Bundling and superclassing

[daylily]

Two disadvantages of type-level-list-based effect systems are that they do not support

• Bundling, as in type MonadStateWriter s m = (MonadState s m, MonadWriter s m)
• Superclassing, as in class MonadGet r m => MonadReader r m

Some possibilities of solving this:

-- A pseudo effect representing a bundle of several effects
data Bundle (es :: [Effect]) :: Effect

-- This will stuck if the user uses a polymorphic effect type
-- Most of the times they don't, but it is possible
type family e :> es :: Constraint where
  Bundle es :> xs = es :>> xs
  e :> xs = (Elem e xs, Super e xs)

-- A possibly better way is to just modify the (:>) typeclass
class e :> es where
instance {-# INCOHERENT #-} xs :>> es => Bundle xs :> es
instance Elem e es => e :> es

class Super e es => Elem e es

-- Users can define custom instances of this typeclass 
-- to override the default behavior (no superclasses)
class Super e xs
instance {-# OVERLAPPABLE #-} Super e xs

[daylily]

Some initial tests show that while superclassing is easy to add as a nonbreaking change, bundling is largely breaking, bringing worse ergonomics.

[daylily]

Closed; wontfix