Change in Haskell using an array for memoization

Change.hs

module Change (findFewestCoins) where

import Data.Array (listArray, (!))
import Data.Function (on)
import Safe.Foldable (minimumByMay)

findFewestCoins :: Integer -> [Integer] -> Maybe [Integer]
findFewestCoins target coins =
  let solutions = listArray (0, target) $ map formSolution [0 .. target]
      lookupSolution x = case compare x 0 of
        LT -> Nothing
        EQ -> Just []
        GT -> solutions ! fromInteger x
      formSolution x =
        let solns = map (lookupSolution . (x -)) coins
            candidates = [coin : soln | (coin, Just soln) <- zip coins solns] in
        minimumByMay (compare `on` length) candidates in
  lookupSolution target