perlsub - Perl subroutines
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.
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-byreference
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.
The return value of a subroutine is the value of the last
expression evaluated by that sub, or the empty list in the
case of an empty sub. More explicitly, a "return" statement
may be used to exit the 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.
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 =~ /^[ ]/) {
$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 inplace
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.
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 looselyheld
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", "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 explicitely. 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, 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 $n0;
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
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 $::x0;
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 [Toc] [Back]
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, 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 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 $hash{key} = "val"; # sets a local value for
this hash entry
local ($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 = andal; # just alias $merlyn, not
@merlyn etc
A "local" modifies its listed variables to be "local" to
the enclosing block, "eval", or "do FILE"--and to any sub-
routine called from within that block. A "local" just
gives temporary values to global (meaning package) variables.
It does not create a local variable. This is
known as dynamic scoping. Lexical scoping is done with
"my", which works more like C's auto declarations.
Some types of lvalues can be localized as well : hash and
array elements and slices, conditionals (provided that
their result is always localizable), and symbolic references.
As for simple variables, this creates new, dynamically
scoped values.
If more than one variable or expression is given to
"local", they must be placed in parentheses. This operator
works by saving the current values of those variables
in its argument list on a hidden stack and restoring them
upon exiting the block, subroutine, or eval. This means
that called subroutines can also reference the local variable,
but not the global one. The argument list may be
assigned to if desired, which allows you to initialize
your local variables. (If no initializer is given for a
particular variable, it is created with an undefined
value.)
Because "local" is a run-time operator, it gets executed
each time through a loop. Consequently, it's more efficient
to localize your variables outside the loop.
Grammatical note on local()
A "local" is simply a modifier on an lvalue expression.
When you assign to a "local"ized variable, the "local"
doesn't change whether its list is viewed as a scalar or
an array. So
local($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;
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 $1, thus is magic and readonly
f();
}
See next section for an alternative to this situation.
WARNING: Localization of tied arrays and hashes does not
currently work as described. This will be fixed in a
future release of Perl; in the meantime, avoid code that
relies on any particular behaviour of localising tied
arrays or hashes (localising individual elements is still
okay). See "Localising Tied Arrays and Hashes Is Broken"
in perl58delta for more details.
Localization of globs
The construct
local *name;
creates a whole new symbol table entry for the glob "name"
in the current package. That means that all variables in
its glob slot ($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.
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 . . .0;
last unless $e > 3;
}
if (@ary) {
$hash{'only a'} = 'test';
delete $hash{'a'};
}
}
print join(' ', map { "$_ $hash{$_}" } sort keys
%hash),".0;
print "The array has ",scalar(@ary)," elements: ",
join(', ', map { defined $_ ? $_ : 'undef' }
@ary),"0;
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 [Toc] [Back]
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 {
# 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-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 $_ = <>;
@Fields = split /^=+$/;
}
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 [Toc] [Back]
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 @$bref0;
sub func {
my ($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 @b0;
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(TDOUT);
sub splutter {
my $fh = shift;
print $fh "her um well a hmmm0;
}
$rec = get_rec(TDIN);
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, $path) ? *FH : undef;
}
Prototypes [Toc] [Back]
Perl supports a very limited kind of compile-time argument
checking using function prototyping. If you declare
sub mypush (@@)
then "mypush()" takes arguments exactly like "push()"
does. The function declaration must be visible at compile
time. The prototype affects only interpretation of newstyle
calls to the function, where new-style is defined as
not using the "&" character. In other words, if you call
it like a built-in function, then it behaves like a builtin
function. If you call it like an old-fashioned subroutine,
then it behaves like an old-fashioned subroutine.
It naturally falls out from this rule that prototypes have
no influence on subroutine references like "foo" or on
indirect subroutine calls like "&{$subref}" 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, @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 "
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 reference to a
typeglob in that slot. The value will be available to the
subroutine either as a simple scalar, or (in the latter
two cases) as a reference to the typeglob. If you wish to
always convert such arguments to a typeglob reference, use
Symbol::qualify_to_ref() as follows:
use Symbol 'qualify_to_ref';
sub foo (*) {
my $fh = qualify_to_ref(shift, caller);
...
}
A semicolon separates mandatory arguments from optional
arguments. It is redundant before "@" or "%", which gobble
up everything else.
Note how the last three examples in the table above are
treated specially by the parser. "mygrep()" is parsed as
a true list operator, "myrand()" is parsed as a true unary
operator with unary precedence the same as "rand()", and
"mytime()" is truly without arguments, just like "time()".
That is, if you say
mytime +2;
you'll get "mytime() + 2", not mytime(2), which is how it
would be parsed without a prototype.
The interesting thing about "&" is that you can generate
new syntax with it, provided it's in the initial position:
sub try (&@) {
my($try,$catch) = @_;
eval { &$try };
if ($@) {
local $_ = $@;
&$catch;
}
}
sub catch (&) { $_[0] }
try {
die "phooey";
} catch {
/phooey/ and print "unphooey0;
};
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 $n0;
}
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 [Toc] [Back]
Functions with a prototype of "()" are potential candidates
for inlining. If the result after optimization and
constant folding is either a constant or a lexicallyscoped
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 [Toc] [Back]
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: you can't take a reference to it, such as the
incorrect "CORE::open" might appear to produce.
Library modules should not in general export built-in
names like "open" or "chdir" as part of their default
@EXPORT list, because these may sneak into someone else's
namespace and change the semantics unexpectedly. Instead,
if the module adds that name to @EXPORT_OK, then it's possible
for a user to import the name explicitly, but not
implicitly. That is, they could say
use Module 'open';
and it would import the "open" override. But if they said
use Module;
they would get the default imports without overrides.
The foregoing mechanism for overriding built-in is
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 $pat = shift;
my @got;
local *D;
if (opendir D, '.') {
@got = grep /$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, 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>".
Finally, some built-ins (e.g. "exists" or "grep") can't be
overridden.
Autoloading [Toc] [Back]
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...
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 modules in SelfLoader,
and the document on adding C functions to Perl code in
perlxs.
Subroutine Attributes [Toc] [Back]
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 "Function
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