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, checkValidMonoType,
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 -- (type vars (excl coercion vars), preds (incl equalities), rho)
108 tcInstType inst_tyvars ty
109 = case tcSplitForAllTys ty of
110 ([], rho) -> let -- There may be overloading despite no type variables;
111 -- (?x :: Int) => Int -> Int
112 (theta, tau) = tcSplitPhiTy rho
114 return ([], theta, tau)
116 (tyvars, rho) -> do { tyvars' <- inst_tyvars tyvars
118 ; let tenv = zipTopTvSubst tyvars (mkTyVarTys tyvars')
119 -- Either the tyvars are freshly made, by inst_tyvars,
120 -- or (in the call from tcSkolSigType) any nested foralls
121 -- have different binders. Either way, zipTopTvSubst is ok
123 ; let (theta, tau) = tcSplitPhiTy (substTy tenv rho)
124 ; return (tyvars', theta, tau) }
128 %************************************************************************
132 %************************************************************************
134 Can't be in TcUnify, as we also need it in TcTyFuns.
138 -- False <=> the two args are (actual, expected) respectively
139 -- True <=> the two args are (expected, actual) respectively
141 checkUpdateMeta :: SwapFlag
142 -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
143 -- Update tv1, which is flexi; occurs check is alrady done
144 -- The 'check' version does a kind check too
145 -- We do a sub-kind check here: we might unify (a b) with (c d)
146 -- where b::*->* and d::*; this should fail
148 checkUpdateMeta swapped tv1 ref1 ty2
149 = do { checkKinds swapped tv1 ty2
150 ; updateMeta tv1 ref1 ty2 }
152 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
153 updateMeta tv1 ref1 ty2
154 = ASSERT( isMetaTyVar tv1 )
155 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
156 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
157 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
158 ; writeMutVar ref1 (Indirect ty2)
162 checkKinds swapped tv1 ty2
163 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
164 -- ty2 has been zonked at this stage, which ensures that
165 -- its kind has as much boxity information visible as possible.
166 | tk2 `isSubKind` tk1 = return ()
169 -- Either the kinds aren't compatible
170 -- (can happen if we unify (a b) with (c d))
171 -- or we are unifying a lifted type variable with an
172 -- unlifted type: e.g. (id 3#) is illegal
173 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
174 unifyKindMisMatch k1 k2
176 (k1,k2) | swapped = (tk2,tk1)
177 | otherwise = (tk1,tk2)
182 checkTauTvUpdate :: TcTyVar -> TcType -> TcM (Maybe TcType)
183 -- (checkTauTvUpdate tv ty)
184 -- We are about to update the TauTv tv with ty.
185 -- Check (a) that tv doesn't occur in ty (occurs check)
186 -- (b) that ty is a monotype
187 -- Furthermore, in the interest of (b), if you find an
188 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
190 -- We have three possible outcomes:
191 -- (1) Return the (non-boxy) type to update the type variable with,
192 -- [we know the update is ok!]
193 -- (2) return Nothing, or
194 -- [we cannot tell whether the update is ok right now]
196 -- [the update is definitely invalid]
197 -- We return Nothing in case the tv occurs in ty *under* a type family
198 -- application. In this case, we must not update tv (to avoid a cyclic type
199 -- term), but we also cannot fail claiming an infinite type. Given
201 -- type instance F Int = Int
204 -- This is perfectly reasonable, if we later get a ~ Int.
206 checkTauTvUpdate orig_tv orig_ty
207 = do { result <- go orig_ty
209 Right ty -> return $ Just ty
210 Left True -> return $ Nothing
211 Left False -> occurCheckErr (mkTyVarTy orig_tv) orig_ty
214 go :: TcType -> TcM (Either Bool TcType)
216 -- Right ty if everything is fine
217 -- Left True if orig_tv occurs in orig_ty, but under a type family app
218 -- Left False if orig_tv occurs in orig_ty (with no type family app)
219 -- It fails if it encounters a forall type, except as an argument for a
220 -- closed type synonym that expands to a tau type.
222 | isSynTyCon tc = go_syn tc tys
223 | otherwise = do { tys' <- mapM go tys
224 ; return $ occurs (TyConApp tc) tys' }
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 = do
347 ty1' <- zonkTcKind ty1
348 ty2' <- zonkTcKind ty2
350 msg = hang (ptext SLIT("Couldn't match kind"))
351 2 (sep [quotes (ppr ty1'),
352 ptext SLIT("against"),
356 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
357 -- tv1 and ty2 are zonked already
360 msg = (env2, ptext SLIT("When matching the kinds of") <+>
361 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
363 (pp_expected, pp_actual) | swapped = (pp2, pp1)
364 | otherwise = (pp1, pp2)
365 (env1, tv1') = tidyOpenTyVar tidy_env tv1
366 (env2, ty2') = tidyOpenType env1 ty2
367 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
368 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
371 Error message for failure due to an occurs check.
374 occurCheckErr :: TcType -> TcType -> TcM a
375 occurCheckErr ty containingTy
376 = do { env0 <- tcInitTidyEnv
377 ; ty' <- zonkTcType ty
378 ; containingTy' <- zonkTcType containingTy
379 ; let (env1, tidy_ty1) = tidyOpenType env0 ty'
380 (env2, tidy_ty2) = tidyOpenType env1 containingTy'
381 extra = sep [ppr tidy_ty1, char '=', ppr tidy_ty2]
382 ; failWithTcM (env2, hang msg 2 extra) }
384 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
387 %************************************************************************
391 %************************************************************************
394 newCoVars :: [(TcType,TcType)] -> TcM [CoVar]
396 = do { us <- newUniqueSupply
397 ; return [ mkCoVar (mkSysTvName uniq FSLIT("co"))
399 | ((ty1,ty2), uniq) <- spec `zip` uniqsFromSupply us] }
401 newMetaCoVar :: TcType -> TcType -> TcM TcTyVar
402 newMetaCoVar ty1 ty2 = newMetaTyVar TauTv (mkCoKind ty1 ty2)
404 newKindVar :: TcM TcKind
405 newKindVar = do { uniq <- newUnique
406 ; ref <- newMutVar Flexi
407 ; return (mkTyVarTy (mkKindVar uniq ref)) }
409 newKindVars :: Int -> TcM [TcKind]
410 newKindVars n = mapM (\ _ -> newKindVar) (nOfThem n ())
414 %************************************************************************
416 SkolemTvs (immutable)
418 %************************************************************************
421 mkSkolTyVar :: Name -> Kind -> SkolemInfo -> TcTyVar
422 mkSkolTyVar name kind info = mkTcTyVar name kind (SkolemTv info)
424 tcSkolSigType :: SkolemInfo -> Type -> TcM ([TcTyVar], TcThetaType, TcType)
425 -- Instantiate a type signature with skolem constants, but
426 -- do *not* give them fresh names, because we want the name to
427 -- be in the type environment -- it is lexically scoped.
428 tcSkolSigType info ty = tcInstType (\tvs -> return (tcSkolSigTyVars info tvs)) ty
430 tcSkolSigTyVars :: SkolemInfo -> [TyVar] -> [TcTyVar]
431 -- Make skolem constants, but do *not* give them new names, as above
432 tcSkolSigTyVars info tyvars = [ mkSkolTyVar (tyVarName tv) (tyVarKind tv) info
435 tcInstSkolTyVar :: SkolemInfo -> Maybe SrcSpan -> TyVar -> TcM TcTyVar
436 -- Instantiate the tyvar, using
437 -- * the occ-name and kind of the supplied tyvar,
438 -- * the unique from the monad,
439 -- * the location either from the tyvar (mb_loc = Nothing)
440 -- or from mb_loc (Just loc)
441 tcInstSkolTyVar info mb_loc tyvar
442 = do { uniq <- newUnique
443 ; let old_name = tyVarName tyvar
444 kind = tyVarKind tyvar
445 loc = mb_loc `orElse` getSrcSpan old_name
446 new_name = mkInternalName uniq (nameOccName old_name) loc
447 ; return (mkSkolTyVar new_name kind info) }
449 tcInstSkolTyVars :: SkolemInfo -> [TyVar] -> TcM [TcTyVar]
450 -- Get the location from the monad
451 tcInstSkolTyVars info tyvars
452 = do { span <- getSrcSpanM
453 ; mapM (tcInstSkolTyVar info (Just span)) tyvars }
455 tcInstSkolType :: SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcType)
456 -- Instantiate a type with fresh skolem constants
457 -- Binding location comes from the monad
458 tcInstSkolType info ty = tcInstType (tcInstSkolTyVars info) ty
462 %************************************************************************
464 MetaTvs (meta type variables; mutable)
466 %************************************************************************
469 newMetaTyVar :: BoxInfo -> Kind -> TcM TcTyVar
470 -- Make a new meta tyvar out of thin air
471 newMetaTyVar box_info kind
472 = do { uniq <- newUnique
473 ; ref <- newMutVar Flexi
474 ; let name = mkSysTvName uniq fs
475 fs = case box_info of
478 SigTv _ -> FSLIT("a")
479 -- We give BoxTv and TauTv the same string, because
480 -- otherwise we get user-visible differences in error
481 -- messages, which are confusing. If you want to see
482 -- the box_info of each tyvar, use -dppr-debug
483 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
485 instMetaTyVar :: BoxInfo -> TyVar -> TcM TcTyVar
486 -- Make a new meta tyvar whose Name and Kind
487 -- come from an existing TyVar
488 instMetaTyVar box_info tyvar
489 = do { uniq <- newUnique
490 ; ref <- newMutVar Flexi
491 ; let name = setNameUnique (tyVarName tyvar) uniq
492 kind = tyVarKind tyvar
493 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
495 readMetaTyVar :: TyVar -> TcM MetaDetails
496 readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
497 readMutVar (metaTvRef tyvar)
499 isFilledMetaTyVar :: TyVar -> TcM Bool
500 -- True of a filled-in (Indirect) meta type variable
502 | not (isTcTyVar tv) = return False
503 | MetaTv _ ref <- tcTyVarDetails tv
504 = do { details <- readMutVar ref
505 ; return (isIndirect details) }
506 | otherwise = return False
508 writeMetaTyVar :: TcTyVar -> TcType -> TcM ()
510 writeMetaTyVar tyvar ty = writeMutVar (metaTvRef tyvar) (Indirect ty)
512 writeMetaTyVar tyvar ty
513 | not (isMetaTyVar tyvar)
514 = pprTrace "writeMetaTyVar" (ppr tyvar) $
518 = ASSERT( isMetaTyVar tyvar )
519 -- TOM: It should also work for coercions
520 -- ASSERT2( k2 `isSubKind` k1, (ppr tyvar <+> ppr k1) $$ (ppr ty <+> ppr k2) )
521 do { ASSERTM2( do { details <- readMetaTyVar tyvar; return (isFlexi details) }, ppr tyvar )
522 ; writeMutVar (metaTvRef tyvar) (Indirect ty) }
530 %************************************************************************
534 %************************************************************************
537 newFlexiTyVar :: Kind -> TcM TcTyVar
538 newFlexiTyVar kind = newMetaTyVar TauTv kind
540 newFlexiTyVarTy :: Kind -> TcM TcType
541 newFlexiTyVarTy kind = do
542 tc_tyvar <- newFlexiTyVar kind
543 return (TyVarTy tc_tyvar)
545 newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
546 newFlexiTyVarTys n kind = mapM newFlexiTyVarTy (nOfThem n kind)
548 tcInstTyVar :: TyVar -> TcM TcTyVar
549 -- Instantiate with a META type variable
550 tcInstTyVar tyvar = instMetaTyVar TauTv tyvar
552 tcInstTyVars :: [TyVar] -> TcM ([TcTyVar], [TcType], TvSubst)
553 -- Instantiate with META type variables
555 = do { tc_tvs <- mapM tcInstTyVar tyvars
556 ; let tys = mkTyVarTys tc_tvs
557 ; return (tc_tvs, tys, zipTopTvSubst tyvars tys) }
558 -- Since the tyvars are freshly made,
559 -- they cannot possibly be captured by
560 -- any existing for-alls. Hence zipTopTvSubst
564 %************************************************************************
568 %************************************************************************
571 tcInstSigTyVars :: Bool -> SkolemInfo -> [TyVar] -> TcM [TcTyVar]
572 -- Instantiate with skolems or meta SigTvs; depending on use_skols
573 -- Always take location info from the supplied tyvars
574 tcInstSigTyVars use_skols skol_info tyvars
576 = mapM (tcInstSkolTyVar skol_info Nothing) tyvars
579 = mapM (instMetaTyVar (SigTv skol_info)) tyvars
581 zonkSigTyVar :: TcTyVar -> TcM TcTyVar
583 | isSkolemTyVar sig_tv
584 = return sig_tv -- Happens in the call in TcBinds.checkDistinctTyVars
586 = ASSERT( isSigTyVar sig_tv )
587 do { ty <- zonkTcTyVar sig_tv
588 ; return (tcGetTyVar "zonkSigTyVar" ty) }
589 -- 'ty' is bound to be a type variable, because SigTvs
590 -- can only be unified with type variables
594 %************************************************************************
598 %************************************************************************
601 newBoxyTyVar :: Kind -> TcM BoxyTyVar
602 newBoxyTyVar kind = newMetaTyVar BoxTv kind
604 newBoxyTyVars :: [Kind] -> TcM [BoxyTyVar]
605 newBoxyTyVars kinds = mapM newBoxyTyVar kinds
607 newBoxyTyVarTys :: [Kind] -> TcM [BoxyType]
608 newBoxyTyVarTys kinds = do { tvs <- mapM newBoxyTyVar kinds; return (mkTyVarTys tvs) }
610 readFilledBox :: BoxyTyVar -> TcM TcType
611 -- Read the contents of the box, which should be filled in by now
612 readFilledBox box_tv = ASSERT( isBoxyTyVar box_tv )
613 do { cts <- readMetaTyVar box_tv
615 Flexi -> pprPanic "readFilledBox" (ppr box_tv)
616 Indirect ty -> return ty }
618 tcInstBoxyTyVar :: TyVar -> TcM BoxyTyVar
619 -- Instantiate with a BOXY type variable
620 tcInstBoxyTyVar tyvar = instMetaTyVar BoxTv tyvar
624 %************************************************************************
626 \subsection{Putting and getting mutable type variables}
628 %************************************************************************
630 But it's more fun to short out indirections on the way: If this
631 version returns a TyVar, then that TyVar is unbound. If it returns
632 any other type, then there might be bound TyVars embedded inside it.
634 We return Nothing iff the original box was unbound.
637 data LookupTyVarResult -- The result of a lookupTcTyVar call
638 = DoneTv TcTyVarDetails -- SkolemTv or virgin MetaTv
641 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
643 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
645 SkolemTv _ -> return (DoneTv details)
646 MetaTv _ ref -> do { meta_details <- readMutVar ref
647 ; case meta_details of
648 Indirect ty -> return (IndirectTv ty)
649 Flexi -> return (DoneTv details) }
651 details = tcTyVarDetails tyvar
654 -- gaw 2004 We aren't shorting anything out anymore, at least for now
656 | not (isTcTyVar tyvar)
657 = pprTrace "getTcTyVar" (ppr tyvar) $
658 return (Just (mkTyVarTy tyvar))
661 = ASSERT2( isTcTyVar tyvar, ppr tyvar ) do
662 maybe_ty <- readMetaTyVar tyvar
664 Just ty -> do ty' <- short_out ty
665 writeMetaTyVar tyvar (Just ty')
668 Nothing -> return Nothing
670 short_out :: TcType -> TcM TcType
671 short_out ty@(TyVarTy tyvar)
672 | not (isTcTyVar tyvar)
676 maybe_ty <- readMetaTyVar tyvar
678 Just ty' -> do ty' <- short_out ty'
679 writeMetaTyVar tyvar (Just ty')
684 short_out other_ty = return other_ty
689 %************************************************************************
691 \subsection{Zonking -- the exernal interfaces}
693 %************************************************************************
695 ----------------- Type variables
698 zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
699 zonkTcTyVars tyvars = mapM zonkTcTyVar tyvars
701 zonkTcTyVarsAndFV :: [TcTyVar] -> TcM TcTyVarSet
702 zonkTcTyVarsAndFV tyvars = tyVarsOfTypes <$> mapM zonkTcTyVar tyvars
704 zonkTcTyVar :: TcTyVar -> TcM TcType
705 zonkTcTyVar tyvar = ASSERT2( isTcTyVar tyvar, ppr tyvar)
706 zonk_tc_tyvar (\ tv -> return (TyVarTy tv)) tyvar
709 ----------------- Types
712 zonkTcType :: TcType -> TcM TcType
713 zonkTcType ty = zonkType (\ tv -> return (TyVarTy tv)) ty
715 zonkTcTypes :: [TcType] -> TcM [TcType]
716 zonkTcTypes tys = mapM zonkTcType tys
718 zonkTcClassConstraints cts = mapM zonk cts
719 where zonk (clas, tys) = do
720 new_tys <- zonkTcTypes tys
721 return (clas, new_tys)
723 zonkTcThetaType :: TcThetaType -> TcM TcThetaType
724 zonkTcThetaType theta = mapM zonkTcPredType theta
726 zonkTcPredType :: TcPredType -> TcM TcPredType
727 zonkTcPredType (ClassP c ts) = ClassP c <$> zonkTcTypes ts
728 zonkTcPredType (IParam n t) = IParam n <$> zonkTcType t
729 zonkTcPredType (EqPred t1 t2) = EqPred <$> zonkTcType t1 <*> zonkTcType t2
732 ------------------- These ...ToType, ...ToKind versions
733 are used at the end of type checking
736 zonkTopTyVar :: TcTyVar -> TcM TcTyVar
737 -- zonkTopTyVar is used, at the top level, on any un-instantiated meta type variables
738 -- to default the kind of ? and ?? etc to *. This is important to ensure that
739 -- instance declarations match. For example consider
740 -- instance Show (a->b)
741 -- foo x = show (\_ -> True)
742 -- Then we'll get a constraint (Show (p ->q)) where p has argTypeKind (printed ??),
743 -- and that won't match the typeKind (*) in the instance decl.
745 -- Because we are at top level, no further constraints are going to affect these
746 -- type variables, so it's time to do it by hand. However we aren't ready
747 -- to default them fully to () or whatever, because the type-class defaulting
748 -- rules have yet to run.
751 | k `eqKind` default_k = return tv
753 = do { tv' <- newFlexiTyVar default_k
754 ; writeMetaTyVar tv (mkTyVarTy tv')
758 default_k = defaultKind k
760 zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TcTyVar]
761 zonkQuantifiedTyVars = mapM zonkQuantifiedTyVar
763 zonkQuantifiedTyVar :: TcTyVar -> TcM TcTyVar
764 -- zonkQuantifiedTyVar is applied to the a TcTyVar when quantifying over it.
766 -- The quantified type variables often include meta type variables
767 -- we want to freeze them into ordinary type variables, and
768 -- default their kind (e.g. from OpenTypeKind to TypeKind)
769 -- -- see notes with Kind.defaultKind
770 -- The meta tyvar is updated to point to the new skolem TyVar. Now any
771 -- bound occurences of the original type variable will get zonked to
772 -- the immutable version.
774 -- We leave skolem TyVars alone; they are immutable.
775 zonkQuantifiedTyVar tv
776 | ASSERT( isTcTyVar tv )
777 isSkolemTyVar tv = return tv
778 -- It might be a skolem type variable,
779 -- for example from a user type signature
781 | otherwise -- It's a meta-type-variable
782 = do { details <- readMetaTyVar tv
784 -- Create the new, frozen, skolem type variable
785 -- We zonk to a skolem, not to a regular TcVar
786 -- See Note [Zonking to Skolem]
787 ; let final_kind = defaultKind (tyVarKind tv)
788 final_tv = mkSkolTyVar (tyVarName tv) final_kind UnkSkol
790 -- Bind the meta tyvar to the new tyvar
792 Indirect ty -> WARN( True, ppr tv $$ ppr ty )
794 -- [Sept 04] I don't think this should happen
795 -- See note [Silly Type Synonym]
797 Flexi -> writeMetaTyVar tv (mkTyVarTy final_tv)
799 -- Return the new tyvar
803 Note [Silly Type Synonyms]
804 ~~~~~~~~~~~~~~~~~~~~~~~~~~
806 type C u a = u -- Note 'a' unused
808 foo :: (forall a. C u a -> C u a) -> u
812 bar = foo (\t -> t + t)
814 * From the (\t -> t+t) we get type {Num d} => d -> d
817 * Now unify with type of foo's arg, and we get:
818 {Num (C d a)} => C d a -> C d a
821 * Now abstract over the 'a', but float out the Num (C d a) constraint
822 because it does not 'really' mention a. (see exactTyVarsOfType)
823 The arg to foo becomes
826 * So we get a dict binding for Num (C d a), which is zonked to give
828 [Note Sept 04: now that we are zonking quantified type variables
829 on construction, the 'a' will be frozen as a regular tyvar on
830 quantification, so the floated dict will still have type (C d a).
831 Which renders this whole note moot; happily!]
833 * Then the /\a abstraction has a zonked 'a' in it.
835 All very silly. I think its harmless to ignore the problem. We'll end up with
836 a /\a in the final result but all the occurrences of a will be zonked to ()
838 Note [Zonking to Skolem]
839 ~~~~~~~~~~~~~~~~~~~~~~~~
840 We used to zonk quantified type variables to regular TyVars. However, this
841 leads to problems. Consider this program from the regression test suite:
843 eval :: Int -> String -> String -> String
844 eval 0 root actual = evalRHS 0 root actual
847 evalRHS 0 root actual = eval 0 root actual
849 It leads to the deferral of an equality
851 (String -> String -> String) ~ a
853 which is propagated up to the toplevel (see TcSimplify.tcSimplifyInferCheck).
854 In the meantime `a' is zonked and quantified to form `evalRHS's signature.
855 This has the *side effect* of also zonking the `a' in the deferred equality
856 (which at this point is being handed around wrapped in an implication
859 Finally, the equality (with the zonked `a') will be handed back to the
860 simplifier by TcRnDriver.tcRnSrcDecls calling TcSimplify.tcSimplifyTop.
861 If we zonk `a' with a regular type variable, we will have this regular type
862 variable now floating around in the simplifier, which in many places assumes to
863 only see proper TcTyVars.
865 We can avoid this problem by zonking with a skolem. The skolem is rigid
866 (which we requirefor a quantified variable), but is still a TcTyVar that the
867 simplifier knows how to deal with.
870 %************************************************************************
872 \subsection{Zonking -- the main work-horses: zonkType, zonkTyVar}
874 %* For internal use only! *
876 %************************************************************************
879 -- For unbound, mutable tyvars, zonkType uses the function given to it
880 -- For tyvars bound at a for-all, zonkType zonks them to an immutable
881 -- type variable and zonks the kind too
883 zonkType :: (TcTyVar -> TcM Type) -- What to do with unbound mutable type variables
884 -- see zonkTcType, and zonkTcTypeToType
887 zonkType unbound_var_fn ty
890 go (TyConApp tc tys) = do tys' <- mapM go tys
891 return (TyConApp tc tys')
893 go (PredTy p) = do p' <- go_pred p
896 go (FunTy arg res) = do arg' <- go arg
898 return (FunTy arg' res')
900 go (AppTy fun arg) = do fun' <- go fun
902 return (mkAppTy fun' arg')
903 -- NB the mkAppTy; we might have instantiated a
904 -- type variable to a type constructor, so we need
905 -- to pull the TyConApp to the top.
907 -- The two interesting cases!
908 go (TyVarTy tyvar) | isTcTyVar tyvar = zonk_tc_tyvar unbound_var_fn tyvar
909 | otherwise = return (TyVarTy tyvar)
910 -- Ordinary (non Tc) tyvars occur inside quantified types
912 go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar ) do
914 return (ForAllTy tyvar ty')
916 go_pred (ClassP c tys) = do tys' <- mapM go tys
917 return (ClassP c tys')
918 go_pred (IParam n ty) = do ty' <- go ty
919 return (IParam n ty')
920 go_pred (EqPred ty1 ty2) = do ty1' <- go ty1
922 return (EqPred ty1' ty2')
924 zonk_tc_tyvar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable variable
925 -> TcTyVar -> TcM TcType
926 zonk_tc_tyvar unbound_var_fn tyvar
927 | not (isMetaTyVar tyvar) -- Skolems
928 = return (TyVarTy tyvar)
930 | otherwise -- Mutables
931 = do { cts <- readMetaTyVar tyvar
933 Flexi -> unbound_var_fn tyvar -- Unbound meta type variable
934 Indirect ty -> zonkType unbound_var_fn ty }
939 %************************************************************************
943 %************************************************************************
946 readKindVar :: KindVar -> TcM (MetaDetails)
947 writeKindVar :: KindVar -> TcKind -> TcM ()
948 readKindVar kv = readMutVar (kindVarRef kv)
949 writeKindVar kv val = writeMutVar (kindVarRef kv) (Indirect val)
952 zonkTcKind :: TcKind -> TcM TcKind
953 zonkTcKind k = zonkTcType k
956 zonkTcKindToKind :: TcKind -> TcM Kind
957 -- When zonking a TcKind to a kind, we need to instantiate kind variables,
958 -- Haskell specifies that * is to be used, so we follow that.
959 zonkTcKindToKind k = zonkType (\ _ -> return liftedTypeKind) k
962 %************************************************************************
964 \subsection{Checking a user type}
966 %************************************************************************
968 When dealing with a user-written type, we first translate it from an HsType
969 to a Type, performing kind checking, and then check various things that should
970 be true about it. We don't want to perform these checks at the same time
971 as the initial translation because (a) they are unnecessary for interface-file
972 types and (b) when checking a mutually recursive group of type and class decls,
973 we can't "look" at the tycons/classes yet. Also, the checks are are rather
974 diverse, and used to really mess up the other code.
976 One thing we check for is 'rank'.
978 Rank 0: monotypes (no foralls)
979 Rank 1: foralls at the front only, Rank 0 inside
980 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
982 basic ::= tyvar | T basic ... basic
984 r2 ::= forall tvs. cxt => r2a
985 r2a ::= r1 -> r2a | basic
986 r1 ::= forall tvs. cxt => r0
987 r0 ::= r0 -> r0 | basic
989 Another thing is to check that type synonyms are saturated.
990 This might not necessarily show up in kind checking.
992 data T k = MkT (k Int)
997 checkValidType :: UserTypeCtxt -> Type -> TcM ()
998 -- Checks that the type is valid for the given context
999 checkValidType ctxt ty = do
1000 traceTc (text "checkValidType" <+> ppr ty)
1001 unboxed <- doptM Opt_UnboxedTuples
1002 rank2 <- doptM Opt_Rank2Types
1003 rankn <- doptM Opt_RankNTypes
1004 polycomp <- doptM Opt_PolymorphicComponents
1006 rank | rankn = Arbitrary
1009 = case ctxt of -- Haskell 98
1010 GenPatCtxt -> Rank 0
1011 LamPatSigCtxt -> Rank 0
1012 BindPatSigCtxt -> Rank 0
1013 DefaultDeclCtxt-> Rank 0
1014 ResSigCtxt -> Rank 0
1015 TySynCtxt _ -> Rank 0
1016 ExprSigCtxt -> Rank 1
1017 FunSigCtxt _ -> Rank 1
1018 ConArgCtxt _ -> if polycomp
1020 -- We are given the type of the entire
1021 -- constructor, hence rank 1
1023 ForSigCtxt _ -> Rank 1
1024 SpecInstCtxt -> Rank 1
1026 actual_kind = typeKind ty
1028 kind_ok = case ctxt of
1029 TySynCtxt _ -> True -- Any kind will do
1030 ResSigCtxt -> isSubOpenTypeKind actual_kind
1031 ExprSigCtxt -> isSubOpenTypeKind actual_kind
1032 GenPatCtxt -> isLiftedTypeKind actual_kind
1033 ForSigCtxt _ -> isLiftedTypeKind actual_kind
1034 other -> isSubArgTypeKind actual_kind
1036 ubx_tup = case ctxt of
1037 TySynCtxt _ | unboxed -> UT_Ok
1038 ExprSigCtxt | unboxed -> UT_Ok
1041 -- Check that the thing has kind Type, and is lifted if necessary
1042 checkTc kind_ok (kindErr actual_kind)
1044 -- Check the internal validity of the type itself
1045 check_type rank ubx_tup ty
1047 traceTc (text "checkValidType done" <+> ppr ty)
1049 checkValidMonoType :: Type -> TcM ()
1050 checkValidMonoType ty = check_mono_type ty
1055 data Rank = Rank Int | Arbitrary
1057 decRank :: Rank -> Rank
1058 decRank Arbitrary = Arbitrary
1059 decRank (Rank n) = Rank (n-1)
1061 nonZeroRank :: Rank -> Bool
1062 nonZeroRank (Rank 0) = False
1063 nonZeroRank _ = True
1065 ----------------------------------------
1066 data UbxTupFlag = UT_Ok | UT_NotOk
1067 -- The "Ok" version means "ok if -fglasgow-exts is on"
1069 ----------------------------------------
1070 check_mono_type :: Type -> TcM () -- No foralls anywhere
1071 -- No unlifted types of any kind
1073 = do { check_type (Rank 0) UT_NotOk ty
1074 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1076 check_type :: Rank -> UbxTupFlag -> Type -> TcM ()
1077 -- The args say what the *type* context requires, independent
1078 -- of *flag* settings. You test the flag settings at usage sites.
1080 -- Rank is allowed rank for function args
1081 -- Rank 0 means no for-alls anywhere
1083 check_type rank ubx_tup ty
1084 | not (null tvs && null theta)
1085 = do { checkTc (nonZeroRank rank) (forAllTyErr ty)
1086 -- Reject e.g. (Maybe (?x::Int => Int)),
1087 -- with a decent error message
1088 ; check_valid_theta SigmaCtxt theta
1089 ; check_type rank ubx_tup tau -- Allow foralls to right of arrow
1090 ; checkFreeness tvs theta
1091 ; checkAmbiguity tvs theta (tyVarsOfType tau) }
1093 (tvs, theta, tau) = tcSplitSigmaTy ty
1095 -- Naked PredTys don't usually show up, but they can as a result of
1096 -- {-# SPECIALISE instance Ord Char #-}
1097 -- The Right Thing would be to fix the way that SPECIALISE instance pragmas
1098 -- are handled, but the quick thing is just to permit PredTys here.
1099 check_type rank ubx_tup (PredTy sty)
1100 = do { dflags <- getDOpts
1101 ; check_pred_ty dflags TypeCtxt sty }
1103 check_type rank ubx_tup (TyVarTy _) = return ()
1104 check_type rank ubx_tup ty@(FunTy arg_ty res_ty)
1105 = do { check_type (decRank rank) UT_NotOk arg_ty
1106 ; check_type rank UT_Ok res_ty }
1108 check_type rank ubx_tup (AppTy ty1 ty2)
1109 = do { check_arg_type rank ty1
1110 ; check_arg_type rank ty2 }
1112 check_type rank ubx_tup ty@(TyConApp tc tys)
1114 = do { -- Check that the synonym has enough args
1115 -- This applies equally to open and closed synonyms
1116 -- It's OK to have an *over-applied* type synonym
1117 -- data Tree a b = ...
1118 -- type Foo a = Tree [a]
1119 -- f :: Foo a b -> ...
1120 checkTc (tyConArity tc <= length tys) arity_msg
1122 -- See Note [Liberal type synonyms]
1123 ; liberal <- doptM Opt_LiberalTypeSynonyms
1124 ; if not liberal || isOpenSynTyCon tc then
1125 -- For H98 and synonym families, do check the type args
1126 mapM_ check_mono_type tys
1128 else -- In the liberal case (only for closed syns), expand then check
1130 Just ty' -> check_type rank ubx_tup ty'
1131 Nothing -> pprPanic "check_tau_type" (ppr ty)
1134 | isUnboxedTupleTyCon tc
1135 = do { ub_tuples_allowed <- doptM Opt_UnboxedTuples
1136 ; checkTc (ubx_tup_ok ub_tuples_allowed) ubx_tup_msg
1138 ; impred <- doptM Opt_ImpredicativeTypes
1139 ; let rank' = if impred then rank else Rank 0
1140 -- c.f. check_arg_type
1141 -- However, args are allowed to be unlifted, or
1142 -- more unboxed tuples, so can't use check_arg_ty
1143 ; mapM_ (check_type rank' UT_Ok) tys }
1146 = mapM_ (check_arg_type rank) tys
1149 ubx_tup_ok ub_tuples_allowed = case ubx_tup of { UT_Ok -> ub_tuples_allowed; other -> False }
1152 tc_arity = tyConArity tc
1154 arity_msg = arityErr "Type synonym" (tyConName tc) tc_arity n_args
1155 ubx_tup_msg = ubxArgTyErr ty
1157 ----------------------------------------
1158 check_arg_type :: Rank -> Type -> TcM ()
1159 -- The sort of type that can instantiate a type variable,
1160 -- or be the argument of a type constructor.
1161 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
1162 -- Other unboxed types are very occasionally allowed as type
1163 -- arguments depending on the kind of the type constructor
1165 -- For example, we want to reject things like:
1167 -- instance Ord a => Ord (forall s. T s a)
1169 -- g :: T s (forall b.b)
1171 -- NB: unboxed tuples can have polymorphic or unboxed args.
1172 -- This happens in the workers for functions returning
1173 -- product types with polymorphic components.
1174 -- But not in user code.
1175 -- Anyway, they are dealt with by a special case in check_tau_type
1177 check_arg_type rank ty
1178 = do { impred <- doptM Opt_ImpredicativeTypes
1179 ; let rank' = if impred then rank else Rank 0 -- Monotype unless impredicative
1180 ; check_type rank' UT_NotOk ty
1181 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1183 ----------------------------------------
1184 forAllTyErr ty = ptext SLIT("Illegal polymorphic or qualified type:") <+> ppr ty
1185 unliftedArgErr ty = ptext SLIT("Illegal unlifted type:") <+> ppr ty
1186 ubxArgTyErr ty = ptext SLIT("Illegal unboxed tuple type as function argument:") <+> ppr ty
1187 kindErr kind = ptext SLIT("Expecting an ordinary type, but found a type of kind") <+> ppr kind
1190 Note [Liberal type synonyms]
1191 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1192 If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
1193 doing validity checking. This allows us to instantiate a synonym defn
1194 with a for-all type, or with a partially-applied type synonym.
1198 Here, T is partially applied, so it's illegal in H98. But if you
1199 expand S first, then T we get just
1203 IMPORTANT: suppose T is a type synonym. Then we must do validity
1204 checking on an appliation (T ty1 ty2)
1206 *either* before expansion (i.e. check ty1, ty2)
1207 *or* after expansion (i.e. expand T ty1 ty2, and then check)
1210 If we do both, we get exponential behaviour!!
1212 data TIACons1 i r c = c i ::: r c
1213 type TIACons2 t x = TIACons1 t (TIACons1 t x)
1214 type TIACons3 t x = TIACons2 t (TIACons1 t x)
1215 type TIACons4 t x = TIACons2 t (TIACons2 t x)
1216 type TIACons7 t x = TIACons4 t (TIACons3 t x)
1219 %************************************************************************
1221 \subsection{Checking a theta or source type}
1223 %************************************************************************
1226 -- Enumerate the contexts in which a "source type", <S>, can occur
1230 -- or (N a) where N is a newtype
1233 = ClassSCCtxt Name -- Superclasses of clas
1234 -- class <S> => C a where ...
1235 | SigmaCtxt -- Theta part of a normal for-all type
1236 -- f :: <S> => a -> a
1237 | DataTyCtxt Name -- Theta part of a data decl
1238 -- data <S> => T a = MkT a
1239 | TypeCtxt -- Source type in an ordinary type
1241 | InstThetaCtxt -- Context of an instance decl
1242 -- instance <S> => C [a] where ...
1244 pprSourceTyCtxt (ClassSCCtxt c) = ptext SLIT("the super-classes of class") <+> quotes (ppr c)
1245 pprSourceTyCtxt SigmaCtxt = ptext SLIT("the context of a polymorphic type")
1246 pprSourceTyCtxt (DataTyCtxt tc) = ptext SLIT("the context of the data type declaration for") <+> quotes (ppr tc)
1247 pprSourceTyCtxt InstThetaCtxt = ptext SLIT("the context of an instance declaration")
1248 pprSourceTyCtxt TypeCtxt = ptext SLIT("the context of a type")
1252 checkValidTheta :: SourceTyCtxt -> ThetaType -> TcM ()
1253 checkValidTheta ctxt theta
1254 = addErrCtxt (checkThetaCtxt ctxt theta) (check_valid_theta ctxt theta)
1256 -------------------------
1257 check_valid_theta ctxt []
1259 check_valid_theta ctxt theta = do
1261 warnTc (notNull dups) (dupPredWarn dups)
1262 mapM_ (check_pred_ty dflags ctxt) theta
1264 (_,dups) = removeDups tcCmpPred theta
1266 -------------------------
1267 check_pred_ty :: DynFlags -> SourceTyCtxt -> PredType -> TcM ()
1268 check_pred_ty dflags ctxt pred@(ClassP cls tys)
1269 = do { -- Class predicates are valid in all contexts
1270 ; checkTc (arity == n_tys) arity_err
1272 -- Check the form of the argument types
1273 ; mapM_ check_mono_type tys
1274 ; checkTc (check_class_pred_tys dflags ctxt tys)
1275 (predTyVarErr pred $$ how_to_allow)
1278 class_name = className cls
1279 arity = classArity cls
1281 arity_err = arityErr "Class" class_name arity n_tys
1282 how_to_allow = parens (ptext SLIT("Use -XFlexibleContexts to permit this"))
1284 check_pred_ty dflags ctxt pred@(EqPred ty1 ty2)
1285 = do { -- Equational constraints are valid in all contexts if type
1286 -- families are permitted
1287 ; checkTc (dopt Opt_TypeFamilies dflags) (eqPredTyErr pred)
1289 -- Check the form of the argument types
1290 ; check_mono_type ty1
1291 ; check_mono_type ty2
1294 check_pred_ty dflags SigmaCtxt (IParam _ ty) = check_mono_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 = mapM_ 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 $ mapM_ 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) ;
1470 Just (clas,tys) -> do
1473 mapM_ check_mono_type tys
1474 check_inst_head dflags clas tys
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)
1489 mapM_ check_mono_type tys
1490 -- For now, I only allow tau-types (not polytypes) in
1491 -- the head of an instance decl.
1492 -- E.g. instance C (forall a. a->a) is rejected
1493 -- One could imagine generalising that, but I'm not sure
1494 -- what all the consequences might be
1497 head_type_synonym_msg = parens (
1498 text "All instance types must be of the form (T t1 ... tn)" $$
1499 text "where T is not a synonym." $$
1500 text "Use -XTypeSynonymInstances if you want to disable this.")
1502 head_type_args_tyvars_msg = parens (vcat [
1503 text "All instance types must be of the form (T a1 ... an)",
1504 text "where a1 ... an are type *variables*,",
1505 text "and each type variable appears at most once in the instance head.",
1506 text "Use -XFlexibleInstances if you want to disable this."])
1508 head_one_type_msg = parens (
1509 text "Only one type can be given in an instance head." $$
1510 text "Use -XMultiParamTypeClasses if you want to allow more.")
1512 instTypeErr pp_ty msg
1513 = sep [ptext SLIT("Illegal instance declaration for") <+> quotes pp_ty,
1518 %************************************************************************
1520 \subsection{Checking instance for termination}
1522 %************************************************************************
1526 checkValidInstance :: [TyVar] -> ThetaType -> Class -> [TcType] -> TcM ()
1527 checkValidInstance tyvars theta clas inst_tys
1528 = do { undecidable_ok <- doptM Opt_UndecidableInstances
1530 ; checkValidTheta InstThetaCtxt theta
1531 ; checkAmbiguity tyvars theta (tyVarsOfTypes inst_tys)
1533 -- Check that instance inference will terminate (if we care)
1534 -- For Haskell 98 this will already have been done by checkValidTheta,
1535 -- but as we may be using other extensions we need to check.
1536 ; unless undecidable_ok $
1537 mapM_ addErrTc (checkInstTermination inst_tys theta)
1539 -- The Coverage Condition
1540 ; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
1541 (instTypeErr (pprClassPred clas inst_tys) msg)
1544 msg = parens (vcat [ptext SLIT("the Coverage Condition fails for one of the functional dependencies;"),
1548 Termination test: the so-called "Paterson conditions" (see Section 5 of
1549 "Understanding functionsl dependencies via Constraint Handling Rules,
1552 We check that each assertion in the context satisfies:
1553 (1) no variable has more occurrences in the assertion than in the head, and
1554 (2) the assertion has fewer constructors and variables (taken together
1555 and counting repetitions) than the head.
1556 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1557 (which have already been checked) guarantee termination.
1559 The underlying idea is that
1561 for any ground substitution, each assertion in the
1562 context has fewer type constructors than the head.
1566 checkInstTermination :: [TcType] -> ThetaType -> [Message]
1567 checkInstTermination tys theta
1568 = mapCatMaybes check theta
1571 size = sizeTypes tys
1573 | not (null (fvPred pred \\ fvs))
1574 = Just (predUndecErr pred nomoreMsg $$ parens undecidableMsg)
1575 | sizePred pred >= size
1576 = Just (predUndecErr pred smallerMsg $$ parens undecidableMsg)
1580 predUndecErr pred msg = sep [msg,
1581 nest 2 (ptext SLIT("in the constraint:") <+> pprPred pred)]
1583 nomoreMsg = ptext SLIT("Variable occurs more often in a constraint than in the instance head")
1584 smallerMsg = ptext SLIT("Constraint is no smaller than the instance head")
1585 undecidableMsg = ptext SLIT("Use -fallow-undecidable-instances to permit this")
1589 %************************************************************************
1591 Checking the context of a derived instance declaration
1593 %************************************************************************
1595 Note [Exotic derived instance contexts]
1596 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1597 In a 'derived' instance declaration, we *infer* the context. It's a
1598 bit unclear what rules we should apply for this; the Haskell report is
1599 silent. Obviously, constraints like (Eq a) are fine, but what about
1600 data T f a = MkT (f a) deriving( Eq )
1601 where we'd get an Eq (f a) constraint. That's probably fine too.
1603 One could go further: consider
1604 data T a b c = MkT (Foo a b c) deriving( Eq )
1605 instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
1607 Notice that this instance (just) satisfies the Paterson termination
1608 conditions. Then we *could* derive an instance decl like this:
1610 instance (C Int a, Eq b, Eq c) => Eq (T a b c)
1612 even though there is no instance for (C Int a), because there just
1613 *might* be an instance for, say, (C Int Bool) at a site where we
1614 need the equality instance for T's.
1616 However, this seems pretty exotic, and it's quite tricky to allow
1617 this, and yet give sensible error messages in the (much more common)
1618 case where we really want that instance decl for C.
1620 So for now we simply require that the derived instance context
1621 should have only type-variable constraints.
1623 Here is another example:
1624 data Fix f = In (f (Fix f)) deriving( Eq )
1625 Here, if we are prepared to allow -fallow-undecidable-instances we
1626 could derive the instance
1627 instance Eq (f (Fix f)) => Eq (Fix f)
1628 but this is so delicate that I don't think it should happen inside
1629 'deriving'. If you want this, write it yourself!
1631 NB: if you want to lift this condition, make sure you still meet the
1632 termination conditions! If not, the deriving mechanism generates
1633 larger and larger constraints. Example:
1635 data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
1637 Note the lack of a Show instance for Succ. First we'll generate
1638 instance (Show (Succ a), Show a) => Show (Seq a)
1640 instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
1641 and so on. Instead we want to complain of no instance for (Show (Succ a)).
1645 Allow constraints which consist only of type variables, with no repeats.
1648 validDerivPred :: PredType -> Bool
1649 validDerivPred (ClassP cls tys) = hasNoDups fvs && sizeTypes tys == length fvs
1650 where fvs = fvTypes tys
1651 validDerivPred otehr = False
1654 %************************************************************************
1656 Checking type instance well-formedness and termination
1658 %************************************************************************
1661 -- Check that a "type instance" is well-formed (which includes decidability
1662 -- unless -fallow-undecidable-instances is given).
1664 checkValidTypeInst :: [Type] -> Type -> TcM ()
1665 checkValidTypeInst typats rhs
1666 = do { -- left-hand side contains no type family applications
1667 -- (vanilla synonyms are fine, though)
1668 ; mapM_ checkTyFamFreeness typats
1670 -- the right-hand side is a tau type
1671 ; checkTc (isTauTy rhs) $
1674 -- we have a decidable instance unless otherwise permitted
1675 ; undecidable_ok <- doptM Opt_UndecidableInstances
1676 ; unless undecidable_ok $
1677 mapM_ addErrTc (checkFamInst typats (tyFamInsts rhs))
1680 -- Make sure that each type family instance is
1681 -- (1) strictly smaller than the lhs,
1682 -- (2) mentions no type variable more often than the lhs, and
1683 -- (3) does not contain any further type family instances.
1685 checkFamInst :: [Type] -- lhs
1686 -> [(TyCon, [Type])] -- type family instances
1688 checkFamInst lhsTys famInsts
1689 = mapCatMaybes check famInsts
1691 size = sizeTypes lhsTys
1692 fvs = fvTypes lhsTys
1694 | not (all isTyFamFree tys)
1695 = Just (famInstUndecErr famInst nestedMsg $$ parens undecidableMsg)
1696 | not (null (fvTypes tys \\ fvs))
1697 = Just (famInstUndecErr famInst nomoreVarMsg $$ parens undecidableMsg)
1698 | size <= sizeTypes tys
1699 = Just (famInstUndecErr famInst smallerAppMsg $$ parens undecidableMsg)
1703 famInst = TyConApp tc tys
1705 -- Ensure that no type family instances occur in a type.
1707 checkTyFamFreeness :: Type -> TcM ()
1708 checkTyFamFreeness ty
1709 = checkTc (isTyFamFree ty) $
1710 tyFamInstInIndexErr ty
1712 -- Check that a type does not contain any type family applications.
1714 isTyFamFree :: Type -> Bool
1715 isTyFamFree = null . tyFamInsts
1719 tyFamInstInIndexErr ty
1720 = hang (ptext SLIT("Illegal type family application in type instance") <>
1725 = hang (ptext SLIT("Illegal polymorphic type in type instance") <> colon) 4 $
1728 famInstUndecErr ty msg
1730 nest 2 (ptext SLIT("in the type family application:") <+>
1733 nestedMsg = ptext SLIT("Nested type family application")
1734 nomoreVarMsg = ptext SLIT("Variable occurs more often than in instance head")
1735 smallerAppMsg = ptext SLIT("Application is no smaller than the instance head")
1739 %************************************************************************
1741 \subsection{Auxiliary functions}
1743 %************************************************************************
1746 -- Free variables of a type, retaining repetitions, and expanding synonyms
1747 fvType :: Type -> [TyVar]
1748 fvType ty | Just exp_ty <- tcView ty = fvType exp_ty
1749 fvType (TyVarTy tv) = [tv]
1750 fvType (TyConApp _ tys) = fvTypes tys
1751 fvType (PredTy pred) = fvPred pred
1752 fvType (FunTy arg res) = fvType arg ++ fvType res
1753 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1754 fvType (ForAllTy tyvar ty) = filter (/= tyvar) (fvType ty)
1756 fvTypes :: [Type] -> [TyVar]
1757 fvTypes tys = concat (map fvType tys)
1759 fvPred :: PredType -> [TyVar]
1760 fvPred (ClassP _ tys') = fvTypes tys'
1761 fvPred (IParam _ ty) = fvType ty
1762 fvPred (EqPred ty1 ty2) = fvType ty1 ++ fvType ty2
1764 -- Size of a type: the number of variables and constructors
1765 sizeType :: Type -> Int
1766 sizeType ty | Just exp_ty <- tcView ty = sizeType exp_ty
1767 sizeType (TyVarTy _) = 1
1768 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1769 sizeType (PredTy pred) = sizePred pred
1770 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1771 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1772 sizeType (ForAllTy _ ty) = sizeType ty
1774 sizeTypes :: [Type] -> Int
1775 sizeTypes xs = sum (map sizeType xs)
1777 sizePred :: PredType -> Int
1778 sizePred (ClassP _ tys') = sizeTypes tys'
1779 sizePred (IParam _ ty) = sizeType ty
1780 sizePred (EqPred ty1 ty2) = sizeType ty1 + sizeType ty2