2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcMonoType]{Typechecking user-specified @MonoTypes@}
7 module TcMonoType ( tcHsType, tcHsSigType, tcHsBoxedSigType,
8 tcContext, tcClassContext,
11 kcHsTyVar, kcHsTyVars, mkTyClTyVars,
12 kcHsType, kcHsSigType, kcHsBoxedSigType, kcHsContext,
13 tcTyVars, tcHsTyVars, mkImmutTyVars,
15 TcSigInfo(..), tcTySig, mkTcSig, maybeSig,
16 checkSigTyVars, sigCtxt, sigPatCtxt
19 #include "HsVersions.h"
21 import HsSyn ( HsType(..), HsTyVarBndr(..), HsUsageAnn(..),
22 Sig(..), HsPred(..), pprParendHsType, HsTupCon(..), hsTyVarNames )
23 import RnHsSyn ( RenamedHsType, RenamedHsPred, RenamedContext, RenamedSig )
24 import TcHsSyn ( TcId )
27 import TcEnv ( tcExtendTyVarEnv, tcLookupTy, tcGetValueEnv, tcGetInScopeTyVars,
28 tcExtendUVarEnv, tcLookupUVar,
29 tcGetGlobalTyVars, valueEnvIds,
30 TyThing(..), tcExtendKindEnv
32 import TcType ( 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, UsageAnn(..),
41 mkTyVarTy, mkTyVarTys, mkFunTy, mkSynTy, mkUsgTy,
42 mkUsForAllTy, 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
51 import PprType ( pprType, pprPred )
52 import Subst ( mkTopTyVarSubst, substTy )
53 import Id ( mkVanillaId, idName, idType, idFreeTyVars )
54 import Var ( TyVar, mkTyVar, tyVarKind, mkNamedUVar )
57 import ErrUtils ( Message )
58 import TyCon ( TyCon, isSynTyCon, tyConArity, tyConKind, tyConName )
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(..) )
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 s a -- The kind checker
122 -> ([TyVar] -> TcM s 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 s 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 s (name, TcKind)
152 kcHsTyVars :: [HsTyVarBndr name] -> NF_TcM s [(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 s ()
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 s ()
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 s ()
180 -- Used for type signatures
181 kcHsSigType = kcTypeType
182 kcHsBoxedSigType = kcBoxedType
184 ---------------------------
185 kcHsType :: RenamedHsType -> TcM s TcKind
186 kcHsType (HsTyVar name) = kcTyVar name
187 kcHsType (HsUsgTy _ ty) = kcHsType ty
188 kcHsType (HsUsgForAllTy _ ty) = kcHsType ty
190 kcHsType (HsListTy ty)
191 = kcBoxedType ty `thenTc` \ tau_ty ->
192 returnTc boxedTypeKind
194 kcHsType (HsTupleTy (HsTupCon _ Boxed) tys)
195 = mapTc kcBoxedType tys `thenTc_`
196 returnTc boxedTypeKind
198 kcHsType ty@(HsTupleTy (HsTupCon _ Unboxed) tys)
199 = failWithTc (unboxedTupleErr ty)
200 -- Unboxed tuples are illegal everywhere except
201 -- just after a function arrow (see kcFunResType)
203 kcHsType (HsFunTy ty1 ty2)
204 = kcTypeType ty1 `thenTc_`
205 kcFunResType ty2 `thenTc_`
206 returnTc boxedTypeKind
208 kcHsType ty@(HsOpTy ty1 op ty2)
209 = kcTyVar op `thenTc` \ op_kind ->
210 kcHsType ty1 `thenTc` \ ty1_kind ->
211 kcHsType ty2 `thenTc` \ ty2_kind ->
212 tcAddErrCtxt (appKindCtxt (ppr ty)) $
213 kcAppKind op_kind ty1_kind `thenTc` \ op_kind' ->
214 kcAppKind op_kind' ty2_kind
216 kcHsType (HsPredTy pred)
217 = kcHsPred pred `thenTc_`
218 returnTc boxedTypeKind
220 kcHsType ty@(HsAppTy ty1 ty2)
221 = kcHsType ty1 `thenTc` \ tc_kind ->
222 kcHsType ty2 `thenTc` \ arg_kind ->
223 tcAddErrCtxt (appKindCtxt (ppr ty)) $
224 kcAppKind tc_kind arg_kind
226 kcHsType (HsForAllTy (Just tv_names) context ty)
227 = kcHsTyVars tv_names `thenNF_Tc` \ kind_env ->
228 tcExtendKindEnv kind_env $
229 kcHsContext context `thenTc_`
231 -- Context behaves like a function type
232 -- This matters. Return-unboxed-tuple analysis can
233 -- give overloaded functions like
234 -- f :: forall a. Num a => (# a->a, a->a #)
235 -- And we want these to get through the type checker
239 kcFunResType ty `thenTc_`
240 returnTc boxedTypeKind
242 ---------------------------
244 = tcLookupTy name `thenTc` \ thing ->
246 ATyVar tv -> returnTc (tyVarKind tv)
247 ATyCon tc -> returnTc (tyConKind tc)
248 AThing k -> returnTc k
249 other -> failWithTc (wrongThingErr "type" thing name)
251 ---------------------------
252 kcFunResType :: RenamedHsType -> TcM s TcKind
253 -- The only place an unboxed tuple type is allowed
254 -- is at the right hand end of an arrow
255 kcFunResType (HsTupleTy (HsTupCon _ Unboxed) tys)
256 = mapTc kcTypeType tys `thenTc_`
257 returnTc unboxedTypeKind
259 kcFunResType ty = kcHsType ty
261 ---------------------------
262 kcAppKind fun_kind arg_kind
263 = case splitFunTy_maybe fun_kind of
264 Just (arg_kind', res_kind)
265 -> unifyKind arg_kind arg_kind' `thenTc_`
268 Nothing -> newKindVar `thenNF_Tc` \ res_kind ->
269 unifyKind fun_kind (mkArrowKind arg_kind res_kind) `thenTc_`
273 ---------------------------
274 kcHsContext ctxt = mapTc_ kcHsPred ctxt
276 kcHsPred :: RenamedHsPred -> TcM s ()
277 kcHsPred pred@(HsPIParam name ty)
278 = tcAddErrCtxt (appKindCtxt (ppr pred)) $
281 kcHsPred pred@(HsPClass cls tys)
282 = tcAddErrCtxt (appKindCtxt (ppr pred)) $
283 tcLookupTy cls `thenNF_Tc` \ thing ->
285 AClass cls -> returnTc (tyConKind (classTyCon cls))
286 AThing kind -> returnTc kind
287 other -> failWithTc (wrongThingErr "class" thing cls)) `thenTc` \ kind ->
288 mapTc kcHsType tys `thenTc` \ arg_kinds ->
289 unifyKind kind (mkArrowKinds arg_kinds boxedTypeKind)
292 %************************************************************************
294 \subsection{Checking types}
296 %************************************************************************
298 tcHsSigType and tcHsBoxedSigType
299 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
301 tcHsSigType and tcHsBoxedSigType are used for type signatures written by the programmer
303 * We hoist any inner for-alls to the top
305 * Notice that we kind-check first, because the type-check assumes
306 that the kinds are already checked.
308 * They are only called when there are no kind vars in the environment
309 so the kind returned is indeed a Kind not a TcKind
312 tcHsSigType :: RenamedHsType -> TcM s TcType
314 = kcTypeType ty `thenTc_`
315 tcHsType ty `thenTc` \ ty' ->
316 returnTc (hoistForAllTys ty')
318 tcHsBoxedSigType :: RenamedHsType -> TcM s Type
320 = kcBoxedType ty `thenTc_`
321 tcHsType ty `thenTc` \ ty' ->
322 returnTc (hoistForAllTys ty')
326 tcHsType, the main work horse
327 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
330 tcHsType :: RenamedHsType -> TcM s Type
331 tcHsType ty@(HsTyVar name)
334 tcHsType (HsListTy ty)
335 = tcHsType ty `thenTc` \ tau_ty ->
336 returnTc (mkListTy tau_ty)
338 tcHsType (HsTupleTy (HsTupCon _ boxity) tys)
339 = mapTc tcHsType tys `thenTc` \ tau_tys ->
340 returnTc (mkTupleTy boxity (length tys) tau_tys)
342 tcHsType (HsFunTy ty1 ty2)
343 = tcHsType ty1 `thenTc` \ tau_ty1 ->
344 tcHsType ty2 `thenTc` \ tau_ty2 ->
345 returnTc (mkFunTy tau_ty1 tau_ty2)
349 returnTc (mkTyConApp genUnitTyCon [])
351 tcHsType (HsOpTy ty1 op ty2) =
352 tcHsType ty1 `thenTc` \ tau_ty1 ->
353 tcHsType ty2 `thenTc` \ tau_ty2 ->
354 tc_fun_type op [tau_ty1,tau_ty2]
356 tcHsType (HsAppTy ty1 ty2)
359 tcHsType (HsPredTy pred)
360 = tcClassAssertion True pred `thenTc` \ pred' ->
361 returnTc (mkPredTy pred')
363 tcHsType (HsUsgTy usg ty)
364 = newUsg usg `thenTc` \ usg' ->
365 tcHsType ty `thenTc` \ tc_ty ->
366 returnTc (mkUsgTy usg' tc_ty)
368 newUsg usg = case usg of
369 HsUsOnce -> returnTc UsOnce
370 HsUsMany -> returnTc UsMany
371 HsUsVar uv_name -> tcLookupUVar uv_name `thenTc` \ uv ->
374 tcHsType (HsUsgForAllTy uv_name ty)
376 uv = mkNamedUVar uv_name
378 tcExtendUVarEnv uv_name uv $
379 tcHsType ty `thenTc` \ tc_ty ->
380 returnTc (mkUsForAllTy uv tc_ty)
382 tcHsType full_ty@(HsForAllTy (Just tv_names) ctxt ty)
384 kind_check = kcHsContext ctxt `thenTc_` kcFunResType ty
386 tcHsTyVars tv_names kind_check $ \ tyvars ->
387 tcContext ctxt `thenTc` \ theta ->
388 tcHsType ty `thenTc` \ tau ->
389 checkAmbiguity full_ty tyvars theta tau `thenTc_`
390 returnTc (mkSigmaTy tyvars theta tau)
392 -- Check for ambiguity
393 -- forall V. P => tau
394 -- is ambiguous if P contains generic variables
395 -- (i.e. one of the Vs) that are not mentioned in tau
397 -- However, we need to take account of functional dependencies
398 -- when we speak of 'mentioned in tau'. Example:
399 -- class C a b | a -> b where ...
401 -- forall x y. (C x y) => x
402 -- is not ambiguous because x is mentioned and x determines y
404 -- NOTE: In addition, GHC insists that at least one type variable
405 -- in each constraint is in V. So we disallow a type like
406 -- forall a. Eq b => b -> b
407 -- even in a scope where b is in scope.
408 -- This is the is_free test below.
410 checkAmbiguity full_ty forall_tyvars theta tau
411 = mapTc check_pred theta
413 tau_vars = tyVarsOfType tau
414 fds = instFunDepsOfTheta theta
415 tvFundep = tyVarFunDep fds
416 extended_tau_vars = oclose tvFundep tau_vars
418 is_ambig ct_var = (ct_var `elem` forall_tyvars) &&
419 not (ct_var `elemUFM` extended_tau_vars)
420 is_free ct_var = not (ct_var `elem` forall_tyvars)
422 check_pred pred = checkTc (not any_ambig) (ambigErr pred full_ty) `thenTc_`
423 checkTc (not all_free) (freeErr pred full_ty)
425 ct_vars = varSetElems (tyVarsOfPred pred)
426 all_free = all is_free ct_vars
427 any_ambig = is_source_polytype && any is_ambig ct_vars
429 -- Notes on the 'is_source_polytype' test above
430 -- Check ambiguity only for source-program types, not
431 -- for types coming from inteface files. The latter can
432 -- legitimately have ambiguous types. Example
433 -- class S a where s :: a -> (Int,Int)
434 -- instance S Char where s _ = (1,1)
435 -- f:: S a => [a] -> Int -> (Int,Int)
436 -- f (_::[a]) x = (a*x,b)
437 -- where (a,b) = s (undefined::a)
438 -- Here the worker for f gets the type
439 -- fw :: forall a. S a => Int -> (# Int, Int #)
441 -- If the list of tv_names is empty, we have a monotype,
442 -- and then we don't need to check for ambiguity either,
443 -- because the test can't fail (see is_ambig).
446 HsForAllTy (Just (UserTyVar _ : _)) _ _ -> True
450 Help functions for type applications
451 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
454 tc_app :: RenamedHsType -> [RenamedHsType] -> TcM s Type
455 tc_app (HsAppTy ty1 ty2) tys
456 = tc_app ty1 (ty2:tys)
459 = tcAddErrCtxt (appKindCtxt pp_app) $
460 mapTc tcHsType tys `thenTc` \ arg_tys ->
462 HsTyVar fun -> tc_fun_type fun arg_tys
463 other -> tcHsType ty `thenTc` \ fun_ty ->
464 returnNF_Tc (mkAppTys fun_ty arg_tys)
466 pp_app = ppr ty <+> sep (map pprParendHsType tys)
468 -- (tc_fun_type ty arg_tys) returns (mkAppTys ty arg_tys)
469 -- But not quite; for synonyms it checks the correct arity, and builds a SynTy
470 -- hence the rather strange functionality.
472 tc_fun_type name arg_tys
473 = tcLookupTy name `thenTc` \ thing ->
475 ATyVar tv -> returnTc (mkAppTys (mkTyVarTy tv) arg_tys)
477 ATyCon tc | isSynTyCon tc -> checkTc arity_ok err_msg `thenTc_`
478 returnTc (mkAppTys (mkSynTy tc (take arity arg_tys))
479 (drop arity arg_tys))
481 | otherwise -> returnTc (mkTyConApp tc arg_tys)
484 arity_ok = arity <= n_args
485 arity = tyConArity tc
486 -- It's OK to have an *over-applied* type synonym
487 -- data Tree a b = ...
488 -- type Foo a = Tree [a]
489 -- f :: Foo a b -> ...
490 err_msg = arityErr "Type synonym" name arity n_args
491 n_args = length arg_tys
493 other -> failWithTc (wrongThingErr "type constructor" thing name)
500 tcClassContext :: RenamedContext -> TcM s ClassContext
501 -- Used when we are expecting a ClassContext (i.e. no implicit params)
502 tcClassContext context
503 = tcContext context `thenTc` \ theta ->
504 returnTc (classesOfPreds theta)
506 tcContext :: RenamedContext -> TcM s ThetaType
507 tcContext context = mapTc (tcClassAssertion False) context
509 tcClassAssertion ccall_ok assn@(HsPClass class_name tys)
510 = tcAddErrCtxt (appKindCtxt (ppr assn)) $
511 mapTc tcHsType tys `thenTc` \ arg_tys ->
512 tcLookupTy class_name `thenTc` \ thing ->
514 AClass clas -> checkTc (arity == n_tys) err `thenTc_`
515 returnTc (Class clas arg_tys)
517 arity = classArity clas
519 err = arityErr "Class" class_name arity n_tys
521 other -> failWithTc (wrongThingErr "class" thing class_name)
523 tcClassAssertion ccall_ok assn@(HsPIParam name ty)
524 = tcAddErrCtxt (appKindCtxt (ppr assn)) $
525 tcHsType ty `thenTc` \ arg_ty ->
526 returnTc (IParam name arg_ty)
530 %************************************************************************
532 \subsection{Type variables, with knot tying!}
534 %************************************************************************
537 mkImmutTyVars :: [(Name,Kind)] -> [TyVar]
538 mkImmutTyVars pairs = [mkTyVar name kind | (name, kind) <- pairs]
540 mkTyClTyVars :: Kind -- Kind of the tycon or class
541 -> [HsTyVarBndr Name]
543 mkTyClTyVars kind tyvar_names
544 = mkImmutTyVars tyvars_w_kinds
546 (tyvars_w_kinds, _) = zipFunTys (hsTyVarNames tyvar_names) kind
550 %************************************************************************
552 \subsection{Signatures}
554 %************************************************************************
556 @tcSigs@ checks the signatures for validity, and returns a list of
557 {\em freshly-instantiated} signatures. That is, the types are already
558 split up, and have fresh type variables installed. All non-type-signature
559 "RenamedSigs" are ignored.
561 The @TcSigInfo@ contains @TcTypes@ because they are unified with
562 the variable's type, and after that checked to see whether they've
568 Name -- N, the Name in corresponding binding
570 TcId -- *Polymorphic* binder for this value...
577 TcId -- *Monomorphic* binder for this value
578 -- Does *not* have name = N
581 [Inst] -- Empty if theta is null, or
582 -- (method mono_id) otherwise
584 SrcLoc -- Of the signature
586 instance Outputable TcSigInfo where
587 ppr (TySigInfo nm id tyvars theta tau _ inst loc) =
588 ppr nm <+> ptext SLIT("::") <+> ppr tyvars <+> ppr theta <+> ptext SLIT("=>") <+> ppr tau
590 maybeSig :: [TcSigInfo] -> Name -> Maybe (TcSigInfo)
591 -- Search for a particular signature
592 maybeSig [] name = Nothing
593 maybeSig (sig@(TySigInfo sig_name _ _ _ _ _ _ _) : sigs) name
594 | name == sig_name = Just sig
595 | otherwise = maybeSig sigs name
600 tcTySig :: RenamedSig -> TcM s TcSigInfo
602 tcTySig (Sig v ty src_loc)
603 = tcAddSrcLoc src_loc $
604 tcAddErrCtxt (tcsigCtxt v) $
605 tcHsSigType ty `thenTc` \ sigma_tc_ty ->
606 mkTcSig (mkVanillaId v sigma_tc_ty) src_loc `thenNF_Tc` \ sig ->
609 mkTcSig :: TcId -> SrcLoc -> NF_TcM s TcSigInfo
610 mkTcSig poly_id src_loc
611 = -- Instantiate this type
612 -- It's important to do this even though in the error-free case
613 -- we could just split the sigma_tc_ty (since the tyvars don't
614 -- unified with anything). But in the case of an error, when
615 -- the tyvars *do* get unified with something, we want to carry on
616 -- typechecking the rest of the program with the function bound
617 -- to a pristine type, namely sigma_tc_ty
619 (tyvars, rho) = splitForAllTys (idType poly_id)
621 mapNF_Tc tcInstSigVar tyvars `thenNF_Tc` \ tyvars' ->
622 -- Make *signature* type variables
625 tyvar_tys' = mkTyVarTys tyvars'
626 rho' = substTy (mkTopTyVarSubst tyvars tyvar_tys') rho
627 -- mkTopTyVarSubst because the tyvars' are fresh
628 (theta', tau') = splitRhoTy rho'
629 -- This splitRhoTy tries hard to make sure that tau' is a type synonym
630 -- wherever possible, which can improve interface files.
632 newMethodWithGivenTy SignatureOrigin
635 theta' tau' `thenNF_Tc` \ inst ->
636 -- We make a Method even if it's not overloaded; no harm
637 instFunDeps SignatureOrigin theta' `thenNF_Tc` \ fds ->
639 returnNF_Tc (TySigInfo name poly_id tyvars' theta' tau' (instToIdBndr inst) (inst : fds) src_loc)
641 name = idName poly_id
646 %************************************************************************
648 \subsection{Checking signature type variables}
650 %************************************************************************
652 @checkSigTyVars@ is used after the type in a type signature has been unified with
653 the actual type found. It then checks that the type variables of the type signature
655 (a) Still all type variables
656 eg matching signature [a] against inferred type [(p,q)]
657 [then a will be unified to a non-type variable]
659 (b) Still all distinct
660 eg matching signature [(a,b)] against inferred type [(p,p)]
661 [then a and b will be unified together]
663 (c) Not mentioned in the environment
664 eg the signature for f in this:
670 Here, f is forced to be monorphic by the free occurence of x.
672 (d) Not (unified with another type variable that is) in scope.
673 eg f x :: (r->r) = (\y->y) :: forall a. a->r
674 when checking the expression type signature, we find that
675 even though there is nothing in scope whose type mentions r,
676 nevertheless the type signature for the expression isn't right.
678 Another example is in a class or instance declaration:
680 op :: forall b. a -> b
682 Here, b gets unified with a
684 Before doing this, the substitution is applied to the signature type variable.
686 We used to have the notion of a "DontBind" type variable, which would
687 only be bound to itself or nothing. Then points (a) and (b) were
688 self-checking. But it gave rise to bogus consequential error messages.
691 f = (*) -- Monomorphic
696 Here, we get a complaint when checking the type signature for g,
697 that g isn't polymorphic enough; but then we get another one when
698 dealing with the (Num x) context arising from f's definition;
699 we try to unify x with Int (to default it), but find that x has already
700 been unified with the DontBind variable "a" from g's signature.
701 This is really a problem with side-effecting unification; we'd like to
702 undo g's effects when its type signature fails, but unification is done
703 by side effect, so we can't (easily).
705 So we revert to ordinary type variables for signatures, and try to
706 give a helpful message in checkSigTyVars.
709 checkSigTyVars :: [TcTyVar] -- Universally-quantified type variables in the signature
710 -> TcTyVarSet -- Tyvars that are free in the type signature
711 -- These should *already* be in the global-var set, and are
712 -- used here only to improve the error message
713 -> TcM s [TcTyVar] -- Zonked signature type variables
715 checkSigTyVars [] free = returnTc []
717 checkSigTyVars sig_tyvars free_tyvars
718 = zonkTcTyVars sig_tyvars `thenNF_Tc` \ sig_tys ->
719 tcGetGlobalTyVars `thenNF_Tc` \ globals ->
721 checkTcM (all_ok sig_tys globals)
722 (complain sig_tys globals) `thenTc_`
724 returnTc (map (getTyVar "checkSigTyVars") sig_tys)
728 all_ok (ty:tys) acc = case getTyVar_maybe ty of
729 Nothing -> False -- Point (a)
730 Just tv | tv `elemVarSet` acc -> False -- Point (b) or (c)
731 | otherwise -> all_ok tys (acc `extendVarSet` tv)
734 complain sig_tys globals
735 = -- For the in-scope ones, zonk them and construct a map
736 -- from the zonked tyvar to the in-scope one
737 -- If any of the in-scope tyvars zonk to a type, then ignore them;
738 -- that'll be caught later when we back up to their type sig
739 tcGetInScopeTyVars `thenNF_Tc` \ in_scope_tvs ->
740 zonkTcTyVars in_scope_tvs `thenNF_Tc` \ in_scope_tys ->
742 in_scope_assoc = [ (zonked_tv, in_scope_tv)
743 | (z_ty, in_scope_tv) <- in_scope_tys `zip` in_scope_tvs,
744 Just zonked_tv <- [getTyVar_maybe z_ty]
746 in_scope_env = mkVarEnv in_scope_assoc
749 -- "check" checks each sig tyvar in turn
751 (env2, in_scope_env, [])
752 (tidy_tvs `zip` tidy_tys) `thenNF_Tc` \ (env3, _, msgs) ->
754 failWithTcM (env3, main_msg $$ nest 4 (vcat msgs))
756 (env1, tidy_tvs) = mapAccumL tidyTyVar emptyTidyEnv sig_tyvars
757 (env2, tidy_tys) = tidyOpenTypes env1 sig_tys
759 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
761 check (env, acc, msgs) (sig_tyvar,ty)
762 -- sig_tyvar is from the signature;
763 -- ty is what you get if you zonk sig_tyvar and then tidy it
765 -- acc maps a zonked type variable back to a signature type variable
766 = case getTyVar_maybe ty of {
767 Nothing -> -- Error (a)!
768 returnNF_Tc (env, acc, unify_msg sig_tyvar (ppr ty) : msgs) ;
772 case lookupVarEnv acc tv of {
773 Just sig_tyvar' -> -- Error (b) or (d)!
774 returnNF_Tc (env, acc, unify_msg sig_tyvar (ppr sig_tyvar') : msgs) ;
778 if tv `elemVarSet` globals -- Error (c)! Type variable escapes
779 -- The least comprehensible, so put it last
780 then tcGetValueEnv `thenNF_Tc` \ ve ->
781 find_globals tv env [] (valueEnvIds ve) `thenNF_Tc` \ (env1, globs) ->
782 find_frees tv env1 [] (varSetElems free_tyvars) `thenNF_Tc` \ (env2, frees) ->
783 returnNF_Tc (env2, acc, escape_msg sig_tyvar tv globs frees : msgs)
786 returnNF_Tc (env, extendVarEnv acc tv sig_tyvar, msgs)
789 -- find_globals looks at the value environment and finds values
790 -- whose types mention the offending type variable. It has to be
791 -- careful to zonk the Id's type first, so it has to be in the monad.
792 -- We must be careful to pass it a zonked type variable, too.
793 find_globals tv tidy_env acc []
794 = returnNF_Tc (tidy_env, acc)
796 find_globals tv tidy_env acc (id:ids)
797 | not (isLocallyDefined id) ||
798 isEmptyVarSet (idFreeTyVars id)
799 = find_globals tv tidy_env acc ids
802 = zonkTcType (idType id) `thenNF_Tc` \ id_ty ->
803 if tv `elemVarSet` tyVarsOfType id_ty then
805 (tidy_env', id_ty') = tidyOpenType tidy_env id_ty
806 acc' = (idName id, id_ty') : acc
808 find_globals tv tidy_env' acc' ids
810 find_globals tv tidy_env acc ids
812 find_frees tv tidy_env acc []
813 = returnNF_Tc (tidy_env, acc)
814 find_frees tv tidy_env acc (ftv:ftvs)
815 = zonkTcTyVar ftv `thenNF_Tc` \ ty ->
816 if tv `elemVarSet` tyVarsOfType ty then
818 (tidy_env', ftv') = tidyTyVar tidy_env ftv
820 find_frees tv tidy_env' (ftv':acc) ftvs
822 find_frees tv tidy_env acc ftvs
825 escape_msg sig_tv tv globs frees
826 = mk_msg sig_tv <+> ptext SLIT("escapes") $$
827 if not (null globs) then
828 vcat [pp_it <+> ptext SLIT("is mentioned in the environment"),
829 ptext SLIT("The following variables in the environment mention") <+> quotes (ppr tv),
830 nest 2 (vcat_first 10 [ppr name <+> dcolon <+> ppr ty | (name,ty) <- globs])
832 else if not (null frees) then
833 vcat [ptext SLIT("It is reachable from the type variable(s)") <+> pprQuotedList frees,
834 nest 2 (ptext SLIT("which") <+> is_are <+> ptext SLIT("free in the signature"))
837 empty -- Sigh. It's really hard to give a good error message
838 -- all the time. One bad case is an existential pattern match
840 is_are | isSingleton frees = ptext SLIT("is")
841 | otherwise = ptext SLIT("are")
842 pp_it | sig_tv /= tv = ptext SLIT("It unifies with") <+> quotes (ppr tv) <> comma <+> ptext SLIT("which")
843 | otherwise = ptext SLIT("It")
845 vcat_first :: Int -> [SDoc] -> SDoc
846 vcat_first n [] = empty
847 vcat_first 0 (x:xs) = text "...others omitted..."
848 vcat_first n (x:xs) = x $$ vcat_first (n-1) xs
850 unify_msg tv thing = mk_msg tv <+> ptext SLIT("is unified with") <+> quotes thing
851 mk_msg tv = ptext SLIT("Quantified type variable") <+> quotes (ppr tv)
854 These two context are used with checkSigTyVars
857 sigCtxt :: Message -> [TcTyVar] -> TcThetaType -> TcTauType
858 -> TidyEnv -> NF_TcM s (TidyEnv, Message)
859 sigCtxt when sig_tyvars sig_theta sig_tau tidy_env
860 = zonkTcType sig_tau `thenNF_Tc` \ actual_tau ->
862 (env1, tidy_sig_tyvars) = tidyTyVars tidy_env sig_tyvars
863 (env2, tidy_sig_rho) = tidyOpenType env1 (mkRhoTy sig_theta sig_tau)
864 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
865 msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tyvars tidy_sig_rho),
866 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau,
870 returnNF_Tc (env3, msg)
872 sigPatCtxt bound_tvs bound_ids tidy_env
874 sep [ptext SLIT("When checking a pattern that binds"),
875 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys))])
877 show_ids = filter is_interesting bound_ids
878 is_interesting id = any (`elemVarSet` idFreeTyVars id) bound_tvs
880 (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
881 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
882 -- Don't zonk the types so we get the separate, un-unified versions
886 %************************************************************************
888 \subsection{Errors and contexts}
890 %************************************************************************
893 tcsigCtxt v = ptext SLIT("In a type signature for") <+> quotes (ppr v)
895 typeKindCtxt :: RenamedHsType -> Message
896 typeKindCtxt ty = sep [ptext SLIT("When checking that"),
897 nest 2 (quotes (ppr ty)),
898 ptext SLIT("is a type")]
900 appKindCtxt :: SDoc -> Message
901 appKindCtxt pp = ptext SLIT("When checking kinds in") <+> quotes pp
903 wrongThingErr expected actual name
904 = pp_actual actual <+> quotes (ppr name) <+> ptext SLIT("used as a") <+> text expected
906 pp_actual (ATyCon _) = ptext SLIT("Type constructor")
907 pp_actual (AClass _) = ptext SLIT("Class")
908 pp_actual (ATyVar _) = ptext SLIT("Type variable")
909 pp_actual (AThing _) = ptext SLIT("Utterly bogus")
912 = sep [ptext SLIT("Ambiguous constraint") <+> quotes (pprPred pred),
913 nest 4 (ptext SLIT("for the type:") <+> ppr ty),
914 nest 4 (ptext SLIT("Each forall'd type variable mentioned by the constraint must appear after the =>"))]
917 = sep [ptext SLIT("The constraint") <+> quotes (pprPred pred) <+>
918 ptext SLIT("does not mention any of the universally quantified type variables"),
919 nest 4 (ptext SLIT("in the type") <+> quotes (ppr ty))
923 = sep [ptext (SLIT("Illegal unboxed tuple as a function or contructor argument:")), nest 4 (ppr ty)]