[GHC] #13128: Refactor AvailInfo to be more sensible

[GHC]

#13128: Refactor AvailInfo to be more sensible
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        Reporter:  mpickering        |                Owner:  mpickering
            Type:  task              |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:  12662
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by mpickering):

 The new problem with this ticket now is that backpack signature files use
 the same interface files as regular modules. There are operations which
 merge signature files together and in doing so perform operations like
 monadically mapping over the names in `AvailTC`. I don't understand this
 well enough yet to refactor things properly.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13128#comment:4>
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