First-order effects admit an automatic definition of handle

[robrix]

To wit, handle state handler = coerce . fmap (handler . (<$ state)).

We should use this in the README for Teletype.

[mitchellwrosen]

This is possible to derive with DerivingVia:

newtype FirstOrderEffect
          (sig :: (Type -> Type) -> Type -> Type)
          (m :: Type -> Type)
          (k :: Type)
  = FirstOrderEffect (sig m k)
  deriving stock Functor

instance
     ( forall m. Functor (sig m)
     , forall m n a. Coercible (sig m a) (sig n a)
     )
  => HFunctor (FirstOrderEffect sig) where

  hmap _ =
    coerce

instance
     ( forall m n a. Coercible (sig m a) (sig n a)
     , forall m. Functor (sig m)
     )
  => Effect (FirstOrderEffect sig) where

  handle state handler =
    coerce . fmap (handler . (<$ state))

[mitchellwrosen]

data MyEffect a b c m k = ...
  deriving (HFunctor, Effect) via (FirstOrderEffect (MyEffect a b c))

[mitchellwrosen]

Whoops, ignore that, it's just an infinite loop... 😅 Ok, I fixed the original snippet, it works

[robrix]

Nice one! Unfortunately, the dependency on -XDerivingVia and -XQuantifiedConstraints puts it out of reach of some of our supported versions of ghc for a while. (Tho we could always use -XCPP or w/e to do this conditionally in 8.6+.)

Seems like we could also do this with -XDefaultSignatures, tho that would probably require -XQuantifiedConstraints too.