PERLSUB(1) Perl Programmers Reference Guide PERLSUB(1)
NAME
perlsub - Perl subroutines (user-defined functions)
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
use feature 'signatures';
sub NAME(SIG) BLOCK # with signature
sub NAME :ATTRS (SIG) BLOCK # with signature, attributes
sub NAME :prototype(PROTO) (SIG) BLOCK # with signature, prototype
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
use feature 'signatures';
$subref = sub (SIG) BLOCK; # with signature
$subref = sub : ATTRS(SIG) BLOCK; # with signature, attributes
To import subroutines:
use MODULE qw(NAME1 NAME2 NAME3);
To call subroutines:
NAME(LIST); # Regular subroutine call.
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.)
In a subroutine that uses signatures (see "Signatures" below), arguments
are assigned into lexical variables introduced by the signature. In the
current implementation of Perl they are also accessible in the @_ array
in the same way as for non-signature subroutines, but accessing them in
this manner is now discouraged inside such a signature-using subroutine.
In a subroutine that does not use signatures, 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.
When not using signatures, Perl does not otherwise provide a means to
create 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.
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.
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 /:/, $var );
Do not, however, be tempted to do this:
(@x, @y) = upcase(@list1, @list2);
Like the flattened incoming parameter list, the return list is also
flattened on return. So all you have managed to do here is stored
everything in @x and made @y empty. See "Pass by Reference" for
alternatives.
A subroutine may be called using an explicit "&" prefix. The "&" is
optional in modern Perl, as are parentheses if the subroutine has been
predeclared. The "&" is not optional when just naming the subroutine,
such as when it's used as an argument to defined() or undef(). Nor is
it optional when you want to do an indirect subroutine call with a
subroutine name or reference using the &$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 an empty argument list
&foo(); # the same
&foo; # foo() gets current args, like foo(@_)!
use strict 'subs';
foo; # like foo() iff sub foo predeclared, else
# a compile-time error
no strict 'subs';
foo; # like foo() iff sub foo predeclared, else
# a literal string "foo"
Not only does the "&" form make the argument list optional, it also
disables any prototype checking on arguments you do provide. This is
partly for historical reasons, and partly for having a convenient way to
cheat if you know what you're doing. See "Prototypes" below.
Since Perl 5.16.0, the "__SUB__" token is available under use feature
'current_sub' and "use v5.16". It will evaluate to a reference to the
currently-running sub, which allows for recursive calls without knowing
your subroutine's name.
use v5.16;
my $factorial = sub {
my ($x) = @_;
return 1 if $x == 1;
return($x * __SUB__->( $x - 1 ) );
};
The behavior of "__SUB__" within a regex code block (such as
"/(?{...})/") is subject to change.
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 whose name start with a left parenthesis are also
reserved the same way. The following is a list of some subroutines that
currently do special, pre-defined things.
documented later in this document
"AUTOLOAD"
documented in perlmod
"CLONE", "CLONE_SKIP"
documented in perlobj
"DESTROY", "DOES"
documented in perltie
"BINMODE", "CLEAR", "CLOSE", "DELETE", "DESTROY", "EOF", "EXISTS",
"EXTEND", "FETCH", "FETCHSIZE", "FILENO", "FIRSTKEY", "GETC",
"NEXTKEY", "OPEN", "POP", "PRINT", "PRINTF", "PUSH", "READ",
"READLINE", "SCALAR", "SEEK", "SHIFT", "SPLICE", "STORE",
"STORESIZE", "TELL", "TIEARRAY", "TIEHANDLE", "TIEHASH",
"TIESCALAR", "UNSHIFT", "UNTIE", "WRITE"
documented in PerlIO::via
"BINMODE", "CLEARERR", "CLOSE", "EOF", "ERROR", "FDOPEN", "FILENO",
"FILL", "FLUSH", "OPEN", "POPPED", "PUSHED", "READ", "SEEK",
"SETLINEBUF", "SYSOPEN", "TELL", "UNREAD", "UTF8", "WRITE"
documented in perlfunc
"import", "unimport", "INC"
documented in UNIVERSAL
"VERSION"
documented in perldebguts
"DB::DB", "DB::sub", "DB::lsub", "DB::goto", "DB::postponed"
undocumented, used internally by the overload feature
any starting with "("
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
Signatures
Perl has a facility to allow a subroutine's formal parameters to be
declared by special syntax, separate from the procedural code of the
subroutine body. The formal parameter list is known as a signature.
This facility must be enabled before it can be used. It is enabled
automatically by a "use v5.36" (or higher) declaration, or more directly
by "use feature 'signatures'", in the current scope.
The signature is part of a subroutine's body. Normally the body of a
subroutine is simply a braced block of code, but when using a signature,
the signature is a parenthesised list that goes immediately before the
block, after any name or attributes.
For example,
sub foo :lvalue ($x, $y = 1, @z) { .... }
The signature declares lexical variables that are in scope for the
block. When the subroutine is called, the signature takes control
first. It populates the signature variables from the list of arguments
that were passed. If the argument list doesn't meet the requirements of
the signature, then it will throw an exception. When the signature
processing is complete, control passes to the block.
Positional parameters are handled by simply naming scalar variables in
the signature. For example,
sub foo ($left, $right) {
return $left + $right;
}
takes two positional parameters, which must be filled at runtime by two
arguments. By default the parameters are mandatory, and it is not
permitted to pass more arguments than expected. So the above is
equivalent to
sub foo {
die "Too many arguments for subroutine" unless @_ <= 2;
die "Too few arguments for subroutine" unless @_ >= 2;
my $left = $_[0];
my $right = $_[1];
return $left + $right;
}
An argument can be ignored by omitting the main part of the name from a
parameter declaration, leaving just a bare "$" sigil. For example,
sub foo ($first, $, $third) {
return "first=$first, third=$third";
}
Although the ignored argument doesn't go into a variable, it is still
mandatory for the caller to pass it.
A positional parameter is made optional by giving a default value,
separated from the parameter name by "=":
sub foo ($left, $right = 0) {
return $left + $right;
}
The above subroutine may be called with either one or two arguments.
The default value expression is evaluated when the subroutine is called,
so it may provide different default values for different calls. It is
only evaluated if the argument was actually omitted from the call. For
example,
my $auto_id = 0;
sub foo ($thing, $id = $auto_id++) {
print "$thing has ID $id";
}
automatically assigns distinct sequential IDs to things for which no ID
was supplied by the caller. A default value expression may also refer
to parameters earlier in the signature, making the default for one
parameter vary according to the earlier parameters. For example,
sub foo ($first_name, $surname, $nickname = $first_name) {
print "$first_name $surname is known as \"$nickname\"";
}
A default value expression can also be written using the "//=" operator,
where it will be evaluated and used if the caller omitted a value or the
value provided was "undef".
sub foo ($name //= "world") {
print "Hello, $name";
}
foo(undef); # will print "Hello, world"
Similarly, the "||=" operator can be used to provide a default
expression to be used whenever the caller provided a false value (and
remember that a missing or "undef" value are also false).
sub foo ($x ||= 10) {
return 5 + $x;
}
An optional parameter can be nameless just like a mandatory parameter.
For example,
sub foo ($thing, $ = 1) {
print $thing;
}
The parameter's default value will still be evaluated if the
corresponding argument isn't supplied, even though the value won't be
stored anywhere. This is in case evaluating it has important side
effects. However, it will be evaluated in void context, so if it
doesn't have side effects and is not trivial it will generate a warning
if the "void" warning category is enabled. If a nameless optional
parameter's default value is not important, it may be omitted just as
the parameter's name was:
sub foo ($thing, $=) {
print $thing;
}
Optional positional parameters must come after all mandatory positional
parameters. (If there are no mandatory positional parameters then an
optional positional parameters can be the first thing in the signature.)
If there are multiple optional positional parameters and not enough
arguments are supplied to fill them all, they will be filled from left
to right.
After positional parameters, additional arguments may be captured in a
slurpy parameter. The simplest form of this is just an array variable:
sub foo ($filter, @inputs) {
print $filter->($_) foreach @inputs;
}
With a slurpy parameter in the signature, there is no upper limit on how
many arguments may be passed. A slurpy array parameter may be nameless
just like a positional parameter, in which case its only effect is to
turn off the argument limit that would otherwise apply:
sub foo ($thing, @) {
print $thing;
}
A slurpy parameter may instead be a hash, in which case the arguments
available to it are interpreted as alternating keys and values. There
must be as many keys as values: if there is an odd argument then an
exception will be thrown. Keys will be stringified, and if there are
duplicates then the later instance takes precedence over the earlier, as
with standard hash construction.
sub foo ($filter, %inputs) {
print $filter->($_, $inputs{$_}) foreach sort keys %inputs;
}
A slurpy hash parameter may be nameless just like other kinds of
parameter. It still insists that the number of arguments available to
it be even, even though they're not being put into a variable.
sub foo ($thing, %) {
print $thing;
}
A slurpy parameter, either array or hash, must be the last thing in the
signature. It may follow mandatory and optional positional parameters;
it may also be the only thing in the signature. Slurpy parameters
cannot have default values: if no arguments are supplied for them then
you get an empty array or empty hash.
A signature may be entirely empty, in which case all it does is check
that the caller passed no arguments:
sub foo () {
return 123;
}
Prior to Perl 5.36 these were considered experimental, and emitted a
warning in the "experimental::signatures" category. From Perl 5.36
onwards this no longer happens, though the warning category still exists
for back-compatibility with code that attempts to disable it with a
statement such as:
no warnings 'experimental::signatures';
In the current Perl implementation, when using a signature the arguments
are still also available in the special array variable @_. However,
accessing them via this array is now discouraged, and should not be
relied upon in newly-written code as this ability may change in a future
version. Code that attempts to access the @_ array will produce
warnings in the "experimental::args_array_with_signatures" category when
compiled:
sub f ($x) {
# This line emits the warning seen below
print "Arguments are @_";
}
Use of @_ in join or string with signatured subroutine is
experimental at ...
There is a difference between the two ways of accessing the arguments:
@_ aliases the arguments, but the signature variables get copies of the
arguments. So writing to a signature variable only changes that
variable, and has no effect on the caller's variables, but writing to an
element of @_ modifies whatever the caller used to supply that argument.
There is a potential syntactic ambiguity between signatures and
prototypes (see "Prototypes"), because both start with an opening
parenthesis and both can appear in some of the same places, such as just
after the name in a subroutine declaration. For historical reasons,
when signatures are not enabled, any opening parenthesis in such a
context will trigger very forgiving prototype parsing. Most signatures
will be interpreted as prototypes in those circumstances, but won't be
valid prototypes. (A valid prototype cannot contain any alphabetic
character.) This will lead to somewhat confusing error messages.
To avoid ambiguity, when signatures are enabled the special syntax for
prototypes is disabled. There is no attempt to guess whether a
parenthesised group was intended to be a prototype or a signature. To
give a subroutine a prototype under these circumstances, use a prototype
attribute. For example,
sub foo :prototype($) { $_[0] }
It is entirely possible for a subroutine to have both a prototype and a
signature. They do different jobs: the prototype affects compilation of
calls to the subroutine, and the signature puts argument values into
lexical variables at runtime. You can therefore write
sub foo :prototype($$) ($left, $right) {
return $left + $right;
}
The prototype attribute, and any other attributes, must come before the
signature. The signature always immediately precedes the block of the
subroutine's body.
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 to
limit their scope dynamically.
Unlike global or package variables localized 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";
# outputs: 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 package or global variable is still accessible using the
fully qualified "::" notation even while a lexical of the same name is
also visible:
package main;
our $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 5.10.0, you can declare variables with the "state"
keyword in place of "my". For that to work, though, you must have
enabled that feature beforehand, either by using the "feature" pragma,
or by using "-E" on one-liners (see feature). Beginning with Perl 5.16,
the "CORE::state" form does not require the "feature" pragma.
The "state" keyword creates a lexical variable (following the same
scoping rules as "my") that persists from one subroutine call to the
next. If a state variable resides inside an anonymous subroutine, then
each copy of the subroutine has its own copy of the state variable.
However, the value of the state variable will still persist between
calls to the same copy of the anonymous subroutine. (Don't forget that
"sub { ... }" creates a new subroutine each time it is executed.)
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 }
And this example uses anonymous subroutines to create separate counters:
use feature 'state';
sub create_counter {
return sub { 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 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 assignment to "state" declarations where the
left hand side of the assignment involves any parentheses is currently
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 5.10.0, 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 behavior 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. The behavior of local() on array elements
specified using negative indexes is particularly surprising, and is very
likely to change.
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 $x = delete local $hash{a};
# $x is [ 7, 8, 9 ]
# %hash is (b => 1)
{
my @nums = delete local @$x[0, 2]
# @nums is (7, 9)
# $x is [ undef, 8 ]
$x[0] = 999; # will be erased when the scope ends
}
# $x is back to [ 7, 8, 9 ]
}
# %hash is back to its original state
This construct is supported since Perl v5.12.
Lvalue subroutines
It is possible to return a modifiable value from a subroutine. To do
this, you have to declare the subroutine to return an lvalue.
my $val;
sub canmod : lvalue {
$val; # or: return $val;
}
sub nomod {
$val;
}
canmod() = 5; # assigns to $val
nomod() = 5; # ERROR
The scalar/list context for the subroutine and for the right-hand side
of assignment is determined as if the subroutine call is replaced by a
scalar. For example, consider:
data(2,3) = get_data(3,4);
Both subroutines here are called in a scalar context, while in:
(data(2,3)) = get_data(3,4);
and in:
(data(2),data(3)) = get_data(3,4);
all the subroutines are called in a list context.
Lvalue subroutines are convenient, but you have to keep in mind that,
when used with objects, they may violate encapsulation. A normal
mutator can check the supplied argument before setting the attribute it
is protecting, an lvalue subroutine cannot. If you require any special
processing when storing and retrieving the values, consider using the
CPAN module Sentinel or something similar.
Lexical Subroutines
Beginning with Perl 5.18, you can declare a private subroutine with "my"
or "state". As with state variables, the "state" keyword is only
available under "use feature 'state'" or "use v5.10" or higher.
Prior to Perl 5.26, lexical subroutines were deemed experimental and
were available only under the "use feature 'lexical_subs'" pragma. They
also produced a warning unless the "experimental::lexical_subs" warnings
category was disabled.
These subroutines are only visible within the block in which they are
declared, and only after that declaration:
# Include these two lines if your code is intended to run under Perl
# versions earlier than 5.26.
no warnings "experimental::lexical_subs";
use feature 'lexical_subs';
foo(); # calls the package/global subroutine
state sub foo {
foo(); # also calls the package subroutine
}
foo(); # calls "state" sub
my $ref = \&foo; # take a reference to "state" sub
my sub bar { ... }
bar(); # calls "my" sub
You can't (directly) write a recursive lexical subroutine:
# WRONG
my sub baz {
baz();
}
This example fails because baz() refers to the package/global subroutine
"baz", not the lexical subroutine currently being defined.
The solution is to use "__SUB__":
my sub baz {
__SUB__->(); # calls itself
}
It is possible to predeclare a lexical subroutine. The "sub foo {...}"
subroutine definition syntax respects any previous "my sub;" or "state
sub;" declaration. Using this to define recursive subroutines is a bad
idea, however:
my sub baz; # predeclaration
sub baz { # define the "my" sub
baz(); # WRONG: calls itself, but leaks memory
}
Just like "my $f; $f = sub { $f->() }", this example leaks memory. The
name "baz" is a reference to the subroutine, and the subroutine uses the
name "baz"; they keep each other alive (see "Circular References" in
perlref).
"state sub" vs "my sub"
What is the difference between "state" subs and "my" subs? Each time
that execution enters a block when "my" subs are declared, a new copy of
each sub is created. "State" subroutines persist from one execution of
the containing block to the next.
So, in general, "state" subroutines are faster. But "my" subs are
necessary if you want to create closures:
sub whatever {
my $x = shift;
my sub inner {
... do something with $x ...
}
inner();
}
In this example, a new $x is created when "whatever" is called, and also
a new "inner", which can see the new $x. A "state" sub will only see
the $x from the first call to "whatever".
"our" subroutines
Like "our $variable", "our sub" creates a lexical alias to the package
subroutine of the same name.
The two main uses for this are to switch back to using the package sub
inside an inner scope:
sub foo { ... }
sub bar {
my sub foo { ... }
{
# need to use the outer foo here
our sub foo;
foo();
}
}
and to make a subroutine visible to other packages in the same scope:
package MySneakyModule;
our sub do_something { ... }
sub do_something_with_caller {
package DB;
() = caller 1; # sets @DB::args
do_something(@args); # uses MySneakyModule::do_something
}
Passing Symbol Table Entries (typeglobs)
WARNING: The mechanism described in this section was originally the only
way to simulate pass-by-reference in older versions of Perl. While it
still works fine in modern versions, the new reference mechanism is
generally easier to work with. See below.
Sometimes you don't want to pass the value of an array to a subroutine
but rather the name of it, so that the subroutine can modify the global
copy of it rather than working with a local copy. In Perl you can refer
to all objects of a particular name by prefixing the name with a star:
*foo. This is often known as a "typeglob", because the star on the
front can be thought of as a wildcard match for all the funny prefix
characters on variables and subroutines and such.
When evaluated, the typeglob produces a scalar value that represents all
the objects of that name, including any filehandle, format, or
subroutine. When assigned to, it causes the name mentioned to refer to
whatever "*" value was assigned to it. Example:
sub doubleary {
local(*someary) = @_;
foreach $elem (@someary) {
$elem *= 2;
}
}
doubleary(*foo);
doubleary(*bar);
Scalars are already passed by reference, so you can modify scalar
arguments without using this mechanism by referring explicitly to $_[0]
etc. You can modify all the elements of an array by passing all the
elements as scalars, but you have to use the "*" mechanism (or the
equivalent reference mechanism) to "push", "pop", or change the size of
an array. It will certainly be faster to pass the typeglob (or
reference).
Even if you don't want to modify an array, this mechanism is useful for
passing multiple arrays in a single LIST, because normally the LIST
mechanism will merge all the array values so that you can't extract out
the individual arrays. For more on typeglobs, see "Typeglobs and
Filehandles" in perldata.
When to Still Use local()
Despite the existence of "my", there are still three places where the
"local" operator still shines. In fact, in these three places, you must
use "local" instead of "my".
1. You need to give a global variable a temporary value, especially $_.
The global variables, like @ARGV or the punctuation variables, must
be "local"ized with local(). This block reads in /etc/motd, and
splits it up into chunks separated by lines of equal signs, which
are placed in @Fields.
{
local @ARGV = ("/etc/motd");
local $/ = undef;
local $_ = <>;
@Fields = split /^\s*=+\s*$/;
}
It particular, it's important to "local"ize $_ in any routine that
assigns to it. Look out for implicit assignments in "while"
conditionals.
2. You need to create a local file or directory handle or a local
function.
A function that needs a filehandle of its own must use local() on a
complete typeglob. This can be used to create new symbol table
entries:
sub ioqueue {
local (*READER, *WRITER); # not my!
pipe (READER, WRITER) or die "pipe: $!";
return (*READER, *WRITER);
}
($head, $tail) = ioqueue();
See the Symbol module for a way to create anonymous symbol table
entries.
Because assignment of a reference to a typeglob creates an alias,
this can be used to create what is effectively a local function, or
at least, a local alias.
{
local *grow = \&shrink; # only until this block exits
grow(); # really calls shrink()
move(); # if move() grow()s, it shrink()s too
}
grow(); # get the real grow() again
See "Function Templates" in perlref for more about manipulating
functions by name in this way.
3. You want to temporarily change just one element of an array or hash.
You can "local"ize just one element of an aggregate. Usually this
is done on dynamics:
{
local $SIG{INT} = 'IGNORE';
funct(); # uninterruptible
}
# interruptibility automatically restored here
But it also works on lexically declared aggregates.
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 ( \@w, \@x, \@y, \@z );
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:
(@w, @x) = func(@y, @z);
or
(%w, %x) = func(%y, %z);
That syntax simply won't work. It sets just @w or %w and clears the @x
or %x. 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:
($wref, $xref) = func(\@y, \@z);
print "@$wref has more than @$xref\n";
sub func {
my ($yref, $zref) = @_;
if (@$yref > @$zref) {
return ($yref, $zref);
} else {
return ($zref, $yref);
}
}
It turns out that you can actually do this also:
(*w, *x) = func(\@y, \@z);
print "@w has more than @x\n";
sub func {
local (*y, *z) = @_;
if (@y > @z) {
return (\@y, \@z);
} else {
return (\@z, \@y);
}
}
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. This can be declared in either the PROTO
section or with a prototype attribute. If you declare either of
sub mypush (\@@)
sub mypush :prototype(\@@)
then mypush() takes arguments exactly like push() does.
If subroutine signatures are enabled (see "Signatures"), then the
shorter PROTO syntax is unavailable, because it would clash with
signatures. In that case, a prototype can only be declared in the form
of an attribute.
The function declaration must be visible at compile time. The prototype
affects only interpretation of regular calls to the function, where
regular is defined as not using the "&" sigil. 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 (perl4) 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 ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
sub myreverse (@) myreverse $x, $y, $z
sub myjoin ($@) myjoin ":", $x, $y, $z
sub mypop (\@) mypop @array
sub mysplice (\@$$@) mysplice @array, 0, 2, @pushme
sub mykeys (\[%@]) mykeys $hashref->%*
sub myopen (*;$) myopen HANDLE, $name
sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
sub mygrep (&@) mygrep { /foo/ } $x, $y, $z
sub myrand (;$) myrand 42
sub mytime () mytime
Any backslashed prototype character represents an actual argument that
must start with that character (optionally preceded by "my", "our" or
"local"), with the exception of "$", which will accept any scalar lvalue
expression, such as "$foo = 7" or "my_function()->[0]". The value
passed as part of @_ will be a reference to the actual argument given in
the subroutine call, obtained by applying "\" to that argument.
You can use the "\[]" backslash group notation to specify more than one
allowed argument type. For example:
sub myref (\[$@%&*])
will allow calling myref() as
myref $var
myref @array
myref %hash
myref &sub
myref *glob
and the first argument of myref() will be a reference to a scalar, an
array, a hash, a subroutine, or a glob.
Unbackslashed prototype characters have special meanings. Any
unbackslashed "@" or "%" eats all remaining arguments, and forces list
context. An argument represented by "$" forces scalar context. An "&"
requires an anonymous subroutine, which, if passed as the first
argument, may look like a bare block: It does not require the "sub"
keyword or a subsequent comma.
A "*" allows the subroutine to accept a bareword, constant, scalar
expression, typeglob, or a reference to a typeglob in that slot. The
value will be available to the subroutine either as a simple scalar, or
(in the latter two cases) as a reference to the typeglob. If you wish
to always convert such arguments to a typeglob reference, use
Symbol::qualify_to_ref() as follows:
use Symbol 'qualify_to_ref';
sub foo (*) {
my $fh = qualify_to_ref(shift, caller);
...
}
The "+" prototype is a special alternative to "$" that will act like
"\[@%]" when given a literal array or hash variable, but will otherwise
force scalar context on the argument. This is useful for functions
which should accept either a literal array or an array reference as the
argument:
sub mypush (+@) {
my $aref = shift;
die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
push @$aref, @_;
}
When using the "+" prototype, your function must check that the argument
is of an acceptable type.
A semicolon (";") separates mandatory arguments from optional arguments.
It is redundant before "@" or "%", which gobble up everything else.
As the last character of a prototype, or just before a semicolon, a "@"
or a "%", you can use "_" in place of "$": if this argument is not
provided, $_ will be used instead.
Note how the last three examples in the table above are treated
specially by the parser. mygrep() is parsed as a true list operator,
myrand() is parsed as a true unary operator with unary precedence the
same as rand(), and mytime() is truly without arguments, just like
time(). That is, if you say
mytime +2;
you'll get "mytime() + 2", not mytime(2), which is how it would be
parsed without a prototype. If you want to force a unary function to
have the same precedence as a list operator, add ";" to the end of the
prototype:
sub mygetprotobynumber($;);
mygetprotobynumber $x > $y; # parsed as mygetprotobynumber($x > $y)
The interesting thing about "&" is that you can generate new syntax with
it, provided it's in the initial position:
sub try (&@) {
my($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( $text =~ /\w+/g );
Then you've just supplied an automatic "scalar" in front of their
argument, which can be more than a bit surprising. The old @foo which
used to hold one thing doesn't get passed in. Instead, func() now gets
passed in a 1; that is, the number of elements in @foo. And the "m//g"
gets called in scalar context so instead of a list of words it returns a
boolean result and advances pos($text). Ouch!
If a sub has both a PROTO and a BLOCK, the prototype is not applied
until after the BLOCK is completely defined. This means that a
recursive function with a prototype has to be predeclared for the
prototype to take effect, like so:
sub foo($$);
sub foo($$) {
foo 1, 2;
}
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 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 }
sub FOO_SET2 () { if (FLAG_MASK & FLAG_FOO) { 1 } }
(Be aware that the last example was not always inlined in Perl 5.20 and
earlier, which did not behave consistently with subroutines containing
inner scopes.) You can countermand inlining by using an explicit
"return":
sub baz_val () {
if (OPT_BAZ) {
return 23;
}
else {
return 42;
}
}
sub bonk_val () { return 12345 }
As alluded to earlier you can also declare inlined subs dynamically at
BEGIN time if their body consists of a lexically-scoped scalar which has
no other references. Only the first example here will be inlined:
BEGIN {
my $var = 1;
no strict 'refs';
*INLINED = sub () { $var };
}
BEGIN {
my $var = 1;
my $ref = \$var;
no strict 'refs';
*NOT_INLINED = sub () { $var };
}
A not so obvious caveat with this (see [RT #79908]) is what happens if
the variable is potentially modifiable. For example:
BEGIN {
my $x = 10;
*FOO = sub () { $x };
$x++;
}
print FOO(); # printed 10 prior to 5.32.0
From Perl 5.22 onwards this gave a deprecation warning, and from Perl
5.32 onwards it became a run-time error. Previously the variable was
immediately inlined, and stopped behaving like a normal lexical
variable; so it printed 10, not 11.
If you still want such a subroutine to be inlined (with no warning),
make sure the variable is not used in a context where it could be
modified aside from where it is declared.
# Fine, no warning
BEGIN {
my $x = 54321;
*INLINED = sub () { $x };
}
# Error
BEGIN {
my $x;
$x = 54321;
*ALSO_INLINED = sub () { $x };
}
Perl 5.22 also introduced the "const" attribute as an alternative. It
was initially experimental, but made stable in Perl 5.40. When applied
to an anonymous subroutine, it forces the sub to be called when the
"sub" expression is evaluated. The return value is captured and turned
into a constant subroutine:
my $x = 54321;
*INLINED = sub : const { $x };
$x++;
The return value of "INLINED" in this example will always be 54321,
regardless of later modifications to $x. You can also put any arbitrary
code inside the sub, at it will be executed immediately and its return
value captured the same way.
If you really want a subroutine with a "()" prototype that returns a
lexical variable you can easily force it to not be inlined by adding an
explicit "return":
BEGIN {
my $x = 10;
*FOO = sub () { return $x };
$x++;
}
print FOO(); # prints 11
The easiest way to tell if a subroutine was inlined is by using
B::Deparse. Consider this example of two subroutines returning 1, one
with a "()" prototype causing it to be inlined, and one without (with
deparse output truncated for clarity):
$ perl -MO=Deparse -e 'sub ONE { 1 } if (ONE) { print ONE if ONE }'
sub ONE {
1;
}
if (ONE ) {
print ONE() if ONE ;
}
$ perl -MO=Deparse -e 'sub ONE () { 1 } if (ONE) { print ONE if ONE }'
sub ONE () { 1 }
do {
print 1
};
If you redefine a subroutine that was eligible for inlining, you'll get
a warning by default. You can use this warning to tell whether or not a
particular subroutine is considered inlinable, since it's different than
the warning for overriding non-inlined subroutines:
$ perl -e 'sub one () {1} sub one () {2}'
Constant subroutine one redefined at -e line 1.
$ perl -we 'sub one {1} sub one {2}'
Subroutine one redefined at -e line 1.
The warning is considered severe enough not to be affected by the -w
switch (or its absence) because previously compiled invocations of the
function will still be using the old value of the function. If you need
to be able to redefine the subroutine, you need to ensure that it isn't
inlined, either by dropping the "()" prototype (which changes calling
semantics, so beware) or by thwarting the inlining mechanism in some
other way, e.g. by adding an explicit "return", as mentioned above:
sub not_inlined () { return 23 }
Overriding Built-in Functions
Many built-in functions may be overridden, though this should be tried
only occasionally and for good reason. Typically this might be done by
a package attempting to emulate missing built-in functionality on a non-
Unix system.
Overriding may be done only by importing the name from a module at
compile time--ordinary predeclaration isn't good enough. However, the
"use subs" pragma lets you, in effect, predeclare subs via the import
syntax, and these names may then override built-in ones:
use subs 'chdir', 'chroot', 'chmod', 'chown';
chdir $somewhere;
sub chdir { ... }
To unambiguously refer to the built-in form, precede the built-in name
with the special package qualifier "CORE::". For example, saying
CORE::open() always refers to the built-in open(), even if the current
package has imported some other subroutine called &open() from
elsewhere. Even though it looks like a regular function call, it isn't:
the "CORE::" prefix in that case is part of Perl's syntax, and works for
any keyword, regardless of what is in the "CORE" package. Taking a
reference to it, that is, "\&CORE::open", only works for some keywords.
See CORE.
Library modules should not in general export built-in names like "open"
or "chdir" as part of their default @EXPORT list, because these may
sneak into someone else's namespace and change the semantics
unexpectedly. Instead, if the module adds that name to @EXPORT_OK, then
it's possible for a user to import the name explicitly, but not
implicitly. That is, they could say
use Module 'open';
and it would import the "open" override. But if they said
use Module;
they would get the default imports without overrides.
The foregoing mechanism for overriding built-in is restricted, quite
deliberately, to the package that requests the import. There is a
second method that is sometimes applicable when you wish to override a
built-in everywhere, without regard to namespace boundaries. This is
achieved by importing a sub into the special namespace "CORE::GLOBAL::".
Here is an example that quite brazenly replaces the "glob" operator with
something that understands regular expressions.
package REGlob;
require Exporter;
@ISA = 'Exporter';
@EXPORT_OK = 'glob';
sub import {
my $pkg = shift;
return unless @_;
my $sym = shift;
my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
$pkg->export($where, $sym, @_);
}
sub glob {
my $pat = shift;
my @got;
if (opendir my $d, '.') {
@got = grep /$pat/, readdir $d;
closedir $d;
}
return @got;
}
1;
And here's how it could be (ab)used:
#use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
package Foo;
use REGlob 'glob'; # override glob() in Foo:: only
print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
The initial comment shows a contrived, even dangerous example. By
overriding "glob" globally, you would be forcing the new (and
subversive) behavior for the "glob" operator for every namespace,
without the complete cognizance or cooperation of the modules that own
those namespaces. Naturally, this should be done with extreme
caution--if it must be done at all.
The "REGlob" example above does not implement all the support needed to
cleanly override Perl's "glob" operator. The built-in "glob" has
different behaviors depending on whether it appears in a scalar or list
context, but our "REGlob" doesn't. Indeed, many Perl built-ins have
such context sensitive behaviors, and these must be adequately supported
by a properly written override. For a fully functional example of
overriding "glob", study the implementation of "File::DosGlob" in the
standard library.
When you override a built-in, your replacement should be consistent (if
possible) with the built-in native syntax. You can achieve this by
using a suitable prototype. To get the prototype of an overridable
built-in, use the "prototype" function with an argument of
"CORE::builtin_name" (see "prototype" in perlfunc).
Note however that some built-ins can't have their syntax expressed by a
prototype (such as "system" or "chomp"). If you override them you won't
be able to fully mimic their original syntax.
The built-ins "do", "require" and "glob" can also be overridden, but due
to special magic, their original syntax is preserved, and you don't have
to define a prototype for their replacements. (You can't override the
"do BLOCK" syntax, though).
"require" has special additional dark magic: if you invoke your
"require" replacement as "require Foo::Bar", it will actually receive
the argument "Foo/Bar.pm" in @_. See "require" in perlfunc.
And, as you'll have noticed from the previous example, if you override
"glob", the "<*>" glob operator is overridden as well.
In a similar fashion, overriding the "readline" function also overrides
the equivalent I/O operator "<FILEHANDLE>". Also, overriding "readpipe"
also overrides the operators `` and "qx//".
Finally, some built-ins (e.g. "exists" or "grep") can't be overridden.
Autoloading
If you call a subroutine that is undefined, you would ordinarily get an
immediate, fatal error complaining that the subroutine doesn't exist.
(Likewise for subroutines being used as methods, when the method doesn't
exist in any base class of the class's package.) However, if an
"AUTOLOAD" subroutine is defined in the package or packages used to
locate the original subroutine, then that "AUTOLOAD" subroutine is
called with the arguments that would have been passed to the original
subroutine. The fully qualified name of the original subroutine
magically appears in the global $AUTOLOAD variable of the same package
as the "AUTOLOAD" routine. The name is not passed as an ordinary
argument because, er, well, just because, that's why. (As an exception,
a method call to a nonexistent "import" or "unimport" method is just
skipped instead. Also, if the AUTOLOAD subroutine is an XSUB, there are
other ways to retrieve the subroutine name. See "Autoloading with
XSUBs" in perlguts for details.)
Many "AUTOLOAD" routines load in a definition for the requested
subroutine using eval(), then execute that subroutine using a special
form of goto() that erases the stack frame of the "AUTOLOAD" routine
without a trace. (See the source to the standard module documented in
AutoLoader, for example.) But an "AUTOLOAD" routine can also just
emulate the routine and never define it. For example, let's pretend
that a function that wasn't defined should just invoke "system" with
those arguments. All you'd do is:
sub AUTOLOAD {
our $AUTOLOAD; # keep 'use strict' happy
my $program = $AUTOLOAD;
$program =~ s/.*:://;
system($program, @_);
}
date();
who();
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;
ls '-l';
A more complete example of this is the Shell module on CPAN, which can
treat undefined subroutine calls as calls to external programs.
Mechanisms are available to help modules writers split their modules
into autoloadable files. See the standard AutoLoader module described
in AutoLoader and in AutoSplit, the standard SelfLoader modules in
SelfLoader, and the document on adding C functions to Perl code in
perlxs.
Subroutine Attributes
A subroutine declaration or definition may have a list of attributes
associated with it. If such an attribute list is present, it is broken
up at space or colon boundaries and treated as though a "use attributes"
had been seen. See attributes for details about what attributes are
currently supported. Unlike the limitation with the obsolescent "use
attrs", the "sub : ATTRLIST" syntax works to associate the attributes
with a pre-declaration, and not just with a subroutine definition.
The attributes must be valid as simple identifier names (without any
punctuation other than the '_' character). They may have a parameter
list appended, which is only checked for whether its parentheses
('(',')') nest properly.
Examples of valid syntax (even though the attributes are unknown):
sub fnord (&\%) : switch(10,foo(7,3)) : expensive;
sub plugh () : Ugly('\(") :Bad;
sub xyzzy : _5x5 { ... }
Examples of invalid syntax:
sub fnord : switch(10,foo(); # ()-string not balanced
sub snoid : Ugly('('); # ()-string not balanced
sub xyzzy : 5x5; # "5x5" not a valid identifier
sub plugh : Y2::north; # "Y2::north" not a simple identifier
sub snurt : foo + bar; # "+" not a colon or space
The attribute list is passed as a list of constant strings to the code
which associates them with the subroutine. In particular, the second
example of valid syntax above currently looks like this in terms of how
it's parsed and invoked:
use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
For further details on attribute lists and their manipulation, see
attributes and Attribute::Handlers.
SEE ALSO
See "Function Templates" in perlref for more about references and
closures. See perlxs if you'd like to learn about calling C subroutines
from Perl. See perlembed if you'd like to learn about calling Perl
subroutines from C. See perlmod to learn about bundling up your
functions in separate files. See perlmodlib to learn what library
modules come standard on your system. See perlootut to learn how to
make object method calls.
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