PERLSUB(1) PERLSUB(1)
perlsub - Perl subroutines
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.
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-byreference
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
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(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;
}
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PERLSUB(1) PERLSUB(1)
$lookahead = <STDIN>; # get first line
while ($_ = 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 /:/, $var );
Do not, however, be tempted to do this:
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PERLSUB(1) PERLSUB(1)
(@a, @b) = upcase(@list1, @list2);
Because like its flat incoming parameter list, the return list is also
flat. So all you have managed to do here is stored everything in @a and
made @b an empty list. See the section on /"Pass by Reference for
alternatives.
A subroutine may be called using the "&" prefix. The "&" is optional in
modern Perls, and so are the parentheses if the subroutine has been
predeclared. (Note, however, that the "&" is NOT optional when you're
just naming the subroutine, such as when it's used as an argument to
defined() or undef(). Nor is it optional when you want to do an indirect
subroutine call with a subroutine name or reference using the &$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.
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
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.
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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
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PERLSUB(1) PERLSUB(1)
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
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PERLSUB(1) PERLSUB(1)
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.
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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.
Temporary Values via local()
NOTE: 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
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PERLSUB(1) PERLSUB(1)
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
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PERLSUB(1) PERLSUB(1)
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..]
Passing Symbol Table Entries (typeglobs) [Toc] [Back]
[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.
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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.
Pass by Reference [Toc] [Back]
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:
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@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:
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PERLSUB(1) PERLSUB(1)
(*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, $path) ? *FH : undef;
}
Although that will actually produce a small memory leak. See the bottom
of the open() entry in the perlfunc manpage for a somewhat cleaner way
using the IO::Handle package.
Prototypes [Toc] [Back]
As of the 5.002 release of perl, if you declare
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sub mypush (\@@)
then mypush() takes arguments exactly like push() does. The declaration
of the function to be called must be visible at compile time. The
prototype affects only the interpretation of new-style calls to the
function, where new-style is defined as not using the & character. In
other words, if you call it like a builtin function, then it behaves like
a builtin function. If you call it like an old-fashioned subroutine,
then it behaves like an old-fashioned subroutine. It naturally falls out
from this rule that prototypes have no influence on subroutine references
like \&foo or on indirect subroutine calls like &{$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.
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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
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PERLSUB(1) PERLSUB(1)
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.
Constant Functions [Toc] [Back]
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 }
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PERLSUB(1) PERLSUB(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 $];
}
Overriding Builtin Functions
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 { ... }
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PERLSUB(1) PERLSUB(1)
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.
Autoloading [Toc] [Back]
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:
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PERLSUB(1) PERLSUB(1)
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.
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.
PPPPaaaaggggeeee 11119999 [ Back ]
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