[GHC] #9204: Conflicting definition in hs-boot file

[GHC]

#9204: Conflicting definition in hs-boot file
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        Reporter:  Feuerbach         |            Owner:  goldfire
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Documentation     |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Changes (by goldfire):

 * owner:   => goldfire
 * component:  Compiler => Documentation


Comment:

 Replying to [comment:1 Feuerbach]:
 > If I add a role annotation to the hs-boot file, this works

 Yes, that's the solution.
 .
 > My guess is that since the hs-boot doesn't have the information about
 the data constructors, it cannot infer the phantom role of the parameter.

 Yes. As documented
 [http://www.haskell.org/ghc/docs/latest/html/users_guide/separate-
 compilation.html#mutual-recursion here], in the last bullet point before
 4.7.10, role inference in hs-boot files defaults to representational, the
 most common case.

 > Still, I wish I didn't have to deal with this. But if this isn't
 possible, at least the error message should suggest adding the role
 annotation to the hs-boot file.

 Yes, absolutely. I will fix in due course, and clarify some of this in the
 manual as well.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9204#comment:2>
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