Ralf Lammel wrote:
Does anyone want to speak up and mention scenarios that would benefit from kind polymorphism? (In Haskell, we are likely to see kind polymorphism, if at all, in the form of type classes whose type parameters can be of different, perhaps of all kinds.)
Here are two possible simple examples. The first is from "real life": I once needed a polymorphic function "replace" that replaces an element of an n-dimensional list given its coordinates and a replacement value: replace :: Int -> a -> [a] -> [a] replace :: Int -> Int -> a -> [[a]] -> [[a]] etc. Also, trivially, in dimension zero: replace :: a -> a -> a So we have: dim 0: replace = const dim 1: replace i x (y:ys) | i == 0 = replace x y : ys | otherwise = y : replace (i-1) x ys replace _ _ _ = [] dim 2: replace i j x (y:ys) | i == 0 = replace j x y : ys | otherwise = y : replace (i-1) j x ys replace _ _ _ _ = [] etc. Intuitively, this ought to be simple. But I leave it as an exercise for the reader to implement it using the current type system. What a mess! Second example: It seems intuitive that the State monad should be isomorphic to the lazy ST monad with STRef, in the sense that it should be possible to implement each monad in terms of the other. (For the purpose of this discussion, let us ignore differences in strictness due to the execution strategies of any given compiler, though that also may be an interesting topic.) Well, in one direction that is trivial - it is easy to implement State using lazy ST and STRef. As for the other direction - yuck! Again, I leave it as an exercise for the reader. Regards, Yitz