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Haskell Quiz/Maximum Sub-Array/Solution Kristof

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This solution uses a left fold to find the maximum sum. It keeps track of the maximum sublist found so far (the list
max
and it's sum
sum
), and the maximum sublist that contains the current position (
r
and
rsum
).
max_sublist l = reverse $ fst $ foldl find_max ([],(0,[],0)) l where
  find_max (max, (sum, r, rsum)) x = (max', (sum', r', rsum'))
    where
    (r', rsum')  = if (rsum > 0)
                   then (x:r, rsum+x)
                   else ([x], x)
    (max', sum') = if (rsum' > sum)
                   then (r', rsum')
                   else (max, sum)