[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):

 OK -- so it seems you want to be able to fuzz the `*`/`Constraint`
 distinction. That is, for example, make explicit dictionaries of `Show`
 and pass them as parameters to functions with a declared `Show`
 constraint. Another way to think about this is that you want the ability
 to define local instances, much like what is done in that blog post. The
 problem here is that local instances threaten coherence properties. Local
 instances are really useful, but I think a fair amount of design work
 needs to be done to integrate local instances with Haskell.

 Or, simply to automate building `Def` instances, I would suggest Template
 Haskell. Without a general plan toward local instances, it seems too
 specific to bake in deriving `Def` instances into GHC, when it can easily
 be done with TH.

 [https://groups.google.com/forum/#!msg/haskell-
 cafe/JR8qd6RBKW4/xDcdtmuE9gsJ This haskell-cafe thread] may be of
 interest. It explores some of this design space.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9819#comment:5>
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