[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):

 The intent of this issue was to provide an alternative `stacked` db
 composing policy in addition to the default `union` composing policy.

 Though, as you suggested, it can be implemented in terms of existing
 primitives by first specifying a list of dbs and then explicitly
 specifying the overriding packages using `-package pkg-x.y.z` flags in an
 environment file. That might mean specifying a lot of `-package` flags in
 the environment file though. We will have to explicitly identify and
 specify all the packages which override other packages in any of the lower
 dbs in the db stack. I guess it should do the job if we do not want to
 have a built-in stacked db policy.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12518#comment:11>
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