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"
38 import TcRnMonad -- TcType, amongst others
58 %************************************************************************
60 \subsection{'hole' type variables}
62 %************************************************************************
65 tcInfer :: (BoxyType -> TcM a) -> TcM (a, TcType)
67 = do { box <- newBoxyTyVar openTypeKind
68 ; res <- tc_infer (mkTyVarTy box)
69 ; res_ty <- {- pprTrace "tcInfer" (ppr (mkTyVarTy box)) $ -} readFilledBox box -- Guaranteed filled-in by now
70 ; return (res, res_ty) }
74 %************************************************************************
78 %************************************************************************
81 subFunTys :: SDoc -- Somthing like "The function f has 3 arguments"
82 -- or "The abstraction (\x.e) takes 1 argument"
83 -> Arity -- Expected # of args
84 -> BoxyRhoType -- res_ty
85 -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
87 -- Attempt to decompse res_ty to have enough top-level arrows to
88 -- match the number of patterns in the match group
90 -- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
91 -- and the inner call to thing_inside passes args: [a1,...,an], b
92 -- then co_fn :: (a1 -> ... -> an -> b) ~ res_ty
94 -- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
97 {- Error messages from subFunTys
99 The abstraction `\Just 1 -> ...' has two arguments
100 but its type `Maybe a -> a' has only one
102 The equation(s) for `f' have two arguments
103 but its type `Maybe a -> a' has only one
105 The section `(f 3)' requires 'f' to take two arguments
106 but its type `Int -> Int' has only one
108 The function 'f' is applied to two arguments
109 but its type `Int -> Int' has only one
113 subFunTys error_herald n_pats res_ty thing_inside
114 = loop n_pats [] res_ty
116 -- In 'loop', the parameter 'arg_tys' accumulates
117 -- the arg types so far, in *reverse order*
118 -- INVARIANT: res_ty :: *
119 loop n args_so_far res_ty
120 | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
122 loop n args_so_far res_ty
123 | isSigmaTy res_ty -- Do this before checking n==0, because we
124 -- guarantee to return a BoxyRhoType, not a BoxySigmaType
125 = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet $ \ _ res_ty' ->
126 loop n args_so_far res_ty'
127 ; return (gen_fn <.> co_fn, res) }
129 loop 0 args_so_far res_ty
130 = do { res <- thing_inside (reverse args_so_far) res_ty
131 ; return (idHsWrapper, res) }
133 loop n args_so_far (FunTy arg_ty res_ty)
134 = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
135 ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
136 ; return (co_fn', res) }
138 -- res_ty might have a type variable at the head, such as (a b c),
139 -- in which case we must fill in with (->). Simplest thing to do
140 -- is to use boxyUnify, but we catch failure and generate our own
141 -- error message on failure
142 loop n args_so_far res_ty@(AppTy _ _)
143 = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
144 ; (_, mb_coi) <- tryTcErrs $ boxyUnify res_ty (FunTy arg_ty' res_ty')
145 ; if isNothing mb_coi then bale_out args_so_far
146 else do { case expectJust "subFunTys" mb_coi of
148 ACo co -> traceTc (text "you're dropping a coercion: " <+> ppr co)
149 ; loop n args_so_far (FunTy arg_ty' res_ty')
153 loop n args_so_far (TyVarTy tv)
154 | isTyConableTyVar tv
155 = do { cts <- readMetaTyVar tv
157 Indirect ty -> loop n args_so_far ty
158 Flexi -> do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
159 ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty
160 ; return (idHsWrapper, res) } }
162 mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
163 mk_res_ty [] = panic "TcUnify.mk_res_ty1"
164 kinds = openTypeKind : take n (repeat argTypeKind)
165 -- Note argTypeKind: the args can have an unboxed type,
166 -- but not an unboxed tuple.
168 loop n args_so_far res_ty = bale_out args_so_far
171 = do { env0 <- tcInitTidyEnv
172 ; res_ty' <- zonkTcType res_ty
173 ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
174 ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
176 mk_msg res_ty n_actual
177 = error_herald <> comma $$
178 sep [ptext SLIT("but its type") <+> quotes (pprType res_ty),
179 if n_actual == 0 then ptext SLIT("has none")
180 else ptext SLIT("has only") <+> speakN n_actual]
184 ----------------------
185 boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
186 -> BoxyRhoType -- Expected type (T a b c)
187 -> TcM [BoxySigmaType] -- Element types, a b c
188 -- It's used for wired-in tycons, so we call checkWiredInTyCOn
189 -- Precondition: never called with FunTyCon
190 -- Precondition: input type :: *
192 boxySplitTyConApp tc orig_ty
193 = do { checkWiredInTyCon tc
194 ; loop (tyConArity tc) [] orig_ty }
196 loop n_req args_so_far ty
197 | Just ty' <- tcView ty = loop n_req args_so_far ty'
199 loop n_req args_so_far (TyConApp tycon args)
201 = ASSERT( n_req == length args) -- ty::*
202 return (args ++ args_so_far)
204 loop n_req args_so_far (AppTy fun arg)
206 = loop (n_req - 1) (arg:args_so_far) fun
208 loop n_req args_so_far (TyVarTy tv)
209 | isTyConableTyVar tv
210 , res_kind `isSubKind` tyVarKind tv
211 = do { cts <- readMetaTyVar tv
213 Indirect ty -> loop n_req args_so_far ty
214 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
215 ; return (arg_tys ++ args_so_far) }
218 mk_res_ty arg_tys' = mkTyConApp tc arg_tys'
219 (arg_kinds, res_kind) = splitKindFunTysN n_req (tyConKind tc)
221 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc))) orig_ty
223 ----------------------
224 boxySplitListTy :: BoxyRhoType -> TcM BoxySigmaType -- Special case for lists
225 boxySplitListTy exp_ty = do { [elt_ty] <- boxySplitTyConApp listTyCon exp_ty
229 ----------------------
230 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
231 -> TcM (BoxySigmaType, BoxySigmaType) -- Returns m, a
232 -- If the incoming type is a mutable type variable of kind k, then
233 -- boxySplitAppTy returns a new type variable (m: * -> k); note the *.
234 -- If the incoming type is boxy, then so are the result types; and vice versa
236 boxySplitAppTy orig_ty
240 | Just ty' <- tcView ty = loop ty'
243 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
244 = return (fun_ty, arg_ty)
247 | isTyConableTyVar tv
248 = do { cts <- readMetaTyVar tv
250 Indirect ty -> loop ty
251 Flexi -> do { [fun_ty,arg_ty] <- withMetaTvs tv kinds mk_res_ty
252 ; return (fun_ty, arg_ty) } }
254 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
255 mk_res_ty other = panic "TcUnify.mk_res_ty2"
256 tv_kind = tyVarKind tv
257 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
259 liftedTypeKind] -- arg type :: *
260 -- The defaultKind is a bit smelly. If you remove it,
261 -- try compiling f x = do { x }
262 -- and you'll get a kind mis-match. It smells, but
263 -- not enough to lose sleep over.
265 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
268 boxySplitFailure actual_ty expected_ty
269 = unifyMisMatch False False actual_ty expected_ty
270 -- "outer" is False, so we don't pop the context
271 -- which is what we want since we have not pushed one!
275 --------------------------------
276 -- withBoxes: the key utility function
277 --------------------------------
280 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
281 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
282 -> ([BoxySigmaType] -> BoxySigmaType)
283 -- Constructs the type to assign
284 -- to the original var
285 -> TcM [BoxySigmaType] -- Return the fresh boxes
287 -- It's entirely possible for the [kind] to be empty.
288 -- For example, when pattern-matching on True,
289 -- we call boxySplitTyConApp passing a boolTyCon
291 -- Invariant: tv is still Flexi
293 withMetaTvs tv kinds mk_res_ty
295 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
296 ; let box_tys = mkTyVarTys box_tvs
297 ; writeMetaTyVar tv (mk_res_ty box_tys)
300 | otherwise -- Non-boxy meta type variable
301 = do { tau_tys <- mapM newFlexiTyVarTy kinds
302 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
303 -- Sure to be a tau-type
306 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
307 -- Allocate a *boxy* tyvar
308 withBox kind thing_inside
309 = do { box_tv <- newMetaTyVar BoxTv kind
310 ; res <- thing_inside (mkTyVarTy box_tv)
311 ; ty <- {- pprTrace "with_box" (ppr (mkTyVarTy box_tv)) $ -} readFilledBox box_tv
316 %************************************************************************
318 Approximate boxy matching
320 %************************************************************************
323 preSubType :: [TcTyVar] -- Quantified type variables
324 -> TcTyVarSet -- Subset of quantified type variables
325 -- see Note [Pre-sub boxy]
326 -> TcType -- The rho-type part; quantified tyvars scopes over this
327 -> BoxySigmaType -- Matching type from the context
328 -> TcM [TcType] -- Types to instantiate the tyvars
329 -- Perform pre-subsumption, and return suitable types
330 -- to instantiate the quantified type varibles:
331 -- info from the pre-subsumption, if there is any
332 -- a boxy type variable otherwise
334 -- Note [Pre-sub boxy]
335 -- The 'btvs' are a subset of 'qtvs'. They are the ones we can
336 -- instantiate to a boxy type variable, because they'll definitely be
337 -- filled in later. This isn't always the case; sometimes we have type
338 -- variables mentioned in the context of the type, but not the body;
339 -- f :: forall a b. C a b => a -> a
340 -- Then we may land up with an unconstrained 'b', so we want to
341 -- instantiate it to a monotype (non-boxy) type variable
343 -- The 'qtvs' that are *neither* fixed by the pre-subsumption, *nor* are in 'btvs',
344 -- are instantiated to TauTv meta variables.
346 preSubType qtvs btvs qty expected_ty
347 = do { tys <- mapM inst_tv qtvs
348 ; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
351 pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
353 | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
354 | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
355 ; return (mkTyVarTy tv') }
356 | otherwise = do { tv' <- tcInstTyVar tv
357 ; return (mkTyVarTy tv') }
360 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
361 -> BoxyRhoType -- Type to match (note a *Rho* type)
362 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
364 -- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
365 -- "Boxy types: inference for higher rank types and impredicativity"
367 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
368 = go tmpl_tvs tmpl_ty emptyVarSet boxy_ty
370 go t_tvs t_ty b_tvs b_ty
371 | Just t_ty' <- tcView t_ty = go t_tvs t_ty' b_tvs b_ty
372 | Just b_ty' <- tcView b_ty = go t_tvs t_ty b_tvs b_ty'
374 go t_tvs (TyVarTy _) b_tvs b_ty = emptyTvSubst -- Rule S-ANY; no bindings
375 -- Rule S-ANY covers (a) type variables and (b) boxy types
376 -- in the template. Both look like a TyVarTy.
377 -- See Note [Sub-match] below
379 go t_tvs t_ty b_tvs b_ty
380 | isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
381 = go (t_tvs `delVarSetList` tvs) t_tau b_tvs b_ty -- Rule S-SPEC
382 -- Under a forall on the left, if there is shadowing,
383 -- do not bind! Hence the delVarSetList.
384 | isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
385 = go t_tvs t_ty (extendVarSetList b_tvs tvs) b_tau -- Rule S-SKOL
386 -- Add to the variables we must not bind to
387 -- NB: it's *important* to discard the theta part. Otherwise
388 -- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
389 -- and end up with a completely bogus binding (b |-> Bool), by lining
390 -- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
391 -- This pre-subsumption stuff can return too few bindings, but it
392 -- must *never* return bogus info.
394 go t_tvs (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
395 = boxy_match t_tvs arg1 b_tvs arg2 (go t_tvs res1 b_tvs res2)
396 -- Match the args, and sub-match the results
398 go t_tvs t_ty b_tvs b_ty = boxy_match t_tvs t_ty b_tvs b_ty emptyTvSubst
399 -- Otherwise defer to boxy matching
400 -- This covers TyConApp, AppTy, PredTy
407 |- head xs : <rhobox>
408 We will do a boxySubMatchType between a ~ <rhobox>
409 But we *don't* want to match [a |-> <rhobox>] because
410 (a) The box should be filled in with a rho-type, but
411 but the returned substitution maps TyVars to boxy
413 (b) In any case, the right final answer might be *either*
414 instantiate 'a' with a rho-type or a sigma type
415 head xs : Int vs head xs : forall b. b->b
416 So the matcher MUST NOT make a choice here. In general, we only
417 bind a template type variable in boxyMatchType, not in boxySubMatchType.
422 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
423 -> [BoxySigmaType] -- Type to match
424 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
426 -- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
427 -- "Boxy types: inference for higher rank types and impredicativity"
429 -- Find a *boxy* substitution that makes the template look as much
430 -- like the BoxySigmaType as possible.
431 -- It's always ok to return an empty substitution;
432 -- anything more is jam on the pudding
434 -- NB1: This is a pure, non-monadic function.
435 -- It does no unification, and cannot fail
437 -- Precondition: the arg lengths are equal
438 -- Precondition: none of the template type variables appear anywhere in the [BoxySigmaType]
442 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
443 = ASSERT( length tmpl_tys == length boxy_tys )
444 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
445 -- ToDo: add error context?
447 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
449 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
450 = boxy_match tmpl_tvs t_ty boxy_tvs b_ty $
451 boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys subst
452 boxy_match_s tmpl_tvs _ boxy_tvs _ subst
453 = panic "boxy_match_s" -- Lengths do not match
457 boxy_match :: TcTyVarSet -> TcType -- Template
458 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
459 -> BoxySigmaType -- Match against this type
463 -- The boxy_tvs argument prevents this match:
464 -- [a] forall b. a ~ forall b. b
465 -- We don't want to bind the template variable 'a'
466 -- to the quantified type variable 'b'!
468 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
469 = go orig_tmpl_ty orig_boxy_ty
472 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
473 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
475 go ty1 ty2 -- C.f. the isSigmaTy case for boxySubMatchType
477 , (tvs1, _, tau1) <- tcSplitSigmaTy ty1
478 , (tvs2, _, tau2) <- tcSplitSigmaTy ty2
479 , equalLength tvs1 tvs2
480 = boxy_match (tmpl_tvs `delVarSetList` tvs1) tau1
481 (boxy_tvs `extendVarSetList` tvs2) tau2 subst
483 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
485 , not $ isOpenSynTyCon tc1
488 go (FunTy arg1 res1) (FunTy arg2 res2)
489 = go_s [arg1,res1] [arg2,res2]
492 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
493 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
494 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
495 = go_s [s1,t1] [s2,t2]
498 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
499 , boxy_tvs `disjointVarSet` tyVarsOfType orig_boxy_ty
500 , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
501 = extendTvSubst subst tv boxy_ty'
503 = subst -- Ignore others
505 boxy_ty' = case lookupTyVar subst tv of
506 Nothing -> orig_boxy_ty
507 Just ty -> ty `boxyLub` orig_boxy_ty
509 go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
510 -- Example: Tree a ~ Maybe Int
511 -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
512 -- misleading error messages. An even more confusing case is
513 -- a -> b ~ Maybe Int
514 -- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
515 -- from this pre-matching phase.
518 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
521 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
522 -- Combine boxy information from the two types
523 -- If there is a conflict, return the first
524 boxyLub orig_ty1 orig_ty2
525 = go orig_ty1 orig_ty2
527 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
528 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
529 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
530 | tc1 == tc2, length ts1 == length ts2
531 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
533 go (TyVarTy tv1) ty2 -- This is the whole point;
534 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
537 -- Look inside type synonyms, but only if the naive version fails
538 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
539 | Just ty2' <- tcView ty1 = go ty1 ty2'
541 -- For now, we don't look inside ForAlls, PredTys
542 go ty1 ty2 = orig_ty1 -- Default
545 Note [Matching kinds]
546 ~~~~~~~~~~~~~~~~~~~~~
547 The target type might legitimately not be a sub-kind of template.
548 For example, suppose the target is simply a box with an OpenTypeKind,
549 and the template is a type variable with LiftedTypeKind.
550 Then it's ok (because the target type will later be refined).
551 We simply don't bind the template type variable.
553 It might also be that the kind mis-match is an error. For example,
554 suppose we match the template (a -> Int) against (Int# -> Int),
555 where the template type variable 'a' has LiftedTypeKind. This
556 matching function does not fail; it simply doesn't bind the template.
557 Later stuff will fail.
559 %************************************************************************
563 %************************************************************************
565 All the tcSub calls have the form
567 tcSub expected_ty offered_ty
569 offered_ty <= expected_ty
571 That is, that a value of type offered_ty is acceptable in
572 a place expecting a value of type expected_ty.
574 It returns a coercion function
575 co_fn :: offered_ty ~ expected_ty
576 which takes an HsExpr of type offered_ty into one of type
581 tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
582 -- (tcSub act exp) checks that
584 tcSubExp actual_ty expected_ty
585 = -- addErrCtxtM (unifyCtxt actual_ty expected_ty) $
586 -- Adding the error context here leads to some very confusing error
587 -- messages, such as "can't match forall a. a->a with forall a. a->a"
588 -- Example is tcfail165:
589 -- do var <- newEmptyMVar :: IO (MVar (forall a. Show a => a -> String))
590 -- putMVar var (show :: forall a. Show a => a -> String)
591 -- Here the info does not flow from the 'var' arg of putMVar to its 'show' arg
592 -- but after zonking it looks as if it does!
594 -- So instead I'm adding the error context when moving from tc_sub to u_tys
596 traceTc (text "tcSubExp" <+> ppr actual_ty <+> ppr expected_ty) >>
597 tc_sub SubOther actual_ty actual_ty False expected_ty expected_ty
599 tcFunResTy :: Name -> BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
600 tcFunResTy fun actual_ty expected_ty
601 = traceTc (text "tcFunResTy" <+> ppr actual_ty <+> ppr expected_ty) >>
602 tc_sub (SubFun fun) actual_ty actual_ty False expected_ty expected_ty
605 data SubCtxt = SubDone -- Error-context already pushed
606 | SubFun Name -- Context is tcFunResTy
607 | SubOther -- Context is something else
609 tc_sub :: SubCtxt -- How to add an error-context
610 -> BoxySigmaType -- actual_ty, before expanding synonyms
611 -> BoxySigmaType -- ..and after
612 -> InBox -- True <=> expected_ty is inside a box
613 -> BoxySigmaType -- expected_ty, before
614 -> BoxySigmaType -- ..and after
616 -- The acual_ty is never inside a box
617 -- IMPORTANT pre-condition: if the args contain foralls, the bound type
618 -- variables are visible non-monadically
619 -- (i.e. tha args are sufficiently zonked)
620 -- This invariant is needed so that we can "see" the foralls, ad
621 -- e.g. in the SPEC rule where we just use splitSigmaTy
623 tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
624 = traceTc (text "tc_sub" <+> ppr act_ty $$ ppr exp_ty) >>
625 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
626 -- This indirection is just here to make
627 -- it easy to insert a debug trace!
629 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
630 | Just exp_ty' <- tcView exp_ty = tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty'
631 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
632 | Just act_ty' <- tcView act_ty = tc_sub sub_ctxt act_sty act_ty' exp_ib exp_sty exp_ty
634 -----------------------------------
635 -- Rule SBOXY, plus other cases when act_ty is a type variable
636 -- Just defer to boxy matching
637 -- This rule takes precedence over SKOL!
638 tc_sub1 sub_ctxt act_sty (TyVarTy tv) exp_ib exp_sty exp_ty
639 = do { traceTc (text "tc_sub1 - case 1")
640 ; coi <- addSubCtxt sub_ctxt act_sty exp_sty $
641 uVar True False tv exp_ib exp_sty exp_ty
642 ; traceTc (case coi of
643 IdCo -> text "tc_sub1 (Rule SBOXY) IdCo"
644 ACo co -> text "tc_sub1 (Rule SBOXY) ACo" <+> ppr co)
645 ; return $ case coi of
650 -----------------------------------
651 -- Skolemisation case (rule SKOL)
652 -- actual_ty: d:Eq b => b->b
653 -- expected_ty: forall a. Ord a => a->a
654 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
656 -- It is essential to do this *before* the specialisation case
657 -- Example: f :: (Eq a => a->a) -> ...
658 -- g :: Ord b => b->b
661 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
663 = do { traceTc (text "tc_sub1 - case 2") ;
664 if exp_ib then -- SKOL does not apply if exp_ty is inside a box
665 defer_to_boxy_matching sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
667 { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ _ body_exp_ty ->
668 tc_sub sub_ctxt act_sty act_ty False body_exp_ty body_exp_ty
669 ; return (gen_fn <.> co_fn) }
672 act_tvs = tyVarsOfType act_ty
673 -- It's really important to check for escape wrt
674 -- the free vars of both expected_ty *and* actual_ty
676 -----------------------------------
677 -- Specialisation case (rule ASPEC):
678 -- actual_ty: forall a. Ord a => a->a
679 -- expected_ty: Int -> Int
680 -- co_fn e = e Int dOrdInt
682 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
683 -- Implements the new SPEC rule in the Appendix of the paper
684 -- "Boxy types: inference for higher rank types and impredicativity"
685 -- (This appendix isn't in the published version.)
686 -- The idea is to *first* do pre-subsumption, and then full subsumption
687 -- Example: forall a. a->a <= Int -> (forall b. Int)
688 -- Pre-subsumpion finds a|->Int, and that works fine, whereas
689 -- just running full subsumption would fail.
690 | isSigmaTy actual_ty
691 = do { traceTc (text "tc_sub1 - case 3")
692 ; -- Perform pre-subsumption, and instantiate
693 -- the type with info from the pre-subsumption;
694 -- boxy tyvars if pre-subsumption gives no info
695 let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
696 tau_tvs = exactTyVarsOfType tau
697 ; inst_tys <- if exp_ib then -- Inside a box, do not do clever stuff
698 do { tyvars' <- mapM tcInstBoxyTyVar tyvars
699 ; return (mkTyVarTys tyvars') }
700 else -- Outside, do clever stuff
701 preSubType tyvars tau_tvs tau expected_ty
702 ; let subst' = zipOpenTvSubst tyvars inst_tys
703 tau' = substTy subst' tau
705 -- Perform a full subsumption check
706 ; traceTc (text "tc_sub_spec" <+> vcat [ppr actual_ty,
707 ppr tyvars <+> ppr theta <+> ppr tau,
709 ; co_fn2 <- tc_sub sub_ctxt tau' tau' exp_ib exp_sty expected_ty
711 -- Deal with the dictionaries
712 -- The origin gives a helpful origin when we have
713 -- a function with type f :: Int -> forall a. Num a => ...
714 -- This way the (Num a) dictionary gets an OccurrenceOf f origin
715 ; let orig = case sub_ctxt of
716 SubFun n -> OccurrenceOf n
717 other -> InstSigOrigin -- Unhelpful
718 ; co_fn1 <- instCall orig inst_tys (substTheta subst' theta)
719 ; return (co_fn2 <.> co_fn1) }
721 -----------------------------------
722 -- Function case (rule F1)
723 tc_sub1 sub_ctxt act_sty (FunTy act_arg act_res) exp_ib exp_sty (FunTy exp_arg exp_res)
724 = do { traceTc (text "tc_sub1 - case 4")
725 ; addSubCtxt sub_ctxt act_sty exp_sty $
726 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
729 -- Function case (rule F2)
730 tc_sub1 sub_ctxt act_sty act_ty@(FunTy act_arg act_res) _ exp_sty (TyVarTy exp_tv)
732 = addSubCtxt sub_ctxt act_sty exp_sty $
733 do { traceTc (text "tc_sub1 - case 5")
734 ; cts <- readMetaTyVar exp_tv
736 Indirect ty -> tc_sub SubDone act_sty act_ty True exp_sty ty
737 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
738 ; tc_sub_funs act_arg act_res True arg_ty res_ty } }
740 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
741 mk_res_ty other = panic "TcUnify.mk_res_ty3"
742 fun_kinds = [argTypeKind, openTypeKind]
744 -- Everything else: defer to boxy matching
745 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty@(TyVarTy exp_tv)
746 = do { traceTc (text "tc_sub1 - case 6a" <+> ppr [isBoxyTyVar exp_tv, isMetaTyVar exp_tv, isSkolemTyVar exp_tv, isExistentialTyVar exp_tv,isSigTyVar exp_tv] )
747 ; defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
750 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
751 = do { traceTc (text "tc_sub1 - case 6")
752 ; defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
755 -----------------------------------
756 defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
757 = do { coi <- addSubCtxt sub_ctxt act_sty exp_sty $
758 u_tys outer False act_sty actual_ty exp_ib exp_sty expected_ty
759 ; return $ case coi of
764 outer = case sub_ctxt of -- Ugh
768 -----------------------------------
769 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
770 = do { arg_coi <- uTys False act_arg exp_ib exp_arg
771 ; co_fn_res <- tc_sub SubDone act_res act_res exp_ib exp_res exp_res
772 ; wrapper1 <- wrapFunResCoercion [exp_arg] co_fn_res
773 ; let wrapper2 = case arg_coi of
775 ACo co -> WpCo $ FunTy co act_res
776 ; return (wrapper1 <.> wrapper2)
779 -----------------------------------
781 :: [TcType] -- Type of args
782 -> HsWrapper -- HsExpr a -> HsExpr b
783 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
784 wrapFunResCoercion arg_tys co_fn_res
785 | isIdHsWrapper co_fn_res
790 = do { arg_ids <- newSysLocalIds FSLIT("sub") arg_tys
791 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpApps arg_ids) }
796 %************************************************************************
798 \subsection{Generalisation}
800 %************************************************************************
803 tcGen :: BoxySigmaType -- expected_ty
804 -> TcTyVarSet -- Extra tyvars that the universally
805 -- quantified tyvars of expected_ty
806 -- must not be unified
807 -> ([TcTyVar] -> BoxyRhoType -> TcM result)
808 -> TcM (HsWrapper, result)
809 -- The expression has type: spec_ty -> expected_ty
811 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
812 -- If not, the call is a no-op
813 = do { traceTc (text "tcGen")
814 -- We want the GenSkol info in the skolemised type variables to
815 -- mention the *instantiated* tyvar names, so that we get a
816 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
817 -- Hence the tiresome but innocuous fixM
818 ; ((tvs', theta', rho'), skol_info) <- fixM (\ ~(_, skol_info) ->
819 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
820 -- Get loation from monad, not from expected_ty
821 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty)
822 ; return ((forall_tvs, theta, rho_ty), skol_info) })
825 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
826 text "expected_ty" <+> ppr expected_ty,
827 text "inst ty" <+> ppr tvs' <+> ppr theta' <+> ppr rho',
828 text "free_tvs" <+> ppr free_tvs])
831 -- Type-check the arg and unify with poly type
832 ; (result, lie) <- getLIE (thing_inside tvs' rho')
834 -- Check that the "forall_tvs" havn't been constrained
835 -- The interesting bit here is that we must include the free variables
836 -- of the expected_ty. Here's an example:
837 -- runST (newVar True)
838 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
839 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
840 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
841 -- So now s' isn't unconstrained because it's linked to a.
842 -- Conclusion: include the free vars of the expected_ty in the
843 -- list of "free vars" for the signature check.
845 ; loc <- getInstLoc (SigOrigin skol_info)
846 ; dicts <- newDictBndrs loc theta'
847 ; inst_binds <- tcSimplifyCheck loc tvs' dicts lie
849 ; checkSigTyVarsWrt free_tvs tvs'
850 ; traceTc (text "tcGen:done")
853 -- The WpLet binds any Insts which came out of the simplification.
854 dict_ids = map instToId dicts
855 co_fn = mkWpTyLams tvs' <.> mkWpLams dict_ids <.> WpLet inst_binds
856 ; returnM (co_fn, result) }
858 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
863 %************************************************************************
867 %************************************************************************
869 The exported functions are all defined as versions of some
870 non-exported generic functions.
873 boxyUnify :: BoxyType -> BoxyType -> TcM CoercionI
874 -- Acutal and expected, respectively
876 = addErrCtxtM (unifyCtxt ty1 ty2) $
877 uTysOuter False ty1 False ty2
880 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM [CoercionI]
881 -- Arguments should have equal length
882 -- Acutal and expected types
883 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
886 unifyType :: TcTauType -> TcTauType -> TcM CoercionI
887 -- No boxes expected inside these types
888 -- Acutal and expected types
889 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
890 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
891 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
892 addErrCtxtM (unifyCtxt ty1 ty2) $
893 uTysOuter True ty1 True ty2
896 unifyPred :: PredType -> PredType -> TcM CoercionI
897 -- Acutal and expected types
898 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
899 uPred True True p1 True p2
901 unifyTheta :: TcThetaType -> TcThetaType -> TcM [CoercionI]
902 -- Acutal and expected types
903 unifyTheta theta1 theta2
904 = do { checkTc (equalLength theta1 theta2)
905 (vcat [ptext SLIT("Contexts differ in length"),
906 nest 2 $ parens $ ptext SLIT("Use -fglasgow-exts to allow this")])
907 ; uList unifyPred theta1 theta2
911 uList :: (a -> a -> TcM b)
912 -> [a] -> [a] -> TcM [b]
913 -- Unify corresponding elements of two lists of types, which
914 -- should be of equal length. We charge down the list explicitly so that
915 -- we can complain if their lengths differ.
916 uList unify [] [] = return []
917 uList unify (ty1:tys1) (ty2:tys2) = do { x <- unify ty1 ty2;
918 ; xs <- uList unify tys1 tys2
921 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
924 @unifyTypeList@ takes a single list of @TauType@s and unifies them
925 all together. It is used, for example, when typechecking explicit
926 lists, when all the elts should be of the same type.
929 unifyTypeList :: [TcTauType] -> TcM ()
930 unifyTypeList [] = returnM ()
931 unifyTypeList [ty] = returnM ()
932 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
933 ; unifyTypeList tys }
936 %************************************************************************
938 \subsection[Unify-uTys]{@uTys@: getting down to business}
940 %************************************************************************
942 @uTys@ is the heart of the unifier. Each arg occurs twice, because
943 we want to report errors in terms of synomyms if possible. The first of
944 the pair is used in error messages only; it is always the same as the
945 second, except that if the first is a synonym then the second may be a
946 de-synonym'd version. This way we get better error messages.
948 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
952 -- False <=> the two args are (actual, expected) respectively
953 -- True <=> the two args are (expected, actual) respectively
955 type InBox = Bool -- True <=> we are inside a box
956 -- False <=> we are outside a box
957 -- The importance of this is that if we get "filled-box meets
958 -- filled-box", we'll look into the boxes and unify... but
959 -- we must not allow polytypes. But if we are in a box on
960 -- just one side, then we can allow polytypes
962 type Outer = Bool -- True <=> this is the outer level of a unification
963 -- so that the types being unified are the
964 -- very ones we began with, not some sub
965 -- component or synonym expansion
966 -- The idea is that if Outer is true then unifyMisMatch should
967 -- pop the context to remove the "Expected/Acutal" context
970 :: InBox -> TcType -- ty1 is the *actual* type
971 -> InBox -> TcType -- ty2 is the *expected* type
973 uTysOuter nb1 ty1 nb2 ty2
974 = do { traceTc (text "uTysOuter" <+> ppr ty1 <+> ppr ty2)
975 ; u_tys True nb1 ty1 ty1 nb2 ty2 ty2 }
977 = do { traceTc (text "uTys" <+> ppr ty1 <+> ppr ty2)
978 ; u_tys False nb1 ty1 ty1 nb2 ty2 ty2 }
982 uTys_s :: InBox -> [TcType] -- tys1 are the *actual* types
983 -> InBox -> [TcType] -- tys2 are the *expected* types
985 uTys_s nb1 [] nb2 [] = returnM []
986 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { coi <- uTys nb1 ty1 nb2 ty2
987 ; cois <- uTys_s nb1 tys1 nb2 tys2
990 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
994 -> InBox -> TcType -> TcType -- ty1 is the *actual* type
995 -> InBox -> TcType -> TcType -- ty2 is the *expected* type
998 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
999 = do { traceTc (text "u_tys " <+> ppr ty1 <+> text " " <+> ppr ty2)
1000 ; coi <- go outer ty1 ty2
1001 ; traceTc (case coi of
1002 ACo co -> text "u_tys yields coercion: " <+> ppr co
1003 IdCo -> text "u_tys yields no coercion")
1008 go :: Outer -> TcType -> TcType -> TcM CoercionI
1010 do { traceTc (text "go " <+> ppr orig_ty1 <+> text "/" <+> ppr ty1
1011 <+> ppr orig_ty2 <+> text "/" <+> ppr ty2)
1015 go1 :: Outer -> TcType -> TcType -> TcM CoercionI
1016 -- Always expand synonyms: see Note [Unification and synonyms]
1017 -- (this also throws away FTVs)
1019 | Just ty1' <- tcView ty1 = go False ty1' ty2
1020 | Just ty2' <- tcView ty2 = go False ty1 ty2'
1022 -- Variables; go for uVar
1023 go1 outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
1024 go1 outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
1025 -- "True" means args swapped
1027 -- The case for sigma-types must *follow* the variable cases
1028 -- because a boxy variable can be filed with a polytype;
1029 -- but must precede FunTy, because ((?x::Int) => ty) look
1030 -- like a FunTy; there isn't necy a forall at the top
1032 | isSigmaTy ty1 || isSigmaTy ty2
1033 = do { traceTc (text "We have sigma types: equalLength" <+> ppr tvs1 <+> ppr tvs2)
1034 ; checkM (equalLength tvs1 tvs2)
1035 (unifyMisMatch outer False orig_ty1 orig_ty2)
1036 ; traceTc (text "We're past the first length test")
1037 ; tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
1038 -- Get location from monad, not from tvs1
1039 ; let tys = mkTyVarTys tvs
1040 in_scope = mkInScopeSet (mkVarSet tvs)
1041 phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
1042 phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
1043 (theta1,tau1) = tcSplitPhiTy phi1
1044 (theta2,tau2) = tcSplitPhiTy phi2
1046 ; addErrCtxtM (unifyForAllCtxt tvs phi1 phi2) $ do
1047 { checkM (equalLength theta1 theta2)
1048 (unifyMisMatch outer False orig_ty1 orig_ty2)
1050 ; cois <- uPreds False nb1 theta1 nb2 theta2 -- TOMDO: do something with these pred_cois
1051 ; traceTc (text "TOMDO!")
1052 ; coi <- uTys nb1 tau1 nb2 tau2
1054 -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
1055 ; free_tvs <- zonkTcTyVarsAndFV (varSetElems (tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2))
1056 ; ifM (any (`elemVarSet` free_tvs) tvs)
1057 (bleatEscapedTvs free_tvs tvs tvs)
1059 -- If both sides are inside a box, we are in a "box-meets-box"
1060 -- situation, and we should not have a polytype at all.
1061 -- If we get here we have two boxes, already filled with
1062 -- the same polytype... but it should be a monotype.
1063 -- This check comes last, because the error message is
1064 -- extremely unhelpful.
1065 ; ifM (nb1 && nb2) (notMonoType ty1)
1069 (tvs1, body1) = tcSplitForAllTys ty1
1070 (tvs2, body2) = tcSplitForAllTys ty2
1073 go1 outer (PredTy p1) (PredTy p2)
1074 = uPred False nb1 p1 nb2 p2
1076 -- Type constructors must match
1077 go1 _ (TyConApp con1 tys1) (TyConApp con2 tys2)
1078 | con1 == con2 && not (isOpenSynTyCon con1)
1079 = do { cois <- uTys_s nb1 tys1 nb2 tys2
1080 ; return $ mkTyConAppCoI con1 tys1 cois
1082 -- See Note [TyCon app]
1083 | con1 == con2 && identicalOpenSynTyConApp
1084 = do { cois <- uTys_s nb1 tys1' nb2 tys2'
1085 ; return $ mkTyConAppCoI con1 tys1 (replicate n IdCo ++ cois)
1089 (idxTys1, tys1') = splitAt n tys1
1090 (idxTys2, tys2') = splitAt n tys2
1091 identicalOpenSynTyConApp = idxTys1 `tcEqTypes` idxTys2
1092 -- See Note [OpenSynTyCon app]
1094 -- Functions; just check the two parts
1095 go1 _ (FunTy fun1 arg1) (FunTy fun2 arg2)
1096 = do { coi_l <- uTys nb1 fun1 nb2 fun2
1097 ; coi_r <- uTys nb1 arg1 nb2 arg2
1098 ; return $ mkFunTyCoI fun1 coi_l arg1 coi_r
1101 -- Applications need a bit of care!
1102 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
1103 -- NB: we've already dealt with type variables and Notes,
1104 -- so if one type is an App the other one jolly well better be too
1105 go1 outer (AppTy s1 t1) ty2
1106 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
1107 = do { coi_s <- uTys nb1 s1 nb2 s2; coi_t <- uTys nb1 t1 nb2 t2
1108 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1110 -- Now the same, but the other way round
1111 -- Don't swap the types, because the error messages get worse
1112 go1 outer ty1 (AppTy s2 t2)
1113 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
1114 = do { coi_s <- uTys nb1 s1 nb2 s2; coi_t <- uTys nb1 t1 nb2 t2
1115 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1117 -- One or both outermost constructors are type family applications.
1118 -- If we can normalise them away, proceed as usual; otherwise, we
1119 -- need to defer unification by generating a wanted equality constraint.
1121 | ty1_is_fun || ty2_is_fun
1122 = do { (coi1, ty1') <- if ty1_is_fun then tcNormalizeFamInst ty1
1123 else return (IdCo, ty1)
1124 ; (coi2, ty2') <- if ty2_is_fun then tcNormalizeFamInst ty2
1125 else return (IdCo, ty2)
1126 ; coi <- if isOpenSynTyConApp ty1' || isOpenSynTyConApp ty2'
1127 then do { -- One type family app can't be reduced yet
1129 ; ty1'' <- zonkTcType ty1'
1130 ; ty2'' <- zonkTcType ty2'
1131 ; if tcEqType ty1'' ty2''
1133 else -- see [Deferred Unification]
1134 defer_unification outer False orig_ty1 orig_ty2
1136 else -- unification can proceed
1138 ; return $ coi1 `mkTransCoI` coi `mkTransCoI` (mkSymCoI coi2)
1141 ty1_is_fun = isOpenSynTyConApp ty1
1142 ty2_is_fun = isOpenSynTyConApp ty2
1144 -- Anything else fails
1145 go1 outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
1149 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
1151 do { coi <- uTys nb1 t1 nb2 t2
1152 ; return $ mkIParamPredCoI n1 coi
1154 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
1156 do { cois <- uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
1157 ; return $ mkClassPPredCoI c1 tys1 cois
1159 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
1161 uPreds outer nb1 [] nb2 [] = return []
1162 uPreds outer nb1 (p1:ps1) nb2 (p2:ps2) =
1163 do { coi <- uPred outer nb1 p1 nb2 p2
1164 ; cois <- uPreds outer nb1 ps1 nb2 ps2
1167 uPreds outer nb1 ps1 nb2 ps2 = panic "uPreds"
1172 When we find two TyConApps, the argument lists are guaranteed equal
1173 length. Reason: intially the kinds of the two types to be unified is
1174 the same. The only way it can become not the same is when unifying two
1175 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
1176 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
1177 which we do, that ensures that f1,f2 have the same kind; and that
1178 means a1,a2 have the same kind. And now the argument repeats.
1180 Note [OpenSynTyCon app]
1181 ~~~~~~~~~~~~~~~~~~~~~~~
1184 type family T a :: * -> *
1186 the two types (T () a) and (T () Int) must unify, even if there are
1187 no type instances for T at all. Should we just turn them into an
1188 equality (T () a ~ T () Int)? I don't think so. We currently try to
1189 eagerly unify everything we can before generating equalities; otherwise,
1190 we could turn the unification of [Int] with [a] into an equality, too.
1192 Note [Unification and synonyms]
1193 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1194 If you are tempted to make a short cut on synonyms, as in this
1198 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
1199 -- NO = if (con1 == con2) then
1200 -- NO -- Good news! Same synonym constructors, so we can shortcut
1201 -- NO -- by unifying their arguments and ignoring their expansions.
1202 -- NO unifyTypepeLists args1 args2
1204 -- NO -- Never mind. Just expand them and try again
1208 then THINK AGAIN. Here is the whole story, as detected and reported
1209 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1211 Here's a test program that should detect the problem:
1215 x = (1 :: Bogus Char) :: Bogus Bool
1218 The problem with [the attempted shortcut code] is that
1222 is not a sufficient condition to be able to use the shortcut!
1223 You also need to know that the type synonym actually USES all
1224 its arguments. For example, consider the following type synonym
1225 which does not use all its arguments.
1230 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1231 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1232 would fail, even though the expanded forms (both \tr{Int}) should
1235 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1236 unnecessarily bind \tr{t} to \tr{Char}.
1238 ... You could explicitly test for the problem synonyms and mark them
1239 somehow as needing expansion, perhaps also issuing a warning to the
1244 %************************************************************************
1246 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1248 %************************************************************************
1250 @uVar@ is called when at least one of the types being unified is a
1251 variable. It does {\em not} assume that the variable is a fixed point
1252 of the substitution; rather, notice that @uVar@ (defined below) nips
1253 back into @uTys@ if it turns out that the variable is already bound.
1257 -> SwapFlag -- False => tyvar is the "actual" (ty is "expected")
1258 -- True => ty is the "actual" (tyvar is "expected")
1260 -> InBox -- True <=> definitely no boxes in t2
1261 -> TcTauType -> TcTauType -- printing and real versions
1264 uVar outer swapped tv1 nb2 ps_ty2 ty2
1265 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1266 | otherwise = brackets (equals <+> ppr ty2)
1267 ; traceTc (text "uVar" <+> ppr swapped <+>
1268 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1269 nest 2 (ptext SLIT(" <-> ")),
1270 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1271 ; details <- lookupTcTyVar tv1
1274 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1275 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1276 -- The 'True' here says that ty1 is now inside a box
1277 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1281 uUnfilledVar :: Outer
1283 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1284 -> TcTauType -> TcTauType -- Type 2
1286 -- Invariant: tyvar 1 is not unified with anything
1288 uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1289 | Just ty2' <- tcView ty2
1290 = -- Expand synonyms; ignore FTVs
1291 uUnfilledVar False swapped tv1 details1 ps_ty2 ty2'
1293 uUnfilledVar outer swapped tv1 details1 ps_ty2 (TyVarTy tv2)
1294 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1296 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1297 -- this is box-meets-box, so fill in with a tau-type
1298 -> do { tau_tv <- tcInstTyVar tv1
1299 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv)
1302 other -> returnM IdCo -- No-op
1304 | otherwise -- Distinct type variables
1305 = do { lookup2 <- lookupTcTyVar tv2
1307 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
1308 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1311 uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2
1312 = -- ty2 is not a type variable
1314 MetaTv (SigTv _) _ -> rigid_variable
1316 do { uMetaVar swapped tv1 info ref1 ps_ty2 non_var_ty2
1319 SkolemTv _ -> rigid_variable
1322 | isOpenSynTyConApp non_var_ty2
1323 = -- 'non_var_ty2's outermost constructor is a type family,
1324 -- which we may may be able to normalise
1325 do { (coi2, ty2') <- tcNormalizeFamInst non_var_ty2
1327 IdCo -> -- no progress, but maybe after other instantiations
1328 defer_unification outer swapped (TyVarTy tv1) ps_ty2
1329 ACo co -> -- progress: so lets try again
1331 ppr co <+> text "::"<+> ppr non_var_ty2 <+> text "~" <+>
1333 ; coi <- uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2'
1334 ; let coi2' = (if swapped then id else mkSymCoI) coi2
1335 ; return $ coi2' `mkTransCoI` coi
1338 | SkolemTv RuntimeUnkSkol <- details1
1339 -- runtime unknown will never match
1340 = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1341 | otherwise -- defer as a given equality may still resolve this
1342 = defer_unification outer swapped (TyVarTy tv1) ps_ty2
1345 Note [Deferred Unification]
1346 ~~~~~~~~~~~~~~~~~~~~
1347 We may encounter a unification ty1 = ty2 that cannot be performed syntactically,
1348 and yet its consistency is undetermined. Previously, there was no way to still
1349 make it consistent. So a mismatch error was issued.
1351 Now these unfications are deferred until constraint simplification, where type
1352 family instances and given equations may (or may not) establish the consistency.
1353 Deferred unifications are of the form
1356 where F is a type function and x is a type variable.
1358 id :: x ~ y => x -> y
1361 involves the unfication x = y. It is deferred until we bring into account the
1362 context x ~ y to establish that it holds.
1364 If available, we defer original types (rather than those where closed type
1365 synonyms have already been expanded via tcCoreView). This is as usual, to
1366 improve error messages.
1369 defer_unification :: Bool -- pop innermost context?
1374 defer_unification outer True ty1 ty2
1375 = defer_unification outer False ty2 ty1
1376 defer_unification outer False ty1 ty2
1377 = do { ty1' <- zonkTcType ty1
1378 ; ty2' <- zonkTcType ty2
1379 ; traceTc $ text "deferring:" <+> ppr ty1 <+> text "~" <+> ppr ty2
1380 ; cotv <- newMetaTyVar TauTv (mkCoKind ty1' ty2')
1381 -- put ty1 ~ ty2 in LIE
1382 -- Left means "wanted"
1383 ; inst <- (if outer then popErrCtxt else id) $
1384 mkEqInst (EqPred ty1' ty2') (Left cotv)
1386 ; return $ ACo $ TyVarTy cotv }
1389 uMetaVar :: SwapFlag
1390 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1393 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1394 -- ty2 is not a type variable
1396 uMetaVar swapped tv1 BoxTv ref1 ps_ty2 non_var_ty2
1397 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1398 -- that any boxes in ty2 are filled with monotypes
1400 -- It should not be the case that tv1 occurs in ty2
1401 -- (i.e. no occurs check should be needed), but if perchance
1402 -- it does, the unbox operation will fill it, and the DEBUG
1404 do { final_ty <- unBox ps_ty2
1406 ; meta_details <- readMutVar ref1
1407 ; case meta_details of
1408 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1409 return () -- This really should *not* happen
1412 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1414 uMetaVar swapped tv1 info1 ref1 ps_ty2 non_var_ty2
1415 = do { final_ty <- checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
1416 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1419 uUnfilledVars :: Outer
1421 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1422 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1424 -- Invarant: The type variables are distinct,
1425 -- Neither is filled in yet
1426 -- They might be boxy or not
1428 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1429 = -- see [Deferred Unification]
1430 defer_unification outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1432 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1433 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2) >> return IdCo
1434 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1435 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1) >> return IdCo
1437 -- ToDo: this function seems too long for what it acutally does!
1438 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1439 = case (info1, info2) of
1440 (BoxTv, BoxTv) -> box_meets_box >> return IdCo
1442 -- If a box meets a TauTv, but the fomer has the smaller kind
1443 -- then we must create a fresh TauTv with the smaller kind
1444 (_, BoxTv) | k1_sub_k2 -> update_tv2 >> return IdCo
1445 | otherwise -> box_meets_box >> return IdCo
1446 (BoxTv, _ ) | k2_sub_k1 -> update_tv1 >> return IdCo
1447 | otherwise -> box_meets_box >> return IdCo
1449 -- Avoid SigTvs if poss
1450 (SigTv _, _ ) | k1_sub_k2 -> update_tv2 >> return IdCo
1451 (_, SigTv _) | k2_sub_k1 -> update_tv1 >> return IdCo
1453 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1454 then update_tv1 >> return IdCo -- Same kinds
1455 else update_tv2 >> return IdCo
1456 | k2_sub_k1 -> update_tv1 >> return IdCo
1457 | otherwise -> kind_err >> return IdCo
1459 -- Update the variable with least kind info
1460 -- See notes on type inference in Kind.lhs
1461 -- The "nicer to" part only applies if the two kinds are the same,
1462 -- so we can choose which to do.
1464 -- Kinds should be guaranteed ok at this point
1465 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1466 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1468 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1471 | k2_sub_k1 = fill_from tv2
1472 | otherwise = kind_err
1474 -- Update *both* tyvars with a TauTv whose name and kind
1475 -- are gotten from tv (avoid losing nice names is poss)
1476 fill_from tv = do { tv' <- tcInstTyVar tv
1477 ; let tau_ty = mkTyVarTy tv'
1478 ; updateMeta tv1 ref1 tau_ty
1479 ; updateMeta tv2 ref2 tau_ty }
1481 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1482 unifyKindMisMatch k1 k2
1486 k1_sub_k2 = k1 `isSubKind` k2
1487 k2_sub_k1 = k2 `isSubKind` k1
1489 nicer_to_update_tv1 = isSystemName (Var.varName tv1)
1490 -- Try to update sys-y type variables in preference to ones
1491 -- gotten (say) by instantiating a polymorphic function with
1492 -- a user-written type sig
1495 checkUpdateMeta :: SwapFlag
1496 -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1497 -- Update tv1, which is flexi; occurs check is alrady done
1498 -- The 'check' version does a kind check too
1499 -- We do a sub-kind check here: we might unify (a b) with (c d)
1500 -- where b::*->* and d::*; this should fail
1502 checkUpdateMeta swapped tv1 ref1 ty2
1503 = do { checkKinds swapped tv1 ty2
1504 ; updateMeta tv1 ref1 ty2 }
1506 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1507 updateMeta tv1 ref1 ty2
1508 = ASSERT( isMetaTyVar tv1 )
1509 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
1510 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
1511 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
1512 ; writeMutVar ref1 (Indirect ty2)
1516 checkKinds swapped tv1 ty2
1517 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
1518 -- ty2 has been zonked at this stage, which ensures that
1519 -- its kind has as much boxity information visible as possible.
1520 | tk2 `isSubKind` tk1 = returnM ()
1523 -- Either the kinds aren't compatible
1524 -- (can happen if we unify (a b) with (c d))
1525 -- or we are unifying a lifted type variable with an
1526 -- unlifted type: e.g. (id 3#) is illegal
1527 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1528 unifyKindMisMatch k1 k2
1530 (k1,k2) | swapped = (tk2,tk1)
1531 | otherwise = (tk1,tk2)
1536 checkTauTvUpdate :: TcTyVar -> TcType -> TcM TcType
1537 -- (checkTauTvUpdate tv ty)
1538 -- We are about to update the TauTv tv with ty.
1539 -- Check (a) that tv doesn't occur in ty (occurs check)
1540 -- (b) that ty is a monotype
1541 -- Furthermore, in the interest of (b), if you find an
1542 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
1544 -- Returns the (non-boxy) type to update the type variable with, or fails
1546 checkTauTvUpdate orig_tv orig_ty
1549 go (TyConApp tc tys)
1550 | isSynTyCon tc = go_syn tc tys
1551 | otherwise = do { tys' <- mappM go tys; return (TyConApp tc tys') }
1552 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
1553 go (PredTy p) = do { p' <- go_pred p; return (PredTy p') }
1554 go (FunTy arg res) = do { arg' <- go arg; res' <- go res; return (FunTy arg' res') }
1555 go (AppTy fun arg) = do { fun' <- go fun; arg' <- go arg; return (mkAppTy fun' arg') }
1556 -- NB the mkAppTy; we might have instantiated a
1557 -- type variable to a type constructor, so we need
1558 -- to pull the TyConApp to the top.
1559 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
1562 | orig_tv == tv = occurCheck tv orig_ty -- (a)
1563 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
1564 | otherwise = return (TyVarTy tv)
1565 -- Ordinary (non Tc) tyvars
1566 -- occur inside quantified types
1568 go_pred (ClassP c tys) = do { tys' <- mapM go tys; return (ClassP c tys') }
1569 go_pred (IParam n ty) = do { ty' <- go ty; return (IParam n ty') }
1570 go_pred (EqPred t1 t2) = do { t1' <- go t1; t2' <- go t2; return (EqPred t1' t2') }
1572 go_tyvar tv (SkolemTv _) = return (TyVarTy tv)
1573 go_tyvar tv (MetaTv box ref)
1574 = do { cts <- readMutVar ref
1576 Indirect ty -> go ty
1577 Flexi -> case box of
1578 BoxTv -> fillBoxWithTau tv ref
1579 other -> return (TyVarTy tv)
1582 -- go_syn is called for synonyms only
1583 -- See Note [Type synonyms and the occur check]
1585 | not (isTauTyCon tc)
1586 = notMonoType orig_ty -- (b) again
1588 = do { (msgs, mb_tys') <- tryTc (mapM go tys)
1590 Just tys' -> return (TyConApp tc tys')
1591 -- Retain the synonym (the common case)
1592 Nothing | isOpenTyCon tc
1593 -> notMonoArgs (TyConApp tc tys)
1594 -- Synonym families must have monotype args
1596 -> go (expectJust "checkTauTvUpdate"
1597 (tcView (TyConApp tc tys)))
1598 -- Try again, expanding the synonym
1601 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
1602 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
1603 -- tau-type meta-variable, whose print-name is the same as tv
1604 -- Choosing the same name is good: when we instantiate a function
1605 -- we allocate boxy tyvars with the same print-name as the quantified
1606 -- tyvar; and then we often fill the box with a tau-tyvar, and again
1607 -- we want to choose the same name.
1608 fillBoxWithTau tv ref
1609 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
1610 ; let tau = mkTyVarTy tv' -- name of the type variable
1611 ; writeMutVar ref (Indirect tau)
1615 Note [Type synonyms and the occur check]
1616 ~~~~~~~~~~~~~~~~~~~~
1617 Basically we want to update tv1 := ps_ty2
1618 because ps_ty2 has type-synonym info, which improves later error messages
1623 f :: (A a -> a -> ()) -> ()
1627 x = f (\ x p -> p x)
1629 In the application (p x), we try to match "t" with "A t". If we go
1630 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
1631 an infinite loop later.
1632 But we should not reject the program, because A t = ().
1633 Rather, we should bind t to () (= non_var_ty2).
1636 refineBox :: TcType -> TcM TcType
1637 -- Unbox the outer box of a boxy type (if any)
1638 refineBox ty@(TyVarTy box_tv)
1639 | isMetaTyVar box_tv
1640 = do { cts <- readMetaTyVar box_tv
1643 Indirect ty -> return ty }
1644 refineBox other_ty = return other_ty
1646 refineBoxToTau :: TcType -> TcM TcType
1647 -- Unbox the outer box of a boxy type, filling with a monotype if it is empty
1648 -- Like refineBox except for the "fill with monotype" part.
1649 refineBoxToTau ty@(TyVarTy box_tv)
1650 | isMetaTyVar box_tv
1651 , MetaTv BoxTv ref <- tcTyVarDetails box_tv
1652 = do { cts <- readMutVar ref
1654 Flexi -> fillBoxWithTau box_tv ref
1655 Indirect ty -> return ty }
1656 refineBoxToTau other_ty = return other_ty
1658 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1659 -- Subtle... we must zap the boxy res_ty
1660 -- to kind * before using it to instantiate a LitInst
1661 -- Calling unBox instead doesn't do the job, because the box
1662 -- often has an openTypeKind, and we don't want to instantiate
1664 zapToMonotype res_ty
1665 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1666 ; boxyUnify res_tau res_ty
1669 unBox :: BoxyType -> TcM TcType
1670 -- unBox implements the judgement
1672 -- with input s', and result s
1674 -- It removes all boxes from the input type, returning a non-boxy type.
1675 -- A filled box in the type can only contain a monotype; unBox fails if not
1676 -- The type can have empty boxes, which unBox fills with a monotype
1678 -- Compare this wth checkTauTvUpdate
1680 -- For once, it's safe to treat synonyms as opaque!
1682 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1683 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1684 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1685 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1686 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1687 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1688 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1690 | isTcTyVar tv -- It's a boxy type variable
1691 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1692 = do { cts <- readMutVar ref -- under nested quantifiers
1694 Flexi -> fillBoxWithTau tv ref
1695 Indirect ty -> do { non_boxy_ty <- unBox ty
1696 ; if isTauTy non_boxy_ty
1697 then return non_boxy_ty
1698 else notMonoType non_boxy_ty }
1700 | otherwise -- Skolems, and meta-tau-variables
1701 = return (TyVarTy tv)
1703 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1704 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1705 unBoxPred (EqPred ty1 ty2) = do { ty1' <- unBox ty1; ty2' <- unBox ty2; return (EqPred ty1' ty2') }
1710 %************************************************************************
1712 \subsection[Unify-context]{Errors and contexts}
1714 %************************************************************************
1720 unifyCtxt act_ty exp_ty tidy_env
1721 = do { act_ty' <- zonkTcType act_ty
1722 ; exp_ty' <- zonkTcType exp_ty
1723 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1724 (env2, act_ty'') = tidyOpenType env1 act_ty'
1725 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1728 mkExpectedActualMsg act_ty exp_ty
1729 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1730 text "Inferred type" <> colon <+> ppr act_ty ])
1733 -- If an error happens we try to figure out whether the function
1734 -- function has been given too many or too few arguments, and say so.
1735 addSubCtxt SubDone actual_res_ty expected_res_ty thing_inside
1737 addSubCtxt sub_ctxt actual_res_ty expected_res_ty thing_inside
1738 = addErrCtxtM mk_err thing_inside
1741 = do { exp_ty' <- zonkTcType expected_res_ty
1742 ; act_ty' <- zonkTcType actual_res_ty
1743 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1744 (env2, act_ty'') = tidyOpenType env1 act_ty'
1745 (exp_args, _) = tcSplitFunTys exp_ty''
1746 (act_args, _) = tcSplitFunTys act_ty''
1748 len_act_args = length act_args
1749 len_exp_args = length exp_args
1751 message = case sub_ctxt of
1752 SubFun fun | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1753 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1754 other -> mkExpectedActualMsg act_ty'' exp_ty''
1755 ; return (env2, message) }
1757 wrongArgsCtxt too_many_or_few fun
1758 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1759 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1760 <+> ptext SLIT("arguments")
1763 unifyForAllCtxt tvs phi1 phi2 env
1764 = returnM (env2, msg)
1766 (env', tvs') = tidyOpenTyVars env tvs -- NB: not tidyTyVarBndrs
1767 (env1, phi1') = tidyOpenType env' phi1
1768 (env2, phi2') = tidyOpenType env1 phi2
1769 msg = vcat [ptext SLIT("When matching") <+> quotes (ppr (mkForAllTys tvs' phi1')),
1770 ptext SLIT(" and") <+> quotes (ppr (mkForAllTys tvs' phi2'))]
1773 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1774 -- tv1 and ty2 are zonked already
1777 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1778 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1780 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1781 | otherwise = (pp1, pp2)
1782 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1783 (env2, ty2') = tidyOpenType env1 ty2
1784 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1785 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1787 unifyMisMatch outer swapped ty1 ty2
1788 = do { (env, msg) <- if swapped then misMatchMsg ty2 ty1
1789 else misMatchMsg ty1 ty2
1791 -- This is the whole point of the 'outer' stuff
1792 ; if outer then popErrCtxt (failWithTcM (env, msg))
1793 else failWithTcM (env, msg)
1796 -----------------------
1798 = do { ty' <- zonkTcType ty
1799 ; env0 <- tcInitTidyEnv
1800 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1801 msg = ptext SLIT("Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1802 ; failWithTcM (env1, msg) }
1805 = do { ty' <- zonkTcType ty
1806 ; env0 <- tcInitTidyEnv
1807 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1808 msg = ptext SLIT("Arguments of synonym family must be monotypes") <+> quotes (ppr tidy_ty)
1809 ; failWithTcM (env1, msg) }
1812 = do { env0 <- tcInitTidyEnv
1813 ; ty' <- zonkTcType ty
1814 ; let (env1, tidy_tyvar) = tidyOpenTyVar env0 tyvar
1815 (env2, tidy_ty) = tidyOpenType env1 ty'
1816 extra = sep [ppr tidy_tyvar, char '=', ppr tidy_ty]
1817 ; failWithTcM (env2, hang msg 2 extra) }
1819 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
1823 %************************************************************************
1827 %************************************************************************
1829 Unifying kinds is much, much simpler than unifying types.
1832 unifyKind :: TcKind -- Expected
1835 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1836 | isSubKindCon kc2 kc1 = returnM ()
1838 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1839 = do { unifyKind a2 a1; unifyKind r1 r2 }
1840 -- Notice the flip in the argument,
1841 -- so that the sub-kinding works right
1842 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1843 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1844 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1846 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1847 unifyKinds [] [] = returnM ()
1848 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1850 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1853 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1854 uKVar swapped kv1 k2
1855 = do { mb_k1 <- readKindVar kv1
1857 Flexi -> uUnboundKVar swapped kv1 k2
1858 Indirect k1 | swapped -> unifyKind k2 k1
1859 | otherwise -> unifyKind k1 k2 }
1862 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1863 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1864 | kv1 == kv2 = returnM ()
1865 | otherwise -- Distinct kind variables
1866 = do { mb_k2 <- readKindVar kv2
1868 Indirect k2 -> uUnboundKVar swapped kv1 k2
1869 Flexi -> writeKindVar kv1 k2 }
1871 uUnboundKVar swapped kv1 non_var_k2
1872 = do { k2' <- zonkTcKind non_var_k2
1873 ; kindOccurCheck kv1 k2'
1874 ; k2'' <- kindSimpleKind swapped k2'
1875 -- KindVars must be bound only to simple kinds
1876 -- Polarities: (kindSimpleKind True ?) succeeds
1877 -- returning *, corresponding to unifying
1880 ; writeKindVar kv1 k2'' }
1883 kindOccurCheck kv1 k2 -- k2 is zonked
1884 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1886 not_in (TyVarTy kv2) = kv1 /= kv2
1887 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1890 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1891 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1892 -- If the flag is False, it requires k <: sk
1893 -- E.g. kindSimpleKind False ?? = *
1894 -- What about (kv -> *) :=: ?? -> *
1895 kindSimpleKind orig_swapped orig_kind
1896 = go orig_swapped orig_kind
1898 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1900 ; return (mkArrowKind k1' k2') }
1902 | isOpenTypeKind k = return liftedTypeKind
1903 | isArgTypeKind k = return liftedTypeKind
1905 | isLiftedTypeKind k = return liftedTypeKind
1906 | isUnliftedTypeKind k = return unliftedTypeKind
1907 go sw k@(TyVarTy _) = return k -- KindVars are always simple
1908 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1909 <+> ppr orig_swapped <+> ppr orig_kind)
1910 -- I think this can't actually happen
1912 -- T v = MkT v v must be a type
1913 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1916 kindOccurCheckErr tyvar ty
1917 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1918 2 (sep [ppr tyvar, char '=', ppr ty])
1920 unifyKindMisMatch ty1 ty2
1921 = zonkTcKind ty1 `thenM` \ ty1' ->
1922 zonkTcKind ty2 `thenM` \ ty2' ->
1924 msg = hang (ptext SLIT("Couldn't match kind"))
1925 2 (sep [quotes (ppr ty1'),
1926 ptext SLIT("against"),
1933 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1934 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1936 unifyFunKind (TyVarTy kvar)
1937 = readKindVar kvar `thenM` \ maybe_kind ->
1939 Indirect fun_kind -> unifyFunKind fun_kind
1941 do { arg_kind <- newKindVar
1942 ; res_kind <- newKindVar
1943 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1944 ; returnM (Just (arg_kind,res_kind)) }
1946 unifyFunKind (FunTy arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1947 unifyFunKind other = returnM Nothing
1950 %************************************************************************
1954 %************************************************************************
1956 ---------------------------
1957 -- We would like to get a decent error message from
1958 -- (a) Under-applied type constructors
1959 -- f :: (Maybe, Maybe)
1960 -- (b) Over-applied type constructors
1961 -- f :: Int x -> Int x
1965 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1966 -- A fancy wrapper for 'unifyKind', which tries
1967 -- to give decent error messages.
1968 -- (checkExpectedKind ty act_kind exp_kind)
1969 -- checks that the actual kind act_kind is compatible
1970 -- with the expected kind exp_kind
1971 -- The first argument, ty, is used only in the error message generation
1972 checkExpectedKind ty act_kind exp_kind
1973 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1976 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1978 Just r -> returnM () ; -- Unification succeeded
1981 -- So there's definitely an error
1982 -- Now to find out what sort
1983 zonkTcKind exp_kind `thenM` \ exp_kind ->
1984 zonkTcKind act_kind `thenM` \ act_kind ->
1986 tcInitTidyEnv `thenM` \ env0 ->
1987 let (exp_as, _) = splitKindFunTys exp_kind
1988 (act_as, _) = splitKindFunTys act_kind
1989 n_exp_as = length exp_as
1990 n_act_as = length act_as
1992 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1993 (env2, tidy_act_kind) = tidyKind env1 act_kind
1995 err | n_exp_as < n_act_as -- E.g. [Maybe]
1996 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1998 -- Now n_exp_as >= n_act_as. In the next two cases,
1999 -- n_exp_as == 0, and hence so is n_act_as
2000 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
2001 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
2002 <+> ptext SLIT("is unlifted")
2004 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
2005 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
2006 <+> ptext SLIT("is lifted")
2008 | otherwise -- E.g. Monad [Int]
2009 = ptext SLIT("Kind mis-match")
2011 more_info = sep [ ptext SLIT("Expected kind") <+>
2012 quotes (pprKind tidy_exp_kind) <> comma,
2013 ptext SLIT("but") <+> quotes (ppr ty) <+>
2014 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
2016 failWithTcM (env2, err $$ more_info)
2020 %************************************************************************
2022 \subsection{Checking signature type variables}
2024 %************************************************************************
2026 @checkSigTyVars@ checks that a set of universally quantified type varaibles
2027 are not mentioned in the environment. In particular:
2029 (a) Not mentioned in the type of a variable in the envt
2030 eg the signature for f in this:
2036 Here, f is forced to be monorphic by the free occurence of x.
2038 (d) Not (unified with another type variable that is) in scope.
2039 eg f x :: (r->r) = (\y->y) :: forall a. a->r
2040 when checking the expression type signature, we find that
2041 even though there is nothing in scope whose type mentions r,
2042 nevertheless the type signature for the expression isn't right.
2044 Another example is in a class or instance declaration:
2046 op :: forall b. a -> b
2048 Here, b gets unified with a
2050 Before doing this, the substitution is applied to the signature type variable.
2053 checkSigTyVars :: [TcTyVar] -> TcM ()
2054 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
2056 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
2057 -- The extra_tvs can include boxy type variables;
2058 -- e.g. TcMatches.tcCheckExistentialPat
2059 checkSigTyVarsWrt extra_tvs sig_tvs
2060 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
2061 ; check_sig_tyvars extra_tvs' sig_tvs }
2064 :: TcTyVarSet -- Global type variables. The universally quantified
2065 -- tyvars should not mention any of these
2066 -- Guaranteed already zonked.
2067 -> [TcTyVar] -- Universally-quantified type variables in the signature
2068 -- Guaranteed to be skolems
2070 check_sig_tyvars extra_tvs []
2072 check_sig_tyvars extra_tvs sig_tvs
2073 = ASSERT( all isSkolemTyVar sig_tvs )
2074 do { gbl_tvs <- tcGetGlobalTyVars
2075 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
2076 text "gbl_tvs" <+> ppr gbl_tvs,
2077 text "extra_tvs" <+> ppr extra_tvs]))
2079 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
2080 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
2081 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
2084 bleatEscapedTvs :: TcTyVarSet -- The global tvs
2085 -> [TcTyVar] -- The possibly-escaping type variables
2086 -> [TcTyVar] -- The zonked versions thereof
2088 -- Complain about escaping type variables
2089 -- We pass a list of type variables, at least one of which
2090 -- escapes. The first list contains the original signature type variable,
2091 -- while the second contains the type variable it is unified to (usually itself)
2092 bleatEscapedTvs globals sig_tvs zonked_tvs
2093 = do { env0 <- tcInitTidyEnv
2094 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
2095 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
2097 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
2098 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
2100 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
2102 check (tidy_env, msgs) (sig_tv, zonked_tv)
2103 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
2105 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
2106 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
2108 -----------------------
2109 escape_msg sig_tv zonked_tv globs
2111 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
2112 nest 2 (vcat globs)]
2114 = msg <+> ptext SLIT("escapes")
2115 -- Sigh. It's really hard to give a good error message
2116 -- all the time. One bad case is an existential pattern match.
2117 -- We rely on the "When..." context to help.
2119 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
2121 | sig_tv == zonked_tv = empty
2122 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
2125 These two context are used with checkSigTyVars
2128 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
2129 -> TidyEnv -> TcM (TidyEnv, Message)
2130 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
2131 = zonkTcType sig_tau `thenM` \ actual_tau ->
2133 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
2134 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
2135 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
2136 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
2137 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
2139 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),