完全数

煮詰まったときの頭の体操．

divisors :: Int -> [Int]

divisors n = divisor' n 1

    where

      divisor' n m | n >= m * 2 && n `mod` m == 0 = m : divisor' n (m + 1)

                   | n >= m * 2 && n `mod` m /= 0 = divisor' n (m + 1)

                   | n <  m * 2                   = []

isPerfectNumber :: Int -> Bool

isPerfectNumber n = let l = divisors n in 

                    foldr (+) 0 l == n

リストモナド使おうと思ったけど使うところが無かった．