typecheck/TcEnv.hs

    -- (c) The University of Glasgow 2006

    {-# LANGUAGE CPP, FlexibleInstances #-}
    {-# OPTIONS_GHC -fno-warn-orphans #-}  -- instance MonadThings is necessarily an orphan

    module TcEnv(
            TyThing(..), TcTyThing(..), TcId,

            -- Instance environment, and InstInfo type
            InstInfo(..), iDFunId, pprInstInfoDetails,
            simpleInstInfoClsTy, simpleInstInfoTy, simpleInstInfoTyCon,
            InstBindings(..),

            -- Global environment
            tcExtendGlobalEnv, tcExtendGlobalEnvImplicit, setGlobalTypeEnv,
            tcExtendGlobalValEnv,
            tcLookupLocatedGlobal, tcLookupGlobal,
            tcLookupField, tcLookupTyCon, tcLookupClass,
            tcLookupDataCon, tcLookupPatSyn, tcLookupConLike,
            tcLookupLocatedGlobalId, tcLookupLocatedTyCon,
            tcLookupLocatedClass, tcLookupAxiom,

            -- Local environment
            tcExtendKindEnv, tcExtendKindEnv2,
            tcExtendTyVarEnv, tcExtendTyVarEnv2,
            tcExtendLetEnv,
            tcExtendIdEnv, tcExtendIdEnv1, tcExtendIdEnv2, tcExtendIdEnv3,
            tcExtendIdBndrs, tcExtendGhciIdEnv,

            tcLookup, tcLookupLocated, tcLookupLocalIds,
            tcLookupId, tcLookupTyVar,
            tcLookupLcl_maybe,
            getScopedTyVarBinds, getInLocalScope,
            wrongThingErr, pprBinders,

            tcExtendRecEnv,         -- For knot-tying

            -- Instances
            tcLookupInstance, tcGetInstEnvs,

            -- Rules
            tcExtendRules,

            -- Defaults
            tcGetDefaultTys,

            -- Global type variables
            tcGetGlobalTyVars, zapLclTypeEnv,

            -- Template Haskell stuff
            checkWellStaged, tcMetaTy, thLevel,
            topIdLvl, isBrackStage,

            -- New Ids
            newLocalName, newDFunName, newFamInstTyConName, newFamInstAxiomName,
            mkStableIdFromString, mkStableIdFromName,
            mkWrapperName
      ) where

    #include "HsVersions.h"

    import HsSyn
    import IfaceEnv
    import TcRnMonad
    import TcMType
    import TcType
    import LoadIface
    import PrelNames
    import TysWiredIn
    import Id
    import IdInfo( IdDetails(VanillaId) )
    import Var
    import VarSet
    import RdrName
    import InstEnv
    import DataCon ( DataCon )
    import PatSyn  ( PatSyn )
    import ConLike
    import TyCon
    import CoAxiom
    import TypeRep
    import Class
    import Name
    import NameEnv
    import VarEnv
    import HscTypes
    import DynFlags
    import SrcLoc
    import BasicTypes hiding( SuccessFlag(..) )
    import Module
    import Outputable
    import Encoding
    import FastString
    import ListSetOps
    import Util
    import Maybes( MaybeErr(..) )
    import Data.IORef
    import Data.List

    {-
    ************************************************************************
    *                                                                      *
    *                      tcLookupGlobal                                  *
    *                                                                      *
    ************************************************************************

    Using the Located versions (eg. tcLookupLocatedGlobal) is preferred,
    unless you know that the SrcSpan in the monad is already set to the
    span of the Name.
    -}

    tcLookupLocatedGlobal :: Located Name -> TcM TyThing
    -- c.f. IfaceEnvEnv.tcIfaceGlobal
    tcLookupLocatedGlobal name
      = addLocM tcLookupGlobal name

    tcLookupGlobal :: Name -> TcM TyThing
    -- The Name is almost always an ExternalName, but not always
    -- In GHCi, we may make command-line bindings (ghci> let x = True)
    -- that bind a GlobalId, but with an InternalName
    tcLookupGlobal name
      = do  {    -- Try local envt
              env <- getGblEnv
            ; case lookupNameEnv (tcg_type_env env) name of {
                    Just thing -> return thing ;
                    Nothing    ->

                    -- Should it have been in the local envt?
              if nameIsLocalOrFrom (tcg_mod env) name
              then notFound name  -- Internal names can happen in GHCi
              else

               -- Try home package table and external package table
        do  { mb_thing <- tcLookupImported_maybe name
            ; case mb_thing of
                Succeeded thing -> return thing
                Failed msg      -> failWithTc msg
            }}}

    tcLookupField :: Name -> TcM Id         -- Returns the selector Id
    tcLookupField name
      = tcLookupId name     -- Note [Record field lookup]

    {- Note [Record field lookup]
       ~~~~~~~~~~~~~~~~~~~~~~~~~~
    You might think we should have tcLookupGlobal here, since record fields
    are always top level.  But consider
            f = e { f = True }
    Then the renamer (which does not keep track of what is a record selector
    and what is not) will rename the definition thus
            f_7 = e { f_7 = True }
    Now the type checker will find f_7 in the *local* type environment, not
    the global (imported) one. It's wrong, of course, but we want to report a tidy
    error, not in TcEnv.notFound.  -}

    tcLookupDataCon :: Name -> TcM DataCon
    tcLookupDataCon name = do
        thing <- tcLookupGlobal name
        case thing of
            AConLike (RealDataCon con) -> return con
            _                          -> wrongThingErr "data constructor" (AGlobal thing) name

    tcLookupPatSyn :: Name -> TcM PatSyn
    tcLookupPatSyn name = do
        thing <- tcLookupGlobal name
        case thing of
            AConLike (PatSynCon ps) -> return ps
            _                       -> wrongThingErr "pattern synonym" (AGlobal thing) name

    tcLookupConLike :: Name -> TcM ConLike
    tcLookupConLike name = do
        thing <- tcLookupGlobal name
        case thing of
            AConLike cl -> return cl
            _           -> wrongThingErr "constructor-like thing" (AGlobal thing) name

    tcLookupClass :: Name -> TcM Class
    tcLookupClass name = do
        thing <- tcLookupGlobal name
        case thing of
            ATyCon tc | Just cls <- tyConClass_maybe tc -> return cls
            _                                           -> wrongThingErr "class" (AGlobal thing) name

    tcLookupTyCon :: Name -> TcM TyCon
    tcLookupTyCon name = do
        thing <- tcLookupGlobal name
        case thing of
            ATyCon tc -> return tc
            _         -> wrongThingErr "type constructor" (AGlobal thing) name

    tcLookupAxiom :: Name -> TcM (CoAxiom Branched)
    tcLookupAxiom name = do
        thing <- tcLookupGlobal name
        case thing of
            ACoAxiom ax -> return ax
            _           -> wrongThingErr "axiom" (AGlobal thing) name

    tcLookupLocatedGlobalId :: Located Name -> TcM Id
    tcLookupLocatedGlobalId = addLocM tcLookupId

    tcLookupLocatedClass :: Located Name -> TcM Class
    tcLookupLocatedClass = addLocM tcLookupClass

    tcLookupLocatedTyCon :: Located Name -> TcM TyCon
    tcLookupLocatedTyCon = addLocM tcLookupTyCon

    -- Find the instance that exactly matches a type class application.  The class arguments must be precisely
    -- the same as in the instance declaration (modulo renaming).
    --
    tcLookupInstance :: Class -> [Type] -> TcM ClsInst
    tcLookupInstance cls tys
      = do { instEnv <- tcGetInstEnvs
           ; case lookupUniqueInstEnv instEnv cls tys of
               Left err             -> failWithTc $ ptext (sLit "Couldn't match instance:") <+> err
               Right (inst, tys)
                 | uniqueTyVars tys -> return inst
                 | otherwise        -> failWithTc errNotExact
           }
      where
        errNotExact = ptext (sLit "Not an exact match (i.e., some variables get instantiated)")

        uniqueTyVars tys = all isTyVarTy tys && hasNoDups (map extractTyVar tys)
          where
            extractTyVar (TyVarTy tv) = tv
            extractTyVar _            = panic "TcEnv.tcLookupInstance: extractTyVar"

    tcGetInstEnvs :: TcM InstEnvs
    -- Gets both the external-package inst-env
    -- and the home-pkg inst env (includes module being compiled)
    tcGetInstEnvs = do { eps <- getEps
                       ; env <- getGblEnv
                       ; return (InstEnvs { ie_global  = eps_inst_env eps
                                          , ie_local   = tcg_inst_env env
                                          , ie_visible = tcg_visible_orphan_mods env }) }

    instance MonadThings (IOEnv (Env TcGblEnv TcLclEnv)) where
        lookupThing = tcLookupGlobal

    {-
    ************************************************************************
    *                                                                      *
                    Extending the global environment
    *                                                                      *
    ************************************************************************
    -}

    setGlobalTypeEnv :: TcGblEnv -> TypeEnv -> TcM TcGblEnv
    -- Use this to update the global type env
    -- It updates both  * the normal tcg_type_env field
    --                  * the tcg_type_env_var field seen by interface files
    setGlobalTypeEnv tcg_env new_type_env
      = do  {     -- Sync the type-envt variable seen by interface files
               writeMutVar (tcg_type_env_var tcg_env) new_type_env
             ; return (tcg_env { tcg_type_env = new_type_env }) }

    tcExtendGlobalEnvImplicit :: [TyThing] -> TcM r -> TcM r
      -- Extend the global environment with some TyThings that can be obtained
      -- via implicitTyThings from other entities in the environment.  Examples
      -- are dfuns, famInstTyCons, data cons, etc.
      -- These TyThings are not added to tcg_tcs.
    tcExtendGlobalEnvImplicit things thing_inside
       = do { tcg_env <- getGblEnv
            ; let ge'  = extendTypeEnvList (tcg_type_env tcg_env) things
            ; tcg_env' <- setGlobalTypeEnv tcg_env ge'
            ; setGblEnv tcg_env' thing_inside }

    tcExtendGlobalEnv :: [TyThing] -> TcM r -> TcM r
      -- Given a mixture of Ids, TyCons, Classes, all defined in the
      -- module being compiled, extend the global environment
    tcExtendGlobalEnv things thing_inside
      = do { env <- getGblEnv
           ; let env' = env { tcg_tcs = [tc | ATyCon tc <- things] ++ tcg_tcs env,
                              tcg_patsyns = [ps | AConLike (PatSynCon ps) <- things] ++ tcg_patsyns env }
           ; setGblEnv env' $
                tcExtendGlobalEnvImplicit things thing_inside
           }

    tcExtendGlobalValEnv :: [Id] -> TcM a -> TcM a
      -- Same deal as tcExtendGlobalEnv, but for Ids
    tcExtendGlobalValEnv ids thing_inside
      = tcExtendGlobalEnvImplicit [AnId id | id <- ids] thing_inside

    tcExtendRecEnv :: [(Name,TyThing)] -> TcM r -> TcM r
    -- Extend the global environments for the type/class knot tying game
    -- Just like tcExtendGlobalEnv, except the argument is a list of pairs
    tcExtendRecEnv gbl_stuff thing_inside
     = do  { tcg_env <- getGblEnv
           ; let ge' = extendNameEnvList (tcg_type_env tcg_env) gbl_stuff
           ; tcg_env' <- setGlobalTypeEnv tcg_env ge'
           ; setGblEnv tcg_env' thing_inside }

    {-
    ************************************************************************
    *                                                                      *
    \subsection{The local environment}
    *                                                                      *
    ************************************************************************
    -}

    tcLookupLocated :: Located Name -> TcM TcTyThing
    tcLookupLocated = addLocM tcLookup

    tcLookupLcl_maybe :: Name -> TcM (Maybe TcTyThing)
    tcLookupLcl_maybe name
      = do { local_env <- getLclTypeEnv
           ; return (lookupNameEnv local_env name) }

    tcLookup :: Name -> TcM TcTyThing
    tcLookup name = do
        local_env <- getLclTypeEnv
        case lookupNameEnv local_env name of
            Just thing -> return thing
            Nothing    -> AGlobal <$> tcLookupGlobal name

    tcLookupTyVar :: Name -> TcM TcTyVar
    tcLookupTyVar name
      = do { thing <- tcLookup name
           ; case thing of
               ATyVar _ tv -> return tv
               _           -> pprPanic "tcLookupTyVar" (ppr name) }

    tcLookupId :: Name -> TcM Id
    -- Used when we aren't interested in the binding level, nor refinement.
    -- The "no refinement" part means that we return the un-refined Id regardless
    --
    -- The Id is never a DataCon. (Why does that matter? see TcExpr.tcId)
    tcLookupId name = do
        thing <- tcLookup name
        case thing of
            ATcId { tct_id = id} -> return id
            AGlobal (AnId id)    -> return id
            _                    -> pprPanic "tcLookupId" (ppr name)

    tcLookupLocalIds :: [Name] -> TcM [TcId]
    -- We expect the variables to all be bound, and all at
    -- the same level as the lookup.  Only used in one place...
    tcLookupLocalIds ns
      = do { env <- getLclEnv
           ; return (map (lookup (tcl_env env)) ns) }
      where
        lookup lenv name
            = case lookupNameEnv lenv name of
                    Just (ATcId { tct_id = id }) ->  id
                    _ -> pprPanic "tcLookupLocalIds" (ppr name)

    getInLocalScope :: TcM (Name -> Bool)
      -- Ids only
    getInLocalScope = do { lcl_env <- getLclTypeEnv
                         ; return (`elemNameEnv` lcl_env) }

    tcExtendKindEnv2 :: [(Name, TcTyThing)] -> TcM r -> TcM r
    -- Used only during kind checking, for TcThings that are
    --      AThing or APromotionErr
    -- No need to update the global tyvars, or tcl_th_bndrs, or tcl_rdr
    tcExtendKindEnv2 things thing_inside
      = updLclEnv upd_env thing_inside
      where
        upd_env env = env { tcl_env = extendNameEnvList (tcl_env env) things }

    tcExtendKindEnv :: [(Name, TcKind)] -> TcM r -> TcM r
    tcExtendKindEnv name_kind_prs
      = tcExtendKindEnv2 [(n, AThing k) | (n,k) <- name_kind_prs]

    -----------------------
    -- Scoped type and kind variables
    tcExtendTyVarEnv :: [TyVar] -> TcM r -> TcM r
    tcExtendTyVarEnv tvs thing_inside
      = tcExtendTyVarEnv2 [(tyVarName tv, tv) | tv <- tvs] thing_inside

    tcExtendTyVarEnv2 :: [(Name,TcTyVar)] -> TcM r -> TcM r
    tcExtendTyVarEnv2 binds thing_inside
      = do { stage <- getStage
           ; tc_extend_local_env (NotTopLevel, thLevel stage)
                        [(name, ATyVar name tv) | (name, tv) <- binds] $
             do { env <- getLclEnv
                ; let env' = env { tcl_tidy = add_tidy_tvs (tcl_tidy env) }
                ; setLclEnv env' thing_inside }}
      where
        add_tidy_tvs env = foldl add env binds

        -- We initialise the "tidy-env", used for tidying types before printing,
        -- by building a reverse map from the in-scope type variables to the
        -- OccName that the programmer originally used for them
        add :: TidyEnv -> (Name, TcTyVar) -> TidyEnv
        add (env,subst) (name, tyvar)
            = case tidyOccName env (nameOccName name) of
                (env', occ') ->  (env', extendVarEnv subst tyvar tyvar')
                    where
                      tyvar' = setTyVarName tyvar name'
                      name'  = tidyNameOcc name occ'

    getScopedTyVarBinds :: TcM [(Name, TcTyVar)]
    getScopedTyVarBinds
      = do  { lcl_env <- getLclEnv
            ; return [(name, tv) | ATyVar name tv <- nameEnvElts (tcl_env lcl_env)] }

    {-
    Note [Initialising the type environment for GHCi]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    tcExtendGhciIdEnv extends the local type environemnt with GHCi
    identifiers (from ic_tythings), bound earlier in the interaction.
    They may have free type variables (RuntimeUnk things), and if we don't
    register these free TyVars as global TyVars then the typechecker will
    try to quantify over them and fall over in zonkQuantifiedTyVar.
    So we must add any free TyVars to the typechecker's global
    TyVar set.  That is most conveniently done here, using the local function
    tcExtendLocalTypeEnv.

    Note especially that

     * tcExtendGhciIdEnv extends the local type env, tcl_env
       That's important because some are not closed (ie have free tyvars)
       and the compiler assumes that the global type env (tcg_type_env) has
       no free tyvars.  Actually, only ones with Internal names can be non-closed
       so we jsut add those

     * The tct_closed flag depends on whether the thing has free (RuntimeUnk)
       type variables

     * It will also does tcExtendGlobalTyVars; this is important
       because of those RuntimeUnk variables

     * It does not extend the local RdrEnv (tcl_rdr), because the things are
       already in the GlobalRdrEnv.  Extending the local RdrEnv isn't terrible,
       but it means there is an entry for the same Name in both global and local
       RdrEnvs, and that lead to duplicate "perhaps you meant..." suggestions
       (e.g. T5564).

       We don't bother with the tcl_th_bndrs environment either.

     * NB: all these TcTyThings will be in the global type envt (tcg_type_env) as
           well.  We are just shadowing them here to deal with the global tyvar
           stuff.  That's why we can simply drop the External-Name ones; they
           will be found in the global envt
    -}

    tcExtendGhciIdEnv :: [TyThing] -> TcM a -> TcM a
    -- Used to bind Ids for GHCi identifiers bound earlier in the user interaction
    -- See Note [Initialising the type environment for GHCi]
    tcExtendGhciIdEnv ids thing_inside
      = do { lcl_env <- tcExtendLocalTypeEnv tc_ty_things emptyVarSet
           ; setLclEnv lcl_env thing_inside }
      where
        tc_ty_things =  [ (name, ATcId { tct_id     = id
                                       , tct_closed = is_top id })
                        | AnId id <- ids
                        , let name = idName id
                        , isInternalName name ]
        is_top id | isEmptyVarSet (tyVarsOfType (idType id)) = TopLevel
                  | otherwise                                = NotTopLevel

    tcExtendLetEnv :: TopLevelFlag -> TopLevelFlag -> [TcId] -> TcM a -> TcM a
    -- Used for both top-level value bindings and and nested let/where-bindings
    tcExtendLetEnv top_lvl closed ids thing_inside
      = do  { stage <- getStage
            ; tc_extend_local_env (top_lvl, thLevel stage)
                                  [ (idName id, ATcId { tct_id = id
                                                      , tct_closed = closed })
                                  | id <- ids] $
              tcExtendIdBndrs [TcIdBndr id top_lvl | id <- ids] thing_inside }

    tcExtendIdEnv :: [TcId] -> TcM a -> TcM a
    tcExtendIdEnv ids thing_inside
      = tcExtendIdEnv2 [(idName id, id) | id <- ids] $
        tcExtendIdBndrs [TcIdBndr id NotTopLevel | id <- ids]
        thing_inside

    tcExtendIdEnv1 :: Name -> TcId -> TcM a -> TcM a
    tcExtendIdEnv1 name id thing_inside
      = tcExtendIdEnv2 [(name,id)] $
        tcExtendIdBndrs [TcIdBndr id NotTopLevel]
        thing_inside

    tcExtendIdEnv2 :: [(Name,TcId)] -> TcM a -> TcM a
    -- Do *not* extend the tcl_bndrs stack
    -- The tct_closed flag really doesn't matter
    -- Invariant: the TcIds are fully zonked (see tcExtendIdEnv above)
    tcExtendIdEnv2 names_w_ids thing_inside
      = tcExtendIdEnv3 names_w_ids emptyVarSet thing_inside

    -- | 'tcExtendIdEnv2', but don't bind the 'TcId's in the 'TyVarSet' argument.
    tcExtendIdEnv3 :: [(Name,TcId)] -> TyVarSet -> TcM a -> TcM a
    -- Invariant: the TcIds are fully zonked (see tcExtendIdEnv above)
    tcExtendIdEnv3 names_w_ids not_actually_free thing_inside
      = do  { stage <- getStage
            ; tc_extend_local_env2 (NotTopLevel, thLevel stage)
                                  [ (name, ATcId { tct_id = id
                                                 , tct_closed = NotTopLevel })
                                  | (name,id) <- names_w_ids] not_actually_free $
              thing_inside }

    tcExtendIdBndrs :: [TcIdBinder] -> TcM a -> TcM a
    tcExtendIdBndrs bndrs = updLclEnv (\env -> env { tcl_bndrs = bndrs ++ tcl_bndrs env })

    tc_extend_local_env :: (TopLevelFlag, ThLevel) -> [(Name, TcTyThing)] -> TcM a -> TcM a
    tc_extend_local_env thlvl extra_env thing_inside =
      tc_extend_local_env2 thlvl extra_env emptyVarSet thing_inside

    tc_extend_local_env2 :: (TopLevelFlag, ThLevel) -> [(Name, TcTyThing)]
                         -> TyVarSet -> TcM a -> TcM a
    tc_extend_local_env2 thlvl extra_env not_actually_free thing_inside
    -- Precondition: the argument list extra_env has TcTyThings
    --               that ATcId or ATyVar, but nothing else
    --
    -- Invariant: the ATcIds are fully zonked. Reasons:
    --      (a) The kinds of the forall'd type variables are defaulted
    --          (see Kind.defaultKind, done in zonkQuantifiedTyVar)
    --      (b) There are no via-Indirect occurrences of the bound variables
    --          in the types, because instantiation does not look through such things
    --      (c) The call to tyVarsOfTypes is ok without looking through refs

    -- The second argument of type TyVarSet is a set of type variables
    -- that are bound together with extra_env and should not be regarded
    -- as free in the types of extra_env.
      = do  { traceTc "env2" (ppr extra_env)
            ; env1 <- tcExtendLocalTypeEnv extra_env not_actually_free
            ; let env2 = extend_local_env thlvl extra_env env1
            ; setLclEnv env2 thing_inside }
      where
        extend_local_env :: (TopLevelFlag, ThLevel) -> [(Name, TcTyThing)] -> TcLclEnv -> TcLclEnv
        -- Extend the local LocalRdrEnv and Template Haskell staging env simultaneously
        -- Reason for extending LocalRdrEnv: after running a TH splice we need
        -- to do renaming.
        extend_local_env thlvl pairs env@(TcLclEnv { tcl_rdr = rdr_env
                                                   , tcl_th_bndrs = th_bndrs })
          = env { tcl_rdr      = extendLocalRdrEnvList rdr_env
                                    [ n | (n, _) <- pairs, isInternalName n ]
                                    -- The LocalRdrEnv contains only non-top-level names
                                    -- (GlobalRdrEnv handles the top level)
                , tcl_th_bndrs = extendNameEnvList th_bndrs  -- We only track Ids in tcl_th_bndrs
                                     [(n, thlvl) | (n, ATcId {}) <- pairs] }

    tcExtendLocalTypeEnv :: [(Name, TcTyThing)] -> TyVarSet -> TcM TcLclEnv
    tcExtendLocalTypeEnv tc_ty_things not_actually_free
      | isEmptyVarSet extra_tvs
      = do { lcl_env@(TcLclEnv { tcl_env = lcl_type_env }) <- getLclEnv
           ; return (lcl_env { tcl_env = extendNameEnvList lcl_type_env tc_ty_things } ) }
      | otherwise
      = do { lcl_env@(TcLclEnv { tcl_env = lcl_type_env }) <- getLclEnv
           ; global_tvs <- readMutVar (tcl_tyvars lcl_env)
           ; new_g_var  <- newMutVar (global_tvs `unionVarSet` extra_tvs)
           ; return (lcl_env { tcl_tyvars = new_g_var
                             , tcl_env = extendNameEnvList lcl_type_env tc_ty_things } ) }
      where
        extra_tvs = foldr get_tvs emptyVarSet tc_ty_things `minusVarSet` not_actually_free

        get_tvs (_, ATcId { tct_id = id, tct_closed = closed }) tvs
          = case closed of
              TopLevel    -> ASSERT2( isEmptyVarSet id_tvs, ppr id $$ ppr (idType id) )
                             tvs
              NotTopLevel -> tvs `unionVarSet` id_tvs
            where id_tvs = tyVarsOfType (idType id)

        get_tvs (_, ATyVar _ tv) tvs          -- See Note [Global TyVars]
          = tvs `unionVarSet` tyVarsOfType (tyVarKind tv) `extendVarSet` tv

        get_tvs (_, AThing k) tvs = tvs `unionVarSet` tyVarsOfType k

        get_tvs (_, AGlobal {})       tvs = tvs
        get_tvs (_, APromotionErr {}) tvs = tvs

            -- Note [Global TyVars]
            -- It's important to add the in-scope tyvars to the global tyvar set
            -- as well.  Consider
            --      f (_::r) = let g y = y::r in ...
            -- Here, g mustn't be generalised.  This is also important during
            -- class and instance decls, when we mustn't generalise the class tyvars
            -- when typechecking the methods.
            --
            -- Nor must we generalise g over any kind variables free in r's kind

    zapLclTypeEnv :: TcM a -> TcM a
    zapLclTypeEnv thing_inside
      = do { tvs_var <- newTcRef emptyVarSet
           ; let upd env = env { tcl_env = emptyNameEnv
                               , tcl_rdr = emptyLocalRdrEnv
                               , tcl_tyvars = tvs_var }
           ; updLclEnv upd thing_inside }

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Rules}
    *                                                                      *
    ************************************************************************
    -}

    tcExtendRules :: [LRuleDecl Id] -> TcM a -> TcM a
            -- Just pop the new rules into the EPS and envt resp
            -- All the rules come from an interface file, not source
            -- Nevertheless, some may be for this module, if we read
            -- its interface instead of its source code
    tcExtendRules lcl_rules thing_inside
     = do { env <- getGblEnv
          ; let
              env' = env { tcg_rules = lcl_rules ++ tcg_rules env }
          ; setGblEnv env' thing_inside }

    {-
    ************************************************************************
    *                                                                      *
                    Meta level
    *                                                                      *
    ************************************************************************
    -}

    checkWellStaged :: SDoc         -- What the stage check is for
                    -> ThLevel      -- Binding level (increases inside brackets)
                    -> ThLevel      -- Use stage
                    -> TcM ()       -- Fail if badly staged, adding an error
    checkWellStaged pp_thing bind_lvl use_lvl
      | use_lvl >= bind_lvl         -- OK! Used later than bound
      = return ()                   -- E.g.  \x -> [| $(f x) |]

      | bind_lvl == outerLevel      -- GHC restriction on top level splices
      = stageRestrictionError pp_thing

      | otherwise                   -- Badly staged
      = failWithTc $                -- E.g.  \x -> $(f x)
        ptext (sLit "Stage error:") <+> pp_thing <+>
            hsep   [ptext (sLit "is bound at stage") <+> ppr bind_lvl,
                    ptext (sLit "but used at stage") <+> ppr use_lvl]

    stageRestrictionError :: SDoc -> TcM a
    stageRestrictionError pp_thing
      = failWithTc $
        sep [ ptext (sLit "GHC stage restriction:")
            , nest 2 (vcat [ pp_thing <+> ptext (sLit "is used in a top-level splice or annotation,")
                           , ptext (sLit "and must be imported, not defined locally")])]

    topIdLvl :: Id -> ThLevel
    -- Globals may either be imported, or may be from an earlier "chunk"
    -- (separated by declaration splices) of this module.  The former
    --  *can* be used inside a top-level splice, but the latter cannot.
    -- Hence we give the former impLevel, but the latter topLevel
    -- E.g. this is bad:
    --      x = [| foo |]
    --      $( f x )
    -- By the time we are prcessing the $(f x), the binding for "x"
    -- will be in the global env, not the local one.
    topIdLvl id | isLocalId id = outerLevel
                | otherwise    = impLevel

    tcMetaTy :: Name -> TcM Type
    -- Given the name of a Template Haskell data type,
    -- return the type
    -- E.g. given the name "Expr" return the type "Expr"
    tcMetaTy tc_name = do
        t <- tcLookupTyCon tc_name
        return (mkTyConApp t [])

    isBrackStage :: ThStage -> Bool
    isBrackStage (Brack {}) = True
    isBrackStage _other     = False

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

    tcGetDefaultTys :: TcM ([Type], -- Default types
                            (Bool,  -- True <=> Use overloaded strings
                             Bool)) -- True <=> Use extended defaulting rules
    tcGetDefaultTys
      = do  { dflags <- getDynFlags
            ; let ovl_strings = xopt Opt_OverloadedStrings dflags
                  extended_defaults = xopt Opt_ExtendedDefaultRules dflags
                                            -- See also Trac #1974
                  flags = (ovl_strings, extended_defaults)

            ; mb_defaults <- getDeclaredDefaultTys
            ; case mb_defaults of {
               Just tys -> return (tys, flags) ;
                                    -- User-supplied defaults
               Nothing  -> do

            -- No use-supplied default
            -- Use [Integer, Double], plus modifications
            { integer_ty <- tcMetaTy integerTyConName
            ; checkWiredInTyCon doubleTyCon
            ; string_ty <- tcMetaTy stringTyConName
            ; let deflt_tys = opt_deflt extended_defaults unitTy  -- Note [Default unitTy]
                              ++ [integer_ty, doubleTy]
                              ++ opt_deflt ovl_strings string_ty
            ; return (deflt_tys, flags) } } }
      where
        opt_deflt True  ty = [ty]
        opt_deflt False _  = []

    {-
    Note [Default unitTy]
    ~~~~~~~~~~~~~~~~~~~~~
    In interative mode (or with -XExtendedDefaultRules) we add () as the first type we
    try when defaulting.  This has very little real impact, except in the following case.
    Consider:
            Text.Printf.printf "hello"
    This has type (forall a. IO a); it prints "hello", and returns 'undefined'.  We don't
    want the GHCi repl loop to try to print that 'undefined'.  The neatest thing is to
    default the 'a' to (), rather than to Integer (which is what would otherwise happen;
    and then GHCi doesn't attempt to print the ().  So in interactive mode, we add
    () to the list of defaulting types.  See Trac #1200.

    ************************************************************************
    *                                                                      *
    \subsection{The InstInfo type}
    *                                                                      *
    ************************************************************************

    The InstInfo type summarises the information in an instance declaration

        instance c => k (t tvs) where b

    It is used just for *local* instance decls (not ones from interface files).
    But local instance decls includes
            - derived ones
            - generic ones
    as well as explicit user written ones.
    -}

    data InstInfo a
      = InstInfo {
          iSpec   :: ClsInst,        -- Includes the dfun id.  Its forall'd type
          iBinds  :: InstBindings a   -- variables scope over the stuff in InstBindings!
        }

    iDFunId :: InstInfo a -> DFunId
    iDFunId info = instanceDFunId (iSpec info)

    data InstBindings a
      = InstBindings
          { ib_tyvars  :: [Name]        -- Names of the tyvars from the instance head
                                        -- that are lexically in scope in the bindings

          , ib_binds   :: (LHsBinds a)  -- Bindings for the instance methods

          , ib_pragmas :: [LSig a]      -- User pragmas recorded for generating
                                        -- specialised instances

          , ib_extensions :: [ExtensionFlag] -- Any extra extensions that should
                                             -- be enabled when type-checking this
                                             -- instance; needed for
                                             -- GeneralizedNewtypeDeriving

          , ib_derived :: Bool
               -- True <=> This code was generated by GHC from a deriving clause
               --          or standalone deriving declaration
               -- Used only to improve error messages
          }

    instance OutputableBndr a => Outputable (InstInfo a) where
        ppr = pprInstInfoDetails

    pprInstInfoDetails :: OutputableBndr a => InstInfo a -> SDoc
    pprInstInfoDetails info
       = hang (pprInstanceHdr (iSpec info) <+> ptext (sLit "where"))
            2 (details (iBinds info))
      where
        details (InstBindings { ib_binds = b }) = pprLHsBinds b

    simpleInstInfoClsTy :: InstInfo a -> (Class, Type)
    simpleInstInfoClsTy info = case instanceHead (iSpec info) of
                               (_, cls, [ty]) -> (cls, ty)
                               _ -> panic "simpleInstInfoClsTy"

    simpleInstInfoTy :: InstInfo a -> Type
    simpleInstInfoTy info = snd (simpleInstInfoClsTy info)

    simpleInstInfoTyCon :: InstInfo a -> TyCon
      -- Gets the type constructor for a simple instance declaration,
      -- i.e. one of the form       instance (...) => C (T a b c) where ...
    simpleInstInfoTyCon inst = tcTyConAppTyCon (simpleInstInfoTy inst)

    {-
    Make a name for the dict fun for an instance decl.  It's an *external*
    name, like otber top-level names, and hence must be made with newGlobalBinder.
    -}

    newDFunName :: Class -> [Type] -> SrcSpan -> TcM Name
    newDFunName clas tys loc
      = do  { is_boot <- tcIsHsBootOrSig
            ; mod     <- getModule
            ; let info_string = occNameString (getOccName clas) ++
                                concatMap (occNameString.getDFunTyKey) tys
            ; dfun_occ <- chooseUniqueOccTc (mkDFunOcc info_string is_boot)
            ; newGlobalBinder mod dfun_occ loc }

    {-
    Make a name for the representation tycon of a family instance.  It's an
    *external* name, like other top-level names, and hence must be made with
    newGlobalBinder.
    -}

    newFamInstTyConName :: Located Name -> [Type] -> TcM Name
    newFamInstTyConName (L loc name) tys = mk_fam_inst_name id loc name [tys]

    newFamInstAxiomName :: SrcSpan -> Name -> [CoAxBranch] -> TcM Name
    newFamInstAxiomName loc name branches
      = mk_fam_inst_name mkInstTyCoOcc loc name (map coAxBranchLHS branches)

    mk_fam_inst_name :: (OccName -> OccName) -> SrcSpan -> Name -> [[Type]] -> TcM Name
    mk_fam_inst_name adaptOcc loc tc_name tyss
      = do  { mod   <- getModule
            ; let info_string = occNameString (getOccName tc_name) ++
                                intercalate "|" ty_strings
            ; occ   <- chooseUniqueOccTc (mkInstTyTcOcc info_string)
            ; newGlobalBinder mod (adaptOcc occ) loc }
      where
        ty_strings = map (concatMap (occNameString . getDFunTyKey)) tyss

    {-
    Stable names used for foreign exports and annotations.
    For stable names, the name must be unique (see #1533).  If the
    same thing has several stable Ids based on it, the
    top-level bindings generated must not have the same name.
    Hence we create an External name (doesn't change), and we
    append a Unique to the string right here.
    -}

    mkStableIdFromString :: String -> Type -> SrcSpan -> (OccName -> OccName) -> TcM TcId
    mkStableIdFromString str sig_ty loc occ_wrapper = do
        uniq <- newUnique
        mod <- getModule
        name <- mkWrapperName "stable" str
        let occ = mkVarOccFS name :: OccName
            gnm = mkExternalName uniq mod (occ_wrapper occ) loc :: Name
            id  = mkExportedLocalId VanillaId gnm sig_ty :: Id
        return id

    mkStableIdFromName :: Name -> Type -> SrcSpan -> (OccName -> OccName) -> TcM TcId
    mkStableIdFromName nm = mkStableIdFromString (getOccString nm)

    mkWrapperName :: (MonadIO m, HasDynFlags m, HasModule m)
                  => String -> String -> m FastString
    mkWrapperName what nameBase
        = do dflags <- getDynFlags
             thisMod <- getModule
             let -- Note [Generating fresh names for ccall wrapper]
                 wrapperRef = nextWrapperNum dflags
                 pkg = packageKeyString  (modulePackageKey thisMod)
                 mod = moduleNameString (moduleName      thisMod)
             wrapperNum <- liftIO $ atomicModifyIORef wrapperRef $ \mod_env ->
                 let num = lookupWithDefaultModuleEnv mod_env 0 thisMod
                     mod_env' = extendModuleEnv mod_env thisMod (num+1)
                 in (mod_env', num)
             let components = [what, show wrapperNum, pkg, mod, nameBase]
             return $ mkFastString $ zEncodeString $ intercalate ":" components

    {-
    Note [Generating fresh names for FFI wrappers]

    We used to use a unique, rather than nextWrapperNum, to distinguish
    between FFI wrapper functions. However, the wrapper names that we
    generate are external names. This means that if a call to them ends up
    in an unfolding, then we can't alpha-rename them, and thus if the
    unique randomly changes from one compile to another then we get a
    spurious ABI change (#4012).

    The wrapper counter has to be per-module, not global, so that the number we end
    up using is not dependent on the modules compiled before the current one.
    -}

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Errors}
    *                                                                      *
    ************************************************************************
    -}

    pprBinders :: [Name] -> SDoc
    -- Used in error messages
    -- Use quotes for a single one; they look a bit "busy" for several
    pprBinders [bndr] = quotes (ppr bndr)
    pprBinders bndrs  = pprWithCommas ppr bndrs

    notFound :: Name -> TcM TyThing
    notFound name
      = do { lcl_env <- getLclEnv
           ; let stage = tcl_th_ctxt lcl_env
           ; case stage of   -- See Note [Out of scope might be a staging error]
               Splice {} -> stageRestrictionError (quotes (ppr name))
               _ -> failWithTc $
                    vcat[ptext (sLit "GHC internal error:") <+> quotes (ppr name) <+>
                         ptext (sLit "is not in scope during type checking, but it passed the renamer"),
                         ptext (sLit "tcl_env of environment:") <+> ppr (tcl_env lcl_env)]
                           -- Take case: printing the whole gbl env can
                           -- cause an infinite loop, in the case where we
                           -- are in the middle of a recursive TyCon/Class group;
                           -- so let's just not print it!  Getting a loop here is
                           -- very unhelpful, because it hides one compiler bug with another
           }

    wrongThingErr :: String -> TcTyThing -> Name -> TcM a
    -- It's important that this only calls pprTcTyThingCategory, which in
    -- turn does not look at the details of the TcTyThing.
    -- See Note [Placeholder PatSyn kinds] in TcBinds
    wrongThingErr expected thing name
      = failWithTc (pprTcTyThingCategory thing <+> quotes (ppr name) <+>
                    ptext (sLit "used as a") <+> text expected)

    {-
    Note [Out of scope might be a staging error]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
      x = 3
      data T = MkT $(foo x)

    This is really a staging error, because we can't run code involving 'x'.
    But in fact the type checker processes types first, so 'x' won't even be
    in the type envt when we look for it in $(foo x).  So inside splices we
    report something missing from the type env as a staging error.
    See Trac #5752 and #5795.
    -}