Scala: Combine Either per the whole List with Either per elements

Question

I have a list of Either, which represents error:

type ErrorType = List[String]
type FailFast[A] = Either[ErrorType, A]

import cats.syntax.either._
val l = List(1.asRight[ErrorType], 5.asRight[ErrorType])

If all of them are right, I want to get a list of [A], in this case - List[Int]

If any Either is left, I want to combine all errors of all either and return it.

I've found a similar topic at [How to reduce a Seq[Either[A,B]] to a Either[A,Seq[B]]

But it was quite long ago. For instance, one of the answers offers to use partitionMap, which I cannot find at this moment. Probably there is a better, more elegant solution. Example with scala-cats would be great.

How I would like to use it:

for {
  listWithEihers <- someFunction
  //if this list contains one or more errors, return Left[List[String]]
  //if everything is fine, convert it to:
  correctItems <- //returns list of List[Int] as right
} yield correctItems

Return type of this for-comprehension must be:

Either[List[String], List[Int]]

Answer 1

As already mentioned in the comments, Either is good for fail-fast behavior. For accumulating multiple errors, you probably want something like Validated. Moreover:

• List is traversable (has instance of Traverse)
• Validated is applicative
• Validated.fromEither maps Either[List[String], X] to Validated[List[String], X], that's exactly what you need as function in traverse.

Therefore, you might try:

• l.traverse(Validated.fromEither) if you are OK with a Validated
• l.traverse(Validated.fromEither).toEither if you really want an Either in the end.

Full example with all imports:

import cats.data.Validated
import cats.syntax.validated._
import cats.syntax.either._
import cats.syntax.traverse._
import cats.instances.list._
import cats.Traverse
import scala.util.Either

type ErrorType = List[String]
type FailFast[A] = Either[ErrorType, A]
val l: List[Either[ErrorType, Int]] = List(1.asRight[ErrorType], 5.asRight[ErrorType])

// solution if you want to keep a `Validated`
val validatedList: Validated[ErrorType, List[Int]] =
  l.traverse(Validated.fromEither)

// solution if you want to transform it back to `Either`
val eitherList: Either[ErrorType, List[Int]] =    
  l.traverse(Validated.fromEither).toEither