Maarten wrote:
Only update (see code below) is a bit ugly (I have no idea why I need fixCastUpdate) class (Show a, Typeable a) => ICustom a where [snip] update :: a -> (forall b. (ICustom b) => b -> b) -> a update a f = f a
instance ICustom Node where getVal (Node n) f = getVal n f update (Node n) f = Node (update n f)
updateM :: forall a b. (ICustom a, ICustom b) => (a -> b) -> NodeState () updateM f = do s <- get let s' = update s (fixCastUpdate f) put s'
Hi Maarten - Looking at this again, I wonder if the following changes would work: -- this change is not strictly necessary update :: a -> (a -> a) -> a updateM :: (forall a. ICustom a => a -> a) -> NodeState () updateM f = do s <- get let s' = update s f put s' I think the reason why fixCastUpdate was needed in your original definition of updateM is because the type of f seems to be too general (a->b) compared to the type of f in the update method of ICustom (b->b) Regards, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com