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 parentheses.
NAME LIST; # Parentheses optional if pre-declared/imported.
&NAME; # Passes current @_ to subroutine.
$var = \&function.
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 $_[0]
and $_[1]. The array @_ is a local array, but its values are implicit
references (predating the perlref manpage) 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
Private Variables via my() and Temporary Values via local(). To create protected environments for a set of functions in a separate
package (and probably a separate file), see 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,
because the assignment copies the values. Otherwise a function is free to
do in-place modifications of @_ and change its caller's
values.
upcase_in($v1, $v2); # this changes $v1 and $v2
sub upcase_in {
for (@_) { tr/a-z/A-Z/ }
}
You aren't allowed to modify constants in this way, of course. If an argument were actually literal and you tried to change it, you'd take a (presumably fatal) exception. For example, this won't work:
upcase_in("frederick");
It would be much safer if the upcase_in function were written
to return a copy of its parameters instead of changing them in place:
($v3, $v4) = upcase($v1, $v2); # this doesn't
sub upcase {
my @parms = @_;
for (@parms) { tr/a-z/A-Z/ }
# wantarray checks if we were called in list context
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:
@newlist = upcase(@list1, @list2);
@newlist = upcase( split /:/, $var );
Do not, however, be tempted to do this:
(@a, @b) = upcase(@list1, @list2);
Because like its flat incoming parameter list, the return list is also
flat. So all you have managed to do here is stored everything in
@a and made @b an empty list. See Pass by Reference for alternatives.
A subroutine may be called using the ``&'' prefix.
The ``&'' is optional in modern Perls, and so are the parentheses if
the subroutine has been pre-declared. (Note, however, that the ``&'' is NOT optional when you're just naming the subroutine, such as when it's used as
an argument to defined or undef. Nor is it
optional when you want to do an indirect subroutine call with a subroutine
name or reference using the &$subref or &{$subref} constructs. See the perlref manpage
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"
Not only does the ``&'' form make the argument list optional, but it also disables any prototype checking on the 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.
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, conditional (if/unless/elsif/else), loop (for/foreach/while/until/continue), subroutine, eval, or
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 the perlref manpage.)
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 right-hand side, while
my $foo = <STDIN>;
supplies a scalar context. But the following declares only 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.
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
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 continue clause), but not beyond it. Similarly, in the conditional
if ((my $answer = <STDIN>) =~ /^yes$/i) {
user_agrees();
} elsif ($answer =~ /^no$/i) {
user_disagrees();
} else {
chomp $answer;
die "'$answer' is neither 'yes' nor 'no'";
}
the scope of $answer extends from its declaration throughout
the rest of the conditional (including elsif and else clauses, if any), but not beyond it.
(None of the foregoing applies to if/unless or while/until
modifiers appended to simple statements. Such modifiers are not control
structures and have no effect on scoping.)
The foreach loop defaults to scoping its index variable dynamically (in the manner of local; see below). However, if the index variable is prefixed with the keyword
``my'', then it is lexically scoped instead. Thus in the loop
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.
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 "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 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 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 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, because its name is not in any package's symbol table. Remember that it's not 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 require or 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, placing merely 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 the perlrun manpage about the BEGIN function.
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, eval{}, or do) and 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 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, 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 as a scalar or an array. So
local($foo) = <STDIN>;
local @FOO = <STDIN>;
both supply a list context to the right-hand side, while
local $foo = <STDIN>;
supplies a scalar context.
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: *foo. This is often known as a ``typeglob'', because 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 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:
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, because 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 Typeglobs and Filehandles.
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 using just the normal list return mechanism. What happens if you want to pass or return a hash? Well, if you're using only 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 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.
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, $dref);
} else {
return ($dref, $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, because only globals (well, and locals) 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
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>;
}
Another way to do this is using *HANDLE{IO}, see the perlref manpage for usage and caveats.
If you're planning on generating new filehandles, you could do this:
sub openit {
my $name = shift;
local *FH;
return open (FH, $path) ? *FH : undef;
}
Although that will actually produce a small memory leak. See the bottom of open() for a somewhat cleaner way using the IO::Handle package.
sub mypush (\@@)
then mypush takes arguments exactly like push
does. The declaration of the function to be called must be visible at
compile time. The prototype affects only the interpretation of new-style
calls to the function, where new-style is defined as not using the & character. In other words, if you call it like a builtin function, then it
behaves like a builtin function. If you call it like an old-fashioned
subroutine, then it behaves like an old-fashioned subroutine. It naturally
falls out from this rule that prototypes have no influence on subroutine
references like \&foo or on indirect subroutine calls like &{$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
Any backslashed prototype character represents an actual argument that
absolutely must start with that character. The value passed to the
subroutine (as part of @_) will be a reference to the actual argument given in the subroutine call,
obtained by applying
\ to that argument.
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
mytime +2;
you'll get mytime + 2, not mytime, 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 (&) { $_[0] }
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? (Never mind.))))
And here's a reimplementation of grep:
sub mygrep (&@) {
my $code = shift;
my @result;
foreach $_ (@_) {
push(@result, $_) if &$code;
}
@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
elements 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 be used only in moderation to make the world a better place.
&) are not affected.
All of the following functions would be inlined.
sub PI () { 3.14159 }
sub ST_DEV () { 0 }
sub ST_INO () { 1 }
sub FLAG_FOO () { 1 << 8 }
sub FLAG_BAR () { 1 << 9 }
sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
sub OPT_BAZ () { 1 }
sub BAZ_VAL () {
if (OPT_BAZ) {
return 23;
}
else {
return 42;
}
}
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 prototype (which changes the calling semantics, so beware) or by thwarting
the inlining mechanism in some other way, such as
my $dummy;
sub not_inlined () {
$dummy || 23
}
Overriding may be done only by importing the name from a module--ordinary
predeclaration isn't good enough. However, the
subs pragma (compiler directive) lets you, in effect, pre-declare 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, 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 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.
AUTOLOAD subroutine defined in the package or packages that were searched for the
original subroutine, then that
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 AUTOLOAD routine. The name is not passed as an ordinary argument because, er, well,
just because, that's why...
Most 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 AUTOLOAD routine without a trace. (See the standard AutoLoader module, for example.) But an 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 pre-declare 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 the AutoLoader manpage and in the AutoSplit manpage, the standard SelfLoader modules in the SelfLoader manpage, and the document on adding C functions to perl code in the perlxs manpage.