Updated.

[p5sagit/p5-mst-13.2.git] / pod / perlsub.pod

=head1 NAME

perlsub - Perl subroutines

=head1 SYNOPSIS

To declare subroutines:

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

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

To define an anonymous subroutine at runtime:

    $subref = sub BLOCK;

To import subroutines:

    use PACKAGE qw(NAME1 NAME2 NAME3);

To call subroutines:

    NAME(LIST);	   # & is optional with parens.
    NAME LIST;	   # Parens optional if predeclared/imported.
    &NAME;	   # Passes current @_ to 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.

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 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 values are implicit
references (predating L<perlref>) to the actual scalar parameters.  The
return value of the subroutine is the value of the last expression
evaluated.  Alternatively, a return statement may be used to specify the
returned value and exit the subroutine.  If you return one or more arrays
and/or 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 the sections below on 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 = $lookahead;  # GLOBAL VARIABLES!!
	LINE: while ($lookahead = <STDIN>) {
	    if ($lookahead =~ /^[ \t]/) {
		$thisline .= $lookahead;
	    }
	    else {
		last LINE;
	    }
	}
	$thisline;
    }

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

Use array assignment to a local list to name your formal 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,
since the assignment copies the values.  Otherwise a function is free to
do in-place modifications of @_ and change its callers 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 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
    sub upcase {
	my @parms = @_;
	for (@parms) { tr/a-z/A-Z/ } 
	return @parms;
    } 

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:

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

Do not, however, be tempted to do this:

    (@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
Perl 5, and so are the parens if the subroutine has been predeclared.
(Note, however, that the "&" is I<NOT> optional when you're just naming
the subroutine, such as when it's used as an argument to defined() or
undef().  Nor is it optional when you want to do an indirect subroutine
call with a subroutine name or reference using the C<&$subref()> or
C<&{$subref}()> constructs.  See L<perlref> for more on that.)

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.

    &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 pre-declared, else "foo"

=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

A "my" declares the listed variables to be confined (lexically) to the
enclosing block, subroutine, C<eval>, or C<do/require/use>'d file.  If
more than one value is listed, the list must be placed in parens.  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>.)

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:

    $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 "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

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

both supply a list context to the righthand side, while

    my $foo = <STDIN>;

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

    my $foo, $bar = 1;

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 the 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.

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

    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 doesn't happen
until run time, so gets executed every time through a loop.

Variables declared with "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
lexical of the same name is also visible:

    package main;
    local $x = 10;
    my    $x = 20;
    print "$x and $::x\n";

That will print out 20 and 10.

You may declare "my" variables at the outer most scope of a file to
totally hide any such identifiers from the outside world.  This is similar
to a 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:

    my $secret_version = '1.001-beta';
    my $secret_sub = sub { print $secret_version };
    &$secret_sub();

As long as the reference is never returned by any function within the
module, no outside module can see the subroutine, since 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,
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.

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.

    {
	my $secret_val = 0; 
	sub gimme_another {
	    return ++$secret_val;
	} 
    } 
    # $secret_val now becomes unreachable by the outside
    # world, but retains its value between calls to gimme_another

If this function is being sourced in from a separate file 
via C<require> or C<use>, then this is probably just fine.  If it's
all in the main program, you'll need to arrange for the my() 
to be executed early, either by putting the whole block above
your pain program, or more likely, merely placing a BEGIN 
sub around it to make sure it gets executed before your program
starts to run:

    sub BEGIN {
	my $secret_val = 0; 
	sub gimme_another {
	    return ++$secret_val;
	} 
    } 

See L<perlrun> about the BEGIN function.

=head2 Temporary Values via local()

B<NOTE>: In general, you should be using "my" instead of "local", because
it's faster and safer.  Execeptions 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.

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<the 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
parens.  All listed elements must be legal lvalues.  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:

    for $i ( 0 .. 9 ) {
	$digits{$i} = $i;
    } 
    # assume this function uses global %digits hash
    parse_num();  

    # 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

Because local() is a run-time command, and so 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
as a scalar or an array.  So

    local($foo) = <STDIN>;
    local @FOO = <STDIN>;

both supply a list context to the righthand side, while

    local $foo = <STDIN>;

supplies a scalar context.

=head2 Passing Symbol Table Entries (typeglobs)

[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.]

Sometimes you don't want to pass the value of an array to a subroutine
but rather the name of it, so that the subroutine can modify the global
copy of it rather than working with a local copy.  In perl you can
refer to all objects of a particular name by prefixing the name
with a star: C<*foo>.  This is often known as a "type glob", since the
star on the front can be thought of as a wildcard match for all the
funny prefix characters on variables and subroutines and such.

When evaluated, the type glob produces a scalar value that represents
all the objects of that name, including any filehandle, format or
subroutine.  When assigned to, it causes the name mentioned to refer to
whatever "*" value was assigned to it.  Example:

    sub doubleary {
	local(*someary) = @_;
	foreach $elem (@someary) {
	    $elem *= 2;
	}
    }
    doubleary(*foo);
    doubleary(*bar);

Note that scalars are already passed by reference, so you can modify
scalar arguments without using this mechanism by referring explicitly
to $_[0] etc.  You can modify all the elements of an array by passing
all the elements as scalars, but you have to use the * mechanism (or
the equivalent reference mechanism) to push, pop or change the size of
an array.  It will certainly be faster to pass the typeglob (or reference).

Even if you don't want to modify an array, this mechanism is useful for
passing multiple arrays in a single LIST, since normally the LIST
mechanism will merge all the array values so that you can't extract out
the individual arrays.  For more on typeglobs, see L<perldata/"Typeglobs">.

=head2 Pass by Reference

If you want to pass more than one array or hash into a function--or 
return them from it--and have them maintain their integrity,
then you're going to have to use an explicit pass-by-reference.
Before you do that, you need to understand references; see L<perlref>.

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:

    @tailings = popmany ( \@a, \@b, \@c, \@d );

    sub popmany {
	my $aref;
	my @retlist = ();
	foreach $aref ( @_ ) {
	    push @retlist, pop @$aref;
	} 
	return @retlist;
    } 

Here's how you might write a function that returns a 
list of keys occurring in all the hashes passed to it:

    @common = inter( \%foo, \%bar, \%joe ); 
    sub inter {
	my ($k, $href, %seen); # locals
	foreach $href (@_) {
	    while ( $k = each %$href ) {
		$seen{$k}++;
	    } 
	} 
	return grep { $seen{$_} == @_ } keys %seen;
    } 

So far, we're just using the normal list return mechanism.
What happens if you want to pass or return a hash?  Well, 
if you're only using one of them, or you don't mind them 
concatenating, then the normal calling convention is ok, although
a little expensive.  

Where people get into trouble is here:

    (@a, @b) = func(@c, @d);
or
    (%a, %b) = func(%c, %d);

That syntax simply won't work.  It just sets @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.

If you can arrange for everyone to deal with this through references, it's
cleaner code, although not so nice to look at.  Here's a function that
takes two array references as arguments, returning the two array elements
in order of how many elements they have in them:

    ($aref, $bref) = func(\@c, \@d);
    print "@$aref has more than @$bref\n";
    sub func {
	my ($cref, $dref) = @_;
	if (@$cref > @$dref) {
	    return ($cref, $dref);
	} else {
	    return ($cref, $cref);
	} 
    } 

It turns out that you can actually do this also:

    (*a, *b) = func(\@c, \@d);
    print "@a has more than @b\n";
    sub func {
	local (*c, *d) = @_;
	if (@c > @d) {
	    return (\@c, \@d);
	} else {
	    return (\@d, \@c);
	} 
    } 

Here we're using the typeglobs to do symbol table aliasing.  It's
a tad subtle, though, and also won't work if you're using my()
variables, since only globals (well, and 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:

    splutter(\*STDOUT);
    sub splutter {
	my $fh = shift;
	print $fh "her um well a hmmm\n";
    }

    $rec = get_rec(\*STDIN);
    sub get_rec {
	my $fh = shift;
	return scalar <$fh>;
    }

If you're planning on generating new filehandles, you could do this:

    sub openit {
	my $name = 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 FileHandle
functions supplied with the POSIX package.

=head2 Prototypes

As of the 5.002 release of perl, if you declare

    sub mypush (\@@)

then mypush() takes arguments exactly like push() does.  (This only works
for function calls that are visible at compile time, not indirect function
calls through a C<&$func> reference nor for method calls as described in
L<perlobj>.)

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

Any backslashed prototype character must be passed something starting
with that character.  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, and *
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.

Note that the last three are syntactically distinguished by the lexer.
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 argumentless, just like 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.

The interesting thing about & is that you can generate new syntax with it:

    sub try (&$) {
	my($try,$catch) = @_;
	eval { &$try };
	if ($@) {
	    local $_ = $@;
	    &$catch;
	}
    }
    sub catch (&) { @_ }

    try {
	die "phooey";
    } 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
scoped, those anonymous subroutines can act like closures... (Gee,
is this sounding a little Lispish?  (Nevermind.))))

And here's a reimplementation of grep:

    sub mygrep (&@) {
	my $code = shift;
	my @result;
	foreach $_ (@_) {
	    push(@result, $_) if &$ref;
	}
	@result;
    }

Some folks would prefer full alphanumeric prototypes.  Alphanumerics have
been intentionally left out of prototypes for the express purpose of
someday in the future adding named, formal parameters.  The current
mechanism's main goal is to let module writers provide better diagnostics
for module users.  Larry feels the notation quite understandable to Perl
programmers, and that it will not intrude greatly upon the meat of the
module, nor make it harder to read.  The line noise is visually
encapsulated into a small pill that's easy to swallow.

It's probably best to prototype new functions, not retrofit prototyping
into older ones.  That's because you must be especially careful about
silent impositions of differing list versus scalar contexts.  For example,
if you decide that a function should take just one parameter, like this:

    sub func ($) {
	my $n = shift;
	print "you gave me $n\n";
    } 

and someone has been calling it with an array or expression
returning a list:

    func(@foo);
    func( split /:/ );

Then you've just supplied an automatic scalar() in front of their
argument, which can be more than a bit surprising.  The old @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 elments
in @foo.  And the split() gets called in a scalar context and
starts scribbling on your @_ parameter list.

This is all very powerful, of course, and should only be used in moderation
to make the world a better place.  

=head2 Overriding Builtin Functions

Many builtin functions may be overridden, though this should only be
tried occasionally and for good reason.  Typically this might be
done by a package attempting to emulate missing builtin functionality
on a non-Unix system.

Overriding may only be done by importing the name from a
module--ordinary predeclaration isn't good enough.  However, the
C<subs> pragma (compiler directive) lets you, in effect, predeclare subs
via the import syntax, and these names may then override the builtin ones:

    use subs 'chdir', 'chroot', 'chmod', 'chown';
    chdir $somewhere;
    sub chdir { ... }

Library modules should not in general export builtin names like "open"
or "chdir" as part of their default @EXPORT list, since these may
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
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

    use Module;

they would get the default imports without the overrides.

=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:

    sub AUTOLOAD {
	my $program = $AUTOLOAD;
	$program =~ s/.*:://;
	system($program, @_);
    } 
    date();
    who('am', i');
    ls('-l');

In fact, if you preclare the functions you want to call that way, you don't
even need the parentheses:

    use subs qw(date who ls);
    date;
    who "am", "i";
    ls -l;

A more complete example of this is the standard Shell module, which
can treat undefined subroutine calls as calls to Unix programs.

Mechanisms are available for modules writers to help split the modules
up into autoloadable files.  See the standard AutoLoader module described
in L<Autoloader>, the standard SelfLoader modules in L<SelfLoader>, and
the document on adding C functions to perl code in L<perlxs>.

=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.