Monad Transformer instances

[gvolpe]

I was working on some examples using Console and Monad Transformers and I figured it makes sense to provide default instances, given some F[_]: Applicative: Console].

[kubukoz]

I would normally just mapK when defining an instance (or use .stdio[EitherT[...]], but I guess there's nothing dangerous about providing inductive instances for transformers. Let's do it :)

[gvolpe]

It comes in handy when you already have the IO instance in scope using import cats.effect.Console.implicits._. You get the transformer instance for free :)

In my case, I have two programs in the same file:

def program[F[_]: Console](implicit M: MonadState[F, FooState]): F[Unit] =
  for {
    current <- M.get
    _ <- Console[F].putStrLn(current)
    _ <- M.set(FooState("foo"))
    updated <- M.get
    _ <- Console[F].putStrLn(updated)
  } yield ()

val p1: IO[Unit] =
  program[StateT[IO, FooState, *]].run(FooState("mt")).void

val p2: IO[Unit] =
  Ref.of[IO, FooState](FooState("bar")).flatMap { ref =>
    ref.runState { implicit ms =>
      program[IO]
    }
  }

In this way, we can be sure the underlying Console[IO] instance is the same one the transformer gets.