Continuing in my quest to understand type design in Haskell, here's another episode that leaves me scratching my head: module Foo where class Vect v a where (<+>) :: Floating a => v a -> v a -> v a data Vector a = Vector a a a instance Vect Vector a where (<+>) (Vector x1 y1 z1) (Vector x2 y2 z2) = Vector (x1+x2) (y1+y2) (z1+z2) instance Vect [Vector a] a where (<+>) l1 l2 = zipWith (<+>) l1 l2 The problem is the last instance declaration. Hugs says "Illegal type in class constraint", which is not very explicit (which class constraint?). GHCI is a bit more verbose: Kind error: Expecting kind `* -> *', but `[Vector a]' has kind `*' When checking kinds in `Vect [Vector a] a' In the instance declaration for `Vect [Vector a] a' I have vague memories of seeing mentioned the concept of "kind", but I can't remember where. Any help would be appreciated! Konrad.