Data.List.mapAccumR のつかいどころ ref: http://qiita.com/items/389da409cabceebd8ab7

pascal.hs

import Data.List (mapAccumR)

pascal :: [[Integer]]
pascal = [1] : [f cs | cs <- pascal ]
  where
    f xs = snd $ mapAccumR g xs' xs'
           where xs'        = 0:xs
                 g (y:ys) z = (ys, y+z)

comb :: Int -> Int -> Integer
comb = (!!) . (pascal !!)