PERLSUB(1) Perl Programmers Reference Guide PERLSUB(1)
NAME
perlsub - Perl subroutines
SYNOPSIS
To declare subroutines:
sub NAME; # A "forward" declaration.
sub NAME(PROTO); # ditto, but with prototypes
sub NAME : ATTRS; # with attributes
sub NAME(PROTO) : ATTRS; # with attributes and prototypes
sub NAME BLOCK # A declaration and a definition.
sub NAME(PROTO) BLOCK # ditto, but with prototypes
sub NAME : ATTRS BLOCK # with attributes
sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes
To define an anonymous subroutine at runtime:
$subref = sub BLOCK; # no proto
$subref = sub (PROTO) BLOCK; # with proto
$subref = sub : ATTRS BLOCK; # with attributes
$subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
To import subroutines:
use MODULE qw(NAME1 NAME2 NAME3);
To call subroutines:
NAME(LIST); # & is optional with parentheses.
NAME LIST; # Parentheses optional if predeclared/imported.
&NAME(LIST); # Circumvent prototypes.
&NAME; # Makes current @_ visible to called subroutine.
DESCRIPTION
Like many languages, Perl provides for user-defined subroutines. These
may be located anywhere in the main program, loaded in from other files
via the "do", "require", or "use" keywords, or generated on the fly
using "eval" or anonymous subroutines. You can even call a function
indirectly using a variable containing its name or a CODE reference.
The Perl model for function call and return values is simple: all
functions are passed as parameters one single flat list of scalars, and
all functions likewise return to their caller one single flat list of
scalars. Any arrays or hashes in these call and return lists will
collapse, losing their identities--but you may always use pass-by-
reference instead to avoid this. Both call and return lists may
contain as many or as few scalar elements as you'd like. (Often a
function without an explicit return statement is called a subroutine,
but there's really no difference from Perl's perspective.)
Any arguments passed in show up in the array @_. Therefore, if you
called a function with two arguments, those would be stored in $_[0]
and $_[1]. The array @_ is a local array, but its elements are aliases
for the actual scalar parameters. In particular, if an element $_[0]
is updated, the corresponding argument is updated (or an error occurs
if it is not updatable). If an argument is an array or hash element
which did not exist when the function was called, that element is
created only when (and if) it is modified or a reference to it is
taken. (Some earlier versions of Perl created the element whether or
not the element was assigned to.) Assigning to the whole array @_
removes that aliasing, and does not update any arguments.
A "return" statement may be used to exit a subroutine, optionally
specifying the returned value, which will be 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 "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 convenient way
to cheat if you know what you're doing. See "Prototypes" below.
Since Perl 5.16.0, the "__SUB__" token is available under "use feature
'current_sub'" and "use 5.16.0". It will evaluate to a reference to
the currently-running sub, which allows for recursive calls without
knowing your subroutine's name.
use 5.16.0;
my $factorial = sub {
my ($x) = @_;
return 1 if $x == 1;
return($x * __SUB__->( $x - 1 ) );
};
Subroutines whose names are in all upper case are reserved to the Perl
core, as are modules whose names are in all lower case. A subroutine
in all capitals is a loosely-held convention meaning it will be called
indirectly by the run-time system itself, usually due to a triggered
event. Subroutines that do special, pre-defined things include
"AUTOLOAD", "CLONE", "DESTROY" plus all functions mentioned in perltie
and PerlIO::via.
The "BEGIN", "UNITCHECK", "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, UNITCHECK, 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, conditional ("if/unless/elsif/else"),
loop ("for/foreach/while/until/continue"), subroutine, "eval", or
"do/require/use"'d file. If more than one value is listed, the list
must be placed in parentheses. All listed elements must be legal
lvalues. Only alphanumeric identifiers may be lexically
scoped--magical built-ins like $/ must currently be "local"ized with
"local" instead.
Unlike dynamic variables created by the "local" operator, lexical
variables declared with "my" are totally hidden from the outside world,
including any called subroutines. This is true if it's the same
subroutine called from itself or elsewhere--every call gets its own
copy.
This doesn't mean that a "my" variable declared in a statically
enclosing lexical scope would be invisible. Only dynamic scopes are
cut off. For example, the "bumpx()" function below has access to the
lexical $x variable because both the "my" and the "sub" occurred at the
same scope, presumably file scope.
my $x = 10;
sub bumpx { $x++ }
An "eval()", however, can see lexical variables of the scope it is
being evaluated in, so long as the names aren't hidden by declarations
within the "eval()" itself. See perlref.
The parameter list to my() may be assigned to if desired, which allows
you to initialize your variables. (If no initializer is given for a
particular variable, it is created with the undefined value.) Commonly
this is used to name input parameters to a subroutine. Examples:
$arg = "fred"; # "global" variable
$n = cube_root(27);
print "$arg thinks the root is $n\n";
fred thinks the root is 3
sub cube_root {
my $arg = shift; # name doesn't matter
$arg **= 1/3;
return $arg;
}
The "my" is simply a modifier on something you might assign to. So
when you do assign to variables in its argument list, "my" doesn't
change whether those variables are viewed as a scalar or an array. So
my ($foo) = <STDIN>; # WRONG?
my @FOO = <STDIN>;
both supply a list context to the right-hand side, while
my $foo = <STDIN>;
supplies a scalar context. But the following declares only one
variable:
my $foo, $bar = 1; # WRONG
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 controlled blocks; control expressions are
part of that scope, too. Thus in the loop
while (my $line = <>) {
$line = lc $line;
} continue {
print $line;
}
the scope of $line extends from its declaration throughout the rest of
the loop construct (including the "continue" clause), but not beyond
it. Similarly, in the conditional
if ((my $answer = <STDIN>) =~ /^yes$/i) {
user_agrees();
} elsif ($answer =~ /^no$/i) {
user_disagrees();
} else {
chomp $answer;
die "'$answer' is neither 'yes' nor 'no'";
}
the scope of $answer extends from its declaration through the rest of
that conditional, including any "elsif" and "else" clauses, but not
beyond it. See "Simple Statements" in perlsyn for information on the
scope of variables in statements with modifiers.
The "foreach" loop defaults to scoping its index variable dynamically
in the manner of "local". However, if the index variable is prefixed
with the keyword "my", or if there is already a lexical by that name in
scope, then a new lexical is created instead. Thus in the loop
for my $i (1, 2, 3) {
some_function();
}
the scope of $i extends to the end of the loop, but not beyond it,
rendering the value of $i inaccessible within "some_function()".
Some users may wish to encourage the use of lexically scoped variables.
As an aid to catching implicit uses to package variables, which are
always global, if you say
use strict 'vars';
then any variable mentioned from there to the end of the enclosing
block must either refer to a lexical variable, be predeclared via "our"
or "use vars", or else must be fully qualified with the package name.
A compilation error results otherwise. An inner block may countermand
this with "no strict 'vars'".
A "my" has both a compile-time and a run-time effect. At compile time,
the compiler takes notice of it. The principal usefulness of this is
to quiet "use strict 'vars'", but it is also essential for generation
of closures as detailed in perlref. Actual initialization is delayed
until run time, though, so it gets executed at the appropriate time,
such as each time through a loop, for example.
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
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
There are two ways to build persistent private variables in Perl 5.10.
First, you can simply use the "state" feature. Or, you can use
closures, if you want to stay compatible with releases older than 5.10.
Persistent variables via state()
Beginning with Perl 5.9.4, you can declare variables with the "state"
keyword in place of "my". For that to work, though, you must have
enabled that feature beforehand, either by using the "feature" pragma,
or by using "-E" on one-liners (see feature). Beginning with Perl
5.16, the "CORE::state" form does not require the "feature" pragma.
For example, the following code maintains a private counter,
incremented each time the gimme_another() function is called:
use feature 'state';
sub gimme_another { state $x; return ++$x }
Also, since $x is lexical, it can't be reached or modified by any Perl
code outside.
When combined with variable declaration, simple scalar assignment to
"state" variables (as in "state $x = 42") is executed only the first
time. When such statements are evaluated subsequent times, the
assignment is ignored. The behavior of this sort of assignment to non-
scalar variables is undefined.
Persistent variables with closures
Just because a lexical variable is lexically (also called statically)
scoped to its enclosing block, "eval", or "do" FILE, this doesn't mean
that within a function it works like a C static. It normally works
more like a C auto, but with implicit garbage collection.
Unlike local variables in C or C++, Perl's lexical variables don't
necessarily get recycled just because their scope has exited. If
something more permanent is still aware of the lexical, it will stick
around. So long as something else references a lexical, that lexical
won't be freed--which is as it should be. You wouldn't want memory
being free until you were done using it, or kept around once you were
done. Automatic garbage collection takes care of this for you.
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 function but in the
block.
{
my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
}
}
# $secret_val now becomes unreachable by the outside
# world, but retains its value between calls to gimme_another
If this function is being sourced in from a separate file via "require"
or "use", then this is probably just fine. If it's all in the main
program, you'll need to arrange for the "my" to be executed early,
either by putting the whole block above your main program, or more
likely, placing merely a "BEGIN" code block around it to make sure it
gets executed before your program starts to run:
BEGIN {
my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
}
}
See "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod about the
special triggered code blocks, "BEGIN", "UNITCHECK", "CHECK", "INIT"
and "END".
If declared at the outermost scope (the file scope), then lexicals work
somewhat like C's file statics. They are available to all functions in
that same file declared below them, but are inaccessible from outside
that file. This strategy is sometimes used in modules to create
private variables that the whole module can see.
Temporary Values via local()
WARNING: In general, you should be using "my" instead of "local",
because it's faster and safer. Exceptions to this include the global
punctuation variables, global filehandles and formats, and direct
manipulation of the Perl symbol table itself. "local" is mostly used
when the current value of a variable must be visible to called
subroutines.
Synopsis:
# localization of values
local $foo; # make $foo dynamically local
local (@wid, %get); # make list of variables local
local $foo = "flurp"; # make $foo dynamic, and init it
local @oof = @bar; # make @oof dynamic, and init it
local $hash{key} = "val"; # sets a local value for this hash entry
delete local $hash{key}; # delete this entry for the current block
local ($cond ? $v1 : $v2); # several types of lvalues support
# localization
# localization of symbols
local *FH; # localize $FH, @FH, %FH, &FH ...
local *merlyn = *randal; # now $merlyn is really $randal, plus
# @merlyn is really @randal, etc
local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
A "local" modifies its listed variables to be "local" to the enclosing
block, "eval", or "do FILE"--and to any subroutine called from within
that block. A "local" just gives temporary values to global (meaning
package) variables. It does not create a local variable. This is
known as dynamic scoping. Lexical scoping is done with "my", which
works more like C's auto declarations.
Some types of lvalues can be localized as well: hash and array elements
and slices, conditionals (provided that their result is always
localizable), and symbolic references. As for simple variables, this
creates new, dynamically scoped values.
If more than one variable or expression is given to "local", they must
be placed in parentheses. This operator works by saving the current
values of those variables in its argument list on a hidden stack and
restoring them upon exiting the block, subroutine, or eval. This means
that called subroutines can also reference the local variable, but not
the global one. The argument list may be assigned to if desired, which
allows you to initialize your local variables. (If no initializer is
given for a particular variable, it is created with an undefined
value.)
Because "local" is a run-time operator, it gets executed each time
through a loop. Consequently, it's more efficient to localize your
variables outside the loop.
Grammatical note on local()
A "local" is simply a modifier on an lvalue expression. When you
assign to a "local"ized variable, the "local" doesn't change whether
its list is viewed as a scalar or an array. So
local($foo) = <STDIN>;
local @FOO = <STDIN>;
both supply a list context to the right-hand side, while
local $foo = <STDIN>;
supplies a scalar context.
Localization of special variables
If you localize a special variable, you'll be giving a new value to it,
but its magic won't go away. That means that all side-effects related
to this magic still work with the localized value.
This feature allows code like this to work :
# Read the whole contents of FILE in $slurp
{ local $/ = undef; $slurp = <FILE>; }
Note, however, that this restricts localization of some values ; for
example, the following statement dies, as of perl 5.9.0, with an error
Modification of a read-only value attempted, because the $1 variable is
magical and read-only :
local $1 = 2;
One exception is the default scalar variable: starting with perl 5.14
"local($_)" will always strip all magic from $_, to make it possible to
safely reuse $_ in a subroutine.
WARNING: Localization of tied arrays and hashes does not currently work
as described. This will be fixed in a future release of Perl; in the
meantime, avoid code that relies on any particular behaviour of
localising tied arrays or hashes (localising individual elements is
still okay). See "Localising Tied Arrays and Hashes Is Broken" in
perl58delta for more details.
Localization of globs
The construct
local *name;
creates a whole new symbol table entry for the glob "name" in the
current package. That means that all variables in its glob slot
($name, @name, %name, &name, and the "name" filehandle) are dynamically
reset.
This implies, among other things, that any magic eventually carried by
those variables is locally lost. In other words, saying "local */"
will not have any effect on the internal value of the input record
separator.
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.
Localized deletion of elements of composite types
You can use the "delete local $array[$idx]" and "delete local
$hash{key}" constructs to delete a composite type entry for the current
block and restore it when it ends. They return the array/hash value
before the localization, which means that they are respectively
equivalent to
do {
my $val = $array[$idx];
local $array[$idx];
delete $array[$idx];
$val
}
and
do {
my $val = $hash{key};
local $hash{key};
delete $hash{key};
$val
}
except that for those the "local" is scoped to the "do" block. Slices
are also accepted.
my %hash = (
a => [ 7, 8, 9 ],
b => 1,
)
{
my $a = delete local $hash{a};
# $a is [ 7, 8, 9 ]
# %hash is (b => 1)
{
my @nums = delete local @$a[0, 2]
# @nums is (7, 9)
# $a is [ undef, 8 ]
$a[0] = 999; # will be erased when the scope ends
}
# $a is back to [ 7, 8, 9 ]
}
# %hash is back to its original state
Lvalue subroutines
WARNING: Lvalue subroutines are still experimental and the
implementation may change in future versions of Perl.
It is possible to return a modifiable value from a subroutine. To do
this, you have to declare the subroutine to return an lvalue.
my $val;
sub canmod : lvalue {
$val; # or: return $val;
}
sub nomod {
$val;
}
canmod() = 5; # assigns to $val
nomod() = 5; # ERROR
The scalar/list context for the subroutine and for the right-hand side
of assignment is determined as if the subroutine call is replaced by a
scalar. For example, consider:
data(2,3) = get_data(3,4);
Both subroutines here are called in a scalar context, while in:
(data(2,3)) = get_data(3,4);
and in:
(data(2),data(3)) = get_data(3,4);
all the subroutines are called in a list context.
Lvalue subroutines are EXPERIMENTAL
They appear to be convenient, but there is at least one reason to
be circumspect.
They violate encapsulation. A normal mutator can check the
supplied argument before setting the attribute it is protecting, an
lvalue subroutine never gets that chance. Consider;
my $some_array_ref = []; # protected by mutators ??
sub set_arr { # normal mutator
my $val = shift;
die("expected array, you supplied ", ref $val)
unless ref $val eq 'ARRAY';
$some_array_ref = $val;
}
sub set_arr_lv : lvalue { # lvalue mutator
$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-reference in older versions of Perl.
While it still works fine in modern versions, the new reference
mechanism is generally easier to work with. See below.
Sometimes you don't want to pass the value of an array to a subroutine
but rather the name of it, so that the subroutine can modify the global
copy of it rather than working with a local copy. In perl you can
refer to all objects of a particular name by prefixing the name with a
star: *foo. This is often known as a "typeglob", because the star on
the front can be thought of as a wildcard match for all the funny
prefix characters on variables and subroutines and such.
When evaluated, the typeglob produces a scalar value that represents
all the objects of that name, including any filehandle, format, or
subroutine. When assigned to, it causes the name mentioned to refer to
whatever "*" value was assigned to it. Example:
sub doubleary {
local(*someary) = @_;
foreach $elem (@someary) {
$elem *= 2;
}
}
doubleary(*foo);
doubleary(*bar);
Scalars are already passed by reference, so you can modify scalar
arguments without using this mechanism by referring explicitly to $_[0]
etc. You can modify all the elements of an array by passing all the
elements as scalars, but you have to use the "*" mechanism (or the
equivalent reference mechanism) to "push", "pop", or change the size of
an array. It will certainly be faster to pass the typeglob (or
reference).
Even if you don't want to modify an array, this mechanism is useful for
passing multiple arrays in a single LIST, because normally the LIST
mechanism will merge all the array values so that you can't extract out
the individual arrays. For more on typeglobs, see "Typeglobs and
Filehandles" in perldata.
When to Still Use local()
Despite the existence of "my", there are still three places where the
"local" operator still shines. In fact, in these three places, you
must use "local" instead of "my".
1. You need to give a global variable a temporary value, especially
$_.
The global variables, like @ARGV or the punctuation variables, must
be "local"ized with "local()". This block reads in /etc/motd, and
splits it up into chunks separated by lines of equal signs, which
are placed in @Fields.
{
local @ARGV = ("/etc/motd");
local $/ = undef;
local $_ = <>;
@Fields = split /^\s*=+\s*$/;
}
It particular, it's important to "local"ize $_ in any routine that
assigns to it. Look out for implicit assignments in "while"
conditionals.
2. You need to create a local file or directory handle or a local
function.
A function that needs a filehandle of its own must use "local()" on
a complete typeglob. This can be used to create new symbol table
entries:
sub ioqueue {
local (*READER, *WRITER); # not my!
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 exits
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, $path) ? *FH : undef;
}
Prototypes
Perl supports a very limited kind of compile-time argument checking
using function prototyping. If you declare
sub mypush (+@)
then "mypush()" takes arguments exactly like "push()" does. The
function declaration must be visible at compile time. The prototype
affects only interpretation of new-style calls to the function, where
new-style is defined as not using the "&" character. In other words,
if you call it like a built-in function, then it behaves like a built-
in function. If you call it like an old-fashioned subroutine, then it
behaves like an old-fashioned subroutine. It naturally falls out from
this rule that prototypes have no influence on subroutine references
like "\&foo" or on indirect subroutine calls like "&{$subref}" or
"$subref->()".
Method calls are not influenced by prototypes either, because the
function to be called is indeterminate at compile time, since the exact
code called depends on inheritance.
Because the intent of this feature is primarily to let you define
subroutines that work like built-in functions, here are prototypes for
some other functions that parse almost exactly like the corresponding
built-in.
Declared as Called as
sub mylink ($$) mylink $old, $new
sub myvec ($$$) myvec $var, $offset, 1
sub myindex ($$;$) myindex &getstring, "substr"
sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
sub myreverse (@) myreverse $a, $b, $c
sub myjoin ($@) myjoin ":", $a, $b, $c
sub mypop (+) mypop @array
sub mysplice (+$$@) mysplice @array, 0, 2, @pushme
sub mykeys (+) mykeys %{$hashref}
sub myopen (*;$) myopen HANDLE, $name
sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
sub myrand (;$) myrand 42
sub mytime () mytime
Any backslashed prototype character represents an actual argument that
must start with that character (optionally preceded by "my", "our" or
"local"), with the exception of "$", which will accept any scalar
lvalue expression, such as "$foo = 7" or "my_function()->[0]". The
value passed as part of @_ will be a reference to the actual argument
given in the subroutine call, obtained by applying "\" to that
argument.
You can use the "\[]" backslash group notation to specify more than one
allowed argument type. For example:
sub myref (\[$@%&*])
will allow calling myref() as
myref $var
myref @array
myref %hash
myref &sub
myref *glob
and the first argument of myref() will be a reference to a scalar, an
array, a hash, a code, or a glob.
Unbackslashed prototype characters have special meanings. Any
unbackslashed "@" or "%" eats all remaining arguments, and forces list
context. An argument represented by "$" forces scalar context. An "&"
requires an anonymous subroutine, which, if passed as the first
argument, does not require the "sub" keyword or a subsequent comma.
A "*" allows the subroutine to accept a bareword, constant, scalar
expression, typeglob, or a reference to a typeglob in that slot. The
value will be available to the subroutine either as a simple scalar, or
(in the latter two cases) as a reference to the typeglob. If you wish
to always convert such arguments to a typeglob reference, use
Symbol::qualify_to_ref() as follows:
use Symbol 'qualify_to_ref';
sub foo (*) {
my $fh = qualify_to_ref(shift, caller);
...
}
The "+" prototype is a special alternative to "$" that will act like
"\[@%]" when given a literal array or hash variable, but will otherwise
force scalar context on the argument. This is useful for functions
which should accept either a literal array or an array reference as the
argument:
sub mypush (+@) {
my $aref = shift;
die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
push @$aref, @_;
}
When using the "+" prototype, your function must check that the
argument is of an acceptable type.
A semicolon (";") separates mandatory arguments from optional
arguments. It is redundant before "@" or "%", which gobble up
everything else.
As the last character of a prototype, or just before a semicolon, a "@"
or a "%", you can use "_" in place of "$": if this argument is not
provided, $_ will be used instead.
Note how the last three examples in the table above are treated
specially by the parser. "mygrep()" is parsed as a true list operator,
"myrand()" is parsed as a true unary operator with unary precedence the
same as "rand()", and "mytime()" is truly without arguments, just like
"time()". That is, if you say
mytime +2;
you'll get "mytime() + 2", not mytime(2), which is how it would be
parsed without a prototype. If you want to force a unary function to
have the same precedence as a list operator, add ";" to the end of the
prototype:
sub mygetprotobynumber($;);
mygetprotobynumber $a > $b; # parsed as mygetprotobynumber($a > $b)
The interesting thing about "&" is that you can generate new syntax
with it, provided it's in the initial position:
sub try (&@) {
my($try,$catch) = @_;
eval { &$try };
if ($@) {
local $_ = $@;
&$catch;
}
}
sub catch (&) { $_[0] }
try {
die "phooey";
} catch {
/phooey/ and print "unphooey\n";
};
That prints "unphooey". (Yes, there are still unresolved issues having
to do with visibility of @_. I'm ignoring that question for the
moment. (But note that if we make @_ lexically scoped, those anonymous
subroutines can act like closures... (Gee, is this sounding a little
Lispish? (Never mind.))))
And here's a reimplementation of the Perl "grep" operator:
sub mygrep (&@) {
my $code = shift;
my @result;
foreach $_ (@_) {
push(@result, $_) if &$code;
}
@result;
}
Some folks would prefer full alphanumeric prototypes. Alphanumerics
have been intentionally left out of prototypes for the express purpose
of someday in the future adding named, formal parameters. The current
mechanism's main goal is to let module writers provide better
diagnostics for module users. Larry feels the notation quite
understandable to Perl programmers, and that it will not intrude
greatly upon the meat of the module, nor make it harder to read. The
line noise is visually encapsulated into a small pill that's easy to
swallow.
If you try to use an alphanumeric sequence in a prototype you will
generate an optional warning - "Illegal character in prototype...".
Unfortunately earlier versions of Perl allowed the prototype to be used
as long as its prefix was a valid prototype. The warning may be
upgraded to a fatal error in a future version of Perl once the majority
of offending code is fixed.
It's probably best to prototype new functions, not retrofit prototyping
into older ones. That's because you must be especially careful about
silent impositions of differing list versus scalar contexts. For
example, if you decide that a function should take just one parameter,
like this:
sub func ($) {
my $n = shift;
print "you gave me $n\n";
}
and someone has been calling it with an array or expression returning a
list:
func(@foo);
func( split /:/ );
Then you've just supplied an automatic "scalar" in front of their
argument, which can be more than a bit surprising. The old @foo which
used to hold one thing doesn't get passed in. Instead, "func()" now
gets passed in a 1; that is, the number of elements in @foo. And the
"split" gets called in scalar context so it starts scribbling on your
@_ parameter list. Ouch!
This is all very powerful, of course, and should be used only in
moderation to make the world a better place.
Constant Functions
Functions with a prototype of "()" are potential candidates for
inlining. If the result after optimization and constant folding is
either a constant or a lexically-scoped scalar which has no other
references, then it will be used in place of function calls made
without "&". Calls made using "&" are never inlined. (See constant.pm
for an easy way to declare most constants.)
The following functions would all be inlined:
sub pi () { 3.14159 } # Not exact, but close.
sub PI () { 4 * atan2 1, 1 } # As good as it gets,
# and it's inlined, too!
sub ST_DEV () { 0 }
sub ST_INO () { 1 }
sub FLAG_FOO () { 1 << 8 }
sub FLAG_BAR () { 1 << 9 }
sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
sub N () { int(OPT_BAZ) / 3 }
sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
Be aware that these will not be inlined; as they contain inner scopes,
the constant folding doesn't reduce them to a single constant:
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 warning by default. (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 affected by the -w switch (or its
absence) because previously compiled invocations of the function will
still be using the old value of the function. If you need to be able
to redefine the subroutine, you need to ensure that it isn't inlined,
either by dropping the "()" prototype (which changes calling semantics,
so beware) or by thwarting the inlining mechanism in some other way,
such as
sub not_inlined () {
23 if $];
}
Overriding Built-in Functions
Many built-in functions may be overridden, though this should be tried
only occasionally and for good reason. Typically this might be done by
a package attempting to emulate missing built-in functionality on a
non-Unix system.
Overriding may be done only by importing the name from a module at
compile time--ordinary predeclaration isn't good enough. However, the
"use subs" pragma lets you, in effect, predeclare subs via the import
syntax, and these names may then override built-in ones:
use subs 'chdir', 'chroot', 'chmod', 'chown';
chdir $somewhere;
sub chdir { ... }
To unambiguously refer to the built-in form, precede the built-in name
with the special package qualifier "CORE::". For example, saying
"CORE::open()" always refers to the built-in "open()", even if the
current package has imported some other subroutine called "&open()"
from elsewhere. Even though it looks like a regular function call, it
isn't: the CORE:: prefix in that case is part of Perl's syntax, and
works for any keyword, regardless of what is in the CORE package.
Taking a reference to it, that is, "\&CORE::open", only works for some
keywords. See CORE.
Library modules should not in general export built-in names like "open"
or "chdir" as part of their default @EXPORT list, because these may
sneak into someone else's namespace and change the semantics
unexpectedly. Instead, if the module adds that name to @EXPORT_OK,
then it's possible for a user to import the name explicitly, but not
implicitly. That is, they could say
use Module 'open';
and it would import the "open" override. But if they said
use Module;
they would get the default imports without overrides.
The foregoing mechanism for overriding built-in is restricted, quite
deliberately, to the package that requests the import. There is a
second method that is sometimes applicable when you wish to override a
built-in everywhere, without regard to namespace boundaries. This is
achieved by importing a sub into the special namespace
"CORE::GLOBAL::". Here is an example that quite brazenly replaces the
"glob" operator with something that understands regular expressions.
package REGlob;
require Exporter;
@ISA = 'Exporter';
@EXPORT_OK = 'glob';
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;
if (opendir my $d, '.') {
@got = grep /$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,
without the complete cognizance or cooperation of the modules that own
those namespaces. Naturally, this should be done with extreme
caution--if it must be done at all.
The "REGlob" example above does not implement all the support needed to
cleanly override perl's "glob" operator. The built-in "glob" has
different behaviors depending on whether it appears in a scalar or list
context, but our "REGlob" doesn't. Indeed, many perl built-in have
such context sensitive behaviors, and these must be adequately
supported by a properly written override. For a fully functional
example of overriding "glob", study the implementation of
"File::DosGlob" in the standard library.
When you override a built-in, your replacement should be consistent (if
possible) with the built-in native syntax. You can achieve this by
using a suitable prototype. To get the prototype of an overridable
built-in, use the "prototype" function with an argument of
"CORE::builtin_name" (see "prototype" in perlfunc).
Note however that some built-ins can't have their syntax expressed by a
prototype (such as "system" or "chomp"). If you override them you
won't be able to fully mimic their original syntax.
The built-ins "do", "require" and "glob" can also be overridden, but
due to special magic, their original syntax is preserved, and you don't
have to define a prototype for their replacements. (You can't override
the "do BLOCK" syntax, though).
"require" has special additional dark magic: if you invoke your
"require" replacement as "require Foo::Bar", it will actually receive
the argument "Foo/Bar.pm" in @_. See "require" in perlfunc.
And, as you'll have noticed from the previous example, if you override
"glob", the "<*>" glob operator is overridden as well.
In a similar fashion, overriding the "readline" function also overrides
the equivalent I/O operator "<FILEHANDLE>". Also, overriding "readpipe"
also overrides the operators "``" and "qx//".
Finally, some built-ins (e.g. "exists" or "grep") can't be overridden.
Autoloading
If you call a subroutine that is undefined, you would ordinarily get an
immediate, fatal error complaining that the subroutine doesn't exist.
(Likewise for subroutines being used as methods, when the method
doesn't exist in any base class of the class's package.) However, if
an "AUTOLOAD" subroutine is defined in the package or packages used to
locate the original subroutine, then that "AUTOLOAD" subroutine is
called with the arguments that would have been passed to the original
subroutine. The fully qualified name of the original subroutine
magically appears in the global $AUTOLOAD variable of the same package
as the "AUTOLOAD" routine. The name is not passed as an ordinary
argument because, er, well, just because, that's why. (As an
exception, a method call to a nonexistent "import" or "unimport" method
is just skipped instead. Also, if the AUTOLOAD subroutine is an XSUB,
there are other ways to retrieve the subroutine name. See "Autoloading
with XSUBs" in perlguts for details.)
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 Shell module on CPAN, which can
treat undefined subroutine calls as calls to external programs.
Mechanisms are available to help modules writers split their modules
into autoloadable files. See the standard AutoLoader module described
in AutoLoader and in AutoSplit, the standard SelfLoader modules in
SelfLoader, and the document on adding C functions to Perl code in
perlxs.
Subroutine Attributes
A subroutine declaration or definition may have a list of attributes
associated with it. If such an attribute list is present, it is broken
up at space or colon boundaries and treated as though a "use
attributes" had been seen. See attributes for details about what
attributes are currently supported. Unlike the limitation with the
obsolescent "use attrs", the "sub : ATTRLIST" syntax works to associate
the attributes with a pre-declaration, and not just with a subroutine
definition.
The attributes must be valid as simple identifier names (without any
punctuation other than the '_' character). They may have a parameter
list appended, which is only checked for whether its parentheses
('(',')') nest properly.
Examples of valid syntax (even though the attributes are unknown):
sub fnord (&\%) : switch(10,foo(7,3)) : expensive;
sub plugh () : Ugly('\(") :Bad;
sub xyzzy : _5x5 { ... }
Examples of invalid syntax:
sub fnord : switch(10,foo(); # ()-string not balanced
sub snoid : Ugly('('); # ()-string not balanced
sub xyzzy : 5x5; # "5x5" not a valid identifier
sub plugh : Y2::north; # "Y2::north" not a simple identifier
sub snurt : foo + bar; # "+" not a colon or space
The attribute list is passed as a list of constant strings to the code
which associates them with the subroutine. In particular, the second
example of valid syntax above currently looks like this in terms of how
it's parsed and invoked:
use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
For further details on attribute lists and their manipulation, see
attributes and Attribute::Handlers.
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 perlootut to learn
how to make object method calls.
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