[hs] Solve 2020_01

hs/app/Main.hs

@@ -3,6 +3,7 @@ module Main where
 import Options.Applicative
 
 import Aoc.Y2019
+import Aoc.Y2020
 
 data Settings = Settings
   { function :: FilePath -> IO ()
@@ -16,6 +17,7 @@ solutions = subparser $ mconcat $ map (\(name, func) -> command name (info (pure
   , ("2019_03", solve201903)
   , ("2019_04", solve201904)
   , ("2019_05", solve201905)
+  , ("2020_01", solve202001)
   ]
 
 parser :: Parser Settings

hs/src/Aoc/Y2020.hs

@@ -0,0 +1,5 @@
+module Aoc.Y2020
+  ( solve202001
+  ) where
+
+import Aoc.Y2020.A01

hs/src/Aoc/Y2020/A01.hs

@@ -0,0 +1,32 @@
+module Aoc.Y2020.A01
+  ( solve202001
+  ) where
+
+import Control.Monad
+
+findPair :: [Integer] -> (Integer, Integer)
+findPair l = head $ do
+  a <- l
+  b <- l
+  guard $ a + b == 2020
+  pure (a, b)
+
+findTriple :: [Integer] -> (Integer, Integer, Integer)
+findTriple l = head $ do
+  a <- l
+  b <- l
+  c <- l
+  guard $ a + b + c == 2020
+  pure (a, b, c)
+
+solve202001 :: FilePath -> IO ()
+solve202001 f = do
+  values <- map read . lines <$> readFile f
+
+  putStrLn ">> Part 1"
+  let (x1, x2) = findPair values
+  putStrLn $ show x1 ++ " * " ++ show x2 ++ " = " ++ show (x1 * x2)
+
+  let (y1, y2, y3) = findTriple values
+  putStrLn ">> Part 2"
+  putStrLn $ show y1 ++ " * " ++ show y2 ++ " * " ++ show y3 ++ " = " ++ show (y1 * y2 * y3)