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diff --git a/2020/18-Operation_Order/second.hs b/2020/18-Operation_Order/second.hs
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+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+import Control.Monad (void, when)
+import Data.Either
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 231 + 51
+
+data Operation = Add | Mul deriving (Eq, Show)
+data Chain = Chain [Either Int Chain] [Operation] deriving Show
+type Input = [Chain]
+
+type Parser = Parsec Void String
+
+parseInt :: Parser Int
+parseInt = do
+  n <- some digitChar
+  void $ optional (char ' ')
+  return $ read n
+
+parseOp :: Parser Operation
+parseOp = do
+  op <- (char '+' *> return Add) <|> (char '*' *> return Mul)
+  void $ char ' '
+  return op
+
+parseParenthesis :: Parser Chain
+parseParenthesis = do
+  void $ char '('
+  a <- parseChain
+  void $ char ')'
+  void $ optional (char ' ')
+  return $ a
+
+parseChain :: Parser Chain
+parseChain = do
+  a <- ((Left <$> parseInt) <|> (Right <$> parseParenthesis))
+  next <- (Just <$> parseOp) <|> (return Nothing)
+  case next of
+    Nothing -> return $ Chain [a] []
+    Just op -> do
+      Chain elems ops <- parseChain
+      return $ Chain (a:elems) (op:ops)
+
+parseInput' :: Parser Input
+parseInput' = do
+  input <- some (parseChain <* void (optional (char '\n')))
+  void eof
+  return $ input
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseInput' filename input of
+    Left bundle -> die $ errorBundlePretty bundle
+    Right input' -> return input'
+
+compute' :: Chain -> Int
+compute' (Chain [Left x] []) = x
+compute' (Chain [Right x] []) = compute' x
+compute' (Chain (x:y:xs) (op:ops))
+  | op == Add = compute' $ Chain (Left (cx+cy):xs) ops
+  | op == Mul = cx * (compute' $ Chain (Left cy:xs) ops)
+  where
+    cx = if isLeft x then fromLeft 0 x else compute' (fromRight (Chain [] []) x)
+    cy = if isLeft y then fromLeft 0 y else compute' (fromRight (Chain [] []) y)
+
+compute :: Input -> Int
+compute input = sum $ map compute' input
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input