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 per-spective.) perspective.)
spective.)
Any arguments passed in show up in the array @_. Therefore, if you called a function with two argu-ments, arguments,
ments, 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 corre-sponding corresponding
sponding 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 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 indistinguish-able indistinguishable
able 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 cre-ating creating
ating private variables, see "Private Variables via my()" and "Temporary Values via local()". To
create protected environments for a set of functions in a separate package (and probably a separate
file), see "Packages" 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 $v2
sub upcase_in {
for (@_) { tr/a-z/A-Z/ }
}
You aren't allowed to modify constants in this way, of course. If an argument were actually literal
and you tried to change it, you'd take a (presumably fatal) exception. For example, this won't
work:
upcase_in("frederick");
It would be much safer if the "upcase_in()" function were written to return a copy of its parameters
instead of changing them in place:
($v3, $v4) = upcase($v1, $v2); # this doesn't 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 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 conve-nient convenient
nient 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. Subrou-tines Subroutines
tines that do special, pre-defined things include "AUTOLOAD", "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 lexically confined to the enclosing block, con-ditional conditional
ditional ("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 vari-ables. variables.
ables. (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
That has the same effect as
my $foo;
$bar = 1;
The declared variable is not introduced (is not visible) until after the current statement. Thus,
my $x = $x;
can be used to initialize 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 precisely by the braces that delimit their con-trolled controlled
trolled 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". How-ever, However,
ever, 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 inacces-sible inaccessible
sible 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 lexi-cal lexical
cal variable, be predeclared via "our" or "use vars", or else must be fully qualified with the pack-age package
age 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.
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 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 func-tion function
tion in an extra block, and put the static variable outside the function but in the block.
{
my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
}
}
# $secret_val now becomes unreachable by the outside
# world, but retains its value between calls to gimme_another
If this function is being sourced in from a separate file via "require" or "use", then this is proba-bly probably
bly 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 special 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 stat-ics. statics.
ics. 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 $hash{key} = "val"; # sets a local value for this hash entry
local ($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 $merlyn, not @merlyn etc
A "local" modifies its listed variables to be "local" to the enclosing block, "eval", or "do
FILE"--and to any subroutine called from within that block. A "local" just gives temporary values to
global (meaning package) variables. It does not create a local variable. This is known as dynamic
scoping. Lexical scoping is done with "my", which works more like C's auto declarations.
Some types of lvalues can be localized as well : hash and array elements and slices, conditionals
(provided that their result is always localizable), and symbolic references. As for simple vari-ables, variables,
ables, this creates new, dynamically scoped values.
If more than one variable or expression is given to "local", they must be placed in parentheses.
This operator works by saving the current values of those variables in its argument list on a hidden
stack and restoring them upon exiting the block, subroutine, or eval. This means that called subrou-tines subroutines
tines can also reference the local variable, but not the global one. The argument list may be
assigned to if desired, which allows you to initialize your local variables. (If no initializer is
given for a particular variable, it is created with an undefined value.)
Because "local" is a run-time operator, it gets executed each time through a loop. Consequently,
it's more efficient to localize your variables outside the loop.
Grammatical note on local()
A "local" is simply a modifier on an lvalue expression. When you assign to a "local"ized variable,
the "local" doesn't change whether its list is viewed as a scalar or an array. So
local($foo) = <STDIN>;
local @FOO = <STDIN>;
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 following snippet will die in perl 5.9.0 :
sub f { local $_ = "foo"; print }
for ($1) {
# now $_ is aliased to $1, thus is magic and readonly
f();
}
See next section for an alternative to this situation.
WARNING: Localization of tied arrays and hashes does not currently work as described. This will be
fixed in a future release of Perl; in the meantime, avoid code that relies on any particular behav-iour behaviour
iour of localising tied arrays or hashes (localising individual elements is still okay). See "Local-ising "Localising
ising Tied Arrays and Hashes Is Broken" in perl58delta for more details.
Localization of globs
The construct
local *name;
creates a whole new symbol table entry for the glob "name" in the current package. That means that
all variables in its glob slot ($name, @name, %name, &name, and the "name" filehandle) are dynami-cally dynamically
cally 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
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 ver-sions versions
sions 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 {
# 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 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 are several reasons to be circumspect.
You can't use the return keyword, you must pass out the value before falling out of subroutine
scope. (see comment 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 originally the only way to simulate pass-by-ref-erence pass-by-reference
erence in older versions of Perl. While it still works fine in modern versions, the new reference
mechanism is generally easier to work with. See below.
Sometimes you don't want to pass the value of an array to a subroutine but rather the name of it, so
that the subroutine can modify the global copy of it rather than working with a local copy. In perl
you can refer to all objects of a particular name by prefixing the name with a star: *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 mecha-nism mechanism
nism 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 mech-anism) mechanism)
anism) 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 perl-data. perldata.
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 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 symbol 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 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? Well, if you're using only one of them, or you don't mind them concatenating, then
the normal calling convention is ok, although a little expensive.
Where people get into trouble is here:
(@a, @b) = func(@c, @d);
or
(%a, %b) = func(%c, %d);
That syntax simply won't work. It sets just @a or %a and clears the @b or %b. Plus the function
didn't get passed into two separate arrays or hashes: it got one long list in @_, as always.
If you can arrange for everyone to deal with this through references, it's cleaner code, although not
so nice to look at. Here's a function that takes two array references as arguments, returning the
two array elements in order of how many elements they have in them:
($aref, $bref) = func(\@c, \@d);
print "@$aref has more than @$bref\n";
sub func {
my ($cref, $dref) = @_;
if (@$cref > @$dref) {
return ($cref, $dref);
} else {
return ($dref, $cref);
}
}
It turns out that you can actually do this also:
(*a, *b) = func(\@c, \@d);
print "@a has more than @b\n";
sub func {
local (*c, *d) = @_;
if (@c > @d) {
return (\@c, \@d);
} else {
return (\@d, \@c);
}
}
Here we're using the typeglobs to do symbol table aliasing. It's a tad subtle, though, and also
won't work if you're using "my" variables, because only globals (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, $path) ? *FH : undef;
}
Prototypes
Perl supports a very limited kind of compile-time argument checking using function prototyping. If
you declare
sub mypush (\@@)
then "mypush()" takes arguments exactly like "push()" does. The function declaration must be visible
at compile time. The prototype affects only interpretation of new-style calls to the function, where
new-style is defined as not using the "&" character. In other words, if you call it like a built-in
function, then it behaves like a built-in function. If you call it like an old-fashioned subroutine,
then it behaves like an old-fashioned subroutine. It naturally falls out from this rule that proto-types prototypes
types have no influence on subroutine references like "\&foo" or on indirect subroutine calls like
"&{$subref}" or "$subref->()".
Method calls are not influenced by prototypes either, because the function to be called is indetermi-nate indeterminate
nate 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 corre-sponding corresponding
sponding 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
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 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 refer-ence reference
ence 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 $fh = qualify_to_ref(shift, caller);
...
}
A semicolon separates mandatory arguments from optional arguments. It is redundant before "@" or
"%", which gobble up everything else.
Note how the last three examples in the table above are treated specially by the parser. "mygrep()"
is parsed as a true list operator, "myrand()" is parsed as a true unary operator with unary prece-dence precedence
dence the same as "rand()", and "mytime()" is truly without arguments, just like "time()". That is,
if you say
mytime +2;
you'll get "mytime() + 2", not mytime(2), which is how it would be parsed without a prototype.
The interesting thing about "&" is that you can generate new syntax with it, provided it's in the
initial position:
sub try (&@) {
my($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 upon the meat of the module, nor make it harder to read. The line noise is visually encapsu-lated encapsulated
lated 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 warning"Illegal
"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 con-texts. contexts.
texts. 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 bet-ter better
ter place.
Constant Functions
Functions with a prototype of "()" are potential candidates for inlining. If the result after opti-mization optimization
mization 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:
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. Typically this might be done by a package attempting to emulate missing built-in functional-
ity on a non-Unix system.
Overriding may be done only by importing the name from a module at compile time--ordinary predeclara-tion predeclaration
tion 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 qual-ifier qualifier
ifier "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 incor-rect incorrect
rect "\&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 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 import-ing importing
ing 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';
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 example. By overriding "glob" globally, you
would be forcing the new (and subversive) behavior for the "glob" operator for every namespace, with-out without
out 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 sen-sitive sensitive
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 over-ridable overridable
ridable built-in, use the "prototype" function with an argument of "CORE::builtin_name" (see "proto-type" "prototype"
type" 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 opera-tor operator
tor 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 ordinarily get an immediate, fatal error com-plaining complaining
plaining 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" sub-routine subroutine
routine 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 sub-routine. subroutine.
routine. 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 ordi-nary ordinary
nary 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 subroutine 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
pretend that a function that wasn't defined should just invoke "system" with those arguments. All
you'd do is:
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 AutoLoader module described in AutoLoader and in AutoSplit, the standard SelfLoader mod-ules modules
ules 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 asso-ciate associate
ciate 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 parenthe-ses parentheses
ses ('(',')') 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::Han-dlers. Attribute::Handlers.
dlers.
SEE ALSO
See "Function Templates" in perlref for more about references 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 2006-01-07 PERLSUB(1)
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