2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[TcMonoType]{Typechecking user-specified @MonoTypes@}
7 module TcMonoType ( tcHsType, tcHsRecType, tcIfaceType,
8 tcHsSigType, tcHsLiftedSigType,
9 tcRecTheta, 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 isUnboxedTupleType, isForAllTy, isIPPred
51 import PprType ( pprType, pprTheta, pprPred )
52 import Subst ( mkTopTyVarSubst, substTy )
53 import CoreFVs ( idFreeTyVars )
54 import Id ( mkLocalId, idName, idType )
55 import Var ( Id, Var, TyVar, mkTyVar, tyVarKind )
58 import ErrUtils ( Message )
59 import TyCon ( TyCon, isSynTyCon, tyConArity, tyConKind )
60 import Class ( 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@(HsIParam name ty)
245 = tcAddErrCtxt (appKindCtxt (ppr pred)) $
248 kcHsPred pred@(HsClassP 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 -- These are used in type and class decls, where kinding is
301 tcHsType ty = tc_type NonRecursive ty `thenTc` \ ty' -> returnTc (hoistForAllTys ty')
302 tcHsRecType wimp_out ty = tc_type wimp_out ty `thenTc` \ ty' -> returnTc (hoistForAllTys ty')
304 -- In interface files the type is already kinded,
305 -- and we definitely don't want to hoist for-alls.
306 -- Otherwise we'll change
307 -- dmfail :: forall m:(*->*) Monad m => forall a:* => String -> m a
309 -- dmfail :: forall m:(*->*) a:* Monad m => String -> m a
310 -- which definitely isn't right!
311 tcIfaceType ty = tc_type NonRecursive ty
315 %************************************************************************
319 %************************************************************************
321 tc_type, the main work horse
322 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
328 tc_type is used to typecheck the types in the RHS of data
329 constructors. In the case of recursive data types, that means that
330 the type constructors themselves are (partly) black holes. e.g.
332 data T a = MkT a [T a]
334 While typechecking the [T a] on the RHS, T itself is not yet fully
335 defined. That in turn places restrictions on what you can check in
336 tcHsType; if you poke on too much you get a black hole. I keep
337 forgetting this, hence this warning!
339 The wimp_out argument tells when we are in a mutually-recursive
340 group of type declarations, so omit various checks else we
341 get a black hole. They'll be done again later, in TcTyClDecls.tcGroup.
343 --------------------------
344 *** END OF BIG WARNING ***
345 --------------------------
349 tc_type :: RecFlag -> RenamedHsType -> TcM Type
351 tc_type wimp_out ty@(HsTyVar name)
352 = tc_app wimp_out ty []
354 tc_type wimp_out (HsListTy ty)
355 = tc_arg_type wimp_out ty `thenTc` \ tau_ty ->
356 returnTc (mkListTy tau_ty)
358 tc_type wimp_out (HsTupleTy (HsTupCon _ boxity arity) tys)
359 = ASSERT( arity == length tys )
360 mapTc tc_tup_arg tys `thenTc` \ tau_tys ->
361 returnTc (mkTupleTy boxity arity tau_tys)
363 tc_tup_arg = case boxity of
364 Boxed -> tc_arg_type wimp_out
365 Unboxed -> tc_type wimp_out
366 -- Unboxed tuples can have polymorphic or unboxed args.
367 -- This happens in the workers for functions returning
368 -- product types with polymorphic components
370 tc_type wimp_out (HsFunTy ty1 ty2)
371 = tc_type wimp_out ty1 `thenTc` \ tau_ty1 ->
372 -- Function argument can be polymorphic, but
373 -- must not be an unboxed tuple
375 -- In a recursive loop we can't ask whether the thing is
376 -- unboxed -- might be a synonym inside a synonym inside a group
377 checkTc (isRec wimp_out || not (isUnboxedTupleType tau_ty1))
378 (ubxArgTyErr ty1) `thenTc_`
379 tc_type wimp_out ty2 `thenTc` \ tau_ty2 ->
380 returnTc (mkFunTy tau_ty1 tau_ty2)
382 tc_type wimp_out (HsNumTy n)
384 returnTc (mkTyConApp genUnitTyCon [])
386 tc_type wimp_out (HsOpTy ty1 op ty2) =
387 tc_arg_type wimp_out ty1 `thenTc` \ tau_ty1 ->
388 tc_arg_type wimp_out ty2 `thenTc` \ tau_ty2 ->
389 tc_fun_type op [tau_ty1,tau_ty2]
391 tc_type wimp_out (HsAppTy ty1 ty2)
392 = tc_app wimp_out ty1 [ty2]
394 tc_type wimp_out (HsPredTy pred)
395 = tc_pred wimp_out pred `thenTc` \ pred' ->
396 returnTc (mkPredTy pred')
398 tc_type wimp_out full_ty@(HsForAllTy (Just tv_names) ctxt ty)
400 kind_check = kcHsContext ctxt `thenTc_` kcHsType ty
402 tcHsTyVars tv_names kind_check $ \ tyvars ->
403 tcRecTheta wimp_out ctxt `thenTc` \ theta ->
405 -- Context behaves like a function type
406 -- This matters. Return-unboxed-tuple analysis can
407 -- give overloaded functions like
408 -- f :: forall a. Num a => (# a->a, a->a #)
409 -- And we want these to get through the type checker
411 tc_arg_type wimp_out ty
416 checkAmbiguity wimp_out is_source tyvars theta tau
418 is_source = case tv_names of
419 (UserTyVar _ : _) -> True
423 -- tc_arg_type checks that the argument of a
424 -- type appplication isn't a for-all type or an unboxed tuple type
425 -- For example, we want to reject things like:
427 -- instance Ord a => Ord (forall s. T s a)
429 -- g :: T s (forall b.b)
431 -- Other unboxed types are very occasionally allowed as type
432 -- arguments depending on the kind of the type constructor
434 tc_arg_type wimp_out arg_ty
436 = tc_type wimp_out arg_ty
439 = tc_type wimp_out arg_ty `thenTc` \ arg_ty' ->
440 checkTc (isRec wimp_out || not (isForAllTy arg_ty')) (polyArgTyErr arg_ty) `thenTc_`
441 checkTc (isRec wimp_out || not (isUnboxedTupleType arg_ty')) (ubxArgTyErr arg_ty) `thenTc_`
444 tc_arg_types wimp_out arg_tys = mapTc (tc_arg_type wimp_out) arg_tys
447 Help functions for type applications
448 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
451 tc_app :: RecFlag -> RenamedHsType -> [RenamedHsType] -> TcM Type
452 tc_app wimp_out (HsAppTy ty1 ty2) tys
453 = tc_app wimp_out ty1 (ty2:tys)
455 tc_app wimp_out ty tys
456 = tcAddErrCtxt (appKindCtxt pp_app) $
457 tc_arg_types wimp_out tys `thenTc` \ arg_tys ->
459 HsTyVar fun -> tc_fun_type fun arg_tys
460 other -> tc_type wimp_out ty `thenTc` \ fun_ty ->
461 returnNF_Tc (mkAppTys fun_ty arg_tys)
463 pp_app = ppr ty <+> sep (map pprParendHsType tys)
465 -- (tc_fun_type ty arg_tys) returns (mkAppTys ty arg_tys)
466 -- But not quite; for synonyms it checks the correct arity, and builds a SynTy
467 -- hence the rather strange functionality.
469 tc_fun_type name arg_tys
470 = tcLookup name `thenTc` \ thing ->
472 ATyVar tv -> returnTc (mkAppTys (mkTyVarTy tv) arg_tys)
475 | isSynTyCon tc -> checkTc arity_ok err_msg `thenTc_`
476 returnTc (mkAppTys (mkSynTy tc (take arity arg_tys))
477 (drop arity arg_tys))
479 | otherwise -> returnTc (mkTyConApp tc arg_tys)
482 arity_ok = arity <= n_args
483 arity = tyConArity tc
484 -- It's OK to have an *over-applied* type synonym
485 -- data Tree a b = ...
486 -- type Foo a = Tree [a]
487 -- f :: Foo a b -> ...
488 err_msg = arityErr "Type synonym" name arity n_args
489 n_args = length arg_tys
491 other -> failWithTc (wrongThingErr "type constructor" thing name)
498 tcRecTheta :: RecFlag -> RenamedContext -> TcM ThetaType
499 -- Used when we are expecting a ClassContext (i.e. no implicit params)
500 tcRecTheta wimp_out context = mapTc (tc_pred wimp_out) context
502 tc_pred wimp_out assn@(HsClassP class_name tys)
503 = tcAddErrCtxt (appKindCtxt (ppr assn)) $
504 tc_arg_types wimp_out tys `thenTc` \ arg_tys ->
505 tcLookupGlobal class_name `thenTc` \ thing ->
507 AClass clas -> checkTc (arity == n_tys) err `thenTc_`
508 returnTc (ClassP clas arg_tys)
510 arity = classArity clas
512 err = arityErr "Class" class_name arity n_tys
514 other -> failWithTc (wrongThingErr "class" (AGlobal thing) class_name)
516 tc_pred wimp_out assn@(HsIParam name ty)
517 = tcAddErrCtxt (appKindCtxt (ppr assn)) $
518 tc_arg_type wimp_out ty `thenTc` \ arg_ty ->
519 returnTc (IParam name arg_ty)
526 is ambiguous if P contains generic variables
527 (i.e. one of the Vs) that are not mentioned in tau
529 However, we need to take account of functional dependencies
530 when we speak of 'mentioned in tau'. Example:
531 class C a b | a -> b where ...
533 forall x y. (C x y) => x
534 is not ambiguous because x is mentioned and x determines y
536 NOTE: In addition, GHC insists that at least one type variable
537 in each constraint is in V. So we disallow a type like
538 forall a. Eq b => b -> b
539 even in a scope where b is in scope.
540 This is the is_free test below.
542 Notes on the 'is_source_polytype' test above
543 Check ambiguity only for source-program types, not
544 for types coming from inteface files. The latter can
545 legitimately have ambiguous types. Example
546 class S a where s :: a -> (Int,Int)
547 instance S Char where s _ = (1,1)
548 f:: S a => [a] -> Int -> (Int,Int)
549 f (_::[a]) x = (a*x,b)
550 where (a,b) = s (undefined::a)
551 Here the worker for f gets the type
552 fw :: forall a. S a => Int -> (# Int, Int #)
554 If the list of tv_names is empty, we have a monotype,
555 and then we don't need to check for ambiguity either,
556 because the test can't fail (see is_ambig).
559 checkAmbiguity :: RecFlag -> Bool
560 -> [TyVar] -> ThetaType -> TauType
562 checkAmbiguity wimp_out is_source_polytype forall_tyvars theta tau
563 | isRec wimp_out = returnTc sigma_ty
564 | otherwise = mapTc_ check_pred theta `thenTc_`
567 sigma_ty = mkSigmaTy forall_tyvars theta tau
568 tau_vars = tyVarsOfType tau
569 extended_tau_vars = grow theta tau_vars
571 -- Hack alert. If there are no tyvars, (ppr sigma_ty) will print
572 -- something strange like {Eq k} -> k -> k, because there is no
573 -- ForAll at the top of the type. Since this is going to the user
574 -- we want it to look like a proper Haskell type even then; hence the hack
576 -- This shows up in the complaint about
578 -- op :: Eq a => a -> a
579 ppr_sigma | null forall_tyvars = pprTheta theta <+> ptext SLIT("=>") <+> ppr tau
580 | otherwise = ppr sigma_ty
582 is_ambig ct_var = (ct_var `elem` forall_tyvars) &&
583 not (ct_var `elemVarSet` extended_tau_vars)
584 is_free ct_var = not (ct_var `elem` forall_tyvars)
586 check_pred pred = checkTc (not any_ambig) (ambigErr pred ppr_sigma) `thenTc_`
587 checkTc (isIPPred pred || not all_free) (freeErr pred ppr_sigma)
589 ct_vars = varSetElems (tyVarsOfPred pred)
590 all_free = all is_free ct_vars
591 any_ambig = is_source_polytype && any is_ambig ct_vars
594 %************************************************************************
596 \subsection{Type variables, with knot tying!}
598 %************************************************************************
601 mkImmutTyVars :: [(Name,Kind)] -> [TyVar]
602 mkImmutTyVars pairs = [mkTyVar name kind | (name, kind) <- pairs]
604 mkTyClTyVars :: Kind -- Kind of the tycon or class
605 -> [HsTyVarBndr Name]
607 mkTyClTyVars kind tyvar_names
608 = mkImmutTyVars tyvars_w_kinds
610 (tyvars_w_kinds, _) = zipFunTys (hsTyVarNames tyvar_names) kind
614 %************************************************************************
616 \subsection{Signatures}
618 %************************************************************************
620 @tcSigs@ checks the signatures for validity, and returns a list of
621 {\em freshly-instantiated} signatures. That is, the types are already
622 split up, and have fresh type variables installed. All non-type-signature
623 "RenamedSigs" are ignored.
625 The @TcSigInfo@ contains @TcTypes@ because they are unified with
626 the variable's type, and after that checked to see whether they've
632 Name -- N, the Name in corresponding binding
634 TcId -- *Polymorphic* binder for this value...
641 TcId -- *Monomorphic* binder for this value
642 -- Does *not* have name = N
645 [Inst] -- Empty if theta is null, or
646 -- (method mono_id) otherwise
648 SrcLoc -- Of the signature
650 instance Outputable TcSigInfo where
651 ppr (TySigInfo nm id tyvars theta tau _ inst loc) =
652 ppr nm <+> ptext SLIT("::") <+> ppr tyvars <+> ppr theta <+> ptext SLIT("=>") <+> ppr tau
654 maybeSig :: [TcSigInfo] -> Name -> Maybe (TcSigInfo)
655 -- Search for a particular signature
656 maybeSig [] name = Nothing
657 maybeSig (sig@(TySigInfo sig_name _ _ _ _ _ _ _) : sigs) name
658 | name == sig_name = Just sig
659 | otherwise = maybeSig sigs name
664 tcTySig :: RenamedSig -> TcM TcSigInfo
666 tcTySig (Sig v ty src_loc)
667 = tcAddSrcLoc src_loc $
668 tcAddErrCtxt (tcsigCtxt v) $
669 tcHsSigType ty `thenTc` \ sigma_tc_ty ->
670 mkTcSig (mkLocalId v sigma_tc_ty) src_loc `thenNF_Tc` \ sig ->
673 mkTcSig :: TcId -> SrcLoc -> NF_TcM TcSigInfo
674 mkTcSig poly_id src_loc
675 = -- Instantiate this type
676 -- It's important to do this even though in the error-free case
677 -- we could just split the sigma_tc_ty (since the tyvars don't
678 -- unified with anything). But in the case of an error, when
679 -- the tyvars *do* get unified with something, we want to carry on
680 -- typechecking the rest of the program with the function bound
681 -- to a pristine type, namely sigma_tc_ty
683 (tyvars, rho) = splitForAllTys (idType poly_id)
685 mapNF_Tc tcInstSigVar tyvars `thenNF_Tc` \ tyvars' ->
686 -- Make *signature* type variables
689 tyvar_tys' = mkTyVarTys tyvars'
690 rho' = substTy (mkTopTyVarSubst tyvars tyvar_tys') rho
691 -- mkTopTyVarSubst because the tyvars' are fresh
692 (theta', tau') = splitRhoTy rho'
693 -- This splitRhoTy tries hard to make sure that tau' is a type synonym
694 -- wherever possible, which can improve interface files.
696 newMethodWithGivenTy SignatureOrigin
699 theta' tau' `thenNF_Tc` \ inst ->
700 -- We make a Method even if it's not overloaded; no harm
702 returnNF_Tc (TySigInfo name poly_id tyvars' theta' tau' (instToId inst) [inst] src_loc)
704 name = idName poly_id
709 %************************************************************************
711 \subsection{Checking signature type variables}
713 %************************************************************************
715 @checkSigTyVars@ is used after the type in a type signature has been unified with
716 the actual type found. It then checks that the type variables of the type signature
718 (a) Still all type variables
719 eg matching signature [a] against inferred type [(p,q)]
720 [then a will be unified to a non-type variable]
722 (b) Still all distinct
723 eg matching signature [(a,b)] against inferred type [(p,p)]
724 [then a and b will be unified together]
726 (c) Not mentioned in the environment
727 eg the signature for f in this:
733 Here, f is forced to be monorphic by the free occurence of x.
735 (d) Not (unified with another type variable that is) in scope.
736 eg f x :: (r->r) = (\y->y) :: forall a. a->r
737 when checking the expression type signature, we find that
738 even though there is nothing in scope whose type mentions r,
739 nevertheless the type signature for the expression isn't right.
741 Another example is in a class or instance declaration:
743 op :: forall b. a -> b
745 Here, b gets unified with a
747 Before doing this, the substitution is applied to the signature type variable.
749 We used to have the notion of a "DontBind" type variable, which would
750 only be bound to itself or nothing. Then points (a) and (b) were
751 self-checking. But it gave rise to bogus consequential error messages.
754 f = (*) -- Monomorphic
759 Here, we get a complaint when checking the type signature for g,
760 that g isn't polymorphic enough; but then we get another one when
761 dealing with the (Num x) context arising from f's definition;
762 we try to unify x with Int (to default it), but find that x has already
763 been unified with the DontBind variable "a" from g's signature.
764 This is really a problem with side-effecting unification; we'd like to
765 undo g's effects when its type signature fails, but unification is done
766 by side effect, so we can't (easily).
768 So we revert to ordinary type variables for signatures, and try to
769 give a helpful message in checkSigTyVars.
772 checkSigTyVars :: [TcTyVar] -- Universally-quantified type variables in the signature
773 -> TcTyVarSet -- Tyvars that are free in the type signature
774 -- Not necessarily zonked
775 -- These should *already* be in the free-in-env set,
776 -- and are used here only to improve the error message
777 -> TcM [TcTyVar] -- Zonked signature type variables
779 checkSigTyVars [] free = returnTc []
780 checkSigTyVars sig_tyvars free_tyvars
781 = zonkTcTyVars sig_tyvars `thenNF_Tc` \ sig_tys ->
782 tcGetGlobalTyVars `thenNF_Tc` \ globals ->
784 checkTcM (allDistinctTyVars sig_tys globals)
785 (complain sig_tys globals) `thenTc_`
787 returnTc (map (getTyVar "checkSigTyVars") sig_tys)
790 complain sig_tys globals
791 = -- For the in-scope ones, zonk them and construct a map
792 -- from the zonked tyvar to the in-scope one
793 -- If any of the in-scope tyvars zonk to a type, then ignore them;
794 -- that'll be caught later when we back up to their type sig
795 tcGetEnv `thenNF_Tc` \ env ->
797 in_scope_tvs = tcEnvTyVars env
799 zonkTcTyVars in_scope_tvs `thenNF_Tc` \ in_scope_tys ->
801 in_scope_assoc = [ (zonked_tv, in_scope_tv)
802 | (z_ty, in_scope_tv) <- in_scope_tys `zip` in_scope_tvs,
803 Just zonked_tv <- [getTyVar_maybe z_ty]
805 in_scope_env = mkVarEnv in_scope_assoc
808 -- "check" checks each sig tyvar in turn
810 (env2, in_scope_env, [])
811 (tidy_tvs `zip` tidy_tys) `thenNF_Tc` \ (env3, _, msgs) ->
813 failWithTcM (env3, main_msg $$ nest 4 (vcat msgs))
815 (env1, tidy_tvs) = mapAccumL tidyTyVar emptyTidyEnv sig_tyvars
816 (env2, tidy_tys) = tidyOpenTypes env1 sig_tys
818 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
820 check (tidy_env, acc, msgs) (sig_tyvar,ty)
821 -- sig_tyvar is from the signature;
822 -- ty is what you get if you zonk sig_tyvar and then tidy it
824 -- acc maps a zonked type variable back to a signature type variable
825 = case getTyVar_maybe ty of {
826 Nothing -> -- Error (a)!
827 returnNF_Tc (tidy_env, acc, unify_msg sig_tyvar (quotes (ppr ty)) : msgs) ;
831 case lookupVarEnv acc tv of {
832 Just sig_tyvar' -> -- Error (b) or (d)!
833 returnNF_Tc (tidy_env, acc, unify_msg sig_tyvar thing : msgs)
835 thing = ptext SLIT("another quantified type variable") <+> quotes (ppr sig_tyvar')
839 if tv `elemVarSet` globals -- Error (c)! Type variable escapes
840 -- The least comprehensible, so put it last
842 -- a) get the local TcIds from the environment,
843 -- and pass them to find_globals (they might have tv free)
844 -- b) similarly, find any free_tyvars that mention tv
845 then tcGetEnv `thenNF_Tc` \ ve ->
846 find_globals tv tidy_env [] (tcEnvTcIds ve) `thenNF_Tc` \ (tidy_env1, globs) ->
847 find_frees tv tidy_env1 [] (varSetElems free_tyvars) `thenNF_Tc` \ (tidy_env2, frees) ->
848 returnNF_Tc (tidy_env2, acc, escape_msg sig_tyvar tv globs frees : msgs)
851 returnNF_Tc (tidy_env, extendVarEnv acc tv sig_tyvar, msgs)
854 -- find_globals looks at the value environment and finds values
855 -- whose types mention the offending type variable. It has to be
856 -- careful to zonk the Id's type first, so it has to be in the monad.
857 -- We must be careful to pass it a zonked type variable, too.
863 -> NF_TcM (TidyEnv,[(Name,Type)])
865 find_globals tv tidy_env acc []
866 = returnNF_Tc (tidy_env, acc)
868 find_globals tv tidy_env acc (id:ids)
869 | isEmptyVarSet (idFreeTyVars id)
870 = find_globals tv tidy_env acc ids
873 = zonkTcType (idType id) `thenNF_Tc` \ id_ty ->
874 if tv `elemVarSet` tyVarsOfType id_ty then
876 (tidy_env', id_ty') = tidyOpenType tidy_env id_ty
877 acc' = (idName id, id_ty') : acc
879 find_globals tv tidy_env' acc' ids
881 find_globals tv tidy_env acc ids
883 find_frees tv tidy_env acc []
884 = returnNF_Tc (tidy_env, acc)
885 find_frees tv tidy_env acc (ftv:ftvs)
886 = zonkTcTyVar ftv `thenNF_Tc` \ ty ->
887 if tv `elemVarSet` tyVarsOfType ty then
889 (tidy_env', ftv') = tidyTyVar tidy_env ftv
891 find_frees tv tidy_env' (ftv':acc) ftvs
893 find_frees tv tidy_env acc ftvs
896 escape_msg sig_tv tv globs frees
897 = mk_msg sig_tv <+> ptext SLIT("escapes") $$
898 if not (null globs) then
899 vcat [pp_it <+> ptext SLIT("is mentioned in the environment"),
900 ptext SLIT("The following variables in the environment mention") <+> quotes (ppr tv),
901 nest 2 (vcat_first 10 [ppr name <+> dcolon <+> ppr ty | (name,ty) <- globs])
903 else if not (null frees) then
904 vcat [ptext SLIT("It is reachable from the type variable(s)") <+> pprQuotedList frees,
905 nest 2 (ptext SLIT("which") <+> is_are <+> ptext SLIT("free in the signature"))
908 empty -- Sigh. It's really hard to give a good error message
909 -- all the time. One bad case is an existential pattern match
911 is_are | isSingleton frees = ptext SLIT("is")
912 | otherwise = ptext SLIT("are")
913 pp_it | sig_tv /= tv = ptext SLIT("It unifies with") <+> quotes (ppr tv) <> comma <+> ptext SLIT("which")
914 | otherwise = ptext SLIT("It")
916 vcat_first :: Int -> [SDoc] -> SDoc
917 vcat_first n [] = empty
918 vcat_first 0 (x:xs) = text "...others omitted..."
919 vcat_first n (x:xs) = x $$ vcat_first (n-1) xs
921 unify_msg tv thing = mk_msg tv <+> ptext SLIT("is unified with") <+> thing
922 mk_msg tv = ptext SLIT("Quantified type variable") <+> quotes (ppr tv)
925 These two context are used with checkSigTyVars
928 sigCtxt :: Message -> [TcTyVar] -> TcThetaType -> TcTauType
929 -> TidyEnv -> NF_TcM (TidyEnv, Message)
930 sigCtxt when sig_tyvars sig_theta sig_tau tidy_env
931 = zonkTcType sig_tau `thenNF_Tc` \ actual_tau ->
933 (env1, tidy_sig_tyvars) = tidyTyVars tidy_env sig_tyvars
934 (env2, tidy_sig_rho) = tidyOpenType env1 (mkRhoTy sig_theta sig_tau)
935 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
936 msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tyvars tidy_sig_rho),
937 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau,
941 returnNF_Tc (env3, msg)
943 sigPatCtxt bound_tvs bound_ids tidy_env
945 sep [ptext SLIT("When checking a pattern that binds"),
946 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys))])
948 show_ids = filter is_interesting bound_ids
949 is_interesting id = any (`elemVarSet` idFreeTyVars id) bound_tvs
951 (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
952 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
953 -- Don't zonk the types so we get the separate, un-unified versions
957 %************************************************************************
959 \subsection{Errors and contexts}
961 %************************************************************************
964 tcsigCtxt v = ptext SLIT("In a type signature for") <+> quotes (ppr v)
966 typeKindCtxt :: RenamedHsType -> Message
967 typeKindCtxt ty = sep [ptext SLIT("When checking that"),
968 nest 2 (quotes (ppr ty)),
969 ptext SLIT("is a type")]
971 appKindCtxt :: SDoc -> Message
972 appKindCtxt pp = ptext SLIT("When checking kinds in") <+> quotes pp
974 wrongThingErr expected thing name
975 = pp_thing thing <+> quotes (ppr name) <+> ptext SLIT("used as a") <+> text expected
977 pp_thing (AGlobal (ATyCon _)) = ptext SLIT("Type constructor")
978 pp_thing (AGlobal (AClass _)) = ptext SLIT("Class")
979 pp_thing (AGlobal (AnId _)) = ptext SLIT("Identifier")
980 pp_thing (ATyVar _) = ptext SLIT("Type variable")
981 pp_thing (ATcId _) = ptext SLIT("Local identifier")
982 pp_thing (AThing _) = ptext SLIT("Utterly bogus")
985 = sep [ptext SLIT("Ambiguous constraint") <+> quotes (pprPred pred),
986 nest 4 (ptext SLIT("for the type:") <+> ppr_ty),
987 nest 4 (ptext SLIT("At least one of the forall'd type variables mentioned by the constraint") $$
988 ptext SLIT("must be reachable from the type after the =>"))]
991 = sep [ptext SLIT("All of the type variables in the constraint") <+> quotes (pprPred pred) <+>
992 ptext SLIT("are already in scope"),
993 nest 4 (ptext SLIT("At least one must be universally quantified here")),
994 ptext SLIT("In the type") <+> quotes ppr_ty
997 polyArgTyErr ty = ptext SLIT("Illegal polymorphic type as argument:") <+> ppr ty
998 ubxArgTyErr ty = ptext SLIT("Illegal unboxed tuple type as argument:") <+> ppr ty