2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section{Type subsumption and unification}
8 -- Full-blown subsumption
9 tcSubPat, tcSubExp, tcGen,
10 checkSigTyVars, checkSigTyVarsWrt, bleatEscapedTvs, sigCtxt,
12 -- Various unifications
13 unifyTauTy, unifyTauTyList, unifyTheta,
14 unifyKind, unifyKinds, unifyFunKind,
17 --------------------------------
19 Expected(..), tcInfer, readExpectedType,
20 zapExpectedType, zapExpectedTo, zapExpectedBranches,
21 subFunTys, unifyFunTys,
22 zapToListTy, unifyListTy,
23 zapToTyConApp, unifyTyConApp,
27 #include "HsVersions.h"
30 import HsSyn ( HsExpr(..) , MatchGroup(..), hsLMatchPats )
31 import TcHsSyn ( mkHsLet, mkHsDictLam,
32 ExprCoFn, idCoercion, isIdCoercion, mkCoercion, (<.>), (<$>) )
33 import TypeRep ( Type(..), PredType(..), TyNote(..) )
35 import TcRnMonad -- TcType, amongst others
36 import TcType ( TcKind, TcType, TcSigmaType, TcRhoType, TcTyVar, TcTauType,
37 TcTyVarSet, TcThetaType, Expected(..), TcTyVarDetails(..),
38 SkolemInfo( GenSkol ), MetaDetails(..),
39 pprTcTyVar, isTauTy, isSigmaTy, mkFunTys, mkTyConApp,
40 tcSplitAppTy_maybe, tcSplitTyConApp_maybe,
41 tyVarsOfType, mkPhiTy, mkTyVarTy, mkPredTy,
42 typeKind, tcSplitFunTy_maybe, mkForAllTys, mkAppTy,
43 tidyOpenType, tidyOpenTypes, tidyOpenTyVar, tidyOpenTyVars,
44 pprType, tidySkolemTyVar, isSkolemTyVar )
45 import Kind ( Kind(..), SimpleKind, KindVar, isArgTypeKind,
46 openTypeKind, liftedTypeKind, mkArrowKind, kindFunResult,
47 isOpenTypeKind, argTypeKind, isLiftedTypeKind, isUnliftedTypeKind,
48 isSubKind, pprKind, splitKindFunTys )
49 import Inst ( newDicts, instToId, tcInstCall )
50 import TcMType ( condLookupTcTyVar, LookupTyVarResult(..),
51 tcSkolType, newKindVar, tcInstTyVars, newMetaTyVar,
52 newTyFlexiVarTy, zonkTcKind, zonkType, zonkTcType, zonkTcTyVarsAndFV,
53 readKindVar, writeKindVar )
54 import TcSimplify ( tcSimplifyCheck )
55 import TcEnv ( tcGetGlobalTyVars, findGlobals )
56 import TyCon ( TyCon, tyConArity, tyConTyVars )
57 import TysWiredIn ( listTyCon )
58 import Id ( Id, mkSysLocal )
59 import Var ( Var, varName, tyVarKind )
60 import VarSet ( emptyVarSet, unitVarSet, unionVarSet, elemVarSet, varSetElems )
62 import Name ( isSystemName, mkSysTvName )
63 import ErrUtils ( Message )
64 import SrcLoc ( noLoc )
65 import BasicTypes ( Arity )
66 import Util ( notNull, equalLength )
72 * A hole is always filled in with an ordinary type, not another hole.
74 %************************************************************************
76 \subsection{'hole' type variables}
78 %************************************************************************
81 newHole = newMutVar (error "Empty hole in typechecker")
83 tcInfer :: (Expected ty -> TcM a) -> TcM (a,ty)
85 = do { hole <- newHole
86 ; res <- tc_infer (Infer hole)
87 ; res_ty <- readMutVar hole
88 ; return (res, res_ty) }
90 readExpectedType :: Expected ty -> TcM ty
91 readExpectedType (Infer hole) = readMutVar hole
92 readExpectedType (Check ty) = returnM ty
94 zapExpectedType :: Expected TcType -> Kind -> TcM TcTauType
95 -- In the inference case, ensure we have a monotype
96 -- (including an unboxed tuple)
97 zapExpectedType (Infer hole) kind
98 = do { ty <- newTyFlexiVarTy kind ;
102 zapExpectedType (Check ty) kind
103 | typeKind ty `isSubKind` kind = return ty
104 | otherwise = do { ty1 <- newTyFlexiVarTy kind
107 -- The unify is to ensure that 'ty' has the desired kind
108 -- For example, in (case e of r -> b) we push an OpenTypeKind
111 zapExpectedBranches :: MatchGroup id -> Expected TcRhoType -> TcM (Expected TcRhoType)
112 -- If there is more than one branch in a case expression,
113 -- and exp_ty is a 'hole', all branches must be types, not type schemes,
114 -- otherwise the order in which we check them would affect the result.
115 zapExpectedBranches (MatchGroup [match] _) exp_ty
116 = return exp_ty -- One branch
117 zapExpectedBranches matches (Check ty)
119 zapExpectedBranches matches (Infer hole)
120 = do { -- Many branches, and inference mode,
121 -- so switch to checking mode with a monotype
122 ty <- newTyFlexiVarTy openTypeKind
123 ; writeMutVar hole ty
124 ; return (Check ty) }
126 zapExpectedTo :: Expected TcType -> TcTauType -> TcM ()
127 zapExpectedTo (Check ty1) ty2 = unifyTauTy ty1 ty2
128 zapExpectedTo (Infer hole) ty2 = do { ty2' <- zonkTcType ty2; writeMutVar hole ty2' }
129 -- See Note [Zonk return type]
131 instance Outputable ty => Outputable (Expected ty) where
132 ppr (Check ty) = ptext SLIT("Expected type") <+> ppr ty
133 ppr (Infer hole) = ptext SLIT("Inferring type")
137 %************************************************************************
139 \subsection[Unify-fun]{@unifyFunTy@}
141 %************************************************************************
143 @subFunTy@ and @unifyFunTy@ is used to avoid the fruitless
144 creation of type variables.
146 * subFunTy is used when we might be faced with a "hole" type variable,
147 in which case we should create two new holes.
149 * unifyFunTy is used when we expect to encounter only "ordinary"
150 type variables, so we should create new ordinary type variables
153 subFunTys :: MatchGroup name
154 -> Expected TcRhoType -- Fail if ty isn't a function type
155 -> ([Expected TcRhoType] -> Expected TcRhoType -> TcM a)
158 subFunTys (MatchGroup (match:null_matches) _) (Infer hole) thing_inside
159 = -- This is the interesting case
160 ASSERT( null null_matches )
161 do { pat_holes <- mapM (\ _ -> newHole) (hsLMatchPats match)
162 ; res_hole <- newHole
165 ; res <- thing_inside (map Infer pat_holes) (Infer res_hole)
167 -- Extract the answers
168 ; arg_tys <- mapM readMutVar pat_holes
169 ; res_ty <- readMutVar res_hole
171 -- Write the answer into the incoming hole
172 ; writeMutVar hole (mkFunTys arg_tys res_ty)
174 -- And return the answer
177 subFunTys (MatchGroup (match:matches) _) (Check ty) thing_inside
178 = ASSERT( all ((== length (hsLMatchPats match)) . length . hsLMatchPats) matches )
179 -- Assertion just checks that all the matches have the same number of pats
180 do { (pat_tys, res_ty) <- unifyFunTys (length (hsLMatchPats match)) ty
181 ; thing_inside (map Check pat_tys) (Check res_ty) }
183 unifyFunTys :: Arity -> TcRhoType -> TcM ([TcSigmaType], TcRhoType)
184 -- Fail if ty isn't a function type, otherwise return arg and result types
185 -- The result types are guaranteed wobbly if the argument is wobbly
187 -- Does not allocate unnecessary meta variables: if the input already is
188 -- a function, we just take it apart. Not only is this efficient, it's important
189 -- for (a) higher rank: the argument might be of form
190 -- (forall a. ty) -> other
191 -- If allocated (fresh-meta-var1 -> fresh-meta-var2) and unified, we'd
192 -- blow up with the meta var meets the forall
194 -- (b) GADTs: if the argument is not wobbly we do not want the result to be
196 unifyFunTys arity ty = unify_fun_ty True arity ty
198 unify_fun_ty use_refinement arity ty
200 = do { res_ty <- wobblify use_refinement ty
203 unify_fun_ty use_refinement arity (NoteTy _ ty)
204 = unify_fun_ty use_refinement arity ty
206 unify_fun_ty use_refinement arity ty@(TyVarTy tv)
207 = do { details <- condLookupTcTyVar use_refinement tv
209 IndirectTv use' ty' -> unify_fun_ty use' arity ty'
210 other -> unify_fun_help arity ty
213 unify_fun_ty use_refinement arity ty
214 = case tcSplitFunTy_maybe ty of
215 Just (arg,res) -> do { arg' <- wobblify use_refinement arg
216 ; (args', res') <- unify_fun_ty use_refinement (arity-1) res
217 ; return (arg':args', res') }
219 Nothing -> unify_fun_help arity ty
220 -- Usually an error, but ty could be (a Int Bool), which can match
222 unify_fun_help :: Arity -> TcRhoType -> TcM ([TcSigmaType], TcRhoType)
223 unify_fun_help arity ty
224 = do { args <- mappM newTyFlexiVarTy (replicate arity argTypeKind)
225 ; res <- newTyFlexiVarTy openTypeKind
226 ; unifyTauTy ty (mkFunTys args res)
227 ; return (args, res) }
231 ----------------------
232 zapToTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
233 -> Expected TcSigmaType -- Expected type (T a b c)
234 -> TcM [TcType] -- Element types, a b c
235 -- Insists that the Expected type is not a forall-type
237 zapToTyConApp tc (Check ty)
238 = unifyTyConApp tc ty -- NB: fails for a forall-type
239 zapToTyConApp tc (Infer hole)
240 = do { (tc_app, elt_tys) <- newTyConApp tc
241 ; writeMutVar hole tc_app
244 zapToListTy :: Expected TcType -> TcM TcType -- Special case for lists
245 zapToListTy exp_ty = do { [elt_ty] <- zapToTyConApp listTyCon exp_ty
248 ----------------------
249 unifyTyConApp :: TyCon -> TcType -> TcM [TcType]
250 unifyTyConApp tc ty = unify_tc_app True tc ty
251 -- Add a boolean flag to remember whether to use
252 -- the type refinement or not
254 unifyListTy :: TcType -> TcM TcType -- Special case for lists
255 unifyListTy exp_ty = do { [elt_ty] <- unifyTyConApp listTyCon exp_ty
259 unify_tc_app use_refinement tc (NoteTy _ ty)
260 = unify_tc_app use_refinement tc ty
262 unify_tc_app use_refinement tc ty@(TyVarTy tyvar)
263 = do { details <- condLookupTcTyVar use_refinement tyvar
265 IndirectTv use' ty' -> unify_tc_app use' tc ty'
266 other -> unify_tc_app_help tc ty
269 unify_tc_app use_refinement tc ty
270 | Just (tycon, arg_tys) <- tcSplitTyConApp_maybe ty,
272 = ASSERT( tyConArity tycon == length arg_tys ) -- ty::*
273 mapM (wobblify use_refinement) arg_tys
275 unify_tc_app use_refinement tc ty = unify_tc_app_help tc ty
278 unify_tc_app_help tc ty -- Revert to ordinary unification
279 = do { (tc_app, arg_tys) <- newTyConApp tc
280 ; if not (isTauTy ty) then -- Can happen if we call zapToTyConApp tc (forall a. ty)
281 unifyMisMatch ty tc_app
283 { unifyTauTy ty tc_app
284 ; returnM arg_tys } }
287 ----------------------
288 unifyAppTy :: TcType -- Expected type function: m
289 -> TcType -- Type to split: m a
290 -> TcM TcType -- Type arg: a
291 unifyAppTy tc ty = unify_app_ty True tc ty
293 unify_app_ty use tc (NoteTy _ ty) = unify_app_ty use tc ty
295 unify_app_ty use tc ty@(TyVarTy tyvar)
296 = do { details <- condLookupTcTyVar use tyvar
298 IndirectTv use' ty' -> unify_app_ty use' tc ty'
299 other -> unify_app_ty_help tc ty
302 unify_app_ty use tc ty
303 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
304 = do { unifyTauTy tc fun_ty
305 ; wobblify use arg_ty }
307 | otherwise = unify_app_ty_help tc ty
309 unify_app_ty_help tc ty -- Revert to ordinary unification
310 = do { arg_ty <- newTyFlexiVarTy (kindFunResult (typeKind tc))
311 ; unifyTauTy (mkAppTy tc arg_ty) ty
315 ----------------------
316 wobblify :: Bool -- True <=> don't wobblify
319 -- Return a wobbly type. At the moment we do that by
320 -- allocating a fresh meta type variable.
321 wobblify True ty = return ty
322 wobblify False ty = do { uniq <- newUnique
323 ; tv <- newMetaTyVar (mkSysTvName uniq FSLIT("w"))
326 ; return (mkTyVarTy tv) }
328 ----------------------
329 newTyConApp :: TyCon -> TcM (TcTauType, [TcTauType])
330 newTyConApp tc = do { (tvs, args, _) <- tcInstTyVars (tyConTyVars tc)
331 ; return (mkTyConApp tc args, args) }
335 %************************************************************************
337 \subsection{Subsumption}
339 %************************************************************************
341 All the tcSub calls have the form
343 tcSub expected_ty offered_ty
345 offered_ty <= expected_ty
347 That is, that a value of type offered_ty is acceptable in
348 a place expecting a value of type expected_ty.
350 It returns a coercion function
351 co_fn :: offered_ty -> expected_ty
352 which takes an HsExpr of type offered_ty into one of type
356 -----------------------
357 -- tcSubExp is used for expressions
358 tcSubExp :: Expected TcRhoType -> TcRhoType -> TcM ExprCoFn
360 tcSubExp (Infer hole) offered_ty
361 = do { offered' <- zonkTcType offered_ty
362 -- Note [Zonk return type]
363 -- zonk to take advantage of the current GADT type refinement.
364 -- If we don't we get spurious "existential type variable escapes":
365 -- case (x::Maybe a) of
366 -- Just b (y::b) -> y
367 -- We need the refinement [b->a] to be applied to the result type
368 ; writeMutVar hole offered'
369 ; return idCoercion }
371 tcSubExp (Check expected_ty) offered_ty
372 = tcSub expected_ty offered_ty
374 -----------------------
375 -- tcSubPat is used for patterns
376 tcSubPat :: TcSigmaType -- Pattern type signature
377 -> Expected TcSigmaType -- Type from context
379 -- In patterns we insist on an exact match; hence no CoFn returned
380 -- See Note [Pattern coercions] in TcPat
381 -- However, we can't call unify directly, because both types might be
382 -- polymorphic; hence the call to tcSub, followed by a check for
383 -- the identity coercion
385 tcSubPat sig_ty (Infer hole)
386 = do { sig_ty' <- zonkTcType sig_ty
387 ; writeMutVar hole sig_ty' -- See notes with tcSubExp above
390 tcSubPat sig_ty (Check exp_ty)
391 = do { co_fn <- tcSub sig_ty exp_ty
393 ; if isIdCoercion co_fn then
396 unifyMisMatch sig_ty exp_ty }
401 %************************************************************************
403 tcSub: main subsumption-check code
405 %************************************************************************
407 No holes expected now. Add some error-check context info.
411 tcSub :: TcSigmaType -> TcSigmaType -> TcM ExprCoFn -- Locally used only
412 -- tcSub exp act checks that
414 tcSub expected_ty actual_ty
415 = traceTc (text "tcSub" <+> details) `thenM_`
416 addErrCtxtM (unifyCtxt "type" expected_ty actual_ty)
417 (tc_sub expected_ty expected_ty actual_ty actual_ty)
419 details = vcat [text "Expected:" <+> ppr expected_ty,
420 text "Actual: " <+> ppr actual_ty]
423 tc_sub :: TcSigmaType -- expected_ty, before expanding synonyms
424 -> TcSigmaType -- ..and after
425 -> TcSigmaType -- actual_ty, before
426 -> TcSigmaType -- ..and after
429 -----------------------------------
431 tc_sub exp_sty (NoteTy _ exp_ty) act_sty act_ty = tc_sub exp_sty exp_ty act_sty act_ty
432 tc_sub exp_sty exp_ty act_sty (NoteTy _ act_ty) = tc_sub exp_sty exp_ty act_sty act_ty
434 -----------------------------------
435 -- Generalisation case
436 -- actual_ty: d:Eq b => b->b
437 -- expected_ty: forall a. Ord a => a->a
438 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
440 -- It is essential to do this *before* the specialisation case
441 -- Example: f :: (Eq a => a->a) -> ...
442 -- g :: Ord b => b->b
445 tc_sub exp_sty expected_ty act_sty actual_ty
446 | isSigmaTy expected_ty
447 = tcGen expected_ty (tyVarsOfType actual_ty) (
448 -- It's really important to check for escape wrt the free vars of
449 -- both expected_ty *and* actual_ty
450 \ body_exp_ty -> tc_sub body_exp_ty body_exp_ty act_sty actual_ty
451 ) `thenM` \ (gen_fn, co_fn) ->
452 returnM (gen_fn <.> co_fn)
454 -----------------------------------
455 -- Specialisation case:
456 -- actual_ty: forall a. Ord a => a->a
457 -- expected_ty: Int -> Int
458 -- co_fn e = e Int dOrdInt
460 tc_sub exp_sty expected_ty act_sty actual_ty
461 | isSigmaTy actual_ty
462 = tcInstCall InstSigOrigin actual_ty `thenM` \ (inst_fn, _, body_ty) ->
463 tc_sub exp_sty expected_ty body_ty body_ty `thenM` \ co_fn ->
464 returnM (co_fn <.> inst_fn)
466 -----------------------------------
469 tc_sub _ (FunTy exp_arg exp_res) _ (FunTy act_arg act_res)
470 = tcSub_fun exp_arg exp_res act_arg act_res
472 -----------------------------------
473 -- Type variable meets function: imitate
475 -- NB 1: we can't just unify the type variable with the type
476 -- because the type might not be a tau-type, and we aren't
477 -- allowed to instantiate an ordinary type variable with
480 -- NB 2: can we short-cut to an error case?
481 -- when the arg/res is not a tau-type?
482 -- NO! e.g. f :: ((forall a. a->a) -> Int) -> Int
484 -- is perfectly fine, because we can instantiate f's type to a monotype
486 -- However, we get can get jolly unhelpful error messages.
487 -- e.g. foo = id runST
489 -- Inferred type is less polymorphic than expected
490 -- Quantified type variable `s' escapes
491 -- Expected type: ST s a -> t
492 -- Inferred type: (forall s1. ST s1 a) -> a
493 -- In the first argument of `id', namely `runST'
494 -- In a right-hand side of function `foo': id runST
496 -- I'm not quite sure what to do about this!
498 tc_sub exp_sty exp_ty@(FunTy exp_arg exp_res) _ act_ty
499 = do { ([act_arg], act_res) <- unifyFunTys 1 act_ty
500 ; tcSub_fun exp_arg exp_res act_arg act_res }
502 tc_sub _ exp_ty act_sty act_ty@(FunTy act_arg act_res)
503 = do { ([exp_arg], exp_res) <- unifyFunTys 1 exp_ty
504 ; tcSub_fun exp_arg exp_res act_arg act_res }
506 -----------------------------------
508 -- If none of the above match, we revert to the plain unifier
509 tc_sub exp_sty expected_ty act_sty actual_ty
510 = uTys True exp_sty expected_ty True act_sty actual_ty `thenM_`
515 tcSub_fun exp_arg exp_res act_arg act_res
516 = tc_sub act_arg act_arg exp_arg exp_arg `thenM` \ co_fn_arg ->
517 tc_sub exp_res exp_res act_res act_res `thenM` \ co_fn_res ->
518 newUnique `thenM` \ uniq ->
520 -- co_fn_arg :: HsExpr exp_arg -> HsExpr act_arg
521 -- co_fn_res :: HsExpr act_res -> HsExpr exp_res
522 -- co_fn :: HsExpr (act_arg -> act_res) -> HsExpr (exp_arg -> exp_res)
523 arg_id = mkSysLocal FSLIT("sub") uniq exp_arg
524 coercion | isIdCoercion co_fn_arg,
525 isIdCoercion co_fn_res = idCoercion
526 | otherwise = mkCoercion co_fn
528 co_fn e = DictLam [arg_id]
529 (noLoc (co_fn_res <$> (HsApp (noLoc e) (noLoc (co_fn_arg <$> HsVar arg_id)))))
530 -- Slight hack; using a "DictLam" to get an ordinary simple lambda
531 -- HsVar arg_id :: HsExpr exp_arg
532 -- co_fn_arg $it :: HsExpr act_arg
533 -- HsApp e $it :: HsExpr act_res
534 -- co_fn_res $it :: HsExpr exp_res
540 %************************************************************************
542 \subsection{Generalisation}
544 %************************************************************************
547 tcGen :: TcSigmaType -- expected_ty
548 -> TcTyVarSet -- Extra tyvars that the universally
549 -- quantified tyvars of expected_ty
550 -- must not be unified
551 -> (TcRhoType -> TcM result) -- spec_ty
552 -> TcM (ExprCoFn, result)
553 -- The expression has type: spec_ty -> expected_ty
555 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
556 -- If not, the call is a no-op
557 = do { -- We want the GenSkol info in the skolemised type variables to
558 -- mention the *instantiated* tyvar names, so that we get a
559 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
560 -- Hence the tiresome but innocuous fixM
561 ((forall_tvs, theta, rho_ty), skol_info) <- fixM (\ ~(_, skol_info) ->
562 do { (forall_tvs, theta, rho_ty) <- tcSkolType skol_info expected_ty
563 ; span <- getSrcSpanM
564 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty) span
565 ; return ((forall_tvs, theta, rho_ty), skol_info) })
568 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
569 text "expected_ty" <+> ppr expected_ty,
570 text "inst ty" <+> ppr forall_tvs <+> ppr theta <+> ppr rho_ty,
571 text "free_tvs" <+> ppr free_tvs,
572 text "forall_tvs" <+> ppr forall_tvs])
575 -- Type-check the arg and unify with poly type
576 ; (result, lie) <- getLIE (thing_inside rho_ty)
578 -- Check that the "forall_tvs" havn't been constrained
579 -- The interesting bit here is that we must include the free variables
580 -- of the expected_ty. Here's an example:
581 -- runST (newVar True)
582 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
583 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
584 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
585 -- So now s' isn't unconstrained because it's linked to a.
586 -- Conclusion: include the free vars of the expected_ty in the
587 -- list of "free vars" for the signature check.
589 ; dicts <- newDicts (SigOrigin skol_info) theta
590 ; inst_binds <- tcSimplifyCheck sig_msg forall_tvs dicts lie
592 ; checkSigTyVarsWrt free_tvs forall_tvs
593 ; traceTc (text "tcGen:done")
596 -- This HsLet binds any Insts which came out of the simplification.
597 -- It's a bit out of place here, but using AbsBind involves inventing
598 -- a couple of new names which seems worse.
599 dict_ids = map instToId dicts
600 co_fn e = TyLam forall_tvs (mkHsDictLam dict_ids (mkHsLet inst_binds (noLoc e)))
601 ; returnM (mkCoercion co_fn, result) }
603 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
604 sig_msg = ptext SLIT("expected type of an expression")
609 %************************************************************************
611 \subsection[Unify-exported]{Exported unification functions}
613 %************************************************************************
615 The exported functions are all defined as versions of some
616 non-exported generic functions.
618 Unify two @TauType@s. Dead straightforward.
621 unifyTauTy :: TcTauType -> TcTauType -> TcM ()
622 unifyTauTy ty1 ty2 -- ty1 expected, ty2 inferred
623 = -- The unifier should only ever see tau-types
624 -- (no quantification whatsoever)
625 ASSERT2( isTauTy ty1, ppr ty1 )
626 ASSERT2( isTauTy ty2, ppr ty2 )
627 addErrCtxtM (unifyCtxt "type" ty1 ty2) $
628 uTys True ty1 ty1 True ty2 ty2
630 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
631 unifyTheta theta1 theta2
632 = do { checkTc (equalLength theta1 theta2)
633 (ptext SLIT("Contexts differ in length"))
634 ; unifyTauTyLists True (map mkPredTy theta1) True (map mkPredTy theta2) }
637 @unifyTauTyList@ unifies corresponding elements of two lists of
638 @TauType@s. It uses @uTys@ to do the real work. The lists should be
639 of equal length. We charge down the list explicitly so that we can
640 complain if their lengths differ.
643 unifyTauTyLists :: Bool -> -- Allow refinements on tys1
645 Bool -> -- Allow refinements on tys2
646 [TcTauType] -> TcM ()
647 -- Precondition: lists must be same length
648 -- Having the caller check gives better error messages
649 -- Actually the caller neve does need to check; see Note [Tycon app]
650 unifyTauTyLists r1 [] r2 [] = returnM ()
651 unifyTauTyLists r1 (ty1:tys1) r2 (ty2:tys2) = uTys r1 ty1 ty1 r2 ty2 ty2 `thenM_`
652 unifyTauTyLists r1 tys1 r2 tys2
653 unifyTauTyLists r1 ty1s r2 ty2s = panic "Unify.unifyTauTyLists: mismatched type lists!"
656 @unifyTauTyList@ takes a single list of @TauType@s and unifies them
657 all together. It is used, for example, when typechecking explicit
658 lists, when all the elts should be of the same type.
661 unifyTauTyList :: [TcTauType] -> TcM ()
662 unifyTauTyList [] = returnM ()
663 unifyTauTyList [ty] = returnM ()
664 unifyTauTyList (ty1:tys@(ty2:_)) = unifyTauTy ty1 ty2 `thenM_`
668 %************************************************************************
670 \subsection[Unify-uTys]{@uTys@: getting down to business}
672 %************************************************************************
674 @uTys@ is the heart of the unifier. Each arg happens twice, because
675 we want to report errors in terms of synomyms if poss. The first of
676 the pair is used in error messages only; it is always the same as the
677 second, except that if the first is a synonym then the second may be a
678 de-synonym'd version. This way we get better error messages.
680 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
683 uTys :: Bool -- Allow refinements to ty1
684 -> TcTauType -> TcTauType -- Error reporting ty1 and real ty1
685 -- ty1 is the *expected* type
686 -> Bool -- Allow refinements to ty2
687 -> TcTauType -> TcTauType -- Error reporting ty2 and real ty2
688 -- ty2 is the *actual* type
691 -- Always expand synonyms (see notes at end)
692 -- (this also throws away FTVs)
693 uTys r1 ps_ty1 (NoteTy n1 ty1) r2 ps_ty2 ty2 = uTys r1 ps_ty1 ty1 r2 ps_ty2 ty2
694 uTys r1 ps_ty1 ty1 r2 ps_ty2 (NoteTy n2 ty2) = uTys r1 ps_ty1 ty1 r2 ps_ty2 ty2
696 -- Variables; go for uVar
697 uTys r1 ps_ty1 (TyVarTy tyvar1) r2 ps_ty2 ty2 = uVar False r1 tyvar1 r2 ps_ty2 ty2
698 uTys r1 ps_ty1 ty1 r2 ps_ty2 (TyVarTy tyvar2) = uVar True r2 tyvar2 r1 ps_ty1 ty1
699 -- "True" means args swapped
702 uTys r1 _ (PredTy (IParam n1 t1)) r2 _ (PredTy (IParam n2 t2))
703 | n1 == n2 = uTys r1 t1 t1 r2 t2 t2
704 uTys r1 _ (PredTy (ClassP c1 tys1)) r2 _ (PredTy (ClassP c2 tys2))
705 | c1 == c2 = unifyTauTyLists r1 tys1 r2 tys2
706 -- Guaranteed equal lengths because the kinds check
708 -- Functions; just check the two parts
709 uTys r1 _ (FunTy fun1 arg1) r2 _ (FunTy fun2 arg2)
710 = uTys r1 fun1 fun1 r2 fun2 fun2 `thenM_` uTys r1 arg1 arg1 r2 arg2 arg2
712 -- Type constructors must match
713 uTys r1 ps_ty1 (TyConApp con1 tys1) r2 ps_ty2 (TyConApp con2 tys2)
714 | con1 == con2 = unifyTauTyLists r1 tys1 r2 tys2
715 -- See Note [TyCon app]
717 -- Applications need a bit of care!
718 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
719 -- NB: we've already dealt with type variables and Notes,
720 -- so if one type is an App the other one jolly well better be too
721 uTys r1 ps_ty1 (AppTy s1 t1) r2 ps_ty2 ty2
722 = case tcSplitAppTy_maybe ty2 of
723 Just (s2,t2) -> uTys r1 s1 s1 r2 s2 s2 `thenM_` uTys r1 t1 t1 r2 t2 t2
724 Nothing -> unifyMisMatch ps_ty1 ps_ty2
726 -- Now the same, but the other way round
727 -- Don't swap the types, because the error messages get worse
728 uTys r1 ps_ty1 ty1 r2 ps_ty2 (AppTy s2 t2)
729 = case tcSplitAppTy_maybe ty1 of
730 Just (s1,t1) -> uTys r1 s1 s1 r2 s2 s2 `thenM_` uTys r1 t1 t1 r2 t2 t2
731 Nothing -> unifyMisMatch ps_ty1 ps_ty2
733 -- Not expecting for-alls in unification
734 -- ... but the error message from the unifyMisMatch more informative
735 -- than a panic message!
737 -- Anything else fails
738 uTys r1 ps_ty1 ty1 r2 ps_ty2 ty2 = unifyMisMatch ps_ty1 ps_ty2
743 When we find two TyConApps, the argument lists are guaranteed equal
744 length. Reason: intially the kinds of the two types to be unified is
745 the same. The only way it can become not the same is when unifying two
746 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
747 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
748 which we do, that ensures that f1,f2 have the same kind; and that
749 means a1,a2 have the same kind. And now the argument repeats.
754 If you are tempted to make a short cut on synonyms, as in this
758 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
759 -- NO = if (con1 == con2) then
760 -- NO -- Good news! Same synonym constructors, so we can shortcut
761 -- NO -- by unifying their arguments and ignoring their expansions.
762 -- NO unifyTauTypeLists args1 args2
764 -- NO -- Never mind. Just expand them and try again
768 then THINK AGAIN. Here is the whole story, as detected and reported
769 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
771 Here's a test program that should detect the problem:
775 x = (1 :: Bogus Char) :: Bogus Bool
778 The problem with [the attempted shortcut code] is that
782 is not a sufficient condition to be able to use the shortcut!
783 You also need to know that the type synonym actually USES all
784 its arguments. For example, consider the following type synonym
785 which does not use all its arguments.
790 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
791 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
792 would fail, even though the expanded forms (both \tr{Int}) should
795 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
796 unnecessarily bind \tr{t} to \tr{Char}.
798 ... You could explicitly test for the problem synonyms and mark them
799 somehow as needing expansion, perhaps also issuing a warning to the
804 %************************************************************************
806 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
808 %************************************************************************
810 @uVar@ is called when at least one of the types being unified is a
811 variable. It does {\em not} assume that the variable is a fixed point
812 of the substitution; rather, notice that @uVar@ (defined below) nips
813 back into @uTys@ if it turns out that the variable is already bound.
816 uVar :: Bool -- False => tyvar is the "expected"
817 -- True => ty is the "expected" thing
818 -> Bool -- True, allow refinements to tv1, False don't
820 -> Bool -- Allow refinements to ty2?
821 -> TcTauType -> TcTauType -- printing and real versions
824 uVar swapped r1 tv1 r2 ps_ty2 ty2
825 = traceTc (text "uVar" <+> ppr swapped <+> ppr tv1 <+> (ppr ps_ty2 $$ ppr ty2)) `thenM_`
826 condLookupTcTyVar r1 tv1 `thenM` \ details ->
828 IndirectTv r1' ty1 | swapped -> uTys r2 ps_ty2 ty2 r1' ty1 ty1 -- Swap back
829 | otherwise -> uTys r1' ty1 ty1 r2 ps_ty2 ty2 -- Same order
830 DoneTv details1 -> uDoneVar swapped tv1 details1 r2 ps_ty2 ty2
833 uDoneVar :: Bool -- Args are swapped
834 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
835 -> Bool -- Allow refinements to ty2
836 -> TcTauType -> TcTauType -- Type 2
838 -- Invariant: tyvar 1 is not unified with anything
840 uDoneVar swapped tv1 details1 r2 ps_ty2 (NoteTy n2 ty2)
841 = -- Expand synonyms; ignore FTVs
842 uDoneVar swapped tv1 details1 r2 ps_ty2 ty2
844 uDoneVar swapped tv1 details1 r2 ps_ty2 ty2@(TyVarTy tv2)
845 -- Same type variable => no-op
849 -- Distinct type variables
851 = do { lookup2 <- condLookupTcTyVar r2 tv2
853 IndirectTv b ty2' -> uDoneVar swapped tv1 details1 b ty2' ty2'
854 DoneTv details2 -> uDoneVars swapped tv1 details1 tv2 details2
857 uDoneVar swapped tv1 details1 r2 ps_ty2 non_var_ty2 -- ty2 is not a type variable
859 MetaTv ref1 -> do { -- Do the occurs check, and check that we are not
860 -- unifying a type variable with a polytype
861 -- Returns a zonked type ready for the update
862 ty2 <- checkValue tv1 r2 ps_ty2 non_var_ty2
863 ; updateMeta swapped tv1 ref1 ty2 }
865 skolem_details -> unifyMisMatch (TyVarTy tv1) ps_ty2
869 uDoneVars :: Bool -- Args are swapped
870 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
871 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
873 -- Invarant: the type variables are distinct,
874 -- and are not already unified with anything
876 uDoneVars swapped tv1 (MetaTv ref1) tv2 details2
878 MetaTv ref2 | update_tv2 -> updateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
879 other -> updateMeta swapped tv1 ref1 (mkTyVarTy tv2)
880 -- Note that updateMeta does a sub-kind check
881 -- We might unify (a b) with (c d) where b::*->* and d::*; this should fail
885 update_tv2 = k1 `isSubKind` k2 && (k1 /= k2 || nicer_to_update_tv2)
886 -- Update the variable with least kind info
887 -- See notes on type inference in Kind.lhs
888 -- The "nicer to" part only applies if the two kinds are the same,
889 -- so we can choose which to do.
891 nicer_to_update_tv2 = isSystemName (varName tv2)
892 -- Try to update sys-y type variables in preference to ones
893 -- gotten (say) by instantiating a polymorphic function with
894 -- a user-written type sig
896 uDoneVars swapped tv1 (SkolemTv _) tv2 details2
898 MetaTv ref2 -> updateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
899 other -> unifyMisMatch (mkTyVarTy tv1) (mkTyVarTy tv2)
901 uDoneVars swapped tv1 (SigSkolTv _ ref1) tv2 details2
903 MetaTv ref2 -> updateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
904 SigSkolTv _ _ -> updateMeta swapped tv1 ref1 (mkTyVarTy tv2)
905 other -> unifyMisMatch (mkTyVarTy tv1) (mkTyVarTy tv2)
908 updateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
909 -- Update tv1, which is flexi; occurs check is alrady done
910 updateMeta swapped tv1 ref1 ty2
911 = do { checkKinds swapped tv1 ty2
912 ; writeMutVar ref1 (Indirect ty2) }
916 checkKinds swapped tv1 ty2
917 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
918 -- ty2 has been zonked at this stage, which ensures that
919 -- its kind has as much boxity information visible as possible.
920 | tk2 `isSubKind` tk1 = returnM ()
923 -- Either the kinds aren't compatible
924 -- (can happen if we unify (a b) with (c d))
925 -- or we are unifying a lifted type variable with an
926 -- unlifted type: e.g. (id 3#) is illegal
927 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
928 unifyKindMisMatch k1 k2
930 (k1,k2) | swapped = (tk2,tk1)
931 | otherwise = (tk1,tk2)
937 checkValue tv1 r2 ps_ty2 non_var_ty2
938 -- Do the occurs check, and check that we are not
939 -- unifying a type variable with a polytype
940 -- Return the type to update the type variable with, or fail
942 -- Basically we want to update tv1 := ps_ty2
943 -- because ps_ty2 has type-synonym info, which improves later error messages
948 -- f :: (A a -> a -> ()) -> ()
952 -- x = f (\ x p -> p x)
954 -- In the application (p x), we try to match "t" with "A t". If we go
955 -- ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
956 -- an infinite loop later.
957 -- But we should not reject the program, because A t = ().
958 -- Rather, we should bind t to () (= non_var_ty2).
960 -- That's why we have this two-state occurs-check
961 = zonk_tc_type r2 ps_ty2 `thenM` \ ps_ty2' ->
962 case okToUnifyWith tv1 ps_ty2' of {
963 Nothing -> returnM ps_ty2' ; -- Success
966 zonk_tc_type r2 non_var_ty2 `thenM` \ non_var_ty2' ->
967 case okToUnifyWith tv1 non_var_ty2' of
968 Nothing -> -- This branch rarely succeeds, except in strange cases
969 -- like that in the example above
972 Just problem -> failWithTcM (unifyCheck problem tv1 ps_ty2')
975 zonk_tc_type refine ty
976 = zonkType (\tv -> return (TyVarTy tv)) refine ty
977 -- We may already be inside a wobbly type t2, and
978 -- should take that into account here
980 data Problem = OccurCheck | NotMonoType
982 okToUnifyWith :: TcTyVar -> TcType -> Maybe Problem
983 -- (okToUnifyWith tv ty) checks whether it's ok to unify
986 -- Just p => not ok, problem p
991 ok (TyVarTy tv') | tv == tv' = Just OccurCheck
992 | otherwise = Nothing
993 ok (AppTy t1 t2) = ok t1 `and` ok t2
994 ok (FunTy t1 t2) = ok t1 `and` ok t2
995 ok (TyConApp _ ts) = oks ts
996 ok (ForAllTy _ _) = Just NotMonoType
997 ok (PredTy st) = ok_st st
998 ok (NoteTy (FTVNote _) t) = ok t
999 ok (NoteTy (SynNote t1) t2) = ok t1 `and` ok t2
1000 -- Type variables may be free in t1 but not t2
1001 -- A forall may be in t2 but not t1
1003 oks ts = foldr (and . ok) Nothing ts
1005 ok_st (ClassP _ ts) = oks ts
1006 ok_st (IParam _ t) = ok t
1009 Just p `and` m = Just p
1013 %************************************************************************
1017 %************************************************************************
1019 Unifying kinds is much, much simpler than unifying types.
1022 unifyKind :: TcKind -- Expected
1025 unifyKind LiftedTypeKind LiftedTypeKind = returnM ()
1026 unifyKind UnliftedTypeKind UnliftedTypeKind = returnM ()
1028 unifyKind OpenTypeKind k2 | isOpenTypeKind k2 = returnM ()
1029 unifyKind ArgTypeKind k2 | isArgTypeKind k2 = returnM ()
1030 -- Respect sub-kinding
1032 unifyKind (FunKind a1 r1) (FunKind a2 r2)
1033 = do { unifyKind a2 a1; unifyKind r1 r2 }
1034 -- Notice the flip in the argument,
1035 -- so that the sub-kinding works right
1037 unifyKind (KindVar kv1) k2 = uKVar False kv1 k2
1038 unifyKind k1 (KindVar kv2) = uKVar True kv2 k1
1039 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1041 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1042 unifyKinds [] [] = returnM ()
1043 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1045 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1048 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1049 uKVar swapped kv1 k2
1050 = do { mb_k1 <- readKindVar kv1
1052 Nothing -> uUnboundKVar swapped kv1 k2
1053 Just k1 | swapped -> unifyKind k2 k1
1054 | otherwise -> unifyKind k1 k2 }
1057 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1058 uUnboundKVar swapped kv1 k2@(KindVar kv2)
1059 | kv1 == kv2 = returnM ()
1060 | otherwise -- Distinct kind variables
1061 = do { mb_k2 <- readKindVar kv2
1063 Just k2 -> uUnboundKVar swapped kv1 k2
1064 Nothing -> writeKindVar kv1 k2 }
1066 uUnboundKVar swapped kv1 non_var_k2
1067 = do { k2' <- zonkTcKind non_var_k2
1068 ; kindOccurCheck kv1 k2'
1069 ; k2'' <- kindSimpleKind swapped k2'
1070 -- KindVars must be bound only to simple kinds
1071 -- Polarities: (kindSimpleKind True ?) succeeds
1072 -- returning *, corresponding to unifying
1075 ; writeKindVar kv1 k2'' }
1078 kindOccurCheck kv1 k2 -- k2 is zonked
1079 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1081 not_in (KindVar kv2) = kv1 /= kv2
1082 not_in (FunKind a2 r2) = not_in a2 && not_in r2
1085 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1086 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1087 -- If the flag is False, it requires k <: sk
1088 -- E.g. kindSimpleKind False ?? = *
1089 -- What about (kv -> *) :=: ?? -> *
1090 kindSimpleKind orig_swapped orig_kind
1091 = go orig_swapped orig_kind
1093 go sw (FunKind k1 k2) = do { k1' <- go (not sw) k1
1095 ; return (FunKind k1' k2') }
1096 go True OpenTypeKind = return liftedTypeKind
1097 go True ArgTypeKind = return liftedTypeKind
1098 go sw LiftedTypeKind = return liftedTypeKind
1099 go sw k@(KindVar _) = return k -- KindVars are always simple
1100 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1101 <+> ppr orig_swapped <+> ppr orig_kind)
1102 -- I think this can't actually happen
1104 -- T v = MkT v v must be a type
1105 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1108 kindOccurCheckErr tyvar ty
1109 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1110 2 (sep [ppr tyvar, char '=', ppr ty])
1112 unifyKindMisMatch ty1 ty2
1113 = zonkTcKind ty1 `thenM` \ ty1' ->
1114 zonkTcKind ty2 `thenM` \ ty2' ->
1116 msg = hang (ptext SLIT("Couldn't match kind"))
1117 2 (sep [quotes (ppr ty1'),
1118 ptext SLIT("against"),
1125 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1126 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1128 unifyFunKind (KindVar kvar)
1129 = readKindVar kvar `thenM` \ maybe_kind ->
1131 Just fun_kind -> unifyFunKind fun_kind
1132 Nothing -> do { arg_kind <- newKindVar
1133 ; res_kind <- newKindVar
1134 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1135 ; returnM (Just (arg_kind,res_kind)) }
1137 unifyFunKind (FunKind arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1138 unifyFunKind other = returnM Nothing
1141 %************************************************************************
1143 \subsection[Unify-context]{Errors and contexts}
1145 %************************************************************************
1151 unifyCtxt s ty1 ty2 tidy_env -- ty1 expected, ty2 inferred
1152 = zonkTcType ty1 `thenM` \ ty1' ->
1153 zonkTcType ty2 `thenM` \ ty2' ->
1154 returnM (err ty1' ty2')
1156 err ty1 ty2 = (env1,
1159 text "Expected" <+> text s <> colon <+> ppr tidy_ty1,
1160 text "Inferred" <+> text s <> colon <+> ppr tidy_ty2
1163 (env1, [tidy_ty1,tidy_ty2]) = tidyOpenTypes tidy_env [ty1,ty2]
1165 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1166 -- tv1 and ty2 are zonked already
1169 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1170 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1172 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1173 | otherwise = (pp1, pp2)
1174 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1175 (env2, ty2') = tidyOpenType env1 ty2
1176 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1177 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1179 unifyMisMatch ty1 ty2
1180 = do { (env1, pp1, extra1) <- ppr_ty emptyTidyEnv ty1
1181 ; (env2, pp2, extra2) <- ppr_ty env1 ty2
1182 ; let msg = sep [sep [ptext SLIT("Couldn't match") <+> pp1, nest 7 (ptext SLIT("against") <+> pp2)],
1183 nest 2 extra1, nest 2 extra2]
1185 failWithTcM (env2, msg) }
1187 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
1189 = do { ty' <- zonkTcType ty
1190 ; let (env1,tidy_ty) = tidyOpenType env ty'
1191 simple_result = (env1, quotes (ppr tidy_ty), empty)
1194 | isSkolemTyVar tv -> return (env2, pp_rigid tv',
1196 | otherwise -> return simple_result
1198 (env2, tv') = tidySkolemTyVar env1 tv
1199 other -> return simple_result }
1201 pp_rigid tv = ptext SLIT("the rigid variable") <+> quotes (ppr tv)
1203 unifyCheck problem tyvar ty
1205 2 (sep [ppr tidy_tyvar, char '=', ppr tidy_ty]))
1207 (env1, tidy_tyvar) = tidyOpenTyVar emptyTidyEnv tyvar
1208 (env2, tidy_ty) = tidyOpenType env1 ty
1210 msg = case problem of
1211 OccurCheck -> ptext SLIT("Occurs check: cannot construct the infinite type:")
1212 NotMonoType -> ptext SLIT("Cannot unify a type variable with a type scheme:")
1216 %************************************************************************
1220 %************************************************************************
1222 ---------------------------
1223 -- We would like to get a decent error message from
1224 -- (a) Under-applied type constructors
1225 -- f :: (Maybe, Maybe)
1226 -- (b) Over-applied type constructors
1227 -- f :: Int x -> Int x
1231 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1232 -- A fancy wrapper for 'unifyKind', which tries
1233 -- to give decent error messages.
1234 checkExpectedKind ty act_kind exp_kind
1235 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1238 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (errs, mb_r) ->
1240 Just _ -> returnM () ; -- Unification succeeded
1243 -- So there's definitely an error
1244 -- Now to find out what sort
1245 zonkTcKind exp_kind `thenM` \ exp_kind ->
1246 zonkTcKind act_kind `thenM` \ act_kind ->
1248 let (exp_as, _) = splitKindFunTys exp_kind
1249 (act_as, _) = splitKindFunTys act_kind
1250 n_exp_as = length exp_as
1251 n_act_as = length act_as
1253 err | n_exp_as < n_act_as -- E.g. [Maybe]
1254 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1256 -- Now n_exp_as >= n_act_as. In the next two cases,
1257 -- n_exp_as == 0, and hence so is n_act_as
1258 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1259 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1260 <+> ptext SLIT("is unlifted")
1262 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1263 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1264 <+> ptext SLIT("is lifted")
1266 | otherwise -- E.g. Monad [Int]
1267 = sep [ ptext SLIT("Expecting kind") <+> quotes (pprKind exp_kind) <> comma,
1268 ptext SLIT("but") <+> quotes (ppr ty) <+>
1269 ptext SLIT("has kind") <+> quotes (pprKind act_kind)]
1271 failWithTc (ptext SLIT("Kind error:") <+> err)
1275 %************************************************************************
1277 \subsection{Checking signature type variables}
1279 %************************************************************************
1281 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1282 are not mentioned in the environment. In particular:
1284 (a) Not mentioned in the type of a variable in the envt
1285 eg the signature for f in this:
1291 Here, f is forced to be monorphic by the free occurence of x.
1293 (d) Not (unified with another type variable that is) in scope.
1294 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1295 when checking the expression type signature, we find that
1296 even though there is nothing in scope whose type mentions r,
1297 nevertheless the type signature for the expression isn't right.
1299 Another example is in a class or instance declaration:
1301 op :: forall b. a -> b
1303 Here, b gets unified with a
1305 Before doing this, the substitution is applied to the signature type variable.
1308 checkSigTyVars :: [TcTyVar] -> TcM ()
1309 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1311 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1312 checkSigTyVarsWrt extra_tvs sig_tvs
1313 = zonkTcTyVarsAndFV (varSetElems extra_tvs) `thenM` \ extra_tvs' ->
1314 check_sig_tyvars extra_tvs' sig_tvs
1317 :: TcTyVarSet -- Global type variables. The universally quantified
1318 -- tyvars should not mention any of these
1319 -- Guaranteed already zonked.
1320 -> [TcTyVar] -- Universally-quantified type variables in the signature
1321 -- Guaranteed to be skolems
1323 check_sig_tyvars extra_tvs []
1325 check_sig_tyvars extra_tvs sig_tvs
1326 = ASSERT( all isSkolemTyVar sig_tvs )
1327 do { gbl_tvs <- tcGetGlobalTyVars
1328 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1329 text "gbl_tvs" <+> ppr gbl_tvs,
1330 text "extra_tvs" <+> ppr extra_tvs]))
1332 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1333 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1334 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1337 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1338 -> [TcTyVar] -- The possibly-escaping type variables
1339 -> [TcTyVar] -- The zonked versions thereof
1341 -- Complain about escaping type variables
1342 -- We pass a list of type variables, at least one of which
1343 -- escapes. The first list contains the original signature type variable,
1344 -- while the second contains the type variable it is unified to (usually itself)
1345 bleatEscapedTvs globals sig_tvs zonked_tvs
1346 = do { (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1347 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1349 (env1, tidy_tvs) = tidyOpenTyVars emptyTidyEnv sig_tvs
1350 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1352 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1354 check (tidy_env, msgs) (sig_tv, zonked_tv)
1355 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1357 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1358 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1360 -----------------------
1361 escape_msg sig_tv zonked_tv globs
1363 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1364 nest 2 (vcat globs)]
1366 = msg <+> ptext SLIT("escapes")
1367 -- Sigh. It's really hard to give a good error message
1368 -- all the time. One bad case is an existential pattern match.
1369 -- We rely on the "When..." context to help.
1371 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1373 | sig_tv == zonked_tv = empty
1374 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1377 These two context are used with checkSigTyVars
1380 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1381 -> TidyEnv -> TcM (TidyEnv, Message)
1382 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1383 = zonkTcType sig_tau `thenM` \ actual_tau ->
1385 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1386 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1387 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1388 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1389 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1391 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),