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, extendVarSetList )
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
359 = do { tys <- mapM inst_tv qtvs
360 ; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
363 pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
365 | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
366 | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
367 ; return (mkTyVarTy tv') }
368 | otherwise = do { tv' <- tcInstTyVar tv
369 ; return (mkTyVarTy tv') }
372 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
373 -> BoxyRhoType -- Type to match (note a *Rho* type)
374 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
376 -- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
377 -- "Boxy types: inference for higher rank types and impredicativity"
379 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
380 = go tmpl_ty emptyVarSet boxy_ty
383 | Just t_ty' <- tcView t_ty = go t_ty' b_tvs b_ty
384 | Just b_ty' <- tcView b_ty = go t_ty b_tvs b_ty'
386 go (TyVarTy _) b_tvs b_ty = emptyTvSubst -- Rule S-ANY; no bindings
387 -- Rule S-ANY covers (a) type variables and (b) boxy types
388 -- in the template. Both look like a TyVarTy.
389 -- See Note [Sub-match] below
392 | isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
393 = go t_tau b_tvs b_ty -- Rule S-SPEC
394 | isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
395 = go t_ty (extendVarSetList b_tvs tvs) b_ty -- Rule S-SKOL
396 -- NB: it's *important* to discard the theta part. Otherwise
397 -- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
398 -- and end up with a completely bogus binding (b |-> Bool), by lining
399 -- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
400 -- This pre-subsumption stuff can return too few bindings, but it
401 -- must *never* return bogus info.
403 go (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
404 = boxy_match tmpl_tvs arg1 b_tvs arg2 (go res1 b_tvs res2)
405 -- Match the args, and sub-match the results
407 go t_ty b_tvs b_ty = boxy_match tmpl_tvs t_ty b_tvs b_ty emptyTvSubst
408 -- Otherwise defer to boxy matching
409 -- This covers TyConApp, AppTy, PredTy
416 |- head xs : <rhobox>
417 We will do a boxySubMatchType between a ~ <rhobox>
418 But we *don't* want to match [a |-> <rhobox>] because
419 (a) The box should be filled in with a rho-type, but
420 but the returned substitution maps TyVars to boxy
422 (b) In any case, the right final answer might be *either*
423 instantiate 'a' with a rho-type or a sigma type
424 head xs : Int vs head xs : forall b. b->b
425 So the matcher MUST NOT make a choice here. In general, we only
426 bind a template type variable in boxyMatchType, not in boxySubMatchType.
431 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
432 -> [BoxySigmaType] -- Type to match
433 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
435 -- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
436 -- "Boxy types: inference for higher rank types and impredicativity"
438 -- Find a *boxy* substitution that makes the template look as much
439 -- like the BoxySigmaType as possible.
440 -- It's always ok to return an empty substitution;
441 -- anything more is jam on the pudding
443 -- NB1: This is a pure, non-monadic function.
444 -- It does no unification, and cannot fail
446 -- Precondition: the arg lengths are equal
447 -- Precondition: none of the template type variables appear in the [BoxySigmaType]
448 -- Precondition: any nested quantifiers in either type differ from
449 -- the template type variables passed as arguments
453 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
454 = ASSERT( length tmpl_tys == length boxy_tys )
455 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
456 -- ToDo: add error context?
458 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
460 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
461 = boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys $
462 boxy_match tmpl_tvs t_ty boxy_tvs b_ty subst
465 boxy_match :: TcTyVarSet -> TcType -- Template
466 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
467 -> BoxySigmaType -- Match against this type
471 -- The boxy_tvs argument prevents this match:
472 -- [a] forall b. a ~ forall b. b
473 -- We don't want to bind the template variable 'a'
474 -- to the quantified type variable 'b'!
476 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
477 = go orig_tmpl_ty orig_boxy_ty
480 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
481 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
483 go (ForAllTy _ ty1) (ForAllTy tv2 ty2)
484 = boxy_match tmpl_tvs ty1 (boxy_tvs `extendVarSet` tv2) ty2 subst
486 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
487 | tc1 == tc2 = go_s tys1 tys2
489 go (FunTy arg1 res1) (FunTy arg2 res2)
490 = go_s [arg1,res1] [arg2,res2]
493 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
494 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
495 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
496 = go_s [s1,t1] [s2,t2]
499 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
500 , not (intersectsVarSet boxy_tvs (tyVarsOfType orig_boxy_ty))
501 , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
502 = extendTvSubst subst tv boxy_ty'
504 = subst -- Ignore others
506 boxy_ty' = case lookupTyVar subst tv of
507 Nothing -> orig_boxy_ty
508 Just ty -> ty `boxyLub` orig_boxy_ty
510 go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
511 -- Example: Tree a ~ Maybe Int
512 -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
513 -- misleading error messages. An even more confusing case is
514 -- a -> b ~ Maybe Int
515 -- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
516 -- from this pre-matching phase.
519 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
522 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
523 -- Combine boxy information from the two types
524 -- If there is a conflict, return the first
525 boxyLub orig_ty1 orig_ty2
526 = go orig_ty1 orig_ty2
528 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
529 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
530 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
531 | tc1 == tc2, length ts1 == length ts2
532 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
534 go (TyVarTy tv1) ty2 -- This is the whole point;
535 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
538 -- Look inside type synonyms, but only if the naive version fails
539 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
540 | Just ty2' <- tcView ty1 = go ty1 ty2'
542 -- For now, we don't look inside ForAlls, PredTys
543 go ty1 ty2 = orig_ty1 -- Default
546 Note [Matching kinds]
547 ~~~~~~~~~~~~~~~~~~~~~
548 The target type might legitimately not be a sub-kind of template.
549 For example, suppose the target is simply a box with an OpenTypeKind,
550 and the template is a type variable with LiftedTypeKind.
551 Then it's ok (because the target type will later be refined).
552 We simply don't bind the template type variable.
554 It might also be that the kind mis-match is an error. For example,
555 suppose we match the template (a -> Int) against (Int# -> Int),
556 where the template type variable 'a' has LiftedTypeKind. This
557 matching function does not fail; it simply doesn't bind the template.
558 Later stuff will fail.
560 %************************************************************************
564 %************************************************************************
566 All the tcSub calls have the form
568 tcSub expected_ty offered_ty
570 offered_ty <= expected_ty
572 That is, that a value of type offered_ty is acceptable in
573 a place expecting a value of type expected_ty.
575 It returns a coercion function
576 co_fn :: offered_ty -> expected_ty
577 which takes an HsExpr of type offered_ty into one of type
582 tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM ExprCoFn -- Locally used only
583 -- (tcSub act exp) checks that
585 tcSubExp actual_ty expected_ty
586 = addErrCtxtM (unifyCtxt actual_ty expected_ty)
587 (tc_sub True actual_ty actual_ty expected_ty expected_ty)
589 tcFunResTy :: Name -> BoxySigmaType -> BoxySigmaType -> TcM ExprCoFn -- Locally used only
590 tcFunResTy fun actual_ty expected_ty
591 = addErrCtxtM (checkFunResCtxt fun actual_ty expected_ty) $
592 (tc_sub True actual_ty actual_ty expected_ty expected_ty)
595 tc_sub :: Outer -- See comments with uTys
596 -> BoxySigmaType -- actual_ty, before expanding synonyms
597 -> BoxySigmaType -- ..and after
598 -> BoxySigmaType -- expected_ty, before
599 -> BoxySigmaType -- ..and after
602 tc_sub outer act_sty act_ty exp_sty exp_ty
603 | Just exp_ty' <- tcView exp_ty = tc_sub False act_sty act_ty exp_sty exp_ty'
604 tc_sub outer act_sty act_ty exp_sty exp_ty
605 | Just act_ty' <- tcView act_ty = tc_sub False act_sty act_ty' exp_sty exp_ty
607 -----------------------------------
608 -- Rule SBOXY, plus other cases when act_ty is a type variable
609 -- Just defer to boxy matching
610 -- This rule takes precedence over SKOL!
611 tc_sub outer act_sty (TyVarTy tv) exp_sty exp_ty
612 = do { uVar outer False tv False exp_sty exp_ty
613 ; return idCoercion }
615 -----------------------------------
616 -- Skolemisation case (rule SKOL)
617 -- actual_ty: d:Eq b => b->b
618 -- expected_ty: forall a. Ord a => a->a
619 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
621 -- It is essential to do this *before* the specialisation case
622 -- Example: f :: (Eq a => a->a) -> ...
623 -- g :: Ord b => b->b
626 tc_sub outer act_sty act_ty exp_sty exp_ty
628 = do { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ body_exp_ty ->
629 tc_sub False act_sty act_ty body_exp_ty body_exp_ty
630 ; return (gen_fn <.> co_fn) }
632 act_tvs = tyVarsOfType act_ty
633 -- It's really important to check for escape wrt
634 -- the free vars of both expected_ty *and* actual_ty
636 -----------------------------------
637 -- Specialisation case (rule ASPEC):
638 -- actual_ty: forall a. Ord a => a->a
639 -- expected_ty: Int -> Int
640 -- co_fn e = e Int dOrdInt
642 tc_sub outer act_sty actual_ty exp_sty expected_ty
643 -- Implements the new SPEC rule in the Appendix of the paper
644 -- "Boxy types: inference for higher rank types and impredicativity"
645 -- (This appendix isn't in the published version.)
646 -- The idea is to *first* do pre-subsumption, and then full subsumption
647 -- Example: forall a. a->a <= Int -> (forall b. Int)
648 -- Pre-subsumpion finds a|->Int, and that works fine, whereas
649 -- just running full subsumption would fail.
650 | isSigmaTy actual_ty
651 = do { -- Perform pre-subsumption, and instantiate
652 -- the type with info from the pre-subsumption;
653 -- boxy tyvars if pre-subsumption gives no info
654 let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
655 tau_tvs = exactTyVarsOfType tau
656 ; inst_tys <- preSubType tyvars tau_tvs tau expected_ty
657 ; let subst' = zipOpenTvSubst tyvars inst_tys
658 tau' = substTy subst' tau
660 -- Perform a full subsumption check
661 ; co_fn <- tc_sub False tau' tau' exp_sty expected_ty
663 -- Deal with the dictionaries
664 ; dicts <- newDicts InstSigOrigin (substTheta subst' theta)
666 ; let inst_fn = CoApps (CoTyApps CoHole inst_tys)
668 ; return (co_fn <.> inst_fn) }
670 -----------------------------------
671 -- Function case (rule F1)
672 tc_sub _ _ (FunTy act_arg act_res) _ (FunTy exp_arg exp_res)
673 = tc_sub_funs act_arg act_res exp_arg exp_res
675 -- Function case (rule F2)
676 tc_sub outer act_sty act_ty@(FunTy act_arg act_res) exp_sty (TyVarTy exp_tv)
678 = do { cts <- readMetaTyVar exp_tv
680 Indirect ty -> do { u_tys outer False act_sty act_ty True exp_sty ty
681 ; return idCoercion }
682 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
683 ; tc_sub_funs act_arg act_res arg_ty res_ty } }
685 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
686 fun_kinds = [argTypeKind, openTypeKind]
688 -- Everything else: defer to boxy matching
689 tc_sub outer act_sty actual_ty exp_sty expected_ty
690 = do { u_tys outer False act_sty actual_ty False exp_sty expected_ty
691 ; return idCoercion }
694 -----------------------------------
695 tc_sub_funs act_arg act_res exp_arg exp_res
696 = do { uTys False act_arg False exp_arg
697 ; co_fn_res <- tc_sub False act_res act_res exp_res exp_res
698 ; wrapFunResCoercion [exp_arg] co_fn_res }
700 -----------------------------------
702 :: [TcType] -- Type of args
703 -> ExprCoFn -- HsExpr a -> HsExpr b
704 -> TcM ExprCoFn -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
705 wrapFunResCoercion arg_tys co_fn_res
706 | isIdCoercion co_fn_res = return idCoercion
707 | null arg_tys = return co_fn_res
709 = do { us <- newUniqueSupply
710 ; let arg_ids = zipWith (mkSysLocal FSLIT("sub")) (uniqsFromSupply us) arg_tys
711 ; return (CoLams arg_ids (co_fn_res <.> (CoApps CoHole arg_ids))) }
716 %************************************************************************
718 \subsection{Generalisation}
720 %************************************************************************
723 tcGen :: BoxySigmaType -- expected_ty
724 -> TcTyVarSet -- Extra tyvars that the universally
725 -- quantified tyvars of expected_ty
726 -- must not be unified
727 -> (BoxyRhoType -> TcM result) -- spec_ty
728 -> TcM (ExprCoFn, result)
729 -- The expression has type: spec_ty -> expected_ty
731 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
732 -- If not, the call is a no-op
733 = do { -- We want the GenSkol info in the skolemised type variables to
734 -- mention the *instantiated* tyvar names, so that we get a
735 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
736 -- Hence the tiresome but innocuous fixM
737 ((forall_tvs, theta, rho_ty), skol_info) <- fixM (\ ~(_, skol_info) ->
738 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
739 ; span <- getSrcSpanM
740 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty) span
741 ; return ((forall_tvs, theta, rho_ty), skol_info) })
744 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
745 text "expected_ty" <+> ppr expected_ty,
746 text "inst ty" <+> ppr forall_tvs <+> ppr theta <+> ppr rho_ty,
747 text "free_tvs" <+> ppr free_tvs,
748 text "forall_tvs" <+> ppr forall_tvs])
751 -- Type-check the arg and unify with poly type
752 ; (result, lie) <- getLIE (thing_inside rho_ty)
754 -- Check that the "forall_tvs" havn't been constrained
755 -- The interesting bit here is that we must include the free variables
756 -- of the expected_ty. Here's an example:
757 -- runST (newVar True)
758 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
759 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
760 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
761 -- So now s' isn't unconstrained because it's linked to a.
762 -- Conclusion: include the free vars of the expected_ty in the
763 -- list of "free vars" for the signature check.
765 ; dicts <- newDicts (SigOrigin skol_info) theta
766 ; inst_binds <- tcSimplifyCheck sig_msg forall_tvs dicts lie
768 ; checkSigTyVarsWrt free_tvs forall_tvs
769 ; traceTc (text "tcGen:done")
772 -- This HsLet binds any Insts which came out of the simplification.
773 -- It's a bit out of place here, but using AbsBind involves inventing
774 -- a couple of new names which seems worse.
775 dict_ids = map instToId dicts
776 co_fn = CoTyLams forall_tvs $ CoLams dict_ids $ CoLet inst_binds CoHole
777 ; returnM (co_fn, result) }
779 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
780 sig_msg = ptext SLIT("expected type of an expression")
785 %************************************************************************
789 %************************************************************************
791 The exported functions are all defined as versions of some
792 non-exported generic functions.
795 boxyUnify :: BoxyType -> BoxyType -> TcM ()
796 -- Acutal and expected, respectively
798 = addErrCtxtM (unifyCtxt ty1 ty2) $
799 uTysOuter False ty1 False ty2
802 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM ()
803 -- Arguments should have equal length
804 -- Acutal and expected types
805 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
808 unifyType :: TcTauType -> TcTauType -> TcM ()
809 -- No boxes expected inside these types
810 -- Acutal and expected types
811 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
812 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
813 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
814 addErrCtxtM (unifyCtxt ty1 ty2) $
815 uTysOuter True ty1 True ty2
818 unifyPred :: PredType -> PredType -> TcM ()
819 -- Acutal and expected types
820 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
821 uPred True True p1 True p2
823 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
824 -- Acutal and expected types
825 unifyTheta theta1 theta2
826 = do { checkTc (equalLength theta1 theta2)
827 (ptext SLIT("Contexts differ in length"))
828 ; uList unifyPred theta1 theta2 }
831 uList :: (a -> a -> TcM ())
832 -> [a] -> [a] -> TcM ()
833 -- Unify corresponding elements of two lists of types, which
834 -- should be f equal length. We charge down the list explicitly so that
835 -- we can complain if their lengths differ.
836 uList unify [] [] = return ()
837 uList unify (ty1:tys1) (ty2:tys2) = do { unify ty1 ty2; uList unify tys1 tys2 }
838 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
841 @unifyTypeList@ takes a single list of @TauType@s and unifies them
842 all together. It is used, for example, when typechecking explicit
843 lists, when all the elts should be of the same type.
846 unifyTypeList :: [TcTauType] -> TcM ()
847 unifyTypeList [] = returnM ()
848 unifyTypeList [ty] = returnM ()
849 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
850 ; unifyTypeList tys }
853 %************************************************************************
855 \subsection[Unify-uTys]{@uTys@: getting down to business}
857 %************************************************************************
859 @uTys@ is the heart of the unifier. Each arg happens twice, because
860 we want to report errors in terms of synomyms if poss. The first of
861 the pair is used in error messages only; it is always the same as the
862 second, except that if the first is a synonym then the second may be a
863 de-synonym'd version. This way we get better error messages.
865 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
868 type NoBoxes = Bool -- True <=> definitely no boxes in this type
869 -- False <=> there might be boxes (always safe)
871 type Outer = Bool -- True <=> this is the outer level of a unification
872 -- so that the types being unified are the
873 -- very ones we began with, not some sub
874 -- component or synonym expansion
875 -- The idea is that if Outer is true then unifyMisMatch should
876 -- pop the context to remove the "Expected/Acutal" context
879 :: NoBoxes -> TcType -- ty1 is the *expected* type
880 -> NoBoxes -> TcType -- ty2 is the *actual* type
882 uTysOuter nb1 ty1 nb2 ty2 = u_tys True nb1 ty1 ty1 nb2 ty2 ty2
883 uTys nb1 ty1 nb2 ty2 = u_tys False nb1 ty1 ty1 nb2 ty2 ty2
887 uTys_s :: NoBoxes -> [TcType] -- ty1 is the *actual* types
888 -> NoBoxes -> [TcType] -- ty2 is the *expected* types
890 uTys_s nb1 [] nb2 [] = returnM ()
891 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { uTys nb1 ty1 nb2 ty2
892 ; uTys_s nb1 tys1 nb2 tys2 }
893 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
897 -> NoBoxes -> TcType -> TcType -- ty1 is the *actual* type
898 -> NoBoxes -> TcType -> TcType -- ty2 is the *expected* type
901 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
905 -- Always expand synonyms (see notes at end)
906 -- (this also throws away FTVs)
908 | Just ty1' <- tcView ty1 = go False ty1' ty2
909 | Just ty2' <- tcView ty2 = go False ty1 ty2'
911 -- Variables; go for uVar
912 go outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
913 go outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
914 -- "True" means args swapped
916 go outer (PredTy p1) (PredTy p2) = uPred outer nb1 p1 nb2 p2
918 -- Type constructors must match
919 go _ (TyConApp con1 tys1) (TyConApp con2 tys2)
920 | con1 == con2 = uTys_s nb1 tys1 nb2 tys2
921 -- See Note [TyCon app]
923 -- Functions; just check the two parts
924 go _ (FunTy fun1 arg1) (FunTy fun2 arg2)
925 = do { uTys nb1 fun1 nb2 fun2
926 ; uTys nb1 arg1 nb2 arg2 }
928 -- Applications need a bit of care!
929 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
930 -- NB: we've already dealt with type variables and Notes,
931 -- so if one type is an App the other one jolly well better be too
932 go outer (AppTy s1 t1) ty2
933 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
934 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
936 -- Now the same, but the other way round
937 -- Don't swap the types, because the error messages get worse
938 go outer ty1 (AppTy s2 t2)
939 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
940 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
942 go _ ty1@(ForAllTy _ _) ty2@(ForAllTy _ _)
943 | length tvs1 == length tvs2
944 = do { tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
945 ; let tys = mkTyVarTys tvs
946 in_scope = mkInScopeSet (mkVarSet tvs)
947 subst1 = mkTvSubst in_scope (zipTyEnv tvs1 tys)
948 subst2 = mkTvSubst in_scope (zipTyEnv tvs2 tys)
949 ; uTys nb1 (substTy subst1 body1) nb2 (substTy subst2 body2)
951 -- If both sides are inside a box, we should not have
952 -- a polytype at all. This check comes last, because
953 -- the error message is extremely unhelpful.
954 ; ifM (nb1 && nb2) (notMonoType ty1)
957 (tvs1, body1) = tcSplitForAllTys ty1
958 (tvs2, body2) = tcSplitForAllTys ty2
960 -- Anything else fails
961 go outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
964 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
965 | n1 == n2 = uTys nb1 t1 nb2 t2
966 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
967 | c1 == c2 = uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
968 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
973 When we find two TyConApps, the argument lists are guaranteed equal
974 length. Reason: intially the kinds of the two types to be unified is
975 the same. The only way it can become not the same is when unifying two
976 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
977 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
978 which we do, that ensures that f1,f2 have the same kind; and that
979 means a1,a2 have the same kind. And now the argument repeats.
984 If you are tempted to make a short cut on synonyms, as in this
988 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
989 -- NO = if (con1 == con2) then
990 -- NO -- Good news! Same synonym constructors, so we can shortcut
991 -- NO -- by unifying their arguments and ignoring their expansions.
992 -- NO unifyTypepeLists args1 args2
994 -- NO -- Never mind. Just expand them and try again
998 then THINK AGAIN. Here is the whole story, as detected and reported
999 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1001 Here's a test program that should detect the problem:
1005 x = (1 :: Bogus Char) :: Bogus Bool
1008 The problem with [the attempted shortcut code] is that
1012 is not a sufficient condition to be able to use the shortcut!
1013 You also need to know that the type synonym actually USES all
1014 its arguments. For example, consider the following type synonym
1015 which does not use all its arguments.
1020 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1021 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1022 would fail, even though the expanded forms (both \tr{Int}) should
1025 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1026 unnecessarily bind \tr{t} to \tr{Char}.
1028 ... You could explicitly test for the problem synonyms and mark them
1029 somehow as needing expansion, perhaps also issuing a warning to the
1034 %************************************************************************
1036 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1038 %************************************************************************
1040 @uVar@ is called when at least one of the types being unified is a
1041 variable. It does {\em not} assume that the variable is a fixed point
1042 of the substitution; rather, notice that @uVar@ (defined below) nips
1043 back into @uTys@ if it turns out that the variable is already bound.
1047 -> Bool -- False => tyvar is the "expected"
1048 -- True => ty is the "expected" thing
1050 -> NoBoxes -- True <=> definitely no boxes in t2
1051 -> TcTauType -> TcTauType -- printing and real versions
1054 uVar outer swapped tv1 nb2 ps_ty2 ty2
1055 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1056 | otherwise = brackets (equals <+> ppr ty2)
1057 ; traceTc (text "uVar" <+> ppr swapped <+>
1058 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1059 nest 2 (ptext SLIT(" :=: ")),
1060 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1061 ; details <- lookupTcTyVar tv1
1064 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1065 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1066 -- The 'True' here says that ty1
1067 -- is definitely box-free
1068 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 ty2
1072 uUnfilledVar :: Outer
1073 -> Bool -- Args are swapped
1074 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1075 -> NoBoxes -> TcTauType -> TcTauType -- Type 2
1077 -- Invariant: tyvar 1 is not unified with anything
1079 uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 ty2
1080 | Just ty2' <- tcView ty2
1081 = -- Expand synonyms; ignore FTVs
1082 uUnfilledVar False swapped tv1 details1 nb2 ps_ty2 ty2'
1084 uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 (TyVarTy tv2)
1085 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1087 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1088 -- this is box-meets-box, so fill in with a tau-type
1089 -> do { tau_tv <- tcInstTyVar tv1
1090 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv) }
1091 other -> returnM () -- No-op
1093 -- Distinct type variables
1095 = do { lookup2 <- lookupTcTyVar tv2
1097 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 True ty2' ty2'
1098 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1101 uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 non_var_ty2 -- ty2 is not a type variable
1103 MetaTv (SigTv _) ref1 -> mis_match -- Can't update a skolem with a non-type-variable
1104 MetaTv info ref1 -> uMetaVar swapped tv1 info ref1 nb2 ps_ty2 non_var_ty2
1105 skolem_details -> mis_match
1107 mis_match = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1111 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1112 -> NoBoxes -> TcType -> TcType
1114 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1115 -- ty2 is not a type variable
1117 uMetaVar swapped tv1 BoxTv ref1 nb2 ps_ty2 non_var_ty2
1118 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1119 -- that any boxes in ty2 are filled with monotypes
1121 -- It should not be the case that tv1 occurs in ty2
1122 -- (i.e. no occurs check should be needed), but if perchance
1123 -- it does, the unbox operation will fill it, and the DEBUG
1125 do { final_ty <- unBox ps_ty2
1127 ; meta_details <- readMutVar ref1
1128 ; case meta_details of
1129 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1130 return () -- This really should *not* happen
1133 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1135 uMetaVar swapped tv1 info1 ref1 nb2 ps_ty2 non_var_ty2
1136 = do { final_ty <- checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
1137 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1140 uUnfilledVars :: Outer
1141 -> Bool -- Args are swapped
1142 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1143 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1145 -- Invarant: The type variables are distinct,
1146 -- Neither is filled in yet
1147 -- They might be boxy or not
1149 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1150 = unifyMisMatch outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1152 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1153 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2)
1154 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1155 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
1157 -- ToDo: this function seems too long for what it acutally does!
1158 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1159 = case (info1, info2) of
1160 (BoxTv, BoxTv) -> box_meets_box
1162 -- If a box meets a TauTv, but the fomer has the smaller kind
1163 -- then we must create a fresh TauTv with the smaller kind
1164 (_, BoxTv) | k1_sub_k2 -> update_tv2
1165 | otherwise -> box_meets_box
1166 (BoxTv, _ ) | k2_sub_k1 -> update_tv1
1167 | otherwise -> box_meets_box
1169 -- Avoid SigTvs if poss
1170 (SigTv _, _ ) | k1_sub_k2 -> update_tv2
1171 (_, SigTv _) | k2_sub_k1 -> update_tv1
1173 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1174 then update_tv1 -- Same kinds
1176 | k2_sub_k1 -> update_tv1
1177 | otherwise -> kind_err
1179 -- Update the variable with least kind info
1180 -- See notes on type inference in Kind.lhs
1181 -- The "nicer to" part only applies if the two kinds are the same,
1182 -- so we can choose which to do.
1184 -- Kinds should be guaranteed ok at this point
1185 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1186 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1188 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1191 | k2_sub_k1 = fill_from tv2
1192 | otherwise = kind_err
1194 -- Update *both* tyvars with a TauTv whose name and kind
1195 -- are gotten from tv (avoid losing nice names is poss)
1196 fill_from tv = do { tv' <- tcInstTyVar tv
1197 ; let tau_ty = mkTyVarTy tv'
1198 ; updateMeta tv1 ref1 tau_ty
1199 ; updateMeta tv2 ref2 tau_ty }
1201 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1202 unifyKindMisMatch k1 k2
1206 k1_sub_k2 = k1 `isSubKind` k2
1207 k2_sub_k1 = k2 `isSubKind` k1
1209 nicer_to_update_tv1 = isSystemName (varName tv1)
1210 -- Try to update sys-y type variables in preference to ones
1211 -- gotten (say) by instantiating a polymorphic function with
1212 -- a user-written type sig
1215 checkUpdateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1216 -- Update tv1, which is flexi; occurs check is alrady done
1217 -- The 'check' version does a kind check too
1218 -- We do a sub-kind check here: we might unify (a b) with (c d)
1219 -- where b::*->* and d::*; this should fail
1221 checkUpdateMeta swapped tv1 ref1 ty2
1222 = do { checkKinds swapped tv1 ty2
1223 ; updateMeta tv1 ref1 ty2 }
1225 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1226 updateMeta tv1 ref1 ty2
1227 = ASSERT( isMetaTyVar tv1 )
1228 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
1229 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
1230 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
1231 ; writeMutVar ref1 (Indirect ty2) }
1234 checkKinds swapped tv1 ty2
1235 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
1236 -- ty2 has been zonked at this stage, which ensures that
1237 -- its kind has as much boxity information visible as possible.
1238 | tk2 `isSubKind` tk1 = returnM ()
1241 -- Either the kinds aren't compatible
1242 -- (can happen if we unify (a b) with (c d))
1243 -- or we are unifying a lifted type variable with an
1244 -- unlifted type: e.g. (id 3#) is illegal
1245 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1246 unifyKindMisMatch k1 k2
1248 (k1,k2) | swapped = (tk2,tk1)
1249 | otherwise = (tk1,tk2)
1254 checkTauTvUpdate :: TcTyVar -> TcType -> TcM TcType
1255 -- (checkTauTvUpdate tv ty)
1256 -- We are about to update the TauTv tv with ty.
1257 -- Check (a) that tv doesn't occur in ty (occurs check)
1258 -- (b) that ty is a monotype
1259 -- Furthermore, in the interest of (b), if you find an
1260 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
1262 -- Returns the (non-boxy) type to update the type variable with, or fails
1264 checkTauTvUpdate orig_tv orig_ty
1267 go (TyConApp tc tys)
1268 | isSynTyCon tc = go_syn tc tys
1269 | otherwise = do { tys' <- mappM go tys; return (TyConApp tc tys') }
1270 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
1271 go (PredTy p) = do { p' <- go_pred p; return (PredTy p') }
1272 go (FunTy arg res) = do { arg' <- go arg; res' <- go res; return (FunTy arg' res') }
1273 go (AppTy fun arg) = do { fun' <- go fun; arg' <- go arg; return (mkAppTy fun' arg') }
1274 -- NB the mkAppTy; we might have instantiated a
1275 -- type variable to a type constructor, so we need
1276 -- to pull the TyConApp to the top.
1277 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
1280 | orig_tv == tv = occurCheck tv orig_ty -- (a)
1281 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
1282 | otherwise = return (TyVarTy tv)
1283 -- Ordinary (non Tc) tyvars
1284 -- occur inside quantified types
1286 go_pred (ClassP c tys) = do { tys' <- mapM go tys; return (ClassP c tys') }
1287 go_pred (IParam n ty) = do { ty' <- go ty; return (IParam n ty') }
1289 go_tyvar tv (SkolemTv _) = return (TyVarTy tv)
1290 go_tyvar tv (MetaTv box ref)
1291 = do { cts <- readMutVar ref
1293 Indirect ty -> go ty
1294 Flexi -> case box of
1295 BoxTv -> fillBoxWithTau tv ref
1296 other -> return (TyVarTy tv)
1299 -- go_syn is called for synonyms only
1300 -- See Note [Type synonyms and the occur check]
1302 | not (isTauTyCon tc)
1303 = notMonoType orig_ty -- (b) again
1305 = do { (msgs, mb_tys') <- tryTc (mapM go tys)
1307 Just tys' -> return (TyConApp tc tys')
1308 -- Retain the synonym (the common case)
1309 Nothing -> go (expectJust "checkTauTvUpdate"
1310 (tcView (TyConApp tc tys)))
1311 -- Try again, expanding the synonym
1314 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
1315 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
1316 -- tau-type meta-variable, whose print-name is the same as tv
1317 -- Choosing the same name is good: when we instantiate a function
1318 -- we allocate boxy tyvars with the same print-name as the quantified
1319 -- tyvar; and then we often fill the box with a tau-tyvar, and again
1320 -- we want to choose the same name.
1321 fillBoxWithTau tv ref
1322 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
1323 ; let tau = mkTyVarTy tv' -- name of the type variable
1324 ; writeMutVar ref (Indirect tau)
1328 Note [Type synonyms and the occur check]
1329 ~~~~~~~~~~~~~~~~~~~~
1330 Basically we want to update tv1 := ps_ty2
1331 because ps_ty2 has type-synonym info, which improves later error messages
1336 f :: (A a -> a -> ()) -> ()
1340 x = f (\ x p -> p x)
1342 In the application (p x), we try to match "t" with "A t". If we go
1343 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
1344 an infinite loop later.
1345 But we should not reject the program, because A t = ().
1346 Rather, we should bind t to () (= non_var_ty2).
1349 stripBoxyType :: BoxyType -> TcM TcType
1350 -- Strip all boxes from the input type, returning a non-boxy type.
1351 -- It's fine for there to be a polytype inside a box (c.f. unBox)
1352 -- All of the boxes should have been filled in by now;
1353 -- hence we return a TcType
1354 stripBoxyType ty = zonkType strip_tv ty
1356 strip_tv tv = ASSERT( not (isBoxyTyVar tv) ) return (TyVarTy tv)
1357 -- strip_tv will be called for *Flexi* meta-tyvars
1358 -- There should not be any Boxy ones; hence the ASSERT
1360 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1361 -- Subtle... we must zap the boxy res_ty
1362 -- to kind * before using it to instantiate a LitInst
1363 -- Calling unBox instead doesn't do the job, because the box
1364 -- often has an openTypeKind, and we don't want to instantiate
1366 zapToMonotype res_ty
1367 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1368 ; boxyUnify res_tau res_ty
1371 unBox :: BoxyType -> TcM TcType
1372 -- unBox implements the judgement
1374 -- with input s', and result s
1376 -- It removes all boxes from the input type, returning a non-boxy type.
1377 -- A filled box in the type can only contain a monotype; unBox fails if not
1378 -- The type can have empty boxes, which unBox fills with a monotype
1380 -- Compare this wth checkTauTvUpdate
1382 -- For once, it's safe to treat synonyms as opaque!
1384 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1385 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1386 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1387 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1388 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1389 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1390 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1392 | isTcTyVar tv -- It's a boxy type variable
1393 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1394 = do { cts <- readMutVar ref -- under nested quantifiers
1396 Flexi -> fillBoxWithTau tv ref
1397 Indirect ty -> do { non_boxy_ty <- unBox ty
1398 ; if isTauTy non_boxy_ty
1399 then return non_boxy_ty
1400 else notMonoType non_boxy_ty }
1402 | otherwise -- Skolems, and meta-tau-variables
1403 = return (TyVarTy tv)
1405 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1406 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1411 %************************************************************************
1413 \subsection[Unify-context]{Errors and contexts}
1415 %************************************************************************
1421 unifyCtxt act_ty exp_ty tidy_env
1422 = do { act_ty' <- zonkTcType act_ty
1423 ; exp_ty' <- zonkTcType exp_ty
1424 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1425 (env2, act_ty'') = tidyOpenType env1 act_ty'
1426 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1429 mkExpectedActualMsg act_ty exp_ty
1430 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1431 text "Inferred type" <> colon <+> ppr act_ty ])
1434 -- If an error happens we try to figure out whether the function
1435 -- function has been given too many or too few arguments, and say so.
1436 checkFunResCtxt fun actual_res_ty expected_res_ty tidy_env
1437 = do { exp_ty' <- zonkTcType expected_res_ty
1438 ; act_ty' <- zonkTcType actual_res_ty
1440 (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1441 (env2, act_ty'') = tidyOpenType env1 act_ty'
1442 (exp_args, _) = tcSplitFunTys exp_ty''
1443 (act_args, _) = tcSplitFunTys act_ty''
1445 len_act_args = length act_args
1446 len_exp_args = length exp_args
1448 message | len_exp_args < len_act_args = wrongArgsCtxt "too few" fun
1449 | len_exp_args > len_act_args = wrongArgsCtxt "too many" fun
1450 | otherwise = mkExpectedActualMsg act_ty'' exp_ty''
1451 ; return (env2, message) }
1454 wrongArgsCtxt too_many_or_few fun
1455 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1456 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1457 <+> ptext SLIT("arguments")
1460 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1461 -- tv1 and ty2 are zonked already
1464 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1465 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1467 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1468 | otherwise = (pp1, pp2)
1469 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1470 (env2, ty2') = tidyOpenType env1 ty2
1471 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1472 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1474 unifyMisMatch outer swapped ty1 ty2
1475 = do { (env, msg) <- if swapped then misMatchMsg ty1 ty2
1476 else misMatchMsg ty2 ty1
1478 -- This is the whole point of the 'outer' stuff
1479 ; if outer then popErrCtxt (failWithTcM (env, msg))
1480 else failWithTcM (env, msg)
1484 = do { env0 <- tcInitTidyEnv
1485 ; (env1, pp1, extra1) <- ppr_ty env0 ty1
1486 ; (env2, pp2, extra2) <- ppr_ty env1 ty2
1487 ; return (env2, sep [sep [ptext SLIT("Couldn't match expected type") <+> pp1,
1488 nest 7 (ptext SLIT("against inferred type") <+> pp2)],
1489 nest 2 extra1, nest 2 extra2]) }
1491 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
1493 = do { ty' <- zonkTcType ty
1494 ; let (env1,tidy_ty) = tidyOpenType env ty'
1495 simple_result = (env1, quotes (ppr tidy_ty), empty)
1498 | isSkolemTyVar tv -> return (env2, pp_rigid tv',
1499 pprSkolTvBinding tv')
1500 | otherwise -> return simple_result
1502 (env2, tv') = tidySkolemTyVar env1 tv
1503 other -> return simple_result }
1505 pp_rigid tv = quotes (ppr tv) <+> parens (ptext SLIT("a rigid variable"))
1509 = do { ty' <- zonkTcType ty
1510 ; env0 <- tcInitTidyEnv
1511 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1512 msg = ptext SLIT("Cannot match a monotype with") <+> ppr tidy_ty
1513 ; failWithTcM (env1, msg) }
1516 = do { env0 <- tcInitTidyEnv
1517 ; ty' <- zonkTcType ty
1518 ; let (env1, tidy_tyvar) = tidyOpenTyVar env0 tyvar
1519 (env2, tidy_ty) = tidyOpenType env1 ty'
1520 extra = sep [ppr tidy_tyvar, char '=', ppr tidy_ty]
1521 ; failWithTcM (env2, hang msg 2 extra) }
1523 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
1527 %************************************************************************
1531 %************************************************************************
1533 Unifying kinds is much, much simpler than unifying types.
1536 unifyKind :: TcKind -- Expected
1539 unifyKind LiftedTypeKind LiftedTypeKind = returnM ()
1540 unifyKind UnliftedTypeKind UnliftedTypeKind = returnM ()
1542 unifyKind OpenTypeKind k2 | isOpenTypeKind k2 = returnM ()
1543 unifyKind ArgTypeKind k2 | isArgTypeKind k2 = returnM ()
1544 -- Respect sub-kinding
1546 unifyKind (FunKind a1 r1) (FunKind a2 r2)
1547 = do { unifyKind a2 a1; unifyKind r1 r2 }
1548 -- Notice the flip in the argument,
1549 -- so that the sub-kinding works right
1551 unifyKind (KindVar kv1) k2 = uKVar False kv1 k2
1552 unifyKind k1 (KindVar kv2) = uKVar True kv2 k1
1553 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1555 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1556 unifyKinds [] [] = returnM ()
1557 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1559 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1562 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1563 uKVar swapped kv1 k2
1564 = do { mb_k1 <- readKindVar kv1
1566 Nothing -> uUnboundKVar swapped kv1 k2
1567 Just k1 | swapped -> unifyKind k2 k1
1568 | otherwise -> unifyKind k1 k2 }
1571 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1572 uUnboundKVar swapped kv1 k2@(KindVar kv2)
1573 | kv1 == kv2 = returnM ()
1574 | otherwise -- Distinct kind variables
1575 = do { mb_k2 <- readKindVar kv2
1577 Just k2 -> uUnboundKVar swapped kv1 k2
1578 Nothing -> writeKindVar kv1 k2 }
1580 uUnboundKVar swapped kv1 non_var_k2
1581 = do { k2' <- zonkTcKind non_var_k2
1582 ; kindOccurCheck kv1 k2'
1583 ; k2'' <- kindSimpleKind swapped k2'
1584 -- KindVars must be bound only to simple kinds
1585 -- Polarities: (kindSimpleKind True ?) succeeds
1586 -- returning *, corresponding to unifying
1589 ; writeKindVar kv1 k2'' }
1592 kindOccurCheck kv1 k2 -- k2 is zonked
1593 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1595 not_in (KindVar kv2) = kv1 /= kv2
1596 not_in (FunKind a2 r2) = not_in a2 && not_in r2
1599 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1600 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1601 -- If the flag is False, it requires k <: sk
1602 -- E.g. kindSimpleKind False ?? = *
1603 -- What about (kv -> *) :=: ?? -> *
1604 kindSimpleKind orig_swapped orig_kind
1605 = go orig_swapped orig_kind
1607 go sw (FunKind k1 k2) = do { k1' <- go (not sw) k1
1609 ; return (FunKind k1' k2') }
1610 go True OpenTypeKind = return liftedTypeKind
1611 go True ArgTypeKind = return liftedTypeKind
1612 go sw LiftedTypeKind = return liftedTypeKind
1613 go sw UnliftedTypeKind = return unliftedTypeKind
1614 go sw k@(KindVar _) = return k -- KindVars are always simple
1615 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1616 <+> ppr orig_swapped <+> ppr orig_kind)
1617 -- I think this can't actually happen
1619 -- T v = MkT v v must be a type
1620 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1623 kindOccurCheckErr tyvar ty
1624 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1625 2 (sep [ppr tyvar, char '=', ppr ty])
1627 unifyKindMisMatch ty1 ty2
1628 = zonkTcKind ty1 `thenM` \ ty1' ->
1629 zonkTcKind ty2 `thenM` \ ty2' ->
1631 msg = hang (ptext SLIT("Couldn't match kind"))
1632 2 (sep [quotes (ppr ty1'),
1633 ptext SLIT("against"),
1640 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1641 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1643 unifyFunKind (KindVar kvar)
1644 = readKindVar kvar `thenM` \ maybe_kind ->
1646 Just fun_kind -> unifyFunKind fun_kind
1647 Nothing -> do { arg_kind <- newKindVar
1648 ; res_kind <- newKindVar
1649 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1650 ; returnM (Just (arg_kind,res_kind)) }
1652 unifyFunKind (FunKind arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1653 unifyFunKind other = returnM Nothing
1656 %************************************************************************
1660 %************************************************************************
1662 ---------------------------
1663 -- We would like to get a decent error message from
1664 -- (a) Under-applied type constructors
1665 -- f :: (Maybe, Maybe)
1666 -- (b) Over-applied type constructors
1667 -- f :: Int x -> Int x
1671 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1672 -- A fancy wrapper for 'unifyKind', which tries
1673 -- to give decent error messages.
1674 checkExpectedKind ty act_kind exp_kind
1675 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1678 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1680 Just r -> returnM () ; -- Unification succeeded
1683 -- So there's definitely an error
1684 -- Now to find out what sort
1685 zonkTcKind exp_kind `thenM` \ exp_kind ->
1686 zonkTcKind act_kind `thenM` \ act_kind ->
1688 tcInitTidyEnv `thenM` \ env0 ->
1689 let (exp_as, _) = splitKindFunTys exp_kind
1690 (act_as, _) = splitKindFunTys act_kind
1691 n_exp_as = length exp_as
1692 n_act_as = length act_as
1694 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1695 (env2, tidy_act_kind) = tidyKind env1 act_kind
1697 err | n_exp_as < n_act_as -- E.g. [Maybe]
1698 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1700 -- Now n_exp_as >= n_act_as. In the next two cases,
1701 -- n_exp_as == 0, and hence so is n_act_as
1702 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1703 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1704 <+> ptext SLIT("is unlifted")
1706 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1707 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1708 <+> ptext SLIT("is lifted")
1710 | otherwise -- E.g. Monad [Int]
1711 = ptext SLIT("Kind mis-match")
1713 more_info = sep [ ptext SLIT("Expected kind") <+>
1714 quotes (pprKind tidy_exp_kind) <> comma,
1715 ptext SLIT("but") <+> quotes (ppr ty) <+>
1716 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1718 failWithTcM (env2, err $$ more_info)
1722 %************************************************************************
1724 \subsection{Checking signature type variables}
1726 %************************************************************************
1728 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1729 are not mentioned in the environment. In particular:
1731 (a) Not mentioned in the type of a variable in the envt
1732 eg the signature for f in this:
1738 Here, f is forced to be monorphic by the free occurence of x.
1740 (d) Not (unified with another type variable that is) in scope.
1741 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1742 when checking the expression type signature, we find that
1743 even though there is nothing in scope whose type mentions r,
1744 nevertheless the type signature for the expression isn't right.
1746 Another example is in a class or instance declaration:
1748 op :: forall b. a -> b
1750 Here, b gets unified with a
1752 Before doing this, the substitution is applied to the signature type variable.
1755 checkSigTyVars :: [TcTyVar] -> TcM ()
1756 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1758 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1759 -- The extra_tvs can include boxy type variables;
1760 -- e.g. TcMatches.tcCheckExistentialPat
1761 checkSigTyVarsWrt extra_tvs sig_tvs
1762 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1763 ; check_sig_tyvars extra_tvs' sig_tvs }
1766 :: TcTyVarSet -- Global type variables. The universally quantified
1767 -- tyvars should not mention any of these
1768 -- Guaranteed already zonked.
1769 -> [TcTyVar] -- Universally-quantified type variables in the signature
1770 -- Guaranteed to be skolems
1772 check_sig_tyvars extra_tvs []
1774 check_sig_tyvars extra_tvs sig_tvs
1775 = ASSERT( all isSkolemTyVar sig_tvs )
1776 do { gbl_tvs <- tcGetGlobalTyVars
1777 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1778 text "gbl_tvs" <+> ppr gbl_tvs,
1779 text "extra_tvs" <+> ppr extra_tvs]))
1781 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1782 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1783 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1786 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1787 -> [TcTyVar] -- The possibly-escaping type variables
1788 -> [TcTyVar] -- The zonked versions thereof
1790 -- Complain about escaping type variables
1791 -- We pass a list of type variables, at least one of which
1792 -- escapes. The first list contains the original signature type variable,
1793 -- while the second contains the type variable it is unified to (usually itself)
1794 bleatEscapedTvs globals sig_tvs zonked_tvs
1795 = do { env0 <- tcInitTidyEnv
1796 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1797 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1799 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1800 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1802 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1804 check (tidy_env, msgs) (sig_tv, zonked_tv)
1805 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1807 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1808 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1810 -----------------------
1811 escape_msg sig_tv zonked_tv globs
1813 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1814 nest 2 (vcat globs)]
1816 = msg <+> ptext SLIT("escapes")
1817 -- Sigh. It's really hard to give a good error message
1818 -- all the time. One bad case is an existential pattern match.
1819 -- We rely on the "When..." context to help.
1821 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1823 | sig_tv == zonked_tv = empty
1824 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1827 These two context are used with checkSigTyVars
1830 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1831 -> TidyEnv -> TcM (TidyEnv, Message)
1832 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1833 = zonkTcType sig_tau `thenM` \ actual_tau ->
1835 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1836 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1837 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1838 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1839 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1841 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),