On , oleg@okmij.org wrote:
[...] Exactly this code is implemented in Example.hs. Here's an excerpt:
-- Default instance
instance (Monad (tm), MonadState m, MonadTrans t)
=> MonadState' (tm) HFalse where
type MState' (tm) HFalse = MState m
get' _ = trace "Default get" $ lift get
put' _ = lift . put
-- Special instances
instance (Monad m)
=> MonadState' (StateT sm) HTrue where
type MState' (StateT sm) HTrue = s
get' _ = trace "Special get" . StateT $ \s -> return (s, s)
put' _ s = StateT $ \_ -> return ((), s)
-- add more special instances if needed ...
plus one more general dispatching instance. Because of the additional
flag, HTrue vs HFalse, the above instances do not overlap.
Can you add more special instances for types that match (tm) without using OverlappingInstances and without modifying the instances you have above?