From 34f5a2612ac06d1c3d2bbd2e27390ee0e46ade80 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?St=C3=A9phane=20Derosiaux?= <derste@gmail.com>
Date: Tue, 12 Jun 2018 22:07:42 +0200
Subject: [PATCH] Add empty to State and StateT objects

Co-authored-by: Lars Hupel <lars.hupel@mytum.de>
---
 core/src/main/scala/cats/data/IndexedStateT.scala    | 12 +++++++++++-
 core/src/main/scala/cats/data/package.scala          |  2 +-
 .../test/scala/cats/tests/IndexedStateTSuite.scala   | 11 +++++++++++
 3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/core/src/main/scala/cats/data/IndexedStateT.scala b/core/src/main/scala/cats/data/IndexedStateT.scala
index 1a9fe1278fb..c7c8721fc23 100644
--- a/core/src/main/scala/cats/data/IndexedStateT.scala
+++ b/core/src/main/scala/cats/data/IndexedStateT.scala
@@ -203,7 +203,7 @@ private[data] trait CommonStateTConstructors {
     IndexedStateT(s => F.pure((s, s)))
 }
 
-object IndexedStateT extends IndexedStateTInstances with CommonStateTConstructors {
+object IndexedStateT extends IndexedStateTInstances with CommonStateTConstructors0 {
   def apply[F[_], SA, SB, A](f: SA => F[(SB, A)])(implicit F: Applicative[F]): IndexedStateT[F, SA, SB, A] =
     new IndexedStateT(F.pure(f))
 
@@ -223,6 +223,11 @@ object IndexedStateT extends IndexedStateTInstances with CommonStateTConstructor
     IndexedStateT(_ => F.map(fsb)(s => (s, ())))
 }
 
+private[data] trait CommonStateTConstructors0 extends CommonStateTConstructors {
+  def empty[F[_], S, A](implicit A: Monoid[A], F: Applicative[F]): IndexedStateT[F, S, S, A] =
+    pure(A.empty)
+}
+
 private[data] abstract class StateTFunctions extends CommonStateTConstructors {
   def apply[F[_], S, A](f: S => F[(S, A)])(implicit F: Applicative[F]): StateT[F, S, A] =
     IndexedStateT(f)
@@ -299,6 +304,11 @@ private[data] abstract class StateFunctions {
    */
   def pure[S, A](a: A): State[S, A] = State(s => (s, a))
 
+  /**
+    * Return `A`'s empty monoid value and maintain the input state.
+    */
+  def empty[S, A: Monoid]: State[S, A] = pure(Monoid[A].empty)
+
   /**
    * Modify the input state and return Unit.
    */
diff --git a/core/src/main/scala/cats/data/package.scala b/core/src/main/scala/cats/data/package.scala
index 218300dbc00..77c60f0b8f4 100644
--- a/core/src/main/scala/cats/data/package.scala
+++ b/core/src/main/scala/cats/data/package.scala
@@ -49,7 +49,7 @@ package object data {
    * context along with the `A` value.
    */
   type StateT[F[_], S, A] = IndexedStateT[F, S, S, A]
-  object StateT extends StateTFunctions
+  object StateT extends StateTFunctions with CommonStateTConstructors0
 
   type State[S, A] = StateT[Eval, S, A]
   object State extends StateFunctions
diff --git a/tests/src/test/scala/cats/tests/IndexedStateTSuite.scala b/tests/src/test/scala/cats/tests/IndexedStateTSuite.scala
index 5134563a8cd..88589251c35 100644
--- a/tests/src/test/scala/cats/tests/IndexedStateTSuite.scala
+++ b/tests/src/test/scala/cats/tests/IndexedStateTSuite.scala
@@ -55,6 +55,17 @@ class IndexedStateTSuite extends CatsSuite {
     }
   }
 
+  test("State.empty, StateT.empty and IndexedStateT.empty are consistent"){
+    forAll { (s: String) =>
+      val state: State[String, Int] = State.empty
+      val stateT: State[String, Int] = StateT.empty
+      val indexedStateT: State[String, Int] = IndexedStateT.empty
+
+      state.run(s) should === (stateT.run(s))
+      state.run(s) should === (indexedStateT.run(s))
+    }
+  }
+
   test("State.get, StateT.get and IndexedStateT.get are consistent") {
     forAll{ (s: String) =>
       val state: State[String, String] = State.get