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(
      -- * Initalisation
      initTc, initTcWithGbl, initTcInteractive, initTcForLookup, initTcRnIf,

      -- * Simple accessors
      discardResult,
      getTopEnv, updTopEnv, getGblEnv, updGblEnv,
      setGblEnv, getLclEnv, updLclEnv, setLclEnv,
      getEnvs, setEnvs,
      xoptM, doptM, goptM, woptM,
      setXOptM, unsetXOptM, unsetGOptM, unsetWOptM,
      whenDOptM, whenGOptM, whenWOptM, whenXOptM,
      getGhcMode,
      withDoDynamicToo,
      getEpsVar,
      getEps,
      updateEps, updateEps_,
      getHpt, getEpsAndHpt,

      -- * Arrow scopes
      newArrowScope, escapeArrowScope,

      -- * Unique supply
      newUnique, newUniqueSupply, newName, newNameAt, cloneLocalName,
      newSysName, newSysLocalId, newSysLocalIds,

      -- * Accessing input/output
      newTcRef, readTcRef, writeTcRef, updTcRef,

      -- * Debugging
      traceTc, traceRn, traceOptTcRn, traceTcRn,
      getPrintUnqualified,
      printForUserTcRn,
      traceIf, traceHiDiffs, traceOptIf,
      debugTc,

      -- * Typechecker global environment
      getIsGHCi, getGHCiMonad, getInteractivePrintName,
      tcIsHsBootOrSig, tcSelfBootInfo, getGlobalRdrEnv,
      getRdrEnvs, getImports,
      getFixityEnv, extendFixityEnv, getRecFieldEnv,
      getDeclaredDefaultTys,
      addDependentFiles,

      -- * Error management
      getSrcSpanM, setSrcSpan, addLocM,
      wrapLocM, wrapLocFstM, wrapLocSndM,
      getErrsVar, setErrsVar,
      addErr,
      failWith, failAt,
      addErrAt, addErrs,
      checkErr,
      addMessages,
      discardWarnings,

      -- * Shared error message stuff: renamer and typechecker
      mkLongErrAt, mkErrDocAt, addLongErrAt, reportErrors, reportError,
      reportWarning, recoverM, mapAndRecoverM, mapAndReportM, foldAndRecoverM,
      tryTc,
      askNoErrs, discardErrs, tryTcDiscardingErrs,
      checkNoErrs, whenNoErrs,
      ifErrsM, failIfErrsM,
      checkTH, failTH,

      -- * Context management for the type checker
      getErrCtxt, setErrCtxt, addErrCtxt, addErrCtxtM, addLandmarkErrCtxt,
      addLandmarkErrCtxtM, updCtxt, popErrCtxt, getCtLocM, setCtLocM,

      -- * Error message generation (type checker)
      addErrTc, addErrsTc,
      addErrTcM, mkErrTcM,
      failWithTc, failWithTcM,
      checkTc, checkTcM,
      failIfTc, failIfTcM,
      warnIf, warnTc, warnTcM,
      addWarnTc, addWarnTcM, addWarn, addWarnAt, add_warn,
      tcInitTidyEnv, tcInitOpenTidyEnv, mkErrInfo,

      -- * Type constraints
      newTcEvBinds,
      addTcEvBind,
      getTcEvTyCoVars, getTcEvBindsMap,
      chooseUniqueOccTc,
      getConstraintVar, setConstraintVar,
      emitConstraints, emitStaticConstraints, emitSimple, emitSimples,
      emitImplication, emitImplications, emitInsoluble,
      discardConstraints, captureConstraints, tryCaptureConstraints,
      pushLevelAndCaptureConstraints,
      pushTcLevelM_, pushTcLevelM,
      getTcLevel, setTcLevel, isTouchableTcM,
      getLclTypeEnv, setLclTypeEnv,
      traceTcConstraints, emitWildCardHoleConstraints,

      -- * Template Haskell context
      recordThUse, recordThSpliceUse, recordTopLevelSpliceLoc,
      getTopLevelSpliceLocs, keepAlive, getStage, getStageAndBindLevel, setStage,
      addModFinalizersWithLclEnv,

      -- * Safe Haskell context
      recordUnsafeInfer, finalSafeMode, fixSafeInstances,

      -- * Stuff for the renamer's local env
      getLocalRdrEnv, setLocalRdrEnv,

      -- * Stuff for interface decls
      mkIfLclEnv,
      initIfaceTcRn,
      initIfaceCheck,
      initIfaceLcl,
      initIfaceLclWithSubst,
      initIfaceLoad,
      getIfModule,
      failIfM,
      forkM_maybe,
      forkM,
      setImplicitEnvM,

      withException,

      -- * Types etc.
      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 DynFlags
    import FastString
    import Panic
    import Util
    import Annotations
    import BasicTypes( TopLevelFlag )
    import Maybes

    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

    import {-# SOURCE #-} TcSplice ( runRemoteModFinalizers )
    import qualified Data.Map as Map

    {-
    ************************************************************************
    *                                                                      *
                            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 { 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) ;
            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 ;
            th_topdecls_var      <- newIORef [] ;
            th_foreign_files_var <- newIORef [] ;
            th_topnames_var      <- newIORef emptyNameSet ;
            th_modfinalizers_var <- newIORef [] ;
            th_state_var         <- newIORef Map.empty ;
            th_remote_state_var  <- newIORef Nothing ;
            let {
                 dflags = hsc_dflags hsc_env ;

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

                 gbl_env = TcGblEnv {
                    tcg_th_topdecls      = th_topdecls_var,
                    tcg_th_foreign_files = th_foreign_files_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,

                    tcg_mod            = mod,
                    tcg_semantic_mod   =
                        if thisPackage dflags == moduleUnitId mod
                            then canonicalizeHomeModule dflags (moduleName mod)
                            else mod,
                    tcg_src            = hsc_src,
                    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_pending_fam_checks = emptyNameEnv,
                    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     =
                        if hsc_src == HsigFile
                            -- Always retain renamed syntax, so that we can give
                            -- better errors.  (TODO: how?)
                            then Just []
                            else 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_merged         = [],
                    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_top_loc        = loc,
                    tcg_static_wc      = static_wc_var,
                    tcg_complete_matches = []
                 } ;
            } ;

            -- OK, here's the business end!
            initTcWithGbl hsc_env gbl_env loc do_this
        }

    -- | Run a 'TcM' action in the context of an existing 'GblEnv'.
    initTcWithGbl :: HscEnv
                  -> TcGblEnv
                  -> RealSrcSpan
                  -> TcM r
                  -> IO (Messages, Maybe r)
    initTcWithGbl hsc_env gbl_env loc do_this
     = do { tvs_var      <- newIORef emptyVarSet
          ; lie_var      <- newIORef emptyWC
          ; errs_var     <- newIORef (emptyBag, emptyBag)
          ; let 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
                    }

          ; 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
          -- If we succeed (maybe_res = Just r), there should be
          -- no unsolved constraints.  But if we exit via an
          -- exception (maybe_res = Nothing), we may have skipped
          -- solving, so don't panic then (Trac #13466)
          ; lie <- readIORef (tcl_lie lcl_env)
          ; when (isJust maybe_res && not (isEmptyWC lie)) $
            pprPanic "initTc: unsolved constraints" (ppr lie)

            -- Collect any error messages
          ; msgs <- readIORef (tcl_errs lcl_env)

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

          ; return (msgs, final_res)
          }
      where dflags = hsc_dflags hsc_env

    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})

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

    -- | A convenient wrapper for taking a @MaybeErr MsgDoc a@ and throwing
    -- an exception if it is an error.
    withException :: TcRnIf gbl lcl (MaybeErr MsgDoc a) -> TcRnIf gbl lcl a
    withException do_this = do
        r <- do_this
        dflags <- getDynFlags
        case r of
            Failed err -> liftIO $ throwGhcExceptionIO (ProgramError (showSDoc dflags err))
            Succeeded result -> return result

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

    cloneLocalName :: Name -> TcM Name
    -- Make a fresh Internal name with the same OccName and SrcSpan
    cloneLocalName name = newNameAt (nameOccName name) (nameSrcSpan name)

    newName :: OccName -> TcM Name
    newName occ = do { loc  <- getSrcSpanM
                     ; newNameAt occ loc }

    newNameAt :: OccName -> SrcSpan -> TcM Name
    newNameAt occ span
      = do { uniq <- newUnique
           ; return (mkInternalName uniq occ span) }

    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) }

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

    -- Typechecker trace
    traceTc :: String -> SDoc -> TcRn ()
    traceTc =
      labelledTraceOptTcRn Opt_D_dump_tc_trace

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

    -- | Trace when a certain flag is enabled. This is like `traceOptTcRn`
    -- but accepts a string as a label and formats the trace message uniformly.
    labelledTraceOptTcRn :: DumpFlag -> String -> SDoc -> TcRn ()
    labelledTraceOptTcRn flag herald doc = do
       traceOptTcRn flag (formatTraceMsg herald doc)

    formatTraceMsg :: String -> SDoc -> SDoc
    formatTraceMsg herald doc = hang (text herald) 2 doc

    -- | 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 { dflags   <- getDynFlags
           ; real_doc <- prettyDoc dflags doc
           ; printer  <- getPrintUnqualified dflags
           ; liftIO $ dumpSDoc dflags printer flag "" real_doc  }
      where
        -- Add current location if -dppr-debug
        prettyDoc :: DynFlags -> SDoc -> TcRn SDoc
        prettyDoc dflags doc = if hasPprDebug dflags
           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) }

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

    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)

    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, lie) <- tryCaptureConstraints thing
           ; emitConstraints lie

           -- Debug trace
           ; case mb_r of
                Left exn -> traceTc "tryTc/recoverM recovering from" $
                            text (showException exn)
                Right {} -> return ()

           ; return mb_r }

    -----------------------
    recoverM :: TcRn r      -- Recovery action; do this if the main one fails
             -> TcRn r      -- Main action: do this first;
                            --  if it generates errors, propagate them all
             -> 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 }

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

    -- | Drop elements of the input that fail, so the result
    -- list can be shorter than the argument list
    mapAndRecoverM :: (a -> TcRn b) -> [a] -> TcRn [b]
    mapAndRecoverM f = fmap reverse . foldAndRecoverM (\xs x -> (:xs) <$> f x ) []

    -- | The accumulator is not updated if the action fails
    foldAndRecoverM :: (b -> a -> TcRn b) -> b -> [a] -> TcRn b
    foldAndRecoverM _ acc []     = return acc
    foldAndRecoverM f acc (x:xs) =
                              do { mb_r <- try_m (f acc x)
                                 ; case mb_r of
                                    Left _  -> foldAndRecoverM f acc xs
                                    Right acc' -> foldAndRecoverM f acc' xs  }

    -- | 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 thing_inside
     = do { errs_var <- newTcRef emptyMessages ;

            res  <- try_m $  -- Be sure to catch exceptions, so that
                             -- we guaranteed to read the messages out
                             -- of that brand-new errs_var!
                    setErrsVar errs_var $
                    thing_inside ;

            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
       }

    -----------------------
    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 }

    -----------------------
    tryTcDiscardingErrs :: TcM r -> TcM r -> TcM r
    -- (tryTcDiscardingErrs recover main) tries 'main';
    --      if 'main' succeeds with no error messages, it's the answer
    --      otherwise discard everything from 'main', including errors,
    --          and try 'recover' instead.
    tryTcDiscardingErrs recover main
      = do  { (msgs, mb_res) <- tryTc main
            ; dflags <- getDynFlags
            ; case mb_res of
                Just res | not (errorsFound dflags msgs)
                  -> -- 'main' succeeed with no error messages
                     do { addMessages msgs  -- msgs might still have warnings
                        ; return res }

                _ -> -- 'main' failed, or produced an error message
                     recover     -- Discard all errors and warnings entirely
            }

    -----------------------
    -- (askNoErrs m) runs m
    -- If m fails,
    --    then (askNoErrs m) fails
    -- If m succeeds with result r,
    --    then (askNoErrs m) succeeds with result (r, b),
    --         where b is True iff m generated no errors
    -- Regardless of success or failure,
    --   propagate any errors/warnings generated by m
    askNoErrs :: TcRn a -> TcRn (a, Bool)
    askNoErrs m
      = do { (msgs, mb_res) <- tryTc m
           ; addMessages msgs  -- Always propagate errors
           ; case mb_res of
               Nothing  -> failM
               Just res -> do { dflags <- getDynFlags
                              ; let errs_found = errorsFound dflags msgs
                              ; return (res, not errs_found) } }
    -----------------------
    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  { (res, no_errs) <- askNoErrs main
            ; unless no_errs failM
            ; return res }

    -----------------------
    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 ())

    checkTH :: a -> String -> TcRn ()
    checkTH _ _ = return () -- OK

    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.
    --   and the warning is enabled
    warnIf :: WarnReason -> Bool -> MsgDoc -> TcRn ()
    warnIf reason is_bad msg
      = do { warn_on <- case reason of
                           NoReason         -> return True
                           Reason warn_flag -> woptM warn_flag
           ; when (warn_on && is_bad) $
             addWarn reason msg }

    -- | 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 :: [TyCoVar] -> 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
    --  = do
    --       dbg <- hasPprDebug <$> getDynFlags
    --       if dbg                -- In -dppr-debug style the output
    --          then return empty  -- just becomes too voluminous
    --          else go dbg 0 env ctxts
     = go False 0 env ctxts
     where
       go :: Bool -> Int -> TidyEnv -> [ErrCtxt] -> TcM SDoc
       go _ _ _   [] = return empty
       go dbg n env ((is_landmark, ctxt) : ctxts)
         | is_landmark || n < mAX_CONTEXTS -- Too verbose || dbg
         = do { (env', msg) <- ctxt env
              ; let n' = if is_landmark then n else n+1
              ; rest <- go dbg n' env' ctxts
              ; return (msg $$ rest) }
         | otherwise
         = go dbg 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 { binds_ref <- newTcRef emptyEvBindMap
                      ; tcvs_ref  <- newTcRef emptyVarSet
                      ; uniq <- newUnique
                      ; traceTc "newTcEvBinds" (text "unique =" <+> ppr uniq)
                      ; return (EvBindsVar { ebv_binds = binds_ref
                                           , ebv_tcvs = tcvs_ref
                                           , ebv_uniq = uniq }) }

    getTcEvTyCoVars :: EvBindsVar -> TcM TyCoVarSet
    getTcEvTyCoVars (EvBindsVar { ebv_tcvs = ev_ref })
      = readTcRef ev_ref

    getTcEvBindsMap :: EvBindsVar -> TcM EvBindMap
    getTcEvBindsMap (EvBindsVar { ebv_binds = ev_ref })
      = readTcRef ev_ref

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

    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 })

    emitStaticConstraints :: WantedConstraints -> TcM ()
    emitStaticConstraints static_lie
      = do { gbl_env <- getGblEnv
           ; updTcRef (tcg_static_wc gbl_env) (`andWC` static_lie) }

    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 { traceTc "emitInsoluble" (ppr ct)
           ; lie_var <- getConstraintVar
           ; updTcRef lie_var (`addInsols` unitBag ct) }

    emitInsolubles :: Cts -> TcM ()
    emitInsolubles cts
      | isEmptyBag cts = return ()
      | otherwise      = do { traceTc "emitInsolubles" (ppr cts)
                            ; lie_var <- getConstraintVar
                            ; updTcRef lie_var (`addInsols` cts) }

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

    tryCaptureConstraints :: TcM a -> TcM (Either IOEnvFailure a, WantedConstraints)
    -- (captureConstraints_maybe m) runs m,
    -- and returns the type constraints it generates
    -- It never throws an exception; instead if thing_inside fails,
    --   it returns Left exn and the insoluble constraints
    tryCaptureConstraints thing_inside
      = do { lie_var <- newTcRef emptyWC
           ; mb_res <- tryM $
                       updLclEnv (\ env -> env { tcl_lie = lie_var }) $
                       thing_inside
           ; lie <- readTcRef lie_var

           -- See Note [Constraints and errors]
           ; let lie_to_keep = case mb_res of
                                 Left {}  -> insolublesOnly lie
                                 Right {} -> lie

           ; return (mb_res, lie_to_keep) }

    captureConstraints :: TcM a -> TcM (a, WantedConstraints)
    -- (captureConstraints m) runs m, and returns the type constraints it generates
    captureConstraints thing_inside
      = do { (mb_res, lie) <- tryCaptureConstraints thing_inside

                -- See Note [Constraints and errors]
                -- If the thing_inside threw an exception, emit the insoluble
                -- constraints only (returned by tryCaptureConstraints)
                -- so that they are not lost
           ; case mb_res of
               Left _    -> do { emitConstraints lie; failM }
               Right res -> return (res, lie) }

    pushLevelAndCaptureConstraints :: TcM a -> TcM (TcLevel, WantedConstraints, a)
    pushLevelAndCaptureConstraints thing_inside
      = do { env <- getLclEnv
           ; let tclvl' = pushTcLevel (tcl_tclvl env)
           ; (res, lie) <- setLclEnv (env { tcl_tclvl = tclvl' }) $
                           captureConstraints thing_inside
           ; 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)
    -- See Note [TcLevel assignment] in TcType
    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
           ; traceOptTcRn Opt_D_dump_tc_trace $
             hang (text (msg ++ ": LIE:")) 2 (ppr lie)
           }

    emitWildCardHoleConstraints :: [(Name, TcTyVar)] -> TcM ()
    emitWildCardHoleConstraints wcs
      = do { ct_loc <- getCtLocM HoleOrigin Nothing
           ; emitInsolubles $ listToBag $
             map (do_one ct_loc) wcs }
      where
        do_one :: CtLoc -> (Name, TcTyVar) -> Ct
        do_one ct_loc (name, tv)
           = CHoleCan { cc_ev = CtDerived { ctev_pred = mkTyVarTy tv
                                          , ctev_loc  = ct_loc' }
                      , cc_hole = TypeHole (occName name) }
           where
             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
             ct_loc' = setCtLocSpan ct_loc real_span

    {- Note [Constraints and errors]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this (Trac #12124):

      foo :: Maybe Int
      foo = return (case Left 3 of
                      Left -> 1  -- Hard error here!
                      _    -> 0)

    The call to 'return' will generate a (Monad m) wanted constraint; but
    then there'll be "hard error" (i.e. an exception in the TcM monad), from
    the unsaturated Left constructor pattern.

    We'll recover in tcPolyBinds, using recoverM.  But then the final
    tcSimplifyTop will see that (Monad m) constraint, with 'm' utterly
    un-filled-in, and will emit a misleading error message.

    The underlying problem is that an exception interrupts the constraint
    gathering process. Bottom line: if we have an exception, it's best
    simply to discard any gathered constraints.  Hence in 'try_m' we
    capture the constraints in a fresh variable, and only emit them into
    the surrounding context if we exit normally.  If an exception is
    raised, simply discard the collected constraints... we have a hard
    error to report.  So this capture-the-emit dance isn't as stupid as it
    looks :-).

    However suppose we throw an exception inside an invocation of
    captureConstraints, and discard all the constraints. Some of those
    contraints might be "variable out of scope" Hole constraints, and that
    might have been the actual original cause of the exception!  For
    example (Trac #12529):
       f = p @ Int
    Here 'p' is out of scope, so we get an insolube Hole constraint. But
    the visible type application fails in the monad (thows an exception).
    We must not discard the out-of-scope error.

    So we /retain the insoluble constraints/ if there is an exception.
    Hence:
      - insolublesOnly in tryCaptureConstraints
      - emitConstraints in the Left case of captureConstraints

    ************************************************************************
    *                                                                      *
                 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 "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 })

    -- | Adds the given modFinalizers to the global environment and set them to use
    -- the current local environment.
    addModFinalizersWithLclEnv :: ThModFinalizers -> TcM ()
    addModFinalizersWithLclEnv mod_finalizers
      = do lcl_env <- getLclEnv
           th_modfinalizers_var <- fmap tcg_th_modfinalizers getGblEnv
           updTcRef th_modfinalizers_var $ \fins ->
             setLclEnv lcl_env (runRemoteModFinalizers mod_finalizers)
             : fins

    {-
    ************************************************************************
    *                                                                      *
                 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 -> Bool -> IfLclEnv
    mkIfLclEnv mod loc boot
                       = IfLclEnv { if_mod     = mod,
                                    if_loc     = loc,
                                    if_boot    = boot,
                                    if_nsubst  = Nothing,
                                    if_implicits_env = Nothing,
                                    if_tv_env  = emptyFsEnv,
                                    if_id_env  = emptyFsEnv }

    -- | 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
            ; dflags <- getDynFlags
            ; let mod = tcg_semantic_mod tcg_env
                  -- When we are instantiating a signature, we DEFINITELY
                  -- do not want to knot tie.
                  is_instantiate = unitIdIsDefinite (thisPackage dflags) &&
                                   not (null (thisUnitIdInsts dflags))
            ; let { if_env = IfGblEnv {
                                if_doc = text "initIfaceTcRn",
                                if_rec_types =
                                    if is_instantiate
                                        then Nothing
                                        else Just (mod, get_type_env)
                             }
                  ; get_type_env = readTcRef (tcg_type_env_var tcg_env) }
            ; setEnvs (if_env, ()) thing_inside }

    -- Used when sucking in a ModIface into a ModDetails to put in
    -- the HPT.  Notably, unlike initIfaceCheck, this does NOT use
    -- hsc_type_env_var (since we're not actually going to typecheck,
    -- so this variable will never get updated!)
    initIfaceLoad :: HscEnv -> IfG a -> IO a
    initIfaceLoad hsc_env do_this
     = do let gbl_env = IfGblEnv {
                            if_doc = text "initIfaceLoad",
                            if_rec_types = Nothing
                        }
          initTcRnIf 'i' hsc_env gbl_env () do_this

    initIfaceCheck :: SDoc -> 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 doc 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_doc = text "initIfaceCheck" <+> doc,
                            if_rec_types = rec_types
                        }
          initTcRnIf 'i' hsc_env gbl_env () do_this

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

    -- | Initialize interface typechecking, but with a 'NameShape'
    -- to apply when typechecking top-level 'OccName's (see
    -- 'lookupIfaceTop')
    initIfaceLclWithSubst :: Module -> SDoc -> Bool -> NameShape -> IfL a -> IfM lcl a
    initIfaceLclWithSubst mod loc_doc hi_boot_file nsubst thing_inside
      = setLclEnv ((mkIfLclEnv mod loc_doc hi_boot_file) { if_nsubst = Just nsubst }) 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 "shouldn't 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 (putLogMsg 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 $ putLogMsg 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) }

    setImplicitEnvM :: TypeEnv -> IfL a -> IfL a
    setImplicitEnvM tenv m = updLclEnv (\lcl -> lcl { if_implicits_env = Just tenv }) m

    {-
    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.
    -}