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1 | =head1 NAME |
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2 | X<syntax> |
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3 | |
4 | perlsyn - Perl syntax |
5 | |
6 | =head1 DESCRIPTION |
7 | |
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8 | A Perl program consists of a sequence of declarations and statements |
9 | which run from the top to the bottom. Loops, subroutines and other |
10 | control structures allow you to jump around within the code. |
11 | |
12 | Perl is a B<free-form> language, you can format and indent it however |
13 | you like. Whitespace mostly serves to separate tokens, unlike |
14 | languages like Python where it is an important part of the syntax. |
15 | |
16 | Many of Perl's syntactic elements are B<optional>. Rather than |
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17 | requiring you to put parentheses around every function call and |
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18 | declare every variable, you can often leave such explicit elements off |
19 | and Perl will figure out what you meant. This is known as B<Do What I |
20 | Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to |
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21 | code in a style with which they are comfortable. |
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22 | |
23 | Perl B<borrows syntax> and concepts from many languages: awk, sed, C, |
24 | Bourne Shell, Smalltalk, Lisp and even English. Other |
25 | languages have borrowed syntax from Perl, particularly its regular |
26 | expression extensions. So if you have programmed in another language |
27 | you will see familiar pieces in Perl. They often work the same, but |
28 | see L<perltrap> for information about how they differ. |
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29 | |
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30 | =head2 Declarations |
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31 | X<declaration> X<undef> X<undefined> X<uninitialized> |
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32 | |
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33 | The only things you need to declare in Perl are report formats and |
34 | subroutines (and sometimes not even subroutines). A variable holds |
35 | the undefined value (C<undef>) until it has been assigned a defined |
36 | value, which is anything other than C<undef>. When used as a number, |
37 | C<undef> is treated as C<0>; when used as a string, it is treated as |
38 | the empty string, C<"">; and when used as a reference that isn't being |
39 | assigned to, it is treated as an error. If you enable warnings, |
40 | you'll be notified of an uninitialized value whenever you treat |
41 | C<undef> as a string or a number. Well, usually. Boolean contexts, |
42 | such as: |
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43 | |
44 | my $a; |
45 | if ($a) {} |
46 | |
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47 | are exempt from warnings (because they care about truth rather than |
48 | definedness). Operators such as C<++>, C<-->, C<+=>, |
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49 | C<-=>, and C<.=>, that operate on undefined left values such as: |
50 | |
51 | my $a; |
52 | $a++; |
53 | |
54 | are also always exempt from such warnings. |
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55 | |
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56 | A declaration can be put anywhere a statement can, but has no effect on |
57 | the execution of the primary sequence of statements--declarations all |
58 | take effect at compile time. Typically all the declarations are put at |
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59 | the beginning or the end of the script. However, if you're using |
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60 | lexically-scoped private variables created with C<my()>, you'll |
61 | have to make sure |
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62 | your format or subroutine definition is within the same block scope |
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63 | as the my if you expect to be able to access those private variables. |
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64 | |
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65 | Declaring a subroutine allows a subroutine name to be used as if it were a |
66 | list operator from that point forward in the program. You can declare a |
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67 | subroutine without defining it by saying C<sub name>, thus: |
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68 | X<subroutine, declaration> |
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69 | |
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70 | sub myname; |
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71 | $me = myname $0 or die "can't get myname"; |
72 | |
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73 | Note that myname() functions as a list operator, not as a unary operator; |
74 | so be careful to use C<or> instead of C<||> in this case. However, if |
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75 | you were to declare the subroutine as C<sub myname ($)>, then |
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76 | C<myname> would function as a unary operator, so either C<or> or |
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77 | C<||> would work. |
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78 | |
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79 | Subroutines declarations can also be loaded up with the C<require> statement |
80 | or both loaded and imported into your namespace with a C<use> statement. |
81 | See L<perlmod> for details on this. |
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82 | |
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83 | A statement sequence may contain declarations of lexically-scoped |
84 | variables, but apart from declaring a variable name, the declaration acts |
85 | like an ordinary statement, and is elaborated within the sequence of |
86 | statements as if it were an ordinary statement. That means it actually |
87 | has both compile-time and run-time effects. |
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88 | |
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89 | =head2 Comments |
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90 | X<comment> X<#> |
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91 | |
92 | Text from a C<"#"> character until the end of the line is a comment, |
93 | and is ignored. Exceptions include C<"#"> inside a string or regular |
94 | expression. |
95 | |
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96 | =head2 Simple Statements |
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97 | X<statement> X<semicolon> X<expression> X<;> |
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98 | |
99 | The only kind of simple statement is an expression evaluated for its |
100 | side effects. Every simple statement must be terminated with a |
101 | semicolon, unless it is the final statement in a block, in which case |
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102 | the semicolon is optional. (A semicolon is still encouraged if the |
103 | block takes up more than one line, because you may eventually add |
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104 | another line.) Note that there are some operators like C<eval {}> and |
105 | C<do {}> that look like compound statements, but aren't (they're just |
106 | TERMs in an expression), and thus need an explicit termination if used |
107 | as the last item in a statement. |
108 | |
109 | =head2 Truth and Falsehood |
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110 | X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0> |
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111 | |
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112 | The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and |
113 | C<undef> are all false in a boolean context. All other values are true. |
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114 | Negation of a true value by C<!> or C<not> returns a special false value. |
115 | When evaluated as a string it is treated as C<''>, but as a number, it |
116 | is treated as 0. |
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117 | |
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118 | =head2 Statement Modifiers |
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119 | X<statement modifier> X<modifier> X<if> X<unless> X<while> |
120 | X<until> X<foreach> X<for> |
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121 | |
122 | Any simple statement may optionally be followed by a I<SINGLE> modifier, |
123 | just before the terminating semicolon (or block ending). The possible |
124 | modifiers are: |
125 | |
126 | if EXPR |
127 | unless EXPR |
128 | while EXPR |
129 | until EXPR |
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130 | foreach LIST |
131 | |
132 | The C<EXPR> following the modifier is referred to as the "condition". |
133 | Its truth or falsehood determines how the modifier will behave. |
134 | |
135 | C<if> executes the statement once I<if> and only if the condition is |
136 | true. C<unless> is the opposite, it executes the statement I<unless> |
137 | the condition is true (i.e., if the condition is false). |
138 | |
139 | print "Basset hounds got long ears" if length $ear >= 10; |
140 | go_outside() and play() unless $is_raining; |
141 | |
142 | The C<foreach> modifier is an iterator: it executes the statement once |
143 | for each item in the LIST (with C<$_> aliased to each item in turn). |
144 | |
145 | print "Hello $_!\n" foreach qw(world Dolly nurse); |
146 | |
147 | C<while> repeats the statement I<while> the condition is true. |
148 | C<until> does the opposite, it repeats the statement I<until> the |
149 | condition is true (or while the condition is false): |
150 | |
151 | # Both of these count from 0 to 10. |
152 | print $i++ while $i <= 10; |
153 | print $j++ until $j > 10; |
154 | |
155 | The C<while> and C<until> modifiers have the usual "C<while> loop" |
156 | semantics (conditional evaluated first), except when applied to a |
157 | C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in |
158 | which case the block executes once before the conditional is |
159 | evaluated. This is so that you can write loops like: |
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160 | |
161 | do { |
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162 | $line = <STDIN>; |
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163 | ... |
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164 | } until $line eq ".\n"; |
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165 | |
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166 | See L<perlfunc/do>. Note also that the loop control statements described |
167 | later will I<NOT> work in this construct, because modifiers don't take |
168 | loop labels. Sorry. You can always put another block inside of it |
169 | (for C<next>) or around it (for C<last>) to do that sort of thing. |
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170 | For C<next>, just double the braces: |
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171 | X<next> X<last> X<redo> |
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172 | |
173 | do {{ |
174 | next if $x == $y; |
175 | # do something here |
176 | }} until $x++ > $z; |
177 | |
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178 | For C<last>, you have to be more elaborate: |
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179 | X<last> |
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180 | |
181 | LOOP: { |
182 | do { |
183 | last if $x = $y**2; |
184 | # do something here |
185 | } while $x++ <= $z; |
186 | } |
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187 | |
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188 | B<NOTE:> The behaviour of a C<my> statement modified with a statement |
189 | modifier conditional or loop construct (e.g. C<my $x if ...>) is |
190 | B<undefined>. The value of the C<my> variable may be C<undef>, any |
191 | previously assigned value, or possibly anything else. Don't rely on |
192 | it. Future versions of perl might do something different from the |
193 | version of perl you try it out on. Here be dragons. |
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194 | X<my> |
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195 | |
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196 | =head2 Compound Statements |
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197 | X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace> |
198 | X<{> X<}> X<if> X<unless> X<while> X<until> X<foreach> X<for> X<continue> |
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199 | |
200 | In Perl, a sequence of statements that defines a scope is called a block. |
201 | Sometimes a block is delimited by the file containing it (in the case |
202 | of a required file, or the program as a whole), and sometimes a block |
203 | is delimited by the extent of a string (in the case of an eval). |
204 | |
205 | But generally, a block is delimited by curly brackets, also known as braces. |
206 | We will call this syntactic construct a BLOCK. |
207 | |
208 | The following compound statements may be used to control flow: |
209 | |
210 | if (EXPR) BLOCK |
211 | if (EXPR) BLOCK else BLOCK |
212 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK |
213 | LABEL while (EXPR) BLOCK |
214 | LABEL while (EXPR) BLOCK continue BLOCK |
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215 | LABEL until (EXPR) BLOCK |
216 | LABEL until (EXPR) BLOCK continue BLOCK |
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217 | LABEL for (EXPR; EXPR; EXPR) BLOCK |
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218 | LABEL foreach VAR (LIST) BLOCK |
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219 | LABEL foreach VAR (LIST) BLOCK continue BLOCK |
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220 | LABEL BLOCK continue BLOCK |
221 | |
222 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, |
223 | not statements. This means that the curly brackets are I<required>--no |
224 | dangling statements allowed. If you want to write conditionals without |
225 | curly brackets there are several other ways to do it. The following |
226 | all do the same thing: |
227 | |
228 | if (!open(FOO)) { die "Can't open $FOO: $!"; } |
229 | die "Can't open $FOO: $!" unless open(FOO); |
230 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! |
231 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; |
232 | # a bit exotic, that last one |
233 | |
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234 | The C<if> statement is straightforward. Because BLOCKs are always |
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235 | bounded by curly brackets, there is never any ambiguity about which |
236 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, |
237 | the sense of the test is reversed. |
238 | |
239 | The C<while> statement executes the block as long as the expression is |
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240 | true (does not evaluate to the null string C<""> or C<0> or C<"0">). |
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241 | The C<until> statement executes the block as long as the expression is |
242 | false. |
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243 | The LABEL is optional, and if present, consists of an identifier followed |
244 | by a colon. The LABEL identifies the loop for the loop control |
245 | statements C<next>, C<last>, and C<redo>. |
246 | If the LABEL is omitted, the loop control statement |
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247 | refers to the innermost enclosing loop. This may include dynamically |
248 | looking back your call-stack at run time to find the LABEL. Such |
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249 | desperate behavior triggers a warning if you use the C<use warnings> |
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250 | pragma or the B<-w> flag. |
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251 | |
252 | If there is a C<continue> BLOCK, it is always executed just before the |
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253 | conditional is about to be evaluated again. Thus it can be used to |
254 | increment a loop variable, even when the loop has been continued via |
255 | the C<next> statement. |
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256 | |
257 | =head2 Loop Control |
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258 | X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue> |
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259 | |
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260 | The C<next> command starts the next iteration of the loop: |
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261 | |
262 | LINE: while (<STDIN>) { |
263 | next LINE if /^#/; # discard comments |
264 | ... |
265 | } |
266 | |
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267 | The C<last> command immediately exits the loop in question. The |
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268 | C<continue> block, if any, is not executed: |
269 | |
270 | LINE: while (<STDIN>) { |
271 | last LINE if /^$/; # exit when done with header |
272 | ... |
273 | } |
274 | |
275 | The C<redo> command restarts the loop block without evaluating the |
276 | conditional again. The C<continue> block, if any, is I<not> executed. |
277 | This command is normally used by programs that want to lie to themselves |
278 | about what was just input. |
279 | |
280 | For example, when processing a file like F</etc/termcap>. |
281 | If your input lines might end in backslashes to indicate continuation, you |
282 | want to skip ahead and get the next record. |
283 | |
284 | while (<>) { |
285 | chomp; |
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286 | if (s/\\$//) { |
287 | $_ .= <>; |
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288 | redo unless eof(); |
289 | } |
290 | # now process $_ |
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291 | } |
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292 | |
293 | which is Perl short-hand for the more explicitly written version: |
294 | |
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295 | LINE: while (defined($line = <ARGV>)) { |
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296 | chomp($line); |
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297 | if ($line =~ s/\\$//) { |
298 | $line .= <ARGV>; |
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299 | redo LINE unless eof(); # not eof(ARGV)! |
300 | } |
301 | # now process $line |
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302 | } |
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303 | |
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304 | Note that if there were a C<continue> block on the above code, it would |
305 | get executed only on lines discarded by the regex (since redo skips the |
306 | continue block). A continue block is often used to reset line counters |
307 | or C<?pat?> one-time matches: |
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308 | |
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309 | # inspired by :1,$g/fred/s//WILMA/ |
310 | while (<>) { |
311 | ?(fred)? && s//WILMA $1 WILMA/; |
312 | ?(barney)? && s//BETTY $1 BETTY/; |
313 | ?(homer)? && s//MARGE $1 MARGE/; |
314 | } continue { |
315 | print "$ARGV $.: $_"; |
316 | close ARGV if eof(); # reset $. |
317 | reset if eof(); # reset ?pat? |
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318 | } |
319 | |
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320 | If the word C<while> is replaced by the word C<until>, the sense of the |
321 | test is reversed, but the conditional is still tested before the first |
322 | iteration. |
323 | |
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324 | The loop control statements don't work in an C<if> or C<unless>, since |
325 | they aren't loops. You can double the braces to make them such, though. |
326 | |
327 | if (/pattern/) {{ |
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328 | last if /fred/; |
329 | next if /barney/; # same effect as "last", but doesn't document as well |
330 | # do something here |
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331 | }} |
332 | |
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333 | This is caused by the fact that a block by itself acts as a loop that |
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334 | executes once, see L<"Basic BLOCKs">. |
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335 | |
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336 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer |
337 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. |
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338 | |
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339 | =head2 For Loops |
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340 | X<for> X<foreach> |
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341 | |
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342 | Perl's C-style C<for> loop works like the corresponding C<while> loop; |
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343 | that means that this: |
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344 | |
345 | for ($i = 1; $i < 10; $i++) { |
346 | ... |
347 | } |
348 | |
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349 | is the same as this: |
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350 | |
351 | $i = 1; |
352 | while ($i < 10) { |
353 | ... |
354 | } continue { |
355 | $i++; |
356 | } |
357 | |
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358 | There is one minor difference: if variables are declared with C<my> |
359 | in the initialization section of the C<for>, the lexical scope of |
360 | those variables is exactly the C<for> loop (the body of the loop |
361 | and the control sections). |
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362 | X<my> |
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363 | |
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364 | Besides the normal array index looping, C<for> can lend itself |
365 | to many other interesting applications. Here's one that avoids the |
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366 | problem you get into if you explicitly test for end-of-file on |
367 | an interactive file descriptor causing your program to appear to |
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368 | hang. |
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369 | X<eof> X<end-of-file> X<end of file> |
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370 | |
371 | $on_a_tty = -t STDIN && -t STDOUT; |
372 | sub prompt { print "yes? " if $on_a_tty } |
373 | for ( prompt(); <STDIN>; prompt() ) { |
374 | # do something |
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375 | } |
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376 | |
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377 | Using C<readline> (or the operator form, C<< <EXPR> >>) as the |
378 | conditional of a C<for> loop is shorthand for the following. This |
379 | behaviour is the same as a C<while> loop conditional. |
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380 | X<readline> X<< <> >> |
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381 | |
382 | for ( prompt(); defined( $_ = <STDIN> ); prompt() ) { |
383 | # do something |
384 | } |
385 | |
cb1a09d0 |
386 | =head2 Foreach Loops |
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387 | X<for> X<foreach> |
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388 | |
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389 | The C<foreach> loop iterates over a normal list value and sets the |
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390 | variable VAR to be each element of the list in turn. If the variable |
391 | is preceded with the keyword C<my>, then it is lexically scoped, and |
392 | is therefore visible only within the loop. Otherwise, the variable is |
393 | implicitly local to the loop and regains its former value upon exiting |
394 | the loop. If the variable was previously declared with C<my>, it uses |
395 | that variable instead of the global one, but it's still localized to |
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396 | the loop. This implicit localisation occurs I<only> in a C<foreach> |
397 | loop. |
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398 | X<my> X<local> |
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399 | |
400 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so |
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401 | you can use C<foreach> for readability or C<for> for brevity. (Or because |
402 | the Bourne shell is more familiar to you than I<csh>, so writing C<for> |
f86cebdf |
403 | comes more naturally.) If VAR is omitted, C<$_> is set to each value. |
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404 | X<$_> |
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405 | |
406 | If any element of LIST is an lvalue, you can modify it by modifying |
407 | VAR inside the loop. Conversely, if any element of LIST is NOT an |
408 | lvalue, any attempt to modify that element will fail. In other words, |
409 | the C<foreach> loop index variable is an implicit alias for each item |
410 | in the list that you're looping over. |
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411 | X<alias> |
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412 | |
413 | If any part of LIST is an array, C<foreach> will get very confused if |
414 | you add or remove elements within the loop body, for example with |
415 | C<splice>. So don't do that. |
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416 | X<splice> |
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417 | |
418 | C<foreach> probably won't do what you expect if VAR is a tied or other |
419 | special variable. Don't do that either. |
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420 | |
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421 | Examples: |
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422 | |
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423 | for (@ary) { s/foo/bar/ } |
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424 | |
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425 | for my $elem (@elements) { |
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426 | $elem *= 2; |
427 | } |
428 | |
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429 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { |
430 | print $count, "\n"; sleep(1); |
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431 | } |
432 | |
433 | for (1..15) { print "Merry Christmas\n"; } |
434 | |
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435 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
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436 | print "Item: $item\n"; |
437 | } |
438 | |
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439 | Here's how a C programmer might code up a particular algorithm in Perl: |
440 | |
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441 | for (my $i = 0; $i < @ary1; $i++) { |
442 | for (my $j = 0; $j < @ary2; $j++) { |
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443 | if ($ary1[$i] > $ary2[$j]) { |
444 | last; # can't go to outer :-( |
445 | } |
446 | $ary1[$i] += $ary2[$j]; |
447 | } |
cb1a09d0 |
448 | # this is where that last takes me |
4633a7c4 |
449 | } |
450 | |
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451 | Whereas here's how a Perl programmer more comfortable with the idiom might |
cb1a09d0 |
452 | do it: |
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453 | |
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454 | OUTER: for my $wid (@ary1) { |
455 | INNER: for my $jet (@ary2) { |
cb1a09d0 |
456 | next OUTER if $wid > $jet; |
457 | $wid += $jet; |
54310121 |
458 | } |
459 | } |
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460 | |
cb1a09d0 |
461 | See how much easier this is? It's cleaner, safer, and faster. It's |
462 | cleaner because it's less noisy. It's safer because if code gets added |
c07a80fd |
463 | between the inner and outer loops later on, the new code won't be |
5f05dabc |
464 | accidentally executed. The C<next> explicitly iterates the other loop |
c07a80fd |
465 | rather than merely terminating the inner one. And it's faster because |
466 | Perl executes a C<foreach> statement more rapidly than it would the |
467 | equivalent C<for> loop. |
4633a7c4 |
468 | |
0d863452 |
469 | =head2 Basic BLOCKs |
470 | X<block> |
4633a7c4 |
471 | |
55497cff |
472 | A BLOCK by itself (labeled or not) is semantically equivalent to a |
473 | loop that executes once. Thus you can use any of the loop control |
474 | statements in it to leave or restart the block. (Note that this is |
475 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief |
476 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> |
477 | block is optional. |
4633a7c4 |
478 | |
27cec4bd |
479 | The BLOCK construct can be used to emulate case structures. |
a0d0e21e |
480 | |
481 | SWITCH: { |
482 | if (/^abc/) { $abc = 1; last SWITCH; } |
483 | if (/^def/) { $def = 1; last SWITCH; } |
484 | if (/^xyz/) { $xyz = 1; last SWITCH; } |
485 | $nothing = 1; |
486 | } |
487 | |
0d863452 |
488 | Such constructs are quite frequently used, because older versions |
489 | of Perl had no official C<switch> statement. |
83df6a1d |
490 | |
0d863452 |
491 | =head2 Switch statements |
492 | X<switch> X<case> X<given> X<when> X<default> |
83df6a1d |
493 | |
27cec4bd |
494 | Starting from Perl 5.10, you can say |
83df6a1d |
495 | |
27cec4bd |
496 | use feature "switch"; |
a0d0e21e |
497 | |
0d863452 |
498 | which enables a switch feature that is closely based on the |
499 | Perl 6 proposal. |
500 | |
501 | The keywords C<given> and C<when> are analogous |
502 | to C<switch> and C<case> in other languages, so the code |
503 | above could be written as |
504 | |
27cec4bd |
505 | given($_) { |
506 | when (/^abc/) { $abc = 1; } |
507 | when (/^def/) { $def = 1; } |
508 | when (/^xyz/) { $xyz = 1; } |
509 | default { $nothing = 1; } |
a0d0e21e |
510 | } |
511 | |
0d863452 |
512 | This construct is very flexible and powerful. For example: |
a0d0e21e |
513 | |
4b7b0ae4 |
514 | use feature ":5.10"; |
515 | given($foo) { |
516 | when (undef) { |
517 | say '$foo is undefined'; |
518 | } |
9f435386 |
519 | |
4b7b0ae4 |
520 | when ("foo") { |
521 | say '$foo is the string "foo"'; |
522 | } |
523 | |
524 | when ([1,3,5,7,9]) { |
525 | say '$foo is an odd digit'; |
526 | continue; # Fall through |
9f435386 |
527 | } |
528 | |
4b7b0ae4 |
529 | when ($_ < 100) { |
530 | say '$foo is numerically less than 100'; |
531 | } |
532 | |
533 | when (\&complicated_check) { |
534 | say 'complicated_check($foo) is true'; |
535 | } |
9f435386 |
536 | |
4b7b0ae4 |
537 | default { |
538 | die q(I don't know what to do with $foo); |
539 | } |
540 | } |
541 | |
542 | C<given(EXPR)> will assign the value of EXPR to C<$_> |
543 | within the lexical scope of the block, so it's similar to |
544 | |
545 | do { my $_ = EXPR; ... } |
546 | |
547 | except that the block is automatically broken out of by a |
548 | successful C<when> or an explicit C<break>. |
549 | |
550 | Most of the power comes from implicit smart matching: |
a0d0e21e |
551 | |
4b7b0ae4 |
552 | when($foo) |
a0d0e21e |
553 | |
0d863452 |
554 | is exactly equivalent to |
a0d0e21e |
555 | |
4b7b0ae4 |
556 | when($_ ~~ $foo) |
a0d0e21e |
557 | |
0d863452 |
558 | (though you need to enable the "~~" feature before you |
559 | can use the C<~~> operator directly). In fact C<when(EXPR)> |
560 | is treated as an implicit smart match most of the time. The |
561 | exceptions are that when EXPR is: |
562 | |
563 | =over 4 |
564 | |
565 | =item o |
566 | |
567 | a subroutine or method call |
568 | |
569 | =item o |
570 | |
571 | a regular expression match, i.e. C</REGEX/> or C<$foo =~ /REGEX/>, |
572 | or a negated regular expression match C<$foo !~ /REGEX/>. |
573 | |
574 | =item o |
575 | |
4b7b0ae4 |
576 | a comparison such as C<$_ E<lt> 10> or C<$x eq "abc"> |
577 | (or of course C<$_ ~~ $c>) |
0d863452 |
578 | |
579 | =item o |
580 | |
581 | C<defined(...)>, C<exists(...)>, or C<eof(...)> |
582 | |
583 | =item o |
4633a7c4 |
584 | |
0d863452 |
585 | A negated expression C<!(...)> or C<not (...)>, or a logical |
586 | exclusive-or C<(...) xor (...)>. |
cb1a09d0 |
587 | |
0d863452 |
588 | =back |
589 | |
590 | then the value of EXPR is used directly as a boolean. |
591 | Furthermore: |
592 | |
593 | =over 4 |
594 | |
595 | =item o |
596 | |
597 | If EXPR is C<... && ...> or C<... and ...>, the test |
598 | is applied recursively to both arguments. If I<both> |
599 | arguments pass the test, then the argument is treated |
600 | as boolean. |
601 | |
602 | =item o |
603 | |
604 | If EXPR is C<... || ...> or C<... or ...>, the test |
605 | is applied recursively to the first argument. |
606 | |
607 | =back |
608 | |
609 | These rules look complicated, but usually they will do what |
610 | you want. For example you could write: |
611 | |
27cec4bd |
612 | when (/^\d$/ && $_ < 75) { ... } |
0d863452 |
613 | |
4b7b0ae4 |
614 | Another useful shortcut is that, if you use a literal array |
615 | or hash as the argument to C<when>, it is turned into a |
616 | reference. So C<given(@foo)> is the same as C<given(\@foo)>, |
617 | for example. |
618 | |
0d863452 |
619 | C<default> behaves exactly like C<when(1 == 1)>, which is |
620 | to say that it always matches. |
621 | |
622 | See L</"Smart matching in detail"> for more information |
623 | on smart matching. |
624 | |
4b7b0ae4 |
625 | =head3 Breaking out |
626 | |
627 | You can use the C<break> keyword to break out of the enclosing |
628 | C<given> block. Every C<when> block is implicitly ended with |
629 | a C<break>. |
630 | |
0d863452 |
631 | =head3 Fall-through |
632 | |
633 | You can use the C<continue> keyword to fall through from one |
634 | case to the next: |
635 | |
27cec4bd |
636 | given($foo) { |
4b7b0ae4 |
637 | when (/x/) { say '$foo contains an x'; continue } |
638 | when (/y/) { say '$foo contains a y' } |
639 | default { say '$foo contains neither an x nor a y' } |
27cec4bd |
640 | } |
0d863452 |
641 | |
642 | =head3 Switching in a loop |
643 | |
644 | Instead of using C<given()>, you can use a C<foreach()> loop. |
645 | For example, here's one way to count how many times a particular |
646 | string occurs in an array: |
647 | |
27cec4bd |
648 | my $count = 0; |
649 | for (@array) { |
650 | when ("foo") { ++$count } |
5a964f20 |
651 | } |
27cec4bd |
652 | print "\@array contains $count copies of 'foo'\n"; |
0d863452 |
653 | |
654 | On exit from the C<when> block, there is an implicit C<next>. |
655 | You can override that with an explicit C<last> if you're only |
656 | interested in the first match. |
657 | |
658 | This doesn't work if you explicitly specify a loop variable, |
659 | as in C<for $item (@array)>. You have to use the default |
660 | variable C<$_>. (You can use C<for my $_ (@array)>.) |
661 | |
662 | =head3 Smart matching in detail |
663 | |
4b7b0ae4 |
664 | The behaviour of a smart match depends on what type of thing |
665 | its arguments are. It is always commutative, i.e. C<$a ~~ $b> |
666 | behaves the same as C<$b ~~ $a>. The behaviour is determined |
667 | by the following table: the first row that applies, in either |
668 | order, determines the match behaviour. |
669 | |
670 | |
671 | $a $b Type of Match Implied Matching Code |
672 | ====== ===== ===================== ============= |
673 | (overloading trumps everything) |
674 | |
675 | Code[+] Code[+] referential equality $a == $b |
676 | Any Code[+] scalar sub truth $b->($a) |
677 | |
678 | Hash Hash hash keys identical [sort keys %$a]~~[sort keys %$b] |
679 | Hash Array hash value slice truth grep $_, @$a{@$b} |
680 | Hash Regex hash key grep grep /$b/, keys %$a |
681 | Hash Any hash entry existence exists $a->{$b} |
682 | |
683 | Array Array arrays are identical[*] |
684 | Array Regex array grep grep /$b/, @$a |
685 | Array Num array contains number grep $_ == $b, @$a |
686 | Array Any array contains string grep $_ eq $b, @$a |
687 | |
688 | Any undef undefined !defined $a |
689 | Any Regex pattern match $a =~ /$b/ |
690 | Code() Code() results are equal $a->() eq $b->() |
691 | Any Code() simple closure truth $b->() # ignoring $a |
692 | Num numish[!] numeric equality $a == $b |
693 | Any Str string equality $a eq $b |
694 | Any Num numeric equality $a == $b |
695 | |
696 | Any Any string equality $a eq $b |
697 | |
698 | |
699 | + - this must be a code reference whose prototype (if present) is not "" |
700 | (subs with a "" prototype are dealt with by the 'Code()' entry lower down) |
701 | * - if a circular reference is found, we fall back to referential equality |
702 | ! - either a real number, or a string that looks like a number |
0d863452 |
703 | |
4b7b0ae4 |
704 | The "matching code" doesn't represent the I<real> matching code, |
705 | of course: it's just there to explain the intended meaning. Unlike |
706 | C<grep>, the smart match operator will short-circuit whenever it can. |
5a964f20 |
707 | |
0d863452 |
708 | =head3 Custom matching via overloading |
5a964f20 |
709 | |
0d863452 |
710 | You can change the way that an object is matched by overloading |
4b7b0ae4 |
711 | the C<~~> operator. This trumps the usual smart match semantics. |
712 | See L<overload>. |
5a964f20 |
713 | |
54a85b95 |
714 | =head3 Differences from Perl 6 |
715 | |
716 | The Perl 5 smart match and C<given>/C<when> constructs are not |
717 | absolutely identical to their Perl 6 analogues. The most visible |
718 | difference is that, in Perl 5, parentheses are required around |
719 | the argument to C<given()> and C<when()>. Parentheses in Perl 6 |
720 | are always optional in a control construct such as C<if()>, |
721 | C<while()>, or C<when()>; they can't be made optional in Perl |
722 | 5 without a great deal of potential confusion, because Perl 5 |
723 | would parse the expression |
724 | |
725 | given $foo { |
726 | ... |
727 | } |
728 | |
729 | as though the argument to C<given> were an element of the hash |
730 | C<%foo>, interpreting the braces as hash-element syntax. |
731 | |
732 | The table of smart matches is not identical to that proposed |
733 | by the Perl 6 specification Synopsis 4. Some of the differences |
734 | are simply a consequence of Perl 5's different data model, while |
735 | other changes have been made to address problems with the Perl 6 |
736 | proposal. For example, the Perl 6 specification implies that |
737 | C<$string ~~ qr/regex/> would test string equality, rather than |
738 | doing a regular expression match. On the other hand, informal |
739 | examples elsewhere make it clear that a regular expression |
740 | match is the intended behaviour. Thus the Synopsis 4 smart |
741 | match specification cannot yet be regarded as definitive. |
742 | |
743 | In Perl 6, C<when()> will always do an implicit smart match |
744 | with its argument, whilst it is convenient in Perl 5 to |
745 | suppress this implicit smart match in certain situations, |
746 | as documented above. (The difference is largely because Perl 5 |
747 | does not, even internally, have a boolean type.) |
748 | |
4633a7c4 |
749 | =head2 Goto |
d74e8afc |
750 | X<goto> |
4633a7c4 |
751 | |
19799a22 |
752 | Although not for the faint of heart, Perl does support a C<goto> |
753 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and |
754 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for |
755 | a C<goto>; it's just the name of the loop. |
4633a7c4 |
756 | |
f86cebdf |
757 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes |
4633a7c4 |
758 | execution there. It may not be used to go into any construct that |
f86cebdf |
759 | requires initialization, such as a subroutine or a C<foreach> loop. It |
4633a7c4 |
760 | also can't be used to go into a construct that is optimized away. It |
761 | can be used to go almost anywhere else within the dynamic scope, |
762 | including out of subroutines, but it's usually better to use some other |
f86cebdf |
763 | construct such as C<last> or C<die>. The author of Perl has never felt the |
764 | need to use this form of C<goto> (in Perl, that is--C is another matter). |
4633a7c4 |
765 | |
f86cebdf |
766 | The C<goto>-EXPR form expects a label name, whose scope will be resolved |
767 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't |
4633a7c4 |
768 | necessarily recommended if you're optimizing for maintainability: |
769 | |
96f2dc66 |
770 | goto(("FOO", "BAR", "GLARCH")[$i]); |
4633a7c4 |
771 | |
f86cebdf |
772 | The C<goto>-&NAME form is highly magical, and substitutes a call to the |
4633a7c4 |
773 | named subroutine for the currently running subroutine. This is used by |
f86cebdf |
774 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then |
4633a7c4 |
775 | pretend that the other subroutine had been called in the first place |
f86cebdf |
776 | (except that any modifications to C<@_> in the current subroutine are |
777 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> |
4633a7c4 |
778 | will be able to tell that this routine was called first. |
779 | |
c07a80fd |
780 | In almost all cases like this, it's usually a far, far better idea to use the |
781 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of |
4633a7c4 |
782 | resorting to a C<goto>. For certain applications, the catch and throw pair of |
783 | C<eval{}> and die() for exception processing can also be a prudent approach. |
cb1a09d0 |
784 | |
785 | =head2 PODs: Embedded Documentation |
d74e8afc |
786 | X<POD> X<documentation> |
cb1a09d0 |
787 | |
788 | Perl has a mechanism for intermixing documentation with source code. |
c07a80fd |
789 | While it's expecting the beginning of a new statement, if the compiler |
cb1a09d0 |
790 | encounters a line that begins with an equal sign and a word, like this |
791 | |
792 | =head1 Here There Be Pods! |
793 | |
794 | Then that text and all remaining text up through and including a line |
795 | beginning with C<=cut> will be ignored. The format of the intervening |
54310121 |
796 | text is described in L<perlpod>. |
cb1a09d0 |
797 | |
798 | This allows you to intermix your source code |
799 | and your documentation text freely, as in |
800 | |
801 | =item snazzle($) |
802 | |
54310121 |
803 | The snazzle() function will behave in the most spectacular |
cb1a09d0 |
804 | form that you can possibly imagine, not even excepting |
805 | cybernetic pyrotechnics. |
806 | |
807 | =cut back to the compiler, nuff of this pod stuff! |
808 | |
809 | sub snazzle($) { |
810 | my $thingie = shift; |
811 | ......... |
54310121 |
812 | } |
cb1a09d0 |
813 | |
54310121 |
814 | Note that pod translators should look at only paragraphs beginning |
184e9718 |
815 | with a pod directive (it makes parsing easier), whereas the compiler |
54310121 |
816 | actually knows to look for pod escapes even in the middle of a |
cb1a09d0 |
817 | paragraph. This means that the following secret stuff will be |
818 | ignored by both the compiler and the translators. |
819 | |
820 | $a=3; |
821 | =secret stuff |
822 | warn "Neither POD nor CODE!?" |
823 | =cut back |
824 | print "got $a\n"; |
825 | |
f86cebdf |
826 | You probably shouldn't rely upon the C<warn()> being podded out forever. |
cb1a09d0 |
827 | Not all pod translators are well-behaved in this regard, and perhaps |
828 | the compiler will become pickier. |
774d564b |
829 | |
830 | One may also use pod directives to quickly comment out a section |
831 | of code. |
832 | |
833 | =head2 Plain Old Comments (Not!) |
d74e8afc |
834 | X<comment> X<line> X<#> X<preprocessor> X<eval> |
774d564b |
835 | |
6ec4bd10 |
836 | Perl can process line directives, much like the C preprocessor. Using |
5a964f20 |
837 | this, one can control Perl's idea of filenames and line numbers in |
774d564b |
838 | error or warning messages (especially for strings that are processed |
f86cebdf |
839 | with C<eval()>). The syntax for this mechanism is the same as for most |
774d564b |
840 | C preprocessors: it matches the regular expression |
6ec4bd10 |
841 | |
842 | # example: '# line 42 "new_filename.plx"' |
82d4537c |
843 | /^\# \s* |
6ec4bd10 |
844 | line \s+ (\d+) \s* |
7b6e93a8 |
845 | (?:\s("?)([^"]+)\2)? \s* |
6ec4bd10 |
846 | $/x |
847 | |
7b6e93a8 |
848 | with C<$1> being the line number for the next line, and C<$3> being |
849 | the optional filename (specified with or without quotes). |
774d564b |
850 | |
003183f2 |
851 | There is a fairly obvious gotcha included with the line directive: |
852 | Debuggers and profilers will only show the last source line to appear |
853 | at a particular line number in a given file. Care should be taken not |
854 | to cause line number collisions in code you'd like to debug later. |
855 | |
774d564b |
856 | Here are some examples that you should be able to type into your command |
857 | shell: |
858 | |
859 | % perl |
860 | # line 200 "bzzzt" |
861 | # the `#' on the previous line must be the first char on line |
862 | die 'foo'; |
863 | __END__ |
864 | foo at bzzzt line 201. |
54310121 |
865 | |
774d564b |
866 | % perl |
867 | # line 200 "bzzzt" |
868 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; |
869 | __END__ |
870 | foo at - line 2001. |
54310121 |
871 | |
774d564b |
872 | % perl |
873 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; |
874 | __END__ |
875 | foo at foo bar line 200. |
54310121 |
876 | |
774d564b |
877 | % perl |
878 | # line 345 "goop" |
879 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; |
880 | print $@; |
881 | __END__ |
882 | foo at goop line 345. |
883 | |
884 | =cut |