2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Type subsumption and unification
10 -- Full-blown subsumption
11 tcSubExp, tcFunResTy, tcGen,
12 checkSigTyVars, checkSigTyVarsWrt, bleatEscapedTvs, sigCtxt,
14 -- Various unifications
15 unifyType, unifyTypeList, unifyTheta,
16 unifyKind, unifyKinds, unifyFunKind,
18 preSubType, boxyMatchTypes,
20 --------------------------------
22 tcInfer, subFunTys, unBox, stripBoxyType, withBox,
23 boxyUnify, boxyUnifyList, zapToMonotype,
24 boxySplitListTy, boxySplitTyConApp, boxySplitAppTy,
28 #include "HsVersions.h"
37 import TcRnMonad -- TcType, amongst others
55 %************************************************************************
57 \subsection{'hole' type variables}
59 %************************************************************************
62 tcInfer :: (BoxyType -> TcM a) -> TcM (a, TcType)
64 = do { box <- newBoxyTyVar openTypeKind
65 ; res <- tc_infer (mkTyVarTy box)
66 ; res_ty <- readFilledBox box -- Guaranteed filled-in by now
67 ; return (res, res_ty) }
71 %************************************************************************
75 %************************************************************************
78 subFunTys :: SDoc -- Somthing like "The function f has 3 arguments"
79 -- or "The abstraction (\x.e) takes 1 argument"
80 -> Arity -- Expected # of args
81 -> BoxyRhoType -- res_ty
82 -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
84 -- Attempt to decompse res_ty to have enough top-level arrows to
85 -- match the number of patterns in the match group
87 -- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
88 -- and the inner call to thing_inside passes args: [a1,...,an], b
89 -- then co_fn :: (a1 -> ... -> an -> b) -> res_ty
91 -- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
94 {- Error messages from subFunTys
96 The abstraction `\Just 1 -> ...' has two arguments
97 but its type `Maybe a -> a' has only one
99 The equation(s) for `f' have two arguments
100 but its type `Maybe a -> a' has only one
102 The section `(f 3)' requires 'f' to take two arguments
103 but its type `Int -> Int' has only one
105 The function 'f' is applied to two arguments
106 but its type `Int -> Int' has only one
110 subFunTys error_herald n_pats res_ty thing_inside
111 = loop n_pats [] res_ty
113 -- In 'loop', the parameter 'arg_tys' accumulates
114 -- the arg types so far, in *reverse order*
115 loop n args_so_far res_ty
116 | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
118 loop n args_so_far res_ty
119 | isSigmaTy res_ty -- Do this before checking n==0, because we
120 -- guarantee to return a BoxyRhoType, not a BoxySigmaType
121 = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet $ \ _ res_ty' ->
122 loop n args_so_far res_ty'
123 ; return (gen_fn <.> co_fn, res) }
125 loop 0 args_so_far res_ty
126 = do { res <- thing_inside (reverse args_so_far) res_ty
127 ; return (idHsWrapper, res) }
129 loop n args_so_far (FunTy arg_ty res_ty)
130 = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
131 ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
132 ; return (co_fn', res) }
134 -- res_ty might have a type variable at the head, such as (a b c),
135 -- in which case we must fill in with (->). Simplest thing to do
136 -- is to use boxyUnify, but we catch failure and generate our own
137 -- error message on failure
138 loop n args_so_far res_ty@(AppTy _ _)
139 = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
140 ; (_, mb_unit) <- tryTcErrs $ boxyUnify res_ty (FunTy arg_ty' res_ty')
141 ; if isNothing mb_unit then bale_out args_so_far
142 else loop n args_so_far (FunTy arg_ty' res_ty') }
144 loop n args_so_far (TyVarTy tv)
145 | not (isImmutableTyVar tv)
146 = do { cts <- readMetaTyVar tv
148 Indirect ty -> loop n args_so_far ty
149 Flexi -> do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
150 ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty
151 ; return (idHsWrapper, res) } }
153 mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
154 mk_res_ty [] = panic "TcUnify.mk_res_ty1"
155 kinds = openTypeKind : take n (repeat argTypeKind)
156 -- Note argTypeKind: the args can have an unboxed type,
157 -- but not an unboxed tuple.
159 loop n args_so_far res_ty = bale_out args_so_far
162 = do { env0 <- tcInitTidyEnv
163 ; res_ty' <- zonkTcType res_ty
164 ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
165 ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
167 mk_msg res_ty n_actual
168 = error_herald <> comma $$
169 sep [ptext SLIT("but its type") <+> quotes (pprType res_ty),
170 if n_actual == 0 then ptext SLIT("has none")
171 else ptext SLIT("has only") <+> speakN n_actual]
175 ----------------------
176 boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
177 -> BoxyRhoType -- Expected type (T a b c)
178 -> TcM [BoxySigmaType] -- Element types, a b c
179 -- It's used for wired-in tycons, so we call checkWiredInTyCOn
180 -- Precondition: never called with FunTyCon
181 -- Precondition: input type :: *
183 boxySplitTyConApp tc orig_ty
184 = do { checkWiredInTyCon tc
185 ; loop (tyConArity tc) [] orig_ty }
187 loop n_req args_so_far ty
188 | Just ty' <- tcView ty = loop n_req args_so_far ty'
190 loop n_req args_so_far (TyConApp tycon args)
192 = ASSERT( n_req == length args) -- ty::*
193 return (args ++ args_so_far)
195 loop n_req args_so_far (AppTy fun arg)
196 = loop (n_req - 1) (arg:args_so_far) fun
198 loop n_req args_so_far (TyVarTy tv)
199 | not (isImmutableTyVar tv)
200 = do { cts <- readMetaTyVar tv
202 Indirect ty -> loop n_req args_so_far ty
203 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
204 ; return (arg_tys ++ args_so_far) }
207 mk_res_ty arg_tys' = mkTyConApp tc arg_tys'
208 arg_kinds = map tyVarKind (take n_req (tyConTyVars tc))
210 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc))) orig_ty
212 ----------------------
213 boxySplitListTy :: BoxyRhoType -> TcM BoxySigmaType -- Special case for lists
214 boxySplitListTy exp_ty = do { [elt_ty] <- boxySplitTyConApp listTyCon exp_ty
218 ----------------------
219 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
220 -> TcM (BoxySigmaType, BoxySigmaType) -- Returns m, a
221 -- Assumes (m: * -> k), where k is the kind of the incoming type
222 -- If the incoming type is boxy, then so are the result types; and vice versa
224 boxySplitAppTy orig_ty
228 | Just ty' <- tcView ty = loop ty'
231 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
232 = return (fun_ty, arg_ty)
235 | not (isImmutableTyVar tv)
236 = do { cts <- readMetaTyVar tv
238 Indirect ty -> loop ty
239 Flexi -> do { [fun_ty,arg_ty] <- withMetaTvs tv kinds mk_res_ty
240 ; return (fun_ty, arg_ty) } }
242 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
243 mk_res_ty other = panic "TcUnify.mk_res_ty2"
244 tv_kind = tyVarKind tv
245 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
247 liftedTypeKind] -- arg type :: *
248 -- The defaultKind is a bit smelly. If you remove it,
249 -- try compiling f x = do { x }
250 -- and you'll get a kind mis-match. It smells, but
251 -- not enough to lose sleep over.
253 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
256 boxySplitFailure actual_ty expected_ty
257 = unifyMisMatch False False actual_ty expected_ty
258 -- "outer" is False, so we don't pop the context
259 -- which is what we want since we have not pushed one!
263 --------------------------------
264 -- withBoxes: the key utility function
265 --------------------------------
268 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
269 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
270 -> ([BoxySigmaType] -> BoxySigmaType)
271 -- Constructs the type to assign
272 -- to the original var
273 -> TcM [BoxySigmaType] -- Return the fresh boxes
275 -- It's entirely possible for the [kind] to be empty.
276 -- For example, when pattern-matching on True,
277 -- we call boxySplitTyConApp passing a boolTyCon
279 -- Invariant: tv is still Flexi
281 withMetaTvs tv kinds mk_res_ty
283 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
284 ; let box_tys = mkTyVarTys box_tvs
285 ; writeMetaTyVar tv (mk_res_ty box_tys)
288 | otherwise -- Non-boxy meta type variable
289 = do { tau_tys <- mapM newFlexiTyVarTy kinds
290 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
291 -- Sure to be a tau-type
294 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
295 -- Allocate a *boxy* tyvar
296 withBox kind thing_inside
297 = do { box_tv <- newMetaTyVar BoxTv kind
298 ; res <- thing_inside (mkTyVarTy box_tv)
299 ; ty <- readFilledBox box_tv
304 %************************************************************************
306 Approximate boxy matching
308 %************************************************************************
311 preSubType :: [TcTyVar] -- Quantified type variables
312 -> TcTyVarSet -- Subset of quantified type variables
313 -- see Note [Pre-sub boxy]
314 -> TcType -- The rho-type part; quantified tyvars scopes over this
315 -> BoxySigmaType -- Matching type from the context
316 -> TcM [TcType] -- Types to instantiate the tyvars
317 -- Perform pre-subsumption, and return suitable types
318 -- to instantiate the quantified type varibles:
319 -- info from the pre-subsumption, if there is any
320 -- a boxy type variable otherwise
322 -- Note [Pre-sub boxy]
323 -- The 'btvs' are a subset of 'qtvs'. They are the ones we can
324 -- instantiate to a boxy type variable, because they'll definitely be
325 -- filled in later. This isn't always the case; sometimes we have type
326 -- variables mentioned in the context of the type, but not the body;
327 -- f :: forall a b. C a b => a -> a
328 -- Then we may land up with an unconstrained 'b', so we want to
329 -- instantiate it to a monotype (non-boxy) type variable
331 -- The 'qtvs' that are *neither* fixed by the pre-subsumption, *nor* are in 'btvs',
332 -- are instantiated to TauTv meta variables.
334 preSubType qtvs btvs qty expected_ty
335 = do { tys <- mapM inst_tv qtvs
336 ; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
339 pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
341 | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
342 | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
343 ; return (mkTyVarTy tv') }
344 | otherwise = do { tv' <- tcInstTyVar tv
345 ; return (mkTyVarTy tv') }
348 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
349 -> BoxyRhoType -- Type to match (note a *Rho* type)
350 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
352 -- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
353 -- "Boxy types: inference for higher rank types and impredicativity"
355 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
356 = go tmpl_tvs tmpl_ty emptyVarSet boxy_ty
358 go t_tvs t_ty b_tvs b_ty
359 | Just t_ty' <- tcView t_ty = go t_tvs t_ty' b_tvs b_ty
360 | Just b_ty' <- tcView b_ty = go t_tvs t_ty b_tvs b_ty'
362 go t_tvs (TyVarTy _) b_tvs b_ty = emptyTvSubst -- Rule S-ANY; no bindings
363 -- Rule S-ANY covers (a) type variables and (b) boxy types
364 -- in the template. Both look like a TyVarTy.
365 -- See Note [Sub-match] below
367 go t_tvs t_ty b_tvs b_ty
368 | isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
369 = go (t_tvs `delVarSetList` tvs) t_tau b_tvs b_ty -- Rule S-SPEC
370 -- Under a forall on the left, if there is shadowing,
371 -- do not bind! Hence the delVarSetList.
372 | isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
373 = go t_tvs t_ty (extendVarSetList b_tvs tvs) b_tau -- Rule S-SKOL
374 -- Add to the variables we must not bind to
375 -- NB: it's *important* to discard the theta part. Otherwise
376 -- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
377 -- and end up with a completely bogus binding (b |-> Bool), by lining
378 -- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
379 -- This pre-subsumption stuff can return too few bindings, but it
380 -- must *never* return bogus info.
382 go t_tvs (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
383 = boxy_match t_tvs arg1 b_tvs arg2 (go t_tvs res1 b_tvs res2)
384 -- Match the args, and sub-match the results
386 go t_tvs t_ty b_tvs b_ty = boxy_match t_tvs t_ty b_tvs b_ty emptyTvSubst
387 -- Otherwise defer to boxy matching
388 -- This covers TyConApp, AppTy, PredTy
395 |- head xs : <rhobox>
396 We will do a boxySubMatchType between a ~ <rhobox>
397 But we *don't* want to match [a |-> <rhobox>] because
398 (a) The box should be filled in with a rho-type, but
399 but the returned substitution maps TyVars to boxy
401 (b) In any case, the right final answer might be *either*
402 instantiate 'a' with a rho-type or a sigma type
403 head xs : Int vs head xs : forall b. b->b
404 So the matcher MUST NOT make a choice here. In general, we only
405 bind a template type variable in boxyMatchType, not in boxySubMatchType.
410 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
411 -> [BoxySigmaType] -- Type to match
412 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
414 -- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
415 -- "Boxy types: inference for higher rank types and impredicativity"
417 -- Find a *boxy* substitution that makes the template look as much
418 -- like the BoxySigmaType as possible.
419 -- It's always ok to return an empty substitution;
420 -- anything more is jam on the pudding
422 -- NB1: This is a pure, non-monadic function.
423 -- It does no unification, and cannot fail
425 -- Precondition: the arg lengths are equal
426 -- Precondition: none of the template type variables appear anywhere in the [BoxySigmaType]
430 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
431 = ASSERT( length tmpl_tys == length boxy_tys )
432 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
433 -- ToDo: add error context?
435 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
437 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
438 = boxy_match tmpl_tvs t_ty boxy_tvs b_ty $
439 boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys subst
440 boxy_match_s tmpl_tvs _ boxy_tvs _ subst
441 = panic "boxy_match_s" -- Lengths do not match
445 boxy_match :: TcTyVarSet -> TcType -- Template
446 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
447 -> BoxySigmaType -- Match against this type
451 -- The boxy_tvs argument prevents this match:
452 -- [a] forall b. a ~ forall b. b
453 -- We don't want to bind the template variable 'a'
454 -- to the quantified type variable 'b'!
456 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
457 = go orig_tmpl_ty orig_boxy_ty
460 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
461 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
463 go ty1 ty2 -- C.f. the isSigmaTy case for boxySubMatchType
465 , (tvs1, _, tau1) <- tcSplitSigmaTy ty1
466 , (tvs2, _, tau2) <- tcSplitSigmaTy ty2
467 , equalLength tvs1 tvs2
468 = boxy_match (tmpl_tvs `delVarSetList` tvs1) tau1
469 (boxy_tvs `extendVarSetList` tvs2) tau2 subst
471 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
472 | tc1 == tc2 = go_s tys1 tys2
474 go (FunTy arg1 res1) (FunTy arg2 res2)
475 = go_s [arg1,res1] [arg2,res2]
478 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
479 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
480 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
481 = go_s [s1,t1] [s2,t2]
484 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
485 , boxy_tvs `disjointVarSet` tyVarsOfType orig_boxy_ty
486 , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
487 = extendTvSubst subst tv boxy_ty'
489 = subst -- Ignore others
491 boxy_ty' = case lookupTyVar subst tv of
492 Nothing -> orig_boxy_ty
493 Just ty -> ty `boxyLub` orig_boxy_ty
495 go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
496 -- Example: Tree a ~ Maybe Int
497 -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
498 -- misleading error messages. An even more confusing case is
499 -- a -> b ~ Maybe Int
500 -- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
501 -- from this pre-matching phase.
504 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
507 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
508 -- Combine boxy information from the two types
509 -- If there is a conflict, return the first
510 boxyLub orig_ty1 orig_ty2
511 = go orig_ty1 orig_ty2
513 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
514 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
515 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
516 | tc1 == tc2, length ts1 == length ts2
517 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
519 go (TyVarTy tv1) ty2 -- This is the whole point;
520 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
523 -- Look inside type synonyms, but only if the naive version fails
524 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
525 | Just ty2' <- tcView ty1 = go ty1 ty2'
527 -- For now, we don't look inside ForAlls, PredTys
528 go ty1 ty2 = orig_ty1 -- Default
531 Note [Matching kinds]
532 ~~~~~~~~~~~~~~~~~~~~~
533 The target type might legitimately not be a sub-kind of template.
534 For example, suppose the target is simply a box with an OpenTypeKind,
535 and the template is a type variable with LiftedTypeKind.
536 Then it's ok (because the target type will later be refined).
537 We simply don't bind the template type variable.
539 It might also be that the kind mis-match is an error. For example,
540 suppose we match the template (a -> Int) against (Int# -> Int),
541 where the template type variable 'a' has LiftedTypeKind. This
542 matching function does not fail; it simply doesn't bind the template.
543 Later stuff will fail.
545 %************************************************************************
549 %************************************************************************
551 All the tcSub calls have the form
553 tcSub expected_ty offered_ty
555 offered_ty <= expected_ty
557 That is, that a value of type offered_ty is acceptable in
558 a place expecting a value of type expected_ty.
560 It returns a coercion function
561 co_fn :: offered_ty -> expected_ty
562 which takes an HsExpr of type offered_ty into one of type
567 tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
568 -- (tcSub act exp) checks that
570 tcSubExp actual_ty expected_ty
571 = -- addErrCtxtM (unifyCtxt actual_ty expected_ty) $
572 -- Adding the error context here leads to some very confusing error
573 -- messages, such as "can't match forall a. a->a with forall a. a->a"
574 -- Example is tcfail165:
575 -- do var <- newEmptyMVar :: IO (MVar (forall a. Show a => a -> String))
576 -- putMVar var (show :: forall a. Show a => a -> String)
577 -- Here the info does not flow from the 'var' arg of putMVar to its 'show' arg
578 -- but after zonking it looks as if it does!
580 -- So instead I'm adding the error context when moving from tc_sub to u_tys
582 traceTc (text "tcSubExp" <+> ppr actual_ty <+> ppr expected_ty) >>
583 tc_sub SubOther actual_ty actual_ty False expected_ty expected_ty
585 tcFunResTy :: Name -> BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
586 tcFunResTy fun actual_ty expected_ty
587 = traceTc (text "tcFunResTy" <+> ppr actual_ty <+> ppr expected_ty) >>
588 tc_sub (SubFun fun) actual_ty actual_ty False expected_ty expected_ty
591 data SubCtxt = SubDone -- Error-context already pushed
592 | SubFun Name -- Context is tcFunResTy
593 | SubOther -- Context is something else
595 tc_sub :: SubCtxt -- How to add an error-context
596 -> BoxySigmaType -- actual_ty, before expanding synonyms
597 -> BoxySigmaType -- ..and after
598 -> InBox -- True <=> expected_ty is inside a box
599 -> BoxySigmaType -- expected_ty, before
600 -> BoxySigmaType -- ..and after
602 -- The acual_ty is never inside a box
603 -- IMPORTANT pre-condition: if the args contain foralls, the bound type
604 -- variables are visible non-monadically
605 -- (i.e. tha args are sufficiently zonked)
606 -- This invariant is needed so that we can "see" the foralls, ad
607 -- e.g. in the SPEC rule where we just use splitSigmaTy
609 tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
610 = tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
611 -- This indirection is just here to make
612 -- it easy to insert a debug trace!
614 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
615 | Just exp_ty' <- tcView exp_ty = tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty'
616 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
617 | Just act_ty' <- tcView act_ty = tc_sub sub_ctxt act_sty act_ty' exp_ib exp_sty exp_ty
619 -----------------------------------
620 -- Rule SBOXY, plus other cases when act_ty is a type variable
621 -- Just defer to boxy matching
622 -- This rule takes precedence over SKOL!
623 tc_sub1 sub_ctxt act_sty (TyVarTy tv) exp_ib exp_sty exp_ty
624 = do { addSubCtxt sub_ctxt act_sty exp_sty $
625 uVar True False tv exp_ib exp_sty exp_ty
626 ; return idHsWrapper }
628 -----------------------------------
629 -- Skolemisation case (rule SKOL)
630 -- actual_ty: d:Eq b => b->b
631 -- expected_ty: forall a. Ord a => a->a
632 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
634 -- It is essential to do this *before* the specialisation case
635 -- Example: f :: (Eq a => a->a) -> ...
636 -- g :: Ord b => b->b
639 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
640 | not exp_ib, -- SKOL does not apply if exp_ty is inside a box
642 = do { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ _ body_exp_ty ->
643 tc_sub sub_ctxt act_sty act_ty False body_exp_ty body_exp_ty
644 ; return (gen_fn <.> co_fn) }
646 act_tvs = tyVarsOfType act_ty
647 -- It's really important to check for escape wrt
648 -- the free vars of both expected_ty *and* actual_ty
650 -----------------------------------
651 -- Specialisation case (rule ASPEC):
652 -- actual_ty: forall a. Ord a => a->a
653 -- expected_ty: Int -> Int
654 -- co_fn e = e Int dOrdInt
656 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
657 -- Implements the new SPEC rule in the Appendix of the paper
658 -- "Boxy types: inference for higher rank types and impredicativity"
659 -- (This appendix isn't in the published version.)
660 -- The idea is to *first* do pre-subsumption, and then full subsumption
661 -- Example: forall a. a->a <= Int -> (forall b. Int)
662 -- Pre-subsumpion finds a|->Int, and that works fine, whereas
663 -- just running full subsumption would fail.
664 | isSigmaTy actual_ty
665 = do { -- Perform pre-subsumption, and instantiate
666 -- the type with info from the pre-subsumption;
667 -- boxy tyvars if pre-subsumption gives no info
668 let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
669 tau_tvs = exactTyVarsOfType tau
670 ; inst_tys <- if exp_ib then -- Inside a box, do not do clever stuff
671 do { tyvars' <- mapM tcInstBoxyTyVar tyvars
672 ; return (mkTyVarTys tyvars') }
673 else -- Outside, do clever stuff
674 preSubType tyvars tau_tvs tau expected_ty
675 ; let subst' = zipOpenTvSubst tyvars inst_tys
676 tau' = substTy subst' tau
678 -- Perform a full subsumption check
679 ; traceTc (text "tc_sub_spec" <+> vcat [ppr actual_ty,
680 ppr tyvars <+> ppr theta <+> ppr tau,
682 ; co_fn2 <- tc_sub sub_ctxt tau' tau' exp_ib exp_sty expected_ty
684 -- Deal with the dictionaries
685 ; co_fn1 <- instCall InstSigOrigin inst_tys (substTheta subst' theta)
686 ; return (co_fn2 <.> co_fn1) }
688 -----------------------------------
689 -- Function case (rule F1)
690 tc_sub1 sub_ctxt act_sty (FunTy act_arg act_res) exp_ib exp_sty (FunTy exp_arg exp_res)
691 = addSubCtxt sub_ctxt act_sty exp_sty $
692 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
694 -- Function case (rule F2)
695 tc_sub1 sub_ctxt act_sty act_ty@(FunTy act_arg act_res) _ exp_sty (TyVarTy exp_tv)
697 = addSubCtxt sub_ctxt act_sty exp_sty $
698 do { cts <- readMetaTyVar exp_tv
700 Indirect ty -> tc_sub SubDone act_sty act_ty True exp_sty ty
701 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
702 ; tc_sub_funs act_arg act_res True arg_ty res_ty } }
704 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
705 mk_res_ty other = panic "TcUnify.mk_res_ty3"
706 fun_kinds = [argTypeKind, openTypeKind]
708 -- Everything else: defer to boxy matching
709 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
710 = do { addSubCtxt sub_ctxt act_sty exp_sty $
711 u_tys True False act_sty actual_ty exp_ib exp_sty expected_ty
712 ; return idHsWrapper }
715 -----------------------------------
716 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
717 = do { uTys False act_arg exp_ib exp_arg
718 ; co_fn_res <- tc_sub SubDone act_res act_res exp_ib exp_res exp_res
719 ; wrapFunResCoercion [exp_arg] co_fn_res }
721 -----------------------------------
723 :: [TcType] -- Type of args
724 -> HsWrapper -- HsExpr a -> HsExpr b
725 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
726 wrapFunResCoercion arg_tys co_fn_res
727 | isIdHsWrapper co_fn_res = return idHsWrapper
728 | null arg_tys = return co_fn_res
730 = do { arg_ids <- newSysLocalIds FSLIT("sub") arg_tys
731 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpApps arg_ids) }
736 %************************************************************************
738 \subsection{Generalisation}
740 %************************************************************************
743 tcGen :: BoxySigmaType -- expected_ty
744 -> TcTyVarSet -- Extra tyvars that the universally
745 -- quantified tyvars of expected_ty
746 -- must not be unified
747 -> ([TcTyVar] -> BoxyRhoType -> TcM result)
748 -> TcM (HsWrapper, result)
749 -- The expression has type: spec_ty -> expected_ty
751 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
752 -- If not, the call is a no-op
753 = do { -- We want the GenSkol info in the skolemised type variables to
754 -- mention the *instantiated* tyvar names, so that we get a
755 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
756 -- Hence the tiresome but innocuous fixM
757 ((tvs', theta', rho'), skol_info) <- fixM (\ ~(_, skol_info) ->
758 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
759 ; span <- getSrcSpanM
760 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty) span
761 ; return ((forall_tvs, theta, rho_ty), skol_info) })
764 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
765 text "expected_ty" <+> ppr expected_ty,
766 text "inst ty" <+> ppr tvs' <+> ppr theta' <+> ppr rho',
767 text "free_tvs" <+> ppr free_tvs])
770 -- Type-check the arg and unify with poly type
771 ; (result, lie) <- getLIE (thing_inside tvs' rho')
773 -- Check that the "forall_tvs" havn't been constrained
774 -- The interesting bit here is that we must include the free variables
775 -- of the expected_ty. Here's an example:
776 -- runST (newVar True)
777 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
778 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
779 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
780 -- So now s' isn't unconstrained because it's linked to a.
781 -- Conclusion: include the free vars of the expected_ty in the
782 -- list of "free vars" for the signature check.
784 ; dicts <- newDictBndrsO (SigOrigin skol_info) theta'
785 ; inst_binds <- tcSimplifyCheck sig_msg tvs' dicts lie
787 ; checkSigTyVarsWrt free_tvs tvs'
788 ; traceTc (text "tcGen:done")
791 -- The WpLet binds any Insts which came out of the simplification.
792 dict_ids = map instToId dicts
793 co_fn = mkWpTyLams tvs' <.> mkWpLams dict_ids <.> WpLet inst_binds
794 ; returnM (co_fn, result) }
796 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
797 sig_msg = ptext SLIT("expected type of an expression")
802 %************************************************************************
806 %************************************************************************
808 The exported functions are all defined as versions of some
809 non-exported generic functions.
812 boxyUnify :: BoxyType -> BoxyType -> TcM ()
813 -- Acutal and expected, respectively
815 = addErrCtxtM (unifyCtxt ty1 ty2) $
816 uTysOuter False ty1 False ty2
819 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM ()
820 -- Arguments should have equal length
821 -- Acutal and expected types
822 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
825 unifyType :: TcTauType -> TcTauType -> TcM ()
826 -- No boxes expected inside these types
827 -- Acutal and expected types
828 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
829 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
830 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
831 addErrCtxtM (unifyCtxt ty1 ty2) $
832 uTysOuter True ty1 True ty2
835 unifyPred :: PredType -> PredType -> TcM ()
836 -- Acutal and expected types
837 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
838 uPred True True p1 True p2
840 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
841 -- Acutal and expected types
842 unifyTheta theta1 theta2
843 = do { checkTc (equalLength theta1 theta2)
844 (ptext SLIT("Contexts differ in length"))
845 ; uList unifyPred theta1 theta2 }
848 uList :: (a -> a -> TcM ())
849 -> [a] -> [a] -> TcM ()
850 -- Unify corresponding elements of two lists of types, which
851 -- should be f equal length. We charge down the list explicitly so that
852 -- we can complain if their lengths differ.
853 uList unify [] [] = return ()
854 uList unify (ty1:tys1) (ty2:tys2) = do { unify ty1 ty2; uList unify tys1 tys2 }
855 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
858 @unifyTypeList@ takes a single list of @TauType@s and unifies them
859 all together. It is used, for example, when typechecking explicit
860 lists, when all the elts should be of the same type.
863 unifyTypeList :: [TcTauType] -> TcM ()
864 unifyTypeList [] = returnM ()
865 unifyTypeList [ty] = returnM ()
866 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
867 ; unifyTypeList tys }
870 %************************************************************************
872 \subsection[Unify-uTys]{@uTys@: getting down to business}
874 %************************************************************************
876 @uTys@ is the heart of the unifier. Each arg happens twice, because
877 we want to report errors in terms of synomyms if poss. The first of
878 the pair is used in error messages only; it is always the same as the
879 second, except that if the first is a synonym then the second may be a
880 de-synonym'd version. This way we get better error messages.
882 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
885 type InBox = Bool -- True <=> we are inside a box
886 -- False <=> we are outside a box
887 -- The importance of this is that if we get "filled-box meets
888 -- filled-box", we'll look into the boxes and unify... but
889 -- we must not allow polytypes. But if we are in a box on
890 -- just one side, then we can allow polytypes
892 type Outer = Bool -- True <=> this is the outer level of a unification
893 -- so that the types being unified are the
894 -- very ones we began with, not some sub
895 -- component or synonym expansion
896 -- The idea is that if Outer is true then unifyMisMatch should
897 -- pop the context to remove the "Expected/Acutal" context
900 :: InBox -> TcType -- ty1 is the *expected* type
901 -> InBox -> TcType -- ty2 is the *actual* type
903 uTysOuter nb1 ty1 nb2 ty2 = do { traceTc (text "uTysOuter" <+> ppr ty1 <+> ppr ty2)
904 ; u_tys True nb1 ty1 ty1 nb2 ty2 ty2 }
905 uTys nb1 ty1 nb2 ty2 = do { traceTc (text "uTys" <+> ppr ty1 <+> ppr ty2)
906 ; u_tys False nb1 ty1 ty1 nb2 ty2 ty2 }
910 uTys_s :: InBox -> [TcType] -- ty1 is the *actual* types
911 -> InBox -> [TcType] -- ty2 is the *expected* types
913 uTys_s nb1 [] nb2 [] = returnM ()
914 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { uTys nb1 ty1 nb2 ty2
915 ; uTys_s nb1 tys1 nb2 tys2 }
916 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
920 -> InBox -> TcType -> TcType -- ty1 is the *actual* type
921 -> InBox -> TcType -> TcType -- ty2 is the *expected* type
924 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
928 -- Always expand synonyms (see notes at end)
929 -- (this also throws away FTVs)
931 | Just ty1' <- tcView ty1 = go False ty1' ty2
932 | Just ty2' <- tcView ty2 = go False ty1 ty2'
934 -- Variables; go for uVar
935 go outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
936 go outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
937 -- "True" means args swapped
939 go outer (PredTy p1) (PredTy p2) = uPred outer nb1 p1 nb2 p2
941 -- Type constructors must match
942 go _ (TyConApp con1 tys1) (TyConApp con2 tys2)
943 | con1 == con2 = uTys_s nb1 tys1 nb2 tys2
944 -- See Note [TyCon app]
946 -- Functions; just check the two parts
947 go _ (FunTy fun1 arg1) (FunTy fun2 arg2)
948 = do { uTys nb1 fun1 nb2 fun2
949 ; uTys nb1 arg1 nb2 arg2 }
951 -- Applications need a bit of care!
952 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
953 -- NB: we've already dealt with type variables and Notes,
954 -- so if one type is an App the other one jolly well better be too
955 go outer (AppTy s1 t1) ty2
956 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
957 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
959 -- Now the same, but the other way round
960 -- Don't swap the types, because the error messages get worse
961 go outer ty1 (AppTy s2 t2)
962 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
963 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
965 go _ ty1@(ForAllTy _ _) ty2@(ForAllTy _ _)
966 | length tvs1 == length tvs2
967 = do { tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
968 ; let tys = mkTyVarTys tvs
969 in_scope = mkInScopeSet (mkVarSet tvs)
970 subst1 = mkTvSubst in_scope (zipTyEnv tvs1 tys)
971 subst2 = mkTvSubst in_scope (zipTyEnv tvs2 tys)
972 ; uTys nb1 (substTy subst1 body1) nb2 (substTy subst2 body2)
974 -- If both sides are inside a box, we are in a "box-meets-box"
975 -- situation, and we should not have a polytype at all.
976 -- If we get here we have two boxes, already filled with
977 -- the same polytype... but it should be a monotype.
978 -- This check comes last, because the error message is
979 -- extremely unhelpful.
980 ; ifM (nb1 && nb2) (notMonoType ty1)
983 (tvs1, body1) = tcSplitForAllTys ty1
984 (tvs2, body2) = tcSplitForAllTys ty2
986 -- Anything else fails
987 go outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
990 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
991 | n1 == n2 = uTys nb1 t1 nb2 t2
992 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
993 | c1 == c2 = uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
994 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
999 When we find two TyConApps, the argument lists are guaranteed equal
1000 length. Reason: intially the kinds of the two types to be unified is
1001 the same. The only way it can become not the same is when unifying two
1002 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
1003 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
1004 which we do, that ensures that f1,f2 have the same kind; and that
1005 means a1,a2 have the same kind. And now the argument repeats.
1010 If you are tempted to make a short cut on synonyms, as in this
1014 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
1015 -- NO = if (con1 == con2) then
1016 -- NO -- Good news! Same synonym constructors, so we can shortcut
1017 -- NO -- by unifying their arguments and ignoring their expansions.
1018 -- NO unifyTypepeLists args1 args2
1020 -- NO -- Never mind. Just expand them and try again
1024 then THINK AGAIN. Here is the whole story, as detected and reported
1025 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1027 Here's a test program that should detect the problem:
1031 x = (1 :: Bogus Char) :: Bogus Bool
1034 The problem with [the attempted shortcut code] is that
1038 is not a sufficient condition to be able to use the shortcut!
1039 You also need to know that the type synonym actually USES all
1040 its arguments. For example, consider the following type synonym
1041 which does not use all its arguments.
1046 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1047 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1048 would fail, even though the expanded forms (both \tr{Int}) should
1051 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1052 unnecessarily bind \tr{t} to \tr{Char}.
1054 ... You could explicitly test for the problem synonyms and mark them
1055 somehow as needing expansion, perhaps also issuing a warning to the
1060 %************************************************************************
1062 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1064 %************************************************************************
1066 @uVar@ is called when at least one of the types being unified is a
1067 variable. It does {\em not} assume that the variable is a fixed point
1068 of the substitution; rather, notice that @uVar@ (defined below) nips
1069 back into @uTys@ if it turns out that the variable is already bound.
1073 -> Bool -- False => tyvar is the "expected"
1074 -- True => ty is the "expected" thing
1076 -> InBox -- True <=> definitely no boxes in t2
1077 -> TcTauType -> TcTauType -- printing and real versions
1080 uVar outer swapped tv1 nb2 ps_ty2 ty2
1081 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1082 | otherwise = brackets (equals <+> ppr ty2)
1083 ; traceTc (text "uVar" <+> ppr swapped <+>
1084 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1085 nest 2 (ptext SLIT(" <-> ")),
1086 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1087 ; details <- lookupTcTyVar tv1
1090 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1091 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1092 -- The 'True' here says that ty1 is now inside a box
1093 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1097 uUnfilledVar :: Outer
1098 -> Bool -- Args are swapped
1099 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1100 -> TcTauType -> TcTauType -- Type 2
1102 -- Invariant: tyvar 1 is not unified with anything
1104 uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1105 | Just ty2' <- tcView ty2
1106 = -- Expand synonyms; ignore FTVs
1107 uUnfilledVar False swapped tv1 details1 ps_ty2 ty2'
1109 uUnfilledVar outer swapped tv1 details1 ps_ty2 (TyVarTy tv2)
1110 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1112 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1113 -- this is box-meets-box, so fill in with a tau-type
1114 -> do { tau_tv <- tcInstTyVar tv1
1115 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv) }
1116 other -> returnM () -- No-op
1118 -- Distinct type variables
1120 = do { lookup2 <- lookupTcTyVar tv2
1122 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
1123 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1126 uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2 -- ty2 is not a type variable
1128 MetaTv (SigTv _) ref1 -> mis_match -- Can't update a skolem with a non-type-variable
1129 MetaTv info ref1 -> uMetaVar swapped tv1 info ref1 ps_ty2 non_var_ty2
1130 skolem_details -> mis_match
1132 mis_match = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1136 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1139 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1140 -- ty2 is not a type variable
1142 uMetaVar swapped tv1 BoxTv ref1 ps_ty2 non_var_ty2
1143 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1144 -- that any boxes in ty2 are filled with monotypes
1146 -- It should not be the case that tv1 occurs in ty2
1147 -- (i.e. no occurs check should be needed), but if perchance
1148 -- it does, the unbox operation will fill it, and the DEBUG
1150 do { final_ty <- unBox ps_ty2
1152 ; meta_details <- readMutVar ref1
1153 ; case meta_details of
1154 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1155 return () -- This really should *not* happen
1158 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1160 uMetaVar swapped tv1 info1 ref1 ps_ty2 non_var_ty2
1161 = do { final_ty <- checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
1162 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1165 uUnfilledVars :: Outer
1166 -> Bool -- Args are swapped
1167 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1168 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1170 -- Invarant: The type variables are distinct,
1171 -- Neither is filled in yet
1172 -- They might be boxy or not
1174 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1175 = unifyMisMatch outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1177 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1178 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2)
1179 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1180 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
1182 -- ToDo: this function seems too long for what it acutally does!
1183 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1184 = case (info1, info2) of
1185 (BoxTv, BoxTv) -> box_meets_box
1187 -- If a box meets a TauTv, but the fomer has the smaller kind
1188 -- then we must create a fresh TauTv with the smaller kind
1189 (_, BoxTv) | k1_sub_k2 -> update_tv2
1190 | otherwise -> box_meets_box
1191 (BoxTv, _ ) | k2_sub_k1 -> update_tv1
1192 | otherwise -> box_meets_box
1194 -- Avoid SigTvs if poss
1195 (SigTv _, _ ) | k1_sub_k2 -> update_tv2
1196 (_, SigTv _) | k2_sub_k1 -> update_tv1
1198 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1199 then update_tv1 -- Same kinds
1201 | k2_sub_k1 -> update_tv1
1202 | otherwise -> kind_err
1204 -- Update the variable with least kind info
1205 -- See notes on type inference in Kind.lhs
1206 -- The "nicer to" part only applies if the two kinds are the same,
1207 -- so we can choose which to do.
1209 -- Kinds should be guaranteed ok at this point
1210 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1211 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1213 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1216 | k2_sub_k1 = fill_from tv2
1217 | otherwise = kind_err
1219 -- Update *both* tyvars with a TauTv whose name and kind
1220 -- are gotten from tv (avoid losing nice names is poss)
1221 fill_from tv = do { tv' <- tcInstTyVar tv
1222 ; let tau_ty = mkTyVarTy tv'
1223 ; updateMeta tv1 ref1 tau_ty
1224 ; updateMeta tv2 ref2 tau_ty }
1226 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1227 unifyKindMisMatch k1 k2
1231 k1_sub_k2 = k1 `isSubKind` k2
1232 k2_sub_k1 = k2 `isSubKind` k1
1234 nicer_to_update_tv1 = isSystemName (Var.varName tv1)
1235 -- Try to update sys-y type variables in preference to ones
1236 -- gotten (say) by instantiating a polymorphic function with
1237 -- a user-written type sig
1240 checkUpdateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1241 -- Update tv1, which is flexi; occurs check is alrady done
1242 -- The 'check' version does a kind check too
1243 -- We do a sub-kind check here: we might unify (a b) with (c d)
1244 -- where b::*->* and d::*; this should fail
1246 checkUpdateMeta swapped tv1 ref1 ty2
1247 = do { checkKinds swapped tv1 ty2
1248 ; updateMeta tv1 ref1 ty2 }
1250 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1251 updateMeta tv1 ref1 ty2
1252 = ASSERT( isMetaTyVar tv1 )
1253 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
1254 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
1255 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
1256 ; writeMutVar ref1 (Indirect ty2) }
1259 checkKinds swapped tv1 ty2
1260 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
1261 -- ty2 has been zonked at this stage, which ensures that
1262 -- its kind has as much boxity information visible as possible.
1263 | tk2 `isSubKind` tk1 = returnM ()
1266 -- Either the kinds aren't compatible
1267 -- (can happen if we unify (a b) with (c d))
1268 -- or we are unifying a lifted type variable with an
1269 -- unlifted type: e.g. (id 3#) is illegal
1270 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1271 unifyKindMisMatch k1 k2
1273 (k1,k2) | swapped = (tk2,tk1)
1274 | otherwise = (tk1,tk2)
1279 checkTauTvUpdate :: TcTyVar -> TcType -> TcM TcType
1280 -- (checkTauTvUpdate tv ty)
1281 -- We are about to update the TauTv tv with ty.
1282 -- Check (a) that tv doesn't occur in ty (occurs check)
1283 -- (b) that ty is a monotype
1284 -- Furthermore, in the interest of (b), if you find an
1285 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
1287 -- Returns the (non-boxy) type to update the type variable with, or fails
1289 checkTauTvUpdate orig_tv orig_ty
1292 go (TyConApp tc tys)
1293 | isSynTyCon tc = go_syn tc tys
1294 | otherwise = do { tys' <- mappM go tys; return (TyConApp tc tys') }
1295 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
1296 go (PredTy p) = do { p' <- go_pred p; return (PredTy p') }
1297 go (FunTy arg res) = do { arg' <- go arg; res' <- go res; return (FunTy arg' res') }
1298 go (AppTy fun arg) = do { fun' <- go fun; arg' <- go arg; return (mkAppTy fun' arg') }
1299 -- NB the mkAppTy; we might have instantiated a
1300 -- type variable to a type constructor, so we need
1301 -- to pull the TyConApp to the top.
1302 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
1305 | orig_tv == tv = occurCheck tv orig_ty -- (a)
1306 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
1307 | otherwise = return (TyVarTy tv)
1308 -- Ordinary (non Tc) tyvars
1309 -- occur inside quantified types
1311 go_pred (ClassP c tys) = do { tys' <- mapM go tys; return (ClassP c tys') }
1312 go_pred (IParam n ty) = do { ty' <- go ty; return (IParam n ty') }
1313 go_pred (EqPred t1 t2) = do { t1' <- go t1; t2' <- go t2; return (EqPred t1' t2') }
1315 go_tyvar tv (SkolemTv _) = return (TyVarTy tv)
1316 go_tyvar tv (MetaTv box ref)
1317 = do { cts <- readMutVar ref
1319 Indirect ty -> go ty
1320 Flexi -> case box of
1321 BoxTv -> fillBoxWithTau tv ref
1322 other -> return (TyVarTy tv)
1325 -- go_syn is called for synonyms only
1326 -- See Note [Type synonyms and the occur check]
1328 | not (isTauTyCon tc)
1329 = notMonoType orig_ty -- (b) again
1331 = do { (msgs, mb_tys') <- tryTc (mapM go tys)
1333 Just tys' -> return (TyConApp tc tys')
1334 -- Retain the synonym (the common case)
1335 Nothing -> go (expectJust "checkTauTvUpdate"
1336 (tcView (TyConApp tc tys)))
1337 -- Try again, expanding the synonym
1340 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
1341 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
1342 -- tau-type meta-variable, whose print-name is the same as tv
1343 -- Choosing the same name is good: when we instantiate a function
1344 -- we allocate boxy tyvars with the same print-name as the quantified
1345 -- tyvar; and then we often fill the box with a tau-tyvar, and again
1346 -- we want to choose the same name.
1347 fillBoxWithTau tv ref
1348 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
1349 ; let tau = mkTyVarTy tv' -- name of the type variable
1350 ; writeMutVar ref (Indirect tau)
1354 Note [Type synonyms and the occur check]
1355 ~~~~~~~~~~~~~~~~~~~~
1356 Basically we want to update tv1 := ps_ty2
1357 because ps_ty2 has type-synonym info, which improves later error messages
1362 f :: (A a -> a -> ()) -> ()
1366 x = f (\ x p -> p x)
1368 In the application (p x), we try to match "t" with "A t". If we go
1369 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
1370 an infinite loop later.
1371 But we should not reject the program, because A t = ().
1372 Rather, we should bind t to () (= non_var_ty2).
1375 stripBoxyType :: BoxyType -> TcM TcType
1376 -- Strip all boxes from the input type, returning a non-boxy type.
1377 -- It's fine for there to be a polytype inside a box (c.f. unBox)
1378 -- All of the boxes should have been filled in by now;
1379 -- hence we return a TcType
1380 stripBoxyType ty = zonkType strip_tv ty
1382 strip_tv tv = ASSERT( not (isBoxyTyVar tv) ) return (TyVarTy tv)
1383 -- strip_tv will be called for *Flexi* meta-tyvars
1384 -- There should not be any Boxy ones; hence the ASSERT
1386 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1387 -- Subtle... we must zap the boxy res_ty
1388 -- to kind * before using it to instantiate a LitInst
1389 -- Calling unBox instead doesn't do the job, because the box
1390 -- often has an openTypeKind, and we don't want to instantiate
1392 zapToMonotype res_ty
1393 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1394 ; boxyUnify res_tau res_ty
1397 unBox :: BoxyType -> TcM TcType
1398 -- unBox implements the judgement
1400 -- with input s', and result s
1402 -- It removes all boxes from the input type, returning a non-boxy type.
1403 -- A filled box in the type can only contain a monotype; unBox fails if not
1404 -- The type can have empty boxes, which unBox fills with a monotype
1406 -- Compare this wth checkTauTvUpdate
1408 -- For once, it's safe to treat synonyms as opaque!
1410 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1411 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1412 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1413 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1414 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1415 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1416 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1418 | isTcTyVar tv -- It's a boxy type variable
1419 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1420 = do { cts <- readMutVar ref -- under nested quantifiers
1422 Flexi -> fillBoxWithTau tv ref
1423 Indirect ty -> do { non_boxy_ty <- unBox ty
1424 ; if isTauTy non_boxy_ty
1425 then return non_boxy_ty
1426 else notMonoType non_boxy_ty }
1428 | otherwise -- Skolems, and meta-tau-variables
1429 = return (TyVarTy tv)
1431 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1432 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1433 unBoxPred (EqPred ty1 ty2) = do { ty1' <- unBox ty1; ty2' <- unBox ty2; return (EqPred ty1' ty2') }
1438 %************************************************************************
1440 \subsection[Unify-context]{Errors and contexts}
1442 %************************************************************************
1448 unifyCtxt act_ty exp_ty tidy_env
1449 = do { act_ty' <- zonkTcType act_ty
1450 ; exp_ty' <- zonkTcType exp_ty
1451 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1452 (env2, act_ty'') = tidyOpenType env1 act_ty'
1453 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1456 mkExpectedActualMsg act_ty exp_ty
1457 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1458 text "Inferred type" <> colon <+> ppr act_ty ])
1461 -- If an error happens we try to figure out whether the function
1462 -- function has been given too many or too few arguments, and say so.
1463 addSubCtxt SubDone actual_res_ty expected_res_ty thing_inside
1465 addSubCtxt sub_ctxt actual_res_ty expected_res_ty thing_inside
1466 = addErrCtxtM mk_err thing_inside
1469 = do { exp_ty' <- zonkTcType expected_res_ty
1470 ; act_ty' <- zonkTcType actual_res_ty
1471 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1472 (env2, act_ty'') = tidyOpenType env1 act_ty'
1473 (exp_args, _) = tcSplitFunTys exp_ty''
1474 (act_args, _) = tcSplitFunTys act_ty''
1476 len_act_args = length act_args
1477 len_exp_args = length exp_args
1479 message = case sub_ctxt of
1480 SubFun fun | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1481 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1482 other -> mkExpectedActualMsg act_ty'' exp_ty''
1483 ; return (env2, message) }
1485 wrongArgsCtxt too_many_or_few fun
1486 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1487 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1488 <+> ptext SLIT("arguments")
1491 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1492 -- tv1 and ty2 are zonked already
1495 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1496 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1498 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1499 | otherwise = (pp1, pp2)
1500 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1501 (env2, ty2') = tidyOpenType env1 ty2
1502 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1503 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1505 unifyMisMatch outer swapped ty1 ty2
1506 = do { (env, msg) <- if swapped then misMatchMsg ty1 ty2
1507 else misMatchMsg ty2 ty1
1509 -- This is the whole point of the 'outer' stuff
1510 ; if outer then popErrCtxt (failWithTcM (env, msg))
1511 else failWithTcM (env, msg)
1515 = do { env0 <- tcInitTidyEnv
1516 ; (env1, pp1, extra1) <- ppr_ty env0 ty1
1517 ; (env2, pp2, extra2) <- ppr_ty env1 ty2
1518 ; return (env2, sep [sep [ptext SLIT("Couldn't match expected type") <+> pp1,
1519 nest 7 (ptext SLIT("against inferred type") <+> pp2)],
1520 nest 2 extra1, nest 2 extra2]) }
1522 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
1524 = do { ty' <- zonkTcType ty
1525 ; let (env1,tidy_ty) = tidyOpenType env ty'
1526 simple_result = (env1, quotes (ppr tidy_ty), empty)
1529 | isSkolemTyVar tv || isSigTyVar tv
1530 -> return (env2, pp_rigid tv', pprSkolTvBinding tv')
1531 | otherwise -> return simple_result
1533 (env2, tv') = tidySkolemTyVar env1 tv
1534 other -> return simple_result }
1536 pp_rigid tv = quotes (ppr tv) <+> parens (ptext SLIT("a rigid variable"))
1540 = do { ty' <- zonkTcType ty
1541 ; env0 <- tcInitTidyEnv
1542 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1543 msg = ptext SLIT("Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1544 ; failWithTcM (env1, msg) }
1547 = do { env0 <- tcInitTidyEnv
1548 ; ty' <- zonkTcType ty
1549 ; let (env1, tidy_tyvar) = tidyOpenTyVar env0 tyvar
1550 (env2, tidy_ty) = tidyOpenType env1 ty'
1551 extra = sep [ppr tidy_tyvar, char '=', ppr tidy_ty]
1552 ; failWithTcM (env2, hang msg 2 extra) }
1554 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
1558 %************************************************************************
1562 %************************************************************************
1564 Unifying kinds is much, much simpler than unifying types.
1567 unifyKind :: TcKind -- Expected
1570 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1571 | isSubKindCon kc2 kc1 = returnM ()
1573 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1574 = do { unifyKind a2 a1; unifyKind r1 r2 }
1575 -- Notice the flip in the argument,
1576 -- so that the sub-kinding works right
1577 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1578 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1579 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1581 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1582 unifyKinds [] [] = returnM ()
1583 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1585 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1588 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1589 uKVar swapped kv1 k2
1590 = do { mb_k1 <- readKindVar kv1
1592 Flexi -> uUnboundKVar swapped kv1 k2
1593 Indirect k1 | swapped -> unifyKind k2 k1
1594 | otherwise -> unifyKind k1 k2 }
1597 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1598 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1599 | kv1 == kv2 = returnM ()
1600 | otherwise -- Distinct kind variables
1601 = do { mb_k2 <- readKindVar kv2
1603 Indirect k2 -> uUnboundKVar swapped kv1 k2
1604 Flexi -> writeKindVar kv1 k2 }
1606 uUnboundKVar swapped kv1 non_var_k2
1607 = do { k2' <- zonkTcKind non_var_k2
1608 ; kindOccurCheck kv1 k2'
1609 ; k2'' <- kindSimpleKind swapped k2'
1610 -- KindVars must be bound only to simple kinds
1611 -- Polarities: (kindSimpleKind True ?) succeeds
1612 -- returning *, corresponding to unifying
1615 ; writeKindVar kv1 k2'' }
1618 kindOccurCheck kv1 k2 -- k2 is zonked
1619 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1621 not_in (TyVarTy kv2) = kv1 /= kv2
1622 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1625 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1626 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1627 -- If the flag is False, it requires k <: sk
1628 -- E.g. kindSimpleKind False ?? = *
1629 -- What about (kv -> *) :=: ?? -> *
1630 kindSimpleKind orig_swapped orig_kind
1631 = go orig_swapped orig_kind
1633 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1635 ; return (mkArrowKind k1' k2') }
1637 | isOpenTypeKind k = return liftedTypeKind
1638 | isArgTypeKind k = return liftedTypeKind
1640 | isLiftedTypeKind k = return liftedTypeKind
1641 | isUnliftedTypeKind k = return unliftedTypeKind
1642 go sw k@(TyVarTy _) = return k -- KindVars are always simple
1643 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1644 <+> ppr orig_swapped <+> ppr orig_kind)
1645 -- I think this can't actually happen
1647 -- T v = MkT v v must be a type
1648 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1651 kindOccurCheckErr tyvar ty
1652 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1653 2 (sep [ppr tyvar, char '=', ppr ty])
1655 unifyKindMisMatch ty1 ty2
1656 = zonkTcKind ty1 `thenM` \ ty1' ->
1657 zonkTcKind ty2 `thenM` \ ty2' ->
1659 msg = hang (ptext SLIT("Couldn't match kind"))
1660 2 (sep [quotes (ppr ty1'),
1661 ptext SLIT("against"),
1668 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1669 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1671 unifyFunKind (TyVarTy kvar)
1672 = readKindVar kvar `thenM` \ maybe_kind ->
1674 Indirect fun_kind -> unifyFunKind fun_kind
1676 do { arg_kind <- newKindVar
1677 ; res_kind <- newKindVar
1678 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1679 ; returnM (Just (arg_kind,res_kind)) }
1681 unifyFunKind (FunTy arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1682 unifyFunKind other = returnM Nothing
1685 %************************************************************************
1689 %************************************************************************
1691 ---------------------------
1692 -- We would like to get a decent error message from
1693 -- (a) Under-applied type constructors
1694 -- f :: (Maybe, Maybe)
1695 -- (b) Over-applied type constructors
1696 -- f :: Int x -> Int x
1700 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1701 -- A fancy wrapper for 'unifyKind', which tries
1702 -- to give decent error messages.
1703 checkExpectedKind ty act_kind exp_kind
1704 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1707 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1709 Just r -> returnM () ; -- Unification succeeded
1712 -- So there's definitely an error
1713 -- Now to find out what sort
1714 zonkTcKind exp_kind `thenM` \ exp_kind ->
1715 zonkTcKind act_kind `thenM` \ act_kind ->
1717 tcInitTidyEnv `thenM` \ env0 ->
1718 let (exp_as, _) = splitKindFunTys exp_kind
1719 (act_as, _) = splitKindFunTys act_kind
1720 n_exp_as = length exp_as
1721 n_act_as = length act_as
1723 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1724 (env2, tidy_act_kind) = tidyKind env1 act_kind
1726 err | n_exp_as < n_act_as -- E.g. [Maybe]
1727 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1729 -- Now n_exp_as >= n_act_as. In the next two cases,
1730 -- n_exp_as == 0, and hence so is n_act_as
1731 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1732 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1733 <+> ptext SLIT("is unlifted")
1735 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1736 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1737 <+> ptext SLIT("is lifted")
1739 | otherwise -- E.g. Monad [Int]
1740 = ptext SLIT("Kind mis-match")
1742 more_info = sep [ ptext SLIT("Expected kind") <+>
1743 quotes (pprKind tidy_exp_kind) <> comma,
1744 ptext SLIT("but") <+> quotes (ppr ty) <+>
1745 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1747 failWithTcM (env2, err $$ more_info)
1751 %************************************************************************
1753 \subsection{Checking signature type variables}
1755 %************************************************************************
1757 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1758 are not mentioned in the environment. In particular:
1760 (a) Not mentioned in the type of a variable in the envt
1761 eg the signature for f in this:
1767 Here, f is forced to be monorphic by the free occurence of x.
1769 (d) Not (unified with another type variable that is) in scope.
1770 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1771 when checking the expression type signature, we find that
1772 even though there is nothing in scope whose type mentions r,
1773 nevertheless the type signature for the expression isn't right.
1775 Another example is in a class or instance declaration:
1777 op :: forall b. a -> b
1779 Here, b gets unified with a
1781 Before doing this, the substitution is applied to the signature type variable.
1784 checkSigTyVars :: [TcTyVar] -> TcM ()
1785 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1787 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1788 -- The extra_tvs can include boxy type variables;
1789 -- e.g. TcMatches.tcCheckExistentialPat
1790 checkSigTyVarsWrt extra_tvs sig_tvs
1791 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1792 ; check_sig_tyvars extra_tvs' sig_tvs }
1795 :: TcTyVarSet -- Global type variables. The universally quantified
1796 -- tyvars should not mention any of these
1797 -- Guaranteed already zonked.
1798 -> [TcTyVar] -- Universally-quantified type variables in the signature
1799 -- Guaranteed to be skolems
1801 check_sig_tyvars extra_tvs []
1803 check_sig_tyvars extra_tvs sig_tvs
1804 = ASSERT( all isSkolemTyVar sig_tvs )
1805 do { gbl_tvs <- tcGetGlobalTyVars
1806 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1807 text "gbl_tvs" <+> ppr gbl_tvs,
1808 text "extra_tvs" <+> ppr extra_tvs]))
1810 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1811 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1812 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1815 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1816 -> [TcTyVar] -- The possibly-escaping type variables
1817 -> [TcTyVar] -- The zonked versions thereof
1819 -- Complain about escaping type variables
1820 -- We pass a list of type variables, at least one of which
1821 -- escapes. The first list contains the original signature type variable,
1822 -- while the second contains the type variable it is unified to (usually itself)
1823 bleatEscapedTvs globals sig_tvs zonked_tvs
1824 = do { env0 <- tcInitTidyEnv
1825 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1826 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1828 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1829 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1831 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1833 check (tidy_env, msgs) (sig_tv, zonked_tv)
1834 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1836 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1837 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1839 -----------------------
1840 escape_msg sig_tv zonked_tv globs
1842 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1843 nest 2 (vcat globs)]
1845 = msg <+> ptext SLIT("escapes")
1846 -- Sigh. It's really hard to give a good error message
1847 -- all the time. One bad case is an existential pattern match.
1848 -- We rely on the "When..." context to help.
1850 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1852 | sig_tv == zonked_tv = empty
1853 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1856 These two context are used with checkSigTyVars
1859 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1860 -> TidyEnv -> TcM (TidyEnv, Message)
1861 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1862 = zonkTcType sig_tau `thenM` \ actual_tau ->
1864 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1865 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1866 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1867 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1868 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1870 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),