Re: [GHC] #10514: Generic for existential types

12 Jun 2015

      #10514: Generic for existential types
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        Reporter:  andreas.abel      |                   Owner:  ekmett
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Core Libraries    |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Description changed by andreas.abel:

Old description:
...
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?
}}}
New description:

 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)  -- TRY 1
     -- 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 -- TRY 2
     -- 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 -- TRY 3
   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?

 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10514#comment:1
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