023.hs

import Data.Ord()
import Data.List.Ordered (member)
import Data.List (sort)


main :: IO()
main = do
    let limit = 28123
    let sumsOfTwoAbundantNumbers = sort $ pairwiseSums $ takeWhile (<limit) abundantNumbers
    let notASum = not . flip member sumsOfTwoAbundantNumbers

    -- TODO(btc): will |notASum| be recomputed for every element in [1..limit]?

    print $ sum $ filter notASum [1..limit]

pairwiseSums :: [Int] -> [Int]
pairwiseSums xs = map (uncurry (+)) (unorderedPairs xs xs)

unorderedPairs :: Ord a => [a] -> [a] -> [(a,a)]
unorderedPairs as bs = [(a, b) | a <- as, b <-bs, a <= b]

abundantNumbers :: [Int]
abundantNumbers = filter isAbundant [0..]

isAbundant :: Integral a => a -> Bool
isAbundant x = x < sum (divisors x)

divisors :: Integral a => a -> [a]
divisors x = 1 : filter (\n -> x `mod` n == 0) [2..x `div` 2]