PERLSUB(1)
NNAAMMEE
perlsub - Perl subroutines
SSYYNNOOPPSSIISS
To declare subroutines:
sub NAME; # A "forward" declaration.
sub NAME(PROTO); # ditto, but with prototypes
sub NAME BLOCK # A declaration and a definition.
sub NAME(PROTO) BLOCK # ditto, but with prototypes
To define an anonymous subroutine at runtime:
$subref = sub BLOCK;
To import subroutines:
use PACKAGE qw(NAME1 NAME2 NAME3);
To call subroutines:
NAME(LIST); # & is optional with parentheses.
NAME LIST; # Parentheses optional if predeclared/imported.
&NAME; # Passes current @_ to subroutine.
DDEESSCCRRIIPPTTIIOONN
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 even generated on the fly using eval
or anonymous subroutines (closures). You can even call a
function indirectly using a variable containing its name
or a CODE reference to it, as in $var = \&function.
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 the language's perspective.)
Any arguments passed to the routine come in as the array
@_. Thus 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 if a reference to it is taken.
(Some earlier versions of Perl created the element whether
or not it was assigned to.) Note that assigning to the
whole array @_ removes the aliasing, and does not update
any arguments.
The return value of the subroutine is the value of the
last expression evaluated. Alternatively, 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 will return an empty list in
a list context, an undefined value in a scalar context, or
nothing in a void context. If you return one or more
arrays and/or hashes, these will be flattened together
into one large indistinguishable list.
Perl does not have named formal parameters, but in
practice all you do is assign to a my() list of these.
Any variables you use in the function that aren't declared
private are global variables. For the 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 section on 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:
# 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;
}
}
$thisline;
}
$lookahead = <STDIN>; # get first line
while ($_ = get_line()) {
...
}
Use array assignment to a local list to name your formal
arguments:
sub maybeset {
my($key, $value) = @_;
$Foo{$key} = $value unless $Foo{$key};
}
This also has the effect of turning call-by-reference into
call-by-value, because the assignment copies the values.
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
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 will
see everything as one big long flat @_ parameter list.
This is one of the ways 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 /:/, $var );
Do not, however, be tempted to do this:
(@a, @b) = upcase(@list1, @list2);
Because like its flat incoming parameter list, the return
list is also flat. So all you have managed to do here is
stored everything in @a and made @b an empty list. See
the section on /"Pass by Reference for alternatives.
A subroutine may be called using the "&" prefix. The "&"
is optional in modern Perls, and so are the parentheses if
the subroutine has been predeclared. (Note, however, that
the "&" is NOT optional when you're just naming the
subroutine, such as when it's used as an argument to
defined() or undef(). Nor is it optional when you want to
do an indirect subroutine call with a subroutine name or
reference using the &$subref() or &{$subref}() constructs.
See the perlref manpage for more on 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, but it also disables any prototype checking on
the 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 section
on Prototypes below.
PPrriivvaattee VVaarriiaabblleess vviiaa 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
A "my" declares the listed variables to be confined
(lexically) 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 builtins like
$/ must currently be localized with "local" instead.
Unlike dynamic variables created by the "local" statement,
lexical variables declared with "my" are totally hidden
from the outside world, including any called subroutines
(even if it's the same subroutine called from itself or
elsewhere--every call gets its own copy).
(An eval(), however, can see the 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 the 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 $arg;
}
The "my" is simply a modifier on something you might
assign to. So when you do assign to the variables in its
argument list, the "my" doesn't change whether those
variables is viewed as a scalar or an array. So
my ($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 the 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 the scope, too.
Thus in the loop
while (defined(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
throughout the rest of the conditional (including elsif
and else clauses, if any), but not beyond it.
(None of the foregoing 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; see below). However,
if the index variable is prefixed with the keyword "my",
then it is lexically scoped 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, and so the value of $i is unavailable in
some_function().
Some users may wish to encourage the use of lexically
scoped variables. As an aid to catching implicit
references to package variables, if you say
use strict 'vars';
then any variable reference from there to the end of the
enclosing block must either refer to a lexical variable,
or 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
principle usefulness of this is to quiet use strict
'vars'. The actual initialization is delayed until run
time, so it gets executed appropriately; every 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 totally from the outside
world. This is similar to C's static variables at the
file level. To do this with a subroutine requires the use
of a closure (anonymous function). If a block (such as an
eval(), function, or package) wants 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.
Just because the lexical variable is lexically (also
called statically) scoped doesn't mean that within a
function it works like a C static. It normally works more
like a C auto. But 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 perlrun manpage about the BEGIN function.
TTeemmppoorraarryy VVaalluueess vviiaa local()
NNOOTTEE: 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, (or subroutine, eval{}, or do) and any
called from within that block. A local() just gives
temporary values to global (meaning package) variables.
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 command, it gets executed
every time through a loop. In releases of Perl previous
to 5.0, this used more stack storage each time until the
loop was exited. Perl now reclaims the space each time
through, but it's still more efficient to declare your
variables outside the loop.
A local is simply a modifier on an lvalue expression.
When you assign to a localized variable, the local doesn't
change whether its list is viewed as a scalar or an array.
So
local($foo) = <STDIN>;
local @FOO = <STDIN>;
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..]
PPaassssiinngg SSyymmbbooll TTaabbllee EEnnttrriieess ((ttyyppeegglloobbss))
[Note: 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);
Note that 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 section on
Typeglobs and Filehandles in the perldata manpage.
PPaassss bbyy RReeffeerreennccee
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,
return 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
(well, and 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 would be better because they'll still work
properly under use strict 'refs'. 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>;
}
Another way to do this is using *HANDLE{IO}, see the
perlref manpage for usage and caveats.
If you're planning on generating new filehandles, you
could do this:
sub openit {
my $name = shift;
local *FH;
return open (FH, $path) ? *FH : undef;
}
Although that will actually produce a small memory leak.
See the bottom of the open() entry in the perlfunc manpage
for a somewhat cleaner way using the IO::Handle package.
PPrroottoottyyppeess
As of the 5.002 release of perl, if you declare
sub mypush (\@@)
then mypush() takes arguments exactly like push() does.
The declaration of the function to be called must be
visible at compile time. The prototype affects only the
interpretation of new-style calls to the function, where
new-style is defined as not using the & character. In
other words, if you call it like a builtin function, then
it behaves like a builtin function. If you call it like
an old-fashioned subroutine, then it behaves like an old-
fashioned subroutine. It naturally falls out from this
rule that prototypes have no influence on subroutine
references like \&foo or on indirect subroutine calls like
&{$subref}.
Method calls are not influenced by prototypes either,
because the function to be called is indeterminate at
compile time, because it depends on inheritance.
Because the intent is primarily to let you define
subroutines that work like builtin commands, here are the
prototypes for some other functions that parse almost
exactly like the corresponding builtins.
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 to the subroutine (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 the rest of the
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 * does whatever it has to do to turn
the argument into a reference to a symbol table entry.
A semicolon separates mandatory arguments from optional
arguments. It is redundant before @ or %.
Note how the last three examples 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 the prototype.
The interesting thing about & is that you can generate new
syntax with it:
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 the 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 grep:
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, the func() now gets
passed in 1, that is, the number of elements in @foo. And
the split() gets called in a scalar context and starts
scribbling on your @_ parameter list.
This is all very powerful, of course, and should be used
only in moderation to make the world a better place.
CCoonnssttaanntt FFuunnccttiioonnss
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 & or do.
Calls made using & or do are never inlined. (See
constant.pm for an easy way to declare most constants.)
All of the following functions would 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 which 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 the calling semantics, so beware) or by thwarting
the inlining mechanism in some other way, such as
sub not_inlined () {
23 if $];
}
OOvveerrrriiddiinngg BBuuiillttiinn FFuunnccttiioonnss
Many builtin 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 builtin 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 subs pragma (compiler directive) lets you, in
effect, predeclare subs via the import syntax, and these
names may then override the builtin ones:
use subs 'chdir', 'chroot', 'chmod', 'chown';
chdir $somewhere;
sub chdir { ... }
To unambiguously refer to the builtin form, one may
precede the builtin name with the special package
qualifier CORE::. For example, saying CORE::open() will
always refer to the builtin open(), even if the current
package has imported some other subroutine called &open()
from elsewhere.
Library modules should not in general export builtin 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 the name to the @EXPORT_OK list, 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 the overrides.
Note that such overriding is restricted to the package
that requests the import. Some means of "globally"
overriding builtins may become available in future.
AAuuttoollooaaddiinngg
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 of the base classes of the class package.) If,
however, there is an AUTOLOAD subroutine defined in the
package or packages that were searched for 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 $AUTOLOAD variable in
the same package as the AUTOLOAD routine. The name is not
passed as an ordinary argument because, er, well, just
because, that's why...
Most AUTOLOAD routines will load in a definition for the
subroutine in question using eval, and then execute that
subroutine using a special form of "goto" that erases the
stack frame of the AUTOLOAD routine without a trace. (See
the standard AutoLoader module, 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 call system()
with those arguments. All you'd do is this:
sub AUTOLOAD {
my $program = $AUTOLOAD;
$program =~ s/.*:://;
system($program, @_);
}
date();
who('am', 'i');
ls('-l');
In fact, if you predeclare the functions you want to call
that way, you don't even need the 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 Unix programs.
Mechanisms are available for modules writers to help split
the modules up 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.
SSEEEE AALLSSOO
See the perlref manpage for more on references. See the
perlxs manpage if you'd like to learn about calling C
subroutines from perl. See the perlmod manpage to learn
about bundling up your functions in separate files.