[GHC] #11164: No way to import a data instance

[GHC]

#11164: No way to import a data instance
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        Reporter:  Feuerbach         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      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 goldfire):

 Agreed on all fronts.

 Would it be better to have this rule?
 * Whenever a module omits an export list, if that module declares and data
 instances, the data family is also (re-)exported.

 That would seem to fix your problem. Note that there is no difficulty if
 the exporting module specifies an export list, as it has to either export
 the data family itself, or use `pattern` to export the data instance
 constructor. In the latter case, we assume the module author has a reason
 not to export the data family (and, the exact same scenario happens with
 ordinary datatypes, requiring importers with import lists to also use
 `pattern`).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11164#comment:3>
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