one large indistinguishable list.
If no C<return> is found and if the last statement is an expression, its
-value is returned. Otherwise, if the last statement is a control structure
-like a C<foreach>, the returned value is unspecified. The empty sub
-returns the empty list.
+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
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.
+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
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<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, CHECK, INIT and END">
+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>
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
+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
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 C<::> notation even while a
this.
=head2 Persistent Private Variables
-X<static> X<variable, persistent> X<variable, static> X<closure>
+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>). Beginning with Perl 5.16,
+you can also write it as C<CORE::state>, which does not require the
+C<feature> pragma.
+
+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.
+
+When combined with variable declaration, simple scalar 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 this sort of assignment to non-scalar variables is 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
}
}
-See L<perlmod/"BEGIN, CHECK, INIT and END"> about the
-special triggered code blocks, C<BEGIN>, C<CHECK>, C<INIT> and C<END>.
+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
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
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
+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.
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 $1, thus is magic and readonly
- f();
- }
-
-See next section for an alternative to this situation.
+One exception is the default scalar variable: starting with perl 5.14
+C<local($_)> will always strip all magic from $_, to make it possible
+to safely reuse $_ in a subroutine.
B<WARNING>: Localization of tied arrays and hashes does not currently
work as described.
will not have any effect on the internal value of the input record
separator.
-Notably, if you want to work with a brand new value of the default scalar
-$_, 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>
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>
my $val;
sub canmod : lvalue {
- # return $val; this doesn't work, don't say "return"
- $val;
+ $val; # or: return $val;
}
sub nomod {
$val;
=item Lvalue subroutines are EXPERIMENTAL
-They appear to be convenient, but there are several reasons to be
+They appear to be convenient, but there is at least one reason 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;
a local alias.
{
- local *grow = \&shrink; # only until this block exists
+ local *grow = \&shrink; # only until this block exits
grow(); # really calls shrink()
move(); # if move() grow()s, it shrink()s too
}
Perl supports a very limited kind of compile-time argument checking
using function prototyping. If you declare
- sub mypush (\@@)
+ sub mypush (+@)
then C<mypush()> takes arguments exactly like C<push()> does. The
function declaration must be visible at compile time. The prototype
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 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 myrand (;$) myrand 42
sub mytime () mytime
Any backslashed prototype character represents an actual argument
-that absolutely must start with that character. The value passed
-as part of C<@_> will be a reference to the actual argument given
-in the subroutine call, obtained by applying C<\> to that argument.
+that must start with that character (optionally preceded by C<my>,
+C<our> or C<local>), with the exception of C<$>, 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 also backslash several argument types simultaneously by using
-the C<\[]> notation:
+You can use the C<\[]> backslash group notation to specify more than one
+allowed argument type. For example:
sub myref (\[$@%&*])
...
}
-A semicolon separates mandatory arguments from optional arguments.
+The C<+> prototype is a special alternative to C<$> 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, 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
mytime +2;
you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
-without a prototype.
+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:
+
+ 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:
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.
+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
sub glob {
my $pat = shift;
my @got;
- local *D;
- if (opendir D, '.') {
- @got = grep /$pat/, readdir D;
- closedir D;
+ if (opendir my $d, '.') {
+ @got = grep /$pat/, readdir $d;
+ closedir $d;
}
return @got;
}
C<glob>, the C<< <*> >> glob operator is overridden as well.
In a similar fashion, overriding the C<readline> function also overrides
-the equivalent I/O operator C<< <FILEHANDLE> >>.
+the equivalent I/O operator C<< <FILEHANDLE> >>. Also, overriding
+C<readpipe> also overrides the operators C<``> and C<qx//>.
Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
variable of the same package as the C<AUTOLOAD> routine. The name
is not passed as an ordinary argument because, er, well, just
because, that's why. (As an exception, a method call to a nonexistent
-C<import> or C<unimport> method is just skipped instead.)
+C<import> or C<unimport> method is just skipped instead. Also, if
+the AUTOLOAD subroutine is an XSUB, C<$AUTOLOAD> is not populated;
+instead, you should call L<< C<SvPVX>E<sol>C<SvCUR>|perlapi >> on the
+C<CV> for C<AUTOLOAD> to retrieve the method name.)
+
Many C<AUTOLOAD> routines load in a definition for the requested
subroutine using eval(), then execute that subroutine using a special
Examples of valid syntax (even though the attributes are unknown):
- sub fnord (&\%) : switch(10,foo(7,3)) : expensive ;
- sub plugh () : Ugly('\(") :Bad ;
+ 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
+ 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
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.
+See L<perlootut> to learn how to make object method calls.
https://perl5.git.perl.org / perl5.git / blobdiff