[GHC] #14030: Implement the "Derive Lift instances for data types in template-haskell" proposal

[GHC]

#14030: Implement the "Derive Lift instances for data types in template-haskell"
proposal
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        Reporter:  RyanGlScott       |                Owner:  RyanGlScott
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Template Haskell  |              Version:  8.3
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 At one point in time (a release of GHC or two ago), I attempted to
 implement this, but ran into interface file-related issues. Unfortunately,
 I no longer have the exact errors in front of me, but I recall that there
 was some difficulty due to the fact that:

 1. We were deriving `Lift` instances in `Language.Haskell.TH.Syntax`
 2. The derived `Lift` code references functions from
 `Language.Haskell.TH.Lib.Internal`
 3. Because `Language.Haskell.TH.Lib.Internal` imports
 `Language.Haskell.TH.Syntax`, there were (seemingly) some issues with
 import cycles

 That being said, I just tried implementing this afresh with GHC 8.6.2 as
 my bootstrapping compiler, and I did not hit any issues at all! This is
 quite strange, and I'm somewhat paranoid that there //is// an issue
 lurking underneath the surface that I just haven't exposed yet.

 While typing this out, I realized one thing. I think the last time I tried
 this, I might have been using GHC 8.2.2 as my bootstrapping compiler.
 There is a notable difference between 8.2.2 and future major releases:
 8.2.2 does //not// have `Language.Haskell.TH.Lib.Internal` (it wouldn't be
 until 8.4 that that would come into existence). I think GHC might have
 been confused because the `DeriveLift`-generated code that the
 bootstrapping compiler (8.2.2) generated mentioned things from
 `Language.Haskell.TH.Lib`, but the stage-1 compiler generated `DeriveLift`
 code that mentioned `Language.Haskell.TH.Lib.Internal`.

 If that's the case, then that would explain why I can't reproduce the
 issue now (with 8.6.2). Still, this has me a bit worried that what we're
 doing here is a bit fragile, and that perhaps moving functions from
 `Language.Haskell.TH.Lib.Internal` to some other, hypothetical module in
 the future might cause this issue to surface once more...

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14030#comment:6>
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