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  1. 11 Oct, 2015 2 commits
  2. 09 Oct, 2015 2 commits
    • Edward Z. Yang's avatar
    • Edward Z. Yang's avatar
      Implement ComponentId, replacing PackageKey and InstalledPackageId. · b083151f
      Edward Z. Yang authored
      
      
      Today in Cabal, when you build and install a package, it is
      uniquely identified using an InstalledPackageId which is computed
      using the ABI hash of the library that was installed.  There
      are few problems with doing it this way:
      
          - In a Nix-like world, we should instead uniquely identify
            build products by some sort of hash on the inputs to the
            compilation (source files, dependencies, flags).  The ABI
            hash doesn't capture any of this!
      
          - An InstalledPackageId suggests that we can uniquely identify
            build products by hashing the source and dependencies of
            a package as a whole.  But Cabal packages contain many components:
            a library, test suite, executables, etc.  Currently, when
            we say InstalledPackageId, we are really just talking about
            the dependencies of the library; however, this is unacceptable
            if a Cabal package can install multiple libraries; we need
            different identifiers for each.
      
          - We've also needed to compute another ID, which we've called
            the "package key", which is to be used for linker symbols
            and type equality GHC-side.  It is confusing what the distinction
            between this ID and InstalledPackageIds are; the main reason
            we needed another ID was because the package key was needed
            prior to compilation, whereas the ABI hash was only available
            afterwards.
      
      This patch replaces InstalledPackageId and PackageKey with a
      new identifier called ComponentId, which has the following
      properties:
      
          - It is computed per-component, and consists of a package
            name, package version, hash of the ComponentIds
            of the dependencies it is built against, and the name
            of the component.  For example, "foo-0.1-abcdef" continues
            to identify the library of package foo-0.1, but
            "foo-0.1-123455-foo.exe" would identify the executable,
            and "foo-0.1-abcdef-bar" would identify a private sub-library
            named bar.
      
          - It is passed to GHC to be used for linker symbols and
            type equality.  So as far as GHC is concerned, this is
            the end-all be-all identifier.
      
          - Cabal the library has a simple, default routine for computing
            a ComponentId which DOES NOT hash source code;
            in a later patch Duncan is working on, cabal-install can
            specify a more detailed ComponentId for a package
            to be built with.
      
      Here are some knock-on effects:
      
          - 'id' is a ComponentId
      
          - 'depends' is now a list of ComponentIds
      
          - New 'abi' field to record what the ABI of a unit is (as it is no longer
            computed by looking at the output of ghc --abi-hash).
      
          - The 'HasInstalledPackageId' typeclass is renamed to
            'HasComponentId'.
      
          - GHC 7.10 has explicit compatibility handling with
            a 'compatPackageKey' (an 'ComponentId') which is
            in a compatible format.  The value of this is read out
            from the 'key' field.
      Signed-off-by: default avatarEdward Z. Yang <ezyang@cs.stanford.edu>
      b083151f
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