[GHC] #8503: New GeneralizedNewtypeDeriving check still isn't permissive enough

4 Nov 2013

      #8503: New GeneralizedNewtypeDeriving check still isn't permissive enough
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       Reporter:  goldfire          |             Owner:  goldfire
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Roman Cheplyaka writes:

 I just tried compiling smallcheck with GHC HEAD, and it didn't work out:

 {{{
  Test/SmallCheck/SeriesMonad.hs:41:7:
      Can't make a derived instance of ‛MonadLogic (Series m)’
        (even with cunning newtype deriving):
        it is not type-safe to use GeneralizedNewtypeDeriving on this
 class;
        ‛msplit’, at type ‛forall a.
                           m a -> m (Maybe (a, m a))’, cannot be converted
 safely
      In the newtype declaration for ‛Series’
 }}}

 So GHC now looks at the methods, but the problem is still there.

 I would agree that `msplit`'s type is problematic (due to the nested
 `m`'s),
 but Simon and Richard previously said that it should work, so I'm
 confused.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8503
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