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 = returnM ()
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' <- mappM go tys
224 ; return $ occurs (TyConApp tc) tys' }
225 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
226 go (PredTy p) = do { p' <- go_pred p
227 ; return $ occurs1 PredTy p' }
228 go (FunTy arg res) = do { arg' <- go arg
230 ; return $ occurs2 FunTy arg' res' }
231 go (AppTy fun arg) = do { fun' <- go fun
233 ; return $ occurs2 mkAppTy fun' arg' }
234 -- NB the mkAppTy; we might have instantiated a
235 -- type variable to a type constructor, so we need
236 -- to pull the TyConApp to the top.
237 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
240 | orig_tv == tv = return $ Left False -- (a)
241 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
242 | otherwise = return $ Right (TyVarTy tv)
243 -- Ordinary (non Tc) tyvars
244 -- occur inside quantified types
246 go_pred (ClassP c tys) = do { tys' <- mapM go tys
247 ; return $ occurs (ClassP c) tys' }
248 go_pred (IParam n ty) = do { ty' <- go ty
249 ; return $ occurs1 (IParam n) ty' }
250 go_pred (EqPred t1 t2) = do { t1' <- go t1
252 ; return $ occurs2 EqPred t1' t2' }
254 go_tyvar tv (SkolemTv _) = return $ Right (TyVarTy tv)
255 go_tyvar tv (MetaTv box ref)
256 = do { cts <- readMutVar ref
260 BoxTv -> do { ty <- fillBoxWithTau tv ref
261 ; return $ Right ty }
262 other -> return $ Right (TyVarTy tv)
265 -- go_syn is called for synonyms only
266 -- See Note [Type synonyms and the occur check]
268 | not (isTauTyCon tc)
269 = notMonoType orig_ty -- (b) again
271 = do { (msgs, mb_tys') <- tryTc (mapM go tys)
274 -- we had a type error => forall in type parameters
276 | isOpenTyCon tc -> notMonoArgs (TyConApp tc tys)
277 -- Synonym families must have monotype args
278 | otherwise -> go (expectJust "checkTauTvUpdate(1)"
279 (tcView (TyConApp tc tys)))
280 -- Try again, expanding the synonym
282 -- no type error, but need to test whether occurs check happend
284 case occurs id tys' of
286 | isOpenTyCon tc -> return $ Left True
287 -- Variable occured under type family application
288 | otherwise -> go (expectJust "checkTauTvUpdate(2)"
289 (tcView (TyConApp tc tys)))
290 -- Try again, expanding the synonym
291 Right raw_tys' -> return $ Right (TyConApp tc raw_tys')
292 -- Retain the synonym (the common case)
295 -- Left results (= occurrence of orig_ty) dominate and
296 -- (Left False) (= fatal occurrence) dominates over (Left True)
297 occurs :: ([a] -> b) -> [Either Bool a] -> Either Bool b
298 occurs c = either Left (Right . c) . foldr combine (Right [])
300 combine (Left famInst1) (Left famInst2) = Left (famInst1 && famInst2)
301 combine (Right _ ) (Left famInst) = Left famInst
302 combine (Left famInst) (Right _) = Left famInst
303 combine (Right arg) (Right args) = Right (arg:args)
305 occurs1 c x = occurs (\[x'] -> c x') [x]
306 occurs2 c x y = occurs (\[x', y'] -> c x' y') [x, y]
308 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
309 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
310 -- tau-type meta-variable, whose print-name is the same as tv
311 -- Choosing the same name is good: when we instantiate a function
312 -- we allocate boxy tyvars with the same print-name as the quantified
313 -- tyvar; and then we often fill the box with a tau-tyvar, and again
314 -- we want to choose the same name.
315 fillBoxWithTau tv ref
316 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
317 ; let tau = mkTyVarTy tv' -- name of the type variable
318 ; writeMutVar ref (Indirect tau)
322 Note [Type synonyms and the occur check]
324 Basically we want to update tv1 := ps_ty2
325 because ps_ty2 has type-synonym info, which improves later error messages
330 f :: (A a -> a -> ()) -> ()
336 In the application (p x), we try to match "t" with "A t". If we go
337 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
338 an infinite loop later.
339 But we should not reject the program, because A t = ().
340 Rather, we should bind t to () (= non_var_ty2).
344 Error mesages in case of kind mismatch.
347 unifyKindMisMatch ty1 ty2
348 = zonkTcKind ty1 `thenM` \ ty1' ->
349 zonkTcKind ty2 `thenM` \ ty2' ->
351 msg = hang (ptext SLIT("Couldn't match kind"))
352 2 (sep [quotes (ppr ty1'),
353 ptext SLIT("against"),
358 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
359 -- tv1 and ty2 are zonked already
362 msg = (env2, ptext SLIT("When matching the kinds of") <+>
363 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
365 (pp_expected, pp_actual) | swapped = (pp2, pp1)
366 | otherwise = (pp1, pp2)
367 (env1, tv1') = tidyOpenTyVar tidy_env tv1
368 (env2, ty2') = tidyOpenType env1 ty2
369 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
370 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
373 Error message for failure due to an occurs check.
376 occurCheckErr :: TcType -> TcType -> TcM a
377 occurCheckErr ty containingTy
378 = do { env0 <- tcInitTidyEnv
379 ; ty' <- zonkTcType ty
380 ; containingTy' <- zonkTcType containingTy
381 ; let (env1, tidy_ty1) = tidyOpenType env0 ty'
382 (env2, tidy_ty2) = tidyOpenType env1 containingTy'
383 extra = sep [ppr tidy_ty1, char '=', ppr tidy_ty2]
384 ; failWithTcM (env2, hang msg 2 extra) }
386 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
389 %************************************************************************
393 %************************************************************************
396 newCoVars :: [(TcType,TcType)] -> TcM [CoVar]
398 = do { us <- newUniqueSupply
399 ; return [ mkCoVar (mkSysTvName uniq FSLIT("co"))
401 | ((ty1,ty2), uniq) <- spec `zip` uniqsFromSupply us] }
403 newMetaCoVar :: TcType -> TcType -> TcM TcTyVar
404 newMetaCoVar ty1 ty2 = newMetaTyVar TauTv (mkCoKind ty1 ty2)
406 newKindVar :: TcM TcKind
407 newKindVar = do { uniq <- newUnique
408 ; ref <- newMutVar Flexi
409 ; return (mkTyVarTy (mkKindVar uniq ref)) }
411 newKindVars :: Int -> TcM [TcKind]
412 newKindVars n = mappM (\ _ -> newKindVar) (nOfThem n ())
416 %************************************************************************
418 SkolemTvs (immutable)
420 %************************************************************************
423 mkSkolTyVar :: Name -> Kind -> SkolemInfo -> TcTyVar
424 mkSkolTyVar name kind info = mkTcTyVar name kind (SkolemTv info)
426 tcSkolSigType :: SkolemInfo -> Type -> TcM ([TcTyVar], TcThetaType, TcType)
427 -- Instantiate a type signature with skolem constants, but
428 -- do *not* give them fresh names, because we want the name to
429 -- be in the type environment -- it is lexically scoped.
430 tcSkolSigType info ty = tcInstType (\tvs -> return (tcSkolSigTyVars info tvs)) ty
432 tcSkolSigTyVars :: SkolemInfo -> [TyVar] -> [TcTyVar]
433 -- Make skolem constants, but do *not* give them new names, as above
434 tcSkolSigTyVars info tyvars = [ mkSkolTyVar (tyVarName tv) (tyVarKind tv) info
437 tcInstSkolTyVar :: SkolemInfo -> Maybe SrcSpan -> TyVar -> TcM TcTyVar
438 -- Instantiate the tyvar, using
439 -- * the occ-name and kind of the supplied tyvar,
440 -- * the unique from the monad,
441 -- * the location either from the tyvar (mb_loc = Nothing)
442 -- or from mb_loc (Just loc)
443 tcInstSkolTyVar info mb_loc tyvar
444 = do { uniq <- newUnique
445 ; let old_name = tyVarName tyvar
446 kind = tyVarKind tyvar
447 loc = mb_loc `orElse` getSrcSpan old_name
448 new_name = mkInternalName uniq (nameOccName old_name) loc
449 ; return (mkSkolTyVar new_name kind info) }
451 tcInstSkolTyVars :: SkolemInfo -> [TyVar] -> TcM [TcTyVar]
452 -- Get the location from the monad
453 tcInstSkolTyVars info tyvars
454 = do { span <- getSrcSpanM
455 ; mapM (tcInstSkolTyVar info (Just span)) tyvars }
457 tcInstSkolType :: SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcType)
458 -- Instantiate a type with fresh skolem constants
459 -- Binding location comes from the monad
460 tcInstSkolType info ty = tcInstType (tcInstSkolTyVars info) ty
464 %************************************************************************
466 MetaTvs (meta type variables; mutable)
468 %************************************************************************
471 newMetaTyVar :: BoxInfo -> Kind -> TcM TcTyVar
472 -- Make a new meta tyvar out of thin air
473 newMetaTyVar box_info kind
474 = do { uniq <- newUnique
475 ; ref <- newMutVar Flexi
476 ; let name = mkSysTvName uniq fs
477 fs = case box_info of
480 SigTv _ -> FSLIT("a")
481 -- We give BoxTv and TauTv the same string, because
482 -- otherwise we get user-visible differences in error
483 -- messages, which are confusing. If you want to see
484 -- the box_info of each tyvar, use -dppr-debug
485 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
487 instMetaTyVar :: BoxInfo -> TyVar -> TcM TcTyVar
488 -- Make a new meta tyvar whose Name and Kind
489 -- come from an existing TyVar
490 instMetaTyVar box_info tyvar
491 = do { uniq <- newUnique
492 ; ref <- newMutVar Flexi
493 ; let name = setNameUnique (tyVarName tyvar) uniq
494 kind = tyVarKind tyvar
495 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
497 readMetaTyVar :: TyVar -> TcM MetaDetails
498 readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
499 readMutVar (metaTvRef tyvar)
501 isFilledMetaTyVar :: TyVar -> TcM Bool
502 -- True of a filled-in (Indirect) meta type variable
504 | not (isTcTyVar tv) = return False
505 | MetaTv _ ref <- tcTyVarDetails tv
506 = do { details <- readMutVar ref
507 ; return (isIndirect details) }
508 | otherwise = return False
510 writeMetaTyVar :: TcTyVar -> TcType -> TcM ()
512 writeMetaTyVar tyvar ty = writeMutVar (metaTvRef tyvar) (Indirect ty)
514 writeMetaTyVar tyvar ty
515 | not (isMetaTyVar tyvar)
516 = pprTrace "writeMetaTyVar" (ppr tyvar) $
520 = ASSERT( isMetaTyVar tyvar )
521 -- TOM: It should also work for coercions
522 -- ASSERT2( k2 `isSubKind` k1, (ppr tyvar <+> ppr k1) $$ (ppr ty <+> ppr k2) )
523 do { ASSERTM2( do { details <- readMetaTyVar tyvar; return (isFlexi details) }, ppr tyvar )
524 ; writeMutVar (metaTvRef tyvar) (Indirect ty) }
532 %************************************************************************
536 %************************************************************************
539 newFlexiTyVar :: Kind -> TcM TcTyVar
540 newFlexiTyVar kind = newMetaTyVar TauTv kind
542 newFlexiTyVarTy :: Kind -> TcM TcType
544 = newFlexiTyVar kind `thenM` \ tc_tyvar ->
545 returnM (TyVarTy tc_tyvar)
547 newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
548 newFlexiTyVarTys n kind = mappM newFlexiTyVarTy (nOfThem n kind)
550 tcInstTyVar :: TyVar -> TcM TcTyVar
551 -- Instantiate with a META type variable
552 tcInstTyVar tyvar = instMetaTyVar TauTv tyvar
554 tcInstTyVars :: [TyVar] -> TcM ([TcTyVar], [TcType], TvSubst)
555 -- Instantiate with META type variables
557 = do { tc_tvs <- mapM tcInstTyVar tyvars
558 ; let tys = mkTyVarTys tc_tvs
559 ; returnM (tc_tvs, tys, zipTopTvSubst tyvars tys) }
560 -- Since the tyvars are freshly made,
561 -- they cannot possibly be captured by
562 -- any existing for-alls. Hence zipTopTvSubst
566 %************************************************************************
570 %************************************************************************
573 tcInstSigTyVars :: Bool -> SkolemInfo -> [TyVar] -> TcM [TcTyVar]
574 -- Instantiate with skolems or meta SigTvs; depending on use_skols
575 -- Always take location info from the supplied tyvars
576 tcInstSigTyVars use_skols skol_info tyvars
578 = mapM (tcInstSkolTyVar skol_info Nothing) tyvars
581 = mapM (instMetaTyVar (SigTv skol_info)) tyvars
583 zonkSigTyVar :: TcTyVar -> TcM TcTyVar
585 | isSkolemTyVar sig_tv
586 = return sig_tv -- Happens in the call in TcBinds.checkDistinctTyVars
588 = ASSERT( isSigTyVar sig_tv )
589 do { ty <- zonkTcTyVar sig_tv
590 ; return (tcGetTyVar "zonkSigTyVar" ty) }
591 -- 'ty' is bound to be a type variable, because SigTvs
592 -- can only be unified with type variables
596 %************************************************************************
600 %************************************************************************
603 newBoxyTyVar :: Kind -> TcM BoxyTyVar
604 newBoxyTyVar kind = newMetaTyVar BoxTv kind
606 newBoxyTyVars :: [Kind] -> TcM [BoxyTyVar]
607 newBoxyTyVars kinds = mapM newBoxyTyVar kinds
609 newBoxyTyVarTys :: [Kind] -> TcM [BoxyType]
610 newBoxyTyVarTys kinds = do { tvs <- mapM newBoxyTyVar kinds; return (mkTyVarTys tvs) }
612 readFilledBox :: BoxyTyVar -> TcM TcType
613 -- Read the contents of the box, which should be filled in by now
614 readFilledBox box_tv = ASSERT( isBoxyTyVar box_tv )
615 do { cts <- readMetaTyVar box_tv
617 Flexi -> pprPanic "readFilledBox" (ppr box_tv)
618 Indirect ty -> return ty }
620 tcInstBoxyTyVar :: TyVar -> TcM BoxyTyVar
621 -- Instantiate with a BOXY type variable
622 tcInstBoxyTyVar tyvar = instMetaTyVar BoxTv tyvar
626 %************************************************************************
628 \subsection{Putting and getting mutable type variables}
630 %************************************************************************
632 But it's more fun to short out indirections on the way: If this
633 version returns a TyVar, then that TyVar is unbound. If it returns
634 any other type, then there might be bound TyVars embedded inside it.
636 We return Nothing iff the original box was unbound.
639 data LookupTyVarResult -- The result of a lookupTcTyVar call
640 = DoneTv TcTyVarDetails -- SkolemTv or virgin MetaTv
643 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
645 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
647 SkolemTv _ -> return (DoneTv details)
648 MetaTv _ ref -> do { meta_details <- readMutVar ref
649 ; case meta_details of
650 Indirect ty -> return (IndirectTv ty)
651 Flexi -> return (DoneTv details) }
653 details = tcTyVarDetails tyvar
656 -- gaw 2004 We aren't shorting anything out anymore, at least for now
658 | not (isTcTyVar tyvar)
659 = pprTrace "getTcTyVar" (ppr tyvar) $
660 returnM (Just (mkTyVarTy tyvar))
663 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
664 readMetaTyVar tyvar `thenM` \ maybe_ty ->
666 Just ty -> short_out ty `thenM` \ ty' ->
667 writeMetaTyVar tyvar (Just ty') `thenM_`
670 Nothing -> returnM Nothing
672 short_out :: TcType -> TcM TcType
673 short_out ty@(TyVarTy tyvar)
674 | not (isTcTyVar tyvar)
678 = readMetaTyVar tyvar `thenM` \ maybe_ty ->
680 Just ty' -> short_out ty' `thenM` \ ty' ->
681 writeMetaTyVar tyvar (Just ty') `thenM_`
686 short_out other_ty = returnM other_ty
691 %************************************************************************
693 \subsection{Zonking -- the exernal interfaces}
695 %************************************************************************
697 ----------------- Type variables
700 zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
701 zonkTcTyVars tyvars = mappM zonkTcTyVar tyvars
703 zonkTcTyVarsAndFV :: [TcTyVar] -> TcM TcTyVarSet
704 zonkTcTyVarsAndFV tyvars = mappM zonkTcTyVar tyvars `thenM` \ tys ->
705 returnM (tyVarsOfTypes tys)
707 zonkTcTyVar :: TcTyVar -> TcM TcType
708 zonkTcTyVar tyvar = ASSERT2( isTcTyVar tyvar, ppr tyvar)
709 zonk_tc_tyvar (\ tv -> returnM (TyVarTy tv)) tyvar
712 ----------------- Types
715 zonkTcType :: TcType -> TcM TcType
716 zonkTcType ty = zonkType (\ tv -> returnM (TyVarTy tv)) ty
718 zonkTcTypes :: [TcType] -> TcM [TcType]
719 zonkTcTypes tys = mappM zonkTcType tys
721 zonkTcClassConstraints cts = mappM zonk cts
722 where zonk (clas, tys)
723 = zonkTcTypes tys `thenM` \ new_tys ->
724 returnM (clas, new_tys)
726 zonkTcThetaType :: TcThetaType -> TcM TcThetaType
727 zonkTcThetaType theta = mappM zonkTcPredType theta
729 zonkTcPredType :: TcPredType -> TcM TcPredType
730 zonkTcPredType (ClassP c ts)
731 = zonkTcTypes ts `thenM` \ new_ts ->
732 returnM (ClassP c new_ts)
733 zonkTcPredType (IParam n t)
734 = zonkTcType t `thenM` \ new_t ->
735 returnM (IParam n new_t)
736 zonkTcPredType (EqPred t1 t2)
737 = zonkTcType t1 `thenM` \ new_t1 ->
738 zonkTcType t2 `thenM` \ new_t2 ->
739 returnM (EqPred new_t1 new_t2)
742 ------------------- These ...ToType, ...ToKind versions
743 are used at the end of type checking
746 zonkTopTyVar :: TcTyVar -> TcM TcTyVar
747 -- zonkTopTyVar is used, at the top level, on any un-instantiated meta type variables
748 -- to default the kind of ? and ?? etc to *. This is important to ensure that
749 -- instance declarations match. For example consider
750 -- instance Show (a->b)
751 -- foo x = show (\_ -> True)
752 -- Then we'll get a constraint (Show (p ->q)) where p has argTypeKind (printed ??),
753 -- and that won't match the typeKind (*) in the instance decl.
755 -- Because we are at top level, no further constraints are going to affect these
756 -- type variables, so it's time to do it by hand. However we aren't ready
757 -- to default them fully to () or whatever, because the type-class defaulting
758 -- rules have yet to run.
761 | k `eqKind` default_k = return tv
763 = do { tv' <- newFlexiTyVar default_k
764 ; writeMetaTyVar tv (mkTyVarTy tv')
768 default_k = defaultKind k
770 zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TcTyVar]
771 zonkQuantifiedTyVars = mappM zonkQuantifiedTyVar
773 zonkQuantifiedTyVar :: TcTyVar -> TcM TcTyVar
774 -- zonkQuantifiedTyVar is applied to the a TcTyVar when quantifying over it.
776 -- The quantified type variables often include meta type variables
777 -- we want to freeze them into ordinary type variables, and
778 -- default their kind (e.g. from OpenTypeKind to TypeKind)
779 -- -- see notes with Kind.defaultKind
780 -- The meta tyvar is updated to point to the new skolem TyVar. Now any
781 -- bound occurences of the original type variable will get zonked to
782 -- the immutable version.
784 -- We leave skolem TyVars alone; they are immutable.
785 zonkQuantifiedTyVar tv
786 | ASSERT( isTcTyVar tv )
787 isSkolemTyVar tv = return tv
788 -- It might be a skolem type variable,
789 -- for example from a user type signature
791 | otherwise -- It's a meta-type-variable
792 = do { details <- readMetaTyVar tv
794 -- Create the new, frozen, skolem type variable
795 -- We zonk to a skolem, not to a regular TcVar
796 -- See Note [Zonking to Skolem]
797 ; let final_kind = defaultKind (tyVarKind tv)
798 final_tv = mkSkolTyVar (tyVarName tv) final_kind UnkSkol
800 -- Bind the meta tyvar to the new tyvar
802 Indirect ty -> WARN( True, ppr tv $$ ppr ty )
804 -- [Sept 04] I don't think this should happen
805 -- See note [Silly Type Synonym]
807 Flexi -> writeMetaTyVar tv (mkTyVarTy final_tv)
809 -- Return the new tyvar
813 Note [Silly Type Synonyms]
814 ~~~~~~~~~~~~~~~~~~~~~~~~~~
816 type C u a = u -- Note 'a' unused
818 foo :: (forall a. C u a -> C u a) -> u
822 bar = foo (\t -> t + t)
824 * From the (\t -> t+t) we get type {Num d} => d -> d
827 * Now unify with type of foo's arg, and we get:
828 {Num (C d a)} => C d a -> C d a
831 * Now abstract over the 'a', but float out the Num (C d a) constraint
832 because it does not 'really' mention a. (see exactTyVarsOfType)
833 The arg to foo becomes
836 * So we get a dict binding for Num (C d a), which is zonked to give
838 [Note Sept 04: now that we are zonking quantified type variables
839 on construction, the 'a' will be frozen as a regular tyvar on
840 quantification, so the floated dict will still have type (C d a).
841 Which renders this whole note moot; happily!]
843 * Then the /\a abstraction has a zonked 'a' in it.
845 All very silly. I think its harmless to ignore the problem. We'll end up with
846 a /\a in the final result but all the occurrences of a will be zonked to ()
848 Note [Zonking to Skolem]
849 ~~~~~~~~~~~~~~~~~~~~~~~~
850 We used to zonk quantified type variables to regular TyVars. However, this
851 leads to problems. Consider this program from the regression test suite:
853 eval :: Int -> String -> String -> String
854 eval 0 root actual = evalRHS 0 root actual
857 evalRHS 0 root actual = eval 0 root actual
859 It leads to the deferral of an equality
861 (String -> String -> String) ~ a
863 which is propagated up to the toplevel (see TcSimplify.tcSimplifyInferCheck).
864 In the meantime `a' is zonked and quantified to form `evalRHS's signature.
865 This has the *side effect* of also zonking the `a' in the deferred equality
866 (which at this point is being handed around wrapped in an implication
869 Finally, the equality (with the zonked `a') will be handed back to the
870 simplifier by TcRnDriver.tcRnSrcDecls calling TcSimplify.tcSimplifyTop.
871 If we zonk `a' with a regular type variable, we will have this regular type
872 variable now floating around in the simplifier, which in many places assumes to
873 only see proper TcTyVars.
875 We can avoid this problem by zonking with a skolem. The skolem is rigid
876 (which we requirefor a quantified variable), but is still a TcTyVar that the
877 simplifier knows how to deal with.
880 %************************************************************************
882 \subsection{Zonking -- the main work-horses: zonkType, zonkTyVar}
884 %* For internal use only! *
886 %************************************************************************
889 -- For unbound, mutable tyvars, zonkType uses the function given to it
890 -- For tyvars bound at a for-all, zonkType zonks them to an immutable
891 -- type variable and zonks the kind too
893 zonkType :: (TcTyVar -> TcM Type) -- What to do with unbound mutable type variables
894 -- see zonkTcType, and zonkTcTypeToType
897 zonkType unbound_var_fn ty
900 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
902 go (TyConApp tc tys) = mappM go tys `thenM` \ tys' ->
903 returnM (TyConApp tc tys')
905 go (PredTy p) = go_pred p `thenM` \ p' ->
908 go (FunTy arg res) = go arg `thenM` \ arg' ->
909 go res `thenM` \ res' ->
910 returnM (FunTy arg' res')
912 go (AppTy fun arg) = go fun `thenM` \ fun' ->
913 go arg `thenM` \ arg' ->
914 returnM (mkAppTy fun' arg')
915 -- NB the mkAppTy; we might have instantiated a
916 -- type variable to a type constructor, so we need
917 -- to pull the TyConApp to the top.
919 -- The two interesting cases!
920 go (TyVarTy tyvar) | isTcTyVar tyvar = zonk_tc_tyvar unbound_var_fn tyvar
921 | otherwise = return (TyVarTy tyvar)
922 -- Ordinary (non Tc) tyvars occur inside quantified types
924 go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar )
925 go ty `thenM` \ ty' ->
926 returnM (ForAllTy tyvar ty')
928 go_pred (ClassP c tys) = mappM go tys `thenM` \ tys' ->
929 returnM (ClassP c tys')
930 go_pred (IParam n ty) = go ty `thenM` \ ty' ->
931 returnM (IParam n ty')
932 go_pred (EqPred ty1 ty2) = go ty1 `thenM` \ ty1' ->
933 go ty2 `thenM` \ ty2' ->
934 returnM (EqPred ty1' ty2')
936 zonk_tc_tyvar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable variable
937 -> TcTyVar -> TcM TcType
938 zonk_tc_tyvar unbound_var_fn tyvar
939 | not (isMetaTyVar tyvar) -- Skolems
940 = returnM (TyVarTy tyvar)
942 | otherwise -- Mutables
943 = do { cts <- readMetaTyVar tyvar
945 Flexi -> unbound_var_fn tyvar -- Unbound meta type variable
946 Indirect ty -> zonkType unbound_var_fn ty }
951 %************************************************************************
955 %************************************************************************
958 readKindVar :: KindVar -> TcM (MetaDetails)
959 writeKindVar :: KindVar -> TcKind -> TcM ()
960 readKindVar kv = readMutVar (kindVarRef kv)
961 writeKindVar kv val = writeMutVar (kindVarRef kv) (Indirect val)
964 zonkTcKind :: TcKind -> TcM TcKind
965 zonkTcKind k = zonkTcType k
968 zonkTcKindToKind :: TcKind -> TcM Kind
969 -- When zonking a TcKind to a kind, we need to instantiate kind variables,
970 -- Haskell specifies that * is to be used, so we follow that.
971 zonkTcKindToKind k = zonkType (\ _ -> return liftedTypeKind) k
974 %************************************************************************
976 \subsection{Checking a user type}
978 %************************************************************************
980 When dealing with a user-written type, we first translate it from an HsType
981 to a Type, performing kind checking, and then check various things that should
982 be true about it. We don't want to perform these checks at the same time
983 as the initial translation because (a) they are unnecessary for interface-file
984 types and (b) when checking a mutually recursive group of type and class decls,
985 we can't "look" at the tycons/classes yet. Also, the checks are are rather
986 diverse, and used to really mess up the other code.
988 One thing we check for is 'rank'.
990 Rank 0: monotypes (no foralls)
991 Rank 1: foralls at the front only, Rank 0 inside
992 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
994 basic ::= tyvar | T basic ... basic
996 r2 ::= forall tvs. cxt => r2a
997 r2a ::= r1 -> r2a | basic
998 r1 ::= forall tvs. cxt => r0
999 r0 ::= r0 -> r0 | basic
1001 Another thing is to check that type synonyms are saturated.
1002 This might not necessarily show up in kind checking.
1004 data T k = MkT (k Int)
1009 checkValidType :: UserTypeCtxt -> Type -> TcM ()
1010 -- Checks that the type is valid for the given context
1011 checkValidType ctxt ty
1012 = traceTc (text "checkValidType" <+> ppr ty) `thenM_`
1013 doptM Opt_UnboxedTuples `thenM` \ unboxed ->
1014 doptM Opt_Rank2Types `thenM` \ rank2 ->
1015 doptM Opt_RankNTypes `thenM` \ rankn ->
1016 doptM Opt_PolymorphicComponents `thenM` \ polycomp ->
1018 rank | rankn = Arbitrary
1021 = case ctxt of -- Haskell 98
1022 GenPatCtxt -> Rank 0
1023 LamPatSigCtxt -> Rank 0
1024 BindPatSigCtxt -> Rank 0
1025 DefaultDeclCtxt-> Rank 0
1026 ResSigCtxt -> Rank 0
1027 TySynCtxt _ -> Rank 0
1028 ExprSigCtxt -> Rank 1
1029 FunSigCtxt _ -> Rank 1
1030 ConArgCtxt _ -> if polycomp
1032 -- We are given the type of the entire
1033 -- constructor, hence rank 1
1035 ForSigCtxt _ -> Rank 1
1036 SpecInstCtxt -> Rank 1
1038 actual_kind = typeKind ty
1040 kind_ok = case ctxt of
1041 TySynCtxt _ -> True -- Any kind will do
1042 ResSigCtxt -> isSubOpenTypeKind actual_kind
1043 ExprSigCtxt -> isSubOpenTypeKind actual_kind
1044 GenPatCtxt -> isLiftedTypeKind actual_kind
1045 ForSigCtxt _ -> isLiftedTypeKind actual_kind
1046 other -> isSubArgTypeKind actual_kind
1048 ubx_tup = case ctxt of
1049 TySynCtxt _ | unboxed -> UT_Ok
1050 ExprSigCtxt | unboxed -> UT_Ok
1053 -- Check that the thing has kind Type, and is lifted if necessary
1054 checkTc kind_ok (kindErr actual_kind) `thenM_`
1056 -- Check the internal validity of the type itself
1057 check_type rank ubx_tup ty `thenM_`
1059 traceTc (text "checkValidType done" <+> ppr ty)
1061 checkValidMonoType :: Type -> TcM ()
1062 checkValidMonoType ty = check_mono_type ty
1067 data Rank = Rank Int | Arbitrary
1069 decRank :: Rank -> Rank
1070 decRank Arbitrary = Arbitrary
1071 decRank (Rank n) = Rank (n-1)
1073 nonZeroRank :: Rank -> Bool
1074 nonZeroRank (Rank 0) = False
1075 nonZeroRank _ = True
1077 ----------------------------------------
1078 data UbxTupFlag = UT_Ok | UT_NotOk
1079 -- The "Ok" version means "ok if -fglasgow-exts is on"
1081 ----------------------------------------
1082 check_mono_type :: Type -> TcM () -- No foralls anywhere
1083 -- No unlifted types of any kind
1085 = do { check_type (Rank 0) UT_NotOk ty
1086 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1088 check_type :: Rank -> UbxTupFlag -> Type -> TcM ()
1089 -- The args say what the *type* context requires, independent
1090 -- of *flag* settings. You test the flag settings at usage sites.
1092 -- Rank is allowed rank for function args
1093 -- Rank 0 means no for-alls anywhere
1095 check_type rank ubx_tup ty
1096 | not (null tvs && null theta)
1097 = do { checkTc (nonZeroRank rank) (forAllTyErr ty)
1098 -- Reject e.g. (Maybe (?x::Int => Int)),
1099 -- with a decent error message
1100 ; check_valid_theta SigmaCtxt theta
1101 ; check_type rank ubx_tup tau -- Allow foralls to right of arrow
1102 ; checkFreeness tvs theta
1103 ; checkAmbiguity tvs theta (tyVarsOfType tau) }
1105 (tvs, theta, tau) = tcSplitSigmaTy ty
1107 -- Naked PredTys don't usually show up, but they can as a result of
1108 -- {-# SPECIALISE instance Ord Char #-}
1109 -- The Right Thing would be to fix the way that SPECIALISE instance pragmas
1110 -- are handled, but the quick thing is just to permit PredTys here.
1111 check_type rank ubx_tup (PredTy sty)
1112 = do { dflags <- getDOpts
1113 ; check_pred_ty dflags TypeCtxt sty }
1115 check_type rank ubx_tup (TyVarTy _) = returnM ()
1116 check_type rank ubx_tup ty@(FunTy arg_ty res_ty)
1117 = do { check_type (decRank rank) UT_NotOk arg_ty
1118 ; check_type rank UT_Ok res_ty }
1120 check_type rank ubx_tup (AppTy ty1 ty2)
1121 = do { check_arg_type rank ty1
1122 ; check_arg_type rank ty2 }
1124 check_type rank ubx_tup (NoteTy other_note ty)
1125 = check_type rank ubx_tup ty
1127 check_type rank ubx_tup ty@(TyConApp tc tys)
1129 = do { -- Check that the synonym has enough args
1130 -- This applies equally to open and closed synonyms
1131 -- It's OK to have an *over-applied* type synonym
1132 -- data Tree a b = ...
1133 -- type Foo a = Tree [a]
1134 -- f :: Foo a b -> ...
1135 checkTc (tyConArity tc <= length tys) arity_msg
1137 -- See Note [Liberal type synonyms]
1138 ; liberal <- doptM Opt_LiberalTypeSynonyms
1139 ; if not liberal || isOpenSynTyCon tc then
1140 -- For H98 and synonym families, do check the type args
1141 mappM_ check_mono_type tys
1143 else -- In the liberal case (only for closed syns), expand then check
1145 Just ty' -> check_type rank ubx_tup ty'
1146 Nothing -> pprPanic "check_tau_type" (ppr ty)
1149 | isUnboxedTupleTyCon tc
1150 = do { ub_tuples_allowed <- doptM Opt_UnboxedTuples
1151 ; checkTc (ubx_tup_ok ub_tuples_allowed) ubx_tup_msg
1153 ; impred <- doptM Opt_ImpredicativeTypes
1154 ; let rank' = if impred then rank else Rank 0
1155 -- c.f. check_arg_type
1156 -- However, args are allowed to be unlifted, or
1157 -- more unboxed tuples, so can't use check_arg_ty
1158 ; mappM_ (check_type rank' UT_Ok) tys }
1161 = mappM_ (check_arg_type rank) tys
1164 ubx_tup_ok ub_tuples_allowed = case ubx_tup of { UT_Ok -> ub_tuples_allowed; other -> False }
1167 tc_arity = tyConArity tc
1169 arity_msg = arityErr "Type synonym" (tyConName tc) tc_arity n_args
1170 ubx_tup_msg = ubxArgTyErr ty
1172 ----------------------------------------
1173 check_arg_type :: Rank -> Type -> TcM ()
1174 -- The sort of type that can instantiate a type variable,
1175 -- or be the argument of a type constructor.
1176 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
1177 -- Other unboxed types are very occasionally allowed as type
1178 -- arguments depending on the kind of the type constructor
1180 -- For example, we want to reject things like:
1182 -- instance Ord a => Ord (forall s. T s a)
1184 -- g :: T s (forall b.b)
1186 -- NB: unboxed tuples can have polymorphic or unboxed args.
1187 -- This happens in the workers for functions returning
1188 -- product types with polymorphic components.
1189 -- But not in user code.
1190 -- Anyway, they are dealt with by a special case in check_tau_type
1192 check_arg_type rank ty
1193 = do { impred <- doptM Opt_ImpredicativeTypes
1194 ; let rank' = if impred then rank else Rank 0 -- Monotype unless impredicative
1195 ; check_type rank' UT_NotOk ty
1196 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1198 ----------------------------------------
1199 forAllTyErr ty = ptext SLIT("Illegal polymorphic or qualified type:") <+> ppr ty
1200 unliftedArgErr ty = ptext SLIT("Illegal unlifted type:") <+> ppr ty
1201 ubxArgTyErr ty = ptext SLIT("Illegal unboxed tuple type as function argument:") <+> ppr ty
1202 kindErr kind = ptext SLIT("Expecting an ordinary type, but found a type of kind") <+> ppr kind
1205 Note [Liberal type synonyms]
1206 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1207 If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
1208 doing validity checking. This allows us to instantiate a synonym defn
1209 with a for-all type, or with a partially-applied type synonym.
1213 Here, T is partially applied, so it's illegal in H98. But if you
1214 expand S first, then T we get just
1218 IMPORTANT: suppose T is a type synonym. Then we must do validity
1219 checking on an appliation (T ty1 ty2)
1221 *either* before expansion (i.e. check ty1, ty2)
1222 *or* after expansion (i.e. expand T ty1 ty2, and then check)
1225 If we do both, we get exponential behaviour!!
1227 data TIACons1 i r c = c i ::: r c
1228 type TIACons2 t x = TIACons1 t (TIACons1 t x)
1229 type TIACons3 t x = TIACons2 t (TIACons1 t x)
1230 type TIACons4 t x = TIACons2 t (TIACons2 t x)
1231 type TIACons7 t x = TIACons4 t (TIACons3 t x)
1234 %************************************************************************
1236 \subsection{Checking a theta or source type}
1238 %************************************************************************
1241 -- Enumerate the contexts in which a "source type", <S>, can occur
1245 -- or (N a) where N is a newtype
1248 = ClassSCCtxt Name -- Superclasses of clas
1249 -- class <S> => C a where ...
1250 | SigmaCtxt -- Theta part of a normal for-all type
1251 -- f :: <S> => a -> a
1252 | DataTyCtxt Name -- Theta part of a data decl
1253 -- data <S> => T a = MkT a
1254 | TypeCtxt -- Source type in an ordinary type
1256 | InstThetaCtxt -- Context of an instance decl
1257 -- instance <S> => C [a] where ...
1259 pprSourceTyCtxt (ClassSCCtxt c) = ptext SLIT("the super-classes of class") <+> quotes (ppr c)
1260 pprSourceTyCtxt SigmaCtxt = ptext SLIT("the context of a polymorphic type")
1261 pprSourceTyCtxt (DataTyCtxt tc) = ptext SLIT("the context of the data type declaration for") <+> quotes (ppr tc)
1262 pprSourceTyCtxt InstThetaCtxt = ptext SLIT("the context of an instance declaration")
1263 pprSourceTyCtxt TypeCtxt = ptext SLIT("the context of a type")
1267 checkValidTheta :: SourceTyCtxt -> ThetaType -> TcM ()
1268 checkValidTheta ctxt theta
1269 = addErrCtxt (checkThetaCtxt ctxt theta) (check_valid_theta ctxt theta)
1271 -------------------------
1272 check_valid_theta ctxt []
1274 check_valid_theta ctxt theta
1275 = getDOpts `thenM` \ dflags ->
1276 warnTc (notNull dups) (dupPredWarn dups) `thenM_`
1277 mappM_ (check_pred_ty dflags ctxt) theta
1279 (_,dups) = removeDups tcCmpPred theta
1281 -------------------------
1282 check_pred_ty :: DynFlags -> SourceTyCtxt -> PredType -> TcM ()
1283 check_pred_ty dflags ctxt pred@(ClassP cls tys)
1284 = do { -- Class predicates are valid in all contexts
1285 ; checkTc (arity == n_tys) arity_err
1287 -- Check the form of the argument types
1288 ; mappM_ check_mono_type tys
1289 ; checkTc (check_class_pred_tys dflags ctxt tys)
1290 (predTyVarErr pred $$ how_to_allow)
1293 class_name = className cls
1294 arity = classArity cls
1296 arity_err = arityErr "Class" class_name arity n_tys
1297 how_to_allow = parens (ptext SLIT("Use -XFlexibleContexts to permit this"))
1299 check_pred_ty dflags ctxt pred@(EqPred ty1 ty2)
1300 = do { -- Equational constraints are valid in all contexts if type
1301 -- families are permitted
1302 ; checkTc (dopt Opt_TypeFamilies dflags) (eqPredTyErr pred)
1304 -- Check the form of the argument types
1305 ; check_mono_type ty1
1306 ; check_mono_type ty2
1309 check_pred_ty dflags SigmaCtxt (IParam _ ty) = check_mono_type ty
1310 -- Implicit parameters only allowed in type
1311 -- signatures; not in instance decls, superclasses etc
1312 -- The reason for not allowing implicit params in instances is a bit
1314 -- If we allowed instance (?x::Int, Eq a) => Foo [a] where ...
1315 -- then when we saw (e :: (?x::Int) => t) it would be unclear how to
1316 -- discharge all the potential usas of the ?x in e. For example, a
1317 -- constraint Foo [Int] might come out of e,and applying the
1318 -- instance decl would show up two uses of ?x.
1321 check_pred_ty dflags ctxt sty = failWithTc (badPredTyErr sty)
1323 -------------------------
1324 check_class_pred_tys :: DynFlags -> SourceTyCtxt -> [Type] -> Bool
1325 check_class_pred_tys dflags ctxt tys
1327 TypeCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
1328 InstThetaCtxt -> flexible_contexts || undecidable_ok || all tcIsTyVarTy tys
1329 -- Further checks on head and theta in
1330 -- checkInstTermination
1331 other -> flexible_contexts || all tyvar_head tys
1333 flexible_contexts = dopt Opt_FlexibleContexts dflags
1334 undecidable_ok = dopt Opt_UndecidableInstances dflags
1336 -------------------------
1337 tyvar_head ty -- Haskell 98 allows predicates of form
1338 | tcIsTyVarTy ty = True -- C (a ty1 .. tyn)
1339 | otherwise -- where a is a type variable
1340 = case tcSplitAppTy_maybe ty of
1341 Just (ty, _) -> tyvar_head ty
1348 is ambiguous if P contains generic variables
1349 (i.e. one of the Vs) that are not mentioned in tau
1351 However, we need to take account of functional dependencies
1352 when we speak of 'mentioned in tau'. Example:
1353 class C a b | a -> b where ...
1355 forall x y. (C x y) => x
1356 is not ambiguous because x is mentioned and x determines y
1358 NB; the ambiguity check is only used for *user* types, not for types
1359 coming from inteface files. The latter can legitimately have
1360 ambiguous types. Example
1362 class S a where s :: a -> (Int,Int)
1363 instance S Char where s _ = (1,1)
1364 f:: S a => [a] -> Int -> (Int,Int)
1365 f (_::[a]) x = (a*x,b)
1366 where (a,b) = s (undefined::a)
1368 Here the worker for f gets the type
1369 fw :: forall a. S a => Int -> (# Int, Int #)
1371 If the list of tv_names is empty, we have a monotype, and then we
1372 don't need to check for ambiguity either, because the test can't fail
1377 checkAmbiguity :: [TyVar] -> ThetaType -> TyVarSet -> TcM ()
1378 checkAmbiguity forall_tyvars theta tau_tyvars
1379 = mappM_ complain (filter is_ambig theta)
1381 complain pred = addErrTc (ambigErr pred)
1382 extended_tau_vars = grow theta tau_tyvars
1384 -- See Note [Implicit parameters and ambiguity] in TcSimplify
1385 is_ambig pred = isClassPred pred &&
1386 any ambig_var (varSetElems (tyVarsOfPred pred))
1388 ambig_var ct_var = (ct_var `elem` forall_tyvars) &&
1389 not (ct_var `elemVarSet` extended_tau_vars)
1392 = sep [ptext SLIT("Ambiguous constraint") <+> quotes (pprPred pred),
1393 nest 4 (ptext SLIT("At least one of the forall'd type variables mentioned by the constraint") $$
1394 ptext SLIT("must be reachable from the type after the '=>'"))]
1397 In addition, GHC insists that at least one type variable
1398 in each constraint is in V. So we disallow a type like
1399 forall a. Eq b => b -> b
1400 even in a scope where b is in scope.
1403 checkFreeness forall_tyvars theta
1404 = do { flexible_contexts <- doptM Opt_FlexibleContexts
1405 ; unless flexible_contexts $ mappM_ complain (filter is_free theta) }
1407 is_free pred = not (isIPPred pred)
1408 && not (any bound_var (varSetElems (tyVarsOfPred pred)))
1409 bound_var ct_var = ct_var `elem` forall_tyvars
1410 complain pred = addErrTc (freeErr pred)
1413 = sep [ ptext SLIT("All of the type variables in the constraint") <+>
1414 quotes (pprPred pred)
1415 , ptext SLIT("are already in scope") <+>
1416 ptext SLIT("(at least one must be universally quantified here)")
1418 ptext SLIT("(Use -XFlexibleContexts to lift this restriction)")
1423 checkThetaCtxt ctxt theta
1424 = vcat [ptext SLIT("In the context:") <+> pprTheta theta,
1425 ptext SLIT("While checking") <+> pprSourceTyCtxt ctxt ]
1427 badPredTyErr sty = ptext SLIT("Illegal constraint") <+> pprPred sty
1428 eqPredTyErr sty = ptext SLIT("Illegal equational constraint") <+> pprPred sty
1430 parens (ptext SLIT("Use -XTypeFamilies to permit this"))
1431 predTyVarErr pred = sep [ptext SLIT("Non type-variable argument"),
1432 nest 2 (ptext SLIT("in the constraint:") <+> pprPred pred)]
1433 dupPredWarn dups = ptext SLIT("Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
1435 arityErr kind name n m
1436 = hsep [ text kind, quotes (ppr name), ptext SLIT("should have"),
1437 n_arguments <> comma, text "but has been given", int m]
1439 n_arguments | n == 0 = ptext SLIT("no arguments")
1440 | n == 1 = ptext SLIT("1 argument")
1441 | True = hsep [int n, ptext SLIT("arguments")]
1445 = do { ty' <- zonkTcType ty
1446 ; env0 <- tcInitTidyEnv
1447 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1448 msg = ptext SLIT("Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1449 ; failWithTcM (env1, msg) }
1452 = do { ty' <- zonkTcType ty
1453 ; env0 <- tcInitTidyEnv
1454 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1455 msg = ptext SLIT("Arguments of type synonym families must be monotypes") <+> quotes (ppr tidy_ty)
1456 ; failWithTcM (env1, msg) }
1460 %************************************************************************
1462 \subsection{Checking for a decent instance head type}
1464 %************************************************************************
1466 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1467 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1469 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1470 flag is on, or (2)~the instance is imported (they must have been
1471 compiled elsewhere). In these cases, we let them go through anyway.
1473 We can also have instances for functions: @instance Foo (a -> b) ...@.
1476 checkValidInstHead :: Type -> TcM (Class, [TcType])
1478 checkValidInstHead ty -- Should be a source type
1479 = case tcSplitPredTy_maybe ty of {
1480 Nothing -> failWithTc (instTypeErr (ppr ty) empty) ;
1483 case getClassPredTys_maybe pred of {
1484 Nothing -> failWithTc (instTypeErr (pprPred pred) empty) ;
1487 getDOpts `thenM` \ dflags ->
1488 mappM_ check_mono_type tys `thenM_`
1489 check_inst_head dflags clas tys `thenM_`
1493 check_inst_head dflags clas tys
1494 -- If GlasgowExts then check at least one isn't a type variable
1495 = do checkTc (dopt Opt_TypeSynonymInstances dflags ||
1496 all tcInstHeadTyNotSynonym tys)
1497 (instTypeErr (pprClassPred clas tys) head_type_synonym_msg)
1498 checkTc (dopt Opt_FlexibleInstances dflags ||
1499 all tcInstHeadTyAppAllTyVars tys)
1500 (instTypeErr (pprClassPred clas tys) head_type_args_tyvars_msg)
1501 checkTc (dopt Opt_MultiParamTypeClasses dflags ||
1503 (instTypeErr (pprClassPred clas tys) head_one_type_msg)
1504 mapM_ check_mono_type tys
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
1512 head_type_synonym_msg = parens (
1513 text "All instance types must be of the form (T t1 ... tn)" $$
1514 text "where T is not a synonym." $$
1515 text "Use -XTypeSynonymInstances if you want to disable this.")
1517 head_type_args_tyvars_msg = parens (
1518 text "All instance types must be of the form (T a1 ... an)" $$
1519 text "where a1 ... an are distinct type *variables*" $$
1520 text "Use -XFlexibleInstances if you want to disable this.")
1522 head_one_type_msg = parens (
1523 text "Only one type can be given in an instance head." $$
1524 text "Use -XMultiParamTypeClasses if you want to allow more.")
1526 instTypeErr pp_ty msg
1527 = sep [ptext SLIT("Illegal instance declaration for") <+> quotes pp_ty,
1532 %************************************************************************
1534 \subsection{Checking instance for termination}
1536 %************************************************************************
1540 checkValidInstance :: [TyVar] -> ThetaType -> Class -> [TcType] -> TcM ()
1541 checkValidInstance tyvars theta clas inst_tys
1542 = do { undecidable_ok <- doptM Opt_UndecidableInstances
1544 ; checkValidTheta InstThetaCtxt theta
1545 ; checkAmbiguity tyvars theta (tyVarsOfTypes inst_tys)
1547 -- Check that instance inference will terminate (if we care)
1548 -- For Haskell 98 this will already have been done by checkValidTheta,
1549 -- but as we may be using other extensions we need to check.
1550 ; unless undecidable_ok $
1551 mapM_ addErrTc (checkInstTermination inst_tys theta)
1553 -- The Coverage Condition
1554 ; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
1555 (instTypeErr (pprClassPred clas inst_tys) msg)
1558 msg = parens (vcat [ptext SLIT("the Coverage Condition fails for one of the functional dependencies;"),
1562 Termination test: the so-called "Paterson conditions" (see Section 5 of
1563 "Understanding functionsl dependencies via Constraint Handling Rules,
1566 We check that each assertion in the context satisfies:
1567 (1) no variable has more occurrences in the assertion than in the head, and
1568 (2) the assertion has fewer constructors and variables (taken together
1569 and counting repetitions) than the head.
1570 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1571 (which have already been checked) guarantee termination.
1573 The underlying idea is that
1575 for any ground substitution, each assertion in the
1576 context has fewer type constructors than the head.
1580 checkInstTermination :: [TcType] -> ThetaType -> [Message]
1581 checkInstTermination tys theta
1582 = mapCatMaybes check theta
1585 size = sizeTypes tys
1587 | not (null (fvPred pred \\ fvs))
1588 = Just (predUndecErr pred nomoreMsg $$ parens undecidableMsg)
1589 | sizePred pred >= size
1590 = Just (predUndecErr pred smallerMsg $$ parens undecidableMsg)
1594 predUndecErr pred msg = sep [msg,
1595 nest 2 (ptext SLIT("in the constraint:") <+> pprPred pred)]
1597 nomoreMsg = ptext SLIT("Variable occurs more often in a constraint than in the instance head")
1598 smallerMsg = ptext SLIT("Constraint is no smaller than the instance head")
1599 undecidableMsg = ptext SLIT("Use -fallow-undecidable-instances to permit this")
1603 %************************************************************************
1605 Checking the context of a derived instance declaration
1607 %************************************************************************
1609 Note [Exotic derived instance contexts]
1610 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1611 In a 'derived' instance declaration, we *infer* the context. It's a
1612 bit unclear what rules we should apply for this; the Haskell report is
1613 silent. Obviously, constraints like (Eq a) are fine, but what about
1614 data T f a = MkT (f a) deriving( Eq )
1615 where we'd get an Eq (f a) constraint. That's probably fine too.
1617 One could go further: consider
1618 data T a b c = MkT (Foo a b c) deriving( Eq )
1619 instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
1621 Notice that this instance (just) satisfies the Paterson termination
1622 conditions. Then we *could* derive an instance decl like this:
1624 instance (C Int a, Eq b, Eq c) => Eq (T a b c)
1626 even though there is no instance for (C Int a), because there just
1627 *might* be an instance for, say, (C Int Bool) at a site where we
1628 need the equality instance for T's.
1630 However, this seems pretty exotic, and it's quite tricky to allow
1631 this, and yet give sensible error messages in the (much more common)
1632 case where we really want that instance decl for C.
1634 So for now we simply require that the derived instance context
1635 should have only type-variable constraints.
1637 Here is another example:
1638 data Fix f = In (f (Fix f)) deriving( Eq )
1639 Here, if we are prepared to allow -fallow-undecidable-instances we
1640 could derive the instance
1641 instance Eq (f (Fix f)) => Eq (Fix f)
1642 but this is so delicate that I don't think it should happen inside
1643 'deriving'. If you want this, write it yourself!
1645 NB: if you want to lift this condition, make sure you still meet the
1646 termination conditions! If not, the deriving mechanism generates
1647 larger and larger constraints. Example:
1649 data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
1651 Note the lack of a Show instance for Succ. First we'll generate
1652 instance (Show (Succ a), Show a) => Show (Seq a)
1654 instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
1655 and so on. Instead we want to complain of no instance for (Show (Succ a)).
1659 Allow constraints which consist only of type variables, with no repeats.
1662 validDerivPred :: PredType -> Bool
1663 validDerivPred (ClassP cls tys) = hasNoDups fvs && sizeTypes tys == length fvs
1664 where fvs = fvTypes tys
1665 validDerivPred otehr = False
1668 %************************************************************************
1670 Checking type instance well-formedness and termination
1672 %************************************************************************
1675 -- Check that a "type instance" is well-formed (which includes decidability
1676 -- unless -fallow-undecidable-instances is given).
1678 checkValidTypeInst :: [Type] -> Type -> TcM ()
1679 checkValidTypeInst typats rhs
1680 = do { -- left-hand side contains no type family applications
1681 -- (vanilla synonyms are fine, though)
1682 ; mappM_ checkTyFamFreeness typats
1684 -- the right-hand side is a tau type
1685 ; checkTc (isTauTy rhs) $
1688 -- we have a decidable instance unless otherwise permitted
1689 ; undecidable_ok <- doptM Opt_UndecidableInstances
1690 ; unless undecidable_ok $
1691 mapM_ addErrTc (checkFamInst typats (tyFamInsts rhs))
1694 -- Make sure that each type family instance is
1695 -- (1) strictly smaller than the lhs,
1696 -- (2) mentions no type variable more often than the lhs, and
1697 -- (3) does not contain any further type family instances.
1699 checkFamInst :: [Type] -- lhs
1700 -> [(TyCon, [Type])] -- type family instances
1702 checkFamInst lhsTys famInsts
1703 = mapCatMaybes check famInsts
1705 size = sizeTypes lhsTys
1706 fvs = fvTypes lhsTys
1708 | not (all isTyFamFree tys)
1709 = Just (famInstUndecErr famInst nestedMsg $$ parens undecidableMsg)
1710 | not (null (fvTypes tys \\ fvs))
1711 = Just (famInstUndecErr famInst nomoreVarMsg $$ parens undecidableMsg)
1712 | size <= sizeTypes tys
1713 = Just (famInstUndecErr famInst smallerAppMsg $$ parens undecidableMsg)
1717 famInst = TyConApp tc tys
1719 -- Ensure that no type family instances occur in a type.
1721 checkTyFamFreeness :: Type -> TcM ()
1722 checkTyFamFreeness ty
1723 = checkTc (isTyFamFree ty) $
1724 tyFamInstInIndexErr ty
1726 -- Check that a type does not contain any type family applications.
1728 isTyFamFree :: Type -> Bool
1729 isTyFamFree = null . tyFamInsts
1733 tyFamInstInIndexErr ty
1734 = hang (ptext SLIT("Illegal type family application in type instance") <>
1739 = hang (ptext SLIT("Illegal polymorphic type in type instance") <> colon) 4 $
1742 famInstUndecErr ty msg
1744 nest 2 (ptext SLIT("in the type family application:") <+>
1747 nestedMsg = ptext SLIT("Nested type family application")
1748 nomoreVarMsg = ptext SLIT("Variable occurs more often than in instance head")
1749 smallerAppMsg = ptext SLIT("Application is no smaller than the instance head")
1753 %************************************************************************
1755 \subsection{Auxiliary functions}
1757 %************************************************************************
1760 -- Free variables of a type, retaining repetitions, and expanding synonyms
1761 fvType :: Type -> [TyVar]
1762 fvType ty | Just exp_ty <- tcView ty = fvType exp_ty
1763 fvType (TyVarTy tv) = [tv]
1764 fvType (TyConApp _ tys) = fvTypes tys
1765 fvType (NoteTy _ ty) = fvType ty
1766 fvType (PredTy pred) = fvPred pred
1767 fvType (FunTy arg res) = fvType arg ++ fvType res
1768 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1769 fvType (ForAllTy tyvar ty) = filter (/= tyvar) (fvType ty)
1771 fvTypes :: [Type] -> [TyVar]
1772 fvTypes tys = concat (map fvType tys)
1774 fvPred :: PredType -> [TyVar]
1775 fvPred (ClassP _ tys') = fvTypes tys'
1776 fvPred (IParam _ ty) = fvType ty
1777 fvPred (EqPred ty1 ty2) = fvType ty1 ++ fvType ty2
1779 -- Size of a type: the number of variables and constructors
1780 sizeType :: Type -> Int
1781 sizeType ty | Just exp_ty <- tcView ty = sizeType exp_ty
1782 sizeType (TyVarTy _) = 1
1783 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1784 sizeType (NoteTy _ ty) = sizeType ty
1785 sizeType (PredTy pred) = sizePred pred
1786 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1787 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1788 sizeType (ForAllTy _ ty) = sizeType ty
1790 sizeTypes :: [Type] -> Int
1791 sizeTypes xs = sum (map sizeType xs)
1793 sizePred :: PredType -> Int
1794 sizePred (ClassP _ tys') = sizeTypes tys'
1795 sizePred (IParam _ ty) = sizeType ty
1796 sizePred (EqPred ty1 ty2) = sizeType ty1 + sizeType ty2