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.
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