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Add Kleisli.fromFunction, WriterT.liftFunctionK #3644

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Nov 8, 2020
Merged

Add Kleisli.fromFunction, WriterT.liftFunctionK #3644

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8f03ca9
Add Kleisli.fromFunction, WriterT.liftFunctionK
kubukoz Oct 21, 2020
089a67e
Use a partially applied type, reorder parameters
kubukoz Nov 3, 2020
35452fb
Fix inference by moving implicit to later
kubukoz Nov 3, 2020
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16 changes: 16 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 @@ -257,6 +257,18 @@ sealed private[data] trait KleisliFunctions {
*/
def local[M[_], A, R](f: R => R)(fa: Kleisli[M, R, A]): Kleisli[M, R, A] =
Kleisli(r => fa.run(f(r)))

/**
* Lifts a function to a Kleisli.
* {{{
* scala> import cats.data.Kleisli
* scala> val stringify = Kleisli.fromFunction[Option, Int].apply(_.toString)
* scala> stringify.run(42)
* res0: Option[String] = Some(42)
* }}}
*/
def fromFunction[M[_], R]: KleisliFromFunctionPartiallyApplied[M, R] =
new KleisliFromFunctionPartiallyApplied[M, R]
}

sealed private[data] trait KleisliFunctionsBinCompat {
Expand Down Expand Up @@ -284,6 +296,10 @@ sealed private[data] trait KleisliFunctionsBinCompat {
new (Kleisli[F, A, *] ~> Kleisli[G, A, *]) { def apply[B](k: Kleisli[F, A, B]): Kleisli[G, A, B] = k.mapK(f) }
}

final class KleisliFromFunctionPartiallyApplied[M[_], R] {
def apply[A](f: R => A)(implicit M: Applicative[M]): Kleisli[M, R, A] = Kleisli(r => M.pure(f(r)))
}

sealed private[data] trait KleisliExplicitInstances {

def endoSemigroupK[F[_]](implicit FM: FlatMap[F]): SemigroupK[λ[α => Kleisli[F, α, α]]] =
Expand Down
6 changes: 6 additions & 0 deletions core/src/main/scala/cats/data/WriterT.scala
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Expand Up @@ -812,4 +812,10 @@ private[data] trait WriterTFunctions {

def valueT[F[_], L, V](vf: F[V])(implicit functorF: Functor[F], monoidL: Monoid[L]): WriterT[F, L, V] =
WriterT.putT[F, L, V](vf)(monoidL.empty)

/**
* Lifts a FunctionK operating on effects to a FunctionK operating on WriterT with these effects.
*/
def liftFunctionK[F[_], G[_], A](f: F ~> G): WriterT[F, A, *] ~> WriterT[G, A, *] =
new (WriterT[F, A, *] ~> WriterT[G, A, *]) { def apply[B](k: WriterT[F, A, B]): WriterT[G, A, B] = k.mapK(f) }
}