Merge pull request #1169 from zainab-ali/state

Adding lift and inspect to StateT
typelevel · Jul 13, 2016 · fe361a3 · fe361a3
2 parents 0d36ff3 + 90a2280
commit fe361a3
Show file tree

Hide file tree

Showing 2 changed files with 55 additions and 0 deletions.
diff --git a/core/src/main/scala/cats/data/StateT.scala b/core/src/main/scala/cats/data/StateT.scala
@@ -133,6 +133,21 @@ object StateT extends StateTInstances {
 
   def pure[F[_], S, A](a: A)(implicit F: Applicative[F]): StateT[F, S, A] =
     StateT(s => F.pure((s, a)))
+
+  def lift[F[_], S, A](fa: F[A])(implicit F: Applicative[F]): StateT[F, S, A] =
+    StateT(s => F.map(fa)(a => (s, a)))
+
+  def inspect[F[_], S, A](f: S => A)(implicit F: Applicative[F]): StateT[F, S, A] =
+    StateT(s => F.pure((s, f(s))))
+
+  def inspectF[F[_], S, A](f: S => F[A])(implicit F: Applicative[F]): StateT[F, S, A] =
+    StateT(s => F.map(f(s))(a => (s, a)))
+
+  def modify[F[_], S](f: S => S)(implicit F: Applicative[F]): StateT[F, S, Unit] =
+    StateT(s => F.pure((f(s), ())))
+
+  def modifyF[F[_], S](f: S => F[S])(implicit F: Applicative[F]): StateT[F, S, Unit] =
+    StateT(s => F.map(f(s))(s => (s, ())))
 }
 
 private[data] sealed trait StateTInstances extends StateTInstances1 {

diff --git a/tests/src/test/scala/cats/tests/StateTTests.scala b/tests/src/test/scala/cats/tests/StateTTests.scala
@@ -29,6 +29,46 @@ class StateTTests extends CatsSuite {
     }
   }
 
+  test("State.inspect and StateT.inspect are consistent") {
+    forAll { (s: String, f: String => Int) =>
+      val state: State[String, Int] = State.inspect(f)
+      val stateT: State[String, Int] = StateT.inspect(f)
+      state.run(s) should === (stateT.run(s))
+    }
+  }
+
+  test("State.inspect and StateT.inspectF are consistent") {
+    forAll { (s: String, f: String => Int) =>
+      val state: State[String, Int] = State.inspect(f)
+      val stateT: State[String, Int] = StateT.inspectF(f.andThen(Eval.now))
+      state.run(s) should === (stateT.run(s))
+    }
+  }
+
+  test("State.modify and StateT.modify are consistent") {
+    forAll { (s: String, f: String => String) =>
+      val state: State[String, Unit] = State.modify(f)
+      val stateT: State[String, Unit] = StateT.modify(f)
+      state.run(s) should === (stateT.run(s))
+    }
+  }
+
+  test("State.modify and StateT.modifyF are consistent") {
+    forAll { (s: String, f: String => String) =>
+      val state: State[String, Unit] = State.modify(f)
+      val stateT: State[String, Unit] = StateT.modifyF(f.andThen(Eval.now))
+      state.run(s) should === (stateT.run(s))
+    }
+  }
+
+  test("State.pure and StateT.lift are consistent") {
+    forAll { (s: String, i: Int) =>
+      val state: State[String, Int] = State.pure(i)
+      val stateT: State[String, Int] = StateT.lift(Eval.now(i))
+      state.run(s) should === (stateT.run(s))
+    }
+  }
+
   test("Cartesian syntax is usable on State") {
     val x = add1 *> add1
     x.runS(0).value should === (2)