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 preSubType, 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, tcInstBoxyTyVar, newKindVar, newMetaTyVar,
33 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 tcSplitSigmaTy, tyVarsOfType, mkPhiTy, mkTyVarTy, mkPredTy,
50 typeKind, mkForAllTys, mkAppTy, isBoxyTyVar,
51 tcView, exactTyVarsOfType,
52 tidyOpenType, tidyOpenTyVar, tidyOpenTyVars,
53 pprType, tidyKind, tidySkolemTyVar, isSkolemTyVar, isSigTyVar,
54 TvSubst, mkTvSubst, zipTyEnv, zipOpenTvSubst, emptyTvSubst,
56 lookupTyVar, extendTvSubst )
57 import Kind ( Kind(..), SimpleKind, KindVar, isArgTypeKind,
58 openTypeKind, liftedTypeKind, unliftedTypeKind,
59 mkArrowKind, defaultKind,
60 isOpenTypeKind, argTypeKind, isLiftedTypeKind, isUnliftedTypeKind,
61 isSubKind, pprKind, splitKindFunTys )
62 import TysPrim ( alphaTy, betaTy )
63 import Inst ( newDicts, instToId )
64 import TyCon ( TyCon, tyConArity, tyConTyVars, isSynTyCon )
65 import TysWiredIn ( listTyCon )
66 import Id ( Id, mkSysLocal )
67 import Var ( Var, varName, tyVarKind, isTcTyVar, tcTyVarDetails )
70 import Name ( Name, isSystemName )
71 import ErrUtils ( Message )
72 import Maybes ( expectJust, isNothing )
73 import BasicTypes ( Arity )
74 import UniqSupply ( uniqsFromSupply )
75 import Util ( notNull, equalLength )
80 import TcType ( isBoxyTy, isFlexi )
84 %************************************************************************
86 \subsection{'hole' type variables}
88 %************************************************************************
91 tcInfer :: (BoxyType -> TcM a) -> TcM (a, TcType)
93 = do { box <- newBoxyTyVar openTypeKind
94 ; res <- tc_infer (mkTyVarTy box)
95 ; res_ty <- readFilledBox box -- Guaranteed filled-in by now
96 ; return (res, res_ty) }
100 %************************************************************************
104 %************************************************************************
107 subFunTys :: SDoc -- Somthing like "The function f has 3 arguments"
108 -- or "The abstraction (\x.e) takes 1 argument"
109 -> Arity -- Expected # of args
110 -> BoxyRhoType -- res_ty
111 -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
113 -- Attempt to decompse res_ty to have enough top-level arrows to
114 -- match the number of patterns in the match group
116 -- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
117 -- and the inner call to thing_inside passes args: [a1,...,an], b
118 -- then co_fn :: (a1 -> ... -> an -> b) -> res_ty
120 -- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
123 {- Error messages from subFunTys
125 The abstraction `\Just 1 -> ...' has two arguments
126 but its type `Maybe a -> a' has only one
128 The equation(s) for `f' have two arguments
129 but its type `Maybe a -> a' has only one
131 The section `(f 3)' requires 'f' to take two arguments
132 but its type `Int -> Int' has only one
134 The function 'f' is applied to two arguments
135 but its type `Int -> Int' has only one
139 subFunTys error_herald n_pats res_ty thing_inside
140 = loop n_pats [] res_ty
142 -- In 'loop', the parameter 'arg_tys' accumulates
143 -- the arg types so far, in *reverse order*
144 loop n args_so_far res_ty
145 | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
147 loop n args_so_far res_ty
148 | isSigmaTy res_ty -- Do this before checking n==0, because we
149 -- guarantee to return a BoxyRhoType, not a BoxySigmaType
150 = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet $ \ res_ty' ->
151 loop n args_so_far res_ty'
152 ; return (gen_fn <.> co_fn, res) }
154 loop 0 args_so_far res_ty
155 = do { res <- thing_inside (reverse args_so_far) res_ty
156 ; return (idCoercion, res) }
158 loop n args_so_far (FunTy arg_ty res_ty)
159 = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
160 ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
161 ; return (co_fn', res) }
163 -- res_ty might have a type variable at the head, such as (a b c),
164 -- in which case we must fill in with (->). Simplest thing to do
165 -- is to use boxyUnify, but we catch failure and generate our own
166 -- error message on failure
167 loop n args_so_far res_ty@(AppTy _ _)
168 = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
169 ; (_, mb_unit) <- tryTcErrs $ boxyUnify res_ty (FunTy arg_ty' res_ty')
170 ; if isNothing mb_unit then bale_out args_so_far
171 else loop n args_so_far (FunTy arg_ty' res_ty') }
173 loop n args_so_far (TyVarTy tv)
174 | not (isImmutableTyVar tv)
175 = do { cts <- readMetaTyVar tv
177 Indirect ty -> loop n args_so_far ty
178 Flexi -> do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
179 ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty
180 ; return (idCoercion, res) } }
182 mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
183 mk_res_ty [] = panic "TcUnify.mk_res_ty1"
184 kinds = openTypeKind : take n (repeat argTypeKind)
185 -- Note argTypeKind: the args can have an unboxed type,
186 -- but not an unboxed tuple.
188 loop n args_so_far res_ty = bale_out args_so_far
191 = do { env0 <- tcInitTidyEnv
192 ; res_ty' <- zonkTcType res_ty
193 ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
194 ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
196 mk_msg res_ty n_actual
197 = error_herald <> comma $$
198 sep [ptext SLIT("but its type") <+> quotes (pprType res_ty),
199 if n_actual == 0 then ptext SLIT("has none")
200 else ptext SLIT("has only") <+> speakN n_actual]
204 ----------------------
205 boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
206 -> BoxyRhoType -- Expected type (T a b c)
207 -> TcM [BoxySigmaType] -- Element types, a b c
208 -- It's used for wired-in tycons, so we call checkWiredInTyCOn
209 -- Precondition: never called with FunTyCon
210 -- Precondition: input type :: *
212 boxySplitTyConApp tc orig_ty
213 = do { checkWiredInTyCon tc
214 ; loop (tyConArity tc) [] orig_ty }
216 loop n_req args_so_far ty
217 | Just ty' <- tcView ty = loop n_req args_so_far ty'
219 loop n_req args_so_far (TyConApp tycon args)
221 = ASSERT( n_req == length args) -- ty::*
222 return (args ++ args_so_far)
224 loop n_req args_so_far (AppTy fun arg)
225 = loop (n_req - 1) (arg:args_so_far) fun
227 loop n_req args_so_far (TyVarTy tv)
228 | not (isImmutableTyVar tv)
229 = do { cts <- readMetaTyVar tv
231 Indirect ty -> loop n_req args_so_far ty
232 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
233 ; return (arg_tys ++ args_so_far) }
236 mk_res_ty arg_tys' = mkTyConApp tc arg_tys'
237 arg_kinds = map tyVarKind (take n_req (tyConTyVars tc))
239 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc))) orig_ty
241 ----------------------
242 boxySplitListTy :: BoxyRhoType -> TcM BoxySigmaType -- Special case for lists
243 boxySplitListTy exp_ty = do { [elt_ty] <- boxySplitTyConApp listTyCon exp_ty
247 ----------------------
248 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
249 -> TcM (BoxySigmaType, BoxySigmaType) -- Returns m, a
250 -- Assumes (m: * -> k), where k is the kind of the incoming type
251 -- If the incoming type is boxy, then so are the result types; and vice versa
253 boxySplitAppTy orig_ty
257 | Just ty' <- tcView ty = loop ty'
260 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
261 = return (fun_ty, arg_ty)
264 | not (isImmutableTyVar tv)
265 = do { cts <- readMetaTyVar tv
267 Indirect ty -> loop ty
268 Flexi -> do { [fun_ty,arg_ty] <- withMetaTvs tv kinds mk_res_ty
269 ; return (fun_ty, arg_ty) } }
271 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
272 mk_res_ty other = panic "TcUnify.mk_res_ty2"
273 tv_kind = tyVarKind tv
274 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
276 liftedTypeKind] -- arg type :: *
277 -- The defaultKind is a bit smelly. If you remove it,
278 -- try compiling f x = do { x }
279 -- and you'll get a kind mis-match. It smells, but
280 -- not enough to lose sleep over.
282 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
285 boxySplitFailure actual_ty expected_ty
286 = unifyMisMatch False False actual_ty expected_ty
287 -- "outer" is False, so we don't pop the context
288 -- which is what we want since we have not pushed one!
292 --------------------------------
293 -- withBoxes: the key utility function
294 --------------------------------
297 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
298 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
299 -> ([BoxySigmaType] -> BoxySigmaType)
300 -- Constructs the type to assign
301 -- to the original var
302 -> TcM [BoxySigmaType] -- Return the fresh boxes
304 -- It's entirely possible for the [kind] to be empty.
305 -- For example, when pattern-matching on True,
306 -- we call boxySplitTyConApp passing a boolTyCon
308 -- Invariant: tv is still Flexi
310 withMetaTvs tv kinds mk_res_ty
312 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
313 ; let box_tys = mkTyVarTys box_tvs
314 ; writeMetaTyVar tv (mk_res_ty box_tys)
317 | otherwise -- Non-boxy meta type variable
318 = do { tau_tys <- mapM newFlexiTyVarTy kinds
319 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
320 -- Sure to be a tau-type
323 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
324 -- Allocate a *boxy* tyvar
325 withBox kind thing_inside
326 = do { box_tv <- newMetaTyVar BoxTv kind
327 ; res <- thing_inside (mkTyVarTy box_tv)
328 ; ty <- readFilledBox box_tv
333 %************************************************************************
335 Approximate boxy matching
337 %************************************************************************
340 preSubType :: [TcTyVar] -- Quantified type variables
341 -> TcTyVarSet -- Subset of quantified type variables
342 -- see Note [Pre-sub boxy]
343 -> TcType -- The rho-type part; quantified tyvars scopes over this
344 -> BoxySigmaType -- Matching type from the context
345 -> TcM [TcType] -- Types to instantiate the tyvars
346 -- Perform pre-subsumption, and return suitable types
347 -- to instantiate the quantified type varibles:
348 -- info from the pre-subsumption, if there is any
349 -- a boxy type variable otherwise
351 -- Note [Pre-sub boxy]
352 -- The 'btvs' are a subset of 'qtvs'. They are the ones we can
353 -- instantiate to a boxy type variable, because they'll definitely be
354 -- filled in later. This isn't always the case; sometimes we have type
355 -- variables mentioned in the context of the type, but not the body;
356 -- f :: forall a b. C a b => a -> a
357 -- Then we may land up with an unconstrained 'b', so we want to
358 -- instantiate it to a monotype (non-boxy) type variable
360 -- The 'qtvs' that are *neither* fixed by the pre-subsumption, *nor* are in 'btvs',
361 -- are instantiated to TauTv meta variables.
363 preSubType qtvs btvs qty expected_ty
364 = do { tys <- mapM inst_tv qtvs
365 ; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
368 pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
370 | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
371 | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
372 ; return (mkTyVarTy tv') }
373 | otherwise = do { tv' <- tcInstTyVar tv
374 ; return (mkTyVarTy tv') }
377 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
378 -> BoxyRhoType -- Type to match (note a *Rho* type)
379 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
381 -- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
382 -- "Boxy types: inference for higher rank types and impredicativity"
384 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
385 = go tmpl_tvs tmpl_ty emptyVarSet boxy_ty
387 go t_tvs t_ty b_tvs b_ty
388 | Just t_ty' <- tcView t_ty = go t_tvs t_ty' b_tvs b_ty
389 | Just b_ty' <- tcView b_ty = go t_tvs t_ty b_tvs b_ty'
391 go t_tvs (TyVarTy _) b_tvs b_ty = emptyTvSubst -- Rule S-ANY; no bindings
392 -- Rule S-ANY covers (a) type variables and (b) boxy types
393 -- in the template. Both look like a TyVarTy.
394 -- See Note [Sub-match] below
396 go t_tvs t_ty b_tvs b_ty
397 | isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
398 = go (t_tvs `delVarSetList` tvs) t_tau b_tvs b_ty -- Rule S-SPEC
399 -- Under a forall on the left, if there is shadowing,
400 -- do not bind! Hence the delVarSetList.
401 | isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
402 = go t_tvs t_ty (extendVarSetList b_tvs tvs) b_tau -- Rule S-SKOL
403 -- Add to the variables we must not bind to
404 -- NB: it's *important* to discard the theta part. Otherwise
405 -- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
406 -- and end up with a completely bogus binding (b |-> Bool), by lining
407 -- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
408 -- This pre-subsumption stuff can return too few bindings, but it
409 -- must *never* return bogus info.
411 go t_tvs (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
412 = boxy_match t_tvs arg1 b_tvs arg2 (go t_tvs res1 b_tvs res2)
413 -- Match the args, and sub-match the results
415 go t_tvs t_ty b_tvs b_ty = boxy_match t_tvs t_ty b_tvs b_ty emptyTvSubst
416 -- Otherwise defer to boxy matching
417 -- This covers TyConApp, AppTy, PredTy
424 |- head xs : <rhobox>
425 We will do a boxySubMatchType between a ~ <rhobox>
426 But we *don't* want to match [a |-> <rhobox>] because
427 (a) The box should be filled in with a rho-type, but
428 but the returned substitution maps TyVars to boxy
430 (b) In any case, the right final answer might be *either*
431 instantiate 'a' with a rho-type or a sigma type
432 head xs : Int vs head xs : forall b. b->b
433 So the matcher MUST NOT make a choice here. In general, we only
434 bind a template type variable in boxyMatchType, not in boxySubMatchType.
439 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
440 -> [BoxySigmaType] -- Type to match
441 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
443 -- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
444 -- "Boxy types: inference for higher rank types and impredicativity"
446 -- Find a *boxy* substitution that makes the template look as much
447 -- like the BoxySigmaType as possible.
448 -- It's always ok to return an empty substitution;
449 -- anything more is jam on the pudding
451 -- NB1: This is a pure, non-monadic function.
452 -- It does no unification, and cannot fail
454 -- Precondition: the arg lengths are equal
455 -- Precondition: none of the template type variables appear anywhere in the [BoxySigmaType]
459 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
460 = ASSERT( length tmpl_tys == length boxy_tys )
461 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
462 -- ToDo: add error context?
464 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
466 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
467 = boxy_match tmpl_tvs t_ty boxy_tvs b_ty $
468 boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys subst
469 boxy_match_s tmpl_tvs _ boxy_tvs _ subst
470 = panic "boxy_match_s" -- Lengths do not match
474 boxy_match :: TcTyVarSet -> TcType -- Template
475 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
476 -> BoxySigmaType -- Match against this type
480 -- The boxy_tvs argument prevents this match:
481 -- [a] forall b. a ~ forall b. b
482 -- We don't want to bind the template variable 'a'
483 -- to the quantified type variable 'b'!
485 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
486 = go orig_tmpl_ty orig_boxy_ty
489 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
490 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
492 go ty1 ty2 -- C.f. the isSigmaTy case for boxySubMatchType
494 , (tvs1, _, tau1) <- tcSplitSigmaTy ty1
495 , (tvs2, _, tau2) <- tcSplitSigmaTy ty2
496 , equalLength tvs1 tvs2
497 = boxy_match (tmpl_tvs `delVarSetList` tvs1) tau1
498 (boxy_tvs `extendVarSetList` tvs2) tau2 subst
500 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
501 | tc1 == tc2 = go_s tys1 tys2
503 go (FunTy arg1 res1) (FunTy arg2 res2)
504 = go_s [arg1,res1] [arg2,res2]
507 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
508 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
509 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
510 = go_s [s1,t1] [s2,t2]
513 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
514 , not (intersectsVarSet boxy_tvs (tyVarsOfType orig_boxy_ty))
515 , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
516 = extendTvSubst subst tv boxy_ty'
518 = subst -- Ignore others
520 boxy_ty' = case lookupTyVar subst tv of
521 Nothing -> orig_boxy_ty
522 Just ty -> ty `boxyLub` orig_boxy_ty
524 go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
525 -- Example: Tree a ~ Maybe Int
526 -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
527 -- misleading error messages. An even more confusing case is
528 -- a -> b ~ Maybe Int
529 -- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
530 -- from this pre-matching phase.
533 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
536 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
537 -- Combine boxy information from the two types
538 -- If there is a conflict, return the first
539 boxyLub orig_ty1 orig_ty2
540 = go orig_ty1 orig_ty2
542 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
543 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
544 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
545 | tc1 == tc2, length ts1 == length ts2
546 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
548 go (TyVarTy tv1) ty2 -- This is the whole point;
549 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
552 -- Look inside type synonyms, but only if the naive version fails
553 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
554 | Just ty2' <- tcView ty1 = go ty1 ty2'
556 -- For now, we don't look inside ForAlls, PredTys
557 go ty1 ty2 = orig_ty1 -- Default
560 Note [Matching kinds]
561 ~~~~~~~~~~~~~~~~~~~~~
562 The target type might legitimately not be a sub-kind of template.
563 For example, suppose the target is simply a box with an OpenTypeKind,
564 and the template is a type variable with LiftedTypeKind.
565 Then it's ok (because the target type will later be refined).
566 We simply don't bind the template type variable.
568 It might also be that the kind mis-match is an error. For example,
569 suppose we match the template (a -> Int) against (Int# -> Int),
570 where the template type variable 'a' has LiftedTypeKind. This
571 matching function does not fail; it simply doesn't bind the template.
572 Later stuff will fail.
574 %************************************************************************
578 %************************************************************************
580 All the tcSub calls have the form
582 tcSub expected_ty offered_ty
584 offered_ty <= expected_ty
586 That is, that a value of type offered_ty is acceptable in
587 a place expecting a value of type expected_ty.
589 It returns a coercion function
590 co_fn :: offered_ty -> expected_ty
591 which takes an HsExpr of type offered_ty into one of type
596 tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM ExprCoFn -- Locally used only
597 -- (tcSub act exp) checks that
599 tcSubExp actual_ty expected_ty
600 = -- addErrCtxtM (unifyCtxt actual_ty expected_ty) $
601 -- Adding the error context here leads to some very confusing error
602 -- messages, such as "can't match foarall a. a->a with forall a. a->a"
603 -- So instead I'm adding it when moving from tc_sub to u_tys
604 tc_sub Nothing actual_ty actual_ty False expected_ty expected_ty
606 tcFunResTy :: Name -> BoxySigmaType -> BoxySigmaType -> TcM ExprCoFn -- Locally used only
607 tcFunResTy fun actual_ty expected_ty
608 = tc_sub (Just fun) actual_ty actual_ty False expected_ty expected_ty
611 tc_sub :: Maybe Name -- Just fun => we're looking at a function result type
612 -> BoxySigmaType -- actual_ty, before expanding synonyms
613 -> BoxySigmaType -- ..and after
614 -> InBox -- True <=> expected_ty is inside a box
615 -> BoxySigmaType -- expected_ty, before
616 -> BoxySigmaType -- ..and after
618 -- The acual_ty is never inside a box
619 -- IMPORTANT pre-condition: if the args contain foralls, the bound type
620 -- variables are visible non-monadically
621 -- (i.e. tha args are sufficiently zonked)
622 -- This invariant is needed so that we can "see" the foralls, ad
623 -- e.g. in the SPEC rule where we just use splitSigmaTy
625 tc_sub mb_fun act_sty act_ty exp_ib exp_sty exp_ty
626 = tc_sub1 mb_fun act_sty act_ty exp_ib exp_sty exp_ty
627 -- This indirection is just here to make
628 -- it easy to insert a debug trace!
630 tc_sub1 mb_fun act_sty act_ty exp_ib exp_sty exp_ty
631 | Just exp_ty' <- tcView exp_ty = tc_sub mb_fun act_sty act_ty exp_ib exp_sty exp_ty'
632 tc_sub1 mb_fun act_sty act_ty exp_ib exp_sty exp_ty
633 | Just act_ty' <- tcView act_ty = tc_sub mb_fun act_sty act_ty' exp_ib exp_sty exp_ty
635 -----------------------------------
636 -- Rule SBOXY, plus other cases when act_ty is a type variable
637 -- Just defer to boxy matching
638 -- This rule takes precedence over SKOL!
639 tc_sub1 mb_fun act_sty (TyVarTy tv) exp_ib exp_sty exp_ty
640 = do { addErrCtxtM (subCtxt mb_fun act_sty exp_sty) $
641 uVar True False tv exp_ib exp_sty exp_ty
642 ; return idCoercion }
644 -----------------------------------
645 -- Skolemisation case (rule SKOL)
646 -- actual_ty: d:Eq b => b->b
647 -- expected_ty: forall a. Ord a => a->a
648 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
650 -- It is essential to do this *before* the specialisation case
651 -- Example: f :: (Eq a => a->a) -> ...
652 -- g :: Ord b => b->b
655 tc_sub1 mb_fun act_sty act_ty exp_ib exp_sty exp_ty
656 | not exp_ib, -- SKOL does not apply if exp_ty is inside a box
658 = do { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ body_exp_ty ->
659 tc_sub mb_fun act_sty act_ty False body_exp_ty body_exp_ty
660 ; return (gen_fn <.> co_fn) }
662 act_tvs = tyVarsOfType act_ty
663 -- It's really important to check for escape wrt
664 -- the free vars of both expected_ty *and* actual_ty
666 -----------------------------------
667 -- Specialisation case (rule ASPEC):
668 -- actual_ty: forall a. Ord a => a->a
669 -- expected_ty: Int -> Int
670 -- co_fn e = e Int dOrdInt
672 tc_sub1 mb_fun act_sty actual_ty exp_ib exp_sty expected_ty
673 -- Implements the new SPEC rule in the Appendix of the paper
674 -- "Boxy types: inference for higher rank types and impredicativity"
675 -- (This appendix isn't in the published version.)
676 -- The idea is to *first* do pre-subsumption, and then full subsumption
677 -- Example: forall a. a->a <= Int -> (forall b. Int)
678 -- Pre-subsumpion finds a|->Int, and that works fine, whereas
679 -- just running full subsumption would fail.
680 | isSigmaTy actual_ty
681 = do { -- Perform pre-subsumption, and instantiate
682 -- the type with info from the pre-subsumption;
683 -- boxy tyvars if pre-subsumption gives no info
684 let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
685 tau_tvs = exactTyVarsOfType tau
686 ; inst_tys <- if exp_ib then -- Inside a box, do not do clever stuff
687 do { tyvars' <- mapM tcInstBoxyTyVar tyvars
688 ; return (mkTyVarTys tyvars') }
689 else -- Outside, do clever stuff
690 preSubType tyvars tau_tvs tau expected_ty
691 ; let subst' = zipOpenTvSubst tyvars inst_tys
692 tau' = substTy subst' tau
694 -- Perform a full subsumption check
695 ; traceTc (text "tc_sub_spec" <+> vcat [ppr actual_ty,
696 ppr tyvars <+> ppr theta <+> ppr tau,
698 ; co_fn <- tc_sub mb_fun tau' tau' exp_ib exp_sty expected_ty
700 -- Deal with the dictionaries
701 ; dicts <- newDicts InstSigOrigin (substTheta subst' theta)
703 ; let inst_fn = CoApps (CoTyApps CoHole inst_tys)
705 ; return (co_fn <.> inst_fn) }
707 -----------------------------------
708 -- Function case (rule F1)
709 tc_sub1 mb_fun _ (FunTy act_arg act_res) exp_ib _ (FunTy exp_arg exp_res)
710 = tc_sub_funs mb_fun act_arg act_res exp_ib exp_arg exp_res
712 -- Function case (rule F2)
713 tc_sub1 mb_fun act_sty act_ty@(FunTy act_arg act_res) _ exp_sty (TyVarTy exp_tv)
715 = do { cts <- readMetaTyVar exp_tv
717 Indirect ty -> tc_sub mb_fun act_sty act_ty True exp_sty ty
718 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
719 ; tc_sub_funs mb_fun act_arg act_res True arg_ty res_ty } }
721 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
722 mk_res_ty other = panic "TcUnify.mk_res_ty3"
723 fun_kinds = [argTypeKind, openTypeKind]
725 -- Everything else: defer to boxy matching
726 tc_sub1 mb_fun act_sty actual_ty exp_ib exp_sty expected_ty
727 = do { addErrCtxtM (subCtxt mb_fun act_sty exp_sty) $
728 u_tys True False act_sty actual_ty exp_ib exp_sty expected_ty
729 ; return idCoercion }
732 -----------------------------------
733 tc_sub_funs mb_fun act_arg act_res exp_ib exp_arg exp_res
734 = do { uTys False act_arg exp_ib exp_arg
735 ; co_fn_res <- tc_sub mb_fun act_res act_res exp_ib exp_res exp_res
736 ; wrapFunResCoercion [exp_arg] co_fn_res }
738 -----------------------------------
740 :: [TcType] -- Type of args
741 -> ExprCoFn -- HsExpr a -> HsExpr b
742 -> TcM ExprCoFn -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
743 wrapFunResCoercion arg_tys co_fn_res
744 | isIdCoercion co_fn_res = return idCoercion
745 | null arg_tys = return co_fn_res
747 = do { us <- newUniqueSupply
748 ; let arg_ids = zipWith (mkSysLocal FSLIT("sub")) (uniqsFromSupply us) arg_tys
749 ; return (CoLams arg_ids (co_fn_res <.> (CoApps CoHole arg_ids))) }
754 %************************************************************************
756 \subsection{Generalisation}
758 %************************************************************************
761 tcGen :: BoxySigmaType -- expected_ty
762 -> TcTyVarSet -- Extra tyvars that the universally
763 -- quantified tyvars of expected_ty
764 -- must not be unified
765 -> (BoxyRhoType -> TcM result) -- spec_ty
766 -> TcM (ExprCoFn, result)
767 -- The expression has type: spec_ty -> expected_ty
769 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
770 -- If not, the call is a no-op
771 = do { -- We want the GenSkol info in the skolemised type variables to
772 -- mention the *instantiated* tyvar names, so that we get a
773 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
774 -- Hence the tiresome but innocuous fixM
775 ((forall_tvs, theta, rho_ty), skol_info) <- fixM (\ ~(_, skol_info) ->
776 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
777 ; span <- getSrcSpanM
778 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty) span
779 ; return ((forall_tvs, theta, rho_ty), skol_info) })
782 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
783 text "expected_ty" <+> ppr expected_ty,
784 text "inst ty" <+> ppr forall_tvs <+> ppr theta <+> ppr rho_ty,
785 text "free_tvs" <+> ppr free_tvs,
786 text "forall_tvs" <+> ppr forall_tvs])
789 -- Type-check the arg and unify with poly type
790 ; (result, lie) <- getLIE (thing_inside rho_ty)
792 -- Check that the "forall_tvs" havn't been constrained
793 -- The interesting bit here is that we must include the free variables
794 -- of the expected_ty. Here's an example:
795 -- runST (newVar True)
796 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
797 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
798 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
799 -- So now s' isn't unconstrained because it's linked to a.
800 -- Conclusion: include the free vars of the expected_ty in the
801 -- list of "free vars" for the signature check.
803 ; dicts <- newDicts (SigOrigin skol_info) theta
804 ; inst_binds <- tcSimplifyCheck sig_msg forall_tvs dicts lie
806 ; checkSigTyVarsWrt free_tvs forall_tvs
807 ; traceTc (text "tcGen:done")
810 -- This HsLet binds any Insts which came out of the simplification.
811 -- It's a bit out of place here, but using AbsBind involves inventing
812 -- a couple of new names which seems worse.
813 dict_ids = map instToId dicts
814 co_fn = CoTyLams forall_tvs $ CoLams dict_ids $ CoLet inst_binds CoHole
815 ; returnM (co_fn, result) }
817 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
818 sig_msg = ptext SLIT("expected type of an expression")
823 %************************************************************************
827 %************************************************************************
829 The exported functions are all defined as versions of some
830 non-exported generic functions.
833 boxyUnify :: BoxyType -> BoxyType -> TcM ()
834 -- Acutal and expected, respectively
836 = addErrCtxtM (unifyCtxt ty1 ty2) $
837 uTysOuter False ty1 False ty2
840 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM ()
841 -- Arguments should have equal length
842 -- Acutal and expected types
843 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
846 unifyType :: TcTauType -> TcTauType -> TcM ()
847 -- No boxes expected inside these types
848 -- Acutal and expected types
849 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
850 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
851 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
852 addErrCtxtM (unifyCtxt ty1 ty2) $
853 uTysOuter True ty1 True ty2
856 unifyPred :: PredType -> PredType -> TcM ()
857 -- Acutal and expected types
858 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
859 uPred True True p1 True p2
861 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
862 -- Acutal and expected types
863 unifyTheta theta1 theta2
864 = do { checkTc (equalLength theta1 theta2)
865 (ptext SLIT("Contexts differ in length"))
866 ; uList unifyPred theta1 theta2 }
869 uList :: (a -> a -> TcM ())
870 -> [a] -> [a] -> TcM ()
871 -- Unify corresponding elements of two lists of types, which
872 -- should be f equal length. We charge down the list explicitly so that
873 -- we can complain if their lengths differ.
874 uList unify [] [] = return ()
875 uList unify (ty1:tys1) (ty2:tys2) = do { unify ty1 ty2; uList unify tys1 tys2 }
876 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
879 @unifyTypeList@ takes a single list of @TauType@s and unifies them
880 all together. It is used, for example, when typechecking explicit
881 lists, when all the elts should be of the same type.
884 unifyTypeList :: [TcTauType] -> TcM ()
885 unifyTypeList [] = returnM ()
886 unifyTypeList [ty] = returnM ()
887 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
888 ; unifyTypeList tys }
891 %************************************************************************
893 \subsection[Unify-uTys]{@uTys@: getting down to business}
895 %************************************************************************
897 @uTys@ is the heart of the unifier. Each arg happens twice, because
898 we want to report errors in terms of synomyms if poss. The first of
899 the pair is used in error messages only; it is always the same as the
900 second, except that if the first is a synonym then the second may be a
901 de-synonym'd version. This way we get better error messages.
903 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
906 type InBox = Bool -- True <=> we are inside a box
907 -- False <=> we are outside a box
908 -- The importance of this is that if we get "filled-box meets
909 -- filled-box", we'll look into the boxes and unify... but
910 -- we must not allow polytypes. But if we are in a box on
911 -- just one side, then we can allow polytypes
913 type Outer = Bool -- True <=> this is the outer level of a unification
914 -- so that the types being unified are the
915 -- very ones we began with, not some sub
916 -- component or synonym expansion
917 -- The idea is that if Outer is true then unifyMisMatch should
918 -- pop the context to remove the "Expected/Acutal" context
921 :: InBox -> TcType -- ty1 is the *expected* type
922 -> InBox -> TcType -- ty2 is the *actual* type
924 uTysOuter nb1 ty1 nb2 ty2 = u_tys True nb1 ty1 ty1 nb2 ty2 ty2
925 uTys nb1 ty1 nb2 ty2 = u_tys False nb1 ty1 ty1 nb2 ty2 ty2
929 uTys_s :: InBox -> [TcType] -- ty1 is the *actual* types
930 -> InBox -> [TcType] -- ty2 is the *expected* types
932 uTys_s nb1 [] nb2 [] = returnM ()
933 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { uTys nb1 ty1 nb2 ty2
934 ; uTys_s nb1 tys1 nb2 tys2 }
935 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
939 -> InBox -> TcType -> TcType -- ty1 is the *actual* type
940 -> InBox -> TcType -> TcType -- ty2 is the *expected* type
943 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
947 -- Always expand synonyms (see notes at end)
948 -- (this also throws away FTVs)
950 | Just ty1' <- tcView ty1 = go False ty1' ty2
951 | Just ty2' <- tcView ty2 = go False ty1 ty2'
953 -- Variables; go for uVar
954 go outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
955 go outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
956 -- "True" means args swapped
958 go outer (PredTy p1) (PredTy p2) = uPred outer nb1 p1 nb2 p2
960 -- Type constructors must match
961 go _ (TyConApp con1 tys1) (TyConApp con2 tys2)
962 | con1 == con2 = uTys_s nb1 tys1 nb2 tys2
963 -- See Note [TyCon app]
965 -- Functions; just check the two parts
966 go _ (FunTy fun1 arg1) (FunTy fun2 arg2)
967 = do { uTys nb1 fun1 nb2 fun2
968 ; uTys nb1 arg1 nb2 arg2 }
970 -- Applications need a bit of care!
971 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
972 -- NB: we've already dealt with type variables and Notes,
973 -- so if one type is an App the other one jolly well better be too
974 go outer (AppTy s1 t1) ty2
975 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
976 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
978 -- Now the same, but the other way round
979 -- Don't swap the types, because the error messages get worse
980 go outer ty1 (AppTy s2 t2)
981 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
982 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
984 go _ ty1@(ForAllTy _ _) ty2@(ForAllTy _ _)
985 | length tvs1 == length tvs2
986 = do { tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
987 ; let tys = mkTyVarTys tvs
988 in_scope = mkInScopeSet (mkVarSet tvs)
989 subst1 = mkTvSubst in_scope (zipTyEnv tvs1 tys)
990 subst2 = mkTvSubst in_scope (zipTyEnv tvs2 tys)
991 ; uTys nb1 (substTy subst1 body1) nb2 (substTy subst2 body2)
993 -- If both sides are inside a box, we are in a "box-meets-box"
994 -- situation, and we should not have a polytype at all.
995 -- If we get here we have two boxes, already filled with
996 -- the same polytype... but it should be a monotype.
997 -- This check comes last, because the error message is
998 -- extremely unhelpful.
999 ; ifM (nb1 && nb2) (notMonoType ty1)
1002 (tvs1, body1) = tcSplitForAllTys ty1
1003 (tvs2, body2) = tcSplitForAllTys ty2
1005 -- Anything else fails
1006 go outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
1009 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
1010 | n1 == n2 = uTys nb1 t1 nb2 t2
1011 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
1012 | c1 == c2 = uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
1013 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
1018 When we find two TyConApps, the argument lists are guaranteed equal
1019 length. Reason: intially the kinds of the two types to be unified is
1020 the same. The only way it can become not the same is when unifying two
1021 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
1022 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
1023 which we do, that ensures that f1,f2 have the same kind; and that
1024 means a1,a2 have the same kind. And now the argument repeats.
1029 If you are tempted to make a short cut on synonyms, as in this
1033 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
1034 -- NO = if (con1 == con2) then
1035 -- NO -- Good news! Same synonym constructors, so we can shortcut
1036 -- NO -- by unifying their arguments and ignoring their expansions.
1037 -- NO unifyTypepeLists args1 args2
1039 -- NO -- Never mind. Just expand them and try again
1043 then THINK AGAIN. Here is the whole story, as detected and reported
1044 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1046 Here's a test program that should detect the problem:
1050 x = (1 :: Bogus Char) :: Bogus Bool
1053 The problem with [the attempted shortcut code] is that
1057 is not a sufficient condition to be able to use the shortcut!
1058 You also need to know that the type synonym actually USES all
1059 its arguments. For example, consider the following type synonym
1060 which does not use all its arguments.
1065 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1066 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1067 would fail, even though the expanded forms (both \tr{Int}) should
1070 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1071 unnecessarily bind \tr{t} to \tr{Char}.
1073 ... You could explicitly test for the problem synonyms and mark them
1074 somehow as needing expansion, perhaps also issuing a warning to the
1079 %************************************************************************
1081 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1083 %************************************************************************
1085 @uVar@ is called when at least one of the types being unified is a
1086 variable. It does {\em not} assume that the variable is a fixed point
1087 of the substitution; rather, notice that @uVar@ (defined below) nips
1088 back into @uTys@ if it turns out that the variable is already bound.
1092 -> Bool -- False => tyvar is the "expected"
1093 -- True => ty is the "expected" thing
1095 -> InBox -- True <=> definitely no boxes in t2
1096 -> TcTauType -> TcTauType -- printing and real versions
1099 uVar outer swapped tv1 nb2 ps_ty2 ty2
1100 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1101 | otherwise = brackets (equals <+> ppr ty2)
1102 ; traceTc (text "uVar" <+> ppr swapped <+>
1103 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1104 nest 2 (ptext SLIT(" :=: ")),
1105 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1106 ; details <- lookupTcTyVar tv1
1109 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1110 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1111 -- The 'True' here says that ty1 is now inside a box
1112 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1116 uUnfilledVar :: Outer
1117 -> Bool -- Args are swapped
1118 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1119 -> TcTauType -> TcTauType -- Type 2
1121 -- Invariant: tyvar 1 is not unified with anything
1123 uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1124 | Just ty2' <- tcView ty2
1125 = -- Expand synonyms; ignore FTVs
1126 uUnfilledVar False swapped tv1 details1 ps_ty2 ty2'
1128 uUnfilledVar outer swapped tv1 details1 ps_ty2 (TyVarTy tv2)
1129 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1131 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1132 -- this is box-meets-box, so fill in with a tau-type
1133 -> do { tau_tv <- tcInstTyVar tv1
1134 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv) }
1135 other -> returnM () -- No-op
1137 -- Distinct type variables
1139 = do { lookup2 <- lookupTcTyVar tv2
1141 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
1142 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1145 uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2 -- ty2 is not a type variable
1147 MetaTv (SigTv _) ref1 -> mis_match -- Can't update a skolem with a non-type-variable
1148 MetaTv info ref1 -> uMetaVar swapped tv1 info ref1 ps_ty2 non_var_ty2
1149 skolem_details -> mis_match
1151 mis_match = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1155 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1158 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1159 -- ty2 is not a type variable
1161 uMetaVar swapped tv1 BoxTv ref1 ps_ty2 non_var_ty2
1162 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1163 -- that any boxes in ty2 are filled with monotypes
1165 -- It should not be the case that tv1 occurs in ty2
1166 -- (i.e. no occurs check should be needed), but if perchance
1167 -- it does, the unbox operation will fill it, and the DEBUG
1169 do { final_ty <- unBox ps_ty2
1171 ; meta_details <- readMutVar ref1
1172 ; case meta_details of
1173 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1174 return () -- This really should *not* happen
1177 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1179 uMetaVar swapped tv1 info1 ref1 ps_ty2 non_var_ty2
1180 = do { final_ty <- checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
1181 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1184 uUnfilledVars :: Outer
1185 -> Bool -- Args are swapped
1186 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1187 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1189 -- Invarant: The type variables are distinct,
1190 -- Neither is filled in yet
1191 -- They might be boxy or not
1193 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1194 = unifyMisMatch outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1196 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1197 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2)
1198 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1199 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
1201 -- ToDo: this function seems too long for what it acutally does!
1202 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1203 = case (info1, info2) of
1204 (BoxTv, BoxTv) -> box_meets_box
1206 -- If a box meets a TauTv, but the fomer has the smaller kind
1207 -- then we must create a fresh TauTv with the smaller kind
1208 (_, BoxTv) | k1_sub_k2 -> update_tv2
1209 | otherwise -> box_meets_box
1210 (BoxTv, _ ) | k2_sub_k1 -> update_tv1
1211 | otherwise -> box_meets_box
1213 -- Avoid SigTvs if poss
1214 (SigTv _, _ ) | k1_sub_k2 -> update_tv2
1215 (_, SigTv _) | k2_sub_k1 -> update_tv1
1217 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1218 then update_tv1 -- Same kinds
1220 | k2_sub_k1 -> update_tv1
1221 | otherwise -> kind_err
1223 -- Update the variable with least kind info
1224 -- See notes on type inference in Kind.lhs
1225 -- The "nicer to" part only applies if the two kinds are the same,
1226 -- so we can choose which to do.
1228 -- Kinds should be guaranteed ok at this point
1229 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1230 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1232 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1235 | k2_sub_k1 = fill_from tv2
1236 | otherwise = kind_err
1238 -- Update *both* tyvars with a TauTv whose name and kind
1239 -- are gotten from tv (avoid losing nice names is poss)
1240 fill_from tv = do { tv' <- tcInstTyVar tv
1241 ; let tau_ty = mkTyVarTy tv'
1242 ; updateMeta tv1 ref1 tau_ty
1243 ; updateMeta tv2 ref2 tau_ty }
1245 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1246 unifyKindMisMatch k1 k2
1250 k1_sub_k2 = k1 `isSubKind` k2
1251 k2_sub_k1 = k2 `isSubKind` k1
1253 nicer_to_update_tv1 = isSystemName (varName tv1)
1254 -- Try to update sys-y type variables in preference to ones
1255 -- gotten (say) by instantiating a polymorphic function with
1256 -- a user-written type sig
1259 checkUpdateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1260 -- Update tv1, which is flexi; occurs check is alrady done
1261 -- The 'check' version does a kind check too
1262 -- We do a sub-kind check here: we might unify (a b) with (c d)
1263 -- where b::*->* and d::*; this should fail
1265 checkUpdateMeta swapped tv1 ref1 ty2
1266 = do { checkKinds swapped tv1 ty2
1267 ; updateMeta tv1 ref1 ty2 }
1269 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1270 updateMeta tv1 ref1 ty2
1271 = ASSERT( isMetaTyVar tv1 )
1272 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
1273 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
1274 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
1275 ; writeMutVar ref1 (Indirect ty2) }
1278 checkKinds swapped tv1 ty2
1279 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
1280 -- ty2 has been zonked at this stage, which ensures that
1281 -- its kind has as much boxity information visible as possible.
1282 | tk2 `isSubKind` tk1 = returnM ()
1285 -- Either the kinds aren't compatible
1286 -- (can happen if we unify (a b) with (c d))
1287 -- or we are unifying a lifted type variable with an
1288 -- unlifted type: e.g. (id 3#) is illegal
1289 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1290 unifyKindMisMatch k1 k2
1292 (k1,k2) | swapped = (tk2,tk1)
1293 | otherwise = (tk1,tk2)
1298 checkTauTvUpdate :: TcTyVar -> TcType -> TcM TcType
1299 -- (checkTauTvUpdate tv ty)
1300 -- We are about to update the TauTv tv with ty.
1301 -- Check (a) that tv doesn't occur in ty (occurs check)
1302 -- (b) that ty is a monotype
1303 -- Furthermore, in the interest of (b), if you find an
1304 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
1306 -- Returns the (non-boxy) type to update the type variable with, or fails
1308 checkTauTvUpdate orig_tv orig_ty
1311 go (TyConApp tc tys)
1312 | isSynTyCon tc = go_syn tc tys
1313 | otherwise = do { tys' <- mappM go tys; return (TyConApp tc tys') }
1314 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
1315 go (PredTy p) = do { p' <- go_pred p; return (PredTy p') }
1316 go (FunTy arg res) = do { arg' <- go arg; res' <- go res; return (FunTy arg' res') }
1317 go (AppTy fun arg) = do { fun' <- go fun; arg' <- go arg; return (mkAppTy fun' arg') }
1318 -- NB the mkAppTy; we might have instantiated a
1319 -- type variable to a type constructor, so we need
1320 -- to pull the TyConApp to the top.
1321 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
1324 | orig_tv == tv = occurCheck tv orig_ty -- (a)
1325 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
1326 | otherwise = return (TyVarTy tv)
1327 -- Ordinary (non Tc) tyvars
1328 -- occur inside quantified types
1330 go_pred (ClassP c tys) = do { tys' <- mapM go tys; return (ClassP c tys') }
1331 go_pred (IParam n ty) = do { ty' <- go ty; return (IParam n ty') }
1333 go_tyvar tv (SkolemTv _) = return (TyVarTy tv)
1334 go_tyvar tv (MetaTv box ref)
1335 = do { cts <- readMutVar ref
1337 Indirect ty -> go ty
1338 Flexi -> case box of
1339 BoxTv -> fillBoxWithTau tv ref
1340 other -> return (TyVarTy tv)
1343 -- go_syn is called for synonyms only
1344 -- See Note [Type synonyms and the occur check]
1346 | not (isTauTyCon tc)
1347 = notMonoType orig_ty -- (b) again
1349 = do { (msgs, mb_tys') <- tryTc (mapM go tys)
1351 Just tys' -> return (TyConApp tc tys')
1352 -- Retain the synonym (the common case)
1353 Nothing -> go (expectJust "checkTauTvUpdate"
1354 (tcView (TyConApp tc tys)))
1355 -- Try again, expanding the synonym
1358 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
1359 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
1360 -- tau-type meta-variable, whose print-name is the same as tv
1361 -- Choosing the same name is good: when we instantiate a function
1362 -- we allocate boxy tyvars with the same print-name as the quantified
1363 -- tyvar; and then we often fill the box with a tau-tyvar, and again
1364 -- we want to choose the same name.
1365 fillBoxWithTau tv ref
1366 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
1367 ; let tau = mkTyVarTy tv' -- name of the type variable
1368 ; writeMutVar ref (Indirect tau)
1372 Note [Type synonyms and the occur check]
1373 ~~~~~~~~~~~~~~~~~~~~
1374 Basically we want to update tv1 := ps_ty2
1375 because ps_ty2 has type-synonym info, which improves later error messages
1380 f :: (A a -> a -> ()) -> ()
1384 x = f (\ x p -> p x)
1386 In the application (p x), we try to match "t" with "A t". If we go
1387 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
1388 an infinite loop later.
1389 But we should not reject the program, because A t = ().
1390 Rather, we should bind t to () (= non_var_ty2).
1393 stripBoxyType :: BoxyType -> TcM TcType
1394 -- Strip all boxes from the input type, returning a non-boxy type.
1395 -- It's fine for there to be a polytype inside a box (c.f. unBox)
1396 -- All of the boxes should have been filled in by now;
1397 -- hence we return a TcType
1398 stripBoxyType ty = zonkType strip_tv ty
1400 strip_tv tv = ASSERT( not (isBoxyTyVar tv) ) return (TyVarTy tv)
1401 -- strip_tv will be called for *Flexi* meta-tyvars
1402 -- There should not be any Boxy ones; hence the ASSERT
1404 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1405 -- Subtle... we must zap the boxy res_ty
1406 -- to kind * before using it to instantiate a LitInst
1407 -- Calling unBox instead doesn't do the job, because the box
1408 -- often has an openTypeKind, and we don't want to instantiate
1410 zapToMonotype res_ty
1411 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1412 ; boxyUnify res_tau res_ty
1415 unBox :: BoxyType -> TcM TcType
1416 -- unBox implements the judgement
1418 -- with input s', and result s
1420 -- It removes all boxes from the input type, returning a non-boxy type.
1421 -- A filled box in the type can only contain a monotype; unBox fails if not
1422 -- The type can have empty boxes, which unBox fills with a monotype
1424 -- Compare this wth checkTauTvUpdate
1426 -- For once, it's safe to treat synonyms as opaque!
1428 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1429 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1430 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1431 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1432 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1433 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1434 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1436 | isTcTyVar tv -- It's a boxy type variable
1437 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1438 = do { cts <- readMutVar ref -- under nested quantifiers
1440 Flexi -> fillBoxWithTau tv ref
1441 Indirect ty -> do { non_boxy_ty <- unBox ty
1442 ; if isTauTy non_boxy_ty
1443 then return non_boxy_ty
1444 else notMonoType non_boxy_ty }
1446 | otherwise -- Skolems, and meta-tau-variables
1447 = return (TyVarTy tv)
1449 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1450 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1455 %************************************************************************
1457 \subsection[Unify-context]{Errors and contexts}
1459 %************************************************************************
1465 unifyCtxt act_ty exp_ty tidy_env
1466 = do { act_ty' <- zonkTcType act_ty
1467 ; exp_ty' <- zonkTcType exp_ty
1468 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1469 (env2, act_ty'') = tidyOpenType env1 act_ty'
1470 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1473 mkExpectedActualMsg act_ty exp_ty
1474 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1475 text "Inferred type" <> colon <+> ppr act_ty ])
1478 -- If an error happens we try to figure out whether the function
1479 -- function has been given too many or too few arguments, and say so.
1480 subCtxt mb_fun actual_res_ty expected_res_ty tidy_env
1481 = do { exp_ty' <- zonkTcType expected_res_ty
1482 ; act_ty' <- zonkTcType actual_res_ty
1484 (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1485 (env2, act_ty'') = tidyOpenType env1 act_ty'
1486 (exp_args, _) = tcSplitFunTys exp_ty''
1487 (act_args, _) = tcSplitFunTys act_ty''
1489 len_act_args = length act_args
1490 len_exp_args = length exp_args
1492 message = case mb_fun of
1493 Just fun | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1494 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1495 other -> mkExpectedActualMsg act_ty'' exp_ty''
1496 ; return (env2, message) }
1499 wrongArgsCtxt too_many_or_few fun
1500 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1501 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1502 <+> ptext SLIT("arguments")
1505 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1506 -- tv1 and ty2 are zonked already
1509 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1510 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1512 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1513 | otherwise = (pp1, pp2)
1514 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1515 (env2, ty2') = tidyOpenType env1 ty2
1516 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1517 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1519 unifyMisMatch outer swapped ty1 ty2
1520 = do { (env, msg) <- if swapped then misMatchMsg ty1 ty2
1521 else misMatchMsg ty2 ty1
1523 -- This is the whole point of the 'outer' stuff
1524 ; if outer then popErrCtxt (failWithTcM (env, msg))
1525 else failWithTcM (env, msg)
1529 = do { env0 <- tcInitTidyEnv
1530 ; (env1, pp1, extra1) <- ppr_ty env0 ty1
1531 ; (env2, pp2, extra2) <- ppr_ty env1 ty2
1532 ; return (env2, sep [sep [ptext SLIT("Couldn't match expected type") <+> pp1,
1533 nest 7 (ptext SLIT("against inferred type") <+> pp2)],
1534 nest 2 extra1, nest 2 extra2]) }
1536 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
1538 = do { ty' <- zonkTcType ty
1539 ; let (env1,tidy_ty) = tidyOpenType env ty'
1540 simple_result = (env1, quotes (ppr tidy_ty), empty)
1543 | isSkolemTyVar tv || isSigTyVar tv
1544 -> return (env2, pp_rigid tv', pprSkolTvBinding tv')
1545 | otherwise -> return simple_result
1547 (env2, tv') = tidySkolemTyVar env1 tv
1548 other -> return simple_result }
1550 pp_rigid tv = quotes (ppr tv) <+> parens (ptext SLIT("a rigid variable"))
1554 = do { ty' <- zonkTcType ty
1555 ; env0 <- tcInitTidyEnv
1556 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1557 msg = ptext SLIT("Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1558 ; failWithTcM (env1, msg) }
1561 = do { env0 <- tcInitTidyEnv
1562 ; ty' <- zonkTcType ty
1563 ; let (env1, tidy_tyvar) = tidyOpenTyVar env0 tyvar
1564 (env2, tidy_ty) = tidyOpenType env1 ty'
1565 extra = sep [ppr tidy_tyvar, char '=', ppr tidy_ty]
1566 ; failWithTcM (env2, hang msg 2 extra) }
1568 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
1572 %************************************************************************
1576 %************************************************************************
1578 Unifying kinds is much, much simpler than unifying types.
1581 unifyKind :: TcKind -- Expected
1584 unifyKind LiftedTypeKind LiftedTypeKind = returnM ()
1585 unifyKind UnliftedTypeKind UnliftedTypeKind = returnM ()
1587 unifyKind OpenTypeKind k2 | isOpenTypeKind k2 = returnM ()
1588 unifyKind ArgTypeKind k2 | isArgTypeKind k2 = returnM ()
1589 -- Respect sub-kinding
1591 unifyKind (FunKind a1 r1) (FunKind a2 r2)
1592 = do { unifyKind a2 a1; unifyKind r1 r2 }
1593 -- Notice the flip in the argument,
1594 -- so that the sub-kinding works right
1596 unifyKind (KindVar kv1) k2 = uKVar False kv1 k2
1597 unifyKind k1 (KindVar kv2) = uKVar True kv2 k1
1598 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1600 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1601 unifyKinds [] [] = returnM ()
1602 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1604 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1607 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1608 uKVar swapped kv1 k2
1609 = do { mb_k1 <- readKindVar kv1
1611 Nothing -> uUnboundKVar swapped kv1 k2
1612 Just k1 | swapped -> unifyKind k2 k1
1613 | otherwise -> unifyKind k1 k2 }
1616 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1617 uUnboundKVar swapped kv1 k2@(KindVar kv2)
1618 | kv1 == kv2 = returnM ()
1619 | otherwise -- Distinct kind variables
1620 = do { mb_k2 <- readKindVar kv2
1622 Just k2 -> uUnboundKVar swapped kv1 k2
1623 Nothing -> writeKindVar kv1 k2 }
1625 uUnboundKVar swapped kv1 non_var_k2
1626 = do { k2' <- zonkTcKind non_var_k2
1627 ; kindOccurCheck kv1 k2'
1628 ; k2'' <- kindSimpleKind swapped k2'
1629 -- KindVars must be bound only to simple kinds
1630 -- Polarities: (kindSimpleKind True ?) succeeds
1631 -- returning *, corresponding to unifying
1634 ; writeKindVar kv1 k2'' }
1637 kindOccurCheck kv1 k2 -- k2 is zonked
1638 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1640 not_in (KindVar kv2) = kv1 /= kv2
1641 not_in (FunKind a2 r2) = not_in a2 && not_in r2
1644 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1645 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1646 -- If the flag is False, it requires k <: sk
1647 -- E.g. kindSimpleKind False ?? = *
1648 -- What about (kv -> *) :=: ?? -> *
1649 kindSimpleKind orig_swapped orig_kind
1650 = go orig_swapped orig_kind
1652 go sw (FunKind k1 k2) = do { k1' <- go (not sw) k1
1654 ; return (FunKind k1' k2') }
1655 go True OpenTypeKind = return liftedTypeKind
1656 go True ArgTypeKind = return liftedTypeKind
1657 go sw LiftedTypeKind = return liftedTypeKind
1658 go sw UnliftedTypeKind = return unliftedTypeKind
1659 go sw k@(KindVar _) = return k -- KindVars are always simple
1660 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1661 <+> ppr orig_swapped <+> ppr orig_kind)
1662 -- I think this can't actually happen
1664 -- T v = MkT v v must be a type
1665 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1668 kindOccurCheckErr tyvar ty
1669 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1670 2 (sep [ppr tyvar, char '=', ppr ty])
1672 unifyKindMisMatch ty1 ty2
1673 = zonkTcKind ty1 `thenM` \ ty1' ->
1674 zonkTcKind ty2 `thenM` \ ty2' ->
1676 msg = hang (ptext SLIT("Couldn't match kind"))
1677 2 (sep [quotes (ppr ty1'),
1678 ptext SLIT("against"),
1685 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1686 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1688 unifyFunKind (KindVar kvar)
1689 = readKindVar kvar `thenM` \ maybe_kind ->
1691 Just fun_kind -> unifyFunKind fun_kind
1692 Nothing -> do { arg_kind <- newKindVar
1693 ; res_kind <- newKindVar
1694 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1695 ; returnM (Just (arg_kind,res_kind)) }
1697 unifyFunKind (FunKind arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1698 unifyFunKind other = returnM Nothing
1701 %************************************************************************
1705 %************************************************************************
1707 ---------------------------
1708 -- We would like to get a decent error message from
1709 -- (a) Under-applied type constructors
1710 -- f :: (Maybe, Maybe)
1711 -- (b) Over-applied type constructors
1712 -- f :: Int x -> Int x
1716 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1717 -- A fancy wrapper for 'unifyKind', which tries
1718 -- to give decent error messages.
1719 checkExpectedKind ty act_kind exp_kind
1720 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1723 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1725 Just r -> returnM () ; -- Unification succeeded
1728 -- So there's definitely an error
1729 -- Now to find out what sort
1730 zonkTcKind exp_kind `thenM` \ exp_kind ->
1731 zonkTcKind act_kind `thenM` \ act_kind ->
1733 tcInitTidyEnv `thenM` \ env0 ->
1734 let (exp_as, _) = splitKindFunTys exp_kind
1735 (act_as, _) = splitKindFunTys act_kind
1736 n_exp_as = length exp_as
1737 n_act_as = length act_as
1739 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1740 (env2, tidy_act_kind) = tidyKind env1 act_kind
1742 err | n_exp_as < n_act_as -- E.g. [Maybe]
1743 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1745 -- Now n_exp_as >= n_act_as. In the next two cases,
1746 -- n_exp_as == 0, and hence so is n_act_as
1747 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1748 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1749 <+> ptext SLIT("is unlifted")
1751 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1752 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1753 <+> ptext SLIT("is lifted")
1755 | otherwise -- E.g. Monad [Int]
1756 = ptext SLIT("Kind mis-match")
1758 more_info = sep [ ptext SLIT("Expected kind") <+>
1759 quotes (pprKind tidy_exp_kind) <> comma,
1760 ptext SLIT("but") <+> quotes (ppr ty) <+>
1761 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1763 failWithTcM (env2, err $$ more_info)
1767 %************************************************************************
1769 \subsection{Checking signature type variables}
1771 %************************************************************************
1773 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1774 are not mentioned in the environment. In particular:
1776 (a) Not mentioned in the type of a variable in the envt
1777 eg the signature for f in this:
1783 Here, f is forced to be monorphic by the free occurence of x.
1785 (d) Not (unified with another type variable that is) in scope.
1786 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1787 when checking the expression type signature, we find that
1788 even though there is nothing in scope whose type mentions r,
1789 nevertheless the type signature for the expression isn't right.
1791 Another example is in a class or instance declaration:
1793 op :: forall b. a -> b
1795 Here, b gets unified with a
1797 Before doing this, the substitution is applied to the signature type variable.
1800 checkSigTyVars :: [TcTyVar] -> TcM ()
1801 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1803 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1804 -- The extra_tvs can include boxy type variables;
1805 -- e.g. TcMatches.tcCheckExistentialPat
1806 checkSigTyVarsWrt extra_tvs sig_tvs
1807 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1808 ; check_sig_tyvars extra_tvs' sig_tvs }
1811 :: TcTyVarSet -- Global type variables. The universally quantified
1812 -- tyvars should not mention any of these
1813 -- Guaranteed already zonked.
1814 -> [TcTyVar] -- Universally-quantified type variables in the signature
1815 -- Guaranteed to be skolems
1817 check_sig_tyvars extra_tvs []
1819 check_sig_tyvars extra_tvs sig_tvs
1820 = ASSERT( all isSkolemTyVar sig_tvs )
1821 do { gbl_tvs <- tcGetGlobalTyVars
1822 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1823 text "gbl_tvs" <+> ppr gbl_tvs,
1824 text "extra_tvs" <+> ppr extra_tvs]))
1826 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1827 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1828 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1831 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1832 -> [TcTyVar] -- The possibly-escaping type variables
1833 -> [TcTyVar] -- The zonked versions thereof
1835 -- Complain about escaping type variables
1836 -- We pass a list of type variables, at least one of which
1837 -- escapes. The first list contains the original signature type variable,
1838 -- while the second contains the type variable it is unified to (usually itself)
1839 bleatEscapedTvs globals sig_tvs zonked_tvs
1840 = do { env0 <- tcInitTidyEnv
1841 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1842 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1844 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1845 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1847 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1849 check (tidy_env, msgs) (sig_tv, zonked_tv)
1850 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1852 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1853 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1855 -----------------------
1856 escape_msg sig_tv zonked_tv globs
1858 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1859 nest 2 (vcat globs)]
1861 = msg <+> ptext SLIT("escapes")
1862 -- Sigh. It's really hard to give a good error message
1863 -- all the time. One bad case is an existential pattern match.
1864 -- We rely on the "When..." context to help.
1866 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1868 | sig_tv == zonked_tv = empty
1869 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1872 These two context are used with checkSigTyVars
1875 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1876 -> TidyEnv -> TcM (TidyEnv, Message)
1877 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1878 = zonkTcType sig_tau `thenM` \ actual_tau ->
1880 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1881 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1882 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1883 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1884 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1886 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),