[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 simonmar):

 Could you give a specific example?  From the description it looks like
 environments are exactly what you want:

 > Stack implements this by passing explicit package-ids of the packages to
 GHC. This scheme works well for cabal projects where we know ALL the
 packages used by the project in advance. But it does not work for scripts
 run using runghc. In that case we do not know the packages required by the
 script in advance and therefore cannot pass the package-ids to GHC. That
 means we cannot make GHC use the packages in the right way. GHC will
 choose the latest version even though we want it to choose a possibly
 older version from the top of the db stack.

 Environments are designed for exactly this situation: you're using
 `runghc` or `ghci`, and you want `GHC` to make a specific set of packages
 visible.  In particular, you're saying you want to deliberately use older
 versions of some package: you could just omit the newer versions from the
 environment, and GHC will ignore them.  Alternatively you can specifically
 request the older version using a `-package` flag.

 One reason I'm pushing on this because I think we should deprecate the
 idea of DB stacks (see #12485).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12518#comment:10>
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