NAME¶
perlsub - Perl subroutines
SYNOPSIS¶
To declare subroutines:
sub NAME; # A "forward" declaration.
sub NAME(PROTO); # ditto, but with prototypes
sub NAME : ATTRS; # with attributes
sub NAME(PROTO) : ATTRS; # with attributes and prototypes
sub NAME BLOCK # A declaration and a definition.
sub NAME(PROTO) BLOCK # ditto, but with prototypes
sub NAME : ATTRS BLOCK # with attributes
sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes
To define an anonymous subroutine at runtime:
$subref = sub BLOCK; # no proto
$subref = sub (PROTO) BLOCK; # with proto
$subref = sub : ATTRS BLOCK; # with attributes
$subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
To import subroutines:
use MODULE qw(NAME1 NAME2 NAME3);
To call subroutines:
NAME(LIST); # & is optional with parentheses.
NAME LIST; # Parentheses optional if predeclared/imported.
&NAME(LIST); # Circumvent prototypes.
&NAME; # Makes current @_ visible to called subroutine.
DESCRIPTION¶
Like many languages, Perl provides for user-defined subroutines. These may be
located anywhere in the main program, loaded in from other files via the
"do", "require", or "use" keywords, or generated
on the fly using "eval" or anonymous subroutines. You can even call
a function indirectly using a variable containing its name or a CODE
reference.
The Perl model for function call and return values is simple: all functions are
passed as parameters one single flat list of scalars, and all functions
likewise return to their caller one single flat list of scalars. Any arrays or
hashes in these call and return lists will collapse, losing their
identities--but you may always use pass-by-reference instead to avoid this.
Both call and return lists may contain as many or as few scalar elements as
you'd like. (Often a function without an explicit return statement is called a
subroutine, but there's really no difference from Perl's perspective.)
Any arguments passed in show up in the array @_. Therefore, if you called a
function with two arguments, those would be stored in $_[0] and $_[1]. The
array @_ is a local array, but its elements are aliases for the actual scalar
parameters. In particular, if an element $_[0] is updated, the corresponding
argument is updated (or an error occurs if it is not updatable). If an
argument is an array or hash element which did not exist when the function was
called, that element is created only when (and if) it is modified or a
reference to it is taken. (Some earlier versions of Perl created the element
whether or not the element was assigned to.) Assigning to the whole array @_
removes that aliasing, and does not update any arguments.
A "return" statement may be used to exit a subroutine, optionally
specifying the returned value, which will be evaluated in the appropriate
context (list, scalar, or void) depending on the context of the subroutine
call. If you specify no return value, the subroutine returns an empty list in
list context, the undefined value in scalar context, or nothing in void
context. If you return one or more aggregates (arrays and hashes), these will
be flattened together into one large indistinguishable list.
If no "return" is found and if the last statement is an expression,
its value is returned. If the last statement is a loop control structure like
a "foreach" or a "while", the returned value is
unspecified. The empty sub returns the empty list.
Perl does not have named formal parameters. In practice all you do is assign to
a "my()" list of these. Variables that aren't declared to be private
are global variables. For gory details on creating private variables, see
"Private Variables via
my()" and "Temporary Values via
local()". To create protected environments for a set of functions
in a separate package (and probably a separate file), see "Packages"
in perlmod.
Example:
sub max {
my $max = shift(@_);
foreach $foo (@_) {
$max = $foo if $max < $foo;
}
return $max;
}
$bestday = max($mon,$tue,$wed,$thu,$fri);
Example:
# get a line, combining continuation lines
# that start with whitespace
sub get_line {
$thisline = $lookahead; # global variables!
LINE: while (defined($lookahead = <STDIN>)) {
if ($lookahead =~ /^[ \t]/) {
$thisline .= $lookahead;
}
else {
last LINE;
}
}
return $thisline;
}
$lookahead = <STDIN>; # get first line
while (defined($line = get_line())) {
...
}
Assigning to a list of private variables to name your arguments:
sub maybeset {
my($key, $value) = @_;
$Foo{$key} = $value unless $Foo{$key};
}
Because the assignment copies the values, this also has the effect of turning
call-by-reference into call-by-value. Otherwise a function is free to do
in-place modifications of @_ and change its caller's values.
upcase_in($v1, $v2); # this changes $v1 and $v2
sub upcase_in {
for (@_) { tr/a-z/A-Z/ }
}
You aren't allowed to modify constants in this way, of course. If an argument
were actually literal and you tried to change it, you'd take a (presumably
fatal) exception. For example, this won't work:
upcase_in("frederick");
It would be much safer if the "upcase_in()" function were written to
return a copy of its parameters instead of changing them in place:
($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2
sub upcase {
return unless defined wantarray; # void context, do nothing
my @parms = @_;
for (@parms) { tr/a-z/A-Z/ }
return wantarray ? @parms : $parms[0];
}
Notice how this (unprototyped) function doesn't care whether it was passed real
scalars or arrays. Perl sees all arguments as one big, long, flat parameter
list in @_. This is one area where Perl's simple argument-passing style
shines. The "upcase()" function would work perfectly well without
changing the "upcase()" definition even if we fed it things like
this:
@newlist = upcase(@list1, @list2);
@newlist = upcase( split /:/, $var );
Do not, however, be tempted to do this:
(@a, @b) = upcase(@list1, @list2);
Like the flattened incoming parameter list, the return list is also flattened on
return. So all you have managed to do here is stored everything in @a and made
@b empty. See "Pass by Reference" for alternatives.
A subroutine may be called using an explicit "&" prefix. The
"&" is optional in modern Perl, as are parentheses if the
subroutine has been predeclared. The "&" is
not optional
when just naming the subroutine, such as when it's used as an argument to
defined() or
undef(). Nor is it optional when you want to do an
indirect subroutine call with a subroutine name or reference using the
"&$subref()" or "&{$subref}()" constructs,
although the "$subref->()" notation solves that problem. See
perlref for more about all that.
Subroutines may be called recursively. If a subroutine is called using the
"&" form, the argument list is optional, and if omitted, no @_
array is set up for the subroutine: the @_ array at the time of the call is
visible to subroutine instead. This is an efficiency mechanism that new users
may wish to avoid.
&foo(1,2,3); # pass three arguments
foo(1,2,3); # the same
foo(); # pass a null list
&foo(); # the same
&foo; # foo() get current args, like foo(@_) !!
foo; # like foo() IFF sub foo predeclared, else "foo"
Not only does the "&" form make the argument list optional, it
also disables any prototype checking on arguments you do provide. This is
partly for historical reasons, and partly for having a convenient way to cheat
if you know what you're doing. See Prototypes below.
Subroutines whose names are in all upper case are reserved to the Perl core, as
are modules whose names are in all lower case. A subroutine in all capitals is
a loosely-held convention meaning it will be called indirectly by the run-time
system itself, usually due to a triggered event. Subroutines that do special,
pre-defined things include "AUTOLOAD", "CLONE",
"DESTROY" plus all functions mentioned in perltie and PerlIO::via.
The "BEGIN", "UNITCHECK", "CHECK",
"INIT" and "END" subroutines are not so much subroutines
as named special code blocks, of which you can have more than one in a
package, and which you can
not call explicitly. See "BEGIN,
UNITCHECK, CHECK, INIT and END" in perlmod
Private Variables via my()¶
Synopsis:
my $foo; # declare $foo lexically local
my (@wid, %get); # declare list of variables local
my $foo = "flurp"; # declare $foo lexical, and init it
my @oof = @bar; # declare @oof lexical, and init it
my $x : Foo = $y; # similar, with an attribute applied
WARNING: The use of attribute lists on "my" declarations is
still evolving. The current semantics and interface are subject to change. See
attributes and Attribute::Handlers.
The "my" operator declares the listed variables to be lexically
confined to the enclosing block, conditional
("if/unless/elsif/else"), loop
("for/foreach/while/until/continue"), subroutine, "eval",
or "do/require/use"'d file. If more than one value is listed, the
list must be placed in parentheses. All listed elements must be legal lvalues.
Only alphanumeric identifiers may be lexically scoped--magical built-ins like
$/ must currently be "local"ized with "local" instead.
Unlike dynamic variables created by the "local" operator, lexical
variables declared with "my" are totally hidden from the outside
world, including any called subroutines. This is true if it's the same
subroutine called from itself or elsewhere--every call gets its own copy.
This doesn't mean that a "my" variable declared in a statically
enclosing lexical scope would be invisible. Only dynamic scopes are cut off.
For example, the "bumpx()" function below has access to the lexical
$x variable because both the "my" and the "sub" occurred
at the same scope, presumably file scope.
my $x = 10;
sub bumpx { $x++ }
An "eval()", however, can see lexical variables of the scope it is
being evaluated in, so long as the names aren't hidden by declarations within
the "eval()" itself. See perlref.
The parameter list to
my() may be assigned to if desired, which allows
you to initialize your variables. (If no initializer is given for a particular
variable, it is created with the undefined value.) Commonly this is used to
name input parameters to a subroutine. Examples:
$arg = "fred"; # "global" variable
$n = cube_root(27);
print "$arg thinks the root is $n\n";
fred thinks the root is 3
sub cube_root {
my $arg = shift; # name doesn't matter
$arg **= 1/3;
return $arg;
}
The "my" is simply a modifier on something you might assign to. So
when you do assign to variables in its argument list, "my" doesn't
change whether those variables are viewed as a scalar or an array. So
my ($foo) = <STDIN>; # WRONG?
my @FOO = <STDIN>;
both supply a list context to the right-hand side, while
my $foo = <STDIN>;
supplies a scalar context. But the following declares only one variable:
my $foo, $bar = 1; # WRONG
That has the same effect as
my $foo;
$bar = 1;
The declared variable is not introduced (is not visible) until after the current
statement. Thus,
my $x = $x;
can be used to initialize a new $x with the value of the old $x, and the
expression
my $x = 123 and $x == 123
is false unless the old $x happened to have the value 123.
Lexical scopes of control structures are not bounded precisely by the braces
that delimit their controlled blocks; control expressions are part of that
scope, too. Thus in the loop
while (my $line = <>) {
$line = lc $line;
} continue {
print $line;
}
the scope of $line extends from its declaration throughout the rest of the loop
construct (including the "continue" clause), but not beyond it.
Similarly, in the conditional
if ((my $answer = <STDIN>) =~ /^yes$/i) {
user_agrees();
} elsif ($answer =~ /^no$/i) {
user_disagrees();
} else {
chomp $answer;
die "'$answer' is neither 'yes' nor 'no'";
}
the scope of $answer extends from its declaration through the rest of that
conditional, including any "elsif" and "else" clauses, but
not beyond it. See "Simple Statements" in perlsyn for information on
the scope of variables in statements with modifiers.
The "foreach" loop defaults to scoping its index variable dynamically
in the manner of "local". However, if the index variable is prefixed
with the keyword "my", or if there is already a lexical by that name
in scope, then a new lexical is created instead. Thus in the loop
for my $i (1, 2, 3) {
some_function();
}
the scope of $i extends to the end of the loop, but not beyond it, rendering the
value of $i inaccessible within "some_function()".
Some users may wish to encourage the use of lexically scoped variables. As an
aid to catching implicit uses to package variables, which are always global,
if you say
use strict 'vars';
then any variable mentioned from there to the end of the enclosing block must
either refer to a lexical variable, be predeclared via "our" or
"use vars", or else must be fully qualified with the package name. A
compilation error results otherwise. An inner block may countermand this with
"no strict 'vars'".
A "my" has both a compile-time and a run-time effect. At compile time,
the compiler takes notice of it. The principal usefulness of this is to quiet
"use strict 'vars'", but it is also essential for generation of
closures as detailed in perlref. Actual initialization is delayed until run
time, though, so it gets executed at the appropriate time, such as each time
through a loop, for example.
Variables declared with "my" are not part of any package and are
therefore never fully qualified with the package name. In particular, you're
not allowed to try to make a package variable (or other global) lexical:
my $pack::var; # ERROR! Illegal syntax
In fact, a dynamic variable (also known as package or global variables) are
still accessible using the fully qualified "::" notation even while
a lexical of the same name is also visible:
package main;
local $x = 10;
my $x = 20;
print "$x and $::x\n";
That will print out 20 and 10.
You may declare "my" variables at the outermost scope of a file to
hide any such identifiers from the world outside that file. This is similar in
spirit to C's static variables when they are used at the file level. To do
this with a subroutine requires the use of a closure (an anonymous function
that accesses enclosing lexicals). If you want to create a private subroutine
that cannot be called from outside that block, it can declare a lexical
variable containing an anonymous sub reference:
my $secret_version = '1.001-beta';
my $secret_sub = sub { print $secret_version };
&$secret_sub();
As long as the reference is never returned by any function within the module, no
outside module can see the subroutine, because its name is not in any
package's symbol table. Remember that it's not
REALLY called
$some_pack::secret_version or anything; it's just $secret_version, unqualified
and unqualifiable.
This does not work with object methods, however; all object methods have to be
in the symbol table of some package to be found. See "Function
Templates" in perlref for something of a work-around to this.
Persistent Private Variables¶
There are two ways to build persistent private variables in Perl 5.10. First,
you can simply use the "state" feature. Or, you can use closures, if
you want to stay compatible with releases older than 5.10.
Persistent variables via state()
Beginning with perl 5.9.4, you can declare variables with the "state"
keyword in place of "my". For that to work, though, you must have
enabled that feature beforehand, either by using the "feature"
pragma, or by using "-E" on one-liners. (see feature)
For example, the following code maintains a private counter, incremented each
time the
gimme_another() function is called:
use feature 'state';
sub gimme_another { state $x; return ++$x }
Also, since $x is lexical, it can't be reached or modified by any Perl code
outside.
When combined with variable declaration, simple scalar assignment to
"state" variables (as in "state $x = 42") is executed only
the first time. When such statements are evaluated subsequent times, the
assignment is ignored. The behavior of this sort of assignment to non-scalar
variables is undefined.
Persistent variables with closures
Just because a lexical variable is lexically (also called statically) scoped to
its enclosing block, "eval", or "do" FILE, this doesn't
mean that within a function it works like a C static. It normally works more
like a C auto, but with implicit garbage collection.
Unlike local variables in C or C++, Perl's lexical variables don't necessarily
get recycled just because their scope has exited. If something more permanent
is still aware of the lexical, it will stick around. So long as something else
references a lexical, that lexical won't be freed--which is as it should be.
You wouldn't want memory being free until you were done using it, or kept
around once you were done. Automatic garbage collection takes care of this for
you.
This means that you can pass back or save away references to lexical variables,
whereas to return a pointer to a C auto is a grave error. It also gives us a
way to simulate C's function statics. Here's a mechanism for giving a function
private variables with both lexical scoping and a static lifetime. If you do
want to create something like C's static variables, just enclose the whole
function in an extra block, and put the static variable outside the function
but in the block.
{
my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
}
}
# $secret_val now becomes unreachable by the outside
# world, but retains its value between calls to gimme_another
If this function is being sourced in from a separate file via
"require" or "use", then this is probably just fine. If
it's all in the main program, you'll need to arrange for the "my" to
be executed early, either by putting the whole block above your main program,
or more likely, placing merely a "BEGIN" code block around it to
make sure it gets executed before your program starts to run:
BEGIN {
my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
}
}
See "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod about the
special triggered code blocks, "BEGIN", "UNITCHECK",
"CHECK", "INIT" and "END".
If declared at the outermost scope (the file scope), then lexicals work somewhat
like C's file statics. They are available to all functions in that same file
declared below them, but are inaccessible from outside that file. This
strategy is sometimes used in modules to create private variables that the
whole module can see.
Temporary Values via local()¶
WARNING: In general, you should be using "my" instead of
"local", because it's faster and safer. Exceptions to this include
the global punctuation variables, global filehandles and formats, and direct
manipulation of the Perl symbol table itself. "local" is mostly used
when the current value of a variable must be visible to called subroutines.
Synopsis:
# localization of values
local $foo; # make $foo dynamically local
local (@wid, %get); # make list of variables local
local $foo = "flurp"; # make $foo dynamic, and init it
local @oof = @bar; # make @oof dynamic, and init it
local $hash{key} = "val"; # sets a local value for this hash entry
delete local $hash{key}; # delete this entry for the current block
local ($cond ? $v1 : $v2); # several types of lvalues support
# localization
# localization of symbols
local *FH; # localize $FH, @FH, %FH, &FH ...
local *merlyn = *randal; # now $merlyn is really $randal, plus
# @merlyn is really @randal, etc
local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
A "local" modifies its listed variables to be "local" to the
enclosing block, "eval", or "do FILE"--and to
any
subroutine called from within that block. A "local" just
gives temporary values to global (meaning package) variables. It does
not create a local variable. This is known as dynamic scoping. Lexical
scoping is done with "my", which works more like C's auto
declarations.
Some types of lvalues can be localized as well : hash and array elements and
slices, conditionals (provided that their result is always localizable), and
symbolic references. As for simple variables, this creates new, dynamically
scoped values.
If more than one variable or expression is given to "local", they must
be placed in parentheses. This operator works by saving the current values of
those variables in its argument list on a hidden stack and restoring them upon
exiting the block, subroutine, or eval. This means that called subroutines can
also reference the local variable, but not the global one. The argument list
may be assigned to if desired, which allows you to initialize your local
variables. (If no initializer is given for a particular variable, it is
created with an undefined value.)
Because "local" is a run-time operator, it gets executed each time
through a loop. Consequently, it's more efficient to localize your variables
outside the loop.
Grammatical note on local()
A "local" is simply a modifier on an lvalue expression. When you
assign to a "local"ized variable, the "local" doesn't
change whether its list is viewed as a scalar or an array. So
local($foo) = <STDIN>;
local @FOO = <STDIN>;
both supply a list context to the right-hand side, while
local $foo = <STDIN>;
supplies a scalar context.
Localization of special variables
If you localize a special variable, you'll be giving a new value to it, but its
magic won't go away. That means that all side-effects related to this magic
still work with the localized value.
This feature allows code like this to work :
# Read the whole contents of FILE in $slurp
{ local $/ = undef; $slurp = <FILE>; }
Note, however, that this restricts localization of some values ; for example,
the following statement dies, as of perl 5.9.0, with an error
Modification
of a read-only value attempted, because the $1 variable is magical and
read-only :
local $1 = 2;
One exception is the default scalar variable: starting with perl 5.14
"local($_)" will always strip all magic from $_, to make it possible
to safely reuse $_ in a subroutine.
WARNING: Localization of tied arrays and hashes does not currently work
as described. This will be fixed in a future release of Perl; in the meantime,
avoid code that relies on any particular behaviour of localising tied arrays
or hashes (localising individual elements is still okay). See "Localising
Tied Arrays and Hashes Is Broken" in perl58delta for more details.
Localization of globs
The construct
local *name;
creates a whole new symbol table entry for the glob "name" in the
current package. That means that all variables in its glob slot ($name, @name,
%name, &name, and the "name" filehandle) are dynamically reset.
This implies, among other things, that any magic eventually carried by those
variables is locally lost. In other words, saying "local */" will
not have any effect on the internal value of the input record separator.
Localization of elements of composite types
It's also worth taking a moment to explain what happens when you
"local"ize a member of a composite type (i.e. an array or hash
element). In this case, the element is "local"ized
by name.
This means that when the scope of the "local()" ends, the saved
value will be restored to the hash element whose key was named in the
"local()", or the array element whose index was named in the
"local()". If that element was deleted while the "local()"
was in effect (e.g. by a "delete()" from a hash or a
"shift()" of an array), it will spring back into existence, possibly
extending an array and filling in the skipped elements with "undef".
For instance, if you say
%hash = ( 'This' => 'is', 'a' => 'test' );
@ary = ( 0..5 );
{
local($ary[5]) = 6;
local($hash{'a'}) = 'drill';
while (my $e = pop(@ary)) {
print "$e . . .\n";
last unless $e > 3;
}
if (@ary) {
$hash{'only a'} = 'test';
delete $hash{'a'};
}
}
print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
print "The array has ",scalar(@ary)," elements: ",
join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
Perl will print
6 . . .
4 . . .
3 . . .
This is a test only a test.
The array has 6 elements: 0, 1, 2, undef, undef, 5
The behavior of
local() on non-existent members of composite types is
subject to change in future.
Localized deletion of elements of composite types
You can use the "delete local $array[$idx]" and "delete local
$hash{key}" constructs to delete a composite type entry for the current
block and restore it when it ends. They return the array/hash value before the
localization, which means that they are respectively equivalent to
do {
my $val = $array[$idx];
local $array[$idx];
delete $array[$idx];
$val
}
and
do {
my $val = $hash{key};
local $hash{key};
delete $hash{key};
$val
}
except that for those the "local" is scoped to the "do"
block. Slices are also accepted.
my %hash = (
a => [ 7, 8, 9 ],
b => 1,
)
{
my $a = delete local $hash{a};
# $a is [ 7, 8, 9 ]
# %hash is (b => 1)
{
my @nums = delete local @$a[0, 2]
# @nums is (7, 9)
# $a is [ undef, 8 ]
$a[0] = 999; # will be erased when the scope ends
}
# $a is back to [ 7, 8, 9 ]
}
# %hash is back to its original state
Lvalue subroutines¶
WARNING: Lvalue subroutines are still experimental and the implementation
may change in future versions of Perl.
It is possible to return a modifiable value from a subroutine. To do this, you
have to declare the subroutine to return an lvalue.
my $val;
sub canmod : lvalue {
# 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 $_ = <>;
@Fields = split /^\s*=+\s*$/;
}
It particular, it's important to "local"ize $_ in any routine that
assigns to it. Look out for implicit assignments in "while"
conditionals.
- 2.
- You need to create a local file or directory handle or a
local function.
A function that needs a filehandle of its own must use "local()"
on a complete typeglob. This can be used to create new symbol table
entries:
sub ioqueue {
local (*READER, *WRITER); # not my!
pipe (READER, WRITER) or die "pipe: $!";
return (*READER, *WRITER);
}
($head, $tail) = ioqueue();
See the Symbol module for a way to create anonymous symbol table entries.
Because assignment of a reference to a typeglob creates an alias, this can
be used to create what is effectively a local function, or at least, a
local alias.
{
local *grow = \&shrink; # only until this block exists
grow(); # really calls shrink()
move(); # if move() grow()s, it shrink()s too
}
grow(); # get the real grow() again
See "Function Templates" in perlref for more about manipulating
functions by name in this way.
- 3.
- You want to temporarily change just one element of an array
or hash.
You can "local"ize just one element of an aggregate. Usually this
is done on dynamics:
{
local $SIG{INT} = 'IGNORE';
funct(); # uninterruptible
}
# interruptibility automatically restored here
But it also works on lexically declared aggregates. Prior to 5.005, this
operation could on occasion misbehave.
Pass by Reference¶
If you want to pass more than one array or hash into a function--or return them
from it--and have them maintain their integrity, then you're going to have to
use an explicit pass-by-reference. Before you do that, you need to understand
references as detailed in perlref. This section may not make much sense to you
otherwise.
Here are a few simple examples. First, let's pass in several arrays to a
function and have it "pop" all of then, returning a new list of all
their former last elements:
@tailings = popmany ( \@a, \@b, \@c, \@d );
sub popmany {
my $aref;
my @retlist = ();
foreach $aref ( @_ ) {
push @retlist, pop @$aref;
}
return @retlist;
}
Here's how you might write a function that returns a list of keys occurring in
all the hashes passed to it:
@common = inter( \%foo, \%bar, \%joe );
sub inter {
my ($k, $href, %seen); # locals
foreach $href (@_) {
while ( $k = each %$href ) {
$seen{$k}++;
}
}
return grep { $seen{$_} == @_ } keys %seen;
}
So far, we're using just the normal list return mechanism. What happens if you
want to pass or return a hash? Well, if you're using only one of them, or you
don't mind them concatenating, then the normal calling convention is ok,
although a little expensive.
Where people get into trouble is here:
(@a, @b) = func(@c, @d);
or
(%a, %b) = func(%c, %d);
That syntax simply won't work. It sets just @a or %a and clears the @b or %b.
Plus the function didn't get passed into two separate arrays or hashes: it got
one long list in @_, as always.
If you can arrange for everyone to deal with this through references, it's
cleaner code, although not so nice to look at. Here's a function that takes
two array references as arguments, returning the two array elements in order
of how many elements they have in them:
($aref, $bref) = func(\@c, \@d);
print "@$aref has more than @$bref\n";
sub func {
my ($cref, $dref) = @_;
if (@$cref > @$dref) {
return ($cref, $dref);
} else {
return ($dref, $cref);
}
}
It turns out that you can actually do this also:
(*a, *b) = func(\@c, \@d);
print "@a has more than @b\n";
sub func {
local (*c, *d) = @_;
if (@c > @d) {
return (\@c, \@d);
} else {
return (\@d, \@c);
}
}
Here we're using the typeglobs to do symbol table aliasing. It's a tad subtle,
though, and also won't work if you're using "my" variables, because
only globals (even in disguise as "local"s) are in the symbol table.
If you're passing around filehandles, you could usually just use the bare
typeglob, like *STDOUT, but typeglobs references work, too. For example:
splutter(\*STDOUT);
sub splutter {
my $fh = shift;
print $fh "her um well a hmmm\n";
}
$rec = get_rec(\*STDIN);
sub get_rec {
my $fh = shift;
return scalar <$fh>;
}
If you're planning on generating new filehandles, you could do this. Notice to
pass back just the bare *FH, not its reference.
sub openit {
my $path = shift;
local *FH;
return open (FH, $path) ? *FH : undef;
}
Prototypes¶
Perl supports a very limited kind of compile-time argument checking using
function prototyping. If you declare
sub mypush (+@)
then "mypush()" takes arguments exactly like "push()" does.
The function declaration must be visible at compile time. The prototype
affects only interpretation of new-style calls to the function, where
new-style is defined as not using the "&" character. In other
words, if you call it like a built-in function, then it behaves like a
built-in function. If you call it like an old-fashioned subroutine, then it
behaves like an old-fashioned subroutine. It naturally falls out from this
rule that prototypes have no influence on subroutine references like
"\&foo" or on indirect subroutine calls like
"&{$subref}" or "$subref->()".
Method calls are not influenced by prototypes either, because the function to be
called is indeterminate at compile time, since the exact code called depends
on inheritance.
Because the intent of this feature is primarily to let you define subroutines
that work like built-in functions, here are prototypes for some other
functions that parse almost exactly like the corresponding built-in.
Declared as Called as
sub mylink ($$) mylink $old, $new
sub myvec ($$$) myvec $var, $offset, 1
sub myindex ($$;$) myindex &getstring, "substr"
sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
sub myreverse (@) myreverse $a, $b, $c
sub myjoin ($@) myjoin ":", $a, $b, $c
sub mypop (+) mypop @array
sub mysplice (+$$@) mysplice @array, 0, 2, @pushme
sub mykeys (+) mykeys %{$hashref}
sub myopen (*;$) myopen HANDLE, $name
sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
sub myrand (;$) myrand 42
sub mytime () mytime
Any backslashed prototype character represents an actual argument that must
start with that character (optionally preceded by "my",
"our" or "local"), with the exception of "$",
which will accept a hash or array element even without a dollar sign, such as
"my_function()->[0]". The value passed as part of @_ will be a
reference to the actual argument given in the subroutine call, obtained by
applying "\" to that argument.
You can use the "\[]" backslash group notation to specify more than
one allowed argument type. For example:
sub myref (\[$@%&*])
will allow calling
myref() as
myref $var
myref @array
myref %hash
myref &sub
myref *glob
and the first argument of
myref() will be a reference to a scalar, an
array, a hash, a code, or a glob.
Unbackslashed prototype characters have special meanings. Any unbackslashed
"@" or "%" eats all remaining arguments, and forces list
context. An argument represented by "$" forces scalar context. An
"&" requires an anonymous subroutine, which, if passed as the
first argument, does not require the "sub" keyword or a subsequent
comma.
A "*" allows the subroutine to accept a bareword, constant, scalar
expression, typeglob, or a reference to a typeglob in that slot. The value
will be available to the subroutine either as a simple scalar, or (in the
latter two cases) as a reference to the typeglob. If you wish to always
convert such arguments to a typeglob reference, use
Symbol::qualify_to_ref() as follows:
use Symbol 'qualify_to_ref';
sub foo (*) {
my $fh = qualify_to_ref(shift, caller);
...
}
The "+" prototype is a special alternative to "$" that will
act like "\[@%]" when given a literal array or hash variable, but
will otherwise force scalar context on the argument. This is useful for
functions which should accept either a literal array or an array reference as
the argument:
sub mypush (+@) {
my $aref = shift;
die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
push @$aref, @_;
}
When using the "+" prototype, your function must check that the
argument is of an acceptable type.
A semicolon (";") separates mandatory arguments from optional
arguments. It is redundant before "@" or "%", which gobble
up everything else.
As the last character of a prototype, or just before a semicolon, you can use
"_" in place of "$": if this argument is not provided, $_
will be used instead.
Note how the last three examples in the table above are treated specially by the
parser. "mygrep()" is parsed as a true list operator,
"myrand()" is parsed as a true unary operator with unary precedence
the same as "rand()", and "mytime()" is truly without
arguments, just like "time()". That is, if you say
mytime +2;
you'll get "mytime() + 2", not mytime(2), which is how it would be
parsed without a prototype.
The interesting thing about "&" is that you can generate new
syntax with it, provided it's in the initial position:
sub try (&@) {
my($try,$catch) = @_;
eval { &$try };
if ($@) {
local $_ = $@;
&$catch;
}
}
sub catch (&) { $_[0] }
try {
die "phooey";
} catch {
/phooey/ and print "unphooey\n";
};
That prints "unphooey". (Yes, there are still unresolved issues having
to do with visibility of @_. I'm ignoring that question for the moment. (But
note that if we make @_ lexically scoped, those anonymous subroutines can act
like closures... (Gee, is this sounding a little Lispish? (Never mind.))))
And here's a reimplementation of the Perl "grep" operator:
sub mygrep (&@) {
my $code = shift;
my @result;
foreach $_ (@_) {
push(@result, $_) if &$code;
}
@result;
}
Some folks would prefer full alphanumeric prototypes. Alphanumerics have been
intentionally left out of prototypes for the express purpose of someday in the
future adding named, formal parameters. The current mechanism's main goal is
to let module writers provide better diagnostics for module users. Larry feels
the notation quite understandable to Perl programmers, and that it will not
intrude greatly upon the meat of the module, nor make it harder to read. The
line noise is visually encapsulated into a small pill that's easy to swallow.
If you try to use an alphanumeric sequence in a prototype you will generate an
optional warning - "Illegal character in prototype...".
Unfortunately earlier versions of Perl allowed the prototype to be used as
long as its prefix was a valid prototype. The warning may be upgraded to a
fatal error in a future version of Perl once the majority of offending code is
fixed.
It's probably best to prototype new functions, not retrofit prototyping into
older ones. That's because you must be especially careful about silent
impositions of differing list versus scalar contexts. For example, if you
decide that a function should take just one parameter, like this:
sub func ($) {
my $n = shift;
print "you gave me $n\n";
}
and someone has been calling it with an array or expression returning a list:
func(@foo);
func( split /:/ );
Then you've just supplied an automatic "scalar" in front of their
argument, which can be more than a bit surprising. The old @foo which used to
hold one thing doesn't get passed in. Instead, "func()" now gets
passed in a 1; that is, the number of elements in @foo. And the
"split" gets called in scalar context so it starts scribbling on
your @_ parameter list. Ouch!
This is all very powerful, of course, and should be used only in moderation to
make the world a better place.
Constant Functions¶
Functions with a prototype of "()" are potential candidates for
inlining. If the result after optimization and constant folding is either a
constant or a lexically-scoped scalar which has no other references, then it
will be used in place of function calls made without "&". Calls
made using "&" are never inlined. (See
constant.pm for an
easy way to declare most constants.)
The following functions would all be inlined:
sub pi () { 3.14159 } # Not exact, but close.
sub PI () { 4 * atan2 1, 1 } # As good as it gets,
# and it's inlined, too!
sub ST_DEV () { 0 }
sub ST_INO () { 1 }
sub FLAG_FOO () { 1 << 8 }
sub FLAG_BAR () { 1 << 9 }
sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
sub N () { int(OPT_BAZ) / 3 }
sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
Be aware that these will not be inlined; as they contain inner scopes, the
constant folding doesn't reduce them to a single constant:
sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }
sub baz_val () {
if (OPT_BAZ) {
return 23;
}
else {
return 42;
}
}
If you redefine a subroutine that was eligible for inlining, you'll get a
mandatory warning. (You can use this warning to tell whether or not a
particular subroutine is considered constant.) The warning is considered
severe enough not to be optional because previously compiled invocations of
the function will still be using the old value of the function. If you need to
be able to redefine the subroutine, you need to ensure that it isn't inlined,
either by dropping the "()" prototype (which changes calling
semantics, so beware) or by thwarting the inlining mechanism in some other
way, such as
sub not_inlined () {
23 if $];
}
Overriding Built-in Functions¶
Many built-in functions may be overridden, though this should be tried only
occasionally and for good reason. Typically this might be done by a package
attempting to emulate missing built-in 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;
if (opendir my $d, '.') {
@got = grep /$pat/, readdir $d;
closedir $d;
}
return @got;
}
1;
And here's how it could be (ab)used:
#use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
package Foo;
use REGlob 'glob'; # override glob() in Foo:: only
print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
The initial comment shows a contrived, even dangerous example. By overriding
"glob" globally, you would be forcing the new (and subversive)
behavior for the "glob" operator for
every namespace, without
the complete cognizance or cooperation of the modules that own those
namespaces. Naturally, this should be done with extreme caution--if it must be
done at all.
The "REGlob" example above does not implement all the support needed
to cleanly override perl's "glob" operator. The built-in
"glob" has different behaviors depending on whether it appears in a
scalar or list context, but our "REGlob" doesn't. Indeed, many perl
built-in have such context sensitive behaviors, and these must be adequately
supported by a properly written override. For a fully functional example of
overriding "glob", study the implementation of
"File::DosGlob" in the standard library.
When you override a built-in, your replacement should be consistent (if
possible) with the built-in native syntax. You can achieve this by using a
suitable prototype. To get the prototype of an overridable built-in, use the
"prototype" function with an argument of
"CORE::builtin_name" (see "prototype" in perlfunc).
Note however that some built-ins can't have their syntax expressed by a
prototype (such as "system" or "chomp"). If you override
them you won't be able to fully mimic their original syntax.
The built-ins "do", "require" and "glob" can also
be overridden, but due to special magic, their original syntax is preserved,
and you don't have to define a prototype for their replacements. (You can't
override the "do BLOCK" syntax, though).
"require" has special additional dark magic: if you invoke your
"require" replacement as "require Foo::Bar", it will
actually receive the argument "Foo/Bar.pm" in @_. See
"require" in perlfunc.
And, as you'll have noticed from the previous example, if you override
"glob", the "<*>" glob operator is overridden as
well.
In a similar fashion, overriding the "readline" function also
overrides the equivalent I/O operator "<FILEHANDLE>". Also,
overriding "readpipe" also overrides the operators "``"
and "qx//".
Finally, some built-ins (e.g. "exists" or "grep") can't be
overridden.
Autoloading¶
If you call a subroutine that is undefined, you would ordinarily get an
immediate, fatal error complaining that the subroutine doesn't exist.
(Likewise for subroutines being used as methods, when the method doesn't exist
in any base class of the class's package.) However, if an "AUTOLOAD"
subroutine is defined in the package or packages used to locate the original
subroutine, then that "AUTOLOAD" subroutine is called with the
arguments that would have been passed to the original subroutine. The fully
qualified name of the original subroutine magically appears in the global
$AUTOLOAD variable of the same package as the "AUTOLOAD" routine.
The name is not passed as an ordinary argument because, er, well, just
because, that's why. (As an exception, a method call to a nonexistent
"import" or "unimport" method is just skipped instead.
Also, if the AUTOLOAD subroutine is an XSUB, $AUTOLOAD is not populated;
instead, you should call "SvPVX"/"SvCUR" on the
"CV" for "AUTOLOAD" to retrieve the method name.)
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¶
A subroutine declaration or definition may have a list of attributes associated
with it. If such an attribute list is present, it is broken up at space or
colon boundaries and treated as though a "use attributes" had been
seen. See attributes for details about what attributes are currently
supported. Unlike the limitation with the obsolescent "use attrs",
the "sub : ATTRLIST" syntax works to associate the attributes with a
pre-declaration, and not just with a subroutine definition.
The attributes must be valid as simple identifier names (without any punctuation
other than the '_' character). They may have a parameter list appended, which
is only checked for whether its parentheses ('(',')') nest properly.
Examples of valid syntax (even though the attributes are unknown):
sub fnord (&\%) : switch(10,foo(7,3)) : expensive;
sub plugh () : Ugly('\(") :Bad;
sub xyzzy : _5x5 { ... }
Examples of invalid syntax:
sub fnord : switch(10,foo(); # ()-string not balanced
sub snoid : Ugly('('); # ()-string not balanced
sub xyzzy : 5x5; # "5x5" not a valid identifier
sub plugh : Y2::north; # "Y2::north" not a simple identifier
sub snurt : foo + bar; # "+" not a colon or space
The attribute list is passed as a list of constant strings to the code which
associates them with the subroutine. In particular, the second example of
valid syntax above currently looks like this in terms of how it's parsed and
invoked:
use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
For further details on attribute lists and their manipulation, see attributes
and Attribute::Handlers.
SEE ALSO¶
See "Function Templates" in perlref for more about references and
closures. See perlxs if you'd like to learn about calling C subroutines from
Perl. See perlembed if you'd like to learn about calling Perl subroutines from
C. See perlmod to learn about bundling up your functions in separate files.
See perlmodlib to learn what library modules come standard on your system. See
perltoot to learn how to make object method calls.