bicovariant either

either.scala

package pi.prelude

import pi.prelude.either.\/._
import zio.ZIO
import zio.prelude.{Bicovariant, Covariant}

package object either {
	sealed trait \/[+E, +A] { self =>
		private final def map0[B](f: A => B): \/[E, B] =
			self match {
				case \/-(a)           => \/-(f(a))
				case error @ -\/(_) => error
			}

		private final def mapError0[F](f: E => F): \/[F, A] =
			self match {
				case -\/(e)          => -\/(f(e))
				case success @ \/-(_) => success
			}
		
		def toEither: Either[E, A] = self match {
			case \/.-\/(e) => Left(e)
			case \/.\/-(x) => Right(x)
		}
	}

	object \/ {
		final case class -\/[+E](e: E) extends (E \/ Nothing)
		final case class \/-[+A](x: A) extends (Nothing \/ A)

		def fromEither[E, A](e: Either[E, A]): E \/ A = e match {
			case Left(value) => -\/(value)
			case Right(value) => \/-(value)
		}

		def fromOption[E, A](ifNone: => E)(o: Option[A]): E \/ A = o.map(\/-(_)).getOrElse(-\/(ifNone))

		implicit def EitherBicovariant: Bicovariant[\/] = new Bicovariant[\/] {
			override def bimap[A, B, AA, BB](f: A => AA, g: B => BB): A \/ B => AA \/ BB =
				_.mapError0(f).map0(g)
		}

		implicit def EitherCovariant[E]: Covariant[({ type lambda[+x] = \/[E, x] })#lambda] =
			new Covariant[({ type lambda[+x] = \/[E, x] })#lambda] {
				override def map[A, B](f: A => B): E \/ A => E \/ B = _.map0(f)
			}

		/*
		implicit def ValidationFailureCovariant[A]
    : Covariant[({ type lambda[+x] = newtypes.Failure[Validation[x, A]] })#lambda] =
    new Covariant[({ type lambda[+x] = newtypes.Failure[Validation[x, A]] })#lambda] {
      def map[E, E1](f: E => E1): newtypes.Failure[Validation[E, A]] => newtypes.Failure[Validation[E1, A]] =
        validation => newtypes.Failure.wrap(newtypes.Failure.unwrap(validation).mapError(f))
    }
		 */

		implicit class ZEitherOps[A](private val x: A) extends AnyVal {
			def left[B]: A \/ B = -\/(x)
			def right[B]: B \/ A = \/-(x)
		}

		implicit class EitherToZIOOps[E, A](private val x: E \/ A) extends AnyVal {
			def io: ZIO[Any, E, A] = ZIO.fromEither(x.toEither)
		}

		implicit class ZIOAbsolveOps[R, E, A](private val z: ZIO[R, E, E \/ A]) extends AnyVal {
			def absolve: ZIO[R, E, A] = z.map(_.toEither).absolve
		}
	}
}

Bah j'ai juste fait pareil que ta 1ere proposition mais currifié non ?

Mais la curryfication n'est pas polymorphique en Scala, cela t'oblige à connaître ton A, ce qui tue un peu l'usage.
Et si dans un call tu as déjà ton e et que tu connais ton A, tu peux toujours placer un : fromOption(e, _)

Ok tu m'as convaincu alors :D