2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section{Type subsumption and unification}
8 -- Full-blown subsumption
9 tcSubExp, tcFunResTy, tcGen,
10 checkSigTyVars, checkSigTyVarsWrt, bleatEscapedTvs, sigCtxt,
12 -- Various unifications
13 unifyType, unifyTypeList, unifyTheta,
14 unifyKind, unifyKinds, unifyFunKind,
16 boxySubMatchType, boxyMatchTypes,
18 --------------------------------
20 tcInfer, subFunTys, unBox, stripBoxyType, withBox,
21 boxyUnify, boxyUnifyList, zapToMonotype,
22 boxySplitListTy, boxySplitTyConApp, boxySplitAppTy,
26 #include "HsVersions.h"
28 import HsSyn ( ExprCoFn(..), idCoercion, isIdCoercion, (<.>) )
29 import TypeRep ( Type(..), PredType(..) )
31 import TcMType ( lookupTcTyVar, LookupTyVarResult(..),
32 tcInstSkolType, newKindVar, newMetaTyVar,
33 tcInstBoxy, newBoxyTyVar, newBoxyTyVarTys, readFilledBox,
34 readMetaTyVar, writeMetaTyVar, newFlexiTyVarTy,
35 tcInstSkolTyVars, tcInstTyVar,
36 zonkTcKind, zonkType, zonkTcType, zonkTcTyVarsAndFV,
37 readKindVar, writeKindVar )
38 import TcSimplify ( tcSimplifyCheck )
39 import TcEnv ( tcGetGlobalTyVars, findGlobals )
40 import TcIface ( checkWiredInTyCon )
41 import TcRnMonad -- TcType, amongst others
42 import TcType ( TcKind, TcType, TcTyVar, BoxyTyVar, TcTauType,
43 BoxySigmaType, BoxyRhoType, BoxyType,
44 TcTyVarSet, TcThetaType, TcTyVarDetails(..), BoxInfo(..),
45 SkolemInfo( GenSkol, UnkSkol ), MetaDetails(..), isImmutableTyVar,
46 pprSkolTvBinding, isTauTy, isTauTyCon, isSigmaTy,
47 mkFunTy, mkFunTys, mkTyConApp, isMetaTyVar,
48 tcSplitForAllTys, tcSplitAppTy_maybe, tcSplitFunTys, mkTyVarTys,
49 tyVarsOfType, mkPhiTy, mkTyVarTy, mkPredTy,
50 typeKind, mkForAllTys, mkAppTy, isBoxyTyVar,
51 tidyOpenType, tidyOpenTyVar, tidyOpenTyVars,
52 pprType, tidyKind, tidySkolemTyVar, isSkolemTyVar, tcView,
53 TvSubst, mkTvSubst, zipTyEnv, substTy, emptyTvSubst,
54 lookupTyVar, extendTvSubst )
55 import Kind ( Kind(..), SimpleKind, KindVar, isArgTypeKind,
56 openTypeKind, liftedTypeKind, mkArrowKind, defaultKind,
57 isOpenTypeKind, argTypeKind, isLiftedTypeKind, isUnliftedTypeKind,
58 isSubKind, pprKind, splitKindFunTys )
59 import TysPrim ( alphaTy, betaTy )
60 import Inst ( newDicts, instToId )
61 import TyCon ( TyCon, tyConArity, tyConTyVars, isSynTyCon )
62 import TysWiredIn ( listTyCon )
63 import Id ( Id, mkSysLocal )
64 import Var ( Var, varName, tyVarKind, isTcTyVar, tcTyVarDetails )
65 import VarSet ( emptyVarSet, mkVarSet, unitVarSet, unionVarSet, elemVarSet, varSetElems,
66 extendVarSet, intersectsVarSet )
68 import Name ( Name, isSystemName )
69 import ErrUtils ( Message )
70 import Maybes ( expectJust, isNothing )
71 import BasicTypes ( Arity )
72 import UniqSupply ( uniqsFromSupply )
73 import Util ( notNull, equalLength )
78 import TcType ( isBoxyTy, isFlexi )
82 %************************************************************************
84 \subsection{'hole' type variables}
86 %************************************************************************
89 tcInfer :: (BoxyType -> TcM a) -> TcM (a, TcType)
91 = do { box <- newBoxyTyVar openTypeKind
92 ; res <- tc_infer (mkTyVarTy box)
93 ; res_ty <- readFilledBox box -- Guaranteed filled-in by now
94 ; return (res, res_ty) }
98 %************************************************************************
102 %************************************************************************
105 subFunTys :: SDoc -- Somthing like "The function f has 3 arguments"
106 -- or "The abstraction (\x.e) takes 1 argument"
107 -> Arity -- Expected # of args
108 -> BoxyRhoType -- res_ty
109 -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
111 -- Attempt to decompse res_ty to have enough top-level arrows to
112 -- match the number of patterns in the match group
114 -- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
115 -- and the inner call to thing_inside passes args: [a1,...,an], b
116 -- then co_fn :: (a1 -> ... -> an -> b) -> res_ty
118 -- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
121 {- Error messages from subFunTys
123 The abstraction `\Just 1 -> ...' has two arguments
124 but its type `Maybe a -> a' has only one
126 The equation(s) for `f' have two arguments
127 but its type `Maybe a -> a' has only one
129 The section `(f 3)' requires 'f' to take two arguments
130 but its type `Int -> Int' has only one
132 The function 'f' is applied to two arguments
133 but its type `Int -> Int' has only one
137 subFunTys error_herald n_pats res_ty thing_inside
138 = loop n_pats [] res_ty
140 -- In 'loop', the parameter 'arg_tys' accumulates
141 -- the arg types so far, in *reverse order*
142 loop n args_so_far res_ty
143 | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
145 loop n args_so_far res_ty
146 | isSigmaTy res_ty -- Do this before checking n==0, because we
147 -- guarantee to return a BoxyRhoType, not a BoxySigmaType
148 = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet $ \ res_ty' ->
149 loop n args_so_far res_ty'
150 ; return (gen_fn <.> co_fn, res) }
152 loop 0 args_so_far res_ty
153 = do { res <- thing_inside (reverse args_so_far) res_ty
154 ; return (idCoercion, res) }
156 loop n args_so_far (FunTy arg_ty res_ty)
157 = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
158 ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
159 ; return (co_fn', res) }
161 -- res_ty might have a type variable at the head, such as (a b c),
162 -- in which case we must fill in with (->). Simplest thing to do
163 -- is to use boxyUnify, but we catch failure and generate our own
164 -- error message on failure
165 loop n args_so_far res_ty@(AppTy _ _)
166 = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
167 ; (_, mb_unit) <- tryTcErrs $ boxyUnify res_ty (FunTy arg_ty' res_ty')
168 ; if isNothing mb_unit then bale_out args_so_far res_ty
169 else loop n args_so_far (FunTy arg_ty' res_ty') }
171 loop n args_so_far (TyVarTy tv)
172 | not (isImmutableTyVar tv)
173 = do { cts <- readMetaTyVar tv
175 Indirect ty -> loop n args_so_far ty
176 Flexi -> do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
177 ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty
178 ; return (idCoercion, res) } }
180 mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
181 kinds = openTypeKind : take n (repeat argTypeKind)
182 -- Note argTypeKind: the args can have an unboxed type,
183 -- but not an unboxed tuple.
185 loop n args_so_far res_ty = bale_out args_so_far res_ty
187 bale_out args_so_far res_ty
188 = do { env0 <- tcInitTidyEnv
189 ; res_ty' <- zonkTcType res_ty
190 ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
191 ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
193 mk_msg res_ty n_actual
194 = error_herald <> comma $$
195 sep [ptext SLIT("but its type") <+> quotes (pprType res_ty),
196 if n_actual == 0 then ptext SLIT("has none")
197 else ptext SLIT("has only") <+> speakN n_actual]
201 ----------------------
202 boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
203 -> BoxyRhoType -- Expected type (T a b c)
204 -> TcM [BoxySigmaType] -- Element types, a b c
205 -- It's used for wired-in tycons, so we call checkWiredInTyCOn
206 -- Precondition: never called with FunTyCon
207 -- Precondition: input type :: *
209 boxySplitTyConApp tc orig_ty
210 = do { checkWiredInTyCon tc
211 ; loop (tyConArity tc) [] orig_ty }
213 loop n_req args_so_far ty
214 | Just ty' <- tcView ty = loop n_req args_so_far ty'
216 loop n_req args_so_far (TyConApp tycon args)
218 = ASSERT( n_req == length args) -- ty::*
219 return (args ++ args_so_far)
221 loop n_req args_so_far (AppTy fun arg)
222 = loop (n_req - 1) (arg:args_so_far) fun
224 loop n_req args_so_far (TyVarTy tv)
225 | not (isImmutableTyVar tv)
226 = do { cts <- readMetaTyVar tv
228 Indirect ty -> loop n_req args_so_far ty
229 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
230 ; return (arg_tys ++ args_so_far) }
233 mk_res_ty arg_tys' = mkTyConApp tc arg_tys'
234 arg_kinds = map tyVarKind (take n_req (tyConTyVars tc))
236 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc))) orig_ty
238 ----------------------
239 boxySplitListTy :: BoxyRhoType -> TcM BoxySigmaType -- Special case for lists
240 boxySplitListTy exp_ty = do { [elt_ty] <- boxySplitTyConApp listTyCon exp_ty
244 ----------------------
245 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
246 -> TcM (BoxySigmaType, BoxySigmaType) -- Returns m, a
247 -- Assumes (m: * -> k), where k is the kind of the incoming type
248 -- If the incoming type is boxy, then so are the result types; and vice versa
250 boxySplitAppTy orig_ty
254 | Just ty' <- tcView ty = loop ty'
257 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
258 = return (fun_ty, arg_ty)
261 | not (isImmutableTyVar tv)
262 = do { cts <- readMetaTyVar tv
264 Indirect ty -> loop ty
265 Flexi -> do { [fun_ty,arg_ty] <- withMetaTvs tv kinds mk_res_ty
266 ; return (fun_ty, arg_ty) } }
268 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
269 tv_kind = tyVarKind tv
270 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
272 liftedTypeKind] -- arg type :: *
273 -- The defaultKind is a bit smelly. If you remove it,
274 -- try compiling f x = do { x }
275 -- and you'll get a kind mis-match. It smells, but
276 -- not enough to lose sleep over.
278 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
281 boxySplitFailure actual_ty expected_ty
282 = unifyMisMatch False False actual_ty expected_ty
283 -- "outer" is False, so we don't pop the context
284 -- which is what we want since we have not pushed one!
288 --------------------------------
289 -- withBoxes: the key utility function
290 --------------------------------
293 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
294 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
295 -> ([BoxySigmaType] -> BoxySigmaType)
296 -- Constructs the type to assign
297 -- to the original var
298 -> TcM [BoxySigmaType] -- Return the fresh boxes
300 -- It's entirely possible for the [kind] to be empty.
301 -- For example, when pattern-matching on True,
302 -- we call boxySplitTyConApp passing a boolTyCon
304 -- Invariant: tv is still Flexi
306 withMetaTvs tv kinds mk_res_ty
308 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
309 ; let box_tys = mkTyVarTys box_tvs
310 ; writeMetaTyVar tv (mk_res_ty box_tys)
313 | otherwise -- Non-boxy meta type variable
314 = do { tau_tys <- mapM newFlexiTyVarTy kinds
315 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
316 -- Sure to be a tau-type
319 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
320 -- Allocate a *boxy* tyvar
321 withBox kind thing_inside
322 = do { box_tv <- newMetaTyVar BoxTv kind
323 ; res <- thing_inside (mkTyVarTy box_tv)
324 ; ty <- readFilledBox box_tv
329 %************************************************************************
331 Approximate boxy matching
333 %************************************************************************
337 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
338 -> BoxyRhoType -- Type to match (note a *Rho* type)
339 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
342 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
343 -> [BoxySigmaType] -- Type to match
344 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
346 -- Find a *boxy* substitution that makes the template look as much
347 -- like the BoxySigmaType as possible.
348 -- It's always ok to return an empty substitution;
349 -- anything more is jam on the pudding
351 -- NB1: This is a pure, non-monadic function.
352 -- It does no unification, and cannot fail
354 -- Note [Matching kinds]
355 -- The target type might legitimately not be a sub-kind of template.
356 -- For example, suppose the target is simply a box with an OpenTypeKind,
357 -- and the template is a type variable with LiftedTypeKind.
358 -- Then it's ok (because the target type will later be refined).
359 -- We simply don't bind the template type variable.
361 -- It might also be that the kind mis-match is an error. For example,
362 -- suppose we match the template (a -> Int) against (Int# -> Int),
363 -- where the template type variable 'a' has LiftedTypeKind. This
364 -- matching function does not fail; it simply doesn't bind the template.
365 -- Later stuff will fail.
367 -- Precondition: the arg lengths are equal
368 -- Precondition: none of the template type variables appear in the [BoxySigmaType]
369 -- Precondition: any nested quantifiers in either type differ from
370 -- the template type variables passed as arguments
376 -- |- head xs : <rhobox>
377 -- We will do a boxySubMatchType between a ~ <rhobox>
378 -- But we *don't* want to match [a |-> <rhobox>] because
379 -- (a) The box should be filled in with a rho-type, but
380 -- but the returned substitution maps TyVars to boxy *sigma*
382 -- (b) In any case, the right final answer might be *either*
383 -- instantiate 'a' with a rho-type or a sigma type
384 -- head xs : Int vs head xs : forall b. b->b
385 -- So the matcher MUST NOT make a choice here. In general, we only
386 -- bind a template type variable in boxyMatchType, not in boxySubMatchType.
388 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
392 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
393 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
395 go (FunTy arg1 res1) (FunTy arg2 res2)
396 = do_match arg1 arg2 (go res1 res2)
397 -- Match the args, and sub-match the results
399 go (TyVarTy _) b_ty = emptyTvSubst -- Do not bind! See Note [Sub-match]
401 go t_ty b_ty = do_match t_ty b_ty emptyTvSubst -- Otherwise we are safe to bind
403 do_match t_ty b_ty subst = boxy_match tmpl_tvs t_ty emptyVarSet b_ty subst
406 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
407 = ASSERT( length tmpl_tys == length boxy_tys )
408 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
409 -- ToDo: add error context?
411 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
413 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
414 = boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys $
415 boxy_match tmpl_tvs t_ty boxy_tvs b_ty subst
418 boxy_match :: TcTyVarSet -> TcType -- Template
419 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
420 -> BoxySigmaType -- Match against this type
424 -- The boxy_tvs argument prevents this match:
425 -- [a] forall b. a ~ forall b. b
426 -- We don't want to bind the template variable 'a'
427 -- to the quantified type variable 'b'!
429 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
430 = go orig_tmpl_ty orig_boxy_ty
433 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
434 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
436 go (ForAllTy _ ty1) (ForAllTy tv2 ty2)
437 = boxy_match tmpl_tvs ty1 (boxy_tvs `extendVarSet` tv2) ty2 subst
439 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
440 | tc1 == tc2 = go_s tys1 tys2
442 go (FunTy arg1 res1) (FunTy arg2 res2)
443 = go_s [arg1,res1] [arg2,res2]
446 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
447 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
448 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
449 = go_s [s1,t1] [s2,t2]
452 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
453 , not (intersectsVarSet boxy_tvs (tyVarsOfType orig_boxy_ty))
454 , typeKind b_ty `isSubKind` tyVarKind tv
455 = extendTvSubst subst tv boxy_ty'
457 boxy_ty' = case lookupTyVar subst tv of
458 Nothing -> orig_boxy_ty
459 Just ty -> ty `boxyLub` orig_boxy_ty
461 go _ _ = subst -- Always safe
464 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
467 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
468 -- Combine boxy information from the two types
469 -- If there is a conflict, return the first
470 boxyLub orig_ty1 orig_ty2
471 = go orig_ty1 orig_ty2
473 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
474 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
475 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
476 | tc1 == tc2, length ts1 == length ts2
477 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
479 go (TyVarTy tv1) ty2 -- This is the whole point;
480 | isTcTyVar tv1, isMetaTyVar tv1 -- choose ty2 if ty2 is a box
483 -- Look inside type synonyms, but only if the naive version fails
484 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
485 | Just ty2' <- tcView ty1 = go ty1 ty2'
487 -- For now, we don't look inside ForAlls, PredTys
488 go ty1 ty2 = orig_ty1 -- Default
492 %************************************************************************
496 %************************************************************************
498 All the tcSub calls have the form
500 tcSub expected_ty offered_ty
502 offered_ty <= expected_ty
504 That is, that a value of type offered_ty is acceptable in
505 a place expecting a value of type expected_ty.
507 It returns a coercion function
508 co_fn :: offered_ty -> expected_ty
509 which takes an HsExpr of type offered_ty into one of type
514 tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM ExprCoFn -- Locally used only
515 -- (tcSub act exp) checks that
517 tcSubExp actual_ty expected_ty
518 = addErrCtxtM (unifyCtxt actual_ty expected_ty)
519 (tc_sub True actual_ty actual_ty expected_ty expected_ty)
521 tcFunResTy :: Name -> BoxySigmaType -> BoxySigmaType -> TcM ExprCoFn -- Locally used only
522 tcFunResTy fun actual_ty expected_ty
523 = addErrCtxtM (checkFunResCtxt fun actual_ty expected_ty) $
524 (tc_sub True actual_ty actual_ty expected_ty expected_ty)
527 tc_sub :: Outer -- See comments with uTys
528 -> BoxySigmaType -- actual_ty, before expanding synonyms
529 -> BoxySigmaType -- ..and after
530 -> BoxySigmaType -- expected_ty, before
531 -> BoxySigmaType -- ..and after
534 tc_sub outer act_sty act_ty exp_sty exp_ty
535 | Just exp_ty' <- tcView exp_ty = tc_sub False act_sty act_ty exp_sty exp_ty'
536 tc_sub outer act_sty act_ty exp_sty exp_ty
537 | Just act_ty' <- tcView act_ty = tc_sub False act_sty act_ty' exp_sty exp_ty
539 -----------------------------------
540 -- Rule SBOXY, plus other cases when act_ty is a type variable
541 -- Just defer to boxy matching
542 -- This rule takes precedence over SKOL!
543 tc_sub outer act_sty (TyVarTy tv) exp_sty exp_ty
544 = do { uVar outer False tv False exp_sty exp_ty
545 ; return idCoercion }
547 -----------------------------------
548 -- Skolemisation case (rule SKOL)
549 -- actual_ty: d:Eq b => b->b
550 -- expected_ty: forall a. Ord a => a->a
551 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
553 -- It is essential to do this *before* the specialisation case
554 -- Example: f :: (Eq a => a->a) -> ...
555 -- g :: Ord b => b->b
558 tc_sub outer act_sty act_ty exp_sty exp_ty
560 = do { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ body_exp_ty ->
561 tc_sub False act_sty act_ty body_exp_ty body_exp_ty
562 ; return (gen_fn <.> co_fn) }
564 act_tvs = tyVarsOfType act_ty
565 -- It's really important to check for escape wrt the free vars of
566 -- both expected_ty *and* actual_ty
568 -----------------------------------
569 -- Specialisation case (rule ASPEC):
570 -- actual_ty: forall a. Ord a => a->a
571 -- expected_ty: Int -> Int
572 -- co_fn e = e Int dOrdInt
574 tc_sub outer act_sty actual_ty exp_sty expected_ty
575 | isSigmaTy actual_ty
576 = do { (tyvars, theta, tau) <- tcInstBoxy actual_ty
577 ; dicts <- newDicts InstSigOrigin theta
579 ; let inst_fn = CoApps (CoTyApps CoHole (mkTyVarTys tyvars))
581 ; co_fn <- tc_sub False tau tau exp_sty expected_ty
582 ; return (co_fn <.> inst_fn) }
584 -----------------------------------
585 -- Function case (rule F1)
586 tc_sub _ _ (FunTy act_arg act_res) _ (FunTy exp_arg exp_res)
587 = tc_sub_funs act_arg act_res exp_arg exp_res
589 -- Function case (rule F2)
590 tc_sub outer act_sty act_ty@(FunTy act_arg act_res) exp_sty (TyVarTy exp_tv)
592 = do { cts <- readMetaTyVar exp_tv
594 Indirect ty -> do { u_tys outer False act_sty act_ty True exp_sty ty
595 ; return idCoercion }
596 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
597 ; tc_sub_funs act_arg act_res arg_ty res_ty } }
599 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
600 fun_kinds = [argTypeKind, openTypeKind]
602 -- Everything else: defer to boxy matching
603 tc_sub outer act_sty actual_ty exp_sty expected_ty
604 = do { u_tys outer False act_sty actual_ty False exp_sty expected_ty
605 ; return idCoercion }
608 -----------------------------------
609 tc_sub_funs act_arg act_res exp_arg exp_res
610 = do { uTys False act_arg False exp_arg
611 ; co_fn_res <- tc_sub False act_res act_res exp_res exp_res
612 ; wrapFunResCoercion [exp_arg] co_fn_res }
614 -----------------------------------
616 :: [TcType] -- Type of args
617 -> ExprCoFn -- HsExpr a -> HsExpr b
618 -> TcM ExprCoFn -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
619 wrapFunResCoercion arg_tys co_fn_res
620 | isIdCoercion co_fn_res = return idCoercion
621 | null arg_tys = return co_fn_res
623 = do { us <- newUniqueSupply
624 ; let arg_ids = zipWith (mkSysLocal FSLIT("sub")) (uniqsFromSupply us) arg_tys
625 ; return (CoLams arg_ids (co_fn_res <.> (CoApps CoHole arg_ids))) }
630 %************************************************************************
632 \subsection{Generalisation}
634 %************************************************************************
637 tcGen :: BoxySigmaType -- expected_ty
638 -> TcTyVarSet -- Extra tyvars that the universally
639 -- quantified tyvars of expected_ty
640 -- must not be unified
641 -> (BoxyRhoType -> TcM result) -- spec_ty
642 -> TcM (ExprCoFn, result)
643 -- The expression has type: spec_ty -> expected_ty
645 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
646 -- If not, the call is a no-op
647 = do { -- We want the GenSkol info in the skolemised type variables to
648 -- mention the *instantiated* tyvar names, so that we get a
649 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
650 -- Hence the tiresome but innocuous fixM
651 ((forall_tvs, theta, rho_ty), skol_info) <- fixM (\ ~(_, skol_info) ->
652 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
653 ; span <- getSrcSpanM
654 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty) span
655 ; return ((forall_tvs, theta, rho_ty), skol_info) })
658 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
659 text "expected_ty" <+> ppr expected_ty,
660 text "inst ty" <+> ppr forall_tvs <+> ppr theta <+> ppr rho_ty,
661 text "free_tvs" <+> ppr free_tvs,
662 text "forall_tvs" <+> ppr forall_tvs])
665 -- Type-check the arg and unify with poly type
666 ; (result, lie) <- getLIE (thing_inside rho_ty)
668 -- Check that the "forall_tvs" havn't been constrained
669 -- The interesting bit here is that we must include the free variables
670 -- of the expected_ty. Here's an example:
671 -- runST (newVar True)
672 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
673 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
674 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
675 -- So now s' isn't unconstrained because it's linked to a.
676 -- Conclusion: include the free vars of the expected_ty in the
677 -- list of "free vars" for the signature check.
679 ; dicts <- newDicts (SigOrigin skol_info) theta
680 ; inst_binds <- tcSimplifyCheck sig_msg forall_tvs dicts lie
682 ; checkSigTyVarsWrt free_tvs forall_tvs
683 ; traceTc (text "tcGen:done")
686 -- This HsLet binds any Insts which came out of the simplification.
687 -- It's a bit out of place here, but using AbsBind involves inventing
688 -- a couple of new names which seems worse.
689 dict_ids = map instToId dicts
690 co_fn = CoTyLams forall_tvs $ CoLams dict_ids $ CoLet inst_binds CoHole
691 ; returnM (co_fn, result) }
693 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
694 sig_msg = ptext SLIT("expected type of an expression")
699 %************************************************************************
703 %************************************************************************
705 The exported functions are all defined as versions of some
706 non-exported generic functions.
709 boxyUnify :: BoxyType -> BoxyType -> TcM ()
710 -- Acutal and expected, respectively
712 = addErrCtxtM (unifyCtxt ty1 ty2) $
713 uTysOuter False ty1 False ty2
716 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM ()
717 -- Arguments should have equal length
718 -- Acutal and expected types
719 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
722 unifyType :: TcTauType -> TcTauType -> TcM ()
723 -- No boxes expected inside these types
724 -- Acutal and expected types
725 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
726 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
727 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
728 addErrCtxtM (unifyCtxt ty1 ty2) $
729 uTysOuter True ty1 True ty2
732 unifyPred :: PredType -> PredType -> TcM ()
733 -- Acutal and expected types
734 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
735 uPred True True p1 True p2
737 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
738 -- Acutal and expected types
739 unifyTheta theta1 theta2
740 = do { checkTc (equalLength theta1 theta2)
741 (ptext SLIT("Contexts differ in length"))
742 ; uList unifyPred theta1 theta2 }
745 uList :: (a -> a -> TcM ())
746 -> [a] -> [a] -> TcM ()
747 -- Unify corresponding elements of two lists of types, which
748 -- should be f equal length. We charge down the list explicitly so that
749 -- we can complain if their lengths differ.
750 uList unify [] [] = return ()
751 uList unify (ty1:tys1) (ty2:tys2) = do { unify ty1 ty2; uList unify tys1 tys2 }
752 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
755 @unifyTypeList@ takes a single list of @TauType@s and unifies them
756 all together. It is used, for example, when typechecking explicit
757 lists, when all the elts should be of the same type.
760 unifyTypeList :: [TcTauType] -> TcM ()
761 unifyTypeList [] = returnM ()
762 unifyTypeList [ty] = returnM ()
763 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
764 ; unifyTypeList tys }
767 %************************************************************************
769 \subsection[Unify-uTys]{@uTys@: getting down to business}
771 %************************************************************************
773 @uTys@ is the heart of the unifier. Each arg happens twice, because
774 we want to report errors in terms of synomyms if poss. The first of
775 the pair is used in error messages only; it is always the same as the
776 second, except that if the first is a synonym then the second may be a
777 de-synonym'd version. This way we get better error messages.
779 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
782 type NoBoxes = Bool -- True <=> definitely no boxes in this type
783 -- False <=> there might be boxes (always safe)
785 type Outer = Bool -- True <=> this is the outer level of a unification
786 -- so that the types being unified are the
787 -- very ones we began with, not some sub
788 -- component or synonym expansion
789 -- The idea is that if Outer is true then unifyMisMatch should
790 -- pop the context to remove the "Expected/Acutal" context
793 :: NoBoxes -> TcType -- ty1 is the *expected* type
794 -> NoBoxes -> TcType -- ty2 is the *actual* type
796 uTysOuter nb1 ty1 nb2 ty2 = u_tys True nb1 ty1 ty1 nb2 ty2 ty2
797 uTys nb1 ty1 nb2 ty2 = u_tys False nb1 ty1 ty1 nb2 ty2 ty2
801 uTys_s :: NoBoxes -> [TcType] -- ty1 is the *actual* types
802 -> NoBoxes -> [TcType] -- ty2 is the *expected* types
804 uTys_s nb1 [] nb2 [] = returnM ()
805 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { uTys nb1 ty1 nb2 ty2
806 ; uTys_s nb1 tys1 nb2 tys2 }
807 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
811 -> NoBoxes -> TcType -> TcType -- ty1 is the *actual* type
812 -> NoBoxes -> TcType -> TcType -- ty2 is the *expected* type
815 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
819 -- Always expand synonyms (see notes at end)
820 -- (this also throws away FTVs)
822 | Just ty1' <- tcView ty1 = go False ty1' ty2
823 | Just ty2' <- tcView ty2 = go False ty1 ty2'
825 -- Variables; go for uVar
826 go outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
827 go outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
828 -- "True" means args swapped
830 go outer (PredTy p1) (PredTy p2) = uPred outer nb1 p1 nb2 p2
832 -- Type constructors must match
833 go _ (TyConApp con1 tys1) (TyConApp con2 tys2)
834 | con1 == con2 = uTys_s nb1 tys1 nb2 tys2
835 -- See Note [TyCon app]
837 -- Functions; just check the two parts
838 go _ (FunTy fun1 arg1) (FunTy fun2 arg2)
839 = do { uTys nb1 fun1 nb2 fun2
840 ; uTys nb1 arg1 nb2 arg2 }
842 -- Applications need a bit of care!
843 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
844 -- NB: we've already dealt with type variables and Notes,
845 -- so if one type is an App the other one jolly well better be too
846 go outer (AppTy s1 t1) ty2
847 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
848 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
850 -- Now the same, but the other way round
851 -- Don't swap the types, because the error messages get worse
852 go outer ty1 (AppTy s2 t2)
853 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
854 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
856 go _ ty1@(ForAllTy _ _) ty2@(ForAllTy _ _)
857 | length tvs1 == length tvs2
858 = do { tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
859 ; let tys = mkTyVarTys tvs
860 in_scope = mkInScopeSet (mkVarSet tvs)
861 subst1 = mkTvSubst in_scope (zipTyEnv tvs1 tys)
862 subst2 = mkTvSubst in_scope (zipTyEnv tvs2 tys)
863 ; uTys nb1 (substTy subst1 body1) nb2 (substTy subst2 body2)
865 -- If both sides are inside a box, we should not have
866 -- a polytype at all. This check comes last, because
867 -- the error message is extremely unhelpful.
868 ; ifM (nb1 && nb2) (notMonoType ty1)
871 (tvs1, body1) = tcSplitForAllTys ty1
872 (tvs2, body2) = tcSplitForAllTys ty2
874 -- Anything else fails
875 go outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
878 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
879 | n1 == n2 = uTys nb1 t1 nb2 t2
880 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
881 | c1 == c2 = uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
882 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
887 When we find two TyConApps, the argument lists are guaranteed equal
888 length. Reason: intially the kinds of the two types to be unified is
889 the same. The only way it can become not the same is when unifying two
890 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
891 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
892 which we do, that ensures that f1,f2 have the same kind; and that
893 means a1,a2 have the same kind. And now the argument repeats.
898 If you are tempted to make a short cut on synonyms, as in this
902 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
903 -- NO = if (con1 == con2) then
904 -- NO -- Good news! Same synonym constructors, so we can shortcut
905 -- NO -- by unifying their arguments and ignoring their expansions.
906 -- NO unifyTypepeLists args1 args2
908 -- NO -- Never mind. Just expand them and try again
912 then THINK AGAIN. Here is the whole story, as detected and reported
913 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
915 Here's a test program that should detect the problem:
919 x = (1 :: Bogus Char) :: Bogus Bool
922 The problem with [the attempted shortcut code] is that
926 is not a sufficient condition to be able to use the shortcut!
927 You also need to know that the type synonym actually USES all
928 its arguments. For example, consider the following type synonym
929 which does not use all its arguments.
934 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
935 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
936 would fail, even though the expanded forms (both \tr{Int}) should
939 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
940 unnecessarily bind \tr{t} to \tr{Char}.
942 ... You could explicitly test for the problem synonyms and mark them
943 somehow as needing expansion, perhaps also issuing a warning to the
948 %************************************************************************
950 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
952 %************************************************************************
954 @uVar@ is called when at least one of the types being unified is a
955 variable. It does {\em not} assume that the variable is a fixed point
956 of the substitution; rather, notice that @uVar@ (defined below) nips
957 back into @uTys@ if it turns out that the variable is already bound.
961 -> Bool -- False => tyvar is the "expected"
962 -- True => ty is the "expected" thing
964 -> NoBoxes -- True <=> definitely no boxes in t2
965 -> TcTauType -> TcTauType -- printing and real versions
968 uVar outer swapped tv1 nb2 ps_ty2 ty2
969 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
970 | otherwise = brackets (equals <+> ppr ty2)
971 ; traceTc (text "uVar" <+> ppr swapped <+>
972 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
973 nest 2 (ptext SLIT(" :=: ")),
974 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
975 ; details <- lookupTcTyVar tv1
978 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
979 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
980 -- The 'True' here says that ty1
981 -- is definitely box-free
982 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 ty2
986 uUnfilledVar :: Outer
987 -> Bool -- Args are swapped
988 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
989 -> NoBoxes -> TcTauType -> TcTauType -- Type 2
991 -- Invariant: tyvar 1 is not unified with anything
993 uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 ty2
994 | Just ty2' <- tcView ty2
995 = -- Expand synonyms; ignore FTVs
996 uUnfilledVar False swapped tv1 details1 nb2 ps_ty2 ty2'
998 uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 ty2@(TyVarTy tv2)
999 -- Same type variable => no-op
1003 -- Distinct type variables
1005 = do { lookup2 <- lookupTcTyVar tv2
1007 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 True ty2' ty2'
1008 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1011 uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 non_var_ty2 -- ty2 is not a type variable
1013 MetaTv (SigTv _) ref1 -> mis_match -- Can't update a skolem with a non-type-variable
1014 MetaTv info ref1 -> uMetaVar swapped tv1 info ref1 nb2 ps_ty2 non_var_ty2
1015 skolem_details -> mis_match
1017 mis_match = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1021 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1022 -> NoBoxes -> TcType -> TcType
1024 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1025 -- ty2 is not a type variable
1027 uMetaVar swapped tv1 info1 ref1 nb2 ps_ty2 non_var_ty2
1028 = do { final_ty <- case info1 of
1029 BoxTv -> unBox ps_ty2 -- No occurs check
1030 other -> checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
1031 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1034 uUnfilledVars :: Outer
1035 -> Bool -- Args are swapped
1036 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1037 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1039 -- Invarant: The type variables are distinct,
1040 -- Neither is filled in yet
1041 -- They might be boxy or not
1043 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1044 = unifyMisMatch outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1046 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1047 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2)
1048 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1049 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
1051 -- ToDo: this function seems too long for what it acutally does!
1052 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1053 = case (info1, info2) of
1054 (BoxTv, BoxTv) -> box_meets_box
1056 -- If a box meets a TauTv, but the fomer has the smaller kind
1057 -- then we must create a fresh TauTv with the smaller kind
1058 (_, BoxTv) | k1_sub_k2 -> update_tv2
1059 | otherwise -> box_meets_box
1060 (BoxTv, _ ) | k2_sub_k1 -> update_tv1
1061 | otherwise -> box_meets_box
1063 -- Avoid SigTvs if poss
1064 (SigTv _, _ ) | k1_sub_k2 -> update_tv2
1065 (_, SigTv _) | k2_sub_k1 -> update_tv1
1067 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1068 then update_tv1 -- Same kinds
1070 | k2_sub_k1 -> update_tv1
1071 | otherwise -> kind_err
1073 -- Update the variable with least kind info
1074 -- See notes on type inference in Kind.lhs
1075 -- The "nicer to" part only applies if the two kinds are the same,
1076 -- so we can choose which to do.
1078 -- Kinds should be guaranteed ok at this point
1079 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1080 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1082 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1085 | k2_sub_k1 = fill_from tv2
1086 | otherwise = kind_err
1088 -- Update *both* tyvars with a TauTv whose name and kind
1089 -- are gotten from tv (avoid losing nice names is poss)
1090 fill_from tv = do { tv' <- tcInstTyVar tv
1091 ; let tau_ty = mkTyVarTy tv'
1092 ; updateMeta tv1 ref1 tau_ty
1093 ; updateMeta tv2 ref2 tau_ty }
1095 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1096 unifyKindMisMatch k1 k2
1100 k1_sub_k2 = k1 `isSubKind` k2
1101 k2_sub_k1 = k2 `isSubKind` k1
1103 nicer_to_update_tv1 = isSystemName (varName tv1)
1104 -- Try to update sys-y type variables in preference to ones
1105 -- gotten (say) by instantiating a polymorphic function with
1106 -- a user-written type sig
1109 checkUpdateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1110 -- Update tv1, which is flexi; occurs check is alrady done
1111 -- The 'check' version does a kind check too
1112 -- We do a sub-kind check here: we might unify (a b) with (c d)
1113 -- where b::*->* and d::*; this should fail
1115 checkUpdateMeta swapped tv1 ref1 ty2
1116 = do { checkKinds swapped tv1 ty2
1117 ; updateMeta tv1 ref1 ty2 }
1119 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1120 updateMeta tv1 ref1 ty2
1121 = ASSERT( isMetaTyVar tv1 )
1122 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
1123 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
1124 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
1125 ; writeMutVar ref1 (Indirect ty2) }
1128 checkKinds swapped tv1 ty2
1129 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
1130 -- ty2 has been zonked at this stage, which ensures that
1131 -- its kind has as much boxity information visible as possible.
1132 | tk2 `isSubKind` tk1 = returnM ()
1135 -- Either the kinds aren't compatible
1136 -- (can happen if we unify (a b) with (c d))
1137 -- or we are unifying a lifted type variable with an
1138 -- unlifted type: e.g. (id 3#) is illegal
1139 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1140 unifyKindMisMatch k1 k2
1142 (k1,k2) | swapped = (tk2,tk1)
1143 | otherwise = (tk1,tk2)
1148 checkTauTvUpdate :: TcTyVar -> TcType -> TcM TcType
1149 -- (checkTauTvUpdate tv ty)
1150 -- We are about to update the TauTv tv with ty.
1151 -- Check (a) that tv doesn't occur in ty (occurs check)
1152 -- (b) that ty is a monotype
1153 -- Furthermore, in the interest of (b), if you find an
1154 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
1156 -- Returns the (non-boxy) type to update the type variable with, or fails
1158 checkTauTvUpdate orig_tv orig_ty
1161 go (TyConApp tc tys)
1162 | isSynTyCon tc = go_syn tc tys
1163 | otherwise = do { tys' <- mappM go tys; return (TyConApp tc tys') }
1164 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
1165 go (PredTy p) = do { p' <- go_pred p; return (PredTy p') }
1166 go (FunTy arg res) = do { arg' <- go arg; res' <- go res; return (FunTy arg' res') }
1167 go (AppTy fun arg) = do { fun' <- go fun; arg' <- go arg; return (mkAppTy fun' arg') }
1168 -- NB the mkAppTy; we might have instantiated a
1169 -- type variable to a type constructor, so we need
1170 -- to pull the TyConApp to the top.
1171 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
1174 | orig_tv == tv = occurCheck tv orig_ty -- (a)
1175 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
1176 | otherwise = return (TyVarTy tv)
1177 -- Ordinary (non Tc) tyvars
1178 -- occur inside quantified types
1180 go_pred (ClassP c tys) = do { tys' <- mapM go tys; return (ClassP c tys') }
1181 go_pred (IParam n ty) = do { ty' <- go ty; return (IParam n ty') }
1183 go_tyvar tv (SkolemTv _) = return (TyVarTy tv)
1184 go_tyvar tv (MetaTv box ref)
1185 = do { cts <- readMutVar ref
1187 Indirect ty -> go ty
1188 Flexi -> case box of
1189 BoxTv -> fillBoxWithTau tv ref
1190 other -> return (TyVarTy tv)
1193 -- go_syn is called for synonyms only
1194 -- See Note [Type synonyms and the occur check]
1196 | not (isTauTyCon tc)
1197 = notMonoType orig_ty -- (b) again
1199 = do { (msgs, mb_tys') <- tryTc (mapM go tys)
1201 Just tys' -> return (TyConApp tc tys')
1202 -- Retain the synonym (the common case)
1203 Nothing -> go (expectJust "checkTauTvUpdate"
1204 (tcView (TyConApp tc tys)))
1205 -- Try again, expanding the synonym
1208 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
1209 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
1210 -- tau-type meta-variable, whose print-name is the same as tv
1211 -- Choosing the same name is good: when we instantiate a function
1212 -- we allocate boxy tyvars with the same print-name as the quantified
1213 -- tyvar; and then we often fill the box with a tau-tyvar, and again
1214 -- we want to choose the same name.
1215 fillBoxWithTau tv ref
1216 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
1217 ; let tau = mkTyVarTy tv' -- name of the type variable
1218 ; writeMutVar ref (Indirect tau)
1222 Note [Type synonyms and the occur check]
1223 ~~~~~~~~~~~~~~~~~~~~
1224 Basically we want to update tv1 := ps_ty2
1225 because ps_ty2 has type-synonym info, which improves later error messages
1230 f :: (A a -> a -> ()) -> ()
1234 x = f (\ x p -> p x)
1236 In the application (p x), we try to match "t" with "A t". If we go
1237 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
1238 an infinite loop later.
1239 But we should not reject the program, because A t = ().
1240 Rather, we should bind t to () (= non_var_ty2).
1243 stripBoxyType :: BoxyType -> TcM TcType
1244 -- Strip all boxes from the input type, returning a non-boxy type.
1245 -- It's fine for there to be a polytype inside a box (c.f. unBox)
1246 -- All of the boxes should have been filled in by now;
1247 -- hence we return a TcType
1248 stripBoxyType ty = zonkType strip_tv ty
1250 strip_tv tv = ASSERT( not (isBoxyTyVar tv) ) return (TyVarTy tv)
1251 -- strip_tv will be called for *Flexi* meta-tyvars
1252 -- There should not be any Boxy ones; hence the ASSERT
1254 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1255 -- Subtle... we must zap the boxy res_ty
1256 -- to kind * before using it to instantiate a LitInst
1257 -- Calling unBox instead doesn't do the job, because the box
1258 -- often has an openTypeKind, and we don't want to instantiate
1260 zapToMonotype res_ty
1261 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1262 ; boxyUnify res_tau res_ty
1265 unBox :: BoxyType -> TcM TcType
1266 -- unBox implements the judgement
1268 -- with input s', and result s
1270 -- It remove all boxes from the input type, returning a non-boxy type.
1271 -- A filled box in the type can only contain a monotype; unBox fails if not
1272 -- The type can have empty boxes, which unBox fills with a monotype
1274 -- Compare this wth checkTauTvUpdate
1276 -- For once, it's safe to treat synonyms as opaque!
1278 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1279 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1280 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1281 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1282 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1283 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1284 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1286 | isTcTyVar tv -- It's a boxy type variable
1287 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1288 = do { cts <- readMutVar ref -- under nested quantifiers
1290 Flexi -> fillBoxWithTau tv ref
1291 Indirect ty -> do { non_boxy_ty <- unBox ty
1292 ; if isTauTy non_boxy_ty
1293 then return non_boxy_ty
1294 else notMonoType non_boxy_ty }
1296 | otherwise -- Skolems, and meta-tau-variables
1297 = return (TyVarTy tv)
1299 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1300 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1305 %************************************************************************
1307 \subsection[Unify-context]{Errors and contexts}
1309 %************************************************************************
1315 unifyCtxt act_ty exp_ty tidy_env
1316 = do { act_ty' <- zonkTcType act_ty
1317 ; exp_ty' <- zonkTcType exp_ty
1318 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1319 (env2, act_ty'') = tidyOpenType env1 act_ty'
1320 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1323 mkExpectedActualMsg act_ty exp_ty
1324 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1325 text "Inferred type" <> colon <+> ppr act_ty ])
1328 -- If an error happens we try to figure out whether the function
1329 -- function has been given too many or too few arguments, and say so.
1330 checkFunResCtxt fun actual_res_ty expected_res_ty tidy_env
1331 = do { exp_ty' <- zonkTcType expected_res_ty
1332 ; act_ty' <- zonkTcType actual_res_ty
1334 (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1335 (env2, act_ty'') = tidyOpenType env1 act_ty'
1336 (exp_args, _) = tcSplitFunTys exp_ty''
1337 (act_args, _) = tcSplitFunTys act_ty''
1339 len_act_args = length act_args
1340 len_exp_args = length exp_args
1342 message | len_exp_args < len_act_args = wrongArgsCtxt "too few" fun
1343 | len_exp_args > len_act_args = wrongArgsCtxt "too many" fun
1344 | otherwise = mkExpectedActualMsg act_ty'' exp_ty''
1345 ; return (env2, message) }
1348 wrongArgsCtxt too_many_or_few fun
1349 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1350 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1351 <+> ptext SLIT("arguments")
1354 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1355 -- tv1 and ty2 are zonked already
1358 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1359 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1361 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1362 | otherwise = (pp1, pp2)
1363 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1364 (env2, ty2') = tidyOpenType env1 ty2
1365 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1366 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1368 unifyMisMatch outer swapped ty1 ty2
1369 = do { (env, msg) <- if swapped then misMatchMsg ty1 ty2
1370 else misMatchMsg ty2 ty1
1372 -- This is the whole point of the 'outer' stuff
1373 ; if outer then popErrCtxt (failWithTcM (env, msg))
1374 else failWithTcM (env, msg)
1378 = do { env0 <- tcInitTidyEnv
1379 ; (env1, pp1, extra1) <- ppr_ty env0 ty1
1380 ; (env2, pp2, extra2) <- ppr_ty env1 ty2
1381 ; return (env2, sep [sep [ptext SLIT("Couldn't match expected type") <+> pp1,
1382 nest 7 (ptext SLIT("against inferred type") <+> pp2)],
1383 nest 2 extra1, nest 2 extra2]) }
1385 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
1387 = do { ty' <- zonkTcType ty
1388 ; let (env1,tidy_ty) = tidyOpenType env ty'
1389 simple_result = (env1, quotes (ppr tidy_ty), empty)
1392 | isSkolemTyVar tv -> return (env2, pp_rigid tv',
1393 pprSkolTvBinding tv')
1394 | otherwise -> return simple_result
1396 (env2, tv') = tidySkolemTyVar env1 tv
1397 other -> return simple_result }
1399 pp_rigid tv = quotes (ppr tv) <+> parens (ptext SLIT("a rigid variable"))
1403 = do { ty' <- zonkTcType ty
1404 ; env0 <- tcInitTidyEnv
1405 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1406 msg = ptext SLIT("Cannot match a monotype with") <+> ppr tidy_ty
1407 ; failWithTcM (env1, msg) }
1410 = do { env0 <- tcInitTidyEnv
1411 ; ty' <- zonkTcType ty
1412 ; let (env1, tidy_tyvar) = tidyOpenTyVar env0 tyvar
1413 (env2, tidy_ty) = tidyOpenType env1 ty'
1414 extra = sep [ppr tidy_tyvar, char '=', ppr tidy_ty]
1415 ; failWithTcM (env2, hang msg 2 extra) }
1417 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
1421 %************************************************************************
1425 %************************************************************************
1427 Unifying kinds is much, much simpler than unifying types.
1430 unifyKind :: TcKind -- Expected
1433 unifyKind LiftedTypeKind LiftedTypeKind = returnM ()
1434 unifyKind UnliftedTypeKind UnliftedTypeKind = returnM ()
1436 unifyKind OpenTypeKind k2 | isOpenTypeKind k2 = returnM ()
1437 unifyKind ArgTypeKind k2 | isArgTypeKind k2 = returnM ()
1438 -- Respect sub-kinding
1440 unifyKind (FunKind a1 r1) (FunKind a2 r2)
1441 = do { unifyKind a2 a1; unifyKind r1 r2 }
1442 -- Notice the flip in the argument,
1443 -- so that the sub-kinding works right
1445 unifyKind (KindVar kv1) k2 = uKVar False kv1 k2
1446 unifyKind k1 (KindVar kv2) = uKVar True kv2 k1
1447 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1449 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1450 unifyKinds [] [] = returnM ()
1451 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1453 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1456 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1457 uKVar swapped kv1 k2
1458 = do { mb_k1 <- readKindVar kv1
1460 Nothing -> uUnboundKVar swapped kv1 k2
1461 Just k1 | swapped -> unifyKind k2 k1
1462 | otherwise -> unifyKind k1 k2 }
1465 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1466 uUnboundKVar swapped kv1 k2@(KindVar kv2)
1467 | kv1 == kv2 = returnM ()
1468 | otherwise -- Distinct kind variables
1469 = do { mb_k2 <- readKindVar kv2
1471 Just k2 -> uUnboundKVar swapped kv1 k2
1472 Nothing -> writeKindVar kv1 k2 }
1474 uUnboundKVar swapped kv1 non_var_k2
1475 = do { k2' <- zonkTcKind non_var_k2
1476 ; kindOccurCheck kv1 k2'
1477 ; k2'' <- kindSimpleKind swapped k2'
1478 -- KindVars must be bound only to simple kinds
1479 -- Polarities: (kindSimpleKind True ?) succeeds
1480 -- returning *, corresponding to unifying
1483 ; writeKindVar kv1 k2'' }
1486 kindOccurCheck kv1 k2 -- k2 is zonked
1487 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1489 not_in (KindVar kv2) = kv1 /= kv2
1490 not_in (FunKind a2 r2) = not_in a2 && not_in r2
1493 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1494 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1495 -- If the flag is False, it requires k <: sk
1496 -- E.g. kindSimpleKind False ?? = *
1497 -- What about (kv -> *) :=: ?? -> *
1498 kindSimpleKind orig_swapped orig_kind
1499 = go orig_swapped orig_kind
1501 go sw (FunKind k1 k2) = do { k1' <- go (not sw) k1
1503 ; return (FunKind k1' k2') }
1504 go True OpenTypeKind = return liftedTypeKind
1505 go True ArgTypeKind = return liftedTypeKind
1506 go sw LiftedTypeKind = return liftedTypeKind
1507 go sw k@(KindVar _) = return k -- KindVars are always simple
1508 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1509 <+> ppr orig_swapped <+> ppr orig_kind)
1510 -- I think this can't actually happen
1512 -- T v = MkT v v must be a type
1513 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1516 kindOccurCheckErr tyvar ty
1517 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1518 2 (sep [ppr tyvar, char '=', ppr ty])
1520 unifyKindMisMatch ty1 ty2
1521 = zonkTcKind ty1 `thenM` \ ty1' ->
1522 zonkTcKind ty2 `thenM` \ ty2' ->
1524 msg = hang (ptext SLIT("Couldn't match kind"))
1525 2 (sep [quotes (ppr ty1'),
1526 ptext SLIT("against"),
1533 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1534 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1536 unifyFunKind (KindVar kvar)
1537 = readKindVar kvar `thenM` \ maybe_kind ->
1539 Just fun_kind -> unifyFunKind fun_kind
1540 Nothing -> do { arg_kind <- newKindVar
1541 ; res_kind <- newKindVar
1542 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1543 ; returnM (Just (arg_kind,res_kind)) }
1545 unifyFunKind (FunKind arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1546 unifyFunKind other = returnM Nothing
1549 %************************************************************************
1553 %************************************************************************
1555 ---------------------------
1556 -- We would like to get a decent error message from
1557 -- (a) Under-applied type constructors
1558 -- f :: (Maybe, Maybe)
1559 -- (b) Over-applied type constructors
1560 -- f :: Int x -> Int x
1564 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1565 -- A fancy wrapper for 'unifyKind', which tries
1566 -- to give decent error messages.
1567 checkExpectedKind ty act_kind exp_kind
1568 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1571 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1573 Just r -> returnM () ; -- Unification succeeded
1576 -- So there's definitely an error
1577 -- Now to find out what sort
1578 zonkTcKind exp_kind `thenM` \ exp_kind ->
1579 zonkTcKind act_kind `thenM` \ act_kind ->
1581 tcInitTidyEnv `thenM` \ env0 ->
1582 let (exp_as, _) = splitKindFunTys exp_kind
1583 (act_as, _) = splitKindFunTys act_kind
1584 n_exp_as = length exp_as
1585 n_act_as = length act_as
1587 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1588 (env2, tidy_act_kind) = tidyKind env1 act_kind
1590 err | n_exp_as < n_act_as -- E.g. [Maybe]
1591 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1593 -- Now n_exp_as >= n_act_as. In the next two cases,
1594 -- n_exp_as == 0, and hence so is n_act_as
1595 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1596 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1597 <+> ptext SLIT("is unlifted")
1599 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1600 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1601 <+> ptext SLIT("is lifted")
1603 | otherwise -- E.g. Monad [Int]
1604 = ptext SLIT("Kind mis-match")
1606 more_info = sep [ ptext SLIT("Expected kind") <+>
1607 quotes (pprKind tidy_exp_kind) <> comma,
1608 ptext SLIT("but") <+> quotes (ppr ty) <+>
1609 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1611 failWithTcM (env2, err $$ more_info)
1615 %************************************************************************
1617 \subsection{Checking signature type variables}
1619 %************************************************************************
1621 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1622 are not mentioned in the environment. In particular:
1624 (a) Not mentioned in the type of a variable in the envt
1625 eg the signature for f in this:
1631 Here, f is forced to be monorphic by the free occurence of x.
1633 (d) Not (unified with another type variable that is) in scope.
1634 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1635 when checking the expression type signature, we find that
1636 even though there is nothing in scope whose type mentions r,
1637 nevertheless the type signature for the expression isn't right.
1639 Another example is in a class or instance declaration:
1641 op :: forall b. a -> b
1643 Here, b gets unified with a
1645 Before doing this, the substitution is applied to the signature type variable.
1648 checkSigTyVars :: [TcTyVar] -> TcM ()
1649 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1651 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1652 -- The extra_tvs can include boxy type variables;
1653 -- e.g. TcMatches.tcCheckExistentialPat
1654 checkSigTyVarsWrt extra_tvs sig_tvs
1655 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1656 ; check_sig_tyvars extra_tvs' sig_tvs }
1659 :: TcTyVarSet -- Global type variables. The universally quantified
1660 -- tyvars should not mention any of these
1661 -- Guaranteed already zonked.
1662 -> [TcTyVar] -- Universally-quantified type variables in the signature
1663 -- Guaranteed to be skolems
1665 check_sig_tyvars extra_tvs []
1667 check_sig_tyvars extra_tvs sig_tvs
1668 = ASSERT( all isSkolemTyVar sig_tvs )
1669 do { gbl_tvs <- tcGetGlobalTyVars
1670 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1671 text "gbl_tvs" <+> ppr gbl_tvs,
1672 text "extra_tvs" <+> ppr extra_tvs]))
1674 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1675 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1676 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1679 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1680 -> [TcTyVar] -- The possibly-escaping type variables
1681 -> [TcTyVar] -- The zonked versions thereof
1683 -- Complain about escaping type variables
1684 -- We pass a list of type variables, at least one of which
1685 -- escapes. The first list contains the original signature type variable,
1686 -- while the second contains the type variable it is unified to (usually itself)
1687 bleatEscapedTvs globals sig_tvs zonked_tvs
1688 = do { env0 <- tcInitTidyEnv
1689 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1690 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1692 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1693 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1695 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1697 check (tidy_env, msgs) (sig_tv, zonked_tv)
1698 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1700 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1701 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1703 -----------------------
1704 escape_msg sig_tv zonked_tv globs
1706 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1707 nest 2 (vcat globs)]
1709 = msg <+> ptext SLIT("escapes")
1710 -- Sigh. It's really hard to give a good error message
1711 -- all the time. One bad case is an existential pattern match.
1712 -- We rely on the "When..." context to help.
1714 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1716 | sig_tv == zonked_tv = empty
1717 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1720 These two context are used with checkSigTyVars
1723 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1724 -> TidyEnv -> TcM (TidyEnv, Message)
1725 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1726 = zonkTcType sig_tau `thenM` \ actual_tau ->
1728 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1729 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1730 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1731 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1732 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1734 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),