[GHC] #13242: Panic "StgCmmEnv: variable not found" with ApplicativeDo and ExistentialQuantification
GHC
ghc-devs at haskell.org
Tue Feb 21 00:11:52 UTC 2017
#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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