On 23 June 2010 13:46, Evan Laforge
I have a parameterized data type:
data Val result = VNum Double | VThunk (SomeMonad result) type Environ result = Map Symbol (Val result)
I have a class to make it easier to typecheck Vals:
class Typecheck a where from_val :: Val result -> Maybe a
I think your problem here is that there's no mention of `a' on the left-hand size of from_val's type signature; you either need to use MPTC+fundep to associate what result is compared to a, or else use a phantom type parameter of Val to make it "data Val result a = ..." and then "from_val :: Val result a -> Maybe a". SPJ's paper on type families has this situation arising in the section on defining a graph class. -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com