[GHC] #9819: Create typesafe method of obtaining dictionary types from class definitions, and constraint objects from dictionary types

[GHC]

#9819: Create typesafe method of obtaining dictionary types from class
definitions, and constraint objects from dictionary types
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              Reporter:              |            Owner:
  spacekitteh                        |           Status:  new
                  Type:  feature     |        Milestone:
  request                            |          Version:
              Priority:  normal      |         Keywords:
             Component:  Compiler    |     Architecture:  Unknown/Multiple
            Resolution:              |       Difficulty:  Moderate (less
      Operating System:              |  than a day)
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by goldfire):

 But, if you had local instances, then there wouldn't be just one, say,
 `Ord Int` out there. There would be the global one, and potentially many
 local ones. It now becomes very confused to try to use a `Set Int`, when
 the `Ord Int` instance might be changing.

 I'm not saying that local instances are Wrong, but that they require a
 fair amount of design work before we can begin to think about
 implementing.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9819#comment:8>
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