State.hs 91.9 KB
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-- (c) The University of Glasgow, 2006

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{-# LANGUAGE CPP, ScopedTypeVariables, BangPatterns, FlexibleContexts #-}
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-- | Unit manipulation
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module GHC.Unit.State (
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        module GHC.Unit.Info,
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        -- * Reading the package config, and processing cmdline args
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        UnitState(..),
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        UnitDatabase (..),
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        emptyUnitState,
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        initUnits,
        readUnitDatabases,
        readUnitDatabase,
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        getUnitDbRefs,
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        resolveUnitDatabase,
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        listUnitInfo,
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        -- * Querying the package config
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        lookupUnit,
        lookupUnit',
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        unsafeLookupUnit,
        lookupUnitId,
        lookupUnitId',
        unsafeLookupUnitId,

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        lookupPackageName,
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        improveUnit,
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        searchPackageId,
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        displayUnitId,
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        listVisibleModuleNames,
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        lookupModuleInAllUnits,
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        lookupModuleWithSuggestions,
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        lookupPluginModuleWithSuggestions,
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        LookupResult(..),
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        ModuleSuggestion(..),
        ModuleOrigin(..),
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        UnusableUnitReason(..),
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        pprReason,
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        -- * Inspecting the set of packages in scope
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        getUnitIncludePath,
        getUnitLibraryPath,
        getUnitLinkOpts,
        getUnitExtraCcOpts,
        getUnitFrameworkPath,
        getUnitFrameworks,
        getPreloadUnitsAnd,
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        collectArchives,
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        collectIncludeDirs, collectLibraryPaths, collectLinkOpts,
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        packageHsLibs, getLibs,
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        -- * Module hole substitution
        ShHoleSubst,
        renameHoleUnit,
        renameHoleModule,
        renameHoleUnit',
        renameHoleModule',
        instUnitToUnit,
        instModuleToModule,

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        -- * Utils
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        mkIndefUnitId,
        updateIndefUnitId,
        unwireUnit,
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        pprFlag,
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        pprUnits,
        pprUnitsSimple,
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        pprModuleMap,
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        homeUnitIsIndefinite,
        homeUnitIsDefinite,
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    )
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where

#include "HsVersions.h"
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import GHC.Prelude
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import GHC.Unit.Database
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import GHC.Unit.Info
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import GHC.Unit.Types
import GHC.Unit.Module
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import GHC.Driver.Session
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import GHC.Driver.Ways
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import GHC.Types.Unique.FM
import GHC.Types.Unique.DFM
import GHC.Types.Unique.Set
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import GHC.Types.Unique.DSet
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import GHC.Utils.Misc
import GHC.Utils.Panic
import GHC.Utils.Outputable as Outputable
import GHC.Data.Maybe
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import System.Environment ( getEnv )
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import GHC.Data.FastString
import GHC.Utils.Error  ( debugTraceMsg, MsgDoc, dumpIfSet_dyn,
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                          withTiming, DumpFormat (..) )
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import GHC.Utils.Exception
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import System.Directory
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import System.FilePath as FilePath
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import Control.Monad
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import Data.Graph (stronglyConnComp, SCC(..))
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import Data.Char ( toUpper )
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import Data.List as List
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import Data.Map (Map)
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import Data.Set (Set)
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import Data.Monoid (First(..))
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import qualified Data.Semigroup as Semigroup
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import qualified Data.Map as Map
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import qualified Data.Map.Strict as MapStrict
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import qualified Data.Set as Set
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-- ---------------------------------------------------------------------------
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-- The Unit state
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-- | Unit state is all stored in 'DynFlags', including the details of
-- all units, which units are exposed, and which modules they
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-- provide.
--
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-- The unit state is computed by 'initUnits', and kept in DynFlags.
-- It is influenced by various command-line flags:
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--
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--   * @-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.
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--
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--   * @-hide-package <pkg>@ causes @<pkg>@ to become hidden.
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--
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--   * (there are a few more flags, check below for their semantics)
--
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-- The unit state has the following properties.
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--
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--   * Let @exposedUnits@ be the set of packages thus exposed.
--     Let @depExposedUnits@ be the transitive closure from @exposedUnits@ of
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--     their dependencies.
--
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--   * When searching for a module from a preload import declaration,
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--     only the exposed modules in @exposedUnits@ are valid.
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--
--   * When searching for a module from an implicit import, all modules
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--     from @depExposedUnits@ are valid.
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--
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--   * When linking in a compilation manager mode, we link in packages the
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--     program depends on (the compiler knows this list by the
--     time it gets to the link step).  Also, we link in all packages
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--     which were mentioned with preload @-package@ flags on the command-line,
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--     or are a transitive dependency of same, or are \"base\"\/\"rts\".
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--     The reason for this is that we might need packages which don't
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--     contain any Haskell modules, and therefore won't be discovered
--     by the normal mechanism of dependency tracking.

-- Notes on DLLs
-- ~~~~~~~~~~~~~
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-- 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
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-- When compiling A, we record in B's Module value whether it's
-- in a different DLL, by setting the DLL flag.

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-- | Given a module name, there may be multiple ways it came into scope,
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-- 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!
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data ModuleOrigin =
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    -- | 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
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    -- | Module is unavailable because the package is unusable.
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  | ModUnusable UnusableUnitReason
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    -- | 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.
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        fromOrigUnit :: Maybe Bool
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        -- | Is the module available from a reexport of an exposed package?
        -- There could be multiple.
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      , fromExposedReexport :: [UnitInfo]
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        -- | Is the module available from a reexport of a hidden package?
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      , fromHiddenReexport :: [UnitInfo]
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        -- | Did the module export come from a package flag? (ToDo: track
        -- more information.
      , fromPackageFlag :: Bool
      }

instance Outputable ModuleOrigin where
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    ppr ModHidden = text "hidden module"
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    ppr (ModUnusable _) = text "unusable module"
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    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" <+>
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                    sep (map (ppr . mkUnit) res)]) ++
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        (if null rhs
            then []
            else [text "hidden reexport by" <+>
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                    sep (map (ppr . mkUnit) res)]) ++
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        (if f then [text "package flag"] else [])
        ))

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

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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"

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instance Monoid ModuleOrigin where
    mempty = ModOrigin Nothing [] [] False
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    mappend = (Semigroup.<>)
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-- | Is the name from the import actually visible? (i.e. does it cause
-- ambiguity, or is it only relevant when we're making suggestions?)
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originVisible :: ModuleOrigin -> Bool
originVisible ModHidden = False
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originVisible (ModUnusable _) = False
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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
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type PreloadUnitClosure = UniqSet UnitId

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-- | 'UniqFM' map from 'Unit' to a 'UnitVisibility'.
type VisibilityMap = Map Unit UnitVisibility
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-- | 'UnitVisibility' records the various aspects of visibility of a particular
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-- 'Unit'.
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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
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      -- ^ The package name associated with the 'Unit'.  This is used
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      -- to implement legacy behavior where @-package foo-0.1@ implicitly
      -- hides any packages named @foo@
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    , uv_requirements :: Map ModuleName (Set InstantiatedModule)
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      -- ^ 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.)
    }
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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)
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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
          }

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instance Monoid UnitVisibility where
    mempty = UnitVisibility
             { uv_expose_all = False
             , uv_renamings = []
             , uv_package_name = First Nothing
             , uv_requirements = Map.empty
             , uv_explicit = False
             }
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    mappend = (Semigroup.<>)
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-- | Map from 'ModuleName' to a set of of module providers (i.e. a 'Module' and
-- its 'ModuleOrigin').
--
-- NB: the set is in fact a 'Map Module ModuleOrigin', probably to keep only one
-- origin for a given 'Module'
type ModuleNameProvidersMap =
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    Map ModuleName (Map Module ModuleOrigin)
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data UnitState = UnitState {
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  -- | A mapping of 'Unit' to 'UnitInfo'.  This list is adjusted
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  -- so that only valid units are here.  'UnitInfo' reflects
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  -- what was stored *on disk*, except for the 'trusted' flag, which
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  -- is adjusted at runtime.  (In particular, some units in this map
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  -- may have the 'exposed' flag be 'False'.)
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  unitInfoMap :: UnitInfoMap,

  -- | The set of transitively reachable units according
  -- to the explicitly provided command line arguments.
  -- A fully instantiated VirtUnit may only be replaced by a RealUnit from
  -- this set.
  -- See Note [VirtUnit to RealUnit improvement]
  preloadClosure :: PreloadUnitClosure,
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  -- | A mapping of 'PackageName' to 'IndefUnitId'.  This is used when
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  -- users refer to packages in Backpack includes.
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  packageNameMap            :: Map PackageName IndefUnitId,
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  -- | A mapping from database unit keys to wired in unit ids.
  wireMap :: Map UnitId UnitId,

  -- | A mapping from wired in unit ids to unit keys from the database.
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  unwireMap :: Map UnitId UnitId,
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  -- | 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.
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  preloadUnits      :: [UnitId],
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  -- | Packages which we explicitly depend on (from a command line flag).
  -- We'll use this to generate version macros.
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  explicitUnits      :: [Unit],
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  -- | 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.
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  moduleNameProvidersMap    :: !ModuleNameProvidersMap,
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  -- | A map, like 'moduleNameProvidersMap', but controlling plugin visibility.
  pluginModuleNameProvidersMap    :: !ModuleNameProvidersMap,
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  -- | 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.
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  requirementContext :: Map ModuleName [InstantiatedModule],

  -- | Indicate if we can instantiate units on-the-fly.
  --
  -- This should only be true when we are type-checking an indefinite unit.
  -- See Note [About units] in GHC.Unit.
  allowVirtualUnits :: !Bool
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  }

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emptyUnitState :: UnitState
emptyUnitState = UnitState {
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    unitInfoMap = Map.empty,
    preloadClosure = emptyUniqSet,
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    packageNameMap = Map.empty,
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    wireMap   = Map.empty,
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    unwireMap = Map.empty,
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    preloadUnits = [],
    explicitUnits = [],
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    moduleNameProvidersMap = Map.empty,
    pluginModuleNameProvidersMap = Map.empty,
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    requirementContext = Map.empty,
    allowVirtualUnits = False
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    }

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-- | Unit database
data UnitDatabase unit = UnitDatabase
   { unitDatabasePath  :: FilePath
   , unitDatabaseUnits :: [GenUnitInfo unit]
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   }

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type UnitInfoMap = Map UnitId UnitInfo
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-- | Find the unit we know about with the given unit, if any
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lookupUnit :: UnitState -> Unit -> Maybe UnitInfo
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lookupUnit pkgs = lookupUnit' (allowVirtualUnits pkgs) (unitInfoMap pkgs) (preloadClosure pkgs)

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-- | A more specialized interface, which doesn't require a 'UnitState' (so it
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-- can be used while we're initializing 'DynFlags')
--
-- Parameters:
--    * a boolean specifying whether or not to look for on-the-fly renamed interfaces
--    * a 'UnitInfoMap'
--    * a 'PreloadUnitClosure'
lookupUnit' :: Bool -> UnitInfoMap -> PreloadUnitClosure -> Unit -> Maybe UnitInfo
lookupUnit' allowOnTheFlyInst pkg_map closure u = case u of
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   HoleUnit   -> error "Hole unit"
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   RealUnit i -> Map.lookup (unDefinite i) pkg_map
   VirtUnit i
      | allowOnTheFlyInst
      -> -- lookup UnitInfo of the indefinite unit to be instantiated and
         -- instantiate it on-the-fly
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         fmap (renameUnitInfo pkg_map closure (instUnitInsts i))
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           (Map.lookup (indefUnit (instUnitInstanceOf i)) pkg_map)

      | otherwise
      -> -- lookup UnitInfo by virtual UnitId. This is used to find indefinite
         -- units. Even if they are real, installed units, they can't use the
         -- `RealUnit` constructor (it is reserved for definite units) so we use
         -- the `VirtUnit` constructor.
         Map.lookup (virtualUnitId i) pkg_map
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-- | Find the unit we know about with the given unit id, if any
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lookupUnitId :: UnitState -> UnitId -> Maybe UnitInfo
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lookupUnitId state uid = lookupUnitId' (unitInfoMap state) uid

-- | Find the unit we know about with the given unit id, if any
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lookupUnitId' :: UnitInfoMap -> UnitId -> Maybe UnitInfo
lookupUnitId' db uid = Map.lookup uid db
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-- | Looks up the given unit in the package state, panicing if it is not found
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unsafeLookupUnit :: HasDebugCallStack => UnitState -> Unit -> UnitInfo
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unsafeLookupUnit state u = case lookupUnit state u of
   Just info -> info
   Nothing   -> pprPanic "unsafeLookupUnit" (ppr u)

-- | Looks up the given unit id in the package state, panicing if it is not found
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unsafeLookupUnitId :: HasDebugCallStack => UnitState -> UnitId -> UnitInfo
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unsafeLookupUnitId state uid = case lookupUnitId state uid of
   Just info -> info
   Nothing   -> pprPanic "unsafeLookupUnitId" (ppr uid)


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-- | 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)
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lookupPackageName :: UnitState -> PackageName -> Maybe IndefUnitId
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lookupPackageName pkgstate n = Map.lookup n (packageNameMap pkgstate)
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-- | Search for packages with a given package ID (e.g. \"foo-0.1\")
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searchPackageId :: UnitState -> PackageId -> [UnitInfo]
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searchPackageId pkgstate pid = filter ((pid ==) . unitPackageId)
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                               (listUnitInfo pkgstate)
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-- | Create a Map UnitId UnitInfo
--
-- For each instantiated unit, we add two map keys:
--    * the real unit id
--    * the virtual unit id made from its instantiation
--
-- We do the same thing for fully indefinite units (which are "instantiated"
-- with module holes).
--
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mkUnitInfoMap :: [UnitInfo] -> UnitInfoMap
mkUnitInfoMap infos = foldl' add Map.empty infos
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  where
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   mkVirt      p = virtualUnitId (mkInstantiatedUnit (unitInstanceOf p) (unitInstantiations p))
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   add pkg_map p
      | not (null (unitInstantiations p))
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      = Map.insert (mkVirt p) p
         $ Map.insert (unitId p) p
         $ pkg_map
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      | otherwise
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      = Map.insert (unitId p) p pkg_map
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-- | Get a list of entries from the package database.  NB: be careful with
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-- 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).
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listUnitInfo :: UnitState -> [UnitInfo]
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listUnitInfo state = Map.elems (unitInfoMap state)
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-- ----------------------------------------------------------------------------
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-- Loading the package db files and building up the package state
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-- | Read the package database files, and sets up various internal tables of
-- package information, according to the package-related flags on the
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-- command-line (@-package@, @-hide-package@ etc.)
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--
-- 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
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-- @-package@ flags.
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--
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-- 'initUnits' can be called again subsequently after updating the
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-- 'packageFlags' field of the 'DynFlags', and it will update the
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-- 'unitState' in 'DynFlags' and return a list of packages to
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-- link in.
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initUnits :: DynFlags -> IO (DynFlags, [UnitId])
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initUnits dflags = do
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  let forcePkgDb (state, _) = unitInfoMap state `seq` ()
  (state,raw_dbs) <- withTiming dflags
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                                (text "initializing package database")
                                forcePkgDb $ do
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    -- read the databases if they have not been already read
    raw_dbs <- case unitDatabases dflags of
          Nothing  -> readUnitDatabases dflags
          Just dbs -> return dbs
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    -- distrust all units if the flag is set
    let distrust_all db = db { unitDatabaseUnits = distrustAllUnits (unitDatabaseUnits db) }
        dbs
           | gopt Opt_DistrustAllPackages dflags
           = map distrust_all raw_dbs
           | otherwise
           = raw_dbs

    -- create the UnitState
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    state <- mkUnitState dflags dbs
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    return (state, raw_dbs)
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  dumpIfSet_dyn (dflags { pprCols = 200 }) Opt_D_dump_mod_map "Mod Map"
    FormatText
    (pprModuleMap (moduleNameProvidersMap state))

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  -- Some wired units can be used to instantiate the home unit. We need to
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  -- replace their unit keys by their wired unit ids.
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  let wiringMap    = wireMap state
      unwiredInsts = homeUnitInstantiations dflags
      wiredInsts   = map (fmap (upd_wired_in_mod wiringMap)) unwiredInsts

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  return (dflags{ unitDatabases          = Just raw_dbs,
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                  unitState              = state,
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                  homeUnitInstantiations = wiredInsts },
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          (preloadUnits state))
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-- -----------------------------------------------------------------------------
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-- Reading the unit database(s)
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readUnitDatabases :: DynFlags -> IO [UnitDatabase UnitId]
readUnitDatabases dflags = do
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  conf_refs <- getUnitDbRefs dflags
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  confs     <- liftM catMaybes $ mapM (resolveUnitDatabase dflags) conf_refs
  mapM (readUnitDatabase dflags) confs
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getUnitDbRefs :: DynFlags -> IO [PkgDbRef]
getUnitDbRefs dflags = do
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  let system_conf_refs = [UserPkgDb, GlobalPkgDb]
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  e_pkg_path <- tryIO (getEnv $ map toUpper (programName dflags) ++ "_PACKAGE_PATH")
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  let base_conf_refs = case e_pkg_path of
        Left _ -> system_conf_refs
        Right path
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         | not (null path) && isSearchPathSeparator (last path)
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         -> map PkgDbPath (splitSearchPath (init path)) ++ system_conf_refs
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         | otherwise
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         -> map PkgDbPath (splitSearchPath path)
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  -- 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 _ = []

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  isNotUser UserPkgDb = False
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  isNotUser _ = True

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  isNotGlobal GlobalPkgDb = False
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  isNotGlobal _ = True
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-- | Return the path of a package database from a 'PkgDbRef'. Return 'Nothing'
-- when the user database filepath is expected but the latter doesn't exist.
--
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-- NB: This logic is reimplemented in Cabal, so if you change it,
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-- make sure you update Cabal. (Or, better yet, dump it in the
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-- compiler info so Cabal can use the info.)
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resolveUnitDatabase :: DynFlags -> PkgDbRef -> IO (Maybe FilePath)
resolveUnitDatabase dflags GlobalPkgDb = return $ Just (globalPackageDatabasePath dflags)
resolveUnitDatabase dflags UserPkgDb = runMaybeT $ do
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  dir <- versionedAppDir dflags
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  let pkgconf = dir </> "package.conf.d"
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  exist <- tryMaybeT $ doesDirectoryExist pkgconf
  if exist then return pkgconf else mzero
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resolveUnitDatabase _ (PkgDbPath name) = return $ Just name
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readUnitDatabase :: DynFlags -> FilePath -> IO (UnitDatabase UnitId)
readUnitDatabase dflags conf_file = do
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  isdir <- doesDirectoryExist conf_file

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  proto_pkg_configs <-
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    if isdir
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       then readDirStyleUnitInfo conf_file
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       else do
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            isfile <- doesFileExist conf_file
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            if isfile
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               then do
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                 mpkgs <- tryReadOldFileStyleUnitInfo
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                 case mpkgs of
                   Just pkgs -> return pkgs
                   Nothing   -> throwGhcExceptionIO $ InstallationError $
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                      "ghc no longer supports single-file style package " ++
                      "databases (" ++ conf_file ++
                      ") use 'ghc-pkg init' to create the database with " ++
                      "the correct format."
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               else throwGhcExceptionIO $ InstallationError $
                      "can't find a package database at " ++ conf_file
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  let
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      -- Fix #16360: remove trailing slash from conf_file before calculating pkgroot
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      conf_file' = dropTrailingPathSeparator conf_file
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      top_dir = topDir dflags
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      pkgroot = takeDirectory conf_file'
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      pkg_configs1 = map (mungeUnitInfo top_dir pkgroot . mapUnitInfo (\(UnitKey x) -> UnitId x) unitIdFS . mkUnitKeyInfo)
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                         proto_pkg_configs
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  --
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  return $ UnitDatabase conf_file' pkg_configs1
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  where
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    readDirStyleUnitInfo conf_dir = do
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      let filename = conf_dir </> "package.cache"
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      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"

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    -- 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.
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    tryReadOldFileStyleUnitInfo = do
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      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)
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                     liftM Just (readDirStyleUnitInfo conf_dir)
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             else return (Just []) -- ghc-pkg will create it when it's updated
        else return Nothing
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distrustAllUnits :: [UnitInfo] -> [UnitInfo]
distrustAllUnits pkgs = map distrust pkgs
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  where
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    distrust pkg = pkg{ unitIsTrusted = False }
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mungeUnitInfo :: FilePath -> FilePath
                   -> UnitInfo -> UnitInfo
mungeUnitInfo top_dir pkgroot =
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    mungeDynLibFields
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  . mungeUnitInfoPaths top_dir pkgroot
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mungeDynLibFields :: UnitInfo -> UnitInfo
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mungeDynLibFields pkg =
    pkg {
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      unitLibraryDynDirs = case unitLibraryDynDirs pkg of
         [] -> unitLibraryDirs pkg
         ds -> ds
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    }

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

applyTrustFlag
   :: DynFlags
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   -> UnitPrecedenceMap
   -> UnusableUnits
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   -> [UnitInfo]
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   -> TrustFlag
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   -> IO [UnitInfo]
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applyTrustFlag dflags prec_map unusable pkgs flag =
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  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 ->
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       case selectPackages prec_map (PackageArg str) pkgs unusable of
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         Left ps       -> trustFlagErr dflags flag ps
         Right (ps,qs) -> return (map trust ps ++ qs)
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          where trust p = p {unitIsTrusted=True}
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    DistrustPackage str ->
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       case selectPackages prec_map (PackageArg str) pkgs unusable of
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         Left ps       -> trustFlagErr dflags flag ps
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         Right (ps,qs) -> return (distrustAllUnits ps ++ qs)
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-- | A little utility to tell if the home unit is indefinite
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-- (if it is not, we should never use on-the-fly renaming.)
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homeUnitIsIndefinite :: DynFlags -> Bool
homeUnitIsIndefinite dflags = not (homeUnitIsDefinite dflags)

-- | A little utility to tell if the home unit is definite
-- (if it is, we should never use on-the-fly renaming.)
homeUnitIsDefinite :: DynFlags -> Bool
homeUnitIsDefinite dflags = unitIsDefinite (homeUnit dflags)
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applyPackageFlag
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   :: DynFlags
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   -> UnitPrecedenceMap
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   -> UnitInfoMap
   -> PreloadUnitClosure
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   -> UnusableUnits
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   -> Bool -- if False, if you expose a package, it implicitly hides
           -- any previously exposed packages with the same name
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   -> [UnitInfo]
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   -> VisibilityMap           -- Initially exposed
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   -> PackageFlag               -- flag to apply
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   -> IO VisibilityMap        -- Now exposed
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applyPackageFlag dflags prec_map pkg_map closure unusable no_hide_others pkgs vm flag =
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  case flag of
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    ExposePackage _ arg (ModRenaming b rns) ->
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       case findPackages prec_map pkg_map closure arg pkgs unusable of
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         Left ps         -> packageFlagErr dflags flag ps
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         Right (p:_) -> return vm'
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          where
           n = fsPackageName p
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           -- 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

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           collectHoles uid = case uid of
             HoleUnit       -> Map.empty
             RealUnit {}    -> Map.empty -- definite units don't have holes
             VirtUnit indef ->
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                  let local = [ Map.singleton
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                                  (moduleName mod)
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                                  (Set.singleton $ Module indef mod_name)
                              | (mod_name, mod) <- instUnitInsts indef
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                              , isHoleModule mod ]
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                      recurse = [ collectHoles (moduleUnit mod)
                                | (_, mod) <- instUnitInsts indef ]
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                  in Map.unionsWith Set.union $ local ++ recurse

           uv = UnitVisibility
                { uv_expose_all = b
                , uv_renamings = rns
                , uv_package_name = First (Just n)
                , uv_requirements = reqs
                , uv_explicit = True
                }
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           vm' = Map.insertWith mappend (mkUnit p) uv vm_cleared
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           -- 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
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                      -- NB: renamings never clear
                      | (_:_) <- rns = vm
                      | otherwise = Map.filterWithKey
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                            (\k uv -> k == mkUnit p
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                                   || First (Just n) /= uv_package_name uv) vm
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         _ -> panic "applyPackageFlag"

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    HidePackage str ->
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       case findPackages prec_map pkg_map closure (PackageArg str) pkgs unusable of
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         Left ps  -> packageFlagErr dflags flag ps
         Right ps -> return vm'
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          where vm' = foldl' (flip Map.delete) vm (map mkUnit ps)
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-- | 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.
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findPackages :: UnitPrecedenceMap
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             -> UnitInfoMap
             -> PreloadUnitClosure
             -> PackageArg -> [UnitInfo]
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             -> UnusableUnits
             -> Either [(UnitInfo, UnusableUnitReason)]
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                [UnitInfo]
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findPackages prec_map pkg_map closure arg pkgs unusable
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  = 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))
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        else Right (sortByPreference prec_map ps)
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  where
    finder (PackageArg str) p
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      = if str == unitPackageIdString p || str == unitPackageNameString p
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          then Just p
          else Nothing
    finder (UnitIdArg uid) p
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      = case uid of
          RealUnit (Definite iuid)
            | iuid == unitId p
            -> Just p
          VirtUnit inst
            | indefUnit (instUnitInstanceOf inst) == unitId p
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            -> Just (renameUnitInfo pkg_map closure (instUnitInsts inst) p)
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          _ -> Nothing
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selectPackages :: UnitPrecedenceMap -> PackageArg -> [UnitInfo]
               -> UnusableUnits
               -> Either [(UnitInfo, UnusableUnitReason)]
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                  ([UnitInfo], [UnitInfo])
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selectPackages prec_map arg pkgs unusable
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  = let matches = matching arg
        (ps,rest) = partition matches pkgs
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    in if null ps
        then Left (filter (matches.fst) (Map.elems unusable))
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        else Right (sortByPreference prec_map ps, rest)
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-- | Rename a 'UnitInfo' according to some module instantiation.
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renameUnitInfo :: UnitInfoMap -> PreloadUnitClosure -> [(ModuleName, Module)] -> UnitInfo -> UnitInfo
renameUnitInfo pkg_map closure insts conf =
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    let hsubst = listToUFM insts
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        smod  = renameHoleModule' pkg_map closure hsubst
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        new_insts = map (\(k,v) -> (k,smod v)) (unitInstantiations conf)
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    in conf {
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        unitInstantiations = new_insts,
        unitExposedModules = map (\(mod_name, mb_mod) -> (mod_name, fmap smod mb_mod))
                             (unitExposedModules conf)
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    }


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-- A package named on the command line can either include the
-- version, or just the name if it is unambiguous.
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matchingStr :: String -> UnitInfo -> Bool
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matchingStr str p
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        =  str == unitPackageIdString p
        || str == unitPackageNameString p
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matchingId :: UnitId -> UnitInfo -> Bool
matchingId uid p = uid == unitId p
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matching :: PackageArg -> UnitInfo -> Bool
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matching (PackageArg str) = matchingStr str
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matching (UnitIdArg (RealUnit (Definite uid))) = matchingId uid
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matching (UnitIdArg _)  = \_ -> False -- TODO: warn in this case
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-- | This sorts a list of packages, putting "preferred" packages first.
-- See 'compareByPreference' for the semantics of "preference".
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sortByPreference :: UnitPrecedenceMap -> [UnitInfo] -> [UnitInfo]
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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
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-- in later package database.
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--
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-- Instead, we use that preference based policy only when one of the packages
-- is integer-gmp and the other is integer-simple.
-- This currently only happens when we're looking up which concrete
-- package to use in place of @integer-wired-in@ and that two different
-- package databases supply a different integer library. For more about
-- the fake @integer-wired-in@ package, see Note [The integer library]
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-- in the @GHC.Builtin.Names@ module.
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compareByPreference
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    :: UnitPrecedenceMap
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    -> UnitInfo
    -> UnitInfo
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    -> Ordering
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compareByPreference prec_map pkg pkg'
  | Just prec  <- Map.lookup (unitId pkg)  prec_map
  , Just prec' <- Map.lookup (unitId pkg') prec_map
  , differentIntegerPkgs pkg pkg'
  = compare prec prec'

  | otherwise
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  = case comparing unitPackageVersion pkg pkg' of
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        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
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  where isIntegerPkg p = unitPackageNameString p `elem`
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          ["integer-simple", "integer-gmp"]
        differentIntegerPkgs p p' =
          isIntegerPkg p && isIntegerPkg p' &&
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          (unitPackageName p /= unitPackageName p')
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comparing :: Ord a => (t -> a) -> t -> t -> Ordering
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comparing f a b = f a `compare` f b

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packageFlagErr :: DynFlags
               -> PackageFlag
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               -> [(UnitInfo, UnusableUnitReason)]
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               -> IO a
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packageFlagErr dflags flag reasons
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  = packageFlagErr' dflags (pprFlag flag) reasons

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

packageFlagErr' :: DynFlags
               -> SDoc
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               -> [(UnitInfo, UnusableUnitReason)]
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               -> IO a
packageFlagErr' dflags flag_doc reasons
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  = throwGhcExceptionIO (CmdLineError (showSDoc dflags $ err))
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  where err = text "cannot satisfy " <> flag_doc <>
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                (if null reasons then Outputable.empty else text ": ") $$
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              nest 4 (ppr_reasons $$
                      text "(use -v for more information)")
        ppr_reasons = vcat (map ppr_reason reasons)
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        ppr_reason (p, reason) =
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            pprReason (ppr (unitId p) <+> text "is") reason
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pprFlag :: PackageFlag -> SDoc
pprFlag flag = case flag of
    HidePackage p   -> text "-hide-package " <> text p
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    ExposePackage doc _ _ -> text doc
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pprTrustFlag :: TrustFlag -> SDoc
pprTrustFlag flag = case flag of
    TrustPackage p    -> text "-trust " <> text p
    DistrustPackage p -> text "-distrust " <> text p

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