add unzip to Functor (#3062)

* added unzip method to Functor typeClass

* fixed formatting

* added @noop on unzip, to fix issue with Simulacrum-generated extension method

* remove exclusion from build.sbt

* updated unzip declaration to include implicit pair derivation, removed noop

* changed signature back to the original proposal

core/src/main/scala/cats/Functor.scala

@@ -1,6 +1,6 @@
 package cats
 
-import simulacrum.typeclass
+import simulacrum.{noop, typeclass}
 
 /**
  * Functor.
@@ -144,6 +144,23 @@ import simulacrum.typeclass
    */
   def tupleRight[A, B](fa: F[A], b: B): F[(A, B)] = map(fa)(a => (a, b))
 
+  /**
+   * Un-zips an `F[(A, B)]` consisting of element pairs or Tuple2 into two separate F's tupled.
+   *
+   * NOTE: Check for effect duplication, possibly memoize before
+   *
+   * {{{
+   * scala> import cats.Functor
+   * scala> import cats.implicits.catsStdInstancesForList
+   *
+   * scala> Functor[List].unzip(List((1,2), (3, 4)))
+   * res0: (List[Int], List[Int]) = (List(1, 3),List(2, 4))
+   * }}}
+   *
+   */
+  @noop
+  def unzip[A, B](fab: F[(A, B)]): (F[A], F[B]) = (map(fab)(_._1), map(fab)(_._2))
+
   /**
    * Lifts `if` to Functor
    *

tests/src/test/scala/cats/tests/FunctorSuite.scala

@@ -29,6 +29,14 @@ class FunctorSuite extends CatsSuite {
     }
   }
 
+  test("unzip preserves structure") {
+    forAll { (l: List[Int], o: Option[Int], m: Map[String, Int]) =>
+      Functor[List].unzip(l.map(i => (i, i))) === ((l, l))
+      Functor[Option].unzip(o.map(i => (i, i))) === ((o, o))
+      Functor[Map[String, *]].unzip(m.map { case (k, v) => (k, (v, v)) }) === ((m, m))
+    }
+  }
+
   test("widen equals map(identity)") {
     forAll { (i: Int) =>
       val list: List[Some[Int]] = List(Some(i))