module TcHsSyn (
	mkHsConApp, mkHsDictLet, mkHsApp,
	hsLitType, hsLPatType, hsPatType, 
	mkHsAppTy, mkSimpleHsAlt,
	nlHsIntLit, 
	shortCutLit, hsOverLitName,
	
	-- re-exported from TcMonad
	TcId, TcIdSet, 

	zonkTopDecls, zonkTopExpr, zonkTopLExpr,
	zonkId, zonkTopBndrs
  ) where

#include "HsVersions.h"

-- friends:
import HsSyn	-- oodles of it

-- others:
import Id

import TcRnMonad
import PrelNames
import TcType
import TcMType
import TysPrim
import TysWiredIn
import DataCon
import Name
import NameSet
import Var
import VarSet
import VarEnv
import Literal
import BasicTypes
import Maybes
import SrcLoc
import DynFlags( DynFlag(..) )
import Bag
import FastString
import Outputable

-- XXX
thenM :: Monad a => a b -> (b -> a c) -> a c
thenM = (>>=)

returnM :: Monad m => a -> m a
returnM = return

mappM :: (Monad m) => (a -> m b) -> [a] -> m [b]
mappM = mapM

hsLPatType :: OutPat Id -> Type
hsLPatType (L _ pat) = hsPatType pat

hsPatType :: Pat Id -> Type
hsPatType (ParPat pat)                = hsLPatType pat
hsPatType (WildPat ty)                = ty
hsPatType (VarPat var)                = idType var
hsPatType (VarPatOut var _)           = idType var
hsPatType (BangPat pat)               = hsLPatType pat
hsPatType (LazyPat pat)               = hsLPatType pat
hsPatType (LitPat lit)                = hsLitType lit
hsPatType (AsPat var _)               = idType (unLoc var)
hsPatType (ViewPat _ _ ty)            = ty
hsPatType (ListPat _ ty)              = mkListTy ty
hsPatType (PArrPat _ ty)              = mkPArrTy ty
hsPatType (TuplePat _ _ ty)           = ty
hsPatType (ConPatOut { pat_ty = ty }) = ty
hsPatType (SigPatOut _ ty)            = ty
hsPatType (NPat lit _ _)              = overLitType lit
hsPatType (NPlusKPat id _ _ _)        = idType (unLoc id)
hsPatType (CoPat _ _ ty)              = ty
hsPatType p                           = pprPanic "hsPatType" (ppr p)

hsLitType :: HsLit -> TcType
hsLitType (HsChar _)       = charTy
hsLitType (HsCharPrim _)   = charPrimTy
hsLitType (HsString _)     = stringTy
hsLitType (HsStringPrim _) = addrPrimTy
hsLitType (HsInt _)        = intTy
hsLitType (HsIntPrim _)    = intPrimTy
hsLitType (HsWordPrim _)   = wordPrimTy
hsLitType (HsInteger _ ty) = ty
hsLitType (HsRat _ ty)     = ty
hsLitType (HsFloatPrim _)  = floatPrimTy
hsLitType (HsDoublePrim _) = doublePrimTy

shortCutLit :: OverLitVal -> TcType -> Maybe (HsExpr TcId)
shortCutLit (HsIntegral i) ty
  | isIntTy ty && inIntRange i   = Just (HsLit (HsInt i))
  | isWordTy ty && inWordRange i = Just (mkLit wordDataCon (HsWordPrim i))
  | isIntegerTy ty 	       	 = Just (HsLit (HsInteger i ty))
  | otherwise		       	 = shortCutLit (HsFractional (fromInteger i)) ty
	-- The 'otherwise' case is important
	-- Consider (3 :: Float).  Syntactically it looks like an IntLit,
	-- so we'll call shortCutIntLit, but of course it's a float
	-- This can make a big difference for programs with a lot of
	-- literals, compiled without -O

shortCutLit (HsFractional f) ty
  | isFloatTy ty  = Just (mkLit floatDataCon  (HsFloatPrim f))
  | isDoubleTy ty = Just (mkLit doubleDataCon (HsDoublePrim f))
  | otherwise     = Nothing

shortCutLit (HsIsString s) ty
  | isStringTy ty = Just (HsLit (HsString s))
  | otherwise     = Nothing

mkLit :: DataCon -> HsLit -> HsExpr Id
mkLit con lit = HsApp (nlHsVar (dataConWrapId con)) (nlHsLit lit)

------------------------------
hsOverLitName :: OverLitVal -> Name
-- Get the canonical 'fromX' name for a particular OverLitVal
hsOverLitName (HsIntegral {})   = fromIntegerName
hsOverLitName (HsFractional {}) = fromRationalName
hsOverLitName (HsIsString {})   = fromStringName

-- zonkId is used *during* typechecking just to zonk the Id's type
zonkId :: TcId -> TcM TcId
zonkId id
  = zonkTcType (idType id) `thenM` \ ty' ->
    returnM (Id.setIdType id ty')

data ZonkEnv = ZonkEnv	(TcType -> TcM Type) 	-- How to zonk a type
			(VarEnv Var)		-- What variables are in scope
	-- Maps an Id or EvVar to its zonked version; both have the same Name
	-- Note that all evidence (coercion variables as well as dictionaries)
	-- 	are kept in the ZonkEnv
	-- Only *type* abstraction is done by side effect
	-- Is only consulted lazily; hence knot-tying

emptyZonkEnv :: ZonkEnv
emptyZonkEnv = ZonkEnv zonkTypeZapping emptyVarEnv

extendZonkEnv :: ZonkEnv -> [Var] -> ZonkEnv
extendZonkEnv (ZonkEnv zonk_ty env) ids 
  = ZonkEnv zonk_ty (extendVarEnvList env [(id,id) | id <- ids])

extendZonkEnv1 :: ZonkEnv -> Var -> ZonkEnv
extendZonkEnv1 (ZonkEnv zonk_ty env) id 
  = ZonkEnv zonk_ty (extendVarEnv env id id)

setZonkType :: ZonkEnv -> (TcType -> TcM Type) -> ZonkEnv
setZonkType (ZonkEnv _ env) zonk_ty = ZonkEnv zonk_ty env

zonkEnvIds :: ZonkEnv -> [Id]
zonkEnvIds (ZonkEnv _ env) = varEnvElts env

zonkIdOcc :: ZonkEnv -> TcId -> Id
-- Ids defined in this module should be in the envt; 
-- ignore others.  (Actually, data constructors are also
-- not LocalVars, even when locally defined, but that is fine.)
-- (Also foreign-imported things aren't currently in the ZonkEnv;
--  that's ok because they don't need zonking.)
--
-- Actually, Template Haskell works in 'chunks' of declarations, and
-- an earlier chunk won't be in the 'env' that the zonking phase 
-- carries around.  Instead it'll be in the tcg_gbl_env, already fully
-- zonked.  There's no point in looking it up there (except for error 
-- checking), and it's not conveniently to hand; hence the simple
-- 'orElse' case in the LocalVar branch.
--
-- Even without template splices, in module Main, the checking of
-- 'main' is done as a separate chunk.
zonkIdOcc (ZonkEnv _zonk_ty env) id 
  | isLocalVar id = lookupVarEnv env id `orElse` id
  | otherwise	  = id

zonkIdOccs :: ZonkEnv -> [TcId] -> [Id]
zonkIdOccs env ids = map (zonkIdOcc env) ids

-- zonkIdBndr is used *after* typechecking to get the Id's type
-- to its final form.  The TyVarEnv give 
zonkIdBndr :: ZonkEnv -> TcId -> TcM Id
zonkIdBndr env id
  = zonkTcTypeToType env (idType id)	`thenM` \ ty' ->
    returnM (Id.setIdType id ty')

zonkIdBndrs :: ZonkEnv -> [TcId] -> TcM [Id]
zonkIdBndrs env ids = mappM (zonkIdBndr env) ids

zonkTopBndrs :: [TcId] -> TcM [Id]
zonkTopBndrs ids = zonkIdBndrs emptyZonkEnv ids

zonkEvBndrsX :: ZonkEnv -> [EvVar] -> TcM (ZonkEnv, [Var])
zonkEvBndrsX = mapAccumLM zonkEvBndrX 

zonkEvBndrX :: ZonkEnv -> EvVar -> TcM (ZonkEnv, EvVar)
-- Works for dictionaries and coercions
zonkEvBndrX env var
  = do { var' <- zonkEvBndr env var
       ; return (extendZonkEnv1 env var', var') }

zonkEvBndr :: ZonkEnv -> EvVar -> TcM EvVar
-- Works for dictionaries and coercions
-- Does not extend the ZonkEnv
zonkEvBndr env var 
  = do { ty' <- zonkTcTypeToType env (varType var)
       ; return (setVarType var ty') }

zonkEvVarOcc :: ZonkEnv -> EvVar -> EvVar
zonkEvVarOcc env v = zonkIdOcc env v

zonkTopExpr :: HsExpr TcId -> TcM (HsExpr Id)
zonkTopExpr e = zonkExpr emptyZonkEnv e

zonkTopLExpr :: LHsExpr TcId -> TcM (LHsExpr Id)
zonkTopLExpr e = zonkLExpr emptyZonkEnv e

zonkTopDecls :: Bag EvBind 
             -> LHsBinds TcId -> NameSet
             -> [LRuleDecl TcId] -> [LTcSpecPrag] -> [LForeignDecl TcId]
	     -> TcM ([Id], 
	             Bag EvBind,
		     Bag (LHsBind  Id),
		     [LForeignDecl Id],
		     [LTcSpecPrag],
		     [LRuleDecl    Id])
zonkTopDecls ev_binds binds sig_ns rules imp_specs fords
  = do	{ (env1, ev_binds') <- zonkEvBinds emptyZonkEnv ev_binds

	 -- Warn about missing signatures
	 -- Do this only when we we have a type to offer
        ; warn_missing_sigs <- doptM Opt_WarnMissingSigs
        ; let sig_warn | warn_missing_sigs = topSigWarn sig_ns
                       | otherwise         = noSigWarn

        ; (env2, binds') <- zonkRecMonoBinds env1 sig_warn binds
			-- Top level is implicitly recursive
	; rules' <- zonkRules env2 rules
        ; specs' <- zonkLTcSpecPrags env2 imp_specs
	; fords' <- zonkForeignExports env2 fords
	; return (zonkEnvIds env2, ev_binds', binds', fords', specs', rules') }

---------------------------------------------
zonkLocalBinds :: ZonkEnv -> HsLocalBinds TcId -> TcM (ZonkEnv, HsLocalBinds Id)
zonkLocalBinds env EmptyLocalBinds
  = return (env, EmptyLocalBinds)

zonkLocalBinds _ (HsValBinds (ValBindsIn {}))
  = panic "zonkLocalBinds" -- Not in typechecker output

zonkLocalBinds env (HsValBinds vb@(ValBindsOut binds sigs))
  = do	{ warn_missing_sigs <- doptM Opt_WarnMissingLocalSigs
        ; let sig_warn | not warn_missing_sigs = noSigWarn
                       | otherwise             = localSigWarn sig_ns
              sig_ns = getTypeSigNames vb
	; (env1, new_binds) <- go env sig_warn binds
        ; return (env1, HsValBinds (ValBindsOut new_binds sigs)) }
  where
    go env _ []
      = return (env, [])
    go env sig_warn ((r,b):bs) 
      = do { (env1, b')  <- zonkRecMonoBinds env sig_warn b
	   ; (env2, bs') <- go env1 sig_warn bs
	   ; return (env2, (r,b'):bs') }

zonkLocalBinds env (HsIPBinds (IPBinds binds dict_binds))
  = mappM (wrapLocM zonk_ip_bind) binds	`thenM` \ new_binds ->
    let
	env1 = extendZonkEnv env [ipNameName n | L _ (IPBind n _) <- new_binds]
    in
    zonkTcEvBinds env1 dict_binds 	`thenM` \ (env2, new_dict_binds) -> 
    returnM (env2, HsIPBinds (IPBinds new_binds new_dict_binds))
  where
    zonk_ip_bind (IPBind n e)
	= mapIPNameTc (zonkIdBndr env) n	`thenM` \ n' ->
	  zonkLExpr env e			`thenM` \ e' ->
	  returnM (IPBind n' e')

---------------------------------------------
zonkRecMonoBinds :: ZonkEnv -> SigWarn -> LHsBinds TcId -> TcM (ZonkEnv, LHsBinds Id)
zonkRecMonoBinds env sig_warn binds 
 = fixM (\ ~(_, new_binds) -> do 
	{ let env1 = extendZonkEnv env (collectHsBindsBinders new_binds)
        ; binds' <- zonkMonoBinds env1 sig_warn binds
        ; return (env1, binds') })

---------------------------------------------
type SigWarn = Bool -> [Id] -> TcM ()	
     -- Missing-signature warning
     -- The Bool is True for an AbsBinds, False otherwise

noSigWarn :: SigWarn
noSigWarn _ _ = return ()

topSigWarn :: NameSet -> SigWarn
topSigWarn sig_ns _ ids = mapM_ (topSigWarnId sig_ns) ids

topSigWarnId :: NameSet -> Id -> TcM ()
-- The NameSet is the Ids that *lack* a signature
-- We have to do it this way round because there are
-- lots of top-level bindings that are generated by GHC
-- and that don't have signatures
topSigWarnId sig_ns id
  | idName id `elemNameSet` sig_ns = warnMissingSig msg id
  | otherwise                      = return ()
  where
    msg = ptext (sLit "Top-level binding with no type signature:")

localSigWarn :: NameSet -> SigWarn
localSigWarn sig_ns is_abs_bind ids
  | not is_abs_bind = return ()
  | otherwise       = mapM_ (localSigWarnId sig_ns) ids

localSigWarnId :: NameSet -> Id -> TcM ()
-- NameSet are the Ids that *have* type signatures
localSigWarnId sig_ns id
  | not (isSigmaTy (idType id))    = return ()
  | idName id `elemNameSet` sig_ns = return ()
  | otherwise                      = warnMissingSig msg id
  where
    msg = ptext (sLit "Polymophic local binding with no type signature:")

warnMissingSig :: SDoc -> Id -> TcM ()
warnMissingSig msg id
  = do  { env0 <- tcInitTidyEnv
        ; let (env1, tidy_ty) = tidyOpenType env0 (idType id)
        ; addWarnTcM (env1, mk_msg tidy_ty) }
  where
    mk_msg ty = sep [ msg, nest 2 $ pprHsVar (idName id) <+> dcolon <+> ppr ty ]

---------------------------------------------
zonkMonoBinds :: ZonkEnv -> SigWarn -> LHsBinds TcId -> TcM (LHsBinds Id)
zonkMonoBinds env sig_warn binds = mapBagM (wrapLocM (zonk_bind env sig_warn)) binds

zonk_bind :: ZonkEnv -> SigWarn -> HsBind TcId -> TcM (HsBind Id)
zonk_bind env sig_warn bind@(PatBind { pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty})
  = do	{ (_env, new_pat) <- zonkPat env pat		-- Env already extended
        ; sig_warn False (collectPatBinders new_pat)
	; new_grhss <- zonkGRHSs env grhss
	; new_ty    <- zonkTcTypeToType env ty
	; return (bind { pat_lhs = new_pat, pat_rhs = new_grhss, pat_rhs_ty = new_ty }) }

zonk_bind env sig_warn (VarBind { var_id = var, var_rhs = expr, var_inline = inl })
  = do { new_var  <- zonkIdBndr env var
       ; sig_warn False [new_var]
       ; new_expr <- zonkLExpr env expr
       ; return (VarBind { var_id = new_var, var_rhs = new_expr, var_inline = inl }) }

zonk_bind env sig_warn bind@(FunBind { fun_id = L loc var, fun_matches = ms
                                     , fun_co_fn = co_fn })
  = do { new_var <- zonkIdBndr env var
       ; sig_warn False [new_var]
       ; (env1, new_co_fn) <- zonkCoFn env co_fn
       ; new_ms <- zonkMatchGroup env1 ms
       ; return (bind { fun_id = L loc new_var, fun_matches = new_ms
                      , fun_co_fn = new_co_fn }) }

zonk_bind env sig_warn (AbsBinds { abs_tvs = tyvars, abs_ev_vars = evs
                                 , abs_ev_binds = ev_binds
			         , abs_exports = exports
                                 , abs_binds = val_binds })
  = ASSERT( all isImmutableTyVar tyvars )
    do { (env1, new_evs) <- zonkEvBndrsX env evs
       ; (env2, new_ev_binds) <- zonkTcEvBinds env1 ev_binds
       ; (new_val_bind, new_exports) <- fixM $ \ ~(new_val_binds, _) ->
    	 do { let env3 = extendZonkEnv env2 (collectHsBindsBinders new_val_binds)
    	    ; new_val_binds <- zonkMonoBinds env3 noSigWarn val_binds
    	    ; new_exports   <- mapM (zonkExport env3) exports
    	    ; return (new_val_binds, new_exports) } 
       ; sig_warn True [b | (_,b,_,_) <- new_exports]
       ; return (AbsBinds { abs_tvs = tyvars, abs_ev_vars = new_evs, abs_ev_binds = new_ev_binds
			  , abs_exports = new_exports, abs_binds = new_val_bind }) }
  where
    zonkExport env (tyvars, global, local, prags)
	-- The tyvars are already zonked
	= zonkIdBndr env global			`thenM` \ new_global ->
	  zonkSpecPrags env prags		`thenM` \ new_prags -> 
	  returnM (tyvars, new_global, zonkIdOcc env local, new_prags)

zonkSpecPrags :: ZonkEnv -> TcSpecPrags -> TcM TcSpecPrags
zonkSpecPrags _   IsDefaultMethod = return IsDefaultMethod
zonkSpecPrags env (SpecPrags ps)  = do { ps' <- zonkLTcSpecPrags env ps
                                       ; return (SpecPrags ps') }

zonkLTcSpecPrags :: ZonkEnv -> [LTcSpecPrag] -> TcM [LTcSpecPrag]
zonkLTcSpecPrags env ps
  = mapM zonk_prag ps
  where
    zonk_prag (L loc (SpecPrag id co_fn inl))
	= do { (_, co_fn') <- zonkCoFn env co_fn
	     ; return (L loc (SpecPrag (zonkIdOcc env id) co_fn' inl)) }

zonkMatchGroup :: ZonkEnv -> MatchGroup TcId-> TcM (MatchGroup Id)
zonkMatchGroup env (MatchGroup ms ty) 
  = do	{ ms' <- mapM (zonkMatch env) ms
	; ty' <- zonkTcTypeToType env ty
	; return (MatchGroup ms' ty') }

zonkMatch :: ZonkEnv -> LMatch TcId-> TcM (LMatch Id)
zonkMatch env (L loc (Match pats _ grhss))
  = do	{ (env1, new_pats) <- zonkPats env pats
	; new_grhss <- zonkGRHSs env1 grhss
	; return (L loc (Match new_pats Nothing new_grhss)) }

-------------------------------------------------------------------------
zonkGRHSs :: ZonkEnv -> GRHSs TcId -> TcM (GRHSs Id)

zonkGRHSs env (GRHSs grhss binds)
  = zonkLocalBinds env binds   	`thenM` \ (new_env, new_binds) ->
    let
	zonk_grhs (GRHS guarded rhs)
	  = zonkStmts new_env guarded	`thenM` \ (env2, new_guarded) ->
	    zonkLExpr env2 rhs		`thenM` \ new_rhs ->
	    returnM (GRHS new_guarded new_rhs)
    in
    mappM (wrapLocM zonk_grhs) grhss 	`thenM` \ new_grhss ->
    returnM (GRHSs new_grhss new_binds)

zonkLExprs :: ZonkEnv -> [LHsExpr TcId] -> TcM [LHsExpr Id]
zonkLExpr  :: ZonkEnv -> LHsExpr TcId   -> TcM (LHsExpr Id)
zonkExpr   :: ZonkEnv -> HsExpr TcId    -> TcM (HsExpr Id)

zonkLExprs env exprs = mappM (zonkLExpr env) exprs
zonkLExpr  env expr  = wrapLocM (zonkExpr env) expr

zonkExpr env (HsVar id)
  = returnM (HsVar (zonkIdOcc env id))

zonkExpr env (HsIPVar id)
  = returnM (HsIPVar (mapIPName (zonkIdOcc env) id))

zonkExpr env (HsLit (HsRat f ty))
  = zonkTcTypeToType env ty	   `thenM` \ new_ty  ->
    returnM (HsLit (HsRat f new_ty))

zonkExpr _ (HsLit lit)
  = returnM (HsLit lit)

zonkExpr env (HsOverLit lit)
  = do	{ lit' <- zonkOverLit env lit
	; return (HsOverLit lit') }

zonkExpr env (HsLam matches)
  = zonkMatchGroup env matches	`thenM` \ new_matches ->
    returnM (HsLam new_matches)

zonkExpr env (HsApp e1 e2)
  = zonkLExpr env e1	`thenM` \ new_e1 ->
    zonkLExpr env e2	`thenM` \ new_e2 ->
    returnM (HsApp new_e1 new_e2)

zonkExpr env (HsBracketOut body bs) 
  = mappM zonk_b bs	`thenM` \ bs' ->
    returnM (HsBracketOut body bs')
  where
    zonk_b (n,e) = zonkLExpr env e	`thenM` \ e' ->
		   returnM (n,e')

zonkExpr _ (HsSpliceE s) = WARN( True, ppr s ) -- Should not happen
			     returnM (HsSpliceE s)

zonkExpr env (OpApp e1 op fixity e2)
  = zonkLExpr env e1	`thenM` \ new_e1 ->
    zonkLExpr env op	`thenM` \ new_op ->
    zonkLExpr env e2	`thenM` \ new_e2 ->
    returnM (OpApp new_e1 new_op fixity new_e2)

zonkExpr env (NegApp expr op)
  = zonkLExpr env expr	`thenM` \ new_expr ->
    zonkExpr env op	`thenM` \ new_op ->
    returnM (NegApp new_expr new_op)

zonkExpr env (HsPar e)    
  = zonkLExpr env e	`thenM` \new_e ->
    returnM (HsPar new_e)

zonkExpr env (SectionL expr op)
  = zonkLExpr env expr	`thenM` \ new_expr ->
    zonkLExpr env op		`thenM` \ new_op ->
    returnM (SectionL new_expr new_op)

zonkExpr env (SectionR op expr)
  = zonkLExpr env op		`thenM` \ new_op ->
    zonkLExpr env expr		`thenM` \ new_expr ->
    returnM (SectionR new_op new_expr)

zonkExpr env (ExplicitTuple tup_args boxed)
  = do { new_tup_args <- mapM zonk_tup_arg tup_args
       ; return (ExplicitTuple new_tup_args boxed) }
  where
    zonk_tup_arg (Present e) = do { e' <- zonkLExpr env e; return (Present e') }
    zonk_tup_arg (Missing t) = do { t' <- zonkTcTypeToType env t; return (Missing t') }

zonkExpr env (HsCase expr ms)
  = zonkLExpr env expr    	`thenM` \ new_expr ->
    zonkMatchGroup env ms	`thenM` \ new_ms ->
    returnM (HsCase new_expr new_ms)

zonkExpr env (HsIf e0 e1 e2 e3)
  = do { new_e0 <- fmapMaybeM (zonkExpr env) e0
       ; new_e1 <- zonkLExpr env e1
       ; new_e2 <- zonkLExpr env e2
       ; new_e3 <- zonkLExpr env e3
       ; returnM (HsIf new_e0 new_e1 new_e2 new_e3) }

zonkExpr env (HsLet binds expr)
  = zonkLocalBinds env binds	`thenM` \ (new_env, new_binds) ->
    zonkLExpr new_env expr	`thenM` \ new_expr ->
    returnM (HsLet new_binds new_expr)

zonkExpr env (HsDo do_or_lc stmts body ty)
  = zonkStmts env stmts 	`thenM` \ (new_env, new_stmts) ->
    zonkLExpr new_env body	`thenM` \ new_body ->
    zonkTcTypeToType env ty	`thenM` \ new_ty   ->
    zonkDo env do_or_lc	 	`thenM` \ new_do_or_lc ->
    returnM (HsDo new_do_or_lc new_stmts new_body new_ty)

zonkExpr env (ExplicitList ty exprs)
  = zonkTcTypeToType env ty	`thenM` \ new_ty ->
    zonkLExprs env exprs	`thenM` \ new_exprs ->
    returnM (ExplicitList new_ty new_exprs)

zonkExpr env (ExplicitPArr ty exprs)
  = zonkTcTypeToType env ty	`thenM` \ new_ty ->
    zonkLExprs env exprs	`thenM` \ new_exprs ->
    returnM (ExplicitPArr new_ty new_exprs)

zonkExpr env (RecordCon data_con con_expr rbinds)
  = do	{ new_con_expr <- zonkExpr env con_expr
	; new_rbinds   <- zonkRecFields env rbinds
	; return (RecordCon data_con new_con_expr new_rbinds) }

zonkExpr env (RecordUpd expr rbinds cons in_tys out_tys)
  = do	{ new_expr    <- zonkLExpr env expr
	; new_in_tys  <- mapM (zonkTcTypeToType env) in_tys
	; new_out_tys <- mapM (zonkTcTypeToType env) out_tys
	; new_rbinds  <- zonkRecFields env rbinds
	; return (RecordUpd new_expr new_rbinds cons new_in_tys new_out_tys) }

zonkExpr env (ExprWithTySigOut e ty) 
  = do { e' <- zonkLExpr env e
       ; return (ExprWithTySigOut e' ty) }

zonkExpr _ (ExprWithTySig _ _) = panic "zonkExpr env:ExprWithTySig"

zonkExpr env (ArithSeq expr info)
  = zonkExpr env expr		`thenM` \ new_expr ->
    zonkArithSeq env info	`thenM` \ new_info ->
    returnM (ArithSeq new_expr new_info)

zonkExpr env (PArrSeq expr info)
  = zonkExpr env expr		`thenM` \ new_expr ->
    zonkArithSeq env info	`thenM` \ new_info ->
    returnM (PArrSeq new_expr new_info)

zonkExpr env (HsSCC lbl expr)
  = zonkLExpr env expr	`thenM` \ new_expr ->
    returnM (HsSCC lbl new_expr)

zonkExpr env (HsTickPragma info expr)
  = zonkLExpr env expr	`thenM` \ new_expr ->
    returnM (HsTickPragma info new_expr)

-- hdaume: core annotations
zonkExpr env (HsCoreAnn lbl expr)
  = zonkLExpr env expr   `thenM` \ new_expr ->
    returnM (HsCoreAnn lbl new_expr)

-- arrow notation extensions
zonkExpr env (HsProc pat body)
  = do	{ (env1, new_pat) <- zonkPat env pat
	; new_body <- zonkCmdTop env1 body
	; return (HsProc new_pat new_body) }

zonkExpr env (HsArrApp e1 e2 ty ho rl)
  = zonkLExpr env e1	    	    	`thenM` \ new_e1 ->
    zonkLExpr env e2	    	    	`thenM` \ new_e2 ->
    zonkTcTypeToType env ty 		`thenM` \ new_ty ->
    returnM (HsArrApp new_e1 new_e2 new_ty ho rl)

zonkExpr env (HsArrForm op fixity args)
  = zonkLExpr env op	    	    	`thenM` \ new_op ->
    mappM (zonkCmdTop env) args		`thenM` \ new_args ->
    returnM (HsArrForm new_op fixity new_args)

zonkExpr env (HsWrap co_fn expr)
  = zonkCoFn env co_fn	`thenM` \ (env1, new_co_fn) ->
    zonkExpr env1 expr	`thenM` \ new_expr ->
    return (HsWrap new_co_fn new_expr)

zonkExpr _ expr = pprPanic "zonkExpr" (ppr expr)

zonkCmdTop :: ZonkEnv -> LHsCmdTop TcId -> TcM (LHsCmdTop Id)
zonkCmdTop env cmd = wrapLocM (zonk_cmd_top env) cmd

zonk_cmd_top :: ZonkEnv -> HsCmdTop TcId -> TcM (HsCmdTop Id)
zonk_cmd_top env (HsCmdTop cmd stack_tys ty ids)
  = zonkLExpr env cmd	    		`thenM` \ new_cmd ->
    zonkTcTypeToTypes env stack_tys	`thenM` \ new_stack_tys ->
    zonkTcTypeToType env ty 		`thenM` \ new_ty ->
    mapSndM (zonkExpr env) ids		`thenM` \ new_ids ->
    returnM (HsCmdTop new_cmd new_stack_tys new_ty new_ids)

-------------------------------------------------------------------------
zonkCoFn :: ZonkEnv -> HsWrapper -> TcM (ZonkEnv, HsWrapper)
zonkCoFn env WpHole   = return (env, WpHole)
zonkCoFn env (WpCompose c1 c2) = do { (env1, c1') <- zonkCoFn env c1
				    ; (env2, c2') <- zonkCoFn env1 c2
				    ; return (env2, WpCompose c1' c2') }
zonkCoFn env (WpCast co)    = do { co' <- zonkTcTypeToType env co
				 ; return (env, WpCast co') }
zonkCoFn env (WpEvLam ev)   = do { (env', ev') <- zonkEvBndrX env ev
				 ; return (env', WpEvLam ev') }
zonkCoFn env (WpEvApp arg)  = do { arg' <- zonkEvTerm env arg 
                                 ; return (env, WpEvApp arg') }
zonkCoFn env (WpTyLam tv)   = ASSERT( isImmutableTyVar tv )
                              return (env, WpTyLam tv) 
zonkCoFn env (WpTyApp ty)   = do { ty' <- zonkTcTypeToType env ty
				 ; return (env, WpTyApp ty') }
zonkCoFn env (WpLet bs)     = do { (env1, bs') <- zonkTcEvBinds env bs
				 ; return (env1, WpLet bs') }

-------------------------------------------------------------------------
zonkDo :: ZonkEnv -> HsStmtContext Name -> TcM (HsStmtContext Name)
-- Only used for 'do', so the only Ids are in a MDoExpr table
zonkDo env (MDoExpr tbl) = do { tbl' <- mapSndM (zonkExpr env) tbl
                              ; return (MDoExpr tbl') }
zonkDo _   do_or_lc      = return do_or_lc

-------------------------------------------------------------------------
zonkOverLit :: ZonkEnv -> HsOverLit TcId -> TcM (HsOverLit Id)
zonkOverLit env lit@(OverLit { ol_witness = e, ol_type = ty })
  = do	{ ty' <- zonkTcTypeToType env ty
	; e' <- zonkExpr env e
 	; return (lit { ol_witness = e', ol_type = ty' }) }

-------------------------------------------------------------------------
zonkArithSeq :: ZonkEnv -> ArithSeqInfo TcId -> TcM (ArithSeqInfo Id)

zonkArithSeq env (From e)
  = zonkLExpr env e		`thenM` \ new_e ->
    returnM (From new_e)

zonkArithSeq env (FromThen e1 e2)
  = zonkLExpr env e1	`thenM` \ new_e1 ->
    zonkLExpr env e2	`thenM` \ new_e2 ->
    returnM (FromThen new_e1 new_e2)

zonkArithSeq env (FromTo e1 e2)
  = zonkLExpr env e1	`thenM` \ new_e1 ->
    zonkLExpr env e2	`thenM` \ new_e2 ->
    returnM (FromTo new_e1 new_e2)

zonkArithSeq env (FromThenTo e1 e2 e3)
  = zonkLExpr env e1	`thenM` \ new_e1 ->
    zonkLExpr env e2	`thenM` \ new_e2 ->
    zonkLExpr env e3	`thenM` \ new_e3 ->
    returnM (FromThenTo new_e1 new_e2 new_e3)


-------------------------------------------------------------------------
zonkStmts :: ZonkEnv -> [LStmt TcId] -> TcM (ZonkEnv, [LStmt Id])
zonkStmts env []     = return (env, [])
zonkStmts env (s:ss) = do { (env1, s')  <- wrapLocSndM (zonkStmt env) s
			  ; (env2, ss') <- zonkStmts env1 ss
			  ; return (env2, s' : ss') }

zonkStmt :: ZonkEnv -> Stmt TcId -> TcM (ZonkEnv, Stmt Id)
zonkStmt env (ParStmt stmts_w_bndrs)
  = mappM zonk_branch stmts_w_bndrs	`thenM` \ new_stmts_w_bndrs ->
    let 
	new_binders = concat (map snd new_stmts_w_bndrs)
	env1 = extendZonkEnv env new_binders
    in
    return (env1, ParStmt new_stmts_w_bndrs)
  where
    zonk_branch (stmts, bndrs) = zonkStmts env stmts	`thenM` \ (env1, new_stmts) ->
				 returnM (new_stmts, zonkIdOccs env1 bndrs)

zonkStmt env (RecStmt { recS_stmts = segStmts, recS_later_ids = lvs, recS_rec_ids = rvs
                      , recS_ret_fn = ret_id, recS_mfix_fn = mfix_id, recS_bind_fn = bind_id
                      , recS_rec_rets = rets, recS_dicts = binds })
  = do { new_rvs <- zonkIdBndrs env rvs
       ; new_lvs <- zonkIdBndrs env lvs
       ; new_ret_id  <- zonkExpr env ret_id
       ; new_mfix_id <- zonkExpr env mfix_id
       ; new_bind_id <- zonkExpr env bind_id
       ; let env1 = extendZonkEnv env new_rvs
       ; (env2, new_segStmts) <- zonkStmts env1 segStmts
	-- Zonk the ret-expressions in an envt that 
	-- has the polymorphic bindings in the envt
       ; new_rets <- mapM (zonkExpr env2) rets
       ; let env3 = extendZonkEnv env new_lvs	-- Only the lvs are needed
       ; (env4, new_binds) <- zonkTcEvBinds env3 binds
       ; return (env4,
                 RecStmt { recS_stmts = new_segStmts, recS_later_ids = new_lvs
                         , recS_rec_ids = new_rvs, recS_ret_fn = new_ret_id
                         , recS_mfix_fn = new_mfix_id, recS_bind_fn = new_bind_id
                         , recS_rec_rets = new_rets, recS_dicts = new_binds }) }

zonkStmt env (ExprStmt expr then_op ty)
  = zonkLExpr env expr		`thenM` \ new_expr ->
    zonkExpr env then_op	`thenM` \ new_then ->
    zonkTcTypeToType env ty	`thenM` \ new_ty ->
    returnM (env, ExprStmt new_expr new_then new_ty)

zonkStmt env (TransformStmt stmts binders usingExpr maybeByExpr)
  = do { (env', stmts') <- zonkStmts env stmts 
    ; let binders' = zonkIdOccs env' binders
    ; usingExpr' <- zonkLExpr env' usingExpr
    ; maybeByExpr' <- zonkMaybeLExpr env' maybeByExpr
    ; return (env', TransformStmt stmts' binders' usingExpr' maybeByExpr') }
    
zonkStmt env (GroupStmt stmts binderMap by using)
  = do { (env', stmts') <- zonkStmts env stmts 
    ; binderMap' <- mappM (zonkBinderMapEntry env') binderMap
    ; by' <- fmapMaybeM (zonkLExpr env') by
    ; using' <- fmapEitherM (zonkLExpr env) (zonkExpr env) using
    ; let env'' = extendZonkEnv env' (map snd binderMap')
    ; return (env'', GroupStmt stmts' binderMap' by' using') }
  where
    zonkBinderMapEntry env (oldBinder, newBinder) = do 
        let oldBinder' = zonkIdOcc env oldBinder
        newBinder' <- zonkIdBndr env newBinder
        return (oldBinder', newBinder') 

zonkStmt env (LetStmt binds)
  = zonkLocalBinds env binds	`thenM` \ (env1, new_binds) ->
    returnM (env1, LetStmt new_binds)

zonkStmt env (BindStmt pat expr bind_op fail_op)
  = do	{ new_expr <- zonkLExpr env expr
	; (env1, new_pat) <- zonkPat env pat
	; new_bind <- zonkExpr env bind_op
	; new_fail <- zonkExpr env fail_op
	; return (env1, BindStmt new_pat new_expr new_bind new_fail) }

zonkMaybeLExpr :: ZonkEnv -> Maybe (LHsExpr TcId) -> TcM (Maybe (LHsExpr Id))
zonkMaybeLExpr _   Nothing  = return Nothing
zonkMaybeLExpr env (Just e) = (zonkLExpr env e) >>= (return . Just)


-------------------------------------------------------------------------
zonkRecFields :: ZonkEnv -> HsRecordBinds TcId -> TcM (HsRecordBinds TcId)
zonkRecFields env (HsRecFields flds dd)
  = do	{ flds' <- mappM zonk_rbind flds
	; return (HsRecFields flds' dd) }
  where
    zonk_rbind fld
      = do { new_id   <- wrapLocM (zonkIdBndr env) (hsRecFieldId fld)
	   ; new_expr <- zonkLExpr env (hsRecFieldArg fld)
	   ; return (fld { hsRecFieldId = new_id, hsRecFieldArg = new_expr }) }

-------------------------------------------------------------------------
mapIPNameTc :: (a -> TcM b) -> IPName a -> TcM (IPName b)
mapIPNameTc f (IPName n) = f n  `thenM` \ r -> returnM (IPName r)

zonkPat :: ZonkEnv -> OutPat TcId -> TcM (ZonkEnv, OutPat Id)
-- Extend the environment as we go, because it's possible for one
-- pattern to bind something that is used in another (inside or
-- to the right)
zonkPat env pat = wrapLocSndM (zonk_pat env) pat

zonk_pat :: ZonkEnv -> Pat TcId -> TcM (ZonkEnv, Pat Id)
zonk_pat env (ParPat p)
  = do	{ (env', p') <- zonkPat env p
  	; return (env', ParPat p') }

zonk_pat env (WildPat ty)
  = do	{ ty' <- zonkTcTypeToType env ty
	; return (env, WildPat ty') }

zonk_pat env (VarPat v)
  = do	{ v' <- zonkIdBndr env v
	; return (extendZonkEnv1 env v', VarPat v') }

zonk_pat env (VarPatOut v binds)
  = do	{ v' <- zonkIdBndr env v
	; (env', binds') <- zonkTcEvBinds (extendZonkEnv1 env v') binds
  	; returnM (env', VarPatOut v' binds') }

zonk_pat env (LazyPat pat)
  = do	{ (env', pat') <- zonkPat env pat
	; return (env',  LazyPat pat') }

zonk_pat env (BangPat pat)
  = do	{ (env', pat') <- zonkPat env pat
	; return (env',  BangPat pat') }

zonk_pat env (AsPat (L loc v) pat)
  = do	{ v' <- zonkIdBndr env v
	; (env', pat') <- zonkPat (extendZonkEnv1 env v') pat
 	; return (env', AsPat (L loc v') pat') }

zonk_pat env (ViewPat expr pat ty)
  = do	{ expr' <- zonkLExpr env expr
	; (env', pat') <- zonkPat env pat
 	; ty' <- zonkTcTypeToType env ty
	; return (env', ViewPat expr' pat' ty') }

zonk_pat env (ListPat pats ty)
  = do	{ ty' <- zonkTcTypeToType env ty
	; (env', pats') <- zonkPats env pats
	; return (env', ListPat pats' ty') }

zonk_pat env (PArrPat pats ty)
  = do	{ ty' <- zonkTcTypeToType env ty
	; (env', pats') <- zonkPats env pats
	; return (env', PArrPat pats' ty') }

zonk_pat env (TuplePat pats boxed ty)
  = do	{ ty' <- zonkTcTypeToType env ty
	; (env', pats') <- zonkPats env pats
	; return (env', TuplePat pats' boxed ty') }

zonk_pat env p@(ConPatOut { pat_ty = ty, pat_dicts = evs, pat_binds = binds, pat_args = args })
  = ASSERT( all isImmutableTyVar (pat_tvs p) ) 
    do	{ new_ty <- zonkTcTypeToType env ty
	; (env1, new_evs) <- zonkEvBndrsX env evs
	; (env2, new_binds) <- zonkTcEvBinds env1 binds
	; (env', new_args) <- zonkConStuff env2 args
	; returnM (env', p { pat_ty = new_ty, pat_dicts = new_evs, 
			     pat_binds = new_binds, pat_args = new_args }) }

zonk_pat env (LitPat lit) = return (env, LitPat lit)

zonk_pat env (SigPatOut pat ty)
  = do	{ ty' <- zonkTcTypeToType env ty
	; (env', pat') <- zonkPat env pat
	; return (env', SigPatOut pat' ty') }

zonk_pat env (NPat lit mb_neg eq_expr)
  = do	{ lit' <- zonkOverLit env lit
 	; mb_neg' <- fmapMaybeM (zonkExpr env) mb_neg
 	; eq_expr' <- zonkExpr env eq_expr
	; return (env, NPat lit' mb_neg' eq_expr') }

zonk_pat env (NPlusKPat (L loc n) lit e1 e2)
  = do	{ n' <- zonkIdBndr env n
	; lit' <- zonkOverLit env lit
 	; e1' <- zonkExpr env e1
	; e2' <- zonkExpr env e2
	; return (extendZonkEnv1 env n', NPlusKPat (L loc n') lit' e1' e2') }

zonk_pat env (CoPat co_fn pat ty) 
  = do { (env', co_fn') <- zonkCoFn env co_fn
       ; (env'', pat') <- zonkPat env' (noLoc pat)
       ; ty' <- zonkTcTypeToType env'' ty
       ; return (env'', CoPat co_fn' (unLoc pat') ty') }

zonk_pat _ pat = pprPanic "zonk_pat" (ppr pat)

---------------------------
zonkConStuff :: ZonkEnv
             -> HsConDetails (OutPat TcId) (HsRecFields id (OutPat TcId))
             -> TcM (ZonkEnv,
                     HsConDetails (OutPat Id) (HsRecFields id (OutPat Id)))
zonkConStuff env (PrefixCon pats)
  = do	{ (env', pats') <- zonkPats env pats
	; return (env', PrefixCon pats') }

zonkConStuff env (InfixCon p1 p2)
  = do	{ (env1, p1') <- zonkPat env  p1
	; (env', p2') <- zonkPat env1 p2
	; return (env', InfixCon p1' p2') }

zonkConStuff env (RecCon (HsRecFields rpats dd))
  = do	{ (env', pats') <- zonkPats env (map hsRecFieldArg rpats)
	; let rpats' = zipWith (\rp p' -> rp { hsRecFieldArg = p' }) rpats pats'
	; returnM (env', RecCon (HsRecFields rpats' dd)) }
	-- Field selectors have declared types; hence no zonking

---------------------------
zonkPats :: ZonkEnv -> [OutPat TcId] -> TcM (ZonkEnv, [OutPat Id])
zonkPats env []		= return (env, [])
zonkPats env (pat:pats) = do { (env1, pat') <- zonkPat env pat
		     ; (env', pats') <- zonkPats env1 pats
		     ; return (env', pat':pats') }

zonkForeignExports :: ZonkEnv -> [LForeignDecl TcId] -> TcM [LForeignDecl Id]
zonkForeignExports env ls = mappM (wrapLocM (zonkForeignExport env)) ls

zonkForeignExport :: ZonkEnv -> ForeignDecl TcId -> TcM (ForeignDecl Id)
zonkForeignExport env (ForeignExport i _hs_ty spec) =
   returnM (ForeignExport (fmap (zonkIdOcc env) i) undefined spec)
zonkForeignExport _ for_imp 
  = returnM for_imp	-- Foreign imports don't need zonking

zonkRules :: ZonkEnv -> [LRuleDecl TcId] -> TcM [LRuleDecl Id]
zonkRules env rs = mappM (wrapLocM (zonkRule env)) rs

zonkRule :: ZonkEnv -> RuleDecl TcId -> TcM (RuleDecl Id)
zonkRule env (HsRule name act (vars{-::[RuleBndr TcId]-}) lhs fv_lhs rhs fv_rhs)
  = do { (env_rhs, new_bndrs) <- mapAccumLM zonk_bndr env vars

       ; unbound_tv_set <- newMutVar emptyVarSet
       ; let env_lhs = setZonkType env_rhs (zonkTypeCollecting unbound_tv_set)
	-- We need to gather the type variables mentioned on the LHS so we can 
	-- quantify over them.  Example:
	--   data T a = C
	-- 
	--   foo :: T a -> Int
	--   foo C = 1
	--
	--   {-# RULES "myrule"  foo C = 1 #-}
	-- 
	-- After type checking the LHS becomes (foo a (C a))
	-- and we do not want to zap the unbound tyvar 'a' to (), because
	-- that limits the applicability of the rule.  Instead, we
	-- want to quantify over it!  
	--
	-- It's easiest to find the free tyvars here. Attempts to do so earlier
	-- are tiresome, because (a) the data type is big and (b) finding the 
	-- free type vars of an expression is necessarily monadic operation.
	--	(consider /\a -> f @ b, where b is side-effected to a)

       ; new_lhs <- zonkLExpr env_lhs lhs
       ; new_rhs <- zonkLExpr env_rhs rhs

       ; unbound_tvs <- readMutVar unbound_tv_set
       ; let final_bndrs :: [RuleBndr Var]
	     final_bndrs = map (RuleBndr . noLoc) (varSetElems unbound_tvs) ++ new_bndrs

       ; return (HsRule name act final_bndrs new_lhs fv_lhs new_rhs fv_rhs) }
  where
   zonk_bndr env (RuleBndr (L loc v)) 
      = do { (env', v') <- zonk_it env v; return (env', RuleBndr (L loc v')) }
   zonk_bndr _ (RuleBndrSig {}) = panic "zonk_bndr RuleBndrSig"

   zonk_it env v
     | isId v     = do { v' <- zonkIdBndr env v; return (extendZonkEnv1 env v', v') }
     | isCoVar v  = do { v' <- zonkEvBndr env v; return (extendZonkEnv1 env v', v') }
     | otherwise  = ASSERT( isImmutableTyVar v) return (env, v)

zonkEvTerm :: ZonkEnv -> EvTerm -> TcM EvTerm
zonkEvTerm env (EvId v)           = ASSERT2( isId v, ppr v ) 
                                    return (EvId (zonkIdOcc env v))
zonkEvTerm env (EvCoercion co)    = do { co' <- zonkTcTypeToType env co
                                       ; return (EvCoercion co') }
zonkEvTerm env (EvCast v co)      = ASSERT( isId v) 
                                    do { co' <- zonkTcTypeToType env co
                                       ; return (EvCast (zonkIdOcc env v) co') }
zonkEvTerm env (EvSuperClass d n) = return (EvSuperClass (zonkIdOcc env d) n)
zonkEvTerm env (EvDFunApp df tys tms)
  = do { tys' <- zonkTcTypeToTypes env tys
       ; let tms' = map (zonkEvVarOcc env) tms
       ; return (EvDFunApp (zonkIdOcc env df) tys' tms') }

zonkTcEvBinds :: ZonkEnv -> TcEvBinds -> TcM (ZonkEnv, TcEvBinds)
zonkTcEvBinds env (TcEvBinds var) = do { (env', bs') <- zonkEvBindsVar env var
				       ; return (env', EvBinds bs') }
zonkTcEvBinds env (EvBinds bs)    = do { (env', bs') <- zonkEvBinds env bs
				       ; return (env', EvBinds bs') }

zonkEvBindsVar :: ZonkEnv -> EvBindsVar -> TcM (ZonkEnv, Bag EvBind)
zonkEvBindsVar env (EvBindsVar ref _) = do { bs <- readMutVar ref
                                           ; zonkEvBinds env (evBindMapBinds bs) }

zonkEvBinds :: ZonkEnv -> Bag EvBind -> TcM (ZonkEnv, Bag EvBind)
zonkEvBinds env binds
  = fixM (\ ~( _, new_binds) -> do
	 { let env1 = extendZonkEnv env (collect_ev_bndrs new_binds)
         ; binds' <- mapBagM (zonkEvBind env1) binds
         ; return (env1, binds') })
  where
    collect_ev_bndrs :: Bag EvBind -> [EvVar]
    collect_ev_bndrs = foldrBag add [] 
    add (EvBind var _) vars = var : vars

zonkEvBind :: ZonkEnv -> EvBind -> TcM EvBind
zonkEvBind env (EvBind var term)
  = do { var' <- zonkEvBndr env var
       ; term' <- zonkEvTerm env term
       ; return (EvBind var' term') }

zonkTcTypeToType :: ZonkEnv -> TcType -> TcM Type
zonkTcTypeToType (ZonkEnv zonk_ty _) ty = zonk_ty ty

zonkTcTypeToTypes :: ZonkEnv -> [TcType] -> TcM [Type]
zonkTcTypeToTypes env tys = mapM (zonkTcTypeToType env) tys

zonkTypeCollecting :: TcRef TyVarSet -> TcType -> TcM Type
-- This variant collects unbound type variables in a mutable variable
zonkTypeCollecting unbound_tv_set
  = zonkType (mkZonkTcTyVar zonk_unbound_tyvar)
  where
    zonk_unbound_tyvar tv 
        = do { tv' <- zonkQuantifiedTyVar tv
	     ; tv_set <- readMutVar unbound_tv_set
	     ; writeMutVar unbound_tv_set (extendVarSet tv_set tv')
	     ; return (mkTyVarTy tv') }

zonkTypeZapping :: TcType -> TcM Type
-- This variant is used for everything except the LHS of rules
-- It zaps unbound type variables to (), or some other arbitrary type
zonkTypeZapping ty 
  = zonkType (mkZonkTcTyVar zonk_unbound_tyvar) ty 
  where
	-- Zonk a mutable but unbound type variable to an arbitrary type
	-- We know it's unbound even though we don't carry an environment,
	-- because at the binding site for a type variable we bind the
	-- mutable tyvar to a fresh immutable one.  So the mutable store
	-- plays the role of an environment.  If we come across a mutable
	-- type variable that isn't so bound, it must be completely free.
    zonk_unbound_tyvar tv = do { let ty = anyTypeOfKind (tyVarKind tv)
			       ; writeMetaTyVar tv ty
			       ; return ty }