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
56 %************************************************************************
58 \subsection{'hole' type variables}
60 %************************************************************************
63 tcInfer :: (BoxyType -> TcM a) -> TcM (a, TcType)
65 = do { box <- newBoxyTyVar openTypeKind
66 ; res <- tc_infer (mkTyVarTy box)
67 ; res_ty <- readFilledBox box -- Guaranteed filled-in by now
68 ; return (res, res_ty) }
72 %************************************************************************
76 %************************************************************************
79 subFunTys :: SDoc -- Somthing like "The function f has 3 arguments"
80 -- or "The abstraction (\x.e) takes 1 argument"
81 -> Arity -- Expected # of args
82 -> BoxyRhoType -- res_ty
83 -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
85 -- Attempt to decompse res_ty to have enough top-level arrows to
86 -- match the number of patterns in the match group
88 -- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
89 -- and the inner call to thing_inside passes args: [a1,...,an], b
90 -- then co_fn :: (a1 -> ... -> an -> b) -> res_ty
92 -- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
95 {- Error messages from subFunTys
97 The abstraction `\Just 1 -> ...' has two arguments
98 but its type `Maybe a -> a' has only one
100 The equation(s) for `f' have two arguments
101 but its type `Maybe a -> a' has only one
103 The section `(f 3)' requires 'f' to take two arguments
104 but its type `Int -> Int' has only one
106 The function 'f' is applied to two arguments
107 but its type `Int -> Int' has only one
111 subFunTys error_herald n_pats res_ty thing_inside
112 = loop n_pats [] res_ty
114 -- In 'loop', the parameter 'arg_tys' accumulates
115 -- the arg types so far, in *reverse order*
116 -- INVARIANT: res_ty :: *
117 loop n args_so_far res_ty
118 | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
120 loop n args_so_far res_ty
121 | isSigmaTy res_ty -- Do this before checking n==0, because we
122 -- guarantee to return a BoxyRhoType, not a BoxySigmaType
123 = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet $ \ _ res_ty' ->
124 loop n args_so_far res_ty'
125 ; return (gen_fn <.> co_fn, res) }
127 loop 0 args_so_far res_ty
128 = do { res <- thing_inside (reverse args_so_far) res_ty
129 ; return (idHsWrapper, res) }
131 loop n args_so_far (FunTy arg_ty res_ty)
132 = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
133 ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
134 ; return (co_fn', res) }
136 -- res_ty might have a type variable at the head, such as (a b c),
137 -- in which case we must fill in with (->). Simplest thing to do
138 -- is to use boxyUnify, but we catch failure and generate our own
139 -- error message on failure
140 loop n args_so_far res_ty@(AppTy _ _)
141 = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
142 ; (_, mb_unit) <- tryTcErrs $ boxyUnify res_ty (FunTy arg_ty' res_ty')
143 ; if isNothing mb_unit then bale_out args_so_far
144 else loop n args_so_far (FunTy arg_ty' res_ty') }
146 loop n args_so_far (TyVarTy tv)
147 | isTyConableTyVar tv
148 = do { cts <- readMetaTyVar tv
150 Indirect ty -> loop n args_so_far ty
151 Flexi -> do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
152 ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty
153 ; return (idHsWrapper, res) } }
155 mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
156 mk_res_ty [] = panic "TcUnify.mk_res_ty1"
157 kinds = openTypeKind : take n (repeat argTypeKind)
158 -- Note argTypeKind: the args can have an unboxed type,
159 -- but not an unboxed tuple.
161 loop n args_so_far res_ty = bale_out args_so_far
164 = do { env0 <- tcInitTidyEnv
165 ; res_ty' <- zonkTcType res_ty
166 ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
167 ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
169 mk_msg res_ty n_actual
170 = error_herald <> comma $$
171 sep [ptext SLIT("but its type") <+> quotes (pprType res_ty),
172 if n_actual == 0 then ptext SLIT("has none")
173 else ptext SLIT("has only") <+> speakN n_actual]
177 ----------------------
178 boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
179 -> BoxyRhoType -- Expected type (T a b c)
180 -> TcM [BoxySigmaType] -- Element types, a b c
181 -- It's used for wired-in tycons, so we call checkWiredInTyCOn
182 -- Precondition: never called with FunTyCon
183 -- Precondition: input type :: *
185 boxySplitTyConApp tc orig_ty
186 = do { checkWiredInTyCon tc
187 ; loop (tyConArity tc) [] orig_ty }
189 loop n_req args_so_far ty
190 | Just ty' <- tcView ty = loop n_req args_so_far ty'
192 loop n_req args_so_far (TyConApp tycon args)
194 = ASSERT( n_req == length args) -- ty::*
195 return (args ++ args_so_far)
197 loop n_req args_so_far (AppTy fun arg)
199 = loop (n_req - 1) (arg:args_so_far) fun
201 loop n_req args_so_far (TyVarTy tv)
202 | isTyConableTyVar tv
203 , res_kind `isSubKind` tyVarKind tv
204 = do { cts <- readMetaTyVar tv
206 Indirect ty -> loop n_req args_so_far ty
207 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
208 ; return (arg_tys ++ args_so_far) }
211 mk_res_ty arg_tys' = mkTyConApp tc arg_tys'
212 (arg_kinds, res_kind) = splitKindFunTysN n_req (tyConKind tc)
214 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc))) orig_ty
216 ----------------------
217 boxySplitListTy :: BoxyRhoType -> TcM BoxySigmaType -- Special case for lists
218 boxySplitListTy exp_ty = do { [elt_ty] <- boxySplitTyConApp listTyCon exp_ty
222 ----------------------
223 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
224 -> TcM (BoxySigmaType, BoxySigmaType) -- Returns m, a
225 -- If the incoming type is a mutable type variable of kind k, then
226 -- boxySplitAppTy returns a new type variable (m: * -> k); note the *.
227 -- If the incoming type is boxy, then so are the result types; and vice versa
229 boxySplitAppTy orig_ty
233 | Just ty' <- tcView ty = loop ty'
236 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
237 = return (fun_ty, arg_ty)
240 | isTyConableTyVar tv
241 = do { cts <- readMetaTyVar tv
243 Indirect ty -> loop ty
244 Flexi -> do { [fun_ty,arg_ty] <- withMetaTvs tv kinds mk_res_ty
245 ; return (fun_ty, arg_ty) } }
247 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
248 mk_res_ty other = panic "TcUnify.mk_res_ty2"
249 tv_kind = tyVarKind tv
250 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
252 liftedTypeKind] -- arg type :: *
253 -- The defaultKind is a bit smelly. If you remove it,
254 -- try compiling f x = do { x }
255 -- and you'll get a kind mis-match. It smells, but
256 -- not enough to lose sleep over.
258 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
261 boxySplitFailure actual_ty expected_ty
262 = unifyMisMatch False False actual_ty expected_ty
263 -- "outer" is False, so we don't pop the context
264 -- which is what we want since we have not pushed one!
268 --------------------------------
269 -- withBoxes: the key utility function
270 --------------------------------
273 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
274 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
275 -> ([BoxySigmaType] -> BoxySigmaType)
276 -- Constructs the type to assign
277 -- to the original var
278 -> TcM [BoxySigmaType] -- Return the fresh boxes
280 -- It's entirely possible for the [kind] to be empty.
281 -- For example, when pattern-matching on True,
282 -- we call boxySplitTyConApp passing a boolTyCon
284 -- Invariant: tv is still Flexi
286 withMetaTvs tv kinds mk_res_ty
288 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
289 ; let box_tys = mkTyVarTys box_tvs
290 ; writeMetaTyVar tv (mk_res_ty box_tys)
293 | otherwise -- Non-boxy meta type variable
294 = do { tau_tys <- mapM newFlexiTyVarTy kinds
295 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
296 -- Sure to be a tau-type
299 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
300 -- Allocate a *boxy* tyvar
301 withBox kind thing_inside
302 = do { box_tv <- newMetaTyVar BoxTv kind
303 ; res <- thing_inside (mkTyVarTy box_tv)
304 ; ty <- readFilledBox box_tv
309 %************************************************************************
311 Approximate boxy matching
313 %************************************************************************
316 preSubType :: [TcTyVar] -- Quantified type variables
317 -> TcTyVarSet -- Subset of quantified type variables
318 -- see Note [Pre-sub boxy]
319 -> TcType -- The rho-type part; quantified tyvars scopes over this
320 -> BoxySigmaType -- Matching type from the context
321 -> TcM [TcType] -- Types to instantiate the tyvars
322 -- Perform pre-subsumption, and return suitable types
323 -- to instantiate the quantified type varibles:
324 -- info from the pre-subsumption, if there is any
325 -- a boxy type variable otherwise
327 -- Note [Pre-sub boxy]
328 -- The 'btvs' are a subset of 'qtvs'. They are the ones we can
329 -- instantiate to a boxy type variable, because they'll definitely be
330 -- filled in later. This isn't always the case; sometimes we have type
331 -- variables mentioned in the context of the type, but not the body;
332 -- f :: forall a b. C a b => a -> a
333 -- Then we may land up with an unconstrained 'b', so we want to
334 -- instantiate it to a monotype (non-boxy) type variable
336 -- The 'qtvs' that are *neither* fixed by the pre-subsumption, *nor* are in 'btvs',
337 -- are instantiated to TauTv meta variables.
339 preSubType qtvs btvs qty expected_ty
340 = do { tys <- mapM inst_tv qtvs
341 ; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
344 pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
346 | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
347 | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
348 ; return (mkTyVarTy tv') }
349 | otherwise = do { tv' <- tcInstTyVar tv
350 ; return (mkTyVarTy tv') }
353 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
354 -> BoxyRhoType -- Type to match (note a *Rho* type)
355 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
357 -- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
358 -- "Boxy types: inference for higher rank types and impredicativity"
360 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
361 = go tmpl_tvs tmpl_ty emptyVarSet boxy_ty
363 go t_tvs t_ty b_tvs b_ty
364 | Just t_ty' <- tcView t_ty = go t_tvs t_ty' b_tvs b_ty
365 | Just b_ty' <- tcView b_ty = go t_tvs t_ty b_tvs b_ty'
367 go t_tvs (TyVarTy _) b_tvs b_ty = emptyTvSubst -- Rule S-ANY; no bindings
368 -- Rule S-ANY covers (a) type variables and (b) boxy types
369 -- in the template. Both look like a TyVarTy.
370 -- See Note [Sub-match] below
372 go t_tvs t_ty b_tvs b_ty
373 | isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
374 = go (t_tvs `delVarSetList` tvs) t_tau b_tvs b_ty -- Rule S-SPEC
375 -- Under a forall on the left, if there is shadowing,
376 -- do not bind! Hence the delVarSetList.
377 | isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
378 = go t_tvs t_ty (extendVarSetList b_tvs tvs) b_tau -- Rule S-SKOL
379 -- Add to the variables we must not bind to
380 -- NB: it's *important* to discard the theta part. Otherwise
381 -- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
382 -- and end up with a completely bogus binding (b |-> Bool), by lining
383 -- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
384 -- This pre-subsumption stuff can return too few bindings, but it
385 -- must *never* return bogus info.
387 go t_tvs (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
388 = boxy_match t_tvs arg1 b_tvs arg2 (go t_tvs res1 b_tvs res2)
389 -- Match the args, and sub-match the results
391 go t_tvs t_ty b_tvs b_ty = boxy_match t_tvs t_ty b_tvs b_ty emptyTvSubst
392 -- Otherwise defer to boxy matching
393 -- This covers TyConApp, AppTy, PredTy
400 |- head xs : <rhobox>
401 We will do a boxySubMatchType between a ~ <rhobox>
402 But we *don't* want to match [a |-> <rhobox>] because
403 (a) The box should be filled in with a rho-type, but
404 but the returned substitution maps TyVars to boxy
406 (b) In any case, the right final answer might be *either*
407 instantiate 'a' with a rho-type or a sigma type
408 head xs : Int vs head xs : forall b. b->b
409 So the matcher MUST NOT make a choice here. In general, we only
410 bind a template type variable in boxyMatchType, not in boxySubMatchType.
415 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
416 -> [BoxySigmaType] -- Type to match
417 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
419 -- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
420 -- "Boxy types: inference for higher rank types and impredicativity"
422 -- Find a *boxy* substitution that makes the template look as much
423 -- like the BoxySigmaType as possible.
424 -- It's always ok to return an empty substitution;
425 -- anything more is jam on the pudding
427 -- NB1: This is a pure, non-monadic function.
428 -- It does no unification, and cannot fail
430 -- Precondition: the arg lengths are equal
431 -- Precondition: none of the template type variables appear anywhere in the [BoxySigmaType]
435 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
436 = ASSERT( length tmpl_tys == length boxy_tys )
437 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
438 -- ToDo: add error context?
440 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
442 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
443 = boxy_match tmpl_tvs t_ty boxy_tvs b_ty $
444 boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys subst
445 boxy_match_s tmpl_tvs _ boxy_tvs _ subst
446 = panic "boxy_match_s" -- Lengths do not match
450 boxy_match :: TcTyVarSet -> TcType -- Template
451 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
452 -> BoxySigmaType -- Match against this type
456 -- The boxy_tvs argument prevents this match:
457 -- [a] forall b. a ~ forall b. b
458 -- We don't want to bind the template variable 'a'
459 -- to the quantified type variable 'b'!
461 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
462 = go orig_tmpl_ty orig_boxy_ty
465 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
466 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
468 go ty1 ty2 -- C.f. the isSigmaTy case for boxySubMatchType
470 , (tvs1, _, tau1) <- tcSplitSigmaTy ty1
471 , (tvs2, _, tau2) <- tcSplitSigmaTy ty2
472 , equalLength tvs1 tvs2
473 = boxy_match (tmpl_tvs `delVarSetList` tvs1) tau1
474 (boxy_tvs `extendVarSetList` tvs2) tau2 subst
476 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
477 | tc1 == tc2 = go_s tys1 tys2
479 go (FunTy arg1 res1) (FunTy arg2 res2)
480 = go_s [arg1,res1] [arg2,res2]
483 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
484 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
485 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
486 = go_s [s1,t1] [s2,t2]
489 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
490 , boxy_tvs `disjointVarSet` tyVarsOfType orig_boxy_ty
491 , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
492 = extendTvSubst subst tv boxy_ty'
494 = subst -- Ignore others
496 boxy_ty' = case lookupTyVar subst tv of
497 Nothing -> orig_boxy_ty
498 Just ty -> ty `boxyLub` orig_boxy_ty
500 go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
501 -- Example: Tree a ~ Maybe Int
502 -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
503 -- misleading error messages. An even more confusing case is
504 -- a -> b ~ Maybe Int
505 -- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
506 -- from this pre-matching phase.
509 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
512 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
513 -- Combine boxy information from the two types
514 -- If there is a conflict, return the first
515 boxyLub orig_ty1 orig_ty2
516 = go orig_ty1 orig_ty2
518 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
519 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
520 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
521 | tc1 == tc2, length ts1 == length ts2
522 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
524 go (TyVarTy tv1) ty2 -- This is the whole point;
525 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
528 -- Look inside type synonyms, but only if the naive version fails
529 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
530 | Just ty2' <- tcView ty2 = go ty1 ty2'
532 -- For now, we don't look inside ForAlls, PredTys
533 go ty1 ty2 = orig_ty1 -- Default
536 Note [Matching kinds]
537 ~~~~~~~~~~~~~~~~~~~~~
538 The target type might legitimately not be a sub-kind of template.
539 For example, suppose the target is simply a box with an OpenTypeKind,
540 and the template is a type variable with LiftedTypeKind.
541 Then it's ok (because the target type will later be refined).
542 We simply don't bind the template type variable.
544 It might also be that the kind mis-match is an error. For example,
545 suppose we match the template (a -> Int) against (Int# -> Int),
546 where the template type variable 'a' has LiftedTypeKind. This
547 matching function does not fail; it simply doesn't bind the template.
548 Later stuff will fail.
550 %************************************************************************
554 %************************************************************************
556 All the tcSub calls have the form
558 tcSub expected_ty offered_ty
560 offered_ty <= expected_ty
562 That is, that a value of type offered_ty is acceptable in
563 a place expecting a value of type expected_ty.
565 It returns a coercion function
566 co_fn :: offered_ty -> expected_ty
567 which takes an HsExpr of type offered_ty into one of type
572 tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
573 -- (tcSub act exp) checks that
575 tcSubExp actual_ty expected_ty
576 = -- addErrCtxtM (unifyCtxt actual_ty expected_ty) $
577 -- Adding the error context here leads to some very confusing error
578 -- messages, such as "can't match forall a. a->a with forall a. a->a"
579 -- Example is tcfail165:
580 -- do var <- newEmptyMVar :: IO (MVar (forall a. Show a => a -> String))
581 -- putMVar var (show :: forall a. Show a => a -> String)
582 -- Here the info does not flow from the 'var' arg of putMVar to its 'show' arg
583 -- but after zonking it looks as if it does!
585 -- So instead I'm adding the error context when moving from tc_sub to u_tys
587 traceTc (text "tcSubExp" <+> ppr actual_ty <+> ppr expected_ty) >>
588 tc_sub SubOther actual_ty actual_ty False expected_ty expected_ty
590 tcFunResTy :: Name -> BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
591 tcFunResTy fun actual_ty expected_ty
592 = traceTc (text "tcFunResTy" <+> ppr actual_ty <+> ppr expected_ty) >>
593 tc_sub (SubFun fun) actual_ty actual_ty False expected_ty expected_ty
596 data SubCtxt = SubDone -- Error-context already pushed
597 | SubFun Name -- Context is tcFunResTy
598 | SubOther -- Context is something else
600 tc_sub :: SubCtxt -- How to add an error-context
601 -> BoxySigmaType -- actual_ty, before expanding synonyms
602 -> BoxySigmaType -- ..and after
603 -> InBox -- True <=> expected_ty is inside a box
604 -> BoxySigmaType -- expected_ty, before
605 -> BoxySigmaType -- ..and after
607 -- The acual_ty is never inside a box
608 -- IMPORTANT pre-condition: if the args contain foralls, the bound type
609 -- variables are visible non-monadically
610 -- (i.e. tha args are sufficiently zonked)
611 -- This invariant is needed so that we can "see" the foralls, ad
612 -- e.g. in the SPEC rule where we just use splitSigmaTy
614 tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
615 = traceTc (text "tc_sub" <+> ppr act_ty $$ ppr exp_ty) >>
616 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
617 -- This indirection is just here to make
618 -- it easy to insert a debug trace!
620 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
621 | Just exp_ty' <- tcView exp_ty = tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty'
622 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
623 | Just act_ty' <- tcView act_ty = tc_sub sub_ctxt act_sty act_ty' exp_ib exp_sty exp_ty
625 -----------------------------------
626 -- Rule SBOXY, plus other cases when act_ty is a type variable
627 -- Just defer to boxy matching
628 -- This rule takes precedence over SKOL!
629 tc_sub1 sub_ctxt act_sty (TyVarTy tv) exp_ib exp_sty exp_ty
630 = do { addSubCtxt sub_ctxt act_sty exp_sty $
631 uVar True False tv exp_ib exp_sty exp_ty
632 ; return idHsWrapper }
634 -----------------------------------
635 -- Skolemisation case (rule SKOL)
636 -- actual_ty: d:Eq b => b->b
637 -- expected_ty: forall a. Ord a => a->a
638 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
640 -- It is essential to do this *before* the specialisation case
641 -- Example: f :: (Eq a => a->a) -> ...
642 -- g :: Ord b => b->b
645 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
647 = if exp_ib then -- SKOL does not apply if exp_ty is inside a box
648 defer_to_boxy_matching sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
650 { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ _ body_exp_ty ->
651 tc_sub sub_ctxt act_sty act_ty False body_exp_ty body_exp_ty
652 ; return (gen_fn <.> co_fn) }
654 act_tvs = tyVarsOfType act_ty
655 -- It's really important to check for escape wrt
656 -- the free vars of both expected_ty *and* actual_ty
658 -----------------------------------
659 -- Specialisation case (rule ASPEC):
660 -- actual_ty: forall a. Ord a => a->a
661 -- expected_ty: Int -> Int
662 -- co_fn e = e Int dOrdInt
664 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
665 -- Implements the new SPEC rule in the Appendix of the paper
666 -- "Boxy types: inference for higher rank types and impredicativity"
667 -- (This appendix isn't in the published version.)
668 -- The idea is to *first* do pre-subsumption, and then full subsumption
669 -- Example: forall a. a->a <= Int -> (forall b. Int)
670 -- Pre-subsumpion finds a|->Int, and that works fine, whereas
671 -- just running full subsumption would fail.
672 | isSigmaTy actual_ty
673 = do { -- Perform pre-subsumption, and instantiate
674 -- the type with info from the pre-subsumption;
675 -- boxy tyvars if pre-subsumption gives no info
676 let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
677 tau_tvs = exactTyVarsOfType tau
678 ; inst_tys <- if exp_ib then -- Inside a box, do not do clever stuff
679 do { tyvars' <- mapM tcInstBoxyTyVar tyvars
680 ; return (mkTyVarTys tyvars') }
681 else -- Outside, do clever stuff
682 preSubType tyvars tau_tvs tau expected_ty
683 ; let subst' = zipOpenTvSubst tyvars inst_tys
684 tau' = substTy subst' tau
686 -- Perform a full subsumption check
687 ; traceTc (text "tc_sub_spec" <+> vcat [ppr actual_ty,
688 ppr tyvars <+> ppr theta <+> ppr tau,
690 ; co_fn2 <- tc_sub sub_ctxt tau' tau' exp_ib exp_sty expected_ty
692 -- Deal with the dictionaries
693 -- The origin gives a helpful origin when we have
694 -- a function with type f :: Int -> forall a. Num a => ...
695 -- This way the (Num a) dictionary gets an OccurrenceOf f origin
696 ; let orig = case sub_ctxt of
697 SubFun n -> OccurrenceOf n
698 other -> InstSigOrigin -- Unhelpful
699 ; co_fn1 <- instCall orig inst_tys (substTheta subst' theta)
700 ; return (co_fn2 <.> co_fn1) }
702 -----------------------------------
703 -- Function case (rule F1)
704 tc_sub1 sub_ctxt act_sty (FunTy act_arg act_res) exp_ib exp_sty (FunTy exp_arg exp_res)
705 = addSubCtxt sub_ctxt act_sty exp_sty $
706 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
708 -- Function case (rule F2)
709 tc_sub1 sub_ctxt act_sty act_ty@(FunTy act_arg act_res) _ exp_sty (TyVarTy exp_tv)
711 = addSubCtxt sub_ctxt act_sty exp_sty $
712 do { cts <- readMetaTyVar exp_tv
714 Indirect ty -> tc_sub SubDone act_sty act_ty True exp_sty ty
715 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
716 ; tc_sub_funs act_arg act_res True arg_ty res_ty } }
718 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
719 mk_res_ty other = panic "TcUnify.mk_res_ty3"
720 fun_kinds = [argTypeKind, openTypeKind]
722 -- Everything else: defer to boxy matching
723 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
724 = defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
726 -----------------------------------
727 defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
728 = do { addSubCtxt sub_ctxt act_sty exp_sty $
729 u_tys outer False act_sty actual_ty exp_ib exp_sty expected_ty
730 ; return idHsWrapper }
732 outer = case sub_ctxt of -- Ugh
736 -----------------------------------
737 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
738 = do { uTys False act_arg exp_ib exp_arg
739 ; co_fn_res <- tc_sub SubDone act_res act_res exp_ib exp_res exp_res
740 ; wrapFunResCoercion [exp_arg] co_fn_res }
742 -----------------------------------
744 :: [TcType] -- Type of args
745 -> HsWrapper -- HsExpr a -> HsExpr b
746 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
747 wrapFunResCoercion arg_tys co_fn_res
748 | isIdHsWrapper co_fn_res = return idHsWrapper
749 | null arg_tys = return co_fn_res
751 = do { arg_ids <- newSysLocalIds FSLIT("sub") arg_tys
752 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpApps arg_ids) }
757 %************************************************************************
759 \subsection{Generalisation}
761 %************************************************************************
764 tcGen :: BoxySigmaType -- expected_ty
765 -> TcTyVarSet -- Extra tyvars that the universally
766 -- quantified tyvars of expected_ty
767 -- must not be unified
768 -> ([TcTyVar] -> BoxyRhoType -> TcM result)
769 -> TcM (HsWrapper, result)
770 -- The expression has type: spec_ty -> expected_ty
772 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
773 -- If not, the call is a no-op
774 = do { -- We want the GenSkol info in the skolemised type variables to
775 -- mention the *instantiated* tyvar names, so that we get a
776 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
777 -- Hence the tiresome but innocuous fixM
778 ((tvs', theta', rho'), skol_info) <- fixM (\ ~(_, skol_info) ->
779 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
780 -- Get loation from monad, not from expected_ty
781 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty)
782 ; return ((forall_tvs, theta, rho_ty), skol_info) })
785 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
786 text "expected_ty" <+> ppr expected_ty,
787 text "inst ty" <+> ppr tvs' <+> ppr theta' <+> ppr rho',
788 text "free_tvs" <+> ppr free_tvs])
791 -- Type-check the arg and unify with poly type
792 ; (result, lie) <- getLIE (thing_inside tvs' rho')
794 -- Check that the "forall_tvs" havn't been constrained
795 -- The interesting bit here is that we must include the free variables
796 -- of the expected_ty. Here's an example:
797 -- runST (newVar True)
798 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
799 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
800 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
801 -- So now s' isn't unconstrained because it's linked to a.
802 -- Conclusion: include the free vars of the expected_ty in the
803 -- list of "free vars" for the signature check.
805 ; loc <- getInstLoc (SigOrigin skol_info)
806 ; dicts <- newDictBndrs loc theta'
807 ; inst_binds <- tcSimplifyCheck loc tvs' dicts lie
809 ; checkSigTyVarsWrt free_tvs tvs'
810 ; traceTc (text "tcGen:done")
813 -- The WpLet binds any Insts which came out of the simplification.
814 dict_ids = map instToId dicts
815 co_fn = mkWpTyLams tvs' <.> mkWpLams dict_ids <.> WpLet inst_binds
816 ; returnM (co_fn, result) }
818 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
823 %************************************************************************
827 %************************************************************************
829 The exported functions are all defined as versions of some
830 non-exported generic functions.
833 boxyUnify :: BoxyType -> BoxyType -> TcM ()
834 -- Acutal and expected, respectively
836 = addErrCtxtM (unifyCtxt ty1 ty2) $
837 uTysOuter False ty1 False ty2
840 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM ()
841 -- Arguments should have equal length
842 -- Acutal and expected types
843 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
846 unifyType :: TcTauType -> TcTauType -> TcM ()
847 -- No boxes expected inside these types
848 -- Acutal and expected types
849 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
850 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
851 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
852 addErrCtxtM (unifyCtxt ty1 ty2) $
853 uTysOuter True ty1 True ty2
856 unifyPred :: PredType -> PredType -> TcM ()
857 -- Acutal and expected types
858 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
859 uPred True True p1 True p2
861 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
862 -- Acutal and expected types
863 unifyTheta theta1 theta2
864 = do { checkTc (equalLength theta1 theta2)
865 (vcat [ptext SLIT("Contexts differ in length"),
866 nest 2 $ parens $ ptext SLIT("Use -fglasgow-exts to allow this")])
867 ; uList unifyPred theta1 theta2 }
870 uList :: (a -> a -> TcM ())
871 -> [a] -> [a] -> TcM ()
872 -- Unify corresponding elements of two lists of types, which
873 -- should be f equal length. We charge down the list explicitly so that
874 -- we can complain if their lengths differ.
875 uList unify [] [] = return ()
876 uList unify (ty1:tys1) (ty2:tys2) = do { unify ty1 ty2; uList unify tys1 tys2 }
877 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
880 @unifyTypeList@ takes a single list of @TauType@s and unifies them
881 all together. It is used, for example, when typechecking explicit
882 lists, when all the elts should be of the same type.
885 unifyTypeList :: [TcTauType] -> TcM ()
886 unifyTypeList [] = returnM ()
887 unifyTypeList [ty] = returnM ()
888 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
889 ; unifyTypeList tys }
892 %************************************************************************
894 \subsection[Unify-uTys]{@uTys@: getting down to business}
896 %************************************************************************
898 @uTys@ is the heart of the unifier. Each arg happens twice, because
899 we want to report errors in terms of synomyms if poss. The first of
900 the pair is used in error messages only; it is always the same as the
901 second, except that if the first is a synonym then the second may be a
902 de-synonym'd version. This way we get better error messages.
904 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
907 type InBox = Bool -- True <=> we are inside a box
908 -- False <=> we are outside a box
909 -- The importance of this is that if we get "filled-box meets
910 -- filled-box", we'll look into the boxes and unify... but
911 -- we must not allow polytypes. But if we are in a box on
912 -- just one side, then we can allow polytypes
914 type Outer = Bool -- True <=> this is the outer level of a unification
915 -- so that the types being unified are the
916 -- very ones we began with, not some sub
917 -- component or synonym expansion
918 -- The idea is that if Outer is true then unifyMisMatch should
919 -- pop the context to remove the "Expected/Acutal" context
922 :: InBox -> TcType -- ty1 is the *expected* type
923 -> InBox -> TcType -- ty2 is the *actual* type
925 uTysOuter nb1 ty1 nb2 ty2 = do { traceTc (text "uTysOuter" <+> ppr ty1 <+> ppr ty2)
926 ; u_tys True nb1 ty1 ty1 nb2 ty2 ty2 }
927 uTys nb1 ty1 nb2 ty2 = do { traceTc (text "uTys" <+> ppr ty1 <+> ppr ty2)
928 ; u_tys False nb1 ty1 ty1 nb2 ty2 ty2 }
932 uTys_s :: InBox -> [TcType] -- ty1 is the *actual* types
933 -> InBox -> [TcType] -- ty2 is the *expected* types
935 uTys_s nb1 [] nb2 [] = returnM ()
936 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { uTys nb1 ty1 nb2 ty2
937 ; uTys_s nb1 tys1 nb2 tys2 }
938 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
942 -> InBox -> TcType -> TcType -- ty1 is the *actual* type
943 -> InBox -> TcType -> TcType -- ty2 is the *expected* type
946 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
950 -- Always expand synonyms (see notes at end)
951 -- (this also throws away FTVs)
953 | Just ty1' <- tcView ty1 = go False ty1' ty2
954 | Just ty2' <- tcView ty2 = go False ty1 ty2'
956 -- Variables; go for uVar
957 go outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
958 go outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
959 -- "True" means args swapped
961 -- The case for sigma-types must *follow* the variable cases
962 -- because a boxy variable can be filed with a polytype;
963 -- but must precede FunTy, because ((?x::Int) => ty) look
964 -- like a FunTy; there isn't necy a forall at the top
966 | isSigmaTy ty1 || isSigmaTy ty2
967 = do { checkM (equalLength tvs1 tvs2)
968 (unifyMisMatch outer False orig_ty1 orig_ty2)
970 ; tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
971 -- Get location from monad, not from tvs1
972 ; let tys = mkTyVarTys tvs
973 in_scope = mkInScopeSet (mkVarSet tvs)
974 phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
975 phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
976 (theta1,tau1) = tcSplitPhiTy phi1
977 (theta2,tau2) = tcSplitPhiTy phi2
979 ; addErrCtxtM (unifyForAllCtxt tvs phi1 phi2) $ do
980 { checkM (equalLength theta1 theta2)
981 (unifyMisMatch outer False orig_ty1 orig_ty2)
983 ; uPreds False nb1 theta1 nb2 theta2
984 ; uTys nb1 tau1 nb2 tau2
986 -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
987 ; free_tvs <- zonkTcTyVarsAndFV (varSetElems (tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2))
988 ; ifM (any (`elemVarSet` free_tvs) tvs)
989 (bleatEscapedTvs free_tvs tvs tvs)
991 -- If both sides are inside a box, we are in a "box-meets-box"
992 -- situation, and we should not have a polytype at all.
993 -- If we get here we have two boxes, already filled with
994 -- the same polytype... but it should be a monotype.
995 -- This check comes last, because the error message is
996 -- extremely unhelpful.
997 ; ifM (nb1 && nb2) (notMonoType ty1)
1000 (tvs1, body1) = tcSplitForAllTys ty1
1001 (tvs2, body2) = tcSplitForAllTys ty2
1004 go outer (PredTy p1) (PredTy p2) = uPred False nb1 p1 nb2 p2
1006 -- Type constructors must match
1007 go _ (TyConApp con1 tys1) (TyConApp con2 tys2)
1008 | con1 == con2 = uTys_s nb1 tys1 nb2 tys2
1009 -- See Note [TyCon app]
1011 -- Functions; just check the two parts
1012 go _ (FunTy fun1 arg1) (FunTy fun2 arg2)
1013 = do { uTys nb1 fun1 nb2 fun2
1014 ; uTys nb1 arg1 nb2 arg2 }
1016 -- Applications need a bit of care!
1017 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
1018 -- NB: we've already dealt with type variables and Notes,
1019 -- so if one type is an App the other one jolly well better be too
1020 go outer (AppTy s1 t1) ty2
1021 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
1022 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
1024 -- Now the same, but the other way round
1025 -- Don't swap the types, because the error messages get worse
1026 go outer ty1 (AppTy s2 t2)
1027 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
1028 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
1031 -- Anything else fails
1032 go outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
1035 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
1036 | n1 == n2 = uTys nb1 t1 nb2 t2
1037 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
1038 | c1 == c2 = uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
1039 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
1041 uPreds outer nb1 [] nb2 [] = return ()
1042 uPreds outer nb1 (p1:ps1) nb2 (p2:ps2) = uPred outer nb1 p1 nb2 p2 >> uPreds outer nb1 ps1 nb2 ps2
1043 uPreds outer nb1 ps1 nb2 ps2 = panic "uPreds"
1048 When we find two TyConApps, the argument lists are guaranteed equal
1049 length. Reason: intially the kinds of the two types to be unified is
1050 the same. The only way it can become not the same is when unifying two
1051 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
1052 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
1053 which we do, that ensures that f1,f2 have the same kind; and that
1054 means a1,a2 have the same kind. And now the argument repeats.
1059 If you are tempted to make a short cut on synonyms, as in this
1063 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
1064 -- NO = if (con1 == con2) then
1065 -- NO -- Good news! Same synonym constructors, so we can shortcut
1066 -- NO -- by unifying their arguments and ignoring their expansions.
1067 -- NO unifyTypepeLists args1 args2
1069 -- NO -- Never mind. Just expand them and try again
1073 then THINK AGAIN. Here is the whole story, as detected and reported
1074 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1076 Here's a test program that should detect the problem:
1080 x = (1 :: Bogus Char) :: Bogus Bool
1083 The problem with [the attempted shortcut code] is that
1087 is not a sufficient condition to be able to use the shortcut!
1088 You also need to know that the type synonym actually USES all
1089 its arguments. For example, consider the following type synonym
1090 which does not use all its arguments.
1095 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1096 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1097 would fail, even though the expanded forms (both \tr{Int}) should
1100 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1101 unnecessarily bind \tr{t} to \tr{Char}.
1103 ... You could explicitly test for the problem synonyms and mark them
1104 somehow as needing expansion, perhaps also issuing a warning to the
1109 %************************************************************************
1111 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1113 %************************************************************************
1115 @uVar@ is called when at least one of the types being unified is a
1116 variable. It does {\em not} assume that the variable is a fixed point
1117 of the substitution; rather, notice that @uVar@ (defined below) nips
1118 back into @uTys@ if it turns out that the variable is already bound.
1122 -> Bool -- False => tyvar is the "expected"
1123 -- True => ty is the "expected" thing
1125 -> InBox -- True <=> definitely no boxes in t2
1126 -> TcTauType -> TcTauType -- printing and real versions
1129 uVar outer swapped tv1 nb2 ps_ty2 ty2
1130 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1131 | otherwise = brackets (equals <+> ppr ty2)
1132 ; traceTc (text "uVar" <+> ppr swapped <+>
1133 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1134 nest 2 (ptext SLIT(" <-> ")),
1135 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1136 ; details <- lookupTcTyVar tv1
1139 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1140 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1141 -- The 'True' here says that ty1 is now inside a box
1142 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1146 uUnfilledVar :: Outer
1147 -> Bool -- Args are swapped
1148 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1149 -> TcTauType -> TcTauType -- Type 2
1151 -- Invariant: tyvar 1 is not unified with anything
1153 uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1154 | Just ty2' <- tcView ty2
1155 = -- Expand synonyms; ignore FTVs
1156 uUnfilledVar False swapped tv1 details1 ps_ty2 ty2'
1158 uUnfilledVar outer swapped tv1 details1 ps_ty2 (TyVarTy tv2)
1159 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1161 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1162 -- this is box-meets-box, so fill in with a tau-type
1163 -> do { tau_tv <- tcInstTyVar tv1
1164 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv) }
1165 other -> returnM () -- No-op
1167 -- Distinct type variables
1169 = do { lookup2 <- lookupTcTyVar tv2
1171 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
1172 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1175 uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2 -- ty2 is not a type variable
1177 MetaTv (SigTv _) ref1 -> mis_match -- Can't update a skolem with a non-type-variable
1178 MetaTv info ref1 -> uMetaVar swapped tv1 info ref1 ps_ty2 non_var_ty2
1179 skolem_details -> mis_match
1181 mis_match = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1185 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1188 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1189 -- ty2 is not a type variable
1191 uMetaVar swapped tv1 BoxTv ref1 ps_ty2 non_var_ty2
1192 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1193 -- that any boxes in ty2 are filled with monotypes
1195 -- It should not be the case that tv1 occurs in ty2
1196 -- (i.e. no occurs check should be needed), but if perchance
1197 -- it does, the unbox operation will fill it, and the DEBUG
1199 do { final_ty <- unBox ps_ty2
1201 ; meta_details <- readMutVar ref1
1202 ; case meta_details of
1203 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1204 return () -- This really should *not* happen
1207 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1209 uMetaVar swapped tv1 info1 ref1 ps_ty2 non_var_ty2
1210 = do { final_ty <- checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
1211 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1214 uUnfilledVars :: Outer
1215 -> Bool -- Args are swapped
1216 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1217 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1219 -- Invarant: The type variables are distinct,
1220 -- Neither is filled in yet
1221 -- They might be boxy or not
1223 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1224 = unifyMisMatch outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1226 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1227 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2)
1228 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1229 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
1231 -- ToDo: this function seems too long for what it acutally does!
1232 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1233 = case (info1, info2) of
1234 (BoxTv, BoxTv) -> box_meets_box
1236 -- If a box meets a TauTv, but the fomer has the smaller kind
1237 -- then we must create a fresh TauTv with the smaller kind
1238 (_, BoxTv) | k1_sub_k2 -> update_tv2
1239 | otherwise -> box_meets_box
1240 (BoxTv, _ ) | k2_sub_k1 -> update_tv1
1241 | otherwise -> box_meets_box
1243 -- Avoid SigTvs if poss
1244 (SigTv _, _ ) | k1_sub_k2 -> update_tv2
1245 (_, SigTv _) | k2_sub_k1 -> update_tv1
1247 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1248 then update_tv1 -- Same kinds
1250 | k2_sub_k1 -> update_tv1
1251 | otherwise -> kind_err
1253 -- Update the variable with least kind info
1254 -- See notes on type inference in Kind.lhs
1255 -- The "nicer to" part only applies if the two kinds are the same,
1256 -- so we can choose which to do.
1258 -- Kinds should be guaranteed ok at this point
1259 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1260 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1262 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1265 | k2_sub_k1 = fill_from tv2
1266 | otherwise = kind_err
1268 -- Update *both* tyvars with a TauTv whose name and kind
1269 -- are gotten from tv (avoid losing nice names is poss)
1270 fill_from tv = do { tv' <- tcInstTyVar tv
1271 ; let tau_ty = mkTyVarTy tv'
1272 ; updateMeta tv1 ref1 tau_ty
1273 ; updateMeta tv2 ref2 tau_ty }
1275 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1276 unifyKindMisMatch k1 k2
1280 k1_sub_k2 = k1 `isSubKind` k2
1281 k2_sub_k1 = k2 `isSubKind` k1
1283 nicer_to_update_tv1 = isSystemName (Var.varName tv1)
1284 -- Try to update sys-y type variables in preference to ones
1285 -- gotten (say) by instantiating a polymorphic function with
1286 -- a user-written type sig
1289 checkUpdateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1290 -- Update tv1, which is flexi; occurs check is alrady done
1291 -- The 'check' version does a kind check too
1292 -- We do a sub-kind check here: we might unify (a b) with (c d)
1293 -- where b::*->* and d::*; this should fail
1295 checkUpdateMeta swapped tv1 ref1 ty2
1296 = do { checkKinds swapped tv1 ty2
1297 ; updateMeta tv1 ref1 ty2 }
1299 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1300 updateMeta tv1 ref1 ty2
1301 = ASSERT( isMetaTyVar tv1 )
1302 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
1303 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
1304 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
1305 ; writeMutVar ref1 (Indirect ty2) }
1308 checkKinds swapped tv1 ty2
1309 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
1310 -- ty2 has been zonked at this stage, which ensures that
1311 -- its kind has as much boxity information visible as possible.
1312 | tk2 `isSubKind` tk1 = returnM ()
1315 -- Either the kinds aren't compatible
1316 -- (can happen if we unify (a b) with (c d))
1317 -- or we are unifying a lifted type variable with an
1318 -- unlifted type: e.g. (id 3#) is illegal
1319 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1320 unifyKindMisMatch k1 k2
1322 (k1,k2) | swapped = (tk2,tk1)
1323 | otherwise = (tk1,tk2)
1328 checkTauTvUpdate :: TcTyVar -> TcType -> TcM TcType
1329 -- (checkTauTvUpdate tv ty)
1330 -- We are about to update the TauTv tv with ty.
1331 -- Check (a) that tv doesn't occur in ty (occurs check)
1332 -- (b) that ty is a monotype
1333 -- Furthermore, in the interest of (b), if you find an
1334 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
1336 -- Returns the (non-boxy) type to update the type variable with, or fails
1338 checkTauTvUpdate orig_tv orig_ty
1341 go (TyConApp tc tys)
1342 | isSynTyCon tc = go_syn tc tys
1343 | otherwise = do { tys' <- mappM go tys; return (TyConApp tc tys') }
1344 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
1345 go (PredTy p) = do { p' <- go_pred p; return (PredTy p') }
1346 go (FunTy arg res) = do { arg' <- go arg; res' <- go res; return (FunTy arg' res') }
1347 go (AppTy fun arg) = do { fun' <- go fun; arg' <- go arg; return (mkAppTy fun' arg') }
1348 -- NB the mkAppTy; we might have instantiated a
1349 -- type variable to a type constructor, so we need
1350 -- to pull the TyConApp to the top.
1351 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
1354 | orig_tv == tv = occurCheck tv orig_ty -- (a)
1355 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
1356 | otherwise = return (TyVarTy tv)
1357 -- Ordinary (non Tc) tyvars
1358 -- occur inside quantified types
1360 go_pred (ClassP c tys) = do { tys' <- mapM go tys; return (ClassP c tys') }
1361 go_pred (IParam n ty) = do { ty' <- go ty; return (IParam n ty') }
1362 go_pred (EqPred t1 t2) = do { t1' <- go t1; t2' <- go t2; return (EqPred t1' t2') }
1364 go_tyvar tv (SkolemTv _) = return (TyVarTy tv)
1365 go_tyvar tv (MetaTv box ref)
1366 = do { cts <- readMutVar ref
1368 Indirect ty -> go ty
1369 Flexi -> case box of
1370 BoxTv -> fillBoxWithTau tv ref
1371 other -> return (TyVarTy tv)
1374 -- go_syn is called for synonyms only
1375 -- See Note [Type synonyms and the occur check]
1377 | not (isTauTyCon tc)
1378 = notMonoType orig_ty -- (b) again
1380 = do { (msgs, mb_tys') <- tryTc (mapM go tys)
1382 Just tys' -> return (TyConApp tc tys')
1383 -- Retain the synonym (the common case)
1384 Nothing | isOpenTyCon tc
1385 -> notMonoArgs (TyConApp tc tys)
1386 -- Synonym families must have monotype args
1388 -> go (expectJust "checkTauTvUpdate"
1389 (tcView (TyConApp tc tys)))
1390 -- Try again, expanding the synonym
1393 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
1394 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
1395 -- tau-type meta-variable, whose print-name is the same as tv
1396 -- Choosing the same name is good: when we instantiate a function
1397 -- we allocate boxy tyvars with the same print-name as the quantified
1398 -- tyvar; and then we often fill the box with a tau-tyvar, and again
1399 -- we want to choose the same name.
1400 fillBoxWithTau tv ref
1401 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
1402 ; let tau = mkTyVarTy tv' -- name of the type variable
1403 ; writeMutVar ref (Indirect tau)
1407 Note [Type synonyms and the occur check]
1408 ~~~~~~~~~~~~~~~~~~~~
1409 Basically we want to update tv1 := ps_ty2
1410 because ps_ty2 has type-synonym info, which improves later error messages
1415 f :: (A a -> a -> ()) -> ()
1419 x = f (\ x p -> p x)
1421 In the application (p x), we try to match "t" with "A t". If we go
1422 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
1423 an infinite loop later.
1424 But we should not reject the program, because A t = ().
1425 Rather, we should bind t to () (= non_var_ty2).
1428 refineBox :: TcType -> TcM TcType
1429 -- Unbox the outer box of a boxy type (if any)
1430 refineBox ty@(TyVarTy box_tv)
1431 | isMetaTyVar box_tv
1432 = do { cts <- readMetaTyVar box_tv
1435 Indirect ty -> return ty }
1436 refineBox other_ty = return other_ty
1438 refineBoxToTau :: TcType -> TcM TcType
1439 -- Unbox the outer box of a boxy type, filling with a monotype if it is empty
1440 -- Like refineBox except for the "fill with monotype" part.
1441 refineBoxToTau ty@(TyVarTy box_tv)
1442 | isMetaTyVar box_tv
1443 , MetaTv BoxTv ref <- tcTyVarDetails box_tv
1444 = do { cts <- readMutVar ref
1446 Flexi -> fillBoxWithTau box_tv ref
1447 Indirect ty -> return ty }
1448 refineBoxToTau other_ty = return other_ty
1450 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1451 -- Subtle... we must zap the boxy res_ty
1452 -- to kind * before using it to instantiate a LitInst
1453 -- Calling unBox instead doesn't do the job, because the box
1454 -- often has an openTypeKind, and we don't want to instantiate
1456 zapToMonotype res_ty
1457 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1458 ; boxyUnify res_tau res_ty
1461 unBox :: BoxyType -> TcM TcType
1462 -- unBox implements the judgement
1464 -- with input s', and result s
1466 -- It removes all boxes from the input type, returning a non-boxy type.
1467 -- A filled box in the type can only contain a monotype; unBox fails if not
1468 -- The type can have empty boxes, which unBox fills with a monotype
1470 -- Compare this wth checkTauTvUpdate
1472 -- For once, it's safe to treat synonyms as opaque!
1474 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1475 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1476 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1477 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1478 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1479 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1480 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1482 | isTcTyVar tv -- It's a boxy type variable
1483 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1484 = do { cts <- readMutVar ref -- under nested quantifiers
1486 Flexi -> fillBoxWithTau tv ref
1487 Indirect ty -> do { non_boxy_ty <- unBox ty
1488 ; if isTauTy non_boxy_ty
1489 then return non_boxy_ty
1490 else notMonoType non_boxy_ty }
1492 | otherwise -- Skolems, and meta-tau-variables
1493 = return (TyVarTy tv)
1495 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1496 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1497 unBoxPred (EqPred ty1 ty2) = do { ty1' <- unBox ty1; ty2' <- unBox ty2; return (EqPred ty1' ty2') }
1502 %************************************************************************
1504 \subsection[Unify-context]{Errors and contexts}
1506 %************************************************************************
1512 unifyCtxt act_ty exp_ty tidy_env
1513 = do { act_ty' <- zonkTcType act_ty
1514 ; exp_ty' <- zonkTcType exp_ty
1515 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1516 (env2, act_ty'') = tidyOpenType env1 act_ty'
1517 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1520 mkExpectedActualMsg act_ty exp_ty
1521 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1522 text "Inferred type" <> colon <+> ppr act_ty ])
1525 -- If an error happens we try to figure out whether the function
1526 -- function has been given too many or too few arguments, and say so.
1527 addSubCtxt SubDone actual_res_ty expected_res_ty thing_inside
1529 addSubCtxt sub_ctxt actual_res_ty expected_res_ty thing_inside
1530 = addErrCtxtM mk_err thing_inside
1533 = do { exp_ty' <- zonkTcType expected_res_ty
1534 ; act_ty' <- zonkTcType actual_res_ty
1535 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1536 (env2, act_ty'') = tidyOpenType env1 act_ty'
1537 (exp_args, _) = tcSplitFunTys exp_ty''
1538 (act_args, _) = tcSplitFunTys act_ty''
1540 len_act_args = length act_args
1541 len_exp_args = length exp_args
1543 message = case sub_ctxt of
1544 SubFun fun | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1545 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1546 other -> mkExpectedActualMsg act_ty'' exp_ty''
1547 ; return (env2, message) }
1549 wrongArgsCtxt too_many_or_few fun
1550 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1551 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1552 <+> ptext SLIT("arguments")
1555 unifyForAllCtxt tvs phi1 phi2 env
1556 = returnM (env2, msg)
1558 (env', tvs') = tidyOpenTyVars env tvs -- NB: not tidyTyVarBndrs
1559 (env1, phi1') = tidyOpenType env' phi1
1560 (env2, phi2') = tidyOpenType env1 phi2
1561 msg = vcat [ptext SLIT("When matching") <+> quotes (ppr (mkForAllTys tvs' phi1')),
1562 ptext SLIT(" and") <+> quotes (ppr (mkForAllTys tvs' phi2'))]
1565 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1566 -- tv1 and ty2 are zonked already
1569 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1570 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1572 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1573 | otherwise = (pp1, pp2)
1574 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1575 (env2, ty2') = tidyOpenType env1 ty2
1576 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1577 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1579 unifyMisMatch outer swapped ty1 ty2
1580 = do { (env, msg) <- if swapped then misMatchMsg ty1 ty2
1581 else misMatchMsg ty2 ty1
1583 -- This is the whole point of the 'outer' stuff
1584 ; if outer then popErrCtxt (failWithTcM (env, msg))
1585 else failWithTcM (env, msg)
1588 -----------------------
1589 misMatchMsg :: TcType -> TcType -> TcM (TidyEnv, SDoc)
1590 -- Generate the message when two types fail to match,
1591 -- going to some trouble to make it helpful
1593 = do { env0 <- tcInitTidyEnv
1594 ; (env1, pp1, extra1) <- ppr_ty env0 ty1 ty2
1595 ; (env2, pp2, extra2) <- ppr_ty env1 ty2 ty1
1596 ; return (env2, sep [sep [ptext SLIT("Couldn't match expected type") <+> pp1,
1597 nest 7 (ptext SLIT("against inferred type") <+> pp2)],
1598 nest 2 (extra1 $$ extra2)]) }
1600 ppr_ty :: TidyEnv -> TcType -> TcType -> TcM (TidyEnv, SDoc, SDoc)
1601 ppr_ty env ty other_ty
1602 = do { ty' <- zonkTcType ty
1603 ; let (env1, tidy_ty) = tidyOpenType env ty'
1604 ; (env2, extra) <- ppr_extra env1 tidy_ty other_ty
1605 ; return (env2, quotes (ppr tidy_ty), extra) }
1607 -- (ppr_extra env ty other_ty) shows extra info about 'ty'
1608 ppr_extra env (TyVarTy tv) other_ty
1609 | isSkolemTyVar tv || isSigTyVar tv
1610 = return (env1, pprSkolTvBinding tv1)
1612 (env1, tv1) = tidySkolemTyVar env tv
1614 ppr_extra env (TyConApp tc1 _) (TyConApp tc2 _)
1615 | getOccName tc1 == getOccName tc2
1616 = -- This case helps with messages that would otherwise say
1617 -- Could not match 'T' does not match 'M.T'
1618 -- which is not helpful
1619 do { this_mod <- getModule
1620 ; return (env, quotes (ppr tc1) <+> ptext SLIT("is defined") <+> mk_mod this_mod) }
1622 tc_mod = nameModule (getName tc1)
1623 tc_pkg = modulePackageId tc_mod
1624 tc2_pkg = modulePackageId (nameModule (getName tc2))
1626 | tc_mod == this_mod = ptext SLIT("in this module")
1628 | not home_pkg && tc2_pkg /= tc_pkg = pp_pkg
1629 -- Suppress the module name if (a) it's from another package
1630 -- (b) other_ty isn't from that same package
1632 | otherwise = ptext SLIT("in module") <+> quotes (ppr tc_mod) <+> pp_pkg
1634 home_pkg = tc_pkg == modulePackageId this_mod
1635 pp_pkg | home_pkg = empty
1636 | otherwise = ptext SLIT("in package") <+> quotes (ppr tc_pkg)
1638 ppr_extra env ty other_ty = return (env, empty) -- Normal case
1640 -----------------------
1642 = do { ty' <- zonkTcType ty
1643 ; env0 <- tcInitTidyEnv
1644 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1645 msg = ptext SLIT("Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1646 ; failWithTcM (env1, msg) }
1649 = do { ty' <- zonkTcType ty
1650 ; env0 <- tcInitTidyEnv
1651 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1652 msg = ptext SLIT("Arguments of synonym family must be monotypes") <+> quotes (ppr tidy_ty)
1653 ; failWithTcM (env1, msg) }
1656 = do { env0 <- tcInitTidyEnv
1657 ; ty' <- zonkTcType ty
1658 ; let (env1, tidy_tyvar) = tidyOpenTyVar env0 tyvar
1659 (env2, tidy_ty) = tidyOpenType env1 ty'
1660 extra = sep [ppr tidy_tyvar, char '=', ppr tidy_ty]
1661 ; failWithTcM (env2, hang msg 2 extra) }
1663 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
1667 %************************************************************************
1671 %************************************************************************
1673 Unifying kinds is much, much simpler than unifying types.
1676 unifyKind :: TcKind -- Expected
1679 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1680 | isSubKindCon kc2 kc1 = returnM ()
1682 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1683 = do { unifyKind a2 a1; unifyKind r1 r2 }
1684 -- Notice the flip in the argument,
1685 -- so that the sub-kinding works right
1686 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1687 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1688 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1690 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1691 unifyKinds [] [] = returnM ()
1692 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1694 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1697 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1698 uKVar swapped kv1 k2
1699 = do { mb_k1 <- readKindVar kv1
1701 Flexi -> uUnboundKVar swapped kv1 k2
1702 Indirect k1 | swapped -> unifyKind k2 k1
1703 | otherwise -> unifyKind k1 k2 }
1706 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1707 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1708 | kv1 == kv2 = returnM ()
1709 | otherwise -- Distinct kind variables
1710 = do { mb_k2 <- readKindVar kv2
1712 Indirect k2 -> uUnboundKVar swapped kv1 k2
1713 Flexi -> writeKindVar kv1 k2 }
1715 uUnboundKVar swapped kv1 non_var_k2
1716 = do { k2' <- zonkTcKind non_var_k2
1717 ; kindOccurCheck kv1 k2'
1718 ; k2'' <- kindSimpleKind swapped k2'
1719 -- KindVars must be bound only to simple kinds
1720 -- Polarities: (kindSimpleKind True ?) succeeds
1721 -- returning *, corresponding to unifying
1724 ; writeKindVar kv1 k2'' }
1727 kindOccurCheck kv1 k2 -- k2 is zonked
1728 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1730 not_in (TyVarTy kv2) = kv1 /= kv2
1731 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1734 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1735 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1736 -- If the flag is False, it requires k <: sk
1737 -- E.g. kindSimpleKind False ?? = *
1738 -- What about (kv -> *) :=: ?? -> *
1739 kindSimpleKind orig_swapped orig_kind
1740 = go orig_swapped orig_kind
1742 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1744 ; return (mkArrowKind k1' k2') }
1746 | isOpenTypeKind k = return liftedTypeKind
1747 | isArgTypeKind k = return liftedTypeKind
1749 | isLiftedTypeKind k = return liftedTypeKind
1750 | isUnliftedTypeKind k = return unliftedTypeKind
1751 go sw k@(TyVarTy _) = return k -- KindVars are always simple
1752 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1753 <+> ppr orig_swapped <+> ppr orig_kind)
1754 -- I think this can't actually happen
1756 -- T v = MkT v v must be a type
1757 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1760 kindOccurCheckErr tyvar ty
1761 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1762 2 (sep [ppr tyvar, char '=', ppr ty])
1764 unifyKindMisMatch ty1 ty2
1765 = zonkTcKind ty1 `thenM` \ ty1' ->
1766 zonkTcKind ty2 `thenM` \ ty2' ->
1768 msg = hang (ptext SLIT("Couldn't match kind"))
1769 2 (sep [quotes (ppr ty1'),
1770 ptext SLIT("against"),
1777 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1778 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1780 unifyFunKind (TyVarTy kvar)
1781 = readKindVar kvar `thenM` \ maybe_kind ->
1783 Indirect fun_kind -> unifyFunKind fun_kind
1785 do { arg_kind <- newKindVar
1786 ; res_kind <- newKindVar
1787 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1788 ; returnM (Just (arg_kind,res_kind)) }
1790 unifyFunKind (FunTy arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1791 unifyFunKind other = returnM Nothing
1794 %************************************************************************
1798 %************************************************************************
1800 ---------------------------
1801 -- We would like to get a decent error message from
1802 -- (a) Under-applied type constructors
1803 -- f :: (Maybe, Maybe)
1804 -- (b) Over-applied type constructors
1805 -- f :: Int x -> Int x
1809 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1810 -- A fancy wrapper for 'unifyKind', which tries
1811 -- to give decent error messages.
1812 -- (checkExpectedKind ty act_kind exp_kind)
1813 -- checks that the actual kind act_kind is compatible
1814 -- with the expected kind exp_kind
1815 -- The first argument, ty, is used only in the error message generation
1816 checkExpectedKind ty act_kind exp_kind
1817 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1820 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1822 Just r -> returnM () ; -- Unification succeeded
1825 -- So there's definitely an error
1826 -- Now to find out what sort
1827 zonkTcKind exp_kind `thenM` \ exp_kind ->
1828 zonkTcKind act_kind `thenM` \ act_kind ->
1830 tcInitTidyEnv `thenM` \ env0 ->
1831 let (exp_as, _) = splitKindFunTys exp_kind
1832 (act_as, _) = splitKindFunTys act_kind
1833 n_exp_as = length exp_as
1834 n_act_as = length act_as
1836 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1837 (env2, tidy_act_kind) = tidyKind env1 act_kind
1839 err | n_exp_as < n_act_as -- E.g. [Maybe]
1840 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1842 -- Now n_exp_as >= n_act_as. In the next two cases,
1843 -- n_exp_as == 0, and hence so is n_act_as
1844 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1845 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1846 <+> ptext SLIT("is unlifted")
1848 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1849 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1850 <+> ptext SLIT("is lifted")
1852 | otherwise -- E.g. Monad [Int]
1853 = ptext SLIT("Kind mis-match")
1855 more_info = sep [ ptext SLIT("Expected kind") <+>
1856 quotes (pprKind tidy_exp_kind) <> comma,
1857 ptext SLIT("but") <+> quotes (ppr ty) <+>
1858 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1860 failWithTcM (env2, err $$ more_info)
1864 %************************************************************************
1866 \subsection{Checking signature type variables}
1868 %************************************************************************
1870 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1871 are not mentioned in the environment. In particular:
1873 (a) Not mentioned in the type of a variable in the envt
1874 eg the signature for f in this:
1880 Here, f is forced to be monorphic by the free occurence of x.
1882 (d) Not (unified with another type variable that is) in scope.
1883 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1884 when checking the expression type signature, we find that
1885 even though there is nothing in scope whose type mentions r,
1886 nevertheless the type signature for the expression isn't right.
1888 Another example is in a class or instance declaration:
1890 op :: forall b. a -> b
1892 Here, b gets unified with a
1894 Before doing this, the substitution is applied to the signature type variable.
1897 checkSigTyVars :: [TcTyVar] -> TcM ()
1898 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1900 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1901 -- The extra_tvs can include boxy type variables;
1902 -- e.g. TcMatches.tcCheckExistentialPat
1903 checkSigTyVarsWrt extra_tvs sig_tvs
1904 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1905 ; check_sig_tyvars extra_tvs' sig_tvs }
1908 :: TcTyVarSet -- Global type variables. The universally quantified
1909 -- tyvars should not mention any of these
1910 -- Guaranteed already zonked.
1911 -> [TcTyVar] -- Universally-quantified type variables in the signature
1912 -- Guaranteed to be skolems
1914 check_sig_tyvars extra_tvs []
1916 check_sig_tyvars extra_tvs sig_tvs
1917 = ASSERT( all isSkolemTyVar sig_tvs )
1918 do { gbl_tvs <- tcGetGlobalTyVars
1919 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1920 text "gbl_tvs" <+> ppr gbl_tvs,
1921 text "extra_tvs" <+> ppr extra_tvs]))
1923 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1924 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1925 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1928 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1929 -> [TcTyVar] -- The possibly-escaping type variables
1930 -> [TcTyVar] -- The zonked versions thereof
1932 -- Complain about escaping type variables
1933 -- We pass a list of type variables, at least one of which
1934 -- escapes. The first list contains the original signature type variable,
1935 -- while the second contains the type variable it is unified to (usually itself)
1936 bleatEscapedTvs globals sig_tvs zonked_tvs
1937 = do { env0 <- tcInitTidyEnv
1938 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1939 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1941 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1942 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1944 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1946 check (tidy_env, msgs) (sig_tv, zonked_tv)
1947 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1949 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1950 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1952 -----------------------
1953 escape_msg sig_tv zonked_tv globs
1955 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1956 nest 2 (vcat globs)]
1958 = msg <+> ptext SLIT("escapes")
1959 -- Sigh. It's really hard to give a good error message
1960 -- all the time. One bad case is an existential pattern match.
1961 -- We rely on the "When..." context to help.
1963 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1965 | sig_tv == zonked_tv = empty
1966 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1969 These two context are used with checkSigTyVars
1972 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1973 -> TidyEnv -> TcM (TidyEnv, Message)
1974 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1975 = zonkTcType sig_tau `thenM` \ actual_tau ->
1977 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1978 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1979 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1980 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1981 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1983 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),