day2: update - use implement ordering like normal people

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

day2.hs

@@ -2,54 +2,32 @@
 
 main :: IO ()
 main = do
-  input <- readFile "day2.in"
+  choices <- parse <$> readFile "day2.in"
   putStr "Q1: "
-  print $ q1 input
+  print $ q1 choices
   putStr "Q2: "
-  print $ q2 input
+  print $ q2 choices
 
 data Choice = Rock | Paper | Scissors deriving (Enum, Eq)
 
-q1 :: String -> Int
-q1 =
-  sum
-    . map
-      ( \[x, _, y] ->
-          let i = val x 'A'
-              j = val y 'X'
-           in fromEnum j + 1 + case () of
-                _
-                  | i == j -> 3
-                  | winCond i == j -> 6
-                  | otherwise -> 0
-      )
-    . lines
+instance Ord Choice where
+  compare x y | x == y = EQ
+  compare Rock Paper = LT
+  compare Paper Scissors = LT
+  compare Scissors Rock = LT
+  compare _ _ = GT
 
-q2 :: String -> Int
-q2 =
-  sum
-    . map
-      ( \[x, _, y] ->
-          let i = val x 'A'
-              j = val y 'X'
-           in 1 + case fromEnum j of
-                0 -> 0 + fromEnum (loseCond i)
-                1 -> 3 + fromEnum i
-                2 -> 6 + fromEnum (winCond i)
-      )
-    . lines
+parse :: String -> [(Choice, Choice)]
+parse = map (\[x, _, y] -> (val x 'A', val y 'X')) . lines
+
+score :: (Choice, Choice) -> Int
+score (i, j) = 1 + fromEnum j + 3 * fromEnum (compare j i)
+
+q1 :: [(Choice, Choice)] -> Int
+q1 = sum . map score
+
+q2 :: [(Choice, Choice)] -> Int
+q2 = sum . map (score . \(i, j) -> (i, toEnum $ mod (fromEnum i + fromEnum j + 2) 3))
 
 val :: Char -> Char -> Choice
 val a b = toEnum (fromEnum a - fromEnum b) :: Choice
-
-winCond :: Choice -> Choice
-winCond c = case c of
-  Rock -> Paper
-  Paper -> Scissors
-  Scissors -> Rock
-
-loseCond :: Choice -> Choice
-loseCond c = case c of
-  Paper -> Rock
-  Scissors -> Paper
-  Rock -> Scissors

flake.nix

@@ -13,18 +13,18 @@
           pkgs = import nixpkgs {
             inherit system;
           };
+          src = ./.;
         in
-        {
-          devShells = with pkgs; rec {
-            default = mkShell {
-              buildInputs = [ ghc ];
-            };
-            withLsp = mkShell {
+        with pkgs; {
+          devShells.default = mkShell {
             buildInputs = [
               ghc
               haskell-language-server
             ];
           };
+          apps.default = {
+            type = "app";
+            program = "${ghc}/bin/runhaskell";
           };
         }
       );