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Distributive instance for Representable Functor #2775

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Apr 8, 2019
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Distributive instance for Representable Functor #2775

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20 changes: 19 additions & 1 deletion core/src/main/scala/cats/Representable.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -93,6 +93,16 @@ private trait RepresentableBimonad[F[_], R] extends RepresentableMonad[F, R] wit
R.index(fa)(M.empty)
}

private trait RepresentableDistributive[F[_], R] extends Distributive[F] {

def R: Representable.Aux[F, R]

override def distribute[G[_], A, B](ga: G[A])(f: A => F[B])(implicit G: Functor[G]): F[G[B]] =
R.tabulate(r => G.map(ga)(a => R.index(f(a))(r)))

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

object Representable {
type Aux[F[_], R] = Representable[F] { type Representation = R }

Expand All @@ -119,12 +129,20 @@ object Representable {
}

/**
* Derives a `Bimonad` instance for any `Representable` functor whos representation
* Derives a `Bimonad` instance for any `Representable` functor whose representation
* has a `Monoid` instance.
*/
def bimonad[F[_], R](implicit Rep: Representable.Aux[F, R], Mon: Monoid[R]): Bimonad[F] =
new RepresentableBimonad[F, R] {
override def R: Representable.Aux[F, R] = Rep
override def M: Monoid[R] = Mon
}

/**
* Derives a `Distributive` instance for any `Representable` functor
*/
def distributive[F[_]](implicit Rep: Representable[F]): Distributive[F] =
new RepresentableDistributive[F, Rep.Representation] {
override def R: Aux[F, Rep.Representation] = Rep
}
}
16 changes: 14 additions & 2 deletions tests/src/test/scala/cats/tests/RepresentableSuite.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -3,7 +3,14 @@ package cats.tests
import cats.laws.discipline.SemigroupalTests.Isomorphisms
import cats.laws.discipline.arbitrary._
import cats.laws.discipline.eq._
import cats.laws.discipline.{BimonadTests, MiniInt, MonadTests, RepresentableTests, SerializableTests}
import cats.laws.discipline.{
BimonadTests,
DistributiveTests,
MiniInt,
MonadTests,
RepresentableTests,
SerializableTests
}
import cats.{Bimonad, Eq, Eval, Id, Representable}
import org.scalacheck.Arbitrary
import cats.data.Kleisli
Expand Down Expand Up @@ -60,14 +67,19 @@ class RepresentableSuite extends CatsSuite {
}

{
//the monadInstance below made a conflict to resolve this one.
// the monadInstance below made a conflict to resolve this one.
// TODO ceedubs is this needed?
implicit val isoFun1: Isomorphisms[MiniInt => ?] = Isomorphisms.invariant[MiniInt => ?]

implicit val monadInstance = Representable.monad[MiniInt => ?]
checkAll("MiniInt => ?", MonadTests[MiniInt => ?].monad[String, String, String])
}

{
implicit val distributiveInstance = Representable.distributive[Pair]
checkAll("Pair[Int]", DistributiveTests[Pair].distributive[Int, Int, Int, Option, MiniInt => ?])
}

// Syntax tests. If it compiles is "passes"
{
// Pair
Expand Down