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 -- The above warning supression flag is a temporary kludge.
14 -- While working on this module you are encouraged to remove it and fix
15 -- any warnings in the module. See
16 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
20 TcTyVar, TcKind, TcType, TcTauType, TcThetaType, TcTyVarSet,
22 --------------------------------
23 -- Creating new mutable type variables
25 newFlexiTyVarTy, -- Kind -> TcM TcType
26 newFlexiTyVarTys, -- Int -> Kind -> TcM [TcType]
27 newKindVar, newKindVars,
28 lookupTcTyVar, LookupTyVarResult(..),
30 newMetaTyVar, readMetaTyVar, writeMetaTyVar, isFilledMetaTyVar,
32 --------------------------------
33 -- Boxy type variables
34 newBoxyTyVar, newBoxyTyVars, newBoxyTyVarTys, readFilledBox,
36 --------------------------------
37 -- Creating new coercion variables
38 newCoVars, newMetaCoVar,
40 --------------------------------
42 tcInstTyVar, tcInstType, tcInstTyVars, tcInstBoxyTyVar,
44 tcInstSkolTyVar, tcInstSkolTyVars, tcInstSkolType,
45 tcSkolSigType, tcSkolSigTyVars, occurCheckErr,
47 --------------------------------
48 -- Checking type validity
49 Rank, UserTypeCtxt(..), checkValidType,
50 SourceTyCtxt(..), checkValidTheta, checkFreeness,
51 checkValidInstHead, checkValidInstance,
52 checkInstTermination, checkValidTypeInst, checkTyFamFreeness,
53 checkUpdateMeta, updateMeta, checkTauTvUpdate, fillBoxWithTau, unifyKindCtxt,
54 unifyKindMisMatch, validDerivPred, arityErr, notMonoType, notMonoArgs,
56 --------------------------------
58 zonkType, zonkTcPredType,
59 zonkTcTyVar, zonkTcTyVars, zonkTcTyVarsAndFV, zonkSigTyVar,
60 zonkQuantifiedTyVar, zonkQuantifiedTyVars,
61 zonkTcType, zonkTcTypes, zonkTcClassConstraints, zonkTcThetaType,
62 zonkTcKindToKind, zonkTcKind, zonkTopTyVar,
64 readKindVar, writeKindVar
67 #include "HsVersions.h"
79 import TcRnMonad -- TcType, amongst others
92 import Control.Monad ( when, unless )
93 import Data.List ( (\\) )
97 %************************************************************************
99 Instantiation in general
101 %************************************************************************
104 tcInstType :: ([TyVar] -> TcM [TcTyVar]) -- How to instantiate the type variables
105 -> TcType -- Type to instantiate
106 -> TcM ([TcTyVar], TcThetaType, TcType) -- Result
107 tcInstType inst_tyvars ty
108 = case tcSplitForAllTys ty of
109 ([], rho) -> let -- There may be overloading despite no type variables;
110 -- (?x :: Int) => Int -> Int
111 (theta, tau) = tcSplitPhiTy rho
113 return ([], theta, tau)
115 (tyvars, rho) -> do { tyvars' <- inst_tyvars tyvars
117 ; let tenv = zipTopTvSubst tyvars (mkTyVarTys tyvars')
118 -- Either the tyvars are freshly made, by inst_tyvars,
119 -- or (in the call from tcSkolSigType) any nested foralls
120 -- have different binders. Either way, zipTopTvSubst is ok
122 ; let (theta, tau) = tcSplitPhiTy (substTy tenv rho)
123 ; return (tyvars', theta, tau) }
127 %************************************************************************
131 %************************************************************************
133 Can't be in TcUnify, as we also need it in TcTyFuns.
137 -- False <=> the two args are (actual, expected) respectively
138 -- True <=> the two args are (expected, actual) respectively
140 checkUpdateMeta :: SwapFlag
141 -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
142 -- Update tv1, which is flexi; occurs check is alrady done
143 -- The 'check' version does a kind check too
144 -- We do a sub-kind check here: we might unify (a b) with (c d)
145 -- where b::*->* and d::*; this should fail
147 checkUpdateMeta swapped tv1 ref1 ty2
148 = do { checkKinds swapped tv1 ty2
149 ; updateMeta tv1 ref1 ty2 }
151 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
152 updateMeta tv1 ref1 ty2
153 = ASSERT( isMetaTyVar tv1 )
154 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
155 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
156 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
157 ; writeMutVar ref1 (Indirect ty2)
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 = returnM ()
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' <- mappM go tys
223 ; return $ occurs (TyConApp tc) tys' }
224 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
225 go (PredTy p) = do { p' <- go_pred p
226 ; return $ occurs1 PredTy p' }
227 go (FunTy arg res) = do { arg' <- go arg
229 ; return $ occurs2 FunTy arg' res' }
230 go (AppTy fun arg) = do { fun' <- go fun
232 ; return $ occurs2 mkAppTy fun' arg' }
233 -- NB the mkAppTy; we might have instantiated a
234 -- type variable to a type constructor, so we need
235 -- to pull the TyConApp to the top.
236 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
239 | orig_tv == tv = return $ Left False -- (a)
240 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
241 | otherwise = return $ Right (TyVarTy tv)
242 -- Ordinary (non Tc) tyvars
243 -- occur inside quantified types
245 go_pred (ClassP c tys) = do { tys' <- mapM go tys
246 ; return $ occurs (ClassP c) tys' }
247 go_pred (IParam n ty) = do { ty' <- go ty
248 ; return $ occurs1 (IParam n) ty' }
249 go_pred (EqPred t1 t2) = do { t1' <- go t1
251 ; return $ occurs2 EqPred t1' t2' }
253 go_tyvar tv (SkolemTv _) = return $ Right (TyVarTy tv)
254 go_tyvar tv (MetaTv box ref)
255 = do { cts <- readMutVar ref
259 BoxTv -> do { ty <- fillBoxWithTau tv ref
260 ; return $ Right ty }
261 other -> return $ Right (TyVarTy tv)
264 -- go_syn is called for synonyms only
265 -- See Note [Type synonyms and the occur check]
267 | not (isTauTyCon tc)
268 = notMonoType orig_ty -- (b) again
270 = do { (msgs, mb_tys') <- tryTc (mapM go tys)
273 -- we had a type error => forall in type parameters
275 | isOpenTyCon tc -> notMonoArgs (TyConApp tc tys)
276 -- Synonym families must have monotype args
277 | otherwise -> go (expectJust "checkTauTvUpdate(1)"
278 (tcView (TyConApp tc tys)))
279 -- Try again, expanding the synonym
281 -- no type error, but need to test whether occurs check happend
283 case occurs id tys' of
285 | isOpenTyCon tc -> return $ Left True
286 -- Variable occured under type family application
287 | otherwise -> go (expectJust "checkTauTvUpdate(2)"
288 (tcView (TyConApp tc tys)))
289 -- Try again, expanding the synonym
290 Right raw_tys' -> return $ Right (TyConApp tc raw_tys')
291 -- Retain the synonym (the common case)
294 -- Left results (= occurrence of orig_ty) dominate and
295 -- (Left False) (= fatal occurrence) dominates over (Left True)
296 occurs :: ([a] -> b) -> [Either Bool a] -> Either Bool b
297 occurs c = either Left (Right . c) . foldr combine (Right [])
299 combine (Left famInst1) (Left famInst2) = Left (famInst1 && famInst2)
300 combine (Right _ ) (Left famInst) = Left famInst
301 combine (Left famInst) (Right _) = Left famInst
302 combine (Right arg) (Right args) = Right (arg:args)
304 occurs1 c x = occurs (\[x'] -> c x') [x]
305 occurs2 c x y = occurs (\[x', y'] -> c x' y') [x, y]
307 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
308 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
309 -- tau-type meta-variable, whose print-name is the same as tv
310 -- Choosing the same name is good: when we instantiate a function
311 -- we allocate boxy tyvars with the same print-name as the quantified
312 -- tyvar; and then we often fill the box with a tau-tyvar, and again
313 -- we want to choose the same name.
314 fillBoxWithTau tv ref
315 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
316 ; let tau = mkTyVarTy tv' -- name of the type variable
317 ; writeMutVar ref (Indirect tau)
321 Note [Type synonyms and the occur check]
323 Basically we want to update tv1 := ps_ty2
324 because ps_ty2 has type-synonym info, which improves later error messages
329 f :: (A a -> a -> ()) -> ()
335 In the application (p x), we try to match "t" with "A t". If we go
336 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
337 an infinite loop later.
338 But we should not reject the program, because A t = ().
339 Rather, we should bind t to () (= non_var_ty2).
343 Error mesages in case of kind mismatch.
346 unifyKindMisMatch ty1 ty2
347 = zonkTcKind ty1 `thenM` \ ty1' ->
348 zonkTcKind ty2 `thenM` \ ty2' ->
350 msg = hang (ptext SLIT("Couldn't match kind"))
351 2 (sep [quotes (ppr ty1'),
352 ptext SLIT("against"),
357 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
358 -- tv1 and ty2 are zonked already
361 msg = (env2, ptext SLIT("When matching the kinds of") <+>
362 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
364 (pp_expected, pp_actual) | swapped = (pp2, pp1)
365 | otherwise = (pp1, pp2)
366 (env1, tv1') = tidyOpenTyVar tidy_env tv1
367 (env2, ty2') = tidyOpenType env1 ty2
368 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
369 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
372 Error message for failure due to an occurs check.
375 occurCheckErr :: TcType -> TcType -> TcM a
376 occurCheckErr ty containingTy
377 = do { env0 <- tcInitTidyEnv
378 ; ty' <- zonkTcType ty
379 ; containingTy' <- zonkTcType containingTy
380 ; let (env1, tidy_ty1) = tidyOpenType env0 ty'
381 (env2, tidy_ty2) = tidyOpenType env1 containingTy'
382 extra = sep [ppr tidy_ty1, char '=', ppr tidy_ty2]
383 ; failWithTcM (env2, hang msg 2 extra) }
385 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
388 %************************************************************************
392 %************************************************************************
395 newCoVars :: [(TcType,TcType)] -> TcM [CoVar]
397 = do { us <- newUniqueSupply
398 ; return [ mkCoVar (mkSysTvName uniq FSLIT("co"))
400 | ((ty1,ty2), uniq) <- spec `zip` uniqsFromSupply us] }
402 newMetaCoVar :: TcType -> TcType -> TcM TcTyVar
403 newMetaCoVar ty1 ty2 = newMetaTyVar TauTv (mkCoKind ty1 ty2)
405 newKindVar :: TcM TcKind
406 newKindVar = do { uniq <- newUnique
407 ; ref <- newMutVar Flexi
408 ; return (mkTyVarTy (mkKindVar uniq ref)) }
410 newKindVars :: Int -> TcM [TcKind]
411 newKindVars n = mappM (\ _ -> newKindVar) (nOfThem n ())
415 %************************************************************************
417 SkolemTvs (immutable)
419 %************************************************************************
422 mkSkolTyVar :: Name -> Kind -> SkolemInfo -> TcTyVar
423 mkSkolTyVar name kind info = mkTcTyVar name kind (SkolemTv info)
425 tcSkolSigType :: SkolemInfo -> Type -> TcM ([TcTyVar], TcThetaType, TcType)
426 -- Instantiate a type signature with skolem constants, but
427 -- do *not* give them fresh names, because we want the name to
428 -- be in the type environment -- it is lexically scoped.
429 tcSkolSigType info ty = tcInstType (\tvs -> return (tcSkolSigTyVars info tvs)) ty
431 tcSkolSigTyVars :: SkolemInfo -> [TyVar] -> [TcTyVar]
432 -- Make skolem constants, but do *not* give them new names, as above
433 tcSkolSigTyVars info tyvars = [ mkSkolTyVar (tyVarName tv) (tyVarKind tv) info
436 tcInstSkolTyVar :: SkolemInfo -> Maybe SrcSpan -> TyVar -> TcM TcTyVar
437 -- Instantiate the tyvar, using
438 -- * the occ-name and kind of the supplied tyvar,
439 -- * the unique from the monad,
440 -- * the location either from the tyvar (mb_loc = Nothing)
441 -- or from mb_loc (Just loc)
442 tcInstSkolTyVar info mb_loc tyvar
443 = do { uniq <- newUnique
444 ; let old_name = tyVarName tyvar
445 kind = tyVarKind tyvar
446 loc = mb_loc `orElse` getSrcSpan old_name
447 new_name = mkInternalName uniq (nameOccName old_name) loc
448 ; return (mkSkolTyVar new_name kind info) }
450 tcInstSkolTyVars :: SkolemInfo -> [TyVar] -> TcM [TcTyVar]
451 -- Get the location from the monad
452 tcInstSkolTyVars info tyvars
453 = do { span <- getSrcSpanM
454 ; mapM (tcInstSkolTyVar info (Just span)) tyvars }
456 tcInstSkolType :: SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcType)
457 -- Instantiate a type with fresh skolem constants
458 -- Binding location comes from the monad
459 tcInstSkolType info ty = tcInstType (tcInstSkolTyVars info) ty
463 %************************************************************************
465 MetaTvs (meta type variables; mutable)
467 %************************************************************************
470 newMetaTyVar :: BoxInfo -> Kind -> TcM TcTyVar
471 -- Make a new meta tyvar out of thin air
472 newMetaTyVar box_info kind
473 = do { uniq <- newUnique
474 ; ref <- newMutVar Flexi
475 ; let name = mkSysTvName uniq fs
476 fs = case box_info of
479 SigTv _ -> FSLIT("a")
480 -- We give BoxTv and TauTv the same string, because
481 -- otherwise we get user-visible differences in error
482 -- messages, which are confusing. If you want to see
483 -- the box_info of each tyvar, use -dppr-debug
484 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
486 instMetaTyVar :: BoxInfo -> TyVar -> TcM TcTyVar
487 -- Make a new meta tyvar whose Name and Kind
488 -- come from an existing TyVar
489 instMetaTyVar box_info tyvar
490 = do { uniq <- newUnique
491 ; ref <- newMutVar Flexi
492 ; let name = setNameUnique (tyVarName tyvar) uniq
493 kind = tyVarKind tyvar
494 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
496 readMetaTyVar :: TyVar -> TcM MetaDetails
497 readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
498 readMutVar (metaTvRef tyvar)
500 isFilledMetaTyVar :: TyVar -> TcM Bool
501 -- True of a filled-in (Indirect) meta type variable
503 | not (isTcTyVar tv) = return False
504 | MetaTv _ ref <- tcTyVarDetails tv
505 = do { details <- readMutVar ref
506 ; return (isIndirect details) }
507 | otherwise = return False
509 writeMetaTyVar :: TcTyVar -> TcType -> TcM ()
511 writeMetaTyVar tyvar ty = writeMutVar (metaTvRef tyvar) (Indirect ty)
513 writeMetaTyVar tyvar ty
514 | not (isMetaTyVar tyvar)
515 = pprTrace "writeMetaTyVar" (ppr tyvar) $
519 = ASSERT( isMetaTyVar tyvar )
520 -- TOM: It should also work for coercions
521 -- ASSERT2( k2 `isSubKind` k1, (ppr tyvar <+> ppr k1) $$ (ppr ty <+> ppr k2) )
522 do { ASSERTM2( do { details <- readMetaTyVar tyvar; return (isFlexi details) }, ppr tyvar )
523 ; writeMutVar (metaTvRef tyvar) (Indirect ty) }
531 %************************************************************************
535 %************************************************************************
538 newFlexiTyVar :: Kind -> TcM TcTyVar
539 newFlexiTyVar kind = newMetaTyVar TauTv kind
541 newFlexiTyVarTy :: Kind -> TcM TcType
543 = newFlexiTyVar kind `thenM` \ tc_tyvar ->
544 returnM (TyVarTy tc_tyvar)
546 newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
547 newFlexiTyVarTys n kind = mappM newFlexiTyVarTy (nOfThem n kind)
549 tcInstTyVar :: TyVar -> TcM TcTyVar
550 -- Instantiate with a META type variable
551 tcInstTyVar tyvar = instMetaTyVar TauTv tyvar
553 tcInstTyVars :: [TyVar] -> TcM ([TcTyVar], [TcType], TvSubst)
554 -- Instantiate with META type variables
556 = do { tc_tvs <- mapM tcInstTyVar tyvars
557 ; let tys = mkTyVarTys tc_tvs
558 ; returnM (tc_tvs, tys, zipTopTvSubst tyvars tys) }
559 -- Since the tyvars are freshly made,
560 -- they cannot possibly be captured by
561 -- any existing for-alls. Hence zipTopTvSubst
565 %************************************************************************
569 %************************************************************************
572 tcInstSigTyVars :: Bool -> SkolemInfo -> [TyVar] -> TcM [TcTyVar]
573 -- Instantiate with skolems or meta SigTvs; depending on use_skols
574 -- Always take location info from the supplied tyvars
575 tcInstSigTyVars use_skols skol_info tyvars
577 = mapM (tcInstSkolTyVar skol_info Nothing) tyvars
580 = mapM (instMetaTyVar (SigTv skol_info)) tyvars
582 zonkSigTyVar :: TcTyVar -> TcM TcTyVar
584 | isSkolemTyVar sig_tv
585 = return sig_tv -- Happens in the call in TcBinds.checkDistinctTyVars
587 = ASSERT( isSigTyVar sig_tv )
588 do { ty <- zonkTcTyVar sig_tv
589 ; return (tcGetTyVar "zonkSigTyVar" ty) }
590 -- 'ty' is bound to be a type variable, because SigTvs
591 -- can only be unified with type variables
595 %************************************************************************
599 %************************************************************************
602 newBoxyTyVar :: Kind -> TcM BoxyTyVar
603 newBoxyTyVar kind = newMetaTyVar BoxTv kind
605 newBoxyTyVars :: [Kind] -> TcM [BoxyTyVar]
606 newBoxyTyVars kinds = mapM newBoxyTyVar kinds
608 newBoxyTyVarTys :: [Kind] -> TcM [BoxyType]
609 newBoxyTyVarTys kinds = do { tvs <- mapM newBoxyTyVar kinds; return (mkTyVarTys tvs) }
611 readFilledBox :: BoxyTyVar -> TcM TcType
612 -- Read the contents of the box, which should be filled in by now
613 readFilledBox box_tv = ASSERT( isBoxyTyVar box_tv )
614 do { cts <- readMetaTyVar box_tv
616 Flexi -> pprPanic "readFilledBox" (ppr box_tv)
617 Indirect ty -> return ty }
619 tcInstBoxyTyVar :: TyVar -> TcM BoxyTyVar
620 -- Instantiate with a BOXY type variable
621 tcInstBoxyTyVar tyvar = instMetaTyVar BoxTv tyvar
625 %************************************************************************
627 \subsection{Putting and getting mutable type variables}
629 %************************************************************************
631 But it's more fun to short out indirections on the way: If this
632 version returns a TyVar, then that TyVar is unbound. If it returns
633 any other type, then there might be bound TyVars embedded inside it.
635 We return Nothing iff the original box was unbound.
638 data LookupTyVarResult -- The result of a lookupTcTyVar call
639 = DoneTv TcTyVarDetails -- SkolemTv or virgin MetaTv
642 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
644 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
646 SkolemTv _ -> return (DoneTv details)
647 MetaTv _ ref -> do { meta_details <- readMutVar ref
648 ; case meta_details of
649 Indirect ty -> return (IndirectTv ty)
650 Flexi -> return (DoneTv details) }
652 details = tcTyVarDetails tyvar
655 -- gaw 2004 We aren't shorting anything out anymore, at least for now
657 | not (isTcTyVar tyvar)
658 = pprTrace "getTcTyVar" (ppr tyvar) $
659 returnM (Just (mkTyVarTy tyvar))
662 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
663 readMetaTyVar tyvar `thenM` \ maybe_ty ->
665 Just ty -> short_out ty `thenM` \ ty' ->
666 writeMetaTyVar tyvar (Just ty') `thenM_`
669 Nothing -> returnM Nothing
671 short_out :: TcType -> TcM TcType
672 short_out ty@(TyVarTy tyvar)
673 | not (isTcTyVar tyvar)
677 = readMetaTyVar tyvar `thenM` \ maybe_ty ->
679 Just ty' -> short_out ty' `thenM` \ ty' ->
680 writeMetaTyVar tyvar (Just ty') `thenM_`
685 short_out other_ty = returnM other_ty
690 %************************************************************************
692 \subsection{Zonking -- the exernal interfaces}
694 %************************************************************************
696 ----------------- Type variables
699 zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
700 zonkTcTyVars tyvars = mappM zonkTcTyVar tyvars
702 zonkTcTyVarsAndFV :: [TcTyVar] -> TcM TcTyVarSet
703 zonkTcTyVarsAndFV tyvars = mappM zonkTcTyVar tyvars `thenM` \ tys ->
704 returnM (tyVarsOfTypes tys)
706 zonkTcTyVar :: TcTyVar -> TcM TcType
707 zonkTcTyVar tyvar = ASSERT2( isTcTyVar tyvar, ppr tyvar)
708 zonk_tc_tyvar (\ tv -> returnM (TyVarTy tv)) tyvar
711 ----------------- Types
714 zonkTcType :: TcType -> TcM TcType
715 zonkTcType ty = zonkType (\ tv -> returnM (TyVarTy tv)) ty
717 zonkTcTypes :: [TcType] -> TcM [TcType]
718 zonkTcTypes tys = mappM zonkTcType tys
720 zonkTcClassConstraints cts = mappM zonk cts
721 where zonk (clas, tys)
722 = zonkTcTypes tys `thenM` \ new_tys ->
723 returnM (clas, new_tys)
725 zonkTcThetaType :: TcThetaType -> TcM TcThetaType
726 zonkTcThetaType theta = mappM zonkTcPredType theta
728 zonkTcPredType :: TcPredType -> TcM TcPredType
729 zonkTcPredType (ClassP c ts)
730 = zonkTcTypes ts `thenM` \ new_ts ->
731 returnM (ClassP c new_ts)
732 zonkTcPredType (IParam n t)
733 = zonkTcType t `thenM` \ new_t ->
734 returnM (IParam n new_t)
735 zonkTcPredType (EqPred t1 t2)
736 = zonkTcType t1 `thenM` \ new_t1 ->
737 zonkTcType t2 `thenM` \ new_t2 ->
738 returnM (EqPred new_t1 new_t2)
741 ------------------- These ...ToType, ...ToKind versions
742 are used at the end of type checking
745 zonkTopTyVar :: TcTyVar -> TcM TcTyVar
746 -- zonkTopTyVar is used, at the top level, on any un-instantiated meta type variables
747 -- to default the kind of ? and ?? etc to *. This is important to ensure that
748 -- instance declarations match. For example consider
749 -- instance Show (a->b)
750 -- foo x = show (\_ -> True)
751 -- Then we'll get a constraint (Show (p ->q)) where p has argTypeKind (printed ??),
752 -- and that won't match the typeKind (*) in the instance decl.
754 -- Because we are at top level, no further constraints are going to affect these
755 -- type variables, so it's time to do it by hand. However we aren't ready
756 -- to default them fully to () or whatever, because the type-class defaulting
757 -- rules have yet to run.
760 | k `eqKind` default_k = return tv
762 = do { tv' <- newFlexiTyVar default_k
763 ; writeMetaTyVar tv (mkTyVarTy tv')
767 default_k = defaultKind k
769 zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TcTyVar]
770 zonkQuantifiedTyVars = mappM zonkQuantifiedTyVar
772 zonkQuantifiedTyVar :: TcTyVar -> TcM TcTyVar
773 -- zonkQuantifiedTyVar is applied to the a TcTyVar when quantifying over it.
775 -- The quantified type variables often include meta type variables
776 -- we want to freeze them into ordinary type variables, and
777 -- default their kind (e.g. from OpenTypeKind to TypeKind)
778 -- -- see notes with Kind.defaultKind
779 -- The meta tyvar is updated to point to the new skolem TyVar. Now any
780 -- bound occurences of the original type variable will get zonked to
781 -- the immutable version.
783 -- We leave skolem TyVars alone; they are immutable.
784 zonkQuantifiedTyVar tv
785 | ASSERT( isTcTyVar tv )
786 isSkolemTyVar tv = return tv
787 -- It might be a skolem type variable,
788 -- for example from a user type signature
790 | otherwise -- It's a meta-type-variable
791 = do { details <- readMetaTyVar tv
793 -- Create the new, frozen, skolem type variable
794 -- We zonk to a skolem, not to a regular TcVar
795 -- See Note [Zonking to Skolem]
796 ; let final_kind = defaultKind (tyVarKind tv)
797 final_tv = mkSkolTyVar (tyVarName tv) final_kind UnkSkol
799 -- Bind the meta tyvar to the new tyvar
801 Indirect ty -> WARN( True, ppr tv $$ ppr ty )
803 -- [Sept 04] I don't think this should happen
804 -- See note [Silly Type Synonym]
806 Flexi -> writeMetaTyVar tv (mkTyVarTy final_tv)
808 -- Return the new tyvar
812 Note [Silly Type Synonyms]
813 ~~~~~~~~~~~~~~~~~~~~~~~~~~
815 type C u a = u -- Note 'a' unused
817 foo :: (forall a. C u a -> C u a) -> u
821 bar = foo (\t -> t + t)
823 * From the (\t -> t+t) we get type {Num d} => d -> d
826 * Now unify with type of foo's arg, and we get:
827 {Num (C d a)} => C d a -> C d a
830 * Now abstract over the 'a', but float out the Num (C d a) constraint
831 because it does not 'really' mention a. (see exactTyVarsOfType)
832 The arg to foo becomes
835 * So we get a dict binding for Num (C d a), which is zonked to give
837 [Note Sept 04: now that we are zonking quantified type variables
838 on construction, the 'a' will be frozen as a regular tyvar on
839 quantification, so the floated dict will still have type (C d a).
840 Which renders this whole note moot; happily!]
842 * Then the /\a abstraction has a zonked 'a' in it.
844 All very silly. I think its harmless to ignore the problem. We'll end up with
845 a /\a in the final result but all the occurrences of a will be zonked to ()
847 Note [Zonking to Skolem]
848 ~~~~~~~~~~~~~~~~~~~~~~~~
849 We used to zonk quantified type variables to regular TyVars. However, this
850 leads to problems. Consider this program from the regression test suite:
852 eval :: Int -> String -> String -> String
853 eval 0 root actual = evalRHS 0 root actual
856 evalRHS 0 root actual = eval 0 root actual
858 It leads to the deferral of an equality
860 (String -> String -> String) ~ a
862 which is propagated up to the toplevel (see TcSimplify.tcSimplifyInferCheck).
863 In the meantime `a' is zonked and quantified to form `evalRHS's signature.
864 This has the *side effect* of also zonking the `a' in the deferred equality
865 (which at this point is being handed around wrapped in an implication
868 Finally, the equality (with the zonked `a') will be handed back to the
869 simplifier by TcRnDriver.tcRnSrcDecls calling TcSimplify.tcSimplifyTop.
870 If we zonk `a' with a regular type variable, we will have this regular type
871 variable now floating around in the simplifier, which in many places assumes to
872 only see proper TcTyVars.
874 We can avoid this problem by zonking with a skolem. The skolem is rigid
875 (which we requirefor a quantified variable), but is still a TcTyVar that the
876 simplifier knows how to deal with.
879 %************************************************************************
881 \subsection{Zonking -- the main work-horses: zonkType, zonkTyVar}
883 %* For internal use only! *
885 %************************************************************************
888 -- For unbound, mutable tyvars, zonkType uses the function given to it
889 -- For tyvars bound at a for-all, zonkType zonks them to an immutable
890 -- type variable and zonks the kind too
892 zonkType :: (TcTyVar -> TcM Type) -- What to do with unbound mutable type variables
893 -- see zonkTcType, and zonkTcTypeToType
896 zonkType unbound_var_fn ty
899 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
901 go (TyConApp tc tys) = mappM go tys `thenM` \ tys' ->
902 returnM (TyConApp tc tys')
904 go (PredTy p) = go_pred p `thenM` \ p' ->
907 go (FunTy arg res) = go arg `thenM` \ arg' ->
908 go res `thenM` \ res' ->
909 returnM (FunTy arg' res')
911 go (AppTy fun arg) = go fun `thenM` \ fun' ->
912 go arg `thenM` \ arg' ->
913 returnM (mkAppTy fun' arg')
914 -- NB the mkAppTy; we might have instantiated a
915 -- type variable to a type constructor, so we need
916 -- to pull the TyConApp to the top.
918 -- The two interesting cases!
919 go (TyVarTy tyvar) | isTcTyVar tyvar = zonk_tc_tyvar unbound_var_fn tyvar
920 | otherwise = return (TyVarTy tyvar)
921 -- Ordinary (non Tc) tyvars occur inside quantified types
923 go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar )
924 go ty `thenM` \ ty' ->
925 returnM (ForAllTy tyvar ty')
927 go_pred (ClassP c tys) = mappM go tys `thenM` \ tys' ->
928 returnM (ClassP c tys')
929 go_pred (IParam n ty) = go ty `thenM` \ ty' ->
930 returnM (IParam n ty')
931 go_pred (EqPred ty1 ty2) = go ty1 `thenM` \ ty1' ->
932 go ty2 `thenM` \ ty2' ->
933 returnM (EqPred ty1' ty2')
935 zonk_tc_tyvar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable variable
936 -> TcTyVar -> TcM TcType
937 zonk_tc_tyvar unbound_var_fn tyvar
938 | not (isMetaTyVar tyvar) -- Skolems
939 = returnM (TyVarTy tyvar)
941 | otherwise -- Mutables
942 = do { cts <- readMetaTyVar tyvar
944 Flexi -> unbound_var_fn tyvar -- Unbound meta type variable
945 Indirect ty -> zonkType unbound_var_fn ty }
950 %************************************************************************
954 %************************************************************************
957 readKindVar :: KindVar -> TcM (MetaDetails)
958 writeKindVar :: KindVar -> TcKind -> TcM ()
959 readKindVar kv = readMutVar (kindVarRef kv)
960 writeKindVar kv val = writeMutVar (kindVarRef kv) (Indirect val)
963 zonkTcKind :: TcKind -> TcM TcKind
964 zonkTcKind k = zonkTcType k
967 zonkTcKindToKind :: TcKind -> TcM Kind
968 -- When zonking a TcKind to a kind, we need to instantiate kind variables,
969 -- Haskell specifies that * is to be used, so we follow that.
970 zonkTcKindToKind k = zonkType (\ _ -> return liftedTypeKind) k
973 %************************************************************************
975 \subsection{Checking a user type}
977 %************************************************************************
979 When dealing with a user-written type, we first translate it from an HsType
980 to a Type, performing kind checking, and then check various things that should
981 be true about it. We don't want to perform these checks at the same time
982 as the initial translation because (a) they are unnecessary for interface-file
983 types and (b) when checking a mutually recursive group of type and class decls,
984 we can't "look" at the tycons/classes yet. Also, the checks are are rather
985 diverse, and used to really mess up the other code.
987 One thing we check for is 'rank'.
989 Rank 0: monotypes (no foralls)
990 Rank 1: foralls at the front only, Rank 0 inside
991 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
993 basic ::= tyvar | T basic ... basic
995 r2 ::= forall tvs. cxt => r2a
996 r2a ::= r1 -> r2a | basic
997 r1 ::= forall tvs. cxt => r0
998 r0 ::= r0 -> r0 | basic
1000 Another thing is to check that type synonyms are saturated.
1001 This might not necessarily show up in kind checking.
1003 data T k = MkT (k Int)
1008 checkValidType :: UserTypeCtxt -> Type -> TcM ()
1009 -- Checks that the type is valid for the given context
1010 checkValidType ctxt ty
1011 = traceTc (text "checkValidType" <+> ppr ty) `thenM_`
1012 doptM Opt_UnboxedTuples `thenM` \ unboxed ->
1013 doptM Opt_Rank2Types `thenM` \ rank2 ->
1014 doptM Opt_RankNTypes `thenM` \ rankn ->
1015 doptM Opt_PolymorphicComponents `thenM` \ polycomp ->
1017 rank | rankn = Arbitrary
1020 = case ctxt of -- Haskell 98
1021 GenPatCtxt -> Rank 0
1022 LamPatSigCtxt -> Rank 0
1023 BindPatSigCtxt -> Rank 0
1024 DefaultDeclCtxt-> Rank 0
1025 ResSigCtxt -> Rank 0
1026 TySynCtxt _ -> Rank 0
1027 ExprSigCtxt -> Rank 1
1028 FunSigCtxt _ -> Rank 1
1029 ConArgCtxt _ -> if polycomp
1031 -- We are given the type of the entire
1032 -- constructor, hence rank 1
1034 ForSigCtxt _ -> Rank 1
1035 SpecInstCtxt -> Rank 1
1037 actual_kind = typeKind ty
1039 kind_ok = case ctxt of
1040 TySynCtxt _ -> True -- Any kind will do
1041 ResSigCtxt -> isSubOpenTypeKind actual_kind
1042 ExprSigCtxt -> isSubOpenTypeKind actual_kind
1043 GenPatCtxt -> isLiftedTypeKind actual_kind
1044 ForSigCtxt _ -> isLiftedTypeKind actual_kind
1045 other -> isSubArgTypeKind actual_kind
1047 ubx_tup = case ctxt of
1048 TySynCtxt _ | unboxed -> UT_Ok
1049 ExprSigCtxt | unboxed -> UT_Ok
1052 -- Check that the thing has kind Type, and is lifted if necessary
1053 checkTc kind_ok (kindErr actual_kind) `thenM_`
1055 -- Check the internal validity of the type itself
1056 check_poly_type rank ubx_tup ty `thenM_`
1058 traceTc (text "checkValidType done" <+> ppr ty)
1063 data Rank = Rank Int | Arbitrary
1065 decRank :: Rank -> Rank
1066 decRank Arbitrary = Arbitrary
1067 decRank (Rank n) = Rank (n-1)
1069 ----------------------------------------
1070 data UbxTupFlag = UT_Ok | UT_NotOk
1071 -- The "Ok" version means "ok if -fglasgow-exts is on"
1073 ----------------------------------------
1074 check_poly_type :: Rank -> UbxTupFlag -> Type -> TcM ()
1075 check_poly_type (Rank 0) ubx_tup ty
1076 = check_tau_type (Rank 0) ubx_tup ty
1078 check_poly_type rank ubx_tup ty
1079 | null tvs && null theta
1080 = check_tau_type rank ubx_tup ty
1082 = do { check_valid_theta SigmaCtxt theta
1083 ; check_poly_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 ----------------------------------------
1090 check_arg_type :: Type -> TcM ()
1091 -- The sort of type that can instantiate a type variable,
1092 -- or be the argument of a type constructor.
1093 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
1094 -- Other unboxed types are very occasionally allowed as type
1095 -- arguments depending on the kind of the type constructor
1097 -- For example, we want to reject things like:
1099 -- instance Ord a => Ord (forall s. T s a)
1101 -- g :: T s (forall b.b)
1103 -- NB: unboxed tuples can have polymorphic or unboxed args.
1104 -- This happens in the workers for functions returning
1105 -- product types with polymorphic components.
1106 -- But not in user code.
1107 -- Anyway, they are dealt with by a special case in check_tau_type
1110 = check_poly_type Arbitrary UT_NotOk ty `thenM_`
1111 checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty)
1113 ----------------------------------------
1114 check_tau_type :: Rank -> UbxTupFlag -> Type -> TcM ()
1115 -- Rank is allowed rank for function args
1116 -- No foralls otherwise
1118 check_tau_type rank ubx_tup ty@(ForAllTy _ _) = failWithTc (forAllTyErr ty)
1119 check_tau_type rank ubx_tup ty@(FunTy (PredTy _) _) = failWithTc (forAllTyErr ty)
1120 -- Reject e.g. (Maybe (?x::Int => Int)), with a decent error message
1122 -- Naked PredTys don't usually show up, but they can as a result of
1123 -- {-# SPECIALISE instance Ord Char #-}
1124 -- The Right Thing would be to fix the way that SPECIALISE instance pragmas
1125 -- are handled, but the quick thing is just to permit PredTys here.
1126 check_tau_type rank ubx_tup (PredTy sty) = getDOpts `thenM` \ dflags ->
1127 check_pred_ty dflags TypeCtxt sty
1129 check_tau_type rank ubx_tup (TyVarTy _) = returnM ()
1130 check_tau_type rank ubx_tup ty@(FunTy arg_ty res_ty)
1131 = check_poly_type (decRank rank) UT_NotOk arg_ty `thenM_`
1132 check_poly_type rank UT_Ok res_ty
1134 check_tau_type rank ubx_tup (AppTy ty1 ty2)
1135 = check_arg_type ty1 `thenM_` check_arg_type ty2
1137 check_tau_type rank ubx_tup (NoteTy other_note ty)
1138 = check_tau_type rank ubx_tup ty
1140 check_tau_type rank ubx_tup ty@(TyConApp tc tys)
1142 = do { -- Check that the synonym has enough args
1143 -- This applies equally to open and closed synonyms
1144 -- It's OK to have an *over-applied* type synonym
1145 -- data Tree a b = ...
1146 -- type Foo a = Tree [a]
1147 -- f :: Foo a b -> ...
1148 checkTc (tyConArity tc <= length tys) arity_msg
1150 -- See Note [Liberal type synonyms]
1151 ; liberal <- doptM Opt_LiberalTypeSynonyms
1152 ; if not liberal || isOpenSynTyCon tc then
1153 -- For H98 and synonym families, do check the type args
1154 mappM_ check_arg_type tys
1156 else -- In the liberal case (only for closed syns), expand then check
1158 Just ty' -> check_tau_type rank ubx_tup ty'
1159 Nothing -> pprPanic "check_tau_type" (ppr ty)
1162 | isUnboxedTupleTyCon tc
1163 = doptM Opt_UnboxedTuples `thenM` \ ub_tuples_allowed ->
1164 checkTc (ubx_tup_ok ub_tuples_allowed) ubx_tup_msg `thenM_`
1165 mappM_ (check_tau_type (Rank 0) UT_Ok) tys
1166 -- Args are allowed to be unlifted, or
1167 -- more unboxed tuples, so can't use check_arg_ty
1170 = mappM_ check_arg_type tys
1173 ubx_tup_ok ub_tuples_allowed = case ubx_tup of { UT_Ok -> ub_tuples_allowed; other -> False }
1176 tc_arity = tyConArity tc
1178 arity_msg = arityErr "Type synonym" (tyConName tc) tc_arity n_args
1179 ubx_tup_msg = ubxArgTyErr ty
1181 ----------------------------------------
1182 forAllTyErr ty = ptext SLIT("Illegal polymorphic or qualified type:") <+> ppr ty
1183 unliftedArgErr ty = ptext SLIT("Illegal unlifted type argument:") <+> ppr ty
1184 ubxArgTyErr ty = ptext SLIT("Illegal unboxed tuple type as function argument:") <+> ppr ty
1185 kindErr kind = ptext SLIT("Expecting an ordinary type, but found a type of kind") <+> ppr kind
1188 Note [Liberal type synonyms]
1189 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1190 If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
1191 doing validity checking. This allows us to instantiate a synonym defn
1192 with a for-all type, or with a partially-applied type synonym.
1196 Here, T is partially applied, so it's illegal in H98. But if you
1197 expand S first, then T we get just
1201 IMPORTANT: suppose T is a type synonym. Then we must do validity
1202 checking on an appliation (T ty1 ty2)
1204 *either* before expansion (i.e. check ty1, ty2)
1205 *or* after expansion (i.e. expand T ty1 ty2, and then check)
1208 If we do both, we get exponential behaviour!!
1210 data TIACons1 i r c = c i ::: r c
1211 type TIACons2 t x = TIACons1 t (TIACons1 t x)
1212 type TIACons3 t x = TIACons2 t (TIACons1 t x)
1213 type TIACons4 t x = TIACons2 t (TIACons2 t x)
1214 type TIACons7 t x = TIACons4 t (TIACons3 t x)
1217 %************************************************************************
1219 \subsection{Checking a theta or source type}
1221 %************************************************************************
1224 -- Enumerate the contexts in which a "source type", <S>, can occur
1228 -- or (N a) where N is a newtype
1231 = ClassSCCtxt Name -- Superclasses of clas
1232 -- class <S> => C a where ...
1233 | SigmaCtxt -- Theta part of a normal for-all type
1234 -- f :: <S> => a -> a
1235 | DataTyCtxt Name -- Theta part of a data decl
1236 -- data <S> => T a = MkT a
1237 | TypeCtxt -- Source type in an ordinary type
1239 | InstThetaCtxt -- Context of an instance decl
1240 -- instance <S> => C [a] where ...
1242 pprSourceTyCtxt (ClassSCCtxt c) = ptext SLIT("the super-classes of class") <+> quotes (ppr c)
1243 pprSourceTyCtxt SigmaCtxt = ptext SLIT("the context of a polymorphic type")
1244 pprSourceTyCtxt (DataTyCtxt tc) = ptext SLIT("the context of the data type declaration for") <+> quotes (ppr tc)
1245 pprSourceTyCtxt InstThetaCtxt = ptext SLIT("the context of an instance declaration")
1246 pprSourceTyCtxt TypeCtxt = ptext SLIT("the context of a type")
1250 checkValidTheta :: SourceTyCtxt -> ThetaType -> TcM ()
1251 checkValidTheta ctxt theta
1252 = addErrCtxt (checkThetaCtxt ctxt theta) (check_valid_theta ctxt theta)
1254 -------------------------
1255 check_valid_theta ctxt []
1257 check_valid_theta ctxt theta
1258 = getDOpts `thenM` \ dflags ->
1259 warnTc (notNull dups) (dupPredWarn dups) `thenM_`
1260 mappM_ (check_pred_ty dflags ctxt) theta
1262 (_,dups) = removeDups tcCmpPred theta
1264 -------------------------
1265 check_pred_ty :: DynFlags -> SourceTyCtxt -> PredType -> TcM ()
1266 check_pred_ty dflags ctxt pred@(ClassP cls tys)
1267 = do { -- Class predicates are valid in all contexts
1268 ; checkTc (arity == n_tys) arity_err
1270 -- Check the form of the argument types
1271 ; mappM_ check_arg_type tys
1272 ; checkTc (check_class_pred_tys dflags ctxt tys)
1273 (predTyVarErr pred $$ how_to_allow)
1276 class_name = className cls
1277 arity = classArity cls
1279 arity_err = arityErr "Class" class_name arity n_tys
1280 how_to_allow = parens (ptext SLIT("Use -XFlexibleContexts to permit this"))
1282 check_pred_ty dflags ctxt pred@(EqPred ty1 ty2)
1283 = do { -- Equational constraints are valid in all contexts if type
1284 -- families are permitted
1285 ; checkTc (dopt Opt_TypeFamilies dflags) (eqPredTyErr pred)
1287 -- Check the form of the argument types
1288 ; check_eq_arg_type ty1
1289 ; check_eq_arg_type ty2
1292 check_eq_arg_type = check_poly_type (Rank 0) UT_NotOk
1294 check_pred_ty dflags SigmaCtxt (IParam _ ty) = check_arg_type ty
1295 -- Implicit parameters only allowed in type
1296 -- signatures; not in instance decls, superclasses etc
1297 -- The reason for not allowing implicit params in instances is a bit
1299 -- If we allowed instance (?x::Int, Eq a) => Foo [a] where ...
1300 -- then when we saw (e :: (?x::Int) => t) it would be unclear how to
1301 -- discharge all the potential usas of the ?x in e. For example, a
1302 -- constraint Foo [Int] might come out of e,and applying the
1303 -- instance decl would show up two uses of ?x.
1306 check_pred_ty dflags ctxt sty = failWithTc (badPredTyErr sty)
1308 -------------------------
1309 check_class_pred_tys :: DynFlags -> SourceTyCtxt -> [Type] -> Bool
1310 check_class_pred_tys dflags ctxt tys
1312 TypeCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
1313 InstThetaCtxt -> flexible_contexts || undecidable_ok || all tcIsTyVarTy tys
1314 -- Further checks on head and theta in
1315 -- checkInstTermination
1316 other -> flexible_contexts || all tyvar_head tys
1318 flexible_contexts = dopt Opt_FlexibleContexts dflags
1319 undecidable_ok = dopt Opt_UndecidableInstances dflags
1321 -------------------------
1322 tyvar_head ty -- Haskell 98 allows predicates of form
1323 | tcIsTyVarTy ty = True -- C (a ty1 .. tyn)
1324 | otherwise -- where a is a type variable
1325 = case tcSplitAppTy_maybe ty of
1326 Just (ty, _) -> tyvar_head ty
1333 is ambiguous if P contains generic variables
1334 (i.e. one of the Vs) that are not mentioned in tau
1336 However, we need to take account of functional dependencies
1337 when we speak of 'mentioned in tau'. Example:
1338 class C a b | a -> b where ...
1340 forall x y. (C x y) => x
1341 is not ambiguous because x is mentioned and x determines y
1343 NB; the ambiguity check is only used for *user* types, not for types
1344 coming from inteface files. The latter can legitimately have
1345 ambiguous types. Example
1347 class S a where s :: a -> (Int,Int)
1348 instance S Char where s _ = (1,1)
1349 f:: S a => [a] -> Int -> (Int,Int)
1350 f (_::[a]) x = (a*x,b)
1351 where (a,b) = s (undefined::a)
1353 Here the worker for f gets the type
1354 fw :: forall a. S a => Int -> (# Int, Int #)
1356 If the list of tv_names is empty, we have a monotype, and then we
1357 don't need to check for ambiguity either, because the test can't fail
1362 checkAmbiguity :: [TyVar] -> ThetaType -> TyVarSet -> TcM ()
1363 checkAmbiguity forall_tyvars theta tau_tyvars
1364 = mappM_ complain (filter is_ambig theta)
1366 complain pred = addErrTc (ambigErr pred)
1367 extended_tau_vars = grow theta tau_tyvars
1369 -- See Note [Implicit parameters and ambiguity] in TcSimplify
1370 is_ambig pred = isClassPred pred &&
1371 any ambig_var (varSetElems (tyVarsOfPred pred))
1373 ambig_var ct_var = (ct_var `elem` forall_tyvars) &&
1374 not (ct_var `elemVarSet` extended_tau_vars)
1377 = sep [ptext SLIT("Ambiguous constraint") <+> quotes (pprPred pred),
1378 nest 4 (ptext SLIT("At least one of the forall'd type variables mentioned by the constraint") $$
1379 ptext SLIT("must be reachable from the type after the '=>'"))]
1382 In addition, GHC insists that at least one type variable
1383 in each constraint is in V. So we disallow a type like
1384 forall a. Eq b => b -> b
1385 even in a scope where b is in scope.
1388 checkFreeness forall_tyvars theta
1389 = do { flexible_contexts <- doptM Opt_FlexibleContexts
1390 ; unless flexible_contexts $ mappM_ complain (filter is_free theta) }
1392 is_free pred = not (isIPPred pred)
1393 && not (any bound_var (varSetElems (tyVarsOfPred pred)))
1394 bound_var ct_var = ct_var `elem` forall_tyvars
1395 complain pred = addErrTc (freeErr pred)
1398 = sep [ ptext SLIT("All of the type variables in the constraint") <+>
1399 quotes (pprPred pred)
1400 , ptext SLIT("are already in scope") <+>
1401 ptext SLIT("(at least one must be universally quantified here)")
1403 ptext SLIT("(Use -XFlexibleContexts to lift this restriction)")
1408 checkThetaCtxt ctxt theta
1409 = vcat [ptext SLIT("In the context:") <+> pprTheta theta,
1410 ptext SLIT("While checking") <+> pprSourceTyCtxt ctxt ]
1412 badPredTyErr sty = ptext SLIT("Illegal constraint") <+> pprPred sty
1413 eqPredTyErr sty = ptext SLIT("Illegal equational constraint") <+> pprPred sty
1415 parens (ptext SLIT("Use -XTypeFamilies to permit this"))
1416 predTyVarErr pred = sep [ptext SLIT("Non type-variable argument"),
1417 nest 2 (ptext SLIT("in the constraint:") <+> pprPred pred)]
1418 dupPredWarn dups = ptext SLIT("Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
1420 arityErr kind name n m
1421 = hsep [ text kind, quotes (ppr name), ptext SLIT("should have"),
1422 n_arguments <> comma, text "but has been given", int m]
1424 n_arguments | n == 0 = ptext SLIT("no arguments")
1425 | n == 1 = ptext SLIT("1 argument")
1426 | True = hsep [int n, ptext SLIT("arguments")]
1430 = do { ty' <- zonkTcType ty
1431 ; env0 <- tcInitTidyEnv
1432 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1433 msg = ptext SLIT("Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1434 ; failWithTcM (env1, msg) }
1437 = do { ty' <- zonkTcType ty
1438 ; env0 <- tcInitTidyEnv
1439 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1440 msg = ptext SLIT("Arguments of type synonym families must be monotypes") <+> quotes (ppr tidy_ty)
1441 ; failWithTcM (env1, msg) }
1445 %************************************************************************
1447 \subsection{Checking for a decent instance head type}
1449 %************************************************************************
1451 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1452 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1454 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1455 flag is on, or (2)~the instance is imported (they must have been
1456 compiled elsewhere). In these cases, we let them go through anyway.
1458 We can also have instances for functions: @instance Foo (a -> b) ...@.
1461 checkValidInstHead :: Type -> TcM (Class, [TcType])
1463 checkValidInstHead ty -- Should be a source type
1464 = case tcSplitPredTy_maybe ty of {
1465 Nothing -> failWithTc (instTypeErr (ppr ty) empty) ;
1468 case getClassPredTys_maybe pred of {
1469 Nothing -> failWithTc (instTypeErr (pprPred pred) empty) ;
1472 getDOpts `thenM` \ dflags ->
1473 mappM_ check_arg_type tys `thenM_`
1474 check_inst_head dflags clas tys `thenM_`
1478 check_inst_head dflags clas tys
1479 -- If GlasgowExts then check at least one isn't a type variable
1480 = do checkTc (dopt Opt_TypeSynonymInstances dflags ||
1481 all tcInstHeadTyNotSynonym tys)
1482 (instTypeErr (pprClassPred clas tys) head_type_synonym_msg)
1483 checkTc (dopt Opt_FlexibleInstances dflags ||
1484 all tcInstHeadTyAppAllTyVars tys)
1485 (instTypeErr (pprClassPred clas tys) head_type_args_tyvars_msg)
1486 checkTc (dopt Opt_MultiParamTypeClasses dflags ||
1488 (instTypeErr (pprClassPred clas tys) head_one_type_msg)
1491 head_type_synonym_msg = parens (
1492 text "All instance types must be of the form (T t1 ... tn)" $$
1493 text "where T is not a synonym." $$
1494 text "Use -XTypeSynonymInstances if you want to disable this.")
1496 head_type_args_tyvars_msg = parens (
1497 text "All instance types must be of the form (T a1 ... an)" $$
1498 text "where a1 ... an are distinct type *variables*" $$
1499 text "Use -XFlexibleInstances if you want to disable this.")
1501 head_one_type_msg = parens (
1502 text "Only one type can be given in an instance head." $$
1503 text "Use -XMultiParamTypeClasses if you want to allow more.")
1505 -- For now, I only allow tau-types (not polytypes) in
1506 -- the head of an instance decl.
1507 -- E.g. instance C (forall a. a->a) is rejected
1508 -- One could imagine generalising that, but I'm not sure
1509 -- what all the consequences might be
1510 check_one ty = do { check_tau_type (Rank 0) UT_NotOk ty
1511 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1513 instTypeErr pp_ty msg
1514 = sep [ptext SLIT("Illegal instance declaration for") <+> quotes pp_ty,
1519 %************************************************************************
1521 \subsection{Checking instance for termination}
1523 %************************************************************************
1527 checkValidInstance :: [TyVar] -> ThetaType -> Class -> [TcType] -> TcM ()
1528 checkValidInstance tyvars theta clas inst_tys
1529 = do { undecidable_ok <- doptM Opt_UndecidableInstances
1531 ; checkValidTheta InstThetaCtxt theta
1532 ; checkAmbiguity tyvars theta (tyVarsOfTypes inst_tys)
1534 -- Check that instance inference will terminate (if we care)
1535 -- For Haskell 98 this will already have been done by checkValidTheta,
1536 -- but as we may be using other extensions we need to check.
1537 ; unless undecidable_ok $
1538 mapM_ addErrTc (checkInstTermination inst_tys theta)
1540 -- The Coverage Condition
1541 ; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
1542 (instTypeErr (pprClassPred clas inst_tys) msg)
1545 msg = parens (vcat [ptext SLIT("the Coverage Condition fails for one of the functional dependencies;"),
1549 Termination test: the so-called "Paterson conditions" (see Section 5 of
1550 "Understanding functionsl dependencies via Constraint Handling Rules,
1553 We check that each assertion in the context satisfies:
1554 (1) no variable has more occurrences in the assertion than in the head, and
1555 (2) the assertion has fewer constructors and variables (taken together
1556 and counting repetitions) than the head.
1557 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1558 (which have already been checked) guarantee termination.
1560 The underlying idea is that
1562 for any ground substitution, each assertion in the
1563 context has fewer type constructors than the head.
1567 checkInstTermination :: [TcType] -> ThetaType -> [Message]
1568 checkInstTermination tys theta
1569 = mapCatMaybes check theta
1572 size = sizeTypes tys
1574 | not (null (fvPred pred \\ fvs))
1575 = Just (predUndecErr pred nomoreMsg $$ parens undecidableMsg)
1576 | sizePred pred >= size
1577 = Just (predUndecErr pred smallerMsg $$ parens undecidableMsg)
1581 predUndecErr pred msg = sep [msg,
1582 nest 2 (ptext SLIT("in the constraint:") <+> pprPred pred)]
1584 nomoreMsg = ptext SLIT("Variable occurs more often in a constraint than in the instance head")
1585 smallerMsg = ptext SLIT("Constraint is no smaller than the instance head")
1586 undecidableMsg = ptext SLIT("Use -fallow-undecidable-instances to permit this")
1590 %************************************************************************
1592 Checking the context of a derived instance declaration
1594 %************************************************************************
1596 Note [Exotic derived instance contexts]
1597 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1598 In a 'derived' instance declaration, we *infer* the context. It's a
1599 bit unclear what rules we should apply for this; the Haskell report is
1600 silent. Obviously, constraints like (Eq a) are fine, but what about
1601 data T f a = MkT (f a) deriving( Eq )
1602 where we'd get an Eq (f a) constraint. That's probably fine too.
1604 One could go further: consider
1605 data T a b c = MkT (Foo a b c) deriving( Eq )
1606 instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
1608 Notice that this instance (just) satisfies the Paterson termination
1609 conditions. Then we *could* derive an instance decl like this:
1611 instance (C Int a, Eq b, Eq c) => Eq (T a b c)
1613 even though there is no instance for (C Int a), because there just
1614 *might* be an instance for, say, (C Int Bool) at a site where we
1615 need the equality instance for T's.
1617 However, this seems pretty exotic, and it's quite tricky to allow
1618 this, and yet give sensible error messages in the (much more common)
1619 case where we really want that instance decl for C.
1621 So for now we simply require that the derived instance context
1622 should have only type-variable constraints.
1624 Here is another example:
1625 data Fix f = In (f (Fix f)) deriving( Eq )
1626 Here, if we are prepared to allow -fallow-undecidable-instances we
1627 could derive the instance
1628 instance Eq (f (Fix f)) => Eq (Fix f)
1629 but this is so delicate that I don't think it should happen inside
1630 'deriving'. If you want this, write it yourself!
1632 NB: if you want to lift this condition, make sure you still meet the
1633 termination conditions! If not, the deriving mechanism generates
1634 larger and larger constraints. Example:
1636 data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
1638 Note the lack of a Show instance for Succ. First we'll generate
1639 instance (Show (Succ a), Show a) => Show (Seq a)
1641 instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
1642 and so on. Instead we want to complain of no instance for (Show (Succ a)).
1646 Allow constraints which consist only of type variables, with no repeats.
1649 validDerivPred :: PredType -> Bool
1650 validDerivPred (ClassP cls tys) = hasNoDups fvs && sizeTypes tys == length fvs
1651 where fvs = fvTypes tys
1652 validDerivPred otehr = False
1655 %************************************************************************
1657 Checking type instance well-formedness and termination
1659 %************************************************************************
1662 -- Check that a "type instance" is well-formed (which includes decidability
1663 -- unless -fallow-undecidable-instances is given).
1665 checkValidTypeInst :: [Type] -> Type -> TcM ()
1666 checkValidTypeInst typats rhs
1667 = do { -- left-hand side contains no type family applications
1668 -- (vanilla synonyms are fine, though)
1669 ; mappM_ checkTyFamFreeness typats
1671 -- the right-hand side is a tau type
1672 ; checkTc (isTauTy rhs) $
1675 -- we have a decidable instance unless otherwise permitted
1676 ; undecidable_ok <- doptM Opt_UndecidableInstances
1677 ; unless undecidable_ok $
1678 mapM_ addErrTc (checkFamInst typats (tyFamInsts rhs))
1681 -- Make sure that each type family instance is
1682 -- (1) strictly smaller than the lhs,
1683 -- (2) mentions no type variable more often than the lhs, and
1684 -- (3) does not contain any further type family instances.
1686 checkFamInst :: [Type] -- lhs
1687 -> [(TyCon, [Type])] -- type family instances
1689 checkFamInst lhsTys famInsts
1690 = mapCatMaybes check famInsts
1692 size = sizeTypes lhsTys
1693 fvs = fvTypes lhsTys
1695 | not (all isTyFamFree tys)
1696 = Just (famInstUndecErr famInst nestedMsg $$ parens undecidableMsg)
1697 | not (null (fvTypes tys \\ fvs))
1698 = Just (famInstUndecErr famInst nomoreVarMsg $$ parens undecidableMsg)
1699 | size <= sizeTypes tys
1700 = Just (famInstUndecErr famInst smallerAppMsg $$ parens undecidableMsg)
1704 famInst = TyConApp tc tys
1706 -- Ensure that no type family instances occur in a type.
1708 checkTyFamFreeness :: Type -> TcM ()
1709 checkTyFamFreeness ty
1710 = checkTc (isTyFamFree ty) $
1711 tyFamInstInIndexErr ty
1713 -- Check that a type does not contain any type family applications.
1715 isTyFamFree :: Type -> Bool
1716 isTyFamFree = null . tyFamInsts
1720 tyFamInstInIndexErr ty
1721 = hang (ptext SLIT("Illegal type family application in type instance") <>
1726 = hang (ptext SLIT("Illegal polymorphic type in type instance") <> colon) 4 $
1729 famInstUndecErr ty msg
1731 nest 2 (ptext SLIT("in the type family application:") <+>
1734 nestedMsg = ptext SLIT("Nested type family application")
1735 nomoreVarMsg = ptext SLIT("Variable occurs more often than in instance head")
1736 smallerAppMsg = ptext SLIT("Application is no smaller than the instance head")
1740 %************************************************************************
1742 \subsection{Auxiliary functions}
1744 %************************************************************************
1747 -- Free variables of a type, retaining repetitions, and expanding synonyms
1748 fvType :: Type -> [TyVar]
1749 fvType ty | Just exp_ty <- tcView ty = fvType exp_ty
1750 fvType (TyVarTy tv) = [tv]
1751 fvType (TyConApp _ tys) = fvTypes tys
1752 fvType (NoteTy _ ty) = fvType ty
1753 fvType (PredTy pred) = fvPred pred
1754 fvType (FunTy arg res) = fvType arg ++ fvType res
1755 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1756 fvType (ForAllTy tyvar ty) = filter (/= tyvar) (fvType ty)
1758 fvTypes :: [Type] -> [TyVar]
1759 fvTypes tys = concat (map fvType tys)
1761 fvPred :: PredType -> [TyVar]
1762 fvPred (ClassP _ tys') = fvTypes tys'
1763 fvPred (IParam _ ty) = fvType ty
1764 fvPred (EqPred ty1 ty2) = fvType ty1 ++ fvType ty2
1766 -- Size of a type: the number of variables and constructors
1767 sizeType :: Type -> Int
1768 sizeType ty | Just exp_ty <- tcView ty = sizeType exp_ty
1769 sizeType (TyVarTy _) = 1
1770 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1771 sizeType (NoteTy _ ty) = sizeType ty
1772 sizeType (PredTy pred) = sizePred pred
1773 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1774 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1775 sizeType (ForAllTy _ ty) = sizeType ty
1777 sizeTypes :: [Type] -> Int
1778 sizeTypes xs = sum (map sizeType xs)
1780 sizePred :: PredType -> Int
1781 sizePred (ClassP _ tys') = sizeTypes tys'
1782 sizePred (IParam _ ty) = sizeType ty
1783 sizePred (EqPred ty1 ty2) = sizeType ty1 + sizeType ty2