From @mscha

  my @​l = (0,1,2),(3,4,5),(6,7);
  say roundrobin(|@​l);

outputs

  (((0 1 2) (3 4 5) (6 7)))

instead of the expected

  ((0 3 6) (1 4 7) (2 5))

The following *does* work​:

  my $l = ((1,2,3),(4,5,6),(7,8));
  say roundrobin(|$l);

Note that zip (the function) has the same issue, but Z (the operator)
does not​:

  my @​l = (0,1,2),(3,4,5),(6,7);
  my $l = ((0,1,2),(3,4,5),(6,7));
  say zip(|@​l);
  say zip(|$l);
  say [Z] @​l;
  say [Z] $l;

outputs​:

  (((0 1 2) (3 4 5) (6 7)))
  ((0 3 6) (1 4 7))
  ((0 3 6) (1 4 7))
  ((0 3 6) (1 4 7))

Unfortunately, there is no operator for roundrobin (right?) that I can
use as a workaround.

Latest Rakudo Star​:

This is Rakudo version 2016.11 built on MoarVM version 2016.11
implementing Perl 6.c.

From @zoffixznet

On IRC I said this was a bug, because the behaviour seemed so surprising, but now that I made a fix for it (diff attached), I find breakage in t/spec/S03-metaops/zip.t and t/spec/S32-container/roundrobin.t that test for what I see is a contradictory (to this ticket) behaviour for RT#​126522.

Basically roundrobin, zip, and a few others aren't meant to treat itemized lists as lists and instead treat them as a single item. This is why the behaviour in this ticket is observed​: array's elements are itemized and so get treated differently.

So.... this is just a pitfall of sorts and isn't a bug?

From @zoffixznet

diff --git a/src/core/List.pm b/src/core/List.pm
index 4edd254..172009b 100644
--- a/src/core/List.pm
+++ b/src/core/List.pm
@@ -1593,14 +1593,14 @@ multi sub infix:<Z>(+lol) {
     return Seq.new(Rakudo::Internals.EmptyIterator) if $arity == 0;
     eager my @l = (^$arity).map: -> $i {
         my \elem = lol[$i];
-        if nqp::iscont(elem) {
-            $laze = False;
-            Rakudo::Internals.WhateverIterator((elem,))
-        }
-        else {
+        if nqp::istype(elem, Iterable) {
             $laze = False unless elem.is-lazy;
             Rakudo::Internals.WhateverIterator(elem)
         }
+        else {
+            $laze = False;
+            Rakudo::Internals.WhateverIterator((elem,))
+        }
     }
 
     gather {
@@ -1622,13 +1622,13 @@ my &zip := &infix:<Z>;
 sub roundrobin(**@lol is raw) {
     my $laze = False;
     my @iters = do for @lol -> \elem {
-        if nqp::iscont(elem) {
-            (elem,).iterator
-        }
-        else {
+        if nqp::istype(elem, Iterable) {
             $laze = True if elem.is-lazy;
             elem.iterator
         }
+        else {
+            (elem,).iterator
+        }
     }
     gather {
         while @iters {

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

From @zoffixznet

per jnthn++ https://irclog.perlgeek.de/perl6-dev/2017-01-04#i_13854591

Turns out this is not a bug, but a consequence of arrays itemizing their contents. You can still use the array, but you need to ».list on it it to de-itemize the contents; same with `zip`​:

<ZoffixW> m​: my @​l = (0,1,2),(3,4,5),(6,7); say zip @​l».list.Slip;
<camelia> rakudo-moar 40d7de​: OUTPUT«((0 3 6) (1 4 7))␤»
<ZoffixW> m​: my @​l = (0,1,2),(3,4,5),(6,7); say roundrobin @​l».list.Slip;
<camelia> rakudo-moar 40d7de​: OUTPUT«((0 3 6) (1 4 7) (2 5))␤»

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