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1 =head1 NAME 2 X<subroutine> X<function> 3 4 perlsub - Perl subroutines 5 6 =head1 SYNOPSIS 7 8 To declare subroutines: 9 X<subroutine, declaration> X<sub> 10 11 sub NAME; # A "forward" declaration. 12 sub NAME(PROTO); # ditto, but with prototypes 13 sub NAME : ATTRS; # with attributes 14 sub NAME(PROTO) : ATTRS; # with attributes and prototypes 15 16 sub NAME BLOCK # A declaration and a definition. 17 sub NAME(PROTO) BLOCK # ditto, but with prototypes 18 sub NAME : ATTRS BLOCK # with attributes 19 sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes 20 21 To define an anonymous subroutine at runtime: 22 X<subroutine, anonymous> 23 24 $subref = sub BLOCK; # no proto 25 $subref = sub (PROTO) BLOCK; # with proto 26 $subref = sub : ATTRS BLOCK; # with attributes 27 $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes 28 29 To import subroutines: 30 X<import> 31 32 use MODULE qw(NAME1 NAME2 NAME3); 33 34 To call subroutines: 35 X<subroutine, call> X<call> 36 37 NAME(LIST); # & is optional with parentheses. 38 NAME LIST; # Parentheses optional if predeclared/imported. 39 &NAME(LIST); # Circumvent prototypes. 40 &NAME; # Makes current @_ visible to called subroutine. 41 42 =head1 DESCRIPTION 43 44 Like many languages, Perl provides for user-defined subroutines. 45 These may be located anywhere in the main program, loaded in from 46 other files via the C<do>, C<require>, or C<use> keywords, or 47 generated on the fly using C<eval> or anonymous subroutines. 48 You can even call a function indirectly using a variable containing 49 its name or a CODE reference. 50 51 The Perl model for function call and return values is simple: all 52 functions are passed as parameters one single flat list of scalars, and 53 all functions likewise return to their caller one single flat list of 54 scalars. Any arrays or hashes in these call and return lists will 55 collapse, losing their identities--but you may always use 56 pass-by-reference instead to avoid this. Both call and return lists may 57 contain as many or as few scalar elements as you'd like. (Often a 58 function without an explicit return statement is called a subroutine, but 59 there's really no difference from Perl's perspective.) 60 X<subroutine, parameter> X<parameter> 61 62 Any arguments passed in show up in the array C<@_>. Therefore, if 63 you called a function with two arguments, those would be stored in 64 C<$_[0]> and C<$_[1]>. The array C<@_> is a local array, but its 65 elements are aliases for the actual scalar parameters. In particular, 66 if an element C<$_[0]> is updated, the corresponding argument is 67 updated (or an error occurs if it is not updatable). If an argument 68 is an array or hash element which did not exist when the function 69 was called, that element is created only when (and if) it is modified 70 or a reference to it is taken. (Some earlier versions of Perl 71 created the element whether or not the element was assigned to.) 72 Assigning to the whole array C<@_> removes that aliasing, and does 73 not update any arguments. 74 X<subroutine, argument> X<argument> X<@_> 75 76 A C<return> statement may be used to exit a subroutine, optionally 77 specifying the returned value, which will be evaluated in the 78 appropriate context (list, scalar, or void) depending on the context of 79 the subroutine call. If you specify no return value, the subroutine 80 returns an empty list in list context, the undefined value in scalar 81 context, or nothing in void context. If you return one or more 82 aggregates (arrays and hashes), these will be flattened together into 83 one large indistinguishable list. 84 85 If no C<return> is found and if the last statement is an expression, its 86 value is returned. If the last statement is a loop control structure 87 like a C<foreach> or a C<while>, the returned value is unspecified. The 88 empty sub returns the empty list. 89 X<subroutine, return value> X<return value> X<return> 90 91 Perl does not have named formal parameters. In practice all you 92 do is assign to a C<my()> list of these. Variables that aren't 93 declared to be private are global variables. For gory details 94 on creating private variables, see L<"Private Variables via my()"> 95 and L<"Temporary Values via local()">. To create protected 96 environments for a set of functions in a separate package (and 97 probably a separate file), see L<perlmod/"Packages">. 98 X<formal parameter> X<parameter, formal> 99 100 Example: 101 102 sub max { 103 my $max = shift(@_); 104 foreach $foo (@_) { 105 $max = $foo if $max < $foo; 106 } 107 return $max; 108 } 109 $bestday = max($mon,$tue,$wed,$thu,$fri); 110 111 Example: 112 113 # get a line, combining continuation lines 114 # that start with whitespace 115 116 sub get_line { 117 $thisline = $lookahead; # global variables! 118 LINE: while (defined($lookahead = <STDIN>)) { 119 if ($lookahead =~ /^[ \t]/) { 120 $thisline .= $lookahead; 121 } 122 else { 123 last LINE; 124 } 125 } 126 return $thisline; 127 } 128 129 $lookahead = <STDIN>; # get first line 130 while (defined($line = get_line())) { 131 ... 132 } 133 134 Assigning to a list of private variables to name your arguments: 135 136 sub maybeset { 137 my($key, $value) = @_; 138 $Foo{$key} = $value unless $Foo{$key}; 139 } 140 141 Because the assignment copies the values, this also has the effect 142 of turning call-by-reference into call-by-value. Otherwise a 143 function is free to do in-place modifications of C<@_> and change 144 its caller's values. 145 X<call-by-reference> X<call-by-value> 146 147 upcase_in($v1, $v2); # this changes $v1 and $v2 148 sub upcase_in { 149 for (@_) { tr/a-z/A-Z/ } 150 } 151 152 You aren't allowed to modify constants in this way, of course. If an 153 argument were actually literal and you tried to change it, you'd take a 154 (presumably fatal) exception. For example, this won't work: 155 X<call-by-reference> X<call-by-value> 156 157 upcase_in("frederick"); 158 159 It would be much safer if the C<upcase_in()> function 160 were written to return a copy of its parameters instead 161 of changing them in place: 162 163 ($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2 164 sub upcase { 165 return unless defined wantarray; # void context, do nothing 166 my @parms = @_; 167 for (@parms) { tr/a-z/A-Z/ } 168 return wantarray ? @parms : $parms[0]; 169 } 170 171 Notice how this (unprototyped) function doesn't care whether it was 172 passed real scalars or arrays. Perl sees all arguments as one big, 173 long, flat parameter list in C<@_>. This is one area where 174 Perl's simple argument-passing style shines. The C<upcase()> 175 function would work perfectly well without changing the C<upcase()> 176 definition even if we fed it things like this: 177 178 @newlist = upcase(@list1, @list2); 179 @newlist = upcase( split /:/, $var ); 180 181 Do not, however, be tempted to do this: 182 183 (@a, @b) = upcase(@list1, @list2); 184 185 Like the flattened incoming parameter list, the return list is also 186 flattened on return. So all you have managed to do here is stored 187 everything in C<@a> and made C<@b> empty. See 188 L<Pass by Reference> for alternatives. 189 190 A subroutine may be called using an explicit C<&> prefix. The 191 C<&> is optional in modern Perl, as are parentheses if the 192 subroutine has been predeclared. The C<&> is I<not> optional 193 when just naming the subroutine, such as when it's used as 194 an argument to defined() or undef(). Nor is it optional when you 195 want to do an indirect subroutine call with a subroutine name or 196 reference using the C<&$subref()> or C<&{$subref}()> constructs, 197 although the C<< $subref->() >> notation solves that problem. 198 See L<perlref> for more about all that. 199 X<&> 200 201 Subroutines may be called recursively. If a subroutine is called 202 using the C<&> form, the argument list is optional, and if omitted, 203 no C<@_> array is set up for the subroutine: the C<@_> array at the 204 time of the call is visible to subroutine instead. This is an 205 efficiency mechanism that new users may wish to avoid. 206 X<recursion> 207 208 &foo(1,2,3); # pass three arguments 209 foo(1,2,3); # the same 210 211 foo(); # pass a null list 212 &foo(); # the same 213 214 &foo; # foo() get current args, like foo(@_) !! 215 foo; # like foo() IFF sub foo predeclared, else "foo" 216 217 Not only does the C<&> form make the argument list optional, it also 218 disables any prototype checking on arguments you do provide. This 219 is partly for historical reasons, and partly for having a convenient way 220 to cheat if you know what you're doing. See L<Prototypes> below. 221 X<&> 222 223 Subroutines whose names are in all upper case are reserved to the Perl 224 core, as are modules whose names are in all lower case. A subroutine in 225 all capitals is a loosely-held convention meaning it will be called 226 indirectly by the run-time system itself, usually due to a triggered event. 227 Subroutines that do special, pre-defined things include C<AUTOLOAD>, C<CLONE>, 228 C<DESTROY> plus all functions mentioned in L<perltie> and L<PerlIO::via>. 229 230 The C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> and C<END> subroutines 231 are not so much subroutines as named special code blocks, of which you 232 can have more than one in a package, and which you can B<not> call 233 explicitly. See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END"> 234 235 =head2 Private Variables via my() 236 X<my> X<variable, lexical> X<lexical> X<lexical variable> X<scope, lexical> 237 X<lexical scope> X<attributes, my> 238 239 Synopsis: 240 241 my $foo; # declare $foo lexically local 242 my (@wid, %get); # declare list of variables local 243 my $foo = "flurp"; # declare $foo lexical, and init it 244 my @oof = @bar; # declare @oof lexical, and init it 245 my $x : Foo = $y; # similar, with an attribute applied 246 247 B<WARNING>: The use of attribute lists on C<my> declarations is still 248 evolving. The current semantics and interface are subject to change. 249 See L<attributes> and L<Attribute::Handlers>. 250 251 The C<my> operator declares the listed variables to be lexically 252 confined to the enclosing block, conditional (C<if/unless/elsif/else>), 253 loop (C<for/foreach/while/until/continue>), subroutine, C<eval>, 254 or C<do/require/use>'d file. If more than one value is listed, the 255 list must be placed in parentheses. All listed elements must be 256 legal lvalues. Only alphanumeric identifiers may be lexically 257 scoped--magical built-ins like C<$/> must currently be C<local>ized 258 with C<local> instead. 259 260 Unlike dynamic variables created by the C<local> operator, lexical 261 variables declared with C<my> are totally hidden from the outside 262 world, including any called subroutines. This is true if it's the 263 same subroutine called from itself or elsewhere--every call gets 264 its own copy. 265 X<local> 266 267 This doesn't mean that a C<my> variable declared in a statically 268 enclosing lexical scope would be invisible. Only dynamic scopes 269 are cut off. For example, the C<bumpx()> function below has access 270 to the lexical $x variable because both the C<my> and the C<sub> 271 occurred at the same scope, presumably file scope. 272 273 my $x = 10; 274 sub bumpx { $x++ } 275 276 An C<eval()>, however, can see lexical variables of the scope it is 277 being evaluated in, so long as the names aren't hidden by declarations within 278 the C<eval()> itself. See L<perlref>. 279 X<eval, scope of> 280 281 The parameter list to my() may be assigned to if desired, which allows you 282 to initialize your variables. (If no initializer is given for a 283 particular variable, it is created with the undefined value.) Commonly 284 this is used to name input parameters to a subroutine. Examples: 285 286 $arg = "fred"; # "global" variable 287 $n = cube_root(27); 288 print "$arg thinks the root is $n\n"; 289 fred thinks the root is 3 290 291 sub cube_root { 292 my $arg = shift; # name doesn't matter 293 $arg **= 1/3; 294 return $arg; 295 } 296 297 The C<my> is simply a modifier on something you might assign to. So when 298 you do assign to variables in its argument list, C<my> doesn't 299 change whether those variables are viewed as a scalar or an array. So 300 301 my ($foo) = <STDIN>; # WRONG? 302 my @FOO = <STDIN>; 303 304 both supply a list context to the right-hand side, while 305 306 my $foo = <STDIN>; 307 308 supplies a scalar context. But the following declares only one variable: 309 310 my $foo, $bar = 1; # WRONG 311 312 That has the same effect as 313 314 my $foo; 315 $bar = 1; 316 317 The declared variable is not introduced (is not visible) until after 318 the current statement. Thus, 319 320 my $x = $x; 321 322 can be used to initialize a new $x with the value of the old $x, and 323 the expression 324 325 my $x = 123 and $x == 123 326 327 is false unless the old $x happened to have the value C<123>. 328 329 Lexical scopes of control structures are not bounded precisely by the 330 braces that delimit their controlled blocks; control expressions are 331 part of that scope, too. Thus in the loop 332 333 while (my $line = <>) { 334 $line = lc $line; 335 } continue { 336 print $line; 337 } 338 339 the scope of $line extends from its declaration throughout the rest of 340 the loop construct (including the C<continue> clause), but not beyond 341 it. Similarly, in the conditional 342 343 if ((my $answer = <STDIN>) =~ /^yes$/i) { 344 user_agrees(); 345 } elsif ($answer =~ /^no$/i) { 346 user_disagrees(); 347 } else { 348 chomp $answer; 349 die "'$answer' is neither 'yes' nor 'no'"; 350 } 351 352 the scope of $answer extends from its declaration through the rest 353 of that conditional, including any C<elsif> and C<else> clauses, 354 but not beyond it. See L<perlsyn/"Simple statements"> for information 355 on the scope of variables in statements with modifiers. 356 357 The C<foreach> loop defaults to scoping its index variable dynamically 358 in the manner of C<local>. However, if the index variable is 359 prefixed with the keyword C<my>, or if there is already a lexical 360 by that name in scope, then a new lexical is created instead. Thus 361 in the loop 362 X<foreach> X<for> 363 364 for my $i (1, 2, 3) { 365 some_function(); 366 } 367 368 the scope of $i extends to the end of the loop, but not beyond it, 369 rendering the value of $i inaccessible within C<some_function()>. 370 X<foreach> X<for> 371 372 Some users may wish to encourage the use of lexically scoped variables. 373 As an aid to catching implicit uses to package variables, 374 which are always global, if you say 375 376 use strict 'vars'; 377 378 then any variable mentioned from there to the end of the enclosing 379 block must either refer to a lexical variable, be predeclared via 380 C<our> or C<use vars>, or else must be fully qualified with the package name. 381 A compilation error results otherwise. An inner block may countermand 382 this with C<no strict 'vars'>. 383 384 A C<my> has both a compile-time and a run-time effect. At compile 385 time, the compiler takes notice of it. The principal usefulness 386 of this is to quiet C<use strict 'vars'>, but it is also essential 387 for generation of closures as detailed in L<perlref>. Actual 388 initialization is delayed until run time, though, so it gets executed 389 at the appropriate time, such as each time through a loop, for 390 example. 391 392 Variables declared with C<my> are not part of any package and are therefore 393 never fully qualified with the package name. In particular, you're not 394 allowed to try to make a package variable (or other global) lexical: 395 396 my $pack::var; # ERROR! Illegal syntax 397 398 In fact, a dynamic variable (also known as package or global variables) 399 are still accessible using the fully qualified C<::> notation even while a 400 lexical of the same name is also visible: 401 402 package main; 403 local $x = 10; 404 my $x = 20; 405 print "$x and $::x\n"; 406 407 That will print out C<20> and C<10>. 408 409 You may declare C<my> variables at the outermost scope of a file 410 to hide any such identifiers from the world outside that file. This 411 is similar in spirit to C's static variables when they are used at 412 the file level. To do this with a subroutine requires the use of 413 a closure (an anonymous function that accesses enclosing lexicals). 414 If you want to create a private subroutine that cannot be called 415 from outside that block, it can declare a lexical variable containing 416 an anonymous sub reference: 417 418 my $secret_version = '1.001-beta'; 419 my $secret_sub = sub { print $secret_version }; 420 &$secret_sub(); 421 422 As long as the reference is never returned by any function within the 423 module, no outside module can see the subroutine, because its name is not in 424 any package's symbol table. Remember that it's not I<REALLY> called 425 C<$some_pack::secret_version> or anything; it's just $secret_version, 426 unqualified and unqualifiable. 427 428 This does not work with object methods, however; all object methods 429 have to be in the symbol table of some package to be found. See 430 L<perlref/"Function Templates"> for something of a work-around to 431 this. 432 433 =head2 Persistent Private Variables 434 X<state> X<state variable> X<static> X<variable, persistent> X<variable, static> X<closure> 435 436 There are two ways to build persistent private variables in Perl 5.10. 437 First, you can simply use the C<state> feature. Or, you can use closures, 438 if you want to stay compatible with releases older than 5.10. 439 440 =head3 Persistent variables via state() 441 442 Beginning with perl 5.9.4, you can declare variables with the C<state> 443 keyword in place of C<my>. For that to work, though, you must have 444 enabled that feature beforehand, either by using the C<feature> pragma, or 445 by using C<-E> on one-liners. (see L<feature>) 446 447 For example, the following code maintains a private counter, incremented 448 each time the gimme_another() function is called: 449 450 use feature 'state'; 451 sub gimme_another { state $x; return ++$x } 452 453 Also, since C<$x> is lexical, it can't be reached or modified by any Perl 454 code outside. 455 456 When combined with variable declaration, simple scalar assignment to C<state> 457 variables (as in C<state $x = 42>) is executed only the first time. When such 458 statements are evaluated subsequent times, the assignment is ignored. The 459 behavior of this sort of assignment to non-scalar variables is undefined. 460 461 =head3 Persistent variables with closures 462 463 Just because a lexical variable is lexically (also called statically) 464 scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that 465 within a function it works like a C static. It normally works more 466 like a C auto, but with implicit garbage collection. 467 468 Unlike local variables in C or C++, Perl's lexical variables don't 469 necessarily get recycled just because their scope has exited. 470 If something more permanent is still aware of the lexical, it will 471 stick around. So long as something else references a lexical, that 472 lexical won't be freed--which is as it should be. You wouldn't want 473 memory being free until you were done using it, or kept around once you 474 were done. Automatic garbage collection takes care of this for you. 475 476 This means that you can pass back or save away references to lexical 477 variables, whereas to return a pointer to a C auto is a grave error. 478 It also gives us a way to simulate C's function statics. Here's a 479 mechanism for giving a function private variables with both lexical 480 scoping and a static lifetime. If you do want to create something like 481 C's static variables, just enclose the whole function in an extra block, 482 and put the static variable outside the function but in the block. 483 484 { 485 my $secret_val = 0; 486 sub gimme_another { 487 return ++$secret_val; 488 } 489 } 490 # $secret_val now becomes unreachable by the outside 491 # world, but retains its value between calls to gimme_another 492 493 If this function is being sourced in from a separate file 494 via C<require> or C<use>, then this is probably just fine. If it's 495 all in the main program, you'll need to arrange for the C<my> 496 to be executed early, either by putting the whole block above 497 your main program, or more likely, placing merely a C<BEGIN> 498 code block around it to make sure it gets executed before your program 499 starts to run: 500 501 BEGIN { 502 my $secret_val = 0; 503 sub gimme_another { 504 return ++$secret_val; 505 } 506 } 507 508 See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END"> about the 509 special triggered code blocks, C<BEGIN>, C<UNITCHECK>, C<CHECK>, 510 C<INIT> and C<END>. 511 512 If declared at the outermost scope (the file scope), then lexicals 513 work somewhat like C's file statics. They are available to all 514 functions in that same file declared below them, but are inaccessible 515 from outside that file. This strategy is sometimes used in modules 516 to create private variables that the whole module can see. 517 518 =head2 Temporary Values via local() 519 X<local> X<scope, dynamic> X<dynamic scope> X<variable, local> 520 X<variable, temporary> 521 522 B<WARNING>: In general, you should be using C<my> instead of C<local>, because 523 it's faster and safer. Exceptions to this include the global punctuation 524 variables, global filehandles and formats, and direct manipulation of the 525 Perl symbol table itself. C<local> is mostly used when the current value 526 of a variable must be visible to called subroutines. 527 528 Synopsis: 529 530 # localization of values 531 532 local $foo; # make $foo dynamically local 533 local (@wid, %get); # make list of variables local 534 local $foo = "flurp"; # make $foo dynamic, and init it 535 local @oof = @bar; # make @oof dynamic, and init it 536 537 local $hash{key} = "val"; # sets a local value for this hash entry 538 local ($cond ? $v1 : $v2); # several types of lvalues support 539 # localization 540 541 # localization of symbols 542 543 local *FH; # localize $FH, @FH, %FH, &FH ... 544 local *merlyn = *randal; # now $merlyn is really $randal, plus 545 # @merlyn is really @randal, etc 546 local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal 547 local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc 548 549 A C<local> modifies its listed variables to be "local" to the 550 enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine 551 called from within that block>. A C<local> just gives temporary 552 values to global (meaning package) variables. It does I<not> create 553 a local variable. This is known as dynamic scoping. Lexical scoping 554 is done with C<my>, which works more like C's auto declarations. 555 556 Some types of lvalues can be localized as well : hash and array elements 557 and slices, conditionals (provided that their result is always 558 localizable), and symbolic references. As for simple variables, this 559 creates new, dynamically scoped values. 560 561 If more than one variable or expression is given to C<local>, they must be 562 placed in parentheses. This operator works 563 by saving the current values of those variables in its argument list on a 564 hidden stack and restoring them upon exiting the block, subroutine, or 565 eval. This means that called subroutines can also reference the local 566 variable, but not the global one. The argument list may be assigned to if 567 desired, which allows you to initialize your local variables. (If no 568 initializer is given for a particular variable, it is created with an 569 undefined value.) 570 571 Because C<local> is a run-time operator, it gets executed each time 572 through a loop. Consequently, it's more efficient to localize your 573 variables outside the loop. 574 575 =head3 Grammatical note on local() 576 X<local, context> 577 578 A C<local> is simply a modifier on an lvalue expression. When you assign to 579 a C<local>ized variable, the C<local> doesn't change whether its list is viewed 580 as a scalar or an array. So 581 582 local($foo) = <STDIN>; 583 local @FOO = <STDIN>; 584 585 both supply a list context to the right-hand side, while 586 587 local $foo = <STDIN>; 588 589 supplies a scalar context. 590 591 =head3 Localization of special variables 592 X<local, special variable> 593 594 If you localize a special variable, you'll be giving a new value to it, 595 but its magic won't go away. That means that all side-effects related 596 to this magic still work with the localized value. 597 598 This feature allows code like this to work : 599 600 # Read the whole contents of FILE in $slurp 601 { local $/ = undef; $slurp = <FILE>; } 602 603 Note, however, that this restricts localization of some values ; for 604 example, the following statement dies, as of perl 5.9.0, with an error 605 I<Modification of a read-only value attempted>, because the $1 variable is 606 magical and read-only : 607 608 local $1 = 2; 609 610 Similarly, but in a way more difficult to spot, the following snippet will 611 die in perl 5.9.0 : 612 613 sub f { local $_ = "foo"; print } 614 for ($1) { 615 # now $_ is aliased to $1, thus is magic and readonly 616 f(); 617 } 618 619 See next section for an alternative to this situation. 620 621 B<WARNING>: Localization of tied arrays and hashes does not currently 622 work as described. 623 This will be fixed in a future release of Perl; in the meantime, avoid 624 code that relies on any particular behaviour of localising tied arrays 625 or hashes (localising individual elements is still okay). 626 See L<perl58delta/"Localising Tied Arrays and Hashes Is Broken"> for more 627 details. 628 X<local, tie> 629 630 =head3 Localization of globs 631 X<local, glob> X<glob> 632 633 The construct 634 635 local *name; 636 637 creates a whole new symbol table entry for the glob C<name> in the 638 current package. That means that all variables in its glob slot ($name, 639 @name, %name, &name, and the C<name> filehandle) are dynamically reset. 640 641 This implies, among other things, that any magic eventually carried by 642 those variables is locally lost. In other words, saying C<local */> 643 will not have any effect on the internal value of the input record 644 separator. 645 646 Notably, if you want to work with a brand new value of the default scalar 647 $_, and avoid the potential problem listed above about $_ previously 648 carrying a magic value, you should use C<local *_> instead of C<local $_>. 649 As of perl 5.9.1, you can also use the lexical form of C<$_> (declaring it 650 with C<my $_>), which avoids completely this problem. 651 652 =head3 Localization of elements of composite types 653 X<local, composite type element> X<local, array element> X<local, hash element> 654 655 It's also worth taking a moment to explain what happens when you 656 C<local>ize a member of a composite type (i.e. an array or hash element). 657 In this case, the element is C<local>ized I<by name>. This means that 658 when the scope of the C<local()> ends, the saved value will be 659 restored to the hash element whose key was named in the C<local()>, or 660 the array element whose index was named in the C<local()>. If that 661 element was deleted while the C<local()> was in effect (e.g. by a 662 C<delete()> from a hash or a C<shift()> of an array), it will spring 663 back into existence, possibly extending an array and filling in the 664 skipped elements with C<undef>. For instance, if you say 665 666 %hash = ( 'This' => 'is', 'a' => 'test' ); 667 @ary = ( 0..5 ); 668 { 669 local($ary[5]) = 6; 670 local($hash{'a'}) = 'drill'; 671 while (my $e = pop(@ary)) { 672 print "$e . . .\n"; 673 last unless $e > 3; 674 } 675 if (@ary) { 676 $hash{'only a'} = 'test'; 677 delete $hash{'a'}; 678 } 679 } 680 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n"; 681 print "The array has ",scalar(@ary)," elements: ", 682 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n"; 683 684 Perl will print 685 686 6 . . . 687 4 . . . 688 3 . . . 689 This is a test only a test. 690 The array has 6 elements: 0, 1, 2, undef, undef, 5 691 692 The behavior of local() on non-existent members of composite 693 types is subject to change in future. 694 695 =head2 Lvalue subroutines 696 X<lvalue> X<subroutine, lvalue> 697 698 B<WARNING>: Lvalue subroutines are still experimental and the 699 implementation may change in future versions of Perl. 700 701 It is possible to return a modifiable value from a subroutine. 702 To do this, you have to declare the subroutine to return an lvalue. 703 704 my $val; 705 sub canmod : lvalue { 706 # return $val; this doesn't work, don't say "return" 707 $val; 708 } 709 sub nomod { 710 $val; 711 } 712 713 canmod() = 5; # assigns to $val 714 nomod() = 5; # ERROR 715 716 The scalar/list context for the subroutine and for the right-hand 717 side of assignment is determined as if the subroutine call is replaced 718 by a scalar. For example, consider: 719 720 data(2,3) = get_data(3,4); 721 722 Both subroutines here are called in a scalar context, while in: 723 724 (data(2,3)) = get_data(3,4); 725 726 and in: 727 728 (data(2),data(3)) = get_data(3,4); 729 730 all the subroutines are called in a list context. 731 732 =over 4 733 734 =item Lvalue subroutines are EXPERIMENTAL 735 736 They appear to be convenient, but there are several reasons to be 737 circumspect. 738 739 You can't use the return keyword, you must pass out the value before 740 falling out of subroutine scope. (see comment in example above). This 741 is usually not a problem, but it disallows an explicit return out of a 742 deeply nested loop, which is sometimes a nice way out. 743 744 They violate encapsulation. A normal mutator can check the supplied 745 argument before setting the attribute it is protecting, an lvalue 746 subroutine never gets that chance. Consider; 747 748 my $some_array_ref = []; # protected by mutators ?? 749 750 sub set_arr { # normal mutator 751 my $val = shift; 752 die("expected array, you supplied ", ref $val) 753 unless ref $val eq 'ARRAY'; 754 $some_array_ref = $val; 755 } 756 sub set_arr_lv : lvalue { # lvalue mutator 757 $some_array_ref; 758 } 759 760 # set_arr_lv cannot stop this ! 761 set_arr_lv() = { a => 1 }; 762 763 =back 764 765 =head2 Passing Symbol Table Entries (typeglobs) 766 X<typeglob> X<*> 767 768 B<WARNING>: The mechanism described in this section was originally 769 the only way to simulate pass-by-reference in older versions of 770 Perl. While it still works fine in modern versions, the new reference 771 mechanism is generally easier to work with. See below. 772 773 Sometimes you don't want to pass the value of an array to a subroutine 774 but rather the name of it, so that the subroutine can modify the global 775 copy of it rather than working with a local copy. In perl you can 776 refer to all objects of a particular name by prefixing the name 777 with a star: C<*foo>. This is often known as a "typeglob", because the 778 star on the front can be thought of as a wildcard match for all the 779 funny prefix characters on variables and subroutines and such. 780 781 When evaluated, the typeglob produces a scalar value that represents 782 all the objects of that name, including any filehandle, format, or 783 subroutine. When assigned to, it causes the name mentioned to refer to 784 whatever C<*> value was assigned to it. Example: 785 786 sub doubleary { 787 local(*someary) = @_; 788 foreach $elem (@someary) { 789 $elem *= 2; 790 } 791 } 792 doubleary(*foo); 793 doubleary(*bar); 794 795 Scalars are already passed by reference, so you can modify 796 scalar arguments without using this mechanism by referring explicitly 797 to C<$_[0]> etc. You can modify all the elements of an array by passing 798 all the elements as scalars, but you have to use the C<*> mechanism (or 799 the equivalent reference mechanism) to C<push>, C<pop>, or change the size of 800 an array. It will certainly be faster to pass the typeglob (or reference). 801 802 Even if you don't want to modify an array, this mechanism is useful for 803 passing multiple arrays in a single LIST, because normally the LIST 804 mechanism will merge all the array values so that you can't extract out 805 the individual arrays. For more on typeglobs, see 806 L<perldata/"Typeglobs and Filehandles">. 807 808 =head2 When to Still Use local() 809 X<local> X<variable, local> 810 811 Despite the existence of C<my>, there are still three places where the 812 C<local> operator still shines. In fact, in these three places, you 813 I<must> use C<local> instead of C<my>. 814 815 =over 4 816 817 =item 1. 818 819 You need to give a global variable a temporary value, especially $_. 820 821 The global variables, like C<@ARGV> or the punctuation variables, must be 822 C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits 823 it up into chunks separated by lines of equal signs, which are placed 824 in C<@Fields>. 825 826 { 827 local @ARGV = ("/etc/motd"); 828 local $/ = undef; 829 local $_ = <>; 830 @Fields = split /^\s*=+\s*$/; 831 } 832 833 It particular, it's important to C<local>ize $_ in any routine that assigns 834 to it. Look out for implicit assignments in C<while> conditionals. 835 836 =item 2. 837 838 You need to create a local file or directory handle or a local function. 839 840 A function that needs a filehandle of its own must use 841 C<local()> on a complete typeglob. This can be used to create new symbol 842 table entries: 843 844 sub ioqueue { 845 local (*READER, *WRITER); # not my! 846 pipe (READER, WRITER) or die "pipe: $!"; 847 return (*READER, *WRITER); 848 } 849 ($head, $tail) = ioqueue(); 850 851 See the Symbol module for a way to create anonymous symbol table 852 entries. 853 854 Because assignment of a reference to a typeglob creates an alias, this 855 can be used to create what is effectively a local function, or at least, 856 a local alias. 857 858 { 859 local *grow = \&shrink; # only until this block exists 860 grow(); # really calls shrink() 861 move(); # if move() grow()s, it shrink()s too 862 } 863 grow(); # get the real grow() again 864 865 See L<perlref/"Function Templates"> for more about manipulating 866 functions by name in this way. 867 868 =item 3. 869 870 You want to temporarily change just one element of an array or hash. 871 872 You can C<local>ize just one element of an aggregate. Usually this 873 is done on dynamics: 874 875 { 876 local $SIG{INT} = 'IGNORE'; 877 funct(); # uninterruptible 878 } 879 # interruptibility automatically restored here 880 881 But it also works on lexically declared aggregates. Prior to 5.005, 882 this operation could on occasion misbehave. 883 884 =back 885 886 =head2 Pass by Reference 887 X<pass by reference> X<pass-by-reference> X<reference> 888 889 If you want to pass more than one array or hash into a function--or 890 return them from it--and have them maintain their integrity, then 891 you're going to have to use an explicit pass-by-reference. Before you 892 do that, you need to understand references as detailed in L<perlref>. 893 This section may not make much sense to you otherwise. 894 895 Here are a few simple examples. First, let's pass in several arrays 896 to a function and have it C<pop> all of then, returning a new list 897 of all their former last elements: 898 899 @tailings = popmany ( \@a, \@b, \@c, \@d ); 900 901 sub popmany { 902 my $aref; 903 my @retlist = (); 904 foreach $aref ( @_ ) { 905 push @retlist, pop @$aref; 906 } 907 return @retlist; 908 } 909 910 Here's how you might write a function that returns a 911 list of keys occurring in all the hashes passed to it: 912 913 @common = inter( \%foo, \%bar, \%joe ); 914 sub inter { 915 my ($k, $href, %seen); # locals 916 foreach $href (@_) { 917 while ( $k = each %$href ) { 918 $seen{$k}++; 919 } 920 } 921 return grep { $seen{$_} == @_ } keys %seen; 922 } 923 924 So far, we're using just the normal list return mechanism. 925 What happens if you want to pass or return a hash? Well, 926 if you're using only one of them, or you don't mind them 927 concatenating, then the normal calling convention is ok, although 928 a little expensive. 929 930 Where people get into trouble is here: 931 932 (@a, @b) = func(@c, @d); 933 or 934 (%a, %b) = func(%c, %d); 935 936 That syntax simply won't work. It sets just C<@a> or C<%a> and 937 clears the C<@b> or C<%b>. Plus the function didn't get passed 938 into two separate arrays or hashes: it got one long list in C<@_>, 939 as always. 940 941 If you can arrange for everyone to deal with this through references, it's 942 cleaner code, although not so nice to look at. Here's a function that 943 takes two array references as arguments, returning the two array elements 944 in order of how many elements they have in them: 945 946 ($aref, $bref) = func(\@c, \@d); 947 print "@$aref has more than @$bref\n"; 948 sub func { 949 my ($cref, $dref) = @_; 950 if (@$cref > @$dref) { 951 return ($cref, $dref); 952 } else { 953 return ($dref, $cref); 954 } 955 } 956 957 It turns out that you can actually do this also: 958 959 (*a, *b) = func(\@c, \@d); 960 print "@a has more than @b\n"; 961 sub func { 962 local (*c, *d) = @_; 963 if (@c > @d) { 964 return (\@c, \@d); 965 } else { 966 return (\@d, \@c); 967 } 968 } 969 970 Here we're using the typeglobs to do symbol table aliasing. It's 971 a tad subtle, though, and also won't work if you're using C<my> 972 variables, because only globals (even in disguise as C<local>s) 973 are in the symbol table. 974 975 If you're passing around filehandles, you could usually just use the bare 976 typeglob, like C<*STDOUT>, but typeglobs references work, too. 977 For example: 978 979 splutter(\*STDOUT); 980 sub splutter { 981 my $fh = shift; 982 print $fh "her um well a hmmm\n"; 983 } 984 985 $rec = get_rec(\*STDIN); 986 sub get_rec { 987 my $fh = shift; 988 return scalar <$fh>; 989 } 990 991 If you're planning on generating new filehandles, you could do this. 992 Notice to pass back just the bare *FH, not its reference. 993 994 sub openit { 995 my $path = shift; 996 local *FH; 997 return open (FH, $path) ? *FH : undef; 998 } 999 1000 =head2 Prototypes 1001 X<prototype> X<subroutine, prototype> 1002 1003 Perl supports a very limited kind of compile-time argument checking 1004 using function prototyping. If you declare 1005 1006 sub mypush (\@@) 1007 1008 then C<mypush()> takes arguments exactly like C<push()> does. The 1009 function declaration must be visible at compile time. The prototype 1010 affects only interpretation of new-style calls to the function, 1011 where new-style is defined as not using the C<&> character. In 1012 other words, if you call it like a built-in function, then it behaves 1013 like a built-in function. If you call it like an old-fashioned 1014 subroutine, then it behaves like an old-fashioned subroutine. It 1015 naturally falls out from this rule that prototypes have no influence 1016 on subroutine references like C<\&foo> or on indirect subroutine 1017 calls like C<&{$subref}> or C<< $subref->() >>. 1018 1019 Method calls are not influenced by prototypes either, because the 1020 function to be called is indeterminate at compile time, since 1021 the exact code called depends on inheritance. 1022 1023 Because the intent of this feature is primarily to let you define 1024 subroutines that work like built-in functions, here are prototypes 1025 for some other functions that parse almost exactly like the 1026 corresponding built-in. 1027 1028 Declared as Called as 1029 1030 sub mylink ($$) mylink $old, $new 1031 sub myvec ($$$) myvec $var, $offset, 1 1032 sub myindex ($$;$) myindex &getstring, "substr" 1033 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off 1034 sub myreverse (@) myreverse $a, $b, $c 1035 sub myjoin ($@) myjoin ":", $a, $b, $c 1036 sub mypop (\@) mypop @array 1037 sub mysplice (\@$$@) mysplice @array, @array, 0, @pushme 1038 sub mykeys (\%) mykeys %{$hashref} 1039 sub myopen (*;$) myopen HANDLE, $name 1040 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE 1041 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c 1042 sub myrand (;$) myrand 42 1043 sub mytime () mytime 1044 1045 Any backslashed prototype character represents an actual argument 1046 that absolutely must start with that character. The value passed 1047 as part of C<@_> will be a reference to the actual argument given 1048 in the subroutine call, obtained by applying C<\> to that argument. 1049 1050 You can also backslash several argument types simultaneously by using 1051 the C<\[]> notation: 1052 1053 sub myref (\[$@%&*]) 1054 1055 will allow calling myref() as 1056 1057 myref $var 1058 myref @array 1059 myref %hash 1060 myref &sub 1061 myref *glob 1062 1063 and the first argument of myref() will be a reference to 1064 a scalar, an array, a hash, a code, or a glob. 1065 1066 Unbackslashed prototype characters have special meanings. Any 1067 unbackslashed C<@> or C<%> eats all remaining arguments, and forces 1068 list context. An argument represented by C<$> forces scalar context. An 1069 C<&> requires an anonymous subroutine, which, if passed as the first 1070 argument, does not require the C<sub> keyword or a subsequent comma. 1071 1072 A C<*> allows the subroutine to accept a bareword, constant, scalar expression, 1073 typeglob, or a reference to a typeglob in that slot. The value will be 1074 available to the subroutine either as a simple scalar, or (in the latter 1075 two cases) as a reference to the typeglob. If you wish to always convert 1076 such arguments to a typeglob reference, use Symbol::qualify_to_ref() as 1077 follows: 1078 1079 use Symbol 'qualify_to_ref'; 1080 1081 sub foo (*) { 1082 my $fh = qualify_to_ref(shift, caller); 1083 ... 1084 } 1085 1086 A semicolon (C<;>) separates mandatory arguments from optional arguments. 1087 It is redundant before C<@> or C<%>, which gobble up everything else. 1088 1089 As the last character of a prototype, or just before a semicolon, you can 1090 use C<_> in place of C<$>: if this argument is not provided, C<$_> will be 1091 used instead. 1092 1093 Note how the last three examples in the table above are treated 1094 specially by the parser. C<mygrep()> is parsed as a true list 1095 operator, C<myrand()> is parsed as a true unary operator with unary 1096 precedence the same as C<rand()>, and C<mytime()> is truly without 1097 arguments, just like C<time()>. That is, if you say 1098 1099 mytime +2; 1100 1101 you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed 1102 without a prototype. 1103 1104 The interesting thing about C<&> is that you can generate new syntax with it, 1105 provided it's in the initial position: 1106 X<&> 1107 1108 sub try (&@) { 1109 my($try,$catch) = @_; 1110 eval { &$try }; 1111 if ($@) { 1112 local $_ = $@; 1113 &$catch; 1114 } 1115 } 1116 sub catch (&) { $_[0] } 1117 1118 try { 1119 die "phooey"; 1120 } catch { 1121 /phooey/ and print "unphooey\n"; 1122 }; 1123 1124 That prints C<"unphooey">. (Yes, there are still unresolved 1125 issues having to do with visibility of C<@_>. I'm ignoring that 1126 question for the moment. (But note that if we make C<@_> lexically 1127 scoped, those anonymous subroutines can act like closures... (Gee, 1128 is this sounding a little Lispish? (Never mind.)))) 1129 1130 And here's a reimplementation of the Perl C<grep> operator: 1131 X<grep> 1132 1133 sub mygrep (&@) { 1134 my $code = shift; 1135 my @result; 1136 foreach $_ (@_) { 1137 push(@result, $_) if &$code; 1138 } 1139 @result; 1140 } 1141 1142 Some folks would prefer full alphanumeric prototypes. Alphanumerics have 1143 been intentionally left out of prototypes for the express purpose of 1144 someday in the future adding named, formal parameters. The current 1145 mechanism's main goal is to let module writers provide better diagnostics 1146 for module users. Larry feels the notation quite understandable to Perl 1147 programmers, and that it will not intrude greatly upon the meat of the 1148 module, nor make it harder to read. The line noise is visually 1149 encapsulated into a small pill that's easy to swallow. 1150 1151 If you try to use an alphanumeric sequence in a prototype you will 1152 generate an optional warning - "Illegal character in prototype...". 1153 Unfortunately earlier versions of Perl allowed the prototype to be 1154 used as long as its prefix was a valid prototype. The warning may be 1155 upgraded to a fatal error in a future version of Perl once the 1156 majority of offending code is fixed. 1157 1158 It's probably best to prototype new functions, not retrofit prototyping 1159 into older ones. That's because you must be especially careful about 1160 silent impositions of differing list versus scalar contexts. For example, 1161 if you decide that a function should take just one parameter, like this: 1162 1163 sub func ($) { 1164 my $n = shift; 1165 print "you gave me $n\n"; 1166 } 1167 1168 and someone has been calling it with an array or expression 1169 returning a list: 1170 1171 func(@foo); 1172 func( split /:/ ); 1173 1174 Then you've just supplied an automatic C<scalar> in front of their 1175 argument, which can be more than a bit surprising. The old C<@foo> 1176 which used to hold one thing doesn't get passed in. Instead, 1177 C<func()> now gets passed in a C<1>; that is, the number of elements 1178 in C<@foo>. And the C<split> gets called in scalar context so it 1179 starts scribbling on your C<@_> parameter list. Ouch! 1180 1181 This is all very powerful, of course, and should be used only in moderation 1182 to make the world a better place. 1183 1184 =head2 Constant Functions 1185 X<constant> 1186 1187 Functions with a prototype of C<()> are potential candidates for 1188 inlining. If the result after optimization and constant folding 1189 is either a constant or a lexically-scoped scalar which has no other 1190 references, then it will be used in place of function calls made 1191 without C<&>. Calls made using C<&> are never inlined. (See 1192 F<constant.pm> for an easy way to declare most constants.) 1193 1194 The following functions would all be inlined: 1195 1196 sub pi () { 3.14159 } # Not exact, but close. 1197 sub PI () { 4 * atan2 1, 1 } # As good as it gets, 1198 # and it's inlined, too! 1199 sub ST_DEV () { 0 } 1200 sub ST_INO () { 1 } 1201 1202 sub FLAG_FOO () { 1 << 8 } 1203 sub FLAG_BAR () { 1 << 9 } 1204 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR } 1205 1206 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) } 1207 1208 sub N () { int(OPT_BAZ) / 3 } 1209 1210 sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO } 1211 1212 Be aware that these will not be inlined; as they contain inner scopes, 1213 the constant folding doesn't reduce them to a single constant: 1214 1215 sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } } 1216 1217 sub baz_val () { 1218 if (OPT_BAZ) { 1219 return 23; 1220 } 1221 else { 1222 return 42; 1223 } 1224 } 1225 1226 If you redefine a subroutine that was eligible for inlining, you'll get 1227 a mandatory warning. (You can use this warning to tell whether or not a 1228 particular subroutine is considered constant.) The warning is 1229 considered severe enough not to be optional because previously compiled 1230 invocations of the function will still be using the old value of the 1231 function. If you need to be able to redefine the subroutine, you need to 1232 ensure that it isn't inlined, either by dropping the C<()> prototype 1233 (which changes calling semantics, so beware) or by thwarting the 1234 inlining mechanism in some other way, such as 1235 1236 sub not_inlined () { 1237 23 if $]; 1238 } 1239 1240 =head2 Overriding Built-in Functions 1241 X<built-in> X<override> X<CORE> X<CORE::GLOBAL> 1242 1243 Many built-in functions may be overridden, though this should be tried 1244 only occasionally and for good reason. Typically this might be 1245 done by a package attempting to emulate missing built-in functionality 1246 on a non-Unix system. 1247 1248 Overriding may be done only by importing the name from a module at 1249 compile time--ordinary predeclaration isn't good enough. However, the 1250 C<use subs> pragma lets you, in effect, predeclare subs 1251 via the import syntax, and these names may then override built-in ones: 1252 1253 use subs 'chdir', 'chroot', 'chmod', 'chown'; 1254 chdir $somewhere; 1255 sub chdir { ... } 1256 1257 To unambiguously refer to the built-in form, precede the 1258 built-in name with the special package qualifier C<CORE::>. For example, 1259 saying C<CORE::open()> always refers to the built-in C<open()>, even 1260 if the current package has imported some other subroutine called 1261 C<&open()> from elsewhere. Even though it looks like a regular 1262 function call, it isn't: you can't take a reference to it, such as 1263 the incorrect C<\&CORE::open> might appear to produce. 1264 1265 Library modules should not in general export built-in names like C<open> 1266 or C<chdir> as part of their default C<@EXPORT> list, because these may 1267 sneak into someone else's namespace and change the semantics unexpectedly. 1268 Instead, if the module adds that name to C<@EXPORT_OK>, then it's 1269 possible for a user to import the name explicitly, but not implicitly. 1270 That is, they could say 1271 1272 use Module 'open'; 1273 1274 and it would import the C<open> override. But if they said 1275 1276 use Module; 1277 1278 they would get the default imports without overrides. 1279 1280 The foregoing mechanism for overriding built-in is restricted, quite 1281 deliberately, to the package that requests the import. There is a second 1282 method that is sometimes applicable when you wish to override a built-in 1283 everywhere, without regard to namespace boundaries. This is achieved by 1284 importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an 1285 example that quite brazenly replaces the C<glob> operator with something 1286 that understands regular expressions. 1287 1288 package REGlob; 1289 require Exporter; 1290 @ISA = 'Exporter'; 1291 @EXPORT_OK = 'glob'; 1292 1293 sub import { 1294 my $pkg = shift; 1295 return unless @_; 1296 my $sym = shift; 1297 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0)); 1298 $pkg->export($where, $sym, @_); 1299 } 1300 1301 sub glob { 1302 my $pat = shift; 1303 my @got; 1304 if (opendir my $d, '.') { 1305 @got = grep /$pat/, readdir $d; 1306 closedir $d; 1307 } 1308 return @got; 1309 } 1310 1; 1311 1312 And here's how it could be (ab)used: 1313 1314 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces 1315 package Foo; 1316 use REGlob 'glob'; # override glob() in Foo:: only 1317 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules 1318 1319 The initial comment shows a contrived, even dangerous example. 1320 By overriding C<glob> globally, you would be forcing the new (and 1321 subversive) behavior for the C<glob> operator for I<every> namespace, 1322 without the complete cognizance or cooperation of the modules that own 1323 those namespaces. Naturally, this should be done with extreme caution--if 1324 it must be done at all. 1325 1326 The C<REGlob> example above does not implement all the support needed to 1327 cleanly override perl's C<glob> operator. The built-in C<glob> has 1328 different behaviors depending on whether it appears in a scalar or list 1329 context, but our C<REGlob> doesn't. Indeed, many perl built-in have such 1330 context sensitive behaviors, and these must be adequately supported by 1331 a properly written override. For a fully functional example of overriding 1332 C<glob>, study the implementation of C<File::DosGlob> in the standard 1333 library. 1334 1335 When you override a built-in, your replacement should be consistent (if 1336 possible) with the built-in native syntax. You can achieve this by using 1337 a suitable prototype. To get the prototype of an overridable built-in, 1338 use the C<prototype> function with an argument of C<"CORE::builtin_name"> 1339 (see L<perlfunc/prototype>). 1340 1341 Note however that some built-ins can't have their syntax expressed by a 1342 prototype (such as C<system> or C<chomp>). If you override them you won't 1343 be able to fully mimic their original syntax. 1344 1345 The built-ins C<do>, C<require> and C<glob> can also be overridden, but due 1346 to special magic, their original syntax is preserved, and you don't have 1347 to define a prototype for their replacements. (You can't override the 1348 C<do BLOCK> syntax, though). 1349 1350 C<require> has special additional dark magic: if you invoke your 1351 C<require> replacement as C<require Foo::Bar>, it will actually receive 1352 the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>. 1353 1354 And, as you'll have noticed from the previous example, if you override 1355 C<glob>, the C<< <*> >> glob operator is overridden as well. 1356 1357 In a similar fashion, overriding the C<readline> function also overrides 1358 the equivalent I/O operator C<< <FILEHANDLE> >>. Also, overriding 1359 C<readpipe> also overrides the operators C<``> and C<qx//>. 1360 1361 Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden. 1362 1363 =head2 Autoloading 1364 X<autoloading> X<AUTOLOAD> 1365 1366 If you call a subroutine that is undefined, you would ordinarily 1367 get an immediate, fatal error complaining that the subroutine doesn't 1368 exist. (Likewise for subroutines being used as methods, when the 1369 method doesn't exist in any base class of the class's package.) 1370 However, if an C<AUTOLOAD> subroutine is defined in the package or 1371 packages used to locate the original subroutine, then that 1372 C<AUTOLOAD> subroutine is called with the arguments that would have 1373 been passed to the original subroutine. The fully qualified name 1374 of the original subroutine magically appears in the global $AUTOLOAD 1375 variable of the same package as the C<AUTOLOAD> routine. The name 1376 is not passed as an ordinary argument because, er, well, just 1377 because, that's why. (As an exception, a method call to a nonexistent 1378 C<import> or C<unimport> method is just skipped instead.) 1379 1380 Many C<AUTOLOAD> routines load in a definition for the requested 1381 subroutine using eval(), then execute that subroutine using a special 1382 form of goto() that erases the stack frame of the C<AUTOLOAD> routine 1383 without a trace. (See the source to the standard module documented 1384 in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can 1385 also just emulate the routine and never define it. For example, 1386 let's pretend that a function that wasn't defined should just invoke 1387 C<system> with those arguments. All you'd do is: 1388 1389 sub AUTOLOAD { 1390 my $program = $AUTOLOAD; 1391 $program =~ s/.*:://; 1392 system($program, @_); 1393 } 1394 date(); 1395 who('am', 'i'); 1396 ls('-l'); 1397 1398 In fact, if you predeclare functions you want to call that way, you don't 1399 even need parentheses: 1400 1401 use subs qw(date who ls); 1402 date; 1403 who "am", "i"; 1404 ls '-l'; 1405 1406 A more complete example of this is the standard Shell module, which 1407 can treat undefined subroutine calls as calls to external programs. 1408 1409 Mechanisms are available to help modules writers split their modules 1410 into autoloadable files. See the standard AutoLoader module 1411 described in L<AutoLoader> and in L<AutoSplit>, the standard 1412 SelfLoader modules in L<SelfLoader>, and the document on adding C 1413 functions to Perl code in L<perlxs>. 1414 1415 =head2 Subroutine Attributes 1416 X<attribute> X<subroutine, attribute> X<attrs> 1417 1418 A subroutine declaration or definition may have a list of attributes 1419 associated with it. If such an attribute list is present, it is 1420 broken up at space or colon boundaries and treated as though a 1421 C<use attributes> had been seen. See L<attributes> for details 1422 about what attributes are currently supported. 1423 Unlike the limitation with the obsolescent C<use attrs>, the 1424 C<sub : ATTRLIST> syntax works to associate the attributes with 1425 a pre-declaration, and not just with a subroutine definition. 1426 1427 The attributes must be valid as simple identifier names (without any 1428 punctuation other than the '_' character). They may have a parameter 1429 list appended, which is only checked for whether its parentheses ('(',')') 1430 nest properly. 1431 1432 Examples of valid syntax (even though the attributes are unknown): 1433 1434 sub fnord (&\%) : switch(10,foo(7,3)) : expensive; 1435 sub plugh () : Ugly('\(") :Bad; 1436 sub xyzzy : _5x5 { ... } 1437 1438 Examples of invalid syntax: 1439 1440 sub fnord : switch(10,foo(); # ()-string not balanced 1441 sub snoid : Ugly('('); # ()-string not balanced 1442 sub xyzzy : 5x5; # "5x5" not a valid identifier 1443 sub plugh : Y2::north; # "Y2::north" not a simple identifier 1444 sub snurt : foo + bar; # "+" not a colon or space 1445 1446 The attribute list is passed as a list of constant strings to the code 1447 which associates them with the subroutine. In particular, the second example 1448 of valid syntax above currently looks like this in terms of how it's 1449 parsed and invoked: 1450 1451 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad'; 1452 1453 For further details on attribute lists and their manipulation, 1454 see L<attributes> and L<Attribute::Handlers>. 1455 1456 =head1 SEE ALSO 1457 1458 See L<perlref/"Function Templates"> for more about references and closures. 1459 See L<perlxs> if you'd like to learn about calling C subroutines from Perl. 1460 See L<perlembed> if you'd like to learn about calling Perl subroutines from C. 1461 See L<perlmod> to learn about bundling up your functions in separate files. 1462 See L<perlmodlib> to learn what library modules come standard on your system. 1463 See L<perltoot> to learn how to make object method calls.
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