Preventing return of a resource from a bracket

Main.hs

{-# LANGUAGE
    RankNTypes
#-}

module Main where

data MyResource a = MyResource {unMyResource :: String}


withMyResource :: String -> (forall a. MyResource a -> IO r) -> IO r
withMyResource s f = f (MyResource s)


main :: IO ()
main = do
  withMyResource "hello"
    $ \res -> putStrLn $ unMyResource res
  -- Does not compile:
  -- sneakyRes <- withMyResource "world"
  --   $ \res -> return res
  -- putStrLn $ unMyResource sneakyRes

The trick is the rank 2 type of withMyResource that lets it dictate the type of the passed function. You should get the following warning if you try to compile the commented section:

Bracket.hs:19:15: error:
    • Couldn't match type ‘a0’ with ‘a’
        because type variable ‘a’ would escape its scope
      This (rigid, skolem) type variable is bound by
        a type expected by the context:
          MyResource a -> IO (MyResource a0)
        at Bracket.hs:(18,16)-(19,24)
      Expected type: IO (MyResource a0)
        Actual type: IO (MyResource a)
    • In the expression: return res
      In the second argument of ‘($)’, namely ‘\ res -> return res’
      In a stmt of a 'do' block:
        sneakyRes <- withMyResource "world" $ \ res -> return res
    • Relevant bindings include