From tim.bollman@live.com

It appears that when iterating lazily through a list that's been flattened (the first time), sometimes certain elements are not reached. For background I was writing some code that used a trie, and was attempting to find the word "inclusive" out of input trigrams. I have cleared away as much of the surrounding code as I could to provide a minimal test case. If you enable the commented line (or otherwise iterate through the @​partials list at least once first), the output will be correct.

Thanks,
Tim Bollman

use v6;

sub find-words(@​partials) {
  state $output = '';
  # my @​a = @​partials.kv;
  for @​partials.kv -> $i, $word {
  my $backup = $output;
  $output ~= $word;
  take $output;
  find-words(@​partials[0 ..^ $i, $i ^..^ +@​partials].flat());
  $output = $backup;
  }
}

my @​found = gather find-words(<ive lus inc>);
.say for @​found;
# ive
# ivelus
# lus
# lusive
# lusiveinc
# lusinc
# lusincive
# inc
# incive

From tim.bollman@live.com

I've determined that the issue is the +@​partials. Sometimes that is returning 1 as the length of the list instead of the full list. It looks to be whenever the first index set returns the empty set, the size calculation fails.

________________________________
From​: perl6 via RT <perl6-bugs-followup@​perl.org>
Sent​: Wednesday, November 23, 2016 5​:15 PM
To​: tim.bollman@​live.com
Subject​: [perl #​130160] AutoReply​: [BUG] Flattened list iteration

Greetings,

This message has been automatically generated in response to the
creation of a trouble ticket regarding​:
  "[BUG] Flattened list iteration",
a summary of which appears below.

There is no need to reply to this message right now. Your ticket has been
assigned an ID of [perl #​130160].

Please include the string​:

  [perl #​130160]

in the subject line of all future correspondence about this issue. To do so,
you may reply to this message.

  Thank you,
  perl6-bugs-followup@​perl.org

It appears that when iterating lazily through a list that's been flattened (the first time), sometimes certain elements are not reached. For background I was writing some code that used a trie, and was attempting to find the word "inclusive" out of input trigrams. I have cleared away as much of the surrounding code as I could to provide a minimal test case. If you enable the commented line (or otherwise iterate through the @​partials list at least once first), the output will be correct.

Thanks,
Tim Bollman

use v6;

sub find-words(@​partials) {
  state $output = '';
  # my @​a = @​partials.kv;
  for @​partials.kv -> $i, $word {
  my $backup = $output;
  $output ~= $word;
  take $output;
  find-words(@​partials[0 ..^ $i, $i ^..^ +@​partials].flat());
  $output = $backup;
  }
}

my @​found = gather find-words(<ive lus inc>);
.say for @​found;
# ive
# ivelus
# lus
# lusive
# lusiveinc
# lusinc
# lusincive
# inc
# incive

From lucasbuchala@gmail.com

I found this results that may be related to this issue.
Maybe there's some strange interaction between these List methods?

  > my @​a := <a b c>[0..2, 0..2].flat.cache; say @​a
  (a b c a b c)

  > my @​a := <a b c>[0..2, 0..2].flat.cache; @​a.Bool; @​a.elems; say @​a
  (a a b c)

  > my @​a := <a b c>[0..2, 0..2].flat.cache; @​a.eager; @​a.Bool; @​a.elems; say @​a
  (a b c a b c)

The RT System itself - Status changed from 'new' to 'open'

From @zoffixznet

On Wed, 23 Nov 2016 17​:15​:41 -0800, TimTom wrote​:

It appears that when iterating lazily through a list that's been
flattened (the first time), sometimes certain elements are not
reached.

Thanks for the report.

Fixed in rakudo/rakudo@61a18c0
Tests added in Raku/roast@10f39438be

@zoffixznet - Status changed from 'open' to 'resolved'