    -- (c) The University of Glasgow, 2006

    {-# LANGUAGE CPP, ScopedTypeVariables, BangPatterns, FlexibleContexts #-}

    -- | Package manipulation
    module Packages (
            module PackageConfig,

            -- * Reading the package config, and processing cmdline args
            PackageState(preloadPackages, explicitPackages, requirementContext),
            PackageConfigMap,
            emptyPackageState,
            initPackages,
            readPackageConfigs,
            getPackageConfRefs,
            resolvePackageConfig,
            readPackageConfig,
            listPackageConfigMap,

            -- * Querying the package config
            lookupPackage,
            lookupPackage',
            lookupInstalledPackage,
            lookupPackageName,
            improveUnitId,
            searchPackageId,
            getPackageDetails,
            getInstalledPackageDetails,
            componentIdString,
            displayInstalledUnitId,
            listVisibleModuleNames,
            lookupModuleInAllPackages,
            lookupModuleWithSuggestions,
            lookupPluginModuleWithSuggestions,
            LookupResult(..),
            ModuleSuggestion(..),
            ModuleOrigin(..),

            -- * Inspecting the set of packages in scope
            getPackageIncludePath,
            getPackageLibraryPath,
            getPackageLinkOpts,
            getPackageExtraCcOpts,
            getPackageFrameworkPath,
            getPackageFrameworks,
            getPackageConfigMap,
            getPreloadPackagesAnd,

            collectArchives,
            collectIncludeDirs, collectLibraryPaths, collectLinkOpts,
            packageHsLibs,

            -- * Utils
            unwireUnitId,
            pprFlag,
            pprPackages,
            pprPackagesSimple,
            pprModuleMap,
            isDllName
        )
    where

    #include "HsVersions.h"

    import GhcPrelude

    import GHC.PackageDb
    import PackageConfig
    import DynFlags
    import Name             ( Name, nameModule_maybe )
    import UniqFM
    import UniqDFM
    import UniqSet
    import Module
    import Util
    import Panic
    import Platform
    import Outputable
    import Maybes

    import System.Environment ( getEnv )
    import FastString
    import ErrUtils         ( debugTraceMsg, MsgDoc, printInfoForUser )
    import Exception

    import System.Directory
    import System.FilePath as FilePath
    import qualified System.FilePath.Posix as FilePath.Posix
    import Control.Monad
    import Data.Graph (stronglyConnComp, SCC(..))
    import Data.Char ( toUpper )
    import Data.List as List
    import Data.Map (Map)
    import Data.Set (Set)
    import Data.Monoid (First(..))
    import Data.Semigroup   ( Semigroup )
    import qualified Data.Semigroup as Semigroup
    import qualified Data.Map as Map
    import qualified Data.Map.Strict as MapStrict
    import qualified Data.Set as Set
    import Data.Version

    -- ---------------------------------------------------------------------------
    -- The Package state

    -- | Package state is all stored in 'DynFlags', including the details of
    -- all packages, which packages are exposed, and which modules they
    -- provide.
    --
    -- The package state is computed by 'initPackages', and kept in DynFlags.
    -- It is influenced by various package flags:
    --
    --   * @-package <pkg>@ and @-package-id <pkg>@ cause @<pkg>@ to become exposed.
    --     If @-hide-all-packages@ was not specified, these commands also cause
    --      all other packages with the same name to become hidden.
    --
    --   * @-hide-package <pkg>@ causes @<pkg>@ to become hidden.
    --
    --   * (there are a few more flags, check below for their semantics)
    --
    -- The package state has the following properties.
    --
    --   * Let @exposedPackages@ be the set of packages thus exposed.
    --     Let @depExposedPackages@ be the transitive closure from @exposedPackages@ of
    --     their dependencies.
    --
    --   * When searching for a module from a preload import declaration,
    --     only the exposed modules in @exposedPackages@ are valid.
    --
    --   * When searching for a module from an implicit import, all modules
    --     from @depExposedPackages@ are valid.
    --
    --   * When linking in a compilation manager mode, we link in packages the
    --     program depends on (the compiler knows this list by the
    --     time it gets to the link step).  Also, we link in all packages
    --     which were mentioned with preload @-package@ flags on the command-line,
    --     or are a transitive dependency of same, or are \"base\"\/\"rts\".
    --     The reason for this is that we might need packages which don't
    --     contain any Haskell modules, and therefore won't be discovered
    --     by the normal mechanism of dependency tracking.

    -- Notes on DLLs
    -- ~~~~~~~~~~~~~
    -- When compiling module A, which imports module B, we need to
    -- know whether B will be in the same DLL as A.
    --      If it's in the same DLL, we refer to B_f_closure
    --      If it isn't, we refer to _imp__B_f_closure
    -- When compiling A, we record in B's Module value whether it's
    -- in a different DLL, by setting the DLL flag.

    -- | Given a module name, there may be multiple ways it came into scope,
    -- possibly simultaneously.  This data type tracks all the possible ways
    -- it could have come into scope.  Warning: don't use the record functions,
    -- they're partial!
    data ModuleOrigin =
        -- | Module is hidden, and thus never will be available for import.
        -- (But maybe the user didn't realize), so we'll still keep track
        -- of these modules.)
        ModHidden
        -- | Module is public, and could have come from some places.
      | ModOrigin {
            -- | @Just False@ means that this module is in
            -- someone's @exported-modules@ list, but that package is hidden;
            -- @Just True@ means that it is available; @Nothing@ means neither
            -- applies.
            fromOrigPackage :: Maybe Bool
            -- | Is the module available from a reexport of an exposed package?
            -- There could be multiple.
          , fromExposedReexport :: [PackageConfig]
            -- | Is the module available from a reexport of a hidden package?
          , fromHiddenReexport :: [PackageConfig]
            -- | Did the module export come from a package flag? (ToDo: track
            -- more information.
          , fromPackageFlag :: Bool
          }

    instance Outputable ModuleOrigin where
        ppr ModHidden = text "hidden module"
        ppr (ModOrigin e res rhs f) = sep (punctuate comma (
            (case e of
                Nothing -> []
                Just False -> [text "hidden package"]
                Just True -> [text "exposed package"]) ++
            (if null res
                then []
                else [text "reexport by" <+>
                        sep (map (ppr . packageConfigId) res)]) ++
            (if null rhs
                then []
                else [text "hidden reexport by" <+>
                        sep (map (ppr . packageConfigId) res)]) ++
            (if f then [text "package flag"] else [])
            ))

    -- | Smart constructor for a module which is in @exposed-modules@.  Takes
    -- as an argument whether or not the defining package is exposed.
    fromExposedModules :: Bool -> ModuleOrigin
    fromExposedModules e = ModOrigin (Just e) [] [] False

    -- | Smart constructor for a module which is in @reexported-modules@.  Takes
    -- as an argument whether or not the reexporting package is expsed, and
    -- also its 'PackageConfig'.
    fromReexportedModules :: Bool -> PackageConfig -> ModuleOrigin
    fromReexportedModules True pkg = ModOrigin Nothing [pkg] [] False
    fromReexportedModules False pkg = ModOrigin Nothing [] [pkg] False

    -- | Smart constructor for a module which was bound by a package flag.
    fromFlag :: ModuleOrigin
    fromFlag = ModOrigin Nothing [] [] True

    instance Semigroup ModuleOrigin where
        ModOrigin e res rhs f <> ModOrigin e' res' rhs' f' =
            ModOrigin (g e e') (res ++ res') (rhs ++ rhs') (f || f')
          where g (Just b) (Just b')
                    | b == b'   = Just b
                    | otherwise = panic "ModOrigin: package both exposed/hidden"
                g Nothing x = x
                g x Nothing = x
        _x <> _y = panic "ModOrigin: hidden module redefined"

    instance Monoid ModuleOrigin where
        mempty = ModOrigin Nothing [] [] False
        mappend = (Semigroup.<>)

    -- | Is the name from the import actually visible? (i.e. does it cause
    -- ambiguity, or is it only relevant when we're making suggestions?)
    originVisible :: ModuleOrigin -> Bool
    originVisible ModHidden = False
    originVisible (ModOrigin b res _ f) = b == Just True || not (null res) || f

    -- | Are there actually no providers for this module?  This will never occur
    -- except when we're filtering based on package imports.
    originEmpty :: ModuleOrigin -> Bool
    originEmpty (ModOrigin Nothing [] [] False) = True
    originEmpty _ = False

    -- | 'UniqFM' map from 'InstalledUnitId'
    type InstalledUnitIdMap = UniqDFM

    -- | 'UniqFM' map from 'UnitId' to 'PackageConfig', plus
    -- the transitive closure of preload packages.
    data PackageConfigMap = PackageConfigMap {
            unPackageConfigMap :: InstalledUnitIdMap PackageConfig,
            -- | The set of transitively reachable packages according
            -- to the explicitly provided command line arguments.
            -- See Note [UnitId to InstalledUnitId improvement]
            preloadClosure :: UniqSet InstalledUnitId
        }

    -- | 'UniqFM' map from 'UnitId' to a 'UnitVisibility'.
    type VisibilityMap = Map UnitId UnitVisibility

    -- | 'UnitVisibility' records the various aspects of visibility of a particular
    -- 'UnitId'.
    data UnitVisibility = UnitVisibility
        { uv_expose_all :: Bool
          --  ^ Should all modules in exposed-modules should be dumped into scope?
        , uv_renamings :: [(ModuleName, ModuleName)]
          -- ^ Any custom renamings that should bring extra 'ModuleName's into
          -- scope.
        , uv_package_name :: First FastString
          -- ^ The package name is associated with the 'UnitId'.  This is used
          -- to implement legacy behavior where @-package foo-0.1@ implicitly
          -- hides any packages named @foo@
        , uv_requirements :: Map ModuleName (Set IndefModule)
          -- ^ The signatures which are contributed to the requirements context
          -- from this unit ID.
        , uv_explicit :: Bool
          -- ^ Whether or not this unit was explicitly brought into scope,
          -- as opposed to implicitly via the 'exposed' fields in the
          -- package database (when @-hide-all-packages@ is not passed.)
        }

    instance Outputable UnitVisibility where
        ppr (UnitVisibility {
            uv_expose_all = b,
            uv_renamings = rns,
            uv_package_name = First mb_pn,
            uv_requirements = reqs,
            uv_explicit = explicit
        }) = ppr (b, rns, mb_pn, reqs, explicit)

    instance Semigroup UnitVisibility where
        uv1 <> uv2
            = UnitVisibility
              { uv_expose_all = uv_expose_all uv1 || uv_expose_all uv2
              , uv_renamings = uv_renamings uv1 ++ uv_renamings uv2
              , uv_package_name = mappend (uv_package_name uv1) (uv_package_name uv2)
              , uv_requirements = Map.unionWith Set.union (uv_requirements uv1) (uv_requirements uv2)
              , uv_explicit = uv_explicit uv1 || uv_explicit uv2
              }

    instance Monoid UnitVisibility where
        mempty = UnitVisibility
                 { uv_expose_all = False
                 , uv_renamings = []
                 , uv_package_name = First Nothing
                 , uv_requirements = Map.empty
                 , uv_explicit = False
                 }
        mappend = (Semigroup.<>)

    type WiredUnitId = DefUnitId
    type PreloadUnitId = InstalledUnitId

    -- | Map from 'ModuleName' to 'Module' to all the origins of the bindings
    -- in scope.  The 'PackageConf' is not cached, mostly for convenience reasons
    -- (since this is the slow path, we'll just look it up again).
    type ModuleToPkgConfAll =
        Map ModuleName (Map Module ModuleOrigin)

    data PackageState = PackageState {
      -- | A mapping of 'UnitId' to 'PackageConfig'.  This list is adjusted
      -- so that only valid packages are here.  'PackageConfig' reflects
      -- what was stored *on disk*, except for the 'trusted' flag, which
      -- is adjusted at runtime.  (In particular, some packages in this map
      -- may have the 'exposed' flag be 'False'.)
      pkgIdMap              :: PackageConfigMap,

      -- | A mapping of 'PackageName' to 'ComponentId'.  This is used when
      -- users refer to packages in Backpack includes.
      packageNameMap            :: Map PackageName ComponentId,

      -- | A mapping from wired in names to the original names from the
      -- package database.
      unwireMap :: Map WiredUnitId WiredUnitId,

      -- | The packages we're going to link in eagerly.  This list
      -- should be in reverse dependency order; that is, a package
      -- is always mentioned before the packages it depends on.
      preloadPackages      :: [PreloadUnitId],

      -- | Packages which we explicitly depend on (from a command line flag).
      -- We'll use this to generate version macros.
      explicitPackages      :: [UnitId],

      -- | This is a full map from 'ModuleName' to all modules which may possibly
      -- be providing it.  These providers may be hidden (but we'll still want
      -- to report them in error messages), or it may be an ambiguous import.
      moduleToPkgConfAll    :: !ModuleToPkgConfAll,

      -- | A map, like 'moduleToPkgConfAll', but controlling plugin visibility.
      pluginModuleToPkgConfAll    :: !ModuleToPkgConfAll,

      -- | A map saying, for each requirement, what interfaces must be merged
      -- together when we use them.  For example, if our dependencies
      -- are @p[A=<A>]@ and @q[A=<A>,B=r[C=<A>]:B]@, then the interfaces
      -- to merge for A are @p[A=<A>]:A@, @q[A=<A>,B=r[C=<A>]:B]:A@
      -- and @r[C=<A>]:C@.
      --
      -- There's an entry in this map for each hole in our home library.
      requirementContext :: Map ModuleName [IndefModule]
      }

    emptyPackageState :: PackageState
    emptyPackageState = PackageState {
        pkgIdMap = emptyPackageConfigMap,
        packageNameMap = Map.empty,
        unwireMap = Map.empty,
        preloadPackages = [],
        explicitPackages = [],
        moduleToPkgConfAll = Map.empty,
        pluginModuleToPkgConfAll = Map.empty,
        requirementContext = Map.empty
        }

    type InstalledPackageIndex = Map InstalledUnitId PackageConfig

    -- | Empty package configuration map
    emptyPackageConfigMap :: PackageConfigMap
    emptyPackageConfigMap = PackageConfigMap emptyUDFM emptyUniqSet

    -- | Find the package we know about with the given unit id, if any
    lookupPackage :: DynFlags -> UnitId -> Maybe PackageConfig
    lookupPackage dflags = lookupPackage' (isIndefinite dflags) (pkgIdMap (pkgState dflags))

    -- | A more specialized interface, which takes a boolean specifying
    -- whether or not to look for on-the-fly renamed interfaces, and
    -- just a 'PackageConfigMap' rather than a 'DynFlags' (so it can
    -- be used while we're initializing 'DynFlags'
    lookupPackage' :: Bool -> PackageConfigMap -> UnitId -> Maybe PackageConfig
    lookupPackage' False (PackageConfigMap pkg_map _) uid = lookupUDFM pkg_map uid
    lookupPackage' True m@(PackageConfigMap pkg_map _) uid =
        case splitUnitIdInsts uid of
            (iuid, Just indef) ->
                fmap (renamePackage m (indefUnitIdInsts indef))
                     (lookupUDFM pkg_map iuid)
            (_, Nothing) -> lookupUDFM pkg_map uid

    {-
    -- | Find the indefinite package for a given 'ComponentId'.
    -- The way this works is just by fiat'ing that every indefinite package's
    -- unit key is precisely its component ID; and that they share uniques.
    lookupComponentId :: DynFlags -> ComponentId -> Maybe PackageConfig
    lookupComponentId dflags (ComponentId cid_fs) = lookupUDFM pkg_map cid_fs
      where
        PackageConfigMap pkg_map = pkgIdMap (pkgState dflags)
    -}

    -- | Find the package we know about with the given package name (e.g. @foo@), if any
    -- (NB: there might be a locally defined unit name which overrides this)
    lookupPackageName :: DynFlags -> PackageName -> Maybe ComponentId
    lookupPackageName dflags n = Map.lookup n (packageNameMap (pkgState dflags))

    -- | Search for packages with a given package ID (e.g. \"foo-0.1\")
    searchPackageId :: DynFlags -> SourcePackageId -> [PackageConfig]
    searchPackageId dflags pid = filter ((pid ==) . sourcePackageId)
                                   (listPackageConfigMap dflags)

    -- | Extends the package configuration map with a list of package configs.
    extendPackageConfigMap
       :: PackageConfigMap -> [PackageConfig] -> PackageConfigMap
    extendPackageConfigMap (PackageConfigMap pkg_map closure) new_pkgs
      = PackageConfigMap (foldl add pkg_map new_pkgs) closure
        -- We also add the expanded version of the packageConfigId, so that
        -- 'improveUnitId' can find it.
      where add pkg_map p = addToUDFM (addToUDFM pkg_map (expandedPackageConfigId p) p)
                                      (installedPackageConfigId p) p

    -- | Looks up the package with the given id in the package state, panicing if it is
    -- not found
    getPackageDetails :: DynFlags -> UnitId -> PackageConfig
    getPackageDetails dflags pid =
        expectJust "getPackageDetails" (lookupPackage dflags pid)

    lookupInstalledPackage :: DynFlags -> InstalledUnitId -> Maybe PackageConfig
    lookupInstalledPackage dflags uid = lookupInstalledPackage' (pkgIdMap (pkgState dflags)) uid

    lookupInstalledPackage' :: PackageConfigMap -> InstalledUnitId -> Maybe PackageConfig
    lookupInstalledPackage' (PackageConfigMap db _) uid = lookupUDFM db uid

    getInstalledPackageDetails :: DynFlags -> InstalledUnitId -> PackageConfig
    getInstalledPackageDetails dflags uid =
        expectJust "getInstalledPackageDetails" $
            lookupInstalledPackage dflags uid

    -- | Get a list of entries from the package database.  NB: be careful with
    -- this function, although all packages in this map are "visible", this
    -- does not imply that the exposed-modules of the package are available
    -- (they may have been thinned or renamed).
    listPackageConfigMap :: DynFlags -> [PackageConfig]
    listPackageConfigMap dflags = eltsUDFM pkg_map
      where
        PackageConfigMap pkg_map _ = pkgIdMap (pkgState dflags)

    -- ----------------------------------------------------------------------------
    -- Loading the package db files and building up the package state

    -- | Call this after 'DynFlags.parseDynFlags'.  It reads the package
    -- database files, and sets up various internal tables of package
    -- information, according to the package-related flags on the
    -- command-line (@-package@, @-hide-package@ etc.)
    --
    -- Returns a list of packages to link in if we're doing dynamic linking.
    -- This list contains the packages that the user explicitly mentioned with
    -- @-package@ flags.
    --
    -- 'initPackages' can be called again subsequently after updating the
    -- 'packageFlags' field of the 'DynFlags', and it will update the
    -- 'pkgState' in 'DynFlags' and return a list of packages to
    -- link in.
    initPackages :: DynFlags -> IO (DynFlags, [PreloadUnitId])
    initPackages dflags0 = do
      dflags <- interpretPackageEnv dflags0
      pkg_db <-
        case pkgDatabase dflags of
            Nothing -> readPackageConfigs dflags
            Just db -> return $ map (\(p, pkgs)
                                        -> (p, setBatchPackageFlags dflags pkgs)) db
      (pkg_state, preload, insts)
            <- mkPackageState dflags pkg_db []
      return (dflags{ pkgDatabase = Just pkg_db,
                      pkgState = pkg_state,
                      thisUnitIdInsts_ = insts },
              preload)

    -- -----------------------------------------------------------------------------
    -- Reading the package database(s)

    readPackageConfigs :: DynFlags -> IO [(FilePath, [PackageConfig])]
    readPackageConfigs dflags = do
      conf_refs <- getPackageConfRefs dflags
      confs     <- liftM catMaybes $ mapM (resolvePackageConfig dflags) conf_refs
      mapM (readPackageConfig dflags) confs

    getPackageConfRefs :: DynFlags -> IO [PkgConfRef]
    getPackageConfRefs dflags = do
      let system_conf_refs = [UserPkgConf, GlobalPkgConf]

      e_pkg_path <- tryIO (getEnv $ map toUpper (programName dflags) ++ "_PACKAGE_PATH")
      let base_conf_refs = case e_pkg_path of
            Left _ -> system_conf_refs
            Right path
             | not (null path) && isSearchPathSeparator (last path)
             -> map PkgConfFile (splitSearchPath (init path)) ++ system_conf_refs
             | otherwise
             -> map PkgConfFile (splitSearchPath path)

      -- Apply the package DB-related flags from the command line to get the
      -- final list of package DBs.
      --
      -- Notes on ordering:
      --  * The list of flags is reversed (later ones first)
      --  * We work with the package DB list in "left shadows right" order
      --  * and finally reverse it at the end, to get "right shadows left"
      --
      return $ reverse (foldr doFlag base_conf_refs (packageDBFlags dflags))
     where
      doFlag (PackageDB p) dbs = p : dbs
      doFlag NoUserPackageDB dbs = filter isNotUser dbs
      doFlag NoGlobalPackageDB dbs = filter isNotGlobal dbs
      doFlag ClearPackageDBs _ = []

      isNotUser UserPkgConf = False
      isNotUser _ = True

      isNotGlobal GlobalPkgConf = False
      isNotGlobal _ = True

    resolvePackageConfig :: DynFlags -> PkgConfRef -> IO (Maybe FilePath)
    resolvePackageConfig dflags GlobalPkgConf = return $ Just (systemPackageConfig dflags)
    -- NB: This logic is reimplemented in Cabal, so if you change it,
    -- make sure you update Cabal.  (Or, better yet, dump it in the
    -- compiler info so Cabal can use the info.)
    resolvePackageConfig dflags UserPkgConf = runMaybeT $ do
      dir <- versionedAppDir dflags
      let pkgconf = dir </> "package.conf.d"
      exist <- tryMaybeT $ doesDirectoryExist pkgconf
      if exist then return pkgconf else mzero
    resolvePackageConfig _ (PkgConfFile name) = return $ Just name

    readPackageConfig :: DynFlags -> FilePath -> IO (FilePath, [PackageConfig])
    readPackageConfig dflags conf_file = do
      isdir <- doesDirectoryExist conf_file

      proto_pkg_configs <-
        if isdir
           then readDirStylePackageConfig conf_file
           else do
                isfile <- doesFileExist conf_file
                if isfile
                   then do
                     mpkgs <- tryReadOldFileStylePackageConfig
                     case mpkgs of
                       Just pkgs -> return pkgs
                       Nothing   -> throwGhcExceptionIO $ InstallationError $
                          "ghc no longer supports single-file style package " ++
                          "databases (" ++ conf_file ++
                          ") use 'ghc-pkg init' to create the database with " ++
                          "the correct format."
                   else throwGhcExceptionIO $ InstallationError $
                          "can't find a package database at " ++ conf_file

      let
          top_dir = topDir dflags
          pkgroot = takeDirectory conf_file
          pkg_configs1 = map (mungePackageConfig top_dir pkgroot)
                             proto_pkg_configs
          pkg_configs2 = setBatchPackageFlags dflags pkg_configs1
      --
      return (conf_file, pkg_configs2)
      where
        readDirStylePackageConfig conf_dir = do
          let filename = conf_dir </> "package.cache"
          cache_exists <- doesFileExist filename
          if cache_exists
            then do
              debugTraceMsg dflags 2 $ text "Using binary package database:"
                                        <+> text filename
              readPackageDbForGhc filename
            else do
              -- If there is no package.cache file, we check if the database is not
              -- empty by inspecting if the directory contains any .conf file. If it
              -- does, something is wrong and we fail. Otherwise we assume that the
              -- database is empty.
              debugTraceMsg dflags 2 $ text "There is no package.cache in"
                                   <+> text conf_dir
                                    <> text ", checking if the database is empty"
              db_empty <- all (not . isSuffixOf ".conf")
                       <$> getDirectoryContents conf_dir
              if db_empty
                then do
                  debugTraceMsg dflags 3 $ text "There are no .conf files in"
                                       <+> text conf_dir <> text ", treating"
                                       <+> text "package database as empty"
                  return []
                else do
                  throwGhcExceptionIO $ InstallationError $
                    "there is no package.cache in " ++ conf_dir ++
                    " even though package database is not empty"

        -- Single-file style package dbs have been deprecated for some time, but
        -- it turns out that Cabal was using them in one place. So this is a
        -- workaround to allow older Cabal versions to use this newer ghc.
        -- We check if the file db contains just "[]" and if so, we look for a new
        -- dir-style db in conf_file.d/, ie in a dir next to the given file.
        -- We cannot just replace the file with a new dir style since Cabal still
        -- assumes it's a file and tries to overwrite with 'writeFile'.
        -- ghc-pkg also cooperates with this workaround.
        tryReadOldFileStylePackageConfig = do
          content <- readFile conf_file `catchIO` \_ -> return ""
          if take 2 content == "[]"
            then do
              let conf_dir = conf_file <.> "d"
              direxists <- doesDirectoryExist conf_dir
              if direxists
                 then do debugTraceMsg dflags 2 (text "Ignoring old file-style db and trying:" <+> text conf_dir)
                         liftM Just (readDirStylePackageConfig conf_dir)
                 else return (Just []) -- ghc-pkg will create it when it's updated
            else return Nothing

    setBatchPackageFlags :: DynFlags -> [PackageConfig] -> [PackageConfig]
    setBatchPackageFlags dflags pkgs = maybeDistrustAll pkgs
      where
        maybeDistrustAll pkgs'
          | gopt Opt_DistrustAllPackages dflags = map distrust pkgs'
          | otherwise                           = pkgs'

        distrust pkg = pkg{ trusted = False }

    mungePackageConfig :: FilePath -> FilePath
                       -> PackageConfig -> PackageConfig
    mungePackageConfig top_dir pkgroot =
        mungeDynLibFields
      . mungePackagePaths top_dir pkgroot

    mungeDynLibFields :: PackageConfig -> PackageConfig
    mungeDynLibFields pkg =
        pkg {
          libraryDynDirs     = libraryDynDirs pkg
                    `orIfNull` libraryDirs pkg
        }
      where
        orIfNull [] flags = flags
        orIfNull flags _  = flags

    -- TODO: This code is duplicated in utils/ghc-pkg/Main.hs
    mungePackagePaths :: FilePath -> FilePath -> PackageConfig -> PackageConfig
    -- Perform path/URL variable substitution as per the Cabal ${pkgroot} spec
    -- (http://www.haskell.org/pipermail/libraries/2009-May/011772.html)
    -- Paths/URLs can be relative to ${pkgroot} or ${pkgrooturl}.
    -- The "pkgroot" is the directory containing the package database.
    --
    -- Also perform a similar substitution for the older GHC-specific
    -- "$topdir" variable. The "topdir" is the location of the ghc
    -- installation (obtained from the -B option).
    mungePackagePaths top_dir pkgroot pkg =
        pkg {
          importDirs  = munge_paths (importDirs pkg),
          includeDirs = munge_paths (includeDirs pkg),
          libraryDirs = munge_paths (libraryDirs pkg),
          libraryDynDirs = munge_paths (libraryDynDirs pkg),
          frameworkDirs = munge_paths (frameworkDirs pkg),
          haddockInterfaces = munge_paths (haddockInterfaces pkg),
          haddockHTMLs = munge_urls (haddockHTMLs pkg)
        }
      where
        munge_paths = map munge_path
        munge_urls  = map munge_url

        munge_path p
          | Just p' <- stripVarPrefix "${pkgroot}" p = pkgroot ++ p'
          | Just p' <- stripVarPrefix "$topdir"    p = top_dir ++ p'
          | otherwise                                = p

        munge_url p
          | Just p' <- stripVarPrefix "${pkgrooturl}" p = toUrlPath pkgroot p'
          | Just p' <- stripVarPrefix "$httptopdir"   p = toUrlPath top_dir p'
          | otherwise                                   = p

        toUrlPath r p = "file:///"
                     -- URLs always use posix style '/' separators:
                     ++ FilePath.Posix.joinPath
                            (r : -- We need to drop a leading "/" or "\\"
                                 -- if there is one:
                                 dropWhile (all isPathSeparator)
                                           (FilePath.splitDirectories p))

        -- We could drop the separator here, and then use </> above. However,
        -- by leaving it in and using ++ we keep the same path separator
        -- rather than letting FilePath change it to use \ as the separator
        stripVarPrefix var path = case stripPrefix var path of
                                  Just [] -> Just []
                                  Just cs@(c : _) | isPathSeparator c -> Just cs
                                  _ -> Nothing

    -- -----------------------------------------------------------------------------
    -- Modify our copy of the package database based on trust flags,
    -- -trust and -distrust.

    applyTrustFlag
       :: DynFlags
       -> PackagePrecedenceIndex
       -> UnusablePackages
       -> [PackageConfig]
       -> TrustFlag
       -> IO [PackageConfig]
    applyTrustFlag dflags prec_map unusable pkgs flag =
      case flag of
        -- we trust all matching packages. Maybe should only trust first one?
        -- and leave others the same or set them untrusted
        TrustPackage str ->
           case selectPackages prec_map (PackageArg str) pkgs unusable of
             Left ps       -> trustFlagErr dflags flag ps
             Right (ps,qs) -> return (map trust ps ++ qs)
              where trust p = p {trusted=True}

        DistrustPackage str ->
           case selectPackages prec_map (PackageArg str) pkgs unusable of
             Left ps       -> trustFlagErr dflags flag ps
             Right (ps,qs) -> return (map distrust ps ++ qs)
              where distrust p = p {trusted=False}

    -- | A little utility to tell if the 'thisPackage' is indefinite
    -- (if it is not, we should never use on-the-fly renaming.)
    isIndefinite :: DynFlags -> Bool
    isIndefinite dflags = not (unitIdIsDefinite (thisPackage dflags))

    applyPackageFlag
       :: DynFlags
       -> PackagePrecedenceIndex
       -> PackageConfigMap
       -> UnusablePackages
       -> Bool -- if False, if you expose a package, it implicitly hides
               -- any previously exposed packages with the same name
       -> [PackageConfig]
       -> VisibilityMap           -- Initially exposed
       -> PackageFlag               -- flag to apply
       -> IO VisibilityMap        -- Now exposed

    applyPackageFlag dflags prec_map pkg_db unusable no_hide_others pkgs vm flag =
      case flag of
        ExposePackage _ arg (ModRenaming b rns) ->
           case findPackages prec_map pkg_db arg pkgs unusable of
             Left ps         -> packageFlagErr dflags flag ps
             Right (p:_) -> return vm'
              where
               n = fsPackageName p

               -- If a user says @-unit-id p[A=<A>]@, this imposes
               -- a requirement on us: whatever our signature A is,
               -- it must fulfill all of p[A=<A>]:A's requirements.
               -- This method is responsible for computing what our
               -- inherited requirements are.
               reqs | UnitIdArg orig_uid <- arg = collectHoles orig_uid
                    | otherwise                 = Map.empty

               collectHoles uid = case splitUnitIdInsts uid of
                    (_, Just indef) ->
                      let local = [ Map.singleton
                                      (moduleName mod)
                                      (Set.singleton $ IndefModule indef mod_name)
                                  | (mod_name, mod) <- indefUnitIdInsts indef
                                  , isHoleModule mod ]
                          recurse = [ collectHoles (moduleUnitId mod)
                                    | (_, mod) <- indefUnitIdInsts indef ]
                      in Map.unionsWith Set.union $ local ++ recurse
                    -- Other types of unit identities don't have holes
                    (_, Nothing) -> Map.empty

               uv = UnitVisibility
                    { uv_expose_all = b
                    , uv_renamings = rns
                    , uv_package_name = First (Just n)
                    , uv_requirements = reqs
                    , uv_explicit = True
                    }
               vm' = Map.insertWith mappend (packageConfigId p) uv vm_cleared
               -- In the old days, if you said `ghc -package p-0.1 -package p-0.2`
               -- (or if p-0.1 was registered in the pkgdb as exposed: True),
               -- the second package flag would override the first one and you
               -- would only see p-0.2 in exposed modules.  This is good for
               -- usability.
               --
               -- However, with thinning and renaming (or Backpack), there might be
               -- situations where you legitimately want to see two versions of a
               -- package at the same time, and this behavior would make it
               -- impossible to do so.  So we decided that if you pass
               -- -hide-all-packages, this should turn OFF the overriding behavior
               -- where an exposed package hides all other packages with the same
               -- name.  This should not affect Cabal at all, which only ever
               -- exposes one package at a time.
               --
               -- NB: Why a variable no_hide_others?  We have to apply this logic to
               -- -plugin-package too, and it's more consistent if the switch in
               -- behavior is based off of
               -- -hide-all-packages/-hide-all-plugin-packages depending on what
               -- flag is in question.
               vm_cleared | no_hide_others = vm
                          -- NB: renamings never clear
                          | (_:_) <- rns = vm
                          | otherwise = Map.filterWithKey
                                (\k uv -> k == packageConfigId p
                                       || First (Just n) /= uv_package_name uv) vm
             _ -> panic "applyPackageFlag"

        HidePackage str ->
           case findPackages prec_map pkg_db (PackageArg str) pkgs unusable of
             Left ps  -> packageFlagErr dflags flag ps
             Right ps -> return vm'
              where vm' = foldl' (flip Map.delete) vm (map packageConfigId ps)

    -- | Like 'selectPackages', but doesn't return a list of unmatched
    -- packages.  Furthermore, any packages it returns are *renamed*
    -- if the 'UnitArg' has a renaming associated with it.
    findPackages :: PackagePrecedenceIndex
                 -> PackageConfigMap -> PackageArg -> [PackageConfig]
                 -> UnusablePackages
                 -> Either [(PackageConfig, UnusablePackageReason)]
                    [PackageConfig]
    findPackages prec_map pkg_db arg pkgs unusable
      = let ps = mapMaybe (finder arg) pkgs
        in if null ps
            then Left (mapMaybe (\(x,y) -> finder arg x >>= \x' -> return (x',y))
                                (Map.elems unusable))
            else Right (sortByPreference prec_map ps)
      where
        finder (PackageArg str) p
          = if str == sourcePackageIdString p || str == packageNameString p
              then Just p
              else Nothing
        finder (UnitIdArg uid) p
          = let (iuid, mb_indef) = splitUnitIdInsts uid
            in if iuid == installedPackageConfigId p
                  then Just (case mb_indef of
                                Nothing    -> p
                                Just indef -> renamePackage pkg_db (indefUnitIdInsts indef) p)
                  else Nothing

    selectPackages :: PackagePrecedenceIndex -> PackageArg -> [PackageConfig]
                   -> UnusablePackages
                   -> Either [(PackageConfig, UnusablePackageReason)]
                      ([PackageConfig], [PackageConfig])
    selectPackages prec_map arg pkgs unusable
      = let matches = matching arg
            (ps,rest) = partition matches pkgs
        in if null ps
            then Left (filter (matches.fst) (Map.elems unusable))
            else Right (sortByPreference prec_map ps, rest)

    -- | Rename a 'PackageConfig' according to some module instantiation.
    renamePackage :: PackageConfigMap -> [(ModuleName, Module)]
                  -> PackageConfig -> PackageConfig
    renamePackage pkg_map insts conf =
        let hsubst = listToUFM insts
            smod  = renameHoleModule' pkg_map hsubst
            new_insts = map (\(k,v) -> (k,smod v)) (instantiatedWith conf)
        in conf {
            instantiatedWith = new_insts,
            exposedModules = map (\(mod_name, mb_mod) -> (mod_name, fmap smod mb_mod))
                                 (exposedModules conf)
        }

    -- A package named on the command line can either include the
    -- version, or just the name if it is unambiguous.
    matchingStr :: String -> PackageConfig -> Bool
    matchingStr str p
            =  str == sourcePackageIdString p
            || str == packageNameString p

    matchingId :: InstalledUnitId -> PackageConfig -> Bool
    matchingId uid p = uid == installedPackageConfigId p

    matching :: PackageArg -> PackageConfig -> Bool
    matching (PackageArg str) = matchingStr str
    matching (UnitIdArg (DefiniteUnitId (DefUnitId uid)))  = matchingId uid
    matching (UnitIdArg _)  = \_ -> False -- TODO: warn in this case

    -- | This sorts a list of packages, putting "preferred" packages first.
    -- See 'compareByPreference' for the semantics of "preference".
    sortByPreference :: PackagePrecedenceIndex -> [PackageConfig] -> [PackageConfig]
    sortByPreference prec_map = sortBy (flip (compareByPreference prec_map))

    -- | Returns 'GT' if @pkg@ should be preferred over @pkg'@ when picking
    -- which should be "active".  Here is the order of preference:
    --
    --      1. First, prefer the latest version
    --      2. If the versions are the same, prefer the package that
    --      came in the latest package database.
    --
    -- Pursuant to #12518, we could change this policy to, for example, remove
    -- the version preference, meaning that we would always prefer the packages
    -- in alter package database.
    --
    compareByPreference
        :: PackagePrecedenceIndex
        -> PackageConfig
        -> PackageConfig
        -> Ordering
    compareByPreference prec_map pkg pkg' =
        case comparing packageVersion pkg pkg' of
            GT -> GT
            EQ | Just prec  <- Map.lookup (unitId pkg)  prec_map
               , Just prec' <- Map.lookup (unitId pkg') prec_map
               -- Prefer the package from the later DB flag (i.e., higher
               -- precedence)
               -> compare prec prec'
               | otherwise
               -> EQ
            LT -> LT

    comparing :: Ord a => (t -> a) -> t -> t -> Ordering
    comparing f a b = f a `compare` f b

    packageFlagErr :: DynFlags
                   -> PackageFlag
                   -> [(PackageConfig, UnusablePackageReason)]
                   -> IO a

    -- for missing DPH package we emit a more helpful error message, because
    -- this may be the result of using -fdph-par or -fdph-seq.
    packageFlagErr dflags (ExposePackage _ (PackageArg pkg) _) []
      | is_dph_package pkg
      = throwGhcExceptionIO (CmdLineError (showSDoc dflags $ dph_err))
      where dph_err = text "the " <> text pkg <> text " package is not installed."
                      $$ text "To install it: \"cabal install dph\"."
            is_dph_package pkg = "dph" `isPrefixOf` pkg
    packageFlagErr dflags flag reasons
      = packageFlagErr' dflags (pprFlag flag) reasons

    trustFlagErr :: DynFlags
                 -> TrustFlag
                 -> [(PackageConfig, UnusablePackageReason)]
                 -> IO a
    trustFlagErr dflags flag reasons
      = packageFlagErr' dflags (pprTrustFlag flag) reasons

    packageFlagErr' :: DynFlags
                   -> SDoc
                   -> [(PackageConfig, UnusablePackageReason)]
                   -> IO a
    packageFlagErr' dflags flag_doc reasons
      = throwGhcExceptionIO (CmdLineError (showSDoc dflags $ err))
      where err = text "cannot satisfy " <> flag_doc <>
                    (if null reasons then Outputable.empty else text ": ") $$
                  nest 4 (ppr_reasons $$
                          text "(use -v for more information)")
            ppr_reasons = vcat (map ppr_reason reasons)
            ppr_reason (p, reason) =
                pprReason (ppr (unitId p) <+> text "is") reason

    pprFlag :: PackageFlag -> SDoc
    pprFlag flag = case flag of
        HidePackage p   -> text "-hide-package " <> text p
        ExposePackage doc _ _ -> text doc

    pprTrustFlag :: TrustFlag -> SDoc
    pprTrustFlag flag = case flag of
        TrustPackage p    -> text "-trust " <> text p
        DistrustPackage p -> text "-distrust " <> text p

    -- -----------------------------------------------------------------------------
    -- Wired-in packages

    wired_in_pkgids :: [String]
    wired_in_pkgids = map unitIdString wiredInUnitIds

    type WiredPackagesMap = Map WiredUnitId WiredUnitId

    findWiredInPackages
       :: DynFlags
       -> PackagePrecedenceIndex
       -> [PackageConfig]           -- database
       -> VisibilityMap             -- info on what packages are visible
                                    -- for wired in selection
       -> IO ([PackageConfig],  -- package database updated for wired in
              WiredPackagesMap) -- map from unit id to wired identity

    findWiredInPackages dflags prec_map pkgs vis_map = do
      --
      -- Now we must find our wired-in packages, and rename them to
      -- their canonical names (eg. base-1.0 ==> base).
      --
      let
            matches :: PackageConfig -> String -> Bool
            pc `matches` pid = packageNameString pc == pid

            -- find which package corresponds to each wired-in package
            -- delete any other packages with the same name
            -- update the package and any dependencies to point to the new
            -- one.
            --
            -- When choosing which package to map to a wired-in package
            -- name, we try to pick the latest version of exposed packages.
            -- However, if there are no exposed wired in packages available
            -- (e.g. -hide-all-packages was used), we can't bail: we *have*
            -- to assign a package for the wired-in package: so we try again
            -- with hidden packages included to (and pick the latest
            -- version).
            --
            -- You can also override the default choice by using -ignore-package:
            -- this works even when there is no exposed wired in package
            -- available.
            --
            findWiredInPackage :: [PackageConfig] -> String
                               -> IO (Maybe PackageConfig)
            findWiredInPackage pkgs wired_pkg =
               let all_ps = [ p | p <- pkgs, p `matches` wired_pkg ]
                   all_exposed_ps =
                        [ p | p <- all_ps
                            , Map.member (packageConfigId p) vis_map ] in
               case all_exposed_ps of
                [] -> case all_ps of
                           []   -> notfound
                           many -> pick (head (sortByPreference prec_map many))
                many -> pick (head (sortByPreference prec_map many))
              where
                    notfound = do
                              debugTraceMsg dflags 2 $
                                text "wired-in package "
                                     <> text wired_pkg
                                     <> text " not found."
                              return Nothing
                    pick :: PackageConfig
                         -> IO (Maybe PackageConfig)
                    pick pkg = do
                            debugTraceMsg dflags 2 $
                                text "wired-in package "
                                     <> text wired_pkg
                                     <> text " mapped to "
                                     <> ppr (unitId pkg)
                            return (Just pkg)

      mb_wired_in_pkgs <- mapM (findWiredInPackage pkgs) wired_in_pkgids
      let
            wired_in_pkgs = catMaybes mb_wired_in_pkgs
            wired_in_ids = mapMaybe definitePackageConfigId wired_in_pkgs

            -- this is old: we used to assume that if there were
            -- multiple versions of wired-in packages installed that
            -- they were mutually exclusive.  Now we're assuming that
            -- you have one "main" version of each wired-in package
            -- (the latest version), and the others are backward-compat
            -- wrappers that depend on this one.  e.g. base-4.0 is the
            -- latest, base-3.0 is a compat wrapper depending on base-4.0.
            {-
            deleteOtherWiredInPackages pkgs = filterOut bad pkgs
              where bad p = any (p `matches`) wired_in_pkgids
                          && package p `notElem` map fst wired_in_ids
            -}

            wiredInMap :: Map WiredUnitId WiredUnitId
            wiredInMap = foldl' add_mapping Map.empty pkgs
              where add_mapping m pkg
                      | Just key <- definitePackageConfigId pkg
                      , key `elem` wired_in_ids
                      = Map.insert key (DefUnitId (stringToInstalledUnitId (packageNameString pkg))) m
                      | otherwise = m

            updateWiredInDependencies pkgs = map (upd_deps . upd_pkg) pkgs
              where upd_pkg pkg
                      | Just def_uid <- definitePackageConfigId pkg
                      , def_uid `elem` wired_in_ids
                      = let PackageName fs = packageName pkg
                        in pkg {
                          unitId = fsToInstalledUnitId fs,
                          componentId = ComponentId fs
                        }
                      | otherwise
                      = pkg
                    upd_deps pkg = pkg {
                          -- temporary harmless DefUnitId invariant violation
                          depends = map (unDefUnitId . upd_wired_in wiredInMap . DefUnitId) (depends pkg),
                          exposedModules
                            = map (\(k,v) -> (k, fmap (upd_wired_in_mod wiredInMap) v))
                                  (exposedModules pkg)
                        }

      return (updateWiredInDependencies pkgs, wiredInMap)

    -- Helper functions for rewiring Module and UnitId.  These
    -- rewrite UnitIds of modules in wired-in packages to the form known to the
    -- compiler. For instance, base-4.9.0.0 will be rewritten to just base, to match
    -- what appears in PrelNames.

    upd_wired_in_mod :: WiredPackagesMap -> Module -> Module
    upd_wired_in_mod wiredInMap (Module uid m) = Module (upd_wired_in_uid wiredInMap uid) m

    upd_wired_in_uid :: WiredPackagesMap -> UnitId -> UnitId
    upd_wired_in_uid wiredInMap (DefiniteUnitId def_uid) =
        DefiniteUnitId (upd_wired_in wiredInMap def_uid)
    upd_wired_in_uid wiredInMap (IndefiniteUnitId indef_uid) =
        IndefiniteUnitId $ newIndefUnitId
            (indefUnitIdComponentId indef_uid)
            (map (\(x,y) -> (x,upd_wired_in_mod wiredInMap y)) (indefUnitIdInsts indef_uid))

    upd_wired_in :: WiredPackagesMap -> DefUnitId -> DefUnitId
    upd_wired_in wiredInMap key
        | Just key' <- Map.lookup key wiredInMap = key'
        | otherwise = key

    updateVisibilityMap :: WiredPackagesMap -> VisibilityMap -> VisibilityMap
    updateVisibilityMap wiredInMap vis_map = foldl' f vis_map (Map.toList wiredInMap)
      where f vm (from, to) = case Map.lookup (DefiniteUnitId from) vis_map of
                        Nothing -> vm
                        Just r -> Map.insert (DefiniteUnitId to) r
                                    (Map.delete (DefiniteUnitId from) vm)

    -- ----------------------------------------------------------------------------

    -- | The reason why a package is unusable.
    data UnusablePackageReason
      = -- | We ignored it explicitly using @-ignore-package@.
        IgnoredWithFlag
        -- | This package transitively depends on a package that was never present
        -- in any of the provided databases.
      | BrokenDependencies   [InstalledUnitId]
        -- | This package transitively depends on a package involved in a cycle.
        -- Note that the list of 'InstalledUnitId' reports the direct dependencies
        -- of this package that (transitively) depended on the cycle, and not
        -- the actual cycle itself (which we report separately at high verbosity.)
      | CyclicDependencies   [InstalledUnitId]
        -- | This package transitively depends on a package which was ignored.
      | IgnoredDependencies  [InstalledUnitId]
        -- | This package transitively depends on a package which was
        -- shadowed by an ABI-incompatible package.
      | ShadowedDependencies [InstalledUnitId]

    instance Outputable UnusablePackageReason where
        ppr IgnoredWithFlag = text "[ignored with flag]"
        ppr (BrokenDependencies uids)   = brackets (text "broken" <+> ppr uids)
        ppr (CyclicDependencies uids)   = brackets (text "cyclic" <+> ppr uids)
        ppr (IgnoredDependencies uids)  = brackets (text "ignored" <+> ppr uids)
        ppr (ShadowedDependencies uids) = brackets (text "shadowed" <+> ppr uids)

    type UnusablePackages = Map InstalledUnitId
                                (PackageConfig, UnusablePackageReason)

    pprReason :: SDoc -> UnusablePackageReason -> SDoc
    pprReason pref reason = case reason of
      IgnoredWithFlag ->
          pref <+> text "ignored due to an -ignore-package flag"
      BrokenDependencies deps ->
          pref <+> text "unusable due to missing dependencies:" $$
            nest 2 (hsep (map ppr deps))
      CyclicDependencies deps ->
          pref <+> text "unusable due to cyclic dependencies:" $$
            nest 2 (hsep (map ppr deps))
      IgnoredDependencies deps ->
          pref <+> text "unusable due to ignored dependencies:" $$
            nest 2 (hsep (map ppr deps))
      ShadowedDependencies deps ->
          pref <+> text "unusable due to shadowed dependencies:" $$
            nest 2 (hsep (map ppr deps))

    reportCycles :: DynFlags -> [SCC PackageConfig] -> IO ()
    reportCycles dflags sccs = mapM_ report sccs
      where
        report (AcyclicSCC _) = return ()
        report (CyclicSCC vs) =
            debugTraceMsg dflags 2 $
              text "these packages are involved in a cycle:" $$
                nest 2 (hsep (map (ppr . unitId) vs))

    reportUnusable :: DynFlags -> UnusablePackages -> IO ()
    reportUnusable dflags pkgs = mapM_ report (Map.toList pkgs)
      where
        report (ipid, (_, reason)) =
           debugTraceMsg dflags 2 $
             pprReason
               (text "package" <+> ppr ipid <+> text "is") reason

    -- ----------------------------------------------------------------------------
    --
    -- Utilities on the database
    --

    -- | A reverse dependency index, mapping an 'InstalledUnitId' to
    -- the 'InstalledUnitId's which have a dependency on it.
    type RevIndex = Map InstalledUnitId [InstalledUnitId]

    -- | Compute the reverse dependency index of a package database.
    reverseDeps :: InstalledPackageIndex -> RevIndex
    reverseDeps db = Map.foldl' go Map.empty db
      where
        go r pkg = foldl' (go' (unitId pkg)) r (depends pkg)
        go' from r to = Map.insertWith (++) to [from] r

    -- | Given a list of 'InstalledUnitId's to remove, a database,
    -- and a reverse dependency index (as computed by 'reverseDeps'),
    -- remove those packages, plus any packages which depend on them.
    -- Returns the pruned database, as well as a list of 'PackageConfig's
    -- that was removed.
    removePackages :: [InstalledUnitId] -> RevIndex
                   -> InstalledPackageIndex
                   -> (InstalledPackageIndex, [PackageConfig])
    removePackages uids index m = go uids (m,[])
      where
        go [] (m,pkgs) = (m,pkgs)
        go (uid:uids) (m,pkgs)
            | Just pkg <- Map.lookup uid m
            = case Map.lookup uid index of
                Nothing    -> go uids (Map.delete uid m, pkg:pkgs)
                Just rdeps -> go (rdeps ++ uids) (Map.delete uid m, pkg:pkgs)
            | otherwise
            = go uids (m,pkgs)

    -- | Given a 'PackageConfig' from some 'InstalledPackageIndex',
    -- return all entries in 'depends' which correspond to packages
    -- that do not exist in the index.
    depsNotAvailable :: InstalledPackageIndex
                     -> PackageConfig
                     -> [InstalledUnitId]
    depsNotAvailable pkg_map pkg = filter (not . (`Map.member` pkg_map)) (depends pkg)

    -- | Given a 'PackageConfig' from some 'InstalledPackageIndex'
    -- return all entries in 'abiDepends' which correspond to packages
    -- that do not exist, OR have mismatching ABIs.
    depsAbiMismatch :: InstalledPackageIndex
                    -> PackageConfig
                    -> [InstalledUnitId]
    depsAbiMismatch pkg_map pkg = map fst . filter (not . abiMatch) $ abiDepends pkg
      where
        abiMatch (dep_uid, abi)
            | Just dep_pkg <- Map.lookup dep_uid pkg_map
            = abiHash dep_pkg == abi
            | otherwise
            = False

    -- -----------------------------------------------------------------------------
    -- Ignore packages

    ignorePackages :: [IgnorePackageFlag] -> [PackageConfig] -> UnusablePackages
    ignorePackages flags pkgs = Map.fromList (concatMap doit flags)
      where
      doit (IgnorePackage str) =
         case partition (matchingStr str) pkgs of
             (ps, _) -> [ (unitId p, (p, IgnoredWithFlag))
                        | p <- ps ]
            -- missing package is not an error for -ignore-package,
            -- because a common usage is to -ignore-package P as
            -- a preventative measure just in case P exists.

    -- ----------------------------------------------------------------------------
    --
    -- Merging databases
    --

    -- | For each package, a mapping from uid -> i indicates that this
    -- package was brought into GHC by the ith @-package-db@ flag on
    -- the command line.  We use this mapping to make sure we prefer
    -- packages that were defined later on the command line, if there
    -- is an ambiguity.
    type PackagePrecedenceIndex = Map InstalledUnitId Int

    -- | Given a list of databases, merge them together, where
    -- packages with the same unit id in later databases override
    -- earlier ones.  This does NOT check if the resulting database
    -- makes sense (that's done by 'validateDatabase').
    mergeDatabases :: DynFlags -> [(FilePath, [PackageConfig])]
                   -> IO (InstalledPackageIndex, PackagePrecedenceIndex)
    mergeDatabases dflags = foldM merge (Map.empty, Map.empty) . zip [1..]
      where
        merge (pkg_map, prec_map) (i, (db_path, db)) = do
          debugTraceMsg dflags 2 $
              text "loading package database" <+> text db_path
          forM_ (Set.toList override_set) $ \pkg ->
              debugTraceMsg dflags 2 $
                  text "package" <+> ppr pkg <+>
                  text "overrides a previously defined package"
          return (pkg_map', prec_map')
         where
          db_map = mk_pkg_map db
          mk_pkg_map = Map.fromList . map (\p -> (unitId p, p))

          -- The set of UnitIds which appear in both db and pkgs.  These are the
          -- ones that get overridden.  Compute this just to give some
          -- helpful debug messages at -v2
          override_set :: Set InstalledUnitId
          override_set = Set.intersection (Map.keysSet db_map)
                                          (Map.keysSet pkg_map)

          -- Now merge the sets together (NB: in case of duplicate,
          -- first argument preferred)
          pkg_map' :: InstalledPackageIndex
          pkg_map' = Map.union db_map pkg_map

          prec_map' :: PackagePrecedenceIndex
          prec_map' = Map.union (Map.map (const i) db_map) prec_map

    -- | Validates a database, removing unusable packages from it
    -- (this includes removing packages that the user has explicitly
    -- ignored.)  Our general strategy:
    --
    -- 1. Remove all broken packages (dangling dependencies)
    -- 2. Remove all packages that are cyclic
    -- 3. Apply ignore flags
    -- 4. Remove all packages which have deps with mismatching ABIs
    --
    validateDatabase :: DynFlags -> InstalledPackageIndex
                     -> (InstalledPackageIndex, UnusablePackages, [SCC PackageConfig])
    validateDatabase dflags pkg_map1 =
        (pkg_map5, unusable, sccs)
      where
        ignore_flags = reverse (ignorePackageFlags dflags)

        -- Compute the reverse dependency index
        index = reverseDeps pkg_map1

        -- Helper function
        mk_unusable mk_err dep_matcher m uids =
          Map.fromList [ (unitId pkg, (pkg, mk_err (dep_matcher m pkg)))
                       | pkg <- uids ]

        -- Find broken packages
        directly_broken = filter (not . null . depsNotAvailable pkg_map1)
                                 (Map.elems pkg_map1)
        (pkg_map2, broken) = removePackages (map unitId directly_broken) index pkg_map1
        unusable_broken = mk_unusable BrokenDependencies depsNotAvailable pkg_map2 broken

        -- Find recursive packages
        sccs = stronglyConnComp [ (pkg, unitId pkg, depends pkg)
                                | pkg <- Map.elems pkg_map2 ]
        getCyclicSCC (CyclicSCC vs) = map unitId vs
        getCyclicSCC (AcyclicSCC _) = []
        (pkg_map3, cyclic) = removePackages (concatMap getCyclicSCC sccs) index pkg_map2
        unusable_cyclic = mk_unusable CyclicDependencies depsNotAvailable pkg_map3 cyclic

        -- Apply ignore flags
        directly_ignored = ignorePackages ignore_flags (Map.elems pkg_map3)
        (pkg_map4, ignored) = removePackages (Map.keys directly_ignored) index pkg_map3
        unusable_ignored = mk_unusable IgnoredDependencies depsNotAvailable pkg_map4 ignored

        -- Knock out packages whose dependencies don't agree with ABI
        -- (i.e., got invalidated due to shadowing)
        directly_shadowed = filter (not . null . depsAbiMismatch pkg_map4)
                                   (Map.elems pkg_map4)
        (pkg_map5, shadowed) = removePackages (map unitId directly_shadowed) index pkg_map4
        unusable_shadowed = mk_unusable ShadowedDependencies depsAbiMismatch pkg_map5 shadowed

        unusable = directly_ignored `Map.union` unusable_ignored
                                    `Map.union` unusable_broken
                                    `Map.union` unusable_cyclic
                                    `Map.union` unusable_shadowed

    -- -----------------------------------------------------------------------------
    -- When all the command-line options are in, we can process our package
    -- settings and populate the package state.

    mkPackageState
        :: DynFlags
        -- initial databases, in the order they were specified on
        -- the command line (later databases shadow earlier ones)
        -> [(FilePath, [PackageConfig])]
        -> [PreloadUnitId]              -- preloaded packages
        -> IO (PackageState,
               [PreloadUnitId],         -- new packages to preload
               Maybe [(ModuleName, Module)])

    mkPackageState dflags dbs preload0 = do
    {-
       Plan.

       There are two main steps for making the package state:

        1. We want to build a single, unified package database based
           on all of the input databases, which upholds the invariant that
           there is only one package per any UnitId and there are no
           dangling dependencies.  We'll do this by merging, and
           then successively filtering out bad dependencies.

           a) Merge all the databases together.
              If an input database defines unit ID that is already in
              the unified database, that package SHADOWS the existing
              package in the current unified database.  Note that
              order is important: packages defined later in the list of
              command line arguments shadow those defined earlier.

           b) Remove all packages with missing dependencies, or
              mutually recursive dependencies.

           b) Remove packages selected by -ignore-package from input database

           c) Remove all packages which depended on packages that are now
              shadowed by an ABI-incompatible package

           d) report (with -v) any packages that were removed by steps 1-3

        2. We want to look at the flags controlling package visibility,
           and build a mapping of what module names are in scope and
           where they live.

           a) on the final, unified database, we apply -trust/-distrust
              flags directly, modifying the database so that the 'trusted'
              field has the correct value.

           b) we use the -package/-hide-package flags to compute a
              visibility map, stating what packages are "exposed" for
              the purposes of computing the module map.
              * if any flag refers to a package which was removed by 1-5, then
                we can give an error message explaining why
              * if -hide-all-packages what not specified, this step also
                hides packages which are superseded by later exposed packages
              * this step is done TWICE if -plugin-package/-hide-all-plugin-packages
                are used

           c) based on the visibility map, we pick wired packages and rewrite
              them to have the expected unitId.

           d) finally, using the visibility map and the package database,
              we build a mapping saying what every in scope module name points to.
    -}

      -- This, and the other reverse's that you will see, are due to the face that
      -- packageFlags, pluginPackageFlags, etc. are all specified in *reverse* order
      -- than they are on the command line.
      let other_flags = reverse (packageFlags dflags)
      debugTraceMsg dflags 2 $
          text "package flags" <+> ppr other_flags

      -- Merge databases together, without checking validity
      (pkg_map1, prec_map) <- mergeDatabases dflags dbs

      -- Now that we've merged everything together, prune out unusable
      -- packages.
      let (pkg_map2, unusable, sccs) = validateDatabase dflags pkg_map1

      reportCycles dflags sccs
      reportUnusable dflags unusable

      -- Apply trust flags (these flags apply regardless of whether
      -- or not packages are visible or not)
      pkgs1 <- foldM (applyTrustFlag dflags prec_map unusable)
                     (Map.elems pkg_map2) (reverse (trustFlags dflags))
      let prelim_pkg_db = extendPackageConfigMap emptyPackageConfigMap pkgs1

      --
      -- Calculate the initial set of packages, prior to any package flags.
      -- This set contains the latest version of all valid (not unusable) packages,
      -- or is empty if we have -hide-all-packages
      --
      let preferLater pkg pkg' =
            case compareByPreference prec_map pkg pkg' of
                GT -> pkg
                _  -> pkg'
          calcInitial m pkg = addToUDFM_C preferLater m (fsPackageName pkg) pkg
          initial = if gopt Opt_HideAllPackages dflags
                        then emptyUDFM
                        else foldl' calcInitial emptyUDFM pkgs1
          vis_map1 = foldUDFM (\p vm ->
                                -- Note: we NEVER expose indefinite packages by
                                -- default, because it's almost assuredly not
                                -- what you want (no mix-in linking has occurred).
                                if exposed p && unitIdIsDefinite (packageConfigId p)
                                   then Map.insert (packageConfigId p)
                                                   UnitVisibility {
                                                     uv_expose_all = True,
                                                     uv_renamings = [],
                                                     uv_package_name = First (Just (fsPackageName p)),
                                                     uv_requirements = Map.empty,
                                                     uv_explicit = False
                                                   }
                                                   vm
                                   else vm)
                             Map.empty initial

      --
      -- Compute a visibility map according to the command-line flags (-package,
      -- -hide-package).  This needs to know about the unusable packages, since if a
      -- user tries to enable an unusable package, we should let them know.
      --
      vis_map2 <- foldM (applyPackageFlag dflags prec_map prelim_pkg_db unusable
                            (gopt Opt_HideAllPackages dflags) pkgs1)
                                vis_map1 other_flags

      --
      -- Sort out which packages are wired in. This has to be done last, since
      -- it modifies the unit ids of wired in packages, but when we process
      -- package arguments we need to key against the old versions.
      --
      (pkgs2, wired_map) <- findWiredInPackages dflags prec_map pkgs1 vis_map2
      let pkg_db = extendPackageConfigMap emptyPackageConfigMap pkgs2

      -- Update the visibility map, so we treat wired packages as visible.
      let vis_map = updateVisibilityMap wired_map vis_map2

      let hide_plugin_pkgs = gopt Opt_HideAllPluginPackages dflags
      plugin_vis_map <-
        case pluginPackageFlags dflags of
            -- common case; try to share the old vis_map
            [] | not hide_plugin_pkgs -> return vis_map
               | otherwise -> return Map.empty
            _ -> do let plugin_vis_map1
                            | hide_plugin_pkgs = Map.empty
                            -- Use the vis_map PRIOR to wired in,
                            -- because otherwise applyPackageFlag
                            -- won't work.
                            | otherwise = vis_map2
                    plugin_vis_map2
                        <- foldM (applyPackageFlag dflags prec_map prelim_pkg_db unusable
                                    (gopt Opt_HideAllPluginPackages dflags) pkgs1)
                                 plugin_vis_map1
                                 (reverse (pluginPackageFlags dflags))
                    -- Updating based on wired in packages is mostly
                    -- good hygiene, because it won't matter: no wired in
                    -- package has a compiler plugin.
                    -- TODO: If a wired in package had a compiler plugin,
                    -- and you tried to pick different wired in packages
                    -- with the plugin flags and the normal flags... what
                    -- would happen?  I don't know!  But this doesn't seem
                    -- likely to actually happen.
                    return (updateVisibilityMap wired_map plugin_vis_map2)

      --
      -- Here we build up a set of the packages mentioned in -package
      -- flags on the command line; these are called the "preload"
      -- packages.  we link these packages in eagerly.  The preload set
      -- should contain at least rts & base, which is why we pretend that
      -- the command line contains -package rts & -package base.
      --
      -- NB: preload IS important even for type-checking, because we
      -- need the correct include path to be set.
      --
      let preload1 = Map.keys (Map.filter uv_explicit vis_map)

      let pkgname_map = foldl add Map.empty pkgs2
            where add pn_map p
                    = Map.insert (packageName p) (componentId p) pn_map

      -- The explicitPackages accurately reflects the set of packages we have turned
      -- on; as such, it also is the only way one can come up with requirements.
      -- The requirement context is directly based off of this: we simply
      -- look for nested unit IDs that are directly fed holes: the requirements
      -- of those units are precisely the ones we need to track
      let explicit_pkgs = Map.keys vis_map
          req_ctx = Map.map (Set.toList)
                  $ Map.unionsWith Set.union (map uv_requirements (Map.elems vis_map))

      let preload2 = preload1

      let
          -- add base & rts to the preload packages
          basicLinkedPackages
           | gopt Opt_AutoLinkPackages dflags
              = filter (flip elemUDFM (unPackageConfigMap pkg_db))
                    [baseUnitId, rtsUnitId]
           | otherwise = []
          -- but in any case remove the current package from the set of
          -- preloaded packages so that base/rts does not end up in the
          -- set up preloaded package when we are just building it
          -- (NB: since this is only relevant for base/rts it doesn't matter
          -- that thisUnitIdInsts_ is not wired yet)
          --
          preload3 = nub $ filter (/= thisPackage dflags)
                         $ (basicLinkedPackages ++ preload2)

      -- Close the preload packages with their dependencies
      dep_preload <- closeDeps dflags pkg_db (zip (map toInstalledUnitId preload3) (repeat Nothing))
      let new_dep_preload = filter (`notElem` preload0) dep_preload

      let mod_map = mkModuleToPkgConfAll dflags pkg_db vis_map
      when (dopt Opt_D_dump_mod_map dflags) $
          printInfoForUser (dflags { pprCols = 200 })
                           alwaysQualify (pprModuleMap mod_map)

      -- Force pstate to avoid leaking the dflags0 passed to mkPackageState
      let !pstate = PackageState{
        preloadPackages     = dep_preload,
        explicitPackages    = explicit_pkgs,
        pkgIdMap            = pkg_db,
        moduleToPkgConfAll  = mod_map,
        pluginModuleToPkgConfAll = mkModuleToPkgConfAll dflags pkg_db plugin_vis_map,
        packageNameMap          = pkgname_map,
        unwireMap = Map.fromList [ (v,k) | (k,v) <- Map.toList wired_map ],
        requirementContext = req_ctx
        }
      let new_insts = fmap (map (fmap (upd_wired_in_mod wired_map))) (thisUnitIdInsts_ dflags)
      return (pstate, new_dep_preload, new_insts)

    -- | Given a wired-in 'UnitId', "unwire" it into the 'UnitId'
    -- that it was recorded as in the package database.
    unwireUnitId :: DynFlags -> UnitId -> UnitId
    unwireUnitId dflags uid@(DefiniteUnitId def_uid) =
        maybe uid DefiniteUnitId (Map.lookup def_uid (unwireMap (pkgState dflags)))
    unwireUnitId _ uid = uid

    -- -----------------------------------------------------------------------------
    -- | Makes the mapping from module to package info

    -- Slight irritation: we proceed by leafing through everything
    -- in the installed package database, which makes handling indefinite
    -- packages a bit bothersome.

    mkModuleToPkgConfAll
      :: DynFlags
      -> PackageConfigMap
      -> VisibilityMap
      -> ModuleToPkgConfAll
    mkModuleToPkgConfAll dflags pkg_db vis_map =
        Map.foldlWithKey extend_modmap emptyMap vis_map
     where
      emptyMap = Map.empty
      sing pk m _ = Map.singleton (mkModule pk m)
      addListTo = foldl' merge
      merge m (k, v) = MapStrict.insertWith (Map.unionWith mappend) k v m
      setOrigins m os = fmap (const os) m
      extend_modmap modmap uid
        UnitVisibility { uv_expose_all = b, uv_renamings = rns }
        = addListTo modmap theBindings
       where
        pkg = pkg_lookup uid

        theBindings :: [(ModuleName, Map Module ModuleOrigin)]
        theBindings = newBindings b rns

        newBindings :: Bool
                    -> [(ModuleName, ModuleName)]
                    -> [(ModuleName, Map Module ModuleOrigin)]
        newBindings e rns  = es e ++ hiddens ++ map rnBinding rns

        rnBinding :: (ModuleName, ModuleName)
                  -> (ModuleName, Map Module ModuleOrigin)
        rnBinding (orig, new) = (new, setOrigins origEntry fromFlag)
         where origEntry = case lookupUFM esmap orig of
                Just r -> r
                Nothing -> throwGhcException (CmdLineError (showSDoc dflags
                            (text "package flag: could not find module name" <+>
                                ppr orig <+> text "in package" <+> ppr pk)))

        es :: Bool -> [(ModuleName, Map Module ModuleOrigin)]
        es e = do
         (m, exposedReexport) <- exposed_mods
         let (pk', m', pkg', origin') =
              case exposedReexport of
               Nothing -> (pk, m, pkg, fromExposedModules e)
               Just (Module pk' m') ->
                let pkg' = pkg_lookup pk'
                in (pk', m', pkg', fromReexportedModules e pkg')
         return (m, sing pk' m' pkg' origin')

        esmap :: UniqFM (Map Module ModuleOrigin)
        esmap = listToUFM (es False) -- parameter here doesn't matter, orig will
                                     -- be overwritten

        hiddens = [(m, sing pk m pkg ModHidden) | m <- hidden_mods]

        pk = packageConfigId pkg
        pkg_lookup uid = lookupPackage' (isIndefinite dflags) pkg_db uid
                            `orElse` pprPanic "pkg_lookup" (ppr uid)

        exposed_mods = exposedModules pkg
        hidden_mods = hiddenModules pkg

    -- -----------------------------------------------------------------------------
    -- Extracting information from the packages in scope

    -- Many of these functions take a list of packages: in those cases,
    -- the list is expected to contain the "dependent packages",
    -- i.e. those packages that were found to be depended on by the
    -- current module/program.  These can be auto or non-auto packages, it
    -- doesn't really matter.  The list is always combined with the list
    -- of preload (command-line) packages to determine which packages to
    -- use.

    -- | Find all the include directories in these and the preload packages
    getPackageIncludePath :: DynFlags -> [PreloadUnitId] -> IO [String]
    getPackageIncludePath dflags pkgs =
      collectIncludeDirs `fmap` getPreloadPackagesAnd dflags pkgs

    collectIncludeDirs :: [PackageConfig] -> [FilePath]
    collectIncludeDirs ps = nub (filter notNull (concatMap includeDirs ps))

    -- | Find all the library paths in these and the preload packages
    getPackageLibraryPath :: DynFlags -> [PreloadUnitId] -> IO [String]
    getPackageLibraryPath dflags pkgs =
      collectLibraryPaths dflags `fmap` getPreloadPackagesAnd dflags pkgs

    collectLibraryPaths :: DynFlags -> [PackageConfig] -> [FilePath]
    collectLibraryPaths dflags = nub . filter notNull
                               . concatMap (libraryDirsForWay dflags)

    -- | Find all the link options in these and the preload packages,
    -- returning (package hs lib options, extra library options, other flags)
    getPackageLinkOpts :: DynFlags -> [PreloadUnitId] -> IO ([String], [String], [String])
    getPackageLinkOpts dflags pkgs =
      collectLinkOpts dflags `fmap` getPreloadPackagesAnd dflags pkgs

    collectLinkOpts :: DynFlags -> [PackageConfig] -> ([String], [String], [String])
    collectLinkOpts dflags ps =
        (
            concatMap (map ("-l" ++) . packageHsLibs dflags) ps,
            concatMap (map ("-l" ++) . extraLibraries) ps,
            concatMap ldOptions ps
        )
    collectArchives :: DynFlags -> PackageConfig -> IO [FilePath]
    collectArchives dflags pc =
      filterM doesFileExist [ searchPath </> ("lib" ++ lib ++ ".a")
                            | searchPath <- searchPaths
                            , lib <- libs ]
      where searchPaths = nub . filter notNull . libraryDirsForWay dflags $ pc
            libs        = packageHsLibs dflags pc ++ extraLibraries pc

    packageHsLibs :: DynFlags -> PackageConfig -> [String]
    packageHsLibs dflags p = map (mkDynName . addSuffix) (hsLibraries p)
      where
            ways0 = ways dflags

            ways1 = filter (/= WayDyn) ways0
            -- the name of a shared library is libHSfoo-ghc<version>.so
            -- we leave out the _dyn, because it is superfluous

            -- debug and profiled RTSs include support for -eventlog
            ways2 | WayDebug `elem` ways1 || WayProf `elem` ways1
                  = filter (/= WayEventLog) ways1
                  | otherwise
                  = ways1

            tag     = mkBuildTag (filter (not . wayRTSOnly) ways2)
            rts_tag = mkBuildTag ways2

            mkDynName x
             | WayDyn `notElem` ways dflags = x
             | "HS" `isPrefixOf` x          =
                  x ++ '-':programName dflags ++ projectVersion dflags
               -- For non-Haskell libraries, we use the name "Cfoo". The .a
               -- file is libCfoo.a, and the .so is libfoo.so. That way the
               -- linker knows what we mean for the vanilla (-lCfoo) and dyn
               -- (-lfoo) ways. We therefore need to strip the 'C' off here.
             | Just x' <- stripPrefix "C" x = x'
             | otherwise
                = panic ("Don't understand library name " ++ x)

            -- Add _thr and other rts suffixes to packages named
            -- `rts` or `rts-1.0`. Why both?  Traditionally the rts
            -- package is called `rts` only.  However the tooling
            -- usually expects a package name to have a version.
            -- As such we will gradually move towards the `rts-1.0`
            -- package name, at which point the `rts` package name
            -- will eventually be unused.
            --
            -- This change elevates the need to add custom hooks
            -- and handling specifically for the `rts` package for
            -- example in ghc-cabal.
            addSuffix rts@"HSrts"    = rts       ++ (expandTag rts_tag)
            addSuffix rts@"HSrts-1.0"= rts       ++ (expandTag rts_tag)
            addSuffix other_lib      = other_lib ++ (expandTag tag)

            expandTag t | null t = ""
                        | otherwise = '_':t

    -- | Either the 'libraryDirs' or 'libraryDynDirs' as appropriate for the way.
    libraryDirsForWay :: DynFlags -> PackageConfig -> [String]
    libraryDirsForWay dflags
      | WayDyn `elem` ways dflags = libraryDynDirs
      | otherwise                 = libraryDirs

    -- | Find all the C-compiler options in these and the preload packages
    getPackageExtraCcOpts :: DynFlags -> [PreloadUnitId] -> IO [String]
    getPackageExtraCcOpts dflags pkgs = do
      ps <- getPreloadPackagesAnd dflags pkgs
      return (concatMap ccOptions ps)

    -- | Find all the package framework paths in these and the preload packages
    getPackageFrameworkPath  :: DynFlags -> [PreloadUnitId] -> IO [String]
    getPackageFrameworkPath dflags pkgs = do
      ps <- getPreloadPackagesAnd dflags pkgs
      return (nub (filter notNull (concatMap frameworkDirs ps)))

    -- | Find all the package frameworks in these and the preload packages
    getPackageFrameworks  :: DynFlags -> [PreloadUnitId] -> IO [String]
    getPackageFrameworks dflags pkgs = do
      ps <- getPreloadPackagesAnd dflags pkgs
      return (concatMap frameworks ps)

    -- -----------------------------------------------------------------------------
    -- Package Utils

    -- | Takes a 'ModuleName', and if the module is in any package returns
    -- list of modules which take that name.
    lookupModuleInAllPackages :: DynFlags
                              -> ModuleName
                              -> [(Module, PackageConfig)]
    lookupModuleInAllPackages dflags m
      = case lookupModuleWithSuggestions dflags m Nothing of
          LookupFound a b -> [(a,b)]
          LookupMultiple rs -> map f rs
            where f (m,_) = (m, expectJust "lookupModule" (lookupPackage dflags
                                                             (moduleUnitId m)))
          _ -> []

    -- | The result of performing a lookup
    data LookupResult =
        -- | Found the module uniquely, nothing else to do
        LookupFound Module PackageConfig
        -- | Multiple modules with the same name in scope
      | LookupMultiple [(Module, ModuleOrigin)]
        -- | No modules found, but there were some hidden ones with
        -- an exact name match.  First is due to package hidden, second
        -- is due to module being hidden
      | LookupHidden [(Module, ModuleOrigin)] [(Module, ModuleOrigin)]
        -- | Nothing found, here are some suggested different names
      | LookupNotFound [ModuleSuggestion] -- suggestions

    data ModuleSuggestion = SuggestVisible ModuleName Module ModuleOrigin
                          | SuggestHidden ModuleName Module ModuleOrigin

    lookupModuleWithSuggestions :: DynFlags
                                -> ModuleName
                                -> Maybe FastString
                                -> LookupResult
    lookupModuleWithSuggestions dflags
      = lookupModuleWithSuggestions' dflags
            (moduleToPkgConfAll (pkgState dflags))

    lookupPluginModuleWithSuggestions :: DynFlags
                                      -> ModuleName
                                      -> Maybe FastString
                                      -> LookupResult
    lookupPluginModuleWithSuggestions dflags
      = lookupModuleWithSuggestions' dflags
            (pluginModuleToPkgConfAll (pkgState dflags))

    lookupModuleWithSuggestions' :: DynFlags
                                -> ModuleToPkgConfAll
                                -> ModuleName
                                -> Maybe FastString
                                -> LookupResult
    lookupModuleWithSuggestions' dflags mod_map m mb_pn
      = case Map.lookup m mod_map of
            Nothing -> LookupNotFound suggestions
            Just xs ->
              case foldl' classify ([],[],[]) (Map.toList xs) of
                ([], [], []) -> LookupNotFound suggestions
                (_, _, [(m, _)])             -> LookupFound m (mod_pkg m)
                (_, _, exposed@(_:_))        -> LookupMultiple exposed
                (hidden_pkg, hidden_mod, []) -> LookupHidden hidden_pkg hidden_mod
      where
        classify (hidden_pkg, hidden_mod, exposed) (m, origin0) =
          let origin = filterOrigin mb_pn (mod_pkg m) origin0
              x = (m, origin)
          in case origin of
              ModHidden                  -> (hidden_pkg,   x:hidden_mod, exposed)
              _ | originEmpty origin     -> (hidden_pkg,   hidden_mod,   exposed)
                | originVisible origin   -> (hidden_pkg,   hidden_mod,   x:exposed)
                | otherwise              -> (x:hidden_pkg, hidden_mod,   exposed)

        pkg_lookup p = lookupPackage dflags p `orElse` pprPanic "lookupModuleWithSuggestions" (ppr p <+> ppr m)
        mod_pkg = pkg_lookup . moduleUnitId

        -- Filters out origins which are not associated with the given package
        -- qualifier.  No-op if there is no package qualifier.  Test if this
        -- excluded all origins with 'originEmpty'.
        filterOrigin :: Maybe FastString
                     -> PackageConfig
                     -> ModuleOrigin
                     -> ModuleOrigin
        filterOrigin Nothing _ o = o
        filterOrigin (Just pn) pkg o =
          case o of
              ModHidden -> if go pkg then ModHidden else mempty
              ModOrigin { fromOrigPackage = e, fromExposedReexport = res,
                          fromHiddenReexport = rhs }
                -> ModOrigin {
                      fromOrigPackage = if go pkg then e else Nothing
                    , fromExposedReexport = filter go res
                    , fromHiddenReexport = filter go rhs
                    , fromPackageFlag = False -- always excluded
                    }
          where go pkg = pn == fsPackageName pkg

        suggestions
          | gopt Opt_HelpfulErrors dflags =
               fuzzyLookup (moduleNameString m) all_mods
          | otherwise = []

        all_mods :: [(String, ModuleSuggestion)]     -- All modules
        all_mods = sortBy (comparing fst) $
            [ (moduleNameString m, suggestion)
            | (m, e) <- Map.toList (moduleToPkgConfAll (pkgState dflags))
            , suggestion <- map (getSuggestion m) (Map.toList e)
            ]
        getSuggestion name (mod, origin) =
            (if originVisible origin then SuggestVisible else SuggestHidden)
                name mod origin

    listVisibleModuleNames :: DynFlags -> [ModuleName]
    listVisibleModuleNames dflags =
        map fst (filter visible (Map.toList (moduleToPkgConfAll (pkgState dflags))))
      where visible (_, ms) = any originVisible (Map.elems ms)

    -- | Find all the 'PackageConfig' in both the preload packages from 'DynFlags' and corresponding to the list of
    -- 'PackageConfig's
    getPreloadPackagesAnd :: DynFlags -> [PreloadUnitId] -> IO [PackageConfig]
    getPreloadPackagesAnd dflags pkgids0 =
      let
          pkgids  = pkgids0 ++
                      -- An indefinite package will have insts to HOLE,
                      -- which is not a real package. Don't look it up.
                      -- Fixes #14525
                      if isIndefinite dflags
                        then []
                        else map (toInstalledUnitId . moduleUnitId . snd)
                                 (thisUnitIdInsts dflags)
          state   = pkgState dflags
          pkg_map = pkgIdMap state
          preload = preloadPackages state
          pairs = zip pkgids (repeat Nothing)
      in do
      all_pkgs <- throwErr dflags (foldM (add_package dflags pkg_map) preload pairs)
      return (map (getInstalledPackageDetails dflags) all_pkgs)

    -- Takes a list of packages, and returns the list with dependencies included,
    -- in reverse dependency order (a package appears before those it depends on).
    closeDeps :: DynFlags
              -> PackageConfigMap
              -> [(InstalledUnitId, Maybe InstalledUnitId)]
              -> IO [InstalledUnitId]
    closeDeps dflags pkg_map ps
        = throwErr dflags (closeDepsErr dflags pkg_map ps)

    throwErr :: DynFlags -> MaybeErr MsgDoc a -> IO a
    throwErr dflags m
                  = case m of
                    Failed e    -> throwGhcExceptionIO (CmdLineError (showSDoc dflags e))
                    Succeeded r -> return r

    closeDepsErr :: DynFlags
                 -> PackageConfigMap
                 -> [(InstalledUnitId,Maybe InstalledUnitId)]
                 -> MaybeErr MsgDoc [InstalledUnitId]
    closeDepsErr dflags pkg_map ps = foldM (add_package dflags pkg_map) [] ps

    -- internal helper
    add_package :: DynFlags
                -> PackageConfigMap
                -> [PreloadUnitId]
                -> (PreloadUnitId,Maybe PreloadUnitId)
                -> MaybeErr MsgDoc [PreloadUnitId]
    add_package dflags pkg_db ps (p, mb_parent)
      | p `elem` ps = return ps     -- Check if we've already added this package
      | otherwise =
          case lookupInstalledPackage' pkg_db p of
            Nothing -> Failed (missingPackageMsg p <>
                               missingDependencyMsg mb_parent)
            Just pkg -> do
               -- Add the package's dependents also
               ps' <- foldM add_unit_key ps (depends pkg)
               return (p : ps')
              where
                add_unit_key ps key
                  = add_package dflags pkg_db ps (key, Just p)

    missingPackageMsg :: Outputable pkgid => pkgid -> SDoc
    missingPackageMsg p = text "unknown package:" <+> ppr p

    missingDependencyMsg :: Maybe InstalledUnitId -> SDoc
    missingDependencyMsg Nothing = Outputable.empty
    missingDependencyMsg (Just parent)
      = space <> parens (text "dependency of" <+> ftext (installedUnitIdFS parent))

    -- -----------------------------------------------------------------------------

    componentIdString :: DynFlags -> ComponentId -> Maybe String
    componentIdString dflags cid = do
        conf <- lookupInstalledPackage dflags (componentIdToInstalledUnitId cid)
        return $
            case sourceLibName conf of
                Nothing -> sourcePackageIdString conf
                Just (PackageName libname) ->
                    packageNameString conf
                        ++ "-" ++ showVersion (packageVersion conf)
                        ++ ":" ++ unpackFS libname

    displayInstalledUnitId :: DynFlags -> InstalledUnitId -> Maybe String
    displayInstalledUnitId dflags uid =
        fmap sourcePackageIdString (lookupInstalledPackage dflags uid)

    -- | Will the 'Name' come from a dynamically linked library?
    isDllName :: DynFlags -> Module -> Name -> Bool
    -- Despite the "dll", I think this function just means that
    -- the symbol comes from another dynamically-linked package,
    -- and applies on all platforms, not just Windows
    isDllName dflags this_mod name
      | WayDyn `notElem` ways dflags = False
      | Just mod <- nameModule_maybe name
        -- Issue #8696 - when GHC is dynamically linked, it will attempt
        -- to load the dynamic dependencies of object files at compile
        -- time for things like QuasiQuotes or
        -- TemplateHaskell. Unfortunately, this interacts badly with
        -- intra-package linking, because we don't generate indirect
        -- (dynamic) symbols for intra-package calls. This means that if a
        -- module with an intra-package call is loaded without its
        -- dependencies, then GHC fails to link. This is the cause of #
        --
        -- In the mean time, always force dynamic indirections to be
        -- generated: when the module name isn't the module being
        -- compiled, references are dynamic.
        = case platformOS $ targetPlatform dflags of
            -- On Windows the hack for #8696 makes it unlinkable.
            -- As the entire setup of the code from Cmm down to the RTS expects
            -- the use of trampolines for the imported functions only when
            -- doing intra-package linking, e.g. refering to a symbol defined in the same
            -- package should not use a trampoline.
            -- I much rather have dynamic TH not supported than the entire Dynamic linking
            -- not due to a hack.
            -- Also not sure this would break on Windows anyway.
            OSMinGW32 -> moduleUnitId mod /= moduleUnitId this_mod

            -- For the other platforms, still perform the hack
            _         -> mod /= this_mod

      | otherwise = False  -- no, it is not even an external name

    -- -----------------------------------------------------------------------------
    -- Displaying packages

    -- | Show (very verbose) package info
    pprPackages :: DynFlags -> SDoc
    pprPackages = pprPackagesWith pprPackageConfig

    pprPackagesWith :: (PackageConfig -> SDoc) -> DynFlags -> SDoc
    pprPackagesWith pprIPI dflags =
        vcat (intersperse (text "---") (map pprIPI (listPackageConfigMap dflags)))

    -- | Show simplified package info.
    --
    -- The idea is to only print package id, and any information that might
    -- be different from the package databases (exposure, trust)
    pprPackagesSimple :: DynFlags -> SDoc
    pprPackagesSimple = pprPackagesWith pprIPI
        where pprIPI ipi = let i = installedUnitIdFS (unitId ipi)
                               e = if exposed ipi then text "E" else text " "
                               t = if trusted ipi then text "T" else text " "
                           in e <> t <> text "  " <> ftext i

    -- | Show the mapping of modules to where they come from.
    pprModuleMap :: ModuleToPkgConfAll -> SDoc
    pprModuleMap mod_map =
      vcat (map pprLine (Map.toList mod_map))
        where
          pprLine (m,e) = ppr m $$ nest 50 (vcat (map (pprEntry m) (Map.toList e)))
          pprEntry :: Outputable a => ModuleName -> (Module, a) -> SDoc
          pprEntry m (m',o)
            | m == moduleName m' = ppr (moduleUnitId m') <+> parens (ppr o)
            | otherwise = ppr m' <+> parens (ppr o)

    fsPackageName :: PackageConfig -> FastString
    fsPackageName = mkFastString . packageNameString

    -- | Given a fully instantiated 'UnitId', improve it into a
    -- 'InstalledUnitId' if we can find it in the package database.
    improveUnitId :: PackageConfigMap -> UnitId -> UnitId
    improveUnitId _ uid@(DefiniteUnitId _) = uid -- short circuit
    improveUnitId pkg_map uid =
        -- Do NOT lookup indefinite ones, they won't be useful!
        case lookupPackage' False pkg_map uid of
            Nothing  -> uid
            Just pkg ->
                -- Do NOT improve if the indefinite unit id is not
                -- part of the closure unique set.  See
                -- Note [UnitId to InstalledUnitId improvement]
                if installedPackageConfigId pkg `elementOfUniqSet` preloadClosure pkg_map
                    then packageConfigId pkg
                    else uid

    -- | Retrieve the 'PackageConfigMap' from 'DynFlags'; used
    -- in the @hs-boot@ loop-breaker.
    getPackageConfigMap :: DynFlags -> PackageConfigMap
    getPackageConfigMap = pkgIdMap . pkgState