Solution to Euler problem 67 in Haskell

euler67.hs

main :: IO ()
main = do
  print (maxTotal [[3], [7, 4], [2, 4, 6], [8, 5, 9, 3]])
  print . maxTotal . parseTable =<< readFile "p067_triangle.txt"

maxTotal :: [[Int]] -> Int
maxTotal tree = case triangularFold getMax tree of
  [x] -> x
  _   -> error "maxTotal: invalid input"
  where getMax x t t' = x + max t t'

triangularFold :: (a -> a -> a -> a) -> [[a]] -> [a]
triangularFold _ []       = []
triangularFold _ [r]      = r
triangularFold f (r : rs) = [f x t t' | (x, t, t') <- zip3 r ts (tail ts)]
  where ts = triangularFold f rs

parseTable :: Read a => String -> [[a]]
parseTable = map ((map read) . words) . lines