[GHC] #9813: Error when reifying type constructor

[GHC]

#9813: Error when reifying type constructor
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        Reporter:  owst              |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  7.8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1899
       Wiki Page:                    |
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Comment (by owst):

 Replying to [comment:27 rwbarton]:
 > This all sounds great except for one thing. Do we have a plan for
 actually loosening the stage restriction? The stage restriction exists for
 technical reasons, not conceptual reasons.
 >
 > The stage restriction currently applies to the module as a whole. It
 says you cannot use an identifier that was defined in a module within a
 splice in the same module. Clearly if the use is in a later declaration
 group than the definition, there is no problem in principle with lifting
 the restriction. But in order to do so, we would have to run the
 declaration group containing the definition through the entire compiler
 pipeline, right up to linking, so that we can load it to run the splice in
 the latter declaration group to begin type checking it. GHC just isn't
 organized to work that way currently.

 An incredibly good point. To answer the first question simply: I don't,
 no.

 I haven't hacked on GHC before, so was attempting to flesh out my original
 bug report to find a change that I might get my teeth into. I have to
 admit I hadn't thought about the technical barriers and thus rather
 haphazardly threw about the "let's loosen the stage restriction" part!

 > The user's guide has this to say on the subject: ...

 Indeed - I pasted that very snippet in comment:14, but I hadn't
 appreciated what the technical barriers were.

 What do you think is a sensible step forward? Change 2.b. in my previous
 comment to "Process NS first, then S. Splices within S can refer (via
 reify but ''not'' evaluation) to all names declared in NS or previous
 DGs), but no names within S"? That way, my original program in this ticket
 would work, but I wouldn't need any of the complexity of changing the
 stage restriction too.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9813#comment:28>
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