[GHC] #13140: Handle subtyping relation for roles in Backpack

[GHC]

#13140: Handle subtyping relation for roles in Backpack
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        Reporter:  ezyang            |                Owner:  ezyang
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |             Keywords:  backpack hs-
      Resolution:                    |  boot
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3123
       Wiki Page:                    |
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Comment (by goldfire):

 If, from your perspective, the added complexity is worth it, then this
 looks OK to me.

 What's frustrating here is that we don't really need a new `Abstract`
 equality relation: the current `Phantom` one works fine. All we need is a
 new role ''annotation''. Is it worth creating a new `data RoleAnnotation =
 Abstract | Concrete Role`? Perhaps. You would also then need to be careful
 about uses of `tyConRoles` and `tyConRolesX`, as they are currently used
 both for construction and decomposition -- up until this proposal, both
 directions were happy with the same roles.

 I do agree that your approach is type safe. But I have to wonder if there
 isn't a simpler solution somehow, as I'm hesitant to advocate yet another
 wrinkle to the role system.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13140#comment:4>
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