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 { let extra_tvs = tyVarsOfType ty_actual
308 ; (sk_wrap, inst_wrap)
309 <- tcGen skol_info extra_tvs ty_expected $ \ _ sk_rho -> do
310 { (in_wrap, in_rho) <- deeplyInstantiate origin ty_actual
311 ; coi <- unifyType in_rho sk_rho
312 ; return (coiToHsWrapper coi <.> in_wrap) }
313 ; return (sk_wrap <.> inst_wrap) }
315 | otherwise -- Urgh! It seems deeply weird to have equality
316 -- when actual is not a polytype, and it makes a big
317 -- difference e.g. tcfail104
318 = do { coi <- unifyType ty_actual ty_expected
319 ; return (coiToHsWrapper coi) }
321 tcInfer :: (TcType -> TcM a) -> TcM (a, TcType)
322 tcInfer tc_infer = do { ty <- newFlexiTyVarTy openTypeKind
327 tcWrapResult :: HsExpr TcId -> TcRhoType -> TcRhoType -> TcM (HsExpr TcId)
328 tcWrapResult expr actual_ty res_ty
329 = do { coi <- unifyType actual_ty res_ty
330 -- Both types are deeply skolemised
331 ; return (mkHsWrapCoI coi expr) }
333 -----------------------------------
335 :: [TcType] -- Type of args
336 -> HsWrapper -- HsExpr a -> HsExpr b
337 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
338 wrapFunResCoercion arg_tys co_fn_res
339 | isIdHsWrapper co_fn_res
344 = do { arg_ids <- newSysLocalIds (fsLit "sub") arg_tys
345 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpEvVarApps arg_ids) }
350 %************************************************************************
352 \subsection{Generalisation}
354 %************************************************************************
357 tcGen :: SkolemInfo -> TcTyVarSet -> TcType
358 -> ([TcTyVar] -> TcRhoType -> TcM result)
359 -> TcM (HsWrapper, result)
360 -- The expression has type: spec_ty -> expected_ty
362 tcGen skol_info extra_tvs
363 expected_ty thing_inside -- We expect expected_ty to be a forall-type
364 -- If not, the call is a no-op
365 = do { traceTc "tcGen" empty
366 ; (wrap, tvs', given, rho') <- deeplySkolemise skol_info expected_ty
369 traceTc "tcGen" $ vcat [
370 text "expected_ty" <+> ppr expected_ty,
371 text "inst ty" <+> ppr tvs' <+> ppr rho' ]
373 -- In 'free_tvs' we must check that the "forall_tvs" havn't been constrained
374 -- The interesting bit here is that we must include the free variables
375 -- of the expected_ty. Here's an example:
376 -- runST (newVar True)
377 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
378 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
379 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
380 -- So now s' isn't unconstrained because it's linked to a.
381 -- Conclusion: pass the free vars of the expected_ty to checkConsraints
382 ; let free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
384 ; (ev_binds, result) <- checkConstraints skol_info free_tvs tvs' given $
385 thing_inside tvs' rho'
387 ; return (wrap <.> mkWpLet ev_binds, result) }
388 -- The ev_binds returned by checkConstraints is very
389 -- often empty, in which case mkWpLet is a no-op
391 checkConstraints :: SkolemInfo
392 -> TcTyVarSet -- Free variables (other than the type envt)
393 -- for the skolem escape check
394 -> [TcTyVar] -- Skolems
397 -> TcM (TcEvBinds, result)
399 checkConstraints skol_info free_tvs skol_tvs given thing_inside
400 | null skol_tvs && null given
401 = do { res <- thing_inside; return (emptyTcEvBinds, res) }
402 -- Just for efficiency. We check every function argument with
403 -- tcPolyExpr, which uses tcGen and hence checkConstraints.
406 = do { (ev_binds, wanted, result) <- newImplication skol_info free_tvs
407 skol_tvs given thing_inside
408 ; emitConstraints wanted
409 ; return (ev_binds, result) }
411 newImplication :: SkolemInfo -> TcTyVarSet -> [TcTyVar]
412 -> [EvVar] -> TcM result
413 -> TcM (TcEvBinds, WantedConstraints, result)
414 newImplication skol_info _free_tvs skol_tvs given thing_inside
415 = ASSERT2( all isTcTyVar skol_tvs, ppr skol_tvs )
416 ASSERT2( all isSkolemTyVar skol_tvs, ppr skol_tvs )
417 do { -- gbl_tvs <- tcGetGlobalTyVars
418 -- ; free_tvs <- zonkTcTyVarsAndFV free_tvs
419 -- ; let untch = gbl_tvs `unionVarSet` free_tvs
421 ; ((result, untch), wanted) <- captureConstraints $
422 captureUntouchables $
425 ; if isEmptyBag wanted && not (hasEqualities given)
426 -- Optimisation : if there are no wanteds, and the givens
427 -- are sufficiently simple, don't generate an implication
428 -- at all. Reason for the hasEqualities test:
429 -- we don't want to lose the "inaccessible alternative"
432 return (emptyTcEvBinds, emptyWanteds, result)
434 { ev_binds_var <- newTcEvBinds
435 ; lcl_env <- getLclTypeEnv
436 ; loc <- getCtLoc skol_info
437 ; let implic = Implic { ic_untch = untch
439 , ic_skols = mkVarSet skol_tvs
440 , ic_scoped = panic "emitImplication"
443 , ic_binds = ev_binds_var
446 ; return (TcEvBinds ev_binds_var, unitBag (WcImplic implic), result) } }
449 %************************************************************************
453 %************************************************************************
455 The exported functions are all defined as versions of some
456 non-exported generic functions.
460 unifyType :: TcTauType -> TcTauType -> TcM CoercionI
461 -- Actual and expected types
462 -- Returns a coercion : ty1 ~ ty2
463 unifyType ty1 ty2 = uType [] ty1 ty2
466 unifyPred :: PredType -> PredType -> TcM CoercionI
467 -- Actual and expected types
468 unifyPred p1 p2 = uPred [UnifyOrigin (mkPredTy p1) (mkPredTy p2)] p1 p2
471 unifyTheta :: TcThetaType -> TcThetaType -> TcM [CoercionI]
472 -- Actual and expected types
473 unifyTheta theta1 theta2
474 = do { checkTc (equalLength theta1 theta2)
475 (vcat [ptext (sLit "Contexts differ in length"),
476 nest 2 $ parens $ ptext (sLit "Use -XRelaxedPolyRec to allow this")])
477 ; zipWithM unifyPred theta1 theta2 }
480 @unifyTypeList@ takes a single list of @TauType@s and unifies them
481 all together. It is used, for example, when typechecking explicit
482 lists, when all the elts should be of the same type.
485 unifyTypeList :: [TcTauType] -> TcM ()
486 unifyTypeList [] = return ()
487 unifyTypeList [_] = return ()
488 unifyTypeList (ty1:tys@(ty2:_)) = do { _ <- unifyType ty1 ty2
489 ; unifyTypeList tys }
492 %************************************************************************
496 %************************************************************************
498 uType is the heart of the unifier. Each arg occurs twice, because
499 we want to report errors in terms of synomyms if possible. The first of
500 the pair is used in error messages only; it is always the same as the
501 second, except that if the first is a synonym then the second may be a
502 de-synonym'd version. This way we get better error messages.
506 = NotSwapped -- Args are: actual, expected
507 | IsSwapped -- Args are: expected, actual
509 instance Outputable SwapFlag where
510 ppr IsSwapped = ptext (sLit "Is-swapped")
511 ppr NotSwapped = ptext (sLit "Not-swapped")
513 unSwap :: SwapFlag -> (a->a->b) -> a -> a -> b
514 unSwap NotSwapped f a b = f a b
515 unSwap IsSwapped f a b = f b a
518 uType, uType_np, uType_defer
520 -> TcType -- ty1 is the *actual* type
521 -> TcType -- ty2 is the *expected* type
525 -- It is always safe to defer unification to the main constraint solver
526 -- See Note [Deferred unification]
527 uType_defer (item : origin) ty1 ty2
528 = do { co_var <- newWantedCoVar ty1 ty2
529 ; traceTc "utype_defer" (vcat [ppr co_var, ppr ty1, ppr ty2, ppr origin])
530 ; loc <- getCtLoc (TypeEqOrigin item)
531 ; wrapEqCtxt origin $
532 emitConstraint (WcEvVar (WantedEvVar co_var loc))
533 ; return $ ACo $ mkTyVarTy co_var }
535 = panic "uType_defer"
538 -- Push a new item on the origin stack (the most common case)
539 uType origin ty1 ty2 -- Push a new item on the origin stack
540 = uType_np (pushOrigin ty1 ty2 origin) ty1 ty2
543 -- unify_np (short for "no push" on the origin stack) does the work
544 uType_np origin orig_ty1 orig_ty2
545 = do { traceTc "u_tys " $ vcat
546 [ sep [ ppr orig_ty1, text "~", ppr orig_ty2]
548 ; coi <- go origin orig_ty1 orig_ty2
550 ACo co -> traceTc "u_tys yields coercion:" (ppr co)
551 IdCo _ -> traceTc "u_tys yields no coercion" empty
554 bale_out :: [EqOrigin] -> TcM a
555 bale_out origin = failWithMisMatch origin
557 go :: [EqOrigin] -> TcType -> TcType -> TcM CoercionI
558 -- The arguments to 'go' are always semantically identical
559 -- to orig_ty{1,2} except for looking through type synonyms
561 -- Variables; go for uVar
562 -- Note that we pass in *original* (before synonym expansion),
563 -- so that type variables tend to get filled in with
564 -- the most informative version of the type
565 go origin (TyVarTy tyvar1) ty2 = uVar origin NotSwapped tyvar1 ty2
566 go origin ty1 (TyVarTy tyvar2) = uVar origin IsSwapped tyvar2 ty1
569 -- see Note [Unification and synonyms]
570 -- Do this after the variable case so that we tend to unify
571 -- variables with un-expended type synonym
573 | Just ty1' <- tcView ty1 = uType origin ty1' ty2
574 | Just ty2' <- tcView ty2 = uType origin ty1 ty2'
577 go origin (PredTy p1) (PredTy p2) = uPred origin p1 p2
579 -- Coercion functions: (t1a ~ t1b) => t1c ~ (t2a ~ t2b) => t2c
581 | Just (t1a,t1b,t1c) <- splitCoPredTy_maybe ty1,
582 Just (t2a,t2b,t2c) <- splitCoPredTy_maybe ty2
583 = do { co1 <- uType origin t1a t2a
584 ; co2 <- uType origin t1b t2b
585 ; co3 <- uType origin t1c t2c
586 ; return $ mkCoPredCoI co1 co2 co3 }
588 -- Functions (or predicate functions) just check the two parts
589 go origin (FunTy fun1 arg1) (FunTy fun2 arg2)
590 = do { coi_l <- uType origin fun1 fun2
591 ; coi_r <- uType origin arg1 arg2
592 ; return $ mkFunTyCoI coi_l coi_r }
594 -- Always defer if a type synonym family (type function)
595 -- is involved. (Data families behave rigidly.)
596 go origin ty1@(TyConApp tc1 _) ty2
597 | isSynFamilyTyCon tc1 = uType_defer origin ty1 ty2
598 go origin ty1 ty2@(TyConApp tc2 _)
599 | isSynFamilyTyCon tc2 = uType_defer origin ty1 ty2
601 go origin (TyConApp tc1 tys1) (TyConApp tc2 tys2)
602 | tc1 == tc2 -- See Note [TyCon app]
603 = do { cois <- uList origin uType tys1 tys2
604 ; return $ mkTyConAppCoI tc1 cois }
606 -- See Note [Care with type applications]
607 go origin (AppTy s1 t1) ty2
608 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
609 = do { coi_s <- uType_np origin s1 s2 -- See Note [Unifying AppTy]
610 ; coi_t <- uType origin t1 t2
611 ; return $ mkAppTyCoI coi_s coi_t }
613 go origin ty1 (AppTy s2 t2)
614 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
615 = do { coi_s <- uType_np origin s1 s2
616 ; coi_t <- uType origin t1 t2
617 ; return $ mkAppTyCoI coi_s coi_t }
620 | tcIsForAllTy ty1 || tcIsForAllTy ty2
621 = unifySigmaTy origin ty1 ty2
623 -- Anything else fails
624 go origin _ _ = bale_out origin
626 unifySigmaTy :: [EqOrigin] -> TcType -> TcType -> TcM CoercionI
627 unifySigmaTy origin ty1 ty2
628 = do { let (tvs1, body1) = tcSplitForAllTys ty1
629 (tvs2, body2) = tcSplitForAllTys ty2
630 ; unless (equalLength tvs1 tvs2) (failWithMisMatch origin)
631 ; skol_tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
632 -- Get location from monad, not from tvs1
633 ; let tys = mkTyVarTys skol_tvs
634 in_scope = mkInScopeSet (mkVarSet skol_tvs)
635 phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
636 phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
637 -- untch = tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2
639 ; ((coi, _untch), lie) <- captureConstraints $
640 captureUntouchables $
641 uType origin phi1 phi2
642 -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
643 ; let bad_lie = filterBag is_bad lie
644 is_bad w = any (`elemVarSet` tyVarsOfWanted w) skol_tvs
645 ; when (not (isEmptyBag bad_lie))
646 (failWithMisMatch origin) -- ToDo: give details from bad_lie
648 ; emitConstraints lie
649 ; return (foldr mkForAllTyCoI coi skol_tvs) }
652 uPred :: [EqOrigin] -> PredType -> PredType -> TcM CoercionI
653 uPred origin (IParam n1 t1) (IParam n2 t2)
655 = do { coi <- uType origin t1 t2
656 ; return $ mkIParamPredCoI n1 coi }
657 uPred origin (ClassP c1 tys1) (ClassP c2 tys2)
659 = do { cois <- uList origin uType tys1 tys2
660 -- Guaranteed equal lengths because the kinds check
661 ; return $ mkClassPPredCoI c1 cois }
662 uPred origin (EqPred ty1a ty1b) (EqPred ty2a ty2b)
663 = do { coia <- uType origin ty1a ty2a
664 ; coib <- uType origin ty1b ty2b
665 ; return $ mkEqPredCoI coia coib }
667 uPred origin _ _ = failWithMisMatch origin
671 -> ([EqOrigin] -> a -> a -> TcM b)
672 -> [a] -> [a] -> TcM [b]
673 -- Unify corresponding elements of two lists of types, which
674 -- should be of equal length. We charge down the list explicitly so that
675 -- we can complain if their lengths differ.
676 uList _ _ [] [] = return []
677 uList origin unify (ty1:tys1) (ty2:tys2) = do { x <- unify origin ty1 ty2;
678 ; xs <- uList origin unify tys1 tys2
680 uList origin _ _ _ = failWithMisMatch origin
681 -- See Note [Mismatched type lists and application decomposition]
685 Note [Care with type applications]
686 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
687 Note: type applications need a bit of care!
688 They can match FunTy and TyConApp, so use splitAppTy_maybe
689 NB: we've already dealt with type variables and Notes,
690 so if one type is an App the other one jolly well better be too
692 Note [Unifying AppTy]
693 ~~~~~~~~~~~~~~~~~~~~~
694 Considerm unifying (m Int) ~ (IO Int) where m is a unification variable
695 that is now bound to (say) (Bool ->). Then we want to report
696 "Can't unify (Bool -> Int) with (IO Int)
698 "Can't unify ((->) Bool) with IO"
699 That is why we use the "_np" variant of uType, which does not alter the error
704 When we find two TyConApps, the argument lists are guaranteed equal
705 length. Reason: intially the kinds of the two types to be unified is
706 the same. The only way it can become not the same is when unifying two
707 AppTys (f1 a1)~(f2 a2). In that case there can't be a TyConApp in
708 the f1,f2 (because it'd absorb the app). If we unify f1~f2 first,
709 which we do, that ensures that f1,f2 have the same kind; and that
710 means a1,a2 have the same kind. And now the argument repeats.
712 Note [Mismatched type lists and application decomposition]
713 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
714 When we find two TyConApps, you might think that the argument lists
715 are guaranteed equal length. But they aren't. Consider matching
716 w (T x) ~ Foo (T x y)
717 We do match (w ~ Foo) first, but in some circumstances we simply create
718 a deferred constraint; and then go ahead and match (T x ~ T x y).
719 This came up in Trac #3950.
722 (a) either we must check for identical argument kinds
723 when decomposing applications,
725 (b) or we must be prepared for ill-kinded unification sub-problems
727 Currently we adopt (b) since it seems more robust -- no need to maintain
730 Note [Unification and synonyms]
731 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
732 If you are tempted to make a short cut on synonyms, as in this
735 uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
736 = if (con1 == con2) then
737 -- Good news! Same synonym constructors, so we can shortcut
738 -- by unifying their arguments and ignoring their expansions.
739 unifyTypepeLists args1 args2
741 -- Never mind. Just expand them and try again
744 then THINK AGAIN. Here is the whole story, as detected and reported
747 Here's a test program that should detect the problem:
750 x = (1 :: Bogus Char) :: Bogus Bool
752 The problem with [the attempted shortcut code] is that
756 is not a sufficient condition to be able to use the shortcut!
757 You also need to know that the type synonym actually USES all
758 its arguments. For example, consider the following type synonym
759 which does not use all its arguments.
763 If you ever tried unifying, say, (Bogus Char) with )Bogus Bool), the
764 unifier would blithely try to unify Char with Bool and would fail,
765 even though the expanded forms (both Int) should match. Similarly,
766 unifying (Bogus Char) with (Bogus t) would unnecessarily bind t to
769 ... You could explicitly test for the problem synonyms and mark them
770 somehow as needing expansion, perhaps also issuing a warning to the
773 Note [Deferred Unification]
774 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
775 We may encounter a unification ty1 ~ ty2 that cannot be performed syntactically,
776 and yet its consistency is undetermined. Previously, there was no way to still
777 make it consistent. So a mismatch error was issued.
779 Now these unfications are deferred until constraint simplification, where type
780 family instances and given equations may (or may not) establish the consistency.
781 Deferred unifications are of the form
784 where F is a type function and x is a type variable.
786 id :: x ~ y => x -> y
789 involves the unfication x = y. It is deferred until we bring into account the
790 context x ~ y to establish that it holds.
792 If available, we defer original types (rather than those where closed type
793 synonyms have already been expanded via tcCoreView). This is, as usual, to
794 improve error messages.
797 %************************************************************************
801 %************************************************************************
803 @uVar@ is called when at least one of the types being unified is a
804 variable. It does {\em not} assume that the variable is a fixed point
805 of the substitution; rather, notice that @uVar@ (defined below) nips
806 back into @uTys@ if it turns out that the variable is already bound.
809 uVar :: [EqOrigin] -> SwapFlag -> TcTyVar -> TcTauType -> TcM CoercionI
810 uVar origin swapped tv1 ty2
811 = do { traceTc "uVar" (vcat [ ppr origin
813 , ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1)
814 , nest 2 (ptext (sLit " ~ "))
815 , ppr ty2 <+> dcolon <+> ppr (typeKind ty2)])
816 ; details <- lookupTcTyVar tv1
818 Filled ty1 -> unSwap swapped (uType_np origin) ty1 ty2
819 Unfilled details1 -> uUnfilledVar origin swapped tv1 details1 ty2
823 uUnfilledVar :: [EqOrigin]
825 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
826 -> TcTauType -- Type 2
828 -- "Unfilled" means that the variable is definitely not a filled-in meta tyvar
829 -- It might be a skolem, or untouchable, or meta
831 uUnfilledVar origin swapped tv1 details1 (TyVarTy tv2)
832 | tv1 == tv2 -- Same type variable => no-op
833 = return (IdCo (mkTyVarTy tv1))
835 | otherwise -- Distinct type variables
836 = do { lookup2 <- lookupTcTyVar tv2
838 Filled ty2' -> uUnfilledVar origin swapped tv1 details1 ty2'
839 Unfilled details2 -> uUnfilledVars origin swapped tv1 details1 tv2 details2
842 uUnfilledVar origin swapped tv1 details1 non_var_ty2 -- ty2 is not a type variable
845 -> do { mb_ty2' <- checkTauTvUpdate tv1 non_var_ty2
847 Nothing -> do { traceTc "Occ/kind defer" (ppr tv1); defer }
848 Just ty2' -> updateMeta tv1 ref1 ty2'
851 _other -> do { traceTc "Skolem defer" (ppr tv1); defer } -- Skolems of all sorts
853 defer = unSwap swapped (uType_defer origin) (mkTyVarTy tv1) non_var_ty2
854 -- Occurs check or an untouchable: just defer
855 -- NB: occurs check isn't necessarily fatal:
856 -- eg tv1 occured in type family parameter
859 uUnfilledVars :: [EqOrigin]
861 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
862 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
864 -- Invarant: The type variables are distinct,
865 -- Neither is filled in yet
867 uUnfilledVars origin swapped tv1 details1 tv2 details2
868 = case (details1, details2) of
869 (MetaTv i1 ref1, MetaTv i2 ref2)
870 | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1 i1 i2
871 then updateMeta tv1 ref1 ty2
872 else updateMeta tv2 ref2 ty1
873 | k2_sub_k1 -> updateMeta tv1 ref1 ty2
875 (_, MetaTv _ ref2) | k1_sub_k2 -> updateMeta tv2 ref2 ty1
876 (MetaTv _ ref1, _) | k2_sub_k1 -> updateMeta tv1 ref1 ty2
878 (_, _) -> unSwap swapped (uType_defer origin) ty1 ty2
879 -- Defer for skolems of all sorts
883 k1_sub_k2 = k1 `isSubKind` k2
884 k2_sub_k1 = k2 `isSubKind` k1
888 nicer_to_update_tv1 _ (SigTv _) = True
889 nicer_to_update_tv1 (SigTv _) _ = False
890 nicer_to_update_tv1 _ _ = isSystemName (Var.varName tv1)
891 -- Try not to update SigTvs; and try to update sys-y type
892 -- variables in preference to ones gotten (say) by
893 -- instantiating a polymorphic function with a user-written
897 checkTauTvUpdate :: TcTyVar -> TcType -> TcM (Maybe TcType)
898 -- (checkTauTvUpdate tv ty)
899 -- We are about to update the TauTv tv with ty.
900 -- Check (a) that tv doesn't occur in ty (occurs check)
901 -- (b) that kind(ty) is a sub-kind of kind(tv)
902 -- (c) that ty does not contain any type families, see Note [Type family sharing]
904 -- We have two possible outcomes:
905 -- (1) Return the type to update the type variable with,
906 -- [we know the update is ok]
907 -- (2) Return Nothing,
908 -- [the update might be dodgy]
910 -- Note that "Nothing" does not mean "definite error". For example
912 -- type instance F Int = Int
915 -- This is perfectly reasonable, if we later get a ~ Int. For now, though,
916 -- we return Nothing, leaving it to the later constraint simplifier to
919 checkTauTvUpdate tv ty
920 = do { ty' <- zonkTcType ty
921 ; if typeKind ty' `isSubKind` tyVarKind tv then
923 Nothing -> return Nothing
924 Just ty'' -> return (Just ty'')
925 else return Nothing }
927 where ok :: TcType -> Maybe TcType
928 ok (TyVarTy tv') | not (tv == tv') = Just (TyVarTy tv')
929 ok this_ty@(TyConApp tc tys)
930 | not (isSynFamilyTyCon tc), Just tys' <- allMaybes (map ok tys)
931 = Just (TyConApp tc tys')
932 | isSynTyCon tc, Just ty_expanded <- tcView this_ty
933 = ok ty_expanded -- See Note [Type synonyms and the occur check]
934 ok (PredTy sty) | Just sty' <- ok_pred sty = Just (PredTy sty')
935 ok (FunTy arg res) | Just arg' <- ok arg, Just res' <- ok res
936 = Just (FunTy arg' res')
937 ok (AppTy fun arg) | Just fun' <- ok fun, Just arg' <- ok arg
938 = Just (AppTy fun' arg')
939 ok (ForAllTy tv1 ty1) | Just ty1' <- ok ty1 = Just (ForAllTy tv1 ty1')
943 ok_pred (IParam nm ty) | Just ty' <- ok ty = Just (IParam nm ty')
944 ok_pred (ClassP cl tys)
945 | Just tys' <- allMaybes (map ok tys)
946 = Just (ClassP cl tys')
947 ok_pred (EqPred ty1 ty2)
948 | Just ty1' <- ok ty1, Just ty2' <- ok ty2
949 = Just (EqPred ty1' ty2')
951 ok_pred _pty = Nothing
955 Note [Type synonyms and the occur check]
957 Generally speaking we need to update a variable with type synonyms not expanded, which
958 improves later error messages, except for when looking inside a type synonym may help resolve
959 a spurious occurs check error. Consider:
962 f :: (A a -> a -> ()) -> ()
968 We will eventually get a constraint of the form t ~ A t. The ok function above will
969 properly expand the type (A t) to just (), which is ok to be unified with t. If we had
970 unified with the original type A t, we would lead the type checker into an infinite loop.
972 Hence, if the occurs check fails for a type synonym application, then (and *only* then),
973 the ok function expands the synonym to detect opportunities for occurs check success using
974 the underlying definition of the type synonym.
976 The same applies later on in the constraint interaction code; see TcInteract,
977 function @occ_check_ok@.
980 Note [Type family sharing]
982 We must avoid eagerly unifying type variables to types that contain function symbols,
983 because this may lead to loss of sharing, and in turn, in very poor performance of the
984 constraint simplifier. Assume that we have a wanted constraint:
993 where D is some type class. If we eagerly unify m1 := [F m2], m2 := [F m3], m3 := [F m2],
994 then, after zonking, our constraint simplifier will be faced with the following wanted
1001 which has to be flattened by the constraint solver. However, because the sharing is lost,
1002 an polynomially larger number of flatten skolems will be created and the constraint sets
1003 we are working with will be polynomially larger.
1005 Instead, if we defer the unifications m1 := [F m2], etc. we will only be generating three
1006 flatten skolems, which is the maximum possible sharing arising from the original constraint.
1009 data LookupTyVarResult -- The result of a lookupTcTyVar call
1010 = Unfilled TcTyVarDetails -- SkolemTv or virgin MetaTv
1013 lookupTcTyVar :: TcTyVar -> TcM LookupTyVarResult
1015 | MetaTv _ ref <- details
1016 = do { meta_details <- readMutVar ref
1017 ; case meta_details of
1018 Indirect ty -> return (Filled ty)
1019 Flexi -> do { is_untch <- isUntouchable tyvar
1020 ; let -- Note [Unifying untouchables]
1021 ret_details | is_untch = SkolemTv UnkSkol
1022 | otherwise = details
1023 ; return (Unfilled ret_details) } }
1025 = return (Unfilled details)
1027 details = ASSERT2( isTcTyVar tyvar, ppr tyvar )
1028 tcTyVarDetails tyvar
1030 updateMeta :: TcTyVar -> TcRef MetaDetails -> TcType -> TcM CoercionI
1031 updateMeta tv1 ref1 ty2
1032 = do { writeMetaTyVarRef tv1 ref1 ty2
1033 ; return (IdCo ty2) }
1036 Note [Unifying untouchables]
1037 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1038 We treat an untouchable type variable as if it was a skolem. That
1039 ensures it won't unify with anything. It's a slight had, because
1040 we return a made-up TcTyVarDetails, but I think it works smoothly.
1043 %************************************************************************
1047 %************************************************************************
1050 pushOrigin :: TcType -> TcType -> [EqOrigin] -> [EqOrigin]
1051 pushOrigin ty_act ty_exp origin
1052 = UnifyOrigin { uo_actual = ty_act, uo_expected = ty_exp } : origin
1055 wrapEqCtxt :: [EqOrigin] -> TcM a -> TcM a
1056 -- Build a suitable error context from the origin and do the thing inside
1057 -- The "couldn't match" error comes from the innermost item on the stack,
1058 -- and, if there is more than one item, the "Expected/inferred" part
1059 -- comes from the outermost item
1060 wrapEqCtxt [] thing_inside = thing_inside
1061 wrapEqCtxt items thing_inside = addErrCtxtM (unifyCtxt (last items)) thing_inside
1064 failWithMisMatch :: [EqOrigin] -> TcM a
1065 -- Generate the message when two types fail to match,
1066 -- going to some trouble to make it helpful.
1067 -- We take the failing types from the top of the origin stack
1068 -- rather than reporting the particular ones we are looking
1070 failWithMisMatch (item:origin)
1071 = wrapEqCtxt origin $
1072 do { ty_act <- zonkTcType (uo_actual item)
1073 ; ty_exp <- zonkTcType (uo_expected item)
1074 ; env0 <- tcInitTidyEnv
1075 ; let (env1, pp_exp) = tidyOpenType env0 ty_exp
1076 (env2, pp_act) = tidyOpenType env1 ty_act
1077 ; failWithTcM (misMatchMsg env2 pp_act pp_exp) }
1079 = panic "failWithMisMatch"
1081 misMatchMsg :: TidyEnv -> TcType -> TcType -> (TidyEnv, SDoc)
1082 misMatchMsg env ty_act ty_exp
1083 = (env2, sep [sep [ ptext (sLit "Couldn't match expected type") <+> quotes (ppr ty_exp)
1084 , nest 12 $ ptext (sLit "with actual type") <+> quotes (ppr ty_act)]
1085 , nest 2 (extra1 $$ extra2) ])
1087 (env1, extra1) = typeExtraInfoMsg env ty_exp
1088 (env2, extra2) = typeExtraInfoMsg env1 ty_act
1092 -----------------------------------------
1094 -----------------------------------------
1098 -- If an error happens we try to figure out whether the function
1099 -- function has been given too many or too few arguments, and say so.
1100 addSubCtxt :: InstOrigin -> TcType -> TcType -> TcM a -> TcM a
1101 addSubCtxt orig actual_res_ty expected_res_ty thing_inside
1102 = addErrCtxtM mk_err thing_inside
1105 = do { exp_ty' <- zonkTcType expected_res_ty
1106 ; act_ty' <- zonkTcType actual_res_ty
1107 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1108 (env2, act_ty'') = tidyOpenType env1 act_ty'
1109 (exp_args, _) = tcSplitFunTys exp_ty''
1110 (act_args, _) = tcSplitFunTys act_ty''
1112 len_act_args = length act_args
1113 len_exp_args = length exp_args
1115 message = case orig of
1117 | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1118 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1119 _ -> mkExpectedActualMsg act_ty'' exp_ty''
1120 ; return (env2, message) }
1123 %************************************************************************
1127 %************************************************************************
1129 Unifying kinds is much, much simpler than unifying types.
1132 matchExpectedFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1133 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1135 matchExpectedFunKind (TyVarTy kvar) = do
1136 maybe_kind <- readKindVar kvar
1138 Indirect fun_kind -> matchExpectedFunKind fun_kind
1140 do { arg_kind <- newKindVar
1141 ; res_kind <- newKindVar
1142 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1143 ; return (Just (arg_kind,res_kind)) }
1145 matchExpectedFunKind (FunTy arg_kind res_kind) = return (Just (arg_kind,res_kind))
1146 matchExpectedFunKind _ = return Nothing
1149 unifyKind :: TcKind -- Expected
1153 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1154 | isSubKindCon kc2 kc1 = return ()
1156 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1157 = do { unifyKind a2 a1; unifyKind r1 r2 }
1158 -- Notice the flip in the argument,
1159 -- so that the sub-kinding works right
1160 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1161 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1162 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1165 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1166 uKVar swapped kv1 k2
1167 = do { mb_k1 <- readKindVar kv1
1169 Flexi -> uUnboundKVar swapped kv1 k2
1170 Indirect k1 | swapped -> unifyKind k2 k1
1171 | otherwise -> unifyKind k1 k2 }
1174 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1175 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1176 | kv1 == kv2 = return ()
1177 | otherwise -- Distinct kind variables
1178 = do { mb_k2 <- readKindVar kv2
1180 Indirect k2 -> uUnboundKVar swapped kv1 k2
1181 Flexi -> writeKindVar kv1 k2 }
1183 uUnboundKVar swapped kv1 non_var_k2
1184 = do { k2' <- zonkTcKind non_var_k2
1185 ; kindOccurCheck kv1 k2'
1186 ; k2'' <- kindSimpleKind swapped k2'
1187 -- KindVars must be bound only to simple kinds
1188 -- Polarities: (kindSimpleKind True ?) succeeds
1189 -- returning *, corresponding to unifying
1192 ; writeKindVar kv1 k2'' }
1195 kindOccurCheck :: TyVar -> Type -> TcM ()
1196 kindOccurCheck kv1 k2 -- k2 is zonked
1197 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1199 not_in (TyVarTy kv2) = kv1 /= kv2
1200 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1203 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1204 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1205 -- If the flag is False, it requires k <: sk
1206 -- E.g. kindSimpleKind False ?? = *
1207 -- What about (kv -> *) ~ ?? -> *
1208 kindSimpleKind orig_swapped orig_kind
1209 = go orig_swapped orig_kind
1211 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1213 ; return (mkArrowKind k1' k2') }
1215 | isOpenTypeKind k = return liftedTypeKind
1216 | isArgTypeKind k = return liftedTypeKind
1218 | isLiftedTypeKind k = return liftedTypeKind
1219 | isUnliftedTypeKind k = return unliftedTypeKind
1220 go _ k@(TyVarTy _) = return k -- KindVars are always simple
1221 go _ _ = failWithTc (ptext (sLit "Unexpected kind unification failure:")
1222 <+> ppr orig_swapped <+> ppr orig_kind)
1223 -- I think this can't actually happen
1225 -- T v = MkT v v must be a type
1226 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1228 unifyKindMisMatch :: TcKind -> TcKind -> TcM ()
1229 unifyKindMisMatch ty1 ty2 = do
1230 ty1' <- zonkTcKind ty1
1231 ty2' <- zonkTcKind ty2
1233 msg = hang (ptext (sLit "Couldn't match kind"))
1234 2 (sep [quotes (ppr ty1'),
1235 ptext (sLit "against"),
1240 kindOccurCheckErr :: Var -> Type -> SDoc
1241 kindOccurCheckErr tyvar ty
1242 = hang (ptext (sLit "Occurs check: cannot construct the infinite kind:"))
1243 2 (sep [ppr tyvar, char '=', ppr ty])
1246 %************************************************************************
1248 \subsection{Checking signature type variables}
1250 %************************************************************************
1252 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1253 are not mentioned in the environment. In particular:
1255 (a) Not mentioned in the type of a variable in the envt
1256 eg the signature for f in this:
1262 Here, f is forced to be monorphic by the free occurence of x.
1264 (d) Not (unified with another type variable that is) in scope.
1265 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1266 when checking the expression type signature, we find that
1267 even though there is nothing in scope whose type mentions r,
1268 nevertheless the type signature for the expression isn't right.
1270 Another example is in a class or instance declaration:
1272 op :: forall b. a -> b
1274 Here, b gets unified with a
1276 Before doing this, the substitution is applied to the signature type variable.
1279 checkSigTyVars :: [TcTyVar] -> TcM ()
1280 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1282 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1283 -- The extra_tvs can include boxy type variables;
1284 -- e.g. TcMatches.tcCheckExistentialPat
1285 checkSigTyVarsWrt extra_tvs sig_tvs
1286 = do { extra_tvs' <- zonkTcTyVarsAndFV extra_tvs
1287 ; check_sig_tyvars extra_tvs' sig_tvs }
1290 :: TcTyVarSet -- Global type variables. The universally quantified
1291 -- tyvars should not mention any of these
1292 -- Guaranteed already zonked.
1293 -> [TcTyVar] -- Universally-quantified type variables in the signature
1294 -- Guaranteed to be skolems
1296 check_sig_tyvars _ []
1298 check_sig_tyvars extra_tvs sig_tvs
1299 = ASSERT( all isTcTyVar sig_tvs && all isSkolemTyVar sig_tvs )
1300 do { gbl_tvs <- tcGetGlobalTyVars
1301 ; traceTc "check_sig_tyvars" $ vcat
1302 [ text "sig_tys" <+> ppr sig_tvs
1303 , text "gbl_tvs" <+> ppr gbl_tvs
1304 , text "extra_tvs" <+> ppr extra_tvs]
1306 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1307 ; when (any (`elemVarSet` env_tvs) sig_tvs)
1308 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1311 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1312 -> [TcTyVar] -- The possibly-escaping type variables
1313 -> [TcTyVar] -- The zonked versions thereof
1315 -- Complain about escaping type variables
1316 -- We pass a list of type variables, at least one of which
1317 -- escapes. The first list contains the original signature type variable,
1318 -- while the second contains the type variable it is unified to (usually itself)
1319 bleatEscapedTvs globals sig_tvs zonked_tvs
1320 = do { env0 <- tcInitTidyEnv
1321 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1322 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1324 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1325 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1327 main_msg = ptext (sLit "Inferred type is less polymorphic than expected")
1329 check (tidy_env, msgs) (sig_tv, zonked_tv)
1330 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1332 = do { lcl_env <- getLclTypeEnv
1333 ; (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) lcl_env tidy_env
1334 ; return (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1336 -----------------------
1337 escape_msg :: Var -> Var -> [SDoc] -> SDoc
1338 escape_msg sig_tv zonked_tv globs
1340 = vcat [sep [msg, ptext (sLit "is mentioned in the environment:")],
1341 nest 2 (vcat globs)]
1343 = msg <+> ptext (sLit "escapes")
1344 -- Sigh. It's really hard to give a good error message
1345 -- all the time. One bad case is an existential pattern match.
1346 -- We rely on the "When..." context to help.
1348 msg = ptext (sLit "Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1350 | sig_tv == zonked_tv = empty
1351 | otherwise = ptext (sLit "is unified with") <+> quotes (ppr zonked_tv) <+> ptext (sLit "which")
1354 These two context are used with checkSigTyVars
1357 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1358 -> TidyEnv -> TcM (TidyEnv, Message)
1359 sigCtxt id sig_tvs sig_theta sig_tau tidy_env = do
1360 actual_tau <- zonkTcType sig_tau
1362 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1363 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1364 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1365 sub_msg = vcat [ptext (sLit "Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1366 ptext (sLit "Type to generalise:") <+> pprType tidy_actual_tau
1368 msg = vcat [ptext (sLit "When trying to generalise the type inferred for") <+> quotes (ppr id),