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 { ASSERTM2( do { details <- readMetaTyVar tyvar; return (isFlexi details) }, ppr tyvar )
527 ; traceTc (text "writeMetaTyVar" <+> ppr tyvar <+> text ":=" <+> ppr ty)
528 ; writeMutVar (metaTvRef tyvar) (Indirect ty) }
530 _k1 = tyVarKind tyvar
535 %************************************************************************
539 %************************************************************************
542 newFlexiTyVar :: Kind -> TcM TcTyVar
543 newFlexiTyVar kind = newMetaTyVar TauTv kind
545 newFlexiTyVarTy :: Kind -> TcM TcType
546 newFlexiTyVarTy kind = do
547 tc_tyvar <- newFlexiTyVar kind
548 return (TyVarTy tc_tyvar)
550 newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
551 newFlexiTyVarTys n kind = mapM newFlexiTyVarTy (nOfThem n kind)
553 tcInstTyVar :: TyVar -> TcM TcTyVar
554 -- Instantiate with a META type variable
555 tcInstTyVar tyvar = instMetaTyVar TauTv tyvar
557 tcInstTyVars :: [TyVar] -> TcM ([TcTyVar], [TcType], TvSubst)
558 -- Instantiate with META type variables
560 = do { tc_tvs <- mapM tcInstTyVar tyvars
561 ; let tys = mkTyVarTys tc_tvs
562 ; return (tc_tvs, tys, zipTopTvSubst tyvars tys) }
563 -- Since the tyvars are freshly made,
564 -- they cannot possibly be captured by
565 -- any existing for-alls. Hence zipTopTvSubst
569 %************************************************************************
573 %************************************************************************
576 zonkSigTyVar :: TcTyVar -> TcM TcTyVar
578 | isSkolemTyVar sig_tv
579 = return sig_tv -- Happens in the call in TcBinds.checkDistinctTyVars
581 = ASSERT( isSigTyVar sig_tv )
582 do { ty <- zonkTcTyVar sig_tv
583 ; return (tcGetTyVar "zonkSigTyVar" ty) }
584 -- 'ty' is bound to be a type variable, because SigTvs
585 -- can only be unified with type variables
589 %************************************************************************
593 %************************************************************************
596 newBoxyTyVar :: Kind -> TcM BoxyTyVar
597 newBoxyTyVar kind = newMetaTyVar BoxTv kind
599 newBoxyTyVars :: [Kind] -> TcM [BoxyTyVar]
600 newBoxyTyVars kinds = mapM newBoxyTyVar kinds
602 newBoxyTyVarTys :: [Kind] -> TcM [BoxyType]
603 newBoxyTyVarTys kinds = do { tvs <- mapM newBoxyTyVar kinds; return (mkTyVarTys tvs) }
605 readFilledBox :: BoxyTyVar -> TcM TcType
606 -- Read the contents of the box, which should be filled in by now
607 readFilledBox box_tv = ASSERT( isBoxyTyVar box_tv )
608 do { cts <- readMetaTyVar box_tv
610 Flexi -> pprPanic "readFilledBox" (ppr box_tv)
611 Indirect ty -> return ty }
613 tcInstBoxyTyVar :: TyVar -> TcM BoxyTyVar
614 -- Instantiate with a BOXY type variable
615 tcInstBoxyTyVar tyvar = instMetaTyVar BoxTv tyvar
619 %************************************************************************
621 \subsection{Putting and getting mutable type variables}
623 %************************************************************************
625 But it's more fun to short out indirections on the way: If this
626 version returns a TyVar, then that TyVar is unbound. If it returns
627 any other type, then there might be bound TyVars embedded inside it.
629 We return Nothing iff the original box was unbound.
632 data LookupTyVarResult -- The result of a lookupTcTyVar call
633 = DoneTv TcTyVarDetails -- SkolemTv or virgin MetaTv
636 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
638 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
640 SkolemTv _ -> return (DoneTv details)
641 MetaTv _ ref -> do { meta_details <- readMutVar ref
642 ; case meta_details of
643 Indirect ty -> return (IndirectTv ty)
644 Flexi -> return (DoneTv details) }
646 details = tcTyVarDetails tyvar
649 -- gaw 2004 We aren't shorting anything out anymore, at least for now
651 | not (isTcTyVar tyvar)
652 = pprTrace "getTcTyVar" (ppr tyvar) $
653 return (Just (mkTyVarTy tyvar))
656 = ASSERT2( isTcTyVar tyvar, ppr tyvar ) do
657 maybe_ty <- readMetaTyVar tyvar
659 Just ty -> do ty' <- short_out ty
660 writeMetaTyVar tyvar (Just ty')
663 Nothing -> return Nothing
665 short_out :: TcType -> TcM TcType
666 short_out ty@(TyVarTy tyvar)
667 | not (isTcTyVar tyvar)
671 maybe_ty <- readMetaTyVar tyvar
673 Just ty' -> do ty' <- short_out ty'
674 writeMetaTyVar tyvar (Just ty')
679 short_out other_ty = return other_ty
684 %************************************************************************
686 \subsection{Zonking -- the exernal interfaces}
688 %************************************************************************
690 ----------------- Type variables
693 zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
694 zonkTcTyVars tyvars = mapM zonkTcTyVar tyvars
696 zonkTcTyVarsAndFV :: [TcTyVar] -> TcM TcTyVarSet
697 zonkTcTyVarsAndFV tyvars = tyVarsOfTypes <$> mapM zonkTcTyVar tyvars
699 zonkTcTyVar :: TcTyVar -> TcM TcType
700 zonkTcTyVar tyvar = ASSERT2( isTcTyVar tyvar, ppr tyvar)
701 zonk_tc_tyvar (\ tv -> return (TyVarTy tv)) tyvar
704 ----------------- Types
707 zonkTcType :: TcType -> TcM TcType
708 zonkTcType ty = zonkType (\ tv -> return (TyVarTy tv)) ty
710 zonkTcTypes :: [TcType] -> TcM [TcType]
711 zonkTcTypes tys = mapM zonkTcType tys
713 zonkTcThetaType :: TcThetaType -> TcM TcThetaType
714 zonkTcThetaType theta = mapM zonkTcPredType theta
716 zonkTcPredType :: TcPredType -> TcM TcPredType
717 zonkTcPredType (ClassP c ts) = ClassP c <$> zonkTcTypes ts
718 zonkTcPredType (IParam n t) = IParam n <$> zonkTcType t
719 zonkTcPredType (EqPred t1 t2) = EqPred <$> zonkTcType t1 <*> zonkTcType t2
722 ------------------- These ...ToType, ...ToKind versions
723 are used at the end of type checking
726 zonkTopTyVar :: TcTyVar -> TcM TcTyVar
727 -- zonkTopTyVar is used, at the top level, on any un-instantiated meta type variables
728 -- to default the kind of ? and ?? etc to *. This is important to ensure that
729 -- instance declarations match. For example consider
730 -- instance Show (a->b)
731 -- foo x = show (\_ -> True)
732 -- Then we'll get a constraint (Show (p ->q)) where p has argTypeKind (printed ??),
733 -- and that won't match the typeKind (*) in the instance decl.
735 -- Because we are at top level, no further constraints are going to affect these
736 -- type variables, so it's time to do it by hand. However we aren't ready
737 -- to default them fully to () or whatever, because the type-class defaulting
738 -- rules have yet to run.
741 | k `eqKind` default_k = return tv
743 = do { tv' <- newFlexiTyVar default_k
744 ; writeMetaTyVar tv (mkTyVarTy tv')
748 default_k = defaultKind k
750 zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TcTyVar]
751 zonkQuantifiedTyVars = mapM zonkQuantifiedTyVar
753 zonkQuantifiedTyVar :: TcTyVar -> TcM TcTyVar
754 -- zonkQuantifiedTyVar is applied to the a TcTyVar when quantifying over it.
756 -- The quantified type variables often include meta type variables
757 -- we want to freeze them into ordinary type variables, and
758 -- default their kind (e.g. from OpenTypeKind to TypeKind)
759 -- -- see notes with Kind.defaultKind
760 -- The meta tyvar is updated to point to the new skolem TyVar. Now any
761 -- bound occurences of the original type variable will get zonked to
762 -- the immutable version.
764 -- We leave skolem TyVars alone; they are immutable.
765 zonkQuantifiedTyVar tv
766 | ASSERT( isTcTyVar tv )
767 isSkolemTyVar tv = return tv
768 -- It might be a skolem type variable,
769 -- for example from a user type signature
771 | otherwise -- It's a meta-type-variable
772 = do { details <- readMetaTyVar tv
774 -- Create the new, frozen, skolem type variable
775 -- We zonk to a skolem, not to a regular TcVar
776 -- See Note [Zonking to Skolem]
777 ; let final_kind = defaultKind (tyVarKind tv)
778 final_tv = mkSkolTyVar (tyVarName tv) final_kind UnkSkol
780 -- Bind the meta tyvar to the new tyvar
782 Indirect ty -> WARN( True, ppr tv $$ ppr ty )
784 -- [Sept 04] I don't think this should happen
785 -- See note [Silly Type Synonym]
787 Flexi -> writeMetaTyVar tv (mkTyVarTy final_tv)
789 -- Return the new tyvar
793 Note [Silly Type Synonyms]
794 ~~~~~~~~~~~~~~~~~~~~~~~~~~
796 type C u a = u -- Note 'a' unused
798 foo :: (forall a. C u a -> C u a) -> u
802 bar = foo (\t -> t + t)
804 * From the (\t -> t+t) we get type {Num d} => d -> d
807 * Now unify with type of foo's arg, and we get:
808 {Num (C d a)} => C d a -> C d a
811 * Now abstract over the 'a', but float out the Num (C d a) constraint
812 because it does not 'really' mention a. (see exactTyVarsOfType)
813 The arg to foo becomes
816 * So we get a dict binding for Num (C d a), which is zonked to give
818 [Note Sept 04: now that we are zonking quantified type variables
819 on construction, the 'a' will be frozen as a regular tyvar on
820 quantification, so the floated dict will still have type (C d a).
821 Which renders this whole note moot; happily!]
823 * Then the \/\a abstraction has a zonked 'a' in it.
825 All very silly. I think its harmless to ignore the problem. We'll end up with
826 a \/\a in the final result but all the occurrences of a will be zonked to ()
828 Note [Zonking to Skolem]
829 ~~~~~~~~~~~~~~~~~~~~~~~~
830 We used to zonk quantified type variables to regular TyVars. However, this
831 leads to problems. Consider this program from the regression test suite:
833 eval :: Int -> String -> String -> String
834 eval 0 root actual = evalRHS 0 root actual
837 evalRHS 0 root actual = eval 0 root actual
839 It leads to the deferral of an equality
841 (String -> String -> String) ~ a
843 which is propagated up to the toplevel (see TcSimplify.tcSimplifyInferCheck).
844 In the meantime `a' is zonked and quantified to form `evalRHS's signature.
845 This has the *side effect* of also zonking the `a' in the deferred equality
846 (which at this point is being handed around wrapped in an implication
849 Finally, the equality (with the zonked `a') will be handed back to the
850 simplifier by TcRnDriver.tcRnSrcDecls calling TcSimplify.tcSimplifyTop.
851 If we zonk `a' with a regular type variable, we will have this regular type
852 variable now floating around in the simplifier, which in many places assumes to
853 only see proper TcTyVars.
855 We can avoid this problem by zonking with a skolem. The skolem is rigid
856 (which we requirefor a quantified variable), but is still a TcTyVar that the
857 simplifier knows how to deal with.
860 %************************************************************************
862 \subsection{Zonking -- the main work-horses: zonkType, zonkTyVar}
864 %* For internal use only! *
866 %************************************************************************
869 -- For unbound, mutable tyvars, zonkType uses the function given to it
870 -- For tyvars bound at a for-all, zonkType zonks them to an immutable
871 -- type variable and zonks the kind too
873 zonkType :: (TcTyVar -> TcM Type) -- What to do with unbound mutable type variables
874 -- see zonkTcType, and zonkTcTypeToType
877 zonkType unbound_var_fn ty
880 go (TyConApp tc tys) = do tys' <- mapM go tys
881 return (TyConApp tc tys')
883 go (PredTy p) = do p' <- go_pred p
886 go (FunTy arg res) = do arg' <- go arg
888 return (FunTy arg' res')
890 go (AppTy fun arg) = do fun' <- go fun
892 return (mkAppTy fun' arg')
893 -- NB the mkAppTy; we might have instantiated a
894 -- type variable to a type constructor, so we need
895 -- to pull the TyConApp to the top.
897 -- The two interesting cases!
898 go (TyVarTy tyvar) | isTcTyVar tyvar = zonk_tc_tyvar unbound_var_fn tyvar
899 | otherwise = return (TyVarTy tyvar)
900 -- Ordinary (non Tc) tyvars occur inside quantified types
902 go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar ) do
904 return (ForAllTy tyvar ty')
906 go_pred (ClassP c tys) = do tys' <- mapM go tys
907 return (ClassP c tys')
908 go_pred (IParam n ty) = do ty' <- go ty
909 return (IParam n ty')
910 go_pred (EqPred ty1 ty2) = do ty1' <- go ty1
912 return (EqPred ty1' ty2')
914 zonk_tc_tyvar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable variable
915 -> TcTyVar -> TcM TcType
916 zonk_tc_tyvar unbound_var_fn tyvar
917 | not (isMetaTyVar tyvar) -- Skolems
918 = return (TyVarTy tyvar)
920 | otherwise -- Mutables
921 = do { cts <- readMetaTyVar tyvar
923 Flexi -> unbound_var_fn tyvar -- Unbound meta type variable
924 Indirect ty -> zonkType unbound_var_fn ty }
929 %************************************************************************
933 %************************************************************************
936 readKindVar :: KindVar -> TcM (MetaDetails)
937 writeKindVar :: KindVar -> TcKind -> TcM ()
938 readKindVar kv = readMutVar (kindVarRef kv)
939 writeKindVar kv val = writeMutVar (kindVarRef kv) (Indirect val)
942 zonkTcKind :: TcKind -> TcM TcKind
943 zonkTcKind k = zonkTcType k
946 zonkTcKindToKind :: TcKind -> TcM Kind
947 -- When zonking a TcKind to a kind, we need to instantiate kind variables,
948 -- Haskell specifies that * is to be used, so we follow that.
949 zonkTcKindToKind k = zonkType (\ _ -> return liftedTypeKind) k
952 %************************************************************************
954 \subsection{Checking a user type}
956 %************************************************************************
958 When dealing with a user-written type, we first translate it from an HsType
959 to a Type, performing kind checking, and then check various things that should
960 be true about it. We don't want to perform these checks at the same time
961 as the initial translation because (a) they are unnecessary for interface-file
962 types and (b) when checking a mutually recursive group of type and class decls,
963 we can't "look" at the tycons/classes yet. Also, the checks are are rather
964 diverse, and used to really mess up the other code.
966 One thing we check for is 'rank'.
968 Rank 0: monotypes (no foralls)
969 Rank 1: foralls at the front only, Rank 0 inside
970 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
972 basic ::= tyvar | T basic ... basic
974 r2 ::= forall tvs. cxt => r2a
975 r2a ::= r1 -> r2a | basic
976 r1 ::= forall tvs. cxt => r0
977 r0 ::= r0 -> r0 | basic
979 Another thing is to check that type synonyms are saturated.
980 This might not necessarily show up in kind checking.
982 data T k = MkT (k Int)
987 checkValidType :: UserTypeCtxt -> Type -> TcM ()
988 -- Checks that the type is valid for the given context
989 checkValidType ctxt ty = do
990 traceTc (text "checkValidType" <+> ppr ty)
991 unboxed <- doptM Opt_UnboxedTuples
992 rank2 <- doptM Opt_Rank2Types
993 rankn <- doptM Opt_RankNTypes
994 polycomp <- doptM Opt_PolymorphicComponents
996 gen_rank n | rankn = ArbitraryRank
1001 DefaultDeclCtxt-> MustBeMonoType
1002 ResSigCtxt -> MustBeMonoType
1003 LamPatSigCtxt -> gen_rank 0
1004 BindPatSigCtxt -> gen_rank 0
1005 TySynCtxt _ -> gen_rank 0
1006 GenPatCtxt -> gen_rank 1
1007 -- This one is a bit of a hack
1008 -- See the forall-wrapping in TcClassDcl.mkGenericInstance
1010 ExprSigCtxt -> gen_rank 1
1011 FunSigCtxt _ -> gen_rank 1
1012 ConArgCtxt _ | polycomp -> gen_rank 2
1013 -- We are given the type of the entire
1014 -- constructor, hence rank 1
1015 | otherwise -> gen_rank 1
1017 ForSigCtxt _ -> gen_rank 1
1018 SpecInstCtxt -> gen_rank 1
1020 actual_kind = typeKind ty
1022 kind_ok = case ctxt of
1023 TySynCtxt _ -> True -- Any kind will do
1024 ResSigCtxt -> isSubOpenTypeKind actual_kind
1025 ExprSigCtxt -> isSubOpenTypeKind actual_kind
1026 GenPatCtxt -> isLiftedTypeKind actual_kind
1027 ForSigCtxt _ -> isLiftedTypeKind actual_kind
1028 _ -> isSubArgTypeKind actual_kind
1030 ubx_tup = case ctxt of
1031 TySynCtxt _ | unboxed -> UT_Ok
1032 ExprSigCtxt | unboxed -> UT_Ok
1035 -- Check that the thing has kind Type, and is lifted if necessary
1036 checkTc kind_ok (kindErr actual_kind)
1038 -- Check the internal validity of the type itself
1039 check_type rank ubx_tup ty
1041 traceTc (text "checkValidType done" <+> ppr ty)
1043 checkValidMonoType :: Type -> TcM ()
1044 checkValidMonoType ty = check_mono_type MustBeMonoType ty
1049 data Rank = ArbitraryRank -- Any rank ok
1050 | MustBeMonoType -- Monotype regardless of flags
1051 | TyConArgMonoType -- Monotype but could be poly if -XImpredicativeTypes
1052 | Rank Int -- Rank n, but could be more with -XRankNTypes
1054 decRank :: Rank -> Rank -- Function arguments
1055 decRank (Rank 0) = Rank 0
1056 decRank (Rank n) = Rank (n-1)
1057 decRank other_rank = other_rank
1059 nonZeroRank :: Rank -> Bool
1060 nonZeroRank ArbitraryRank = True
1061 nonZeroRank (Rank n) = n>0
1062 nonZeroRank _ = False
1064 ----------------------------------------
1065 data UbxTupFlag = UT_Ok | UT_NotOk
1066 -- The "Ok" version means "ok if UnboxedTuples is on"
1068 ----------------------------------------
1069 check_mono_type :: Rank -> Type -> TcM () -- No foralls anywhere
1070 -- No unlifted types of any kind
1071 check_mono_type rank ty
1072 = do { check_type rank UT_NotOk ty
1073 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1075 check_type :: Rank -> UbxTupFlag -> Type -> TcM ()
1076 -- The args say what the *type context* requires, independent
1077 -- of *flag* settings. You test the flag settings at usage sites.
1079 -- Rank is allowed rank for function args
1080 -- Rank 0 means no for-alls anywhere
1082 check_type rank ubx_tup ty
1083 | not (null tvs && null theta)
1084 = do { checkTc (nonZeroRank rank) (forAllTyErr rank ty)
1085 -- Reject e.g. (Maybe (?x::Int => Int)),
1086 -- with a decent error message
1087 ; check_valid_theta SigmaCtxt theta
1088 ; check_type rank ubx_tup tau -- Allow foralls to right of arrow
1089 ; checkFreeness tvs theta
1090 ; checkAmbiguity tvs theta (tyVarsOfType tau) }
1092 (tvs, theta, tau) = tcSplitSigmaTy ty
1094 -- Naked PredTys don't usually show up, but they can as a result of
1095 -- {-# SPECIALISE instance Ord Char #-}
1096 -- The Right Thing would be to fix the way that SPECIALISE instance pragmas
1097 -- are handled, but the quick thing is just to permit PredTys here.
1098 check_type _ _ (PredTy sty)
1099 = do { dflags <- getDOpts
1100 ; check_pred_ty dflags TypeCtxt sty }
1102 check_type _ _ (TyVarTy _) = return ()
1103 check_type rank _ (FunTy arg_ty res_ty)
1104 = do { check_type (decRank rank) UT_NotOk arg_ty
1105 ; check_type rank UT_Ok res_ty }
1107 check_type rank _ (AppTy ty1 ty2)
1108 = do { check_arg_type rank ty1
1109 ; check_arg_type rank ty2 }
1111 check_type rank ubx_tup ty@(TyConApp tc tys)
1113 = do { -- Check that the synonym has enough args
1114 -- This applies equally to open and closed synonyms
1115 -- It's OK to have an *over-applied* type synonym
1116 -- data Tree a b = ...
1117 -- type Foo a = Tree [a]
1118 -- f :: Foo a b -> ...
1119 checkTc (tyConArity tc <= length tys) arity_msg
1121 -- See Note [Liberal type synonyms]
1122 ; liberal <- doptM Opt_LiberalTypeSynonyms
1123 ; if not liberal || isOpenSynTyCon tc then
1124 -- For H98 and synonym families, do check the type args
1125 mapM_ (check_mono_type TyConArgMonoType) tys
1127 else -- In the liberal case (only for closed syns), expand then check
1129 Just ty' -> check_type rank ubx_tup ty'
1130 Nothing -> pprPanic "check_tau_type" (ppr ty)
1133 | isUnboxedTupleTyCon tc
1134 = do { ub_tuples_allowed <- doptM Opt_UnboxedTuples
1135 ; checkTc (ubx_tup_ok ub_tuples_allowed) ubx_tup_msg
1137 ; impred <- doptM Opt_ImpredicativeTypes
1138 ; let rank' = if impred then ArbitraryRank else TyConArgMonoType
1139 -- c.f. check_arg_type
1140 -- However, args are allowed to be unlifted, or
1141 -- more unboxed tuples, so can't use check_arg_ty
1142 ; mapM_ (check_type rank' UT_Ok) tys }
1145 = mapM_ (check_arg_type rank) tys
1148 ubx_tup_ok ub_tuples_allowed = case ubx_tup of
1149 UT_Ok -> ub_tuples_allowed
1153 tc_arity = tyConArity tc
1155 arity_msg = arityErr "Type synonym" (tyConName tc) tc_arity n_args
1156 ubx_tup_msg = ubxArgTyErr ty
1158 check_type _ _ ty = pprPanic "check_type" (ppr ty)
1160 ----------------------------------------
1161 check_arg_type :: Rank -> Type -> TcM ()
1162 -- The sort of type that can instantiate a type variable,
1163 -- or be the argument of a type constructor.
1164 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
1165 -- Other unboxed types are very occasionally allowed as type
1166 -- arguments depending on the kind of the type constructor
1168 -- For example, we want to reject things like:
1170 -- instance Ord a => Ord (forall s. T s a)
1172 -- g :: T s (forall b.b)
1174 -- NB: unboxed tuples can have polymorphic or unboxed args.
1175 -- This happens in the workers for functions returning
1176 -- product types with polymorphic components.
1177 -- But not in user code.
1178 -- Anyway, they are dealt with by a special case in check_tau_type
1180 check_arg_type rank ty
1181 = do { impred <- doptM Opt_ImpredicativeTypes
1182 ; let rank' = if impred then ArbitraryRank -- Arg of tycon can have arby rank, regardless
1183 else case rank of -- Predictive => must be monotype
1184 MustBeMonoType -> MustBeMonoType
1185 _ -> TyConArgMonoType
1186 -- Make sure that MustBeMonoType is propagated,
1187 -- so that we don't suggest -XImpredicativeTypes in
1188 -- (Ord (forall a.a)) => a -> a
1190 ; check_type rank' UT_NotOk ty
1191 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1193 ----------------------------------------
1194 forAllTyErr :: Rank -> Type -> SDoc
1196 = vcat [ hang (ptext (sLit "Illegal polymorphic or qualified type:")) 2 (ppr ty)
1199 suggestion = case rank of
1200 Rank _ -> ptext (sLit "Perhaps you intended to use -XRankNTypes or -XRank2Types")
1201 TyConArgMonoType -> ptext (sLit "Perhaps you intended to use -XImpredicativeTypes")
1202 _ -> empty -- Polytype is always illegal
1204 unliftedArgErr, ubxArgTyErr :: Type -> SDoc
1205 unliftedArgErr ty = sep [ptext (sLit "Illegal unlifted type:"), ppr ty]
1206 ubxArgTyErr ty = sep [ptext (sLit "Illegal unboxed tuple type as function argument:"), ppr ty]
1208 kindErr :: Kind -> SDoc
1209 kindErr kind = sep [ptext (sLit "Expecting an ordinary type, but found a type of kind"), ppr kind]
1212 Note [Liberal type synonyms]
1213 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1214 If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
1215 doing validity checking. This allows us to instantiate a synonym defn
1216 with a for-all type, or with a partially-applied type synonym.
1220 Here, T is partially applied, so it's illegal in H98. But if you
1221 expand S first, then T we get just
1225 IMPORTANT: suppose T is a type synonym. Then we must do validity
1226 checking on an appliation (T ty1 ty2)
1228 *either* before expansion (i.e. check ty1, ty2)
1229 *or* after expansion (i.e. expand T ty1 ty2, and then check)
1232 If we do both, we get exponential behaviour!!
1234 data TIACons1 i r c = c i ::: r c
1235 type TIACons2 t x = TIACons1 t (TIACons1 t x)
1236 type TIACons3 t x = TIACons2 t (TIACons1 t x)
1237 type TIACons4 t x = TIACons2 t (TIACons2 t x)
1238 type TIACons7 t x = TIACons4 t (TIACons3 t x)
1241 %************************************************************************
1243 \subsection{Checking a theta or source type}
1245 %************************************************************************
1248 -- Enumerate the contexts in which a "source type", <S>, can occur
1252 -- or (N a) where N is a newtype
1255 = ClassSCCtxt Name -- Superclasses of clas
1256 -- class <S> => C a where ...
1257 | SigmaCtxt -- Theta part of a normal for-all type
1258 -- f :: <S> => a -> a
1259 | DataTyCtxt Name -- Theta part of a data decl
1260 -- data <S> => T a = MkT a
1261 | TypeCtxt -- Source type in an ordinary type
1263 | InstThetaCtxt -- Context of an instance decl
1264 -- instance <S> => C [a] where ...
1266 pprSourceTyCtxt :: SourceTyCtxt -> SDoc
1267 pprSourceTyCtxt (ClassSCCtxt c) = ptext (sLit "the super-classes of class") <+> quotes (ppr c)
1268 pprSourceTyCtxt SigmaCtxt = ptext (sLit "the context of a polymorphic type")
1269 pprSourceTyCtxt (DataTyCtxt tc) = ptext (sLit "the context of the data type declaration for") <+> quotes (ppr tc)
1270 pprSourceTyCtxt InstThetaCtxt = ptext (sLit "the context of an instance declaration")
1271 pprSourceTyCtxt TypeCtxt = ptext (sLit "the context of a type")
1275 checkValidTheta :: SourceTyCtxt -> ThetaType -> TcM ()
1276 checkValidTheta ctxt theta
1277 = addErrCtxt (checkThetaCtxt ctxt theta) (check_valid_theta ctxt theta)
1279 -------------------------
1280 check_valid_theta :: SourceTyCtxt -> [PredType] -> TcM ()
1281 check_valid_theta _ []
1283 check_valid_theta ctxt theta = do
1285 warnTc (notNull dups) (dupPredWarn dups)
1286 mapM_ (check_pred_ty dflags ctxt) theta
1288 (_,dups) = removeDups tcCmpPred theta
1290 -------------------------
1291 check_pred_ty :: DynFlags -> SourceTyCtxt -> PredType -> TcM ()
1292 check_pred_ty dflags ctxt pred@(ClassP cls tys)
1293 = do { -- Class predicates are valid in all contexts
1294 ; checkTc (arity == n_tys) arity_err
1296 -- Check the form of the argument types
1297 ; mapM_ checkValidMonoType tys
1298 ; checkTc (check_class_pred_tys dflags ctxt tys)
1299 (predTyVarErr pred $$ how_to_allow)
1302 class_name = className cls
1303 arity = classArity cls
1305 arity_err = arityErr "Class" class_name arity n_tys
1306 how_to_allow = parens (ptext (sLit "Use -XFlexibleContexts to permit this"))
1308 check_pred_ty dflags _ pred@(EqPred ty1 ty2)
1309 = do { -- Equational constraints are valid in all contexts if type
1310 -- families are permitted
1311 ; checkTc (dopt Opt_TypeFamilies dflags) (eqPredTyErr pred)
1313 -- Check the form of the argument types
1314 ; checkValidMonoType ty1
1315 ; checkValidMonoType ty2
1318 check_pred_ty _ SigmaCtxt (IParam _ ty) = checkValidMonoType ty
1319 -- Implicit parameters only allowed in type
1320 -- signatures; not in instance decls, superclasses etc
1321 -- The reason for not allowing implicit params in instances is a bit
1323 -- If we allowed instance (?x::Int, Eq a) => Foo [a] where ...
1324 -- then when we saw (e :: (?x::Int) => t) it would be unclear how to
1325 -- discharge all the potential usas of the ?x in e. For example, a
1326 -- constraint Foo [Int] might come out of e,and applying the
1327 -- instance decl would show up two uses of ?x.
1330 check_pred_ty _ _ sty = failWithTc (badPredTyErr sty)
1332 -------------------------
1333 check_class_pred_tys :: DynFlags -> SourceTyCtxt -> [Type] -> Bool
1334 check_class_pred_tys dflags ctxt tys
1336 TypeCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
1337 InstThetaCtxt -> flexible_contexts || undecidable_ok || all tcIsTyVarTy tys
1338 -- Further checks on head and theta in
1339 -- checkInstTermination
1340 _ -> flexible_contexts || all tyvar_head tys
1342 flexible_contexts = dopt Opt_FlexibleContexts dflags
1343 undecidable_ok = dopt Opt_UndecidableInstances dflags
1345 -------------------------
1346 tyvar_head :: Type -> Bool
1347 tyvar_head ty -- Haskell 98 allows predicates of form
1348 | tcIsTyVarTy ty = True -- C (a ty1 .. tyn)
1349 | otherwise -- where a is a type variable
1350 = case tcSplitAppTy_maybe ty of
1351 Just (ty, _) -> tyvar_head ty
1358 is ambiguous if P contains generic variables
1359 (i.e. one of the Vs) that are not mentioned in tau
1361 However, we need to take account of functional dependencies
1362 when we speak of 'mentioned in tau'. Example:
1363 class C a b | a -> b where ...
1365 forall x y. (C x y) => x
1366 is not ambiguous because x is mentioned and x determines y
1368 NB; the ambiguity check is only used for *user* types, not for types
1369 coming from inteface files. The latter can legitimately have
1370 ambiguous types. Example
1372 class S a where s :: a -> (Int,Int)
1373 instance S Char where s _ = (1,1)
1374 f:: S a => [a] -> Int -> (Int,Int)
1375 f (_::[a]) x = (a*x,b)
1376 where (a,b) = s (undefined::a)
1378 Here the worker for f gets the type
1379 fw :: forall a. S a => Int -> (# Int, Int #)
1381 If the list of tv_names is empty, we have a monotype, and then we
1382 don't need to check for ambiguity either, because the test can't fail
1387 checkAmbiguity :: [TyVar] -> ThetaType -> TyVarSet -> TcM ()
1388 checkAmbiguity forall_tyvars theta tau_tyvars
1389 = mapM_ complain (filter is_ambig theta)
1391 complain pred = addErrTc (ambigErr pred)
1392 extended_tau_vars = grow theta tau_tyvars
1394 -- See Note [Implicit parameters and ambiguity] in TcSimplify
1395 is_ambig pred = isClassPred pred &&
1396 any ambig_var (varSetElems (tyVarsOfPred pred))
1398 ambig_var ct_var = (ct_var `elem` forall_tyvars) &&
1399 not (ct_var `elemVarSet` extended_tau_vars)
1401 ambigErr :: PredType -> SDoc
1403 = sep [ptext (sLit "Ambiguous constraint") <+> quotes (pprPred pred),
1404 nest 4 (ptext (sLit "At least one of the forall'd type variables mentioned by the constraint") $$
1405 ptext (sLit "must be reachable from the type after the '=>'"))]
1408 In addition, GHC insists that at least one type variable
1409 in each constraint is in V. So we disallow a type like
1410 forall a. Eq b => b -> b
1411 even in a scope where b is in scope.
1414 checkFreeness :: [Var] -> [PredType] -> TcM ()
1415 checkFreeness forall_tyvars theta
1416 = do { flexible_contexts <- doptM Opt_FlexibleContexts
1417 ; unless flexible_contexts $ mapM_ complain (filter is_free theta) }
1419 is_free pred = not (isIPPred pred)
1420 && not (any bound_var (varSetElems (tyVarsOfPred pred)))
1421 bound_var ct_var = ct_var `elem` forall_tyvars
1422 complain pred = addErrTc (freeErr pred)
1424 freeErr :: PredType -> SDoc
1426 = sep [ ptext (sLit "All of the type variables in the constraint") <+>
1427 quotes (pprPred pred)
1428 , ptext (sLit "are already in scope") <+>
1429 ptext (sLit "(at least one must be universally quantified here)")
1431 ptext (sLit "(Use -XFlexibleContexts to lift this restriction)")
1436 checkThetaCtxt :: SourceTyCtxt -> ThetaType -> SDoc
1437 checkThetaCtxt ctxt theta
1438 = vcat [ptext (sLit "In the context:") <+> pprTheta theta,
1439 ptext (sLit "While checking") <+> pprSourceTyCtxt ctxt ]
1441 badPredTyErr, eqPredTyErr, predTyVarErr :: PredType -> SDoc
1442 badPredTyErr sty = ptext (sLit "Illegal constraint") <+> pprPred sty
1443 eqPredTyErr sty = ptext (sLit "Illegal equational constraint") <+> pprPred sty
1445 parens (ptext (sLit "Use -XTypeFamilies to permit this"))
1446 predTyVarErr pred = sep [ptext (sLit "Non type-variable argument"),
1447 nest 2 (ptext (sLit "in the constraint:") <+> pprPred pred)]
1448 dupPredWarn :: [[PredType]] -> SDoc
1449 dupPredWarn dups = ptext (sLit "Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
1451 arityErr :: Outputable a => String -> a -> Int -> Int -> SDoc
1452 arityErr kind name n m
1453 = hsep [ text kind, quotes (ppr name), ptext (sLit "should have"),
1454 n_arguments <> comma, text "but has been given", int m]
1456 n_arguments | n == 0 = ptext (sLit "no arguments")
1457 | n == 1 = ptext (sLit "1 argument")
1458 | True = hsep [int n, ptext (sLit "arguments")]
1461 notMonoType :: TcType -> TcM a
1463 = do { ty' <- zonkTcType ty
1464 ; env0 <- tcInitTidyEnv
1465 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1466 msg = ptext (sLit "Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1467 ; failWithTcM (env1, msg) }
1469 notMonoArgs :: TcType -> TcM a
1471 = do { ty' <- zonkTcType ty
1472 ; env0 <- tcInitTidyEnv
1473 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1474 msg = ptext (sLit "Arguments of type synonym families must be monotypes") <+> quotes (ppr tidy_ty)
1475 ; failWithTcM (env1, msg) }
1479 %************************************************************************
1481 \subsection{Checking for a decent instance head type}
1483 %************************************************************************
1485 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1486 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1488 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1489 flag is on, or (2)~the instance is imported (they must have been
1490 compiled elsewhere). In these cases, we let them go through anyway.
1492 We can also have instances for functions: @instance Foo (a -> b) ...@.
1495 checkValidInstHead :: Type -> TcM (Class, [TcType])
1497 checkValidInstHead ty -- Should be a source type
1498 = case tcSplitPredTy_maybe ty of {
1499 Nothing -> failWithTc (instTypeErr (ppr ty) empty) ;
1502 case getClassPredTys_maybe pred of {
1503 Nothing -> failWithTc (instTypeErr (pprPred pred) empty) ;
1504 Just (clas,tys) -> do
1507 check_inst_head dflags clas tys
1511 check_inst_head :: DynFlags -> Class -> [Type] -> TcM ()
1512 check_inst_head dflags clas tys
1513 = do { -- If GlasgowExts then check at least one isn't a type variable
1514 ; checkTc (dopt Opt_TypeSynonymInstances dflags ||
1515 all tcInstHeadTyNotSynonym tys)
1516 (instTypeErr (pprClassPred clas tys) head_type_synonym_msg)
1517 ; checkTc (dopt Opt_FlexibleInstances dflags ||
1518 all tcInstHeadTyAppAllTyVars tys)
1519 (instTypeErr (pprClassPred clas tys) head_type_args_tyvars_msg)
1520 ; checkTc (dopt Opt_MultiParamTypeClasses dflags ||
1522 (instTypeErr (pprClassPred clas tys) head_one_type_msg)
1523 -- May not contain type family applications
1524 ; mapM_ checkTyFamFreeness tys
1526 ; mapM_ checkValidMonoType tys
1527 -- For now, I only allow tau-types (not polytypes) in
1528 -- the head of an instance decl.
1529 -- E.g. instance C (forall a. a->a) is rejected
1530 -- One could imagine generalising that, but I'm not sure
1531 -- what all the consequences might be
1535 head_type_synonym_msg = parens (
1536 text "All instance types must be of the form (T t1 ... tn)" $$
1537 text "where T is not a synonym." $$
1538 text "Use -XTypeSynonymInstances if you want to disable this.")
1540 head_type_args_tyvars_msg = parens (vcat [
1541 text "All instance types must be of the form (T a1 ... an)",
1542 text "where a1 ... an are type *variables*,",
1543 text "and each type variable appears at most once in the instance head.",
1544 text "Use -XFlexibleInstances if you want to disable this."])
1546 head_one_type_msg = parens (
1547 text "Only one type can be given in an instance head." $$
1548 text "Use -XMultiParamTypeClasses if you want to allow more.")
1550 instTypeErr :: SDoc -> SDoc -> SDoc
1551 instTypeErr pp_ty msg
1552 = sep [ptext (sLit "Illegal instance declaration for") <+> quotes pp_ty,
1557 %************************************************************************
1559 \subsection{Checking instance for termination}
1561 %************************************************************************
1565 checkValidInstance :: [TyVar] -> ThetaType -> Class -> [TcType] -> TcM ()
1566 checkValidInstance tyvars theta clas inst_tys
1567 = do { undecidable_ok <- doptM Opt_UndecidableInstances
1569 ; checkValidTheta InstThetaCtxt theta
1570 ; checkAmbiguity tyvars theta (tyVarsOfTypes inst_tys)
1572 -- Check that instance inference will terminate (if we care)
1573 -- For Haskell 98 this will already have been done by checkValidTheta,
1574 -- but as we may be using other extensions we need to check.
1575 ; unless undecidable_ok $
1576 mapM_ addErrTc (checkInstTermination inst_tys theta)
1578 -- The Coverage Condition
1579 ; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
1580 (instTypeErr (pprClassPred clas inst_tys) msg)
1583 msg = parens (vcat [ptext (sLit "the Coverage Condition fails for one of the functional dependencies;"),
1587 Termination test: the so-called "Paterson conditions" (see Section 5 of
1588 "Understanding functionsl dependencies via Constraint Handling Rules,
1591 We check that each assertion in the context satisfies:
1592 (1) no variable has more occurrences in the assertion than in the head, and
1593 (2) the assertion has fewer constructors and variables (taken together
1594 and counting repetitions) than the head.
1595 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1596 (which have already been checked) guarantee termination.
1598 The underlying idea is that
1600 for any ground substitution, each assertion in the
1601 context has fewer type constructors than the head.
1605 checkInstTermination :: [TcType] -> ThetaType -> [Message]
1606 checkInstTermination tys theta
1607 = mapCatMaybes check theta
1610 size = sizeTypes tys
1612 | not (null (fvPred pred \\ fvs))
1613 = Just (predUndecErr pred nomoreMsg $$ parens undecidableMsg)
1614 | sizePred pred >= size
1615 = Just (predUndecErr pred smallerMsg $$ parens undecidableMsg)
1619 predUndecErr :: PredType -> SDoc -> SDoc
1620 predUndecErr pred msg = sep [msg,
1621 nest 2 (ptext (sLit "in the constraint:") <+> pprPred pred)]
1623 nomoreMsg, smallerMsg, undecidableMsg :: SDoc
1624 nomoreMsg = ptext (sLit "Variable occurs more often in a constraint than in the instance head")
1625 smallerMsg = ptext (sLit "Constraint is no smaller than the instance head")
1626 undecidableMsg = ptext (sLit "Use -XUndecidableInstances to permit this")
1630 %************************************************************************
1632 Checking the context of a derived instance declaration
1634 %************************************************************************
1636 Note [Exotic derived instance contexts]
1637 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1638 In a 'derived' instance declaration, we *infer* the context. It's a
1639 bit unclear what rules we should apply for this; the Haskell report is
1640 silent. Obviously, constraints like (Eq a) are fine, but what about
1641 data T f a = MkT (f a) deriving( Eq )
1642 where we'd get an Eq (f a) constraint. That's probably fine too.
1644 One could go further: consider
1645 data T a b c = MkT (Foo a b c) deriving( Eq )
1646 instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
1648 Notice that this instance (just) satisfies the Paterson termination
1649 conditions. Then we *could* derive an instance decl like this:
1651 instance (C Int a, Eq b, Eq c) => Eq (T a b c)
1653 even though there is no instance for (C Int a), because there just
1654 *might* be an instance for, say, (C Int Bool) at a site where we
1655 need the equality instance for T's.
1657 However, this seems pretty exotic, and it's quite tricky to allow
1658 this, and yet give sensible error messages in the (much more common)
1659 case where we really want that instance decl for C.
1661 So for now we simply require that the derived instance context
1662 should have only type-variable constraints.
1664 Here is another example:
1665 data Fix f = In (f (Fix f)) deriving( Eq )
1666 Here, if we are prepared to allow -XUndecidableInstances we
1667 could derive the instance
1668 instance Eq (f (Fix f)) => Eq (Fix f)
1669 but this is so delicate that I don't think it should happen inside
1670 'deriving'. If you want this, write it yourself!
1672 NB: if you want to lift this condition, make sure you still meet the
1673 termination conditions! If not, the deriving mechanism generates
1674 larger and larger constraints. Example:
1676 data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
1678 Note the lack of a Show instance for Succ. First we'll generate
1679 instance (Show (Succ a), Show a) => Show (Seq a)
1681 instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
1682 and so on. Instead we want to complain of no instance for (Show (Succ a)).
1686 Allow constraints which consist only of type variables, with no repeats.
1689 validDerivPred :: PredType -> Bool
1690 validDerivPred (ClassP _ tys) = hasNoDups fvs && sizeTypes tys == length fvs
1691 where fvs = fvTypes tys
1692 validDerivPred _ = False
1695 %************************************************************************
1697 Checking type instance well-formedness and termination
1699 %************************************************************************
1702 -- Check that a "type instance" is well-formed (which includes decidability
1703 -- unless -XUndecidableInstances is given).
1705 checkValidTypeInst :: [Type] -> Type -> TcM ()
1706 checkValidTypeInst typats rhs
1707 = do { -- left-hand side contains no type family applications
1708 -- (vanilla synonyms are fine, though)
1709 ; mapM_ checkTyFamFreeness typats
1711 -- the right-hand side is a tau type
1712 ; checkValidMonoType rhs
1714 -- we have a decidable instance unless otherwise permitted
1715 ; undecidable_ok <- doptM Opt_UndecidableInstances
1716 ; unless undecidable_ok $
1717 mapM_ addErrTc (checkFamInst typats (tyFamInsts rhs))
1720 -- Make sure that each type family instance is
1721 -- (1) strictly smaller than the lhs,
1722 -- (2) mentions no type variable more often than the lhs, and
1723 -- (3) does not contain any further type family instances.
1725 checkFamInst :: [Type] -- lhs
1726 -> [(TyCon, [Type])] -- type family instances
1728 checkFamInst lhsTys famInsts
1729 = mapCatMaybes check famInsts
1731 size = sizeTypes lhsTys
1732 fvs = fvTypes lhsTys
1734 | not (all isTyFamFree tys)
1735 = Just (famInstUndecErr famInst nestedMsg $$ parens undecidableMsg)
1736 | not (null (fvTypes tys \\ fvs))
1737 = Just (famInstUndecErr famInst nomoreVarMsg $$ parens undecidableMsg)
1738 | size <= sizeTypes tys
1739 = Just (famInstUndecErr famInst smallerAppMsg $$ parens undecidableMsg)
1743 famInst = TyConApp tc tys
1745 -- Ensure that no type family instances occur in a type.
1747 checkTyFamFreeness :: Type -> TcM ()
1748 checkTyFamFreeness ty
1749 = checkTc (isTyFamFree ty) $
1750 tyFamInstIllegalErr ty
1752 -- Check that a type does not contain any type family applications.
1754 isTyFamFree :: Type -> Bool
1755 isTyFamFree = null . tyFamInsts
1759 tyFamInstIllegalErr :: Type -> SDoc
1760 tyFamInstIllegalErr ty
1761 = hang (ptext (sLit "Illegal type synonym family application in instance") <>
1765 famInstUndecErr :: Type -> SDoc -> SDoc
1766 famInstUndecErr ty msg
1768 nest 2 (ptext (sLit "in the type family application:") <+>
1771 nestedMsg, nomoreVarMsg, smallerAppMsg :: SDoc
1772 nestedMsg = ptext (sLit "Nested type family application")
1773 nomoreVarMsg = ptext (sLit "Variable occurs more often than in instance head")
1774 smallerAppMsg = ptext (sLit "Application is no smaller than the instance head")
1778 %************************************************************************
1780 \subsection{Auxiliary functions}
1782 %************************************************************************
1785 -- Free variables of a type, retaining repetitions, and expanding synonyms
1786 fvType :: Type -> [TyVar]
1787 fvType ty | Just exp_ty <- tcView ty = fvType exp_ty
1788 fvType (TyVarTy tv) = [tv]
1789 fvType (TyConApp _ tys) = fvTypes tys
1790 fvType (PredTy pred) = fvPred pred
1791 fvType (FunTy arg res) = fvType arg ++ fvType res
1792 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1793 fvType (ForAllTy tyvar ty) = filter (/= tyvar) (fvType ty)
1795 fvTypes :: [Type] -> [TyVar]
1796 fvTypes tys = concat (map fvType tys)
1798 fvPred :: PredType -> [TyVar]
1799 fvPred (ClassP _ tys') = fvTypes tys'
1800 fvPred (IParam _ ty) = fvType ty
1801 fvPred (EqPred ty1 ty2) = fvType ty1 ++ fvType ty2
1803 -- Size of a type: the number of variables and constructors
1804 sizeType :: Type -> Int
1805 sizeType ty | Just exp_ty <- tcView ty = sizeType exp_ty
1806 sizeType (TyVarTy _) = 1
1807 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1808 sizeType (PredTy pred) = sizePred pred
1809 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1810 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1811 sizeType (ForAllTy _ ty) = sizeType ty
1813 sizeTypes :: [Type] -> Int
1814 sizeTypes xs = sum (map sizeType xs)
1816 sizePred :: PredType -> Int
1817 sizePred (ClassP _ tys') = sizeTypes tys'
1818 sizePred (IParam _ ty) = sizeType ty
1819 sizePred (EqPred ty1 ty2) = sizeType ty1 + sizeType ty2