Daniel Fischer wrote:
My Oleg rating isn't high either, and certainly you can do it more elegant, but
class Concat l1 l2 l3 | l1 l2 -> l3, l1 l3 -> l2 where pConcat :: l1 a -> l2 a -> l3 a
instance Concat (Plist Zero) (Plist n) (Plist n) where pConcat _ ys = ys
instance Concat (Plist n1) (Plist n2) (Plist t) => Concat (Plist (S n1)) (Plist n2) (Plist (S t)) where pConcat (x :| xs) ys = x :| pConcat xs ys
works, you don't even need the Add class then - btw, you'd want instance Add n1 n2 t => Add (S n1) n2 (S t) anyway.
Thanks, and also thanks to Dan Doel who showed how to do it with the new type families. I'll stick with the Fundeps solution here for the moment until type families settle down, but that method is cleaner. I was also able to write this: class Concat p1 p2 p3 | p1 p2 -> p3, p1 p3 -> p2 where pConcat :: p1 a -> p2 a -> p3 a pBogus :: p1 a -> p2 a -> p3 a instance Concat (Plist Zero) (Plist n) (Plist n) where pConcat _ ys = ys pBogus _ ys = ys instance Concat (Plist n1) (Plist n2) (Plist t) => Concat (Plist (S n1)) (Plist n2) (Plist (S t)) where pConcat (x :| xs) ys = x :| pConcat xs ys pBogus xs ys = ys And indeed the second definition of pBogus gave me a compile-time type error because the length of the result didn't agree with the type length. I'm going to be doing a presentation on Haskell for my boss soon, and this should definitely impress (he has a solid coding background). Thanks again, Paul.