[GHC] #9725: Constraint deduction failure
GHC
ghc-devs at haskell.org
Wed Oct 29 17:19:45 UTC 2014
#9725: Constraint deduction failure
-------------------------------------+-------------------------------------
Reporter: heisenbug | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.8.3
(Type checker) | Keywords:
Resolution: | Architecture: Unknown/Multiple
Operating System: | Difficulty: Unknown
Unknown/Multiple | Blocked By:
Type of failure: GHC | Related Tickets:
rejects valid program |
Test Case: |
Blocking: |
Differential Revisions: |
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Comment (by goldfire):
But you don't have to decompose the given equality to get the wanted. We
have
{{{
[G] e1 :: ent k1 ~ M b
[G] e2 :: Kn (M b)
[W] e3 :: Kn (ent k1)
}}}
Solve with
{{{
e3 := e2 |> Kn (sym e1)
}}}
The problem is that GHC tries to decompose the given, fails utterly, and
then has no way forward. So, I'd say this shows a completeness bug in the
solver. It's one we may not want to address, as I agree my kind equality
stuff makes this straightforward and I don't see an easy way of solving
this problem.
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9725#comment:10>
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