[GHC] #14880: GHC panic: updateRole

[GHC]

#14880: GHC panic: updateRole
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        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 The `updateRole` panic triggers on various invalid-tycon problems. I'm not
 surprised that roles aren't involved.

 Yes, a `forall` to the right of the `::` in a data declaration makes fine
 sense. For example, here is one way to define heterogeneous equality:

 {{{#!hs
 data (:~~:) :: forall k1. k1 -> forall k2. k2 -> Type where
   HRefl :: a :~~: a
 }}}

 This definition is actually a touch more general than one where `k1` and
 `k2` are quantified prenex, as the "Practical Type Inference" paper
 explains.

 I don't see a need to document this specially.

 As for the toposorting suggestions: I've considered it a design principle
 that implicit quantification comes before all explicit quantification. Of
 course, we can't panic when something goes wrong here, but I think this
 design is a good one, just to have some rules that users can rely on.

 My existing patch still panics on this case, but I agree that no one
 should spend time on this until that patch lands.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14880#comment:5>
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