From 3e1a053d1f3a8b1d86972099429f297d34ba0e09 Mon Sep 17 00:00:00 2001
From: "Kai(luo) Wang" <kailuo.wang@gmail.com>
Date: Sat, 9 Dec 2017 22:37:25 -0500
Subject: [PATCH] Add  `Endo` type alias for (`A =>A`) (#2076)

* added reduceRightK and Endo

* added mima excpetion

* removed reduceRightK

* minor
---
 core/src/main/scala/cats/instances/function.scala    | 2 +-
 core/src/main/scala/cats/package.scala               | 1 +
 tests/src/test/scala/cats/tests/CategorySuite.scala  | 1 -
 tests/src/test/scala/cats/tests/ComposeSuite.scala   | 1 -
 tests/src/test/scala/cats/tests/FoldableSuite.scala  | 4 +++-
 tests/src/test/scala/cats/tests/ReducibleSuite.scala | 6 ++++--
 6 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/core/src/main/scala/cats/instances/function.scala b/core/src/main/scala/cats/instances/function.scala
index f6841aab70..19bbd678e9 100644
--- a/core/src/main/scala/cats/instances/function.scala
+++ b/core/src/main/scala/cats/instances/function.scala
@@ -88,7 +88,7 @@ private[instances] sealed trait Function1Instances extends Function1Instances0 {
       def compose[A, B, C](f: B => C, g: A => B): A => C = f.compose(g)
     }
 
-  implicit val catsStdMonoidKForFunction1: MonoidK[λ[α => Function1[α, α]]] =
+  implicit val catsStdMonoidKForFunction1: MonoidK[Endo] =
     Category[Function1].algebraK
 }
 
diff --git a/core/src/main/scala/cats/package.scala b/core/src/main/scala/cats/package.scala
index 46a24e9837..cab9ea844f 100644
--- a/core/src/main/scala/cats/package.scala
+++ b/core/src/main/scala/cats/package.scala
@@ -32,6 +32,7 @@ package object cats {
  * encodes pure unary function application.
  */
   type Id[A] = A
+  type Endo[A] = A => A
   implicit val catsInstancesForId: Bimonad[Id] with CommutativeMonad[Id] with Comonad[Id] with NonEmptyTraverse[Id] =
     new Bimonad[Id] with CommutativeMonad[Id] with Comonad[Id] with NonEmptyTraverse[Id] {
       def pure[A](a: A): A = a
diff --git a/tests/src/test/scala/cats/tests/CategorySuite.scala b/tests/src/test/scala/cats/tests/CategorySuite.scala
index d4e215d997..5eb98a484b 100644
--- a/tests/src/test/scala/cats/tests/CategorySuite.scala
+++ b/tests/src/test/scala/cats/tests/CategorySuite.scala
@@ -9,7 +9,6 @@ import cats.laws.discipline.eq.catsLawsEqForFn1
 
 class CategorySuite extends CatsSuite {
   val functionCategory = Category[Function1]
-  type Endo[A] = Function1[A, A]
 
   checkAll("Category[Function1].algebraK", MonoidKTests[Endo](functionCategory.algebraK).monoidK[Int])
   checkAll("Category[Function1].algebraK", SerializableTests.serializable(functionCategory.algebraK))
diff --git a/tests/src/test/scala/cats/tests/ComposeSuite.scala b/tests/src/test/scala/cats/tests/ComposeSuite.scala
index 10f839f893..1c6ca48343 100644
--- a/tests/src/test/scala/cats/tests/ComposeSuite.scala
+++ b/tests/src/test/scala/cats/tests/ComposeSuite.scala
@@ -8,7 +8,6 @@ import cats.laws.discipline.eq.catsLawsEqForFn1
 
 class ComposeSuite extends CatsSuite {
   val functionCompose = Compose[Function1]
-  type Endo[A] = Function1[A, A]
 
   checkAll("Compose[Function1].algebraK", SemigroupKTests[Endo](functionCompose.algebraK).semigroupK[Int])
   checkAll("Compose[Function1].algebraK", SerializableTests.serializable(functionCompose.algebraK))
diff --git a/tests/src/test/scala/cats/tests/FoldableSuite.scala b/tests/src/test/scala/cats/tests/FoldableSuite.scala
index 487e8f32f2..7928469ad0 100644
--- a/tests/src/test/scala/cats/tests/FoldableSuite.scala
+++ b/tests/src/test/scala/cats/tests/FoldableSuite.scala
@@ -9,7 +9,9 @@ import cats.instances.all._
 import cats.data._
 import cats.laws.discipline.arbitrary._
 
-abstract class FoldableSuite[F[_]: Foldable](name: String)(implicit ArbFInt: Arbitrary[F[Int]], ArbFString: Arbitrary[F[String]]) extends CatsSuite with PropertyChecks {
+abstract class FoldableSuite[F[_]: Foldable](name: String)(
+  implicit ArbFInt: Arbitrary[F[Int]],
+     ArbFString: Arbitrary[F[String]]) extends CatsSuite with PropertyChecks {
 
   def iterator[T](fa: F[T]): Iterator[T]
 
diff --git a/tests/src/test/scala/cats/tests/ReducibleSuite.scala b/tests/src/test/scala/cats/tests/ReducibleSuite.scala
index a92fdd25b5..b7d3afe4f5 100644
--- a/tests/src/test/scala/cats/tests/ReducibleSuite.scala
+++ b/tests/src/test/scala/cats/tests/ReducibleSuite.scala
@@ -2,7 +2,6 @@ package cats
 package tests
 
 import org.scalacheck.Arbitrary
-
 import cats.data.NonEmptyList
 
 class ReducibleSuiteAdditional extends CatsSuite {
@@ -71,7 +70,10 @@ class ReducibleSuiteAdditional extends CatsSuite {
 
 }
 
-abstract class ReducibleSuite[F[_]: Reducible](name: String)(implicit ArbFInt: Arbitrary[F[Int]], ArbFString: Arbitrary[F[String]]) extends FoldableSuite[F](name) {
+abstract class ReducibleSuite[F[_]: Reducible](name: String)(
+  implicit ArbFInt: Arbitrary[F[Int]],
+    ArbFString: Arbitrary[F[String]]) extends FoldableSuite[F](name) {
+
   def range(start: Long, endInclusive: Long): F[Long]
 
   test(s"Reducible[$name].reduceLeftM stack safety") {