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(..),
29 newMetaTyVar, readMetaTyVar, writeMetaTyVar,
31 --------------------------------
32 -- Boxy type variables
33 newBoxyTyVar, newBoxyTyVars, newBoxyTyVarTys, readFilledBox,
35 --------------------------------
36 -- Creating new coercion variables
37 newCoVars, newMetaCoVar,
39 --------------------------------
41 tcInstTyVar, tcInstType, tcInstTyVars, tcInstBoxyTyVar,
42 tcInstSigTyVars, zonkSigTyVar,
43 tcInstSkolTyVar, tcInstSkolTyVars, tcInstSkolType,
44 tcSkolSigType, tcSkolSigTyVars, occurCheckErr,
46 --------------------------------
47 -- Checking type validity
48 Rank, UserTypeCtxt(..), checkValidType,
49 SourceTyCtxt(..), checkValidTheta, checkFreeness,
50 checkValidInstHead, checkValidInstance,
51 checkInstTermination, checkValidTypeInst, checkTyFamFreeness,
52 checkUpdateMeta, updateMeta, checkTauTvUpdate, fillBoxWithTau, unifyKindCtxt,
53 unifyKindMisMatch, validDerivPred, arityErr, notMonoType, notMonoArgs,
55 --------------------------------
57 zonkType, zonkTcPredType,
58 zonkTcTyVar, zonkTcTyVars, zonkTcTyVarsAndFV,
59 zonkQuantifiedTyVar, zonkQuantifiedTyVars,
60 zonkTcType, zonkTcTypes, zonkTcClassConstraints, zonkTcThetaType,
61 zonkTcKindToKind, zonkTcKind, zonkTopTyVar,
63 readKindVar, writeKindVar
66 #include "HsVersions.h"
78 import TcRnMonad -- TcType, amongst others
91 import Control.Monad ( when, unless )
92 import Data.List ( (\\) )
96 %************************************************************************
98 Instantiation in general
100 %************************************************************************
103 tcInstType :: ([TyVar] -> TcM [TcTyVar]) -- How to instantiate the type variables
104 -> TcType -- Type to instantiate
105 -> TcM ([TcTyVar], TcThetaType, TcType) -- Result
106 tcInstType inst_tyvars ty
107 = case tcSplitForAllTys ty of
108 ([], rho) -> let -- There may be overloading despite no type variables;
109 -- (?x :: Int) => Int -> Int
110 (theta, tau) = tcSplitPhiTy rho
112 return ([], theta, tau)
114 (tyvars, rho) -> do { tyvars' <- inst_tyvars tyvars
116 ; let tenv = zipTopTvSubst tyvars (mkTyVarTys tyvars')
117 -- Either the tyvars are freshly made, by inst_tyvars,
118 -- or (in the call from tcSkolSigType) any nested foralls
119 -- have different binders. Either way, zipTopTvSubst is ok
121 ; let (theta, tau) = tcSplitPhiTy (substTy tenv rho)
122 ; return (tyvars', theta, tau) }
126 %************************************************************************
130 %************************************************************************
132 Can't be in TcUnify, as we also need it in TcTyFuns.
136 -- False <=> the two args are (actual, expected) respectively
137 -- True <=> the two args are (expected, actual) respectively
139 checkUpdateMeta :: SwapFlag
140 -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
141 -- Update tv1, which is flexi; occurs check is alrady done
142 -- The 'check' version does a kind check too
143 -- We do a sub-kind check here: we might unify (a b) with (c d)
144 -- where b::*->* and d::*; this should fail
146 checkUpdateMeta swapped tv1 ref1 ty2
147 = do { checkKinds swapped tv1 ty2
148 ; updateMeta tv1 ref1 ty2 }
150 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
151 updateMeta tv1 ref1 ty2
152 = ASSERT( isMetaTyVar tv1 )
153 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
154 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
155 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
156 ; writeMutVar ref1 (Indirect ty2)
160 checkKinds swapped tv1 ty2
161 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
162 -- ty2 has been zonked at this stage, which ensures that
163 -- its kind has as much boxity information visible as possible.
164 | tk2 `isSubKind` tk1 = returnM ()
167 -- Either the kinds aren't compatible
168 -- (can happen if we unify (a b) with (c d))
169 -- or we are unifying a lifted type variable with an
170 -- unlifted type: e.g. (id 3#) is illegal
171 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
172 unifyKindMisMatch k1 k2
174 (k1,k2) | swapped = (tk2,tk1)
175 | otherwise = (tk1,tk2)
180 checkTauTvUpdate :: TcTyVar -> TcType -> TcM (Maybe TcType)
181 -- (checkTauTvUpdate tv ty)
182 -- We are about to update the TauTv tv with ty.
183 -- Check (a) that tv doesn't occur in ty (occurs check)
184 -- (b) that ty is a monotype
185 -- Furthermore, in the interest of (b), if you find an
186 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
188 -- We have three possible outcomes:
189 -- (1) Return the (non-boxy) type to update the type variable with,
190 -- [we know the update is ok!]
191 -- (2) return Nothing, or
192 -- [we cannot tell whether the update is ok right now]
194 -- [the update is definitely invalid]
195 -- We return Nothing in case the tv occurs in ty *under* a type family
196 -- application. In this case, we must not update tv (to avoid a cyclic type
197 -- term), but we also cannot fail claiming an infinite type. Given
199 -- type instance F Int = Int
202 -- This is perfectly reasonable, if we later get a ~ Int.
204 checkTauTvUpdate orig_tv orig_ty
205 = do { result <- go orig_ty
207 Right ty -> return $ Just ty
208 Left True -> return $ Nothing
209 Left False -> occurCheckErr (mkTyVarTy orig_tv) orig_ty
212 go :: TcType -> TcM (Either Bool TcType)
214 -- Right ty if everything is fine
215 -- Left True if orig_tv occurs in orig_ty, but under a type family app
216 -- Left False if orig_tv occurs in orig_ty (with no type family app)
217 -- It fails if it encounters a forall type, except as an argument for a
218 -- closed type synonym that expands to a tau type.
220 | isSynTyCon tc = go_syn tc tys
221 | otherwise = do { tys' <- mappM go tys
222 ; return $ occurs (TyConApp tc) tys' }
223 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
224 go (PredTy p) = do { p' <- go_pred p
225 ; return $ occurs1 PredTy p' }
226 go (FunTy arg res) = do { arg' <- go arg
228 ; return $ occurs2 FunTy arg' res' }
229 go (AppTy fun arg) = do { fun' <- go fun
231 ; return $ occurs2 mkAppTy fun' arg' }
232 -- NB the mkAppTy; we might have instantiated a
233 -- type variable to a type constructor, so we need
234 -- to pull the TyConApp to the top.
235 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
238 | orig_tv == tv = return $ Left False -- (a)
239 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
240 | otherwise = return $ Right (TyVarTy tv)
241 -- Ordinary (non Tc) tyvars
242 -- occur inside quantified types
244 go_pred (ClassP c tys) = do { tys' <- mapM go tys
245 ; return $ occurs (ClassP c) tys' }
246 go_pred (IParam n ty) = do { ty' <- go ty
247 ; return $ occurs1 (IParam n) ty' }
248 go_pred (EqPred t1 t2) = do { t1' <- go t1
250 ; return $ occurs2 EqPred t1' t2' }
252 go_tyvar tv (SkolemTv _) = return $ Right (TyVarTy tv)
253 go_tyvar tv (MetaTv box ref)
254 = do { cts <- readMutVar ref
258 BoxTv -> do { ty <- fillBoxWithTau tv ref
259 ; return $ Right ty }
260 other -> return $ Right (TyVarTy tv)
263 -- go_syn is called for synonyms only
264 -- See Note [Type synonyms and the occur check]
266 | not (isTauTyCon tc)
267 = notMonoType orig_ty -- (b) again
269 = do { (msgs, mb_tys') <- tryTc (mapM go tys)
272 -- we had a type error => forall in type parameters
274 | isOpenTyCon tc -> notMonoArgs (TyConApp tc tys)
275 -- Synonym families must have monotype args
276 | otherwise -> go (expectJust "checkTauTvUpdate(1)"
277 (tcView (TyConApp tc tys)))
278 -- Try again, expanding the synonym
280 -- no type error, but need to test whether occurs check happend
282 case occurs id tys' of
284 | isOpenTyCon tc -> return $ Left True
285 -- Variable occured under type family application
286 | otherwise -> go (expectJust "checkTauTvUpdate(2)"
287 (tcView (TyConApp tc tys)))
288 -- Try again, expanding the synonym
289 Right raw_tys' -> return $ Right (TyConApp tc raw_tys')
290 -- Retain the synonym (the common case)
293 -- Left results (= occurrence of orig_ty) dominate and
294 -- (Left False) (= fatal occurrence) dominates over (Left True)
295 occurs :: ([a] -> b) -> [Either Bool a] -> Either Bool b
296 occurs c = either Left (Right . c) . foldr combine (Right [])
298 combine (Left famInst1) (Left famInst2) = Left (famInst1 && famInst2)
299 combine (Right _ ) (Left famInst) = Left famInst
300 combine (Left famInst) (Right _) = Left famInst
301 combine (Right arg) (Right args) = Right (arg:args)
303 occurs1 c x = occurs (\[x'] -> c x') [x]
304 occurs2 c x y = occurs (\[x', y'] -> c x' y') [x, y]
306 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
307 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
308 -- tau-type meta-variable, whose print-name is the same as tv
309 -- Choosing the same name is good: when we instantiate a function
310 -- we allocate boxy tyvars with the same print-name as the quantified
311 -- tyvar; and then we often fill the box with a tau-tyvar, and again
312 -- we want to choose the same name.
313 fillBoxWithTau tv ref
314 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
315 ; let tau = mkTyVarTy tv' -- name of the type variable
316 ; writeMutVar ref (Indirect tau)
320 Note [Type synonyms and the occur check]
322 Basically we want to update tv1 := ps_ty2
323 because ps_ty2 has type-synonym info, which improves later error messages
328 f :: (A a -> a -> ()) -> ()
334 In the application (p x), we try to match "t" with "A t". If we go
335 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
336 an infinite loop later.
337 But we should not reject the program, because A t = ().
338 Rather, we should bind t to () (= non_var_ty2).
342 Error mesages in case of kind mismatch.
345 unifyKindMisMatch ty1 ty2
346 = zonkTcKind ty1 `thenM` \ ty1' ->
347 zonkTcKind ty2 `thenM` \ ty2' ->
349 msg = hang (ptext SLIT("Couldn't match kind"))
350 2 (sep [quotes (ppr ty1'),
351 ptext SLIT("against"),
356 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
357 -- tv1 and ty2 are zonked already
360 msg = (env2, ptext SLIT("When matching the kinds of") <+>
361 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
363 (pp_expected, pp_actual) | swapped = (pp2, pp1)
364 | otherwise = (pp1, pp2)
365 (env1, tv1') = tidyOpenTyVar tidy_env tv1
366 (env2, ty2') = tidyOpenType env1 ty2
367 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
368 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
371 Error message for failure due to an occurs check.
374 occurCheckErr :: TcType -> TcType -> TcM a
375 occurCheckErr ty containingTy
376 = do { env0 <- tcInitTidyEnv
377 ; ty' <- zonkTcType ty
378 ; containingTy' <- zonkTcType containingTy
379 ; let (env1, tidy_ty1) = tidyOpenType env0 ty'
380 (env2, tidy_ty2) = tidyOpenType env1 containingTy'
381 extra = sep [ppr tidy_ty1, char '=', ppr tidy_ty2]
382 ; failWithTcM (env2, hang msg 2 extra) }
384 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
387 %************************************************************************
391 %************************************************************************
394 newCoVars :: [(TcType,TcType)] -> TcM [CoVar]
396 = do { us <- newUniqueSupply
397 ; return [ mkCoVar (mkSysTvName uniq FSLIT("co"))
399 | ((ty1,ty2), uniq) <- spec `zip` uniqsFromSupply us] }
401 newMetaCoVar :: TcType -> TcType -> TcM TcTyVar
402 newMetaCoVar ty1 ty2 = newMetaTyVar TauTv (mkCoKind ty1 ty2)
404 newKindVar :: TcM TcKind
405 newKindVar = do { uniq <- newUnique
406 ; ref <- newMutVar Flexi
407 ; return (mkTyVarTy (mkKindVar uniq ref)) }
409 newKindVars :: Int -> TcM [TcKind]
410 newKindVars n = mappM (\ _ -> newKindVar) (nOfThem n ())
414 %************************************************************************
416 SkolemTvs (immutable)
418 %************************************************************************
421 mkSkolTyVar :: Name -> Kind -> SkolemInfo -> TcTyVar
422 mkSkolTyVar name kind info = mkTcTyVar name kind (SkolemTv info)
424 tcSkolSigType :: SkolemInfo -> Type -> TcM ([TcTyVar], TcThetaType, TcType)
425 -- Instantiate a type signature with skolem constants, but
426 -- do *not* give them fresh names, because we want the name to
427 -- be in the type environment -- it is lexically scoped.
428 tcSkolSigType info ty = tcInstType (\tvs -> return (tcSkolSigTyVars info tvs)) ty
430 tcSkolSigTyVars :: SkolemInfo -> [TyVar] -> [TcTyVar]
431 -- Make skolem constants, but do *not* give them new names, as above
432 tcSkolSigTyVars info tyvars = [ mkSkolTyVar (tyVarName tv) (tyVarKind tv) info
435 tcInstSkolTyVar :: SkolemInfo -> Maybe SrcSpan -> TyVar -> TcM TcTyVar
436 -- Instantiate the tyvar, using
437 -- * the occ-name and kind of the supplied tyvar,
438 -- * the unique from the monad,
439 -- * the location either from the tyvar (mb_loc = Nothing)
440 -- or from mb_loc (Just loc)
441 tcInstSkolTyVar info mb_loc tyvar
442 = do { uniq <- newUnique
443 ; let old_name = tyVarName tyvar
444 kind = tyVarKind tyvar
445 loc = mb_loc `orElse` getSrcSpan old_name
446 new_name = mkInternalName uniq (nameOccName old_name) loc
447 ; return (mkSkolTyVar new_name kind info) }
449 tcInstSkolTyVars :: SkolemInfo -> [TyVar] -> TcM [TcTyVar]
450 -- Get the location from the monad
451 tcInstSkolTyVars info tyvars
452 = do { span <- getSrcSpanM
453 ; mapM (tcInstSkolTyVar info (Just span)) tyvars }
455 tcInstSkolType :: SkolemInfo -> TcType -> TcM ([TcTyVar], TcThetaType, TcType)
456 -- Instantiate a type with fresh skolem constants
457 -- Binding location comes from the monad
458 tcInstSkolType info ty = tcInstType (tcInstSkolTyVars info) ty
462 %************************************************************************
464 MetaTvs (meta type variables; mutable)
466 %************************************************************************
469 newMetaTyVar :: BoxInfo -> Kind -> TcM TcTyVar
470 -- Make a new meta tyvar out of thin air
471 newMetaTyVar box_info kind
472 = do { uniq <- newUnique
473 ; ref <- newMutVar Flexi
474 ; let name = mkSysTvName uniq fs
475 fs = case box_info of
478 SigTv _ -> FSLIT("a")
479 -- We give BoxTv and TauTv the same string, because
480 -- otherwise we get user-visible differences in error
481 -- messages, which are confusing. If you want to see
482 -- the box_info of each tyvar, use -dppr-debug
483 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
485 instMetaTyVar :: BoxInfo -> TyVar -> TcM TcTyVar
486 -- Make a new meta tyvar whose Name and Kind
487 -- come from an existing TyVar
488 instMetaTyVar box_info tyvar
489 = do { uniq <- newUnique
490 ; ref <- newMutVar Flexi
491 ; let name = setNameUnique (tyVarName tyvar) uniq
492 kind = tyVarKind tyvar
493 ; return (mkTcTyVar name kind (MetaTv box_info ref)) }
495 readMetaTyVar :: TyVar -> TcM MetaDetails
496 readMetaTyVar tyvar = ASSERT2( isMetaTyVar tyvar, ppr tyvar )
497 readMutVar (metaTvRef tyvar)
499 writeMetaTyVar :: TcTyVar -> TcType -> TcM ()
501 writeMetaTyVar tyvar ty = writeMutVar (metaTvRef tyvar) (Indirect ty)
503 writeMetaTyVar tyvar ty
504 | not (isMetaTyVar tyvar)
505 = pprTrace "writeMetaTyVar" (ppr tyvar) $
509 = ASSERT( isMetaTyVar tyvar )
510 -- TOM: It should also work for coercions
511 -- ASSERT2( k2 `isSubKind` k1, (ppr tyvar <+> ppr k1) $$ (ppr ty <+> ppr k2) )
512 do { ASSERTM2( do { details <- readMetaTyVar tyvar; return (isFlexi details) }, ppr tyvar )
513 ; writeMutVar (metaTvRef tyvar) (Indirect ty) }
521 %************************************************************************
525 %************************************************************************
528 newFlexiTyVar :: Kind -> TcM TcTyVar
529 newFlexiTyVar kind = newMetaTyVar TauTv kind
531 newFlexiTyVarTy :: Kind -> TcM TcType
533 = newFlexiTyVar kind `thenM` \ tc_tyvar ->
534 returnM (TyVarTy tc_tyvar)
536 newFlexiTyVarTys :: Int -> Kind -> TcM [TcType]
537 newFlexiTyVarTys n kind = mappM newFlexiTyVarTy (nOfThem n kind)
539 tcInstTyVar :: TyVar -> TcM TcTyVar
540 -- Instantiate with a META type variable
541 tcInstTyVar tyvar = instMetaTyVar TauTv tyvar
543 tcInstTyVars :: [TyVar] -> TcM ([TcTyVar], [TcType], TvSubst)
544 -- Instantiate with META type variables
546 = do { tc_tvs <- mapM tcInstTyVar tyvars
547 ; let tys = mkTyVarTys tc_tvs
548 ; returnM (tc_tvs, tys, zipTopTvSubst tyvars tys) }
549 -- Since the tyvars are freshly made,
550 -- they cannot possibly be captured by
551 -- any existing for-alls. Hence zipTopTvSubst
555 %************************************************************************
559 %************************************************************************
562 tcInstSigTyVars :: Bool -> SkolemInfo -> [TyVar] -> TcM [TcTyVar]
563 -- Instantiate with skolems or meta SigTvs; depending on use_skols
564 -- Always take location info from the supplied tyvars
565 tcInstSigTyVars use_skols skol_info tyvars
567 = mapM (tcInstSkolTyVar skol_info Nothing) tyvars
570 = mapM (instMetaTyVar (SigTv skol_info)) tyvars
572 zonkSigTyVar :: TcTyVar -> TcM TcTyVar
574 | isSkolemTyVar sig_tv
575 = return sig_tv -- Happens in the call in TcBinds.checkDistinctTyVars
577 = ASSERT( isSigTyVar sig_tv )
578 do { ty <- zonkTcTyVar sig_tv
579 ; return (tcGetTyVar "zonkSigTyVar" ty) }
580 -- 'ty' is bound to be a type variable, because SigTvs
581 -- can only be unified with type variables
585 %************************************************************************
589 %************************************************************************
592 newBoxyTyVar :: Kind -> TcM BoxyTyVar
593 newBoxyTyVar kind = newMetaTyVar BoxTv kind
595 newBoxyTyVars :: [Kind] -> TcM [BoxyTyVar]
596 newBoxyTyVars kinds = mapM newBoxyTyVar kinds
598 newBoxyTyVarTys :: [Kind] -> TcM [BoxyType]
599 newBoxyTyVarTys kinds = do { tvs <- mapM newBoxyTyVar kinds; return (mkTyVarTys tvs) }
601 readFilledBox :: BoxyTyVar -> TcM TcType
602 -- Read the contents of the box, which should be filled in by now
603 readFilledBox box_tv = ASSERT( isBoxyTyVar box_tv )
604 do { cts <- readMetaTyVar box_tv
606 Flexi -> pprPanic "readFilledBox" (ppr box_tv)
607 Indirect ty -> return ty }
609 tcInstBoxyTyVar :: TyVar -> TcM BoxyTyVar
610 -- Instantiate with a BOXY type variable
611 tcInstBoxyTyVar tyvar = instMetaTyVar BoxTv tyvar
615 %************************************************************************
617 \subsection{Putting and getting mutable type variables}
619 %************************************************************************
621 But it's more fun to short out indirections on the way: If this
622 version returns a TyVar, then that TyVar is unbound. If it returns
623 any other type, then there might be bound TyVars embedded inside it.
625 We return Nothing iff the original box was unbound.
628 data LookupTyVarResult -- The result of a lookupTcTyVar call
629 = DoneTv TcTyVarDetails -- SkolemTv or virgin MetaTv
632 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
634 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
636 SkolemTv _ -> return (DoneTv details)
637 MetaTv _ ref -> do { meta_details <- readMutVar ref
638 ; case meta_details of
639 Indirect ty -> return (IndirectTv ty)
640 Flexi -> return (DoneTv details) }
642 details = tcTyVarDetails tyvar
645 -- gaw 2004 We aren't shorting anything out anymore, at least for now
647 | not (isTcTyVar tyvar)
648 = pprTrace "getTcTyVar" (ppr tyvar) $
649 returnM (Just (mkTyVarTy tyvar))
652 = ASSERT2( isTcTyVar tyvar, ppr tyvar )
653 readMetaTyVar tyvar `thenM` \ maybe_ty ->
655 Just ty -> short_out ty `thenM` \ ty' ->
656 writeMetaTyVar tyvar (Just ty') `thenM_`
659 Nothing -> returnM Nothing
661 short_out :: TcType -> TcM TcType
662 short_out ty@(TyVarTy tyvar)
663 | not (isTcTyVar tyvar)
667 = readMetaTyVar tyvar `thenM` \ maybe_ty ->
669 Just ty' -> short_out ty' `thenM` \ ty' ->
670 writeMetaTyVar tyvar (Just ty') `thenM_`
675 short_out other_ty = returnM other_ty
680 %************************************************************************
682 \subsection{Zonking -- the exernal interfaces}
684 %************************************************************************
686 ----------------- Type variables
689 zonkTcTyVars :: [TcTyVar] -> TcM [TcType]
690 zonkTcTyVars tyvars = mappM zonkTcTyVar tyvars
692 zonkTcTyVarsAndFV :: [TcTyVar] -> TcM TcTyVarSet
693 zonkTcTyVarsAndFV tyvars = mappM zonkTcTyVar tyvars `thenM` \ tys ->
694 returnM (tyVarsOfTypes tys)
696 zonkTcTyVar :: TcTyVar -> TcM TcType
697 zonkTcTyVar tyvar = ASSERT2( isTcTyVar tyvar, ppr tyvar)
698 zonk_tc_tyvar (\ tv -> returnM (TyVarTy tv)) tyvar
701 ----------------- Types
704 zonkTcType :: TcType -> TcM TcType
705 zonkTcType ty = zonkType (\ tv -> returnM (TyVarTy tv)) ty
707 zonkTcTypes :: [TcType] -> TcM [TcType]
708 zonkTcTypes tys = mappM zonkTcType tys
710 zonkTcClassConstraints cts = mappM zonk cts
711 where zonk (clas, tys)
712 = zonkTcTypes tys `thenM` \ new_tys ->
713 returnM (clas, new_tys)
715 zonkTcThetaType :: TcThetaType -> TcM TcThetaType
716 zonkTcThetaType theta = mappM zonkTcPredType theta
718 zonkTcPredType :: TcPredType -> TcM TcPredType
719 zonkTcPredType (ClassP c ts)
720 = zonkTcTypes ts `thenM` \ new_ts ->
721 returnM (ClassP c new_ts)
722 zonkTcPredType (IParam n t)
723 = zonkTcType t `thenM` \ new_t ->
724 returnM (IParam n new_t)
725 zonkTcPredType (EqPred t1 t2)
726 = zonkTcType t1 `thenM` \ new_t1 ->
727 zonkTcType t2 `thenM` \ new_t2 ->
728 returnM (EqPred new_t1 new_t2)
731 ------------------- These ...ToType, ...ToKind versions
732 are used at the end of type checking
735 zonkTopTyVar :: TcTyVar -> TcM TcTyVar
736 -- zonkTopTyVar is used, at the top level, on any un-instantiated meta type variables
737 -- to default the kind of ? and ?? etc to *. This is important to ensure that
738 -- instance declarations match. For example consider
739 -- instance Show (a->b)
740 -- foo x = show (\_ -> True)
741 -- Then we'll get a constraint (Show (p ->q)) where p has argTypeKind (printed ??),
742 -- and that won't match the typeKind (*) in the instance decl.
744 -- Because we are at top level, no further constraints are going to affect these
745 -- type variables, so it's time to do it by hand. However we aren't ready
746 -- to default them fully to () or whatever, because the type-class defaulting
747 -- rules have yet to run.
750 | k `eqKind` default_k = return tv
752 = do { tv' <- newFlexiTyVar default_k
753 ; writeMetaTyVar tv (mkTyVarTy tv')
757 default_k = defaultKind k
759 zonkQuantifiedTyVars :: [TcTyVar] -> TcM [TcTyVar]
760 zonkQuantifiedTyVars = mappM zonkQuantifiedTyVar
762 zonkQuantifiedTyVar :: TcTyVar -> TcM TcTyVar
763 -- zonkQuantifiedTyVar is applied to the a TcTyVar when quantifying over it.
765 -- The quantified type variables often include meta type variables
766 -- we want to freeze them into ordinary type variables, and
767 -- default their kind (e.g. from OpenTypeKind to TypeKind)
768 -- -- see notes with Kind.defaultKind
769 -- The meta tyvar is updated to point to the new skolem TyVar. Now any
770 -- bound occurences of the original type variable will get zonked to
771 -- the immutable version.
773 -- We leave skolem TyVars alone; they are immutable.
774 zonkQuantifiedTyVar tv
775 | ASSERT( isTcTyVar tv )
776 isSkolemTyVar tv = return tv
777 -- It might be a skolem type variable,
778 -- for example from a user type signature
780 | otherwise -- It's a meta-type-variable
781 = do { details <- readMetaTyVar tv
783 -- Create the new, frozen, skolem type variable
784 -- We zonk to a skolem, not to a regular TcVar
785 -- See Note [Zonking to Skolem]
786 ; let final_kind = defaultKind (tyVarKind tv)
787 final_tv = mkSkolTyVar (tyVarName tv) final_kind UnkSkol
789 -- Bind the meta tyvar to the new tyvar
791 Indirect ty -> WARN( True, ppr tv $$ ppr ty )
793 -- [Sept 04] I don't think this should happen
794 -- See note [Silly Type Synonym]
796 Flexi -> writeMetaTyVar tv (mkTyVarTy final_tv)
798 -- Return the new tyvar
802 Note [Silly Type Synonyms]
803 ~~~~~~~~~~~~~~~~~~~~~~~~~~
805 type C u a = u -- Note 'a' unused
807 foo :: (forall a. C u a -> C u a) -> u
811 bar = foo (\t -> t + t)
813 * From the (\t -> t+t) we get type {Num d} => d -> d
816 * Now unify with type of foo's arg, and we get:
817 {Num (C d a)} => C d a -> C d a
820 * Now abstract over the 'a', but float out the Num (C d a) constraint
821 because it does not 'really' mention a. (see exactTyVarsOfType)
822 The arg to foo becomes
825 * So we get a dict binding for Num (C d a), which is zonked to give
827 [Note Sept 04: now that we are zonking quantified type variables
828 on construction, the 'a' will be frozen as a regular tyvar on
829 quantification, so the floated dict will still have type (C d a).
830 Which renders this whole note moot; happily!]
832 * Then the /\a abstraction has a zonked 'a' in it.
834 All very silly. I think its harmless to ignore the problem. We'll end up with
835 a /\a in the final result but all the occurrences of a will be zonked to ()
837 Note [Zonking to Skolem]
838 ~~~~~~~~~~~~~~~~~~~~~~~~
839 We used to zonk quantified type variables to regular TyVars. However, this
840 leads to problems. Consider this program from the regression test suite:
842 eval :: Int -> String -> String -> String
843 eval 0 root actual = evalRHS 0 root actual
846 evalRHS 0 root actual = eval 0 root actual
848 It leads to the deferral of an equality
850 (String -> String -> String) ~ a
852 which is propagated up to the toplevel (see TcSimplify.tcSimplifyInferCheck).
853 In the meantime `a' is zonked and quantified to form `evalRHS's signature.
854 This has the *side effect* of also zonking the `a' in the deferred equality
855 (which at this point is being handed around wrapped in an implication
858 Finally, the equality (with the zonked `a') will be handed back to the
859 simplifier by TcRnDriver.tcRnSrcDecls calling TcSimplify.tcSimplifyTop.
860 If we zonk `a' with a regular type variable, we will have this regular type
861 variable now floating around in the simplifier, which in many places assumes to
862 only see proper TcTyVars.
864 We can avoid this problem by zonking with a skolem. The skolem is rigid
865 (which we requirefor a quantified variable), but is still a TcTyVar that the
866 simplifier knows how to deal with.
869 %************************************************************************
871 \subsection{Zonking -- the main work-horses: zonkType, zonkTyVar}
873 %* For internal use only! *
875 %************************************************************************
878 -- For unbound, mutable tyvars, zonkType uses the function given to it
879 -- For tyvars bound at a for-all, zonkType zonks them to an immutable
880 -- type variable and zonks the kind too
882 zonkType :: (TcTyVar -> TcM Type) -- What to do with unbound mutable type variables
883 -- see zonkTcType, and zonkTcTypeToType
886 zonkType unbound_var_fn ty
889 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
891 go (TyConApp tc tys) = mappM go tys `thenM` \ tys' ->
892 returnM (TyConApp tc tys')
894 go (PredTy p) = go_pred p `thenM` \ p' ->
897 go (FunTy arg res) = go arg `thenM` \ arg' ->
898 go res `thenM` \ res' ->
899 returnM (FunTy arg' res')
901 go (AppTy fun arg) = go fun `thenM` \ fun' ->
902 go arg `thenM` \ arg' ->
903 returnM (mkAppTy fun' arg')
904 -- NB the mkAppTy; we might have instantiated a
905 -- type variable to a type constructor, so we need
906 -- to pull the TyConApp to the top.
908 -- The two interesting cases!
909 go (TyVarTy tyvar) | isTcTyVar tyvar = zonk_tc_tyvar unbound_var_fn tyvar
910 | otherwise = return (TyVarTy tyvar)
911 -- Ordinary (non Tc) tyvars occur inside quantified types
913 go (ForAllTy tyvar ty) = ASSERT( isImmutableTyVar tyvar )
914 go ty `thenM` \ ty' ->
915 returnM (ForAllTy tyvar ty')
917 go_pred (ClassP c tys) = mappM go tys `thenM` \ tys' ->
918 returnM (ClassP c tys')
919 go_pred (IParam n ty) = go ty `thenM` \ ty' ->
920 returnM (IParam n ty')
921 go_pred (EqPred ty1 ty2) = go ty1 `thenM` \ ty1' ->
922 go ty2 `thenM` \ ty2' ->
923 returnM (EqPred ty1' ty2')
925 zonk_tc_tyvar :: (TcTyVar -> TcM Type) -- What to do for an unbound mutable variable
926 -> TcTyVar -> TcM TcType
927 zonk_tc_tyvar unbound_var_fn tyvar
928 | not (isMetaTyVar tyvar) -- Skolems
929 = returnM (TyVarTy tyvar)
931 | otherwise -- Mutables
932 = do { cts <- readMetaTyVar tyvar
934 Flexi -> unbound_var_fn tyvar -- Unbound meta type variable
935 Indirect ty -> zonkType unbound_var_fn ty }
940 %************************************************************************
944 %************************************************************************
947 readKindVar :: KindVar -> TcM (MetaDetails)
948 writeKindVar :: KindVar -> TcKind -> TcM ()
949 readKindVar kv = readMutVar (kindVarRef kv)
950 writeKindVar kv val = writeMutVar (kindVarRef kv) (Indirect val)
953 zonkTcKind :: TcKind -> TcM TcKind
954 zonkTcKind k = zonkTcType k
957 zonkTcKindToKind :: TcKind -> TcM Kind
958 -- When zonking a TcKind to a kind, we need to instantiate kind variables,
959 -- Haskell specifies that * is to be used, so we follow that.
960 zonkTcKindToKind k = zonkType (\ _ -> return liftedTypeKind) k
963 %************************************************************************
965 \subsection{Checking a user type}
967 %************************************************************************
969 When dealing with a user-written type, we first translate it from an HsType
970 to a Type, performing kind checking, and then check various things that should
971 be true about it. We don't want to perform these checks at the same time
972 as the initial translation because (a) they are unnecessary for interface-file
973 types and (b) when checking a mutually recursive group of type and class decls,
974 we can't "look" at the tycons/classes yet. Also, the checks are are rather
975 diverse, and used to really mess up the other code.
977 One thing we check for is 'rank'.
979 Rank 0: monotypes (no foralls)
980 Rank 1: foralls at the front only, Rank 0 inside
981 Rank 2: foralls at the front, Rank 1 on left of fn arrow,
983 basic ::= tyvar | T basic ... basic
985 r2 ::= forall tvs. cxt => r2a
986 r2a ::= r1 -> r2a | basic
987 r1 ::= forall tvs. cxt => r0
988 r0 ::= r0 -> r0 | basic
990 Another thing is to check that type synonyms are saturated.
991 This might not necessarily show up in kind checking.
993 data T k = MkT (k Int)
998 checkValidType :: UserTypeCtxt -> Type -> TcM ()
999 -- Checks that the type is valid for the given context
1000 checkValidType ctxt ty
1001 = traceTc (text "checkValidType" <+> ppr ty) `thenM_`
1002 doptM Opt_UnboxedTuples `thenM` \ unboxed ->
1003 doptM Opt_Rank2Types `thenM` \ rank2 ->
1004 doptM Opt_RankNTypes `thenM` \ rankn ->
1005 doptM Opt_PolymorphicComponents `thenM` \ polycomp ->
1007 rank | rankn = Arbitrary
1010 = case ctxt of -- Haskell 98
1011 GenPatCtxt -> Rank 0
1012 LamPatSigCtxt -> Rank 0
1013 BindPatSigCtxt -> Rank 0
1014 DefaultDeclCtxt-> Rank 0
1015 ResSigCtxt -> Rank 0
1016 TySynCtxt _ -> Rank 0
1017 ExprSigCtxt -> Rank 1
1018 FunSigCtxt _ -> Rank 1
1019 ConArgCtxt _ -> if polycomp
1021 -- We are given the type of the entire
1022 -- constructor, hence rank 1
1024 ForSigCtxt _ -> Rank 1
1025 SpecInstCtxt -> Rank 1
1027 actual_kind = typeKind ty
1029 kind_ok = case ctxt of
1030 TySynCtxt _ -> True -- Any kind will do
1031 ResSigCtxt -> isSubOpenTypeKind actual_kind
1032 ExprSigCtxt -> isSubOpenTypeKind actual_kind
1033 GenPatCtxt -> isLiftedTypeKind actual_kind
1034 ForSigCtxt _ -> isLiftedTypeKind actual_kind
1035 other -> isSubArgTypeKind actual_kind
1037 ubx_tup = case ctxt of
1038 TySynCtxt _ | unboxed -> UT_Ok
1039 ExprSigCtxt | unboxed -> UT_Ok
1042 -- Check that the thing has kind Type, and is lifted if necessary
1043 checkTc kind_ok (kindErr actual_kind) `thenM_`
1045 -- Check the internal validity of the type itself
1046 check_poly_type rank ubx_tup ty `thenM_`
1048 traceTc (text "checkValidType done" <+> ppr ty)
1053 data Rank = Rank Int | Arbitrary
1055 decRank :: Rank -> Rank
1056 decRank Arbitrary = Arbitrary
1057 decRank (Rank n) = Rank (n-1)
1059 ----------------------------------------
1060 data UbxTupFlag = UT_Ok | UT_NotOk
1061 -- The "Ok" version means "ok if -fglasgow-exts is on"
1063 ----------------------------------------
1064 check_poly_type :: Rank -> UbxTupFlag -> Type -> TcM ()
1065 check_poly_type (Rank 0) ubx_tup ty
1066 = check_tau_type (Rank 0) ubx_tup ty
1068 check_poly_type rank ubx_tup ty
1069 | null tvs && null theta
1070 = check_tau_type rank ubx_tup ty
1072 = do { check_valid_theta SigmaCtxt theta
1073 ; check_poly_type rank ubx_tup tau -- Allow foralls to right of arrow
1074 ; checkFreeness tvs theta
1075 ; checkAmbiguity tvs theta (tyVarsOfType tau) }
1077 (tvs, theta, tau) = tcSplitSigmaTy ty
1079 ----------------------------------------
1080 check_arg_type :: Type -> TcM ()
1081 -- The sort of type that can instantiate a type variable,
1082 -- or be the argument of a type constructor.
1083 -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
1084 -- Other unboxed types are very occasionally allowed as type
1085 -- arguments depending on the kind of the type constructor
1087 -- For example, we want to reject things like:
1089 -- instance Ord a => Ord (forall s. T s a)
1091 -- g :: T s (forall b.b)
1093 -- NB: unboxed tuples can have polymorphic or unboxed args.
1094 -- This happens in the workers for functions returning
1095 -- product types with polymorphic components.
1096 -- But not in user code.
1097 -- Anyway, they are dealt with by a special case in check_tau_type
1100 = check_poly_type Arbitrary UT_NotOk ty `thenM_`
1101 checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty)
1103 ----------------------------------------
1104 check_tau_type :: Rank -> UbxTupFlag -> Type -> TcM ()
1105 -- Rank is allowed rank for function args
1106 -- No foralls otherwise
1108 check_tau_type rank ubx_tup ty@(ForAllTy _ _) = failWithTc (forAllTyErr ty)
1109 check_tau_type rank ubx_tup ty@(FunTy (PredTy _) _) = failWithTc (forAllTyErr ty)
1110 -- Reject e.g. (Maybe (?x::Int => Int)), with a decent error message
1112 -- Naked PredTys don't usually show up, but they can as a result of
1113 -- {-# SPECIALISE instance Ord Char #-}
1114 -- The Right Thing would be to fix the way that SPECIALISE instance pragmas
1115 -- are handled, but the quick thing is just to permit PredTys here.
1116 check_tau_type rank ubx_tup (PredTy sty) = getDOpts `thenM` \ dflags ->
1117 check_pred_ty dflags TypeCtxt sty
1119 check_tau_type rank ubx_tup (TyVarTy _) = returnM ()
1120 check_tau_type rank ubx_tup ty@(FunTy arg_ty res_ty)
1121 = check_poly_type (decRank rank) UT_NotOk arg_ty `thenM_`
1122 check_poly_type rank UT_Ok res_ty
1124 check_tau_type rank ubx_tup (AppTy ty1 ty2)
1125 = check_arg_type ty1 `thenM_` check_arg_type ty2
1127 check_tau_type rank ubx_tup (NoteTy other_note ty)
1128 = check_tau_type rank ubx_tup ty
1130 check_tau_type rank ubx_tup ty@(TyConApp tc tys)
1132 = do { -- Check that the synonym has enough args
1133 -- This applies equally to open and closed synonyms
1134 -- It's OK to have an *over-applied* type synonym
1135 -- data Tree a b = ...
1136 -- type Foo a = Tree [a]
1137 -- f :: Foo a b -> ...
1138 checkTc (tyConArity tc <= length tys) arity_msg
1140 -- See Note [Liberal type synonyms]
1141 ; liberal <- doptM Opt_LiberalTypeSynonyms
1142 ; if not liberal || isOpenSynTyCon tc then
1143 -- For H98 and synonym families, do check the type args
1144 mappM_ check_arg_type tys
1146 else -- In the liberal case (only for closed syns), expand then check
1148 Just ty' -> check_tau_type rank ubx_tup ty'
1149 Nothing -> pprPanic "check_tau_type" (ppr ty)
1152 | isUnboxedTupleTyCon tc
1153 = doptM Opt_UnboxedTuples `thenM` \ ub_tuples_allowed ->
1154 checkTc (ubx_tup_ok ub_tuples_allowed) ubx_tup_msg `thenM_`
1155 mappM_ (check_tau_type (Rank 0) UT_Ok) tys
1156 -- Args are allowed to be unlifted, or
1157 -- more unboxed tuples, so can't use check_arg_ty
1160 = mappM_ check_arg_type tys
1163 ubx_tup_ok ub_tuples_allowed = case ubx_tup of { UT_Ok -> ub_tuples_allowed; other -> False }
1166 tc_arity = tyConArity tc
1168 arity_msg = arityErr "Type synonym" (tyConName tc) tc_arity n_args
1169 ubx_tup_msg = ubxArgTyErr ty
1171 ----------------------------------------
1172 forAllTyErr ty = ptext SLIT("Illegal polymorphic or qualified type:") <+> ppr ty
1173 unliftedArgErr ty = ptext SLIT("Illegal unlifted type argument:") <+> ppr ty
1174 ubxArgTyErr ty = ptext SLIT("Illegal unboxed tuple type as function argument:") <+> ppr ty
1175 kindErr kind = ptext SLIT("Expecting an ordinary type, but found a type of kind") <+> ppr kind
1178 Note [Liberal type synonyms]
1179 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1180 If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
1181 doing validity checking. This allows us to instantiate a synonym defn
1182 with a for-all type, or with a partially-applied type synonym.
1186 Here, T is partially applied, so it's illegal in H98. But if you
1187 expand S first, then T we get just
1191 IMPORTANT: suppose T is a type synonym. Then we must do validity
1192 checking on an appliation (T ty1 ty2)
1194 *either* before expansion (i.e. check ty1, ty2)
1195 *or* after expansion (i.e. expand T ty1 ty2, and then check)
1198 If we do both, we get exponential behaviour!!
1200 data TIACons1 i r c = c i ::: r c
1201 type TIACons2 t x = TIACons1 t (TIACons1 t x)
1202 type TIACons3 t x = TIACons2 t (TIACons1 t x)
1203 type TIACons4 t x = TIACons2 t (TIACons2 t x)
1204 type TIACons7 t x = TIACons4 t (TIACons3 t x)
1207 %************************************************************************
1209 \subsection{Checking a theta or source type}
1211 %************************************************************************
1214 -- Enumerate the contexts in which a "source type", <S>, can occur
1218 -- or (N a) where N is a newtype
1221 = ClassSCCtxt Name -- Superclasses of clas
1222 -- class <S> => C a where ...
1223 | SigmaCtxt -- Theta part of a normal for-all type
1224 -- f :: <S> => a -> a
1225 | DataTyCtxt Name -- Theta part of a data decl
1226 -- data <S> => T a = MkT a
1227 | TypeCtxt -- Source type in an ordinary type
1229 | InstThetaCtxt -- Context of an instance decl
1230 -- instance <S> => C [a] where ...
1232 pprSourceTyCtxt (ClassSCCtxt c) = ptext SLIT("the super-classes of class") <+> quotes (ppr c)
1233 pprSourceTyCtxt SigmaCtxt = ptext SLIT("the context of a polymorphic type")
1234 pprSourceTyCtxt (DataTyCtxt tc) = ptext SLIT("the context of the data type declaration for") <+> quotes (ppr tc)
1235 pprSourceTyCtxt InstThetaCtxt = ptext SLIT("the context of an instance declaration")
1236 pprSourceTyCtxt TypeCtxt = ptext SLIT("the context of a type")
1240 checkValidTheta :: SourceTyCtxt -> ThetaType -> TcM ()
1241 checkValidTheta ctxt theta
1242 = addErrCtxt (checkThetaCtxt ctxt theta) (check_valid_theta ctxt theta)
1244 -------------------------
1245 check_valid_theta ctxt []
1247 check_valid_theta ctxt theta
1248 = getDOpts `thenM` \ dflags ->
1249 warnTc (notNull dups) (dupPredWarn dups) `thenM_`
1250 mappM_ (check_pred_ty dflags ctxt) theta
1252 (_,dups) = removeDups tcCmpPred theta
1254 -------------------------
1255 check_pred_ty :: DynFlags -> SourceTyCtxt -> PredType -> TcM ()
1256 check_pred_ty dflags ctxt pred@(ClassP cls tys)
1257 = do { -- Class predicates are valid in all contexts
1258 ; checkTc (arity == n_tys) arity_err
1260 -- Check the form of the argument types
1261 ; mappM_ check_arg_type tys
1262 ; checkTc (check_class_pred_tys dflags ctxt tys)
1263 (predTyVarErr pred $$ how_to_allow)
1266 class_name = className cls
1267 arity = classArity cls
1269 arity_err = arityErr "Class" class_name arity n_tys
1270 how_to_allow = parens (ptext SLIT("Use -XFlexibleContexts to permit this"))
1272 check_pred_ty dflags ctxt pred@(EqPred ty1 ty2)
1273 = do { -- Equational constraints are valid in all contexts if type
1274 -- families are permitted
1275 ; checkTc (dopt Opt_TypeFamilies dflags) (eqPredTyErr pred)
1277 -- Check the form of the argument types
1278 ; check_eq_arg_type ty1
1279 ; check_eq_arg_type ty2
1282 check_eq_arg_type = check_poly_type (Rank 0) UT_NotOk
1284 check_pred_ty dflags SigmaCtxt (IParam _ ty) = check_arg_type ty
1285 -- Implicit parameters only allowed in type
1286 -- signatures; not in instance decls, superclasses etc
1287 -- The reason for not allowing implicit params in instances is a bit
1289 -- If we allowed instance (?x::Int, Eq a) => Foo [a] where ...
1290 -- then when we saw (e :: (?x::Int) => t) it would be unclear how to
1291 -- discharge all the potential usas of the ?x in e. For example, a
1292 -- constraint Foo [Int] might come out of e,and applying the
1293 -- instance decl would show up two uses of ?x.
1296 check_pred_ty dflags ctxt sty = failWithTc (badPredTyErr sty)
1298 -------------------------
1299 check_class_pred_tys :: DynFlags -> SourceTyCtxt -> [Type] -> Bool
1300 check_class_pred_tys dflags ctxt tys
1302 TypeCtxt -> True -- {-# SPECIALISE instance Eq (T Int) #-} is fine
1303 InstThetaCtxt -> flexible_contexts || undecidable_ok || all tcIsTyVarTy tys
1304 -- Further checks on head and theta in
1305 -- checkInstTermination
1306 other -> flexible_contexts || all tyvar_head tys
1308 flexible_contexts = dopt Opt_FlexibleContexts dflags
1309 undecidable_ok = dopt Opt_UndecidableInstances dflags
1311 -------------------------
1312 tyvar_head ty -- Haskell 98 allows predicates of form
1313 | tcIsTyVarTy ty = True -- C (a ty1 .. tyn)
1314 | otherwise -- where a is a type variable
1315 = case tcSplitAppTy_maybe ty of
1316 Just (ty, _) -> tyvar_head ty
1323 is ambiguous if P contains generic variables
1324 (i.e. one of the Vs) that are not mentioned in tau
1326 However, we need to take account of functional dependencies
1327 when we speak of 'mentioned in tau'. Example:
1328 class C a b | a -> b where ...
1330 forall x y. (C x y) => x
1331 is not ambiguous because x is mentioned and x determines y
1333 NB; the ambiguity check is only used for *user* types, not for types
1334 coming from inteface files. The latter can legitimately have
1335 ambiguous types. Example
1337 class S a where s :: a -> (Int,Int)
1338 instance S Char where s _ = (1,1)
1339 f:: S a => [a] -> Int -> (Int,Int)
1340 f (_::[a]) x = (a*x,b)
1341 where (a,b) = s (undefined::a)
1343 Here the worker for f gets the type
1344 fw :: forall a. S a => Int -> (# Int, Int #)
1346 If the list of tv_names is empty, we have a monotype, and then we
1347 don't need to check for ambiguity either, because the test can't fail
1352 checkAmbiguity :: [TyVar] -> ThetaType -> TyVarSet -> TcM ()
1353 checkAmbiguity forall_tyvars theta tau_tyvars
1354 = mappM_ complain (filter is_ambig theta)
1356 complain pred = addErrTc (ambigErr pred)
1357 extended_tau_vars = grow theta tau_tyvars
1359 -- See Note [Implicit parameters and ambiguity] in TcSimplify
1360 is_ambig pred = isClassPred pred &&
1361 any ambig_var (varSetElems (tyVarsOfPred pred))
1363 ambig_var ct_var = (ct_var `elem` forall_tyvars) &&
1364 not (ct_var `elemVarSet` extended_tau_vars)
1367 = sep [ptext SLIT("Ambiguous constraint") <+> quotes (pprPred pred),
1368 nest 4 (ptext SLIT("At least one of the forall'd type variables mentioned by the constraint") $$
1369 ptext SLIT("must be reachable from the type after the '=>'"))]
1372 In addition, GHC insists that at least one type variable
1373 in each constraint is in V. So we disallow a type like
1374 forall a. Eq b => b -> b
1375 even in a scope where b is in scope.
1378 checkFreeness forall_tyvars theta
1379 = do { flexible_contexts <- doptM Opt_FlexibleContexts
1380 ; unless flexible_contexts $ mappM_ complain (filter is_free theta) }
1382 is_free pred = not (isIPPred pred)
1383 && not (any bound_var (varSetElems (tyVarsOfPred pred)))
1384 bound_var ct_var = ct_var `elem` forall_tyvars
1385 complain pred = addErrTc (freeErr pred)
1388 = sep [ ptext SLIT("All of the type variables in the constraint") <+>
1389 quotes (pprPred pred)
1390 , ptext SLIT("are already in scope") <+>
1391 ptext SLIT("(at least one must be universally quantified here)")
1393 ptext SLIT("(Use -XFlexibleContexts to lift this restriction)")
1398 checkThetaCtxt ctxt theta
1399 = vcat [ptext SLIT("In the context:") <+> pprTheta theta,
1400 ptext SLIT("While checking") <+> pprSourceTyCtxt ctxt ]
1402 badPredTyErr sty = ptext SLIT("Illegal constraint") <+> pprPred sty
1403 eqPredTyErr sty = ptext SLIT("Illegal equational constraint") <+> pprPred sty
1405 parens (ptext SLIT("Use -XTypeFamilies to permit this"))
1406 predTyVarErr pred = sep [ptext SLIT("Non type-variable argument"),
1407 nest 2 (ptext SLIT("in the constraint:") <+> pprPred pred)]
1408 dupPredWarn dups = ptext SLIT("Duplicate constraint(s):") <+> pprWithCommas pprPred (map head dups)
1410 arityErr kind name n m
1411 = hsep [ text kind, quotes (ppr name), ptext SLIT("should have"),
1412 n_arguments <> comma, text "but has been given", int m]
1414 n_arguments | n == 0 = ptext SLIT("no arguments")
1415 | n == 1 = ptext SLIT("1 argument")
1416 | True = hsep [int n, ptext SLIT("arguments")]
1420 = do { ty' <- zonkTcType ty
1421 ; env0 <- tcInitTidyEnv
1422 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1423 msg = ptext SLIT("Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1424 ; failWithTcM (env1, msg) }
1427 = do { ty' <- zonkTcType ty
1428 ; env0 <- tcInitTidyEnv
1429 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1430 msg = ptext SLIT("Arguments of type synonym families must be monotypes") <+> quotes (ppr tidy_ty)
1431 ; failWithTcM (env1, msg) }
1435 %************************************************************************
1437 \subsection{Checking for a decent instance head type}
1439 %************************************************************************
1441 @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
1442 it must normally look like: @instance Foo (Tycon a b c ...) ...@
1444 The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
1445 flag is on, or (2)~the instance is imported (they must have been
1446 compiled elsewhere). In these cases, we let them go through anyway.
1448 We can also have instances for functions: @instance Foo (a -> b) ...@.
1451 checkValidInstHead :: Type -> TcM (Class, [TcType])
1453 checkValidInstHead ty -- Should be a source type
1454 = case tcSplitPredTy_maybe ty of {
1455 Nothing -> failWithTc (instTypeErr (ppr ty) empty) ;
1458 case getClassPredTys_maybe pred of {
1459 Nothing -> failWithTc (instTypeErr (pprPred pred) empty) ;
1462 getDOpts `thenM` \ dflags ->
1463 mappM_ check_arg_type tys `thenM_`
1464 check_inst_head dflags clas tys `thenM_`
1468 check_inst_head dflags clas tys
1469 -- If GlasgowExts then check at least one isn't a type variable
1470 = do checkTc (dopt Opt_TypeSynonymInstances dflags ||
1471 all tcInstHeadTyNotSynonym tys)
1472 (instTypeErr (pprClassPred clas tys) head_type_synonym_msg)
1473 checkTc (dopt Opt_FlexibleInstances dflags ||
1474 all tcInstHeadTyAppAllTyVars tys)
1475 (instTypeErr (pprClassPred clas tys) head_type_args_tyvars_msg)
1476 checkTc (dopt Opt_MultiParamTypeClasses dflags ||
1478 (instTypeErr (pprClassPred clas tys) head_one_type_msg)
1481 head_type_synonym_msg = parens (
1482 text "All instance types must be of the form (T t1 ... tn)" $$
1483 text "where T is not a synonym." $$
1484 text "Use -XTypeSynonymInstances if you want to disable this.")
1486 head_type_args_tyvars_msg = parens (
1487 text "All instance types must be of the form (T a1 ... an)" $$
1488 text "where a1 ... an are distinct type *variables*" $$
1489 text "Use -XFlexibleInstances if you want to disable this.")
1491 head_one_type_msg = parens (
1492 text "Only one type can be given in an instance head." $$
1493 text "Use -XMultiParamTypeClasses if you want to allow more.")
1495 -- For now, I only allow tau-types (not polytypes) in
1496 -- the head of an instance decl.
1497 -- E.g. instance C (forall a. a->a) is rejected
1498 -- One could imagine generalising that, but I'm not sure
1499 -- what all the consequences might be
1500 check_one ty = do { check_tau_type (Rank 0) UT_NotOk ty
1501 ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
1503 instTypeErr pp_ty msg
1504 = sep [ptext SLIT("Illegal instance declaration for") <+> quotes pp_ty,
1509 %************************************************************************
1511 \subsection{Checking instance for termination}
1513 %************************************************************************
1517 checkValidInstance :: [TyVar] -> ThetaType -> Class -> [TcType] -> TcM ()
1518 checkValidInstance tyvars theta clas inst_tys
1519 = do { undecidable_ok <- doptM Opt_UndecidableInstances
1521 ; checkValidTheta InstThetaCtxt theta
1522 ; checkAmbiguity tyvars theta (tyVarsOfTypes inst_tys)
1524 -- Check that instance inference will terminate (if we care)
1525 -- For Haskell 98 this will already have been done by checkValidTheta,
1526 -- but as we may be using other extensions we need to check.
1527 ; unless undecidable_ok $
1528 mapM_ addErrTc (checkInstTermination inst_tys theta)
1530 -- The Coverage Condition
1531 ; checkTc (undecidable_ok || checkInstCoverage clas inst_tys)
1532 (instTypeErr (pprClassPred clas inst_tys) msg)
1535 msg = parens (vcat [ptext SLIT("the Coverage Condition fails for one of the functional dependencies;"),
1539 Termination test: the so-called "Paterson conditions" (see Section 5 of
1540 "Understanding functionsl dependencies via Constraint Handling Rules,
1543 We check that each assertion in the context satisfies:
1544 (1) no variable has more occurrences in the assertion than in the head, and
1545 (2) the assertion has fewer constructors and variables (taken together
1546 and counting repetitions) than the head.
1547 This is only needed with -fglasgow-exts, as Haskell 98 restrictions
1548 (which have already been checked) guarantee termination.
1550 The underlying idea is that
1552 for any ground substitution, each assertion in the
1553 context has fewer type constructors than the head.
1557 checkInstTermination :: [TcType] -> ThetaType -> [Message]
1558 checkInstTermination tys theta
1559 = mapCatMaybes check theta
1562 size = sizeTypes tys
1564 | not (null (fvPred pred \\ fvs))
1565 = Just (predUndecErr pred nomoreMsg $$ parens undecidableMsg)
1566 | sizePred pred >= size
1567 = Just (predUndecErr pred smallerMsg $$ parens undecidableMsg)
1571 predUndecErr pred msg = sep [msg,
1572 nest 2 (ptext SLIT("in the constraint:") <+> pprPred pred)]
1574 nomoreMsg = ptext SLIT("Variable occurs more often in a constraint than in the instance head")
1575 smallerMsg = ptext SLIT("Constraint is no smaller than the instance head")
1576 undecidableMsg = ptext SLIT("Use -fallow-undecidable-instances to permit this")
1580 %************************************************************************
1582 Checking the context of a derived instance declaration
1584 %************************************************************************
1586 Note [Exotic derived instance contexts]
1587 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1588 In a 'derived' instance declaration, we *infer* the context. It's a
1589 bit unclear what rules we should apply for this; the Haskell report is
1590 silent. Obviously, constraints like (Eq a) are fine, but what about
1591 data T f a = MkT (f a) deriving( Eq )
1592 where we'd get an Eq (f a) constraint. That's probably fine too.
1594 One could go further: consider
1595 data T a b c = MkT (Foo a b c) deriving( Eq )
1596 instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
1598 Notice that this instance (just) satisfies the Paterson termination
1599 conditions. Then we *could* derive an instance decl like this:
1601 instance (C Int a, Eq b, Eq c) => Eq (T a b c)
1603 even though there is no instance for (C Int a), because there just
1604 *might* be an instance for, say, (C Int Bool) at a site where we
1605 need the equality instance for T's.
1607 However, this seems pretty exotic, and it's quite tricky to allow
1608 this, and yet give sensible error messages in the (much more common)
1609 case where we really want that instance decl for C.
1611 So for now we simply require that the derived instance context
1612 should have only type-variable constraints.
1614 Here is another example:
1615 data Fix f = In (f (Fix f)) deriving( Eq )
1616 Here, if we are prepared to allow -fallow-undecidable-instances we
1617 could derive the instance
1618 instance Eq (f (Fix f)) => Eq (Fix f)
1619 but this is so delicate that I don't think it should happen inside
1620 'deriving'. If you want this, write it yourself!
1622 NB: if you want to lift this condition, make sure you still meet the
1623 termination conditions! If not, the deriving mechanism generates
1624 larger and larger constraints. Example:
1626 data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
1628 Note the lack of a Show instance for Succ. First we'll generate
1629 instance (Show (Succ a), Show a) => Show (Seq a)
1631 instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
1632 and so on. Instead we want to complain of no instance for (Show (Succ a)).
1636 Allow constraints which consist only of type variables, with no repeats.
1639 validDerivPred :: PredType -> Bool
1640 validDerivPred (ClassP cls tys) = hasNoDups fvs && sizeTypes tys == length fvs
1641 where fvs = fvTypes tys
1642 validDerivPred otehr = False
1645 %************************************************************************
1647 Checking type instance well-formedness and termination
1649 %************************************************************************
1652 -- Check that a "type instance" is well-formed (which includes decidability
1653 -- unless -fallow-undecidable-instances is given).
1655 checkValidTypeInst :: [Type] -> Type -> TcM ()
1656 checkValidTypeInst typats rhs
1657 = do { -- left-hand side contains no type family applications
1658 -- (vanilla synonyms are fine, though)
1659 ; mappM_ checkTyFamFreeness typats
1661 -- the right-hand side is a tau type
1662 ; checkTc (isTauTy rhs) $
1665 -- we have a decidable instance unless otherwise permitted
1666 ; undecidable_ok <- doptM Opt_UndecidableInstances
1667 ; unless undecidable_ok $
1668 mapM_ addErrTc (checkFamInst typats (tyFamInsts rhs))
1671 -- Make sure that each type family instance is
1672 -- (1) strictly smaller than the lhs,
1673 -- (2) mentions no type variable more often than the lhs, and
1674 -- (3) does not contain any further type family instances.
1676 checkFamInst :: [Type] -- lhs
1677 -> [(TyCon, [Type])] -- type family instances
1679 checkFamInst lhsTys famInsts
1680 = mapCatMaybes check famInsts
1682 size = sizeTypes lhsTys
1683 fvs = fvTypes lhsTys
1685 | not (all isTyFamFree tys)
1686 = Just (famInstUndecErr famInst nestedMsg $$ parens undecidableMsg)
1687 | not (null (fvTypes tys \\ fvs))
1688 = Just (famInstUndecErr famInst nomoreVarMsg $$ parens undecidableMsg)
1689 | size <= sizeTypes tys
1690 = Just (famInstUndecErr famInst smallerAppMsg $$ parens undecidableMsg)
1694 famInst = TyConApp tc tys
1696 -- Ensure that no type family instances occur in a type.
1698 checkTyFamFreeness :: Type -> TcM ()
1699 checkTyFamFreeness ty
1700 = checkTc (isTyFamFree ty) $
1701 tyFamInstInIndexErr ty
1703 -- Check that a type does not contain any type family applications.
1705 isTyFamFree :: Type -> Bool
1706 isTyFamFree = null . tyFamInsts
1710 tyFamInstInIndexErr ty
1711 = hang (ptext SLIT("Illegal type family application in type instance") <>
1716 = hang (ptext SLIT("Illegal polymorphic type in type instance") <> colon) 4 $
1719 famInstUndecErr ty msg
1721 nest 2 (ptext SLIT("in the type family application:") <+>
1724 nestedMsg = ptext SLIT("Nested type family application")
1725 nomoreVarMsg = ptext SLIT("Variable occurs more often than in instance head")
1726 smallerAppMsg = ptext SLIT("Application is no smaller than the instance head")
1730 %************************************************************************
1732 \subsection{Auxiliary functions}
1734 %************************************************************************
1737 -- Free variables of a type, retaining repetitions, and expanding synonyms
1738 fvType :: Type -> [TyVar]
1739 fvType ty | Just exp_ty <- tcView ty = fvType exp_ty
1740 fvType (TyVarTy tv) = [tv]
1741 fvType (TyConApp _ tys) = fvTypes tys
1742 fvType (NoteTy _ ty) = fvType ty
1743 fvType (PredTy pred) = fvPred pred
1744 fvType (FunTy arg res) = fvType arg ++ fvType res
1745 fvType (AppTy fun arg) = fvType fun ++ fvType arg
1746 fvType (ForAllTy tyvar ty) = filter (/= tyvar) (fvType ty)
1748 fvTypes :: [Type] -> [TyVar]
1749 fvTypes tys = concat (map fvType tys)
1751 fvPred :: PredType -> [TyVar]
1752 fvPred (ClassP _ tys') = fvTypes tys'
1753 fvPred (IParam _ ty) = fvType ty
1754 fvPred (EqPred ty1 ty2) = fvType ty1 ++ fvType ty2
1756 -- Size of a type: the number of variables and constructors
1757 sizeType :: Type -> Int
1758 sizeType ty | Just exp_ty <- tcView ty = sizeType exp_ty
1759 sizeType (TyVarTy _) = 1
1760 sizeType (TyConApp _ tys) = sizeTypes tys + 1
1761 sizeType (NoteTy _ ty) = sizeType ty
1762 sizeType (PredTy pred) = sizePred pred
1763 sizeType (FunTy arg res) = sizeType arg + sizeType res + 1
1764 sizeType (AppTy fun arg) = sizeType fun + sizeType arg
1765 sizeType (ForAllTy _ ty) = sizeType ty
1767 sizeTypes :: [Type] -> Int
1768 sizeTypes xs = sum (map sizeType xs)
1770 sizePred :: PredType -> Int
1771 sizePred (ClassP _ tys') = sizeTypes tys'
1772 sizePred (IParam _ ty) = sizeType ty
1773 sizePred (EqPred ty1 ty2) = sizeType ty1 + sizeType ty2