How do I execute a list of effects? - scala

Question

I have a value of this type:
val effects : List[EitherT[Future, Error, Foo]] = ...
How do I run through the list and execute those effects? I did
effects.traverse
But that method is not defined for that type.

Answer 1

As mentioned in comments, it's better to get rid of Future when you are declaring some list of effects and want to execute them and gather effects in one list after. It's better to use IO or another lazy evaluated effect.
So, if you just need to traverse on Future[List[A]]:
import cats.data.EitherT
import scala.concurrent.{ExecutionContext, Future}
import cats.syntax.traverse._
import cats.instances.list._
import cats.instances.future._
implicit val ec: ExecutionContext = scala.concurrent.ExecutionContext.global
case class Foo()
val effects : List[EitherT[Future, Error, Foo]] = ???
def f(e: EitherT[Future, Error, Foo]): Future[Either[Error, Foo]] = ???
val result: Future[List[Either[Error, Foo]]] = effects.traverse(f)
this code compiles and all you need is implement f. You need to add import cats.instances.future._ to have traverse function in Furure.
but better way to construct you process on something like IO:
import cats.data.EitherT
import cats.syntax.traverse._
import cats.instances.list._
import cats.effect.IO
val effects : List[EitherT[IO, Error, Foo]] = ???
def f(e: EitherT[IO, Error, Foo]): IO[Either[Error, Foo]] = ???
val result: IO[List[Either[Error, Foo]]] = effects.traverse(f)
and call it using unsafeRunAsync.