# More type design questions

**Konrad Hinsen
**
hinsen@cnrs-orleans.fr

*Mon, 18 Aug 2003 08:56:07 +0200*

Continuing in my quest to understand type design in Haskell, here's anoth=
er=20
episode that leaves me scratching my head:
module Foo where
class Vect v a where
(<+>) :: Floating a =3D> v a -> v a -> v a
data Vector a =3D Vector a a a
instance Vect Vector a where
(<+>) (Vector x1 y1 z1) (Vector x2 y2 z2)
=3D Vector (x1+x2) (y1+y2) (z1+z2)
instance Vect [Vector a] a where
(<+>) l1 l2 =3D zipWith (<+>) l1 l2
The problem is the last instance declaration. Hugs says "Illegal type in =
class=20
constraint", which is not very explicit (which class constraint?). GHCI i=
s a=20
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 ca=
n't=20
remember where. Any help would be appreciated!
Konrad.