CODEWITHSUNDEEP
scalatypeseitherhigher-kinded-types
Lando-L
Lando-L

Reputation: 853

Scala Either with List

I wanted to implement a similar type called Result to the Either type. The main difference is, that the Left side of the Result type should always be a List of something. What would be the right type definition for this? I tried having something like that:

sealed trait Result[List, +A] {
    def map[B](f: A => B): Result[List, B] = this match {
        case Failure(err) => Failure(err)
        case Success(value) => Success(f(value))
    }
    def apply[B](f: Result[List, A => B]): Result[List, B] = (f, this) match {
        case (Failure(fE), Failure(aE)) => Failure(fE ::: aE)
        case ...
    }
}

final case class Failure[+E](errors: List[E]) extends Result[List[E], Nothing]
final case class Success[+A](value: A) extends Result[Nothing, A]

But this runs in an error in the map function saying Failure[Any] does not equal Result[List, B] and Success[B] does not equal Result[List, B]. Is the type definition Result[List, +A] already wrong, should I use a higher-kinded type like List[_] instead?

Upvotes: 0

Views: 272

Answers (1)

Andrey Tyukin
Andrey Tyukin

Reputation: 44977

If you want that the left side is always List of something, then there is no need to abstract over the List. Abstract over E:

sealed trait Result[+E, +A] {
    def map[B](f: A => B): Result[E, B] = this match {
        case Failure(err) => Failure(err)
        case Success(value) => Success(f(value))
    }
    def apply[B, F >: E](f: Result[F, A => B]): Result[F, B] = (f, this) match {
        case (Failure(fE), Failure(aE)) => Failure(fE ::: aE)
        case (Failure(fE), _) => Failure(fE)
        case (_, Failure(aE)) => Failure(aE)
        case (Success(fS), Success(tS)) => Success(fS(tS))
    }
}

case class Failure[+E](errors: List[E]) extends Result[E, Nothing]
case class Success[+A](value: A) extends Result[Nothing, A]

Note that it looks as if you are reinventing some standard Applicatives from Scalaz/Cats.

Upvotes: 2

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