[GHC] #14061: `instance Eq Dyn`

30 Jul 2017


      #14061: `instance Eq Dyn`
-------------------------------------+-------------------------------------
           Reporter:  zaoqi          |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 {-# LANGUAGE KindSignatures, GADTs #-}
 data Dyn :: * where
     Dyn :: a -> Dyn
 }}}
 How to write the correct `instance Eq Dyn`, do not change `data Dyn`?

 {{{#!hs
 {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}

 class GEq a b where
     geq :: a -> b -> Bool

 instance {-# OVERLAPPABLE #-} (Eq a) => GEq a a where
     geq = (==)

 instance {-# OVERLAPPING #-} GEq a b where
     geq _ _ = False
 }}}
 {{{#!hs
 instance Eq Dyn where
     Dyn a == Dyn b = geq a b
 }}}

 {{{
 GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
 Prelude> :load Dyn.hs
 [1 of 1] Compiling Main             ( Dyn.hs, interpreted )
 Ok, modules loaded: Main.
 *Main> Dyn () == Dyn ()
 False
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14061
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