cats-effect: How to transform `List[IO]` to `IO[List]`

Question

I created a list of IO[Unit] in order to retrieve data from a list of URL. But now how I convert it back to a single IO[Unit] ?

score 33 · Accepted Answer · answered Apr 12 '18 at 16:15

33

You can do this in the following way

val x: List[IO[Unit]] = ???

import cats.implicits._

val y: IO[List[Unit]] = x.sequence

val z: IO[Unit] = y.map(_ => ())

answered Apr 12 '18 at 16:15

Dmytro Mitin

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Also, you can substitute `.map(_ => ())` with `.void` (though `sequence_` mentioned below is shorter). see the [scaladoc](https://typelevel.org/cats/api/cats/Functor.html#void[A](fa:F[A]):F[Unit]) – Haemin Yoo Oct 16 '18 at 05:25

score 16 · Answer 2 · answered Apr 16 '18 at 11:17

This is just in addition to what Dmytro already said, you can actually do it in a single step by using traverse_ or sequence_. Both of these are really useful if you just don't care about the result. Code would look like this:

import cats.implicits._

val x: List[IO[Unit]] = ???

val y: IO[Unit] = x.sequence_

cats-effect: How to transform `List[IO]` to `IO[List]`

2 Answers2