typecheck/TcRnMonad.hs

    {-
    (c) The University of Glasgow 2006

    Functions for working with the typechecker environment (setters, getters...).
    -}

    {-# LANGUAGE CPP, ExplicitForAll, FlexibleInstances #-}
    {-# OPTIONS_GHC -fno-warn-orphans #-}

    module TcRnMonad(
            module TcRnMonad,
            module TcRnTypes,
            module IOEnv
      ) where

    #include "HsVersions.h"

    import TcRnTypes        -- Re-export all
    import IOEnv            -- Re-export all
    import TcEvidence

    import HsSyn hiding (LIE)
    import HscTypes
    import Module
    import RdrName
    import Name
    import Type

    import TcType
    import InstEnv
    import FamInstEnv
    import PrelNames

    import Id
    import VarSet
    import VarEnv
    import ErrUtils
    import SrcLoc
    import NameEnv
    import NameSet
    import Bag
    import Outputable
    import UniqSupply
    import UniqFM
    import DynFlags
    import StaticFlags
    import FastString
    import Panic
    import Util
    import Annotations
    import BasicTypes( TopLevelFlag )

    import qualified GHC.LanguageExtensions as LangExt

    import Control.Exception
    import Data.IORef
    import Control.Monad
    import Data.Set ( Set )
    import qualified Data.Set as Set

    #ifdef GHCI
    import qualified Data.Map as Map
    #endif

    {-
    ************************************************************************
    *                                                                      *
                            initTc
    *                                                                      *
    ************************************************************************
    -}

    -- | Setup the initial typechecking environment
    initTc :: HscEnv
           -> HscSource
           -> Bool          -- True <=> retain renamed syntax trees
           -> Module
           -> RealSrcSpan
           -> TcM r
           -> IO (Messages, Maybe r)
                    -- Nothing => error thrown by the thing inside
                    -- (error messages should have been printed already)

    initTc hsc_env hsc_src keep_rn_syntax mod loc do_this
     = do { errs_var     <- newIORef (emptyBag, emptyBag) ;
            tvs_var      <- newIORef emptyVarSet ;
            keep_var     <- newIORef emptyNameSet ;
            used_gre_var <- newIORef [] ;
            th_var       <- newIORef False ;
            th_splice_var<- newIORef False ;
            th_locs_var  <- newIORef Set.empty ;
            infer_var    <- newIORef (True, emptyBag) ;
            lie_var      <- newIORef emptyWC ;
            dfun_n_var   <- newIORef emptyOccSet ;
            type_env_var <- case hsc_type_env_var hsc_env of {
                               Just (_mod, te_var) -> return te_var ;
                               Nothing             -> newIORef emptyNameEnv } ;

            dependent_files_var <- newIORef [] ;
            static_wc_var       <- newIORef emptyWC ;
    #ifdef GHCI
            th_topdecls_var      <- newIORef [] ;
            th_topnames_var      <- newIORef emptyNameSet ;
            th_modfinalizers_var <- newIORef [] ;
            th_state_var         <- newIORef Map.empty ;
            th_remote_state_var  <- newIORef Nothing ;
    #endif /* GHCI */
            let {
                 dflags = hsc_dflags hsc_env ;

                 maybe_rn_syntax :: forall a. a -> Maybe a ;
                 maybe_rn_syntax empty_val
                    | keep_rn_syntax = Just empty_val
                    | otherwise      = Nothing ;

                 gbl_env = TcGblEnv {
    #ifdef GHCI
                    tcg_th_topdecls      = th_topdecls_var,
                    tcg_th_topnames      = th_topnames_var,
                    tcg_th_modfinalizers = th_modfinalizers_var,
                    tcg_th_state         = th_state_var,
                    tcg_th_remote_state  = th_remote_state_var,
    #endif /* GHCI */

                    tcg_mod            = mod,
                    tcg_src            = hsc_src,
                    tcg_sig_of         = getSigOf dflags (moduleName mod),
                    tcg_impl_rdr_env   = Nothing,
                    tcg_rdr_env        = emptyGlobalRdrEnv,
                    tcg_fix_env        = emptyNameEnv,
                    tcg_field_env      = emptyNameEnv,
                    tcg_default        = if moduleUnitId mod == primUnitId
                                         then Just []  -- See Note [Default types]
                                         else Nothing,
                    tcg_type_env       = emptyNameEnv,
                    tcg_type_env_var   = type_env_var,
                    tcg_inst_env       = emptyInstEnv,
                    tcg_fam_inst_env   = emptyFamInstEnv,
                    tcg_ann_env        = emptyAnnEnv,
                    tcg_th_used        = th_var,
                    tcg_th_splice_used = th_splice_var,
                    tcg_th_top_level_locs
                                       = th_locs_var,
                    tcg_exports        = [],
                    tcg_imports        = emptyImportAvails,
                    tcg_used_gres     = used_gre_var,
                    tcg_dus            = emptyDUs,

                    tcg_rn_imports     = [],
                    tcg_rn_exports     = maybe_rn_syntax [],
                    tcg_rn_decls       = maybe_rn_syntax emptyRnGroup,
                    tcg_tr_module      = Nothing,
                    tcg_binds          = emptyLHsBinds,
                    tcg_imp_specs      = [],
                    tcg_sigs           = emptyNameSet,
                    tcg_ev_binds       = emptyBag,
                    tcg_warns          = NoWarnings,
                    tcg_anns           = [],
                    tcg_tcs            = [],
                    tcg_insts          = [],
                    tcg_fam_insts      = [],
                    tcg_rules          = [],
                    tcg_fords          = [],
                    tcg_vects          = [],
                    tcg_patsyns        = [],
                    tcg_dfun_n         = dfun_n_var,
                    tcg_keep           = keep_var,
                    tcg_doc_hdr        = Nothing,
                    tcg_hpc            = False,
                    tcg_main           = Nothing,
                    tcg_self_boot      = NoSelfBoot,
                    tcg_safeInfer      = infer_var,
                    tcg_dependent_files = dependent_files_var,
                    tcg_tc_plugins     = [],
                    tcg_static_wc      = static_wc_var
                 } ;
                 lcl_env = TcLclEnv {
                    tcl_errs       = errs_var,
                    tcl_loc        = loc,     -- Should be over-ridden very soon!
                    tcl_ctxt       = [],
                    tcl_rdr        = emptyLocalRdrEnv,
                    tcl_th_ctxt    = topStage,
                    tcl_th_bndrs   = emptyNameEnv,
                    tcl_arrow_ctxt = NoArrowCtxt,
                    tcl_env        = emptyNameEnv,
                    tcl_bndrs      = [],
                    tcl_tidy       = emptyTidyEnv,
                    tcl_tyvars     = tvs_var,
                    tcl_lie        = lie_var,
                    tcl_tclvl      = topTcLevel
                 } ;
            } ;

            -- OK, here's the business end!
            maybe_res <- initTcRnIf 'a' hsc_env gbl_env lcl_env $
                         do { r <- tryM do_this
                            ; case r of
                              Right res -> return (Just res)
                              Left _    -> return Nothing } ;

            -- Check for unsolved constraints
            lie <- readIORef lie_var ;
            if isEmptyWC lie
               then return ()
               else pprPanic "initTc: unsolved constraints" (ppr lie) ;

            -- Collect any error messages
            msgs <- readIORef errs_var ;

            let { final_res | errorsFound dflags msgs = Nothing
                            | otherwise               = maybe_res } ;

            return (msgs, final_res)
        }

    initTcInteractive :: HscEnv -> TcM a -> IO (Messages, Maybe a)
    -- Initialise the type checker monad for use in GHCi
    initTcInteractive hsc_env thing_inside
      = initTc hsc_env HsSrcFile False
               (icInteractiveModule (hsc_IC hsc_env))
               (realSrcLocSpan interactive_src_loc)
               thing_inside
      where
        interactive_src_loc = mkRealSrcLoc (fsLit "<interactive>") 1 1

    initTcForLookup :: HscEnv -> TcM a -> IO a
    -- The thing_inside is just going to look up something
    -- in the environment, so we don't need much setup
    initTcForLookup hsc_env thing_inside
      = do { (msgs, m) <- initTcInteractive hsc_env thing_inside
           ; case m of
                 Nothing -> throwIO $ mkSrcErr $ snd msgs
                 Just x -> return x }

    {- Note [Default types]
    ~~~~~~~~~~~~~~~~~~~~~~~
    The Integer type is simply not available in package ghc-prim (it is
    declared in integer-gmp).  So we set the defaulting types to (Just
    []), meaning there are no default types, rather then Nothing, which
    means "use the default default types of Integer, Double".

    If you don't do this, attempted defaulting in package ghc-prim causes
    an actual crash (attempting to look up the Integer type).

    ************************************************************************
    *                                                                      *
                    Initialisation
    *                                                                      *
    ************************************************************************
    -}

    initTcRnIf :: Char              -- Tag for unique supply
               -> HscEnv
               -> gbl -> lcl
               -> TcRnIf gbl lcl a
               -> IO a
    initTcRnIf uniq_tag hsc_env gbl_env lcl_env thing_inside
       = do { us     <- mkSplitUniqSupply uniq_tag ;
            ; us_var <- newIORef us ;

            ; let { env = Env { env_top = hsc_env,
                                env_us  = us_var,
                                env_gbl = gbl_env,
                                env_lcl = lcl_env} }

            ; runIOEnv env thing_inside
            }

    {-
    ************************************************************************
    *                                                                      *
                    Simple accessors
    *                                                                      *
    ************************************************************************
    -}

    discardResult :: TcM a -> TcM ()
    discardResult a = a >> return ()

    getTopEnv :: TcRnIf gbl lcl HscEnv
    getTopEnv = do { env <- getEnv; return (env_top env) }

    updTopEnv :: (HscEnv -> HscEnv) -> TcRnIf gbl lcl a -> TcRnIf gbl lcl a
    updTopEnv upd = updEnv (\ env@(Env { env_top = top }) ->
                              env { env_top = upd top })

    getGblEnv :: TcRnIf gbl lcl gbl
    getGblEnv = do { env <- getEnv; return (env_gbl env) }

    updGblEnv :: (gbl -> gbl) -> TcRnIf gbl lcl a -> TcRnIf gbl lcl a
    updGblEnv upd = updEnv (\ env@(Env { env_gbl = gbl }) ->
                              env { env_gbl = upd gbl })

    setGblEnv :: gbl -> TcRnIf gbl lcl a -> TcRnIf gbl lcl a
    setGblEnv gbl_env = updEnv (\ env -> env { env_gbl = gbl_env })

    getLclEnv :: TcRnIf gbl lcl lcl
    getLclEnv = do { env <- getEnv; return (env_lcl env) }

    updLclEnv :: (lcl -> lcl) -> TcRnIf gbl lcl a -> TcRnIf gbl lcl a
    updLclEnv upd = updEnv (\ env@(Env { env_lcl = lcl }) ->
                              env { env_lcl = upd lcl })

    setLclEnv :: lcl' -> TcRnIf gbl lcl' a -> TcRnIf gbl lcl a
    setLclEnv lcl_env = updEnv (\ env -> env { env_lcl = lcl_env })

    getEnvs :: TcRnIf gbl lcl (gbl, lcl)
    getEnvs = do { env <- getEnv; return (env_gbl env, env_lcl env) }

    setEnvs :: (gbl', lcl') -> TcRnIf gbl' lcl' a -> TcRnIf gbl lcl a
    setEnvs (gbl_env, lcl_env) = updEnv (\ env -> env { env_gbl = gbl_env, env_lcl = lcl_env })

    -- Command-line flags

    xoptM :: LangExt.Extension -> TcRnIf gbl lcl Bool
    xoptM flag = do { dflags <- getDynFlags; return (xopt flag dflags) }

    doptM :: DumpFlag -> TcRnIf gbl lcl Bool
    doptM flag = do { dflags <- getDynFlags; return (dopt flag dflags) }

    goptM :: GeneralFlag -> TcRnIf gbl lcl Bool
    goptM flag = do { dflags <- getDynFlags; return (gopt flag dflags) }

    woptM :: WarningFlag -> TcRnIf gbl lcl Bool
    woptM flag = do { dflags <- getDynFlags; return (wopt flag dflags) }

    setXOptM :: LangExt.Extension -> TcRnIf gbl lcl a -> TcRnIf gbl lcl a
    setXOptM flag =
      updTopEnv (\top -> top { hsc_dflags = xopt_set (hsc_dflags top) flag})

    unsetGOptM :: GeneralFlag -> TcRnIf gbl lcl a -> TcRnIf gbl lcl a
    unsetGOptM flag =
      updTopEnv (\top -> top { hsc_dflags = gopt_unset (hsc_dflags top) flag})

    unsetWOptM :: WarningFlag -> TcRnIf gbl lcl a -> TcRnIf gbl lcl a
    unsetWOptM flag =
      updTopEnv (\top -> top { hsc_dflags = wopt_unset (hsc_dflags top) flag})

    -- | Do it flag is true
    whenDOptM :: DumpFlag -> TcRnIf gbl lcl () -> TcRnIf gbl lcl ()
    whenDOptM flag thing_inside = do b <- doptM flag
                                     when b thing_inside

    whenGOptM :: GeneralFlag -> TcRnIf gbl lcl () -> TcRnIf gbl lcl ()
    whenGOptM flag thing_inside = do b <- goptM flag
                                     when b thing_inside

    whenWOptM :: WarningFlag -> TcRnIf gbl lcl () -> TcRnIf gbl lcl ()
    whenWOptM flag thing_inside = do b <- woptM flag
                                     when b thing_inside

    whenXOptM :: LangExt.Extension -> TcRnIf gbl lcl () -> TcRnIf gbl lcl ()
    whenXOptM flag thing_inside = do b <- xoptM flag
                                     when b thing_inside

    getGhcMode :: TcRnIf gbl lcl GhcMode
    getGhcMode = do { env <- getTopEnv; return (ghcMode (hsc_dflags env)) }

    withDoDynamicToo :: TcRnIf gbl lcl a -> TcRnIf gbl lcl a
    withDoDynamicToo =
      updTopEnv (\top@(HscEnv { hsc_dflags = dflags }) ->
                  top { hsc_dflags = dynamicTooMkDynamicDynFlags dflags })

    getEpsVar :: TcRnIf gbl lcl (TcRef ExternalPackageState)
    getEpsVar = do { env <- getTopEnv; return (hsc_EPS env) }

    getEps :: TcRnIf gbl lcl ExternalPackageState
    getEps = do { env <- getTopEnv; readMutVar (hsc_EPS env) }

    -- | Update the external package state.  Returns the second result of the
    -- modifier function.
    --
    -- This is an atomic operation and forces evaluation of the modified EPS in
    -- order to avoid space leaks.
    updateEps :: (ExternalPackageState -> (ExternalPackageState, a))
              -> TcRnIf gbl lcl a
    updateEps upd_fn = do
      traceIf (text "updating EPS")
      eps_var <- getEpsVar
      atomicUpdMutVar' eps_var upd_fn

    -- | Update the external package state.
    --
    -- This is an atomic operation and forces evaluation of the modified EPS in
    -- order to avoid space leaks.
    updateEps_ :: (ExternalPackageState -> ExternalPackageState)
               -> TcRnIf gbl lcl ()
    updateEps_ upd_fn = do
      traceIf (text "updating EPS_")
      eps_var <- getEpsVar
      atomicUpdMutVar' eps_var (\eps -> (upd_fn eps, ()))

    getHpt :: TcRnIf gbl lcl HomePackageTable
    getHpt = do { env <- getTopEnv; return (hsc_HPT env) }

    getEpsAndHpt :: TcRnIf gbl lcl (ExternalPackageState, HomePackageTable)
    getEpsAndHpt = do { env <- getTopEnv; eps <- readMutVar (hsc_EPS env)
                      ; return (eps, hsc_HPT env) }

    {-
    ************************************************************************
    *                                                                      *
                    Arrow scopes
    *                                                                      *
    ************************************************************************
    -}

    newArrowScope :: TcM a -> TcM a
    newArrowScope
      = updLclEnv $ \env -> env { tcl_arrow_ctxt = ArrowCtxt (tcl_rdr env) (tcl_lie env) }

    -- Return to the stored environment (from the enclosing proc)
    escapeArrowScope :: TcM a -> TcM a
    escapeArrowScope
      = updLclEnv $ \ env ->
        case tcl_arrow_ctxt env of
          NoArrowCtxt       -> env
          ArrowCtxt rdr_env lie -> env { tcl_arrow_ctxt = NoArrowCtxt
                                       , tcl_lie = lie
                                       , tcl_rdr = rdr_env }

    {-
    ************************************************************************
    *                                                                      *
                    Unique supply
    *                                                                      *
    ************************************************************************
    -}

    newUnique :: TcRnIf gbl lcl Unique
    newUnique
     = do { env <- getEnv ;
            let { u_var = env_us env } ;
            us <- readMutVar u_var ;
            case takeUniqFromSupply us of { (uniq, us') -> do {
            writeMutVar u_var us' ;
            return $! uniq }}}
       -- NOTE 1: we strictly split the supply, to avoid the possibility of leaving
       -- a chain of unevaluated supplies behind.
       -- NOTE 2: we use the uniq in the supply from the MutVar directly, and
       -- throw away one half of the new split supply.  This is safe because this
       -- is the only place we use that unique.  Using the other half of the split
       -- supply is safer, but slower.

    newUniqueSupply :: TcRnIf gbl lcl UniqSupply
    newUniqueSupply
     = do { env <- getEnv ;
            let { u_var = env_us env } ;
            us <- readMutVar u_var ;
            case splitUniqSupply us of { (us1,us2) -> do {
            writeMutVar u_var us1 ;
            return us2 }}}

    newLocalName :: Name -> TcM Name
    newLocalName name = newName (nameOccName name)

    newName :: OccName -> TcM Name
    newName occ
      = do { uniq <- newUnique
           ; loc  <- getSrcSpanM
           ; return (mkInternalName uniq occ loc) }

    newSysName :: OccName -> TcRnIf gbl lcl Name
    newSysName occ
      = do { uniq <- newUnique
           ; return (mkSystemName uniq occ) }

    newSysLocalId :: FastString -> TcType -> TcRnIf gbl lcl TcId
    newSysLocalId fs ty
      = do  { u <- newUnique
            ; return (mkSysLocalOrCoVar fs u ty) }

    newSysLocalIds :: FastString -> [TcType] -> TcRnIf gbl lcl [TcId]
    newSysLocalIds fs tys
      = do  { us <- newUniqueSupply
            ; return (zipWith (mkSysLocalOrCoVar fs) (uniqsFromSupply us) tys) }

    instance MonadUnique (IOEnv (Env gbl lcl)) where
            getUniqueM = newUnique
            getUniqueSupplyM = newUniqueSupply

    {-
    ************************************************************************
    *                                                                      *
                    Accessing input/output
    *                                                                      *
    ************************************************************************
    -}

    newTcRef :: a -> TcRnIf gbl lcl (TcRef a)
    newTcRef = newMutVar

    readTcRef :: TcRef a -> TcRnIf gbl lcl a
    readTcRef = readMutVar

    writeTcRef :: TcRef a -> a -> TcRnIf gbl lcl ()
    writeTcRef = writeMutVar

    updTcRef :: TcRef a -> (a -> a) -> TcRnIf gbl lcl ()
    -- Returns ()
    updTcRef ref fn = liftIO $ do { old <- readIORef ref
                                  ; writeIORef ref (fn old) }

    updTcRefX :: TcRef a -> (a -> a) -> TcRnIf gbl lcl a
    -- Returns previous value
    updTcRefX ref fn = liftIO $ do { old <- readIORef ref
                                  ; writeIORef ref (fn old)
                                  ; return old }

    {-
    ************************************************************************
    *                                                                      *
                    Debugging
    *                                                                      *
    ************************************************************************
    -}

    traceTc :: String -> SDoc -> TcRn ()
    traceTc herald doc = traceTcN 1 (hang (text herald) 2 doc)

    -- | Typechecker trace
    traceTcN :: Int -> SDoc -> TcRn ()
    traceTcN level doc
        = do dflags <- getDynFlags
             when (level <= traceLevel dflags && not opt_NoDebugOutput) $
                 traceOptTcRn Opt_D_dump_tc_trace doc

    traceRn :: SDoc -> TcRn ()
    traceRn = traceOptTcRn Opt_D_dump_rn_trace -- Renamer Trace

    -- | Output a doc if the given 'DumpFlag' is set.
    --
    -- By default this logs to stdout
    -- However, if the `-ddump-to-file` flag is set,
    -- then this will dump output to a file
    --
    -- Just a wrapper for 'dumpSDoc'
    traceOptTcRn :: DumpFlag -> SDoc -> TcRn ()
    traceOptTcRn flag doc
      = do { dflags <- getDynFlags
           ; when (dopt flag dflags) (traceTcRn flag doc)
        }

    traceTcRn :: DumpFlag -> SDoc -> TcRn ()
    -- ^ Unconditionally dump some trace output
    --
    -- The DumpFlag is used only to set the output filename
    -- for --dump-to-file, not to decide whether or not to output
    -- That part is done by the caller
    traceTcRn flag doc
      = do { real_doc <- prettyDoc doc
           ; dflags   <- getDynFlags
           ; printer  <- getPrintUnqualified dflags
           ; liftIO $ dumpSDoc dflags printer flag "" real_doc  }
      where
        -- Add current location if opt_PprStyle_Debug
        prettyDoc :: SDoc -> TcRn SDoc
        prettyDoc doc = if opt_PprStyle_Debug
           then do { loc  <- getSrcSpanM; return $ mkLocMessage SevOutput loc doc }
           else return doc -- The full location is usually way too much

    getPrintUnqualified :: DynFlags -> TcRn PrintUnqualified
    getPrintUnqualified dflags
      = do { rdr_env <- getGlobalRdrEnv
           ; return $ mkPrintUnqualified dflags rdr_env }

    -- | Like logInfoTcRn, but for user consumption
    printForUserTcRn :: SDoc -> TcRn ()
    printForUserTcRn doc
      = do { dflags <- getDynFlags
           ; printer <- getPrintUnqualified dflags
           ; liftIO (printOutputForUser dflags printer doc) }

    -- | Typechecker debug
    debugDumpTcRn :: SDoc -> TcRn ()
    debugDumpTcRn doc = unless opt_NoDebugOutput $
                        traceOptTcRn Opt_D_dump_tc doc

    {-
    traceIf and traceHiDiffs work in the TcRnIf monad, where no RdrEnv is
    available.  Alas, they behave inconsistently with the other stuff;
    e.g. are unaffected by -dump-to-file.
    -}

    traceIf, traceHiDiffs :: SDoc -> TcRnIf m n ()
    traceIf      = traceOptIf Opt_D_dump_if_trace
    traceHiDiffs = traceOptIf Opt_D_dump_hi_diffs

    traceOptIf :: DumpFlag -> SDoc -> TcRnIf m n ()
    traceOptIf flag doc
      = whenDOptM flag $    -- No RdrEnv available, so qualify everything
        do { dflags <- getDynFlags
           ; liftIO (putMsg dflags doc) }

    {-
    ************************************************************************
    *                                                                      *
                    Typechecker global environment
    *                                                                      *
    ************************************************************************
    -}

    setModule :: Module -> TcRn a -> TcRn a
    setModule mod thing_inside = updGblEnv (\env -> env { tcg_mod = mod }) thing_inside

    getIsGHCi :: TcRn Bool
    getIsGHCi = do { mod <- getModule
                   ; return (isInteractiveModule mod) }

    getGHCiMonad :: TcRn Name
    getGHCiMonad = do { hsc <- getTopEnv; return (ic_monad $ hsc_IC hsc) }

    getInteractivePrintName :: TcRn Name
    getInteractivePrintName = do { hsc <- getTopEnv; return (ic_int_print $ hsc_IC hsc) }

    tcIsHsBootOrSig :: TcRn Bool
    tcIsHsBootOrSig = do { env <- getGblEnv; return (isHsBootOrSig (tcg_src env)) }

    tcSelfBootInfo :: TcRn SelfBootInfo
    tcSelfBootInfo = do { env <- getGblEnv; return (tcg_self_boot env) }

    getGlobalRdrEnv :: TcRn GlobalRdrEnv
    getGlobalRdrEnv = do { env <- getGblEnv; return (tcg_rdr_env env) }

    getRdrEnvs :: TcRn (GlobalRdrEnv, LocalRdrEnv)
    getRdrEnvs = do { (gbl,lcl) <- getEnvs; return (tcg_rdr_env gbl, tcl_rdr lcl) }

    getImports :: TcRn ImportAvails
    getImports = do { env <- getGblEnv; return (tcg_imports env) }

    getFixityEnv :: TcRn FixityEnv
    getFixityEnv = do { env <- getGblEnv; return (tcg_fix_env env) }

    extendFixityEnv :: [(Name,FixItem)] -> RnM a -> RnM a
    extendFixityEnv new_bit
      = updGblEnv (\env@(TcGblEnv { tcg_fix_env = old_fix_env }) ->
                    env {tcg_fix_env = extendNameEnvList old_fix_env new_bit})

    getRecFieldEnv :: TcRn RecFieldEnv
    getRecFieldEnv = do { env <- getGblEnv; return (tcg_field_env env) }

    getDeclaredDefaultTys :: TcRn (Maybe [Type])
    getDeclaredDefaultTys = do { env <- getGblEnv; return (tcg_default env) }

    addDependentFiles :: [FilePath] -> TcRn ()
    addDependentFiles fs = do
      ref <- fmap tcg_dependent_files getGblEnv
      dep_files <- readTcRef ref
      writeTcRef ref (fs ++ dep_files)

    {-
    ************************************************************************
    *                                                                      *
                    Error management
    *                                                                      *
    ************************************************************************
    -}

    getSrcSpanM :: TcRn SrcSpan
            -- Avoid clash with Name.getSrcLoc
    getSrcSpanM = do { env <- getLclEnv; return (RealSrcSpan (tcl_loc env)) }

    setSrcSpan :: SrcSpan -> TcRn a -> TcRn a
    setSrcSpan (RealSrcSpan real_loc) thing_inside
        = updLclEnv (\env -> env { tcl_loc = real_loc }) thing_inside
    -- Don't overwrite useful info with useless:
    setSrcSpan (UnhelpfulSpan _) thing_inside = thing_inside

    addLocM :: (a -> TcM b) -> Located a -> TcM b
    addLocM fn (L loc a) = setSrcSpan loc $ fn a

    wrapLocM :: (a -> TcM b) -> Located a -> TcM (Located b)
    wrapLocM fn (L loc a) = setSrcSpan loc $ do b <- fn a; return (L loc b)

    wrapLocFstM :: (a -> TcM (b,c)) -> Located a -> TcM (Located b, c)
    wrapLocFstM fn (L loc a) =
      setSrcSpan loc $ do
        (b,c) <- fn a
        return (L loc b, c)

    wrapLocSndM :: (a -> TcM (b,c)) -> Located a -> TcM (b, Located c)
    wrapLocSndM fn (L loc a) =
      setSrcSpan loc $ do
        (b,c) <- fn a
        return (b, L loc c)

    -- Reporting errors

    getErrsVar :: TcRn (TcRef Messages)
    getErrsVar = do { env <- getLclEnv; return (tcl_errs env) }

    setErrsVar :: TcRef Messages -> TcRn a -> TcRn a
    setErrsVar v = updLclEnv (\ env -> env { tcl_errs =  v })

    addErr :: MsgDoc -> TcRn ()
    addErr msg = do { loc <- getSrcSpanM; addErrAt loc msg }

    failWith :: MsgDoc -> TcRn a
    failWith msg = addErr msg >> failM

    failAt :: SrcSpan -> MsgDoc -> TcRn a
    failAt loc msg = addErrAt loc msg >> failM

    addErrAt :: SrcSpan -> MsgDoc -> TcRn ()
    -- addErrAt is mainly (exclusively?) used by the renamer, where
    -- tidying is not an issue, but it's all lazy so the extra
    -- work doesn't matter
    addErrAt loc msg = do { ctxt <- getErrCtxt
                          ; tidy_env <- tcInitTidyEnv
                          ; err_info <- mkErrInfo tidy_env ctxt
                          ; addLongErrAt loc msg err_info }

    addErrs :: [(SrcSpan,MsgDoc)] -> TcRn ()
    addErrs msgs = mapM_ add msgs
                 where
                   add (loc,msg) = addErrAt loc msg

    checkErr :: Bool -> MsgDoc -> TcRn ()
    -- Add the error if the bool is False
    checkErr ok msg = unless ok (addErr msg)

    -- | Display a warning if a condition is met.
    warnIf :: WarnReason -> Bool -> MsgDoc -> TcRn ()
    warnIf reason True  msg = addWarn reason msg
    warnIf _      False _   = return ()

    addMessages :: Messages -> TcRn ()
    addMessages msgs1
      = do { errs_var <- getErrsVar ;
             msgs0 <- readTcRef errs_var ;
             writeTcRef errs_var (unionMessages msgs0 msgs1) }

    discardWarnings :: TcRn a -> TcRn a
    -- Ignore warnings inside the thing inside;
    -- used to ignore-unused-variable warnings inside derived code
    discardWarnings thing_inside
      = do  { errs_var <- getErrsVar
            ; (old_warns, _) <- readTcRef errs_var

            ; result <- thing_inside

            -- Revert warnings to old_warns
            ; (_new_warns, new_errs) <- readTcRef errs_var
            ; writeTcRef errs_var (old_warns, new_errs)

            ; return result }

    {-
    ************************************************************************
    *                                                                      *
            Shared error message stuff: renamer and typechecker
    *                                                                      *
    ************************************************************************
    -}

    mkLongErrAt :: SrcSpan -> MsgDoc -> MsgDoc -> TcRn ErrMsg
    mkLongErrAt loc msg extra
      = do { dflags <- getDynFlags ;
             printer <- getPrintUnqualified dflags ;
             return $ mkLongErrMsg dflags loc printer msg extra }

    mkErrDocAt :: SrcSpan -> ErrDoc -> TcRn ErrMsg
    mkErrDocAt loc errDoc
      = do { dflags <- getDynFlags ;
             printer <- getPrintUnqualified dflags ;
             return $ mkErrDoc dflags loc printer errDoc }

    addLongErrAt :: SrcSpan -> MsgDoc -> MsgDoc -> TcRn ()
    addLongErrAt loc msg extra = mkLongErrAt loc msg extra >>= reportError

    reportErrors :: [ErrMsg] -> TcM ()
    reportErrors = mapM_ reportError

    reportError :: ErrMsg -> TcRn ()
    reportError err
      = do { traceTc "Adding error:" (pprLocErrMsg err) ;
             errs_var <- getErrsVar ;
             (warns, errs) <- readTcRef errs_var ;
             writeTcRef errs_var (warns, errs `snocBag` err) }

    reportWarning :: WarnReason -> ErrMsg -> TcRn ()
    reportWarning reason err
      = do { let warn = makeIntoWarning reason err
                        -- 'err' was built by mkLongErrMsg or something like that,
                        -- so it's of error severity.  For a warning we downgrade
                        -- its severity to SevWarning

           ; traceTc "Adding warning:" (pprLocErrMsg warn)
           ; errs_var <- getErrsVar
           ; (warns, errs) <- readTcRef errs_var
           ; writeTcRef errs_var (warns `snocBag` warn, errs) }

    try_m :: TcRn r -> TcRn (Either IOEnvFailure r)
    -- Does tryM, with a debug-trace on failure
    try_m thing
      = do { mb_r <- tryM thing ;
             case mb_r of
                 Left exn -> do { traceTc "tryTc/recoverM recovering from" $
                                          text (showException exn)
                                ; return mb_r }
                 Right _  -> return mb_r }

    -----------------------
    recoverM :: TcRn r      -- Recovery action; do this if the main one fails
             -> TcRn r      -- Main action: do this first
             -> TcRn r
    -- Errors in 'thing' are retained
    recoverM recover thing
      = do { mb_res <- try_m thing ;
             case mb_res of
               Left _    -> recover
               Right res -> return res }

    -----------------------
    mapAndRecoverM :: (a -> TcRn b) -> [a] -> TcRn [b]
    -- Drop elements of the input that fail, so the result
    -- list can be shorter than the argument list
    mapAndRecoverM _ []     = return []
    mapAndRecoverM f (x:xs) = do { mb_r <- try_m (f x)
                                 ; rs <- mapAndRecoverM f xs
                                 ; return (case mb_r of
                                              Left _  -> rs
                                              Right r -> r:rs) }

    -- | Succeeds if applying the argument to all members of the lists succeeds,
    --   but nevertheless runs it on all arguments, to collect all errors.
    mapAndReportM :: (a -> TcRn b) -> [a] -> TcRn [b]
    mapAndReportM f xs = checkNoErrs (mapAndRecoverM f xs)

    -----------------------
    tryTc :: TcRn a -> TcRn (Messages, Maybe a)
    -- (tryTc m) executes m, and returns
    --      Just r,  if m succeeds (returning r)
    --      Nothing, if m fails
    -- It also returns all the errors and warnings accumulated by m
    -- It always succeeds (never raises an exception)
    tryTc m
     = do { errs_var <- newTcRef emptyMessages ;
            res  <- try_m (setErrsVar errs_var m) ;
            msgs <- readTcRef errs_var ;
            return (msgs, case res of
                                Left _  -> Nothing
                                Right val -> Just val)
            -- The exception is always the IOEnv built-in
            -- in exception; see IOEnv.failM
       }

    -- (askNoErrs m) runs m
    -- If m fails, (askNoErrs m) fails
    -- If m succeeds with result r, (askNoErrs m) succeeds with result (r, b),
    --  where b is True iff m generated no errors
    -- Regardless of success or failure, any errors generated by m are propagated
    askNoErrs :: TcRn a -> TcRn (a, Bool)
    askNoErrs m
     = do { errs_var <- newTcRef emptyMessages
          ; res  <- setErrsVar errs_var m
          ; (warns, errs) <- readTcRef errs_var
          ; addMessages (warns, errs)
          ; return (res, isEmptyBag errs) }

    discardErrs :: TcRn a -> TcRn a
    -- (discardErrs m) runs m,
    --   discarding all error messages and warnings generated by m
    -- If m fails, discardErrs fails, and vice versa
    discardErrs m
     = do { errs_var <- newTcRef emptyMessages
          ; setErrsVar errs_var m }

    -----------------------
    tryTcErrs :: TcRn a -> TcRn (Messages, Maybe a)
    -- Run the thing, returning
    --      Just r,  if m succceeds with no error messages
    --      Nothing, if m fails, or if it succeeds but has error messages
    -- Either way, the messages are returned;
    -- even in the Just case there might be warnings
    tryTcErrs thing
      = do  { (msgs, res) <- tryTc thing
            ; dflags <- getDynFlags
            ; let errs_found = errorsFound dflags msgs
            ; return (msgs, case res of
                              Nothing -> Nothing
                              Just val | errs_found -> Nothing
                                       | otherwise  -> Just val)
            }

    -----------------------
    tryTcLIE :: TcM a -> TcM (Messages, Maybe a)
    -- Just like tryTcErrs, except that it ensures that the LIE
    -- for the thing is propagated only if there are no errors
    -- Hence it's restricted to the type-check monad
    tryTcLIE thing_inside
      = do  { ((msgs, mb_res), lie) <- captureConstraints (tryTcErrs thing_inside) ;
            ; case mb_res of
                Nothing  -> return (msgs, Nothing)
                Just val -> do { emitConstraints lie; return (msgs, Just val) }
            }

    -----------------------
    tryTcLIE_ :: TcM r -> TcM r -> TcM r
    -- (tryTcLIE_ r m) tries m;
    --      if m succeeds with no error messages, it's the answer
    --      otherwise tryTcLIE_ drops everything from m and tries r instead.
    tryTcLIE_ recover main
      = do  { (msgs, mb_res) <- tryTcLIE main
            ; case mb_res of
                 Just val -> do { addMessages msgs  -- There might be warnings
                                 ; return val }
                 Nothing  -> recover                -- Discard all msgs
            }

    -----------------------
    checkNoErrs :: TcM r -> TcM r
    -- (checkNoErrs m) succeeds iff m succeeds and generates no errors
    -- If m fails then (checkNoErrsTc m) fails.
    -- If m succeeds, it checks whether m generated any errors messages
    --      (it might have recovered internally)
    --      If so, it fails too.
    -- Regardless, any errors generated by m are propagated to the enclosing context.
    checkNoErrs main
      = do  { (msgs, mb_res) <- tryTcLIE main
            ; addMessages msgs
            ; case mb_res of
                Nothing  -> failM
                Just val -> return val
            }

    whenNoErrs :: TcM () -> TcM ()
    whenNoErrs thing = ifErrsM (return ()) thing

    ifErrsM :: TcRn r -> TcRn r -> TcRn r
    --      ifErrsM bale_out normal
    -- does 'bale_out' if there are errors in errors collection
    -- otherwise does 'normal'
    ifErrsM bale_out normal
     = do { errs_var <- getErrsVar ;
            msgs <- readTcRef errs_var ;
            dflags <- getDynFlags ;
            if errorsFound dflags msgs then
               bale_out
            else
               normal }

    failIfErrsM :: TcRn ()
    -- Useful to avoid error cascades
    failIfErrsM = ifErrsM failM (return ())

    #ifdef GHCI
    checkTH :: a -> String -> TcRn ()
    checkTH _ _ = return () -- OK
    #else
    checkTH :: Outputable a => a -> String -> TcRn ()
    checkTH e what = failTH e what  -- Raise an error in a stage-1 compiler
    #endif

    failTH :: Outputable a => a -> String -> TcRn x
    failTH e what  -- Raise an error in a stage-1 compiler
      = failWithTc (vcat [ hang (char 'A' <+> text what
                                 <+> text "requires GHC with interpreter support:")
                              2 (ppr e)
                         , text "Perhaps you are using a stage-1 compiler?" ])

    {-
    ************************************************************************
    *                                                                      *
            Context management for the type checker
    *                                                                      *
    ************************************************************************
    -}

    getErrCtxt :: TcM [ErrCtxt]
    getErrCtxt = do { env <- getLclEnv; return (tcl_ctxt env) }

    setErrCtxt :: [ErrCtxt] -> TcM a -> TcM a
    setErrCtxt ctxt = updLclEnv (\ env -> env { tcl_ctxt = ctxt })

    -- | Add a fixed message to the error context. This message should not
    -- do any tidying.
    addErrCtxt :: MsgDoc -> TcM a -> TcM a
    addErrCtxt msg = addErrCtxtM (\env -> return (env, msg))

    -- | Add a message to the error context. This message may do tidying.
    addErrCtxtM :: (TidyEnv -> TcM (TidyEnv, MsgDoc)) -> TcM a -> TcM a
    addErrCtxtM ctxt = updCtxt (\ ctxts -> (False, ctxt) : ctxts)

    -- | Add a fixed landmark message to the error context. A landmark
    -- message is always sure to be reported, even if there is a lot of
    -- context. It also doesn't count toward the maximum number of contexts
    -- reported.
    addLandmarkErrCtxt :: MsgDoc -> TcM a -> TcM a
    addLandmarkErrCtxt msg = addLandmarkErrCtxtM (\env -> return (env, msg))

    -- | Variant of 'addLandmarkErrCtxt' that allows for monadic operations
    -- and tidying.
    addLandmarkErrCtxtM :: (TidyEnv -> TcM (TidyEnv, MsgDoc)) -> TcM a -> TcM a
    addLandmarkErrCtxtM ctxt = updCtxt (\ctxts -> (True, ctxt) : ctxts)

    -- Helper function for the above
    updCtxt :: ([ErrCtxt] -> [ErrCtxt]) -> TcM a -> TcM a
    updCtxt upd = updLclEnv (\ env@(TcLclEnv { tcl_ctxt = ctxt }) ->
                               env { tcl_ctxt = upd ctxt })

    popErrCtxt :: TcM a -> TcM a
    popErrCtxt = updCtxt (\ msgs -> case msgs of { [] -> []; (_ : ms) -> ms })

    getCtLocM :: CtOrigin -> Maybe TypeOrKind -> TcM CtLoc
    getCtLocM origin t_or_k
      = do { env <- getLclEnv
           ; return (CtLoc { ctl_origin = origin
                           , ctl_env    = env
                           , ctl_t_or_k = t_or_k
                           , ctl_depth  = initialSubGoalDepth }) }

    setCtLocM :: CtLoc -> TcM a -> TcM a
    -- Set the SrcSpan and error context from the CtLoc
    setCtLocM (CtLoc { ctl_env = lcl }) thing_inside
      = updLclEnv (\env -> env { tcl_loc   = tcl_loc lcl
                               , tcl_bndrs = tcl_bndrs lcl
                               , tcl_ctxt  = tcl_ctxt lcl })
                  thing_inside

    {-
    ************************************************************************
    *                                                                      *
                 Error message generation (type checker)
    *                                                                      *
    ************************************************************************

        The addErrTc functions add an error message, but do not cause failure.
        The 'M' variants pass a TidyEnv that has already been used to
        tidy up the message; we then use it to tidy the context messages
    -}

    addErrTc :: MsgDoc -> TcM ()
    addErrTc err_msg = do { env0 <- tcInitTidyEnv
                          ; addErrTcM (env0, err_msg) }

    addErrsTc :: [MsgDoc] -> TcM ()
    addErrsTc err_msgs = mapM_ addErrTc err_msgs

    addErrTcM :: (TidyEnv, MsgDoc) -> TcM ()
    addErrTcM (tidy_env, err_msg)
      = do { ctxt <- getErrCtxt ;
             loc  <- getSrcSpanM ;
             add_err_tcm tidy_env err_msg loc ctxt }

    -- Return the error message, instead of reporting it straight away
    mkErrTcM :: (TidyEnv, MsgDoc) -> TcM ErrMsg
    mkErrTcM (tidy_env, err_msg)
      = do { ctxt <- getErrCtxt ;
             loc  <- getSrcSpanM ;
             err_info <- mkErrInfo tidy_env ctxt ;
             mkLongErrAt loc err_msg err_info }

    -- The failWith functions add an error message and cause failure

    failWithTc :: MsgDoc -> TcM a               -- Add an error message and fail
    failWithTc err_msg
      = addErrTc err_msg >> failM

    failWithTcM :: (TidyEnv, MsgDoc) -> TcM a   -- Add an error message and fail
    failWithTcM local_and_msg
      = addErrTcM local_and_msg >> failM

    checkTc :: Bool -> MsgDoc -> TcM ()         -- Check that the boolean is true
    checkTc True  _   = return ()
    checkTc False err = failWithTc err

    checkTcM :: Bool -> (TidyEnv, MsgDoc) -> TcM ()
    checkTcM True  _   = return ()
    checkTcM False err = failWithTcM err

    failIfTc :: Bool -> MsgDoc -> TcM ()         -- Check that the boolean is false
    failIfTc False _   = return ()
    failIfTc True  err = failWithTc err

    failIfTcM :: Bool -> (TidyEnv, MsgDoc) -> TcM ()
       -- Check that the boolean is false
    failIfTcM False _   = return ()
    failIfTcM True  err = failWithTcM err

    --         Warnings have no 'M' variant, nor failure

    -- | Display a warning if a condition is met.
    warnTc :: WarnReason -> Bool -> MsgDoc -> TcM ()
    warnTc reason warn_if_true warn_msg
      | warn_if_true = addWarnTc reason warn_msg
      | otherwise    = return ()

    -- | Display a warning if a condition is met.
    warnTcM :: WarnReason -> Bool -> (TidyEnv, MsgDoc) -> TcM ()
    warnTcM reason warn_if_true warn_msg
      | warn_if_true = addWarnTcM reason warn_msg
      | otherwise    = return ()

    -- | Display a warning in the current context.
    addWarnTc :: WarnReason -> MsgDoc -> TcM ()
    addWarnTc reason msg
     = do { env0 <- tcInitTidyEnv ;
          addWarnTcM reason (env0, msg) }

    -- | Display a warning in a given context.
    addWarnTcM :: WarnReason -> (TidyEnv, MsgDoc) -> TcM ()
    addWarnTcM reason (env0, msg)
     = do { ctxt <- getErrCtxt ;
            err_info <- mkErrInfo env0 ctxt ;
            add_warn reason msg err_info }

    -- | Display a warning for the current source location.
    addWarn :: WarnReason -> MsgDoc -> TcRn ()
    addWarn reason msg = add_warn reason msg Outputable.empty

    -- | Display a warning for a given source location.
    addWarnAt :: WarnReason -> SrcSpan -> MsgDoc -> TcRn ()
    addWarnAt reason loc msg = add_warn_at reason loc msg Outputable.empty

    -- | Display a warning, with an optional flag, for the current source
    -- location.
    add_warn :: WarnReason -> MsgDoc -> MsgDoc -> TcRn ()
    add_warn reason msg extra_info
      = do { loc <- getSrcSpanM
           ; add_warn_at reason loc msg extra_info }

    -- | Display a warning, with an optional flag, for a given location.
    add_warn_at :: WarnReason -> SrcSpan -> MsgDoc -> MsgDoc -> TcRn ()
    add_warn_at reason loc msg extra_info
      = do { dflags <- getDynFlags ;
             printer <- getPrintUnqualified dflags ;
             let { warn = mkLongWarnMsg dflags loc printer
                                        msg extra_info } ;
             reportWarning reason warn }

    tcInitTidyEnv :: TcM TidyEnv
    tcInitTidyEnv
      = do  { lcl_env <- getLclEnv
            ; return (tcl_tidy lcl_env) }

    -- | Get a 'TidyEnv' that includes mappings for all vars free in the given
    -- type. Useful when tidying open types.
    tcInitOpenTidyEnv :: TyCoVarSet -> TcM TidyEnv
    tcInitOpenTidyEnv tvs
      = do { env1 <- tcInitTidyEnv
           ; let env2 = tidyFreeTyCoVars env1 tvs
           ; return env2 }

    {-
    -----------------------------------
            Other helper functions
    -}

    add_err_tcm :: TidyEnv -> MsgDoc -> SrcSpan
                -> [ErrCtxt]
                -> TcM ()
    add_err_tcm tidy_env err_msg loc ctxt
     = do { err_info <- mkErrInfo tidy_env ctxt ;
            addLongErrAt loc err_msg err_info }

    mkErrInfo :: TidyEnv -> [ErrCtxt] -> TcM SDoc
    -- Tidy the error info, trimming excessive contexts
    mkErrInfo env ctxts
    --  | opt_PprStyle_Debug     -- In -dppr-debug style the output
    --  = return empty           -- just becomes too voluminous
     | otherwise
     = go 0 env ctxts
     where
       go :: Int -> TidyEnv -> [ErrCtxt] -> TcM SDoc
       go _ _   [] = return empty
       go n env ((is_landmark, ctxt) : ctxts)
         | is_landmark || n < mAX_CONTEXTS -- Too verbose || opt_PprStyle_Debug
         = do { (env', msg) <- ctxt env
              ; let n' = if is_landmark then n else n+1
              ; rest <- go n' env' ctxts
              ; return (msg $$ rest) }
         | otherwise
         = go n env ctxts

    mAX_CONTEXTS :: Int     -- No more than this number of non-landmark contexts
    mAX_CONTEXTS = 3

    -- debugTc is useful for monadic debugging code

    debugTc :: TcM () -> TcM ()
    debugTc thing
     | debugIsOn = thing
     | otherwise = return ()

    {-
    ************************************************************************
    *                                                                      *
                 Type constraints
    *                                                                      *
    ************************************************************************
    -}

    newTcEvBinds :: TcM EvBindsVar
    newTcEvBinds = do { ref <- newTcRef emptyEvBindMap
                      ; uniq <- newUnique
                      ; traceTc "newTcEvBinds" (text "unique =" <+> ppr uniq)
                      ; return (EvBindsVar ref uniq) }

    addTcEvBind :: EvBindsVar -> EvBind -> TcM ()
    -- Add a binding to the TcEvBinds by side effect
    addTcEvBind (EvBindsVar ev_ref u) ev_bind
      = do { traceTc "addTcEvBind" $ ppr u $$
                                     ppr ev_bind
           ; bnds <- readTcRef ev_ref
           ; writeTcRef ev_ref (extendEvBinds bnds ev_bind) }

    getTcEvBinds :: EvBindsVar -> TcM (Bag EvBind)
    getTcEvBinds (EvBindsVar ev_ref _)
      = do { bnds <- readTcRef ev_ref
           ; return (evBindMapBinds bnds) }

    getTcEvBindsMap :: EvBindsVar -> TcM EvBindMap
    getTcEvBindsMap (EvBindsVar ev_ref _)
      = readTcRef ev_ref

    chooseUniqueOccTc :: (OccSet -> OccName) -> TcM OccName
    chooseUniqueOccTc fn =
      do { env <- getGblEnv
         ; let dfun_n_var = tcg_dfun_n env
         ; set <- readTcRef dfun_n_var
         ; let occ = fn set
         ; writeTcRef dfun_n_var (extendOccSet set occ)
         ; return occ }

    getConstraintVar :: TcM (TcRef WantedConstraints)
    getConstraintVar = do { env <- getLclEnv; return (tcl_lie env) }

    setConstraintVar :: TcRef WantedConstraints -> TcM a -> TcM a
    setConstraintVar lie_var = updLclEnv (\ env -> env { tcl_lie = lie_var })

    emitConstraints :: WantedConstraints -> TcM ()
    emitConstraints ct
      = do { lie_var <- getConstraintVar ;
             updTcRef lie_var (`andWC` ct) }

    emitSimple :: Ct -> TcM ()
    emitSimple ct
      = do { lie_var <- getConstraintVar ;
             updTcRef lie_var (`addSimples` unitBag ct) }

    emitSimples :: Cts -> TcM ()
    emitSimples cts
      = do { lie_var <- getConstraintVar ;
             updTcRef lie_var (`addSimples` cts) }

    emitImplication :: Implication -> TcM ()
    emitImplication ct
      = do { lie_var <- getConstraintVar ;
             updTcRef lie_var (`addImplics` unitBag ct) }

    emitImplications :: Bag Implication -> TcM ()
    emitImplications ct
      = unless (isEmptyBag ct) $
        do { lie_var <- getConstraintVar ;
             updTcRef lie_var (`addImplics` ct) }

    emitInsoluble :: Ct -> TcM ()
    emitInsoluble ct
      = do { lie_var <- getConstraintVar ;
             updTcRef lie_var (`addInsols` unitBag ct) ;
             v <- readTcRef lie_var ;
             traceTc "emitInsoluble" (ppr v) }

    -- | Throw out any constraints emitted by the thing_inside
    discardConstraints :: TcM a -> TcM a
    discardConstraints thing_inside = fst <$> captureConstraints thing_inside

    captureConstraints :: TcM a -> TcM (a, WantedConstraints)
    -- (captureConstraints m) runs m, and returns the type constraints it generates
    captureConstraints thing_inside
      = do { lie_var <- newTcRef emptyWC ;
             res <- updLclEnv (\ env -> env { tcl_lie = lie_var })
                              thing_inside ;
             lie <- readTcRef lie_var ;
             return (res, lie) }

    pushLevelAndCaptureConstraints :: TcM a -> TcM (TcLevel, WantedConstraints, a)
    pushLevelAndCaptureConstraints thing_inside
      = do { env <- getLclEnv
           ; lie_var <- newTcRef emptyWC
           ; let tclvl' = pushTcLevel (tcl_tclvl env)
           ; res <- setLclEnv (env { tcl_tclvl = tclvl'
                                   , tcl_lie   = lie_var })
                    thing_inside
           ; lie <- readTcRef lie_var
           ; return (tclvl', lie, res) }

    pushTcLevelM_ :: TcM a -> TcM a
    pushTcLevelM_ x = updLclEnv (\ env -> env { tcl_tclvl = pushTcLevel (tcl_tclvl env) }) x

    pushTcLevelM :: TcM a -> TcM (a, TcLevel)
    pushTcLevelM thing_inside
      = do { env <- getLclEnv
           ; let tclvl' = pushTcLevel (tcl_tclvl env)
           ; res <- setLclEnv (env { tcl_tclvl = tclvl' })
                              thing_inside
           ; return (res, tclvl') }

    getTcLevel :: TcM TcLevel
    getTcLevel = do { env <- getLclEnv
                    ; return (tcl_tclvl env) }

    setTcLevel :: TcLevel -> TcM a -> TcM a
    setTcLevel tclvl thing_inside
      = updLclEnv (\env -> env { tcl_tclvl = tclvl }) thing_inside

    isTouchableTcM :: TcTyVar -> TcM Bool
    isTouchableTcM tv
      = do { env <- getLclEnv
           ; return (isTouchableMetaTyVar (tcl_tclvl env) tv) }

    getLclTypeEnv :: TcM TcTypeEnv
    getLclTypeEnv = do { env <- getLclEnv; return (tcl_env env) }

    setLclTypeEnv :: TcLclEnv -> TcM a -> TcM a
    -- Set the local type envt, but do *not* disturb other fields,
    -- notably the lie_var
    setLclTypeEnv lcl_env thing_inside
      = updLclEnv upd thing_inside
      where
        upd env = env { tcl_env = tcl_env lcl_env,
                        tcl_tyvars = tcl_tyvars lcl_env }

    traceTcConstraints :: String -> TcM ()
    traceTcConstraints msg
      = do { lie_var <- getConstraintVar
           ; lie     <- readTcRef lie_var
           ; traceTc (msg ++ ": LIE:") (ppr lie)
           }

    emitWildCardHoleConstraints :: [(Name, TcTyVar)] -> TcM ()
    emitWildCardHoleConstraints wcs
      = do { ctLoc <- getCtLocM HoleOrigin Nothing
           ; forM_ wcs $ \(name, tv) -> do {
           ; let real_span = case nameSrcSpan name of
                               RealSrcSpan span  -> span
                               UnhelpfulSpan str -> pprPanic "emitWildCardHoleConstraints"
                                                          (ppr name <+> quotes (ftext str))
                   -- Wildcards are defined locally, and so have RealSrcSpans
                 ctLoc' = setCtLocSpan ctLoc real_span
                 ty     = mkTyVarTy tv
                 can    = CHoleCan { cc_ev   = CtDerived { ctev_pred = ty
                                                         , ctev_loc  = ctLoc' }
                                   , cc_hole = TypeHole (occName name) }
           ; emitInsoluble can } }

    {-
    ************************************************************************
    *                                                                      *
                 Template Haskell context
    *                                                                      *
    ************************************************************************
    -}

    recordThUse :: TcM ()
    recordThUse = do { env <- getGblEnv; writeTcRef (tcg_th_used env) True }

    recordThSpliceUse :: TcM ()
    recordThSpliceUse = do { env <- getGblEnv; writeTcRef (tcg_th_splice_used env) True }

    -- | When generating an out-of-scope error message for a variable matching a
    -- binding in a later inter-splice group, the typechecker uses the splice
    -- locations to provide details in the message about the scope of that binding.
    recordTopLevelSpliceLoc :: SrcSpan -> TcM ()
    recordTopLevelSpliceLoc (RealSrcSpan real_loc)
      = do { env <- getGblEnv
           ; let locs_var = tcg_th_top_level_locs env
           ; locs0 <- readTcRef locs_var
           ; writeTcRef locs_var (Set.insert real_loc locs0) }
    recordTopLevelSpliceLoc (UnhelpfulSpan _) = return ()

    getTopLevelSpliceLocs :: TcM (Set RealSrcSpan)
    getTopLevelSpliceLocs
      = do { env <- getGblEnv
           ; readTcRef (tcg_th_top_level_locs env) }

    keepAlive :: Name -> TcRn ()     -- Record the name in the keep-alive set
    keepAlive name
      = do { env <- getGblEnv
           ; traceRn (text "keep alive" <+> ppr name)
           ; updTcRef (tcg_keep env) (`extendNameSet` name) }

    getStage :: TcM ThStage
    getStage = do { env <- getLclEnv; return (tcl_th_ctxt env) }

    getStageAndBindLevel :: Name -> TcRn (Maybe (TopLevelFlag, ThLevel, ThStage))
    getStageAndBindLevel name
      = do { env <- getLclEnv;
           ; case lookupNameEnv (tcl_th_bndrs env) name of
               Nothing                  -> return Nothing
               Just (top_lvl, bind_lvl) -> return (Just (top_lvl, bind_lvl, tcl_th_ctxt env)) }

    setStage :: ThStage -> TcM a -> TcRn a
    setStage s = updLclEnv (\ env -> env { tcl_th_ctxt = s })

    {-
    ************************************************************************
    *                                                                      *
                 Safe Haskell context
    *                                                                      *
    ************************************************************************
    -}

    -- | Mark that safe inference has failed
    -- See Note [Safe Haskell Overlapping Instances Implementation]
    -- although this is used for more than just that failure case.
    recordUnsafeInfer :: WarningMessages -> TcM ()
    recordUnsafeInfer warns =
        getGblEnv >>= \env -> writeTcRef (tcg_safeInfer env) (False, warns)

    -- | Figure out the final correct safe haskell mode
    finalSafeMode :: DynFlags -> TcGblEnv -> IO SafeHaskellMode
    finalSafeMode dflags tcg_env = do
        safeInf <- fst <$> readIORef (tcg_safeInfer tcg_env)
        return $ case safeHaskell dflags of
            Sf_None | safeInferOn dflags && safeInf -> Sf_Safe
                    | otherwise                     -> Sf_None
            s -> s

    -- | Switch instances to safe instances if we're in Safe mode.
    fixSafeInstances :: SafeHaskellMode -> [ClsInst] -> [ClsInst]
    fixSafeInstances sfMode | sfMode /= Sf_Safe = id
    fixSafeInstances _ = map fixSafe
      where fixSafe inst = let new_flag = (is_flag inst) { isSafeOverlap = True }
                           in inst { is_flag = new_flag }

    {-
    ************************************************************************
    *                                                                      *
                 Stuff for the renamer's local env
    *                                                                      *
    ************************************************************************
    -}

    getLocalRdrEnv :: RnM LocalRdrEnv
    getLocalRdrEnv = do { env <- getLclEnv; return (tcl_rdr env) }

    setLocalRdrEnv :: LocalRdrEnv -> RnM a -> RnM a
    setLocalRdrEnv rdr_env thing_inside
      = updLclEnv (\env -> env {tcl_rdr = rdr_env}) thing_inside

    {-
    ************************************************************************
    *                                                                      *
                 Stuff for interface decls
    *                                                                      *
    ************************************************************************
    -}

    mkIfLclEnv :: Module -> SDoc -> IfLclEnv
    mkIfLclEnv mod loc = IfLclEnv { if_mod     = mod,
                                    if_loc     = loc,
                                    if_tv_env  = emptyUFM,
                                    if_id_env  = emptyUFM }

    -- | Run an 'IfG' (top-level interface monad) computation inside an existing
    -- 'TcRn' (typecheck-renaming monad) computation by initializing an 'IfGblEnv'
    -- based on 'TcGblEnv'.
    initIfaceTcRn :: IfG a -> TcRn a
    initIfaceTcRn thing_inside
      = do  { tcg_env <- getGblEnv
            ; let { if_env = IfGblEnv {
                                if_rec_types = Just (tcg_mod tcg_env, get_type_env)
                             }
                  ; get_type_env = readTcRef (tcg_type_env_var tcg_env) }
            ; setEnvs (if_env, ()) thing_inside }

    initIfaceCheck :: HscEnv -> IfG a -> IO a
    -- Used when checking the up-to-date-ness of the old Iface
    -- Initialise the environment with no useful info at all
    initIfaceCheck hsc_env do_this
     = do let rec_types = case hsc_type_env_var hsc_env of
                             Just (mod,var) -> Just (mod, readTcRef var)
                             Nothing        -> Nothing
              gbl_env = IfGblEnv { if_rec_types = rec_types }
          initTcRnIf 'i' hsc_env gbl_env () do_this

    initIfaceTc :: ModIface
                -> (TcRef TypeEnv -> IfL a) -> TcRnIf gbl lcl a
    -- Used when type-checking checking an up-to-date interface file
    -- No type envt from the current module, but we do know the module dependencies
    initIfaceTc iface do_this
     = do   { tc_env_var <- newTcRef emptyTypeEnv
            ; let { gbl_env = IfGblEnv {
                                if_rec_types = Just (mod, readTcRef tc_env_var)
                              } ;
                  ; if_lenv = mkIfLclEnv mod doc
               }
            ; setEnvs (gbl_env, if_lenv) (do_this tc_env_var)
        }
      where
        mod = mi_module iface
        doc = text "The interface for" <+> quotes (ppr mod)

    initIfaceLcl :: Module -> SDoc -> IfL a -> IfM lcl a
    initIfaceLcl mod loc_doc thing_inside
      = setLclEnv (mkIfLclEnv mod loc_doc) thing_inside

    getIfModule :: IfL Module
    getIfModule = do { env <- getLclEnv; return (if_mod env) }

    --------------------
    failIfM :: MsgDoc -> IfL a
    -- The Iface monad doesn't have a place to accumulate errors, so we
    -- just fall over fast if one happens; it "shouldnt happen".
    -- We use IfL here so that we can get context info out of the local env
    failIfM msg
      = do  { env <- getLclEnv
            ; let full_msg = (if_loc env <> colon) $$ nest 2 msg
            ; dflags <- getDynFlags
            ; liftIO (log_action dflags dflags NoReason SevFatal
                       noSrcSpan (defaultErrStyle dflags) full_msg)
            ; failM }

    --------------------
    forkM_maybe :: SDoc -> IfL a -> IfL (Maybe a)
    -- Run thing_inside in an interleaved thread.
    -- It shares everything with the parent thread, so this is DANGEROUS.
    --
    -- It returns Nothing if the computation fails
    --
    -- It's used for lazily type-checking interface
    -- signatures, which is pretty benign

    forkM_maybe doc thing_inside
     -- NB: Don't share the mutable env_us with the interleaved thread since env_us
     --     does not get updated atomically (e.g. in newUnique and newUniqueSupply).
     = do { child_us <- newUniqueSupply
          ; child_env_us <- newMutVar child_us
            -- see Note [Masking exceptions in forkM_maybe]
          ; unsafeInterleaveM $ uninterruptibleMaskM_ $ updEnv (\env -> env { env_us = child_env_us }) $
            do { traceIf (text "Starting fork {" <+> doc)
               ; mb_res <- tryM $
                           updLclEnv (\env -> env { if_loc = if_loc env $$ doc }) $
                           thing_inside
               ; case mb_res of
                    Right r  -> do  { traceIf (text "} ending fork" <+> doc)
                                    ; return (Just r) }
                    Left exn -> do {

                        -- Bleat about errors in the forked thread, if -ddump-if-trace is on
                        -- Otherwise we silently discard errors. Errors can legitimately
                        -- happen when compiling interface signatures (see tcInterfaceSigs)
                          whenDOptM Opt_D_dump_if_trace $ do
                              dflags <- getDynFlags
                              let msg = hang (text "forkM failed:" <+> doc)
                                           2 (text (show exn))
                              liftIO $ log_action dflags
                                                  dflags
                                                  NoReason
                                                  SevFatal
                                                  noSrcSpan
                                                  (defaultErrStyle dflags)
                                                  msg

                        ; traceIf (text "} ending fork (badly)" <+> doc)
                        ; return Nothing }
            }}

    forkM :: SDoc -> IfL a -> IfL a
    forkM doc thing_inside
     = do   { mb_res <- forkM_maybe doc thing_inside
            ; return (case mb_res of
                            Nothing -> pgmError "Cannot continue after interface file error"
                                       -- pprPanic "forkM" doc
                            Just r  -> r) }

    {-
    Note [Masking exceptions in forkM_maybe]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    When using GHC-as-API it must be possible to interrupt snippets of code
    executed using runStmt (#1381). Since commit 02c4ab04 this is almost possible
    by throwing an asynchronous interrupt to the GHC thread. However, there is a
    subtle problem: runStmt first typechecks the code before running it, and the
    exception might interrupt the type checker rather than the code. Moreover, the
    typechecker might be inside an unsafeInterleaveIO (through forkM_maybe), and
    more importantly might be inside an exception handler inside that
    unsafeInterleaveIO. If that is the case, the exception handler will rethrow the
    asynchronous exception as a synchronous exception, and the exception will end
    up as the value of the unsafeInterleaveIO thunk (see #8006 for a detailed
    discussion).  We don't currently know a general solution to this problem, but
    we can use uninterruptibleMask_ to avoid the situation.
    -}