2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Monadic type operations
8 This module contains monadic operations over types that contain
13 TcTyVar, TcKind, TcType, TcTauType, TcThetaType, TcTyVarSet,
15 --------------------------------
16 -- Creating new mutable type variables
18 newFlexiTyVarTy, -- Kind -> TcM TcType
19 newFlexiTyVarTys, -- Int -> Kind -> TcM [TcType]
20 newKindVar, newKindVars,
21 lookupTcTyVar, LookupTyVarResult(..),
23 newMetaTyVar, readMetaTyVar, writeMetaTyVar, isFilledMetaTyVar,
25 --------------------------------
26 -- Boxy type variables
27 newBoxyTyVar, newBoxyTyVars, newBoxyTyVarTys, readFilledBox,
29 --------------------------------
30 -- Creating new coercion variables
31 newCoVars, newMetaCoVar,
33 --------------------------------
35 tcInstTyVar, tcInstType, tcInstTyVars, tcInstBoxyTyVar,
37 tcInstSkolTyVars, tcInstSkolType,
38 tcSkolSigType, tcSkolSigTyVars, occurCheckErr,
40 --------------------------------
41 -- Checking type validity
42 Rank, UserTypeCtxt(..), checkValidType, checkValidMonoType,
43 SourceTyCtxt(..), checkValidTheta, checkFreeness,
44 checkValidInstHead, checkValidInstance,
45 checkInstTermination, checkValidTypeInst, checkTyFamFreeness,
46 checkUpdateMeta, updateMeta, checkTauTvUpdate, fillBoxWithTau, unifyKindCtxt,
47 unifyKindMisMatch, validDerivPred, arityErr, notMonoType, notMonoArgs,
49 --------------------------------
51 zonkType, zonkTcPredType,
52 zonkTcTyVar, zonkTcTyVars, zonkTcTyVarsAndFV, zonkSigTyVar,
53 zonkQuantifiedTyVar, zonkQuantifiedTyVars,
54 zonkTcType, zonkTcTypes, zonkTcThetaType,
55 zonkTcKindToKind, zonkTcKind, zonkTopTyVar,
57 readKindVar, writeKindVar
60 #include "HsVersions.h"
72 import TcRnMonad -- TcType, amongst others
88 import Data.List ( (\\) )
92 %************************************************************************
94 Instantiation in general
96 %************************************************************************
99 tcInstType :: ([TyVar] -> TcM [TcTyVar]) -- How to instantiate the type variables
100 -> TcType -- Type to instantiate
101 -> TcM ([TcTyVar], TcThetaType, TcType) -- Result
102 -- (type vars (excl coercion vars), preds (incl equalities), rho)
103 tcInstType inst_tyvars ty
104 = case tcSplitForAllTys ty of
105 ([], rho) -> let -- There may be overloading despite no type variables;
106 -- (?x :: Int) => Int -> Int
107 (theta, tau) = tcSplitPhiTy rho
109 return ([], theta, tau)
111 (tyvars, rho) -> do { tyvars' <- inst_tyvars tyvars
113 ; let tenv = zipTopTvSubst tyvars (mkTyVarTys tyvars')
114 -- Either the tyvars are freshly made, by inst_tyvars,
115 -- or (in the call from tcSkolSigType) any nested foralls
116 -- have different binders. Either way, zipTopTvSubst is ok
118 ; let (theta, tau) = tcSplitPhiTy (substTy tenv rho)
119 ; return (tyvars', theta, tau) }
123 %************************************************************************
127 %************************************************************************
129 Can't be in TcUnify, as we also need it in TcTyFuns.
133 -- False <=> the two args are (actual, expected) respectively
134 -- True <=> the two args are (expected, actual) respectively
136 checkUpdateMeta :: SwapFlag
137 -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
138 -- Update tv1, which is flexi; occurs check is alrady done
139 -- The 'check' version does a kind check too
140 -- We do a sub-kind check here: we might unify (a b) with (c d)
141 -- where b::*->* and d::*; this should fail
143 checkUpdateMeta swapped tv1 ref1 ty2
144 = do { checkKinds swapped tv1 ty2
145 ; updateMeta tv1 ref1 ty2 }
147 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
148 updateMeta tv1 ref1 ty2
149 = ASSERT( isMetaTyVar tv1 )
150 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
151 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
152 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
153 ; writeMutVar ref1 (Indirect ty2)
157 checkKinds :: Bool -> TyVar -> Type -> TcM ()
158 checkKinds swapped tv1 ty2
159 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
160 -- ty2 has been zonked at this stage, which ensures that
161 -- its kind has as much boxity information visible as possible.
162 | tk2 `isSubKind` tk1 = return ()
165 -- Either the kinds aren't compatible
166 -- (can happen if we unify (a b) with (c d))
167 -- or we are unifying a lifted type variable with an
168 -- unlifted type: e.g. (id 3#) is illegal
169 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
170 unifyKindMisMatch k1 k2
172 (k1,k2) | swapped = (tk2,tk1)
173 | otherwise = (tk1,tk2)
178 checkTauTvUpdate :: TcTyVar -> TcType -> TcM (Maybe TcType)
179 -- (checkTauTvUpdate tv ty)
180 -- We are about to update the TauTv tv with ty.
181 -- Check (a) that tv doesn't occur in ty (occurs check)
182 -- (b) that ty is a monotype
183 -- Furthermore, in the interest of (b), if you find an
184 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
186 -- We have three possible outcomes:
187 -- (1) Return the (non-boxy) type to update the type variable with,
188 -- [we know the update is ok!]
189 -- (2) return Nothing, or
190 -- [we cannot tell whether the update is ok right now]
192 -- [the update is definitely invalid]
193 -- We return Nothing in case the tv occurs in ty *under* a type family
194 -- application. In this case, we must not update tv (to avoid a cyclic type
195 -- term), but we also cannot fail claiming an infinite type. Given
197 -- type instance F Int = Int
200 -- This is perfectly reasonable, if we later get a ~ Int.
202 checkTauTvUpdate orig_tv orig_ty
203 = do { result <- go orig_ty
205 Right ty -> return $ Just ty
206 Left True -> return $ Nothing
207 Left False -> occurCheckErr (mkTyVarTy orig_tv) orig_ty
210 go :: TcType -> TcM (Either Bool TcType)
212 -- Right ty if everything is fine
213 -- Left True if orig_tv occurs in orig_ty, but under a type family app
214 -- Left False if orig_tv occurs in orig_ty (with no type family app)
215 -- It fails if it encounters a forall type, except as an argument for a
216 -- closed type synonym that expands to a tau type.
218 | isSynTyCon tc = go_syn tc tys
219 | otherwise = do { tys' <- mapM go tys
220 ; return $ occurs (TyConApp tc) tys' }
221 go (PredTy p) = do { p' <- go_pred p
222 ; return $ occurs1 PredTy p' }
223 go (FunTy arg res) = do { arg' <- go arg
225 ; return $ occurs2 FunTy arg' res' }
226 go (AppTy fun arg) = do { fun' <- go fun
228 ; return $ occurs2 mkAppTy fun' arg' }
229 -- NB the mkAppTy; we might have instantiated a
230 -- type variable to a type constructor, so we need
231 -- to pull the TyConApp to the top.
232 go (ForAllTy _ _) = notMonoType orig_ty -- (b)
235 | orig_tv == tv = return $ Left False -- (a)
236 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
237 | otherwise = return $ Right (TyVarTy tv)
238 -- Ordinary (non Tc) tyvars
239 -- occur inside quantified types
241 go_pred (ClassP c tys) = do { tys' <- mapM go tys
242 ; return $ occurs (ClassP c) tys' }
243 go_pred (IParam n ty) = do { ty' <- go ty
244 ; return $ occurs1 (IParam n) ty' }
245 go_pred (EqPred t1 t2) = do { t1' <- go t1
247 ; return $ occurs2 EqPred t1' t2' }
249 go_tyvar tv (SkolemTv _) = return $ Right (TyVarTy tv)
250 go_tyvar tv (MetaTv box ref)
251 = do { cts <- readMutVar ref
255 BoxTv -> do { ty <- fillBoxWithTau tv ref
256 ; return $ Right ty }
257 _ -> return $ Right (TyVarTy tv)
260 -- go_syn is called for synonyms only
261 -- See Note [Type synonyms and the occur check]
263 | not (isTauTyCon tc)
264 = notMonoType orig_ty -- (b) again
266 = do { (_msgs, mb_tys') <- tryTc (mapM go tys)
269 -- we had a type error => forall in type parameters
271 | isOpenTyCon tc -> notMonoArgs (TyConApp tc tys)
272 -- Synonym families must have monotype args
273 | otherwise -> go (expectJust "checkTauTvUpdate(1)"
274 (tcView (TyConApp tc tys)))
275 -- Try again, expanding the synonym
277 -- no type error, but need to test whether occurs check happend
279 case occurs id tys' of
281 | isOpenTyCon tc -> return $ Left True
282 -- Variable occured under type family application
283 | otherwise -> go (expectJust "checkTauTvUpdate(2)"
284 (tcView (TyConApp tc tys)))
285 -- Try again, expanding the synonym
286 Right raw_tys' -> return $ Right (TyConApp tc raw_tys')
287 -- Retain the synonym (the common case)
290 -- Left results (= occurrence of orig_ty) dominate and
291 -- (Left False) (= fatal occurrence) dominates over (Left True)
292 occurs :: ([a] -> b) -> [Either Bool a] -> Either Bool b
293 occurs c = either Left (Right . c) . foldr combine (Right [])
295 combine (Left famInst1) (Left famInst2) = Left (famInst1 && famInst2)
296 combine (Right _ ) (Left famInst) = Left famInst
297 combine (Left famInst) (Right _) = Left famInst
298 combine (Right arg) (Right args) = Right (arg:args)
300 occurs1 c x = occurs (\[x'] -> c x') [x]
301 occurs2 c x y = occurs (\[x', y'] -> c x' y') [x, y]
303 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
304 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
305 -- tau-type meta-variable, whose print-name is the same as tv
306 -- Choosing the same name is good: when we instantiate a function
307 -- we allocate boxy tyvars with the same print-name as the quantified
308 -- tyvar; and then we often fill the box with a tau-tyvar, and again
309 -- we want to choose the same name.
310 fillBoxWithTau tv ref
311 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
312 ; let tau = mkTyVarTy tv' -- name of the type variable
313 ; writeMutVar ref (Indirect tau)
317 Note [Type synonyms and the occur check]
319 Basically we want to update tv1 := ps_ty2
320 because ps_ty2 has type-synonym info, which improves later error messages
325 f :: (A a -> a -> ()) -> ()
331 In the application (p x), we try to match "t" with "A t". If we go
332 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
333 an infinite loop later.
334 But we should not reject the program, because A t = ().
335 Rather, we should bind t to () (= non_var_ty2).
339 Error mesages in case of kind mismatch.
342 unifyKindMisMatch :: TcKind -> TcKind -> TcM ()
343 unifyKindMisMatch ty1 ty2 = do
344 ty1' <- zonkTcKind ty1
345 ty2' <- zonkTcKind ty2
347 msg = hang (ptext (sLit "Couldn't match kind"))
348 2 (sep [quotes (ppr ty1'),
349 ptext (sLit "against"),
353 unifyKindCtxt :: Bool -> TyVar -> Type -> TidyEnv -> TcM (TidyEnv, SDoc)
354 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
355 -- tv1 and ty2 are zonked already
358 msg = (env2, ptext (sLit "When matching the kinds of") <+>
359 sep [quotes pp_expected <+> ptext (sLit "and"), quotes pp_actual])
361 (pp_expected, pp_actual) | swapped = (pp2, pp1)
362 | otherwise = (pp1, pp2)
363 (env1, tv1') = tidyOpenTyVar tidy_env tv1
364 (env2, ty2') = tidyOpenType env1 ty2
365 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
366 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
369 Error message for failure due to an occurs check.
372 occurCheckErr :: TcType -> TcType -> TcM a
373 occurCheckErr ty containingTy
374 = do { env0 <- tcInitTidyEnv
375 ; ty' <- zonkTcType ty
376 ; containingTy' <- zonkTcType containingTy
377 ; let (env1, tidy_ty1) = tidyOpenType env0 ty'
378 (env2, tidy_ty2) = tidyOpenType env1 containingTy'
379 extra = sep [ppr tidy_ty1, char '=', ppr tidy_ty2]
380 ; failWithTcM (env2, hang msg 2 extra) }
382 msg = ptext (sLit "Occurs check: cannot construct the infinite type:")
385 %************************************************************************
389 %************************************************************************
392 newCoVars :: [(TcType,TcType)] -> TcM [CoVar]
394 = do { us <- newUniqueSupply
395 ; return [ mkCoVar (mkSysTvName uniq (fsLit "co"))
397 | ((ty1,ty2), uniq) <- spec `zip` uniqsFromSupply us] }
399 newMetaCoVar :: TcType -> TcType -> TcM TcTyVar
400 newMetaCoVar ty1 ty2 = newMetaTyVar TauTv (mkCoKind ty1 ty2)
402 newKindVar :: TcM TcKind
403 newKindVar = do { uniq <- newUnique
404 ; ref <- newMutVar Flexi
405 ; return (mkTyVarTy (mkKindVar uniq ref)) }
407 newKindVars :: Int -> TcM [TcKind]
408 newKindVars n = mapM (\ _ -> newKindVar) (nOfThem n ())
412 %************************************************************************
414 SkolemTvs (immutable)
416 %************************************************************************
419 mkSkolTyVar :: Name -> Kind -> SkolemInfo -> TcTyVar
420 mkSkolTyVar name kind info = mkTcTyVar name kind (SkolemTv info)
422 tcSkolSigType :: SkolemInfo -> Type -> TcM ([TcTyVar], TcThetaType, TcType)
423 -- Instantiate a type signature with skolem constants, but
424 -- do *not* give them fresh names, because we want the name to
425 -- be in the type environment -- it is lexically scoped.
426 tcSkolSigType info ty = tcInstType (\tvs -> return (tcSkolSigTyVars info tvs)) ty
428 tcSkolSigTyVars :: SkolemInfo -> [TyVar] -> [TcTyVar]
429 -- Make skolem constants, but do *not* give them new names, as above
430 tcSkolSigTyVars info tyvars = [ mkSkolTyVar (tyVarName tv) (tyVarKind tv) info
433 tcInstSkolTyVar :: SkolemInfo -> (Name -> SrcSpan) -> TyVar -> TcM TcTyVar
434 -- Instantiate the tyvar, using
435 -- * the occ-name and kind of the supplied tyvar,
436 -- * the unique from the monad,
437 -- * the location either from the tyvar (mb_loc = Nothing)
438 -- or from mb_loc (Just loc)
439 tcInstSkolTyVar info get_loc tyvar
440 = do { uniq <- newUnique
441 ; let old_name = tyVarName tyvar
442 kind = tyVarKind tyvar
443 loc = get_loc old_name
444 new_name = mkInternalName uniq (nameOccName old_name) loc
445 ; return (mkSkolTyVar new_name kind info) }
447 tcInstSkolTyVars :: SkolemInfo -> [TyVar] -> TcM [TcTyVar]
448 -- Get the location from the monad
449 tcInstSkolTyVars info tyvars
450 = do { span <- getSrcSpanM
451 ; mapM (tcInstSkolTyVar info (const span)) tyvars }
453 tcInstSkolType :: SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcType)
454 -- Instantiate a type with fresh skolem constants
455 -- Binding location comes from the monad
456 tcInstSkolType info ty = tcInstType (tcInstSkolTyVars info) ty
458 tcInstSigType :: Bool -> SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcRhoType)
459 -- Instantiate with skolems or meta SigTvs; depending on use_skols
460 -- Always take location info from the supplied tyvars
461 tcInstSigType use_skols skol_info ty
462 = tcInstType (mapM inst_tyvar) ty
464 inst_tyvar | use_skols = tcInstSkolTyVar skol_info getSrcSpan
465 | otherwise = instMetaTyVar (SigTv skol_info)
469 %************************************************************************
471 MetaTvs (meta type variables; mutable)
473 %************************************************************************
476 newMetaTyVar :: BoxInfo -> Kind -> TcM TcTyVar
477 -- Make a new meta tyvar out of thin air
478 newMetaTyVar box_info kind
479 = do { uniq <- newUnique
480 ; ref <- newMutVar Flexi
481 ; let name = mkSysTvName uniq fs
482 fs = case box_info of
486 -- We give BoxTv and TauTv the same string, because
487 -- otherwise we get user-visible differences in error
488 -- messages, which are confusing. If you want to see
489 -- the box_info of each tyvar, use -dppr-debug
490 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
492 instMetaTyVar :: BoxInfo -> TyVar -> TcM TcTyVar
493 -- Make a new meta tyvar whose Name and Kind
494 -- come from an existing TyVar
495 instMetaTyVar box_info tyvar
496 = do { uniq <- newUnique
497 ; ref <- newMutVar Flexi
498 ; let name = setNameUnique (tyVarName tyvar) uniq
499 kind = tyVarKind tyvar
500 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
502 readMetaTyVar :: TyVar -> TcM MetaDetails
503 readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
504 readMutVar (metaTvRef tyvar)
506 isFilledMetaTyVar :: TyVar -> TcM Bool
507 -- True of a filled-in (Indirect) meta type variable
509 | not (isTcTyVar tv) = return False
510 | MetaTv _ ref <- tcTyVarDetails tv
511 = do { details <- readMutVar ref
512 ; return (isIndirect details) }
513 | otherwise = return False
515 writeMetaTyVar :: TcTyVar -> TcType -> TcM ()
516 writeMetaTyVar tyvar ty
517 | not debugIsOn = writeMutVar (metaTvRef tyvar) (Indirect ty)
518 writeMetaTyVar tyvar ty
519 | not (isMetaTyVar tyvar)
520 = pprTrace "writeMetaTyVar" (ppr tyvar) $
523 = ASSERT( isMetaTyVar tyvar )
524 -- TOM: It should also work for coercions
525 -- ASSERT2( k2 `isSubKind` k1, (ppr tyvar <+> ppr k1) $$ (ppr ty <+> ppr k2) )
526 do { ASSERTM2( do { details <- readMetaTyVar tyvar; return (isFlexi details) }, ppr tyvar )
527 ; writeMutVar (metaTvRef tyvar) (Indirect ty) }
529 _k1 = tyVarKind tyvar
534 %************************************************************************
538 %************************************************************************
541 newFlexiTyVar :: Kind -> TcM TcTyVar
542 newFlexiTyVar kind = newMetaTyVar TauTv kind
544 newFlexiTyVarTy :: Kind -> TcM TcType
545 newFlexiTyVarTy kind = do
546 tc_tyvar <- newFlexiTyVar kind
547 return (TyVarTy tc_tyvar)
549 newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
550 newFlexiTyVarTys n kind = mapM newFlexiTyVarTy (nOfThem n kind)
552 tcInstTyVar :: TyVar -> TcM TcTyVar
553 -- Instantiate with a META type variable
554 tcInstTyVar tyvar = instMetaTyVar TauTv tyvar
556 tcInstTyVars :: [TyVar] -> TcM ([TcTyVar], [TcType], TvSubst)
557 -- Instantiate with META type variables
559 = do { tc_tvs <- mapM tcInstTyVar tyvars
560 ; let tys = mkTyVarTys tc_tvs
561 ; return (tc_tvs, tys, zipTopTvSubst tyvars tys) }
562 -- Since the tyvars are freshly made,
563 -- they cannot possibly be captured by
564 -- any existing for-alls. Hence zipTopTvSubst
568 %************************************************************************
572 %************************************************************************
575 zonkSigTyVar :: TcTyVar -> TcM TcTyVar
577 | isSkolemTyVar sig_tv
578 = return sig_tv -- Happens in the call in TcBinds.checkDistinctTyVars
580 = ASSERT( isSigTyVar sig_tv )
581 do { ty <- zonkTcTyVar sig_tv
582 ; return (tcGetTyVar "zonkSigTyVar" ty) }
583 -- 'ty' is bound to be a type variable, because SigTvs
584 -- can only be unified with type variables
588 %************************************************************************
592 %************************************************************************
595 newBoxyTyVar :: Kind -> TcM BoxyTyVar
596 newBoxyTyVar kind = newMetaTyVar BoxTv kind
598 newBoxyTyVars :: [Kind] -> TcM [BoxyTyVar]
599 newBoxyTyVars kinds = mapM newBoxyTyVar kinds
601 newBoxyTyVarTys :: [Kind] -> TcM [BoxyType]
602 newBoxyTyVarTys kinds = do { tvs <- mapM newBoxyTyVar kinds; return (mkTyVarTys tvs) }
604 readFilledBox :: BoxyTyVar -> TcM TcType
605 -- Read the contents of the box, which should be filled in by now
606 readFilledBox box_tv = ASSERT( isBoxyTyVar box_tv )
607 do { cts <- readMetaTyVar box_tv
609 Flexi -> pprPanic "readFilledBox" (ppr box_tv)
610 Indirect ty -> return ty }
612 tcInstBoxyTyVar :: TyVar -> TcM BoxyTyVar
613 -- Instantiate with a BOXY type variable
614 tcInstBoxyTyVar tyvar = instMetaTyVar BoxTv tyvar
618 %************************************************************************
620 \subsection{Putting and getting mutable type variables}
622 %************************************************************************
624 But it's more fun to short out indirections on the way: If this
625 version returns a TyVar, then that TyVar is unbound. If it returns
626 any other type, then there might be bound TyVars embedded inside it.
628 We return Nothing iff the original box was unbound.
631 data LookupTyVarResult -- The result of a lookupTcTyVar call
632 = DoneTv TcTyVarDetails -- SkolemTv or virgin MetaTv
635 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
637 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
639 SkolemTv _ -> return (DoneTv details)
640 MetaTv _ ref -> do { meta_details <- readMutVar ref
641 ; case meta_details of
642 Indirect ty -> return (IndirectTv ty)
643 Flexi -> return (DoneTv details) }
645 details = tcTyVarDetails tyvar
648 -- gaw 2004 We aren't shorting anything out anymore, at least for now
650 | not (isTcTyVar tyvar)
651 = pprTrace "getTcTyVar" (ppr tyvar) $
652 return (Just (mkTyVarTy tyvar))
655 = ASSERT2( isTcTyVar tyvar, ppr tyvar ) do
656 maybe_ty <- readMetaTyVar tyvar
658 Just ty -> do ty' <- short_out ty
659 writeMetaTyVar tyvar (Just ty')
662 Nothing -> return Nothing
664 short_out :: TcType -> TcM TcType
665 short_out ty@(TyVarTy tyvar)
666 | not (isTcTyVar tyvar)
670 maybe_ty <- readMetaTyVar tyvar
672 Just ty' -> do ty' <- short_out ty'
673 writeMetaTyVar tyvar (Just ty')
678 short_out other_ty = return other_ty
683 %************************************************************************
685 \subsection{Zonking -- the exernal interfaces}
687 %************************************************************************
689 ----------------- Type variables
692 zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
693 zonkTcTyVars tyvars = mapM zonkTcTyVar tyvars
695 zonkTcTyVarsAndFV :: [TcTyVar] -> TcM TcTyVarSet
696 zonkTcTyVarsAndFV tyvars = tyVarsOfTypes <$> mapM zonkTcTyVar tyvars
698 zonkTcTyVar :: TcTyVar -> TcM TcType
699 zonkTcTyVar tyvar = ASSERT2( isTcTyVar tyvar, ppr tyvar)
700 zonk_tc_tyvar (\ tv -> return (TyVarTy tv)) tyvar
703 ----------------- Types
706 zonkTcType :: TcType -> TcM TcType
707 zonkTcType ty = zonkType (\ tv -> return (TyVarTy tv)) ty
709 zonkTcTypes :: [TcType] -> TcM [TcType]
710 zonkTcTypes tys = mapM zonkTcType tys
712 zonkTcThetaType :: TcThetaType -> TcM TcThetaType
713 zonkTcThetaType theta = mapM zonkTcPredType theta
715 zonkTcPredType :: TcPredType -> TcM TcPredType
716 zonkTcPredType (ClassP c ts) = ClassP c <$> zonkTcTypes ts
717 zonkTcPredType (IParam n t) = IParam n <$> zonkTcType t
718 zonkTcPredType (EqPred t1 t2) = EqPred <$> zonkTcType t1 <*> zonkTcType t2
721 ------------------- These ...ToType, ...ToKind versions
722 are used at the end of type checking
725 zonkTopTyVar :: TcTyVar -> TcM TcTyVar
726 -- zonkTopTyVar is used, at the top level, on any un-instantiated meta type variables
727 -- to default the kind of ? and ?? etc to *. This is important to ensure that
728 -- instance declarations match. For example consider
729 -- instance Show (a->b)
730 -- foo x = show (\_ -> True)
731 -- Then we'll get a constraint (Show (p ->q)) where p has argTypeKind (printed ??),
732 -- and that won't match the typeKind (*) in the instance decl.
734 -- Because we are at top level, no further constraints are going to affect these
735 -- type variables, so it's time to do it by hand. However we aren't ready
736 -- to default them fully to () or whatever, because the type-class defaulting
737 -- rules have yet to run.
740 | k `eqKind` default_k = return tv
742 = do { tv' <- newFlexiTyVar default_k
743 ; writeMetaTyVar tv (mkTyVarTy tv')
747 default_k = defaultKind k
749 zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TcTyVar]
750 zonkQuantifiedTyVars = mapM zonkQuantifiedTyVar
752 zonkQuantifiedTyVar :: TcTyVar -> TcM TcTyVar
753 -- zonkQuantifiedTyVar is applied to the a TcTyVar when quantifying over it.
755 -- The quantified type variables often include meta type variables
756 -- we want to freeze them into ordinary type variables, and
757 -- default their kind (e.g. from OpenTypeKind to TypeKind)
758 -- -- see notes with Kind.defaultKind
759 -- The meta tyvar is updated to point to the new skolem TyVar. Now any
760 -- bound occurences of the original type variable will get zonked to
761 -- the immutable version.
763 -- We leave skolem TyVars alone; they are immutable.
764 zonkQuantifiedTyVar tv
765 | ASSERT( isTcTyVar tv )
766 isSkolemTyVar tv = return tv
767 -- It might be a skolem type variable,
768 -- for example from a user type signature
770 | otherwise -- It's a meta-type-variable
771 = do { details <- readMetaTyVar tv
773 -- Create the new, frozen, skolem type variable
774 -- We zonk to a skolem, not to a regular TcVar
775 -- See Note [Zonking to Skolem]
776 ; let final_kind = defaultKind (tyVarKind tv)
777 final_tv = mkSkolTyVar (tyVarName tv) final_kind UnkSkol
779 -- Bind the meta tyvar to the new tyvar
781 Indirect ty -> WARN( True, ppr tv $$ ppr ty )
783 -- [Sept 04] I don't think this should happen
784 -- See note [Silly Type Synonym]
786 Flexi -> writeMetaTyVar tv (mkTyVarTy final_tv)
788 -- Return the new tyvar
792 Note [Silly Type Synonyms]
793 ~~~~~~~~~~~~~~~~~~~~~~~~~~
795 type C u a = u -- Note 'a' unused
797 foo :: (forall a. C u a -> C u a) -> u
801 bar = foo (\t -> t + t)
803 * From the (\t -> t+t) we get type {Num d} => d -> d
806 * Now unify with type of foo's arg, and we get:
807 {Num (C d a)} => C d a -> C d a
810 * Now abstract over the 'a', but float out the Num (C d a) constraint
811 because it does not 'really' mention a. (see exactTyVarsOfType)
812 The arg to foo becomes
815 * So we get a dict binding for Num (C d a), which is zonked to give
817 [Note Sept 04: now that we are zonking quantified type variables
818 on construction, the 'a' will be frozen as a regular tyvar on
819 quantification, so the floated dict will still have type (C d a).
820 Which renders this whole note moot; happily!]
822 * Then the \/\a abstraction has a zonked 'a' in it.
824 All very silly. I think its harmless to ignore the problem. We'll end up with
825 a \/\a in the final result but all the occurrences of a will be zonked to ()
827 Note [Zonking to Skolem]
828 ~~~~~~~~~~~~~~~~~~~~~~~~
829 We used to zonk quantified type variables to regular TyVars. However, this
830 leads to problems. Consider this program from the regression test suite:
832 eval :: Int -> String -> String -> String
833 eval 0 root actual = evalRHS 0 root actual
836 evalRHS 0 root actual = eval 0 root actual
838 It leads to the deferral of an equality
840 (String -> String -> String) ~ a
842 which is propagated up to the toplevel (see TcSimplify.tcSimplifyInferCheck).
843 In the meantime `a' is zonked and quantified to form `evalRHS's signature.
844 This has the *side effect* of also zonking the `a' in the deferred equality
845 (which at this point is being handed around wrapped in an implication
848 Finally, the equality (with the zonked `a') will be handed back to the
849 simplifier by TcRnDriver.tcRnSrcDecls calling TcSimplify.tcSimplifyTop.
850 If we zonk `a' with a regular type variable, we will have this regular type
851 variable now floating around in the simplifier, which in many places assumes to
852 only see proper TcTyVars.
854 We can avoid this problem by zonking with a skolem. The skolem is rigid
855 (which we requirefor a quantified variable), but is still a TcTyVar that the
856 simplifier knows how to deal with.
859 %************************************************************************
861 \subsection{Zonking -- the main work-horses: zonkType, zonkTyVar}
863 %* For internal use only! *
865 %************************************************************************
868 -- For unbound, mutable tyvars, zonkType uses the function given to it
869 -- For tyvars bound at a for-all, zonkType zonks them to an immutable
870 -- type variable and zonks the kind too
872 zonkType :: (TcTyVar -> TcM Type) -- What to do with unbound mutable type variables
873 -- see zonkTcType, and zonkTcTypeToType
876 zonkType unbound_var_fn ty
879 go (TyConApp tc tys) = do tys' <- mapM go tys
880 return (TyConApp tc tys')
882 go (PredTy p) = do p' <- go_pred p
885 go (FunTy arg res) = do arg' <- go arg
887 return (FunTy arg' res')
889 go (AppTy fun arg) = do fun' <- go fun
891 return (mkAppTy fun' arg')
892 -- NB the mkAppTy; we might have instantiated a
893 -- type variable to a type constructor, so we need
894 -- to pull the TyConApp to the top.
896 -- The two interesting cases!
897 go (TyVarTy tyvar) | isTcTyVar tyvar = zonk_tc_tyvar unbound_var_fn tyvar
898 | otherwise = return (TyVarTy tyvar)
899 -- Ordinary (non Tc) tyvars occur inside quantified types
901 go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar ) do
903 return (ForAllTy tyvar ty')
905 go_pred (ClassP c tys) = do tys' <- mapM go tys
906 return (ClassP c tys')
907 go_pred (IParam n ty) = do ty' <- go ty
908 return (IParam n ty')
909 go_pred (EqPred ty1 ty2) = do ty1' <- go ty1
911 return (EqPred ty1' ty2')
913 zonk_tc_tyvar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable variable
914 -> TcTyVar -> TcM TcType
915 zonk_tc_tyvar unbound_var_fn tyvar
916 | not (isMetaTyVar tyvar) -- Skolems
917 = return (TyVarTy tyvar)
919 | otherwise -- Mutables
920 = do { cts <- readMetaTyVar tyvar
922 Flexi -> unbound_var_fn tyvar -- Unbound meta type variable
923 Indirect ty -> zonkType unbound_var_fn ty }
928 %************************************************************************
932 %************************************************************************
935 readKindVar :: KindVar -> TcM (MetaDetails)
936 writeKindVar :: KindVar -> TcKind -> TcM ()
937 readKindVar kv = readMutVar (kindVarRef kv)
938 writeKindVar kv val = writeMutVar (kindVarRef kv) (Indirect val)
941 zonkTcKind :: TcKind -> TcM TcKind
942 zonkTcKind k = zonkTcType k
945 zonkTcKindToKind :: TcKind -> TcM Kind
946 -- When zonking a TcKind to a kind, we need to instantiate kind variables,
947 -- Haskell specifies that * is to be used, so we follow that.
948 zonkTcKindToKind k = zonkType (\ _ -> return liftedTypeKind) k
951 %************************************************************************
953 \subsection{Checking a user type}
955 %************************************************************************
957 When dealing with a user-written type, we first translate it from an HsType
958 to a Type, performing kind checking, and then check various things that should
959 be true about it. We don't want to perform these checks at the same time
960 as the initial translation because (a) they are unnecessary for interface-file
961 types and (b) when checking a mutually recursive group of type and class decls,
962 we can't "look" at the tycons/classes yet. Also, the checks are are rather
963 diverse, and used to really mess up the other code.
965 One thing we check for is 'rank'.
967 Rank 0: monotypes (no foralls)
968 Rank 1: foralls at the front only, Rank 0 inside
969 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
971 basic ::= tyvar | T basic ... basic
973 r2 ::= forall tvs. cxt => r2a
974 r2a ::= r1 -> r2a | basic
975 r1 ::= forall tvs. cxt => r0
976 r0 ::= r0 -> r0 | basic
978 Another thing is to check that type synonyms are saturated.
979 This might not necessarily show up in kind checking.
981 data T k = MkT (k Int)
986 checkValidType :: UserTypeCtxt -> Type -> TcM ()
987 -- Checks that the type is valid for the given context
988 checkValidType ctxt ty = do
989 traceTc (text "checkValidType" <+> ppr ty)
990 unboxed <- doptM Opt_UnboxedTuples
991 rank2 <- doptM Opt_Rank2Types
992 rankn <- doptM Opt_RankNTypes
993 polycomp <- doptM Opt_PolymorphicComponents
995 gen_rank n | rankn = ArbitraryRank
1000 GenPatCtxt -> MustBeMonoType
1001 DefaultDeclCtxt-> MustBeMonoType
1002 ResSigCtxt -> MustBeMonoType
1003 LamPatSigCtxt -> gen_rank 0
1004 BindPatSigCtxt -> gen_rank 0
1005 TySynCtxt _ -> gen_rank 0
1006 ExprSigCtxt -> gen_rank 1
1007 FunSigCtxt _ -> gen_rank 1
1008 ConArgCtxt _ | polycomp -> gen_rank 2
1009 -- We are given the type of the entire
1010 -- constructor, hence rank 1
1011 | otherwise -> gen_rank 1
1012 ForSigCtxt _ -> gen_rank 1
1013 SpecInstCtxt -> gen_rank 1
1015 actual_kind = typeKind ty
1017 kind_ok = case ctxt of
1018 TySynCtxt _ -> True -- Any kind will do
1019 ResSigCtxt -> isSubOpenTypeKind actual_kind
1020 ExprSigCtxt -> isSubOpenTypeKind actual_kind
1021 GenPatCtxt -> isLiftedTypeKind actual_kind
1022 ForSigCtxt _ -> isLiftedTypeKind actual_kind
1023 _ -> isSubArgTypeKind actual_kind
1025 ubx_tup = case ctxt of
1026 TySynCtxt _ | unboxed -> UT_Ok
1027 ExprSigCtxt | unboxed -> UT_Ok
1030 -- Check that the thing has kind Type, and is lifted if necessary
1031 checkTc kind_ok (kindErr actual_kind)
1033 -- Check the internal validity of the type itself
1034 check_type rank ubx_tup ty
1036 traceTc (text "checkValidType done" <+> ppr ty)
1038 checkValidMonoType :: Type -> TcM ()
1039 checkValidMonoType ty = check_mono_type MustBeMonoType ty
1044 data Rank = ArbitraryRank -- Any rank ok
1045 | MustBeMonoType -- Monotype regardless of flags
1046 | TyConArgMonoType -- Monotype but could be poly if -XImpredicativeTypes
1047 | Rank Int -- Rank n, but could be more with -XRankNTypes
1049 decRank :: Rank -> Rank -- Function arguments
1050 decRank (Rank 0) = Rank 0
1051 decRank (Rank n) = Rank (n-1)
1052 decRank other_rank = other_rank
1054 nonZeroRank :: Rank -> Bool
1055 nonZeroRank ArbitraryRank = True
1056 nonZeroRank (Rank n) = n>0
1057 nonZeroRank _ = False
1059 ----------------------------------------
1060 data UbxTupFlag = UT_Ok | UT_NotOk
1061 -- The "Ok" version means "ok if UnboxedTuples is on"
1063 ----------------------------------------
1064 check_mono_type :: Rank -> Type -> TcM () -- No foralls anywhere
1065 -- No unlifted types of any kind
1066 check_mono_type rank ty
1067 = do { check_type rank UT_NotOk ty
1068 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1070 check_type :: Rank -> UbxTupFlag -> Type -> TcM ()
1071 -- The args say what the *type context* requires, independent
1072 -- of *flag* settings. You test the flag settings at usage sites.
1074 -- Rank is allowed rank for function args
1075 -- Rank 0 means no for-alls anywhere
1077 check_type rank ubx_tup ty
1078 | not (null tvs && null theta)
1079 = do { checkTc (nonZeroRank rank) (forAllTyErr rank ty)
1080 -- Reject e.g. (Maybe (?x::Int => Int)),
1081 -- with a decent error message
1082 ; check_valid_theta SigmaCtxt theta
1083 ; check_type rank ubx_tup tau -- Allow foralls to right of arrow
1084 ; checkFreeness tvs theta
1085 ; checkAmbiguity tvs theta (tyVarsOfType tau) }
1087 (tvs, theta, tau) = tcSplitSigmaTy ty
1089 -- Naked PredTys don't usually show up, but they can as a result of
1090 -- {-# SPECIALISE instance Ord Char #-}
1091 -- The Right Thing would be to fix the way that SPECIALISE instance pragmas
1092 -- are handled, but the quick thing is just to permit PredTys here.
1093 check_type _ _ (PredTy sty)
1094 = do { dflags <- getDOpts
1095 ; check_pred_ty dflags TypeCtxt sty }
1097 check_type _ _ (TyVarTy _) = return ()
1098 check_type rank _ (FunTy arg_ty res_ty)
1099 = do { check_type (decRank rank) UT_NotOk arg_ty
1100 ; check_type rank UT_Ok res_ty }
1102 check_type rank _ (AppTy ty1 ty2)
1103 = do { check_arg_type rank ty1
1104 ; check_arg_type rank ty2 }
1106 check_type rank ubx_tup ty@(TyConApp tc tys)
1108 = do { -- Check that the synonym has enough args
1109 -- This applies equally to open and closed synonyms
1110 -- It's OK to have an *over-applied* type synonym
1111 -- data Tree a b = ...
1112 -- type Foo a = Tree [a]
1113 -- f :: Foo a b -> ...
1114 checkTc (tyConArity tc <= length tys) arity_msg
1116 -- See Note [Liberal type synonyms]
1117 ; liberal <- doptM Opt_LiberalTypeSynonyms
1118 ; if not liberal || isOpenSynTyCon tc then
1119 -- For H98 and synonym families, do check the type args
1120 mapM_ (check_mono_type TyConArgMonoType) tys
1122 else -- In the liberal case (only for closed syns), expand then check
1124 Just ty' -> check_type rank ubx_tup ty'
1125 Nothing -> pprPanic "check_tau_type" (ppr ty)
1128 | isUnboxedTupleTyCon tc
1129 = do { ub_tuples_allowed <- doptM Opt_UnboxedTuples
1130 ; checkTc (ubx_tup_ok ub_tuples_allowed) ubx_tup_msg
1132 ; impred <- doptM Opt_ImpredicativeTypes
1133 ; let rank' = if impred then ArbitraryRank else TyConArgMonoType
1134 -- c.f. check_arg_type
1135 -- However, args are allowed to be unlifted, or
1136 -- more unboxed tuples, so can't use check_arg_ty
1137 ; mapM_ (check_type rank' UT_Ok) tys }
1140 = mapM_ (check_arg_type rank) tys
1143 ubx_tup_ok ub_tuples_allowed = case ubx_tup of
1144 UT_Ok -> ub_tuples_allowed
1148 tc_arity = tyConArity tc
1150 arity_msg = arityErr "Type synonym" (tyConName tc) tc_arity n_args
1151 ubx_tup_msg = ubxArgTyErr ty
1153 check_type _ _ ty = pprPanic "check_type" (ppr ty)
1155 ----------------------------------------
1156 check_arg_type :: Rank -> Type -> TcM ()
1157 -- The sort of type that can instantiate a type variable,
1158 -- or be the argument of a type constructor.
1159 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
1160 -- Other unboxed types are very occasionally allowed as type
1161 -- arguments depending on the kind of the type constructor
1163 -- For example, we want to reject things like:
1165 -- instance Ord a => Ord (forall s. T s a)
1167 -- g :: T s (forall b.b)
1169 -- NB: unboxed tuples can have polymorphic or unboxed args.
1170 -- This happens in the workers for functions returning
1171 -- product types with polymorphic components.
1172 -- But not in user code.
1173 -- Anyway, they are dealt with by a special case in check_tau_type
1175 check_arg_type rank ty
1176 = do { impred <- doptM Opt_ImpredicativeTypes
1177 ; let rank' = if impred then ArbitraryRank -- Arg of tycon can have arby rank, regardless
1178 else case rank of -- Predictive => must be monotype
1179 MustBeMonoType -> MustBeMonoType
1180 _ -> TyConArgMonoType
1181 -- Make sure that MustBeMonoType is propagated,
1182 -- so that we don't suggest -XImpredicativeTypes in
1183 -- (Ord (forall a.a)) => a -> a
1185 ; check_type rank' UT_NotOk ty
1186 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1188 ----------------------------------------
1189 forAllTyErr :: Rank -> Type -> SDoc
1191 = vcat [ hang (ptext (sLit "Illegal polymorphic or qualified type:")) 2 (ppr ty)
1194 suggestion = case rank of
1195 Rank _ -> ptext (sLit "Perhaps you intended to use -XRankNTypes or -XRank2Types")
1196 TyConArgMonoType -> ptext (sLit "Perhaps you intended to use -XImpredicativeTypes")
1197 _ -> empty -- Polytype is always illegal
1199 unliftedArgErr, ubxArgTyErr :: Type -> SDoc
1200 unliftedArgErr ty = sep [ptext (sLit "Illegal unlifted type:"), ppr ty]
1201 ubxArgTyErr ty = sep [ptext (sLit "Illegal unboxed tuple type as function argument:"), ppr ty]
1203 kindErr :: Kind -> SDoc
1204 kindErr kind = sep [ptext (sLit "Expecting an ordinary type, but found a type of kind"), ppr kind]
1207 Note [Liberal type synonyms]
1208 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1209 If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
1210 doing validity checking. This allows us to instantiate a synonym defn
1211 with a for-all type, or with a partially-applied type synonym.
1215 Here, T is partially applied, so it's illegal in H98. But if you
1216 expand S first, then T we get just
1220 IMPORTANT: suppose T is a type synonym. Then we must do validity
1221 checking on an appliation (T ty1 ty2)
1223 *either* before expansion (i.e. check ty1, ty2)
1224 *or* after expansion (i.e. expand T ty1 ty2, and then check)
1227 If we do both, we get exponential behaviour!!
1229 data TIACons1 i r c = c i ::: r c
1230 type TIACons2 t x = TIACons1 t (TIACons1 t x)
1231 type TIACons3 t x = TIACons2 t (TIACons1 t x)
1232 type TIACons4 t x = TIACons2 t (TIACons2 t x)
1233 type TIACons7 t x = TIACons4 t (TIACons3 t x)
1236 %************************************************************************
1238 \subsection{Checking a theta or source type}
1240 %************************************************************************
1243 -- Enumerate the contexts in which a "source type", <S>, can occur
1247 -- or (N a) where N is a newtype
1250 = ClassSCCtxt Name -- Superclasses of clas
1251 -- class <S> => C a where ...
1252 | SigmaCtxt -- Theta part of a normal for-all type
1253 -- f :: <S> => a -> a
1254 | DataTyCtxt Name -- Theta part of a data decl
1255 -- data <S> => T a = MkT a
1256 | TypeCtxt -- Source type in an ordinary type
1258 | InstThetaCtxt -- Context of an instance decl
1259 -- instance <S> => C [a] where ...
1261 pprSourceTyCtxt :: SourceTyCtxt -> SDoc
1262 pprSourceTyCtxt (ClassSCCtxt c) = ptext (sLit "the super-classes of class") <+> quotes (ppr c)
1263 pprSourceTyCtxt SigmaCtxt = ptext (sLit "the context of a polymorphic type")
1264 pprSourceTyCtxt (DataTyCtxt tc) = ptext (sLit "the context of the data type declaration for") <+> quotes (ppr tc)
1265 pprSourceTyCtxt InstThetaCtxt = ptext (sLit "the context of an instance declaration")
1266 pprSourceTyCtxt TypeCtxt = ptext (sLit "the context of a type")
1270 checkValidTheta :: SourceTyCtxt -> ThetaType -> TcM ()
1271 checkValidTheta ctxt theta
1272 = addErrCtxt (checkThetaCtxt ctxt theta) (check_valid_theta ctxt theta)
1274 -------------------------
1275 check_valid_theta :: SourceTyCtxt -> [PredType] -> TcM ()
1276 check_valid_theta _ []
1278 check_valid_theta ctxt theta = do
1280 warnTc (notNull dups) (dupPredWarn dups)
1281 mapM_ (check_pred_ty dflags ctxt) theta
1283 (_,dups) = removeDups tcCmpPred theta
1285 -------------------------
1286 check_pred_ty :: DynFlags -> SourceTyCtxt -> PredType -> TcM ()
1287 check_pred_ty dflags ctxt pred@(ClassP cls tys)
1288 = do { -- Class predicates are valid in all contexts
1289 ; checkTc (arity == n_tys) arity_err
1291 -- Check the form of the argument types
1292 ; mapM_ checkValidMonoType tys
1293 ; checkTc (check_class_pred_tys dflags ctxt tys)
1294 (predTyVarErr pred $$ how_to_allow)
1297 class_name = className cls
1298 arity = classArity cls
1300 arity_err = arityErr "Class" class_name arity n_tys
1301 how_to_allow = parens (ptext (sLit "Use -XFlexibleContexts to permit this"))
1303 check_pred_ty dflags _ pred@(EqPred ty1 ty2)
1304 = do { -- Equational constraints are valid in all contexts if type
1305 -- families are permitted
1306 ; checkTc (dopt Opt_TypeFamilies dflags) (eqPredTyErr pred)
1308 -- Check the form of the argument types
1309 ; checkValidMonoType ty1
1310 ; checkValidMonoType ty2
1313 check_pred_ty _ SigmaCtxt (IParam _ ty) = checkValidMonoType ty
1314 -- Implicit parameters only allowed in type
1315 -- signatures; not in instance decls, superclasses etc
1316 -- The reason for not allowing implicit params in instances is a bit
1318 -- If we allowed instance (?x::Int, Eq a) => Foo [a] where ...
1319 -- then when we saw (e :: (?x::Int) => t) it would be unclear how to
1320 -- discharge all the potential usas of the ?x in e. For example, a
1321 -- constraint Foo [Int] might come out of e,and applying the
1322 -- instance decl would show up two uses of ?x.
1325 check_pred_ty _ _ sty = failWithTc (badPredTyErr sty)
1327 -------------------------
1328 check_class_pred_tys :: DynFlags -> SourceTyCtxt -> [Type] -> Bool
1329 check_class_pred_tys dflags ctxt tys
1331 TypeCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
1332 InstThetaCtxt -> flexible_contexts || undecidable_ok || all tcIsTyVarTy tys
1333 -- Further checks on head and theta in
1334 -- checkInstTermination
1335 _ -> flexible_contexts || all tyvar_head tys
1337 flexible_contexts = dopt Opt_FlexibleContexts dflags
1338 undecidable_ok = dopt Opt_UndecidableInstances dflags
1340 -------------------------
1341 tyvar_head :: Type -> Bool
1342 tyvar_head ty -- Haskell 98 allows predicates of form
1343 | tcIsTyVarTy ty = True -- C (a ty1 .. tyn)
1344 | otherwise -- where a is a type variable
1345 = case tcSplitAppTy_maybe ty of
1346 Just (ty, _) -> tyvar_head ty
1353 is ambiguous if P contains generic variables
1354 (i.e. one of the Vs) that are not mentioned in tau
1356 However, we need to take account of functional dependencies
1357 when we speak of 'mentioned in tau'. Example:
1358 class C a b | a -> b where ...
1360 forall x y. (C x y) => x
1361 is not ambiguous because x is mentioned and x determines y
1363 NB; the ambiguity check is only used for *user* types, not for types
1364 coming from inteface files. The latter can legitimately have
1365 ambiguous types. Example
1367 class S a where s :: a -> (Int,Int)
1368 instance S Char where s _ = (1,1)
1369 f:: S a => [a] -> Int -> (Int,Int)
1370 f (_::[a]) x = (a*x,b)
1371 where (a,b) = s (undefined::a)
1373 Here the worker for f gets the type
1374 fw :: forall a. S a => Int -> (# Int, Int #)
1376 If the list of tv_names is empty, we have a monotype, and then we
1377 don't need to check for ambiguity either, because the test can't fail
1382 checkAmbiguity :: [TyVar] -> ThetaType -> TyVarSet -> TcM ()
1383 checkAmbiguity forall_tyvars theta tau_tyvars
1384 = mapM_ complain (filter is_ambig theta)
1386 complain pred = addErrTc (ambigErr pred)
1387 extended_tau_vars = grow theta tau_tyvars
1389 -- See Note [Implicit parameters and ambiguity] in TcSimplify
1390 is_ambig pred = isClassPred pred &&
1391 any ambig_var (varSetElems (tyVarsOfPred pred))
1393 ambig_var ct_var = (ct_var `elem` forall_tyvars) &&
1394 not (ct_var `elemVarSet` extended_tau_vars)
1396 ambigErr :: PredType -> SDoc
1398 = sep [ptext (sLit "Ambiguous constraint") <+> quotes (pprPred pred),
1399 nest 4 (ptext (sLit "At least one of the forall'd type variables mentioned by the constraint") $$
1400 ptext (sLit "must be reachable from the type after the '=>'"))]
1403 In addition, GHC insists that at least one type variable
1404 in each constraint is in V. So we disallow a type like
1405 forall a. Eq b => b -> b
1406 even in a scope where b is in scope.
1409 checkFreeness :: [Var] -> [PredType] -> TcM ()
1410 checkFreeness forall_tyvars theta
1411 = do { flexible_contexts <- doptM Opt_FlexibleContexts
1412 ; unless flexible_contexts $ mapM_ complain (filter is_free theta) }
1414 is_free pred = not (isIPPred pred)
1415 && not (any bound_var (varSetElems (tyVarsOfPred pred)))
1416 bound_var ct_var = ct_var `elem` forall_tyvars
1417 complain pred = addErrTc (freeErr pred)
1419 freeErr :: PredType -> SDoc
1421 = sep [ ptext (sLit "All of the type variables in the constraint") <+>
1422 quotes (pprPred pred)
1423 , ptext (sLit "are already in scope") <+>
1424 ptext (sLit "(at least one must be universally quantified here)")
1426 ptext (sLit "(Use -XFlexibleContexts to lift this restriction)")
1431 checkThetaCtxt :: SourceTyCtxt -> ThetaType -> SDoc
1432 checkThetaCtxt ctxt theta
1433 = vcat [ptext (sLit "In the context:") <+> pprTheta theta,
1434 ptext (sLit "While checking") <+> pprSourceTyCtxt ctxt ]
1436 badPredTyErr, eqPredTyErr, predTyVarErr :: PredType -> SDoc
1437 badPredTyErr sty = ptext (sLit "Illegal constraint") <+> pprPred sty
1438 eqPredTyErr sty = ptext (sLit "Illegal equational constraint") <+> pprPred sty
1440 parens (ptext (sLit "Use -XTypeFamilies to permit this"))
1441 predTyVarErr pred = sep [ptext (sLit "Non type-variable argument"),
1442 nest 2 (ptext (sLit "in the constraint:") <+> pprPred pred)]
1443 dupPredWarn :: [[PredType]] -> SDoc
1444 dupPredWarn dups = ptext (sLit "Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
1446 arityErr :: Outputable a => String -> a -> Int -> Int -> SDoc
1447 arityErr kind name n m
1448 = hsep [ text kind, quotes (ppr name), ptext (sLit "should have"),
1449 n_arguments <> comma, text "but has been given", int m]
1451 n_arguments | n == 0 = ptext (sLit "no arguments")
1452 | n == 1 = ptext (sLit "1 argument")
1453 | True = hsep [int n, ptext (sLit "arguments")]
1456 notMonoType :: TcType -> TcM a
1458 = do { ty' <- zonkTcType ty
1459 ; env0 <- tcInitTidyEnv
1460 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1461 msg = ptext (sLit "Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1462 ; failWithTcM (env1, msg) }
1464 notMonoArgs :: TcType -> TcM a
1466 = do { ty' <- zonkTcType ty
1467 ; env0 <- tcInitTidyEnv
1468 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1469 msg = ptext (sLit "Arguments of type synonym families must be monotypes") <+> quotes (ppr tidy_ty)
1470 ; failWithTcM (env1, msg) }
1474 %************************************************************************
1476 \subsection{Checking for a decent instance head type}
1478 %************************************************************************
1480 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1481 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1483 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1484 flag is on, or (2)~the instance is imported (they must have been
1485 compiled elsewhere). In these cases, we let them go through anyway.
1487 We can also have instances for functions: @instance Foo (a -> b) ...@.
1490 checkValidInstHead :: Type -> TcM (Class, [TcType])
1492 checkValidInstHead ty -- Should be a source type
1493 = case tcSplitPredTy_maybe ty of {
1494 Nothing -> failWithTc (instTypeErr (ppr ty) empty) ;
1497 case getClassPredTys_maybe pred of {
1498 Nothing -> failWithTc (instTypeErr (pprPred pred) empty) ;
1499 Just (clas,tys) -> do
1502 check_inst_head dflags clas tys
1506 check_inst_head :: DynFlags -> Class -> [Type] -> TcM ()
1507 check_inst_head dflags clas tys
1508 = do { -- If GlasgowExts then check at least one isn't a type variable
1509 ; checkTc (dopt Opt_TypeSynonymInstances dflags ||
1510 all tcInstHeadTyNotSynonym tys)
1511 (instTypeErr (pprClassPred clas tys) head_type_synonym_msg)
1512 ; checkTc (dopt Opt_FlexibleInstances dflags ||
1513 all tcInstHeadTyAppAllTyVars tys)
1514 (instTypeErr (pprClassPred clas tys) head_type_args_tyvars_msg)
1515 ; checkTc (dopt Opt_MultiParamTypeClasses dflags ||
1517 (instTypeErr (pprClassPred clas tys) head_one_type_msg)
1518 -- May not contain type family applications
1519 ; mapM_ checkTyFamFreeness tys
1521 ; mapM_ checkValidMonoType tys
1522 -- For now, I only allow tau-types (not polytypes) in
1523 -- the head of an instance decl.
1524 -- E.g. instance C (forall a. a->a) is rejected
1525 -- One could imagine generalising that, but I'm not sure
1526 -- what all the consequences might be
1530 head_type_synonym_msg = parens (
1531 text "All instance types must be of the form (T t1 ... tn)" $$
1532 text "where T is not a synonym." $$
1533 text "Use -XTypeSynonymInstances if you want to disable this.")
1535 head_type_args_tyvars_msg = parens (vcat [
1536 text "All instance types must be of the form (T a1 ... an)",
1537 text "where a1 ... an are type *variables*,",
1538 text "and each type variable appears at most once in the instance head.",
1539 text "Use -XFlexibleInstances if you want to disable this."])
1541 head_one_type_msg = parens (
1542 text "Only one type can be given in an instance head." $$
1543 text "Use -XMultiParamTypeClasses if you want to allow more.")
1545 instTypeErr :: SDoc -> SDoc -> SDoc
1546 instTypeErr pp_ty msg
1547 = sep [ptext (sLit "Illegal instance declaration for") <+> quotes pp_ty,
1552 %************************************************************************
1554 \subsection{Checking instance for termination}
1556 %************************************************************************
1560 checkValidInstance :: [TyVar] -> ThetaType -> Class -> [TcType] -> TcM ()
1561 checkValidInstance tyvars theta clas inst_tys
1562 = do { undecidable_ok <- doptM Opt_UndecidableInstances
1564 ; checkValidTheta InstThetaCtxt theta
1565 ; checkAmbiguity tyvars theta (tyVarsOfTypes inst_tys)
1567 -- Check that instance inference will terminate (if we care)
1568 -- For Haskell 98 this will already have been done by checkValidTheta,
1569 -- but as we may be using other extensions we need to check.
1570 ; unless undecidable_ok $
1571 mapM_ addErrTc (checkInstTermination inst_tys theta)
1573 -- The Coverage Condition
1574 ; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
1575 (instTypeErr (pprClassPred clas inst_tys) msg)
1578 msg = parens (vcat [ptext (sLit "the Coverage Condition fails for one of the functional dependencies;"),
1582 Termination test: the so-called "Paterson conditions" (see Section 5 of
1583 "Understanding functionsl dependencies via Constraint Handling Rules,
1586 We check that each assertion in the context satisfies:
1587 (1) no variable has more occurrences in the assertion than in the head, and
1588 (2) the assertion has fewer constructors and variables (taken together
1589 and counting repetitions) than the head.
1590 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1591 (which have already been checked) guarantee termination.
1593 The underlying idea is that
1595 for any ground substitution, each assertion in the
1596 context has fewer type constructors than the head.
1600 checkInstTermination :: [TcType] -> ThetaType -> [Message]
1601 checkInstTermination tys theta
1602 = mapCatMaybes check theta
1605 size = sizeTypes tys
1607 | not (null (fvPred pred \\ fvs))
1608 = Just (predUndecErr pred nomoreMsg $$ parens undecidableMsg)
1609 | sizePred pred >= size
1610 = Just (predUndecErr pred smallerMsg $$ parens undecidableMsg)
1614 predUndecErr :: PredType -> SDoc -> SDoc
1615 predUndecErr pred msg = sep [msg,
1616 nest 2 (ptext (sLit "in the constraint:") <+> pprPred pred)]
1618 nomoreMsg, smallerMsg, undecidableMsg :: SDoc
1619 nomoreMsg = ptext (sLit "Variable occurs more often in a constraint than in the instance head")
1620 smallerMsg = ptext (sLit "Constraint is no smaller than the instance head")
1621 undecidableMsg = ptext (sLit "Use -XUndecidableInstances to permit this")
1625 %************************************************************************
1627 Checking the context of a derived instance declaration
1629 %************************************************************************
1631 Note [Exotic derived instance contexts]
1632 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1633 In a 'derived' instance declaration, we *infer* the context. It's a
1634 bit unclear what rules we should apply for this; the Haskell report is
1635 silent. Obviously, constraints like (Eq a) are fine, but what about
1636 data T f a = MkT (f a) deriving( Eq )
1637 where we'd get an Eq (f a) constraint. That's probably fine too.
1639 One could go further: consider
1640 data T a b c = MkT (Foo a b c) deriving( Eq )
1641 instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
1643 Notice that this instance (just) satisfies the Paterson termination
1644 conditions. Then we *could* derive an instance decl like this:
1646 instance (C Int a, Eq b, Eq c) => Eq (T a b c)
1648 even though there is no instance for (C Int a), because there just
1649 *might* be an instance for, say, (C Int Bool) at a site where we
1650 need the equality instance for T's.
1652 However, this seems pretty exotic, and it's quite tricky to allow
1653 this, and yet give sensible error messages in the (much more common)
1654 case where we really want that instance decl for C.
1656 So for now we simply require that the derived instance context
1657 should have only type-variable constraints.
1659 Here is another example:
1660 data Fix f = In (f (Fix f)) deriving( Eq )
1661 Here, if we are prepared to allow -XUndecidableInstances we
1662 could derive the instance
1663 instance Eq (f (Fix f)) => Eq (Fix f)
1664 but this is so delicate that I don't think it should happen inside
1665 'deriving'. If you want this, write it yourself!
1667 NB: if you want to lift this condition, make sure you still meet the
1668 termination conditions! If not, the deriving mechanism generates
1669 larger and larger constraints. Example:
1671 data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
1673 Note the lack of a Show instance for Succ. First we'll generate
1674 instance (Show (Succ a), Show a) => Show (Seq a)
1676 instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
1677 and so on. Instead we want to complain of no instance for (Show (Succ a)).
1681 Allow constraints which consist only of type variables, with no repeats.
1684 validDerivPred :: PredType -> Bool
1685 validDerivPred (ClassP _ tys) = hasNoDups fvs && sizeTypes tys == length fvs
1686 where fvs = fvTypes tys
1687 validDerivPred _ = False
1690 %************************************************************************
1692 Checking type instance well-formedness and termination
1694 %************************************************************************
1697 -- Check that a "type instance" is well-formed (which includes decidability
1698 -- unless -XUndecidableInstances is given).
1700 checkValidTypeInst :: [Type] -> Type -> TcM ()
1701 checkValidTypeInst typats rhs
1702 = do { -- left-hand side contains no type family applications
1703 -- (vanilla synonyms are fine, though)
1704 ; mapM_ checkTyFamFreeness typats
1706 -- the right-hand side is a tau type
1707 ; checkValidMonoType rhs
1709 -- we have a decidable instance unless otherwise permitted
1710 ; undecidable_ok <- doptM Opt_UndecidableInstances
1711 ; unless undecidable_ok $
1712 mapM_ addErrTc (checkFamInst typats (tyFamInsts rhs))
1715 -- Make sure that each type family instance is
1716 -- (1) strictly smaller than the lhs,
1717 -- (2) mentions no type variable more often than the lhs, and
1718 -- (3) does not contain any further type family instances.
1720 checkFamInst :: [Type] -- lhs
1721 -> [(TyCon, [Type])] -- type family instances
1723 checkFamInst lhsTys famInsts
1724 = mapCatMaybes check famInsts
1726 size = sizeTypes lhsTys
1727 fvs = fvTypes lhsTys
1729 | not (all isTyFamFree tys)
1730 = Just (famInstUndecErr famInst nestedMsg $$ parens undecidableMsg)
1731 | not (null (fvTypes tys \\ fvs))
1732 = Just (famInstUndecErr famInst nomoreVarMsg $$ parens undecidableMsg)
1733 | size <= sizeTypes tys
1734 = Just (famInstUndecErr famInst smallerAppMsg $$ parens undecidableMsg)
1738 famInst = TyConApp tc tys
1740 -- Ensure that no type family instances occur in a type.
1742 checkTyFamFreeness :: Type -> TcM ()
1743 checkTyFamFreeness ty
1744 = checkTc (isTyFamFree ty) $
1745 tyFamInstIllegalErr ty
1747 -- Check that a type does not contain any type family applications.
1749 isTyFamFree :: Type -> Bool
1750 isTyFamFree = null . tyFamInsts
1754 tyFamInstIllegalErr :: Type -> SDoc
1755 tyFamInstIllegalErr ty
1756 = hang (ptext (sLit "Illegal type synonym family application in instance") <>
1760 famInstUndecErr :: Type -> SDoc -> SDoc
1761 famInstUndecErr ty msg
1763 nest 2 (ptext (sLit "in the type family application:") <+>
1766 nestedMsg, nomoreVarMsg, smallerAppMsg :: SDoc
1767 nestedMsg = ptext (sLit "Nested type family application")
1768 nomoreVarMsg = ptext (sLit "Variable occurs more often than in instance head")
1769 smallerAppMsg = ptext (sLit "Application is no smaller than the instance head")
1773 %************************************************************************
1775 \subsection{Auxiliary functions}
1777 %************************************************************************
1780 -- Free variables of a type, retaining repetitions, and expanding synonyms
1781 fvType :: Type -> [TyVar]
1782 fvType ty | Just exp_ty <- tcView ty = fvType exp_ty
1783 fvType (TyVarTy tv) = [tv]
1784 fvType (TyConApp _ tys) = fvTypes tys
1785 fvType (PredTy pred) = fvPred pred
1786 fvType (FunTy arg res) = fvType arg ++ fvType res
1787 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1788 fvType (ForAllTy tyvar ty) = filter (/= tyvar) (fvType ty)
1790 fvTypes :: [Type] -> [TyVar]
1791 fvTypes tys = concat (map fvType tys)
1793 fvPred :: PredType -> [TyVar]
1794 fvPred (ClassP _ tys') = fvTypes tys'
1795 fvPred (IParam _ ty) = fvType ty
1796 fvPred (EqPred ty1 ty2) = fvType ty1 ++ fvType ty2
1798 -- Size of a type: the number of variables and constructors
1799 sizeType :: Type -> Int
1800 sizeType ty | Just exp_ty <- tcView ty = sizeType exp_ty
1801 sizeType (TyVarTy _) = 1
1802 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1803 sizeType (PredTy pred) = sizePred pred
1804 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1805 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1806 sizeType (ForAllTy _ ty) = sizeType ty
1808 sizeTypes :: [Type] -> Int
1809 sizeTypes xs = sum (map sizeType xs)
1811 sizePred :: PredType -> Int
1812 sizePred (ClassP _ tys') = sizeTypes tys'
1813 sizePred (IParam _ ty) = sizeType ty
1814 sizePred (EqPred ty1 ty2) = sizeType ty1 + sizeType ty2