Hei. I was wondering if I can achieve something si...

# arrow

[Satyam Agarwal]

Hei. I was wondering if I can achieve something similar with IO like we have in Validated by using Semigroup with applicative, we can get a list of all the failures.
I see we have IOSemiGroup which has combine method, but I don’t it will give me the desired effect right ? Because IO encapsulates Throwable, so it will be always IO<A> while Throwable will be first failure while combining.
I have seen the same thing with parTraverse on IO. I got the desired effect by giving myIOOp.attempt() to parTraverse. Then I get IO<List<Either<Throwable, A>>>, so with this I can extract out all failures.
But it becomes complicated quickly.
Validated applicative is neat, and uses tupledN which is nice so I don’t have to resort to coverting to List first.
So, is there a way ?

[simon.vergauwen]

Using Arrow Fx Coroutines this can become quite simple. There is parTraverse for Iterable<A>.

val List<ValidatedNel<Throwable, A>> = listOf(1, 2, 3).parTraverse { i -> Validated.catch { f(i) } }

And then you can sequence List<ValidatedNel<Throwable, A>> to ValidatedNel<Throwable, List<A>>.

Looking at the implementation of parTraverse and the impl of List.sequence you can probably flatten these 2 operations into a single one.

[simon.vergauwen]

With IO you can follow the same pattern, but it'll involve a 3rd wrapper IO instead of suspend.

[Satyam Agarwal]

Right. Thanks 🙂