2020-01 part 2 in haskell

2020/01/second.hs

@@ -0,0 +1,32 @@
+module Main where
+
+import Control.Monad (when)
+import Data.Maybe (catMaybes, fromJust)
+import System.Exit (die)
+
+exampleExpectedOutput = 241861950
+
+compute2020 :: Int -> Int -> Int -> Maybe Int
+compute2020 a b c
+  | a + b + c == 2020 = Just (a * b * c)
+  | otherwise = Nothing
+
+compute :: String -> Int
+compute input = head . catMaybes $ processInput inputList
+  where
+    inputList :: [Int]
+    inputList = map read $ lines input
+    processInput :: [Int] -> [Maybe Int]
+    processInput (_:[]) = [Nothing]
+    processInput (x:xs) = (processA $ compute2020 x) xs ++ processInput xs
+    processA :: (Int -> Int -> Maybe Int) -> [Int] -> [Maybe Int]
+    processA f (_:[]) = [Nothing]
+    processA f (x:xs) = map (f x) xs ++ processA f xs
+
+main :: IO ()
+main = do
+  example <- readFile "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- readFile "input"
+  print $ compute input