perlsub.pod

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=head1 NAME

perlsub - Perl subroutines

=head1 SYNOPSIS

To declare subroutines:

    sub NAME;			  # A "forward" declaration.
    sub NAME(PROTO);		  #  ditto, but with prototypes
    sub NAME : ATTRS;		  #  with attributes
    sub NAME(PROTO) : ATTRS;	  #  with attributes and prototypes

    sub NAME BLOCK		  # A declaration and a definition.
    sub NAME(PROTO) BLOCK	  #  ditto, but with prototypes
    sub NAME : ATTRS BLOCK	  #  with attributes
    sub NAME(PROTO) : ATTRS BLOCK #  with prototypes and attributes

To define an anonymous subroutine at runtime:

    $subref = sub BLOCK;		 # no proto
    $subref = sub (PROTO) BLOCK;	 # with proto
    $subref = sub : ATTRS BLOCK;	 # with attributes
    $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes

To import subroutines:

    use MODULE qw(NAME1 NAME2 NAME3);

To call subroutines:

    NAME(LIST);	   # & is optional with parentheses.
    NAME LIST;	   # Parentheses optional if predeclared/imported.
    &NAME(LIST);   # Circumvent prototypes.
    &NAME;	   # Makes current @_ visible to called subroutine.

=head1 DESCRIPTION

Like many languages, Perl provides for user-defined subroutines.
These may be located anywhere in the main program, loaded in from
other files via the C<do>, C<require>, or C<use> keywords, or
generated on the fly using C<eval> or anonymous subroutines.
You can even call a function indirectly using a variable containing
its name or a CODE reference.

The Perl model for function call and return values is simple: all
functions are passed as parameters one single flat list of scalars, and
all functions likewise return to their caller one single flat list of
scalars.  Any arrays or hashes in these call and return lists will
collapse, losing their identities--but you may always use
pass-by-reference instead to avoid this.  Both call and return lists may
contain as many or as few scalar elements as you'd like.  (Often a
function without an explicit return statement is called a subroutine, but
there's really no difference from Perl's perspective.)

Any arguments passed in show up in the array C<@_>.  Therefore, if
you called a function with two arguments, those would be stored in
C<$_[0]> and C<$_[1]>.  The array C<@_> is a local array, but its
elements are aliases for the actual scalar parameters.  In particular,
if an element C<$_[0]> is updated, the corresponding argument is
updated (or an error occurs if it is not updatable).  If an argument
is an array or hash element which did not exist when the function
was called, that element is created only when (and if) it is modified
or a reference to it is taken.  (Some earlier versions of Perl
created the element whether or not the element was assigned to.)
Assigning to the whole array C<@_> removes that aliasing, and does
not update any arguments.

The return value of a subroutine is the value of the last expression
evaluated.  More explicitly, a C<return> statement may be used to exit the
subroutine, optionally specifying the returned value, which will be
evaluated in the appropriate context (list, scalar, or void) depending
on the context of the subroutine call.  If you specify no return value,
the subroutine returns an empty list in list context, the undefined
value in scalar context, or nothing in void context.  If you return
one or more aggregates (arrays and hashes), these will be flattened
together into one large indistinguishable list.

Perl does not have named formal parameters.  In practice all you
do is assign to a C<my()> list of these.  Variables that aren't
declared to be private are global variables.  For gory details
on creating private variables, see L<"Private Variables via my()">
and L<"Temporary Values via local()">.  To create protected
environments for a set of functions in a separate package (and
probably a separate file), see L<perlmod/"Packages">.

Example:

    sub max {
	my $max = shift(@_);
	foreach $foo (@_) {
	    $max = $foo if $max < $foo;
	}
	return $max;
    }
    $bestday = max($mon,$tue,$wed,$thu,$fri);

Example:

    # get a line, combining continuation lines
    #  that start with whitespace

    sub get_line {
	$thisline = $lookahead;  # global variables!
	LINE: while (defined($lookahead = <STDIN>)) {
	    if ($lookahead =~ /^[ \t]/) {
		$thisline .= $lookahead;
	    }
	    else {
		last LINE;
	    }
	}
	return $thisline;
    }

    $lookahead = <STDIN>;	# get first line
    while (defined($line = get_line())) {
	...
    }

Assigning to a list of private variables to name your arguments:

    sub maybeset {
	my($key, $value) = @_;
	$Foo{$key} = $value unless $Foo{$key};
    }

Because the assignment copies the values, this also has the effect
of turning call-by-reference into call-by-value.  Otherwise a
function is free to do in-place modifications of C<@_> and change
its caller's values.

    upcase_in($v1, $v2);  # this changes $v1 and $v2
    sub upcase_in {
	for (@_) { tr/a-z/A-Z/ }
    }

You aren't allowed to modify constants in this way, of course.  If an
argument were actually literal and you tried to change it, you'd take a
(presumably fatal) exception.   For example, this won't work:

    upcase_in("frederick");

It would be much safer if the C<upcase_in()> function
were written to return a copy of its parameters instead
of changing them in place:

    ($v3, $v4) = upcase($v1, $v2);  # this doesn't change $v1 and $v2
    sub upcase {
	return unless defined wantarray;  # void context, do nothing
	my @parms = @_;
	for (@parms) { tr/a-z/A-Z/ }
  	return wantarray ? @parms : $parms[0];
    }

Notice how this (unprototyped) function doesn't care whether it was
passed real scalars or arrays.  Perl sees all arguments as one big,
long, flat parameter list in C<@_>.  This is one area where
Perl's simple argument-passing style shines.  The C<upcase()>
function would work perfectly well without changing the C<upcase()>
definition even if we fed it things like this:

    @newlist   = upcase(@list1, @list2);
    @newlist   = upcase( split /:/, $var );

Do not, however, be tempted to do this:

    (@a, @b)   = upcase(@list1, @list2);

Like the flattened incoming parameter list, the return list is also
flattened on return.  So all you have managed to do here is stored
everything in C<@a> and made C<@b> an empty list.  See 
L<Pass by Reference> for alternatives.

A subroutine may be called using an explicit C<&> prefix.  The
C<&> is optional in modern Perl, as are parentheses if the
subroutine has been predeclared.  The C<&> is I<not> optional
when just naming the subroutine, such as when it's used as
an argument to defined() or undef().  Nor is it optional when you
want to do an indirect subroutine call with a subroutine name or
reference using the C<&$subref()> or C<&{$subref}()> constructs,
although the C<< $subref->() >> notation solves that problem.
See L<perlref> for more about all that.

Subroutines may be called recursively.  If a subroutine is called
using the C<&> form, the argument list is optional, and if omitted,
no C<@_> array is set up for the subroutine: the C<@_> array at the
time of the call is visible to subroutine instead.  This is an
efficiency mechanism that new users may wish to avoid.

    &foo(1,2,3);	# pass three arguments
    foo(1,2,3);		# the same

    foo();		# pass a null list
    &foo();		# the same

    &foo;		# foo() get current args, like foo(@_) !!
    foo;		# like foo() IFF sub foo predeclared, else "foo"

Not only does the C<&> form make the argument list optional, it also
disables any prototype checking on arguments you do provide.  This
is partly for historical reasons, and partly for having a convenient way
to cheat if you know what you're doing.  See L<Prototypes> below.

Functions whose names are in all upper case are reserved to the Perl
core, as are modules whose names are in all lower case.  A
function in all capitals is a loosely-held convention meaning it
will be called indirectly by the run-time system itself, usually
due to a triggered event.  Functions that do special, pre-defined
things include C<BEGIN>, C<CHECK>, C<INIT>, C<END>, C<AUTOLOAD>, and
C<DESTROY>--plus all functions mentioned in L<perltie>.

=head2 Private Variables via my()

Synopsis:

    my $foo;	    	# declare $foo lexically local
    my (@wid, %get); 	# declare list of variables local
    my $foo = "flurp";	# declare $foo lexical, and init it
    my @oof = @bar;	# declare @oof lexical, and init it
    my $x : Foo = $y;	# similar, with an attribute applied

B<WARNING>: The use of attribute lists on C<my> declarations is
experimental.  This feature should not be relied upon.  It may
change or disappear in future releases of Perl.  See L<attributes>.

The C<my> operator declares the listed variables to be lexically
confined to the enclosing block, conditional (C<if/unless/elsif/else>),
loop (C<for/foreach/while/until/continue>), subroutine, C<eval>,
or C<do/require/use>'d file.  If more than one value is listed, the
list must be placed in parentheses.  All listed elements must be
legal lvalues.  Only alphanumeric identifiers may be lexically
scoped--magical built-ins like C<$/> must currently be C<local>ize
with C<local> instead.

Unlike dynamic variables created by the C<local> operator, lexical
variables declared with C<my> are totally hidden from the outside
world, including any called subroutines.  This is true if it's the
same subroutine called from itself or elsewhere--every call gets
its own copy.

This doesn't mean that a C<my> variable declared in a statically
enclosing lexical scope would be invisible.  Only dynamic scopes
are cut off.   For example, the C<bumpx()> function below has access
to the lexical $x variable because both the C<my> and the C<sub>
occurred at the same scope, presumably file scope.

    my $x = 10;
    sub bumpx { $x++ } 

An C<eval()>, however, can see lexical variables of the scope it is
being evaluated in, so long as the names aren't hidden by declarations within
the C<eval()> itself.  See L<perlref>.

The parameter list to my() may be assigned to if desired, which allows you
to initialize your variables.  (If no initializer is given for a
particular variable, it is created with the undefined value.)  Commonly
this is used to name input parameters to a subroutine.  Examples:

    $arg = "fred";	  # "global" variable
    $n = cube_root(27);
    print "$arg thinks the root is $n\n";
 fred thinks the root is 3

    sub cube_root {
	my $arg = shift;  # name doesn't matter
	$arg **= 1/3;
	return $arg;
    }

The C<my> is simply a modifier on something you might assign to.  So when
you do assign to variables in its argument list, C<my> doesn't
change whether those variables are viewed as a scalar or an array.  So

    my ($foo) = <STDIN>;		# WRONG?
    my @FOO = <STDIN>;

both supply a list context to the right-hand side, while

    my $foo = <STDIN>;

supplies a scalar context.  But the following declares only one variable:

    my $foo, $bar = 1;			# WRONG

That has the same effect as

    my $foo;
    $bar = 1;

The declared variable is not introduced (is not visible) until after
the current statement.  Thus,

    my $x = $x;

can be used to initialize a new $x with the value of the old $x, and
the expression

    my $x = 123 and $x == 123

is false unless the old $x happened to have the value C<123>.

Lexical scopes of control structures are not bounded precisely by the
braces that delimit their controlled blocks; control expressions are
part of that scope, too.  Thus in the loop

    while (my $line = <>) {
        $line = lc $line;
    } continue {
        print $line;
    }

the scope of $line extends from its declaration throughout the rest of
the loop construct (including the C<continue> clause), but not beyond
it.  Similarly, in the conditional

    if ((my $answer = <STDIN>) =~ /^yes$/i) {
        user_agrees();
    } elsif ($answer =~ /^no$/i) {
        user_disagrees();