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Add Coproduct fold #1032

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May 15, 2016
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Add Coproduct fold #1032

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24 changes: 23 additions & 1 deletion core/src/main/scala/cats/data/Coproduct.scala
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
package cats
package data

import cats.arrow.NaturalTransformation
import cats.functor.Contravariant

/** `F` on the left and `G` on the right of [[Xor]].
Expand Down Expand Up @@ -57,6 +58,28 @@ final case class Coproduct[F[_], G[_], A](run: F[A] Xor G[A]) {
def toValidated: Validated[F[A], G[A]] =
run.toValidated

/**
* Fold this coproduct into a new type constructor using two natural transformations.
*
* Example:
* {{{
* scala> import cats.arrow.NaturalTransformation
* scala> import cats.data.Coproduct
* scala> val listToOption =
* | new NaturalTransformation[List, Option] {
* | def apply[A](fa: List[A]): Option[A] = fa.headOption
* | }
* scala> val optionToOption = NaturalTransformation.id[Option]
* scala> val cp1: Coproduct[List, Option, Int] = Coproduct.leftc(List(1,2,3))
* scala> val cp2: Coproduct[List, Option, Int] = Coproduct.rightc(Some(4))
* scala> cp1.fold(listToOption, optionToOption)
* res0: Option[Int] = Some(1)
* scala> cp2.fold(listToOption, optionToOption)
* res1: Option[Int] = Some(4)
* }}}
*/
def fold[H[_]](f: NaturalTransformation[F, H], g: NaturalTransformation[G, H]): H[A] =
run.fold(f.apply, g.apply)
}

object Coproduct extends CoproductInstances {
Expand All @@ -78,7 +101,6 @@ object Coproduct extends CoproductInstances {
def left[G[_]]: CoproductLeft[G] = new CoproductLeft[G]

def right[F[_]]: CoproductRight[F] = new CoproductRight[F]

}

private[data] sealed abstract class CoproductInstances3 {
Expand Down