2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
5 \section[TcMonoType]{Typechecking user-specified @MonoTypes@}
9 tcHsSigType, tcHsDeriv,
13 kcHsTyVars, kcHsSigType, kcHsLiftedSigType,
14 kcCheckHsType, kcHsContext, kcHsType,
16 -- Typechecking kinded types
17 tcHsKindedContext, tcHsKindedType, tcHsBangType,
18 tcTyVarBndrs, dsHsType, tcLHsConResTy,
21 -- Pattern type signatures
22 tcHsPatSigType, tcPatSig
25 #include "HsVersions.h"
35 import {- Kind parts of -} Type
51 ----------------------------
53 ----------------------------
55 Generally speaking we now type-check types in three phases
57 1. kcHsType: kind check the HsType
58 *includes* performing any TH type splices;
59 so it returns a translated, and kind-annotated, type
61 2. dsHsType: convert from HsType to Type:
63 expand type synonyms [mkGenTyApps]
64 hoist the foralls [tcHsType]
66 3. checkValidType: check the validity of the resulting type
68 Often these steps are done one after the other (tcHsSigType).
69 But in mutually recursive groups of type and class decls we do
70 1 kind-check the whole group
71 2 build TyCons/Classes in a knot-tied way
72 3 check the validity of types in the now-unknotted TyCons/Classes
74 For example, when we find
75 (forall a m. m a -> m a)
76 we bind a,m to kind varibles and kind-check (m a -> m a). This makes
77 a get kind *, and m get kind *->*. Now we typecheck (m a -> m a) in
78 an environment that binds a and m suitably.
80 The kind checker passed to tcHsTyVars needs to look at enough to
81 establish the kind of the tyvar:
82 * For a group of type and class decls, it's just the group, not
83 the rest of the program
84 * For a tyvar bound in a pattern type signature, its the types
85 mentioned in the other type signatures in that bunch of patterns
86 * For a tyvar bound in a RULE, it's the type signatures on other
87 universally quantified variables in the rule
89 Note that this may occasionally give surprising results. For example:
91 data T a b = MkT (a b)
93 Here we deduce a::*->*, b::*
94 But equally valid would be a::(*->*)-> *, b::*->*
99 Some of the validity check could in principle be done by the kind checker,
102 - During desugaring, we normalise by expanding type synonyms. Only
103 after this step can we check things like type-synonym saturation
104 e.g. type T k = k Int
106 Then (T S) is ok, because T is saturated; (T S) expands to (S Int);
107 and then S is saturated. This is a GHC extension.
109 - Similarly, also a GHC extension, we look through synonyms before complaining
110 about the form of a class or instance declaration
112 - Ambiguity checks involve functional dependencies, and it's easier to wait
113 until knots have been resolved before poking into them
115 Also, in a mutually recursive group of types, we can't look at the TyCon until we've
116 finished building the loop. So to keep things simple, we postpone most validity
117 checking until step (3).
121 During step (1) we might fault in a TyCon defined in another module, and it might
122 (via a loop) refer back to a TyCon defined in this module. So when we tie a big
123 knot around type declarations with ARecThing, so that the fault-in code can get
124 the TyCon being defined.
127 %************************************************************************
129 \subsection{Checking types}
131 %************************************************************************
134 tcHsSigType :: UserTypeCtxt -> LHsType Name -> TcM Type
135 -- Do kind checking, and hoist for-alls to the top
136 -- NB: it's important that the foralls that come from the top-level
137 -- HsForAllTy in hs_ty occur *first* in the returned type.
138 -- See Note [Scoped] with TcSigInfo
139 tcHsSigType ctxt hs_ty
140 = addErrCtxt (pprHsSigCtxt ctxt hs_ty) $
141 do { kinded_ty <- kcTypeType hs_ty
142 ; ty <- tcHsKindedType kinded_ty
143 ; checkValidType ctxt ty
146 -- Used for the deriving(...) items
147 tcHsDeriv :: LHsType Name -> TcM ([TyVar], Class, [Type])
148 tcHsDeriv = addLocM (tc_hs_deriv [])
150 tc_hs_deriv tv_names (HsPredTy (HsClassP cls_name hs_tys))
151 = kcHsTyVars tv_names $ \ tv_names' ->
152 do { cls_kind <- kcClass cls_name
153 ; (tys, res_kind) <- kcApps cls_kind (ppr cls_name) hs_tys
154 ; tcTyVarBndrs tv_names' $ \ tyvars ->
155 do { arg_tys <- dsHsTypes tys
156 ; cls <- tcLookupClass cls_name
157 ; return (tyvars, cls, arg_tys) }}
159 tc_hs_deriv tv_names1 (HsForAllTy _ tv_names2 (L _ []) (L _ ty))
160 = -- Funny newtype deriving form
162 -- where C has arity 2. Hence can't use regular functions
163 tc_hs_deriv (tv_names1 ++ tv_names2) ty
166 = failWithTc (ptext SLIT("Illegal deriving item") <+> ppr other)
169 These functions are used during knot-tying in
170 type and class declarations, when we have to
171 separate kind-checking, desugaring, and validity checking
174 kcHsSigType, kcHsLiftedSigType :: LHsType Name -> TcM (LHsType Name)
175 -- Used for type signatures
176 kcHsSigType ty = kcTypeType ty
177 kcHsLiftedSigType ty = kcLiftedType ty
179 tcHsKindedType :: LHsType Name -> TcM Type
180 -- Don't do kind checking, nor validity checking.
181 -- This is used in type and class decls, where kinding is
182 -- done in advance, and validity checking is done later
183 -- [Validity checking done later because of knot-tying issues.]
184 tcHsKindedType hs_ty = dsHsType hs_ty
186 tcHsBangType :: LHsType Name -> TcM Type
187 -- Permit a bang, but discard it
188 tcHsBangType (L span (HsBangTy b ty)) = tcHsKindedType ty
189 tcHsBangType ty = tcHsKindedType ty
191 tcHsKindedContext :: LHsContext Name -> TcM ThetaType
192 -- Used when we are expecting a ClassContext (i.e. no implicit params)
193 -- Does not do validity checking, like tcHsKindedType
194 tcHsKindedContext hs_theta = addLocM (mappM dsHsLPred) hs_theta
198 %************************************************************************
200 The main kind checker: kcHsType
202 %************************************************************************
204 First a couple of simple wrappers for kcHsType
207 ---------------------------
208 kcLiftedType :: LHsType Name -> TcM (LHsType Name)
209 -- The type ty must be a *lifted* *type*
210 kcLiftedType ty = kcCheckHsType ty liftedTypeKind
212 ---------------------------
213 kcTypeType :: LHsType Name -> TcM (LHsType Name)
214 -- The type ty must be a *type*, but it can be lifted or
215 -- unlifted or an unboxed tuple.
216 kcTypeType ty = kcCheckHsType ty openTypeKind
218 ---------------------------
219 kcCheckHsType :: LHsType Name -> TcKind -> TcM (LHsType Name)
220 -- Check that the type has the specified kind
221 -- Be sure to use checkExpectedKind, rather than simply unifying
222 -- with OpenTypeKind, because it gives better error messages
223 kcCheckHsType (L span ty) exp_kind
225 do { (ty', act_kind) <- add_ctxt ty (kc_hs_type ty)
226 -- Add the context round the inner check only
227 -- because checkExpectedKind already mentions
228 -- 'ty' by name in any error message
230 ; checkExpectedKind (strip ty) act_kind exp_kind
231 ; return (L span ty') }
233 -- Wrap a context around only if we want to show that contexts.
234 add_ctxt (HsPredTy p) thing = thing
235 -- Omit invisble ones and ones user's won't grok (HsPred p).
236 add_ctxt (HsForAllTy _ _ (L _ []) _) thing = thing
237 -- Omit wrapping if the theta-part is empty
238 -- Reason: the recursive call to kcLiftedType, in the ForAllTy
239 -- case of kc_hs_type, will do the wrapping instead
240 -- and we don't want to duplicate
241 add_ctxt other_ty thing = addErrCtxt (typeCtxt other_ty) thing
243 -- We infer the kind of the type, and then complain if it's
244 -- not right. But we don't want to complain about
245 -- (ty) or !(ty) or forall a. ty
246 -- when the real difficulty is with the 'ty' part.
247 strip (HsParTy (L _ ty)) = strip ty
248 strip (HsBangTy _ (L _ ty)) = strip ty
249 strip (HsForAllTy _ _ _ (L _ ty)) = strip ty
253 Here comes the main function
256 kcHsType :: LHsType Name -> TcM (LHsType Name, TcKind)
257 kcHsType ty = wrapLocFstM kc_hs_type ty
258 -- kcHsType *returns* the kind of the type, rather than taking an expected
259 -- kind as argument as tcExpr does.
261 -- (a) the kind of (->) is
262 -- forall bx1 bx2. Type bx1 -> Type bx2 -> Type Boxed
263 -- so we'd need to generate huge numbers of bx variables.
264 -- (b) kinds are so simple that the error messages are fine
266 -- The translated type has explicitly-kinded type-variable binders
268 kc_hs_type (HsParTy ty)
269 = kcHsType ty `thenM` \ (ty', kind) ->
270 returnM (HsParTy ty', kind)
272 kc_hs_type (HsTyVar name)
273 = kcTyVar name `thenM` \ kind ->
274 returnM (HsTyVar name, kind)
276 kc_hs_type (HsListTy ty)
277 = kcLiftedType ty `thenM` \ ty' ->
278 returnM (HsListTy ty', liftedTypeKind)
280 kc_hs_type (HsPArrTy ty)
281 = kcLiftedType ty `thenM` \ ty' ->
282 returnM (HsPArrTy ty', liftedTypeKind)
284 kc_hs_type (HsNumTy n)
285 = returnM (HsNumTy n, liftedTypeKind)
287 kc_hs_type (HsKindSig ty k)
288 = kcCheckHsType ty k `thenM` \ ty' ->
289 returnM (HsKindSig ty' k, k)
291 kc_hs_type (HsTupleTy Boxed tys)
292 = mappM kcLiftedType tys `thenM` \ tys' ->
293 returnM (HsTupleTy Boxed tys', liftedTypeKind)
295 kc_hs_type (HsTupleTy Unboxed tys)
296 = mappM kcTypeType tys `thenM` \ tys' ->
297 returnM (HsTupleTy Unboxed tys', ubxTupleKind)
299 kc_hs_type (HsFunTy ty1 ty2)
300 = kcCheckHsType ty1 argTypeKind `thenM` \ ty1' ->
301 kcTypeType ty2 `thenM` \ ty2' ->
302 returnM (HsFunTy ty1' ty2', liftedTypeKind)
304 kc_hs_type ty@(HsOpTy ty1 op ty2)
305 = addLocM kcTyVar op `thenM` \ op_kind ->
306 kcApps op_kind (ppr op) [ty1,ty2] `thenM` \ ([ty1',ty2'], res_kind) ->
307 returnM (HsOpTy ty1' op ty2', res_kind)
309 kc_hs_type ty@(HsAppTy ty1 ty2)
310 = kcHsType fun_ty `thenM` \ (fun_ty', fun_kind) ->
311 kcApps fun_kind (ppr fun_ty) arg_tys `thenM` \ ((arg_ty':arg_tys'), res_kind) ->
312 returnM (foldl mk_app (HsAppTy fun_ty' arg_ty') arg_tys', res_kind)
314 (fun_ty, arg_tys) = split ty1 [ty2]
315 split (L _ (HsAppTy f a)) as = split f (a:as)
317 mk_app fun arg = HsAppTy (noLoc fun) arg -- Add noLocs for inner nodes of
318 -- the application; they are never used
320 kc_hs_type (HsPredTy pred)
321 = kcHsPred pred `thenM` \ pred' ->
322 returnM (HsPredTy pred', liftedTypeKind)
324 kc_hs_type (HsForAllTy exp tv_names context ty)
325 = kcHsTyVars tv_names $ \ tv_names' ->
326 do { ctxt' <- kcHsContext context
327 ; ty' <- kcLiftedType ty
328 -- The body of a forall is usually a type, but in principle
329 -- there's no reason to prohibit *unlifted* types.
330 -- In fact, GHC can itself construct a function with an
331 -- unboxed tuple inside a for-all (via CPR analyis; see
332 -- typecheck/should_compile/tc170)
334 -- Still, that's only for internal interfaces, which aren't
335 -- kind-checked, so we only allow liftedTypeKind here
337 ; return (HsForAllTy exp tv_names' ctxt' ty', liftedTypeKind) }
339 kc_hs_type (HsBangTy b ty)
340 = do { (ty', kind) <- kcHsType ty
341 ; return (HsBangTy b ty', kind) }
343 kc_hs_type ty@(HsSpliceTy _)
344 = failWithTc (ptext SLIT("Unexpected type splice:") <+> ppr ty)
346 -- remove the doc nodes here, no need to worry about the location since
347 -- its the same for a doc node and it's child type node
348 kc_hs_type (HsDocTy ty _)
349 = kc_hs_type (unLoc ty)
351 ---------------------------
352 kcApps :: TcKind -- Function kind
354 -> [LHsType Name] -- Arg types
355 -> TcM ([LHsType Name], TcKind) -- Kind-checked args
356 kcApps fun_kind ppr_fun args
357 = split_fk fun_kind (length args) `thenM` \ (arg_kinds, res_kind) ->
358 zipWithM kc_arg args arg_kinds `thenM` \ args' ->
359 returnM (args', res_kind)
361 split_fk fk 0 = returnM ([], fk)
362 split_fk fk n = unifyFunKind fk `thenM` \ mb_fk ->
364 Nothing -> failWithTc too_many_args
365 Just (ak,fk') -> split_fk fk' (n-1) `thenM` \ (aks, rk) ->
368 kc_arg arg arg_kind = kcCheckHsType arg arg_kind
370 too_many_args = ptext SLIT("Kind error:") <+> quotes ppr_fun <+>
371 ptext SLIT("is applied to too many type arguments")
373 ---------------------------
374 kcHsContext :: LHsContext Name -> TcM (LHsContext Name)
375 kcHsContext ctxt = wrapLocM (mappM kcHsLPred) ctxt
377 kcHsLPred :: LHsPred Name -> TcM (LHsPred Name)
378 kcHsLPred = wrapLocM kcHsPred
380 kcHsPred :: HsPred Name -> TcM (HsPred Name)
381 kcHsPred pred -- Checks that the result is of kind liftedType
382 = kc_pred pred `thenM` \ (pred', kind) ->
383 checkExpectedKind pred kind liftedTypeKind `thenM_`
386 ---------------------------
387 kc_pred :: HsPred Name -> TcM (HsPred Name, TcKind)
388 -- Does *not* check for a saturated
389 -- application (reason: used from TcDeriv)
390 kc_pred pred@(HsIParam name ty)
391 = kcHsType ty `thenM` \ (ty', kind) ->
392 returnM (HsIParam name ty', kind)
394 kc_pred pred@(HsClassP cls tys)
395 = kcClass cls `thenM` \ kind ->
396 kcApps kind (ppr cls) tys `thenM` \ (tys', res_kind) ->
397 returnM (HsClassP cls tys', res_kind)
399 ---------------------------
400 kcTyVar :: Name -> TcM TcKind
401 kcTyVar name -- Could be a tyvar or a tycon
402 = traceTc (text "lk1" <+> ppr name) `thenM_`
403 tcLookup name `thenM` \ thing ->
404 traceTc (text "lk2" <+> ppr name <+> ppr thing) `thenM_`
406 ATyVar _ ty -> returnM (typeKind ty)
407 AThing kind -> returnM kind
408 AGlobal (ATyCon tc) -> returnM (tyConKind tc)
409 other -> wrongThingErr "type" thing name
411 kcClass :: Name -> TcM TcKind
412 kcClass cls -- Must be a class
413 = tcLookup cls `thenM` \ thing ->
415 AThing kind -> returnM kind
416 AGlobal (AClass cls) -> returnM (tyConKind (classTyCon cls))
417 other -> wrongThingErr "class" thing cls
421 %************************************************************************
425 %************************************************************************
429 * Transforms from HsType to Type
432 It cannot fail, and does no validity checking, except for
433 structural matters, such as
434 (a) spurious ! annotations.
435 (b) a class used as a type
438 dsHsType :: LHsType Name -> TcM Type
439 -- All HsTyVarBndrs in the intput type are kind-annotated
440 dsHsType ty = ds_type (unLoc ty)
442 ds_type ty@(HsTyVar name)
445 ds_type (HsParTy ty) -- Remove the parentheses markers
448 ds_type ty@(HsBangTy _ _) -- No bangs should be here
449 = failWithTc (ptext SLIT("Unexpected strictness annotation:") <+> ppr ty)
451 ds_type (HsKindSig ty k)
452 = dsHsType ty -- Kind checking done already
454 ds_type (HsListTy ty)
455 = dsHsType ty `thenM` \ tau_ty ->
456 checkWiredInTyCon listTyCon `thenM_`
457 returnM (mkListTy tau_ty)
459 ds_type (HsPArrTy ty)
460 = dsHsType ty `thenM` \ tau_ty ->
461 checkWiredInTyCon parrTyCon `thenM_`
462 returnM (mkPArrTy tau_ty)
464 ds_type (HsTupleTy boxity tys)
465 = dsHsTypes tys `thenM` \ tau_tys ->
466 checkWiredInTyCon tycon `thenM_`
467 returnM (mkTyConApp tycon tau_tys)
469 tycon = tupleTyCon boxity (length tys)
471 ds_type (HsFunTy ty1 ty2)
472 = dsHsType ty1 `thenM` \ tau_ty1 ->
473 dsHsType ty2 `thenM` \ tau_ty2 ->
474 returnM (mkFunTy tau_ty1 tau_ty2)
476 ds_type (HsOpTy ty1 (L span op) ty2)
477 = dsHsType ty1 `thenM` \ tau_ty1 ->
478 dsHsType ty2 `thenM` \ tau_ty2 ->
479 setSrcSpan span (ds_var_app op [tau_ty1,tau_ty2])
483 tcLookupTyCon genUnitTyConName `thenM` \ tc ->
484 returnM (mkTyConApp tc [])
486 ds_type ty@(HsAppTy _ _)
489 ds_type (HsPredTy pred)
490 = dsHsPred pred `thenM` \ pred' ->
491 returnM (mkPredTy pred')
493 ds_type full_ty@(HsForAllTy exp tv_names ctxt ty)
494 = tcTyVarBndrs tv_names $ \ tyvars ->
495 mappM dsHsLPred (unLoc ctxt) `thenM` \ theta ->
496 dsHsType ty `thenM` \ tau ->
497 returnM (mkSigmaTy tyvars theta tau)
499 ds_type (HsSpliceTy {}) = panic "ds_type: HsSpliceTy"
501 dsHsTypes arg_tys = mappM dsHsType arg_tys
504 Help functions for type applications
505 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
508 ds_app :: HsType Name -> [LHsType Name] -> TcM Type
509 ds_app (HsAppTy ty1 ty2) tys
510 = ds_app (unLoc ty1) (ty2:tys)
513 = dsHsTypes tys `thenM` \ arg_tys ->
515 HsTyVar fun -> ds_var_app fun arg_tys
516 other -> ds_type ty `thenM` \ fun_ty ->
517 returnM (mkAppTys fun_ty arg_tys)
519 ds_var_app :: Name -> [Type] -> TcM Type
520 ds_var_app name arg_tys
521 = tcLookup name `thenM` \ thing ->
523 ATyVar _ ty -> returnM (mkAppTys ty arg_tys)
524 AGlobal (ATyCon tc) -> returnM (mkTyConApp tc arg_tys)
525 other -> wrongThingErr "type" thing name
533 dsHsLPred :: LHsPred Name -> TcM PredType
534 dsHsLPred pred = dsHsPred (unLoc pred)
536 dsHsPred pred@(HsClassP class_name tys)
537 = dsHsTypes tys `thenM` \ arg_tys ->
538 tcLookupClass class_name `thenM` \ clas ->
539 returnM (ClassP clas arg_tys)
541 dsHsPred (HsIParam name ty)
542 = dsHsType ty `thenM` \ arg_ty ->
543 returnM (IParam name arg_ty)
546 GADT constructor signatures
549 tcLHsConResTy :: LHsType Name -> TcM (TyCon, [TcType])
551 = addErrCtxt (gadtResCtxt res_ty) $
552 case get_largs res_ty [] of
553 (HsTyVar tc_name, args)
554 -> do { args' <- mapM dsHsType args
555 ; thing <- tcLookup tc_name
557 AGlobal (ATyCon tc) -> return (tc, args')
558 other -> failWithTc (badGadtDecl res_ty) }
559 other -> failWithTc (badGadtDecl res_ty)
561 -- We can't call dsHsType on res_ty, and then do tcSplitTyConApp_maybe
562 -- because that causes a black hole, and for good reason. Building
563 -- the type means expanding type synonyms, and we can't do that
564 -- inside the "knot". So we have to work by steam.
565 get_largs (L _ ty) args = get_args ty args
566 get_args (HsAppTy fun arg) args = get_largs fun (arg:args)
567 get_args (HsParTy ty) args = get_largs ty args
568 get_args (HsOpTy ty1 (L span tc) ty2) args = (HsTyVar tc, ty1:ty2:args)
569 get_args ty args = (ty, args)
572 = hang (ptext SLIT("In the result type of a data constructor:"))
575 = hang (ptext SLIT("Malformed constructor result type:"))
578 typeCtxt ty = ptext SLIT("In the type") <+> quotes (ppr ty)
581 %************************************************************************
583 Type-variable binders
585 %************************************************************************
589 kcHsTyVars :: [LHsTyVarBndr Name]
590 -> ([LHsTyVarBndr Name] -> TcM r) -- These binders are kind-annotated
591 -- They scope over the thing inside
593 kcHsTyVars tvs thing_inside
594 = mappM (wrapLocM kcHsTyVar) tvs `thenM` \ bndrs ->
595 tcExtendKindEnvTvs bndrs (thing_inside bndrs)
597 kcHsTyVar :: HsTyVarBndr Name -> TcM (HsTyVarBndr Name)
598 -- Return a *kind-annotated* binder, and a tyvar with a mutable kind in it
599 kcHsTyVar (UserTyVar name) = newKindVar `thenM` \ kind ->
600 returnM (KindedTyVar name kind)
601 kcHsTyVar (KindedTyVar name kind) = returnM (KindedTyVar name kind)
604 tcTyVarBndrs :: [LHsTyVarBndr Name] -- Kind-annotated binders, which need kind-zonking
605 -> ([TyVar] -> TcM r)
607 -- Used when type-checking types/classes/type-decls
608 -- Brings into scope immutable TyVars, not mutable ones that require later zonking
609 tcTyVarBndrs bndrs thing_inside
610 = mapM (zonk . unLoc) bndrs `thenM` \ tyvars ->
611 tcExtendTyVarEnv tyvars (thing_inside tyvars)
613 zonk (KindedTyVar name kind) = do { kind' <- zonkTcKindToKind kind
614 ; return (mkTyVar name kind') }
615 zonk (UserTyVar name) = pprTrace "Un-kinded tyvar" (ppr name) $
616 return (mkTyVar name liftedTypeKind)
618 -----------------------------------
619 tcDataKindSig :: Maybe Kind -> TcM [TyVar]
620 -- GADT decls can have a (perhaps partial) kind signature
621 -- e.g. data T :: * -> * -> * where ...
622 -- This function makes up suitable (kinded) type variables for
623 -- the argument kinds, and checks that the result kind is indeed *.
624 -- We use it also to make up argument type variables for for data instances.
625 tcDataKindSig Nothing = return []
626 tcDataKindSig (Just kind)
627 = do { checkTc (isLiftedTypeKind res_kind) (badKindSig kind)
628 ; span <- getSrcSpanM
629 ; us <- newUniqueSupply
630 ; let loc = srcSpanStart span
631 uniqs = uniqsFromSupply us
632 ; return [ mk_tv loc uniq str kind
633 | ((kind, str), uniq) <- arg_kinds `zip` names `zip` uniqs ] }
635 (arg_kinds, res_kind) = splitKindFunTys kind
636 mk_tv loc uniq str kind = mkTyVar name kind
638 name = mkInternalName uniq occ loc
639 occ = mkOccName tvName str
641 names :: [String] -- a,b,c...aa,ab,ac etc
642 names = [ c:cs | cs <- "" : names, c <- ['a'..'z'] ]
644 badKindSig :: Kind -> SDoc
646 = hang (ptext SLIT("Kind signature on data type declaration has non-* return kind"))
651 %************************************************************************
653 Scoped type variables
655 %************************************************************************
658 tcAddScopedTyVars is used for scoped type variables added by pattern
660 e.g. \ ((x::a), (y::a)) -> x+y
661 They never have explicit kinds (because this is source-code only)
662 They are mutable (because they can get bound to a more specific type).
664 Usually we kind-infer and expand type splices, and then
665 tupecheck/desugar the type. That doesn't work well for scoped type
666 variables, because they scope left-right in patterns. (e.g. in the
667 example above, the 'a' in (y::a) is bound by the 'a' in (x::a).
669 The current not-very-good plan is to
670 * find all the types in the patterns
671 * find their free tyvars
673 * bring the kinded type vars into scope
674 * BUT throw away the kind-checked type
675 (we'll kind-check it again when we type-check the pattern)
677 This is bad because throwing away the kind checked type throws away
678 its splices. But too bad for now. [July 03]
681 We no longer specify that these type variables must be univerally
682 quantified (lots of email on the subject). If you want to put that
684 a) Do a checkSigTyVars after thing_inside
685 b) More insidiously, don't pass in expected_ty, else
686 we unify with it too early and checkSigTyVars barfs
687 Instead you have to pass in a fresh ty var, and unify
688 it with expected_ty afterwards
691 tcHsPatSigType :: UserTypeCtxt
692 -> LHsType Name -- The type signature
693 -> TcM ([TyVar], -- Newly in-scope type variables
694 Type) -- The signature
695 -- Used for type-checking type signatures in
696 -- (a) patterns e.g f (x::Int) = e
697 -- (b) result signatures e.g. g x :: Int = e
698 -- (c) RULE forall bndrs e.g. forall (x::Int). f x = x
700 tcHsPatSigType ctxt hs_ty
701 = addErrCtxt (pprHsSigCtxt ctxt hs_ty) $
702 do { -- Find the type variables that are mentioned in the type
703 -- but not already in scope. These are the ones that
704 -- should be bound by the pattern signature
705 in_scope <- getInLocalScope
706 ; let span = getLoc hs_ty
707 sig_tvs = [ L span (UserTyVar n)
708 | n <- nameSetToList (extractHsTyVars hs_ty),
711 -- Behave very like type-checking (HsForAllTy sig_tvs hs_ty),
712 -- except that we want to keep the tvs separate
713 ; (kinded_tvs, kinded_ty) <- kcHsTyVars sig_tvs $ \ kinded_tvs -> do
714 { kinded_ty <- kcTypeType hs_ty
715 ; return (kinded_tvs, kinded_ty) }
716 ; tcTyVarBndrs kinded_tvs $ \ tyvars -> do
717 { sig_ty <- dsHsType kinded_ty
718 ; checkValidType ctxt sig_ty
719 ; return (tyvars, sig_ty)
722 tcPatSig :: UserTypeCtxt
725 -> TcM (TcType, -- The type to use for "inside" the signature
726 [(Name,TcType)]) -- The new bit of type environment, binding
727 -- the scoped type variables
728 tcPatSig ctxt sig res_ty
729 = do { (sig_tvs, sig_ty) <- tcHsPatSigType ctxt sig
731 ; if null sig_tvs then do {
732 -- The type signature binds no type variables,
733 -- and hence is rigid, so use it to zap the res_ty
734 boxyUnify sig_ty res_ty
735 ; return (sig_ty, [])
738 -- Type signature binds at least one scoped type variable
740 -- A pattern binding cannot bind scoped type variables
741 -- The renamer fails with a name-out-of-scope error
742 -- if a pattern binding tries to bind a type variable,
743 -- So we just have an ASSERT here
744 ; let in_pat_bind = case ctxt of
745 BindPatSigCtxt -> True
747 ; ASSERT( not in_pat_bind || null sig_tvs ) return ()
749 -- Check that pat_ty is rigid
750 ; checkTc (isRigidTy res_ty) (wobblyPatSig sig_tvs)
752 -- Now match the pattern signature against res_ty
753 -- For convenience, and uniform-looking error messages
754 -- we do the matching by allocating meta type variables,
755 -- unifying, and reading out the results.
756 -- This is a strictly local operation.
757 ; box_tvs <- mapM tcInstBoxyTyVar sig_tvs
758 ; boxyUnify (substTyWith sig_tvs (mkTyVarTys box_tvs) sig_ty) res_ty
759 ; sig_tv_tys <- mapM readFilledBox box_tvs
761 -- Check that each is bound to a distinct type variable,
762 -- and one that is not already in scope
763 ; let tv_binds = map tyVarName sig_tvs `zip` sig_tv_tys
764 ; binds_in_scope <- getScopedTyVarBinds
765 ; check binds_in_scope tv_binds
768 ; return (res_ty, tv_binds)
771 check in_scope [] = return ()
772 check in_scope ((n,ty):rest) = do { check_one in_scope n ty
773 ; check ((n,ty):in_scope) rest }
775 check_one in_scope n ty
776 = do { checkTc (tcIsTyVarTy ty) (scopedNonVar n ty)
777 -- Must bind to a type variable
779 ; checkTc (null dups) (dupInScope n (head dups) ty)
780 -- Must not bind to the same type variable
781 -- as some other in-scope type variable
785 dups = [n' | (n',ty') <- in_scope, tcEqType ty' ty]
789 %************************************************************************
791 Scoped type variables
793 %************************************************************************
796 pprHsSigCtxt :: UserTypeCtxt -> LHsType Name -> SDoc
797 pprHsSigCtxt ctxt hs_ty = vcat [ ptext SLIT("In") <+> pprUserTypeCtxt ctxt <> colon,
798 nest 2 (pp_sig ctxt) ]
800 pp_sig (FunSigCtxt n) = pp_n_colon n
801 pp_sig (ConArgCtxt n) = pp_n_colon n
802 pp_sig (ForSigCtxt n) = pp_n_colon n
803 pp_sig (RuleSigCtxt n) = pp_n_colon n
804 pp_sig other = ppr (unLoc hs_ty)
806 pp_n_colon n = ppr n <+> dcolon <+> ppr (unLoc hs_ty)
810 = hang (ptext SLIT("A pattern type signature cannot bind scoped type variables")
811 <+> pprQuotedList sig_tvs)
812 2 (ptext SLIT("unless the pattern has a rigid type context"))
815 = vcat [sep [ptext SLIT("The scoped type variable") <+> quotes (ppr n),
816 nest 2 (ptext SLIT("is bound to the type") <+> quotes (ppr ty))],
817 nest 2 (ptext SLIT("You can only bind scoped type variables to type variables"))]
820 = hang (ptext SLIT("The scoped type variables") <+> quotes (ppr n) <+> ptext SLIT("and") <+> quotes (ppr n'))
821 2 (vcat [ptext SLIT("are bound to the same type (variable)"),
822 ptext SLIT("Distinct scoped type variables must be distinct")])