Your problem is that next_state :: forall s v. VertexState s v => s -> s So an application of next_state to an argument of type sty will require the constraint VertexState sty v but that says NOTHING about v. It's a bit like calling a function with type forall a b. Foo a => b -> b Anyway, I think what you want is a functional dependency class VertesState s v | s -> v Then fixing s will fix v, which is what you want I think Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Ketil Malde | Sent: 25 August 2004 14:45 | To: glasgow-haskell-users@haskell.org | Subject: GHC and MPTCs | | | Hi, | | I recently tried to do the following code: | | class VertexState s v where | new_point :: s -> v -> v -> v | next_state :: s -> s | | -- interleave :: (VertexState s a) => s -> [a] -> [a] -> [a] | interleave s (t:ts) (b:bs) = | new_point s t b : interleave (next_state s) bs (t:ts) | interleave _ [] [b] = [] | | Firstly, GHC doesn't deal all too gracefully with this if the type | signature is uncommented, it tells me: | | Could not deduce (VertexState s v) | from the context (VertexState s a) | arising from use of `next_state' at Grid.lhs:25 | Probable fix: | Add (VertexState s v) to the type signature(s) for `interleave' | | Unfortunately, doing the probable fix just results in a new request | for (VertexState s v1), and so on. | | I suppose the real problem is that GHC can't decide from next_state | that it is supposed to use exactly this instance of VertexState, is | there a good way to do that without adding an explicit dummy parameter | of type v? | | (I really would like to be able to have instances with the same s but | different v's, which I think adding functional deps would make | difficult.) | | -kzm | -- | If I haven't seen further, it is by standing in the footprints of giants | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users