[GHC] #10514: Generic for existential types

12 Jun 2015


      #10514: Generic for existential types
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              Reporter:              |             Owner:  ekmett
  andreas.abel                       |            Status:  new
                  Type:  feature     |         Milestone:
  request                            |           Version:  7.10.1
              Priority:  normal      |  Operating System:  Unknown/Multiple
             Component:  Core        |   Type of failure:  None/Unknown
  Libraries                          |        Blocked By:
              Keywords:              |   Related Tickets:
          Architecture:              |
  Unknown/Multiple                   |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 I have some use for Generic for an existential type which is constraint to
 be Generic.

 {{{!#hs

 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE ExistentialQuantification #-}
 {-# LANGUAGE StandaloneDeriving #-}

 import GHC.Generics

 data U = forall a. (Generic a) => U a
   -- deriving (Generic)
     -- Can't make a derived instance of ‘Generic U’:
     --   Constructor ‘U’ has existentials or constraints in its type
     --   Possible fix: use a standalone deriving declaration instead

 -- deriving instance Generic U
     -- Can't make a derived instance of ‘Generic U’:
     --   U must be a vanilla data constructor
     -- In the stand-alone deriving instance for ‘Generic U’

 data D1Ser
 data C1_0Ser

 instance Generic U where
   type Rep U = D D1Ser (C1 C1_0Ser (S1 NoSelector (Rep a)))
     -- Not in scope: type variable ‘a’

 -- How to bring the existential type `a' into scope?

 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10514
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