[GHC] #10823: Expose keepAlive to Template Haskell

[GHC]

#10823: Expose keepAlive to Template Haskell
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        Reporter:  spinda            |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Template Haskell  |                 Version:  7.10.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by goldfire):

 It's a bit of a hack, but there's a workaround here.

 Define this in some available module:

 {{{
 data Anything = forall a. Mk a

 class KeepAlive (what :: Symbol) where
   whatToKeep :: proxy what -> Anything
 }}}

 And then generate this using existing TH:

 {{{
 instance KeepAlive "gt" where
   whatToKeep _ = Mk gt
 }}}

 Then GHC will mark `gt` as used.

 This is ugly, but it should work well in practice. In the effort to keep
 TH from bloating too much, I would lean against adding new features to
 support operations that can be simulated well today. Feel free to try to
 convince me otherwise -- I'm not dead set in this position. But I'm
 worried about feature creep here.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10823#comment:2>
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