Stream.flat_map influences the behaviour of preceeding Stream operations in a pipe

[Paul Balomiri]

Hi,

Today i banged my head against this one :

iex(13)> [[true,false],[false],[true]]|>Stream.filter(fn([x|tail])-> IO.inspect([filter: x]); x end) |>Stream.take(1) |> Stream.flat_map(fn(x)-> IO.inspect([flat_map: x]); x end)  |> Enum.to_list()

[filter: true]

[flat_map: [true, false]]

[filter: false]

[filter: true]

[true, false]

taking away the culprit `Stream.flat_map` prevents the execution of `Stream.filter`'s function after the first element:

[[true,false],[false],[true]]|>Stream.filter(fn([x|tail])-> IO.inspect([filter: x]); x end) |>Stream.take(1) |> Enum.to_list()

[filter: true]

[[true, false]]

It seems to me that this is a bug, but what i find more puzzling is that i do not know how `Stream.flat_map` has even any influence on the `Stream.filter` function.

Did i miss here how `Stream.flat_map` should work or shall file a bug request?

Paul

[Ben Wilson]

It would probably be best to post this on either the mailing list OR the forum, but not both. Many people use both and having the discussion happen in 2 places is not supre helpful.

[José Valim]

Hi Paul,

You got the bug backwards. The issue is that flat_map continues even after take(1). Streams are by design meant to halt when items will no longer be consumed.

I will investigate it soon.

PS: I will send an email that retires this mailing list later today.

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José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

[José Valim]

It has been fixed in master.