[GHC] #7991: stm: add mkWeakTVar

[GHC]

#7991: stm: add mkWeakTVar
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        Reporter:  errge              |            Owner:  simonmar
            Type:  feature request    |           Status:  patch
        Priority:  highest            |        Milestone:  7.8.1
       Component:  libraries (other)  |          Version:  7.6.3
      Resolution:                     |         Keywords:
Operating System:  Unknown/Multiple   |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown       |       Difficulty:  Unknown
       Test Case:                     |       Blocked By:
        Blocking:                     |  Related Tickets:
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Comment (by errge):

 Sorry, I broke the link in the description by deleting the pull request
 repository.

 Here is an updated link:

 https://github.com/Gabriel439/Haskell-Pipes-Concurrency-
 Library/blob/master/src/Pipes/Concurrent.hs#L211

 The only reason why Tekmo is doing unsafeIOToSTM here is because there is
 no mkWeakTVar.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7991#comment:4>
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