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 )
49 import Maybes ( allMaybes )
58 %************************************************************************
60 matchExpected functions
62 %************************************************************************
64 Note [Herald for matchExpectedFunTys]
65 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
66 The 'herald' always looks like:
67 "The equation(s) for 'f' have"
68 "The abstraction (\x.e) takes"
69 "The section (+ x) expects"
70 "The function 'f' is applied to"
72 This is used to construct a message of form
74 The abstraction `\Just 1 -> ...' takes two arguments
75 but its type `Maybe a -> a' has only one
77 The equation(s) for `f' have two arguments
78 but its type `Maybe a -> a' has only one
80 The section `(f 3)' requires 'f' to take two arguments
81 but its type `Int -> Int' has only one
83 The function 'f' is applied to two arguments
84 but its type `Int -> Int' has only one
86 Note [matchExpectedFunTys]
87 ~~~~~~~~~~~~~~~~~~~~~~~~~~
88 matchExpectedFunTys checks that an (Expected rho) has the form
89 of an n-ary function. It passes the decomposed type to the
90 thing_inside, and returns a wrapper to coerce between the two types
92 It's used wherever a language construct must have a functional type,
98 This is not (currently) where deep skolemisation occurs;
99 matchExpectedFunTys does not skolmise nested foralls in the
100 expected type, becuase it expects that to have been done already
104 matchExpectedFunTys :: SDoc -- See Note [Herald for matchExpectedFunTys]
107 -> TcM (CoercionI, [TcSigmaType], TcRhoType)
109 -- If matchExpectFunTys n ty = (co, [t1,..,tn], ty_r)
110 -- then co : ty ~ (t1 -> ... -> tn -> ty_r)
112 -- Does not allocate unnecessary meta variables: if the input already is
113 -- a function, we just take it apart. Not only is this efficient,
114 -- it's important for higher rank: the argument might be of form
115 -- (forall a. ty) -> other
116 -- If allocated (fresh-meta-var1 -> fresh-meta-var2) and unified, we'd
117 -- hide the forall inside a meta-variable
119 matchExpectedFunTys herald arity orig_ty
122 -- If go n ty = (co, [t1,..,tn], ty_r)
123 -- then co : ty ~ t1 -> .. -> tn -> ty_r
126 | n_req == 0 = return (IdCo ty, [], ty)
129 | Just ty' <- tcView ty = go n_req ty'
131 go n_req (FunTy arg_ty res_ty)
132 | not (isPredTy arg_ty)
133 = do { (coi, tys, ty_r) <- go (n_req-1) res_ty
134 ; return (mkFunTyCoI (IdCo arg_ty) coi, arg_ty:tys, ty_r) }
136 go _ (TyConApp tc _) -- A common case
137 | not (isSynFamilyTyCon tc)
138 = do { (env,msg) <- mk_ctxt emptyTidyEnv
139 ; failWithTcM (env,msg) }
141 go n_req ty@(TyVarTy tv)
142 | ASSERT( isTcTyVar tv) isMetaTyVar tv
143 = do { cts <- readMetaTyVar tv
145 Indirect ty' -> go n_req ty'
146 Flexi -> defer n_req ty }
148 -- In all other cases we bale out into ordinary unification
149 go n_req ty = defer n_req ty
153 = addErrCtxtM mk_ctxt $
154 do { arg_tys <- newFlexiTyVarTys n_req argTypeKind
155 ; res_ty <- newFlexiTyVarTy openTypeKind
156 ; coi <- unifyType fun_ty (mkFunTys arg_tys res_ty)
157 ; return (coi, arg_tys, res_ty) }
160 mk_ctxt :: TidyEnv -> TcM (TidyEnv, Message)
161 mk_ctxt env = do { orig_ty1 <- zonkTcType orig_ty
162 ; let (env', orig_ty2) = tidyOpenType env orig_ty1
163 (args, _) = tcSplitFunTys orig_ty2
164 n_actual = length args
165 ; return (env', mk_msg orig_ty2 n_actual) }
168 = herald <+> speakNOf arity (ptext (sLit "argument")) <> comma $$
169 sep [ptext (sLit "but its type") <+> quotes (pprType ty),
170 if n_args == 0 then ptext (sLit "has none")
171 else ptext (sLit "has only") <+> speakN n_args]
176 ----------------------
177 matchExpectedListTy :: TcRhoType -> TcM (CoercionI, TcRhoType)
178 -- Special case for lists
179 matchExpectedListTy exp_ty
180 = do { (coi, [elt_ty]) <- matchExpectedTyConApp listTyCon exp_ty
181 ; return (coi, elt_ty) }
183 ----------------------
184 matchExpectedPArrTy :: TcRhoType -> TcM (CoercionI, TcRhoType)
185 -- Special case for parrs
186 matchExpectedPArrTy exp_ty
187 = do { (coi, [elt_ty]) <- matchExpectedTyConApp parrTyCon exp_ty
188 ; return (coi, elt_ty) }
190 ----------------------
191 matchExpectedTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
192 -> TcRhoType -- orig_ty
193 -> TcM (CoercionI, -- T a b c ~ orig_ty
194 [TcSigmaType]) -- Element types, a b c
196 -- It's used for wired-in tycons, so we call checkWiredInTyCon
197 -- Precondition: never called with FunTyCon
198 -- Precondition: input type :: *
200 matchExpectedTyConApp tc orig_ty
201 = do { checkWiredInTyCon tc
202 ; go (tyConArity tc) orig_ty [] }
204 go :: Int -> TcRhoType -> [TcSigmaType] -> TcM (CoercionI, [TcSigmaType])
205 -- If go n ty tys = (co, [t1..tn] ++ tys)
206 -- then co : T t1..tn ~ ty
209 | Just ty' <- tcView ty = go n_req ty' tys
211 go n_req ty@(TyVarTy tv) tys
212 | ASSERT( isTcTyVar tv) isMetaTyVar tv
213 = do { cts <- readMetaTyVar tv
215 Indirect ty -> go n_req ty tys
216 Flexi -> defer n_req ty tys }
218 go n_req ty@(TyConApp tycon args) tys
220 = ASSERT( n_req == length args) -- ty::*
221 return (IdCo ty, args ++ tys)
223 go n_req (AppTy fun arg) tys
225 = do { (coi, args) <- go (n_req - 1) fun (arg : tys)
226 ; return (mkAppTyCoI coi (IdCo arg), args) }
228 go n_req ty tys = defer n_req ty tys
232 = do { tau_tys <- mapM newFlexiTyVarTy arg_kinds
233 ; coi <- unifyType (mkTyConApp tc tau_tys) ty
234 ; return (coi, tau_tys ++ tys) }
236 (arg_kinds, _) = splitKindFunTysN n_req (tyConKind tc)
238 ----------------------
239 matchExpectedAppTy :: TcRhoType -- orig_ty
240 -> TcM (CoercionI, -- m a ~ orig_ty
241 (TcSigmaType, TcSigmaType)) -- Returns m, a
242 -- If the incoming type is a mutable type variable of kind k, then
243 -- matchExpectedAppTy returns a new type variable (m: * -> k); note the *.
245 matchExpectedAppTy orig_ty
249 | Just ty' <- tcView ty = go ty'
251 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
252 = return (IdCo orig_ty, (fun_ty, arg_ty))
255 | ASSERT( isTcTyVar tv) isMetaTyVar tv
256 = do { cts <- readMetaTyVar tv
263 -- Defer splitting by generating an equality constraint
264 defer = do { ty1 <- newFlexiTyVarTy kind1
265 ; ty2 <- newFlexiTyVarTy kind2
266 ; coi <- unifyType (mkAppTy ty1 ty2) orig_ty
267 ; return (coi, (ty1, ty2)) }
269 orig_kind = typeKind orig_ty
270 kind1 = mkArrowKind liftedTypeKind (defaultKind orig_kind)
271 kind2 = liftedTypeKind -- m :: * -> k
273 -- The defaultKind is a bit smelly. If you remove it,
274 -- try compiling f x = do { x }
275 -- and you'll get a kind mis-match. It smells, but
276 -- not enough to lose sleep over.
280 %************************************************************************
284 %************************************************************************
286 All the tcSub calls have the form
288 tcSub actual_ty expected_ty
290 actual_ty <= expected_ty
292 That is, that a value of type actual_ty is acceptable in
293 a place expecting a value of type expected_ty.
295 It returns a coercion function
296 co_fn :: actual_ty ~ expected_ty
297 which takes an HsExpr of type actual_ty into one of type
301 tcSubType :: CtOrigin -> SkolemInfo -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
302 -- Check that ty_actual is more polymorphic than ty_expected
303 -- Both arguments might be polytypes, so we must instantiate and skolemise
304 -- Returns a wrapper of shape ty_actual ~ ty_expected
305 tcSubType origin skol_info ty_actual ty_expected
306 | isSigmaTy ty_actual
307 = do { (sk_wrap, inst_wrap)
308 <- tcGen skol_info 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 -> TcType
357 -> ([TcTyVar] -> TcRhoType -> TcM result)
358 -> TcM (HsWrapper, result)
359 -- The expression has type: spec_ty -> expected_ty
361 tcGen skol_info expected_ty thing_inside
362 -- 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 -- Generally 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.
381 -- However [Oct 10] now that the untouchables are a range of
382 -- TcTyVars, all tihs is handled automatically with no need for
383 -- extra faffing around
385 ; (ev_binds, result) <- checkConstraints skol_info tvs' given $
386 thing_inside tvs' rho'
388 ; return (wrap <.> mkWpLet ev_binds, result) }
389 -- The ev_binds returned by checkConstraints is very
390 -- often empty, in which case mkWpLet is a no-op
392 checkConstraints :: SkolemInfo
393 -> [TcTyVar] -- Skolems
396 -> TcM (TcEvBinds, result)
398 checkConstraints skol_info 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 = newImplication skol_info skol_tvs given thing_inside
407 newImplication :: SkolemInfo -> [TcTyVar]
408 -> [EvVar] -> TcM result
409 -> TcM (TcEvBinds, result)
410 newImplication skol_info skol_tvs given thing_inside
411 = ASSERT2( all isTcTyVar skol_tvs, ppr skol_tvs )
412 ASSERT2( all isSkolemTyVar skol_tvs, ppr skol_tvs )
413 do { ((result, untch), wanted) <- captureConstraints $
414 captureUntouchables $
417 ; if isEmptyBag wanted && not (hasEqualities given)
418 -- Optimisation : if there are no wanteds, and the givens
419 -- are sufficiently simple, don't generate an implication
420 -- at all. Reason for the hasEqualities test:
421 -- we don't want to lose the "inaccessible alternative"
424 return (emptyTcEvBinds, result)
426 { ev_binds_var <- newTcEvBinds
427 ; lcl_env <- getLclTypeEnv
428 ; loc <- getCtLoc skol_info
429 ; let implic = Implic { ic_untch = untch
431 , ic_skols = mkVarSet skol_tvs
432 , ic_scoped = panic "emitImplication"
435 , ic_binds = ev_binds_var
438 ; emitConstraint (WcImplic implic)
439 ; return (TcEvBinds ev_binds_var, result) } }
442 %************************************************************************
446 %************************************************************************
448 The exported functions are all defined as versions of some
449 non-exported generic functions.
453 unifyType :: TcTauType -> TcTauType -> TcM CoercionI
454 -- Actual and expected types
455 -- Returns a coercion : ty1 ~ ty2
456 unifyType ty1 ty2 = uType [] ty1 ty2
459 unifyPred :: PredType -> PredType -> TcM CoercionI
460 -- Actual and expected types
461 unifyPred p1 p2 = uPred [UnifyOrigin (mkPredTy p1) (mkPredTy p2)] p1 p2
464 unifyTheta :: TcThetaType -> TcThetaType -> TcM [CoercionI]
465 -- Actual and expected types
466 unifyTheta theta1 theta2
467 = do { checkTc (equalLength theta1 theta2)
468 (vcat [ptext (sLit "Contexts differ in length"),
469 nest 2 $ parens $ ptext (sLit "Use -XRelaxedPolyRec to allow this")])
470 ; zipWithM unifyPred theta1 theta2 }
473 @unifyTypeList@ takes a single list of @TauType@s and unifies them
474 all together. It is used, for example, when typechecking explicit
475 lists, when all the elts should be of the same type.
478 unifyTypeList :: [TcTauType] -> TcM ()
479 unifyTypeList [] = return ()
480 unifyTypeList [_] = return ()
481 unifyTypeList (ty1:tys@(ty2:_)) = do { _ <- unifyType ty1 ty2
482 ; unifyTypeList tys }
485 %************************************************************************
489 %************************************************************************
491 uType is the heart of the unifier. Each arg occurs twice, because
492 we want to report errors in terms of synomyms if possible. The first of
493 the pair is used in error messages only; it is always the same as the
494 second, except that if the first is a synonym then the second may be a
495 de-synonym'd version. This way we get better error messages.
499 = NotSwapped -- Args are: actual, expected
500 | IsSwapped -- Args are: expected, actual
502 instance Outputable SwapFlag where
503 ppr IsSwapped = ptext (sLit "Is-swapped")
504 ppr NotSwapped = ptext (sLit "Not-swapped")
506 unSwap :: SwapFlag -> (a->a->b) -> a -> a -> b
507 unSwap NotSwapped f a b = f a b
508 unSwap IsSwapped f a b = f b a
511 uType, uType_np, uType_defer
513 -> TcType -- ty1 is the *actual* type
514 -> TcType -- ty2 is the *expected* type
518 -- It is always safe to defer unification to the main constraint solver
519 -- See Note [Deferred unification]
520 uType_defer (item : origin) ty1 ty2
521 = do { co_var <- newWantedCoVar ty1 ty2
522 ; traceTc "utype_defer" (vcat [ppr co_var, ppr ty1, ppr ty2, ppr origin])
523 ; loc <- getCtLoc (TypeEqOrigin item)
524 ; wrapEqCtxt origin $
525 emitConstraint (WcEvVar (WantedEvVar co_var loc))
526 ; return $ ACo $ mkTyVarTy co_var }
528 = panic "uType_defer"
531 -- Push a new item on the origin stack (the most common case)
532 uType origin ty1 ty2 -- Push a new item on the origin stack
533 = uType_np (pushOrigin ty1 ty2 origin) ty1 ty2
536 -- unify_np (short for "no push" on the origin stack) does the work
537 uType_np origin orig_ty1 orig_ty2
538 = do { traceTc "u_tys " $ vcat
539 [ sep [ ppr orig_ty1, text "~", ppr orig_ty2]
541 ; coi <- go orig_ty1 orig_ty2
543 ACo co -> traceTc "u_tys yields coercion:" (ppr co)
544 IdCo _ -> traceTc "u_tys yields no coercion" empty
547 bale_out :: [EqOrigin] -> TcM a
548 bale_out origin = failWithMisMatch origin
550 go :: TcType -> TcType -> TcM CoercionI
551 -- The arguments to 'go' are always semantically identical
552 -- to orig_ty{1,2} except for looking through type synonyms
554 -- Variables; go for uVar
555 -- Note that we pass in *original* (before synonym expansion),
556 -- so that type variables tend to get filled in with
557 -- the most informative version of the type
558 go (TyVarTy tyvar1) ty2 = uVar origin NotSwapped tyvar1 ty2
559 go ty1 (TyVarTy tyvar2) = uVar origin IsSwapped tyvar2 ty1
562 -- see Note [Unification and synonyms]
563 -- Do this after the variable case so that we tend to unify
564 -- variables with un-expanded type synonym
566 -- Also NB that we recurse to 'go' so that we don't push a
567 -- new item on the origin stack. As a result if we have
569 -- and we try to unify Foo ~ Bool
570 -- we'll end up saying "can't match Foo with Bool"
571 -- rather than "can't match "Int with Bool". See Trac #4535.
573 | Just ty1' <- tcView ty1 = go ty1' ty2
574 | Just ty2' <- tcView ty2 = go ty1 ty2'
578 go (PredTy p1) (PredTy p2) = uPred origin p1 p2
580 -- Coercion functions: (t1a ~ t1b) => t1c ~ (t2a ~ t2b) => t2c
582 | Just (t1a,t1b,t1c) <- splitCoPredTy_maybe ty1,
583 Just (t2a,t2b,t2c) <- splitCoPredTy_maybe ty2
584 = do { co1 <- uType origin t1a t2a
585 ; co2 <- uType origin t1b t2b
586 ; co3 <- uType origin t1c t2c
587 ; return $ mkCoPredCoI co1 co2 co3 }
589 -- Functions (or predicate functions) just check the two parts
590 go (FunTy fun1 arg1) (FunTy fun2 arg2)
591 = do { coi_l <- uType origin fun1 fun2
592 ; coi_r <- uType origin arg1 arg2
593 ; return $ mkFunTyCoI coi_l coi_r }
595 -- Always defer if a type synonym family (type function)
596 -- is involved. (Data families behave rigidly.)
597 go ty1@(TyConApp tc1 _) ty2
598 | isSynFamilyTyCon tc1 = uType_defer origin ty1 ty2
599 go ty1 ty2@(TyConApp tc2 _)
600 | isSynFamilyTyCon tc2 = uType_defer origin ty1 ty2
602 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
603 | tc1 == tc2 -- See Note [TyCon app]
604 = do { cois <- uList origin uType tys1 tys2
605 ; return $ mkTyConAppCoI tc1 cois }
607 -- See Note [Care with type applications]
609 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
610 = do { coi_s <- uType_np origin s1 s2 -- See Note [Unifying AppTy]
611 ; coi_t <- uType origin t1 t2
612 ; return $ mkAppTyCoI coi_s coi_t }
615 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
616 = do { coi_s <- uType_np origin s1 s2
617 ; coi_t <- uType origin t1 t2
618 ; return $ mkAppTyCoI coi_s coi_t }
621 | tcIsForAllTy ty1 || tcIsForAllTy ty2
622 = unifySigmaTy origin ty1 ty2
624 -- Anything else fails
625 go _ _ = bale_out origin
627 unifySigmaTy :: [EqOrigin] -> TcType -> TcType -> TcM CoercionI
628 unifySigmaTy origin ty1 ty2
629 = do { let (tvs1, body1) = tcSplitForAllTys ty1
630 (tvs2, body2) = tcSplitForAllTys ty2
631 ; unless (equalLength tvs1 tvs2) (failWithMisMatch origin)
632 ; skol_tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
633 -- Get location from monad, not from tvs1
634 ; let tys = mkTyVarTys skol_tvs
635 in_scope = mkInScopeSet (mkVarSet skol_tvs)
636 phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
637 phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
638 -- untch = tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2
640 ; ((coi, _untch), lie) <- captureConstraints $
641 captureUntouchables $
642 uType origin phi1 phi2
643 -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
644 ; let bad_lie = filterBag is_bad lie
645 is_bad w = any (`elemVarSet` tyVarsOfWanted w) skol_tvs
646 ; when (not (isEmptyBag bad_lie))
647 (failWithMisMatch origin) -- ToDo: give details from bad_lie
649 ; emitConstraints lie
650 ; return (foldr mkForAllTyCoI coi skol_tvs) }
653 uPred :: [EqOrigin] -> PredType -> PredType -> TcM CoercionI
654 uPred origin (IParam n1 t1) (IParam n2 t2)
656 = do { coi <- uType origin t1 t2
657 ; return $ mkIParamPredCoI n1 coi }
658 uPred origin (ClassP c1 tys1) (ClassP c2 tys2)
660 = do { cois <- uList origin uType tys1 tys2
661 -- Guaranteed equal lengths because the kinds check
662 ; return $ mkClassPPredCoI c1 cois }
663 uPred origin (EqPred ty1a ty1b) (EqPred ty2a ty2b)
664 = do { coia <- uType origin ty1a ty2a
665 ; coib <- uType origin ty1b ty2b
666 ; return $ mkEqPredCoI coia coib }
668 uPred origin _ _ = failWithMisMatch origin
672 -> ([EqOrigin] -> a -> a -> TcM b)
673 -> [a] -> [a] -> TcM [b]
674 -- Unify corresponding elements of two lists of types, which
675 -- should be of equal length. We charge down the list explicitly so that
676 -- we can complain if their lengths differ.
677 uList _ _ [] [] = return []
678 uList origin unify (ty1:tys1) (ty2:tys2) = do { x <- unify origin ty1 ty2;
679 ; xs <- uList origin unify tys1 tys2
681 uList origin _ _ _ = failWithMisMatch origin
682 -- See Note [Mismatched type lists and application decomposition]
686 Note [Care with type applications]
687 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
688 Note: type applications need a bit of care!
689 They can match FunTy and TyConApp, so use splitAppTy_maybe
690 NB: we've already dealt with type variables and Notes,
691 so if one type is an App the other one jolly well better be too
693 Note [Unifying AppTy]
694 ~~~~~~~~~~~~~~~~~~~~~
695 Considerm unifying (m Int) ~ (IO Int) where m is a unification variable
696 that is now bound to (say) (Bool ->). Then we want to report
697 "Can't unify (Bool -> Int) with (IO Int)
699 "Can't unify ((->) Bool) with IO"
700 That is why we use the "_np" variant of uType, which does not alter the error
705 When we find two TyConApps, the argument lists are guaranteed equal
706 length. Reason: intially the kinds of the two types to be unified is
707 the same. The only way it can become not the same is when unifying two
708 AppTys (f1 a1)~(f2 a2). In that case there can't be a TyConApp in
709 the f1,f2 (because it'd absorb the app). If we unify f1~f2 first,
710 which we do, that ensures that f1,f2 have the same kind; and that
711 means a1,a2 have the same kind. And now the argument repeats.
713 Note [Mismatched type lists and application decomposition]
714 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
715 When we find two TyConApps, you might think that the argument lists
716 are guaranteed equal length. But they aren't. Consider matching
717 w (T x) ~ Foo (T x y)
718 We do match (w ~ Foo) first, but in some circumstances we simply create
719 a deferred constraint; and then go ahead and match (T x ~ T x y).
720 This came up in Trac #3950.
723 (a) either we must check for identical argument kinds
724 when decomposing applications,
726 (b) or we must be prepared for ill-kinded unification sub-problems
728 Currently we adopt (b) since it seems more robust -- no need to maintain
731 Note [Unification and synonyms]
732 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
733 If you are tempted to make a short cut on synonyms, as in this
736 uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
737 = if (con1 == con2) then
738 -- Good news! Same synonym constructors, so we can shortcut
739 -- by unifying their arguments and ignoring their expansions.
740 unifyTypepeLists args1 args2
742 -- Never mind. Just expand them and try again
745 then THINK AGAIN. Here is the whole story, as detected and reported
748 Here's a test program that should detect the problem:
751 x = (1 :: Bogus Char) :: Bogus Bool
753 The problem with [the attempted shortcut code] is that
757 is not a sufficient condition to be able to use the shortcut!
758 You also need to know that the type synonym actually USES all
759 its arguments. For example, consider the following type synonym
760 which does not use all its arguments.
764 If you ever tried unifying, say, (Bogus Char) with )Bogus Bool), the
765 unifier would blithely try to unify Char with Bool and would fail,
766 even though the expanded forms (both Int) should match. Similarly,
767 unifying (Bogus Char) with (Bogus t) would unnecessarily bind t to
770 ... You could explicitly test for the problem synonyms and mark them
771 somehow as needing expansion, perhaps also issuing a warning to the
774 Note [Deferred Unification]
775 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
776 We may encounter a unification ty1 ~ ty2 that cannot be performed syntactically,
777 and yet its consistency is undetermined. Previously, there was no way to still
778 make it consistent. So a mismatch error was issued.
780 Now these unfications are deferred until constraint simplification, where type
781 family instances and given equations may (or may not) establish the consistency.
782 Deferred unifications are of the form
785 where F is a type function and x is a type variable.
787 id :: x ~ y => x -> y
790 involves the unfication x = y. It is deferred until we bring into account the
791 context x ~ y to establish that it holds.
793 If available, we defer original types (rather than those where closed type
794 synonyms have already been expanded via tcCoreView). This is, as usual, to
795 improve error messages.
798 %************************************************************************
802 %************************************************************************
804 @uVar@ is called when at least one of the types being unified is a
805 variable. It does {\em not} assume that the variable is a fixed point
806 of the substitution; rather, notice that @uVar@ (defined below) nips
807 back into @uTys@ if it turns out that the variable is already bound.
810 uVar :: [EqOrigin] -> SwapFlag -> TcTyVar -> TcTauType -> TcM CoercionI
811 uVar origin swapped tv1 ty2
812 = do { traceTc "uVar" (vcat [ ppr origin
814 , ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1)
815 , nest 2 (ptext (sLit " ~ "))
816 , ppr ty2 <+> dcolon <+> ppr (typeKind ty2)])
817 ; details <- lookupTcTyVar tv1
819 Filled ty1 -> unSwap swapped (uType_np origin) ty1 ty2
820 Unfilled details1 -> uUnfilledVar origin swapped tv1 details1 ty2
824 uUnfilledVar :: [EqOrigin]
826 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
827 -> TcTauType -- Type 2
829 -- "Unfilled" means that the variable is definitely not a filled-in meta tyvar
830 -- It might be a skolem, or untouchable, or meta
832 uUnfilledVar origin swapped tv1 details1 (TyVarTy tv2)
833 | tv1 == tv2 -- Same type variable => no-op
834 = return (IdCo (mkTyVarTy tv1))
836 | otherwise -- Distinct type variables
837 = do { lookup2 <- lookupTcTyVar tv2
839 Filled ty2' -> uUnfilledVar origin swapped tv1 details1 ty2'
840 Unfilled details2 -> uUnfilledVars origin swapped tv1 details1 tv2 details2
843 uUnfilledVar origin swapped tv1 details1 non_var_ty2 -- ty2 is not a type variable
846 -> do { mb_ty2' <- checkTauTvUpdate tv1 non_var_ty2
848 Nothing -> do { traceTc "Occ/kind defer" (ppr tv1); defer }
849 Just ty2' -> updateMeta tv1 ref1 ty2'
852 _other -> do { traceTc "Skolem defer" (ppr tv1); defer } -- Skolems of all sorts
854 defer = unSwap swapped (uType_defer origin) (mkTyVarTy tv1) non_var_ty2
855 -- Occurs check or an untouchable: just defer
856 -- NB: occurs check isn't necessarily fatal:
857 -- eg tv1 occured in type family parameter
860 uUnfilledVars :: [EqOrigin]
862 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
863 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
865 -- Invarant: The type variables are distinct,
866 -- Neither is filled in yet
868 uUnfilledVars origin swapped tv1 details1 tv2 details2
869 = case (details1, details2) of
870 (MetaTv i1 ref1, MetaTv i2 ref2)
871 | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1 i1 i2
872 then updateMeta tv1 ref1 ty2
873 else updateMeta tv2 ref2 ty1
874 | k2_sub_k1 -> updateMeta tv1 ref1 ty2
876 (_, MetaTv _ ref2) | k1_sub_k2 -> updateMeta tv2 ref2 ty1
877 (MetaTv _ ref1, _) | k2_sub_k1 -> updateMeta tv1 ref1 ty2
879 (_, _) -> unSwap swapped (uType_defer origin) ty1 ty2
880 -- Defer for skolems of all sorts
884 k1_sub_k2 = k1 `isSubKind` k2
885 k2_sub_k1 = k2 `isSubKind` k1
889 nicer_to_update_tv1 _ (SigTv _) = True
890 nicer_to_update_tv1 (SigTv _) _ = False
891 nicer_to_update_tv1 _ _ = isSystemName (Var.varName tv1)
892 -- Try not to update SigTvs; and try to update sys-y type
893 -- variables in preference to ones gotten (say) by
894 -- instantiating a polymorphic function with a user-written
898 checkTauTvUpdate :: TcTyVar -> TcType -> TcM (Maybe TcType)
899 -- (checkTauTvUpdate tv ty)
900 -- We are about to update the TauTv tv with ty.
901 -- Check (a) that tv doesn't occur in ty (occurs check)
902 -- (b) that kind(ty) is a sub-kind of kind(tv)
903 -- (c) that ty does not contain any type families, see Note [Type family sharing]
905 -- We have two possible outcomes:
906 -- (1) Return the type to update the type variable with,
907 -- [we know the update is ok]
908 -- (2) Return Nothing,
909 -- [the update might be dodgy]
911 -- Note that "Nothing" does not mean "definite error". For example
913 -- type instance F Int = Int
916 -- This is perfectly reasonable, if we later get a ~ Int. For now, though,
917 -- we return Nothing, leaving it to the later constraint simplifier to
920 checkTauTvUpdate tv ty
921 = do { ty' <- zonkTcType ty
922 ; if typeKind ty' `isSubKind` tyVarKind tv then
924 Nothing -> return Nothing
925 Just ty'' -> return (Just ty'')
926 else return Nothing }
928 where ok :: TcType -> Maybe TcType
929 ok (TyVarTy tv') | not (tv == tv') = Just (TyVarTy tv')
930 ok this_ty@(TyConApp tc tys)
931 | not (isSynFamilyTyCon tc), Just tys' <- allMaybes (map ok tys)
932 = Just (TyConApp tc tys')
933 | isSynTyCon tc, Just ty_expanded <- tcView this_ty
934 = ok ty_expanded -- See Note [Type synonyms and the occur check]
935 ok (PredTy sty) | Just sty' <- ok_pred sty = Just (PredTy sty')
936 ok (FunTy arg res) | Just arg' <- ok arg, Just res' <- ok res
937 = Just (FunTy arg' res')
938 ok (AppTy fun arg) | Just fun' <- ok fun, Just arg' <- ok arg
939 = Just (AppTy fun' arg')
940 ok (ForAllTy tv1 ty1) | Just ty1' <- ok ty1 = Just (ForAllTy tv1 ty1')
944 ok_pred (IParam nm ty) | Just ty' <- ok ty = Just (IParam nm ty')
945 ok_pred (ClassP cl tys)
946 | Just tys' <- allMaybes (map ok tys)
947 = Just (ClassP cl tys')
948 ok_pred (EqPred ty1 ty2)
949 | Just ty1' <- ok ty1, Just ty2' <- ok ty2
950 = Just (EqPred ty1' ty2')
952 ok_pred _pty = Nothing
956 Note [Type synonyms and the occur check]
958 Generally speaking we need to update a variable with type synonyms not expanded, which
959 improves later error messages, except for when looking inside a type synonym may help resolve
960 a spurious occurs check error. Consider:
963 f :: (A a -> a -> ()) -> ()
969 We will eventually get a constraint of the form t ~ A t. The ok function above will
970 properly expand the type (A t) to just (), which is ok to be unified with t. If we had
971 unified with the original type A t, we would lead the type checker into an infinite loop.
973 Hence, if the occurs check fails for a type synonym application, then (and *only* then),
974 the ok function expands the synonym to detect opportunities for occurs check success using
975 the underlying definition of the type synonym.
977 The same applies later on in the constraint interaction code; see TcInteract,
978 function @occ_check_ok@.
981 Note [Type family sharing]
983 We must avoid eagerly unifying type variables to types that contain function symbols,
984 because this may lead to loss of sharing, and in turn, in very poor performance of the
985 constraint simplifier. Assume that we have a wanted constraint:
994 where D is some type class. If we eagerly unify m1 := [F m2], m2 := [F m3], m3 := [F m2],
995 then, after zonking, our constraint simplifier will be faced with the following wanted
1002 which has to be flattened by the constraint solver. However, because the sharing is lost,
1003 an polynomially larger number of flatten skolems will be created and the constraint sets
1004 we are working with will be polynomially larger.
1006 Instead, if we defer the unifications m1 := [F m2], etc. we will only be generating three
1007 flatten skolems, which is the maximum possible sharing arising from the original constraint.
1010 data LookupTyVarResult -- The result of a lookupTcTyVar call
1011 = Unfilled TcTyVarDetails -- SkolemTv or virgin MetaTv
1014 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
1016 | MetaTv _ ref <- details
1017 = do { meta_details <- readMutVar ref
1018 ; case meta_details of
1019 Indirect ty -> return (Filled ty)
1020 Flexi -> do { is_untch <- isUntouchable tyvar
1021 ; let -- Note [Unifying untouchables]
1022 ret_details | is_untch = SkolemTv UnkSkol
1023 | otherwise = details
1024 ; return (Unfilled ret_details) } }
1026 = return (Unfilled details)
1028 details = ASSERT2( isTcTyVar tyvar, ppr tyvar )
1029 tcTyVarDetails tyvar
1031 updateMeta :: TcTyVar -> TcRef MetaDetails -> TcType -> TcM CoercionI
1032 updateMeta tv1 ref1 ty2
1033 = do { writeMetaTyVarRef tv1 ref1 ty2
1034 ; return (IdCo ty2) }
1037 Note [Unifying untouchables]
1038 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1039 We treat an untouchable type variable as if it was a skolem. That
1040 ensures it won't unify with anything. It's a slight had, because
1041 we return a made-up TcTyVarDetails, but I think it works smoothly.
1044 %************************************************************************
1048 %************************************************************************
1051 pushOrigin :: TcType -> TcType -> [EqOrigin] -> [EqOrigin]
1052 pushOrigin ty_act ty_exp origin
1053 = UnifyOrigin { uo_actual = ty_act, uo_expected = ty_exp } : origin
1056 wrapEqCtxt :: [EqOrigin] -> TcM a -> TcM a
1057 -- Build a suitable error context from the origin and do the thing inside
1058 -- The "couldn't match" error comes from the innermost item on the stack,
1059 -- and, if there is more than one item, the "Expected/inferred" part
1060 -- comes from the outermost item
1061 wrapEqCtxt [] thing_inside = thing_inside
1062 wrapEqCtxt items thing_inside = addErrCtxtM (unifyCtxt (last items)) thing_inside
1065 failWithMisMatch :: [EqOrigin] -> TcM a
1066 -- Generate the message when two types fail to match,
1067 -- going to some trouble to make it helpful.
1068 -- We take the failing types from the top of the origin stack
1069 -- rather than reporting the particular ones we are looking
1071 failWithMisMatch (item:origin)
1072 = wrapEqCtxt origin $
1073 do { ty_act <- zonkTcType (uo_actual item)
1074 ; ty_exp <- zonkTcType (uo_expected item)
1075 ; env0 <- tcInitTidyEnv
1076 ; let (env1, pp_exp) = tidyOpenType env0 ty_exp
1077 (env2, pp_act) = tidyOpenType env1 ty_act
1078 ; failWithTcM (misMatchMsg env2 pp_act pp_exp) }
1080 = panic "failWithMisMatch"
1082 misMatchMsg :: TidyEnv -> TcType -> TcType -> (TidyEnv, SDoc)
1083 misMatchMsg env ty_act ty_exp
1084 = (env2, sep [sep [ ptext (sLit "Couldn't match expected type") <+> quotes (ppr ty_exp)
1085 , nest 12 $ ptext (sLit "with actual type") <+> quotes (ppr ty_act)]
1086 , nest 2 (extra1 $$ extra2) ])
1088 (env1, extra1) = typeExtraInfoMsg env ty_exp
1089 (env2, extra2) = typeExtraInfoMsg env1 ty_act
1093 -----------------------------------------
1095 -----------------------------------------
1099 -- If an error happens we try to figure out whether the function
1100 -- function has been given too many or too few arguments, and say so.
1101 addSubCtxt :: InstOrigin -> TcType -> TcType -> TcM a -> TcM a
1102 addSubCtxt orig actual_res_ty expected_res_ty thing_inside
1103 = addErrCtxtM mk_err thing_inside
1106 = do { exp_ty' <- zonkTcType expected_res_ty
1107 ; act_ty' <- zonkTcType actual_res_ty
1108 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1109 (env2, act_ty'') = tidyOpenType env1 act_ty'
1110 (exp_args, _) = tcSplitFunTys exp_ty''
1111 (act_args, _) = tcSplitFunTys act_ty''
1113 len_act_args = length act_args
1114 len_exp_args = length exp_args
1116 message = case orig of
1118 | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1119 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1120 _ -> mkExpectedActualMsg act_ty'' exp_ty''
1121 ; return (env2, message) }
1124 %************************************************************************
1128 %************************************************************************
1130 Unifying kinds is much, much simpler than unifying types.
1133 matchExpectedFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1134 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1136 matchExpectedFunKind (TyVarTy kvar) = do
1137 maybe_kind <- readKindVar kvar
1139 Indirect fun_kind -> matchExpectedFunKind fun_kind
1141 do { arg_kind <- newKindVar
1142 ; res_kind <- newKindVar
1143 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1144 ; return (Just (arg_kind,res_kind)) }
1146 matchExpectedFunKind (FunTy arg_kind res_kind) = return (Just (arg_kind,res_kind))
1147 matchExpectedFunKind _ = return Nothing
1150 unifyKind :: TcKind -- Expected
1154 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1155 | isSubKindCon kc2 kc1 = return ()
1157 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1158 = do { unifyKind a2 a1; unifyKind r1 r2 }
1159 -- Notice the flip in the argument,
1160 -- so that the sub-kinding works right
1161 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1162 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1163 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1166 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1167 uKVar swapped kv1 k2
1168 = do { mb_k1 <- readKindVar kv1
1170 Flexi -> uUnboundKVar swapped kv1 k2
1171 Indirect k1 | swapped -> unifyKind k2 k1
1172 | otherwise -> unifyKind k1 k2 }
1175 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1176 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1177 | kv1 == kv2 = return ()
1178 | otherwise -- Distinct kind variables
1179 = do { mb_k2 <- readKindVar kv2
1181 Indirect k2 -> uUnboundKVar swapped kv1 k2
1182 Flexi -> writeKindVar kv1 k2 }
1184 uUnboundKVar swapped kv1 non_var_k2
1185 = do { k2' <- zonkTcKind non_var_k2
1186 ; kindOccurCheck kv1 k2'
1187 ; k2'' <- kindSimpleKind swapped k2'
1188 -- KindVars must be bound only to simple kinds
1189 -- Polarities: (kindSimpleKind True ?) succeeds
1190 -- returning *, corresponding to unifying
1193 ; writeKindVar kv1 k2'' }
1196 kindOccurCheck :: TyVar -> Type -> TcM ()
1197 kindOccurCheck kv1 k2 -- k2 is zonked
1198 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1200 not_in (TyVarTy kv2) = kv1 /= kv2
1201 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1204 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1205 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1206 -- If the flag is False, it requires k <: sk
1207 -- E.g. kindSimpleKind False ?? = *
1208 -- What about (kv -> *) ~ ?? -> *
1209 kindSimpleKind orig_swapped orig_kind
1210 = go orig_swapped orig_kind
1212 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1214 ; return (mkArrowKind k1' k2') }
1216 | isOpenTypeKind k = return liftedTypeKind
1217 | isArgTypeKind k = return liftedTypeKind
1219 | isLiftedTypeKind k = return liftedTypeKind
1220 | isUnliftedTypeKind k = return unliftedTypeKind
1221 go _ k@(TyVarTy _) = return k -- KindVars are always simple
1222 go _ _ = failWithTc (ptext (sLit "Unexpected kind unification failure:")
1223 <+> ppr orig_swapped <+> ppr orig_kind)
1224 -- I think this can't actually happen
1226 -- T v = MkT v v must be a type
1227 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1229 unifyKindMisMatch :: TcKind -> TcKind -> TcM ()
1230 unifyKindMisMatch ty1 ty2 = do
1231 ty1' <- zonkTcKind ty1
1232 ty2' <- zonkTcKind ty2
1234 msg = hang (ptext (sLit "Couldn't match kind"))
1235 2 (sep [quotes (ppr ty1'),
1236 ptext (sLit "against"),
1241 kindOccurCheckErr :: Var -> Type -> SDoc
1242 kindOccurCheckErr tyvar ty
1243 = hang (ptext (sLit "Occurs check: cannot construct the infinite kind:"))
1244 2 (sep [ppr tyvar, char '=', ppr ty])
1247 %************************************************************************
1249 \subsection{Checking signature type variables}
1251 %************************************************************************
1253 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1254 are not mentioned in the environment. In particular:
1256 (a) Not mentioned in the type of a variable in the envt
1257 eg the signature for f in this:
1263 Here, f is forced to be monorphic by the free occurence of x.
1265 (d) Not (unified with another type variable that is) in scope.
1266 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1267 when checking the expression type signature, we find that
1268 even though there is nothing in scope whose type mentions r,
1269 nevertheless the type signature for the expression isn't right.
1271 Another example is in a class or instance declaration:
1273 op :: forall b. a -> b
1275 Here, b gets unified with a
1277 Before doing this, the substitution is applied to the signature type variable.
1280 checkSigTyVars :: [TcTyVar] -> TcM ()
1281 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1283 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1284 -- The extra_tvs can include boxy type variables;
1285 -- e.g. TcMatches.tcCheckExistentialPat
1286 checkSigTyVarsWrt extra_tvs sig_tvs
1287 = do { extra_tvs' <- zonkTcTyVarsAndFV extra_tvs
1288 ; check_sig_tyvars extra_tvs' sig_tvs }
1291 :: TcTyVarSet -- Global type variables. The universally quantified
1292 -- tyvars should not mention any of these
1293 -- Guaranteed already zonked.
1294 -> [TcTyVar] -- Universally-quantified type variables in the signature
1295 -- Guaranteed to be skolems
1297 check_sig_tyvars _ []
1299 check_sig_tyvars extra_tvs sig_tvs
1300 = ASSERT( all isTcTyVar sig_tvs && all isSkolemTyVar sig_tvs )
1301 do { gbl_tvs <- tcGetGlobalTyVars
1302 ; traceTc "check_sig_tyvars" $ vcat
1303 [ text "sig_tys" <+> ppr sig_tvs
1304 , text "gbl_tvs" <+> ppr gbl_tvs
1305 , text "extra_tvs" <+> ppr extra_tvs]
1307 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1308 ; when (any (`elemVarSet` env_tvs) sig_tvs)
1309 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1312 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1313 -> [TcTyVar] -- The possibly-escaping type variables
1314 -> [TcTyVar] -- The zonked versions thereof
1316 -- Complain about escaping type variables
1317 -- We pass a list of type variables, at least one of which
1318 -- escapes. The first list contains the original signature type variable,
1319 -- while the second contains the type variable it is unified to (usually itself)
1320 bleatEscapedTvs globals sig_tvs zonked_tvs
1321 = do { env0 <- tcInitTidyEnv
1322 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1323 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1325 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1326 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1328 main_msg = ptext (sLit "Inferred type is less polymorphic than expected")
1330 check (tidy_env, msgs) (sig_tv, zonked_tv)
1331 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1333 = do { lcl_env <- getLclTypeEnv
1334 ; (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) lcl_env tidy_env
1335 ; return (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1337 -----------------------
1338 escape_msg :: Var -> Var -> [SDoc] -> SDoc
1339 escape_msg sig_tv zonked_tv globs
1341 = vcat [sep [msg, ptext (sLit "is mentioned in the environment:")],
1342 nest 2 (vcat globs)]
1344 = msg <+> ptext (sLit "escapes")
1345 -- Sigh. It's really hard to give a good error message
1346 -- all the time. One bad case is an existential pattern match.
1347 -- We rely on the "When..." context to help.
1349 msg = ptext (sLit "Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1351 | sig_tv == zonked_tv = empty
1352 | otherwise = ptext (sLit "is unified with") <+> quotes (ppr zonked_tv) <+> ptext (sLit "which")
1355 These two context are used with checkSigTyVars
1358 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1359 -> TidyEnv -> TcM (TidyEnv, Message)
1360 sigCtxt id sig_tvs sig_theta sig_tau tidy_env = do
1361 actual_tau <- zonkTcType sig_tau
1363 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1364 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1365 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1366 sub_msg = vcat [ptext (sLit "Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1367 ptext (sLit "Type to generalise:") <+> pprType tidy_actual_tau
1369 msg = vcat [ptext (sLit "When trying to generalise the type inferred for") <+> quotes (ppr id),