |
d38ca171 |
107 | if ($source =~ m/\G\s*($pod_or_DATA)/gc) { |
108 | next component; |
109 | } |
3ed9f206 |
110 | @pos = Text::Balanced::_match_variable(\$source,qr/\s*/); |
111 | if (defined $pos[0]) |
112 | { |
3961318e |
113 | $text .= " " . substr($source,$pos[0],$pos[4]-$pos[0]); |
3ed9f206 |
114 | next component; |
115 | } |
116 | |
74a6a946 |
117 | if ($Perl5 && $source =~ m/\G(\n*)(\s*)(switch)\b(?=\s*[(])/gc |
6596d39b |
118 | || $Perl6 && $source =~ m/\G(\n*)(\s*)(given)\b(?=\s*[(])/gc |
119 | || $Perl6 && $source =~ m/\G(\n*)(\s*)(given)\b(.*)(?=\{)/gc) |
3ed9f206 |
120 | { |
74a6a946 |
121 | my $keyword = $3; |
6596d39b |
122 | my $arg = $4; |
9c291939 |
123 | # print STDERR "[$arg]\n"; |
3ed9f206 |
124 | $text .= $1.$2.'S_W_I_T_C_H: while (1) '; |
6596d39b |
125 | unless ($arg) { |
126 | @pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\(/,qr/\)/,qr/[[{(<]/,qr/[]})>]/,undef) |
127 | or do { |
128 | die "Bad $keyword statement (problem in the parentheses?) near $Switch::file line ", line(substr($source,0,pos $source),$line), "\n"; |
129 | }; |
130 | $arg = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
131 | } |
3ed9f206 |
132 | $arg =~ s {^\s*[(]\s*%} { ( \\\%} || |
133 | $arg =~ s {^\s*[(]\s*m\b} { ( qr} || |
134 | $arg =~ s {^\s*[(]\s*/} { ( qr/} || |
135 | $arg =~ s {^\s*[(]\s*qw} { ( \\qw}; |
136 | @pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef) |
137 | or do { |
74a6a946 |
138 | die "Bad $keyword statement (problem in the code block?) near $Switch::file line ", line(substr($source,0, pos $source), $line), "\n"; |
3ed9f206 |
139 | }; |
140 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
141 | $code =~ s/{/{ local \$::_S_W_I_T_C_H; Switch::switch $arg;/; |
142 | $text .= $code . 'continue {last}'; |
143 | next component; |
144 | } |
74a6a946 |
145 | elsif ($Perl5 && $source =~ m/\G(\s*)(case\b)(?!\s*=>)/gc |
146 | || $Perl6 && $source =~ m/\G(\s*)(when\b)(?!\s*=>)/gc) |
3ed9f206 |
147 | { |
74a6a946 |
148 | my $keyword = $2; |
3ed9f206 |
149 | $text .= $1."if (Switch::case"; |
150 | if (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)) { |
151 | my $code = substr($source,$pos[0],$pos[4]-$pos[0]); |
152 | $text .= " sub" if is_block $code; |
153 | $text .= " " . filter_blocks($code,line(substr($source,0,$pos[0]),$line)) . ")"; |
154 | } |
155 | elsif (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/[[(]/,qr/[])]/,qr/[[({]/,qr/[])}]/,undef)) { |
156 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
157 | $code =~ s {^\s*[(]\s*%} { ( \\\%} || |
158 | $code =~ s {^\s*[(]\s*m\b} { ( qr} || |
159 | $code =~ s {^\s*[(]\s*/} { ( qr/} || |
160 | $code =~ s {^\s*[(]\s*qw} { ( \\qw}; |
161 | $text .= " $code)"; |
162 | } |
74a6a946 |
163 | elsif ($Perl6 && do{@pos = Text::Balanced::_match_variable(\$source,qr/\s*/)}) { |
164 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
165 | $code =~ s {^\s*%} { \%} || |
166 | $code =~ s {^\s*@} { \@}; |
167 | $text .= " $code)"; |
168 | } |
d38ca171 |
169 | elsif ( @pos = Text::Balanced::_match_quotelike(\$source,qr/\s*/,1,0)) { |
3ed9f206 |
170 | my $code = substr($source,$pos[2],$pos[18]-$pos[2]); |
171 | $code = filter_blocks($code,line(substr($source,0,$pos[2]),$line)); |
172 | $code =~ s {^\s*m} { qr} || |
173 | $code =~ s {^\s*/} { qr/} || |
174 | $code =~ s {^\s*qw} { \\qw}; |
175 | $text .= " $code)"; |
176 | } |
74a6a946 |
177 | elsif ($Perl5 && $source =~ m/\G\s*(([^\$\@{])[^\$\@{]*)(?=\s*{)/gc |
6596d39b |
178 | || $Perl6 && $source =~ m/\G\s*([^;{]*)()/gc) { |
3ed9f206 |
179 | my $code = filter_blocks($1,line(substr($source,0,pos $source),$line)); |
180 | $text .= ' \\' if $2 eq '%'; |
181 | $text .= " $code)"; |
182 | } |
183 | else { |
74a6a946 |
184 | die "Bad $keyword statement (invalid $keyword value?) near $Switch::file line ", line(substr($source,0,pos $source), $line), "\n"; |
3ed9f206 |
185 | } |
186 | |
6596d39b |
187 | die "Missing opening brace or semi-colon after 'when' value near $Switch::file line ", line(substr($source,0,pos $source), $line), "\n" |
188 | unless !$Perl6 || $source =~ m/\G(\s*)(?=;|\{)/gc; |
74a6a946 |
189 | |
190 | do{@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)} |
3ed9f206 |
191 | or do { |
192 | if ($source =~ m/\G\s*(?=([};]|\Z))/gc) { |
193 | $casecounter++; |
194 | next component; |
195 | } |
74a6a946 |
196 | die "Bad $keyword statement (problem in the code block?) near $Switch::file line ", line(substr($source,0,pos $source),$line), "\n"; |
3ed9f206 |
197 | }; |
198 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
199 | $code =~ s/}(?=\s*\Z)/;last S_W_I_T_C_H }/ |
200 | unless $fallthrough; |
201 | $text .= "{ while (1) $code continue { goto C_A_S_E_$casecounter } last S_W_I_T_C_H; C_A_S_E_$casecounter: }"; |
202 | $casecounter++; |
203 | next component; |
204 | } |
205 | |
d38ca171 |
206 | $source =~ m/\G(\s*(-[sm]\s+|\w+|#.*\n|\W))/gc; |
3ed9f206 |
207 | $text .= $1; |
208 | } |
209 | $text; |
210 | } |
211 | |
212 | |
213 | |
214 | sub in |
215 | { |
216 | my ($x,$y) = @_; |
217 | my @numy; |
218 | for my $nextx ( @$x ) |
219 | { |
a1813bef |
220 | my $numx = ref($nextx) || defined $nextx && (~$nextx&$nextx) eq 0; |
3ed9f206 |
221 | for my $j ( 0..$#$y ) |
222 | { |
223 | my $nexty = $y->[$j]; |
a1813bef |
224 | push @numy, ref($nexty) || defined $nexty && (~$nexty&$nexty) eq 0 |
3ed9f206 |
225 | if @numy <= $j; |
226 | return 1 if $numx && $numy[$j] && $nextx==$nexty |
227 | || $nextx eq $nexty; |
228 | |
229 | } |
230 | } |
231 | return ""; |
232 | } |
233 | |
234 | sub on_exists |
235 | { |
236 | my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ }; |
237 | [ keys %$ref ] |
238 | } |
239 | |
240 | sub on_defined |
241 | { |
242 | my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ }; |
243 | [ grep { defined $ref->{$_} } keys %$ref ] |
244 | } |
245 | |
246 | sub switch(;$) |
247 | { |
248 | my ($s_val) = @_ ? $_[0] : $_; |
249 | my $s_ref = ref $s_val; |
250 | |
251 | if ($s_ref eq 'CODE') |
252 | { |
253 | $::_S_W_I_T_C_H = |
254 | sub { my $c_val = $_[0]; |
255 | return $s_val == $c_val if ref $c_val eq 'CODE'; |
256 | return $s_val->(@$c_val) if ref $c_val eq 'ARRAY'; |
257 | return $s_val->($c_val); |
258 | }; |
259 | } |
a1813bef |
260 | elsif ($s_ref eq "" && defined $s_val && (~$s_val&$s_val) eq 0) # NUMERIC SCALAR |
3ed9f206 |
261 | { |
262 | $::_S_W_I_T_C_H = |
263 | sub { my $c_val = $_[0]; |
264 | my $c_ref = ref $c_val; |
265 | return $s_val == $c_val if $c_ref eq "" |
a1813bef |
266 | && defined $c_val |
3ed9f206 |
267 | && (~$c_val&$c_val) eq 0; |
268 | return $s_val eq $c_val if $c_ref eq ""; |
269 | return in([$s_val],$c_val) if $c_ref eq 'ARRAY'; |
270 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
271 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
272 | return scalar $s_val=~/$c_val/ |
273 | if $c_ref eq 'Regexp'; |
274 | return scalar $c_val->{$s_val} |
275 | if $c_ref eq 'HASH'; |
276 | return; |
277 | }; |
278 | } |
279 | elsif ($s_ref eq "") # STRING SCALAR |
280 | { |
281 | $::_S_W_I_T_C_H = |
282 | sub { my $c_val = $_[0]; |
283 | my $c_ref = ref $c_val; |
284 | return $s_val eq $c_val if $c_ref eq ""; |
285 | return in([$s_val],$c_val) if $c_ref eq 'ARRAY'; |
286 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
287 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
288 | return scalar $s_val=~/$c_val/ |
289 | if $c_ref eq 'Regexp'; |
290 | return scalar $c_val->{$s_val} |
291 | if $c_ref eq 'HASH'; |
292 | return; |
293 | }; |
294 | } |
295 | elsif ($s_ref eq 'ARRAY') |
296 | { |
297 | $::_S_W_I_T_C_H = |
298 | sub { my $c_val = $_[0]; |
299 | my $c_ref = ref $c_val; |
300 | return in($s_val,[$c_val]) if $c_ref eq ""; |
301 | return in($s_val,$c_val) if $c_ref eq 'ARRAY'; |
302 | return $c_val->(@$s_val) if $c_ref eq 'CODE'; |
303 | return $c_val->call(@$s_val) |
304 | if $c_ref eq 'Switch'; |
305 | return scalar grep {$_=~/$c_val/} @$s_val |
306 | if $c_ref eq 'Regexp'; |
307 | return scalar grep {$c_val->{$_}} @$s_val |
308 | if $c_ref eq 'HASH'; |
309 | return; |
310 | }; |
311 | } |
312 | elsif ($s_ref eq 'Regexp') |
313 | { |
314 | $::_S_W_I_T_C_H = |
315 | sub { my $c_val = $_[0]; |
316 | my $c_ref = ref $c_val; |
317 | return $c_val=~/s_val/ if $c_ref eq ""; |
318 | return scalar grep {$_=~/s_val/} @$c_val |
319 | if $c_ref eq 'ARRAY'; |
320 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
321 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
322 | return $s_val eq $c_val if $c_ref eq 'Regexp'; |
323 | return grep {$_=~/$s_val/ && $c_val->{$_}} keys %$c_val |
324 | if $c_ref eq 'HASH'; |
325 | return; |
326 | }; |
327 | } |
328 | elsif ($s_ref eq 'HASH') |
329 | { |
330 | $::_S_W_I_T_C_H = |
331 | sub { my $c_val = $_[0]; |
332 | my $c_ref = ref $c_val; |
333 | return $s_val->{$c_val} if $c_ref eq ""; |
334 | return scalar grep {$s_val->{$_}} @$c_val |
335 | if $c_ref eq 'ARRAY'; |
336 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
337 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
338 | return grep {$_=~/$c_val/ && $s_val->{"$_"}} keys %$s_val |
339 | if $c_ref eq 'Regexp'; |
340 | return $s_val==$c_val if $c_ref eq 'HASH'; |
341 | return; |
342 | }; |
343 | } |
344 | elsif ($s_ref eq 'Switch') |
345 | { |
346 | $::_S_W_I_T_C_H = |
347 | sub { my $c_val = $_[0]; |
348 | return $s_val == $c_val if ref $c_val eq 'Switch'; |
349 | return $s_val->call(@$c_val) |
350 | if ref $c_val eq 'ARRAY'; |
351 | return $s_val->call($c_val); |
352 | }; |
353 | } |
354 | else |
355 | { |
356 | croak "Cannot switch on $s_ref"; |
357 | } |
358 | return 1; |
359 | } |
360 | |
d38ca171 |
361 | sub case($) { local $SIG{__WARN__} = \&carp; |
362 | $::_S_W_I_T_C_H->(@_); } |
3ed9f206 |
363 | |
364 | # IMPLEMENT __ |
365 | |
366 | my $placeholder = bless { arity=>1, impl=>sub{$_[1+$_[0]]} }; |
367 | |
368 | sub __() { $placeholder } |
369 | |
370 | sub __arg($) |
371 | { |
372 | my $index = $_[0]+1; |
373 | bless { arity=>0, impl=>sub{$_[$index]} }; |
374 | } |
375 | |
376 | sub hosub(&@) |
377 | { |
378 | # WRITE THIS |
379 | } |
380 | |
381 | sub call |
382 | { |
383 | my ($self,@args) = @_; |
384 | return $self->{impl}->(0,@args); |
385 | } |
386 | |
387 | sub meta_bop(&) |
388 | { |
389 | my ($op) = @_; |
390 | sub |
391 | { |
392 | my ($left, $right, $reversed) = @_; |
393 | ($right,$left) = @_ if $reversed; |
394 | |
395 | my $rop = ref $right eq 'Switch' |
396 | ? $right |
397 | : bless { arity=>0, impl=>sub{$right} }; |
398 | |
399 | my $lop = ref $left eq 'Switch' |
400 | ? $left |
401 | : bless { arity=>0, impl=>sub{$left} }; |
402 | |
403 | my $arity = $lop->{arity} + $rop->{arity}; |
404 | |
405 | return bless { |
406 | arity => $arity, |
407 | impl => sub { my $start = shift; |
408 | return $op->($lop->{impl}->($start,@_), |
409 | $rop->{impl}->($start+$lop->{arity},@_)); |
410 | } |
411 | }; |
412 | }; |
413 | } |
414 | |
415 | sub meta_uop(&) |
416 | { |
417 | my ($op) = @_; |
418 | sub |
419 | { |
420 | my ($left) = @_; |
421 | |
422 | my $lop = ref $left eq 'Switch' |
423 | ? $left |
424 | : bless { arity=>0, impl=>sub{$left} }; |
425 | |
426 | my $arity = $lop->{arity}; |
427 | |
428 | return bless { |
429 | arity => $arity, |
430 | impl => sub { $op->($lop->{impl}->(@_)) } |
431 | }; |
432 | }; |
433 | } |
434 | |
435 | |
436 | use overload |
437 | "+" => meta_bop {$_[0] + $_[1]}, |
438 | "-" => meta_bop {$_[0] - $_[1]}, |
439 | "*" => meta_bop {$_[0] * $_[1]}, |
440 | "/" => meta_bop {$_[0] / $_[1]}, |
441 | "%" => meta_bop {$_[0] % $_[1]}, |
442 | "**" => meta_bop {$_[0] ** $_[1]}, |
443 | "<<" => meta_bop {$_[0] << $_[1]}, |
444 | ">>" => meta_bop {$_[0] >> $_[1]}, |
445 | "x" => meta_bop {$_[0] x $_[1]}, |
446 | "." => meta_bop {$_[0] . $_[1]}, |
447 | "<" => meta_bop {$_[0] < $_[1]}, |
448 | "<=" => meta_bop {$_[0] <= $_[1]}, |
449 | ">" => meta_bop {$_[0] > $_[1]}, |
450 | ">=" => meta_bop {$_[0] >= $_[1]}, |
451 | "==" => meta_bop {$_[0] == $_[1]}, |
452 | "!=" => meta_bop {$_[0] != $_[1]}, |
453 | "<=>" => meta_bop {$_[0] <=> $_[1]}, |
454 | "lt" => meta_bop {$_[0] lt $_[1]}, |
455 | "le" => meta_bop {$_[0] le $_[1]}, |
456 | "gt" => meta_bop {$_[0] gt $_[1]}, |
457 | "ge" => meta_bop {$_[0] ge $_[1]}, |
458 | "eq" => meta_bop {$_[0] eq $_[1]}, |
459 | "ne" => meta_bop {$_[0] ne $_[1]}, |
460 | "cmp" => meta_bop {$_[0] cmp $_[1]}, |
461 | "\&" => meta_bop {$_[0] & $_[1]}, |
462 | "^" => meta_bop {$_[0] ^ $_[1]}, |
463 | "|" => meta_bop {$_[0] | $_[1]}, |
464 | "atan2" => meta_bop {atan2 $_[0], $_[1]}, |
465 | |
466 | "neg" => meta_uop {-$_[0]}, |
467 | "!" => meta_uop {!$_[0]}, |
468 | "~" => meta_uop {~$_[0]}, |
469 | "cos" => meta_uop {cos $_[0]}, |
470 | "sin" => meta_uop {sin $_[0]}, |
471 | "exp" => meta_uop {exp $_[0]}, |
472 | "abs" => meta_uop {abs $_[0]}, |
473 | "log" => meta_uop {log $_[0]}, |
474 | "sqrt" => meta_uop {sqrt $_[0]}, |
475 | "bool" => sub { croak "Can't use && or || in expression containing __" }, |
476 | |
477 | # "&()" => sub { $_[0]->{impl} }, |
478 | |
479 | # "||" => meta_bop {$_[0] || $_[1]}, |
480 | # "&&" => meta_bop {$_[0] && $_[1]}, |
481 | # fallback => 1, |
482 | ; |
483 | 1; |
484 | |
485 | __END__ |
486 | |
487 | |
488 | =head1 NAME |
489 | |
490 | Switch - A switch statement for Perl |
491 | |
492 | =head1 VERSION |
493 | |
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494 | This document describes version 2.09 of Switch, |
495 | released June 12, 2002. |
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496 | |
497 | =head1 SYNOPSIS |
498 | |
499 | use Switch; |
500 | |
501 | switch ($val) { |
502 | |
503 | case 1 { print "number 1" } |
504 | case "a" { print "string a" } |
505 | case [1..10,42] { print "number in list" } |
506 | case (@array) { print "number in list" } |
507 | case /\w+/ { print "pattern" } |
508 | case qr/\w+/ { print "pattern" } |
509 | case (%hash) { print "entry in hash" } |
510 | case (\%hash) { print "entry in hash" } |
511 | case (\&sub) { print "arg to subroutine" } |
512 | else { print "previous case not true" } |
513 | } |
514 | |
515 | =head1 BACKGROUND |
516 | |
517 | [Skip ahead to L<"DESCRIPTION"> if you don't care about the whys |
518 | and wherefores of this control structure] |
519 | |
520 | In seeking to devise a "Swiss Army" case mechanism suitable for Perl, |
521 | it is useful to generalize this notion of distributed conditional |
522 | testing as far as possible. Specifically, the concept of "matching" |
523 | between the switch value and the various case values need not be |
524 | restricted to numeric (or string or referential) equality, as it is in other |
525 | languages. Indeed, as Table 1 illustrates, Perl |
526 | offers at least eighteen different ways in which two values could |
527 | generate a match. |
528 | |
529 | Table 1: Matching a switch value ($s) with a case value ($c) |
530 | |
531 | Switch Case Type of Match Implied Matching Code |
532 | Value Value |
533 | ====== ===== ===================== ============= |
534 | |
535 | number same numeric or referential match if $s == $c; |
536 | or ref equality |
537 | |
538 | object method result of method call match if $s->$c(); |
539 | ref name match if defined $s->$c(); |
540 | or ref |
541 | |
542 | other other string equality match if $s eq $c; |
543 | non-ref non-ref |
544 | scalar scalar |
545 | |
546 | string regexp pattern match match if $s =~ /$c/; |
547 | |
548 | array scalar array entry existence match if 0<=$c && $c<@$s; |
549 | ref array entry definition match if defined $s->[$c]; |
550 | array entry truth match if $s->[$c]; |
551 | |
552 | array array array intersection match if intersects(@$s, @$c); |
553 | ref ref (apply this table to |
554 | all pairs of elements |
555 | $s->[$i] and |
556 | $c->[$j]) |
557 | |
558 | array regexp array grep match if grep /$c/, @$s; |
559 | ref |
560 | |
561 | hash scalar hash entry existence match if exists $s->{$c}; |
562 | ref hash entry definition match if defined $s->{$c}; |
563 | hash entry truth match if $s->{$c}; |
564 | |
565 | hash regexp hash grep match if grep /$c/, keys %$s; |
566 | ref |
567 | |
568 | sub scalar return value defn match if defined $s->($c); |
569 | ref return value truth match if $s->($c); |
570 | |
571 | sub array return value defn match if defined $s->(@$c); |
572 | ref ref return value truth match if $s->(@$c); |
573 | |
574 | |
575 | In reality, Table 1 covers 31 alternatives, because only the equality and |
576 | intersection tests are commutative; in all other cases, the roles of |
577 | the C<$s> and C<$c> variables could be reversed to produce a |
578 | different test. For example, instead of testing a single hash for |
579 | the existence of a series of keys (C<match if exists $s-E<gt>{$c}>), |
580 | one could test for the existence of a single key in a series of hashes |
581 | (C<match if exists $c-E<gt>{$s}>). |
582 | |
583 | As L<perltodo> observes, a Perl case mechanism must support all these |
584 | "ways to do it". |
585 | |
586 | |
587 | =head1 DESCRIPTION |
588 | |
589 | The Switch.pm module implements a generalized case mechanism that covers |
590 | the numerous possible combinations of switch and case values described above. |
591 | |
592 | The module augments the standard Perl syntax with two new control |
593 | statements: C<switch> and C<case>. The C<switch> statement takes a |
594 | single scalar argument of any type, specified in parentheses. |
595 | C<switch> stores this value as the |
596 | current switch value in a (localized) control variable. |
597 | The value is followed by a block which may contain one or more |
598 | Perl statements (including the C<case> statement described below). |
599 | The block is unconditionally executed once the switch value has |
600 | been cached. |
601 | |
602 | A C<case> statement takes a single scalar argument (in mandatory |
603 | parentheses if it's a variable; otherwise the parens are optional) and |
604 | selects the appropriate type of matching between that argument and the |
605 | current switch value. The type of matching used is determined by the |
606 | respective types of the switch value and the C<case> argument, as |
607 | specified in Table 1. If the match is successful, the mandatory |
608 | block associated with the C<case> statement is executed. |
609 | |
610 | In most other respects, the C<case> statement is semantically identical |
611 | to an C<if> statement. For example, it can be followed by an C<else> |
612 | clause, and can be used as a postfix statement qualifier. |
613 | |
614 | However, when a C<case> block has been executed control is automatically |
615 | transferred to the statement after the immediately enclosing C<switch> |
616 | block, rather than to the next statement within the block. In other |
617 | words, the success of any C<case> statement prevents other cases in the |
618 | same scope from executing. But see L<"Allowing fall-through"> below. |
619 | |
620 | Together these two new statements provide a fully generalized case |
621 | mechanism: |
622 | |
623 | use Switch; |
624 | |
625 | # AND LATER... |
626 | |
627 | %special = ( woohoo => 1, d'oh => 1 ); |
628 | |
629 | while (<>) { |
630 | switch ($_) { |
631 | |
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632 | case (%special) { print "homer\n"; } # if $special{$_} |
633 | case /a-z/i { print "alpha\n"; } # if $_ =~ /a-z/i |
634 | case [1..9] { print "small num\n"; } # if $_ in [1..9] |
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635 | |
636 | case { $_[0] >= 10 } { # if $_ >= 10 |
637 | my $age = <>; |
638 | switch (sub{ $_[0] < $age } ) { |
639 | |
640 | case 20 { print "teens\n"; } # if 20 < $age |
641 | case 30 { print "twenties\n"; } # if 30 < $age |
642 | else { print "history\n"; } |
643 | } |
644 | } |
645 | |
646 | print "must be punctuation\n" case /\W/; # if $_ ~= /\W/ |
647 | } |
648 | |
649 | Note that C<switch>es can be nested within C<case> (or any other) blocks, |
650 | and a series of C<case> statements can try different types of matches |
651 | -- hash membership, pattern match, array intersection, simple equality, |
652 | etc. -- against the same switch value. |
653 | |
654 | The use of intersection tests against an array reference is particularly |
655 | useful for aggregating integral cases: |
656 | |
657 | sub classify_digit |
658 | { |
659 | switch ($_[0]) { case 0 { return 'zero' } |
660 | case [2,4,6,8] { return 'even' } |
661 | case [1,3,4,7,9] { return 'odd' } |
662 | case /[A-F]/i { return 'hex' } |
663 | } |
664 | } |
665 | |
666 | |
667 | =head2 Allowing fall-through |
668 | |
669 | Fall-though (trying another case after one has already succeeded) |
670 | is usually a Bad Idea in a switch statement. However, this |
671 | is Perl, not a police state, so there I<is> a way to do it, if you must. |
672 | |
673 | If a C<case> block executes an untargetted C<next>, control is |
674 | immediately transferred to the statement I<after> the C<case> statement |
675 | (i.e. usually another case), rather than out of the surrounding |
676 | C<switch> block. |
677 | |
678 | For example: |
679 | |
680 | switch ($val) { |
681 | case 1 { handle_num_1(); next } # and try next case... |
682 | case "1" { handle_str_1(); next } # and try next case... |
683 | case [0..9] { handle_num_any(); } # and we're done |
684 | case /\d/ { handle_dig_any(); next } # and try next case... |
685 | case /.*/ { handle_str_any(); next } # and try next case... |
686 | } |
687 | |
688 | If $val held the number C<1>, the above C<switch> block would call the |
689 | first three C<handle_...> subroutines, jumping to the next case test |
690 | each time it encountered a C<next>. After the thrid C<case> block |
691 | was executed, control would jump to the end of the enclosing |
692 | C<switch> block. |
693 | |
694 | On the other hand, if $val held C<10>, then only the last two C<handle_...> |
695 | subroutines would be called. |
696 | |
697 | Note that this mechanism allows the notion of I<conditional fall-through>. |
698 | For example: |
699 | |
700 | switch ($val) { |
701 | case [0..9] { handle_num_any(); next if $val < 7; } |
702 | case /\d/ { handle_dig_any(); } |
703 | } |
704 | |
705 | If an untargetted C<last> statement is executed in a case block, this |
706 | immediately transfers control out of the enclosing C<switch> block |
707 | (in other words, there is an implicit C<last> at the end of each |
708 | normal C<case> block). Thus the previous example could also have been |
709 | written: |
710 | |
711 | switch ($val) { |
712 | case [0..9] { handle_num_any(); last if $val >= 7; next; } |
713 | case /\d/ { handle_dig_any(); } |
714 | } |
715 | |
716 | |
717 | =head2 Automating fall-through |
718 | |
719 | In situations where case fall-through should be the norm, rather than an |
720 | exception, an endless succession of terminal C<next>s is tedious and ugly. |
721 | Hence, it is possible to reverse the default behaviour by specifying |
722 | the string "fallthrough" when importing the module. For example, the |
723 | following code is equivalent to the first example in L<"Allowing fall-through">: |
724 | |
725 | use Switch 'fallthrough'; |
726 | |
727 | switch ($val) { |
728 | case 1 { handle_num_1(); } |
729 | case "1" { handle_str_1(); } |
730 | case [0..9] { handle_num_any(); last } |
731 | case /\d/ { handle_dig_any(); } |
732 | case /.*/ { handle_str_any(); } |
733 | } |
734 | |
735 | Note the explicit use of a C<last> to preserve the non-fall-through |
736 | behaviour of the third case. |
737 | |
738 | |
739 | |
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740 | =head2 Alternative syntax |
741 | |
742 | Perl 6 will provide a built-in switch statement with essentially the |
743 | same semantics as those offered by Switch.pm, but with a different |
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744 | pair of keywords. In Perl 6 C<switch> will be spelled C<given>, and |
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745 | C<case> will be pronounced C<when>. In addition, the C<when> statement |
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746 | will not require switch or case values to be parenthesized. |
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747 | |
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748 | This future syntax is also (largely) available via the Switch.pm module, by |
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749 | importing it with the argument C<"Perl6">. For example: |
750 | |
751 | use Switch 'Perl6'; |
752 | |
753 | given ($val) { |
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754 | when 1 { handle_num_1(); } |
755 | when ($str1) { handle_str_1(); } |
756 | when [0..9] { handle_num_any(); last } |
757 | when /\d/ { handle_dig_any(); } |
758 | when /.*/ { handle_str_any(); } |
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759 | } |
760 | |
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761 | Note that scalars still need to be parenthesized, since they would be |
762 | ambiguous in Perl 5. |
763 | |
764 | Note too that you can mix and match both syntaxes by importing the module |
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765 | with: |
766 | |
767 | use Switch 'Perl5', 'Perl6'; |
768 | |
769 | |
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770 | =head2 Higher-order Operations |
771 | |
772 | One situation in which C<switch> and C<case> do not provide a good |
773 | substitute for a cascaded C<if>, is where a switch value needs to |
774 | be tested against a series of conditions. For example: |
775 | |
776 | sub beverage { |
777 | switch (shift) { |
778 | |
779 | case sub { $_[0] < 10 } { return 'milk' } |
780 | case sub { $_[0] < 20 } { return 'coke' } |
781 | case sub { $_[0] < 30 } { return 'beer' } |
782 | case sub { $_[0] < 40 } { return 'wine' } |
783 | case sub { $_[0] < 50 } { return 'malt' } |
784 | case sub { $_[0] < 60 } { return 'Moet' } |
785 | else { return 'milk' } |
786 | } |
787 | } |
788 | |
789 | The need to specify each condition as a subroutine block is tiresome. To |
790 | overcome this, when importing Switch.pm, a special "placeholder" |
791 | subroutine named C<__> [sic] may also be imported. This subroutine |
792 | converts (almost) any expression in which it appears to a reference to a |
793 | higher-order function. That is, the expression: |
794 | |
795 | use Switch '__'; |
796 | |
797 | __ < 2 + __ |
798 | |
799 | is equivalent to: |
800 | |
801 | sub { $_[0] < 2 + $_[1] } |
802 | |
803 | With C<__>, the previous ugly case statements can be rewritten: |
804 | |
805 | case __ < 10 { return 'milk' } |
806 | case __ < 20 { return 'coke' } |
807 | case __ < 30 { return 'beer' } |
808 | case __ < 40 { return 'wine' } |
809 | case __ < 50 { return 'malt' } |
810 | case __ < 60 { return 'Moet' } |
811 | else { return 'milk' } |
812 | |
813 | The C<__> subroutine makes extensive use of operator overloading to |
814 | perform its magic. All operations involving __ are overloaded to |
815 | produce an anonymous subroutine that implements a lazy version |
816 | of the original operation. |
817 | |
818 | The only problem is that operator overloading does not allow the |
819 | boolean operators C<&&> and C<||> to be overloaded. So a case statement |
820 | like this: |
821 | |
822 | case 0 <= __ && __ < 10 { return 'digit' } |
823 | |
824 | doesn't act as expected, because when it is |
825 | executed, it constructs two higher order subroutines |
826 | and then treats the two resulting references as arguments to C<&&>: |
827 | |
828 | sub { 0 <= $_[0] } && sub { $_[0] < 10 } |
829 | |
830 | This boolean expression is inevitably true, since both references are |
831 | non-false. Fortunately, the overloaded C<'bool'> operator catches this |
832 | situation and flags it as a error. |
833 | |
834 | =head1 DEPENDENCIES |
835 | |
836 | The module is implemented using Filter::Util::Call and Text::Balanced |
837 | and requires both these modules to be installed. |
838 | |
839 | =head1 AUTHOR |
840 | |
841 | Damian Conway (damian@conway.org) |
842 | |
843 | =head1 BUGS |
844 | |
845 | There are undoubtedly serious bugs lurking somewhere in code this funky :-) |
846 | Bug reports and other feedback are most welcome. |
847 | |
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848 | =head1 LIMITATION |
849 | |
850 | Due to the heuristic nature of Switch.pm's source parsing, the presence |
851 | of regexes specified with raw C<?...?> delimiters may cause mysterious |
852 | errors. The workaround is to use C<m?...?> instead. |
853 | |
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854 | =head1 COPYRIGHT |
855 | |
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856 | Copyright (c) 1997-2001, Damian Conway. All Rights Reserved. |
857 | This module is free software. It may be used, redistributed |
858 | and/or modified under the same terms as Perl itself. |