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Add Foldable, Traverse and Comonad instances to WriterT (#2182)
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* Added foldLeft and foldRight to WriterT and Foldable instance

* Added tests for Foldable instance

* Added traverse. Fixed priority of the implicit search for isntances

* Added traverse implementation to WriterT

* Added test for WriterT Traverse

* Refactored names

* Changed priority of Traverse instance

* Added comonad instance for WrtiterT and Const. Added tests

* Completed Comonad tests using tuple instead of Const

* Refactored extract derivation to avoid implicit search

* Added instances and reolution tests for Id. Removed unneeded type
parameters.

* Minor refactoring

* Attempt to increase coverage

* Removed unneeded serialization test

* Refactored traverse derivation

* Fixed improper inheritance of WriterTFunctor
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@barambani @kailuowang
barambani authored and kailuowang committed Mar 12, 2018
1 parent 2ee184e commit 23e0a72
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Showing 2 changed files with 121 additions and 13 deletions.
86 changes: 73 additions & 13 deletions core/src/main/scala/cats/data/WriterT.scala
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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.tupleLeft(f(lv._2), lv._1))
)(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, ?]] =
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 All @@ -251,7 +290,7 @@ private[data] sealed abstract class WriterTInstances10 {
private[data] sealed trait WriterTFunctor[F[_], L] extends Functor[WriterT[F, L, ?]] {
implicit def F0: Functor[F]

def map[A, B](fa: WriterT[F, L, A])(f: A => B): WriterT[F, L, B] =
override def map[A, B](fa: WriterT[F, L, A])(f: A => B): WriterT[F, L, B] =
fa.map(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] with WriterTFunctor[F, L] {

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
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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, ?]]))
}

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