[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 have realised another problem with `ApplicativeDo`, not covered by
 our paper.  Consider
 {{{
 data A where
   A :: a -> (a->Int) -> A

 st :: A -> A -> IO (Int,Int)
 st p q = do
   p1 <- pure p
   A x1 f1 <- pure p1
   q1 <- pure q
   A x2 f2 <- pure q1
   pure (f1 x1, f2 x2)
 }}}
 We'll try to gather `(x1,f1)` in a pair, and `(x2,f2)`; but that won't
 work because they each mention an existential type variable.  Our
 desugaring just doesn't work at all for that case.

 What to do?

 Another awkward wrinkle: what are the dependencies induced by view
 patterns?  And do we account for them?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13242#comment:6>
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