Re: [Haskell-cafe] Monad instance for Data.Set, again

On 3/28/08, Dan Weston wrote:

I'm having trouble embedding unconstrained monads into the NewMonad:

Is there some trick (e.g. newtype boxing/unboxing) to get all the unconstrained monads automatically instanced? Then the do notation could be presumably remapped to the new class structure.

The usual trick here is to use newtypes. (Yes, it sucks)

newtype OldMonad m = OldMonad m unwrapMonad :: OldMonad m -> m unwrapMonad (OldMonad m) = m

instance Monad m => Suitable (OldMonad m) v where data Constraints (OldMonad m) v = NoConstraints constraints _ = NoConstraints instance Monad m => NewMonad (OldMonad m) where newReturn = OldMonad . return newBind x k = OldMonad $ unwrapMonad x >>= unwrapMonad . k