partitionProb124.hs

import Data.List
import Data.Ord

partitions :: [a] -> [([a], [a])]
partitions [] = [([], [])]
partitions (x : xs) = let ps = partitions xs in
    [(x : ys, zs) | (ys, zs) <- ps] ++ [(ys, x : zs) | (ys, zs) <- ps]

unbalance :: Num a => ([a], [a]) -> a
unbalance (ys, zs) = abs (sum ys - sum zs)

balanced :: (Num a, Ord a) => [a] -> ([a], [a])
balanced = minimumBy (comparing unbalance) . partitions