content: {-# LANGUAGE RankNTypes #-}
  
  {-
          From: Ralf Hinze <ralf@uran.informatik.uni-bonn.de>
          Date: Fri, 15 Aug 1997 15:20:51 +0200 (MET DST)
  
  I *suppose* that there is a bug in GHC's type checker. The following
  program, which I think is ill-typed, passes silently the type checker.
  Needless to say that it uses some of GHC's arcane type extensions.
  -}
  
  module ShouldFail where
  
  data ContT m a          =  KContT (forall res. (a -> m res) -> m res)
  unKContT (KContT x)     =  x
  
  callcc                  :: ((a -> ContT m b) -> ContT m a) -> ContT m a
  callcc f                =  KContT (\cont -> unKContT (f (\a -> KContT (\cont' -> cont a))) cont)
  
  {-
  `ContT' is a continuation monad transformer. Note that we locally
  qualify over the result type `res' (sometimes called answer or
  output).  IMHO this make it impossible to define control constructs
  like `callcc'. Let's have a closer look: the code of `callcc' contains
  the subexpression `KContT (\cont' -> cont a)'. To be well-typed the
  argument of `KContT' must have the type `(All res) => (a -> m res) -> m
  res'. Quantification is not possible, however, since the type variable
  in `cont's type cannot be forall'd, since it also appears at an outer
  level.  Right? Or wrong?
  -}
  