2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Type subsumption and unification
10 -- Full-blown subsumption
11 tcSubExp, tcFunResTy, tcGen,
12 checkSigTyVars, checkSigTyVarsWrt, bleatEscapedTvs, sigCtxt,
14 -- Various unifications
15 unifyType, unifyTypeList, unifyTheta,
16 unifyKind, unifyKinds, unifyFunKind,
18 preSubType, boxyMatchTypes,
20 --------------------------------
22 tcInfer, subFunTys, unBox, stripBoxyType, withBox,
23 boxyUnify, boxyUnifyList, zapToMonotype,
24 boxySplitListTy, boxySplitTyConApp, boxySplitAppTy,
28 #include "HsVersions.h"
37 import TcRnMonad -- TcType, amongst others
56 %************************************************************************
58 \subsection{'hole' type variables}
60 %************************************************************************
63 tcInfer :: (BoxyType -> TcM a) -> TcM (a, TcType)
65 = do { box <- newBoxyTyVar openTypeKind
66 ; res <- tc_infer (mkTyVarTy box)
67 ; res_ty <- readFilledBox box -- Guaranteed filled-in by now
68 ; return (res, res_ty) }
72 %************************************************************************
76 %************************************************************************
79 subFunTys :: SDoc -- Somthing like "The function f has 3 arguments"
80 -- or "The abstraction (\x.e) takes 1 argument"
81 -> Arity -- Expected # of args
82 -> BoxyRhoType -- res_ty
83 -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
85 -- Attempt to decompse res_ty to have enough top-level arrows to
86 -- match the number of patterns in the match group
88 -- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
89 -- and the inner call to thing_inside passes args: [a1,...,an], b
90 -- then co_fn :: (a1 -> ... -> an -> b) -> res_ty
92 -- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
95 {- Error messages from subFunTys
97 The abstraction `\Just 1 -> ...' has two arguments
98 but its type `Maybe a -> a' has only one
100 The equation(s) for `f' have two arguments
101 but its type `Maybe a -> a' has only one
103 The section `(f 3)' requires 'f' to take two arguments
104 but its type `Int -> Int' has only one
106 The function 'f' is applied to two arguments
107 but its type `Int -> Int' has only one
111 subFunTys error_herald n_pats res_ty thing_inside
112 = loop n_pats [] res_ty
114 -- In 'loop', the parameter 'arg_tys' accumulates
115 -- the arg types so far, in *reverse order*
116 loop n args_so_far res_ty
117 | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
119 loop n args_so_far res_ty
120 | isSigmaTy res_ty -- Do this before checking n==0, because we
121 -- guarantee to return a BoxyRhoType, not a BoxySigmaType
122 = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet $ \ _ res_ty' ->
123 loop n args_so_far res_ty'
124 ; return (gen_fn <.> co_fn, res) }
126 loop 0 args_so_far res_ty
127 = do { res <- thing_inside (reverse args_so_far) res_ty
128 ; return (idHsWrapper, res) }
130 loop n args_so_far (FunTy arg_ty res_ty)
131 = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
132 ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
133 ; return (co_fn', res) }
135 -- res_ty might have a type variable at the head, such as (a b c),
136 -- in which case we must fill in with (->). Simplest thing to do
137 -- is to use boxyUnify, but we catch failure and generate our own
138 -- error message on failure
139 loop n args_so_far res_ty@(AppTy _ _)
140 = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
141 ; (_, mb_unit) <- tryTcErrs $ boxyUnify res_ty (FunTy arg_ty' res_ty')
142 ; if isNothing mb_unit then bale_out args_so_far
143 else loop n args_so_far (FunTy arg_ty' res_ty') }
145 loop n args_so_far (TyVarTy tv)
146 | not (isImmutableTyVar tv)
147 = do { cts <- readMetaTyVar tv
149 Indirect ty -> loop n args_so_far ty
150 Flexi -> do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
151 ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty
152 ; return (idHsWrapper, res) } }
154 mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
155 mk_res_ty [] = panic "TcUnify.mk_res_ty1"
156 kinds = openTypeKind : take n (repeat argTypeKind)
157 -- Note argTypeKind: the args can have an unboxed type,
158 -- but not an unboxed tuple.
160 loop n args_so_far res_ty = bale_out args_so_far
163 = do { env0 <- tcInitTidyEnv
164 ; res_ty' <- zonkTcType res_ty
165 ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
166 ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
168 mk_msg res_ty n_actual
169 = error_herald <> comma $$
170 sep [ptext SLIT("but its type") <+> quotes (pprType res_ty),
171 if n_actual == 0 then ptext SLIT("has none")
172 else ptext SLIT("has only") <+> speakN n_actual]
176 ----------------------
177 boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
178 -> BoxyRhoType -- Expected type (T a b c)
179 -> TcM [BoxySigmaType] -- Element types, a b c
180 -- It's used for wired-in tycons, so we call checkWiredInTyCOn
181 -- Precondition: never called with FunTyCon
182 -- Precondition: input type :: *
184 boxySplitTyConApp tc orig_ty
185 = do { checkWiredInTyCon tc
186 ; loop (tyConArity tc) [] orig_ty }
188 loop n_req args_so_far ty
189 | Just ty' <- tcView ty = loop n_req args_so_far ty'
191 loop n_req args_so_far (TyConApp tycon args)
193 = ASSERT( n_req == length args) -- ty::*
194 return (args ++ args_so_far)
196 loop n_req args_so_far (AppTy fun arg)
197 = loop (n_req - 1) (arg:args_so_far) fun
199 loop n_req args_so_far (TyVarTy tv)
200 | not (isImmutableTyVar tv)
201 = do { cts <- readMetaTyVar tv
203 Indirect ty -> loop n_req args_so_far ty
204 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
205 ; return (arg_tys ++ args_so_far) }
208 mk_res_ty arg_tys' = mkTyConApp tc arg_tys'
209 arg_kinds = map tyVarKind (take n_req (tyConTyVars tc))
211 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc))) orig_ty
213 ----------------------
214 boxySplitListTy :: BoxyRhoType -> TcM BoxySigmaType -- Special case for lists
215 boxySplitListTy exp_ty = do { [elt_ty] <- boxySplitTyConApp listTyCon exp_ty
219 ----------------------
220 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
221 -> TcM (BoxySigmaType, BoxySigmaType) -- Returns m, a
222 -- Assumes (m: * -> k), where k is the kind of the incoming type
223 -- If the incoming type is boxy, then so are the result types; and vice versa
225 boxySplitAppTy orig_ty
229 | Just ty' <- tcView ty = loop ty'
232 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
233 = return (fun_ty, arg_ty)
236 | not (isImmutableTyVar tv)
237 = do { cts <- readMetaTyVar tv
239 Indirect ty -> loop ty
240 Flexi -> do { [fun_ty,arg_ty] <- withMetaTvs tv kinds mk_res_ty
241 ; return (fun_ty, arg_ty) } }
243 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
244 mk_res_ty other = panic "TcUnify.mk_res_ty2"
245 tv_kind = tyVarKind tv
246 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
248 liftedTypeKind] -- arg type :: *
249 -- The defaultKind is a bit smelly. If you remove it,
250 -- try compiling f x = do { x }
251 -- and you'll get a kind mis-match. It smells, but
252 -- not enough to lose sleep over.
254 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
257 boxySplitFailure actual_ty expected_ty
258 = unifyMisMatch False False actual_ty expected_ty
259 -- "outer" is False, so we don't pop the context
260 -- which is what we want since we have not pushed one!
264 --------------------------------
265 -- withBoxes: the key utility function
266 --------------------------------
269 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
270 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
271 -> ([BoxySigmaType] -> BoxySigmaType)
272 -- Constructs the type to assign
273 -- to the original var
274 -> TcM [BoxySigmaType] -- Return the fresh boxes
276 -- It's entirely possible for the [kind] to be empty.
277 -- For example, when pattern-matching on True,
278 -- we call boxySplitTyConApp passing a boolTyCon
280 -- Invariant: tv is still Flexi
282 withMetaTvs tv kinds mk_res_ty
284 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
285 ; let box_tys = mkTyVarTys box_tvs
286 ; writeMetaTyVar tv (mk_res_ty box_tys)
289 | otherwise -- Non-boxy meta type variable
290 = do { tau_tys <- mapM newFlexiTyVarTy kinds
291 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
292 -- Sure to be a tau-type
295 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
296 -- Allocate a *boxy* tyvar
297 withBox kind thing_inside
298 = do { box_tv <- newMetaTyVar BoxTv kind
299 ; res <- thing_inside (mkTyVarTy box_tv)
300 ; ty <- readFilledBox box_tv
305 %************************************************************************
307 Approximate boxy matching
309 %************************************************************************
312 preSubType :: [TcTyVar] -- Quantified type variables
313 -> TcTyVarSet -- Subset of quantified type variables
314 -- see Note [Pre-sub boxy]
315 -> TcType -- The rho-type part; quantified tyvars scopes over this
316 -> BoxySigmaType -- Matching type from the context
317 -> TcM [TcType] -- Types to instantiate the tyvars
318 -- Perform pre-subsumption, and return suitable types
319 -- to instantiate the quantified type varibles:
320 -- info from the pre-subsumption, if there is any
321 -- a boxy type variable otherwise
323 -- Note [Pre-sub boxy]
324 -- The 'btvs' are a subset of 'qtvs'. They are the ones we can
325 -- instantiate to a boxy type variable, because they'll definitely be
326 -- filled in later. This isn't always the case; sometimes we have type
327 -- variables mentioned in the context of the type, but not the body;
328 -- f :: forall a b. C a b => a -> a
329 -- Then we may land up with an unconstrained 'b', so we want to
330 -- instantiate it to a monotype (non-boxy) type variable
332 -- The 'qtvs' that are *neither* fixed by the pre-subsumption, *nor* are in 'btvs',
333 -- are instantiated to TauTv meta variables.
335 preSubType qtvs btvs qty expected_ty
336 = do { tys <- mapM inst_tv qtvs
337 ; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
340 pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
342 | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
343 | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
344 ; return (mkTyVarTy tv') }
345 | otherwise = do { tv' <- tcInstTyVar tv
346 ; return (mkTyVarTy tv') }
349 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
350 -> BoxyRhoType -- Type to match (note a *Rho* type)
351 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
353 -- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
354 -- "Boxy types: inference for higher rank types and impredicativity"
356 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
357 = go tmpl_tvs tmpl_ty emptyVarSet boxy_ty
359 go t_tvs t_ty b_tvs b_ty
360 | Just t_ty' <- tcView t_ty = go t_tvs t_ty' b_tvs b_ty
361 | Just b_ty' <- tcView b_ty = go t_tvs t_ty b_tvs b_ty'
363 go t_tvs (TyVarTy _) b_tvs b_ty = emptyTvSubst -- Rule S-ANY; no bindings
364 -- Rule S-ANY covers (a) type variables and (b) boxy types
365 -- in the template. Both look like a TyVarTy.
366 -- See Note [Sub-match] below
368 go t_tvs t_ty b_tvs b_ty
369 | isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
370 = go (t_tvs `delVarSetList` tvs) t_tau b_tvs b_ty -- Rule S-SPEC
371 -- Under a forall on the left, if there is shadowing,
372 -- do not bind! Hence the delVarSetList.
373 | isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
374 = go t_tvs t_ty (extendVarSetList b_tvs tvs) b_tau -- Rule S-SKOL
375 -- Add to the variables we must not bind to
376 -- NB: it's *important* to discard the theta part. Otherwise
377 -- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
378 -- and end up with a completely bogus binding (b |-> Bool), by lining
379 -- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
380 -- This pre-subsumption stuff can return too few bindings, but it
381 -- must *never* return bogus info.
383 go t_tvs (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
384 = boxy_match t_tvs arg1 b_tvs arg2 (go t_tvs res1 b_tvs res2)
385 -- Match the args, and sub-match the results
387 go t_tvs t_ty b_tvs b_ty = boxy_match t_tvs t_ty b_tvs b_ty emptyTvSubst
388 -- Otherwise defer to boxy matching
389 -- This covers TyConApp, AppTy, PredTy
396 |- head xs : <rhobox>
397 We will do a boxySubMatchType between a ~ <rhobox>
398 But we *don't* want to match [a |-> <rhobox>] because
399 (a) The box should be filled in with a rho-type, but
400 but the returned substitution maps TyVars to boxy
402 (b) In any case, the right final answer might be *either*
403 instantiate 'a' with a rho-type or a sigma type
404 head xs : Int vs head xs : forall b. b->b
405 So the matcher MUST NOT make a choice here. In general, we only
406 bind a template type variable in boxyMatchType, not in boxySubMatchType.
411 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
412 -> [BoxySigmaType] -- Type to match
413 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
415 -- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
416 -- "Boxy types: inference for higher rank types and impredicativity"
418 -- Find a *boxy* substitution that makes the template look as much
419 -- like the BoxySigmaType as possible.
420 -- It's always ok to return an empty substitution;
421 -- anything more is jam on the pudding
423 -- NB1: This is a pure, non-monadic function.
424 -- It does no unification, and cannot fail
426 -- Precondition: the arg lengths are equal
427 -- Precondition: none of the template type variables appear anywhere in the [BoxySigmaType]
431 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
432 = ASSERT( length tmpl_tys == length boxy_tys )
433 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
434 -- ToDo: add error context?
436 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
438 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
439 = boxy_match tmpl_tvs t_ty boxy_tvs b_ty $
440 boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys subst
441 boxy_match_s tmpl_tvs _ boxy_tvs _ subst
442 = panic "boxy_match_s" -- Lengths do not match
446 boxy_match :: TcTyVarSet -> TcType -- Template
447 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
448 -> BoxySigmaType -- Match against this type
452 -- The boxy_tvs argument prevents this match:
453 -- [a] forall b. a ~ forall b. b
454 -- We don't want to bind the template variable 'a'
455 -- to the quantified type variable 'b'!
457 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
458 = go orig_tmpl_ty orig_boxy_ty
461 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
462 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
464 go ty1 ty2 -- C.f. the isSigmaTy case for boxySubMatchType
466 , (tvs1, _, tau1) <- tcSplitSigmaTy ty1
467 , (tvs2, _, tau2) <- tcSplitSigmaTy ty2
468 , equalLength tvs1 tvs2
469 = boxy_match (tmpl_tvs `delVarSetList` tvs1) tau1
470 (boxy_tvs `extendVarSetList` tvs2) tau2 subst
472 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
473 | tc1 == tc2 = go_s tys1 tys2
475 go (FunTy arg1 res1) (FunTy arg2 res2)
476 = go_s [arg1,res1] [arg2,res2]
479 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
480 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
481 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
482 = go_s [s1,t1] [s2,t2]
485 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
486 , boxy_tvs `disjointVarSet` tyVarsOfType orig_boxy_ty
487 , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
488 = extendTvSubst subst tv boxy_ty'
490 = subst -- Ignore others
492 boxy_ty' = case lookupTyVar subst tv of
493 Nothing -> orig_boxy_ty
494 Just ty -> ty `boxyLub` orig_boxy_ty
496 go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
497 -- Example: Tree a ~ Maybe Int
498 -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
499 -- misleading error messages. An even more confusing case is
500 -- a -> b ~ Maybe Int
501 -- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
502 -- from this pre-matching phase.
505 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
508 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
509 -- Combine boxy information from the two types
510 -- If there is a conflict, return the first
511 boxyLub orig_ty1 orig_ty2
512 = go orig_ty1 orig_ty2
514 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
515 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
516 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
517 | tc1 == tc2, length ts1 == length ts2
518 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
520 go (TyVarTy tv1) ty2 -- This is the whole point;
521 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
524 -- Look inside type synonyms, but only if the naive version fails
525 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
526 | Just ty2' <- tcView ty1 = go ty1 ty2'
528 -- For now, we don't look inside ForAlls, PredTys
529 go ty1 ty2 = orig_ty1 -- Default
532 Note [Matching kinds]
533 ~~~~~~~~~~~~~~~~~~~~~
534 The target type might legitimately not be a sub-kind of template.
535 For example, suppose the target is simply a box with an OpenTypeKind,
536 and the template is a type variable with LiftedTypeKind.
537 Then it's ok (because the target type will later be refined).
538 We simply don't bind the template type variable.
540 It might also be that the kind mis-match is an error. For example,
541 suppose we match the template (a -> Int) against (Int# -> Int),
542 where the template type variable 'a' has LiftedTypeKind. This
543 matching function does not fail; it simply doesn't bind the template.
544 Later stuff will fail.
546 %************************************************************************
550 %************************************************************************
552 All the tcSub calls have the form
554 tcSub expected_ty offered_ty
556 offered_ty <= expected_ty
558 That is, that a value of type offered_ty is acceptable in
559 a place expecting a value of type expected_ty.
561 It returns a coercion function
562 co_fn :: offered_ty -> expected_ty
563 which takes an HsExpr of type offered_ty into one of type
568 tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
569 -- (tcSub act exp) checks that
571 tcSubExp actual_ty expected_ty
572 = -- addErrCtxtM (unifyCtxt actual_ty expected_ty) $
573 -- Adding the error context here leads to some very confusing error
574 -- messages, such as "can't match forall a. a->a with forall a. a->a"
575 -- Example is tcfail165:
576 -- do var <- newEmptyMVar :: IO (MVar (forall a. Show a => a -> String))
577 -- putMVar var (show :: forall a. Show a => a -> String)
578 -- Here the info does not flow from the 'var' arg of putMVar to its 'show' arg
579 -- but after zonking it looks as if it does!
581 -- So instead I'm adding the error context when moving from tc_sub to u_tys
583 traceTc (text "tcSubExp" <+> ppr actual_ty <+> ppr expected_ty) >>
584 tc_sub SubOther actual_ty actual_ty False expected_ty expected_ty
586 tcFunResTy :: Name -> BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
587 tcFunResTy fun actual_ty expected_ty
588 = traceTc (text "tcFunResTy" <+> ppr actual_ty <+> ppr expected_ty) >>
589 tc_sub (SubFun fun) actual_ty actual_ty False expected_ty expected_ty
592 data SubCtxt = SubDone -- Error-context already pushed
593 | SubFun Name -- Context is tcFunResTy
594 | SubOther -- Context is something else
596 tc_sub :: SubCtxt -- How to add an error-context
597 -> BoxySigmaType -- actual_ty, before expanding synonyms
598 -> BoxySigmaType -- ..and after
599 -> InBox -- True <=> expected_ty is inside a box
600 -> BoxySigmaType -- expected_ty, before
601 -> BoxySigmaType -- ..and after
603 -- The acual_ty is never inside a box
604 -- IMPORTANT pre-condition: if the args contain foralls, the bound type
605 -- variables are visible non-monadically
606 -- (i.e. tha args are sufficiently zonked)
607 -- This invariant is needed so that we can "see" the foralls, ad
608 -- e.g. in the SPEC rule where we just use splitSigmaTy
610 tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
611 = tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
612 -- This indirection is just here to make
613 -- it easy to insert a debug trace!
615 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
616 | Just exp_ty' <- tcView exp_ty = tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty'
617 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
618 | Just act_ty' <- tcView act_ty = tc_sub sub_ctxt act_sty act_ty' exp_ib exp_sty exp_ty
620 -----------------------------------
621 -- Rule SBOXY, plus other cases when act_ty is a type variable
622 -- Just defer to boxy matching
623 -- This rule takes precedence over SKOL!
624 tc_sub1 sub_ctxt act_sty (TyVarTy tv) exp_ib exp_sty exp_ty
625 = do { addSubCtxt sub_ctxt act_sty exp_sty $
626 uVar True False tv exp_ib exp_sty exp_ty
627 ; return idHsWrapper }
629 -----------------------------------
630 -- Skolemisation case (rule SKOL)
631 -- actual_ty: d:Eq b => b->b
632 -- expected_ty: forall a. Ord a => a->a
633 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
635 -- It is essential to do this *before* the specialisation case
636 -- Example: f :: (Eq a => a->a) -> ...
637 -- g :: Ord b => b->b
640 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
641 | not exp_ib, -- SKOL does not apply if exp_ty is inside a box
643 = do { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ _ body_exp_ty ->
644 tc_sub sub_ctxt act_sty act_ty False body_exp_ty body_exp_ty
645 ; return (gen_fn <.> co_fn) }
647 act_tvs = tyVarsOfType act_ty
648 -- It's really important to check for escape wrt
649 -- the free vars of both expected_ty *and* actual_ty
651 -----------------------------------
652 -- Specialisation case (rule ASPEC):
653 -- actual_ty: forall a. Ord a => a->a
654 -- expected_ty: Int -> Int
655 -- co_fn e = e Int dOrdInt
657 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
658 -- Implements the new SPEC rule in the Appendix of the paper
659 -- "Boxy types: inference for higher rank types and impredicativity"
660 -- (This appendix isn't in the published version.)
661 -- The idea is to *first* do pre-subsumption, and then full subsumption
662 -- Example: forall a. a->a <= Int -> (forall b. Int)
663 -- Pre-subsumpion finds a|->Int, and that works fine, whereas
664 -- just running full subsumption would fail.
665 | isSigmaTy actual_ty
666 = do { -- Perform pre-subsumption, and instantiate
667 -- the type with info from the pre-subsumption;
668 -- boxy tyvars if pre-subsumption gives no info
669 let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
670 tau_tvs = exactTyVarsOfType tau
671 ; inst_tys <- if exp_ib then -- Inside a box, do not do clever stuff
672 do { tyvars' <- mapM tcInstBoxyTyVar tyvars
673 ; return (mkTyVarTys tyvars') }
674 else -- Outside, do clever stuff
675 preSubType tyvars tau_tvs tau expected_ty
676 ; let subst' = zipOpenTvSubst tyvars inst_tys
677 tau' = substTy subst' tau
679 -- Perform a full subsumption check
680 ; traceTc (text "tc_sub_spec" <+> vcat [ppr actual_ty,
681 ppr tyvars <+> ppr theta <+> ppr tau,
683 ; co_fn2 <- tc_sub sub_ctxt tau' tau' exp_ib exp_sty expected_ty
685 -- Deal with the dictionaries
686 ; co_fn1 <- instCall InstSigOrigin inst_tys (substTheta subst' theta)
687 ; return (co_fn2 <.> co_fn1) }
689 -----------------------------------
690 -- Function case (rule F1)
691 tc_sub1 sub_ctxt act_sty (FunTy act_arg act_res) exp_ib exp_sty (FunTy exp_arg exp_res)
692 = addSubCtxt sub_ctxt act_sty exp_sty $
693 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
695 -- Function case (rule F2)
696 tc_sub1 sub_ctxt act_sty act_ty@(FunTy act_arg act_res) _ exp_sty (TyVarTy exp_tv)
698 = addSubCtxt sub_ctxt act_sty exp_sty $
699 do { cts <- readMetaTyVar exp_tv
701 Indirect ty -> tc_sub SubDone act_sty act_ty True exp_sty ty
702 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
703 ; tc_sub_funs act_arg act_res True arg_ty res_ty } }
705 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
706 mk_res_ty other = panic "TcUnify.mk_res_ty3"
707 fun_kinds = [argTypeKind, openTypeKind]
709 -- Everything else: defer to boxy matching
710 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
711 = do { addSubCtxt sub_ctxt act_sty exp_sty $
712 u_tys True False act_sty actual_ty exp_ib exp_sty expected_ty
713 ; return idHsWrapper }
716 -----------------------------------
717 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
718 = do { uTys False act_arg exp_ib exp_arg
719 ; co_fn_res <- tc_sub SubDone act_res act_res exp_ib exp_res exp_res
720 ; wrapFunResCoercion [exp_arg] co_fn_res }
722 -----------------------------------
724 :: [TcType] -- Type of args
725 -> HsWrapper -- HsExpr a -> HsExpr b
726 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
727 wrapFunResCoercion arg_tys co_fn_res
728 | isIdHsWrapper co_fn_res = return idHsWrapper
729 | null arg_tys = return co_fn_res
731 = do { arg_ids <- newSysLocalIds FSLIT("sub") arg_tys
732 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpApps arg_ids) }
737 %************************************************************************
739 \subsection{Generalisation}
741 %************************************************************************
744 tcGen :: BoxySigmaType -- expected_ty
745 -> TcTyVarSet -- Extra tyvars that the universally
746 -- quantified tyvars of expected_ty
747 -- must not be unified
748 -> ([TcTyVar] -> BoxyRhoType -> TcM result)
749 -> TcM (HsWrapper, result)
750 -- The expression has type: spec_ty -> expected_ty
752 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
753 -- If not, the call is a no-op
754 = do { -- We want the GenSkol info in the skolemised type variables to
755 -- mention the *instantiated* tyvar names, so that we get a
756 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
757 -- Hence the tiresome but innocuous fixM
758 ((tvs', theta', rho'), skol_info) <- fixM (\ ~(_, skol_info) ->
759 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
760 ; span <- getSrcSpanM
761 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty) span
762 ; return ((forall_tvs, theta, rho_ty), skol_info) })
765 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
766 text "expected_ty" <+> ppr expected_ty,
767 text "inst ty" <+> ppr tvs' <+> ppr theta' <+> ppr rho',
768 text "free_tvs" <+> ppr free_tvs])
771 -- Type-check the arg and unify with poly type
772 ; (result, lie) <- getLIE (thing_inside tvs' rho')
774 -- Check that the "forall_tvs" havn't been constrained
775 -- The interesting bit here is that we must include the free variables
776 -- of the expected_ty. Here's an example:
777 -- runST (newVar True)
778 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
779 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
780 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
781 -- So now s' isn't unconstrained because it's linked to a.
782 -- Conclusion: include the free vars of the expected_ty in the
783 -- list of "free vars" for the signature check.
785 ; dicts <- newDictBndrsO (SigOrigin skol_info) theta'
786 ; inst_binds <- tcSimplifyCheck sig_msg tvs' dicts lie
788 ; checkSigTyVarsWrt free_tvs tvs'
789 ; traceTc (text "tcGen:done")
792 -- The WpLet binds any Insts which came out of the simplification.
793 dict_ids = map instToId dicts
794 co_fn = mkWpTyLams tvs' <.> mkWpLams dict_ids <.> WpLet inst_binds
795 ; returnM (co_fn, result) }
797 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
798 sig_msg = ptext SLIT("expected type of an expression")
803 %************************************************************************
807 %************************************************************************
809 The exported functions are all defined as versions of some
810 non-exported generic functions.
813 boxyUnify :: BoxyType -> BoxyType -> TcM ()
814 -- Acutal and expected, respectively
816 = addErrCtxtM (unifyCtxt ty1 ty2) $
817 uTysOuter False ty1 False ty2
820 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM ()
821 -- Arguments should have equal length
822 -- Acutal and expected types
823 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
826 unifyType :: TcTauType -> TcTauType -> TcM ()
827 -- No boxes expected inside these types
828 -- Acutal and expected types
829 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
830 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
831 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
832 addErrCtxtM (unifyCtxt ty1 ty2) $
833 uTysOuter True ty1 True ty2
836 unifyPred :: PredType -> PredType -> TcM ()
837 -- Acutal and expected types
838 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
839 uPred True True p1 True p2
841 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
842 -- Acutal and expected types
843 unifyTheta theta1 theta2
844 = do { checkTc (equalLength theta1 theta2)
845 (ptext SLIT("Contexts differ in length"))
846 ; uList unifyPred theta1 theta2 }
849 uList :: (a -> a -> TcM ())
850 -> [a] -> [a] -> TcM ()
851 -- Unify corresponding elements of two lists of types, which
852 -- should be f equal length. We charge down the list explicitly so that
853 -- we can complain if their lengths differ.
854 uList unify [] [] = return ()
855 uList unify (ty1:tys1) (ty2:tys2) = do { unify ty1 ty2; uList unify tys1 tys2 }
856 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
859 @unifyTypeList@ takes a single list of @TauType@s and unifies them
860 all together. It is used, for example, when typechecking explicit
861 lists, when all the elts should be of the same type.
864 unifyTypeList :: [TcTauType] -> TcM ()
865 unifyTypeList [] = returnM ()
866 unifyTypeList [ty] = returnM ()
867 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
868 ; unifyTypeList tys }
871 %************************************************************************
873 \subsection[Unify-uTys]{@uTys@: getting down to business}
875 %************************************************************************
877 @uTys@ is the heart of the unifier. Each arg happens twice, because
878 we want to report errors in terms of synomyms if poss. The first of
879 the pair is used in error messages only; it is always the same as the
880 second, except that if the first is a synonym then the second may be a
881 de-synonym'd version. This way we get better error messages.
883 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
886 type InBox = Bool -- True <=> we are inside a box
887 -- False <=> we are outside a box
888 -- The importance of this is that if we get "filled-box meets
889 -- filled-box", we'll look into the boxes and unify... but
890 -- we must not allow polytypes. But if we are in a box on
891 -- just one side, then we can allow polytypes
893 type Outer = Bool -- True <=> this is the outer level of a unification
894 -- so that the types being unified are the
895 -- very ones we began with, not some sub
896 -- component or synonym expansion
897 -- The idea is that if Outer is true then unifyMisMatch should
898 -- pop the context to remove the "Expected/Acutal" context
901 :: InBox -> TcType -- ty1 is the *expected* type
902 -> InBox -> TcType -- ty2 is the *actual* type
904 uTysOuter nb1 ty1 nb2 ty2 = do { traceTc (text "uTysOuter" <+> ppr ty1 <+> ppr ty2)
905 ; u_tys True nb1 ty1 ty1 nb2 ty2 ty2 }
906 uTys nb1 ty1 nb2 ty2 = do { traceTc (text "uTys" <+> ppr ty1 <+> ppr ty2)
907 ; u_tys False nb1 ty1 ty1 nb2 ty2 ty2 }
911 uTys_s :: InBox -> [TcType] -- ty1 is the *actual* types
912 -> InBox -> [TcType] -- ty2 is the *expected* types
914 uTys_s nb1 [] nb2 [] = returnM ()
915 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { uTys nb1 ty1 nb2 ty2
916 ; uTys_s nb1 tys1 nb2 tys2 }
917 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
921 -> InBox -> TcType -> TcType -- ty1 is the *actual* type
922 -> InBox -> TcType -> TcType -- ty2 is the *expected* type
925 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
929 -- Always expand synonyms (see notes at end)
930 -- (this also throws away FTVs)
932 | Just ty1' <- tcView ty1 = go False ty1' ty2
933 | Just ty2' <- tcView ty2 = go False ty1 ty2'
935 -- Variables; go for uVar
936 go outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
937 go outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
938 -- "True" means args swapped
940 go outer (PredTy p1) (PredTy p2) = uPred outer nb1 p1 nb2 p2
942 -- Type constructors must match
943 go _ (TyConApp con1 tys1) (TyConApp con2 tys2)
944 | con1 == con2 = uTys_s nb1 tys1 nb2 tys2
945 -- See Note [TyCon app]
947 -- Functions; just check the two parts
948 go _ (FunTy fun1 arg1) (FunTy fun2 arg2)
949 = do { uTys nb1 fun1 nb2 fun2
950 ; uTys nb1 arg1 nb2 arg2 }
952 -- Applications need a bit of care!
953 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
954 -- NB: we've already dealt with type variables and Notes,
955 -- so if one type is an App the other one jolly well better be too
956 go outer (AppTy s1 t1) ty2
957 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
958 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
960 -- Now the same, but the other way round
961 -- Don't swap the types, because the error messages get worse
962 go outer ty1 (AppTy s2 t2)
963 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
964 = do { uTys nb1 s1 nb2 s2; uTys nb1 t1 nb2 t2 }
966 go _ ty1@(ForAllTy _ _) ty2@(ForAllTy _ _)
967 | length tvs1 == length tvs2
968 = do { tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
969 ; let tys = mkTyVarTys tvs
970 in_scope = mkInScopeSet (mkVarSet tvs)
971 subst1 = mkTvSubst in_scope (zipTyEnv tvs1 tys)
972 subst2 = mkTvSubst in_scope (zipTyEnv tvs2 tys)
973 ; uTys nb1 (substTy subst1 body1) nb2 (substTy subst2 body2)
975 -- If both sides are inside a box, we are in a "box-meets-box"
976 -- situation, and we should not have a polytype at all.
977 -- If we get here we have two boxes, already filled with
978 -- the same polytype... but it should be a monotype.
979 -- This check comes last, because the error message is
980 -- extremely unhelpful.
981 ; ifM (nb1 && nb2) (notMonoType ty1)
984 (tvs1, body1) = tcSplitForAllTys ty1
985 (tvs2, body2) = tcSplitForAllTys ty2
987 -- Anything else fails
988 go outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
991 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
992 | n1 == n2 = uTys nb1 t1 nb2 t2
993 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
994 | c1 == c2 = uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
995 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
1000 When we find two TyConApps, the argument lists are guaranteed equal
1001 length. Reason: intially the kinds of the two types to be unified is
1002 the same. The only way it can become not the same is when unifying two
1003 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
1004 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
1005 which we do, that ensures that f1,f2 have the same kind; and that
1006 means a1,a2 have the same kind. And now the argument repeats.
1011 If you are tempted to make a short cut on synonyms, as in this
1015 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
1016 -- NO = if (con1 == con2) then
1017 -- NO -- Good news! Same synonym constructors, so we can shortcut
1018 -- NO -- by unifying their arguments and ignoring their expansions.
1019 -- NO unifyTypepeLists args1 args2
1021 -- NO -- Never mind. Just expand them and try again
1025 then THINK AGAIN. Here is the whole story, as detected and reported
1026 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1028 Here's a test program that should detect the problem:
1032 x = (1 :: Bogus Char) :: Bogus Bool
1035 The problem with [the attempted shortcut code] is that
1039 is not a sufficient condition to be able to use the shortcut!
1040 You also need to know that the type synonym actually USES all
1041 its arguments. For example, consider the following type synonym
1042 which does not use all its arguments.
1047 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1048 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1049 would fail, even though the expanded forms (both \tr{Int}) should
1052 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1053 unnecessarily bind \tr{t} to \tr{Char}.
1055 ... You could explicitly test for the problem synonyms and mark them
1056 somehow as needing expansion, perhaps also issuing a warning to the
1061 %************************************************************************
1063 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1065 %************************************************************************
1067 @uVar@ is called when at least one of the types being unified is a
1068 variable. It does {\em not} assume that the variable is a fixed point
1069 of the substitution; rather, notice that @uVar@ (defined below) nips
1070 back into @uTys@ if it turns out that the variable is already bound.
1074 -> Bool -- False => tyvar is the "expected"
1075 -- True => ty is the "expected" thing
1077 -> InBox -- True <=> definitely no boxes in t2
1078 -> TcTauType -> TcTauType -- printing and real versions
1081 uVar outer swapped tv1 nb2 ps_ty2 ty2
1082 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1083 | otherwise = brackets (equals <+> ppr ty2)
1084 ; traceTc (text "uVar" <+> ppr swapped <+>
1085 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1086 nest 2 (ptext SLIT(" <-> ")),
1087 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1088 ; details <- lookupTcTyVar tv1
1091 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1092 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1093 -- The 'True' here says that ty1 is now inside a box
1094 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1098 uUnfilledVar :: Outer
1099 -> Bool -- Args are swapped
1100 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1101 -> TcTauType -> TcTauType -- Type 2
1103 -- Invariant: tyvar 1 is not unified with anything
1105 uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1106 | Just ty2' <- tcView ty2
1107 = -- Expand synonyms; ignore FTVs
1108 uUnfilledVar False swapped tv1 details1 ps_ty2 ty2'
1110 uUnfilledVar outer swapped tv1 details1 ps_ty2 (TyVarTy tv2)
1111 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1113 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1114 -- this is box-meets-box, so fill in with a tau-type
1115 -> do { tau_tv <- tcInstTyVar tv1
1116 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv) }
1117 other -> returnM () -- No-op
1119 -- Distinct type variables
1121 = do { lookup2 <- lookupTcTyVar tv2
1123 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
1124 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1127 uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2 -- ty2 is not a type variable
1129 MetaTv (SigTv _) ref1 -> mis_match -- Can't update a skolem with a non-type-variable
1130 MetaTv info ref1 -> uMetaVar swapped tv1 info ref1 ps_ty2 non_var_ty2
1131 skolem_details -> mis_match
1133 mis_match = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1137 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1140 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1141 -- ty2 is not a type variable
1143 uMetaVar swapped tv1 BoxTv ref1 ps_ty2 non_var_ty2
1144 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1145 -- that any boxes in ty2 are filled with monotypes
1147 -- It should not be the case that tv1 occurs in ty2
1148 -- (i.e. no occurs check should be needed), but if perchance
1149 -- it does, the unbox operation will fill it, and the DEBUG
1151 do { final_ty <- unBox ps_ty2
1153 ; meta_details <- readMutVar ref1
1154 ; case meta_details of
1155 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1156 return () -- This really should *not* happen
1159 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1161 uMetaVar swapped tv1 info1 ref1 ps_ty2 non_var_ty2
1162 = do { final_ty <- checkTauTvUpdate tv1 ps_ty2 -- Occurs check + monotype check
1163 ; checkUpdateMeta swapped tv1 ref1 final_ty }
1166 uUnfilledVars :: Outer
1167 -> Bool -- Args are swapped
1168 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1169 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1171 -- Invarant: The type variables are distinct,
1172 -- Neither is filled in yet
1173 -- They might be boxy or not
1175 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1176 = unifyMisMatch outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1178 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1179 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2)
1180 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1181 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
1183 -- ToDo: this function seems too long for what it acutally does!
1184 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1185 = case (info1, info2) of
1186 (BoxTv, BoxTv) -> box_meets_box
1188 -- If a box meets a TauTv, but the fomer has the smaller kind
1189 -- then we must create a fresh TauTv with the smaller kind
1190 (_, BoxTv) | k1_sub_k2 -> update_tv2
1191 | otherwise -> box_meets_box
1192 (BoxTv, _ ) | k2_sub_k1 -> update_tv1
1193 | otherwise -> box_meets_box
1195 -- Avoid SigTvs if poss
1196 (SigTv _, _ ) | k1_sub_k2 -> update_tv2
1197 (_, SigTv _) | k2_sub_k1 -> update_tv1
1199 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1200 then update_tv1 -- Same kinds
1202 | k2_sub_k1 -> update_tv1
1203 | otherwise -> kind_err
1205 -- Update the variable with least kind info
1206 -- See notes on type inference in Kind.lhs
1207 -- The "nicer to" part only applies if the two kinds are the same,
1208 -- so we can choose which to do.
1210 -- Kinds should be guaranteed ok at this point
1211 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1212 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1214 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1217 | k2_sub_k1 = fill_from tv2
1218 | otherwise = kind_err
1220 -- Update *both* tyvars with a TauTv whose name and kind
1221 -- are gotten from tv (avoid losing nice names is poss)
1222 fill_from tv = do { tv' <- tcInstTyVar tv
1223 ; let tau_ty = mkTyVarTy tv'
1224 ; updateMeta tv1 ref1 tau_ty
1225 ; updateMeta tv2 ref2 tau_ty }
1227 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1228 unifyKindMisMatch k1 k2
1232 k1_sub_k2 = k1 `isSubKind` k2
1233 k2_sub_k1 = k2 `isSubKind` k1
1235 nicer_to_update_tv1 = isSystemName (Var.varName tv1)
1236 -- Try to update sys-y type variables in preference to ones
1237 -- gotten (say) by instantiating a polymorphic function with
1238 -- a user-written type sig
1241 checkUpdateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1242 -- Update tv1, which is flexi; occurs check is alrady done
1243 -- The 'check' version does a kind check too
1244 -- We do a sub-kind check here: we might unify (a b) with (c d)
1245 -- where b::*->* and d::*; this should fail
1247 checkUpdateMeta swapped tv1 ref1 ty2
1248 = do { checkKinds swapped tv1 ty2
1249 ; updateMeta tv1 ref1 ty2 }
1251 updateMeta :: TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1252 updateMeta tv1 ref1 ty2
1253 = ASSERT( isMetaTyVar tv1 )
1254 ASSERT( isBoxyTyVar tv1 || isTauTy ty2 )
1255 do { ASSERTM2( do { details <- readMetaTyVar tv1; return (isFlexi details) }, ppr tv1 )
1256 ; traceTc (text "updateMeta" <+> ppr tv1 <+> text ":=" <+> ppr ty2)
1257 ; writeMutVar ref1 (Indirect ty2) }
1260 checkKinds swapped tv1 ty2
1261 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
1262 -- ty2 has been zonked at this stage, which ensures that
1263 -- its kind has as much boxity information visible as possible.
1264 | tk2 `isSubKind` tk1 = returnM ()
1267 -- Either the kinds aren't compatible
1268 -- (can happen if we unify (a b) with (c d))
1269 -- or we are unifying a lifted type variable with an
1270 -- unlifted type: e.g. (id 3#) is illegal
1271 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1272 unifyKindMisMatch k1 k2
1274 (k1,k2) | swapped = (tk2,tk1)
1275 | otherwise = (tk1,tk2)
1280 checkTauTvUpdate :: TcTyVar -> TcType -> TcM TcType
1281 -- (checkTauTvUpdate tv ty)
1282 -- We are about to update the TauTv tv with ty.
1283 -- Check (a) that tv doesn't occur in ty (occurs check)
1284 -- (b) that ty is a monotype
1285 -- Furthermore, in the interest of (b), if you find an
1286 -- empty box (BoxTv that is Flexi), fill it in with a TauTv
1288 -- Returns the (non-boxy) type to update the type variable with, or fails
1290 checkTauTvUpdate orig_tv orig_ty
1293 go (TyConApp tc tys)
1294 | isSynTyCon tc = go_syn tc tys
1295 | otherwise = do { tys' <- mappM go tys; return (TyConApp tc tys') }
1296 go (NoteTy _ ty2) = go ty2 -- Discard free-tyvar annotations
1297 go (PredTy p) = do { p' <- go_pred p; return (PredTy p') }
1298 go (FunTy arg res) = do { arg' <- go arg; res' <- go res; return (FunTy arg' res') }
1299 go (AppTy fun arg) = do { fun' <- go fun; arg' <- go arg; return (mkAppTy fun' arg') }
1300 -- NB the mkAppTy; we might have instantiated a
1301 -- type variable to a type constructor, so we need
1302 -- to pull the TyConApp to the top.
1303 go (ForAllTy tv ty) = notMonoType orig_ty -- (b)
1306 | orig_tv == tv = occurCheck tv orig_ty -- (a)
1307 | isTcTyVar tv = go_tyvar tv (tcTyVarDetails tv)
1308 | otherwise = return (TyVarTy tv)
1309 -- Ordinary (non Tc) tyvars
1310 -- occur inside quantified types
1312 go_pred (ClassP c tys) = do { tys' <- mapM go tys; return (ClassP c tys') }
1313 go_pred (IParam n ty) = do { ty' <- go ty; return (IParam n ty') }
1314 go_pred (EqPred t1 t2) = do { t1' <- go t1; t2' <- go t2; return (EqPred t1' t2') }
1316 go_tyvar tv (SkolemTv _) = return (TyVarTy tv)
1317 go_tyvar tv (MetaTv box ref)
1318 = do { cts <- readMutVar ref
1320 Indirect ty -> go ty
1321 Flexi -> case box of
1322 BoxTv -> fillBoxWithTau tv ref
1323 other -> return (TyVarTy tv)
1326 -- go_syn is called for synonyms only
1327 -- See Note [Type synonyms and the occur check]
1329 | not (isTauTyCon tc)
1330 = notMonoType orig_ty -- (b) again
1332 = do { (msgs, mb_tys') <- tryTc (mapM go tys)
1334 Just tys' -> return (TyConApp tc tys')
1335 -- Retain the synonym (the common case)
1336 Nothing -> go (expectJust "checkTauTvUpdate"
1337 (tcView (TyConApp tc tys)))
1338 -- Try again, expanding the synonym
1341 fillBoxWithTau :: BoxyTyVar -> IORef MetaDetails -> TcM TcType
1342 -- (fillBoxWithTau tv ref) fills ref with a freshly allocated
1343 -- tau-type meta-variable, whose print-name is the same as tv
1344 -- Choosing the same name is good: when we instantiate a function
1345 -- we allocate boxy tyvars with the same print-name as the quantified
1346 -- tyvar; and then we often fill the box with a tau-tyvar, and again
1347 -- we want to choose the same name.
1348 fillBoxWithTau tv ref
1349 = do { tv' <- tcInstTyVar tv -- Do not gratuitously forget
1350 ; let tau = mkTyVarTy tv' -- name of the type variable
1351 ; writeMutVar ref (Indirect tau)
1355 Note [Type synonyms and the occur check]
1356 ~~~~~~~~~~~~~~~~~~~~
1357 Basically we want to update tv1 := ps_ty2
1358 because ps_ty2 has type-synonym info, which improves later error messages
1363 f :: (A a -> a -> ()) -> ()
1367 x = f (\ x p -> p x)
1369 In the application (p x), we try to match "t" with "A t". If we go
1370 ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
1371 an infinite loop later.
1372 But we should not reject the program, because A t = ().
1373 Rather, we should bind t to () (= non_var_ty2).
1376 stripBoxyType :: BoxyType -> TcM TcType
1377 -- Strip all boxes from the input type, returning a non-boxy type.
1378 -- It's fine for there to be a polytype inside a box (c.f. unBox)
1379 -- All of the boxes should have been filled in by now;
1380 -- hence we return a TcType
1381 stripBoxyType ty = zonkType strip_tv ty
1383 strip_tv tv = ASSERT( not (isBoxyTyVar tv) ) return (TyVarTy tv)
1384 -- strip_tv will be called for *Flexi* meta-tyvars
1385 -- There should not be any Boxy ones; hence the ASSERT
1387 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1388 -- Subtle... we must zap the boxy res_ty
1389 -- to kind * before using it to instantiate a LitInst
1390 -- Calling unBox instead doesn't do the job, because the box
1391 -- often has an openTypeKind, and we don't want to instantiate
1393 zapToMonotype res_ty
1394 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1395 ; boxyUnify res_tau res_ty
1398 unBox :: BoxyType -> TcM TcType
1399 -- unBox implements the judgement
1401 -- with input s', and result s
1403 -- It removes all boxes from the input type, returning a non-boxy type.
1404 -- A filled box in the type can only contain a monotype; unBox fails if not
1405 -- The type can have empty boxes, which unBox fills with a monotype
1407 -- Compare this wth checkTauTvUpdate
1409 -- For once, it's safe to treat synonyms as opaque!
1411 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1412 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1413 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1414 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1415 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1416 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1417 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1419 | isTcTyVar tv -- It's a boxy type variable
1420 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1421 = do { cts <- readMutVar ref -- under nested quantifiers
1423 Flexi -> fillBoxWithTau tv ref
1424 Indirect ty -> do { non_boxy_ty <- unBox ty
1425 ; if isTauTy non_boxy_ty
1426 then return non_boxy_ty
1427 else notMonoType non_boxy_ty }
1429 | otherwise -- Skolems, and meta-tau-variables
1430 = return (TyVarTy tv)
1432 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1433 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1434 unBoxPred (EqPred ty1 ty2) = do { ty1' <- unBox ty1; ty2' <- unBox ty2; return (EqPred ty1' ty2') }
1439 %************************************************************************
1441 \subsection[Unify-context]{Errors and contexts}
1443 %************************************************************************
1449 unifyCtxt act_ty exp_ty tidy_env
1450 = do { act_ty' <- zonkTcType act_ty
1451 ; exp_ty' <- zonkTcType exp_ty
1452 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1453 (env2, act_ty'') = tidyOpenType env1 act_ty'
1454 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1457 mkExpectedActualMsg act_ty exp_ty
1458 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1459 text "Inferred type" <> colon <+> ppr act_ty ])
1462 -- If an error happens we try to figure out whether the function
1463 -- function has been given too many or too few arguments, and say so.
1464 addSubCtxt SubDone actual_res_ty expected_res_ty thing_inside
1466 addSubCtxt sub_ctxt actual_res_ty expected_res_ty thing_inside
1467 = addErrCtxtM mk_err thing_inside
1470 = do { exp_ty' <- zonkTcType expected_res_ty
1471 ; act_ty' <- zonkTcType actual_res_ty
1472 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1473 (env2, act_ty'') = tidyOpenType env1 act_ty'
1474 (exp_args, _) = tcSplitFunTys exp_ty''
1475 (act_args, _) = tcSplitFunTys act_ty''
1477 len_act_args = length act_args
1478 len_exp_args = length exp_args
1480 message = case sub_ctxt of
1481 SubFun fun | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1482 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1483 other -> mkExpectedActualMsg act_ty'' exp_ty''
1484 ; return (env2, message) }
1486 wrongArgsCtxt too_many_or_few fun
1487 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1488 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1489 <+> ptext SLIT("arguments")
1492 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1493 -- tv1 and ty2 are zonked already
1496 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1497 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1499 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1500 | otherwise = (pp1, pp2)
1501 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1502 (env2, ty2') = tidyOpenType env1 ty2
1503 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1504 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1506 unifyMisMatch outer swapped ty1 ty2
1507 = do { (env, msg) <- if swapped then misMatchMsg ty1 ty2
1508 else misMatchMsg ty2 ty1
1510 -- This is the whole point of the 'outer' stuff
1511 ; if outer then popErrCtxt (failWithTcM (env, msg))
1512 else failWithTcM (env, msg)
1516 = do { env0 <- tcInitTidyEnv
1517 ; (env1, pp1, extra1) <- ppr_ty env0 ty1
1518 ; (env2, pp2, extra2) <- ppr_ty env1 ty2
1519 ; return (env2, sep [sep [ptext SLIT("Couldn't match expected type") <+> pp1,
1520 nest 7 (ptext SLIT("against inferred type") <+> pp2)],
1521 nest 2 extra1, nest 2 extra2]) }
1523 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
1525 = do { ty' <- zonkTcType ty
1526 ; let (env1,tidy_ty) = tidyOpenType env ty'
1527 simple_result = (env1, quotes (ppr tidy_ty), empty)
1530 | isSkolemTyVar tv || isSigTyVar tv
1531 -> return (env2, pp_rigid tv', pprSkolTvBinding tv')
1532 | otherwise -> return simple_result
1534 (env2, tv') = tidySkolemTyVar env1 tv
1535 other -> return simple_result }
1537 pp_rigid tv = quotes (ppr tv) <+> parens (ptext SLIT("a rigid variable"))
1541 = do { ty' <- zonkTcType ty
1542 ; env0 <- tcInitTidyEnv
1543 ; let (env1, tidy_ty) = tidyOpenType env0 ty'
1544 msg = ptext SLIT("Cannot match a monotype with") <+> quotes (ppr tidy_ty)
1545 ; failWithTcM (env1, msg) }
1548 = do { env0 <- tcInitTidyEnv
1549 ; ty' <- zonkTcType ty
1550 ; let (env1, tidy_tyvar) = tidyOpenTyVar env0 tyvar
1551 (env2, tidy_ty) = tidyOpenType env1 ty'
1552 extra = sep [ppr tidy_tyvar, char '=', ppr tidy_ty]
1553 ; failWithTcM (env2, hang msg 2 extra) }
1555 msg = ptext SLIT("Occurs check: cannot construct the infinite type:")
1559 %************************************************************************
1563 %************************************************************************
1565 Unifying kinds is much, much simpler than unifying types.
1568 unifyKind :: TcKind -- Expected
1571 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1572 | isSubKindCon kc2 kc1 = returnM ()
1574 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1575 = do { unifyKind a2 a1; unifyKind r1 r2 }
1576 -- Notice the flip in the argument,
1577 -- so that the sub-kinding works right
1578 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1579 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1580 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1582 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1583 unifyKinds [] [] = returnM ()
1584 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1586 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1589 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1590 uKVar swapped kv1 k2
1591 = do { mb_k1 <- readKindVar kv1
1593 Flexi -> uUnboundKVar swapped kv1 k2
1594 Indirect k1 | swapped -> unifyKind k2 k1
1595 | otherwise -> unifyKind k1 k2 }
1598 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1599 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1600 | kv1 == kv2 = returnM ()
1601 | otherwise -- Distinct kind variables
1602 = do { mb_k2 <- readKindVar kv2
1604 Indirect k2 -> uUnboundKVar swapped kv1 k2
1605 Flexi -> writeKindVar kv1 k2 }
1607 uUnboundKVar swapped kv1 non_var_k2
1608 = do { k2' <- zonkTcKind non_var_k2
1609 ; kindOccurCheck kv1 k2'
1610 ; k2'' <- kindSimpleKind swapped k2'
1611 -- KindVars must be bound only to simple kinds
1612 -- Polarities: (kindSimpleKind True ?) succeeds
1613 -- returning *, corresponding to unifying
1616 ; writeKindVar kv1 k2'' }
1619 kindOccurCheck kv1 k2 -- k2 is zonked
1620 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1622 not_in (TyVarTy kv2) = kv1 /= kv2
1623 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1626 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1627 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1628 -- If the flag is False, it requires k <: sk
1629 -- E.g. kindSimpleKind False ?? = *
1630 -- What about (kv -> *) :=: ?? -> *
1631 kindSimpleKind orig_swapped orig_kind
1632 = go orig_swapped orig_kind
1634 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1636 ; return (mkArrowKind k1' k2') }
1638 | isOpenTypeKind k = return liftedTypeKind
1639 | isArgTypeKind k = return liftedTypeKind
1641 | isLiftedTypeKind k = return liftedTypeKind
1642 | isUnliftedTypeKind k = return unliftedTypeKind
1643 go sw k@(TyVarTy _) = return k -- KindVars are always simple
1644 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1645 <+> ppr orig_swapped <+> ppr orig_kind)
1646 -- I think this can't actually happen
1648 -- T v = MkT v v must be a type
1649 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1652 kindOccurCheckErr tyvar ty
1653 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1654 2 (sep [ppr tyvar, char '=', ppr ty])
1656 unifyKindMisMatch ty1 ty2
1657 = zonkTcKind ty1 `thenM` \ ty1' ->
1658 zonkTcKind ty2 `thenM` \ ty2' ->
1660 msg = hang (ptext SLIT("Couldn't match kind"))
1661 2 (sep [quotes (ppr ty1'),
1662 ptext SLIT("against"),
1669 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1670 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1672 unifyFunKind (TyVarTy kvar)
1673 = readKindVar kvar `thenM` \ maybe_kind ->
1675 Indirect fun_kind -> unifyFunKind fun_kind
1677 do { arg_kind <- newKindVar
1678 ; res_kind <- newKindVar
1679 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1680 ; returnM (Just (arg_kind,res_kind)) }
1682 unifyFunKind (FunTy arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1683 unifyFunKind other = returnM Nothing
1686 %************************************************************************
1690 %************************************************************************
1692 ---------------------------
1693 -- We would like to get a decent error message from
1694 -- (a) Under-applied type constructors
1695 -- f :: (Maybe, Maybe)
1696 -- (b) Over-applied type constructors
1697 -- f :: Int x -> Int x
1701 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1702 -- A fancy wrapper for 'unifyKind', which tries
1703 -- to give decent error messages.
1704 checkExpectedKind ty act_kind exp_kind
1705 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1708 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1710 Just r -> returnM () ; -- Unification succeeded
1713 -- So there's definitely an error
1714 -- Now to find out what sort
1715 zonkTcKind exp_kind `thenM` \ exp_kind ->
1716 zonkTcKind act_kind `thenM` \ act_kind ->
1718 tcInitTidyEnv `thenM` \ env0 ->
1719 let (exp_as, _) = splitKindFunTys exp_kind
1720 (act_as, _) = splitKindFunTys act_kind
1721 n_exp_as = length exp_as
1722 n_act_as = length act_as
1724 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1725 (env2, tidy_act_kind) = tidyKind env1 act_kind
1727 err | n_exp_as < n_act_as -- E.g. [Maybe]
1728 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1730 -- Now n_exp_as >= n_act_as. In the next two cases,
1731 -- n_exp_as == 0, and hence so is n_act_as
1732 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1733 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1734 <+> ptext SLIT("is unlifted")
1736 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1737 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1738 <+> ptext SLIT("is lifted")
1740 | otherwise -- E.g. Monad [Int]
1741 = ptext SLIT("Kind mis-match")
1743 more_info = sep [ ptext SLIT("Expected kind") <+>
1744 quotes (pprKind tidy_exp_kind) <> comma,
1745 ptext SLIT("but") <+> quotes (ppr ty) <+>
1746 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1748 failWithTcM (env2, err $$ more_info)
1752 %************************************************************************
1754 \subsection{Checking signature type variables}
1756 %************************************************************************
1758 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1759 are not mentioned in the environment. In particular:
1761 (a) Not mentioned in the type of a variable in the envt
1762 eg the signature for f in this:
1768 Here, f is forced to be monorphic by the free occurence of x.
1770 (d) Not (unified with another type variable that is) in scope.
1771 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1772 when checking the expression type signature, we find that
1773 even though there is nothing in scope whose type mentions r,
1774 nevertheless the type signature for the expression isn't right.
1776 Another example is in a class or instance declaration:
1778 op :: forall b. a -> b
1780 Here, b gets unified with a
1782 Before doing this, the substitution is applied to the signature type variable.
1785 checkSigTyVars :: [TcTyVar] -> TcM ()
1786 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1788 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1789 -- The extra_tvs can include boxy type variables;
1790 -- e.g. TcMatches.tcCheckExistentialPat
1791 checkSigTyVarsWrt extra_tvs sig_tvs
1792 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1793 ; check_sig_tyvars extra_tvs' sig_tvs }
1796 :: TcTyVarSet -- Global type variables. The universally quantified
1797 -- tyvars should not mention any of these
1798 -- Guaranteed already zonked.
1799 -> [TcTyVar] -- Universally-quantified type variables in the signature
1800 -- Guaranteed to be skolems
1802 check_sig_tyvars extra_tvs []
1804 check_sig_tyvars extra_tvs sig_tvs
1805 = ASSERT( all isSkolemTyVar sig_tvs )
1806 do { gbl_tvs <- tcGetGlobalTyVars
1807 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1808 text "gbl_tvs" <+> ppr gbl_tvs,
1809 text "extra_tvs" <+> ppr extra_tvs]))
1811 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1812 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1813 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1816 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1817 -> [TcTyVar] -- The possibly-escaping type variables
1818 -> [TcTyVar] -- The zonked versions thereof
1820 -- Complain about escaping type variables
1821 -- We pass a list of type variables, at least one of which
1822 -- escapes. The first list contains the original signature type variable,
1823 -- while the second contains the type variable it is unified to (usually itself)
1824 bleatEscapedTvs globals sig_tvs zonked_tvs
1825 = do { env0 <- tcInitTidyEnv
1826 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1827 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1829 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1830 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1832 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1834 check (tidy_env, msgs) (sig_tv, zonked_tv)
1835 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1837 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1838 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1840 -----------------------
1841 escape_msg sig_tv zonked_tv globs
1843 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1844 nest 2 (vcat globs)]
1846 = msg <+> ptext SLIT("escapes")
1847 -- Sigh. It's really hard to give a good error message
1848 -- all the time. One bad case is an existential pattern match.
1849 -- We rely on the "When..." context to help.
1851 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1853 | sig_tv == zonked_tv = empty
1854 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1857 These two context are used with checkSigTyVars
1860 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1861 -> TidyEnv -> TcM (TidyEnv, Message)
1862 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1863 = zonkTcType sig_tau `thenM` \ actual_tau ->
1865 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1866 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1867 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1868 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1869 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1871 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),