Re: [Haskell-cafe] Improving MTL instances (was: Overlapping/Incoherent instances)

On Mon, Oct 13, 2008 at 3:29 AM, Ryan Ingram wrote:

On Mon, Oct 13, 2008 at 2:04 AM, J. Garrett Morris wrote:

...

Indeed - MTL seems to have been rewritten at some point in the past to prefer exhaustive enumeration to overlap.

Indeed, and I actually think this is a weakness of the current implementation. Anyone who comes up with a new transformer that provides different functionality than what is there needs to explicitly provide all the relevant instances, instead of letting MonadTrans do its thing.

(First of all, sorry for the double reply.) I'm certainly way out of my depth here, but would something like associated classes help here? I'm imagining something like this (I'm sure my syntax is all wrong, though):

class TypedMonad m where class MonadType m

instance (MonadTrans m, TypedMonad n) => (MonadType n) (m n)

So then you could write something like

instance Monad m => TypedMonad (ReaderT i m) where class MonadType (ReaderT i m) = MonadReader

and likewise for Reader, Writer(T), State(T), IO, etc... Then, for instance

instance MonadWriter (StateT s (ReaderT r (WriterT w IO)))

is fully-automatic... Or wouldn't this work, at least once associated classes is implemented? Cheers, steve