Question

data Vector a = Vector a a a deriving (Eq, Show)

instance Functor Vector where
    fmap f (Vector x y z) = Vector (f x) (f y) (f z)

So far so good.

instance Num ((Num a) => Vector a) where
    negate = fmap negate

Doesn’t work. I tried many different variations on that first line but GHC keeps complaining. I want to make a Vector containing numbers an instance of Num; surely this should be possible? Otherwise I would have to make an instance for Int, Integer, Float, Double, etc. all with the same definition.

Answer 1

instance Num a => Num (Vector a) where
   negate = fmap negate

Consider writing other methods too.

(You can write deriving (Eq, Show, Functor) if you turn on -XDeriveFunctor.)