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 | { |
52d8c818 |
104 | $text .= " " if $pos[0] < $pos[2]; |
105 | $text .= substr($source,$pos[2],$pos[18]-$pos[2]); |
3ed9f206 |
106 | next component; |
107 | } |
d38ca171 |
108 | if ($source =~ m/\G\s*($pod_or_DATA)/gc) { |
109 | next component; |
110 | } |
3ed9f206 |
111 | @pos = Text::Balanced::_match_variable(\$source,qr/\s*/); |
112 | if (defined $pos[0]) |
113 | { |
52d8c818 |
114 | $text .= " " if $pos[0] < $pos[2]; |
115 | $text .= substr($source,$pos[0],$pos[4]-$pos[0]); |
3ed9f206 |
116 | next component; |
117 | } |
118 | |
74a6a946 |
119 | if ($Perl5 && $source =~ m/\G(\n*)(\s*)(switch)\b(?=\s*[(])/gc |
6596d39b |
120 | || $Perl6 && $source =~ m/\G(\n*)(\s*)(given)\b(?=\s*[(])/gc |
121 | || $Perl6 && $source =~ m/\G(\n*)(\s*)(given)\b(.*)(?=\{)/gc) |
3ed9f206 |
122 | { |
74a6a946 |
123 | my $keyword = $3; |
6596d39b |
124 | my $arg = $4; |
9c291939 |
125 | # print STDERR "[$arg]\n"; |
3ed9f206 |
126 | $text .= $1.$2.'S_W_I_T_C_H: while (1) '; |
6596d39b |
127 | unless ($arg) { |
128 | @pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\(/,qr/\)/,qr/[[{(<]/,qr/[]})>]/,undef) |
129 | or do { |
130 | die "Bad $keyword statement (problem in the parentheses?) near $Switch::file line ", line(substr($source,0,pos $source),$line), "\n"; |
131 | }; |
132 | $arg = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
133 | } |
3ed9f206 |
134 | $arg =~ s {^\s*[(]\s*%} { ( \\\%} || |
135 | $arg =~ s {^\s*[(]\s*m\b} { ( qr} || |
136 | $arg =~ s {^\s*[(]\s*/} { ( qr/} || |
137 | $arg =~ s {^\s*[(]\s*qw} { ( \\qw}; |
138 | @pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef) |
139 | or do { |
74a6a946 |
140 | die "Bad $keyword statement (problem in the code block?) near $Switch::file line ", line(substr($source,0, pos $source), $line), "\n"; |
3ed9f206 |
141 | }; |
142 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
143 | $code =~ s/{/{ local \$::_S_W_I_T_C_H; Switch::switch $arg;/; |
144 | $text .= $code . 'continue {last}'; |
145 | next component; |
146 | } |
74a6a946 |
147 | elsif ($Perl5 && $source =~ m/\G(\s*)(case\b)(?!\s*=>)/gc |
148 | || $Perl6 && $source =~ m/\G(\s*)(when\b)(?!\s*=>)/gc) |
3ed9f206 |
149 | { |
74a6a946 |
150 | my $keyword = $2; |
3ed9f206 |
151 | $text .= $1."if (Switch::case"; |
152 | if (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)) { |
153 | my $code = substr($source,$pos[0],$pos[4]-$pos[0]); |
52d8c818 |
154 | $text .= " " if $pos[0] < $pos[2]; |
155 | $text .= "sub " if is_block $code; |
156 | $text .= filter_blocks($code,line(substr($source,0,$pos[0]),$line)) . ")"; |
3ed9f206 |
157 | } |
158 | elsif (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/[[(]/,qr/[])]/,qr/[[({]/,qr/[])}]/,undef)) { |
159 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
160 | $code =~ s {^\s*[(]\s*%} { ( \\\%} || |
161 | $code =~ s {^\s*[(]\s*m\b} { ( qr} || |
162 | $code =~ s {^\s*[(]\s*/} { ( qr/} || |
163 | $code =~ s {^\s*[(]\s*qw} { ( \\qw}; |
52d8c818 |
164 | $text .= " " if $pos[0] < $pos[2]; |
165 | $text .= "$code)"; |
3ed9f206 |
166 | } |
74a6a946 |
167 | elsif ($Perl6 && do{@pos = Text::Balanced::_match_variable(\$source,qr/\s*/)}) { |
168 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
169 | $code =~ s {^\s*%} { \%} || |
170 | $code =~ s {^\s*@} { \@}; |
52d8c818 |
171 | $text .= " " if $pos[0] < $pos[2]; |
172 | $text .= "$code)"; |
74a6a946 |
173 | } |
d38ca171 |
174 | elsif ( @pos = Text::Balanced::_match_quotelike(\$source,qr/\s*/,1,0)) { |
3ed9f206 |
175 | my $code = substr($source,$pos[2],$pos[18]-$pos[2]); |
176 | $code = filter_blocks($code,line(substr($source,0,$pos[2]),$line)); |
177 | $code =~ s {^\s*m} { qr} || |
178 | $code =~ s {^\s*/} { qr/} || |
179 | $code =~ s {^\s*qw} { \\qw}; |
52d8c818 |
180 | $text .= " " if $pos[0] < $pos[2]; |
181 | $text .= "$code)"; |
3ed9f206 |
182 | } |
74a6a946 |
183 | elsif ($Perl5 && $source =~ m/\G\s*(([^\$\@{])[^\$\@{]*)(?=\s*{)/gc |
6596d39b |
184 | || $Perl6 && $source =~ m/\G\s*([^;{]*)()/gc) { |
3ed9f206 |
185 | my $code = filter_blocks($1,line(substr($source,0,pos $source),$line)); |
186 | $text .= ' \\' if $2 eq '%'; |
187 | $text .= " $code)"; |
188 | } |
189 | else { |
74a6a946 |
190 | die "Bad $keyword statement (invalid $keyword value?) near $Switch::file line ", line(substr($source,0,pos $source), $line), "\n"; |
3ed9f206 |
191 | } |
192 | |
6596d39b |
193 | die "Missing opening brace or semi-colon after 'when' value near $Switch::file line ", line(substr($source,0,pos $source), $line), "\n" |
194 | unless !$Perl6 || $source =~ m/\G(\s*)(?=;|\{)/gc; |
74a6a946 |
195 | |
196 | do{@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)} |
3ed9f206 |
197 | or do { |
198 | if ($source =~ m/\G\s*(?=([};]|\Z))/gc) { |
199 | $casecounter++; |
200 | next component; |
201 | } |
74a6a946 |
202 | die "Bad $keyword statement (problem in the code block?) near $Switch::file line ", line(substr($source,0,pos $source),$line), "\n"; |
3ed9f206 |
203 | }; |
204 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
205 | $code =~ s/}(?=\s*\Z)/;last S_W_I_T_C_H }/ |
206 | unless $fallthrough; |
207 | $text .= "{ while (1) $code continue { goto C_A_S_E_$casecounter } last S_W_I_T_C_H; C_A_S_E_$casecounter: }"; |
208 | $casecounter++; |
209 | next component; |
210 | } |
211 | |
d38ca171 |
212 | $source =~ m/\G(\s*(-[sm]\s+|\w+|#.*\n|\W))/gc; |
3ed9f206 |
213 | $text .= $1; |
214 | } |
215 | $text; |
216 | } |
217 | |
218 | |
219 | |
220 | sub in |
221 | { |
222 | my ($x,$y) = @_; |
223 | my @numy; |
224 | for my $nextx ( @$x ) |
225 | { |
a1813bef |
226 | my $numx = ref($nextx) || defined $nextx && (~$nextx&$nextx) eq 0; |
3ed9f206 |
227 | for my $j ( 0..$#$y ) |
228 | { |
229 | my $nexty = $y->[$j]; |
a1813bef |
230 | push @numy, ref($nexty) || defined $nexty && (~$nexty&$nexty) eq 0 |
3ed9f206 |
231 | if @numy <= $j; |
232 | return 1 if $numx && $numy[$j] && $nextx==$nexty |
233 | || $nextx eq $nexty; |
234 | |
235 | } |
236 | } |
237 | return ""; |
238 | } |
239 | |
240 | sub on_exists |
241 | { |
242 | my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ }; |
243 | [ keys %$ref ] |
244 | } |
245 | |
246 | sub on_defined |
247 | { |
248 | my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ }; |
249 | [ grep { defined $ref->{$_} } keys %$ref ] |
250 | } |
251 | |
252 | sub switch(;$) |
253 | { |
254 | my ($s_val) = @_ ? $_[0] : $_; |
255 | my $s_ref = ref $s_val; |
256 | |
257 | if ($s_ref eq 'CODE') |
258 | { |
259 | $::_S_W_I_T_C_H = |
260 | sub { my $c_val = $_[0]; |
261 | return $s_val == $c_val if ref $c_val eq 'CODE'; |
262 | return $s_val->(@$c_val) if ref $c_val eq 'ARRAY'; |
263 | return $s_val->($c_val); |
264 | }; |
265 | } |
a1813bef |
266 | elsif ($s_ref eq "" && defined $s_val && (~$s_val&$s_val) eq 0) # NUMERIC SCALAR |
3ed9f206 |
267 | { |
268 | $::_S_W_I_T_C_H = |
269 | sub { my $c_val = $_[0]; |
270 | my $c_ref = ref $c_val; |
271 | return $s_val == $c_val if $c_ref eq "" |
a1813bef |
272 | && defined $c_val |
3ed9f206 |
273 | && (~$c_val&$c_val) eq 0; |
274 | return $s_val eq $c_val if $c_ref eq ""; |
275 | return in([$s_val],$c_val) if $c_ref eq 'ARRAY'; |
276 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
277 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
278 | return scalar $s_val=~/$c_val/ |
279 | if $c_ref eq 'Regexp'; |
280 | return scalar $c_val->{$s_val} |
281 | if $c_ref eq 'HASH'; |
282 | return; |
283 | }; |
284 | } |
285 | elsif ($s_ref eq "") # STRING SCALAR |
286 | { |
287 | $::_S_W_I_T_C_H = |
288 | sub { my $c_val = $_[0]; |
289 | my $c_ref = ref $c_val; |
290 | return $s_val eq $c_val if $c_ref eq ""; |
291 | return in([$s_val],$c_val) if $c_ref eq 'ARRAY'; |
292 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
293 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
294 | return scalar $s_val=~/$c_val/ |
295 | if $c_ref eq 'Regexp'; |
296 | return scalar $c_val->{$s_val} |
297 | if $c_ref eq 'HASH'; |
298 | return; |
299 | }; |
300 | } |
301 | elsif ($s_ref eq 'ARRAY') |
302 | { |
303 | $::_S_W_I_T_C_H = |
304 | sub { my $c_val = $_[0]; |
305 | my $c_ref = ref $c_val; |
306 | return in($s_val,[$c_val]) if $c_ref eq ""; |
307 | return in($s_val,$c_val) if $c_ref eq 'ARRAY'; |
308 | return $c_val->(@$s_val) if $c_ref eq 'CODE'; |
309 | return $c_val->call(@$s_val) |
310 | if $c_ref eq 'Switch'; |
311 | return scalar grep {$_=~/$c_val/} @$s_val |
312 | if $c_ref eq 'Regexp'; |
313 | return scalar grep {$c_val->{$_}} @$s_val |
314 | if $c_ref eq 'HASH'; |
315 | return; |
316 | }; |
317 | } |
318 | elsif ($s_ref eq 'Regexp') |
319 | { |
320 | $::_S_W_I_T_C_H = |
321 | sub { my $c_val = $_[0]; |
322 | my $c_ref = ref $c_val; |
323 | return $c_val=~/s_val/ if $c_ref eq ""; |
324 | return scalar grep {$_=~/s_val/} @$c_val |
325 | if $c_ref eq 'ARRAY'; |
326 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
327 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
328 | return $s_val eq $c_val if $c_ref eq 'Regexp'; |
329 | return grep {$_=~/$s_val/ && $c_val->{$_}} keys %$c_val |
330 | if $c_ref eq 'HASH'; |
331 | return; |
332 | }; |
333 | } |
334 | elsif ($s_ref eq 'HASH') |
335 | { |
336 | $::_S_W_I_T_C_H = |
337 | sub { my $c_val = $_[0]; |
338 | my $c_ref = ref $c_val; |
339 | return $s_val->{$c_val} if $c_ref eq ""; |
340 | return scalar grep {$s_val->{$_}} @$c_val |
341 | if $c_ref eq 'ARRAY'; |
342 | return $c_val->($s_val) if $c_ref eq 'CODE'; |
343 | return $c_val->call($s_val) if $c_ref eq 'Switch'; |
344 | return grep {$_=~/$c_val/ && $s_val->{"$_"}} keys %$s_val |
345 | if $c_ref eq 'Regexp'; |
346 | return $s_val==$c_val if $c_ref eq 'HASH'; |
347 | return; |
348 | }; |
349 | } |
350 | elsif ($s_ref eq 'Switch') |
351 | { |
352 | $::_S_W_I_T_C_H = |
353 | sub { my $c_val = $_[0]; |
354 | return $s_val == $c_val if ref $c_val eq 'Switch'; |
355 | return $s_val->call(@$c_val) |
356 | if ref $c_val eq 'ARRAY'; |
357 | return $s_val->call($c_val); |
358 | }; |
359 | } |
360 | else |
361 | { |
362 | croak "Cannot switch on $s_ref"; |
363 | } |
364 | return 1; |
365 | } |
366 | |
d38ca171 |
367 | sub case($) { local $SIG{__WARN__} = \&carp; |
368 | $::_S_W_I_T_C_H->(@_); } |
3ed9f206 |
369 | |
370 | # IMPLEMENT __ |
371 | |
372 | my $placeholder = bless { arity=>1, impl=>sub{$_[1+$_[0]]} }; |
373 | |
374 | sub __() { $placeholder } |
375 | |
376 | sub __arg($) |
377 | { |
378 | my $index = $_[0]+1; |
379 | bless { arity=>0, impl=>sub{$_[$index]} }; |
380 | } |
381 | |
382 | sub hosub(&@) |
383 | { |
384 | # WRITE THIS |
385 | } |
386 | |
387 | sub call |
388 | { |
389 | my ($self,@args) = @_; |
390 | return $self->{impl}->(0,@args); |
391 | } |
392 | |
393 | sub meta_bop(&) |
394 | { |
395 | my ($op) = @_; |
396 | sub |
397 | { |
398 | my ($left, $right, $reversed) = @_; |
399 | ($right,$left) = @_ if $reversed; |
400 | |
401 | my $rop = ref $right eq 'Switch' |
402 | ? $right |
403 | : bless { arity=>0, impl=>sub{$right} }; |
404 | |
405 | my $lop = ref $left eq 'Switch' |
406 | ? $left |
407 | : bless { arity=>0, impl=>sub{$left} }; |
408 | |
409 | my $arity = $lop->{arity} + $rop->{arity}; |
410 | |
411 | return bless { |
412 | arity => $arity, |
413 | impl => sub { my $start = shift; |
414 | return $op->($lop->{impl}->($start,@_), |
415 | $rop->{impl}->($start+$lop->{arity},@_)); |
416 | } |
417 | }; |
418 | }; |
419 | } |
420 | |
421 | sub meta_uop(&) |
422 | { |
423 | my ($op) = @_; |
424 | sub |
425 | { |
426 | my ($left) = @_; |
427 | |
428 | my $lop = ref $left eq 'Switch' |
429 | ? $left |
430 | : bless { arity=>0, impl=>sub{$left} }; |
431 | |
432 | my $arity = $lop->{arity}; |
433 | |
434 | return bless { |
435 | arity => $arity, |
436 | impl => sub { $op->($lop->{impl}->(@_)) } |
437 | }; |
438 | }; |
439 | } |
440 | |
441 | |
442 | use overload |
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] << $_[1]}, |
450 | ">>" => meta_bop {$_[0] >> $_[1]}, |
451 | "x" => meta_bop {$_[0] x $_[1]}, |
452 | "." => meta_bop {$_[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] != $_[1]}, |
459 | "<=>" => meta_bop {$_[0] <=> $_[1]}, |
460 | "lt" => meta_bop {$_[0] lt $_[1]}, |
461 | "le" => meta_bop {$_[0] le $_[1]}, |
462 | "gt" => meta_bop {$_[0] gt $_[1]}, |
463 | "ge" => meta_bop {$_[0] ge $_[1]}, |
464 | "eq" => meta_bop {$_[0] eq $_[1]}, |
465 | "ne" => meta_bop {$_[0] ne $_[1]}, |
466 | "cmp" => meta_bop {$_[0] cmp $_[1]}, |
467 | "\&" => meta_bop {$_[0] & $_[1]}, |
468 | "^" => meta_bop {$_[0] ^ $_[1]}, |
469 | "|" => meta_bop {$_[0] | $_[1]}, |
470 | "atan2" => meta_bop {atan2 $_[0], $_[1]}, |
471 | |
472 | "neg" => meta_uop {-$_[0]}, |
473 | "!" => meta_uop {!$_[0]}, |
474 | "~" => meta_uop {~$_[0]}, |
475 | "cos" => meta_uop {cos $_[0]}, |
476 | "sin" => meta_uop {sin $_[0]}, |
477 | "exp" => meta_uop {exp $_[0]}, |
478 | "abs" => meta_uop {abs $_[0]}, |
479 | "log" => meta_uop {log $_[0]}, |
480 | "sqrt" => meta_uop {sqrt $_[0]}, |
481 | "bool" => sub { croak "Can't use && or || in expression containing __" }, |
482 | |
483 | # "&()" => sub { $_[0]->{impl} }, |
484 | |
485 | # "||" => meta_bop {$_[0] || $_[1]}, |
486 | # "&&" => meta_bop {$_[0] && $_[1]}, |
487 | # fallback => 1, |
488 | ; |
489 | 1; |
490 | |
491 | __END__ |
492 | |
493 | |
494 | =head1 NAME |
495 | |
496 | Switch - A switch statement for Perl |
497 | |
498 | =head1 VERSION |
499 | |
218ab4da |
500 | This document describes version 2.09 of Switch, |
501 | released June 12, 2002. |
3ed9f206 |
502 | |
503 | =head1 SYNOPSIS |
504 | |
505 | use Switch; |
506 | |
507 | switch ($val) { |
508 | |
509 | case 1 { print "number 1" } |
510 | case "a" { print "string a" } |
511 | case [1..10,42] { print "number in list" } |
512 | case (@array) { print "number in list" } |
513 | case /\w+/ { print "pattern" } |
514 | case qr/\w+/ { print "pattern" } |
515 | case (%hash) { print "entry in hash" } |
516 | case (\%hash) { print "entry in hash" } |
517 | case (\&sub) { print "arg to subroutine" } |
518 | else { print "previous case not true" } |
519 | } |
520 | |
521 | =head1 BACKGROUND |
522 | |
523 | [Skip ahead to L<"DESCRIPTION"> if you don't care about the whys |
524 | and wherefores of this control structure] |
525 | |
526 | In seeking to devise a "Swiss Army" case mechanism suitable for Perl, |
527 | it is useful to generalize this notion of distributed conditional |
528 | testing as far as possible. Specifically, the concept of "matching" |
529 | between the switch value and the various case values need not be |
530 | restricted to numeric (or string or referential) equality, as it is in other |
531 | languages. Indeed, as Table 1 illustrates, Perl |
532 | offers at least eighteen different ways in which two values could |
533 | generate a match. |
534 | |
535 | Table 1: Matching a switch value ($s) with a case value ($c) |
536 | |
537 | Switch Case Type of Match Implied Matching Code |
538 | Value Value |
539 | ====== ===== ===================== ============= |
540 | |
541 | number same numeric or referential match if $s == $c; |
542 | or ref equality |
543 | |
544 | object method result of method call match if $s->$c(); |
545 | ref name match if defined $s->$c(); |
546 | or ref |
547 | |
548 | other other string equality match if $s eq $c; |
549 | non-ref non-ref |
550 | scalar scalar |
551 | |
552 | string regexp pattern match match if $s =~ /$c/; |
553 | |
554 | array scalar array entry existence match if 0<=$c && $c<@$s; |
555 | ref array entry definition match if defined $s->[$c]; |
556 | array entry truth match if $s->[$c]; |
557 | |
558 | array array array intersection match if intersects(@$s, @$c); |
559 | ref ref (apply this table to |
560 | all pairs of elements |
561 | $s->[$i] and |
562 | $c->[$j]) |
563 | |
564 | array regexp array grep match if grep /$c/, @$s; |
565 | ref |
566 | |
567 | hash scalar hash entry existence match if exists $s->{$c}; |
568 | ref hash entry definition match if defined $s->{$c}; |
569 | hash entry truth match if $s->{$c}; |
570 | |
571 | hash regexp hash grep match if grep /$c/, keys %$s; |
572 | ref |
573 | |
574 | sub scalar return value defn match if defined $s->($c); |
575 | ref return value truth match if $s->($c); |
576 | |
577 | sub array return value defn match if defined $s->(@$c); |
578 | ref ref return value truth match if $s->(@$c); |
579 | |
580 | |
581 | In reality, Table 1 covers 31 alternatives, because only the equality and |
582 | intersection tests are commutative; in all other cases, the roles of |
583 | the C<$s> and C<$c> variables could be reversed to produce a |
584 | different test. For example, instead of testing a single hash for |
585 | the existence of a series of keys (C<match if exists $s-E<gt>{$c}>), |
586 | one could test for the existence of a single key in a series of hashes |
587 | (C<match if exists $c-E<gt>{$s}>). |
588 | |
589 | As L<perltodo> observes, a Perl case mechanism must support all these |
590 | "ways to do it". |
591 | |
592 | |
593 | =head1 DESCRIPTION |
594 | |
595 | The Switch.pm module implements a generalized case mechanism that covers |
596 | the numerous possible combinations of switch and case values described above. |
597 | |
598 | The module augments the standard Perl syntax with two new control |
599 | statements: C<switch> and C<case>. The C<switch> statement takes a |
600 | single scalar argument of any type, specified in parentheses. |
601 | C<switch> stores this value as the |
602 | current switch value in a (localized) control variable. |
603 | The value is followed by a block which may contain one or more |
604 | Perl statements (including the C<case> statement described below). |
605 | The block is unconditionally executed once the switch value has |
606 | been cached. |
607 | |
608 | A C<case> statement takes a single scalar argument (in mandatory |
609 | parentheses if it's a variable; otherwise the parens are optional) and |
610 | selects the appropriate type of matching between that argument and the |
611 | current switch value. The type of matching used is determined by the |
612 | respective types of the switch value and the C<case> argument, as |
613 | specified in Table 1. If the match is successful, the mandatory |
614 | block associated with the C<case> statement is executed. |
615 | |
616 | In most other respects, the C<case> statement is semantically identical |
617 | to an C<if> statement. For example, it can be followed by an C<else> |
618 | clause, and can be used as a postfix statement qualifier. |
619 | |
620 | However, when a C<case> block has been executed control is automatically |
621 | transferred to the statement after the immediately enclosing C<switch> |
622 | block, rather than to the next statement within the block. In other |
623 | words, the success of any C<case> statement prevents other cases in the |
624 | same scope from executing. But see L<"Allowing fall-through"> below. |
625 | |
626 | Together these two new statements provide a fully generalized case |
627 | mechanism: |
628 | |
629 | use Switch; |
630 | |
631 | # AND LATER... |
632 | |
633 | %special = ( woohoo => 1, d'oh => 1 ); |
634 | |
635 | while (<>) { |
636 | switch ($_) { |
637 | |
74a6a946 |
638 | case (%special) { print "homer\n"; } # if $special{$_} |
639 | case /a-z/i { print "alpha\n"; } # if $_ =~ /a-z/i |
640 | case [1..9] { print "small num\n"; } # if $_ in [1..9] |
3ed9f206 |
641 | |
642 | case { $_[0] >= 10 } { # if $_ >= 10 |
643 | my $age = <>; |
644 | switch (sub{ $_[0] < $age } ) { |
645 | |
646 | case 20 { print "teens\n"; } # if 20 < $age |
647 | case 30 { print "twenties\n"; } # if 30 < $age |
648 | else { print "history\n"; } |
649 | } |
650 | } |
651 | |
652 | print "must be punctuation\n" case /\W/; # if $_ ~= /\W/ |
653 | } |
654 | |
655 | Note that C<switch>es can be nested within C<case> (or any other) blocks, |
656 | and a series of C<case> statements can try different types of matches |
657 | -- hash membership, pattern match, array intersection, simple equality, |
658 | etc. -- against the same switch value. |
659 | |
660 | The use of intersection tests against an array reference is particularly |
661 | useful for aggregating integral cases: |
662 | |
663 | sub classify_digit |
664 | { |
665 | switch ($_[0]) { case 0 { return 'zero' } |
666 | case [2,4,6,8] { return 'even' } |
667 | case [1,3,4,7,9] { return 'odd' } |
668 | case /[A-F]/i { return 'hex' } |
669 | } |
670 | } |
671 | |
672 | |
673 | =head2 Allowing fall-through |
674 | |
675 | Fall-though (trying another case after one has already succeeded) |
676 | is usually a Bad Idea in a switch statement. However, this |
677 | is Perl, not a police state, so there I<is> a way to do it, if you must. |
678 | |
679 | If a C<case> block executes an untargetted C<next>, control is |
680 | immediately transferred to the statement I<after> the C<case> statement |
681 | (i.e. usually another case), rather than out of the surrounding |
682 | C<switch> block. |
683 | |
684 | For example: |
685 | |
686 | switch ($val) { |
687 | case 1 { handle_num_1(); next } # and try next case... |
688 | case "1" { handle_str_1(); next } # and try next case... |
689 | case [0..9] { handle_num_any(); } # and we're done |
690 | case /\d/ { handle_dig_any(); next } # and try next case... |
691 | case /.*/ { handle_str_any(); next } # and try next case... |
692 | } |
693 | |
694 | If $val held the number C<1>, the above C<switch> block would call the |
695 | first three C<handle_...> subroutines, jumping to the next case test |
696 | each time it encountered a C<next>. After the thrid C<case> block |
697 | was executed, control would jump to the end of the enclosing |
698 | C<switch> block. |
699 | |
700 | On the other hand, if $val held C<10>, then only the last two C<handle_...> |
701 | subroutines would be called. |
702 | |
703 | Note that this mechanism allows the notion of I<conditional fall-through>. |
704 | For example: |
705 | |
706 | switch ($val) { |
707 | case [0..9] { handle_num_any(); next if $val < 7; } |
708 | case /\d/ { handle_dig_any(); } |
709 | } |
710 | |
711 | If an untargetted C<last> statement is executed in a case block, this |
712 | immediately transfers control out of the enclosing C<switch> block |
713 | (in other words, there is an implicit C<last> at the end of each |
714 | normal C<case> block). Thus the previous example could also have been |
715 | written: |
716 | |
717 | switch ($val) { |
718 | case [0..9] { handle_num_any(); last if $val >= 7; next; } |
719 | case /\d/ { handle_dig_any(); } |
720 | } |
721 | |
722 | |
723 | =head2 Automating fall-through |
724 | |
725 | In situations where case fall-through should be the norm, rather than an |
726 | exception, an endless succession of terminal C<next>s is tedious and ugly. |
727 | Hence, it is possible to reverse the default behaviour by specifying |
728 | the string "fallthrough" when importing the module. For example, the |
729 | following code is equivalent to the first example in L<"Allowing fall-through">: |
730 | |
731 | use Switch 'fallthrough'; |
732 | |
733 | switch ($val) { |
734 | case 1 { handle_num_1(); } |
735 | case "1" { handle_str_1(); } |
736 | case [0..9] { handle_num_any(); last } |
737 | case /\d/ { handle_dig_any(); } |
738 | case /.*/ { handle_str_any(); } |
739 | } |
740 | |
741 | Note the explicit use of a C<last> to preserve the non-fall-through |
742 | behaviour of the third case. |
743 | |
744 | |
745 | |
74a6a946 |
746 | =head2 Alternative syntax |
747 | |
748 | Perl 6 will provide a built-in switch statement with essentially the |
749 | same semantics as those offered by Switch.pm, but with a different |
693b9afd |
750 | pair of keywords. In Perl 6 C<switch> will be spelled C<given>, and |
74a6a946 |
751 | C<case> will be pronounced C<when>. In addition, the C<when> statement |
6596d39b |
752 | will not require switch or case values to be parenthesized. |
74a6a946 |
753 | |
6596d39b |
754 | This future syntax is also (largely) available via the Switch.pm module, by |
74a6a946 |
755 | importing it with the argument C<"Perl6">. For example: |
756 | |
757 | use Switch 'Perl6'; |
758 | |
759 | given ($val) { |
6596d39b |
760 | when 1 { handle_num_1(); } |
761 | when ($str1) { handle_str_1(); } |
762 | when [0..9] { handle_num_any(); last } |
763 | when /\d/ { handle_dig_any(); } |
764 | when /.*/ { handle_str_any(); } |
74a6a946 |
765 | } |
766 | |
6596d39b |
767 | Note that scalars still need to be parenthesized, since they would be |
768 | ambiguous in Perl 5. |
769 | |
770 | Note too that you can mix and match both syntaxes by importing the module |
74a6a946 |
771 | with: |
772 | |
773 | use Switch 'Perl5', 'Perl6'; |
774 | |
775 | |
3ed9f206 |
776 | =head2 Higher-order Operations |
777 | |
778 | One situation in which C<switch> and C<case> do not provide a good |
779 | substitute for a cascaded C<if>, is where a switch value needs to |
780 | be tested against a series of conditions. For example: |
781 | |
782 | sub beverage { |
783 | switch (shift) { |
784 | |
785 | case sub { $_[0] < 10 } { return 'milk' } |
786 | case sub { $_[0] < 20 } { return 'coke' } |
787 | case sub { $_[0] < 30 } { return 'beer' } |
788 | case sub { $_[0] < 40 } { return 'wine' } |
789 | case sub { $_[0] < 50 } { return 'malt' } |
790 | case sub { $_[0] < 60 } { return 'Moet' } |
791 | else { return 'milk' } |
792 | } |
793 | } |
794 | |
795 | The need to specify each condition as a subroutine block is tiresome. To |
796 | overcome this, when importing Switch.pm, a special "placeholder" |
797 | subroutine named C<__> [sic] may also be imported. This subroutine |
798 | converts (almost) any expression in which it appears to a reference to a |
799 | higher-order function. That is, the expression: |
800 | |
801 | use Switch '__'; |
802 | |
803 | __ < 2 + __ |
804 | |
805 | is equivalent to: |
806 | |
807 | sub { $_[0] < 2 + $_[1] } |
808 | |
809 | With C<__>, the previous ugly case statements can be rewritten: |
810 | |
811 | case __ < 10 { return 'milk' } |
812 | case __ < 20 { return 'coke' } |
813 | case __ < 30 { return 'beer' } |
814 | case __ < 40 { return 'wine' } |
815 | case __ < 50 { return 'malt' } |
816 | case __ < 60 { return 'Moet' } |
817 | else { return 'milk' } |
818 | |
819 | The C<__> subroutine makes extensive use of operator overloading to |
820 | perform its magic. All operations involving __ are overloaded to |
821 | produce an anonymous subroutine that implements a lazy version |
822 | of the original operation. |
823 | |
824 | The only problem is that operator overloading does not allow the |
825 | boolean operators C<&&> and C<||> to be overloaded. So a case statement |
826 | like this: |
827 | |
828 | case 0 <= __ && __ < 10 { return 'digit' } |
829 | |
830 | doesn't act as expected, because when it is |
831 | executed, it constructs two higher order subroutines |
832 | and then treats the two resulting references as arguments to C<&&>: |
833 | |
834 | sub { 0 <= $_[0] } && sub { $_[0] < 10 } |
835 | |
836 | This boolean expression is inevitably true, since both references are |
837 | non-false. Fortunately, the overloaded C<'bool'> operator catches this |
838 | situation and flags it as a error. |
839 | |
840 | =head1 DEPENDENCIES |
841 | |
842 | The module is implemented using Filter::Util::Call and Text::Balanced |
843 | and requires both these modules to be installed. |
844 | |
845 | =head1 AUTHOR |
846 | |
847 | Damian Conway (damian@conway.org) |
848 | |
849 | =head1 BUGS |
850 | |
851 | There are undoubtedly serious bugs lurking somewhere in code this funky :-) |
852 | Bug reports and other feedback are most welcome. |
853 | |
d38ca171 |
854 | =head1 LIMITATION |
855 | |
856 | Due to the heuristic nature of Switch.pm's source parsing, the presence |
857 | of regexes specified with raw C<?...?> delimiters may cause mysterious |
858 | errors. The workaround is to use C<m?...?> instead. |
859 | |
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860 | =head1 COPYRIGHT |
861 | |
55a1c97c |
862 | Copyright (c) 1997-2001, Damian Conway. All Rights Reserved. |
863 | This module is free software. It may be used, redistributed |
864 | and/or modified under the same terms as Perl itself. |