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Add Foldable, Traverse and Comonad instances to WriterT #2182

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Mar 12, 2018
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84 changes: 72 additions & 12 deletions core/src/main/scala/cats/data/WriterT.scala
Original file line number Diff line number Diff line change
@@ -1,8 +1,8 @@
package cats
package data

import cats.Foldable
import cats.kernel.instances.tuple._

import cats.kernel.CommutativeMonoid
import cats.syntax.semigroup._

Expand Down Expand Up @@ -63,6 +63,17 @@ final case class WriterT[F[_], L, V](run: F[(L, V)]) {
mapWritten(_ => monoidL.empty)

def show(implicit F: Show[F[(L, V)]]): String = F.show(run)

def foldLeft[C](c: C)(f: (C, V) => C)(implicit F: Foldable[F]): C =
F.foldLeft(run, c)((z, lv) => f(z, lv._2))

def foldRight[C](lc: Eval[C])(f: (V, Eval[C]) => Eval[C])(implicit F: Foldable[F]): Eval[C] =
F.foldRight(run, lc)((lv, z) => f(lv._2, z))

def traverse[G[_], V1](f: V => G[V1])(implicit F: Traverse[F], G: Applicative[G]): G[WriterT[F, L, V1]] =
G.map(
F.traverse(run)(lv => G.product(G.pure(lv._1), f(lv._2)))
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Can this be G.tupleLeft(f(lv._2), lv._1) instead of G.product(G.pure(lv._1), f(lv._2))?

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sure changing it 👍

)(WriterT.apply)
}

object WriterT extends WriterTInstances with WriterTFunctions {
Expand Down Expand Up @@ -95,9 +106,23 @@ private[data] sealed abstract class WriterTInstances extends WriterTInstances0 {
implicit val F0: Monad[F] = F
implicit val L0: Monoid[L] = L
}

implicit def catsDataTraverseForWriterTId[L](implicit F: Traverse[Id]): Traverse[WriterT[Id, L, ?]] =
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@barambani you can't have any instance that potentially conflicts in the same trait level.
I think there is Functor instance conflict here for [WriterT[Id, L, ?]. I think you might need to create quite a few more priority layers to separate the instances.

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to be clear I didn't see the actually error message on travis. But I think separating them out might solve the problem.

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@barambani barambani Mar 9, 2018

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Sorry for the confusion, my bad for the lack of clarity. This prioritisation should work (let's see when the CI completes). My question was more in general about the reason why we needed to create instances for Id. I would have expected that something like Comonad[WriterT[Id, ListWrapper[Int], ?]] could summon the instance for F[_] and I was wondering if you had resources about why that's not the case.
This doesn't mean that I'm not happy to add more priority levels if you think that's better. Thanks for the great help here.

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@barambani I don't know the underlying reason that Id needs to be special-cased, but it's a weakness in scalac's type inference. This is a related Stack Overflow post that I've seen before. Something like Id might be a tricky one because A =:= Id[A] =:= Id[Id[A]], so there may be some logic to short-circuit inference when applying a type results in the same type so you don't end up in an infinite loop or something. This is pure speculation :)

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@ceedubs thanks a lot, I didn't want for you to waste time on this. I was just taking advantage of this discussion in case you knew it out of your head 😃 👍

catsDataTraverseForWriterT[Id, L](F)
}

private[data] sealed abstract class WriterTInstances0 extends WriterTInstances1 {

implicit def catsDataTraverseForWriterT[F[_], L](implicit F: Traverse[F]): Traverse[WriterT[F, L, ?]] =
new WriterTTraverse[F, L] {
val F0: Traverse[F] = F
}

implicit def catsDataFoldableForWriterTId[L](implicit F: Foldable[Id]): Foldable[WriterT[Id, L, ?]] =
catsDataFoldableForWriterT[Id, L](F)
}

private[data] sealed abstract class WriterTInstances1 extends WriterTInstances2 {
implicit def catsDataMonadErrorForWriterT[F[_], L, E](implicit F: MonadError[F, E], L: Monoid[L]): MonadError[WriterT[F, L, ?], E] =
new WriterTMonadError[F, L, E] {
implicit val F0: MonadError[F, E] = F
Expand Down Expand Up @@ -134,9 +159,14 @@ private[data] sealed abstract class WriterTInstances0 extends WriterTInstances1

implicit def catsDataMonoidForWriterTId[L:Monoid, V:Monoid]: Monoid[WriterT[Id, L, V]] =
catsDataMonoidForWriterT[Id, L, V]

implicit def catsDataFoldableForWriterT[F[_], L](implicit F: Foldable[F]): Foldable[WriterT[F, L, ?]] =
new WriterTFoldable[F, L]{
val F0: Foldable[F] = F
}
}

private[data] sealed abstract class WriterTInstances1 extends WriterTInstances2 {
private[data] sealed abstract class WriterTInstances2 extends WriterTInstances3 {
implicit def catsDataMonadForWriterTId[L:Monoid]: Monad[WriterT[Id, L, ?]] =
catsDataMonadForWriterT[Id, L]

Expand All @@ -145,9 +175,12 @@ private[data] sealed abstract class WriterTInstances1 extends WriterTInstances2

implicit def catsDataSemigroupForWriterTId[L:Semigroup, V:Semigroup]: Semigroup[WriterT[Id, L, V]] =
catsDataSemigroupForWriterT[Id, L, V]

implicit def catsDataComonadForWriterTId[L](implicit F: Comonad[Id]): Comonad[WriterT[Id, L, ?]] =
catsDataComonadForWriterT[Id, L](F)
}

private[data] sealed abstract class WriterTInstances2 extends WriterTInstances3 {
private[data] sealed abstract class WriterTInstances3 extends WriterTInstances4 {
implicit def catsDataMonadForWriterT[F[_], L](implicit F: Monad[F], L: Monoid[L]): Monad[WriterT[F, L, ?]] =
new WriterTMonad[F, L] {
implicit val F0: Monad[F] = F
Expand All @@ -163,12 +196,12 @@ private[data] sealed abstract class WriterTInstances2 extends WriterTInstances3
catsDataCoflatMapForWriterT[Id, L]
}

private[data] sealed abstract class WriterTInstances3 extends WriterTInstances4 {
private[data] sealed abstract class WriterTInstances4 extends WriterTInstances5 {
implicit def catsDataFlatMapForWriterTId[L:Semigroup]: FlatMap[WriterT[Id, L, ?]] =
catsDataFlatMapForWriterT2[Id, L]
}

private[data] sealed abstract class WriterTInstances4 extends WriterTInstances5 {
private[data] sealed abstract class WriterTInstances5 extends WriterTInstances6 {
implicit def catsDataFlatMapForWriterT1[F[_], L](implicit F: FlatMap[F], L: Monoid[L]): FlatMap[WriterT[F, L, ?]] =
new WriterTFlatMap1[F, L] {
implicit val F0: FlatMap[F] = F
Expand All @@ -181,15 +214,15 @@ private[data] sealed abstract class WriterTInstances4 extends WriterTInstances5
}
}

private[data] sealed abstract class WriterTInstances5 extends WriterTInstances6 {
private[data] sealed abstract class WriterTInstances6 extends WriterTInstances7 {
implicit def catsDataApplicativeErrorForWriterT[F[_], L, E](implicit F: ApplicativeError[F, E], L: Monoid[L]): ApplicativeError[WriterT[F, L, ?], E] =
new WriterTApplicativeError[F, L, E] {
implicit val F0: ApplicativeError[F, E] = F
implicit val L0: Monoid[L] = L
}
}

private[data] sealed abstract class WriterTInstances6 extends WriterTInstances7 {
private[data] sealed abstract class WriterTInstances7 extends WriterTInstances8 {
implicit def catsDataAlternativeForWriterT[F[_], L](implicit F: Alternative[F], L: Monoid[L]): Alternative[WriterT[F, L, ?]] =
new WriterTAlternative[F, L] {
implicit val F0: Alternative[F] = F
Expand All @@ -202,7 +235,7 @@ private[data] sealed abstract class WriterTInstances6 extends WriterTInstances7
}
}

private[data] sealed abstract class WriterTInstances7 extends WriterTInstances8 {
private[data] sealed abstract class WriterTInstances8 extends WriterTInstances9 {
implicit def catsDataMonoidKForWriterT[F[_], L](implicit F: MonoidK[F]): MonoidK[WriterT[F, L, ?]] =
new WriterTMonoidK[F, L] {
implicit val F0: MonoidK[F] = F
Expand All @@ -219,7 +252,7 @@ private[data] sealed abstract class WriterTInstances7 extends WriterTInstances8
}
}

private[data] sealed abstract class WriterTInstances8 extends WriterTInstances9 {
private[data] sealed abstract class WriterTInstances9 extends WriterTInstances10 {
implicit def catsDataSemigroupKForWriterT[F[_], L](implicit F: SemigroupK[F]): SemigroupK[WriterT[F, L, ?]] =
new WriterTSemigroupK[F, L] {
implicit val F0: SemigroupK[F] = F
Expand All @@ -230,18 +263,24 @@ private[data] sealed abstract class WriterTInstances8 extends WriterTInstances9
implicit val F0: Applicative[F] = F
implicit val L0: Monoid[L] = L
}

}

private[data] sealed abstract class WriterTInstances9 extends WriterTInstances10 {
private[data] sealed abstract class WriterTInstances10 extends WriterTInstances11 {
implicit def catsDataApplyForWriterT[F[_], L](implicit F: Apply[F], L: Semigroup[L]): Apply[WriterT[F, L, ?]] =
new WriterTApply[F, L] {
implicit val F0: Apply[F] = F
implicit val L0: Semigroup[L] = L
}
}

private[data] sealed abstract class WriterTInstances10 {
private[data] sealed abstract class WriterTInstances11 extends WriterTInstances12 {
implicit def catsDataComonadForWriterT[F[_], L](implicit F: Comonad[F]): Comonad[WriterT[F, L, ?]] =
new WriterTComonad[F, L] {
implicit val F0: Comonad[F] = F
}
}

private[data] sealed abstract class WriterTInstances12 {
implicit def catsDataCoflatMapForWriterT[F[_], L](implicit F: Functor[F]): CoflatMap[WriterT[F, L, ?]] =
new WriterTCoflatMap[F, L] {
implicit val F0: Functor[F] = F
Expand Down Expand Up @@ -410,6 +449,27 @@ private[data] sealed trait WriterTCoflatMap[F[_], L] extends CoflatMap[WriterT[F
def coflatMap[A, B](fa: WriterT[F, L, A])(f: WriterT[F, L, A] => B): WriterT[F, L, B] = fa.map(_ => f(fa))
}

private[data] sealed trait WriterTFoldable[F[_], L] extends Foldable[WriterT[F, L, ?]] {

implicit def F0: Foldable[F]

def foldLeft[A, B](fa: WriterT[F, L, A], b: B)(f: (B, A) => B): B = fa.foldLeft(b)(f)
def foldRight[A, B](fa: WriterT[F, L, A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] = fa.foldRight(lb)(f)
}

private[data] sealed trait WriterTTraverse[F[_], L] extends Traverse[WriterT[F, L, ?]] with WriterTFoldable[F, L] {
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If you extends WriterTFunctor here as well, you get its map implementation instead of the default traverse[Id] from Traverse.

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I will change it thanks 👍


override implicit def F0: Traverse[F]

def traverse[G[_]: Applicative, A, B](fa: WriterT[F, L, A])(f: A => G[B]): G[WriterT[F, L, B]] = fa.traverse(f)
}

private[data] sealed trait WriterTComonad[F[_], L] extends Comonad[WriterT[F, L, ?]] with WriterTCoflatMap[F, L] {

override implicit def F0: Comonad[F]

def extract[A](fa: WriterT[F, L, A]): A = F0.extract(F0.map(fa.run)(_._2))
}

private[data] trait WriterTFunctions {
def putT[F[_], L, V](vf: F[V])(l: L)(implicit functorF: Functor[F]): WriterT[F, L, V] =
Expand Down
48 changes: 48 additions & 0 deletions tests/src/test/scala/cats/tests/WriterTSuite.scala
Original file line number Diff line number Diff line change
Expand Up @@ -370,6 +370,54 @@ class WriterTSuite extends CatsSuite {
checkAll("ContravariantMonoidal[WriterT[Const[String, ?], Int, ?]]", SerializableTests.serializable(ContravariantMonoidal[WriterT[Const[String, ?], Int, ?]]))
}

{
// F has a Foldable and L has a Monoid
implicit val L: Monoid[ListWrapper[Int]] = ListWrapper.monoid[Int]
Foldable[Const[String, ?]]
Foldable[WriterT[Const[String, ?], ListWrapper[Int], ?]]

checkAll("WriterT[Const[String, ?], ListWrapper[Int], ?]", FoldableTests[WriterT[Const[String, ?], ListWrapper[Int], ?]].foldable[Int, Int])
checkAll("Foldable[WriterT[Const[String, ?], ListWrapper[Int], ?]]", SerializableTests.serializable(Foldable[WriterT[Const[String, ?], ListWrapper[Int], ?]]))

Foldable[Id]
Foldable[WriterT[Id, ListWrapper[Int], ?]]
Foldable[Writer[ListWrapper[Int], ?]]

checkAll("WriterT[Id, ListWrapper[Int], ?]", FoldableTests[WriterT[Id, ListWrapper[Int], ?]].foldable[Int, Int])
}

{
// F has a Traverse and L has a Monoid
implicit val L: Monoid[ListWrapper[Int]] = ListWrapper.monoid[Int]
Traverse[Const[String, ?]]
Traverse[WriterT[Const[String, ?], ListWrapper[Int], ?]]

checkAll("WriterT[Const[String, ?], ListWrapper[Int], ?]", TraverseTests[WriterT[Const[String, ?], ListWrapper[Int], ?]].traverse[Int, Int, Int, Int, Option, Option])
checkAll("Traverse[WriterT[Const[String, ?], ListWrapper[Int], ?]]", SerializableTests.serializable(Traverse[WriterT[Const[String, ?], ListWrapper[Int], ?]]))

Traverse[Id]
Traverse[WriterT[Id, ListWrapper[Int], ?]]
Traverse[Writer[ListWrapper[Int], ?]]

checkAll("WriterT[Id, ListWrapper[Int], ?]", TraverseTests[WriterT[Id, ListWrapper[Int], ?]].traverse[Int, Int, Int, Int, Option, Option])
}

{
// F has a Comonad and L has a Monoid
implicit val L: Monoid[ListWrapper[Int]] = ListWrapper.monoid[Int]
Comonad[(String, ?)]
Comonad[WriterT[(String, ?), ListWrapper[Int], ?]]

checkAll("WriterT[(String, ?), ListWrapper[Int], ?]", ComonadTests[WriterT[(String, ?), ListWrapper[Int], ?]].comonad[Int, Int, Int])
checkAll("Comonad[WriterT[(String, ?), ListWrapper[Int], ?]]", SerializableTests.serializable(Comonad[WriterT[(String, ?), ListWrapper[Int], ?]]))

Comonad[Id]
Comonad[WriterT[Id, ListWrapper[Int], ?]]
Comonad[Writer[ListWrapper[Int], ?]]

checkAll("WriterT[Id, ListWrapper[Int], ?]", ComonadTests[WriterT[Id, ListWrapper[Int], ?]].comonad[Int, Int, Int])
}

checkAll("WriterT[Option, Int, ?]", CommutativeMonadTests[WriterT[Option, Int, ?]].commutativeMonad[Int, Int, Int])
checkAll("CommutativeMonad[WriterT[Option, Int, ?]]",SerializableTests.serializable(CommutativeMonad[WriterT[Option, Int, ?]]))
}