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Add MonadError instance for StateT. #1413

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Oct 25, 2016
Merged

Add MonadError instance for StateT. #1413

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20 changes: 17 additions & 3 deletions core/src/main/scala/cats/data/StateT.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -174,14 +174,19 @@ private[data] sealed trait StateTInstances1 extends StateTInstances2 {
new StateTMonadCombine[F, S] { implicit def F = F0 }
}

private[data] sealed trait StateTInstances2 {
implicit def catsDataMonadForStateT[F[_], S](implicit F0: Monad[F]): Monad[StateT[F, S, ?]] =
new StateTMonad[F, S] { implicit def F = F0 }
private[data] sealed trait StateTInstances2 extends StateTInstances3 {
implicit def catsDataMonadErrorForStateT[F[_], S, E](implicit F0: MonadError[F, E]): MonadError[StateT[F, S, ?], E] =
new StateTMonadError[F, S, E] { implicit def F = F0 }

implicit def catsDataSemigroupKForStateT[F[_], S](implicit F0: Monad[F], G0: SemigroupK[F]): SemigroupK[StateT[F, S, ?]] =
new StateTSemigroupK[F, S] { implicit def F = F0; implicit def G = G0 }
}

private[data] sealed trait StateTInstances3 {
implicit def catsDataMonadForStateT[F[_], S](implicit F0: Monad[F]): Monad[StateT[F, S, ?]] =
new StateTMonad[F, S] { implicit def F = F0 }
}

// To workaround SI-7139 `object State` needs to be defined inside the package object
// together with the type alias.
private[data] abstract class StateFunctions {
Expand Down Expand Up @@ -258,3 +263,12 @@ private[data] sealed trait StateTMonadCombine[F[_], S] extends MonadCombine[Stat

def empty[A]: StateT[F, S, A] = liftT[F, A](F.empty[A])
}

private[data] sealed trait StateTMonadError[F[_], S, E] extends StateTMonad[F, S] with MonadError[StateT[F, S, ?], E] {
implicit def F: MonadError[F, E]

def raiseError[A](e: E): StateT[F, S, A] = StateT.lift(F.raiseError(e))

def handleErrorWith[A](fa: StateT[F, S, A])(f: E => StateT[F, S, A]): StateT[F, S, A] =
StateT(s => F.handleErrorWith(fa.run(s))(e => f(e).run(s)))
}
12 changes: 11 additions & 1 deletion tests/src/test/scala/cats/tests/StateTTests.scala
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Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
package cats
package tests

import cats.data.{State, StateT}
import cats.data.{State, StateT, EitherT}
import cats.kernel.instances.tuple._
import cats.laws.discipline._
import cats.laws.discipline.eq._
Expand Down Expand Up @@ -260,6 +260,16 @@ class StateTTests extends CatsSuite {
checkAll("State[Long, ?]", MonadTests[State[Long, ?]].monad[Int, Int, Int])
checkAll("Monad[State[Long, ?]]", SerializableTests.serializable(Monad[State[Long, ?]]))
}

{
// F has a MonadError
implicit val iso = CartesianTests.Isomorphisms.invariant[StateT[Option, Int, ?]]
implicit val eqEitherTFA: Eq[EitherT[StateT[Option, Int , ?], Unit, Int]] = EitherT.catsDataEqForEitherT[StateT[Option, Int , ?], Unit, Int]

checkAll("StateT[Option, Int, Int]", MonadErrorTests[StateT[Option, Int , ?], Unit].monadError[Int, Int, Int])
checkAll("MonadError[StateT[Option, Int , ?], Unit]", SerializableTests.serializable(MonadError[StateT[Option, Int , ?], Unit]))
}

}

object StateTTests extends StateTTestsInstances {
Expand Down