9 use List::Util qw/reduce sum/;
11 use B qw/class ppname svref_2object OPf_KIDS/;
15 Sub::Nary - Try to count how many elements a subroutine can return in list context.
32 my $sn = Sub::Nary->new();
33 my $r = $sn->nary(\&hlagh);
37 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).
43 The usual constructor. Currently takes no argument.
45 =head2 C<nary $coderf>
47 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
49 { 1 => 0.2, 2 => 0.4, 4 => 0.3, list => 0.1 }
51 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.
55 Flushes the L<Sub::Nary> object cache. Returns the object itself.
57 =head1 PROBABILITY OF RETURN
59 The probability is computed as such :
63 =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.
65 For example, the subroutine
75 is seen returning one or two arguments each with probability C<1/2>.
87 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>.
89 =item * The total probability law for a given returning point is the convolution product of the probabilities of its list elements.
97 returns C<3> or C<4> arguments with probability C<1/2> ; and
100 return simple(), simple()
103 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.
105 =item * The C<'list'> state is absorbant in regard of all the other ones.
107 This is just a pedantic way to say that "list + fixed length = list".
111 return 1, simple(), @_
114 is considered as always returning an unbounded list.
115 The convolution law also does not behave the same when C<list> elements are involved : in the following example,
126 return oneorlist(), oneorlist()
129 C<composed> returns C<2> scalars with probability C<1/2 * 1/2 = 1/4> and a C<list> with probability C<3/4>.
137 XSLoader::load(__PACKAGE__, $VERSION);
141 croak 'First argument isn\'t a valid ' . __PACKAGE__ . ' object'
142 unless ref $_[0] and $_[0]->isa(__PACKAGE__);
147 $class = ref($class) || $class || __PACKAGE__;
148 bless { cache => { } }, $class;
154 $self->{cache} = { };
163 return $self->enter(svref_2object($sub));
168 $n eq 'null' ? substr(ppname($_[0]->targ), 3) : $n
174 my $la = delete $a->{list};
175 my $lb = delete $b->{list};
176 if (defined $la || defined $lb) {
179 $res{list} = $la + $lb - $la * $lb;
181 while (my ($ka, $va) = each %$a) {
183 while (my ($kb, $vb) = each %$b) {
184 my $key = $ka + int $kb;
185 $res{$key} += $va * $vb;
189 }} map { (ref) ? $_ : { $_ => 1 } } grep defined, @_;
194 $a->{$_} += $b->{$_} for keys %$b;
196 } map { (ref) ? $_ : { $_ => 1 } } grep defined, @_;
200 $ops{$_} = 1 for scalops;
201 $ops{$_} = 0 for qw/stub nextstate/;
202 $ops{$_} = 1 for qw/padsv/;
203 $ops{$_} = 'list' for qw/padav/;
204 $ops{$_} = 'list' for qw/padhv rv2hv/;
205 $ops{$_} = 'list' for qw/padany flip match/;
208 my ($self, $cv) = @_;
210 return 'list' if class($cv) ne 'CV';
214 return { %{$self->{cache}->{$tag}} } if exists $self->{cache}->{$tag};
216 # Anything can happen with recursion
217 for (@{$self->{cv}}) {
218 return 'list' if $tag == tag($_->ROOT);
221 unshift @{$self->{cv}}, $cv;
222 (my $r, undef) = $self->expect_any($op->first);
223 shift @{$self->{cv}};
225 $r = { $r => 1} unless ref $r;
226 my $total = sum values %$r;
227 $r = { map { $_ => $r->{$_} / $total } keys %$r };
228 $self->{cache}->{$tag} = { %$r };
233 my ($self, $op) = @_;
235 return ($self->expect_list($op))[0] => 1 if name($op) eq 'return';
237 if ($op->flags & OPf_KIDS) {
238 for ($op = $op->first; not null $op; $op = $op->sibling) {
239 my ($p, $r) = $self->expect_return($op);
240 return $p => 1 if $r;
248 my ($self, $op) = @_;
251 my $meth = $self->can('pp_' . $n);
252 return $self->$meth($op) if $meth;
253 return $ops{$n} => 0 if exists $ops{$n};
255 if ($op->flags & OPf_KIDS) {
258 for ($op = $op->first; not null $op; $op = $op->sibling) {
260 next if $n eq 'pushmark';
261 if ($n eq 'nextstate'
262 and not null(($op = $op->sibling)->sibling)) {
263 ($p, $r) = $self->expect_return($op);
264 return $p => 1 if $r;
266 ($p, $r) = $self->expect_any($op);
267 return $p => 1 if $r;
271 return (combine @res) => 0;
278 my ($self, $op) = @_;
280 return ($self->expect_list($op))[0] => 1 if name($op) eq 'return';
282 if (class($op) eq 'LOGOP' and not null $op->first) {
287 ($p, $r) = $self->expect_return($op);
288 return $p => 1 if $r;
291 push @res, ($self->expect_any($op))[0];
293 # If the logop has no else branch, it can also return the *scalar* result of
299 push @res, ($self->expect_any($op))[0];
302 return (add @res) => 0;
305 return $self->expect_list($op);
308 # Stolen from B::Deparse
310 sub padval { $_[0]->{cv}->[0]->PADLIST->ARRAYelt(1)->ARRAYelt($_[1]) }
313 my ($self, $op) = @_;
314 if (class($op) eq 'PADOP') {
315 return $self->padval($op->padix)
316 } else { # class($op) eq "SVOP"
322 my ($self, $op) = @_;
324 # the constant could be in the pad (under useithreads)
325 $sv = $self->padval($op->targ) unless $$sv;
330 my ($self, $op, $exp) = @_;
333 while ($next->flags & OPf_KIDS) {
334 $next = $next->first;
336 while (not null $next) {
338 my ($p, $r) = $self->expect_return($op, $exp);
339 return $p => 1 if $r;
340 $next = $op->sibling;
343 if (name($op) eq 'rv2cv') {
347 my $next = $op->sibling;
348 while (not null $next) {
350 $next = $next->sibling;
353 } while ($op->flags & OPf_KIDS and { map { $_ => 1 } qw/null leave/ }->{$n});
354 return 'list' unless { map { $_ => 1 } qw/gv refgen/ }->{$n};
355 local $self->{sub} = 1;
356 return $self->expect_any($op, $exp);
364 my ($self, $op) = @_;
366 return $self->{sub} ? $self->enter($self->gv_or_padgv($op)->CV) : 1
370 my ($self, $op) = @_;
372 return $self->{sub} ? $self->enter($self->const_sv($op)) : 1
376 my ($self, $op) = @_;
379 while ($op->flags & OPf_KIDS) {
380 my $nop = $op->first;
382 if ($nn eq 'pushmark') {
383 $nop = $nop->sibling;
386 if ($n eq 'rv2cv' and $nn eq 'gv') {
387 return $self->enter($self->gv_or_padgv($nop)->CV);
397 my ($self, $op) = @_;
399 if (class($op) eq 'SVOP' and (my $sv = $self->const_sv($op))) {
402 return $sv->FILL + 1;
403 } elsif ($c eq 'HV') {
404 return 2 * $sv->FILL;
411 sub pp_aslice { $_[0]->expect_any($_[1]->first->sibling) }
414 *pp_hslice = *pp_aslice{CODE};
416 sub pp_lslice { $_[0]->expect_any($_[1]->first) }
419 my ($self, $op) = @_;
422 return (name($op) eq 'const') ? $self->expect_any($op) : 'list';
425 sub pp_aassign { $_[0]->expect_any($_[1]->first) }
427 sub pp_leaveloop { $_[0]->expect_return($_[1]->first->sibling) }
431 An object-oriented module shouldn't export any function, and so does this one.
435 The algorithm may be pessimistic (things seen as C<list> while they are of fixed length) but not optimistic (the opposite, duh).
437 C<wantarray> isn't specialized when encountered in the optree.
443 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).
447 Vincent Pit, C<< <perl at profvince.com> >>, L<http://www.profvince.com>.
449 You can contact me by mail or on #perl @ FreeNode (vincent or Prof_Vince).
453 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.
457 You can find documentation for this module with the perldoc command.
461 Tests code coverage report is available at L<http://www.profvince.com/perl/cover/Sub-Nary>.
463 =head1 ACKNOWLEDGEMENTS
465 Thanks to Sebastien Aperghis-Tramoni for helping to name this module.
467 =head1 COPYRIGHT & LICENSE
469 Copyright 2008 Vincent Pit, all rights reserved.
471 This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
475 1; # End of Sub::Nary