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diff --git a/2020/11-Seating_System/first.hs b/2020/11-Seating_System/first.hs
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+++ b/2020/11-Seating_System/first.hs
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+module Main (main) where
+import Control.Monad (when)
+import Data.List (foldl')
+import Data.Maybe (mapMaybe)
+import qualified Data.Map as M
+import System.Exit (die)
+
+exampleExpectedOutput = 37
+
+type Seating = M.Map (Int, Int) Bool
+
+inputToList :: String -> [((Int, Int), Bool)]
+inputToList = snd . foldl'  nextSeat  ((0, 0), [])
+  where
+    nextSeat :: ((Int, Int), [((Int, Int), Bool)]) -> Char -> ((Int, Int), [((Int, Int), Bool)])
+    nextSeat ((x, y), acc) '\n' = ((0, y+1), acc)
+    nextSeat ((x, y), acc) '.' = ((x+1, y), acc)
+    nextSeat ((x, y), acc) 'L' = ((x+1, y), acc ++ [((x, y), False)])
+
+parseInput :: String -> IO Seating
+parseInput filename = do
+  input <- readFile filename
+  return $ M.fromList (inputToList input)
+
+compute :: Seating -> Int
+compute seating
+  | seating == seating' = length $ M.filter id seating
+  | otherwise = compute seating'
+  where
+    seating' = M.mapWithKey next seating
+    next :: (Int, Int) -> Bool -> Bool
+    next (x, y) False = around (x, y) == []
+    next (x, y) True = length (around (x,y)) < 4
+    around :: (Int, Int) -> [Bool]
+    around (x, y) = filter id $ mapMaybe (\v -> M.lookup v seating) [(x-1, y-1), (x, y-1), (x+1, y-1), (x-1, y), (x+1, y), (x-1, y+1), (x, y+1), (x+1, y+1)]
+
+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