To declare subroutines:
- sub NAME; # A "forward" declaration.
- sub NAME(PROTO); # ditto, but with prototypes
+ sub NAME; # A "forward" declaration.
+ sub NAME(PROTO); # ditto, but with prototypes
+ sub NAME : ATTRS; # with attributes
+ sub NAME(PROTO) : ATTRS; # with attributes and prototypes
- sub NAME BLOCK # A declaration and a definition.
- sub NAME(PROTO) BLOCK # ditto, but with prototypes
+ sub NAME BLOCK # A declaration and a definition.
+ sub NAME(PROTO) BLOCK # ditto, but with prototypes
+ sub NAME : ATTRS BLOCK # with attributes
+ sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes
To define an anonymous subroutine at runtime:
- $subref = sub BLOCK;
+ $subref = sub BLOCK; # no proto
+ $subref = sub (PROTO) BLOCK; # with proto
+ $subref = sub : ATTRS BLOCK; # with attributes
+ $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
To import subroutines:
- use PACKAGE qw(NAME1 NAME2 NAME3);
+ use MODULE qw(NAME1 NAME2 NAME3);
To call subroutines:
NAME(LIST); # & is optional with parentheses.
- NAME LIST; # Parentheses optional if pre-declared/imported.
- &NAME; # Passes current @_ to subroutine.
+ NAME LIST; # Parentheses optional if predeclared/imported.
+ &NAME(LIST); # Circumvent prototypes.
+ &NAME; # Makes current @_ visible to called subroutine.
=head1 DESCRIPTION
-Like many languages, Perl provides for user-defined subroutines. These
-may be located anywhere in the main program, loaded in from other files
-via the C<do>, C<require>, or C<use> keywords, or even generated on the
-fly using C<eval> or anonymous subroutines (closures). You can even call
-a function indirectly using a variable containing its name or a CODE reference
-to it, as in C<$var = \&function>.
+Like many languages, Perl provides for user-defined subroutines.
+These may be located anywhere in the main program, loaded in from
+other files via the C<do>, C<require>, or C<use> keywords, or
+generated on the fly using C<eval> or anonymous subroutines (closures).
+You can even call a function indirectly using a variable containing
+its name or a CODE reference.
The Perl model for function call and return values is simple: all
functions are passed as parameters one single flat list of scalars, and
pass-by-reference instead to avoid this. Both call and return lists may
contain as many or as few scalar elements as you'd like. (Often a
function without an explicit return statement is called a subroutine, but
-there's really no difference from the language's perspective.)
-
-Any arguments passed to the routine come in as the array @_. Thus if you
-called a function with two arguments, those would be stored in C<$_[0]>
-and C<$_[1]>. The array @_ is a local array, but its values are implicit
-references (predating L<perlref>) to the actual scalar parameters. The
-return value of the subroutine is the value of the last expression
-evaluated. Alternatively, a return statement may be used to specify the
-returned value and exit the subroutine. If you return one or more arrays
-and/or hashes, these will be flattened together into one large
-indistinguishable list.
-
-Perl does not have named formal parameters, but in practice all you do is
-assign to a my() list of these. Any variables you use in the function
-that aren't declared private are global variables. For the gory details
-on creating private variables, see
-L<"Private Variables via my()"> and L<"Temporary Values via local()">.
-To create protected environments for a set of functions in a separate
-package (and probably a separate file), see L<perlmod/"Packages">.
+there's really no difference from Perl's perspective.)
+
+Any arguments passed in show up in the array C<@_>. Therefore, if
+you called a function with two arguments, those would be stored in
+C<$_[0]> and C<$_[1]>. The array C<@_> is a local array, but its
+elements are aliases for the actual scalar parameters. In particular,
+if an element C<$_[0]> is updated, the corresponding argument is
+updated (or an error occurs if it is not updatable). If an argument
+is an array or hash element which did not exist when the function
+was called, that element is created only when (and if) it is modified
+or a reference to it is taken. (Some earlier versions of Perl
+created the element whether or not the element was assigned to.)
+Assigning to the whole array C<@_> removes that aliasing, and does
+not update any arguments.
+
+The return value of a subroutine is the value of the last expression
+evaluated. More explicitly, a C<return> statement may be used to exit the
+subroutine, optionally specifying the returned value, which will be
+evaluated in the appropriate context (list, scalar, or void) depending
+on the context of the subroutine call. If you specify no return value,
+the subroutine returns an empty list in list context, the undefined
+value in scalar context, or nothing in void context. If you return
+one or more aggregates (arrays and hashes), these will be flattened
+together into one large indistinguishable list.
+
+Perl does not have named formal parameters. In practice all you
+do is assign to a C<my()> list of these. Variables that aren't
+declared to be private are global variables. For gory details
+on creating private variables, see L<"Private Variables via my()">
+and L<"Temporary Values via local()">. To create protected
+environments for a set of functions in a separate package (and
+probably a separate file), see L<perlmod/"Packages">.
Example:
# that start with whitespace
sub get_line {
- $thisline = $lookahead; # GLOBAL VARIABLES!!
- LINE: while ($lookahead = <STDIN>) {
+ $thisline = $lookahead; # global variables!
+ LINE: while (defined($lookahead = <STDIN>)) {
if ($lookahead =~ /^[ \t]/) {
$thisline .= $lookahead;
}
last LINE;
}
}
- $thisline;
+ return $thisline;
}
$lookahead = <STDIN>; # get first line
- while ($_ = get_line()) {
+ while (defined($line = get_line())) {
...
}
-Use array assignment to a local list to name your formal arguments:
+Assigning to a list of private variables to name your arguments:
sub maybeset {
my($key, $value) = @_;
$Foo{$key} = $value unless $Foo{$key};
}
-This also has the effect of turning call-by-reference into call-by-value,
-because the assignment copies the values. Otherwise a function is free to
-do in-place modifications of @_ and change its caller's values.
+Because the assignment copies the values, this also has the effect
+of turning call-by-reference into call-by-value. Otherwise a
+function is free to do in-place modifications of C<@_> and change
+its caller's values.
upcase_in($v1, $v2); # this changes $v1 and $v2
sub upcase_in {
- for (@_) { tr/a-z/A-Z/ }
- }
+ for (@_) { tr/a-z/A-Z/ }
+ }
You aren't allowed to modify constants in this way, of course. If an
argument were actually literal and you tried to change it, you'd take a
upcase_in("frederick");
-It would be much safer if the upcase_in() function
+It would be much safer if the C<upcase_in()> function
were written to return a copy of its parameters instead
of changing them in place:
- ($v3, $v4) = upcase($v1, $v2); # this doesn't
+ ($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2
sub upcase {
+ return unless defined wantarray; # void context, do nothing
my @parms = @_;
- for (@parms) { tr/a-z/A-Z/ }
- # wantarray checks if we were called in list context
+ for (@parms) { tr/a-z/A-Z/ }
return wantarray ? @parms : $parms[0];
- }
+ }
-Notice how this (unprototyped) function doesn't care whether it was passed
-real scalars or arrays. Perl will see everything as one big long flat @_
-parameter list. This is one of the ways where Perl's simple
-argument-passing style shines. The upcase() function would work perfectly
-well without changing the upcase() definition even if we fed it things
-like this:
+Notice how this (unprototyped) function doesn't care whether it was
+passed real scalars or arrays. Perl sees all arugments as one big,
+long, flat parameter list in C<@_>. This is one area where
+Perl's simple argument-passing style shines. The C<upcase()>
+function would work perfectly well without changing the C<upcase()>
+definition even if we fed it things like this:
@newlist = upcase(@list1, @list2);
@newlist = upcase( split /:/, $var );
(@a, @b) = upcase(@list1, @list2);
-Because like its flat incoming parameter list, the return list is also
-flat. So all you have managed to do here is stored everything in @a and
-made @b an empty list. See L</"Pass by Reference"> for alternatives.
-
-A subroutine may be called using the "&" prefix. The "&" is optional in
-Perl 5, and so are the parentheses if the subroutine has been pre-declared.
-(Note, however, that the "&" is I<NOT> optional when you're just naming
-the subroutine, such as when it's used as an argument to defined() or
-undef(). Nor is it optional when you want to do an indirect subroutine
-call with a subroutine name or reference using the C<&$subref()> or
-C<&{$subref}()> constructs. See L<perlref> for more on that.)
-
-Subroutines may be called recursively. If a subroutine is called using
-the "&" form, the argument list is optional, and if omitted, no @_ array is
-set up for the subroutine: the @_ array at the time of the call is
-visible to subroutine instead. This is an efficiency mechanism that
-new users may wish to avoid.
+Like the flattened incoming parameter list, the return list is also
+flattened on return. So all you have managed to do here is stored
+everything in C<@a> and made C<@b> an empty list. See L<Pass by
+Reference> for alternatives.
+
+A subroutine may be called using an explicit C<&> prefix. The
+C<&> is optional in modern Perl, as are parentheses if the
+subroutine has been predeclared. The C<&> is I<not> optional
+when just naming the subroutine, such as when it's used as
+an argument to defined() or undef(). Nor is it optional when you
+want to do an indirect subroutine call with a subroutine name or
+reference using the C<&$subref()> or C<&{$subref}()> constructs,
+although the C<$subref-E<gt>()> notation solves that problem.
+See L<perlref> for more about all that.
+
+Subroutines may be called recursively. If a subroutine is called
+using the C<&> form, the argument list is optional, and if omitted,
+no C<@_> array is set up for the subroutine: the C<@_> array at the
+time of the call is visible to subroutine instead. This is an
+efficiency mechanism that new users may wish to avoid.
&foo(1,2,3); # pass three arguments
foo(1,2,3); # the same
&foo(); # the same
&foo; # foo() get current args, like foo(@_) !!
- foo; # like foo() IFF sub foo pre-declared, else "foo"
+ foo; # like foo() IFF sub foo predeclared, else "foo"
-Not only does the "&" form make the argument list optional, but it also
-disables any prototype checking on the arguments you do provide. This
+Not only does the C<&> form make the argument list optional, it also
+disables any prototype checking on arguments you do provide. This
is partly for historical reasons, and partly for having a convenient way
-to cheat if you know what you're doing. See the section on Prototypes below.
+to cheat if you know what you're doing. See L<Prototypes> below.
+
+Functions whose names are in all upper case are reserved to the Perl
+core, as are modules whose names are in all lower case. A
+function in all capitals is a loosely-held convention meaning it
+will be called indirectly by the run-time system itself, usually
+due to a triggered event. Functions that do special, pre-defined
+things include C<BEGIN>, C<END>, C<AUTOLOAD>, and C<DESTROY>--plus
+all functions mentioned in L<perltie>. The 5.005 release adds
+C<INIT> to this list.
=head2 Private Variables via my()
my (@wid, %get); # declare list of variables local
my $foo = "flurp"; # declare $foo lexical, and init it
my @oof = @bar; # declare @oof lexical, and init it
-
-A "my" declares the listed variables to be confined (lexically) to the
-enclosing block, conditional (C<if/unless/elsif/else>), loop
-(C<for/foreach/while/until/continue>), subroutine, C<eval>, or
-C<do/require/use>'d file. If more than one value is listed, the list
-must be placed in parentheses. All listed elements must be legal lvalues.
-Only alphanumeric identifiers may be lexically scoped--magical
-builtins like $/ must currently be localized with "local" instead.
-
-Unlike dynamic variables created by the "local" statement, lexical
-variables declared with "my" are totally hidden from the outside world,
-including any called subroutines (even if it's the same subroutine called
-from itself or elsewhere--every call gets its own copy).
-
-(An eval(), however, can see the lexical variables of the scope it is
-being evaluated in so long as the names aren't hidden by declarations within
-the eval() itself. See L<perlref>.)
+ my $x : Foo = $y; # similar, with an attribute applied
+
+B<WARNING>: The use of attribute lists on C<my> declarations is
+experimental. This feature should not be relied upon. It may
+change or disappear in future releases of Perl. See L<attributes>.
+
+The C<my> operator declares the listed variables to be lexically
+confined to the enclosing block, conditional (C<if/unless/elsif/else>),
+loop (C<for/foreach/while/until/continue>), subroutine, C<eval>,
+or C<do/require/use>'d file. If more than one value is listed, the
+list must be placed in parentheses. All listed elements must be
+legal lvalues. Only alphanumeric identifiers may be lexically
+scoped--magical built-ins like C<$/> must currently be C<local>ize
+with C<local> instead.
+
+Unlike dynamic variables created by the C<local> operator, lexical
+variables declared with C<my> are totally hidden from the outside
+world, including any called subroutines. This is true if it's the
+same subroutine called from itself or elsewhere--every call gets
+its own copy.
+
+This doesn't mean that a C<my> variable declared in a statically
+enclosing lexical scope would be invisible. Only dynamic scopes
+are cut off. For example, the C<bumpx()> function below has access
+to the lexical $x variable because both the C<my> and the C<sub>
+occurred at the same scope, presumably file scope.
+
+ my $x = 10;
+ sub bumpx { $x++ }
+
+An C<eval()>, however, can see lexical variables of the scope it is
+being evaluated in, so long as the names aren't hidden by declarations within
+the C<eval()> itself. See L<perlref>.
The parameter list to my() may be assigned to if desired, which allows you
to initialize your variables. (If no initializer is given for a
particular variable, it is created with the undefined value.) Commonly
-this is used to name the parameters to a subroutine. Examples:
+this is used to name input parameters to a subroutine. Examples:
$arg = "fred"; # "global" variable
$n = cube_root(27);
my $arg = shift; # name doesn't matter
$arg **= 1/3;
return $arg;
- }
+ }
-The "my" is simply a modifier on something you might assign to. So when
-you do assign to the variables in its argument list, the "my" doesn't
-change whether those variables is viewed as a scalar or an array. So
+The C<my> is simply a modifier on something you might assign to. So when
+you do assign to variables in its argument list, C<my> doesn't
+change whether those variables are viewed as a scalar or an array. So
- my ($foo) = <STDIN>;
+ my ($foo) = <STDIN>; # WRONG?
my @FOO = <STDIN>;
both supply a list context to the right-hand side, while
supplies a scalar context. But the following declares only one variable:
- my $foo, $bar = 1;
+ my $foo, $bar = 1; # WRONG
That has the same effect as
my $x = $x;
-can be used to initialize the new $x with the value of the old $x, and
+can be used to initialize a new $x with the value of the old $x, and
the expression
my $x = 123 and $x == 123
-is false unless the old $x happened to have the value 123.
+is false unless the old $x happened to have the value C<123>.
Lexical scopes of control structures are not bounded precisely by the
braces that delimit their controlled blocks; control expressions are
-part of the scope, too. Thus in the loop
+part of that scope, too. Thus in the loop
while (my $line = <>) {
$line = lc $line;
die "'$answer' is neither 'yes' nor 'no'";
}
-the scope of $answer extends from its declaration throughout the rest
-of the conditional (including C<elsif> and C<else> clauses, if any),
+the scope of $answer extends from its declaration through the rest
+of that conditional, including any C<elsif> and C<else> clauses,
but not beyond it.
-(None of the foregoing applies to C<if/unless> or C<while/until>
+None of the foregoing text applies to C<if/unless> or C<while/until>
modifiers appended to simple statements. Such modifiers are not
-control structures and have no effect on scoping.)
+control structures and have no effect on scoping.
The C<foreach> loop defaults to scoping its index variable dynamically
-(in the manner of C<local>; see below). However, if the index
-variable is prefixed with the keyword "my", then it is lexically
-scoped instead. Thus in the loop
+in the manner of C<local>. However, if the index variable is
+prefixed with the keyword C<my>, or if there is already a lexical
+by that name in scope, then a new lexical is created instead. Thus
+in the loop
for my $i (1, 2, 3) {
some_function();
}
-the scope of $i extends to the end of the loop, but not beyond it, and
-so the value of $i is unavailable in some_function().
+the scope of $i extends to the end of the loop, but not beyond it,
+rendering the value of $i inaccessible within C<some_function()>.
Some users may wish to encourage the use of lexically scoped variables.
-As an aid to catching implicit references to package variables,
-if you say
+As an aid to catching implicit uses to package variables,
+which are always global, if you say
use strict 'vars';
-then any variable reference from there to the end of the enclosing
-block must either refer to a lexical variable, or must be fully
-qualified with the package name. A compilation error results
-otherwise. An inner block may countermand this with S<"no strict 'vars'">.
-
-A my() has both a compile-time and a run-time effect. At compile time,
-the compiler takes notice of it; the principle usefulness of this is to
-quiet C<use strict 'vars'>. The actual initialization doesn't happen
-until run time, so gets executed every time through a loop.
-
-Variables declared with "my" are not part of any package and are therefore
+then any variable mentioned from there to the end of the enclosing
+block must either refer to a lexical variable, be predeclared via
+C<use vars>, or else must be fully qualified with the package name.
+A compilation error results otherwise. An inner block may countermand
+this with C<no strict 'vars'>.
+
+A C<my> has both a compile-time and a run-time effect. At compile
+time, the compiler takes notice of it. The principle usefulness
+of this is to quiet C<use strict 'vars'>, but it is also essential
+for generation of closures as detailed in L<perlref>. Actual
+initialization is delayed until run time, though, so it gets executed
+at the appropriate time, such as each time through a loop, for
+example.
+
+Variables declared with C<my> are not part of any package and are therefore
never fully qualified with the package name. In particular, you're not
allowed to try to make a package variable (or other global) lexical:
my $_; # also illegal (currently)
In fact, a dynamic variable (also known as package or global variables)
-are still accessible using the fully qualified :: notation even while a
+are still accessible using the fully qualified C<::> notation even while a
lexical of the same name is also visible:
package main;
my $x = 20;
print "$x and $::x\n";
-That will print out 20 and 10.
+That will print out C<20> and C<10>.
-You may declare "my" variables at the outermost scope of a file to
-hide any such identifiers totally from the outside world. This is similar
-to C's static variables at the file level. To do this with a subroutine
-requires the use of a closure (anonymous function). If a block (such as
-an eval(), function, or C<package>) wants to create a private subroutine
-that cannot be called from outside that block, it can declare a lexical
-variable containing an anonymous sub reference:
+You may declare C<my> variables at the outermost scope of a file
+to hide any such identifiers from the world outside that file. This
+is similar in spirit to C's static variables when they are used at
+the file level. To do this with a subroutine requires the use of
+a closure (an anonymous function that accesses enclosing lexicals).
+If you want to create a private subroutine that cannot be called
+from outside that block, it can declare a lexical variable containing
+an anonymous sub reference:
my $secret_version = '1.001-beta';
my $secret_sub = sub { print $secret_version };
As long as the reference is never returned by any function within the
module, no outside module can see the subroutine, because its name is not in
any package's symbol table. Remember that it's not I<REALLY> called
-$some_pack::secret_version or anything; it's just $secret_version,
+C<$some_pack::secret_version> or anything; it's just $secret_version,
unqualified and unqualifiable.
-This does not work with object methods, however; all object methods have
-to be in the symbol table of some package to be found.
-
-Just because the lexical variable is lexically (also called statically)
-scoped doesn't mean that within a function it works like a C static. It
-normally works more like a C auto. But here's a mechanism for giving a
-function private variables with both lexical scoping and a static
-lifetime. If you do want to create something like C's static variables,
-just enclose the whole function in an extra block, and put the
-static variable outside the function but in the block.
+This does not work with object methods, however; all object methods
+have to be in the symbol table of some package to be found. See
+L<perlref/"Function Templates"> for something of a work-around to
+this.
+
+=head2 Persistent Private Variables
+
+Just because a lexical variable is lexically (also called statically)
+scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that
+within a function it works like a C static. It normally works more
+like a C auto, but with implicit garbage collection.
+
+Unlike local variables in C or C++, Perl's lexical variables don't
+necessarily get recycled just because their scope has exited.
+If something more permanent is still aware of the lexical, it will
+stick around. So long as something else references a lexical, that
+lexical won't be freed--which is as it should be. You wouldn't want
+memory being free until you were done using it, or kept around once you
+were done. Automatic garbage collection takes care of this for you.
+
+This means that you can pass back or save away references to lexical
+variables, whereas to return a pointer to a C auto is a grave error.
+It also gives us a way to simulate C's function statics. Here's a
+mechanism for giving a function private variables with both lexical
+scoping and a static lifetime. If you do want to create something like
+C's static variables, just enclose the whole function in an extra block,
+and put the static variable outside the function but in the block.
{
- my $secret_val = 0;
+ my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
- }
- }
+ }
+ }
# $secret_val now becomes unreachable by the outside
# world, but retains its value between calls to gimme_another
-If this function is being sourced in from a separate file
+If this function is being sourced in from a separate file
via C<require> or C<use>, then this is probably just fine. If it's
-all in the main program, you'll need to arrange for the my()
+all in the main program, you'll need to arrange for the C<my>
to be executed early, either by putting the whole block above
-your pain program, or more likely, placing merely a BEGIN
+your main program, or more likely, placing merely a C<BEGIN>
sub around it to make sure it gets executed before your program
starts to run:
sub BEGIN {
- my $secret_val = 0;
+ my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
- }
- }
+ }
+ }
-See L<perlrun> about the BEGIN function.
+See L<perlmod/"Package Constructors and Destructors"> about the
+special triggered functions, C<BEGIN> and C<INIT>.
+
+If declared at the outermost scope (the file scope), then lexicals
+work somewhat like C's file statics. They are available to all
+functions in that same file declared below them, but are inaccessible
+from outside that file. This strategy is sometimes used in modules
+to create private variables that the whole module can see.
=head2 Temporary Values via local()
-B<NOTE>: In general, you should be using "my" instead of "local", because
+B<WARNING>: In general, you should be using C<my> instead of C<local>, because
it's faster and safer. Exceptions to this include the global punctuation
variables, filehandles and formats, and direct manipulation of the Perl
-symbol table itself. Format variables often use "local" though, as do
+symbol table itself. Format variables often use C<local> though, as do
other variables whose current value must be visible to called
subroutines.
local *merlyn = *randal; # now $merlyn is really $randal, plus
# @merlyn is really @randal, etc
local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
- local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
+ local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
-A local() modifies its listed variables to be local to the enclosing
-block, (or subroutine, C<eval{}> or C<do>) and I<any called from
-within that block>. A local() just gives temporary values to global
-(meaning package) variables. This is known as dynamic scoping. Lexical
-scoping is done with "my", which works more like C's auto declarations.
+A C<local> modifies its listed variables to be "local" to the
+enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine
+called from within that block>. A C<local> just gives temporary
+values to global (meaning package) variables. It does I<not> create
+a local variable. This is known as dynamic scoping. Lexical scoping
+is done with C<my>, which works more like C's auto declarations.
-If more than one variable is given to local(), they must be placed in
+If more than one variable is given to C<local>, they must be placed in
parentheses. All listed elements must be legal lvalues. This operator works
by saving the current values of those variables in its argument list on a
-hidden stack and restoring them upon exiting the block, subroutine or
+hidden stack and restoring them upon exiting the block, subroutine, or
eval. This means that called subroutines can also reference the local
variable, but not the global one. The argument list may be assigned to if
desired, which allows you to initialize your local variables. (If no
for $i ( 0 .. 9 ) {
$digits{$i} = $i;
- }
+ }
# assume this function uses global %digits hash
- parse_num();
+ parse_num();
# now temporarily add to %digits hash
if ($base12) {
}
# old %digits restored here
-Because local() is a run-time command, it gets executed every time
+Because C<local> is a run-time operator, it gets executed each time
through a loop. In releases of Perl previous to 5.0, this used more stack
storage each time until the loop was exited. Perl now reclaims the space
each time through, but it's still more efficient to declare your variables
outside the loop.
-A local is simply a modifier on an lvalue expression. When you assign to
-a localized variable, the local doesn't change whether its list is viewed
+A C<local> is simply a modifier on an lvalue expression. When you assign to
+a C<local>ized variable, the C<local> doesn't change whether its list is viewed
as a scalar or an array. So
local($foo) = <STDIN>;
supplies a scalar context.
+A note about C<local()> and composite types is in order. Something
+like C<local(%foo)> works by temporarily placing a brand new hash in
+the symbol table. The old hash is left alone, but is hidden "behind"
+the new one.
+
+This means the old variable is completely invisible via the symbol
+table (i.e. the hash entry in the C<*foo> typeglob) for the duration
+of the dynamic scope within which the C<local()> was seen. This
+has the effect of allowing one to temporarily occlude any magic on
+composite types. For instance, this will briefly alter a tied
+hash to some other implementation:
+
+ tie %ahash, 'APackage';
+ [...]
+ {
+ local %ahash;
+ tie %ahash, 'BPackage';
+ [..called code will see %ahash tied to 'BPackage'..]
+ {
+ local %ahash;
+ [..%ahash is a normal (untied) hash here..]
+ }
+ }
+ [..%ahash back to its initial tied self again..]
+
+As another example, a custom implementation of C<%ENV> might look
+like this:
+
+ {
+ local %ENV;
+ tie %ENV, 'MyOwnEnv';
+ [..do your own fancy %ENV manipulation here..]
+ }
+ [..normal %ENV behavior here..]
+
+It's also worth taking a moment to explain what happens when you
+C<local>ize a member of a composite type (i.e. an array or hash element).
+In this case, the element is C<local>ized I<by name>. This means that
+when the scope of the C<local()> ends, the saved value will be
+restored to the hash element whose key was named in the C<local()>, or
+the array element whose index was named in the C<local()>. If that
+element was deleted while the C<local()> was in effect (e.g. by a
+C<delete()> from a hash or a C<shift()> of an array), it will spring
+back into existence, possibly extending an array and filling in the
+skipped elements with C<undef>. For instance, if you say
+
+ %hash = ( 'This' => 'is', 'a' => 'test' );
+ @ary = ( 0..5 );
+ {
+ local($ary[5]) = 6;
+ local($hash{'a'}) = 'drill';
+ while (my $e = pop(@ary)) {
+ print "$e . . .\n";
+ last unless $e > 3;
+ }
+ if (@ary) {
+ $hash{'only a'} = 'test';
+ delete $hash{'a'};
+ }
+ }
+ print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
+ print "The array has ",scalar(@ary)," elements: ",
+ join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
+
+Perl will print
+
+ 6 . . .
+ 4 . . .
+ 3 . . .
+ This is a test only a test.
+ The array has 6 elements: 0, 1, 2, undef, undef, 5
+
+The behavior of local() on non-existent members of composite
+types is subject to change in future.
+
+=head2 Lvalue subroutines
+
+B<WARNING>: Lvalue subroutines are still experimental and the implementation
+may change in future versions of Perl.
+
+It is possible to return a modifiable value from a subroutine.
+To do this, you have to declare the subroutine to return an lvalue.
+
+ my $val;
+ sub canmod : lvalue {
+ $val;
+ }
+ sub nomod {
+ $val;
+ }
+
+ canmod() = 5; # assigns to $val
+ nomod() = 5; # ERROR
+
+The scalar/list context for the subroutine and for the right-hand
+side of assignment is determined as if the subroutine call is replaced
+by a scalar. For example, consider:
+
+ data(2,3) = get_data(3,4);
+
+Both subroutines here are called in a scalar context, while in:
+
+ (data(2,3)) = get_data(3,4);
+
+and in:
+
+ (data(2),data(3)) = get_data(3,4);
+
+all the subroutines are called in a list context.
+
+The current implementation does not allow arrays and hashes to be
+returned from lvalue subroutines directly. You may return a
+reference instead. This restriction may be lifted in future.
+
=head2 Passing Symbol Table Entries (typeglobs)
-[Note: The mechanism described in this section was originally the only
-way to simulate pass-by-reference in older versions of Perl. While it
-still works fine in modern versions, the new reference mechanism is
-generally easier to work with. See below.]
+B<WARNING>: The mechanism described in this section was originally
+the only way to simulate pass-by-reference in older versions of
+Perl. While it still works fine in modern versions, the new reference
+mechanism is generally easier to work with. See below.
Sometimes you don't want to pass the value of an array to a subroutine
but rather the name of it, so that the subroutine can modify the global
funny prefix characters on variables and subroutines and such.
When evaluated, the typeglob produces a scalar value that represents
-all the objects of that name, including any filehandle, format or
+all the objects of that name, including any filehandle, format, or
subroutine. When assigned to, it causes the name mentioned to refer to
-whatever "*" value was assigned to it. Example:
+whatever C<*> value was assigned to it. Example:
sub doubleary {
local(*someary) = @_;
doubleary(*foo);
doubleary(*bar);
-Note that scalars are already passed by reference, so you can modify
+Scalars are already passed by reference, so you can modify
scalar arguments without using this mechanism by referring explicitly
to C<$_[0]> etc. You can modify all the elements of an array by passing
-all the elements as scalars, but you have to use the * mechanism (or
-the equivalent reference mechanism) to push, pop or change the size of
+all the elements as scalars, but you have to use the C<*> mechanism (or
+the equivalent reference mechanism) to C<push>, C<pop>, or change the size of
an array. It will certainly be faster to pass the typeglob (or reference).
Even if you don't want to modify an array, this mechanism is useful for
passing multiple arrays in a single LIST, because normally the LIST
mechanism will merge all the array values so that you can't extract out
the individual arrays. For more on typeglobs, see
-L<perldata/"Typeglobs and FileHandles">.
+L<perldata/"Typeglobs and Filehandles">.
+
+=head2 When to Still Use local()
+
+Despite the existence of C<my>, there are still three places where the
+C<local> operator still shines. In fact, in these three places, you
+I<must> use C<local> instead of C<my>.
+
+=over
+
+=item 1. You need to give a global variable a temporary value, especially $_.
+
+The global variables, like C<@ARGV> or the punctuation variables, must be
+C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
+it up into chunks separated by lines of equal signs, which are placed
+in C<@Fields>.
+
+ {
+ local @ARGV = ("/etc/motd");
+ local $/ = undef;
+ local $_ = <>;
+ @Fields = split /^\s*=+\s*$/;
+ }
+
+It particular, it's important to C<local>ize $_ in any routine that assigns
+to it. Look out for implicit assignments in C<while> conditionals.
+
+=item 2. You need to create a local file or directory handle or a local function.
+
+A function that needs a filehandle of its own must use
+C<local()> on a complete typeglob. This can be used to create new symbol
+table entries:
+
+ sub ioqueue {
+ local (*READER, *WRITER); # not my!
+ pipe (READER, WRITER); or die "pipe: $!";
+ return (*READER, *WRITER);
+ }
+ ($head, $tail) = ioqueue();
+
+See the Symbol module for a way to create anonymous symbol table
+entries.
+
+Because assignment of a reference to a typeglob creates an alias, this
+can be used to create what is effectively a local function, or at least,
+a local alias.
+
+ {
+ local *grow = \&shrink; # only until this block exists
+ grow(); # really calls shrink()
+ move(); # if move() grow()s, it shrink()s too
+ }
+ grow(); # get the real grow() again
+
+See L<perlref/"Function Templates"> for more about manipulating
+functions by name in this way.
+
+=item 3. You want to temporarily change just one element of an array or hash.
+
+You can C<local>ize just one element of an aggregate. Usually this
+is done on dynamics:
+
+ {
+ local $SIG{INT} = 'IGNORE';
+ funct(); # uninterruptible
+ }
+ # interruptibility automatically restored here
+
+But it also works on lexically declared aggregates. Prior to 5.005,
+this operation could on occasion misbehave.
+
+=back
=head2 Pass by Reference
do that, you need to understand references as detailed in L<perlref>.
This section may not make much sense to you otherwise.
-Here are a few simple examples. First, let's pass in several
-arrays to a function and have it pop all of then, return a new
-list of all their former last elements:
+Here are a few simple examples. First, let's pass in several arrays
+to a function and have it C<pop> all of then, returning a new list
+of all their former last elements:
@tailings = popmany ( \@a, \@b, \@c, \@d );
my @retlist = ();
foreach $aref ( @_ ) {
push @retlist, pop @$aref;
- }
+ }
return @retlist;
- }
+ }
-Here's how you might write a function that returns a
+Here's how you might write a function that returns a
list of keys occurring in all the hashes passed to it:
- @common = inter( \%foo, \%bar, \%joe );
+ @common = inter( \%foo, \%bar, \%joe );
sub inter {
my ($k, $href, %seen); # locals
foreach $href (@_) {
while ( $k = each %$href ) {
$seen{$k}++;
- }
- }
+ }
+ }
return grep { $seen{$_} == @_ } keys %seen;
- }
+ }
So far, we're using just the normal list return mechanism.
-What happens if you want to pass or return a hash? Well,
-if you're using only one of them, or you don't mind them
+What happens if you want to pass or return a hash? Well,
+if you're using only one of them, or you don't mind them
concatenating, then the normal calling convention is ok, although
-a little expensive.
+a little expensive.
Where people get into trouble is here:
or
(%a, %b) = func(%c, %d);
-That syntax simply won't work. It sets just @a or %a and clears the @b or
-%b. Plus the function didn't get passed into two separate arrays or
-hashes: it got one long list in @_, as always.
+That syntax simply won't work. It sets just C<@a> or C<%a> and
+clears the C<@b> or C<%b>. Plus the function didn't get passed
+into two separate arrays or hashes: it got one long list in C<@_>,
+as always.
If you can arrange for everyone to deal with this through references, it's
cleaner code, although not so nice to look at. Here's a function that
return ($cref, $dref);
} else {
return ($dref, $cref);
- }
- }
+ }
+ }
It turns out that you can actually do this also:
return (\@c, \@d);
} else {
return (\@d, \@c);
- }
- }
+ }
+ }
Here we're using the typeglobs to do symbol table aliasing. It's
-a tad subtle, though, and also won't work if you're using my()
-variables, because only globals (well, and local()s) are in the symbol table.
+a tad subtle, though, and also won't work if you're using C<my>
+variables, because only globals (even in disguise as C<local>s)
+are in the symbol table.
+
+If you're passing around filehandles, you could usually just use the bare
+typeglob, like C<*STDOUT>, but typeglobs references work, too.
+For example:
+
+ splutter(\*STDOUT);
+ sub splutter {
+ my $fh = shift;
+ print $fh "her um well a hmmm\n";
+ }
+
+ $rec = get_rec(\*STDIN);
+ sub get_rec {
+ my $fh = shift;
+ return scalar <$fh>;
+ }
+
+If you're planning on generating new filehandles, you could do this.
+Notice to pass back just the bare *FH, not its reference.
+
+ sub openit {
+ my $path = shift;
+ local *FH;
+ return open (FH, $path) ? *FH : undef;
+ }
=head2 Prototypes
-As of the 5.002 release of perl, if you declare
+Perl supports a very limited kind of compile-time argument checking
+using function prototyping. If you declare
sub mypush (\@@)
-then mypush() takes arguments exactly like push() does. The declaration
-of the function to be called must be visible at compile time. The prototype
-affects only the interpretation of new-style calls to the function, where
-new-style is defined as not using the C<&> character. In other words,
-if you call it like a builtin function, then it behaves like a builtin
-function. If you call it like an old-fashioned subroutine, then it
-behaves like an old-fashioned subroutine. It naturally falls out from
-this rule that prototypes have no influence on subroutine references
-like C<\&foo> or on indirect subroutine calls like C<&{$subref}>.
+then C<mypush()> takes arguments exactly like C<push()> does. The
+function declaration must be visible at compile time. The prototype
+affects only interpretation of new-style calls to the function,
+where new-style is defined as not using the C<&> character. In
+other words, if you call it like a built-in function, then it behaves
+like a built-in function. If you call it like an old-fashioned
+subroutine, then it behaves like an old-fashioned subroutine. It
+naturally falls out from this rule that prototypes have no influence
+on subroutine references like C<\&foo> or on indirect subroutine
+calls like C<&{$subref}> or C<$subref-E<gt>()>.
Method calls are not influenced by prototypes either, because the
-function to be called is indeterminate at compile time, because it depends
-on inheritance.
+function to be called is indeterminate at compile time, since
+the exact code called depends on inheritance.
-Because the intent is primarily to let you define subroutines that work
-like builtin commands, here are the prototypes for some other functions
-that parse almost exactly like the corresponding builtins.
+Because the intent of this feature is primarily to let you define
+subroutines that work like built-in functions, here are prototypes
+for some other functions that parse almost exactly like the
+corresponding built-in.
Declared as Called as
- sub mylink ($$) mylink $old, $new
- sub myvec ($$$) myvec $var, $offset, 1
- sub myindex ($$;$) myindex &getstring, "substr"
- sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
- sub myreverse (@) myreverse $a,$b,$c
- sub myjoin ($@) myjoin ":",$a,$b,$c
- sub mypop (\@) mypop @array
- sub mysplice (\@$$@) mysplice @array,@array,0,@pushme
- sub mykeys (\%) mykeys %{$hashref}
- sub myopen (*;$) myopen HANDLE, $name
- sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
- sub mygrep (&@) mygrep { /foo/ } $a,$b,$c
- sub myrand ($) myrand 42
- sub mytime () mytime
+ sub mylink ($$) mylink $old, $new
+ sub myvec ($$$) myvec $var, $offset, 1
+ sub myindex ($$;$) myindex &getstring, "substr"
+ sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
+ sub myreverse (@) myreverse $a, $b, $c
+ sub myjoin ($@) myjoin ":", $a, $b, $c
+ sub mypop (\@) mypop @array
+ sub mysplice (\@$$@) mysplice @array, @array, 0, @pushme
+ sub mykeys (\%) mykeys %{$hashref}
+ sub myopen (*;$) myopen HANDLE, $name
+ sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
+ sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
+ sub myrand ($) myrand 42
+ sub mytime () mytime
Any backslashed prototype character represents an actual argument
that absolutely must start with that character. The value passed
-to the subroutine (as part of C<@_>) will be a reference to the
-actual argument given in the subroutine call, obtained by applying
-C<\> to that argument.
+as part of C<@_> will be a reference to the actual argument given
+in the subroutine call, obtained by applying C<\> to that argument.
Unbackslashed prototype characters have special meanings. Any
-unbackslashed @ or % eats all the rest of the arguments, and forces
-list context. An argument represented by $ forces scalar context. An
-& requires an anonymous subroutine, which, if passed as the first
-argument, does not require the "sub" keyword or a subsequent comma. A
-* does whatever it has to do to turn the argument into a reference to a
-symbol table entry.
+unbackslashed C<@> or C<%> eats all remaining arguments, and forces
+list context. An argument represented by C<$> forces scalar context. An
+C<&> requires an anonymous subroutine, which, if passed as the first
+argument, does not require the C<sub> keyword or a subsequent comma. A
+C<*> allows the subroutine to accept a bareword, constant, scalar expression,
+typeglob, or a reference to a typeglob in that slot. The value will be
+available to the subroutine either as a simple scalar, or (in the latter
+two cases) as a reference to the typeglob.
A semicolon separates mandatory arguments from optional arguments.
-(It is redundant before @ or %.)
+It is redundant before C<@> or C<%>, which gobble up everything else.
-Note how the last three examples above are treated specially by the parser.
-mygrep() is parsed as a true list operator, myrand() is parsed as a
-true unary operator with unary precedence the same as rand(), and
-mytime() is truly without arguments, just like time(). That is, if you
-say
+Note how the last three examples in the table above are treated
+specially by the parser. C<mygrep()> is parsed as a true list
+operator, C<myrand()> is parsed as a true unary operator with unary
+precedence the same as C<rand()>, and C<mytime()> is truly without
+arguments, just like C<time()>. That is, if you say
mytime +2;
-you'll get mytime() + 2, not mytime(2), which is how it would be parsed
-without the prototype.
+you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
+without a prototype.
-The interesting thing about & is that you can generate new syntax with it:
+The interesting thing about C<&> is that you can generate new syntax with it,
+provided it's in the initial position:
sub try (&@) {
my($try,$catch) = @_;
/phooey/ and print "unphooey\n";
};
-That prints "unphooey". (Yes, there are still unresolved
-issues having to do with the visibility of @_. I'm ignoring that
-question for the moment. (But note that if we make @_ lexically
+That prints C<"unphooey">. (Yes, there are still unresolved
+issues having to do with visibility of C<@_>. I'm ignoring that
+question for the moment. (But note that if we make C<@_> lexically
scoped, those anonymous subroutines can act like closures... (Gee,
is this sounding a little Lispish? (Never mind.))))
-And here's a reimplementation of grep:
+And here's a reimplementation of the Perl C<grep> operator:
sub mygrep (&@) {
my $code = shift;
sub func ($) {
my $n = shift;
print "you gave me $n\n";
- }
+ }
and someone has been calling it with an array or expression
returning a list:
func(@foo);
func( split /:/ );
-Then you've just supplied an automatic scalar() in front of their
-argument, which can be more than a bit surprising. The old @foo
+Then you've just supplied an automatic C<scalar> in front of their
+argument, which can be more than a bit surprising. The old C<@foo>
which used to hold one thing doesn't get passed in. Instead,
-the func() now gets passed in 1, that is, the number of elements
-in @foo. And the split() gets called in a scalar context and
-starts scribbling on your @_ parameter list.
+C<func()> now gets passed in a C<1>; that is, the number of elements
+in C<@foo>. And the C<split> gets called in scalar context so it
+starts scribbling on your C<@_> parameter list. Ouch!
This is all very powerful, of course, and should be used only in moderation
-to make the world a better place.
+to make the world a better place.
+
+=head2 Constant Functions
+
+Functions with a prototype of C<()> are potential candidates for
+inlining. If the result after optimization and constant folding
+is either a constant or a lexically-scoped scalar which has no other
+references, then it will be used in place of function calls made
+without C<&>. Calls made using C<&> are never inlined. (See
+F<constant.pm> for an easy way to declare most constants.)
+
+The following functions would all be inlined:
+
+ sub pi () { 3.14159 } # Not exact, but close.
+ sub PI () { 4 * atan2 1, 1 } # As good as it gets,
+ # and it's inlined, too!
+ sub ST_DEV () { 0 }
+ sub ST_INO () { 1 }
+
+ sub FLAG_FOO () { 1 << 8 }
+ sub FLAG_BAR () { 1 << 9 }
+ sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
+
+ sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
+ sub BAZ_VAL () {
+ if (OPT_BAZ) {
+ return 23;
+ }
+ else {
+ return 42;
+ }
+ }
+
+ sub N () { int(BAZ_VAL) / 3 }
+ BEGIN {
+ my $prod = 1;
+ for (1..N) { $prod *= $_ }
+ sub N_FACTORIAL () { $prod }
+ }
+
+If you redefine a subroutine that was eligible for inlining, you'll get
+a mandatory warning. (You can use this warning to tell whether or not a
+particular subroutine is considered constant.) The warning is
+considered severe enough not to be optional because previously compiled
+invocations of the function will still be using the old value of the
+function. If you need to be able to redefine the subroutine, you need to
+ensure that it isn't inlined, either by dropping the C<()> prototype
+(which changes calling semantics, so beware) or by thwarting the
+inlining mechanism in some other way, such as
+
+ sub not_inlined () {
+ 23 if $];
+ }
-=head2 Overriding Builtin Functions
+=head2 Overriding Built-in Functions
-Many builtin functions may be overridden, though this should be tried
+Many built-in functions may be overridden, though this should be tried
only occasionally and for good reason. Typically this might be
-done by a package attempting to emulate missing builtin functionality
+done by a package attempting to emulate missing built-in functionality
on a non-Unix system.
Overriding may be done only by importing the name from a
module--ordinary predeclaration isn't good enough. However, the
-C<subs> pragma (compiler directive) lets you, in effect, pre-declare subs
-via the import syntax, and these names may then override the builtin ones:
+C<use subs> pragma lets you, in effect, predeclare subs
+via the import syntax, and these names may then override built-in ones:
use subs 'chdir', 'chroot', 'chmod', 'chown';
chdir $somewhere;
sub chdir { ... }
-Library modules should not in general export builtin names like "open"
-or "chdir" as part of their default @EXPORT list, because these may
+To unambiguously refer to the built-in form, precede the
+built-in name with the special package qualifier C<CORE::>. For example,
+saying C<CORE::open()> always refers to the built-in C<open()>, even
+if the current package has imported some other subroutine called
+C<&open()> from elsewhere. Even though it looks like a regular
+function call, it isn't: you can't take a reference to it, such as
+the incorrect C<\&CORE::open> might appear to produce.
+
+Library modules should not in general export built-in names like C<open>
+or C<chdir> as part of their default C<@EXPORT> list, because these may
sneak into someone else's namespace and change the semantics unexpectedly.
-Instead, if the module adds the name to the @EXPORT_OK list, then it's
+Instead, if the module adds that name to C<@EXPORT_OK>, then it's
possible for a user to import the name explicitly, but not implicitly.
That is, they could say
use Module 'open';
-and it would import the open override, but if they said
+and it would import the C<open> override. But if they said
use Module;
-they would get the default imports without the overrides.
+they would get the default imports without overrides.
+
+The foregoing mechanism for overriding built-in is restricted, quite
+deliberately, to the package that requests the import. There is a second
+method that is sometimes applicable when you wish to override a built-in
+everywhere, without regard to namespace boundaries. This is achieved by
+importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an
+example that quite brazenly replaces the C<glob> operator with something
+that understands regular expressions.
+
+ package REGlob;
+ require Exporter;
+ @ISA = 'Exporter';
+ @EXPORT_OK = 'glob';
+
+ sub import {
+ my $pkg = shift;
+ return unless @_;
+ my $sym = shift;
+ my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
+ $pkg->export($where, $sym, @_);
+ }
+
+ sub glob {
+ my $pat = shift;
+ my @got;
+ local *D;
+ if (opendir D, '.') {
+ @got = grep /$pat/, readdir D;
+ closedir D;
+ }
+ return @got;
+ }
+ 1;
+
+And here's how it could be (ab)used:
+
+ #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
+ package Foo;
+ use REGlob 'glob'; # override glob() in Foo:: only
+ print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
+
+The initial comment shows a contrived, even dangerous example.
+By overriding C<glob> globally, you would be forcing the new (and
+subversive) behavior for the C<glob> operator for I<every> namespace,
+without the complete cognizance or cooperation of the modules that own
+those namespaces. Naturally, this should be done with extreme caution--if
+it must be done at all.
+
+The C<REGlob> example above does not implement all the support needed to
+cleanly override perl's C<glob> operator. The built-in C<glob> has
+different behaviors depending on whether it appears in a scalar or list
+context, but our C<REGlob> doesn't. Indeed, many perl built-in have such
+context sensitive behaviors, and these must be adequately supported by
+a properly written override. For a fully functional example of overriding
+C<glob>, study the implementation of C<File::DosGlob> in the standard
+library.
=head2 Autoloading
-If you call a subroutine that is undefined, you would ordinarily get an
-immediate fatal error complaining that the subroutine doesn't exist.
-(Likewise for subroutines being used as methods, when the method
-doesn't exist in any of the base classes of the class package.) If,
-however, there is an C<AUTOLOAD> subroutine defined in the package or
-packages that were searched for the original subroutine, then that
-C<AUTOLOAD> subroutine is called with the arguments that would have been
-passed to the original subroutine. The fully qualified name of the
-original subroutine magically appears in the $AUTOLOAD variable in the
-same package as the C<AUTOLOAD> routine. The name is not passed as an
-ordinary argument because, er, well, just because, that's why...
-
-Most C<AUTOLOAD> routines will load in a definition for the subroutine in
-question using eval, and then execute that subroutine using a special
-form of "goto" that erases the stack frame of the C<AUTOLOAD> routine
-without a trace. (See the standard C<AutoLoader> module, for example.)
-But an C<AUTOLOAD> routine can also just emulate the routine and never
-define it. For example, let's pretend that a function that wasn't defined
-should just call system() with those arguments. All you'd do is this:
+If you call a subroutine that is undefined, you would ordinarily
+get an immediate, fatal error complaining that the subroutine doesn't
+exist. (Likewise for subroutines being used as methods, when the
+method doesn't exist in any base class of the class's package.)
+However, if an C<AUTOLOAD> subroutine is defined in the package or
+packages used to locate the original subroutine, then that
+C<AUTOLOAD> subroutine is called with the arguments that would have
+been passed to the original subroutine. The fully qualified name
+of the original subroutine magically appears in the global $AUTOLOAD
+variable of the same package as the C<AUTOLOAD> routine. The name
+is not passed as an ordinary argument because, er, well, just
+because, that's why...
+
+Many C<AUTOLOAD> routines load in a definition for the requested
+subroutine using eval(), then execute that subroutine using a special
+form of goto() that erases the stack frame of the C<AUTOLOAD> routine
+without a trace. (See the source to the standard module documented
+in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can
+also just emulate the routine and never define it. For example,
+let's pretend that a function that wasn't defined should just invoke
+C<system> with those arguments. All you'd do is:
sub AUTOLOAD {
my $program = $AUTOLOAD;
$program =~ s/.*:://;
system($program, @_);
- }
+ }
date();
who('am', 'i');
ls('-l');
-In fact, if you pre-declare the functions you want to call that way, you don't
-even need the parentheses:
+In fact, if you predeclare functions you want to call that way, you don't
+even need parentheses:
use subs qw(date who ls);
date;
ls -l;
A more complete example of this is the standard Shell module, which
-can treat undefined subroutine calls as calls to Unix programs.
+can treat undefined subroutine calls as calls to external programs.
-Mechanisms are available for modules writers to help split the modules
-up into autoloadable files. See the standard AutoLoader module
+Mechanisms are available to help modules writers split their modules
+into autoloadable files. See the standard AutoLoader module
described in L<AutoLoader> and in L<AutoSplit>, the standard
SelfLoader modules in L<SelfLoader>, and the document on adding C
-functions to perl code in L<perlxs>.
+functions to Perl code in L<perlxs>.
+
+=head2 Subroutine Attributes
+
+A subroutine declaration or definition may have a list of attributes
+associated with it. If such an attribute list is present, it is
+broken up at space or comma boundaries and treated as though a
+C<use attributes> had been seen. See L<attributes> for details
+about what attributes are currently supported.
+Unlike the limitation with the obsolescent C<use attrs>, the
+C<sub : ATTRLIST> syntax works to associate the attributes with
+a pre-declaration, and not just with a subroutine definition.
+
+The attributes must be valid as simple identifier names (without any
+punctuation other than the '_' character). They may have a parameter
+list appended, which is only checked for whether its parentheses ('(',')')
+nest properly.
+
+Examples of valid syntax (even though the attributes are unknown):
+
+ sub fnord (&\%) : switch(10,foo(7,3)) , , expensive ;
+ sub plugh () : Ugly('\(") , Bad ;
+ sub xyzzy : _5x5 { ... }
+
+Examples of invalid syntax:
+
+ sub fnord : switch(10,foo() ; # ()-string not balanced
+ sub snoid : Ugly('(') ; # ()-string not balanced
+ sub xyzzy : 5x5 ; # "5x5" not a valid identifier
+ sub plugh : Y2::north ; # "Y2::north" not a simple identifier
+ sub snurt : foo + bar ; # "+" not a comma or space
+
+The attribute list is passed as a list of constant strings to the code
+which associates them with the subroutine. In particular, the second example
+of valid syntax above currently looks like this in terms of how it's
+parsed and invoked:
+
+ use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
+
+For further details on attribute lists and their manipulation,
+see L<attributes>.
=head1 SEE ALSO
-See L<perlref> for more on references. See L<perlxs> if you'd
-like to learn about calling C subroutines from perl. See
-L<perlmod> to learn about bundling up your functions in
-separate files.
+See L<perlref/"Function Templates"> for more about references and closures.
+See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
+See L<perlembed> if you'd like to learn about calling PErl subroutines from C.
+See L<perlmod> to learn about bundling up your functions in separate files.
+See L<perlmodlib> to learn what library modules come standard on your system.
+See L<perltoot> to learn how to make object method calls.