path: root/compiler/typecheck/TcRnMonad.hs

{-
(c) The University of Glasgow 2006


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

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


module TcRnMonad(
  -- * Initalisation
  initTc, initTcWithGbl, initTcInteractive, 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, unlessXOptM,
  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, traceTcRnForUser,
  traceTcRnWithStyle,
  getPrintUnqualified,
  printForUserTcRn,
  traceIf, traceHiDiffs, traceOptIf,
  debugTc,

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

  -- * Error management
  getSrcSpanM, setSrcSpan, addLocM,
  wrapLocM, wrapLocFstM, wrapLocSndM,wrapLocM_,
  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,
  attemptM, tryTc,
  askNoErrs, discardErrs, tryTcDiscardingErrs,
  checkNoErrs, whenNoErrs,
  ifErrsM, failIfErrsM,

  -- * 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, mkErrTc,
  failWithTc, failWithTcM,
  checkTc, checkTcM,
  failIfTc, failIfTcM,
  warnIfFlag, warnIf, warnTc, warnTcM,
  addWarnTc, addWarnTcM, addWarn, addWarnAt, add_warn,
  mkErrInfo,

  -- * Type constraints
  newTcEvBinds, newNoTcEvBinds, cloneEvBindsVar,
  addTcEvBind, addTopEvBinds,
  getTcEvTyCoVars, getTcEvBindsMap, setTcEvBindsMap,
  chooseUniqueOccTc,
  getConstraintVar, setConstraintVar,
  emitConstraints, emitStaticConstraints, emitSimple, emitSimples,
  emitImplication, emitImplications, emitInsoluble,
  discardConstraints, captureConstraints, tryCaptureConstraints,
  pushLevelAndCaptureConstraints,
  pushTcLevelM_, pushTcLevelM, pushTcLevelsM,
  getTcLevel, setTcLevel, isTouchableTcM,
  getLclTypeEnv, setLclTypeEnv,
  traceTcConstraints,
  emitNamedWildCardHoleConstraints, emitAnonWildCardHoleConstraint,

  -- * 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,

  -- * Stuff for cost centres.
  ContainsCostCentreState(..), getCCIndexM,

  -- * Types etc.
  module TcRnTypes,
  module IOEnv
  ) where

#include "HsVersions.h"

import GhcPrelude

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

import GHC.Hs 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, TypeOrKind(..) )
import Maybes
import CostCentreState

import qualified GHC.LanguageExtensions as LangExt

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

import {-# SOURCE #-} TcEnv    ( tcInitTidyEnv )

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 ;
        cc_st_var           <- newIORef newCostCentreState ;
        th_topdecls_var      <- newIORef [] ;
        th_foreign_files_var <- newIORef [] ;
        th_topnames_var      <- newIORef emptyNameSet ;
        th_modfinalizers_var <- newIORef [] ;
        th_coreplugins_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

                | gopt Opt_WriteHie dflags       = Just empty_val

                  -- We want to serialize the documentation in the .hi-files,
                  -- and need to extract it from the renamed syntax first.
                  -- See 'ExtractDocs.extractDocs'.
                | gopt Opt_Haddock 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_coreplugins = th_coreplugins_var,
                tcg_th_state         = th_state_var,
                tcg_th_remote_state  = th_remote_state_var,

                tcg_mod            = mod,
                tcg_semantic_mod   =
                    canonicalizeModuleIfHome dflags 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_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_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_hf_plugins     = [],
                tcg_top_loc        = loc,
                tcg_static_wc      = static_wc_var,
                tcg_complete_matches = [],
                tcg_cc_st          = cc_st_var
             } ;
        } ;

        -- 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 { 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_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 (#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

{- 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 }

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 }

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

unlessXOptM :: LangExt.Extension -> TcRnIf gbl lcl () -> TcRnIf gbl lcl ()
unlessXOptM flag thing_inside = do b <- xoptM flag
                                   unless 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)
       }

-- Certain tests (T3017, Roles3, T12763 etc.) expect part of the
-- output generated by `-ddump-types` to be in 'PprUser' style. However,
-- generally we want all other debugging output to use 'PprDump'
-- style. 'traceTcRn' and 'traceTcRnForUser' help us accomplish this.

-- | A wrapper around 'traceTcRnWithStyle' which uses 'PprDump' style.
traceTcRn :: DumpFlag -> SDoc -> TcRn ()
traceTcRn flag doc
  = do { dflags  <- getDynFlags
       ; printer <- getPrintUnqualified dflags
       ; let dump_style = mkDumpStyle dflags printer
       ; traceTcRnWithStyle dump_style dflags flag doc }

-- | A wrapper around 'traceTcRnWithStyle' which uses 'PprUser' style.
traceTcRnForUser :: DumpFlag -> SDoc -> TcRn ()
-- Used by 'TcRnDriver.tcDump'.
traceTcRnForUser flag doc
  = do { dflags  <- getDynFlags
       ; printer <- getPrintUnqualified dflags
       ; let user_style = mkUserStyle dflags printer AllTheWay
       ; traceTcRnWithStyle user_style dflags flag doc }

traceTcRnWithStyle :: PprStyle -> DynFlags -> 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
traceTcRnWithStyle sty dflags flag doc
  = do { real_doc <- prettyDoc dflags doc
       ; liftIO $ dumpSDocWithStyle sty dflags 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)) }

tcIsHsig :: TcRn Bool
tcIsHsig = do { env <- getGblEnv; return (isHsigFile (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 :: HasSrcSpan a => (SrcSpanLess a -> TcM b) -> a -> TcM b
addLocM fn (dL->L loc a) = setSrcSpan loc $ fn a

wrapLocM :: (HasSrcSpan a, HasSrcSpan b) =>
            (SrcSpanLess a -> TcM (SrcSpanLess b)) -> a -> TcM b
-- wrapLocM :: (a -> TcM b) -> Located a -> TcM (Located b)
wrapLocM fn (dL->L loc a) = setSrcSpan loc $ do { b <- fn a
                                                ; return (cL loc b) }
wrapLocFstM :: (HasSrcSpan a, HasSrcSpan b) =>
               (SrcSpanLess a -> TcM (SrcSpanLess b,c)) -> a -> TcM (b, c)
wrapLocFstM fn (dL->L loc a) =
  setSrcSpan loc $ do
    (b,c) <- fn a
    return (cL loc b, c)

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

wrapLocM_ :: HasSrcSpan a =>
             (SrcSpanLess a -> TcM ()) -> a -> TcM ()
wrapLocM_ fn (dL->L loc a) = setSrcSpan loc (fn a)

-- 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) }


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

{- *********************************************************************
*                                                                      *
        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 recovery and exceptions
*                                                                      *
********************************************************************* -}

tcTryM :: TcRn r -> TcRn (Maybe r)
-- The most basic function: catch the exception
--   Nothing => an exception happened
--   Just r  => no exception, result R
-- Errors and constraints are propagated in both cases
-- Never throws an exception
tcTryM thing_inside
  = do { either_res <- tryM thing_inside
       ; return (case either_res of
                    Left _  -> Nothing
                    Right r -> Just r) }
         -- In the Left case the exception is always the IOEnv
         -- built-in in exception; see IOEnv.failM

-----------------------
capture_constraints :: TcM r -> TcM (r, WantedConstraints)
-- capture_constraints simply captures and returns the
--                     constraints generated by thing_inside
-- Precondition: thing_inside must not throw an exception!
-- Reason for precondition: an exception would blow past the place
-- where we read the lie_var, and we'd lose the constraints altogether
capture_constraints thing_inside
  = do { lie_var <- newTcRef emptyWC
       ; res <- updLclEnv (\ env -> env { tcl_lie = lie_var }) $
                thing_inside
       ; lie <- readTcRef lie_var
       ; return (res, lie) }

capture_messages :: TcM r -> TcM (r, Messages)
-- capture_messages simply captures and returns the
--                  errors arnd warnings generated by thing_inside
-- Precondition: thing_inside must not throw an exception!
-- Reason for precondition: an exception would blow past the place
-- where we read the msg_var, and we'd lose the constraints altogether
capture_messages thing_inside
  = do { msg_var <- newTcRef emptyMessages
       ; res     <- setErrsVar msg_var thing_inside
       ; msgs    <- readTcRef msg_var
       ; return (res, msgs) }

-----------------------
-- (askNoErrs m) runs m
-- If m fails,
--    then (askNoErrs m) fails, propagating only
--         insoluble constraints
--
-- 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 thing_inside
  = do { ((mb_res, lie), msgs) <- capture_messages    $
                                  capture_constraints $
                                  tcTryM thing_inside
       ; addMessages msgs

       ; case mb_res of
           Nothing  -> do { emitConstraints (insolublesOnly lie)
                          ; failM }

           Just res -> do { emitConstraints lie
                          ; dflags <- getDynFlags
                          ; let errs_found = errorsFound dflags msgs
                                          || insolubleWC lie
                          ; return (res, not errs_found) } }

-----------------------
tryCaptureConstraints :: TcM a -> TcM (Maybe a, WantedConstraints)
-- (tryCaptureConstraints_maybe m) runs m,
--   and returns the type constraints it generates
-- It never throws an exception; instead if thing_inside fails,
--   it returns Nothing and the /insoluble/ constraints
-- Error messages are propagated
tryCaptureConstraints thing_inside
  = do { (mb_res, lie) <- capture_constraints $
                          tcTryM thing_inside

       -- See Note [Constraints and errors]
       ; let lie_to_keep = case mb_res of
                             Nothing -> insolublesOnly lie
                             Just {} -> lie

       ; return (mb_res, lie_to_keep) }

captureConstraints :: TcM a -> TcM (a, WantedConstraints)
-- (captureConstraints m) runs m, and returns the type constraints it generates
-- If thing_inside fails (throwing an exception),
--   then (captureConstraints thing_inside) fails too
--   propagating the insoluble constraints only
-- Error messages are propagated in either case
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
           Nothing  -> do { emitConstraints lie; failM }
           Just res -> return (res, lie) }

-----------------------
attemptM :: TcRn r -> TcRn (Maybe r)
-- (attemptM thing_inside) runs thing_inside
-- If thing_inside succeeds, returning r,
--   we return (Just r), and propagate all constraints and errors
-- If thing_inside fail, throwing an exception,
--   we return Nothing, propagating insoluble constraints,
--                      and all errors
-- attemptM never throws an exception
attemptM thing_inside
  = do { (mb_r, lie) <- tryCaptureConstraints thing_inside
       ; emitConstraints lie

       -- Debug trace
       ; when (isNothing mb_r) $
         traceTc "attemptM recovering with insoluble constraints" $
                 (ppr lie)

       ; 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
-- (recoverM recover thing_inside) runs thing_inside
-- If thing_inside fails, propagate its errors and insoluble constraints
--                        and run 'recover'
-- If thing_inside succeeds, propagate all its errors and constraints
--
-- Can fail, if 'recover' fails
recoverM recover thing
  = do { mb_res <- attemptM thing ;
         case mb_res of
           Nothing  -> recover
           Just 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 xs
  = do { mb_rs <- mapM (attemptM . f) xs
       ; return [r | Just r <- mb_rs] }

-- | Apply the function to all elements on the input list
-- If all succeed, return the list of results
-- Othewise fail, propagating all errors
mapAndReportM :: (a -> TcRn b) -> [a] -> TcRn [b]
mapAndReportM f xs
  = do { mb_rs <- mapM (attemptM . f) xs
       ; when (any isNothing mb_rs) failM
       ; return [r | Just r <- mb_rs] }

-- | 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 <- attemptM (f acc x)
                             ; case mb_r of
                                Nothing   -> foldAndRecoverM f acc xs
                                Just acc' -> foldAndRecoverM f acc' xs  }

-----------------------
tryTc :: TcRn a -> TcRn (Maybe a, Messages)
-- (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
 = capture_messages (attemptM thing_inside)

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

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

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

mkErrTc :: MsgDoc -> TcM ErrMsg
mkErrTc msg = do { env0 <- tcInitTidyEnv
                 ; mkErrTcM (env0, msg) }

-- 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
warnIfFlag :: WarningFlag -> Bool -> MsgDoc -> TcRn ()
warnIfFlag warn_flag is_bad msg
  = do { warn_on <- woptM warn_flag
       ; when (warn_on && is_bad) $
         addWarn (Reason warn_flag) msg }

-- | Display a warning if a condition is met.
warnIf :: Bool -> MsgDoc -> TcRn ()
warnIf is_bad msg
  = when is_bad (addWarn NoReason 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 }


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

addTopEvBinds :: Bag EvBind -> TcM a -> TcM a
addTopEvBinds new_ev_binds thing_inside
  =updGblEnv upd_env thing_inside
  where
    upd_env tcg_env = tcg_env { tcg_ev_binds = tcg_ev_binds tcg_env
                                               `unionBags` new_ev_binds }

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 }) }

-- | Creates an EvBindsVar incapable of holding any bindings. It still
-- tracks covar usages (see comments on ebv_tcvs in TcEvidence), thus
-- must be made monadically
newNoTcEvBinds :: TcM EvBindsVar
newNoTcEvBinds
  = do { tcvs_ref  <- newTcRef emptyVarSet
       ; uniq <- newUnique
       ; traceTc "newNoTcEvBinds" (text "unique =" <+> ppr uniq)
       ; return (CoEvBindsVar { ebv_tcvs = tcvs_ref
                              , ebv_uniq = uniq }) }

cloneEvBindsVar :: EvBindsVar -> TcM EvBindsVar
-- Clone the refs, so that any binding created when
-- solving don't pollute the original
cloneEvBindsVar ebv@(EvBindsVar {})
  = do { binds_ref <- newTcRef emptyEvBindMap
       ; tcvs_ref  <- newTcRef emptyVarSet
       ; return (ebv { ebv_binds = binds_ref
                     , ebv_tcvs = tcvs_ref }) }
cloneEvBindsVar ebv@(CoEvBindsVar {})
  = do { tcvs_ref  <- newTcRef emptyVarSet
       ; return (ebv { ebv_tcvs = tcvs_ref }) }

getTcEvTyCoVars :: EvBindsVar -> TcM TyCoVarSet
getTcEvTyCoVars ev_binds_var
  = readTcRef (ebv_tcvs ev_binds_var)

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

setTcEvBindsMap :: EvBindsVar -> EvBindMap -> TcM ()
setTcEvBindsMap (EvBindsVar { ebv_binds = ev_ref }) binds
  = writeTcRef ev_ref binds
setTcEvBindsMap v@(CoEvBindsVar {}) ev_binds
  | isEmptyEvBindMap ev_binds
  = return ()
  | otherwise
  = pprPanic "setTcEvBindsMap" (ppr v $$ ppr ev_binds)

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 $$
                                 ppr ev_bind
       ; bnds <- readTcRef ev_ref
       ; writeTcRef ev_ref (extendEvBinds bnds ev_bind) }
addTcEvBind (CoEvBindsVar { ebv_uniq = u }) ev_bind
  = pprPanic "addTcEvBind CoEvBindsVar" (ppr ev_bind $$ ppr u)

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
  | isEmptyWC ct
  = return ()
  | otherwise
  = 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

-- | The name says it all. The returned TcLevel is the *inner* TcLevel.
pushLevelAndCaptureConstraints :: TcM a -> TcM (TcLevel, WantedConstraints, a)
pushLevelAndCaptureConstraints thing_inside
  = do { env <- getLclEnv
       ; let tclvl' = pushTcLevel (tcl_tclvl env)
       ; traceTc "pushLevelAndCaptureConstraints {" (ppr tclvl')
       ; (res, lie) <- setLclEnv (env { tcl_tclvl = tclvl' }) $
                       captureConstraints thing_inside
       ; traceTc "pushLevelAndCaptureConstraints }" (ppr tclvl')
       ; 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 (TcLevel, a)
-- 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 (tclvl', res) }

-- Returns pushed TcLevel
pushTcLevelsM :: Int -> TcM a -> TcM (a, TcLevel)
pushTcLevelsM num_levels thing_inside
  = do { env <- getLclEnv
       ; let tclvl' = nTimes num_levels 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 { lvl <- getTcLevel
       ; return (isTouchableMetaTyVar lvl 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 }

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)
       }

emitAnonWildCardHoleConstraint :: TcTyVar -> TcM ()
emitAnonWildCardHoleConstraint tv
  = do { ct_loc <- getCtLocM HoleOrigin Nothing
       ; emitInsolubles $ unitBag $
         CHoleCan { cc_ev = CtDerived { ctev_pred = mkTyVarTy tv
                                      , ctev_loc  = ct_loc }
                  , cc_hole = TypeHole (mkTyVarOcc "_") } }

emitNamedWildCardHoleConstraints :: [(Name, TcTyVar)] -> TcM ()
emitNamedWildCardHoleConstraints 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 "emitNamedWildCardHoleConstraints"
                                                      (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 (#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 'attemptM' 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
constraints might be "variable out of scope" Hole constraints, and that
might have been the actual original cause of the exception!  For
example (#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

However note that freshly-generated constraints like (Int ~ Bool), or
((a -> b) ~ Int) are all CNonCanonical, and hence won't be flagged as
insoluble.  The constraint solver does that.  So they'll be discarded.
That's probably ok; but see th/5358 as a not-so-good example:
   t1 :: Int
   t1 x = x   -- Manifestly wrong

   foo = $(...raises exception...)
We report the exception, but not the bug in t1.  Oh well.  Possible
solution: make TcUnify.uType spot manifestly-insoluble constraints.


************************************************************************
*                                                                      *
             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 ->
         (lcl_env, 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_SafeInferred
                | 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 && sfMode /= Sf_SafeInferred = 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.
-}

-- | Environments which track 'CostCentreState'
class ContainsCostCentreState e where
  extractCostCentreState :: e -> TcRef CostCentreState

instance ContainsCostCentreState TcGblEnv where
  extractCostCentreState = tcg_cc_st

instance ContainsCostCentreState DsGblEnv where
  extractCostCentreState = ds_cc_st

-- | Get the next cost centre index associated with a given name.
getCCIndexM :: (ContainsCostCentreState gbl)
            => FastString -> TcRnIf gbl lcl CostCentreIndex
getCCIndexM nm = do
  env <- getGblEnv
  let cc_st_ref = extractCostCentreState env
  cc_st <- readTcRef cc_st_ref
  let (idx, cc_st') = getCCIndex nm cc_st
  writeTcRef cc_st_ref cc_st'
  return idx