A method StateT.fromState turning State[A, F[B]] into StateT[F,A, B] is added. (#3524)

* A method `StateT.fromState` turning `State[A, F[B]]` into `StateT[F, A, B]` is added.

* Usage of Applicative typeclass is desugared in the method `StateT.fromState` for the sake of consistency with the rest of the file.

Author: akopich

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

@@ -199,6 +199,15 @@ private[data] trait CommonStateTConstructors {
 
   def get[F[_], S](implicit F: Applicative[F]): IndexedStateT[F, S, S, S] =
     IndexedStateT(s => F.pure((s, s)))
+
+  /**
+   * Turn `State[A, F[B]]` into `StateT[F, A, B]`
+   */
+  def fromState[F[_], A, B](s: State[A, F[B]])(implicit F: Applicative[F]): StateT[F, A, B] =
+    s.transformF { eval =>
+      val (a, fb) = eval.value
+      F.map(fb)((a, _))
+    }
 }
 
 object IndexedStateT extends IndexedStateTInstances with CommonStateTConstructors0 {

tests/src/test/scala/cats/tests/IndexedStateTSuite.scala

@@ -324,6 +324,17 @@ class IndexedStateTSuite extends CatsSuite {
     }
   }
 
+  test("fromState correctly turns State[A, F[B]] into StateT[F, A, B]") {
+    val state: State[Int, Option[Int]] = add1.map(Some.apply)
+    import cats.implicits.catsStdInstancesForOption
+    forAll { (initial: Int) =>
+      StateT.fromState(state).run(initial).get should === {
+        val (s, Some(result)) = state.run(initial).value
+        (s, result)
+      }
+    }
+  }
+
   implicit val iso: Isomorphisms[IndexedStateT[ListWrapper, String, Int, *]] =
     Isomorphisms.invariant[IndexedStateT[ListWrapper, String, Int, *]](
       IndexedStateT.catsDataFunctorForIndexedStateT(ListWrapper.monad)