    {-
    (c) The University of Glasgow 2006
    (c) The AQUA Project, Glasgow University, 1996-1998

    TcHsSyn: Specialisations of the @HsSyn@ syntax for the typechecker

    This module is an extension of @HsSyn@ syntax, for use in the type
    checker.
    -}

    {-# LANGUAGE CPP #-}

    module TcHsSyn (
            mkHsConApp, mkHsDictLet, mkHsApp,
            hsLitType, hsLPatType, hsPatType,
            mkHsAppTy, mkSimpleHsAlt,
            nlHsIntLit,
            shortCutLit, hsOverLitName,
            conLikeResTy,

            -- re-exported from TcMonad
            TcId, TcIdSet,

            zonkTopDecls, zonkTopExpr, zonkTopLExpr,
            zonkTopBndrs, zonkTyBndrsX,
            emptyZonkEnv, mkEmptyZonkEnv, mkTyVarZonkEnv,
            zonkTcTypeToType, zonkTcTypeToTypes, zonkTyVarOcc,
      ) where

    #include "HsVersions.h"

    import HsSyn
    import Id
    import TcRnMonad
    import PrelNames
    import TypeRep     -- We can see the representation of types
    import TcType
    import TcMType ( defaultKindVarToStar, zonkQuantifiedTyVar, writeMetaTyVar )
    import TcEvidence
    import Coercion
    import TysPrim
    import TysWiredIn
    import Type
    import ConLike
    import DataCon
    import PatSyn( patSynInstResTy )
    import Name
    import NameSet
    import Var
    import VarSet
    import VarEnv
    import DynFlags
    import Literal
    import BasicTypes
    import Maybes
    import SrcLoc
    import Bag
    import FastString
    import Outputable
    import Util
    #if __GLASGOW_HASKELL__ < 709
    import Data.Traversable ( traverse )
    #endif

    {-
    ************************************************************************
    *                                                                      *
    \subsection[mkFailurePair]{Code for pattern-matching and other failures}
    *                                                                      *
    ************************************************************************

    Note: If @hsLPatType@ doesn't bear a strong resemblance to @exprType@,
    then something is wrong.
    -}

    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 (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 Nothing)      = mkListTy ty
    hsPatType (ListPat _ _ (Just (ty,_))) = ty
    hsPatType (PArrPat _ ty)              = mkPArrTy ty
    hsPatType (TuplePat _ bx tys)         = mkTupleTy (boxityNormalTupleSort bx) tys
    hsPatType (ConPatOut { pat_con = L _ con, pat_arg_tys = tys })
                                          = conLikeResTy con tys
    hsPatType (SigPatOut _ ty)            = ty
    hsPatType (NPat (L _ lit) _ _)        = overLitType lit
    hsPatType (NPlusKPat id _ _ _)        = idType (unLoc id)
    hsPatType (CoPat _ _ ty)              = ty
    hsPatType p                           = pprPanic "hsPatType" (ppr p)

    conLikeResTy :: ConLike -> [Type] -> Type
    conLikeResTy (RealDataCon con) tys = mkTyConApp (dataConTyCon con) tys
    conLikeResTy (PatSynCon ps)    tys = patSynInstResTy ps tys

    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 (HsInt64Prim _ _)  = int64PrimTy
    hsLitType (HsWord64Prim _ _) = word64PrimTy
    hsLitType (HsInteger _ _ ty) = ty
    hsLitType (HsRat _ ty)       = ty
    hsLitType (HsFloatPrim _)    = floatPrimTy
    hsLitType (HsDoublePrim _)   = doublePrimTy

    -- Overloaded literals. Here mainly because it uses isIntTy etc

    shortCutLit :: DynFlags -> OverLitVal -> TcType -> Maybe (HsExpr TcId)
    shortCutLit dflags (HsIntegral src i) ty
      | isIntTy ty  && inIntRange  dflags i = Just (HsLit (HsInt src i))
      | isWordTy ty && inWordRange dflags i
                                       = Just (mkLit wordDataCon (HsWordPrim src i))
      | isIntegerTy ty = Just (HsLit (HsInteger src i ty))
      | otherwise = shortCutLit dflags (HsFractional (integralFractionalLit 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 src s) ty
      | isStringTy ty = Just (HsLit (HsString src 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

    {-
    ************************************************************************
    *                                                                      *
    \subsection[BackSubst-HsBinds]{Running a substitution over @HsBinds@}
    *                                                                      *
    ************************************************************************

    The rest of the zonking is done *after* typechecking.
    The main zonking pass runs over the bindings

     a) to convert TcTyVars to TyVars etc, dereferencing any bindings etc
     b) convert unbound TcTyVar to Void
     c) convert each TcId to an Id by zonking its type

    The type variables are converted by binding mutable tyvars to immutable ones
    and then zonking as normal.

    The Ids are converted by binding them in the normal Tc envt; that
    way we maintain sharing; eg an Id is zonked at its binding site and they
    all occurrences of that Id point to the common zonked copy

    It's all pretty boring stuff, because HsSyn is such a large type, and
    the environment manipulation is tiresome.
    -}

    type UnboundTyVarZonker = TcTyVar-> TcM Type
            -- How to zonk an unbound type variable
            -- Note [Zonking the LHS of a RULE]

    data ZonkEnv
      = ZonkEnv
          UnboundTyVarZonker
          (TyVarEnv TyVar)          --
          (IdEnv    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

    instance Outputable ZonkEnv where
      ppr (ZonkEnv _ _ty_env var_env) = vcat (map ppr (varEnvElts var_env))

    emptyZonkEnv :: ZonkEnv
    emptyZonkEnv = mkEmptyZonkEnv zonkTypeZapping

    mkEmptyZonkEnv :: UnboundTyVarZonker -> ZonkEnv
    mkEmptyZonkEnv zonker = ZonkEnv zonker emptyVarEnv emptyVarEnv

    extendIdZonkEnv :: ZonkEnv -> [Var] -> ZonkEnv
    extendIdZonkEnv (ZonkEnv zonk_ty ty_env id_env) ids
      = ZonkEnv zonk_ty ty_env (extendVarEnvList id_env [(id,id) | id <- ids])

    extendIdZonkEnv1 :: ZonkEnv -> Var -> ZonkEnv
    extendIdZonkEnv1 (ZonkEnv zonk_ty ty_env id_env) id
      = ZonkEnv zonk_ty ty_env (extendVarEnv id_env id id)

    extendTyZonkEnv1 :: ZonkEnv -> TyVar -> ZonkEnv
    extendTyZonkEnv1 (ZonkEnv zonk_ty ty_env id_env) ty
      = ZonkEnv zonk_ty (extendVarEnv ty_env ty ty) id_env

    mkTyVarZonkEnv :: [TyVar] -> ZonkEnv
    mkTyVarZonkEnv tvs = ZonkEnv zonkTypeZapping (mkVarEnv [(tv,tv) | tv <- tvs]) emptyVarEnv

    setZonkType :: ZonkEnv -> UnboundTyVarZonker -> ZonkEnv
    setZonkType (ZonkEnv _ ty_env id_env) zonk_ty = ZonkEnv zonk_ty ty_env id_env

    zonkEnvIds :: ZonkEnv -> [Id]
    zonkEnvIds (ZonkEnv _ _ id_env) = varEnvElts id_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 _ty_env 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
      = do ty' <- zonkTcTypeToType env (idType id)
           return (Id.setIdType id ty')

    zonkIdBndrs :: ZonkEnv -> [TcId] -> TcM [Id]
    zonkIdBndrs env ids = mapM (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 (extendIdZonkEnv1 env var', var') }

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

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

    zonkTyBndrsX :: ZonkEnv -> [TyVar] -> TcM (ZonkEnv, [TyVar])
    zonkTyBndrsX = mapAccumLM zonkTyBndrX

    zonkTyBndrX :: ZonkEnv -> TyVar -> TcM (ZonkEnv, TyVar)
    -- This guarantees to return a TyVar (not a TcTyVar)
    -- then we add it to the envt, so all occurrences are replaced
    zonkTyBndrX env tv
      = do { ki <- zonkTcTypeToType env (tyVarKind tv)
           ; let tv' = mkTyVar (tyVarName tv) ki
           ; return (extendTyZonkEnv1 env tv', tv') }

    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 -> Bag OccName -> NameSet
                 -> [LRuleDecl TcId] -> [LVectDecl TcId] -> [LTcSpecPrag] -> [LForeignDecl TcId]
                 -> TcM ([Id],
                         Bag EvBind,
                         LHsBinds Id,
                         [LForeignDecl Id],
                         [LTcSpecPrag],
                         [LRuleDecl    Id],
                         [LVectDecl    Id])
    zonkTopDecls ev_binds binds exports sig_ns rules vects 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 <- woptM Opt_WarnMissingSigs
            ; warn_only_exported <- woptM Opt_WarnMissingExportedSigs
            ; let sig_warn
                    | warn_only_exported = topSigWarnIfExported exports sig_ns
                    | warn_missing_sigs  = topSigWarn sig_ns
                    | otherwise          = noSigWarn

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

    ---------------------------------------------
    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 <- woptM 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)) = do
        new_binds <- mapM (wrapLocM zonk_ip_bind) binds
        let
            env1 = extendIdZonkEnv env [ n | L _ (IPBind (Right n) _) <- new_binds]
        (env2, new_dict_binds) <- zonkTcEvBinds env1 dict_binds
        return (env2, HsIPBinds (IPBinds new_binds new_dict_binds))
      where
        zonk_ip_bind (IPBind n e)
            = do n' <- mapIPNameTc (zonkIdBndr env) n
                 e' <- zonkLExpr env e
                 return (IPBind n' e')

    ---------------------------------------------
    zonkRecMonoBinds :: ZonkEnv -> SigWarn -> LHsBinds TcId -> TcM (ZonkEnv, LHsBinds Id)
    zonkRecMonoBinds env sig_warn binds
     = fixM (\ ~(_, new_binds) -> do
            { let env1 = extendIdZonkEnv 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 ()

    topSigWarnIfExported :: Bag OccName -> NameSet -> SigWarn
    topSigWarnIfExported exported sig_ns _ ids
      = mapM_ (topSigWarnIdIfExported exported sig_ns) ids

    topSigWarnIdIfExported :: Bag OccName -> NameSet -> Id -> TcM ()
    topSigWarnIdIfExported exported sig_ns id
      | getOccName id `elemBag` exported
      = topSigWarnId sig_ns id
      | otherwise
      = 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 "Polymorphic 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 $ pprPrefixName (idName id) <+> dcolon <+> ppr ty ]

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

    zonk_lbind :: ZonkEnv -> SigWarn -> LHsBind TcId -> TcM (LHsBind Id)
    zonk_lbind env sig_warn = wrapLocM (zonk_bind env sig_warn)

    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 zonkLExpr 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 zonkLExpr 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 { (env0, new_tyvars) <- zonkTyBndrsX env tyvars
           ; (env1, new_evs) <- zonkEvBndrsX env0 evs
           ; (env2, new_ev_binds) <- zonkTcEvBinds env1 ev_binds
           ; (new_val_bind, new_exports) <- fixM $ \ ~(new_val_binds, _) ->
             do { let env3 = extendIdZonkEnv 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 (map abe_poly new_exports)
           ; return (AbsBinds { abs_tvs = new_tyvars, abs_ev_vars = new_evs
                              , abs_ev_binds = new_ev_binds
                              , abs_exports = new_exports, abs_binds = new_val_bind }) }
      where
        zonkExport env (ABE{ abe_wrap = wrap, abe_poly = poly_id
                           , abe_mono = mono_id, abe_prags = prags })
            = do new_poly_id <- zonkIdBndr env poly_id
                 (_, new_wrap) <- zonkCoFn env wrap
                 new_prags <- zonkSpecPrags env prags
                 return (ABE{ abe_wrap = new_wrap, abe_poly = new_poly_id
                            , abe_mono = zonkIdOcc env mono_id
                            , abe_prags = new_prags })

    zonk_bind env _sig_warn (PatSynBind bind@(PSB { psb_id = L loc id
                                                  , psb_args = details
                                                  , psb_def = lpat
                                                  , psb_dir = dir }))
      = do { id' <- zonkIdBndr env id
           ; details' <- zonkPatSynDetails env details
           ;(env1, lpat') <- zonkPat env lpat
           ; (_env2, dir') <- zonkPatSynDir env1 dir
           ; return $ PatSynBind $
                      bind { psb_id = L loc id'
                           , psb_args = details'
                           , psb_def = lpat'
                           , psb_dir = dir' } }

    zonkPatSynDetails :: ZonkEnv
                      -> HsPatSynDetails (Located TcId)
                      -> TcM (HsPatSynDetails (Located Id))
    zonkPatSynDetails env = traverse (wrapLocM $ zonkIdBndr env)

    zonkPatSynDir :: ZonkEnv -> HsPatSynDir TcId -> TcM (ZonkEnv, HsPatSynDir Id)
    zonkPatSynDir env Unidirectional = return (env, Unidirectional)
    zonkPatSynDir env ImplicitBidirectional = return (env, ImplicitBidirectional)
    zonkPatSynDir env (ExplicitBidirectional mg) = do
        mg' <- zonkMatchGroup env zonkLExpr mg
        return (env, ExplicitBidirectional mg')

    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)) }

    {-
    ************************************************************************
    *                                                                      *
    \subsection[BackSubst-Match-GRHSs]{Match and GRHSs}
    *                                                                      *
    ************************************************************************
    -}

    zonkMatchGroup :: ZonkEnv
                   -> (ZonkEnv -> Located (body TcId) -> TcM (Located (body Id)))
                   -> MatchGroup TcId (Located (body TcId)) -> TcM (MatchGroup Id (Located (body Id)))
    zonkMatchGroup env zBody (MG { mg_alts = ms, mg_arg_tys = arg_tys, mg_res_ty = res_ty, mg_origin = origin })
      = do  { ms' <- mapM (zonkMatch env zBody) ms
            ; arg_tys' <- zonkTcTypeToTypes env arg_tys
            ; res_ty'  <- zonkTcTypeToType env res_ty
            ; return (MG { mg_alts = ms', mg_arg_tys = arg_tys', mg_res_ty = res_ty', mg_origin = origin }) }

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

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

    zonkGRHSs env zBody (GRHSs grhss binds) = do
        (new_env, new_binds) <- zonkLocalBinds env binds
        let
            zonk_grhs (GRHS guarded rhs)
              = do (env2, new_guarded) <- zonkStmts new_env zonkLExpr guarded
                   new_rhs <- zBody env2 rhs
                   return (GRHS new_guarded new_rhs)
        new_grhss <- mapM (wrapLocM zonk_grhs) grhss
        return (GRHSs new_grhss new_binds)

    {-
    ************************************************************************
    *                                                                      *
    \subsection[BackSubst-HsExpr]{Running a zonkitution over a TypeCheckedExpr}
    *                                                                      *
    ************************************************************************
    -}

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

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

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

    zonkExpr _ (HsIPVar id)
      = return (HsIPVar id)

    zonkExpr env (HsLit (HsRat f ty))
      = do new_ty <- zonkTcTypeToType env ty
           return (HsLit (HsRat f new_ty))

    zonkExpr _ (HsLit lit)
      = return (HsLit lit)

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

    zonkExpr env (HsLam matches)
      = do new_matches <- zonkMatchGroup env zonkLExpr matches
           return (HsLam new_matches)

    zonkExpr env (HsLamCase arg matches)
      = do new_arg <- zonkTcTypeToType env arg
           new_matches <- zonkMatchGroup env zonkLExpr matches
           return (HsLamCase new_arg new_matches)

    zonkExpr env (HsApp e1 e2)
      = do new_e1 <- zonkLExpr env e1
           new_e2 <- zonkLExpr env e2
           return (HsApp new_e1 new_e2)

    zonkExpr _ e@(HsRnBracketOut _ _)
      = pprPanic "zonkExpr: HsRnBracketOut" (ppr e)

    zonkExpr env (HsTcBracketOut body bs)
      = do bs' <- mapM zonk_b bs
           return (HsTcBracketOut body bs')
      where
        zonk_b (PendSplice n e) = do e' <- zonkLExpr env e
                                     return (PendSplice n e')

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

    zonkExpr env (OpApp e1 op fixity e2)
      = do new_e1 <- zonkLExpr env e1
           new_op <- zonkLExpr env op
           new_e2 <- zonkLExpr env e2
           return (OpApp new_e1 new_op fixity new_e2)

    zonkExpr env (NegApp expr op)
      = do new_expr <- zonkLExpr env expr
           new_op <- zonkExpr env op
           return (NegApp new_expr new_op)

    zonkExpr env (HsPar e)
      = do new_e <- zonkLExpr env e
           return (HsPar new_e)

    zonkExpr env (SectionL expr op)
      = do new_expr <- zonkLExpr env expr
           new_op   <- zonkLExpr env op
           return (SectionL new_expr new_op)

    zonkExpr env (SectionR op expr)
      = do new_op   <- zonkLExpr env op
           new_expr <- zonkLExpr env expr
           return (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 (L l (Present e)) = do { e' <- zonkLExpr env e
                                            ; return (L l (Present e')) }
        zonk_tup_arg (L l (Missing t)) = do { t' <- zonkTcTypeToType env t
                                            ; return (L l (Missing t')) }

    zonkExpr env (HsCase expr ms)
      = do new_expr <- zonkLExpr env expr
           new_ms <- zonkMatchGroup env zonkLExpr ms
           return (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
           ; return (HsIf new_e0 new_e1 new_e2 new_e3) }

    zonkExpr env (HsMultiIf ty alts)
      = do { alts' <- mapM (wrapLocM zonk_alt) alts
           ; ty'   <- zonkTcTypeToType env ty
           ; return $ HsMultiIf ty' alts' }
      where zonk_alt (GRHS guard expr)
              = do { (env', guard') <- zonkStmts env zonkLExpr guard
                   ; expr'          <- zonkLExpr env' expr
                   ; return $ GRHS guard' expr' }

    zonkExpr env (HsLet binds expr)
      = do (new_env, new_binds) <- zonkLocalBinds env binds
           new_expr <- zonkLExpr new_env expr
           return (HsLet new_binds new_expr)

    zonkExpr env (HsDo do_or_lc stmts ty)
      = do (_, new_stmts) <- zonkStmts env zonkLExpr stmts
           new_ty <- zonkTcTypeToType env ty
           return (HsDo do_or_lc new_stmts new_ty)

    zonkExpr env (ExplicitList ty wit exprs)
      = do new_ty <- zonkTcTypeToType env ty
           new_wit <- zonkWit env wit
           new_exprs <- zonkLExprs env exprs
           return (ExplicitList new_ty new_wit new_exprs)
       where zonkWit _ Nothing = return Nothing
             zonkWit env (Just fln) = do new_fln <- zonkExpr env fln
                                         return (Just new_fln)

    zonkExpr env (ExplicitPArr ty exprs)
      = do new_ty <- zonkTcTypeToType env ty
           new_exprs <- zonkLExprs env exprs
           return (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 wit info)
      = do new_expr <- zonkExpr env expr
           new_wit <- zonkWit env wit
           new_info <- zonkArithSeq env info
           return (ArithSeq new_expr new_wit new_info)
       where zonkWit _ Nothing = return Nothing
             zonkWit env (Just fln) = do new_fln <- zonkExpr env fln
                                         return (Just new_fln)

    zonkExpr env (PArrSeq expr info)
      = do new_expr <- zonkExpr env expr
           new_info <- zonkArithSeq env info
           return (PArrSeq new_expr new_info)

    zonkExpr env (HsSCC src lbl expr)
      = do new_expr <- zonkLExpr env expr
           return (HsSCC src lbl new_expr)

    zonkExpr env (HsTickPragma src info expr)
      = do new_expr <- zonkLExpr env expr
           return (HsTickPragma src info new_expr)

    -- hdaume: core annotations
    zonkExpr env (HsCoreAnn src lbl expr)
      = do new_expr <- zonkLExpr env expr
           return (HsCoreAnn src 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) }

    -- StaticPointers extension
    zonkExpr env (HsStatic expr)
      = HsStatic <$> zonkLExpr env expr

    zonkExpr env (HsWrap co_fn expr)
      = do (env1, new_co_fn) <- zonkCoFn env co_fn
           new_expr <- zonkExpr env1 expr
           return (HsWrap new_co_fn new_expr)

    zonkExpr _ (HsUnboundVar v)
      = return (HsUnboundVar v)

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

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

    zonkLCmd  :: ZonkEnv -> LHsCmd TcId   -> TcM (LHsCmd Id)
    zonkCmd   :: ZonkEnv -> HsCmd TcId    -> TcM (HsCmd Id)

    zonkLCmd  env cmd  = wrapLocM (zonkCmd env) cmd

    zonkCmd env (HsCmdCast co cmd)
      = do { co' <- zonkTcCoToCo env co
           ; cmd' <- zonkCmd env cmd
           ; return (HsCmdCast co' cmd') }
    zonkCmd env (HsCmdArrApp e1 e2 ty ho rl)
      = do new_e1 <- zonkLExpr env e1
           new_e2 <- zonkLExpr env e2
           new_ty <- zonkTcTypeToType env ty
           return (HsCmdArrApp new_e1 new_e2 new_ty ho rl)

    zonkCmd env (HsCmdArrForm op fixity args)
      = do new_op <- zonkLExpr env op
           new_args <- mapM (zonkCmdTop env) args
           return (HsCmdArrForm new_op fixity new_args)

    zonkCmd env (HsCmdApp c e)
      = do new_c <- zonkLCmd env c
           new_e <- zonkLExpr env e
           return (HsCmdApp new_c new_e)

    zonkCmd env (HsCmdLam matches)
      = do new_matches <- zonkMatchGroup env zonkLCmd matches
           return (HsCmdLam new_matches)

    zonkCmd env (HsCmdPar c)
      = do new_c <- zonkLCmd env c
           return (HsCmdPar new_c)

    zonkCmd env (HsCmdCase expr ms)
      = do new_expr <- zonkLExpr env expr
           new_ms <- zonkMatchGroup env zonkLCmd ms
           return (HsCmdCase new_expr new_ms)

    zonkCmd env (HsCmdIf eCond ePred cThen cElse)
      = do { new_eCond <- fmapMaybeM (zonkExpr env) eCond
           ; new_ePred <- zonkLExpr env ePred
           ; new_cThen <- zonkLCmd env cThen
           ; new_cElse <- zonkLCmd env cElse
           ; return (HsCmdIf new_eCond new_ePred new_cThen new_cElse) }

    zonkCmd env (HsCmdLet binds cmd)
      = do (new_env, new_binds) <- zonkLocalBinds env binds
           new_cmd <- zonkLCmd new_env cmd
           return (HsCmdLet new_binds new_cmd)

    zonkCmd env (HsCmdDo stmts ty)
      = do (_, new_stmts) <- zonkStmts env zonkLCmd stmts
           new_ty <- zonkTcTypeToType env ty
           return (HsCmdDo new_stmts new_ty)

    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)
      = do new_cmd <- zonkLCmd env cmd
           new_stack_tys <- zonkTcTypeToType env stack_tys
           new_ty <- zonkTcTypeToType env ty
           new_ids <- mapSndM (zonkExpr env) ids
           return (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 (WpFun c1 c2 t1 t2) = do { (env1, c1') <- zonkCoFn env c1
                                          ; (env2, c2') <- zonkCoFn env1 c2
                                          ; t1'         <- zonkTcTypeToType env2 t1
                                          ; t2'         <- zonkTcTypeToType env2 t2
                                          ; return (env2, WpFun c1' c2' t1' t2') }
    zonkCoFn env (WpCast co) = do { co' <- zonkTcCoToCo 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 )
                                  do { (env', tv') <- zonkTyBndrX env 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') }

    -------------------------------------------------------------------------
    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)
      = do new_e <- zonkLExpr env e
           return (From new_e)

    zonkArithSeq env (FromThen e1 e2)
      = do new_e1 <- zonkLExpr env e1
           new_e2 <- zonkLExpr env e2
           return (FromThen new_e1 new_e2)

    zonkArithSeq env (FromTo e1 e2)
      = do new_e1 <- zonkLExpr env e1
           new_e2 <- zonkLExpr env e2
           return (FromTo new_e1 new_e2)

    zonkArithSeq env (FromThenTo e1 e2 e3)
      = do new_e1 <- zonkLExpr env e1
           new_e2 <- zonkLExpr env e2
           new_e3 <- zonkLExpr env e3
           return (FromThenTo new_e1 new_e2 new_e3)

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

    zonkStmt :: ZonkEnv
             -> (ZonkEnv -> Located (body TcId) -> TcM (Located (body Id)))
             -> Stmt TcId (Located (body TcId)) -> TcM (ZonkEnv, Stmt Id (Located (body Id)))
    zonkStmt env _ (ParStmt stmts_w_bndrs mzip_op bind_op)
      = do { new_stmts_w_bndrs <- mapM zonk_branch stmts_w_bndrs
           ; let new_binders = [b | ParStmtBlock _ bs _ <- new_stmts_w_bndrs, b <- bs]
                 env1 = extendIdZonkEnv env new_binders
           ; new_mzip <- zonkExpr env1 mzip_op
           ; new_bind <- zonkExpr env1 bind_op
           ; return (env1, ParStmt new_stmts_w_bndrs new_mzip new_bind) }
      where
        zonk_branch (ParStmtBlock stmts bndrs return_op)
           = do { (env1, new_stmts) <- zonkStmts env zonkLExpr stmts
                ; new_return <- zonkExpr env1 return_op
                ; return (ParStmtBlock new_stmts (zonkIdOccs env1 bndrs) new_return) }

    zonkStmt env zBody (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_later_rets = later_rets, recS_rec_rets = rec_rets
                                , recS_ret_ty = ret_ty })
      = do { new_rvs <- zonkIdBndrs env rvs
           ; new_lvs <- zonkIdBndrs env lvs
           ; new_ret_ty  <- zonkTcTypeToType env ret_ty
           ; new_ret_id  <- zonkExpr env ret_id
           ; new_mfix_id <- zonkExpr env mfix_id
           ; new_bind_id <- zonkExpr env bind_id
           ; let env1 = extendIdZonkEnv env new_rvs
           ; (env2, new_segStmts) <- zonkStmts env1 zBody segStmts
            -- Zonk the ret-expressions in an envt that
            -- has the polymorphic bindings in the envt
           ; new_later_rets <- mapM (zonkExpr env2) later_rets
           ; new_rec_rets <- mapM (zonkExpr env2) rec_rets
           ; return (extendIdZonkEnv env new_lvs,     -- Only the lvs are needed
                     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_later_rets = new_later_rets
                             , recS_rec_rets = new_rec_rets, recS_ret_ty = new_ret_ty }) }

    zonkStmt env zBody (BodyStmt body then_op guard_op ty)
      = do new_body <- zBody env body
           new_then <- zonkExpr env then_op
           new_guard <- zonkExpr env guard_op
           new_ty <- zonkTcTypeToType env ty
           return (env, BodyStmt new_body new_then new_guard new_ty)

    zonkStmt env zBody (LastStmt body ret_op)
      = do new_body <- zBody env body
           new_ret <- zonkExpr env ret_op
           return (env, LastStmt new_body new_ret)

    zonkStmt env _ (TransStmt { trS_stmts = stmts, trS_bndrs = binderMap
                                  , trS_by = by, trS_form = form, trS_using = using
                                  , trS_ret = return_op, trS_bind = bind_op, trS_fmap = liftM_op })
      = do { (env', stmts') <- zonkStmts env zonkLExpr stmts
        ; binderMap' <- mapM (zonkBinderMapEntry env') binderMap
        ; by'        <- fmapMaybeM (zonkLExpr env') by
        ; using'     <- zonkLExpr env using
        ; return_op' <- zonkExpr env' return_op
        ; bind_op'   <- zonkExpr env' bind_op
        ; liftM_op'  <- zonkExpr env' liftM_op
        ; let env'' = extendIdZonkEnv env' (map snd binderMap')
        ; return (env'', TransStmt { trS_stmts = stmts', trS_bndrs = binderMap'
                                   , trS_by = by', trS_form = form, trS_using = using'
                                   , trS_ret = return_op', trS_bind = bind_op', trS_fmap = liftM_op' }) }
      where
        zonkBinderMapEntry env (oldBinder, newBinder) = do
            let oldBinder' = zonkIdOcc env oldBinder
            newBinder' <- zonkIdBndr env newBinder
            return (oldBinder', newBinder')

    zonkStmt env _ (LetStmt binds)
      = do (env1, new_binds) <- zonkLocalBinds env binds
           return (env1, LetStmt new_binds)

    zonkStmt env zBody (BindStmt pat body bind_op fail_op)
      = do  { new_body <- zBody env body
            ; (env1, new_pat) <- zonkPat env pat
            ; new_bind <- zonkExpr env bind_op
            ; new_fail <- zonkExpr env fail_op
            ; return (env1, BindStmt new_pat new_body new_bind new_fail) }

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

    -------------------------------------------------------------------------
    mapIPNameTc :: (a -> TcM b) -> Either (Located HsIPName) a
                -> TcM (Either (Located HsIPName) b)
    mapIPNameTc _ (Left x)  = return (Left x)
    mapIPNameTc f (Right x) = do r <- f x
                                 return (Right r)

    {-
    ************************************************************************
    *                                                                      *
    \subsection[BackSubst-Pats]{Patterns}
    *                                                                      *
    ************************************************************************
    -}

    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 (extendIdZonkEnv1 env v', VarPat v') }

    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 (extendIdZonkEnv1 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 Nothing)
      = do  { ty' <- zonkTcTypeToType env ty
            ; (env', pats') <- zonkPats env pats
            ; return (env', ListPat pats' ty' Nothing) }

    zonk_pat env (ListPat pats ty (Just (ty2,wit)))
      = do  { wit' <- zonkExpr env wit
            ; ty2' <- zonkTcTypeToType env ty2
            ; ty' <- zonkTcTypeToType env ty
            ; (env', pats') <- zonkPats env pats
            ; return (env', ListPat pats' ty' (Just (ty2',wit'))) }

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

    zonk_pat env p@(ConPatOut { pat_arg_tys = tys, pat_tvs = tyvars
                              , pat_dicts = evs, pat_binds = binds
                              , pat_args = args, pat_wrap = wrapper })
      = ASSERT( all isImmutableTyVar tyvars )
        do  { new_tys <- mapM (zonkTcTypeToType env) tys
            ; (env0, new_tyvars) <- zonkTyBndrsX env tyvars
              -- Must zonk the existential variables, because their
              -- /kind/ need potential zonking.
              -- cf typecheck/should_compile/tc221.hs
            ; (env1, new_evs) <- zonkEvBndrsX env0 evs
            ; (env2, new_binds) <- zonkTcEvBinds env1 binds
            ; (env3, new_wrapper) <- zonkCoFn env2 wrapper
            ; (env', new_args) <- zonkConStuff env3 args
            ; return (env', p { pat_arg_tys = new_tys,
                                pat_tvs = new_tyvars,
                                pat_dicts = new_evs,
                                pat_binds = new_binds,
                                pat_args = new_args,
                                pat_wrap = new_wrapper}) }

    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 (L l 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 (L l lit') mb_neg' eq_expr') }

    zonk_pat env (NPlusKPat (L loc n) (L l lit) e1 e2)
      = do  { n' <- zonkIdBndr env n
            ; lit' <- zonkOverLit env lit
            ; e1' <- zonkExpr env e1
            ; e2' <- zonkExpr env e2
            ; return (extendIdZonkEnv1 env n',
                      NPlusKPat (L loc n') (L l 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 . unLoc) rpats)
            ; let rpats' = zipWith (\(L l rp) p' -> L l (rp { hsRecFieldArg = p' }))
                                   rpats pats'
            ; return (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') }

    {-
    ************************************************************************
    *                                                                      *
    \subsection[BackSubst-Foreign]{Foreign exports}
    *                                                                      *
    ************************************************************************
    -}

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

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

    zonkRules :: ZonkEnv -> [LRuleDecl TcId] -> TcM [LRuleDecl Id]
    zonkRules env rs = mapM (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 { unbound_tkv_set <- newMutVar emptyVarSet
           ; let env_rule = setZonkType env (zonkTvCollecting unbound_tkv_set)
                  -- See Note [Zonking the LHS of a RULE]

           ; (env_inside, new_bndrs) <- mapAccumLM zonk_bndr env_rule vars

           ; new_lhs <- zonkLExpr env_inside lhs
           ; new_rhs <- zonkLExpr env_inside rhs

           ; unbound_tkvs <- readMutVar unbound_tkv_set

           ; let final_bndrs :: [LRuleBndr Var]
                 final_bndrs = map (noLoc . RuleBndr . noLoc)
                                   (varSetElemsKvsFirst unbound_tkvs)
                               ++ new_bndrs

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

       zonk_it env v
         | isId v     = do { v' <- zonkIdBndr env v
                           ; return (extendIdZonkEnv1 env v', v') }
         | otherwise  = ASSERT( isImmutableTyVar v)
                        zonkTyBndrX env v
                        -- DV: used to be return (env,v) but that is plain
                        -- wrong because we may need to go inside the kind
                        -- of v and zonk there!

    zonkVects :: ZonkEnv -> [LVectDecl TcId] -> TcM [LVectDecl Id]
    zonkVects env = mapM (wrapLocM (zonkVect env))

    zonkVect :: ZonkEnv -> VectDecl TcId -> TcM (VectDecl Id)
    zonkVect env (HsVect s v e)
      = do { v' <- wrapLocM (zonkIdBndr env) v
           ; e' <- zonkLExpr env e
           ; return $ HsVect s v' e'
           }
    zonkVect env (HsNoVect s v)
      = do { v' <- wrapLocM (zonkIdBndr env) v
           ; return $ HsNoVect s v'
           }
    zonkVect _env (HsVectTypeOut s t rt)
      = return $ HsVectTypeOut s t rt
    zonkVect _ (HsVectTypeIn _ _ _ _) = panic "TcHsSyn.zonkVect: HsVectTypeIn"
    zonkVect _env (HsVectClassOut c)
      = return $ HsVectClassOut c
    zonkVect _ (HsVectClassIn _ _) = panic "TcHsSyn.zonkVect: HsVectClassIn"
    zonkVect _env (HsVectInstOut i)
      = return $ HsVectInstOut i
    zonkVect _ (HsVectInstIn _) = panic "TcHsSyn.zonkVect: HsVectInstIn"

    {-
    ************************************************************************
    *                                                                      *
                  Constraints and evidence
    *                                                                      *
    ************************************************************************
    -}

    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' <- zonkTcCoToCo env co
                                           ; return (EvCoercion co') }
    zonkEvTerm env (EvCast tm co)     = do { tm' <- zonkEvTerm env tm
                                           ; co' <- zonkTcCoToCo env co
                                           ; return (mkEvCast tm' co') }
    zonkEvTerm env (EvTupleSel tm n)  = do { tm' <- zonkEvTerm env tm
                                           ; return (EvTupleSel tm' n) }
    zonkEvTerm env (EvTupleMk tms)    = do { tms' <- mapM (zonkEvTerm env) tms
                                           ; return (EvTupleMk tms') }
    zonkEvTerm _   (EvLit l)          = return (EvLit l)

    zonkEvTerm env (EvTypeable ev) =
      fmap EvTypeable $
      case ev of
        EvTypeableTyCon tc ks    -> return (EvTypeableTyCon tc ks)
        EvTypeableTyApp t1 t2    -> do e1 <- zonk t1
                                       e2 <- zonk t2
                                       return (EvTypeableTyApp e1 e2)
        EvTypeableTyLit t        -> EvTypeableTyLit `fmap` zonkTcTypeToType env t
      where
      zonk (ev,t) = do ev' <- zonkEvTerm env ev
                       t'  <- zonkTcTypeToType env t
                       return (ev',t')

    zonkEvTerm env (EvCallStack cs)
      = case cs of
          EvCsEmpty -> return (EvCallStack cs)
          EvCsTop n l tm -> do { tm' <- zonkEvTerm env tm
                               ; return (EvCallStack (EvCsTop n l tm')) }
          EvCsPushCall n l tm -> do { tm' <- zonkEvTerm env tm
                                    ; return (EvCallStack (EvCsPushCall n l tm')) }

    zonkEvTerm env (EvSuperClass d n) = do { d' <- zonkEvTerm env d
                                           ; return (EvSuperClass d' n) }
    zonkEvTerm env (EvDFunApp df tys tms)
      = do { tys' <- zonkTcTypeToTypes env tys
           ; tms' <- mapM (zonkEvTerm env) tms
           ; return (EvDFunApp (zonkIdOcc env df) tys' tms') }
    zonkEvTerm env (EvDelayedError ty msg)
      = do { ty' <- zonkTcTypeToType env ty
           ; return (EvDelayedError ty' msg) }

    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
      = {-# SCC "zonkEvBinds" #-}
        fixM (\ ~( _, new_binds) -> do
             { let env1 = extendIdZonkEnv 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'  <- {-# SCC "zonkEvBndr" #-} zonkEvBndr env var

             -- Optimise the common case of Refl coercions
             -- See Note [Optimise coercion zonking]
             -- This has a very big effect on some programs (eg Trac #5030)
           ; let ty' = idType var'

           ; case getEqPredTys_maybe ty' of
               Just (r, ty1, ty2) | ty1 `eqType` ty2
                      -> return (EvBind var' (EvCoercion (mkTcReflCo r ty1)))
               _other -> do { term' <- zonkEvTerm env term
                            ; return (EvBind var' term') } }

    {-
    ************************************************************************
    *                                                                      *
                             Zonking types
    *                                                                      *
    ************************************************************************

    Note [Zonking the LHS of a RULE]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    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 get zonkTvCollecting to gather 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)

    And that in turn is why ZonkEnv carries the function to use for
    type variables!

    Note [Zonking mutable unbound type or kind variables]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In zonkTypeZapping, we zonk mutable but unbound type or kind variables to an
    arbitrary type. We know if they are unbound even though we don't carry an
    environment, because at the binding site for a variable we bind the mutable
    var to a fresh immutable one.  So the mutable store plays the role of an
    environment.  If we come across a mutable variable that isn't so bound, it
    must be completely free. We zonk the expected kind to make sure we don't get
    some unbound meta variable as the kind.

    Note that since we have kind polymorphism, zonk_unbound_tyvar will handle both
    type and kind variables. Consider the following datatype:

      data Phantom a = Phantom Int

    The type of Phantom is (forall (k : BOX). forall (a : k). Int). Both `a` and
    `k` are unbound variables. We want to zonk this to
    (forall (k : AnyK). forall (a : Any AnyK). Int). For that we have to check if
    we have a type or a kind variable; for kind variables we just return AnyK (and
    not the ill-kinded Any BOX).

    Note [Optimise coercion zonkind]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When optimising evidence binds we may come across situations where
    a coercion looks like
          cv = ReflCo ty
    or    cv1 = cv2
    where the type 'ty' is big.  In such cases it is a waste of time to zonk both
      * The variable on the LHS
      * The coercion on the RHS
    Rather, we can zonk the variable, and if its type is (ty ~ ty), we can just
    use Refl on the right, ignoring the actual coercion on the RHS.

    This can have a very big effect, because the constraint solver sometimes does go
    to a lot of effort to prove Refl!  (Eg when solving  10+3 = 10+3; cf Trac #5030)
    -}

    zonkTyVarOcc :: ZonkEnv -> TyVar -> TcM TcType
    zonkTyVarOcc env@(ZonkEnv zonk_unbound_tyvar tv_env _) tv
      | isTcTyVar tv
      = case tcTyVarDetails tv of
             SkolemTv {}    -> lookup_in_env
             RuntimeUnk {}  -> lookup_in_env
             FlatSkol ty    -> zonkTcTypeToType env ty
             MetaTv { mtv_ref = ref }
               -> do { cts <- readMutVar ref
                     ; case cts of
                          Flexi -> do { kind <- {-# SCC "zonkKind1" #-}
                                                zonkTcTypeToType env (tyVarKind tv)
                                      ; zonk_unbound_tyvar (setTyVarKind tv kind) }
                          Indirect ty -> do { zty <- zonkTcTypeToType env ty
                                            -- Small optimisation: shortern-out indirect steps
                                            -- so that the old type may be more easily collected.
                                            ; writeMutVar ref (Indirect zty)
                                            ; return zty } }
      | otherwise
      = lookup_in_env
      where
        lookup_in_env    -- Look up in the env just as we do for Ids
          = case lookupVarEnv tv_env tv of
              Nothing  -> return (mkTyVarTy tv)
              Just tv' -> return (mkTyVarTy tv')

    zonkTcTypeToType :: ZonkEnv -> TcType -> TcM Type
    zonkTcTypeToType env ty
      = go ty
      where
        go (TyConApp tc tys) = do tys' <- mapM go tys
                                  return (mkTyConApp tc tys')
                    -- Establish Type invariants
                    -- 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.

            -- The two interesting cases!
        go (TyVarTy tv) = zonkTyVarOcc env tv

        go (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
                              do { (env', tv') <- zonkTyBndrX env tv
                                 ; ty' <- zonkTcTypeToType env' ty
                                 ; return (ForAllTy tv' ty') }

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

    zonkCoToCo :: ZonkEnv -> Coercion -> TcM Coercion
    zonkCoToCo env co
      = go co
      where
        go (Refl r ty)               = mkReflCo r <$> zonkTcTypeToType env ty
        go (TyConAppCo r tc args)    = mkTyConAppCo r tc <$> mapM go args
        go (AppCo co arg)            = mkAppCo <$> go co <*> go arg
        go (AxiomInstCo ax ind args) = AxiomInstCo ax ind <$> mapM go args
        go (UnivCo s r ty1 ty2)      = mkUnivCo s r <$> zonkTcTypeToType env ty1
                                                    <*> zonkTcTypeToType env ty2
        go (SymCo co)                = mkSymCo <$> go co
        go (TransCo co1 co2)         = mkTransCo <$> go co1 <*> go co2
        go (NthCo n co)              = mkNthCo n <$> go co
        go (LRCo lr co)              = mkLRCo lr <$> go co
        go (InstCo co arg)           = mkInstCo <$> go co <*> zonkTcTypeToType env arg
        go (SubCo co)                = mkSubCo <$> go co
        go (AxiomRuleCo ax ts cs)    = AxiomRuleCo ax <$> mapM (zonkTcTypeToType env) ts
                                                      <*> mapM go cs

        -- The two interesting cases!
        go (CoVarCo cv)              = return (mkCoVarCo $ zonkIdOcc env cv)
        go (ForAllCo tv co)          = ASSERT( isImmutableTyVar tv )
                                       do { (env', tv') <- zonkTyBndrX env tv
                                          ; co' <- zonkCoToCo env' co
                                          ; return (mkForAllCo tv' co') }

    zonkTvCollecting :: TcRef TyVarSet -> UnboundTyVarZonker
    -- This variant collects unbound type variables in a mutable variable
    -- Works on both types and kinds
    zonkTvCollecting unbound_tv_set tv
      = do { poly_kinds <- xoptM Opt_PolyKinds
           ; if isKindVar tv && not poly_kinds then defaultKindVarToStar tv
             else do
           { tv' <- zonkQuantifiedTyVar tv
           ; tv_set <- readMutVar unbound_tv_set
           ; writeMutVar unbound_tv_set (extendVarSet tv_set tv')
           ; return (mkTyVarTy tv') } }

    zonkTypeZapping :: UnboundTyVarZonker
    -- This variant is used for everything except the LHS of rules
    -- It zaps unbound type variables to (), or some other arbitrary type
    -- Works on both types and kinds
    zonkTypeZapping tv
      = do { let ty = if isKindVar tv
                      -- ty is actually a kind, zonk to AnyK
                      then anyKind
                      else anyTypeOfKind (defaultKind (tyVarKind tv))
           ; writeMetaTyVar tv ty
           ; return ty }

    zonkTcCoToCo :: ZonkEnv -> TcCoercion -> TcM TcCoercion
    -- NB: zonking often reveals that the coercion is an identity
    --     in which case the Refl-ness can propagate up to the top
    --     which in turn gives more efficient desugaring.  So it's
    --     worth using the 'mk' smart constructors on the RHS
    zonkTcCoToCo env co
      = go co
      where
        go (TcLetCo bs co)        = do { (env', bs') <- zonkTcEvBinds env bs
                                       ; co' <- zonkTcCoToCo env' co
                                       ; return (TcLetCo bs' co') }
        go (TcCoVarCo cv)         = return (mkTcCoVarCo (zonkEvVarOcc env cv))
        go (TcRefl r ty)          = do { ty' <- zonkTcTypeToType env ty
                                       ; return (TcRefl r ty') }
        go (TcTyConAppCo r tc cos)
                                  = do { cos' <- mapM go cos; return (mkTcTyConAppCo r tc cos') }
        go (TcAxiomInstCo ax ind cos)
                                  = do { cos' <- mapM go cos; return (TcAxiomInstCo ax ind cos') }
        go (TcAppCo co1 co2)      = do { co1' <- go co1; co2' <- go co2
                                       ; return (mkTcAppCo co1' co2') }
        go (TcCastCo co1 co2)     = do { co1' <- go co1; co2' <- go co2
                                       ; return (TcCastCo co1' co2') }
        go (TcPhantomCo ty1 ty2)  = do { ty1' <- zonkTcTypeToType env ty1
                                       ; ty2' <- zonkTcTypeToType env ty2
                                       ; return (TcPhantomCo ty1' ty2') }
        go (TcSymCo co)           = do { co' <- go co; return (mkTcSymCo co') }
        go (TcNthCo n co)         = do { co' <- go co; return (mkTcNthCo n co') }
        go (TcLRCo lr co)         = do { co' <- go co; return (mkTcLRCo lr co') }
        go (TcTransCo co1 co2)    = do { co1' <- go co1; co2' <- go co2
                                       ; return (mkTcTransCo co1' co2') }
        go (TcForAllCo tv co)     = ASSERT( isImmutableTyVar tv )
                                    do { co' <- go co; return (mkTcForAllCo tv co') }
        go (TcSubCo co)           = do { co' <- go co; return (mkTcSubCo co') }
        go (TcAxiomRuleCo co ts cs) = do { ts' <- zonkTcTypeToTypes env ts
                                         ; cs' <- mapM go cs
                                         ; return (TcAxiomRuleCo co ts' cs')
                                         }
        go (TcCoercion co)        = do { co' <- zonkCoToCo env co
                                       ; return (TcCoercion co') }