[GHC] #7991: stm: add mkWeakTVar

[GHC]

#7991: stm: add mkWeakTVar
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Reporter:  errge             |          Owner:                   
    Type:  feature request   |         Status:  new              
Priority:  normal            |      Component:  libraries (other)
 Version:  7.6.3             |       Keywords:                   
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple 
 Failure:  None/Unknown      |      Blockedby:                   
Blocking:                    |        Related:                   
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 The motivation is to get rid of an unsafeIOToSTM from pipes-concurrency.

 https://github.com/errge/Haskell-Pipes-Concurrency-
 Library/commit/356a295f2b8da3bcc89ffe8e121647237f472d0f

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7991>
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