[GHC] #12518: Allow customizing immutable package dbs by stacking

[GHC]

#12518: Allow customizing immutable package dbs by stacking
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        Reporter:  harendra          |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Package system    |              Version:  8.0.1
      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 harendra):

 We can also add `--include-hidden-pkgs` to this list to address the
 handling of `exposed` flag that you raised. That will complete the whole
 story. If we combine these with the `environment file` it will become a
 powerful way to compose package databases and can be widely useful. Let me
 know what you think, does it make sense to make it so flexible?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12518#comment:6>
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