Enum.zip([], Stream.cycle(1..10)) returns error

[pejrich]

Elixir and Erlang/OTP versions

OTP 27.2.1
Elixir 1.18.2-otp-27

Operating system

macOS

Current behavior

Not sure if this is intended but it seems a bit inconsistent to me.

# zip works with empty lists, as well as cycling over a list
iex> Enum.zip([], [])
[]
iex> Enum.zip([], Stream.cycle([1,2,3,4,5]))
[]
# The output of cycling a list, and cycling a range is as expected, equal.
iex> Stream.cycle([1,2,3,4,5]) |> Enum.take(10)
[1, 2, 3, 4, 5, 1, 2, 3, 4, 5]
iex> Stream.cycle(1..5) |> Enum.take(10)
[1, 2, 3, 4, 5, 1, 2, 3, 4, 5]
# But zipping an empty list against a cycle over a range throws an error
iex> Enum.zip([], Stream.cycle(1..5))
** (ArgumentError) cannot cycle over an empty enumerable
    (elixir 1.18.2) lib/stream.ex:1448: anonymous fn/2 in Stream.check_cycle_first_element/1
    (elixir 1.18.2) lib/stream.ex:1419: Stream.do_cycle/5
    (stdlib 6.2) lists.erl:2310: :lists.foreach_1/2
    (elixir 1.18.2) lib/stream/reducers.ex:74: Stream.Reducers.do_zip_enum/4
    (elixir 1.18.2) lib/enum.ex:4174: Enum.zip_reduce/3
    (elixir 1.18.2) lib/enum.ex:4028: Enum.zip/1
    iex:57: (file)

Expected behavior

Intuitively, based on the other results, I would expect the last one to return [] too, rather than throw an error, since I'm never actually needing to cycle that stream, and it's a valid stream to cycle, so I'm unsure why it would throw an error, so I'm not sure if this is a bug or intended.

[v0idpwn]

Gave it a stab on v0idpwn@165ecc0. Works, but not happy with my solution, probably there's a better way to solve it.

[sabiwara]

Thanks @pejrich for the report and @v0idpwn for the investigation! 💜

What seems to be happening is:

• we reach this point when the first enumerable is empty
• we try to close the remaining enums, that is our Stream.cycle, with {:halt, []}
• this causes a {:halted, []} explicitly guarded against here

I couldn't find any test that fails if we remove this guard, sent a PR fixing it this way.

[josevalim]

Fixed by 71da1a2