{-# LANGUAGE OverloadedStrings, ImpredicativeTypes #-}

data ImpredicativeList = Empty | forall a. Some a ImpredicativeList

countThem :: ImpredicativeList -> Int
countThem Empty = 0
countThem (Some _ xs) = 1 + countThem xs

main :: IO ()
main = do
  print (countThem (Some True (Some 1 (Some 'a' Empty))))