Merge pull request #1388 from tixxit/topic/statet-set

Add StateT.set and StateT.setF.

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

@@ -150,6 +150,12 @@ object StateT extends StateTInstances {
 
   def modifyF[F[_], S](f: S => F[S])(implicit F: Applicative[F]): StateT[F, S, Unit] =
     StateT(s => F.map(f(s))(s => (s, ())))
+
+  def set[F[_], S](s: S)(implicit F: Applicative[F]): StateT[F, S, Unit] =
+    StateT(_ => F.pure((s, ())))
+
+  def setF[F[_], S](fs: F[S])(implicit F: Applicative[F]): StateT[F, S, Unit] =
+    StateT(_ => F.map(fs)(s => (s, ())))
 }
 
 private[data] sealed trait StateTInstances extends StateTInstances1 {

tests/src/test/scala/cats/tests/StateTTests.scala

@@ -69,6 +69,22 @@ class StateTTests extends CatsSuite {
     }
   }
 
+  test("State.set and StateT.set are consistent") {
+    forAll { (init: String, s: String) =>
+      val state: State[String, Unit] = State.set(s)
+      val stateT: StateT[Eval, String, Unit] = StateT.set(s)
+      state.run(init) should === (stateT.run(init))
+    }
+  }
+
+  test("State.set and StateT.setF are consistent") {
+    forAll { (init: String, s: String) =>
+      val state: State[String, Unit] = State.set(s)
+      val stateT: StateT[Eval, String, Unit] = StateT.setF(Eval.now(s))
+      state.run(init) should === (stateT.run(init))
+    }
+  }
+
   test("Cartesian syntax is usable on State") {
     val x = add1 *> add1
     x.runS(0).value should === (2)
@@ -124,6 +140,22 @@ class StateTTests extends CatsSuite {
     }
   }
 
+  test("StateT.set equivalent to modify ignoring first param") {
+    forAll { (init: String, update: String) =>
+      val s1 = StateT.modify[Eval, String](_ => update)
+      val s2 = StateT.set[Eval, String](update)
+      s1.run(init) should === (s2.run(init))
+    }
+  }
+
+  test("StateT.setF equivalent to modifyF ignoring first param") {
+    forAll { (init: String, update: String) =>
+      val s1 = StateT.modifyF[Eval, String](_ => Eval.now(update))
+      val s2 = StateT.setF(Eval.now(update))
+      s1.run(init) should === (s2.run(init))
+    }
+  }
+
   test(".get and then .run produces same state as value"){
     forAll { (s: State[Long, Int], initial: Long) =>
       val (finalS, finalA) = s.get.run(initial).value