aoc2020/day1.hs

module Main where

main :: IO ()
main = do
  raw <- readFile "day1.txt"
  let ls = map read $ lines raw :: [Integer]
   in do
     case (solveA ls) of
       Nothing -> error "no answer"
       (Just x) -> putStrLn $ "day1a: " ++ (show x)
     case (solveB ls) of
       Nothing -> error "no answer"
       (Just x) -> putStrLn $ "day1b: " ++ (show x)

solveA :: [Integer] -> Maybe Integer
solveA [] = Nothing
solveA xs = Just $ (\(x,y) -> x * y) lsa
  where
    lsa = if lsa' == [] then error "no answer" else head lsa'
    lsa' = [ (x,y) | x <- xs, y <- xs, x + y == 2020 ]

solveB :: [Integer] -> Maybe Integer
solveB [] = Nothing
solveB xs = Just $ (\(x,y,z) -> x * y * z) lsb
  where
    lsb = if lsb' == [] then error "no answer" else head lsb'
    lsb' = [ (x,y,z) | x <- xs, y <- xs, z <- xs, x + y + z == 2020 ]