Division-free determinant for Integer matrices

Determinant.hs

-- This is a direct copy&paste from Bird's Pearls of Functional Algorithm Design
import Data.List (transpose)

trimult xss yss = zipWith (map . dp) xss (submats (transpose yss))

submats :: [[a]] -> [[[a]]]
submats [[x ]] = [[[x ]]]
submats xss = xss : submats (map tail (tail xss))

dp xs ys = sum (zipWith (*) xs ys)

mut xss = zipWith (:) ys (map tail xss)
  where ys = map negate (tail (scanr (+) 0 (map head xss)))

det :: [[Integer ]] -> Integer
det ass = head (head bss)
  where
  bss = foldl (trimult . mut) ass' (replicate (n - 1) ass)
  ass' = if odd n then upper ass else map (map negate) (upper ass)
  n = length ass

upper = zipWith drop [0..]