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Add Foldable, Traverse and Comonad instances to WriterT #2182

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merged 16 commits into from
Mar 12, 2018
Merged

Add Foldable, Traverse and Comonad instances to WriterT #2182

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0f3b16c
Added foldLeft and foldRight to WriterT and Foldable instance
barambani Mar 3, 2018
4d1d7f7
Added tests for Foldable instance
barambani Mar 3, 2018
241f213
Added traverse. Fixed priority of the implicit search for isntances
barambani Mar 4, 2018
e68849d
Added traverse implementation to WriterT
barambani Mar 4, 2018
6729608
Added test for WriterT Traverse
barambani Mar 4, 2018
2a7f5f2
Refactored names
barambani Mar 5, 2018
454cc8f
Changed priority of Traverse instance
barambani Mar 5, 2018
4cf8304
Added comonad instance for WrtiterT and Const. Added tests
barambani Mar 7, 2018
0c2c002
Completed Comonad tests using tuple instead of Const
barambani Mar 7, 2018
e63a7e3
Refactored extract derivation to avoid implicit search
barambani Mar 7, 2018
fba0f7a
Added instances and reolution tests for Id. Removed unneeded type
barambani Mar 9, 2018
2e4b2b5
Minor refactoring
barambani Mar 9, 2018
fd077c3
Attempt to increase coverage
barambani Mar 9, 2018
a40e9f7
Removed unneeded serialization test
barambani Mar 9, 2018
b3ab664
Refactored traverse derivation
barambani Mar 12, 2018
152ee7a
Fixed improper inheritance of WriterTFunctor
barambani Mar 12, 2018
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59 changes: 48 additions & 11 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.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 +106,21 @@ private[data] sealed abstract class WriterTInstances extends WriterTInstances0 {
implicit val F0: Monad[F] = F
implicit val L0: Monoid[L] = L
}

implicit def catsDataFoldableForWriterT[F[_], L, V](implicit F: Foldable[F]): Foldable[WriterT[F, L, ?]] =
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The Traverse instance is more specific and thus should have a higher priority than the Foldable instance.
In case you are curious, here is the guideline in this regard.
https://typelevel.org/cats/guidelines.html#a-idimplicit-priority-hrefimplicit-prioritya-implicit-instance-priority
I guess you can leave the instance of Traverse where you have now and move the Foldable instance down to WriterTInstance1

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thanks, makes perfectly sense. I wasn't sure about the precedence there, I should have read the guidelines myself. I'm changing it.

new WriterTFoldable[F, L]{
val F0: Foldable[F] = F
}
}

private[data] sealed abstract class WriterTInstances0 extends WriterTInstances1 {
implicit def catsDataTraverseForWriterT[F[_], L, V](implicit F: Traverse[F]): Traverse[WriterT[F, L, ?]] =
new WriterTTraverse[F, L] {
val F0: Traverse[F] = 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 @@ -136,7 +159,7 @@ private[data] sealed abstract class WriterTInstances0 extends WriterTInstances1
catsDataMonoidForWriterT[Id, L, V]
}

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 @@ -147,7 +170,7 @@ private[data] sealed abstract class WriterTInstances1 extends WriterTInstances2
catsDataSemigroupForWriterT[Id, L, V]
}

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 +186,12 @@ 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 +204,15 @@ 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 +225,7 @@ 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 +242,7 @@ 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 @@ -233,15 +256,15 @@ private[data] sealed abstract class WriterTInstances8 extends WriterTInstances9

}

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 {
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 +433,20 @@ 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 👍


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] trait WriterTFunctions {
def putT[F[_], L, V](vf: F[V])(l: L)(implicit functorF: Functor[F]): WriterT[F, L, V] =
Expand Down
18 changes: 18 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 +370,24 @@ 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[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], ?]]))
}

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

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