Drop noop for Functor unzip with Liskov evidence (#3318)

Co-authored-by: Gagandeep kalra <gagandeepkalra1994@gmail.com>
Co-authored-by: Lars Hupel <lars.hupel@mytum.de>

core/src/main/scala/cats/syntax/functor.scala

@@ -1,4 +1,64 @@
 package cats
 package syntax
 
-trait FunctorSyntax extends Functor.ToFunctorOps
+trait FunctorSyntax extends Functor.ToFunctorOps {
+  implicit final def catsSyntaxFunctorTuple2Ops[F[_], A, B](fab: F[(A, B)]): FunctorTuple2Ops[F, A, B] =
+    new FunctorTuple2Ops[F, A, B](fab)
+}
+
+final class FunctorTuple2Ops[F[_], A, B](private val fab: F[(A, B)]) extends AnyVal {
+
+  /**
+   * Lifts `Tuple2#_1` to Functor
+   *
+   * {{{
+   * scala> import cats.data.Chain
+   * scala> import cats.syntax.functor._
+   *
+   * scala> Chain((1, 2), (3, 4), (5, 6))._1F == Chain(1, 3, 5)
+   * res0: Boolean = true
+   * }}}
+   */
+  def _1F(implicit F: Functor[F]): F[A] = F.map(fab)(_._1)
+
+  /**
+   * Lifts `Tuple2#_2` to Functor
+   *
+   * {{{
+   * scala> import cats.data.Chain
+   * scala> import cats.syntax.functor._
+   *
+   * scala> Chain((1, 2), (3, 4), (5, 6))._2F == Chain(2, 4, 6)
+   * res0: Boolean = true
+   * }}}
+   */
+  def _2F(implicit F: Functor[F]): F[B] = F.map(fab)(_._2)
+
+  /**
+   * Lifts `Tuple2#swap` to Functor
+   *
+   * {{{
+   * scala> import cats.data.Chain
+   * scala> import cats.syntax.functor._
+   *
+   * scala> Chain((1, 2), (3, 4), (5, 6)).swapF == Chain((2, 1), (4, 3), (6, 5))
+   * res0: Boolean = true
+   * }}}
+   */
+  def swapF(implicit F: Functor[F]): F[(B, A)] = F.map(fab)(_.swap)
+
+  /**
+   * 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.data.Chain
+   * scala> import cats.syntax.functor._
+   *
+   * scala> Chain((1, 2), (3, 4), (5, 6)).unzip == ((Chain(1, 3, 5), Chain(2, 4, 6)))
+   * res0: Boolean = true
+   * }}}
+   */
+  def unzip(implicit F: Functor[F]): (F[A], F[B]) = F.unzip(fab)
+}

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

@@ -2,6 +2,8 @@ package cats.tests
 
 import cats.Functor
 import cats.syntax.functor._
+import cats.data.{NonEmptyList, NonEmptyMap}
+import cats.laws.discipline.arbitrary._
 import cats.syntax.eq._
 import org.scalacheck.Prop._
 
@@ -34,10 +36,33 @@ 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))
+    forAll { (nel: NonEmptyList[Int], o: Option[Int], nem: NonEmptyMap[String, Int]) =>
+      val l = nel.toList
+      val m = nem.toSortedMap
+
+      assert(Functor[List].unzip(l.map(i => (i, i))) === ((l, l)))
+      assert(Functor[Option].unzip(o.map(i => (i, i))) === ((o, o)))
+      assert(Functor[Map[String, *]].unzip(m.map { case (k, v) => (k, (v, v)) }) === ((m, m)))
+
+      //postfix test for Cats datatypes
+      assert(nel.map(i => (i, i)).unzip === ((nel, nel)))
+      assert(nem.map(v => (v, v)).unzip === ((nem, nem)))
+    }
+
+    //empty test for completeness
+    val emptyL = List.empty[Int]
+    val emptyM = Map.empty[String, Int]
+
+    assert(Functor[List].unzip(List.empty[(Int, Int)]) === ((emptyL, emptyL)))
+    assert(Functor[Map[String, *]].unzip(Map.empty[String, (Int, Int)]) === ((emptyM, emptyM)))
+  }
+
+  test("_1F, _2F and swapF form correct lists for concrete list of tuples") {
+    forAll { (l: List[(Int, Int)]) =>
+      val (l1, l2) = l.unzip
+      assertEquals(l._1F, l1)
+      assertEquals(l._2F, l2)
+      assertEquals(l.swapF, l2.zip(l1))
     }
   }