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(..),
23 newMetaTyVar, readMetaTyVar, writeMetaTyVar, isFilledMetaTyVar,
25 --------------------------------
26 -- Boxy type variables
27 newBoxyTyVar, newBoxyTyVars, newBoxyTyVarTys, readFilledBox,
29 --------------------------------
30 -- Creating new coercion variables
31 newCoVars, newMetaCoVar,
33 --------------------------------
35 tcInstTyVar, tcInstType, tcInstTyVars, tcInstBoxyTyVar,
37 tcInstSkolTyVars, tcInstSkolType,
38 tcSkolSigType, tcSkolSigTyVars, occurCheckErr,
40 --------------------------------
41 -- Checking type validity
42 Rank, UserTypeCtxt(..), checkValidType, checkValidMonoType,
43 SourceTyCtxt(..), checkValidTheta, checkFreeness,
44 checkValidInstHead, checkValidInstance,
45 checkInstTermination, checkValidTypeInst, checkTyFamFreeness,
46 checkUpdateMeta, updateMeta, checkTauTvUpdate, fillBoxWithTau, unifyKindCtxt,
47 unifyKindMisMatch, validDerivPred, arityErr, notMonoType, notMonoArgs,
49 --------------------------------
51 zonkType, zonkTcPredType,
52 zonkTcTyVar, zonkTcTyVars, zonkTcTyVarsAndFV, zonkSigTyVar,
53 zonkQuantifiedTyVar, zonkQuantifiedTyVars,
54 zonkTcType, zonkTcTypes, zonkTcThetaType,
55 zonkTcKindToKind, zonkTcKind, zonkTopTyVar,
57 readKindVar, writeKindVar
60 #include "HsVersions.h"
72 import TcRnMonad -- TcType, amongst others
88 import Data.List ( (\\) )
92 %************************************************************************
94 Instantiation in general
96 %************************************************************************
99 tcInstType :: ([TyVar] -> TcM [TcTyVar]) -- How to instantiate the type variables
100 -> TcType -- Type to instantiate
101 -> TcM ([TcTyVar], TcThetaType, TcType) -- Result
102 -- (type vars (excl coercion vars), preds (incl equalities), rho)
103 tcInstType inst_tyvars ty
104 = case tcSplitForAllTys ty of
105 ([], rho) -> let -- There may be overloading despite no type variables;
106 -- (?x :: Int) => Int -> Int
107 (theta, tau) = tcSplitPhiTy rho
109 return ([], theta, tau)
111 (tyvars, rho) -> do { tyvars' <- inst_tyvars tyvars
113 ; let tenv = zipTopTvSubst tyvars (mkTyVarTys tyvars')
114 -- Either the tyvars are freshly made, by inst_tyvars,
115 -- or (in the call from tcSkolSigType) any nested foralls
116 -- have different binders. Either way, zipTopTvSubst is ok
118 ; let (theta, tau) = tcSplitPhiTy (substTy tenv rho)
119 ; return (tyvars', theta, tau) }
123 %************************************************************************
127 %************************************************************************
129 Can't be in TcUnify, as we also need it in TcTyFuns.
133 -- False <=> the two args are (actual, expected) respectively
134 -- True <=> the two args are (expected, actual) respectively
136 checkUpdateMeta :: SwapFlag
137 -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
138 -- Update tv1, which is flexi; occurs check is alrady done
139 -- The 'check' version does a kind check too
140 -- We do a sub-kind check here: we might unify (a b) with (c d)
141 -- where b::*->* and d::*; this should fail
143 checkUpdateMeta swapped tv1 ref1 ty2
144 = do { checkKinds swapped tv1 ty2
145 ; updateMeta tv1 ref1 ty2 }
147 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
148 updateMeta tv1 ref1 ty2
149 = ASSERT( isMetaTyVar tv1 )
150 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
151 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
152 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
153 ; writeMutVar ref1 (Indirect ty2)
157 checkKinds :: Bool -> TyVar -> Type -> TcM ()
158 checkKinds swapped tv1 ty2
159 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
160 -- ty2 has been zonked at this stage, which ensures that
161 -- its kind has as much boxity information visible as possible.
162 | tk2 `isSubKind` tk1 = return ()
165 -- Either the kinds aren't compatible
166 -- (can happen if we unify (a b) with (c d))
167 -- or we are unifying a lifted type variable with an
168 -- unlifted type: e.g. (id 3#) is illegal
169 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
170 unifyKindMisMatch k1 k2
172 (k1,k2) | swapped = (tk2,tk1)
173 | otherwise = (tk1,tk2)
178 checkTauTvUpdate :: TcTyVar -> TcType -> TcM (Maybe TcType)
179 -- (checkTauTvUpdate tv ty)
180 -- We are about to update the TauTv tv with ty.
181 -- Check (a) that tv doesn't occur in ty (occurs check)
182 -- (b) that ty is a monotype
183 -- Furthermore, in the interest of (b), if you find an
184 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
186 -- We have three possible outcomes:
187 -- (1) Return the (non-boxy) type to update the type variable with,
188 -- [we know the update is ok!]
189 -- (2) return Nothing, or
190 -- [we cannot tell whether the update is ok right now]
192 -- [the update is definitely invalid]
193 -- We return Nothing in case the tv occurs in ty *under* a type family
194 -- application. In this case, we must not update tv (to avoid a cyclic type
195 -- term), but we also cannot fail claiming an infinite type. Given
197 -- type instance F Int = Int
200 -- This is perfectly reasonable, if we later get a ~ Int.
202 checkTauTvUpdate orig_tv orig_ty
203 = do { result <- go orig_ty
205 Right ty -> return $ Just ty
206 Left True -> return $ Nothing
207 Left False -> occurCheckErr (mkTyVarTy orig_tv) orig_ty
210 go :: TcType -> TcM (Either Bool TcType)
212 -- Right ty if everything is fine
213 -- Left True if orig_tv occurs in orig_ty, but under a type family app
214 -- Left False if orig_tv occurs in orig_ty (with no type family app)
215 -- It fails if it encounters a forall type, except as an argument for a
216 -- closed type synonym that expands to a tau type.
218 | isSynTyCon tc = go_syn tc tys
219 | otherwise = do { tys' <- mapM go tys
220 ; return $ occurs (TyConApp tc) tys' }
221 go (PredTy p) = do { p' <- go_pred p
222 ; return $ occurs1 PredTy p' }
223 go (FunTy arg res) = do { arg' <- go arg
225 ; return $ occurs2 FunTy arg' res' }
226 go (AppTy fun arg) = do { fun' <- go fun
228 ; return $ occurs2 mkAppTy fun' arg' }
229 -- NB the mkAppTy; we might have instantiated a
230 -- type variable to a type constructor, so we need
231 -- to pull the TyConApp to the top.
232 go (ForAllTy _ _) = notMonoType orig_ty -- (b)
235 | orig_tv == tv = return $ Left False -- (a)
236 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
237 | otherwise = return $ Right (TyVarTy tv)
238 -- Ordinary (non Tc) tyvars
239 -- occur inside quantified types
241 go_pred (ClassP c tys) = do { tys' <- mapM go tys
242 ; return $ occurs (ClassP c) tys' }
243 go_pred (IParam n ty) = do { ty' <- go ty
244 ; return $ occurs1 (IParam n) ty' }
245 go_pred (EqPred t1 t2) = do { t1' <- go t1
247 ; return $ occurs2 EqPred t1' t2' }
249 go_tyvar tv (SkolemTv _) = return $ Right (TyVarTy tv)
250 go_tyvar tv (MetaTv box ref)
251 = do { cts <- readMutVar ref
255 BoxTv -> do { ty <- fillBoxWithTau tv ref
256 ; return $ Right ty }
257 _ -> return $ Right (TyVarTy tv)
260 -- go_syn is called for synonyms only
261 -- See Note [Type synonyms and the occur check]
263 | not (isTauTyCon tc)
264 = notMonoType orig_ty -- (b) again
266 = do { (_msgs, mb_tys') <- tryTc (mapM go tys)
269 -- we had a type error => forall in type parameters
271 | isOpenTyCon tc -> notMonoArgs (TyConApp tc tys)
272 -- Synonym families must have monotype args
273 | otherwise -> go (expectJust "checkTauTvUpdate(1)"
274 (tcView (TyConApp tc tys)))
275 -- Try again, expanding the synonym
277 -- no type error, but need to test whether occurs check happend
279 case occurs id tys' of
281 | isOpenTyCon tc -> return $ Left True
282 -- Variable occured under type family application
283 | otherwise -> go (expectJust "checkTauTvUpdate(2)"
284 (tcView (TyConApp tc tys)))
285 -- Try again, expanding the synonym
286 Right raw_tys' -> return $ Right (TyConApp tc raw_tys')
287 -- Retain the synonym (the common case)
290 -- Left results (= occurrence of orig_ty) dominate and
291 -- (Left False) (= fatal occurrence) dominates over (Left True)
292 occurs :: ([a] -> b) -> [Either Bool a] -> Either Bool b
293 occurs c = either Left (Right . c) . foldr combine (Right [])
295 combine (Left famInst1) (Left famInst2) = Left (famInst1 && famInst2)
296 combine (Right _ ) (Left famInst) = Left famInst
297 combine (Left famInst) (Right _) = Left famInst
298 combine (Right arg) (Right args) = Right (arg:args)
300 occurs1 c x = occurs (\[x'] -> c x') [x]
301 occurs2 c x y = occurs (\[x', y'] -> c x' y') [x, y]
303 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
304 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
305 -- tau-type meta-variable, whose print-name is the same as tv
306 -- Choosing the same name is good: when we instantiate a function
307 -- we allocate boxy tyvars with the same print-name as the quantified
308 -- tyvar; and then we often fill the box with a tau-tyvar, and again
309 -- we want to choose the same name.
310 fillBoxWithTau tv ref
311 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
312 ; let tau = mkTyVarTy tv' -- name of the type variable
313 ; writeMutVar ref (Indirect tau)
317 Note [Type synonyms and the occur check]
319 Basically we want to update tv1 := ps_ty2
320 because ps_ty2 has type-synonym info, which improves later error messages
325 f :: (A a -> a -> ()) -> ()
331 In the application (p x), we try to match "t" with "A t". If we go
332 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
333 an infinite loop later.
334 But we should not reject the program, because A t = ().
335 Rather, we should bind t to () (= non_var_ty2).
339 Error mesages in case of kind mismatch.
342 unifyKindMisMatch :: TcKind -> TcKind -> TcM ()
343 unifyKindMisMatch ty1 ty2 = do
344 ty1' <- zonkTcKind ty1
345 ty2' <- zonkTcKind ty2
347 msg = hang (ptext (sLit "Couldn't match kind"))
348 2 (sep [quotes (ppr ty1'),
349 ptext (sLit "against"),
353 unifyKindCtxt :: Bool -> TyVar -> Type -> TidyEnv -> TcM (TidyEnv, SDoc)
354 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
355 -- tv1 and ty2 are zonked already
358 msg = (env2, ptext (sLit "When matching the kinds of") <+>
359 sep [quotes pp_expected <+> ptext (sLit "and"), quotes pp_actual])
361 (pp_expected, pp_actual) | swapped = (pp2, pp1)
362 | otherwise = (pp1, pp2)
363 (env1, tv1') = tidyOpenTyVar tidy_env tv1
364 (env2, ty2') = tidyOpenType env1 ty2
365 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
366 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
369 Error message for failure due to an occurs check.
372 occurCheckErr :: TcType -> TcType -> TcM a
373 occurCheckErr ty containingTy
374 = do { env0 <- tcInitTidyEnv
375 ; ty' <- zonkTcType ty
376 ; containingTy' <- zonkTcType containingTy
377 ; let (env1, tidy_ty1) = tidyOpenType env0 ty'
378 (env2, tidy_ty2) = tidyOpenType env1 containingTy'
379 extra = sep [ppr tidy_ty1, char '=', ppr tidy_ty2]
380 ; failWithTcM (env2, hang msg 2 extra) }
382 msg = ptext (sLit "Occurs check: cannot construct the infinite type:")
385 %************************************************************************
389 %************************************************************************
392 newCoVars :: [(TcType,TcType)] -> TcM [CoVar]
394 = do { us <- newUniqueSupply
395 ; return [ mkCoVar (mkSysTvName uniq (fsLit "co"))
397 | ((ty1,ty2), uniq) <- spec `zip` uniqsFromSupply us] }
399 newMetaCoVar :: TcType -> TcType -> TcM TcTyVar
400 newMetaCoVar ty1 ty2 = newMetaTyVar TauTv (mkCoKind ty1 ty2)
402 newKindVar :: TcM TcKind
403 newKindVar = do { uniq <- newUnique
404 ; ref <- newMutVar Flexi
405 ; return (mkTyVarTy (mkKindVar uniq ref)) }
407 newKindVars :: Int -> TcM [TcKind]
408 newKindVars n = mapM (\ _ -> newKindVar) (nOfThem n ())
412 %************************************************************************
414 SkolemTvs (immutable)
416 %************************************************************************
419 mkSkolTyVar :: Name -> Kind -> SkolemInfo -> TcTyVar
420 mkSkolTyVar name kind info = mkTcTyVar name kind (SkolemTv info)
422 tcSkolSigType :: SkolemInfo -> Type -> TcM ([TcTyVar], TcThetaType, TcType)
423 -- Instantiate a type signature with skolem constants, but
424 -- do *not* give them fresh names, because we want the name to
425 -- be in the type environment -- it is lexically scoped.
426 tcSkolSigType info ty = tcInstType (\tvs -> return (tcSkolSigTyVars info tvs)) ty
428 tcSkolSigTyVars :: SkolemInfo -> [TyVar] -> [TcTyVar]
429 -- Make skolem constants, but do *not* give them new names, as above
430 tcSkolSigTyVars info tyvars = [ mkSkolTyVar (tyVarName tv) (tyVarKind tv) info
433 tcInstSkolTyVar :: SkolemInfo -> (Name -> SrcSpan) -> TyVar -> TcM TcTyVar
434 -- Instantiate the tyvar, using
435 -- * the occ-name and kind of the supplied tyvar,
436 -- * the unique from the monad,
437 -- * the location either from the tyvar (mb_loc = Nothing)
438 -- or from mb_loc (Just loc)
439 tcInstSkolTyVar info get_loc tyvar
440 = do { uniq <- newUnique
441 ; let old_name = tyVarName tyvar
442 kind = tyVarKind tyvar
443 loc = get_loc old_name
444 new_name = mkInternalName uniq (nameOccName old_name) loc
445 ; return (mkSkolTyVar new_name kind info) }
447 tcInstSkolTyVars :: SkolemInfo -> [TyVar] -> TcM [TcTyVar]
448 -- Get the location from the monad
449 tcInstSkolTyVars info tyvars
450 = do { span <- getSrcSpanM
451 ; mapM (tcInstSkolTyVar info (const span)) tyvars }
453 tcInstSkolType :: SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcType)
454 -- Instantiate a type with fresh skolem constants
455 -- Binding location comes from the monad
456 tcInstSkolType info ty = tcInstType (tcInstSkolTyVars info) ty
458 tcInstSigType :: Bool -> SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcRhoType)
459 -- Instantiate with skolems or meta SigTvs; depending on use_skols
460 -- Always take location info from the supplied tyvars
461 tcInstSigType use_skols skol_info ty
462 = tcInstType (mapM inst_tyvar) ty
464 inst_tyvar | use_skols = tcInstSkolTyVar skol_info getSrcSpan
465 | otherwise = instMetaTyVar (SigTv skol_info)
469 %************************************************************************
471 MetaTvs (meta type variables; mutable)
473 %************************************************************************
476 newMetaTyVar :: BoxInfo -> Kind -> TcM TcTyVar
477 -- Make a new meta tyvar out of thin air
478 newMetaTyVar box_info kind
479 = do { uniq <- newUnique
480 ; ref <- newMutVar Flexi
481 ; let name = mkSysTvName uniq fs
482 fs = case box_info of
486 -- We give BoxTv and TauTv the same string, because
487 -- otherwise we get user-visible differences in error
488 -- messages, which are confusing. If you want to see
489 -- the box_info of each tyvar, use -dppr-debug
490 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
492 instMetaTyVar :: BoxInfo -> TyVar -> TcM TcTyVar
493 -- Make a new meta tyvar whose Name and Kind
494 -- come from an existing TyVar
495 instMetaTyVar box_info tyvar
496 = do { uniq <- newUnique
497 ; ref <- newMutVar Flexi
498 ; let name = setNameUnique (tyVarName tyvar) uniq
499 kind = tyVarKind tyvar
500 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
502 readMetaTyVar :: TyVar -> TcM MetaDetails
503 readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
504 readMutVar (metaTvRef tyvar)
506 isFilledMetaTyVar :: TyVar -> TcM Bool
507 -- True of a filled-in (Indirect) meta type variable
509 | not (isTcTyVar tv) = return False
510 | MetaTv _ ref <- tcTyVarDetails tv
511 = do { details <- readMutVar ref
512 ; return (isIndirect details) }
513 | otherwise = return False
515 writeMetaTyVar :: TcTyVar -> TcType -> TcM ()
516 writeMetaTyVar tyvar ty
517 | not debugIsOn = writeMutVar (metaTvRef tyvar) (Indirect ty)
518 writeMetaTyVar tyvar ty
519 | not (isMetaTyVar tyvar)
520 = pprTrace "writeMetaTyVar" (ppr tyvar) $
523 = ASSERT( isMetaTyVar tyvar )
524 -- TOM: It should also work for coercions
525 -- ASSERT2( k2 `isSubKind` k1, (ppr tyvar <+> ppr k1) $$ (ppr ty <+> ppr k2) )
526 do { if debugIsOn then do { details <- readMetaTyVar tyvar;
527 ; WARN( not (isFlexi details), ppr tyvar )
530 -- Temporarily make this a warning, until we fix Trac #2999
532 ; traceTc (text "writeMetaTyVar" <+> ppr tyvar <+> text ":=" <+> ppr ty)
533 ; writeMutVar (metaTvRef tyvar) (Indirect ty) }
535 _k1 = tyVarKind tyvar
540 %************************************************************************
544 %************************************************************************
547 newFlexiTyVar :: Kind -> TcM TcTyVar
548 newFlexiTyVar kind = newMetaTyVar TauTv kind
550 newFlexiTyVarTy :: Kind -> TcM TcType
551 newFlexiTyVarTy kind = do
552 tc_tyvar <- newFlexiTyVar kind
553 return (TyVarTy tc_tyvar)
555 newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
556 newFlexiTyVarTys n kind = mapM newFlexiTyVarTy (nOfThem n kind)
558 tcInstTyVar :: TyVar -> TcM TcTyVar
559 -- Instantiate with a META type variable
560 tcInstTyVar tyvar = instMetaTyVar TauTv tyvar
562 tcInstTyVars :: [TyVar] -> TcM ([TcTyVar], [TcType], TvSubst)
563 -- Instantiate with META type variables
565 = do { tc_tvs <- mapM tcInstTyVar tyvars
566 ; let tys = mkTyVarTys tc_tvs
567 ; return (tc_tvs, tys, zipTopTvSubst tyvars tys) }
568 -- Since the tyvars are freshly made,
569 -- they cannot possibly be captured by
570 -- any existing for-alls. Hence zipTopTvSubst
574 %************************************************************************
578 %************************************************************************
581 zonkSigTyVar :: TcTyVar -> TcM TcTyVar
583 | isSkolemTyVar sig_tv
584 = return sig_tv -- Happens in the call in TcBinds.checkDistinctTyVars
586 = ASSERT( isSigTyVar sig_tv )
587 do { ty <- zonkTcTyVar sig_tv
588 ; return (tcGetTyVar "zonkSigTyVar" ty) }
589 -- 'ty' is bound to be a type variable, because SigTvs
590 -- can only be unified with type variables
594 %************************************************************************
598 %************************************************************************
601 newBoxyTyVar :: Kind -> TcM BoxyTyVar
602 newBoxyTyVar kind = newMetaTyVar BoxTv kind
604 newBoxyTyVars :: [Kind] -> TcM [BoxyTyVar]
605 newBoxyTyVars kinds = mapM newBoxyTyVar kinds
607 newBoxyTyVarTys :: [Kind] -> TcM [BoxyType]
608 newBoxyTyVarTys kinds = do { tvs <- mapM newBoxyTyVar kinds; return (mkTyVarTys tvs) }
610 readFilledBox :: BoxyTyVar -> TcM TcType
611 -- Read the contents of the box, which should be filled in by now
612 readFilledBox box_tv = ASSERT( isBoxyTyVar box_tv )
613 do { cts <- readMetaTyVar box_tv
615 Flexi -> pprPanic "readFilledBox" (ppr box_tv)
616 Indirect ty -> return ty }
618 tcInstBoxyTyVar :: TyVar -> TcM BoxyTyVar
619 -- Instantiate with a BOXY type variable
620 tcInstBoxyTyVar tyvar = instMetaTyVar BoxTv tyvar
624 %************************************************************************
626 \subsection{Putting and getting mutable type variables}
628 %************************************************************************
630 But it's more fun to short out indirections on the way: If this
631 version returns a TyVar, then that TyVar is unbound. If it returns
632 any other type, then there might be bound TyVars embedded inside it.
634 We return Nothing iff the original box was unbound.
637 data LookupTyVarResult -- The result of a lookupTcTyVar call
638 = DoneTv TcTyVarDetails -- SkolemTv or virgin MetaTv
641 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
643 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
645 SkolemTv _ -> return (DoneTv details)
646 MetaTv _ ref -> do { meta_details <- readMutVar ref
647 ; case meta_details of
648 Indirect ty -> return (IndirectTv ty)
649 Flexi -> return (DoneTv details) }
651 details = tcTyVarDetails tyvar
654 -- gaw 2004 We aren't shorting anything out anymore, at least for now
656 | not (isTcTyVar tyvar)
657 = pprTrace "getTcTyVar" (ppr tyvar) $
658 return (Just (mkTyVarTy tyvar))
661 = ASSERT2( isTcTyVar tyvar, ppr tyvar ) do
662 maybe_ty <- readMetaTyVar tyvar
664 Just ty -> do ty' <- short_out ty
665 writeMetaTyVar tyvar (Just ty')
668 Nothing -> return Nothing
670 short_out :: TcType -> TcM TcType
671 short_out ty@(TyVarTy tyvar)
672 | not (isTcTyVar tyvar)
676 maybe_ty <- readMetaTyVar tyvar
678 Just ty' -> do ty' <- short_out ty'
679 writeMetaTyVar tyvar (Just ty')
684 short_out other_ty = return other_ty
689 %************************************************************************
691 \subsection{Zonking -- the exernal interfaces}
693 %************************************************************************
695 ----------------- Type variables
698 zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
699 zonkTcTyVars tyvars = mapM zonkTcTyVar tyvars
701 zonkTcTyVarsAndFV :: [TcTyVar] -> TcM TcTyVarSet
702 zonkTcTyVarsAndFV tyvars = tyVarsOfTypes <$> mapM zonkTcTyVar tyvars
704 zonkTcTyVar :: TcTyVar -> TcM TcType
705 zonkTcTyVar tyvar = ASSERT2( isTcTyVar tyvar, ppr tyvar)
706 zonk_tc_tyvar (\ tv -> return (TyVarTy tv)) tyvar
709 ----------------- Types
712 zonkTcType :: TcType -> TcM TcType
713 zonkTcType ty = zonkType (\ tv -> return (TyVarTy tv)) ty
715 zonkTcTypes :: [TcType] -> TcM [TcType]
716 zonkTcTypes tys = mapM zonkTcType tys
718 zonkTcThetaType :: TcThetaType -> TcM TcThetaType
719 zonkTcThetaType theta = mapM zonkTcPredType theta
721 zonkTcPredType :: TcPredType -> TcM TcPredType
722 zonkTcPredType (ClassP c ts) = ClassP c <$> zonkTcTypes ts
723 zonkTcPredType (IParam n t) = IParam n <$> zonkTcType t
724 zonkTcPredType (EqPred t1 t2) = EqPred <$> zonkTcType t1 <*> zonkTcType t2
727 ------------------- These ...ToType, ...ToKind versions
728 are used at the end of type checking
731 zonkTopTyVar :: TcTyVar -> TcM TcTyVar
732 -- zonkTopTyVar is used, at the top level, on any un-instantiated meta type variables
733 -- to default the kind of ? and ?? etc to *. This is important to ensure that
734 -- instance declarations match. For example consider
735 -- instance Show (a->b)
736 -- foo x = show (\_ -> True)
737 -- Then we'll get a constraint (Show (p ->q)) where p has argTypeKind (printed ??),
738 -- and that won't match the typeKind (*) in the instance decl.
740 -- Because we are at top level, no further constraints are going to affect these
741 -- type variables, so it's time to do it by hand. However we aren't ready
742 -- to default them fully to () or whatever, because the type-class defaulting
743 -- rules have yet to run.
746 | k `eqKind` default_k = return tv
748 = do { tv' <- newFlexiTyVar default_k
749 ; writeMetaTyVar tv (mkTyVarTy tv')
753 default_k = defaultKind k
755 zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TcTyVar]
756 zonkQuantifiedTyVars = mapM zonkQuantifiedTyVar
758 zonkQuantifiedTyVar :: TcTyVar -> TcM TcTyVar
759 -- zonkQuantifiedTyVar is applied to the a TcTyVar when quantifying over it.
761 -- The quantified type variables often include meta type variables
762 -- we want to freeze them into ordinary type variables, and
763 -- default their kind (e.g. from OpenTypeKind to TypeKind)
764 -- -- see notes with Kind.defaultKind
765 -- The meta tyvar is updated to point to the new skolem TyVar. Now any
766 -- bound occurences of the original type variable will get zonked to
767 -- the immutable version.
769 -- We leave skolem TyVars alone; they are immutable.
770 zonkQuantifiedTyVar tv
771 | ASSERT2( isTcTyVar tv, ppr tv )
773 = do { kind <- zonkTcType (tyVarKind tv)
774 ; return $ setTyVarKind tv kind
776 -- It might be a skolem type variable,
777 -- for example from a user type signature
779 | otherwise -- It's a meta-type-variable
780 = do { details <- readMetaTyVar tv
782 -- Create the new, frozen, skolem type variable
783 -- We zonk to a skolem, not to a regular TcVar
784 -- See Note [Zonking to Skolem]
785 ; let final_kind = defaultKind (tyVarKind tv)
786 final_tv = mkSkolTyVar (tyVarName tv) final_kind UnkSkol
788 -- Bind the meta tyvar to the new tyvar
790 Indirect ty -> WARN( True, ppr tv $$ ppr ty )
792 -- [Sept 04] I don't think this should happen
793 -- See note [Silly Type Synonym]
795 Flexi -> writeMetaTyVar tv (mkTyVarTy final_tv)
797 -- Return the new tyvar
801 Note [Silly Type Synonyms]
802 ~~~~~~~~~~~~~~~~~~~~~~~~~~
804 type C u a = u -- Note 'a' unused
806 foo :: (forall a. C u a -> C u a) -> u
810 bar = foo (\t -> t + t)
812 * From the (\t -> t+t) we get type {Num d} => d -> d
815 * Now unify with type of foo's arg, and we get:
816 {Num (C d a)} => C d a -> C d a
819 * Now abstract over the 'a', but float out the Num (C d a) constraint
820 because it does not 'really' mention a. (see exactTyVarsOfType)
821 The arg to foo becomes
824 * So we get a dict binding for Num (C d a), which is zonked to give
826 [Note Sept 04: now that we are zonking quantified type variables
827 on construction, the 'a' will be frozen as a regular tyvar on
828 quantification, so the floated dict will still have type (C d a).
829 Which renders this whole note moot; happily!]
831 * Then the \/\a abstraction has a zonked 'a' in it.
833 All very silly. I think its harmless to ignore the problem. We'll end up with
834 a \/\a in the final result but all the occurrences of a will be zonked to ()
836 Note [Zonking to Skolem]
837 ~~~~~~~~~~~~~~~~~~~~~~~~
838 We used to zonk quantified type variables to regular TyVars. However, this
839 leads to problems. Consider this program from the regression test suite:
841 eval :: Int -> String -> String -> String
842 eval 0 root actual = evalRHS 0 root actual
845 evalRHS 0 root actual = eval 0 root actual
847 It leads to the deferral of an equality
849 (String -> String -> String) ~ a
851 which is propagated up to the toplevel (see TcSimplify.tcSimplifyInferCheck).
852 In the meantime `a' is zonked and quantified to form `evalRHS's signature.
853 This has the *side effect* of also zonking the `a' in the deferred equality
854 (which at this point is being handed around wrapped in an implication
857 Finally, the equality (with the zonked `a') will be handed back to the
858 simplifier by TcRnDriver.tcRnSrcDecls calling TcSimplify.tcSimplifyTop.
859 If we zonk `a' with a regular type variable, we will have this regular type
860 variable now floating around in the simplifier, which in many places assumes to
861 only see proper TcTyVars.
863 We can avoid this problem by zonking with a skolem. The skolem is rigid
864 (which we requirefor a quantified variable), but is still a TcTyVar that the
865 simplifier knows how to deal with.
868 %************************************************************************
870 \subsection{Zonking -- the main work-horses: zonkType, zonkTyVar}
872 %* For internal use only! *
874 %************************************************************************
877 -- For unbound, mutable tyvars, zonkType uses the function given to it
878 -- For tyvars bound at a for-all, zonkType zonks them to an immutable
879 -- type variable and zonks the kind too
881 zonkType :: (TcTyVar -> TcM Type) -- What to do with unbound mutable type variables
882 -- see zonkTcType, and zonkTcTypeToType
885 zonkType unbound_var_fn ty
888 go (TyConApp tc tys) = do tys' <- mapM go tys
889 return (TyConApp tc tys')
891 go (PredTy p) = do p' <- go_pred p
894 go (FunTy arg res) = do arg' <- go arg
896 return (FunTy arg' res')
898 go (AppTy fun arg) = do fun' <- go fun
900 return (mkAppTy fun' arg')
901 -- NB the mkAppTy; we might have instantiated a
902 -- type variable to a type constructor, so we need
903 -- to pull the TyConApp to the top.
905 -- The two interesting cases!
906 go (TyVarTy tyvar) | isTcTyVar tyvar = zonk_tc_tyvar unbound_var_fn tyvar
907 | otherwise = liftM TyVarTy $
908 zonkTyVar unbound_var_fn tyvar
909 -- Ordinary (non Tc) tyvars occur inside quantified types
911 go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar ) do
913 tyvar' <- zonkTyVar unbound_var_fn tyvar
914 return (ForAllTy tyvar' ty')
916 go_pred (ClassP c tys) = do tys' <- mapM go tys
917 return (ClassP c tys')
918 go_pred (IParam n ty) = do ty' <- go ty
919 return (IParam n ty')
920 go_pred (EqPred ty1 ty2) = do ty1' <- go ty1
922 return (EqPred ty1' ty2')
924 zonk_tc_tyvar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable var
925 -> TcTyVar -> TcM TcType
926 zonk_tc_tyvar unbound_var_fn tyvar
927 | not (isMetaTyVar tyvar) -- Skolems
928 = return (TyVarTy tyvar)
930 | otherwise -- Mutables
931 = do { cts <- readMetaTyVar tyvar
933 Flexi -> unbound_var_fn tyvar -- Unbound meta type variable
934 Indirect ty -> zonkType unbound_var_fn ty }
936 -- Zonk the kind of a non-TC tyvar in case it is a coercion variable (their
937 -- kind contains types).
939 zonkTyVar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable var
940 -> TyVar -> TcM TyVar
941 zonkTyVar unbound_var_fn tv
943 = do { kind <- zonkType unbound_var_fn (tyVarKind tv)
944 ; return $ setTyVarKind tv kind
946 | otherwise = return tv
951 %************************************************************************
955 %************************************************************************
958 readKindVar :: KindVar -> TcM (MetaDetails)
959 writeKindVar :: KindVar -> TcKind -> TcM ()
960 readKindVar kv = readMutVar (kindVarRef kv)
961 writeKindVar kv val = writeMutVar (kindVarRef kv) (Indirect val)
964 zonkTcKind :: TcKind -> TcM TcKind
965 zonkTcKind k = zonkTcType k
968 zonkTcKindToKind :: TcKind -> TcM Kind
969 -- When zonking a TcKind to a kind, we need to instantiate kind variables,
970 -- Haskell specifies that * is to be used, so we follow that.
971 zonkTcKindToKind k = zonkType (\ _ -> return liftedTypeKind) k
974 %************************************************************************
976 \subsection{Checking a user type}
978 %************************************************************************
980 When dealing with a user-written type, we first translate it from an HsType
981 to a Type, performing kind checking, and then check various things that should
982 be true about it. We don't want to perform these checks at the same time
983 as the initial translation because (a) they are unnecessary for interface-file
984 types and (b) when checking a mutually recursive group of type and class decls,
985 we can't "look" at the tycons/classes yet. Also, the checks are are rather
986 diverse, and used to really mess up the other code.
988 One thing we check for is 'rank'.
990 Rank 0: monotypes (no foralls)
991 Rank 1: foralls at the front only, Rank 0 inside
992 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
994 basic ::= tyvar | T basic ... basic
996 r2 ::= forall tvs. cxt => r2a
997 r2a ::= r1 -> r2a | basic
998 r1 ::= forall tvs. cxt => r0
999 r0 ::= r0 -> r0 | basic
1001 Another thing is to check that type synonyms are saturated.
1002 This might not necessarily show up in kind checking.
1004 data T k = MkT (k Int)
1009 checkValidType :: UserTypeCtxt -> Type -> TcM ()
1010 -- Checks that the type is valid for the given context
1011 checkValidType ctxt ty = do
1012 traceTc (text "checkValidType" <+> ppr ty)
1013 unboxed <- doptM Opt_UnboxedTuples
1014 rank2 <- doptM Opt_Rank2Types
1015 rankn <- doptM Opt_RankNTypes
1016 polycomp <- doptM Opt_PolymorphicComponents
1018 gen_rank n | rankn = ArbitraryRank
1020 | otherwise = Rank n
1023 DefaultDeclCtxt-> MustBeMonoType
1024 ResSigCtxt -> MustBeMonoType
1025 LamPatSigCtxt -> gen_rank 0
1026 BindPatSigCtxt -> gen_rank 0
1027 TySynCtxt _ -> gen_rank 0
1028 GenPatCtxt -> gen_rank 1
1029 -- This one is a bit of a hack
1030 -- See the forall-wrapping in TcClassDcl.mkGenericInstance
1032 ExprSigCtxt -> gen_rank 1
1033 FunSigCtxt _ -> gen_rank 1
1034 ConArgCtxt _ | polycomp -> gen_rank 2
1035 -- We are given the type of the entire
1036 -- constructor, hence rank 1
1037 | otherwise -> gen_rank 1
1039 ForSigCtxt _ -> gen_rank 1
1040 SpecInstCtxt -> gen_rank 1
1042 actual_kind = typeKind ty
1044 kind_ok = case ctxt of
1045 TySynCtxt _ -> True -- Any kind will do
1046 ResSigCtxt -> isSubOpenTypeKind actual_kind
1047 ExprSigCtxt -> isSubOpenTypeKind actual_kind
1048 GenPatCtxt -> isLiftedTypeKind actual_kind
1049 ForSigCtxt _ -> isLiftedTypeKind actual_kind
1050 _ -> isSubArgTypeKind actual_kind
1052 ubx_tup = case ctxt of
1053 TySynCtxt _ | unboxed -> UT_Ok
1054 ExprSigCtxt | unboxed -> UT_Ok
1057 -- Check that the thing has kind Type, and is lifted if necessary
1058 checkTc kind_ok (kindErr actual_kind)
1060 -- Check the internal validity of the type itself
1061 check_type rank ubx_tup ty
1063 traceTc (text "checkValidType done" <+> ppr ty)
1065 checkValidMonoType :: Type -> TcM ()
1066 checkValidMonoType ty = check_mono_type MustBeMonoType ty
1071 data Rank = ArbitraryRank -- Any rank ok
1072 | MustBeMonoType -- Monotype regardless of flags
1073 | TyConArgMonoType -- Monotype but could be poly if -XImpredicativeTypes
1074 | SynArgMonoType -- Monotype but could be poly if -XLiberalTypeSynonyms
1075 | Rank Int -- Rank n, but could be more with -XRankNTypes
1077 decRank :: Rank -> Rank -- Function arguments
1078 decRank (Rank 0) = Rank 0
1079 decRank (Rank n) = Rank (n-1)
1080 decRank other_rank = other_rank
1082 nonZeroRank :: Rank -> Bool
1083 nonZeroRank ArbitraryRank = True
1084 nonZeroRank (Rank n) = n>0
1085 nonZeroRank _ = False
1087 ----------------------------------------
1088 data UbxTupFlag = UT_Ok | UT_NotOk
1089 -- The "Ok" version means "ok if UnboxedTuples is on"
1091 ----------------------------------------
1092 check_mono_type :: Rank -> Type -> TcM () -- No foralls anywhere
1093 -- No unlifted types of any kind
1094 check_mono_type rank ty
1095 = do { check_type rank UT_NotOk ty
1096 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1098 check_type :: Rank -> UbxTupFlag -> Type -> TcM ()
1099 -- The args say what the *type context* requires, independent
1100 -- of *flag* settings. You test the flag settings at usage sites.
1102 -- Rank is allowed rank for function args
1103 -- Rank 0 means no for-alls anywhere
1105 check_type rank ubx_tup ty
1106 | not (null tvs && null theta)
1107 = do { checkTc (nonZeroRank rank) (forAllTyErr rank ty)
1108 -- Reject e.g. (Maybe (?x::Int => Int)),
1109 -- with a decent error message
1110 ; check_valid_theta SigmaCtxt theta
1111 ; check_type rank ubx_tup tau -- Allow foralls to right of arrow
1112 ; checkFreeness tvs theta
1113 ; checkAmbiguity tvs theta (tyVarsOfType tau) }
1115 (tvs, theta, tau) = tcSplitSigmaTy ty
1117 -- Naked PredTys don't usually show up, but they can as a result of
1118 -- {-# SPECIALISE instance Ord Char #-}
1119 -- The Right Thing would be to fix the way that SPECIALISE instance pragmas
1120 -- are handled, but the quick thing is just to permit PredTys here.
1121 check_type _ _ (PredTy sty)
1122 = do { dflags <- getDOpts
1123 ; check_pred_ty dflags TypeCtxt sty }
1125 check_type _ _ (TyVarTy _) = return ()
1126 check_type rank _ (FunTy arg_ty res_ty)
1127 = do { check_type (decRank rank) UT_NotOk arg_ty
1128 ; check_type rank UT_Ok res_ty }
1130 check_type rank _ (AppTy ty1 ty2)
1131 = do { check_arg_type rank ty1
1132 ; check_arg_type rank ty2 }
1134 check_type rank ubx_tup ty@(TyConApp tc tys)
1136 = do { -- Check that the synonym has enough args
1137 -- This applies equally to open and closed synonyms
1138 -- It's OK to have an *over-applied* type synonym
1139 -- data Tree a b = ...
1140 -- type Foo a = Tree [a]
1141 -- f :: Foo a b -> ...
1142 checkTc (tyConArity tc <= length tys) arity_msg
1144 -- See Note [Liberal type synonyms]
1145 ; liberal <- doptM Opt_LiberalTypeSynonyms
1146 ; if not liberal || isOpenSynTyCon tc then
1147 -- For H98 and synonym families, do check the type args
1148 mapM_ (check_mono_type SynArgMonoType) tys
1150 else -- In the liberal case (only for closed syns), expand then check
1152 Just ty' -> check_type rank ubx_tup ty'
1153 Nothing -> pprPanic "check_tau_type" (ppr ty)
1156 | isUnboxedTupleTyCon tc
1157 = do { ub_tuples_allowed <- doptM Opt_UnboxedTuples
1158 ; checkTc (ubx_tup_ok ub_tuples_allowed) ubx_tup_msg
1160 ; impred <- doptM Opt_ImpredicativeTypes
1161 ; let rank' = if impred then ArbitraryRank else TyConArgMonoType
1162 -- c.f. check_arg_type
1163 -- However, args are allowed to be unlifted, or
1164 -- more unboxed tuples, so can't use check_arg_ty
1165 ; mapM_ (check_type rank' UT_Ok) tys }
1168 = mapM_ (check_arg_type rank) tys
1171 ubx_tup_ok ub_tuples_allowed = case ubx_tup of
1172 UT_Ok -> ub_tuples_allowed
1176 tc_arity = tyConArity tc
1178 arity_msg = arityErr "Type synonym" (tyConName tc) tc_arity n_args
1179 ubx_tup_msg = ubxArgTyErr ty
1181 check_type _ _ ty = pprPanic "check_type" (ppr ty)
1183 ----------------------------------------
1184 check_arg_type :: Rank -> Type -> TcM ()
1185 -- The sort of type that can instantiate a type variable,
1186 -- or be the argument of a type constructor.
1187 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
1188 -- Other unboxed types are very occasionally allowed as type
1189 -- arguments depending on the kind of the type constructor
1191 -- For example, we want to reject things like:
1193 -- instance Ord a => Ord (forall s. T s a)
1195 -- g :: T s (forall b.b)
1197 -- NB: unboxed tuples can have polymorphic or unboxed args.
1198 -- This happens in the workers for functions returning
1199 -- product types with polymorphic components.
1200 -- But not in user code.
1201 -- Anyway, they are dealt with by a special case in check_tau_type
1203 check_arg_type rank ty
1204 = do { impred <- doptM Opt_ImpredicativeTypes
1205 ; let rank' = if impred then ArbitraryRank -- Arg of tycon can have arby rank, regardless
1206 else case rank of -- Predictive => must be monotype
1207 MustBeMonoType -> MustBeMonoType
1208 _ -> TyConArgMonoType
1209 -- Make sure that MustBeMonoType is propagated,
1210 -- so that we don't suggest -XImpredicativeTypes in
1211 -- (Ord (forall a.a)) => a -> a
1213 ; check_type rank' UT_NotOk ty
1214 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1216 ----------------------------------------
1217 forAllTyErr :: Rank -> Type -> SDoc
1219 = vcat [ hang (ptext (sLit "Illegal polymorphic or qualified type:")) 2 (ppr ty)
1222 suggestion = case rank of
1223 Rank _ -> ptext (sLit "Perhaps you intended to use -XRankNTypes or -XRank2Types")
1224 TyConArgMonoType -> ptext (sLit "Perhaps you intended to use -XImpredicativeTypes")
1225 SynArgMonoType -> ptext (sLit "Perhaps you intended to use -XLiberalTypeSynonyms")
1226 _ -> empty -- Polytype is always illegal
1228 unliftedArgErr, ubxArgTyErr :: Type -> SDoc
1229 unliftedArgErr ty = sep [ptext (sLit "Illegal unlifted type:"), ppr ty]
1230 ubxArgTyErr ty = sep [ptext (sLit "Illegal unboxed tuple type as function argument:"), ppr ty]
1232 kindErr :: Kind -> SDoc
1233 kindErr kind = sep [ptext (sLit "Expecting an ordinary type, but found a type of kind"), ppr kind]
1236 Note [Liberal type synonyms]
1237 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1238 If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
1239 doing validity checking. This allows us to instantiate a synonym defn
1240 with a for-all type, or with a partially-applied type synonym.
1244 Here, T is partially applied, so it's illegal in H98. But if you
1245 expand S first, then T we get just
1249 IMPORTANT: suppose T is a type synonym. Then we must do validity
1250 checking on an appliation (T ty1 ty2)
1252 *either* before expansion (i.e. check ty1, ty2)
1253 *or* after expansion (i.e. expand T ty1 ty2, and then check)
1256 If we do both, we get exponential behaviour!!
1258 data TIACons1 i r c = c i ::: r c
1259 type TIACons2 t x = TIACons1 t (TIACons1 t x)
1260 type TIACons3 t x = TIACons2 t (TIACons1 t x)
1261 type TIACons4 t x = TIACons2 t (TIACons2 t x)
1262 type TIACons7 t x = TIACons4 t (TIACons3 t x)
1265 %************************************************************************
1267 \subsection{Checking a theta or source type}
1269 %************************************************************************
1272 -- Enumerate the contexts in which a "source type", <S>, can occur
1276 -- or (N a) where N is a newtype
1279 = ClassSCCtxt Name -- Superclasses of clas
1280 -- class <S> => C a where ...
1281 | SigmaCtxt -- Theta part of a normal for-all type
1282 -- f :: <S> => a -> a
1283 | DataTyCtxt Name -- Theta part of a data decl
1284 -- data <S> => T a = MkT a
1285 | TypeCtxt -- Source type in an ordinary type
1287 | InstThetaCtxt -- Context of an instance decl
1288 -- instance <S> => C [a] where ...
1290 pprSourceTyCtxt :: SourceTyCtxt -> SDoc
1291 pprSourceTyCtxt (ClassSCCtxt c) = ptext (sLit "the super-classes of class") <+> quotes (ppr c)
1292 pprSourceTyCtxt SigmaCtxt = ptext (sLit "the context of a polymorphic type")
1293 pprSourceTyCtxt (DataTyCtxt tc) = ptext (sLit "the context of the data type declaration for") <+> quotes (ppr tc)
1294 pprSourceTyCtxt InstThetaCtxt = ptext (sLit "the context of an instance declaration")
1295 pprSourceTyCtxt TypeCtxt = ptext (sLit "the context of a type")
1299 checkValidTheta :: SourceTyCtxt -> ThetaType -> TcM ()
1300 checkValidTheta ctxt theta
1301 = addErrCtxt (checkThetaCtxt ctxt theta) (check_valid_theta ctxt theta)
1303 -------------------------
1304 check_valid_theta :: SourceTyCtxt -> [PredType] -> TcM ()
1305 check_valid_theta _ []
1307 check_valid_theta ctxt theta = do
1309 warnTc (notNull dups) (dupPredWarn dups)
1310 mapM_ (check_pred_ty dflags ctxt) theta
1312 (_,dups) = removeDups tcCmpPred theta
1314 -------------------------
1315 check_pred_ty :: DynFlags -> SourceTyCtxt -> PredType -> TcM ()
1316 check_pred_ty dflags ctxt pred@(ClassP cls tys)
1317 = do { -- Class predicates are valid in all contexts
1318 ; checkTc (arity == n_tys) arity_err
1320 -- Check the form of the argument types
1321 ; mapM_ checkValidMonoType tys
1322 ; checkTc (check_class_pred_tys dflags ctxt tys)
1323 (predTyVarErr pred $$ how_to_allow)
1326 class_name = className cls
1327 arity = classArity cls
1329 arity_err = arityErr "Class" class_name arity n_tys
1330 how_to_allow = parens (ptext (sLit "Use -XFlexibleContexts to permit this"))
1332 check_pred_ty _ (ClassSCCtxt _) (EqPred _ _)
1333 = -- We do not yet support superclass equalities.
1335 sep [ ptext (sLit "The current implementation of type families does not")
1336 , ptext (sLit "support equality constraints in superclass contexts.")
1337 , ptext (sLit "They are planned for a future release.")
1340 check_pred_ty dflags _ pred@(EqPred ty1 ty2)
1341 = do { -- Equational constraints are valid in all contexts if type
1342 -- families are permitted
1343 ; checkTc (dopt Opt_TypeFamilies dflags) (eqPredTyErr pred)
1345 -- Check the form of the argument types
1346 ; checkValidMonoType ty1
1347 ; checkValidMonoType ty2
1350 check_pred_ty _ SigmaCtxt (IParam _ ty) = checkValidMonoType ty
1351 -- Implicit parameters only allowed in type
1352 -- signatures; not in instance decls, superclasses etc
1353 -- The reason for not allowing implicit params in instances is a bit
1355 -- If we allowed instance (?x::Int, Eq a) => Foo [a] where ...
1356 -- then when we saw (e :: (?x::Int) => t) it would be unclear how to
1357 -- discharge all the potential usas of the ?x in e. For example, a
1358 -- constraint Foo [Int] might come out of e,and applying the
1359 -- instance decl would show up two uses of ?x.
1362 check_pred_ty _ _ sty = failWithTc (badPredTyErr sty)
1364 -------------------------
1365 check_class_pred_tys :: DynFlags -> SourceTyCtxt -> [Type] -> Bool
1366 check_class_pred_tys dflags ctxt tys
1368 TypeCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
1369 InstThetaCtxt -> flexible_contexts || undecidable_ok || all tcIsTyVarTy tys
1370 -- Further checks on head and theta in
1371 -- checkInstTermination
1372 _ -> flexible_contexts || all tyvar_head tys
1374 flexible_contexts = dopt Opt_FlexibleContexts dflags
1375 undecidable_ok = dopt Opt_UndecidableInstances dflags
1377 -------------------------
1378 tyvar_head :: Type -> Bool
1379 tyvar_head ty -- Haskell 98 allows predicates of form
1380 | tcIsTyVarTy ty = True -- C (a ty1 .. tyn)
1381 | otherwise -- where a is a type variable
1382 = case tcSplitAppTy_maybe ty of
1383 Just (ty, _) -> tyvar_head ty
1390 is ambiguous if P contains generic variables
1391 (i.e. one of the Vs) that are not mentioned in tau
1393 However, we need to take account of functional dependencies
1394 when we speak of 'mentioned in tau'. Example:
1395 class C a b | a -> b where ...
1397 forall x y. (C x y) => x
1398 is not ambiguous because x is mentioned and x determines y
1400 NB; the ambiguity check is only used for *user* types, not for types
1401 coming from inteface files. The latter can legitimately have
1402 ambiguous types. Example
1404 class S a where s :: a -> (Int,Int)
1405 instance S Char where s _ = (1,1)
1406 f:: S a => [a] -> Int -> (Int,Int)
1407 f (_::[a]) x = (a*x,b)
1408 where (a,b) = s (undefined::a)
1410 Here the worker for f gets the type
1411 fw :: forall a. S a => Int -> (# Int, Int #)
1413 If the list of tv_names is empty, we have a monotype, and then we
1414 don't need to check for ambiguity either, because the test can't fail
1419 checkAmbiguity :: [TyVar] -> ThetaType -> TyVarSet -> TcM ()
1420 checkAmbiguity forall_tyvars theta tau_tyvars
1421 = mapM_ complain (filter is_ambig theta)
1423 complain pred = addErrTc (ambigErr pred)
1424 extended_tau_vars = grow theta tau_tyvars
1426 -- See Note [Implicit parameters and ambiguity] in TcSimplify
1427 is_ambig pred = isClassPred pred &&
1428 any ambig_var (varSetElems (tyVarsOfPred pred))
1430 ambig_var ct_var = (ct_var `elem` forall_tyvars) &&
1431 not (ct_var `elemVarSet` extended_tau_vars)
1433 ambigErr :: PredType -> SDoc
1435 = sep [ptext (sLit "Ambiguous constraint") <+> quotes (pprPred pred),
1436 nest 4 (ptext (sLit "At least one of the forall'd type variables mentioned by the constraint") $$
1437 ptext (sLit "must be reachable from the type after the '=>'"))]
1440 In addition, GHC insists that at least one type variable
1441 in each constraint is in V. So we disallow a type like
1442 forall a. Eq b => b -> b
1443 even in a scope where b is in scope.
1446 checkFreeness :: [Var] -> [PredType] -> TcM ()
1447 checkFreeness forall_tyvars theta
1448 = do { flexible_contexts <- doptM Opt_FlexibleContexts
1449 ; unless flexible_contexts $ mapM_ complain (filter is_free theta) }
1451 is_free pred = not (isIPPred pred)
1452 && not (any bound_var (varSetElems (tyVarsOfPred pred)))
1453 bound_var ct_var = ct_var `elem` forall_tyvars
1454 complain pred = addErrTc (freeErr pred)
1456 freeErr :: PredType -> SDoc
1458 = sep [ ptext (sLit "All of the type variables in the constraint") <+>
1459 quotes (pprPred pred)
1460 , ptext (sLit "are already in scope") <+>
1461 ptext (sLit "(at least one must be universally quantified here)")
1463 ptext (sLit "(Use -XFlexibleContexts to lift this restriction)")
1468 checkThetaCtxt :: SourceTyCtxt -> ThetaType -> SDoc
1469 checkThetaCtxt ctxt theta
1470 = vcat [ptext (sLit "In the context:") <+> pprTheta theta,
1471 ptext (sLit "While checking") <+> pprSourceTyCtxt ctxt ]
1473 badPredTyErr, eqPredTyErr, predTyVarErr :: PredType -> SDoc
1474 badPredTyErr sty = ptext (sLit "Illegal constraint") <+> pprPred sty
1475 eqPredTyErr sty = ptext (sLit "Illegal equational constraint") <+> pprPred sty
1477 parens (ptext (sLit "Use -XTypeFamilies to permit this"))
1478 predTyVarErr pred = sep [ptext (sLit "Non type-variable argument"),
1479 nest 2 (ptext (sLit "in the constraint:") <+> pprPred pred)]
1480 dupPredWarn :: [[PredType]] -> SDoc
1481 dupPredWarn dups = ptext (sLit "Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
1483 arityErr :: Outputable a => String -> a -> Int -> Int -> SDoc
1484 arityErr kind name n m
1485 = hsep [ text kind, quotes (ppr name), ptext (sLit "should have"),
1486 n_arguments <> comma, text "but has been given", int m]
1488 n_arguments | n == 0 = ptext (sLit "no arguments")
1489 | n == 1 = ptext (sLit "1 argument")
1490 | True = hsep [int n, ptext (sLit "arguments")]
1493 notMonoType :: TcType -> TcM a
1495 = do { ty' <- zonkTcType ty
1496 ; env0 <- tcInitTidyEnv
1497 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1498 msg = ptext (sLit "Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1499 ; failWithTcM (env1, msg) }
1501 notMonoArgs :: TcType -> TcM a
1503 = do { ty' <- zonkTcType ty
1504 ; env0 <- tcInitTidyEnv
1505 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1506 msg = ptext (sLit "Arguments of type synonym families must be monotypes") <+> quotes (ppr tidy_ty)
1507 ; failWithTcM (env1, msg) }
1511 %************************************************************************
1513 \subsection{Checking for a decent instance head type}
1515 %************************************************************************
1517 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1518 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1520 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1521 flag is on, or (2)~the instance is imported (they must have been
1522 compiled elsewhere). In these cases, we let them go through anyway.
1524 We can also have instances for functions: @instance Foo (a -> b) ...@.
1527 checkValidInstHead :: Type -> TcM (Class, [TcType])
1529 checkValidInstHead ty -- Should be a source type
1530 = case tcSplitPredTy_maybe ty of {
1531 Nothing -> failWithTc (instTypeErr (ppr ty) empty) ;
1534 case getClassPredTys_maybe pred of {
1535 Nothing -> failWithTc (instTypeErr (pprPred pred) empty) ;
1536 Just (clas,tys) -> do
1539 check_inst_head dflags clas tys
1543 check_inst_head :: DynFlags -> Class -> [Type] -> TcM ()
1544 check_inst_head dflags clas tys
1545 = do { -- If GlasgowExts then check at least one isn't a type variable
1546 ; checkTc (dopt Opt_TypeSynonymInstances dflags ||
1547 all tcInstHeadTyNotSynonym tys)
1548 (instTypeErr (pprClassPred clas tys) head_type_synonym_msg)
1549 ; checkTc (dopt Opt_FlexibleInstances dflags ||
1550 all tcInstHeadTyAppAllTyVars tys)
1551 (instTypeErr (pprClassPred clas tys) head_type_args_tyvars_msg)
1552 ; checkTc (dopt Opt_MultiParamTypeClasses dflags ||
1554 (instTypeErr (pprClassPred clas tys) head_one_type_msg)
1555 -- May not contain type family applications
1556 ; mapM_ checkTyFamFreeness tys
1558 ; mapM_ checkValidMonoType tys
1559 -- For now, I only allow tau-types (not polytypes) in
1560 -- the head of an instance decl.
1561 -- E.g. instance C (forall a. a->a) is rejected
1562 -- One could imagine generalising that, but I'm not sure
1563 -- what all the consequences might be
1567 head_type_synonym_msg = parens (
1568 text "All instance types must be of the form (T t1 ... tn)" $$
1569 text "where T is not a synonym." $$
1570 text "Use -XTypeSynonymInstances if you want to disable this.")
1572 head_type_args_tyvars_msg = parens (vcat [
1573 text "All instance types must be of the form (T a1 ... an)",
1574 text "where a1 ... an are type *variables*,",
1575 text "and each type variable appears at most once in the instance head.",
1576 text "Use -XFlexibleInstances if you want to disable this."])
1578 head_one_type_msg = parens (
1579 text "Only one type can be given in an instance head." $$
1580 text "Use -XMultiParamTypeClasses if you want to allow more.")
1582 instTypeErr :: SDoc -> SDoc -> SDoc
1583 instTypeErr pp_ty msg
1584 = sep [ptext (sLit "Illegal instance declaration for") <+> quotes pp_ty,
1589 %************************************************************************
1591 \subsection{Checking instance for termination}
1593 %************************************************************************
1597 checkValidInstance :: [TyVar] -> ThetaType -> Class -> [TcType] -> TcM ()
1598 checkValidInstance tyvars theta clas inst_tys
1599 = do { undecidable_ok <- doptM Opt_UndecidableInstances
1601 ; checkValidTheta InstThetaCtxt theta
1602 ; checkAmbiguity tyvars theta (tyVarsOfTypes inst_tys)
1604 -- Check that instance inference will terminate (if we care)
1605 -- For Haskell 98 this will already have been done by checkValidTheta,
1606 -- but as we may be using other extensions we need to check.
1607 ; unless undecidable_ok $
1608 mapM_ addErrTc (checkInstTermination inst_tys theta)
1610 -- The Coverage Condition
1611 ; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
1612 (instTypeErr (pprClassPred clas inst_tys) msg)
1615 msg = parens (vcat [ptext (sLit "the Coverage Condition fails for one of the functional dependencies;"),
1619 Termination test: the so-called "Paterson conditions" (see Section 5 of
1620 "Understanding functionsl dependencies via Constraint Handling Rules,
1623 We check that each assertion in the context satisfies:
1624 (1) no variable has more occurrences in the assertion than in the head, and
1625 (2) the assertion has fewer constructors and variables (taken together
1626 and counting repetitions) than the head.
1627 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1628 (which have already been checked) guarantee termination.
1630 The underlying idea is that
1632 for any ground substitution, each assertion in the
1633 context has fewer type constructors than the head.
1637 checkInstTermination :: [TcType] -> ThetaType -> [Message]
1638 checkInstTermination tys theta
1639 = mapCatMaybes check theta
1642 size = sizeTypes tys
1644 | not (null (fvPred pred \\ fvs))
1645 = Just (predUndecErr pred nomoreMsg $$ parens undecidableMsg)
1646 | sizePred pred >= size
1647 = Just (predUndecErr pred smallerMsg $$ parens undecidableMsg)
1651 predUndecErr :: PredType -> SDoc -> SDoc
1652 predUndecErr pred msg = sep [msg,
1653 nest 2 (ptext (sLit "in the constraint:") <+> pprPred pred)]
1655 nomoreMsg, smallerMsg, undecidableMsg :: SDoc
1656 nomoreMsg = ptext (sLit "Variable occurs more often in a constraint than in the instance head")
1657 smallerMsg = ptext (sLit "Constraint is no smaller than the instance head")
1658 undecidableMsg = ptext (sLit "Use -XUndecidableInstances to permit this")
1662 %************************************************************************
1664 Checking the context of a derived instance declaration
1666 %************************************************************************
1668 Note [Exotic derived instance contexts]
1669 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1670 In a 'derived' instance declaration, we *infer* the context. It's a
1671 bit unclear what rules we should apply for this; the Haskell report is
1672 silent. Obviously, constraints like (Eq a) are fine, but what about
1673 data T f a = MkT (f a) deriving( Eq )
1674 where we'd get an Eq (f a) constraint. That's probably fine too.
1676 One could go further: consider
1677 data T a b c = MkT (Foo a b c) deriving( Eq )
1678 instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
1680 Notice that this instance (just) satisfies the Paterson termination
1681 conditions. Then we *could* derive an instance decl like this:
1683 instance (C Int a, Eq b, Eq c) => Eq (T a b c)
1685 even though there is no instance for (C Int a), because there just
1686 *might* be an instance for, say, (C Int Bool) at a site where we
1687 need the equality instance for T's.
1689 However, this seems pretty exotic, and it's quite tricky to allow
1690 this, and yet give sensible error messages in the (much more common)
1691 case where we really want that instance decl for C.
1693 So for now we simply require that the derived instance context
1694 should have only type-variable constraints.
1696 Here is another example:
1697 data Fix f = In (f (Fix f)) deriving( Eq )
1698 Here, if we are prepared to allow -XUndecidableInstances we
1699 could derive the instance
1700 instance Eq (f (Fix f)) => Eq (Fix f)
1701 but this is so delicate that I don't think it should happen inside
1702 'deriving'. If you want this, write it yourself!
1704 NB: if you want to lift this condition, make sure you still meet the
1705 termination conditions! If not, the deriving mechanism generates
1706 larger and larger constraints. Example:
1708 data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
1710 Note the lack of a Show instance for Succ. First we'll generate
1711 instance (Show (Succ a), Show a) => Show (Seq a)
1713 instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
1714 and so on. Instead we want to complain of no instance for (Show (Succ a)).
1718 Allow constraints which consist only of type variables, with no repeats.
1721 validDerivPred :: PredType -> Bool
1722 validDerivPred (ClassP _ tys) = hasNoDups fvs && sizeTypes tys == length fvs
1723 where fvs = fvTypes tys
1724 validDerivPred _ = False
1727 %************************************************************************
1729 Checking type instance well-formedness and termination
1731 %************************************************************************
1734 -- Check that a "type instance" is well-formed (which includes decidability
1735 -- unless -XUndecidableInstances is given).
1737 checkValidTypeInst :: [Type] -> Type -> TcM ()
1738 checkValidTypeInst typats rhs
1739 = do { -- left-hand side contains no type family applications
1740 -- (vanilla synonyms are fine, though)
1741 ; mapM_ checkTyFamFreeness typats
1743 -- the right-hand side is a tau type
1744 ; checkValidMonoType rhs
1746 -- we have a decidable instance unless otherwise permitted
1747 ; undecidable_ok <- doptM Opt_UndecidableInstances
1748 ; unless undecidable_ok $
1749 mapM_ addErrTc (checkFamInst typats (tyFamInsts rhs))
1752 -- Make sure that each type family instance is
1753 -- (1) strictly smaller than the lhs,
1754 -- (2) mentions no type variable more often than the lhs, and
1755 -- (3) does not contain any further type family instances.
1757 checkFamInst :: [Type] -- lhs
1758 -> [(TyCon, [Type])] -- type family instances
1760 checkFamInst lhsTys famInsts
1761 = mapCatMaybes check famInsts
1763 size = sizeTypes lhsTys
1764 fvs = fvTypes lhsTys
1766 | not (all isTyFamFree tys)
1767 = Just (famInstUndecErr famInst nestedMsg $$ parens undecidableMsg)
1768 | not (null (fvTypes tys \\ fvs))
1769 = Just (famInstUndecErr famInst nomoreVarMsg $$ parens undecidableMsg)
1770 | size <= sizeTypes tys
1771 = Just (famInstUndecErr famInst smallerAppMsg $$ parens undecidableMsg)
1775 famInst = TyConApp tc tys
1777 -- Ensure that no type family instances occur in a type.
1779 checkTyFamFreeness :: Type -> TcM ()
1780 checkTyFamFreeness ty
1781 = checkTc (isTyFamFree ty) $
1782 tyFamInstIllegalErr ty
1784 -- Check that a type does not contain any type family applications.
1786 isTyFamFree :: Type -> Bool
1787 isTyFamFree = null . tyFamInsts
1791 tyFamInstIllegalErr :: Type -> SDoc
1792 tyFamInstIllegalErr ty
1793 = hang (ptext (sLit "Illegal type synonym family application in instance") <>
1797 famInstUndecErr :: Type -> SDoc -> SDoc
1798 famInstUndecErr ty msg
1800 nest 2 (ptext (sLit "in the type family application:") <+>
1803 nestedMsg, nomoreVarMsg, smallerAppMsg :: SDoc
1804 nestedMsg = ptext (sLit "Nested type family application")
1805 nomoreVarMsg = ptext (sLit "Variable occurs more often than in instance head")
1806 smallerAppMsg = ptext (sLit "Application is no smaller than the instance head")
1810 %************************************************************************
1812 \subsection{Auxiliary functions}
1814 %************************************************************************
1817 -- Free variables of a type, retaining repetitions, and expanding synonyms
1818 fvType :: Type -> [TyVar]
1819 fvType ty | Just exp_ty <- tcView ty = fvType exp_ty
1820 fvType (TyVarTy tv) = [tv]
1821 fvType (TyConApp _ tys) = fvTypes tys
1822 fvType (PredTy pred) = fvPred pred
1823 fvType (FunTy arg res) = fvType arg ++ fvType res
1824 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1825 fvType (ForAllTy tyvar ty) = filter (/= tyvar) (fvType ty)
1827 fvTypes :: [Type] -> [TyVar]
1828 fvTypes tys = concat (map fvType tys)
1830 fvPred :: PredType -> [TyVar]
1831 fvPred (ClassP _ tys') = fvTypes tys'
1832 fvPred (IParam _ ty) = fvType ty
1833 fvPred (EqPred ty1 ty2) = fvType ty1 ++ fvType ty2
1835 -- Size of a type: the number of variables and constructors
1836 sizeType :: Type -> Int
1837 sizeType ty | Just exp_ty <- tcView ty = sizeType exp_ty
1838 sizeType (TyVarTy _) = 1
1839 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1840 sizeType (PredTy pred) = sizePred pred
1841 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1842 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1843 sizeType (ForAllTy _ ty) = sizeType ty
1845 sizeTypes :: [Type] -> Int
1846 sizeTypes xs = sum (map sizeType xs)
1848 sizePred :: PredType -> Int
1849 sizePred (ClassP _ tys') = sizeTypes tys'
1850 sizePred (IParam _ ty) = sizeType ty
1851 sizePred (EqPred ty1 ty2) = sizeType ty1 + sizeType ty2