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PERLSUB(1)
NAME
perlsub - Perl subroutines
SYNOPSIS
To declare subroutines:
sub NAME; # A "forward" declaration.
sub NAME(PROTO); # ditto, but with prototypes
sub NAME : ATTRS; # with attributes
sub NAME(PROTO) : ATTRS; # with attributes and prototypes
sub NAME BLOCK # A declaration and a definition.
sub NAME(PROTO) BLOCK # ditto, but with prototypes
sub NAME : ATTRS BLOCK # with attributes
sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes
To define an anonymous subroutine at runtime:
$subref = sub BLOCK; # no proto
$subref = sub (PROTO) BLOCK; # with proto
$subref = sub : ATTRS BLOCK; # with attributes
$subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
To import subroutines:
use MODULE qw(NAME1 NAME2 NAME3);
To call subroutines:
NAME(LIST); # & is optional with parentheses.
NAME LIST; # Parentheses optional if predeclared/imported.
&NAME(LIST); # Circumvent prototypes.
&NAME; # Makes current @_ visible to called subroutine.
DESCRIPTION
Like many languages, Perl provides for user-defined subroutines. These may
be located anywhere in the main program, loaded in from other files via the
"do", "require", or "use" keywords, or generated on the fly using "eval" or
anonymous subroutines. You can even call a function indirectly using a
variable containing its name or a CODE reference.
The Perl model for function call and return values is simple: all functions
are passed as parameters one single flat list of scalars, and all functions
likewise return to their caller one single flat list of scalars. Any
arrays or hashes in these call and return lists will collapse, losing their
identities--but you may always use pass-by-reference instead to avoid this.
Both call and return lists may contain as many or as few scalar elements as
you'd like. (Often a function without an explicit return statement is
called a subroutine, but there's really no difference from Perl's
perspective.)
Any arguments passed in show up in the array "@_". Therefore, if you
called a function with two arguments, those would be stored in "$_[0]" and
"$_[1]". The array "@_" is a local array, but its elements are aliases for
the actual scalar parameters. In particular, if an element "$_[0]" is
updated, the corresponding argument is updated (or an error occurs if it is
not updatable). If an argument is an array or hash element which did not
exist when the function was called, that element is created only when (and
if) it is modified or a reference to it is taken. (Some earlier versions
of Perl created the element whether or not the element was assigned to.)
Assigning to the whole array "@_" removes that aliasing, and does not
update any arguments.
The return value of a subroutine is the value of the last expression
evaluated. More explicitly, a "return" statement may be used to exit the
subroutine, optionally specifying the returned value, which will be
evaluated in the appropriate context (list, scalar, or void) depending on
the context of the subroutine call. If you specify no return value, the
subroutine returns an empty list in list context, the undefined value in
scalar context, or nothing in void context. If you return one or more
aggregates (arrays and hashes), these will be flattened together into one
large indistinguishable list.
Perl does not have named formal parameters. In practice all you do is
assign to a "my()" list of these. Variables that aren't declared to be
private are global variables. For gory details on creating private
variables, see the section on "Private Variables via my()" and the section
on "Temporary Values via local()". To create protected environments for a
set of functions in a separate package (and probably a separate file), see
the Packages entry 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:
# get a line, combining continuation lines
# that start with whitespace
sub get_line {
$thisline = $lookahead; # global variables!
LINE: while (defined($lookahead = <STDIN>)) {
if ($lookahead =~ /^[ \t]/) {
$thisline .= $lookahead;
}
else {
last LINE;
}
}
return $thisline;
}
$lookahead = <STDIN>; # get first line
while (defined($line = get_line())) {
...
}
Assigning to a list of private variables to name your arguments:
sub maybeset {
my($key, $value) = @_;
$Foo{$key} = $value unless $Foo{$key};
}
Because the assignment copies the values, this also has the effect of
turning call-by-reference into call-by-value. Otherwise a function is free
to do in-place modifications of "@_" and change its caller's values.
upcase_in($v1, $v2); # this changes $v1 and $v2
sub upcase_in {
for (@_) { tr/a-z/A-Z/ }
}
You aren't allowed to modify constants in this way, of course. If an
argument were actually literal and you tried to change it, you'd take a
(presumably fatal) exception. For example, this won't work:
upcase_in("frederick");
It would be much safer if the "upcase_in()" function were written to return
a copy of its parameters instead of changing them in place:
($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2
sub upcase {
return unless defined wantarray; # void context, do nothing
my @parms = @_;
for (@parms) { tr/a-z/A-Z/ }
return wantarray ? @parms : $parms[0];
}
Notice how this (unprototyped) function doesn't care whether it was passed
real scalars or arrays. Perl sees all arguments as one big, long, flat
parameter list in "@_". This is one area where Perl's simple argument-
passing style shines. The "upcase()" function would work perfectly well
without changing the "upcase()" definition even if we fed it things like
this:
@newlist = upcase(@list1, @list2);
@newlist = upcase( split /:/, $var );
Do not, however, be tempted to do this:
(@a, @b) = upcase(@list1, @list2);
Like the flattened incoming parameter list, the return list is also
flattened on return. So all you have managed to do here is stored
everything in "@a" and made "@b" an empty list. See the Pass by Reference
entry elsewhere in this document for alternatives.
A subroutine may be called using an explicit "&" prefix. The "&" is
optional in modern Perl, as are parentheses if the subroutine has been
predeclared. The "&" is not optional when just naming the subroutine, such
as when it's used as an argument to defined() or undef(). Nor is it
optional when you want to do an indirect subroutine call with a subroutine
name or reference using the "&$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); # pass three arguments
foo(1,2,3); # the same
foo(); # pass a null list
&foo(); # the same
&foo; # foo() get current args, like foo(@_) !!
foo; # like foo() IFF sub foo predeclared, else "foo"
Not only does the "&" form make the argument list optional, it also
disables any prototype checking on arguments you do provide. This is
partly for historical reasons, and partly for having a convenient way to
cheat if you know what you're doing. See the Prototypes manpage below.
Functions whose names are in all upper case are reserved to the Perl core,
as are modules whose names are in all lower case. A function in all
capitals is a loosely-held convention meaning it will be called indirectly
by the run-time system itself, usually due to a triggered event. Functions
that do special, pre-defined things include "BEGIN", "CHECK", "INIT",
"END", "AUTOLOAD", and "DESTROY"--plus all functions mentioned in the
perltie manpage.
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 = $y; # similar, with an attribute applied
WARNING: The use of attribute lists on "my" declarations is experimental.
This feature should not be relied upon. It may change or disappear in
future releases of Perl. See the attributes manpage.
The "my" operator declares the listed variables to be lexically confined to
the enclosing block, conditional ("if/unless/elsif/else"), loop
("for/foreach/while/until/continue"), subroutine, "eval", or
"do/require/use"'d file. If more than one value is listed, the list must
be placed in parentheses. All listed elements must be legal lvalues. Only
alphanumeric identifiers may be lexically scoped--magical built-ins like
"$/" must currently be "local"ize 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"; # "global" variable
$n = cube_root(27);
print "$arg thinks the root is $n\n";
fred thinks the root is 3
sub cube_root {
my $arg = shift; # name doesn't matter
$arg **= 1/3;
return $arg;
}
The "my" is simply a modifier on something you might assign to. So when
you do assign to variables in its argument list, "my" doesn't change
whether those variables are viewed as a scalar or an array. So
my ($foo) = <STDIN>; # WRONG?
my @FOO = <STDIN>;
both supply a list context to the right-hand side, while
my $foo = <STDIN>;
supplies a scalar context. But the following declares only one variable:
my $foo, $bar = 1; # WRONG
That has the same effect as
my $foo;
$bar = 1;
The declared variable is not introduced (is not visible) until after the
current statement. Thus,
my $x = $x;
can be used to initialize a new $x with the value of the old $x, and the
expression
my $x = 123 and $x == 123
is false unless the old $x happened to have the value "123".
Lexical scopes of control structures are not bounded precisely by the
braces that delimit their controlled blocks; control expressions are part
of that scope, too. Thus in the loop
while (my $line = <>) {
$line = lc $line;
} continue {
print $line;
}
the scope of $line extends from its declaration throughout the rest of the
loop construct (including the "continue" clause), but not beyond it.
Similarly, in the conditional
if ((my $answer = <STDIN>) =~ /^yes$/i) {
user_agrees();
} elsif ($answer =~ /^no$/i) {
user_disagrees();
} else {
chomp $answer;
die "'$answer' is neither 'yes' nor 'no'";
}
the scope of $answer extends from its declaration through the rest of that
conditional, including any "elsif" and "else" clauses, but not beyond it.
None of the foregoing text applies to "if/unless" or "while/until"
modifiers appended to simple statements. Such modifiers are not control
structures and have no effect on scoping.
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; # ERROR! Illegal syntax
my $_; # also illegal (currently)
In fact, a dynamic variable (also known as package or global variables) are
still accessible using the fully qualified "::" notation even while a
lexical of the same name is also visible:
package main;
local $x = 10;
my $x = 20;
print "$x and $::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 the Function
Templates entry in the perlref manpage for something of a work-around to
this.
Persistent Private Variables
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" sub around it to make sure it gets executed before your program
starts to run:
sub BEGIN {
my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
}
}
See the Package Constructors and Destructors entry in the perlmod manpage
about the special triggered functions, "BEGIN", "CHECK", "INIT" and "END".
If declared at the outermost scope (the file scope), then lexicals work
somewhat like C's file statics. They are available to all functions in
that same file declared below them, but are inaccessible from outside that
file. This strategy is sometimes used in modules to create private
variables that the whole module can see.
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, filehandles and formats, and direct manipulation of the Perl
symbol table itself. Format variables often use "local" though, as do
other variables whose current value must be visible to called subroutines.
Synopsis:
local $foo; # declare $foo dynamically local
local (@wid, %get); # declare list of variables local
local $foo = "flurp"; # declare $foo dynamic, and init it
local @oof = @bar; # declare @oof dynamic, and init it
local *FH; # localize $FH, @FH, %FH, &FH ...
local *merlyn = *randal; # now $merlyn is really $randal, plus
# @merlyn is really @randal, etc
local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
A "local" modifies its listed variables to be "local" to the enclosing
block, "eval", or "do FILE"--and to any subroutine called from within that
block. A "local" just gives temporary values to global (meaning package)
variables. It does not create a local variable. This is known as dynamic
scoping. Lexical scoping is done with "my", which works more like C's auto
declarations.
If more than one variable is given to "local", they must be placed in
parentheses. All listed elements must be legal lvalues. This operator
works by saving the current values of those variables in its argument list
on a hidden stack and restoring them upon exiting the block, subroutine, or
eval. This means that called subroutines can also reference the local
variable, but not the global one. The argument list may be assigned to if
desired, which allows you to initialize your local variables. (If no
initializer is given for a particular variable, it is created with an
undefined value.) Commonly this is used to name the parameters to a
subroutine. Examples:
for $i ( 0 .. 9 ) {
$digits{$i} = $i;
}
# assume this function uses global %digits hash
parse_num();
# now temporarily add to %digits hash
if ($base12) {
# (NOTE: not claiming this is efficient!)
local %digits = (%digits, 't' => 10, 'e' => 11);
parse_num(); # parse_num gets this new %digits!
}
# old %digits restored here
Because "local" is a run-time operator, it gets executed each time through
a loop. In releases of Perl previous to 5.0, this used more stack storage
each time until the loop was exited. Perl now reclaims the space each time
through, but it's still more efficient to declare your variables outside
the loop.
A "local" is simply a modifier on an lvalue expression. When you assign to
a "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 $foo = <STDIN>;
supplies a scalar context.
A note about "local()" and composite types is in order. Something like
"local(%foo)" works by temporarily placing a brand new hash in the symbol
table. The old hash is left alone, but is hidden "behind" the new one.
This means the old variable is completely invisible via the symbol table
(i.e. the hash entry in the "*foo" typeglob) for the duration of the
dynamic scope within which the "local()" was seen. This has the effect of
allowing one to temporarily occlude any magic on composite types. For
instance, this will briefly alter a tied hash to some other implementation:
tie %ahash, 'APackage';
[...]
{
local %ahash;
tie %ahash, 'BPackage';
[..called code will see %ahash tied to 'BPackage'..]
{
local %ahash;
[..%ahash is a normal (untied) hash here..]
}
}
[..%ahash back to its initial tied self again..]
As another example, a custom implementation of "%ENV" might look like this:
{
local %ENV;
tie %ENV, 'MyOwnEnv';
[..do your own fancy %ENV manipulation here..]
}
[..normal %ENV behavior here..]
It's also worth taking a moment to explain what happens when you "local"ize
a member of a composite type (i.e. an array or hash element). In this
case, the element is "local"ized by name. This means that when the scope of
the "local()" ends, the saved value will be restored to the hash element
whose key was named in the "local()", or the array element whose index was
named in the "local()". If that element was deleted while the "local()"
was in effect (e.g. by a "delete()" from a hash or a "shift()" of an
array), it will spring back into existence, possibly extending an array and
filling in the skipped elements with "undef". For instance, if you say
%hash = ( 'This' => 'is', 'a' => 'test' );
@ary = ( 0..5 );
{
local($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.
Lvalue subroutines
WARNING: Lvalue subroutines are still experimental and the implementation
may change in future versions of Perl.
It is possible to return a modifiable value from a subroutine. To do this,
you have to declare the subroutine to return an lvalue.
my $val;
sub canmod : lvalue {
$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.
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 the Typeglobs and
Filehandles entry in the perldata manpage.
When to Still Use local()
Despite the existence of "my", there are still three places where the
"local" operator still shines. In fact, in these three places, you must
use "local" instead of "my".
1. You need to give a global variable a temporary value, especially $_.
The global variables, like "@ARGV" or the punctuation variables, must
be "local"ized with "local()". This block reads in /etc/motd, and
splits it up into chunks separated by lines of equal signs, which are
placed in "@Fields".
{
local @ARGV = ("/etc/motd");
local $/ = undef;
local $_ = <>;
@Fields = split /^\s*=+\s*$/;
}
It particular, it's important to "local"ize $_ in any routine that
assigns to it. Look out for implicit assignments in "while"
conditionals.
2. You need to create a local file or directory handle or a local
function.
A function that needs a filehandle of its own must use "local()" on a
complete typeglob. This can be used to create new symbol table
entries:
sub ioqueue {
local (*READER, *WRITER); # not my!
pipe (READER, WRITER); or die "pipe: $!";
return (*READER, *WRITER);
}
($head, $tail) = ioqueue();
See the Symbol module for a way to create anonymous symbol table
entries.
Because assignment of a reference to a typeglob creates an alias, this
can be used to create what is effectively a local function, or at
least, a local alias.
{
local *grow = <!>shrink; # only until this block exists
grow(); # really calls shrink()
move(); # if move() grow()s, it shrink()s too
}
grow(); # get the real grow() again
See the Function Templates entry in the perlref manpage for more about
manipulating functions by name in this way.
3. You want to temporarily change just one element of an array or hash.
You can "local"ize just one element of an aggregate. Usually this is
done on dynamics:
{
local $SIG{INT} = 'IGNORE';
funct(); # uninterruptible
}
# interruptibility automatically restored here
But it also works on lexically declared aggregates. Prior to 5.005,
this operation could on occasion misbehave.
Pass by Reference
If you want to pass more than one array or hash into a function--or return
them from it--and have them maintain their integrity, then you're going to
have to use an explicit pass-by-reference. Before you do that, you need to
understand references as detailed in the perlref manpage. This section may
not make much sense to you otherwise.
Here are a few simple examples. First, let's pass in several arrays to a
function and have it "pop" all of then, returning a new list of all their
former last elements:
@tailings = popmany ( \@a, \@b, \@c, \@d );
sub popmany {
my $aref;
my @retlist = ();
foreach $aref ( @_ ) {
push @retlist, pop @$aref;
}
return @retlist;
}
Here's how you might write a function that returns a list of keys occurring
in all the hashes passed to it:
@common = inter( \%foo, \%bar, \%joe );
sub inter {
my ($k, $href, %seen); # locals
foreach $href (@_) {
while ( $k = each %$href ) {
$seen{$k}++;
}
}
return grep { $seen{$_} == @_ } keys %seen;
}
So far, we're using just the normal list return mechanism. What happens if
you want to pass or return a hash? Well, if you're using only one of them,
or you don't mind them concatenating, then the normal calling convention is
ok, although a little expensive.
Where people get into trouble is here:
(@a, @b) = func(@c, @d);
or
(%a, %b) = func(%c, %d);
That syntax simply won't work. It sets just "@a" or "%a" and clears the
"@b" or "%b". Plus the function didn't get passed into two separate arrays
or hashes: it got one long list in "@_", as always.
If you can arrange for everyone to deal with this through references, it's
cleaner code, although not so nice to look at. Here's a function that
takes two array references as arguments, returning the two array elements
in order of how many elements they have in them:
($aref, $bref) = func(\@c, \@d);
print "@$aref has more than @$bref\n";
sub func {
my ($cref, $dref) = @_;
if (@$cref > @$dref) {
return ($cref, $dref);
} else {
return ($dref, $cref);
}
}
It turns out that you can actually do this also:
(*a, *b) = func(\@c, \@d);
print "@a has more than @b\n";
sub func {
local (*c, *d) = @_;
if (@c > @d) {
return (\@c, \@d);
} else {
return (\@d, \@c);
}
}
Here we're using the typeglobs to do symbol table aliasing. It's a tad
subtle, though, and also won't work if you're using "my" variables, because
only globals (even in disguise as "local"s) are in the symbol table.
If you're passing around filehandles, you could usually just use the bare
typeglob, like "*STDOUT", but typeglobs references work, too. For example:
splutter(\*STDOUT);
sub splutter {
my $fh = shift;
print $fh "her um well a hmmm\n";
}
$rec = get_rec(\*STDIN);
sub get_rec {
my $fh = shift;
return scalar <$fh>;
}
If you're planning on generating new filehandles, you could do this.
Notice to pass back just the bare *FH, not its reference.
sub openit {
my $path = shift;
local *FH;
return open (FH, $path) ? *FH : undef;
}
Prototypes
Perl supports a very limited kind of compile-time argument checking using
function prototyping. If you declare
sub mypush (\@@)
then "mypush()" takes arguments exactly like "push()" does. The function
declaration must be visible at compile time. The prototype affects only
interpretation of new-style calls to the function, where new-style is
defined as not using the "&" character. In other words, if you call it
like a built-in function, then it behaves like a built-in function. If you
call it like an old-fashioned subroutine, then it behaves like an old-
fashioned subroutine. It naturally falls out from this rule that
prototypes have no influence on subroutine references like "\!>foo" or on
indirect subroutine calls like "&{$subref}" or "$subref->()".
Method calls are not influenced by prototypes either, because the function
to be called is indeterminate at compile time, since the exact code called
depends on inheritance.
Because the intent of this feature is primarily to let you define
subroutines that work like built-in functions, here are prototypes for some
other functions that parse almost exactly like the corresponding built-in.
Declared as Called as
sub mylink ($$) mylink $old, $new
sub myvec ($$$) myvec $var, $offset, 1
sub myindex ($$;$) myindex &getstring, "substr"
sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
sub myreverse (@) myreverse $a, $b, $c
sub myjoin ($@) myjoin ":", $a, $b, $c
sub mypop (\@) mypop @array
sub mysplice (\@$$@) mysplice @array, @array, 0, @pushme
sub mykeys (\%) mykeys %{$hashref}
sub myopen (*;$) myopen HANDLE, $name
sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
sub myrand ($) myrand 42
sub mytime () mytime
Any backslashed prototype character represents an actual argument that
absolutely must start with that character. The value passed as part of
"@_" will be a reference to the actual argument given in the subroutine
call, obtained by applying "\" to that argument.
Unbackslashed prototype characters have special meanings. Any
unbackslashed "@" or "%" eats all remaining arguments, and forces list
context. An argument represented by "$" forces scalar context. An "&"
requires an anonymous subroutine, which, if passed as the first argument,
does not require the "sub" keyword or a subsequent comma.
A "*" allows the subroutine to accept a bareword, constant, scalar
expression, typeglob, or a reference to a typeglob in that slot. The value
will be available to the subroutine either as a simple scalar, or (in the
latter two cases) as a reference to the typeglob. If you wish to always
convert such arguments to a typeglob reference, use
Symbol::qualify_to_ref() as follows:
use Symbol 'qualify_to_ref';
sub foo (*) {
my $fh = qualify_to_ref(shift, caller);
...
}
A semicolon separates mandatory arguments from optional arguments. It is
redundant before "@" or "%", which gobble up everything else.
Note how the last three examples in the table above are treated specially
by the parser. "mygrep()" is parsed as a true list operator, "myrand()" is
parsed as a true unary operator with unary precedence the same as "rand()",
and "mytime()" is truly without arguments, just like "time()". That is, if
you say
mytime +2;
you'll get "mytime() + 2", not "mytime(2)", which is how it would be parsed
without a prototype.
The interesting thing about "&" is that you can generate new syntax with
it, provided it's in the initial position:
sub try (&@) {
my($try,$catch) = @_;
eval { &$try };
if ($@) {
local $_ = $@;
&$catch;
}
}
sub catch (&) { $_[0] }
try {
die "phooey";
} catch {
/phooey/ and print "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.
It's probably best to prototype new functions, not retrofit prototyping
into older ones. That's because you must be especially careful about
silent impositions of differing list versus scalar contexts. For example,
if you decide that a function should take just one parameter, like this:
sub func ($) {
my $n = shift;
print "you gave me $n\n";
}
and someone has been calling it with an array or expression returning a
list:
func(@foo);
func( split /:/ );
Then you've just supplied an automatic "scalar" in front of their argument,
which can be more than a bit surprising. The old "@foo" which used to hold
one thing doesn't get passed in. Instead, "func()" now gets passed in a
"1"; that is, the number of elements in "@foo". And the "split" gets
called in scalar context so it starts scribbling on your "@_" parameter
list. Ouch!
This is all very powerful, of course, and should be used only in moderation
to make the world a better place.
Constant Functions
Functions with a prototype of "()" are potential candidates for inlining.
If the result after optimization and constant folding is either a constant
or a lexically-scoped scalar which has no other references, then it will be
used in place of function calls made without "&". Calls made using "&" are
never inlined. (See constant.pm for an easy way to declare most
constants.)
The following functions would all be inlined:
sub pi () { 3.14159 } # Not exact, but close.
sub PI () { 4 * atan2 1, 1 } # As good as it gets,
# and it's inlined, too!
sub ST_DEV () { 0 }
sub ST_INO () { 1 }
sub FLAG_FOO () { 1 << 8 }
sub FLAG_BAR () { 1 << 9 }
sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
sub BAZ_VAL () {
if (OPT_BAZ) {
return 23;
}
else {
return 42;
}
}
sub N () { int(BAZ_VAL) / 3 }
BEGIN {
my $prod = 1;
for (1..N) { $prod *= $_ }
sub N_FACTORIAL () { $prod }
}
If you redefine a subroutine that was eligible for inlining, you'll get a
mandatory warning. (You can use this warning to tell whether or not a
particular subroutine is considered constant.) The warning is considered
severe enough not to be optional because previously compiled invocations of
the function will still be using the old value of the function. If you
need to be able to redefine the subroutine, you need to ensure that it
isn't inlined, either by dropping the "()" prototype (which changes calling
semantics, so beware) or by thwarting the inlining mechanism in some other
way, such as
sub not_inlined () {
23 if $];
}
Overriding Built-in Functions
Many built-in functions may be overridden, though this should be tried only
occasionally and for good reason. Typically this might be done by a
package attempting to emulate missing built-in functionality on a non-Unix
system.
Overriding may be done only by importing the name from a module--ordinary
predeclaration isn't good enough. However, the "use subs" pragma lets you,
in effect, predeclare subs via the import syntax, and these names may then
override built-in ones:
use subs 'chdir', 'chroot', 'chmod', 'chown';
chdir $somewhere;
sub chdir { ... }
To unambiguously refer to the built-in form, precede the built-in name with
the special package qualifier "CORE::". For example, saying "CORE::open()"
always refers to the built-in "open()", even if the current package has
imported some other subroutine called "&open()" from elsewhere. Even
though it looks like a regular function call, it isn't: you can't take a
reference to it, such as the incorrect "\!>CORE::open" might appear to
produce.
Library modules should not in general export built-in names like "open" or
"chdir" as part of their default "@EXPORT" list, because these may sneak
into someone else's namespace and change the semantics unexpectedly.
Instead, if the module adds that name to "@EXPORT_OK", then it's possible
for a user to import the name explicitly, but not implicitly. That is,
they could say
use Module 'open';
and it would import the "open" override. But if they said
use Module;
they would get the default imports without overrides.
The foregoing mechanism for overriding built-in is restricted, quite
deliberately, to the package that requests the import. There is a second
method that is sometimes applicable when you wish to override a built-in
everywhere, without regard to namespace boundaries. This is achieved by
importing a sub into the special namespace "CORE::GLOBAL::". Here is an
example that quite brazenly replaces the "glob" operator with something
that understands regular expressions.
package REGlob;
require Exporter;
@ISA = 'Exporter';
@EXPORT_OK = 'glob';
sub import {
my $pkg = shift;
return unless @_;
my $sym = shift;
my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
$pkg->export($where, $sym, @_);
}
sub glob {
my $pat = shift;
my @got;
local *D;
if (opendir D, '.') {
@got = grep /$pat/, readdir D;
closedir D;
}
return @got;
}
1;
And here's how it could be (ab)used:
#use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
package Foo;
use REGlob 'glob'; # override glob() in Foo:: only
print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
The initial comment shows a contrived, even dangerous example. By
overriding "glob" globally, you would be forcing the new (and subversive)
behavior for the "glob" operator for every namespace, without the complete
cognizance or cooperation of the modules that own those namespaces.
Naturally, this should be done with extreme caution--if it must be done at
all.
The "REGlob" example above does not implement all the support needed to
cleanly override perl's "glob" operator. The built-in "glob" has different
behaviors depending on whether it appears in a scalar or list context, but
our "REGlob" doesn't. Indeed, many perl built-in have such context
sensitive behaviors, and these must be adequately supported by a properly
written override. For a fully functional example of overriding "glob",
study the implementation of "File::DosGlob" in the standard library.
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...
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 the AutoLoader
manpage, for example.) But an "AUTOLOAD" routine can also just emulate the
routine and never define it. For example, let's pretend that a function
that wasn't defined should just invoke "system" with those arguments. All
you'd do is:
sub AUTOLOAD {
my $program = $AUTOLOAD;
$program =~ s/.*:://;
system($program, @_);
}
date();
who('am', 'i');
ls('-l');
In fact, if you predeclare functions you want to call that way, you don't
even need parentheses:
use subs qw(date who ls);
date;
who "am", "i";
ls -l;
A more complete example of this is the standard Shell module, which can
treat undefined subroutine calls as calls to external programs.
Mechanisms are available to help modules writers split their modules into
autoloadable files. See the standard AutoLoader module described in the
AutoLoader manpage and in the AutoSplit manpage, the standard SelfLoader
modules in the SelfLoader manpage, and the document on adding C functions
to Perl code in the perlxs manpage.
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 the attributes manpage 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 the
attributes manpage.
SEE ALSO
See the Function Templates entry in the perlref manpage for more about
references and closures. See the perlxs manpage if you'd like to learn
about calling C subroutines from Perl. See the perlembed manpage if you'd
like to learn about calling Perl subroutines from C. See the perlmod
manpage to learn about bundling up your functions in separate files. See
the perlmodlib manpage to learn what library modules come standard on your
system. See the perltoot manpage to learn how to make object method calls.
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