[GHC] #4329: GHC.Conc modifyTVar primitive

[GHC]

#4329: GHC.Conc modifyTVar primitive
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    Reporter:  dmbarbour         |       Owner:                                    
        Type:  feature request   |      Status:  new                               
    Priority:  normal            |   Component:  Compiler                          
     Version:  6.12.3            |    Keywords:  TVar, STM, modifyTVar, Concurrency
    Testcase:                    |   Blockedby:                                    
          Os:  Unknown/Multiple  |    Blocking:                                    
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown                      
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 I would like modifyTVar as a 'write-only' primitive supported by GHC's
 STM.

 The semantic definition is:

 > modifyTVar :: TVar a -> (a -> a) -> STM ()
 > modifyTVar v f = readTVar v >>= writeTVar v . f

 However, explicitly reading then writing the TVar introduces unnecessary
 interference between transactions. The value held by the TVar does not
 affect the behavior of the transaction. modifyTVar should not interfere
 with other transactions that only writeTVar or modifyTVar.

 Even if a non-interfering implementation isn't feasible due to the
 underlying implementation of GHC's STM, providing the function would
 provide a point for a transparent upgrade in the future.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4329>
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