2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcMonoType]{Typechecking user-specified @MonoTypes@}
7 module TcMonoType ( tcHsType, tcHsRecType,
8 tcHsSigType, tcHsLiftedSigType,
9 tcRecClassContext, checkAmbiguity,
12 kcHsTyVar, kcHsTyVars, mkTyClTyVars,
13 kcHsType, kcHsSigType, kcHsLiftedSigType, 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, instToId )
37 import FunDeps ( grow )
38 import TcUnify ( unifyKind, unifyOpenTypeKind )
39 import Unify ( allDistinctTyVars )
40 import Type ( Type, Kind, PredType(..), ThetaType, SigmaType, TauType,
41 mkTyVarTy, mkTyVarTys, mkFunTy, mkSynTy,
42 zipFunTys, hoistForAllTys,
43 mkSigmaTy, mkPredTy, mkTyConApp,
44 mkAppTys, splitForAllTys, splitRhoTy, mkRhoTy,
45 liftedTypeKind, unliftedTypeKind, 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 CoreFVs ( idFreeTyVars )
54 import Id ( mkVanillaId, idName, idType )
55 import Var ( Id, Var, TyVar, mkTyVar, tyVarKind )
58 import ErrUtils ( Message )
59 import TyCon ( TyCon, isSynTyCon, tyConArity, tyConKind )
60 import Class ( ClassContext, classArity, classTyCon )
62 import TysWiredIn ( mkListTy, mkTupleTy, genUnitTyCon )
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 kcLiftedType :: RenamedHsType -> TcM ()
164 -- The type ty must be a *lifted* *type*
166 = kcHsType ty `thenTc` \ kind ->
167 tcAddErrCtxt (typeKindCtxt ty) $
168 unifyKind liftedTypeKind kind
170 ---------------------------
171 kcTypeType :: RenamedHsType -> TcM ()
172 -- The type ty must be a *type*, but it can be lifted or unlifted.
174 = kcHsType ty `thenTc` \ kind ->
175 tcAddErrCtxt (typeKindCtxt ty) $
176 unifyOpenTypeKind kind
178 ---------------------------
179 kcHsSigType, kcHsLiftedSigType :: RenamedHsType -> TcM ()
180 -- Used for type signatures
181 kcHsSigType = kcTypeType
182 kcHsLiftedSigType = kcLiftedType
184 ---------------------------
185 kcHsType :: RenamedHsType -> TcM TcKind
186 kcHsType (HsTyVar name) = kcTyVar name
188 kcHsType (HsListTy ty)
189 = kcLiftedType ty `thenTc` \ tau_ty ->
190 returnTc liftedTypeKind
192 kcHsType (HsTupleTy (HsTupCon _ boxity _) tys)
193 = mapTc kcTypeType tys `thenTc_`
194 returnTc (case boxity of
195 Boxed -> liftedTypeKind
196 Unboxed -> unliftedTypeKind)
198 kcHsType (HsFunTy ty1 ty2)
199 = kcTypeType ty1 `thenTc_`
200 kcTypeType ty2 `thenTc_`
201 returnTc liftedTypeKind
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 liftedTypeKind
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 liftedTypeKind
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 liftedTypeKind)
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 tcHsLiftedSigType
278 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
280 tcHsSigType and tcHsLiftedSigType 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, tcHsLiftedSigType :: RenamedHsType -> TcM Type
292 -- Do kind checking, and hoist for-alls to the top
293 tcHsSigType ty = kcTypeType ty `thenTc_` tcHsType ty
294 tcHsLiftedSigType ty = kcLiftedType 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 arity) tys)
348 = ASSERT( arity == length tys )
349 mapTc tc_tup_arg tys `thenTc` \ tau_tys ->
350 returnTc (mkTupleTy boxity arity tau_tys)
352 tc_tup_arg = case boxity of
353 Boxed -> tc_arg_type wimp_out
354 Unboxed -> tc_type wimp_out
355 -- Unboxed tuples can have polymorphic or unboxed args.
356 -- This happens in the workers for functions returning
357 -- product types with polymorphic components
359 tc_type wimp_out (HsFunTy ty1 ty2)
360 = tc_type wimp_out ty1 `thenTc` \ tau_ty1 ->
361 -- Function argument can be polymorphic, but
362 -- must not be an unboxed tuple
363 checkTc (not (isUnboxedTupleType tau_ty1))
364 (ubxArgTyErr ty1) `thenTc_`
365 tc_type wimp_out ty2 `thenTc` \ tau_ty2 ->
366 returnTc (mkFunTy tau_ty1 tau_ty2)
368 tc_type wimp_out (HsNumTy n)
370 returnTc (mkTyConApp genUnitTyCon [])
372 tc_type wimp_out (HsOpTy ty1 op ty2) =
373 tc_arg_type wimp_out ty1 `thenTc` \ tau_ty1 ->
374 tc_arg_type wimp_out ty2 `thenTc` \ tau_ty2 ->
375 tc_fun_type op [tau_ty1,tau_ty2]
377 tc_type wimp_out (HsAppTy ty1 ty2)
378 = tc_app wimp_out ty1 [ty2]
380 tc_type wimp_out (HsPredTy pred)
381 = tc_pred wimp_out pred `thenTc` \ pred' ->
382 returnTc (mkPredTy pred')
384 tc_type wimp_out full_ty@(HsForAllTy (Just tv_names) ctxt ty)
386 kind_check = kcHsContext ctxt `thenTc_` kcHsType ty
388 tcHsTyVars tv_names kind_check $ \ tyvars ->
389 tc_context wimp_out ctxt `thenTc` \ theta ->
391 -- Context behaves like a function type
392 -- This matters. Return-unboxed-tuple analysis can
393 -- give overloaded functions like
394 -- f :: forall a. Num a => (# a->a, a->a #)
395 -- And we want these to get through the type checker
397 tc_arg_type wimp_out ty
402 checkAmbiguity wimp_out is_source tyvars theta tau
404 is_source = case tv_names of
405 (UserTyVar _ : _) -> True
409 -- tc_arg_type checks that the argument of a
410 -- type appplication isn't a for-all type or an unboxed tuple type
411 -- For example, we want to reject things like:
413 -- instance Ord a => Ord (forall s. T s a)
415 -- g :: T s (forall b.b)
417 -- Other unboxed types are very occasionally allowed as type
418 -- arguments depending on the kind of the type constructor
420 tc_arg_type wimp_out arg_ty
422 = tc_type wimp_out arg_ty
425 = tc_type wimp_out arg_ty `thenTc` \ arg_ty' ->
426 checkTc (not (isForAllTy arg_ty')) (polyArgTyErr arg_ty) `thenTc_`
427 checkTc (not (isUnboxedTupleType arg_ty')) (ubxArgTyErr arg_ty) `thenTc_`
430 tc_arg_types wimp_out arg_tys = mapTc (tc_arg_type wimp_out) arg_tys
433 Help functions for type applications
434 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
437 tc_app :: RecFlag -> RenamedHsType -> [RenamedHsType] -> TcM Type
438 tc_app wimp_out (HsAppTy ty1 ty2) tys
439 = tc_app wimp_out ty1 (ty2:tys)
441 tc_app wimp_out ty tys
442 = tcAddErrCtxt (appKindCtxt pp_app) $
443 tc_arg_types wimp_out tys `thenTc` \ arg_tys ->
445 HsTyVar fun -> tc_fun_type fun arg_tys
446 other -> tc_type wimp_out ty `thenTc` \ fun_ty ->
447 returnNF_Tc (mkAppTys fun_ty arg_tys)
449 pp_app = ppr ty <+> sep (map pprParendHsType tys)
451 -- (tc_fun_type ty arg_tys) returns (mkAppTys ty arg_tys)
452 -- But not quite; for synonyms it checks the correct arity, and builds a SynTy
453 -- hence the rather strange functionality.
455 tc_fun_type name arg_tys
456 = tcLookup name `thenTc` \ thing ->
458 ATyVar tv -> returnTc (mkAppTys (mkTyVarTy tv) arg_tys)
461 | isSynTyCon tc -> checkTc arity_ok err_msg `thenTc_`
462 returnTc (mkAppTys (mkSynTy tc (take arity arg_tys))
463 (drop arity arg_tys))
465 | otherwise -> returnTc (mkTyConApp tc arg_tys)
468 arity_ok = arity <= n_args
469 arity = tyConArity tc
470 -- It's OK to have an *over-applied* type synonym
471 -- data Tree a b = ...
472 -- type Foo a = Tree [a]
473 -- f :: Foo a b -> ...
474 err_msg = arityErr "Type synonym" name arity n_args
475 n_args = length arg_tys
477 other -> failWithTc (wrongThingErr "type constructor" thing name)
484 tcRecClassContext :: RecFlag -> RenamedContext -> TcM ClassContext
485 -- Used when we are expecting a ClassContext (i.e. no implicit params)
486 tcRecClassContext wimp_out context
487 = tc_context wimp_out context `thenTc` \ theta ->
488 returnTc (classesOfPreds theta)
490 tc_context :: RecFlag -> RenamedContext -> TcM ThetaType
491 tc_context wimp_out context = mapTc (tc_pred wimp_out) context
493 tc_pred wimp_out assn@(HsPClass class_name tys)
494 = tcAddErrCtxt (appKindCtxt (ppr assn)) $
495 tc_arg_types wimp_out tys `thenTc` \ arg_tys ->
496 tcLookupGlobal class_name `thenTc` \ thing ->
498 AClass clas -> checkTc (arity == n_tys) err `thenTc_`
499 returnTc (Class clas arg_tys)
501 arity = classArity clas
503 err = arityErr "Class" class_name arity n_tys
505 other -> failWithTc (wrongThingErr "class" (AGlobal thing) class_name)
507 tc_pred wimp_out assn@(HsPIParam name ty)
508 = tcAddErrCtxt (appKindCtxt (ppr assn)) $
509 tc_arg_type wimp_out ty `thenTc` \ arg_ty ->
510 returnTc (IParam name arg_ty)
517 is ambiguous if P contains generic variables
518 (i.e. one of the Vs) that are not mentioned in tau
520 However, we need to take account of functional dependencies
521 when we speak of 'mentioned in tau'. Example:
522 class C a b | a -> b where ...
524 forall x y. (C x y) => x
525 is not ambiguous because x is mentioned and x determines y
527 NOTE: In addition, GHC insists that at least one type variable
528 in each constraint is in V. So we disallow a type like
529 forall a. Eq b => b -> b
530 even in a scope where b is in scope.
531 This is the is_free test below.
533 Notes on the 'is_source_polytype' test above
534 Check ambiguity only for source-program types, not
535 for types coming from inteface files. The latter can
536 legitimately have ambiguous types. Example
537 class S a where s :: a -> (Int,Int)
538 instance S Char where s _ = (1,1)
539 f:: S a => [a] -> Int -> (Int,Int)
540 f (_::[a]) x = (a*x,b)
541 where (a,b) = s (undefined::a)
542 Here the worker for f gets the type
543 fw :: forall a. S a => Int -> (# Int, Int #)
545 If the list of tv_names is empty, we have a monotype,
546 and then we don't need to check for ambiguity either,
547 because the test can't fail (see is_ambig).
550 checkAmbiguity :: RecFlag -> Bool
551 -> [TyVar] -> ThetaType -> TauType
553 checkAmbiguity wimp_out is_source_polytype forall_tyvars theta tau
554 | isRec wimp_out = returnTc sigma_ty
555 | otherwise = mapTc_ check_pred theta `thenTc_`
558 sigma_ty = mkSigmaTy forall_tyvars theta tau
559 tau_vars = tyVarsOfType tau
560 extended_tau_vars = grow theta tau_vars
562 is_ambig ct_var = (ct_var `elem` forall_tyvars) &&
563 not (ct_var `elemVarSet` extended_tau_vars)
564 is_free ct_var = not (ct_var `elem` forall_tyvars)
566 check_pred pred = checkTc (not any_ambig) (ambigErr pred sigma_ty) `thenTc_`
567 checkTc (is_ip pred || not all_free) (freeErr pred sigma_ty)
569 ct_vars = varSetElems (tyVarsOfPred pred)
570 all_free = all is_free ct_vars
571 any_ambig = is_source_polytype && any is_ambig ct_vars
572 is_ip (IParam _ _) = True
576 %************************************************************************
578 \subsection{Type variables, with knot tying!}
580 %************************************************************************
583 mkImmutTyVars :: [(Name,Kind)] -> [TyVar]
584 mkImmutTyVars pairs = [mkTyVar name kind | (name, kind) <- pairs]
586 mkTyClTyVars :: Kind -- Kind of the tycon or class
587 -> [HsTyVarBndr Name]
589 mkTyClTyVars kind tyvar_names
590 = mkImmutTyVars tyvars_w_kinds
592 (tyvars_w_kinds, _) = zipFunTys (hsTyVarNames tyvar_names) kind
596 %************************************************************************
598 \subsection{Signatures}
600 %************************************************************************
602 @tcSigs@ checks the signatures for validity, and returns a list of
603 {\em freshly-instantiated} signatures. That is, the types are already
604 split up, and have fresh type variables installed. All non-type-signature
605 "RenamedSigs" are ignored.
607 The @TcSigInfo@ contains @TcTypes@ because they are unified with
608 the variable's type, and after that checked to see whether they've
614 Name -- N, the Name in corresponding binding
616 TcId -- *Polymorphic* binder for this value...
623 TcId -- *Monomorphic* binder for this value
624 -- Does *not* have name = N
627 [Inst] -- Empty if theta is null, or
628 -- (method mono_id) otherwise
630 SrcLoc -- Of the signature
632 instance Outputable TcSigInfo where
633 ppr (TySigInfo nm id tyvars theta tau _ inst loc) =
634 ppr nm <+> ptext SLIT("::") <+> ppr tyvars <+> ppr theta <+> ptext SLIT("=>") <+> ppr tau
636 maybeSig :: [TcSigInfo] -> Name -> Maybe (TcSigInfo)
637 -- Search for a particular signature
638 maybeSig [] name = Nothing
639 maybeSig (sig@(TySigInfo sig_name _ _ _ _ _ _ _) : sigs) name
640 | name == sig_name = Just sig
641 | otherwise = maybeSig sigs name
646 tcTySig :: RenamedSig -> TcM TcSigInfo
648 tcTySig (Sig v ty src_loc)
649 = tcAddSrcLoc src_loc $
650 tcAddErrCtxt (tcsigCtxt v) $
651 tcHsSigType ty `thenTc` \ sigma_tc_ty ->
652 mkTcSig (mkVanillaId v sigma_tc_ty) src_loc `thenNF_Tc` \ sig ->
655 mkTcSig :: TcId -> SrcLoc -> NF_TcM TcSigInfo
656 mkTcSig poly_id src_loc
657 = -- Instantiate this type
658 -- It's important to do this even though in the error-free case
659 -- we could just split the sigma_tc_ty (since the tyvars don't
660 -- unified with anything). But in the case of an error, when
661 -- the tyvars *do* get unified with something, we want to carry on
662 -- typechecking the rest of the program with the function bound
663 -- to a pristine type, namely sigma_tc_ty
665 (tyvars, rho) = splitForAllTys (idType poly_id)
667 mapNF_Tc tcInstSigVar tyvars `thenNF_Tc` \ tyvars' ->
668 -- Make *signature* type variables
671 tyvar_tys' = mkTyVarTys tyvars'
672 rho' = substTy (mkTopTyVarSubst tyvars tyvar_tys') rho
673 -- mkTopTyVarSubst because the tyvars' are fresh
674 (theta', tau') = splitRhoTy rho'
675 -- This splitRhoTy tries hard to make sure that tau' is a type synonym
676 -- wherever possible, which can improve interface files.
678 newMethodWithGivenTy SignatureOrigin
681 theta' tau' `thenNF_Tc` \ inst ->
682 -- We make a Method even if it's not overloaded; no harm
684 returnNF_Tc (TySigInfo name poly_id tyvars' theta' tau' (instToId inst) [inst] src_loc)
686 name = idName poly_id
691 %************************************************************************
693 \subsection{Checking signature type variables}
695 %************************************************************************
697 @checkSigTyVars@ is used after the type in a type signature has been unified with
698 the actual type found. It then checks that the type variables of the type signature
700 (a) Still all type variables
701 eg matching signature [a] against inferred type [(p,q)]
702 [then a will be unified to a non-type variable]
704 (b) Still all distinct
705 eg matching signature [(a,b)] against inferred type [(p,p)]
706 [then a and b will be unified together]
708 (c) Not mentioned in the environment
709 eg the signature for f in this:
715 Here, f is forced to be monorphic by the free occurence of x.
717 (d) Not (unified with another type variable that is) in scope.
718 eg f x :: (r->r) = (\y->y) :: forall a. a->r
719 when checking the expression type signature, we find that
720 even though there is nothing in scope whose type mentions r,
721 nevertheless the type signature for the expression isn't right.
723 Another example is in a class or instance declaration:
725 op :: forall b. a -> b
727 Here, b gets unified with a
729 Before doing this, the substitution is applied to the signature type variable.
731 We used to have the notion of a "DontBind" type variable, which would
732 only be bound to itself or nothing. Then points (a) and (b) were
733 self-checking. But it gave rise to bogus consequential error messages.
736 f = (*) -- Monomorphic
741 Here, we get a complaint when checking the type signature for g,
742 that g isn't polymorphic enough; but then we get another one when
743 dealing with the (Num x) context arising from f's definition;
744 we try to unify x with Int (to default it), but find that x has already
745 been unified with the DontBind variable "a" from g's signature.
746 This is really a problem with side-effecting unification; we'd like to
747 undo g's effects when its type signature fails, but unification is done
748 by side effect, so we can't (easily).
750 So we revert to ordinary type variables for signatures, and try to
751 give a helpful message in checkSigTyVars.
754 checkSigTyVars :: [TcTyVar] -- Universally-quantified type variables in the signature
755 -> TcTyVarSet -- Tyvars that are free in the type signature
756 -- Not necessarily zonked
757 -- These should *already* be in the free-in-env set,
758 -- and are used here only to improve the error message
759 -> TcM [TcTyVar] -- Zonked signature type variables
761 checkSigTyVars [] free = returnTc []
762 checkSigTyVars sig_tyvars free_tyvars
763 = zonkTcTyVars sig_tyvars `thenNF_Tc` \ sig_tys ->
764 tcGetGlobalTyVars `thenNF_Tc` \ globals ->
766 checkTcM (allDistinctTyVars sig_tys globals)
767 (complain sig_tys globals) `thenTc_`
769 returnTc (map (getTyVar "checkSigTyVars") sig_tys)
772 complain sig_tys globals
773 = -- For the in-scope ones, zonk them and construct a map
774 -- from the zonked tyvar to the in-scope one
775 -- If any of the in-scope tyvars zonk to a type, then ignore them;
776 -- that'll be caught later when we back up to their type sig
777 tcGetEnv `thenNF_Tc` \ env ->
779 in_scope_tvs = tcEnvTyVars env
781 zonkTcTyVars in_scope_tvs `thenNF_Tc` \ in_scope_tys ->
783 in_scope_assoc = [ (zonked_tv, in_scope_tv)
784 | (z_ty, in_scope_tv) <- in_scope_tys `zip` in_scope_tvs,
785 Just zonked_tv <- [getTyVar_maybe z_ty]
787 in_scope_env = mkVarEnv in_scope_assoc
790 -- "check" checks each sig tyvar in turn
792 (env2, in_scope_env, [])
793 (tidy_tvs `zip` tidy_tys) `thenNF_Tc` \ (env3, _, msgs) ->
795 failWithTcM (env3, main_msg $$ nest 4 (vcat msgs))
797 (env1, tidy_tvs) = mapAccumL tidyTyVar emptyTidyEnv sig_tyvars
798 (env2, tidy_tys) = tidyOpenTypes env1 sig_tys
800 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
802 check (tidy_env, acc, msgs) (sig_tyvar,ty)
803 -- sig_tyvar is from the signature;
804 -- ty is what you get if you zonk sig_tyvar and then tidy it
806 -- acc maps a zonked type variable back to a signature type variable
807 = case getTyVar_maybe ty of {
808 Nothing -> -- Error (a)!
809 returnNF_Tc (tidy_env, acc, unify_msg sig_tyvar (ppr ty) : msgs) ;
813 case lookupVarEnv acc tv of {
814 Just sig_tyvar' -> -- Error (b) or (d)!
815 returnNF_Tc (tidy_env, acc, unify_msg sig_tyvar (ppr sig_tyvar') : msgs) ;
819 if tv `elemVarSet` globals -- Error (c)! Type variable escapes
820 -- The least comprehensible, so put it last
822 -- a) get the local TcIds from the environment,
823 -- and pass them to find_globals (they might have tv free)
824 -- b) similarly, find any free_tyvars that mention tv
825 then tcGetEnv `thenNF_Tc` \ ve ->
826 find_globals tv tidy_env [] (tcEnvTcIds ve) `thenNF_Tc` \ (tidy_env1, globs) ->
827 find_frees tv tidy_env1 [] (varSetElems free_tyvars) `thenNF_Tc` \ (tidy_env2, frees) ->
828 returnNF_Tc (tidy_env2, acc, escape_msg sig_tyvar tv globs frees : msgs)
831 returnNF_Tc (tidy_env, extendVarEnv acc tv sig_tyvar, msgs)
834 -- find_globals looks at the value environment and finds values
835 -- whose types mention the offending type variable. It has to be
836 -- careful to zonk the Id's type first, so it has to be in the monad.
837 -- We must be careful to pass it a zonked type variable, too.
843 -> NF_TcM (TidyEnv,[(Name,Type)])
845 find_globals tv tidy_env acc []
846 = returnNF_Tc (tidy_env, acc)
848 find_globals tv tidy_env acc (id:ids)
849 | isEmptyVarSet (idFreeTyVars id)
850 = find_globals tv tidy_env acc ids
853 = zonkTcType (idType id) `thenNF_Tc` \ id_ty ->
854 if tv `elemVarSet` tyVarsOfType id_ty then
856 (tidy_env', id_ty') = tidyOpenType tidy_env id_ty
857 acc' = (idName id, id_ty') : acc
859 find_globals tv tidy_env' acc' ids
861 find_globals tv tidy_env acc ids
863 find_frees tv tidy_env acc []
864 = returnNF_Tc (tidy_env, acc)
865 find_frees tv tidy_env acc (ftv:ftvs)
866 = zonkTcTyVar ftv `thenNF_Tc` \ ty ->
867 if tv `elemVarSet` tyVarsOfType ty then
869 (tidy_env', ftv') = tidyTyVar tidy_env ftv
871 find_frees tv tidy_env' (ftv':acc) ftvs
873 find_frees tv tidy_env acc ftvs
876 escape_msg sig_tv tv globs frees
877 = mk_msg sig_tv <+> ptext SLIT("escapes") $$
878 if not (null globs) then
879 vcat [pp_it <+> ptext SLIT("is mentioned in the environment"),
880 ptext SLIT("The following variables in the environment mention") <+> quotes (ppr tv),
881 nest 2 (vcat_first 10 [ppr name <+> dcolon <+> ppr ty | (name,ty) <- globs])
883 else if not (null frees) then
884 vcat [ptext SLIT("It is reachable from the type variable(s)") <+> pprQuotedList frees,
885 nest 2 (ptext SLIT("which") <+> is_are <+> ptext SLIT("free in the signature"))
888 empty -- Sigh. It's really hard to give a good error message
889 -- all the time. One bad case is an existential pattern match
891 is_are | isSingleton frees = ptext SLIT("is")
892 | otherwise = ptext SLIT("are")
893 pp_it | sig_tv /= tv = ptext SLIT("It unifies with") <+> quotes (ppr tv) <> comma <+> ptext SLIT("which")
894 | otherwise = ptext SLIT("It")
896 vcat_first :: Int -> [SDoc] -> SDoc
897 vcat_first n [] = empty
898 vcat_first 0 (x:xs) = text "...others omitted..."
899 vcat_first n (x:xs) = x $$ vcat_first (n-1) xs
901 unify_msg tv thing = mk_msg tv <+> ptext SLIT("is unified with") <+> quotes thing
902 mk_msg tv = ptext SLIT("Quantified type variable") <+> quotes (ppr tv)
905 These two context are used with checkSigTyVars
908 sigCtxt :: Message -> [TcTyVar] -> TcThetaType -> TcTauType
909 -> TidyEnv -> NF_TcM (TidyEnv, Message)
910 sigCtxt when sig_tyvars sig_theta sig_tau tidy_env
911 = zonkTcType sig_tau `thenNF_Tc` \ actual_tau ->
913 (env1, tidy_sig_tyvars) = tidyTyVars tidy_env sig_tyvars
914 (env2, tidy_sig_rho) = tidyOpenType env1 (mkRhoTy sig_theta sig_tau)
915 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
916 msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tyvars tidy_sig_rho),
917 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau,
921 returnNF_Tc (env3, msg)
923 sigPatCtxt bound_tvs bound_ids tidy_env
925 sep [ptext SLIT("When checking a pattern that binds"),
926 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys))])
928 show_ids = filter is_interesting bound_ids
929 is_interesting id = any (`elemVarSet` idFreeTyVars id) bound_tvs
931 (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
932 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
933 -- Don't zonk the types so we get the separate, un-unified versions
937 %************************************************************************
939 \subsection{Errors and contexts}
941 %************************************************************************
944 tcsigCtxt v = ptext SLIT("In a type signature for") <+> quotes (ppr v)
946 typeKindCtxt :: RenamedHsType -> Message
947 typeKindCtxt ty = sep [ptext SLIT("When checking that"),
948 nest 2 (quotes (ppr ty)),
949 ptext SLIT("is a type")]
951 appKindCtxt :: SDoc -> Message
952 appKindCtxt pp = ptext SLIT("When checking kinds in") <+> quotes pp
954 wrongThingErr expected thing name
955 = pp_thing thing <+> quotes (ppr name) <+> ptext SLIT("used as a") <+> text expected
957 pp_thing (AGlobal (ATyCon _)) = ptext SLIT("Type constructor")
958 pp_thing (AGlobal (AClass _)) = ptext SLIT("Class")
959 pp_thing (AGlobal (AnId _)) = ptext SLIT("Identifier")
960 pp_thing (ATyVar _) = ptext SLIT("Type variable")
961 pp_thing (ATcId _) = ptext SLIT("Local identifier")
962 pp_thing (AThing _) = ptext SLIT("Utterly bogus")
965 = sep [ptext SLIT("Ambiguous constraint") <+> quotes (pprPred pred),
966 nest 4 (ptext SLIT("for the type:") <+> ppr ty),
967 nest 4 (ptext SLIT("At least one of the forall'd type variables mentioned by the constraint") $$
968 ptext SLIT("must be reachable from the type after the =>"))]
971 = sep [ptext SLIT("The constraint") <+> quotes (pprPred pred) <+>
972 ptext SLIT("does not mention any of the universally quantified type variables"),
973 nest 4 (ptext SLIT("in the type") <+> quotes (ppr ty))
976 polyArgTyErr ty = ptext SLIT("Illegal polymorphic type as argument:") <+> ppr ty
977 ubxArgTyErr ty = ptext SLIT("Illegal unboxed tuple type as argument:") <+> ppr ty