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2228-add MonadError instance for optionT where F is a Monad #2299

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Jun 19, 2018
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2228-add MonadError instance for optionT where F is a Monad #2299

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793f2cc
added MonadError instance for OptionT where F is a Monad
heyrutvik Jun 18, 2018
a46f889
test added
heyrutvik Jun 18, 2018
50cd9e9
removed duplicate code using OptionTMonad[F]
heyrutvik Jun 18, 2018
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26 changes: 26 additions & 0 deletions core/src/main/scala/cats/data/OptionT.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -254,6 +254,9 @@ private[data] sealed abstract class OptionTInstances1 extends OptionTInstances2

implicit def catsDataEqForOptionT[F[_], A](implicit F0: Eq[F[Option[A]]]): Eq[OptionT[F, A]] =
new OptionTEq[F, A] { implicit val F = F0 }

implicit def catsDataMonadErrorMonadForOptionT[F[_]](implicit F0: Monad[F]): MonadError[OptionT[F, ?], Unit] =
new OptionTMonadErrorMonad[F] { implicit val F = F0 }
}

private[data] sealed abstract class OptionTInstances2 extends OptionTInstances3 {
Expand Down Expand Up @@ -287,6 +290,29 @@ private[data] trait OptionTMonad[F[_]] extends Monad[OptionT[F, ?]] {
)))
}

private[data] trait OptionTMonadErrorMonad[F[_]] extends MonadError[OptionT[F, ?], Unit] {
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Wondering if it'd be possible to extend OptionTMonad[F] here to reduce duplication (like the trait below does).

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I see.. You are right. I should have mixin OptionTMonad[F]. Changing it..

implicit def F: Monad[F]

def pure[A](a: A): OptionT[F, A] = OptionT.pure(a)

def flatMap[A, B](fa: OptionT[F, A])(f: A => OptionT[F, B]): OptionT[F, B] = fa.flatMap(f)

override def map[A, B](fa: OptionT[F, A])(f: A => B): OptionT[F, B] = fa.map(f)

def tailRecM[A, B](a: A)(f: A => OptionT[F, Either[A, B]]): OptionT[F, B] =
OptionT(F.tailRecM(a)(a0 => F.map(f(a0).value)(
_.fold(Either.right[A, Option[B]](None))(_.map(b => Some(b): Option[B]))
)))

def raiseError[A](e: Unit): OptionT[F, A] = OptionT.none

def handleErrorWith[A](fa: OptionT[F, A])(f: Unit => OptionT[F, A]): OptionT[F, A] =
OptionT(F.flatMap(fa.value) {
case s @ Some(_) => F.pure(s)
case None => f(()).value
})
}

private trait OptionTMonadError[F[_], E] extends MonadError[OptionT[F, ?], E] with OptionTMonad[F] {
override def F: MonadError[F, E]

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7 changes: 7 additions & 0 deletions tests/src/test/scala/cats/tests/OptionTSuite.scala
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Expand Up @@ -130,6 +130,13 @@ class OptionTSuite extends CatsSuite {
checkAll("Semigroup[OptionT[ListWrapper, Int]]", SerializableTests.serializable(Semigroup[OptionT[ListWrapper, Int]]))
}

{
// MonadError instance where F has a Monad
implicit val F = ListWrapper.monad
checkAll("OptionT[ListWrapper, Int]", MonadErrorTests[OptionT[ListWrapper, ?], Unit].monadError[Int, Int, Int])
checkAll("MonadError[OptionT[List, ?]]", SerializableTests.serializable(MonadError[OptionT[ListWrapper, ?], Unit]))
}

test("fold and cata consistent") {
forAll { (o: OptionT[List, Int], s: String, f: Int => String) =>
o.fold(s)(f) should === (o.cata(s, f))
Expand Down