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Add an applyK function to Kleisli. #2563

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Jan 14, 2019
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Add an applyK function to Kleisli. #2563

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27 changes: 27 additions & 0 deletions core/src/main/scala/cats/data/Kleisli.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -113,6 +113,33 @@ object Kleisli
case _ =>
Kleisli(run)
}

/**
* Creates a `FunctionK` that transforms a `Kleisli[F, A, B]` into an `F[B]` by applying the value of type `a:A`.
* {{{
* scala> import cats.{~>}, cats.data.{Kleisli, EitherT}
*
* scala> def f(i: Int): Option[Either[Char, Char]] = if (i > 0) Some(Right('n')) else if (i < 0) Some(Left('z')) else None
*
* scala> type KOI[A] = Kleisli[Option, Int, A]
* scala> val b: KOI[Either[Char, Char]] = Kleisli[Option, Int, Either[Char, Char]](f _)
* scala> val nt: Kleisli[Option, Int, ?] ~> Option = Kleisli.applyK[Option, Int](1)
* scala> nt(b)
* res0: Option[Either[Char, Char]] = Some(Right('n'))
*
* scala> type EKOIC[A] = EitherT[KOI, Char, A]
* scala> val c: EKOIC[Char] = EitherT[KOI, Char, Char](b)
* scala> c.mapK(nt).value
* res1: Option[Either[Char, Char]] = Some(Right('n'))
*
* scala> val ntz = Kleisli.applyK[Option, Int](0)
* scala> c.mapK(ntz).value
* res2: Option[Either[Char, Char]] = None
* }}}
*/
def applyK[F[_], A](a: A): Kleisli[F, A, ?] ~> F =
λ[Kleisli[F, A, ?] ~> F](_.apply(a))

}

sealed private[data] trait KleisliFunctions {
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