[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):

 No, the issue is at a lower level than environments. With existing GHC
 options, it is not possible to express what is required here. We want GHC
 to choose packages in a way so as to stack the dbs on top of each other
 (upper dbs in the stack overriding the lower ones) rather than union them
 which is the default behavior.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12518#comment:9>
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