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 origin 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 :: [EqOrigin] -> 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 origin (TyVarTy tyvar1) ty2 = uVar origin NotSwapped tyvar1 ty2
559 go origin 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-expended type synonym
566 | Just ty1' <- tcView ty1 = uType origin ty1' ty2
567 | Just ty2' <- tcView ty2 = uType origin ty1 ty2'
570 go origin (PredTy p1) (PredTy p2) = uPred origin p1 p2
572 -- Coercion functions: (t1a ~ t1b) => t1c ~ (t2a ~ t2b) => t2c
574 | Just (t1a,t1b,t1c) <- splitCoPredTy_maybe ty1,
575 Just (t2a,t2b,t2c) <- splitCoPredTy_maybe ty2
576 = do { co1 <- uType origin t1a t2a
577 ; co2 <- uType origin t1b t2b
578 ; co3 <- uType origin t1c t2c
579 ; return $ mkCoPredCoI co1 co2 co3 }
581 -- Functions (or predicate functions) just check the two parts
582 go origin (FunTy fun1 arg1) (FunTy fun2 arg2)
583 = do { coi_l <- uType origin fun1 fun2
584 ; coi_r <- uType origin arg1 arg2
585 ; return $ mkFunTyCoI coi_l coi_r }
587 -- Always defer if a type synonym family (type function)
588 -- is involved. (Data families behave rigidly.)
589 go origin ty1@(TyConApp tc1 _) ty2
590 | isSynFamilyTyCon tc1 = uType_defer origin ty1 ty2
591 go origin ty1 ty2@(TyConApp tc2 _)
592 | isSynFamilyTyCon tc2 = uType_defer origin ty1 ty2
594 go origin (TyConApp tc1 tys1) (TyConApp tc2 tys2)
595 | tc1 == tc2 -- See Note [TyCon app]
596 = do { cois <- uList origin uType tys1 tys2
597 ; return $ mkTyConAppCoI tc1 cois }
599 -- See Note [Care with type applications]
600 go origin (AppTy s1 t1) ty2
601 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
602 = do { coi_s <- uType_np origin s1 s2 -- See Note [Unifying AppTy]
603 ; coi_t <- uType origin t1 t2
604 ; return $ mkAppTyCoI coi_s coi_t }
606 go origin ty1 (AppTy s2 t2)
607 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
608 = do { coi_s <- uType_np origin s1 s2
609 ; coi_t <- uType origin t1 t2
610 ; return $ mkAppTyCoI coi_s coi_t }
613 | tcIsForAllTy ty1 || tcIsForAllTy ty2
614 = unifySigmaTy origin ty1 ty2
616 -- Anything else fails
617 go origin _ _ = bale_out origin
619 unifySigmaTy :: [EqOrigin] -> TcType -> TcType -> TcM CoercionI
620 unifySigmaTy origin ty1 ty2
621 = do { let (tvs1, body1) = tcSplitForAllTys ty1
622 (tvs2, body2) = tcSplitForAllTys ty2
623 ; unless (equalLength tvs1 tvs2) (failWithMisMatch origin)
624 ; skol_tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
625 -- Get location from monad, not from tvs1
626 ; let tys = mkTyVarTys skol_tvs
627 in_scope = mkInScopeSet (mkVarSet skol_tvs)
628 phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
629 phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
630 -- untch = tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2
632 ; ((coi, _untch), lie) <- captureConstraints $
633 captureUntouchables $
634 uType origin phi1 phi2
635 -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
636 ; let bad_lie = filterBag is_bad lie
637 is_bad w = any (`elemVarSet` tyVarsOfWanted w) skol_tvs
638 ; when (not (isEmptyBag bad_lie))
639 (failWithMisMatch origin) -- ToDo: give details from bad_lie
641 ; emitConstraints lie
642 ; return (foldr mkForAllTyCoI coi skol_tvs) }
645 uPred :: [EqOrigin] -> PredType -> PredType -> TcM CoercionI
646 uPred origin (IParam n1 t1) (IParam n2 t2)
648 = do { coi <- uType origin t1 t2
649 ; return $ mkIParamPredCoI n1 coi }
650 uPred origin (ClassP c1 tys1) (ClassP c2 tys2)
652 = do { cois <- uList origin uType tys1 tys2
653 -- Guaranteed equal lengths because the kinds check
654 ; return $ mkClassPPredCoI c1 cois }
655 uPred origin (EqPred ty1a ty1b) (EqPred ty2a ty2b)
656 = do { coia <- uType origin ty1a ty2a
657 ; coib <- uType origin ty1b ty2b
658 ; return $ mkEqPredCoI coia coib }
660 uPred origin _ _ = failWithMisMatch origin
664 -> ([EqOrigin] -> a -> a -> TcM b)
665 -> [a] -> [a] -> TcM [b]
666 -- Unify corresponding elements of two lists of types, which
667 -- should be of equal length. We charge down the list explicitly so that
668 -- we can complain if their lengths differ.
669 uList _ _ [] [] = return []
670 uList origin unify (ty1:tys1) (ty2:tys2) = do { x <- unify origin ty1 ty2;
671 ; xs <- uList origin unify tys1 tys2
673 uList origin _ _ _ = failWithMisMatch origin
674 -- See Note [Mismatched type lists and application decomposition]
678 Note [Care with type applications]
679 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
680 Note: type applications need a bit of care!
681 They can match FunTy and TyConApp, so use splitAppTy_maybe
682 NB: we've already dealt with type variables and Notes,
683 so if one type is an App the other one jolly well better be too
685 Note [Unifying AppTy]
686 ~~~~~~~~~~~~~~~~~~~~~
687 Considerm unifying (m Int) ~ (IO Int) where m is a unification variable
688 that is now bound to (say) (Bool ->). Then we want to report
689 "Can't unify (Bool -> Int) with (IO Int)
691 "Can't unify ((->) Bool) with IO"
692 That is why we use the "_np" variant of uType, which does not alter the error
697 When we find two TyConApps, the argument lists are guaranteed equal
698 length. Reason: intially the kinds of the two types to be unified is
699 the same. The only way it can become not the same is when unifying two
700 AppTys (f1 a1)~(f2 a2). In that case there can't be a TyConApp in
701 the f1,f2 (because it'd absorb the app). If we unify f1~f2 first,
702 which we do, that ensures that f1,f2 have the same kind; and that
703 means a1,a2 have the same kind. And now the argument repeats.
705 Note [Mismatched type lists and application decomposition]
706 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
707 When we find two TyConApps, you might think that the argument lists
708 are guaranteed equal length. But they aren't. Consider matching
709 w (T x) ~ Foo (T x y)
710 We do match (w ~ Foo) first, but in some circumstances we simply create
711 a deferred constraint; and then go ahead and match (T x ~ T x y).
712 This came up in Trac #3950.
715 (a) either we must check for identical argument kinds
716 when decomposing applications,
718 (b) or we must be prepared for ill-kinded unification sub-problems
720 Currently we adopt (b) since it seems more robust -- no need to maintain
723 Note [Unification and synonyms]
724 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
725 If you are tempted to make a short cut on synonyms, as in this
728 uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
729 = if (con1 == con2) then
730 -- Good news! Same synonym constructors, so we can shortcut
731 -- by unifying their arguments and ignoring their expansions.
732 unifyTypepeLists args1 args2
734 -- Never mind. Just expand them and try again
737 then THINK AGAIN. Here is the whole story, as detected and reported
740 Here's a test program that should detect the problem:
743 x = (1 :: Bogus Char) :: Bogus Bool
745 The problem with [the attempted shortcut code] is that
749 is not a sufficient condition to be able to use the shortcut!
750 You also need to know that the type synonym actually USES all
751 its arguments. For example, consider the following type synonym
752 which does not use all its arguments.
756 If you ever tried unifying, say, (Bogus Char) with )Bogus Bool), the
757 unifier would blithely try to unify Char with Bool and would fail,
758 even though the expanded forms (both Int) should match. Similarly,
759 unifying (Bogus Char) with (Bogus t) would unnecessarily bind t to
762 ... You could explicitly test for the problem synonyms and mark them
763 somehow as needing expansion, perhaps also issuing a warning to the
766 Note [Deferred Unification]
767 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
768 We may encounter a unification ty1 ~ ty2 that cannot be performed syntactically,
769 and yet its consistency is undetermined. Previously, there was no way to still
770 make it consistent. So a mismatch error was issued.
772 Now these unfications are deferred until constraint simplification, where type
773 family instances and given equations may (or may not) establish the consistency.
774 Deferred unifications are of the form
777 where F is a type function and x is a type variable.
779 id :: x ~ y => x -> y
782 involves the unfication x = y. It is deferred until we bring into account the
783 context x ~ y to establish that it holds.
785 If available, we defer original types (rather than those where closed type
786 synonyms have already been expanded via tcCoreView). This is, as usual, to
787 improve error messages.
790 %************************************************************************
794 %************************************************************************
796 @uVar@ is called when at least one of the types being unified is a
797 variable. It does {\em not} assume that the variable is a fixed point
798 of the substitution; rather, notice that @uVar@ (defined below) nips
799 back into @uTys@ if it turns out that the variable is already bound.
802 uVar :: [EqOrigin] -> SwapFlag -> TcTyVar -> TcTauType -> TcM CoercionI
803 uVar origin swapped tv1 ty2
804 = do { traceTc "uVar" (vcat [ ppr origin
806 , ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1)
807 , nest 2 (ptext (sLit " ~ "))
808 , ppr ty2 <+> dcolon <+> ppr (typeKind ty2)])
809 ; details <- lookupTcTyVar tv1
811 Filled ty1 -> unSwap swapped (uType_np origin) ty1 ty2
812 Unfilled details1 -> uUnfilledVar origin swapped tv1 details1 ty2
816 uUnfilledVar :: [EqOrigin]
818 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
819 -> TcTauType -- Type 2
821 -- "Unfilled" means that the variable is definitely not a filled-in meta tyvar
822 -- It might be a skolem, or untouchable, or meta
824 uUnfilledVar origin swapped tv1 details1 (TyVarTy tv2)
825 | tv1 == tv2 -- Same type variable => no-op
826 = return (IdCo (mkTyVarTy tv1))
828 | otherwise -- Distinct type variables
829 = do { lookup2 <- lookupTcTyVar tv2
831 Filled ty2' -> uUnfilledVar origin swapped tv1 details1 ty2'
832 Unfilled details2 -> uUnfilledVars origin swapped tv1 details1 tv2 details2
835 uUnfilledVar origin swapped tv1 details1 non_var_ty2 -- ty2 is not a type variable
838 -> do { mb_ty2' <- checkTauTvUpdate tv1 non_var_ty2
840 Nothing -> do { traceTc "Occ/kind defer" (ppr tv1); defer }
841 Just ty2' -> updateMeta tv1 ref1 ty2'
844 _other -> do { traceTc "Skolem defer" (ppr tv1); defer } -- Skolems of all sorts
846 defer = unSwap swapped (uType_defer origin) (mkTyVarTy tv1) non_var_ty2
847 -- Occurs check or an untouchable: just defer
848 -- NB: occurs check isn't necessarily fatal:
849 -- eg tv1 occured in type family parameter
852 uUnfilledVars :: [EqOrigin]
854 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
855 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
857 -- Invarant: The type variables are distinct,
858 -- Neither is filled in yet
860 uUnfilledVars origin swapped tv1 details1 tv2 details2
861 = case (details1, details2) of
862 (MetaTv i1 ref1, MetaTv i2 ref2)
863 | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1 i1 i2
864 then updateMeta tv1 ref1 ty2
865 else updateMeta tv2 ref2 ty1
866 | k2_sub_k1 -> updateMeta tv1 ref1 ty2
868 (_, MetaTv _ ref2) | k1_sub_k2 -> updateMeta tv2 ref2 ty1
869 (MetaTv _ ref1, _) | k2_sub_k1 -> updateMeta tv1 ref1 ty2
871 (_, _) -> unSwap swapped (uType_defer origin) ty1 ty2
872 -- Defer for skolems of all sorts
876 k1_sub_k2 = k1 `isSubKind` k2
877 k2_sub_k1 = k2 `isSubKind` k1
881 nicer_to_update_tv1 _ (SigTv _) = True
882 nicer_to_update_tv1 (SigTv _) _ = False
883 nicer_to_update_tv1 _ _ = isSystemName (Var.varName tv1)
884 -- Try not to update SigTvs; and try to update sys-y type
885 -- variables in preference to ones gotten (say) by
886 -- instantiating a polymorphic function with a user-written
890 checkTauTvUpdate :: TcTyVar -> TcType -> TcM (Maybe TcType)
891 -- (checkTauTvUpdate tv ty)
892 -- We are about to update the TauTv tv with ty.
893 -- Check (a) that tv doesn't occur in ty (occurs check)
894 -- (b) that kind(ty) is a sub-kind of kind(tv)
895 -- (c) that ty does not contain any type families, see Note [Type family sharing]
897 -- We have two possible outcomes:
898 -- (1) Return the type to update the type variable with,
899 -- [we know the update is ok]
900 -- (2) Return Nothing,
901 -- [the update might be dodgy]
903 -- Note that "Nothing" does not mean "definite error". For example
905 -- type instance F Int = Int
908 -- This is perfectly reasonable, if we later get a ~ Int. For now, though,
909 -- we return Nothing, leaving it to the later constraint simplifier to
912 checkTauTvUpdate tv ty
913 = do { ty' <- zonkTcType ty
914 ; if typeKind ty' `isSubKind` tyVarKind tv then
916 Nothing -> return Nothing
917 Just ty'' -> return (Just ty'')
918 else return Nothing }
920 where ok :: TcType -> Maybe TcType
921 ok (TyVarTy tv') | not (tv == tv') = Just (TyVarTy tv')
922 ok this_ty@(TyConApp tc tys)
923 | not (isSynFamilyTyCon tc), Just tys' <- allMaybes (map ok tys)
924 = Just (TyConApp tc tys')
925 | isSynTyCon tc, Just ty_expanded <- tcView this_ty
926 = ok ty_expanded -- See Note [Type synonyms and the occur check]
927 ok (PredTy sty) | Just sty' <- ok_pred sty = Just (PredTy sty')
928 ok (FunTy arg res) | Just arg' <- ok arg, Just res' <- ok res
929 = Just (FunTy arg' res')
930 ok (AppTy fun arg) | Just fun' <- ok fun, Just arg' <- ok arg
931 = Just (AppTy fun' arg')
932 ok (ForAllTy tv1 ty1) | Just ty1' <- ok ty1 = Just (ForAllTy tv1 ty1')
936 ok_pred (IParam nm ty) | Just ty' <- ok ty = Just (IParam nm ty')
937 ok_pred (ClassP cl tys)
938 | Just tys' <- allMaybes (map ok tys)
939 = Just (ClassP cl tys')
940 ok_pred (EqPred ty1 ty2)
941 | Just ty1' <- ok ty1, Just ty2' <- ok ty2
942 = Just (EqPred ty1' ty2')
944 ok_pred _pty = Nothing
948 Note [Type synonyms and the occur check]
950 Generally speaking we need to update a variable with type synonyms not expanded, which
951 improves later error messages, except for when looking inside a type synonym may help resolve
952 a spurious occurs check error. Consider:
955 f :: (A a -> a -> ()) -> ()
961 We will eventually get a constraint of the form t ~ A t. The ok function above will
962 properly expand the type (A t) to just (), which is ok to be unified with t. If we had
963 unified with the original type A t, we would lead the type checker into an infinite loop.
965 Hence, if the occurs check fails for a type synonym application, then (and *only* then),
966 the ok function expands the synonym to detect opportunities for occurs check success using
967 the underlying definition of the type synonym.
969 The same applies later on in the constraint interaction code; see TcInteract,
970 function @occ_check_ok@.
973 Note [Type family sharing]
975 We must avoid eagerly unifying type variables to types that contain function symbols,
976 because this may lead to loss of sharing, and in turn, in very poor performance of the
977 constraint simplifier. Assume that we have a wanted constraint:
986 where D is some type class. If we eagerly unify m1 := [F m2], m2 := [F m3], m3 := [F m2],
987 then, after zonking, our constraint simplifier will be faced with the following wanted
994 which has to be flattened by the constraint solver. However, because the sharing is lost,
995 an polynomially larger number of flatten skolems will be created and the constraint sets
996 we are working with will be polynomially larger.
998 Instead, if we defer the unifications m1 := [F m2], etc. we will only be generating three
999 flatten skolems, which is the maximum possible sharing arising from the original constraint.
1002 data LookupTyVarResult -- The result of a lookupTcTyVar call
1003 = Unfilled TcTyVarDetails -- SkolemTv or virgin MetaTv
1006 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
1008 | MetaTv _ ref <- details
1009 = do { meta_details <- readMutVar ref
1010 ; case meta_details of
1011 Indirect ty -> return (Filled ty)
1012 Flexi -> do { is_untch <- isUntouchable tyvar
1013 ; let -- Note [Unifying untouchables]
1014 ret_details | is_untch = SkolemTv UnkSkol
1015 | otherwise = details
1016 ; return (Unfilled ret_details) } }
1018 = return (Unfilled details)
1020 details = ASSERT2( isTcTyVar tyvar, ppr tyvar )
1021 tcTyVarDetails tyvar
1023 updateMeta :: TcTyVar -> TcRef MetaDetails -> TcType -> TcM CoercionI
1024 updateMeta tv1 ref1 ty2
1025 = do { writeMetaTyVarRef tv1 ref1 ty2
1026 ; return (IdCo ty2) }
1029 Note [Unifying untouchables]
1030 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1031 We treat an untouchable type variable as if it was a skolem. That
1032 ensures it won't unify with anything. It's a slight had, because
1033 we return a made-up TcTyVarDetails, but I think it works smoothly.
1036 %************************************************************************
1040 %************************************************************************
1043 pushOrigin :: TcType -> TcType -> [EqOrigin] -> [EqOrigin]
1044 pushOrigin ty_act ty_exp origin
1045 = UnifyOrigin { uo_actual = ty_act, uo_expected = ty_exp } : origin
1048 wrapEqCtxt :: [EqOrigin] -> TcM a -> TcM a
1049 -- Build a suitable error context from the origin and do the thing inside
1050 -- The "couldn't match" error comes from the innermost item on the stack,
1051 -- and, if there is more than one item, the "Expected/inferred" part
1052 -- comes from the outermost item
1053 wrapEqCtxt [] thing_inside = thing_inside
1054 wrapEqCtxt items thing_inside = addErrCtxtM (unifyCtxt (last items)) thing_inside
1057 failWithMisMatch :: [EqOrigin] -> TcM a
1058 -- Generate the message when two types fail to match,
1059 -- going to some trouble to make it helpful.
1060 -- We take the failing types from the top of the origin stack
1061 -- rather than reporting the particular ones we are looking
1063 failWithMisMatch (item:origin)
1064 = wrapEqCtxt origin $
1065 do { ty_act <- zonkTcType (uo_actual item)
1066 ; ty_exp <- zonkTcType (uo_expected item)
1067 ; env0 <- tcInitTidyEnv
1068 ; let (env1, pp_exp) = tidyOpenType env0 ty_exp
1069 (env2, pp_act) = tidyOpenType env1 ty_act
1070 ; failWithTcM (misMatchMsg env2 pp_act pp_exp) }
1072 = panic "failWithMisMatch"
1074 misMatchMsg :: TidyEnv -> TcType -> TcType -> (TidyEnv, SDoc)
1075 misMatchMsg env ty_act ty_exp
1076 = (env2, sep [sep [ ptext (sLit "Couldn't match expected type") <+> quotes (ppr ty_exp)
1077 , nest 12 $ ptext (sLit "with actual type") <+> quotes (ppr ty_act)]
1078 , nest 2 (extra1 $$ extra2) ])
1080 (env1, extra1) = typeExtraInfoMsg env ty_exp
1081 (env2, extra2) = typeExtraInfoMsg env1 ty_act
1085 -----------------------------------------
1087 -----------------------------------------
1091 -- If an error happens we try to figure out whether the function
1092 -- function has been given too many or too few arguments, and say so.
1093 addSubCtxt :: InstOrigin -> TcType -> TcType -> TcM a -> TcM a
1094 addSubCtxt orig actual_res_ty expected_res_ty thing_inside
1095 = addErrCtxtM mk_err thing_inside
1098 = do { exp_ty' <- zonkTcType expected_res_ty
1099 ; act_ty' <- zonkTcType actual_res_ty
1100 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1101 (env2, act_ty'') = tidyOpenType env1 act_ty'
1102 (exp_args, _) = tcSplitFunTys exp_ty''
1103 (act_args, _) = tcSplitFunTys act_ty''
1105 len_act_args = length act_args
1106 len_exp_args = length exp_args
1108 message = case orig of
1110 | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1111 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1112 _ -> mkExpectedActualMsg act_ty'' exp_ty''
1113 ; return (env2, message) }
1116 %************************************************************************
1120 %************************************************************************
1122 Unifying kinds is much, much simpler than unifying types.
1125 matchExpectedFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1126 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1128 matchExpectedFunKind (TyVarTy kvar) = do
1129 maybe_kind <- readKindVar kvar
1131 Indirect fun_kind -> matchExpectedFunKind fun_kind
1133 do { arg_kind <- newKindVar
1134 ; res_kind <- newKindVar
1135 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1136 ; return (Just (arg_kind,res_kind)) }
1138 matchExpectedFunKind (FunTy arg_kind res_kind) = return (Just (arg_kind,res_kind))
1139 matchExpectedFunKind _ = return Nothing
1142 unifyKind :: TcKind -- Expected
1146 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1147 | isSubKindCon kc2 kc1 = return ()
1149 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1150 = do { unifyKind a2 a1; unifyKind r1 r2 }
1151 -- Notice the flip in the argument,
1152 -- so that the sub-kinding works right
1153 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1154 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1155 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1158 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1159 uKVar swapped kv1 k2
1160 = do { mb_k1 <- readKindVar kv1
1162 Flexi -> uUnboundKVar swapped kv1 k2
1163 Indirect k1 | swapped -> unifyKind k2 k1
1164 | otherwise -> unifyKind k1 k2 }
1167 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1168 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1169 | kv1 == kv2 = return ()
1170 | otherwise -- Distinct kind variables
1171 = do { mb_k2 <- readKindVar kv2
1173 Indirect k2 -> uUnboundKVar swapped kv1 k2
1174 Flexi -> writeKindVar kv1 k2 }
1176 uUnboundKVar swapped kv1 non_var_k2
1177 = do { k2' <- zonkTcKind non_var_k2
1178 ; kindOccurCheck kv1 k2'
1179 ; k2'' <- kindSimpleKind swapped k2'
1180 -- KindVars must be bound only to simple kinds
1181 -- Polarities: (kindSimpleKind True ?) succeeds
1182 -- returning *, corresponding to unifying
1185 ; writeKindVar kv1 k2'' }
1188 kindOccurCheck :: TyVar -> Type -> TcM ()
1189 kindOccurCheck kv1 k2 -- k2 is zonked
1190 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1192 not_in (TyVarTy kv2) = kv1 /= kv2
1193 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1196 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1197 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1198 -- If the flag is False, it requires k <: sk
1199 -- E.g. kindSimpleKind False ?? = *
1200 -- What about (kv -> *) ~ ?? -> *
1201 kindSimpleKind orig_swapped orig_kind
1202 = go orig_swapped orig_kind
1204 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1206 ; return (mkArrowKind k1' k2') }
1208 | isOpenTypeKind k = return liftedTypeKind
1209 | isArgTypeKind k = return liftedTypeKind
1211 | isLiftedTypeKind k = return liftedTypeKind
1212 | isUnliftedTypeKind k = return unliftedTypeKind
1213 go _ k@(TyVarTy _) = return k -- KindVars are always simple
1214 go _ _ = failWithTc (ptext (sLit "Unexpected kind unification failure:")
1215 <+> ppr orig_swapped <+> ppr orig_kind)
1216 -- I think this can't actually happen
1218 -- T v = MkT v v must be a type
1219 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1221 unifyKindMisMatch :: TcKind -> TcKind -> TcM ()
1222 unifyKindMisMatch ty1 ty2 = do
1223 ty1' <- zonkTcKind ty1
1224 ty2' <- zonkTcKind ty2
1226 msg = hang (ptext (sLit "Couldn't match kind"))
1227 2 (sep [quotes (ppr ty1'),
1228 ptext (sLit "against"),
1233 kindOccurCheckErr :: Var -> Type -> SDoc
1234 kindOccurCheckErr tyvar ty
1235 = hang (ptext (sLit "Occurs check: cannot construct the infinite kind:"))
1236 2 (sep [ppr tyvar, char '=', ppr ty])
1239 %************************************************************************
1241 \subsection{Checking signature type variables}
1243 %************************************************************************
1245 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1246 are not mentioned in the environment. In particular:
1248 (a) Not mentioned in the type of a variable in the envt
1249 eg the signature for f in this:
1255 Here, f is forced to be monorphic by the free occurence of x.
1257 (d) Not (unified with another type variable that is) in scope.
1258 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1259 when checking the expression type signature, we find that
1260 even though there is nothing in scope whose type mentions r,
1261 nevertheless the type signature for the expression isn't right.
1263 Another example is in a class or instance declaration:
1265 op :: forall b. a -> b
1267 Here, b gets unified with a
1269 Before doing this, the substitution is applied to the signature type variable.
1272 checkSigTyVars :: [TcTyVar] -> TcM ()
1273 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1275 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1276 -- The extra_tvs can include boxy type variables;
1277 -- e.g. TcMatches.tcCheckExistentialPat
1278 checkSigTyVarsWrt extra_tvs sig_tvs
1279 = do { extra_tvs' <- zonkTcTyVarsAndFV extra_tvs
1280 ; check_sig_tyvars extra_tvs' sig_tvs }
1283 :: TcTyVarSet -- Global type variables. The universally quantified
1284 -- tyvars should not mention any of these
1285 -- Guaranteed already zonked.
1286 -> [TcTyVar] -- Universally-quantified type variables in the signature
1287 -- Guaranteed to be skolems
1289 check_sig_tyvars _ []
1291 check_sig_tyvars extra_tvs sig_tvs
1292 = ASSERT( all isTcTyVar sig_tvs && all isSkolemTyVar sig_tvs )
1293 do { gbl_tvs <- tcGetGlobalTyVars
1294 ; traceTc "check_sig_tyvars" $ vcat
1295 [ text "sig_tys" <+> ppr sig_tvs
1296 , text "gbl_tvs" <+> ppr gbl_tvs
1297 , text "extra_tvs" <+> ppr extra_tvs]
1299 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1300 ; when (any (`elemVarSet` env_tvs) sig_tvs)
1301 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1304 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1305 -> [TcTyVar] -- The possibly-escaping type variables
1306 -> [TcTyVar] -- The zonked versions thereof
1308 -- Complain about escaping type variables
1309 -- We pass a list of type variables, at least one of which
1310 -- escapes. The first list contains the original signature type variable,
1311 -- while the second contains the type variable it is unified to (usually itself)
1312 bleatEscapedTvs globals sig_tvs zonked_tvs
1313 = do { env0 <- tcInitTidyEnv
1314 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1315 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1317 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1318 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1320 main_msg = ptext (sLit "Inferred type is less polymorphic than expected")
1322 check (tidy_env, msgs) (sig_tv, zonked_tv)
1323 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1325 = do { lcl_env <- getLclTypeEnv
1326 ; (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) lcl_env tidy_env
1327 ; return (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1329 -----------------------
1330 escape_msg :: Var -> Var -> [SDoc] -> SDoc
1331 escape_msg sig_tv zonked_tv globs
1333 = vcat [sep [msg, ptext (sLit "is mentioned in the environment:")],
1334 nest 2 (vcat globs)]
1336 = msg <+> ptext (sLit "escapes")
1337 -- Sigh. It's really hard to give a good error message
1338 -- all the time. One bad case is an existential pattern match.
1339 -- We rely on the "When..." context to help.
1341 msg = ptext (sLit "Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1343 | sig_tv == zonked_tv = empty
1344 | otherwise = ptext (sLit "is unified with") <+> quotes (ppr zonked_tv) <+> ptext (sLit "which")
1347 These two context are used with checkSigTyVars
1350 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1351 -> TidyEnv -> TcM (TidyEnv, Message)
1352 sigCtxt id sig_tvs sig_theta sig_tau tidy_env = do
1353 actual_tau <- zonkTcType sig_tau
1355 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1356 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1357 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1358 sub_msg = vcat [ptext (sLit "Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1359 ptext (sLit "Type to generalise:") <+> pprType tidy_actual_tau
1361 msg = vcat [ptext (sLit "When trying to generalise the type inferred for") <+> quotes (ppr id),