Commit | Line | Data |
3ed9f206 |
1 | package Switch; |
2 | |
3 | use strict; |
4 | use vars qw($VERSION); |
5 | use Carp; |
6 | |
f43acd3e |
7 | $VERSION = '2.10'; |
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 = ""; |
93 | component: while (pos $source < length $source) |
94 | { |
3961318e |
95 | if ($source =~ m/(\G\s*use\s+Switch\b)/gc) |
3ed9f206 |
96 | { |
97 | $text .= q{use Switch 'noimport'}; |
98 | next component; |
99 | } |
d38ca171 |
100 | my @pos = Text::Balanced::_match_quotelike(\$source,qr/\s*/,1,0); |
3ed9f206 |
101 | if (defined $pos[0]) |
102 | { |
52d8c818 |
103 | $text .= " " if $pos[0] < $pos[2]; |
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 | { |
52d8c818 |
113 | $text .= " " if $pos[0] < $pos[2]; |
114 | $text .= substr($source,$pos[0],$pos[4]-$pos[0]); |
3ed9f206 |
115 | next component; |
116 | } |
117 | |
74a6a946 |
118 | if ($Perl5 && $source =~ m/\G(\n*)(\s*)(switch)\b(?=\s*[(])/gc |
6596d39b |
119 | || $Perl6 && $source =~ m/\G(\n*)(\s*)(given)\b(?=\s*[(])/gc |
120 | || $Perl6 && $source =~ m/\G(\n*)(\s*)(given)\b(.*)(?=\{)/gc) |
3ed9f206 |
121 | { |
74a6a946 |
122 | my $keyword = $3; |
6596d39b |
123 | my $arg = $4; |
9c291939 |
124 | # print STDERR "[$arg]\n"; |
3ed9f206 |
125 | $text .= $1.$2.'S_W_I_T_C_H: while (1) '; |
6596d39b |
126 | unless ($arg) { |
127 | @pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\(/,qr/\)/,qr/[[{(<]/,qr/[]})>]/,undef) |
128 | or do { |
129 | die "Bad $keyword statement (problem in the parentheses?) near $Switch::file line ", line(substr($source,0,pos $source),$line), "\n"; |
130 | }; |
131 | $arg = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
132 | } |
3ed9f206 |
133 | $arg =~ s {^\s*[(]\s*%} { ( \\\%} || |
134 | $arg =~ s {^\s*[(]\s*m\b} { ( qr} || |
135 | $arg =~ s {^\s*[(]\s*/} { ( qr/} || |
136 | $arg =~ s {^\s*[(]\s*qw} { ( \\qw}; |
137 | @pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef) |
138 | or do { |
74a6a946 |
139 | die "Bad $keyword statement (problem in the code block?) near $Switch::file line ", line(substr($source,0, pos $source), $line), "\n"; |
3ed9f206 |
140 | }; |
141 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
142 | $code =~ s/{/{ local \$::_S_W_I_T_C_H; Switch::switch $arg;/; |
143 | $text .= $code . 'continue {last}'; |
144 | next component; |
145 | } |
74a6a946 |
146 | elsif ($Perl5 && $source =~ m/\G(\s*)(case\b)(?!\s*=>)/gc |
147 | || $Perl6 && $source =~ m/\G(\s*)(when\b)(?!\s*=>)/gc) |
3ed9f206 |
148 | { |
74a6a946 |
149 | my $keyword = $2; |
3ed9f206 |
150 | $text .= $1."if (Switch::case"; |
151 | if (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)) { |
152 | my $code = substr($source,$pos[0],$pos[4]-$pos[0]); |
52d8c818 |
153 | $text .= " " if $pos[0] < $pos[2]; |
154 | $text .= "sub " if is_block $code; |
155 | $text .= filter_blocks($code,line(substr($source,0,$pos[0]),$line)) . ")"; |
3ed9f206 |
156 | } |
157 | elsif (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/[[(]/,qr/[])]/,qr/[[({]/,qr/[])}]/,undef)) { |
158 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
159 | $code =~ s {^\s*[(]\s*%} { ( \\\%} || |
160 | $code =~ s {^\s*[(]\s*m\b} { ( qr} || |
161 | $code =~ s {^\s*[(]\s*/} { ( qr/} || |
162 | $code =~ s {^\s*[(]\s*qw} { ( \\qw}; |
52d8c818 |
163 | $text .= " " if $pos[0] < $pos[2]; |
164 | $text .= "$code)"; |
3ed9f206 |
165 | } |
74a6a946 |
166 | elsif ($Perl6 && do{@pos = Text::Balanced::_match_variable(\$source,qr/\s*/)}) { |
167 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
168 | $code =~ s {^\s*%} { \%} || |
169 | $code =~ s {^\s*@} { \@}; |
52d8c818 |
170 | $text .= " " if $pos[0] < $pos[2]; |
171 | $text .= "$code)"; |
74a6a946 |
172 | } |
d38ca171 |
173 | elsif ( @pos = Text::Balanced::_match_quotelike(\$source,qr/\s*/,1,0)) { |
3ed9f206 |
174 | my $code = substr($source,$pos[2],$pos[18]-$pos[2]); |
175 | $code = filter_blocks($code,line(substr($source,0,$pos[2]),$line)); |
176 | $code =~ s {^\s*m} { qr} || |
177 | $code =~ s {^\s*/} { qr/} || |
178 | $code =~ s {^\s*qw} { \\qw}; |
52d8c818 |
179 | $text .= " " if $pos[0] < $pos[2]; |
180 | $text .= "$code)"; |
3ed9f206 |
181 | } |
74a6a946 |
182 | elsif ($Perl5 && $source =~ m/\G\s*(([^\$\@{])[^\$\@{]*)(?=\s*{)/gc |
6596d39b |
183 | || $Perl6 && $source =~ m/\G\s*([^;{]*)()/gc) { |
3ed9f206 |
184 | my $code = filter_blocks($1,line(substr($source,0,pos $source),$line)); |
185 | $text .= ' \\' if $2 eq '%'; |
186 | $text .= " $code)"; |
187 | } |
188 | else { |
74a6a946 |
189 | die "Bad $keyword statement (invalid $keyword value?) near $Switch::file line ", line(substr($source,0,pos $source), $line), "\n"; |
3ed9f206 |
190 | } |
191 | |
6596d39b |
192 | die "Missing opening brace or semi-colon after 'when' value near $Switch::file line ", line(substr($source,0,pos $source), $line), "\n" |
193 | unless !$Perl6 || $source =~ m/\G(\s*)(?=;|\{)/gc; |
74a6a946 |
194 | |
195 | do{@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)} |
3ed9f206 |
196 | or do { |
197 | if ($source =~ m/\G\s*(?=([};]|\Z))/gc) { |
198 | $casecounter++; |
199 | next component; |
200 | } |
74a6a946 |
201 | die "Bad $keyword statement (problem in the code block?) near $Switch::file line ", line(substr($source,0,pos $source),$line), "\n"; |
3ed9f206 |
202 | }; |
203 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
204 | $code =~ s/}(?=\s*\Z)/;last S_W_I_T_C_H }/ |
205 | unless $fallthrough; |
206 | $text .= "{ while (1) $code continue { goto C_A_S_E_$casecounter } last S_W_I_T_C_H; C_A_S_E_$casecounter: }"; |
207 | $casecounter++; |
208 | next component; |
209 | } |
210 | |
d38ca171 |
211 | $source =~ m/\G(\s*(-[sm]\s+|\w+|#.*\n|\W))/gc; |
3ed9f206 |
212 | $text .= $1; |
213 | } |
214 | $text; |
215 | } |
216 | |
217 | |
218 | |
219 | sub in |
220 | { |
221 | my ($x,$y) = @_; |
222 | my @numy; |
223 | for my $nextx ( @$x ) |
224 | { |
a1813bef |
225 | my $numx = ref($nextx) || defined $nextx && (~$nextx&$nextx) eq 0; |
3ed9f206 |
226 | for my $j ( 0..$#$y ) |
227 | { |
228 | my $nexty = $y->[$j]; |
a1813bef |
229 | push @numy, ref($nexty) || defined $nexty && (~$nexty&$nexty) eq 0 |
3ed9f206 |
230 | if @numy <= $j; |
231 | return 1 if $numx && $numy[$j] && $nextx==$nexty |
232 | || $nextx eq $nexty; |
233 | |
234 | } |
235 | } |
236 | return ""; |
237 | } |
238 | |
239 | sub on_exists |
240 | { |
241 | my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ }; |
242 | [ keys %$ref ] |
243 | } |
244 | |
245 | sub on_defined |
246 | { |
247 | my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ }; |
248 | [ grep { defined $ref->{$_} } keys %$ref ] |
249 | } |
250 | |
251 | sub switch(;$) |
252 | { |
253 | my ($s_val) = @_ ? $_[0] : $_; |
254 | my $s_ref = ref $s_val; |
255 | |
256 | if ($s_ref eq 'CODE') |
257 | { |
258 | $::_S_W_I_T_C_H = |
259 | sub { my $c_val = $_[0]; |
260 | return $s_val == $c_val if ref $c_val eq 'CODE'; |
261 | return $s_val->(@$c_val) if ref $c_val eq 'ARRAY'; |
262 | return $s_val->($c_val); |
263 | }; |
264 | } |
a1813bef |
265 | elsif ($s_ref eq "" && defined $s_val && (~$s_val&$s_val) eq 0) # NUMERIC SCALAR |
3ed9f206 |
266 | { |
267 | $::_S_W_I_T_C_H = |
268 | sub { my $c_val = $_[0]; |
269 | my $c_ref = ref $c_val; |
270 | return $s_val == $c_val if $c_ref eq "" |
a1813bef |
271 | && defined $c_val |
3ed9f206 |
272 | && (~$c_val&$c_val) eq 0; |
273 | return $s_val eq $c_val if $c_ref eq ""; |
274 | return in([$s_val],$c_val) if $c_ref eq 'ARRAY'; |
275 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
276 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
277 | return scalar $s_val=~/$c_val/ |
278 | if $c_ref eq 'Regexp'; |
279 | return scalar $c_val->{$s_val} |
280 | if $c_ref eq 'HASH'; |
281 | return; |
282 | }; |
283 | } |
284 | elsif ($s_ref eq "") # STRING SCALAR |
285 | { |
286 | $::_S_W_I_T_C_H = |
287 | sub { my $c_val = $_[0]; |
288 | my $c_ref = ref $c_val; |
289 | return $s_val eq $c_val if $c_ref eq ""; |
290 | return in([$s_val],$c_val) if $c_ref eq 'ARRAY'; |
291 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
292 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
293 | return scalar $s_val=~/$c_val/ |
294 | if $c_ref eq 'Regexp'; |
295 | return scalar $c_val->{$s_val} |
296 | if $c_ref eq 'HASH'; |
297 | return; |
298 | }; |
299 | } |
300 | elsif ($s_ref eq 'ARRAY') |
301 | { |
302 | $::_S_W_I_T_C_H = |
303 | sub { my $c_val = $_[0]; |
304 | my $c_ref = ref $c_val; |
305 | return in($s_val,[$c_val]) if $c_ref eq ""; |
306 | return in($s_val,$c_val) if $c_ref eq 'ARRAY'; |
307 | return $c_val->(@$s_val) if $c_ref eq 'CODE'; |
308 | return $c_val->call(@$s_val) |
309 | if $c_ref eq 'Switch'; |
310 | return scalar grep {$_=~/$c_val/} @$s_val |
311 | if $c_ref eq 'Regexp'; |
312 | return scalar grep {$c_val->{$_}} @$s_val |
313 | if $c_ref eq 'HASH'; |
314 | return; |
315 | }; |
316 | } |
317 | elsif ($s_ref eq 'Regexp') |
318 | { |
319 | $::_S_W_I_T_C_H = |
320 | sub { my $c_val = $_[0]; |
321 | my $c_ref = ref $c_val; |
322 | return $c_val=~/s_val/ if $c_ref eq ""; |
323 | return scalar grep {$_=~/s_val/} @$c_val |
324 | if $c_ref eq 'ARRAY'; |
325 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
326 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
327 | return $s_val eq $c_val if $c_ref eq 'Regexp'; |
328 | return grep {$_=~/$s_val/ && $c_val->{$_}} keys %$c_val |
329 | if $c_ref eq 'HASH'; |
330 | return; |
331 | }; |
332 | } |
333 | elsif ($s_ref eq 'HASH') |
334 | { |
335 | $::_S_W_I_T_C_H = |
336 | sub { my $c_val = $_[0]; |
337 | my $c_ref = ref $c_val; |
338 | return $s_val->{$c_val} if $c_ref eq ""; |
339 | return scalar grep {$s_val->{$_}} @$c_val |
340 | if $c_ref eq 'ARRAY'; |
341 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
342 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
343 | return grep {$_=~/$c_val/ && $s_val->{"$_"}} keys %$s_val |
344 | if $c_ref eq 'Regexp'; |
345 | return $s_val==$c_val if $c_ref eq 'HASH'; |
346 | return; |
347 | }; |
348 | } |
349 | elsif ($s_ref eq 'Switch') |
350 | { |
351 | $::_S_W_I_T_C_H = |
352 | sub { my $c_val = $_[0]; |
353 | return $s_val == $c_val if ref $c_val eq 'Switch'; |
354 | return $s_val->call(@$c_val) |
355 | if ref $c_val eq 'ARRAY'; |
356 | return $s_val->call($c_val); |
357 | }; |
358 | } |
359 | else |
360 | { |
361 | croak "Cannot switch on $s_ref"; |
362 | } |
363 | return 1; |
364 | } |
365 | |
d38ca171 |
366 | sub case($) { local $SIG{__WARN__} = \&carp; |
367 | $::_S_W_I_T_C_H->(@_); } |
3ed9f206 |
368 | |
369 | # IMPLEMENT __ |
370 | |
371 | my $placeholder = bless { arity=>1, impl=>sub{$_[1+$_[0]]} }; |
372 | |
373 | sub __() { $placeholder } |
374 | |
375 | sub __arg($) |
376 | { |
377 | my $index = $_[0]+1; |
378 | bless { arity=>0, impl=>sub{$_[$index]} }; |
379 | } |
380 | |
381 | sub hosub(&@) |
382 | { |
383 | # WRITE THIS |
384 | } |
385 | |
386 | sub call |
387 | { |
388 | my ($self,@args) = @_; |
389 | return $self->{impl}->(0,@args); |
390 | } |
391 | |
392 | sub meta_bop(&) |
393 | { |
394 | my ($op) = @_; |
395 | sub |
396 | { |
397 | my ($left, $right, $reversed) = @_; |
398 | ($right,$left) = @_ if $reversed; |
399 | |
400 | my $rop = ref $right eq 'Switch' |
401 | ? $right |
402 | : bless { arity=>0, impl=>sub{$right} }; |
403 | |
404 | my $lop = ref $left eq 'Switch' |
405 | ? $left |
406 | : bless { arity=>0, impl=>sub{$left} }; |
407 | |
408 | my $arity = $lop->{arity} + $rop->{arity}; |
409 | |
410 | return bless { |
411 | arity => $arity, |
412 | impl => sub { my $start = shift; |
413 | return $op->($lop->{impl}->($start,@_), |
414 | $rop->{impl}->($start+$lop->{arity},@_)); |
415 | } |
416 | }; |
417 | }; |
418 | } |
419 | |
420 | sub meta_uop(&) |
421 | { |
422 | my ($op) = @_; |
423 | sub |
424 | { |
425 | my ($left) = @_; |
426 | |
427 | my $lop = ref $left eq 'Switch' |
428 | ? $left |
429 | : bless { arity=>0, impl=>sub{$left} }; |
430 | |
431 | my $arity = $lop->{arity}; |
432 | |
433 | return bless { |
434 | arity => $arity, |
435 | impl => sub { $op->($lop->{impl}->(@_)) } |
436 | }; |
437 | }; |
438 | } |
439 | |
440 | |
441 | use overload |
442 | "+" => meta_bop {$_[0] + $_[1]}, |
443 | "-" => meta_bop {$_[0] - $_[1]}, |
444 | "*" => meta_bop {$_[0] * $_[1]}, |
445 | "/" => meta_bop {$_[0] / $_[1]}, |
446 | "%" => meta_bop {$_[0] % $_[1]}, |
447 | "**" => meta_bop {$_[0] ** $_[1]}, |
448 | "<<" => meta_bop {$_[0] << $_[1]}, |
449 | ">>" => meta_bop {$_[0] >> $_[1]}, |
450 | "x" => meta_bop {$_[0] x $_[1]}, |
451 | "." => meta_bop {$_[0] . $_[1]}, |
452 | "<" => meta_bop {$_[0] < $_[1]}, |
453 | "<=" => meta_bop {$_[0] <= $_[1]}, |
454 | ">" => meta_bop {$_[0] > $_[1]}, |
455 | ">=" => meta_bop {$_[0] >= $_[1]}, |
456 | "==" => meta_bop {$_[0] == $_[1]}, |
457 | "!=" => meta_bop {$_[0] != $_[1]}, |
458 | "<=>" => meta_bop {$_[0] <=> $_[1]}, |
459 | "lt" => meta_bop {$_[0] lt $_[1]}, |
460 | "le" => meta_bop {$_[0] le $_[1]}, |
461 | "gt" => meta_bop {$_[0] gt $_[1]}, |
462 | "ge" => meta_bop {$_[0] ge $_[1]}, |
463 | "eq" => meta_bop {$_[0] eq $_[1]}, |
464 | "ne" => meta_bop {$_[0] ne $_[1]}, |
465 | "cmp" => meta_bop {$_[0] cmp $_[1]}, |
466 | "\&" => meta_bop {$_[0] & $_[1]}, |
467 | "^" => meta_bop {$_[0] ^ $_[1]}, |
468 | "|" => meta_bop {$_[0] | $_[1]}, |
469 | "atan2" => meta_bop {atan2 $_[0], $_[1]}, |
470 | |
471 | "neg" => meta_uop {-$_[0]}, |
472 | "!" => meta_uop {!$_[0]}, |
473 | "~" => meta_uop {~$_[0]}, |
474 | "cos" => meta_uop {cos $_[0]}, |
475 | "sin" => meta_uop {sin $_[0]}, |
476 | "exp" => meta_uop {exp $_[0]}, |
477 | "abs" => meta_uop {abs $_[0]}, |
478 | "log" => meta_uop {log $_[0]}, |
479 | "sqrt" => meta_uop {sqrt $_[0]}, |
480 | "bool" => sub { croak "Can't use && or || in expression containing __" }, |
481 | |
482 | # "&()" => sub { $_[0]->{impl} }, |
483 | |
484 | # "||" => meta_bop {$_[0] || $_[1]}, |
485 | # "&&" => meta_bop {$_[0] && $_[1]}, |
486 | # fallback => 1, |
487 | ; |
488 | 1; |
489 | |
490 | __END__ |
491 | |
492 | |
493 | =head1 NAME |
494 | |
495 | Switch - A switch statement for Perl |
496 | |
497 | =head1 VERSION |
498 | |
218ab4da |
499 | This document describes version 2.09 of Switch, |
500 | released June 12, 2002. |
3ed9f206 |
501 | |
502 | =head1 SYNOPSIS |
503 | |
504 | use Switch; |
505 | |
506 | switch ($val) { |
507 | |
508 | case 1 { print "number 1" } |
509 | case "a" { print "string a" } |
510 | case [1..10,42] { print "number in list" } |
511 | case (@array) { print "number in list" } |
512 | case /\w+/ { print "pattern" } |
513 | case qr/\w+/ { print "pattern" } |
514 | case (%hash) { print "entry in hash" } |
515 | case (\%hash) { print "entry in hash" } |
516 | case (\&sub) { print "arg to subroutine" } |
517 | else { print "previous case not true" } |
518 | } |
519 | |
520 | =head1 BACKGROUND |
521 | |
522 | [Skip ahead to L<"DESCRIPTION"> if you don't care about the whys |
523 | and wherefores of this control structure] |
524 | |
525 | In seeking to devise a "Swiss Army" case mechanism suitable for Perl, |
526 | it is useful to generalize this notion of distributed conditional |
527 | testing as far as possible. Specifically, the concept of "matching" |
528 | between the switch value and the various case values need not be |
529 | restricted to numeric (or string or referential) equality, as it is in other |
530 | languages. Indeed, as Table 1 illustrates, Perl |
531 | offers at least eighteen different ways in which two values could |
532 | generate a match. |
533 | |
534 | Table 1: Matching a switch value ($s) with a case value ($c) |
535 | |
536 | Switch Case Type of Match Implied Matching Code |
537 | Value Value |
538 | ====== ===== ===================== ============= |
539 | |
540 | number same numeric or referential match if $s == $c; |
541 | or ref equality |
542 | |
543 | object method result of method call match if $s->$c(); |
544 | ref name match if defined $s->$c(); |
545 | or ref |
546 | |
547 | other other string equality match if $s eq $c; |
548 | non-ref non-ref |
549 | scalar scalar |
550 | |
551 | string regexp pattern match match if $s =~ /$c/; |
552 | |
553 | array scalar array entry existence match if 0<=$c && $c<@$s; |
554 | ref array entry definition match if defined $s->[$c]; |
555 | array entry truth match if $s->[$c]; |
556 | |
557 | array array array intersection match if intersects(@$s, @$c); |
558 | ref ref (apply this table to |
559 | all pairs of elements |
560 | $s->[$i] and |
561 | $c->[$j]) |
562 | |
563 | array regexp array grep match if grep /$c/, @$s; |
564 | ref |
565 | |
566 | hash scalar hash entry existence match if exists $s->{$c}; |
567 | ref hash entry definition match if defined $s->{$c}; |
568 | hash entry truth match if $s->{$c}; |
569 | |
570 | hash regexp hash grep match if grep /$c/, keys %$s; |
571 | ref |
572 | |
573 | sub scalar return value defn match if defined $s->($c); |
574 | ref return value truth match if $s->($c); |
575 | |
576 | sub array return value defn match if defined $s->(@$c); |
577 | ref ref return value truth match if $s->(@$c); |
578 | |
579 | |
580 | In reality, Table 1 covers 31 alternatives, because only the equality and |
581 | intersection tests are commutative; in all other cases, the roles of |
582 | the C<$s> and C<$c> variables could be reversed to produce a |
583 | different test. For example, instead of testing a single hash for |
584 | the existence of a series of keys (C<match if exists $s-E<gt>{$c}>), |
585 | one could test for the existence of a single key in a series of hashes |
586 | (C<match if exists $c-E<gt>{$s}>). |
587 | |
588 | As L<perltodo> observes, a Perl case mechanism must support all these |
589 | "ways to do it". |
590 | |
591 | |
592 | =head1 DESCRIPTION |
593 | |
594 | The Switch.pm module implements a generalized case mechanism that covers |
595 | the numerous possible combinations of switch and case values described above. |
596 | |
597 | The module augments the standard Perl syntax with two new control |
598 | statements: C<switch> and C<case>. The C<switch> statement takes a |
599 | single scalar argument of any type, specified in parentheses. |
600 | C<switch> stores this value as the |
601 | current switch value in a (localized) control variable. |
602 | The value is followed by a block which may contain one or more |
603 | Perl statements (including the C<case> statement described below). |
604 | The block is unconditionally executed once the switch value has |
605 | been cached. |
606 | |
607 | A C<case> statement takes a single scalar argument (in mandatory |
608 | parentheses if it's a variable; otherwise the parens are optional) and |
609 | selects the appropriate type of matching between that argument and the |
610 | current switch value. The type of matching used is determined by the |
611 | respective types of the switch value and the C<case> argument, as |
612 | specified in Table 1. If the match is successful, the mandatory |
613 | block associated with the C<case> statement is executed. |
614 | |
615 | In most other respects, the C<case> statement is semantically identical |
616 | to an C<if> statement. For example, it can be followed by an C<else> |
617 | clause, and can be used as a postfix statement qualifier. |
618 | |
619 | However, when a C<case> block has been executed control is automatically |
620 | transferred to the statement after the immediately enclosing C<switch> |
621 | block, rather than to the next statement within the block. In other |
622 | words, the success of any C<case> statement prevents other cases in the |
623 | same scope from executing. But see L<"Allowing fall-through"> below. |
624 | |
625 | Together these two new statements provide a fully generalized case |
626 | mechanism: |
627 | |
628 | use Switch; |
629 | |
630 | # AND LATER... |
631 | |
632 | %special = ( woohoo => 1, d'oh => 1 ); |
633 | |
634 | while (<>) { |
635 | switch ($_) { |
636 | |
74a6a946 |
637 | case (%special) { print "homer\n"; } # if $special{$_} |
638 | case /a-z/i { print "alpha\n"; } # if $_ =~ /a-z/i |
639 | case [1..9] { print "small num\n"; } # if $_ in [1..9] |
3ed9f206 |
640 | |
641 | case { $_[0] >= 10 } { # if $_ >= 10 |
642 | my $age = <>; |
643 | switch (sub{ $_[0] < $age } ) { |
644 | |
645 | case 20 { print "teens\n"; } # if 20 < $age |
646 | case 30 { print "twenties\n"; } # if 30 < $age |
647 | else { print "history\n"; } |
648 | } |
649 | } |
650 | |
651 | print "must be punctuation\n" case /\W/; # if $_ ~= /\W/ |
652 | } |
653 | |
654 | Note that C<switch>es can be nested within C<case> (or any other) blocks, |
655 | and a series of C<case> statements can try different types of matches |
656 | -- hash membership, pattern match, array intersection, simple equality, |
657 | etc. -- against the same switch value. |
658 | |
659 | The use of intersection tests against an array reference is particularly |
660 | useful for aggregating integral cases: |
661 | |
662 | sub classify_digit |
663 | { |
664 | switch ($_[0]) { case 0 { return 'zero' } |
665 | case [2,4,6,8] { return 'even' } |
666 | case [1,3,4,7,9] { return 'odd' } |
667 | case /[A-F]/i { return 'hex' } |
668 | } |
669 | } |
670 | |
671 | |
672 | =head2 Allowing fall-through |
673 | |
674 | Fall-though (trying another case after one has already succeeded) |
675 | is usually a Bad Idea in a switch statement. However, this |
676 | is Perl, not a police state, so there I<is> a way to do it, if you must. |
677 | |
678 | If a C<case> block executes an untargetted C<next>, control is |
679 | immediately transferred to the statement I<after> the C<case> statement |
680 | (i.e. usually another case), rather than out of the surrounding |
681 | C<switch> block. |
682 | |
683 | For example: |
684 | |
685 | switch ($val) { |
686 | case 1 { handle_num_1(); next } # and try next case... |
687 | case "1" { handle_str_1(); next } # and try next case... |
688 | case [0..9] { handle_num_any(); } # and we're done |
689 | case /\d/ { handle_dig_any(); next } # and try next case... |
690 | case /.*/ { handle_str_any(); next } # and try next case... |
691 | } |
692 | |
693 | If $val held the number C<1>, the above C<switch> block would call the |
694 | first three C<handle_...> subroutines, jumping to the next case test |
695 | each time it encountered a C<next>. After the thrid C<case> block |
696 | was executed, control would jump to the end of the enclosing |
697 | C<switch> block. |
698 | |
699 | On the other hand, if $val held C<10>, then only the last two C<handle_...> |
700 | subroutines would be called. |
701 | |
702 | Note that this mechanism allows the notion of I<conditional fall-through>. |
703 | For example: |
704 | |
705 | switch ($val) { |
706 | case [0..9] { handle_num_any(); next if $val < 7; } |
707 | case /\d/ { handle_dig_any(); } |
708 | } |
709 | |
710 | If an untargetted C<last> statement is executed in a case block, this |
711 | immediately transfers control out of the enclosing C<switch> block |
712 | (in other words, there is an implicit C<last> at the end of each |
713 | normal C<case> block). Thus the previous example could also have been |
714 | written: |
715 | |
716 | switch ($val) { |
717 | case [0..9] { handle_num_any(); last if $val >= 7; next; } |
718 | case /\d/ { handle_dig_any(); } |
719 | } |
720 | |
721 | |
722 | =head2 Automating fall-through |
723 | |
724 | In situations where case fall-through should be the norm, rather than an |
725 | exception, an endless succession of terminal C<next>s is tedious and ugly. |
726 | Hence, it is possible to reverse the default behaviour by specifying |
727 | the string "fallthrough" when importing the module. For example, the |
728 | following code is equivalent to the first example in L<"Allowing fall-through">: |
729 | |
730 | use Switch 'fallthrough'; |
731 | |
732 | switch ($val) { |
733 | case 1 { handle_num_1(); } |
734 | case "1" { handle_str_1(); } |
735 | case [0..9] { handle_num_any(); last } |
736 | case /\d/ { handle_dig_any(); } |
737 | case /.*/ { handle_str_any(); } |
738 | } |
739 | |
740 | Note the explicit use of a C<last> to preserve the non-fall-through |
741 | behaviour of the third case. |
742 | |
743 | |
744 | |
74a6a946 |
745 | =head2 Alternative syntax |
746 | |
747 | Perl 6 will provide a built-in switch statement with essentially the |
748 | same semantics as those offered by Switch.pm, but with a different |
693b9afd |
749 | pair of keywords. In Perl 6 C<switch> will be spelled C<given>, and |
74a6a946 |
750 | C<case> will be pronounced C<when>. In addition, the C<when> statement |
6596d39b |
751 | will not require switch or case values to be parenthesized. |
74a6a946 |
752 | |
6596d39b |
753 | This future syntax is also (largely) available via the Switch.pm module, by |
74a6a946 |
754 | importing it with the argument C<"Perl6">. For example: |
755 | |
756 | use Switch 'Perl6'; |
757 | |
758 | given ($val) { |
6596d39b |
759 | when 1 { handle_num_1(); } |
760 | when ($str1) { handle_str_1(); } |
761 | when [0..9] { handle_num_any(); last } |
762 | when /\d/ { handle_dig_any(); } |
763 | when /.*/ { handle_str_any(); } |
74a6a946 |
764 | } |
765 | |
6596d39b |
766 | Note that scalars still need to be parenthesized, since they would be |
767 | ambiguous in Perl 5. |
768 | |
769 | Note too that you can mix and match both syntaxes by importing the module |
74a6a946 |
770 | with: |
771 | |
772 | use Switch 'Perl5', 'Perl6'; |
773 | |
774 | |
3ed9f206 |
775 | =head2 Higher-order Operations |
776 | |
777 | One situation in which C<switch> and C<case> do not provide a good |
778 | substitute for a cascaded C<if>, is where a switch value needs to |
779 | be tested against a series of conditions. For example: |
780 | |
781 | sub beverage { |
782 | switch (shift) { |
783 | |
784 | case sub { $_[0] < 10 } { return 'milk' } |
785 | case sub { $_[0] < 20 } { return 'coke' } |
786 | case sub { $_[0] < 30 } { return 'beer' } |
787 | case sub { $_[0] < 40 } { return 'wine' } |
788 | case sub { $_[0] < 50 } { return 'malt' } |
789 | case sub { $_[0] < 60 } { return 'Moet' } |
790 | else { return 'milk' } |
791 | } |
792 | } |
793 | |
794 | The need to specify each condition as a subroutine block is tiresome. To |
795 | overcome this, when importing Switch.pm, a special "placeholder" |
796 | subroutine named C<__> [sic] may also be imported. This subroutine |
797 | converts (almost) any expression in which it appears to a reference to a |
798 | higher-order function. That is, the expression: |
799 | |
800 | use Switch '__'; |
801 | |
802 | __ < 2 + __ |
803 | |
804 | is equivalent to: |
805 | |
806 | sub { $_[0] < 2 + $_[1] } |
807 | |
808 | With C<__>, the previous ugly case statements can be rewritten: |
809 | |
810 | case __ < 10 { return 'milk' } |
811 | case __ < 20 { return 'coke' } |
812 | case __ < 30 { return 'beer' } |
813 | case __ < 40 { return 'wine' } |
814 | case __ < 50 { return 'malt' } |
815 | case __ < 60 { return 'Moet' } |
816 | else { return 'milk' } |
817 | |
818 | The C<__> subroutine makes extensive use of operator overloading to |
819 | perform its magic. All operations involving __ are overloaded to |
820 | produce an anonymous subroutine that implements a lazy version |
821 | of the original operation. |
822 | |
823 | The only problem is that operator overloading does not allow the |
824 | boolean operators C<&&> and C<||> to be overloaded. So a case statement |
825 | like this: |
826 | |
827 | case 0 <= __ && __ < 10 { return 'digit' } |
828 | |
829 | doesn't act as expected, because when it is |
830 | executed, it constructs two higher order subroutines |
831 | and then treats the two resulting references as arguments to C<&&>: |
832 | |
833 | sub { 0 <= $_[0] } && sub { $_[0] < 10 } |
834 | |
835 | This boolean expression is inevitably true, since both references are |
836 | non-false. Fortunately, the overloaded C<'bool'> operator catches this |
837 | situation and flags it as a error. |
838 | |
839 | =head1 DEPENDENCIES |
840 | |
841 | The module is implemented using Filter::Util::Call and Text::Balanced |
842 | and requires both these modules to be installed. |
843 | |
844 | =head1 AUTHOR |
845 | |
846 | Damian Conway (damian@conway.org) |
847 | |
848 | =head1 BUGS |
849 | |
850 | There are undoubtedly serious bugs lurking somewhere in code this funky :-) |
851 | Bug reports and other feedback are most welcome. |
852 | |
d38ca171 |
853 | =head1 LIMITATION |
854 | |
855 | Due to the heuristic nature of Switch.pm's source parsing, the presence |
856 | of regexes specified with raw C<?...?> delimiters may cause mysterious |
857 | errors. The workaround is to use C<m?...?> instead. |
858 | |
3ed9f206 |
859 | =head1 COPYRIGHT |
860 | |
55a1c97c |
861 | Copyright (c) 1997-2001, Damian Conway. All Rights Reserved. |
862 | This module is free software. It may be used, redistributed |
863 | and/or modified under the same terms as Perl itself. |