    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
    -}

    {-# LANGUAGE CPP, DeriveDataTypeable #-}

    -- |
    -- #name_types#
    -- GHC uses several kinds of name internally:
    --
    -- * 'OccName.OccName': see "OccName#name_types"
    --
    -- * 'RdrName.RdrName' is the type of names that come directly from the parser. They
    --   have not yet had their scoping and binding resolved by the renamer and can be
    --   thought of to a first approximation as an 'OccName.OccName' with an optional module
    --   qualifier
    --
    -- * 'Name.Name': see "Name#name_types"
    --
    -- * 'Id.Id': see "Id#name_types"
    --
    -- * 'Var.Var': see "Var#name_types"

    module RdrName (
            -- * The main type
            RdrName(..),    -- Constructors exported only to BinIface

            -- ** Construction
            mkRdrUnqual, mkRdrQual,
            mkUnqual, mkVarUnqual, mkQual, mkOrig,
            nameRdrName, getRdrName,

            -- ** Destruction
            rdrNameOcc, rdrNameSpace, setRdrNameSpace, demoteRdrName,
            isRdrDataCon, isRdrTyVar, isRdrTc, isQual, isQual_maybe, isUnqual,
            isOrig, isOrig_maybe, isExact, isExact_maybe, isSrcRdrName,

            -- * Local mapping of 'RdrName' to 'Name.Name'
            LocalRdrEnv, emptyLocalRdrEnv, extendLocalRdrEnv, extendLocalRdrEnvList,
            lookupLocalRdrEnv, lookupLocalRdrOcc,
            elemLocalRdrEnv, inLocalRdrEnvScope,
            localRdrEnvElts, delLocalRdrEnvList,

            -- * Global mapping of 'RdrName' to 'GlobalRdrElt's
            GlobalRdrEnv, emptyGlobalRdrEnv, mkGlobalRdrEnv, plusGlobalRdrEnv,
            lookupGlobalRdrEnv, extendGlobalRdrEnv,
            pprGlobalRdrEnv, globalRdrEnvElts,
            lookupGRE_RdrName, lookupGRE_Name, getGRE_NameQualifier_maybes,
            transformGREs, findLocalDupsRdrEnv, pickGREs,

            -- * GlobalRdrElts
            gresFromAvails, gresFromAvail,

            -- ** Global 'RdrName' mapping elements: 'GlobalRdrElt', 'Provenance', 'ImportSpec'
            GlobalRdrElt(..), isLocalGRE, unQualOK, qualSpecOK, unQualSpecOK,
            Provenance(..), pprNameProvenance,
            Parent(..),
            ImportSpec(..), ImpDeclSpec(..), ImpItemSpec(..),
            importSpecLoc, importSpecModule, isExplicitItem
      ) where

    #include "HsVersions.h"

    import Module
    import Name
    import Avail
    import NameSet
    import Maybes
    import SrcLoc
    import FastString
    import Outputable
    import Unique
    import Util
    import StaticFlags( opt_PprStyle_Debug )

    import Data.Data

    {-
    ************************************************************************
    *                                                                      *
    \subsection{The main data type}
    *                                                                      *
    ************************************************************************
    -}

    -- | Do not use the data constructors of RdrName directly: prefer the family
    -- of functions that creates them, such as 'mkRdrUnqual'
    --
    -- - Note: A Located RdrName will only have API Annotations if it is a
    --         compound one,
    --   e.g.
    --
    -- > `bar`
    -- > ( ~ )
    --
    -- - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnType',
    --           'ApiAnnotation.AnnOpen'  @'('@ or @'['@ or @'[:'@,
    --           'ApiAnnotation.AnnClose' @')'@ or @']'@ or @':]'@,,
    --           'ApiAnnotation.AnnBackquote' @'`'@,
    --           'ApiAnnotation.AnnVal','ApiAnnotation.AnnTildehsh',
    --           'ApiAnnotation.AnnTilde',

    -- For details on above see note [Api annotations] in ApiAnnotation
    data RdrName
      = Unqual OccName
            -- ^ Used for ordinary, unqualified occurrences, e.g. @x@, @y@ or @Foo@.
            -- Create such a 'RdrName' with 'mkRdrUnqual'

      | Qual ModuleName OccName
            -- ^ A qualified name written by the user in
            -- /source/ code.  The module isn't necessarily
            -- the module where the thing is defined;
            -- just the one from which it is imported.
            -- Examples are @Bar.x@, @Bar.y@ or @Bar.Foo@.
            -- Create such a 'RdrName' with 'mkRdrQual'

      | Orig Module OccName
            -- ^ An original name; the module is the /defining/ module.
            -- This is used when GHC generates code that will be fed
            -- into the renamer (e.g. from deriving clauses), but where
            -- we want to say \"Use Prelude.map dammit\". One of these
            -- can be created with 'mkOrig'

      | Exact Name
            -- ^ We know exactly the 'Name'. This is used:
            --
            --  (1) When the parser parses built-in syntax like @[]@
            --      and @(,)@, but wants a 'RdrName' from it
            --
            --  (2) By Template Haskell, when TH has generated a unique name
            --
            -- Such a 'RdrName' can be created by using 'getRdrName' on a 'Name'
      deriving (Data, Typeable)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Simple functions}
    *                                                                      *
    ************************************************************************
    -}

    instance HasOccName RdrName where
      occName = rdrNameOcc

    rdrNameOcc :: RdrName -> OccName
    rdrNameOcc (Qual _ occ) = occ
    rdrNameOcc (Unqual occ) = occ
    rdrNameOcc (Orig _ occ) = occ
    rdrNameOcc (Exact name) = nameOccName name

    rdrNameSpace :: RdrName -> NameSpace
    rdrNameSpace = occNameSpace . rdrNameOcc

    setRdrNameSpace :: RdrName -> NameSpace -> RdrName
    -- ^ This rather gruesome function is used mainly by the parser.
    -- When parsing:
    --
    -- > data T a = T | T1 Int
    --
    -- we parse the data constructors as /types/ because of parser ambiguities,
    -- so then we need to change the /type constr/ to a /data constr/
    --
    -- The exact-name case /can/ occur when parsing:
    --
    -- > data [] a = [] | a : [a]
    --
    -- For the exact-name case we return an original name.
    setRdrNameSpace (Unqual occ) ns = Unqual (setOccNameSpace ns occ)
    setRdrNameSpace (Qual m occ) ns = Qual m (setOccNameSpace ns occ)
    setRdrNameSpace (Orig m occ) ns = Orig m (setOccNameSpace ns occ)
    setRdrNameSpace (Exact n)    ns
      | isExternalName n
      = Orig (nameModule n) occ
      | otherwise   -- This can happen when quoting and then splicing a fixity
                    -- declaration for a type
      = Exact $ mkSystemNameAt (nameUnique n) occ (nameSrcSpan n)
      where
        occ = setOccNameSpace ns (nameOccName n)

    -- demoteRdrName lowers the NameSpace of RdrName.
    -- see Note [Demotion] in OccName
    demoteRdrName :: RdrName -> Maybe RdrName
    demoteRdrName (Unqual occ) = fmap Unqual (demoteOccName occ)
    demoteRdrName (Qual m occ) = fmap (Qual m) (demoteOccName occ)
    demoteRdrName (Orig _ _) = panic "demoteRdrName"
    demoteRdrName (Exact _) = panic "demoteRdrName"

            -- These two are the basic constructors
    mkRdrUnqual :: OccName -> RdrName
    mkRdrUnqual occ = Unqual occ

    mkRdrQual :: ModuleName -> OccName -> RdrName
    mkRdrQual mod occ = Qual mod occ

    mkOrig :: Module -> OccName -> RdrName
    mkOrig mod occ = Orig mod occ

    ---------------
            -- These two are used when parsing source files
            -- They do encode the module and occurrence names
    mkUnqual :: NameSpace -> FastString -> RdrName
    mkUnqual sp n = Unqual (mkOccNameFS sp n)

    mkVarUnqual :: FastString -> RdrName
    mkVarUnqual n = Unqual (mkVarOccFS n)

    -- | Make a qualified 'RdrName' in the given namespace and where the 'ModuleName' and
    -- the 'OccName' are taken from the first and second elements of the tuple respectively
    mkQual :: NameSpace -> (FastString, FastString) -> RdrName
    mkQual sp (m, n) = Qual (mkModuleNameFS m) (mkOccNameFS sp n)

    getRdrName :: NamedThing thing => thing -> RdrName
    getRdrName name = nameRdrName (getName name)

    nameRdrName :: Name -> RdrName
    nameRdrName name = Exact name
    -- Keep the Name even for Internal names, so that the
    -- unique is still there for debug printing, particularly
    -- of Types (which are converted to IfaceTypes before printing)

    nukeExact :: Name -> RdrName
    nukeExact n
      | isExternalName n = Orig (nameModule n) (nameOccName n)
      | otherwise        = Unqual (nameOccName n)

    isRdrDataCon :: RdrName -> Bool
    isRdrTyVar   :: RdrName -> Bool
    isRdrTc      :: RdrName -> Bool

    isRdrDataCon rn = isDataOcc (rdrNameOcc rn)
    isRdrTyVar   rn = isTvOcc   (rdrNameOcc rn)
    isRdrTc      rn = isTcOcc   (rdrNameOcc rn)

    isSrcRdrName :: RdrName -> Bool
    isSrcRdrName (Unqual _) = True
    isSrcRdrName (Qual _ _) = True
    isSrcRdrName _          = False

    isUnqual :: RdrName -> Bool
    isUnqual (Unqual _) = True
    isUnqual _          = False

    isQual :: RdrName -> Bool
    isQual (Qual _ _) = True
    isQual _          = False

    isQual_maybe :: RdrName -> Maybe (ModuleName, OccName)
    isQual_maybe (Qual m n) = Just (m,n)
    isQual_maybe _          = Nothing

    isOrig :: RdrName -> Bool
    isOrig (Orig _ _) = True
    isOrig _          = False

    isOrig_maybe :: RdrName -> Maybe (Module, OccName)
    isOrig_maybe (Orig m n) = Just (m,n)
    isOrig_maybe _          = Nothing

    isExact :: RdrName -> Bool
    isExact (Exact _) = True
    isExact _         = False

    isExact_maybe :: RdrName -> Maybe Name
    isExact_maybe (Exact n) = Just n
    isExact_maybe _         = Nothing

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Instances}
    *                                                                      *
    ************************************************************************
    -}

    instance Outputable RdrName where
        ppr (Exact name)   = ppr name
        ppr (Unqual occ)   = ppr occ
        ppr (Qual mod occ) = ppr mod <> dot <> ppr occ
        ppr (Orig mod occ) = getPprStyle (\sty -> pprModulePrefix sty mod occ <> ppr occ)

    instance OutputableBndr RdrName where
        pprBndr _ n
            | isTvOcc (rdrNameOcc n) = char '@' <+> ppr n
            | otherwise              = ppr n

        pprInfixOcc  rdr = pprInfixVar  (isSymOcc (rdrNameOcc rdr)) (ppr rdr)
        pprPrefixOcc rdr
          | Just name <- isExact_maybe rdr = pprPrefixName name
                 -- pprPrefixName has some special cases, so
                 -- we delegate to them rather than reproduce them
          | otherwise = pprPrefixVar (isSymOcc (rdrNameOcc rdr)) (ppr rdr)

    instance Eq RdrName where
        (Exact n1)    == (Exact n2)    = n1==n2
            -- Convert exact to orig
        (Exact n1)    == r2@(Orig _ _) = nukeExact n1 == r2
        r1@(Orig _ _) == (Exact n2)    = r1 == nukeExact n2

        (Orig m1 o1)  == (Orig m2 o2)  = m1==m2 && o1==o2
        (Qual m1 o1)  == (Qual m2 o2)  = m1==m2 && o1==o2
        (Unqual o1)   == (Unqual o2)   = o1==o2
        _             == _             = False

    instance Ord RdrName where
        a <= b = case (a `compare` b) of { LT -> True;  EQ -> True;  GT -> False }
        a <  b = case (a `compare` b) of { LT -> True;  EQ -> False; GT -> False }
        a >= b = case (a `compare` b) of { LT -> False; EQ -> True;  GT -> True  }
        a >  b = case (a `compare` b) of { LT -> False; EQ -> False; GT -> True  }

            -- Exact < Unqual < Qual < Orig
            -- [Note: Apr 2004] We used to use nukeExact to convert Exact to Orig
            --      before comparing so that Prelude.map == the exact Prelude.map, but
            --      that meant that we reported duplicates when renaming bindings
            --      generated by Template Haskell; e.g
            --      do { n1 <- newName "foo"; n2 <- newName "foo";
            --           <decl involving n1,n2> }
            --      I think we can do without this conversion
        compare (Exact n1) (Exact n2) = n1 `compare` n2
        compare (Exact _)  _          = LT

        compare (Unqual _)   (Exact _)    = GT
        compare (Unqual o1)  (Unqual  o2) = o1 `compare` o2
        compare (Unqual _)   _            = LT

        compare (Qual _ _)   (Exact _)    = GT
        compare (Qual _ _)   (Unqual _)   = GT
        compare (Qual m1 o1) (Qual m2 o2) = (o1 `compare` o2) `thenCmp` (m1 `compare` m2)
        compare (Qual _ _)   (Orig _ _)   = LT

        compare (Orig m1 o1) (Orig m2 o2) = (o1 `compare` o2) `thenCmp` (m1 `compare` m2)
        compare (Orig _ _)   _            = GT

    {-
    ************************************************************************
    *                                                                      *
                            LocalRdrEnv
    *                                                                      *
    ************************************************************************
    -}

    -- | This environment is used to store local bindings (@let@, @where@, lambda, @case@).
    -- It is keyed by OccName, because we never use it for qualified names
    -- We keep the current mapping, *and* the set of all Names in scope
    -- Reason: see Note [Splicing Exact Names] in RnEnv
    data LocalRdrEnv = LRE { lre_env      :: OccEnv Name
                           , lre_in_scope :: NameSet }

    instance Outputable LocalRdrEnv where
      ppr (LRE {lre_env = env, lre_in_scope = ns})
        = hang (ptext (sLit "LocalRdrEnv {"))
             2 (vcat [ ptext (sLit "env =") <+> pprOccEnv ppr_elt env
                     , ptext (sLit "in_scope =") <+> braces (pprWithCommas ppr (nameSetElems ns))
                     ] <+> char '}')
        where
          ppr_elt name = parens (ppr (getUnique (nameOccName name))) <+> ppr name
                         -- So we can see if the keys line up correctly

    emptyLocalRdrEnv :: LocalRdrEnv
    emptyLocalRdrEnv = LRE { lre_env = emptyOccEnv, lre_in_scope = emptyNameSet }

    extendLocalRdrEnv :: LocalRdrEnv -> Name -> LocalRdrEnv
    -- The Name should be a non-top-level thing
    extendLocalRdrEnv (LRE { lre_env = env, lre_in_scope = ns }) name
      = WARN( isExternalName name, ppr name )
        LRE { lre_env      = extendOccEnv env (nameOccName name) name
            , lre_in_scope = extendNameSet ns name }

    extendLocalRdrEnvList :: LocalRdrEnv -> [Name] -> LocalRdrEnv
    extendLocalRdrEnvList (LRE { lre_env = env, lre_in_scope = ns }) names
      = WARN( any isExternalName names, ppr names )
        LRE { lre_env = extendOccEnvList env [(nameOccName n, n) | n <- names]
            , lre_in_scope = extendNameSetList ns names }

    lookupLocalRdrEnv :: LocalRdrEnv -> RdrName -> Maybe Name
    lookupLocalRdrEnv (LRE { lre_env = env }) (Unqual occ) = lookupOccEnv env occ
    lookupLocalRdrEnv _                       _            = Nothing

    lookupLocalRdrOcc :: LocalRdrEnv -> OccName -> Maybe Name
    lookupLocalRdrOcc (LRE { lre_env = env }) occ = lookupOccEnv env occ

    elemLocalRdrEnv :: RdrName -> LocalRdrEnv -> Bool
    elemLocalRdrEnv rdr_name (LRE { lre_env = env, lre_in_scope = ns })
      = case rdr_name of
          Unqual occ -> occ  `elemOccEnv` env
          Exact name -> name `elemNameSet` ns  -- See Note [Local bindings with Exact Names]
          Qual {} -> False
          Orig {} -> False

    localRdrEnvElts :: LocalRdrEnv -> [Name]
    localRdrEnvElts (LRE { lre_env = env }) = occEnvElts env

    inLocalRdrEnvScope :: Name -> LocalRdrEnv -> Bool
    -- This is the point of the NameSet
    inLocalRdrEnvScope name (LRE { lre_in_scope = ns }) = name `elemNameSet` ns

    delLocalRdrEnvList :: LocalRdrEnv -> [OccName] -> LocalRdrEnv
    delLocalRdrEnvList (LRE { lre_env = env, lre_in_scope = ns }) occs
      = LRE { lre_env = delListFromOccEnv env occs
            , lre_in_scope = ns }

    {-
    Note [Local bindings with Exact Names]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    With Template Haskell we can make local bindings that have Exact Names.
    Computing shadowing etc may use elemLocalRdrEnv (at least it certainly
    does so in RnTpes.bindHsTyVars), so for an Exact Name we must consult
    the in-scope-name-set.

    ************************************************************************
    *                                                                      *
                            GlobalRdrEnv
    *                                                                      *
    ************************************************************************
    -}

    type GlobalRdrEnv = OccEnv [GlobalRdrElt]
    -- ^ Keyed by 'OccName'; when looking up a qualified name
    -- we look up the 'OccName' part, and then check the 'Provenance'
    -- to see if the appropriate qualification is valid.  This
    -- saves routinely doubling the size of the env by adding both
    -- qualified and unqualified names to the domain.
    --
    -- The list in the codomain is required because there may be name clashes
    -- These only get reported on lookup, not on construction
    --
    -- INVARIANT: All the members of the list have distinct
    --            'gre_name' fields; that is, no duplicate Names
    --
    -- INVARIANT: Imported provenance => Name is an ExternalName
    --            However LocalDefs can have an InternalName.  This
    --            happens only when type-checking a [d| ... |] Template
    --            Haskell quotation; see this note in RnNames
    --            Note [Top-level Names in Template Haskell decl quotes]

    -- | An element of the 'GlobalRdrEnv'
    data GlobalRdrElt
      = GRE { gre_name :: Name,
              gre_par  :: Parent,
              gre_prov :: Provenance        -- ^ Why it's in scope
        }

    -- | The children of a Name are the things that are abbreviated by the ".."
    --   notation in export lists.  See Note [Parents]
    data Parent = NoParent | ParentIs Name
                  deriving (Eq)

    instance Outputable Parent where
       ppr NoParent     = empty
       ppr (ParentIs n) = ptext (sLit "parent:") <> ppr n

    plusParent :: Parent -> Parent -> Parent
    -- See Note [Combining parents]
    plusParent (ParentIs n) p2 = hasParent n p2
    plusParent p1 (ParentIs n) = hasParent n p1
    plusParent _ _ = NoParent

    hasParent :: Name -> Parent -> Parent
    #ifdef DEBUG
    hasParent n (ParentIs n')
      | n /= n' = pprPanic "hasParent" (ppr n <+> ppr n')  -- Parents should agree
    #endif
    hasParent n _  = ParentIs n

    {-
    Note [Parents]
    ~~~~~~~~~~~~~~~~~
      Parent           Children
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      data T           Data constructors
                       Record-field ids

      data family T    Data constructors and record-field ids
                       of all visible data instances of T

      class C          Class operations
                       Associated type constructors

    Note [Combining parents]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    With an associated type we might have
       module M where
         class C a where
           data T a
           op :: T a -> a
         instance C Int where
           data T Int = TInt
         instance C Bool where
           data T Bool = TBool

    Then:   C is the parent of T
            T is the parent of TInt and TBool
    So: in an export list
        C(..) is short for C( op, T )
        T(..) is short for T( TInt, TBool )

    Module M exports everything, so its exports will be
       AvailTC C [C,T,op]
       AvailTC T [T,TInt,TBool]
    On import we convert to GlobalRdrElt and the combine
    those.  For T that will mean we have
      one GRE with Parent C
      one GRE with NoParent
    That's why plusParent picks the "best" case.
    -}

    -- | make a 'GlobalRdrEnv' where all the elements point to the same
    -- Provenance (useful for "hiding" imports, or imports with
    -- no details).
    gresFromAvails :: Provenance -> [AvailInfo] -> [GlobalRdrElt]
    gresFromAvails prov avails
      = concatMap (gresFromAvail (const prov)) avails

    gresFromAvail :: (Name -> Provenance) -> AvailInfo -> [GlobalRdrElt]
    gresFromAvail prov_fn avail
      = [ GRE {gre_name = n,
               gre_par = mkParent n avail,
               gre_prov = prov_fn n}
        | n <- availNames avail ]
      where

    mkParent :: Name -> AvailInfo -> Parent
    mkParent _ (Avail _)                 = NoParent
    mkParent n (AvailTC m _) | n == m    = NoParent
                             | otherwise = ParentIs m

    emptyGlobalRdrEnv :: GlobalRdrEnv
    emptyGlobalRdrEnv = emptyOccEnv

    globalRdrEnvElts :: GlobalRdrEnv -> [GlobalRdrElt]
    globalRdrEnvElts env = foldOccEnv (++) [] env

    instance Outputable GlobalRdrElt where
      ppr gre = hang (ppr (gre_name gre) <+> ppr (gre_par gre))
                   2 (pprNameProvenance gre)

    pprGlobalRdrEnv :: Bool -> GlobalRdrEnv -> SDoc
    pprGlobalRdrEnv locals_only env
      = vcat [ ptext (sLit "GlobalRdrEnv") <+> ppWhen locals_only (ptext (sLit "(locals only)"))
                 <+> lbrace
             , nest 2 (vcat [ pp (remove_locals gre_list) | gre_list <- occEnvElts env ]
                 <+> rbrace) ]
      where
        remove_locals gres | locals_only = filter isLocalGRE gres
                           | otherwise   = gres
        pp []   = empty
        pp gres = hang (ppr occ
                         <+> parens (ptext (sLit "unique") <+> ppr (getUnique occ))
                         <> colon)
                     2 (vcat (map ppr gres))
          where
            occ = nameOccName (gre_name (head gres))

    lookupGlobalRdrEnv :: GlobalRdrEnv -> OccName -> [GlobalRdrElt]
    lookupGlobalRdrEnv env occ_name = case lookupOccEnv env occ_name of
                                      Nothing   -> []
                                      Just gres -> gres

    lookupGRE_RdrName :: RdrName -> GlobalRdrEnv -> [GlobalRdrElt]
    lookupGRE_RdrName rdr_name env
      = case lookupOccEnv env (rdrNameOcc rdr_name) of
        Nothing   -> []
        Just gres -> pickGREs rdr_name gres

    lookupGRE_Name :: GlobalRdrEnv -> Name -> [GlobalRdrElt]
    lookupGRE_Name env name
      = [ gre | gre <- lookupGlobalRdrEnv env (nameOccName name),
                gre_name gre == name ]

    getGRE_NameQualifier_maybes :: GlobalRdrEnv -> Name -> [Maybe [ModuleName]]
    -- Returns all the qualifiers by which 'x' is in scope
    -- Nothing means "the unqualified version is in scope"
    -- [] means the thing is not in scope at all
    getGRE_NameQualifier_maybes env
      = map (qualifier_maybe . gre_prov) . lookupGRE_Name env
      where
        qualifier_maybe LocalDef       = Nothing
        qualifier_maybe (Imported iss) = Just $ map (is_as . is_decl) iss

    isLocalGRE :: GlobalRdrElt -> Bool
    isLocalGRE (GRE {gre_prov = LocalDef}) = True
    isLocalGRE _                           = False

    unQualOK :: GlobalRdrElt -> Bool
    -- ^ Test if an unqualifed version of this thing would be in scope
    unQualOK (GRE {gre_prov = LocalDef})    = True
    unQualOK (GRE {gre_prov = Imported is}) = any unQualSpecOK is

    pickGREs :: RdrName -> [GlobalRdrElt] -> [GlobalRdrElt]
    -- ^ Take a list of GREs which have the right OccName
    -- Pick those GREs that are suitable for this RdrName
    -- And for those, keep only only the Provenances that are suitable
    -- Only used for Qual and Unqual, not Orig or Exact
    --
    -- Consider:
    --
    -- @
    --       module A ( f ) where
    --       import qualified Foo( f )
    --       import Baz( f )
    --       f = undefined
    -- @
    --
    -- Let's suppose that @Foo.f@ and @Baz.f@ are the same entity really.
    -- The export of @f@ is ambiguous because it's in scope from the local def
    -- and the import.  The lookup of @Unqual f@ should return a GRE for
    -- the locally-defined @f@, and a GRE for the imported @f@, with a /single/
    -- provenance, namely the one for @Baz(f)@.
    pickGREs rdr_name gres
      | (_ : _ : _) <- candidates  -- This is usually false, so we don't have to
                                   -- even look at internal_candidates
      , (gre : _)   <- internal_candidates
      = [gre]  -- For this internal_candidate stuff,
               -- see Note [Template Haskell binders in the GlobalRdrEnv]
               -- If there are multiple Internal candidates, pick the
               -- first one (ie with the (innermost binding)
      | otherwise
      = ASSERT2( isSrcRdrName rdr_name, ppr rdr_name )
        candidates
      where
        candidates = mapMaybe pick gres
        internal_candidates = filter (isInternalName . gre_name) candidates

        rdr_is_unqual = isUnqual rdr_name
        rdr_is_qual   = isQual_maybe rdr_name

        pick :: GlobalRdrElt -> Maybe GlobalRdrElt
        pick gre@(GRE {gre_prov = LocalDef, gre_name = n})  -- Local def
            | rdr_is_unqual                    = Just gre
            | Just (mod,_) <- rdr_is_qual        -- Qualified name
            , Just n_mod <- nameModule_maybe n   -- Binder is External
            , mod == moduleName n_mod          = Just gre
            | otherwise                        = Nothing
        pick gre@(GRE {gre_prov = Imported [is]})   -- Single import (efficiency)
            | rdr_is_unqual,
              not (is_qual (is_decl is))    = Just gre
            | Just (mod,_) <- rdr_is_qual,
              mod == is_as (is_decl is)     = Just gre
            | otherwise                     = Nothing
        pick gre@(GRE {gre_prov = Imported is})     -- Multiple import
            | null filtered_is = Nothing
            | otherwise        = Just (gre {gre_prov = Imported filtered_is})
            where
              filtered_is | rdr_is_unqual
                          = filter (not . is_qual    . is_decl) is
                          | Just (mod,_) <- rdr_is_qual
                          = filter ((== mod) . is_as . is_decl) is
                          | otherwise
                          = []

    -- Building GlobalRdrEnvs

    plusGlobalRdrEnv :: GlobalRdrEnv -> GlobalRdrEnv -> GlobalRdrEnv
    plusGlobalRdrEnv env1 env2 = plusOccEnv_C (foldr insertGRE) env1 env2

    mkGlobalRdrEnv :: [GlobalRdrElt] -> GlobalRdrEnv
    mkGlobalRdrEnv gres
      = foldr add emptyGlobalRdrEnv gres
      where
        add gre env = extendOccEnv_Acc insertGRE singleton env
                                       (nameOccName (gre_name gre))
                                       gre

    insertGRE :: GlobalRdrElt -> [GlobalRdrElt] -> [GlobalRdrElt]
    insertGRE new_g [] = [new_g]
    insertGRE new_g (old_g : old_gs)
            | gre_name new_g == gre_name old_g
            = new_g `plusGRE` old_g : old_gs
            | otherwise
            = old_g : insertGRE new_g old_gs

    plusGRE :: GlobalRdrElt -> GlobalRdrElt -> GlobalRdrElt
    -- Used when the gre_name fields match
    plusGRE g1 g2
      = GRE { gre_name = gre_name g1,
              gre_prov = gre_prov g1 `plusProv`   gre_prov g2,
              gre_par  = gre_par  g1 `plusParent` gre_par  g2 }

    transformGREs :: (GlobalRdrElt -> GlobalRdrElt)
                  -> [OccName]
                  -> GlobalRdrEnv -> GlobalRdrEnv
    -- ^ Apply a transformation function to the GREs for these OccNames
    transformGREs trans_gre occs rdr_env
      = foldr trans rdr_env occs
      where
        trans occ env
          = case lookupOccEnv env occ of
               Just gres -> extendOccEnv env occ (map trans_gre gres)
               Nothing   -> env

    extendGlobalRdrEnv :: Bool -> GlobalRdrEnv -> [AvailInfo] -> GlobalRdrEnv
    -- Extend with new LocalDef GREs from the AvailInfos.
    --
    -- If do_shadowing is True, first remove name clashes between the new
    -- AvailInfos and the existing GlobalRdrEnv.
    -- This is used by the GHCi top-level
    --
    -- E.g.  Adding a LocalDef "x" when there is an existing GRE for Q.x
    --       should remove any unqualified import of Q.x,
    --       leaving only the qualified one
    --
    -- However do *not* remove name clashes between the AvailInfos themselves,
    -- so that (say)   data T = A | A
    -- will still give a duplicate-binding error.
    -- Same thing if there are multiple AvailInfos (don't remove clashes),
    -- though I'm not sure this ever happens with do_shadowing=True

    extendGlobalRdrEnv do_shadowing env avails
      = foldl add_avail env1 avails
      where
        names = concatMap availNames avails
        env1 | do_shadowing = foldl shadow_name env names
             | otherwise    = env
             -- By doing the removal first, we ensure that the new AvailInfos
             -- don't shadow each other; that would conceal genuine errors
             -- E.g. in GHCi   data T = A | A

        add_avail env avail = foldl (add_name avail) env (availNames avail)

        add_name avail env name
           = extendOccEnv_Acc (:) singleton env occ gre
           where
             occ = nameOccName name
             gre = GRE { gre_name = name
                       , gre_par = mkParent name avail
                       , gre_prov = LocalDef }

    shadow_name :: GlobalRdrEnv -> Name -> GlobalRdrEnv
    shadow_name env name
      = alterOccEnv (fmap alter_fn) env (nameOccName name)
      where
        alter_fn :: [GlobalRdrElt] -> [GlobalRdrElt]
        alter_fn gres = mapMaybe (shadow_with name) gres

        shadow_with :: Name -> GlobalRdrElt -> Maybe GlobalRdrElt
        shadow_with new_name old_gre@(GRE { gre_name = old_name, gre_prov = LocalDef })
           = case (nameModule_maybe old_name, nameModule_maybe new_name) of
               (Nothing,      _)                                 -> Nothing
               (Just old_mod, Just new_mod) | new_mod == old_mod -> Nothing
               (Just old_mod, _) -> Just (old_gre { gre_prov = Imported [fake_imp_spec] })
                  where
                     fake_imp_spec = ImpSpec id_spec ImpAll  -- Urgh!
                     old_mod_name = moduleName old_mod
                     id_spec = ImpDeclSpec { is_mod = old_mod_name
                                           , is_as = old_mod_name
                                           , is_qual = True
                                           , is_dloc = nameSrcSpan old_name }
        shadow_with new_name old_gre@(GRE { gre_prov = Imported imp_specs })
           | null imp_specs' = Nothing
           | otherwise       = Just (old_gre { gre_prov = Imported imp_specs' })
           where
             imp_specs' = mapMaybe (shadow_is new_name) imp_specs

        shadow_is :: Name -> ImportSpec -> Maybe ImportSpec
        shadow_is new_name is@(ImpSpec { is_decl = id_spec })
           | Just new_mod <- nameModule_maybe new_name
           , is_as id_spec == moduleName new_mod
           = Nothing   -- Shadow both qualified and unqualified
           | otherwise -- Shadow unqualified only
           = Just (is { is_decl = id_spec { is_qual = True } })

    {-
    Note [Template Haskell binders in the GlobalRdrEnv]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    For reasons described in Note [Top-level Names in Template Haskell decl quotes]
    in RnNames, a GRE with an Internal gre_name (i.e. one generated by a TH decl
    quote) should *shadow* a GRE with an External gre_name.  Hence some faffing
    around in pickGREs and findLocalDupsRdrEnv
    -}

    findLocalDupsRdrEnv :: GlobalRdrEnv -> [Name] -> [[GlobalRdrElt]]
    -- ^ For each 'OccName', see if there are multiple local definitions
    -- for it; return a list of all such
    -- and return a list of the duplicate bindings
    findLocalDupsRdrEnv rdr_env occs
      = go rdr_env [] occs
      where
        go _       dups [] = dups
        go rdr_env dups (name:names)
          = case filter (pick name) gres of
              []       -> go rdr_env  dups              names
              [_]      -> go rdr_env  dups              names   -- The common case
              dup_gres -> go rdr_env' (dup_gres : dups) names
          where
            occ      = nameOccName name
            gres     = lookupOccEnv rdr_env occ `orElse` []
            rdr_env' = delFromOccEnv rdr_env occ
                -- The delFromOccEnv avoids repeating the same
                -- complaint twice, when names itself has a duplicate
                -- which is a common case

        -- See Note [Template Haskell binders in the GlobalRdrEnv]
        pick name (GRE { gre_name = n, gre_prov = LocalDef })
          | isInternalName name = isInternalName n
          | otherwise           = True
        pick _ _ = False

    {-
    ************************************************************************
    *                                                                      *
                            Provenance
    *                                                                      *
    ************************************************************************
    -}

    -- | The 'Provenance' of something says how it came to be in scope.
    -- It's quite elaborate so that we can give accurate unused-name warnings.
    data Provenance
      = LocalDef            -- ^ The thing was defined locally
      | Imported
            [ImportSpec]    -- ^ The thing was imported.
                            --
                            -- INVARIANT: the list of 'ImportSpec' is non-empty

    data ImportSpec = ImpSpec { is_decl :: ImpDeclSpec,
                                is_item :: ImpItemSpec }
                    deriving( Eq, Ord )

    -- | Describes a particular import declaration and is
    -- shared among all the 'Provenance's for that decl
    data ImpDeclSpec
      = ImpDeclSpec {
            is_mod      :: ModuleName, -- ^ Module imported, e.g. @import Muggle@
                                       -- Note the @Muggle@ may well not be
                                       -- the defining module for this thing!

                                       -- TODO: either should be Module, or there
                                       -- should be a Maybe PackageKey here too.
            is_as       :: ModuleName, -- ^ Import alias, e.g. from @as M@ (or @Muggle@ if there is no @as@ clause)
            is_qual     :: Bool,       -- ^ Was this import qualified?
            is_dloc     :: SrcSpan     -- ^ The location of the entire import declaration
        }

    -- | Describes import info a particular Name
    data ImpItemSpec
      = ImpAll              -- ^ The import had no import list,
                            -- or had a hiding list

      | ImpSome {
            is_explicit :: Bool,
            is_iloc     :: SrcSpan  -- Location of the import item
        }   -- ^ The import had an import list.
            -- The 'is_explicit' field is @True@ iff the thing was named
            -- /explicitly/ in the import specs rather
            -- than being imported as part of a "..." group. Consider:
            --
            -- > import C( T(..) )
            --
            -- Here the constructors of @T@ are not named explicitly;
            -- only @T@ is named explicitly.

    unQualSpecOK :: ImportSpec -> Bool
    -- ^ Is in scope unqualified?
    unQualSpecOK is = not (is_qual (is_decl is))

    qualSpecOK :: ModuleName -> ImportSpec -> Bool
    -- ^ Is in scope qualified with the given module?
    qualSpecOK mod is = mod == is_as (is_decl is)

    importSpecLoc :: ImportSpec -> SrcSpan
    importSpecLoc (ImpSpec decl ImpAll) = is_dloc decl
    importSpecLoc (ImpSpec _    item)   = is_iloc item

    importSpecModule :: ImportSpec -> ModuleName
    importSpecModule is = is_mod (is_decl is)

    isExplicitItem :: ImpItemSpec -> Bool
    isExplicitItem ImpAll                        = False
    isExplicitItem (ImpSome {is_explicit = exp}) = exp

    -- Note [Comparing provenance]
    -- Comparison of provenance is just used for grouping
    -- error messages (in RnEnv.warnUnusedBinds)
    instance Eq Provenance where
      p1 == p2 = case p1 `compare` p2 of EQ -> True; _ -> False

    instance Eq ImpDeclSpec where
      p1 == p2 = case p1 `compare` p2 of EQ -> True; _ -> False

    instance Eq ImpItemSpec where
      p1 == p2 = case p1 `compare` p2 of EQ -> True; _ -> False

    instance Ord Provenance where
       compare LocalDef      LocalDef        = EQ
       compare LocalDef      (Imported _)    = LT
       compare (Imported _ ) LocalDef        = GT
       compare (Imported is1) (Imported is2) = compare (head is1)
            {- See Note [Comparing provenance] -}      (head is2)

    instance Ord ImpDeclSpec where
       compare is1 is2 = (is_mod is1 `compare` is_mod is2) `thenCmp`
                         (is_dloc is1 `compare` is_dloc is2)

    instance Ord ImpItemSpec where
       compare is1 is2 = is_iloc is1 `compare` is_iloc is2

    plusProv :: Provenance -> Provenance -> Provenance
    -- Choose LocalDef over Imported
    -- There is an obscure bug lurking here; in the presence
    -- of recursive modules, something can be imported *and* locally
    -- defined, and one might refer to it with a qualified name from
    -- the import -- but I'm going to ignore that because it makes
    -- the isLocalGRE predicate so much nicer this way
    plusProv LocalDef        LocalDef        = panic "plusProv"
    plusProv LocalDef        _               = LocalDef
    plusProv _               LocalDef        = LocalDef
    plusProv (Imported is1)  (Imported is2)  = Imported (is1++is2)

    pprNameProvenance :: GlobalRdrElt -> SDoc
    -- ^ Print out the place where the name was imported
    pprNameProvenance (GRE {gre_name = name, gre_prov = LocalDef})
      = ptext (sLit "defined at") <+> ppr (nameSrcLoc name)
    pprNameProvenance (GRE {gre_name = name, gre_prov = Imported whys})
      = case whys of
            (why:_) | opt_PprStyle_Debug -> vcat (map pp_why whys)
                    | otherwise          -> pp_why why
            [] -> panic "pprNameProvenance"
      where
        pp_why why = sep [ppr why, ppr_defn_site why name]

    -- If we know the exact definition point (which we may do with GHCi)
    -- then show that too.  But not if it's just "imported from X".
    ppr_defn_site :: ImportSpec -> Name -> SDoc
    ppr_defn_site imp_spec name
      | same_module && not (isGoodSrcSpan loc)
      = empty              -- Nothing interesting to say
      | otherwise
      = parens $ hang (ptext (sLit "and originally defined") <+> pp_mod)
                    2 (pprLoc loc)
      where
        loc = nameSrcSpan name
        defining_mod = nameModule name
        same_module = importSpecModule imp_spec == moduleName defining_mod
        pp_mod | same_module = empty
               | otherwise   = ptext (sLit "in") <+> quotes (ppr defining_mod)

    instance Outputable ImportSpec where
       ppr imp_spec
         = ptext (sLit "imported") <+> qual
            <+> ptext (sLit "from") <+> quotes (ppr (importSpecModule imp_spec))
            <+> pprLoc (importSpecLoc imp_spec)
         where
           qual | is_qual (is_decl imp_spec) = ptext (sLit "qualified")
                | otherwise                  = empty

    pprLoc :: SrcSpan -> SDoc
    pprLoc (RealSrcSpan s)    = ptext (sLit "at") <+> ppr s
    pprLoc (UnhelpfulSpan {}) = empty