Merge pull request #875 from ceedubs/state-get

Add .get method to StateT
typelevel · Feb 17, 2016 · 82b6650 · 82b6650
2 parents 769efe7 + c081bdb
commit 82b6650
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diff --git a/core/src/main/scala/cats/data/StateT.scala b/core/src/main/scala/cats/data/StateT.scala
@@ -108,6 +108,11 @@ final class StateT[F[_], S, A](val runF: F[S => F[(S, A)]]) extends Serializable
   def inspect[B](f: S => B)(implicit F: Monad[F]): StateT[F, S, B] =
     transform((s, _) => (s, f(s)))
 
+  /**
+    * Get the input state, without modifying the state.
+    */
+  def get(implicit F: Monad[F]): StateT[F, S, S] =
+    inspect(identity)
 }
 
 object StateT extends StateTInstances {

diff --git a/tests/src/test/scala/cats/tests/StateTTests.scala b/tests/src/test/scala/cats/tests/StateTTests.scala
@@ -77,6 +77,19 @@ class StateTTests extends CatsSuite {
     }
   }
 
+  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
+      finalS should === (finalA)
+    }
+  }
+
+  test(".get equivalent to flatMap with State.get"){
+    forAll { (s: State[Long, Int]) =>
+      s.get should === (s.flatMap(_ => State.get))
+    }
+  }
+
   test("StateT#transformS with identity is identity") {
     forAll { (s: StateT[List, Long, Int]) =>
       s.transformS[Long](identity, (s, i) => i) should === (s)