[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):

 I'm not sure what you mean by "grant access"... but perhaps this will
 help: When you pattern match on the `Dict` constructor, you get the `c`
 constraint in the context. So, the following is conceivable:

 {{{
 -- this type is actually from ekmett's "constraints" package
 data Dict c where
   Dict :: c => Dict c

 class IntLike a where
   fromInt :: Int -> a
   toInt :: a -> Int

 dictForIntLikeString :: Dict (IntLike String)
 dictForIntLikeString = ...

 stringToInt :: String -> Int
 stringToInt s = case dictForIntLikeString of
   Dict -> toInt s
 }}}

 Note that there is no `instance IntLike String` around -- the reason
 `toInt` can be called in `stringToInt` is because of the pattern-match on
 `Dict`. Now, this example falls apart because the implementation of
 `dictForIntLikeString` seems impossible without a `instance IntLike
 String` dictionary available. But perhaps this answers your question about
 what `Dict` can do.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9819#comment:3>
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