Alternative instance MonadState s (ContT r m) about mtl HOT 5 CLOSED

This would violate user expectation somewhat severely.

from mtl.

Just as a datapoint, it happened to be the instance I wanted, inasmuch as MonadReader is "simpler" than MonadState: indeed you can kinda/sorta make MonadReader a superclass of MonadState, though I'm not actually proposing this.

instance MonadReader s m where
    ask = get
    -- | imperfect, but you get the idea
    local ss ma = do
        s <- get
        put (ss s)
        a <- ma
        put s
        return a

from mtl.

Reader and State do have a sort of incestuous relationship.

Codensity (Reader s) ~ State s, because State is made up of the composition of Reader s and its adjoint functor: (, s). Using this you can always embed reader effects into State.

In an almost equally universal manner you can embed writer effects into state, but there are some minor complications on that front, and while the reader embedding costs you commutativity the latter costs you productivity.

from mtl.

Would it be worth the trouble to come up with a ContT wrapper offering this instance?

from mtl.

Possibly. If so it should belong in transformers though, not mtl, and then mtl would inherit support for it.

from mtl.