content: {-
  (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 Eta.TypeCheck.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 fresh type variables for pm checking
    genInstSkolTyVarsX,
  
    --------------------------------
    -- Creating new evidence variables
    newEvVar, newEvVars, newEq, newDict,
    newTcEvBinds, addTcEvBind,
    newSimpleWanted, newWanted, 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
  
  -- friends:
  import Eta.Types.TypeRep
  import Eta.TypeCheck.TcType
  import Eta.Types.Type
  import Eta.Types.Class
  import Eta.BasicTypes.Var
  import Eta.BasicTypes.VarEnv
  import qualified Eta.LanguageExtensions as LangExt
  -- others:
  import Eta.TypeCheck.TcRnMonad        -- TcType, amongst others
  import Eta.BasicTypes.Id
  import qualified Eta.BasicTypes.Id as Id
  import Eta.BasicTypes.Name
  import Eta.BasicTypes.VarSet
  import Eta.Prelude.PrelNames
  import Eta.Utils.Util
  import Eta.Utils.Outputable
  import Eta.Utils.FastString
  import Eta.BasicTypes.SrcLoc
  import Eta.Utils.Bag
  
  import Control.Monad
  import Data.List        ( partition, mapAccumL )
  
  #include "HsVersions.h"
  
  {-
  ************************************************************************
  *                                                                      *
          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 }
  
  newWanted :: CtOrigin -> PredType -> TcM CtEvidence
  -- Deals with both equality and non-equality predicates
  newWanted orig pty
   = do loc <- getCtLoc orig
        v <- newEvVar pty
        return $ 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
  
  {-
  % Generating fresh variables for pattern match check
  -}
  
  -- UNINSTANTIATED VERSION OF tcInstSkolTyVars
  genInstSkolTyVarsX :: SrcSpan -> TvSubst -> [TyVar] -> TcRnIf gbl lcl (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
  genInstSkolTyVarsX loc subst tvs = instSkolTyVarsX (mkTcSkolTyVar loc False) subst tvs
  
  {-
  ************************************************************************
  *                                                                      *
          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 LangExt.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 _ -> --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
  