2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section{Type subsumption and unification}
8 -- Full-blown subsumption
9 tcSubExp, tcFunResTy, tcGen,
10 checkSigTyVars, checkSigTyVarsWrt, bleatEscapedTvs, sigCtxt,
12 -- Various unifications
13 unifyType, unifyTypeList, unifyTheta,
14 unifyKind, unifyKinds, unifyFunKind,
16 boxySubMatchType, boxyMatchTypes,
18 --------------------------------
20 tcInfer, subFunTys, unBox, stripBoxyType, withBox,
21 boxyUnify, boxyUnifyList, zapToMonotype,
22 boxySplitListTy, boxySplitTyConApp, boxySplitAppTy,
26 #include "HsVersions.h"
28 import HsSyn ( ExprCoFn(..), idCoercion, isIdCoercion, (<.>) )
29 import TypeRep ( Type(..), PredType(..) )
31 import TcMType ( lookupTcTyVar, LookupTyVarResult(..),
32 tcInstSkolType, newKindVar, newMetaTyVar,
33 tcInstBoxy, newBoxyTyVar, newBoxyTyVarTys, readFilledBox,
34 readMetaTyVar, writeMetaTyVar, newFlexiTyVarTy,
35 tcInstSkolTyVars, tcInstTyVar,
36 zonkTcKind, zonkType, zonkTcType, zonkTcTyVarsAndFV,
37 readKindVar, writeKindVar )
38 import TcSimplify ( tcSimplifyCheck )
39 import TcEnv ( tcGetGlobalTyVars, findGlobals )
40 import TcIface ( checkWiredInTyCon )
41 import TcRnMonad -- TcType, amongst others
42 import TcType ( TcKind, TcType, TcTyVar, BoxyTyVar, TcTauType,
43 BoxySigmaType, BoxyRhoType, BoxyType,
44 TcTyVarSet, TcThetaType, TcTyVarDetails(..), BoxInfo(..),
45 SkolemInfo( GenSkol, UnkSkol ), MetaDetails(..), isImmutableTyVar,
46 pprSkolTvBinding, isTauTy, isTauTyCon, isSigmaTy,
47 mkFunTy, mkFunTys, mkTyConApp, isMetaTyVar,
48 tcSplitForAllTys, tcSplitAppTy_maybe, tcSplitFunTys, mkTyVarTys,
49 tyVarsOfType, mkPhiTy, mkTyVarTy, mkPredTy,
50 typeKind, mkForAllTys, mkAppTy, isBoxyTyVar,
51 tidyOpenType, tidyOpenTyVar, tidyOpenTyVars,
52 pprType, tidyKind, tidySkolemTyVar, isSkolemTyVar, tcView,
53 TvSubst, mkTvSubst, zipTyEnv, substTy, emptyTvSubst,
54 lookupTyVar, extendTvSubst )
55 import Kind ( Kind(..), SimpleKind, KindVar, isArgTypeKind,
56 openTypeKind, liftedTypeKind, unliftedTypeKind,
57 mkArrowKind, defaultKind,
58 isOpenTypeKind, argTypeKind, isLiftedTypeKind, isUnliftedTypeKind,
59 isSubKind, pprKind, splitKindFunTys )
60 import TysPrim ( alphaTy, betaTy )
61 import Inst ( newDicts, instToId )
62 import TyCon ( TyCon, tyConArity, tyConTyVars, isSynTyCon )
63 import TysWiredIn ( listTyCon )
64 import Id ( Id, mkSysLocal )
65 import Var ( Var, varName, tyVarKind, isTcTyVar, tcTyVarDetails )
66 import VarSet ( emptyVarSet, mkVarSet, unitVarSet, unionVarSet, elemVarSet, varSetElems,
67 extendVarSet, intersectsVarSet )
69 import Name ( Name, isSystemName )
70 import ErrUtils ( Message )
71 import Maybes ( expectJust, isNothing )
72 import BasicTypes ( Arity )
73 import UniqSupply ( uniqsFromSupply )
74 import Util ( notNull, equalLength )
79 import TcType ( isBoxyTy, isFlexi )
83 %************************************************************************
85 \subsection{'hole' type variables}
87 %************************************************************************
90 tcInfer :: (BoxyType -> TcM a) -> TcM (a, TcType)
92 = do { box <- newBoxyTyVar openTypeKind
93 ; res <- tc_infer (mkTyVarTy box)
94 ; res_ty <- readFilledBox box -- Guaranteed filled-in by now
95 ; return (res, res_ty) }
99 %************************************************************************
103 %************************************************************************
106 subFunTys :: SDoc -- Somthing like "The function f has 3 arguments"
107 -- or "The abstraction (\x.e) takes 1 argument"
108 -> Arity -- Expected # of args
109 -> BoxyRhoType -- res_ty
110 -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
112 -- Attempt to decompse res_ty to have enough top-level arrows to
113 -- match the number of patterns in the match group
115 -- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
116 -- and the inner call to thing_inside passes args: [a1,...,an], b
117 -- then co_fn :: (a1 -> ... -> an -> b) -> res_ty
119 -- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
122 {- Error messages from subFunTys
124 The abstraction `\Just 1 -> ...' has two arguments
125 but its type `Maybe a -> a' has only one
127 The equation(s) for `f' have two arguments
128 but its type `Maybe a -> a' has only one
130 The section `(f 3)' requires 'f' to take two arguments
131 but its type `Int -> Int' has only one
133 The function 'f' is applied to two arguments
134 but its type `Int -> Int' has only one
138 subFunTys error_herald n_pats res_ty thing_inside
139 = loop n_pats [] res_ty
141 -- In 'loop', the parameter 'arg_tys' accumulates
142 -- the arg types so far, in *reverse order*
143 loop n args_so_far res_ty
144 | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
146 loop n args_so_far res_ty
147 | isSigmaTy res_ty -- Do this before checking n==0, because we
148 -- guarantee to return a BoxyRhoType, not a BoxySigmaType
149 = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet $ \ res_ty' ->
150 loop n args_so_far res_ty'
151 ; return (gen_fn <.> co_fn, res) }
153 loop 0 args_so_far res_ty
154 = do { res <- thing_inside (reverse args_so_far) res_ty
155 ; return (idCoercion, res) }
157 loop n args_so_far (FunTy arg_ty res_ty)
158 = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
159 ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
160 ; return (co_fn', res) }
162 -- res_ty might have a type variable at the head, such as (a b c),
163 -- in which case we must fill in with (->). Simplest thing to do
164 -- is to use boxyUnify, but we catch failure and generate our own
165 -- error message on failure
166 loop n args_so_far res_ty@(AppTy _ _)
167 = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
168 ; (_, mb_unit) <- tryTcErrs $ boxyUnify res_ty (FunTy arg_ty' res_ty')
169 ; if isNothing mb_unit then bale_out args_so_far res_ty
170 else loop n args_so_far (FunTy arg_ty' res_ty') }
172 loop n args_so_far (TyVarTy tv)
173 | not (isImmutableTyVar tv)
174 = do { cts <- readMetaTyVar tv
176 Indirect ty -> loop n args_so_far ty
177 Flexi -> do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
178 ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty
179 ; return (idCoercion, res) } }
181 mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
182 kinds = openTypeKind : take n (repeat argTypeKind)
183 -- Note argTypeKind: the args can have an unboxed type,
184 -- but not an unboxed tuple.
186 loop n args_so_far res_ty = bale_out args_so_far res_ty
188 bale_out args_so_far res_ty
189 = do { env0 <- tcInitTidyEnv
190 ; res_ty' <- zonkTcType res_ty
191 ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
192 ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
194 mk_msg res_ty n_actual
195 = error_herald <> comma $$
196 sep [ptext SLIT("but its type") <+> quotes (pprType res_ty),
197 if n_actual == 0 then ptext SLIT("has none")
198 else ptext SLIT("has only") <+> speakN n_actual]
202 ----------------------
203 boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
204 -> BoxyRhoType -- Expected type (T a b c)
205 -> TcM [BoxySigmaType] -- Element types, a b c
206 -- It's used for wired-in tycons, so we call checkWiredInTyCOn
207 -- Precondition: never called with FunTyCon
208 -- Precondition: input type :: *
210 boxySplitTyConApp tc orig_ty
211 = do { checkWiredInTyCon tc
212 ; loop (tyConArity tc) [] orig_ty }
214 loop n_req args_so_far ty
215 | Just ty' <- tcView ty = loop n_req args_so_far ty'
217 loop n_req args_so_far (TyConApp tycon args)
219 = ASSERT( n_req == length args) -- ty::*
220 return (args ++ args_so_far)
222 loop n_req args_so_far (AppTy fun arg)
223 = loop (n_req - 1) (arg:args_so_far) fun
225 loop n_req args_so_far (TyVarTy tv)
226 | not (isImmutableTyVar tv)
227 = do { cts <- readMetaTyVar tv
229 Indirect ty -> loop n_req args_so_far ty
230 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
231 ; return (arg_tys ++ args_so_far) }
234 mk_res_ty arg_tys' = mkTyConApp tc arg_tys'
235 arg_kinds = map tyVarKind (take n_req (tyConTyVars tc))
237 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc))) orig_ty
239 ----------------------
240 boxySplitListTy :: BoxyRhoType -> TcM BoxySigmaType -- Special case for lists
241 boxySplitListTy exp_ty = do { [elt_ty] <- boxySplitTyConApp listTyCon exp_ty
245 ----------------------
246 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
247 -> TcM (BoxySigmaType, BoxySigmaType) -- Returns m, a
248 -- Assumes (m: * -> k), where k is the kind of the incoming type
249 -- If the incoming type is boxy, then so are the result types; and vice versa
251 boxySplitAppTy orig_ty
255 | Just ty' <- tcView ty = loop ty'
258 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
259 = return (fun_ty, arg_ty)
262 | not (isImmutableTyVar tv)
263 = do { cts <- readMetaTyVar tv
265 Indirect ty -> loop ty
266 Flexi -> do { [fun_ty,arg_ty] <- withMetaTvs tv kinds mk_res_ty
267 ; return (fun_ty, arg_ty) } }
269 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
270 tv_kind = tyVarKind tv
271 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
273 liftedTypeKind] -- arg type :: *
274 -- The defaultKind is a bit smelly. If you remove it,
275 -- try compiling f x = do { x }
276 -- and you'll get a kind mis-match. It smells, but
277 -- not enough to lose sleep over.
279 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
282 boxySplitFailure actual_ty expected_ty
283 = unifyMisMatch False False actual_ty expected_ty
284 -- "outer" is False, so we don't pop the context
285 -- which is what we want since we have not pushed one!
289 --------------------------------
290 -- withBoxes: the key utility function
291 --------------------------------
294 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
295 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
296 -> ([BoxySigmaType] -> BoxySigmaType)
297 -- Constructs the type to assign
298 -- to the original var
299 -> TcM [BoxySigmaType] -- Return the fresh boxes
301 -- It's entirely possible for the [kind] to be empty.
302 -- For example, when pattern-matching on True,
303 -- we call boxySplitTyConApp passing a boolTyCon
305 -- Invariant: tv is still Flexi
307 withMetaTvs tv kinds mk_res_ty
309 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
310 ; let box_tys = mkTyVarTys box_tvs
311 ; writeMetaTyVar tv (mk_res_ty box_tys)
314 | otherwise -- Non-boxy meta type variable
315 = do { tau_tys <- mapM newFlexiTyVarTy kinds
316 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
317 -- Sure to be a tau-type
320 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
321 -- Allocate a *boxy* tyvar
322 withBox kind thing_inside
323 = do { box_tv <- newMetaTyVar BoxTv kind
324 ; res <- thing_inside (mkTyVarTy box_tv)
325 ; ty <- readFilledBox box_tv
330 %************************************************************************
332 Approximate boxy matching
334 %************************************************************************
338 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
339 -> BoxyRhoType -- Type to match (note a *Rho* type)
340 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
343 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
344 -> [BoxySigmaType] -- Type to match
345 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
347 -- Find a *boxy* substitution that makes the template look as much
348 -- like the BoxySigmaType as possible.
349 -- It's always ok to return an empty substitution;
350 -- anything more is jam on the pudding
352 -- NB1: This is a pure, non-monadic function.
353 -- It does no unification, and cannot fail
355 -- Note [Matching kinds]
356 -- The target type might legitimately not be a sub-kind of template.
357 -- For example, suppose the target is simply a box with an OpenTypeKind,
358 -- and the template is a type variable with LiftedTypeKind.
359 -- Then it's ok (because the target type will later be refined).
360 -- We simply don't bind the template type variable.
362 -- It might also be that the kind mis-match is an error. For example,
363 -- suppose we match the template (a -> Int) against (Int# -> Int),
364 -- where the template type variable 'a' has LiftedTypeKind. This
365 -- matching function does not fail; it simply doesn't bind the template.
366 -- Later stuff will fail.
368 -- Precondition: the arg lengths are equal
369 -- Precondition: none of the template type variables appear in the [BoxySigmaType]
370 -- Precondition: any nested quantifiers in either type differ from
371 -- the template type variables passed as arguments
377 -- |- head xs : <rhobox>
378 -- We will do a boxySubMatchType between a ~ <rhobox>
379 -- But we *don't* want to match [a |-> <rhobox>] because
380 -- (a) The box should be filled in with a rho-type, but
381 -- but the returned substitution maps TyVars to boxy *sigma*
383 -- (b) In any case, the right final answer might be *either*
384 -- instantiate 'a' with a rho-type or a sigma type
385 -- head xs : Int vs head xs : forall b. b->b
386 -- So the matcher MUST NOT make a choice here. In general, we only
387 -- bind a template type variable in boxyMatchType, not in boxySubMatchType.
389 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
393 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
394 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
396 go (FunTy arg1 res1) (FunTy arg2 res2)
397 = do_match arg1 arg2 (go res1 res2)
398 -- Match the args, and sub-match the results
400 go (TyVarTy _) b_ty = emptyTvSubst -- Do not bind! See Note [Sub-match]
402 go t_ty b_ty = do_match t_ty b_ty emptyTvSubst -- Otherwise we are safe to bind
404 do_match t_ty b_ty subst = boxy_match tmpl_tvs t_ty emptyVarSet b_ty subst
407 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
408 = ASSERT( length tmpl_tys == length boxy_tys )
409 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
410 -- ToDo: add error context?
412 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
414 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
415 = boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys $
416 boxy_match tmpl_tvs t_ty boxy_tvs b_ty subst
419 boxy_match :: TcTyVarSet -> TcType -- Template
420 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
421 -> BoxySigmaType -- Match against this type
425 -- The boxy_tvs argument prevents this match:
426 -- [a] forall b. a ~ forall b. b
427 -- We don't want to bind the template variable 'a'
428 -- to the quantified type variable 'b'!
430 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
431 = go orig_tmpl_ty orig_boxy_ty
434 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
435 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
437 go (ForAllTy _ ty1) (ForAllTy tv2 ty2)
438 = boxy_match tmpl_tvs ty1 (boxy_tvs `extendVarSet` tv2) ty2 subst
440 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
441 | tc1 == tc2 = go_s tys1 tys2
443 go (FunTy arg1 res1) (FunTy arg2 res2)
444 = go_s [arg1,res1] [arg2,res2]
447 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
448 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
449 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
450 = go_s [s1,t1] [s2,t2]
453 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
454 , not (intersectsVarSet boxy_tvs (tyVarsOfType orig_boxy_ty))
455 , typeKind b_ty `isSubKind` tyVarKind tv
456 = extendTvSubst subst tv boxy_ty'
458 boxy_ty' = case lookupTyVar subst tv of
459 Nothing -> orig_boxy_ty
460 Just ty -> ty `boxyLub` orig_boxy_ty
462 go _ _ = subst -- Always safe
465 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
468 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
469 -- Combine boxy information from the two types
470 -- If there is a conflict, return the first
471 boxyLub orig_ty1 orig_ty2
472 = go orig_ty1 orig_ty2
474 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
475 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
476 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
477 | tc1 == tc2, length ts1 == length ts2
478 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
480 go (TyVarTy tv1) ty2 -- This is the whole point;
481 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
484 -- Look inside type synonyms, but only if the naive version fails
485 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
486 | Just ty2' <- tcView ty1 = go ty1 ty2'
488 -- For now, we don't look inside ForAlls, PredTys
489 go ty1 ty2 = orig_ty1 -- Default
493 %************************************************************************
497 %************************************************************************
499 All the tcSub calls have the form
501 tcSub expected_ty offered_ty
503 offered_ty <= expected_ty
505 That is, that a value of type offered_ty is acceptable in
506 a place expecting a value of type expected_ty.
508 It returns a coercion function
509 co_fn :: offered_ty -> expected_ty
510 which takes an HsExpr of type offered_ty into one of type
515 tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM ExprCoFn -- Locally used only
516 -- (tcSub act exp) checks that
518 tcSubExp actual_ty expected_ty
519 = addErrCtxtM (unifyCtxt actual_ty expected_ty)
520 (tc_sub True actual_ty actual_ty expected_ty expected_ty)
522 tcFunResTy :: Name -> BoxySigmaType -> BoxySigmaType -> TcM ExprCoFn -- Locally used only
523 tcFunResTy fun actual_ty expected_ty
524 = addErrCtxtM (checkFunResCtxt fun actual_ty expected_ty) $
525 (tc_sub True actual_ty actual_ty expected_ty expected_ty)
528 tc_sub :: Outer -- See comments with uTys
529 -> BoxySigmaType -- actual_ty, before expanding synonyms
530 -> BoxySigmaType -- ..and after
531 -> BoxySigmaType -- expected_ty, before
532 -> BoxySigmaType -- ..and after
535 tc_sub outer act_sty act_ty exp_sty exp_ty
536 | Just exp_ty' <- tcView exp_ty = tc_sub False act_sty act_ty exp_sty exp_ty'
537 tc_sub outer act_sty act_ty exp_sty exp_ty
538 | Just act_ty' <- tcView act_ty = tc_sub False act_sty act_ty' exp_sty exp_ty
540 -----------------------------------
541 -- Rule SBOXY, plus other cases when act_ty is a type variable
542 -- Just defer to boxy matching
543 -- This rule takes precedence over SKOL!
544 tc_sub outer act_sty (TyVarTy tv) exp_sty exp_ty
545 = do { uVar outer False tv False exp_sty exp_ty
546 ; return idCoercion }
548 -----------------------------------
549 -- Skolemisation case (rule SKOL)
550 -- actual_ty: d:Eq b => b->b
551 -- expected_ty: forall a. Ord a => a->a
552 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
554 -- It is essential to do this *before* the specialisation case
555 -- Example: f :: (Eq a => a->a) -> ...
556 -- g :: Ord b => b->b
559 tc_sub outer act_sty act_ty exp_sty exp_ty
561 = do { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ body_exp_ty ->
562 tc_sub False act_sty act_ty body_exp_ty body_exp_ty
563 ; return (gen_fn <.> co_fn) }
565 act_tvs = tyVarsOfType act_ty
566 -- It's really important to check for escape wrt the free vars of
567 -- both expected_ty *and* actual_ty
569 -----------------------------------
570 -- Specialisation case (rule ASPEC):
571 -- actual_ty: forall a. Ord a => a->a
572 -- expected_ty: Int -> Int
573 -- co_fn e = e Int dOrdInt
575 tc_sub outer act_sty actual_ty exp_sty expected_ty
576 | isSigmaTy actual_ty
577 = do { (tyvars, theta, tau) <- tcInstBoxy actual_ty
578 ; dicts <- newDicts InstSigOrigin theta
580 ; let inst_fn = CoApps (CoTyApps CoHole (mkTyVarTys tyvars))
582 ; co_fn <- tc_sub False tau tau exp_sty expected_ty
583 ; return (co_fn <.> inst_fn) }
585 -----------------------------------
586 -- Function case (rule F1)
587 tc_sub _ _ (FunTy act_arg act_res) _ (FunTy exp_arg exp_res)
588 = tc_sub_funs act_arg act_res exp_arg exp_res
590 -- Function case (rule F2)
591 tc_sub outer act_sty act_ty@(FunTy act_arg act_res) exp_sty (TyVarTy exp_tv)
593 = do { cts <- readMetaTyVar exp_tv
595 Indirect ty -> do { u_tys outer False act_sty act_ty True exp_sty ty
596 ; return idCoercion }
597 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
598 ; tc_sub_funs act_arg act_res arg_ty res_ty } }
600 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
601 fun_kinds = [argTypeKind, openTypeKind]
603 -- Everything else: defer to boxy matching
604 tc_sub outer act_sty actual_ty exp_sty expected_ty
605 = do { u_tys outer False act_sty actual_ty False exp_sty expected_ty
606 ; return idCoercion }
609 -----------------------------------
610 tc_sub_funs act_arg act_res exp_arg exp_res
611 = do { uTys False act_arg False exp_arg
612 ; co_fn_res <- tc_sub False act_res act_res exp_res exp_res
613 ; wrapFunResCoercion [exp_arg] co_fn_res }
615 -----------------------------------
617 :: [TcType] -- Type of args
618 -> ExprCoFn -- HsExpr a -> HsExpr b
619 -> TcM ExprCoFn -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
620 wrapFunResCoercion arg_tys co_fn_res
621 | isIdCoercion co_fn_res = return idCoercion
622 | null arg_tys = return co_fn_res
624 = do { us <- newUniqueSupply
625 ; let arg_ids = zipWith (mkSysLocal FSLIT("sub")) (uniqsFromSupply us) arg_tys
626 ; return (CoLams arg_ids (co_fn_res <.> (CoApps CoHole arg_ids))) }
631 %************************************************************************
633 \subsection{Generalisation}
635 %************************************************************************
638 tcGen :: BoxySigmaType -- expected_ty
639 -> TcTyVarSet -- Extra tyvars that the universally
640 -- quantified tyvars of expected_ty
641 -- must not be unified
642 -> (BoxyRhoType -> TcM result) -- spec_ty
643 -> TcM (ExprCoFn, result)
644 -- The expression has type: spec_ty -> expected_ty
646 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
647 -- If not, the call is a no-op
648 = do { -- We want the GenSkol info in the skolemised type variables to
649 -- mention the *instantiated* tyvar names, so that we get a
650 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
651 -- Hence the tiresome but innocuous fixM
652 ((forall_tvs, theta, rho_ty), skol_info) <- fixM (\ ~(_, skol_info) ->
653 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
654 ; span <- getSrcSpanM
655 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty) span
656 ; return ((forall_tvs, theta, rho_ty), skol_info) })
659 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
660 text "expected_ty" <+> ppr expected_ty,
661 text "inst ty" <+> ppr forall_tvs <+> ppr theta <+> ppr rho_ty,
662 text "free_tvs" <+> ppr free_tvs,
663 text "forall_tvs" <+> ppr forall_tvs])
666 -- Type-check the arg and unify with poly type
667 ; (result, lie) <- getLIE (thing_inside rho_ty)
669 -- Check that the "forall_tvs" havn't been constrained
670 -- The interesting bit here is that we must include the free variables
671 -- of the expected_ty. Here's an example:
672 -- runST (newVar True)
673 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
674 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
675 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
676 -- So now s' isn't unconstrained because it's linked to a.
677 -- Conclusion: include the free vars of the expected_ty in the
678 -- list of "free vars" for the signature check.
680 ; dicts <- newDicts (SigOrigin skol_info) theta
681 ; inst_binds <- tcSimplifyCheck sig_msg forall_tvs dicts lie
683 ; checkSigTyVarsWrt free_tvs forall_tvs
684 ; traceTc (text "tcGen:done")
687 -- This HsLet binds any Insts which came out of the simplification.
688 -- It's a bit out of place here, but using AbsBind involves inventing
689 -- a couple of new names which seems worse.
690 dict_ids = map instToId dicts
691 co_fn = CoTyLams forall_tvs $ CoLams dict_ids $ CoLet inst_binds CoHole
692 ; returnM (co_fn, result) }
694 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
695 sig_msg = ptext SLIT("expected type of an expression")
700 %************************************************************************
704 %************************************************************************
706 The exported functions are all defined as versions of some
707 non-exported generic functions.
710 boxyUnify :: BoxyType -> BoxyType -> TcM ()
711 -- Acutal and expected, respectively
713 = addErrCtxtM (unifyCtxt ty1 ty2) $
714 uTysOuter False ty1 False ty2
717 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM ()
718 -- Arguments should have equal length
719 -- Acutal and expected types
720 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
723 unifyType :: TcTauType -> TcTauType -> TcM ()
724 -- No boxes expected inside these types
725 -- Acutal and expected types
726 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
727 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
728 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
729 addErrCtxtM (unifyCtxt ty1 ty2) $
730 uTysOuter True ty1 True ty2
733 unifyPred :: PredType -> PredType -> TcM ()
734 -- Acutal and expected types
735 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
736 uPred True True p1 True p2
738 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
739 -- Acutal and expected types
740 unifyTheta theta1 theta2
741 = do { checkTc (equalLength theta1 theta2)
742 (ptext SLIT("Contexts differ in length"))
743 ; uList unifyPred theta1 theta2 }
746 uList :: (a -> a -> TcM ())
747 -> [a] -> [a] -> TcM ()
748 -- Unify corresponding elements of two lists of types, which
749 -- should be f equal length. We charge down the list explicitly so that
750 -- we can complain if their lengths differ.
751 uList unify [] [] = return ()
752 uList unify (ty1:tys1) (ty2:tys2) = do { unify ty1 ty2; uList unify tys1 tys2 }
753 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
756 @unifyTypeList@ takes a single list of @TauType@s and unifies them
757 all together. It is used, for example, when typechecking explicit
758 lists, when all the elts should be of the same type.
761 unifyTypeList :: [TcTauType] -> TcM ()
762 unifyTypeList [] = returnM ()
763 unifyTypeList [ty] = returnM ()
764 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
765 ; unifyTypeList tys }
768 %************************************************************************
770 \subsection[Unify-uTys]{@uTys@: getting down to business}
772 %************************************************************************
774 @uTys@ is the heart of the unifier. Each arg happens twice, because
775 we want to report errors in terms of synomyms if poss. The first of
776 the pair is used in error messages only; it is always the same as the
777 second, except that if the first is a synonym then the second may be a
778 de-synonym'd version. This way we get better error messages.
780 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
783 type NoBoxes = Bool -- True <=> definitely no boxes in this type
784 -- False <=> there might be boxes (always safe)
786 type Outer = Bool -- True <=> this is the outer level of a unification
787 -- so that the types being unified are the
788 -- very ones we began with, not some sub
789 -- component or synonym expansion
790 -- The idea is that if Outer is true then unifyMisMatch should
791 -- pop the context to remove the "Expected/Acutal" context
794 :: NoBoxes -> TcType -- ty1 is the *expected* type
795 -> NoBoxes -> TcType -- ty2 is the *actual* type
797 uTysOuter nb1 ty1 nb2 ty2 = u_tys True nb1 ty1 ty1 nb2 ty2 ty2
798 uTys nb1 ty1 nb2 ty2 = u_tys False nb1 ty1 ty1 nb2 ty2 ty2
802 uTys_s :: NoBoxes -> [TcType] -- ty1 is the *actual* types
803 -> NoBoxes -> [TcType] -- ty2 is the *expected* types
805 uTys_s nb1 [] nb2 [] = returnM ()
806 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { uTys nb1 ty1 nb2 ty2
807 ; uTys_s nb1 tys1 nb2 tys2 }
808 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
812 -> NoBoxes -> TcType -> TcType -- ty1 is the *actual* type
813 -> NoBoxes -> TcType -> TcType -- ty2 is the *expected* type
816 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
820 -- Always expand synonyms (see notes at end)
821 -- (this also throws away FTVs)
823 | Just ty1' <- tcView ty1 = go False ty1' ty2
824 | Just ty2' <- tcView ty2 = go False ty1 ty2'
826 -- Variables; go for uVar
827 go outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
828 go outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
829 -- "True" means args swapped
831 go outer (PredTy p1) (PredTy p2) = uPred outer nb1 p1 nb2 p2
833 -- Type constructors must match
834 go _ (TyConApp con1 tys1) (TyConApp con2 tys2)
835 | con1 == con2 = uTys_s nb1 tys1 nb2 tys2
836 -- See Note [TyCon app]
838 -- Functions; just check the two parts
839 go _ (FunTy fun1 arg1) (FunTy fun2 arg2)
840 = do { uTys nb1 fun1 nb2 fun2
841 ; uTys nb1 arg1 nb2 arg2 }
843 -- Applications need a bit of care!
844 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
845 -- NB: we've already dealt with type variables and Notes,
846 -- so if one type is an App the other one jolly well better be too
847 go outer (AppTy s1 t1) ty2
848 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
849 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
851 -- Now the same, but the other way round
852 -- Don't swap the types, because the error messages get worse
853 go outer ty1 (AppTy s2 t2)
854 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
855 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
857 go _ ty1@(ForAllTy _ _) ty2@(ForAllTy _ _)
858 | length tvs1 == length tvs2
859 = do { tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
860 ; let tys = mkTyVarTys tvs
861 in_scope = mkInScopeSet (mkVarSet tvs)
862 subst1 = mkTvSubst in_scope (zipTyEnv tvs1 tys)
863 subst2 = mkTvSubst in_scope (zipTyEnv tvs2 tys)
864 ; uTys nb1 (substTy subst1 body1) nb2 (substTy subst2 body2)
866 -- If both sides are inside a box, we should not have
867 -- a polytype at all. This check comes last, because
868 -- the error message is extremely unhelpful.
869 ; ifM (nb1 && nb2) (notMonoType ty1)
872 (tvs1, body1) = tcSplitForAllTys ty1
873 (tvs2, body2) = tcSplitForAllTys ty2
875 -- Anything else fails
876 go outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
879 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
880 | n1 == n2 = uTys nb1 t1 nb2 t2
881 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
882 | c1 == c2 = uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
883 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
888 When we find two TyConApps, the argument lists are guaranteed equal
889 length. Reason: intially the kinds of the two types to be unified is
890 the same. The only way it can become not the same is when unifying two
891 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
892 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
893 which we do, that ensures that f1,f2 have the same kind; and that
894 means a1,a2 have the same kind. And now the argument repeats.
899 If you are tempted to make a short cut on synonyms, as in this
903 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
904 -- NO = if (con1 == con2) then
905 -- NO -- Good news! Same synonym constructors, so we can shortcut
906 -- NO -- by unifying their arguments and ignoring their expansions.
907 -- NO unifyTypepeLists args1 args2
909 -- NO -- Never mind. Just expand them and try again
913 then THINK AGAIN. Here is the whole story, as detected and reported
914 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
916 Here's a test program that should detect the problem:
920 x = (1 :: Bogus Char) :: Bogus Bool
923 The problem with [the attempted shortcut code] is that
927 is not a sufficient condition to be able to use the shortcut!
928 You also need to know that the type synonym actually USES all
929 its arguments. For example, consider the following type synonym
930 which does not use all its arguments.
935 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
936 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
937 would fail, even though the expanded forms (both \tr{Int}) should
940 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
941 unnecessarily bind \tr{t} to \tr{Char}.
943 ... You could explicitly test for the problem synonyms and mark them
944 somehow as needing expansion, perhaps also issuing a warning to the
949 %************************************************************************
951 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
953 %************************************************************************
955 @uVar@ is called when at least one of the types being unified is a
956 variable. It does {\em not} assume that the variable is a fixed point
957 of the substitution; rather, notice that @uVar@ (defined below) nips
958 back into @uTys@ if it turns out that the variable is already bound.
962 -> Bool -- False => tyvar is the "expected"
963 -- True => ty is the "expected" thing
965 -> NoBoxes -- True <=> definitely no boxes in t2
966 -> TcTauType -> TcTauType -- printing and real versions
969 uVar outer swapped tv1 nb2 ps_ty2 ty2
970 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
971 | otherwise = brackets (equals <+> ppr ty2)
972 ; traceTc (text "uVar" <+> ppr swapped <+>
973 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
974 nest 2 (ptext SLIT(" :=: ")),
975 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
976 ; details <- lookupTcTyVar tv1
979 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
980 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
981 -- The 'True' here says that ty1
982 -- is definitely box-free
983 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 ty2
987 uUnfilledVar :: Outer
988 -> Bool -- Args are swapped
989 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
990 -> NoBoxes -> TcTauType -> TcTauType -- Type 2
992 -- Invariant: tyvar 1 is not unified with anything
994 uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 ty2
995 | Just ty2' <- tcView ty2
996 = -- Expand synonyms; ignore FTVs
997 uUnfilledVar False swapped tv1 details1 nb2 ps_ty2 ty2'
999 uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 (TyVarTy tv2)
1000 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1002 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1003 -- this is box-meets-box, so fill in with a tau-type
1004 -> do { tau_tv <- tcInstTyVar tv1
1005 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv) }
1006 other -> returnM () -- No-op
1008 -- Distinct type variables
1010 = do { lookup2 <- lookupTcTyVar tv2
1012 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 True ty2' ty2'
1013 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1016 uUnfilledVar outer swapped tv1 details1 nb2 ps_ty2 non_var_ty2 -- ty2 is not a type variable
1018 MetaTv (SigTv _) ref1 -> mis_match -- Can't update a skolem with a non-type-variable
1019 MetaTv info ref1 -> uMetaVar swapped tv1 info ref1 nb2 ps_ty2 non_var_ty2
1020 skolem_details -> mis_match
1022 mis_match = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1026 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1027 -> NoBoxes -> TcType -> TcType
1029 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1030 -- ty2 is not a type variable
1032 uMetaVar swapped tv1 BoxTv ref1 nb2 ps_ty2 non_var_ty2
1033 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1034 -- that any boxes in ty2 are filled with monotypes
1036 -- It should not be the case that tv1 occurs in ty2
1037 -- (i.e. no occurs check should be needed), but if perchance
1038 -- it does, the unbox operation will fill it, and the DEBUG
1040 do { final_ty <- unBox ps_ty2
1042 ; meta_details <- readMutVar ref1
1043 ; case meta_details of
1044 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1045 return () -- This really should *not* happen
1048 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1050 uMetaVar swapped tv1 info1 ref1 nb2 ps_ty2 non_var_ty2
1051 = do { final_ty <- checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
1052 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1055 uUnfilledVars :: Outer
1056 -> Bool -- Args are swapped
1057 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1058 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1060 -- Invarant: The type variables are distinct,
1061 -- Neither is filled in yet
1062 -- They might be boxy or not
1064 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1065 = unifyMisMatch outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1067 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1068 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2)
1069 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1070 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
1072 -- ToDo: this function seems too long for what it acutally does!
1073 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1074 = case (info1, info2) of
1075 (BoxTv, BoxTv) -> box_meets_box
1077 -- If a box meets a TauTv, but the fomer has the smaller kind
1078 -- then we must create a fresh TauTv with the smaller kind
1079 (_, BoxTv) | k1_sub_k2 -> update_tv2
1080 | otherwise -> box_meets_box
1081 (BoxTv, _ ) | k2_sub_k1 -> update_tv1
1082 | otherwise -> box_meets_box
1084 -- Avoid SigTvs if poss
1085 (SigTv _, _ ) | k1_sub_k2 -> update_tv2
1086 (_, SigTv _) | k2_sub_k1 -> update_tv1
1088 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1089 then update_tv1 -- Same kinds
1091 | k2_sub_k1 -> update_tv1
1092 | otherwise -> kind_err
1094 -- Update the variable with least kind info
1095 -- See notes on type inference in Kind.lhs
1096 -- The "nicer to" part only applies if the two kinds are the same,
1097 -- so we can choose which to do.
1099 -- Kinds should be guaranteed ok at this point
1100 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1101 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1103 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1106 | k2_sub_k1 = fill_from tv2
1107 | otherwise = kind_err
1109 -- Update *both* tyvars with a TauTv whose name and kind
1110 -- are gotten from tv (avoid losing nice names is poss)
1111 fill_from tv = do { tv' <- tcInstTyVar tv
1112 ; let tau_ty = mkTyVarTy tv'
1113 ; updateMeta tv1 ref1 tau_ty
1114 ; updateMeta tv2 ref2 tau_ty }
1116 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1117 unifyKindMisMatch k1 k2
1121 k1_sub_k2 = k1 `isSubKind` k2
1122 k2_sub_k1 = k2 `isSubKind` k1
1124 nicer_to_update_tv1 = isSystemName (varName tv1)
1125 -- Try to update sys-y type variables in preference to ones
1126 -- gotten (say) by instantiating a polymorphic function with
1127 -- a user-written type sig
1130 checkUpdateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1131 -- Update tv1, which is flexi; occurs check is alrady done
1132 -- The 'check' version does a kind check too
1133 -- We do a sub-kind check here: we might unify (a b) with (c d)
1134 -- where b::*->* and d::*; this should fail
1136 checkUpdateMeta swapped tv1 ref1 ty2
1137 = do { checkKinds swapped tv1 ty2
1138 ; updateMeta tv1 ref1 ty2 }
1140 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1141 updateMeta tv1 ref1 ty2
1142 = ASSERT( isMetaTyVar tv1 )
1143 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
1144 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
1145 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
1146 ; writeMutVar ref1 (Indirect ty2) }
1149 checkKinds swapped tv1 ty2
1150 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
1151 -- ty2 has been zonked at this stage, which ensures that
1152 -- its kind has as much boxity information visible as possible.
1153 | tk2 `isSubKind` tk1 = returnM ()
1156 -- Either the kinds aren't compatible
1157 -- (can happen if we unify (a b) with (c d))
1158 -- or we are unifying a lifted type variable with an
1159 -- unlifted type: e.g. (id 3#) is illegal
1160 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1161 unifyKindMisMatch k1 k2
1163 (k1,k2) | swapped = (tk2,tk1)
1164 | otherwise = (tk1,tk2)
1169 checkTauTvUpdate :: TcTyVar -> TcType -> TcM TcType
1170 -- (checkTauTvUpdate tv ty)
1171 -- We are about to update the TauTv tv with ty.
1172 -- Check (a) that tv doesn't occur in ty (occurs check)
1173 -- (b) that ty is a monotype
1174 -- Furthermore, in the interest of (b), if you find an
1175 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
1177 -- Returns the (non-boxy) type to update the type variable with, or fails
1179 checkTauTvUpdate orig_tv orig_ty
1182 go (TyConApp tc tys)
1183 | isSynTyCon tc = go_syn tc tys
1184 | otherwise = do { tys' <- mappM go tys; return (TyConApp tc tys') }
1185 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
1186 go (PredTy p) = do { p' <- go_pred p; return (PredTy p') }
1187 go (FunTy arg res) = do { arg' <- go arg; res' <- go res; return (FunTy arg' res') }
1188 go (AppTy fun arg) = do { fun' <- go fun; arg' <- go arg; return (mkAppTy fun' arg') }
1189 -- NB the mkAppTy; we might have instantiated a
1190 -- type variable to a type constructor, so we need
1191 -- to pull the TyConApp to the top.
1192 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
1195 | orig_tv == tv = occurCheck tv orig_ty -- (a)
1196 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
1197 | otherwise = return (TyVarTy tv)
1198 -- Ordinary (non Tc) tyvars
1199 -- occur inside quantified types
1201 go_pred (ClassP c tys) = do { tys' <- mapM go tys; return (ClassP c tys') }
1202 go_pred (IParam n ty) = do { ty' <- go ty; return (IParam n ty') }
1204 go_tyvar tv (SkolemTv _) = return (TyVarTy tv)
1205 go_tyvar tv (MetaTv box ref)
1206 = do { cts <- readMutVar ref
1208 Indirect ty -> go ty
1209 Flexi -> case box of
1210 BoxTv -> fillBoxWithTau tv ref
1211 other -> return (TyVarTy tv)
1214 -- go_syn is called for synonyms only
1215 -- See Note [Type synonyms and the occur check]
1217 | not (isTauTyCon tc)
1218 = notMonoType orig_ty -- (b) again
1220 = do { (msgs, mb_tys') <- tryTc (mapM go tys)
1222 Just tys' -> return (TyConApp tc tys')
1223 -- Retain the synonym (the common case)
1224 Nothing -> go (expectJust "checkTauTvUpdate"
1225 (tcView (TyConApp tc tys)))
1226 -- Try again, expanding the synonym
1229 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
1230 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
1231 -- tau-type meta-variable, whose print-name is the same as tv
1232 -- Choosing the same name is good: when we instantiate a function
1233 -- we allocate boxy tyvars with the same print-name as the quantified
1234 -- tyvar; and then we often fill the box with a tau-tyvar, and again
1235 -- we want to choose the same name.
1236 fillBoxWithTau tv ref
1237 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
1238 ; let tau = mkTyVarTy tv' -- name of the type variable
1239 ; writeMutVar ref (Indirect tau)
1243 Note [Type synonyms and the occur check]
1244 ~~~~~~~~~~~~~~~~~~~~
1245 Basically we want to update tv1 := ps_ty2
1246 because ps_ty2 has type-synonym info, which improves later error messages
1251 f :: (A a -> a -> ()) -> ()
1255 x = f (\ x p -> p x)
1257 In the application (p x), we try to match "t" with "A t". If we go
1258 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
1259 an infinite loop later.
1260 But we should not reject the program, because A t = ().
1261 Rather, we should bind t to () (= non_var_ty2).
1264 stripBoxyType :: BoxyType -> TcM TcType
1265 -- Strip all boxes from the input type, returning a non-boxy type.
1266 -- It's fine for there to be a polytype inside a box (c.f. unBox)
1267 -- All of the boxes should have been filled in by now;
1268 -- hence we return a TcType
1269 stripBoxyType ty = zonkType strip_tv ty
1271 strip_tv tv = ASSERT( not (isBoxyTyVar tv) ) return (TyVarTy tv)
1272 -- strip_tv will be called for *Flexi* meta-tyvars
1273 -- There should not be any Boxy ones; hence the ASSERT
1275 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1276 -- Subtle... we must zap the boxy res_ty
1277 -- to kind * before using it to instantiate a LitInst
1278 -- Calling unBox instead doesn't do the job, because the box
1279 -- often has an openTypeKind, and we don't want to instantiate
1281 zapToMonotype res_ty
1282 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1283 ; boxyUnify res_tau res_ty
1286 unBox :: BoxyType -> TcM TcType
1287 -- unBox implements the judgement
1289 -- with input s', and result s
1291 -- It remove all boxes from the input type, returning a non-boxy type.
1292 -- A filled box in the type can only contain a monotype; unBox fails if not
1293 -- The type can have empty boxes, which unBox fills with a monotype
1295 -- Compare this wth checkTauTvUpdate
1297 -- For once, it's safe to treat synonyms as opaque!
1299 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1300 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1301 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1302 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1303 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1304 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1305 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1307 | isTcTyVar tv -- It's a boxy type variable
1308 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1309 = do { cts <- readMutVar ref -- under nested quantifiers
1311 Flexi -> fillBoxWithTau tv ref
1312 Indirect ty -> do { non_boxy_ty <- unBox ty
1313 ; if isTauTy non_boxy_ty
1314 then return non_boxy_ty
1315 else notMonoType non_boxy_ty }
1317 | otherwise -- Skolems, and meta-tau-variables
1318 = return (TyVarTy tv)
1320 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1321 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1326 %************************************************************************
1328 \subsection[Unify-context]{Errors and contexts}
1330 %************************************************************************
1336 unifyCtxt act_ty exp_ty tidy_env
1337 = do { act_ty' <- zonkTcType act_ty
1338 ; exp_ty' <- zonkTcType exp_ty
1339 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1340 (env2, act_ty'') = tidyOpenType env1 act_ty'
1341 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1344 mkExpectedActualMsg act_ty exp_ty
1345 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1346 text "Inferred type" <> colon <+> ppr act_ty ])
1349 -- If an error happens we try to figure out whether the function
1350 -- function has been given too many or too few arguments, and say so.
1351 checkFunResCtxt fun actual_res_ty expected_res_ty tidy_env
1352 = do { exp_ty' <- zonkTcType expected_res_ty
1353 ; act_ty' <- zonkTcType actual_res_ty
1355 (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1356 (env2, act_ty'') = tidyOpenType env1 act_ty'
1357 (exp_args, _) = tcSplitFunTys exp_ty''
1358 (act_args, _) = tcSplitFunTys act_ty''
1360 len_act_args = length act_args
1361 len_exp_args = length exp_args
1363 message | len_exp_args < len_act_args = wrongArgsCtxt "too few" fun
1364 | len_exp_args > len_act_args = wrongArgsCtxt "too many" fun
1365 | otherwise = mkExpectedActualMsg act_ty'' exp_ty''
1366 ; return (env2, message) }
1369 wrongArgsCtxt too_many_or_few fun
1370 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1371 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1372 <+> ptext SLIT("arguments")
1375 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1376 -- tv1 and ty2 are zonked already
1379 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1380 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1382 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1383 | otherwise = (pp1, pp2)
1384 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1385 (env2, ty2') = tidyOpenType env1 ty2
1386 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1387 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1389 unifyMisMatch outer swapped ty1 ty2
1390 = do { (env, msg) <- if swapped then misMatchMsg ty1 ty2
1391 else misMatchMsg ty2 ty1
1393 -- This is the whole point of the 'outer' stuff
1394 ; if outer then popErrCtxt (failWithTcM (env, msg))
1395 else failWithTcM (env, msg)
1399 = do { env0 <- tcInitTidyEnv
1400 ; (env1, pp1, extra1) <- ppr_ty env0 ty1
1401 ; (env2, pp2, extra2) <- ppr_ty env1 ty2
1402 ; return (env2, sep [sep [ptext SLIT("Couldn't match expected type") <+> pp1,
1403 nest 7 (ptext SLIT("against inferred type") <+> pp2)],
1404 nest 2 extra1, nest 2 extra2]) }
1406 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
1408 = do { ty' <- zonkTcType ty
1409 ; let (env1,tidy_ty) = tidyOpenType env ty'
1410 simple_result = (env1, quotes (ppr tidy_ty), empty)
1413 | isSkolemTyVar tv -> return (env2, pp_rigid tv',
1414 pprSkolTvBinding tv')
1415 | otherwise -> return simple_result
1417 (env2, tv') = tidySkolemTyVar env1 tv
1418 other -> return simple_result }
1420 pp_rigid tv = quotes (ppr tv) <+> parens (ptext SLIT("a rigid variable"))
1424 = do { ty' <- zonkTcType ty
1425 ; env0 <- tcInitTidyEnv
1426 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1427 msg = ptext SLIT("Cannot match a monotype with") <+> ppr tidy_ty
1428 ; failWithTcM (env1, msg) }
1431 = do { env0 <- tcInitTidyEnv
1432 ; ty' <- zonkTcType ty
1433 ; let (env1, tidy_tyvar) = tidyOpenTyVar env0 tyvar
1434 (env2, tidy_ty) = tidyOpenType env1 ty'
1435 extra = sep [ppr tidy_tyvar, char '=', ppr tidy_ty]
1436 ; failWithTcM (env2, hang msg 2 extra) }
1438 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
1442 %************************************************************************
1446 %************************************************************************
1448 Unifying kinds is much, much simpler than unifying types.
1451 unifyKind :: TcKind -- Expected
1454 unifyKind LiftedTypeKind LiftedTypeKind = returnM ()
1455 unifyKind UnliftedTypeKind UnliftedTypeKind = returnM ()
1457 unifyKind OpenTypeKind k2 | isOpenTypeKind k2 = returnM ()
1458 unifyKind ArgTypeKind k2 | isArgTypeKind k2 = returnM ()
1459 -- Respect sub-kinding
1461 unifyKind (FunKind a1 r1) (FunKind a2 r2)
1462 = do { unifyKind a2 a1; unifyKind r1 r2 }
1463 -- Notice the flip in the argument,
1464 -- so that the sub-kinding works right
1466 unifyKind (KindVar kv1) k2 = uKVar False kv1 k2
1467 unifyKind k1 (KindVar kv2) = uKVar True kv2 k1
1468 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1470 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1471 unifyKinds [] [] = returnM ()
1472 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1474 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1477 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1478 uKVar swapped kv1 k2
1479 = do { mb_k1 <- readKindVar kv1
1481 Nothing -> uUnboundKVar swapped kv1 k2
1482 Just k1 | swapped -> unifyKind k2 k1
1483 | otherwise -> unifyKind k1 k2 }
1486 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1487 uUnboundKVar swapped kv1 k2@(KindVar kv2)
1488 | kv1 == kv2 = returnM ()
1489 | otherwise -- Distinct kind variables
1490 = do { mb_k2 <- readKindVar kv2
1492 Just k2 -> uUnboundKVar swapped kv1 k2
1493 Nothing -> writeKindVar kv1 k2 }
1495 uUnboundKVar swapped kv1 non_var_k2
1496 = do { k2' <- zonkTcKind non_var_k2
1497 ; kindOccurCheck kv1 k2'
1498 ; k2'' <- kindSimpleKind swapped k2'
1499 -- KindVars must be bound only to simple kinds
1500 -- Polarities: (kindSimpleKind True ?) succeeds
1501 -- returning *, corresponding to unifying
1504 ; writeKindVar kv1 k2'' }
1507 kindOccurCheck kv1 k2 -- k2 is zonked
1508 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1510 not_in (KindVar kv2) = kv1 /= kv2
1511 not_in (FunKind a2 r2) = not_in a2 && not_in r2
1514 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1515 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1516 -- If the flag is False, it requires k <: sk
1517 -- E.g. kindSimpleKind False ?? = *
1518 -- What about (kv -> *) :=: ?? -> *
1519 kindSimpleKind orig_swapped orig_kind
1520 = go orig_swapped orig_kind
1522 go sw (FunKind k1 k2) = do { k1' <- go (not sw) k1
1524 ; return (FunKind k1' k2') }
1525 go True OpenTypeKind = return liftedTypeKind
1526 go True ArgTypeKind = return liftedTypeKind
1527 go sw LiftedTypeKind = return liftedTypeKind
1528 go sw UnliftedTypeKind = return unliftedTypeKind
1529 go sw k@(KindVar _) = return k -- KindVars are always simple
1530 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1531 <+> ppr orig_swapped <+> ppr orig_kind)
1532 -- I think this can't actually happen
1534 -- T v = MkT v v must be a type
1535 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1538 kindOccurCheckErr tyvar ty
1539 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1540 2 (sep [ppr tyvar, char '=', ppr ty])
1542 unifyKindMisMatch ty1 ty2
1543 = zonkTcKind ty1 `thenM` \ ty1' ->
1544 zonkTcKind ty2 `thenM` \ ty2' ->
1546 msg = hang (ptext SLIT("Couldn't match kind"))
1547 2 (sep [quotes (ppr ty1'),
1548 ptext SLIT("against"),
1555 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1556 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1558 unifyFunKind (KindVar kvar)
1559 = readKindVar kvar `thenM` \ maybe_kind ->
1561 Just fun_kind -> unifyFunKind fun_kind
1562 Nothing -> do { arg_kind <- newKindVar
1563 ; res_kind <- newKindVar
1564 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1565 ; returnM (Just (arg_kind,res_kind)) }
1567 unifyFunKind (FunKind arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1568 unifyFunKind other = returnM Nothing
1571 %************************************************************************
1575 %************************************************************************
1577 ---------------------------
1578 -- We would like to get a decent error message from
1579 -- (a) Under-applied type constructors
1580 -- f :: (Maybe, Maybe)
1581 -- (b) Over-applied type constructors
1582 -- f :: Int x -> Int x
1586 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1587 -- A fancy wrapper for 'unifyKind', which tries
1588 -- to give decent error messages.
1589 checkExpectedKind ty act_kind exp_kind
1590 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1593 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1595 Just r -> returnM () ; -- Unification succeeded
1598 -- So there's definitely an error
1599 -- Now to find out what sort
1600 zonkTcKind exp_kind `thenM` \ exp_kind ->
1601 zonkTcKind act_kind `thenM` \ act_kind ->
1603 tcInitTidyEnv `thenM` \ env0 ->
1604 let (exp_as, _) = splitKindFunTys exp_kind
1605 (act_as, _) = splitKindFunTys act_kind
1606 n_exp_as = length exp_as
1607 n_act_as = length act_as
1609 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1610 (env2, tidy_act_kind) = tidyKind env1 act_kind
1612 err | n_exp_as < n_act_as -- E.g. [Maybe]
1613 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1615 -- Now n_exp_as >= n_act_as. In the next two cases,
1616 -- n_exp_as == 0, and hence so is n_act_as
1617 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1618 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1619 <+> ptext SLIT("is unlifted")
1621 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1622 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1623 <+> ptext SLIT("is lifted")
1625 | otherwise -- E.g. Monad [Int]
1626 = ptext SLIT("Kind mis-match")
1628 more_info = sep [ ptext SLIT("Expected kind") <+>
1629 quotes (pprKind tidy_exp_kind) <> comma,
1630 ptext SLIT("but") <+> quotes (ppr ty) <+>
1631 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1633 failWithTcM (env2, err $$ more_info)
1637 %************************************************************************
1639 \subsection{Checking signature type variables}
1641 %************************************************************************
1643 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1644 are not mentioned in the environment. In particular:
1646 (a) Not mentioned in the type of a variable in the envt
1647 eg the signature for f in this:
1653 Here, f is forced to be monorphic by the free occurence of x.
1655 (d) Not (unified with another type variable that is) in scope.
1656 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1657 when checking the expression type signature, we find that
1658 even though there is nothing in scope whose type mentions r,
1659 nevertheless the type signature for the expression isn't right.
1661 Another example is in a class or instance declaration:
1663 op :: forall b. a -> b
1665 Here, b gets unified with a
1667 Before doing this, the substitution is applied to the signature type variable.
1670 checkSigTyVars :: [TcTyVar] -> TcM ()
1671 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1673 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1674 -- The extra_tvs can include boxy type variables;
1675 -- e.g. TcMatches.tcCheckExistentialPat
1676 checkSigTyVarsWrt extra_tvs sig_tvs
1677 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1678 ; check_sig_tyvars extra_tvs' sig_tvs }
1681 :: TcTyVarSet -- Global type variables. The universally quantified
1682 -- tyvars should not mention any of these
1683 -- Guaranteed already zonked.
1684 -> [TcTyVar] -- Universally-quantified type variables in the signature
1685 -- Guaranteed to be skolems
1687 check_sig_tyvars extra_tvs []
1689 check_sig_tyvars extra_tvs sig_tvs
1690 = ASSERT( all isSkolemTyVar sig_tvs )
1691 do { gbl_tvs <- tcGetGlobalTyVars
1692 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1693 text "gbl_tvs" <+> ppr gbl_tvs,
1694 text "extra_tvs" <+> ppr extra_tvs]))
1696 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1697 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1698 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1701 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1702 -> [TcTyVar] -- The possibly-escaping type variables
1703 -> [TcTyVar] -- The zonked versions thereof
1705 -- Complain about escaping type variables
1706 -- We pass a list of type variables, at least one of which
1707 -- escapes. The first list contains the original signature type variable,
1708 -- while the second contains the type variable it is unified to (usually itself)
1709 bleatEscapedTvs globals sig_tvs zonked_tvs
1710 = do { env0 <- tcInitTidyEnv
1711 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1712 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1714 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1715 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1717 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1719 check (tidy_env, msgs) (sig_tv, zonked_tv)
1720 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1722 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1723 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1725 -----------------------
1726 escape_msg sig_tv zonked_tv globs
1728 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1729 nest 2 (vcat globs)]
1731 = msg <+> ptext SLIT("escapes")
1732 -- Sigh. It's really hard to give a good error message
1733 -- all the time. One bad case is an existential pattern match.
1734 -- We rely on the "When..." context to help.
1736 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1738 | sig_tv == zonked_tv = empty
1739 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1742 These two context are used with checkSigTyVars
1745 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1746 -> TidyEnv -> TcM (TidyEnv, Message)
1747 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1748 = zonkTcType sig_tau `thenM` \ actual_tau ->
1750 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1751 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1752 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1753 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1754 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1756 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),