scala-cats EitherT: chaining futures-CodePudding

I am following this. Structure of my program is:

(for {
        data <- myService.fetch(id).eitherT
        values <- anotherService.fetch(data.id).eitherT
        // todo: process values as part of forcomprehension
      } yield data.id).value

myService.fetch returns Future[...].

anotherService.fetch also returns Future[...], and values is a Seq. I want to process entries in values using foreach

values.foreach(value => service.moreAction(value))

moreAction also returns Future[...]

Hence my aim is to chain it with previous Futures in for-comprehension itself.

What is idiomatic way of doing it?

CodePudding user response：

In Cats if you want to go through all values of some data, apply some effect to them, and then get a data within this effect:

DataOf[A] => (A => Effect[B]) => Effect[DataOf[B]]

you use Traverse type class with its extension methods.

(Think of it like of generalization of Future.sequence which takes Seq[Future[A]] and returns Future[Seq[A]], but one which performs seq.map(toFuture) and Future.sequence in one step).

import cats.syntax.traverse._

for {
  data <- myService.fetch(id).eitherT
  values <- anotherService.fetch(data.id).eitherT
  // Traverse is not available for Seq, it requires some specific data e.g. List
  result <- values.toList.traverse(value => service.moreAction(value).eitherT)
} yield result