typelevel/cats-mtl

Port PrivateInstances from meow-mtl?

cb372 opened this issue · 5 comments

cb372 commented

I always find it a bit surprising that e.g. evidence of MonadState does not provide evidence of Monad.

So if I want to write

def foo[F[_]](...)(implicit F: MonadState[F, MyState]) =
  for {
    a <- firstThing()
    b <- secondThing(a)
  } yield b

then I have to add a : Monad context bound. But that feels redundant - it's a MonadState, so of course it's a Monad!

If you're using meow-mtl, this can be fixed by importing com.olegpy.meow.prelude._, thanks to PrivateInstances.

Is there a good reason not to make that functionality available out of the box in cats-mtl?

They are using shapeless LowPriority, which implies there might be cases where you would want your Monad[F] evidence to be different from the val monad: Monad[F] member of MonadState, but I can't think of a use case where I would want that. Maybe I'm missing something.

LukaJCB commented

Can we provide this without the shapeless dependency? We could publish it as a different module, but I'm not sure how useful that is vs. just using meow-mtl.

cb372 commented

Well we could just get rid of the implicit LP: LowPriority, but I’m sure it must be there for a good reason

catostrophe commented

What if you need F[_] : MonadState[*[_], S] : MonadError[*[_], E]?

catostrophe commented

I thought the whole idea of using scato encoding was for allowing one to have several mtl bounds with the same root typeclass without conflicts.

LukaJCB commented

I think without LowPriority you would get ambiguities. So I'm not sure how feasible this is unfortunately.