[hs] Switch to new day system

hs/src/Aoc/Y2020/D01.hs

@@ -0,0 +1,37 @@
+module Aoc.Y2020.D01
+  ( day
+  ) where
+
+import           Control.Monad
+
+import           Aoc.Day
+
+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)
+
+day :: Day
+day = dayFile "2020_01" solve202001