+package Sub::Nary;
+
+use 5.008001;
+
+use strict;
+use warnings;
+
+use Carp qw/croak/;
+use List::Util qw/reduce sum/;
+
+use B qw/class ppname svref_2object OPf_KIDS/;
+
+=head1 NAME
+
+Sub::Nary - Try to count how many elements a subroutine can return in list context.
+
+=head1 VERSION
+
+Version 0.01
+
+=cut
+
+our $VERSION;
+BEGIN {
+ $VERSION = '0.01';
+}
+
+=head1 SYNOPSIS
+
+ use Sub::Nary;
+
+ my $sn = Sub::Nary->new();
+ my $r = $sn->nary(\&hlagh);
+
+=head1 DESCRIPTION
+
+This module uses the L<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).
+
+=head1 METHODS
+
+=head2 C<new>
+
+The usual constructor. Currently takes no argument.
+
+=head2 C<nary $coderf>
+
+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 C<'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 C<1> scalar C<1> time over C<5> and so on. The sum of all values is C<1>. The returned result, and all the results obtained from intermediate subs, are cached into the object.
+
+=head2 C<flush>
+
+Flushes the L<Sub::Nary> object cache. Returns the object itself.
+
+=head1 PROBABILITY OF RETURN
+
+The probability is computed as such :
+
+=over 4
+
+=item * All the returning points in the same subroutine (i.e. all the explicit C<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 C<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 C<1> (when the two tests fail, the last computed value is returned, which here is C<< $x > 0.9 >> evaluated in the scalar context of the test), C<2> or C<3> arguments each with probability C<1/3>.
+
+=item * 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 C<3> or C<4> arguments with probability C<1/2> ; and
+
+ sub double {
+ return simple(), simple()
+ }
+
+never returns C<1> argument but returns C<2> with probability C<1/2 * 1/2 = 1/4>, C<3> with probability C<1/2 * 1/2 + 1/2 * 1/2 = 1/2> and C<4> with probability C<1/4> too.
+
+=item * The C<'list'> state is absorbant 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.
+The convolution law also does not behave the same when C<list> elements are involved : in the following example,
+
+ sub oneorlist {
+ if (rand < 0.1) {
+ return 1
+ } else {
+ return @_
+ }
+ }
+
+ sub composed {
+ return oneorlist(), oneorlist()
+ }
+
+C<composed> returns C<2> scalars with probability C<1/2 * 1/2 = 1/4> and a C<list> with probability C<3/4>.
+
+=back
+
+=cut
+
+BEGIN {
+ require XSLoader;
+ XSLoader::load(__PACKAGE__, $VERSION);
+}
+
+sub _check_self {
+ croak 'First argument isn\'t a valid ' . __PACKAGE__ . ' object'
+ unless ref $_[0] and $_[0]->isa(__PACKAGE__);
+}
+
+sub new {
+ my $class = shift;
+ $class = ref($class) || $class || __PACKAGE__;
+ bless { cache => { } }, $class;
+}
+
+sub flush {
+ my $self = shift;
+ _check_self($self);
+ $self->{cache} = { };
+ $self;
+}
+
+sub nary {
+ my $self = shift;
+ my $sub = shift;
+
+ $self->{cv} = [ ];
+ return $self->enter(svref_2object($sub));
+}
+
+sub name ($) {
+ my $n = $_[0]->name;
+ $n eq 'null' ? substr(ppname($_[0]->targ), 3) : $n
+}
+
+sub combine {
+ reduce {{
+ my %res;
+ my $la = delete $a->{list};
+ my $lb = delete $b->{list};
+ if (defined $la || defined $lb) {
+ $la ||= 0;
+ $lb ||= 0;
+ $res{list} = $la + $lb - $la * $lb;
+ }
+ while (my ($ka, $va) = each %$a) {
+ $ka = int $ka;
+ while (my ($kb, $vb) = each %$b) {
+ my $key = $ka + int $kb;
+ $res{$key} += $va * $vb;
+ }
+ }
+ \%res
+ }} map { (ref) ? $_ : { $_ => 1 } } grep defined, @_;
+}
+
+sub add {
+ reduce {
+ $a->{$_} += $b->{$_} for keys %$b;
+ $a
+ } map { (ref) ? $_ : { $_ => 1 } } grep defined, @_;
+}
+
+my %ops;
+$ops{$_} = 1 for scalops;
+$ops{$_} = 0 for qw/stub nextstate/;
+$ops{$_} = 1 for qw/padsv/;
+$ops{$_} = 'list' for qw/padav/;
+$ops{$_} = 'list' for qw/padhv rv2hv/;
+$ops{$_} = 'list' for qw/padany flip/;
+
+sub enter {
+ my ($self, $cv) = @_;
+
+ my $op = $cv->ROOT;
+ my $tag = tag($op);
+
+ return { %{$self->{cache}->{$tag}} } if exists $self->{cache}->{$tag};
+
+ # Anything can happen with recursion
+ for (@{$self->{cv}}) {
+ return 'list' if $tag == tag($_->ROOT);
+ }
+
+ unshift @{$self->{cv}}, $cv;
+ (my $r, undef) = $self->expect_any($op->first);
+ shift @{$self->{cv}};
+
+ $r = { $r => 1} unless ref $r;
+ my $total = sum values %$r;
+ $r = { map { $_ => $r->{$_} / $total } keys %$r };
+ $self->{cache}->{$tag} = { %$r };
+ return $r;
+}
+
+sub expect_return {
+ my ($self, $op) = @_;
+
+ return ($self->expect_list($op))[0] => 1 if name($op) eq 'return';
+
+ if ($op->flags & OPf_KIDS) {
+ for ($op = $op->first; not null $op; $op = $op->sibling) {
+ my ($p, $r) = $self->expect_return($op);
+ return $p => 1 if $r;
+ }
+ }
+
+ return;
+}
+
+sub expect_list {
+ my ($self, $op) = @_;
+
+ my $n = name($op);
+ my $meth = $self->can('pp_' . $n);
+ return $self->$meth($op) if $meth;
+ return $ops{$n} => 0 if exists $ops{$n};
+
+ if ($op->flags & OPf_KIDS) {
+ my @res = (0);
+ my ($p, $r);
+ for ($op = $op->first; not null $op; $op = $op->sibling) {
+ my $n = name($op);
+ next if $n eq 'pushmark';
+ if ($n eq 'nextstate'
+ and not null(($op = $op->sibling)->sibling)) {
+ ($p, $r) = $self->expect_return($op);
+ return $p => 1 if $r;
+ } else {
+ ($p, $r) = $self->expect_any($op);
+ return $p => 1 if $r;
+ push @res, $p;
+ }
+ }
+ return (combine @res) => 0;
+ }
+
+ return;
+}
+
+sub expect_any {
+ my ($self, $op) = @_;
+
+ return ($self->expect_list($op))[0] => 1 if name($op) eq 'return';
+
+ if (class($op) eq 'LOGOP') {
+ my @res;
+ my ($p, $r);
+
+ my $op = $op->first;
+ ($p, $r) = $self->expect_return($op);
+ return $p => 1 if $r;
+
+ $op = $op->sibling;
+ push @res, ($self->expect_any($op))[0];
+
+ # If the logop has no else branch, it can also return the *scalar* result of
+ # the conditional
+ $op = $op->sibling;
+ if (null $op) {
+ push @res, 1;
+ } else {
+ push @res, ($self->expect_any($op))[0];
+ }
+
+ return (add @res) => 0;
+ }
+
+ return $self->expect_list($op);
+}
+
+# Stolen from Sub::Deparse
+
+sub padval { $_[0]->{cv}->[0]->PADLIST->ARRAYelt(1)->ARRAYelt($_[1]) }
+
+sub gv_or_padgv {
+ my ($self, $op) = @_;
+ if (class($op) eq 'PADOP') {
+ return $self->padval($op->padix)
+ } else { # class($op) eq "SVOP"
+ return $op->gv;
+ }
+}
+
+sub const_sv {
+ my ($self, $op) = @_;
+ my $sv = $op->sv;
+ # the constant could be in the pad (under useithreads)
+ $sv = $self->padval($op->targ) unless $$sv;
+ return $sv;
+}
+
+sub pp_entersub {
+ my ($self, $op, $exp) = @_;
+
+ my $next = $op;
+ while ($next->flags & OPf_KIDS) {
+ $next = $next->first;
+ }
+ while (not null $next) {
+ $op = $next;
+ my ($p, $r) = $self->expect_return($op, $exp);
+ return $p => 1 if $r;
+ $next = $op->sibling;
+ }
+
+ if (name($op) eq 'rv2cv') {
+ my $n;
+ do {
+ $op = $op->first;
+ my $next = $op->sibling;
+ while (not null $next) {
+ $op = $next;
+ $next = $next->sibling;
+ }
+ $n = name($op)
+ } while ($op->flags & OPf_KIDS and { map { $_ => 1 } qw/null leave/ }->{$n});
+ return 'list' unless { map { $_ => 1 } qw/gv refgen/ }->{$n};
+ local $self->{sub} = 1;
+ return $self->expect_any($op, $exp);
+ } else {
+ # Method call ?
+ return 'list';
+ }
+}
+
+sub pp_gv {
+ my ($self, $op) = @_;
+
+ return $self->{sub} ? $self->enter($self->gv_or_padgv($op)->CV) : 1
+}
+
+sub pp_anoncode {
+ my ($self, $op) = @_;
+
+ return $self->{sub} ? $self->enter($self->const_sv($op)) : 1
+}
+
+sub pp_goto {
+ my ($self, $op) = @_;
+
+ my $n = name($op);
+ while ($op->flags & OPf_KIDS) {
+ my $nop = $op->first;
+ my $nn = name($nop);
+ if ($nn eq 'pushmark') {
+ $nop = $nop->sibling;
+ $nn = name($nop);
+ }
+ if ($n eq 'rv2cv' and $nn eq 'gv') {
+ return $self->enter($self->gv_or_padgv($nop)->CV);
+ }
+ $op = $nop;
+ $n = $nn;
+ }
+
+ return 'list';
+}
+
+sub pp_const {
+ my ($self, $op) = @_;
+
+ if (class($op) eq 'SVOP' and (my $sv = $self->const_sv($op))) {
+ my $c = class($sv);
+ if ($c eq 'AV') {
+ return $sv->FILL + 1;
+ } elsif ($c eq 'HV') {
+ return 2 * $sv->FILL;
+ }
+ }
+
+ return 1;
+}
+
+sub pp_aslice { $_[0]->expect_any($_[1]->first->sibling) }
+
+sub pp_hslice;
+*pp_hslice = *pp_aslice{CODE};
+
+sub pp_lslice { $_[0]->expect_any($_[1]->first) }
+
+sub pp_rv2av {
+ my ($self, $op) = @_;
+ $op = $op->first;
+
+ return (name($op) eq 'const') ? $self->expect_any($op) : 'list';
+}
+
+sub pp_aassign { $_[0]->expect_any($_[1]->first) }
+
+sub pp_leaveloop { $_[0]->expect_return($_[1]->first->sibling) }
+
+=head1 EXPORT
+
+An object-oriented module shouldn't export any function, and so does this one.
+
+=head1 CAVEATS
+
+The algorithm may be pessimistic (things seen as C<list> while they are of fixed length) but not optimistic (the opposite, duh).
+
+C<wantarray> isn't specialized when encountered in the optree.
+
+=head1 DEPENDENCIES
+
+L<perl> 5.8.1.
+
+L<Carp> (standard since perl 5), L<B> (since perl 5.005), L<XSLoader> (since perl 5.006) and L<List::Util> (since perl 5.007003).
+
+=head1 AUTHOR
+
+Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
+
+You can contact me by mail or on #perl @ FreeNode (vincent or Prof_Vince).
+
+=head1 BUGS
+
+Please report any bugs or feature requests to C<bug-b-nary at rt.cpan.org>, or through the web interface at L<http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Sub-Nary>. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
+
+=head1 SUPPORT
+
+You can find documentation for this module with the perldoc command.
+
+ perldoc Sub::Nary
+
+Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Sub-Nary>.
+
+=head1 ACKNOWLEDGEMENTS
+
+Thanks to Sebastien Aperghis-Tramoni for helping to name this module.
+
+=head1 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.
+
+=cut
+
+1; # End of Sub::Nary