[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 gidyn):

 I think we actually have 2 implementation proposals here:

 - tokyo's rules add type constraints, whenever the data type appears in an
 explicit type signature or when the constructor is used
 - use regular type inference

 The second implementation would give different results for `c` and `d`:

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

 No type constraint is added, because none is inferred by the definition of
 `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
 }}}

 `DatatypeContexts` wouldn't have any effect in this case.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8026#comment:20>
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