Haskell のコード断片 (1)

quick sort
 qsort :: Ord a => [a] -> [a]
 qsort []     = []
 qsort (a:xs) = qsort less_than_a ++ [a] ++ qsort not_less_than_a

                where less_than_a     = [ x | x <- xs, x < a ]
                      not_less_than_a = [ x | x <- xs, not (x < a) ]

「How (どうやって計算するか)」ではなく
「What (何を計算するか)」を書く