2 X<subroutine> X<function>
4 perlsub - Perl subroutines
8 To declare subroutines:
9 X<subroutine, declaration> X<sub>
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
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
21 To define an anonymous subroutine at runtime:
22 X<subroutine, anonymous>
24 $subref = sub BLOCK; # no proto
25 $subref = sub (PROTO) BLOCK; # with proto
26 $subref = sub : ATTRS BLOCK; # with attributes
27 $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
29 To import subroutines:
32 use MODULE qw(NAME1 NAME2 NAME3);
35 X<subroutine, call> X<call>
37 NAME(LIST); # & is optional with parentheses.
38 NAME LIST; # Parentheses optional if predeclared/imported.
39 &NAME(LIST); # Circumvent prototypes.
40 &NAME; # Makes current @_ visible to called subroutine.
44 Like many languages, Perl provides for user-defined subroutines.
45 These may be located anywhere in the main program, loaded in from
46 other files via the C<do>, C<require>, or C<use> keywords, or
47 generated on the fly using C<eval> or anonymous subroutines.
48 You can even call a function indirectly using a variable containing
49 its name or a CODE reference.
51 The Perl model for function call and return values is simple: all
52 functions are passed as parameters one single flat list of scalars, and
53 all functions likewise return to their caller one single flat list of
54 scalars. Any arrays or hashes in these call and return lists will
55 collapse, losing their identities--but you may always use
56 pass-by-reference instead to avoid this. Both call and return lists may
57 contain as many or as few scalar elements as you'd like. (Often a
58 function without an explicit return statement is called a subroutine, but
59 there's really no difference from Perl's perspective.)
60 X<subroutine, parameter> X<parameter>
62 Any arguments passed in show up in the array C<@_>. Therefore, if
63 you called a function with two arguments, those would be stored in
64 C<$_[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<@_>
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.
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>
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>
105 $max = $foo if $max < $foo;
109 $bestday = max($mon,$tue,$wed,$thu,$fri);
113 # get a line, combining continuation lines
114 # that start with whitespace
117 $thisline = $lookahead; # global variables!
118 LINE: while (defined($lookahead = <STDIN>)) {
119 if ($lookahead =~ /^[ \t]/) {
120 $thisline .= $lookahead;
129 $lookahead = <STDIN>; # get first line
130 while (defined($line = get_line())) {
134 Assigning to a list of private variables to name your arguments:
137 my($key, $value) = @_;
138 $Foo{$key} = $value unless $Foo{$key};
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
145 X<call-by-reference> X<call-by-value>
147 upcase_in($v1, $v2); # this changes $v1 and $v2
149 for (@_) { tr/a-z/A-Z/ }
152 You aren't allowed to modify constants in this way, of course. If an
153 argument were actually literal and you tried to change it, you'd take a
154 (presumably fatal) exception. For example, this won't work:
155 X<call-by-reference> X<call-by-value>
157 upcase_in("frederick");
159 It would be much safer if the C<upcase_in()> function
160 were written to return a copy of its parameters instead
161 of changing them in place:
163 ($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2
165 return unless defined wantarray; # void context, do nothing
167 for (@parms) { tr/a-z/A-Z/ }
168 return wantarray ? @parms : $parms[0];
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:
178 @newlist = upcase(@list1, @list2);
179 @newlist = upcase( split /:/, $var );
181 Do not, however, be tempted to do this:
183 (@a, @b) = upcase(@list1, @list2);
185 Like the flattened incoming parameter list, the return list is also
186 flattened on return. So all you have managed to do here is stored
187 everything in C<@a> and made C<@b> empty. See
188 L<Pass by Reference> for alternatives.
190 A subroutine may be called using an explicit C<&> prefix. The
191 C<&> is optional in modern Perl, as are parentheses if the
192 subroutine has been predeclared. The C<&> is I<not> optional
193 when just naming the subroutine, such as when it's used as
194 an argument to defined() or undef(). Nor is it optional when you
195 want to do an indirect subroutine call with a subroutine name or
196 reference using the C<&$subref()> or C<&{$subref}()> constructs,
197 although the C<< $subref->() >> notation solves that problem.
198 See L<perlref> for more about all that.
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.
208 &foo(1,2,3); # pass three arguments
209 foo(1,2,3); # the same
211 foo(); # pass a null list
214 &foo; # foo() get current args, like foo(@_) !!
215 foo; # like foo() IFF sub foo predeclared, else "foo"
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.
223 Since Perl 5.16.0, the C<__SUB__> token is available under C<use feature
224 'current_sub'> and C<use 5.16.0>. It will evaluate to a reference to the
225 currently-running sub, which allows for recursive calls without knowing
226 your subroutine's name.
229 my $factorial = sub {
232 return($x * __SUB__->( $x - 1 ) );
235 Subroutines whose names are in all upper case are reserved to the Perl
236 core, as are modules whose names are in all lower case. A subroutine in
237 all capitals is a loosely-held convention meaning it will be called
238 indirectly by the run-time system itself, usually due to a triggered event.
239 Subroutines that do special, pre-defined things include C<AUTOLOAD>, C<CLONE>,
240 C<DESTROY> plus all functions mentioned in L<perltie> and L<PerlIO::via>.
242 The C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> and C<END> subroutines
243 are not so much subroutines as named special code blocks, of which you
244 can have more than one in a package, and which you can B<not> call
245 explicitly. See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">
247 =head2 Private Variables via my()
248 X<my> X<variable, lexical> X<lexical> X<lexical variable> X<scope, lexical>
249 X<lexical scope> X<attributes, my>
253 my $foo; # declare $foo lexically local
254 my (@wid, %get); # declare list of variables local
255 my $foo = "flurp"; # declare $foo lexical, and init it
256 my @oof = @bar; # declare @oof lexical, and init it
257 my $x : Foo = $y; # similar, with an attribute applied
259 B<WARNING>: The use of attribute lists on C<my> declarations is still
260 evolving. The current semantics and interface are subject to change.
261 See L<attributes> and L<Attribute::Handlers>.
263 The C<my> operator declares the listed variables to be lexically
264 confined to the enclosing block, conditional (C<if/unless/elsif/else>),
265 loop (C<for/foreach/while/until/continue>), subroutine, C<eval>,
266 or C<do/require/use>'d file. If more than one value is listed, the
267 list must be placed in parentheses. All listed elements must be
268 legal lvalues. Only alphanumeric identifiers may be lexically
269 scoped--magical built-ins like C<$/> must currently be C<local>ized
270 with C<local> instead.
272 Unlike dynamic variables created by the C<local> operator, lexical
273 variables declared with C<my> are totally hidden from the outside
274 world, including any called subroutines. This is true if it's the
275 same subroutine called from itself or elsewhere--every call gets
279 This doesn't mean that a C<my> variable declared in a statically
280 enclosing lexical scope would be invisible. Only dynamic scopes
281 are cut off. For example, the C<bumpx()> function below has access
282 to the lexical $x variable because both the C<my> and the C<sub>
283 occurred at the same scope, presumably file scope.
288 An C<eval()>, however, can see lexical variables of the scope it is
289 being evaluated in, so long as the names aren't hidden by declarations within
290 the C<eval()> itself. See L<perlref>.
293 The parameter list to my() may be assigned to if desired, which allows you
294 to initialize your variables. (If no initializer is given for a
295 particular variable, it is created with the undefined value.) Commonly
296 this is used to name input parameters to a subroutine. Examples:
298 $arg = "fred"; # "global" variable
300 print "$arg thinks the root is $n\n";
301 fred thinks the root is 3
304 my $arg = shift; # name doesn't matter
309 The C<my> is simply a modifier on something you might assign to. So when
310 you do assign to variables in its argument list, C<my> doesn't
311 change whether those variables are viewed as a scalar or an array. So
313 my ($foo) = <STDIN>; # WRONG?
316 both supply a list context to the right-hand side, while
320 supplies a scalar context. But the following declares only one variable:
322 my $foo, $bar = 1; # WRONG
324 That has the same effect as
329 The declared variable is not introduced (is not visible) until after
330 the current statement. Thus,
334 can be used to initialize a new $x with the value of the old $x, and
337 my $x = 123 and $x == 123
339 is false unless the old $x happened to have the value C<123>.
341 Lexical scopes of control structures are not bounded precisely by the
342 braces that delimit their controlled blocks; control expressions are
343 part of that scope, too. Thus in the loop
345 while (my $line = <>) {
351 the scope of $line extends from its declaration throughout the rest of
352 the loop construct (including the C<continue> clause), but not beyond
353 it. Similarly, in the conditional
355 if ((my $answer = <STDIN>) =~ /^yes$/i) {
357 } elsif ($answer =~ /^no$/i) {
361 die "'$answer' is neither 'yes' nor 'no'";
364 the scope of $answer extends from its declaration through the rest
365 of that conditional, including any C<elsif> and C<else> clauses,
366 but not beyond it. See L<perlsyn/"Simple Statements"> for information
367 on the scope of variables in statements with modifiers.
369 The C<foreach> loop defaults to scoping its index variable dynamically
370 in the manner of C<local>. However, if the index variable is
371 prefixed with the keyword C<my>, or if there is already a lexical
372 by that name in scope, then a new lexical is created instead. Thus
376 for my $i (1, 2, 3) {
380 the scope of $i extends to the end of the loop, but not beyond it,
381 rendering the value of $i inaccessible within C<some_function()>.
384 Some users may wish to encourage the use of lexically scoped variables.
385 As an aid to catching implicit uses to package variables,
386 which are always global, if you say
390 then any variable mentioned from there to the end of the enclosing
391 block must either refer to a lexical variable, be predeclared via
392 C<our> or C<use vars>, or else must be fully qualified with the package name.
393 A compilation error results otherwise. An inner block may countermand
394 this with C<no strict 'vars'>.
396 A C<my> has both a compile-time and a run-time effect. At compile
397 time, the compiler takes notice of it. The principal usefulness
398 of this is to quiet C<use strict 'vars'>, but it is also essential
399 for generation of closures as detailed in L<perlref>. Actual
400 initialization is delayed until run time, though, so it gets executed
401 at the appropriate time, such as each time through a loop, for
404 Variables declared with C<my> are not part of any package and are therefore
405 never fully qualified with the package name. In particular, you're not
406 allowed to try to make a package variable (or other global) lexical:
408 my $pack::var; # ERROR! Illegal syntax
410 In fact, a dynamic variable (also known as package or global variables)
411 are still accessible using the fully qualified C<::> notation even while a
412 lexical of the same name is also visible:
417 print "$x and $::x\n";
419 That will print out C<20> and C<10>.
421 You may declare C<my> variables at the outermost scope of a file
422 to hide any such identifiers from the world outside that file. This
423 is similar in spirit to C's static variables when they are used at
424 the file level. To do this with a subroutine requires the use of
425 a closure (an anonymous function that accesses enclosing lexicals).
426 If you want to create a private subroutine that cannot be called
427 from outside that block, it can declare a lexical variable containing
428 an anonymous sub reference:
430 my $secret_version = '1.001-beta';
431 my $secret_sub = sub { print $secret_version };
434 As long as the reference is never returned by any function within the
435 module, no outside module can see the subroutine, because its name is not in
436 any package's symbol table. Remember that it's not I<REALLY> called
437 C<$some_pack::secret_version> or anything; it's just $secret_version,
438 unqualified and unqualifiable.
440 This does not work with object methods, however; all object methods
441 have to be in the symbol table of some package to be found. See
442 L<perlref/"Function Templates"> for something of a work-around to
445 =head2 Persistent Private Variables
446 X<state> X<state variable> X<static> X<variable, persistent> X<variable, static> X<closure>
448 There are two ways to build persistent private variables in Perl 5.10.
449 First, you can simply use the C<state> feature. Or, you can use closures,
450 if you want to stay compatible with releases older than 5.10.
452 =head3 Persistent variables via state()
454 Beginning with Perl 5.10.0, you can declare variables with the C<state>
455 keyword in place of C<my>. For that to work, though, you must have
456 enabled that feature beforehand, either by using the C<feature> pragma, or
457 by using C<-E> on one-liners (see L<feature>). Beginning with Perl 5.16,
458 the C<CORE::state> form does not require the
461 For example, the following code maintains a private counter, incremented
462 each time the gimme_another() function is called:
465 sub gimme_another { state $x; return ++$x }
467 Also, since C<$x> is lexical, it can't be reached or modified by any Perl
470 When combined with variable declaration, simple scalar assignment to C<state>
471 variables (as in C<state $x = 42>) is executed only the first time. When such
472 statements are evaluated subsequent times, the assignment is ignored. The
473 behavior of this sort of assignment to non-scalar variables is undefined.
475 =head3 Persistent variables with closures
477 Just because a lexical variable is lexically (also called statically)
478 scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that
479 within a function it works like a C static. It normally works more
480 like a C auto, but with implicit garbage collection.
482 Unlike local variables in C or C++, Perl's lexical variables don't
483 necessarily get recycled just because their scope has exited.
484 If something more permanent is still aware of the lexical, it will
485 stick around. So long as something else references a lexical, that
486 lexical won't be freed--which is as it should be. You wouldn't want
487 memory being free until you were done using it, or kept around once you
488 were done. Automatic garbage collection takes care of this for you.
490 This means that you can pass back or save away references to lexical
491 variables, whereas to return a pointer to a C auto is a grave error.
492 It also gives us a way to simulate C's function statics. Here's a
493 mechanism for giving a function private variables with both lexical
494 scoping and a static lifetime. If you do want to create something like
495 C's static variables, just enclose the whole function in an extra block,
496 and put the static variable outside the function but in the block.
501 return ++$secret_val;
504 # $secret_val now becomes unreachable by the outside
505 # world, but retains its value between calls to gimme_another
507 If this function is being sourced in from a separate file
508 via C<require> or C<use>, then this is probably just fine. If it's
509 all in the main program, you'll need to arrange for the C<my>
510 to be executed early, either by putting the whole block above
511 your main program, or more likely, placing merely a C<BEGIN>
512 code block around it to make sure it gets executed before your program
518 return ++$secret_val;
522 See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END"> about the
523 special triggered code blocks, C<BEGIN>, C<UNITCHECK>, C<CHECK>,
526 If declared at the outermost scope (the file scope), then lexicals
527 work somewhat like C's file statics. They are available to all
528 functions in that same file declared below them, but are inaccessible
529 from outside that file. This strategy is sometimes used in modules
530 to create private variables that the whole module can see.
532 =head2 Temporary Values via local()
533 X<local> X<scope, dynamic> X<dynamic scope> X<variable, local>
534 X<variable, temporary>
536 B<WARNING>: In general, you should be using C<my> instead of C<local>, because
537 it's faster and safer. Exceptions to this include the global punctuation
538 variables, global filehandles and formats, and direct manipulation of the
539 Perl symbol table itself. C<local> is mostly used when the current value
540 of a variable must be visible to called subroutines.
544 # localization of values
546 local $foo; # make $foo dynamically local
547 local (@wid, %get); # make list of variables local
548 local $foo = "flurp"; # make $foo dynamic, and init it
549 local @oof = @bar; # make @oof dynamic, and init it
551 local $hash{key} = "val"; # sets a local value for this hash entry
552 delete local $hash{key}; # delete this entry for the current block
553 local ($cond ? $v1 : $v2); # several types of lvalues support
556 # localization of symbols
558 local *FH; # localize $FH, @FH, %FH, &FH ...
559 local *merlyn = *randal; # now $merlyn is really $randal, plus
560 # @merlyn is really @randal, etc
561 local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
562 local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
564 A C<local> modifies its listed variables to be "local" to the
565 enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine
566 called from within that block>. A C<local> just gives temporary
567 values to global (meaning package) variables. It does I<not> create
568 a local variable. This is known as dynamic scoping. Lexical scoping
569 is done with C<my>, which works more like C's auto declarations.
571 Some types of lvalues can be localized as well: hash and array elements
572 and slices, conditionals (provided that their result is always
573 localizable), and symbolic references. As for simple variables, this
574 creates new, dynamically scoped values.
576 If more than one variable or expression is given to C<local>, they must be
577 placed in parentheses. This operator works
578 by saving the current values of those variables in its argument list on a
579 hidden stack and restoring them upon exiting the block, subroutine, or
580 eval. This means that called subroutines can also reference the local
581 variable, but not the global one. The argument list may be assigned to if
582 desired, which allows you to initialize your local variables. (If no
583 initializer is given for a particular variable, it is created with an
586 Because C<local> is a run-time operator, it gets executed each time
587 through a loop. Consequently, it's more efficient to localize your
588 variables outside the loop.
590 =head3 Grammatical note on local()
593 A C<local> is simply a modifier on an lvalue expression. When you assign to
594 a C<local>ized variable, the C<local> doesn't change whether its list is viewed
595 as a scalar or an array. So
597 local($foo) = <STDIN>;
598 local @FOO = <STDIN>;
600 both supply a list context to the right-hand side, while
602 local $foo = <STDIN>;
604 supplies a scalar context.
606 =head3 Localization of special variables
607 X<local, special variable>
609 If you localize a special variable, you'll be giving a new value to it,
610 but its magic won't go away. That means that all side-effects related
611 to this magic still work with the localized value.
613 This feature allows code like this to work :
615 # Read the whole contents of FILE in $slurp
616 { local $/ = undef; $slurp = <FILE>; }
618 Note, however, that this restricts localization of some values ; for
619 example, the following statement dies, as of perl 5.10.0, with an error
620 I<Modification of a read-only value attempted>, because the $1 variable is
621 magical and read-only :
625 One exception is the default scalar variable: starting with perl 5.14
626 C<local($_)> will always strip all magic from $_, to make it possible
627 to safely reuse $_ in a subroutine.
629 B<WARNING>: Localization of tied arrays and hashes does not currently
631 This will be fixed in a future release of Perl; in the meantime, avoid
632 code that relies on any particular behaviour of localising tied arrays
633 or hashes (localising individual elements is still okay).
634 See L<perl58delta/"Localising Tied Arrays and Hashes Is Broken"> for more
638 =head3 Localization of globs
639 X<local, glob> X<glob>
645 creates a whole new symbol table entry for the glob C<name> in the
646 current package. That means that all variables in its glob slot ($name,
647 @name, %name, &name, and the C<name> filehandle) are dynamically reset.
649 This implies, among other things, that any magic eventually carried by
650 those variables is locally lost. In other words, saying C<local */>
651 will not have any effect on the internal value of the input record
654 =head3 Localization of elements of composite types
655 X<local, composite type element> X<local, array element> X<local, hash element>
657 It's also worth taking a moment to explain what happens when you
658 C<local>ize a member of a composite type (i.e. an array or hash element).
659 In this case, the element is C<local>ized I<by name>. This means that
660 when the scope of the C<local()> ends, the saved value will be
661 restored to the hash element whose key was named in the C<local()>, or
662 the array element whose index was named in the C<local()>. If that
663 element was deleted while the C<local()> was in effect (e.g. by a
664 C<delete()> from a hash or a C<shift()> of an array), it will spring
665 back into existence, possibly extending an array and filling in the
666 skipped elements with C<undef>. For instance, if you say
668 %hash = ( 'This' => 'is', 'a' => 'test' );
672 local($hash{'a'}) = 'drill';
673 while (my $e = pop(@ary)) {
678 $hash{'only a'} = 'test';
682 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
683 print "The array has ",scalar(@ary)," elements: ",
684 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
691 This is a test only a test.
692 The array has 6 elements: 0, 1, 2, undef, undef, 5
694 The behavior of local() on non-existent members of composite
695 types is subject to change in future.
697 =head3 Localized deletion of elements of composite types
698 X<delete> X<local, composite type element> X<local, array element> X<local, hash element>
700 You can use the C<delete local $array[$idx]> and C<delete local $hash{key}>
701 constructs to delete a composite type entry for the current block and restore
702 it when it ends. They return the array/hash value before the localization,
703 which means that they are respectively equivalent to
706 my $val = $array[$idx];
715 my $val = $hash{key};
721 except that for those the C<local> is scoped to the C<do> block. Slices are
730 my $a = delete local $hash{a};
735 my @nums = delete local @$a[0, 2]
739 $a[0] = 999; # will be erased when the scope ends
741 # $a is back to [ 7, 8, 9 ]
744 # %hash is back to its original state
746 =head2 Lvalue subroutines
747 X<lvalue> X<subroutine, lvalue>
749 B<WARNING>: Lvalue subroutines are still experimental and the
750 implementation may change in future versions of Perl.
752 It is possible to return a modifiable value from a subroutine.
753 To do this, you have to declare the subroutine to return an lvalue.
756 sub canmod : lvalue {
757 $val; # or: return $val;
763 canmod() = 5; # assigns to $val
766 The scalar/list context for the subroutine and for the right-hand
767 side of assignment is determined as if the subroutine call is replaced
768 by a scalar. For example, consider:
770 data(2,3) = get_data(3,4);
772 Both subroutines here are called in a scalar context, while in:
774 (data(2,3)) = get_data(3,4);
778 (data(2),data(3)) = get_data(3,4);
780 all the subroutines are called in a list context.
784 =item Lvalue subroutines are EXPERIMENTAL
786 They appear to be convenient, but there is at least one reason to be
789 They violate encapsulation. A normal mutator can check the supplied
790 argument before setting the attribute it is protecting, an lvalue
791 subroutine never gets that chance. Consider;
793 my $some_array_ref = []; # protected by mutators ??
795 sub set_arr { # normal mutator
797 die("expected array, you supplied ", ref $val)
798 unless ref $val eq 'ARRAY';
799 $some_array_ref = $val;
801 sub set_arr_lv : lvalue { # lvalue mutator
805 # set_arr_lv cannot stop this !
806 set_arr_lv() = { a => 1 };
810 =head2 Passing Symbol Table Entries (typeglobs)
813 B<WARNING>: The mechanism described in this section was originally
814 the only way to simulate pass-by-reference in older versions of
815 Perl. While it still works fine in modern versions, the new reference
816 mechanism is generally easier to work with. See below.
818 Sometimes you don't want to pass the value of an array to a subroutine
819 but rather the name of it, so that the subroutine can modify the global
820 copy of it rather than working with a local copy. In perl you can
821 refer to all objects of a particular name by prefixing the name
822 with a star: C<*foo>. This is often known as a "typeglob", because the
823 star on the front can be thought of as a wildcard match for all the
824 funny prefix characters on variables and subroutines and such.
826 When evaluated, the typeglob produces a scalar value that represents
827 all the objects of that name, including any filehandle, format, or
828 subroutine. When assigned to, it causes the name mentioned to refer to
829 whatever C<*> value was assigned to it. Example:
832 local(*someary) = @_;
833 foreach $elem (@someary) {
840 Scalars are already passed by reference, so you can modify
841 scalar arguments without using this mechanism by referring explicitly
842 to C<$_[0]> etc. You can modify all the elements of an array by passing
843 all the elements as scalars, but you have to use the C<*> mechanism (or
844 the equivalent reference mechanism) to C<push>, C<pop>, or change the size of
845 an array. It will certainly be faster to pass the typeglob (or reference).
847 Even if you don't want to modify an array, this mechanism is useful for
848 passing multiple arrays in a single LIST, because normally the LIST
849 mechanism will merge all the array values so that you can't extract out
850 the individual arrays. For more on typeglobs, see
851 L<perldata/"Typeglobs and Filehandles">.
853 =head2 When to Still Use local()
854 X<local> X<variable, local>
856 Despite the existence of C<my>, there are still three places where the
857 C<local> operator still shines. In fact, in these three places, you
858 I<must> use C<local> instead of C<my>.
864 You need to give a global variable a temporary value, especially $_.
866 The global variables, like C<@ARGV> or the punctuation variables, must be
867 C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
868 it up into chunks separated by lines of equal signs, which are placed
872 local @ARGV = ("/etc/motd");
875 @Fields = split /^\s*=+\s*$/;
878 It particular, it's important to C<local>ize $_ in any routine that assigns
879 to it. Look out for implicit assignments in C<while> conditionals.
883 You need to create a local file or directory handle or a local function.
885 A function that needs a filehandle of its own must use
886 C<local()> on a complete typeglob. This can be used to create new symbol
890 local (*READER, *WRITER); # not my!
891 pipe (READER, WRITER) or die "pipe: $!";
892 return (*READER, *WRITER);
894 ($head, $tail) = ioqueue();
896 See the Symbol module for a way to create anonymous symbol table
899 Because assignment of a reference to a typeglob creates an alias, this
900 can be used to create what is effectively a local function, or at least,
904 local *grow = \&shrink; # only until this block exits
905 grow(); # really calls shrink()
906 move(); # if move() grow()s, it shrink()s too
908 grow(); # get the real grow() again
910 See L<perlref/"Function Templates"> for more about manipulating
911 functions by name in this way.
915 You want to temporarily change just one element of an array or hash.
917 You can C<local>ize just one element of an aggregate. Usually this
921 local $SIG{INT} = 'IGNORE';
922 funct(); # uninterruptible
924 # interruptibility automatically restored here
926 But it also works on lexically declared aggregates.
930 =head2 Pass by Reference
931 X<pass by reference> X<pass-by-reference> X<reference>
933 If you want to pass more than one array or hash into a function--or
934 return them from it--and have them maintain their integrity, then
935 you're going to have to use an explicit pass-by-reference. Before you
936 do that, you need to understand references as detailed in L<perlref>.
937 This section may not make much sense to you otherwise.
939 Here are a few simple examples. First, let's pass in several arrays
940 to a function and have it C<pop> all of then, returning a new list
941 of all their former last elements:
943 @tailings = popmany ( \@a, \@b, \@c, \@d );
948 foreach $aref ( @_ ) {
949 push @retlist, pop @$aref;
954 Here's how you might write a function that returns a
955 list of keys occurring in all the hashes passed to it:
957 @common = inter( \%foo, \%bar, \%joe );
959 my ($k, $href, %seen); # locals
961 while ( $k = each %$href ) {
965 return grep { $seen{$_} == @_ } keys %seen;
968 So far, we're using just the normal list return mechanism.
969 What happens if you want to pass or return a hash? Well,
970 if you're using only one of them, or you don't mind them
971 concatenating, then the normal calling convention is ok, although
974 Where people get into trouble is here:
976 (@a, @b) = func(@c, @d);
978 (%a, %b) = func(%c, %d);
980 That syntax simply won't work. It sets just C<@a> or C<%a> and
981 clears the C<@b> or C<%b>. Plus the function didn't get passed
982 into two separate arrays or hashes: it got one long list in C<@_>,
985 If you can arrange for everyone to deal with this through references, it's
986 cleaner code, although not so nice to look at. Here's a function that
987 takes two array references as arguments, returning the two array elements
988 in order of how many elements they have in them:
990 ($aref, $bref) = func(\@c, \@d);
991 print "@$aref has more than @$bref\n";
993 my ($cref, $dref) = @_;
994 if (@$cref > @$dref) {
995 return ($cref, $dref);
997 return ($dref, $cref);
1001 It turns out that you can actually do this also:
1003 (*a, *b) = func(\@c, \@d);
1004 print "@a has more than @b\n";
1006 local (*c, *d) = @_;
1014 Here we're using the typeglobs to do symbol table aliasing. It's
1015 a tad subtle, though, and also won't work if you're using C<my>
1016 variables, because only globals (even in disguise as C<local>s)
1017 are in the symbol table.
1019 If you're passing around filehandles, you could usually just use the bare
1020 typeglob, like C<*STDOUT>, but typeglobs references work, too.
1026 print $fh "her um well a hmmm\n";
1029 $rec = get_rec(\*STDIN);
1032 return scalar <$fh>;
1035 If you're planning on generating new filehandles, you could do this.
1036 Notice to pass back just the bare *FH, not its reference.
1041 return open (FH, $path) ? *FH : undef;
1045 X<prototype> X<subroutine, prototype>
1047 Perl supports a very limited kind of compile-time argument checking
1048 using function prototyping. If you declare
1052 then C<mypush()> takes arguments exactly like C<push()> does. The
1053 function declaration must be visible at compile time. The prototype
1054 affects only interpretation of new-style calls to the function,
1055 where new-style is defined as not using the C<&> character. In
1056 other words, if you call it like a built-in function, then it behaves
1057 like a built-in function. If you call it like an old-fashioned
1058 subroutine, then it behaves like an old-fashioned subroutine. It
1059 naturally falls out from this rule that prototypes have no influence
1060 on subroutine references like C<\&foo> or on indirect subroutine
1061 calls like C<&{$subref}> or C<< $subref->() >>.
1063 Method calls are not influenced by prototypes either, because the
1064 function to be called is indeterminate at compile time, since
1065 the exact code called depends on inheritance.
1067 Because the intent of this feature is primarily to let you define
1068 subroutines that work like built-in functions, here are prototypes
1069 for some other functions that parse almost exactly like the
1070 corresponding built-in.
1072 Declared as Called as
1074 sub mylink ($$) mylink $old, $new
1075 sub myvec ($$$) myvec $var, $offset, 1
1076 sub myindex ($$;$) myindex &getstring, "substr"
1077 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
1078 sub myreverse (@) myreverse $a, $b, $c
1079 sub myjoin ($@) myjoin ":", $a, $b, $c
1080 sub mypop (+) mypop @array
1081 sub mysplice (+$$@) mysplice @array, 0, 2, @pushme
1082 sub mykeys (+) mykeys %{$hashref}
1083 sub myopen (*;$) myopen HANDLE, $name
1084 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
1085 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
1086 sub myrand (;$) myrand 42
1087 sub mytime () mytime
1089 Any backslashed prototype character represents an actual argument
1090 that must start with that character (optionally preceded by C<my>,
1091 C<our> or C<local>), with the exception of C<$>, which will
1092 accept any scalar lvalue expression, such as C<$foo = 7> or
1093 C<< my_function()->[0] >>. The value passed as part of C<@_> will be a
1094 reference to the actual argument given in the subroutine call,
1095 obtained by applying C<\> to that argument.
1097 You can use the C<\[]> backslash group notation to specify more than one
1098 allowed argument type. For example:
1100 sub myref (\[$@%&*])
1102 will allow calling myref() as
1110 and the first argument of myref() will be a reference to
1111 a scalar, an array, a hash, a code, or a glob.
1113 Unbackslashed prototype characters have special meanings. Any
1114 unbackslashed C<@> or C<%> eats all remaining arguments, and forces
1115 list context. An argument represented by C<$> forces scalar context. An
1116 C<&> requires an anonymous subroutine, which, if passed as the first
1117 argument, does not require the C<sub> keyword or a subsequent comma.
1119 A C<*> allows the subroutine to accept a bareword, constant, scalar expression,
1120 typeglob, or a reference to a typeglob in that slot. The value will be
1121 available to the subroutine either as a simple scalar, or (in the latter
1122 two cases) as a reference to the typeglob. If you wish to always convert
1123 such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
1126 use Symbol 'qualify_to_ref';
1129 my $fh = qualify_to_ref(shift, caller);
1133 The C<+> prototype is a special alternative to C<$> that will act like
1134 C<\[@%]> when given a literal array or hash variable, but will otherwise
1135 force scalar context on the argument. This is useful for functions which
1136 should accept either a literal array or an array reference as the argument:
1140 die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
1144 When using the C<+> prototype, your function must check that the argument
1145 is of an acceptable type.
1147 A semicolon (C<;>) separates mandatory arguments from optional arguments.
1148 It is redundant before C<@> or C<%>, which gobble up everything else.
1150 As the last character of a prototype, or just before a semicolon, a C<@>
1151 or a C<%>, you can use C<_> in place of C<$>: if this argument is not
1152 provided, C<$_> will be used instead.
1154 Note how the last three examples in the table above are treated
1155 specially by the parser. C<mygrep()> is parsed as a true list
1156 operator, C<myrand()> is parsed as a true unary operator with unary
1157 precedence the same as C<rand()>, and C<mytime()> is truly without
1158 arguments, just like C<time()>. That is, if you say
1162 you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
1163 without a prototype. If you want to force a unary function to have the
1164 same precedence as a list operator, add C<;> to the end of the prototype:
1166 sub mygetprotobynumber($;);
1167 mygetprotobynumber $a > $b; # parsed as mygetprotobynumber($a > $b)
1169 The interesting thing about C<&> is that you can generate new syntax with it,
1170 provided it's in the initial position:
1174 my($try,$catch) = @_;
1181 sub catch (&) { $_[0] }
1186 /phooey/ and print "unphooey\n";
1189 That prints C<"unphooey">. (Yes, there are still unresolved
1190 issues having to do with visibility of C<@_>. I'm ignoring that
1191 question for the moment. (But note that if we make C<@_> lexically
1192 scoped, those anonymous subroutines can act like closures... (Gee,
1193 is this sounding a little Lispish? (Never mind.))))
1195 And here's a reimplementation of the Perl C<grep> operator:
1202 push(@result, $_) if &$code;
1207 Some folks would prefer full alphanumeric prototypes. Alphanumerics have
1208 been intentionally left out of prototypes for the express purpose of
1209 someday in the future adding named, formal parameters. The current
1210 mechanism's main goal is to let module writers provide better diagnostics
1211 for module users. Larry feels the notation quite understandable to Perl
1212 programmers, and that it will not intrude greatly upon the meat of the
1213 module, nor make it harder to read. The line noise is visually
1214 encapsulated into a small pill that's easy to swallow.
1216 If you try to use an alphanumeric sequence in a prototype you will
1217 generate an optional warning - "Illegal character in prototype...".
1218 Unfortunately earlier versions of Perl allowed the prototype to be
1219 used as long as its prefix was a valid prototype. The warning may be
1220 upgraded to a fatal error in a future version of Perl once the
1221 majority of offending code is fixed.
1223 It's probably best to prototype new functions, not retrofit prototyping
1224 into older ones. That's because you must be especially careful about
1225 silent impositions of differing list versus scalar contexts. For example,
1226 if you decide that a function should take just one parameter, like this:
1230 print "you gave me $n\n";
1233 and someone has been calling it with an array or expression
1239 Then you've just supplied an automatic C<scalar> in front of their
1240 argument, which can be more than a bit surprising. The old C<@foo>
1241 which used to hold one thing doesn't get passed in. Instead,
1242 C<func()> now gets passed in a C<1>; that is, the number of elements
1243 in C<@foo>. And the C<split> gets called in scalar context so it
1244 starts scribbling on your C<@_> parameter list. Ouch!
1246 This is all very powerful, of course, and should be used only in moderation
1247 to make the world a better place.
1249 =head2 Constant Functions
1252 Functions with a prototype of C<()> are potential candidates for
1253 inlining. If the result after optimization and constant folding
1254 is either a constant or a lexically-scoped scalar which has no other
1255 references, then it will be used in place of function calls made
1256 without C<&>. Calls made using C<&> are never inlined. (See
1257 F<constant.pm> for an easy way to declare most constants.)
1259 The following functions would all be inlined:
1261 sub pi () { 3.14159 } # Not exact, but close.
1262 sub PI () { 4 * atan2 1, 1 } # As good as it gets,
1263 # and it's inlined, too!
1267 sub FLAG_FOO () { 1 << 8 }
1268 sub FLAG_BAR () { 1 << 9 }
1269 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
1271 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
1273 sub N () { int(OPT_BAZ) / 3 }
1275 sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
1277 Be aware that these will not be inlined; as they contain inner scopes,
1278 the constant folding doesn't reduce them to a single constant:
1280 sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }
1291 If you redefine a subroutine that was eligible for inlining, you'll get
1292 a warning by default. (You can use this warning to tell whether or not a
1293 particular subroutine is considered constant.) The warning is
1294 considered severe enough not to be affected by the B<-w>
1295 switch (or its absence) because previously compiled
1296 invocations of the function will still be using the old value of the
1297 function. If you need to be able to redefine the subroutine, you need to
1298 ensure that it isn't inlined, either by dropping the C<()> prototype
1299 (which changes calling semantics, so beware) or by thwarting the
1300 inlining mechanism in some other way, such as
1302 sub not_inlined () {
1306 =head2 Overriding Built-in Functions
1307 X<built-in> X<override> X<CORE> X<CORE::GLOBAL>
1309 Many built-in functions may be overridden, though this should be tried
1310 only occasionally and for good reason. Typically this might be
1311 done by a package attempting to emulate missing built-in functionality
1312 on a non-Unix system.
1314 Overriding may be done only by importing the name from a module at
1315 compile time--ordinary predeclaration isn't good enough. However, the
1316 C<use subs> pragma lets you, in effect, predeclare subs
1317 via the import syntax, and these names may then override built-in ones:
1319 use subs 'chdir', 'chroot', 'chmod', 'chown';
1323 To unambiguously refer to the built-in form, precede the
1324 built-in name with the special package qualifier C<CORE::>. For example,
1325 saying C<CORE::open()> always refers to the built-in C<open()>, even
1326 if the current package has imported some other subroutine called
1327 C<&open()> from elsewhere. Even though it looks like a regular
1328 function call, it isn't: the CORE:: prefix in that case is part of Perl's
1329 syntax, and works for any keyword, regardless of what is in the CORE
1330 package. Taking a reference to it, that is, C<\&CORE::open>, only works
1331 for some keywords. See L<CORE>.
1333 Library modules should not in general export built-in names like C<open>
1334 or C<chdir> as part of their default C<@EXPORT> list, because these may
1335 sneak into someone else's namespace and change the semantics unexpectedly.
1336 Instead, if the module adds that name to C<@EXPORT_OK>, then it's
1337 possible for a user to import the name explicitly, but not implicitly.
1338 That is, they could say
1342 and it would import the C<open> override. But if they said
1346 they would get the default imports without overrides.
1348 The foregoing mechanism for overriding built-in is restricted, quite
1349 deliberately, to the package that requests the import. There is a second
1350 method that is sometimes applicable when you wish to override a built-in
1351 everywhere, without regard to namespace boundaries. This is achieved by
1352 importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an
1353 example that quite brazenly replaces the C<glob> operator with something
1354 that understands regular expressions.
1359 @EXPORT_OK = 'glob';
1365 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
1366 $pkg->export($where, $sym, @_);
1372 if (opendir my $d, '.') {
1373 @got = grep /$pat/, readdir $d;
1380 And here's how it could be (ab)used:
1382 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
1384 use REGlob 'glob'; # override glob() in Foo:: only
1385 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
1387 The initial comment shows a contrived, even dangerous example.
1388 By overriding C<glob> globally, you would be forcing the new (and
1389 subversive) behavior for the C<glob> operator for I<every> namespace,
1390 without the complete cognizance or cooperation of the modules that own
1391 those namespaces. Naturally, this should be done with extreme caution--if
1392 it must be done at all.
1394 The C<REGlob> example above does not implement all the support needed to
1395 cleanly override perl's C<glob> operator. The built-in C<glob> has
1396 different behaviors depending on whether it appears in a scalar or list
1397 context, but our C<REGlob> doesn't. Indeed, many perl built-in have such
1398 context sensitive behaviors, and these must be adequately supported by
1399 a properly written override. For a fully functional example of overriding
1400 C<glob>, study the implementation of C<File::DosGlob> in the standard
1403 When you override a built-in, your replacement should be consistent (if
1404 possible) with the built-in native syntax. You can achieve this by using
1405 a suitable prototype. To get the prototype of an overridable built-in,
1406 use the C<prototype> function with an argument of C<"CORE::builtin_name">
1407 (see L<perlfunc/prototype>).
1409 Note however that some built-ins can't have their syntax expressed by a
1410 prototype (such as C<system> or C<chomp>). If you override them you won't
1411 be able to fully mimic their original syntax.
1413 The built-ins C<do>, C<require> and C<glob> can also be overridden, but due
1414 to special magic, their original syntax is preserved, and you don't have
1415 to define a prototype for their replacements. (You can't override the
1416 C<do BLOCK> syntax, though).
1418 C<require> has special additional dark magic: if you invoke your
1419 C<require> replacement as C<require Foo::Bar>, it will actually receive
1420 the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>.
1422 And, as you'll have noticed from the previous example, if you override
1423 C<glob>, the C<< <*> >> glob operator is overridden as well.
1425 In a similar fashion, overriding the C<readline> function also overrides
1426 the equivalent I/O operator C<< <FILEHANDLE> >>. Also, overriding
1427 C<readpipe> also overrides the operators C<``> and C<qx//>.
1429 Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
1432 X<autoloading> X<AUTOLOAD>
1434 If you call a subroutine that is undefined, you would ordinarily
1435 get an immediate, fatal error complaining that the subroutine doesn't
1436 exist. (Likewise for subroutines being used as methods, when the
1437 method doesn't exist in any base class of the class's package.)
1438 However, if an C<AUTOLOAD> subroutine is defined in the package or
1439 packages used to locate the original subroutine, then that
1440 C<AUTOLOAD> subroutine is called with the arguments that would have
1441 been passed to the original subroutine. The fully qualified name
1442 of the original subroutine magically appears in the global $AUTOLOAD
1443 variable of the same package as the C<AUTOLOAD> routine. The name
1444 is not passed as an ordinary argument because, er, well, just
1445 because, that's why. (As an exception, a method call to a nonexistent
1446 C<import> or C<unimport> method is just skipped instead. Also, if
1447 the AUTOLOAD subroutine is an XSUB, there are other ways to retrieve the
1448 subroutine name. See L<perlguts/Autoloading with XSUBs> for details.)
1451 Many C<AUTOLOAD> routines load in a definition for the requested
1452 subroutine using eval(), then execute that subroutine using a special
1453 form of goto() that erases the stack frame of the C<AUTOLOAD> routine
1454 without a trace. (See the source to the standard module documented
1455 in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can
1456 also just emulate the routine and never define it. For example,
1457 let's pretend that a function that wasn't defined should just invoke
1458 C<system> with those arguments. All you'd do is:
1461 my $program = $AUTOLOAD;
1462 $program =~ s/.*:://;
1463 system($program, @_);
1469 In fact, if you predeclare functions you want to call that way, you don't
1470 even need parentheses:
1472 use subs qw(date who ls);
1477 A more complete example of this is the Shell module on CPAN, which
1478 can treat undefined subroutine calls as calls to external programs.
1480 Mechanisms are available to help modules writers split their modules
1481 into autoloadable files. See the standard AutoLoader module
1482 described in L<AutoLoader> and in L<AutoSplit>, the standard
1483 SelfLoader modules in L<SelfLoader>, and the document on adding C
1484 functions to Perl code in L<perlxs>.
1486 =head2 Subroutine Attributes
1487 X<attribute> X<subroutine, attribute> X<attrs>
1489 A subroutine declaration or definition may have a list of attributes
1490 associated with it. If such an attribute list is present, it is
1491 broken up at space or colon boundaries and treated as though a
1492 C<use attributes> had been seen. See L<attributes> for details
1493 about what attributes are currently supported.
1494 Unlike the limitation with the obsolescent C<use attrs>, the
1495 C<sub : ATTRLIST> syntax works to associate the attributes with
1496 a pre-declaration, and not just with a subroutine definition.
1498 The attributes must be valid as simple identifier names (without any
1499 punctuation other than the '_' character). They may have a parameter
1500 list appended, which is only checked for whether its parentheses ('(',')')
1503 Examples of valid syntax (even though the attributes are unknown):
1505 sub fnord (&\%) : switch(10,foo(7,3)) : expensive;
1506 sub plugh () : Ugly('\(") :Bad;
1507 sub xyzzy : _5x5 { ... }
1509 Examples of invalid syntax:
1511 sub fnord : switch(10,foo(); # ()-string not balanced
1512 sub snoid : Ugly('('); # ()-string not balanced
1513 sub xyzzy : 5x5; # "5x5" not a valid identifier
1514 sub plugh : Y2::north; # "Y2::north" not a simple identifier
1515 sub snurt : foo + bar; # "+" not a colon or space
1517 The attribute list is passed as a list of constant strings to the code
1518 which associates them with the subroutine. In particular, the second example
1519 of valid syntax above currently looks like this in terms of how it's
1522 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
1524 For further details on attribute lists and their manipulation,
1525 see L<attributes> and L<Attribute::Handlers>.
1529 See L<perlref/"Function Templates"> for more about references and closures.
1530 See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
1531 See L<perlembed> if you'd like to learn about calling Perl subroutines from C.
1532 See L<perlmod> to learn about bundling up your functions in separate files.
1533 See L<perlmodlib> to learn what library modules come standard on your system.
1534 See L<perlootut> to learn how to make object method calls.