2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section{Type subsumption and unification}
8 -- Full-blown subsumption
9 tcSubExp, tcFunResTy, tcGen,
10 checkSigTyVars, checkSigTyVarsWrt, bleatEscapedTvs, sigCtxt,
12 -- Various unifications
13 unifyType, unifyTypeList, unifyTheta,
14 unifyKind, unifyKinds, unifyFunKind,
16 preSubType, boxyMatchTypes,
18 --------------------------------
20 tcInfer, subFunTys, unBox, stripBoxyType, withBox,
21 boxyUnify, boxyUnifyList, zapToMonotype,
22 boxySplitListTy, boxySplitTyConApp, boxySplitAppTy,
26 #include "HsVersions.h"
28 import HsSyn ( ExprCoFn(..), idCoercion, isIdCoercion, (<.>) )
29 import TypeRep ( Type(..), PredType(..) )
31 import TcMType ( lookupTcTyVar, LookupTyVarResult(..),
32 tcInstSkolType, tcInstBoxyTyVar, newKindVar, newMetaTyVar,
33 newBoxyTyVar, newBoxyTyVarTys, readFilledBox,
34 readMetaTyVar, writeMetaTyVar, newFlexiTyVarTy,
35 tcInstSkolTyVars, tcInstTyVar,
36 zonkTcKind, zonkType, zonkTcType, zonkTcTyVarsAndFV,
37 readKindVar, writeKindVar )
38 import TcSimplify ( tcSimplifyCheck )
39 import TcEnv ( tcGetGlobalTyVars, findGlobals )
40 import TcIface ( checkWiredInTyCon )
41 import TcRnMonad -- TcType, amongst others
42 import TcType ( TcKind, TcType, TcTyVar, BoxyTyVar, TcTauType,
43 BoxySigmaType, BoxyRhoType, BoxyType,
44 TcTyVarSet, TcThetaType, TcTyVarDetails(..), BoxInfo(..),
45 SkolemInfo( GenSkol, UnkSkol ), MetaDetails(..), isImmutableTyVar,
46 pprSkolTvBinding, isTauTy, isTauTyCon, isSigmaTy,
47 mkFunTy, mkFunTys, mkTyConApp, isMetaTyVar,
48 tcSplitForAllTys, tcSplitAppTy_maybe, tcSplitFunTys, mkTyVarTys,
49 tcSplitSigmaTy, tyVarsOfType, mkPhiTy, mkTyVarTy, mkPredTy,
50 typeKind, mkForAllTys, mkAppTy, isBoxyTyVar,
52 tidyOpenType, tidyOpenTyVar, tidyOpenTyVars,
53 pprType, tidyKind, tidySkolemTyVar, isSkolemTyVar, tcView,
54 TvSubst, mkTvSubst, zipTyEnv, zipOpenTvSubst, emptyTvSubst,
56 lookupTyVar, extendTvSubst )
57 import Kind ( Kind(..), SimpleKind, KindVar, isArgTypeKind,
58 openTypeKind, liftedTypeKind, unliftedTypeKind,
59 mkArrowKind, defaultKind,
60 isOpenTypeKind, argTypeKind, isLiftedTypeKind, isUnliftedTypeKind,
61 isSubKind, pprKind, splitKindFunTys )
62 import TysPrim ( alphaTy, betaTy )
63 import Inst ( newDicts, instToId )
64 import TyCon ( TyCon, tyConArity, tyConTyVars, isSynTyCon )
65 import TysWiredIn ( listTyCon )
66 import Id ( Id, mkSysLocal )
67 import Var ( Var, varName, tyVarKind, isTcTyVar, tcTyVarDetails )
68 import VarSet ( emptyVarSet, mkVarSet, unitVarSet, unionVarSet, elemVarSet, varSetElems,
69 extendVarSet, intersectsVarSet )
71 import Name ( Name, isSystemName )
72 import ErrUtils ( Message )
73 import Maybes ( expectJust, isNothing )
74 import BasicTypes ( Arity )
75 import UniqSupply ( uniqsFromSupply )
76 import Util ( notNull, equalLength )
81 import TcType ( isBoxyTy, isFlexi )
85 %************************************************************************
87 \subsection{'hole' type variables}
89 %************************************************************************
92 tcInfer :: (BoxyType -> TcM a) -> TcM (a, TcType)
94 = do { box <- newBoxyTyVar openTypeKind
95 ; res <- tc_infer (mkTyVarTy box)
96 ; res_ty <- readFilledBox box -- Guaranteed filled-in by now
97 ; return (res, res_ty) }
101 %************************************************************************
105 %************************************************************************
108 subFunTys :: SDoc -- Somthing like "The function f has 3 arguments"
109 -- or "The abstraction (\x.e) takes 1 argument"
110 -> Arity -- Expected # of args
111 -> BoxyRhoType -- res_ty
112 -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
114 -- Attempt to decompse res_ty to have enough top-level arrows to
115 -- match the number of patterns in the match group
117 -- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
118 -- and the inner call to thing_inside passes args: [a1,...,an], b
119 -- then co_fn :: (a1 -> ... -> an -> b) -> res_ty
121 -- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
124 {- Error messages from subFunTys
126 The abstraction `\Just 1 -> ...' has two arguments
127 but its type `Maybe a -> a' has only one
129 The equation(s) for `f' have two arguments
130 but its type `Maybe a -> a' has only one
132 The section `(f 3)' requires 'f' to take two arguments
133 but its type `Int -> Int' has only one
135 The function 'f' is applied to two arguments
136 but its type `Int -> Int' has only one
140 subFunTys error_herald n_pats res_ty thing_inside
141 = loop n_pats [] res_ty
143 -- In 'loop', the parameter 'arg_tys' accumulates
144 -- the arg types so far, in *reverse order*
145 loop n args_so_far res_ty
146 | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
148 loop n args_so_far res_ty
149 | isSigmaTy res_ty -- Do this before checking n==0, because we
150 -- guarantee to return a BoxyRhoType, not a BoxySigmaType
151 = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet $ \ res_ty' ->
152 loop n args_so_far res_ty'
153 ; return (gen_fn <.> co_fn, res) }
155 loop 0 args_so_far res_ty
156 = do { res <- thing_inside (reverse args_so_far) res_ty
157 ; return (idCoercion, res) }
159 loop n args_so_far (FunTy arg_ty res_ty)
160 = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
161 ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
162 ; return (co_fn', res) }
164 -- res_ty might have a type variable at the head, such as (a b c),
165 -- in which case we must fill in with (->). Simplest thing to do
166 -- is to use boxyUnify, but we catch failure and generate our own
167 -- error message on failure
168 loop n args_so_far res_ty@(AppTy _ _)
169 = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
170 ; (_, mb_unit) <- tryTcErrs $ boxyUnify res_ty (FunTy arg_ty' res_ty')
171 ; if isNothing mb_unit then bale_out args_so_far
172 else loop n args_so_far (FunTy arg_ty' res_ty') }
174 loop n args_so_far (TyVarTy tv)
175 | not (isImmutableTyVar tv)
176 = do { cts <- readMetaTyVar tv
178 Indirect ty -> loop n args_so_far ty
179 Flexi -> do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
180 ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty
181 ; return (idCoercion, res) } }
183 mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
184 kinds = openTypeKind : take n (repeat argTypeKind)
185 -- Note argTypeKind: the args can have an unboxed type,
186 -- but not an unboxed tuple.
188 loop n args_so_far res_ty = bale_out args_so_far
191 = do { env0 <- tcInitTidyEnv
192 ; res_ty' <- zonkTcType res_ty
193 ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
194 ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
196 mk_msg res_ty n_actual
197 = error_herald <> comma $$
198 sep [ptext SLIT("but its type") <+> quotes (pprType res_ty),
199 if n_actual == 0 then ptext SLIT("has none")
200 else ptext SLIT("has only") <+> speakN n_actual]
204 ----------------------
205 boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
206 -> BoxyRhoType -- Expected type (T a b c)
207 -> TcM [BoxySigmaType] -- Element types, a b c
208 -- It's used for wired-in tycons, so we call checkWiredInTyCOn
209 -- Precondition: never called with FunTyCon
210 -- Precondition: input type :: *
212 boxySplitTyConApp tc orig_ty
213 = do { checkWiredInTyCon tc
214 ; loop (tyConArity tc) [] orig_ty }
216 loop n_req args_so_far ty
217 | Just ty' <- tcView ty = loop n_req args_so_far ty'
219 loop n_req args_so_far (TyConApp tycon args)
221 = ASSERT( n_req == length args) -- ty::*
222 return (args ++ args_so_far)
224 loop n_req args_so_far (AppTy fun arg)
225 = loop (n_req - 1) (arg:args_so_far) fun
227 loop n_req args_so_far (TyVarTy tv)
228 | not (isImmutableTyVar tv)
229 = do { cts <- readMetaTyVar tv
231 Indirect ty -> loop n_req args_so_far ty
232 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
233 ; return (arg_tys ++ args_so_far) }
236 mk_res_ty arg_tys' = mkTyConApp tc arg_tys'
237 arg_kinds = map tyVarKind (take n_req (tyConTyVars tc))
239 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc))) orig_ty
241 ----------------------
242 boxySplitListTy :: BoxyRhoType -> TcM BoxySigmaType -- Special case for lists
243 boxySplitListTy exp_ty = do { [elt_ty] <- boxySplitTyConApp listTyCon exp_ty
247 ----------------------
248 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
249 -> TcM (BoxySigmaType, BoxySigmaType) -- Returns m, a
250 -- Assumes (m: * -> k), where k is the kind of the incoming type
251 -- If the incoming type is boxy, then so are the result types; and vice versa
253 boxySplitAppTy orig_ty
257 | Just ty' <- tcView ty = loop ty'
260 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
261 = return (fun_ty, arg_ty)
264 | not (isImmutableTyVar tv)
265 = do { cts <- readMetaTyVar tv
267 Indirect ty -> loop ty
268 Flexi -> do { [fun_ty,arg_ty] <- withMetaTvs tv kinds mk_res_ty
269 ; return (fun_ty, arg_ty) } }
271 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
272 tv_kind = tyVarKind tv
273 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
275 liftedTypeKind] -- arg type :: *
276 -- The defaultKind is a bit smelly. If you remove it,
277 -- try compiling f x = do { x }
278 -- and you'll get a kind mis-match. It smells, but
279 -- not enough to lose sleep over.
281 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
284 boxySplitFailure actual_ty expected_ty
285 = unifyMisMatch False False actual_ty expected_ty
286 -- "outer" is False, so we don't pop the context
287 -- which is what we want since we have not pushed one!
291 --------------------------------
292 -- withBoxes: the key utility function
293 --------------------------------
296 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
297 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
298 -> ([BoxySigmaType] -> BoxySigmaType)
299 -- Constructs the type to assign
300 -- to the original var
301 -> TcM [BoxySigmaType] -- Return the fresh boxes
303 -- It's entirely possible for the [kind] to be empty.
304 -- For example, when pattern-matching on True,
305 -- we call boxySplitTyConApp passing a boolTyCon
307 -- Invariant: tv is still Flexi
309 withMetaTvs tv kinds mk_res_ty
311 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
312 ; let box_tys = mkTyVarTys box_tvs
313 ; writeMetaTyVar tv (mk_res_ty box_tys)
316 | otherwise -- Non-boxy meta type variable
317 = do { tau_tys <- mapM newFlexiTyVarTy kinds
318 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
319 -- Sure to be a tau-type
322 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
323 -- Allocate a *boxy* tyvar
324 withBox kind thing_inside
325 = do { box_tv <- newMetaTyVar BoxTv kind
326 ; res <- thing_inside (mkTyVarTy box_tv)
327 ; ty <- readFilledBox box_tv
332 %************************************************************************
334 Approximate boxy matching
336 %************************************************************************
339 preSubType :: [TcTyVar] -- Quantified type variables
340 -> TcTyVarSet -- Subset of quantified type variables
341 -- that can be instantiated with boxy types
342 -> TcType -- The rho-type part; quantified tyvars scopes over this
343 -> BoxySigmaType -- Matching type from the context
344 -> TcM [TcType] -- Types to instantiate the tyvars
345 -- Perform pre-subsumption, and return suitable types
346 -- to instantiate the quantified type varibles:
347 -- info from the pre-subsumption, if there is any
348 -- a boxy type variable otherwise
350 -- The 'btvs' are a subset of 'qtvs'. They are the ones we can
351 -- instantiate to a boxy type variable, because they'll definitely be
352 -- filled in later. This isn't always the case; sometimes we have type
353 -- variables mentioned in the context of the type, but not the body;
354 -- f :: forall a b. C a b => a -> a
355 -- Then we may land up with an unconstrained 'b', so we want to
356 -- instantiate it to a monotype (non-boxy) type variable
358 preSubType qtvs btvs qty expected_ty
361 pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
363 | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
364 | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
365 ; return (mkTyVarTy tv') }
366 | otherwise = do { tv' <- tcInstTyVar tv
367 ; return (mkTyVarTy tv') }
370 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
371 -> BoxyRhoType -- Type to match (note a *Rho* type)
372 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
374 -- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
375 -- "Boxy types: inference for higher rank types and impredicativity"
377 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
378 = go tmpl_ty emptyVarSet boxy_ty
381 | Just t_ty' <- tcView t_ty = go t_ty' b_tvs b_ty
382 | Just b_ty' <- tcView b_ty = go t_ty b_tvs b_ty'
384 go (TyVarTy _) b_tvs b_ty = emptyTvSubst -- Rule S-ANY; no bindings
385 -- Rule S-ANY covers (a) type variables and (b) boxy types
386 -- in the template. Both look like a TyVarTy.
387 -- See Note [Sub-match] below
389 go (ForAllTy tv t_ty) b_tvs b_ty = go t_ty b_tvs b_ty -- Rule S-SPEC
390 go t_ty b_tvs (ForAllTy tv b_ty) = go t_ty b_tvs' b_ty -- Rule S-SKOL
391 where b_tvs' = extendVarSet b_tvs tv
393 go (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
394 = boxy_match tmpl_tvs arg1 b_tvs arg2 (go res1 b_tvs res2)
395 -- Match the args, and sub-match the results
397 go t_ty b_tvs b_ty = boxy_match tmpl_tvs t_ty b_tvs b_ty emptyTvSubst
398 -- Otherwise defer to boxy matching
399 -- This covers TyConApp, AppTy, PredTy
406 |- head xs : <rhobox>
407 We will do a boxySubMatchType between a ~ <rhobox>
408 But we *don't* want to match [a |-> <rhobox>] because
409 (a) The box should be filled in with a rho-type, but
410 but the returned substitution maps TyVars to boxy
412 (b) In any case, the right final answer might be *either*
413 instantiate 'a' with a rho-type or a sigma type
414 head xs : Int vs head xs : forall b. b->b
415 So the matcher MUST NOT make a choice here. In general, we only
416 bind a template type variable in boxyMatchType, not in boxySubMatchType.
421 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
422 -> [BoxySigmaType] -- Type to match
423 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
425 -- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
426 -- "Boxy types: inference for higher rank types and impredicativity"
428 -- Find a *boxy* substitution that makes the template look as much
429 -- like the BoxySigmaType as possible.
430 -- It's always ok to return an empty substitution;
431 -- anything more is jam on the pudding
433 -- NB1: This is a pure, non-monadic function.
434 -- It does no unification, and cannot fail
436 -- Precondition: the arg lengths are equal
437 -- Precondition: none of the template type variables appear in the [BoxySigmaType]
438 -- Precondition: any nested quantifiers in either type differ from
439 -- the template type variables passed as arguments
443 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
444 = ASSERT( length tmpl_tys == length boxy_tys )
445 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
446 -- ToDo: add error context?
448 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
450 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
451 = boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys $
452 boxy_match tmpl_tvs t_ty boxy_tvs b_ty subst
455 boxy_match :: TcTyVarSet -> TcType -- Template
456 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
457 -> BoxySigmaType -- Match against this type
461 -- The boxy_tvs argument prevents this match:
462 -- [a] forall b. a ~ forall b. b
463 -- We don't want to bind the template variable 'a'
464 -- to the quantified type variable 'b'!
466 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
467 = go orig_tmpl_ty orig_boxy_ty
470 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
471 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
473 go (ForAllTy _ ty1) (ForAllTy tv2 ty2)
474 = boxy_match tmpl_tvs ty1 (boxy_tvs `extendVarSet` tv2) ty2 subst
476 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
477 | tc1 == tc2 = go_s tys1 tys2
479 go (FunTy arg1 res1) (FunTy arg2 res2)
480 = go_s [arg1,res1] [arg2,res2]
483 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
484 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
485 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
486 = go_s [s1,t1] [s2,t2]
489 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
490 , not (intersectsVarSet boxy_tvs (tyVarsOfType orig_boxy_ty))
491 , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
492 = extendTvSubst subst tv boxy_ty'
494 boxy_ty' = case lookupTyVar subst tv of
495 Nothing -> orig_boxy_ty
496 Just ty -> ty `boxyLub` orig_boxy_ty
498 go _ _ = subst -- Always safe
501 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
504 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
505 -- Combine boxy information from the two types
506 -- If there is a conflict, return the first
507 boxyLub orig_ty1 orig_ty2
508 = go orig_ty1 orig_ty2
510 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
511 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
512 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
513 | tc1 == tc2, length ts1 == length ts2
514 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
516 go (TyVarTy tv1) ty2 -- This is the whole point;
517 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
520 -- Look inside type synonyms, but only if the naive version fails
521 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
522 | Just ty2' <- tcView ty1 = go ty1 ty2'
524 -- For now, we don't look inside ForAlls, PredTys
525 go ty1 ty2 = orig_ty1 -- Default
528 Note [Matching kinds]
529 ~~~~~~~~~~~~~~~~~~~~~
530 The target type might legitimately not be a sub-kind of template.
531 For example, suppose the target is simply a box with an OpenTypeKind,
532 and the template is a type variable with LiftedTypeKind.
533 Then it's ok (because the target type will later be refined).
534 We simply don't bind the template type variable.
536 It might also be that the kind mis-match is an error. For example,
537 suppose we match the template (a -> Int) against (Int# -> Int),
538 where the template type variable 'a' has LiftedTypeKind. This
539 matching function does not fail; it simply doesn't bind the template.
540 Later stuff will fail.
542 %************************************************************************
546 %************************************************************************
548 All the tcSub calls have the form
550 tcSub expected_ty offered_ty
552 offered_ty <= expected_ty
554 That is, that a value of type offered_ty is acceptable in
555 a place expecting a value of type expected_ty.
557 It returns a coercion function
558 co_fn :: offered_ty -> expected_ty
559 which takes an HsExpr of type offered_ty into one of type
564 tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM ExprCoFn -- Locally used only
565 -- (tcSub act exp) checks that
567 tcSubExp actual_ty expected_ty
568 = addErrCtxtM (unifyCtxt actual_ty expected_ty)
569 (tc_sub True actual_ty actual_ty expected_ty expected_ty)
571 tcFunResTy :: Name -> BoxySigmaType -> BoxySigmaType -> TcM ExprCoFn -- Locally used only
572 tcFunResTy fun actual_ty expected_ty
573 = addErrCtxtM (checkFunResCtxt fun actual_ty expected_ty) $
574 (tc_sub True actual_ty actual_ty expected_ty expected_ty)
577 tc_sub :: Outer -- See comments with uTys
578 -> BoxySigmaType -- actual_ty, before expanding synonyms
579 -> BoxySigmaType -- ..and after
580 -> BoxySigmaType -- expected_ty, before
581 -> BoxySigmaType -- ..and after
584 tc_sub outer act_sty act_ty exp_sty exp_ty
585 | Just exp_ty' <- tcView exp_ty = tc_sub False act_sty act_ty exp_sty exp_ty'
586 tc_sub outer act_sty act_ty exp_sty exp_ty
587 | Just act_ty' <- tcView act_ty = tc_sub False act_sty act_ty' exp_sty exp_ty
589 -----------------------------------
590 -- Rule SBOXY, plus other cases when act_ty is a type variable
591 -- Just defer to boxy matching
592 -- This rule takes precedence over SKOL!
593 tc_sub outer act_sty (TyVarTy tv) exp_sty exp_ty
594 = do { uVar outer False tv False exp_sty exp_ty
595 ; return idCoercion }
597 -----------------------------------
598 -- Skolemisation case (rule SKOL)
599 -- actual_ty: d:Eq b => b->b
600 -- expected_ty: forall a. Ord a => a->a
601 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
603 -- It is essential to do this *before* the specialisation case
604 -- Example: f :: (Eq a => a->a) -> ...
605 -- g :: Ord b => b->b
608 tc_sub outer act_sty act_ty exp_sty exp_ty
610 = do { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ body_exp_ty ->
611 tc_sub False act_sty act_ty body_exp_ty body_exp_ty
612 ; return (gen_fn <.> co_fn) }
614 act_tvs = tyVarsOfType act_ty
615 -- It's really important to check for escape wrt
616 -- the free vars of both expected_ty *and* actual_ty
618 -----------------------------------
619 -- Specialisation case (rule ASPEC):
620 -- actual_ty: forall a. Ord a => a->a
621 -- expected_ty: Int -> Int
622 -- co_fn e = e Int dOrdInt
624 tc_sub outer act_sty actual_ty exp_sty expected_ty
625 -- Implements the new SPEC rule in the Appendix of the paper
626 -- "Boxy types: inference for higher rank types and impredicativity"
627 -- (This appendix isn't in the published version.)
628 -- The idea is to *first* do pre-subsumption, and then full subsumption
629 -- Example: forall a. a->a <= Int -> (forall b. Int)
630 -- Pre-subsumpion finds a|->Int, and that works fine, whereas
631 -- just running full subsumption would fail.
632 | isSigmaTy actual_ty
633 = do { -- Perform pre-subsumption, and instantiate
634 -- the type with info from the pre-subsumption;
635 -- boxy tyvars if pre-subsumption gives no info
636 let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
637 tau_tvs = exactTyVarsOfType tau
638 ; inst_tys <- preSubType tyvars tau_tvs tau expected_ty
639 ; let subst' = zipOpenTvSubst tyvars inst_tys
640 tau' = substTy subst' tau
642 -- Perform a full subsumption check
643 ; co_fn <- tc_sub False tau' tau' exp_sty expected_ty
645 -- Deal with the dictionaries
646 ; dicts <- newDicts InstSigOrigin (substTheta subst' theta)
648 ; let inst_fn = CoApps (CoTyApps CoHole inst_tys)
650 ; return (co_fn <.> inst_fn) }
652 -----------------------------------
653 -- Function case (rule F1)
654 tc_sub _ _ (FunTy act_arg act_res) _ (FunTy exp_arg exp_res)
655 = tc_sub_funs act_arg act_res exp_arg exp_res
657 -- Function case (rule F2)
658 tc_sub outer act_sty act_ty@(FunTy act_arg act_res) exp_sty (TyVarTy exp_tv)
660 = do { cts <- readMetaTyVar exp_tv
662 Indirect ty -> do { u_tys outer False act_sty act_ty True exp_sty ty
663 ; return idCoercion }
664 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
665 ; tc_sub_funs act_arg act_res arg_ty res_ty } }
667 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
668 fun_kinds = [argTypeKind, openTypeKind]
670 -- Everything else: defer to boxy matching
671 tc_sub outer act_sty actual_ty exp_sty expected_ty
672 = do { u_tys outer False act_sty actual_ty False exp_sty expected_ty
673 ; return idCoercion }
676 -----------------------------------
677 tc_sub_funs act_arg act_res exp_arg exp_res
678 = do { uTys False act_arg False exp_arg
679 ; co_fn_res <- tc_sub False act_res act_res exp_res exp_res
680 ; wrapFunResCoercion [exp_arg] co_fn_res }
682 -----------------------------------
684 :: [TcType] -- Type of args
685 -> ExprCoFn -- HsExpr a -> HsExpr b
686 -> TcM ExprCoFn -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
687 wrapFunResCoercion arg_tys co_fn_res
688 | isIdCoercion co_fn_res = return idCoercion
689 | null arg_tys = return co_fn_res
691 = do { us <- newUniqueSupply
692 ; let arg_ids = zipWith (mkSysLocal FSLIT("sub")) (uniqsFromSupply us) arg_tys
693 ; return (CoLams arg_ids (co_fn_res <.> (CoApps CoHole arg_ids))) }
698 %************************************************************************
700 \subsection{Generalisation}
702 %************************************************************************
705 tcGen :: BoxySigmaType -- expected_ty
706 -> TcTyVarSet -- Extra tyvars that the universally
707 -- quantified tyvars of expected_ty
708 -- must not be unified
709 -> (BoxyRhoType -> TcM result) -- spec_ty
710 -> TcM (ExprCoFn, result)
711 -- The expression has type: spec_ty -> expected_ty
713 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
714 -- If not, the call is a no-op
715 = do { -- We want the GenSkol info in the skolemised type variables to
716 -- mention the *instantiated* tyvar names, so that we get a
717 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
718 -- Hence the tiresome but innocuous fixM
719 ((forall_tvs, theta, rho_ty), skol_info) <- fixM (\ ~(_, skol_info) ->
720 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
721 ; span <- getSrcSpanM
722 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty) span
723 ; return ((forall_tvs, theta, rho_ty), skol_info) })
726 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
727 text "expected_ty" <+> ppr expected_ty,
728 text "inst ty" <+> ppr forall_tvs <+> ppr theta <+> ppr rho_ty,
729 text "free_tvs" <+> ppr free_tvs,
730 text "forall_tvs" <+> ppr forall_tvs])
733 -- Type-check the arg and unify with poly type
734 ; (result, lie) <- getLIE (thing_inside rho_ty)
736 -- Check that the "forall_tvs" havn't been constrained
737 -- The interesting bit here is that we must include the free variables
738 -- of the expected_ty. Here's an example:
739 -- runST (newVar True)
740 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
741 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
742 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
743 -- So now s' isn't unconstrained because it's linked to a.
744 -- Conclusion: include the free vars of the expected_ty in the
745 -- list of "free vars" for the signature check.
747 ; dicts <- newDicts (SigOrigin skol_info) theta
748 ; inst_binds <- tcSimplifyCheck sig_msg forall_tvs dicts lie
750 ; checkSigTyVarsWrt free_tvs forall_tvs
751 ; traceTc (text "tcGen:done")
754 -- This HsLet binds any Insts which came out of the simplification.
755 -- It's a bit out of place here, but using AbsBind involves inventing
756 -- a couple of new names which seems worse.
757 dict_ids = map instToId dicts
758 co_fn = CoTyLams forall_tvs $ CoLams dict_ids $ CoLet inst_binds CoHole
759 ; returnM (co_fn, result) }
761 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
762 sig_msg = ptext SLIT("expected type of an expression")
767 %************************************************************************
771 %************************************************************************
773 The exported functions are all defined as versions of some
774 non-exported generic functions.
777 boxyUnify :: BoxyType -> BoxyType -> TcM ()
778 -- Acutal and expected, respectively
780 = addErrCtxtM (unifyCtxt ty1 ty2) $
781 uTysOuter False ty1 False ty2
784 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM ()
785 -- Arguments should have equal length
786 -- Acutal and expected types
787 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
790 unifyType :: TcTauType -> TcTauType -> TcM ()
791 -- No boxes expected inside these types
792 -- Acutal and expected types
793 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
794 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
795 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
796 addErrCtxtM (unifyCtxt ty1 ty2) $
797 uTysOuter True ty1 True ty2
800 unifyPred :: PredType -> PredType -> TcM ()
801 -- Acutal and expected types
802 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
803 uPred True True p1 True p2
805 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
806 -- Acutal and expected types
807 unifyTheta theta1 theta2
808 = do { checkTc (equalLength theta1 theta2)
809 (ptext SLIT("Contexts differ in length"))
810 ; uList unifyPred theta1 theta2 }
813 uList :: (a -> a -> TcM ())
814 -> [a] -> [a] -> TcM ()
815 -- Unify corresponding elements of two lists of types, which
816 -- should be f equal length. We charge down the list explicitly so that
817 -- we can complain if their lengths differ.
818 uList unify [] [] = return ()
819 uList unify (ty1:tys1) (ty2:tys2) = do { unify ty1 ty2; uList unify tys1 tys2 }
820 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
823 @unifyTypeList@ takes a single list of @TauType@s and unifies them
824 all together. It is used, for example, when typechecking explicit
825 lists, when all the elts should be of the same type.
828 unifyTypeList :: [TcTauType] -> TcM ()
829 unifyTypeList [] = returnM ()
830 unifyTypeList [ty] = returnM ()
831 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
832 ; unifyTypeList tys }
835 %************************************************************************
837 \subsection[Unify-uTys]{@uTys@: getting down to business}
839 %************************************************************************
841 @uTys@ is the heart of the unifier. Each arg happens twice, because
842 we want to report errors in terms of synomyms if poss. The first of
843 the pair is used in error messages only; it is always the same as the
844 second, except that if the first is a synonym then the second may be a
845 de-synonym'd version. This way we get better error messages.
847 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
850 type NoBoxes = Bool -- True <=> definitely no boxes in this type
851 -- False <=> there might be boxes (always safe)
853 type Outer = Bool -- True <=> this is the outer level of a unification
854 -- so that the types being unified are the
855 -- very ones we began with, not some sub
856 -- component or synonym expansion
857 -- The idea is that if Outer is true then unifyMisMatch should
858 -- pop the context to remove the "Expected/Acutal" context
861 :: NoBoxes -> TcType -- ty1 is the *expected* type
862 -> NoBoxes -> TcType -- ty2 is the *actual* type
864 uTysOuter nb1 ty1 nb2 ty2 = u_tys True nb1 ty1 ty1 nb2 ty2 ty2
865 uTys nb1 ty1 nb2 ty2 = u_tys False nb1 ty1 ty1 nb2 ty2 ty2
869 uTys_s :: NoBoxes -> [TcType] -- ty1 is the *actual* types
870 -> NoBoxes -> [TcType] -- ty2 is the *expected* types
872 uTys_s nb1 [] nb2 [] = returnM ()
873 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { uTys nb1 ty1 nb2 ty2
874 ; uTys_s nb1 tys1 nb2 tys2 }
875 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
879 -> NoBoxes -> TcType -> TcType -- ty1 is the *actual* type
880 -> NoBoxes -> TcType -> TcType -- ty2 is the *expected* type
883 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
887 -- Always expand synonyms (see notes at end)
888 -- (this also throws away FTVs)
890 | Just ty1' <- tcView ty1 = go False ty1' ty2
891 | Just ty2' <- tcView ty2 = go False ty1 ty2'
893 -- Variables; go for uVar
894 go outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
895 go outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
896 -- "True" means args swapped
898 go outer (PredTy p1) (PredTy p2) = uPred outer nb1 p1 nb2 p2
900 -- Type constructors must match
901 go _ (TyConApp con1 tys1) (TyConApp con2 tys2)
902 | con1 == con2 = uTys_s nb1 tys1 nb2 tys2
903 -- See Note [TyCon app]
905 -- Functions; just check the two parts
906 go _ (FunTy fun1 arg1) (FunTy fun2 arg2)
907 = do { uTys nb1 fun1 nb2 fun2
908 ; uTys nb1 arg1 nb2 arg2 }
910 -- Applications need a bit of care!
911 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
912 -- NB: we've already dealt with type variables and Notes,
913 -- so if one type is an App the other one jolly well better be too
914 go outer (AppTy s1 t1) ty2
915 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
916 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
918 -- Now the same, but the other way round
919 -- Don't swap the types, because the error messages get worse
920 go outer ty1 (AppTy s2 t2)
921 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
922 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
924 go _ ty1@(ForAllTy _ _) ty2@(ForAllTy _ _)
925 | length tvs1 == length tvs2
926 = do { tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
927 ; let tys = mkTyVarTys tvs
928 in_scope = mkInScopeSet (mkVarSet tvs)
929 subst1 = mkTvSubst in_scope (zipTyEnv tvs1 tys)
930 subst2 = mkTvSubst in_scope (zipTyEnv tvs2 tys)
931 ; uTys nb1 (substTy subst1 body1) nb2 (substTy subst2 body2)
933 -- If both sides are inside a box, we should not have
934 -- a polytype at all. This check comes last, because
935 -- the error message is extremely unhelpful.
936 ; ifM (nb1 && nb2) (notMonoType ty1)
939 (tvs1, body1) = tcSplitForAllTys ty1
940 (tvs2, body2) = tcSplitForAllTys ty2
942 -- Anything else fails
943 go outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
946 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
947 | n1 == n2 = uTys nb1 t1 nb2 t2
948 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
949 | c1 == c2 = uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
950 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
955 When we find two TyConApps, the argument lists are guaranteed equal
956 length. Reason: intially the kinds of the two types to be unified is
957 the same. The only way it can become not the same is when unifying two
958 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
959 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
960 which we do, that ensures that f1,f2 have the same kind; and that
961 means a1,a2 have the same kind. And now the argument repeats.
966 If you are tempted to make a short cut on synonyms, as in this
970 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
971 -- NO = if (con1 == con2) then
972 -- NO -- Good news! Same synonym constructors, so we can shortcut
973 -- NO -- by unifying their arguments and ignoring their expansions.
974 -- NO unifyTypepeLists args1 args2
976 -- NO -- Never mind. Just expand them and try again
980 then THINK AGAIN. Here is the whole story, as detected and reported
981 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
983 Here's a test program that should detect the problem:
987 x = (1 :: Bogus Char) :: Bogus Bool
990 The problem with [the attempted shortcut code] is that
994 is not a sufficient condition to be able to use the shortcut!
995 You also need to know that the type synonym actually USES all
996 its arguments. For example, consider the following type synonym
997 which does not use all its arguments.
1002 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1003 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1004 would fail, even though the expanded forms (both \tr{Int}) should
1007 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1008 unnecessarily bind \tr{t} to \tr{Char}.
1010 ... You could explicitly test for the problem synonyms and mark them
1011 somehow as needing expansion, perhaps also issuing a warning to the
1016 %************************************************************************
1018 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1020 %************************************************************************
1022 @uVar@ is called when at least one of the types being unified is a
1023 variable. It does {\em not} assume that the variable is a fixed point
1024 of the substitution; rather, notice that @uVar@ (defined below) nips
1025 back into @uTys@ if it turns out that the variable is already bound.
1029 -> Bool -- False => tyvar is the "expected"
1030 -- True => ty is the "expected" thing
1032 -> NoBoxes -- True <=> definitely no boxes in t2
1033 -> TcTauType -> TcTauType -- printing and real versions
1036 uVar outer swapped tv1 nb2 ps_ty2 ty2
1037 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1038 | otherwise = brackets (equals <+> ppr ty2)
1039 ; traceTc (text "uVar" <+> ppr swapped <+>
1040 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1041 nest 2 (ptext SLIT(" :=: ")),
1042 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1043 ; details <- lookupTcTyVar tv1
1046 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1047 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1048 -- The 'True' here says that ty1
1049 -- is definitely box-free
1050 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 ty2
1054 uUnfilledVar :: Outer
1055 -> Bool -- Args are swapped
1056 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1057 -> NoBoxes -> TcTauType -> TcTauType -- Type 2
1059 -- Invariant: tyvar 1 is not unified with anything
1061 uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 ty2
1062 | Just ty2' <- tcView ty2
1063 = -- Expand synonyms; ignore FTVs
1064 uUnfilledVar False swapped tv1 details1 nb2 ps_ty2 ty2'
1066 uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 (TyVarTy tv2)
1067 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1069 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1070 -- this is box-meets-box, so fill in with a tau-type
1071 -> do { tau_tv <- tcInstTyVar tv1
1072 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv) }
1073 other -> returnM () -- No-op
1075 -- Distinct type variables
1077 = do { lookup2 <- lookupTcTyVar tv2
1079 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 True ty2' ty2'
1080 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1083 uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 non_var_ty2 -- ty2 is not a type variable
1085 MetaTv (SigTv _) ref1 -> mis_match -- Can't update a skolem with a non-type-variable
1086 MetaTv info ref1 -> uMetaVar swapped tv1 info ref1 nb2 ps_ty2 non_var_ty2
1087 skolem_details -> mis_match
1089 mis_match = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1093 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1094 -> NoBoxes -> TcType -> TcType
1096 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1097 -- ty2 is not a type variable
1099 uMetaVar swapped tv1 BoxTv ref1 nb2 ps_ty2 non_var_ty2
1100 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1101 -- that any boxes in ty2 are filled with monotypes
1103 -- It should not be the case that tv1 occurs in ty2
1104 -- (i.e. no occurs check should be needed), but if perchance
1105 -- it does, the unbox operation will fill it, and the DEBUG
1107 do { final_ty <- unBox ps_ty2
1109 ; meta_details <- readMutVar ref1
1110 ; case meta_details of
1111 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1112 return () -- This really should *not* happen
1115 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1117 uMetaVar swapped tv1 info1 ref1 nb2 ps_ty2 non_var_ty2
1118 = do { final_ty <- checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
1119 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1122 uUnfilledVars :: Outer
1123 -> Bool -- Args are swapped
1124 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1125 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1127 -- Invarant: The type variables are distinct,
1128 -- Neither is filled in yet
1129 -- They might be boxy or not
1131 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1132 = unifyMisMatch outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1134 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1135 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2)
1136 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1137 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
1139 -- ToDo: this function seems too long for what it acutally does!
1140 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1141 = case (info1, info2) of
1142 (BoxTv, BoxTv) -> box_meets_box
1144 -- If a box meets a TauTv, but the fomer has the smaller kind
1145 -- then we must create a fresh TauTv with the smaller kind
1146 (_, BoxTv) | k1_sub_k2 -> update_tv2
1147 | otherwise -> box_meets_box
1148 (BoxTv, _ ) | k2_sub_k1 -> update_tv1
1149 | otherwise -> box_meets_box
1151 -- Avoid SigTvs if poss
1152 (SigTv _, _ ) | k1_sub_k2 -> update_tv2
1153 (_, SigTv _) | k2_sub_k1 -> update_tv1
1155 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1156 then update_tv1 -- Same kinds
1158 | k2_sub_k1 -> update_tv1
1159 | otherwise -> kind_err
1161 -- Update the variable with least kind info
1162 -- See notes on type inference in Kind.lhs
1163 -- The "nicer to" part only applies if the two kinds are the same,
1164 -- so we can choose which to do.
1166 -- Kinds should be guaranteed ok at this point
1167 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1168 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1170 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1173 | k2_sub_k1 = fill_from tv2
1174 | otherwise = kind_err
1176 -- Update *both* tyvars with a TauTv whose name and kind
1177 -- are gotten from tv (avoid losing nice names is poss)
1178 fill_from tv = do { tv' <- tcInstTyVar tv
1179 ; let tau_ty = mkTyVarTy tv'
1180 ; updateMeta tv1 ref1 tau_ty
1181 ; updateMeta tv2 ref2 tau_ty }
1183 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1184 unifyKindMisMatch k1 k2
1188 k1_sub_k2 = k1 `isSubKind` k2
1189 k2_sub_k1 = k2 `isSubKind` k1
1191 nicer_to_update_tv1 = isSystemName (varName tv1)
1192 -- Try to update sys-y type variables in preference to ones
1193 -- gotten (say) by instantiating a polymorphic function with
1194 -- a user-written type sig
1197 checkUpdateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1198 -- Update tv1, which is flexi; occurs check is alrady done
1199 -- The 'check' version does a kind check too
1200 -- We do a sub-kind check here: we might unify (a b) with (c d)
1201 -- where b::*->* and d::*; this should fail
1203 checkUpdateMeta swapped tv1 ref1 ty2
1204 = do { checkKinds swapped tv1 ty2
1205 ; updateMeta tv1 ref1 ty2 }
1207 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1208 updateMeta tv1 ref1 ty2
1209 = ASSERT( isMetaTyVar tv1 )
1210 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
1211 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
1212 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
1213 ; writeMutVar ref1 (Indirect ty2) }
1216 checkKinds swapped tv1 ty2
1217 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
1218 -- ty2 has been zonked at this stage, which ensures that
1219 -- its kind has as much boxity information visible as possible.
1220 | tk2 `isSubKind` tk1 = returnM ()
1223 -- Either the kinds aren't compatible
1224 -- (can happen if we unify (a b) with (c d))
1225 -- or we are unifying a lifted type variable with an
1226 -- unlifted type: e.g. (id 3#) is illegal
1227 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1228 unifyKindMisMatch k1 k2
1230 (k1,k2) | swapped = (tk2,tk1)
1231 | otherwise = (tk1,tk2)
1236 checkTauTvUpdate :: TcTyVar -> TcType -> TcM TcType
1237 -- (checkTauTvUpdate tv ty)
1238 -- We are about to update the TauTv tv with ty.
1239 -- Check (a) that tv doesn't occur in ty (occurs check)
1240 -- (b) that ty is a monotype
1241 -- Furthermore, in the interest of (b), if you find an
1242 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
1244 -- Returns the (non-boxy) type to update the type variable with, or fails
1246 checkTauTvUpdate orig_tv orig_ty
1249 go (TyConApp tc tys)
1250 | isSynTyCon tc = go_syn tc tys
1251 | otherwise = do { tys' <- mappM go tys; return (TyConApp tc tys') }
1252 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
1253 go (PredTy p) = do { p' <- go_pred p; return (PredTy p') }
1254 go (FunTy arg res) = do { arg' <- go arg; res' <- go res; return (FunTy arg' res') }
1255 go (AppTy fun arg) = do { fun' <- go fun; arg' <- go arg; return (mkAppTy fun' arg') }
1256 -- NB the mkAppTy; we might have instantiated a
1257 -- type variable to a type constructor, so we need
1258 -- to pull the TyConApp to the top.
1259 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
1262 | orig_tv == tv = occurCheck tv orig_ty -- (a)
1263 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
1264 | otherwise = return (TyVarTy tv)
1265 -- Ordinary (non Tc) tyvars
1266 -- occur inside quantified types
1268 go_pred (ClassP c tys) = do { tys' <- mapM go tys; return (ClassP c tys') }
1269 go_pred (IParam n ty) = do { ty' <- go ty; return (IParam n ty') }
1271 go_tyvar tv (SkolemTv _) = return (TyVarTy tv)
1272 go_tyvar tv (MetaTv box ref)
1273 = do { cts <- readMutVar ref
1275 Indirect ty -> go ty
1276 Flexi -> case box of
1277 BoxTv -> fillBoxWithTau tv ref
1278 other -> return (TyVarTy tv)
1281 -- go_syn is called for synonyms only
1282 -- See Note [Type synonyms and the occur check]
1284 | not (isTauTyCon tc)
1285 = notMonoType orig_ty -- (b) again
1287 = do { (msgs, mb_tys') <- tryTc (mapM go tys)
1289 Just tys' -> return (TyConApp tc tys')
1290 -- Retain the synonym (the common case)
1291 Nothing -> go (expectJust "checkTauTvUpdate"
1292 (tcView (TyConApp tc tys)))
1293 -- Try again, expanding the synonym
1296 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
1297 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
1298 -- tau-type meta-variable, whose print-name is the same as tv
1299 -- Choosing the same name is good: when we instantiate a function
1300 -- we allocate boxy tyvars with the same print-name as the quantified
1301 -- tyvar; and then we often fill the box with a tau-tyvar, and again
1302 -- we want to choose the same name.
1303 fillBoxWithTau tv ref
1304 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
1305 ; let tau = mkTyVarTy tv' -- name of the type variable
1306 ; writeMutVar ref (Indirect tau)
1310 Note [Type synonyms and the occur check]
1311 ~~~~~~~~~~~~~~~~~~~~
1312 Basically we want to update tv1 := ps_ty2
1313 because ps_ty2 has type-synonym info, which improves later error messages
1318 f :: (A a -> a -> ()) -> ()
1322 x = f (\ x p -> p x)
1324 In the application (p x), we try to match "t" with "A t". If we go
1325 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
1326 an infinite loop later.
1327 But we should not reject the program, because A t = ().
1328 Rather, we should bind t to () (= non_var_ty2).
1331 stripBoxyType :: BoxyType -> TcM TcType
1332 -- Strip all boxes from the input type, returning a non-boxy type.
1333 -- It's fine for there to be a polytype inside a box (c.f. unBox)
1334 -- All of the boxes should have been filled in by now;
1335 -- hence we return a TcType
1336 stripBoxyType ty = zonkType strip_tv ty
1338 strip_tv tv = ASSERT( not (isBoxyTyVar tv) ) return (TyVarTy tv)
1339 -- strip_tv will be called for *Flexi* meta-tyvars
1340 -- There should not be any Boxy ones; hence the ASSERT
1342 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1343 -- Subtle... we must zap the boxy res_ty
1344 -- to kind * before using it to instantiate a LitInst
1345 -- Calling unBox instead doesn't do the job, because the box
1346 -- often has an openTypeKind, and we don't want to instantiate
1348 zapToMonotype res_ty
1349 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1350 ; boxyUnify res_tau res_ty
1353 unBox :: BoxyType -> TcM TcType
1354 -- unBox implements the judgement
1356 -- with input s', and result s
1358 -- It removes all boxes from the input type, returning a non-boxy type.
1359 -- A filled box in the type can only contain a monotype; unBox fails if not
1360 -- The type can have empty boxes, which unBox fills with a monotype
1362 -- Compare this wth checkTauTvUpdate
1364 -- For once, it's safe to treat synonyms as opaque!
1366 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1367 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1368 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1369 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1370 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1371 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1372 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1374 | isTcTyVar tv -- It's a boxy type variable
1375 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1376 = do { cts <- readMutVar ref -- under nested quantifiers
1378 Flexi -> fillBoxWithTau tv ref
1379 Indirect ty -> do { non_boxy_ty <- unBox ty
1380 ; if isTauTy non_boxy_ty
1381 then return non_boxy_ty
1382 else notMonoType non_boxy_ty }
1384 | otherwise -- Skolems, and meta-tau-variables
1385 = return (TyVarTy tv)
1387 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1388 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1393 %************************************************************************
1395 \subsection[Unify-context]{Errors and contexts}
1397 %************************************************************************
1403 unifyCtxt act_ty exp_ty tidy_env
1404 = do { act_ty' <- zonkTcType act_ty
1405 ; exp_ty' <- zonkTcType exp_ty
1406 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1407 (env2, act_ty'') = tidyOpenType env1 act_ty'
1408 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1411 mkExpectedActualMsg act_ty exp_ty
1412 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1413 text "Inferred type" <> colon <+> ppr act_ty ])
1416 -- If an error happens we try to figure out whether the function
1417 -- function has been given too many or too few arguments, and say so.
1418 checkFunResCtxt fun actual_res_ty expected_res_ty tidy_env
1419 = do { exp_ty' <- zonkTcType expected_res_ty
1420 ; act_ty' <- zonkTcType actual_res_ty
1422 (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1423 (env2, act_ty'') = tidyOpenType env1 act_ty'
1424 (exp_args, _) = tcSplitFunTys exp_ty''
1425 (act_args, _) = tcSplitFunTys act_ty''
1427 len_act_args = length act_args
1428 len_exp_args = length exp_args
1430 message | len_exp_args < len_act_args = wrongArgsCtxt "too few" fun
1431 | len_exp_args > len_act_args = wrongArgsCtxt "too many" fun
1432 | otherwise = mkExpectedActualMsg act_ty'' exp_ty''
1433 ; return (env2, message) }
1436 wrongArgsCtxt too_many_or_few fun
1437 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1438 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1439 <+> ptext SLIT("arguments")
1442 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1443 -- tv1 and ty2 are zonked already
1446 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1447 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1449 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1450 | otherwise = (pp1, pp2)
1451 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1452 (env2, ty2') = tidyOpenType env1 ty2
1453 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1454 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1456 unifyMisMatch outer swapped ty1 ty2
1457 = do { (env, msg) <- if swapped then misMatchMsg ty1 ty2
1458 else misMatchMsg ty2 ty1
1460 -- This is the whole point of the 'outer' stuff
1461 ; if outer then popErrCtxt (failWithTcM (env, msg))
1462 else failWithTcM (env, msg)
1466 = do { env0 <- tcInitTidyEnv
1467 ; (env1, pp1, extra1) <- ppr_ty env0 ty1
1468 ; (env2, pp2, extra2) <- ppr_ty env1 ty2
1469 ; return (env2, sep [sep [ptext SLIT("Couldn't match expected type") <+> pp1,
1470 nest 7 (ptext SLIT("against inferred type") <+> pp2)],
1471 nest 2 extra1, nest 2 extra2]) }
1473 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
1475 = do { ty' <- zonkTcType ty
1476 ; let (env1,tidy_ty) = tidyOpenType env ty'
1477 simple_result = (env1, quotes (ppr tidy_ty), empty)
1480 | isSkolemTyVar tv -> return (env2, pp_rigid tv',
1481 pprSkolTvBinding tv')
1482 | otherwise -> return simple_result
1484 (env2, tv') = tidySkolemTyVar env1 tv
1485 other -> return simple_result }
1487 pp_rigid tv = quotes (ppr tv) <+> parens (ptext SLIT("a rigid variable"))
1491 = do { ty' <- zonkTcType ty
1492 ; env0 <- tcInitTidyEnv
1493 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1494 msg = ptext SLIT("Cannot match a monotype with") <+> ppr tidy_ty
1495 ; failWithTcM (env1, msg) }
1498 = do { env0 <- tcInitTidyEnv
1499 ; ty' <- zonkTcType ty
1500 ; let (env1, tidy_tyvar) = tidyOpenTyVar env0 tyvar
1501 (env2, tidy_ty) = tidyOpenType env1 ty'
1502 extra = sep [ppr tidy_tyvar, char '=', ppr tidy_ty]
1503 ; failWithTcM (env2, hang msg 2 extra) }
1505 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
1509 %************************************************************************
1513 %************************************************************************
1515 Unifying kinds is much, much simpler than unifying types.
1518 unifyKind :: TcKind -- Expected
1521 unifyKind LiftedTypeKind LiftedTypeKind = returnM ()
1522 unifyKind UnliftedTypeKind UnliftedTypeKind = returnM ()
1524 unifyKind OpenTypeKind k2 | isOpenTypeKind k2 = returnM ()
1525 unifyKind ArgTypeKind k2 | isArgTypeKind k2 = returnM ()
1526 -- Respect sub-kinding
1528 unifyKind (FunKind a1 r1) (FunKind a2 r2)
1529 = do { unifyKind a2 a1; unifyKind r1 r2 }
1530 -- Notice the flip in the argument,
1531 -- so that the sub-kinding works right
1533 unifyKind (KindVar kv1) k2 = uKVar False kv1 k2
1534 unifyKind k1 (KindVar kv2) = uKVar True kv2 k1
1535 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1537 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1538 unifyKinds [] [] = returnM ()
1539 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1541 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1544 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1545 uKVar swapped kv1 k2
1546 = do { mb_k1 <- readKindVar kv1
1548 Nothing -> uUnboundKVar swapped kv1 k2
1549 Just k1 | swapped -> unifyKind k2 k1
1550 | otherwise -> unifyKind k1 k2 }
1553 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1554 uUnboundKVar swapped kv1 k2@(KindVar kv2)
1555 | kv1 == kv2 = returnM ()
1556 | otherwise -- Distinct kind variables
1557 = do { mb_k2 <- readKindVar kv2
1559 Just k2 -> uUnboundKVar swapped kv1 k2
1560 Nothing -> writeKindVar kv1 k2 }
1562 uUnboundKVar swapped kv1 non_var_k2
1563 = do { k2' <- zonkTcKind non_var_k2
1564 ; kindOccurCheck kv1 k2'
1565 ; k2'' <- kindSimpleKind swapped k2'
1566 -- KindVars must be bound only to simple kinds
1567 -- Polarities: (kindSimpleKind True ?) succeeds
1568 -- returning *, corresponding to unifying
1571 ; writeKindVar kv1 k2'' }
1574 kindOccurCheck kv1 k2 -- k2 is zonked
1575 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1577 not_in (KindVar kv2) = kv1 /= kv2
1578 not_in (FunKind a2 r2) = not_in a2 && not_in r2
1581 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1582 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1583 -- If the flag is False, it requires k <: sk
1584 -- E.g. kindSimpleKind False ?? = *
1585 -- What about (kv -> *) :=: ?? -> *
1586 kindSimpleKind orig_swapped orig_kind
1587 = go orig_swapped orig_kind
1589 go sw (FunKind k1 k2) = do { k1' <- go (not sw) k1
1591 ; return (FunKind k1' k2') }
1592 go True OpenTypeKind = return liftedTypeKind
1593 go True ArgTypeKind = return liftedTypeKind
1594 go sw LiftedTypeKind = return liftedTypeKind
1595 go sw UnliftedTypeKind = return unliftedTypeKind
1596 go sw k@(KindVar _) = return k -- KindVars are always simple
1597 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1598 <+> ppr orig_swapped <+> ppr orig_kind)
1599 -- I think this can't actually happen
1601 -- T v = MkT v v must be a type
1602 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1605 kindOccurCheckErr tyvar ty
1606 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1607 2 (sep [ppr tyvar, char '=', ppr ty])
1609 unifyKindMisMatch ty1 ty2
1610 = zonkTcKind ty1 `thenM` \ ty1' ->
1611 zonkTcKind ty2 `thenM` \ ty2' ->
1613 msg = hang (ptext SLIT("Couldn't match kind"))
1614 2 (sep [quotes (ppr ty1'),
1615 ptext SLIT("against"),
1622 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1623 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1625 unifyFunKind (KindVar kvar)
1626 = readKindVar kvar `thenM` \ maybe_kind ->
1628 Just fun_kind -> unifyFunKind fun_kind
1629 Nothing -> do { arg_kind <- newKindVar
1630 ; res_kind <- newKindVar
1631 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1632 ; returnM (Just (arg_kind,res_kind)) }
1634 unifyFunKind (FunKind arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1635 unifyFunKind other = returnM Nothing
1638 %************************************************************************
1642 %************************************************************************
1644 ---------------------------
1645 -- We would like to get a decent error message from
1646 -- (a) Under-applied type constructors
1647 -- f :: (Maybe, Maybe)
1648 -- (b) Over-applied type constructors
1649 -- f :: Int x -> Int x
1653 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1654 -- A fancy wrapper for 'unifyKind', which tries
1655 -- to give decent error messages.
1656 checkExpectedKind ty act_kind exp_kind
1657 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1660 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1662 Just r -> returnM () ; -- Unification succeeded
1665 -- So there's definitely an error
1666 -- Now to find out what sort
1667 zonkTcKind exp_kind `thenM` \ exp_kind ->
1668 zonkTcKind act_kind `thenM` \ act_kind ->
1670 tcInitTidyEnv `thenM` \ env0 ->
1671 let (exp_as, _) = splitKindFunTys exp_kind
1672 (act_as, _) = splitKindFunTys act_kind
1673 n_exp_as = length exp_as
1674 n_act_as = length act_as
1676 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1677 (env2, tidy_act_kind) = tidyKind env1 act_kind
1679 err | n_exp_as < n_act_as -- E.g. [Maybe]
1680 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1682 -- Now n_exp_as >= n_act_as. In the next two cases,
1683 -- n_exp_as == 0, and hence so is n_act_as
1684 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1685 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1686 <+> ptext SLIT("is unlifted")
1688 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1689 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1690 <+> ptext SLIT("is lifted")
1692 | otherwise -- E.g. Monad [Int]
1693 = ptext SLIT("Kind mis-match")
1695 more_info = sep [ ptext SLIT("Expected kind") <+>
1696 quotes (pprKind tidy_exp_kind) <> comma,
1697 ptext SLIT("but") <+> quotes (ppr ty) <+>
1698 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1700 failWithTcM (env2, err $$ more_info)
1704 %************************************************************************
1706 \subsection{Checking signature type variables}
1708 %************************************************************************
1710 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1711 are not mentioned in the environment. In particular:
1713 (a) Not mentioned in the type of a variable in the envt
1714 eg the signature for f in this:
1720 Here, f is forced to be monorphic by the free occurence of x.
1722 (d) Not (unified with another type variable that is) in scope.
1723 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1724 when checking the expression type signature, we find that
1725 even though there is nothing in scope whose type mentions r,
1726 nevertheless the type signature for the expression isn't right.
1728 Another example is in a class or instance declaration:
1730 op :: forall b. a -> b
1732 Here, b gets unified with a
1734 Before doing this, the substitution is applied to the signature type variable.
1737 checkSigTyVars :: [TcTyVar] -> TcM ()
1738 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1740 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1741 -- The extra_tvs can include boxy type variables;
1742 -- e.g. TcMatches.tcCheckExistentialPat
1743 checkSigTyVarsWrt extra_tvs sig_tvs
1744 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1745 ; check_sig_tyvars extra_tvs' sig_tvs }
1748 :: TcTyVarSet -- Global type variables. The universally quantified
1749 -- tyvars should not mention any of these
1750 -- Guaranteed already zonked.
1751 -> [TcTyVar] -- Universally-quantified type variables in the signature
1752 -- Guaranteed to be skolems
1754 check_sig_tyvars extra_tvs []
1756 check_sig_tyvars extra_tvs sig_tvs
1757 = ASSERT( all isSkolemTyVar sig_tvs )
1758 do { gbl_tvs <- tcGetGlobalTyVars
1759 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1760 text "gbl_tvs" <+> ppr gbl_tvs,
1761 text "extra_tvs" <+> ppr extra_tvs]))
1763 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1764 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1765 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1768 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1769 -> [TcTyVar] -- The possibly-escaping type variables
1770 -> [TcTyVar] -- The zonked versions thereof
1772 -- Complain about escaping type variables
1773 -- We pass a list of type variables, at least one of which
1774 -- escapes. The first list contains the original signature type variable,
1775 -- while the second contains the type variable it is unified to (usually itself)
1776 bleatEscapedTvs globals sig_tvs zonked_tvs
1777 = do { env0 <- tcInitTidyEnv
1778 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1779 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1781 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1782 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1784 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1786 check (tidy_env, msgs) (sig_tv, zonked_tv)
1787 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1789 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1790 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1792 -----------------------
1793 escape_msg sig_tv zonked_tv globs
1795 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1796 nest 2 (vcat globs)]
1798 = msg <+> ptext SLIT("escapes")
1799 -- Sigh. It's really hard to give a good error message
1800 -- all the time. One bad case is an existential pattern match.
1801 -- We rely on the "When..." context to help.
1803 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1805 | sig_tv == zonked_tv = empty
1806 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1809 These two context are used with checkSigTyVars
1812 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1813 -> TidyEnv -> TcM (TidyEnv, Message)
1814 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1815 = zonkTcType sig_tau `thenM` \ actual_tau ->
1817 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1818 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1819 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1820 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1821 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1823 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),