[GHC] #16319: unexpected difference between newtype and data

14 Feb 2019

      #16319: unexpected difference between newtype and data
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           Reporter:  pjljvdlaar        |             Owner:  (none)
               Type:  bug               |            Status:  new
           Priority:  normal            |         Milestone:
          Component:  Compiler          |           Version:  8.2.2
           Keywords:                    |  Operating System:  Windows
       Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
          Test Case:                    |        Blocked By:
           Blocking:                    |   Related Tickets:
Differential Rev(s):                    |         Wiki Page:
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 The following code compiles fine
 {{{#!hs
 -- | A referable class
 -- An object can only have one reference.
 -- For example, a function is either referenced by name of by signature
 but not both.
 class ( Eq (Ref a)
       , Ord (Ref a)
       , Hashable (Ref a)
       , Show (Ref a)
       , Read (Ref a)
       , Data (Ref a)
       ) => Referable a where
     type Ref a
     toRef :: a -> Ref a

 data Referable a =>
             RefMap a = RefMap { -- | the HashMap
                                 toHashMap :: HashMap.Map (Ref a) a
                               } deriving (Eq, Ord, Show, Read, Data)
 }}}
 yet following the hlint hint to replace data by the equivalent newtype
 results in the following issues:
 {{{
   src\TorXakis\Referable.hs:59:43: error:
         * Could not deduce (Eq (Ref a))
             arising from the 'deriving' clause of a data type declaration
           from the context: Eq a
             bound by the deriving clause for `Eq (RefMap a)'
             at src\TorXakis\Referable.hs:59:43-44
           Possible fix:
             use a standalone 'deriving instance' declaration,
               so you can specify the instance context yourself
         * When deriving the instance for (Eq (RefMap a))
        |
     59 |                               } deriving (Eq, Ord, Show, Read,
 Data)
        |                                           ^^

     src\TorXakis\Referable.hs:59:47: error:
         * Could not deduce (Ord (Ref a))
             arising from the 'deriving' clause of a data type declaration
           from the context: Ord a
             bound by the deriving clause for `Ord (RefMap a)'
             at src\TorXakis\Referable.hs:59:47-49
           Possible fix:
             use a standalone 'deriving instance' declaration,
               so you can specify the instance context yourself
         * When deriving the instance for (Ord (RefMap a))
        |
     59 |                               } deriving (Eq, Ord, Show, Read,
 Data)
 }}}

 I assume this is a bug...

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16319
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