Add Representable instance for Eval (#2329)

* Add Representable instance for Eval

* Change F from def to val

core/src/main/scala/cats/Eval.scala

@@ -422,6 +422,22 @@ private[cats] sealed abstract class EvalInstances extends EvalInstances0 {
 
   implicit def catsGroupForEval[A: Group]: Group[Eval[A]] =
     new EvalGroup[A] { val algebra: Group[A] = Group[A] }
+
+  implicit val catsRepresentableForEval: Representable.Aux[Eval, Unit] = new Representable[Eval] {
+    override type Representation = Unit
+
+    override val F: Functor[Eval] = Functor[Eval]
+
+    /**
+     * Create a function that "indexes" into the `F` structure using `Representation`
+     */
+    override def index[A](f: Eval[A]): Unit => A = (_: Unit) => f.value
+
+    /**
+     * Reconstructs the `F` structure using the index function
+     */
+    override def tabulate[A](f: Unit => A): Eval[A] = Eval.later(f(()))
+  }
 }
 
 private[cats] sealed abstract class EvalInstances0 extends EvalInstances1 {

tests/src/test/scala/cats/tests/RepresentableSuite.scala

@@ -4,7 +4,7 @@ import cats.laws.discipline.SemigroupalTests.Isomorphisms
 import cats.laws.discipline.arbitrary._
 import cats.laws.discipline.eq._
 import cats.laws.discipline.{BimonadTests, MonadTests, RepresentableTests, SerializableTests}
-import cats.{Bimonad, Eq, Id, Representable}
+import cats.{Bimonad, Eq, Eval, Id, Representable}
 import org.scalacheck.Arbitrary
 import cats.data.Kleisli
 
@@ -21,6 +21,8 @@ class RepresentableSuite extends CatsSuite {
   checkAll("Pair[String, String] <-> Boolean => String", RepresentableTests[Pair, Boolean].representable[String])
   checkAll("Representable[Pair]", SerializableTests.serializable(Representable[Pair]))
 
+  checkAll("Eval[Int] <-> Unit => Int", RepresentableTests[Eval, Unit].representable[Int])
+  checkAll("Representable[Eval]", SerializableTests.serializable(Representable[Eval]))
 
   {
     implicit val representableKleisliPair = Kleisli.catsDataRepresentableForKleisli[Pair, Boolean, String]