2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcMonoType]{Typechecking user-specified @MonoTypes@}
7 module TcMonoType ( tcHsType, tcHsRecType,
8 tcHsSigType, tcHsBoxedSigType,
9 tcRecClassContext, checkAmbiguity,
12 kcHsTyVar, kcHsTyVars, mkTyClTyVars,
13 kcHsType, kcHsSigType, kcHsBoxedSigType, kcHsContext,
14 tcTyVars, tcHsTyVars, mkImmutTyVars,
16 TcSigInfo(..), tcTySig, mkTcSig, maybeSig,
17 checkSigTyVars, sigCtxt, sigPatCtxt
20 #include "HsVersions.h"
22 import HsSyn ( HsType(..), HsTyVarBndr(..),
23 Sig(..), HsPred(..), pprParendHsType, HsTupCon(..), hsTyVarNames )
24 import RnHsSyn ( RenamedHsType, RenamedHsPred, RenamedContext, RenamedSig )
25 import TcHsSyn ( TcId )
28 import TcEnv ( tcExtendTyVarEnv, tcLookup, tcLookupGlobal,
29 tcGetGlobalTyVars, tcEnvTcIds, tcEnvTyVars,
30 TyThing(..), TcTyThing(..), tcExtendKindEnv
32 import TcType ( TcKind, TcTyVar, TcThetaType, TcTauType,
33 newKindVar, tcInstSigVar,
34 zonkKindEnv, zonkTcType, zonkTcTyVars, zonkTcTyVar
36 import Inst ( Inst, InstOrigin(..), newMethodWithGivenTy, instToIdBndr,
37 instFunDeps, instFunDepsOfTheta )
38 import FunDeps ( tyVarFunDep, oclose )
39 import TcUnify ( unifyKind, unifyOpenTypeKind )
40 import Type ( Type, Kind, PredType(..), ThetaType,
41 mkTyVarTy, mkTyVarTys, mkFunTy, mkSynTy,
42 zipFunTys, hoistForAllTys,
43 mkSigmaTy, mkPredTy, mkTyConApp,
44 mkAppTys, splitForAllTys, splitRhoTy, mkRhoTy,
45 boxedTypeKind, unboxedTypeKind, mkArrowKind,
46 mkArrowKinds, getTyVar_maybe, getTyVar, splitFunTy_maybe,
47 tidyOpenType, tidyOpenTypes, tidyTyVar, tidyTyVars,
48 tyVarsOfType, tyVarsOfPred, mkForAllTys,
49 classesOfPreds, isUnboxedTupleType, isForAllTy
51 import PprType ( pprType, pprPred )
52 import Subst ( mkTopTyVarSubst, substTy )
53 import Id ( mkVanillaId, idName, idType, idFreeTyVars )
54 import Var ( Id, Var, TyVar, mkTyVar, tyVarKind )
57 import ErrUtils ( Message )
58 import TyCon ( TyCon, isSynTyCon, tyConArity, tyConKind )
59 import Class ( ClassContext, classArity, classTyCon )
60 import Name ( Name, isLocallyDefined )
61 import TysWiredIn ( mkListTy, mkTupleTy, genUnitTyCon )
62 import UniqFM ( elemUFM )
63 import BasicTypes ( Boxity(..), RecFlag(..), isRec )
64 import SrcLoc ( SrcLoc )
65 import Util ( mapAccumL, isSingleton )
71 %************************************************************************
73 \subsection{Kind checking}
75 %************************************************************************
79 When we come across the binding site for some type variables, we
82 1. Figure out what kind each tyvar has
84 2. Create suitably-kinded tyvars,
86 and typecheck the body
88 To do step 1, we proceed thus:
90 1a. Bind each type variable to a kind variable
91 1b. Apply the kind checker
92 1c. Zonk the resulting kinds
94 The kind checker is passed to tcHsTyVars as an argument.
96 For example, when we find
97 (forall a m. m a -> m a)
98 we bind a,m to kind varibles and kind-check (m a -> m a). This
99 makes a get kind *, and m get kind *->*. Now we typecheck (m a -> m a)
100 in an environment that binds a and m suitably.
102 The kind checker passed to tcHsTyVars needs to look at enough to
103 establish the kind of the tyvar:
104 * For a group of type and class decls, it's just the group, not
105 the rest of the program
106 * For a tyvar bound in a pattern type signature, its the types
107 mentioned in the other type signatures in that bunch of patterns
108 * For a tyvar bound in a RULE, it's the type signatures on other
109 universally quantified variables in the rule
111 Note that this may occasionally give surprising results. For example:
113 data T a b = MkT (a b)
115 Here we deduce a::*->*, b::*.
116 But equally valid would be
117 a::(*->*)-> *, b::*->*
120 tcHsTyVars :: [HsTyVarBndr Name]
121 -> TcM a -- The kind checker
122 -> ([TyVar] -> TcM b)
125 tcHsTyVars [] kind_check thing_inside = thing_inside []
126 -- A useful short cut for a common case!
128 tcHsTyVars tv_names kind_check thing_inside
129 = kcHsTyVars tv_names `thenNF_Tc` \ tv_names_w_kinds ->
130 tcExtendKindEnv tv_names_w_kinds kind_check `thenTc_`
131 zonkKindEnv tv_names_w_kinds `thenNF_Tc` \ tvs_w_kinds ->
133 tyvars = mkImmutTyVars tvs_w_kinds
135 tcExtendTyVarEnv tyvars (thing_inside tyvars)
138 -> TcM a -- The kind checker
140 tcTyVars [] kind_check = returnTc []
142 tcTyVars tv_names kind_check
143 = mapNF_Tc newNamedKindVar tv_names `thenTc` \ kind_env ->
144 tcExtendKindEnv kind_env kind_check `thenTc_`
145 zonkKindEnv kind_env `thenNF_Tc` \ tvs_w_kinds ->
146 listNF_Tc [tcNewSigTyVar name kind | (name,kind) <- tvs_w_kinds]
151 kcHsTyVar :: HsTyVarBndr name -> NF_TcM (name, TcKind)
152 kcHsTyVars :: [HsTyVarBndr name] -> NF_TcM [(name, TcKind)]
154 kcHsTyVar (UserTyVar name) = newNamedKindVar name
155 kcHsTyVar (IfaceTyVar name kind) = returnNF_Tc (name, kind)
157 kcHsTyVars tvs = mapNF_Tc kcHsTyVar tvs
159 newNamedKindVar name = newKindVar `thenNF_Tc` \ kind ->
160 returnNF_Tc (name, kind)
162 ---------------------------
163 kcBoxedType :: RenamedHsType -> TcM ()
164 -- The type ty must be a *boxed* *type*
166 = kcHsType ty `thenTc` \ kind ->
167 tcAddErrCtxt (typeKindCtxt ty) $
168 unifyKind boxedTypeKind kind
170 ---------------------------
171 kcTypeType :: RenamedHsType -> TcM ()
172 -- The type ty must be a *type*, but it can be boxed or unboxed.
174 = kcHsType ty `thenTc` \ kind ->
175 tcAddErrCtxt (typeKindCtxt ty) $
176 unifyOpenTypeKind kind
178 ---------------------------
179 kcHsSigType, kcHsBoxedSigType :: RenamedHsType -> TcM ()
180 -- Used for type signatures
181 kcHsSigType = kcTypeType
182 kcHsBoxedSigType = kcBoxedType
184 ---------------------------
185 kcHsType :: RenamedHsType -> TcM TcKind
186 kcHsType (HsTyVar name) = kcTyVar name
188 kcHsType (HsListTy ty)
189 = kcBoxedType ty `thenTc` \ tau_ty ->
190 returnTc boxedTypeKind
192 kcHsType (HsTupleTy (HsTupCon _ boxity) tys)
193 = mapTc kcTypeType tys `thenTc_`
194 returnTc (case boxity of
195 Boxed -> boxedTypeKind
196 Unboxed -> unboxedTypeKind)
198 kcHsType (HsFunTy ty1 ty2)
199 = kcTypeType ty1 `thenTc_`
200 kcTypeType ty2 `thenTc_`
201 returnTc boxedTypeKind
203 kcHsType ty@(HsOpTy ty1 op ty2)
204 = kcTyVar op `thenTc` \ op_kind ->
205 kcHsType ty1 `thenTc` \ ty1_kind ->
206 kcHsType ty2 `thenTc` \ ty2_kind ->
207 tcAddErrCtxt (appKindCtxt (ppr ty)) $
208 kcAppKind op_kind ty1_kind `thenTc` \ op_kind' ->
209 kcAppKind op_kind' ty2_kind
211 kcHsType (HsPredTy pred)
212 = kcHsPred pred `thenTc_`
213 returnTc boxedTypeKind
215 kcHsType ty@(HsAppTy ty1 ty2)
216 = kcHsType ty1 `thenTc` \ tc_kind ->
217 kcHsType ty2 `thenTc` \ arg_kind ->
218 tcAddErrCtxt (appKindCtxt (ppr ty)) $
219 kcAppKind tc_kind arg_kind
221 kcHsType (HsForAllTy (Just tv_names) context ty)
222 = kcHsTyVars tv_names `thenNF_Tc` \ kind_env ->
223 tcExtendKindEnv kind_env $
224 kcHsContext context `thenTc_`
225 kcHsType ty `thenTc_`
226 returnTc boxedTypeKind
228 ---------------------------
229 kcAppKind fun_kind arg_kind
230 = case splitFunTy_maybe fun_kind of
231 Just (arg_kind', res_kind)
232 -> unifyKind arg_kind arg_kind' `thenTc_`
235 Nothing -> newKindVar `thenNF_Tc` \ res_kind ->
236 unifyKind fun_kind (mkArrowKind arg_kind res_kind) `thenTc_`
240 ---------------------------
241 kcHsContext ctxt = mapTc_ kcHsPred ctxt
243 kcHsPred :: RenamedHsPred -> TcM ()
244 kcHsPred pred@(HsPIParam name ty)
245 = tcAddErrCtxt (appKindCtxt (ppr pred)) $
248 kcHsPred pred@(HsPClass cls tys)
249 = tcAddErrCtxt (appKindCtxt (ppr pred)) $
250 kcClass cls `thenTc` \ kind ->
251 mapTc kcHsType tys `thenTc` \ arg_kinds ->
252 unifyKind kind (mkArrowKinds arg_kinds boxedTypeKind)
254 ---------------------------
255 kcTyVar name -- Could be a tyvar or a tycon
256 = tcLookup name `thenTc` \ thing ->
258 AThing kind -> returnTc kind
259 ATyVar tv -> returnTc (tyVarKind tv)
260 AGlobal (ATyCon tc) -> returnTc (tyConKind tc)
261 other -> failWithTc (wrongThingErr "type" thing name)
263 kcClass cls -- Must be a class
264 = tcLookup cls `thenNF_Tc` \ thing ->
266 AThing kind -> returnTc kind
267 AGlobal (AClass cls) -> returnTc (tyConKind (classTyCon cls))
268 other -> failWithTc (wrongThingErr "class" thing cls)
271 %************************************************************************
273 \subsection{Checking types}
275 %************************************************************************
277 tcHsSigType and tcHsBoxedSigType
278 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
280 tcHsSigType and tcHsBoxedSigType are used for type signatures written by the programmer
282 * We hoist any inner for-alls to the top
284 * Notice that we kind-check first, because the type-check assumes
285 that the kinds are already checked.
287 * They are only called when there are no kind vars in the environment
288 so the kind returned is indeed a Kind not a TcKind
291 tcHsSigType, tcHsBoxedSigType :: RenamedHsType -> TcM Type
292 -- Do kind checking, and hoist for-alls to the top
293 tcHsSigType ty = kcTypeType ty `thenTc_` tcHsType ty
294 tcHsBoxedSigType ty = kcBoxedType ty `thenTc_` tcHsType ty
296 tcHsType :: RenamedHsType -> TcM Type
297 tcHsRecType :: RecFlag -> RenamedHsType -> TcM Type
298 -- Don't do kind checking, but do hoist for-alls to the top
299 tcHsType ty = tc_type NonRecursive ty `thenTc` \ ty' -> returnTc (hoistForAllTys ty')
300 tcHsRecType wimp_out ty = tc_type wimp_out ty `thenTc` \ ty' -> returnTc (hoistForAllTys ty')
304 %************************************************************************
308 %************************************************************************
310 tc_type, the main work horse
311 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
317 tc_type is used to typecheck the types in the RHS of data
318 constructors. In the case of recursive data types, that means that
319 the type constructors themselves are (partly) black holes. e.g.
321 data T a = MkT a [T a]
323 While typechecking the [T a] on the RHS, T itself is not yet fully
324 defined. That in turn places restrictions on what you can check in
325 tcHsType; if you poke on too much you get a black hole. I keep
326 forgetting this, hence this warning!
328 The wimp_out argument tells when we are in a mutually-recursive
329 group of type declarations, so omit various checks else we
330 get a black hole. They'll be done again later, in TcTyClDecls.tcGroup.
332 --------------------------
333 *** END OF BIG WARNING ***
334 --------------------------
338 tc_type :: RecFlag -> RenamedHsType -> TcM Type
340 tc_type wimp_out ty@(HsTyVar name)
341 = tc_app wimp_out ty []
343 tc_type wimp_out (HsListTy ty)
344 = tc_arg_type wimp_out ty `thenTc` \ tau_ty ->
345 returnTc (mkListTy tau_ty)
347 tc_type wimp_out (HsTupleTy (HsTupCon _ boxity) tys)
348 = mapTc tc_tup_arg tys `thenTc` \ tau_tys ->
349 returnTc (mkTupleTy boxity (length tys) tau_tys)
351 tc_tup_arg = case boxity of
352 Boxed -> tc_arg_type wimp_out
353 Unboxed -> tc_type wimp_out
354 -- Unboxed tuples can have polymorphic or unboxed args.
355 -- This happens in the workers for functions returning
356 -- product types with polymorphic components
358 tc_type wimp_out (HsFunTy ty1 ty2)
359 = tc_type wimp_out ty1 `thenTc` \ tau_ty1 ->
360 -- Function argument can be polymorphic, but
361 -- must not be an unboxed tuple
362 checkTc (not (isUnboxedTupleType tau_ty1))
363 (ubxArgTyErr ty1) `thenTc_`
364 tc_type wimp_out ty2 `thenTc` \ tau_ty2 ->
365 returnTc (mkFunTy tau_ty1 tau_ty2)
367 tc_type wimp_out (HsNumTy n)
369 returnTc (mkTyConApp genUnitTyCon [])
371 tc_type wimp_out (HsOpTy ty1 op ty2) =
372 tc_arg_type wimp_out ty1 `thenTc` \ tau_ty1 ->
373 tc_arg_type wimp_out ty2 `thenTc` \ tau_ty2 ->
374 tc_fun_type op [tau_ty1,tau_ty2]
376 tc_type wimp_out (HsAppTy ty1 ty2)
377 = tc_app wimp_out ty1 [ty2]
379 tc_type wimp_out (HsPredTy pred)
380 = tc_pred wimp_out pred `thenTc` \ pred' ->
381 returnTc (mkPredTy pred')
383 tc_type wimp_out full_ty@(HsForAllTy (Just tv_names) ctxt ty)
385 kind_check = kcHsContext ctxt `thenTc_` kcHsType ty
387 tcHsTyVars tv_names kind_check $ \ tyvars ->
388 tc_context wimp_out ctxt `thenTc` \ theta ->
390 -- Context behaves like a function type
391 -- This matters. Return-unboxed-tuple analysis can
392 -- give overloaded functions like
393 -- f :: forall a. Num a => (# a->a, a->a #)
394 -- And we want these to get through the type checker
396 tc_arg_type wimp_out ty
401 checkAmbiguity wimp_out is_source tyvars theta tau
403 is_source = case tv_names of
404 (UserTyVar _ : _) -> True
408 -- tc_arg_type checks that the argument of a
409 -- type appplication isn't a for-all type or an unboxed tuple type
410 -- For example, we want to reject things like:
412 -- instance Ord a => Ord (forall s. T s a)
414 -- g :: T s (forall b.b)
416 -- Other unboxed types are very occasionally allowed as type
417 -- arguments depending on the kind of the type constructor
419 tc_arg_type wimp_out arg_ty
421 = tc_type wimp_out arg_ty
424 = tc_type wimp_out arg_ty `thenTc` \ arg_ty' ->
425 checkTc (not (isForAllTy arg_ty')) (polyArgTyErr arg_ty) `thenTc_`
426 checkTc (not (isUnboxedTupleType arg_ty')) (ubxArgTyErr arg_ty) `thenTc_`
429 tc_arg_types wimp_out arg_tys = mapTc (tc_arg_type wimp_out) arg_tys
432 Help functions for type applications
433 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
436 tc_app :: RecFlag -> RenamedHsType -> [RenamedHsType] -> TcM Type
437 tc_app wimp_out (HsAppTy ty1 ty2) tys
438 = tc_app wimp_out ty1 (ty2:tys)
440 tc_app wimp_out ty tys
441 = tcAddErrCtxt (appKindCtxt pp_app) $
442 tc_arg_types wimp_out tys `thenTc` \ arg_tys ->
444 HsTyVar fun -> tc_fun_type fun arg_tys
445 other -> tc_type wimp_out ty `thenTc` \ fun_ty ->
446 returnNF_Tc (mkAppTys fun_ty arg_tys)
448 pp_app = ppr ty <+> sep (map pprParendHsType tys)
450 -- (tc_fun_type ty arg_tys) returns (mkAppTys ty arg_tys)
451 -- But not quite; for synonyms it checks the correct arity, and builds a SynTy
452 -- hence the rather strange functionality.
454 tc_fun_type name arg_tys
455 = tcLookup name `thenTc` \ thing ->
457 ATyVar tv -> returnTc (mkAppTys (mkTyVarTy tv) arg_tys)
460 | isSynTyCon tc -> checkTc arity_ok err_msg `thenTc_`
461 returnTc (mkAppTys (mkSynTy tc (take arity arg_tys))
462 (drop arity arg_tys))
464 | otherwise -> returnTc (mkTyConApp tc arg_tys)
467 arity_ok = arity <= n_args
468 arity = tyConArity tc
469 -- It's OK to have an *over-applied* type synonym
470 -- data Tree a b = ...
471 -- type Foo a = Tree [a]
472 -- f :: Foo a b -> ...
473 err_msg = arityErr "Type synonym" name arity n_args
474 n_args = length arg_tys
476 other -> failWithTc (wrongThingErr "type constructor" thing name)
483 tcRecClassContext :: RecFlag -> RenamedContext -> TcM ClassContext
484 -- Used when we are expecting a ClassContext (i.e. no implicit params)
485 tcRecClassContext wimp_out context
486 = tc_context wimp_out context `thenTc` \ theta ->
487 returnTc (classesOfPreds theta)
489 tc_context :: RecFlag -> RenamedContext -> TcM ThetaType
490 tc_context wimp_out context = mapTc (tc_pred wimp_out) context
492 tc_pred wimp_out assn@(HsPClass class_name tys)
493 = tcAddErrCtxt (appKindCtxt (ppr assn)) $
494 tc_arg_types wimp_out tys `thenTc` \ arg_tys ->
495 tcLookupGlobal class_name `thenTc` \ thing ->
497 AClass clas -> checkTc (arity == n_tys) err `thenTc_`
498 returnTc (Class clas arg_tys)
500 arity = classArity clas
502 err = arityErr "Class" class_name arity n_tys
504 other -> failWithTc (wrongThingErr "class" (AGlobal thing) class_name)
506 tc_pred wimp_out assn@(HsPIParam name ty)
507 = tcAddErrCtxt (appKindCtxt (ppr assn)) $
508 tc_arg_type wimp_out ty `thenTc` \ arg_ty ->
509 returnTc (IParam name arg_ty)
516 is ambiguous if P contains generic variables
517 (i.e. one of the Vs) that are not mentioned in tau
519 However, we need to take account of functional dependencies
520 when we speak of 'mentioned in tau'. Example:
521 class C a b | a -> b where ...
523 forall x y. (C x y) => x
524 is not ambiguous because x is mentioned and x determines y
526 NOTE: In addition, GHC insists that at least one type variable
527 in each constraint is in V. So we disallow a type like
528 forall a. Eq b => b -> b
529 even in a scope where b is in scope.
530 This is the is_free test below.
532 Notes on the 'is_source_polytype' test above
533 Check ambiguity only for source-program types, not
534 for types coming from inteface files. The latter can
535 legitimately have ambiguous types. Example
536 class S a where s :: a -> (Int,Int)
537 instance S Char where s _ = (1,1)
538 f:: S a => [a] -> Int -> (Int,Int)
539 f (_::[a]) x = (a*x,b)
540 where (a,b) = s (undefined::a)
541 Here the worker for f gets the type
542 fw :: forall a. S a => Int -> (# Int, Int #)
544 If the list of tv_names is empty, we have a monotype,
545 and then we don't need to check for ambiguity either,
546 because the test can't fail (see is_ambig).
549 checkAmbiguity wimp_out is_source_polytype forall_tyvars theta tau
550 | isRec wimp_out = returnTc sigma_ty
551 | otherwise = mapTc_ check_pred theta `thenTc_`
554 sigma_ty = mkSigmaTy forall_tyvars theta tau
555 tau_vars = tyVarsOfType tau
556 fds = instFunDepsOfTheta theta
557 tvFundep = tyVarFunDep fds
558 extended_tau_vars = oclose tvFundep tau_vars
560 is_ambig ct_var = (ct_var `elem` forall_tyvars) &&
561 not (ct_var `elemUFM` extended_tau_vars)
562 is_free ct_var = not (ct_var `elem` forall_tyvars)
564 check_pred pred = checkTc (not any_ambig) (ambigErr pred sigma_ty) `thenTc_`
565 checkTc (is_ip pred || not all_free) (freeErr pred sigma_ty)
567 ct_vars = varSetElems (tyVarsOfPred pred)
568 all_free = all is_free ct_vars
569 any_ambig = is_source_polytype && any is_ambig ct_vars
570 is_ip (IParam _ _) = True
574 %************************************************************************
576 \subsection{Type variables, with knot tying!}
578 %************************************************************************
581 mkImmutTyVars :: [(Name,Kind)] -> [TyVar]
582 mkImmutTyVars pairs = [mkTyVar name kind | (name, kind) <- pairs]
584 mkTyClTyVars :: Kind -- Kind of the tycon or class
585 -> [HsTyVarBndr Name]
587 mkTyClTyVars kind tyvar_names
588 = mkImmutTyVars tyvars_w_kinds
590 (tyvars_w_kinds, _) = zipFunTys (hsTyVarNames tyvar_names) kind
594 %************************************************************************
596 \subsection{Signatures}
598 %************************************************************************
600 @tcSigs@ checks the signatures for validity, and returns a list of
601 {\em freshly-instantiated} signatures. That is, the types are already
602 split up, and have fresh type variables installed. All non-type-signature
603 "RenamedSigs" are ignored.
605 The @TcSigInfo@ contains @TcTypes@ because they are unified with
606 the variable's type, and after that checked to see whether they've
612 Name -- N, the Name in corresponding binding
614 TcId -- *Polymorphic* binder for this value...
621 TcId -- *Monomorphic* binder for this value
622 -- Does *not* have name = N
625 [Inst] -- Empty if theta is null, or
626 -- (method mono_id) otherwise
628 SrcLoc -- Of the signature
630 instance Outputable TcSigInfo where
631 ppr (TySigInfo nm id tyvars theta tau _ inst loc) =
632 ppr nm <+> ptext SLIT("::") <+> ppr tyvars <+> ppr theta <+> ptext SLIT("=>") <+> ppr tau
634 maybeSig :: [TcSigInfo] -> Name -> Maybe (TcSigInfo)
635 -- Search for a particular signature
636 maybeSig [] name = Nothing
637 maybeSig (sig@(TySigInfo sig_name _ _ _ _ _ _ _) : sigs) name
638 | name == sig_name = Just sig
639 | otherwise = maybeSig sigs name
644 tcTySig :: RenamedSig -> TcM TcSigInfo
646 tcTySig (Sig v ty src_loc)
647 = tcAddSrcLoc src_loc $
648 tcAddErrCtxt (tcsigCtxt v) $
649 tcHsSigType ty `thenTc` \ sigma_tc_ty ->
650 mkTcSig (mkVanillaId v sigma_tc_ty) src_loc `thenNF_Tc` \ sig ->
653 mkTcSig :: TcId -> SrcLoc -> NF_TcM TcSigInfo
654 mkTcSig poly_id src_loc
655 = -- Instantiate this type
656 -- It's important to do this even though in the error-free case
657 -- we could just split the sigma_tc_ty (since the tyvars don't
658 -- unified with anything). But in the case of an error, when
659 -- the tyvars *do* get unified with something, we want to carry on
660 -- typechecking the rest of the program with the function bound
661 -- to a pristine type, namely sigma_tc_ty
663 (tyvars, rho) = splitForAllTys (idType poly_id)
665 mapNF_Tc tcInstSigVar tyvars `thenNF_Tc` \ tyvars' ->
666 -- Make *signature* type variables
669 tyvar_tys' = mkTyVarTys tyvars'
670 rho' = substTy (mkTopTyVarSubst tyvars tyvar_tys') rho
671 -- mkTopTyVarSubst because the tyvars' are fresh
672 (theta', tau') = splitRhoTy rho'
673 -- This splitRhoTy tries hard to make sure that tau' is a type synonym
674 -- wherever possible, which can improve interface files.
676 newMethodWithGivenTy SignatureOrigin
679 theta' tau' `thenNF_Tc` \ inst ->
680 -- We make a Method even if it's not overloaded; no harm
681 instFunDeps SignatureOrigin theta' `thenNF_Tc` \ fds ->
683 returnNF_Tc (TySigInfo name poly_id tyvars' theta' tau' (instToIdBndr inst) (inst : fds) src_loc)
685 name = idName poly_id
690 %************************************************************************
692 \subsection{Checking signature type variables}
694 %************************************************************************
696 @checkSigTyVars@ is used after the type in a type signature has been unified with
697 the actual type found. It then checks that the type variables of the type signature
699 (a) Still all type variables
700 eg matching signature [a] against inferred type [(p,q)]
701 [then a will be unified to a non-type variable]
703 (b) Still all distinct
704 eg matching signature [(a,b)] against inferred type [(p,p)]
705 [then a and b will be unified together]
707 (c) Not mentioned in the environment
708 eg the signature for f in this:
714 Here, f is forced to be monorphic by the free occurence of x.
716 (d) Not (unified with another type variable that is) in scope.
717 eg f x :: (r->r) = (\y->y) :: forall a. a->r
718 when checking the expression type signature, we find that
719 even though there is nothing in scope whose type mentions r,
720 nevertheless the type signature for the expression isn't right.
722 Another example is in a class or instance declaration:
724 op :: forall b. a -> b
726 Here, b gets unified with a
728 Before doing this, the substitution is applied to the signature type variable.
730 We used to have the notion of a "DontBind" type variable, which would
731 only be bound to itself or nothing. Then points (a) and (b) were
732 self-checking. But it gave rise to bogus consequential error messages.
735 f = (*) -- Monomorphic
740 Here, we get a complaint when checking the type signature for g,
741 that g isn't polymorphic enough; but then we get another one when
742 dealing with the (Num x) context arising from f's definition;
743 we try to unify x with Int (to default it), but find that x has already
744 been unified with the DontBind variable "a" from g's signature.
745 This is really a problem with side-effecting unification; we'd like to
746 undo g's effects when its type signature fails, but unification is done
747 by side effect, so we can't (easily).
749 So we revert to ordinary type variables for signatures, and try to
750 give a helpful message in checkSigTyVars.
753 checkSigTyVars :: [TcTyVar] -- Universally-quantified type variables in the signature
754 -> TcTyVarSet -- Tyvars that are free in the type signature
755 -- These should *already* be in the global-var set, and are
756 -- used here only to improve the error message
757 -> TcM [TcTyVar] -- Zonked signature type variables
759 checkSigTyVars [] free = returnTc []
761 checkSigTyVars sig_tyvars free_tyvars
762 = zonkTcTyVars sig_tyvars `thenNF_Tc` \ sig_tys ->
763 tcGetGlobalTyVars `thenNF_Tc` \ globals ->
765 checkTcM (all_ok sig_tys globals)
766 (complain sig_tys globals) `thenTc_`
768 returnTc (map (getTyVar "checkSigTyVars") sig_tys)
772 all_ok (ty:tys) acc = case getTyVar_maybe ty of
773 Nothing -> False -- Point (a)
774 Just tv | tv `elemVarSet` acc -> False -- Point (b) or (c)
775 | otherwise -> all_ok tys (acc `extendVarSet` tv)
778 complain sig_tys globals
779 = -- For the in-scope ones, zonk them and construct a map
780 -- from the zonked tyvar to the in-scope one
781 -- If any of the in-scope tyvars zonk to a type, then ignore them;
782 -- that'll be caught later when we back up to their type sig
783 tcGetEnv `thenNF_Tc` \ env ->
785 in_scope_tvs = tcEnvTyVars env
787 zonkTcTyVars in_scope_tvs `thenNF_Tc` \ in_scope_tys ->
789 in_scope_assoc = [ (zonked_tv, in_scope_tv)
790 | (z_ty, in_scope_tv) <- in_scope_tys `zip` in_scope_tvs,
791 Just zonked_tv <- [getTyVar_maybe z_ty]
793 in_scope_env = mkVarEnv in_scope_assoc
796 -- "check" checks each sig tyvar in turn
798 (env2, in_scope_env, [])
799 (tidy_tvs `zip` tidy_tys) `thenNF_Tc` \ (env3, _, msgs) ->
801 failWithTcM (env3, main_msg $$ nest 4 (vcat msgs))
803 (env1, tidy_tvs) = mapAccumL tidyTyVar emptyTidyEnv sig_tyvars
804 (env2, tidy_tys) = tidyOpenTypes env1 sig_tys
806 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
808 check (tidy_env, acc, msgs) (sig_tyvar,ty)
809 -- sig_tyvar is from the signature;
810 -- ty is what you get if you zonk sig_tyvar and then tidy it
812 -- acc maps a zonked type variable back to a signature type variable
813 = case getTyVar_maybe ty of {
814 Nothing -> -- Error (a)!
815 returnNF_Tc (tidy_env, acc, unify_msg sig_tyvar (ppr ty) : msgs) ;
819 case lookupVarEnv acc tv of {
820 Just sig_tyvar' -> -- Error (b) or (d)!
821 returnNF_Tc (tidy_env, acc, unify_msg sig_tyvar (ppr sig_tyvar') : msgs) ;
825 if tv `elemVarSet` globals -- Error (c)! Type variable escapes
826 -- The least comprehensible, so put it last
828 -- a) get the local TcIds from the environment,
829 -- and pass them to find_globals (they might have tv free)
830 -- b) similarly, find any free_tyvars that mention tv
831 then tcGetEnv `thenNF_Tc` \ ve ->
832 find_globals tv tidy_env [] (tcEnvTcIds ve) `thenNF_Tc` \ (tidy_env1, globs) ->
833 find_frees tv tidy_env1 [] (varSetElems free_tyvars) `thenNF_Tc` \ (tidy_env2, frees) ->
834 returnNF_Tc (tidy_env2, acc, escape_msg sig_tyvar tv globs frees : msgs)
837 returnNF_Tc (tidy_env, extendVarEnv acc tv sig_tyvar, msgs)
840 -- find_globals looks at the value environment and finds values
841 -- whose types mention the offending type variable. It has to be
842 -- careful to zonk the Id's type first, so it has to be in the monad.
843 -- We must be careful to pass it a zonked type variable, too.
849 -> NF_TcM (TidyEnv,[(Name,Type)])
851 find_globals tv tidy_env acc []
852 = returnNF_Tc (tidy_env, acc)
854 find_globals tv tidy_env acc (id:ids)
855 | not (isLocallyDefined id) ||
856 isEmptyVarSet (idFreeTyVars id)
857 = find_globals tv tidy_env acc ids
860 = zonkTcType (idType id) `thenNF_Tc` \ id_ty ->
861 if tv `elemVarSet` tyVarsOfType id_ty then
863 (tidy_env', id_ty') = tidyOpenType tidy_env id_ty
864 acc' = (idName id, id_ty') : acc
866 find_globals tv tidy_env' acc' ids
868 find_globals tv tidy_env acc ids
870 find_frees tv tidy_env acc []
871 = returnNF_Tc (tidy_env, acc)
872 find_frees tv tidy_env acc (ftv:ftvs)
873 = zonkTcTyVar ftv `thenNF_Tc` \ ty ->
874 if tv `elemVarSet` tyVarsOfType ty then
876 (tidy_env', ftv') = tidyTyVar tidy_env ftv
878 find_frees tv tidy_env' (ftv':acc) ftvs
880 find_frees tv tidy_env acc ftvs
883 escape_msg sig_tv tv globs frees
884 = mk_msg sig_tv <+> ptext SLIT("escapes") $$
885 if not (null globs) then
886 vcat [pp_it <+> ptext SLIT("is mentioned in the environment"),
887 ptext SLIT("The following variables in the environment mention") <+> quotes (ppr tv),
888 nest 2 (vcat_first 10 [ppr name <+> dcolon <+> ppr ty | (name,ty) <- globs])
890 else if not (null frees) then
891 vcat [ptext SLIT("It is reachable from the type variable(s)") <+> pprQuotedList frees,
892 nest 2 (ptext SLIT("which") <+> is_are <+> ptext SLIT("free in the signature"))
895 empty -- Sigh. It's really hard to give a good error message
896 -- all the time. One bad case is an existential pattern match
898 is_are | isSingleton frees = ptext SLIT("is")
899 | otherwise = ptext SLIT("are")
900 pp_it | sig_tv /= tv = ptext SLIT("It unifies with") <+> quotes (ppr tv) <> comma <+> ptext SLIT("which")
901 | otherwise = ptext SLIT("It")
903 vcat_first :: Int -> [SDoc] -> SDoc
904 vcat_first n [] = empty
905 vcat_first 0 (x:xs) = text "...others omitted..."
906 vcat_first n (x:xs) = x $$ vcat_first (n-1) xs
908 unify_msg tv thing = mk_msg tv <+> ptext SLIT("is unified with") <+> quotes thing
909 mk_msg tv = ptext SLIT("Quantified type variable") <+> quotes (ppr tv)
912 These two context are used with checkSigTyVars
915 sigCtxt :: Message -> [TcTyVar] -> TcThetaType -> TcTauType
916 -> TidyEnv -> NF_TcM (TidyEnv, Message)
917 sigCtxt when sig_tyvars sig_theta sig_tau tidy_env
918 = zonkTcType sig_tau `thenNF_Tc` \ actual_tau ->
920 (env1, tidy_sig_tyvars) = tidyTyVars tidy_env sig_tyvars
921 (env2, tidy_sig_rho) = tidyOpenType env1 (mkRhoTy sig_theta sig_tau)
922 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
923 msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tyvars tidy_sig_rho),
924 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau,
928 returnNF_Tc (env3, msg)
930 sigPatCtxt bound_tvs bound_ids tidy_env
932 sep [ptext SLIT("When checking a pattern that binds"),
933 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys))])
935 show_ids = filter is_interesting bound_ids
936 is_interesting id = any (`elemVarSet` idFreeTyVars id) bound_tvs
938 (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
939 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
940 -- Don't zonk the types so we get the separate, un-unified versions
944 %************************************************************************
946 \subsection{Errors and contexts}
948 %************************************************************************
951 tcsigCtxt v = ptext SLIT("In a type signature for") <+> quotes (ppr v)
953 typeKindCtxt :: RenamedHsType -> Message
954 typeKindCtxt ty = sep [ptext SLIT("When checking that"),
955 nest 2 (quotes (ppr ty)),
956 ptext SLIT("is a type")]
958 appKindCtxt :: SDoc -> Message
959 appKindCtxt pp = ptext SLIT("When checking kinds in") <+> quotes pp
961 wrongThingErr expected thing name
962 = pp_thing thing <+> quotes (ppr name) <+> ptext SLIT("used as a") <+> text expected
964 pp_thing (AGlobal (ATyCon _)) = ptext SLIT("Type constructor")
965 pp_thing (AGlobal (AClass _)) = ptext SLIT("Class")
966 pp_thing (AGlobal (AnId _)) = ptext SLIT("Identifier")
967 pp_thing (ATyVar _) = ptext SLIT("Type variable")
968 pp_thing (ATcId _) = ptext SLIT("Local identifier")
969 pp_thing (AThing _) = ptext SLIT("Utterly bogus")
972 = sep [ptext SLIT("Ambiguous constraint") <+> quotes (pprPred pred),
973 nest 4 (ptext SLIT("for the type:") <+> ppr ty),
974 nest 4 (ptext SLIT("Each forall'd type variable mentioned by the constraint must appear after the =>"))]
977 = sep [ptext SLIT("The constraint") <+> quotes (pprPred pred) <+>
978 ptext SLIT("does not mention any of the universally quantified type variables"),
979 nest 4 (ptext SLIT("in the type") <+> quotes (ppr ty))
982 polyArgTyErr ty = ptext SLIT("Illegal polymorphic type as argument:") <+> ppr ty
983 ubxArgTyErr ty = ptext SLIT("Illegal unboxed tuple type as argument:") <+> ppr ty