Move contravariant instances to separate trait, in accordance with (t…

…ypelevel#1659)
LukaJCB · Sep 8, 2017 · f645496 · f645496
1 parent bf96364
commit f645496
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diff --git a/core/src/main/scala/cats/instances/all.scala b/core/src/main/scala/cats/instances/all.scala
@@ -11,6 +11,7 @@ trait AllInstances
   with    EquivInstances
   with    FunctionInstances
   with    FutureInstances
+  with    HashInstances
   with    ListInstances
   with    MapInstances
   with    MonoidInstances

diff --git a/core/src/main/scala/cats/instances/hash.scala b/core/src/main/scala/cats/instances/hash.scala
@@ -0,0 +1,18 @@
+package cats
+package instances
+
+import cats.functor._
+
+/**
+ * @author Tongfei Chen
+ */
+trait HashInstances {
+  implicit val catsFunctorContravariantForHash: Contravariant[Hash] =
+    new Contravariant[Hash] {
+      /** Derive an `Hash` for `B` given an `Hash[A]` and a function `B => A`.
+       *
+       * Note: resulting instances are law-abiding only when the functions used are injective (represent a one-to-one mapping)
+       */
+      def contramap[A, B](fa: Hash[A])(f: B => A): Hash[B] = Hash.by(f)(fa)
+    }
+}