Add Semigroup and Monoid instances for Const

(cherry picked from commit 7ed5b32)

core/src/main/scala/scalaz/Const.scala

@@ -2,6 +2,20 @@ package scalaz
 
 final case class Const[A, B](getConst: A)
 
+private sealed trait ConstSemigroup[A, B] extends Semigroup[Const[A, B]] {
+  def A: Semigroup[A]
+
+  override def append(f1: Const[A, B], f2: => Const[A, B]): Const[A, B] =
+    Const(A.append(f1.getConst, f2.getConst))
+}
+
+private sealed trait ConstMonoid[A, B] extends Monoid[Const[A, B]] with ConstSemigroup[A, B] {
+  def A: Monoid[A]
+
+  override def zero: Const[A, B] =
+    Const(A.zero)
+}
+
 private sealed trait ConstFunctor[C] extends Functor[({type λ[α] = Const[C, α]})#λ]{
   override def map[A, B](fa: Const[C, A])(f: A => B): Const[C, B] = Const(fa.getConst)
 }
@@ -52,6 +66,11 @@ sealed abstract class ConstInstances0 extends ConstInstances1 {
     val OA: Equal[A] = implicitly
   }
 
+  implicit def constSemigroup[A: Semigroup, B]: Semigroup[Const[A, B]] =
+    new ConstSemigroup[A, B] {
+      val A: Semigroup[A] = implicitly
+    }
+
   implicit def constApply[C: Semigroup]: Apply[({type λ[α] = Const[C, α]})#λ] = new ConstApply[C] {
     val C: Semigroup[C] = implicitly
   }
@@ -62,6 +81,11 @@ sealed abstract class ConstInstances extends ConstInstances0 {
     val OA: Order[A] = implicitly
   }
 
+  implicit def constMonoid[A: Monoid, B]: Monoid[Const[A, B]] =
+    new ConstMonoid[A, B] {
+      val A: Monoid[A] = implicitly
+    }
+
   implicit def constApplicative[C: Monoid]: Applicative[({type λ[α] = Const[C, α]})#λ] = new ConstApplicative[C] {
     val C: Monoid[C] = implicitly
   }