On Wednesday 23 June 2010 05:46:46, Evan Laforge wrote:
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
Would it work if you made Typecheck a two-parameter type class? class Typecheck result a where from_val :: Val result -> Maybe a instance Typecheck result Double where ... instance Typecheck result (SomeMonad result) where
instance Typecheck Double where from_val (VNum d) = Just d from_val _ = Nothing
Now I can write
lookup_environ :: (Typecheck a) => Symbol -> Environ result -> Maybe a
Now of course there's a question of how to write Typecheck for VThunk. I would like to be able to call 'lookup_environ' expecting a 'SomeMonad result' and get Nothing or Just depending on if it's present.
instance Typecheck (SomeMonad result) where from_val (VThunk f) = Just f
But I need something to say that the 'result' from the instance head is the same as the 'result' from the class declaration, because otherwise I get
Couldn't match expected type `result' against inferred type `result1'