perlsub.1

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.\" ========================================================================
.\"
.IX Title "PERLSUB 1"
.TH PERLSUB 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
perlsub \- Perl subroutines
.IX Xref "subroutine function"
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
To declare subroutines:
.IX Xref "subroutine, declaration sub"
.PP
.Vb 4
\&    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
.Ve
.PP
To define an anonymous subroutine at runtime:
.IX Xref "subroutine, anonymous"
.PP
.Vb 4
\&    $subref = sub BLOCK;                 # no proto
\&    $subref = sub (PROTO) BLOCK;         # with proto
\&    $subref = sub : ATTRS BLOCK;         # with attributes
\&    $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
.Ve
.PP
To import subroutines:
.IX Xref "import"
.PP
.Vb 1
\&    use MODULE qw(NAME1 NAME2 NAME3);
.Ve
.PP
To call subroutines:
.IX Xref "subroutine, call call"
.PP
.Vb 4
\&    NAME(LIST);    # & is optional with parentheses.
\&    NAME LIST;     # Parentheses optional if predeclared/imported.
\&    &NAME(LIST);   # Circumvent prototypes.
\&    &NAME;         # Makes current @_ visible to called subroutine.
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Like many languages, Perl provides for user-defined subroutines.
These may be located anywhere in the main program, loaded in from
other files via the \f(CW\*(C`do\*(C'\fR, \f(CW\*(C`require\*(C'\fR, or \f(CW\*(C`use\*(C'\fR keywords, or
generated on the fly using \f(CW\*(C`eval\*(C'\fR or anonymous subroutines.
You can even call a function indirectly using a variable containing
its name or a \s-1CODE\s0 reference.
.PP
The Perl model for function call and return values is simple: all
functions are passed as parameters one single flat list of scalars, and
all functions likewise return to their caller one single flat list of
scalars.  Any arrays or hashes in these call and return lists will
collapse, losing their identities\*(--but you may always use
pass-by-reference instead to avoid this.  Both call and return lists may
contain as many or as few scalar elements as you'd like.  (Often a
function without an explicit return statement is called a subroutine, but
there's really no difference from Perl's perspective.)
.IX Xref "subroutine, parameter parameter"
.PP
Any arguments passed in show up in the array \f(CW@_\fR.  Therefore, if
you called a function with two arguments, those would be stored in
\&\f(CW$_[0]\fR and \f(CW$_[1]\fR.  The array \f(CW@_\fR is a local array, but its
elements are aliases for the actual scalar parameters.  In particular,
if an element \f(CW$_[0]\fR 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 \f(CW@_\fR removes that aliasing, and does
not update any arguments.
.IX Xref "subroutine, argument argument @_"
.PP
A \f(CW\*(C`return\*(C'\fR 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.
.PP
If no \f(CW\*(C`return\*(C'\fR 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 \f(CW\*(C`foreach\*(C'\fR or a \f(CW\*(C`while\*(C'\fR, the returned value is unspecified. The
empty sub returns the empty list.
.IX Xref "subroutine, return value return value return"
.PP
Perl does not have named formal parameters.  In practice all you
do is assign to a \f(CW\*(C`my()\*(C'\fR 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 \fImy()\fR\*(R"
and \*(L"Temporary Values via \fIlocal()\fR\*(R".  To create protected
environments for a set of functions in a separate package (and
probably a separate file), see \*(L"Packages\*(R" in perlmod.
.IX Xref "formal parameter parameter, formal"
.PP
Example:
.PP
.Vb 8
\&    sub max {
\&        my $max = shift(@_);
\&        foreach $foo (@_) {
\&            $max = $foo if $max < $foo;
\&        }
\&        return $max;
\&    }
\&    $bestday = max($mon,$tue,$wed,$thu,$fri);
.Ve
.PP
Example:
.PP
.Vb 2
\&    # get a line, combining continuation lines
\&    #  that start with whitespace
\&
\&    sub get_line {
\&        $thisline = $lookahead;  # global variables!
\&        LINE: while (defined($lookahead = <STDIN>)) {
\&            if ($lookahead =~ /^[ \et]/) {
\&                $thisline .= $lookahead;
\&            }
\&            else {
\&                last LINE;
\&            }
\&        }
\&        return $thisline;
\&    }
\&
\&    $lookahead = <STDIN>;       # get first line
\&    while (defined($line = get_line())) {
\&        ...
\&    }
.Ve
.PP
Assigning to a list of private variables to name your arguments:
.PP
.Vb 4
\&    sub maybeset {
\&        my($key, $value) = @_;
\&        $Foo{$key} = $value unless $Foo{$key};
\&    }
.Ve
.PP
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 \f(CW@_\fR and change
its caller's values.
.IX Xref "call-by-reference call-by-value"
.PP
.Vb 4
\&    upcase_in($v1, $v2);  # this changes $v1 and $v2
\&    sub upcase_in {
\&        for (@_) { tr/a\-z/A\-Z/ }
\&    }
.Ve
.PP
You aren't allowed to modify constants in this way, of course.  If an
argument were actually literal and you tried to change it, you'd take a
(presumably fatal) exception.   For example, this won't work:
.IX Xref "call-by-reference call-by-value"
.PP
.Vb 1
\&    upcase_in("frederick");
.Ve
.PP
It would be much safer if the \f(CW\*(C`upcase_in()\*(C'\fR function
were written to return a copy of its parameters instead
of changing them in place:
.PP
.Vb 7
\&    ($v3, $v4) = upcase($v1, $v2);  # this doesn\*(Aqt 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];
\&    }
.Ve
.PP
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 \f(CW@_\fR.  This is one area where
Perl's simple argument-passing style shines.  The \f(CW\*(C`upcase()\*(C'\fR
function would work perfectly well without changing the \f(CW\*(C`upcase()\*(C'\fR
definition even if we fed it things like this:
.PP
.Vb 2
\&    @newlist   = upcase(@list1, @list2);
\&    @newlist   = upcase( split /:/, $var );
.Ve
.PP
Do not, however, be tempted to do this: