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 (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 = do
348 ty1' <- zonkTcKind ty1
349 ty2' <- zonkTcKind ty2
351 msg = hang (ptext SLIT("Couldn't match kind"))
352 2 (sep [quotes (ppr ty1'),
353 ptext SLIT("against"),
357 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
358 -- tv1 and ty2 are zonked already
361 msg = (env2, ptext SLIT("When matching the kinds of") <+>
362 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
364 (pp_expected, pp_actual) | swapped = (pp2, pp1)
365 | otherwise = (pp1, pp2)
366 (env1, tv1') = tidyOpenTyVar tidy_env tv1
367 (env2, ty2') = tidyOpenType env1 ty2
368 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
369 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
372 Error message for failure due to an occurs check.
375 occurCheckErr :: TcType -> TcType -> TcM a
376 occurCheckErr ty containingTy
377 = do { env0 <- tcInitTidyEnv
378 ; ty' <- zonkTcType ty
379 ; containingTy' <- zonkTcType containingTy
380 ; let (env1, tidy_ty1) = tidyOpenType env0 ty'
381 (env2, tidy_ty2) = tidyOpenType env1 containingTy'
382 extra = sep [ppr tidy_ty1, char '=', ppr tidy_ty2]
383 ; failWithTcM (env2, hang msg 2 extra) }
385 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
388 %************************************************************************
392 %************************************************************************
395 newCoVars :: [(TcType,TcType)] -> TcM [CoVar]
397 = do { us <- newUniqueSupply
398 ; return [ mkCoVar (mkSysTvName uniq FSLIT("co"))
400 | ((ty1,ty2), uniq) <- spec `zip` uniqsFromSupply us] }
402 newMetaCoVar :: TcType -> TcType -> TcM TcTyVar
403 newMetaCoVar ty1 ty2 = newMetaTyVar TauTv (mkCoKind ty1 ty2)
405 newKindVar :: TcM TcKind
406 newKindVar = do { uniq <- newUnique
407 ; ref <- newMutVar Flexi
408 ; return (mkTyVarTy (mkKindVar uniq ref)) }
410 newKindVars :: Int -> TcM [TcKind]
411 newKindVars n = mapM (\ _ -> newKindVar) (nOfThem n ())
415 %************************************************************************
417 SkolemTvs (immutable)
419 %************************************************************************
422 mkSkolTyVar :: Name -> Kind -> SkolemInfo -> TcTyVar
423 mkSkolTyVar name kind info = mkTcTyVar name kind (SkolemTv info)
425 tcSkolSigType :: SkolemInfo -> Type -> TcM ([TcTyVar], TcThetaType, TcType)
426 -- Instantiate a type signature with skolem constants, but
427 -- do *not* give them fresh names, because we want the name to
428 -- be in the type environment -- it is lexically scoped.
429 tcSkolSigType info ty = tcInstType (\tvs -> return (tcSkolSigTyVars info tvs)) ty
431 tcSkolSigTyVars :: SkolemInfo -> [TyVar] -> [TcTyVar]
432 -- Make skolem constants, but do *not* give them new names, as above
433 tcSkolSigTyVars info tyvars = [ mkSkolTyVar (tyVarName tv) (tyVarKind tv) info
436 tcInstSkolTyVar :: SkolemInfo -> Maybe SrcSpan -> TyVar -> TcM TcTyVar
437 -- Instantiate the tyvar, using
438 -- * the occ-name and kind of the supplied tyvar,
439 -- * the unique from the monad,
440 -- * the location either from the tyvar (mb_loc = Nothing)
441 -- or from mb_loc (Just loc)
442 tcInstSkolTyVar info mb_loc tyvar
443 = do { uniq <- newUnique
444 ; let old_name = tyVarName tyvar
445 kind = tyVarKind tyvar
446 loc = mb_loc `orElse` getSrcSpan old_name
447 new_name = mkInternalName uniq (nameOccName old_name) loc
448 ; return (mkSkolTyVar new_name kind info) }
450 tcInstSkolTyVars :: SkolemInfo -> [TyVar] -> TcM [TcTyVar]
451 -- Get the location from the monad
452 tcInstSkolTyVars info tyvars
453 = do { span <- getSrcSpanM
454 ; mapM (tcInstSkolTyVar info (Just span)) tyvars }
456 tcInstSkolType :: SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcType)
457 -- Instantiate a type with fresh skolem constants
458 -- Binding location comes from the monad
459 tcInstSkolType info ty = tcInstType (tcInstSkolTyVars info) ty
463 %************************************************************************
465 MetaTvs (meta type variables; mutable)
467 %************************************************************************
470 newMetaTyVar :: BoxInfo -> Kind -> TcM TcTyVar
471 -- Make a new meta tyvar out of thin air
472 newMetaTyVar box_info kind
473 = do { uniq <- newUnique
474 ; ref <- newMutVar Flexi
475 ; let name = mkSysTvName uniq fs
476 fs = case box_info of
479 SigTv _ -> FSLIT("a")
480 -- We give BoxTv and TauTv the same string, because
481 -- otherwise we get user-visible differences in error
482 -- messages, which are confusing. If you want to see
483 -- the box_info of each tyvar, use -dppr-debug
484 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
486 instMetaTyVar :: BoxInfo -> TyVar -> TcM TcTyVar
487 -- Make a new meta tyvar whose Name and Kind
488 -- come from an existing TyVar
489 instMetaTyVar box_info tyvar
490 = do { uniq <- newUnique
491 ; ref <- newMutVar Flexi
492 ; let name = setNameUnique (tyVarName tyvar) uniq
493 kind = tyVarKind tyvar
494 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
496 readMetaTyVar :: TyVar -> TcM MetaDetails
497 readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
498 readMutVar (metaTvRef tyvar)
500 isFilledMetaTyVar :: TyVar -> TcM Bool
501 -- True of a filled-in (Indirect) meta type variable
503 | not (isTcTyVar tv) = return False
504 | MetaTv _ ref <- tcTyVarDetails tv
505 = do { details <- readMutVar ref
506 ; return (isIndirect details) }
507 | otherwise = return False
509 writeMetaTyVar :: TcTyVar -> TcType -> TcM ()
511 writeMetaTyVar tyvar ty = writeMutVar (metaTvRef tyvar) (Indirect ty)
513 writeMetaTyVar tyvar ty
514 | not (isMetaTyVar tyvar)
515 = pprTrace "writeMetaTyVar" (ppr tyvar) $
519 = ASSERT( isMetaTyVar tyvar )
520 -- TOM: It should also work for coercions
521 -- ASSERT2( k2 `isSubKind` k1, (ppr tyvar <+> ppr k1) $$ (ppr ty <+> ppr k2) )
522 do { ASSERTM2( do { details <- readMetaTyVar tyvar; return (isFlexi details) }, ppr tyvar )
523 ; writeMutVar (metaTvRef tyvar) (Indirect ty) }
531 %************************************************************************
535 %************************************************************************
538 newFlexiTyVar :: Kind -> TcM TcTyVar
539 newFlexiTyVar kind = newMetaTyVar TauTv kind
541 newFlexiTyVarTy :: Kind -> TcM TcType
542 newFlexiTyVarTy kind = do
543 tc_tyvar <- newFlexiTyVar kind
544 return (TyVarTy tc_tyvar)
546 newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
547 newFlexiTyVarTys n kind = mapM newFlexiTyVarTy (nOfThem n kind)
549 tcInstTyVar :: TyVar -> TcM TcTyVar
550 -- Instantiate with a META type variable
551 tcInstTyVar tyvar = instMetaTyVar TauTv tyvar
553 tcInstTyVars :: [TyVar] -> TcM ([TcTyVar], [TcType], TvSubst)
554 -- Instantiate with META type variables
556 = do { tc_tvs <- mapM tcInstTyVar tyvars
557 ; let tys = mkTyVarTys tc_tvs
558 ; return (tc_tvs, tys, zipTopTvSubst tyvars tys) }
559 -- Since the tyvars are freshly made,
560 -- they cannot possibly be captured by
561 -- any existing for-alls. Hence zipTopTvSubst
565 %************************************************************************
569 %************************************************************************
572 tcInstSigTyVars :: Bool -> SkolemInfo -> [TyVar] -> TcM [TcTyVar]
573 -- Instantiate with skolems or meta SigTvs; depending on use_skols
574 -- Always take location info from the supplied tyvars
575 tcInstSigTyVars use_skols skol_info tyvars
577 = mapM (tcInstSkolTyVar skol_info Nothing) tyvars
580 = mapM (instMetaTyVar (SigTv skol_info)) tyvars
582 zonkSigTyVar :: TcTyVar -> TcM TcTyVar
584 | isSkolemTyVar sig_tv
585 = return sig_tv -- Happens in the call in TcBinds.checkDistinctTyVars
587 = ASSERT( isSigTyVar sig_tv )
588 do { ty <- zonkTcTyVar sig_tv
589 ; return (tcGetTyVar "zonkSigTyVar" ty) }
590 -- 'ty' is bound to be a type variable, because SigTvs
591 -- can only be unified with type variables
595 %************************************************************************
599 %************************************************************************
602 newBoxyTyVar :: Kind -> TcM BoxyTyVar
603 newBoxyTyVar kind = newMetaTyVar BoxTv kind
605 newBoxyTyVars :: [Kind] -> TcM [BoxyTyVar]
606 newBoxyTyVars kinds = mapM newBoxyTyVar kinds
608 newBoxyTyVarTys :: [Kind] -> TcM [BoxyType]
609 newBoxyTyVarTys kinds = do { tvs <- mapM newBoxyTyVar kinds; return (mkTyVarTys tvs) }
611 readFilledBox :: BoxyTyVar -> TcM TcType
612 -- Read the contents of the box, which should be filled in by now
613 readFilledBox box_tv = ASSERT( isBoxyTyVar box_tv )
614 do { cts <- readMetaTyVar box_tv
616 Flexi -> pprPanic "readFilledBox" (ppr box_tv)
617 Indirect ty -> return ty }
619 tcInstBoxyTyVar :: TyVar -> TcM BoxyTyVar
620 -- Instantiate with a BOXY type variable
621 tcInstBoxyTyVar tyvar = instMetaTyVar BoxTv tyvar
625 %************************************************************************
627 \subsection{Putting and getting mutable type variables}
629 %************************************************************************
631 But it's more fun to short out indirections on the way: If this
632 version returns a TyVar, then that TyVar is unbound. If it returns
633 any other type, then there might be bound TyVars embedded inside it.
635 We return Nothing iff the original box was unbound.
638 data LookupTyVarResult -- The result of a lookupTcTyVar call
639 = DoneTv TcTyVarDetails -- SkolemTv or virgin MetaTv
642 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
644 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
646 SkolemTv _ -> return (DoneTv details)
647 MetaTv _ ref -> do { meta_details <- readMutVar ref
648 ; case meta_details of
649 Indirect ty -> return (IndirectTv ty)
650 Flexi -> return (DoneTv details) }
652 details = tcTyVarDetails tyvar
655 -- gaw 2004 We aren't shorting anything out anymore, at least for now
657 | not (isTcTyVar tyvar)
658 = pprTrace "getTcTyVar" (ppr tyvar) $
659 return (Just (mkTyVarTy tyvar))
662 = ASSERT2( isTcTyVar tyvar, ppr tyvar ) do
663 maybe_ty <- readMetaTyVar tyvar
665 Just ty -> do ty' <- short_out ty
666 writeMetaTyVar tyvar (Just ty')
669 Nothing -> return Nothing
671 short_out :: TcType -> TcM TcType
672 short_out ty@(TyVarTy tyvar)
673 | not (isTcTyVar tyvar)
677 maybe_ty <- readMetaTyVar tyvar
679 Just ty' -> do ty' <- short_out ty'
680 writeMetaTyVar tyvar (Just ty')
685 short_out other_ty = return other_ty
690 %************************************************************************
692 \subsection{Zonking -- the exernal interfaces}
694 %************************************************************************
696 ----------------- Type variables
699 zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
700 zonkTcTyVars tyvars = mapM zonkTcTyVar tyvars
702 zonkTcTyVarsAndFV :: [TcTyVar] -> TcM TcTyVarSet
703 zonkTcTyVarsAndFV tyvars = tyVarsOfTypes <$> mapM zonkTcTyVar tyvars
705 zonkTcTyVar :: TcTyVar -> TcM TcType
706 zonkTcTyVar tyvar = ASSERT2( isTcTyVar tyvar, ppr tyvar)
707 zonk_tc_tyvar (\ tv -> return (TyVarTy tv)) tyvar
710 ----------------- Types
713 zonkTcType :: TcType -> TcM TcType
714 zonkTcType ty = zonkType (\ tv -> return (TyVarTy tv)) ty
716 zonkTcTypes :: [TcType] -> TcM [TcType]
717 zonkTcTypes tys = mapM zonkTcType tys
719 zonkTcClassConstraints cts = mapM zonk cts
720 where zonk (clas, tys) = do
721 new_tys <- zonkTcTypes tys
722 return (clas, new_tys)
724 zonkTcThetaType :: TcThetaType -> TcM TcThetaType
725 zonkTcThetaType theta = mapM zonkTcPredType theta
727 zonkTcPredType :: TcPredType -> TcM TcPredType
728 zonkTcPredType (ClassP c ts) = ClassP c <$> zonkTcTypes ts
729 zonkTcPredType (IParam n t) = IParam n <$> zonkTcType t
730 zonkTcPredType (EqPred t1 t2) = EqPred <$> zonkTcType t1 <*> zonkTcType t2
733 ------------------- These ...ToType, ...ToKind versions
734 are used at the end of type checking
737 zonkTopTyVar :: TcTyVar -> TcM TcTyVar
738 -- zonkTopTyVar is used, at the top level, on any un-instantiated meta type variables
739 -- to default the kind of ? and ?? etc to *. This is important to ensure that
740 -- instance declarations match. For example consider
741 -- instance Show (a->b)
742 -- foo x = show (\_ -> True)
743 -- Then we'll get a constraint (Show (p ->q)) where p has argTypeKind (printed ??),
744 -- and that won't match the typeKind (*) in the instance decl.
746 -- Because we are at top level, no further constraints are going to affect these
747 -- type variables, so it's time to do it by hand. However we aren't ready
748 -- to default them fully to () or whatever, because the type-class defaulting
749 -- rules have yet to run.
752 | k `eqKind` default_k = return tv
754 = do { tv' <- newFlexiTyVar default_k
755 ; writeMetaTyVar tv (mkTyVarTy tv')
759 default_k = defaultKind k
761 zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TcTyVar]
762 zonkQuantifiedTyVars = mapM zonkQuantifiedTyVar
764 zonkQuantifiedTyVar :: TcTyVar -> TcM TcTyVar
765 -- zonkQuantifiedTyVar is applied to the a TcTyVar when quantifying over it.
767 -- The quantified type variables often include meta type variables
768 -- we want to freeze them into ordinary type variables, and
769 -- default their kind (e.g. from OpenTypeKind to TypeKind)
770 -- -- see notes with Kind.defaultKind
771 -- The meta tyvar is updated to point to the new skolem TyVar. Now any
772 -- bound occurences of the original type variable will get zonked to
773 -- the immutable version.
775 -- We leave skolem TyVars alone; they are immutable.
776 zonkQuantifiedTyVar tv
777 | ASSERT( isTcTyVar tv )
778 isSkolemTyVar tv = return tv
779 -- It might be a skolem type variable,
780 -- for example from a user type signature
782 | otherwise -- It's a meta-type-variable
783 = do { details <- readMetaTyVar tv
785 -- Create the new, frozen, skolem type variable
786 -- We zonk to a skolem, not to a regular TcVar
787 -- See Note [Zonking to Skolem]
788 ; let final_kind = defaultKind (tyVarKind tv)
789 final_tv = mkSkolTyVar (tyVarName tv) final_kind UnkSkol
791 -- Bind the meta tyvar to the new tyvar
793 Indirect ty -> WARN( True, ppr tv $$ ppr ty )
795 -- [Sept 04] I don't think this should happen
796 -- See note [Silly Type Synonym]
798 Flexi -> writeMetaTyVar tv (mkTyVarTy final_tv)
800 -- Return the new tyvar
804 Note [Silly Type Synonyms]
805 ~~~~~~~~~~~~~~~~~~~~~~~~~~
807 type C u a = u -- Note 'a' unused
809 foo :: (forall a. C u a -> C u a) -> u
813 bar = foo (\t -> t + t)
815 * From the (\t -> t+t) we get type {Num d} => d -> d
818 * Now unify with type of foo's arg, and we get:
819 {Num (C d a)} => C d a -> C d a
822 * Now abstract over the 'a', but float out the Num (C d a) constraint
823 because it does not 'really' mention a. (see exactTyVarsOfType)
824 The arg to foo becomes
827 * So we get a dict binding for Num (C d a), which is zonked to give
829 [Note Sept 04: now that we are zonking quantified type variables
830 on construction, the 'a' will be frozen as a regular tyvar on
831 quantification, so the floated dict will still have type (C d a).
832 Which renders this whole note moot; happily!]
834 * Then the /\a abstraction has a zonked 'a' in it.
836 All very silly. I think its harmless to ignore the problem. We'll end up with
837 a /\a in the final result but all the occurrences of a will be zonked to ()
839 Note [Zonking to Skolem]
840 ~~~~~~~~~~~~~~~~~~~~~~~~
841 We used to zonk quantified type variables to regular TyVars. However, this
842 leads to problems. Consider this program from the regression test suite:
844 eval :: Int -> String -> String -> String
845 eval 0 root actual = evalRHS 0 root actual
848 evalRHS 0 root actual = eval 0 root actual
850 It leads to the deferral of an equality
852 (String -> String -> String) ~ a
854 which is propagated up to the toplevel (see TcSimplify.tcSimplifyInferCheck).
855 In the meantime `a' is zonked and quantified to form `evalRHS's signature.
856 This has the *side effect* of also zonking the `a' in the deferred equality
857 (which at this point is being handed around wrapped in an implication
860 Finally, the equality (with the zonked `a') will be handed back to the
861 simplifier by TcRnDriver.tcRnSrcDecls calling TcSimplify.tcSimplifyTop.
862 If we zonk `a' with a regular type variable, we will have this regular type
863 variable now floating around in the simplifier, which in many places assumes to
864 only see proper TcTyVars.
866 We can avoid this problem by zonking with a skolem. The skolem is rigid
867 (which we requirefor a quantified variable), but is still a TcTyVar that the
868 simplifier knows how to deal with.
871 %************************************************************************
873 \subsection{Zonking -- the main work-horses: zonkType, zonkTyVar}
875 %* For internal use only! *
877 %************************************************************************
880 -- For unbound, mutable tyvars, zonkType uses the function given to it
881 -- For tyvars bound at a for-all, zonkType zonks them to an immutable
882 -- type variable and zonks the kind too
884 zonkType :: (TcTyVar -> TcM Type) -- What to do with unbound mutable type variables
885 -- see zonkTcType, and zonkTcTypeToType
888 zonkType unbound_var_fn ty
891 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
893 go (TyConApp tc tys) = do tys' <- mapM go tys
894 return (TyConApp tc tys')
896 go (PredTy p) = do p' <- go_pred p
899 go (FunTy arg res) = do arg' <- go arg
901 return (FunTy arg' res')
903 go (AppTy fun arg) = do fun' <- go fun
905 return (mkAppTy fun' arg')
906 -- NB the mkAppTy; we might have instantiated a
907 -- type variable to a type constructor, so we need
908 -- to pull the TyConApp to the top.
910 -- The two interesting cases!
911 go (TyVarTy tyvar) | isTcTyVar tyvar = zonk_tc_tyvar unbound_var_fn tyvar
912 | otherwise = return (TyVarTy tyvar)
913 -- Ordinary (non Tc) tyvars occur inside quantified types
915 go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar ) do
917 return (ForAllTy tyvar ty')
919 go_pred (ClassP c tys) = do tys' <- mapM go tys
920 return (ClassP c tys')
921 go_pred (IParam n ty) = do ty' <- go ty
922 return (IParam n ty')
923 go_pred (EqPred ty1 ty2) = do ty1' <- go ty1
925 return (EqPred ty1' ty2')
927 zonk_tc_tyvar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable variable
928 -> TcTyVar -> TcM TcType
929 zonk_tc_tyvar unbound_var_fn tyvar
930 | not (isMetaTyVar tyvar) -- Skolems
931 = return (TyVarTy tyvar)
933 | otherwise -- Mutables
934 = do { cts <- readMetaTyVar tyvar
936 Flexi -> unbound_var_fn tyvar -- Unbound meta type variable
937 Indirect ty -> zonkType unbound_var_fn ty }
942 %************************************************************************
946 %************************************************************************
949 readKindVar :: KindVar -> TcM (MetaDetails)
950 writeKindVar :: KindVar -> TcKind -> TcM ()
951 readKindVar kv = readMutVar (kindVarRef kv)
952 writeKindVar kv val = writeMutVar (kindVarRef kv) (Indirect val)
955 zonkTcKind :: TcKind -> TcM TcKind
956 zonkTcKind k = zonkTcType k
959 zonkTcKindToKind :: TcKind -> TcM Kind
960 -- When zonking a TcKind to a kind, we need to instantiate kind variables,
961 -- Haskell specifies that * is to be used, so we follow that.
962 zonkTcKindToKind k = zonkType (\ _ -> return liftedTypeKind) k
965 %************************************************************************
967 \subsection{Checking a user type}
969 %************************************************************************
971 When dealing with a user-written type, we first translate it from an HsType
972 to a Type, performing kind checking, and then check various things that should
973 be true about it. We don't want to perform these checks at the same time
974 as the initial translation because (a) they are unnecessary for interface-file
975 types and (b) when checking a mutually recursive group of type and class decls,
976 we can't "look" at the tycons/classes yet. Also, the checks are are rather
977 diverse, and used to really mess up the other code.
979 One thing we check for is 'rank'.
981 Rank 0: monotypes (no foralls)
982 Rank 1: foralls at the front only, Rank 0 inside
983 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
985 basic ::= tyvar | T basic ... basic
987 r2 ::= forall tvs. cxt => r2a
988 r2a ::= r1 -> r2a | basic
989 r1 ::= forall tvs. cxt => r0
990 r0 ::= r0 -> r0 | basic
992 Another thing is to check that type synonyms are saturated.
993 This might not necessarily show up in kind checking.
995 data T k = MkT (k Int)
1000 checkValidType :: UserTypeCtxt -> Type -> TcM ()
1001 -- Checks that the type is valid for the given context
1002 checkValidType ctxt ty = do
1003 traceTc (text "checkValidType" <+> ppr ty)
1004 unboxed <- doptM Opt_UnboxedTuples
1005 rank2 <- doptM Opt_Rank2Types
1006 rankn <- doptM Opt_RankNTypes
1007 polycomp <- doptM Opt_PolymorphicComponents
1009 rank | rankn = Arbitrary
1012 = case ctxt of -- Haskell 98
1013 GenPatCtxt -> Rank 0
1014 LamPatSigCtxt -> Rank 0
1015 BindPatSigCtxt -> Rank 0
1016 DefaultDeclCtxt-> Rank 0
1017 ResSigCtxt -> Rank 0
1018 TySynCtxt _ -> Rank 0
1019 ExprSigCtxt -> Rank 1
1020 FunSigCtxt _ -> Rank 1
1021 ConArgCtxt _ -> if polycomp
1023 -- We are given the type of the entire
1024 -- constructor, hence rank 1
1026 ForSigCtxt _ -> Rank 1
1027 SpecInstCtxt -> Rank 1
1029 actual_kind = typeKind ty
1031 kind_ok = case ctxt of
1032 TySynCtxt _ -> True -- Any kind will do
1033 ResSigCtxt -> isSubOpenTypeKind actual_kind
1034 ExprSigCtxt -> isSubOpenTypeKind actual_kind
1035 GenPatCtxt -> isLiftedTypeKind actual_kind
1036 ForSigCtxt _ -> isLiftedTypeKind actual_kind
1037 other -> isSubArgTypeKind actual_kind
1039 ubx_tup = case ctxt of
1040 TySynCtxt _ | unboxed -> UT_Ok
1041 ExprSigCtxt | unboxed -> UT_Ok
1044 -- Check that the thing has kind Type, and is lifted if necessary
1045 checkTc kind_ok (kindErr actual_kind)
1047 -- Check the internal validity of the type itself
1048 check_type rank ubx_tup ty
1050 traceTc (text "checkValidType done" <+> ppr ty)
1052 checkValidMonoType :: Type -> TcM ()
1053 checkValidMonoType ty = check_mono_type ty
1058 data Rank = Rank Int | Arbitrary
1060 decRank :: Rank -> Rank
1061 decRank Arbitrary = Arbitrary
1062 decRank (Rank n) = Rank (n-1)
1064 nonZeroRank :: Rank -> Bool
1065 nonZeroRank (Rank 0) = False
1066 nonZeroRank _ = True
1068 ----------------------------------------
1069 data UbxTupFlag = UT_Ok | UT_NotOk
1070 -- The "Ok" version means "ok if -fglasgow-exts is on"
1072 ----------------------------------------
1073 check_mono_type :: Type -> TcM () -- No foralls anywhere
1074 -- No unlifted types of any kind
1076 = do { check_type (Rank 0) UT_NotOk ty
1077 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1079 check_type :: Rank -> UbxTupFlag -> Type -> TcM ()
1080 -- The args say what the *type* context requires, independent
1081 -- of *flag* settings. You test the flag settings at usage sites.
1083 -- Rank is allowed rank for function args
1084 -- Rank 0 means no for-alls anywhere
1086 check_type rank ubx_tup ty
1087 | not (null tvs && null theta)
1088 = do { checkTc (nonZeroRank rank) (forAllTyErr ty)
1089 -- Reject e.g. (Maybe (?x::Int => Int)),
1090 -- with a decent error message
1091 ; check_valid_theta SigmaCtxt theta
1092 ; check_type rank ubx_tup tau -- Allow foralls to right of arrow
1093 ; checkFreeness tvs theta
1094 ; checkAmbiguity tvs theta (tyVarsOfType tau) }
1096 (tvs, theta, tau) = tcSplitSigmaTy ty
1098 -- Naked PredTys don't usually show up, but they can as a result of
1099 -- {-# SPECIALISE instance Ord Char #-}
1100 -- The Right Thing would be to fix the way that SPECIALISE instance pragmas
1101 -- are handled, but the quick thing is just to permit PredTys here.
1102 check_type rank ubx_tup (PredTy sty)
1103 = do { dflags <- getDOpts
1104 ; check_pred_ty dflags TypeCtxt sty }
1106 check_type rank ubx_tup (TyVarTy _) = return ()
1107 check_type rank ubx_tup ty@(FunTy arg_ty res_ty)
1108 = do { check_type (decRank rank) UT_NotOk arg_ty
1109 ; check_type rank UT_Ok res_ty }
1111 check_type rank ubx_tup (AppTy ty1 ty2)
1112 = do { check_arg_type rank ty1
1113 ; check_arg_type rank ty2 }
1115 check_type rank ubx_tup (NoteTy other_note ty)
1116 = check_type rank ubx_tup ty
1118 check_type rank ubx_tup ty@(TyConApp tc tys)
1120 = do { -- Check that the synonym has enough args
1121 -- This applies equally to open and closed synonyms
1122 -- It's OK to have an *over-applied* type synonym
1123 -- data Tree a b = ...
1124 -- type Foo a = Tree [a]
1125 -- f :: Foo a b -> ...
1126 checkTc (tyConArity tc <= length tys) arity_msg
1128 -- See Note [Liberal type synonyms]
1129 ; liberal <- doptM Opt_LiberalTypeSynonyms
1130 ; if not liberal || isOpenSynTyCon tc then
1131 -- For H98 and synonym families, do check the type args
1132 mapM_ check_mono_type tys
1134 else -- In the liberal case (only for closed syns), expand then check
1136 Just ty' -> check_type rank ubx_tup ty'
1137 Nothing -> pprPanic "check_tau_type" (ppr ty)
1140 | isUnboxedTupleTyCon tc
1141 = do { ub_tuples_allowed <- doptM Opt_UnboxedTuples
1142 ; checkTc (ubx_tup_ok ub_tuples_allowed) ubx_tup_msg
1144 ; impred <- doptM Opt_ImpredicativeTypes
1145 ; let rank' = if impred then rank else Rank 0
1146 -- c.f. check_arg_type
1147 -- However, args are allowed to be unlifted, or
1148 -- more unboxed tuples, so can't use check_arg_ty
1149 ; mapM_ (check_type rank' UT_Ok) tys }
1152 = mapM_ (check_arg_type rank) tys
1155 ubx_tup_ok ub_tuples_allowed = case ubx_tup of { UT_Ok -> ub_tuples_allowed; other -> False }
1158 tc_arity = tyConArity tc
1160 arity_msg = arityErr "Type synonym" (tyConName tc) tc_arity n_args
1161 ubx_tup_msg = ubxArgTyErr ty
1163 ----------------------------------------
1164 check_arg_type :: Rank -> Type -> TcM ()
1165 -- The sort of type that can instantiate a type variable,
1166 -- or be the argument of a type constructor.
1167 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
1168 -- Other unboxed types are very occasionally allowed as type
1169 -- arguments depending on the kind of the type constructor
1171 -- For example, we want to reject things like:
1173 -- instance Ord a => Ord (forall s. T s a)
1175 -- g :: T s (forall b.b)
1177 -- NB: unboxed tuples can have polymorphic or unboxed args.
1178 -- This happens in the workers for functions returning
1179 -- product types with polymorphic components.
1180 -- But not in user code.
1181 -- Anyway, they are dealt with by a special case in check_tau_type
1183 check_arg_type rank ty
1184 = do { impred <- doptM Opt_ImpredicativeTypes
1185 ; let rank' = if impred then rank else Rank 0 -- Monotype unless impredicative
1186 ; check_type rank' UT_NotOk ty
1187 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1189 ----------------------------------------
1190 forAllTyErr ty = ptext SLIT("Illegal polymorphic or qualified type:") <+> ppr ty
1191 unliftedArgErr ty = ptext SLIT("Illegal unlifted type:") <+> ppr ty
1192 ubxArgTyErr ty = ptext SLIT("Illegal unboxed tuple type as function argument:") <+> ppr ty
1193 kindErr kind = ptext SLIT("Expecting an ordinary type, but found a type of kind") <+> ppr kind
1196 Note [Liberal type synonyms]
1197 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1198 If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
1199 doing validity checking. This allows us to instantiate a synonym defn
1200 with a for-all type, or with a partially-applied type synonym.
1204 Here, T is partially applied, so it's illegal in H98. But if you
1205 expand S first, then T we get just
1209 IMPORTANT: suppose T is a type synonym. Then we must do validity
1210 checking on an appliation (T ty1 ty2)
1212 *either* before expansion (i.e. check ty1, ty2)
1213 *or* after expansion (i.e. expand T ty1 ty2, and then check)
1216 If we do both, we get exponential behaviour!!
1218 data TIACons1 i r c = c i ::: r c
1219 type TIACons2 t x = TIACons1 t (TIACons1 t x)
1220 type TIACons3 t x = TIACons2 t (TIACons1 t x)
1221 type TIACons4 t x = TIACons2 t (TIACons2 t x)
1222 type TIACons7 t x = TIACons4 t (TIACons3 t x)
1225 %************************************************************************
1227 \subsection{Checking a theta or source type}
1229 %************************************************************************
1232 -- Enumerate the contexts in which a "source type", <S>, can occur
1236 -- or (N a) where N is a newtype
1239 = ClassSCCtxt Name -- Superclasses of clas
1240 -- class <S> => C a where ...
1241 | SigmaCtxt -- Theta part of a normal for-all type
1242 -- f :: <S> => a -> a
1243 | DataTyCtxt Name -- Theta part of a data decl
1244 -- data <S> => T a = MkT a
1245 | TypeCtxt -- Source type in an ordinary type
1247 | InstThetaCtxt -- Context of an instance decl
1248 -- instance <S> => C [a] where ...
1250 pprSourceTyCtxt (ClassSCCtxt c) = ptext SLIT("the super-classes of class") <+> quotes (ppr c)
1251 pprSourceTyCtxt SigmaCtxt = ptext SLIT("the context of a polymorphic type")
1252 pprSourceTyCtxt (DataTyCtxt tc) = ptext SLIT("the context of the data type declaration for") <+> quotes (ppr tc)
1253 pprSourceTyCtxt InstThetaCtxt = ptext SLIT("the context of an instance declaration")
1254 pprSourceTyCtxt TypeCtxt = ptext SLIT("the context of a type")
1258 checkValidTheta :: SourceTyCtxt -> ThetaType -> TcM ()
1259 checkValidTheta ctxt theta
1260 = addErrCtxt (checkThetaCtxt ctxt theta) (check_valid_theta ctxt theta)
1262 -------------------------
1263 check_valid_theta ctxt []
1265 check_valid_theta ctxt theta = do
1267 warnTc (notNull dups) (dupPredWarn dups)
1268 mapM_ (check_pred_ty dflags ctxt) theta
1270 (_,dups) = removeDups tcCmpPred theta
1272 -------------------------
1273 check_pred_ty :: DynFlags -> SourceTyCtxt -> PredType -> TcM ()
1274 check_pred_ty dflags ctxt pred@(ClassP cls tys)
1275 = do { -- Class predicates are valid in all contexts
1276 ; checkTc (arity == n_tys) arity_err
1278 -- Check the form of the argument types
1279 ; mapM_ check_mono_type tys
1280 ; checkTc (check_class_pred_tys dflags ctxt tys)
1281 (predTyVarErr pred $$ how_to_allow)
1284 class_name = className cls
1285 arity = classArity cls
1287 arity_err = arityErr "Class" class_name arity n_tys
1288 how_to_allow = parens (ptext SLIT("Use -XFlexibleContexts to permit this"))
1290 check_pred_ty dflags ctxt pred@(EqPred ty1 ty2)
1291 = do { -- Equational constraints are valid in all contexts if type
1292 -- families are permitted
1293 ; checkTc (dopt Opt_TypeFamilies dflags) (eqPredTyErr pred)
1295 -- Check the form of the argument types
1296 ; check_mono_type ty1
1297 ; check_mono_type ty2
1300 check_pred_ty dflags SigmaCtxt (IParam _ ty) = check_mono_type ty
1301 -- Implicit parameters only allowed in type
1302 -- signatures; not in instance decls, superclasses etc
1303 -- The reason for not allowing implicit params in instances is a bit
1305 -- If we allowed instance (?x::Int, Eq a) => Foo [a] where ...
1306 -- then when we saw (e :: (?x::Int) => t) it would be unclear how to
1307 -- discharge all the potential usas of the ?x in e. For example, a
1308 -- constraint Foo [Int] might come out of e,and applying the
1309 -- instance decl would show up two uses of ?x.
1312 check_pred_ty dflags ctxt sty = failWithTc (badPredTyErr sty)
1314 -------------------------
1315 check_class_pred_tys :: DynFlags -> SourceTyCtxt -> [Type] -> Bool
1316 check_class_pred_tys dflags ctxt tys
1318 TypeCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
1319 InstThetaCtxt -> flexible_contexts || undecidable_ok || all tcIsTyVarTy tys
1320 -- Further checks on head and theta in
1321 -- checkInstTermination
1322 other -> flexible_contexts || all tyvar_head tys
1324 flexible_contexts = dopt Opt_FlexibleContexts dflags
1325 undecidable_ok = dopt Opt_UndecidableInstances dflags
1327 -------------------------
1328 tyvar_head ty -- Haskell 98 allows predicates of form
1329 | tcIsTyVarTy ty = True -- C (a ty1 .. tyn)
1330 | otherwise -- where a is a type variable
1331 = case tcSplitAppTy_maybe ty of
1332 Just (ty, _) -> tyvar_head ty
1339 is ambiguous if P contains generic variables
1340 (i.e. one of the Vs) that are not mentioned in tau
1342 However, we need to take account of functional dependencies
1343 when we speak of 'mentioned in tau'. Example:
1344 class C a b | a -> b where ...
1346 forall x y. (C x y) => x
1347 is not ambiguous because x is mentioned and x determines y
1349 NB; the ambiguity check is only used for *user* types, not for types
1350 coming from inteface files. The latter can legitimately have
1351 ambiguous types. Example
1353 class S a where s :: a -> (Int,Int)
1354 instance S Char where s _ = (1,1)
1355 f:: S a => [a] -> Int -> (Int,Int)
1356 f (_::[a]) x = (a*x,b)
1357 where (a,b) = s (undefined::a)
1359 Here the worker for f gets the type
1360 fw :: forall a. S a => Int -> (# Int, Int #)
1362 If the list of tv_names is empty, we have a monotype, and then we
1363 don't need to check for ambiguity either, because the test can't fail
1368 checkAmbiguity :: [TyVar] -> ThetaType -> TyVarSet -> TcM ()
1369 checkAmbiguity forall_tyvars theta tau_tyvars
1370 = mapM_ complain (filter is_ambig theta)
1372 complain pred = addErrTc (ambigErr pred)
1373 extended_tau_vars = grow theta tau_tyvars
1375 -- See Note [Implicit parameters and ambiguity] in TcSimplify
1376 is_ambig pred = isClassPred pred &&
1377 any ambig_var (varSetElems (tyVarsOfPred pred))
1379 ambig_var ct_var = (ct_var `elem` forall_tyvars) &&
1380 not (ct_var `elemVarSet` extended_tau_vars)
1383 = sep [ptext SLIT("Ambiguous constraint") <+> quotes (pprPred pred),
1384 nest 4 (ptext SLIT("At least one of the forall'd type variables mentioned by the constraint") $$
1385 ptext SLIT("must be reachable from the type after the '=>'"))]
1388 In addition, GHC insists that at least one type variable
1389 in each constraint is in V. So we disallow a type like
1390 forall a. Eq b => b -> b
1391 even in a scope where b is in scope.
1394 checkFreeness forall_tyvars theta
1395 = do { flexible_contexts <- doptM Opt_FlexibleContexts
1396 ; unless flexible_contexts $ mapM_ complain (filter is_free theta) }
1398 is_free pred = not (isIPPred pred)
1399 && not (any bound_var (varSetElems (tyVarsOfPred pred)))
1400 bound_var ct_var = ct_var `elem` forall_tyvars
1401 complain pred = addErrTc (freeErr pred)
1404 = sep [ ptext SLIT("All of the type variables in the constraint") <+>
1405 quotes (pprPred pred)
1406 , ptext SLIT("are already in scope") <+>
1407 ptext SLIT("(at least one must be universally quantified here)")
1409 ptext SLIT("(Use -XFlexibleContexts to lift this restriction)")
1414 checkThetaCtxt ctxt theta
1415 = vcat [ptext SLIT("In the context:") <+> pprTheta theta,
1416 ptext SLIT("While checking") <+> pprSourceTyCtxt ctxt ]
1418 badPredTyErr sty = ptext SLIT("Illegal constraint") <+> pprPred sty
1419 eqPredTyErr sty = ptext SLIT("Illegal equational constraint") <+> pprPred sty
1421 parens (ptext SLIT("Use -XTypeFamilies to permit this"))
1422 predTyVarErr pred = sep [ptext SLIT("Non type-variable argument"),
1423 nest 2 (ptext SLIT("in the constraint:") <+> pprPred pred)]
1424 dupPredWarn dups = ptext SLIT("Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
1426 arityErr kind name n m
1427 = hsep [ text kind, quotes (ppr name), ptext SLIT("should have"),
1428 n_arguments <> comma, text "but has been given", int m]
1430 n_arguments | n == 0 = ptext SLIT("no arguments")
1431 | n == 1 = ptext SLIT("1 argument")
1432 | True = hsep [int n, ptext SLIT("arguments")]
1436 = do { ty' <- zonkTcType ty
1437 ; env0 <- tcInitTidyEnv
1438 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1439 msg = ptext SLIT("Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1440 ; failWithTcM (env1, msg) }
1443 = do { ty' <- zonkTcType ty
1444 ; env0 <- tcInitTidyEnv
1445 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1446 msg = ptext SLIT("Arguments of type synonym families must be monotypes") <+> quotes (ppr tidy_ty)
1447 ; failWithTcM (env1, msg) }
1451 %************************************************************************
1453 \subsection{Checking for a decent instance head type}
1455 %************************************************************************
1457 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1458 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1460 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1461 flag is on, or (2)~the instance is imported (they must have been
1462 compiled elsewhere). In these cases, we let them go through anyway.
1464 We can also have instances for functions: @instance Foo (a -> b) ...@.
1467 checkValidInstHead :: Type -> TcM (Class, [TcType])
1469 checkValidInstHead ty -- Should be a source type
1470 = case tcSplitPredTy_maybe ty of {
1471 Nothing -> failWithTc (instTypeErr (ppr ty) empty) ;
1474 case getClassPredTys_maybe pred of {
1475 Nothing -> failWithTc (instTypeErr (pprPred pred) empty) ;
1476 Just (clas,tys) -> do
1479 mapM_ check_mono_type tys
1480 check_inst_head dflags clas tys
1484 check_inst_head dflags clas tys
1485 -- If GlasgowExts then check at least one isn't a type variable
1486 = do checkTc (dopt Opt_TypeSynonymInstances dflags ||
1487 all tcInstHeadTyNotSynonym tys)
1488 (instTypeErr (pprClassPred clas tys) head_type_synonym_msg)
1489 checkTc (dopt Opt_FlexibleInstances dflags ||
1490 all tcInstHeadTyAppAllTyVars tys)
1491 (instTypeErr (pprClassPred clas tys) head_type_args_tyvars_msg)
1492 checkTc (dopt Opt_MultiParamTypeClasses dflags ||
1494 (instTypeErr (pprClassPred clas tys) head_one_type_msg)
1495 mapM_ check_mono_type tys
1496 -- For now, I only allow tau-types (not polytypes) in
1497 -- the head of an instance decl.
1498 -- E.g. instance C (forall a. a->a) is rejected
1499 -- One could imagine generalising that, but I'm not sure
1500 -- what all the consequences might be
1503 head_type_synonym_msg = parens (
1504 text "All instance types must be of the form (T t1 ... tn)" $$
1505 text "where T is not a synonym." $$
1506 text "Use -XTypeSynonymInstances if you want to disable this.")
1508 head_type_args_tyvars_msg = parens (
1509 text "All instance types must be of the form (T a1 ... an)" $$
1510 text "where a1 ... an are distinct type *variables*" $$
1511 text "Use -XFlexibleInstances if you want to disable this.")
1513 head_one_type_msg = parens (
1514 text "Only one type can be given in an instance head." $$
1515 text "Use -XMultiParamTypeClasses if you want to allow more.")
1517 instTypeErr pp_ty msg
1518 = sep [ptext SLIT("Illegal instance declaration for") <+> quotes pp_ty,
1523 %************************************************************************
1525 \subsection{Checking instance for termination}
1527 %************************************************************************
1531 checkValidInstance :: [TyVar] -> ThetaType -> Class -> [TcType] -> TcM ()
1532 checkValidInstance tyvars theta clas inst_tys
1533 = do { undecidable_ok <- doptM Opt_UndecidableInstances
1535 ; checkValidTheta InstThetaCtxt theta
1536 ; checkAmbiguity tyvars theta (tyVarsOfTypes inst_tys)
1538 -- Check that instance inference will terminate (if we care)
1539 -- For Haskell 98 this will already have been done by checkValidTheta,
1540 -- but as we may be using other extensions we need to check.
1541 ; unless undecidable_ok $
1542 mapM_ addErrTc (checkInstTermination inst_tys theta)
1544 -- The Coverage Condition
1545 ; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
1546 (instTypeErr (pprClassPred clas inst_tys) msg)
1549 msg = parens (vcat [ptext SLIT("the Coverage Condition fails for one of the functional dependencies;"),
1553 Termination test: the so-called "Paterson conditions" (see Section 5 of
1554 "Understanding functionsl dependencies via Constraint Handling Rules,
1557 We check that each assertion in the context satisfies:
1558 (1) no variable has more occurrences in the assertion than in the head, and
1559 (2) the assertion has fewer constructors and variables (taken together
1560 and counting repetitions) than the head.
1561 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1562 (which have already been checked) guarantee termination.
1564 The underlying idea is that
1566 for any ground substitution, each assertion in the
1567 context has fewer type constructors than the head.
1571 checkInstTermination :: [TcType] -> ThetaType -> [Message]
1572 checkInstTermination tys theta
1573 = mapCatMaybes check theta
1576 size = sizeTypes tys
1578 | not (null (fvPred pred \\ fvs))
1579 = Just (predUndecErr pred nomoreMsg $$ parens undecidableMsg)
1580 | sizePred pred >= size
1581 = Just (predUndecErr pred smallerMsg $$ parens undecidableMsg)
1585 predUndecErr pred msg = sep [msg,
1586 nest 2 (ptext SLIT("in the constraint:") <+> pprPred pred)]
1588 nomoreMsg = ptext SLIT("Variable occurs more often in a constraint than in the instance head")
1589 smallerMsg = ptext SLIT("Constraint is no smaller than the instance head")
1590 undecidableMsg = ptext SLIT("Use -fallow-undecidable-instances to permit this")
1594 %************************************************************************
1596 Checking the context of a derived instance declaration
1598 %************************************************************************
1600 Note [Exotic derived instance contexts]
1601 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1602 In a 'derived' instance declaration, we *infer* the context. It's a
1603 bit unclear what rules we should apply for this; the Haskell report is
1604 silent. Obviously, constraints like (Eq a) are fine, but what about
1605 data T f a = MkT (f a) deriving( Eq )
1606 where we'd get an Eq (f a) constraint. That's probably fine too.
1608 One could go further: consider
1609 data T a b c = MkT (Foo a b c) deriving( Eq )
1610 instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
1612 Notice that this instance (just) satisfies the Paterson termination
1613 conditions. Then we *could* derive an instance decl like this:
1615 instance (C Int a, Eq b, Eq c) => Eq (T a b c)
1617 even though there is no instance for (C Int a), because there just
1618 *might* be an instance for, say, (C Int Bool) at a site where we
1619 need the equality instance for T's.
1621 However, this seems pretty exotic, and it's quite tricky to allow
1622 this, and yet give sensible error messages in the (much more common)
1623 case where we really want that instance decl for C.
1625 So for now we simply require that the derived instance context
1626 should have only type-variable constraints.
1628 Here is another example:
1629 data Fix f = In (f (Fix f)) deriving( Eq )
1630 Here, if we are prepared to allow -fallow-undecidable-instances we
1631 could derive the instance
1632 instance Eq (f (Fix f)) => Eq (Fix f)
1633 but this is so delicate that I don't think it should happen inside
1634 'deriving'. If you want this, write it yourself!
1636 NB: if you want to lift this condition, make sure you still meet the
1637 termination conditions! If not, the deriving mechanism generates
1638 larger and larger constraints. Example:
1640 data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
1642 Note the lack of a Show instance for Succ. First we'll generate
1643 instance (Show (Succ a), Show a) => Show (Seq a)
1645 instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
1646 and so on. Instead we want to complain of no instance for (Show (Succ a)).
1650 Allow constraints which consist only of type variables, with no repeats.
1653 validDerivPred :: PredType -> Bool
1654 validDerivPred (ClassP cls tys) = hasNoDups fvs && sizeTypes tys == length fvs
1655 where fvs = fvTypes tys
1656 validDerivPred otehr = False
1659 %************************************************************************
1661 Checking type instance well-formedness and termination
1663 %************************************************************************
1666 -- Check that a "type instance" is well-formed (which includes decidability
1667 -- unless -fallow-undecidable-instances is given).
1669 checkValidTypeInst :: [Type] -> Type -> TcM ()
1670 checkValidTypeInst typats rhs
1671 = do { -- left-hand side contains no type family applications
1672 -- (vanilla synonyms are fine, though)
1673 ; mapM_ checkTyFamFreeness typats
1675 -- the right-hand side is a tau type
1676 ; checkTc (isTauTy rhs) $
1679 -- we have a decidable instance unless otherwise permitted
1680 ; undecidable_ok <- doptM Opt_UndecidableInstances
1681 ; unless undecidable_ok $
1682 mapM_ addErrTc (checkFamInst typats (tyFamInsts rhs))
1685 -- Make sure that each type family instance is
1686 -- (1) strictly smaller than the lhs,
1687 -- (2) mentions no type variable more often than the lhs, and
1688 -- (3) does not contain any further type family instances.
1690 checkFamInst :: [Type] -- lhs
1691 -> [(TyCon, [Type])] -- type family instances
1693 checkFamInst lhsTys famInsts
1694 = mapCatMaybes check famInsts
1696 size = sizeTypes lhsTys
1697 fvs = fvTypes lhsTys
1699 | not (all isTyFamFree tys)
1700 = Just (famInstUndecErr famInst nestedMsg $$ parens undecidableMsg)
1701 | not (null (fvTypes tys \\ fvs))
1702 = Just (famInstUndecErr famInst nomoreVarMsg $$ parens undecidableMsg)
1703 | size <= sizeTypes tys
1704 = Just (famInstUndecErr famInst smallerAppMsg $$ parens undecidableMsg)
1708 famInst = TyConApp tc tys
1710 -- Ensure that no type family instances occur in a type.
1712 checkTyFamFreeness :: Type -> TcM ()
1713 checkTyFamFreeness ty
1714 = checkTc (isTyFamFree ty) $
1715 tyFamInstInIndexErr ty
1717 -- Check that a type does not contain any type family applications.
1719 isTyFamFree :: Type -> Bool
1720 isTyFamFree = null . tyFamInsts
1724 tyFamInstInIndexErr ty
1725 = hang (ptext SLIT("Illegal type family application in type instance") <>
1730 = hang (ptext SLIT("Illegal polymorphic type in type instance") <> colon) 4 $
1733 famInstUndecErr ty msg
1735 nest 2 (ptext SLIT("in the type family application:") <+>
1738 nestedMsg = ptext SLIT("Nested type family application")
1739 nomoreVarMsg = ptext SLIT("Variable occurs more often than in instance head")
1740 smallerAppMsg = ptext SLIT("Application is no smaller than the instance head")
1744 %************************************************************************
1746 \subsection{Auxiliary functions}
1748 %************************************************************************
1751 -- Free variables of a type, retaining repetitions, and expanding synonyms
1752 fvType :: Type -> [TyVar]
1753 fvType ty | Just exp_ty <- tcView ty = fvType exp_ty
1754 fvType (TyVarTy tv) = [tv]
1755 fvType (TyConApp _ tys) = fvTypes tys
1756 fvType (NoteTy _ ty) = fvType ty
1757 fvType (PredTy pred) = fvPred pred
1758 fvType (FunTy arg res) = fvType arg ++ fvType res
1759 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1760 fvType (ForAllTy tyvar ty) = filter (/= tyvar) (fvType ty)
1762 fvTypes :: [Type] -> [TyVar]
1763 fvTypes tys = concat (map fvType tys)
1765 fvPred :: PredType -> [TyVar]
1766 fvPred (ClassP _ tys') = fvTypes tys'
1767 fvPred (IParam _ ty) = fvType ty
1768 fvPred (EqPred ty1 ty2) = fvType ty1 ++ fvType ty2
1770 -- Size of a type: the number of variables and constructors
1771 sizeType :: Type -> Int
1772 sizeType ty | Just exp_ty <- tcView ty = sizeType exp_ty
1773 sizeType (TyVarTy _) = 1
1774 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1775 sizeType (NoteTy _ ty) = sizeType ty
1776 sizeType (PredTy pred) = sizePred pred
1777 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1778 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1779 sizeType (ForAllTy _ ty) = sizeType ty
1781 sizeTypes :: [Type] -> Int
1782 sizeTypes xs = sum (map sizeType xs)
1784 sizePred :: PredType -> Int
1785 sizePred (ClassP _ tys') = sizeTypes tys'
1786 sizePred (IParam _ ty) = sizeType ty
1787 sizePred (EqPred ty1 ty2) = sizeType ty1 + sizeType ty2