PERLSUB(1) Perl Programmers Reference Guide PERLSUB(1)
NAME
perlsub - Perl subroutines
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.
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 "do", "require", or "use" keywords, or generated on the fly using
"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 @_. Therefore, if you called a function with two arguments,
those would be stored in $_[0] and $_[1]. The array @_ is a local array, but its elements are aliases for the
actual scalar parameters. In particular, if an element $_[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 @_ removes that aliasing, and does not update any arguments.
A "return" statement may be used to exit a subroutine, optionally specifying the returned value, which will be
environments for a set of functions in a separate package (and probably a separate file), see "Packages" in
perlmod.
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 @_ 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:
}
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 @_. This is one area 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 /:/, $var );
Do not, however, be tempted to do this:
(@a, @b) = upcase(@list1, @list2);
Like the flattened incoming parameter list, the return list is also flattened on return. So all you have
managed to do here is stored everything in @a and made @b empty. See "Pass by Reference" for alternatives.
A subroutine may be called using an explicit "&" prefix. The "&" is optional in modern Perl, as are
parentheses if the subroutine has been predeclared. The "&" is not optional when just naming the subroutine,
such as when it's used as an argument to defined() or undef(). Nor is it optional when you want to do an
indirect subroutine call with a subroutine name or reference using the "&$subref()" or "&{$subref}()"
constructs, although the "$subref->()" notation solves that problem. See perlref for more about all 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 predeclared, else "foo"
Not only does the "&" 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 "Prototypes" below.
Since Perl 5.16.0, the "__SUB__" token is available under "use feature 'current_sub'" and "use 5.16.0". It
will evaluate to a reference to the currently-running sub, which allows for recursive calls without knowing
your subroutine's name.
use 5.16.0;
my $factorial = sub {
my ($x) = @_;
return 1 if $x == 1;
return($x * __SUB__->( $x - 1 ) );
};
Subroutines whose names are in all upper case are reserved to the Perl core, as are modules whose names are in
all lower case. A subroutine 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. Subroutines that do special, pre-
my @oof = @bar; # declare @oof lexical, and init it
my $x : Foo = $y; # similar, with an attribute applied
WARNING: The use of attribute lists on "my" declarations is still evolving. The current semantics and
interface are subject to change. See attributes and Attribute::Handlers.
The "my" operator declares the listed variables to be lexically confined 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 built-ins like $/ must
currently be "local"ized with "local" instead.
Unlike dynamic variables created by the "local" operator, lexical variables declared with "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 "my" variable declared in a statically enclosing lexical scope would be invisible.
Only dynamic scopes are cut off. For example, the "bumpx()" function below has access to the lexical $x
variable because both the "my" and the "sub" occurred at the same scope, presumably file scope.
my $x = 10;
sub bumpx { $x++ }
An "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 "eval()" itself. See 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 $arg;
}
The "my" is simply a modifier on something you might assign to. So when you do assign to variables in its
argument list, "my" doesn't change whether those variables are viewed as a scalar or an array. So
my ($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
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 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
"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 through the rest of that conditional, including any "elsif"
and "else" clauses, but not beyond it. See "Simple Statements" in perlsyn for information on the scope of
variables in statements with modifiers.
The "foreach" loop defaults to scoping its index variable dynamically in the manner of "local". However, if
the index variable is prefixed with the keyword "my", or if there is already a lexical by that name in scope,
then a new lexical is created 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, rendering the value of $i inaccessible
within "some_function()".
Some users may wish to encourage the use of lexically scoped variables. As an aid to catching implicit uses
to package variables, which are always global, if you say
use strict 'vars';
then any variable mentioned from there to the end of the enclosing block must either refer to a lexical
variable, be predeclared via "our" or "use vars", or else 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
principal usefulness of this is to quiet "use strict 'vars'", but it is also essential for generation of
closures as detailed in perlref. Actual initialization is delayed until run time, though, so it gets executed
at the appropriate time, such as each time through a loop, for example.
That will print out 20 and 10.
You may declare "my" variables at the outermost scope of a file to hide any such identifiers from the world
outside that file. This is similar in spirit to C's static variables when they are used at the file level.
To do this with a subroutine requires the use of a closure (an anonymous function that accesses enclosing
lexicals). If you want 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. See "Function Templates" in perlref for something of a work-around to this.
Persistent Private Variables
There are two ways to build persistent private variables in Perl 5.10. First, you can simply use the "state"
feature. Or, you can use closures, if you want to stay compatible with releases older than 5.10.
Persistent variables via state()
Beginning with Perl 5.9.4, you can declare variables with the "state" keyword in place of "my". For that to
work, though, you must have enabled that feature beforehand, either by using the "feature" pragma, or by using
"-E" on one-liners (see feature). Beginning with Perl 5.16, the "CORE::state" form does not require the
"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 $x is lexical, it can't be reached or modified by any Perl code outside.
When combined with variable declaration, simple scalar assignment to "state" variables (as in "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.
Persistent variables with closures
Just because a lexical variable is lexically (also called statically) scoped to its enclosing block, "eval",
or "do" FILE, this doesn't mean that within a function it works like a C static. It normally works more like
a C auto, but with implicit garbage collection.
Unlike local variables in C or C++, Perl's lexical variables don't necessarily get recycled just because their
scope has exited. If something more permanent is still aware of the lexical, it will stick around. So long
as something else references a lexical, that lexical won't be freed--which is as it should be. You wouldn't
want memory being free until you were done using it, or kept around once you were done. Automatic garbage
collection takes care of this for you.
}
# $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 main program, or more likely, placing merely a "BEGIN" code block around it
to make sure it gets executed before your program starts to run:
BEGIN {
my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
}
}
See "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod about the special triggered code blocks, "BEGIN",
"UNITCHECK", "CHECK", "INIT" and "END".
If declared at the outermost scope (the file scope), then lexicals work somewhat like C's file statics. They
are available to all functions in that same file declared below them, but are inaccessible from outside that
file. This strategy is sometimes used in modules to create private variables that the whole module can see.
Temporary Values via local()
WARNING: In general, you should be using "my" instead of "local", because it's faster and safer. Exceptions
to this include the global punctuation variables, global filehandles and formats, and direct manipulation of
the Perl symbol table itself. "local" is mostly used when the current value of a variable must be visible to
called subroutines.
Synopsis:
# localization of values
local $foo; # make $foo dynamically local
local (@wid, %get); # make list of variables local
local $foo = "flurp"; # make $foo dynamic, and init it
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
# localization of symbols
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 $merlyn, not @merlyn etc
A "local" modifies its listed variables to be "local" to the enclosing block, "eval", or "do FILE"--and to any
subroutine called from within that block. A "local" just gives temporary values to global (meaning package)
variables. It does not create a local variable. This is known as dynamic scoping. Lexical scoping is done
Because "local" is a run-time operator, it gets executed each time through a loop. Consequently, it's more
efficient to localize your variables outside the loop.
Grammatical note on local()
A "local" is simply a modifier on an lvalue expression. When you assign to a "local"ized variable, the
"local" doesn't change whether its list is viewed as a scalar or an array. So
local($foo) = <STDIN>;
local @FOO = <STDIN>;
both supply a list context to the right-hand side, while
local $foo = <STDIN>;
supplies a scalar context.
Localization of special variables
If you localize a special variable, you'll be giving a new value to it, but its magic won't go away. That
means that all side-effects related to this magic still work with the localized value.
This feature allows code like this to work :
# Read the whole contents of FILE in $slurp
{ local $/ = undef; $slurp = <FILE>; }
Note, however, that this restricts localization of some values ; for example, the following statement dies, as
of perl 5.9.0, with an error Modification of a read-only value attempted, because the $1 variable is magical
and read-only :
local $1 = 2;
One exception is the default scalar variable: starting with perl 5.14 "local($_)" will always strip all magic
from $_, to make it possible to safely reuse $_ in a subroutine.
WARNING: Localization of tied arrays and hashes does not currently work as described. This will be fixed in a
future release of Perl; in the meantime, avoid code that relies on any particular behaviour of localising tied
arrays or hashes (localising individual elements is still okay). See "Localising Tied Arrays and Hashes Is
Broken" in perl58delta for more details.
Localization of globs
The construct
local *name;
creates a whole new symbol table entry for the glob "name" in the current package. That means that all
variables in its glob slot ($name, @name, %name, &name, and the "name" filehandle) are dynamically reset.
This implies, among other things, that any magic eventually carried by those variables is locally lost. In
other words, saying "local */" will not have any effect on the internal value of the input record separator.
{
local($ary[5]) = 6;
local($hash{'a'}) = 'drill';
while (my $e = pop(@ary)) {
print "$e . . .\n";
last unless $e > 3;
}
if (@ary) {
$hash{'only a'} = 'test';
delete $hash{'a'};
}
}
print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
print "The array has ",scalar(@ary)," elements: ",
join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
Perl will print
6 . . .
4 . . .
3 . . .
This is a test only a test.
The array has 6 elements: 0, 1, 2, undef, undef, 5
The behavior of local() on non-existent members of composite types is subject to change in future.
Localized deletion of elements of composite types
You can use the "delete local $array[$idx]" and "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 "local" is scoped to the "do" block. Slices are also accepted.
my %hash = (
a => [ 7, 8, 9 ],
b => 1,
)
}
# $a is back to [ 7, 8, 9 ]
}
# %hash is back to its original state
Lvalue subroutines
WARNING: Lvalue subroutines are still experimental and the implementation may change in future versions of
Perl.
It is possible to return a modifiable value from a subroutine. To do this, you have to declare the subroutine
to return an lvalue.
my $val;
sub canmod : lvalue {
$val; # or: return $val;
}
sub nomod {
$val;
}
canmod() = 5; # assigns to $val
nomod() = 5; # ERROR
The scalar/list context for the subroutine and for the right-hand side of assignment is determined as if the
subroutine call is replaced by a scalar. For example, consider:
data(2,3) = get_data(3,4);
Both subroutines here are called in a scalar context, while in:
(data(2,3)) = get_data(3,4);
and in:
(data(2),data(3)) = get_data(3,4);
all the subroutines are called in a list context.
Lvalue subroutines are EXPERIMENTAL
They appear to be convenient, but there is at least one reason to be circumspect.
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;
my $some_array_ref = []; # protected by mutators ??
sub set_arr { # normal mutator
my $val = shift;
die("expected array, you supplied ", ref $val)
unless ref $val eq 'ARRAY';
$some_array_ref = $val;
}
sub set_arr_lv : lvalue { # lvalue mutator
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);
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" in perldata.
When to Still Use local()
Despite the existence of "my", there are still three places where the "local" operator still shines. In fact,
in these three places, you must use "local" instead of "my".
1. You need to give a global variable a temporary value, especially $_.
The global variables, like @ARGV or the punctuation variables, must be "local"ized with "local()". This
block reads in /etc/motd, and splits it up into chunks separated by lines of equal signs, which are placed
in @Fields.
{
local @ARGV = ("/etc/motd");
local $/ = undef;
local $_ = <>;
@Fields = split /^\s*=+\s*$/;
}
It particular, it's important to "local"ize $_ in any routine that assigns to it. Look out for implicit
assignments in "while" conditionals.
2. You need to create a local file or directory handle or a local function.
A function that needs a filehandle of its own must use "local()" on a complete typeglob. This can be
used to create new symbol table entries:
sub ioqueue {
local (*READER, *WRITER); # not my!
grow(); # really calls shrink()
move(); # if move() grow()s, it shrink()s too
}
grow(); # get the real grow() again
See "Function Templates" in perlref for more about manipulating functions by name in this way.
3. You want to temporarily change just one element of an array or hash.
You can "local"ize just one element of an aggregate. Usually this is done on dynamics:
{
local $SIG{INT} = 'IGNORE';
funct(); # uninterruptible
}
# interruptibility automatically restored here
But it also works on lexically declared aggregates. Prior to 5.005, this operation could on occasion
misbehave.
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 as detailed in perlref. This section may not make much sense to you
otherwise.
Here are a few simple examples. First, let's pass in several arrays to a function and have it "pop" all of
then, returning 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?
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 (even in disguise as "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 work, too. 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. Notice to pass back just the bare *FH,
not its reference.
sub openit {
my $path = shift;
local *FH;
return open (FH, $path) ? *FH : undef;
}
subroutine references like "\&foo" or on indirect subroutine calls like "&{$subref}" or "$subref->()".
Method calls are not influenced by prototypes either, because the function to be called is indeterminate at
compile time, since the exact code called depends on inheritance.
Because the intent of this feature is primarily to let you define subroutines that work like built-in
functions, here are prototypes for some other functions that parse almost exactly like the corresponding
built-in.
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, 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 mytime () mytime
Any backslashed prototype character represents an actual argument that must start with that character
(optionally preceded by "my", "our" or "local"), with the exception of "$", which will accept any scalar
lvalue expression, such as "$foo = 7" or "my_function()->[0]". The value passed as part of @_ will be a
reference to the actual argument given in the subroutine call, obtained by applying "\" to that argument.
You can use the "\[]" backslash group notation to specify more than one allowed argument type. For example:
sub myref (\[$@%&*])
will allow calling myref() as
myref $var
myref @array
myref %hash
myref &sub
myref *glob
and the first argument of myref() will be a reference to a scalar, an array, a hash, a code, or a glob.
Unbackslashed prototype characters have special meanings. Any unbackslashed "@" or "%" eats all remaining
arguments, and forces list context. An argument represented by "$" 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 "*" allows the subroutine to accept a bareword, constant, scalar expression, typeglob, or a reference to a
typeglob in that slot. The value will be available to the subroutine either as a simple scalar, or (in the
latter two cases) as a reference to the typeglob. If you wish to always convert such arguments to a typeglob
reference, use Symbol::qualify_to_ref() as follows:
sub mypush (+@) {
my $aref = shift;
die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
push @$aref, @_;
}
When using the "+" prototype, your function must check that the argument is of an acceptable type.
A semicolon (";") separates mandatory arguments from optional arguments. It is redundant before "@" or "%",
which gobble up everything else.
As the last character of a prototype, or just before a semicolon, a "@" or a "%", you can use "_" in place of
"$": if this argument is not provided, $_ will be used instead.
Note how the last three examples in the table above are treated specially by the parser. "mygrep()" is parsed
as a true list operator, "myrand()" is parsed as a true unary operator with unary precedence the same as
"rand()", and "mytime()" is truly without arguments, just like "time()". That is, if you say
mytime +2;
you'll get "mytime() + 2", not mytime(2), which is how it would be parsed without a prototype. If you want to
force a unary function to have the same precedence as a list operator, add ";" to the end of the prototype:
sub mygetprotobynumber($;);
mygetprotobynumber $a > $b; # parsed as mygetprotobynumber($a > $b)
The interesting thing about "&" is that you can generate new syntax with it, provided it's in the initial
position:
sub try (&@) {
my($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 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 the Perl "grep" operator:
sub mygrep (&@) {
my $code = shift;
my @result;
If you try to use an alphanumeric sequence in a prototype you will generate an optional warning - "Illegal
character in prototype...". Unfortunately earlier versions of Perl allowed the prototype to be used as long
as its prefix was a valid prototype. The warning may be upgraded to a fatal error in a future version of Perl
once the majority of offending code is fixed.
It's probably best to prototype new functions, not retrofit prototyping into older ones. That's because you
must be especially careful about silent impositions of differing list versus scalar contexts. For example, if
you decide that a function should take just one parameter, like this:
sub func ($) {
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, "func()" now gets
passed in a 1; that is, the number of elements in @foo. And the "split" gets called in scalar context so it
starts scribbling on your @_ parameter list. Ouch!
This is all very powerful, of course, and should be used only in moderation to make the world a better place.
Constant Functions
Functions with a prototype of "()" are potential candidates for inlining. If the result after optimization
and constant folding is either a constant or a lexically-scoped scalar which has no other references, then it
will be used in place of function calls made without "&". Calls made using "&" are never inlined. (See
constant.pm for an easy way to declare most constants.)
The following functions would all be inlined:
sub pi () { 3.14159 } # Not exact, but close.
sub PI () { 4 * atan2 1, 1 } # As good as it gets,
# and it's inlined, too!
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 () { not (0x1B58 & FLAG_MASK) }
sub N () { int(OPT_BAZ) / 3 }
sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
Be aware that these will not be inlined; as they contain inner scopes, the constant folding doesn't reduce
them to a single constant:
this warning to tell whether or not a particular subroutine is considered constant.) The warning is
considered severe enough not to be affected by the -w switch (or its absence) 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 calling semantics, so beware) or by thwarting the inlining mechanism in some other way, such as
sub not_inlined () {
23 if $];
}
Overriding Built-in Functions
Many built-in functions may be overridden, though this should be tried only occasionally and for good reason.
Typically this might be done by a package attempting to emulate missing built-in functionality on a non-Unix
system.
Overriding may be done only by importing the name from a module at compile time--ordinary predeclaration isn't
good enough. However, the "use subs" pragma lets you, in effect, predeclare subs via the import syntax, and
these names may then override built-in ones:
use subs 'chdir', 'chroot', 'chmod', 'chown';
chdir $somewhere;
sub chdir { ... }
To unambiguously refer to the built-in form, precede the built-in name with the special package qualifier
"CORE::". For example, saying "CORE::open()" always refers to the built-in "open()", even if the current
package has imported some other subroutine called "&open()" from elsewhere. Even though it looks like a
regular function call, it isn't: the CORE:: prefix in that case is part of Perl's syntax, and works for any
keyword, regardless of what is in the CORE package. Taking a reference to it, that is, "\&CORE::open", only
works for some keywords. See CORE.
Library modules should not in general export built-in names like "open" or "chdir" as part of their default
@EXPORT list, because these may sneak into someone else's namespace and change the semantics unexpectedly.
Instead, if the module adds that name to @EXPORT_OK, then it's possible for a user to import the name
explicitly, but not implicitly. That is, they could say
use Module 'open';
and it would import the "open" override. But if they said
use Module;
they would get the default imports without overrides.
The foregoing mechanism for overriding built-in is restricted, quite deliberately, to the package that
requests the import. There is a second method that is sometimes applicable when you wish to override a built-
in everywhere, without regard to namespace boundaries. This is achieved by importing a sub into the special
namespace "CORE::GLOBAL::". Here is an example that quite brazenly replaces the "glob" operator with
something that understands regular expressions.
package REGlob;
require Exporter;
@ISA = 'Exporter';
@EXPORT_OK = 'glob';
@got = grep /$pat/, readdir $d;
closedir $d;
}
return @got;
}
1;
And here's how it could be (ab)used:
#use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
package Foo;
use REGlob 'glob'; # override glob() in Foo:: only
print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
The initial comment shows a contrived, even dangerous example. By overriding "glob" globally, you would be
forcing the new (and subversive) behavior for the "glob" operator for every namespace, without the complete
cognizance or cooperation of the modules that own those namespaces. Naturally, this should be done with
extreme caution--if it must be done at all.
The "REGlob" example above does not implement all the support needed to cleanly override perl's "glob"
operator. The built-in "glob" has different behaviors depending on whether it appears in a scalar or list
context, but our "REGlob" doesn't. Indeed, many perl built-in have such context sensitive behaviors, and
these must be adequately supported by a properly written override. For a fully functional example of
overriding "glob", study the implementation of "File::DosGlob" in the standard library.
When you override a built-in, your replacement should be consistent (if possible) with the built-in native
syntax. You can achieve this by using a suitable prototype. To get the prototype of an overridable built-in,
use the "prototype" function with an argument of "CORE::builtin_name" (see "prototype" in perlfunc).
Note however that some built-ins can't have their syntax expressed by a prototype (such as "system" or
"chomp"). If you override them you won't be able to fully mimic their original syntax.
The built-ins "do", "require" and "glob" can also be overridden, but due to special magic, their original
syntax is preserved, and you don't have to define a prototype for their replacements. (You can't override the
"do BLOCK" syntax, though).
"require" has special additional dark magic: if you invoke your "require" replacement as "require Foo::Bar",
it will actually receive the argument "Foo/Bar.pm" in @_. See "require" in perlfunc.
And, as you'll have noticed from the previous example, if you override "glob", the "<*>" glob operator is
overridden as well.
In a similar fashion, overriding the "readline" function also overrides the equivalent I/O operator
"<FILEHANDLE>". Also, overriding "readpipe" also overrides the operators "``" and "qx//".
Finally, some built-ins (e.g. "exists" or "grep") can't be overridden.
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 base class of the class's package.) However, if an "AUTOLOAD" subroutine is defined in the
package or packages used to locate 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 global $AUTOLOAD variable of the same package as the "AUTOLOAD" routine.
my $program = $AUTOLOAD;
$program =~ s/.*:://;
system($program, @_);
}
date();
who('am', 'i');
ls('-l');
In fact, if you predeclare functions you want to call that way, you don't even need parentheses:
use subs qw(date who ls);
date;
who "am", "i";
ls '-l';
A more complete example of this is the Shell module on CPAN, which can treat undefined subroutine calls as
calls to external programs.
Mechanisms are available to help modules writers split their modules into autoloadable files. See the
standard AutoLoader module described in AutoLoader and in AutoSplit, the standard SelfLoader modules in
SelfLoader, and the document on adding C functions to Perl code in perlxs.
Subroutine Attributes
A subroutine declaration or definition may have a list of attributes associated with it. If such an attribute
list is present, it is broken up at space or colon boundaries and treated as though a "use attributes" had
been seen. See attributes for details about what attributes are currently supported. Unlike the limitation
with the obsolescent "use attrs", the "sub : ATTRLIST" syntax works to associate the attributes with a pre-
declaration, and not just with a subroutine definition.
The attributes must be valid as simple identifier names (without any punctuation other than the '_'
character). They may have a parameter list appended, which is only checked for whether its parentheses
('(',')') nest properly.
Examples of valid syntax (even though the attributes are unknown):
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
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 of valid syntax above currently looks like this in terms of how
it's parsed and invoked:
use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
For further details on attribute lists and their manipulation, see attributes and Attribute::Handlers.
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