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Adding lift and inspect to StateT #1169

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merged 3 commits into from
Jul 13, 2016
Merged

Adding lift and inspect to StateT #1169

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7d6551f
Adding lift and modify to StateT
zainab-ali Jun 29, 2016
d7c1bb5
Adding inspect instead of modify to StateT
zainab-ali Jun 29, 2016
90a2280
Adding StateT.modify
zainab-ali Jul 8, 2016
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6 changes: 6 additions & 0 deletions core/src/main/scala/cats/data/StateT.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -133,6 +133,12 @@ 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 => F[A])(implicit F: Applicative[F]): StateT[F, S, A] =
StateT(s => F.map(f(s))(a => (s, a)))
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Should we have inspect take S => A and create a separate inspectF for S => F[A]? It seems like the simple S => A case might be pretty common, so it might be nice to make it concise.

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Agreed. I'm thinking of adding a modify taking S => S and a modifyF taking S => F[S] while
I'm at it.

}

private[data] sealed trait StateTInstances extends StateTInstances1 {
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16 changes: 16 additions & 0 deletions tests/src/test/scala/cats/tests/StateTTests.scala
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Expand Up @@ -29,6 +29,22 @@ 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.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)
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