Template Haskell changes to names and package keys

[Edward Z. Yang]

In GHC 7.10, we changed the internal representation of names to
be based on package keys (base_XXXXXX) rather than package IDs
(base-4.7.0.1), however, we forgot to update the Template Haskell
API to track these changes.  This lead to some bugs in TH
code which was synthesizing names by using package name and
version directly, e.g. https://ghc.haskell.org/trac/ghc/ticket/10279

We now propose the following changes to the TH API in order to
track these changes:

    1. Currently, a TH NameG contains a PkgName, defined as:

        newtype PkgName = PkgName String

       This is badly misleading, even in the old world order, since
       these needed version numbers as well. We propose that this be
       renamed to PkgKey:

        newtype PkgKey = PkgKey String
        mkPackageKey :: String -> PackageKey
        mkPackageKey = PkgKey

    2. Package keys are somewhat hard to synthesize, so we also
       offer an API for querying the package database of the GHC which
       is compiling your code for information about packages.  So,
       we introduce a new abstract data type:

        data Package
        packageKey :: Package -> PkgKey

       and some functions for getting packages:

        searchPackage :: String -- Package name
                      -> String -- Version
                      -> Q [Package]

        reifyPackage :: PkgKey -> Q Package

       We could add other functions (e.g., return all packages with a
       package name).

    3. Commonly, a user wants to get the package key of the current
       package.  Following Simon's suggestion, this will be done by
       augmenting ModuleInfo:

        data ModuleInfo =
            ModuleInfo { mi_this_mod :: Module -- new
                       , mi_imports :: [Module] }

       We'll also add a function for accessing the module package key:

        modulePackageKey :: Module -> PkgKey

       And a convenience function for accessing the current module:

        thisPackageKey :: Q PkgKey
        thisPackageKey = fmap (modulePackageKey . mi_this_mod) qReifyModule

        thisPackage :: Q Package
        thisPackage = reifyPackage =<< thisPackageKey

Discussion period: 1 month

Thanks,
Edward

(apologies to cc'd folks, I sent from my wrong email address)