NAME
Sub::Nary - Try to count how many elements a subroutine can return in
list context.
VERSION
Version 0.02
SYNOPSIS
use Sub::Nary;
my $sn = Sub::Nary->new();
my $r = $sn->nary(\&hlagh);
DESCRIPTION
This module uses the B framework to walk into subroutines and try to
guess how many scalars are likely to be returned in list context. It's
not always possible to give a definitive answer to this question at
compile time, so the results are given in terms of "probability of
return" (to be understood in a sense described below).
METHODS
"new"
The usual constructor. Currently takes no argument.
"nary $coderef"
Takes a code reference to a named or anonymous subroutine, and returns a
hash reference whose keys are the possible numbers of returning scalars,
and the corresponding values the "probability" to get them. The special
key 'list' is used to denote a possibly infinite number of returned
arguments. The return value hence would look at
{ 1 => 0.2, 2 => 0.4, 4 => 0.3, list => 0.1 }
that is, we should get 1 scalar 1 time over 5 and so on. The sum of all
values is 1. The returned result, and all the results obtained from
intermediate subs, are cached into the object.
"flush"
Flushes the Sub::Nary object cache. Returns the object itself.
PROBABILITY OF RETURN
The probability is computed as such :
* All the returning points in the same subroutine (i.e. all the explicit
"return" and the last computed value) are considered equally possible.
For example, the subroutine
sub simple {
if (rand < 0.1) {
return 1;
} else {
return 2, 3;
}
}
is seen returning one or two arguments each with probability "1/2".
As for
sub hlagh {
my $x = rand;
if ($x < 0.1) {
return 1, 2, 3;
} elsif ($x > 0.9) {
return 4, 5;
}
}
it is considered to return 1 (when the two tests fail, the last
computed value is returned, which here is "$x > 0.9" evaluated in
the scalar context of the test), 2 or 3 arguments each with
probability "1/3".
* The total probability law for a given returning point is the
convolution product of the probabilities of its list elements.
As such,
sub notsosimple {
return 1, simple(), 2
}
returns 3 or 4 arguments with probability "1/2" ; and
sub double {
return simple(), simple()
}
never returns 1 argument but returns 2 with probability "1/2 * 1/2 =
1/4", 3 with probability "1/2 * 1/2 + 1/2 * 1/2 = 1/2" and 4 with
probability "1/4" too.
* If a core function may return different numbers of scalars, each kind
is considered equally possible.
For example, "stat" returns 13 elements on success and 0 on error.
The according probability will then be "{ 0 => 0.5, 13 => 0.5 }".
* The "list" state is absorbing in regard of all the other ones.
This is just a pedantic way to say that "list + fixed length =
list". That's why
sub listy {
return 1, simple(), @_
}
is considered as always returning an unbounded list.
Also, the convolution law does not behave the same when "list"
elements are involved : in the following example,
sub oneorlist {
if (rand < 0.1) {
return 1
} else {
return @_
}
}
sub composed {
return oneorlist(), oneorlist()
}
"composed" returns 2 scalars with probability "1/2 * 1/2 = 1/4" and
a "list" with probability "3/4".
EXPORT
An object-oriented module shouldn't export any function, and so does
this one.
CAVEATS
The algorithm may be pessimistic (things seen as "list" while they are
of fixed length) but not optimistic (the opposite, duh).
"wantarray" isn't specialized when encountered in the optree.
DEPENDENCIES
perl 5.8.1.
Carp (standard since perl 5), B (since perl 5.005), XSLoader (since perl
5.006) and List::Util (since perl 5.007003).
AUTHOR
Vincent Pit, "", .
You can contact me by mail or on #perl @ FreeNode (vincent or
Prof_Vince).
BUGS
Please report any bugs or feature requests to "bug-b-nary at
rt.cpan.org", or through the web interface at
. I will be
notified, and then you'll automatically be notified of progress on your
bug as I make changes.
SUPPORT
You can find documentation for this module with the perldoc command.
perldoc Sub::Nary
Tests code coverage report is available at
.
ACKNOWLEDGEMENTS
Thanks to Sebastien Aperghis-Tramoni for helping to name this module.
COPYRIGHT & LICENSE
Copyright 2008 Vincent Pit, all rights reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.