[GHC] #9123: Need for higher kinded roles

[GHC]

#9123: Need for higher kinded roles
------------------------------------+-------------------------------------
       Reporter:  simonpj           |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
------------------------------------+-------------------------------------
 This [http://www.haskell.org/pipermail/ghc-devs/2014-May/004964.html
 thread] on ghc-devs identifies a real shortcoming of the new roles system.
 Here's a compact example, from the thread
 {{{
 class C m where
   ret :: a -> m a
   bind :: m a -> (a -> m b) -> m b
   join :: m (m a) -> m a

 newtype T m a = MkT (m a) deriving( C )
 }}}
 The `deriving(C)` is accepted without `join` in the class, but rejected
 when `join` is added.  And the AMP proposal adds `join` to class `Monad`!

 In one way it is rightly rejected: it really would be unsound to derive an
 instance for `C (T K)` where `K`'s argument had nominal (but not
 representational) role.  But we have no way to limit the type constructors
 at which `T` can be used.

 This deficiency is noted in the [http://research.microsoft.com/en-
 us/um/people/simonpj/papers/ext-f/ Safe Coercions paper], but this seems
 to be the first occasion on which it has bitten us badly.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9123>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler