Parallel Mandlebrot set generator using Repa

Mandlebrot

import Data.Array.Repa

data Complex a = Complex { re, im :: !a }

instance Num a => Num (Complex a) where
    (Complex a b) + (Complex c d) = Complex (a + c) (b + d)
    (Complex a b) - (Complex c d) = Complex (a - c) (b - d)
    (Complex a b) * (Complex c d) = Complex (a*c - b*d) (a*d + b*c)
    abs _    = undefined
    signum _ = undefined
    fromInteger i = Complex i 0

mandlebrot :: Num a => Complex a -> Maybe Integer
mandlebrot c = go c 0
    where go z@(Complex a b) iter
            | iter >= maxIter        = Nothing
            | abs a > 2 || abs b > 2 = Just iter
            | otherwise              = go (z^2 + c) (iter + 1)
          maxIter = 255

genMandlebrot :: Array D DIM2 Integer
genMandlebrot = fromFunction DIM2 (mandlebrot . func)
    where func sh = case listOfShape sh of [a, b] -> Complex a b