[GHC] #12096: Attach stacktrace information to SomeException

[GHC]

#12096: Attach stacktrace information to SomeException
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        Reporter:  ndtimofeev        |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              Version:  8.0.1
      Resolution:                    |             Keywords:  Exceptions
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ndtimofeev):

 We can substitute `?callStack` in `catch` from actual value to value from
 caught exception value.
 {{{#!hs
 catch' :: Exception e => IO a -> (HasCallStack => e -> IO a) -> IO a
 catch' eval handler = eval `catch` \err@(SomeExceptionWithStack stack _)
 ->
     let ?callStack = stack in
         case fromException err of
             Just ex -> handler ex
             _       -> throw err
 }}}
 Now all `throw` called from `catch` handler will be implicit rethrow. It
 looks like a ugly hack and required Rank2Types.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12096#comment:18>
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