Traverse Array of Aff to Aff of array

[azhdari]

Hi,

How can we implement this function?

static Aff<RT, Arr<A>> TraverseArray<RT, A>(this Arr<Aff<RT, A>> arr)

Maybe it's possible using LanguageExt.Aff<R, Arr<A>>.Effect();, but I don't know how.

Thank you.

[CK-LinoPro]

Aff is an asynchronous monad while Arr is a synchronous monad.
It is usually not possible to implement sequence :: t (m a) -> m (t a) (the function you want under its usual name) for this case.

Let's consider the simple case of static Option<Task<T>> Sequence(this Task<Option<T>> task) instead.
Task is asynchronous, Option is synchronous.

static Option<Task<T>> Sequence<T>(this Task<Option<T>> task)
    => ??? : Some(???) : None;

The challenge is filling in the blanks (???) but it turns out that's impossible because to know whether the task holds a Some or a None value you have to wait for the task to finish for which you need an asynchronous result type.

The other way around (synchronous -> asynchronous) is trivial.

static async Task<Option<T>> Sequence(this Option<Task<T>> option)
    => option.Case switch
    {
        T t => Some((await t).ConfigureAwait(false)),
        _ => None,
    };

[azhdari]

Thank you for your time. What you mentioned is exactly what I was thinking about. I'm so glad that it has a build-in solution.

[louthy]

You can use the built-in methods:

• SequenceSerial
• SequenceParallel
• TraverseSerial
• TraverseParallel

[azhdari]

WOW, thank you so much. I don't know how I didn't see these methods!