minimum path value through a triangle

Triangle.hs

module Triangle where

-- top-down version
minTriangleFlow :: (Num a, Ord a) => [[a]] -> a
minTriangleFlow = minimum . foldl sift []
    where sift as bs = zipWith (+) bs $ pres as
          pres [] = [0]
          pres as@(ah:at) = ah : zipWith min as at ++ [last as]

-- bottom-up version (not safe if triangle input is misformed)
minTriangleFlow' :: (Num a, Ord a) => [[a]] -> a
minTriangleFlow' = minimum . foldr sift (repeat 0)
    where sift as bs = zipWith (+) as $ zipWith min bs (tail bs)

exampleTriangle =
    [       [1]
    ,     [2 , 4]
    ,   [5 , 1 , 4]
    , [2 , 3 , 4 , 5]
    ]

main = print $ minTriangleFlow exampleTriangle