2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section{Type subsumption and unification}
8 -- Full-blown subsumption
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,
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, 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, mkTyVarTys,
49 tyVarsOfType, mkPhiTy, mkTyVarTy, mkPredTy,
50 typeKind, mkForAllTys, mkAppTy, isBoxyTyVar,
51 tidyOpenType, tidyOpenTypes, 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 ( isSystemName )
69 import ErrUtils ( Message )
70 import Maybes ( fromJust, 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 actual_ty expected_ty
286 --------------------------------
287 -- withBoxes: the key utility function
288 --------------------------------
291 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
292 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
293 -> ([BoxySigmaType] -> BoxySigmaType)
294 -- Constructs the type to assign
295 -- to the original var
296 -> TcM [BoxySigmaType] -- Return the fresh boxes
298 -- It's entirely possible for the [kind] to be empty.
299 -- For example, when pattern-matching on True,
300 -- we call boxySplitTyConApp passing a boolTyCon
302 -- Invariant: tv is still Flexi
304 withMetaTvs tv kinds mk_res_ty
306 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
307 ; let box_tys = mkTyVarTys box_tvs
308 ; writeMetaTyVar tv (mk_res_ty box_tys)
311 | otherwise -- Non-boxy meta type variable
312 = do { tau_tys <- mapM newFlexiTyVarTy kinds
313 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
314 -- Sure to be a tau-type
317 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
318 -- Allocate a *boxy* tyvar
319 withBox kind thing_inside
320 = do { box_tv <- newMetaTyVar BoxTv kind
321 ; res <- thing_inside (mkTyVarTy box_tv)
322 ; ty <- readFilledBox box_tv
327 %************************************************************************
329 Approximate boxy matching
331 %************************************************************************
335 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
336 -> BoxyRhoType -- Type to match (note a *Rho* type)
337 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
340 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
341 -> [BoxySigmaType] -- Type to match
342 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
344 -- Find a *boxy* substitution that makes the template look as much
345 -- like the BoxySigmaType as possible.
346 -- It's always ok to return an empty substitution;
347 -- anything more is jam on the pudding
349 -- NB1: This is a pure, non-monadic function.
350 -- It does no unification, and cannot fail
352 -- Note [Matching kinds]
353 -- The target type might legitimately not be a sub-kind of template.
354 -- For example, suppose the target is simply a box with an OpenTypeKind,
355 -- and the template is a type variable with LiftedTypeKind.
356 -- Then it's ok (because the target type will later be refined).
357 -- We simply don't bind the template type variable.
359 -- It might also be that the kind mis-match is an error. For example,
360 -- suppose we match the template (a -> Int) against (Int# -> Int),
361 -- where the template type variable 'a' has LiftedTypeKind. This
362 -- matching function does not fail; it simply doesn't bind the template.
363 -- Later stuff will fail.
365 -- Precondition: the arg lengths are equal
366 -- Precondition: none of the template type variables appear in the [BoxySigmaType]
367 -- Precondition: any nested quantifiers in either type differ from
368 -- the template type variables passed as arguments
374 -- |- head xs : <rhobox>
375 -- We will do a boxySubMatchType between a ~ <rhobox>
376 -- But we *don't* want to match [a |-> <rhobox>] because
377 -- (a) The box should be filled in with a rho-type, but
378 -- but the returned substitution maps TyVars to boxy *sigma*
380 -- (b) In any case, the right final answer might be *either*
381 -- instantiate 'a' with a rho-type or a sigma type
382 -- head xs : Int vs head xs : forall b. b->b
383 -- So the matcher MUST NOT make a choice here. In general, we only
384 -- bind a template type variable in boxyMatchType, not in boxySubMatchType.
386 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
390 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
391 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
393 go (FunTy arg1 res1) (FunTy arg2 res2)
394 = do_match arg1 arg2 (go res1 res2)
395 -- Match the args, and sub-match the results
397 go (TyVarTy _) b_ty = emptyTvSubst -- Do not bind! See Note [Sub-match]
399 go t_ty b_ty = do_match t_ty b_ty emptyTvSubst -- Otherwise we are safe to bind
401 do_match t_ty b_ty subst = boxy_match tmpl_tvs t_ty emptyVarSet b_ty subst
404 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
405 = ASSERT( length tmpl_tys == length boxy_tys )
406 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
407 -- ToDo: add error context?
409 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
411 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
412 = boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys $
413 boxy_match tmpl_tvs t_ty boxy_tvs b_ty subst
416 boxy_match :: TcTyVarSet -> TcType -- Template
417 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
418 -> BoxySigmaType -- Match against this type
422 -- The boxy_tvs argument prevents this match:
423 -- [a] forall b. a ~ forall b. b
424 -- We don't want to bind the template variable 'a'
425 -- to the quantified type variable 'b'!
427 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
428 = go orig_tmpl_ty orig_boxy_ty
431 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
432 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
434 go (ForAllTy _ ty1) (ForAllTy tv2 ty2)
435 = boxy_match tmpl_tvs ty1 (boxy_tvs `extendVarSet` tv2) ty2 subst
437 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
438 | tc1 == tc2 = go_s tys1 tys2
440 go (FunTy arg1 res1) (FunTy arg2 res2)
441 = go_s [arg1,res1] [arg2,res2]
444 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
445 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
446 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
447 = go_s [s1,t1] [s2,t2]
450 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
451 , not (intersectsVarSet boxy_tvs (tyVarsOfType orig_boxy_ty))
452 , typeKind b_ty `isSubKind` tyVarKind tv
453 = extendTvSubst subst tv boxy_ty'
455 boxy_ty' = case lookupTyVar subst tv of
456 Nothing -> orig_boxy_ty
457 Just ty -> ty `boxyLub` orig_boxy_ty
459 go _ _ = subst -- Always safe
462 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
465 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
466 -- Combine boxy information from the two types
467 -- If there is a conflict, return the first
468 boxyLub orig_ty1 orig_ty2
469 = go orig_ty1 orig_ty2
471 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
472 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
473 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
474 | tc1 == tc2, length ts1 == length ts2
475 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
477 go (TyVarTy tv1) ty2 -- This is the whole point;
478 | isTcTyVar tv1, isMetaTyVar tv1 -- choose ty2 if ty2 is a box
481 -- Look inside type synonyms, but only if the naive version fails
482 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
483 | Just ty2' <- tcView ty1 = go ty1 ty2'
485 -- For now, we don't look inside ForAlls, PredTys
486 go ty1 ty2 = orig_ty1 -- Default
490 %************************************************************************
494 %************************************************************************
496 All the tcSub calls have the form
498 tcSub expected_ty offered_ty
500 offered_ty <= expected_ty
502 That is, that a value of type offered_ty is acceptable in
503 a place expecting a value of type expected_ty.
505 It returns a coercion function
506 co_fn :: offered_ty -> expected_ty
507 which takes an HsExpr of type offered_ty into one of type
512 tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM ExprCoFn -- Locally used only
513 -- (tcSub act exp) checks that
515 tcSubExp actual_ty expected_ty
516 = traceTc (text "tcSub" <+> details) `thenM_`
517 addErrCtxtM (unifyCtxt "type" actual_ty expected_ty)
518 (tc_sub actual_ty actual_ty expected_ty expected_ty)
520 details = vcat [text "Expected:" <+> ppr expected_ty,
521 text "Actual: " <+> ppr actual_ty]
524 tc_sub :: BoxySigmaType -- actual_ty, before expanding synonyms
525 -> BoxySigmaType -- ..and after
526 -> BoxySigmaType -- expected_ty, before
527 -> BoxySigmaType -- ..and after
530 tc_sub act_sty act_ty exp_sty exp_ty
531 | Just exp_ty' <- tcView exp_ty = tc_sub act_sty act_ty exp_sty exp_ty'
532 tc_sub act_sty act_ty exp_sty exp_ty
533 | Just act_ty' <- tcView act_ty = tc_sub act_sty act_ty' exp_sty exp_ty
535 -----------------------------------
536 -- Rule SBOXY, plus other cases when act_ty is a type variable
537 -- Just defer to boxy matching
538 -- This rule takes precedence over SKOL!
539 tc_sub act_sty (TyVarTy tv) exp_sty exp_ty
540 = do { uVar False tv False exp_sty exp_ty
541 ; return idCoercion }
543 -----------------------------------
544 -- Skolemisation case (rule SKOL)
545 -- actual_ty: d:Eq b => b->b
546 -- expected_ty: forall a. Ord a => a->a
547 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
549 -- It is essential to do this *before* the specialisation case
550 -- Example: f :: (Eq a => a->a) -> ...
551 -- g :: Ord b => b->b
554 tc_sub act_sty act_ty exp_sty exp_ty
556 = do { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ body_exp_ty ->
557 tc_sub act_sty act_ty body_exp_ty body_exp_ty
558 ; return (gen_fn <.> co_fn) }
560 act_tvs = tyVarsOfType act_ty
561 -- It's really important to check for escape wrt the free vars of
562 -- both expected_ty *and* actual_ty
564 -----------------------------------
565 -- Specialisation case (rule ASPEC):
566 -- actual_ty: forall a. Ord a => a->a
567 -- expected_ty: Int -> Int
568 -- co_fn e = e Int dOrdInt
570 tc_sub act_sty actual_ty exp_sty expected_ty
571 | isSigmaTy actual_ty
572 = do { (tyvars, theta, tau) <- tcInstBoxy actual_ty
573 ; dicts <- newDicts InstSigOrigin theta
575 ; let inst_fn = CoApps (CoTyApps CoHole (mkTyVarTys tyvars))
577 ; co_fn <- tc_sub tau tau exp_sty expected_ty
578 ; return (co_fn <.> inst_fn) }
580 -----------------------------------
581 -- Function case (rule F1)
582 tc_sub _ (FunTy act_arg act_res) _ (FunTy exp_arg exp_res)
583 = tc_sub_funs act_arg act_res exp_arg exp_res
585 -- Function case (rule F2)
586 tc_sub act_sty act_ty@(FunTy act_arg act_res) exp_sty (TyVarTy exp_tv)
588 = do { cts <- readMetaTyVar exp_tv
590 Indirect ty -> do { u_tys False act_sty act_ty True exp_sty ty
591 ; return idCoercion }
592 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
593 ; tc_sub_funs act_arg act_res arg_ty res_ty } }
595 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
596 fun_kinds = [argTypeKind, openTypeKind]
598 -- Everything else: defer to boxy matching
599 tc_sub act_sty actual_ty exp_sty expected_ty
600 = do { u_tys False act_sty actual_ty False exp_sty expected_ty
601 ; return idCoercion }
604 -----------------------------------
605 tc_sub_funs act_arg act_res exp_arg exp_res
606 = do { uTys False act_arg False exp_arg
607 ; co_fn_res <- tc_sub act_res act_res exp_res exp_res
608 ; wrapFunResCoercion [exp_arg] co_fn_res }
610 -----------------------------------
612 :: [TcType] -- Type of args
613 -> ExprCoFn -- HsExpr a -> HsExpr b
614 -> TcM ExprCoFn -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
615 wrapFunResCoercion arg_tys co_fn_res
616 | isIdCoercion co_fn_res = return idCoercion
617 | null arg_tys = return co_fn_res
619 = do { us <- newUniqueSupply
620 ; let arg_ids = zipWith (mkSysLocal FSLIT("sub")) (uniqsFromSupply us) arg_tys
621 ; return (CoLams arg_ids (co_fn_res <.> (CoApps CoHole arg_ids))) }
626 %************************************************************************
628 \subsection{Generalisation}
630 %************************************************************************
633 tcGen :: BoxySigmaType -- expected_ty
634 -> TcTyVarSet -- Extra tyvars that the universally
635 -- quantified tyvars of expected_ty
636 -- must not be unified
637 -> (BoxyRhoType -> TcM result) -- spec_ty
638 -> TcM (ExprCoFn, result)
639 -- The expression has type: spec_ty -> expected_ty
641 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
642 -- If not, the call is a no-op
643 = do { -- We want the GenSkol info in the skolemised type variables to
644 -- mention the *instantiated* tyvar names, so that we get a
645 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
646 -- Hence the tiresome but innocuous fixM
647 ((forall_tvs, theta, rho_ty), skol_info) <- fixM (\ ~(_, skol_info) ->
648 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
649 ; span <- getSrcSpanM
650 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty) span
651 ; return ((forall_tvs, theta, rho_ty), skol_info) })
654 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
655 text "expected_ty" <+> ppr expected_ty,
656 text "inst ty" <+> ppr forall_tvs <+> ppr theta <+> ppr rho_ty,
657 text "free_tvs" <+> ppr free_tvs,
658 text "forall_tvs" <+> ppr forall_tvs])
661 -- Type-check the arg and unify with poly type
662 ; (result, lie) <- getLIE (thing_inside rho_ty)
664 -- Check that the "forall_tvs" havn't been constrained
665 -- The interesting bit here is that we must include the free variables
666 -- of the expected_ty. Here's an example:
667 -- runST (newVar True)
668 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
669 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
670 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
671 -- So now s' isn't unconstrained because it's linked to a.
672 -- Conclusion: include the free vars of the expected_ty in the
673 -- list of "free vars" for the signature check.
675 ; dicts <- newDicts (SigOrigin skol_info) theta
676 ; inst_binds <- tcSimplifyCheck sig_msg forall_tvs dicts lie
678 ; checkSigTyVarsWrt free_tvs forall_tvs
679 ; traceTc (text "tcGen:done")
682 -- This HsLet binds any Insts which came out of the simplification.
683 -- It's a bit out of place here, but using AbsBind involves inventing
684 -- a couple of new names which seems worse.
685 dict_ids = map instToId dicts
686 co_fn = CoTyLams forall_tvs $ CoLams dict_ids $ CoLet inst_binds CoHole
687 ; returnM (co_fn, result) }
689 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
690 sig_msg = ptext SLIT("expected type of an expression")
695 %************************************************************************
699 %************************************************************************
701 The exported functions are all defined as versions of some
702 non-exported generic functions.
705 boxyUnify :: BoxyType -> BoxyType -> TcM ()
706 -- Acutal and expected, respectively
708 = addErrCtxtM (unifyCtxt "type" ty1 ty2) $
709 uTys False ty1 False ty2
712 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM ()
713 -- Arguments should have equal length
714 -- Acutal and expected types
715 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
718 unifyType :: TcTauType -> TcTauType -> TcM ()
719 -- No boxes expected inside these types
720 -- Acutal and expected types
721 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
722 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
723 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
724 addErrCtxtM (unifyCtxt "type" ty1 ty2) $
725 uTys True ty1 True ty2
728 unifyPred :: PredType -> PredType -> TcM ()
729 -- Acutal and expected types
730 unifyPred p1 p2 = addErrCtxtM (unifyCtxt "type constraint" (mkPredTy p1) (mkPredTy p2)) $
731 uPred True p1 True p2
733 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
734 -- Acutal and expected types
735 unifyTheta theta1 theta2
736 = do { checkTc (equalLength theta1 theta2)
737 (ptext SLIT("Contexts differ in length"))
738 ; uList unifyPred theta1 theta2 }
741 uList :: (a -> a -> TcM ())
742 -> [a] -> [a] -> TcM ()
743 -- Unify corresponding elements of two lists of types, which
744 -- should be f equal length. We charge down the list explicitly so that
745 -- we can complain if their lengths differ.
746 uList unify [] [] = return ()
747 uList unify (ty1:tys1) (ty2:tys2) = do { unify ty1 ty2; uList unify tys1 tys2 }
748 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
751 @unifyTypeList@ takes a single list of @TauType@s and unifies them
752 all together. It is used, for example, when typechecking explicit
753 lists, when all the elts should be of the same type.
756 unifyTypeList :: [TcTauType] -> TcM ()
757 unifyTypeList [] = returnM ()
758 unifyTypeList [ty] = returnM ()
759 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
760 ; unifyTypeList tys }
763 %************************************************************************
765 \subsection[Unify-uTys]{@uTys@: getting down to business}
767 %************************************************************************
769 @uTys@ is the heart of the unifier. Each arg happens twice, because
770 we want to report errors in terms of synomyms if poss. The first of
771 the pair is used in error messages only; it is always the same as the
772 second, except that if the first is a synonym then the second may be a
773 de-synonym'd version. This way we get better error messages.
775 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
778 type NoBoxes = Bool -- True <=> definitely no boxes in this type
779 -- False <=> there might be boxes (always safe)
781 uTys :: NoBoxes -> TcType -- ty1 is the *expected* type
782 -> NoBoxes -> TcType -- ty2 is the *actual* type
784 uTys nb1 ty1 nb2 ty2 = u_tys nb1 ty1 ty1 nb2 ty2 ty2
788 uTys_s :: NoBoxes -> [TcType] -- ty1 is the *actual* types
789 -> NoBoxes -> [TcType] -- ty2 is the *expected* types
791 uTys_s nb1 [] nb2 [] = returnM ()
792 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { uTys nb1 ty1 nb2 ty2
793 ; uTys_s nb1 tys1 nb2 tys2 }
794 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
797 u_tys :: NoBoxes -> TcType -> TcType -- ty1 is the *actual* type
798 -> NoBoxes -> TcType -> TcType -- ty2 is the *expected* type
801 u_tys nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
805 -- Always expand synonyms (see notes at end)
806 -- (this also throws away FTVs)
808 | Just ty1' <- tcView ty1 = go ty1' ty2
809 | Just ty2' <- tcView ty2 = go ty1 ty2'
811 -- Variables; go for uVar
812 go (TyVarTy tyvar1) ty2 = uVar False tyvar1 nb2 orig_ty2 ty2
813 go ty1 (TyVarTy tyvar2) = uVar True tyvar2 nb1 orig_ty1 ty1
814 -- "True" means args swapped
816 go (PredTy p1) (PredTy p2) = uPred nb1 p1 nb2 p2
818 -- Type constructors must match
819 go (TyConApp con1 tys1) (TyConApp con2 tys2)
820 | con1 == con2 = uTys_s nb1 tys1 nb2 tys2
821 -- See Note [TyCon app]
823 -- Functions; just check the two parts
824 go (FunTy fun1 arg1) (FunTy fun2 arg2)
825 = do { uTys nb1 fun1 nb2 fun2
826 ; uTys nb1 arg1 nb2 arg2 }
828 -- Applications need a bit of care!
829 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
830 -- NB: we've already dealt with type variables and Notes,
831 -- so if one type is an App the other one jolly well better be too
833 = case tcSplitAppTy_maybe ty2 of
834 Just (s2,t2) -> do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
835 Nothing -> unifyMisMatch False orig_ty1 orig_ty2
837 -- Now the same, but the other way round
838 -- Don't swap the types, because the error messages get worse
840 = case tcSplitAppTy_maybe ty1 of
841 Just (s1,t1) -> do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
842 Nothing -> unifyMisMatch False orig_ty1 orig_ty2
844 go ty1@(ForAllTy _ _) ty2@(ForAllTy _ _)
845 | length tvs1 == length tvs2
846 = do { tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
847 ; let tys = mkTyVarTys tvs
848 in_scope = mkInScopeSet (mkVarSet tvs)
849 subst1 = mkTvSubst in_scope (zipTyEnv tvs1 tys)
850 subst2 = mkTvSubst in_scope (zipTyEnv tvs2 tys)
851 ; uTys nb1 (substTy subst1 body1) nb2 (substTy subst2 body2)
853 -- If both sides are inside a box, we should not have
854 -- a polytype at all. This check comes last, because
855 -- the error message is extremely unhelpful.
856 ; ifM (nb1 && nb2) (notMonoType ty1)
859 (tvs1, body1) = tcSplitForAllTys ty1
860 (tvs2, body2) = tcSplitForAllTys ty2
862 -- Anything else fails
863 go _ _ = unifyMisMatch False orig_ty1 orig_ty2
866 uPred nb1 (IParam n1 t1) nb2 (IParam n2 t2)
867 | n1 == n2 = uTys nb1 t1 nb2 t2
868 uPred nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
869 | c1 == c2 = uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
870 uPred _ p1 _ p2 = unifyMisMatch False (mkPredTy p1) (mkPredTy p2)
875 When we find two TyConApps, the argument lists are guaranteed equal
876 length. Reason: intially the kinds of the two types to be unified is
877 the same. The only way it can become not the same is when unifying two
878 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
879 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
880 which we do, that ensures that f1,f2 have the same kind; and that
881 means a1,a2 have the same kind. And now the argument repeats.
886 If you are tempted to make a short cut on synonyms, as in this
890 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
891 -- NO = if (con1 == con2) then
892 -- NO -- Good news! Same synonym constructors, so we can shortcut
893 -- NO -- by unifying their arguments and ignoring their expansions.
894 -- NO unifyTypepeLists args1 args2
896 -- NO -- Never mind. Just expand them and try again
900 then THINK AGAIN. Here is the whole story, as detected and reported
901 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
903 Here's a test program that should detect the problem:
907 x = (1 :: Bogus Char) :: Bogus Bool
910 The problem with [the attempted shortcut code] is that
914 is not a sufficient condition to be able to use the shortcut!
915 You also need to know that the type synonym actually USES all
916 its arguments. For example, consider the following type synonym
917 which does not use all its arguments.
922 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
923 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
924 would fail, even though the expanded forms (both \tr{Int}) should
927 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
928 unnecessarily bind \tr{t} to \tr{Char}.
930 ... You could explicitly test for the problem synonyms and mark them
931 somehow as needing expansion, perhaps also issuing a warning to the
936 %************************************************************************
938 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
940 %************************************************************************
942 @uVar@ is called when at least one of the types being unified is a
943 variable. It does {\em not} assume that the variable is a fixed point
944 of the substitution; rather, notice that @uVar@ (defined below) nips
945 back into @uTys@ if it turns out that the variable is already bound.
948 uVar :: Bool -- False => tyvar is the "expected"
949 -- True => ty is the "expected" thing
951 -> NoBoxes -- True <=> definitely no boxes in t2
952 -> TcTauType -> TcTauType -- printing and real versions
955 uVar swapped tv1 nb2 ps_ty2 ty2
956 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
957 | otherwise = brackets (equals <+> ppr ty2)
958 ; traceTc (text "uVar" <+> ppr swapped <+>
959 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
960 nest 2 (ptext SLIT(" :=: ")),
961 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
962 ; details <- lookupTcTyVar tv1
965 | swapped -> u_tys nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
966 | otherwise -> u_tys True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
967 -- The 'True' here says that ty1
968 -- is definitely box-free
969 DoneTv details1 -> uUnfilledVar swapped tv1 details1 nb2 ps_ty2 ty2
973 uUnfilledVar :: Bool -- Args are swapped
974 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
975 -> NoBoxes -> TcTauType -> TcTauType -- Type 2
977 -- Invariant: tyvar 1 is not unified with anything
979 uUnfilledVar swapped tv1 details1 nb2 ps_ty2 ty2
980 | Just ty2' <- tcView ty2
981 = -- Expand synonyms; ignore FTVs
982 uUnfilledVar swapped tv1 details1 nb2 ps_ty2 ty2'
984 uUnfilledVar swapped tv1 details1 nb2 ps_ty2 ty2@(TyVarTy tv2)
985 -- Same type variable => no-op
989 -- Distinct type variables
991 = do { lookup2 <- lookupTcTyVar tv2
993 IndirectTv ty2' -> uUnfilledVar swapped tv1 details1 True ty2' ty2'
994 DoneTv details2 -> uUnfilledVars swapped tv1 details1 tv2 details2
997 uUnfilledVar swapped tv1 details1 nb2 ps_ty2 non_var_ty2 -- ty2 is not a type variable
999 MetaTv (SigTv _) ref1 -> mis_match -- Can't update a skolem with a non-type-variable
1000 MetaTv info ref1 -> uMetaVar swapped tv1 info ref1 nb2 ps_ty2 non_var_ty2
1001 skolem_details -> mis_match
1003 mis_match = unifyMisMatch swapped (TyVarTy tv1) ps_ty2
1007 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1008 -> NoBoxes -> TcType -> TcType
1010 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1011 -- ty2 is not a type variable
1013 uMetaVar swapped tv1 info1 ref1 nb2 ps_ty2 non_var_ty2
1014 = do { final_ty <- case info1 of
1015 BoxTv -> unBox ps_ty2 -- No occurs check
1016 other -> checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
1017 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1020 uUnfilledVars :: Bool -- Args are swapped
1021 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1022 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1024 -- Invarant: The type variables are distinct,
1025 -- Neither is filled in yet
1026 -- They might be boxy or not
1028 uUnfilledVars swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1029 = unifyMisMatch swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1031 uUnfilledVars swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1032 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2)
1033 uUnfilledVars swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1034 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
1036 -- ToDo: this function seems too long for what it acutally does!
1037 uUnfilledVars swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1038 = case (info1, info2) of
1039 (BoxTv, BoxTv) -> box_meets_box
1041 -- If a box meets a TauTv, but the fomer has the smaller kind
1042 -- then we must create a fresh TauTv with the smaller kind
1043 (_, BoxTv) | k1_sub_k2 -> update_tv2
1044 | otherwise -> box_meets_box
1045 (BoxTv, _ ) | k2_sub_k1 -> update_tv1
1046 | otherwise -> box_meets_box
1048 -- Avoid SigTvs if poss
1049 (SigTv _, _ ) | k1_sub_k2 -> update_tv2
1050 (_, SigTv _) | k2_sub_k1 -> update_tv1
1052 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1053 then update_tv1 -- Same kinds
1055 | k2_sub_k1 -> update_tv1
1056 | otherwise -> kind_err
1058 -- Update the variable with least kind info
1059 -- See notes on type inference in Kind.lhs
1060 -- The "nicer to" part only applies if the two kinds are the same,
1061 -- so we can choose which to do.
1063 -- Kinds should be guaranteed ok at this point
1064 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1065 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1067 box_meets_box | k1_sub_k2 = fill_with k1
1068 | k2_sub_k1 = fill_with k2
1069 | otherwise = kind_err
1071 fill_with kind = do { tau_ty <- newFlexiTyVarTy kind
1072 ; updateMeta tv1 ref1 tau_ty
1073 ; updateMeta tv2 ref2 tau_ty }
1075 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1076 unifyKindMisMatch k1 k2
1080 k1_sub_k2 = k1 `isSubKind` k2
1081 k2_sub_k1 = k2 `isSubKind` k1
1083 nicer_to_update_tv1 = isSystemName (varName tv1)
1084 -- Try to update sys-y type variables in preference to ones
1085 -- gotten (say) by instantiating a polymorphic function with
1086 -- a user-written type sig
1089 checkUpdateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1090 -- Update tv1, which is flexi; occurs check is alrady done
1091 -- The 'check' version does a kind check too
1092 -- We do a sub-kind check here: we might unify (a b) with (c d)
1093 -- where b::*->* and d::*; this should fail
1095 checkUpdateMeta swapped tv1 ref1 ty2
1096 = do { checkKinds swapped tv1 ty2
1097 ; updateMeta tv1 ref1 ty2 }
1099 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1100 updateMeta tv1 ref1 ty2
1101 = ASSERT( isMetaTyVar tv1 )
1102 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
1103 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
1104 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
1105 ; writeMutVar ref1 (Indirect ty2) }
1108 checkKinds swapped tv1 ty2
1109 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
1110 -- ty2 has been zonked at this stage, which ensures that
1111 -- its kind has as much boxity information visible as possible.
1112 | tk2 `isSubKind` tk1 = returnM ()
1115 -- Either the kinds aren't compatible
1116 -- (can happen if we unify (a b) with (c d))
1117 -- or we are unifying a lifted type variable with an
1118 -- unlifted type: e.g. (id 3#) is illegal
1119 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1120 unifyKindMisMatch k1 k2
1122 (k1,k2) | swapped = (tk2,tk1)
1123 | otherwise = (tk1,tk2)
1128 checkTauTvUpdate :: TcTyVar -> TcType -> TcM TcType
1129 -- (checkTauTvUpdate tv ty)
1130 -- We are about to update the TauTv tv with ty.
1131 -- Check (a) that tv doesn't occur in ty (occurs check)
1132 -- (b) that ty is a monotype
1133 -- Furthermore, in the interest of (b), if you find an
1134 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
1136 -- Returns the (non-boxy) type to update the type variable with, or fails
1138 checkTauTvUpdate orig_tv orig_ty
1141 go (TyConApp tc tys)
1142 | isSynTyCon tc = go_syn tc tys
1143 | otherwise = do { tys' <- mappM go tys; return (TyConApp tc tys') }
1144 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
1145 go (PredTy p) = do { p' <- go_pred p; return (PredTy p') }
1146 go (FunTy arg res) = do { arg' <- go arg; res' <- go res; return (FunTy arg' res') }
1147 go (AppTy fun arg) = do { fun' <- go fun; arg' <- go arg; return (mkAppTy fun' arg') }
1148 -- NB the mkAppTy; we might have instantiated a
1149 -- type variable to a type constructor, so we need
1150 -- to pull the TyConApp to the top.
1151 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
1154 | orig_tv == tv = occurCheck tv orig_ty -- (a)
1155 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
1156 | otherwise = return (TyVarTy tv)
1157 -- Ordinary (non Tc) tyvars
1158 -- occur inside quantified types
1160 go_pred (ClassP c tys) = do { tys' <- mapM go tys; return (ClassP c tys') }
1161 go_pred (IParam n ty) = do { ty' <- go ty; return (IParam n ty') }
1163 go_tyvar tv (SkolemTv _) = return (TyVarTy tv)
1164 go_tyvar tv (MetaTv box ref)
1165 = do { cts <- readMutVar ref
1167 Indirect ty -> go ty
1168 Flexi -> case box of
1169 BoxTv -> do { tau <- newFlexiTyVarTy (tyVarKind tv)
1170 ; writeMutVar ref (Indirect tau)
1172 other -> return (TyVarTy tv)
1175 -- go_syn is called for synonyms only
1176 -- See Note [Type synonyms and the occur check]
1178 | not (isTauTyCon tc)
1179 = notMonoType orig_ty -- (b) again
1181 = do { (msgs, mb_tys') <- tryTc (mapM go tys)
1183 Just tys' -> return (TyConApp tc tys')
1184 -- Retain the synonym (the common case)
1185 Nothing -> go (fromJust (tcView (TyConApp tc tys)))
1186 -- Try again, expanding the synonym
1190 Note [Type synonyms and the occur check]
1191 ~~~~~~~~~~~~~~~~~~~~
1192 Basically we want to update tv1 := ps_ty2
1193 because ps_ty2 has type-synonym info, which improves later error messages
1198 f :: (A a -> a -> ()) -> ()
1202 x = f (\ x p -> p x)
1204 In the application (p x), we try to match "t" with "A t". If we go
1205 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
1206 an infinite loop later.
1207 But we should not reject the program, because A t = ().
1208 Rather, we should bind t to () (= non_var_ty2).
1211 stripBoxyType :: BoxyType -> TcM TcType
1212 -- Strip all boxes from the input type, returning a non-boxy type.
1213 -- It's fine for there to be a polytype inside a box (c.f. unBox)
1214 -- All of the boxes should have been filled in by now;
1215 -- hence we return a TcType
1216 stripBoxyType ty = zonkType strip_tv ty
1218 strip_tv tv = ASSERT( not (isBoxyTyVar tv) ) return (TyVarTy tv)
1219 -- strip_tv will be called for *Flexi* meta-tyvars
1220 -- There should not be any Boxy ones; hence the ASSERT
1222 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1223 -- Subtle... we must zap the boxy res_ty
1224 -- to kind * before using it to instantiate a LitInst
1225 -- Calling unBox instead doesn't do the job, because the box
1226 -- often has an openTypeKind, and we don't want to instantiate
1228 zapToMonotype res_ty
1229 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1230 ; boxyUnify res_tau res_ty
1233 unBox :: BoxyType -> TcM TcType
1234 -- unBox implements the judgement
1236 -- with input s', and result s
1238 -- It remove all boxes from the input type, returning a non-boxy type.
1239 -- A filled box in the type can only contain a monotype; unBox fails if not
1240 -- The type can have empty boxes, which unBox fills with a monotype
1242 -- Compare this wth checkTauTvUpdate
1244 -- For once, it's safe to treat synonyms as opaque!
1246 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1247 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1248 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1249 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1250 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1251 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1252 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1254 | isTcTyVar tv -- It's a boxy type variable
1255 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1256 = do { cts <- readMutVar ref -- under nested quantifiers
1258 Indirect ty -> do { non_boxy_ty <- unBox ty
1259 ; if isTauTy non_boxy_ty
1260 then return non_boxy_ty
1261 else notMonoType non_boxy_ty }
1262 Flexi -> do { tau <- newFlexiTyVarTy (tyVarKind tv)
1263 ; writeMutVar ref (Indirect tau)
1266 | otherwise -- Skolems, and meta-tau-variables
1267 = return (TyVarTy tv)
1269 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1270 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1275 %************************************************************************
1279 %************************************************************************
1281 Unifying kinds is much, much simpler than unifying types.
1284 unifyKind :: TcKind -- Expected
1287 unifyKind LiftedTypeKind LiftedTypeKind = returnM ()
1288 unifyKind UnliftedTypeKind UnliftedTypeKind = returnM ()
1290 unifyKind OpenTypeKind k2 | isOpenTypeKind k2 = returnM ()
1291 unifyKind ArgTypeKind k2 | isArgTypeKind k2 = returnM ()
1292 -- Respect sub-kinding
1294 unifyKind (FunKind a1 r1) (FunKind a2 r2)
1295 = do { unifyKind a2 a1; unifyKind r1 r2 }
1296 -- Notice the flip in the argument,
1297 -- so that the sub-kinding works right
1299 unifyKind (KindVar kv1) k2 = uKVar False kv1 k2
1300 unifyKind k1 (KindVar kv2) = uKVar True kv2 k1
1301 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1303 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1304 unifyKinds [] [] = returnM ()
1305 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1307 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1310 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1311 uKVar swapped kv1 k2
1312 = do { mb_k1 <- readKindVar kv1
1314 Nothing -> uUnboundKVar swapped kv1 k2
1315 Just k1 | swapped -> unifyKind k2 k1
1316 | otherwise -> unifyKind k1 k2 }
1319 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1320 uUnboundKVar swapped kv1 k2@(KindVar kv2)
1321 | kv1 == kv2 = returnM ()
1322 | otherwise -- Distinct kind variables
1323 = do { mb_k2 <- readKindVar kv2
1325 Just k2 -> uUnboundKVar swapped kv1 k2
1326 Nothing -> writeKindVar kv1 k2 }
1328 uUnboundKVar swapped kv1 non_var_k2
1329 = do { k2' <- zonkTcKind non_var_k2
1330 ; kindOccurCheck kv1 k2'
1331 ; k2'' <- kindSimpleKind swapped k2'
1332 -- KindVars must be bound only to simple kinds
1333 -- Polarities: (kindSimpleKind True ?) succeeds
1334 -- returning *, corresponding to unifying
1337 ; writeKindVar kv1 k2'' }
1340 kindOccurCheck kv1 k2 -- k2 is zonked
1341 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1343 not_in (KindVar kv2) = kv1 /= kv2
1344 not_in (FunKind a2 r2) = not_in a2 && not_in r2
1347 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1348 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1349 -- If the flag is False, it requires k <: sk
1350 -- E.g. kindSimpleKind False ?? = *
1351 -- What about (kv -> *) :=: ?? -> *
1352 kindSimpleKind orig_swapped orig_kind
1353 = go orig_swapped orig_kind
1355 go sw (FunKind k1 k2) = do { k1' <- go (not sw) k1
1357 ; return (FunKind k1' k2') }
1358 go True OpenTypeKind = return liftedTypeKind
1359 go True ArgTypeKind = return liftedTypeKind
1360 go sw LiftedTypeKind = return liftedTypeKind
1361 go sw k@(KindVar _) = return k -- KindVars are always simple
1362 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1363 <+> ppr orig_swapped <+> ppr orig_kind)
1364 -- I think this can't actually happen
1366 -- T v = MkT v v must be a type
1367 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1370 kindOccurCheckErr tyvar ty
1371 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1372 2 (sep [ppr tyvar, char '=', ppr ty])
1374 unifyKindMisMatch ty1 ty2
1375 = zonkTcKind ty1 `thenM` \ ty1' ->
1376 zonkTcKind ty2 `thenM` \ ty2' ->
1378 msg = hang (ptext SLIT("Couldn't match kind"))
1379 2 (sep [quotes (ppr ty1'),
1380 ptext SLIT("against"),
1387 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1388 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1390 unifyFunKind (KindVar kvar)
1391 = readKindVar kvar `thenM` \ maybe_kind ->
1393 Just fun_kind -> unifyFunKind fun_kind
1394 Nothing -> do { arg_kind <- newKindVar
1395 ; res_kind <- newKindVar
1396 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1397 ; returnM (Just (arg_kind,res_kind)) }
1399 unifyFunKind (FunKind arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1400 unifyFunKind other = returnM Nothing
1403 %************************************************************************
1405 \subsection[Unify-context]{Errors and contexts}
1407 %************************************************************************
1413 unifyCtxt s ty1 ty2 tidy_env -- ty1 inferred, ty2 expected
1414 = zonkTcType ty1 `thenM` \ ty1' ->
1415 zonkTcType ty2 `thenM` \ ty2' ->
1416 returnM (err ty1' ty2')
1418 err ty1 ty2 = (env1,
1421 text "Expected" <+> text s <> colon <+> ppr tidy_ty2,
1422 text "Inferred" <+> text s <> colon <+> ppr tidy_ty1
1425 (env1, [tidy_ty1,tidy_ty2]) = tidyOpenTypes tidy_env [ty1,ty2]
1427 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1428 -- tv1 and ty2 are zonked already
1431 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1432 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1434 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1435 | otherwise = (pp1, pp2)
1436 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1437 (env2, ty2') = tidyOpenType env1 ty2
1438 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1439 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1441 unifyMisMatch swapped ty1 ty2
1442 = do { (env, msg) <- if swapped then misMatchMsg ty2 ty1
1443 else misMatchMsg ty1 ty2
1444 ; failWithTcM (env, msg) }
1447 = do { env0 <- tcInitTidyEnv
1448 ; (env1, pp1, extra1) <- ppr_ty env0 ty1
1449 ; (env2, pp2, extra2) <- ppr_ty env1 ty2
1450 ; return (env2, sep [sep [ptext SLIT("Couldn't match") <+> pp1,
1451 nest 7 (ptext SLIT("against") <+> pp2)],
1452 nest 2 extra1, nest 2 extra2]) }
1454 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
1456 = do { ty' <- zonkTcType ty
1457 ; let (env1,tidy_ty) = tidyOpenType env ty'
1458 simple_result = (env1, quotes (ppr tidy_ty), empty)
1461 | isSkolemTyVar tv -> return (env2, pp_rigid tv',
1462 pprSkolTvBinding tv')
1463 | otherwise -> return simple_result
1465 (env2, tv') = tidySkolemTyVar env1 tv
1466 other -> return simple_result }
1468 pp_rigid tv = ptext SLIT("the rigid variable") <+> quotes (ppr tv)
1472 = do { ty' <- zonkTcType ty
1473 ; env0 <- tcInitTidyEnv
1474 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1475 msg = ptext SLIT("Cannot match a monotype with") <+> ppr tidy_ty
1476 ; failWithTcM (env1, msg) }
1479 = do { env0 <- tcInitTidyEnv
1480 ; ty' <- zonkTcType ty
1481 ; let (env1, tidy_tyvar) = tidyOpenTyVar env0 tyvar
1482 (env2, tidy_ty) = tidyOpenType env1 ty
1483 extra = sep [ppr tidy_tyvar, char '=', ppr tidy_ty]
1484 ; failWithTcM (env2, hang msg 2 extra) }
1486 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
1490 %************************************************************************
1494 %************************************************************************
1496 ---------------------------
1497 -- We would like to get a decent error message from
1498 -- (a) Under-applied type constructors
1499 -- f :: (Maybe, Maybe)
1500 -- (b) Over-applied type constructors
1501 -- f :: Int x -> Int x
1505 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1506 -- A fancy wrapper for 'unifyKind', which tries
1507 -- to give decent error messages.
1508 checkExpectedKind ty act_kind exp_kind
1509 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1512 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1514 Just r -> returnM () ; -- Unification succeeded
1517 -- So there's definitely an error
1518 -- Now to find out what sort
1519 zonkTcKind exp_kind `thenM` \ exp_kind ->
1520 zonkTcKind act_kind `thenM` \ act_kind ->
1522 tcInitTidyEnv `thenM` \ env0 ->
1523 let (exp_as, _) = splitKindFunTys exp_kind
1524 (act_as, _) = splitKindFunTys act_kind
1525 n_exp_as = length exp_as
1526 n_act_as = length act_as
1528 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1529 (env2, tidy_act_kind) = tidyKind env1 act_kind
1531 err | n_exp_as < n_act_as -- E.g. [Maybe]
1532 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1534 -- Now n_exp_as >= n_act_as. In the next two cases,
1535 -- n_exp_as == 0, and hence so is n_act_as
1536 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1537 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1538 <+> ptext SLIT("is unlifted")
1540 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1541 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1542 <+> ptext SLIT("is lifted")
1544 | otherwise -- E.g. Monad [Int]
1545 = ptext SLIT("Kind mis-match")
1547 more_info = sep [ ptext SLIT("Expected kind") <+>
1548 quotes (pprKind tidy_exp_kind) <> comma,
1549 ptext SLIT("but") <+> quotes (ppr ty) <+>
1550 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1552 failWithTcM (env2, err $$ more_info)
1556 %************************************************************************
1558 \subsection{Checking signature type variables}
1560 %************************************************************************
1562 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1563 are not mentioned in the environment. In particular:
1565 (a) Not mentioned in the type of a variable in the envt
1566 eg the signature for f in this:
1572 Here, f is forced to be monorphic by the free occurence of x.
1574 (d) Not (unified with another type variable that is) in scope.
1575 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1576 when checking the expression type signature, we find that
1577 even though there is nothing in scope whose type mentions r,
1578 nevertheless the type signature for the expression isn't right.
1580 Another example is in a class or instance declaration:
1582 op :: forall b. a -> b
1584 Here, b gets unified with a
1586 Before doing this, the substitution is applied to the signature type variable.
1589 checkSigTyVars :: [TcTyVar] -> TcM ()
1590 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1592 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1593 -- The extra_tvs can include boxy type variables;
1594 -- e.g. TcMatches.tcCheckExistentialPat
1595 checkSigTyVarsWrt extra_tvs sig_tvs
1596 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1597 ; check_sig_tyvars extra_tvs' sig_tvs }
1600 :: TcTyVarSet -- Global type variables. The universally quantified
1601 -- tyvars should not mention any of these
1602 -- Guaranteed already zonked.
1603 -> [TcTyVar] -- Universally-quantified type variables in the signature
1604 -- Guaranteed to be skolems
1606 check_sig_tyvars extra_tvs []
1608 check_sig_tyvars extra_tvs sig_tvs
1609 = ASSERT( all isSkolemTyVar sig_tvs )
1610 do { gbl_tvs <- tcGetGlobalTyVars
1611 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1612 text "gbl_tvs" <+> ppr gbl_tvs,
1613 text "extra_tvs" <+> ppr extra_tvs]))
1615 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1616 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1617 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1620 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1621 -> [TcTyVar] -- The possibly-escaping type variables
1622 -> [TcTyVar] -- The zonked versions thereof
1624 -- Complain about escaping type variables
1625 -- We pass a list of type variables, at least one of which
1626 -- escapes. The first list contains the original signature type variable,
1627 -- while the second contains the type variable it is unified to (usually itself)
1628 bleatEscapedTvs globals sig_tvs zonked_tvs
1629 = do { env0 <- tcInitTidyEnv
1630 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1631 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1633 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1634 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1636 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1638 check (tidy_env, msgs) (sig_tv, zonked_tv)
1639 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1641 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1642 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1644 -----------------------
1645 escape_msg sig_tv zonked_tv globs
1647 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1648 nest 2 (vcat globs)]
1650 = msg <+> ptext SLIT("escapes")
1651 -- Sigh. It's really hard to give a good error message
1652 -- all the time. One bad case is an existential pattern match.
1653 -- We rely on the "When..." context to help.
1655 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1657 | sig_tv == zonked_tv = empty
1658 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1661 These two context are used with checkSigTyVars
1664 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1665 -> TidyEnv -> TcM (TidyEnv, Message)
1666 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1667 = zonkTcType sig_tau `thenM` \ actual_tau ->
1669 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1670 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1671 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1672 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1673 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1675 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),