Merge pull request #4805 from satorg/traverse-foldMap-consistency

Add `traverse` and `foldMap` consistency rule

Author: satorg

laws/src/main/scala/cats/laws/TraverseLaws.scala

@@ -166,6 +166,9 @@ trait TraverseLaws[F[_]] extends FunctorLaws[F] with FoldableLaws[F] with Unorde
 
     lhs <-> rhs
   }
+
+  def traverseConsistentFoldMap[A, B: Monoid](fa: F[A], fn: A => B): IsEq[B] =
+    F.traverse(fa)(a => Const[B, B](fn(a))).getConst <-> F.foldMap(fa)(fn)
 }
 
 object TraverseLaws {

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

@@ -82,7 +82,8 @@ trait TraverseTests[F[_]] extends FunctorTests[F] with FoldableTests[F] with Uno
         "traverse ref zipWithLongIndex" -> forAll(laws.zipWithLongIndexRef[A, C] _),
         "traverse ref mapWithLongIndex" -> forAll(laws.mapWithLongIndexRef[A, C] _),
         "traverse ref traverseWithLongIndexM" -> forAll(laws.traverseWithLongIndexMRef[Option, A, C] _),
-        "traverse ref updated" -> forAll(laws.updatedRef[A, A](_, _, _))
+        "traverse ref updated" -> forAll(laws.updatedRef[A, A](_, _, _)),
+        "traverse consistent with foldMap" -> forAll(laws.traverseConsistentFoldMap[A, M](_, _))
       )
   }
 }