RnEnv.hs 82.9 KB
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{-
(c) The GRASP/AQUA Project, Glasgow University, 1992-2006

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\section[RnEnv]{Environment manipulation for the renamer monad}
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-}
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{-# LANGUAGE CPP #-}

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module RnEnv (
        newTopSrcBinder,
        lookupLocatedTopBndrRn, lookupTopBndrRn,
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        lookupLocatedOccRn, lookupOccRn, lookupOccRn_maybe,
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        lookupLocalOccRn_maybe, lookupInfoOccRn,
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        lookupLocalOccThLvl_maybe,
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        lookupTypeOccRn, lookupKindOccRn,
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        lookupGlobalOccRn, lookupGlobalOccRn_maybe,
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        lookupOccRn_overloaded, lookupGlobalOccRn_overloaded,
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        reportUnboundName, unknownNameSuggestions,
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        HsSigCtxt(..), lookupLocalTcNames, lookupSigOccRn,
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        lookupSigCtxtOccRn,
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        lookupFixityRn, lookupTyFixityRn,
        lookupInstDeclBndr, lookupSubBndrOcc, lookupFamInstName,
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        lookupSubBndrGREs, lookupConstructorFields,
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        lookupSyntaxName, lookupSyntaxNames, lookupIfThenElse,
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        lookupGreAvailRn,
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        getLookupOccRn,
        addUsedGRE, addUsedGREs, addUsedDataCons,
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        newLocalBndrRn, newLocalBndrsRn,
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        bindLocalNames, bindLocalNamesFV,
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        MiniFixityEnv,
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        addLocalFixities,
        bindLocatedLocalsFV, bindLocatedLocalsRn,
        extendTyVarEnvFVRn,

        checkDupRdrNames, checkShadowedRdrNames,
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        checkDupNames, checkDupAndShadowedNames, checkTupSize,
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        addFvRn, mapFvRn, mapMaybeFvRn, mapFvRnCPS,
        warnUnusedMatches,
        warnUnusedTopBinds, warnUnusedLocalBinds,
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        mkFieldEnv,
        dataTcOccs, kindSigErr, perhapsForallMsg, unknownSubordinateErr,
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        HsDocContext(..), docOfHsDocContext
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    ) where
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#include "HsVersions.h"
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import LoadIface        ( loadInterfaceForName, loadSrcInterface_maybe )
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import IfaceEnv
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import HsSyn
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import RdrName
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import HscTypes
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import TcEnv
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import TcRnMonad
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import RdrHsSyn         ( setRdrNameSpace )
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import Name
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import NameSet
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import NameEnv
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import Avail
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import Module
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import ConLike
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import DataCon
import TyCon
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import PrelNames        ( mkUnboundName, isUnboundName, rOOT_MAIN, forall_tv_RDR )
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import ErrUtils         ( MsgDoc )
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import BasicTypes       ( Fixity(..), FixityDirection(..), minPrecedence, defaultFixity )
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import SrcLoc
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import Outputable
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import Util
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import Maybes
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import BasicTypes       ( TopLevelFlag(..) )
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import ListSetOps       ( removeDups )
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import DynFlags
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import FastString
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import Control.Monad
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import Data.List
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import ListSetOps       ( minusList )
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import Constants        ( mAX_TUPLE_SIZE )
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{-
*********************************************************
*                                                      *
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                Source-code binders
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*                                                      *
*********************************************************
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Note [Signature lazy interface loading]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

GHC's lazy interface loading can be a bit confusing, so this Note is an
empirical description of what happens in one interesting case. When
compiling a signature module against an its implementation, we do NOT
load interface files associated with its names until after the type
checking phase.  For example:

    module ASig where
        data T
        f :: T -> T

Suppose we compile this with -sig-of "A is ASig":

    module B where
        data T = T
        f T = T

    module A(module B) where
        import B

During type checking, we'll load A.hi because we need to know what the
RdrEnv for the module is, but we DO NOT load the interface for B.hi!
It's wholly unnecessary: our local definition 'data T' in ASig is all
the information we need to finish type checking.  This is contrast to
type checking of ordinary Haskell files, in which we would not have the
local definition "data T" and would need to consult B.hi immediately.
(Also, this situation never occurs for hs-boot files, since you're not
allowed to reexport from another module.)

After type checking, we then check that the types we provided are
consistent with the backing implementation (in checkHiBootOrHsigIface).
At this point, B.hi is loaded, because we need something to compare
against.

I discovered this behavior when trying to figure out why type class
instances for Data.Map weren't in the EPS when I was type checking a
test very much like ASig (sigof02dm): the associated interface hadn't
been loaded yet!  (The larger issue is a moot point, since an instance
declared in a signature can never be a duplicate.)

This behavior might change in the future.  Consider this
alternate module B:

    module B where
        {-# DEPRECATED T, f "Don't use" #-}
        data T = T
        f T = T

One might conceivably want to report deprecation warnings when compiling
ASig with -sig-of B, in which case we need to look at B.hi to find the
deprecation warnings during renaming.  At the moment, you don't get any
warning until you use the identifier further downstream.  This would
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require adjusting addUsedGRE so that during signature compilation,
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we do not report deprecation warnings for LocalDef.  See also
Note [Handling of deprecations]
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-}
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newTopSrcBinder :: Located RdrName -> RnM Name
newTopSrcBinder (L loc rdr_name)
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  | Just name <- isExact_maybe rdr_name
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  =     -- This is here to catch
        --   (a) Exact-name binders created by Template Haskell
        --   (b) The PrelBase defn of (say) [] and similar, for which
        --       the parser reads the special syntax and returns an Exact RdrName
        -- We are at a binding site for the name, so check first that it
        -- the current module is the correct one; otherwise GHC can get
        -- very confused indeed. This test rejects code like
        --      data T = (,) Int Int
        -- unless we are in GHC.Tup
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    if isExternalName name then
      do { this_mod <- getModule
         ; unless (this_mod == nameModule name)
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                  (addErrAt loc (badOrigBinding rdr_name))
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         ; return name }
    else   -- See Note [Binders in Template Haskell] in Convert.hs
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      do { this_mod <- getModule
         ; externaliseName this_mod name }
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  | Just (rdr_mod, rdr_occ) <- isOrig_maybe rdr_name
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  = do  { this_mod <- getModule
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        ; unless (rdr_mod == this_mod || rdr_mod == rOOT_MAIN)
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                 (addErrAt loc (badOrigBinding rdr_name))
        -- When reading External Core we get Orig names as binders,
        -- but they should agree with the module gotten from the monad
        --
        -- We can get built-in syntax showing up here too, sadly.  If you type
        --      data T = (,,,)
        -- the constructor is parsed as a type, and then RdrHsSyn.tyConToDataCon
        -- uses setRdrNameSpace to make it into a data constructors.  At that point
        -- the nice Exact name for the TyCon gets swizzled to an Orig name.
        -- Hence the badOrigBinding error message.
        --
        -- Except for the ":Main.main = ..." definition inserted into
        -- the Main module; ugh!

        -- Because of this latter case, we call newGlobalBinder with a module from
        -- the RdrName, not from the environment.  In principle, it'd be fine to
        -- have an arbitrary mixture of external core definitions in a single module,
        -- (apart from module-initialisation issues, perhaps).
        ; newGlobalBinder rdr_mod rdr_occ loc }
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  | otherwise
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  = do  { unless (not (isQual rdr_name))
                 (addErrAt loc (badQualBndrErr rdr_name))
                -- Binders should not be qualified; if they are, and with a different
                -- module name, we we get a confusing "M.T is not in scope" error later

        ; stage <- getStage
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        ; env <- getGblEnv
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        ; if isBrackStage stage then
                -- We are inside a TH bracket, so make an *Internal* name
                -- See Note [Top-level Names in Template Haskell decl quotes] in RnNames
             do { uniq <- newUnique
                ; return (mkInternalName uniq (rdrNameOcc rdr_name) loc) }
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          else case tcg_impl_rdr_env env of
            Just gr ->
                -- We're compiling --sig-of, so resolve with respect to this
                -- module.
                -- See Note [Signature parameters in TcGblEnv and DynFlags]
             do { case lookupGlobalRdrEnv gr (rdrNameOcc rdr_name) of
                    -- Be sure to override the loc so that we get accurate
                    -- information later
                    [GRE{ gre_name = n }] -> do
                      -- NB: Just adding this line will not work:
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                      --    addUsedGRE True gre
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                      -- see Note [Signature lazy interface loading] for
                      -- more details.
                      return (setNameLoc n loc)
                    _ -> do
                      { -- NB: cannot use reportUnboundName rdr_name
                        -- because it looks up in the wrong RdrEnv
                        -- ToDo: more helpful error messages
                      ; addErr (unknownNameErr (pprNonVarNameSpace
                            (occNameSpace (rdrNameOcc rdr_name))) rdr_name)
                      ; return (mkUnboundName rdr_name)
                      }
                }
            Nothing ->
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                -- Normal case
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             do { this_mod <- getModule
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                ; traceRn (text "newTopSrcBinder" <+> (ppr this_mod $$ ppr rdr_name $$ ppr loc))
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                ; newGlobalBinder this_mod (rdrNameOcc rdr_name) loc } }
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{-
*********************************************************
*                                                      *
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        Source code occurrences
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*                                                      *
*********************************************************
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Looking up a name in the RnEnv.

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Note [Type and class operator definitions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We want to reject all of these unless we have -XTypeOperators (Trac #3265)
   data a :*: b  = ...
   class a :*: b where ...
   data (:*:) a b  = ....
   class (:*:) a b where ...
The latter two mean that we are not just looking for a
*syntactically-infix* declaration, but one that uses an operator
OccName.  We use OccName.isSymOcc to detect that case, which isn't
terribly efficient, but there seems to be no better way.
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-}
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lookupTopBndrRn :: RdrName -> RnM Name
lookupTopBndrRn n = do nopt <- lookupTopBndrRn_maybe n
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                       case nopt of
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                         Just n' -> return n'
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                         Nothing -> do traceRn $ (text "lookupTopBndrRn fail" <+> ppr n)
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                                       unboundName WL_LocalTop n
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lookupLocatedTopBndrRn :: Located RdrName -> RnM (Located Name)
lookupLocatedTopBndrRn = wrapLocM lookupTopBndrRn

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lookupTopBndrRn_maybe :: RdrName -> RnM (Maybe Name)
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-- Look up a top-level source-code binder.   We may be looking up an unqualified 'f',
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-- and there may be several imported 'f's too, which must not confuse us.
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-- For example, this is OK:
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--      import Foo( f )
--      infix 9 f       -- The 'f' here does not need to be qualified
--      f x = x         -- Nor here, of course
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-- So we have to filter out the non-local ones.
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--
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-- A separate function (importsFromLocalDecls) reports duplicate top level
-- decls, so here it's safe just to choose an arbitrary one.
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--
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-- There should never be a qualified name in a binding position in Haskell,
-- but there can be if we have read in an external-Core file.
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-- The Haskell parser checks for the illegal qualified name in Haskell
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-- source files, so we don't need to do so here.
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lookupTopBndrRn_maybe rdr_name
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  | Just name <- isExact_maybe rdr_name
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  = do { name' <- lookupExactOcc name; return (Just name') }
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  | Just (rdr_mod, rdr_occ) <- isOrig_maybe rdr_name
        -- This deals with the case of derived bindings, where
        -- we don't bother to call newTopSrcBinder first
        -- We assume there is no "parent" name
  = do  { loc <- getSrcSpanM
        ; n <- newGlobalBinder rdr_mod rdr_occ loc
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        ; return (Just n)}
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  | otherwise
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  = do  {  -- Check for operators in type or class declarations
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           -- See Note [Type and class operator definitions]
          let occ = rdrNameOcc rdr_name
        ; when (isTcOcc occ && isSymOcc occ)
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               (do { op_ok <- xoptM Opt_TypeOperators
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                   ; unless op_ok (addErr (opDeclErr rdr_name)) })

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        ; env <- getGlobalRdrEnv
        ; case filter isLocalGRE (lookupGRE_RdrName rdr_name env) of
            [gre] -> return (Just (gre_name gre))
            _     -> return Nothing  -- Ambiguous (can't happen) or unbound
    }
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-----------------------------------------------
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-- | Lookup an @Exact@ @RdrName@. See Note [Looking up Exact RdrNames].
-- This adds an error if the name cannot be found.
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lookupExactOcc :: Name -> RnM Name
lookupExactOcc name
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  = do { result <- lookupExactOcc_either name
       ; case result of
           Left err -> do { addErr err
                          ; return name }
           Right name' -> return name' }

-- | Lookup an @Exact@ @RdrName@. See Note [Looking up Exact RdrNames].
-- This never adds an error, but it may return one.
lookupExactOcc_either :: Name -> RnM (Either MsgDoc Name)
-- See Note [Looking up Exact RdrNames]
lookupExactOcc_either name
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  | Just thing <- wiredInNameTyThing_maybe name
  , Just tycon <- case thing of
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                    ATyCon tc                 -> Just tc
                    AConLike (RealDataCon dc) -> Just (dataConTyCon dc)
                    _                         -> Nothing
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  , isTupleTyCon tycon
  = do { checkTupSize (tyConArity tycon)
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       ; return (Right name) }
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  | isExternalName name
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  = return (Right name)
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  | otherwise
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  = do { env <- getGlobalRdrEnv
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       ; let -- See Note [Splicing Exact names]
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             main_occ =  nameOccName name
             demoted_occs = case demoteOccName main_occ of
                              Just occ -> [occ]
                              Nothing  -> []
             gres = [ gre | occ <- main_occ : demoted_occs
                          , gre <- lookupGlobalRdrEnv env occ
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                          , gre_name gre == name ]
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       ; case gres of
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           [gre] -> return (Right (gre_name gre))

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           []    -> -- See Note [Splicing Exact names]
                    do { lcl_env <- getLocalRdrEnv
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                       ; if name `inLocalRdrEnvScope` lcl_env
                         then return (Right name)
                         else
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#ifdef GHCI
                         do { th_topnames_var <- fmap tcg_th_topnames getGblEnv
                            ; th_topnames <- readTcRef th_topnames_var
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                            ; if name `elemNameSet` th_topnames
                              then return (Right name)
                              else return (Left exact_nm_err)
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                            }
#else /* !GHCI */
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                         return (Left exact_nm_err)
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#endif /* !GHCI */
                       }
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           gres -> return (Left (sameNameErr gres))   -- Ugh!  See Note [Template Haskell ambiguity]
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       }
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  where
    exact_nm_err = hang (ptext (sLit "The exact Name") <+> quotes (ppr name) <+> ptext (sLit "is not in scope"))
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                      2 (vcat [ ptext (sLit "Probable cause: you used a unique Template Haskell name (NameU), ")
                              , ptext (sLit "perhaps via newName, but did not bind it")
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                              , ptext (sLit "If that's it, then -ddump-splices might be useful") ])
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sameNameErr :: [GlobalRdrElt] -> MsgDoc
sameNameErr [] = panic "addSameNameErr: empty list"
sameNameErr gres@(_ : _)
  = hang (ptext (sLit "Same exact name in multiple name-spaces:"))
       2 (vcat (map pp_one sorted_names) $$ th_hint)
  where
    sorted_names = sortWith nameSrcLoc (map gre_name gres)
    pp_one name
      = hang (pprNameSpace (occNameSpace (getOccName name))
              <+> quotes (ppr name) <> comma)
           2 (ptext (sLit "declared at:") <+> ppr (nameSrcLoc name))

    th_hint = vcat [ ptext (sLit "Probable cause: you bound a unique Template Haskell name (NameU),")
                   , ptext (sLit "perhaps via newName, in different name-spaces.")
                   , ptext (sLit "If that's it, then -ddump-splices might be useful") ]

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-----------------------------------------------
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lookupInstDeclBndr :: Name -> SDoc -> RdrName -> RnM Name
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-- This is called on the method name on the left-hand side of an
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-- instance declaration binding. eg.  instance Functor T where
--                                       fmap = ...
--                                       ^^^^ called on this
-- Regardless of how many unqualified fmaps are in scope, we want
-- the one that comes from the Functor class.
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--
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-- Furthermore, note that we take no account of whether the
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-- name is only in scope qualified.  I.e. even if method op is
-- in scope as M.op, we still allow plain 'op' on the LHS of
-- an instance decl
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--
-- The "what" parameter says "method" or "associated type",
-- depending on what we are looking up
lookupInstDeclBndr cls what rdr
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  = do { when (isQual rdr)
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              (addErr (badQualBndrErr rdr))
                -- In an instance decl you aren't allowed
                -- to use a qualified name for the method
                -- (Although it'd make perfect sense.)
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       ; lookupSubBndrOcc False -- False => we don't give deprecated
                                -- warnings when a deprecated class
                                -- method is defined. We only warn
                                -- when it's used
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                          (Just cls) doc rdr }
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  where
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    doc = what <+> ptext (sLit "of class") <+> quotes (ppr cls)
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-----------------------------------------------
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lookupFamInstName :: Maybe Name -> Located RdrName -> RnM (Located Name)
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-- Used for TyData and TySynonym family instances only,
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-- See Note [Family instance binders]
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lookupFamInstName (Just cls) tc_rdr  -- Associated type; c.f RnBinds.rnMethodBind
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  = wrapLocM (lookupInstDeclBndr cls (ptext (sLit "associated type"))) tc_rdr
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lookupFamInstName Nothing tc_rdr     -- Family instance; tc_rdr is an *occurrence*
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  = lookupLocatedOccRn tc_rdr
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-----------------------------------------------
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lookupConstructorFields :: Name -> RnM [FieldLabel]
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-- Look up the fields of a given constructor
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--   *  For constructors from this module, use the record field env,
--      which is itself gathered from the (as yet un-typechecked)
--      data type decls
--
--    * For constructors from imported modules, use the *type* environment
--      since imported modles are already compiled, the info is conveniently
--      right there
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lookupConstructorFields con_name
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  = do  { this_mod <- getModule
        ; if nameIsLocalOrFrom this_mod con_name then
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          do { field_env <- getRecFieldEnv
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             ; return (lookupNameEnv field_env con_name `orElse` []) }
          else
          do { con <- tcLookupDataCon con_name
             ; return (dataConFieldLabels con) } }
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-----------------------------------------------
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-- Used for record construction and pattern matching
-- When the -XDisambiguateRecordFields flag is on, take account of the
-- constructor name to disambiguate which field to use; it's just the
-- same as for instance decls
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--
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-- NB: Consider this:
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--      module Foo where { data R = R { fld :: Int } }
--      module Odd where { import Foo; fld x = x { fld = 3 } }
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-- Arguably this should work, because the reference to 'fld' is
-- unambiguous because there is only one field id 'fld' in scope.
-- But currently it's rejected.
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lookupSubBndrOcc :: Bool
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                 -> Maybe Name  -- Nothing => just look it up as usual
                                -- Just p  => use parent p to disambiguate
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                 -> SDoc -> RdrName
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                 -> RnM Name
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lookupSubBndrOcc warnIfDeprec parent doc rdr_name
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  | Just n <- isExact_maybe rdr_name   -- This happens in derived code
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  = lookupExactOcc n
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  | Just (rdr_mod, rdr_occ) <- isOrig_maybe rdr_name
  = lookupOrig rdr_mod rdr_occ

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  | otherwise   -- Find all the things the rdr-name maps to
  = do  {       -- and pick the one with the right parent namep
          env <- getGlobalRdrEnv
        ; case lookupSubBndrGREs env parent rdr_name of
                -- NB: lookupGlobalRdrEnv, not lookupGRE_RdrName!
                --     The latter does pickGREs, but we want to allow 'x'
                --     even if only 'M.x' is in scope
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            [gre] -> do { addUsedGRE warnIfDeprec gre
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                          -- Add a usage; this is an *occurrence* site
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                          -- Note [Usage for sub-bndrs]
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                        ; return (gre_name gre) }
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            []    -> do { ns <- lookupQualifiedNameGHCi rdr_name
                        ; case ns of {
                                (n:_) -> return n ;
                                -- Unlikely to be more than one...?
                                [] -> do
                        { addErr (unknownSubordinateErr doc rdr_name)
                        ; return (mkUnboundName rdr_name) } } }
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            gres  -> do { addNameClashErrRn rdr_name gres
                        ; return (gre_name (head gres)) } }
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lookupSubBndrGREs :: GlobalRdrEnv -> Maybe Name -> RdrName -> [GlobalRdrElt]
-- If parent = Nothing, just do a normal lookup
-- If parent = Just p then find all GREs that
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--   (a) have parent p
--   (b) for Unqual, are in scope qualified or unqualified
--       for Qual, are in scope with that qualification
lookupSubBndrGREs env parent rdr_name
  = case parent of
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      Nothing               -> pickGREs rdr_name gres
      Just p
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        | isUnqual rdr_name -> filter (parent_is p) gres
        | otherwise         -> filter (parent_is p) (pickGREs rdr_name gres)

  where
    gres = lookupGlobalRdrEnv env (rdrNameOcc rdr_name)

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    parent_is p (GRE { gre_par = ParentIs p' })             = p == p'
    parent_is p (GRE { gre_par = FldParent { par_is = p'}}) = p == p'
    parent_is _ _                                           = False
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{-
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Note [Family instance binders]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
  data family F a
  data instance F T = X1 | X2

The 'data instance' decl has an *occurrence* of F (and T), and *binds*
X1 and X2.  (This is unlike a normal data type declaration which would
bind F too.)  So we want an AvailTC F [X1,X2].

Now consider a similar pair:
  class C a where
    data G a
  instance C S where
    data G S = Y1 | Y2

The 'data G S' *binds* Y1 and Y2, and has an *occurrence* of G.

But there is a small complication: in an instance decl, we don't use
qualified names on the LHS; instead we use the class to disambiguate.
Thus:
  module M where
    import Blib( G )
    class C a where
      data G a
    instance C S where
      data G S = Y1 | Y2
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Even though there are two G's in scope (M.G and Blib.G), the occurrence
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of 'G' in the 'instance C S' decl is unambiguous, because C has only
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one associated type called G. This is exactly what happens for methods,
and it is only consistent to do the same thing for types. That's the
role of the function lookupTcdName; the (Maybe Name) give the class of
the encloseing instance decl, if any.

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Note [Looking up Exact RdrNames]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Exact RdrNames are generated by Template Haskell.  See Note [Binders
in Template Haskell] in Convert.

For data types and classes have Exact system Names in the binding
positions for constructors, TyCons etc.  For example
    [d| data T = MkT Int |]
when we splice in and Convert to HsSyn RdrName, we'll get
    data (Exact (system Name "T")) = (Exact (system Name "MkT")) ...
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These System names are generated by Convert.thRdrName
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But, constructors and the like need External Names, not System Names!
So we do the following

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 * In RnEnv.newTopSrcBinder we spot Exact RdrNames that wrap a
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   non-External Name, and make an External name for it. This is
   the name that goes in the GlobalRdrEnv

 * When looking up an occurrence of an Exact name, done in
   RnEnv.lookupExactOcc, we find the Name with the right unique in the
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   GlobalRdrEnv, and use the one from the envt -- it will be an
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   External Name in the case of the data type/constructor above.

 * Exact names are also use for purely local binders generated
   by TH, such as    \x_33. x_33
   Both binder and occurrence are Exact RdrNames.  The occurrence
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   gets looked up in the LocalRdrEnv by RnEnv.lookupOccRn, and
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   misses, because lookupLocalRdrEnv always returns Nothing for
   an Exact Name.  Now we fall through to lookupExactOcc, which
   will find the Name is not in the GlobalRdrEnv, so we just use
   the Exact supplied Name.

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Note [Splicing Exact names]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the splice $(do { x <- newName "x"; return (VarE x) })
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This will generate a (HsExpr RdrName) term that mentions the
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Exact RdrName "x_56" (or whatever), but does not bind it.  So
when looking such Exact names we want to check that it's in scope,
otherwise the type checker will get confused.  To do this we need to
keep track of all the Names in scope, and the LocalRdrEnv does just that;
we consult it with RdrName.inLocalRdrEnvScope.
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There is another wrinkle.  With TH and -XDataKinds, consider
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   $( [d| data Nat = Zero
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          data T = MkT (Proxy 'Zero)  |] )
After splicing, but before renaming we get this:
   data Nat_77{tc} = Zero_78{d}
   data T_79{tc} = MkT_80{d} (Proxy 'Zero_78{tc})  |] )
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The occurrence of 'Zero in the data type for T has the right unique,
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but it has a TcClsName name-space in its OccName.  (This is set by
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the ctxt_ns argument of Convert.thRdrName.)  When we check that is
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in scope in the GlobalRdrEnv, we need to look up the DataName namespace
too.  (An alternative would be to make the GlobalRdrEnv also have
a Name -> GRE mapping.)

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Note [Template Haskell ambiguity]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The GlobalRdrEnv invariant says that if
  occ -> [gre1, ..., gren]
then the gres have distinct Names (INVARIANT 1 of GlobalRdrEnv).
This is guaranteed by extendGlobalRdrEnvRn (the dups check in add_gre).

So how can we get multiple gres in lookupExactOcc_maybe?  Because in
TH we might use the same TH NameU in two different name spaces.
eg (Trac #7241):
   $(newName "Foo" >>= \o -> return [DataD [] o [] [RecC o []] [''Show]])
Here we generate a type constructor and data constructor with the same
unique, but differnt name spaces.

It'd be nicer to rule this out in extendGlobalRdrEnvRn, but that would
mean looking up the OccName in every name-space, just in case, and that
seems a bit brutal.  So it's just done here on lookup.  But we might
need to revisit that choice.

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Note [Usage for sub-bndrs]
~~~~~~~~~~~~~~~~~~~~~~~~~~
If you have this
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   import qualified M( C( f ) )
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   instance M.C T where
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     f x = x
then is the qualified import M.f used?  Obviously yes.
But the RdrName used in the instance decl is unqualified.  In effect,
we fill in the qualification by looking for f's whose class is M.C
But when adding to the UsedRdrNames we must make that qualification
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explicit (saying "used  M.f"), otherwise we get "Redundant import of M.f".

So we make up a suitable (fake) RdrName.  But be careful
   import qualifed M
   import M( C(f) )
   instance C T where
     f x = x
Here we want to record a use of 'f', not of 'M.f', otherwise
we'll miss the fact that the qualified import is redundant.
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--------------------------------------------------
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--              Occurrences
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--------------------------------------------------
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-}
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getLookupOccRn :: RnM (Name -> Maybe Name)
getLookupOccRn
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  = do local_env <- getLocalRdrEnv
       return (lookupLocalRdrOcc local_env . nameOccName)
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lookupLocatedOccRn :: Located RdrName -> RnM (Located Name)
lookupLocatedOccRn = wrapLocM lookupOccRn

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lookupLocalOccRn_maybe :: RdrName -> RnM (Maybe Name)
-- Just look in the local environment
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lookupLocalOccRn_maybe rdr_name
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  = do { local_env <- getLocalRdrEnv
       ; return (lookupLocalRdrEnv local_env rdr_name) }

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lookupLocalOccThLvl_maybe :: Name -> RnM (Maybe (TopLevelFlag, ThLevel))
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-- Just look in the local environment
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lookupLocalOccThLvl_maybe name
  = do { lcl_env <- getLclEnv
       ; return (lookupNameEnv (tcl_th_bndrs lcl_env) name) }
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-- lookupOccRn looks up an occurrence of a RdrName
lookupOccRn :: RdrName -> RnM Name
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lookupOccRn rdr_name
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  = do { mb_name <- lookupOccRn_maybe rdr_name
       ; case mb_name of
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           Just name -> return name
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           Nothing   -> reportUnboundName rdr_name }
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lookupKindOccRn :: RdrName -> RnM Name
-- Looking up a name occurring in a kind
lookupKindOccRn rdr_name
  = do { mb_name <- lookupOccRn_maybe rdr_name
       ; case mb_name of
           Just name -> return name
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           Nothing   -> reportUnboundName rdr_name  }
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-- lookupPromotedOccRn looks up an optionally promoted RdrName.
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lookupTypeOccRn :: RdrName -> RnM Name
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-- see Note [Demotion]
lookupTypeOccRn rdr_name
  = do { mb_name <- lookupOccRn_maybe rdr_name
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       ; case mb_name of {
             Just name -> return name ;
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             Nothing   -> lookup_demoted rdr_name } }

lookup_demoted :: RdrName -> RnM Name
lookup_demoted rdr_name
  | Just demoted_rdr <- demoteRdrName rdr_name
    -- Maybe it's the name of a *data* constructor
  = do { data_kinds <- xoptM Opt_DataKinds
       ; mb_demoted_name <- lookupOccRn_maybe demoted_rdr
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       ; case mb_demoted_name of
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           Nothing -> reportUnboundName rdr_name
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           Just demoted_name
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             | data_kinds ->
             do { whenWOptM Opt_WarnUntickedPromotedConstructors $
                  addWarn (untickedPromConstrWarn demoted_name)
                ; return demoted_name }
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             | otherwise  -> unboundNameX WL_Any rdr_name suggest_dk }
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  | otherwise
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  = reportUnboundName rdr_name
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  where
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    suggest_dk = ptext (sLit "A data constructor of that name is in scope; did you mean DataKinds?")
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    untickedPromConstrWarn name =
      text "Unticked promoted constructor" <> colon <+> quotes (ppr name) <> dot
      $$
      hsep [ text "Use"
           , quotes (char '\'' <> ppr name)
           , text "instead of"
           , quotes (ppr name) <> dot ]
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{-
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Note [Demotion]
~~~~~~~~~~~~~~~
When the user writes:
  data Nat = Zero | Succ Nat
  foo :: f Zero -> Int

'Zero' in the type signature of 'foo' is parsed as:
  HsTyVar ("Zero", TcClsName)
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When the renamer hits this occurrence of 'Zero' it's going to realise
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that it's not in scope. But because it is renaming a type, it knows
that 'Zero' might be a promoted data constructor, so it will demote
its namespace to DataName and do a second lookup.

The final result (after the renamer) will be:
  HsTyVar ("Zero", DataName)
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-}
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--              Use this version to get tracing
--
-- lookupOccRn_maybe, lookupOccRn_maybe' :: RdrName -> RnM (Maybe Name)
-- lookupOccRn_maybe rdr_name
--  = do { mb_res <- lookupOccRn_maybe' rdr_name
--       ; gbl_rdr_env   <- getGlobalRdrEnv
--       ; local_rdr_env <- getLocalRdrEnv
--       ; traceRn $ text "lookupOccRn_maybe" <+>
--           vcat [ ppr rdr_name <+> ppr (getUnique (rdrNameOcc rdr_name))
--                , ppr mb_res
--                , text "Lcl env" <+> ppr local_rdr_env
--                , text "Gbl env" <+> ppr [ (getUnique (nameOccName (gre_name (head gres'))),gres') | gres <- occEnvElts gbl_rdr_env
--                                         , let gres' = filter isLocalGRE gres, not (null gres') ] ]
--       ; return mb_res }

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lookupOccRn_maybe :: RdrName -> RnM (Maybe Name)
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-- lookupOccRn looks up an occurrence of a RdrName
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lookupOccRn_maybe rdr_name
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  = do { local_env <- getLocalRdrEnv
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       ; case lookupLocalRdrEnv local_env rdr_name of {
          Just name -> return (Just name) ;
          Nothing   -> do
       { mb_name <- lookupGlobalOccRn_maybe rdr_name
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       ; case mb_name of {
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                Just name  -> return (Just name) ;
                Nothing -> do
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       { ns <- lookupQualifiedNameGHCi rdr_name
                      -- This test is not expensive,
                      -- and only happens for failed lookups
       ; case ns of
           (n:_) -> return (Just n)  -- Unlikely to be more than one...?
           []    -> return Nothing } } } } }
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lookupGlobalOccRn :: RdrName -> RnM Name
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-- lookupGlobalOccRn is like lookupOccRn, except that it looks in the global
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-- environment.  Adds an error message if the RdrName is not in scope.
lookupGlobalOccRn rdr_name
  = do { mb_name <- lookupGlobalOccRn_maybe rdr_name
       ; case mb_name of
           Just n  -> return n
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           Nothing -> do { traceRn (text "lookupGlobalOccRn" <+> ppr rdr_name)
                         ; unboundName WL_Global rdr_name } }
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lookupInfoOccRn :: RdrName -> RnM [Name]
-- lookupInfoOccRn is intended for use in GHCi's ":info" command
-- It finds all the GREs that RdrName could mean, not complaining
-- about ambiguity, but rather returning them all
-- C.f. Trac #9881
lookupInfoOccRn rdr_name
  | Just n <- isExact_maybe rdr_name   -- e.g. (->)
  = return [n]

  | Just (rdr_mod, rdr_occ) <- isOrig_maybe rdr_name
  = do { n <- lookupOrig rdr_mod rdr_occ
       ; return [n] }

  | otherwise
  = do { rdr_env <- getGlobalRdrEnv
       ; let ns = map gre_name (lookupGRE_RdrName rdr_name rdr_env)
       ; qual_ns <- lookupQualifiedNameGHCi rdr_name
       ; return (ns ++ (qual_ns `minusList` ns)) }

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lookupGlobalOccRn_maybe :: RdrName -> RnM (Maybe Name)
-- No filter function; does not report an error on failure

lookupGlobalOccRn_maybe rdr_name
  | Just n <- isExact_maybe rdr_name   -- This happens in derived code
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  = do { n' <- lookupExactOcc n; return (Just n') }
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  | Just (rdr_mod, rdr_occ) <- isOrig_maybe rdr_name
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  = do { n <- lookupOrig rdr_mod rdr_occ
       ; return (Just n) }
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  | otherwise
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  = do  { mb_gre <- lookupGreRn_maybe rdr_name
        ; case mb_gre of
                Nothing  -> return Nothing
                Just gre -> return (Just (gre_name gre)) }
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-- | Like 'lookupOccRn_maybe', but with a more informative result if
-- the 'RdrName' happens to be a record selector:
--
--   * Nothing         -> name not in scope (no error reported)
--   * Just (Left x)   -> name uniquely refers to x,
--                        or there is a name clash (reported)
--   * Just (Right xs) -> name refers to one or more record selectors;
--                        if overload_ok was False, this list will be
--                        a singleton.
lookupOccRn_overloaded  :: Bool -> RdrName -> RnM (Maybe (Either Name [FieldOcc Name]))
lookupOccRn_overloaded overload_ok rdr_name
  = do { local_env <- getLocalRdrEnv
       ; case lookupLocalRdrEnv local_env rdr_name of {
          Just name -> return (Just (Left name)) ;
          Nothing   -> do
       { mb_name <- lookupGlobalOccRn_overloaded overload_ok rdr_name
       ; case mb_name of {
           Just name -> return (Just name) ;
           Nothing   -> do
       { ns <- lookupQualifiedNameGHCi rdr_name
                      -- This test is not expensive,
                      -- and only happens for failed lookups
       ; case ns of
           (n:_) -> return $ Just $ Left n  -- Unlikely to be more than one...?
           []    -> return Nothing  } } } } }

lookupGlobalOccRn_overloaded :: Bool -> RdrName -> RnM (Maybe (Either Name [FieldOcc Name]))
lookupGlobalOccRn_overloaded overload_ok rdr_name
  | Just n <- isExact_maybe rdr_name   -- This happens in derived code
  = do { n' <- lookupExactOcc n; return (Just (Left n')) }

  | Just (rdr_mod, rdr_occ) <- isOrig_maybe rdr_name
  = do { n <- lookupOrig rdr_mod rdr_occ
       ; return (Just (Left n)) }

  | otherwise
  = do  { env <- getGlobalRdrEnv
        ; case lookupGRE_RdrName rdr_name env of
                []    -> return Nothing
                [gre] | isRecFldGRE gre
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                         -> do { addUsedGRE True gre
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                               ; let fld_occ = FieldOcc rdr_name (gre_name gre)
                               ; return (Just (Right [fld_occ])) }
                      | otherwise
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                         -> do { addUsedGRE True gre
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                               ; return (Just (Left (gre_name gre))) }
                gres  | all isRecFldGRE gres && overload_ok
                            -- Don't record usage for ambiguous selectors
                            -- until we know which is meant
                         -> return (Just (Right (map (FieldOcc rdr_name . gre_name) gres)))
                gres     -> do { addNameClashErrRn rdr_name gres
                               ; return (Just (Left (gre_name (head gres)))) } }


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--------------------------------------------------
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--      Lookup in the Global RdrEnv of the module
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--------------------------------------------------

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lookupGreRn_maybe :: RdrName -> RnM (Maybe GlobalRdrElt)
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-- Look up the RdrName in the GlobalRdrEnv
--   Exactly one binding: records it as "used", return (Just gre)
--   No bindings:         return Nothing
--   Many bindings:       report "ambiguous", return an arbitrary (Just gre)
-- (This API is a bit strange; lookupGRERn2_maybe is simpler.
--  But it works and I don't want to fiddle too much.)
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lookupGreRn_maybe rdr_name
  = do  { env <- getGlobalRdrEnv
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        ; case lookupGRE_RdrName rdr_name env of
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            []    -> return Nothing
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            [gre] -> do { addUsedGRE True gre
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                        ; return (Just gre) }
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            gres  -> do { addNameClashErrRn rdr_name gres
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                        ; traceRn (text "name clash" <+> (ppr rdr_name $$ ppr gres $$ ppr env))
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                        ; return (Just (head gres)) } }
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lookupGreRn2_maybe :: RdrName -> RnM (Maybe GlobalRdrElt)
-- Look up the RdrName in the GlobalRdrEnv
--   Exactly one binding: record it as "used",   return (Just gre)
--   No bindings:         report "not in scope", return Nothing
--   Many bindings:       report "ambiguous",    return Nothing
lookupGreRn2_maybe rdr_name
  = do  { env <- getGlobalRdrEnv
        ; case lookupGRE_RdrName rdr_name env of
            []    -> do { _ <- unboundName WL_Global rdr_name
                        ; return Nothing }
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            [gre] -> do { addUsedGRE True gre
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                        ; return (Just gre) }
            gres  -> do { addNameClashErrRn rdr_name gres
                        ; traceRn (text "name clash" <+> (ppr rdr_name $$ ppr gres $$ ppr env))
                        ; return Nothing } }

lookupGreAvailRn :: RdrName -> RnM (Name, AvailInfo)
-- Used in export lists
-- If not found or ambiguous, add error message, and fake with UnboundName
lookupGreAvailRn rdr_name
  = do  { mb_gre <- lookupGreRn2_maybe rdr_name
        ; case mb_gre of {
            Just gre -> return (gre_name gre, availFromGRE gre) ;
            Nothing  ->
    do  { traceRn (text "lookupGreRn" <+> ppr rdr_name)
        ; let name = mkUnboundName rdr_name
        ; return (name, Avail name) } } }

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{-
*********************************************************
*                                                      *
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                Deprecations
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*                                                      *
*********************************************************
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Note [Handling of deprecations]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* We report deprecations at each *occurrence* of the deprecated thing
  (see Trac #5867)

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* We do not report deprecations for locally-defined names. For a
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  start, we may be exporting a deprecated thing. Also we may use a
  deprecated thing in the defn of another deprecated things.  We may
  even use a deprecated thing in the defn of a non-deprecated thing,
  when changing a module's interface.

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* addUsedGREs: we do not report deprecations for sub-binders:
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     - the ".." completion for records
     - the ".." in an export item 'T(..)'
     - the things exported by a module export 'module M'
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-}
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addUsedDataCons :: GlobalRdrEnv -> TyCon -> RnM ()
-- Remember use of in-scope data constructors (Trac #7969)
addUsedDataCons rdr_env tycon
  = addUsedGREs [ gre
                | dc <- tyConDataCons tycon
                , gre : _ <- [lookupGRE_Name rdr_env (dataConName dc) ] ]

addUsedGRE :: Bool -> GlobalRdrElt -> RnM ()
-- Called for both local and imported things
-- Add usage *and* warn if deprecated
addUsedGRE warn_if_deprec gre
  = do { when warn_if_deprec (warnIfDeprecated gre)
       ; unless (isLocalGRE gre) $
         do { env <- getGblEnv
            ; traceRn (text "addUsedGRE" <+> ppr gre)
            ; updMutVar (tcg_used_gres env) (gre :) } }

addUsedGREs :: [GlobalRdrElt] -> RnM ()
-- Record uses of any *imported* GREs
-- Used for recording used sub-bndrs
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-- NB: no call to warnIfDeprecated; see Note [Handling of deprecations]
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addUsedGREs gres
  | null imp_gres = return ()
  | otherwise     = do { env <- getGblEnv
                       ; traceRn (text "addUsedGREs" <+> ppr imp_gres)
                       ; updMutVar (tcg_used_gres env) (imp_gres ++) }
  where
    imp_gres = filterOut isLocalGRE gres
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warnIfDeprecated :: GlobalRdrElt -> RnM ()
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warnIfDeprecated gre@(GRE { gre_name = name, gre_imp = iss })
  | (imp_spec : _) <- iss
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  = do { dflags <- getDynFlags
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       ; this_mod <- getModule
       ; when (wopt Opt_WarnWarningsDeprecations dflags &&
               not (nameIsLocalOrFrom this_mod name)) $
                   -- See Note [Handling of deprecations]
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         do { iface <- loadInterfaceForName doc name
            ; case lookupImpDeprec iface gre of
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                Just txt -> addWarn (mk_msg imp_spec txt)
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                Nothing  -> return () } }
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  | otherwise
  = return ()
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  where
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    occ = greOccName gre
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    name_mod = ASSERT2( isExternalName name, ppr name ) nameModule name
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    doc = ptext (sLit "The name") <+> quotes (ppr occ) <+> ptext (sLit "is mentioned explicitly")
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    mk_msg imp_spec txt
      = sep [ sep [ ptext (sLit "In the use of")
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                    <+> pprNonVarNameSpace (occNameSpace occ)
                    <+> quotes (ppr occ)
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                  , parens imp_msg <> colon ]
            , ppr txt ]
      where
        imp_mod  = importSpecModule imp_spec
        imp_msg  = ptext (sLit "imported from") <+> ppr imp_mod <> extra
        extra | imp_mod == moduleName name_mod = Outputable.empty
              | otherwise = ptext (sLit ", but defined in") <+> ppr name_mod
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lookupImpDeprec :: ModIface -> GlobalRdrElt -> Maybe WarningTxt
lookupImpDeprec iface gre
  = mi_warn_fn iface (gre_name gre) `mplus`  -- Bleat if the thing,
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    case gre_par gre of                      -- or its parent, is warn'd
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       ParentIs  p              -> mi_warn_fn iface p
       FldParent { par_is = p } -> mi_warn_fn iface p
       NoParent                 -> Nothing
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{-