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, refineBox, refineBoxToTau, 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 = traceTc (text "tc_sub" <+> ppr act_ty $$ ppr exp_ty) >>
611 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
612 -- This indirection is just here to make
613 -- it easy to insert a debug trace!
615 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
616 | Just exp_ty' <- tcView exp_ty = tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty'
617 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
618 | Just act_ty' <- tcView act_ty = tc_sub sub_ctxt act_sty act_ty' exp_ib exp_sty exp_ty
620 -----------------------------------
621 -- Rule SBOXY, plus other cases when act_ty is a type variable
622 -- Just defer to boxy matching
623 -- This rule takes precedence over SKOL!
624 tc_sub1 sub_ctxt act_sty (TyVarTy tv) exp_ib exp_sty exp_ty
625 = do { addSubCtxt sub_ctxt act_sty exp_sty $
626 uVar True False tv exp_ib exp_sty exp_ty
627 ; return idHsWrapper }
629 -----------------------------------
630 -- Skolemisation case (rule SKOL)
631 -- actual_ty: d:Eq b => b->b
632 -- expected_ty: forall a. Ord a => a->a
633 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
635 -- It is essential to do this *before* the specialisation case
636 -- Example: f :: (Eq a => a->a) -> ...
637 -- g :: Ord b => b->b
640 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
642 = if exp_ib then -- SKOL does not apply if exp_ty is inside a box
643 defer_to_boxy_matching sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
645 { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ _ body_exp_ty ->
646 tc_sub sub_ctxt act_sty act_ty False body_exp_ty body_exp_ty
647 ; return (gen_fn <.> co_fn) }
649 act_tvs = tyVarsOfType act_ty
650 -- It's really important to check for escape wrt
651 -- the free vars of both expected_ty *and* actual_ty
653 -----------------------------------
654 -- Specialisation case (rule ASPEC):
655 -- actual_ty: forall a. Ord a => a->a
656 -- expected_ty: Int -> Int
657 -- co_fn e = e Int dOrdInt
659 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
660 -- Implements the new SPEC rule in the Appendix of the paper
661 -- "Boxy types: inference for higher rank types and impredicativity"
662 -- (This appendix isn't in the published version.)
663 -- The idea is to *first* do pre-subsumption, and then full subsumption
664 -- Example: forall a. a->a <= Int -> (forall b. Int)
665 -- Pre-subsumpion finds a|->Int, and that works fine, whereas
666 -- just running full subsumption would fail.
667 | isSigmaTy actual_ty
668 = do { -- Perform pre-subsumption, and instantiate
669 -- the type with info from the pre-subsumption;
670 -- boxy tyvars if pre-subsumption gives no info
671 let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
672 tau_tvs = exactTyVarsOfType tau
673 ; inst_tys <- if exp_ib then -- Inside a box, do not do clever stuff
674 do { tyvars' <- mapM tcInstBoxyTyVar tyvars
675 ; return (mkTyVarTys tyvars') }
676 else -- Outside, do clever stuff
677 preSubType tyvars tau_tvs tau expected_ty
678 ; let subst' = zipOpenTvSubst tyvars inst_tys
679 tau' = substTy subst' tau
681 -- Perform a full subsumption check
682 ; traceTc (text "tc_sub_spec" <+> vcat [ppr actual_ty,
683 ppr tyvars <+> ppr theta <+> ppr tau,
685 ; co_fn2 <- tc_sub sub_ctxt tau' tau' exp_ib exp_sty expected_ty
687 -- Deal with the dictionaries
688 -- The origin gives a helpful origin when we have
689 -- a function with type f :: Int -> forall a. Num a => ...
690 -- This way the (Num a) dictionary gets an OccurrenceOf f origin
691 ; let orig = case sub_ctxt of
692 SubFun n -> OccurrenceOf n
693 other -> InstSigOrigin -- Unhelpful
694 ; co_fn1 <- instCall orig inst_tys (substTheta subst' theta)
695 ; return (co_fn2 <.> co_fn1) }
697 -----------------------------------
698 -- Function case (rule F1)
699 tc_sub1 sub_ctxt act_sty (FunTy act_arg act_res) exp_ib exp_sty (FunTy exp_arg exp_res)
700 = addSubCtxt sub_ctxt act_sty exp_sty $
701 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
703 -- Function case (rule F2)
704 tc_sub1 sub_ctxt act_sty act_ty@(FunTy act_arg act_res) _ exp_sty (TyVarTy exp_tv)
706 = addSubCtxt sub_ctxt act_sty exp_sty $
707 do { cts <- readMetaTyVar exp_tv
709 Indirect ty -> tc_sub SubDone act_sty act_ty True exp_sty ty
710 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
711 ; tc_sub_funs act_arg act_res True arg_ty res_ty } }
713 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
714 mk_res_ty other = panic "TcUnify.mk_res_ty3"
715 fun_kinds = [argTypeKind, openTypeKind]
717 -- Everything else: defer to boxy matching
718 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
719 = defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
721 -----------------------------------
722 defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
723 = do { addSubCtxt sub_ctxt act_sty exp_sty $
724 u_tys True False act_sty actual_ty exp_ib exp_sty expected_ty
725 ; return idHsWrapper }
727 -----------------------------------
728 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
729 = do { uTys False act_arg exp_ib exp_arg
730 ; co_fn_res <- tc_sub SubDone act_res act_res exp_ib exp_res exp_res
731 ; wrapFunResCoercion [exp_arg] co_fn_res }
733 -----------------------------------
735 :: [TcType] -- Type of args
736 -> HsWrapper -- HsExpr a -> HsExpr b
737 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
738 wrapFunResCoercion arg_tys co_fn_res
739 | isIdHsWrapper co_fn_res = return idHsWrapper
740 | null arg_tys = return co_fn_res
742 = do { arg_ids <- newSysLocalIds FSLIT("sub") arg_tys
743 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpApps arg_ids) }
748 %************************************************************************
750 \subsection{Generalisation}
752 %************************************************************************
755 tcGen :: BoxySigmaType -- expected_ty
756 -> TcTyVarSet -- Extra tyvars that the universally
757 -- quantified tyvars of expected_ty
758 -- must not be unified
759 -> ([TcTyVar] -> BoxyRhoType -> TcM result)
760 -> TcM (HsWrapper, result)
761 -- The expression has type: spec_ty -> expected_ty
763 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
764 -- If not, the call is a no-op
765 = do { -- We want the GenSkol info in the skolemised type variables to
766 -- mention the *instantiated* tyvar names, so that we get a
767 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
768 -- Hence the tiresome but innocuous fixM
769 ((tvs', theta', rho'), skol_info) <- fixM (\ ~(_, skol_info) ->
770 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
771 -- Get loation from monad, not from expected_ty
772 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty)
773 ; return ((forall_tvs, theta, rho_ty), skol_info) })
776 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
777 text "expected_ty" <+> ppr expected_ty,
778 text "inst ty" <+> ppr tvs' <+> ppr theta' <+> ppr rho',
779 text "free_tvs" <+> ppr free_tvs])
782 -- Type-check the arg and unify with poly type
783 ; (result, lie) <- getLIE (thing_inside tvs' rho')
785 -- Check that the "forall_tvs" havn't been constrained
786 -- The interesting bit here is that we must include the free variables
787 -- of the expected_ty. Here's an example:
788 -- runST (newVar True)
789 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
790 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
791 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
792 -- So now s' isn't unconstrained because it's linked to a.
793 -- Conclusion: include the free vars of the expected_ty in the
794 -- list of "free vars" for the signature check.
796 ; loc <- getInstLoc (SigOrigin skol_info)
797 ; dicts <- newDictBndrs loc theta'
798 ; inst_binds <- tcSimplifyCheck loc tvs' dicts lie
800 ; checkSigTyVarsWrt free_tvs tvs'
801 ; traceTc (text "tcGen:done")
804 -- The WpLet binds any Insts which came out of the simplification.
805 dict_ids = map instToId dicts
806 co_fn = mkWpTyLams tvs' <.> mkWpLams dict_ids <.> WpLet inst_binds
807 ; returnM (co_fn, result) }
809 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
814 %************************************************************************
818 %************************************************************************
820 The exported functions are all defined as versions of some
821 non-exported generic functions.
824 boxyUnify :: BoxyType -> BoxyType -> TcM ()
825 -- Acutal and expected, respectively
827 = addErrCtxtM (unifyCtxt ty1 ty2) $
828 uTysOuter False ty1 False ty2
831 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM ()
832 -- Arguments should have equal length
833 -- Acutal and expected types
834 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
837 unifyType :: TcTauType -> TcTauType -> TcM ()
838 -- No boxes expected inside these types
839 -- Acutal and expected types
840 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
841 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
842 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
843 addErrCtxtM (unifyCtxt ty1 ty2) $
844 uTysOuter True ty1 True ty2
847 unifyPred :: PredType -> PredType -> TcM ()
848 -- Acutal and expected types
849 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
850 uPred True True p1 True p2
852 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
853 -- Acutal and expected types
854 unifyTheta theta1 theta2
855 = do { checkTc (equalLength theta1 theta2)
856 (vcat [ptext SLIT("Contexts differ in length"),
857 nest 2 $ parens $ ptext SLIT("Use -fglasgow-exts to allow this")])
858 ; uList unifyPred theta1 theta2 }
861 uList :: (a -> a -> TcM ())
862 -> [a] -> [a] -> TcM ()
863 -- Unify corresponding elements of two lists of types, which
864 -- should be f equal length. We charge down the list explicitly so that
865 -- we can complain if their lengths differ.
866 uList unify [] [] = return ()
867 uList unify (ty1:tys1) (ty2:tys2) = do { unify ty1 ty2; uList unify tys1 tys2 }
868 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
871 @unifyTypeList@ takes a single list of @TauType@s and unifies them
872 all together. It is used, for example, when typechecking explicit
873 lists, when all the elts should be of the same type.
876 unifyTypeList :: [TcTauType] -> TcM ()
877 unifyTypeList [] = returnM ()
878 unifyTypeList [ty] = returnM ()
879 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
880 ; unifyTypeList tys }
883 %************************************************************************
885 \subsection[Unify-uTys]{@uTys@: getting down to business}
887 %************************************************************************
889 @uTys@ is the heart of the unifier. Each arg happens twice, because
890 we want to report errors in terms of synomyms if poss. The first of
891 the pair is used in error messages only; it is always the same as the
892 second, except that if the first is a synonym then the second may be a
893 de-synonym'd version. This way we get better error messages.
895 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
898 type InBox = Bool -- True <=> we are inside a box
899 -- False <=> we are outside a box
900 -- The importance of this is that if we get "filled-box meets
901 -- filled-box", we'll look into the boxes and unify... but
902 -- we must not allow polytypes. But if we are in a box on
903 -- just one side, then we can allow polytypes
905 type Outer = Bool -- True <=> this is the outer level of a unification
906 -- so that the types being unified are the
907 -- very ones we began with, not some sub
908 -- component or synonym expansion
909 -- The idea is that if Outer is true then unifyMisMatch should
910 -- pop the context to remove the "Expected/Acutal" context
913 :: InBox -> TcType -- ty1 is the *expected* type
914 -> InBox -> TcType -- ty2 is the *actual* type
916 uTysOuter nb1 ty1 nb2 ty2 = do { traceTc (text "uTysOuter" <+> ppr ty1 <+> ppr ty2)
917 ; u_tys True nb1 ty1 ty1 nb2 ty2 ty2 }
918 uTys nb1 ty1 nb2 ty2 = do { traceTc (text "uTys" <+> ppr ty1 <+> ppr ty2)
919 ; u_tys False nb1 ty1 ty1 nb2 ty2 ty2 }
923 uTys_s :: InBox -> [TcType] -- ty1 is the *actual* types
924 -> InBox -> [TcType] -- ty2 is the *expected* types
926 uTys_s nb1 [] nb2 [] = returnM ()
927 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { uTys nb1 ty1 nb2 ty2
928 ; uTys_s nb1 tys1 nb2 tys2 }
929 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
933 -> InBox -> TcType -> TcType -- ty1 is the *actual* type
934 -> InBox -> TcType -> TcType -- ty2 is the *expected* type
937 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
941 -- Always expand synonyms (see notes at end)
942 -- (this also throws away FTVs)
944 | Just ty1' <- tcView ty1 = go False ty1' ty2
945 | Just ty2' <- tcView ty2 = go False ty1 ty2'
947 -- Variables; go for uVar
948 go outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
949 go outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
950 -- "True" means args swapped
952 -- The case for sigma-types must *follow* the variable cases
953 -- because a boxy variable can be filed with a polytype;
954 -- but must precede FunTy, because ((?x::Int) => ty) look
955 -- like a FunTy; there isn't necy a forall at the top
957 | isSigmaTy ty1 || isSigmaTy ty2
958 = do { checkM (equalLength tvs1 tvs2)
959 (unifyMisMatch outer False orig_ty1 orig_ty2)
961 ; tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
962 -- Get location from monad, not from tvs1
963 ; let tys = mkTyVarTys tvs
964 in_scope = mkInScopeSet (mkVarSet tvs)
965 subst1 = mkTvSubst in_scope (zipTyEnv tvs1 tys)
966 subst2 = mkTvSubst in_scope (zipTyEnv tvs2 tys)
967 (theta1,tau1) = tcSplitPhiTy (substTy subst1 body1)
968 (theta2,tau2) = tcSplitPhiTy (substTy subst2 body2)
970 ; checkM (equalLength theta1 theta2)
971 (unifyMisMatch outer False orig_ty1 orig_ty2)
973 ; uPreds False nb1 theta1 nb2 theta2
974 ; uTys nb1 tau1 nb2 tau2
976 -- If both sides are inside a box, we are in a "box-meets-box"
977 -- situation, and we should not have a polytype at all.
978 -- If we get here we have two boxes, already filled with
979 -- the same polytype... but it should be a monotype.
980 -- This check comes last, because the error message is
981 -- extremely unhelpful.
982 ; ifM (nb1 && nb2) (notMonoType ty1)
985 (tvs1, body1) = tcSplitForAllTys ty1
986 (tvs2, body2) = tcSplitForAllTys ty2
989 go outer (PredTy p1) (PredTy p2) = uPred False nb1 p1 nb2 p2
991 -- Type constructors must match
992 go _ (TyConApp con1 tys1) (TyConApp con2 tys2)
993 | con1 == con2 = uTys_s nb1 tys1 nb2 tys2
994 -- See Note [TyCon app]
996 -- Functions; just check the two parts
997 go _ (FunTy fun1 arg1) (FunTy fun2 arg2)
998 = do { uTys nb1 fun1 nb2 fun2
999 ; uTys nb1 arg1 nb2 arg2 }
1001 -- Applications need a bit of care!
1002 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
1003 -- NB: we've already dealt with type variables and Notes,
1004 -- so if one type is an App the other one jolly well better be too
1005 go outer (AppTy s1 t1) ty2
1006 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
1007 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
1009 -- Now the same, but the other way round
1010 -- Don't swap the types, because the error messages get worse
1011 go outer ty1 (AppTy s2 t2)
1012 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
1013 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
1016 -- Anything else fails
1017 go outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
1020 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
1021 | n1 == n2 = uTys nb1 t1 nb2 t2
1022 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
1023 | c1 == c2 = uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
1024 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
1026 uPreds outer nb1 [] nb2 [] = return ()
1027 uPreds outer nb1 (p1:ps1) nb2 (p2:ps2) = uPred outer nb1 p1 nb2 p2 >> uPreds outer nb1 ps1 nb2 ps2
1028 uPreds outer nb1 ps1 nb2 ps2 = panic "uPreds"
1033 When we find two TyConApps, the argument lists are guaranteed equal
1034 length. Reason: intially the kinds of the two types to be unified is
1035 the same. The only way it can become not the same is when unifying two
1036 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
1037 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
1038 which we do, that ensures that f1,f2 have the same kind; and that
1039 means a1,a2 have the same kind. And now the argument repeats.
1044 If you are tempted to make a short cut on synonyms, as in this
1048 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
1049 -- NO = if (con1 == con2) then
1050 -- NO -- Good news! Same synonym constructors, so we can shortcut
1051 -- NO -- by unifying their arguments and ignoring their expansions.
1052 -- NO unifyTypepeLists args1 args2
1054 -- NO -- Never mind. Just expand them and try again
1058 then THINK AGAIN. Here is the whole story, as detected and reported
1059 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1061 Here's a test program that should detect the problem:
1065 x = (1 :: Bogus Char) :: Bogus Bool
1068 The problem with [the attempted shortcut code] is that
1072 is not a sufficient condition to be able to use the shortcut!
1073 You also need to know that the type synonym actually USES all
1074 its arguments. For example, consider the following type synonym
1075 which does not use all its arguments.
1080 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1081 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1082 would fail, even though the expanded forms (both \tr{Int}) should
1085 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1086 unnecessarily bind \tr{t} to \tr{Char}.
1088 ... You could explicitly test for the problem synonyms and mark them
1089 somehow as needing expansion, perhaps also issuing a warning to the
1094 %************************************************************************
1096 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1098 %************************************************************************
1100 @uVar@ is called when at least one of the types being unified is a
1101 variable. It does {\em not} assume that the variable is a fixed point
1102 of the substitution; rather, notice that @uVar@ (defined below) nips
1103 back into @uTys@ if it turns out that the variable is already bound.
1107 -> Bool -- False => tyvar is the "expected"
1108 -- True => ty is the "expected" thing
1110 -> InBox -- True <=> definitely no boxes in t2
1111 -> TcTauType -> TcTauType -- printing and real versions
1114 uVar outer swapped tv1 nb2 ps_ty2 ty2
1115 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1116 | otherwise = brackets (equals <+> ppr ty2)
1117 ; traceTc (text "uVar" <+> ppr swapped <+>
1118 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1119 nest 2 (ptext SLIT(" <-> ")),
1120 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1121 ; details <- lookupTcTyVar tv1
1124 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1125 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1126 -- The 'True' here says that ty1 is now inside a box
1127 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1131 uUnfilledVar :: Outer
1132 -> Bool -- Args are swapped
1133 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1134 -> TcTauType -> TcTauType -- Type 2
1136 -- Invariant: tyvar 1 is not unified with anything
1138 uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1139 | Just ty2' <- tcView ty2
1140 = -- Expand synonyms; ignore FTVs
1141 uUnfilledVar False swapped tv1 details1 ps_ty2 ty2'
1143 uUnfilledVar outer swapped tv1 details1 ps_ty2 (TyVarTy tv2)
1144 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1146 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1147 -- this is box-meets-box, so fill in with a tau-type
1148 -> do { tau_tv <- tcInstTyVar tv1
1149 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv) }
1150 other -> returnM () -- No-op
1152 -- Distinct type variables
1154 = do { lookup2 <- lookupTcTyVar tv2
1156 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
1157 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1160 uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2 -- ty2 is not a type variable
1162 MetaTv (SigTv _) ref1 -> mis_match -- Can't update a skolem with a non-type-variable
1163 MetaTv info ref1 -> uMetaVar swapped tv1 info ref1 ps_ty2 non_var_ty2
1164 skolem_details -> mis_match
1166 mis_match = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1170 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1173 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1174 -- ty2 is not a type variable
1176 uMetaVar swapped tv1 BoxTv ref1 ps_ty2 non_var_ty2
1177 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1178 -- that any boxes in ty2 are filled with monotypes
1180 -- It should not be the case that tv1 occurs in ty2
1181 -- (i.e. no occurs check should be needed), but if perchance
1182 -- it does, the unbox operation will fill it, and the DEBUG
1184 do { final_ty <- unBox ps_ty2
1186 ; meta_details <- readMutVar ref1
1187 ; case meta_details of
1188 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1189 return () -- This really should *not* happen
1192 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1194 uMetaVar swapped tv1 info1 ref1 ps_ty2 non_var_ty2
1195 = do { final_ty <- checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
1196 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1199 uUnfilledVars :: Outer
1200 -> Bool -- Args are swapped
1201 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1202 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1204 -- Invarant: The type variables are distinct,
1205 -- Neither is filled in yet
1206 -- They might be boxy or not
1208 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1209 = unifyMisMatch outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1211 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1212 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2)
1213 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1214 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
1216 -- ToDo: this function seems too long for what it acutally does!
1217 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1218 = case (info1, info2) of
1219 (BoxTv, BoxTv) -> box_meets_box
1221 -- If a box meets a TauTv, but the fomer has the smaller kind
1222 -- then we must create a fresh TauTv with the smaller kind
1223 (_, BoxTv) | k1_sub_k2 -> update_tv2
1224 | otherwise -> box_meets_box
1225 (BoxTv, _ ) | k2_sub_k1 -> update_tv1
1226 | otherwise -> box_meets_box
1228 -- Avoid SigTvs if poss
1229 (SigTv _, _ ) | k1_sub_k2 -> update_tv2
1230 (_, SigTv _) | k2_sub_k1 -> update_tv1
1232 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1233 then update_tv1 -- Same kinds
1235 | k2_sub_k1 -> update_tv1
1236 | otherwise -> kind_err
1238 -- Update the variable with least kind info
1239 -- See notes on type inference in Kind.lhs
1240 -- The "nicer to" part only applies if the two kinds are the same,
1241 -- so we can choose which to do.
1243 -- Kinds should be guaranteed ok at this point
1244 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1245 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1247 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1250 | k2_sub_k1 = fill_from tv2
1251 | otherwise = kind_err
1253 -- Update *both* tyvars with a TauTv whose name and kind
1254 -- are gotten from tv (avoid losing nice names is poss)
1255 fill_from tv = do { tv' <- tcInstTyVar tv
1256 ; let tau_ty = mkTyVarTy tv'
1257 ; updateMeta tv1 ref1 tau_ty
1258 ; updateMeta tv2 ref2 tau_ty }
1260 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1261 unifyKindMisMatch k1 k2
1265 k1_sub_k2 = k1 `isSubKind` k2
1266 k2_sub_k1 = k2 `isSubKind` k1
1268 nicer_to_update_tv1 = isSystemName (Var.varName tv1)
1269 -- Try to update sys-y type variables in preference to ones
1270 -- gotten (say) by instantiating a polymorphic function with
1271 -- a user-written type sig
1274 checkUpdateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1275 -- Update tv1, which is flexi; occurs check is alrady done
1276 -- The 'check' version does a kind check too
1277 -- We do a sub-kind check here: we might unify (a b) with (c d)
1278 -- where b::*->* and d::*; this should fail
1280 checkUpdateMeta swapped tv1 ref1 ty2
1281 = do { checkKinds swapped tv1 ty2
1282 ; updateMeta tv1 ref1 ty2 }
1284 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1285 updateMeta tv1 ref1 ty2
1286 = ASSERT( isMetaTyVar tv1 )
1287 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
1288 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
1289 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
1290 ; writeMutVar ref1 (Indirect ty2) }
1293 checkKinds swapped tv1 ty2
1294 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
1295 -- ty2 has been zonked at this stage, which ensures that
1296 -- its kind has as much boxity information visible as possible.
1297 | tk2 `isSubKind` tk1 = returnM ()
1300 -- Either the kinds aren't compatible
1301 -- (can happen if we unify (a b) with (c d))
1302 -- or we are unifying a lifted type variable with an
1303 -- unlifted type: e.g. (id 3#) is illegal
1304 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1305 unifyKindMisMatch k1 k2
1307 (k1,k2) | swapped = (tk2,tk1)
1308 | otherwise = (tk1,tk2)
1313 checkTauTvUpdate :: TcTyVar -> TcType -> TcM TcType
1314 -- (checkTauTvUpdate tv ty)
1315 -- We are about to update the TauTv tv with ty.
1316 -- Check (a) that tv doesn't occur in ty (occurs check)
1317 -- (b) that ty is a monotype
1318 -- Furthermore, in the interest of (b), if you find an
1319 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
1321 -- Returns the (non-boxy) type to update the type variable with, or fails
1323 checkTauTvUpdate orig_tv orig_ty
1326 go (TyConApp tc tys)
1327 | isSynTyCon tc = go_syn tc tys
1328 | otherwise = do { tys' <- mappM go tys; return (TyConApp tc tys') }
1329 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
1330 go (PredTy p) = do { p' <- go_pred p; return (PredTy p') }
1331 go (FunTy arg res) = do { arg' <- go arg; res' <- go res; return (FunTy arg' res') }
1332 go (AppTy fun arg) = do { fun' <- go fun; arg' <- go arg; return (mkAppTy fun' arg') }
1333 -- NB the mkAppTy; we might have instantiated a
1334 -- type variable to a type constructor, so we need
1335 -- to pull the TyConApp to the top.
1336 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
1339 | orig_tv == tv = occurCheck tv orig_ty -- (a)
1340 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
1341 | otherwise = return (TyVarTy tv)
1342 -- Ordinary (non Tc) tyvars
1343 -- occur inside quantified types
1345 go_pred (ClassP c tys) = do { tys' <- mapM go tys; return (ClassP c tys') }
1346 go_pred (IParam n ty) = do { ty' <- go ty; return (IParam n ty') }
1347 go_pred (EqPred t1 t2) = do { t1' <- go t1; t2' <- go t2; return (EqPred t1' t2') }
1349 go_tyvar tv (SkolemTv _) = return (TyVarTy tv)
1350 go_tyvar tv (MetaTv box ref)
1351 = do { cts <- readMutVar ref
1353 Indirect ty -> go ty
1354 Flexi -> case box of
1355 BoxTv -> fillBoxWithTau tv ref
1356 other -> return (TyVarTy tv)
1359 -- go_syn is called for synonyms only
1360 -- See Note [Type synonyms and the occur check]
1362 | not (isTauTyCon tc)
1363 = notMonoType orig_ty -- (b) again
1365 = do { (msgs, mb_tys') <- tryTc (mapM go tys)
1367 Just tys' -> return (TyConApp tc tys')
1368 -- Retain the synonym (the common case)
1369 Nothing -> go (expectJust "checkTauTvUpdate"
1370 (tcView (TyConApp tc tys)))
1371 -- Try again, expanding the synonym
1374 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
1375 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
1376 -- tau-type meta-variable, whose print-name is the same as tv
1377 -- Choosing the same name is good: when we instantiate a function
1378 -- we allocate boxy tyvars with the same print-name as the quantified
1379 -- tyvar; and then we often fill the box with a tau-tyvar, and again
1380 -- we want to choose the same name.
1381 fillBoxWithTau tv ref
1382 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
1383 ; let tau = mkTyVarTy tv' -- name of the type variable
1384 ; writeMutVar ref (Indirect tau)
1388 Note [Type synonyms and the occur check]
1389 ~~~~~~~~~~~~~~~~~~~~
1390 Basically we want to update tv1 := ps_ty2
1391 because ps_ty2 has type-synonym info, which improves later error messages
1396 f :: (A a -> a -> ()) -> ()
1400 x = f (\ x p -> p x)
1402 In the application (p x), we try to match "t" with "A t". If we go
1403 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
1404 an infinite loop later.
1405 But we should not reject the program, because A t = ().
1406 Rather, we should bind t to () (= non_var_ty2).
1409 refineBox :: TcType -> TcM TcType
1410 -- Unbox the outer box of a boxy type (if any)
1411 refineBox ty@(TyVarTy box_tv)
1412 | isMetaTyVar box_tv
1413 = do { cts <- readMetaTyVar box_tv
1416 Indirect ty -> return ty }
1417 refineBox other_ty = return other_ty
1419 refineBoxToTau :: TcType -> TcM TcType
1420 -- Unbox the outer box of a boxy type, filling with a monotype if it is empty
1421 -- Like refineBox except for the "fill with monotype" part.
1422 refineBoxToTau ty@(TyVarTy box_tv)
1423 | isMetaTyVar box_tv
1424 , MetaTv BoxTv ref <- tcTyVarDetails box_tv
1425 = do { cts <- readMutVar ref
1427 Flexi -> fillBoxWithTau box_tv ref
1428 Indirect ty -> return ty }
1429 refineBoxToTau other_ty = return other_ty
1431 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1432 -- Subtle... we must zap the boxy res_ty
1433 -- to kind * before using it to instantiate a LitInst
1434 -- Calling unBox instead doesn't do the job, because the box
1435 -- often has an openTypeKind, and we don't want to instantiate
1437 zapToMonotype res_ty
1438 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1439 ; boxyUnify res_tau res_ty
1442 unBox :: BoxyType -> TcM TcType
1443 -- unBox implements the judgement
1445 -- with input s', and result s
1447 -- It removes all boxes from the input type, returning a non-boxy type.
1448 -- A filled box in the type can only contain a monotype; unBox fails if not
1449 -- The type can have empty boxes, which unBox fills with a monotype
1451 -- Compare this wth checkTauTvUpdate
1453 -- For once, it's safe to treat synonyms as opaque!
1455 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1456 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1457 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1458 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1459 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1460 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1461 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1463 | isTcTyVar tv -- It's a boxy type variable
1464 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1465 = do { cts <- readMutVar ref -- under nested quantifiers
1467 Flexi -> fillBoxWithTau tv ref
1468 Indirect ty -> do { non_boxy_ty <- unBox ty
1469 ; if isTauTy non_boxy_ty
1470 then return non_boxy_ty
1471 else notMonoType non_boxy_ty }
1473 | otherwise -- Skolems, and meta-tau-variables
1474 = return (TyVarTy tv)
1476 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1477 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1478 unBoxPred (EqPred ty1 ty2) = do { ty1' <- unBox ty1; ty2' <- unBox ty2; return (EqPred ty1' ty2') }
1483 %************************************************************************
1485 \subsection[Unify-context]{Errors and contexts}
1487 %************************************************************************
1493 unifyCtxt act_ty exp_ty tidy_env
1494 = do { act_ty' <- zonkTcType act_ty
1495 ; exp_ty' <- zonkTcType exp_ty
1496 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1497 (env2, act_ty'') = tidyOpenType env1 act_ty'
1498 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1501 mkExpectedActualMsg act_ty exp_ty
1502 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1503 text "Inferred type" <> colon <+> ppr act_ty ])
1506 -- If an error happens we try to figure out whether the function
1507 -- function has been given too many or too few arguments, and say so.
1508 addSubCtxt SubDone actual_res_ty expected_res_ty thing_inside
1510 addSubCtxt sub_ctxt actual_res_ty expected_res_ty thing_inside
1511 = addErrCtxtM mk_err thing_inside
1514 = do { exp_ty' <- zonkTcType expected_res_ty
1515 ; act_ty' <- zonkTcType actual_res_ty
1516 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1517 (env2, act_ty'') = tidyOpenType env1 act_ty'
1518 (exp_args, _) = tcSplitFunTys exp_ty''
1519 (act_args, _) = tcSplitFunTys act_ty''
1521 len_act_args = length act_args
1522 len_exp_args = length exp_args
1524 message = case sub_ctxt of
1525 SubFun fun | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1526 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1527 other -> mkExpectedActualMsg act_ty'' exp_ty''
1528 ; return (env2, message) }
1530 wrongArgsCtxt too_many_or_few fun
1531 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1532 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1533 <+> ptext SLIT("arguments")
1536 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1537 -- tv1 and ty2 are zonked already
1540 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1541 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1543 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1544 | otherwise = (pp1, pp2)
1545 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1546 (env2, ty2') = tidyOpenType env1 ty2
1547 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1548 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1550 unifyMisMatch outer swapped ty1 ty2
1551 = do { (env, msg) <- if swapped then misMatchMsg ty1 ty2
1552 else misMatchMsg ty2 ty1
1554 -- This is the whole point of the 'outer' stuff
1555 ; if outer then popErrCtxt (failWithTcM (env, msg))
1556 else failWithTcM (env, msg)
1560 = do { env0 <- tcInitTidyEnv
1561 ; (env1, pp1, extra1) <- ppr_ty env0 ty1
1562 ; (env2, pp2, extra2) <- ppr_ty env1 ty2
1563 ; return (env2, sep [sep [ptext SLIT("Couldn't match expected type") <+> pp1,
1564 nest 7 (ptext SLIT("against inferred type") <+> pp2)],
1565 nest 2 extra1, nest 2 extra2]) }
1567 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
1569 = do { ty' <- zonkTcType ty
1570 ; let (env1,tidy_ty) = tidyOpenType env ty'
1571 simple_result = (env1, quotes (ppr tidy_ty), empty)
1574 | isSkolemTyVar tv || isSigTyVar tv
1575 -> return (env2, pp_rigid tv', pprSkolTvBinding tv')
1576 | otherwise -> return simple_result
1578 (env2, tv') = tidySkolemTyVar env1 tv
1579 other -> return simple_result }
1581 pp_rigid tv = quotes (ppr tv) <+> parens (ptext SLIT("a rigid variable"))
1585 = do { ty' <- zonkTcType ty
1586 ; env0 <- tcInitTidyEnv
1587 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1588 msg = ptext SLIT("Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1589 ; failWithTcM (env1, msg) }
1592 = do { env0 <- tcInitTidyEnv
1593 ; ty' <- zonkTcType ty
1594 ; let (env1, tidy_tyvar) = tidyOpenTyVar env0 tyvar
1595 (env2, tidy_ty) = tidyOpenType env1 ty'
1596 extra = sep [ppr tidy_tyvar, char '=', ppr tidy_ty]
1597 ; failWithTcM (env2, hang msg 2 extra) }
1599 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
1603 %************************************************************************
1607 %************************************************************************
1609 Unifying kinds is much, much simpler than unifying types.
1612 unifyKind :: TcKind -- Expected
1615 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1616 | isSubKindCon kc2 kc1 = returnM ()
1618 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1619 = do { unifyKind a2 a1; unifyKind r1 r2 }
1620 -- Notice the flip in the argument,
1621 -- so that the sub-kinding works right
1622 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1623 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1624 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1626 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1627 unifyKinds [] [] = returnM ()
1628 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1630 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1633 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1634 uKVar swapped kv1 k2
1635 = do { mb_k1 <- readKindVar kv1
1637 Flexi -> uUnboundKVar swapped kv1 k2
1638 Indirect k1 | swapped -> unifyKind k2 k1
1639 | otherwise -> unifyKind k1 k2 }
1642 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1643 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1644 | kv1 == kv2 = returnM ()
1645 | otherwise -- Distinct kind variables
1646 = do { mb_k2 <- readKindVar kv2
1648 Indirect k2 -> uUnboundKVar swapped kv1 k2
1649 Flexi -> writeKindVar kv1 k2 }
1651 uUnboundKVar swapped kv1 non_var_k2
1652 = do { k2' <- zonkTcKind non_var_k2
1653 ; kindOccurCheck kv1 k2'
1654 ; k2'' <- kindSimpleKind swapped k2'
1655 -- KindVars must be bound only to simple kinds
1656 -- Polarities: (kindSimpleKind True ?) succeeds
1657 -- returning *, corresponding to unifying
1660 ; writeKindVar kv1 k2'' }
1663 kindOccurCheck kv1 k2 -- k2 is zonked
1664 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1666 not_in (TyVarTy kv2) = kv1 /= kv2
1667 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1670 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1671 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1672 -- If the flag is False, it requires k <: sk
1673 -- E.g. kindSimpleKind False ?? = *
1674 -- What about (kv -> *) :=: ?? -> *
1675 kindSimpleKind orig_swapped orig_kind
1676 = go orig_swapped orig_kind
1678 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1680 ; return (mkArrowKind k1' k2') }
1682 | isOpenTypeKind k = return liftedTypeKind
1683 | isArgTypeKind k = return liftedTypeKind
1685 | isLiftedTypeKind k = return liftedTypeKind
1686 | isUnliftedTypeKind k = return unliftedTypeKind
1687 go sw k@(TyVarTy _) = return k -- KindVars are always simple
1688 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1689 <+> ppr orig_swapped <+> ppr orig_kind)
1690 -- I think this can't actually happen
1692 -- T v = MkT v v must be a type
1693 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1696 kindOccurCheckErr tyvar ty
1697 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1698 2 (sep [ppr tyvar, char '=', ppr ty])
1700 unifyKindMisMatch ty1 ty2
1701 = zonkTcKind ty1 `thenM` \ ty1' ->
1702 zonkTcKind ty2 `thenM` \ ty2' ->
1704 msg = hang (ptext SLIT("Couldn't match kind"))
1705 2 (sep [quotes (ppr ty1'),
1706 ptext SLIT("against"),
1713 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1714 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1716 unifyFunKind (TyVarTy kvar)
1717 = readKindVar kvar `thenM` \ maybe_kind ->
1719 Indirect fun_kind -> unifyFunKind fun_kind
1721 do { arg_kind <- newKindVar
1722 ; res_kind <- newKindVar
1723 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1724 ; returnM (Just (arg_kind,res_kind)) }
1726 unifyFunKind (FunTy arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1727 unifyFunKind other = returnM Nothing
1730 %************************************************************************
1734 %************************************************************************
1736 ---------------------------
1737 -- We would like to get a decent error message from
1738 -- (a) Under-applied type constructors
1739 -- f :: (Maybe, Maybe)
1740 -- (b) Over-applied type constructors
1741 -- f :: Int x -> Int x
1745 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1746 -- A fancy wrapper for 'unifyKind', which tries
1747 -- to give decent error messages.
1748 checkExpectedKind ty act_kind exp_kind
1749 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1752 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1754 Just r -> returnM () ; -- Unification succeeded
1757 -- So there's definitely an error
1758 -- Now to find out what sort
1759 zonkTcKind exp_kind `thenM` \ exp_kind ->
1760 zonkTcKind act_kind `thenM` \ act_kind ->
1762 tcInitTidyEnv `thenM` \ env0 ->
1763 let (exp_as, _) = splitKindFunTys exp_kind
1764 (act_as, _) = splitKindFunTys act_kind
1765 n_exp_as = length exp_as
1766 n_act_as = length act_as
1768 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1769 (env2, tidy_act_kind) = tidyKind env1 act_kind
1771 err | n_exp_as < n_act_as -- E.g. [Maybe]
1772 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1774 -- Now n_exp_as >= n_act_as. In the next two cases,
1775 -- n_exp_as == 0, and hence so is n_act_as
1776 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1777 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1778 <+> ptext SLIT("is unlifted")
1780 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1781 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1782 <+> ptext SLIT("is lifted")
1784 | otherwise -- E.g. Monad [Int]
1785 = ptext SLIT("Kind mis-match")
1787 more_info = sep [ ptext SLIT("Expected kind") <+>
1788 quotes (pprKind tidy_exp_kind) <> comma,
1789 ptext SLIT("but") <+> quotes (ppr ty) <+>
1790 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1792 failWithTcM (env2, err $$ more_info)
1796 %************************************************************************
1798 \subsection{Checking signature type variables}
1800 %************************************************************************
1802 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1803 are not mentioned in the environment. In particular:
1805 (a) Not mentioned in the type of a variable in the envt
1806 eg the signature for f in this:
1812 Here, f is forced to be monorphic by the free occurence of x.
1814 (d) Not (unified with another type variable that is) in scope.
1815 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1816 when checking the expression type signature, we find that
1817 even though there is nothing in scope whose type mentions r,
1818 nevertheless the type signature for the expression isn't right.
1820 Another example is in a class or instance declaration:
1822 op :: forall b. a -> b
1824 Here, b gets unified with a
1826 Before doing this, the substitution is applied to the signature type variable.
1829 checkSigTyVars :: [TcTyVar] -> TcM ()
1830 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1832 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1833 -- The extra_tvs can include boxy type variables;
1834 -- e.g. TcMatches.tcCheckExistentialPat
1835 checkSigTyVarsWrt extra_tvs sig_tvs
1836 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1837 ; check_sig_tyvars extra_tvs' sig_tvs }
1840 :: TcTyVarSet -- Global type variables. The universally quantified
1841 -- tyvars should not mention any of these
1842 -- Guaranteed already zonked.
1843 -> [TcTyVar] -- Universally-quantified type variables in the signature
1844 -- Guaranteed to be skolems
1846 check_sig_tyvars extra_tvs []
1848 check_sig_tyvars extra_tvs sig_tvs
1849 = ASSERT( all isSkolemTyVar sig_tvs )
1850 do { gbl_tvs <- tcGetGlobalTyVars
1851 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1852 text "gbl_tvs" <+> ppr gbl_tvs,
1853 text "extra_tvs" <+> ppr extra_tvs]))
1855 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1856 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1857 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1860 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1861 -> [TcTyVar] -- The possibly-escaping type variables
1862 -> [TcTyVar] -- The zonked versions thereof
1864 -- Complain about escaping type variables
1865 -- We pass a list of type variables, at least one of which
1866 -- escapes. The first list contains the original signature type variable,
1867 -- while the second contains the type variable it is unified to (usually itself)
1868 bleatEscapedTvs globals sig_tvs zonked_tvs
1869 = do { env0 <- tcInitTidyEnv
1870 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1871 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1873 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1874 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1876 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1878 check (tidy_env, msgs) (sig_tv, zonked_tv)
1879 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1881 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1882 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1884 -----------------------
1885 escape_msg sig_tv zonked_tv globs
1887 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1888 nest 2 (vcat globs)]
1890 = msg <+> ptext SLIT("escapes")
1891 -- Sigh. It's really hard to give a good error message
1892 -- all the time. One bad case is an existential pattern match.
1893 -- We rely on the "When..." context to help.
1895 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1897 | sig_tv == zonked_tv = empty
1898 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1901 These two context are used with checkSigTyVars
1904 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1905 -> TidyEnv -> TcM (TidyEnv, Message)
1906 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1907 = zonkTcType sig_tau `thenM` \ actual_tau ->
1909 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1910 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1911 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1912 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1913 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1915 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),