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diff --git a/2020/07-Handy_Haversacks/first.hs b/2020/07-Handy_Haversacks/first.hs
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+++ b/2020/07-Handy_Haversacks/first.hs
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+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+import Control.Monad (void, when)
+import qualified Data.Set as S
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 4
+
+data Bag = Bag { name :: String
+               , contents :: [(Int, String)]
+               } deriving (Show)
+
+type Parser = Parsec Void String
+
+parseName :: Parser String
+parseName = do
+  adjective <- some letterChar
+  void (char ' ')
+  color <- some letterChar
+  return $ adjective ++ " " ++ color
+
+parseInt :: Parser Int
+parseInt = do
+  i <- some digitChar
+  return $ read i
+
+parseContent :: Parser (Int, String)
+parseContent = do
+  i <- parseInt
+  void (char ' ')
+  name <- parseName
+  void (string " bag")
+  void (optional $ char 's')
+  void (string ", " <|> string ".")
+  return (i, name)
+
+parseContents :: Parser [(Int, String)]
+parseContents = some parseContent <|> (string "no other bags." *> return [])
+
+parseBag :: Parser Bag
+parseBag = do
+  name <- parseName
+  void $ string " bags contain "
+  contents <- parseContents
+  void (char '\n')
+  return $ Bag { name=name, contents=contents }
+
+parseBags :: Parser [Bag]
+parseBags = some parseBag <* eof
+
+parseInput :: String -> IO [Bag]
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseBags filename input of
+    Left bundle -> die $ errorBundlePretty bundle
+    Right bags -> return bags
+
+compute :: [Bag]-> Int
+compute bags = (length . compute' $ S.fromList ["shiny gold"]) - 1
+  where
+    bagsContaining :: S.Set String -> S.Set String
+    bagsContaining s = S.union s $ S.fromList . (map name) $ filter (\x -> not . S.null . S.intersection s $ S.fromList . (map snd) . contents $ x) bags
+    compute' :: S.Set String -> S.Set String
+    compute' s = if s == s' then s else compute' s'
+      where
+        s' = bagsContaining s
+
+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