[GHC] #12418: Make `MonadCont (ContT r m)` polykinded (r::k), (m::k -> Type)

[GHC]

#12418: Make `MonadCont (ContT r m)` polykinded (r::k), (m::k -> Type)
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 I want to make sure I understand this correctly, this brittleness applies
 to monad transformers and not the instances below `↓` since they are
 already kind polymorphic?

 {{{#!hs
 instance forall k (r :: k) (m :: k -> *). Functor     (ContT r m)
 instance forall k (r :: k) (m :: k -> *). Applicative (ContT r m)
 instance forall k (r :: k) (m :: k -> *). Monad       (ContT r m)
 }}}

 ----

 I have no use for a kind-polymorphic instance (that I know of!), the
 original motivation was allowing

 {{{#!hs
 callCC :: ((a -> ContT r m b) -> ContT r m a) -> ContT r m a

 callCC @(ContT _ _)
 }}}

 to match the instance.

 {{{#!hs
 instance k ~ Type => MonadCont (ContT @k r m)
 }}}

 would allow `callCC @(ContT _ _)` but not `callCC :: ((a -> ContT r m b)
 -> ContT r m a) -> ContT r m a` it seems (I get `Couldn't match type ‘k1’
 with ‘*’ arising from a use of ‘callCC’`).

 I don't understand your point about `MonadState` and `MonadReader`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12418#comment:4>
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