=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(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(SIG) BLOCK # with a signature instead
+ sub NAME : ATTRS BLOCK # with attributes
+ sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes
+ sub NAME(SIG) : ATTRS BLOCK # with a signature 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 (SIG) BLOCK; # with signature
+ $subref = sub : ATTRS BLOCK; # with attributes
+ $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
+ $subref = sub (SIG) : ATTRS BLOCK; # with signature and attributes
To import subroutines:
+X<import>
- use PACKAGE qw(NAME1 NAME2 NAME3);
+ use MODULE qw(NAME1 NAME2 NAME3);
To call subroutines:
+X<subroutine, call> X<call>
NAME(LIST); # & is optional with parentheses.
NAME LIST; # Parentheses optional if predeclared/imported.
- &NAME; # Passes current @_ to subroutine.
+ &NAME(LIST); # Circumvent prototypes.
+ &NAME; # Makes current @_ visible to called subroutine.
=head1 DESCRIPTION
-Like many languages, Perl provides for user-defined subroutines. These
-may be located anywhere in the main program, loaded in from other files
-via the C<do>, C<require>, or C<use> keywords, or even generated on the
-fly using C<eval> or anonymous subroutines (closures). You can even call
-a function indirectly using a variable containing its name or a CODE reference
-to it, as in C<$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.
+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 elements are
-aliases for the actual scalar parameters. In particular, if an element
-C<$_[0]> is updated, the corresponding argument is updated (or an error
-occurs if it is not updatable). If an argument is an array or hash
-element which did not exist when the function was called, that element is
-created only when (and if) it is modified or if a reference to it is
-taken. (Some earlier versions of Perl created the element whether or not
-it was assigned to.) Note that assigning to the whole array @_ removes
-the aliasing, and does not update any arguments.
-
-The return value of the subroutine is the value of the last expression
-evaluated. Alternatively, a return statement may be used to exit the
-subroutine, optionally specifying the returned value, which will be
-evaluated in the appropriate context (list, scalar, or void) depending
-on the context of the subroutine call. If you specify no return value,
-the subroutine will return an empty list in a list context, an undefined
-value in a scalar context, or nothing in a void context. If you return
-one or more arrays and/or hashes, these will be flattened together into
+there's really no difference from Perl's perspective.)
+X<subroutine, parameter> X<parameter>
+
+Any arguments passed in show up in the array C<@_>.
+(They may also show up in lexical variables introduced by a signature;
+see L</Signatures> below.) Therefore, if
+you called a function with two arguments, those would be stored in
+C<$_[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.
-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">.
+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>
+
+Aside from an experimental facility (see L</Signatures> below),
+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:
# that start with whitespace
sub get_line {
- $thisline = $lookahead; # GLOBAL VARIABLES!!
+ $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.
+X<call-by-reference> X<call-by-value>
upcase_in($v1, $v2); # this changes $v1 and $v2
sub upcase_in {
You aren't allowed to modify constants in this way, of course. If an
argument were actually literal and you tried to change it, you'd take a
(presumably fatal) exception. For example, this won't work:
+X<call-by-reference> X<call-by-value>
upcase_in("frederick");
-It would be much safer if the upcase_in() function
+It would be much safer if the C<upcase_in()> function
were written to return a copy of its parameters instead
of changing them in place:
- ($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 = @_;
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 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 /:/, $var );
(@a, @b) = upcase(@list1, @list2);
-Because like its flat incoming parameter list, the return list is also
-flat. So all you have managed to do here is stored everything in @a and
-made @b an empty list. See L</"Pass by Reference"> for alternatives.
-
-A subroutine may be called using the "&" prefix. The "&" is optional
-in modern Perls, and so are the parentheses if the subroutine has been
-predeclared. (Note, however, that the "&" is I<NOT> optional when
-you're just naming the subroutine, such as when it's used as an
-argument to defined() or undef(). Nor is it optional when you want to
-do an indirect subroutine call with a subroutine name or reference
-using the C<&$subref()> or C<&{$subref}()> constructs. See L<perlref>
-for more on that.)
-
-Subroutines may be called recursively. If a subroutine is called using
-the "&" form, the argument list is optional, and if omitted, no @_ array is
-set up for the subroutine: the @_ array at the time of the call is
-visible to subroutine instead. This is an efficiency mechanism that
-new users may wish to avoid.
+Like the flattened incoming parameter list, the return list is also
+flattened on return. So all you have managed to do here is stored
+everything in C<@a> and made C<@b> empty. See
+L</Pass by Reference> for alternatives.
+
+A subroutine may be called using an explicit C<&> prefix. The
+C<&> is optional in modern Perl, as are parentheses if the
+subroutine has been predeclared. The C<&> is I<not> optional
+when just naming the subroutine, such as when it's used as
+an argument to defined() or undef(). Nor is it optional when you
+want to do an indirect subroutine call with a subroutine name or
+reference using the C<&$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 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; # foo() get current args, like 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.
+X<&>
+
+Since Perl 5.16.0, the C<__SUB__> token is available under C<use feature
+'current_sub'> and C<use 5.16.0>. It will evaluate to a reference to the
+currently-running sub, which allows for recursive calls without knowing
+your subroutine's name.
+
+ use 5.16.0;
+ my $factorial = sub {
+ my ($x) = @_;
+ return 1 if $x == 1;
+ return($x * __SUB__->( $x - 1 ) );
+ };
+
+The behavior of C<__SUB__> within a regex code block (such as C</(?{...})/>)
+is subject to change.
+
+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 whose name start with a left parenthesis are also reserved the
+same way. The following is a list of some subroutines that currently do
+special, pre-defined things.
+
+=over
+
+=item documented later in this document
+
+C<AUTOLOAD>
+
+=item documented in L<perlmod>
+
+C<CLONE>, C<CLONE_SKIP>
+
+=item documented in L<perlobj>
+
+C<DESTROY>, C<DOES>
+
+=item documented in L<perltie>
+
+C<BINMODE>, C<CLEAR>, C<CLOSE>, C<DELETE>, C<DESTROY>, C<EOF>, C<EXISTS>,
+C<EXTEND>, C<FETCH>, C<FETCHSIZE>, C<FILENO>, C<FIRSTKEY>, C<GETC>,
+C<NEXTKEY>, C<OPEN>, C<POP>, C<PRINT>, C<PRINTF>, C<PUSH>, C<READ>,
+C<READLINE>, C<SCALAR>, C<SEEK>, C<SHIFT>, C<SPLICE>, C<STORE>,
+C<STORESIZE>, C<TELL>, C<TIEARRAY>, C<TIEHANDLE>, C<TIEHASH>,
+C<TIESCALAR>, C<UNSHIFT>, C<UNTIE>, C<WRITE>
+
+=item documented in L<PerlIO::via>
+
+C<BINMODE>, C<CLEARERR>, C<CLOSE>, C<EOF>, C<ERROR>, C<FDOPEN>, C<FILENO>,
+C<FILL>, C<FLUSH>, C<OPEN>, C<POPPED>, C<PUSHED>, C<READ>, C<SEEK>,
+C<SETLINEBUF>, C<SYSOPEN>, C<TELL>, C<UNREAD>, C<UTF8>, C<WRITE>
+
+=item documented in L<perlfunc>
+
+L<< C<import> | perlfunc/use >>, L<< C<unimport> | perlfunc/use >>,
+L<< C<INC> | perlfunc/require >>
+
+=item documented in L<UNIVERSAL>
+
+C<VERSION>
+
+=item documented in L<perldebguts>
+
+C<DB::DB>, C<DB::sub>, C<DB::lsub>, C<DB::goto>, C<DB::postponed>
+
+=item undocumented, used internally by the L<overload> feature
+
+any starting with C<(>
+
+=back
+
+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 Signatures
+
+B<WARNING>: Subroutine signatures are experimental. The feature may be
+modified or removed in future versions of Perl.
+
+Perl has an experimental facility to allow a subroutine's formal
+parameters to be introduced by special syntax, separate from the
+procedural code of the subroutine body. The formal parameter list
+is known as a I<signature>. The facility must be enabled first by a
+pragmatic declaration, C<use feature 'signatures'>, and it will produce
+a warning unless the "experimental::signatures" warnings category is
+disabled.
+
+The signature is part of a subroutine's body. Normally the body of a
+subroutine is simply a braced block of code. When using a signature,
+the signature is a parenthesised list that goes immediately after
+the subroutine name (or, for anonymous subroutines, immediately after
+the C<sub> keyword). The signature declares lexical variables that are
+in scope for the block. When the subroutine is called, the signature
+takes control first. It populates the signature variables from the
+list of arguments that were passed. If the argument list doesn't meet
+the requirements of the signature, then it will throw an exception.
+When the signature processing is complete, control passes to the block.
+
+Positional parameters are handled by simply naming scalar variables in
+the signature. For example,
+
+ sub foo ($left, $right) {
+ return $left + $right;
+ }
+
+takes two positional parameters, which must be filled at runtime by
+two arguments. By default the parameters are mandatory, and it is
+not permitted to pass more arguments than expected. So the above is
+equivalent to
+
+ sub foo {
+ die "Too many arguments for subroutine" unless @_ <= 2;
+ die "Too few arguments for subroutine" unless @_ >= 2;
+ my $left = $_[0];
+ my $right = $_[1];
+ return $left + $right;
+ }
+
+An argument can be ignored by omitting the main part of the name from
+a parameter declaration, leaving just a bare C<$> sigil. For example,
+
+ sub foo ($first, $, $third) {
+ return "first=$first, third=$third";
+ }
+
+Although the ignored argument doesn't go into a variable, it is still
+mandatory for the caller to pass it.
+
+A positional parameter is made optional by giving a default value,
+separated from the parameter name by C<=>:
+
+ sub foo ($left, $right = 0) {
+ return $left + $right;
+ }
+
+The above subroutine may be called with either one or two arguments.
+The default value expression is evaluated when the subroutine is called,
+so it may provide different default values for different calls. It is
+only evaluated if the argument was actually omitted from the call.
+For example,
+
+ my $auto_id = 0;
+ sub foo ($thing, $id = $auto_id++) {
+ print "$thing has ID $id";
+ }
+
+automatically assigns distinct sequential IDs to things for which no
+ID was supplied by the caller. A default value expression may also
+refer to parameters earlier in the signature, making the default for
+one parameter vary according to the earlier parameters. For example,
+
+ sub foo ($first_name, $surname, $nickname = $first_name) {
+ print "$first_name $surname is known as \"$nickname\"";
+ }
+
+An optional parameter can be nameless just like a mandatory parameter.
+For example,
+
+ sub foo ($thing, $ = 1) {
+ print $thing;
+ }
+
+The parameter's default value will still be evaluated if the corresponding
+argument isn't supplied, even though the value won't be stored anywhere.
+This is in case evaluating it has important side effects. However, it
+will be evaluated in void context, so if it doesn't have side effects
+and is not trivial it will generate a warning if the "void" warning
+category is enabled. If a nameless optional parameter's default value
+is not important, it may be omitted just as the parameter's name was:
+
+ sub foo ($thing, $=) {
+ print $thing;
+ }
+
+Optional positional parameters must come after all mandatory positional
+parameters. (If there are no mandatory positional parameters then an
+optional positional parameters can be the first thing in the signature.)
+If there are multiple optional positional parameters and not enough
+arguments are supplied to fill them all, they will be filled from left
+to right.
+
+After positional parameters, additional arguments may be captured in a
+slurpy parameter. The simplest form of this is just an array variable:
+
+ sub foo ($filter, @inputs) {
+ print $filter->($_) foreach @inputs;
+ }
+
+With a slurpy parameter in the signature, there is no upper limit on how
+many arguments may be passed. A slurpy array parameter may be nameless
+just like a positional parameter, in which case its only effect is to
+turn off the argument limit that would otherwise apply:
+
+ sub foo ($thing, @) {
+ print $thing;
+ }
+
+A slurpy parameter may instead be a hash, in which case the arguments
+available to it are interpreted as alternating keys and values.
+There must be as many keys as values: if there is an odd argument then
+an exception will be thrown. Keys will be stringified, and if there are
+duplicates then the later instance takes precedence over the earlier,
+as with standard hash construction.
+
+ sub foo ($filter, %inputs) {
+ print $filter->($_, $inputs{$_}) foreach sort keys %inputs;
+ }
+
+A slurpy hash parameter may be nameless just like other kinds of
+parameter. It still insists that the number of arguments available to
+it be even, even though they're not being put into a variable.
+
+ sub foo ($thing, %) {
+ print $thing;
+ }
+
+A slurpy parameter, either array or hash, must be the last thing in the
+signature. It may follow mandatory and optional positional parameters;
+it may also be the only thing in the signature. Slurpy parameters cannot
+have default values: if no arguments are supplied for them then you get
+an empty array or empty hash.
+
+A signature may be entirely empty, in which case all it does is check
+that the caller passed no arguments:
+
+ sub foo () {
+ return 123;
+ }
+
+When using a signature, the arguments are still available in the special
+array variable C<@_>, in addition to the lexical variables of the
+signature. There is a difference between the two ways of accessing the
+arguments: C<@_> I<aliases> the arguments, but the signature variables
+get I<copies> of the arguments. So writing to a signature variable
+only changes that variable, and has no effect on the caller's variables,
+but writing to an element of C<@_> modifies whatever the caller used to
+supply that argument.
+
+There is a potential syntactic ambiguity between signatures and prototypes
+(see L</Prototypes>), because both start with an opening parenthesis and
+both can appear in some of the same places, such as just after the name
+in a subroutine declaration. For historical reasons, when signatures
+are not enabled, any opening parenthesis in such a context will trigger
+very forgiving prototype parsing. Most signatures will be interpreted
+as prototypes in those circumstances, but won't be valid prototypes.
+(A valid prototype cannot contain any alphabetic character.) This will
+lead to somewhat confusing error messages.
+
+To avoid ambiguity, when signatures are enabled the special syntax
+for prototypes is disabled. There is no attempt to guess whether a
+parenthesised group was intended to be a prototype or a signature.
+To give a subroutine a prototype under these circumstances, use a
+L<prototype attribute|attributes/Built-in Attributes>. For example,
+
+ sub foo :prototype($) { $_[0] }
+
+It is entirely possible for a subroutine to have both a prototype and
+a signature. They do different jobs: the prototype affects compilation
+of calls to the subroutine, and the signature puts argument values into
+lexical variables at runtime. You can therefore write
+
+ sub foo ($left, $right) : prototype($$) {
+ return $left + $right;
+ }
+
+The prototype attribute, and any other attributes, come after
+the signature.
=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 (@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 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>/C<unless>/C<elsif>/C<else>), loop
+(C<for>/C<foreach>/C<while>/C<until>/C<continue>), subroutine, C<eval>,
+or C<do>/C<require>/C<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 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);
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 (defined(my $line = <>)) {
+ while (my $line = <>) {
$line = lc $line;
} continue {
print $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),
-but not beyond it.
-
-(None of the foregoing 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.)
+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>; 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
+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, 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()>.
+X<foreach> X<for>
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 is delayed until
-run time, so it gets executed appropriately; every time through a loop,
-for example.
-
-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<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
- 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.
+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 5.10.0, 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>). Beginning with Perl 5.16,
+the C<CORE::state> form does not require the
+C<feature> pragma.
+
+The C<state> keyword creates a lexical variable (following the same scoping
+rules as C<my>) that persists from one subroutine call to the next. If a
+state variable resides inside an anonymous subroutine, then each copy of
+the subroutine has its own copy of the state variable. However, the value
+of the state variable will still persist between calls to the same copy of
+the anonymous subroutine. (Don't forget that C<sub { ... }> creates a new
+subroutine each time it is executed.)
-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.
+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 }
+
+And this example uses anonymous subroutines to create separate counters:
+
+ use feature 'state';
+ sub create_counter {
+ return sub { state $x; return ++$x }
+ }
+
+Also, since C<$x> is lexical, it can't be reached or modified by any Perl
+code outside.
+
+When combined with variable declaration, simple assignment to C<state>
+variables (as in C<state $x = 42>) is executed only the first time. When such
+statements are evaluated subsequent times, the assignment is ignored. The
+behavior of assignment to C<state> declarations where the left hand side
+of the assignment involves any parentheses is currently undefined.
+
+=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;
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 main program, or more likely, placing merely a BEGIN
-sub around it to make sure it gets executed before your program
+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:
- sub BEGIN {
+ BEGIN {
my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
}
}
-See L<perlrun> about the BEGIN function.
+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<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
-other variables whose current value must be visible to called
-subroutines.
+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:
- local $foo; # declare $foo dynamically local
- local (@wid, %get); # declare list of variables local
- local $foo = "flurp"; # declare $foo dynamic, and init it
- local @oof = @bar; # declare @oof dynamic, and init it
-
- 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 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.
-
-If more than one variable is given to local(), they must be placed in
-parentheses. All listed elements must be legal lvalues. This operator works
+ # 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
+ delete local $hash{key}; # delete this entry for the current block
+ 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.) Commonly this is used to name the parameters to a
-subroutine. Examples:
+undefined value.)
- for $i ( 0 .. 9 ) {
- $digits{$i} = $i;
- }
- # assume this function uses global %digits hash
- parse_num();
+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.
- # now temporarily add to %digits hash
- if ($base12) {
- # (NOTE: not claiming this is efficient!)
- local %digits = (%digits, 't' => 10, 'e' => 11);
- parse_num(); # parse_num gets this new %digits!
- }
- # old %digits restored here
+=head3 Grammatical note on local()
+X<local, context>
-Because local() is a run-time command, it gets executed every 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.
+=head3 Localization of special variables
+X<local, special variable>
-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:
+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.
- 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..]
+This feature allows code like this to work :
-As another example, a custom implementation of C<%ENV> might look
-like this:
+ # Read the whole contents of FILE in $slurp
+ { local $/ = undef; $slurp = <FILE>; }
- {
- local %ENV;
- tie %ENV, 'MyOwnEnv';
- [..do your own fancy %ENV manipulation here..]
- }
- [..normal %ENV behavior here..]
+Note, however, that this restricts localization of some values ; for
+example, the following statement dies, as of perl 5.10.0, with an error
+I<Modification of a read-only value attempted>, because the $1 variable is
+magical and read-only :
+
+ local $1 = 2;
+
+One exception is the default scalar variable: starting with perl 5.14
+C<local($_)> will always strip all magic from $_, to make it possible
+to safely reuse $_ in a subroutine.
+
+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 behavior 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.
+
+=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
-localize a member of a composite type (i.e. an array or hash element).
-In this case, the element is localized I<by name>. This means that
+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
This is a test only a test.
The array has 6 elements: 0, 1, 2, undef, undef, 5
-In short, be careful when manipulating the containers for composite types
-whose elements have been localized.
+The behavior of local() on non-existent members of composite
+types is subject to change in future.
+
+=head3 Localized deletion of elements of composite types
+X<delete> X<local, composite type element> X<local, array element> X<local, hash element>
+
+You can use the C<delete local $array[$idx]> and C<delete local $hash{key}>
+constructs to delete a composite type entry for the current block and restore
+it when it ends. They return the array/hash value before the localization,
+which means that they are respectively equivalent to
+
+ do {
+ my $val = $array[$idx];
+ local $array[$idx];
+ delete $array[$idx];
+ $val
+ }
+
+and
+
+ do {
+ my $val = $hash{key};
+ local $hash{key};
+ delete $hash{key};
+ $val
+ }
+
+except that for those the C<local> is
+scoped to the C<do> block. Slices are
+also accepted.
+
+ my %hash = (
+ a => [ 7, 8, 9 ],
+ b => 1,
+ )
+
+ {
+ my $a = delete local $hash{a};
+ # $a is [ 7, 8, 9 ]
+ # %hash is (b => 1)
+
+ {
+ my @nums = delete local @$a[0, 2]
+ # @nums is (7, 9)
+ # $a is [ undef, 8 ]
+
+ $a[0] = 999; # will be erased when the scope ends
+ }
+ # $a is back to [ 7, 8, 9 ]
+
+ }
+ # %hash is back to its original state
+
+=head2 Lvalue subroutines
+X<lvalue> X<subroutine, lvalue>
+
+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; # or: 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.
+
+Lvalue subroutines are convenient, but you have to keep in mind that,
+when used with objects, they may violate encapsulation. A normal
+mutator can check the supplied argument before setting the attribute
+it is protecting, an lvalue subroutine cannot. If you require any
+special processing when storing and retrieving the values, consider
+using the CPAN module Sentinel or something similar.
+
+=head2 Lexical Subroutines
+X<my sub> X<state sub> X<our sub> X<subroutine, lexical>
+
+Beginning with Perl 5.18, you can declare a private subroutine with C<my>
+or C<state>. As with state variables, the C<state> keyword is only
+available under C<use feature 'state'> or C<use 5.010> or higher.
+
+Prior to Perl 5.26, lexical subroutines were deemed experimental and were
+available only under the C<use feature 'lexical_subs'> pragma. They also
+produced a warning unless the "experimental::lexical_subs" warnings
+category was disabled.
+
+These subroutines are only visible within the block in which they are
+declared, and only after that declaration:
+
+ # Include these two lines if your code is intended to run under Perl
+ # versions earlier than 5.26.
+ no warnings "experimental::lexical_subs";
+ use feature 'lexical_subs';
+
+ foo(); # calls the package/global subroutine
+ state sub foo {
+ foo(); # also calls the package subroutine
+ }
+ foo(); # calls "state" sub
+ my $ref = \&foo; # take a reference to "state" sub
+
+ my sub bar { ... }
+ bar(); # calls "my" sub
+
+You can't (directly) write a recursive lexical subroutine:
+
+ # WRONG
+ my sub baz {
+ baz();
+ }
+
+This example fails because C<baz()> refers to the package/global subroutine
+C<baz>, not the lexical subroutine currently being defined.
+
+The solution is to use L<C<__SUB__>|perlfunc/__SUB__>:
+
+ my sub baz {
+ __SUB__->(); # calls itself
+ }
+
+It is possible to predeclare a lexical subroutine. The C<sub foo {...}>
+subroutine definition syntax respects any previous C<my sub;> or C<state sub;>
+declaration. Using this to define recursive subroutines is a bad idea,
+however:
+
+ my sub baz; # predeclaration
+ sub baz { # define the "my" sub
+ baz(); # WRONG: calls itself, but leaks memory
+ }
+
+Just like C<< my $f; $f = sub { $f->() } >>, this example leaks memory. The
+name C<baz> is a reference to the subroutine, and the subroutine uses the name
+C<baz>; they keep each other alive (see L<perlref/Circular References>).
+
+=head3 C<state sub> vs C<my sub>
+
+What is the difference between "state" subs and "my" subs? Each time that
+execution enters a block when "my" subs are declared, a new copy of each
+sub is created. "State" subroutines persist from one execution of the
+containing block to the next.
+
+So, in general, "state" subroutines are faster. But "my" subs are
+necessary if you want to create closures:
+
+ sub whatever {
+ my $x = shift;
+ my sub inner {
+ ... do something with $x ...
+ }
+ inner();
+ }
+
+In this example, a new C<$x> is created when C<whatever> is called, and
+also a new C<inner>, which can see the new C<$x>. A "state" sub will only
+see the C<$x> from the first call to C<whatever>.
+
+=head3 C<our> subroutines
+
+Like C<our $variable>, C<our sub> creates a lexical alias to the package
+subroutine of the same name.
+
+The two main uses for this are to switch back to using the package sub
+inside an inner scope:
+
+ sub foo { ... }
+
+ sub bar {
+ my sub foo { ... }
+ {
+ # need to use the outer foo here
+ our sub foo;
+ foo();
+ }
+ }
+
+and to make a subroutine visible to other packages in the same scope:
+
+ package MySneakyModule;
+
+ our sub do_something { ... }
+
+ sub do_something_with_caller {
+ package DB;
+ () = caller 1; # sets @DB::args
+ do_something(@args); # uses MySneakyModule::do_something
+ }
=head2 Passing Symbol Table Entries (typeglobs)
+X<typeglob> X<*>
-[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
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 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
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 exits
+ 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.
+
+=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
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 );
sub popmany {
my $aref;
- my @retlist = ();
+ my @retlist;
foreach $aref ( @_ ) {
push @retlist, pop @$aref;
}
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
}
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 *STDOUT, but typeglobs references would be better because
-they'll still work properly under C<use strict 'refs'>. For example:
+typeglob, like C<*STDOUT>, but typeglobs references work, too.
+For example:
splutter(\*STDOUT);
sub splutter {
return scalar <$fh>;
}
-Another way to do this is using *HANDLE{IO}, see L<perlref> for usage
-and caveats.
-
-If you're planning on generating new filehandles, you could do this:
+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 $name = shift;
+ my $path = shift;
local *FH;
return open (FH, $path) ? *FH : undef;
}
-Although that will actually produce a small memory leak. See the bottom
-of L<perlfunc/open()> for a somewhat cleaner way using the IO::Handle
-package.
-
=head2 Prototypes
+X<prototype> X<subroutine, prototype>
-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. This can be declared in either the PROTO
+section or with a L<prototype attribute|attributes/Built-in Attributes>.
+If you declare either of
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}>.
+ sub mypush :prototype(\@@)
+
+then C<mypush()> takes arguments exactly like C<push()> does.
+
+If subroutine signatures are enabled (see L</Signatures>), then
+the shorter PROTO syntax is unavailable, because it would clash with
+signatures. In that case, a prototype can only be declared in the form
+of an attribute.
+
+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, because it 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.
-
- 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
+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, 0, 2, @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.
+that must start with that character (optionally preceded by C<my>,
+C<our> or C<local>), with the exception of C<$>, which will
+accept any scalar lvalue expression, such as C<$foo = 7> or
+C<< my_function()->[0] >>. 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 use the C<\[]> backslash group notation to specify more than one
+allowed argument type. For example:
+
+ 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 @ 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.
-
-A semicolon separates mandatory arguments from optional arguments.
-(It is redundant before @ or %.)
-
-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
+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 $fh = qualify_to_ref(shift, caller);
+ ...
+ }
+
+The C<+> prototype is a special alternative to C<$> that will act like
+C<\[@%]> when given a literal array or hash variable, but will otherwise
+force scalar context on the argument. This is useful for functions which
+should accept either a literal array or an array reference as the argument:
+
+ sub mypush (+@) {
+ my $aref = shift;
+ die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
+ push @$aref, @_;
+ }
+
+When using the C<+> prototype, your function must check that the argument
+is of an acceptable type.
+
+A semicolon (C<;>) separates mandatory arguments from optional arguments.
+It is redundant before C<@> or C<%>, which gobble up everything else.
+
+As the last character of a prototype, or just before a semicolon, a C<@>
+or a C<%>, 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 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. If you want to force a unary function to have the
+same precedence as a list operator, add C<;> to the end of the prototype:
-The interesting thing about & is that you can generate new syntax with it:
+ sub mygetprotobynumber($;);
+ mygetprotobynumber $a > $b; # parsed as mygetprotobynumber($a > $b)
+
+The interesting thing about C<&> is that you can generate new syntax with it,
+provided it's in the initial position:
+X<&>
sub try (&@) {
my($try,$catch) = @_;
/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:
+X<grep>
sub mygrep (&@) {
my $code = shift;
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,
returning a list:
func(@foo);
- func( split /:/ );
+ func( $text =~ /\w+/g );
-Then you've just supplied an automatic scalar() in front of their
-argument, which can be more than a bit surprising. The old @foo
+Then you've just supplied an automatic C<scalar> in front of their
+argument, which can be more than a bit surprising. The old C<@foo>
which used to hold one thing doesn't get passed in. Instead,
-the func() now gets passed in 1, that is, the number of elements
-in @foo. And the split() gets called in a scalar context and
-starts scribbling on your @_ parameter list.
+C<func()> now gets passed in a C<1>; that is, the number of elements
+in C<@foo>. And the C<m//g> gets called in scalar context so instead of a
+list of words it returns a boolean result and advances C<pos($text)>. Ouch!
+
+If a sub has both a PROTO and a BLOCK, the prototype is not applied
+until after the BLOCK is completely defined. This means that a recursive
+function with a prototype has to be predeclared for the prototype to take
+effect, like so:
+
+ sub foo($$);
+ sub foo($$) {
+ foo 1, 2;
+ }
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
+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<&> or C<do>. Calls made using C<&> or C<do> are never
-inlined. (See constant.pm for an easy way to declare most
-constants.)
+without C<&>. Calls made using C<&> are never inlined. (See
+F<constant.pm> for an easy way to declare most constants.)
-All of the following functions would be inlined.
+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,
sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
- sub BAZ_VAL () {
+
+ sub N () { int(OPT_BAZ) / 3 }
+
+ sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
+ sub FOO_SET2 () { if (FLAG_MASK & FLAG_FOO) { 1 } }
+
+(Be aware that the last example was not always inlined in Perl 5.20 and
+earlier, which did not behave consistently with subroutines containing
+inner scopes.) You can countermand inlining by using an explicit
+C<return>:
+
+ sub baz_val () {
if (OPT_BAZ) {
return 23;
}
return 42;
}
}
+ sub bonk_val () { return 12345 }
+
+As alluded to earlier you can also declare inlined subs dynamically at
+BEGIN time if their body consists of a lexically-scoped scalar which
+has no other references. Only the first example here will be inlined:
+
+ BEGIN {
+ my $var = 1;
+ no strict 'refs';
+ *INLINED = sub () { $var };
+ }
+
+ BEGIN {
+ my $var = 1;
+ my $ref = \$var;
+ no strict 'refs';
+ *NOT_INLINED = sub () { $var };
+ }
+
+A not so obvious caveat with this (see [RT #79908]) is that the
+variable will be immediately inlined, and will stop behaving like a
+normal lexical variable, e.g. this will print C<79907>, not C<79908>:
- sub N () { int(BAZ_VAL) / 3 }
BEGIN {
- my $prod = 1;
- for (1..N) { $prod *= $_ }
- sub N_FACTORIAL () { $prod }
+ my $x = 79907;
+ *RT_79908 = sub () { $x };
+ $x++;
}
+ print RT_79908(); # prints 79907
-If you redefine a subroutine which 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 the calling semantics, so beware) or by thwarting the
-inlining mechanism in some other way, such as
+As of Perl 5.22, this buggy behavior, while preserved for backward
+compatibility, is detected and emits a deprecation warning. If you want
+the subroutine to be inlined (with no warning), make sure the variable is
+not used in a context where it could be modified aside from where it is
+declared.
- sub not_inlined () {
- 23 if $];
+ # Fine, no warning
+ BEGIN {
+ my $x = 54321;
+ *INLINED = sub () { $x };
+ }
+ # Warns. Future Perl versions will stop inlining it.
+ BEGIN {
+ my $x;
+ $x = 54321;
+ *ALSO_INLINED = sub () { $x };
}
-=head2 Overriding Builtin Functions
+Perl 5.22 also introduces the experimental "const" attribute as an
+alternative. (Disable the "experimental::const_attr" warnings if you want
+to use it.) When applied to an anonymous subroutine, it forces the sub to
+be called when the C<sub> expression is evaluated. The return value is
+captured and turned into a constant subroutine:
-Many builtin functions may be overridden, though this should be tried
+ my $x = 54321;
+ *INLINED = sub : const { $x };
+ $x++;
+
+The return value of C<INLINED> in this example will always be 54321,
+regardless of later modifications to $x. You can also put any arbitrary
+code inside the sub, at it will be executed immediately and its return
+value captured the same way.
+
+If you really want a subroutine with a C<()> prototype that returns a
+lexical variable you can easily force it to not be inlined by adding
+an explicit C<return>:
+
+ BEGIN {
+ my $x = 79907;
+ *RT_79908 = sub () { return $x };
+ $x++;
+ }
+ print RT_79908(); # prints 79908
+
+The easiest way to tell if a subroutine was inlined is by using
+L<B::Deparse>. Consider this example of two subroutines returning
+C<1>, one with a C<()> prototype causing it to be inlined, and one
+without (with deparse output truncated for clarity):
+
+ $ perl -MO=Deparse -le 'sub ONE { 1 } if (ONE) { print ONE if ONE }'
+ sub ONE {
+ 1;
+ }
+ if (ONE ) {
+ print ONE() if ONE ;
+ }
+ $ perl -MO=Deparse -le 'sub ONE () { 1 } if (ONE) { print ONE if ONE }'
+ sub ONE () { 1 }
+ do {
+ print 1
+ };
+
+If you redefine a subroutine that was eligible for inlining, you'll
+get a warning by default. You can use this warning to tell whether or
+not a particular subroutine is considered inlinable, since it's
+different than the warning for overriding non-inlined subroutines:
+
+ $ perl -e 'sub one () {1} sub one () {2}'
+ Constant subroutine one redefined at -e line 1.
+ $ perl -we 'sub one {1} sub one {2}'
+ Subroutine one redefined at -e line 1.
+
+The warning is considered severe enough not to be affected by the
+B<-w> switch (or its absence) 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, e.g. by adding an explicit C<return>, as
+mentioned above:
+
+ sub not_inlined () { return 23 }
+
+=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 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, 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 { ... }
-To unambiguously refer to the builtin form, one may precede the
-builtin name with the special package qualifier C<CORE::>. For example,
-saying C<CORE::open()> will always refer to the builtin C<open()>, even
+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.
-
-Library modules should not in general export builtin names like "open"
-or "chdir" as part of their default @EXPORT list, because these may
+C<&open()> from elsewhere. Even though it looks like a regular
+function call, it isn't: the CORE:: prefix in that case is part of Perl's
+syntax, and works for any keyword, regardless of what is in the CORE
+package. Taking a reference to it, that is, C<\&CORE::open>, only works
+for some keywords. See L<CORE>.
+
+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, @_);
+ }
-Note that such overriding is restricted to the package that requests
-the import. Some means of "globally" overriding builtins may become
-available in future.
+ sub glob {
+ my $pat = shift;
+ my @got;
+ if (opendir my $d, '.') {
+ @got = grep /$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
-
-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:
+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. Also, if
+the AUTOLOAD subroutine is an XSUB, there are other ways to retrieve the
+subroutine name. See L<perlguts/Autoloading with XSUBs> for details.)
+
+
+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;
who('am', 'i');
ls('-l');
-In fact, if you predeclare 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;
who "am", "i";
- ls -l;
+ ls '-l';
-A more complete example of this is the standard Shell module, which
-can treat undefined subroutine calls as calls to Unix programs.
+A more complete example of this is the Shell module on CPAN, which
+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
+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>.
=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<perlootut> to learn how to make object method calls.
https://perl5.git.perl.org / perl5.git / blobdiff