Solves problem 21 of Project Euler with list comprehensions.

P21.hs

module P21 (properDivisors, sumDivisors, sumAmicable, isAmicable, amicableList) where



properDivisors :: Int -> [Int]
properDivisors x = [xs | xs <- [1..x-1], x `mod` xs == 0]

sumDivisors :: Int -> Int
sumDivisors x = sum $ properDivisors x

sumAmicable = sum [x | x <- [1..10000], isAmicable x]

amicableList = [x | x <- [1..10000], isAmicable x]

isAmicable x = if sumDivisors otherNumber == x && not (x == otherNumber) then True else False
    where otherNumber = sumDivisors x