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

[Ryan Ingram]

On 3/28/08, Dan Weston <westondan at imageworks.com> 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