ghc-pkg read/write GADT

Main.hs

data ReadMode      -- just type level tags
data ReadWriteMode

-- | Like Bool but statically determined by the mode type.
data OpenMode mode where
  OpenReadMode      :: OpenMode ReadMode
  OpenReadWriteMode :: OpenMode ReadWriteMode

-- | Like Maybe but the case is statically determined by the mode type.
data InReadWriteMode mode a where
  InReadMode      ::      InReadWriteMode ReadMode a
  InReadWriteMode :: a -> InReadWriteMode ReadWriteMode a

deriving instance Show a => Show (InReadWriteMode mode a)

getPkgDatabases :: Verbosity
                -> OpenMode mode -- we are modifying, not reading
                -> Bool    -- use the user db
                -> Bool    -- read caches, if available
                -> Bool    -- expand vars, like ${pkgroot} and $topdir
                -> [Flag]
                -> IO (PackageDBStack,
                          -- the real package DB stack: [global,user] ++
                          -- DBs specified on the command line with -f.
                       InReadWriteMode mode FilePath,
                          -- which one to modify, if any
                       PackageDBStack)

-- then uses look like:

  (db_stack, InReadWriteMode to_modify, _flag_dbs) <-
      getPkgDatabases verbosity OpenReadWriteMode True{-use user-}
                                True{-use cache-} False{-expand vars-} my_flags

-- but still need some more changes so that changeDB takes this to_modify output,
-- rather than what it takes now. That might avoid us having to add a Maybe Handle
-- into the PackageDb type.

No love for DataKinds here?