title: What is the purpose of Rank2Types?
body: <p>I am not really proficient in Haskell, so this might be a very easy question.</p>
  
  <p>What language limitation do <a href="https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#ghc-flag--XRank2Types" rel="noreferrer">Rank2Types</a> solve? Don't functions in Haskell already support polymorphic arguments?</p>