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, execTcTyVarBinds,
40 --------------------------------
41 -- Checking type validity
42 Rank, UserTypeCtxt(..), checkValidType, checkValidMonoType,
43 SourceTyCtxt(..), checkValidTheta, checkFreeness,
44 checkValidInstHead, checkValidInstance,
45 checkInstTermination, checkValidTypeInst, checkTyFamFreeness, checkKinds,
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
90 import Data.List ( (\\) )
94 %************************************************************************
96 Instantiation in general
98 %************************************************************************
101 tcInstType :: ([TyVar] -> TcM [TcTyVar]) -- How to instantiate the type variables
102 -> TcType -- Type to instantiate
103 -> TcM ([TcTyVar], TcThetaType, TcType) -- Result
104 -- (type vars (excl coercion vars), preds (incl equalities), rho)
105 tcInstType inst_tyvars ty
106 = case tcSplitForAllTys ty of
107 ([], rho) -> let -- There may be overloading despite no type variables;
108 -- (?x :: Int) => Int -> Int
109 (theta, tau) = tcSplitPhiTy rho
111 return ([], theta, tau)
113 (tyvars, rho) -> do { tyvars' <- inst_tyvars tyvars
115 ; let tenv = zipTopTvSubst tyvars (mkTyVarTys tyvars')
116 -- Either the tyvars are freshly made, by inst_tyvars,
117 -- or (in the call from tcSkolSigType) any nested foralls
118 -- have different binders. Either way, zipTopTvSubst is ok
120 ; let (theta, tau) = tcSplitPhiTy (substTy tenv rho)
121 ; return (tyvars', theta, tau) }
125 %************************************************************************
129 %************************************************************************
131 Can't be in TcUnify, as we also need it in TcTyFuns.
135 -- False <=> the two args are (actual, expected) respectively
136 -- True <=> the two args are (expected, actual) respectively
138 checkUpdateMeta :: SwapFlag
139 -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
140 -- Update tv1, which is flexi; occurs check is alrady done
141 -- The 'check' version does a kind check too
142 -- We do a sub-kind check here: we might unify (a b) with (c d)
143 -- where b::*->* and d::*; this should fail
145 checkUpdateMeta swapped tv1 ref1 ty2
146 = do { checkKinds swapped tv1 ty2
147 ; updateMeta tv1 ref1 ty2 }
149 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
150 updateMeta tv1 ref1 ty2
151 = ASSERT( isMetaTyVar tv1 )
152 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
153 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
154 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
155 ; writeMutVar ref1 (Indirect ty2)
159 checkKinds :: Bool -> TyVar -> Type -> TcM ()
160 checkKinds swapped tv1 ty2
161 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
162 -- ty2 has been zonked at this stage, which ensures that
163 -- its kind has as much boxity information visible as possible.
164 | tk2 `isSubKind` tk1 = return ()
167 -- Either the kinds aren't compatible
168 -- (can happen if we unify (a b) with (c d))
169 -- or we are unifying a lifted type variable with an
170 -- unlifted type: e.g. (id 3#) is illegal
171 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
172 unifyKindMisMatch k1 k2
174 (k1,k2) | swapped = (tk2,tk1)
175 | otherwise = (tk1,tk2)
180 checkTauTvUpdate :: TcTyVar -> TcType -> TcM (Maybe TcType)
181 -- (checkTauTvUpdate tv ty)
182 -- We are about to update the TauTv tv with ty.
183 -- Check (a) that tv doesn't occur in ty (occurs check)
184 -- (b) that ty is a monotype
185 -- Furthermore, in the interest of (b), if you find an
186 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
188 -- We have three possible outcomes:
189 -- (1) Return the (non-boxy) type to update the type variable with,
190 -- [we know the update is ok!]
191 -- (2) return Nothing, or
192 -- [we cannot tell whether the update is ok right now]
194 -- [the update is definitely invalid]
195 -- We return Nothing in case the tv occurs in ty *under* a type family
196 -- application. In this case, we must not update tv (to avoid a cyclic type
197 -- term), but we also cannot fail claiming an infinite type. Given
199 -- type instance F Int = Int
202 -- This is perfectly reasonable, if we later get a ~ Int.
204 checkTauTvUpdate orig_tv orig_ty
205 = do { result <- go orig_ty
207 Right ty -> return $ Just ty
208 Left True -> return $ Nothing
209 Left False -> occurCheckErr (mkTyVarTy orig_tv) orig_ty
212 go :: TcType -> TcM (Either Bool TcType)
214 -- Right ty if everything is fine
215 -- Left True if orig_tv occurs in orig_ty, but under a type family app
216 -- Left False if orig_tv occurs in orig_ty (with no type family app)
217 -- It fails if it encounters a forall type, except as an argument for a
218 -- closed type synonym that expands to a tau type.
220 | isSynTyCon tc = go_syn tc tys
221 | otherwise = do { tys' <- mapM go tys
222 ; return $ occurs (TyConApp tc) tys' }
223 go (PredTy p) = do { p' <- go_pred p
224 ; return $ occurs1 PredTy p' }
225 go (FunTy arg res) = do { arg' <- go arg
227 ; return $ occurs2 FunTy arg' res' }
228 go (AppTy fun arg) = do { fun' <- go fun
230 ; return $ occurs2 mkAppTy fun' arg' }
231 -- NB the mkAppTy; we might have instantiated a
232 -- type variable to a type constructor, so we need
233 -- to pull the TyConApp to the top.
234 go (ForAllTy _ _) = notMonoType orig_ty -- (b)
237 | orig_tv == tv = return $ Left False -- (a)
238 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
239 | otherwise = return $ Right (TyVarTy tv)
240 -- Ordinary (non Tc) tyvars
241 -- occur inside quantified types
243 go_pred (ClassP c tys) = do { tys' <- mapM go tys
244 ; return $ occurs (ClassP c) tys' }
245 go_pred (IParam n ty) = do { ty' <- go ty
246 ; return $ occurs1 (IParam n) ty' }
247 go_pred (EqPred t1 t2) = do { t1' <- go t1
249 ; return $ occurs2 EqPred t1' t2' }
251 go_tyvar tv (SkolemTv _) = return $ Right (TyVarTy tv)
252 go_tyvar tv (MetaTv box ref)
253 = do { cts <- readMutVar ref
257 BoxTv -> do { ty <- fillBoxWithTau tv ref
258 ; return $ Right ty }
259 _ -> return $ Right (TyVarTy tv)
262 -- go_syn is called for synonyms only
263 -- See Note [Type synonyms and the occur check]
265 | not (isTauTyCon tc)
266 = notMonoType orig_ty -- (b) again
268 = do { (_msgs, mb_tys') <- tryTc (mapM go tys)
271 -- we had a type error => forall in type parameters
273 | isOpenTyCon tc -> notMonoArgs (TyConApp tc tys)
274 -- Synonym families must have monotype args
275 | otherwise -> go (expectJust "checkTauTvUpdate(1)"
276 (tcView (TyConApp tc tys)))
277 -- Try again, expanding the synonym
279 -- no type error, but need to test whether occurs check happend
281 case occurs id tys' of
283 | isOpenTyCon tc -> return $ Left True
284 -- Variable occured under type family application
285 | otherwise -> go (expectJust "checkTauTvUpdate(2)"
286 (tcView (TyConApp tc tys)))
287 -- Try again, expanding the synonym
288 Right raw_tys' -> return $ Right (TyConApp tc raw_tys')
289 -- Retain the synonym (the common case)
292 -- Left results (= occurrence of orig_ty) dominate and
293 -- (Left False) (= fatal occurrence) dominates over (Left True)
294 occurs :: ([a] -> b) -> [Either Bool a] -> Either Bool b
295 occurs c = either Left (Right . c) . foldr combine (Right [])
297 combine (Left famInst1) (Left famInst2) = Left (famInst1 && famInst2)
298 combine (Right _ ) (Left famInst) = Left famInst
299 combine (Left famInst) (Right _) = Left famInst
300 combine (Right arg) (Right args) = Right (arg:args)
302 occurs1 c x = occurs (\[x'] -> c x') [x]
303 occurs2 c x y = occurs (\[x', y'] -> c x' y') [x, y]
305 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
306 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
307 -- tau-type meta-variable, whose print-name is the same as tv
308 -- Choosing the same name is good: when we instantiate a function
309 -- we allocate boxy tyvars with the same print-name as the quantified
310 -- tyvar; and then we often fill the box with a tau-tyvar, and again
311 -- we want to choose the same name.
312 fillBoxWithTau tv ref
313 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
314 ; let tau = mkTyVarTy tv' -- name of the type variable
315 ; writeMutVar ref (Indirect tau)
319 Note [Type synonyms and the occur check]
321 Basically we want to update tv1 := ps_ty2
322 because ps_ty2 has type-synonym info, which improves later error messages
327 f :: (A a -> a -> ()) -> ()
333 In the application (p x), we try to match "t" with "A t". If we go
334 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
335 an infinite loop later.
336 But we should not reject the program, because A t = ().
337 Rather, we should bind t to () (= non_var_ty2).
341 Execute a bag of type variable bindings.
344 execTcTyVarBinds :: TcTyVarBinds -> TcM ()
345 execTcTyVarBinds = mapM_ execTcTyVarBind . bagToList
347 execTcTyVarBind (TcTyVarBind tv ty)
348 = do { ASSERTM2( do { details <- readMetaTyVar tv
349 ; return (isFlexi details) }, ppr tv )
350 ; ty' <- if isCoVar tv
352 else do { maybe_ty <- checkTauTvUpdate tv ty
354 Nothing -> pprPanic "TcRnMonad.execTcTyBind"
355 (ppr tv <+> text ":=" <+> ppr ty)
356 Just ty' -> return ty'
358 ; writeMetaTyVar tv ty'
362 Error mesages in case of kind mismatch.
365 unifyKindMisMatch :: TcKind -> TcKind -> TcM ()
366 unifyKindMisMatch ty1 ty2 = do
367 ty1' <- zonkTcKind ty1
368 ty2' <- zonkTcKind ty2
370 msg = hang (ptext (sLit "Couldn't match kind"))
371 2 (sep [quotes (ppr ty1'),
372 ptext (sLit "against"),
376 unifyKindCtxt :: Bool -> TyVar -> Type -> TidyEnv -> TcM (TidyEnv, SDoc)
377 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
378 -- tv1 and ty2 are zonked already
381 msg = (env2, ptext (sLit "When matching the kinds of") <+>
382 sep [quotes pp_expected <+> ptext (sLit "and"), quotes pp_actual])
384 (pp_expected, pp_actual) | swapped = (pp2, pp1)
385 | otherwise = (pp1, pp2)
386 (env1, tv1') = tidyOpenTyVar tidy_env tv1
387 (env2, ty2') = tidyOpenType env1 ty2
388 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
389 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
392 Error message for failure due to an occurs check.
395 occurCheckErr :: TcType -> TcType -> TcM a
396 occurCheckErr ty containingTy
397 = do { env0 <- tcInitTidyEnv
398 ; ty' <- zonkTcType ty
399 ; containingTy' <- zonkTcType containingTy
400 ; let (env1, tidy_ty1) = tidyOpenType env0 ty'
401 (env2, tidy_ty2) = tidyOpenType env1 containingTy'
402 extra = sep [ppr tidy_ty1, char '=', ppr tidy_ty2]
403 ; failWithTcM (env2, hang msg 2 extra) }
405 msg = ptext (sLit "Occurs check: cannot construct the infinite type:")
408 %************************************************************************
412 %************************************************************************
415 newCoVars :: [(TcType,TcType)] -> TcM [CoVar]
417 = do { us <- newUniqueSupply
418 ; return [ mkCoVar (mkSysTvName uniq (fsLit "co_kv"))
420 | ((ty1,ty2), uniq) <- spec `zip` uniqsFromSupply us] }
422 newMetaCoVar :: TcType -> TcType -> TcM TcTyVar
423 newMetaCoVar ty1 ty2 = newMetaTyVar TauTv (mkCoKind ty1 ty2)
425 newKindVar :: TcM TcKind
426 newKindVar = do { uniq <- newUnique
427 ; ref <- newMutVar Flexi
428 ; return (mkTyVarTy (mkKindVar uniq ref)) }
430 newKindVars :: Int -> TcM [TcKind]
431 newKindVars n = mapM (\ _ -> newKindVar) (nOfThem n ())
435 %************************************************************************
437 SkolemTvs (immutable)
439 %************************************************************************
442 mkSkolTyVar :: Name -> Kind -> SkolemInfo -> TcTyVar
443 mkSkolTyVar name kind info = mkTcTyVar name kind (SkolemTv info)
445 tcSkolSigType :: SkolemInfo -> Type -> TcM ([TcTyVar], TcThetaType, TcType)
446 -- Instantiate a type signature with skolem constants, but
447 -- do *not* give them fresh names, because we want the name to
448 -- be in the type environment -- it is lexically scoped.
449 tcSkolSigType info ty = tcInstType (\tvs -> return (tcSkolSigTyVars info tvs)) ty
451 tcSkolSigTyVars :: SkolemInfo -> [TyVar] -> [TcTyVar]
452 -- Make skolem constants, but do *not* give them new names, as above
453 tcSkolSigTyVars info tyvars = [ mkSkolTyVar (tyVarName tv) (tyVarKind tv) info
456 tcInstSkolTyVar :: SkolemInfo -> (Name -> SrcSpan) -> TyVar -> TcM TcTyVar
457 -- Instantiate the tyvar, using
458 -- * the occ-name and kind of the supplied tyvar,
459 -- * the unique from the monad,
460 -- * the location either from the tyvar (mb_loc = Nothing)
461 -- or from mb_loc (Just loc)
462 tcInstSkolTyVar info get_loc tyvar
463 = do { uniq <- newUnique
464 ; let old_name = tyVarName tyvar
465 kind = tyVarKind tyvar
466 loc = get_loc old_name
467 new_name = mkInternalName uniq (nameOccName old_name) loc
468 ; return (mkSkolTyVar new_name kind info) }
470 tcInstSkolTyVars :: SkolemInfo -> [TyVar] -> TcM [TcTyVar]
471 -- Get the location from the monad
472 tcInstSkolTyVars info tyvars
473 = do { span <- getSrcSpanM
474 ; mapM (tcInstSkolTyVar info (const span)) tyvars }
476 tcInstSkolType :: SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcType)
477 -- Instantiate a type with fresh skolem constants
478 -- Binding location comes from the monad
479 tcInstSkolType info ty = tcInstType (tcInstSkolTyVars info) ty
481 tcInstSigType :: Bool -> SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcRhoType)
482 -- Instantiate with skolems or meta SigTvs; depending on use_skols
483 -- Always take location info from the supplied tyvars
484 tcInstSigType use_skols skol_info ty
485 = tcInstType (mapM inst_tyvar) ty
487 inst_tyvar | use_skols = tcInstSkolTyVar skol_info getSrcSpan
488 | otherwise = instMetaTyVar (SigTv skol_info)
492 %************************************************************************
494 MetaTvs (meta type variables; mutable)
496 %************************************************************************
499 newMetaTyVar :: BoxInfo -> Kind -> TcM TcTyVar
500 -- Make a new meta tyvar out of thin air
501 newMetaTyVar box_info kind
502 = do { uniq <- newUnique
503 ; ref <- newMutVar Flexi
504 ; let name = mkSysTvName uniq fs
505 fs = case box_info of
509 -- We give BoxTv and TauTv the same string, because
510 -- otherwise we get user-visible differences in error
511 -- messages, which are confusing. If you want to see
512 -- the box_info of each tyvar, use -dppr-debug
513 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
515 instMetaTyVar :: BoxInfo -> TyVar -> TcM TcTyVar
516 -- Make a new meta tyvar whose Name and Kind
517 -- come from an existing TyVar
518 instMetaTyVar box_info tyvar
519 = do { uniq <- newUnique
520 ; ref <- newMutVar Flexi
521 ; let name = setNameUnique (tyVarName tyvar) uniq
522 kind = tyVarKind tyvar
523 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
525 readMetaTyVar :: TyVar -> TcM MetaDetails
526 readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
527 readMutVar (metaTvRef tyvar)
529 isFilledMetaTyVar :: TyVar -> TcM Bool
530 -- True of a filled-in (Indirect) meta type variable
532 | not (isTcTyVar tv) = return False
533 | MetaTv _ ref <- tcTyVarDetails tv
534 = do { details <- readMutVar ref
535 ; return (isIndirect details) }
536 | otherwise = return False
538 writeMetaTyVar :: TcTyVar -> TcType -> TcM ()
539 writeMetaTyVar tyvar ty
540 | not debugIsOn = writeMutVar (metaTvRef tyvar) (Indirect ty)
541 writeMetaTyVar tyvar ty
542 | not (isMetaTyVar tyvar)
543 = pprTrace "writeMetaTyVar" (ppr tyvar) $
546 = ASSERT( isMetaTyVar tyvar )
547 ASSERT2( isCoVar tyvar || typeKind ty `isSubKind` tyVarKind tyvar,
548 (ppr tyvar <+> ppr (tyVarKind tyvar))
549 $$ (ppr ty <+> ppr (typeKind ty)) )
550 do { if debugIsOn then do { details <- readMetaTyVar tyvar;
551 -- FIXME ; ASSERT2( not (isFlexi details), ppr tyvar )
552 ; WARN( not (isFlexi details), ppr tyvar )
556 ; traceTc (text "writeMetaTyVar" <+> ppr tyvar <+> text ":=" <+> ppr ty)
557 ; writeMutVar (metaTvRef tyvar) (Indirect ty) }
561 %************************************************************************
565 %************************************************************************
568 newFlexiTyVar :: Kind -> TcM TcTyVar
569 newFlexiTyVar kind = newMetaTyVar TauTv kind
571 newFlexiTyVarTy :: Kind -> TcM TcType
572 newFlexiTyVarTy kind = do
573 tc_tyvar <- newFlexiTyVar kind
574 return (TyVarTy tc_tyvar)
576 newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
577 newFlexiTyVarTys n kind = mapM newFlexiTyVarTy (nOfThem n kind)
579 tcInstTyVar :: TyVar -> TcM TcTyVar
580 -- Instantiate with a META type variable
581 tcInstTyVar tyvar = instMetaTyVar TauTv tyvar
583 tcInstTyVars :: [TyVar] -> TcM ([TcTyVar], [TcType], TvSubst)
584 -- Instantiate with META type variables
586 = do { tc_tvs <- mapM tcInstTyVar tyvars
587 ; let tys = mkTyVarTys tc_tvs
588 ; return (tc_tvs, tys, zipTopTvSubst tyvars tys) }
589 -- Since the tyvars are freshly made,
590 -- they cannot possibly be captured by
591 -- any existing for-alls. Hence zipTopTvSubst
595 %************************************************************************
599 %************************************************************************
602 zonkSigTyVar :: TcTyVar -> TcM TcTyVar
604 | isSkolemTyVar sig_tv
605 = return sig_tv -- Happens in the call in TcBinds.checkDistinctTyVars
607 = ASSERT( isSigTyVar sig_tv )
608 do { ty <- zonkTcTyVar sig_tv
609 ; return (tcGetTyVar "zonkSigTyVar" ty) }
610 -- 'ty' is bound to be a type variable, because SigTvs
611 -- can only be unified with type variables
615 %************************************************************************
619 %************************************************************************
622 newBoxyTyVar :: Kind -> TcM BoxyTyVar
623 newBoxyTyVar kind = newMetaTyVar BoxTv kind
625 newBoxyTyVars :: [Kind] -> TcM [BoxyTyVar]
626 newBoxyTyVars kinds = mapM newBoxyTyVar kinds
628 newBoxyTyVarTys :: [Kind] -> TcM [BoxyType]
629 newBoxyTyVarTys kinds = do { tvs <- mapM newBoxyTyVar kinds; return (mkTyVarTys tvs) }
631 readFilledBox :: BoxyTyVar -> TcM TcType
632 -- Read the contents of the box, which should be filled in by now
633 readFilledBox box_tv = ASSERT( isBoxyTyVar box_tv )
634 do { cts <- readMetaTyVar box_tv
636 Flexi -> pprPanic "readFilledBox" (ppr box_tv)
637 Indirect ty -> return ty }
639 tcInstBoxyTyVar :: TyVar -> TcM BoxyTyVar
640 -- Instantiate with a BOXY type variable
641 tcInstBoxyTyVar tyvar = instMetaTyVar BoxTv tyvar
645 %************************************************************************
647 \subsection{Putting and getting mutable type variables}
649 %************************************************************************
651 But it's more fun to short out indirections on the way: If this
652 version returns a TyVar, then that TyVar is unbound. If it returns
653 any other type, then there might be bound TyVars embedded inside it.
655 We return Nothing iff the original box was unbound.
658 data LookupTyVarResult -- The result of a lookupTcTyVar call
659 = DoneTv TcTyVarDetails -- SkolemTv or virgin MetaTv
662 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
664 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
666 SkolemTv _ -> return (DoneTv details)
667 MetaTv _ ref -> do { meta_details <- readMutVar ref
668 ; case meta_details of
669 Indirect ty -> return (IndirectTv ty)
670 Flexi -> return (DoneTv details) }
672 details = tcTyVarDetails tyvar
675 -- gaw 2004 We aren't shorting anything out anymore, at least for now
677 | not (isTcTyVar tyvar)
678 = pprTrace "getTcTyVar" (ppr tyvar) $
679 return (Just (mkTyVarTy tyvar))
682 = ASSERT2( isTcTyVar tyvar, ppr tyvar ) do
683 maybe_ty <- readMetaTyVar tyvar
685 Just ty -> do ty' <- short_out ty
686 writeMetaTyVar tyvar (Just ty')
689 Nothing -> return Nothing
691 short_out :: TcType -> TcM TcType
692 short_out ty@(TyVarTy tyvar)
693 | not (isTcTyVar tyvar)
697 maybe_ty <- readMetaTyVar tyvar
699 Just ty' -> do ty' <- short_out ty'
700 writeMetaTyVar tyvar (Just ty')
705 short_out other_ty = return other_ty
710 %************************************************************************
712 \subsection{Zonking -- the exernal interfaces}
714 %************************************************************************
716 ----------------- Type variables
719 zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
720 zonkTcTyVars tyvars = mapM zonkTcTyVar tyvars
722 zonkTcTyVarsAndFV :: [TcTyVar] -> TcM TcTyVarSet
723 zonkTcTyVarsAndFV tyvars = tyVarsOfTypes <$> mapM zonkTcTyVar tyvars
725 zonkTcTyVar :: TcTyVar -> TcM TcType
726 zonkTcTyVar tyvar = ASSERT2( isTcTyVar tyvar, ppr tyvar)
727 zonk_tc_tyvar (\ tv -> return (TyVarTy tv)) tyvar
730 ----------------- Types
733 zonkTcType :: TcType -> TcM TcType
734 zonkTcType ty = zonkType (\ tv -> return (TyVarTy tv)) ty
736 zonkTcTypes :: [TcType] -> TcM [TcType]
737 zonkTcTypes tys = mapM zonkTcType tys
739 zonkTcThetaType :: TcThetaType -> TcM TcThetaType
740 zonkTcThetaType theta = mapM zonkTcPredType theta
742 zonkTcPredType :: TcPredType -> TcM TcPredType
743 zonkTcPredType (ClassP c ts) = ClassP c <$> zonkTcTypes ts
744 zonkTcPredType (IParam n t) = IParam n <$> zonkTcType t
745 zonkTcPredType (EqPred t1 t2) = EqPred <$> zonkTcType t1 <*> zonkTcType t2
748 ------------------- These ...ToType, ...ToKind versions
749 are used at the end of type checking
752 zonkTopTyVar :: TcTyVar -> TcM TcTyVar
753 -- zonkTopTyVar is used, at the top level, on any un-instantiated meta type variables
754 -- to default the kind of ? and ?? etc to *. This is important to ensure that
755 -- instance declarations match. For example consider
756 -- instance Show (a->b)
757 -- foo x = show (\_ -> True)
758 -- Then we'll get a constraint (Show (p ->q)) where p has argTypeKind (printed ??),
759 -- and that won't match the typeKind (*) in the instance decl.
761 -- Because we are at top level, no further constraints are going to affect these
762 -- type variables, so it's time to do it by hand. However we aren't ready
763 -- to default them fully to () or whatever, because the type-class defaulting
764 -- rules have yet to run.
767 | k `eqKind` default_k = return tv
769 = do { tv' <- newFlexiTyVar default_k
770 ; writeMetaTyVar tv (mkTyVarTy tv')
774 default_k = defaultKind k
776 zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TcTyVar]
777 zonkQuantifiedTyVars = mapM zonkQuantifiedTyVar
779 zonkQuantifiedTyVar :: TcTyVar -> TcM TcTyVar
780 -- zonkQuantifiedTyVar is applied to the a TcTyVar when quantifying over it.
782 -- The quantified type variables often include meta type variables
783 -- we want to freeze them into ordinary type variables, and
784 -- default their kind (e.g. from OpenTypeKind to TypeKind)
785 -- -- see notes with Kind.defaultKind
786 -- The meta tyvar is updated to point to the new skolem TyVar. Now any
787 -- bound occurences of the original type variable will get zonked to
788 -- the immutable version.
790 -- We leave skolem TyVars alone; they are immutable.
791 zonkQuantifiedTyVar tv
792 | ASSERT2( isTcTyVar tv, ppr tv )
794 = do { kind <- zonkTcType (tyVarKind tv)
795 ; return $ setTyVarKind tv kind
797 -- It might be a skolem type variable,
798 -- for example from a user type signature
800 | otherwise -- It's a meta-type-variable
801 = do { details <- readMetaTyVar tv
803 -- Create the new, frozen, skolem type variable
804 -- We zonk to a skolem, not to a regular TcVar
805 -- See Note [Zonking to Skolem]
806 ; let final_kind = defaultKind (tyVarKind tv)
807 final_tv = mkSkolTyVar (tyVarName tv) final_kind UnkSkol
809 -- Bind the meta tyvar to the new tyvar
811 Indirect ty -> WARN( True, ppr tv $$ ppr ty )
813 -- [Sept 04] I don't think this should happen
814 -- See note [Silly Type Synonym]
816 Flexi -> writeMetaTyVar tv (mkTyVarTy final_tv)
818 -- Return the new tyvar
822 Note [Silly Type Synonyms]
823 ~~~~~~~~~~~~~~~~~~~~~~~~~~
825 type C u a = u -- Note 'a' unused
827 foo :: (forall a. C u a -> C u a) -> u
831 bar = foo (\t -> t + t)
833 * From the (\t -> t+t) we get type {Num d} => d -> d
836 * Now unify with type of foo's arg, and we get:
837 {Num (C d a)} => C d a -> C d a
840 * Now abstract over the 'a', but float out the Num (C d a) constraint
841 because it does not 'really' mention a. (see exactTyVarsOfType)
842 The arg to foo becomes
845 * So we get a dict binding for Num (C d a), which is zonked to give
847 [Note Sept 04: now that we are zonking quantified type variables
848 on construction, the 'a' will be frozen as a regular tyvar on
849 quantification, so the floated dict will still have type (C d a).
850 Which renders this whole note moot; happily!]
852 * Then the \/\a abstraction has a zonked 'a' in it.
854 All very silly. I think its harmless to ignore the problem. We'll end up with
855 a \/\a in the final result but all the occurrences of a will be zonked to ()
857 Note [Zonking to Skolem]
858 ~~~~~~~~~~~~~~~~~~~~~~~~
859 We used to zonk quantified type variables to regular TyVars. However, this
860 leads to problems. Consider this program from the regression test suite:
862 eval :: Int -> String -> String -> String
863 eval 0 root actual = evalRHS 0 root actual
866 evalRHS 0 root actual = eval 0 root actual
868 It leads to the deferral of an equality
870 (String -> String -> String) ~ a
872 which is propagated up to the toplevel (see TcSimplify.tcSimplifyInferCheck).
873 In the meantime `a' is zonked and quantified to form `evalRHS's signature.
874 This has the *side effect* of also zonking the `a' in the deferred equality
875 (which at this point is being handed around wrapped in an implication
878 Finally, the equality (with the zonked `a') will be handed back to the
879 simplifier by TcRnDriver.tcRnSrcDecls calling TcSimplify.tcSimplifyTop.
880 If we zonk `a' with a regular type variable, we will have this regular type
881 variable now floating around in the simplifier, which in many places assumes to
882 only see proper TcTyVars.
884 We can avoid this problem by zonking with a skolem. The skolem is rigid
885 (which we requirefor a quantified variable), but is still a TcTyVar that the
886 simplifier knows how to deal with.
889 %************************************************************************
891 \subsection{Zonking -- the main work-horses: zonkType, zonkTyVar}
893 %* For internal use only! *
895 %************************************************************************
898 -- For unbound, mutable tyvars, zonkType uses the function given to it
899 -- For tyvars bound at a for-all, zonkType zonks them to an immutable
900 -- type variable and zonks the kind too
902 zonkType :: (TcTyVar -> TcM Type) -- What to do with unbound mutable type variables
903 -- see zonkTcType, and zonkTcTypeToType
906 zonkType unbound_var_fn ty
909 go (TyConApp tc tys) = do tys' <- mapM go tys
910 return (TyConApp tc tys')
912 go (PredTy p) = do p' <- go_pred p
915 go (FunTy arg res) = do arg' <- go arg
917 return (FunTy arg' res')
919 go (AppTy fun arg) = do fun' <- go fun
921 return (mkAppTy fun' arg')
922 -- NB the mkAppTy; we might have instantiated a
923 -- type variable to a type constructor, so we need
924 -- to pull the TyConApp to the top.
926 -- The two interesting cases!
927 go (TyVarTy tyvar) | isTcTyVar tyvar = zonk_tc_tyvar unbound_var_fn tyvar
928 | otherwise = liftM TyVarTy $
929 zonkTyVar unbound_var_fn tyvar
930 -- Ordinary (non Tc) tyvars occur inside quantified types
932 go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar ) do
934 tyvar' <- zonkTyVar unbound_var_fn tyvar
935 return (ForAllTy tyvar' ty')
937 go_pred (ClassP c tys) = do tys' <- mapM go tys
938 return (ClassP c tys')
939 go_pred (IParam n ty) = do ty' <- go ty
940 return (IParam n ty')
941 go_pred (EqPred ty1 ty2) = do ty1' <- go ty1
943 return (EqPred ty1' ty2')
945 zonk_tc_tyvar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable var
946 -> TcTyVar -> TcM TcType
947 zonk_tc_tyvar unbound_var_fn tyvar
948 | not (isMetaTyVar tyvar) -- Skolems
949 = return (TyVarTy tyvar)
951 | otherwise -- Mutables
952 = do { cts <- readMetaTyVar tyvar
954 Flexi -> unbound_var_fn tyvar -- Unbound meta type variable
955 Indirect ty -> zonkType unbound_var_fn ty }
957 -- Zonk the kind of a non-TC tyvar in case it is a coercion variable (their
958 -- kind contains types).
960 zonkTyVar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable var
961 -> TyVar -> TcM TyVar
962 zonkTyVar unbound_var_fn tv
964 = do { kind <- zonkType unbound_var_fn (tyVarKind tv)
965 ; return $ setTyVarKind tv kind
967 | otherwise = return tv
972 %************************************************************************
976 %************************************************************************
979 readKindVar :: KindVar -> TcM (MetaDetails)
980 writeKindVar :: KindVar -> TcKind -> TcM ()
981 readKindVar kv = readMutVar (kindVarRef kv)
982 writeKindVar kv val = writeMutVar (kindVarRef kv) (Indirect val)
985 zonkTcKind :: TcKind -> TcM TcKind
986 zonkTcKind k = zonkTcType k
989 zonkTcKindToKind :: TcKind -> TcM Kind
990 -- When zonking a TcKind to a kind, we need to instantiate kind variables,
991 -- Haskell specifies that * is to be used, so we follow that.
992 zonkTcKindToKind k = zonkType (\ _ -> return liftedTypeKind) k
995 %************************************************************************
997 \subsection{Checking a user type}
999 %************************************************************************
1001 When dealing with a user-written type, we first translate it from an HsType
1002 to a Type, performing kind checking, and then check various things that should
1003 be true about it. We don't want to perform these checks at the same time
1004 as the initial translation because (a) they are unnecessary for interface-file
1005 types and (b) when checking a mutually recursive group of type and class decls,
1006 we can't "look" at the tycons/classes yet. Also, the checks are are rather
1007 diverse, and used to really mess up the other code.
1009 One thing we check for is 'rank'.
1011 Rank 0: monotypes (no foralls)
1012 Rank 1: foralls at the front only, Rank 0 inside
1013 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
1015 basic ::= tyvar | T basic ... basic
1017 r2 ::= forall tvs. cxt => r2a
1018 r2a ::= r1 -> r2a | basic
1019 r1 ::= forall tvs. cxt => r0
1020 r0 ::= r0 -> r0 | basic
1022 Another thing is to check that type synonyms are saturated.
1023 This might not necessarily show up in kind checking.
1025 data T k = MkT (k Int)
1030 checkValidType :: UserTypeCtxt -> Type -> TcM ()
1031 -- Checks that the type is valid for the given context
1032 checkValidType ctxt ty = do
1033 traceTc (text "checkValidType" <+> ppr ty)
1034 unboxed <- doptM Opt_UnboxedTuples
1035 rank2 <- doptM Opt_Rank2Types
1036 rankn <- doptM Opt_RankNTypes
1037 polycomp <- doptM Opt_PolymorphicComponents
1039 gen_rank n | rankn = ArbitraryRank
1041 | otherwise = Rank n
1044 DefaultDeclCtxt-> MustBeMonoType
1045 ResSigCtxt -> MustBeMonoType
1046 LamPatSigCtxt -> gen_rank 0
1047 BindPatSigCtxt -> gen_rank 0
1048 TySynCtxt _ -> gen_rank 0
1049 GenPatCtxt -> gen_rank 1
1050 -- This one is a bit of a hack
1051 -- See the forall-wrapping in TcClassDcl.mkGenericInstance
1053 ExprSigCtxt -> gen_rank 1
1054 FunSigCtxt _ -> gen_rank 1
1055 ConArgCtxt _ | polycomp -> gen_rank 2
1056 -- We are given the type of the entire
1057 -- constructor, hence rank 1
1058 | otherwise -> gen_rank 1
1060 ForSigCtxt _ -> gen_rank 1
1061 SpecInstCtxt -> gen_rank 1
1062 ThBrackCtxt -> gen_rank 1
1064 actual_kind = typeKind ty
1066 kind_ok = case ctxt of
1067 TySynCtxt _ -> True -- Any kind will do
1068 ThBrackCtxt -> True -- Any kind will do
1069 ResSigCtxt -> isSubOpenTypeKind actual_kind
1070 ExprSigCtxt -> isSubOpenTypeKind actual_kind
1071 GenPatCtxt -> isLiftedTypeKind actual_kind
1072 ForSigCtxt _ -> isLiftedTypeKind actual_kind
1073 _ -> isSubArgTypeKind actual_kind
1075 ubx_tup = case ctxt of
1076 TySynCtxt _ | unboxed -> UT_Ok
1077 ExprSigCtxt | unboxed -> UT_Ok
1078 ThBrackCtxt | unboxed -> UT_Ok
1081 -- Check the internal validity of the type itself
1082 check_type rank ubx_tup ty
1084 -- Check that the thing has kind Type, and is lifted if necessary
1085 -- Do this second, becuase we can't usefully take the kind of an
1086 -- ill-formed type such as (a~Int)
1087 checkTc kind_ok (kindErr actual_kind)
1089 traceTc (text "checkValidType done" <+> ppr ty)
1091 checkValidMonoType :: Type -> TcM ()
1092 checkValidMonoType ty = check_mono_type MustBeMonoType ty
1097 data Rank = ArbitraryRank -- Any rank ok
1098 | MustBeMonoType -- Monotype regardless of flags
1099 | TyConArgMonoType -- Monotype but could be poly if -XImpredicativeTypes
1100 | SynArgMonoType -- Monotype but could be poly if -XLiberalTypeSynonyms
1101 | Rank Int -- Rank n, but could be more with -XRankNTypes
1103 decRank :: Rank -> Rank -- Function arguments
1104 decRank (Rank 0) = Rank 0
1105 decRank (Rank n) = Rank (n-1)
1106 decRank other_rank = other_rank
1108 nonZeroRank :: Rank -> Bool
1109 nonZeroRank ArbitraryRank = True
1110 nonZeroRank (Rank n) = n>0
1111 nonZeroRank _ = False
1113 ----------------------------------------
1114 data UbxTupFlag = UT_Ok | UT_NotOk
1115 -- The "Ok" version means "ok if UnboxedTuples is on"
1117 ----------------------------------------
1118 check_mono_type :: Rank -> Type -> TcM () -- No foralls anywhere
1119 -- No unlifted types of any kind
1120 check_mono_type rank ty
1121 = do { check_type rank UT_NotOk ty
1122 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1124 check_type :: Rank -> UbxTupFlag -> Type -> TcM ()
1125 -- The args say what the *type context* requires, independent
1126 -- of *flag* settings. You test the flag settings at usage sites.
1128 -- Rank is allowed rank for function args
1129 -- Rank 0 means no for-alls anywhere
1131 check_type rank ubx_tup ty
1132 | not (null tvs && null theta)
1133 = do { checkTc (nonZeroRank rank) (forAllTyErr rank ty)
1134 -- Reject e.g. (Maybe (?x::Int => Int)),
1135 -- with a decent error message
1136 ; check_valid_theta SigmaCtxt theta
1137 ; check_type rank ubx_tup tau -- Allow foralls to right of arrow
1138 ; checkFreeness tvs theta
1139 ; checkAmbiguity tvs theta (tyVarsOfType tau) }
1141 (tvs, theta, tau) = tcSplitSigmaTy ty
1143 -- Naked PredTys should, I think, have been rejected before now
1144 check_type _ _ ty@(PredTy {})
1145 = failWithTc (text "Predicate used as a type:" <+> ppr ty)
1147 check_type _ _ (TyVarTy _) = return ()
1149 check_type rank _ (FunTy arg_ty res_ty)
1150 = do { check_type (decRank rank) UT_NotOk arg_ty
1151 ; check_type rank UT_Ok res_ty }
1153 check_type rank _ (AppTy ty1 ty2)
1154 = do { check_arg_type rank ty1
1155 ; check_arg_type rank ty2 }
1157 check_type rank ubx_tup ty@(TyConApp tc tys)
1159 = do { -- Check that the synonym has enough args
1160 -- This applies equally to open and closed synonyms
1161 -- It's OK to have an *over-applied* type synonym
1162 -- data Tree a b = ...
1163 -- type Foo a = Tree [a]
1164 -- f :: Foo a b -> ...
1165 checkTc (tyConArity tc <= length tys) arity_msg
1167 -- See Note [Liberal type synonyms]
1168 ; liberal <- doptM Opt_LiberalTypeSynonyms
1169 ; if not liberal || isOpenSynTyCon tc then
1170 -- For H98 and synonym families, do check the type args
1171 mapM_ (check_mono_type SynArgMonoType) tys
1173 else -- In the liberal case (only for closed syns), expand then check
1175 Just ty' -> check_type rank ubx_tup ty'
1176 Nothing -> pprPanic "check_tau_type" (ppr ty)
1179 | isUnboxedTupleTyCon tc
1180 = do { ub_tuples_allowed <- doptM Opt_UnboxedTuples
1181 ; checkTc (ubx_tup_ok ub_tuples_allowed) ubx_tup_msg
1183 ; impred <- doptM Opt_ImpredicativeTypes
1184 ; let rank' = if impred then ArbitraryRank else TyConArgMonoType
1185 -- c.f. check_arg_type
1186 -- However, args are allowed to be unlifted, or
1187 -- more unboxed tuples, so can't use check_arg_ty
1188 ; mapM_ (check_type rank' UT_Ok) tys }
1191 = mapM_ (check_arg_type rank) tys
1194 ubx_tup_ok ub_tuples_allowed = case ubx_tup of
1195 UT_Ok -> ub_tuples_allowed
1199 tc_arity = tyConArity tc
1201 arity_msg = arityErr "Type synonym" (tyConName tc) tc_arity n_args
1202 ubx_tup_msg = ubxArgTyErr ty
1204 check_type _ _ ty = pprPanic "check_type" (ppr ty)
1206 ----------------------------------------
1207 check_arg_type :: Rank -> Type -> TcM ()
1208 -- The sort of type that can instantiate a type variable,
1209 -- or be the argument of a type constructor.
1210 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
1211 -- Other unboxed types are very occasionally allowed as type
1212 -- arguments depending on the kind of the type constructor
1214 -- For example, we want to reject things like:
1216 -- instance Ord a => Ord (forall s. T s a)
1218 -- g :: T s (forall b.b)
1220 -- NB: unboxed tuples can have polymorphic or unboxed args.
1221 -- This happens in the workers for functions returning
1222 -- product types with polymorphic components.
1223 -- But not in user code.
1224 -- Anyway, they are dealt with by a special case in check_tau_type
1226 check_arg_type rank ty
1227 = do { impred <- doptM Opt_ImpredicativeTypes
1228 ; let rank' = case rank of -- Predictive => must be monotype
1229 MustBeMonoType -> MustBeMonoType -- Monotype, regardless
1230 _other | impred -> ArbitraryRank
1231 | otherwise -> TyConArgMonoType
1232 -- Make sure that MustBeMonoType is propagated,
1233 -- so that we don't suggest -XImpredicativeTypes in
1234 -- (Ord (forall a.a)) => a -> a
1235 -- and so that if it Must be a monotype, we check that it is!
1237 ; check_type rank' UT_NotOk ty
1238 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1240 ----------------------------------------
1241 forAllTyErr :: Rank -> Type -> SDoc
1243 = vcat [ hang (ptext (sLit "Illegal polymorphic or qualified type:")) 2 (ppr ty)
1246 suggestion = case rank of
1247 Rank _ -> ptext (sLit "Perhaps you intended to use -XRankNTypes or -XRank2Types")
1248 TyConArgMonoType -> ptext (sLit "Perhaps you intended to use -XImpredicativeTypes")
1249 SynArgMonoType -> ptext (sLit "Perhaps you intended to use -XLiberalTypeSynonyms")
1250 _ -> empty -- Polytype is always illegal
1252 unliftedArgErr, ubxArgTyErr :: Type -> SDoc
1253 unliftedArgErr ty = sep [ptext (sLit "Illegal unlifted type:"), ppr ty]
1254 ubxArgTyErr ty = sep [ptext (sLit "Illegal unboxed tuple type as function argument:"), ppr ty]
1256 kindErr :: Kind -> SDoc
1257 kindErr kind = sep [ptext (sLit "Expecting an ordinary type, but found a type of kind"), ppr kind]
1260 Note [Liberal type synonyms]
1261 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1262 If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
1263 doing validity checking. This allows us to instantiate a synonym defn
1264 with a for-all type, or with a partially-applied type synonym.
1268 Here, T is partially applied, so it's illegal in H98. But if you
1269 expand S first, then T we get just
1273 IMPORTANT: suppose T is a type synonym. Then we must do validity
1274 checking on an appliation (T ty1 ty2)
1276 *either* before expansion (i.e. check ty1, ty2)
1277 *or* after expansion (i.e. expand T ty1 ty2, and then check)
1280 If we do both, we get exponential behaviour!!
1282 data TIACons1 i r c = c i ::: r c
1283 type TIACons2 t x = TIACons1 t (TIACons1 t x)
1284 type TIACons3 t x = TIACons2 t (TIACons1 t x)
1285 type TIACons4 t x = TIACons2 t (TIACons2 t x)
1286 type TIACons7 t x = TIACons4 t (TIACons3 t x)
1289 %************************************************************************
1291 \subsection{Checking a theta or source type}
1293 %************************************************************************
1296 -- Enumerate the contexts in which a "source type", <S>, can occur
1300 -- or (N a) where N is a newtype
1303 = ClassSCCtxt Name -- Superclasses of clas
1304 -- class <S> => C a where ...
1305 | SigmaCtxt -- Theta part of a normal for-all type
1306 -- f :: <S> => a -> a
1307 | DataTyCtxt Name -- Theta part of a data decl
1308 -- data <S> => T a = MkT a
1309 | TypeCtxt -- Source type in an ordinary type
1311 | InstThetaCtxt -- Context of an instance decl
1312 -- instance <S> => C [a] where ...
1314 pprSourceTyCtxt :: SourceTyCtxt -> SDoc
1315 pprSourceTyCtxt (ClassSCCtxt c) = ptext (sLit "the super-classes of class") <+> quotes (ppr c)
1316 pprSourceTyCtxt SigmaCtxt = ptext (sLit "the context of a polymorphic type")
1317 pprSourceTyCtxt (DataTyCtxt tc) = ptext (sLit "the context of the data type declaration for") <+> quotes (ppr tc)
1318 pprSourceTyCtxt InstThetaCtxt = ptext (sLit "the context of an instance declaration")
1319 pprSourceTyCtxt TypeCtxt = ptext (sLit "the context of a type")
1323 checkValidTheta :: SourceTyCtxt -> ThetaType -> TcM ()
1324 checkValidTheta ctxt theta
1325 = addErrCtxt (checkThetaCtxt ctxt theta) (check_valid_theta ctxt theta)
1327 -------------------------
1328 check_valid_theta :: SourceTyCtxt -> [PredType] -> TcM ()
1329 check_valid_theta _ []
1331 check_valid_theta ctxt theta = do
1333 warnTc (notNull dups) (dupPredWarn dups)
1334 mapM_ (check_pred_ty dflags ctxt) theta
1336 (_,dups) = removeDups tcCmpPred theta
1338 -------------------------
1339 check_pred_ty :: DynFlags -> SourceTyCtxt -> PredType -> TcM ()
1340 check_pred_ty dflags ctxt pred@(ClassP cls tys)
1341 = do { -- Class predicates are valid in all contexts
1342 ; checkTc (arity == n_tys) arity_err
1344 -- Check the form of the argument types
1345 ; mapM_ checkValidMonoType tys
1346 ; checkTc (check_class_pred_tys dflags ctxt tys)
1347 (predTyVarErr pred $$ how_to_allow)
1350 class_name = className cls
1351 arity = classArity cls
1353 arity_err = arityErr "Class" class_name arity n_tys
1354 how_to_allow = parens (ptext (sLit "Use -XFlexibleContexts to permit this"))
1356 check_pred_ty _ (ClassSCCtxt _) (EqPred _ _)
1357 = -- We do not yet support superclass equalities.
1359 sep [ ptext (sLit "The current implementation of type families does not")
1360 , ptext (sLit "support equality constraints in superclass contexts.")
1361 , ptext (sLit "They are planned for a future release.")
1364 check_pred_ty dflags _ pred@(EqPred ty1 ty2)
1365 = do { -- Equational constraints are valid in all contexts if type
1366 -- families are permitted
1367 ; checkTc (dopt Opt_TypeFamilies dflags) (eqPredTyErr pred)
1369 -- Check the form of the argument types
1370 ; checkValidMonoType ty1
1371 ; checkValidMonoType ty2
1374 check_pred_ty _ SigmaCtxt (IParam _ ty) = checkValidMonoType ty
1375 -- Implicit parameters only allowed in type
1376 -- signatures; not in instance decls, superclasses etc
1377 -- The reason for not allowing implicit params in instances is a bit
1379 -- If we allowed instance (?x::Int, Eq a) => Foo [a] where ...
1380 -- then when we saw (e :: (?x::Int) => t) it would be unclear how to
1381 -- discharge all the potential usas of the ?x in e. For example, a
1382 -- constraint Foo [Int] might come out of e,and applying the
1383 -- instance decl would show up two uses of ?x.
1386 check_pred_ty _ _ sty = failWithTc (badPredTyErr sty)
1388 -------------------------
1389 check_class_pred_tys :: DynFlags -> SourceTyCtxt -> [Type] -> Bool
1390 check_class_pred_tys dflags ctxt tys
1392 TypeCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
1393 InstThetaCtxt -> flexible_contexts || undecidable_ok || all tcIsTyVarTy tys
1394 -- Further checks on head and theta in
1395 -- checkInstTermination
1396 _ -> flexible_contexts || all tyvar_head tys
1398 flexible_contexts = dopt Opt_FlexibleContexts dflags
1399 undecidable_ok = dopt Opt_UndecidableInstances dflags
1401 -------------------------
1402 tyvar_head :: Type -> Bool
1403 tyvar_head ty -- Haskell 98 allows predicates of form
1404 | tcIsTyVarTy ty = True -- C (a ty1 .. tyn)
1405 | otherwise -- where a is a type variable
1406 = case tcSplitAppTy_maybe ty of
1407 Just (ty, _) -> tyvar_head ty
1414 is ambiguous if P contains generic variables
1415 (i.e. one of the Vs) that are not mentioned in tau
1417 However, we need to take account of functional dependencies
1418 when we speak of 'mentioned in tau'. Example:
1419 class C a b | a -> b where ...
1421 forall x y. (C x y) => x
1422 is not ambiguous because x is mentioned and x determines y
1424 NB; the ambiguity check is only used for *user* types, not for types
1425 coming from inteface files. The latter can legitimately have
1426 ambiguous types. Example
1428 class S a where s :: a -> (Int,Int)
1429 instance S Char where s _ = (1,1)
1430 f:: S a => [a] -> Int -> (Int,Int)
1431 f (_::[a]) x = (a*x,b)
1432 where (a,b) = s (undefined::a)
1434 Here the worker for f gets the type
1435 fw :: forall a. S a => Int -> (# Int, Int #)
1437 If the list of tv_names is empty, we have a monotype, and then we
1438 don't need to check for ambiguity either, because the test can't fail
1443 checkAmbiguity :: [TyVar] -> ThetaType -> TyVarSet -> TcM ()
1444 checkAmbiguity forall_tyvars theta tau_tyvars
1445 = mapM_ complain (filter is_ambig theta)
1447 complain pred = addErrTc (ambigErr pred)
1448 extended_tau_vars = growThetaTyVars theta tau_tyvars
1450 -- See Note [Implicit parameters and ambiguity] in TcSimplify
1451 is_ambig pred = isClassPred pred &&
1452 any ambig_var (varSetElems (tyVarsOfPred pred))
1454 ambig_var ct_var = (ct_var `elem` forall_tyvars) &&
1455 not (ct_var `elemVarSet` extended_tau_vars)
1457 ambigErr :: PredType -> SDoc
1459 = sep [ptext (sLit "Ambiguous constraint") <+> quotes (pprPred pred),
1460 nest 4 (ptext (sLit "At least one of the forall'd type variables mentioned by the constraint") $$
1461 ptext (sLit "must be reachable from the type after the '=>'"))]
1463 --------------------------
1464 -- For this 'grow' stuff see Note [Growing the tau-tvs using constraints] in Inst
1466 growThetaTyVars :: TcThetaType -> TyVarSet -> TyVarSet
1468 growThetaTyVars theta tvs
1470 | otherwise = fixVarSet mk_next tvs
1472 mk_next tvs = foldr growPredTyVars tvs theta
1475 growPredTyVars :: TcPredType -> TyVarSet -> TyVarSet
1476 -- Here is where the special case for inplicit parameters happens
1477 growPredTyVars (IParam _ ty) tvs = tvs `unionVarSet` tyVarsOfType ty
1478 growPredTyVars pred tvs = growTyVars (tyVarsOfPred pred) tvs
1480 growTyVars :: TyVarSet -> TyVarSet -> TyVarSet
1481 growTyVars new_tvs tvs
1482 | new_tvs `intersectsVarSet` tvs = tvs `unionVarSet` new_tvs
1486 In addition, GHC insists that at least one type variable
1487 in each constraint is in V. So we disallow a type like
1488 forall a. Eq b => b -> b
1489 even in a scope where b is in scope.
1492 checkFreeness :: [Var] -> [PredType] -> TcM ()
1493 checkFreeness forall_tyvars theta
1494 = do { flexible_contexts <- doptM Opt_FlexibleContexts
1495 ; unless flexible_contexts $ mapM_ complain (filter is_free theta) }
1497 is_free pred = not (isIPPred pred)
1498 && not (any bound_var (varSetElems (tyVarsOfPred pred)))
1499 bound_var ct_var = ct_var `elem` forall_tyvars
1500 complain pred = addErrTc (freeErr pred)
1502 freeErr :: PredType -> SDoc
1504 = sep [ ptext (sLit "All of the type variables in the constraint") <+>
1505 quotes (pprPred pred)
1506 , ptext (sLit "are already in scope") <+>
1507 ptext (sLit "(at least one must be universally quantified here)")
1509 ptext (sLit "(Use -XFlexibleContexts to lift this restriction)")
1514 checkThetaCtxt :: SourceTyCtxt -> ThetaType -> SDoc
1515 checkThetaCtxt ctxt theta
1516 = vcat [ptext (sLit "In the context:") <+> pprTheta theta,
1517 ptext (sLit "While checking") <+> pprSourceTyCtxt ctxt ]
1519 badPredTyErr, eqPredTyErr, predTyVarErr :: PredType -> SDoc
1520 badPredTyErr sty = ptext (sLit "Illegal constraint") <+> pprPred sty
1521 eqPredTyErr sty = ptext (sLit "Illegal equational constraint") <+> pprPred sty
1523 parens (ptext (sLit "Use -XTypeFamilies to permit this"))
1524 predTyVarErr pred = sep [ptext (sLit "Non type-variable argument"),
1525 nest 2 (ptext (sLit "in the constraint:") <+> pprPred pred)]
1526 dupPredWarn :: [[PredType]] -> SDoc
1527 dupPredWarn dups = ptext (sLit "Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
1529 arityErr :: Outputable a => String -> a -> Int -> Int -> SDoc
1530 arityErr kind name n m
1531 = hsep [ text kind, quotes (ppr name), ptext (sLit "should have"),
1532 n_arguments <> comma, text "but has been given", int m]
1534 n_arguments | n == 0 = ptext (sLit "no arguments")
1535 | n == 1 = ptext (sLit "1 argument")
1536 | True = hsep [int n, ptext (sLit "arguments")]
1539 notMonoType :: TcType -> TcM a
1541 = do { ty' <- zonkTcType ty
1542 ; env0 <- tcInitTidyEnv
1543 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1544 msg = ptext (sLit "Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1545 ; failWithTcM (env1, msg) }
1547 notMonoArgs :: TcType -> TcM a
1549 = do { ty' <- zonkTcType ty
1550 ; env0 <- tcInitTidyEnv
1551 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1552 msg = ptext (sLit "Arguments of type synonym families must be monotypes") <+> quotes (ppr tidy_ty)
1553 ; failWithTcM (env1, msg) }
1557 %************************************************************************
1559 \subsection{Checking for a decent instance head type}
1561 %************************************************************************
1563 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1564 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1566 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1567 flag is on, or (2)~the instance is imported (they must have been
1568 compiled elsewhere). In these cases, we let them go through anyway.
1570 We can also have instances for functions: @instance Foo (a -> b) ...@.
1573 checkValidInstHead :: Type -> TcM (Class, [TcType])
1575 checkValidInstHead ty -- Should be a source type
1576 = case tcSplitPredTy_maybe ty of {
1577 Nothing -> failWithTc (instTypeErr (ppr ty) empty) ;
1580 case getClassPredTys_maybe pred of {
1581 Nothing -> failWithTc (instTypeErr (pprPred pred) empty) ;
1582 Just (clas,tys) -> do
1585 check_inst_head dflags clas tys
1589 check_inst_head :: DynFlags -> Class -> [Type] -> TcM ()
1590 check_inst_head dflags clas tys
1591 = do { -- If GlasgowExts then check at least one isn't a type variable
1592 ; checkTc (dopt Opt_TypeSynonymInstances dflags ||
1593 all tcInstHeadTyNotSynonym tys)
1594 (instTypeErr (pprClassPred clas tys) head_type_synonym_msg)
1595 ; checkTc (dopt Opt_FlexibleInstances dflags ||
1596 all tcInstHeadTyAppAllTyVars tys)
1597 (instTypeErr (pprClassPred clas tys) head_type_args_tyvars_msg)
1598 ; checkTc (dopt Opt_MultiParamTypeClasses dflags ||
1600 (instTypeErr (pprClassPred clas tys) head_one_type_msg)
1601 -- May not contain type family applications
1602 ; mapM_ checkTyFamFreeness tys
1604 ; mapM_ checkValidMonoType tys
1605 -- For now, I only allow tau-types (not polytypes) in
1606 -- the head of an instance decl.
1607 -- E.g. instance C (forall a. a->a) is rejected
1608 -- One could imagine generalising that, but I'm not sure
1609 -- what all the consequences might be
1613 head_type_synonym_msg = parens (
1614 text "All instance types must be of the form (T t1 ... tn)" $$
1615 text "where T is not a synonym." $$
1616 text "Use -XTypeSynonymInstances if you want to disable this.")
1618 head_type_args_tyvars_msg = parens (vcat [
1619 text "All instance types must be of the form (T a1 ... an)",
1620 text "where a1 ... an are type *variables*,",
1621 text "and each type variable appears at most once in the instance head.",
1622 text "Use -XFlexibleInstances if you want to disable this."])
1624 head_one_type_msg = parens (
1625 text "Only one type can be given in an instance head." $$
1626 text "Use -XMultiParamTypeClasses if you want to allow more.")
1628 instTypeErr :: SDoc -> SDoc -> SDoc
1629 instTypeErr pp_ty msg
1630 = sep [ptext (sLit "Illegal instance declaration for") <+> quotes pp_ty,
1635 %************************************************************************
1637 \subsection{Checking instance for termination}
1639 %************************************************************************
1643 checkValidInstance :: [TyVar] -> ThetaType -> Class -> [TcType] -> TcM ()
1644 checkValidInstance tyvars theta clas inst_tys
1645 = do { undecidable_ok <- doptM Opt_UndecidableInstances
1647 ; checkValidTheta InstThetaCtxt theta
1648 ; checkAmbiguity tyvars theta (tyVarsOfTypes inst_tys)
1650 -- Check that instance inference will terminate (if we care)
1651 -- For Haskell 98 this will already have been done by checkValidTheta,
1652 -- but as we may be using other extensions we need to check.
1653 ; unless undecidable_ok $
1654 mapM_ addErrTc (checkInstTermination inst_tys theta)
1656 -- The Coverage Condition
1657 ; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
1658 (instTypeErr (pprClassPred clas inst_tys) msg)
1661 msg = parens (vcat [ptext (sLit "the Coverage Condition fails for one of the functional dependencies;"),
1665 Termination test: the so-called "Paterson conditions" (see Section 5 of
1666 "Understanding functionsl dependencies via Constraint Handling Rules,
1669 We check that each assertion in the context satisfies:
1670 (1) no variable has more occurrences in the assertion than in the head, and
1671 (2) the assertion has fewer constructors and variables (taken together
1672 and counting repetitions) than the head.
1673 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1674 (which have already been checked) guarantee termination.
1676 The underlying idea is that
1678 for any ground substitution, each assertion in the
1679 context has fewer type constructors than the head.
1683 checkInstTermination :: [TcType] -> ThetaType -> [Message]
1684 checkInstTermination tys theta
1685 = mapCatMaybes check theta
1688 size = sizeTypes tys
1690 | not (null (fvPred pred \\ fvs))
1691 = Just (predUndecErr pred nomoreMsg $$ parens undecidableMsg)
1692 | sizePred pred >= size
1693 = Just (predUndecErr pred smallerMsg $$ parens undecidableMsg)
1697 predUndecErr :: PredType -> SDoc -> SDoc
1698 predUndecErr pred msg = sep [msg,
1699 nest 2 (ptext (sLit "in the constraint:") <+> pprPred pred)]
1701 nomoreMsg, smallerMsg, undecidableMsg :: SDoc
1702 nomoreMsg = ptext (sLit "Variable occurs more often in a constraint than in the instance head")
1703 smallerMsg = ptext (sLit "Constraint is no smaller than the instance head")
1704 undecidableMsg = ptext (sLit "Use -XUndecidableInstances to permit this")
1708 %************************************************************************
1710 Checking the context of a derived instance declaration
1712 %************************************************************************
1714 Note [Exotic derived instance contexts]
1715 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1716 In a 'derived' instance declaration, we *infer* the context. It's a
1717 bit unclear what rules we should apply for this; the Haskell report is
1718 silent. Obviously, constraints like (Eq a) are fine, but what about
1719 data T f a = MkT (f a) deriving( Eq )
1720 where we'd get an Eq (f a) constraint. That's probably fine too.
1722 One could go further: consider
1723 data T a b c = MkT (Foo a b c) deriving( Eq )
1724 instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
1726 Notice that this instance (just) satisfies the Paterson termination
1727 conditions. Then we *could* derive an instance decl like this:
1729 instance (C Int a, Eq b, Eq c) => Eq (T a b c)
1731 even though there is no instance for (C Int a), because there just
1732 *might* be an instance for, say, (C Int Bool) at a site where we
1733 need the equality instance for T's.
1735 However, this seems pretty exotic, and it's quite tricky to allow
1736 this, and yet give sensible error messages in the (much more common)
1737 case where we really want that instance decl for C.
1739 So for now we simply require that the derived instance context
1740 should have only type-variable constraints.
1742 Here is another example:
1743 data Fix f = In (f (Fix f)) deriving( Eq )
1744 Here, if we are prepared to allow -XUndecidableInstances we
1745 could derive the instance
1746 instance Eq (f (Fix f)) => Eq (Fix f)
1747 but this is so delicate that I don't think it should happen inside
1748 'deriving'. If you want this, write it yourself!
1750 NB: if you want to lift this condition, make sure you still meet the
1751 termination conditions! If not, the deriving mechanism generates
1752 larger and larger constraints. Example:
1754 data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
1756 Note the lack of a Show instance for Succ. First we'll generate
1757 instance (Show (Succ a), Show a) => Show (Seq a)
1759 instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
1760 and so on. Instead we want to complain of no instance for (Show (Succ a)).
1764 Allow constraints which consist only of type variables, with no repeats.
1767 validDerivPred :: PredType -> Bool
1768 validDerivPred (ClassP _ tys) = hasNoDups fvs && sizeTypes tys == length fvs
1769 where fvs = fvTypes tys
1770 validDerivPred _ = False
1773 %************************************************************************
1775 Checking type instance well-formedness and termination
1777 %************************************************************************
1780 -- Check that a "type instance" is well-formed (which includes decidability
1781 -- unless -XUndecidableInstances is given).
1783 checkValidTypeInst :: [Type] -> Type -> TcM ()
1784 checkValidTypeInst typats rhs
1785 = do { -- left-hand side contains no type family applications
1786 -- (vanilla synonyms are fine, though)
1787 ; mapM_ checkTyFamFreeness typats
1789 -- the right-hand side is a tau type
1790 ; checkValidMonoType rhs
1792 -- we have a decidable instance unless otherwise permitted
1793 ; undecidable_ok <- doptM Opt_UndecidableInstances
1794 ; unless undecidable_ok $
1795 mapM_ addErrTc (checkFamInst typats (tyFamInsts rhs))
1798 -- Make sure that each type family instance is
1799 -- (1) strictly smaller than the lhs,
1800 -- (2) mentions no type variable more often than the lhs, and
1801 -- (3) does not contain any further type family instances.
1803 checkFamInst :: [Type] -- lhs
1804 -> [(TyCon, [Type])] -- type family instances
1806 checkFamInst lhsTys famInsts
1807 = mapCatMaybes check famInsts
1809 size = sizeTypes lhsTys
1810 fvs = fvTypes lhsTys
1812 | not (all isTyFamFree tys)
1813 = Just (famInstUndecErr famInst nestedMsg $$ parens undecidableMsg)
1814 | not (null (fvTypes tys \\ fvs))
1815 = Just (famInstUndecErr famInst nomoreVarMsg $$ parens undecidableMsg)
1816 | size <= sizeTypes tys
1817 = Just (famInstUndecErr famInst smallerAppMsg $$ parens undecidableMsg)
1821 famInst = TyConApp tc tys
1823 -- Ensure that no type family instances occur in a type.
1825 checkTyFamFreeness :: Type -> TcM ()
1826 checkTyFamFreeness ty
1827 = checkTc (isTyFamFree ty) $
1828 tyFamInstIllegalErr ty
1830 -- Check that a type does not contain any type family applications.
1832 isTyFamFree :: Type -> Bool
1833 isTyFamFree = null . tyFamInsts
1837 tyFamInstIllegalErr :: Type -> SDoc
1838 tyFamInstIllegalErr ty
1839 = hang (ptext (sLit "Illegal type synonym family application in instance") <>
1843 famInstUndecErr :: Type -> SDoc -> SDoc
1844 famInstUndecErr ty msg
1846 nest 2 (ptext (sLit "in the type family application:") <+>
1849 nestedMsg, nomoreVarMsg, smallerAppMsg :: SDoc
1850 nestedMsg = ptext (sLit "Nested type family application")
1851 nomoreVarMsg = ptext (sLit "Variable occurs more often than in instance head")
1852 smallerAppMsg = ptext (sLit "Application is no smaller than the instance head")
1856 %************************************************************************
1858 \subsection{Auxiliary functions}
1860 %************************************************************************
1863 -- Free variables of a type, retaining repetitions, and expanding synonyms
1864 fvType :: Type -> [TyVar]
1865 fvType ty | Just exp_ty <- tcView ty = fvType exp_ty
1866 fvType (TyVarTy tv) = [tv]
1867 fvType (TyConApp _ tys) = fvTypes tys
1868 fvType (PredTy pred) = fvPred pred
1869 fvType (FunTy arg res) = fvType arg ++ fvType res
1870 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1871 fvType (ForAllTy tyvar ty) = filter (/= tyvar) (fvType ty)
1873 fvTypes :: [Type] -> [TyVar]
1874 fvTypes tys = concat (map fvType tys)
1876 fvPred :: PredType -> [TyVar]
1877 fvPred (ClassP _ tys') = fvTypes tys'
1878 fvPred (IParam _ ty) = fvType ty
1879 fvPred (EqPred ty1 ty2) = fvType ty1 ++ fvType ty2
1881 -- Size of a type: the number of variables and constructors
1882 sizeType :: Type -> Int
1883 sizeType ty | Just exp_ty <- tcView ty = sizeType exp_ty
1884 sizeType (TyVarTy _) = 1
1885 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1886 sizeType (PredTy pred) = sizePred pred
1887 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1888 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1889 sizeType (ForAllTy _ ty) = sizeType ty
1891 sizeTypes :: [Type] -> Int
1892 sizeTypes xs = sum (map sizeType xs)
1894 -- Size of a predicate
1896 -- Equalities are a special case. The equality itself doesn't contribute to the
1897 -- size and as we do not count class predicates, we have to start with one less.
1898 -- This is easy to see considering that, given
1899 -- class C a b | a -> b
1901 -- constraints (C a b) and (F a ~ b) are equivalent in size.
1902 sizePred :: PredType -> Int
1903 sizePred (ClassP _ tys') = sizeTypes tys'
1904 sizePred (IParam _ ty) = sizeType ty
1905 sizePred (EqPred ty1 ty2) = sizeType ty1 + sizeType ty2 - 1