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 mkCoLams, mkCoTyLams, mkCoApps )
30 import TypeRep ( Type(..), PredType(..) )
32 import TcMType ( lookupTcTyVar, LookupTyVarResult(..),
33 tcInstSkolType, tcInstBoxyTyVar, newKindVar, newMetaTyVar,
34 newBoxyTyVar, newBoxyTyVarTys, readFilledBox,
35 readMetaTyVar, writeMetaTyVar, newFlexiTyVarTy,
36 tcInstSkolTyVars, tcInstTyVar,
37 zonkTcKind, zonkType, zonkTcType, zonkTcTyVarsAndFV,
38 readKindVar, writeKindVar )
39 import TcSimplify ( tcSimplifyCheck )
40 import TcEnv ( tcGetGlobalTyVars, findGlobals )
41 import TcIface ( checkWiredInTyCon )
42 import TcRnMonad -- TcType, amongst others
43 import TcType ( TcKind, TcType, TcTyVar, BoxyTyVar, TcTauType,
44 BoxySigmaType, BoxyRhoType, BoxyType,
45 TcTyVarSet, TcThetaType, TcTyVarDetails(..), BoxInfo(..),
46 SkolemInfo( GenSkol, UnkSkol ), MetaDetails(..), isImmutableTyVar,
47 pprSkolTvBinding, isTauTy, isTauTyCon, isSigmaTy,
48 mkFunTy, mkFunTys, mkTyConApp, isMetaTyVar,
49 tcSplitForAllTys, tcSplitAppTy_maybe, tcSplitFunTys, mkTyVarTys,
50 tcSplitSigmaTy, tyVarsOfType, mkPhiTy, mkTyVarTy, mkPredTy,
51 typeKind, mkForAllTys, mkAppTy, isBoxyTyVar,
52 tcView, exactTyVarsOfType,
53 tidyOpenType, tidyOpenTyVar, tidyOpenTyVars,
54 pprType, tidyKind, tidySkolemTyVar, isSkolemTyVar, isSigTyVar,
55 TvSubst, mkTvSubst, zipTyEnv, zipOpenTvSubst, emptyTvSubst,
57 lookupTyVar, extendTvSubst )
58 import Type ( Kind, SimpleKind, KindVar,
59 openTypeKind, liftedTypeKind, unliftedTypeKind,
60 mkArrowKind, defaultKind,
61 argTypeKind, isLiftedTypeKind, isUnliftedTypeKind,
62 isSubKind, pprKind, splitKindFunTys, isSubKindCon,
63 isOpenTypeKind, isArgTypeKind )
64 import TysPrim ( alphaTy, betaTy )
65 import Inst ( newDictBndrsO, instCall, instToId )
66 import TyCon ( TyCon, tyConArity, tyConTyVars, isSynTyCon )
67 import TysWiredIn ( listTyCon )
69 import Var ( Var, varName, tyVarKind, isTcTyVar, tcTyVarDetails )
72 import Name ( Name, isSystemName )
73 import ErrUtils ( Message )
74 import Maybes ( expectJust, isNothing )
75 import BasicTypes ( Arity )
76 import Util ( notNull, equalLength )
81 import TcType ( isBoxyTy, isFlexi )
85 %************************************************************************
87 \subsection{'hole' type variables}
89 %************************************************************************
92 tcInfer :: (BoxyType -> TcM a) -> TcM (a, TcType)
94 = do { box <- newBoxyTyVar openTypeKind
95 ; res <- tc_infer (mkTyVarTy box)
96 ; res_ty <- readFilledBox box -- Guaranteed filled-in by now
97 ; return (res, res_ty) }
101 %************************************************************************
105 %************************************************************************
108 subFunTys :: SDoc -- Somthing like "The function f has 3 arguments"
109 -- or "The abstraction (\x.e) takes 1 argument"
110 -> Arity -- Expected # of args
111 -> BoxyRhoType -- res_ty
112 -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
114 -- Attempt to decompse res_ty to have enough top-level arrows to
115 -- match the number of patterns in the match group
117 -- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
118 -- and the inner call to thing_inside passes args: [a1,...,an], b
119 -- then co_fn :: (a1 -> ... -> an -> b) -> res_ty
121 -- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
124 {- Error messages from subFunTys
126 The abstraction `\Just 1 -> ...' has two arguments
127 but its type `Maybe a -> a' has only one
129 The equation(s) for `f' have two arguments
130 but its type `Maybe a -> a' has only one
132 The section `(f 3)' requires 'f' to take two arguments
133 but its type `Int -> Int' has only one
135 The function 'f' is applied to two arguments
136 but its type `Int -> Int' has only one
140 subFunTys error_herald n_pats res_ty thing_inside
141 = loop n_pats [] res_ty
143 -- In 'loop', the parameter 'arg_tys' accumulates
144 -- the arg types so far, in *reverse order*
145 loop n args_so_far res_ty
146 | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
148 loop n args_so_far res_ty
149 | isSigmaTy res_ty -- Do this before checking n==0, because we
150 -- guarantee to return a BoxyRhoType, not a BoxySigmaType
151 = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet $ \ res_ty' ->
152 loop n args_so_far res_ty'
153 ; return (gen_fn <.> co_fn, res) }
155 loop 0 args_so_far res_ty
156 = do { res <- thing_inside (reverse args_so_far) res_ty
157 ; return (idCoercion, res) }
159 loop n args_so_far (FunTy arg_ty res_ty)
160 = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
161 ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
162 ; return (co_fn', res) }
164 -- res_ty might have a type variable at the head, such as (a b c),
165 -- in which case we must fill in with (->). Simplest thing to do
166 -- is to use boxyUnify, but we catch failure and generate our own
167 -- error message on failure
168 loop n args_so_far res_ty@(AppTy _ _)
169 = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
170 ; (_, mb_unit) <- tryTcErrs $ boxyUnify res_ty (FunTy arg_ty' res_ty')
171 ; if isNothing mb_unit then bale_out args_so_far
172 else loop n args_so_far (FunTy arg_ty' res_ty') }
174 loop n args_so_far (TyVarTy tv)
175 | not (isImmutableTyVar tv)
176 = do { cts <- readMetaTyVar tv
178 Indirect ty -> loop n args_so_far ty
179 Flexi -> do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
180 ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty
181 ; return (idCoercion, res) } }
183 mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
184 mk_res_ty [] = panic "TcUnify.mk_res_ty1"
185 kinds = openTypeKind : take n (repeat argTypeKind)
186 -- Note argTypeKind: the args can have an unboxed type,
187 -- but not an unboxed tuple.
189 loop n args_so_far res_ty = bale_out args_so_far
192 = do { env0 <- tcInitTidyEnv
193 ; res_ty' <- zonkTcType res_ty
194 ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
195 ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
197 mk_msg res_ty n_actual
198 = error_herald <> comma $$
199 sep [ptext SLIT("but its type") <+> quotes (pprType res_ty),
200 if n_actual == 0 then ptext SLIT("has none")
201 else ptext SLIT("has only") <+> speakN n_actual]
205 ----------------------
206 boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
207 -> BoxyRhoType -- Expected type (T a b c)
208 -> TcM [BoxySigmaType] -- Element types, a b c
209 -- It's used for wired-in tycons, so we call checkWiredInTyCOn
210 -- Precondition: never called with FunTyCon
211 -- Precondition: input type :: *
213 boxySplitTyConApp tc orig_ty
214 = do { checkWiredInTyCon tc
215 ; loop (tyConArity tc) [] orig_ty }
217 loop n_req args_so_far ty
218 | Just ty' <- tcView ty = loop n_req args_so_far ty'
220 loop n_req args_so_far (TyConApp tycon args)
222 = ASSERT( n_req == length args) -- ty::*
223 return (args ++ args_so_far)
225 loop n_req args_so_far (AppTy fun arg)
226 = loop (n_req - 1) (arg:args_so_far) fun
228 loop n_req args_so_far (TyVarTy tv)
229 | not (isImmutableTyVar tv)
230 = do { cts <- readMetaTyVar tv
232 Indirect ty -> loop n_req args_so_far ty
233 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
234 ; return (arg_tys ++ args_so_far) }
237 mk_res_ty arg_tys' = mkTyConApp tc arg_tys'
238 arg_kinds = map tyVarKind (take n_req (tyConTyVars tc))
240 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc))) orig_ty
242 ----------------------
243 boxySplitListTy :: BoxyRhoType -> TcM BoxySigmaType -- Special case for lists
244 boxySplitListTy exp_ty = do { [elt_ty] <- boxySplitTyConApp listTyCon exp_ty
248 ----------------------
249 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
250 -> TcM (BoxySigmaType, BoxySigmaType) -- Returns m, a
251 -- Assumes (m: * -> k), where k is the kind of the incoming type
252 -- If the incoming type is boxy, then so are the result types; and vice versa
254 boxySplitAppTy orig_ty
258 | Just ty' <- tcView ty = loop ty'
261 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
262 = return (fun_ty, arg_ty)
265 | not (isImmutableTyVar tv)
266 = do { cts <- readMetaTyVar tv
268 Indirect ty -> loop ty
269 Flexi -> do { [fun_ty,arg_ty] <- withMetaTvs tv kinds mk_res_ty
270 ; return (fun_ty, arg_ty) } }
272 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
273 mk_res_ty other = panic "TcUnify.mk_res_ty2"
274 tv_kind = tyVarKind tv
275 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
277 liftedTypeKind] -- arg type :: *
278 -- The defaultKind is a bit smelly. If you remove it,
279 -- try compiling f x = do { x }
280 -- and you'll get a kind mis-match. It smells, but
281 -- not enough to lose sleep over.
283 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
286 boxySplitFailure actual_ty expected_ty
287 = unifyMisMatch False False actual_ty expected_ty
288 -- "outer" is False, so we don't pop the context
289 -- which is what we want since we have not pushed one!
293 --------------------------------
294 -- withBoxes: the key utility function
295 --------------------------------
298 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
299 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
300 -> ([BoxySigmaType] -> BoxySigmaType)
301 -- Constructs the type to assign
302 -- to the original var
303 -> TcM [BoxySigmaType] -- Return the fresh boxes
305 -- It's entirely possible for the [kind] to be empty.
306 -- For example, when pattern-matching on True,
307 -- we call boxySplitTyConApp passing a boolTyCon
309 -- Invariant: tv is still Flexi
311 withMetaTvs tv kinds mk_res_ty
313 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
314 ; let box_tys = mkTyVarTys box_tvs
315 ; writeMetaTyVar tv (mk_res_ty box_tys)
318 | otherwise -- Non-boxy meta type variable
319 = do { tau_tys <- mapM newFlexiTyVarTy kinds
320 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
321 -- Sure to be a tau-type
324 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
325 -- Allocate a *boxy* tyvar
326 withBox kind thing_inside
327 = do { box_tv <- newMetaTyVar BoxTv kind
328 ; res <- thing_inside (mkTyVarTy box_tv)
329 ; ty <- readFilledBox box_tv
334 %************************************************************************
336 Approximate boxy matching
338 %************************************************************************
341 preSubType :: [TcTyVar] -- Quantified type variables
342 -> TcTyVarSet -- Subset of quantified type variables
343 -- see Note [Pre-sub boxy]
344 -> TcType -- The rho-type part; quantified tyvars scopes over this
345 -> BoxySigmaType -- Matching type from the context
346 -> TcM [TcType] -- Types to instantiate the tyvars
347 -- Perform pre-subsumption, and return suitable types
348 -- to instantiate the quantified type varibles:
349 -- info from the pre-subsumption, if there is any
350 -- a boxy type variable otherwise
352 -- Note [Pre-sub boxy]
353 -- The 'btvs' are a subset of 'qtvs'. They are the ones we can
354 -- instantiate to a boxy type variable, because they'll definitely be
355 -- filled in later. This isn't always the case; sometimes we have type
356 -- variables mentioned in the context of the type, but not the body;
357 -- f :: forall a b. C a b => a -> a
358 -- Then we may land up with an unconstrained 'b', so we want to
359 -- instantiate it to a monotype (non-boxy) type variable
361 -- The 'qtvs' that are *neither* fixed by the pre-subsumption, *nor* are in 'btvs',
362 -- are instantiated to TauTv meta variables.
364 preSubType qtvs btvs qty expected_ty
365 = do { tys <- mapM inst_tv qtvs
366 ; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
369 pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
371 | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
372 | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
373 ; return (mkTyVarTy tv') }
374 | otherwise = do { tv' <- tcInstTyVar tv
375 ; return (mkTyVarTy tv') }
378 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
379 -> BoxyRhoType -- Type to match (note a *Rho* type)
380 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
382 -- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
383 -- "Boxy types: inference for higher rank types and impredicativity"
385 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
386 = go tmpl_tvs tmpl_ty emptyVarSet boxy_ty
388 go t_tvs t_ty b_tvs b_ty
389 | Just t_ty' <- tcView t_ty = go t_tvs t_ty' b_tvs b_ty
390 | Just b_ty' <- tcView b_ty = go t_tvs t_ty b_tvs b_ty'
392 go t_tvs (TyVarTy _) b_tvs b_ty = emptyTvSubst -- Rule S-ANY; no bindings
393 -- Rule S-ANY covers (a) type variables and (b) boxy types
394 -- in the template. Both look like a TyVarTy.
395 -- See Note [Sub-match] below
397 go t_tvs t_ty b_tvs b_ty
398 | isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
399 = go (t_tvs `delVarSetList` tvs) t_tau b_tvs b_ty -- Rule S-SPEC
400 -- Under a forall on the left, if there is shadowing,
401 -- do not bind! Hence the delVarSetList.
402 | isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
403 = go t_tvs t_ty (extendVarSetList b_tvs tvs) b_tau -- Rule S-SKOL
404 -- Add to the variables we must not bind to
405 -- NB: it's *important* to discard the theta part. Otherwise
406 -- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
407 -- and end up with a completely bogus binding (b |-> Bool), by lining
408 -- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
409 -- This pre-subsumption stuff can return too few bindings, but it
410 -- must *never* return bogus info.
412 go t_tvs (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
413 = boxy_match t_tvs arg1 b_tvs arg2 (go t_tvs res1 b_tvs res2)
414 -- Match the args, and sub-match the results
416 go t_tvs t_ty b_tvs b_ty = boxy_match t_tvs t_ty b_tvs b_ty emptyTvSubst
417 -- Otherwise defer to boxy matching
418 -- This covers TyConApp, AppTy, PredTy
425 |- head xs : <rhobox>
426 We will do a boxySubMatchType between a ~ <rhobox>
427 But we *don't* want to match [a |-> <rhobox>] because
428 (a) The box should be filled in with a rho-type, but
429 but the returned substitution maps TyVars to boxy
431 (b) In any case, the right final answer might be *either*
432 instantiate 'a' with a rho-type or a sigma type
433 head xs : Int vs head xs : forall b. b->b
434 So the matcher MUST NOT make a choice here. In general, we only
435 bind a template type variable in boxyMatchType, not in boxySubMatchType.
440 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
441 -> [BoxySigmaType] -- Type to match
442 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
444 -- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
445 -- "Boxy types: inference for higher rank types and impredicativity"
447 -- Find a *boxy* substitution that makes the template look as much
448 -- like the BoxySigmaType as possible.
449 -- It's always ok to return an empty substitution;
450 -- anything more is jam on the pudding
452 -- NB1: This is a pure, non-monadic function.
453 -- It does no unification, and cannot fail
455 -- Precondition: the arg lengths are equal
456 -- Precondition: none of the template type variables appear anywhere in the [BoxySigmaType]
460 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
461 = ASSERT( length tmpl_tys == length boxy_tys )
462 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
463 -- ToDo: add error context?
465 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
467 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
468 = boxy_match tmpl_tvs t_ty boxy_tvs b_ty $
469 boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys subst
470 boxy_match_s tmpl_tvs _ boxy_tvs _ subst
471 = panic "boxy_match_s" -- Lengths do not match
475 boxy_match :: TcTyVarSet -> TcType -- Template
476 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
477 -> BoxySigmaType -- Match against this type
481 -- The boxy_tvs argument prevents this match:
482 -- [a] forall b. a ~ forall b. b
483 -- We don't want to bind the template variable 'a'
484 -- to the quantified type variable 'b'!
486 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
487 = go orig_tmpl_ty orig_boxy_ty
490 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
491 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
493 go ty1 ty2 -- C.f. the isSigmaTy case for boxySubMatchType
495 , (tvs1, _, tau1) <- tcSplitSigmaTy ty1
496 , (tvs2, _, tau2) <- tcSplitSigmaTy ty2
497 , equalLength tvs1 tvs2
498 = boxy_match (tmpl_tvs `delVarSetList` tvs1) tau1
499 (boxy_tvs `extendVarSetList` tvs2) tau2 subst
501 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
502 | tc1 == tc2 = go_s tys1 tys2
504 go (FunTy arg1 res1) (FunTy arg2 res2)
505 = go_s [arg1,res1] [arg2,res2]
508 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
509 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
510 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
511 = go_s [s1,t1] [s2,t2]
514 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
515 , boxy_tvs `disjointVarSet` tyVarsOfType orig_boxy_ty
516 , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
517 = extendTvSubst subst tv boxy_ty'
519 = subst -- Ignore others
521 boxy_ty' = case lookupTyVar subst tv of
522 Nothing -> orig_boxy_ty
523 Just ty -> ty `boxyLub` orig_boxy_ty
525 go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
526 -- Example: Tree a ~ Maybe Int
527 -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
528 -- misleading error messages. An even more confusing case is
529 -- a -> b ~ Maybe Int
530 -- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
531 -- from this pre-matching phase.
534 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
537 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
538 -- Combine boxy information from the two types
539 -- If there is a conflict, return the first
540 boxyLub orig_ty1 orig_ty2
541 = go orig_ty1 orig_ty2
543 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
544 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
545 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
546 | tc1 == tc2, length ts1 == length ts2
547 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
549 go (TyVarTy tv1) ty2 -- This is the whole point;
550 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
553 -- Look inside type synonyms, but only if the naive version fails
554 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
555 | Just ty2' <- tcView ty1 = go ty1 ty2'
557 -- For now, we don't look inside ForAlls, PredTys
558 go ty1 ty2 = orig_ty1 -- Default
561 Note [Matching kinds]
562 ~~~~~~~~~~~~~~~~~~~~~
563 The target type might legitimately not be a sub-kind of template.
564 For example, suppose the target is simply a box with an OpenTypeKind,
565 and the template is a type variable with LiftedTypeKind.
566 Then it's ok (because the target type will later be refined).
567 We simply don't bind the template type variable.
569 It might also be that the kind mis-match is an error. For example,
570 suppose we match the template (a -> Int) against (Int# -> Int),
571 where the template type variable 'a' has LiftedTypeKind. This
572 matching function does not fail; it simply doesn't bind the template.
573 Later stuff will fail.
575 %************************************************************************
579 %************************************************************************
581 All the tcSub calls have the form
583 tcSub expected_ty offered_ty
585 offered_ty <= expected_ty
587 That is, that a value of type offered_ty is acceptable in
588 a place expecting a value of type expected_ty.
590 It returns a coercion function
591 co_fn :: offered_ty -> expected_ty
592 which takes an HsExpr of type offered_ty into one of type
597 tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM ExprCoFn -- Locally used only
598 -- (tcSub act exp) checks that
600 tcSubExp actual_ty expected_ty
601 = -- addErrCtxtM (unifyCtxt actual_ty expected_ty) $
602 -- Adding the error context here leads to some very confusing error
603 -- messages, such as "can't match foarall a. a->a with forall a. a->a"
604 -- So instead I'm adding it when moving from tc_sub to u_tys
605 traceTc (text "tcSubExp" <+> ppr actual_ty <+> ppr expected_ty) >>
606 tc_sub Nothing actual_ty actual_ty False expected_ty expected_ty
608 tcFunResTy :: Name -> BoxySigmaType -> BoxySigmaType -> TcM ExprCoFn -- Locally used only
609 tcFunResTy fun actual_ty expected_ty
610 = traceTc (text "tcFunResTy" <+> ppr actual_ty <+> ppr expected_ty) >>
611 tc_sub (Just fun) actual_ty actual_ty False expected_ty expected_ty
614 tc_sub :: Maybe Name -- Just fun => we're looking at a function result type
615 -> BoxySigmaType -- actual_ty, before expanding synonyms
616 -> BoxySigmaType -- ..and after
617 -> InBox -- True <=> expected_ty is inside a box
618 -> BoxySigmaType -- expected_ty, before
619 -> BoxySigmaType -- ..and after
621 -- The acual_ty is never inside a box
622 -- IMPORTANT pre-condition: if the args contain foralls, the bound type
623 -- variables are visible non-monadically
624 -- (i.e. tha args are sufficiently zonked)
625 -- This invariant is needed so that we can "see" the foralls, ad
626 -- e.g. in the SPEC rule where we just use splitSigmaTy
628 tc_sub mb_fun act_sty act_ty exp_ib exp_sty exp_ty
629 = tc_sub1 mb_fun act_sty act_ty exp_ib exp_sty exp_ty
630 -- This indirection is just here to make
631 -- it easy to insert a debug trace!
633 tc_sub1 mb_fun act_sty act_ty exp_ib exp_sty exp_ty
634 | Just exp_ty' <- tcView exp_ty = tc_sub mb_fun act_sty act_ty exp_ib exp_sty exp_ty'
635 tc_sub1 mb_fun act_sty act_ty exp_ib exp_sty exp_ty
636 | Just act_ty' <- tcView act_ty = tc_sub mb_fun act_sty act_ty' exp_ib exp_sty exp_ty
638 -----------------------------------
639 -- Rule SBOXY, plus other cases when act_ty is a type variable
640 -- Just defer to boxy matching
641 -- This rule takes precedence over SKOL!
642 tc_sub1 mb_fun act_sty (TyVarTy tv) exp_ib exp_sty exp_ty
643 = do { addErrCtxtM (subCtxt mb_fun act_sty exp_sty) $
644 uVar True False tv exp_ib exp_sty exp_ty
645 ; return idCoercion }
647 -----------------------------------
648 -- Skolemisation case (rule SKOL)
649 -- actual_ty: d:Eq b => b->b
650 -- expected_ty: forall a. Ord a => a->a
651 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
653 -- It is essential to do this *before* the specialisation case
654 -- Example: f :: (Eq a => a->a) -> ...
655 -- g :: Ord b => b->b
658 tc_sub1 mb_fun act_sty act_ty exp_ib exp_sty exp_ty
659 | not exp_ib, -- SKOL does not apply if exp_ty is inside a box
661 = do { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ body_exp_ty ->
662 tc_sub mb_fun act_sty act_ty False body_exp_ty body_exp_ty
663 ; return (gen_fn <.> co_fn) }
665 act_tvs = tyVarsOfType act_ty
666 -- It's really important to check for escape wrt
667 -- the free vars of both expected_ty *and* actual_ty
669 -----------------------------------
670 -- Specialisation case (rule ASPEC):
671 -- actual_ty: forall a. Ord a => a->a
672 -- expected_ty: Int -> Int
673 -- co_fn e = e Int dOrdInt
675 tc_sub1 mb_fun act_sty actual_ty exp_ib exp_sty expected_ty
676 -- Implements the new SPEC rule in the Appendix of the paper
677 -- "Boxy types: inference for higher rank types and impredicativity"
678 -- (This appendix isn't in the published version.)
679 -- The idea is to *first* do pre-subsumption, and then full subsumption
680 -- Example: forall a. a->a <= Int -> (forall b. Int)
681 -- Pre-subsumpion finds a|->Int, and that works fine, whereas
682 -- just running full subsumption would fail.
683 | isSigmaTy actual_ty
684 = do { -- Perform pre-subsumption, and instantiate
685 -- the type with info from the pre-subsumption;
686 -- boxy tyvars if pre-subsumption gives no info
687 let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
688 tau_tvs = exactTyVarsOfType tau
689 ; inst_tys <- if exp_ib then -- Inside a box, do not do clever stuff
690 do { tyvars' <- mapM tcInstBoxyTyVar tyvars
691 ; return (mkTyVarTys tyvars') }
692 else -- Outside, do clever stuff
693 preSubType tyvars tau_tvs tau expected_ty
694 ; let subst' = zipOpenTvSubst tyvars inst_tys
695 tau' = substTy subst' tau
697 -- Perform a full subsumption check
698 ; traceTc (text "tc_sub_spec" <+> vcat [ppr actual_ty,
699 ppr tyvars <+> ppr theta <+> ppr tau,
701 ; co_fn2 <- tc_sub mb_fun tau' tau' exp_ib exp_sty expected_ty
703 -- Deal with the dictionaries
704 ; co_fn1 <- instCall InstSigOrigin inst_tys (substTheta subst' theta)
705 ; return (co_fn2 <.> co_fn1) }
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 { arg_ids <- newSysLocalIds FSLIT("sub") arg_tys
748 ; return (mkCoLams arg_ids <.> co_fn_res <.> mkCoApps arg_ids) }
753 %************************************************************************
755 \subsection{Generalisation}
757 %************************************************************************
760 tcGen :: BoxySigmaType -- expected_ty
761 -> TcTyVarSet -- Extra tyvars that the universally
762 -- quantified tyvars of expected_ty
763 -- must not be unified
764 -> (BoxyRhoType -> TcM result) -- spec_ty
765 -> TcM (ExprCoFn, result)
766 -- The expression has type: spec_ty -> expected_ty
768 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
769 -- If not, the call is a no-op
770 = do { -- We want the GenSkol info in the skolemised type variables to
771 -- mention the *instantiated* tyvar names, so that we get a
772 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
773 -- Hence the tiresome but innocuous fixM
774 ((forall_tvs, theta, rho_ty), skol_info) <- fixM (\ ~(_, skol_info) ->
775 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
776 ; span <- getSrcSpanM
777 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty) span
778 ; return ((forall_tvs, theta, rho_ty), skol_info) })
781 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
782 text "expected_ty" <+> ppr expected_ty,
783 text "inst ty" <+> ppr forall_tvs <+> ppr theta <+> ppr rho_ty,
784 text "free_tvs" <+> ppr free_tvs,
785 text "forall_tvs" <+> ppr forall_tvs])
788 -- Type-check the arg and unify with poly type
789 ; (result, lie) <- getLIE (thing_inside rho_ty)
791 -- Check that the "forall_tvs" havn't been constrained
792 -- The interesting bit here is that we must include the free variables
793 -- of the expected_ty. Here's an example:
794 -- runST (newVar True)
795 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
796 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
797 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
798 -- So now s' isn't unconstrained because it's linked to a.
799 -- Conclusion: include the free vars of the expected_ty in the
800 -- list of "free vars" for the signature check.
802 ; dicts <- newDictBndrsO (SigOrigin skol_info) theta
803 ; inst_binds <- tcSimplifyCheck sig_msg forall_tvs dicts lie
805 ; checkSigTyVarsWrt free_tvs forall_tvs
806 ; traceTc (text "tcGen:done")
809 -- The CoLet binds any Insts which came out of the simplification.
810 dict_ids = map instToId dicts
811 co_fn = mkCoTyLams forall_tvs <.> mkCoLams dict_ids <.> CoLet inst_binds
812 ; returnM (co_fn, result) }
814 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
815 sig_msg = ptext SLIT("expected type of an expression")
820 %************************************************************************
824 %************************************************************************
826 The exported functions are all defined as versions of some
827 non-exported generic functions.
830 boxyUnify :: BoxyType -> BoxyType -> TcM ()
831 -- Acutal and expected, respectively
833 = addErrCtxtM (unifyCtxt ty1 ty2) $
834 uTysOuter False ty1 False ty2
837 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM ()
838 -- Arguments should have equal length
839 -- Acutal and expected types
840 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
843 unifyType :: TcTauType -> TcTauType -> TcM ()
844 -- No boxes expected inside these types
845 -- Acutal and expected types
846 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
847 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
848 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
849 addErrCtxtM (unifyCtxt ty1 ty2) $
850 uTysOuter True ty1 True ty2
853 unifyPred :: PredType -> PredType -> TcM ()
854 -- Acutal and expected types
855 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
856 uPred True True p1 True p2
858 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
859 -- Acutal and expected types
860 unifyTheta theta1 theta2
861 = do { checkTc (equalLength theta1 theta2)
862 (ptext SLIT("Contexts differ in length"))
863 ; uList unifyPred theta1 theta2 }
866 uList :: (a -> a -> TcM ())
867 -> [a] -> [a] -> TcM ()
868 -- Unify corresponding elements of two lists of types, which
869 -- should be f equal length. We charge down the list explicitly so that
870 -- we can complain if their lengths differ.
871 uList unify [] [] = return ()
872 uList unify (ty1:tys1) (ty2:tys2) = do { unify ty1 ty2; uList unify tys1 tys2 }
873 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
876 @unifyTypeList@ takes a single list of @TauType@s and unifies them
877 all together. It is used, for example, when typechecking explicit
878 lists, when all the elts should be of the same type.
881 unifyTypeList :: [TcTauType] -> TcM ()
882 unifyTypeList [] = returnM ()
883 unifyTypeList [ty] = returnM ()
884 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
885 ; unifyTypeList tys }
888 %************************************************************************
890 \subsection[Unify-uTys]{@uTys@: getting down to business}
892 %************************************************************************
894 @uTys@ is the heart of the unifier. Each arg happens twice, because
895 we want to report errors in terms of synomyms if poss. The first of
896 the pair is used in error messages only; it is always the same as the
897 second, except that if the first is a synonym then the second may be a
898 de-synonym'd version. This way we get better error messages.
900 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
903 type InBox = Bool -- True <=> we are inside a box
904 -- False <=> we are outside a box
905 -- The importance of this is that if we get "filled-box meets
906 -- filled-box", we'll look into the boxes and unify... but
907 -- we must not allow polytypes. But if we are in a box on
908 -- just one side, then we can allow polytypes
910 type Outer = Bool -- True <=> this is the outer level of a unification
911 -- so that the types being unified are the
912 -- very ones we began with, not some sub
913 -- component or synonym expansion
914 -- The idea is that if Outer is true then unifyMisMatch should
915 -- pop the context to remove the "Expected/Acutal" context
918 :: InBox -> TcType -- ty1 is the *expected* type
919 -> InBox -> TcType -- ty2 is the *actual* type
921 uTysOuter nb1 ty1 nb2 ty2 = do { traceTc (text "uTysOuter" <+> ppr ty1 <+> ppr ty2)
922 ; u_tys True nb1 ty1 ty1 nb2 ty2 ty2 }
923 uTys nb1 ty1 nb2 ty2 = do { traceTc (text "uTys" <+> ppr ty1 <+> ppr ty2)
924 ; u_tys False nb1 ty1 ty1 nb2 ty2 ty2 }
928 uTys_s :: InBox -> [TcType] -- ty1 is the *actual* types
929 -> InBox -> [TcType] -- ty2 is the *expected* types
931 uTys_s nb1 [] nb2 [] = returnM ()
932 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { uTys nb1 ty1 nb2 ty2
933 ; uTys_s nb1 tys1 nb2 tys2 }
934 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
938 -> InBox -> TcType -> TcType -- ty1 is the *actual* type
939 -> InBox -> TcType -> TcType -- ty2 is the *expected* type
942 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
946 -- Always expand synonyms (see notes at end)
947 -- (this also throws away FTVs)
949 | Just ty1' <- tcView ty1 = go False ty1' ty2
950 | Just ty2' <- tcView ty2 = go False ty1 ty2'
952 -- Variables; go for uVar
953 go outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
954 go outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
955 -- "True" means args swapped
957 go outer (PredTy p1) (PredTy p2) = uPred outer nb1 p1 nb2 p2
959 -- Type constructors must match
960 go _ (TyConApp con1 tys1) (TyConApp con2 tys2)
961 | con1 == con2 = uTys_s nb1 tys1 nb2 tys2
962 -- See Note [TyCon app]
964 -- Functions; just check the two parts
965 go _ (FunTy fun1 arg1) (FunTy fun2 arg2)
966 = do { uTys nb1 fun1 nb2 fun2
967 ; uTys nb1 arg1 nb2 arg2 }
969 -- Applications need a bit of care!
970 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
971 -- NB: we've already dealt with type variables and Notes,
972 -- so if one type is an App the other one jolly well better be too
973 go outer (AppTy s1 t1) ty2
974 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
975 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
977 -- Now the same, but the other way round
978 -- Don't swap the types, because the error messages get worse
979 go outer ty1 (AppTy s2 t2)
980 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
981 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
983 go _ ty1@(ForAllTy _ _) ty2@(ForAllTy _ _)
984 | length tvs1 == length tvs2
985 = do { tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
986 ; let tys = mkTyVarTys tvs
987 in_scope = mkInScopeSet (mkVarSet tvs)
988 subst1 = mkTvSubst in_scope (zipTyEnv tvs1 tys)
989 subst2 = mkTvSubst in_scope (zipTyEnv tvs2 tys)
990 ; uTys nb1 (substTy subst1 body1) nb2 (substTy subst2 body2)
992 -- If both sides are inside a box, we are in a "box-meets-box"
993 -- situation, and we should not have a polytype at all.
994 -- If we get here we have two boxes, already filled with
995 -- the same polytype... but it should be a monotype.
996 -- This check comes last, because the error message is
997 -- extremely unhelpful.
998 ; ifM (nb1 && nb2) (notMonoType ty1)
1001 (tvs1, body1) = tcSplitForAllTys ty1
1002 (tvs2, body2) = tcSplitForAllTys ty2
1004 -- Anything else fails
1005 go outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
1008 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
1009 | n1 == n2 = uTys nb1 t1 nb2 t2
1010 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
1011 | c1 == c2 = uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
1012 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
1017 When we find two TyConApps, the argument lists are guaranteed equal
1018 length. Reason: intially the kinds of the two types to be unified is
1019 the same. The only way it can become not the same is when unifying two
1020 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
1021 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
1022 which we do, that ensures that f1,f2 have the same kind; and that
1023 means a1,a2 have the same kind. And now the argument repeats.
1028 If you are tempted to make a short cut on synonyms, as in this
1032 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
1033 -- NO = if (con1 == con2) then
1034 -- NO -- Good news! Same synonym constructors, so we can shortcut
1035 -- NO -- by unifying their arguments and ignoring their expansions.
1036 -- NO unifyTypepeLists args1 args2
1038 -- NO -- Never mind. Just expand them and try again
1042 then THINK AGAIN. Here is the whole story, as detected and reported
1043 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1045 Here's a test program that should detect the problem:
1049 x = (1 :: Bogus Char) :: Bogus Bool
1052 The problem with [the attempted shortcut code] is that
1056 is not a sufficient condition to be able to use the shortcut!
1057 You also need to know that the type synonym actually USES all
1058 its arguments. For example, consider the following type synonym
1059 which does not use all its arguments.
1064 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1065 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1066 would fail, even though the expanded forms (both \tr{Int}) should
1069 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1070 unnecessarily bind \tr{t} to \tr{Char}.
1072 ... You could explicitly test for the problem synonyms and mark them
1073 somehow as needing expansion, perhaps also issuing a warning to the
1078 %************************************************************************
1080 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1082 %************************************************************************
1084 @uVar@ is called when at least one of the types being unified is a
1085 variable. It does {\em not} assume that the variable is a fixed point
1086 of the substitution; rather, notice that @uVar@ (defined below) nips
1087 back into @uTys@ if it turns out that the variable is already bound.
1091 -> Bool -- False => tyvar is the "expected"
1092 -- True => ty is the "expected" thing
1094 -> InBox -- True <=> definitely no boxes in t2
1095 -> TcTauType -> TcTauType -- printing and real versions
1098 uVar outer swapped tv1 nb2 ps_ty2 ty2
1099 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1100 | otherwise = brackets (equals <+> ppr ty2)
1101 ; traceTc (text "uVar" <+> ppr swapped <+>
1102 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1103 nest 2 (ptext SLIT(" <-> ")),
1104 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1105 ; details <- lookupTcTyVar tv1
1108 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1109 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1110 -- The 'True' here says that ty1 is now inside a box
1111 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1115 uUnfilledVar :: Outer
1116 -> Bool -- Args are swapped
1117 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1118 -> TcTauType -> TcTauType -- Type 2
1120 -- Invariant: tyvar 1 is not unified with anything
1122 uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1123 | Just ty2' <- tcView ty2
1124 = -- Expand synonyms; ignore FTVs
1125 uUnfilledVar False swapped tv1 details1 ps_ty2 ty2'
1127 uUnfilledVar outer swapped tv1 details1 ps_ty2 (TyVarTy tv2)
1128 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1130 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1131 -- this is box-meets-box, so fill in with a tau-type
1132 -> do { tau_tv <- tcInstTyVar tv1
1133 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv) }
1134 other -> returnM () -- No-op
1136 -- Distinct type variables
1138 = do { lookup2 <- lookupTcTyVar tv2
1140 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
1141 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1144 uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2 -- ty2 is not a type variable
1146 MetaTv (SigTv _) ref1 -> mis_match -- Can't update a skolem with a non-type-variable
1147 MetaTv info ref1 -> uMetaVar swapped tv1 info ref1 ps_ty2 non_var_ty2
1148 skolem_details -> mis_match
1150 mis_match = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1154 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1157 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1158 -- ty2 is not a type variable
1160 uMetaVar swapped tv1 BoxTv ref1 ps_ty2 non_var_ty2
1161 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1162 -- that any boxes in ty2 are filled with monotypes
1164 -- It should not be the case that tv1 occurs in ty2
1165 -- (i.e. no occurs check should be needed), but if perchance
1166 -- it does, the unbox operation will fill it, and the DEBUG
1168 do { final_ty <- unBox ps_ty2
1170 ; meta_details <- readMutVar ref1
1171 ; case meta_details of
1172 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1173 return () -- This really should *not* happen
1176 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1178 uMetaVar swapped tv1 info1 ref1 ps_ty2 non_var_ty2
1179 = do { final_ty <- checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
1180 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1183 uUnfilledVars :: Outer
1184 -> Bool -- Args are swapped
1185 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1186 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1188 -- Invarant: The type variables are distinct,
1189 -- Neither is filled in yet
1190 -- They might be boxy or not
1192 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1193 = unifyMisMatch outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1195 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1196 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2)
1197 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1198 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
1200 -- ToDo: this function seems too long for what it acutally does!
1201 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1202 = case (info1, info2) of
1203 (BoxTv, BoxTv) -> box_meets_box
1205 -- If a box meets a TauTv, but the fomer has the smaller kind
1206 -- then we must create a fresh TauTv with the smaller kind
1207 (_, BoxTv) | k1_sub_k2 -> update_tv2
1208 | otherwise -> box_meets_box
1209 (BoxTv, _ ) | k2_sub_k1 -> update_tv1
1210 | otherwise -> box_meets_box
1212 -- Avoid SigTvs if poss
1213 (SigTv _, _ ) | k1_sub_k2 -> update_tv2
1214 (_, SigTv _) | k2_sub_k1 -> update_tv1
1216 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1217 then update_tv1 -- Same kinds
1219 | k2_sub_k1 -> update_tv1
1220 | otherwise -> kind_err
1222 -- Update the variable with least kind info
1223 -- See notes on type inference in Kind.lhs
1224 -- The "nicer to" part only applies if the two kinds are the same,
1225 -- so we can choose which to do.
1227 -- Kinds should be guaranteed ok at this point
1228 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1229 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1231 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1234 | k2_sub_k1 = fill_from tv2
1235 | otherwise = kind_err
1237 -- Update *both* tyvars with a TauTv whose name and kind
1238 -- are gotten from tv (avoid losing nice names is poss)
1239 fill_from tv = do { tv' <- tcInstTyVar tv
1240 ; let tau_ty = mkTyVarTy tv'
1241 ; updateMeta tv1 ref1 tau_ty
1242 ; updateMeta tv2 ref2 tau_ty }
1244 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1245 unifyKindMisMatch k1 k2
1249 k1_sub_k2 = k1 `isSubKind` k2
1250 k2_sub_k1 = k2 `isSubKind` k1
1252 nicer_to_update_tv1 = isSystemName (varName tv1)
1253 -- Try to update sys-y type variables in preference to ones
1254 -- gotten (say) by instantiating a polymorphic function with
1255 -- a user-written type sig
1258 checkUpdateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1259 -- Update tv1, which is flexi; occurs check is alrady done
1260 -- The 'check' version does a kind check too
1261 -- We do a sub-kind check here: we might unify (a b) with (c d)
1262 -- where b::*->* and d::*; this should fail
1264 checkUpdateMeta swapped tv1 ref1 ty2
1265 = do { checkKinds swapped tv1 ty2
1266 ; updateMeta tv1 ref1 ty2 }
1268 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1269 updateMeta tv1 ref1 ty2
1270 = ASSERT( isMetaTyVar tv1 )
1271 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
1272 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
1273 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
1274 ; writeMutVar ref1 (Indirect ty2) }
1277 checkKinds swapped tv1 ty2
1278 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
1279 -- ty2 has been zonked at this stage, which ensures that
1280 -- its kind has as much boxity information visible as possible.
1281 | tk2 `isSubKind` tk1 = returnM ()
1284 -- Either the kinds aren't compatible
1285 -- (can happen if we unify (a b) with (c d))
1286 -- or we are unifying a lifted type variable with an
1287 -- unlifted type: e.g. (id 3#) is illegal
1288 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1289 unifyKindMisMatch k1 k2
1291 (k1,k2) | swapped = (tk2,tk1)
1292 | otherwise = (tk1,tk2)
1297 checkTauTvUpdate :: TcTyVar -> TcType -> TcM TcType
1298 -- (checkTauTvUpdate tv ty)
1299 -- We are about to update the TauTv tv with ty.
1300 -- Check (a) that tv doesn't occur in ty (occurs check)
1301 -- (b) that ty is a monotype
1302 -- Furthermore, in the interest of (b), if you find an
1303 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
1305 -- Returns the (non-boxy) type to update the type variable with, or fails
1307 checkTauTvUpdate orig_tv orig_ty
1310 go (TyConApp tc tys)
1311 | isSynTyCon tc = go_syn tc tys
1312 | otherwise = do { tys' <- mappM go tys; return (TyConApp tc tys') }
1313 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
1314 go (PredTy p) = do { p' <- go_pred p; return (PredTy p') }
1315 go (FunTy arg res) = do { arg' <- go arg; res' <- go res; return (FunTy arg' res') }
1316 go (AppTy fun arg) = do { fun' <- go fun; arg' <- go arg; return (mkAppTy fun' arg') }
1317 -- NB the mkAppTy; we might have instantiated a
1318 -- type variable to a type constructor, so we need
1319 -- to pull the TyConApp to the top.
1320 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
1323 | orig_tv == tv = occurCheck tv orig_ty -- (a)
1324 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
1325 | otherwise = return (TyVarTy tv)
1326 -- Ordinary (non Tc) tyvars
1327 -- occur inside quantified types
1329 go_pred (ClassP c tys) = do { tys' <- mapM go tys; return (ClassP c tys') }
1330 go_pred (IParam n ty) = do { ty' <- go ty; return (IParam n ty') }
1331 go_pred (EqPred t1 t2) = do { t1' <- go t1; t2' <- go t2; return (EqPred t1' t2') }
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 (TyConApp kc1 []) (TyConApp kc2 [])
1585 | isSubKindCon kc2 kc1 = returnM ()
1587 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1588 = do { unifyKind a2 a1; unifyKind r1 r2 }
1589 -- Notice the flip in the argument,
1590 -- so that the sub-kinding works right
1591 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1592 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1593 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1595 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1596 unifyKinds [] [] = returnM ()
1597 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1599 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1602 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1603 uKVar swapped kv1 k2
1604 = do { mb_k1 <- readKindVar kv1
1606 Flexi -> uUnboundKVar swapped kv1 k2
1607 Indirect k1 | swapped -> unifyKind k2 k1
1608 | otherwise -> unifyKind k1 k2 }
1611 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1612 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1613 | kv1 == kv2 = returnM ()
1614 | otherwise -- Distinct kind variables
1615 = do { mb_k2 <- readKindVar kv2
1617 Indirect k2 -> uUnboundKVar swapped kv1 k2
1618 Flexi -> writeKindVar kv1 k2 }
1620 uUnboundKVar swapped kv1 non_var_k2
1621 = do { k2' <- zonkTcKind non_var_k2
1622 ; kindOccurCheck kv1 k2'
1623 ; k2'' <- kindSimpleKind swapped k2'
1624 -- KindVars must be bound only to simple kinds
1625 -- Polarities: (kindSimpleKind True ?) succeeds
1626 -- returning *, corresponding to unifying
1629 ; writeKindVar kv1 k2'' }
1632 kindOccurCheck kv1 k2 -- k2 is zonked
1633 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1635 not_in (TyVarTy kv2) = kv1 /= kv2
1636 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1639 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1640 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1641 -- If the flag is False, it requires k <: sk
1642 -- E.g. kindSimpleKind False ?? = *
1643 -- What about (kv -> *) :=: ?? -> *
1644 kindSimpleKind orig_swapped orig_kind
1645 = go orig_swapped orig_kind
1647 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1649 ; return (mkArrowKind k1' k2') }
1651 | isOpenTypeKind k = return liftedTypeKind
1652 | isArgTypeKind k = return liftedTypeKind
1654 | isLiftedTypeKind k = return liftedTypeKind
1655 | isUnliftedTypeKind k = return unliftedTypeKind
1656 go sw k@(TyVarTy _) = return k -- KindVars are always simple
1657 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1658 <+> ppr orig_swapped <+> ppr orig_kind)
1659 -- I think this can't actually happen
1661 -- T v = MkT v v must be a type
1662 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1665 kindOccurCheckErr tyvar ty
1666 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1667 2 (sep [ppr tyvar, char '=', ppr ty])
1669 unifyKindMisMatch ty1 ty2
1670 = zonkTcKind ty1 `thenM` \ ty1' ->
1671 zonkTcKind ty2 `thenM` \ ty2' ->
1673 msg = hang (ptext SLIT("Couldn't match kind"))
1674 2 (sep [quotes (ppr ty1'),
1675 ptext SLIT("against"),
1682 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1683 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1685 unifyFunKind (TyVarTy kvar)
1686 = readKindVar kvar `thenM` \ maybe_kind ->
1688 Indirect fun_kind -> unifyFunKind fun_kind
1690 do { arg_kind <- newKindVar
1691 ; res_kind <- newKindVar
1692 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1693 ; returnM (Just (arg_kind,res_kind)) }
1695 unifyFunKind (FunTy arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1696 unifyFunKind other = returnM Nothing
1699 %************************************************************************
1703 %************************************************************************
1705 ---------------------------
1706 -- We would like to get a decent error message from
1707 -- (a) Under-applied type constructors
1708 -- f :: (Maybe, Maybe)
1709 -- (b) Over-applied type constructors
1710 -- f :: Int x -> Int x
1714 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1715 -- A fancy wrapper for 'unifyKind', which tries
1716 -- to give decent error messages.
1717 checkExpectedKind ty act_kind exp_kind
1718 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1721 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1723 Just r -> returnM () ; -- Unification succeeded
1726 -- So there's definitely an error
1727 -- Now to find out what sort
1728 zonkTcKind exp_kind `thenM` \ exp_kind ->
1729 zonkTcKind act_kind `thenM` \ act_kind ->
1731 tcInitTidyEnv `thenM` \ env0 ->
1732 let (exp_as, _) = splitKindFunTys exp_kind
1733 (act_as, _) = splitKindFunTys act_kind
1734 n_exp_as = length exp_as
1735 n_act_as = length act_as
1737 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1738 (env2, tidy_act_kind) = tidyKind env1 act_kind
1740 err | n_exp_as < n_act_as -- E.g. [Maybe]
1741 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1743 -- Now n_exp_as >= n_act_as. In the next two cases,
1744 -- n_exp_as == 0, and hence so is n_act_as
1745 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1746 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1747 <+> ptext SLIT("is unlifted")
1749 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1750 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1751 <+> ptext SLIT("is lifted")
1753 | otherwise -- E.g. Monad [Int]
1754 = ptext SLIT("Kind mis-match")
1756 more_info = sep [ ptext SLIT("Expected kind") <+>
1757 quotes (pprKind tidy_exp_kind) <> comma,
1758 ptext SLIT("but") <+> quotes (ppr ty) <+>
1759 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1761 failWithTcM (env2, err $$ more_info)
1765 %************************************************************************
1767 \subsection{Checking signature type variables}
1769 %************************************************************************
1771 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1772 are not mentioned in the environment. In particular:
1774 (a) Not mentioned in the type of a variable in the envt
1775 eg the signature for f in this:
1781 Here, f is forced to be monorphic by the free occurence of x.
1783 (d) Not (unified with another type variable that is) in scope.
1784 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1785 when checking the expression type signature, we find that
1786 even though there is nothing in scope whose type mentions r,
1787 nevertheless the type signature for the expression isn't right.
1789 Another example is in a class or instance declaration:
1791 op :: forall b. a -> b
1793 Here, b gets unified with a
1795 Before doing this, the substitution is applied to the signature type variable.
1798 checkSigTyVars :: [TcTyVar] -> TcM ()
1799 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1801 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1802 -- The extra_tvs can include boxy type variables;
1803 -- e.g. TcMatches.tcCheckExistentialPat
1804 checkSigTyVarsWrt extra_tvs sig_tvs
1805 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1806 ; check_sig_tyvars extra_tvs' sig_tvs }
1809 :: TcTyVarSet -- Global type variables. The universally quantified
1810 -- tyvars should not mention any of these
1811 -- Guaranteed already zonked.
1812 -> [TcTyVar] -- Universally-quantified type variables in the signature
1813 -- Guaranteed to be skolems
1815 check_sig_tyvars extra_tvs []
1817 check_sig_tyvars extra_tvs sig_tvs
1818 = ASSERT( all isSkolemTyVar sig_tvs )
1819 do { gbl_tvs <- tcGetGlobalTyVars
1820 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1821 text "gbl_tvs" <+> ppr gbl_tvs,
1822 text "extra_tvs" <+> ppr extra_tvs]))
1824 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1825 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1826 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1829 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1830 -> [TcTyVar] -- The possibly-escaping type variables
1831 -> [TcTyVar] -- The zonked versions thereof
1833 -- Complain about escaping type variables
1834 -- We pass a list of type variables, at least one of which
1835 -- escapes. The first list contains the original signature type variable,
1836 -- while the second contains the type variable it is unified to (usually itself)
1837 bleatEscapedTvs globals sig_tvs zonked_tvs
1838 = do { env0 <- tcInitTidyEnv
1839 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1840 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1842 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1843 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1845 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1847 check (tidy_env, msgs) (sig_tv, zonked_tv)
1848 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1850 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1851 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1853 -----------------------
1854 escape_msg sig_tv zonked_tv globs
1856 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1857 nest 2 (vcat globs)]
1859 = msg <+> ptext SLIT("escapes")
1860 -- Sigh. It's really hard to give a good error message
1861 -- all the time. One bad case is an existential pattern match.
1862 -- We rely on the "When..." context to help.
1864 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1866 | sig_tv == zonked_tv = empty
1867 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1870 These two context are used with checkSigTyVars
1873 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1874 -> TidyEnv -> TcM (TidyEnv, Message)
1875 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1876 = zonkTcType sig_tau `thenM` \ actual_tau ->
1878 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1879 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1880 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1881 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1882 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1884 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),