2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Monadic type operations
8 This module contains monadic operations over types that contain
13 TcTyVar, TcKind, TcType, TcTauType, TcThetaType, TcTyVarSet,
15 --------------------------------
16 -- Creating new mutable type variables
18 newFlexiTyVarTy, -- Kind -> TcM TcType
19 newFlexiTyVarTys, -- Int -> Kind -> TcM [TcType]
20 newKindVar, newKindVars,
21 lookupTcTyVar, LookupTyVarResult(..),
22 newMetaTyVar, readMetaTyVar, writeMetaTyVar,
24 --------------------------------
25 -- Boxy type variables
26 newBoxyTyVar, newBoxyTyVars, newBoxyTyVarTys, readFilledBox,
28 --------------------------------
29 -- Creating new coercion variables
32 --------------------------------
34 tcInstTyVar, tcInstType, tcInstTyVars, tcInstBoxyTyVar,
35 tcInstSigTyVars, zonkSigTyVar,
36 tcInstSkolTyVar, tcInstSkolTyVars, tcInstSkolType,
37 tcSkolSigType, tcSkolSigTyVars,
39 --------------------------------
40 -- Checking type validity
41 Rank, UserTypeCtxt(..), checkValidType,
42 SourceTyCtxt(..), checkValidTheta, checkFreeness,
43 checkValidInstHead, checkValidInstance, checkAmbiguity,
47 --------------------------------
49 zonkType, zonkTcPredType,
50 zonkTcTyVar, zonkTcTyVars, zonkTcTyVarsAndFV, zonkQuantifiedTyVar,
51 zonkTcType, zonkTcTypes, zonkTcClassConstraints, zonkTcThetaType,
52 zonkTcKindToKind, zonkTcKind, zonkTopTyVar,
54 readKindVar, writeKindVar
58 #include "HsVersions.h"
71 import TcRnMonad -- TcType, amongst others
84 import Control.Monad ( when )
85 import Data.List ( (\\) )
89 %************************************************************************
91 Instantiation in general
93 %************************************************************************
96 tcInstType :: ([TyVar] -> TcM [TcTyVar]) -- How to instantiate the type variables
97 -> TcType -- Type to instantiate
98 -> TcM ([TcTyVar], TcThetaType, TcType) -- Result
99 tcInstType inst_tyvars ty
100 = case tcSplitForAllTys ty of
101 ([], rho) -> let -- There may be overloading despite no type variables;
102 -- (?x :: Int) => Int -> Int
103 (theta, tau) = tcSplitPhiTy rho
105 return ([], theta, tau)
107 (tyvars, rho) -> do { tyvars' <- inst_tyvars tyvars
109 ; let tenv = zipTopTvSubst tyvars (mkTyVarTys tyvars')
110 -- Either the tyvars are freshly made, by inst_tyvars,
111 -- or (in the call from tcSkolSigType) any nested foralls
112 -- have different binders. Either way, zipTopTvSubst is ok
114 ; let (theta, tau) = tcSplitPhiTy (substTy tenv rho)
115 ; return (tyvars', theta, tau) }
119 %************************************************************************
123 %************************************************************************
126 newCoVars :: [(TcType,TcType)] -> TcM [CoVar]
128 = do { us <- newUniqueSupply
129 ; return [ mkCoVar (mkSysTvName uniq FSLIT("co"))
131 | ((ty1,ty2), uniq) <- spec `zip` uniqsFromSupply us] }
133 newKindVar :: TcM TcKind
134 newKindVar = do { uniq <- newUnique
135 ; ref <- newMutVar Flexi
136 ; return (mkTyVarTy (mkKindVar uniq ref)) }
138 newKindVars :: Int -> TcM [TcKind]
139 newKindVars n = mappM (\ _ -> newKindVar) (nOfThem n ())
143 %************************************************************************
145 SkolemTvs (immutable)
147 %************************************************************************
150 mkSkolTyVar :: Name -> Kind -> SkolemInfo -> TcTyVar
151 mkSkolTyVar name kind info = mkTcTyVar name kind (SkolemTv info)
153 tcSkolSigType :: SkolemInfo -> Type -> TcM ([TcTyVar], TcThetaType, TcType)
154 -- Instantiate a type signature with skolem constants, but
155 -- do *not* give them fresh names, because we want the name to
156 -- be in the type environment -- it is lexically scoped.
157 tcSkolSigType info ty = tcInstType (\tvs -> return (tcSkolSigTyVars info tvs)) ty
159 tcSkolSigTyVars :: SkolemInfo -> [TyVar] -> [TcTyVar]
160 -- Make skolem constants, but do *not* give them new names, as above
161 tcSkolSigTyVars info tyvars = [ mkSkolTyVar (tyVarName tv) (tyVarKind tv) info
164 tcInstSkolTyVar :: SkolemInfo -> Maybe SrcLoc -> TyVar -> TcM TcTyVar
165 -- Instantiate the tyvar, using
166 -- * the occ-name and kind of the supplied tyvar,
167 -- * the unique from the monad,
168 -- * the location either from the tyvar (mb_loc = Nothing)
169 -- or from mb_loc (Just loc)
170 tcInstSkolTyVar info mb_loc tyvar
171 = do { uniq <- newUnique
172 ; let old_name = tyVarName tyvar
173 kind = tyVarKind tyvar
174 loc = mb_loc `orElse` getSrcLoc old_name
175 new_name = mkInternalName uniq (nameOccName old_name) loc
176 ; return (mkSkolTyVar new_name kind info) }
178 tcInstSkolTyVars :: SkolemInfo -> [TyVar] -> TcM [TcTyVar]
179 -- Get the location from the monad
180 tcInstSkolTyVars info tyvars
181 = do { span <- getSrcSpanM
182 ; mapM (tcInstSkolTyVar info (Just (srcSpanStart span))) tyvars }
184 tcInstSkolType :: SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcType)
185 -- Instantiate a type with fresh skolem constants
186 -- Binding location comes from the monad
187 tcInstSkolType info ty = tcInstType (tcInstSkolTyVars info) ty
191 %************************************************************************
193 MetaTvs (meta type variables; mutable)
195 %************************************************************************
198 newMetaTyVar :: BoxInfo -> Kind -> TcM TcTyVar
199 -- Make a new meta tyvar out of thin air
200 newMetaTyVar box_info kind
201 = do { uniq <- newUnique
202 ; ref <- newMutVar Flexi ;
203 ; let name = mkSysTvName uniq fs
204 fs = case box_info of
207 SigTv _ -> FSLIT("a")
208 -- We give BoxTv and TauTv the same string, because
209 -- otherwise we get user-visible differences in error
210 -- messages, which are confusing. If you want to see
211 -- the box_info of each tyvar, use -dppr-debug
212 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
214 instMetaTyVar :: BoxInfo -> TyVar -> TcM TcTyVar
215 -- Make a new meta tyvar whose Name and Kind
216 -- come from an existing TyVar
217 instMetaTyVar box_info tyvar
218 = do { uniq <- newUnique
219 ; ref <- newMutVar Flexi ;
220 ; let name = setNameUnique (tyVarName tyvar) uniq
221 kind = tyVarKind tyvar
222 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
224 readMetaTyVar :: TyVar -> TcM MetaDetails
225 readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
226 readMutVar (metaTvRef tyvar)
228 writeMetaTyVar :: TcTyVar -> TcType -> TcM ()
230 writeMetaTyVar tyvar ty = writeMutVar (metaTvRef tyvar) (Indirect ty)
232 writeMetaTyVar tyvar ty
233 | not (isMetaTyVar tyvar)
234 = pprTrace "writeMetaTyVar" (ppr tyvar) $
238 = ASSERT( isMetaTyVar tyvar )
239 ASSERT2( k2 `isSubKind` k1, (ppr tyvar <+> ppr k1) $$ (ppr ty <+> ppr k2) )
240 do { ASSERTM2( do { details <- readMetaTyVar tyvar; return (isFlexi details) }, ppr tyvar )
241 ; writeMutVar (metaTvRef tyvar) (Indirect ty) }
249 %************************************************************************
253 %************************************************************************
256 newFlexiTyVar :: Kind -> TcM TcTyVar
257 newFlexiTyVar kind = newMetaTyVar TauTv kind
259 newFlexiTyVarTy :: Kind -> TcM TcType
261 = newFlexiTyVar kind `thenM` \ tc_tyvar ->
262 returnM (TyVarTy tc_tyvar)
264 newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
265 newFlexiTyVarTys n kind = mappM newFlexiTyVarTy (nOfThem n kind)
267 tcInstTyVar :: TyVar -> TcM TcTyVar
268 -- Instantiate with a META type variable
269 tcInstTyVar tyvar = instMetaTyVar TauTv tyvar
271 tcInstTyVars :: [TyVar] -> TcM ([TcTyVar], [TcType], TvSubst)
272 -- Instantiate with META type variables
274 = do { tc_tvs <- mapM tcInstTyVar tyvars
275 ; let tys = mkTyVarTys tc_tvs
276 ; returnM (tc_tvs, tys, zipTopTvSubst tyvars tys) }
277 -- Since the tyvars are freshly made,
278 -- they cannot possibly be captured by
279 -- any existing for-alls. Hence zipTopTvSubst
283 %************************************************************************
287 %************************************************************************
290 tcInstSigTyVars :: Bool -> SkolemInfo -> [TyVar] -> TcM [TcTyVar]
291 -- Instantiate with skolems or meta SigTvs; depending on use_skols
292 -- Always take location info from the supplied tyvars
293 tcInstSigTyVars use_skols skol_info tyvars
295 = mapM (tcInstSkolTyVar skol_info Nothing) tyvars
298 = mapM (instMetaTyVar (SigTv skol_info)) tyvars
300 zonkSigTyVar :: TcTyVar -> TcM TcTyVar
302 | isSkolemTyVar sig_tv
303 = return sig_tv -- Happens in the call in TcBinds.checkDistinctTyVars
305 = ASSERT( isSigTyVar sig_tv )
306 do { ty <- zonkTcTyVar sig_tv
307 ; return (tcGetTyVar "zonkSigTyVar" ty) }
308 -- 'ty' is bound to be a type variable, because SigTvs
309 -- can only be unified with type variables
313 %************************************************************************
317 %************************************************************************
320 newBoxyTyVar :: Kind -> TcM BoxyTyVar
321 newBoxyTyVar kind = newMetaTyVar BoxTv kind
323 newBoxyTyVars :: [Kind] -> TcM [BoxyTyVar]
324 newBoxyTyVars kinds = mapM newBoxyTyVar kinds
326 newBoxyTyVarTys :: [Kind] -> TcM [BoxyType]
327 newBoxyTyVarTys kinds = do { tvs <- mapM newBoxyTyVar kinds; return (mkTyVarTys tvs) }
329 readFilledBox :: BoxyTyVar -> TcM TcType
330 -- Read the contents of the box, which should be filled in by now
331 readFilledBox box_tv = ASSERT( isBoxyTyVar box_tv )
332 do { cts <- readMetaTyVar box_tv
334 Flexi -> pprPanic "readFilledBox" (ppr box_tv)
335 Indirect ty -> return ty }
337 tcInstBoxyTyVar :: TyVar -> TcM BoxyTyVar
338 -- Instantiate with a BOXY type variable
339 tcInstBoxyTyVar tyvar = instMetaTyVar BoxTv tyvar
343 %************************************************************************
345 \subsection{Putting and getting mutable type variables}
347 %************************************************************************
349 But it's more fun to short out indirections on the way: If this
350 version returns a TyVar, then that TyVar is unbound. If it returns
351 any other type, then there might be bound TyVars embedded inside it.
353 We return Nothing iff the original box was unbound.
356 data LookupTyVarResult -- The result of a lookupTcTyVar call
357 = DoneTv TcTyVarDetails -- SkolemTv or virgin MetaTv
360 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
362 = ASSERT( isTcTyVar tyvar )
364 SkolemTv _ -> return (DoneTv details)
365 MetaTv _ ref -> do { meta_details <- readMutVar ref
366 ; case meta_details of
367 Indirect ty -> return (IndirectTv ty)
368 Flexi -> return (DoneTv details) }
370 details = tcTyVarDetails tyvar
373 -- gaw 2004 We aren't shorting anything out anymore, at least for now
375 | not (isTcTyVar tyvar)
376 = pprTrace "getTcTyVar" (ppr tyvar) $
377 returnM (Just (mkTyVarTy tyvar))
380 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
381 readMetaTyVar tyvar `thenM` \ maybe_ty ->
383 Just ty -> short_out ty `thenM` \ ty' ->
384 writeMetaTyVar tyvar (Just ty') `thenM_`
387 Nothing -> returnM Nothing
389 short_out :: TcType -> TcM TcType
390 short_out ty@(TyVarTy tyvar)
391 | not (isTcTyVar tyvar)
395 = readMetaTyVar tyvar `thenM` \ maybe_ty ->
397 Just ty' -> short_out ty' `thenM` \ ty' ->
398 writeMetaTyVar tyvar (Just ty') `thenM_`
403 short_out other_ty = returnM other_ty
408 %************************************************************************
410 \subsection{Zonking -- the exernal interfaces}
412 %************************************************************************
414 ----------------- Type variables
417 zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
418 zonkTcTyVars tyvars = mappM zonkTcTyVar tyvars
420 zonkTcTyVarsAndFV :: [TcTyVar] -> TcM TcTyVarSet
421 zonkTcTyVarsAndFV tyvars = mappM zonkTcTyVar tyvars `thenM` \ tys ->
422 returnM (tyVarsOfTypes tys)
424 zonkTcTyVar :: TcTyVar -> TcM TcType
425 zonkTcTyVar tyvar = ASSERT( isTcTyVar tyvar )
426 zonk_tc_tyvar (\ tv -> returnM (TyVarTy tv)) tyvar
429 ----------------- Types
432 zonkTcType :: TcType -> TcM TcType
433 zonkTcType ty = zonkType (\ tv -> returnM (TyVarTy tv)) ty
435 zonkTcTypes :: [TcType] -> TcM [TcType]
436 zonkTcTypes tys = mappM zonkTcType tys
438 zonkTcClassConstraints cts = mappM zonk cts
439 where zonk (clas, tys)
440 = zonkTcTypes tys `thenM` \ new_tys ->
441 returnM (clas, new_tys)
443 zonkTcThetaType :: TcThetaType -> TcM TcThetaType
444 zonkTcThetaType theta = mappM zonkTcPredType theta
446 zonkTcPredType :: TcPredType -> TcM TcPredType
447 zonkTcPredType (ClassP c ts)
448 = zonkTcTypes ts `thenM` \ new_ts ->
449 returnM (ClassP c new_ts)
450 zonkTcPredType (IParam n t)
451 = zonkTcType t `thenM` \ new_t ->
452 returnM (IParam n new_t)
453 zonkTcPredType (EqPred t1 t2)
454 = zonkTcType t1 `thenM` \ new_t1 ->
455 zonkTcType t2 `thenM` \ new_t2 ->
456 returnM (EqPred new_t1 new_t2)
459 ------------------- These ...ToType, ...ToKind versions
460 are used at the end of type checking
463 zonkTopTyVar :: TcTyVar -> TcM TcTyVar
464 -- zonkTopTyVar is used, at the top level, on any un-instantiated meta type variables
465 -- to default the kind of ? and ?? etc to *. This is important to ensure that
466 -- instance declarations match. For example consider
467 -- instance Show (a->b)
468 -- foo x = show (\_ -> True)
469 -- Then we'll get a constraint (Show (p ->q)) where p has argTypeKind (printed ??),
470 -- and that won't match the typeKind (*) in the instance decl.
472 -- Because we are at top level, no further constraints are going to affect these
473 -- type variables, so it's time to do it by hand. However we aren't ready
474 -- to default them fully to () or whatever, because the type-class defaulting
475 -- rules have yet to run.
478 | k `eqKind` default_k = return tv
480 = do { tv' <- newFlexiTyVar default_k
481 ; writeMetaTyVar tv (mkTyVarTy tv')
485 default_k = defaultKind k
487 zonkQuantifiedTyVar :: TcTyVar -> TcM TyVar
488 -- zonkQuantifiedTyVar is applied to the a TcTyVar when quantifying over it.
489 -- It might be a meta TyVar, in which case we freeze it into an ordinary TyVar.
490 -- When we do this, we also default the kind -- see notes with Kind.defaultKind
491 -- The meta tyvar is updated to point to the new regular TyVar. Now any
492 -- bound occurences of the original type variable will get zonked to
493 -- the immutable version.
495 -- We leave skolem TyVars alone; they are immutable.
496 zonkQuantifiedTyVar tv
497 | isSkolemTyVar tv = return tv
498 -- It might be a skolem type variable,
499 -- for example from a user type signature
501 | otherwise -- It's a meta-type-variable
502 = do { details <- readMetaTyVar tv
504 -- Create the new, frozen, regular type variable
505 ; let final_kind = defaultKind (tyVarKind tv)
506 final_tv = mkTyVar (tyVarName tv) final_kind
508 -- Bind the meta tyvar to the new tyvar
510 Indirect ty -> WARN( True, ppr tv $$ ppr ty )
512 -- [Sept 04] I don't think this should happen
513 -- See note [Silly Type Synonym]
515 Flexi -> writeMetaTyVar tv (mkTyVarTy final_tv)
517 -- Return the new tyvar
521 [Silly Type Synonyms]
524 type C u a = u -- Note 'a' unused
526 foo :: (forall a. C u a -> C u a) -> u
530 bar = foo (\t -> t + t)
532 * From the (\t -> t+t) we get type {Num d} => d -> d
535 * Now unify with type of foo's arg, and we get:
536 {Num (C d a)} => C d a -> C d a
539 * Now abstract over the 'a', but float out the Num (C d a) constraint
540 because it does not 'really' mention a. (see exactTyVarsOfType)
541 The arg to foo becomes
544 * So we get a dict binding for Num (C d a), which is zonked to give
546 [Note Sept 04: now that we are zonking quantified type variables
547 on construction, the 'a' will be frozen as a regular tyvar on
548 quantification, so the floated dict will still have type (C d a).
549 Which renders this whole note moot; happily!]
551 * Then the /\a abstraction has a zonked 'a' in it.
553 All very silly. I think its harmless to ignore the problem. We'll end up with
554 a /\a in the final result but all the occurrences of a will be zonked to ()
557 %************************************************************************
559 \subsection{Zonking -- the main work-horses: zonkType, zonkTyVar}
561 %* For internal use only! *
563 %************************************************************************
566 -- For unbound, mutable tyvars, zonkType uses the function given to it
567 -- For tyvars bound at a for-all, zonkType zonks them to an immutable
568 -- type variable and zonks the kind too
570 zonkType :: (TcTyVar -> TcM Type) -- What to do with unbound mutable type variables
571 -- see zonkTcType, and zonkTcTypeToType
574 zonkType unbound_var_fn ty
577 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
579 go (TyConApp tc tys) = mappM go tys `thenM` \ tys' ->
580 returnM (TyConApp tc tys')
582 go (PredTy p) = go_pred p `thenM` \ p' ->
585 go (FunTy arg res) = go arg `thenM` \ arg' ->
586 go res `thenM` \ res' ->
587 returnM (FunTy arg' res')
589 go (AppTy fun arg) = go fun `thenM` \ fun' ->
590 go arg `thenM` \ arg' ->
591 returnM (mkAppTy fun' arg')
592 -- NB the mkAppTy; we might have instantiated a
593 -- type variable to a type constructor, so we need
594 -- to pull the TyConApp to the top.
596 -- The two interesting cases!
597 go (TyVarTy tyvar) | isTcTyVar tyvar = zonk_tc_tyvar unbound_var_fn tyvar
598 | otherwise = return (TyVarTy tyvar)
599 -- Ordinary (non Tc) tyvars occur inside quantified types
601 go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar )
602 go ty `thenM` \ ty' ->
603 returnM (ForAllTy tyvar ty')
605 go_pred (ClassP c tys) = mappM go tys `thenM` \ tys' ->
606 returnM (ClassP c tys')
607 go_pred (IParam n ty) = go ty `thenM` \ ty' ->
608 returnM (IParam n ty')
609 go_pred (EqPred ty1 ty2) = go ty1 `thenM` \ ty1' ->
610 go ty2 `thenM` \ ty2' ->
611 returnM (EqPred ty1' ty2')
613 zonk_tc_tyvar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable variable
614 -> TcTyVar -> TcM TcType
615 zonk_tc_tyvar unbound_var_fn tyvar
616 | not (isMetaTyVar tyvar) -- Skolems
617 = returnM (TyVarTy tyvar)
619 | otherwise -- Mutables
620 = do { cts <- readMetaTyVar tyvar
622 Flexi -> unbound_var_fn tyvar -- Unbound meta type variable
623 Indirect ty -> zonkType unbound_var_fn ty }
628 %************************************************************************
632 %************************************************************************
635 readKindVar :: KindVar -> TcM (MetaDetails)
636 writeKindVar :: KindVar -> TcKind -> TcM ()
637 readKindVar kv = readMutVar (kindVarRef kv)
638 writeKindVar kv val = writeMutVar (kindVarRef kv) (Indirect val)
641 zonkTcKind :: TcKind -> TcM TcKind
642 zonkTcKind k = zonkTcType k
645 zonkTcKindToKind :: TcKind -> TcM Kind
646 -- When zonking a TcKind to a kind, we need to instantiate kind variables,
647 -- Haskell specifies that * is to be used, so we follow that.
648 zonkTcKindToKind k = zonkType (\ _ -> return liftedTypeKind) k
651 %************************************************************************
653 \subsection{Checking a user type}
655 %************************************************************************
657 When dealing with a user-written type, we first translate it from an HsType
658 to a Type, performing kind checking, and then check various things that should
659 be true about it. We don't want to perform these checks at the same time
660 as the initial translation because (a) they are unnecessary for interface-file
661 types and (b) when checking a mutually recursive group of type and class decls,
662 we can't "look" at the tycons/classes yet. Also, the checks are are rather
663 diverse, and used to really mess up the other code.
665 One thing we check for is 'rank'.
667 Rank 0: monotypes (no foralls)
668 Rank 1: foralls at the front only, Rank 0 inside
669 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
671 basic ::= tyvar | T basic ... basic
673 r2 ::= forall tvs. cxt => r2a
674 r2a ::= r1 -> r2a | basic
675 r1 ::= forall tvs. cxt => r0
676 r0 ::= r0 -> r0 | basic
678 Another thing is to check that type synonyms are saturated.
679 This might not necessarily show up in kind checking.
681 data T k = MkT (k Int)
686 checkValidType :: UserTypeCtxt -> Type -> TcM ()
687 -- Checks that the type is valid for the given context
688 checkValidType ctxt ty
689 = traceTc (text "checkValidType" <+> ppr ty) `thenM_`
690 doptM Opt_GlasgowExts `thenM` \ gla_exts ->
692 rank | gla_exts = Arbitrary
694 = case ctxt of -- Haskell 98
696 LamPatSigCtxt -> Rank 0
697 BindPatSigCtxt -> Rank 0
698 DefaultDeclCtxt-> Rank 0
700 TySynCtxt _ -> Rank 0
701 ExprSigCtxt -> Rank 1
702 FunSigCtxt _ -> Rank 1
703 ConArgCtxt _ -> Rank 1 -- We are given the type of the entire
704 -- constructor, hence rank 1
705 ForSigCtxt _ -> Rank 1
706 SpecInstCtxt -> Rank 1
708 actual_kind = typeKind ty
710 kind_ok = case ctxt of
711 TySynCtxt _ -> True -- Any kind will do
712 ResSigCtxt -> isSubOpenTypeKind actual_kind
713 ExprSigCtxt -> isSubOpenTypeKind actual_kind
714 GenPatCtxt -> isLiftedTypeKind actual_kind
715 ForSigCtxt _ -> isLiftedTypeKind actual_kind
716 other -> isSubArgTypeKind actual_kind
718 ubx_tup | not gla_exts = UT_NotOk
719 | otherwise = case ctxt of
723 -- Unboxed tuples ok in function results,
724 -- but for type synonyms we allow them even at
727 -- Check that the thing has kind Type, and is lifted if necessary
728 checkTc kind_ok (kindErr actual_kind) `thenM_`
730 -- Check the internal validity of the type itself
731 check_poly_type rank ubx_tup ty `thenM_`
733 traceTc (text "checkValidType done" <+> ppr ty)
738 data Rank = Rank Int | Arbitrary
740 decRank :: Rank -> Rank
741 decRank Arbitrary = Arbitrary
742 decRank (Rank n) = Rank (n-1)
744 ----------------------------------------
745 data UbxTupFlag = UT_Ok | UT_NotOk
746 -- The "Ok" version means "ok if -fglasgow-exts is on"
748 ----------------------------------------
749 check_poly_type :: Rank -> UbxTupFlag -> Type -> TcM ()
750 check_poly_type (Rank 0) ubx_tup ty
751 = check_tau_type (Rank 0) ubx_tup ty
753 check_poly_type rank ubx_tup ty
754 | null tvs && null theta
755 = check_tau_type rank ubx_tup ty
757 = do { check_valid_theta SigmaCtxt theta
758 ; check_poly_type rank ubx_tup tau -- Allow foralls to right of arrow
759 ; checkFreeness tvs theta
760 ; checkAmbiguity tvs theta (tyVarsOfType tau) }
762 (tvs, theta, tau) = tcSplitSigmaTy ty
764 ----------------------------------------
765 check_arg_type :: Type -> TcM ()
766 -- The sort of type that can instantiate a type variable,
767 -- or be the argument of a type constructor.
768 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
769 -- Other unboxed types are very occasionally allowed as type
770 -- arguments depending on the kind of the type constructor
772 -- For example, we want to reject things like:
774 -- instance Ord a => Ord (forall s. T s a)
776 -- g :: T s (forall b.b)
778 -- NB: unboxed tuples can have polymorphic or unboxed args.
779 -- This happens in the workers for functions returning
780 -- product types with polymorphic components.
781 -- But not in user code.
782 -- Anyway, they are dealt with by a special case in check_tau_type
785 = check_poly_type Arbitrary UT_NotOk ty `thenM_`
786 checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty)
788 ----------------------------------------
789 check_tau_type :: Rank -> UbxTupFlag -> Type -> TcM ()
790 -- Rank is allowed rank for function args
791 -- No foralls otherwise
793 check_tau_type rank ubx_tup ty@(ForAllTy _ _) = failWithTc (forAllTyErr ty)
794 check_tau_type rank ubx_tup ty@(FunTy (PredTy _) _) = failWithTc (forAllTyErr ty)
795 -- Reject e.g. (Maybe (?x::Int => Int)), with a decent error message
797 -- Naked PredTys don't usually show up, but they can as a result of
798 -- {-# SPECIALISE instance Ord Char #-}
799 -- The Right Thing would be to fix the way that SPECIALISE instance pragmas
800 -- are handled, but the quick thing is just to permit PredTys here.
801 check_tau_type rank ubx_tup (PredTy sty) = getDOpts `thenM` \ dflags ->
802 check_pred_ty dflags TypeCtxt sty
804 check_tau_type rank ubx_tup (TyVarTy _) = returnM ()
805 check_tau_type rank ubx_tup ty@(FunTy arg_ty res_ty)
806 = check_poly_type (decRank rank) UT_NotOk arg_ty `thenM_`
807 check_poly_type rank UT_Ok res_ty
809 check_tau_type rank ubx_tup (AppTy ty1 ty2)
810 = check_arg_type ty1 `thenM_` check_arg_type ty2
812 check_tau_type rank ubx_tup (NoteTy other_note ty)
813 = check_tau_type rank ubx_tup ty
815 check_tau_type rank ubx_tup ty@(TyConApp tc tys)
817 = do { -- It's OK to have an *over-applied* type synonym
818 -- data Tree a b = ...
819 -- type Foo a = Tree [a]
820 -- f :: Foo a b -> ...
822 Just ty' -> check_tau_type rank ubx_tup ty' -- Check expansion
823 Nothing -> failWithTc arity_msg
825 ; gla_exts <- doptM Opt_GlasgowExts
827 -- If -fglasgow-exts then don't check the type arguments
828 -- This allows us to instantiate a synonym defn with a
829 -- for-all type, or with a partially-applied type synonym.
830 -- e.g. type T a b = a
833 -- Here, T is partially applied, so it's illegal in H98.
834 -- But if you expand S first, then T we get just
839 -- For H98, do check the type args
840 mappM_ check_arg_type tys
843 | isUnboxedTupleTyCon tc
844 = doptM Opt_GlasgowExts `thenM` \ gla_exts ->
845 checkTc (ubx_tup_ok gla_exts) ubx_tup_msg `thenM_`
846 mappM_ (check_tau_type (Rank 0) UT_Ok) tys
847 -- Args are allowed to be unlifted, or
848 -- more unboxed tuples, so can't use check_arg_ty
851 = mappM_ check_arg_type tys
854 ubx_tup_ok gla_exts = case ubx_tup of { UT_Ok -> gla_exts; other -> False }
857 tc_arity = tyConArity tc
859 arity_msg = arityErr "Type synonym" (tyConName tc) tc_arity n_args
860 ubx_tup_msg = ubxArgTyErr ty
862 ----------------------------------------
863 forAllTyErr ty = ptext SLIT("Illegal polymorphic or qualified type:") <+> ppr ty
864 unliftedArgErr ty = ptext SLIT("Illegal unlifted type argument:") <+> ppr ty
865 ubxArgTyErr ty = ptext SLIT("Illegal unboxed tuple type as function argument:") <+> ppr ty
866 kindErr kind = ptext SLIT("Expecting an ordinary type, but found a type of kind") <+> ppr kind
871 %************************************************************************
873 \subsection{Checking a theta or source type}
875 %************************************************************************
878 -- Enumerate the contexts in which a "source type", <S>, can occur
882 -- or (N a) where N is a newtype
885 = ClassSCCtxt Name -- Superclasses of clas
886 -- class <S> => C a where ...
887 | SigmaCtxt -- Theta part of a normal for-all type
888 -- f :: <S> => a -> a
889 | DataTyCtxt Name -- Theta part of a data decl
890 -- data <S> => T a = MkT a
891 | TypeCtxt -- Source type in an ordinary type
893 | InstThetaCtxt -- Context of an instance decl
894 -- instance <S> => C [a] where ...
896 pprSourceTyCtxt (ClassSCCtxt c) = ptext SLIT("the super-classes of class") <+> quotes (ppr c)
897 pprSourceTyCtxt SigmaCtxt = ptext SLIT("the context of a polymorphic type")
898 pprSourceTyCtxt (DataTyCtxt tc) = ptext SLIT("the context of the data type declaration for") <+> quotes (ppr tc)
899 pprSourceTyCtxt InstThetaCtxt = ptext SLIT("the context of an instance declaration")
900 pprSourceTyCtxt TypeCtxt = ptext SLIT("the context of a type")
904 checkValidTheta :: SourceTyCtxt -> ThetaType -> TcM ()
905 checkValidTheta ctxt theta
906 = addErrCtxt (checkThetaCtxt ctxt theta) (check_valid_theta ctxt theta)
908 -------------------------
909 check_valid_theta ctxt []
911 check_valid_theta ctxt theta
912 = getDOpts `thenM` \ dflags ->
913 warnTc (notNull dups) (dupPredWarn dups) `thenM_`
914 mappM_ (check_pred_ty dflags ctxt) theta
916 (_,dups) = removeDups tcCmpPred theta
918 -------------------------
919 check_pred_ty dflags ctxt pred@(ClassP cls tys)
920 = -- Class predicates are valid in all contexts
921 checkTc (arity == n_tys) arity_err `thenM_`
923 -- Check the form of the argument types
924 mappM_ check_arg_type tys `thenM_`
925 checkTc (check_class_pred_tys dflags ctxt tys)
926 (predTyVarErr pred $$ how_to_allow)
929 class_name = className cls
930 arity = classArity cls
932 arity_err = arityErr "Class" class_name arity n_tys
933 how_to_allow = parens (ptext SLIT("Use -fglasgow-exts to permit this"))
935 check_pred_ty dflags SigmaCtxt (IParam _ ty) = check_arg_type ty
936 -- Implicit parameters only allows in type
937 -- signatures; not in instance decls, superclasses etc
938 -- The reason for not allowing implicit params in instances is a bit subtle
939 -- If we allowed instance (?x::Int, Eq a) => Foo [a] where ...
940 -- then when we saw (e :: (?x::Int) => t) it would be unclear how to
941 -- discharge all the potential usas of the ?x in e. For example, a
942 -- constraint Foo [Int] might come out of e,and applying the
943 -- instance decl would show up two uses of ?x.
946 check_pred_ty dflags ctxt sty = failWithTc (badPredTyErr sty)
948 -------------------------
949 check_class_pred_tys dflags ctxt tys
951 TypeCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
952 InstThetaCtxt -> gla_exts || undecidable_ok || all tcIsTyVarTy tys
953 -- Further checks on head and theta in
954 -- checkInstTermination
955 other -> gla_exts || all tyvar_head tys
957 gla_exts = dopt Opt_GlasgowExts dflags
958 undecidable_ok = dopt Opt_AllowUndecidableInstances dflags
960 -------------------------
961 tyvar_head ty -- Haskell 98 allows predicates of form
962 | tcIsTyVarTy ty = True -- C (a ty1 .. tyn)
963 | otherwise -- where a is a type variable
964 = case tcSplitAppTy_maybe ty of
965 Just (ty, _) -> tyvar_head ty
972 is ambiguous if P contains generic variables
973 (i.e. one of the Vs) that are not mentioned in tau
975 However, we need to take account of functional dependencies
976 when we speak of 'mentioned in tau'. Example:
977 class C a b | a -> b where ...
979 forall x y. (C x y) => x
980 is not ambiguous because x is mentioned and x determines y
982 NB; the ambiguity check is only used for *user* types, not for types
983 coming from inteface files. The latter can legitimately have
984 ambiguous types. Example
986 class S a where s :: a -> (Int,Int)
987 instance S Char where s _ = (1,1)
988 f:: S a => [a] -> Int -> (Int,Int)
989 f (_::[a]) x = (a*x,b)
990 where (a,b) = s (undefined::a)
992 Here the worker for f gets the type
993 fw :: forall a. S a => Int -> (# Int, Int #)
995 If the list of tv_names is empty, we have a monotype, and then we
996 don't need to check for ambiguity either, because the test can't fail
1000 checkAmbiguity :: [TyVar] -> ThetaType -> TyVarSet -> TcM ()
1001 checkAmbiguity forall_tyvars theta tau_tyvars
1002 = mappM_ complain (filter is_ambig theta)
1004 complain pred = addErrTc (ambigErr pred)
1005 extended_tau_vars = grow theta tau_tyvars
1007 -- Only a *class* predicate can give rise to ambiguity
1008 -- An *implicit parameter* cannot. For example:
1009 -- foo :: (?x :: [a]) => Int
1011 -- is fine. The call site will suppply a particular 'x'
1012 is_ambig pred = isClassPred pred &&
1013 any ambig_var (varSetElems (tyVarsOfPred pred))
1015 ambig_var ct_var = (ct_var `elem` forall_tyvars) &&
1016 not (ct_var `elemVarSet` extended_tau_vars)
1019 = sep [ptext SLIT("Ambiguous constraint") <+> quotes (pprPred pred),
1020 nest 4 (ptext SLIT("At least one of the forall'd type variables mentioned by the constraint") $$
1021 ptext SLIT("must be reachable from the type after the '=>'"))]
1024 In addition, GHC insists that at least one type variable
1025 in each constraint is in V. So we disallow a type like
1026 forall a. Eq b => b -> b
1027 even in a scope where b is in scope.
1030 checkFreeness forall_tyvars theta
1031 = do { gla_exts <- doptM Opt_GlasgowExts
1032 ; if gla_exts then return () -- New! Oct06
1033 else mappM_ complain (filter is_free theta) }
1035 is_free pred = not (isIPPred pred)
1036 && not (any bound_var (varSetElems (tyVarsOfPred pred)))
1037 bound_var ct_var = ct_var `elem` forall_tyvars
1038 complain pred = addErrTc (freeErr pred)
1041 = sep [ptext SLIT("All of the type variables in the constraint") <+> quotes (pprPred pred) <+>
1042 ptext SLIT("are already in scope"),
1043 nest 4 (ptext SLIT("(at least one must be universally quantified here)"))
1048 checkThetaCtxt ctxt theta
1049 = vcat [ptext SLIT("In the context:") <+> pprTheta theta,
1050 ptext SLIT("While checking") <+> pprSourceTyCtxt ctxt ]
1052 badPredTyErr sty = ptext SLIT("Illegal constraint") <+> pprPred sty
1053 predTyVarErr pred = sep [ptext SLIT("Non type-variable argument"),
1054 nest 2 (ptext SLIT("in the constraint:") <+> pprPred pred)]
1055 dupPredWarn dups = ptext SLIT("Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
1057 arityErr kind name n m
1058 = hsep [ text kind, quotes (ppr name), ptext SLIT("should have"),
1059 n_arguments <> comma, text "but has been given", int m]
1061 n_arguments | n == 0 = ptext SLIT("no arguments")
1062 | n == 1 = ptext SLIT("1 argument")
1063 | True = hsep [int n, ptext SLIT("arguments")]
1067 %************************************************************************
1069 \subsection{Checking for a decent instance head type}
1071 %************************************************************************
1073 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1074 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1076 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1077 flag is on, or (2)~the instance is imported (they must have been
1078 compiled elsewhere). In these cases, we let them go through anyway.
1080 We can also have instances for functions: @instance Foo (a -> b) ...@.
1083 checkValidInstHead :: Type -> TcM (Class, [TcType])
1085 checkValidInstHead ty -- Should be a source type
1086 = case tcSplitPredTy_maybe ty of {
1087 Nothing -> failWithTc (instTypeErr (ppr ty) empty) ;
1090 case getClassPredTys_maybe pred of {
1091 Nothing -> failWithTc (instTypeErr (pprPred pred) empty) ;
1094 getDOpts `thenM` \ dflags ->
1095 mappM_ check_arg_type tys `thenM_`
1096 check_inst_head dflags clas tys `thenM_`
1100 check_inst_head dflags clas tys
1101 -- If GlasgowExts then check at least one isn't a type variable
1102 | dopt Opt_GlasgowExts dflags
1103 = mapM_ check_one tys
1105 -- WITH HASKELL 98, MUST HAVE C (T a b c)
1107 = checkTc (isSingleton tys && tcValidInstHeadTy first_ty)
1108 (instTypeErr (pprClassPred clas tys) head_shape_msg)
1111 (first_ty : _) = tys
1113 head_shape_msg = parens (text "The instance type must be of form (T a b c)" $$
1114 text "where T is not a synonym, and a,b,c are distinct type variables")
1116 -- For now, I only allow tau-types (not polytypes) in
1117 -- the head of an instance decl.
1118 -- E.g. instance C (forall a. a->a) is rejected
1119 -- One could imagine generalising that, but I'm not sure
1120 -- what all the consequences might be
1121 check_one ty = do { check_tau_type (Rank 0) UT_NotOk ty
1122 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1124 instTypeErr pp_ty msg
1125 = sep [ptext SLIT("Illegal instance declaration for") <+> quotes pp_ty,
1130 %************************************************************************
1132 \subsection{Checking instance for termination}
1134 %************************************************************************
1138 checkValidInstance :: [TyVar] -> ThetaType -> Class -> [TcType] -> TcM ()
1139 checkValidInstance tyvars theta clas inst_tys
1140 = do { gla_exts <- doptM Opt_GlasgowExts
1141 ; undecidable_ok <- doptM Opt_AllowUndecidableInstances
1143 ; checkValidTheta InstThetaCtxt theta
1144 ; checkAmbiguity tyvars theta (tyVarsOfTypes inst_tys)
1146 -- Check that instance inference will terminate (if we care)
1147 -- For Haskell 98, checkValidTheta has already done that
1148 ; when (gla_exts && not undecidable_ok) $
1149 mapM_ failWithTc (checkInstTermination inst_tys theta)
1151 -- The Coverage Condition
1152 ; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
1153 (instTypeErr (pprClassPred clas inst_tys) msg)
1156 msg = parens (vcat [ptext SLIT("the Coverage Condition fails for one of the functional dependencies;"),
1160 Termination test: each assertion in the context satisfies
1161 (1) no variable has more occurrences in the assertion than in the head, and
1162 (2) the assertion has fewer constructors and variables (taken together
1163 and counting repetitions) than the head.
1164 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1165 (which have already been checked) guarantee termination.
1167 The underlying idea is that
1169 for any ground substitution, each assertion in the
1170 context has fewer type constructors than the head.
1174 checkInstTermination :: [TcType] -> ThetaType -> [Message]
1175 checkInstTermination tys theta
1176 = mapCatMaybes check theta
1179 size = sizeTypes tys
1181 | not (null (fvPred pred \\ fvs))
1182 = Just (predUndecErr pred nomoreMsg $$ parens undecidableMsg)
1183 | sizePred pred >= size
1184 = Just (predUndecErr pred smallerMsg $$ parens undecidableMsg)
1188 predUndecErr pred msg = sep [msg,
1189 nest 2 (ptext SLIT("in the constraint:") <+> pprPred pred)]
1191 nomoreMsg = ptext SLIT("Variable occurs more often in a constraint than in the instance head")
1192 smallerMsg = ptext SLIT("Constraint is no smaller than the instance head")
1193 undecidableMsg = ptext SLIT("Use -fallow-undecidable-instances to permit this")
1195 -- Free variables of a type, retaining repetitions, and expanding synonyms
1196 fvType :: Type -> [TyVar]
1197 fvType ty | Just exp_ty <- tcView ty = fvType exp_ty
1198 fvType (TyVarTy tv) = [tv]
1199 fvType (TyConApp _ tys) = fvTypes tys
1200 fvType (NoteTy _ ty) = fvType ty
1201 fvType (PredTy pred) = fvPred pred
1202 fvType (FunTy arg res) = fvType arg ++ fvType res
1203 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1204 fvType (ForAllTy tyvar ty) = filter (/= tyvar) (fvType ty)
1206 fvTypes :: [Type] -> [TyVar]
1207 fvTypes tys = concat (map fvType tys)
1209 fvPred :: PredType -> [TyVar]
1210 fvPred (ClassP _ tys') = fvTypes tys'
1211 fvPred (IParam _ ty) = fvType ty
1212 fvPred (EqPred ty1 ty2) = fvType ty1 ++ fvType ty2
1214 -- Size of a type: the number of variables and constructors
1215 sizeType :: Type -> Int
1216 sizeType ty | Just exp_ty <- tcView ty = sizeType exp_ty
1217 sizeType (TyVarTy _) = 1
1218 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1219 sizeType (NoteTy _ ty) = sizeType ty
1220 sizeType (PredTy pred) = sizePred pred
1221 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1222 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1223 sizeType (ForAllTy _ ty) = sizeType ty
1225 sizeTypes :: [Type] -> Int
1226 sizeTypes xs = sum (map sizeType xs)
1228 sizePred :: PredType -> Int
1229 sizePred (ClassP _ tys') = sizeTypes tys'
1230 sizePred (IParam _ ty) = sizeType ty
1231 sizePred (EqPred ty1 ty2) = sizeType ty1 + sizeType ty2