PERLSUB(1) Perl Programmers Reference Guide PERLSUB(1)NAMEperlsub - 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 subrou-
tines. 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 sim-
ple: 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 con-
tain 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
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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 parame-
ters. 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 ele-
ment 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
evaluated in the appropriate context (list, scalar, or void)
depending on the context of the subroutine call. If you
specify no return value, the subroutine returns an empty
list in list context, the undefined value in scalar context,
or nothing in void context. If you return one or more
aggregates (arrays and hashes), these will be flattened
together into one large indistinguishable list.
If no "return" is found and if the last statement is an
expression, its value is returned. If the last statement is
a loop control structure like a "foreach" or a "while", the
returned value is unspecified. The empty sub returns the
empty list.
Perl does not have named formal parameters. In practice all
you do is assign to a "my()" list of these. Variables that
aren't declared to be private are global variables. For
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 "Pack-
ages" in perlmod.
Example:
sub max {
my $max = shift(@_);
foreach $foo (@_) {
$max = $foo if $max < $foo;
}
return $max;
}
$bestday = max($mon,$tue,$wed,$thu,$fri);
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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 argu-
ments:
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 $v2
sub upcase_in {
for (@_) { tr/a-z/A-Z/ }
}
You aren't allowed to modify constants in this way, of
course. If an argument were actually literal and you tried
to change it, you'd take a (presumably fatal) exception.
For example, this won't work:
upcase_in("frederick");
It would be much safer if the "upcase_in()" function were
written to return a copy of its parameters instead of chang-
ing them in place:
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($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2
sub upcase {
return unless defined wantarray; # void context, do nothing
my @parms = @_;
for (@parms) { tr/a-z/A-Z/ }
return wantarray ? @parms : $parms[0];
}
Notice how this (unprototyped) function doesn't care whether
it was passed real scalars or arrays. Perl sees all argu-
ments 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 subrou-
tine 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
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&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.
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. Sub-
routines that do special, pre-defined things include "AUTO-
LOAD", "CLONE", "DESTROY" plus all functions mentioned in
perltie and PerlIO::via.
The "BEGIN", "CHECK", "INIT" and "END" subroutines are not
so much subroutines as named special code blocks, of which
you can have more than one in a package, and which you can
not call explicitly. See "BEGIN, CHECK, INIT and END" in
perlmod
Private Variables via my()
Synopsis:
my $foo; # declare $foo lexically local
my (@wid, %get); # declare list of variables local
my $foo = "flurp"; # declare $foo lexical, and init it
my @oof = @bar; # declare @oof lexical, and init it
my $x : Foo = $y; # similar, with an attribute applied
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 lexi-
cally 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
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elsewhere--every call gets its own copy.
This doesn't mean that a "my" variable declared in a stati-
cally 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 ini-
tializer 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
That has the same effect as
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my $foo;
$bar = 1;
The declared variable is not introduced (is not visible)
until after the current statement. Thus,
my $x = $x;
can be used to initialize a new $x with the value of the old
$x, and the expression
my $x = 123 and $x == 123
is false unless the old $x happened to have the value 123.
Lexical scopes of control structures are not bounded pre-
cisely 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
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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 princi-
pal 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.
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 from the world outside
that file. This is similar in spirit to C's static
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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
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.
This means that you can pass back or save away references to
lexical variables, whereas to return a pointer to a C auto
is a grave error. It also gives us a way to simulate C's
function statics. 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 func-
tion but in the block.
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{
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 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, CHECK, INIT and END" in perlmod about the spe-
cial triggered code blocks, "BEGIN", "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
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local $hash{key} = "val"; # sets a local value for this hash entry
local ($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 $merlyn, not @merlyn etc
A "local" modifies its listed variables to be "local" to the
enclosing block, "eval", or "do FILE"--and to any subroutine
called from within that block. A "local" just gives tem-
porary values to global (meaning package) variables. It
does not create a local variable. This is known as dynamic
scoping. Lexical scoping is done with "my", which works
more like C's auto declarations.
Some types of lvalues can be localized as well : hash and
array elements and slices, conditionals (provided that their
result is always localizable), and symbolic references. As
for simple variables, this creates new, dynamically scoped
values.
If more than one variable or expression is given to "local",
they must be placed in parentheses. This operator works by
saving the current values of those variables in its argument
list on a hidden stack and restoring them upon exiting the
block, subroutine, or eval. This means that called subrou-
tines can also reference the local variable, but not the
global one. The argument list may be assigned to if
desired, which allows you to initialize your local vari-
ables. (If no initializer is given for a particular vari-
able, it is created with an undefined value.)
Because "local" is a run-time operator, it gets executed
each time through a loop. Consequently, it's more efficient
to localize your variables outside the loop.
Grammatical note on local()
A "local" is simply a modifier on an lvalue expression.
When you assign to a "local"ized variable, the "local"
doesn't change whether its list is viewed as a scalar or an
array. So
local($foo) = <STDIN>;
local @FOO = <STDIN>;
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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;
Similarly, but in a way more difficult to spot, the follow-
ing snippet will die in perl 5.9.0 :
sub f { local $_ = "foo"; print }
for ($1) {
# now $_ is aliased to $1, thus is magic and readonly
f();
}
See next section for an alternative to this situation.
WARNING: Localization of tied arrays and hashes does not
currently work as described. This will be fixed in a future
release of Perl; in the meantime, avoid code that relies on
any particular behaviour of localising tied arrays or hashes
(localising individual elements is still okay). See
"Localising Tied Arrays and Hashes Is Broken" in perl58delta
for more details.
Localization of globs
The construct
local *name;
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creates a whole new symbol table entry for the glob "name"
in the current package. That means that all variables in
its glob slot ($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.
Notably, if you want to work with a brand new value of the
default scalar $_, and avoid the potential problem listed
above about $_ previously carrying a magic value, you should
use "local *_" instead of "local $_".
Localization of elements of composite types
It's also worth taking a moment to explain what happens when
you "local"ize a member of a composite type (i.e. an array
or hash element). In this case, the element is "local"ized
by name. This means that when the scope of the "local()"
ends, the saved value will be restored to the hash element
whose key was named in the "local()", or the array element
whose index was named in the "local()". If that element was
deleted while the "local()" was in effect (e.g. by a
"delete()" from a hash or a "shift()" of an array), it will
spring back into existence, possibly extending an array and
filling in the skipped elements with "undef". For instance,
if you say
%hash = ( 'This' => 'is', 'a' => 'test' );
@ary = ( 0..5 );
{
local($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
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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.
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 subrou-
tine. To do this, you have to declare the subroutine to
return an lvalue.
my $val;
sub canmod : lvalue {
# return $val; this doesn't work, don't say "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 sub-
routine 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 are several rea-
sons to be circumspect.
You can't use the return keyword, you must pass out the
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value before falling out of subroutine scope. (see com-
ment in example above). This is usually not a problem,
but it disallows an explicit return out of a deeply
nested loop, which is sometimes a nice way out.
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
$some_array_ref;
}
# set_arr_lv cannot stop this !
set_arr_lv() = { a => 1 };
Passing Symbol Table Entries (typeglobs)
WARNING: The mechanism described in this section was origi-
nally the only way to simulate pass-by-reference in older
versions of Perl. While it still works fine in modern ver-
sions, 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 par-
ticular 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:
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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 refer-
ence).
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 perl-
data.
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
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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!
pipe (READER, WRITER) or die "pipe: $!";
return (*READER, *WRITER);
}
($head, $tail) = ioqueue();
See the Symbol module for a way to create anonymous sym-
bol table entries.
Because assignment of a reference to a typeglob creates
an alias, this can be used to create what is effectively
a local function, or at least, a local alias.
{
local *grow = \&shrink; # only until this block exists
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 mani-
pulating 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 mis-
behave.
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 expli-
cit pass-by-reference. Before you do that, you need to
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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? Well, if
you're using only one of them, or you don't mind them con-
catenating, 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
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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 refer-
ences 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.
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sub openit {
my $path = shift;
local *FH;
return open (FH, $path) ? *FH : undef;
}
Prototypes
Perl supports a very limited kind of compile-time argument
checking using function prototyping. If you declare
sub mypush (\@@)
then "mypush()" takes arguments exactly like "push()" does.
The function declaration must be visible at compile time.
The prototype affects only interpretation of new-style calls
to the function, where new-style is defined as not using the
"&" character. In other words, if you call it like a
built-in function, then it behaves like a built-in function.
If you call it like an old-fashioned subroutine, then it
behaves like an old-fashioned subroutine. It naturally
falls out from this rule that prototypes have no influence
on subroutine references like "\&foo" or on indirect subrou-
tine calls like "&{$subref}" or "$subref->()".
Method calls are not influenced by prototypes either,
because the function to be called is indeterminate at com-
pile time, since the exact code called depends on inheri-
tance.
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, @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
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Any backslashed prototype character represents an actual
argument that absolutely must start with that character.
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 also backslash several argument types simultaneously
by using the "\[]" notation:
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 subrou-
tine, 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:
use Symbol 'qualify_to_ref';
sub foo (*) {
my $fh = qualify_to_ref(shift, caller);
...
}
A semicolon separates mandatory arguments from optional
arguments. It is redundant before "@" or "%", which gobble
up everything else.
Note how the last three examples in the table above are
treated specially by the parser. "mygrep()" is parsed as a
true list operator, "myrand()" is parsed as a true unary
operator with unary precedence the same as "rand()", and
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"mytime()" is truly without arguments, just like "time()".
That is, if you say
mytime +2;
you'll get "mytime() + 2", not mytime(2), which is how it
would be parsed without a prototype.
The interesting thing about "&" is that you can generate new
syntax with it, provided it's in the initial position:
sub try (&@) {
my($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;
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
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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.
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:
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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:
sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }
sub baz_val () {
if (OPT_BAZ) {
return 23;
}
else {
return 42;
}
}
If you redefine a subroutine that 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 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. Typ-
ically this might be done by a package attempting to emulate
missing built-in functionality on a non-Unix system.
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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: you
can't take a reference to it, such as the incorrect
"\&CORE::open" might appear to produce.
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 res-
tricted, 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 expres-
sions.
package REGlob;
require Exporter;
@ISA = 'Exporter';
@EXPORT_OK = 'glob';
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sub import {
my $pkg = shift;
return unless @_;
my $sym = shift;
my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
$pkg->export($where, $sym, @_);
}
sub glob {
my $pat = shift;
my @got;
local *D;
if (opendir D, '.') {
@got = grep /$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 exam-
ple. 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 coopera-
tion 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 sup-
port 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 sen-
sitive 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 "proto-
type" function with an argument of "CORE::builtin_name" (see
"prototype" in perlfunc).
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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 over-
ridden, 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" syn-
tax, 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>".
Finally, some built-ins (e.g. "exists" or "grep") can't be
overridden.
Autoloading
If you call a subroutine that is undefined, you would ordi-
narily 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" subrou-
tine 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. The name is not passed as an ordinary argument
because, er, well, just because, that's why...
Many "AUTOLOAD" routines load in a definition for the
requested subroutine using eval(), then execute that subrou-
tine using a special form of goto() that erases the stack
frame of the "AUTOLOAD" routine without a trace. (See the
source to the standard module documented in AutoLoader, for
example.) But an "AUTOLOAD" routine can also just emulate
the routine and never define it. For example, let's pre-
tend that a function that wasn't defined should just invoke
"system" with those arguments. All you'd do is:
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sub AUTOLOAD {
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 standard Shell
module, 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 Auto-
Loader 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 sub-
routine 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 { ... }
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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 manipula-
tion, see attributes and Attribute::Handlers.
SEE ALSO
See "Function Templates" in perlref for more about refer-
ences and closures. See perlxs if you'd like to learn about
calling C subroutines from Perl. See perlembed if you'd like
to learn about calling Perl subroutines from C. See perlmod
to learn about bundling up your functions in separate files.
See perlmodlib to learn what library modules come standard
on your system. See perltoot to learn how to make object
method calls.
perl v5.8.8 2006-06-30 29
