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diff --git a/2020/19-Monster_Messages/first.hs b/2020/19-Monster_Messages/first.hs
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+++ b/2020/19-Monster_Messages/first.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.List (foldl')
+import Data.Maybe (isJust)
+import Data.Map qualified as M
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 2
+
+type Match = Char
+type RuleID = Int
+type Action = [RuleID]
+type Rule = Either Match [Action]
+type Message = String
+data Input = Input { rules :: M.Map RuleID Rule
+                   , messages :: [Message]
+                   } deriving (Show)
+
+type Parser = Parsec Void String
+
+parseInt :: Parser Int
+parseInt = do
+  n <- some digitChar
+  void $ optional (char ' ')
+  return $ read n
+
+parseMatch :: Parser Match
+parseMatch = do
+  void $ char '"'
+  c <- letterChar
+  void $ char '"'
+  return c
+
+parseAction :: Parser Action
+parseAction = some (parseInt <* optional (char ' '))
+
+parseRule :: Parser (RuleID, Rule)
+parseRule = do
+  id <- some digitChar
+  void $ string ": "
+  rule <- (Left <$> parseMatch) <|> (Right <$> some (parseAction <* optional (string "| ")))
+  void $ char '\n'
+  return $ (read id, rule)
+
+parseInput' :: Parser Input
+parseInput' = do
+  rules <- M.fromList <$> some parseRule
+  void $ char '\n'
+  messages <- some (some letterChar <* (optional $ char '\n'))
+  void eof
+  return $ Input rules messages
+
+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 :: Input -> Int
+compute (Input rules messages) = length . filter id $ map isValid messages
+  where
+    isValid :: Message -> Bool
+    isValid msg = case matches 0 msg of
+      Just "" -> True
+      otherwise -> False
+    matches :: RuleID -> Message -> Maybe Message
+    matches ruleId msg = case rules M.! ruleId of
+      Left c -> if head msg == c then (Just $ tail msg) else Nothing
+      Right actions -> case map (processAction msg) actions of
+        Nothing:a:_ -> a
+        a:_ -> a
+        otherwise -> Nothing
+    processAction :: Message -> Action -> Maybe Message
+    processAction msg action = foldl' step (Just msg) action
+    step :: Maybe Message -> RuleID -> Maybe Message
+    step Nothing _ = Nothing
+    step (Just msg) ruleID = matches ruleID msg
+
+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