2 % (c) The GRASP/AQUA Project, Glasgow University, 1998
4 \section[Type]{Type - public interface}
8 -- re-exports from TypeRep
9 TyThing(..), Type, PredType(..), ThetaType,
12 -- Re-exports from Kind
15 -- Re-exports from TyCon
18 mkTyVarTy, mkTyVarTys, getTyVar, getTyVar_maybe, isTyVarTy,
20 mkAppTy, mkAppTys, splitAppTy, splitAppTys, splitAppTy_maybe,
22 mkFunTy, mkFunTys, splitFunTy, splitFunTy_maybe,
23 splitFunTys, splitFunTysN,
24 funResultTy, funArgTy, zipFunTys, isFunTy,
26 mkGenTyConApp, mkTyConApp, mkTyConTy,
27 tyConAppTyCon, tyConAppArgs,
28 splitTyConApp_maybe, splitTyConApp,
32 repType, typePrimRep, coreView, tcView,
34 mkForAllTy, mkForAllTys, splitForAllTy_maybe, splitForAllTys,
35 applyTy, applyTys, isForAllTy, dropForAlls,
38 predTypeRep, mkPredTy, mkPredTys,
41 splitRecNewType_maybe,
44 isUnLiftedType, isUnboxedTupleType, isAlgType, isPrimitiveType,
45 isStrictType, isStrictPred,
48 tyVarsOfType, tyVarsOfTypes, tyVarsOfPred, tyVarsOfTheta,
49 typeKind, addFreeTyVars,
51 -- Tidying up for printing
53 tidyOpenType, tidyOpenTypes,
54 tidyTyVarBndr, tidyFreeTyVars,
55 tidyOpenTyVar, tidyOpenTyVars,
56 tidyTopType, tidyPred,
60 coreEqType, tcEqType, tcEqTypes, tcCmpType, tcCmpTypes,
61 tcEqPred, tcCmpPred, tcEqTypeX,
67 TvSubstEnv, emptyTvSubstEnv, -- Representation widely visible
68 TvSubst(..), emptyTvSubst, -- Representation visible to a few friends
69 mkTvSubst, mkOpenTvSubst, zipOpenTvSubst, zipTopTvSubst, mkTopTvSubst, notElemTvSubst,
70 getTvSubstEnv, setTvSubstEnv, getTvInScope, extendTvInScope,
71 extendTvSubst, extendTvSubstList, isInScope, composeTvSubst,
73 -- Performing substitution on types
74 substTy, substTys, substTyWith, substTheta,
75 substPred, substTyVar, substTyVarBndr, deShadowTy,
78 pprType, pprParendType, pprTyThingCategory,
79 pprPred, pprTheta, pprThetaArrow, pprClassPred
82 #include "HsVersions.h"
84 -- We import the representation and primitive functions from TypeRep.
85 -- Many things are reexported, but not the representation!
91 import Var ( Var, TyVar, tyVarKind, tyVarName, setTyVarName, mkTyVar )
95 import Name ( NamedThing(..), mkInternalName, tidyOccName )
96 import Class ( Class, classTyCon )
97 import TyCon ( TyCon, isRecursiveTyCon, isPrimTyCon,
98 isUnboxedTupleTyCon, isUnLiftedTyCon,
99 isFunTyCon, isNewTyCon, newTyConRep, newTyConRhs,
100 isAlgTyCon, tyConArity,
101 tcExpandTyCon_maybe, coreExpandTyCon_maybe,
102 tyConKind, PrimRep(..), tyConPrimRep,
106 import StaticFlags ( opt_DictsStrict )
107 import SrcLoc ( noSrcLoc )
108 import Unique ( Uniquable(..) )
109 import Util ( mapAccumL, seqList, lengthIs, snocView, thenCmp, isEqual, all2 )
111 import UniqSet ( sizeUniqSet ) -- Should come via VarSet
112 import Maybe ( isJust )
116 %************************************************************************
120 %************************************************************************
122 In Core, we "look through" non-recursive newtypes and PredTypes.
125 {-# INLINE coreView #-}
126 coreView :: Type -> Maybe Type
127 -- Srips off the *top layer only* of a type to give
128 -- its underlying representation type.
129 -- Returns Nothing if there is nothing to look through.
131 -- In the case of newtypes, it returns
132 -- *either* a vanilla TyConApp (recursive newtype, or non-saturated)
133 -- *or* the newtype representation (otherwise), meaning the
134 -- type written in the RHS of the newtype decl,
135 -- which may itself be a newtype
137 -- Example: newtype R = MkR S
139 -- newtype T = MkT (T -> T)
140 -- expandNewTcApp on R gives Just S
142 -- on T gives Nothing (no expansion)
144 -- By being non-recursive and inlined, this case analysis gets efficiently
145 -- joined onto the case analysis that the caller is already doing
146 coreView (NoteTy _ ty) = Just ty
147 coreView (PredTy p) = Just (predTypeRep p)
148 coreView (TyConApp tc tys) | Just (tenv, rhs, tys') <- coreExpandTyCon_maybe tc tys
149 = Just (mkAppTys (substTy (mkTopTvSubst tenv) rhs) tys')
150 -- Its important to use mkAppTys, rather than (foldl AppTy),
151 -- because the function part might well return a
152 -- partially-applied type constructor; indeed, usually will!
153 coreView ty = Nothing
155 -----------------------------------------------
156 {-# INLINE tcView #-}
157 tcView :: Type -> Maybe Type
158 -- Same, but for the type checker, which just looks through synonyms
159 tcView (NoteTy _ ty) = Just ty
160 tcView (TyConApp tc tys) | Just (tenv, rhs, tys') <- tcExpandTyCon_maybe tc tys
161 = Just (mkAppTys (substTy (mkTopTvSubst tenv) rhs) tys')
166 %************************************************************************
168 \subsection{Constructor-specific functions}
170 %************************************************************************
173 ---------------------------------------------------------------------
177 mkTyVarTy :: TyVar -> Type
180 mkTyVarTys :: [TyVar] -> [Type]
181 mkTyVarTys = map mkTyVarTy -- a common use of mkTyVarTy
183 getTyVar :: String -> Type -> TyVar
184 getTyVar msg ty = case getTyVar_maybe ty of
186 Nothing -> panic ("getTyVar: " ++ msg)
188 isTyVarTy :: Type -> Bool
189 isTyVarTy ty = isJust (getTyVar_maybe ty)
191 getTyVar_maybe :: Type -> Maybe TyVar
192 getTyVar_maybe ty | Just ty' <- coreView ty = getTyVar_maybe ty'
193 getTyVar_maybe (TyVarTy tv) = Just tv
194 getTyVar_maybe other = Nothing
198 ---------------------------------------------------------------------
201 We need to be pretty careful with AppTy to make sure we obey the
202 invariant that a TyConApp is always visibly so. mkAppTy maintains the
206 mkAppTy orig_ty1 orig_ty2
209 mk_app (NoteTy _ ty1) = mk_app ty1
210 mk_app (TyConApp tc tys) = mkGenTyConApp tc (tys ++ [orig_ty2])
211 mk_app ty1 = AppTy orig_ty1 orig_ty2
212 -- We call mkGenTyConApp because the TyConApp could be an
213 -- under-saturated type synonym. GHC allows that; e.g.
214 -- type Foo k = k a -> k a
216 -- foo :: Foo Id -> Foo Id
218 -- Here Id is partially applied in the type sig for Foo,
219 -- but once the type synonyms are expanded all is well
221 mkAppTys :: Type -> [Type] -> Type
222 mkAppTys orig_ty1 [] = orig_ty1
223 -- This check for an empty list of type arguments
224 -- avoids the needless loss of a type synonym constructor.
225 -- For example: mkAppTys Rational []
226 -- returns to (Ratio Integer), which has needlessly lost
227 -- the Rational part.
228 mkAppTys orig_ty1 orig_tys2
231 mk_app (NoteTy _ ty1) = mk_app ty1
232 mk_app (TyConApp tc tys) = mkGenTyConApp tc (tys ++ orig_tys2)
233 -- mkGenTyConApp: see notes with mkAppTy
234 mk_app ty1 = foldl AppTy orig_ty1 orig_tys2
236 splitAppTy_maybe :: Type -> Maybe (Type, Type)
237 splitAppTy_maybe ty | Just ty' <- coreView ty = splitAppTy_maybe ty'
238 splitAppTy_maybe (FunTy ty1 ty2) = Just (TyConApp funTyCon [ty1], ty2)
239 splitAppTy_maybe (AppTy ty1 ty2) = Just (ty1, ty2)
240 splitAppTy_maybe (TyConApp tc tys) = case snocView tys of
242 Just (tys',ty') -> Just (TyConApp tc tys', ty')
243 splitAppTy_maybe other = Nothing
245 splitAppTy :: Type -> (Type, Type)
246 splitAppTy ty = case splitAppTy_maybe ty of
248 Nothing -> panic "splitAppTy"
250 splitAppTys :: Type -> (Type, [Type])
251 splitAppTys ty = split ty ty []
253 split orig_ty ty args | Just ty' <- coreView ty = split orig_ty ty' args
254 split orig_ty (AppTy ty arg) args = split ty ty (arg:args)
255 split orig_ty (TyConApp tc tc_args) args = (TyConApp tc [], tc_args ++ args)
256 split orig_ty (FunTy ty1 ty2) args = ASSERT( null args )
257 (TyConApp funTyCon [], [ty1,ty2])
258 split orig_ty ty args = (orig_ty, args)
262 ---------------------------------------------------------------------
267 mkFunTy :: Type -> Type -> Type
268 mkFunTy arg res = FunTy arg res
270 mkFunTys :: [Type] -> Type -> Type
271 mkFunTys tys ty = foldr FunTy ty tys
273 isFunTy :: Type -> Bool
274 isFunTy ty = isJust (splitFunTy_maybe ty)
276 splitFunTy :: Type -> (Type, Type)
277 splitFunTy ty | Just ty' <- coreView ty = splitFunTy ty'
278 splitFunTy (FunTy arg res) = (arg, res)
279 splitFunTy other = pprPanic "splitFunTy" (ppr other)
281 splitFunTy_maybe :: Type -> Maybe (Type, Type)
282 splitFunTy_maybe ty | Just ty' <- coreView ty = splitFunTy_maybe ty'
283 splitFunTy_maybe (FunTy arg res) = Just (arg, res)
284 splitFunTy_maybe other = Nothing
286 splitFunTys :: Type -> ([Type], Type)
287 splitFunTys ty = split [] ty ty
289 split args orig_ty ty | Just ty' <- coreView ty = split args orig_ty ty'
290 split args orig_ty (FunTy arg res) = split (arg:args) res res
291 split args orig_ty ty = (reverse args, orig_ty)
293 splitFunTysN :: Int -> Type -> ([Type], Type)
294 -- Split off exactly n arg tys
295 splitFunTysN 0 ty = ([], ty)
296 splitFunTysN n ty = case splitFunTy ty of { (arg, res) ->
297 case splitFunTysN (n-1) res of { (args, res) ->
300 zipFunTys :: Outputable a => [a] -> Type -> ([(a,Type)], Type)
301 zipFunTys orig_xs orig_ty = split [] orig_xs orig_ty orig_ty
303 split acc [] nty ty = (reverse acc, nty)
305 | Just ty' <- coreView ty = split acc xs nty ty'
306 split acc (x:xs) nty (FunTy arg res) = split ((x,arg):acc) xs res res
307 split acc (x:xs) nty ty = pprPanic "zipFunTys" (ppr orig_xs <+> ppr orig_ty)
309 funResultTy :: Type -> Type
310 funResultTy ty | Just ty' <- coreView ty = funResultTy ty'
311 funResultTy (FunTy arg res) = res
312 funResultTy ty = pprPanic "funResultTy" (ppr ty)
314 funArgTy :: Type -> Type
315 funArgTy ty | Just ty' <- coreView ty = funArgTy ty'
316 funArgTy (FunTy arg res) = arg
317 funArgTy ty = pprPanic "funArgTy" (ppr ty)
321 ---------------------------------------------------------------------
324 @mkTyConApp@ is a key function, because it builds a TyConApp, FunTy or PredTy,
328 mkGenTyConApp :: TyCon -> [Type] -> Type
332 mkTyConApp :: TyCon -> [Type] -> Type
334 | isFunTyCon tycon, [ty1,ty2] <- tys
340 mkTyConTy :: TyCon -> Type
341 mkTyConTy tycon = mkTyConApp tycon []
343 -- splitTyConApp "looks through" synonyms, because they don't
344 -- mean a distinct type, but all other type-constructor applications
345 -- including functions are returned as Just ..
347 tyConAppTyCon :: Type -> TyCon
348 tyConAppTyCon ty = fst (splitTyConApp ty)
350 tyConAppArgs :: Type -> [Type]
351 tyConAppArgs ty = snd (splitTyConApp ty)
353 splitTyConApp :: Type -> (TyCon, [Type])
354 splitTyConApp ty = case splitTyConApp_maybe ty of
356 Nothing -> pprPanic "splitTyConApp" (ppr ty)
358 splitTyConApp_maybe :: Type -> Maybe (TyCon, [Type])
359 splitTyConApp_maybe ty | Just ty' <- coreView ty = splitTyConApp_maybe ty'
360 splitTyConApp_maybe (TyConApp tc tys) = Just (tc, tys)
361 splitTyConApp_maybe (FunTy arg res) = Just (funTyCon, [arg,res])
362 splitTyConApp_maybe other = Nothing
366 ---------------------------------------------------------------------
371 mkSynTy tycon tys = panic "No longer used"
372 {- Delete in due course
373 | n_args == arity -- Exactly saturated
375 | n_args > arity -- Over-saturated
376 = case splitAt arity tys of { (as,bs) -> mkAppTys (mk_syn as) bs }
377 -- Its important to use mkAppTys, rather than (foldl AppTy),
378 -- because (mk_syn as) might well return a partially-applied
379 -- type constructor; indeed, usually will!
380 | otherwise -- Un-saturated
382 -- For the un-saturated case we build TyConApp directly
383 -- (mkTyConApp ASSERTs that the tc isn't a SynTyCon).
384 -- Here we are relying on checkValidType to find
385 -- the error. What we can't do is use mkSynTy with
386 -- too few arg tys, because that is utterly bogus.
389 mk_syn tys = NoteTy (SynNote (TyConApp tycon tys))
390 (substTyWith tyvars tys body)
392 (tyvars, body) = ASSERT( isSynTyCon tycon ) getSynTyConDefn tycon
393 arity = tyConArity tycon
398 Notes on type synonyms
399 ~~~~~~~~~~~~~~~~~~~~~~
400 The various "split" functions (splitFunTy, splitRhoTy, splitForAllTy) try
401 to return type synonyms whereever possible. Thus
406 splitFunTys (a -> Foo a) = ([a], Foo a)
409 The reason is that we then get better (shorter) type signatures in
410 interfaces. Notably this plays a role in tcTySigs in TcBinds.lhs.
415 repType looks through
419 (d) usage annotations
420 (e) all newtypes, including recursive ones
421 It's useful in the back end.
424 repType :: Type -> Type
425 -- Only applied to types of kind *; hence tycons are saturated
426 repType (ForAllTy _ ty) = repType ty
427 repType (NoteTy _ ty) = repType ty
428 repType ty | Just ty' <- coreView ty = repType ty'
431 -- ToDo: this could be moved to the code generator, using splitTyConApp instead
432 -- of inspecting the type directly.
433 typePrimRep :: Type -> PrimRep
434 typePrimRep ty = case repType ty of
435 TyConApp tc _ -> tyConPrimRep tc
437 AppTy _ _ -> PtrRep -- See note below
439 other -> pprPanic "typePrimRep" (ppr ty)
440 -- Types of the form 'f a' must be of kind *, not *#, so
441 -- we are guaranteed that they are represented by pointers.
442 -- The reason is that f must have kind *->*, not *->*#, because
443 -- (we claim) there is no way to constrain f's kind any other
446 -- new_type_rep doesn't ask any questions:
447 -- it just expands newtype, whether recursive or not
448 new_type_rep new_tycon tys = ASSERT( tys `lengthIs` tyConArity new_tycon )
449 case newTyConRep new_tycon of
450 (tvs, rep_ty) -> substTyWith tvs tys rep_ty
454 ---------------------------------------------------------------------
459 mkForAllTy :: TyVar -> Type -> Type
461 = mkForAllTys [tyvar] ty
463 mkForAllTys :: [TyVar] -> Type -> Type
464 mkForAllTys tyvars ty = foldr ForAllTy ty tyvars
466 isForAllTy :: Type -> Bool
467 isForAllTy (NoteTy _ ty) = isForAllTy ty
468 isForAllTy (ForAllTy _ _) = True
469 isForAllTy other_ty = False
471 splitForAllTy_maybe :: Type -> Maybe (TyVar, Type)
472 splitForAllTy_maybe ty = splitFAT_m ty
474 splitFAT_m ty | Just ty' <- coreView ty = splitFAT_m ty'
475 splitFAT_m (ForAllTy tyvar ty) = Just(tyvar, ty)
476 splitFAT_m _ = Nothing
478 splitForAllTys :: Type -> ([TyVar], Type)
479 splitForAllTys ty = split ty ty []
481 split orig_ty ty tvs | Just ty' <- coreView ty = split orig_ty ty' tvs
482 split orig_ty (ForAllTy tv ty) tvs = split ty ty (tv:tvs)
483 split orig_ty t tvs = (reverse tvs, orig_ty)
485 dropForAlls :: Type -> Type
486 dropForAlls ty = snd (splitForAllTys ty)
489 -- (mkPiType now in CoreUtils)
493 Instantiate a for-all type with one or more type arguments.
494 Used when we have a polymorphic function applied to type args:
496 Then we use (applyTys type-of-f [t1,t2]) to compute the type of
500 applyTy :: Type -> Type -> Type
501 applyTy ty arg | Just ty' <- coreView ty = applyTy ty' arg
502 applyTy (ForAllTy tv ty) arg = substTyWith [tv] [arg] ty
503 applyTy other arg = panic "applyTy"
505 applyTys :: Type -> [Type] -> Type
506 -- This function is interesting because
507 -- a) the function may have more for-alls than there are args
508 -- b) less obviously, it may have fewer for-alls
509 -- For case (b) think of
510 -- applyTys (forall a.a) [forall b.b, Int]
511 -- This really can happen, via dressing up polymorphic types with newtype
512 -- clothing. Here's an example:
513 -- newtype R = R (forall a. a->a)
514 -- foo = case undefined :: R of
517 applyTys orig_fun_ty [] = orig_fun_ty
518 applyTys orig_fun_ty arg_tys
519 | n_tvs == n_args -- The vastly common case
520 = substTyWith tvs arg_tys rho_ty
521 | n_tvs > n_args -- Too many for-alls
522 = substTyWith (take n_args tvs) arg_tys
523 (mkForAllTys (drop n_args tvs) rho_ty)
524 | otherwise -- Too many type args
525 = ASSERT2( n_tvs > 0, ppr orig_fun_ty ) -- Zero case gives infnite loop!
526 applyTys (substTyWith tvs (take n_tvs arg_tys) rho_ty)
529 (tvs, rho_ty) = splitForAllTys orig_fun_ty
531 n_args = length arg_tys
535 %************************************************************************
537 \subsection{Source types}
539 %************************************************************************
541 A "source type" is a type that is a separate type as far as the type checker is
542 concerned, but which has low-level representation as far as the back end is concerned.
544 Source types are always lifted.
546 The key function is predTypeRep which gives the representation of a source type:
549 mkPredTy :: PredType -> Type
550 mkPredTy pred = PredTy pred
552 mkPredTys :: ThetaType -> [Type]
553 mkPredTys preds = map PredTy preds
555 predTypeRep :: PredType -> Type
556 -- Convert a PredType to its "representation type";
557 -- the post-type-checking type used by all the Core passes of GHC.
558 -- Unwraps only the outermost level; for example, the result might
559 -- be a newtype application
560 predTypeRep (IParam _ ty) = ty
561 predTypeRep (ClassP clas tys) = mkTyConApp (classTyCon clas) tys
562 -- Result might be a newtype application, but the consumer will
563 -- look through that too if necessary
567 %************************************************************************
571 %************************************************************************
574 splitRecNewType_maybe :: Type -> Maybe Type
575 -- Sometimes we want to look through a recursive newtype, and that's what happens here
576 -- It only strips *one layer* off, so the caller will usually call itself recursively
577 -- Only applied to types of kind *, hence the newtype is always saturated
578 splitRecNewType_maybe ty | Just ty' <- coreView ty = splitRecNewType_maybe ty'
579 splitRecNewType_maybe (TyConApp tc tys)
581 = ASSERT( tys `lengthIs` tyConArity tc ) -- splitRecNewType_maybe only be applied
582 -- to *types* (of kind *)
583 ASSERT( isRecursiveTyCon tc ) -- Guaranteed by coreView
584 case newTyConRhs tc of
585 (tvs, rep_ty) -> ASSERT( length tvs == length tys )
586 Just (substTyWith tvs tys rep_ty)
588 splitRecNewType_maybe other = Nothing
592 %************************************************************************
594 \subsection{Kinds and free variables}
596 %************************************************************************
598 ---------------------------------------------------------------------
599 Finding the kind of a type
600 ~~~~~~~~~~~~~~~~~~~~~~~~~~
602 typeKind :: Type -> Kind
604 typeKind (TyVarTy tyvar) = tyVarKind tyvar
605 typeKind (TyConApp tycon tys) = foldr (\_ k -> kindFunResult k) (tyConKind tycon) tys
606 typeKind (NoteTy _ ty) = typeKind ty
607 typeKind (PredTy _) = liftedTypeKind -- Predicates are always
608 -- represented by lifted types
609 typeKind (AppTy fun arg) = kindFunResult (typeKind fun)
610 typeKind (FunTy arg res) = liftedTypeKind
611 typeKind (ForAllTy tv ty) = typeKind ty
615 ---------------------------------------------------------------------
616 Free variables of a type
617 ~~~~~~~~~~~~~~~~~~~~~~~~
619 tyVarsOfType :: Type -> TyVarSet
620 tyVarsOfType (TyVarTy tv) = unitVarSet tv
621 tyVarsOfType (TyConApp tycon tys) = tyVarsOfTypes tys
622 tyVarsOfType (NoteTy (FTVNote tvs) ty2) = tvs
623 tyVarsOfType (PredTy sty) = tyVarsOfPred sty
624 tyVarsOfType (FunTy arg res) = tyVarsOfType arg `unionVarSet` tyVarsOfType res
625 tyVarsOfType (AppTy fun arg) = tyVarsOfType fun `unionVarSet` tyVarsOfType arg
626 tyVarsOfType (ForAllTy tyvar ty) = tyVarsOfType ty `minusVarSet` unitVarSet tyvar
631 -- What are the free tyvars of (T x)? Empty, of course!
632 -- Here's the example that Ralf Laemmel showed me:
633 -- foo :: (forall a. C u a -> C u a) -> u
634 -- mappend :: Monoid u => u -> u -> u
636 -- bar :: Monoid u => u
637 -- bar = foo (\t -> t `mappend` t)
638 -- We have to generalise at the arg to f, and we don't
639 -- want to capture the constraint (Monad (C u a)) because
640 -- it appears to mention a. Pretty silly, but it was useful to him.
643 tyVarsOfTypes :: [Type] -> TyVarSet
644 tyVarsOfTypes tys = foldr (unionVarSet.tyVarsOfType) emptyVarSet tys
646 tyVarsOfPred :: PredType -> TyVarSet
647 tyVarsOfPred (IParam _ ty) = tyVarsOfType ty
648 tyVarsOfPred (ClassP _ tys) = tyVarsOfTypes tys
650 tyVarsOfTheta :: ThetaType -> TyVarSet
651 tyVarsOfTheta = foldr (unionVarSet . tyVarsOfPred) emptyVarSet
653 -- Add a Note with the free tyvars to the top of the type
654 addFreeTyVars :: Type -> Type
655 addFreeTyVars ty@(NoteTy (FTVNote _) _) = ty
656 addFreeTyVars ty = NoteTy (FTVNote (tyVarsOfType ty)) ty
659 %************************************************************************
661 \subsection{TidyType}
663 %************************************************************************
665 tidyTy tidies up a type for printing in an error message, or in
668 It doesn't change the uniques at all, just the print names.
671 tidyTyVarBndr :: TidyEnv -> TyVar -> (TidyEnv, TyVar)
672 tidyTyVarBndr (tidy_env, subst) tyvar
673 = case tidyOccName tidy_env (getOccName name) of
674 (tidy', occ') -> ((tidy', subst'), tyvar')
676 subst' = extendVarEnv subst tyvar tyvar'
677 tyvar' = setTyVarName tyvar name'
678 name' = mkInternalName (getUnique name) occ' noSrcLoc
679 -- Note: make a *user* tyvar, so it printes nicely
680 -- Could extract src loc, but no need.
682 name = tyVarName tyvar
684 tidyFreeTyVars :: TidyEnv -> TyVarSet -> TidyEnv
685 -- Add the free tyvars to the env in tidy form,
686 -- so that we can tidy the type they are free in
687 tidyFreeTyVars env tyvars = fst (tidyOpenTyVars env (varSetElems tyvars))
689 tidyOpenTyVars :: TidyEnv -> [TyVar] -> (TidyEnv, [TyVar])
690 tidyOpenTyVars env tyvars = mapAccumL tidyOpenTyVar env tyvars
692 tidyOpenTyVar :: TidyEnv -> TyVar -> (TidyEnv, TyVar)
693 -- Treat a new tyvar as a binder, and give it a fresh tidy name
694 tidyOpenTyVar env@(tidy_env, subst) tyvar
695 = case lookupVarEnv subst tyvar of
696 Just tyvar' -> (env, tyvar') -- Already substituted
697 Nothing -> tidyTyVarBndr env tyvar -- Treat it as a binder
699 tidyType :: TidyEnv -> Type -> Type
700 tidyType env@(tidy_env, subst) ty
703 go (TyVarTy tv) = case lookupVarEnv subst tv of
704 Nothing -> TyVarTy tv
705 Just tv' -> TyVarTy tv'
706 go (TyConApp tycon tys) = let args = map go tys
707 in args `seqList` TyConApp tycon args
708 go (NoteTy note ty) = (NoteTy $! (go_note note)) $! (go ty)
709 go (PredTy sty) = PredTy (tidyPred env sty)
710 go (AppTy fun arg) = (AppTy $! (go fun)) $! (go arg)
711 go (FunTy fun arg) = (FunTy $! (go fun)) $! (go arg)
712 go (ForAllTy tv ty) = ForAllTy tvp $! (tidyType envp ty)
714 (envp, tvp) = tidyTyVarBndr env tv
716 go_note note@(FTVNote ftvs) = note -- No need to tidy the free tyvars
718 tidyTypes env tys = map (tidyType env) tys
720 tidyPred :: TidyEnv -> PredType -> PredType
721 tidyPred env (IParam n ty) = IParam n (tidyType env ty)
722 tidyPred env (ClassP clas tys) = ClassP clas (tidyTypes env tys)
726 @tidyOpenType@ grabs the free type variables, tidies them
727 and then uses @tidyType@ to work over the type itself
730 tidyOpenType :: TidyEnv -> Type -> (TidyEnv, Type)
732 = (env', tidyType env' ty)
734 env' = tidyFreeTyVars env (tyVarsOfType ty)
736 tidyOpenTypes :: TidyEnv -> [Type] -> (TidyEnv, [Type])
737 tidyOpenTypes env tys = mapAccumL tidyOpenType env tys
739 tidyTopType :: Type -> Type
740 tidyTopType ty = tidyType emptyTidyEnv ty
744 %************************************************************************
748 %************************************************************************
750 We use a grevious hack for tidying KindVars. A TidyEnv contains
751 a (VarEnv Var) substitution, to express the renaming; but
752 KindVars are not Vars. The Right Thing ultimately is to make them
753 into Vars (and perhaps make Kinds into Types), but I just do a hack
754 here: I make up a TyVar just to remember the new OccName for the
758 tidyKind :: TidyEnv -> Kind -> (TidyEnv, Kind)
759 tidyKind env@(tidy_env, subst) (KindVar kvar)
760 | Just tv <- lookupVarEnv_Directly subst uniq
761 = (env, KindVar (setKindVarOcc kvar (getOccName tv)))
763 = ((tidy', subst'), KindVar kvar')
765 uniq = kindVarUniq kvar
766 (tidy', occ') = tidyOccName tidy_env (kindVarOcc kvar)
767 kvar' = setKindVarOcc kvar occ'
768 fake_tv = mkTyVar tv_name (panic "tidyKind:fake tv kind")
769 tv_name = mkInternalName uniq occ' noSrcLoc
770 subst' = extendVarEnv subst fake_tv fake_tv
772 tidyKind env (FunKind k1 k2)
773 = (env2, FunKind k1' k2')
775 (env1, k1') = tidyKind env k1
776 (env2, k2') = tidyKind env1 k2
778 tidyKind env k = (env, k) -- Atomic kinds
782 %************************************************************************
784 \subsection{Liftedness}
786 %************************************************************************
789 isUnLiftedType :: Type -> Bool
790 -- isUnLiftedType returns True for forall'd unlifted types:
791 -- x :: forall a. Int#
792 -- I found bindings like these were getting floated to the top level.
793 -- They are pretty bogus types, mind you. It would be better never to
796 isUnLiftedType ty | Just ty' <- coreView ty = isUnLiftedType ty'
797 isUnLiftedType (ForAllTy tv ty) = isUnLiftedType ty
798 isUnLiftedType (TyConApp tc _) = isUnLiftedTyCon tc
799 isUnLiftedType other = False
801 isUnboxedTupleType :: Type -> Bool
802 isUnboxedTupleType ty = case splitTyConApp_maybe ty of
803 Just (tc, ty_args) -> isUnboxedTupleTyCon tc
806 -- Should only be applied to *types*; hence the assert
807 isAlgType :: Type -> Bool
808 isAlgType ty = case splitTyConApp_maybe ty of
809 Just (tc, ty_args) -> ASSERT( ty_args `lengthIs` tyConArity tc )
814 @isStrictType@ computes whether an argument (or let RHS) should
815 be computed strictly or lazily, based only on its type.
816 Works just like isUnLiftedType, except that it has a special case
817 for dictionaries. Since it takes account of ClassP, you might think
818 this function should be in TcType, but isStrictType is used by DataCon,
819 which is below TcType in the hierarchy, so it's convenient to put it here.
822 isStrictType (PredTy pred) = isStrictPred pred
823 isStrictType ty | Just ty' <- coreView ty = isStrictType ty'
824 isStrictType (ForAllTy tv ty) = isStrictType ty
825 isStrictType (TyConApp tc _) = isUnLiftedTyCon tc
826 isStrictType other = False
828 isStrictPred (ClassP clas _) = opt_DictsStrict && not (isNewTyCon (classTyCon clas))
829 isStrictPred other = False
830 -- We may be strict in dictionary types, but only if it
831 -- has more than one component.
832 -- [Being strict in a single-component dictionary risks
833 -- poking the dictionary component, which is wrong.]
837 isPrimitiveType :: Type -> Bool
838 -- Returns types that are opaque to Haskell.
839 -- Most of these are unlifted, but now that we interact with .NET, we
840 -- may have primtive (foreign-imported) types that are lifted
841 isPrimitiveType ty = case splitTyConApp_maybe ty of
842 Just (tc, ty_args) -> ASSERT( ty_args `lengthIs` tyConArity tc )
848 %************************************************************************
850 \subsection{Sequencing on types
852 %************************************************************************
855 seqType :: Type -> ()
856 seqType (TyVarTy tv) = tv `seq` ()
857 seqType (AppTy t1 t2) = seqType t1 `seq` seqType t2
858 seqType (FunTy t1 t2) = seqType t1 `seq` seqType t2
859 seqType (NoteTy note t2) = seqNote note `seq` seqType t2
860 seqType (PredTy p) = seqPred p
861 seqType (TyConApp tc tys) = tc `seq` seqTypes tys
862 seqType (ForAllTy tv ty) = tv `seq` seqType ty
864 seqTypes :: [Type] -> ()
866 seqTypes (ty:tys) = seqType ty `seq` seqTypes tys
868 seqNote :: TyNote -> ()
869 seqNote (FTVNote set) = sizeUniqSet set `seq` ()
871 seqPred :: PredType -> ()
872 seqPred (ClassP c tys) = c `seq` seqTypes tys
873 seqPred (IParam n ty) = n `seq` seqType ty
877 %************************************************************************
879 Equality for Core types
880 (We don't use instances so that we know where it happens)
882 %************************************************************************
884 Note that eqType works right even for partial applications of newtypes.
885 See Note [Newtype eta] in TyCon.lhs
888 coreEqType :: Type -> Type -> Bool
892 rn_env = mkRnEnv2 (mkInScopeSet (tyVarsOfType t1 `unionVarSet` tyVarsOfType t2))
894 eq env (TyVarTy tv1) (TyVarTy tv2) = rnOccL env tv1 == rnOccR env tv2
895 eq env (ForAllTy tv1 t1) (ForAllTy tv2 t2) = eq (rnBndr2 env tv1 tv2) t1 t2
896 eq env (AppTy s1 t1) (AppTy s2 t2) = eq env s1 s2 && eq env t1 t2
897 eq env (FunTy s1 t1) (FunTy s2 t2) = eq env s1 s2 && eq env t1 t2
898 eq env (TyConApp tc1 tys1) (TyConApp tc2 tys2)
899 | tc1 == tc2, all2 (eq env) tys1 tys2 = True
900 -- The lengths should be equal because
901 -- the two types have the same kind
902 -- NB: if the type constructors differ that does not
903 -- necessarily mean that the types aren't equal
904 -- (synonyms, newtypes)
905 -- Even if the type constructors are the same, but the arguments
906 -- differ, the two types could be the same (e.g. if the arg is just
907 -- ignored in the RHS). In both these cases we fall through to an
908 -- attempt to expand one side or the other.
910 -- Now deal with newtypes, synonyms, pred-tys
911 eq env t1 t2 | Just t1' <- coreView t1 = eq env t1' t2
912 | Just t2' <- coreView t2 = eq env t1 t2'
914 -- Fall through case; not equal!
919 %************************************************************************
921 Comparision for source types
922 (We don't use instances so that we know where it happens)
924 %************************************************************************
928 do *not* look through newtypes, PredTypes
931 tcEqType :: Type -> Type -> Bool
932 tcEqType t1 t2 = isEqual $ cmpType t1 t2
934 tcEqTypes :: [Type] -> [Type] -> Bool
935 tcEqTypes tys1 tys2 = isEqual $ cmpTypes tys1 tys2
937 tcCmpType :: Type -> Type -> Ordering
938 tcCmpType t1 t2 = cmpType t1 t2
940 tcCmpTypes :: [Type] -> [Type] -> Ordering
941 tcCmpTypes tys1 tys2 = cmpTypes tys1 tys2
943 tcEqPred :: PredType -> PredType -> Bool
944 tcEqPred p1 p2 = isEqual $ cmpPred p1 p2
946 tcCmpPred :: PredType -> PredType -> Ordering
947 tcCmpPred p1 p2 = cmpPred p1 p2
949 tcEqTypeX :: RnEnv2 -> Type -> Type -> Bool
950 tcEqTypeX env t1 t2 = isEqual $ cmpTypeX env t1 t2
953 Now here comes the real worker
956 cmpType :: Type -> Type -> Ordering
957 cmpType t1 t2 = cmpTypeX rn_env t1 t2
959 rn_env = mkRnEnv2 (mkInScopeSet (tyVarsOfType t1 `unionVarSet` tyVarsOfType t2))
961 cmpTypes :: [Type] -> [Type] -> Ordering
962 cmpTypes ts1 ts2 = cmpTypesX rn_env ts1 ts2
964 rn_env = mkRnEnv2 (mkInScopeSet (tyVarsOfTypes ts1 `unionVarSet` tyVarsOfTypes ts2))
966 cmpPred :: PredType -> PredType -> Ordering
967 cmpPred p1 p2 = cmpPredX rn_env p1 p2
969 rn_env = mkRnEnv2 (mkInScopeSet (tyVarsOfPred p1 `unionVarSet` tyVarsOfPred p2))
971 cmpTypeX :: RnEnv2 -> Type -> Type -> Ordering -- Main workhorse
972 cmpTypeX env t1 t2 | Just t1' <- tcView t1 = cmpTypeX env t1' t2
973 | Just t2' <- tcView t2 = cmpTypeX env t1 t2'
975 cmpTypeX env (TyVarTy tv1) (TyVarTy tv2) = rnOccL env tv1 `compare` rnOccR env tv2
976 cmpTypeX env (ForAllTy tv1 t1) (ForAllTy tv2 t2) = cmpTypeX (rnBndr2 env tv1 tv2) t1 t2
977 cmpTypeX env (AppTy s1 t1) (AppTy s2 t2) = cmpTypeX env s1 s2 `thenCmp` cmpTypeX env t1 t2
978 cmpTypeX env (FunTy s1 t1) (FunTy s2 t2) = cmpTypeX env s1 s2 `thenCmp` cmpTypeX env t1 t2
979 cmpTypeX env (PredTy p1) (PredTy p2) = cmpPredX env p1 p2
980 cmpTypeX env (TyConApp tc1 tys1) (TyConApp tc2 tys2) = (tc1 `compare` tc2) `thenCmp` cmpTypesX env tys1 tys2
981 cmpTypeX env t1 (NoteTy _ t2) = cmpTypeX env t1 t2
983 -- Deal with the rest: TyVarTy < AppTy < FunTy < TyConApp < ForAllTy < PredTy
984 cmpTypeX env (AppTy _ _) (TyVarTy _) = GT
986 cmpTypeX env (FunTy _ _) (TyVarTy _) = GT
987 cmpTypeX env (FunTy _ _) (AppTy _ _) = GT
989 cmpTypeX env (TyConApp _ _) (TyVarTy _) = GT
990 cmpTypeX env (TyConApp _ _) (AppTy _ _) = GT
991 cmpTypeX env (TyConApp _ _) (FunTy _ _) = GT
993 cmpTypeX env (ForAllTy _ _) (TyVarTy _) = GT
994 cmpTypeX env (ForAllTy _ _) (AppTy _ _) = GT
995 cmpTypeX env (ForAllTy _ _) (FunTy _ _) = GT
996 cmpTypeX env (ForAllTy _ _) (TyConApp _ _) = GT
998 cmpTypeX env (PredTy _) t2 = GT
1000 cmpTypeX env _ _ = LT
1003 cmpTypesX :: RnEnv2 -> [Type] -> [Type] -> Ordering
1004 cmpTypesX env [] [] = EQ
1005 cmpTypesX env (t1:tys1) (t2:tys2) = cmpTypeX env t1 t2 `thenCmp` cmpTypesX env tys1 tys2
1006 cmpTypesX env [] tys = LT
1007 cmpTypesX env ty [] = GT
1010 cmpPredX :: RnEnv2 -> PredType -> PredType -> Ordering
1011 cmpPredX env (IParam n1 ty1) (IParam n2 ty2) = (n1 `compare` n2) `thenCmp` cmpTypeX env ty1 ty2
1012 -- Compare types as well as names for implicit parameters
1013 -- This comparison is used exclusively (I think) for the
1014 -- finite map built in TcSimplify
1015 cmpPredX env (ClassP c1 tys1) (ClassP c2 tys2) = (c1 `compare` c2) `thenCmp` cmpTypesX env tys1 tys2
1016 cmpPredX env (IParam _ _) (ClassP _ _) = LT
1017 cmpPredX env (ClassP _ _) (IParam _ _) = GT
1020 PredTypes are used as a FM key in TcSimplify,
1021 so we take the easy path and make them an instance of Ord
1024 instance Eq PredType where { (==) = tcEqPred }
1025 instance Ord PredType where { compare = tcCmpPred }
1029 %************************************************************************
1033 %************************************************************************
1037 = TvSubst InScopeSet -- The in-scope type variables
1038 TvSubstEnv -- The substitution itself
1039 -- See Note [Apply Once]
1041 {- ----------------------------------------------------------
1044 We use TvSubsts to instantiate things, and we might instantiate
1048 So the substition might go [a->b, b->a]. A similar situation arises in Core
1049 when we find a beta redex like
1050 (/\ a /\ b -> e) b a
1051 Then we also end up with a substition that permutes type variables. Other
1052 variations happen to; for example [a -> (a, b)].
1054 ***************************************************
1055 *** So a TvSubst must be applied precisely once ***
1056 ***************************************************
1058 A TvSubst is not idempotent, but, unlike the non-idempotent substitution
1059 we use during unifications, it must not be repeatedly applied.
1060 -------------------------------------------------------------- -}
1063 type TvSubstEnv = TyVarEnv Type
1064 -- A TvSubstEnv is used both inside a TvSubst (with the apply-once
1065 -- invariant discussed in Note [Apply Once]), and also independently
1066 -- in the middle of matching, and unification (see Types.Unify)
1067 -- So you have to look at the context to know if it's idempotent or
1068 -- apply-once or whatever
1069 emptyTvSubstEnv :: TvSubstEnv
1070 emptyTvSubstEnv = emptyVarEnv
1072 composeTvSubst :: InScopeSet -> TvSubstEnv -> TvSubstEnv -> TvSubstEnv
1073 -- (compose env1 env2)(x) is env1(env2(x)); i.e. apply env2 then env1
1074 -- It assumes that both are idempotent
1075 -- Typically, env1 is the refinement to a base substitution env2
1076 composeTvSubst in_scope env1 env2
1077 = env1 `plusVarEnv` mapVarEnv (substTy subst1) env2
1078 -- First apply env1 to the range of env2
1079 -- Then combine the two, making sure that env1 loses if
1080 -- both bind the same variable; that's why env1 is the
1081 -- *left* argument to plusVarEnv, because the right arg wins
1083 subst1 = TvSubst in_scope env1
1085 emptyTvSubst = TvSubst emptyInScopeSet emptyVarEnv
1087 isEmptyTvSubst :: TvSubst -> Bool
1088 isEmptyTvSubst (TvSubst _ env) = isEmptyVarEnv env
1090 mkTvSubst :: InScopeSet -> TvSubstEnv -> TvSubst
1093 getTvSubstEnv :: TvSubst -> TvSubstEnv
1094 getTvSubstEnv (TvSubst _ env) = env
1096 getTvInScope :: TvSubst -> InScopeSet
1097 getTvInScope (TvSubst in_scope _) = in_scope
1099 isInScope :: Var -> TvSubst -> Bool
1100 isInScope v (TvSubst in_scope _) = v `elemInScopeSet` in_scope
1102 notElemTvSubst :: TyVar -> TvSubst -> Bool
1103 notElemTvSubst tv (TvSubst _ env) = not (tv `elemVarEnv` env)
1105 setTvSubstEnv :: TvSubst -> TvSubstEnv -> TvSubst
1106 setTvSubstEnv (TvSubst in_scope _) env = TvSubst in_scope env
1108 extendTvInScope :: TvSubst -> [Var] -> TvSubst
1109 extendTvInScope (TvSubst in_scope env) vars = TvSubst (extendInScopeSetList in_scope vars) env
1111 extendTvSubst :: TvSubst -> TyVar -> Type -> TvSubst
1112 extendTvSubst (TvSubst in_scope env) tv ty = TvSubst in_scope (extendVarEnv env tv ty)
1114 extendTvSubstList :: TvSubst -> [TyVar] -> [Type] -> TvSubst
1115 extendTvSubstList (TvSubst in_scope env) tvs tys
1116 = TvSubst in_scope (extendVarEnvList env (tvs `zip` tys))
1118 -- mkOpenTvSubst and zipOpenTvSubst generate the in-scope set from
1119 -- the types given; but it's just a thunk so with a bit of luck
1120 -- it'll never be evaluated
1122 mkOpenTvSubst :: TvSubstEnv -> TvSubst
1123 mkOpenTvSubst env = TvSubst (mkInScopeSet (tyVarsOfTypes (varEnvElts env))) env
1125 zipOpenTvSubst :: [TyVar] -> [Type] -> TvSubst
1126 zipOpenTvSubst tyvars tys
1128 | length tyvars /= length tys
1129 = pprTrace "zipOpenTvSubst" (ppr tyvars $$ ppr tys) emptyTvSubst
1132 = TvSubst (mkInScopeSet (tyVarsOfTypes tys)) (zipTyEnv tyvars tys)
1134 -- mkTopTvSubst is called when doing top-level substitutions.
1135 -- Here we expect that the free vars of the range of the
1136 -- substitution will be empty.
1137 mkTopTvSubst :: [(TyVar, Type)] -> TvSubst
1138 mkTopTvSubst prs = TvSubst emptyInScopeSet (mkVarEnv prs)
1140 zipTopTvSubst :: [TyVar] -> [Type] -> TvSubst
1141 zipTopTvSubst tyvars tys
1143 | length tyvars /= length tys
1144 = pprTrace "zipOpenTvSubst" (ppr tyvars $$ ppr tys) emptyTvSubst
1147 = TvSubst emptyInScopeSet (zipTyEnv tyvars tys)
1149 zipTyEnv :: [TyVar] -> [Type] -> TvSubstEnv
1152 | length tyvars /= length tys
1153 = pprTrace "mkTopTvSubst" (ppr tyvars $$ ppr tys) emptyVarEnv
1156 = zip_ty_env tyvars tys emptyVarEnv
1158 -- Later substitutions in the list over-ride earlier ones,
1159 -- but there should be no loops
1160 zip_ty_env [] [] env = env
1161 zip_ty_env (tv:tvs) (ty:tys) env = zip_ty_env tvs tys (extendVarEnv env tv ty)
1162 -- There used to be a special case for when
1164 -- (a not-uncommon case) in which case the substitution was dropped.
1165 -- But the type-tidier changes the print-name of a type variable without
1166 -- changing the unique, and that led to a bug. Why? Pre-tidying, we had
1167 -- a type {Foo t}, where Foo is a one-method class. So Foo is really a newtype.
1168 -- And it happened that t was the type variable of the class. Post-tiding,
1169 -- it got turned into {Foo t2}. The ext-core printer expanded this using
1170 -- sourceTypeRep, but that said "Oh, t == t2" because they have the same unique,
1171 -- and so generated a rep type mentioning t not t2.
1173 -- Simplest fix is to nuke the "optimisation"
1174 zip_ty_env tvs tys env = pprTrace "Var/Type length mismatch: " (ppr tvs $$ ppr tys) env
1175 -- zip_ty_env _ _ env = env
1177 instance Outputable TvSubst where
1178 ppr (TvSubst ins env)
1179 = sep[ ptext SLIT("<TvSubst"),
1180 nest 2 (ptext SLIT("In scope:") <+> ppr ins),
1181 nest 2 (ptext SLIT("Env:") <+> ppr env) ]
1184 %************************************************************************
1186 Performing type substitutions
1188 %************************************************************************
1191 substTyWith :: [TyVar] -> [Type] -> Type -> Type
1192 substTyWith tvs tys = ASSERT( length tvs == length tys )
1193 substTy (zipOpenTvSubst tvs tys)
1195 substTy :: TvSubst -> Type -> Type
1196 substTy subst ty | isEmptyTvSubst subst = ty
1197 | otherwise = subst_ty subst ty
1199 substTys :: TvSubst -> [Type] -> [Type]
1200 substTys subst tys | isEmptyTvSubst subst = tys
1201 | otherwise = map (subst_ty subst) tys
1203 substTheta :: TvSubst -> ThetaType -> ThetaType
1204 substTheta subst theta
1205 | isEmptyTvSubst subst = theta
1206 | otherwise = map (substPred subst) theta
1208 substPred :: TvSubst -> PredType -> PredType
1209 substPred subst (IParam n ty) = IParam n (subst_ty subst ty)
1210 substPred subst (ClassP clas tys) = ClassP clas (map (subst_ty subst) tys)
1212 deShadowTy :: TyVarSet -> Type -> Type -- Remove any nested binders mentioning tvs
1214 = subst_ty (mkTvSubst in_scope emptyTvSubstEnv) ty
1216 in_scope = mkInScopeSet tvs
1218 -- Note that the in_scope set is poked only if we hit a forall
1219 -- so it may often never be fully computed
1223 go (TyVarTy tv) = substTyVar subst tv
1224 go (TyConApp tc tys) = let args = map go tys
1225 in args `seqList` TyConApp tc args
1227 go (PredTy p) = PredTy $! (substPred subst p)
1229 go (NoteTy (FTVNote _) ty2) = go ty2 -- Discard the free tyvar note
1231 go (FunTy arg res) = (FunTy $! (go arg)) $! (go res)
1232 go (AppTy fun arg) = mkAppTy (go fun) $! (go arg)
1233 -- The mkAppTy smart constructor is important
1234 -- we might be replacing (a Int), represented with App
1235 -- by [Int], represented with TyConApp
1236 go (ForAllTy tv ty) = case substTyVarBndr subst tv of
1237 (subst', tv') -> ForAllTy tv' $! (subst_ty subst' ty)
1239 substTyVar :: TvSubst -> TyVar -> Type
1240 substTyVar (TvSubst in_scope env) tv
1241 = case (lookupVarEnv env tv) of
1242 Nothing -> TyVarTy tv
1243 Just ty' -> ty' -- See Note [Apply Once]
1245 substTyVarBndr :: TvSubst -> TyVar -> (TvSubst, TyVar)
1246 substTyVarBndr subst@(TvSubst in_scope env) old_var
1247 | old_var == new_var -- No need to clone
1248 -- But we *must* zap any current substitution for the variable.
1250 -- (\x.e) with id_subst = [x |-> e']
1251 -- Here we must simply zap the substitution for x
1253 -- The new_id isn't cloned, but it may have a different type
1254 -- etc, so we must return it, not the old id
1255 = (TvSubst (in_scope `extendInScopeSet` new_var)
1256 (delVarEnv env old_var),
1259 | otherwise -- The new binder is in scope so
1260 -- we'd better rename it away from the in-scope variables
1261 -- Extending the substitution to do this renaming also
1262 -- has the (correct) effect of discarding any existing
1263 -- substitution for that variable
1264 = (TvSubst (in_scope `extendInScopeSet` new_var)
1265 (extendVarEnv env old_var (TyVarTy new_var)),
1268 new_var = uniqAway in_scope old_var
1269 -- The uniqAway part makes sure the new variable is not already in scope