[GHC] #8026: DatatypeContexts should be fixed, not deprecated

[GHC]

#8026: DatatypeContexts should be fixed, not deprecated
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              Reporter:  gidyn       |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.6.3
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by goldfire):

 This proposal ''does'' propose a change to the type system, as it proposes
 a change to the set of well-typed programs. The same is true of partial
 type signatures. Type inference is an algorithm that implements a way to
 enforce the constraints of a type system.

 Your example is clear for that simple situation. But, Haskell's type
 system is not that simple! For example:

 {{{
 data Eq a => HasEq a = MkHasEq a
   deriving Typeable
 data Show a => HasShow a = MkHasShow a
 a :: Maybe (HasEq a) -> Maybe (HasEq a) -> Bool
 a (Just x) (Just y) = x == y
 a _ _ = Nothing

 b :: Either (HasEq a) (HasShow a) -> String   -- what constraints are used
 here???
 b (Left x) = show (x == x)
 b (Right x) = show x

 c :: Proxy (HasEq a) -> ()   -- what constraints are used here???
 c _ = ()

 data Dynamic = forall a. Typeable a => MkDyn a

 d :: Dynamic -> Maybe Bool
 d (MkDyn y) | Just (MkHasEq z) <- cast y = Just (z == z)   -- where does
 the Eq constraint come from??
 d _ = Nothing
 }}}

 Your examples seem to address specific, easy cases. But, we need to
 consider all the wide range of compositions with other Haskell features!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8026#comment:18>
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