[GHC] #10841: Run handler on STM retry

[GHC]

#10841: Run handler on STM retry
-------------------------------------+-------------------------------------
              Reporter:  shlevy      |             Owner:
                  Type:  feature     |            Status:  new
  request                            |
              Priority:  normal      |         Milestone:
             Component:  libraries   |           Version:  7.10.2
  (other)                            |
              Keywords:  stm         |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 It would be nice to have a function like:

 {{{#!hs
 -- | Perform a series of STM actions atomically.
 --
 --   Whenever the transaction retries, run the handler. If it doesn't
 return 'Nothing',
 --   stop the retrying and return the value given by the handler.
 atomicallyWithRetryHandler :: STM a -> IO (Maybe a) -> IO a
 }}}

 This would allow tracking retry statistics, printing debug messages on
 each retry, etc. without resorting to unsafeIOToSTM which can't safely do
 things like printing.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10841>
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