2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Type subsumption and unification
10 -- Full-blown subsumption
11 tcWrapResult, tcSubType, tcGen,
12 checkConstraints, newImplication, sigCtxt,
14 -- Various unifications
15 unifyType, unifyTypeList, unifyTheta, unifyKind,
17 --------------------------------
20 matchExpectedListTy, matchExpectedPArrTy,
21 matchExpectedTyConApp, matchExpectedAppTy,
22 matchExpectedFunTys, matchExpectedFunKind,
26 #include "HsVersions.h"
31 import TcErrors ( typeExtraInfoMsg, unifyCtxt )
57 %************************************************************************
59 matchExpected functions
61 %************************************************************************
63 Note [Herald for matchExpectedFunTys]
64 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
65 The 'herald' always looks like:
66 "The equation(s) for 'f' have"
67 "The abstraction (\x.e) takes"
68 "The section (+ x) expects"
69 "The function 'f' is applied to"
71 This is used to construct a message of form
73 The abstraction `\Just 1 -> ...' takes two arguments
74 but its type `Maybe a -> a' has only one
76 The equation(s) for `f' have two arguments
77 but its type `Maybe a -> a' has only one
79 The section `(f 3)' requires 'f' to take two arguments
80 but its type `Int -> Int' has only one
82 The function 'f' is applied to two arguments
83 but its type `Int -> Int' has only one
85 Note [matchExpectedFunTys]
86 ~~~~~~~~~~~~~~~~~~~~~~~~~~
87 matchExpectedFunTys checks that an (Expected rho) has the form
88 of an n-ary function. It passes the decomposed type to the
89 thing_inside, and returns a wrapper to coerce between the two types
91 It's used wherever a language construct must have a functional type,
97 This is not (currently) where deep skolemisation occurs;
98 matchExpectedFunTys does not skolmise nested foralls in the
99 expected type, becuase it expects that to have been done already
103 matchExpectedFunTys :: SDoc -- See Note [Herald for matchExpectedFunTys]
106 -> TcM (CoercionI, [TcSigmaType], TcRhoType)
108 -- If matchExpectFunTys n ty = (co, [t1,..,tn], ty_r)
109 -- then co : ty ~ (t1 -> ... -> tn -> ty_r)
111 -- Does not allocate unnecessary meta variables: if the input already is
112 -- a function, we just take it apart. Not only is this efficient,
113 -- it's important for higher rank: the argument might be of form
114 -- (forall a. ty) -> other
115 -- If allocated (fresh-meta-var1 -> fresh-meta-var2) and unified, we'd
116 -- hide the forall inside a meta-variable
118 matchExpectedFunTys herald arity orig_ty
121 -- If go n ty = (co, [t1,..,tn], ty_r)
122 -- then co : ty ~ t1 -> .. -> tn -> ty_r
125 | n_req == 0 = return (IdCo ty, [], ty)
128 | Just ty' <- tcView ty = go n_req ty'
130 go n_req (FunTy arg_ty res_ty)
131 | not (isPredTy arg_ty)
132 = do { (coi, tys, ty_r) <- go (n_req-1) res_ty
133 ; return (mkFunTyCoI (IdCo arg_ty) coi, arg_ty:tys, ty_r) }
135 go _ (TyConApp tc _) -- A common case
136 | not (isSynFamilyTyCon tc)
137 = do { (env,msg) <- mk_ctxt emptyTidyEnv
138 ; failWithTcM (env,msg) }
140 go n_req ty@(TyVarTy tv)
141 | ASSERT( isTcTyVar tv) isMetaTyVar tv
142 = do { cts <- readMetaTyVar tv
144 Indirect ty' -> go n_req ty'
145 Flexi -> defer n_req ty }
147 -- In all other cases we bale out into ordinary unification
148 go n_req ty = defer n_req ty
152 = addErrCtxtM mk_ctxt $
153 do { arg_tys <- newFlexiTyVarTys n_req argTypeKind
154 ; res_ty <- newFlexiTyVarTy openTypeKind
155 ; coi <- unifyType fun_ty (mkFunTys arg_tys res_ty)
156 ; return (coi, arg_tys, res_ty) }
159 mk_ctxt :: TidyEnv -> TcM (TidyEnv, Message)
160 mk_ctxt env = do { orig_ty1 <- zonkTcType orig_ty
161 ; let (env', orig_ty2) = tidyOpenType env orig_ty1
162 (args, _) = tcSplitFunTys orig_ty2
163 n_actual = length args
164 ; return (env', mk_msg orig_ty2 n_actual) }
167 = herald <+> speakNOf arity (ptext (sLit "argument")) <> comma $$
168 sep [ptext (sLit "but its type") <+> quotes (pprType ty),
169 if n_args == 0 then ptext (sLit "has none")
170 else ptext (sLit "has only") <+> speakN n_args]
175 ----------------------
176 matchExpectedListTy :: TcRhoType -> TcM (CoercionI, TcRhoType)
177 -- Special case for lists
178 matchExpectedListTy exp_ty
179 = do { (coi, [elt_ty]) <- matchExpectedTyConApp listTyCon exp_ty
180 ; return (coi, elt_ty) }
182 ----------------------
183 matchExpectedPArrTy :: TcRhoType -> TcM (CoercionI, TcRhoType)
184 -- Special case for parrs
185 matchExpectedPArrTy exp_ty
186 = do { (coi, [elt_ty]) <- matchExpectedTyConApp parrTyCon exp_ty
187 ; return (coi, elt_ty) }
189 ----------------------
190 matchExpectedTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
191 -> TcRhoType -- orig_ty
192 -> TcM (CoercionI, -- T a b c ~ orig_ty
193 [TcSigmaType]) -- Element types, a b c
195 -- It's used for wired-in tycons, so we call checkWiredInTyCon
196 -- Precondition: never called with FunTyCon
197 -- Precondition: input type :: *
199 matchExpectedTyConApp tc orig_ty
200 = do { checkWiredInTyCon tc
201 ; go (tyConArity tc) orig_ty [] }
203 go :: Int -> TcRhoType -> [TcSigmaType] -> TcM (CoercionI, [TcSigmaType])
204 -- If go n ty tys = (co, [t1..tn] ++ tys)
205 -- then co : T t1..tn ~ ty
208 | Just ty' <- tcView ty = go n_req ty' tys
210 go n_req ty@(TyVarTy tv) tys
211 | ASSERT( isTcTyVar tv) isMetaTyVar tv
212 = do { cts <- readMetaTyVar tv
214 Indirect ty -> go n_req ty tys
215 Flexi -> defer n_req ty tys }
217 go n_req ty@(TyConApp tycon args) tys
219 = ASSERT( n_req == length args) -- ty::*
220 return (IdCo ty, args ++ tys)
222 go n_req (AppTy fun arg) tys
224 = do { (coi, args) <- go (n_req - 1) fun (arg : tys)
225 ; return (mkAppTyCoI coi (IdCo arg), args) }
227 go n_req ty tys = defer n_req ty tys
231 = do { tau_tys <- mapM newFlexiTyVarTy arg_kinds
232 ; coi <- unifyType (mkTyConApp tc tau_tys) ty
233 ; return (coi, tau_tys ++ tys) }
235 (arg_kinds, _) = splitKindFunTysN n_req (tyConKind tc)
237 ----------------------
238 matchExpectedAppTy :: TcRhoType -- orig_ty
239 -> TcM (CoercionI, -- m a ~ orig_ty
240 (TcSigmaType, TcSigmaType)) -- Returns m, a
241 -- If the incoming type is a mutable type variable of kind k, then
242 -- matchExpectedAppTy returns a new type variable (m: * -> k); note the *.
244 matchExpectedAppTy orig_ty
248 | Just ty' <- tcView ty = go ty'
250 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
251 = return (IdCo orig_ty, (fun_ty, arg_ty))
254 | ASSERT( isTcTyVar tv) isMetaTyVar tv
255 = do { cts <- readMetaTyVar tv
262 -- Defer splitting by generating an equality constraint
263 defer = do { ty1 <- newFlexiTyVarTy kind1
264 ; ty2 <- newFlexiTyVarTy kind2
265 ; coi <- unifyType (mkAppTy ty1 ty2) orig_ty
266 ; return (coi, (ty1, ty2)) }
268 orig_kind = typeKind orig_ty
269 kind1 = mkArrowKind liftedTypeKind (defaultKind orig_kind)
270 kind2 = liftedTypeKind -- m :: * -> k
272 -- The defaultKind is a bit smelly. If you remove it,
273 -- try compiling f x = do { x }
274 -- and you'll get a kind mis-match. It smells, but
275 -- not enough to lose sleep over.
279 %************************************************************************
283 %************************************************************************
285 All the tcSub calls have the form
287 tcSub actual_ty expected_ty
289 actual_ty <= expected_ty
291 That is, that a value of type actual_ty is acceptable in
292 a place expecting a value of type expected_ty.
294 It returns a coercion function
295 co_fn :: actual_ty ~ expected_ty
296 which takes an HsExpr of type actual_ty into one of type
300 tcSubType :: CtOrigin -> SkolemInfo -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
301 -- Check that ty_actual is more polymorphic than ty_expected
302 -- Both arguments might be polytypes, so we must instantiate and skolemise
303 -- Returns a wrapper of shape ty_actual ~ ty_expected
304 tcSubType origin skol_info ty_actual ty_expected
305 | isSigmaTy ty_actual
306 = do { let extra_tvs = tyVarsOfType ty_actual
307 ; (sk_wrap, inst_wrap)
308 <- tcGen skol_info extra_tvs ty_expected $ \ _ sk_rho -> do
309 { (in_wrap, in_rho) <- deeplyInstantiate origin ty_actual
310 ; coi <- unifyType in_rho sk_rho
311 ; return (coiToHsWrapper coi <.> in_wrap) }
312 ; return (sk_wrap <.> inst_wrap) }
314 | otherwise -- Urgh! It seems deeply weird to have equality
315 -- when actual is not a polytype, and it makes a big
316 -- difference e.g. tcfail104
317 = do { coi <- unifyType ty_actual ty_expected
318 ; return (coiToHsWrapper coi) }
320 tcInfer :: (TcType -> TcM a) -> TcM (a, TcType)
321 tcInfer tc_infer = do { ty <- newFlexiTyVarTy openTypeKind
326 tcWrapResult :: HsExpr TcId -> TcRhoType -> TcRhoType -> TcM (HsExpr TcId)
327 tcWrapResult expr actual_ty res_ty
328 = do { coi <- unifyType actual_ty res_ty
329 -- Both types are deeply skolemised
330 ; return (mkHsWrapCoI coi expr) }
332 -----------------------------------
334 :: [TcType] -- Type of args
335 -> HsWrapper -- HsExpr a -> HsExpr b
336 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
337 wrapFunResCoercion arg_tys co_fn_res
338 | isIdHsWrapper co_fn_res
343 = do { arg_ids <- newSysLocalIds (fsLit "sub") arg_tys
344 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpEvVarApps arg_ids) }
349 %************************************************************************
351 \subsection{Generalisation}
353 %************************************************************************
356 tcGen :: SkolemInfo -> TcTyVarSet -> TcType
357 -> ([TcTyVar] -> TcRhoType -> TcM result)
358 -> TcM (HsWrapper, result)
359 -- The expression has type: spec_ty -> expected_ty
361 tcGen skol_info extra_tvs
362 expected_ty thing_inside -- We expect expected_ty to be a forall-type
363 -- If not, the call is a no-op
364 = do { traceTc "tcGen" empty
365 ; (wrap, tvs', given, rho') <- deeplySkolemise skol_info expected_ty
368 traceTc "tcGen" $ vcat [
369 text "expected_ty" <+> ppr expected_ty,
370 text "inst ty" <+> ppr tvs' <+> ppr rho' ]
372 -- In 'free_tvs' we must check that the "forall_tvs" havn't been constrained
373 -- The interesting bit here is that we must include the free variables
374 -- of the expected_ty. Here's an example:
375 -- runST (newVar True)
376 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
377 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
378 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
379 -- So now s' isn't unconstrained because it's linked to a.
380 -- Conclusion: pass the free vars of the expected_ty to checkConsraints
381 ; let free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
383 ; (ev_binds, result) <- checkConstraints skol_info free_tvs tvs' given $
384 thing_inside tvs' rho'
386 ; return (wrap <.> mkWpLet ev_binds, result) }
387 -- The ev_binds returned by checkConstraints is very
388 -- often empty, in which case mkWpLet is a no-op
390 checkConstraints :: SkolemInfo
391 -> TcTyVarSet -- Free variables (other than the type envt)
392 -- for the skolem escape check
393 -> [TcTyVar] -- Skolems
396 -> TcM (TcEvBinds, result)
398 checkConstraints skol_info free_tvs skol_tvs given thing_inside
399 | null skol_tvs && null given
400 = do { res <- thing_inside; return (emptyTcEvBinds, res) }
401 -- Just for efficiency. We check every function argument with
402 -- tcPolyExpr, which uses tcGen and hence checkConstraints.
405 = do { (ev_binds, wanted, result) <- newImplication skol_info free_tvs
406 skol_tvs given thing_inside
407 ; emitConstraints wanted
408 ; return (ev_binds, result) }
410 newImplication :: SkolemInfo -> TcTyVarSet -> [TcTyVar]
411 -> [EvVar] -> TcM result
412 -> TcM (TcEvBinds, WantedConstraints, result)
413 newImplication skol_info free_tvs skol_tvs given thing_inside
414 = ASSERT2( all isTcTyVar skol_tvs, ppr skol_tvs )
415 ASSERT2( all isSkolemTyVar skol_tvs, ppr skol_tvs )
416 do { gbl_tvs <- tcGetGlobalTyVars
417 ; lcl_env <- getLclTypeEnv
418 ; let all_free_tvs = gbl_tvs `unionVarSet` free_tvs
420 ; (result, wanted) <- getConstraints $
421 setUntouchables all_free_tvs $
424 ; if isEmptyBag wanted && not (hasEqualities given)
425 -- Optimisation : if there are no wanteds, and the givens
426 -- are sufficiently simple, don't generate an implication
427 -- at all. Reason for the hasEqualities test:
428 -- we don't want to lose the "inaccessible alternative"
431 return (emptyTcEvBinds, emptyWanteds, result)
433 { ev_binds_var <- newTcEvBinds
434 ; loc <- getCtLoc skol_info
435 ; let implic = Implic { ic_env_tvs = all_free_tvs
437 , ic_skols = mkVarSet skol_tvs
438 , ic_scoped = panic "emitImplication"
441 , ic_binds = ev_binds_var
444 ; return (TcEvBinds ev_binds_var, unitBag (WcImplic implic), result) } }
448 %************************************************************************
452 %************************************************************************
454 The exported functions are all defined as versions of some
455 non-exported generic functions.
459 unifyType :: TcTauType -> TcTauType -> TcM CoercionI
460 -- Actual and expected types
461 -- Returns a coercion : ty1 ~ ty2
462 unifyType ty1 ty2 = uType [] ty1 ty2
465 unifyPred :: PredType -> PredType -> TcM CoercionI
466 -- Actual and expected types
467 unifyPred p1 p2 = uPred [UnifyOrigin (mkPredTy p1) (mkPredTy p2)] p1 p2
470 unifyTheta :: TcThetaType -> TcThetaType -> TcM [CoercionI]
471 -- Actual and expected types
472 unifyTheta theta1 theta2
473 = do { checkTc (equalLength theta1 theta2)
474 (vcat [ptext (sLit "Contexts differ in length"),
475 nest 2 $ parens $ ptext (sLit "Use -XRelaxedPolyRec to allow this")])
476 ; zipWithM unifyPred theta1 theta2 }
479 @unifyTypeList@ takes a single list of @TauType@s and unifies them
480 all together. It is used, for example, when typechecking explicit
481 lists, when all the elts should be of the same type.
484 unifyTypeList :: [TcTauType] -> TcM ()
485 unifyTypeList [] = return ()
486 unifyTypeList [_] = return ()
487 unifyTypeList (ty1:tys@(ty2:_)) = do { _ <- unifyType ty1 ty2
488 ; unifyTypeList tys }
491 %************************************************************************
495 %************************************************************************
497 uType is the heart of the unifier. Each arg occurs twice, because
498 we want to report errors in terms of synomyms if possible. The first of
499 the pair is used in error messages only; it is always the same as the
500 second, except that if the first is a synonym then the second may be a
501 de-synonym'd version. This way we get better error messages.
505 = NotSwapped -- Args are: actual, expected
506 | IsSwapped -- Args are: expected, actual
508 instance Outputable SwapFlag where
509 ppr IsSwapped = ptext (sLit "Is-swapped")
510 ppr NotSwapped = ptext (sLit "Not-swapped")
512 unSwap :: SwapFlag -> (a->a->b) -> a -> a -> b
513 unSwap NotSwapped f a b = f a b
514 unSwap IsSwapped f a b = f b a
517 uType, uType_np, uType_defer
519 -> TcType -- ty1 is the *actual* type
520 -> TcType -- ty2 is the *expected* type
524 -- It is always safe to defer unification to the main constraint solver
525 -- See Note [Deferred unification]
526 uType_defer (item : origin) ty1 ty2
527 = do { co_var <- newWantedCoVar ty1 ty2
528 ; traceTc "utype_defer" (vcat [ppr co_var, ppr ty1, ppr ty2, ppr origin])
529 ; loc <- getCtLoc (TypeEqOrigin item)
530 ; wrapEqCtxt origin $
531 emitConstraint (WcEvVar (WantedEvVar co_var loc))
532 ; return $ ACo $ mkTyVarTy co_var }
534 = panic "uType_defer"
537 -- Push a new item on the origin stack (the most common case)
538 uType origin ty1 ty2 -- Push a new item on the origin stack
539 = uType_np (pushOrigin ty1 ty2 origin) ty1 ty2
542 -- unify_np (short for "no push" on the origin stack) does the work
543 uType_np origin orig_ty1 orig_ty2
544 = do { traceTc "u_tys " $ vcat
545 [ sep [ ppr orig_ty1, text "~", ppr orig_ty2]
547 ; coi <- go origin orig_ty1 orig_ty2
549 ACo co -> traceTc "u_tys yields coercion:" (ppr co)
550 IdCo _ -> traceTc "u_tys yields no coercion" empty
553 bale_out :: [EqOrigin] -> TcM a
554 bale_out origin = failWithMisMatch origin
556 go :: [EqOrigin] -> TcType -> TcType -> TcM CoercionI
557 -- The arguments to 'go' are always semantically identical
558 -- to orig_ty{1,2} except for looking through type synonyms
560 -- Variables; go for uVar
561 -- Note that we pass in *original* (before synonym expansion),
562 -- so that type variables tend to get filled in with
563 -- the most informative version of the type
564 go origin (TyVarTy tyvar1) ty2 = uVar origin NotSwapped tyvar1 ty2
565 go origin ty1 (TyVarTy tyvar2) = uVar origin IsSwapped tyvar2 ty1
568 -- see Note [Unification and synonyms]
569 -- Do this after the variable case so that we tend to unify
570 -- variables with un-expended type synonym
572 | Just ty1' <- tcView ty1 = uType origin ty1' ty2
573 | Just ty2' <- tcView ty2 = uType origin ty1 ty2'
576 go origin (PredTy p1) (PredTy p2) = uPred origin p1 p2
578 -- Functions; just check the two parts
579 go origin (FunTy fun1 arg1) (FunTy fun2 arg2)
580 = do { coi_l <- uType origin fun1 fun2
581 ; coi_r <- uType origin arg1 arg2
582 ; return $ mkFunTyCoI coi_l coi_r }
584 -- Always defer if a type synonym family (type function)
585 -- is involved. (Data families behave rigidly.)
586 go origin ty1@(TyConApp tc1 _) ty2
587 | isSynFamilyTyCon tc1 = uType_defer origin ty1 ty2
588 go origin ty1 ty2@(TyConApp tc2 _)
589 | isSynFamilyTyCon tc2 = uType_defer origin ty1 ty2
591 go origin (TyConApp tc1 tys1) (TyConApp tc2 tys2)
592 | tc1 == tc2 -- See Note [TyCon app]
593 = do { cois <- uList origin uType tys1 tys2
594 ; return $ mkTyConAppCoI tc1 cois }
596 -- See Note [Care with type applications]
597 go origin (AppTy s1 t1) ty2
598 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
599 = do { coi_s <- uType_np origin s1 s2 -- See Note [Unifying AppTy]
600 ; coi_t <- uType origin t1 t2
601 ; return $ mkAppTyCoI coi_s coi_t }
603 go origin ty1 (AppTy s2 t2)
604 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
605 = do { coi_s <- uType_np origin s1 s2
606 ; coi_t <- uType origin t1 t2
607 ; return $ mkAppTyCoI coi_s coi_t }
610 | isSigmaTy ty1 || isSigmaTy ty2
611 = unifySigmaTy origin ty1 ty2
613 -- Anything else fails
614 go origin _ _ = bale_out origin
616 unifySigmaTy :: [EqOrigin] -> TcType -> TcType -> TcM CoercionI
617 unifySigmaTy origin ty1 ty2
618 = do { let (tvs1, body1) = tcSplitForAllTys ty1
619 (tvs2, body2) = tcSplitForAllTys ty2
620 ; unless (equalLength tvs1 tvs2) (failWithMisMatch origin)
621 ; skol_tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
622 -- Get location from monad, not from tvs1
623 ; let tys = mkTyVarTys skol_tvs
624 in_scope = mkInScopeSet (mkVarSet skol_tvs)
625 phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
626 phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
627 untch = tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2
629 ; (coi, lie) <- getConstraints $
630 setUntouchables untch $
631 uType origin phi1 phi2
633 -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
634 ; let bad_lie = filterBag is_bad lie
635 is_bad w = any (`elemVarSet` tyVarsOfWanted w) skol_tvs
636 ; when (not (isEmptyBag bad_lie))
637 (failWithMisMatch origin) -- ToDo: give details from bad_lie
639 ; emitConstraints lie
640 ; return (foldr mkForAllTyCoI coi skol_tvs) }
643 uPred :: [EqOrigin] -> PredType -> PredType -> TcM CoercionI
644 uPred origin (IParam n1 t1) (IParam n2 t2)
646 = do { coi <- uType origin t1 t2
647 ; return $ mkIParamPredCoI n1 coi }
648 uPred origin (ClassP c1 tys1) (ClassP c2 tys2)
650 = do { cois <- uList origin uType tys1 tys2
651 -- Guaranteed equal lengths because the kinds check
652 ; return $ mkClassPPredCoI c1 cois }
653 uPred origin (EqPred ty1a ty1b) (EqPred ty2a ty2b)
654 = do { coia <- uType origin ty1a ty2a
655 ; coib <- uType origin ty1b ty2b
656 ; return $ mkEqPredCoI coia coib }
658 uPred origin _ _ = failWithMisMatch origin
662 -> ([EqOrigin] -> a -> a -> TcM b)
663 -> [a] -> [a] -> TcM [b]
664 -- Unify corresponding elements of two lists of types, which
665 -- should be of equal length. We charge down the list explicitly so that
666 -- we can complain if their lengths differ.
667 uList _ _ [] [] = return []
668 uList origin unify (ty1:tys1) (ty2:tys2) = do { x <- unify origin ty1 ty2;
669 ; xs <- uList origin unify tys1 tys2
671 uList origin _ _ _ = failWithMisMatch origin
672 -- See Note [Mismatched type lists and application decomposition]
676 Note [Care with type applications]
677 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
678 Note: type applications need a bit of care!
679 They can match FunTy and TyConApp, so use splitAppTy_maybe
680 NB: we've already dealt with type variables and Notes,
681 so if one type is an App the other one jolly well better be too
683 Note [Unifying AppTy]
684 ~~~~~~~~~~~~~~~~~~~~~
685 Considerm unifying (m Int) ~ (IO Int) where m is a unification variable
686 that is now bound to (say) (Bool ->). Then we want to report
687 "Can't unify (Bool -> Int) with (IO Int)
689 "Can't unify ((->) Bool) with IO"
690 That is why we use the "_np" variant of uType, which does not alter the error
695 When we find two TyConApps, the argument lists are guaranteed equal
696 length. Reason: intially the kinds of the two types to be unified is
697 the same. The only way it can become not the same is when unifying two
698 AppTys (f1 a1)~(f2 a2). In that case there can't be a TyConApp in
699 the f1,f2 (because it'd absorb the app). If we unify f1~f2 first,
700 which we do, that ensures that f1,f2 have the same kind; and that
701 means a1,a2 have the same kind. And now the argument repeats.
703 Note [Mismatched type lists and application decomposition]
704 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
705 When we find two TyConApps, you might think that the argument lists
706 are guaranteed equal length. But they aren't. Consider matching
707 w (T x) ~ Foo (T x y)
708 We do match (w ~ Foo) first, but in some circumstances we simply create
709 a deferred constraint; and then go ahead and match (T x ~ T x y).
710 This came up in Trac #3950.
713 (a) either we must check for identical argument kinds
714 when decomposing applications,
716 (b) or we must be prepared for ill-kinded unification sub-problems
718 Currently we adopt (b) since it seems more robust -- no need to maintain
721 Note [Unification and synonyms]
722 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
723 If you are tempted to make a short cut on synonyms, as in this
726 uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
727 = if (con1 == con2) then
728 -- Good news! Same synonym constructors, so we can shortcut
729 -- by unifying their arguments and ignoring their expansions.
730 unifyTypepeLists args1 args2
732 -- Never mind. Just expand them and try again
735 then THINK AGAIN. Here is the whole story, as detected and reported
738 Here's a test program that should detect the problem:
741 x = (1 :: Bogus Char) :: Bogus Bool
743 The problem with [the attempted shortcut code] is that
747 is not a sufficient condition to be able to use the shortcut!
748 You also need to know that the type synonym actually USES all
749 its arguments. For example, consider the following type synonym
750 which does not use all its arguments.
754 If you ever tried unifying, say, (Bogus Char) with )Bogus Bool), the
755 unifier would blithely try to unify Char with Bool and would fail,
756 even though the expanded forms (both Int) should match. Similarly,
757 unifying (Bogus Char) with (Bogus t) would unnecessarily bind t to
760 ... You could explicitly test for the problem synonyms and mark them
761 somehow as needing expansion, perhaps also issuing a warning to the
764 Note [Deferred Unification]
765 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
766 We may encounter a unification ty1 ~ ty2 that cannot be performed syntactically,
767 and yet its consistency is undetermined. Previously, there was no way to still
768 make it consistent. So a mismatch error was issued.
770 Now these unfications are deferred until constraint simplification, where type
771 family instances and given equations may (or may not) establish the consistency.
772 Deferred unifications are of the form
775 where F is a type function and x is a type variable.
777 id :: x ~ y => x -> y
780 involves the unfication x = y. It is deferred until we bring into account the
781 context x ~ y to establish that it holds.
783 If available, we defer original types (rather than those where closed type
784 synonyms have already been expanded via tcCoreView). This is, as usual, to
785 improve error messages.
788 %************************************************************************
792 %************************************************************************
794 @uVar@ is called when at least one of the types being unified is a
795 variable. It does {\em not} assume that the variable is a fixed point
796 of the substitution; rather, notice that @uVar@ (defined below) nips
797 back into @uTys@ if it turns out that the variable is already bound.
800 uVar :: [EqOrigin] -> SwapFlag -> TcTyVar -> TcTauType -> TcM CoercionI
801 uVar origin swapped tv1 ty2
802 = do { traceTc "uVar" (vcat [ ppr origin
804 , ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1)
805 , nest 2 (ptext (sLit " ~ "))
806 , ppr ty2 <+> dcolon <+> ppr (typeKind ty2)])
807 ; details <- lookupTcTyVar tv1
809 Filled ty1 -> unSwap swapped (uType_np origin) ty1 ty2
810 Unfilled details1 -> uUnfilledVar origin swapped tv1 details1 ty2
814 uUnfilledVar :: [EqOrigin]
816 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
817 -> TcTauType -- Type 2
819 -- "Unfilled" means that the variable is definitely not a filled-in meta tyvar
820 -- It might be a skolem, or untouchable, or meta
822 uUnfilledVar origin swapped tv1 details1 (TyVarTy tv2)
823 | tv1 == tv2 -- Same type variable => no-op
824 = return (IdCo (mkTyVarTy tv1))
826 | otherwise -- Distinct type variables
827 = do { lookup2 <- lookupTcTyVar tv2
829 Filled ty2' -> uUnfilledVar origin swapped tv1 details1 ty2'
830 Unfilled details2 -> uUnfilledVars origin swapped tv1 details1 tv2 details2
833 uUnfilledVar origin swapped tv1 details1 non_var_ty2 -- ty2 is not a type variable
836 -> do { mb_ty2' <- checkTauTvUpdate tv1 non_var_ty2
838 Nothing -> do { traceTc "Occ/kind defer" (ppr tv1); defer }
839 Just ty2' -> updateMeta tv1 ref1 ty2'
842 _other -> do { traceTc "Skolem defer" (ppr tv1); defer } -- Skolems of all sorts
844 defer = unSwap swapped (uType_defer origin) (mkTyVarTy tv1) non_var_ty2
845 -- Occurs check or an untouchable: just defer
846 -- NB: occurs check isn't necessarily fatal:
847 -- eg tv1 occured in type family parameter
850 uUnfilledVars :: [EqOrigin]
852 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
853 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
855 -- Invarant: The type variables are distinct,
856 -- Neither is filled in yet
858 uUnfilledVars origin swapped tv1 details1 tv2 details2
859 = case (details1, details2) of
860 (MetaTv i1 ref1, MetaTv i2 ref2)
861 | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1 i1 i2
862 then updateMeta tv1 ref1 ty2
863 else updateMeta tv2 ref2 ty1
864 | k2_sub_k1 -> updateMeta tv1 ref1 ty2
866 (_, MetaTv _ ref2) | k1_sub_k2 -> updateMeta tv2 ref2 ty1
867 (MetaTv _ ref1, _) | k2_sub_k1 -> updateMeta tv1 ref1 ty2
869 (_, _) -> unSwap swapped (uType_defer origin) ty1 ty2
870 -- Defer for skolems of all sorts
874 k1_sub_k2 = k1 `isSubKind` k2
875 k2_sub_k1 = k2 `isSubKind` k1
879 nicer_to_update_tv1 _ (SigTv _) = True
880 nicer_to_update_tv1 (SigTv _) _ = False
881 nicer_to_update_tv1 _ _ = isSystemName (Var.varName tv1)
882 -- Try not to update SigTvs; and try to update sys-y type
883 -- variables in preference to ones gotten (say) by
884 -- instantiating a polymorphic function with a user-written
888 checkTauTvUpdate :: TcTyVar -> TcType -> TcM (Maybe TcType)
889 -- (checkTauTvUpdate tv ty)
890 -- We are about to update the TauTv tv with ty.
891 -- Check (a) that tv doesn't occur in ty (occurs check)
892 -- (b) that ty is a monotype
893 -- (c) that kind(ty) is a sub-kind of kind(tv)
895 -- We have two possible outcomes:
896 -- (1) Return the type to update the type variable with,
897 -- [we know the update is ok]
898 -- (2) Return Nothing,
899 -- [the update might be dodgy]
901 -- Note that "Nothing" does not mean "definite error". For example
903 -- type instance F Int = Int
906 -- This is perfectly reasonable, if we later get a ~ Int. For now, though,
907 -- we return Nothing, leaving it to the later constraint simplifier to
910 checkTauTvUpdate tv ty
911 = do { ty' <- zonkTcType ty
912 ; if not (tv `elemVarSet` tyVarsOfType ty')
913 && typeKind ty' `isSubKind` tyVarKind tv
914 then return (Just ty')
915 else return Nothing }
920 data LookupTyVarResult -- The result of a lookupTcTyVar call
921 = Unfilled TcTyVarDetails -- SkolemTv or virgin MetaTv
924 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
926 | MetaTv _ ref <- details
927 = do { meta_details <- readMutVar ref
928 ; case meta_details of
929 Indirect ty -> return (Filled ty)
930 Flexi -> do { is_untch <- isUntouchable tyvar
931 ; let -- Note [Unifying untouchables]
932 ret_details | is_untch = SkolemTv UnkSkol
933 | otherwise = details
934 ; return (Unfilled ret_details) } }
936 = return (Unfilled details)
938 details = ASSERT2( isTcTyVar tyvar, ppr tyvar )
941 updateMeta :: TcTyVar -> TcRef MetaDetails -> TcType -> TcM CoercionI
942 updateMeta tv1 ref1 ty2
943 = do { writeMetaTyVarRef tv1 ref1 ty2
944 ; return (IdCo ty2) }
947 Note [Unifying untouchables]
948 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
949 We treat an untouchable type variable as if it was a skolem. That
950 ensures it won't unify with anything. It's a slight had, because
951 we return a made-up TcTyVarDetails, but I think it works smoothly.
954 %************************************************************************
958 %************************************************************************
961 pushOrigin :: TcType -> TcType -> [EqOrigin] -> [EqOrigin]
962 pushOrigin ty_act ty_exp origin
963 = UnifyOrigin { uo_actual = ty_act, uo_expected = ty_exp } : origin
966 wrapEqCtxt :: [EqOrigin] -> TcM a -> TcM a
967 -- Build a suitable error context from the origin and do the thing inside
968 -- The "couldn't match" error comes from the innermost item on the stack,
969 -- and, if there is more than one item, the "Expected/inferred" part
970 -- comes from the outermost item
971 wrapEqCtxt [] thing_inside = thing_inside
972 wrapEqCtxt items thing_inside = addErrCtxtM (unifyCtxt (last items)) thing_inside
975 failWithMisMatch :: [EqOrigin] -> TcM a
976 -- Generate the message when two types fail to match,
977 -- going to some trouble to make it helpful.
978 -- We take the failing types from the top of the origin stack
979 -- rather than reporting the particular ones we are looking
981 failWithMisMatch (item:origin)
982 = wrapEqCtxt origin $
983 do { ty_act <- zonkTcType (uo_actual item)
984 ; ty_exp <- zonkTcType (uo_expected item)
985 ; env0 <- tcInitTidyEnv
986 ; let (env1, pp_exp) = tidyOpenType env0 ty_exp
987 (env2, pp_act) = tidyOpenType env1 ty_act
988 ; failWithTcM (misMatchMsg env2 pp_act pp_exp) }
990 = panic "failWithMisMatch"
992 misMatchMsg :: TidyEnv -> TcType -> TcType -> (TidyEnv, SDoc)
993 misMatchMsg env ty_act ty_exp
994 = (env2, sep [sep [ ptext (sLit "Couldn't match expected type") <+> quotes (ppr ty_exp)
995 , nest 12 $ ptext (sLit "with actual type") <+> quotes (ppr ty_act)]
996 , nest 2 (extra1 $$ extra2) ])
998 (env1, extra1) = typeExtraInfoMsg env ty_exp
999 (env2, extra2) = typeExtraInfoMsg env1 ty_act
1003 -----------------------------------------
1005 -----------------------------------------
1009 -- If an error happens we try to figure out whether the function
1010 -- function has been given too many or too few arguments, and say so.
1011 addSubCtxt :: InstOrigin -> TcType -> TcType -> TcM a -> TcM a
1012 addSubCtxt orig actual_res_ty expected_res_ty thing_inside
1013 = addErrCtxtM mk_err thing_inside
1016 = do { exp_ty' <- zonkTcType expected_res_ty
1017 ; act_ty' <- zonkTcType actual_res_ty
1018 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1019 (env2, act_ty'') = tidyOpenType env1 act_ty'
1020 (exp_args, _) = tcSplitFunTys exp_ty''
1021 (act_args, _) = tcSplitFunTys act_ty''
1023 len_act_args = length act_args
1024 len_exp_args = length exp_args
1026 message = case orig of
1028 | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1029 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1030 _ -> mkExpectedActualMsg act_ty'' exp_ty''
1031 ; return (env2, message) }
1034 %************************************************************************
1038 %************************************************************************
1040 Unifying kinds is much, much simpler than unifying types.
1043 matchExpectedFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1044 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1046 matchExpectedFunKind (TyVarTy kvar) = do
1047 maybe_kind <- readKindVar kvar
1049 Indirect fun_kind -> matchExpectedFunKind fun_kind
1051 do { arg_kind <- newKindVar
1052 ; res_kind <- newKindVar
1053 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1054 ; return (Just (arg_kind,res_kind)) }
1056 matchExpectedFunKind (FunTy arg_kind res_kind) = return (Just (arg_kind,res_kind))
1057 matchExpectedFunKind _ = return Nothing
1060 unifyKind :: TcKind -- Expected
1064 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1065 | isSubKindCon kc2 kc1 = return ()
1067 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1068 = do { unifyKind a2 a1; unifyKind r1 r2 }
1069 -- Notice the flip in the argument,
1070 -- so that the sub-kinding works right
1071 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1072 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1073 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1076 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1077 uKVar swapped kv1 k2
1078 = do { mb_k1 <- readKindVar kv1
1080 Flexi -> uUnboundKVar swapped kv1 k2
1081 Indirect k1 | swapped -> unifyKind k2 k1
1082 | otherwise -> unifyKind k1 k2 }
1085 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1086 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1087 | kv1 == kv2 = return ()
1088 | otherwise -- Distinct kind variables
1089 = do { mb_k2 <- readKindVar kv2
1091 Indirect k2 -> uUnboundKVar swapped kv1 k2
1092 Flexi -> writeKindVar kv1 k2 }
1094 uUnboundKVar swapped kv1 non_var_k2
1095 = do { k2' <- zonkTcKind non_var_k2
1096 ; kindOccurCheck kv1 k2'
1097 ; k2'' <- kindSimpleKind swapped k2'
1098 -- KindVars must be bound only to simple kinds
1099 -- Polarities: (kindSimpleKind True ?) succeeds
1100 -- returning *, corresponding to unifying
1103 ; writeKindVar kv1 k2'' }
1106 kindOccurCheck :: TyVar -> Type -> TcM ()
1107 kindOccurCheck kv1 k2 -- k2 is zonked
1108 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1110 not_in (TyVarTy kv2) = kv1 /= kv2
1111 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1114 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1115 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1116 -- If the flag is False, it requires k <: sk
1117 -- E.g. kindSimpleKind False ?? = *
1118 -- What about (kv -> *) ~ ?? -> *
1119 kindSimpleKind orig_swapped orig_kind
1120 = go orig_swapped orig_kind
1122 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1124 ; return (mkArrowKind k1' k2') }
1126 | isOpenTypeKind k = return liftedTypeKind
1127 | isArgTypeKind k = return liftedTypeKind
1129 | isLiftedTypeKind k = return liftedTypeKind
1130 | isUnliftedTypeKind k = return unliftedTypeKind
1131 go _ k@(TyVarTy _) = return k -- KindVars are always simple
1132 go _ _ = failWithTc (ptext (sLit "Unexpected kind unification failure:")
1133 <+> ppr orig_swapped <+> ppr orig_kind)
1134 -- I think this can't actually happen
1136 -- T v = MkT v v must be a type
1137 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1139 unifyKindMisMatch :: TcKind -> TcKind -> TcM ()
1140 unifyKindMisMatch ty1 ty2 = do
1141 ty1' <- zonkTcKind ty1
1142 ty2' <- zonkTcKind ty2
1144 msg = hang (ptext (sLit "Couldn't match kind"))
1145 2 (sep [quotes (ppr ty1'),
1146 ptext (sLit "against"),
1151 kindOccurCheckErr :: Var -> Type -> SDoc
1152 kindOccurCheckErr tyvar ty
1153 = hang (ptext (sLit "Occurs check: cannot construct the infinite kind:"))
1154 2 (sep [ppr tyvar, char '=', ppr ty])
1157 %************************************************************************
1159 \subsection{Checking signature type variables}
1161 %************************************************************************
1163 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1164 are not mentioned in the environment. In particular:
1166 (a) Not mentioned in the type of a variable in the envt
1167 eg the signature for f in this:
1173 Here, f is forced to be monorphic by the free occurence of x.
1175 (d) Not (unified with another type variable that is) in scope.
1176 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1177 when checking the expression type signature, we find that
1178 even though there is nothing in scope whose type mentions r,
1179 nevertheless the type signature for the expression isn't right.
1181 Another example is in a class or instance declaration:
1183 op :: forall b. a -> b
1185 Here, b gets unified with a
1187 Before doing this, the substitution is applied to the signature type variable.
1190 checkSigTyVars :: [TcTyVar] -> TcM ()
1191 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1193 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1194 -- The extra_tvs can include boxy type variables;
1195 -- e.g. TcMatches.tcCheckExistentialPat
1196 checkSigTyVarsWrt extra_tvs sig_tvs
1197 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1198 ; check_sig_tyvars extra_tvs' sig_tvs }
1201 :: TcTyVarSet -- Global type variables. The universally quantified
1202 -- tyvars should not mention any of these
1203 -- Guaranteed already zonked.
1204 -> [TcTyVar] -- Universally-quantified type variables in the signature
1205 -- Guaranteed to be skolems
1207 check_sig_tyvars _ []
1209 check_sig_tyvars extra_tvs sig_tvs
1210 = ASSERT( all isTcTyVar sig_tvs && all isSkolemTyVar sig_tvs )
1211 do { gbl_tvs <- tcGetGlobalTyVars
1212 ; traceTc "check_sig_tyvars" $ vcat
1213 [ text "sig_tys" <+> ppr sig_tvs
1214 , text "gbl_tvs" <+> ppr gbl_tvs
1215 , text "extra_tvs" <+> ppr extra_tvs]
1217 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1218 ; when (any (`elemVarSet` env_tvs) sig_tvs)
1219 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1222 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1223 -> [TcTyVar] -- The possibly-escaping type variables
1224 -> [TcTyVar] -- The zonked versions thereof
1226 -- Complain about escaping type variables
1227 -- We pass a list of type variables, at least one of which
1228 -- escapes. The first list contains the original signature type variable,
1229 -- while the second contains the type variable it is unified to (usually itself)
1230 bleatEscapedTvs globals sig_tvs zonked_tvs
1231 = do { env0 <- tcInitTidyEnv
1232 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1233 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1235 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1236 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1238 main_msg = ptext (sLit "Inferred type is less polymorphic than expected")
1240 check (tidy_env, msgs) (sig_tv, zonked_tv)
1241 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1243 = do { lcl_env <- getLclTypeEnv
1244 ; (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) lcl_env tidy_env
1245 ; return (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1247 -----------------------
1248 escape_msg :: Var -> Var -> [SDoc] -> SDoc
1249 escape_msg sig_tv zonked_tv globs
1251 = vcat [sep [msg, ptext (sLit "is mentioned in the environment:")],
1252 nest 2 (vcat globs)]
1254 = msg <+> ptext (sLit "escapes")
1255 -- Sigh. It's really hard to give a good error message
1256 -- all the time. One bad case is an existential pattern match.
1257 -- We rely on the "When..." context to help.
1259 msg = ptext (sLit "Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1261 | sig_tv == zonked_tv = empty
1262 | otherwise = ptext (sLit "is unified with") <+> quotes (ppr zonked_tv) <+> ptext (sLit "which")
1265 These two context are used with checkSigTyVars
1268 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1269 -> TidyEnv -> TcM (TidyEnv, Message)
1270 sigCtxt id sig_tvs sig_theta sig_tau tidy_env = do
1271 actual_tau <- zonkTcType sig_tau
1273 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1274 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1275 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1276 sub_msg = vcat [ptext (sLit "Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1277 ptext (sLit "Type to generalise:") <+> pprType tidy_actual_tau
1279 msg = vcat [ptext (sLit "When trying to generalise the type inferred for") <+> quotes (ppr id),