From @awwaiid

Here is an example that works as expected​:

  > for ^2 { my $n = 5; say [+] (^$n X ^$n) }
  50
  50

If we use constants to construct the Seq, it is not re-created on the
second loop iteration​:

  > for ^2 { say [+] (^5 X ^5) }
  50
  This Seq has already been iterated, and its values consumed
  (you might solve this by adding .cache on usages of the Seq, or
  by assigning the Seq into an array)
  in block <unit> at <unknown file> line 1

In my particular case after building the Seq I was applying map and grep to
it which would change on each iteration -- it yells at me for the
constant-Seq re-use even though I am doing different things with it each
time.

  > for ^2 -> $n { say [+] (^5 X ^5).map(* + $n) }
  50
  This Seq has already been iterated, and its values consumed
  (you might solve this by adding .cache on usages of the Seq, or
  by assigning the Seq into an array)
  in block <unit> at <unknown file> line 1

I don't feel that the suggested .cache is appropriate, but maybe I'm wrong?
Seems like if I wanted this Seq to be shared I should have to define it
before the block.

Note that if I manually unrolled this loop I wouldn't get the error (since
the Seq would be written out twice, so not considered the same Seq).

From @zoffixznet

Still present in Rakudo version 2016.07.1-19-g8dff9df built on MoarVM version 2016.07-3-gc01472d

$ ./perl6-m -e 'for ^2 { say [+] (^5 X ^5) }'
50
This Seq has already been iterated, and its values consumed
(you might solve this by adding .cache on usages of the Seq, or
by assigning the Seq into an array)
  in block <unit> at -e line 1

--
Cheers,
ZZ | https://twitter.com/zoffix

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

From @jnthn

On Sun Mar 20 08​:30​:28 2016, awwaiid wrote​:

Here is an example that works as expected​:

> for ^2 \{ my $n = 5; say \[\+\] \(^$n X ^$n\) \}
50
50

If we use constants to construct the Seq, it is not re-created on the
second loop iteration​:

> for ^2 \{ say \[\+\] \(^5 X ^5\) \}
50
This Seq has already been iterated, and its values consumed
\(you might solve this by adding \.cache on usages of the Seq, or
by assigning the Seq into an array\)
  in block \<unit> at \<unknown file> line 1

In my particular case after building the Seq I was applying map and grep to
it which would change on each iteration -- it yells at me for the
constant-Seq re-use even though I am doing different things with it each
time.

Yeah, it was a constant folding issue. I've now made sure we don't try to do that for things that produce a Seq, which fixes the issue. Test added to S03-metaops/cross.t.

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