2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section{Monadic type operations}
6 This module contains monadic operations over types that contain mutable type variables
10 TcTyVar, TcKind, TcType, TcTauType, TcThetaType, TcTyVarSet,
12 --------------------------------
13 -- Creating new mutable type variables
15 newTyFlexiVarTy, -- Kind -> TcM TcType
16 newTyFlexiVarTys, -- Int -> Kind -> TcM [TcType]
17 newKindVar, newKindVars,
18 lookupTcTyVar, condLookupTcTyVar, LookupTyVarResult(..),
19 newMetaTyVar, readMetaTyVar, writeMetaTyVar, putMetaTyVar,
21 --------------------------------
23 tcInstTyVar, tcInstTyVars, tcInstType,
24 tcSkolTyVar, tcSkolTyVars, tcSkolType,
26 --------------------------------
27 -- Checking type validity
28 Rank, UserTypeCtxt(..), checkValidType, pprHsSigCtxt,
29 SourceTyCtxt(..), checkValidTheta, checkFreeness,
30 checkValidInstHead, instTypeErr, checkAmbiguity,
31 arityErr, isRigidType,
33 --------------------------------
35 zonkType, zonkTcPredType,
36 zonkTcTyVar, zonkTcTyVars, zonkTcTyVarsAndFV, zonkQuantifiedTyVar,
37 zonkTcType, zonkTcTypes, zonkTcClassConstraints, zonkTcThetaType,
38 zonkTcKindToKind, zonkTcKind,
40 readKindVar, writeKindVar
44 #include "HsVersions.h"
48 import HsSyn ( LHsType )
49 import TypeRep ( Type(..), PredType(..), TyNote(..), -- Friend; can see representation
52 import TcType ( TcType, TcThetaType, TcTauType, TcPredType,
53 TcTyVarSet, TcKind, TcTyVar, TcTyVarDetails(..),
54 MetaDetails(..), SkolemInfo(..), isMetaTyVar, metaTvRef,
55 tcEqType, tcCmpPred, isClassPred,
56 tcSplitPhiTy, tcSplitPredTy_maybe, tcSplitAppTy_maybe,
57 tcSplitTyConApp_maybe, tcSplitForAllTys,
58 tcIsTyVarTy, tcSplitSigmaTy, tcIsTyVarTy,
59 isUnLiftedType, isIPPred, isImmutableTyVar,
60 typeKind, isFlexi, isSkolemTyVar,
61 mkAppTy, mkTyVarTy, mkTyVarTys,
62 tyVarsOfPred, getClassPredTys_maybe,
63 tyVarsOfType, tyVarsOfTypes,
64 pprPred, pprTheta, pprClassPred )
65 import Kind ( Kind(..), KindVar(..), mkKindVar, isSubKind,
66 isLiftedTypeKind, isArgTypeKind, isOpenTypeKind,
67 liftedTypeKind, defaultKind
69 import Type ( TvSubst, zipTopTvSubst, substTy )
70 import Class ( Class, classArity, className )
71 import TyCon ( TyCon, isSynTyCon, isUnboxedTupleTyCon,
72 tyConArity, tyConName )
73 import Var ( TyVar, tyVarKind, tyVarName, isTyVar,
74 mkTyVar, mkTcTyVar, tcTyVarDetails, isTcTyVar )
77 import TcRnMonad -- TcType, amongst others
78 import FunDeps ( grow )
79 import Name ( Name, setNameUnique, mkSysTvName )
82 import CmdLineOpts ( dopt, DynFlag(..) )
83 import Util ( nOfThem, isSingleton, equalLength, notNull )
84 import ListSetOps ( removeDups )
85 import SrcLoc ( unLoc )
90 %************************************************************************
92 \subsection{New type variables}
94 %************************************************************************
97 newMetaTyVar :: Name -> Kind -> MetaDetails -> TcM TyVar
98 newMetaTyVar name kind details
99 = do { ref <- newMutVar details ;
100 return (mkTcTyVar name kind (MetaTv ref)) }
102 readMetaTyVar :: TyVar -> TcM MetaDetails
103 readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
104 readMutVar (metaTvRef tyvar)
106 writeMetaTyVar :: TyVar -> MetaDetails -> TcM ()
107 writeMetaTyVar tyvar val = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
108 writeMutVar (metaTvRef tyvar) val
110 newFlexiTyVar :: Kind -> TcM TcTyVar
112 = newUnique `thenM` \ uniq ->
113 newMetaTyVar (mkSysTvName uniq FSLIT("t")) kind Flexi
115 newTyFlexiVarTy :: Kind -> TcM TcType
117 = newFlexiTyVar kind `thenM` \ tc_tyvar ->
118 returnM (TyVarTy tc_tyvar)
120 newTyFlexiVarTys :: Int -> Kind -> TcM [TcType]
121 newTyFlexiVarTys n kind = mappM newTyFlexiVarTy (nOfThem n kind)
123 isRigidType :: TcType -> TcM Bool
124 -- Check that the type is rigid, *taking the type refinement into account*
125 -- In other words if a rigid type variable tv is refined to a wobbly type,
126 -- the answer should be False
127 -- ToDo: can this happen?
129 = do { rigids <- mapM is_rigid (varSetElems (tyVarsOfType ty))
130 ; return (and rigids) }
132 is_rigid tv = do { details <- lookupTcTyVar tv
134 RigidTv -> return True
135 IndirectTv True ty -> isRigidType ty
136 other -> return False
139 newKindVar :: TcM TcKind
140 newKindVar = do { uniq <- newUnique
141 ; ref <- newMutVar Nothing
142 ; return (KindVar (mkKindVar uniq ref)) }
144 newKindVars :: Int -> TcM [TcKind]
145 newKindVars n = mappM (\ _ -> newKindVar) (nOfThem n ())
149 %************************************************************************
151 \subsection{Type instantiation}
153 %************************************************************************
155 Instantiating a bunch of type variables
159 Note that we don't change the print-name
160 This won't confuse the type checker but there's a chance
161 that two different tyvars will print the same way
162 in an error message. -dppr-debug will show up the difference
163 Better watch out for this. If worst comes to worst, just
168 -----------------------
169 tcInstTyVars :: [TyVar] -> TcM ([TcTyVar], [TcType], TvSubst)
171 = do { tc_tvs <- mappM tcInstTyVar tyvars
172 ; let tys = mkTyVarTys tc_tvs
173 ; returnM (tc_tvs, tys, zipTopTvSubst tyvars tys) }
174 -- Since the tyvars are freshly made,
175 -- they cannot possibly be captured by
176 -- any existing for-alls. Hence zipTopTvSubst
179 = do { uniq <- newUnique
180 ; let name = setNameUnique (tyVarName tyvar) uniq
181 -- See Note [TyVarName]
182 ; newMetaTyVar name (tyVarKind tyvar) Flexi }
184 tcInstType :: TcType -> TcM ([TcTyVar], TcThetaType, TcType)
185 -- tcInstType instantiates the outer-level for-alls of a TcType with
186 -- fresh (mutable) type variables, splits off the dictionary part,
187 -- and returns the pieces.
189 = case tcSplitForAllTys ty of
190 ([], rho) -> -- There may be overloading despite no type variables;
191 -- (?x :: Int) => Int -> Int
193 (theta, tau) = tcSplitPhiTy rho
195 returnM ([], theta, tau)
197 (tyvars, rho) -> tcInstTyVars tyvars `thenM` \ (tyvars', _, tenv) ->
199 (theta, tau) = tcSplitPhiTy (substTy tenv rho)
201 returnM (tyvars', theta, tau)
203 ---------------------------------------------
204 -- Similar functions but for skolem constants
206 tcSkolTyVars :: SkolemInfo -> [TyVar] -> TcM [TcTyVar]
207 tcSkolTyVars info tyvars = mappM (tcSkolTyVar info) tyvars
209 tcSkolTyVar :: SkolemInfo -> TyVar -> TcM TcTyVar
210 tcSkolTyVar info tyvar
211 = do { uniq <- newUnique
212 ; let name = setNameUnique (tyVarName tyvar) uniq
213 -- See Note [TyVarName]
214 ; return (mkTcTyVar name (tyVarKind tyvar)
217 tcSkolType :: SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcType)
219 = case tcSplitForAllTys ty of
221 (theta, tau) = tcSplitPhiTy rho
223 returnM ([], theta, tau)
225 (tyvars, rho) -> tcSkolTyVars info tyvars `thenM` \ tyvars' ->
227 tenv = zipTopTvSubst tyvars (mkTyVarTys tyvars')
228 (theta, tau) = tcSplitPhiTy (substTy tenv rho)
230 returnM (tyvars', theta, tau)
234 %************************************************************************
236 \subsection{Putting and getting mutable type variables}
238 %************************************************************************
241 putMetaTyVar :: TcTyVar -> TcType -> TcM ()
243 putMetaTyVar tyvar ty = writeMetaTyVar tyvar (Indirect ty)
245 putMetaTyVar tyvar ty
246 | not (isMetaTyVar tyvar)
247 = pprTrace "putTcTyVar" (ppr tyvar) $
251 = ASSERT( isMetaTyVar tyvar )
252 ASSERT2( k2 `isSubKind` k1, (ppr tyvar <+> ppr k1) $$ (ppr ty <+> ppr k2) )
253 do { ASSERTM( do { details <- readMetaTyVar tyvar; return (isFlexi details) } )
254 ; writeMetaTyVar tyvar (Indirect ty) }
261 But it's more fun to short out indirections on the way: If this
262 version returns a TyVar, then that TyVar is unbound. If it returns
263 any other type, then there might be bound TyVars embedded inside it.
265 We return Nothing iff the original box was unbound.
268 data LookupTyVarResult -- The result of a lookupTcTyVar call
271 | IndirectTv Bool TcType
272 -- True => This is a non-wobbly type refinement,
273 -- gotten from GADT match unification
274 -- False => This is a wobbly type,
275 -- gotten from inference unification
277 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
278 -- This function is the ONLY PLACE that we consult the
279 -- type refinement carried by the monad
281 -- The boolean returned with Indirect
283 = case tcTyVarDetails tyvar of
284 SkolemTv _ -> do { type_reft <- getTypeRefinement
285 ; case lookupVarEnv type_reft tyvar of
286 Just ty -> return (IndirectTv True ty)
287 Nothing -> return RigidTv
289 MetaTv ref -> do { details <- readMutVar ref
291 Indirect ty -> return (IndirectTv False ty)
292 Flexi -> return FlexiTv
295 -- Look up a meta type variable, conditionally consulting
296 -- the current type refinement
297 condLookupTcTyVar :: Bool -> TcTyVar -> TcM LookupTyVarResult
298 condLookupTcTyVar use_refinement tyvar
299 | use_refinement = lookupTcTyVar tyvar
301 = case tcTyVarDetails tyvar of
302 SkolemTv _ -> return RigidTv
303 MetaTv ref -> do { details <- readMutVar ref
305 Indirect ty -> return (IndirectTv False ty)
306 Flexi -> return FlexiTv
310 -- gaw 2004 We aren't shorting anything out anymore, at least for now
312 | not (isTcTyVar tyvar)
313 = pprTrace "getTcTyVar" (ppr tyvar) $
314 returnM (Just (mkTyVarTy tyvar))
317 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
318 readMetaTyVar tyvar `thenM` \ maybe_ty ->
320 Just ty -> short_out ty `thenM` \ ty' ->
321 writeMetaTyVar tyvar (Just ty') `thenM_`
324 Nothing -> returnM Nothing
326 short_out :: TcType -> TcM TcType
327 short_out ty@(TyVarTy tyvar)
328 | not (isTcTyVar tyvar)
332 = readMetaTyVar tyvar `thenM` \ maybe_ty ->
334 Just ty' -> short_out ty' `thenM` \ ty' ->
335 writeMetaTyVar tyvar (Just ty') `thenM_`
340 short_out other_ty = returnM other_ty
345 %************************************************************************
347 \subsection{Zonking -- the exernal interfaces}
349 %************************************************************************
351 ----------------- Type variables
354 zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
355 zonkTcTyVars tyvars = mappM zonkTcTyVar tyvars
357 zonkTcTyVarsAndFV :: [TcTyVar] -> TcM TcTyVarSet
358 zonkTcTyVarsAndFV tyvars = mappM zonkTcTyVar tyvars `thenM` \ tys ->
359 returnM (tyVarsOfTypes tys)
361 zonkTcTyVar :: TcTyVar -> TcM TcType
362 zonkTcTyVar tyvar = zonkTyVar (\ tv -> returnM (TyVarTy tv)) True tyvar
365 ----------------- Types
368 zonkTcType :: TcType -> TcM TcType
369 zonkTcType ty = zonkType (\ tv -> returnM (TyVarTy tv)) True ty
371 zonkTcTypes :: [TcType] -> TcM [TcType]
372 zonkTcTypes tys = mappM zonkTcType tys
374 zonkTcClassConstraints cts = mappM zonk cts
375 where zonk (clas, tys)
376 = zonkTcTypes tys `thenM` \ new_tys ->
377 returnM (clas, new_tys)
379 zonkTcThetaType :: TcThetaType -> TcM TcThetaType
380 zonkTcThetaType theta = mappM zonkTcPredType theta
382 zonkTcPredType :: TcPredType -> TcM TcPredType
383 zonkTcPredType (ClassP c ts)
384 = zonkTcTypes ts `thenM` \ new_ts ->
385 returnM (ClassP c new_ts)
386 zonkTcPredType (IParam n t)
387 = zonkTcType t `thenM` \ new_t ->
388 returnM (IParam n new_t)
391 ------------------- These ...ToType, ...ToKind versions
392 are used at the end of type checking
395 zonkQuantifiedTyVar :: TcTyVar -> TcM TyVar
396 -- zonkQuantifiedTyVar is applied to the a TcTyVar when quantifying over it.
397 -- It might be a meta TyVar, in which case we freeze it inot ano ordinary TyVar.
398 -- When we do this, we also default the kind -- see notes with Kind.defaultKind
399 -- The meta tyvar is updated to point to the new regular TyVar. Now any
400 -- bound occurences of the original type variable will get zonked to
401 -- the immutable version.
403 -- We leave skolem TyVars alone; they are imutable.
404 zonkQuantifiedTyVar tv
405 | isSkolemTyVar tv = return tv
406 -- It might be a skolem type variable,
407 -- for example from a user type signature
409 | otherwise -- It's a meta-type-variable
410 = do { details <- readMetaTyVar tv
412 -- Create the new, frozen, regular type variable
413 ; let final_kind = defaultKind (tyVarKind tv)
414 final_tv = mkTyVar (tyVarName tv) final_kind
416 -- Bind the meta tyvar to the new tyvar
418 Indirect ty -> WARN( True, ppr tv $$ ppr ty )
420 -- [Sept 04] I don't think this should happen
421 -- See note [Silly Type Synonym]
423 other -> writeMetaTyVar tv (Indirect (mkTyVarTy final_tv))
425 -- Return the new tyvar
429 [Silly Type Synonyms]
432 type C u a = u -- Note 'a' unused
434 foo :: (forall a. C u a -> C u a) -> u
438 bar = foo (\t -> t + t)
440 * From the (\t -> t+t) we get type {Num d} => d -> d
443 * Now unify with type of foo's arg, and we get:
444 {Num (C d a)} => C d a -> C d a
447 * Now abstract over the 'a', but float out the Num (C d a) constraint
448 because it does not 'really' mention a. (see Type.tyVarsOfType)
449 The arg to foo becomes
452 * So we get a dict binding for Num (C d a), which is zonked to give
454 [Note Sept 04: now that we are zonking quantified type variables
455 on construction, the 'a' will be frozen as a regular tyvar on
456 quantification, so the floated dict will still have type (C d a).
457 Which renders this whole note moot; happily!]
459 * Then the /\a abstraction has a zonked 'a' in it.
461 All very silly. I think its harmless to ignore the problem. We'll end up with
462 a /\a in the final result but all the occurrences of a will be zonked to ()
465 %************************************************************************
467 \subsection{Zonking -- the main work-horses: zonkType, zonkTyVar}
469 %* For internal use only! *
471 %************************************************************************
474 -- For unbound, mutable tyvars, zonkType uses the function given to it
475 -- For tyvars bound at a for-all, zonkType zonks them to an immutable
476 -- type variable and zonks the kind too
478 zonkType :: (TcTyVar -> TcM Type) -- What to do with unbound mutable type variables
479 -- see zonkTcType, and zonkTcTypeToType
480 -> Bool -- Should we consult the current type refinement?
483 zonkType unbound_var_fn rflag ty
486 go (TyConApp tycon tys) = mappM go tys `thenM` \ tys' ->
487 returnM (TyConApp tycon tys')
489 go (NewTcApp tycon tys) = mappM go tys `thenM` \ tys' ->
490 returnM (NewTcApp tycon tys')
492 go (NoteTy (SynNote ty1) ty2) = go ty1 `thenM` \ ty1' ->
493 go ty2 `thenM` \ ty2' ->
494 returnM (NoteTy (SynNote ty1') ty2')
496 go (NoteTy (FTVNote _) ty2) = go ty2 -- Discard free-tyvar annotations
498 go (PredTy p) = go_pred p `thenM` \ p' ->
501 go (FunTy arg res) = go arg `thenM` \ arg' ->
502 go res `thenM` \ res' ->
503 returnM (FunTy arg' res')
505 go (AppTy fun arg) = go fun `thenM` \ fun' ->
506 go arg `thenM` \ arg' ->
507 returnM (mkAppTy fun' arg')
508 -- NB the mkAppTy; we might have instantiated a
509 -- type variable to a type constructor, so we need
510 -- to pull the TyConApp to the top.
512 -- The two interesting cases!
513 go (TyVarTy tyvar) = zonkTyVar unbound_var_fn rflag tyvar
515 go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar )
516 go ty `thenM` \ ty' ->
517 returnM (ForAllTy tyvar ty')
519 go_pred (ClassP c tys) = mappM go tys `thenM` \ tys' ->
520 returnM (ClassP c tys')
521 go_pred (IParam n ty) = go ty `thenM` \ ty' ->
522 returnM (IParam n ty')
524 zonkTyVar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable variable
525 -> Bool -- Consult the type refinement?
526 -> TcTyVar -> TcM TcType
527 zonkTyVar unbound_var_fn rflag tyvar
528 | not (isTcTyVar tyvar) -- This can happen when
529 -- zonking a forall type, when the bound type variable
530 -- needn't be mutable
531 = returnM (TyVarTy tyvar)
534 = condLookupTcTyVar rflag tyvar `thenM` \ details ->
536 -- If b is true, the variable was refined, and therefore it is okay
537 -- to continue refining inside. Otherwise it was wobbly and we should
538 -- not refine further inside.
539 IndirectTv b ty -> zonkType unbound_var_fn b ty -- Bound flexi/refined rigid
540 FlexiTv -> unbound_var_fn tyvar -- Unbound flexi
541 RigidTv -> return (TyVarTy tyvar) -- Rigid, no zonking necessary
546 %************************************************************************
550 %************************************************************************
553 readKindVar :: KindVar -> TcM (Maybe TcKind)
554 writeKindVar :: KindVar -> TcKind -> TcM ()
555 readKindVar (KVar _ ref) = readMutVar ref
556 writeKindVar (KVar _ ref) val = writeMutVar ref (Just val)
559 zonkTcKind :: TcKind -> TcM TcKind
560 zonkTcKind (FunKind k1 k2) = do { k1' <- zonkTcKind k1
561 ; k2' <- zonkTcKind k2
562 ; returnM (FunKind k1' k2') }
563 zonkTcKind k@(KindVar kv) = do { mb_kind <- readKindVar kv
566 Just k -> zonkTcKind k }
567 zonkTcKind other_kind = returnM other_kind
570 zonkTcKindToKind :: TcKind -> TcM Kind
571 zonkTcKindToKind (FunKind k1 k2) = do { k1' <- zonkTcKindToKind k1
572 ; k2' <- zonkTcKindToKind k2
573 ; returnM (FunKind k1' k2') }
575 zonkTcKindToKind (KindVar kv) = do { mb_kind <- readKindVar kv
577 Nothing -> return liftedTypeKind
578 Just k -> zonkTcKindToKind k }
580 zonkTcKindToKind OpenTypeKind = returnM liftedTypeKind -- An "Open" kind defaults to *
581 zonkTcKindToKind other_kind = returnM other_kind
584 %************************************************************************
586 \subsection{Checking a user type}
588 %************************************************************************
590 When dealing with a user-written type, we first translate it from an HsType
591 to a Type, performing kind checking, and then check various things that should
592 be true about it. We don't want to perform these checks at the same time
593 as the initial translation because (a) they are unnecessary for interface-file
594 types and (b) when checking a mutually recursive group of type and class decls,
595 we can't "look" at the tycons/classes yet. Also, the checks are are rather
596 diverse, and used to really mess up the other code.
598 One thing we check for is 'rank'.
600 Rank 0: monotypes (no foralls)
601 Rank 1: foralls at the front only, Rank 0 inside
602 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
604 basic ::= tyvar | T basic ... basic
606 r2 ::= forall tvs. cxt => r2a
607 r2a ::= r1 -> r2a | basic
608 r1 ::= forall tvs. cxt => r0
609 r0 ::= r0 -> r0 | basic
611 Another thing is to check that type synonyms are saturated.
612 This might not necessarily show up in kind checking.
614 data T k = MkT (k Int)
620 = FunSigCtxt Name -- Function type signature
621 | ExprSigCtxt -- Expression type signature
622 | ConArgCtxt Name -- Data constructor argument
623 | TySynCtxt Name -- RHS of a type synonym decl
624 | GenPatCtxt -- Pattern in generic decl
625 -- f{| a+b |} (Inl x) = ...
626 | PatSigCtxt -- Type sig in pattern
628 | ResSigCtxt -- Result type sig
630 | ForSigCtxt Name -- Foreign inport or export signature
631 | RuleSigCtxt Name -- Signature on a forall'd variable in a RULE
632 | DefaultDeclCtxt -- Types in a default declaration
634 -- Notes re TySynCtxt
635 -- We allow type synonyms that aren't types; e.g. type List = []
637 -- If the RHS mentions tyvars that aren't in scope, we'll
638 -- quantify over them:
639 -- e.g. type T = a->a
640 -- will become type T = forall a. a->a
642 -- With gla-exts that's right, but for H98 we should complain.
645 pprHsSigCtxt :: UserTypeCtxt -> LHsType Name -> SDoc
646 pprHsSigCtxt ctxt hs_ty = pprUserTypeCtxt (unLoc hs_ty) ctxt
648 pprUserTypeCtxt ty (FunSigCtxt n) = sep [ptext SLIT("In the type signature:"), pp_sig n ty]
649 pprUserTypeCtxt ty ExprSigCtxt = sep [ptext SLIT("In an expression type signature:"), nest 2 (ppr ty)]
650 pprUserTypeCtxt ty (ConArgCtxt c) = sep [ptext SLIT("In the type of the constructor"), pp_sig c ty]
651 pprUserTypeCtxt ty (TySynCtxt c) = sep [ptext SLIT("In the RHS of the type synonym") <+> quotes (ppr c) <> comma,
652 nest 2 (ptext SLIT(", namely") <+> ppr ty)]
653 pprUserTypeCtxt ty GenPatCtxt = sep [ptext SLIT("In the type pattern of a generic definition:"), nest 2 (ppr ty)]
654 pprUserTypeCtxt ty PatSigCtxt = sep [ptext SLIT("In a pattern type signature:"), nest 2 (ppr ty)]
655 pprUserTypeCtxt ty ResSigCtxt = sep [ptext SLIT("In a result type signature:"), nest 2 (ppr ty)]
656 pprUserTypeCtxt ty (ForSigCtxt n) = sep [ptext SLIT("In the foreign declaration:"), pp_sig n ty]
657 pprUserTypeCtxt ty (RuleSigCtxt n) = sep [ptext SLIT("In the type signature:"), pp_sig n ty]
658 pprUserTypeCtxt ty DefaultDeclCtxt = sep [ptext SLIT("In a type in a `default' declaration:"), nest 2 (ppr ty)]
660 pp_sig n ty = nest 2 (ppr n <+> dcolon <+> ppr ty)
664 checkValidType :: UserTypeCtxt -> Type -> TcM ()
665 -- Checks that the type is valid for the given context
666 checkValidType ctxt ty
667 = traceTc (text "checkValidType" <+> ppr ty) `thenM_`
668 doptM Opt_GlasgowExts `thenM` \ gla_exts ->
670 rank | gla_exts = Arbitrary
672 = case ctxt of -- Haskell 98
675 DefaultDeclCtxt-> Rank 0
677 TySynCtxt _ -> Rank 0
678 ExprSigCtxt -> Rank 1
679 FunSigCtxt _ -> Rank 1
680 ConArgCtxt _ -> Rank 1 -- We are given the type of the entire
681 -- constructor, hence rank 1
682 ForSigCtxt _ -> Rank 1
683 RuleSigCtxt _ -> Rank 1
685 actual_kind = typeKind ty
687 kind_ok = case ctxt of
688 TySynCtxt _ -> True -- Any kind will do
689 ResSigCtxt -> isOpenTypeKind actual_kind
690 ExprSigCtxt -> isOpenTypeKind actual_kind
691 GenPatCtxt -> isLiftedTypeKind actual_kind
692 ForSigCtxt _ -> isLiftedTypeKind actual_kind
693 other -> isArgTypeKind actual_kind
695 ubx_tup | not gla_exts = UT_NotOk
696 | otherwise = case ctxt of
700 -- Unboxed tuples ok in function results,
701 -- but for type synonyms we allow them even at
704 -- Check that the thing has kind Type, and is lifted if necessary
705 checkTc kind_ok (kindErr actual_kind) `thenM_`
707 -- Check the internal validity of the type itself
708 check_poly_type rank ubx_tup ty `thenM_`
710 traceTc (text "checkValidType done" <+> ppr ty)
715 data Rank = Rank Int | Arbitrary
717 decRank :: Rank -> Rank
718 decRank Arbitrary = Arbitrary
719 decRank (Rank n) = Rank (n-1)
721 ----------------------------------------
722 data UbxTupFlag = UT_Ok | UT_NotOk
723 -- The "Ok" version means "ok if -fglasgow-exts is on"
725 ----------------------------------------
726 check_poly_type :: Rank -> UbxTupFlag -> Type -> TcM ()
727 check_poly_type (Rank 0) ubx_tup ty
728 = check_tau_type (Rank 0) ubx_tup ty
730 check_poly_type rank ubx_tup ty
732 (tvs, theta, tau) = tcSplitSigmaTy ty
734 check_valid_theta SigmaCtxt theta `thenM_`
735 check_tau_type (decRank rank) ubx_tup tau `thenM_`
736 checkFreeness tvs theta `thenM_`
737 checkAmbiguity tvs theta (tyVarsOfType tau)
739 ----------------------------------------
740 check_arg_type :: Type -> TcM ()
741 -- The sort of type that can instantiate a type variable,
742 -- or be the argument of a type constructor.
743 -- Not an unboxed tuple, not a forall.
744 -- Other unboxed types are very occasionally allowed as type
745 -- arguments depending on the kind of the type constructor
747 -- For example, we want to reject things like:
749 -- instance Ord a => Ord (forall s. T s a)
751 -- g :: T s (forall b.b)
753 -- NB: unboxed tuples can have polymorphic or unboxed args.
754 -- This happens in the workers for functions returning
755 -- product types with polymorphic components.
756 -- But not in user code.
757 -- Anyway, they are dealt with by a special case in check_tau_type
760 = check_tau_type (Rank 0) UT_NotOk ty `thenM_`
761 checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty)
763 ----------------------------------------
764 check_tau_type :: Rank -> UbxTupFlag -> Type -> TcM ()
765 -- Rank is allowed rank for function args
766 -- No foralls otherwise
768 check_tau_type rank ubx_tup ty@(ForAllTy _ _) = failWithTc (forAllTyErr ty)
769 check_tau_type rank ubx_tup (PredTy sty) = getDOpts `thenM` \ dflags ->
770 check_source_ty dflags TypeCtxt sty
771 check_tau_type rank ubx_tup (TyVarTy _) = returnM ()
772 check_tau_type rank ubx_tup ty@(FunTy arg_ty res_ty)
773 = check_poly_type rank UT_NotOk arg_ty `thenM_`
774 check_tau_type rank UT_Ok res_ty
776 check_tau_type rank ubx_tup (AppTy ty1 ty2)
777 = check_arg_type ty1 `thenM_` check_arg_type ty2
779 check_tau_type rank ubx_tup (NoteTy (SynNote syn) ty)
780 -- Synonym notes are built only when the synonym is
781 -- saturated (see Type.mkSynTy)
782 = doptM Opt_GlasgowExts `thenM` \ gla_exts ->
784 -- If -fglasgow-exts then don't check the 'note' part.
785 -- This allows us to instantiate a synonym defn with a
786 -- for-all type, or with a partially-applied type synonym.
787 -- e.g. type T a b = a
790 -- Here, T is partially applied, so it's illegal in H98.
791 -- But if you expand S first, then T we get just
796 -- For H98, do check the un-expanded part
797 check_tau_type rank ubx_tup syn
800 check_tau_type rank ubx_tup ty
802 check_tau_type rank ubx_tup (NoteTy other_note ty)
803 = check_tau_type rank ubx_tup ty
805 check_tau_type rank ubx_tup (NewTcApp tc tys)
806 = mappM_ check_arg_type tys
808 check_tau_type rank ubx_tup ty@(TyConApp tc tys)
810 = -- NB: Type.mkSynTy builds a TyConApp (not a NoteTy) for an unsaturated
811 -- synonym application, leaving it to checkValidType (i.e. right here)
813 checkTc syn_arity_ok arity_msg `thenM_`
814 mappM_ check_arg_type tys
816 | isUnboxedTupleTyCon tc
817 = doptM Opt_GlasgowExts `thenM` \ gla_exts ->
818 checkTc (ubx_tup_ok gla_exts) ubx_tup_msg `thenM_`
819 mappM_ (check_tau_type (Rank 0) UT_Ok) tys
820 -- Args are allowed to be unlifted, or
821 -- more unboxed tuples, so can't use check_arg_ty
824 = mappM_ check_arg_type tys
827 ubx_tup_ok gla_exts = case ubx_tup of { UT_Ok -> gla_exts; other -> False }
829 syn_arity_ok = tc_arity <= n_args
830 -- It's OK to have an *over-applied* type synonym
831 -- data Tree a b = ...
832 -- type Foo a = Tree [a]
833 -- f :: Foo a b -> ...
835 tc_arity = tyConArity tc
837 arity_msg = arityErr "Type synonym" (tyConName tc) tc_arity n_args
838 ubx_tup_msg = ubxArgTyErr ty
840 ----------------------------------------
841 forAllTyErr ty = ptext SLIT("Illegal polymorphic type:") <+> ppr ty
842 unliftedArgErr ty = ptext SLIT("Illegal unlifted type argument:") <+> ppr ty
843 ubxArgTyErr ty = ptext SLIT("Illegal unboxed tuple type as function argument:") <+> ppr ty
844 kindErr kind = ptext SLIT("Expecting an ordinary type, but found a type of kind") <+> ppr kind
849 %************************************************************************
851 \subsection{Checking a theta or source type}
853 %************************************************************************
856 -- Enumerate the contexts in which a "source type", <S>, can occur
860 -- or (N a) where N is a newtype
863 = ClassSCCtxt Name -- Superclasses of clas
864 -- class <S> => C a where ...
865 | SigmaCtxt -- Theta part of a normal for-all type
866 -- f :: <S> => a -> a
867 | DataTyCtxt Name -- Theta part of a data decl
868 -- data <S> => T a = MkT a
869 | TypeCtxt -- Source type in an ordinary type
871 | InstThetaCtxt -- Context of an instance decl
872 -- instance <S> => C [a] where ...
873 | InstHeadCtxt -- Head of an instance decl
874 -- instance ... => Eq a where ...
876 pprSourceTyCtxt (ClassSCCtxt c) = ptext SLIT("the super-classes of class") <+> quotes (ppr c)
877 pprSourceTyCtxt SigmaCtxt = ptext SLIT("the context of a polymorphic type")
878 pprSourceTyCtxt (DataTyCtxt tc) = ptext SLIT("the context of the data type declaration for") <+> quotes (ppr tc)
879 pprSourceTyCtxt InstThetaCtxt = ptext SLIT("the context of an instance declaration")
880 pprSourceTyCtxt InstHeadCtxt = ptext SLIT("the head of an instance declaration")
881 pprSourceTyCtxt TypeCtxt = ptext SLIT("the context of a type")
885 checkValidTheta :: SourceTyCtxt -> ThetaType -> TcM ()
886 checkValidTheta ctxt theta
887 = addErrCtxt (checkThetaCtxt ctxt theta) (check_valid_theta ctxt theta)
889 -------------------------
890 check_valid_theta ctxt []
892 check_valid_theta ctxt theta
893 = getDOpts `thenM` \ dflags ->
894 warnTc (notNull dups) (dupPredWarn dups) `thenM_`
895 -- Actually, in instance decls and type signatures,
896 -- duplicate constraints are eliminated by TcHsType.hoistForAllTys,
897 -- so this error can only fire for the context of a class or
899 mappM_ (check_source_ty dflags ctxt) theta
901 (_,dups) = removeDups tcCmpPred theta
903 -------------------------
904 check_source_ty dflags ctxt pred@(ClassP cls tys)
905 = -- Class predicates are valid in all contexts
906 checkTc (arity == n_tys) arity_err `thenM_`
908 -- Check the form of the argument types
909 mappM_ check_arg_type tys `thenM_`
910 checkTc (check_class_pred_tys dflags ctxt tys)
911 (predTyVarErr pred $$ how_to_allow)
914 class_name = className cls
915 arity = classArity cls
917 arity_err = arityErr "Class" class_name arity n_tys
919 how_to_allow = case ctxt of
920 InstHeadCtxt -> empty -- Should not happen
921 InstThetaCtxt -> parens undecidableMsg
922 other -> parens (ptext SLIT("Use -fglasgow-exts to permit this"))
924 check_source_ty dflags SigmaCtxt (IParam _ ty) = check_arg_type ty
925 -- Implicit parameters only allows in type
926 -- signatures; not in instance decls, superclasses etc
927 -- The reason for not allowing implicit params in instances is a bit subtle
928 -- If we allowed instance (?x::Int, Eq a) => Foo [a] where ...
929 -- then when we saw (e :: (?x::Int) => t) it would be unclear how to
930 -- discharge all the potential usas of the ?x in e. For example, a
931 -- constraint Foo [Int] might come out of e,and applying the
932 -- instance decl would show up two uses of ?x.
935 check_source_ty dflags ctxt sty = failWithTc (badSourceTyErr sty)
937 -------------------------
938 check_class_pred_tys dflags ctxt tys
940 InstHeadCtxt -> True -- We check for instance-head
941 -- formation in checkValidInstHead
942 InstThetaCtxt -> undecidable_ok || all tcIsTyVarTy tys
943 other -> gla_exts || all tyvar_head tys
945 undecidable_ok = dopt Opt_AllowUndecidableInstances dflags
946 gla_exts = dopt Opt_GlasgowExts dflags
948 -------------------------
949 tyvar_head ty -- Haskell 98 allows predicates of form
950 | tcIsTyVarTy ty = True -- C (a ty1 .. tyn)
951 | otherwise -- where a is a type variable
952 = case tcSplitAppTy_maybe ty of
953 Just (ty, _) -> tyvar_head ty
960 is ambiguous if P contains generic variables
961 (i.e. one of the Vs) that are not mentioned in tau
963 However, we need to take account of functional dependencies
964 when we speak of 'mentioned in tau'. Example:
965 class C a b | a -> b where ...
967 forall x y. (C x y) => x
968 is not ambiguous because x is mentioned and x determines y
970 NB; the ambiguity check is only used for *user* types, not for types
971 coming from inteface files. The latter can legitimately have
972 ambiguous types. Example
974 class S a where s :: a -> (Int,Int)
975 instance S Char where s _ = (1,1)
976 f:: S a => [a] -> Int -> (Int,Int)
977 f (_::[a]) x = (a*x,b)
978 where (a,b) = s (undefined::a)
980 Here the worker for f gets the type
981 fw :: forall a. S a => Int -> (# Int, Int #)
983 If the list of tv_names is empty, we have a monotype, and then we
984 don't need to check for ambiguity either, because the test can't fail
988 checkAmbiguity :: [TyVar] -> ThetaType -> TyVarSet -> TcM ()
989 checkAmbiguity forall_tyvars theta tau_tyvars
990 = mappM_ complain (filter is_ambig theta)
992 complain pred = addErrTc (ambigErr pred)
993 extended_tau_vars = grow theta tau_tyvars
995 -- Only a *class* predicate can give rise to ambiguity
996 -- An *implicit parameter* cannot. For example:
997 -- foo :: (?x :: [a]) => Int
999 -- is fine. The call site will suppply a particular 'x'
1000 is_ambig pred = isClassPred pred &&
1001 any ambig_var (varSetElems (tyVarsOfPred pred))
1003 ambig_var ct_var = (ct_var `elem` forall_tyvars) &&
1004 not (ct_var `elemVarSet` extended_tau_vars)
1007 = sep [ptext SLIT("Ambiguous constraint") <+> quotes (pprPred pred),
1008 nest 4 (ptext SLIT("At least one of the forall'd type variables mentioned by the constraint") $$
1009 ptext SLIT("must be reachable from the type after the '=>'"))]
1012 In addition, GHC insists that at least one type variable
1013 in each constraint is in V. So we disallow a type like
1014 forall a. Eq b => b -> b
1015 even in a scope where b is in scope.
1018 checkFreeness forall_tyvars theta
1019 = mappM_ complain (filter is_free theta)
1021 is_free pred = not (isIPPred pred)
1022 && not (any bound_var (varSetElems (tyVarsOfPred pred)))
1023 bound_var ct_var = ct_var `elem` forall_tyvars
1024 complain pred = addErrTc (freeErr pred)
1027 = sep [ptext SLIT("All of the type variables in the constraint") <+> quotes (pprPred pred) <+>
1028 ptext SLIT("are already in scope"),
1029 nest 4 (ptext SLIT("(at least one must be universally quantified here)"))
1034 checkThetaCtxt ctxt theta
1035 = vcat [ptext SLIT("In the context:") <+> pprTheta theta,
1036 ptext SLIT("While checking") <+> pprSourceTyCtxt ctxt ]
1038 badSourceTyErr sty = ptext SLIT("Illegal constraint") <+> pprPred sty
1039 predTyVarErr pred = ptext SLIT("Non-type variables in constraint:") <+> pprPred pred
1040 dupPredWarn dups = ptext SLIT("Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
1042 arityErr kind name n m
1043 = hsep [ text kind, quotes (ppr name), ptext SLIT("should have"),
1044 n_arguments <> comma, text "but has been given", int m]
1046 n_arguments | n == 0 = ptext SLIT("no arguments")
1047 | n == 1 = ptext SLIT("1 argument")
1048 | True = hsep [int n, ptext SLIT("arguments")]
1052 %************************************************************************
1054 \subsection{Checking for a decent instance head type}
1056 %************************************************************************
1058 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1059 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1061 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1062 flag is on, or (2)~the instance is imported (they must have been
1063 compiled elsewhere). In these cases, we let them go through anyway.
1065 We can also have instances for functions: @instance Foo (a -> b) ...@.
1068 checkValidInstHead :: Type -> TcM (Class, [TcType])
1070 checkValidInstHead ty -- Should be a source type
1071 = case tcSplitPredTy_maybe ty of {
1072 Nothing -> failWithTc (instTypeErr (ppr ty) empty) ;
1075 case getClassPredTys_maybe pred of {
1076 Nothing -> failWithTc (instTypeErr (pprPred pred) empty) ;
1079 getDOpts `thenM` \ dflags ->
1080 mappM_ check_arg_type tys `thenM_`
1081 check_inst_head dflags clas tys `thenM_`
1085 check_inst_head dflags clas tys
1086 -- If GlasgowExts then check at least one isn't a type variable
1087 | dopt Opt_GlasgowExts dflags
1088 = check_tyvars dflags clas tys
1090 -- WITH HASKELL 1.4, MUST HAVE C (T a b c)
1092 Just (tycon, arg_tys) <- tcSplitTyConApp_maybe first_ty,
1093 not (isSynTyCon tycon), -- ...but not a synonym
1094 all tcIsTyVarTy arg_tys, -- Applied to type variables
1095 equalLength (varSetElems (tyVarsOfTypes arg_tys)) arg_tys
1096 -- This last condition checks that all the type variables are distinct
1100 = failWithTc (instTypeErr (pprClassPred clas tys) head_shape_msg)
1103 (first_ty : _) = tys
1105 head_shape_msg = parens (text "The instance type must be of form (T a b c)" $$
1106 text "where T is not a synonym, and a,b,c are distinct type variables")
1108 check_tyvars dflags clas tys
1109 -- Check that at least one isn't a type variable
1110 -- unless -fallow-undecideable-instances
1111 | dopt Opt_AllowUndecidableInstances dflags = returnM ()
1112 | not (all tcIsTyVarTy tys) = returnM ()
1113 | otherwise = failWithTc (instTypeErr (pprClassPred clas tys) msg)
1115 msg = parens (ptext SLIT("There must be at least one non-type-variable in the instance head")
1118 undecidableMsg = ptext SLIT("Use -fallow-undecidable-instances to permit this")
1122 instTypeErr pp_ty msg
1123 = sep [ptext SLIT("Illegal instance declaration for") <+> quotes pp_ty,