NAME
Sub::Recursive - Anonymous memory leak free recursive subroutines
SYNOPSIS
use Sub::Recursive;
# LEAK FREE recursive subroutine.
my $fac = recursive {
my ($n) = @_;
return 1 if $n < 1;
return $n * $REC->($n - 1);
};
# Recursive anonymous definition in one line, plus invocation.
print recursive { $_[0] <= 1 ? 1 : $_[0] * $REC->($_[0] - 1) } -> (5);
# Experimental interface
use Sub::Recursive qw/ mutually_recursive %REC /;
my ($odd, $even) = mutually_recursive(
odd => sub { $_[0] == 0 ? 0 : $REC{even}->($_[0] - 1) },
even => sub { $_[0] == 0 ? 1 : $REC{odd }->($_[0] - 1) },
);
DESCRIPTION
Recursive closures suffer from a severe memory leak. "Sub::Recursive"
makes the problem go away cleanly and at the same time allows you to
write recursive subroutines as expression and can make them truly
anonymous. There's no significant speed difference between using
&recursive and writing the simpler leaking solution.
The problem
The following won't work:
my $fac = sub {
my ($n) = @_;
return 1 if $n < 1;
return $n * $fac->($n - 1);
};
because of the recursive use of $fac which isn't available until after
the statement. The common fix is to do
my $fac;
$fac = sub {
my ($n) = @_;
return 1 if $n < 1;
return $n * $fac->($n - 1);
};
Unfortunately, this introduces another problem.
Because of perl's reference count system, the code above is a memory
leak. $fac references the anonymous sub which references $fac, thus
creating a circular reference. This module does not suffer from that
memory leak.
There are two more reasons why I don't like to write recursive closures
like that: (a) you have to first declare it, then assign it thus
requiring more than a simple expression (b) you have to name it one way
or another.
The solution
This module fixes all those issues. Just change "sub" for "recursive"
and use &$REC for the recursive call:
use Sub::Recursive;
my $fac = recursive {
my ($n) = @_;
return 1 if $n < 1;
return $n * $REC->($n - 1);
};
It also makes it easy to pass it directly to a subroutine,
foo(recursive { ... });
just as any other anonymous subroutine.
EXPORTED FUNCTIONS
If no arguments are given to the "use" statement $REC and &recursive are
exported. If any arguments are given only those given are exported.
":ALL" exports all.
$REC - exported by default
$REC holds a reference to the current subroutine inside subroutines
created with &recursive. Don't ever touch $REC inside or outside the
subroutine except for the recursive call.
"recursive" - exported by default
&recursive takes one argument and that's an anonymous sub defined in the
same package as the call to &recursive is in. It's prototyped with "&"
so bare-block calling style is encouraged.
recursive { ... }
The return value is an anonymous closure that has &$REC working in it.
%REC
This is an experimental part of the API.
%REC holds the subroutine references given to &mutually_recursive, with
the same keys.
Don't ever touch %REC inside or outside the subroutines except for the
recursive calls.
"mutually_recursive"
This is an experimental part of the API.
&mutually_recursive works like &recursive except it takes a list of
key/value pairs where the key names are the names used for the keys in
%REC and the values are the subroutine references. The return values are
the subroutine references, ordered as given to &mutually_recursive.
my ($odd, $even) = mutually_recursive(
odd => sub { $_[0] == 0 ? 0 : $REC{even}->($_[0] - 1) },
even => sub { $_[0] == 0 ? 1 : $REC{odd }->($_[0] - 1) },
);
BUGS
If you follow the rest of the manual you don't have to read this
section. I include this section anyway to make debugging simpler.
$REC is a package global and as such there are some gotchas. You won't
encounter any of these bugs below if you just use
recursive { ... }
and don't mention $REC outside of such an expression. In short: it's
quite unlikely you'll get bitten by any of these bugs.
"my" and "our"
Don't declare $REC with "my". That'll make $REC mean your lexical
variable rather than the global that "Sub::Recursive" uses.
Don't declare $REC with "our". In particular, problem arise the
"our" scopes over several packages. If you do
package Foo;
use Sub::Recursive;
our $REC;
...
package Bar;
my $fatal = recursive { $REC->() };
$REC in &$fatal will be using the value of $Foo::REC but
"Sub::Recursive" has no way of knowing that and will think you use
$Bar::REC.
If you for some reason need to have $REC declared you can as a last
resort get around both these issues by fully qualifying $REC to the
package in which the subroutine is created.
package Foo;
use Sub::Recursive;
my $REC; # Bad.
my $fatal = recursive { $Foo::REC->() }; # Still works.
Subroutine reference defined in another package
This is a really far out edge case.
If the subroutine reference given to &recursive is defined in
another package than the call to &recursive in it then it won't
work.
package Foo;
my $foo = sub { $REC->() };
package Bar;
use Sub::Recursive;
my $bar = &recursive($foo); # Won't work.
The subroutine referenced by $foo is using $Foo::REC but &recursive
thinks it's using $Bar::REC. Note that you have to circumvent
prototyping in order to encounter this bug.
Why you'd want to do this escapes me. Please contact me if you find
a reason for doing this.
EXAMPLE
Some algorithms are perhaps best written recursively. For simplicity,
let's say I have a tree consisting of arrays of array with arbitrary
depth. I want to map over this data structure, translating every value
to another. For this I might use
my $translator = recursive { [ map {
ref() ? $REC->($_) : do {
$translate{$_}
}
} @{$_[0]} ] };
my $bar = $translator->($foo);
Now, a tree mapper isn't perhaps the best example as it's a pretty
general problem to solve, and should perhaps be abstracted but it still
serves as an example of how this module can be handy.
A similar but more specialized task would be to find all men who share
their Y chromosome.
# A person data structure look like this.
my $person = {
name => ...,
sons => [ ... ], # objects like $person
daughters => [ ... ], # objects like $person
};
my @names = recursive {
my ($person) = @_;
$person->{name},
map $REC->($_), @{$person->{sons}}
} -> ($forefather);
This particular example isn't a closure as it doesn't reference any
lexicals outside itself (and thus could've been written as a named
subroutine). It's easy enough to think of a case when it would be a
closure though. For instance if some branches should be excluded. A
simple flag would solve that.
my %exclude = ...;
my @names = recursive {
my ($person) = @_;
return if $exclude{$person};
$person->{name},
map $REC->($_), @{$person->{sons}}
} -> ($forefather);
Hopefully this illustrates how this module allows you to write recursive
algorithms inline like any other algorithm.
AUTHOR
Johan Lodin <
[email protected]>
COPYRIGHT
Copyright 2004-2005 Johan Lodin. All rights reserved.
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
SEE ALSO
perlref
