Hello, I got a funny one today. I had an IO with a...

# arrow

[Yeray Cabello]

Hello, I got a funny one today.
I had an IO with a list of items inside, and I needed to apply an IO operation to each element in the list.
The issue is solved in two ways, but I'm baffled as why the second-to-last and last options are different.
I'm evidently missing something and I would LOVE to get it out of my head.

Untitled.kt

[Yeray Cabello]

I'm seeing now that it's actually wrapped in a flatMap, and not a map so that's probably the piece I'm missing. I still don't fully get it.

[simon.vergauwen]

You’re dealing with 2 HKTs here which both have a fix method.

[simon.vergauwen]

sequence for List<IO<A>> returns IO<Kind<ForListK, A>>

[simon.vergauwen]

What you’re doing in the first snippet is call fix on IO before you return it to flatMap which is no-op.

[simon.vergauwen]

So what you want to do instead is call fix on the nested HKT there instead using map as well.

val appliedFixSkins: IO<ListK<BananaSkin>> = getBananas().flatMap { bananas ->
  bananas.map { banana ->
    eat(banana)
  }.sequence(IO.applicative()).map { it.fix() }
}

[simon.vergauwen]

Luckily this can all disappear completely with Arrow Meta! 🙂

[Yeray Cabello]

Oh, thanks a lot, man. I was fixing the IO instead of the Kind.

[simon.vergauwen]

You’re welcome! 👍

[Attila Domokos]

I bumped into the exact same case last night, had to .map { it.fix() }, which looks and reads awkward. Can't wait for Arrow Meta. 👍