[GHC] #9265: Extend PackageId to include version dependency information

[GHC]

#9265: Extend PackageId to include version dependency information
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       Reporter:  ezyang            |             Owner:  ezyang
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Package system    |           Version:  7.9
       Keywords:  backpack          |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 [MultiInstances Multiple installed instances of a package] has been a long
 sought after feature to alleviate Cabal hell, and was the subject of an
 [GSoCMultipleInstances GSoC project] that ultimately never got merged into
 HEAD.

 This ticket is our latest attack on the problem, summarized as: add a
 version dependency hash to the `PackageId` (NOT the `InstalledPackageId`)
 associated with packages in the database.  This approach is distinguished
 from the prior attempts as follows:

 - We make no attempt to support multiple installations of different ways
 or ABI-compatible versions of a library (e.g. with/without optimizations.)
 This corresponds to supporting multiple InstalledPackageIds in a database;
 in our case, we're supporting multiple `PackageId`.

 - We do not have to deal with conflicting "instances" in GHC's package
 resolver: multiple instances can be simultaneously loaded and used in a
 single compiled program. This is because PackageIds are baked into the
 exported linker symbols, so different versions will have different names
 and can peacefully coexist.

 - We do not have to deal with the delicate ordering problem of calculating
 an ABI hash when configuring a package, which is prior to when we actually
 know it (ABI hash is only known after compilation), because our hash is
 based ONLY on dependency resolution choice.

 - In absence of preference for previously installed packages, our Cabal
 dependency resolution stays exactly the same: Cabal picks versions, and
 from these choices we calculate the dependency hashes. With preference, we
 will have to do a little more work.

 - We do not attempt to garbage collect old packages. Because we are not
 dependent on ABI, there is not an explosion of installed pacakges from
 package development; new installed packages only occur when version
 numbers are bumped up, or packages are installed in a combinatorially
 different fashion.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9265>
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