=head1 NAME
+X<subroutine> X<function>
perlsub - Perl subroutines
=head1 SYNOPSIS
To declare subroutines:
+X<subroutine, declaration> X<sub>
- sub NAME; # A "forward" declaration.
- sub NAME BLOCK # A declaration and a definition.
+ 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 : ATTRS BLOCK # with attributes
+ sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes
To define an anonymous subroutine at runtime:
+X<subroutine, anonymous>
- $subref = sub BLOCK;
+ $subref = sub BLOCK; # no proto
+ $subref = sub (PROTO) BLOCK; # with proto
+ $subref = sub : ATTRS BLOCK; # with attributes
+ $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
To import subroutines:
+X<import>
- use PACKAGE qw(NAME1 NAME2 NAME3);
+ use MODULE qw(NAME1 NAME2 NAME3);
To call subroutines:
+X<subroutine, call> X<call>
- &NAME # Passes current @_ to subroutine.
- &NAME(LIST); # Parens required with & form.
- NAME(LIST); # & is optional with parens.
- NAME LIST; # Parens optional if predeclared/imported.
+ NAME(LIST); # & is optional with parentheses.
+ NAME LIST; # Parentheses optional if predeclared/imported.
+ &NAME(LIST); # Circumvent prototypes.
+ &NAME; # Makes current @_ visible to called subroutine.
=head1 DESCRIPTION
-Any arguments passed to the routine come in as array @_, that is
-($_[0], $_[1], ...). The array @_ is a local array, but its values are
-references to the actual scalar parameters. The return value of the
-subroutine is the value of the last expression evaluated, and can be
-either an array value or a scalar value. Alternatively, a return
-statement may be used to specify the returned value and exit the
-subroutine. To create local variables see the local() and my()
-operators.
-
-A subroutine may be called using the "&" prefix. The "&" is optional in Perl
-5, and so are the parens if the subroutine has been predeclared.
-(Note, however, that the "&" is I<NOT> optional when you're just naming the
-subroutine, such as when it's used as an argument to defined() or
-undef(). Nor is it optional when you want to do an indirect subroutine
-call with a subroutine name or reference using the C<&$subref()> or
-C<&{$subref}()> constructs. See L<perlref> for more on that.)
+Like many languages, Perl provides for user-defined subroutines.
+These may be located anywhere in the main program, loaded in from
+other files via the C<do>, C<require>, or C<use> keywords, or
+generated on the fly using C<eval> or anonymous subroutines.
+You can even call a function indirectly using a variable containing
+its name or a CODE reference.
+
+The Perl model for function call and return values is simple: all
+functions are passed as parameters one single flat list of scalars, and
+all functions likewise return to their caller one single flat list of
+scalars. Any arrays or hashes in these call and return lists will
+collapse, losing their identities--but you may always use
+pass-by-reference instead to avoid this. Both call and return lists may
+contain as many or as few scalar elements as you'd like. (Often a
+function without an explicit return statement is called a subroutine, but
+there's really no difference from Perl's perspective.)
+X<subroutine, parameter> X<parameter>
+
+Any arguments passed in show up in the array C<@_>. Therefore, if
+you called a function with two arguments, those would be stored in
+C<$_[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.
+X<subroutine, argument> X<argument> X<@_>
+
+A C<return> statement may be used to exit a 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.
+
+If no C<return> is found and if the last statement is an expression, its
+value is returned. If the last statement is a loop control structure
+like a C<foreach> or a C<while>, the returned value is unspecified. The
+empty sub returns the empty list.
+X<subroutine, return value> X<return value> X<return>
+
+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">.
+X<formal parameter> X<parameter, formal>
Example:
- sub MAX {
- my $max = pop(@_);
+ sub max {
+ my $max = shift(@_);
foreach $foo (@_) {
$max = $foo if $max < $foo;
}
- $max;
+ return $max;
}
-
- ...
- $bestday = &MAX($mon,$tue,$wed,$thu,$fri);
+ $bestday = max($mon,$tue,$wed,$thu,$fri);
Example:
# that start with whitespace
sub get_line {
- $thisline = $lookahead;
- LINE: while ($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};
+ $Foo{$key} = $value unless $Foo{$key};
+ }
+
+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.
+X<call-by-reference> X<call-by-value>
+
+ upcase_in($v1, $v2); # this changes $v1 and $v2
+ sub upcase_in {
+ for (@_) { tr/a-z/A-Z/ }
}
-This also has the effect of turning call-by-reference into
-call-by-value, since the assignment copies the values.
+You aren't allowed to modify constants in this way, of course. If an
+argument were actually literal and you tried to change it, you'd take a
+(presumably fatal) exception. For example, this won't work:
+X<call-by-reference> X<call-by-value>
+
+ upcase_in("frederick");
+
+It would be much safer if the C<upcase_in()> function
+were written to return a copy of its parameters instead
+of changing them in place:
+
+ ($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2
+ sub upcase {
+ return unless defined wantarray; # void context, do nothing
+ my @parms = @_;
+ 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 sees all arguments 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 /:/, $var );
+
+Do not, however, be tempted to do this:
+
+ (@a, @b) = upcase(@list1, @list2);
+
+Like the flattened incoming parameter list, the return list is also
+flattened on return. So all you have managed to do here is stored
+everything in C<@a> and made C<@b> empty. See
+L<Pass by Reference> for alternatives.
+
+A subroutine may be called using an explicit C<&> prefix. The
+C<&> is optional in modern Perl, as are parentheses if the
+subroutine has been predeclared. The C<&> is I<not> optional
+when just naming the subroutine, such as when it's used as
+an argument to defined() or undef(). Nor is it optional when you
+want to do an indirect subroutine call with a subroutine name or
+reference using the C<&$subref()> or C<&{$subref}()> constructs,
+although the C<< $subref->() >> notation solves that problem.
+See L<perlref> for more about all that.
+X<&>
-Subroutines may be called recursively. If a subroutine is called using
-the "&" form, the argument list is optional. If omitted, no @_ array is
-set up for the subroutine; the @_ array at the time of the call is
-visible to subroutine instead.
+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.
+X<recursion>
&foo(1,2,3); # pass three arguments
foo(1,2,3); # the same
foo(); # pass a null list
&foo(); # the same
- &foo; # pass no arguments--more efficient
-If a module wants to create a private subroutine that cannot be called
-from outside the module, it can declare a lexical variable containing
+ &foo; # foo() get current args, like foo(@_) !!
+ foo; # like foo() IFF sub foo predeclared, else "foo"
+
+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 L<Prototypes> below.
+X<&>
+
+Subroutines whose names are in all upper case are reserved to the Perl
+core, as are modules whose names are in all lower case. A subroutine 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.
+Subroutines that do special, pre-defined things include C<AUTOLOAD>, C<CLONE>,
+C<DESTROY> plus all functions mentioned in L<perltie> and L<PerlIO::via>.
+
+The C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> and C<END> subroutines
+are not so much subroutines as named special code blocks, of which you
+can have more than one in a package, and which you can B<not> call
+explicitly. See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">
+
+=head2 Private Variables via my()
+X<my> X<variable, lexical> X<lexical> X<lexical variable> X<scope, lexical>
+X<lexical scope> X<attributes, my>
+
+Synopsis:
+
+ my $foo; # declare $foo lexically local
+ 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
+ my $x : Foo = $y; # similar, with an attribute applied
+
+B<WARNING>: The use of attribute lists on C<my> declarations is still
+evolving. The current semantics and interface are subject to change.
+See L<attributes> and L<Attribute::Handlers>.
+
+The C<my> operator declares the listed variables to be lexically
+confined to the enclosing block, conditional (C<if/unless/elsif/else>),
+loop (C<for/foreach/while/until/continue>), subroutine, C<eval>,
+or C<do/require/use>'d file. If more than one value is listed, the
+list must be placed in parentheses. All listed elements must be
+legal lvalues. Only alphanumeric identifiers may be lexically
+scoped--magical built-ins like C<$/> must currently be C<local>ized
+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.
+X<local>
+
+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>.
+X<eval, scope of>
+
+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 input parameters to a subroutine. Examples:
+
+ $arg = "fred"; # "global" variable
+ $n = cube_root(27);
+ print "$arg thinks the root is $n\n";
+ fred thinks the root is 3
+
+ sub cube_root {
+ my $arg = shift; # name doesn't matter
+ $arg **= 1/3;
+ return $arg;
+ }
+
+The C<my> is simply a modifier on something you might assign to. So when
+you do assign to variables in its argument list, C<my> doesn't
+change whether those variables are viewed as a scalar or an array. So
+
+ my ($foo) = <STDIN>; # WRONG?
+ my @FOO = <STDIN>;
+
+both supply a list context to the right-hand side, while
+
+ my $foo = <STDIN>;
+
+supplies a scalar context. But the following declares only one variable:
+
+ my $foo, $bar = 1; # WRONG
+
+That has the same effect as
+
+ my $foo;
+ $bar = 1;
+
+The declared variable is not introduced (is not visible) until after
+the current statement. Thus,
+
+ my $x = $x;
+
+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 C<123>.
+
+Lexical scopes of control structures are not bounded precisely by the
+braces that delimit their controlled blocks; control expressions are
+part of that scope, too. Thus in the loop
+
+ while (my $line = <>) {
+ $line = lc $line;
+ } continue {
+ print $line;
+ }
+
+the scope of $line extends from its declaration throughout the rest of
+the loop construct (including the C<continue> clause), but not beyond
+it. Similarly, in the conditional
+
+ if ((my $answer = <STDIN>) =~ /^yes$/i) {
+ user_agrees();
+ } elsif ($answer =~ /^no$/i) {
+ user_disagrees();
+ } else {
+ chomp $answer;
+ die "'$answer' is neither 'yes' nor 'no'";
+ }
+
+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. See L<perlsyn/"Simple statements"> for information
+on the scope of variables in statements with modifiers.
+
+The C<foreach> loop defaults to scoping its index variable dynamically
+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
+X<foreach> X<for>
+
+ for my $i (1, 2, 3) {
+ 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()>.
+X<foreach> X<for>
+
+Some users may wish to encourage the use of lexically scoped variables.
+As an aid to catching implicit uses to package variables,
+which are always global, if you say
+
+ use strict 'vars';
+
+then any variable mentioned from there to the end of the enclosing
+block must either refer to a lexical variable, be predeclared via
+C<our> or 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 principal 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 $pack::var; # ERROR! Illegal syntax
+
+In fact, a dynamic variable (also known as package or global variables)
+are still accessible using the fully qualified C<::> notation even while a
+lexical of the same name is also visible:
+
+ package main;
+ local $x = 10;
+ my $x = 20;
+ print "$x and $::x\n";
+
+That will print out C<20> and C<10>.
+
+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 $subref = sub { ... }
- &$subref(1,2,3);
+ my $secret_version = '1.001-beta';
+ my $secret_sub = sub { print $secret_version };
+ &$secret_sub();
+
+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
+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. See
+L<perlref/"Function Templates"> for something of a work-around to
+this.
+
+=head2 Persistent Private Variables
+X<state> X<state variable> X<static> X<variable, persistent> X<variable, static> X<closure>
+
+There are two ways to build persistent private variables in Perl 5.10.
+First, you can simply use the C<state> feature. Or, you can use closures,
+if you want to stay compatible with releases older than 5.10.
+
+=head3 Persistent variables via state()
+
+Beginning with perl 5.9.4, you can declare variables with the C<state>
+keyword in place of C<my>. For that to work, though, you must have
+enabled that feature beforehand, either by using the C<feature> pragma, or
+by using C<-E> on one-liners. (see L<feature>)
+
+For example, the following code maintains a private counter, incremented
+each time the gimme_another() function is called:
+
+ use feature 'state';
+ sub gimme_another { state $x; return ++$x }
+
+Also, since C<$x> is lexical, it can't be reached or modified by any Perl
+code outside.
-As long as the reference is never returned by any function within the module,
-no outside module can see the subroutine, since its name is not in any
-package's symbol table.
+Be aware that assignment to C<state> variables (as in C<state $x = 42>)
+are executed every time; to initialize (or re-initialize) an undefined
+state scalar, you can use, for example, the defined-or assignment :
+
+ state $x //= initial_value();
+
+=head3 Persistent variables with closures
+
+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 $secret_val = 0;
+ sub gimme_another {
+ return ++$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
+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 C<my>
+to be executed early, either by putting the whole block above
+your main program, or more likely, placing merely a C<BEGIN>
+code block around it to make sure it gets executed before your program
+starts to run:
+
+ BEGIN {
+ my $secret_val = 0;
+ sub gimme_another {
+ return ++$secret_val;
+ }
+ }
+
+See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END"> about the
+special triggered code blocks, C<BEGIN>, C<UNITCHECK>, C<CHECK>,
+C<INIT> and C<END>.
+
+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()
+X<local> X<scope, dynamic> X<dynamic scope> X<variable, local>
+X<variable, temporary>
+
+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, global filehandles and formats, and direct manipulation of the
+Perl symbol table itself. C<local> is mostly used when the current value
+of a variable must be visible to called subroutines.
+
+Synopsis:
+
+ # localization of values
+
+ local $foo; # make $foo dynamically local
+ local (@wid, %get); # make list of variables local
+ local $foo = "flurp"; # make $foo dynamic, and init it
+ local @oof = @bar; # make @oof dynamic, and init it
+
+ local $hash{key} = "val"; # sets a local value for this hash entry
+ local ($cond ? $v1 : $v2); # several types of lvalues support
+ # localization
+
+ # localization of symbols
+
+ local *FH; # localize $FH, @FH, %FH, &FH ...
+ 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 $merlyn, not @merlyn etc
+
+A C<local> modifies its listed variables to be "local" to the
+enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine
+called from within that block>. A C<local> just gives temporary
+values to global (meaning package) variables. It does I<not> create
+a local variable. This is known as dynamic scoping. Lexical scoping
+is done with C<my>, which works more like C's auto declarations.
+
+Some types of lvalues can be localized as well : hash and array elements
+and slices, conditionals (provided that their result is always
+localizable), and symbolic references. As for simple variables, this
+creates new, dynamically scoped values.
+
+If more than one variable or expression is given to C<local>, they must be
+placed in parentheses. This operator works
+by saving the current values of those variables in its argument list on a
+hidden stack and restoring them upon exiting the block, subroutine, or
+eval. This means that called subroutines can also reference the local
+variable, but not the global one. The argument list may be assigned to if
+desired, which allows you to initialize your local variables. (If no
+initializer is given for a particular variable, it is created with an
+undefined value.)
+
+Because C<local> is a run-time operator, it gets executed each time
+through a loop. Consequently, it's more efficient to localize your
+variables outside the loop.
+
+=head3 Grammatical note on local()
+X<local, context>
+
+A C<local> is simply a modifier on an lvalue expression. When you assign to
+a C<local>ized variable, the C<local> doesn't change whether its list is viewed
+as a scalar or an array. So
+
+ local($foo) = <STDIN>;
+ local @FOO = <STDIN>;
+
+both supply a list context to the right-hand side, while
+
+ local $foo = <STDIN>;
+
+supplies a scalar context.
+
+=head3 Localization of special variables
+X<local, special variable>
+
+If you localize a special variable, you'll be giving a new value to it,
+but its magic won't go away. That means that all side-effects related
+to this magic still work with the localized value.
+
+This feature allows code like this to work :
+
+ # Read the whole contents of FILE in $slurp
+ { local $/ = undef; $slurp = <FILE>; }
+
+Note, however, that this restricts localization of some values ; for
+example, the following statement dies, as of perl 5.9.0, with an error
+I<Modification of a read-only value attempted>, because the $1 variable is
+magical and read-only :
+
+ local $1 = 2;
+
+Similarly, but in a way more difficult to spot, the following snippet will
+die in perl 5.9.0 :
+
+ sub f { local $_ = "foo"; print }
+ for ($1) {
+ # now $_ is aliased to $1, thus is magic and readonly
+ f();
+ }
+
+See next section for an alternative to this situation.
+
+B<WARNING>: Localization of tied arrays and hashes does not currently
+work as described.
+This will be fixed in a future release of Perl; in the meantime, avoid
+code that relies on any particular behaviour of localising tied arrays
+or hashes (localising individual elements is still okay).
+See L<perl58delta/"Localising Tied Arrays and Hashes Is Broken"> for more
+details.
+X<local, tie>
+
+=head3 Localization of globs
+X<local, glob> X<glob>
+
+The construct
+
+ local *name;
+
+creates a whole new symbol table entry for the glob C<name> in the
+current package. That means that all variables in its glob slot ($name,
+@name, %name, &name, and the C<name> filehandle) are dynamically reset.
+
+This implies, among other things, that any magic eventually carried by
+those variables is locally lost. In other words, saying C<local */>
+will not have any effect on the internal value of the input record
+separator.
+
+Notably, if you want to work with a brand new value of the default scalar
+$_, and avoid the potential problem listed above about $_ previously
+carrying a magic value, you should use C<local *_> instead of C<local $_>.
+As of perl 5.9.1, you can also use the lexical form of C<$_> (declaring it
+with C<my $_>), which avoids completely this problem.
+
+=head3 Localization of elements of composite types
+X<local, composite type element> X<local, array element> X<local, hash element>
+
+It's also worth taking a moment to explain what happens when you
+C<local>ize a member of a composite type (i.e. an array or hash element).
+In this case, the element is C<local>ized I<by name>. This means that
+when the scope of the C<local()> ends, the saved value will be
+restored to the hash element whose key was named in the C<local()>, or
+the array element whose index was named in the C<local()>. If that
+element was deleted while the C<local()> was in effect (e.g. by a
+C<delete()> from a hash or a C<shift()> of an array), it will spring
+back into existence, possibly extending an array and filling in the
+skipped elements with C<undef>. For instance, if you say
+
+ %hash = ( 'This' => 'is', 'a' => 'test' );
+ @ary = ( 0..5 );
+ {
+ local($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
+X<lvalue> X<subroutine, lvalue>
+
+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 {
+ # return $val; this doesn't work, don't say "return"
+ $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.
+
+=over 4
+
+=item Lvalue subroutines are EXPERIMENTAL
+
+They appear to be convenient, but there are several reasons to be
+circumspect.
+
+You can't use the return keyword, you must pass out the value before
+falling out of subroutine scope. (see comment in example above). This
+is usually not a problem, but it disallows an explicit return out of a
+deeply nested loop, which is sometimes a nice way out.
+
+They violate encapsulation. A normal mutator can check the supplied
+argument before setting the attribute it is protecting, an lvalue
+subroutine never gets that chance. Consider;
+
+ my $some_array_ref = []; # protected by mutators ??
+
+ sub set_arr { # normal mutator
+ my $val = shift;
+ die("expected array, you supplied ", ref $val)
+ unless ref $val eq 'ARRAY';
+ $some_array_ref = $val;
+ }
+ sub set_arr_lv : lvalue { # lvalue mutator
+ $some_array_ref;
+ }
-=head2 Passing Symbol Table Entries
+ # set_arr_lv cannot stop this !
+ set_arr_lv() = { a => 1 };
-[Note: The mechanism described in this section works fine in Perl 5, but
-the new reference mechanism is generally easier to work with. See L<perlref>.]
+=back
+
+=head2 Passing Symbol Table Entries (typeglobs)
+X<typeglob> X<*>
+
+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
copy of it rather than working with a local copy. In perl you can
-refer to all the objects of a particular name by prefixing the name
-with a star: C<*foo>. This is often known as a "type glob", since the
+refer to all objects of a particular name by prefixing the name
+with a star: C<*foo>. This is often known as a "typeglob", because the
star on the front can be thought of as a wildcard match for all the
funny prefix characters on variables and subroutines and such.
-When evaluated, the type glob produces a scalar value that represents
-all the objects of that name, including any filehandle, format or
+When evaluated, the typeglob produces a scalar value that represents
+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 $_[0] etc. You can modify all the elements of an array by passing
-all the elements as scalars, but you have to use the * mechanism (or
-the equivalent reference mechanism) to push, pop or change the size of
+to C<$_[0]> etc. You can modify all the elements of an array by passing
+all the elements as scalars, but you have to use the 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, since normally the LIST
+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.
+the individual arrays. For more on typeglobs, see
+L<perldata/"Typeglobs and Filehandles">.
+
+=head2 When to Still Use local()
+X<local> X<variable, 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 4
+
+=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
+X<pass by reference> X<pass-by-reference> X<reference>
+
+If you want to pass more than one array or hash into a function--or
+return them from it--and have them maintain their integrity, then
+you're going to have to use an explicit pass-by-reference. Before you
+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 C<pop> all of then, returning a new list
+of all their former last elements:
+
+ @tailings = popmany ( \@a, \@b, \@c, \@d );
+
+ sub popmany {
+ my $aref;
+ my @retlist = ();
+ foreach $aref ( @_ ) {
+ push @retlist, pop @$aref;
+ }
+ return @retlist;
+ }
+
+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 );
+ 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
+concatenating, then the normal calling convention is ok, although
+a little expensive.
+
+Where people get into trouble is here:
+
+ (@a, @b) = func(@c, @d);
+or
+ (%a, %b) = func(%c, %d);
+
+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
+takes two array references as arguments, returning the two array elements
+in order of how many elements they have in them:
+
+ ($aref, $bref) = func(\@c, \@d);
+ print "@$aref has more than @$bref\n";
+ sub func {
+ my ($cref, $dref) = @_;
+ if (@$cref > @$dref) {
+ return ($cref, $dref);
+ } else {
+ return ($dref, $cref);
+ }
+ }
+
+It turns out that you can actually do this also:
+
+ (*a, *b) = func(\@c, \@d);
+ print "@a has more than @b\n";
+ sub func {
+ local (*c, *d) = @_;
+ if (@c > @d) {
+ 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 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, $path) ? *FH : undef;
+ }
+
+=head2 Prototypes
+X<prototype> X<subroutine, prototype>
+
+Perl supports a very limited kind of compile-time argument checking
+using function prototyping. If you declare
+
+ sub mypush (\@@)
+
+then C<mypush()> takes arguments exactly like C<push()> does. The
+function declaration must be visible at compile time. The prototype
+affects only interpretation of new-style calls to the function,
+where new-style is defined as not using the C<&> character. In
+other words, if you call it like a built-in function, then it behaves
+like a built-in function. If you call it like an old-fashioned
+subroutine, then it behaves like an old-fashioned subroutine. It
+naturally falls out from this rule that prototypes have no influence
+on subroutine references like C<\&foo> or on indirect subroutine
+calls like C<&{$subref}> or C<< $subref->() >>.
+
+Method calls are not influenced by prototypes either, because the
+function to be called is indeterminate at compile time, since
+the exact code called depends on inheritance.
+
+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
+
+Any backslashed prototype character represents an actual argument
+that absolutely must start with that character. The value passed
+as part of C<@_> will be a reference to the actual argument given
+in the subroutine call, obtained by applying C<\> to that argument.
+
+You can also backslash several argument types simultaneously by using
+the C<\[]> notation:
+
+ sub myref (\[$@%&*])
+
+will allow calling myref() as
+
+ myref $var
+ myref @array
+ myref %hash
+ myref &sub
+ myref *glob
+
+and the first argument of myref() will be a reference to
+a scalar, an array, a hash, a code, or a glob.
+
+Unbackslashed prototype characters have special meanings. Any
+unbackslashed C<@> or C<%> eats all remaining arguments, and forces
+list context. An argument represented by C<$> 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. If you wish to always convert
+such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
+follows:
+
+ use Symbol 'qualify_to_ref';
+
+ sub foo (*) {
+ my $fh = qualify_to_ref(shift, caller);
+ ...
+ }
-=head2 Overriding builtin functions
+A semicolon (C<;>) separates mandatory arguments from optional arguments.
+It is redundant before C<@> or C<%>, which gobble up everything else.
-Many builtin functions may be overridden, though this should only be
-tried occasionally and for good reason. Typically this might be
-done by a package attempting to emulate missing builtin functionality
+As the last character of a prototype, or just before a semicolon, you can
+use C<_> in place of C<$>: if this argument is not provided, C<$_> will be
+used instead.
+
+Note how the last three examples in the table above are treated
+specially by the parser. C<mygrep()> is parsed as a true list
+operator, C<myrand()> is parsed as a true unary operator with unary
+precedence the same as C<rand()>, and C<mytime()> is truly without
+arguments, just like C<time()>. That is, if you say
+
+ mytime +2;
+
+you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
+without a prototype.
+
+The interesting thing about C<&> is that you can generate new syntax with it,
+provided it's in the initial position:
+X<&>
+
+ sub try (&@) {
+ my($try,$catch) = @_;
+ eval { &$try };
+ if ($@) {
+ local $_ = $@;
+ &$catch;
+ }
+ }
+ sub catch (&) { $_[0] }
+
+ try {
+ die "phooey";
+ } catch {
+ /phooey/ and print "unphooey\n";
+ };
+
+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 the Perl C<grep> operator:
+X<grep>
+
+ sub mygrep (&@) {
+ my $code = shift;
+ my @result;
+ foreach $_ (@_) {
+ push(@result, $_) if &$code;
+ }
+ @result;
+ }
+
+Some folks would prefer full alphanumeric prototypes. Alphanumerics have
+been intentionally left out of prototypes for the express purpose of
+someday in the future adding named, formal parameters. The current
+mechanism's main goal is to let module writers provide better diagnostics
+for module users. Larry feels the notation quite understandable to Perl
+programmers, and that it will not intrude greatly upon the meat of the
+module, nor make it harder to read. The line noise is visually
+encapsulated into a small pill that's easy to swallow.
+
+If you try to use an alphanumeric sequence in a prototype you will
+generate an optional warning - "Illegal character in prototype...".
+Unfortunately earlier versions of Perl allowed the prototype to be
+used as long as its prefix was a valid prototype. The warning may be
+upgraded to a fatal error in a future version of Perl once the
+majority of offending code is fixed.
+
+It's probably best to prototype new functions, not retrofit prototyping
+into older ones. That's because you must be especially careful about
+silent impositions of differing list versus scalar contexts. For example,
+if you decide that a function should take just one parameter, like this:
+
+ 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 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,
+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.
+
+=head2 Constant Functions
+X<constant>
+
+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 N () { int(OPT_BAZ) / 3 }
+
+ sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
+
+Be aware that these will not be inlined; as they contain inner scopes,
+the constant folding doesn't reduce them to a single constant:
+
+ sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }
+
+ sub baz_val () {
+ if (OPT_BAZ) {
+ return 23;
+ }
+ else {
+ return 42;
+ }
+ }
+
+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 Built-in Functions
+X<built-in> X<override> X<CORE> X<CORE::GLOBAL>
+
+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 built-in functionality
on a non-Unix system.
-Overriding may only be done by importing the name from a
-module--ordinary predeclaration isn't good enough. However, the
-C<subs> pragma (compiler directive) lets you, in effect, predeclare subs
-via the import syntax, and these names may then override the builtin ones:
+Overriding may be done only by importing the name from a module at
+compile time--ordinary predeclaration isn't good enough. However, the
+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, since 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.
+
+When you override a built-in, your replacement should be consistent (if
+possible) with the built-in native syntax. You can achieve this by using
+a suitable prototype. To get the prototype of an overridable built-in,
+use the C<prototype> function with an argument of C<"CORE::builtin_name">
+(see L<perlfunc/prototype>).
+
+Note however that some built-ins can't have their syntax expressed by a
+prototype (such as C<system> or C<chomp>). If you override them you won't
+be able to fully mimic their original syntax.
+
+The built-ins C<do>, C<require> and C<glob> can also be overridden, but due
+to special magic, their original syntax is preserved, and you don't have
+to define a prototype for their replacements. (You can't override the
+C<do BLOCK> syntax, though).
+
+C<require> has special additional dark magic: if you invoke your
+C<require> replacement as C<require Foo::Bar>, it will actually receive
+the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>.
+
+And, as you'll have noticed from the previous example, if you override
+C<glob>, the C<< <*> >> glob operator is overridden as well.
+
+In a similar fashion, overriding the C<readline> function also overrides
+the equivalent I/O operator C<< <FILEHANDLE> >>. Also, overriding
+C<readpipe> also overrides the operators C<``> and C<qx//>.
+
+Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
=head2 Autoloading
+X<autoloading> X<AUTOLOAD>
+
+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. (As an exception, a method call to a nonexistent
+C<import> or C<unimport> method is just skipped instead.)
+
+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 predeclare functions you want to call that way, you don't
+even need parentheses:
+
+ use subs qw(date who ls);
+ date;
+ who "am", "i";
+ ls '-l';
+
+A more complete example of this is the standard Shell module, which
+can treat undefined subroutine calls as calls to external programs.
+
+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>.
+
+=head2 Subroutine Attributes
+X<attribute> X<subroutine, attribute> X<attrs>
+
+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 colon 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 colon 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> and L<Attribute::Handlers>.
-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. A good example of this is the standard Shell module, which
-can treat undefined subroutine calls as calls to Unix programs.
-
-There are mechanisms available for modules to help them split themselves
-up into autoloadable files to be used with the standard AutoLoader module.
-See the document on extensions.
+=head1 SEE ALSO
+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.
https://perl5.git.perl.org / perl5.git / blobdiff