Edward, On Sat, Sep 26, 2009 at 11:41 AM, Edward Kmett wrote:

I would just like to add that Oleg and Chung-chieh made sure in their finally tagless paper to use monomorphic lifting of literals explicitly to avoid this sort of ambiguity. Using Num or another typeclass is fine as long as all you want to do is evaluate your EDSL. But what about partial evaluation? CPS transformation? Compilation? You might be able to muddle through the first two, but compilation will derail your solution. Ultimately you will not be able to work over arbitrary Num instances if you want to do more than interpret. That was the main point of the monomorphic int :: Int -> r Int, char :: Char -> r Char methods they were using. If all I know about something is that there is a valid Num instance for it I have no way to emit machine code for it. -Edward [...]

If thye type parameter is present in the class head, you can put constraints on it in your instances. E.g., class ENum repr a where constant :: a -> repr a add :: repr a -> repr a -> repr a newtype E a = E { unE :: a } instance (Num a) => ENum E a where constant = E Similarly to the ENum instance for an evaluator, E, you could define a type class for code gen: data UntypedIntermediate = ConstInt Int | ConstFloat Float | Add Intermediate Intermediate class Emittable a where emit :: a -> UntypedIntermediate instance Emittable Int where emit i = ConstInt i instance Emittable Float where emit f = ConstFloat f Then, you could do compilation, given an Emittable constraint: newtype C a = C { unC :: UntypedIntermediate } instance (Emittable a) => ENum C a where constant e = C $ emit $ unC e add e1 e2 = C $ Add (emit $ unC e1) (emit $ unC e2) I think that by using the strategy of lifting type parameters into class head, and by defining the type classes & instances you need for a certain interpretation, you can modularly define tagless EDSLs, i.e. define the language once, as a collection of typeclasses, and then be able to define arbitrary interpretations of that EDSL, in separate modules, without having to modify the EDSL type classes. (Note: I haven't tried running the above code.) Sincerely, Brad