[GHC] #10970: Built in MIN_VERSION macro support

[GHC]

#10970: Built in MIN_VERSION macro support
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        Reporter:  ezyang            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1349,
       Wiki Page:                    |  Phab:D1869
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Comment (by rwbarton):

 Replying to [comment:12 hvr]:
 > Replying to [comment:3 rwbarton]:
 > > [...] I don't understand any more why it should be the case that we
 only create these macros for packages explicitly specified with `-package`
 or equivalent.
 >
 > Well, if you `-hide-all-packages` and unmask a few packages via
 `-package`, then exposing macros for all other packages in the package
 database would be very unsound, as it would destroy modularity.

 I was unclear in comment:3. I thought I was pretty clear in comment:7, but
 let me try again.

 For a given compilation, after processing all package-related flags and
 environment variables, and reading in all package databases, GHC selects
 some set of installed packages, called the exposed packages. These are the
 packages that GHC will use to satisfy import statements.

 My proposal is simply: define `MIN_VERSION_x` macros for exactly the
 exposed packages. If you can import it, then you can check its version
 too.

 So, if you `-hide-all-packages` and unmask a few packages via `-package`,
 you'd only get `MIN_VERSION_x` macros for those packages you exposed with
 `-package`.

 The modularity concerns that Cabal addresses for imports then extend
 directly to `MIN_VERSION_x` macros. In Cabal, you need to state your
 dependency on a module to import it. So, you need to state your dependency
 on a module to check its version with `MIN_VERSION_x`, too. No need to
 reason explicitly about `-hide-all-packages` and `-package` flags to
 arrive at this result.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10970#comment:13>
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