2020 day1/haskell: add solution

2020/day1/day1.hs

@@ -0,0 +1,12 @@
+{-# LANGUAGE TypeApplications #-}
+
+import Data.List
+import Data.Maybe
+import Control.Monad
+
+find_n_summing :: (Num a, Eq a) => a -> Int -> [a] -> Maybe [a]
+find_n_summing to = (find ((to ==) . sum) .) . replicateM
+
+main = do
+    nums <- fmap read <$> lines <$> readFile "input.txt"
+    mapM_ print $ product <$> fromJust <$> flip (find_n_summing 2020) nums <$> [2, 3]