Commit | Line | Data |
a0d0e21e |
1 | =head1 NAME |
2 | |
3 | perlsub - Perl subroutines |
4 | |
5 | =head1 SYNOPSIS |
6 | |
7 | To declare subroutines: |
8 | |
09bef843 |
9 | sub NAME; # A "forward" declaration. |
10 | sub NAME(PROTO); # ditto, but with prototypes |
11 | sub NAME : ATTRS; # with attributes |
12 | sub NAME(PROTO) : ATTRS; # with attributes and prototypes |
cb1a09d0 |
13 | |
09bef843 |
14 | sub NAME BLOCK # A declaration and a definition. |
15 | sub NAME(PROTO) BLOCK # ditto, but with prototypes |
16 | sub NAME : ATTRS BLOCK # with attributes |
17 | sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes |
a0d0e21e |
18 | |
748a9306 |
19 | To define an anonymous subroutine at runtime: |
20 | |
09bef843 |
21 | $subref = sub BLOCK; # no proto |
22 | $subref = sub (PROTO) BLOCK; # with proto |
23 | $subref = sub : ATTRS BLOCK; # with attributes |
24 | $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes |
748a9306 |
25 | |
a0d0e21e |
26 | To import subroutines: |
27 | |
19799a22 |
28 | use MODULE qw(NAME1 NAME2 NAME3); |
a0d0e21e |
29 | |
30 | To call subroutines: |
31 | |
5f05dabc |
32 | NAME(LIST); # & is optional with parentheses. |
54310121 |
33 | NAME LIST; # Parentheses optional if predeclared/imported. |
19799a22 |
34 | &NAME(LIST); # Circumvent prototypes. |
5a964f20 |
35 | &NAME; # Makes current @_ visible to called subroutine. |
a0d0e21e |
36 | |
37 | =head1 DESCRIPTION |
38 | |
19799a22 |
39 | Like many languages, Perl provides for user-defined subroutines. |
40 | These may be located anywhere in the main program, loaded in from |
41 | other files via the C<do>, C<require>, or C<use> keywords, or |
be3174d2 |
42 | generated on the fly using C<eval> or anonymous subroutines. |
19799a22 |
43 | You can even call a function indirectly using a variable containing |
44 | its name or a CODE reference. |
cb1a09d0 |
45 | |
46 | The Perl model for function call and return values is simple: all |
47 | functions are passed as parameters one single flat list of scalars, and |
48 | all functions likewise return to their caller one single flat list of |
49 | scalars. Any arrays or hashes in these call and return lists will |
50 | collapse, losing their identities--but you may always use |
51 | pass-by-reference instead to avoid this. Both call and return lists may |
52 | contain as many or as few scalar elements as you'd like. (Often a |
53 | function without an explicit return statement is called a subroutine, but |
19799a22 |
54 | there's really no difference from Perl's perspective.) |
55 | |
56 | Any arguments passed in show up in the array C<@_>. Therefore, if |
57 | you called a function with two arguments, those would be stored in |
58 | C<$_[0]> and C<$_[1]>. The array C<@_> is a local array, but its |
59 | elements are aliases for the actual scalar parameters. In particular, |
60 | if an element C<$_[0]> is updated, the corresponding argument is |
61 | updated (or an error occurs if it is not updatable). If an argument |
62 | is an array or hash element which did not exist when the function |
63 | was called, that element is created only when (and if) it is modified |
64 | or a reference to it is taken. (Some earlier versions of Perl |
65 | created the element whether or not the element was assigned to.) |
66 | Assigning to the whole array C<@_> removes that aliasing, and does |
67 | not update any arguments. |
68 | |
69 | The return value of a subroutine is the value of the last expression |
70 | evaluated. More explicitly, a C<return> statement may be used to exit the |
54310121 |
71 | subroutine, optionally specifying the returned value, which will be |
72 | evaluated in the appropriate context (list, scalar, or void) depending |
73 | on the context of the subroutine call. If you specify no return value, |
19799a22 |
74 | the subroutine returns an empty list in list context, the undefined |
75 | value in scalar context, or nothing in void context. If you return |
76 | one or more aggregates (arrays and hashes), these will be flattened |
77 | together into one large indistinguishable list. |
78 | |
79 | Perl does not have named formal parameters. In practice all you |
80 | do is assign to a C<my()> list of these. Variables that aren't |
81 | declared to be private are global variables. For gory details |
82 | on creating private variables, see L<"Private Variables via my()"> |
83 | and L<"Temporary Values via local()">. To create protected |
84 | environments for a set of functions in a separate package (and |
85 | probably a separate file), see L<perlmod/"Packages">. |
a0d0e21e |
86 | |
87 | Example: |
88 | |
cb1a09d0 |
89 | sub max { |
90 | my $max = shift(@_); |
a0d0e21e |
91 | foreach $foo (@_) { |
92 | $max = $foo if $max < $foo; |
93 | } |
cb1a09d0 |
94 | return $max; |
a0d0e21e |
95 | } |
cb1a09d0 |
96 | $bestday = max($mon,$tue,$wed,$thu,$fri); |
a0d0e21e |
97 | |
98 | Example: |
99 | |
100 | # get a line, combining continuation lines |
101 | # that start with whitespace |
102 | |
103 | sub get_line { |
19799a22 |
104 | $thisline = $lookahead; # global variables! |
54310121 |
105 | LINE: while (defined($lookahead = <STDIN>)) { |
a0d0e21e |
106 | if ($lookahead =~ /^[ \t]/) { |
107 | $thisline .= $lookahead; |
108 | } |
109 | else { |
110 | last LINE; |
111 | } |
112 | } |
19799a22 |
113 | return $thisline; |
a0d0e21e |
114 | } |
115 | |
116 | $lookahead = <STDIN>; # get first line |
19799a22 |
117 | while (defined($line = get_line())) { |
a0d0e21e |
118 | ... |
119 | } |
120 | |
09bef843 |
121 | Assigning to a list of private variables to name your arguments: |
a0d0e21e |
122 | |
123 | sub maybeset { |
124 | my($key, $value) = @_; |
cb1a09d0 |
125 | $Foo{$key} = $value unless $Foo{$key}; |
a0d0e21e |
126 | } |
127 | |
19799a22 |
128 | Because the assignment copies the values, this also has the effect |
129 | of turning call-by-reference into call-by-value. Otherwise a |
130 | function is free to do in-place modifications of C<@_> and change |
131 | its caller's values. |
cb1a09d0 |
132 | |
133 | upcase_in($v1, $v2); # this changes $v1 and $v2 |
134 | sub upcase_in { |
54310121 |
135 | for (@_) { tr/a-z/A-Z/ } |
136 | } |
cb1a09d0 |
137 | |
138 | You aren't allowed to modify constants in this way, of course. If an |
139 | argument were actually literal and you tried to change it, you'd take a |
140 | (presumably fatal) exception. For example, this won't work: |
141 | |
142 | upcase_in("frederick"); |
143 | |
f86cebdf |
144 | It would be much safer if the C<upcase_in()> function |
cb1a09d0 |
145 | were written to return a copy of its parameters instead |
146 | of changing them in place: |
147 | |
19799a22 |
148 | ($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2 |
cb1a09d0 |
149 | sub upcase { |
54310121 |
150 | return unless defined wantarray; # void context, do nothing |
cb1a09d0 |
151 | my @parms = @_; |
54310121 |
152 | for (@parms) { tr/a-z/A-Z/ } |
c07a80fd |
153 | return wantarray ? @parms : $parms[0]; |
54310121 |
154 | } |
cb1a09d0 |
155 | |
19799a22 |
156 | Notice how this (unprototyped) function doesn't care whether it was |
a2293a43 |
157 | passed real scalars or arrays. Perl sees all arguments as one big, |
19799a22 |
158 | long, flat parameter list in C<@_>. This is one area where |
159 | Perl's simple argument-passing style shines. The C<upcase()> |
160 | function would work perfectly well without changing the C<upcase()> |
161 | definition even if we fed it things like this: |
cb1a09d0 |
162 | |
163 | @newlist = upcase(@list1, @list2); |
164 | @newlist = upcase( split /:/, $var ); |
165 | |
166 | Do not, however, be tempted to do this: |
167 | |
168 | (@a, @b) = upcase(@list1, @list2); |
169 | |
19799a22 |
170 | Like the flattened incoming parameter list, the return list is also |
171 | flattened on return. So all you have managed to do here is stored |
13a2d996 |
172 | everything in C<@a> and made C<@b> an empty list. See |
173 | L<Pass by Reference> for alternatives. |
19799a22 |
174 | |
175 | A subroutine may be called using an explicit C<&> prefix. The |
176 | C<&> is optional in modern Perl, as are parentheses if the |
177 | subroutine has been predeclared. The C<&> is I<not> optional |
178 | when just naming the subroutine, such as when it's used as |
179 | an argument to defined() or undef(). Nor is it optional when you |
180 | want to do an indirect subroutine call with a subroutine name or |
181 | reference using the C<&$subref()> or C<&{$subref}()> constructs, |
c47ff5f1 |
182 | although the C<< $subref->() >> notation solves that problem. |
19799a22 |
183 | See L<perlref> for more about all that. |
184 | |
185 | Subroutines may be called recursively. If a subroutine is called |
186 | using the C<&> form, the argument list is optional, and if omitted, |
187 | no C<@_> array is set up for the subroutine: the C<@_> array at the |
188 | time of the call is visible to subroutine instead. This is an |
189 | efficiency mechanism that new users may wish to avoid. |
a0d0e21e |
190 | |
191 | &foo(1,2,3); # pass three arguments |
192 | foo(1,2,3); # the same |
193 | |
194 | foo(); # pass a null list |
195 | &foo(); # the same |
a0d0e21e |
196 | |
cb1a09d0 |
197 | &foo; # foo() get current args, like foo(@_) !! |
54310121 |
198 | foo; # like foo() IFF sub foo predeclared, else "foo" |
cb1a09d0 |
199 | |
19799a22 |
200 | Not only does the C<&> form make the argument list optional, it also |
201 | disables any prototype checking on arguments you do provide. This |
c07a80fd |
202 | is partly for historical reasons, and partly for having a convenient way |
19799a22 |
203 | to cheat if you know what you're doing. See L<Prototypes> below. |
c07a80fd |
204 | |
09bef843 |
205 | Functions whose names are in all upper case are reserved to the Perl |
19799a22 |
206 | core, as are modules whose names are in all lower case. A |
207 | function in all capitals is a loosely-held convention meaning it |
208 | will be called indirectly by the run-time system itself, usually |
209 | due to a triggered event. Functions that do special, pre-defined |
f2fc0a40 |
210 | things include C<BEGIN>, C<CHECK>, C<INIT>, C<END>, C<AUTOLOAD>, |
211 | C<CLONE> and C<DESTROY>--plus all functions mentioned in L<perltie>. |
5a964f20 |
212 | |
b687b08b |
213 | =head2 Private Variables via my() |
cb1a09d0 |
214 | |
215 | Synopsis: |
216 | |
217 | my $foo; # declare $foo lexically local |
218 | my (@wid, %get); # declare list of variables local |
219 | my $foo = "flurp"; # declare $foo lexical, and init it |
220 | my @oof = @bar; # declare @oof lexical, and init it |
09bef843 |
221 | my $x : Foo = $y; # similar, with an attribute applied |
222 | |
223 | B<WARNING>: The use of attribute lists on C<my> declarations is |
224 | experimental. This feature should not be relied upon. It may |
225 | change or disappear in future releases of Perl. See L<attributes>. |
cb1a09d0 |
226 | |
19799a22 |
227 | The C<my> operator declares the listed variables to be lexically |
228 | confined to the enclosing block, conditional (C<if/unless/elsif/else>), |
229 | loop (C<for/foreach/while/until/continue>), subroutine, C<eval>, |
230 | or C<do/require/use>'d file. If more than one value is listed, the |
231 | list must be placed in parentheses. All listed elements must be |
232 | legal lvalues. Only alphanumeric identifiers may be lexically |
09bef843 |
233 | scoped--magical built-ins like C<$/> must currently be C<local>ize |
19799a22 |
234 | with C<local> instead. |
235 | |
236 | Unlike dynamic variables created by the C<local> operator, lexical |
237 | variables declared with C<my> are totally hidden from the outside |
238 | world, including any called subroutines. This is true if it's the |
239 | same subroutine called from itself or elsewhere--every call gets |
240 | its own copy. |
241 | |
242 | This doesn't mean that a C<my> variable declared in a statically |
243 | enclosing lexical scope would be invisible. Only dynamic scopes |
244 | are cut off. For example, the C<bumpx()> function below has access |
245 | to the lexical $x variable because both the C<my> and the C<sub> |
246 | occurred at the same scope, presumably file scope. |
5a964f20 |
247 | |
248 | my $x = 10; |
249 | sub bumpx { $x++ } |
250 | |
19799a22 |
251 | An C<eval()>, however, can see lexical variables of the scope it is |
252 | being evaluated in, so long as the names aren't hidden by declarations within |
253 | the C<eval()> itself. See L<perlref>. |
cb1a09d0 |
254 | |
19799a22 |
255 | The parameter list to my() may be assigned to if desired, which allows you |
cb1a09d0 |
256 | to initialize your variables. (If no initializer is given for a |
257 | particular variable, it is created with the undefined value.) Commonly |
19799a22 |
258 | this is used to name input parameters to a subroutine. Examples: |
cb1a09d0 |
259 | |
260 | $arg = "fred"; # "global" variable |
261 | $n = cube_root(27); |
262 | print "$arg thinks the root is $n\n"; |
263 | fred thinks the root is 3 |
264 | |
265 | sub cube_root { |
266 | my $arg = shift; # name doesn't matter |
267 | $arg **= 1/3; |
268 | return $arg; |
54310121 |
269 | } |
cb1a09d0 |
270 | |
19799a22 |
271 | The C<my> is simply a modifier on something you might assign to. So when |
272 | you do assign to variables in its argument list, C<my> doesn't |
6cc33c6d |
273 | change whether those variables are viewed as a scalar or an array. So |
cb1a09d0 |
274 | |
5a964f20 |
275 | my ($foo) = <STDIN>; # WRONG? |
cb1a09d0 |
276 | my @FOO = <STDIN>; |
277 | |
5f05dabc |
278 | both supply a list context to the right-hand side, while |
cb1a09d0 |
279 | |
280 | my $foo = <STDIN>; |
281 | |
5f05dabc |
282 | supplies a scalar context. But the following declares only one variable: |
748a9306 |
283 | |
5a964f20 |
284 | my $foo, $bar = 1; # WRONG |
748a9306 |
285 | |
cb1a09d0 |
286 | That has the same effect as |
748a9306 |
287 | |
cb1a09d0 |
288 | my $foo; |
289 | $bar = 1; |
a0d0e21e |
290 | |
cb1a09d0 |
291 | The declared variable is not introduced (is not visible) until after |
292 | the current statement. Thus, |
293 | |
294 | my $x = $x; |
295 | |
19799a22 |
296 | can be used to initialize a new $x with the value of the old $x, and |
cb1a09d0 |
297 | the expression |
298 | |
299 | my $x = 123 and $x == 123 |
300 | |
19799a22 |
301 | is false unless the old $x happened to have the value C<123>. |
cb1a09d0 |
302 | |
55497cff |
303 | Lexical scopes of control structures are not bounded precisely by the |
304 | braces that delimit their controlled blocks; control expressions are |
19799a22 |
305 | part of that scope, too. Thus in the loop |
55497cff |
306 | |
19799a22 |
307 | while (my $line = <>) { |
55497cff |
308 | $line = lc $line; |
309 | } continue { |
310 | print $line; |
311 | } |
312 | |
19799a22 |
313 | the scope of $line extends from its declaration throughout the rest of |
55497cff |
314 | the loop construct (including the C<continue> clause), but not beyond |
315 | it. Similarly, in the conditional |
316 | |
317 | if ((my $answer = <STDIN>) =~ /^yes$/i) { |
318 | user_agrees(); |
319 | } elsif ($answer =~ /^no$/i) { |
320 | user_disagrees(); |
321 | } else { |
322 | chomp $answer; |
323 | die "'$answer' is neither 'yes' nor 'no'"; |
324 | } |
325 | |
19799a22 |
326 | the scope of $answer extends from its declaration through the rest |
327 | of that conditional, including any C<elsif> and C<else> clauses, |
55497cff |
328 | but not beyond it. |
329 | |
5d6b5b93 |
330 | B<NOTE:> None of the foregoing text applies to C<if/unless>, |
e855abca |
331 | C<while/until> or C<for> modifiers appended to simple statements. Such |
332 | modifiers are not control structures and have no effect on scoping. The |
333 | use of such modifiers in conjunction with C<my> is best avoided, as it may |
334 | have unexpected effects. A future release of Perl may define precise |
5d6b5b93 |
335 | semantics for constructs such as C<my $foo = 1 if $bar>. |
55497cff |
336 | |
5f05dabc |
337 | The C<foreach> loop defaults to scoping its index variable dynamically |
19799a22 |
338 | in the manner of C<local>. However, if the index variable is |
339 | prefixed with the keyword C<my>, or if there is already a lexical |
340 | by that name in scope, then a new lexical is created instead. Thus |
341 | in the loop |
55497cff |
342 | |
343 | for my $i (1, 2, 3) { |
344 | some_function(); |
345 | } |
346 | |
19799a22 |
347 | the scope of $i extends to the end of the loop, but not beyond it, |
348 | rendering the value of $i inaccessible within C<some_function()>. |
55497cff |
349 | |
cb1a09d0 |
350 | Some users may wish to encourage the use of lexically scoped variables. |
19799a22 |
351 | As an aid to catching implicit uses to package variables, |
352 | which are always global, if you say |
cb1a09d0 |
353 | |
354 | use strict 'vars'; |
355 | |
19799a22 |
356 | then any variable mentioned from there to the end of the enclosing |
357 | block must either refer to a lexical variable, be predeclared via |
77ca0c92 |
358 | C<our> or C<use vars>, or else must be fully qualified with the package name. |
19799a22 |
359 | A compilation error results otherwise. An inner block may countermand |
360 | this with C<no strict 'vars'>. |
361 | |
362 | A C<my> has both a compile-time and a run-time effect. At compile |
8593bda5 |
363 | time, the compiler takes notice of it. The principal usefulness |
19799a22 |
364 | of this is to quiet C<use strict 'vars'>, but it is also essential |
365 | for generation of closures as detailed in L<perlref>. Actual |
366 | initialization is delayed until run time, though, so it gets executed |
367 | at the appropriate time, such as each time through a loop, for |
368 | example. |
369 | |
370 | Variables declared with C<my> are not part of any package and are therefore |
cb1a09d0 |
371 | never fully qualified with the package name. In particular, you're not |
372 | allowed to try to make a package variable (or other global) lexical: |
373 | |
374 | my $pack::var; # ERROR! Illegal syntax |
375 | my $_; # also illegal (currently) |
376 | |
377 | In fact, a dynamic variable (also known as package or global variables) |
f86cebdf |
378 | are still accessible using the fully qualified C<::> notation even while a |
cb1a09d0 |
379 | lexical of the same name is also visible: |
380 | |
381 | package main; |
382 | local $x = 10; |
383 | my $x = 20; |
384 | print "$x and $::x\n"; |
385 | |
f86cebdf |
386 | That will print out C<20> and C<10>. |
cb1a09d0 |
387 | |
19799a22 |
388 | You may declare C<my> variables at the outermost scope of a file |
389 | to hide any such identifiers from the world outside that file. This |
390 | is similar in spirit to C's static variables when they are used at |
391 | the file level. To do this with a subroutine requires the use of |
392 | a closure (an anonymous function that accesses enclosing lexicals). |
393 | If you want to create a private subroutine that cannot be called |
394 | from outside that block, it can declare a lexical variable containing |
395 | an anonymous sub reference: |
cb1a09d0 |
396 | |
397 | my $secret_version = '1.001-beta'; |
398 | my $secret_sub = sub { print $secret_version }; |
399 | &$secret_sub(); |
400 | |
401 | As long as the reference is never returned by any function within the |
5f05dabc |
402 | module, no outside module can see the subroutine, because its name is not in |
cb1a09d0 |
403 | any package's symbol table. Remember that it's not I<REALLY> called |
19799a22 |
404 | C<$some_pack::secret_version> or anything; it's just $secret_version, |
cb1a09d0 |
405 | unqualified and unqualifiable. |
406 | |
19799a22 |
407 | This does not work with object methods, however; all object methods |
408 | have to be in the symbol table of some package to be found. See |
409 | L<perlref/"Function Templates"> for something of a work-around to |
410 | this. |
cb1a09d0 |
411 | |
c2611fb3 |
412 | =head2 Persistent Private Variables |
5a964f20 |
413 | |
414 | Just because a lexical variable is lexically (also called statically) |
f86cebdf |
415 | scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that |
5a964f20 |
416 | within a function it works like a C static. It normally works more |
417 | like a C auto, but with implicit garbage collection. |
418 | |
419 | Unlike local variables in C or C++, Perl's lexical variables don't |
420 | necessarily get recycled just because their scope has exited. |
421 | If something more permanent is still aware of the lexical, it will |
422 | stick around. So long as something else references a lexical, that |
423 | lexical won't be freed--which is as it should be. You wouldn't want |
424 | memory being free until you were done using it, or kept around once you |
425 | were done. Automatic garbage collection takes care of this for you. |
426 | |
427 | This means that you can pass back or save away references to lexical |
428 | variables, whereas to return a pointer to a C auto is a grave error. |
429 | It also gives us a way to simulate C's function statics. Here's a |
430 | mechanism for giving a function private variables with both lexical |
431 | scoping and a static lifetime. If you do want to create something like |
432 | C's static variables, just enclose the whole function in an extra block, |
433 | and put the static variable outside the function but in the block. |
cb1a09d0 |
434 | |
435 | { |
54310121 |
436 | my $secret_val = 0; |
cb1a09d0 |
437 | sub gimme_another { |
438 | return ++$secret_val; |
54310121 |
439 | } |
440 | } |
cb1a09d0 |
441 | # $secret_val now becomes unreachable by the outside |
442 | # world, but retains its value between calls to gimme_another |
443 | |
54310121 |
444 | If this function is being sourced in from a separate file |
cb1a09d0 |
445 | via C<require> or C<use>, then this is probably just fine. If it's |
19799a22 |
446 | all in the main program, you'll need to arrange for the C<my> |
cb1a09d0 |
447 | to be executed early, either by putting the whole block above |
f86cebdf |
448 | your main program, or more likely, placing merely a C<BEGIN> |
cb1a09d0 |
449 | sub around it to make sure it gets executed before your program |
450 | starts to run: |
451 | |
452 | sub BEGIN { |
54310121 |
453 | my $secret_val = 0; |
cb1a09d0 |
454 | sub gimme_another { |
455 | return ++$secret_val; |
54310121 |
456 | } |
457 | } |
cb1a09d0 |
458 | |
19799a22 |
459 | See L<perlmod/"Package Constructors and Destructors"> about the |
7d30b5c4 |
460 | special triggered functions, C<BEGIN>, C<CHECK>, C<INIT> and C<END>. |
cb1a09d0 |
461 | |
19799a22 |
462 | If declared at the outermost scope (the file scope), then lexicals |
463 | work somewhat like C's file statics. They are available to all |
464 | functions in that same file declared below them, but are inaccessible |
465 | from outside that file. This strategy is sometimes used in modules |
466 | to create private variables that the whole module can see. |
5a964f20 |
467 | |
cb1a09d0 |
468 | =head2 Temporary Values via local() |
469 | |
19799a22 |
470 | B<WARNING>: In general, you should be using C<my> instead of C<local>, because |
6d28dffb |
471 | it's faster and safer. Exceptions to this include the global punctuation |
cb1a09d0 |
472 | variables, filehandles and formats, and direct manipulation of the Perl |
19799a22 |
473 | symbol table itself. Format variables often use C<local> though, as do |
cb1a09d0 |
474 | other variables whose current value must be visible to called |
475 | subroutines. |
476 | |
477 | Synopsis: |
478 | |
479 | local $foo; # declare $foo dynamically local |
480 | local (@wid, %get); # declare list of variables local |
481 | local $foo = "flurp"; # declare $foo dynamic, and init it |
482 | local @oof = @bar; # declare @oof dynamic, and init it |
483 | |
484 | local *FH; # localize $FH, @FH, %FH, &FH ... |
485 | local *merlyn = *randal; # now $merlyn is really $randal, plus |
486 | # @merlyn is really @randal, etc |
487 | local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal |
54310121 |
488 | local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc |
cb1a09d0 |
489 | |
19799a22 |
490 | A C<local> modifies its listed variables to be "local" to the |
491 | enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine |
492 | called from within that block>. A C<local> just gives temporary |
493 | values to global (meaning package) variables. It does I<not> create |
494 | a local variable. This is known as dynamic scoping. Lexical scoping |
495 | is done with C<my>, which works more like C's auto declarations. |
cb1a09d0 |
496 | |
19799a22 |
497 | If more than one variable is given to C<local>, they must be placed in |
5f05dabc |
498 | parentheses. All listed elements must be legal lvalues. This operator works |
cb1a09d0 |
499 | by saving the current values of those variables in its argument list on a |
5f05dabc |
500 | hidden stack and restoring them upon exiting the block, subroutine, or |
cb1a09d0 |
501 | eval. This means that called subroutines can also reference the local |
502 | variable, but not the global one. The argument list may be assigned to if |
503 | desired, which allows you to initialize your local variables. (If no |
504 | initializer is given for a particular variable, it is created with an |
505 | undefined value.) Commonly this is used to name the parameters to a |
506 | subroutine. Examples: |
507 | |
508 | for $i ( 0 .. 9 ) { |
509 | $digits{$i} = $i; |
54310121 |
510 | } |
cb1a09d0 |
511 | # assume this function uses global %digits hash |
54310121 |
512 | parse_num(); |
cb1a09d0 |
513 | |
514 | # now temporarily add to %digits hash |
515 | if ($base12) { |
516 | # (NOTE: not claiming this is efficient!) |
517 | local %digits = (%digits, 't' => 10, 'e' => 11); |
518 | parse_num(); # parse_num gets this new %digits! |
519 | } |
520 | # old %digits restored here |
521 | |
19799a22 |
522 | Because C<local> is a run-time operator, it gets executed each time |
cb1a09d0 |
523 | through a loop. In releases of Perl previous to 5.0, this used more stack |
524 | storage each time until the loop was exited. Perl now reclaims the space |
525 | each time through, but it's still more efficient to declare your variables |
526 | outside the loop. |
527 | |
f86cebdf |
528 | A C<local> is simply a modifier on an lvalue expression. When you assign to |
529 | a C<local>ized variable, the C<local> doesn't change whether its list is viewed |
cb1a09d0 |
530 | as a scalar or an array. So |
531 | |
532 | local($foo) = <STDIN>; |
533 | local @FOO = <STDIN>; |
534 | |
5f05dabc |
535 | both supply a list context to the right-hand side, while |
cb1a09d0 |
536 | |
537 | local $foo = <STDIN>; |
538 | |
539 | supplies a scalar context. |
540 | |
3e3baf6d |
541 | A note about C<local()> and composite types is in order. Something |
542 | like C<local(%foo)> works by temporarily placing a brand new hash in |
543 | the symbol table. The old hash is left alone, but is hidden "behind" |
544 | the new one. |
545 | |
546 | This means the old variable is completely invisible via the symbol |
547 | table (i.e. the hash entry in the C<*foo> typeglob) for the duration |
548 | of the dynamic scope within which the C<local()> was seen. This |
549 | has the effect of allowing one to temporarily occlude any magic on |
550 | composite types. For instance, this will briefly alter a tied |
551 | hash to some other implementation: |
552 | |
553 | tie %ahash, 'APackage'; |
554 | [...] |
555 | { |
556 | local %ahash; |
557 | tie %ahash, 'BPackage'; |
558 | [..called code will see %ahash tied to 'BPackage'..] |
559 | { |
560 | local %ahash; |
561 | [..%ahash is a normal (untied) hash here..] |
562 | } |
563 | } |
564 | [..%ahash back to its initial tied self again..] |
565 | |
566 | As another example, a custom implementation of C<%ENV> might look |
567 | like this: |
568 | |
569 | { |
570 | local %ENV; |
571 | tie %ENV, 'MyOwnEnv'; |
572 | [..do your own fancy %ENV manipulation here..] |
573 | } |
574 | [..normal %ENV behavior here..] |
575 | |
6ee623d5 |
576 | It's also worth taking a moment to explain what happens when you |
f86cebdf |
577 | C<local>ize a member of a composite type (i.e. an array or hash element). |
578 | In this case, the element is C<local>ized I<by name>. This means that |
6ee623d5 |
579 | when the scope of the C<local()> ends, the saved value will be |
580 | restored to the hash element whose key was named in the C<local()>, or |
581 | the array element whose index was named in the C<local()>. If that |
582 | element was deleted while the C<local()> was in effect (e.g. by a |
583 | C<delete()> from a hash or a C<shift()> of an array), it will spring |
584 | back into existence, possibly extending an array and filling in the |
585 | skipped elements with C<undef>. For instance, if you say |
586 | |
587 | %hash = ( 'This' => 'is', 'a' => 'test' ); |
588 | @ary = ( 0..5 ); |
589 | { |
590 | local($ary[5]) = 6; |
591 | local($hash{'a'}) = 'drill'; |
592 | while (my $e = pop(@ary)) { |
593 | print "$e . . .\n"; |
594 | last unless $e > 3; |
595 | } |
596 | if (@ary) { |
597 | $hash{'only a'} = 'test'; |
598 | delete $hash{'a'}; |
599 | } |
600 | } |
601 | print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n"; |
602 | print "The array has ",scalar(@ary)," elements: ", |
603 | join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n"; |
604 | |
605 | Perl will print |
606 | |
607 | 6 . . . |
608 | 4 . . . |
609 | 3 . . . |
610 | This is a test only a test. |
611 | The array has 6 elements: 0, 1, 2, undef, undef, 5 |
612 | |
19799a22 |
613 | The behavior of local() on non-existent members of composite |
7185e5cc |
614 | types is subject to change in future. |
615 | |
cd06dffe |
616 | =head2 Lvalue subroutines |
617 | |
618 | B<WARNING>: Lvalue subroutines are still experimental and the implementation |
619 | may change in future versions of Perl. |
620 | |
621 | It is possible to return a modifiable value from a subroutine. |
622 | To do this, you have to declare the subroutine to return an lvalue. |
623 | |
624 | my $val; |
625 | sub canmod : lvalue { |
626 | $val; |
627 | } |
628 | sub nomod { |
629 | $val; |
630 | } |
631 | |
632 | canmod() = 5; # assigns to $val |
633 | nomod() = 5; # ERROR |
634 | |
635 | The scalar/list context for the subroutine and for the right-hand |
636 | side of assignment is determined as if the subroutine call is replaced |
637 | by a scalar. For example, consider: |
638 | |
639 | data(2,3) = get_data(3,4); |
640 | |
641 | Both subroutines here are called in a scalar context, while in: |
642 | |
643 | (data(2,3)) = get_data(3,4); |
644 | |
645 | and in: |
646 | |
647 | (data(2),data(3)) = get_data(3,4); |
648 | |
649 | all the subroutines are called in a list context. |
650 | |
cb1a09d0 |
651 | =head2 Passing Symbol Table Entries (typeglobs) |
652 | |
19799a22 |
653 | B<WARNING>: The mechanism described in this section was originally |
654 | the only way to simulate pass-by-reference in older versions of |
655 | Perl. While it still works fine in modern versions, the new reference |
656 | mechanism is generally easier to work with. See below. |
a0d0e21e |
657 | |
658 | Sometimes you don't want to pass the value of an array to a subroutine |
659 | but rather the name of it, so that the subroutine can modify the global |
660 | copy of it rather than working with a local copy. In perl you can |
cb1a09d0 |
661 | refer to all objects of a particular name by prefixing the name |
5f05dabc |
662 | with a star: C<*foo>. This is often known as a "typeglob", because the |
a0d0e21e |
663 | star on the front can be thought of as a wildcard match for all the |
664 | funny prefix characters on variables and subroutines and such. |
665 | |
55497cff |
666 | When evaluated, the typeglob produces a scalar value that represents |
5f05dabc |
667 | all the objects of that name, including any filehandle, format, or |
a0d0e21e |
668 | subroutine. When assigned to, it causes the name mentioned to refer to |
19799a22 |
669 | whatever C<*> value was assigned to it. Example: |
a0d0e21e |
670 | |
671 | sub doubleary { |
672 | local(*someary) = @_; |
673 | foreach $elem (@someary) { |
674 | $elem *= 2; |
675 | } |
676 | } |
677 | doubleary(*foo); |
678 | doubleary(*bar); |
679 | |
19799a22 |
680 | Scalars are already passed by reference, so you can modify |
a0d0e21e |
681 | scalar arguments without using this mechanism by referring explicitly |
1fef88e7 |
682 | to C<$_[0]> etc. You can modify all the elements of an array by passing |
f86cebdf |
683 | all the elements as scalars, but you have to use the C<*> mechanism (or |
684 | the equivalent reference mechanism) to C<push>, C<pop>, or change the size of |
a0d0e21e |
685 | an array. It will certainly be faster to pass the typeglob (or reference). |
686 | |
687 | Even if you don't want to modify an array, this mechanism is useful for |
5f05dabc |
688 | passing multiple arrays in a single LIST, because normally the LIST |
a0d0e21e |
689 | mechanism will merge all the array values so that you can't extract out |
55497cff |
690 | the individual arrays. For more on typeglobs, see |
2ae324a7 |
691 | L<perldata/"Typeglobs and Filehandles">. |
cb1a09d0 |
692 | |
5a964f20 |
693 | =head2 When to Still Use local() |
694 | |
19799a22 |
695 | Despite the existence of C<my>, there are still three places where the |
696 | C<local> operator still shines. In fact, in these three places, you |
5a964f20 |
697 | I<must> use C<local> instead of C<my>. |
698 | |
13a2d996 |
699 | =over 4 |
5a964f20 |
700 | |
551e1d92 |
701 | =item 1. |
702 | |
703 | You need to give a global variable a temporary value, especially $_. |
5a964f20 |
704 | |
f86cebdf |
705 | The global variables, like C<@ARGV> or the punctuation variables, must be |
706 | C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits |
5a964f20 |
707 | it up into chunks separated by lines of equal signs, which are placed |
f86cebdf |
708 | in C<@Fields>. |
5a964f20 |
709 | |
710 | { |
711 | local @ARGV = ("/etc/motd"); |
712 | local $/ = undef; |
713 | local $_ = <>; |
714 | @Fields = split /^\s*=+\s*$/; |
715 | } |
716 | |
19799a22 |
717 | It particular, it's important to C<local>ize $_ in any routine that assigns |
5a964f20 |
718 | to it. Look out for implicit assignments in C<while> conditionals. |
719 | |
551e1d92 |
720 | =item 2. |
721 | |
722 | You need to create a local file or directory handle or a local function. |
5a964f20 |
723 | |
09bef843 |
724 | A function that needs a filehandle of its own must use |
725 | C<local()> on a complete typeglob. This can be used to create new symbol |
5a964f20 |
726 | table entries: |
727 | |
728 | sub ioqueue { |
729 | local (*READER, *WRITER); # not my! |
730 | pipe (READER, WRITER); or die "pipe: $!"; |
731 | return (*READER, *WRITER); |
732 | } |
733 | ($head, $tail) = ioqueue(); |
734 | |
735 | See the Symbol module for a way to create anonymous symbol table |
736 | entries. |
737 | |
738 | Because assignment of a reference to a typeglob creates an alias, this |
739 | can be used to create what is effectively a local function, or at least, |
740 | a local alias. |
741 | |
742 | { |
f86cebdf |
743 | local *grow = \&shrink; # only until this block exists |
744 | grow(); # really calls shrink() |
745 | move(); # if move() grow()s, it shrink()s too |
5a964f20 |
746 | } |
f86cebdf |
747 | grow(); # get the real grow() again |
5a964f20 |
748 | |
749 | See L<perlref/"Function Templates"> for more about manipulating |
750 | functions by name in this way. |
751 | |
551e1d92 |
752 | =item 3. |
753 | |
754 | You want to temporarily change just one element of an array or hash. |
5a964f20 |
755 | |
f86cebdf |
756 | You can C<local>ize just one element of an aggregate. Usually this |
5a964f20 |
757 | is done on dynamics: |
758 | |
759 | { |
760 | local $SIG{INT} = 'IGNORE'; |
761 | funct(); # uninterruptible |
762 | } |
763 | # interruptibility automatically restored here |
764 | |
765 | But it also works on lexically declared aggregates. Prior to 5.005, |
766 | this operation could on occasion misbehave. |
767 | |
768 | =back |
769 | |
cb1a09d0 |
770 | =head2 Pass by Reference |
771 | |
55497cff |
772 | If you want to pass more than one array or hash into a function--or |
773 | return them from it--and have them maintain their integrity, then |
774 | you're going to have to use an explicit pass-by-reference. Before you |
775 | do that, you need to understand references as detailed in L<perlref>. |
c07a80fd |
776 | This section may not make much sense to you otherwise. |
cb1a09d0 |
777 | |
19799a22 |
778 | Here are a few simple examples. First, let's pass in several arrays |
779 | to a function and have it C<pop> all of then, returning a new list |
780 | of all their former last elements: |
cb1a09d0 |
781 | |
782 | @tailings = popmany ( \@a, \@b, \@c, \@d ); |
783 | |
784 | sub popmany { |
785 | my $aref; |
786 | my @retlist = (); |
787 | foreach $aref ( @_ ) { |
788 | push @retlist, pop @$aref; |
54310121 |
789 | } |
cb1a09d0 |
790 | return @retlist; |
54310121 |
791 | } |
cb1a09d0 |
792 | |
54310121 |
793 | Here's how you might write a function that returns a |
cb1a09d0 |
794 | list of keys occurring in all the hashes passed to it: |
795 | |
54310121 |
796 | @common = inter( \%foo, \%bar, \%joe ); |
cb1a09d0 |
797 | sub inter { |
798 | my ($k, $href, %seen); # locals |
799 | foreach $href (@_) { |
800 | while ( $k = each %$href ) { |
801 | $seen{$k}++; |
54310121 |
802 | } |
803 | } |
cb1a09d0 |
804 | return grep { $seen{$_} == @_ } keys %seen; |
54310121 |
805 | } |
cb1a09d0 |
806 | |
5f05dabc |
807 | So far, we're using just the normal list return mechanism. |
54310121 |
808 | What happens if you want to pass or return a hash? Well, |
809 | if you're using only one of them, or you don't mind them |
cb1a09d0 |
810 | concatenating, then the normal calling convention is ok, although |
54310121 |
811 | a little expensive. |
cb1a09d0 |
812 | |
813 | Where people get into trouble is here: |
814 | |
815 | (@a, @b) = func(@c, @d); |
816 | or |
817 | (%a, %b) = func(%c, %d); |
818 | |
19799a22 |
819 | That syntax simply won't work. It sets just C<@a> or C<%a> and |
820 | clears the C<@b> or C<%b>. Plus the function didn't get passed |
821 | into two separate arrays or hashes: it got one long list in C<@_>, |
822 | as always. |
cb1a09d0 |
823 | |
824 | If you can arrange for everyone to deal with this through references, it's |
825 | cleaner code, although not so nice to look at. Here's a function that |
826 | takes two array references as arguments, returning the two array elements |
827 | in order of how many elements they have in them: |
828 | |
829 | ($aref, $bref) = func(\@c, \@d); |
830 | print "@$aref has more than @$bref\n"; |
831 | sub func { |
832 | my ($cref, $dref) = @_; |
833 | if (@$cref > @$dref) { |
834 | return ($cref, $dref); |
835 | } else { |
c07a80fd |
836 | return ($dref, $cref); |
54310121 |
837 | } |
838 | } |
cb1a09d0 |
839 | |
840 | It turns out that you can actually do this also: |
841 | |
842 | (*a, *b) = func(\@c, \@d); |
843 | print "@a has more than @b\n"; |
844 | sub func { |
845 | local (*c, *d) = @_; |
846 | if (@c > @d) { |
847 | return (\@c, \@d); |
848 | } else { |
849 | return (\@d, \@c); |
54310121 |
850 | } |
851 | } |
cb1a09d0 |
852 | |
853 | Here we're using the typeglobs to do symbol table aliasing. It's |
19799a22 |
854 | a tad subtle, though, and also won't work if you're using C<my> |
09bef843 |
855 | variables, because only globals (even in disguise as C<local>s) |
19799a22 |
856 | are in the symbol table. |
5f05dabc |
857 | |
858 | If you're passing around filehandles, you could usually just use the bare |
19799a22 |
859 | typeglob, like C<*STDOUT>, but typeglobs references work, too. |
860 | For example: |
5f05dabc |
861 | |
862 | splutter(\*STDOUT); |
863 | sub splutter { |
864 | my $fh = shift; |
865 | print $fh "her um well a hmmm\n"; |
866 | } |
867 | |
868 | $rec = get_rec(\*STDIN); |
869 | sub get_rec { |
870 | my $fh = shift; |
871 | return scalar <$fh>; |
872 | } |
873 | |
19799a22 |
874 | If you're planning on generating new filehandles, you could do this. |
875 | Notice to pass back just the bare *FH, not its reference. |
5f05dabc |
876 | |
877 | sub openit { |
19799a22 |
878 | my $path = shift; |
5f05dabc |
879 | local *FH; |
e05a3a1e |
880 | return open (FH, $path) ? *FH : undef; |
54310121 |
881 | } |
5f05dabc |
882 | |
cb1a09d0 |
883 | =head2 Prototypes |
884 | |
19799a22 |
885 | Perl supports a very limited kind of compile-time argument checking |
886 | using function prototyping. If you declare |
cb1a09d0 |
887 | |
888 | sub mypush (\@@) |
889 | |
19799a22 |
890 | then C<mypush()> takes arguments exactly like C<push()> does. The |
891 | function declaration must be visible at compile time. The prototype |
892 | affects only interpretation of new-style calls to the function, |
893 | where new-style is defined as not using the C<&> character. In |
894 | other words, if you call it like a built-in function, then it behaves |
895 | like a built-in function. If you call it like an old-fashioned |
896 | subroutine, then it behaves like an old-fashioned subroutine. It |
897 | naturally falls out from this rule that prototypes have no influence |
898 | on subroutine references like C<\&foo> or on indirect subroutine |
c47ff5f1 |
899 | calls like C<&{$subref}> or C<< $subref->() >>. |
c07a80fd |
900 | |
901 | Method calls are not influenced by prototypes either, because the |
19799a22 |
902 | function to be called is indeterminate at compile time, since |
903 | the exact code called depends on inheritance. |
cb1a09d0 |
904 | |
19799a22 |
905 | Because the intent of this feature is primarily to let you define |
906 | subroutines that work like built-in functions, here are prototypes |
907 | for some other functions that parse almost exactly like the |
908 | corresponding built-in. |
cb1a09d0 |
909 | |
910 | Declared as Called as |
911 | |
f86cebdf |
912 | sub mylink ($$) mylink $old, $new |
913 | sub myvec ($$$) myvec $var, $offset, 1 |
914 | sub myindex ($$;$) myindex &getstring, "substr" |
915 | sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off |
916 | sub myreverse (@) myreverse $a, $b, $c |
917 | sub myjoin ($@) myjoin ":", $a, $b, $c |
918 | sub mypop (\@) mypop @array |
919 | sub mysplice (\@$$@) mysplice @array, @array, 0, @pushme |
920 | sub mykeys (\%) mykeys %{$hashref} |
921 | sub myopen (*;$) myopen HANDLE, $name |
922 | sub mypipe (**) mypipe READHANDLE, WRITEHANDLE |
923 | sub mygrep (&@) mygrep { /foo/ } $a, $b, $c |
924 | sub myrand ($) myrand 42 |
925 | sub mytime () mytime |
cb1a09d0 |
926 | |
c07a80fd |
927 | Any backslashed prototype character represents an actual argument |
6e47f808 |
928 | that absolutely must start with that character. The value passed |
19799a22 |
929 | as part of C<@_> will be a reference to the actual argument given |
930 | in the subroutine call, obtained by applying C<\> to that argument. |
c07a80fd |
931 | |
5b794e05 |
932 | You can also backslash several argument types simultaneously by using |
933 | the C<\[]> notation: |
934 | |
935 | sub myref (\[$@%&*]) |
936 | |
937 | will allow calling myref() as |
938 | |
939 | myref $var |
940 | myref @array |
941 | myref %hash |
942 | myref &sub |
943 | myref *glob |
944 | |
945 | and the first argument of myref() will be a reference to |
946 | a scalar, an array, a hash, a code, or a glob. |
947 | |
c07a80fd |
948 | Unbackslashed prototype characters have special meanings. Any |
19799a22 |
949 | unbackslashed C<@> or C<%> eats all remaining arguments, and forces |
f86cebdf |
950 | list context. An argument represented by C<$> forces scalar context. An |
951 | C<&> requires an anonymous subroutine, which, if passed as the first |
0df79f0c |
952 | argument, does not require the C<sub> keyword or a subsequent comma. |
953 | |
954 | A C<*> allows the subroutine to accept a bareword, constant, scalar expression, |
648ca4f7 |
955 | typeglob, or a reference to a typeglob in that slot. The value will be |
956 | available to the subroutine either as a simple scalar, or (in the latter |
0df79f0c |
957 | two cases) as a reference to the typeglob. If you wish to always convert |
958 | such arguments to a typeglob reference, use Symbol::qualify_to_ref() as |
959 | follows: |
960 | |
961 | use Symbol 'qualify_to_ref'; |
962 | |
963 | sub foo (*) { |
964 | my $fh = qualify_to_ref(shift, caller); |
965 | ... |
966 | } |
c07a80fd |
967 | |
968 | A semicolon separates mandatory arguments from optional arguments. |
19799a22 |
969 | It is redundant before C<@> or C<%>, which gobble up everything else. |
cb1a09d0 |
970 | |
19799a22 |
971 | Note how the last three examples in the table above are treated |
972 | specially by the parser. C<mygrep()> is parsed as a true list |
973 | operator, C<myrand()> is parsed as a true unary operator with unary |
974 | precedence the same as C<rand()>, and C<mytime()> is truly without |
975 | arguments, just like C<time()>. That is, if you say |
cb1a09d0 |
976 | |
977 | mytime +2; |
978 | |
f86cebdf |
979 | you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed |
19799a22 |
980 | without a prototype. |
cb1a09d0 |
981 | |
19799a22 |
982 | The interesting thing about C<&> is that you can generate new syntax with it, |
983 | provided it's in the initial position: |
cb1a09d0 |
984 | |
6d28dffb |
985 | sub try (&@) { |
cb1a09d0 |
986 | my($try,$catch) = @_; |
987 | eval { &$try }; |
988 | if ($@) { |
989 | local $_ = $@; |
990 | &$catch; |
991 | } |
992 | } |
55497cff |
993 | sub catch (&) { $_[0] } |
cb1a09d0 |
994 | |
995 | try { |
996 | die "phooey"; |
997 | } catch { |
998 | /phooey/ and print "unphooey\n"; |
999 | }; |
1000 | |
f86cebdf |
1001 | That prints C<"unphooey">. (Yes, there are still unresolved |
19799a22 |
1002 | issues having to do with visibility of C<@_>. I'm ignoring that |
f86cebdf |
1003 | question for the moment. (But note that if we make C<@_> lexically |
cb1a09d0 |
1004 | scoped, those anonymous subroutines can act like closures... (Gee, |
5f05dabc |
1005 | is this sounding a little Lispish? (Never mind.)))) |
cb1a09d0 |
1006 | |
19799a22 |
1007 | And here's a reimplementation of the Perl C<grep> operator: |
cb1a09d0 |
1008 | |
1009 | sub mygrep (&@) { |
1010 | my $code = shift; |
1011 | my @result; |
1012 | foreach $_ (@_) { |
6e47f808 |
1013 | push(@result, $_) if &$code; |
cb1a09d0 |
1014 | } |
1015 | @result; |
1016 | } |
a0d0e21e |
1017 | |
cb1a09d0 |
1018 | Some folks would prefer full alphanumeric prototypes. Alphanumerics have |
1019 | been intentionally left out of prototypes for the express purpose of |
1020 | someday in the future adding named, formal parameters. The current |
1021 | mechanism's main goal is to let module writers provide better diagnostics |
1022 | for module users. Larry feels the notation quite understandable to Perl |
1023 | programmers, and that it will not intrude greatly upon the meat of the |
1024 | module, nor make it harder to read. The line noise is visually |
1025 | encapsulated into a small pill that's easy to swallow. |
1026 | |
420cdfc1 |
1027 | If you try to use an alphanumeric sequence in a prototype you will |
1028 | generate an optional warning - "Illegal character in prototype...". |
1029 | Unfortunately earlier versions of Perl allowed the prototype to be |
1030 | used as long as its prefix was a valid prototype. The warning may be |
1031 | upgraded to a fatal error in a future version of Perl once the |
1032 | majority of offending code is fixed. |
1033 | |
cb1a09d0 |
1034 | It's probably best to prototype new functions, not retrofit prototyping |
1035 | into older ones. That's because you must be especially careful about |
1036 | silent impositions of differing list versus scalar contexts. For example, |
1037 | if you decide that a function should take just one parameter, like this: |
1038 | |
1039 | sub func ($) { |
1040 | my $n = shift; |
1041 | print "you gave me $n\n"; |
54310121 |
1042 | } |
cb1a09d0 |
1043 | |
1044 | and someone has been calling it with an array or expression |
1045 | returning a list: |
1046 | |
1047 | func(@foo); |
1048 | func( split /:/ ); |
1049 | |
19799a22 |
1050 | Then you've just supplied an automatic C<scalar> in front of their |
f86cebdf |
1051 | argument, which can be more than a bit surprising. The old C<@foo> |
cb1a09d0 |
1052 | which used to hold one thing doesn't get passed in. Instead, |
19799a22 |
1053 | C<func()> now gets passed in a C<1>; that is, the number of elements |
1054 | in C<@foo>. And the C<split> gets called in scalar context so it |
1055 | starts scribbling on your C<@_> parameter list. Ouch! |
cb1a09d0 |
1056 | |
5f05dabc |
1057 | This is all very powerful, of course, and should be used only in moderation |
54310121 |
1058 | to make the world a better place. |
44a8e56a |
1059 | |
1060 | =head2 Constant Functions |
1061 | |
1062 | Functions with a prototype of C<()> are potential candidates for |
19799a22 |
1063 | inlining. If the result after optimization and constant folding |
1064 | is either a constant or a lexically-scoped scalar which has no other |
54310121 |
1065 | references, then it will be used in place of function calls made |
19799a22 |
1066 | without C<&>. Calls made using C<&> are never inlined. (See |
1067 | F<constant.pm> for an easy way to declare most constants.) |
44a8e56a |
1068 | |
5a964f20 |
1069 | The following functions would all be inlined: |
44a8e56a |
1070 | |
699e6cd4 |
1071 | sub pi () { 3.14159 } # Not exact, but close. |
1072 | sub PI () { 4 * atan2 1, 1 } # As good as it gets, |
1073 | # and it's inlined, too! |
44a8e56a |
1074 | sub ST_DEV () { 0 } |
1075 | sub ST_INO () { 1 } |
1076 | |
1077 | sub FLAG_FOO () { 1 << 8 } |
1078 | sub FLAG_BAR () { 1 << 9 } |
1079 | sub FLAG_MASK () { FLAG_FOO | FLAG_BAR } |
54310121 |
1080 | |
1081 | sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) } |
44a8e56a |
1082 | sub BAZ_VAL () { |
1083 | if (OPT_BAZ) { |
1084 | return 23; |
1085 | } |
1086 | else { |
1087 | return 42; |
1088 | } |
1089 | } |
cb1a09d0 |
1090 | |
54310121 |
1091 | sub N () { int(BAZ_VAL) / 3 } |
1092 | BEGIN { |
1093 | my $prod = 1; |
1094 | for (1..N) { $prod *= $_ } |
1095 | sub N_FACTORIAL () { $prod } |
1096 | } |
1097 | |
5a964f20 |
1098 | If you redefine a subroutine that was eligible for inlining, you'll get |
4cee8e80 |
1099 | a mandatory warning. (You can use this warning to tell whether or not a |
1100 | particular subroutine is considered constant.) The warning is |
1101 | considered severe enough not to be optional because previously compiled |
1102 | invocations of the function will still be using the old value of the |
19799a22 |
1103 | function. If you need to be able to redefine the subroutine, you need to |
4cee8e80 |
1104 | ensure that it isn't inlined, either by dropping the C<()> prototype |
19799a22 |
1105 | (which changes calling semantics, so beware) or by thwarting the |
4cee8e80 |
1106 | inlining mechanism in some other way, such as |
1107 | |
4cee8e80 |
1108 | sub not_inlined () { |
54310121 |
1109 | 23 if $]; |
4cee8e80 |
1110 | } |
1111 | |
19799a22 |
1112 | =head2 Overriding Built-in Functions |
a0d0e21e |
1113 | |
19799a22 |
1114 | Many built-in functions may be overridden, though this should be tried |
5f05dabc |
1115 | only occasionally and for good reason. Typically this might be |
19799a22 |
1116 | done by a package attempting to emulate missing built-in functionality |
a0d0e21e |
1117 | on a non-Unix system. |
1118 | |
5f05dabc |
1119 | Overriding may be done only by importing the name from a |
a0d0e21e |
1120 | module--ordinary predeclaration isn't good enough. However, the |
19799a22 |
1121 | C<use subs> pragma lets you, in effect, predeclare subs |
1122 | via the import syntax, and these names may then override built-in ones: |
a0d0e21e |
1123 | |
1124 | use subs 'chdir', 'chroot', 'chmod', 'chown'; |
1125 | chdir $somewhere; |
1126 | sub chdir { ... } |
1127 | |
19799a22 |
1128 | To unambiguously refer to the built-in form, precede the |
1129 | built-in name with the special package qualifier C<CORE::>. For example, |
1130 | saying C<CORE::open()> always refers to the built-in C<open()>, even |
fb73857a |
1131 | if the current package has imported some other subroutine called |
19799a22 |
1132 | C<&open()> from elsewhere. Even though it looks like a regular |
09bef843 |
1133 | function call, it isn't: you can't take a reference to it, such as |
19799a22 |
1134 | the incorrect C<\&CORE::open> might appear to produce. |
fb73857a |
1135 | |
19799a22 |
1136 | Library modules should not in general export built-in names like C<open> |
1137 | or C<chdir> as part of their default C<@EXPORT> list, because these may |
a0d0e21e |
1138 | sneak into someone else's namespace and change the semantics unexpectedly. |
19799a22 |
1139 | Instead, if the module adds that name to C<@EXPORT_OK>, then it's |
a0d0e21e |
1140 | possible for a user to import the name explicitly, but not implicitly. |
1141 | That is, they could say |
1142 | |
1143 | use Module 'open'; |
1144 | |
19799a22 |
1145 | and it would import the C<open> override. But if they said |
a0d0e21e |
1146 | |
1147 | use Module; |
1148 | |
19799a22 |
1149 | they would get the default imports without overrides. |
a0d0e21e |
1150 | |
19799a22 |
1151 | The foregoing mechanism for overriding built-in is restricted, quite |
95d94a4f |
1152 | deliberately, to the package that requests the import. There is a second |
19799a22 |
1153 | method that is sometimes applicable when you wish to override a built-in |
95d94a4f |
1154 | everywhere, without regard to namespace boundaries. This is achieved by |
1155 | importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an |
1156 | example that quite brazenly replaces the C<glob> operator with something |
1157 | that understands regular expressions. |
1158 | |
1159 | package REGlob; |
1160 | require Exporter; |
1161 | @ISA = 'Exporter'; |
1162 | @EXPORT_OK = 'glob'; |
1163 | |
1164 | sub import { |
1165 | my $pkg = shift; |
1166 | return unless @_; |
1167 | my $sym = shift; |
1168 | my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0)); |
1169 | $pkg->export($where, $sym, @_); |
1170 | } |
1171 | |
1172 | sub glob { |
1173 | my $pat = shift; |
1174 | my @got; |
19799a22 |
1175 | local *D; |
1176 | if (opendir D, '.') { |
1177 | @got = grep /$pat/, readdir D; |
1178 | closedir D; |
1179 | } |
1180 | return @got; |
95d94a4f |
1181 | } |
1182 | 1; |
1183 | |
1184 | And here's how it could be (ab)used: |
1185 | |
1186 | #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces |
1187 | package Foo; |
1188 | use REGlob 'glob'; # override glob() in Foo:: only |
1189 | print for <^[a-z_]+\.pm\$>; # show all pragmatic modules |
1190 | |
19799a22 |
1191 | The initial comment shows a contrived, even dangerous example. |
95d94a4f |
1192 | By overriding C<glob> globally, you would be forcing the new (and |
19799a22 |
1193 | subversive) behavior for the C<glob> operator for I<every> namespace, |
95d94a4f |
1194 | without the complete cognizance or cooperation of the modules that own |
1195 | those namespaces. Naturally, this should be done with extreme caution--if |
1196 | it must be done at all. |
1197 | |
1198 | The C<REGlob> example above does not implement all the support needed to |
19799a22 |
1199 | cleanly override perl's C<glob> operator. The built-in C<glob> has |
95d94a4f |
1200 | different behaviors depending on whether it appears in a scalar or list |
19799a22 |
1201 | context, but our C<REGlob> doesn't. Indeed, many perl built-in have such |
95d94a4f |
1202 | context sensitive behaviors, and these must be adequately supported by |
1203 | a properly written override. For a fully functional example of overriding |
1204 | C<glob>, study the implementation of C<File::DosGlob> in the standard |
1205 | library. |
1206 | |
77bc9082 |
1207 | When you override a built-in, your replacement should be consistent (if |
1208 | possible) with the built-in native syntax. You can achieve this by using |
1209 | a suitable prototype. To get the prototype of an overridable built-in, |
1210 | use the C<prototype> function with an argument of C<"CORE::builtin_name"> |
1211 | (see L<perlfunc/prototype>). |
1212 | |
1213 | Note however that some built-ins can't have their syntax expressed by a |
1214 | prototype (such as C<system> or C<chomp>). If you override them you won't |
1215 | be able to fully mimic their original syntax. |
1216 | |
fe854a6f |
1217 | The built-ins C<do>, C<require> and C<glob> can also be overridden, but due |
77bc9082 |
1218 | to special magic, their original syntax is preserved, and you don't have |
1219 | to define a prototype for their replacements. (You can't override the |
1220 | C<do BLOCK> syntax, though). |
1221 | |
1222 | C<require> has special additional dark magic: if you invoke your |
1223 | C<require> replacement as C<require Foo::Bar>, it will actually receive |
1224 | the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>. |
1225 | |
1226 | And, as you'll have noticed from the previous example, if you override |
fe854a6f |
1227 | C<glob>, the C<E<lt>*E<gt>> glob operator is overridden as well. |
77bc9082 |
1228 | |
9b3023bc |
1229 | In a similar fashion, overriding the C<readline> function also overrides |
1230 | the equivalent I/O operator C<< <FILEHANDLE> >>. |
1231 | |
fe854a6f |
1232 | Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden. |
77bc9082 |
1233 | |
a0d0e21e |
1234 | =head2 Autoloading |
1235 | |
19799a22 |
1236 | If you call a subroutine that is undefined, you would ordinarily |
1237 | get an immediate, fatal error complaining that the subroutine doesn't |
1238 | exist. (Likewise for subroutines being used as methods, when the |
1239 | method doesn't exist in any base class of the class's package.) |
1240 | However, if an C<AUTOLOAD> subroutine is defined in the package or |
1241 | packages used to locate the original subroutine, then that |
1242 | C<AUTOLOAD> subroutine is called with the arguments that would have |
1243 | been passed to the original subroutine. The fully qualified name |
1244 | of the original subroutine magically appears in the global $AUTOLOAD |
1245 | variable of the same package as the C<AUTOLOAD> routine. The name |
1246 | is not passed as an ordinary argument because, er, well, just |
1247 | because, that's why... |
1248 | |
1249 | Many C<AUTOLOAD> routines load in a definition for the requested |
1250 | subroutine using eval(), then execute that subroutine using a special |
1251 | form of goto() that erases the stack frame of the C<AUTOLOAD> routine |
1252 | without a trace. (See the source to the standard module documented |
1253 | in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can |
1254 | also just emulate the routine and never define it. For example, |
1255 | let's pretend that a function that wasn't defined should just invoke |
1256 | C<system> with those arguments. All you'd do is: |
cb1a09d0 |
1257 | |
1258 | sub AUTOLOAD { |
1259 | my $program = $AUTOLOAD; |
1260 | $program =~ s/.*:://; |
1261 | system($program, @_); |
54310121 |
1262 | } |
cb1a09d0 |
1263 | date(); |
6d28dffb |
1264 | who('am', 'i'); |
cb1a09d0 |
1265 | ls('-l'); |
1266 | |
19799a22 |
1267 | In fact, if you predeclare functions you want to call that way, you don't |
1268 | even need parentheses: |
cb1a09d0 |
1269 | |
1270 | use subs qw(date who ls); |
1271 | date; |
1272 | who "am", "i"; |
1273 | ls -l; |
1274 | |
1275 | A more complete example of this is the standard Shell module, which |
19799a22 |
1276 | can treat undefined subroutine calls as calls to external programs. |
a0d0e21e |
1277 | |
19799a22 |
1278 | Mechanisms are available to help modules writers split their modules |
1279 | into autoloadable files. See the standard AutoLoader module |
6d28dffb |
1280 | described in L<AutoLoader> and in L<AutoSplit>, the standard |
1281 | SelfLoader modules in L<SelfLoader>, and the document on adding C |
19799a22 |
1282 | functions to Perl code in L<perlxs>. |
cb1a09d0 |
1283 | |
09bef843 |
1284 | =head2 Subroutine Attributes |
1285 | |
1286 | A subroutine declaration or definition may have a list of attributes |
1287 | associated with it. If such an attribute list is present, it is |
0120eecf |
1288 | broken up at space or colon boundaries and treated as though a |
09bef843 |
1289 | C<use attributes> had been seen. See L<attributes> for details |
1290 | about what attributes are currently supported. |
1291 | Unlike the limitation with the obsolescent C<use attrs>, the |
1292 | C<sub : ATTRLIST> syntax works to associate the attributes with |
1293 | a pre-declaration, and not just with a subroutine definition. |
1294 | |
1295 | The attributes must be valid as simple identifier names (without any |
1296 | punctuation other than the '_' character). They may have a parameter |
1297 | list appended, which is only checked for whether its parentheses ('(',')') |
1298 | nest properly. |
1299 | |
1300 | Examples of valid syntax (even though the attributes are unknown): |
1301 | |
0120eecf |
1302 | sub fnord (&\%) : switch(10,foo(7,3)) : expensive ; |
1303 | sub plugh () : Ugly('\(") :Bad ; |
09bef843 |
1304 | sub xyzzy : _5x5 { ... } |
1305 | |
1306 | Examples of invalid syntax: |
1307 | |
1308 | sub fnord : switch(10,foo() ; # ()-string not balanced |
1309 | sub snoid : Ugly('(') ; # ()-string not balanced |
1310 | sub xyzzy : 5x5 ; # "5x5" not a valid identifier |
1311 | sub plugh : Y2::north ; # "Y2::north" not a simple identifier |
0120eecf |
1312 | sub snurt : foo + bar ; # "+" not a colon or space |
09bef843 |
1313 | |
1314 | The attribute list is passed as a list of constant strings to the code |
1315 | which associates them with the subroutine. In particular, the second example |
1316 | of valid syntax above currently looks like this in terms of how it's |
1317 | parsed and invoked: |
1318 | |
1319 | use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad'; |
1320 | |
1321 | For further details on attribute lists and their manipulation, |
1322 | see L<attributes>. |
1323 | |
cb1a09d0 |
1324 | =head1 SEE ALSO |
a0d0e21e |
1325 | |
19799a22 |
1326 | See L<perlref/"Function Templates"> for more about references and closures. |
1327 | See L<perlxs> if you'd like to learn about calling C subroutines from Perl. |
a2293a43 |
1328 | See L<perlembed> if you'd like to learn about calling Perl subroutines from C. |
19799a22 |
1329 | See L<perlmod> to learn about bundling up your functions in separate files. |
1330 | See L<perlmodlib> to learn what library modules come standard on your system. |
1331 | See L<perltoot> to learn how to make object method calls. |