lisa
Hi,
I have a problem with Stream.chunk_while in combination with Enum.at (elixir version is 1.5.1)
here is the code:
chunk_fun = fn
i, [] ->
{:cont, [i]}
i, chunk ->
if rem(i, 2) == 0 do
{:cont, Enum.reverse(chunk), [i]}
else
{:cont, [i | chunk]}
end
end
after_fun = fn
[] -> {:cont, []}
chunk -> {:cont, Enum.reverse(chunk), []}
end
stream = [1, 2, 3, 4, 5] |> Stream.chunk_while([], chunk_fun, after_fun)
calling stream |> Enum.at(0)
gives me this error:
** (ArithmeticError) bad argument in arithmetic expression
(elixir) lib/enum.ex:793: anonymous fn/3 in Enum.fetch_enumerable/3
(elixir) lib/stream.ex:1413: anonymous fn/3 in Enumerable.Stream.reduce/3
(elixir) lib/stream.ex:234: Stream.after_chunk_while/3
(elixir) lib/stream.ex:1446: Enumerable.Stream.do_done/2
(elixir) lib/enum.ex:789: Enum.fetch_enumerable/3
(elixir) lib/enum.ex:311: Enum.at/3
but it’s ok to run stream with Enum.to_list:
> Enum.to_list(stream)
[[1], [2, 3], [4, 5]]
also it’s ok to get last chunk like so:
> stream |> Enum.at(2)
[4, 5]
Probably I’m doing something wrong?
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aseigo
Enum.atis expecting theEnumerable.reduceto always call the reducer function with two params: the entry and a tuple which it passes through ({:not_found, index}) … however, when a:halttuple is passed toStream.chunk_while, it actually calls the after function (3rd parameter toStream.chunk_while) and that calls the reducer function again!So it is being run one too many times, and as a result instead of an index which it is incrementing with
+ 1, it gets (in your case, at least) a list. This is the source of the arithmetic error:(If your stream was a list of integers, it would also return an incorrect value, fwiw … it would eventually return a
{:not_found, max_index}tuple, I believe..)In HEAD, it does this differently and the search for the correct value almost works .. but not quite. Instead if fails with:
Because it is now doing this:
… it is expecting a list with one entry at the end of its traversal and for the same source reason it doesn’t end up with that. oof.
Paging @josevalim … looks like a bone fide bug?
josevalim
Yes, please file a bug report so we can include a fix on v1.6.
lisa
https://github.com/elixir-lang/elixir/issues/7112