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 HsSyn -- HsType
74 import TcRnMonad -- TcType, amongst others
91 import Data.List ( (\\) )
95 %************************************************************************
97 Instantiation in general
99 %************************************************************************
102 tcInstType :: ([TyVar] -> TcM [TcTyVar]) -- How to instantiate the type variables
103 -> TcType -- Type to instantiate
104 -> TcM ([TcTyVar], TcThetaType, TcType) -- Result
105 -- (type vars (excl coercion vars), preds (incl equalities), rho)
106 tcInstType inst_tyvars ty
107 = case tcSplitForAllTys ty of
108 ([], rho) -> let -- There may be overloading despite no type variables;
109 -- (?x :: Int) => Int -> Int
110 (theta, tau) = tcSplitPhiTy rho
112 return ([], theta, tau)
114 (tyvars, rho) -> do { tyvars' <- inst_tyvars tyvars
116 ; let tenv = zipTopTvSubst tyvars (mkTyVarTys tyvars')
117 -- Either the tyvars are freshly made, by inst_tyvars,
118 -- or (in the call from tcSkolSigType) any nested foralls
119 -- have different binders. Either way, zipTopTvSubst is ok
121 ; let (theta, tau) = tcSplitPhiTy (substTy tenv rho)
122 ; return (tyvars', theta, tau) }
126 %************************************************************************
130 %************************************************************************
132 Can't be in TcUnify, as we also need it in TcTyFuns.
136 -- False <=> the two args are (actual, expected) respectively
137 -- True <=> the two args are (expected, actual) respectively
139 checkUpdateMeta :: SwapFlag
140 -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
141 -- Update tv1, which is flexi; occurs check is alrady done
142 -- The 'check' version does a kind check too
143 -- We do a sub-kind check here: we might unify (a b) with (c d)
144 -- where b::*->* and d::*; this should fail
146 checkUpdateMeta swapped tv1 ref1 ty2
147 = do { checkKinds swapped tv1 ty2
148 ; updateMeta tv1 ref1 ty2 }
150 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
151 updateMeta tv1 ref1 ty2
152 = ASSERT( isMetaTyVar tv1 )
153 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
154 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
155 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
156 ; writeMutVar ref1 (Indirect ty2)
160 checkKinds :: Bool -> TyVar -> Type -> TcM ()
161 checkKinds swapped tv1 ty2
162 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
163 -- ty2 has been zonked at this stage, which ensures that
164 -- its kind has as much boxity information visible as possible.
165 | tk2 `isSubKind` tk1 = return ()
168 -- Either the kinds aren't compatible
169 -- (can happen if we unify (a b) with (c d))
170 -- or we are unifying a lifted type variable with an
171 -- unlifted type: e.g. (id 3#) is illegal
172 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
173 unifyKindMisMatch k1 k2
175 (k1,k2) | swapped = (tk2,tk1)
176 | otherwise = (tk1,tk2)
181 checkTauTvUpdate :: TcTyVar -> TcType -> TcM (Maybe TcType)
182 -- (checkTauTvUpdate tv ty)
183 -- We are about to update the TauTv tv with ty.
184 -- Check (a) that tv doesn't occur in ty (occurs check)
185 -- (b) that ty is a monotype
186 -- Furthermore, in the interest of (b), if you find an
187 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
189 -- We have three possible outcomes:
190 -- (1) Return the (non-boxy) type to update the type variable with,
191 -- [we know the update is ok!]
192 -- (2) return Nothing, or
193 -- [we cannot tell whether the update is ok right now]
195 -- [the update is definitely invalid]
196 -- We return Nothing in case the tv occurs in ty *under* a type family
197 -- application. In this case, we must not update tv (to avoid a cyclic type
198 -- term), but we also cannot fail claiming an infinite type. Given
200 -- type instance F Int = Int
203 -- This is perfectly reasonable, if we later get a ~ Int.
205 checkTauTvUpdate orig_tv orig_ty
206 = do { result <- go orig_ty
208 Right ty -> return $ Just ty
209 Left True -> return $ Nothing
210 Left False -> occurCheckErr (mkTyVarTy orig_tv) orig_ty
213 go :: TcType -> TcM (Either Bool TcType)
215 -- Right ty if everything is fine
216 -- Left True if orig_tv occurs in orig_ty, but under a type family app
217 -- Left False if orig_tv occurs in orig_ty (with no type family app)
218 -- It fails if it encounters a forall type, except as an argument for a
219 -- closed type synonym that expands to a tau type.
221 | isSynTyCon tc = go_syn tc tys
222 | otherwise = do { tys' <- mapM go tys
223 ; return $ occurs (TyConApp tc) tys' }
224 go (PredTy p) = do { p' <- go_pred p
225 ; return $ occurs1 PredTy p' }
226 go (FunTy arg res) = do { arg' <- go arg
228 ; return $ occurs2 FunTy arg' res' }
229 go (AppTy fun arg) = do { fun' <- go fun
231 ; return $ occurs2 mkAppTy fun' arg' }
232 -- NB the mkAppTy; we might have instantiated a
233 -- type variable to a type constructor, so we need
234 -- to pull the TyConApp to the top.
235 go (ForAllTy _ _) = notMonoType orig_ty -- (b)
238 | orig_tv == tv = return $ Left False -- (a)
239 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
240 | otherwise = return $ Right (TyVarTy tv)
241 -- Ordinary (non Tc) tyvars
242 -- occur inside quantified types
244 go_pred (ClassP c tys) = do { tys' <- mapM go tys
245 ; return $ occurs (ClassP c) tys' }
246 go_pred (IParam n ty) = do { ty' <- go ty
247 ; return $ occurs1 (IParam n) ty' }
248 go_pred (EqPred t1 t2) = do { t1' <- go t1
250 ; return $ occurs2 EqPred t1' t2' }
252 go_tyvar tv (SkolemTv _) = return $ Right (TyVarTy tv)
253 go_tyvar tv (MetaTv box ref)
254 = do { cts <- readMutVar ref
258 BoxTv -> do { ty <- fillBoxWithTau tv ref
259 ; return $ Right ty }
260 _ -> return $ Right (TyVarTy tv)
263 -- go_syn is called for synonyms only
264 -- See Note [Type synonyms and the occur check]
266 | not (isTauTyCon tc)
267 = notMonoType orig_ty -- (b) again
269 = do { (_msgs, mb_tys') <- tryTc (mapM go tys)
272 -- we had a type error => forall in type parameters
274 | isOpenTyCon tc -> notMonoArgs (TyConApp tc tys)
275 -- Synonym families must have monotype args
276 | otherwise -> go (expectJust "checkTauTvUpdate(1)"
277 (tcView (TyConApp tc tys)))
278 -- Try again, expanding the synonym
280 -- no type error, but need to test whether occurs check happend
282 case occurs id tys' of
284 | isOpenTyCon tc -> return $ Left True
285 -- Variable occured under type family application
286 | otherwise -> go (expectJust "checkTauTvUpdate(2)"
287 (tcView (TyConApp tc tys)))
288 -- Try again, expanding the synonym
289 Right raw_tys' -> return $ Right (TyConApp tc raw_tys')
290 -- Retain the synonym (the common case)
293 -- Left results (= occurrence of orig_ty) dominate and
294 -- (Left False) (= fatal occurrence) dominates over (Left True)
295 occurs :: ([a] -> b) -> [Either Bool a] -> Either Bool b
296 occurs c = either Left (Right . c) . foldr combine (Right [])
298 combine (Left famInst1) (Left famInst2) = Left (famInst1 && famInst2)
299 combine (Right _ ) (Left famInst) = Left famInst
300 combine (Left famInst) (Right _) = Left famInst
301 combine (Right arg) (Right args) = Right (arg:args)
303 occurs1 c x = occurs (\[x'] -> c x') [x]
304 occurs2 c x y = occurs (\[x', y'] -> c x' y') [x, y]
306 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
307 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
308 -- tau-type meta-variable, whose print-name is the same as tv
309 -- Choosing the same name is good: when we instantiate a function
310 -- we allocate boxy tyvars with the same print-name as the quantified
311 -- tyvar; and then we often fill the box with a tau-tyvar, and again
312 -- we want to choose the same name.
313 fillBoxWithTau tv ref
314 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
315 ; let tau = mkTyVarTy tv' -- name of the type variable
316 ; writeMutVar ref (Indirect tau)
320 Note [Type synonyms and the occur check]
322 Basically we want to update tv1 := ps_ty2
323 because ps_ty2 has type-synonym info, which improves later error messages
328 f :: (A a -> a -> ()) -> ()
334 In the application (p x), we try to match "t" with "A t". If we go
335 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
336 an infinite loop later.
337 But we should not reject the program, because A t = ().
338 Rather, we should bind t to () (= non_var_ty2).
342 Execute a bag of type variable bindings.
345 execTcTyVarBinds :: TcTyVarBinds -> TcM ()
346 execTcTyVarBinds = mapM_ execTcTyVarBind . bagToList
348 execTcTyVarBind (TcTyVarBind tv ty)
349 = do { ASSERTM2( do { details <- readMetaTyVar tv
350 ; return (isFlexi details) }, ppr tv )
351 ; ty' <- if isCoVar tv
353 else do { maybe_ty <- checkTauTvUpdate tv ty
355 Nothing -> pprPanic "TcRnMonad.execTcTyBind"
356 (ppr tv <+> text ":=" <+> ppr ty)
357 Just ty' -> return ty'
359 ; writeMetaTyVar tv ty'
363 Error mesages in case of kind mismatch.
366 unifyKindMisMatch :: TcKind -> TcKind -> TcM ()
367 unifyKindMisMatch ty1 ty2 = do
368 ty1' <- zonkTcKind ty1
369 ty2' <- zonkTcKind ty2
371 msg = hang (ptext (sLit "Couldn't match kind"))
372 2 (sep [quotes (ppr ty1'),
373 ptext (sLit "against"),
377 unifyKindCtxt :: Bool -> TyVar -> Type -> TidyEnv -> TcM (TidyEnv, SDoc)
378 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
379 -- tv1 and ty2 are zonked already
382 msg = (env2, ptext (sLit "When matching the kinds of") <+>
383 sep [quotes pp_expected <+> ptext (sLit "and"), quotes pp_actual])
385 (pp_expected, pp_actual) | swapped = (pp2, pp1)
386 | otherwise = (pp1, pp2)
387 (env1, tv1') = tidyOpenTyVar tidy_env tv1
388 (env2, ty2') = tidyOpenType env1 ty2
389 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
390 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
393 Error message for failure due to an occurs check.
396 occurCheckErr :: TcType -> TcType -> TcM a
397 occurCheckErr ty containingTy
398 = do { env0 <- tcInitTidyEnv
399 ; ty' <- zonkTcType ty
400 ; containingTy' <- zonkTcType containingTy
401 ; let (env1, tidy_ty1) = tidyOpenType env0 ty'
402 (env2, tidy_ty2) = tidyOpenType env1 containingTy'
403 extra = sep [ppr tidy_ty1, char '=', ppr tidy_ty2]
404 ; failWithTcM (env2, hang msg 2 extra) }
406 msg = ptext (sLit "Occurs check: cannot construct the infinite type:")
409 %************************************************************************
413 %************************************************************************
416 newCoVars :: [(TcType,TcType)] -> TcM [CoVar]
418 = do { us <- newUniqueSupply
419 ; return [ mkCoVar (mkSysTvName uniq (fsLit "co_kv"))
421 | ((ty1,ty2), uniq) <- spec `zip` uniqsFromSupply us] }
423 newMetaCoVar :: TcType -> TcType -> TcM TcTyVar
424 newMetaCoVar ty1 ty2 = newMetaTyVar TauTv (mkCoKind ty1 ty2)
426 newKindVar :: TcM TcKind
427 newKindVar = do { uniq <- newUnique
428 ; ref <- newMutVar Flexi
429 ; return (mkTyVarTy (mkKindVar uniq ref)) }
431 newKindVars :: Int -> TcM [TcKind]
432 newKindVars n = mapM (\ _ -> newKindVar) (nOfThem n ())
436 %************************************************************************
438 SkolemTvs (immutable)
440 %************************************************************************
443 mkSkolTyVar :: Name -> Kind -> SkolemInfo -> TcTyVar
444 mkSkolTyVar name kind info = mkTcTyVar name kind (SkolemTv info)
446 tcSkolSigType :: SkolemInfo -> Type -> TcM ([TcTyVar], TcThetaType, TcType)
447 -- Instantiate a type signature with skolem constants, but
448 -- do *not* give them fresh names, because we want the name to
449 -- be in the type environment -- it is lexically scoped.
450 tcSkolSigType info ty = tcInstType (\tvs -> return (tcSkolSigTyVars info tvs)) ty
452 tcSkolSigTyVars :: SkolemInfo -> [TyVar] -> [TcTyVar]
453 -- Make skolem constants, but do *not* give them new names, as above
454 tcSkolSigTyVars info tyvars = [ mkSkolTyVar (tyVarName tv) (tyVarKind tv) info
457 tcInstSkolTyVar :: SkolemInfo -> (Name -> SrcSpan) -> TyVar -> TcM TcTyVar
458 -- Instantiate the tyvar, using
459 -- * the occ-name and kind of the supplied tyvar,
460 -- * the unique from the monad,
461 -- * the location either from the tyvar (mb_loc = Nothing)
462 -- or from mb_loc (Just loc)
463 tcInstSkolTyVar info get_loc tyvar
464 = do { uniq <- newUnique
465 ; let old_name = tyVarName tyvar
466 kind = tyVarKind tyvar
467 loc = get_loc old_name
468 new_name = mkInternalName uniq (nameOccName old_name) loc
469 ; return (mkSkolTyVar new_name kind info) }
471 tcInstSkolTyVars :: SkolemInfo -> [TyVar] -> TcM [TcTyVar]
472 -- Get the location from the monad
473 tcInstSkolTyVars info tyvars
474 = do { span <- getSrcSpanM
475 ; mapM (tcInstSkolTyVar info (const span)) tyvars }
477 tcInstSkolType :: SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcType)
478 -- Instantiate a type with fresh skolem constants
479 -- Binding location comes from the monad
480 tcInstSkolType info ty = tcInstType (tcInstSkolTyVars info) ty
482 tcInstSigType :: Bool -> SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcRhoType)
483 -- Instantiate with skolems or meta SigTvs; depending on use_skols
484 -- Always take location info from the supplied tyvars
485 tcInstSigType use_skols skol_info ty
486 = tcInstType (mapM inst_tyvar) ty
488 inst_tyvar | use_skols = tcInstSkolTyVar skol_info getSrcSpan
489 | otherwise = instMetaTyVar (SigTv skol_info)
493 %************************************************************************
495 MetaTvs (meta type variables; mutable)
497 %************************************************************************
500 newMetaTyVar :: BoxInfo -> Kind -> TcM TcTyVar
501 -- Make a new meta tyvar out of thin air
502 newMetaTyVar box_info kind
503 = do { uniq <- newUnique
504 ; ref <- newMutVar Flexi
505 ; let name = mkSysTvName uniq fs
506 fs = case box_info of
510 -- We give BoxTv and TauTv the same string, because
511 -- otherwise we get user-visible differences in error
512 -- messages, which are confusing. If you want to see
513 -- the box_info of each tyvar, use -dppr-debug
514 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
516 instMetaTyVar :: BoxInfo -> TyVar -> TcM TcTyVar
517 -- Make a new meta tyvar whose Name and Kind
518 -- come from an existing TyVar
519 instMetaTyVar box_info tyvar
520 = do { uniq <- newUnique
521 ; ref <- newMutVar Flexi
522 ; let name = setNameUnique (tyVarName tyvar) uniq
523 kind = tyVarKind tyvar
524 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
526 readMetaTyVar :: TyVar -> TcM MetaDetails
527 readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
528 readMutVar (metaTvRef tyvar)
530 isFilledMetaTyVar :: TyVar -> TcM Bool
531 -- True of a filled-in (Indirect) meta type variable
533 | not (isTcTyVar tv) = return False
534 | MetaTv _ ref <- tcTyVarDetails tv
535 = do { details <- readMutVar ref
536 ; return (isIndirect details) }
537 | otherwise = return False
539 writeMetaTyVar :: TcTyVar -> TcType -> TcM ()
540 writeMetaTyVar tyvar ty
541 | not debugIsOn = writeMutVar (metaTvRef tyvar) (Indirect ty)
542 writeMetaTyVar tyvar ty
543 | not (isMetaTyVar tyvar)
544 = pprTrace "writeMetaTyVar" (ppr tyvar) $
547 = ASSERT( isMetaTyVar tyvar )
548 ASSERT2( isCoVar tyvar || typeKind ty `isSubKind` tyVarKind tyvar,
549 (ppr tyvar <+> ppr (tyVarKind tyvar))
550 $$ (ppr ty <+> ppr (typeKind ty)) )
551 do { if debugIsOn then do { details <- readMetaTyVar tyvar;
552 -- FIXME ; ASSERT2( not (isFlexi details), ppr tyvar )
553 ; WARN( not (isFlexi details), ppr tyvar )
557 ; traceTc (text "writeMetaTyVar" <+> ppr tyvar <+> text ":=" <+> ppr ty)
558 ; writeMutVar (metaTvRef tyvar) (Indirect ty) }
562 %************************************************************************
566 %************************************************************************
569 newFlexiTyVar :: Kind -> TcM TcTyVar
570 newFlexiTyVar kind = newMetaTyVar TauTv kind
572 newFlexiTyVarTy :: Kind -> TcM TcType
573 newFlexiTyVarTy kind = do
574 tc_tyvar <- newFlexiTyVar kind
575 return (TyVarTy tc_tyvar)
577 newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
578 newFlexiTyVarTys n kind = mapM newFlexiTyVarTy (nOfThem n kind)
580 tcInstTyVar :: TyVar -> TcM TcTyVar
581 -- Instantiate with a META type variable
582 tcInstTyVar tyvar = instMetaTyVar TauTv tyvar
584 tcInstTyVars :: [TyVar] -> TcM ([TcTyVar], [TcType], TvSubst)
585 -- Instantiate with META type variables
587 = do { tc_tvs <- mapM tcInstTyVar tyvars
588 ; let tys = mkTyVarTys tc_tvs
589 ; return (tc_tvs, tys, zipTopTvSubst tyvars tys) }
590 -- Since the tyvars are freshly made,
591 -- they cannot possibly be captured by
592 -- any existing for-alls. Hence zipTopTvSubst
596 %************************************************************************
600 %************************************************************************
603 zonkSigTyVar :: TcTyVar -> TcM TcTyVar
605 | isSkolemTyVar sig_tv
606 = return sig_tv -- Happens in the call in TcBinds.checkDistinctTyVars
608 = ASSERT( isSigTyVar sig_tv )
609 do { ty <- zonkTcTyVar sig_tv
610 ; return (tcGetTyVar "zonkSigTyVar" ty) }
611 -- 'ty' is bound to be a type variable, because SigTvs
612 -- can only be unified with type variables
616 %************************************************************************
620 %************************************************************************
623 newBoxyTyVar :: Kind -> TcM BoxyTyVar
624 newBoxyTyVar kind = newMetaTyVar BoxTv kind
626 newBoxyTyVars :: [Kind] -> TcM [BoxyTyVar]
627 newBoxyTyVars kinds = mapM newBoxyTyVar kinds
629 newBoxyTyVarTys :: [Kind] -> TcM [BoxyType]
630 newBoxyTyVarTys kinds = do { tvs <- mapM newBoxyTyVar kinds; return (mkTyVarTys tvs) }
632 readFilledBox :: BoxyTyVar -> TcM TcType
633 -- Read the contents of the box, which should be filled in by now
634 readFilledBox box_tv = ASSERT( isBoxyTyVar box_tv )
635 do { cts <- readMetaTyVar box_tv
637 Flexi -> pprPanic "readFilledBox" (ppr box_tv)
638 Indirect ty -> return ty }
640 tcInstBoxyTyVar :: TyVar -> TcM BoxyTyVar
641 -- Instantiate with a BOXY type variable
642 tcInstBoxyTyVar tyvar = instMetaTyVar BoxTv tyvar
646 %************************************************************************
648 \subsection{Putting and getting mutable type variables}
650 %************************************************************************
652 But it's more fun to short out indirections on the way: If this
653 version returns a TyVar, then that TyVar is unbound. If it returns
654 any other type, then there might be bound TyVars embedded inside it.
656 We return Nothing iff the original box was unbound.
659 data LookupTyVarResult -- The result of a lookupTcTyVar call
660 = DoneTv TcTyVarDetails -- SkolemTv or virgin MetaTv
663 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
665 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
667 SkolemTv _ -> return (DoneTv details)
668 MetaTv _ ref -> do { meta_details <- readMutVar ref
669 ; case meta_details of
670 Indirect ty -> return (IndirectTv ty)
671 Flexi -> return (DoneTv details) }
673 details = tcTyVarDetails tyvar
676 -- gaw 2004 We aren't shorting anything out anymore, at least for now
678 | not (isTcTyVar tyvar)
679 = pprTrace "getTcTyVar" (ppr tyvar) $
680 return (Just (mkTyVarTy tyvar))
683 = ASSERT2( isTcTyVar tyvar, ppr tyvar ) do
684 maybe_ty <- readMetaTyVar tyvar
686 Just ty -> do ty' <- short_out ty
687 writeMetaTyVar tyvar (Just ty')
690 Nothing -> return Nothing
692 short_out :: TcType -> TcM TcType
693 short_out ty@(TyVarTy tyvar)
694 | not (isTcTyVar tyvar)
698 maybe_ty <- readMetaTyVar tyvar
700 Just ty' -> do ty' <- short_out ty'
701 writeMetaTyVar tyvar (Just ty')
706 short_out other_ty = return other_ty
711 %************************************************************************
713 \subsection{Zonking -- the exernal interfaces}
715 %************************************************************************
717 ----------------- Type variables
720 zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
721 zonkTcTyVars tyvars = mapM zonkTcTyVar tyvars
723 zonkTcTyVarsAndFV :: [TcTyVar] -> TcM TcTyVarSet
724 zonkTcTyVarsAndFV tyvars = tyVarsOfTypes <$> mapM zonkTcTyVar tyvars
726 zonkTcTyVar :: TcTyVar -> TcM TcType
727 zonkTcTyVar tyvar = ASSERT2( isTcTyVar tyvar, ppr tyvar)
728 zonk_tc_tyvar (\ tv -> return (TyVarTy tv)) tyvar
731 ----------------- Types
734 zonkTcType :: TcType -> TcM TcType
735 zonkTcType ty = zonkType (\ tv -> return (TyVarTy tv)) ty
737 zonkTcTypes :: [TcType] -> TcM [TcType]
738 zonkTcTypes tys = mapM zonkTcType tys
740 zonkTcThetaType :: TcThetaType -> TcM TcThetaType
741 zonkTcThetaType theta = mapM zonkTcPredType theta
743 zonkTcPredType :: TcPredType -> TcM TcPredType
744 zonkTcPredType (ClassP c ts) = ClassP c <$> zonkTcTypes ts
745 zonkTcPredType (IParam n t) = IParam n <$> zonkTcType t
746 zonkTcPredType (EqPred t1 t2) = EqPred <$> zonkTcType t1 <*> zonkTcType t2
749 ------------------- These ...ToType, ...ToKind versions
750 are used at the end of type checking
753 zonkTopTyVar :: TcTyVar -> TcM TcTyVar
754 -- zonkTopTyVar is used, at the top level, on any un-instantiated meta type variables
755 -- to default the kind of ? and ?? etc to *. This is important to ensure that
756 -- instance declarations match. For example consider
757 -- instance Show (a->b)
758 -- foo x = show (\_ -> True)
759 -- Then we'll get a constraint (Show (p ->q)) where p has argTypeKind (printed ??),
760 -- and that won't match the typeKind (*) in the instance decl.
762 -- Because we are at top level, no further constraints are going to affect these
763 -- type variables, so it's time to do it by hand. However we aren't ready
764 -- to default them fully to () or whatever, because the type-class defaulting
765 -- rules have yet to run.
768 | k `eqKind` default_k = return tv
770 = do { tv' <- newFlexiTyVar default_k
771 ; writeMetaTyVar tv (mkTyVarTy tv')
775 default_k = defaultKind k
777 zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TcTyVar]
778 zonkQuantifiedTyVars = mapM zonkQuantifiedTyVar
780 zonkQuantifiedTyVar :: TcTyVar -> TcM TcTyVar
781 -- zonkQuantifiedTyVar is applied to the a TcTyVar when quantifying over it.
783 -- The quantified type variables often include meta type variables
784 -- we want to freeze them into ordinary type variables, and
785 -- default their kind (e.g. from OpenTypeKind to TypeKind)
786 -- -- see notes with Kind.defaultKind
787 -- The meta tyvar is updated to point to the new skolem TyVar. Now any
788 -- bound occurences of the original type variable will get zonked to
789 -- the immutable version.
791 -- We leave skolem TyVars alone; they are immutable.
792 zonkQuantifiedTyVar tv
793 | ASSERT2( isTcTyVar tv, ppr tv )
795 = do { kind <- zonkTcType (tyVarKind tv)
796 ; return $ setTyVarKind tv kind
798 -- It might be a skolem type variable,
799 -- for example from a user type signature
801 | otherwise -- It's a meta-type-variable
802 = do { details <- readMetaTyVar tv
804 -- Create the new, frozen, skolem type variable
805 -- We zonk to a skolem, not to a regular TcVar
806 -- See Note [Zonking to Skolem]
807 ; let final_kind = defaultKind (tyVarKind tv)
808 final_tv = mkSkolTyVar (tyVarName tv) final_kind UnkSkol
810 -- Bind the meta tyvar to the new tyvar
812 Indirect ty -> WARN( True, ppr tv $$ ppr ty )
814 -- [Sept 04] I don't think this should happen
815 -- See note [Silly Type Synonym]
817 Flexi -> writeMetaTyVar tv (mkTyVarTy final_tv)
819 -- Return the new tyvar
823 Note [Silly Type Synonyms]
824 ~~~~~~~~~~~~~~~~~~~~~~~~~~
826 type C u a = u -- Note 'a' unused
828 foo :: (forall a. C u a -> C u a) -> u
832 bar = foo (\t -> t + t)
834 * From the (\t -> t+t) we get type {Num d} => d -> d
837 * Now unify with type of foo's arg, and we get:
838 {Num (C d a)} => C d a -> C d a
841 * Now abstract over the 'a', but float out the Num (C d a) constraint
842 because it does not 'really' mention a. (see exactTyVarsOfType)
843 The arg to foo becomes
846 * So we get a dict binding for Num (C d a), which is zonked to give
848 [Note Sept 04: now that we are zonking quantified type variables
849 on construction, the 'a' will be frozen as a regular tyvar on
850 quantification, so the floated dict will still have type (C d a).
851 Which renders this whole note moot; happily!]
853 * Then the \/\a abstraction has a zonked 'a' in it.
855 All very silly. I think its harmless to ignore the problem. We'll end up with
856 a \/\a in the final result but all the occurrences of a will be zonked to ()
858 Note [Zonking to Skolem]
859 ~~~~~~~~~~~~~~~~~~~~~~~~
860 We used to zonk quantified type variables to regular TyVars. However, this
861 leads to problems. Consider this program from the regression test suite:
863 eval :: Int -> String -> String -> String
864 eval 0 root actual = evalRHS 0 root actual
867 evalRHS 0 root actual = eval 0 root actual
869 It leads to the deferral of an equality
871 (String -> String -> String) ~ a
873 which is propagated up to the toplevel (see TcSimplify.tcSimplifyInferCheck).
874 In the meantime `a' is zonked and quantified to form `evalRHS's signature.
875 This has the *side effect* of also zonking the `a' in the deferred equality
876 (which at this point is being handed around wrapped in an implication
879 Finally, the equality (with the zonked `a') will be handed back to the
880 simplifier by TcRnDriver.tcRnSrcDecls calling TcSimplify.tcSimplifyTop.
881 If we zonk `a' with a regular type variable, we will have this regular type
882 variable now floating around in the simplifier, which in many places assumes to
883 only see proper TcTyVars.
885 We can avoid this problem by zonking with a skolem. The skolem is rigid
886 (which we requirefor a quantified variable), but is still a TcTyVar that the
887 simplifier knows how to deal with.
890 %************************************************************************
892 \subsection{Zonking -- the main work-horses: zonkType, zonkTyVar}
894 %* For internal use only! *
896 %************************************************************************
899 -- For unbound, mutable tyvars, zonkType uses the function given to it
900 -- For tyvars bound at a for-all, zonkType zonks them to an immutable
901 -- type variable and zonks the kind too
903 zonkType :: (TcTyVar -> TcM Type) -- What to do with unbound mutable type variables
904 -- see zonkTcType, and zonkTcTypeToType
907 zonkType unbound_var_fn ty
910 go (TyConApp tc tys) = do tys' <- mapM go tys
911 return (TyConApp tc tys')
913 go (PredTy p) = do p' <- go_pred p
916 go (FunTy arg res) = do arg' <- go arg
918 return (FunTy arg' res')
920 go (AppTy fun arg) = do fun' <- go fun
922 return (mkAppTy fun' arg')
923 -- NB the mkAppTy; we might have instantiated a
924 -- type variable to a type constructor, so we need
925 -- to pull the TyConApp to the top.
927 -- The two interesting cases!
928 go (TyVarTy tyvar) | isTcTyVar tyvar = zonk_tc_tyvar unbound_var_fn tyvar
929 | otherwise = liftM TyVarTy $
930 zonkTyVar unbound_var_fn tyvar
931 -- Ordinary (non Tc) tyvars occur inside quantified types
933 go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar ) do
935 tyvar' <- zonkTyVar unbound_var_fn tyvar
936 return (ForAllTy tyvar' ty')
938 go_pred (ClassP c tys) = do tys' <- mapM go tys
939 return (ClassP c tys')
940 go_pred (IParam n ty) = do ty' <- go ty
941 return (IParam n ty')
942 go_pred (EqPred ty1 ty2) = do ty1' <- go ty1
944 return (EqPred ty1' ty2')
946 zonk_tc_tyvar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable var
947 -> TcTyVar -> TcM TcType
948 zonk_tc_tyvar unbound_var_fn tyvar
949 | not (isMetaTyVar tyvar) -- Skolems
950 = return (TyVarTy tyvar)
952 | otherwise -- Mutables
953 = do { cts <- readMetaTyVar tyvar
955 Flexi -> unbound_var_fn tyvar -- Unbound meta type variable
956 Indirect ty -> zonkType unbound_var_fn ty }
958 -- Zonk the kind of a non-TC tyvar in case it is a coercion variable (their
959 -- kind contains types).
961 zonkTyVar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable var
962 -> TyVar -> TcM TyVar
963 zonkTyVar unbound_var_fn tv
965 = do { kind <- zonkType unbound_var_fn (tyVarKind tv)
966 ; return $ setTyVarKind tv kind
968 | otherwise = return tv
973 %************************************************************************
977 %************************************************************************
980 readKindVar :: KindVar -> TcM (MetaDetails)
981 writeKindVar :: KindVar -> TcKind -> TcM ()
982 readKindVar kv = readMutVar (kindVarRef kv)
983 writeKindVar kv val = writeMutVar (kindVarRef kv) (Indirect val)
986 zonkTcKind :: TcKind -> TcM TcKind
987 zonkTcKind k = zonkTcType k
990 zonkTcKindToKind :: TcKind -> TcM Kind
991 -- When zonking a TcKind to a kind, we need to instantiate kind variables,
992 -- Haskell specifies that * is to be used, so we follow that.
993 zonkTcKindToKind k = zonkType (\ _ -> return liftedTypeKind) k
996 %************************************************************************
998 \subsection{Checking a user type}
1000 %************************************************************************
1002 When dealing with a user-written type, we first translate it from an HsType
1003 to a Type, performing kind checking, and then check various things that should
1004 be true about it. We don't want to perform these checks at the same time
1005 as the initial translation because (a) they are unnecessary for interface-file
1006 types and (b) when checking a mutually recursive group of type and class decls,
1007 we can't "look" at the tycons/classes yet. Also, the checks are are rather
1008 diverse, and used to really mess up the other code.
1010 One thing we check for is 'rank'.
1012 Rank 0: monotypes (no foralls)
1013 Rank 1: foralls at the front only, Rank 0 inside
1014 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
1016 basic ::= tyvar | T basic ... basic
1018 r2 ::= forall tvs. cxt => r2a
1019 r2a ::= r1 -> r2a | basic
1020 r1 ::= forall tvs. cxt => r0
1021 r0 ::= r0 -> r0 | basic
1023 Another thing is to check that type synonyms are saturated.
1024 This might not necessarily show up in kind checking.
1026 data T k = MkT (k Int)
1031 checkValidType :: UserTypeCtxt -> Type -> TcM ()
1032 -- Checks that the type is valid for the given context
1033 checkValidType ctxt ty = do
1034 traceTc (text "checkValidType" <+> ppr ty)
1035 unboxed <- doptM Opt_UnboxedTuples
1036 rank2 <- doptM Opt_Rank2Types
1037 rankn <- doptM Opt_RankNTypes
1038 polycomp <- doptM Opt_PolymorphicComponents
1040 gen_rank n | rankn = ArbitraryRank
1042 | otherwise = Rank n
1045 DefaultDeclCtxt-> MustBeMonoType
1046 ResSigCtxt -> MustBeMonoType
1047 LamPatSigCtxt -> gen_rank 0
1048 BindPatSigCtxt -> gen_rank 0
1049 TySynCtxt _ -> gen_rank 0
1050 GenPatCtxt -> gen_rank 1
1051 -- This one is a bit of a hack
1052 -- See the forall-wrapping in TcClassDcl.mkGenericInstance
1054 ExprSigCtxt -> gen_rank 1
1055 FunSigCtxt _ -> gen_rank 1
1056 ConArgCtxt _ | polycomp -> gen_rank 2
1057 -- We are given the type of the entire
1058 -- constructor, hence rank 1
1059 | otherwise -> gen_rank 1
1061 ForSigCtxt _ -> gen_rank 1
1062 SpecInstCtxt -> gen_rank 1
1063 ThBrackCtxt -> gen_rank 1
1065 actual_kind = typeKind ty
1067 kind_ok = case ctxt of
1068 TySynCtxt _ -> True -- Any kind will do
1069 ThBrackCtxt -> True -- Any kind will do
1070 ResSigCtxt -> isSubOpenTypeKind actual_kind
1071 ExprSigCtxt -> isSubOpenTypeKind actual_kind
1072 GenPatCtxt -> isLiftedTypeKind actual_kind
1073 ForSigCtxt _ -> isLiftedTypeKind actual_kind
1074 _ -> isSubArgTypeKind actual_kind
1076 ubx_tup = case ctxt of
1077 TySynCtxt _ | unboxed -> UT_Ok
1078 ExprSigCtxt | unboxed -> UT_Ok
1079 ThBrackCtxt | unboxed -> UT_Ok
1082 -- Check the internal validity of the type itself
1083 check_type rank ubx_tup ty
1085 -- Check that the thing has kind Type, and is lifted if necessary
1086 -- Do this second, becuase we can't usefully take the kind of an
1087 -- ill-formed type such as (a~Int)
1088 checkTc kind_ok (kindErr actual_kind)
1090 traceTc (text "checkValidType done" <+> ppr ty)
1092 checkValidMonoType :: Type -> TcM ()
1093 checkValidMonoType ty = check_mono_type MustBeMonoType ty
1098 data Rank = ArbitraryRank -- Any rank ok
1099 | MustBeMonoType -- Monotype regardless of flags
1100 | TyConArgMonoType -- Monotype but could be poly if -XImpredicativeTypes
1101 | SynArgMonoType -- Monotype but could be poly if -XLiberalTypeSynonyms
1102 | Rank Int -- Rank n, but could be more with -XRankNTypes
1104 decRank :: Rank -> Rank -- Function arguments
1105 decRank (Rank 0) = Rank 0
1106 decRank (Rank n) = Rank (n-1)
1107 decRank other_rank = other_rank
1109 nonZeroRank :: Rank -> Bool
1110 nonZeroRank ArbitraryRank = True
1111 nonZeroRank (Rank n) = n>0
1112 nonZeroRank _ = False
1114 ----------------------------------------
1115 data UbxTupFlag = UT_Ok | UT_NotOk
1116 -- The "Ok" version means "ok if UnboxedTuples is on"
1118 ----------------------------------------
1119 check_mono_type :: Rank -> Type -> TcM () -- No foralls anywhere
1120 -- No unlifted types of any kind
1121 check_mono_type rank ty
1122 = do { check_type rank UT_NotOk ty
1123 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1125 check_type :: Rank -> UbxTupFlag -> Type -> TcM ()
1126 -- The args say what the *type context* requires, independent
1127 -- of *flag* settings. You test the flag settings at usage sites.
1129 -- Rank is allowed rank for function args
1130 -- Rank 0 means no for-alls anywhere
1132 check_type rank ubx_tup ty
1133 | not (null tvs && null theta)
1134 = do { checkTc (nonZeroRank rank) (forAllTyErr rank ty)
1135 -- Reject e.g. (Maybe (?x::Int => Int)),
1136 -- with a decent error message
1137 ; check_valid_theta SigmaCtxt theta
1138 ; check_type rank ubx_tup tau -- Allow foralls to right of arrow
1139 ; checkFreeness tvs theta
1140 ; checkAmbiguity tvs theta (tyVarsOfType tau) }
1142 (tvs, theta, tau) = tcSplitSigmaTy ty
1144 -- Naked PredTys should, I think, have been rejected before now
1145 check_type _ _ ty@(PredTy {})
1146 = failWithTc (text "Predicate used as a type:" <+> ppr ty)
1148 check_type _ _ (TyVarTy _) = return ()
1150 check_type rank _ (FunTy arg_ty res_ty)
1151 = do { check_type (decRank rank) UT_NotOk arg_ty
1152 ; check_type rank UT_Ok res_ty }
1154 check_type rank _ (AppTy ty1 ty2)
1155 = do { check_arg_type rank ty1
1156 ; check_arg_type rank ty2 }
1158 check_type rank ubx_tup ty@(TyConApp tc tys)
1160 = do { -- Check that the synonym has enough args
1161 -- This applies equally to open and closed synonyms
1162 -- It's OK to have an *over-applied* type synonym
1163 -- data Tree a b = ...
1164 -- type Foo a = Tree [a]
1165 -- f :: Foo a b -> ...
1166 checkTc (tyConArity tc <= length tys) arity_msg
1168 -- See Note [Liberal type synonyms]
1169 ; liberal <- doptM Opt_LiberalTypeSynonyms
1170 ; if not liberal || isOpenSynTyCon tc then
1171 -- For H98 and synonym families, do check the type args
1172 mapM_ (check_mono_type SynArgMonoType) tys
1174 else -- In the liberal case (only for closed syns), expand then check
1176 Just ty' -> check_type rank ubx_tup ty'
1177 Nothing -> pprPanic "check_tau_type" (ppr ty)
1180 | isUnboxedTupleTyCon tc
1181 = do { ub_tuples_allowed <- doptM Opt_UnboxedTuples
1182 ; checkTc (ubx_tup_ok ub_tuples_allowed) ubx_tup_msg
1184 ; impred <- doptM Opt_ImpredicativeTypes
1185 ; let rank' = if impred then ArbitraryRank else TyConArgMonoType
1186 -- c.f. check_arg_type
1187 -- However, args are allowed to be unlifted, or
1188 -- more unboxed tuples, so can't use check_arg_ty
1189 ; mapM_ (check_type rank' UT_Ok) tys }
1192 = mapM_ (check_arg_type rank) tys
1195 ubx_tup_ok ub_tuples_allowed = case ubx_tup of
1196 UT_Ok -> ub_tuples_allowed
1200 tc_arity = tyConArity tc
1202 arity_msg = arityErr "Type synonym" (tyConName tc) tc_arity n_args
1203 ubx_tup_msg = ubxArgTyErr ty
1205 check_type _ _ ty = pprPanic "check_type" (ppr ty)
1207 ----------------------------------------
1208 check_arg_type :: Rank -> Type -> TcM ()
1209 -- The sort of type that can instantiate a type variable,
1210 -- or be the argument of a type constructor.
1211 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
1212 -- Other unboxed types are very occasionally allowed as type
1213 -- arguments depending on the kind of the type constructor
1215 -- For example, we want to reject things like:
1217 -- instance Ord a => Ord (forall s. T s a)
1219 -- g :: T s (forall b.b)
1221 -- NB: unboxed tuples can have polymorphic or unboxed args.
1222 -- This happens in the workers for functions returning
1223 -- product types with polymorphic components.
1224 -- But not in user code.
1225 -- Anyway, they are dealt with by a special case in check_tau_type
1227 check_arg_type rank ty
1228 = do { impred <- doptM Opt_ImpredicativeTypes
1229 ; let rank' = case rank of -- Predictive => must be monotype
1230 MustBeMonoType -> MustBeMonoType -- Monotype, regardless
1231 _other | impred -> ArbitraryRank
1232 | otherwise -> TyConArgMonoType
1233 -- Make sure that MustBeMonoType is propagated,
1234 -- so that we don't suggest -XImpredicativeTypes in
1235 -- (Ord (forall a.a)) => a -> a
1236 -- and so that if it Must be a monotype, we check that it is!
1238 ; check_type rank' UT_NotOk ty
1239 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1241 ----------------------------------------
1242 forAllTyErr :: Rank -> Type -> SDoc
1244 = vcat [ hang (ptext (sLit "Illegal polymorphic or qualified type:")) 2 (ppr ty)
1247 suggestion = case rank of
1248 Rank _ -> ptext (sLit "Perhaps you intended to use -XRankNTypes or -XRank2Types")
1249 TyConArgMonoType -> ptext (sLit "Perhaps you intended to use -XImpredicativeTypes")
1250 SynArgMonoType -> ptext (sLit "Perhaps you intended to use -XLiberalTypeSynonyms")
1251 _ -> empty -- Polytype is always illegal
1253 unliftedArgErr, ubxArgTyErr :: Type -> SDoc
1254 unliftedArgErr ty = sep [ptext (sLit "Illegal unlifted type:"), ppr ty]
1255 ubxArgTyErr ty = sep [ptext (sLit "Illegal unboxed tuple type as function argument:"), ppr ty]
1257 kindErr :: Kind -> SDoc
1258 kindErr kind = sep [ptext (sLit "Expecting an ordinary type, but found a type of kind"), ppr kind]
1261 Note [Liberal type synonyms]
1262 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1263 If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
1264 doing validity checking. This allows us to instantiate a synonym defn
1265 with a for-all type, or with a partially-applied type synonym.
1269 Here, T is partially applied, so it's illegal in H98. But if you
1270 expand S first, then T we get just
1274 IMPORTANT: suppose T is a type synonym. Then we must do validity
1275 checking on an appliation (T ty1 ty2)
1277 *either* before expansion (i.e. check ty1, ty2)
1278 *or* after expansion (i.e. expand T ty1 ty2, and then check)
1281 If we do both, we get exponential behaviour!!
1283 data TIACons1 i r c = c i ::: r c
1284 type TIACons2 t x = TIACons1 t (TIACons1 t x)
1285 type TIACons3 t x = TIACons2 t (TIACons1 t x)
1286 type TIACons4 t x = TIACons2 t (TIACons2 t x)
1287 type TIACons7 t x = TIACons4 t (TIACons3 t x)
1290 %************************************************************************
1292 \subsection{Checking a theta or source type}
1294 %************************************************************************
1297 -- Enumerate the contexts in which a "source type", <S>, can occur
1301 -- or (N a) where N is a newtype
1304 = ClassSCCtxt Name -- Superclasses of clas
1305 -- class <S> => C a where ...
1306 | SigmaCtxt -- Theta part of a normal for-all type
1307 -- f :: <S> => a -> a
1308 | DataTyCtxt Name -- Theta part of a data decl
1309 -- data <S> => T a = MkT a
1310 | TypeCtxt -- Source type in an ordinary type
1312 | InstThetaCtxt -- Context of an instance decl
1313 -- instance <S> => C [a] where ...
1315 pprSourceTyCtxt :: SourceTyCtxt -> SDoc
1316 pprSourceTyCtxt (ClassSCCtxt c) = ptext (sLit "the super-classes of class") <+> quotes (ppr c)
1317 pprSourceTyCtxt SigmaCtxt = ptext (sLit "the context of a polymorphic type")
1318 pprSourceTyCtxt (DataTyCtxt tc) = ptext (sLit "the context of the data type declaration for") <+> quotes (ppr tc)
1319 pprSourceTyCtxt InstThetaCtxt = ptext (sLit "the context of an instance declaration")
1320 pprSourceTyCtxt TypeCtxt = ptext (sLit "the context of a type")
1324 checkValidTheta :: SourceTyCtxt -> ThetaType -> TcM ()
1325 checkValidTheta ctxt theta
1326 = addErrCtxt (checkThetaCtxt ctxt theta) (check_valid_theta ctxt theta)
1328 -------------------------
1329 check_valid_theta :: SourceTyCtxt -> [PredType] -> TcM ()
1330 check_valid_theta _ []
1332 check_valid_theta ctxt theta = do
1334 warnTc (notNull dups) (dupPredWarn dups)
1335 mapM_ (check_pred_ty dflags ctxt) theta
1337 (_,dups) = removeDups tcCmpPred theta
1339 -------------------------
1340 check_pred_ty :: DynFlags -> SourceTyCtxt -> PredType -> TcM ()
1341 check_pred_ty dflags ctxt pred@(ClassP cls tys)
1342 = do { -- Class predicates are valid in all contexts
1343 ; checkTc (arity == n_tys) arity_err
1345 -- Check the form of the argument types
1346 ; mapM_ checkValidMonoType tys
1347 ; checkTc (check_class_pred_tys dflags ctxt tys)
1348 (predTyVarErr pred $$ how_to_allow)
1351 class_name = className cls
1352 arity = classArity cls
1354 arity_err = arityErr "Class" class_name arity n_tys
1355 how_to_allow = parens (ptext (sLit "Use -XFlexibleContexts to permit this"))
1357 check_pred_ty _ (ClassSCCtxt _) (EqPred _ _)
1358 = -- We do not yet support superclass equalities.
1360 sep [ ptext (sLit "The current implementation of type families does not")
1361 , ptext (sLit "support equality constraints in superclass contexts.")
1362 , ptext (sLit "They are planned for a future release.")
1365 check_pred_ty dflags _ pred@(EqPred ty1 ty2)
1366 = do { -- Equational constraints are valid in all contexts if type
1367 -- families are permitted
1368 ; checkTc (dopt Opt_TypeFamilies dflags) (eqPredTyErr pred)
1370 -- Check the form of the argument types
1371 ; checkValidMonoType ty1
1372 ; checkValidMonoType ty2
1375 check_pred_ty _ SigmaCtxt (IParam _ ty) = checkValidMonoType ty
1376 -- Implicit parameters only allowed in type
1377 -- signatures; not in instance decls, superclasses etc
1378 -- The reason for not allowing implicit params in instances is a bit
1380 -- If we allowed instance (?x::Int, Eq a) => Foo [a] where ...
1381 -- then when we saw (e :: (?x::Int) => t) it would be unclear how to
1382 -- discharge all the potential usas of the ?x in e. For example, a
1383 -- constraint Foo [Int] might come out of e,and applying the
1384 -- instance decl would show up two uses of ?x.
1387 check_pred_ty _ _ sty = failWithTc (badPredTyErr sty)
1389 -------------------------
1390 check_class_pred_tys :: DynFlags -> SourceTyCtxt -> [Type] -> Bool
1391 check_class_pred_tys dflags ctxt tys
1393 TypeCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
1394 InstThetaCtxt -> flexible_contexts || undecidable_ok || all tcIsTyVarTy tys
1395 -- Further checks on head and theta in
1396 -- checkInstTermination
1397 _ -> flexible_contexts || all tyvar_head tys
1399 flexible_contexts = dopt Opt_FlexibleContexts dflags
1400 undecidable_ok = dopt Opt_UndecidableInstances dflags
1402 -------------------------
1403 tyvar_head :: Type -> Bool
1404 tyvar_head ty -- Haskell 98 allows predicates of form
1405 | tcIsTyVarTy ty = True -- C (a ty1 .. tyn)
1406 | otherwise -- where a is a type variable
1407 = case tcSplitAppTy_maybe ty of
1408 Just (ty, _) -> tyvar_head ty
1415 is ambiguous if P contains generic variables
1416 (i.e. one of the Vs) that are not mentioned in tau
1418 However, we need to take account of functional dependencies
1419 when we speak of 'mentioned in tau'. Example:
1420 class C a b | a -> b where ...
1422 forall x y. (C x y) => x
1423 is not ambiguous because x is mentioned and x determines y
1425 NB; the ambiguity check is only used for *user* types, not for types
1426 coming from inteface files. The latter can legitimately have
1427 ambiguous types. Example
1429 class S a where s :: a -> (Int,Int)
1430 instance S Char where s _ = (1,1)
1431 f:: S a => [a] -> Int -> (Int,Int)
1432 f (_::[a]) x = (a*x,b)
1433 where (a,b) = s (undefined::a)
1435 Here the worker for f gets the type
1436 fw :: forall a. S a => Int -> (# Int, Int #)
1438 If the list of tv_names is empty, we have a monotype, and then we
1439 don't need to check for ambiguity either, because the test can't fail
1444 checkAmbiguity :: [TyVar] -> ThetaType -> TyVarSet -> TcM ()
1445 checkAmbiguity forall_tyvars theta tau_tyvars
1446 = mapM_ complain (filter is_ambig theta)
1448 complain pred = addErrTc (ambigErr pred)
1449 extended_tau_vars = growThetaTyVars theta tau_tyvars
1451 -- See Note [Implicit parameters and ambiguity] in TcSimplify
1452 is_ambig pred = isClassPred pred &&
1453 any ambig_var (varSetElems (tyVarsOfPred pred))
1455 ambig_var ct_var = (ct_var `elem` forall_tyvars) &&
1456 not (ct_var `elemVarSet` extended_tau_vars)
1458 ambigErr :: PredType -> SDoc
1460 = sep [ptext (sLit "Ambiguous constraint") <+> quotes (pprPred pred),
1461 nest 4 (ptext (sLit "At least one of the forall'd type variables mentioned by the constraint") $$
1462 ptext (sLit "must be reachable from the type after the '=>'"))]
1464 --------------------------
1465 -- For this 'grow' stuff see Note [Growing the tau-tvs using constraints] in Inst
1467 growThetaTyVars :: TcThetaType -> TyVarSet -> TyVarSet
1469 growThetaTyVars theta tvs
1471 | otherwise = fixVarSet mk_next tvs
1473 mk_next tvs = foldr growPredTyVars tvs theta
1476 growPredTyVars :: TcPredType -> TyVarSet -> TyVarSet
1477 -- Here is where the special case for inplicit parameters happens
1478 growPredTyVars (IParam _ ty) tvs = tvs `unionVarSet` tyVarsOfType ty
1479 growPredTyVars pred tvs = growTyVars (tyVarsOfPred pred) tvs
1481 growTyVars :: TyVarSet -> TyVarSet -> TyVarSet
1482 growTyVars new_tvs tvs
1483 | new_tvs `intersectsVarSet` tvs = tvs `unionVarSet` new_tvs
1487 In addition, GHC insists that at least one type variable
1488 in each constraint is in V. So we disallow a type like
1489 forall a. Eq b => b -> b
1490 even in a scope where b is in scope.
1493 checkFreeness :: [Var] -> [PredType] -> TcM ()
1494 checkFreeness forall_tyvars theta
1495 = do { flexible_contexts <- doptM Opt_FlexibleContexts
1496 ; unless flexible_contexts $ mapM_ complain (filter is_free theta) }
1498 is_free pred = not (isIPPred pred)
1499 && not (any bound_var (varSetElems (tyVarsOfPred pred)))
1500 bound_var ct_var = ct_var `elem` forall_tyvars
1501 complain pred = addErrTc (freeErr pred)
1503 freeErr :: PredType -> SDoc
1505 = sep [ ptext (sLit "All of the type variables in the constraint") <+>
1506 quotes (pprPred pred)
1507 , ptext (sLit "are already in scope") <+>
1508 ptext (sLit "(at least one must be universally quantified here)")
1510 ptext (sLit "(Use -XFlexibleContexts to lift this restriction)")
1515 checkThetaCtxt :: SourceTyCtxt -> ThetaType -> SDoc
1516 checkThetaCtxt ctxt theta
1517 = vcat [ptext (sLit "In the context:") <+> pprTheta theta,
1518 ptext (sLit "While checking") <+> pprSourceTyCtxt ctxt ]
1520 badPredTyErr, eqPredTyErr, predTyVarErr :: PredType -> SDoc
1521 badPredTyErr sty = ptext (sLit "Illegal constraint") <+> pprPred sty
1522 eqPredTyErr sty = ptext (sLit "Illegal equational constraint") <+> pprPred sty
1524 parens (ptext (sLit "Use -XTypeFamilies to permit this"))
1525 predTyVarErr pred = sep [ptext (sLit "Non type-variable argument"),
1526 nest 2 (ptext (sLit "in the constraint:") <+> pprPred pred)]
1527 dupPredWarn :: [[PredType]] -> SDoc
1528 dupPredWarn dups = ptext (sLit "Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
1530 arityErr :: Outputable a => String -> a -> Int -> Int -> SDoc
1531 arityErr kind name n m
1532 = hsep [ text kind, quotes (ppr name), ptext (sLit "should have"),
1533 n_arguments <> comma, text "but has been given", int m]
1535 n_arguments | n == 0 = ptext (sLit "no arguments")
1536 | n == 1 = ptext (sLit "1 argument")
1537 | True = hsep [int n, ptext (sLit "arguments")]
1540 notMonoType :: TcType -> TcM a
1542 = do { ty' <- zonkTcType ty
1543 ; env0 <- tcInitTidyEnv
1544 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1545 msg = ptext (sLit "Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1546 ; failWithTcM (env1, msg) }
1548 notMonoArgs :: TcType -> TcM a
1550 = do { ty' <- zonkTcType ty
1551 ; env0 <- tcInitTidyEnv
1552 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1553 msg = ptext (sLit "Arguments of type synonym families must be monotypes") <+> quotes (ppr tidy_ty)
1554 ; failWithTcM (env1, msg) }
1558 %************************************************************************
1560 \subsection{Checking for a decent instance head type}
1562 %************************************************************************
1564 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1565 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1567 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1568 flag is on, or (2)~the instance is imported (they must have been
1569 compiled elsewhere). In these cases, we let them go through anyway.
1571 We can also have instances for functions: @instance Foo (a -> b) ...@.
1574 checkValidInstHead :: Type -> TcM (Class, [TcType])
1576 checkValidInstHead ty -- Should be a source type
1577 = case tcSplitPredTy_maybe ty of {
1578 Nothing -> failWithTc (instTypeErr (ppr ty) empty) ;
1581 case getClassPredTys_maybe pred of {
1582 Nothing -> failWithTc (instTypeErr (pprPred pred) empty) ;
1583 Just (clas,tys) -> do
1586 check_inst_head dflags clas tys
1590 check_inst_head :: DynFlags -> Class -> [Type] -> TcM ()
1591 check_inst_head dflags clas tys
1592 = do { -- If GlasgowExts then check at least one isn't a type variable
1593 ; checkTc (dopt Opt_TypeSynonymInstances dflags ||
1594 all tcInstHeadTyNotSynonym tys)
1595 (instTypeErr (pprClassPred clas tys) head_type_synonym_msg)
1596 ; checkTc (dopt Opt_FlexibleInstances dflags ||
1597 all tcInstHeadTyAppAllTyVars tys)
1598 (instTypeErr (pprClassPred clas tys) head_type_args_tyvars_msg)
1599 ; checkTc (dopt Opt_MultiParamTypeClasses dflags ||
1601 (instTypeErr (pprClassPred clas tys) head_one_type_msg)
1602 -- May not contain type family applications
1603 ; mapM_ checkTyFamFreeness tys
1605 ; mapM_ checkValidMonoType tys
1606 -- For now, I only allow tau-types (not polytypes) in
1607 -- the head of an instance decl.
1608 -- E.g. instance C (forall a. a->a) is rejected
1609 -- One could imagine generalising that, but I'm not sure
1610 -- what all the consequences might be
1614 head_type_synonym_msg = parens (
1615 text "All instance types must be of the form (T t1 ... tn)" $$
1616 text "where T is not a synonym." $$
1617 text "Use -XTypeSynonymInstances if you want to disable this.")
1619 head_type_args_tyvars_msg = parens (vcat [
1620 text "All instance types must be of the form (T a1 ... an)",
1621 text "where a1 ... an are type *variables*,",
1622 text "and each type variable appears at most once in the instance head.",
1623 text "Use -XFlexibleInstances if you want to disable this."])
1625 head_one_type_msg = parens (
1626 text "Only one type can be given in an instance head." $$
1627 text "Use -XMultiParamTypeClasses if you want to allow more.")
1629 instTypeErr :: SDoc -> SDoc -> SDoc
1630 instTypeErr pp_ty msg
1631 = sep [ptext (sLit "Illegal instance declaration for") <+> quotes pp_ty,
1636 %************************************************************************
1638 \subsection{Checking instance for termination}
1640 %************************************************************************
1643 checkValidInstance :: LHsType Name -> [TyVar] -> ThetaType -> Type
1644 -> TcM (Class, [TcType])
1645 checkValidInstance hs_type tyvars theta tau
1646 = setSrcSpan (getLoc hs_type) $
1647 do { (clas, inst_tys) <- setSrcSpan head_loc $
1648 checkValidInstHead tau
1650 ; undecidable_ok <- doptM Opt_UndecidableInstances
1652 ; checkValidTheta InstThetaCtxt theta
1653 ; checkAmbiguity tyvars theta (tyVarsOfTypes inst_tys)
1655 -- Check that instance inference will terminate (if we care)
1656 -- For Haskell 98 this will already have been done by checkValidTheta,
1657 -- but as we may be using other extensions we need to check.
1658 ; unless undecidable_ok $
1659 mapM_ addErrTc (checkInstTermination inst_tys theta)
1661 -- The Coverage Condition
1662 ; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
1663 (instTypeErr (pprClassPred clas inst_tys) msg)
1665 ; return (clas, inst_tys)
1668 msg = parens (vcat [ptext (sLit "the Coverage Condition fails for one of the functional dependencies;"),
1671 -- The location of the "head" of the instance
1672 head_loc = case hs_type of
1673 L _ (HsForAllTy _ _ _ (L loc _)) -> loc
1677 Termination test: the so-called "Paterson conditions" (see Section 5 of
1678 "Understanding functionsl dependencies via Constraint Handling Rules,
1681 We check that each assertion in the context satisfies:
1682 (1) no variable has more occurrences in the assertion than in the head, and
1683 (2) the assertion has fewer constructors and variables (taken together
1684 and counting repetitions) than the head.
1685 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1686 (which have already been checked) guarantee termination.
1688 The underlying idea is that
1690 for any ground substitution, each assertion in the
1691 context has fewer type constructors than the head.
1695 checkInstTermination :: [TcType] -> ThetaType -> [Message]
1696 checkInstTermination tys theta
1697 = mapCatMaybes check theta
1700 size = sizeTypes tys
1702 | not (null (fvPred pred \\ fvs))
1703 = Just (predUndecErr pred nomoreMsg $$ parens undecidableMsg)
1704 | sizePred pred >= size
1705 = Just (predUndecErr pred smallerMsg $$ parens undecidableMsg)
1709 predUndecErr :: PredType -> SDoc -> SDoc
1710 predUndecErr pred msg = sep [msg,
1711 nest 2 (ptext (sLit "in the constraint:") <+> pprPred pred)]
1713 nomoreMsg, smallerMsg, undecidableMsg :: SDoc
1714 nomoreMsg = ptext (sLit "Variable occurs more often in a constraint than in the instance head")
1715 smallerMsg = ptext (sLit "Constraint is no smaller than the instance head")
1716 undecidableMsg = ptext (sLit "Use -XUndecidableInstances to permit this")
1720 %************************************************************************
1722 Checking the context of a derived instance declaration
1724 %************************************************************************
1726 Note [Exotic derived instance contexts]
1727 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1728 In a 'derived' instance declaration, we *infer* the context. It's a
1729 bit unclear what rules we should apply for this; the Haskell report is
1730 silent. Obviously, constraints like (Eq a) are fine, but what about
1731 data T f a = MkT (f a) deriving( Eq )
1732 where we'd get an Eq (f a) constraint. That's probably fine too.
1734 One could go further: consider
1735 data T a b c = MkT (Foo a b c) deriving( Eq )
1736 instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
1738 Notice that this instance (just) satisfies the Paterson termination
1739 conditions. Then we *could* derive an instance decl like this:
1741 instance (C Int a, Eq b, Eq c) => Eq (T a b c)
1742 even though there is no instance for (C Int a), because there just
1743 *might* be an instance for, say, (C Int Bool) at a site where we
1744 need the equality instance for T's.
1746 However, this seems pretty exotic, and it's quite tricky to allow
1747 this, and yet give sensible error messages in the (much more common)
1748 case where we really want that instance decl for C.
1750 So for now we simply require that the derived instance context
1751 should have only type-variable constraints.
1753 Here is another example:
1754 data Fix f = In (f (Fix f)) deriving( Eq )
1755 Here, if we are prepared to allow -XUndecidableInstances we
1756 could derive the instance
1757 instance Eq (f (Fix f)) => Eq (Fix f)
1758 but this is so delicate that I don't think it should happen inside
1759 'deriving'. If you want this, write it yourself!
1761 NB: if you want to lift this condition, make sure you still meet the
1762 termination conditions! If not, the deriving mechanism generates
1763 larger and larger constraints. Example:
1765 data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
1767 Note the lack of a Show instance for Succ. First we'll generate
1768 instance (Show (Succ a), Show a) => Show (Seq a)
1770 instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
1771 and so on. Instead we want to complain of no instance for (Show (Succ a)).
1775 Allow constraints which consist only of type variables, with no repeats.
1778 validDerivPred :: PredType -> Bool
1779 validDerivPred (ClassP _ tys) = hasNoDups fvs && sizeTypes tys == length fvs
1780 where fvs = fvTypes tys
1781 validDerivPred _ = False
1784 %************************************************************************
1786 Checking type instance well-formedness and termination
1788 %************************************************************************
1791 -- Check that a "type instance" is well-formed (which includes decidability
1792 -- unless -XUndecidableInstances is given).
1794 checkValidTypeInst :: [Type] -> Type -> TcM ()
1795 checkValidTypeInst typats rhs
1796 = do { -- left-hand side contains no type family applications
1797 -- (vanilla synonyms are fine, though)
1798 ; mapM_ checkTyFamFreeness typats
1800 -- the right-hand side is a tau type
1801 ; checkValidMonoType rhs
1803 -- we have a decidable instance unless otherwise permitted
1804 ; undecidable_ok <- doptM Opt_UndecidableInstances
1805 ; unless undecidable_ok $
1806 mapM_ addErrTc (checkFamInst typats (tyFamInsts rhs))
1809 -- Make sure that each type family instance is
1810 -- (1) strictly smaller than the lhs,
1811 -- (2) mentions no type variable more often than the lhs, and
1812 -- (3) does not contain any further type family instances.
1814 checkFamInst :: [Type] -- lhs
1815 -> [(TyCon, [Type])] -- type family instances
1817 checkFamInst lhsTys famInsts
1818 = mapCatMaybes check famInsts
1820 size = sizeTypes lhsTys
1821 fvs = fvTypes lhsTys
1823 | not (all isTyFamFree tys)
1824 = Just (famInstUndecErr famInst nestedMsg $$ parens undecidableMsg)
1825 | not (null (fvTypes tys \\ fvs))
1826 = Just (famInstUndecErr famInst nomoreVarMsg $$ parens undecidableMsg)
1827 | size <= sizeTypes tys
1828 = Just (famInstUndecErr famInst smallerAppMsg $$ parens undecidableMsg)
1832 famInst = TyConApp tc tys
1834 -- Ensure that no type family instances occur in a type.
1836 checkTyFamFreeness :: Type -> TcM ()
1837 checkTyFamFreeness ty
1838 = checkTc (isTyFamFree ty) $
1839 tyFamInstIllegalErr ty
1841 -- Check that a type does not contain any type family applications.
1843 isTyFamFree :: Type -> Bool
1844 isTyFamFree = null . tyFamInsts
1848 tyFamInstIllegalErr :: Type -> SDoc
1849 tyFamInstIllegalErr ty
1850 = hang (ptext (sLit "Illegal type synonym family application in instance") <>
1854 famInstUndecErr :: Type -> SDoc -> SDoc
1855 famInstUndecErr ty msg
1857 nest 2 (ptext (sLit "in the type family application:") <+>
1860 nestedMsg, nomoreVarMsg, smallerAppMsg :: SDoc
1861 nestedMsg = ptext (sLit "Nested type family application")
1862 nomoreVarMsg = ptext (sLit "Variable occurs more often than in instance head")
1863 smallerAppMsg = ptext (sLit "Application is no smaller than the instance head")
1867 %************************************************************************
1869 \subsection{Auxiliary functions}
1871 %************************************************************************
1874 -- Free variables of a type, retaining repetitions, and expanding synonyms
1875 fvType :: Type -> [TyVar]
1876 fvType ty | Just exp_ty <- tcView ty = fvType exp_ty
1877 fvType (TyVarTy tv) = [tv]
1878 fvType (TyConApp _ tys) = fvTypes tys
1879 fvType (PredTy pred) = fvPred pred
1880 fvType (FunTy arg res) = fvType arg ++ fvType res
1881 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1882 fvType (ForAllTy tyvar ty) = filter (/= tyvar) (fvType ty)
1884 fvTypes :: [Type] -> [TyVar]
1885 fvTypes tys = concat (map fvType tys)
1887 fvPred :: PredType -> [TyVar]
1888 fvPred (ClassP _ tys') = fvTypes tys'
1889 fvPred (IParam _ ty) = fvType ty
1890 fvPred (EqPred ty1 ty2) = fvType ty1 ++ fvType ty2
1892 -- Size of a type: the number of variables and constructors
1893 sizeType :: Type -> Int
1894 sizeType ty | Just exp_ty <- tcView ty = sizeType exp_ty
1895 sizeType (TyVarTy _) = 1
1896 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1897 sizeType (PredTy pred) = sizePred pred
1898 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1899 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1900 sizeType (ForAllTy _ ty) = sizeType ty
1902 sizeTypes :: [Type] -> Int
1903 sizeTypes xs = sum (map sizeType xs)
1905 -- Size of a predicate
1907 -- Equalities are a special case. The equality itself doesn't contribute to the
1908 -- size and as we do not count class predicates, we have to start with one less.
1909 -- This is easy to see considering that, given
1910 -- class C a b | a -> b
1912 -- constraints (C a b) and (F a ~ b) are equivalent in size.
1913 sizePred :: PredType -> Int
1914 sizePred (ClassP _ tys') = sizeTypes tys'
1915 sizePred (IParam _ ty) = sizeType ty
1916 sizePred (EqPred ty1 ty2) = sizeType ty1 + sizeType ty2 - 1