2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Type subsumption and unification
10 -- Full-blown subsumption
11 tcSubExp, tcFunResTy, tcGen,
12 checkSigTyVars, checkSigTyVarsWrt, bleatEscapedTvs, sigCtxt,
14 -- Various unifications
15 unifyType, unifyTypeList, unifyTheta,
16 unifyKind, unifyKinds, unifyFunKind,
18 preSubType, boxyMatchTypes,
20 --------------------------------
22 tcInfer, subFunTys, unBox, refineBox, refineBoxToTau, withBox,
23 boxyUnify, boxyUnifyList, zapToMonotype,
24 boxySplitListTy, boxySplitTyConApp, boxySplitAppTy,
28 #include "HsVersions.h"
37 import TcRnMonad -- TcType, amongst others
55 %************************************************************************
57 \subsection{'hole' type variables}
59 %************************************************************************
62 tcInfer :: (BoxyType -> TcM a) -> TcM (a, TcType)
64 = do { box <- newBoxyTyVar openTypeKind
65 ; res <- tc_infer (mkTyVarTy box)
66 ; res_ty <- readFilledBox box -- Guaranteed filled-in by now
67 ; return (res, res_ty) }
71 %************************************************************************
75 %************************************************************************
78 subFunTys :: SDoc -- Somthing like "The function f has 3 arguments"
79 -- or "The abstraction (\x.e) takes 1 argument"
80 -> Arity -- Expected # of args
81 -> BoxyRhoType -- res_ty
82 -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
84 -- Attempt to decompse res_ty to have enough top-level arrows to
85 -- match the number of patterns in the match group
87 -- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
88 -- and the inner call to thing_inside passes args: [a1,...,an], b
89 -- then co_fn :: (a1 -> ... -> an -> b) -> res_ty
91 -- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
94 {- Error messages from subFunTys
96 The abstraction `\Just 1 -> ...' has two arguments
97 but its type `Maybe a -> a' has only one
99 The equation(s) for `f' have two arguments
100 but its type `Maybe a -> a' has only one
102 The section `(f 3)' requires 'f' to take two arguments
103 but its type `Int -> Int' has only one
105 The function 'f' is applied to two arguments
106 but its type `Int -> Int' has only one
110 subFunTys error_herald n_pats res_ty thing_inside
111 = loop n_pats [] res_ty
113 -- In 'loop', the parameter 'arg_tys' accumulates
114 -- the arg types so far, in *reverse order*
115 -- INVARIANT: res_ty :: *
116 loop n args_so_far res_ty
117 | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
119 loop n args_so_far res_ty
120 | isSigmaTy res_ty -- Do this before checking n==0, because we
121 -- guarantee to return a BoxyRhoType, not a BoxySigmaType
122 = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet $ \ _ res_ty' ->
123 loop n args_so_far res_ty'
124 ; return (gen_fn <.> co_fn, res) }
126 loop 0 args_so_far res_ty
127 = do { res <- thing_inside (reverse args_so_far) res_ty
128 ; return (idHsWrapper, res) }
130 loop n args_so_far (FunTy arg_ty res_ty)
131 = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
132 ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
133 ; return (co_fn', res) }
135 -- res_ty might have a type variable at the head, such as (a b c),
136 -- in which case we must fill in with (->). Simplest thing to do
137 -- is to use boxyUnify, but we catch failure and generate our own
138 -- error message on failure
139 loop n args_so_far res_ty@(AppTy _ _)
140 = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
141 ; (_, mb_unit) <- tryTcErrs $ boxyUnify res_ty (FunTy arg_ty' res_ty')
142 ; if isNothing mb_unit then bale_out args_so_far
143 else loop n args_so_far (FunTy arg_ty' res_ty') }
145 loop n args_so_far (TyVarTy tv)
146 | isTyConableTyVar tv
147 = do { cts <- readMetaTyVar tv
149 Indirect ty -> loop n args_so_far ty
150 Flexi -> do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
151 ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty
152 ; return (idHsWrapper, res) } }
154 mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
155 mk_res_ty [] = panic "TcUnify.mk_res_ty1"
156 kinds = openTypeKind : take n (repeat argTypeKind)
157 -- Note argTypeKind: the args can have an unboxed type,
158 -- but not an unboxed tuple.
160 loop n args_so_far res_ty = bale_out args_so_far
163 = do { env0 <- tcInitTidyEnv
164 ; res_ty' <- zonkTcType res_ty
165 ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
166 ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
168 mk_msg res_ty n_actual
169 = error_herald <> comma $$
170 sep [ptext SLIT("but its type") <+> quotes (pprType res_ty),
171 if n_actual == 0 then ptext SLIT("has none")
172 else ptext SLIT("has only") <+> speakN n_actual]
176 ----------------------
177 boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
178 -> BoxyRhoType -- Expected type (T a b c)
179 -> TcM [BoxySigmaType] -- Element types, a b c
180 -- It's used for wired-in tycons, so we call checkWiredInTyCOn
181 -- Precondition: never called with FunTyCon
182 -- Precondition: input type :: *
184 boxySplitTyConApp tc orig_ty
185 = do { checkWiredInTyCon tc
186 ; loop (tyConArity tc) [] orig_ty }
188 loop n_req args_so_far ty
189 | Just ty' <- tcView ty = loop n_req args_so_far ty'
191 loop n_req args_so_far (TyConApp tycon args)
193 = ASSERT( n_req == length args) -- ty::*
194 return (args ++ args_so_far)
196 loop n_req args_so_far (AppTy fun arg)
198 = loop (n_req - 1) (arg:args_so_far) fun
200 loop n_req args_so_far (TyVarTy tv)
201 | isTyConableTyVar tv
202 , res_kind `isSubKind` tyVarKind tv
203 = do { cts <- readMetaTyVar tv
205 Indirect ty -> loop n_req args_so_far ty
206 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
207 ; return (arg_tys ++ args_so_far) }
210 mk_res_ty arg_tys' = mkTyConApp tc arg_tys'
211 (arg_kinds, res_kind) = splitKindFunTysN n_req (tyConKind tc)
213 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc))) orig_ty
215 ----------------------
216 boxySplitListTy :: BoxyRhoType -> TcM BoxySigmaType -- Special case for lists
217 boxySplitListTy exp_ty = do { [elt_ty] <- boxySplitTyConApp listTyCon exp_ty
221 ----------------------
222 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
223 -> TcM (BoxySigmaType, BoxySigmaType) -- Returns m, a
224 -- If the incoming type is a mutable type variable of kind k, then
225 -- boxySplitAppTy returns a new type variable (m: * -> k); note the *.
226 -- If the incoming type is boxy, then so are the result types; and vice versa
228 boxySplitAppTy orig_ty
232 | Just ty' <- tcView ty = loop ty'
235 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
236 = return (fun_ty, arg_ty)
239 | isTyConableTyVar tv
240 = do { cts <- readMetaTyVar tv
242 Indirect ty -> loop ty
243 Flexi -> do { [fun_ty,arg_ty] <- withMetaTvs tv kinds mk_res_ty
244 ; return (fun_ty, arg_ty) } }
246 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
247 mk_res_ty other = panic "TcUnify.mk_res_ty2"
248 tv_kind = tyVarKind tv
249 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
251 liftedTypeKind] -- arg type :: *
252 -- The defaultKind is a bit smelly. If you remove it,
253 -- try compiling f x = do { x }
254 -- and you'll get a kind mis-match. It smells, but
255 -- not enough to lose sleep over.
257 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
260 boxySplitFailure actual_ty expected_ty
261 = unifyMisMatch False False actual_ty expected_ty
262 -- "outer" is False, so we don't pop the context
263 -- which is what we want since we have not pushed one!
267 --------------------------------
268 -- withBoxes: the key utility function
269 --------------------------------
272 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
273 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
274 -> ([BoxySigmaType] -> BoxySigmaType)
275 -- Constructs the type to assign
276 -- to the original var
277 -> TcM [BoxySigmaType] -- Return the fresh boxes
279 -- It's entirely possible for the [kind] to be empty.
280 -- For example, when pattern-matching on True,
281 -- we call boxySplitTyConApp passing a boolTyCon
283 -- Invariant: tv is still Flexi
285 withMetaTvs tv kinds mk_res_ty
287 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
288 ; let box_tys = mkTyVarTys box_tvs
289 ; writeMetaTyVar tv (mk_res_ty box_tys)
292 | otherwise -- Non-boxy meta type variable
293 = do { tau_tys <- mapM newFlexiTyVarTy kinds
294 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
295 -- Sure to be a tau-type
298 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
299 -- Allocate a *boxy* tyvar
300 withBox kind thing_inside
301 = do { box_tv <- newMetaTyVar BoxTv kind
302 ; res <- thing_inside (mkTyVarTy box_tv)
303 ; ty <- readFilledBox box_tv
308 %************************************************************************
310 Approximate boxy matching
312 %************************************************************************
315 preSubType :: [TcTyVar] -- Quantified type variables
316 -> TcTyVarSet -- Subset of quantified type variables
317 -- see Note [Pre-sub boxy]
318 -> TcType -- The rho-type part; quantified tyvars scopes over this
319 -> BoxySigmaType -- Matching type from the context
320 -> TcM [TcType] -- Types to instantiate the tyvars
321 -- Perform pre-subsumption, and return suitable types
322 -- to instantiate the quantified type varibles:
323 -- info from the pre-subsumption, if there is any
324 -- a boxy type variable otherwise
326 -- Note [Pre-sub boxy]
327 -- The 'btvs' are a subset of 'qtvs'. They are the ones we can
328 -- instantiate to a boxy type variable, because they'll definitely be
329 -- filled in later. This isn't always the case; sometimes we have type
330 -- variables mentioned in the context of the type, but not the body;
331 -- f :: forall a b. C a b => a -> a
332 -- Then we may land up with an unconstrained 'b', so we want to
333 -- instantiate it to a monotype (non-boxy) type variable
335 -- The 'qtvs' that are *neither* fixed by the pre-subsumption, *nor* are in 'btvs',
336 -- are instantiated to TauTv meta variables.
338 preSubType qtvs btvs qty expected_ty
339 = do { tys <- mapM inst_tv qtvs
340 ; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
343 pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
345 | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
346 | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
347 ; return (mkTyVarTy tv') }
348 | otherwise = do { tv' <- tcInstTyVar tv
349 ; return (mkTyVarTy tv') }
352 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
353 -> BoxyRhoType -- Type to match (note a *Rho* type)
354 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
356 -- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
357 -- "Boxy types: inference for higher rank types and impredicativity"
359 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
360 = go tmpl_tvs tmpl_ty emptyVarSet boxy_ty
362 go t_tvs t_ty b_tvs b_ty
363 | Just t_ty' <- tcView t_ty = go t_tvs t_ty' b_tvs b_ty
364 | Just b_ty' <- tcView b_ty = go t_tvs t_ty b_tvs b_ty'
366 go t_tvs (TyVarTy _) b_tvs b_ty = emptyTvSubst -- Rule S-ANY; no bindings
367 -- Rule S-ANY covers (a) type variables and (b) boxy types
368 -- in the template. Both look like a TyVarTy.
369 -- See Note [Sub-match] below
371 go t_tvs t_ty b_tvs b_ty
372 | isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
373 = go (t_tvs `delVarSetList` tvs) t_tau b_tvs b_ty -- Rule S-SPEC
374 -- Under a forall on the left, if there is shadowing,
375 -- do not bind! Hence the delVarSetList.
376 | isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
377 = go t_tvs t_ty (extendVarSetList b_tvs tvs) b_tau -- Rule S-SKOL
378 -- Add to the variables we must not bind to
379 -- NB: it's *important* to discard the theta part. Otherwise
380 -- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
381 -- and end up with a completely bogus binding (b |-> Bool), by lining
382 -- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
383 -- This pre-subsumption stuff can return too few bindings, but it
384 -- must *never* return bogus info.
386 go t_tvs (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
387 = boxy_match t_tvs arg1 b_tvs arg2 (go t_tvs res1 b_tvs res2)
388 -- Match the args, and sub-match the results
390 go t_tvs t_ty b_tvs b_ty = boxy_match t_tvs t_ty b_tvs b_ty emptyTvSubst
391 -- Otherwise defer to boxy matching
392 -- This covers TyConApp, AppTy, PredTy
399 |- head xs : <rhobox>
400 We will do a boxySubMatchType between a ~ <rhobox>
401 But we *don't* want to match [a |-> <rhobox>] because
402 (a) The box should be filled in with a rho-type, but
403 but the returned substitution maps TyVars to boxy
405 (b) In any case, the right final answer might be *either*
406 instantiate 'a' with a rho-type or a sigma type
407 head xs : Int vs head xs : forall b. b->b
408 So the matcher MUST NOT make a choice here. In general, we only
409 bind a template type variable in boxyMatchType, not in boxySubMatchType.
414 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
415 -> [BoxySigmaType] -- Type to match
416 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
418 -- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
419 -- "Boxy types: inference for higher rank types and impredicativity"
421 -- Find a *boxy* substitution that makes the template look as much
422 -- like the BoxySigmaType as possible.
423 -- It's always ok to return an empty substitution;
424 -- anything more is jam on the pudding
426 -- NB1: This is a pure, non-monadic function.
427 -- It does no unification, and cannot fail
429 -- Precondition: the arg lengths are equal
430 -- Precondition: none of the template type variables appear anywhere in the [BoxySigmaType]
434 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
435 = ASSERT( length tmpl_tys == length boxy_tys )
436 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
437 -- ToDo: add error context?
439 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
441 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
442 = boxy_match tmpl_tvs t_ty boxy_tvs b_ty $
443 boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys subst
444 boxy_match_s tmpl_tvs _ boxy_tvs _ subst
445 = panic "boxy_match_s" -- Lengths do not match
449 boxy_match :: TcTyVarSet -> TcType -- Template
450 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
451 -> BoxySigmaType -- Match against this type
455 -- The boxy_tvs argument prevents this match:
456 -- [a] forall b. a ~ forall b. b
457 -- We don't want to bind the template variable 'a'
458 -- to the quantified type variable 'b'!
460 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
461 = go orig_tmpl_ty orig_boxy_ty
464 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
465 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
467 go ty1 ty2 -- C.f. the isSigmaTy case for boxySubMatchType
469 , (tvs1, _, tau1) <- tcSplitSigmaTy ty1
470 , (tvs2, _, tau2) <- tcSplitSigmaTy ty2
471 , equalLength tvs1 tvs2
472 = boxy_match (tmpl_tvs `delVarSetList` tvs1) tau1
473 (boxy_tvs `extendVarSetList` tvs2) tau2 subst
475 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
476 | tc1 == tc2 = go_s tys1 tys2
478 go (FunTy arg1 res1) (FunTy arg2 res2)
479 = go_s [arg1,res1] [arg2,res2]
482 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
483 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
484 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
485 = go_s [s1,t1] [s2,t2]
488 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
489 , boxy_tvs `disjointVarSet` tyVarsOfType orig_boxy_ty
490 , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
491 = extendTvSubst subst tv boxy_ty'
493 = subst -- Ignore others
495 boxy_ty' = case lookupTyVar subst tv of
496 Nothing -> orig_boxy_ty
497 Just ty -> ty `boxyLub` orig_boxy_ty
499 go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
500 -- Example: Tree a ~ Maybe Int
501 -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
502 -- misleading error messages. An even more confusing case is
503 -- a -> b ~ Maybe Int
504 -- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
505 -- from this pre-matching phase.
508 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
511 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
512 -- Combine boxy information from the two types
513 -- If there is a conflict, return the first
514 boxyLub orig_ty1 orig_ty2
515 = go orig_ty1 orig_ty2
517 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
518 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
519 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
520 | tc1 == tc2, length ts1 == length ts2
521 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
523 go (TyVarTy tv1) ty2 -- This is the whole point;
524 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
527 -- Look inside type synonyms, but only if the naive version fails
528 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
529 | Just ty2' <- tcView ty2 = go ty1 ty2'
531 -- For now, we don't look inside ForAlls, PredTys
532 go ty1 ty2 = orig_ty1 -- Default
535 Note [Matching kinds]
536 ~~~~~~~~~~~~~~~~~~~~~
537 The target type might legitimately not be a sub-kind of template.
538 For example, suppose the target is simply a box with an OpenTypeKind,
539 and the template is a type variable with LiftedTypeKind.
540 Then it's ok (because the target type will later be refined).
541 We simply don't bind the template type variable.
543 It might also be that the kind mis-match is an error. For example,
544 suppose we match the template (a -> Int) against (Int# -> Int),
545 where the template type variable 'a' has LiftedTypeKind. This
546 matching function does not fail; it simply doesn't bind the template.
547 Later stuff will fail.
549 %************************************************************************
553 %************************************************************************
555 All the tcSub calls have the form
557 tcSub expected_ty offered_ty
559 offered_ty <= expected_ty
561 That is, that a value of type offered_ty is acceptable in
562 a place expecting a value of type expected_ty.
564 It returns a coercion function
565 co_fn :: offered_ty -> expected_ty
566 which takes an HsExpr of type offered_ty into one of type
571 tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
572 -- (tcSub act exp) checks that
574 tcSubExp actual_ty expected_ty
575 = -- addErrCtxtM (unifyCtxt actual_ty expected_ty) $
576 -- Adding the error context here leads to some very confusing error
577 -- messages, such as "can't match forall a. a->a with forall a. a->a"
578 -- Example is tcfail165:
579 -- do var <- newEmptyMVar :: IO (MVar (forall a. Show a => a -> String))
580 -- putMVar var (show :: forall a. Show a => a -> String)
581 -- Here the info does not flow from the 'var' arg of putMVar to its 'show' arg
582 -- but after zonking it looks as if it does!
584 -- So instead I'm adding the error context when moving from tc_sub to u_tys
586 traceTc (text "tcSubExp" <+> ppr actual_ty <+> ppr expected_ty) >>
587 tc_sub SubOther actual_ty actual_ty False expected_ty expected_ty
589 tcFunResTy :: Name -> BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
590 tcFunResTy fun actual_ty expected_ty
591 = traceTc (text "tcFunResTy" <+> ppr actual_ty <+> ppr expected_ty) >>
592 tc_sub (SubFun fun) actual_ty actual_ty False expected_ty expected_ty
595 data SubCtxt = SubDone -- Error-context already pushed
596 | SubFun Name -- Context is tcFunResTy
597 | SubOther -- Context is something else
599 tc_sub :: SubCtxt -- How to add an error-context
600 -> BoxySigmaType -- actual_ty, before expanding synonyms
601 -> BoxySigmaType -- ..and after
602 -> InBox -- True <=> expected_ty is inside a box
603 -> BoxySigmaType -- expected_ty, before
604 -> BoxySigmaType -- ..and after
606 -- The acual_ty is never inside a box
607 -- IMPORTANT pre-condition: if the args contain foralls, the bound type
608 -- variables are visible non-monadically
609 -- (i.e. tha args are sufficiently zonked)
610 -- This invariant is needed so that we can "see" the foralls, ad
611 -- e.g. in the SPEC rule where we just use splitSigmaTy
613 tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
614 = traceTc (text "tc_sub" <+> ppr act_ty $$ ppr exp_ty) >>
615 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
616 -- This indirection is just here to make
617 -- it easy to insert a debug trace!
619 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
620 | Just exp_ty' <- tcView exp_ty = tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty'
621 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
622 | Just act_ty' <- tcView act_ty = tc_sub sub_ctxt act_sty act_ty' exp_ib exp_sty exp_ty
624 -----------------------------------
625 -- Rule SBOXY, plus other cases when act_ty is a type variable
626 -- Just defer to boxy matching
627 -- This rule takes precedence over SKOL!
628 tc_sub1 sub_ctxt act_sty (TyVarTy tv) exp_ib exp_sty exp_ty
629 = do { addSubCtxt sub_ctxt act_sty exp_sty $
630 uVar True False tv exp_ib exp_sty exp_ty
631 ; return idHsWrapper }
633 -----------------------------------
634 -- Skolemisation case (rule SKOL)
635 -- actual_ty: d:Eq b => b->b
636 -- expected_ty: forall a. Ord a => a->a
637 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
639 -- It is essential to do this *before* the specialisation case
640 -- Example: f :: (Eq a => a->a) -> ...
641 -- g :: Ord b => b->b
644 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
646 = if exp_ib then -- SKOL does not apply if exp_ty is inside a box
647 defer_to_boxy_matching sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
649 { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ _ body_exp_ty ->
650 tc_sub sub_ctxt act_sty act_ty False body_exp_ty body_exp_ty
651 ; return (gen_fn <.> co_fn) }
653 act_tvs = tyVarsOfType act_ty
654 -- It's really important to check for escape wrt
655 -- the free vars of both expected_ty *and* actual_ty
657 -----------------------------------
658 -- Specialisation case (rule ASPEC):
659 -- actual_ty: forall a. Ord a => a->a
660 -- expected_ty: Int -> Int
661 -- co_fn e = e Int dOrdInt
663 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
664 -- Implements the new SPEC rule in the Appendix of the paper
665 -- "Boxy types: inference for higher rank types and impredicativity"
666 -- (This appendix isn't in the published version.)
667 -- The idea is to *first* do pre-subsumption, and then full subsumption
668 -- Example: forall a. a->a <= Int -> (forall b. Int)
669 -- Pre-subsumpion finds a|->Int, and that works fine, whereas
670 -- just running full subsumption would fail.
671 | isSigmaTy actual_ty
672 = do { -- Perform pre-subsumption, and instantiate
673 -- the type with info from the pre-subsumption;
674 -- boxy tyvars if pre-subsumption gives no info
675 let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
676 tau_tvs = exactTyVarsOfType tau
677 ; inst_tys <- if exp_ib then -- Inside a box, do not do clever stuff
678 do { tyvars' <- mapM tcInstBoxyTyVar tyvars
679 ; return (mkTyVarTys tyvars') }
680 else -- Outside, do clever stuff
681 preSubType tyvars tau_tvs tau expected_ty
682 ; let subst' = zipOpenTvSubst tyvars inst_tys
683 tau' = substTy subst' tau
685 -- Perform a full subsumption check
686 ; traceTc (text "tc_sub_spec" <+> vcat [ppr actual_ty,
687 ppr tyvars <+> ppr theta <+> ppr tau,
689 ; co_fn2 <- tc_sub sub_ctxt tau' tau' exp_ib exp_sty expected_ty
691 -- Deal with the dictionaries
692 -- The origin gives a helpful origin when we have
693 -- a function with type f :: Int -> forall a. Num a => ...
694 -- This way the (Num a) dictionary gets an OccurrenceOf f origin
695 ; let orig = case sub_ctxt of
696 SubFun n -> OccurrenceOf n
697 other -> InstSigOrigin -- Unhelpful
698 ; co_fn1 <- instCall orig inst_tys (substTheta subst' theta)
699 ; return (co_fn2 <.> co_fn1) }
701 -----------------------------------
702 -- Function case (rule F1)
703 tc_sub1 sub_ctxt act_sty (FunTy act_arg act_res) exp_ib exp_sty (FunTy exp_arg exp_res)
704 = addSubCtxt sub_ctxt act_sty exp_sty $
705 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
707 -- Function case (rule F2)
708 tc_sub1 sub_ctxt act_sty act_ty@(FunTy act_arg act_res) _ exp_sty (TyVarTy exp_tv)
710 = addSubCtxt sub_ctxt act_sty exp_sty $
711 do { cts <- readMetaTyVar exp_tv
713 Indirect ty -> tc_sub SubDone act_sty act_ty True exp_sty ty
714 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
715 ; tc_sub_funs act_arg act_res True arg_ty res_ty } }
717 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
718 mk_res_ty other = panic "TcUnify.mk_res_ty3"
719 fun_kinds = [argTypeKind, openTypeKind]
721 -- Everything else: defer to boxy matching
722 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
723 = defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
725 -----------------------------------
726 defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
727 = do { addSubCtxt sub_ctxt act_sty exp_sty $
728 u_tys outer False act_sty actual_ty exp_ib exp_sty expected_ty
729 ; return idHsWrapper }
731 outer = case sub_ctxt of -- Ugh
735 -----------------------------------
736 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
737 = do { uTys False act_arg exp_ib exp_arg
738 ; co_fn_res <- tc_sub SubDone act_res act_res exp_ib exp_res exp_res
739 ; wrapFunResCoercion [exp_arg] co_fn_res }
741 -----------------------------------
743 :: [TcType] -- Type of args
744 -> HsWrapper -- HsExpr a -> HsExpr b
745 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
746 wrapFunResCoercion arg_tys co_fn_res
747 | isIdHsWrapper co_fn_res = return idHsWrapper
748 | null arg_tys = return co_fn_res
750 = do { arg_ids <- newSysLocalIds FSLIT("sub") arg_tys
751 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpApps arg_ids) }
756 %************************************************************************
758 \subsection{Generalisation}
760 %************************************************************************
763 tcGen :: BoxySigmaType -- expected_ty
764 -> TcTyVarSet -- Extra tyvars that the universally
765 -- quantified tyvars of expected_ty
766 -- must not be unified
767 -> ([TcTyVar] -> BoxyRhoType -> TcM result)
768 -> TcM (HsWrapper, result)
769 -- The expression has type: spec_ty -> expected_ty
771 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
772 -- If not, the call is a no-op
773 = do { -- We want the GenSkol info in the skolemised type variables to
774 -- mention the *instantiated* tyvar names, so that we get a
775 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
776 -- Hence the tiresome but innocuous fixM
777 ((tvs', theta', rho'), skol_info) <- fixM (\ ~(_, skol_info) ->
778 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
779 -- Get loation from monad, not from expected_ty
780 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty)
781 ; return ((forall_tvs, theta, rho_ty), skol_info) })
784 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
785 text "expected_ty" <+> ppr expected_ty,
786 text "inst ty" <+> ppr tvs' <+> ppr theta' <+> ppr rho',
787 text "free_tvs" <+> ppr free_tvs])
790 -- Type-check the arg and unify with poly type
791 ; (result, lie) <- getLIE (thing_inside tvs' rho')
793 -- Check that the "forall_tvs" havn't been constrained
794 -- The interesting bit here is that we must include the free variables
795 -- of the expected_ty. Here's an example:
796 -- runST (newVar True)
797 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
798 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
799 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
800 -- So now s' isn't unconstrained because it's linked to a.
801 -- Conclusion: include the free vars of the expected_ty in the
802 -- list of "free vars" for the signature check.
804 ; loc <- getInstLoc (SigOrigin skol_info)
805 ; dicts <- newDictBndrs loc theta'
806 ; inst_binds <- tcSimplifyCheck loc tvs' dicts lie
808 ; checkSigTyVarsWrt free_tvs tvs'
809 ; traceTc (text "tcGen:done")
812 -- The WpLet binds any Insts which came out of the simplification.
813 dict_ids = map instToId dicts
814 co_fn = mkWpTyLams tvs' <.> mkWpLams dict_ids <.> WpLet inst_binds
815 ; returnM (co_fn, result) }
817 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
822 %************************************************************************
826 %************************************************************************
828 The exported functions are all defined as versions of some
829 non-exported generic functions.
832 boxyUnify :: BoxyType -> BoxyType -> TcM ()
833 -- Acutal and expected, respectively
835 = addErrCtxtM (unifyCtxt ty1 ty2) $
836 uTysOuter False ty1 False ty2
839 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM ()
840 -- Arguments should have equal length
841 -- Acutal and expected types
842 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
845 unifyType :: TcTauType -> TcTauType -> TcM ()
846 -- No boxes expected inside these types
847 -- Acutal and expected types
848 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
849 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
850 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
851 addErrCtxtM (unifyCtxt ty1 ty2) $
852 uTysOuter True ty1 True ty2
855 unifyPred :: PredType -> PredType -> TcM ()
856 -- Acutal and expected types
857 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
858 uPred True True p1 True p2
860 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
861 -- Acutal and expected types
862 unifyTheta theta1 theta2
863 = do { checkTc (equalLength theta1 theta2)
864 (vcat [ptext SLIT("Contexts differ in length"),
865 nest 2 $ parens $ ptext SLIT("Use -fglasgow-exts to allow this")])
866 ; uList unifyPred theta1 theta2 }
869 uList :: (a -> a -> TcM ())
870 -> [a] -> [a] -> TcM ()
871 -- Unify corresponding elements of two lists of types, which
872 -- should be f equal length. We charge down the list explicitly so that
873 -- we can complain if their lengths differ.
874 uList unify [] [] = return ()
875 uList unify (ty1:tys1) (ty2:tys2) = do { unify ty1 ty2; uList unify tys1 tys2 }
876 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
879 @unifyTypeList@ takes a single list of @TauType@s and unifies them
880 all together. It is used, for example, when typechecking explicit
881 lists, when all the elts should be of the same type.
884 unifyTypeList :: [TcTauType] -> TcM ()
885 unifyTypeList [] = returnM ()
886 unifyTypeList [ty] = returnM ()
887 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
888 ; unifyTypeList tys }
891 %************************************************************************
893 \subsection[Unify-uTys]{@uTys@: getting down to business}
895 %************************************************************************
897 @uTys@ is the heart of the unifier. Each arg happens twice, because
898 we want to report errors in terms of synomyms if poss. The first of
899 the pair is used in error messages only; it is always the same as the
900 second, except that if the first is a synonym then the second may be a
901 de-synonym'd version. This way we get better error messages.
903 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
906 type InBox = Bool -- True <=> we are inside a box
907 -- False <=> we are outside a box
908 -- The importance of this is that if we get "filled-box meets
909 -- filled-box", we'll look into the boxes and unify... but
910 -- we must not allow polytypes. But if we are in a box on
911 -- just one side, then we can allow polytypes
913 type Outer = Bool -- True <=> this is the outer level of a unification
914 -- so that the types being unified are the
915 -- very ones we began with, not some sub
916 -- component or synonym expansion
917 -- The idea is that if Outer is true then unifyMisMatch should
918 -- pop the context to remove the "Expected/Acutal" context
921 :: InBox -> TcType -- ty1 is the *expected* type
922 -> InBox -> TcType -- ty2 is the *actual* type
924 uTysOuter nb1 ty1 nb2 ty2 = do { traceTc (text "uTysOuter" <+> ppr ty1 <+> ppr ty2)
925 ; u_tys True nb1 ty1 ty1 nb2 ty2 ty2 }
926 uTys nb1 ty1 nb2 ty2 = do { traceTc (text "uTys" <+> ppr ty1 <+> ppr ty2)
927 ; u_tys False nb1 ty1 ty1 nb2 ty2 ty2 }
931 uTys_s :: InBox -> [TcType] -- ty1 is the *actual* types
932 -> InBox -> [TcType] -- ty2 is the *expected* types
934 uTys_s nb1 [] nb2 [] = returnM ()
935 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { uTys nb1 ty1 nb2 ty2
936 ; uTys_s nb1 tys1 nb2 tys2 }
937 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
941 -> InBox -> TcType -> TcType -- ty1 is the *actual* type
942 -> InBox -> TcType -> TcType -- ty2 is the *expected* type
945 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
949 -- Always expand synonyms (see notes at end)
950 -- (this also throws away FTVs)
952 | Just ty1' <- tcView ty1 = go False ty1' ty2
953 | Just ty2' <- tcView ty2 = go False ty1 ty2'
955 -- Variables; go for uVar
956 go outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
957 go outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
958 -- "True" means args swapped
960 -- The case for sigma-types must *follow* the variable cases
961 -- because a boxy variable can be filed with a polytype;
962 -- but must precede FunTy, because ((?x::Int) => ty) look
963 -- like a FunTy; there isn't necy a forall at the top
965 | isSigmaTy ty1 || isSigmaTy ty2
966 = do { checkM (equalLength tvs1 tvs2)
967 (unifyMisMatch outer False orig_ty1 orig_ty2)
969 ; tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
970 -- Get location from monad, not from tvs1
971 ; let tys = mkTyVarTys tvs
972 in_scope = mkInScopeSet (mkVarSet tvs)
973 phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
974 phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
975 (theta1,tau1) = tcSplitPhiTy phi1
976 (theta2,tau2) = tcSplitPhiTy phi2
978 ; addErrCtxtM (unifyForAllCtxt tvs phi1 phi2) $ do
979 { checkM (equalLength theta1 theta2)
980 (unifyMisMatch outer False orig_ty1 orig_ty2)
982 ; uPreds False nb1 theta1 nb2 theta2
983 ; uTys nb1 tau1 nb2 tau2
985 -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
986 ; free_tvs <- zonkTcTyVarsAndFV (varSetElems (tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2))
987 ; ifM (any (`elemVarSet` free_tvs) tvs)
988 (bleatEscapedTvs free_tvs tvs tvs)
990 -- If both sides are inside a box, we are in a "box-meets-box"
991 -- situation, and we should not have a polytype at all.
992 -- If we get here we have two boxes, already filled with
993 -- the same polytype... but it should be a monotype.
994 -- This check comes last, because the error message is
995 -- extremely unhelpful.
996 ; ifM (nb1 && nb2) (notMonoType ty1)
999 (tvs1, body1) = tcSplitForAllTys ty1
1000 (tvs2, body2) = tcSplitForAllTys ty2
1003 go outer (PredTy p1) (PredTy p2) = uPred False nb1 p1 nb2 p2
1005 -- Type constructors must match
1006 go _ (TyConApp con1 tys1) (TyConApp con2 tys2)
1007 | con1 == con2 = uTys_s nb1 tys1 nb2 tys2
1008 -- See Note [TyCon app]
1010 -- Functions; just check the two parts
1011 go _ (FunTy fun1 arg1) (FunTy fun2 arg2)
1012 = do { uTys nb1 fun1 nb2 fun2
1013 ; uTys nb1 arg1 nb2 arg2 }
1015 -- Applications need a bit of care!
1016 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
1017 -- NB: we've already dealt with type variables and Notes,
1018 -- so if one type is an App the other one jolly well better be too
1019 go outer (AppTy s1 t1) ty2
1020 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
1021 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
1023 -- Now the same, but the other way round
1024 -- Don't swap the types, because the error messages get worse
1025 go outer ty1 (AppTy s2 t2)
1026 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
1027 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
1030 -- Anything else fails
1031 go outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
1034 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
1035 | n1 == n2 = uTys nb1 t1 nb2 t2
1036 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
1037 | c1 == c2 = uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
1038 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
1040 uPreds outer nb1 [] nb2 [] = return ()
1041 uPreds outer nb1 (p1:ps1) nb2 (p2:ps2) = uPred outer nb1 p1 nb2 p2 >> uPreds outer nb1 ps1 nb2 ps2
1042 uPreds outer nb1 ps1 nb2 ps2 = panic "uPreds"
1047 When we find two TyConApps, the argument lists are guaranteed equal
1048 length. Reason: intially the kinds of the two types to be unified is
1049 the same. The only way it can become not the same is when unifying two
1050 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
1051 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
1052 which we do, that ensures that f1,f2 have the same kind; and that
1053 means a1,a2 have the same kind. And now the argument repeats.
1058 If you are tempted to make a short cut on synonyms, as in this
1062 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
1063 -- NO = if (con1 == con2) then
1064 -- NO -- Good news! Same synonym constructors, so we can shortcut
1065 -- NO -- by unifying their arguments and ignoring their expansions.
1066 -- NO unifyTypepeLists args1 args2
1068 -- NO -- Never mind. Just expand them and try again
1072 then THINK AGAIN. Here is the whole story, as detected and reported
1073 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1075 Here's a test program that should detect the problem:
1079 x = (1 :: Bogus Char) :: Bogus Bool
1082 The problem with [the attempted shortcut code] is that
1086 is not a sufficient condition to be able to use the shortcut!
1087 You also need to know that the type synonym actually USES all
1088 its arguments. For example, consider the following type synonym
1089 which does not use all its arguments.
1094 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1095 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1096 would fail, even though the expanded forms (both \tr{Int}) should
1099 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1100 unnecessarily bind \tr{t} to \tr{Char}.
1102 ... You could explicitly test for the problem synonyms and mark them
1103 somehow as needing expansion, perhaps also issuing a warning to the
1108 %************************************************************************
1110 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1112 %************************************************************************
1114 @uVar@ is called when at least one of the types being unified is a
1115 variable. It does {\em not} assume that the variable is a fixed point
1116 of the substitution; rather, notice that @uVar@ (defined below) nips
1117 back into @uTys@ if it turns out that the variable is already bound.
1121 -> Bool -- False => tyvar is the "expected"
1122 -- True => ty is the "expected" thing
1124 -> InBox -- True <=> definitely no boxes in t2
1125 -> TcTauType -> TcTauType -- printing and real versions
1128 uVar outer swapped tv1 nb2 ps_ty2 ty2
1129 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1130 | otherwise = brackets (equals <+> ppr ty2)
1131 ; traceTc (text "uVar" <+> ppr swapped <+>
1132 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1133 nest 2 (ptext SLIT(" <-> ")),
1134 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1135 ; details <- lookupTcTyVar tv1
1138 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1139 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1140 -- The 'True' here says that ty1 is now inside a box
1141 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1145 uUnfilledVar :: Outer
1146 -> Bool -- Args are swapped
1147 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1148 -> TcTauType -> TcTauType -- Type 2
1150 -- Invariant: tyvar 1 is not unified with anything
1152 uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1153 | Just ty2' <- tcView ty2
1154 = -- Expand synonyms; ignore FTVs
1155 uUnfilledVar False swapped tv1 details1 ps_ty2 ty2'
1157 uUnfilledVar outer swapped tv1 details1 ps_ty2 (TyVarTy tv2)
1158 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1160 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1161 -- this is box-meets-box, so fill in with a tau-type
1162 -> do { tau_tv <- tcInstTyVar tv1
1163 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv) }
1164 other -> returnM () -- No-op
1166 -- Distinct type variables
1168 = do { lookup2 <- lookupTcTyVar tv2
1170 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
1171 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1174 uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2 -- ty2 is not a type variable
1176 MetaTv (SigTv _) ref1 -> mis_match -- Can't update a skolem with a non-type-variable
1177 MetaTv info ref1 -> uMetaVar swapped tv1 info ref1 ps_ty2 non_var_ty2
1178 skolem_details -> mis_match
1180 mis_match = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1184 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1187 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1188 -- ty2 is not a type variable
1190 uMetaVar swapped tv1 BoxTv ref1 ps_ty2 non_var_ty2
1191 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1192 -- that any boxes in ty2 are filled with monotypes
1194 -- It should not be the case that tv1 occurs in ty2
1195 -- (i.e. no occurs check should be needed), but if perchance
1196 -- it does, the unbox operation will fill it, and the DEBUG
1198 do { final_ty <- unBox ps_ty2
1200 ; meta_details <- readMutVar ref1
1201 ; case meta_details of
1202 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1203 return () -- This really should *not* happen
1206 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1208 uMetaVar swapped tv1 info1 ref1 ps_ty2 non_var_ty2
1209 = do { final_ty <- checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
1210 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1213 uUnfilledVars :: Outer
1214 -> Bool -- Args are swapped
1215 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1216 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1218 -- Invarant: The type variables are distinct,
1219 -- Neither is filled in yet
1220 -- They might be boxy or not
1222 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1223 = unifyMisMatch outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1225 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1226 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2)
1227 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1228 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
1230 -- ToDo: this function seems too long for what it acutally does!
1231 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1232 = case (info1, info2) of
1233 (BoxTv, BoxTv) -> box_meets_box
1235 -- If a box meets a TauTv, but the fomer has the smaller kind
1236 -- then we must create a fresh TauTv with the smaller kind
1237 (_, BoxTv) | k1_sub_k2 -> update_tv2
1238 | otherwise -> box_meets_box
1239 (BoxTv, _ ) | k2_sub_k1 -> update_tv1
1240 | otherwise -> box_meets_box
1242 -- Avoid SigTvs if poss
1243 (SigTv _, _ ) | k1_sub_k2 -> update_tv2
1244 (_, SigTv _) | k2_sub_k1 -> update_tv1
1246 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1247 then update_tv1 -- Same kinds
1249 | k2_sub_k1 -> update_tv1
1250 | otherwise -> kind_err
1252 -- Update the variable with least kind info
1253 -- See notes on type inference in Kind.lhs
1254 -- The "nicer to" part only applies if the two kinds are the same,
1255 -- so we can choose which to do.
1257 -- Kinds should be guaranteed ok at this point
1258 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1259 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1261 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1264 | k2_sub_k1 = fill_from tv2
1265 | otherwise = kind_err
1267 -- Update *both* tyvars with a TauTv whose name and kind
1268 -- are gotten from tv (avoid losing nice names is poss)
1269 fill_from tv = do { tv' <- tcInstTyVar tv
1270 ; let tau_ty = mkTyVarTy tv'
1271 ; updateMeta tv1 ref1 tau_ty
1272 ; updateMeta tv2 ref2 tau_ty }
1274 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1275 unifyKindMisMatch k1 k2
1279 k1_sub_k2 = k1 `isSubKind` k2
1280 k2_sub_k1 = k2 `isSubKind` k1
1282 nicer_to_update_tv1 = isSystemName (Var.varName tv1)
1283 -- Try to update sys-y type variables in preference to ones
1284 -- gotten (say) by instantiating a polymorphic function with
1285 -- a user-written type sig
1288 checkUpdateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1289 -- Update tv1, which is flexi; occurs check is alrady done
1290 -- The 'check' version does a kind check too
1291 -- We do a sub-kind check here: we might unify (a b) with (c d)
1292 -- where b::*->* and d::*; this should fail
1294 checkUpdateMeta swapped tv1 ref1 ty2
1295 = do { checkKinds swapped tv1 ty2
1296 ; updateMeta tv1 ref1 ty2 }
1298 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1299 updateMeta tv1 ref1 ty2
1300 = ASSERT( isMetaTyVar tv1 )
1301 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
1302 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
1303 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
1304 ; writeMutVar ref1 (Indirect ty2) }
1307 checkKinds swapped tv1 ty2
1308 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
1309 -- ty2 has been zonked at this stage, which ensures that
1310 -- its kind has as much boxity information visible as possible.
1311 | tk2 `isSubKind` tk1 = returnM ()
1314 -- Either the kinds aren't compatible
1315 -- (can happen if we unify (a b) with (c d))
1316 -- or we are unifying a lifted type variable with an
1317 -- unlifted type: e.g. (id 3#) is illegal
1318 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1319 unifyKindMisMatch k1 k2
1321 (k1,k2) | swapped = (tk2,tk1)
1322 | otherwise = (tk1,tk2)
1327 checkTauTvUpdate :: TcTyVar -> TcType -> TcM TcType
1328 -- (checkTauTvUpdate tv ty)
1329 -- We are about to update the TauTv tv with ty.
1330 -- Check (a) that tv doesn't occur in ty (occurs check)
1331 -- (b) that ty is a monotype
1332 -- Furthermore, in the interest of (b), if you find an
1333 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
1335 -- Returns the (non-boxy) type to update the type variable with, or fails
1337 checkTauTvUpdate orig_tv orig_ty
1340 go (TyConApp tc tys)
1341 | isSynTyCon tc = go_syn tc tys
1342 | otherwise = do { tys' <- mappM go tys; return (TyConApp tc tys') }
1343 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
1344 go (PredTy p) = do { p' <- go_pred p; return (PredTy p') }
1345 go (FunTy arg res) = do { arg' <- go arg; res' <- go res; return (FunTy arg' res') }
1346 go (AppTy fun arg) = do { fun' <- go fun; arg' <- go arg; return (mkAppTy fun' arg') }
1347 -- NB the mkAppTy; we might have instantiated a
1348 -- type variable to a type constructor, so we need
1349 -- to pull the TyConApp to the top.
1350 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
1353 | orig_tv == tv = occurCheck tv orig_ty -- (a)
1354 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
1355 | otherwise = return (TyVarTy tv)
1356 -- Ordinary (non Tc) tyvars
1357 -- occur inside quantified types
1359 go_pred (ClassP c tys) = do { tys' <- mapM go tys; return (ClassP c tys') }
1360 go_pred (IParam n ty) = do { ty' <- go ty; return (IParam n ty') }
1361 go_pred (EqPred t1 t2) = do { t1' <- go t1; t2' <- go t2; return (EqPred t1' t2') }
1363 go_tyvar tv (SkolemTv _) = return (TyVarTy tv)
1364 go_tyvar tv (MetaTv box ref)
1365 = do { cts <- readMutVar ref
1367 Indirect ty -> go ty
1368 Flexi -> case box of
1369 BoxTv -> fillBoxWithTau tv ref
1370 other -> return (TyVarTy tv)
1373 -- go_syn is called for synonyms only
1374 -- See Note [Type synonyms and the occur check]
1376 | not (isTauTyCon tc)
1377 = notMonoType orig_ty -- (b) again
1379 = do { (msgs, mb_tys') <- tryTc (mapM go tys)
1381 Just tys' -> return (TyConApp tc tys')
1382 -- Retain the synonym (the common case)
1383 Nothing | isOpenTyCon tc
1384 -> notMonoArgs (TyConApp tc tys)
1385 -- Synonym families must have monotype args
1387 -> go (expectJust "checkTauTvUpdate"
1388 (tcView (TyConApp tc tys)))
1389 -- Try again, expanding the synonym
1392 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
1393 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
1394 -- tau-type meta-variable, whose print-name is the same as tv
1395 -- Choosing the same name is good: when we instantiate a function
1396 -- we allocate boxy tyvars with the same print-name as the quantified
1397 -- tyvar; and then we often fill the box with a tau-tyvar, and again
1398 -- we want to choose the same name.
1399 fillBoxWithTau tv ref
1400 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
1401 ; let tau = mkTyVarTy tv' -- name of the type variable
1402 ; writeMutVar ref (Indirect tau)
1406 Note [Type synonyms and the occur check]
1407 ~~~~~~~~~~~~~~~~~~~~
1408 Basically we want to update tv1 := ps_ty2
1409 because ps_ty2 has type-synonym info, which improves later error messages
1414 f :: (A a -> a -> ()) -> ()
1418 x = f (\ x p -> p x)
1420 In the application (p x), we try to match "t" with "A t". If we go
1421 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
1422 an infinite loop later.
1423 But we should not reject the program, because A t = ().
1424 Rather, we should bind t to () (= non_var_ty2).
1427 refineBox :: TcType -> TcM TcType
1428 -- Unbox the outer box of a boxy type (if any)
1429 refineBox ty@(TyVarTy box_tv)
1430 | isMetaTyVar box_tv
1431 = do { cts <- readMetaTyVar box_tv
1434 Indirect ty -> return ty }
1435 refineBox other_ty = return other_ty
1437 refineBoxToTau :: TcType -> TcM TcType
1438 -- Unbox the outer box of a boxy type, filling with a monotype if it is empty
1439 -- Like refineBox except for the "fill with monotype" part.
1440 refineBoxToTau ty@(TyVarTy box_tv)
1441 | isMetaTyVar box_tv
1442 , MetaTv BoxTv ref <- tcTyVarDetails box_tv
1443 = do { cts <- readMutVar ref
1445 Flexi -> fillBoxWithTau box_tv ref
1446 Indirect ty -> return ty }
1447 refineBoxToTau other_ty = return other_ty
1449 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1450 -- Subtle... we must zap the boxy res_ty
1451 -- to kind * before using it to instantiate a LitInst
1452 -- Calling unBox instead doesn't do the job, because the box
1453 -- often has an openTypeKind, and we don't want to instantiate
1455 zapToMonotype res_ty
1456 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1457 ; boxyUnify res_tau res_ty
1460 unBox :: BoxyType -> TcM TcType
1461 -- unBox implements the judgement
1463 -- with input s', and result s
1465 -- It removes all boxes from the input type, returning a non-boxy type.
1466 -- A filled box in the type can only contain a monotype; unBox fails if not
1467 -- The type can have empty boxes, which unBox fills with a monotype
1469 -- Compare this wth checkTauTvUpdate
1471 -- For once, it's safe to treat synonyms as opaque!
1473 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1474 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1475 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1476 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1477 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1478 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1479 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1481 | isTcTyVar tv -- It's a boxy type variable
1482 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1483 = do { cts <- readMutVar ref -- under nested quantifiers
1485 Flexi -> fillBoxWithTau tv ref
1486 Indirect ty -> do { non_boxy_ty <- unBox ty
1487 ; if isTauTy non_boxy_ty
1488 then return non_boxy_ty
1489 else notMonoType non_boxy_ty }
1491 | otherwise -- Skolems, and meta-tau-variables
1492 = return (TyVarTy tv)
1494 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1495 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1496 unBoxPred (EqPred ty1 ty2) = do { ty1' <- unBox ty1; ty2' <- unBox ty2; return (EqPred ty1' ty2') }
1501 %************************************************************************
1503 \subsection[Unify-context]{Errors and contexts}
1505 %************************************************************************
1511 unifyCtxt act_ty exp_ty tidy_env
1512 = do { act_ty' <- zonkTcType act_ty
1513 ; exp_ty' <- zonkTcType exp_ty
1514 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1515 (env2, act_ty'') = tidyOpenType env1 act_ty'
1516 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1519 mkExpectedActualMsg act_ty exp_ty
1520 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1521 text "Inferred type" <> colon <+> ppr act_ty ])
1524 -- If an error happens we try to figure out whether the function
1525 -- function has been given too many or too few arguments, and say so.
1526 addSubCtxt SubDone actual_res_ty expected_res_ty thing_inside
1528 addSubCtxt sub_ctxt actual_res_ty expected_res_ty thing_inside
1529 = addErrCtxtM mk_err thing_inside
1532 = do { exp_ty' <- zonkTcType expected_res_ty
1533 ; act_ty' <- zonkTcType actual_res_ty
1534 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1535 (env2, act_ty'') = tidyOpenType env1 act_ty'
1536 (exp_args, _) = tcSplitFunTys exp_ty''
1537 (act_args, _) = tcSplitFunTys act_ty''
1539 len_act_args = length act_args
1540 len_exp_args = length exp_args
1542 message = case sub_ctxt of
1543 SubFun fun | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1544 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1545 other -> mkExpectedActualMsg act_ty'' exp_ty''
1546 ; return (env2, message) }
1548 wrongArgsCtxt too_many_or_few fun
1549 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1550 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1551 <+> ptext SLIT("arguments")
1554 unifyForAllCtxt tvs phi1 phi2 env
1555 = returnM (env2, msg)
1557 (env', tvs') = tidyOpenTyVars env tvs -- NB: not tidyTyVarBndrs
1558 (env1, phi1') = tidyOpenType env' phi1
1559 (env2, phi2') = tidyOpenType env1 phi2
1560 msg = vcat [ptext SLIT("When matching") <+> quotes (ppr (mkForAllTys tvs' phi1')),
1561 ptext SLIT(" and") <+> quotes (ppr (mkForAllTys tvs' phi2'))]
1564 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1565 -- tv1 and ty2 are zonked already
1568 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1569 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1571 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1572 | otherwise = (pp1, pp2)
1573 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1574 (env2, ty2') = tidyOpenType env1 ty2
1575 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1576 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1578 unifyMisMatch outer swapped ty1 ty2
1579 = do { (env, msg) <- if swapped then misMatchMsg ty1 ty2
1580 else misMatchMsg ty2 ty1
1582 -- This is the whole point of the 'outer' stuff
1583 ; if outer then popErrCtxt (failWithTcM (env, msg))
1584 else failWithTcM (env, msg)
1588 = do { env0 <- tcInitTidyEnv
1589 ; (env1, pp1, extra1) <- ppr_ty env0 ty1
1590 ; (env2, pp2, extra2) <- ppr_ty env1 ty2
1591 ; return (env2, sep [sep [ptext SLIT("Couldn't match expected type") <+> pp1,
1592 nest 7 (ptext SLIT("against inferred type") <+> pp2)],
1593 nest 2 extra1, nest 2 extra2]) }
1595 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
1597 = do { ty' <- zonkTcType ty
1598 ; let (env1,tidy_ty) = tidyOpenType env ty'
1599 simple_result = (env1, quotes (ppr tidy_ty), empty)
1602 | isSkolemTyVar tv || isSigTyVar tv
1603 -> return (env2, pp_rigid tv', pprSkolTvBinding tv')
1604 | otherwise -> return simple_result
1606 (env2, tv') = tidySkolemTyVar env1 tv
1607 other -> return simple_result }
1609 pp_rigid tv = quotes (ppr tv) <+> parens (ptext SLIT("a rigid variable"))
1613 = do { ty' <- zonkTcType ty
1614 ; env0 <- tcInitTidyEnv
1615 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1616 msg = ptext SLIT("Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1617 ; failWithTcM (env1, msg) }
1620 = do { ty' <- zonkTcType ty
1621 ; env0 <- tcInitTidyEnv
1622 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1623 msg = ptext SLIT("Arguments of synonym family must be monotypes") <+> quotes (ppr tidy_ty)
1624 ; failWithTcM (env1, msg) }
1627 = do { env0 <- tcInitTidyEnv
1628 ; ty' <- zonkTcType ty
1629 ; let (env1, tidy_tyvar) = tidyOpenTyVar env0 tyvar
1630 (env2, tidy_ty) = tidyOpenType env1 ty'
1631 extra = sep [ppr tidy_tyvar, char '=', ppr tidy_ty]
1632 ; failWithTcM (env2, hang msg 2 extra) }
1634 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
1638 %************************************************************************
1642 %************************************************************************
1644 Unifying kinds is much, much simpler than unifying types.
1647 unifyKind :: TcKind -- Expected
1650 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1651 | isSubKindCon kc2 kc1 = returnM ()
1653 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1654 = do { unifyKind a2 a1; unifyKind r1 r2 }
1655 -- Notice the flip in the argument,
1656 -- so that the sub-kinding works right
1657 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1658 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1659 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1661 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1662 unifyKinds [] [] = returnM ()
1663 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1665 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1668 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1669 uKVar swapped kv1 k2
1670 = do { mb_k1 <- readKindVar kv1
1672 Flexi -> uUnboundKVar swapped kv1 k2
1673 Indirect k1 | swapped -> unifyKind k2 k1
1674 | otherwise -> unifyKind k1 k2 }
1677 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1678 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1679 | kv1 == kv2 = returnM ()
1680 | otherwise -- Distinct kind variables
1681 = do { mb_k2 <- readKindVar kv2
1683 Indirect k2 -> uUnboundKVar swapped kv1 k2
1684 Flexi -> writeKindVar kv1 k2 }
1686 uUnboundKVar swapped kv1 non_var_k2
1687 = do { k2' <- zonkTcKind non_var_k2
1688 ; kindOccurCheck kv1 k2'
1689 ; k2'' <- kindSimpleKind swapped k2'
1690 -- KindVars must be bound only to simple kinds
1691 -- Polarities: (kindSimpleKind True ?) succeeds
1692 -- returning *, corresponding to unifying
1695 ; writeKindVar kv1 k2'' }
1698 kindOccurCheck kv1 k2 -- k2 is zonked
1699 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1701 not_in (TyVarTy kv2) = kv1 /= kv2
1702 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1705 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1706 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1707 -- If the flag is False, it requires k <: sk
1708 -- E.g. kindSimpleKind False ?? = *
1709 -- What about (kv -> *) :=: ?? -> *
1710 kindSimpleKind orig_swapped orig_kind
1711 = go orig_swapped orig_kind
1713 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1715 ; return (mkArrowKind k1' k2') }
1717 | isOpenTypeKind k = return liftedTypeKind
1718 | isArgTypeKind k = return liftedTypeKind
1720 | isLiftedTypeKind k = return liftedTypeKind
1721 | isUnliftedTypeKind k = return unliftedTypeKind
1722 go sw k@(TyVarTy _) = return k -- KindVars are always simple
1723 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1724 <+> ppr orig_swapped <+> ppr orig_kind)
1725 -- I think this can't actually happen
1727 -- T v = MkT v v must be a type
1728 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1731 kindOccurCheckErr tyvar ty
1732 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1733 2 (sep [ppr tyvar, char '=', ppr ty])
1735 unifyKindMisMatch ty1 ty2
1736 = zonkTcKind ty1 `thenM` \ ty1' ->
1737 zonkTcKind ty2 `thenM` \ ty2' ->
1739 msg = hang (ptext SLIT("Couldn't match kind"))
1740 2 (sep [quotes (ppr ty1'),
1741 ptext SLIT("against"),
1748 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1749 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1751 unifyFunKind (TyVarTy kvar)
1752 = readKindVar kvar `thenM` \ maybe_kind ->
1754 Indirect fun_kind -> unifyFunKind fun_kind
1756 do { arg_kind <- newKindVar
1757 ; res_kind <- newKindVar
1758 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1759 ; returnM (Just (arg_kind,res_kind)) }
1761 unifyFunKind (FunTy arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1762 unifyFunKind other = returnM Nothing
1765 %************************************************************************
1769 %************************************************************************
1771 ---------------------------
1772 -- We would like to get a decent error message from
1773 -- (a) Under-applied type constructors
1774 -- f :: (Maybe, Maybe)
1775 -- (b) Over-applied type constructors
1776 -- f :: Int x -> Int x
1780 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1781 -- A fancy wrapper for 'unifyKind', which tries
1782 -- to give decent error messages.
1783 -- (checkExpectedKind ty act_kind exp_kind)
1784 -- checks that the actual kind act_kind is compatible
1785 -- with the expected kind exp_kind
1786 -- The first argument, ty, is used only in the error message generation
1787 checkExpectedKind ty act_kind exp_kind
1788 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1791 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1793 Just r -> returnM () ; -- Unification succeeded
1796 -- So there's definitely an error
1797 -- Now to find out what sort
1798 zonkTcKind exp_kind `thenM` \ exp_kind ->
1799 zonkTcKind act_kind `thenM` \ act_kind ->
1801 tcInitTidyEnv `thenM` \ env0 ->
1802 let (exp_as, _) = splitKindFunTys exp_kind
1803 (act_as, _) = splitKindFunTys act_kind
1804 n_exp_as = length exp_as
1805 n_act_as = length act_as
1807 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1808 (env2, tidy_act_kind) = tidyKind env1 act_kind
1810 err | n_exp_as < n_act_as -- E.g. [Maybe]
1811 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1813 -- Now n_exp_as >= n_act_as. In the next two cases,
1814 -- n_exp_as == 0, and hence so is n_act_as
1815 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1816 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1817 <+> ptext SLIT("is unlifted")
1819 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1820 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1821 <+> ptext SLIT("is lifted")
1823 | otherwise -- E.g. Monad [Int]
1824 = ptext SLIT("Kind mis-match")
1826 more_info = sep [ ptext SLIT("Expected kind") <+>
1827 quotes (pprKind tidy_exp_kind) <> comma,
1828 ptext SLIT("but") <+> quotes (ppr ty) <+>
1829 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1831 failWithTcM (env2, err $$ more_info)
1835 %************************************************************************
1837 \subsection{Checking signature type variables}
1839 %************************************************************************
1841 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1842 are not mentioned in the environment. In particular:
1844 (a) Not mentioned in the type of a variable in the envt
1845 eg the signature for f in this:
1851 Here, f is forced to be monorphic by the free occurence of x.
1853 (d) Not (unified with another type variable that is) in scope.
1854 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1855 when checking the expression type signature, we find that
1856 even though there is nothing in scope whose type mentions r,
1857 nevertheless the type signature for the expression isn't right.
1859 Another example is in a class or instance declaration:
1861 op :: forall b. a -> b
1863 Here, b gets unified with a
1865 Before doing this, the substitution is applied to the signature type variable.
1868 checkSigTyVars :: [TcTyVar] -> TcM ()
1869 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1871 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1872 -- The extra_tvs can include boxy type variables;
1873 -- e.g. TcMatches.tcCheckExistentialPat
1874 checkSigTyVarsWrt extra_tvs sig_tvs
1875 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1876 ; check_sig_tyvars extra_tvs' sig_tvs }
1879 :: TcTyVarSet -- Global type variables. The universally quantified
1880 -- tyvars should not mention any of these
1881 -- Guaranteed already zonked.
1882 -> [TcTyVar] -- Universally-quantified type variables in the signature
1883 -- Guaranteed to be skolems
1885 check_sig_tyvars extra_tvs []
1887 check_sig_tyvars extra_tvs sig_tvs
1888 = ASSERT( all isSkolemTyVar sig_tvs )
1889 do { gbl_tvs <- tcGetGlobalTyVars
1890 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1891 text "gbl_tvs" <+> ppr gbl_tvs,
1892 text "extra_tvs" <+> ppr extra_tvs]))
1894 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1895 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1896 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1899 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1900 -> [TcTyVar] -- The possibly-escaping type variables
1901 -> [TcTyVar] -- The zonked versions thereof
1903 -- Complain about escaping type variables
1904 -- We pass a list of type variables, at least one of which
1905 -- escapes. The first list contains the original signature type variable,
1906 -- while the second contains the type variable it is unified to (usually itself)
1907 bleatEscapedTvs globals sig_tvs zonked_tvs
1908 = do { env0 <- tcInitTidyEnv
1909 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1910 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1912 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1913 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1915 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1917 check (tidy_env, msgs) (sig_tv, zonked_tv)
1918 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1920 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1921 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1923 -----------------------
1924 escape_msg sig_tv zonked_tv globs
1926 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1927 nest 2 (vcat globs)]
1929 = msg <+> ptext SLIT("escapes")
1930 -- Sigh. It's really hard to give a good error message
1931 -- all the time. One bad case is an existential pattern match.
1932 -- We rely on the "When..." context to help.
1934 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1936 | sig_tv == zonked_tv = empty
1937 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1940 These two context are used with checkSigTyVars
1943 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1944 -> TidyEnv -> TcM (TidyEnv, Message)
1945 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1946 = zonkTcType sig_tau `thenM` \ actual_tau ->
1948 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1949 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1950 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1951 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1952 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1954 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),