[GHC] #10871: Implement "fat" interface files which can be directly compiled without source

[GHC]

#10871: Implement "fat" interface files which can be directly compiled without
source
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        Reporter:  ezyang            |                Owner:  ezyang
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:  backpack
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):

 First, I absolutely agree that we should be able to use Backpack
 without Cabal, indeed it must be so since Cabal is just a layer on top
 of GHC.

 The original intuition with Backpack was that building fully
 instantiated packages can work exactly as it does now - we build
 everything from the leaves up to the root in dependency order, no
 extra mechanisms are needed.  Furthermore, when dealing with
 indefinite packages, we can generate interface files but not code.
 I'm still not seeing anything in this that requires fat interface
 files.

 Let me re-answer your questions: What does it mean to
 install an indefinite package?  We install just the interface files,
 so that we can typecheck against it.  What does it mean to install a
 package that instantiates an indefinite package?  Just built and
 install it!  Then we have to build the package that it instantiates,
 which is exactly how cabal-install works now.

 I think your story is more complicated.  You said "GHC can manage
 compiling the instantiated units, since it can just resume that
 compilation!" but that's blurring the boundary between Cabal and GHC,
 since suddenly GHC has to go and compile *and install* things.  I can
 imagine this getting really messy.  Better to have Cabal manage
 building and installing things at the package level, like it does now.

 GHC will need to know when it is just typechecking something
 vs. compiling it against definite dependencies.  Hence, Cabal will
 also need a new command (or something) to do this - it's a new mode of
 operation, and the user needs to be in control, so there's no escaping
 this being visible at the Cabal level, I believe.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10871#comment:17>
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