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, checkAmbiguity,
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(..),
22 Sig(..), HsPred(..), pprParendHsType, HsTupCon(..), hsTyVarNames )
23 import RnHsSyn ( RenamedHsType, RenamedHsPred, RenamedContext, RenamedSig )
24 import TcHsSyn ( TcId )
27 import TcEnv ( tcExtendTyVarEnv, tcExtendKindEnv,
28 tcLookupGlobal, tcLookup,
29 tcEnvTcIds, tcEnvTyVars,
31 TyThing(..), TcTyThing(..)
33 import TcType ( TcType, TcKind, TcTyVar, TcThetaType, TcTauType,
34 newKindVar, tcInstSigVar,
35 zonkKindEnv, zonkTcType, zonkTcTyVars, zonkTcTyVar
37 import Inst ( Inst, InstOrigin(..), newMethodWithGivenTy, instToIdBndr,
38 instFunDeps, instFunDepsOfTheta )
39 import FunDeps ( tyVarFunDep, oclose )
40 import TcUnify ( unifyKind, unifyOpenTypeKind )
41 import Type ( Type, Kind, PredType(..), ThetaType,
42 mkTyVarTy, mkTyVarTys, mkFunTy, mkSynTy,
43 zipFunTys, hoistForAllTys,
44 mkSigmaTy, mkPredTy, mkTyConApp,
45 mkAppTys, splitForAllTys, splitRhoTy, mkRhoTy,
46 boxedTypeKind, unboxedTypeKind, mkArrowKind,
47 mkArrowKinds, getTyVar_maybe, getTyVar, splitFunTy_maybe,
48 tidyOpenType, tidyOpenTypes, tidyTyVar, tidyTyVars,
49 tyVarsOfType, tyVarsOfPred, mkForAllTys,
52 import PprType ( pprType, pprPred )
53 import Subst ( mkTopTyVarSubst, substTy )
54 import Id ( Id, mkVanillaId, idName, idType, idFreeTyVars )
55 import Var ( 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 UniqFM ( elemUFM )
64 import BasicTypes ( Boxity(..) )
65 import SrcLoc ( SrcLoc )
66 import Util ( mapAccumL, isSingleton )
68 import HscTypes ( TyThing(..) )
72 %************************************************************************
74 \subsection{Kind checking}
76 %************************************************************************
80 When we come across the binding site for some type variables, we
83 1. Figure out what kind each tyvar has
85 2. Create suitably-kinded tyvars,
87 and typecheck the body
89 To do step 1, we proceed thus:
91 1a. Bind each type variable to a kind variable
92 1b. Apply the kind checker
93 1c. Zonk the resulting kinds
95 The kind checker is passed to tcHsTyVars as an argument.
97 For example, when we find
98 (forall a m. m a -> m a)
99 we bind a,m to kind varibles and kind-check (m a -> m a). This
100 makes a get kind *, and m get kind *->*. Now we typecheck (m a -> m a)
101 in an environment that binds a and m suitably.
103 The kind checker passed to tcHsTyVars needs to look at enough to
104 establish the kind of the tyvar:
105 * For a group of type and class decls, it's just the group, not
106 the rest of the program
107 * For a tyvar bound in a pattern type signature, its the types
108 mentioned in the other type signatures in that bunch of patterns
109 * For a tyvar bound in a RULE, it's the type signatures on other
110 universally quantified variables in the rule
112 Note that this may occasionally give surprising results. For example:
114 data T a b = MkT (a b)
116 Here we deduce a::*->*, b::*.
117 But equally valid would be
118 a::(*->*)-> *, b::*->*
121 tcHsTyVars :: [HsTyVarBndr Name]
122 -> TcM a -- The kind checker
123 -> ([TyVar] -> TcM b)
126 tcHsTyVars [] kind_check thing_inside = thing_inside []
127 -- A useful short cut for a common case!
129 tcHsTyVars tv_names kind_check thing_inside
130 = kcHsTyVars tv_names `thenNF_Tc` \ tv_names_w_kinds ->
131 tcExtendKindEnv tv_names_w_kinds kind_check `thenTc_`
132 zonkKindEnv tv_names_w_kinds `thenNF_Tc` \ tvs_w_kinds ->
134 tyvars = mkImmutTyVars tvs_w_kinds
136 tcExtendTyVarEnv tyvars (thing_inside tyvars)
139 -> TcM a -- The kind checker
141 tcTyVars [] kind_check = returnTc []
143 tcTyVars tv_names kind_check
144 = mapNF_Tc newNamedKindVar tv_names `thenTc` \ kind_env ->
145 tcExtendKindEnv kind_env kind_check `thenTc_`
146 zonkKindEnv kind_env `thenNF_Tc` \ tvs_w_kinds ->
147 listNF_Tc [tcNewSigTyVar name kind | (name,kind) <- tvs_w_kinds]
152 kcHsTyVar :: HsTyVarBndr name -> NF_TcM (name, TcKind)
153 kcHsTyVars :: [HsTyVarBndr name] -> NF_TcM [(name, TcKind)]
155 kcHsTyVar (UserTyVar name) = newNamedKindVar name
156 kcHsTyVar (IfaceTyVar name kind) = returnNF_Tc (name, kind)
158 kcHsTyVars tvs = mapNF_Tc kcHsTyVar tvs
160 newNamedKindVar name = newKindVar `thenNF_Tc` \ kind ->
161 returnNF_Tc (name, kind)
163 ---------------------------
164 kcBoxedType :: RenamedHsType -> TcM ()
165 -- The type ty must be a *boxed* *type*
167 = kcHsType ty `thenTc` \ kind ->
168 tcAddErrCtxt (typeKindCtxt ty) $
169 unifyKind boxedTypeKind kind
171 ---------------------------
172 kcTypeType :: RenamedHsType -> TcM ()
173 -- The type ty must be a *type*, but it can be boxed or unboxed.
175 = kcHsType ty `thenTc` \ kind ->
176 tcAddErrCtxt (typeKindCtxt ty) $
177 unifyOpenTypeKind kind
179 ---------------------------
180 kcHsSigType, kcHsBoxedSigType :: RenamedHsType -> TcM ()
181 -- Used for type signatures
182 kcHsSigType = kcTypeType
183 kcHsBoxedSigType = kcBoxedType
185 ---------------------------
186 kcHsType :: RenamedHsType -> TcM TcKind
187 kcHsType (HsTyVar name) = kcTyVar name
188 kcHsType (HsUsgTy _ ty) = kcHsType ty
189 kcHsType (HsUsgForAllTy _ ty) = kcHsType ty
191 kcHsType (HsListTy ty)
192 = kcBoxedType ty `thenTc` \ tau_ty ->
193 returnTc boxedTypeKind
195 kcHsType (HsTupleTy (HsTupCon _ Boxed) tys)
196 = mapTc kcBoxedType tys `thenTc_`
197 returnTc boxedTypeKind
199 kcHsType ty@(HsTupleTy (HsTupCon _ Unboxed) tys)
200 = failWithTc (unboxedTupleErr ty)
201 -- Unboxed tuples are illegal everywhere except
202 -- just after a function arrow (see kcFunResType)
204 kcHsType (HsFunTy ty1 ty2)
205 = kcTypeType ty1 `thenTc_`
206 kcFunResType ty2 `thenTc_`
207 returnTc boxedTypeKind
209 kcHsType ty@(HsOpTy ty1 op ty2)
210 = kcTyVar op `thenTc` \ op_kind ->
211 kcHsType ty1 `thenTc` \ ty1_kind ->
212 kcHsType ty2 `thenTc` \ ty2_kind ->
213 tcAddErrCtxt (appKindCtxt (ppr ty)) $
214 kcAppKind op_kind ty1_kind `thenTc` \ op_kind' ->
215 kcAppKind op_kind' ty2_kind
217 kcHsType (HsPredTy pred)
218 = kcHsPred pred `thenTc_`
219 returnTc boxedTypeKind
221 kcHsType ty@(HsAppTy ty1 ty2)
222 = kcHsType ty1 `thenTc` \ tc_kind ->
223 kcHsType ty2 `thenTc` \ arg_kind ->
224 tcAddErrCtxt (appKindCtxt (ppr ty)) $
225 kcAppKind tc_kind arg_kind
227 kcHsType (HsForAllTy (Just tv_names) context ty)
228 = kcHsTyVars tv_names `thenNF_Tc` \ kind_env ->
229 tcExtendKindEnv kind_env $
230 kcHsContext context `thenTc_`
232 -- Context behaves like a function type
233 -- This matters. Return-unboxed-tuple analysis can
234 -- give overloaded functions like
235 -- f :: forall a. Num a => (# a->a, a->a #)
236 -- And we want these to get through the type checker
240 kcFunResType ty `thenTc_`
241 returnTc boxedTypeKind
243 ---------------------------
244 kcFunResType :: RenamedHsType -> TcM TcKind
245 -- The only place an unboxed tuple type is allowed
246 -- is at the right hand end of an arrow
247 kcFunResType (HsTupleTy (HsTupCon _ Unboxed) tys)
248 = mapTc kcTypeType tys `thenTc_`
249 returnTc unboxedTypeKind
251 kcFunResType ty = kcHsType ty
253 ---------------------------
254 kcAppKind fun_kind arg_kind
255 = case splitFunTy_maybe fun_kind of
256 Just (arg_kind', res_kind)
257 -> unifyKind arg_kind arg_kind' `thenTc_`
260 Nothing -> newKindVar `thenNF_Tc` \ res_kind ->
261 unifyKind fun_kind (mkArrowKind arg_kind res_kind) `thenTc_`
265 ---------------------------
266 kcHsContext ctxt = mapTc_ kcHsPred ctxt
268 kcHsPred :: RenamedHsPred -> TcM ()
269 kcHsPred pred@(HsPIParam name ty)
270 = tcAddErrCtxt (appKindCtxt (ppr pred)) $
273 kcHsPred pred@(HsPClass cls tys)
274 = tcAddErrCtxt (appKindCtxt (ppr pred)) $
275 kcClass cls `thenTc` \ kind ->
276 mapTc kcHsType tys `thenTc` \ arg_kinds ->
277 unifyKind kind (mkArrowKinds arg_kinds boxedTypeKind)
279 ---------------------------
280 kcTyVar name -- Could be a tyvar or a tycon
281 = tcLookup name `thenTc` \ thing ->
283 AThing kind -> returnTc kind
284 ATyVar tv -> returnTc (tyVarKind tv)
285 AGlobal (ATyCon tc) -> returnTc (tyConKind tc)
286 other -> failWithTc (wrongThingErr "type" thing name)
288 kcClass cls -- Must be a class
289 = tcLookup cls `thenNF_Tc` \ thing ->
291 AThing kind -> returnTc kind
292 AGlobal (AClass cls) -> returnTc (tyConKind (classTyCon cls))
293 other -> failWithTc (wrongThingErr "class" thing cls)
296 %************************************************************************
298 \subsection{Checking types}
300 %************************************************************************
302 tcHsSigType and tcHsBoxedSigType
303 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
305 tcHsSigType and tcHsBoxedSigType are used for type signatures written by the programmer
307 * We hoist any inner for-alls to the top
309 * Notice that we kind-check first, because the type-check assumes
310 that the kinds are already checked.
312 * They are only called when there are no kind vars in the environment
313 so the kind returned is indeed a Kind not a TcKind
316 tcHsSigType :: RenamedHsType -> TcM TcType
318 = kcTypeType ty `thenTc_`
319 tcHsType ty `thenTc` \ ty' ->
320 returnTc (hoistForAllTys ty')
322 tcHsBoxedSigType :: RenamedHsType -> TcM Type
324 = kcBoxedType ty `thenTc_`
325 tcHsType ty `thenTc` \ ty' ->
326 returnTc (hoistForAllTys ty')
330 tcHsType, the main work horse
331 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
334 tcHsType :: RenamedHsType -> TcM Type
335 tcHsType ty@(HsTyVar name)
338 tcHsType (HsListTy ty)
339 = tcHsType ty `thenTc` \ tau_ty ->
340 returnTc (mkListTy tau_ty)
342 tcHsType (HsTupleTy (HsTupCon _ boxity) tys)
343 = mapTc tcHsType tys `thenTc` \ tau_tys ->
344 returnTc (mkTupleTy boxity (length tys) tau_tys)
346 tcHsType (HsFunTy ty1 ty2)
347 = tcHsType ty1 `thenTc` \ tau_ty1 ->
348 tcHsType ty2 `thenTc` \ tau_ty2 ->
349 returnTc (mkFunTy tau_ty1 tau_ty2)
353 returnTc (mkTyConApp genUnitTyCon [])
355 tcHsType (HsOpTy ty1 op ty2) =
356 tcHsType ty1 `thenTc` \ tau_ty1 ->
357 tcHsType ty2 `thenTc` \ tau_ty2 ->
358 tc_fun_type op [tau_ty1,tau_ty2]
360 tcHsType (HsAppTy ty1 ty2)
363 tcHsType (HsPredTy pred)
364 = tcClassAssertion True pred `thenTc` \ pred' ->
365 returnTc (mkPredTy pred')
367 tcHsType full_ty@(HsForAllTy (Just tv_names) ctxt ty)
369 kind_check = kcHsContext ctxt `thenTc_` kcFunResType ty
371 tcHsTyVars tv_names kind_check $ \ tyvars ->
372 tcContext ctxt `thenTc` \ theta ->
373 tcHsType ty `thenTc` \ tau ->
374 checkAmbiguity full_ty tyvars theta tau `thenTc_`
375 returnTc (mkSigmaTy tyvars theta tau)
377 checkAmbiguity :: RenamedHsType -> [TyVar] -> ThetaType -> Type -> TcM ()
378 -- Check for ambiguity
379 -- forall V. P => tau
380 -- is ambiguous if P contains generic variables
381 -- (i.e. one of the Vs) that are not mentioned in tau
383 -- However, we need to take account of functional dependencies
384 -- when we speak of 'mentioned in tau'. Example:
385 -- class C a b | a -> b where ...
387 -- forall x y. (C x y) => x
388 -- is not ambiguous because x is mentioned and x determines y
390 -- NOTE: In addition, GHC insists that at least one type variable
391 -- in each constraint is in V. So we disallow a type like
392 -- forall a. Eq b => b -> b
393 -- even in a scope where b is in scope.
394 -- This is the is_free test below.
396 checkAmbiguity full_ty forall_tyvars theta tau
397 = mapTc_ check_pred theta
399 tau_vars = tyVarsOfType tau
400 fds = instFunDepsOfTheta theta
401 tvFundep = tyVarFunDep fds
402 extended_tau_vars = oclose tvFundep tau_vars
404 is_ambig ct_var = (ct_var `elem` forall_tyvars) &&
405 not (ct_var `elemUFM` extended_tau_vars)
406 is_free ct_var = not (ct_var `elem` forall_tyvars)
408 check_pred pred = checkTc (not any_ambig) (ambigErr pred full_ty) `thenTc_`
409 checkTc (not all_free) (freeErr pred full_ty)
411 ct_vars = varSetElems (tyVarsOfPred pred)
412 all_free = all is_free ct_vars
413 any_ambig = is_source_polytype && any is_ambig ct_vars
415 -- Notes on the 'is_source_polytype' test above
416 -- Check ambiguity only for source-program types, not
417 -- for types coming from inteface files. The latter can
418 -- legitimately have ambiguous types. Example
419 -- class S a where s :: a -> (Int,Int)
420 -- instance S Char where s _ = (1,1)
421 -- f:: S a => [a] -> Int -> (Int,Int)
422 -- f (_::[a]) x = (a*x,b)
423 -- where (a,b) = s (undefined::a)
424 -- Here the worker for f gets the type
425 -- fw :: forall a. S a => Int -> (# Int, Int #)
427 -- If the list of tv_names is empty, we have a monotype,
428 -- and then we don't need to check for ambiguity either,
429 -- because the test can't fail (see is_ambig).
432 HsForAllTy (Just (UserTyVar _ : _)) _ _ -> True
436 Help functions for type applications
437 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
440 tc_app :: RenamedHsType -> [RenamedHsType] -> TcM Type
441 tc_app (HsAppTy ty1 ty2) tys
442 = tc_app ty1 (ty2:tys)
445 = tcAddErrCtxt (appKindCtxt pp_app) $
446 mapTc tcHsType tys `thenTc` \ arg_tys ->
448 HsTyVar fun -> tc_fun_type fun arg_tys
449 other -> tcHsType ty `thenTc` \ fun_ty ->
450 returnNF_Tc (mkAppTys fun_ty arg_tys)
452 pp_app = ppr ty <+> sep (map pprParendHsType tys)
454 -- (tc_fun_type ty arg_tys) returns (mkAppTys ty arg_tys)
455 -- But not quite; for synonyms it checks the correct arity, and builds a SynTy
456 -- hence the rather strange functionality.
458 tc_fun_type name arg_tys
459 = tcLookup name `thenTc` \ thing ->
461 ATyVar tv -> returnTc (mkAppTys (mkTyVarTy tv) arg_tys)
464 | isSynTyCon tc -> checkTc arity_ok err_msg `thenTc_`
465 returnTc (mkAppTys (mkSynTy tc (take arity arg_tys))
466 (drop arity arg_tys))
468 | otherwise -> returnTc (mkTyConApp tc arg_tys)
471 arity_ok = arity <= n_args
472 arity = tyConArity tc
473 -- It's OK to have an *over-applied* type synonym
474 -- data Tree a b = ...
475 -- type Foo a = Tree [a]
476 -- f :: Foo a b -> ...
477 err_msg = arityErr "Type synonym" name arity n_args
478 n_args = length arg_tys
480 other -> failWithTc (wrongThingErr "type constructor" thing name)
487 tcClassContext :: RenamedContext -> TcM ClassContext
488 -- Used when we are expecting a ClassContext (i.e. no implicit params)
489 tcClassContext context
490 = tcContext context `thenTc` \ theta ->
491 returnTc (classesOfPreds theta)
493 tcContext :: RenamedContext -> TcM ThetaType
494 tcContext context = mapTc (tcClassAssertion False) context
496 tcClassAssertion ccall_ok assn@(HsPClass class_name tys)
497 = tcAddErrCtxt (appKindCtxt (ppr assn)) $
498 mapTc tcHsType tys `thenTc` \ arg_tys ->
499 tcLookupGlobal class_name `thenTc` \ thing ->
501 AClass clas -> checkTc (arity == n_tys) err `thenTc_`
502 returnTc (Class clas arg_tys)
504 arity = classArity clas
506 err = arityErr "Class" class_name arity n_tys
508 other -> failWithTc (wrongThingErr "class" (AGlobal thing) class_name)
510 tcClassAssertion ccall_ok assn@(HsPIParam name ty)
511 = tcAddErrCtxt (appKindCtxt (ppr assn)) $
512 tcHsType ty `thenTc` \ arg_ty ->
513 returnTc (IParam name arg_ty)
517 %************************************************************************
519 \subsection{Type variables, with knot tying!}
521 %************************************************************************
524 mkImmutTyVars :: [(Name,Kind)] -> [TyVar]
525 mkImmutTyVars pairs = [mkTyVar name kind | (name, kind) <- pairs]
527 mkTyClTyVars :: Kind -- Kind of the tycon or class
528 -> [HsTyVarBndr Name]
530 mkTyClTyVars kind tyvar_names
531 = mkImmutTyVars tyvars_w_kinds
533 (tyvars_w_kinds, _) = zipFunTys (hsTyVarNames tyvar_names) kind
537 %************************************************************************
539 \subsection{Signatures}
541 %************************************************************************
543 @tcSigs@ checks the signatures for validity, and returns a list of
544 {\em freshly-instantiated} signatures. That is, the types are already
545 split up, and have fresh type variables installed. All non-type-signature
546 "RenamedSigs" are ignored.
548 The @TcSigInfo@ contains @TcTypes@ because they are unified with
549 the variable's type, and after that checked to see whether they've
555 Name -- N, the Name in corresponding binding
557 TcId -- *Polymorphic* binder for this value...
564 TcId -- *Monomorphic* binder for this value
565 -- Does *not* have name = N
568 [Inst] -- Empty if theta is null, or
569 -- (method mono_id) otherwise
571 SrcLoc -- Of the signature
573 instance Outputable TcSigInfo where
574 ppr (TySigInfo nm id tyvars theta tau _ inst loc) =
575 ppr nm <+> ptext SLIT("::") <+> ppr tyvars <+> ppr theta <+> ptext SLIT("=>") <+> ppr tau
577 maybeSig :: [TcSigInfo] -> Name -> Maybe (TcSigInfo)
578 -- Search for a particular signature
579 maybeSig [] name = Nothing
580 maybeSig (sig@(TySigInfo sig_name _ _ _ _ _ _ _) : sigs) name
581 | name == sig_name = Just sig
582 | otherwise = maybeSig sigs name
587 tcTySig :: RenamedSig -> TcM TcSigInfo
589 tcTySig (Sig v ty src_loc)
590 = tcAddSrcLoc src_loc $
591 tcAddErrCtxt (tcsigCtxt v) $
592 tcHsSigType ty `thenTc` \ sigma_tc_ty ->
593 mkTcSig (mkVanillaId v sigma_tc_ty) src_loc `thenNF_Tc` \ sig ->
596 mkTcSig :: TcId -> SrcLoc -> NF_TcM TcSigInfo
597 mkTcSig poly_id src_loc
598 = -- Instantiate this type
599 -- It's important to do this even though in the error-free case
600 -- we could just split the sigma_tc_ty (since the tyvars don't
601 -- unified with anything). But in the case of an error, when
602 -- the tyvars *do* get unified with something, we want to carry on
603 -- typechecking the rest of the program with the function bound
604 -- to a pristine type, namely sigma_tc_ty
606 (tyvars, rho) = splitForAllTys (idType poly_id)
608 mapNF_Tc tcInstSigVar tyvars `thenNF_Tc` \ tyvars' ->
609 -- Make *signature* type variables
612 tyvar_tys' = mkTyVarTys tyvars'
613 rho' = substTy (mkTopTyVarSubst tyvars tyvar_tys') rho
614 -- mkTopTyVarSubst because the tyvars' are fresh
615 (theta', tau') = splitRhoTy rho'
616 -- This splitRhoTy tries hard to make sure that tau' is a type synonym
617 -- wherever possible, which can improve interface files.
619 newMethodWithGivenTy SignatureOrigin
622 theta' tau' `thenNF_Tc` \ inst ->
623 -- We make a Method even if it's not overloaded; no harm
624 instFunDeps SignatureOrigin theta' `thenNF_Tc` \ fds ->
626 returnNF_Tc (TySigInfo name poly_id tyvars' theta' tau' (instToIdBndr inst) (inst : fds) src_loc)
628 name = idName poly_id
633 %************************************************************************
635 \subsection{Checking signature type variables}
637 %************************************************************************
639 @checkSigTyVars@ is used after the type in a type signature has been unified with
640 the actual type found. It then checks that the type variables of the type signature
642 (a) Still all type variables
643 eg matching signature [a] against inferred type [(p,q)]
644 [then a will be unified to a non-type variable]
646 (b) Still all distinct
647 eg matching signature [(a,b)] against inferred type [(p,p)]
648 [then a and b will be unified together]
650 (c) Not mentioned in the environment
651 eg the signature for f in this:
657 Here, f is forced to be monorphic by the free occurence of x.
659 (d) Not (unified with another type variable that is) in scope.
660 eg f x :: (r->r) = (\y->y) :: forall a. a->r
661 when checking the expression type signature, we find that
662 even though there is nothing in scope whose type mentions r,
663 nevertheless the type signature for the expression isn't right.
665 Another example is in a class or instance declaration:
667 op :: forall b. a -> b
669 Here, b gets unified with a
671 Before doing this, the substitution is applied to the signature type variable.
673 We used to have the notion of a "DontBind" type variable, which would
674 only be bound to itself or nothing. Then points (a) and (b) were
675 self-checking. But it gave rise to bogus consequential error messages.
678 f = (*) -- Monomorphic
683 Here, we get a complaint when checking the type signature for g,
684 that g isn't polymorphic enough; but then we get another one when
685 dealing with the (Num x) context arising from f's definition;
686 we try to unify x with Int (to default it), but find that x has already
687 been unified with the DontBind variable "a" from g's signature.
688 This is really a problem with side-effecting unification; we'd like to
689 undo g's effects when its type signature fails, but unification is done
690 by side effect, so we can't (easily).
692 So we revert to ordinary type variables for signatures, and try to
693 give a helpful message in checkSigTyVars.
696 checkSigTyVars :: [TcTyVar] -- Universally-quantified type variables in the signature
697 -> TcTyVarSet -- Tyvars that are free in the type signature
698 -- These should *already* be in the global-var set, and are
699 -- used here only to improve the error message
700 -> TcM [TcTyVar] -- Zonked signature type variables
702 checkSigTyVars [] free = returnTc []
704 checkSigTyVars sig_tyvars free_tyvars
705 = zonkTcTyVars sig_tyvars `thenNF_Tc` \ sig_tys ->
706 tcGetGlobalTyVars `thenNF_Tc` \ globals ->
708 checkTcM (all_ok sig_tys globals)
709 (complain sig_tys globals) `thenTc_`
711 returnTc (map (getTyVar "checkSigTyVars") sig_tys)
715 all_ok (ty:tys) acc = case getTyVar_maybe ty of
716 Nothing -> False -- Point (a)
717 Just tv | tv `elemVarSet` acc -> False -- Point (b) or (c)
718 | otherwise -> all_ok tys (acc `extendVarSet` tv)
721 complain sig_tys globals
722 = -- For the in-scope ones, zonk them and construct a map
723 -- from the zonked tyvar to the in-scope one
724 -- If any of the in-scope tyvars zonk to a type, then ignore them;
725 -- that'll be caught later when we back up to their type sig
726 tcGetEnv `thenNF_Tc` \ env ->
728 in_scope_tvs = tcEnvTyVars env
730 zonkTcTyVars in_scope_tvs `thenNF_Tc` \ in_scope_tys ->
732 in_scope_assoc = [ (zonked_tv, in_scope_tv)
733 | (z_ty, in_scope_tv) <- in_scope_tys `zip` in_scope_tvs,
734 Just zonked_tv <- [getTyVar_maybe z_ty]
736 in_scope_env = mkVarEnv in_scope_assoc
739 -- "check" checks each sig tyvar in turn
741 (env2, in_scope_env, [])
742 (tidy_tvs `zip` tidy_tys) `thenNF_Tc` \ (env3, _, msgs) ->
744 failWithTcM (env3, main_msg $$ nest 4 (vcat msgs))
746 (env1, tidy_tvs) = mapAccumL tidyTyVar emptyTidyEnv sig_tyvars
747 (env2, tidy_tys) = tidyOpenTypes env1 sig_tys
749 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
751 check (tidy_env, acc, msgs) (sig_tyvar,ty)
752 -- sig_tyvar is from the signature;
753 -- ty is what you get if you zonk sig_tyvar and then tidy it
755 -- acc maps a zonked type variable back to a signature type variable
756 = case getTyVar_maybe ty of {
757 Nothing -> -- Error (a)!
758 returnNF_Tc (tidy_env, acc, unify_msg sig_tyvar (ppr ty) : msgs) ;
762 case lookupVarEnv acc tv of {
763 Just sig_tyvar' -> -- Error (b) or (d)!
764 returnNF_Tc (tidy_env, acc, unify_msg sig_tyvar (ppr sig_tyvar') : msgs) ;
768 if tv `elemVarSet` globals -- Error (c)! Type variable escapes
769 -- The least comprehensible, so put it last
771 -- a) get the local TcIds from the environment,
772 -- and pass them to find_globals (they might have tv free)
773 -- b) similarly, find any free_tyvars that mention tv
774 then tcGetEnv `thenNF_Tc` \ tc_env ->
775 find_globals tv tidy_env [] (tcEnvTcIds tc_env) `thenNF_Tc` \ (tidy_env1, globs) ->
776 find_frees tv tidy_env1 [] (varSetElems free_tyvars) `thenNF_Tc` \ (tidy_env2, frees) ->
777 returnNF_Tc (tidy_env2, acc, escape_msg sig_tyvar tv globs frees : msgs)
780 returnNF_Tc (tidy_env, extendVarEnv acc tv sig_tyvar, msgs)
783 -- find_globals looks at the value environment and finds values
784 -- whose types mention the offending type variable. It has to be
785 -- careful to zonk the Id's type first, so it has to be in the monad.
786 -- We must be careful to pass it a zonked type variable, too.
792 -> NF_TcM (TidyEnv,[(Name,Type)])
794 find_globals tv tidy_env acc []
795 = returnNF_Tc (tidy_env, acc)
797 find_globals tv tidy_env acc (id:ids)
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 (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 thing name
904 = pp_thing thing <+> quotes (ppr name) <+> ptext SLIT("used as a") <+> text expected
906 pp_thing (AGlobal (ATyCon _)) = ptext SLIT("Type constructor")
907 pp_thing (AGlobal (AClass _)) = ptext SLIT("Class")
908 pp_thing (AGlobal (AnId _)) = ptext SLIT("Identifier")
909 pp_thing (ATyVar _) = ptext SLIT("Type variable")
910 pp_thing (ATcId _) = ptext SLIT("Local identifier")
911 pp_thing (AThing _) = ptext SLIT("Utterly bogus")
914 = sep [ptext SLIT("Ambiguous constraint") <+> quotes (pprPred pred),
915 nest 4 (ptext SLIT("for the type:") <+> ppr ty),
916 nest 4 (ptext SLIT("Each forall'd type variable mentioned by the constraint must appear after the =>"))]
919 = sep [ptext SLIT("The constraint") <+> quotes (pprPred pred) <+>
920 ptext SLIT("does not mention any of the universally quantified type variables"),
921 nest 4 (ptext SLIT("in the type") <+> quotes (ppr ty))
925 = sep [ptext (SLIT("Illegal unboxed tuple as a function or contructor argument:")), nest 4 (ppr ty)]