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,
48 growPredTyVars, growTyVars, growThetaTyVars,
50 --------------------------------
52 zonkType, zonkTcPredType,
53 zonkTcTyVar, zonkTcTyVars, zonkTcTyVarsAndFV, zonkSigTyVar,
54 zonkQuantifiedTyVar, zonkQuantifiedTyVars,
55 zonkTcType, zonkTcTypes, zonkTcThetaType,
56 zonkTcKindToKind, zonkTcKind, zonkTopTyVar,
58 readKindVar, writeKindVar
61 #include "HsVersions.h"
73 import TcRnMonad -- TcType, amongst others
89 import Data.List ( (\\) )
93 %************************************************************************
95 Instantiation in general
97 %************************************************************************
100 tcInstType :: ([TyVar] -> TcM [TcTyVar]) -- How to instantiate the type variables
101 -> TcType -- Type to instantiate
102 -> TcM ([TcTyVar], TcThetaType, TcType) -- Result
103 -- (type vars (excl coercion vars), preds (incl equalities), rho)
104 tcInstType inst_tyvars ty
105 = case tcSplitForAllTys ty of
106 ([], rho) -> let -- There may be overloading despite no type variables;
107 -- (?x :: Int) => Int -> Int
108 (theta, tau) = tcSplitPhiTy rho
110 return ([], theta, tau)
112 (tyvars, rho) -> do { tyvars' <- inst_tyvars tyvars
114 ; let tenv = zipTopTvSubst tyvars (mkTyVarTys tyvars')
115 -- Either the tyvars are freshly made, by inst_tyvars,
116 -- or (in the call from tcSkolSigType) any nested foralls
117 -- have different binders. Either way, zipTopTvSubst is ok
119 ; let (theta, tau) = tcSplitPhiTy (substTy tenv rho)
120 ; return (tyvars', theta, tau) }
124 %************************************************************************
128 %************************************************************************
130 Can't be in TcUnify, as we also need it in TcTyFuns.
134 -- False <=> the two args are (actual, expected) respectively
135 -- True <=> the two args are (expected, actual) respectively
137 checkUpdateMeta :: SwapFlag
138 -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
139 -- Update tv1, which is flexi; occurs check is alrady done
140 -- The 'check' version does a kind check too
141 -- We do a sub-kind check here: we might unify (a b) with (c d)
142 -- where b::*->* and d::*; this should fail
144 checkUpdateMeta swapped tv1 ref1 ty2
145 = do { checkKinds swapped tv1 ty2
146 ; updateMeta tv1 ref1 ty2 }
148 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
149 updateMeta tv1 ref1 ty2
150 = ASSERT( isMetaTyVar tv1 )
151 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
152 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
153 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
154 ; writeMutVar ref1 (Indirect ty2)
158 checkKinds :: Bool -> TyVar -> Type -> TcM ()
159 checkKinds swapped tv1 ty2
160 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
161 -- ty2 has been zonked at this stage, which ensures that
162 -- its kind has as much boxity information visible as possible.
163 | tk2 `isSubKind` tk1 = return ()
166 -- Either the kinds aren't compatible
167 -- (can happen if we unify (a b) with (c d))
168 -- or we are unifying a lifted type variable with an
169 -- unlifted type: e.g. (id 3#) is illegal
170 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
171 unifyKindMisMatch k1 k2
173 (k1,k2) | swapped = (tk2,tk1)
174 | otherwise = (tk1,tk2)
179 checkTauTvUpdate :: TcTyVar -> TcType -> TcM (Maybe TcType)
180 -- (checkTauTvUpdate tv ty)
181 -- We are about to update the TauTv tv with ty.
182 -- Check (a) that tv doesn't occur in ty (occurs check)
183 -- (b) that ty is a monotype
184 -- Furthermore, in the interest of (b), if you find an
185 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
187 -- We have three possible outcomes:
188 -- (1) Return the (non-boxy) type to update the type variable with,
189 -- [we know the update is ok!]
190 -- (2) return Nothing, or
191 -- [we cannot tell whether the update is ok right now]
193 -- [the update is definitely invalid]
194 -- We return Nothing in case the tv occurs in ty *under* a type family
195 -- application. In this case, we must not update tv (to avoid a cyclic type
196 -- term), but we also cannot fail claiming an infinite type. Given
198 -- type instance F Int = Int
201 -- This is perfectly reasonable, if we later get a ~ Int.
203 checkTauTvUpdate orig_tv orig_ty
204 = do { result <- go orig_ty
206 Right ty -> return $ Just ty
207 Left True -> return $ Nothing
208 Left False -> occurCheckErr (mkTyVarTy orig_tv) orig_ty
211 go :: TcType -> TcM (Either Bool TcType)
213 -- Right ty if everything is fine
214 -- Left True if orig_tv occurs in orig_ty, but under a type family app
215 -- Left False if orig_tv occurs in orig_ty (with no type family app)
216 -- It fails if it encounters a forall type, except as an argument for a
217 -- closed type synonym that expands to a tau type.
219 | isSynTyCon tc = go_syn tc tys
220 | otherwise = do { tys' <- mapM go tys
221 ; return $ occurs (TyConApp tc) tys' }
222 go (PredTy p) = do { p' <- go_pred p
223 ; return $ occurs1 PredTy p' }
224 go (FunTy arg res) = do { arg' <- go arg
226 ; return $ occurs2 FunTy arg' res' }
227 go (AppTy fun arg) = do { fun' <- go fun
229 ; return $ occurs2 mkAppTy fun' arg' }
230 -- NB the mkAppTy; we might have instantiated a
231 -- type variable to a type constructor, so we need
232 -- to pull the TyConApp to the top.
233 go (ForAllTy _ _) = notMonoType orig_ty -- (b)
236 | orig_tv == tv = return $ Left False -- (a)
237 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
238 | otherwise = return $ Right (TyVarTy tv)
239 -- Ordinary (non Tc) tyvars
240 -- occur inside quantified types
242 go_pred (ClassP c tys) = do { tys' <- mapM go tys
243 ; return $ occurs (ClassP c) tys' }
244 go_pred (IParam n ty) = do { ty' <- go ty
245 ; return $ occurs1 (IParam n) ty' }
246 go_pred (EqPred t1 t2) = do { t1' <- go t1
248 ; return $ occurs2 EqPred t1' t2' }
250 go_tyvar tv (SkolemTv _) = return $ Right (TyVarTy tv)
251 go_tyvar tv (MetaTv box ref)
252 = do { cts <- readMutVar ref
256 BoxTv -> do { ty <- fillBoxWithTau tv ref
257 ; return $ Right ty }
258 _ -> return $ Right (TyVarTy tv)
261 -- go_syn is called for synonyms only
262 -- See Note [Type synonyms and the occur check]
264 | not (isTauTyCon tc)
265 = notMonoType orig_ty -- (b) again
267 = do { (_msgs, mb_tys') <- tryTc (mapM go tys)
270 -- we had a type error => forall in type parameters
272 | isOpenTyCon tc -> notMonoArgs (TyConApp tc tys)
273 -- Synonym families must have monotype args
274 | otherwise -> go (expectJust "checkTauTvUpdate(1)"
275 (tcView (TyConApp tc tys)))
276 -- Try again, expanding the synonym
278 -- no type error, but need to test whether occurs check happend
280 case occurs id tys' of
282 | isOpenTyCon tc -> return $ Left True
283 -- Variable occured under type family application
284 | otherwise -> go (expectJust "checkTauTvUpdate(2)"
285 (tcView (TyConApp tc tys)))
286 -- Try again, expanding the synonym
287 Right raw_tys' -> return $ Right (TyConApp tc raw_tys')
288 -- Retain the synonym (the common case)
291 -- Left results (= occurrence of orig_ty) dominate and
292 -- (Left False) (= fatal occurrence) dominates over (Left True)
293 occurs :: ([a] -> b) -> [Either Bool a] -> Either Bool b
294 occurs c = either Left (Right . c) . foldr combine (Right [])
296 combine (Left famInst1) (Left famInst2) = Left (famInst1 && famInst2)
297 combine (Right _ ) (Left famInst) = Left famInst
298 combine (Left famInst) (Right _) = Left famInst
299 combine (Right arg) (Right args) = Right (arg:args)
301 occurs1 c x = occurs (\[x'] -> c x') [x]
302 occurs2 c x y = occurs (\[x', y'] -> c x' y') [x, y]
304 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
305 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
306 -- tau-type meta-variable, whose print-name is the same as tv
307 -- Choosing the same name is good: when we instantiate a function
308 -- we allocate boxy tyvars with the same print-name as the quantified
309 -- tyvar; and then we often fill the box with a tau-tyvar, and again
310 -- we want to choose the same name.
311 fillBoxWithTau tv ref
312 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
313 ; let tau = mkTyVarTy tv' -- name of the type variable
314 ; writeMutVar ref (Indirect tau)
318 Note [Type synonyms and the occur check]
320 Basically we want to update tv1 := ps_ty2
321 because ps_ty2 has type-synonym info, which improves later error messages
326 f :: (A a -> a -> ()) -> ()
332 In the application (p x), we try to match "t" with "A t". If we go
333 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
334 an infinite loop later.
335 But we should not reject the program, because A t = ().
336 Rather, we should bind t to () (= non_var_ty2).
340 Error mesages in case of kind mismatch.
343 unifyKindMisMatch :: TcKind -> TcKind -> TcM ()
344 unifyKindMisMatch ty1 ty2 = do
345 ty1' <- zonkTcKind ty1
346 ty2' <- zonkTcKind ty2
348 msg = hang (ptext (sLit "Couldn't match kind"))
349 2 (sep [quotes (ppr ty1'),
350 ptext (sLit "against"),
354 unifyKindCtxt :: Bool -> TyVar -> Type -> TidyEnv -> TcM (TidyEnv, SDoc)
355 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
356 -- tv1 and ty2 are zonked already
359 msg = (env2, ptext (sLit "When matching the kinds of") <+>
360 sep [quotes pp_expected <+> ptext (sLit "and"), quotes pp_actual])
362 (pp_expected, pp_actual) | swapped = (pp2, pp1)
363 | otherwise = (pp1, pp2)
364 (env1, tv1') = tidyOpenTyVar tidy_env tv1
365 (env2, ty2') = tidyOpenType env1 ty2
366 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
367 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
370 Error message for failure due to an occurs check.
373 occurCheckErr :: TcType -> TcType -> TcM a
374 occurCheckErr ty containingTy
375 = do { env0 <- tcInitTidyEnv
376 ; ty' <- zonkTcType ty
377 ; containingTy' <- zonkTcType containingTy
378 ; let (env1, tidy_ty1) = tidyOpenType env0 ty'
379 (env2, tidy_ty2) = tidyOpenType env1 containingTy'
380 extra = sep [ppr tidy_ty1, char '=', ppr tidy_ty2]
381 ; failWithTcM (env2, hang msg 2 extra) }
383 msg = ptext (sLit "Occurs check: cannot construct the infinite type:")
386 %************************************************************************
390 %************************************************************************
393 newCoVars :: [(TcType,TcType)] -> TcM [CoVar]
395 = do { us <- newUniqueSupply
396 ; return [ mkCoVar (mkSysTvName uniq (fsLit "co"))
398 | ((ty1,ty2), uniq) <- spec `zip` uniqsFromSupply us] }
400 newMetaCoVar :: TcType -> TcType -> TcM TcTyVar
401 newMetaCoVar ty1 ty2 = newMetaTyVar TauTv (mkCoKind ty1 ty2)
403 newKindVar :: TcM TcKind
404 newKindVar = do { uniq <- newUnique
405 ; ref <- newMutVar Flexi
406 ; return (mkTyVarTy (mkKindVar uniq ref)) }
408 newKindVars :: Int -> TcM [TcKind]
409 newKindVars n = mapM (\ _ -> newKindVar) (nOfThem n ())
413 %************************************************************************
415 SkolemTvs (immutable)
417 %************************************************************************
420 mkSkolTyVar :: Name -> Kind -> SkolemInfo -> TcTyVar
421 mkSkolTyVar name kind info = mkTcTyVar name kind (SkolemTv info)
423 tcSkolSigType :: SkolemInfo -> Type -> TcM ([TcTyVar], TcThetaType, TcType)
424 -- Instantiate a type signature with skolem constants, but
425 -- do *not* give them fresh names, because we want the name to
426 -- be in the type environment -- it is lexically scoped.
427 tcSkolSigType info ty = tcInstType (\tvs -> return (tcSkolSigTyVars info tvs)) ty
429 tcSkolSigTyVars :: SkolemInfo -> [TyVar] -> [TcTyVar]
430 -- Make skolem constants, but do *not* give them new names, as above
431 tcSkolSigTyVars info tyvars = [ mkSkolTyVar (tyVarName tv) (tyVarKind tv) info
434 tcInstSkolTyVar :: SkolemInfo -> (Name -> SrcSpan) -> TyVar -> TcM TcTyVar
435 -- Instantiate the tyvar, using
436 -- * the occ-name and kind of the supplied tyvar,
437 -- * the unique from the monad,
438 -- * the location either from the tyvar (mb_loc = Nothing)
439 -- or from mb_loc (Just loc)
440 tcInstSkolTyVar info get_loc tyvar
441 = do { uniq <- newUnique
442 ; let old_name = tyVarName tyvar
443 kind = tyVarKind tyvar
444 loc = get_loc old_name
445 new_name = mkInternalName uniq (nameOccName old_name) loc
446 ; return (mkSkolTyVar new_name kind info) }
448 tcInstSkolTyVars :: SkolemInfo -> [TyVar] -> TcM [TcTyVar]
449 -- Get the location from the monad
450 tcInstSkolTyVars info tyvars
451 = do { span <- getSrcSpanM
452 ; mapM (tcInstSkolTyVar info (const span)) tyvars }
454 tcInstSkolType :: SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcType)
455 -- Instantiate a type with fresh skolem constants
456 -- Binding location comes from the monad
457 tcInstSkolType info ty = tcInstType (tcInstSkolTyVars info) ty
459 tcInstSigType :: Bool -> SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcRhoType)
460 -- Instantiate with skolems or meta SigTvs; depending on use_skols
461 -- Always take location info from the supplied tyvars
462 tcInstSigType use_skols skol_info ty
463 = tcInstType (mapM inst_tyvar) ty
465 inst_tyvar | use_skols = tcInstSkolTyVar skol_info getSrcSpan
466 | otherwise = instMetaTyVar (SigTv skol_info)
470 %************************************************************************
472 MetaTvs (meta type variables; mutable)
474 %************************************************************************
477 newMetaTyVar :: BoxInfo -> Kind -> TcM TcTyVar
478 -- Make a new meta tyvar out of thin air
479 newMetaTyVar box_info kind
480 = do { uniq <- newUnique
481 ; ref <- newMutVar Flexi
482 ; let name = mkSysTvName uniq fs
483 fs = case box_info of
487 -- We give BoxTv and TauTv the same string, because
488 -- otherwise we get user-visible differences in error
489 -- messages, which are confusing. If you want to see
490 -- the box_info of each tyvar, use -dppr-debug
491 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
493 instMetaTyVar :: BoxInfo -> TyVar -> TcM TcTyVar
494 -- Make a new meta tyvar whose Name and Kind
495 -- come from an existing TyVar
496 instMetaTyVar box_info tyvar
497 = do { uniq <- newUnique
498 ; ref <- newMutVar Flexi
499 ; let name = setNameUnique (tyVarName tyvar) uniq
500 kind = tyVarKind tyvar
501 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
503 readMetaTyVar :: TyVar -> TcM MetaDetails
504 readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
505 readMutVar (metaTvRef tyvar)
507 isFilledMetaTyVar :: TyVar -> TcM Bool
508 -- True of a filled-in (Indirect) meta type variable
510 | not (isTcTyVar tv) = return False
511 | MetaTv _ ref <- tcTyVarDetails tv
512 = do { details <- readMutVar ref
513 ; return (isIndirect details) }
514 | otherwise = return False
516 writeMetaTyVar :: TcTyVar -> TcType -> TcM ()
517 writeMetaTyVar tyvar ty
518 | not debugIsOn = writeMutVar (metaTvRef tyvar) (Indirect ty)
519 writeMetaTyVar tyvar ty
520 | not (isMetaTyVar tyvar)
521 = pprTrace "writeMetaTyVar" (ppr tyvar) $
524 = ASSERT( isMetaTyVar tyvar )
525 -- TOM: It should also work for coercions
526 -- ASSERT2( k2 `isSubKind` k1, (ppr tyvar <+> ppr k1) $$ (ppr ty <+> ppr k2) )
527 do { if debugIsOn then do { details <- readMetaTyVar tyvar;
528 ; WARN( not (isFlexi details), ppr tyvar )
531 -- Temporarily make this a warning, until we fix Trac #2999
533 ; traceTc (text "writeMetaTyVar" <+> ppr tyvar <+> text ":=" <+> ppr ty)
534 ; writeMutVar (metaTvRef tyvar) (Indirect ty) }
536 _k1 = tyVarKind tyvar
541 %************************************************************************
545 %************************************************************************
548 newFlexiTyVar :: Kind -> TcM TcTyVar
549 newFlexiTyVar kind = newMetaTyVar TauTv kind
551 newFlexiTyVarTy :: Kind -> TcM TcType
552 newFlexiTyVarTy kind = do
553 tc_tyvar <- newFlexiTyVar kind
554 return (TyVarTy tc_tyvar)
556 newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
557 newFlexiTyVarTys n kind = mapM newFlexiTyVarTy (nOfThem n kind)
559 tcInstTyVar :: TyVar -> TcM TcTyVar
560 -- Instantiate with a META type variable
561 tcInstTyVar tyvar = instMetaTyVar TauTv tyvar
563 tcInstTyVars :: [TyVar] -> TcM ([TcTyVar], [TcType], TvSubst)
564 -- Instantiate with META type variables
566 = do { tc_tvs <- mapM tcInstTyVar tyvars
567 ; let tys = mkTyVarTys tc_tvs
568 ; return (tc_tvs, tys, zipTopTvSubst tyvars tys) }
569 -- Since the tyvars are freshly made,
570 -- they cannot possibly be captured by
571 -- any existing for-alls. Hence zipTopTvSubst
575 %************************************************************************
579 %************************************************************************
582 zonkSigTyVar :: TcTyVar -> TcM TcTyVar
584 | isSkolemTyVar sig_tv
585 = return sig_tv -- Happens in the call in TcBinds.checkDistinctTyVars
587 = ASSERT( isSigTyVar sig_tv )
588 do { ty <- zonkTcTyVar sig_tv
589 ; return (tcGetTyVar "zonkSigTyVar" ty) }
590 -- 'ty' is bound to be a type variable, because SigTvs
591 -- can only be unified with type variables
595 %************************************************************************
599 %************************************************************************
602 newBoxyTyVar :: Kind -> TcM BoxyTyVar
603 newBoxyTyVar kind = newMetaTyVar BoxTv kind
605 newBoxyTyVars :: [Kind] -> TcM [BoxyTyVar]
606 newBoxyTyVars kinds = mapM newBoxyTyVar kinds
608 newBoxyTyVarTys :: [Kind] -> TcM [BoxyType]
609 newBoxyTyVarTys kinds = do { tvs <- mapM newBoxyTyVar kinds; return (mkTyVarTys tvs) }
611 readFilledBox :: BoxyTyVar -> TcM TcType
612 -- Read the contents of the box, which should be filled in by now
613 readFilledBox box_tv = ASSERT( isBoxyTyVar box_tv )
614 do { cts <- readMetaTyVar box_tv
616 Flexi -> pprPanic "readFilledBox" (ppr box_tv)
617 Indirect ty -> return ty }
619 tcInstBoxyTyVar :: TyVar -> TcM BoxyTyVar
620 -- Instantiate with a BOXY type variable
621 tcInstBoxyTyVar tyvar = instMetaTyVar BoxTv tyvar
625 %************************************************************************
627 \subsection{Putting and getting mutable type variables}
629 %************************************************************************
631 But it's more fun to short out indirections on the way: If this
632 version returns a TyVar, then that TyVar is unbound. If it returns
633 any other type, then there might be bound TyVars embedded inside it.
635 We return Nothing iff the original box was unbound.
638 data LookupTyVarResult -- The result of a lookupTcTyVar call
639 = DoneTv TcTyVarDetails -- SkolemTv or virgin MetaTv
642 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
644 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
646 SkolemTv _ -> return (DoneTv details)
647 MetaTv _ ref -> do { meta_details <- readMutVar ref
648 ; case meta_details of
649 Indirect ty -> return (IndirectTv ty)
650 Flexi -> return (DoneTv details) }
652 details = tcTyVarDetails tyvar
655 -- gaw 2004 We aren't shorting anything out anymore, at least for now
657 | not (isTcTyVar tyvar)
658 = pprTrace "getTcTyVar" (ppr tyvar) $
659 return (Just (mkTyVarTy tyvar))
662 = ASSERT2( isTcTyVar tyvar, ppr tyvar ) do
663 maybe_ty <- readMetaTyVar tyvar
665 Just ty -> do ty' <- short_out ty
666 writeMetaTyVar tyvar (Just ty')
669 Nothing -> return Nothing
671 short_out :: TcType -> TcM TcType
672 short_out ty@(TyVarTy tyvar)
673 | not (isTcTyVar tyvar)
677 maybe_ty <- readMetaTyVar tyvar
679 Just ty' -> do ty' <- short_out ty'
680 writeMetaTyVar tyvar (Just ty')
685 short_out other_ty = return other_ty
690 %************************************************************************
692 \subsection{Zonking -- the exernal interfaces}
694 %************************************************************************
696 ----------------- Type variables
699 zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
700 zonkTcTyVars tyvars = mapM zonkTcTyVar tyvars
702 zonkTcTyVarsAndFV :: [TcTyVar] -> TcM TcTyVarSet
703 zonkTcTyVarsAndFV tyvars = tyVarsOfTypes <$> mapM zonkTcTyVar tyvars
705 zonkTcTyVar :: TcTyVar -> TcM TcType
706 zonkTcTyVar tyvar = ASSERT2( isTcTyVar tyvar, ppr tyvar)
707 zonk_tc_tyvar (\ tv -> return (TyVarTy tv)) tyvar
710 ----------------- Types
713 zonkTcType :: TcType -> TcM TcType
714 zonkTcType ty = zonkType (\ tv -> return (TyVarTy tv)) ty
716 zonkTcTypes :: [TcType] -> TcM [TcType]
717 zonkTcTypes tys = mapM zonkTcType tys
719 zonkTcThetaType :: TcThetaType -> TcM TcThetaType
720 zonkTcThetaType theta = mapM zonkTcPredType theta
722 zonkTcPredType :: TcPredType -> TcM TcPredType
723 zonkTcPredType (ClassP c ts) = ClassP c <$> zonkTcTypes ts
724 zonkTcPredType (IParam n t) = IParam n <$> zonkTcType t
725 zonkTcPredType (EqPred t1 t2) = EqPred <$> zonkTcType t1 <*> zonkTcType t2
728 ------------------- These ...ToType, ...ToKind versions
729 are used at the end of type checking
732 zonkTopTyVar :: TcTyVar -> TcM TcTyVar
733 -- zonkTopTyVar is used, at the top level, on any un-instantiated meta type variables
734 -- to default the kind of ? and ?? etc to *. This is important to ensure that
735 -- instance declarations match. For example consider
736 -- instance Show (a->b)
737 -- foo x = show (\_ -> True)
738 -- Then we'll get a constraint (Show (p ->q)) where p has argTypeKind (printed ??),
739 -- and that won't match the typeKind (*) in the instance decl.
741 -- Because we are at top level, no further constraints are going to affect these
742 -- type variables, so it's time to do it by hand. However we aren't ready
743 -- to default them fully to () or whatever, because the type-class defaulting
744 -- rules have yet to run.
747 | k `eqKind` default_k = return tv
749 = do { tv' <- newFlexiTyVar default_k
750 ; writeMetaTyVar tv (mkTyVarTy tv')
754 default_k = defaultKind k
756 zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TcTyVar]
757 zonkQuantifiedTyVars = mapM zonkQuantifiedTyVar
759 zonkQuantifiedTyVar :: TcTyVar -> TcM TcTyVar
760 -- zonkQuantifiedTyVar is applied to the a TcTyVar when quantifying over it.
762 -- The quantified type variables often include meta type variables
763 -- we want to freeze them into ordinary type variables, and
764 -- default their kind (e.g. from OpenTypeKind to TypeKind)
765 -- -- see notes with Kind.defaultKind
766 -- The meta tyvar is updated to point to the new skolem TyVar. Now any
767 -- bound occurences of the original type variable will get zonked to
768 -- the immutable version.
770 -- We leave skolem TyVars alone; they are immutable.
771 zonkQuantifiedTyVar tv
772 | ASSERT2( isTcTyVar tv, ppr tv )
774 = do { kind <- zonkTcType (tyVarKind tv)
775 ; return $ setTyVarKind tv kind
777 -- It might be a skolem type variable,
778 -- for example from a user type signature
780 | otherwise -- It's a meta-type-variable
781 = do { details <- readMetaTyVar tv
783 -- Create the new, frozen, skolem type variable
784 -- We zonk to a skolem, not to a regular TcVar
785 -- See Note [Zonking to Skolem]
786 ; let final_kind = defaultKind (tyVarKind tv)
787 final_tv = mkSkolTyVar (tyVarName tv) final_kind UnkSkol
789 -- Bind the meta tyvar to the new tyvar
791 Indirect ty -> WARN( True, ppr tv $$ ppr ty )
793 -- [Sept 04] I don't think this should happen
794 -- See note [Silly Type Synonym]
796 Flexi -> writeMetaTyVar tv (mkTyVarTy final_tv)
798 -- Return the new tyvar
802 Note [Silly Type Synonyms]
803 ~~~~~~~~~~~~~~~~~~~~~~~~~~
805 type C u a = u -- Note 'a' unused
807 foo :: (forall a. C u a -> C u a) -> u
811 bar = foo (\t -> t + t)
813 * From the (\t -> t+t) we get type {Num d} => d -> d
816 * Now unify with type of foo's arg, and we get:
817 {Num (C d a)} => C d a -> C d a
820 * Now abstract over the 'a', but float out the Num (C d a) constraint
821 because it does not 'really' mention a. (see exactTyVarsOfType)
822 The arg to foo becomes
825 * So we get a dict binding for Num (C d a), which is zonked to give
827 [Note Sept 04: now that we are zonking quantified type variables
828 on construction, the 'a' will be frozen as a regular tyvar on
829 quantification, so the floated dict will still have type (C d a).
830 Which renders this whole note moot; happily!]
832 * Then the \/\a abstraction has a zonked 'a' in it.
834 All very silly. I think its harmless to ignore the problem. We'll end up with
835 a \/\a in the final result but all the occurrences of a will be zonked to ()
837 Note [Zonking to Skolem]
838 ~~~~~~~~~~~~~~~~~~~~~~~~
839 We used to zonk quantified type variables to regular TyVars. However, this
840 leads to problems. Consider this program from the regression test suite:
842 eval :: Int -> String -> String -> String
843 eval 0 root actual = evalRHS 0 root actual
846 evalRHS 0 root actual = eval 0 root actual
848 It leads to the deferral of an equality
850 (String -> String -> String) ~ a
852 which is propagated up to the toplevel (see TcSimplify.tcSimplifyInferCheck).
853 In the meantime `a' is zonked and quantified to form `evalRHS's signature.
854 This has the *side effect* of also zonking the `a' in the deferred equality
855 (which at this point is being handed around wrapped in an implication
858 Finally, the equality (with the zonked `a') will be handed back to the
859 simplifier by TcRnDriver.tcRnSrcDecls calling TcSimplify.tcSimplifyTop.
860 If we zonk `a' with a regular type variable, we will have this regular type
861 variable now floating around in the simplifier, which in many places assumes to
862 only see proper TcTyVars.
864 We can avoid this problem by zonking with a skolem. The skolem is rigid
865 (which we requirefor a quantified variable), but is still a TcTyVar that the
866 simplifier knows how to deal with.
869 %************************************************************************
871 \subsection{Zonking -- the main work-horses: zonkType, zonkTyVar}
873 %* For internal use only! *
875 %************************************************************************
878 -- For unbound, mutable tyvars, zonkType uses the function given to it
879 -- For tyvars bound at a for-all, zonkType zonks them to an immutable
880 -- type variable and zonks the kind too
882 zonkType :: (TcTyVar -> TcM Type) -- What to do with unbound mutable type variables
883 -- see zonkTcType, and zonkTcTypeToType
886 zonkType unbound_var_fn ty
889 go (TyConApp tc tys) = do tys' <- mapM go tys
890 return (TyConApp tc tys')
892 go (PredTy p) = do p' <- go_pred p
895 go (FunTy arg res) = do arg' <- go arg
897 return (FunTy arg' res')
899 go (AppTy fun arg) = do fun' <- go fun
901 return (mkAppTy fun' arg')
902 -- NB the mkAppTy; we might have instantiated a
903 -- type variable to a type constructor, so we need
904 -- to pull the TyConApp to the top.
906 -- The two interesting cases!
907 go (TyVarTy tyvar) | isTcTyVar tyvar = zonk_tc_tyvar unbound_var_fn tyvar
908 | otherwise = liftM TyVarTy $
909 zonkTyVar unbound_var_fn tyvar
910 -- Ordinary (non Tc) tyvars occur inside quantified types
912 go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar ) do
914 tyvar' <- zonkTyVar unbound_var_fn tyvar
915 return (ForAllTy tyvar' ty')
917 go_pred (ClassP c tys) = do tys' <- mapM go tys
918 return (ClassP c tys')
919 go_pred (IParam n ty) = do ty' <- go ty
920 return (IParam n ty')
921 go_pred (EqPred ty1 ty2) = do ty1' <- go ty1
923 return (EqPred ty1' ty2')
925 zonk_tc_tyvar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable var
926 -> TcTyVar -> TcM TcType
927 zonk_tc_tyvar unbound_var_fn tyvar
928 | not (isMetaTyVar tyvar) -- Skolems
929 = return (TyVarTy tyvar)
931 | otherwise -- Mutables
932 = do { cts <- readMetaTyVar tyvar
934 Flexi -> unbound_var_fn tyvar -- Unbound meta type variable
935 Indirect ty -> zonkType unbound_var_fn ty }
937 -- Zonk the kind of a non-TC tyvar in case it is a coercion variable (their
938 -- kind contains types).
940 zonkTyVar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable var
941 -> TyVar -> TcM TyVar
942 zonkTyVar unbound_var_fn tv
944 = do { kind <- zonkType unbound_var_fn (tyVarKind tv)
945 ; return $ setTyVarKind tv kind
947 | otherwise = return tv
952 %************************************************************************
956 %************************************************************************
959 readKindVar :: KindVar -> TcM (MetaDetails)
960 writeKindVar :: KindVar -> TcKind -> TcM ()
961 readKindVar kv = readMutVar (kindVarRef kv)
962 writeKindVar kv val = writeMutVar (kindVarRef kv) (Indirect val)
965 zonkTcKind :: TcKind -> TcM TcKind
966 zonkTcKind k = zonkTcType k
969 zonkTcKindToKind :: TcKind -> TcM Kind
970 -- When zonking a TcKind to a kind, we need to instantiate kind variables,
971 -- Haskell specifies that * is to be used, so we follow that.
972 zonkTcKindToKind k = zonkType (\ _ -> return liftedTypeKind) k
975 %************************************************************************
977 \subsection{Checking a user type}
979 %************************************************************************
981 When dealing with a user-written type, we first translate it from an HsType
982 to a Type, performing kind checking, and then check various things that should
983 be true about it. We don't want to perform these checks at the same time
984 as the initial translation because (a) they are unnecessary for interface-file
985 types and (b) when checking a mutually recursive group of type and class decls,
986 we can't "look" at the tycons/classes yet. Also, the checks are are rather
987 diverse, and used to really mess up the other code.
989 One thing we check for is 'rank'.
991 Rank 0: monotypes (no foralls)
992 Rank 1: foralls at the front only, Rank 0 inside
993 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
995 basic ::= tyvar | T basic ... basic
997 r2 ::= forall tvs. cxt => r2a
998 r2a ::= r1 -> r2a | basic
999 r1 ::= forall tvs. cxt => r0
1000 r0 ::= r0 -> r0 | basic
1002 Another thing is to check that type synonyms are saturated.
1003 This might not necessarily show up in kind checking.
1005 data T k = MkT (k Int)
1010 checkValidType :: UserTypeCtxt -> Type -> TcM ()
1011 -- Checks that the type is valid for the given context
1012 checkValidType ctxt ty = do
1013 traceTc (text "checkValidType" <+> ppr ty)
1014 unboxed <- doptM Opt_UnboxedTuples
1015 rank2 <- doptM Opt_Rank2Types
1016 rankn <- doptM Opt_RankNTypes
1017 polycomp <- doptM Opt_PolymorphicComponents
1019 gen_rank n | rankn = ArbitraryRank
1021 | otherwise = Rank n
1024 DefaultDeclCtxt-> MustBeMonoType
1025 ResSigCtxt -> MustBeMonoType
1026 LamPatSigCtxt -> gen_rank 0
1027 BindPatSigCtxt -> gen_rank 0
1028 TySynCtxt _ -> gen_rank 0
1029 GenPatCtxt -> gen_rank 1
1030 -- This one is a bit of a hack
1031 -- See the forall-wrapping in TcClassDcl.mkGenericInstance
1033 ExprSigCtxt -> gen_rank 1
1034 FunSigCtxt _ -> gen_rank 1
1035 ConArgCtxt _ | polycomp -> gen_rank 2
1036 -- We are given the type of the entire
1037 -- constructor, hence rank 1
1038 | otherwise -> gen_rank 1
1040 ForSigCtxt _ -> gen_rank 1
1041 SpecInstCtxt -> gen_rank 1
1043 actual_kind = typeKind ty
1045 kind_ok = case ctxt of
1046 TySynCtxt _ -> True -- Any kind will do
1047 ResSigCtxt -> isSubOpenTypeKind actual_kind
1048 ExprSigCtxt -> isSubOpenTypeKind actual_kind
1049 GenPatCtxt -> isLiftedTypeKind actual_kind
1050 ForSigCtxt _ -> isLiftedTypeKind actual_kind
1051 _ -> isSubArgTypeKind actual_kind
1053 ubx_tup = case ctxt of
1054 TySynCtxt _ | unboxed -> UT_Ok
1055 ExprSigCtxt | unboxed -> UT_Ok
1058 -- Check that the thing has kind Type, and is lifted if necessary
1059 checkTc kind_ok (kindErr actual_kind)
1061 -- Check the internal validity of the type itself
1062 check_type rank ubx_tup ty
1064 traceTc (text "checkValidType done" <+> ppr ty)
1066 checkValidMonoType :: Type -> TcM ()
1067 checkValidMonoType ty = check_mono_type MustBeMonoType ty
1072 data Rank = ArbitraryRank -- Any rank ok
1073 | MustBeMonoType -- Monotype regardless of flags
1074 | TyConArgMonoType -- Monotype but could be poly if -XImpredicativeTypes
1075 | SynArgMonoType -- Monotype but could be poly if -XLiberalTypeSynonyms
1076 | Rank Int -- Rank n, but could be more with -XRankNTypes
1078 decRank :: Rank -> Rank -- Function arguments
1079 decRank (Rank 0) = Rank 0
1080 decRank (Rank n) = Rank (n-1)
1081 decRank other_rank = other_rank
1083 nonZeroRank :: Rank -> Bool
1084 nonZeroRank ArbitraryRank = True
1085 nonZeroRank (Rank n) = n>0
1086 nonZeroRank _ = False
1088 ----------------------------------------
1089 data UbxTupFlag = UT_Ok | UT_NotOk
1090 -- The "Ok" version means "ok if UnboxedTuples is on"
1092 ----------------------------------------
1093 check_mono_type :: Rank -> Type -> TcM () -- No foralls anywhere
1094 -- No unlifted types of any kind
1095 check_mono_type rank ty
1096 = do { check_type rank UT_NotOk ty
1097 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1099 check_type :: Rank -> UbxTupFlag -> Type -> TcM ()
1100 -- The args say what the *type context* requires, independent
1101 -- of *flag* settings. You test the flag settings at usage sites.
1103 -- Rank is allowed rank for function args
1104 -- Rank 0 means no for-alls anywhere
1106 check_type rank ubx_tup ty
1107 | not (null tvs && null theta)
1108 = do { checkTc (nonZeroRank rank) (forAllTyErr rank ty)
1109 -- Reject e.g. (Maybe (?x::Int => Int)),
1110 -- with a decent error message
1111 ; check_valid_theta SigmaCtxt theta
1112 ; check_type rank ubx_tup tau -- Allow foralls to right of arrow
1113 ; checkFreeness tvs theta
1114 ; checkAmbiguity tvs theta (tyVarsOfType tau) }
1116 (tvs, theta, tau) = tcSplitSigmaTy ty
1118 -- Naked PredTys don't usually show up, but they can as a result of
1119 -- {-# SPECIALISE instance Ord Char #-}
1120 -- The Right Thing would be to fix the way that SPECIALISE instance pragmas
1121 -- are handled, but the quick thing is just to permit PredTys here.
1122 check_type _ _ (PredTy sty)
1123 = do { dflags <- getDOpts
1124 ; check_pred_ty dflags TypeCtxt sty }
1126 check_type _ _ (TyVarTy _) = return ()
1127 check_type rank _ (FunTy arg_ty res_ty)
1128 = do { check_type (decRank rank) UT_NotOk arg_ty
1129 ; check_type rank UT_Ok res_ty }
1131 check_type rank _ (AppTy ty1 ty2)
1132 = do { check_arg_type rank ty1
1133 ; check_arg_type rank ty2 }
1135 check_type rank ubx_tup ty@(TyConApp tc tys)
1137 = do { -- Check that the synonym has enough args
1138 -- This applies equally to open and closed synonyms
1139 -- It's OK to have an *over-applied* type synonym
1140 -- data Tree a b = ...
1141 -- type Foo a = Tree [a]
1142 -- f :: Foo a b -> ...
1143 checkTc (tyConArity tc <= length tys) arity_msg
1145 -- See Note [Liberal type synonyms]
1146 ; liberal <- doptM Opt_LiberalTypeSynonyms
1147 ; if not liberal || isOpenSynTyCon tc then
1148 -- For H98 and synonym families, do check the type args
1149 mapM_ (check_mono_type SynArgMonoType) tys
1151 else -- In the liberal case (only for closed syns), expand then check
1153 Just ty' -> check_type rank ubx_tup ty'
1154 Nothing -> pprPanic "check_tau_type" (ppr ty)
1157 | isUnboxedTupleTyCon tc
1158 = do { ub_tuples_allowed <- doptM Opt_UnboxedTuples
1159 ; checkTc (ubx_tup_ok ub_tuples_allowed) ubx_tup_msg
1161 ; impred <- doptM Opt_ImpredicativeTypes
1162 ; let rank' = if impred then ArbitraryRank else TyConArgMonoType
1163 -- c.f. check_arg_type
1164 -- However, args are allowed to be unlifted, or
1165 -- more unboxed tuples, so can't use check_arg_ty
1166 ; mapM_ (check_type rank' UT_Ok) tys }
1169 = mapM_ (check_arg_type rank) tys
1172 ubx_tup_ok ub_tuples_allowed = case ubx_tup of
1173 UT_Ok -> ub_tuples_allowed
1177 tc_arity = tyConArity tc
1179 arity_msg = arityErr "Type synonym" (tyConName tc) tc_arity n_args
1180 ubx_tup_msg = ubxArgTyErr ty
1182 check_type _ _ ty = pprPanic "check_type" (ppr ty)
1184 ----------------------------------------
1185 check_arg_type :: Rank -> Type -> TcM ()
1186 -- The sort of type that can instantiate a type variable,
1187 -- or be the argument of a type constructor.
1188 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
1189 -- Other unboxed types are very occasionally allowed as type
1190 -- arguments depending on the kind of the type constructor
1192 -- For example, we want to reject things like:
1194 -- instance Ord a => Ord (forall s. T s a)
1196 -- g :: T s (forall b.b)
1198 -- NB: unboxed tuples can have polymorphic or unboxed args.
1199 -- This happens in the workers for functions returning
1200 -- product types with polymorphic components.
1201 -- But not in user code.
1202 -- Anyway, they are dealt with by a special case in check_tau_type
1204 check_arg_type rank ty
1205 = do { impred <- doptM Opt_ImpredicativeTypes
1206 ; let rank' = if impred then ArbitraryRank -- Arg of tycon can have arby rank, regardless
1207 else case rank of -- Predictive => must be monotype
1208 MustBeMonoType -> MustBeMonoType
1209 _ -> TyConArgMonoType
1210 -- Make sure that MustBeMonoType is propagated,
1211 -- so that we don't suggest -XImpredicativeTypes in
1212 -- (Ord (forall a.a)) => a -> a
1214 ; check_type rank' UT_NotOk ty
1215 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1217 ----------------------------------------
1218 forAllTyErr :: Rank -> Type -> SDoc
1220 = vcat [ hang (ptext (sLit "Illegal polymorphic or qualified type:")) 2 (ppr ty)
1223 suggestion = case rank of
1224 Rank _ -> ptext (sLit "Perhaps you intended to use -XRankNTypes or -XRank2Types")
1225 TyConArgMonoType -> ptext (sLit "Perhaps you intended to use -XImpredicativeTypes")
1226 SynArgMonoType -> ptext (sLit "Perhaps you intended to use -XLiberalTypeSynonyms")
1227 _ -> empty -- Polytype is always illegal
1229 unliftedArgErr, ubxArgTyErr :: Type -> SDoc
1230 unliftedArgErr ty = sep [ptext (sLit "Illegal unlifted type:"), ppr ty]
1231 ubxArgTyErr ty = sep [ptext (sLit "Illegal unboxed tuple type as function argument:"), ppr ty]
1233 kindErr :: Kind -> SDoc
1234 kindErr kind = sep [ptext (sLit "Expecting an ordinary type, but found a type of kind"), ppr kind]
1237 Note [Liberal type synonyms]
1238 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1239 If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
1240 doing validity checking. This allows us to instantiate a synonym defn
1241 with a for-all type, or with a partially-applied type synonym.
1245 Here, T is partially applied, so it's illegal in H98. But if you
1246 expand S first, then T we get just
1250 IMPORTANT: suppose T is a type synonym. Then we must do validity
1251 checking on an appliation (T ty1 ty2)
1253 *either* before expansion (i.e. check ty1, ty2)
1254 *or* after expansion (i.e. expand T ty1 ty2, and then check)
1257 If we do both, we get exponential behaviour!!
1259 data TIACons1 i r c = c i ::: r c
1260 type TIACons2 t x = TIACons1 t (TIACons1 t x)
1261 type TIACons3 t x = TIACons2 t (TIACons1 t x)
1262 type TIACons4 t x = TIACons2 t (TIACons2 t x)
1263 type TIACons7 t x = TIACons4 t (TIACons3 t x)
1266 %************************************************************************
1268 \subsection{Checking a theta or source type}
1270 %************************************************************************
1273 -- Enumerate the contexts in which a "source type", <S>, can occur
1277 -- or (N a) where N is a newtype
1280 = ClassSCCtxt Name -- Superclasses of clas
1281 -- class <S> => C a where ...
1282 | SigmaCtxt -- Theta part of a normal for-all type
1283 -- f :: <S> => a -> a
1284 | DataTyCtxt Name -- Theta part of a data decl
1285 -- data <S> => T a = MkT a
1286 | TypeCtxt -- Source type in an ordinary type
1288 | InstThetaCtxt -- Context of an instance decl
1289 -- instance <S> => C [a] where ...
1291 pprSourceTyCtxt :: SourceTyCtxt -> SDoc
1292 pprSourceTyCtxt (ClassSCCtxt c) = ptext (sLit "the super-classes of class") <+> quotes (ppr c)
1293 pprSourceTyCtxt SigmaCtxt = ptext (sLit "the context of a polymorphic type")
1294 pprSourceTyCtxt (DataTyCtxt tc) = ptext (sLit "the context of the data type declaration for") <+> quotes (ppr tc)
1295 pprSourceTyCtxt InstThetaCtxt = ptext (sLit "the context of an instance declaration")
1296 pprSourceTyCtxt TypeCtxt = ptext (sLit "the context of a type")
1300 checkValidTheta :: SourceTyCtxt -> ThetaType -> TcM ()
1301 checkValidTheta ctxt theta
1302 = addErrCtxt (checkThetaCtxt ctxt theta) (check_valid_theta ctxt theta)
1304 -------------------------
1305 check_valid_theta :: SourceTyCtxt -> [PredType] -> TcM ()
1306 check_valid_theta _ []
1308 check_valid_theta ctxt theta = do
1310 warnTc (notNull dups) (dupPredWarn dups)
1311 mapM_ (check_pred_ty dflags ctxt) theta
1313 (_,dups) = removeDups tcCmpPred theta
1315 -------------------------
1316 check_pred_ty :: DynFlags -> SourceTyCtxt -> PredType -> TcM ()
1317 check_pred_ty dflags ctxt pred@(ClassP cls tys)
1318 = do { -- Class predicates are valid in all contexts
1319 ; checkTc (arity == n_tys) arity_err
1321 -- Check the form of the argument types
1322 ; mapM_ checkValidMonoType tys
1323 ; checkTc (check_class_pred_tys dflags ctxt tys)
1324 (predTyVarErr pred $$ how_to_allow)
1327 class_name = className cls
1328 arity = classArity cls
1330 arity_err = arityErr "Class" class_name arity n_tys
1331 how_to_allow = parens (ptext (sLit "Use -XFlexibleContexts to permit this"))
1333 check_pred_ty _ (ClassSCCtxt _) (EqPred _ _)
1334 = -- We do not yet support superclass equalities.
1336 sep [ ptext (sLit "The current implementation of type families does not")
1337 , ptext (sLit "support equality constraints in superclass contexts.")
1338 , ptext (sLit "They are planned for a future release.")
1341 check_pred_ty dflags _ pred@(EqPred ty1 ty2)
1342 = do { -- Equational constraints are valid in all contexts if type
1343 -- families are permitted
1344 ; checkTc (dopt Opt_TypeFamilies dflags) (eqPredTyErr pred)
1346 -- Check the form of the argument types
1347 ; checkValidMonoType ty1
1348 ; checkValidMonoType ty2
1351 check_pred_ty _ SigmaCtxt (IParam _ ty) = checkValidMonoType ty
1352 -- Implicit parameters only allowed in type
1353 -- signatures; not in instance decls, superclasses etc
1354 -- The reason for not allowing implicit params in instances is a bit
1356 -- If we allowed instance (?x::Int, Eq a) => Foo [a] where ...
1357 -- then when we saw (e :: (?x::Int) => t) it would be unclear how to
1358 -- discharge all the potential usas of the ?x in e. For example, a
1359 -- constraint Foo [Int] might come out of e,and applying the
1360 -- instance decl would show up two uses of ?x.
1363 check_pred_ty _ _ sty = failWithTc (badPredTyErr sty)
1365 -------------------------
1366 check_class_pred_tys :: DynFlags -> SourceTyCtxt -> [Type] -> Bool
1367 check_class_pred_tys dflags ctxt tys
1369 TypeCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
1370 InstThetaCtxt -> flexible_contexts || undecidable_ok || all tcIsTyVarTy tys
1371 -- Further checks on head and theta in
1372 -- checkInstTermination
1373 _ -> flexible_contexts || all tyvar_head tys
1375 flexible_contexts = dopt Opt_FlexibleContexts dflags
1376 undecidable_ok = dopt Opt_UndecidableInstances dflags
1378 -------------------------
1379 tyvar_head :: Type -> Bool
1380 tyvar_head ty -- Haskell 98 allows predicates of form
1381 | tcIsTyVarTy ty = True -- C (a ty1 .. tyn)
1382 | otherwise -- where a is a type variable
1383 = case tcSplitAppTy_maybe ty of
1384 Just (ty, _) -> tyvar_head ty
1391 is ambiguous if P contains generic variables
1392 (i.e. one of the Vs) that are not mentioned in tau
1394 However, we need to take account of functional dependencies
1395 when we speak of 'mentioned in tau'. Example:
1396 class C a b | a -> b where ...
1398 forall x y. (C x y) => x
1399 is not ambiguous because x is mentioned and x determines y
1401 NB; the ambiguity check is only used for *user* types, not for types
1402 coming from inteface files. The latter can legitimately have
1403 ambiguous types. Example
1405 class S a where s :: a -> (Int,Int)
1406 instance S Char where s _ = (1,1)
1407 f:: S a => [a] -> Int -> (Int,Int)
1408 f (_::[a]) x = (a*x,b)
1409 where (a,b) = s (undefined::a)
1411 Here the worker for f gets the type
1412 fw :: forall a. S a => Int -> (# Int, Int #)
1414 If the list of tv_names is empty, we have a monotype, and then we
1415 don't need to check for ambiguity either, because the test can't fail
1420 checkAmbiguity :: [TyVar] -> ThetaType -> TyVarSet -> TcM ()
1421 checkAmbiguity forall_tyvars theta tau_tyvars
1422 = mapM_ complain (filter is_ambig theta)
1424 complain pred = addErrTc (ambigErr pred)
1425 extended_tau_vars = growThetaTyVars theta tau_tyvars
1427 -- See Note [Implicit parameters and ambiguity] in TcSimplify
1428 is_ambig pred = isClassPred pred &&
1429 any ambig_var (varSetElems (tyVarsOfPred pred))
1431 ambig_var ct_var = (ct_var `elem` forall_tyvars) &&
1432 not (ct_var `elemVarSet` extended_tau_vars)
1434 ambigErr :: PredType -> SDoc
1436 = sep [ptext (sLit "Ambiguous constraint") <+> quotes (pprPred pred),
1437 nest 4 (ptext (sLit "At least one of the forall'd type variables mentioned by the constraint") $$
1438 ptext (sLit "must be reachable from the type after the '=>'"))]
1440 --------------------------
1441 -- For this 'grow' stuff see Note [Growing the tau-tvs using constraints] in Inst
1443 growThetaTyVars :: TcThetaType -> TyVarSet -> TyVarSet
1445 growThetaTyVars theta tvs
1447 | otherwise = fixVarSet mk_next tvs
1449 mk_next tvs = foldr growPredTyVars tvs theta
1452 growPredTyVars :: TcPredType -> TyVarSet -> TyVarSet
1453 -- Here is where the special case for inplicit parameters happens
1454 growPredTyVars (IParam _ ty) tvs = tvs `unionVarSet` tyVarsOfType ty
1455 growPredTyVars pred tvs = growTyVars (tyVarsOfPred pred) tvs
1457 growTyVars :: TyVarSet -> TyVarSet -> TyVarSet
1458 growTyVars new_tvs tvs
1459 | new_tvs `intersectsVarSet` tvs = tvs `unionVarSet` new_tvs
1463 In addition, GHC insists that at least one type variable
1464 in each constraint is in V. So we disallow a type like
1465 forall a. Eq b => b -> b
1466 even in a scope where b is in scope.
1469 checkFreeness :: [Var] -> [PredType] -> TcM ()
1470 checkFreeness forall_tyvars theta
1471 = do { flexible_contexts <- doptM Opt_FlexibleContexts
1472 ; unless flexible_contexts $ mapM_ complain (filter is_free theta) }
1474 is_free pred = not (isIPPred pred)
1475 && not (any bound_var (varSetElems (tyVarsOfPred pred)))
1476 bound_var ct_var = ct_var `elem` forall_tyvars
1477 complain pred = addErrTc (freeErr pred)
1479 freeErr :: PredType -> SDoc
1481 = sep [ ptext (sLit "All of the type variables in the constraint") <+>
1482 quotes (pprPred pred)
1483 , ptext (sLit "are already in scope") <+>
1484 ptext (sLit "(at least one must be universally quantified here)")
1486 ptext (sLit "(Use -XFlexibleContexts to lift this restriction)")
1491 checkThetaCtxt :: SourceTyCtxt -> ThetaType -> SDoc
1492 checkThetaCtxt ctxt theta
1493 = vcat [ptext (sLit "In the context:") <+> pprTheta theta,
1494 ptext (sLit "While checking") <+> pprSourceTyCtxt ctxt ]
1496 badPredTyErr, eqPredTyErr, predTyVarErr :: PredType -> SDoc
1497 badPredTyErr sty = ptext (sLit "Illegal constraint") <+> pprPred sty
1498 eqPredTyErr sty = ptext (sLit "Illegal equational constraint") <+> pprPred sty
1500 parens (ptext (sLit "Use -XTypeFamilies to permit this"))
1501 predTyVarErr pred = sep [ptext (sLit "Non type-variable argument"),
1502 nest 2 (ptext (sLit "in the constraint:") <+> pprPred pred)]
1503 dupPredWarn :: [[PredType]] -> SDoc
1504 dupPredWarn dups = ptext (sLit "Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
1506 arityErr :: Outputable a => String -> a -> Int -> Int -> SDoc
1507 arityErr kind name n m
1508 = hsep [ text kind, quotes (ppr name), ptext (sLit "should have"),
1509 n_arguments <> comma, text "but has been given", int m]
1511 n_arguments | n == 0 = ptext (sLit "no arguments")
1512 | n == 1 = ptext (sLit "1 argument")
1513 | True = hsep [int n, ptext (sLit "arguments")]
1516 notMonoType :: TcType -> TcM a
1518 = do { ty' <- zonkTcType ty
1519 ; env0 <- tcInitTidyEnv
1520 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1521 msg = ptext (sLit "Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1522 ; failWithTcM (env1, msg) }
1524 notMonoArgs :: TcType -> TcM a
1526 = do { ty' <- zonkTcType ty
1527 ; env0 <- tcInitTidyEnv
1528 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1529 msg = ptext (sLit "Arguments of type synonym families must be monotypes") <+> quotes (ppr tidy_ty)
1530 ; failWithTcM (env1, msg) }
1534 %************************************************************************
1536 \subsection{Checking for a decent instance head type}
1538 %************************************************************************
1540 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1541 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1543 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1544 flag is on, or (2)~the instance is imported (they must have been
1545 compiled elsewhere). In these cases, we let them go through anyway.
1547 We can also have instances for functions: @instance Foo (a -> b) ...@.
1550 checkValidInstHead :: Type -> TcM (Class, [TcType])
1552 checkValidInstHead ty -- Should be a source type
1553 = case tcSplitPredTy_maybe ty of {
1554 Nothing -> failWithTc (instTypeErr (ppr ty) empty) ;
1557 case getClassPredTys_maybe pred of {
1558 Nothing -> failWithTc (instTypeErr (pprPred pred) empty) ;
1559 Just (clas,tys) -> do
1562 check_inst_head dflags clas tys
1566 check_inst_head :: DynFlags -> Class -> [Type] -> TcM ()
1567 check_inst_head dflags clas tys
1568 = do { -- If GlasgowExts then check at least one isn't a type variable
1569 ; checkTc (dopt Opt_TypeSynonymInstances dflags ||
1570 all tcInstHeadTyNotSynonym tys)
1571 (instTypeErr (pprClassPred clas tys) head_type_synonym_msg)
1572 ; checkTc (dopt Opt_FlexibleInstances dflags ||
1573 all tcInstHeadTyAppAllTyVars tys)
1574 (instTypeErr (pprClassPred clas tys) head_type_args_tyvars_msg)
1575 ; checkTc (dopt Opt_MultiParamTypeClasses dflags ||
1577 (instTypeErr (pprClassPred clas tys) head_one_type_msg)
1578 -- May not contain type family applications
1579 ; mapM_ checkTyFamFreeness tys
1581 ; mapM_ checkValidMonoType tys
1582 -- For now, I only allow tau-types (not polytypes) in
1583 -- the head of an instance decl.
1584 -- E.g. instance C (forall a. a->a) is rejected
1585 -- One could imagine generalising that, but I'm not sure
1586 -- what all the consequences might be
1590 head_type_synonym_msg = parens (
1591 text "All instance types must be of the form (T t1 ... tn)" $$
1592 text "where T is not a synonym." $$
1593 text "Use -XTypeSynonymInstances if you want to disable this.")
1595 head_type_args_tyvars_msg = parens (vcat [
1596 text "All instance types must be of the form (T a1 ... an)",
1597 text "where a1 ... an are type *variables*,",
1598 text "and each type variable appears at most once in the instance head.",
1599 text "Use -XFlexibleInstances if you want to disable this."])
1601 head_one_type_msg = parens (
1602 text "Only one type can be given in an instance head." $$
1603 text "Use -XMultiParamTypeClasses if you want to allow more.")
1605 instTypeErr :: SDoc -> SDoc -> SDoc
1606 instTypeErr pp_ty msg
1607 = sep [ptext (sLit "Illegal instance declaration for") <+> quotes pp_ty,
1612 %************************************************************************
1614 \subsection{Checking instance for termination}
1616 %************************************************************************
1620 checkValidInstance :: [TyVar] -> ThetaType -> Class -> [TcType] -> TcM ()
1621 checkValidInstance tyvars theta clas inst_tys
1622 = do { undecidable_ok <- doptM Opt_UndecidableInstances
1624 ; checkValidTheta InstThetaCtxt theta
1625 ; checkAmbiguity tyvars theta (tyVarsOfTypes inst_tys)
1627 -- Check that instance inference will terminate (if we care)
1628 -- For Haskell 98 this will already have been done by checkValidTheta,
1629 -- but as we may be using other extensions we need to check.
1630 ; unless undecidable_ok $
1631 mapM_ addErrTc (checkInstTermination inst_tys theta)
1633 -- The Coverage Condition
1634 ; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
1635 (instTypeErr (pprClassPred clas inst_tys) msg)
1638 msg = parens (vcat [ptext (sLit "the Coverage Condition fails for one of the functional dependencies;"),
1642 Termination test: the so-called "Paterson conditions" (see Section 5 of
1643 "Understanding functionsl dependencies via Constraint Handling Rules,
1646 We check that each assertion in the context satisfies:
1647 (1) no variable has more occurrences in the assertion than in the head, and
1648 (2) the assertion has fewer constructors and variables (taken together
1649 and counting repetitions) than the head.
1650 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1651 (which have already been checked) guarantee termination.
1653 The underlying idea is that
1655 for any ground substitution, each assertion in the
1656 context has fewer type constructors than the head.
1660 checkInstTermination :: [TcType] -> ThetaType -> [Message]
1661 checkInstTermination tys theta
1662 = mapCatMaybes check theta
1665 size = sizeTypes tys
1667 | not (null (fvPred pred \\ fvs))
1668 = Just (predUndecErr pred nomoreMsg $$ parens undecidableMsg)
1669 | sizePred pred >= size
1670 = Just (predUndecErr pred smallerMsg $$ parens undecidableMsg)
1674 predUndecErr :: PredType -> SDoc -> SDoc
1675 predUndecErr pred msg = sep [msg,
1676 nest 2 (ptext (sLit "in the constraint:") <+> pprPred pred)]
1678 nomoreMsg, smallerMsg, undecidableMsg :: SDoc
1679 nomoreMsg = ptext (sLit "Variable occurs more often in a constraint than in the instance head")
1680 smallerMsg = ptext (sLit "Constraint is no smaller than the instance head")
1681 undecidableMsg = ptext (sLit "Use -XUndecidableInstances to permit this")
1685 %************************************************************************
1687 Checking the context of a derived instance declaration
1689 %************************************************************************
1691 Note [Exotic derived instance contexts]
1692 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1693 In a 'derived' instance declaration, we *infer* the context. It's a
1694 bit unclear what rules we should apply for this; the Haskell report is
1695 silent. Obviously, constraints like (Eq a) are fine, but what about
1696 data T f a = MkT (f a) deriving( Eq )
1697 where we'd get an Eq (f a) constraint. That's probably fine too.
1699 One could go further: consider
1700 data T a b c = MkT (Foo a b c) deriving( Eq )
1701 instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
1703 Notice that this instance (just) satisfies the Paterson termination
1704 conditions. Then we *could* derive an instance decl like this:
1706 instance (C Int a, Eq b, Eq c) => Eq (T a b c)
1708 even though there is no instance for (C Int a), because there just
1709 *might* be an instance for, say, (C Int Bool) at a site where we
1710 need the equality instance for T's.
1712 However, this seems pretty exotic, and it's quite tricky to allow
1713 this, and yet give sensible error messages in the (much more common)
1714 case where we really want that instance decl for C.
1716 So for now we simply require that the derived instance context
1717 should have only type-variable constraints.
1719 Here is another example:
1720 data Fix f = In (f (Fix f)) deriving( Eq )
1721 Here, if we are prepared to allow -XUndecidableInstances we
1722 could derive the instance
1723 instance Eq (f (Fix f)) => Eq (Fix f)
1724 but this is so delicate that I don't think it should happen inside
1725 'deriving'. If you want this, write it yourself!
1727 NB: if you want to lift this condition, make sure you still meet the
1728 termination conditions! If not, the deriving mechanism generates
1729 larger and larger constraints. Example:
1731 data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
1733 Note the lack of a Show instance for Succ. First we'll generate
1734 instance (Show (Succ a), Show a) => Show (Seq a)
1736 instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
1737 and so on. Instead we want to complain of no instance for (Show (Succ a)).
1741 Allow constraints which consist only of type variables, with no repeats.
1744 validDerivPred :: PredType -> Bool
1745 validDerivPred (ClassP _ tys) = hasNoDups fvs && sizeTypes tys == length fvs
1746 where fvs = fvTypes tys
1747 validDerivPred _ = False
1750 %************************************************************************
1752 Checking type instance well-formedness and termination
1754 %************************************************************************
1757 -- Check that a "type instance" is well-formed (which includes decidability
1758 -- unless -XUndecidableInstances is given).
1760 checkValidTypeInst :: [Type] -> Type -> TcM ()
1761 checkValidTypeInst typats rhs
1762 = do { -- left-hand side contains no type family applications
1763 -- (vanilla synonyms are fine, though)
1764 ; mapM_ checkTyFamFreeness typats
1766 -- the right-hand side is a tau type
1767 ; checkValidMonoType rhs
1769 -- we have a decidable instance unless otherwise permitted
1770 ; undecidable_ok <- doptM Opt_UndecidableInstances
1771 ; unless undecidable_ok $
1772 mapM_ addErrTc (checkFamInst typats (tyFamInsts rhs))
1775 -- Make sure that each type family instance is
1776 -- (1) strictly smaller than the lhs,
1777 -- (2) mentions no type variable more often than the lhs, and
1778 -- (3) does not contain any further type family instances.
1780 checkFamInst :: [Type] -- lhs
1781 -> [(TyCon, [Type])] -- type family instances
1783 checkFamInst lhsTys famInsts
1784 = mapCatMaybes check famInsts
1786 size = sizeTypes lhsTys
1787 fvs = fvTypes lhsTys
1789 | not (all isTyFamFree tys)
1790 = Just (famInstUndecErr famInst nestedMsg $$ parens undecidableMsg)
1791 | not (null (fvTypes tys \\ fvs))
1792 = Just (famInstUndecErr famInst nomoreVarMsg $$ parens undecidableMsg)
1793 | size <= sizeTypes tys
1794 = Just (famInstUndecErr famInst smallerAppMsg $$ parens undecidableMsg)
1798 famInst = TyConApp tc tys
1800 -- Ensure that no type family instances occur in a type.
1802 checkTyFamFreeness :: Type -> TcM ()
1803 checkTyFamFreeness ty
1804 = checkTc (isTyFamFree ty) $
1805 tyFamInstIllegalErr ty
1807 -- Check that a type does not contain any type family applications.
1809 isTyFamFree :: Type -> Bool
1810 isTyFamFree = null . tyFamInsts
1814 tyFamInstIllegalErr :: Type -> SDoc
1815 tyFamInstIllegalErr ty
1816 = hang (ptext (sLit "Illegal type synonym family application in instance") <>
1820 famInstUndecErr :: Type -> SDoc -> SDoc
1821 famInstUndecErr ty msg
1823 nest 2 (ptext (sLit "in the type family application:") <+>
1826 nestedMsg, nomoreVarMsg, smallerAppMsg :: SDoc
1827 nestedMsg = ptext (sLit "Nested type family application")
1828 nomoreVarMsg = ptext (sLit "Variable occurs more often than in instance head")
1829 smallerAppMsg = ptext (sLit "Application is no smaller than the instance head")
1833 %************************************************************************
1835 \subsection{Auxiliary functions}
1837 %************************************************************************
1840 -- Free variables of a type, retaining repetitions, and expanding synonyms
1841 fvType :: Type -> [TyVar]
1842 fvType ty | Just exp_ty <- tcView ty = fvType exp_ty
1843 fvType (TyVarTy tv) = [tv]
1844 fvType (TyConApp _ tys) = fvTypes tys
1845 fvType (PredTy pred) = fvPred pred
1846 fvType (FunTy arg res) = fvType arg ++ fvType res
1847 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1848 fvType (ForAllTy tyvar ty) = filter (/= tyvar) (fvType ty)
1850 fvTypes :: [Type] -> [TyVar]
1851 fvTypes tys = concat (map fvType tys)
1853 fvPred :: PredType -> [TyVar]
1854 fvPred (ClassP _ tys') = fvTypes tys'
1855 fvPred (IParam _ ty) = fvType ty
1856 fvPred (EqPred ty1 ty2) = fvType ty1 ++ fvType ty2
1858 -- Size of a type: the number of variables and constructors
1859 sizeType :: Type -> Int
1860 sizeType ty | Just exp_ty <- tcView ty = sizeType exp_ty
1861 sizeType (TyVarTy _) = 1
1862 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1863 sizeType (PredTy pred) = sizePred pred
1864 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1865 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1866 sizeType (ForAllTy _ ty) = sizeType ty
1868 sizeTypes :: [Type] -> Int
1869 sizeTypes xs = sum (map sizeType xs)
1871 sizePred :: PredType -> Int
1872 sizePred (ClassP _ tys') = sizeTypes tys'
1873 sizePred (IParam _ ty) = sizeType ty
1874 sizePred (EqPred ty1 ty2) = sizeType ty1 + sizeType ty2