Merge pull request #877 from adelbertc/prod-lazy

Remove required laziness in Prod, fixes #615

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

@@ -5,21 +5,10 @@ package data
  * [[Prod]] is a product to two independent functor values.
  *
  * See: [[https://www.cs.ox.ac.uk/jeremy.gibbons/publications/iterator.pdf The Essence of the Iterator Pattern]]
- */
-sealed trait Prod[F[_], G[_], A] {
-  def first: F[A]
-  def second: G[A]
-}
-object Prod extends ProdInstances {
-  def apply[F[_], G[_], A](first0: => F[A], second0: => G[A]): Prod[F, G, A] = new Prod[F, G, A] {
-    val firstThunk: Eval[F[A]] = Later(first0)
-    val secondThunk: Eval[G[A]] = Later(second0)
-    def first: F[A] = firstThunk.value
-    def second: G[A] = secondThunk.value
-  }
-  def unapply[F[_], G[_], A](x: Prod[F, G, A]): Option[(F[A], G[A])] =
-    Some((x.first, x.second))
-}
+  */
+final case class Prod[F[_], G[_], A](first: F[A], second: G[A])
+
+object Prod extends ProdInstances
 
 private[data] sealed abstract class ProdInstances extends ProdInstances0 {
   implicit def prodAlternative[F[_], G[_]](implicit FF: Alternative[F], GG: Alternative[G]): Alternative[Lambda[X => Prod[F, G, X]]] = new ProdAlternative[F, G] {