Task.andAlwaysThen proposal

[pravdomil]

Hello, I found no way how to do:

if task succeed then do task2
else if task failed then do task3

Someting like:

andThenWithResult : (Result x a -> Task x a) -> Task x a -> Task x a

Any ideas?

EDIT: Also I'm searching for andThenAlways that will do a task regardless if the task failed or not.

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[dullbananas]

use andThen then onError

[pravdomil]

show me how

[avh4]

andThenWithResult : (Result x a -> Task x a) -> Task x a -> Task x a
andThenWithResult next task =
    task
        |> Task.andThen (\a -> next (Ok a))
        |> Task.onError (\x -> next (Err x))

or with function composition,

andThenWithResult : (Result x a -> Task x a) -> Task x a -> Task x a
andThenWithResult next =
    Task.andThen (Ok >> next)
        >> Task.onError (Err >> next)

[pravdomil]

@avh4 I think that there is following problem:

1. task succeed
2. andThen calls nextTask
3. nextTask fails
4. onError calls nextTask second time?

[dullbananas]

onError calls nextTask second time?

next is only executed once.

• Task.andThen only does something if the previous task was successful.
• Task.onError only does something if the previous task failed.

edit: actually you are correct, i didn't pay attention to "nextTask fails"

[avhubtran]

ah yeah, I think you're right @pravdomil I think this could be solved still somehow using andThen/onError, but you'd also wrap the error in another Result so you could tell the difference between the original task failing and next failing. Since I don't quickly have the answer at hand for that, would you mind posting your question on Discourse https://discourse.elm-lang.org/ or asking in Elm Slack as noted by the bot "Ask questions a community forum. You will get an answer quicker that way!"

[pravdomil]

Thanks for feedback I will leave the issue open I think that it can be common code pattern and easy to miss the right implementation

[albertdahlin]

I think this will do what you are asking for:

toResult : Task x a -> Task y (Result x a)
toResult task =
    Task.map Ok task
        |> Task.onError (Task.succeed << Err)

andThenWithResult : (Result x a -> Task x a) -> Task x a -> Task x a
andThenWithResult next task =
  toResult task
      |> Task.andThen next

[pravdomil]

@albertdahlin I think that works.

I endup using:

andAlwaysThen : (Result x a -> Task y b) -> Task x a -> Task y b
andAlwaysThen toTask a =
    a
        |> Task.map Ok
        |> Task.onError (Err >> Task.succeed)
        |> Task.andThen toTask