typecheck/TcMType.hs

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998

    Monadic type operations

    This module contains monadic operations over types that contain
    mutable type variables
    -}

    {-# LANGUAGE CPP #-}

    module TcMType (
      TcTyVar, TcKind, TcType, TcTauType, TcThetaType, TcTyVarSet,

      --------------------------------
      -- Creating new mutable type variables
      newFlexiTyVar,
      newFlexiTyVarTy,              -- Kind -> TcM TcType
      newFlexiTyVarTys,             -- Int -> Kind -> TcM [TcType]
      newReturnTyVar, newReturnTyVarTy,
      newMetaKindVar, newMetaKindVars,
      mkTcTyVarName, cloneMetaTyVar,

      newMetaTyVar, readMetaTyVar, writeMetaTyVar, writeMetaTyVarRef,
      newMetaDetails, isFilledMetaTyVar, isUnfilledMetaTyVar,

      --------------------------------
      -- Creating new evidence variables
      newEvVar, newEvVars, newEq, newDict,
      newTcEvBinds, addTcEvBind,
      newSimpleWanted, newSimpleWanteds,

      --------------------------------
      -- Instantiation
      tcInstTyVars, newSigTyVar,
      tcInstType,
      tcInstSkolTyVars, tcInstSuperSkolTyVarsX,
      tcInstSigTyVarsLoc, tcInstSigTyVars,
      tcInstSkolType,
      tcSkolDFunType, tcSuperSkolTyVars,

      instSkolTyVars, freshenTyVarBndrs,

      --------------------------------
      -- Zonking and tidying
      zonkTcPredType, zonkTidyTcType, zonkTidyOrigin,
      tidyEvVar, tidyCt, tidySkolemInfo,
      skolemiseUnboundMetaTyVar,
      zonkTcTyVar, zonkTcTyVars, zonkTyVarsAndFV, zonkTcTypeAndFV,
      zonkQuantifiedTyVar, quantifyTyVars,
      zonkTcTyVarBndr, zonkTcType, zonkTcTypes, zonkTcThetaType,

      zonkTcKind, defaultKindVarToStar,
      zonkEvVar, zonkWC, zonkSimples, zonkId, zonkCt, zonkSkolemInfo,

      tcGetGlobalTyVars,

      --------------------------------
      -- (Named) Wildcards
      newWildcardVar, newWildcardVarMetaKind
      ) where

    #include "HsVersions.h"

    -- friends:
    import TypeRep
    import TcType
    import Type
    import Class
    import Var
    import VarEnv

    -- others:
    import TcRnMonad        -- TcType, amongst others
    import Id
    import Name
    import VarSet
    import PrelNames
    import DynFlags
    import Util
    import Outputable
    import FastString
    import SrcLoc
    import Bag

    import Control.Monad
    import Data.List        ( partition, mapAccumL )

    {-
    ************************************************************************
    *                                                                      *
            Kind variables
    *                                                                      *
    ************************************************************************
    -}

    mkKindName :: Unique -> Name
    mkKindName unique = mkSystemName unique kind_var_occ

    kind_var_occ :: OccName -- Just one for all MetaKindVars
                            -- They may be jiggled by tidying
    kind_var_occ = mkOccName tvName "k"

    newMetaKindVar :: TcM TcKind
    newMetaKindVar = do { uniq <- newUnique
                        ; details <- newMetaDetails (TauTv False)
                        ; let kv = mkTcTyVar (mkKindName uniq) superKind details
                        ; return (mkTyVarTy kv) }

    newMetaKindVars :: Int -> TcM [TcKind]
    newMetaKindVars n = mapM (\ _ -> newMetaKindVar) (nOfThem n ())

    {-
    ************************************************************************
    *                                                                      *
         Evidence variables; range over constraints we can abstract over
    *                                                                      *
    ************************************************************************
    -}

    newEvVars :: TcThetaType -> TcM [EvVar]
    newEvVars theta = mapM newEvVar theta

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

    newEvVar :: TcPredType -> TcM EvVar
    -- Creates new *rigid* variables for predicates
    newEvVar ty = do { name <- newSysName (predTypeOccName ty)
                     ; return (mkLocalId name ty) }

    newEq :: TcType -> TcType -> TcM EvVar
    newEq ty1 ty2
      = do { name <- newSysName (mkVarOccFS (fsLit "cobox"))
           ; return (mkLocalId name (mkTcEqPred ty1 ty2)) }

    newDict :: Class -> [TcType] -> TcM DictId
    newDict cls tys
      = do { name <- newSysName (mkDictOcc (getOccName cls))
           ; return (mkLocalId name (mkClassPred cls tys)) }

    predTypeOccName :: PredType -> OccName
    predTypeOccName ty = case classifyPredType ty of
        ClassPred cls _ -> mkDictOcc (getOccName cls)
        EqPred _ _ _    -> mkVarOccFS (fsLit "cobox")
        TuplePred _     -> mkVarOccFS (fsLit "tup")
        IrredPred _     -> mkVarOccFS (fsLit "irred")

    {-
    *********************************************************************************
    *                                                                               *
    *                   Wanted constraints
    *                                                                               *
    *********************************************************************************
    -}

    newSimpleWanted :: CtOrigin -> PredType -> TcM Ct
    newSimpleWanted orig pty
      = do loc <- getCtLoc orig
           v <- newEvVar pty
           return $ mkNonCanonical $
                CtWanted { ctev_evar = v
                         , ctev_pred = pty
                         , ctev_loc = loc }

    newSimpleWanteds :: CtOrigin -> ThetaType -> TcM [Ct]
    newSimpleWanteds orig = mapM (newSimpleWanted orig)

    {-
    ************************************************************************
    *                                                                      *
            SkolemTvs (immutable)
    *                                                                      *
    ************************************************************************
    -}

    tcInstType :: ([TyVar] -> TcM (TvSubst, [TcTyVar]))     -- How to instantiate the type variables
               -> TcType                                    -- Type to instantiate
               -> TcM ([TcTyVar], TcThetaType, TcType)      -- Result
                    -- (type vars (excl coercion vars), preds (incl equalities), rho)
    tcInstType inst_tyvars ty
      = case tcSplitForAllTys ty of
            ([],     rho) -> let    -- There may be overloading despite no type variables;
                                    --      (?x :: Int) => Int -> Int
                               (theta, tau) = tcSplitPhiTy rho
                             in
                             return ([], theta, tau)

            (tyvars, rho) -> do { (subst, tyvars') <- inst_tyvars tyvars
                                ; let (theta, tau) = tcSplitPhiTy (substTy subst rho)
                                ; return (tyvars', theta, tau) }

    tcSkolDFunType :: Type -> TcM ([TcTyVar], TcThetaType, TcType)
    -- Instantiate a type signature with skolem constants.
    -- We could give them fresh names, but no need to do so
    tcSkolDFunType ty = tcInstType tcInstSuperSkolTyVars ty

    tcSuperSkolTyVars :: [TyVar] -> (TvSubst, [TcTyVar])
    -- Make skolem constants, but do *not* give them new names, as above
    -- Moreover, make them "super skolems"; see comments with superSkolemTv
    -- see Note [Kind substitution when instantiating]
    -- Precondition: tyvars should be ordered (kind vars first)
    tcSuperSkolTyVars = mapAccumL tcSuperSkolTyVar (mkTopTvSubst [])

    tcSuperSkolTyVar :: TvSubst -> TyVar -> (TvSubst, TcTyVar)
    tcSuperSkolTyVar subst tv
      = (extendTvSubst subst tv (mkTyVarTy new_tv), new_tv)
      where
        kind   = substTy subst (tyVarKind tv)
        new_tv = mkTcTyVar (tyVarName tv) kind superSkolemTv

    tcInstSkolTyVars :: [TyVar] -> TcM (TvSubst, [TcTyVar])
    tcInstSkolTyVars = tcInstSkolTyVars' False emptyTvSubst

    tcInstSuperSkolTyVars :: [TyVar] -> TcM (TvSubst, [TcTyVar])
    tcInstSuperSkolTyVars = tcInstSuperSkolTyVarsX emptyTvSubst

    tcInstSuperSkolTyVarsX :: TvSubst -> [TyVar] -> TcM (TvSubst, [TcTyVar])
    tcInstSuperSkolTyVarsX subst = tcInstSkolTyVars' True subst

    tcInstSkolTyVars' :: Bool -> TvSubst -> [TyVar] -> TcM (TvSubst, [TcTyVar])
    -- Precondition: tyvars should be ordered (kind vars first)
    -- see Note [Kind substitution when instantiating]
    -- Get the location from the monad; this is a complete freshening operation
    tcInstSkolTyVars' overlappable subst tvs
      = do { loc <- getSrcSpanM
           ; instSkolTyVarsX (mkTcSkolTyVar loc overlappable) subst tvs }

    mkTcSkolTyVar :: SrcSpan -> Bool -> Unique -> Name -> Kind -> TcTyVar
    mkTcSkolTyVar loc overlappable uniq old_name kind
      = mkTcTyVar (mkInternalName uniq (getOccName old_name) loc)
                  kind
                  (SkolemTv overlappable)

    tcInstSigTyVarsLoc :: SrcSpan -> [TyVar] -> TcRnIf gbl lcl (TvSubst, [TcTyVar])
    -- We specify the location
    tcInstSigTyVarsLoc loc = instSkolTyVars (mkTcSkolTyVar loc False)

    tcInstSigTyVars :: [TyVar] -> TcRnIf gbl lcl (TvSubst, [TcTyVar])
    -- Get the location from the TyVar itself, not the monad
    tcInstSigTyVars
      = instSkolTyVars mk_tv
      where
        mk_tv uniq old_name kind
           = mkTcTyVar (setNameUnique old_name uniq) kind (SkolemTv False)

    tcInstSkolType :: TcType -> TcM ([TcTyVar], TcThetaType, TcType)
    -- Instantiate a type with fresh skolem constants
    -- Binding location comes from the monad
    tcInstSkolType ty = tcInstType tcInstSkolTyVars ty

    ------------------
    freshenTyVarBndrs :: [TyVar] -> TcRnIf gbl lcl (TvSubst, [TyVar])
    -- ^ Give fresh uniques to a bunch of TyVars, but they stay
    --   as TyVars, rather than becoming TcTyVars
    -- Used in FamInst.newFamInst, and Inst.newClsInst
    freshenTyVarBndrs = instSkolTyVars mk_tv
      where
        mk_tv uniq old_name kind = mkTyVar (setNameUnique old_name uniq) kind

    ------------------
    instSkolTyVars :: (Unique -> Name -> Kind -> TyVar)
                    -> [TyVar] -> TcRnIf gbl lcl (TvSubst, [TyVar])
    instSkolTyVars mk_tv = instSkolTyVarsX mk_tv emptyTvSubst

    instSkolTyVarsX :: (Unique -> Name -> Kind -> TyVar)
                    -> TvSubst -> [TyVar] -> TcRnIf gbl lcl (TvSubst, [TyVar])
    instSkolTyVarsX mk_tv = mapAccumLM (instSkolTyVarX mk_tv)

    instSkolTyVarX :: (Unique -> Name -> Kind -> TyVar)
                   -> TvSubst -> TyVar -> TcRnIf gbl lcl (TvSubst, TyVar)
    instSkolTyVarX mk_tv subst tyvar
      = do  { uniq <- newUnique
            ; let new_tv = mk_tv uniq old_name kind
            ; return (extendTvSubst subst tyvar (mkTyVarTy new_tv), new_tv) }
      where
        old_name = tyVarName tyvar
        kind     = substTy subst (tyVarKind tyvar)

    {-
    Note [Kind substitution when instantiating]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When we instantiate a bunch of kind and type variables, first we
    expect them to be sorted (kind variables first, then type variables).
    Then we have to instantiate the kind variables, build a substitution
    from old variables to the new variables, then instantiate the type
    variables substituting the original kind.

    Exemple: If we want to instantiate
      [(k1 :: BOX), (k2 :: BOX), (a :: k1 -> k2), (b :: k1)]
    we want
      [(?k1 :: BOX), (?k2 :: BOX), (?a :: ?k1 -> ?k2), (?b :: ?k1)]
    instead of the buggous
      [(?k1 :: BOX), (?k2 :: BOX), (?a :: k1 -> k2), (?b :: k1)]

    ************************************************************************
    *                                                                      *
            MetaTvs (meta type variables; mutable)
    *                                                                      *
    ************************************************************************
    -}

    newMetaTyVar :: MetaInfo -> Kind -> TcM TcTyVar
    -- Make a new meta tyvar out of thin air
    newMetaTyVar meta_info kind
      = do  { uniq <- newUnique
            ; let name = mkTcTyVarName uniq s
                  s = case meta_info of
                            ReturnTv    -> fsLit "r"
                            TauTv True  -> fsLit "w"
                            TauTv False -> fsLit "t"
                            FlatMetaTv  -> fsLit "fmv"
                            SigTv       -> fsLit "a"
            ; details <- newMetaDetails meta_info
            ; return (mkTcTyVar name kind details) }

    newNamedMetaTyVar :: Name -> MetaInfo -> Kind -> TcM TcTyVar
    -- Make a new meta tyvar out of thin air
    newNamedMetaTyVar name meta_info kind
      = do { details <- newMetaDetails meta_info
           ; return (mkTcTyVar name kind details) }

    newSigTyVar :: Name -> Kind -> TcM TcTyVar
    newSigTyVar name kind
      = do { uniq <- newUnique
           ; let name' = setNameUnique name uniq
                          -- Use the same OccName so that the tidy-er
                          -- doesn't gratuitously rename 'a' to 'a0' etc
           ; details <- newMetaDetails SigTv
           ; return (mkTcTyVar name' kind details) }

    newMetaDetails :: MetaInfo -> TcM TcTyVarDetails
    newMetaDetails info
      = do { ref <- newMutVar Flexi
           ; tclvl <- getTcLevel
           ; return (MetaTv { mtv_info = info, mtv_ref = ref, mtv_tclvl = tclvl }) }

    cloneMetaTyVar :: TcTyVar -> TcM TcTyVar
    cloneMetaTyVar tv
      = ASSERT( isTcTyVar tv )
        do  { uniq <- newUnique
            ; ref  <- newMutVar Flexi
            ; let name'    = setNameUnique (tyVarName tv) uniq
                  details' = case tcTyVarDetails tv of
                               details@(MetaTv {}) -> details { mtv_ref = ref }
                               _ -> pprPanic "cloneMetaTyVar" (ppr tv)
            ; return (mkTcTyVar name' (tyVarKind tv) details') }

    mkTcTyVarName :: Unique -> FastString -> Name
    mkTcTyVarName uniq str = mkSysTvName uniq str

    -- Works for both type and kind variables
    readMetaTyVar :: TyVar -> TcM MetaDetails
    readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
                          readMutVar (metaTvRef tyvar)

    isFilledMetaTyVar :: TyVar -> TcM Bool
    -- True of a filled-in (Indirect) meta type variable
    isFilledMetaTyVar tv
      | not (isTcTyVar tv) = return False
      | MetaTv { mtv_ref = ref } <- tcTyVarDetails tv
      = do  { details <- readMutVar ref
            ; return (isIndirect details) }
      | otherwise = return False

    isUnfilledMetaTyVar :: TyVar -> TcM Bool
    -- True of a un-filled-in (Flexi) meta type variable
    isUnfilledMetaTyVar tv
      | not (isTcTyVar tv) = return False
      | MetaTv { mtv_ref = ref } <- tcTyVarDetails tv
      = do  { details <- readMutVar ref
            ; return (isFlexi details) }
      | otherwise = return False

    --------------------
    -- Works with both type and kind variables
    writeMetaTyVar :: TcTyVar -> TcType -> TcM ()
    -- Write into a currently-empty MetaTyVar

    writeMetaTyVar tyvar ty
      | not debugIsOn
      = writeMetaTyVarRef tyvar (metaTvRef tyvar) ty

    -- Everything from here on only happens if DEBUG is on
      | not (isTcTyVar tyvar)
      = WARN( True, text "Writing to non-tc tyvar" <+> ppr tyvar )
        return ()

      | MetaTv { mtv_ref = ref } <- tcTyVarDetails tyvar
      = writeMetaTyVarRef tyvar ref ty

      | otherwise
      = WARN( True, text "Writing to non-meta tyvar" <+> ppr tyvar )
        return ()

    --------------------
    writeMetaTyVarRef :: TcTyVar -> TcRef MetaDetails -> TcType -> TcM ()
    -- Here the tyvar is for error checking only;
    -- the ref cell must be for the same tyvar
    writeMetaTyVarRef tyvar ref ty
      | not debugIsOn
      = do { traceTc "writeMetaTyVar" (ppr tyvar <+> text ":=" <+> ppr ty)
           ; writeMutVar ref (Indirect ty) }

    -- Everything from here on only happens if DEBUG is on
      | otherwise
      = do { meta_details <- readMutVar ref;
           -- Zonk kinds to allow the error check to work
           ; zonked_tv_kind <- zonkTcKind tv_kind
           ; zonked_ty_kind <- zonkTcKind ty_kind

           -- Check for double updates
           ; ASSERT2( isFlexi meta_details,
                      hang (text "Double update of meta tyvar")
                       2 (ppr tyvar $$ ppr meta_details) )

             traceTc "writeMetaTyVar" (ppr tyvar <+> text ":=" <+> ppr ty)
           ; writeMutVar ref (Indirect ty)
           ; when (   not (isPredTy tv_kind)
                        -- Don't check kinds for updates to coercion variables
                   && not (zonked_ty_kind `tcIsSubKind` zonked_tv_kind))
           $ WARN( True, hang (text "Ill-kinded update to meta tyvar")
                            2 (    ppr tyvar <+> text "::" <+> (ppr tv_kind $$ ppr zonked_tv_kind)
                               <+> text ":="
                               <+> ppr ty    <+> text "::" <+> (ppr ty_kind $$ ppr zonked_ty_kind) ) )
             (return ()) }
      where
        tv_kind = tyVarKind tyvar
        ty_kind = typeKind ty

    {-
    ************************************************************************
    *                                                                      *
            MetaTvs: TauTvs
    *                                                                      *
    ************************************************************************
    -}

    newFlexiTyVar :: Kind -> TcM TcTyVar
    newFlexiTyVar kind = newMetaTyVar (TauTv False) kind

    newFlexiTyVarTy  :: Kind -> TcM TcType
    newFlexiTyVarTy kind = do
        tc_tyvar <- newFlexiTyVar kind
        return (TyVarTy tc_tyvar)

    newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
    newFlexiTyVarTys n kind = mapM newFlexiTyVarTy (nOfThem n kind)

    newReturnTyVar :: Kind -> TcM TcTyVar
    newReturnTyVar kind = newMetaTyVar ReturnTv kind

    newReturnTyVarTy :: Kind -> TcM TcType
    newReturnTyVarTy kind = TyVarTy <$> newReturnTyVar kind

    tcInstTyVars :: [TKVar] -> TcM (TvSubst, [TcTyVar])
    -- Instantiate with META type variables
    -- Note that this works for a sequence of kind and type
    -- variables.  Eg    [ (k:BOX), (a:k->k) ]
    --             Gives [ (k7:BOX), (a8:k7->k7) ]
    tcInstTyVars tyvars = mapAccumLM tcInstTyVarX emptyTvSubst tyvars
        -- emptyTvSubst has an empty in-scope set, but that's fine here
        -- Since the tyvars are freshly made, they cannot possibly be
        -- captured by any existing for-alls.

    tcInstTyVarX :: TvSubst -> TKVar -> TcM (TvSubst, TcTyVar)
    -- Make a new unification variable tyvar whose Name and Kind come from
    -- an existing TyVar. We substitute kind variables in the kind.
    tcInstTyVarX subst tyvar
      = do  { uniq <- newUnique
            ; details <- newMetaDetails (TauTv False)
            ; let name   = mkSystemName uniq (getOccName tyvar)
                  kind   = substTy subst (tyVarKind tyvar)
                  new_tv = mkTcTyVar name kind details
            ; return (extendTvSubst subst tyvar (mkTyVarTy new_tv), new_tv) }

    {-
    ************************************************************************
    *                                                                      *
                 Quantification
    *                                                                      *
    ************************************************************************

    Note [quantifyTyVars]
    ~~~~~~~~~~~~~~~~~~~~~
    quantifyTyVars is give the free vars of a type that we
    are about to wrap in a forall.

    It takes these free type/kind variables and
      1. Zonks them and remove globals
      2. Partitions into type and kind variables (kvs1, tvs)
      3. Extends kvs1 with free kind vars in the kinds of tvs (removing globals)
      4. Calls zonkQuantifiedTyVar on each

    Step (3) is often unimportant, because the kind variable is often
    also free in the type.  Eg
         Typeable k (a::k)
    has free vars {k,a}.  But the type (see Trac #7916)
        (f::k->*) (a::k)
    has free vars {f,a}, but we must add 'k' as well! Hence step (3).
    -}

    quantifyTyVars :: TcTyVarSet -> TcTyVarSet -> TcM [TcTyVar]
    -- See Note [quantifyTyVars]
    -- The input is a mixture of type and kind variables; a kind variable k
    --   may occur *after* a tyvar mentioning k in its kind
    -- Can be given a mixture of TcTyVars and TyVars, in the case of
    --   associated type declarations

    quantifyTyVars gbl_tvs tkvs
      = do { tkvs    <- zonkTyVarsAndFV tkvs
           ; gbl_tvs <- zonkTyVarsAndFV gbl_tvs
           ; let (kvs, tvs) = partitionVarSet isKindVar (closeOverKinds tkvs `minusVarSet` gbl_tvs)
                                  -- NB kinds of tvs are zonked by zonkTyVarsAndFV
                 kvs2 = varSetElems kvs
                 qtvs = varSetElems tvs

                 -- In the non-PolyKinds case, default the kind variables
                 -- to *, and zonk the tyvars as usual.  Notice that this
                 -- may make quantifyTyVars return a shorter list
                 -- than it was passed, but that's ok
           ; poly_kinds <- xoptM Opt_PolyKinds
           ; qkvs <- if poly_kinds
                     then return kvs2
                     else do { let (meta_kvs, skolem_kvs) = partition is_meta kvs2
                                   is_meta kv = isTcTyVar kv && isMetaTyVar kv
                             ; mapM_ defaultKindVarToStar meta_kvs
                             ; return skolem_kvs }  -- should be empty

           ; mapM zonk_quant (qkvs ++ qtvs) }
               -- Because of the order, any kind variables
               -- mentioned in the kinds of the type variables refer to
               -- the now-quantified versions
      where
        zonk_quant tkv
          | isTcTyVar tkv = zonkQuantifiedTyVar tkv
          | otherwise     = return tkv
          -- For associated types, we have the class variables
          -- in scope, and they are TyVars not TcTyVars

    zonkQuantifiedTyVar :: TcTyVar -> TcM TcTyVar
    -- The quantified type variables often include meta type variables
    -- we want to freeze them into ordinary type variables, and
    -- default their kind (e.g. from OpenTypeKind to TypeKind)
    --                      -- see notes with Kind.defaultKind
    -- The meta tyvar is updated to point to the new skolem TyVar.  Now any
    -- bound occurrences of the original type variable will get zonked to
    -- the immutable version.
    --
    -- We leave skolem TyVars alone; they are immutable.
    --
    -- This function is called on both kind and type variables,
    -- but kind variables *only* if PolyKinds is on.
    zonkQuantifiedTyVar tv
      = ASSERT2( isTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
          SkolemTv {} -> do { kind <- zonkTcKind (tyVarKind tv)
                            ; return $ setTyVarKind tv kind }
            -- It might be a skolem type variable,
            -- for example from a user type signature

          MetaTv { mtv_ref = ref } ->
              do when debugIsOn $ do
                     -- [Sept 04] Check for non-empty.
                     -- See note [Silly Type Synonym]
                     cts <- readMutVar ref
                     case cts of
                         Flexi -> return ()
                         Indirect ty -> WARN( True, ppr tv $$ ppr ty )
                                        return ()
                 skolemiseUnboundMetaTyVar tv vanillaSkolemTv
          _other -> pprPanic "zonkQuantifiedTyVar" (ppr tv) -- FlatSkol, RuntimeUnk

    defaultKindVarToStar :: TcTyVar -> TcM Kind
    -- We have a meta-kind: unify it with '*'
    defaultKindVarToStar kv
      = do { ASSERT( isKindVar kv && isMetaTyVar kv )
             writeMetaTyVar kv liftedTypeKind
           ; return liftedTypeKind }

    skolemiseUnboundMetaTyVar :: TcTyVar -> TcTyVarDetails -> TcM TyVar
    -- We have a Meta tyvar with a ref-cell inside it
    -- Skolemise it, including giving it a new Name, so that
    --   we are totally out of Meta-tyvar-land
    -- We create a skolem TyVar, not a regular TyVar
    --   See Note [Zonking to Skolem]
    skolemiseUnboundMetaTyVar tv details
      = ASSERT2( isMetaTyVar tv, ppr tv )
        do  { span <- getSrcSpanM    -- Get the location from "here"
                                     -- ie where we are generalising
            ; uniq <- newUnique      -- Remove it from TcMetaTyVar unique land
            ; kind <- zonkTcKind (tyVarKind tv)
            ; let tv_name = getOccName tv
                  new_tv_name = if isWildcardVar tv
                                then generaliseWildcardVarName tv_name
                                else tv_name
                  final_name = mkInternalName uniq new_tv_name span
                  final_kind = defaultKind kind
                  final_tv   = mkTcTyVar final_name final_kind details

            ; traceTc "Skolemising" (ppr tv <+> ptext (sLit ":=") <+> ppr final_tv)
            ; writeMetaTyVar tv (mkTyVarTy final_tv)
            ; return final_tv }
      where
        -- If a wildcard type called _a is generalised, we rename it to w_a
        generaliseWildcardVarName :: OccName -> OccName
        generaliseWildcardVarName name | startsWithUnderscore name
          = mkOccNameFS (occNameSpace name) (appendFS (fsLit "w") (occNameFS name))
        generaliseWildcardVarName name = name

    {-
    Note [Zonking to Skolem]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    We used to zonk quantified type variables to regular TyVars.  However, this
    leads to problems.  Consider this program from the regression test suite:

      eval :: Int -> String -> String -> String
      eval 0 root actual = evalRHS 0 root actual

      evalRHS :: Int -> a
      evalRHS 0 root actual = eval 0 root actual

    It leads to the deferral of an equality (wrapped in an implication constraint)

      forall a. () => ((String -> String -> String) ~ a)

    which is propagated up to the toplevel (see TcSimplify.tcSimplifyInferCheck).
    In the meantime `a' is zonked and quantified to form `evalRHS's signature.
    This has the *side effect* of also zonking the `a' in the deferred equality
    (which at this point is being handed around wrapped in an implication
    constraint).

    Finally, the equality (with the zonked `a') will be handed back to the
    simplifier by TcRnDriver.tcRnSrcDecls calling TcSimplify.tcSimplifyTop.
    If we zonk `a' with a regular type variable, we will have this regular type
    variable now floating around in the simplifier, which in many places assumes to
    only see proper TcTyVars.

    We can avoid this problem by zonking with a skolem.  The skolem is rigid
    (which we require for a quantified variable), but is still a TcTyVar that the
    simplifier knows how to deal with.

    Note [Silly Type Synonyms]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this:
            type C u a = u  -- Note 'a' unused

            foo :: (forall a. C u a -> C u a) -> u
            foo x = ...

            bar :: Num u => u
            bar = foo (\t -> t + t)

    * From the (\t -> t+t) we get type  {Num d} =>  d -> d
      where d is fresh.

    * Now unify with type of foo's arg, and we get:
            {Num (C d a)} =>  C d a -> C d a
      where a is fresh.

    * Now abstract over the 'a', but float out the Num (C d a) constraint
      because it does not 'really' mention a.  (see exactTyVarsOfType)
      The arg to foo becomes
            \/\a -> \t -> t+t

    * So we get a dict binding for Num (C d a), which is zonked to give
            a = ()
      [Note Sept 04: now that we are zonking quantified type variables
      on construction, the 'a' will be frozen as a regular tyvar on
      quantification, so the floated dict will still have type (C d a).
      Which renders this whole note moot; happily!]

    * Then the \/\a abstraction has a zonked 'a' in it.

    All very silly.   I think its harmless to ignore the problem.  We'll end up with
    a \/\a in the final result but all the occurrences of a will be zonked to ()

    ************************************************************************
    *                                                                      *
                  Zonking types
    *                                                                      *
    ************************************************************************

    @tcGetGlobalTyVars@ returns a fully-zonked set of tyvars free in the environment.
    To improve subsequent calls to the same function it writes the zonked set back into
    the environment.
    -}

    tcGetGlobalTyVars :: TcM TcTyVarSet
    tcGetGlobalTyVars
      = do { (TcLclEnv {tcl_tyvars = gtv_var}) <- getLclEnv
           ; gbl_tvs  <- readMutVar gtv_var
           ; gbl_tvs' <- zonkTyVarsAndFV gbl_tvs
           ; writeMutVar gtv_var gbl_tvs'
           ; return gbl_tvs' }
      where

    zonkTcTypeAndFV :: TcType -> TcM TyVarSet
    -- Zonk a type and take its free variables
    -- With kind polymorphism it can be essential to zonk *first*
    -- so that we find the right set of free variables.  Eg
    --    forall k1. forall (a:k2). a
    -- where k2:=k1 is in the substitution.  We don't want
    -- k2 to look free in this type!
    zonkTcTypeAndFV ty = do { ty <- zonkTcType ty; return (tyVarsOfType ty) }

    zonkTyVar :: TyVar -> TcM TcType
    -- Works on TyVars and TcTyVars
    zonkTyVar tv | isTcTyVar tv = zonkTcTyVar tv
                 | otherwise    = return (mkTyVarTy tv)
       -- Hackily, when typechecking type and class decls
       -- we have TyVars in scopeadded (only) in
       -- TcHsType.tcTyClTyVars, but it seems
       -- painful to make them into TcTyVars there

    zonkTyVarsAndFV :: TyVarSet -> TcM TyVarSet
    zonkTyVarsAndFV tyvars = tyVarsOfTypes <$> mapM zonkTyVar (varSetElems tyvars)

    zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
    zonkTcTyVars tyvars = mapM zonkTcTyVar tyvars

    -----------------  Types
    zonkTyVarKind :: TyVar -> TcM TyVar
    zonkTyVarKind tv = do { kind' <- zonkTcKind (tyVarKind tv)
                          ; return (setTyVarKind tv kind') }

    zonkTcTypes :: [TcType] -> TcM [TcType]
    zonkTcTypes tys = mapM zonkTcType tys

    zonkTcThetaType :: TcThetaType -> TcM TcThetaType
    zonkTcThetaType theta = mapM zonkTcPredType theta

    zonkTcPredType :: TcPredType -> TcM TcPredType
    zonkTcPredType = zonkTcType

    {-
    ************************************************************************
    *                                                                      *
                  Zonking constraints
    *                                                                      *
    ************************************************************************
    -}

    zonkImplication :: Implication -> TcM (Bag Implication)
    zonkImplication implic@(Implic { ic_skols  = skols
                                   , ic_given  = given
                                   , ic_wanted = wanted
                                   , ic_info   = info })
      = do { skols'  <- mapM zonkTcTyVarBndr skols  -- Need to zonk their kinds!
                                                    -- as Trac #7230 showed
           ; given'  <- mapM zonkEvVar given
           ; info'   <- zonkSkolemInfo info
           ; wanted' <- zonkWCRec wanted
           ; if isEmptyWC wanted'
             then return emptyBag
             else return $ unitBag $
                  implic { ic_skols  = skols'
                         , ic_given  = given'
                         , ic_wanted = wanted'
                         , ic_info   = info' } }

    zonkEvVar :: EvVar -> TcM EvVar
    zonkEvVar var = do { ty' <- zonkTcType (varType var)
                       ; return (setVarType var ty') }

    zonkWC :: WantedConstraints -> TcM WantedConstraints
    zonkWC wc = zonkWCRec wc

    zonkWCRec :: WantedConstraints -> TcM WantedConstraints
    zonkWCRec (WC { wc_simple = simple, wc_impl = implic, wc_insol = insol })
      = do { simple' <- zonkSimples simple
           ; implic' <- flatMapBagM zonkImplication implic
           ; insol'  <- zonkSimples insol
           ; return (WC { wc_simple = simple', wc_impl = implic', wc_insol = insol' }) }

    zonkSimples :: Cts -> TcM Cts
    zonkSimples cts = do { cts' <- mapBagM zonkCt' cts
                         ; traceTc "zonkSimples done:" (ppr cts')
                         ; return cts' }

    zonkCt' :: Ct -> TcM Ct
    zonkCt' ct = zonkCt ct

    zonkCt :: Ct -> TcM Ct
    zonkCt ct@(CHoleCan { cc_ev = ev })
      = do { ev' <- zonkCtEvidence ev
           ; return $ ct { cc_ev = ev' } }
    zonkCt ct
      = do { fl' <- zonkCtEvidence (cc_ev ct)
           ; return (mkNonCanonical fl') }

    zonkCtEvidence :: CtEvidence -> TcM CtEvidence
    zonkCtEvidence ctev@(CtGiven { ctev_pred = pred })
      = do { pred' <- zonkTcType pred
           ; return (ctev { ctev_pred = pred'}) }
    zonkCtEvidence ctev@(CtWanted { ctev_pred = pred })
      = do { pred' <- zonkTcType pred
           ; return (ctev { ctev_pred = pred' }) }
    zonkCtEvidence ctev@(CtDerived { ctev_pred = pred })
      = do { pred' <- zonkTcType pred
           ; return (ctev { ctev_pred = pred' }) }

    zonkSkolemInfo :: SkolemInfo -> TcM SkolemInfo
    zonkSkolemInfo (SigSkol cx ty)  = do { ty' <- zonkTcType ty
                                         ; return (SigSkol cx ty') }
    zonkSkolemInfo (InferSkol ntys) = do { ntys' <- mapM do_one ntys
                                         ; return (InferSkol ntys') }
      where
        do_one (n, ty) = do { ty' <- zonkTcType ty; return (n, ty') }
    zonkSkolemInfo skol_info = return skol_info

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Zonking -- the main work-horses: zonkTcType, zonkTcTyVar}
    *                                                                      *
    *              For internal use only!                                  *
    *                                                                      *
    ************************************************************************
    -}

    -- zonkId is used *during* typechecking just to zonk the Id's type
    zonkId :: TcId -> TcM TcId
    zonkId id
      = do { ty' <- zonkTcType (idType id)
           ; return (Id.setIdType id ty') }

    -- For unbound, mutable tyvars, zonkType uses the function given to it
    -- For tyvars bound at a for-all, zonkType zonks them to an immutable
    --      type variable and zonks the kind too

    zonkTcType :: TcType -> TcM TcType
    zonkTcType ty
      = go ty
      where
        go (TyConApp tc tys) = do tys' <- mapM go tys
                                  return (TyConApp tc tys')
                    -- Do NOT establish Type invariants, because
                    -- doing so is strict in the TyCOn.
                    -- See Note [Zonking inside the knot] in TcHsType

        go (LitTy n)         = return (LitTy n)

        go (FunTy arg res)   = do arg' <- go arg
                                  res' <- go res
                                  return (FunTy arg' res')

        go (AppTy fun arg)   = do fun' <- go fun
                                  arg' <- go arg
                                  return (mkAppTy fun' arg')
                    -- NB the mkAppTy; we might have instantiated a
                    -- type variable to a type constructor, so we need
                    -- to pull the TyConApp to the top.
                    -- OK to do this because only strict in the structure
                    -- not in the TyCon.
                    -- See Note [Zonking inside the knot] in TcHsType

            -- The two interesting cases!
        go (TyVarTy tyvar) | isTcTyVar tyvar = zonkTcTyVar tyvar
                           | otherwise       = TyVarTy <$> updateTyVarKindM go tyvar
                    -- Ordinary (non Tc) tyvars occur inside quantified types

        go (ForAllTy tv ty) = do { tv' <- zonkTcTyVarBndr tv
                                 ; ty' <- go ty
                                 ; return (ForAllTy tv' ty') }

    zonkTcTyVarBndr :: TcTyVar -> TcM TcTyVar
    -- A tyvar binder is never a unification variable (MetaTv),
    -- rather it is always a skolems.  BUT it may have a kind
    -- that has not yet been zonked, and may include kind
    -- unification variables.
    zonkTcTyVarBndr tyvar
      = ASSERT2( isImmutableTyVar tyvar, ppr tyvar ) do
        updateTyVarKindM zonkTcType tyvar

    zonkTcTyVar :: TcTyVar -> TcM TcType
    -- Simply look through all Flexis
    zonkTcTyVar tv
      = ASSERT2( isTcTyVar tv, ppr tv ) do
        case tcTyVarDetails tv of
          SkolemTv {}   -> zonk_kind_and_return
          RuntimeUnk {} -> zonk_kind_and_return
          FlatSkol ty   -> zonkTcType ty
          MetaTv { mtv_ref = ref }
             -> do { cts <- readMutVar ref
                   ; case cts of
                        Flexi       -> zonk_kind_and_return
                        Indirect ty -> zonkTcType ty }
      where
        zonk_kind_and_return = do { z_tv <- zonkTyVarKind tv
                                  ; return (TyVarTy z_tv) }

    {-
    ************************************************************************
    *                                                                      *
                            Zonking kinds
    *                                                                      *
    ************************************************************************
    -}

    zonkTcKind :: TcKind -> TcM TcKind
    zonkTcKind k = zonkTcType k

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

    zonkTidyTcType :: TidyEnv -> TcType -> TcM (TidyEnv, TcType)
    zonkTidyTcType env ty = do { ty' <- zonkTcType ty
                               ; return (tidyOpenType env ty') }

    zonkTidyOrigin :: TidyEnv -> CtOrigin -> TcM (TidyEnv, CtOrigin)
    zonkTidyOrigin env (GivenOrigin skol_info)
      = do { skol_info1 <- zonkSkolemInfo skol_info
           ; let (env1, skol_info2) = tidySkolemInfo env skol_info1
           ; return (env1, GivenOrigin skol_info2) }
    zonkTidyOrigin env (TypeEqOrigin { uo_actual = act, uo_expected = exp })
      = do { (env1, act') <- zonkTidyTcType env  act
           ; (env2, exp') <- zonkTidyTcType env1 exp
           ; return ( env2, TypeEqOrigin { uo_actual = act', uo_expected = exp' }) }
    zonkTidyOrigin env (KindEqOrigin ty1 ty2 orig)
      = do { (env1, ty1') <- zonkTidyTcType env  ty1
           ; (env2, ty2') <- zonkTidyTcType env1 ty2
           ; (env3, orig') <- zonkTidyOrigin env2 orig
           ; return (env3, KindEqOrigin ty1' ty2' orig') }
    zonkTidyOrigin env (CoercibleOrigin ty1 ty2)
      = do { (env1, ty1') <- zonkTidyTcType env  ty1
           ; (env2, ty2') <- zonkTidyTcType env1 ty2
           ; return (env2, CoercibleOrigin ty1' ty2') }
    zonkTidyOrigin env (FunDepOrigin1 p1 l1 p2 l2)
      = do { (env1, p1') <- zonkTidyTcType env  p1
           ; (env2, p2') <- zonkTidyTcType env1 p2
           ; return (env2, FunDepOrigin1 p1' l1 p2' l2) }
    zonkTidyOrigin env (FunDepOrigin2 p1 o1 p2 l2)
      = do { (env1, p1') <- zonkTidyTcType env  p1
           ; (env2, p2') <- zonkTidyTcType env1 p2
           ; (env3, o1') <- zonkTidyOrigin env2 o1
           ; return (env3, FunDepOrigin2 p1' o1' p2' l2) }
    zonkTidyOrigin env orig = return (env, orig)

    ----------------
    tidyCt :: TidyEnv -> Ct -> Ct
    -- Used only in error reporting
    -- Also converts it to non-canonical
    tidyCt env ct
      = case ct of
         CHoleCan { cc_ev = ev }
           -> ct { cc_ev = tidy_ev env ev }
         _ -> mkNonCanonical (tidy_ev env (ctEvidence ct))
      where
        tidy_ev :: TidyEnv -> CtEvidence -> CtEvidence
         -- NB: we do not tidy the ctev_evtm/var field because we don't
         --     show it in error messages
        tidy_ev env ctev@(CtGiven { ctev_pred = pred })
          = ctev { ctev_pred = tidyType env pred }
        tidy_ev env ctev@(CtWanted { ctev_pred = pred })
          = ctev { ctev_pred = tidyType env pred }
        tidy_ev env ctev@(CtDerived { ctev_pred = pred })
          = ctev { ctev_pred = tidyType env pred }

    ----------------
    tidyEvVar :: TidyEnv -> EvVar -> EvVar
    tidyEvVar env var = setVarType var (tidyType env (varType var))

    ----------------
    tidySkolemInfo :: TidyEnv -> SkolemInfo -> (TidyEnv, SkolemInfo)
    tidySkolemInfo env (SigSkol cx ty)
      = (env', SigSkol cx ty')
      where
        (env', ty') = tidyOpenType env ty

    tidySkolemInfo env (InferSkol ids)
      = (env', InferSkol ids')
      where
        (env', ids') = mapAccumL do_one env ids
        do_one env (name, ty) = (env', (name, ty'))
           where
             (env', ty') = tidyOpenType env ty

    tidySkolemInfo env (UnifyForAllSkol skol_tvs ty)
      = (env1, UnifyForAllSkol skol_tvs' ty')
      where
        env1 = tidyFreeTyVars env (tyVarsOfType ty `delVarSetList` skol_tvs)
        (env2, skol_tvs') = tidyTyVarBndrs env1 skol_tvs
        ty'               = tidyType env2 ty

    tidySkolemInfo env info = (env, info)

    {-
    ************************************************************************
    *                                                                      *
            (Named) Wildcards
    *                                                                      *
    ************************************************************************
    -}

    -- | Create a new meta var with the given kind. This meta var should be used
    -- to replace a wildcard in a type. Such a wildcard meta var can be
    -- distinguished from other meta vars with the 'isWildcardVar' function.
    newWildcardVar :: Name -> Kind -> TcM TcTyVar
    newWildcardVar name kind = newNamedMetaTyVar name (TauTv True) kind

    -- | Create a new meta var (which can unify with a type of any kind). This
    -- meta var should be used to replace a wildcard in a type. Such a wildcard
    -- meta var can be distinguished from other meta vars with the 'isWildcardVar'
    -- function.
    newWildcardVarMetaKind :: Name -> TcM TcTyVar
    newWildcardVarMetaKind name = do kind <- newMetaKindVar
                                     newWildcardVar name kind

    -- | Return 'True' if the argument is a meta var created for a wildcard (by
    -- 'newWildcardVar' or 'newWildcardVarMetaKind').
    isWildcardVar :: TcTyVar -> Bool
    isWildcardVar tv | isTcTyVar tv, MetaTv (TauTv True) _ _ <- tcTyVarDetails tv = True
    isWildcardVar _ = False