Henry, Jason pointed out:
You'd get fromEnum and toEnum. Which I think, would give you the int mapping that you are after.
fromEnum :: Enum a => a -> Int toEnum :: Enum a => Int -> a
To me, this would indeed seem the way to go for your particular example. Moreover, as for generic producer functions in general, the pattern suggested by the Prelude would be to have c :: Color c = undefined convert :: Data a => Int -> a convert i x = let c = dataTypeConstrs (dataTypeOf x) !! (i-1) in fromConstr c and then use it as in convert 1 `asTypeOf` c You'll find out that in most cases the (pseudo) "type annotation" isn't really needed and the type of the value to produce can be determined automatically by the context. Cheers, Stefan