perlsub - Perl subroutines
To declare subroutines:
sub NAME;
sub NAME(PROTO);
sub NAME : ATTRS;
sub NAME(PROTO) : ATTRS;
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;
$subref = sub (PROTO) BLOCK;
$subref = sub : ATTRS BLOCK;
$subref = sub (PROTO) : ATTRS BLOCK;
To import subroutines:
use MODULE qw(NAME1 NAME2 NAME3);
To call subroutines:
NAME(LIST);
NAME LIST;
&NAME(LIST);
&NAME;
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 the perlmod manpage.
Example:
sub max {
my $max = shift(@_);
foreach $foo (@_) {
$max = $foo if $max < $foo;
}
return $max;
}
$bestday = max($mon,$tue,$wed,$thu,$fri);
Example:
sub get_line {
$thisline = $lookahead;
LINE: while (defined($lookahead = <STDIN>)) {
if ($lookahead =~ /^[ \t]/) {
$thisline .= $lookahead;
}
else {
last LINE;
}
}
return $thisline;
}
$lookahead = <STDIN>;
while (defined($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, $v2);
sub upcase_in {
for (@_) { tr/a-z/A-Z/ }
}
You aren't allowed to modify constants in this way, of course. If an
argument were actually literal and you tried to change it, you'd take a
(presumably fatal) exception. For example, this won't work:
upcase_in("frederick");
It would be much safer if the upcase_in() function
were written to return a copy of its parameters instead
of changing them in place:
($v3, $v4) = upcase($v1, $v2);
sub upcase {
return unless defined wantarray;
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 the perlref manpage 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);
foo(1,2,3);
foo();
&foo();
&foo;
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 the perltie manpage and the PerlIO::via manpage.
The BEGIN, CHECK, INIT and END subroutines are not so much
subroutines as named special code blocks, of which you can have more
than one in a package, and which you can not call explicitly. See
BEGIN, CHECK, INIT and END in the perlmod manpage
Synopsis:
my $foo;
my (@wid, %get);
my $foo = "flurp";
my @oof = @bar;
my $x : Foo = $y;
WARNING: The use of attribute lists on my declarations is still
evolving. The current semantics and interface are subject to change.
See the attributes manpage and the Attribute::Handlers manpage.
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 localized
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 the perlref manpage.
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";
$n = cube_root(27);
print "$arg thinks the root is $n\n";
fred thinks the root is 3
sub cube_root {
my $arg = shift;
$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>;
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;
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 the perlsyn manpage 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 the perlref manpage. 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;
my $_;
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 the perlref manpage for something of a work-around to
this.
Just because a lexical variable is lexically (also called statically)
scoped to its enclosing block, eval, or do FILE, this doesn't mean that
within a function it works like a C static. It normally works more
like a C auto, but with implicit garbage collection.
Unlike local variables in C or C++, Perl's lexical variables don't
necessarily get recycled just because their scope has exited.
If something more permanent is still aware of the lexical, it will
stick around. So long as something else references a lexical, that
lexical won't be freed--which is as it should be. You wouldn't want
memory being free until you were done using it, or kept around once you
were done. Automatic garbage collection takes care of this for you.
This means that you can pass back or save away references to lexical
variables, whereas to return a pointer to a C auto is a grave error.
It also gives us a way to simulate C's function statics. Here's a
mechanism for giving a function private variables with both lexical
scoping and a static lifetime. If you do want to create something like
C's static variables, just enclose the whole function in an extra block,
and put the static variable outside the function but in the block.
{
my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
}
}
If this function is being sourced in from a separate file
via require or use, then this is probably just fine. If it's
all in the main program, you'll need to arrange for the my
to be executed early, either by putting the whole block above
your main program, or more likely, placing merely a BEGIN
code block around it to make sure it gets executed before your program
starts to run:
BEGIN {
my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
}
}
See BEGIN, CHECK, INIT and END in the perlmod manpage about the
special triggered code blocks, BEGIN, CHECK, INIT and END.
If declared at the outermost scope (the file scope), then lexicals
work somewhat like C's file statics. They are available to all
functions in that same file declared below them, but are inaccessible
from outside that file. This strategy is sometimes used in modules
to create private variables that the whole module can see.
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:
local $foo;
local (@wid, %get);
local $foo = "flurp";
local @oof = @bar;
local $hash{key} = "val";
local ($cond ? $v1 : $v2);
local *FH;
local *merlyn = *randal;
local *merlyn = 'randal';
local *merlyn = \$randal;
A local modifies its listed variables to be "local" to the
enclosing block, eval, or do FILE--and to any subroutine
called from within that block. A local just gives temporary
values to global (meaning package) variables. It does not create
a local variable. This is known as dynamic scoping. Lexical scoping
is done with my, which works more like C's auto declarations.
Some types of lvalues can be localized as well : hash and array elements
and slices, conditionals (provided that their result is always
localizable), and symbolic references. As for simple variables, this
creates new, dynamically scoped values.
If more than one variable or expression is given to local, they must be
placed in parentheses. This operator works
by saving the current values of those variables in its argument list on a
hidden stack and restoring them upon exiting the block, subroutine, or
eval. This means that called subroutines can also reference the local
variable, but not the global one. The argument list may be assigned to if
desired, which allows you to initialize your local variables. (If no
initializer is given for a particular variable, it is created with an
undefined value.)
Because local is a run-time operator, it gets executed each time
through a loop. Consequently, it's more efficient to localize your
variables outside the loop.
A local is simply a modifier on an lvalue expression. When you assign to
a localized variable, the local doesn't change whether its list is viewed
as a scalar or an array. So
local($foo) = <STDIN>;
local @FOO = <STDIN>;
both supply a list context to the right-hand side, while
local $foo = <STDIN>;
supplies a scalar context.
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 :
{ local $/ = undef; $slurp = <FILE>; }
Note, however, that this restricts localization of some values ; for
example, the following statement dies, as of perl 5.9.0, with an error
Modification of a read-only value attempted, because the $1 variable is
magical and read-only :
local $1 = 2;
Similarly, but in a way more difficult to spot, the following snippet will
die in perl 5.9.0 :
sub f { local $_ = "foo"; print }
for ($1) {
f();
}
See next section for an alternative to this situation.
WARNING: Localization of tied arrays and hashes does not currently
work as described.
This will be fixed in a future release of Perl; in the meantime, avoid
code that relies on any particular behaviour of localising tied arrays
or hashes (localising individual elements is still okay).
See Localising Tied Arrays and Hashes Is Broken in the perl58delta manpage for more
details.
The construct
local *name;
creates a whole new symbol table entry for the glob name in the
current package. That means that all variables in its glob slot ($name,
@name, %name, &name, and the name filehandle) are dynamically reset.
This implies, among other things, that any magic eventually carried by
those variables is locally lost. In other words, saying local */
will not have any effect on the internal value of the input record
separator.
Notably, if you want to work with a brand new value of the default scalar
$_, and avoid the potential problem listed above about $_ previously
carrying a magic value, you should use local *_ instead of local $_.
It's also worth taking a moment to explain what happens when you
localize a member of a composite type (i.e. an array or hash element).
In this case, the element is localized by name. This means that
when the scope of the local() ends, the saved value will be
restored to the hash element whose key was named in the local(), or
the array element whose index was named in the local(). If that
element was deleted while the local() was in effect (e.g. by a
delete() from a hash or a shift() of an array), it will spring
back into existence, possibly extending an array and filling in the
skipped elements with undef. For instance, if you say
%hash = ( 'This' => 'is', 'a' => 'test' );
@ary = ( 0..5 );
{
local($ary[5]) = 6;
local($hash{'a'}) = 'drill';
while (my $e = pop(@ary)) {
print "$e . . .\n";
last unless $e > 3;
}
if (@ary) {
$hash{'only a'} = 'test';
delete $hash{'a'};
}
}
print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
print "The array has ",scalar(@ary)," elements: ",
join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
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.
WARNING: Lvalue subroutines are still experimental and the
implementation may change in future versions of Perl.
It is possible to return a modifiable value from a subroutine.
To do this, you have to declare the subroutine to return an lvalue.
my $val;
sub canmod : lvalue {
$val;
}
sub nomod {
$val;
}
canmod() = 5;
nomod() = 5;
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 = [];
sub set_arr {
my $val = shift;
die("expected array, you supplied ", ref $val)
unless ref $val eq 'ARRAY';
$some_array_ref = $val;
}
sub set_arr_lv : lvalue {
$some_array_ref;
}
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