[GHC] #13242: Panic "StgCmmEnv: variable not found" with ApplicativeDo and ExistentialQuantification

[GHC]

#13242: Panic "StgCmmEnv: variable not found" with ApplicativeDo and
ExistentialQuantification
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        Reporter:  ljli              |                Owner:  simonmar
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.3
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
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 simonpj):

 Simon, I also want to discuss the implementation.  Look at `goArgs` in
 `tcApplicativeStmts`.  It uses a CPS structure, so that things brought
 into scope stay in scope.  But that's wrong for constraints (hence this
 ticket), and it's not really right for bindings either (only the
 `thing_inside` needs to see those binders).

 So, proposal:

 * Use a `mapM` to process each of the args indepdendly (mirroring the
 intended parallel semantics)
 * Now bring into scope all the variables bound by the args (which we can
 get from their patterns), and typecheck `thing_inside`.

 Can you see anything wrong with this?  Should not be hard to try.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13242#comment:9>
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