2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section{Type subsumption and unification}
8 -- Full-blown subsumption
9 tcSubOff, tcSubExp, tcGen,
10 checkSigTyVars, checkSigTyVarsWrt, sigCtxt, findGlobals,
12 -- Various unifications
13 unifyTauTy, unifyTauTyList, unifyTauTyLists,
14 unifyKind, unifyKinds, unifyTypeKind, unifyFunKind,
16 --------------------------------
18 Expected(..), newHole, readExpectedType,
19 zapExpectedType, zapExpectedTo, zapExpectedBranches,
20 subFunTys, unifyFunTy,
21 zapToListTy, unifyListTy,
22 zapToPArrTy, unifyPArrTy,
23 zapToTupleTy, unifyTupleTy
27 #include "HsVersions.h"
30 import HsSyn ( HsExpr(..) )
31 import TcHsSyn ( mkHsLet,
32 ExprCoFn, idCoercion, isIdCoercion, mkCoercion, (<.>), (<$>) )
33 import TypeRep ( Type(..), PredType(..), TyNote(..), typeCon, openKindCon )
35 import TcRnMonad -- TcType, amongst others
36 import TcType ( TcKind, TcType, TcSigmaType, TcRhoType, TcTyVar, TcTauType,
37 TcTyVarSet, TcThetaType, TyVarDetails(SigTv),
38 isTauTy, isSigmaTy, mkFunTys, mkTyConApp,
39 tcSplitAppTy_maybe, tcSplitTyConApp_maybe,
40 tcGetTyVar_maybe, tcGetTyVar,
41 mkFunTy, tyVarsOfType, mkPhiTy,
42 typeKind, tcSplitFunTy_maybe, mkForAllTys,
43 isSkolemTyVar, isUserTyVar,
44 tidyOpenType, tidyOpenTypes, tidyOpenTyVar, tidyOpenTyVars,
45 eqKind, openTypeKind, liftedTypeKind, isTypeKind, mkArrowKind,
46 hasMoreBoxityInfo, allDistinctTyVars
48 import Inst ( newDicts, instToId, tcInstCall )
49 import TcMType ( getTcTyVar, putTcTyVar, tcInstType, newKindVar,
50 newTyVarTy, newTyVarTys, newBoxityVar,
51 zonkTcType, zonkTcTyVars, zonkTcTyVarsAndFV )
52 import TcSimplify ( tcSimplifyCheck )
53 import TysWiredIn ( listTyCon, parrTyCon, tupleTyCon )
54 import TcEnv ( tcGetGlobalTyVars, findGlobals )
55 import TyCon ( TyCon, tyConArity, isTupleTyCon, tupleTyConBoxity )
56 import PprType ( pprType )
57 import Id ( Id, mkSysLocal )
58 import Var ( Var, varName, tyVarKind )
59 import VarSet ( emptyVarSet, unitVarSet, unionVarSet, elemVarSet, varSetElems )
61 import Name ( isSystemName )
62 import ErrUtils ( Message )
63 import BasicTypes ( Boxity, Arity, isBoxed )
64 import Util ( equalLength, lengthExceeds, notNull )
70 * A hole is always filled in with an ordinary type, not another hole.
72 %************************************************************************
74 \subsection{'hole' type variables}
76 %************************************************************************
79 data Expected ty = Infer (TcRef ty) -- The hole to fill in for type inference
80 | Check ty -- The type to check during type checking
82 newHole :: TcM (TcRef ty)
83 newHole = newMutVar (error "Empty hole in typechecker")
85 readExpectedType :: Expected ty -> TcM ty
86 readExpectedType (Infer hole) = readMutVar hole
87 readExpectedType (Check ty) = returnM ty
89 zapExpectedType :: Expected TcType -> TcM TcTauType
90 -- In the inference case, ensure we have a monotype
91 zapExpectedType (Infer hole)
92 = do { ty <- newTyVarTy openTypeKind ;
96 zapExpectedType (Check ty) = return ty
98 zapExpectedTo :: Expected TcType -> TcTauType -> TcM ()
99 zapExpectedTo (Infer hole) ty2 = writeMutVar hole ty2
100 zapExpectedTo (Check ty1) ty2 = unifyTauTy ty1 ty2
102 zapExpectedBranches :: [a] -> Expected TcType -> TcM (Expected TcType)
103 -- Zap the expected type to a monotype if there is more than one branch
104 zapExpectedBranches branches exp_ty
105 | lengthExceeds branches 1 = zapExpectedType exp_ty `thenM` \ exp_ty' ->
106 return (Check exp_ty')
107 | otherwise = returnM exp_ty
109 instance Outputable ty => Outputable (Expected ty) where
110 ppr (Check ty) = ptext SLIT("Expected type") <+> ppr ty
111 ppr (Infer hole) = ptext SLIT("Inferring type")
115 %************************************************************************
117 \subsection[Unify-fun]{@unifyFunTy@}
119 %************************************************************************
121 @subFunTy@ and @unifyFunTy@ is used to avoid the fruitless
122 creation of type variables.
124 * subFunTy is used when we might be faced with a "hole" type variable,
125 in which case we should create two new holes.
127 * unifyFunTy is used when we expect to encounter only "ordinary"
128 type variables, so we should create new ordinary type variables
132 -> Expected TcRhoType -- Fail if ty isn't a function type
133 -> ([(pat, Expected TcRhoType)] -> Expected TcRhoType -> TcM a)
136 subFunTys pats (Infer hole) thing_inside
137 = -- This is the interesting case
138 mapM new_pat_hole pats `thenM` \ pats_w_holes ->
139 newHole `thenM` \ res_hole ->
142 thing_inside pats_w_holes (Infer res_hole) `thenM` \ answer ->
144 -- Extract the answers
145 mapM read_pat_hole pats_w_holes `thenM` \ arg_tys ->
146 readMutVar res_hole `thenM` \ res_ty ->
148 -- Write the answer into the incoming hole
149 writeMutVar hole (mkFunTys arg_tys res_ty) `thenM_`
151 -- And return the answer
154 new_pat_hole pat = newHole `thenM` \ hole -> return (pat, Infer hole)
155 read_pat_hole (pat, Infer hole) = readMutVar hole
157 subFunTys pats (Check ty) thing_inside
158 = go pats ty `thenM` \ (pats_w_tys, res_ty) ->
159 thing_inside pats_w_tys res_ty
161 go [] ty = return ([], Check ty)
162 go (pat:pats) ty = unifyFunTy ty `thenM` \ (arg,res) ->
163 go pats res `thenM` \ (pats_w_tys, final_res) ->
164 return ((pat, Check arg) : pats_w_tys, final_res)
166 unifyFunTy :: TcRhoType -- Fail if ty isn't a function type
167 -> TcM (TcType, TcType) -- otherwise return arg and result types
169 unifyFunTy ty@(TyVarTy tyvar)
170 = getTcTyVar tyvar `thenM` \ maybe_ty ->
172 Just ty' -> unifyFunTy ty'
173 Nothing -> unify_fun_ty_help ty
176 = case tcSplitFunTy_maybe ty of
177 Just arg_and_res -> returnM arg_and_res
178 Nothing -> unify_fun_ty_help ty
180 unify_fun_ty_help ty -- Special cases failed, so revert to ordinary unification
181 = newTyVarTy openTypeKind `thenM` \ arg ->
182 newTyVarTy openTypeKind `thenM` \ res ->
183 unifyTauTy ty (mkFunTy arg res) `thenM_`
188 ----------------------
189 zapToListTy, zapToPArrTy :: Expected TcType -- expected list type
190 -> TcM TcType -- list element type
191 unifyListTy, unifyPArrTy :: TcType -> TcM TcType
192 zapToListTy = zapToXTy listTyCon
193 unifyListTy = unifyXTy listTyCon
194 zapToPArrTy = zapToXTy parrTyCon
195 unifyPArrTy = unifyXTy parrTyCon
197 ----------------------
198 zapToXTy :: TyCon -- T :: *->*
199 -> Expected TcType -- Expected type (T a)
200 -> TcM TcType -- Element type, a
202 zapToXTy tc (Check ty) = unifyXTy tc ty
203 zapToXTy tc (Infer hole) = do { elt_ty <- newTyVarTy liftedTypeKind ;
204 writeMutVar hole (mkTyConApp tc [elt_ty]) ;
207 ----------------------
208 unifyXTy :: TyCon -> TcType -> TcM TcType
209 unifyXTy tc ty@(TyVarTy tyvar)
210 = getTcTyVar tyvar `thenM` \ maybe_ty ->
212 Just ty' -> unifyXTy tc ty'
213 other -> unify_x_ty_help tc ty
216 = case tcSplitTyConApp_maybe ty of
217 Just (tycon, [arg_ty]) | tycon == tc -> returnM arg_ty
218 other -> unify_x_ty_help tc ty
220 unify_x_ty_help tc ty -- Revert to ordinary unification
221 = newTyVarTy liftedTypeKind `thenM` \ elt_ty ->
222 unifyTauTy ty (mkTyConApp tc [elt_ty]) `thenM_`
227 ----------------------
228 zapToTupleTy :: Boxity -> Arity -> Expected TcType -> TcM [TcType]
229 zapToTupleTy boxity arity (Check ty) = unifyTupleTy boxity arity ty
230 zapToTupleTy boxity arity (Infer hole) = do { (tup_ty, arg_tys) <- new_tuple_ty boxity arity ;
231 writeMutVar hole tup_ty ;
234 unifyTupleTy boxity arity ty@(TyVarTy tyvar)
235 = getTcTyVar tyvar `thenM` \ maybe_ty ->
237 Just ty' -> unifyTupleTy boxity arity ty'
238 other -> unify_tuple_ty_help boxity arity ty
240 unifyTupleTy boxity arity ty
241 = case tcSplitTyConApp_maybe ty of
242 Just (tycon, arg_tys)
244 && tyConArity tycon == arity
245 && tupleTyConBoxity tycon == boxity
247 other -> unify_tuple_ty_help boxity arity ty
249 unify_tuple_ty_help boxity arity ty
250 = new_tuple_ty boxity arity `thenM` \ (tup_ty, arg_tys) ->
251 unifyTauTy ty tup_ty `thenM_`
254 new_tuple_ty boxity arity
255 = newTyVarTys arity kind `thenM` \ arg_tys ->
256 return (mkTyConApp tup_tc arg_tys, arg_tys)
258 tup_tc = tupleTyCon boxity arity
259 kind | isBoxed boxity = liftedTypeKind
260 | otherwise = openTypeKind
264 %************************************************************************
266 \subsection{Subsumption}
268 %************************************************************************
270 All the tcSub calls have the form
272 tcSub expected_ty offered_ty
274 offered_ty <= expected_ty
276 That is, that a value of type offered_ty is acceptable in
277 a place expecting a value of type expected_ty.
279 It returns a coercion function
280 co_fn :: offered_ty -> expected_ty
281 which takes an HsExpr of type offered_ty into one of type
285 tcSubExp :: Expected TcRhoType -> TcRhoType -> TcM ExprCoFn
286 tcSubOff :: TcSigmaType -> Expected TcSigmaType -> TcM ExprCoFn
289 These two check for holes
292 tcSubExp expected_ty offered_ty
293 = traceTc (text "tcSubExp" <+> (ppr expected_ty $$ ppr offered_ty)) `thenM_`
294 checkHole expected_ty offered_ty tcSub
296 tcSubOff expected_ty offered_ty
297 = checkHole offered_ty expected_ty (\ off exp -> tcSub exp off)
299 -- checkHole looks for a hole in its first arg;
300 -- If so, and it is uninstantiated, it fills in the hole
301 -- with its second arg
302 -- Otherwise it calls thing_inside, passing the two args, looking
303 -- through any instantiated hole
305 checkHole (Infer hole) other_ty thing_inside
306 = do { writeMutVar hole other_ty; return idCoercion }
308 checkHole (Check ty) other_ty thing_inside
309 = thing_inside ty other_ty
312 No holes expected now. Add some error-check context info.
315 tcSub :: TcSigmaType -> TcSigmaType -> TcM ExprCoFn -- Locally used only
316 tcSub expected_ty actual_ty
317 = traceTc (text "tcSub" <+> details) `thenM_`
318 addErrCtxtM (unifyCtxt "type" expected_ty actual_ty)
319 (tc_sub expected_ty expected_ty actual_ty actual_ty)
321 details = vcat [text "Expected:" <+> ppr expected_ty,
322 text "Actual: " <+> ppr actual_ty]
325 tc_sub carries the types before and after expanding type synonyms
328 tc_sub :: TcSigmaType -- expected_ty, before expanding synonyms
329 -> TcSigmaType -- ..and after
330 -> TcSigmaType -- actual_ty, before
331 -> TcSigmaType -- ..and after
334 -----------------------------------
336 tc_sub exp_sty (NoteTy _ exp_ty) act_sty act_ty = tc_sub exp_sty exp_ty act_sty act_ty
337 tc_sub exp_sty exp_ty act_sty (NoteTy _ act_ty) = tc_sub exp_sty exp_ty act_sty act_ty
339 -----------------------------------
340 -- Generalisation case
341 -- actual_ty: d:Eq b => b->b
342 -- expected_ty: forall a. Ord a => a->a
343 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
345 -- It is essential to do this *before* the specialisation case
346 -- Example: f :: (Eq a => a->a) -> ...
347 -- g :: Ord b => b->b
350 tc_sub exp_sty expected_ty act_sty actual_ty
351 | isSigmaTy expected_ty
352 = tcGen expected_ty (tyVarsOfType actual_ty) (
353 -- It's really important to check for escape wrt the free vars of
354 -- both expected_ty *and* actual_ty
355 \ body_exp_ty -> tc_sub body_exp_ty body_exp_ty act_sty actual_ty
356 ) `thenM` \ (gen_fn, co_fn) ->
357 returnM (gen_fn <.> co_fn)
359 -----------------------------------
360 -- Specialisation case:
361 -- actual_ty: forall a. Ord a => a->a
362 -- expected_ty: Int -> Int
363 -- co_fn e = e Int dOrdInt
365 tc_sub exp_sty expected_ty act_sty actual_ty
366 | isSigmaTy actual_ty
367 = tcInstCall Rank2Origin actual_ty `thenM` \ (inst_fn, body_ty) ->
368 tc_sub exp_sty expected_ty body_ty body_ty `thenM` \ co_fn ->
369 returnM (co_fn <.> inst_fn)
371 -----------------------------------
374 tc_sub _ (FunTy exp_arg exp_res) _ (FunTy act_arg act_res)
375 = tcSub_fun exp_arg exp_res act_arg act_res
377 -----------------------------------
378 -- Type variable meets function: imitate
380 -- NB 1: we can't just unify the type variable with the type
381 -- because the type might not be a tau-type, and we aren't
382 -- allowed to instantiate an ordinary type variable with
385 -- NB 2: can we short-cut to an error case?
386 -- when the arg/res is not a tau-type?
387 -- NO! e.g. f :: ((forall a. a->a) -> Int) -> Int
389 -- is perfectly fine, because we can instantiat f's type to a monotype
391 -- However, we get can get jolly unhelpful error messages.
392 -- e.g. foo = id runST
394 -- Inferred type is less polymorphic than expected
395 -- Quantified type variable `s' escapes
396 -- Expected type: ST s a -> t
397 -- Inferred type: (forall s1. ST s1 a) -> a
398 -- In the first argument of `id', namely `runST'
399 -- In a right-hand side of function `foo': id runST
401 -- I'm not quite sure what to do about this!
403 tc_sub exp_sty exp_ty@(FunTy exp_arg exp_res) _ (TyVarTy tv)
404 = getTcTyVar tv `thenM` \ maybe_ty ->
406 Just ty -> tc_sub exp_sty exp_ty ty ty
407 Nothing -> imitateFun tv exp_sty `thenM` \ (act_arg, act_res) ->
408 tcSub_fun exp_arg exp_res act_arg act_res
410 tc_sub _ (TyVarTy tv) act_sty act_ty@(FunTy act_arg act_res)
411 = getTcTyVar tv `thenM` \ maybe_ty ->
413 Just ty -> tc_sub ty ty act_sty act_ty
414 Nothing -> imitateFun tv act_sty `thenM` \ (exp_arg, exp_res) ->
415 tcSub_fun exp_arg exp_res act_arg act_res
417 -----------------------------------
419 -- If none of the above match, we revert to the plain unifier
420 tc_sub exp_sty expected_ty act_sty actual_ty
421 = uTys exp_sty expected_ty act_sty actual_ty `thenM_`
425 %************************************************************************
427 \subsection{Functions}
429 %************************************************************************
432 tcSub_fun exp_arg exp_res act_arg act_res
433 = tc_sub act_arg act_arg exp_arg exp_arg `thenM` \ co_fn_arg ->
434 tc_sub exp_res exp_res act_res act_res `thenM` \ co_fn_res ->
435 newUnique `thenM` \ uniq ->
437 -- co_fn_arg :: HsExpr exp_arg -> HsExpr act_arg
438 -- co_fn_res :: HsExpr act_res -> HsExpr exp_res
439 -- co_fn :: HsExpr (act_arg -> act_res) -> HsExpr (exp_arg -> exp_res)
440 arg_id = mkSysLocal FSLIT("sub") uniq exp_arg
441 coercion | isIdCoercion co_fn_arg,
442 isIdCoercion co_fn_res = idCoercion
443 | otherwise = mkCoercion co_fn
445 co_fn e = DictLam [arg_id]
446 (co_fn_res <$> (HsApp e (co_fn_arg <$> (HsVar arg_id))))
447 -- Slight hack; using a "DictLam" to get an ordinary simple lambda
448 -- HsVar arg_id :: HsExpr exp_arg
449 -- co_fn_arg $it :: HsExpr act_arg
450 -- HsApp e $it :: HsExpr act_res
451 -- co_fn_res $it :: HsExpr exp_res
455 imitateFun :: TcTyVar -> TcType -> TcM (TcType, TcType)
457 = -- NB: tv is an *ordinary* tyvar and so are the new ones
459 -- Check that tv isn't a type-signature type variable
460 -- (This would be found later in checkSigTyVars, but
461 -- we get a better error message if we do it here.)
462 checkM (not (isSkolemTyVar tv))
463 (failWithTcM (unifyWithSigErr tv ty)) `thenM_`
465 newTyVarTy openTypeKind `thenM` \ arg ->
466 newTyVarTy openTypeKind `thenM` \ res ->
467 putTcTyVar tv (mkFunTy arg res) `thenM_`
472 %************************************************************************
474 \subsection{Generalisation}
476 %************************************************************************
479 tcGen :: TcSigmaType -- expected_ty
480 -> TcTyVarSet -- Extra tyvars that the universally
481 -- quantified tyvars of expected_ty
482 -- must not be unified
483 -> (TcRhoType -> TcM result) -- spec_ty
484 -> TcM (ExprCoFn, result)
485 -- The expression has type: spec_ty -> expected_ty
487 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
488 -- If not, the call is a no-op
489 = tcInstType SigTv expected_ty `thenM` \ (forall_tvs, theta, phi_ty) ->
491 -- Type-check the arg and unify with poly type
492 getLIE (thing_inside phi_ty) `thenM` \ (result, lie) ->
494 -- Check that the "forall_tvs" havn't been constrained
495 -- The interesting bit here is that we must include the free variables
496 -- of the expected_ty. Here's an example:
497 -- runST (newVar True)
498 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
499 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
500 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
501 -- So now s' isn't unconstrained because it's linked to a.
502 -- Conclusion: include the free vars of the expected_ty in the
503 -- list of "free vars" for the signature check.
505 newDicts SignatureOrigin theta `thenM` \ dicts ->
506 tcSimplifyCheck sig_msg forall_tvs dicts lie `thenM` \ inst_binds ->
509 zonkTcTyVars forall_tvs `thenM` \ forall_tys ->
510 traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
511 text "expected_ty" <+> ppr expected_ty,
512 text "inst ty" <+> ppr forall_tvs <+> ppr theta <+> ppr phi_ty,
513 text "free_tvs" <+> ppr free_tvs,
514 text "forall_tys" <+> ppr forall_tys]) `thenM_`
517 checkSigTyVarsWrt free_tvs forall_tvs `thenM` \ zonked_tvs ->
519 traceTc (text "tcGen:done") `thenM_`
522 -- This HsLet binds any Insts which came out of the simplification.
523 -- It's a bit out of place here, but using AbsBind involves inventing
524 -- a couple of new names which seems worse.
525 dict_ids = map instToId dicts
526 co_fn e = TyLam zonked_tvs (DictLam dict_ids (mkHsLet inst_binds e))
528 returnM (mkCoercion co_fn, result)
530 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
531 sig_msg = ptext SLIT("expected type of an expression")
536 %************************************************************************
538 \subsection[Unify-exported]{Exported unification functions}
540 %************************************************************************
542 The exported functions are all defined as versions of some
543 non-exported generic functions.
545 Unify two @TauType@s. Dead straightforward.
548 unifyTauTy :: TcTauType -> TcTauType -> TcM ()
549 unifyTauTy ty1 ty2 -- ty1 expected, ty2 inferred
550 = -- The unifier should only ever see tau-types
551 -- (no quantification whatsoever)
552 ASSERT2( isTauTy ty1, ppr ty1 )
553 ASSERT2( isTauTy ty2, ppr ty2 )
554 addErrCtxtM (unifyCtxt "type" ty1 ty2) $
558 @unifyTauTyList@ unifies corresponding elements of two lists of
559 @TauType@s. It uses @uTys@ to do the real work. The lists should be
560 of equal length. We charge down the list explicitly so that we can
561 complain if their lengths differ.
564 unifyTauTyLists :: [TcTauType] -> [TcTauType] -> TcM ()
565 unifyTauTyLists [] [] = returnM ()
566 unifyTauTyLists (ty1:tys1) (ty2:tys2) = uTys ty1 ty1 ty2 ty2 `thenM_`
567 unifyTauTyLists tys1 tys2
568 unifyTauTyLists ty1s ty2s = panic "Unify.unifyTauTyLists: mismatched type lists!"
571 @unifyTauTyList@ takes a single list of @TauType@s and unifies them
572 all together. It is used, for example, when typechecking explicit
573 lists, when all the elts should be of the same type.
576 unifyTauTyList :: [TcTauType] -> TcM ()
577 unifyTauTyList [] = returnM ()
578 unifyTauTyList [ty] = returnM ()
579 unifyTauTyList (ty1:tys@(ty2:_)) = unifyTauTy ty1 ty2 `thenM_`
583 %************************************************************************
585 \subsection[Unify-uTys]{@uTys@: getting down to business}
587 %************************************************************************
589 @uTys@ is the heart of the unifier. Each arg happens twice, because
590 we want to report errors in terms of synomyms if poss. The first of
591 the pair is used in error messages only; it is always the same as the
592 second, except that if the first is a synonym then the second may be a
593 de-synonym'd version. This way we get better error messages.
595 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
598 uTys :: TcTauType -> TcTauType -- Error reporting ty1 and real ty1
599 -- ty1 is the *expected* type
601 -> TcTauType -> TcTauType -- Error reporting ty2 and real ty2
602 -- ty2 is the *actual* type
605 -- Always expand synonyms (see notes at end)
606 -- (this also throws away FTVs)
607 uTys ps_ty1 (NoteTy n1 ty1) ps_ty2 ty2 = uTys ps_ty1 ty1 ps_ty2 ty2
608 uTys ps_ty1 ty1 ps_ty2 (NoteTy n2 ty2) = uTys ps_ty1 ty1 ps_ty2 ty2
610 -- Variables; go for uVar
611 uTys ps_ty1 (TyVarTy tyvar1) ps_ty2 ty2 = uVar False tyvar1 ps_ty2 ty2
612 uTys ps_ty1 ty1 ps_ty2 (TyVarTy tyvar2) = uVar True tyvar2 ps_ty1 ty1
613 -- "True" means args swapped
616 uTys _ (PredTy (IParam n1 t1)) _ (PredTy (IParam n2 t2))
617 | n1 == n2 = uTys t1 t1 t2 t2
618 uTys _ (PredTy (ClassP c1 tys1)) _ (PredTy (ClassP c2 tys2))
619 | c1 == c2 = unifyTauTyLists tys1 tys2
621 -- Functions; just check the two parts
622 uTys _ (FunTy fun1 arg1) _ (FunTy fun2 arg2)
623 = uTys fun1 fun1 fun2 fun2 `thenM_` uTys arg1 arg1 arg2 arg2
625 -- NewType constructors must match
626 uTys _ (NewTcApp tc1 tys1) _ (NewTcApp tc2 tys2)
627 | tc1 == tc2 = unifyTauTyLists tys1 tys2
629 -- Ordinary type constructors must match
630 uTys ps_ty1 (TyConApp con1 tys1) ps_ty2 (TyConApp con2 tys2)
631 | con1 == con2 && equalLength tys1 tys2
632 = unifyTauTyLists tys1 tys2
634 | con1 == openKindCon
635 -- When we are doing kind checking, we might match a kind '?'
636 -- against a kind '*' or '#'. Notably, CCallable :: ? -> *, and
637 -- (CCallable Int) and (CCallable Int#) are both OK
638 = unifyTypeKind ps_ty2
640 -- Applications need a bit of care!
641 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
642 -- NB: we've already dealt with type variables and Notes,
643 -- so if one type is an App the other one jolly well better be too
644 uTys ps_ty1 (AppTy s1 t1) ps_ty2 ty2
645 = case tcSplitAppTy_maybe ty2 of
646 Just (s2,t2) -> uTys s1 s1 s2 s2 `thenM_` uTys t1 t1 t2 t2
647 Nothing -> unifyMisMatch ps_ty1 ps_ty2
649 -- Now the same, but the other way round
650 -- Don't swap the types, because the error messages get worse
651 uTys ps_ty1 ty1 ps_ty2 (AppTy s2 t2)
652 = case tcSplitAppTy_maybe ty1 of
653 Just (s1,t1) -> uTys s1 s1 s2 s2 `thenM_` uTys t1 t1 t2 t2
654 Nothing -> unifyMisMatch ps_ty1 ps_ty2
656 -- Not expecting for-alls in unification
657 -- ... but the error message from the unifyMisMatch more informative
658 -- than a panic message!
660 -- Anything else fails
661 uTys ps_ty1 ty1 ps_ty2 ty2 = unifyMisMatch ps_ty1 ps_ty2
667 If you are tempted to make a short cut on synonyms, as in this
671 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
672 -- NO = if (con1 == con2) then
673 -- NO -- Good news! Same synonym constructors, so we can shortcut
674 -- NO -- by unifying their arguments and ignoring their expansions.
675 -- NO unifyTauTypeLists args1 args2
677 -- NO -- Never mind. Just expand them and try again
681 then THINK AGAIN. Here is the whole story, as detected and reported
682 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
684 Here's a test program that should detect the problem:
688 x = (1 :: Bogus Char) :: Bogus Bool
691 The problem with [the attempted shortcut code] is that
695 is not a sufficient condition to be able to use the shortcut!
696 You also need to know that the type synonym actually USES all
697 its arguments. For example, consider the following type synonym
698 which does not use all its arguments.
703 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
704 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
705 would fail, even though the expanded forms (both \tr{Int}) should
708 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
709 unnecessarily bind \tr{t} to \tr{Char}.
711 ... You could explicitly test for the problem synonyms and mark them
712 somehow as needing expansion, perhaps also issuing a warning to the
717 %************************************************************************
719 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
721 %************************************************************************
723 @uVar@ is called when at least one of the types being unified is a
724 variable. It does {\em not} assume that the variable is a fixed point
725 of the substitution; rather, notice that @uVar@ (defined below) nips
726 back into @uTys@ if it turns out that the variable is already bound.
729 uVar :: Bool -- False => tyvar is the "expected"
730 -- True => ty is the "expected" thing
732 -> TcTauType -> TcTauType -- printing and real versions
735 uVar swapped tv1 ps_ty2 ty2
736 = traceTc (text "uVar" <+> ppr swapped <+> ppr tv1 <+> (ppr ps_ty2 $$ ppr ty2)) `thenM_`
737 getTcTyVar tv1 `thenM` \ maybe_ty1 ->
739 Just ty1 | swapped -> uTys ps_ty2 ty2 ty1 ty1 -- Swap back
740 | otherwise -> uTys ty1 ty1 ps_ty2 ty2 -- Same order
741 other -> uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2
743 -- Expand synonyms; ignore FTVs
744 uUnboundVar swapped tv1 maybe_ty1 ps_ty2 (NoteTy n2 ty2)
745 = uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2
748 -- The both-type-variable case
749 uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2@(TyVarTy tv2)
751 -- Same type variable => no-op
755 -- Distinct type variables
756 -- ASSERT maybe_ty1 /= Just
758 = getTcTyVar tv2 `thenM` \ maybe_ty2 ->
760 Just ty2' -> uUnboundVar swapped tv1 maybe_ty1 ty2' ty2'
764 -> WARN( not (k1 `hasMoreBoxityInfo` k2), (ppr tv1 <+> ppr k1) $$ (ppr tv2 <+> ppr k2) )
765 putTcTyVar tv2 (TyVarTy tv1) `thenM_`
769 -> WARN( not (k2 `hasMoreBoxityInfo` k1), (ppr tv2 <+> ppr k2) $$ (ppr tv1 <+> ppr k1) )
770 putTcTyVar tv1 ps_ty2 `thenM_`
775 update_tv2 = (k2 `eqKind` openTypeKind) || (not (k1 `eqKind` openTypeKind) && nicer_to_update_tv2)
776 -- Try to get rid of open type variables as soon as poss
778 nicer_to_update_tv2 = isUserTyVar tv1
779 -- Don't unify a signature type variable if poss
780 || isSystemName (varName tv2)
781 -- Try to update sys-y type variables in preference to sig-y ones
783 -- Second one isn't a type variable
784 uUnboundVar swapped tv1 maybe_ty1 ps_ty2 non_var_ty2
785 = -- Check that tv1 isn't a type-signature type variable
786 checkM (not (isSkolemTyVar tv1))
787 (failWithTcM (unifyWithSigErr tv1 ps_ty2)) `thenM_`
789 -- Do the occurs check, and check that we are not
790 -- unifying a type variable with a polytype
791 -- Returns a zonked type ready for the update
792 checkValue tv1 ps_ty2 non_var_ty2 `thenM` \ ty2 ->
794 -- Check that the kinds match
795 checkKinds swapped tv1 ty2 `thenM_`
797 -- Perform the update
798 putTcTyVar tv1 ty2 `thenM_`
803 checkKinds swapped tv1 ty2
804 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
805 -- ty2 has been zonked at this stage, which ensures that
806 -- its kind has as much boxity information visible as possible.
807 | tk2 `hasMoreBoxityInfo` tk1 = returnM ()
810 -- Either the kinds aren't compatible
811 -- (can happen if we unify (a b) with (c d))
812 -- or we are unifying a lifted type variable with an
813 -- unlifted type: e.g. (id 3#) is illegal
814 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
818 (k1,k2) | swapped = (tk2,tk1)
819 | otherwise = (tk1,tk2)
825 checkValue tv1 ps_ty2 non_var_ty2
826 -- Do the occurs check, and check that we are not
827 -- unifying a type variable with a polytype
828 -- Return the type to update the type variable with, or fail
830 -- Basically we want to update tv1 := ps_ty2
831 -- because ps_ty2 has type-synonym info, which improves later error messages
836 -- f :: (A a -> a -> ()) -> ()
840 -- x = f (\ x p -> p x)
842 -- In the application (p x), we try to match "t" with "A t". If we go
843 -- ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
844 -- an infinite loop later.
845 -- But we should not reject the program, because A t = ().
846 -- Rather, we should bind t to () (= non_var_ty2).
848 -- That's why we have this two-state occurs-check
849 = zonkTcType ps_ty2 `thenM` \ ps_ty2' ->
850 case okToUnifyWith tv1 ps_ty2' of {
851 Nothing -> returnM ps_ty2' ; -- Success
854 zonkTcType non_var_ty2 `thenM` \ non_var_ty2' ->
855 case okToUnifyWith tv1 non_var_ty2' of
856 Nothing -> -- This branch rarely succeeds, except in strange cases
857 -- like that in the example above
860 Just problem -> failWithTcM (unifyCheck problem tv1 ps_ty2')
863 data Problem = OccurCheck | NotMonoType
865 okToUnifyWith :: TcTyVar -> TcType -> Maybe Problem
866 -- (okToUnifyWith tv ty) checks whether it's ok to unify
869 -- Just p => not ok, problem p
874 ok (TyVarTy tv') | tv == tv' = Just OccurCheck
875 | otherwise = Nothing
876 ok (AppTy t1 t2) = ok t1 `and` ok t2
877 ok (FunTy t1 t2) = ok t1 `and` ok t2
878 ok (TyConApp _ ts) = oks ts
879 ok (NewTcApp _ ts) = oks ts
880 ok (ForAllTy _ _) = Just NotMonoType
881 ok (PredTy st) = ok_st st
882 ok (NoteTy (FTVNote _) t) = ok t
883 ok (NoteTy (SynNote t1) t2) = ok t1 `and` ok t2
884 -- Type variables may be free in t1 but not t2
885 -- A forall may be in t2 but not t1
887 oks ts = foldr (and . ok) Nothing ts
889 ok_st (ClassP _ ts) = oks ts
890 ok_st (IParam _ t) = ok t
893 Just p `and` m = Just p
896 %************************************************************************
898 \subsection{Kind unification}
900 %************************************************************************
903 unifyKind :: TcKind -- Expected
906 unifyKind k1 k2 = uTys k1 k1 k2 k2
908 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
909 unifyKinds [] [] = returnM ()
910 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
912 unifyKinds _ _ = panic "unifyKinds: length mis-match"
916 unifyTypeKind :: TcKind -> TcM ()
917 -- Ensures that the argument kind is a liftedTypeKind or unliftedTypeKind
918 -- If it's a kind variable, make it (Type bx), for a fresh boxity variable bx
920 unifyTypeKind ty@(TyVarTy tyvar)
921 = getTcTyVar tyvar `thenM` \ maybe_ty ->
923 Just ty' -> unifyTypeKind ty'
924 Nothing -> newBoxityVar `thenM` \ bx_var ->
925 putTcTyVar tyvar (mkTyConApp typeCon [bx_var]) `thenM_`
929 | isTypeKind ty = returnM ()
930 | otherwise -- Failure
931 = zonkTcType ty `thenM` \ ty1 ->
932 failWithTc (ptext SLIT("Type expected but") <+> quotes (ppr ty1) <+> ptext SLIT("found"))
936 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
937 -- Like unifyFunTy, but does not fail; instead just returns Nothing
939 unifyFunKind (TyVarTy tyvar)
940 = getTcTyVar tyvar `thenM` \ maybe_ty ->
942 Just fun_kind -> unifyFunKind fun_kind
943 Nothing -> newKindVar `thenM` \ arg_kind ->
944 newKindVar `thenM` \ res_kind ->
945 putTcTyVar tyvar (mkArrowKind arg_kind res_kind) `thenM_`
946 returnM (Just (arg_kind,res_kind))
948 unifyFunKind (FunTy arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
949 unifyFunKind (NoteTy _ ty) = unifyFunKind ty
950 unifyFunKind other = returnM Nothing
953 %************************************************************************
955 \subsection[Unify-context]{Errors and contexts}
957 %************************************************************************
963 unifyCtxt s ty1 ty2 tidy_env -- ty1 expected, ty2 inferred
964 = zonkTcType ty1 `thenM` \ ty1' ->
965 zonkTcType ty2 `thenM` \ ty2' ->
966 returnM (err ty1' ty2')
971 text "Expected" <+> text s <> colon <+> ppr tidy_ty1,
972 text "Inferred" <+> text s <> colon <+> ppr tidy_ty2
975 (env1, [tidy_ty1,tidy_ty2]) = tidyOpenTypes tidy_env [ty1,ty2]
977 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
978 -- tv1 is zonked already
979 = zonkTcType ty2 `thenM` \ ty2' ->
982 err ty2 = (env2, ptext SLIT("When matching types") <+>
983 sep [quotes pp_expected, ptext SLIT("and"), quotes pp_actual])
985 (pp_expected, pp_actual) | swapped = (pp2, pp1)
986 | otherwise = (pp1, pp2)
987 (env1, tv1') = tidyOpenTyVar tidy_env tv1
988 (env2, ty2') = tidyOpenType env1 ty2
992 unifyMisMatch ty1 ty2
993 = zonkTcType ty1 `thenM` \ ty1' ->
994 zonkTcType ty2 `thenM` \ ty2' ->
996 (env, [tidy_ty1, tidy_ty2]) = tidyOpenTypes emptyTidyEnv [ty1',ty2']
997 msg = hang (ptext SLIT("Couldn't match"))
998 4 (sep [quotes (ppr tidy_ty1),
999 ptext SLIT("against"),
1000 quotes (ppr tidy_ty2)])
1002 failWithTcM (env, msg)
1004 unifyWithSigErr tyvar ty
1005 = (env2, hang (ptext SLIT("Cannot unify the type-signature variable") <+> quotes (ppr tidy_tyvar))
1006 4 (ptext SLIT("with the type") <+> quotes (ppr tidy_ty)))
1008 (env1, tidy_tyvar) = tidyOpenTyVar emptyTidyEnv tyvar
1009 (env2, tidy_ty) = tidyOpenType env1 ty
1011 unifyCheck problem tyvar ty
1013 4 (sep [ppr tidy_tyvar, char '=', ppr tidy_ty]))
1015 (env1, tidy_tyvar) = tidyOpenTyVar emptyTidyEnv tyvar
1016 (env2, tidy_ty) = tidyOpenType env1 ty
1018 msg = case problem of
1019 OccurCheck -> ptext SLIT("Occurs check: cannot construct the infinite type:")
1020 NotMonoType -> ptext SLIT("Cannot unify a type variable with a type scheme:")
1025 %************************************************************************
1027 \subsection{Checking signature type variables}
1029 %************************************************************************
1031 @checkSigTyVars@ is used after the type in a type signature has been unified with
1032 the actual type found. It then checks that the type variables of the type signature
1034 (a) Still all type variables
1035 eg matching signature [a] against inferred type [(p,q)]
1036 [then a will be unified to a non-type variable]
1038 (b) Still all distinct
1039 eg matching signature [(a,b)] against inferred type [(p,p)]
1040 [then a and b will be unified together]
1042 (c) Not mentioned in the environment
1043 eg the signature for f in this:
1049 Here, f is forced to be monorphic by the free occurence of x.
1051 (d) Not (unified with another type variable that is) in scope.
1052 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1053 when checking the expression type signature, we find that
1054 even though there is nothing in scope whose type mentions r,
1055 nevertheless the type signature for the expression isn't right.
1057 Another example is in a class or instance declaration:
1059 op :: forall b. a -> b
1061 Here, b gets unified with a
1063 Before doing this, the substitution is applied to the signature type variable.
1065 We used to have the notion of a "DontBind" type variable, which would
1066 only be bound to itself or nothing. Then points (a) and (b) were
1067 self-checking. But it gave rise to bogus consequential error messages.
1070 f = (*) -- Monomorphic
1072 g :: Num a => a -> a
1075 Here, we get a complaint when checking the type signature for g,
1076 that g isn't polymorphic enough; but then we get another one when
1077 dealing with the (Num x) context arising from f's definition;
1078 we try to unify x with Int (to default it), but find that x has already
1079 been unified with the DontBind variable "a" from g's signature.
1080 This is really a problem with side-effecting unification; we'd like to
1081 undo g's effects when its type signature fails, but unification is done
1082 by side effect, so we can't (easily).
1084 So we revert to ordinary type variables for signatures, and try to
1085 give a helpful message in checkSigTyVars.
1088 checkSigTyVars :: [TcTyVar] -> TcM [TcTyVar]
1089 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1091 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM [TcTyVar]
1092 checkSigTyVarsWrt extra_tvs sig_tvs
1093 = zonkTcTyVarsAndFV (varSetElems extra_tvs) `thenM` \ extra_tvs' ->
1094 check_sig_tyvars extra_tvs' sig_tvs
1097 :: TcTyVarSet -- Global type variables. The universally quantified
1098 -- tyvars should not mention any of these
1099 -- Guaranteed already zonked.
1100 -> [TcTyVar] -- Universally-quantified type variables in the signature
1101 -- Not guaranteed zonked.
1102 -> TcM [TcTyVar] -- Zonked signature type variables
1104 check_sig_tyvars extra_tvs []
1106 check_sig_tyvars extra_tvs sig_tvs
1107 = zonkTcTyVars sig_tvs `thenM` \ sig_tys ->
1108 tcGetGlobalTyVars `thenM` \ gbl_tvs ->
1110 env_tvs = gbl_tvs `unionVarSet` extra_tvs
1112 traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tys,
1113 text "gbl_tvs" <+> ppr gbl_tvs,
1114 text "extra_tvs" <+> ppr extra_tvs])) `thenM_`
1116 checkM (allDistinctTyVars sig_tys env_tvs)
1117 (complain sig_tys env_tvs) `thenM_`
1119 returnM (map (tcGetTyVar "checkSigTyVars") sig_tys)
1122 complain sig_tys globals
1123 = -- "check" checks each sig tyvar in turn
1125 (env2, emptyVarEnv, [])
1126 (tidy_tvs `zip` tidy_tys) `thenM` \ (env3, _, msgs) ->
1128 failWithTcM (env3, main_msg $$ nest 4 (vcat msgs))
1130 (env1, tidy_tvs) = tidyOpenTyVars emptyTidyEnv sig_tvs
1131 (env2, tidy_tys) = tidyOpenTypes env1 sig_tys
1133 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1135 check (tidy_env, acc, msgs) (sig_tyvar,ty)
1136 -- sig_tyvar is from the signature;
1137 -- ty is what you get if you zonk sig_tyvar and then tidy it
1139 -- acc maps a zonked type variable back to a signature type variable
1140 = case tcGetTyVar_maybe ty of {
1141 Nothing -> -- Error (a)!
1142 returnM (tidy_env, acc, unify_msg sig_tyvar (quotes (ppr ty)) : msgs) ;
1146 case lookupVarEnv acc tv of {
1147 Just sig_tyvar' -> -- Error (b)!
1148 returnM (tidy_env, acc, unify_msg sig_tyvar thing : msgs)
1150 thing = ptext SLIT("another quantified type variable") <+> quotes (ppr sig_tyvar')
1154 if tv `elemVarSet` globals -- Error (c) or (d)! Type variable escapes
1155 -- The least comprehensible, so put it last
1157 -- get the local TcIds and TyVars from the environment,
1158 -- and pass them to find_globals (they might have tv free)
1159 then findGlobals (unitVarSet tv) tidy_env `thenM` \ (tidy_env1, globs) ->
1160 returnM (tidy_env1, acc, escape_msg sig_tyvar tv globs : msgs)
1163 returnM (tidy_env, extendVarEnv acc tv sig_tyvar, msgs)
1169 -----------------------
1170 escape_msg sig_tv tv globs
1171 = mk_msg sig_tv <+> ptext SLIT("escapes") $$
1172 if notNull globs then
1173 vcat [pp_it <+> ptext SLIT("is mentioned in the environment:"),
1174 nest 2 (vcat globs)]
1176 empty -- Sigh. It's really hard to give a good error message
1177 -- all the time. One bad case is an existential pattern match.
1178 -- We rely on the "When..." context to help.
1180 pp_it | sig_tv /= tv = ptext SLIT("It unifies with") <+> quotes (ppr tv) <> comma <+> ptext SLIT("which")
1181 | otherwise = ptext SLIT("It")
1184 unify_msg tv thing = mk_msg tv <+> ptext SLIT("is unified with") <+> thing
1185 mk_msg tv = ptext SLIT("Quantified type variable") <+> quotes (ppr tv)
1188 These two context are used with checkSigTyVars
1191 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1192 -> TidyEnv -> TcM (TidyEnv, Message)
1193 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1194 = zonkTcType sig_tau `thenM` \ actual_tau ->
1196 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1197 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1198 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1199 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1200 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1202 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),