Adding CoflatMap and tests to WriterT and fixed laws for CoflatMap

core/src/main/scala/cats/data/WriterT.scala

@@ -87,8 +87,7 @@ private[data] sealed abstract class WriterTInstances0 extends WriterTInstances1
       implicit val L0: Monoid[L] = L
     }
 
-  implicit def writerTIdFunctor[L]: Functor[WriterT[Id, L, ?]] =
-    writerTFunctor[Id, L]
+  def writerTIdFunctor[L]: Functor[WriterT[Id, L, ?]] = writerTIdCoflatMap
 
   implicit def writerTIdFlatMap[L:Semigroup]: FlatMap[WriterT[Id, L, ?]] =
     writerTFlatMap[Id, L]
@@ -111,7 +110,11 @@ private[data] sealed abstract class WriterTInstances1 extends WriterTInstances2
     new WriterTMonoid[F, L, V] {
       implicit val F0: Monoid[F[(L, V)]] = W
     }
+
+  implicit def writerTIdCoflatMap[L]: CoflatMap[WriterT[Id, L, ?]] = 
+    writerTCoflatMap[Id, L]
 }
+
 private[data] sealed abstract class WriterTInstances2 extends WriterTInstances3 {
   implicit def writerTMonadWriter[F[_], L](implicit F: Monad[F], L: Monoid[L]): MonadWriter[WriterT[F, L, ?], L] =
     new WriterTMonadWriter[F, L] {
@@ -131,6 +134,7 @@ private[data] sealed abstract class WriterTInstances3 extends WriterTInstances4
       implicit val F0: Alternative[F] = F
       implicit val L0: Monoid[L] = L
     }
+
 }
 
 private[data] sealed abstract class WriterTInstances4 extends WriterTInstances5 {
@@ -168,7 +172,10 @@ private[data] sealed abstract class WriterTInstances6 extends WriterTInstances7
 }
 
 private[data] sealed abstract class WriterTInstances7 {
-  implicit def writerTFunctor[F[_], L](implicit F: Functor[F]): Functor[WriterT[F, L, ?]] = new WriterTFunctor[F, L] {
+
+  def writerTFunctor[F[_], L](implicit F: Functor[F]): Functor[WriterT[F, L, ?]] = writerTCoflatMap
+
+  implicit def writerTCoflatMap[F[_], L](implicit F: Functor[F]): WriterTCoflatMap[F, L] = new WriterTCoflatMap[F, L] {
     implicit val F0: Functor[F] = F
   }
 }
@@ -259,12 +266,18 @@ private[data] sealed trait WriterTSemigroup[F[_], L, A] extends Semigroup[Writer
     WriterT(F0.combine(x.run, y.run))
 }
 
-private[data] sealed trait WriterTMonoid[F[_], L, A] extends Monoid[WriterT[F, L, A]] with WriterTSemigroup[F, L, A]{
+private[data] sealed trait WriterTMonoid[F[_], L, A] extends Monoid[WriterT[F, L, A]] with WriterTSemigroup[F, L, A] {
   override implicit def F0: Monoid[F[(L, A)]]
 
   def empty: WriterT[F, L, A] = WriterT(F0.empty)
 }
 
+private[data] sealed trait WriterTCoflatMap[F[_], L] extends CoflatMap[WriterT[F, L, ?]] with WriterTFunctor[F, L] {
+
+  def coflatMap[A, B](fa: WriterT[F, L, A])(f: WriterT[F, L, A] => B): WriterT[F, L, B] = fa.map(_ => f(fa))
+}
+
+
 trait WriterTFunctions {
   def putT[F[_], L, V](vf: F[V])(l: L)(implicit functorF: Functor[F]): WriterT[F, L, V] =
     WriterT(functorF.map(vf)(v => (l, v)))
@@ -281,6 +294,3 @@ trait WriterTFunctions {
   def valueT[F[_], L, V](vf: F[V])(implicit functorF: Functor[F], monoidL: Monoid[L]): WriterT[F, L, V] =
     WriterT.putT[F, L, V](vf)(monoidL.empty)
 }
-
-
-

laws/src/main/scala/cats/laws/discipline/CoflatMapTests.scala

@@ -7,18 +7,25 @@ import org.scalacheck.Prop
 import Prop._
 import org.typelevel.discipline.Laws
 
-trait CoflatMapTests[F[_]] extends Laws {
+trait CoflatMapTests[F[_]] extends Laws with FunctorTests[F] {
   def laws: CoflatMapLaws[F]
 
   def coflatMap[A: Arbitrary, B: Arbitrary, C: Arbitrary](implicit
     ArbFA: Arbitrary[F[A]],
     EqFA: Eq[F[A]],
-    EqFC: Eq[F[C]]
+    EqFC: Eq[F[C]],
+    EqFFA: Eq[F[F[A]]],
+    EqFB: Eq[F[B]],
+    EqFFFA: Eq[F[F[F[A]]]]
   ): RuleSet = {
     new DefaultRuleSet(
       name = "coflatMap",
-      parent = None,
-      "coflatMap associativity" -> forAll(laws.coflatMapAssociativity[A, B, C] _))
+      parent = Some(functor[A, B, C]),
+      "coflatMap associativity" -> forAll(laws.coflatMapAssociativity[A, B, C] _),
+      "coflatMap identity" -> forAll(laws.coflatMapIdentity[A, B] _),
+      "coflatten coherence" -> forAll(laws.coflattenCoherence[A, B] _),
+      "coflatten throughMap" -> forAll(laws.coflattenThroughMap[A] _)
+    )
   }
 }
 

tests/src/test/scala/cats/tests/WriterTTests.scala

@@ -268,4 +268,15 @@ class WriterTTests extends CatsSuite {
 
     Semigroup[WriterT[Id, Int, Int]]
   }
+
+  {
+    // F has a functor
+    implicit val F: Functor[ListWrapper] = ListWrapper.functor
+
+    CoflatMap[WriterT[ListWrapper, Int, ?]]
+    checkAll("WriterT[Listwrapper, Int, ?]", CoflatMapTests[WriterT[ListWrapper, Int, ?]].coflatMap[Int, Int, Int])
+
+    // Id has a Functor
+    CoflatMap[WriterT[Id, Int, ?]]
+  }
 }