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