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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
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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
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To import subroutines:
use MODULE qw(NAME1 NAME2 NAME3);
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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.
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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.
The return value of a subroutine is the value of the last expression evaluated. 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 perlmod/"Packages".
Example:
sub max {
my $max = shift(@_);
foreach $foo (@_) {
$max = $foo if $max < $foo;
}
return $max;
}
$bestday = max($mon,$tue,$wed,$thu,$fri);
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Example:
# get a line, combining continuation lines
# that start with whitespace
sub get_line {
$thisline = $lookahead; # global variables!
LINE: while (defined($lookahead = <STDIN>)) {
if ($lookahead =~ /^[ \t]/) {
$thisline .= $lookahead;
}
else {
last LINE;
}
}
return $thisline;
}
$lookahead = <STDIN>; # get first line
while (defined($line = get_line())) {
...
}
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Assigning to a list of private variables to name your arguments:
sub maybeset {
my($key, $value) = @_;
$Foo{$key} = $value unless $Foo{$key};
}
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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 $v2
sub upcase_in {
for (@_) { tr/a-z/A-Z/ }
}
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You aren't allowed to modify constants in this way, of course. If an argument were actually
literal and you tried to change it, you'd take a (presumably fatal) exception. For example, this
won't work:
It would be much safer if the upcase_in() function were written to return a copy
of its parameters instead of changing them in place:
($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];
}
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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 /:/, $var );
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Do not, however, be tempted to do this:
(@a, @b) = upcase(@list1, @list2);
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Like the flattened incoming parameter list, the return list is also flattened on return. So
all you have managed to do here is stored everything in @a and made @b
empty. See Pass by Reference for alternatives.
A subroutine may be called using an explicit & prefix. The &
is optional in modern Perl, as are parentheses if the subroutine has been predeclared. The &
is not optional when just naming the subroutine, such as when it's used as an argument to
defined() or undef(). Nor is it optional when you want to do an indirect subroutine call with a
subroutine name or reference using the &$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"
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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.
Functions whose names are in all upper case are reserved to the Perl core, as are modules
whose names are in all lower case. A function 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. Functions that do special, pre-defined things include BEGIN, CHECK,
INIT, END, AUTOLOAD, CLONE and DESTROY--plus
all functions mentioned in perltie.
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
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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 localize 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++ }
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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 $arg;
}
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The my is simply a modifier on something you might assign to. So when you do
assign to variables in its argument list, my doesn't change whether those variables
are viewed as a scalar or an array. So
my ($foo) = <STDIN>; # WRONG?
my @FOO = <STDIN>;
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both supply a list context to the right-hand side, while
supplies a scalar context. But the following declares only one variable:
my $foo, $bar = 1; # WRONG
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That has the same effect as
The declared variable is not introduced (is not visible) until after the current statement.
Thus,
can be used to initialize a new $x with the value of the old $x, and the expression
my $x = 123 and $x == 123
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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;
}
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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'";
}
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the scope of $answer extends from its declaration through the rest of that conditional,
including any elsif and else clauses, but not beyond it.
NOTE: The behaviour of a my statement modified with a statement modifier
conditional or loop construct (e.g. my $x if ...) is undefined. The value of
the my variable may be undef, any previously assigned value, or
possibly anything else. Don't rely on it. Future versions of perl might do something different
from the version of perl you try it out on. Here be dragons.
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();
}
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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
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)
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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";
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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();
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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 perlref/"Function
Templates" for something of a work-around to this.
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
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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 sub around it to make sure it gets executed
before your program starts to run:
sub BEGIN {
my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
}
}
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See perlmod/"Package
Constructors and Destructors" about the special triggered functions, 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.
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,
filehandles and formats, and direct manipulation of the Perl symbol table itself. Format
variables often use local though, as do other variables whose current value must be
visible to called subroutines.
Synopsis:
local $foo; # declare $foo dynamically local
local (@wid, %get); # declare list of variables local
local $foo = "flurp"; # declare $foo dynamic, and init it
local @oof = @bar; # declare @oof dynamic, and init it
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
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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.
If more than one variable is given to local, they must be placed in parentheses.
All listed elements must be legal lvalues. 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.) Commonly this is used to name the parameters to a
subroutine. Examples:
for $i ( 0 .. 9 ) {
$digits{$i} = $i;
}
# assume this function uses global %digits hash
parse_num();
# now temporarily add to %digits hash
if ($base12) {
# (NOTE: not claiming this is efficient!)
local %digits = (%digits, 't' => 10, 'e' => 11);
parse_num(); # parse_num gets this new %digits!
}
# old %digits restored here
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Because local is a run-time operator, it gets executed each time through a loop.
In releases of Perl previous to 5.0, this used more stack storage each time until the loop was
exited. Perl now reclaims the space each time through, but it's still more efficient to declare
your variables outside the loop.
A local is simply a modifier on an lvalue expression. When you assign to a localized
variable, the local doesn't change whether its list is viewed as a scalar or an
array. So
local($foo) = <STDIN>;
local @FOO = <STDIN>;
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both supply a list context to the right-hand side, while
supplies a scalar context.
A note about local() and composite types is in order. Something like local(%foo)
works by temporarily placing a brand new hash in the symbol table. The old hash is left alone,
but is hidden "behind" the new one.
This means the old variable is completely invisible via the symbol table (i.e. the hash entry
in the *foo typeglob) for the duration of the dynamic scope within which the local()
was seen. This has the effect of allowing one to temporarily occlude any magic on composite
types. For instance, this will briefly alter a tied hash to some other implementation:
tie %ahash, 'APackage';
[...]
{
local %ahash;
tie %ahash, 'BPackage';
[..called code will see %ahash tied to 'BPackage'..]
{
local %ahash;
[..%ahash is a normal (untied) hash here..]
}
}
[..%ahash back to its initial tied self again..]
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WARNING The code example above 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 perldelta/"Localising
Tied Arrays and Hashes Is Broken" for more details.
As another example, a custom implementation of %ENV might look like this:
{
local %ENV;
tie %ENV, 'MyOwnEnv';
[..do your own fancy %ENV manipulation here..]
}
[..normal %ENV behavior here..]
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It's also worth taking a moment to explain what happens when you localize a
member of a composite type (i.e. an array or hash element). In this case, the element is localized
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";
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Perl will print
6 . . .
4 . . .
3 . . .
This is a test only a test.
The array has 6 elements: 0, 1, 2, undef, undef, 5
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The behavior of local() on non-existent members of composite types is subject to change in
future.
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
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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);
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Both subroutines here are called in a scalar context, while in:
(data(2,3)) = get_data(3,4);
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and in:
(data(2),data(3)) = get_data(3,4);
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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 };
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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);
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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 perldata/"Typeglobs
and Filehandles".
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 localized
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 $_ = <>;
@Fields = split /^\s*=+\s*$/;
}
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It particular, it's important to localize $_ 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();
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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
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See perlref/"Function
Templates" 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 localize just one element of an aggregate. Usually this is done on
dynamics:
{
local $SIG{INT} = 'IGNORE';
funct(); # uninterruptible
}
# interruptibility automatically restored here
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|
But it also works on lexically declared aggregates. Prior to 5.005, this operation could
on occasion misbehave.
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;
}
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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;
}
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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);
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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);
}
}
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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);
}
}
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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 locals) 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>;
}
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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, $path) ? *FH : undef;
}
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Perl supports a very limited kind of compile-time argument checking using function
prototyping. If you declare
then mypush() takes arguments exactly like push() does. The
function declaration must be visible at compile time. The prototype affects only interpretation
of new-style calls to the function, where new-style is defined as not using the &
character. In other words, if you call it like a built-in function, then it behaves like a
built-in function. If you call it like an old-fashioned subroutine, then it behaves like an
old-fashioned subroutine. It naturally falls out from this rule that prototypes have no
influence on subroutine references like \&foo or on indirect subroutine calls
like &{$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, @array, 0, @pushme
sub mykeys (\%) mykeys %{$hashref}
sub myopen (*;$) myopen HANDLE, $name
sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
sub myrand ($) myrand 42
sub mytime () mytime
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Any backslashed prototype character represents an actual argument that absolutely must start
with that character. The value passed as part of @_ will be a reference to the
actual argument given in the subroutine call, obtained by applying \ to that
argument.
You can also backslash several argument types simultaneously by using the \[]
notation:
will allow calling myref() as
myref $var
myref @array
myref %hash
myref &sub
myref *glob
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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 $fh = qualify_to_ref(shift, caller);
...
}
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A semicolon separates mandatory arguments from optional arguments. It is redundant before @
or %, which gobble up everything else.
Note how the last three examples in the table above are treated specially by the parser. mygrep()
is parsed as a true list operator, myrand() is parsed as a true unary operator with
unary precedence the same as rand(), and mytime() is truly without
arguments, just like time(). That is, if you say
you'll get mytime() + 2, not mytime(2), which is how it would be
parsed without a prototype.
The interesting thing about & is that you can generate new syntax with it,
provided it's in the initial position:
sub try (&@) {
my($try,$catch) = @_;
eval { &$try };
if ($@) {
local $_ = $@;
&$catch;
}
}
sub catch (&) { $_[0] }
try {
die "phooey";
} catch {
/phooey/ and print "unphooey\n";
};
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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;
}
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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";
}
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and someone has been calling it with an array or expression returning a list:
func(@foo);
func( split /:/ );
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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.
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 BAZ_VAL () {
if (OPT_BAZ) {
return 23;
}
else {
return 42;
}
}
sub N () { int(BAZ_VAL) / 3 }
BEGIN {
my $prod = 1;
for (1..N) { $prod *= $_ }
sub N_FACTORIAL () { $prod }
}
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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 $];
}
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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--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 { ... }
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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
and it would import the open override. But if they said
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;
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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
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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 perlfunc/prototype).
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 perlfunc/require.
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.
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');
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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;
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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.
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 { ... }
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Examples of invalid syntax:
sub fnord : switch(10,foo() ; # ()-string not balanced
sub snoid : Ugly('(') ; # ()-string not balanced
sub xyzzy : 5x5 ; # "5x5" not a valid identifier
sub plugh : Y2::north ; # "Y2::north" not a simple identifier
sub snurt : foo + bar ; # "+" not a colon or space
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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';
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For further details on attribute lists and their manipulation, see attributes and Attribute::Handlers.
See perlref/"Function
Templates" for more about references and closures. See perlxs if you'd like to learn about
calling C subroutines from Perl. See perlembed if you'd like to learn
about calling Perl subroutines from C. See perlmod to learn about bundling up
your functions in separate files. See perlmodlib to learn what library
modules come standard on your system. See perltoot to learn how to make
object method calls.
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