2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Type subsumption and unification
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
17 -- Full-blown subsumption
18 tcSubExp, tcFunResTy, tcGen,
19 checkSigTyVars, checkSigTyVarsWrt, bleatEscapedTvs, sigCtxt,
21 -- Various unifications
22 unifyType, unifyTypeList, unifyTheta,
23 unifyKind, unifyKinds, unifyFunKind,
25 preSubType, boxyMatchTypes,
27 --------------------------------
29 tcInfer, subFunTys, unBox, refineBox, refineBoxToTau, withBox,
30 boxyUnify, boxyUnifyList, zapToMonotype,
31 boxySplitListTy, boxySplitPArrTy, boxySplitTyConApp, boxySplitAppTy,
35 #include "HsVersions.h"
45 import TcRnMonad -- TcType, amongst others
65 %************************************************************************
67 \subsection{'hole' type variables}
69 %************************************************************************
72 tcInfer :: (BoxyType -> TcM a) -> TcM (a, TcType)
74 = do { box <- newBoxyTyVar openTypeKind
75 ; res <- tc_infer (mkTyVarTy box)
76 ; res_ty <- {- pprTrace "tcInfer" (ppr (mkTyVarTy box)) $ -} readFilledBox box -- Guaranteed filled-in by now
77 ; return (res, res_ty) }
81 %************************************************************************
85 %************************************************************************
88 subFunTys :: SDoc -- Somthing like "The function f has 3 arguments"
89 -- or "The abstraction (\x.e) takes 1 argument"
90 -> Arity -- Expected # of args
91 -> BoxyRhoType -- res_ty
92 -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
94 -- Attempt to decompse res_ty to have enough top-level arrows to
95 -- match the number of patterns in the match group
97 -- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
98 -- and the inner call to thing_inside passes args: [a1,...,an], b
99 -- then co_fn :: (a1 -> ... -> an -> b) ~ res_ty
101 -- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
104 {- Error messages from subFunTys
106 The abstraction `\Just 1 -> ...' has two arguments
107 but its type `Maybe a -> a' has only one
109 The equation(s) for `f' have two arguments
110 but its type `Maybe a -> a' has only one
112 The section `(f 3)' requires 'f' to take two arguments
113 but its type `Int -> Int' has only one
115 The function 'f' is applied to two arguments
116 but its type `Int -> Int' has only one
120 subFunTys error_herald n_pats res_ty thing_inside
121 = loop n_pats [] res_ty
123 -- In 'loop', the parameter 'arg_tys' accumulates
124 -- the arg types so far, in *reverse order*
125 -- INVARIANT: res_ty :: *
126 loop n args_so_far res_ty
127 | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
129 loop n args_so_far res_ty
130 | isSigmaTy res_ty -- Do this before checking n==0, because we
131 -- guarantee to return a BoxyRhoType, not a
133 = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet $ \ _ res_ty' ->
134 loop n args_so_far res_ty'
135 ; return (gen_fn <.> co_fn, res) }
137 loop 0 args_so_far res_ty
138 = do { res <- thing_inside (reverse args_so_far) res_ty
139 ; return (idHsWrapper, res) }
141 loop n args_so_far (FunTy arg_ty res_ty)
142 = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
143 ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
144 ; return (co_fn', res) }
146 -- Try to normalise synonym families and defer if that's not possible
147 loop n args_so_far ty@(TyConApp tc tys)
149 = do { (coi1, ty') <- tcNormaliseFamInst ty
151 IdCo -> defer n args_so_far ty
152 -- no progress, but maybe solvable => defer
153 ACo _ -> -- progress: so lets try again
154 do { (co_fn, res) <- loop n args_so_far ty'
155 ; return $ (co_fn <.> coiToHsWrapper (mkSymCoI coi1), res)
159 -- res_ty might have a type variable at the head, such as (a b c),
160 -- in which case we must fill in with (->). Simplest thing to do
161 -- is to use boxyUnify, but we catch failure and generate our own
162 -- error message on failure
163 loop n args_so_far res_ty@(AppTy _ _)
164 = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
165 ; (_, mb_coi) <- tryTcErrs $
166 boxyUnify res_ty (FunTy arg_ty' res_ty')
167 ; if isNothing mb_coi then bale_out args_so_far
168 else do { let coi = expectJust "subFunTys" mb_coi
169 ; (co_fn, res) <- loop n args_so_far (FunTy arg_ty'
171 ; return (co_fn <.> coiToHsWrapper coi, res)
175 loop n args_so_far ty@(TyVarTy tv)
176 | isTyConableTyVar tv
177 = do { cts <- readMetaTyVar tv
179 Indirect ty -> loop n args_so_far ty
181 do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
182 ; res <- thing_inside (reverse args_so_far ++ arg_tys)
184 ; return (idHsWrapper, res) } }
185 | otherwise -- defer as tyvar may be refined by equalities
186 = defer n args_so_far ty
188 mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
189 mk_res_ty [] = panic "TcUnify.mk_res_ty1"
190 kinds = openTypeKind : take n (repeat argTypeKind)
191 -- Note argTypeKind: the args can have an unboxed type,
192 -- but not an unboxed tuple.
194 loop n args_so_far res_ty = bale_out args_so_far
196 -- build a template type a1 -> ... -> an -> b and defer an equality
197 -- between that template and the expected result type res_ty; then,
198 -- use the template to type the thing_inside
199 defer n args_so_far ty
200 = do { arg_tys <- newFlexiTyVarTys n argTypeKind
201 ; res_ty' <- newFlexiTyVarTy openTypeKind
202 ; let fun_ty = mkFunTys arg_tys res_ty'
203 ; coi <- defer_unification False False fun_ty ty
204 ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty'
205 ; return (coiToHsWrapper coi, res)
209 = do { env0 <- tcInitTidyEnv
210 ; res_ty' <- zonkTcType res_ty
211 ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
212 ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
214 mk_msg res_ty n_actual
215 = error_herald <> comma $$
216 sep [ptext SLIT("but its type") <+> quotes (pprType res_ty),
217 if n_actual == 0 then ptext SLIT("has none")
218 else ptext SLIT("has only") <+> speakN n_actual]
222 ----------------------
223 boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
224 -> BoxyRhoType -- Expected type (T a b c)
225 -> TcM ([BoxySigmaType], -- Element types, a b c
227 -- It's used for wired-in tycons, so we call checkWiredInTyCon
228 -- Precondition: never called with FunTyCon
229 -- Precondition: input type :: *
231 boxySplitTyConApp tc orig_ty
232 = do { checkWiredInTyCon tc
233 ; loop (tyConArity tc) [] orig_ty }
235 loop n_req args_so_far ty
236 | Just ty' <- tcView ty = loop n_req args_so_far ty'
238 loop n_req args_so_far ty@(TyConApp tycon args)
240 = ASSERT( n_req == length args) -- ty::*
241 return (args ++ args_so_far, IdCo)
243 | isOpenSynTyCon tycon -- try to normalise type family application
244 = do { (coi1, ty') <- tcNormaliseFamInst ty
245 ; traceTc $ text "boxySplitTyConApp:" <+>
246 ppr ty <+> text "==>" <+> ppr ty'
248 IdCo -> defer -- no progress, but maybe solvable => defer
249 ACo _ -> -- progress: so lets try again
250 do { (args, coi2) <- loop n_req args_so_far ty'
251 ; return $ (args, coi2 `mkTransCoI` mkSymCoI coi1)
255 loop n_req args_so_far (AppTy fun arg)
257 = do { (args, coi) <- loop (n_req - 1) (arg:args_so_far) fun
258 ; return (args, mkAppTyCoI fun coi arg IdCo)
261 loop n_req args_so_far (TyVarTy tv)
262 | isTyConableTyVar tv
263 , res_kind `isSubKind` tyVarKind tv
264 = do { cts <- readMetaTyVar tv
266 Indirect ty -> loop n_req args_so_far ty
267 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
268 ; return (arg_tys ++ args_so_far, IdCo) }
270 | otherwise -- defer as tyvar may be refined by equalities
273 (arg_kinds, res_kind) = splitKindFunTysN n_req (tyConKind tc)
275 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc)))
278 -- defer splitting by generating an equality constraint
279 defer = boxySplitDefer arg_kinds mk_res_ty orig_ty
281 (arg_kinds, _) = splitKindFunTys (tyConKind tc)
283 -- apply splitted tycon to arguments
284 mk_res_ty = mkTyConApp tc
286 ----------------------
287 boxySplitListTy :: BoxyRhoType -> TcM (BoxySigmaType, CoercionI)
288 -- Special case for lists
289 boxySplitListTy exp_ty
290 = do { ([elt_ty], coi) <- boxySplitTyConApp listTyCon exp_ty
291 ; return (elt_ty, coi) }
293 ----------------------
294 boxySplitPArrTy :: BoxyRhoType -> TcM (BoxySigmaType, CoercionI)
295 -- Special case for parrs
296 boxySplitPArrTy exp_ty
297 = do { ([elt_ty], coi) <- boxySplitTyConApp parrTyCon exp_ty
298 ; return (elt_ty, coi) }
300 ----------------------
301 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
302 -> TcM ((BoxySigmaType, BoxySigmaType), -- Returns m, a
304 -- If the incoming type is a mutable type variable of kind k, then
305 -- boxySplitAppTy returns a new type variable (m: * -> k); note the *.
306 -- If the incoming type is boxy, then so are the result types; and vice versa
308 boxySplitAppTy orig_ty
312 | Just ty' <- tcView ty = loop ty'
315 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
316 = return ((fun_ty, arg_ty), IdCo)
318 loop ty@(TyConApp tycon args)
319 | isOpenSynTyCon tycon -- try to normalise type family application
320 = do { (coi1, ty') <- tcNormaliseFamInst ty
322 IdCo -> defer -- no progress, but maybe solvable => defer
323 ACo co -> -- progress: so lets try again
324 do { (args, coi2) <- loop ty'
325 ; return $ (args, coi2 `mkTransCoI` mkSymCoI coi1)
330 | isTyConableTyVar tv
331 = do { cts <- readMetaTyVar tv
333 Indirect ty -> loop ty
334 Flexi -> do { [fun_ty, arg_ty] <- withMetaTvs tv kinds mk_res_ty
335 ; return ((fun_ty, arg_ty), IdCo) } }
336 | otherwise -- defer as tyvar may be refined by equalities
339 tv_kind = tyVarKind tv
340 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
342 liftedTypeKind] -- arg type :: *
343 -- The defaultKind is a bit smelly. If you remove it,
344 -- try compiling f x = do { x }
345 -- and you'll get a kind mis-match. It smells, but
346 -- not enough to lose sleep over.
348 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
350 -- defer splitting by generating an equality constraint
351 defer = do { ([ty1, ty2], coi) <- boxySplitDefer arg_kinds mk_res_ty orig_ty
352 ; return ((ty1, ty2), coi)
355 orig_kind = typeKind orig_ty
356 arg_kinds = [mkArrowKind liftedTypeKind (defaultKind orig_kind),
358 liftedTypeKind] -- arg type :: *
360 -- build type application
361 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
362 mk_res_ty _other = panic "TcUnify.mk_res_ty2"
365 boxySplitFailure actual_ty expected_ty
366 = unifyMisMatch False False actual_ty expected_ty
367 -- "outer" is False, so we don't pop the context
368 -- which is what we want since we have not pushed one!
371 boxySplitDefer :: [Kind] -- kinds of required arguments
372 -> ([TcType] -> TcTauType) -- construct lhs from argument tyvars
373 -> BoxyRhoType -- type to split
374 -> TcM ([TcType], CoercionI)
375 boxySplitDefer kinds mkTy orig_ty
376 = do { tau_tys <- mapM newFlexiTyVarTy kinds
377 ; coi <- defer_unification False False (mkTy tau_tys) orig_ty
378 ; return (tau_tys, coi)
383 --------------------------------
384 -- withBoxes: the key utility function
385 --------------------------------
388 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
389 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
390 -> ([BoxySigmaType] -> BoxySigmaType)
391 -- Constructs the type to assign
392 -- to the original var
393 -> TcM [BoxySigmaType] -- Return the fresh boxes
395 -- It's entirely possible for the [kind] to be empty.
396 -- For example, when pattern-matching on True,
397 -- we call boxySplitTyConApp passing a boolTyCon
399 -- Invariant: tv is still Flexi
401 withMetaTvs tv kinds mk_res_ty
403 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
404 ; let box_tys = mkTyVarTys box_tvs
405 ; writeMetaTyVar tv (mk_res_ty box_tys)
408 | otherwise -- Non-boxy meta type variable
409 = do { tau_tys <- mapM newFlexiTyVarTy kinds
410 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
411 -- Sure to be a tau-type
414 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
415 -- Allocate a *boxy* tyvar
416 withBox kind thing_inside
417 = do { box_tv <- newMetaTyVar BoxTv kind
418 ; res <- thing_inside (mkTyVarTy box_tv)
419 ; ty <- {- pprTrace "with_box" (ppr (mkTyVarTy box_tv)) $ -} readFilledBox box_tv
424 %************************************************************************
426 Approximate boxy matching
428 %************************************************************************
431 preSubType :: [TcTyVar] -- Quantified type variables
432 -> TcTyVarSet -- Subset of quantified type variables
433 -- see Note [Pre-sub boxy]
434 -> TcType -- The rho-type part; quantified tyvars scopes over this
435 -> BoxySigmaType -- Matching type from the context
436 -> TcM [TcType] -- Types to instantiate the tyvars
437 -- Perform pre-subsumption, and return suitable types
438 -- to instantiate the quantified type varibles:
439 -- info from the pre-subsumption, if there is any
440 -- a boxy type variable otherwise
442 -- Note [Pre-sub boxy]
443 -- The 'btvs' are a subset of 'qtvs'. They are the ones we can
444 -- instantiate to a boxy type variable, because they'll definitely be
445 -- filled in later. This isn't always the case; sometimes we have type
446 -- variables mentioned in the context of the type, but not the body;
447 -- f :: forall a b. C a b => a -> a
448 -- Then we may land up with an unconstrained 'b', so we want to
449 -- instantiate it to a monotype (non-boxy) type variable
451 -- The 'qtvs' that are *neither* fixed by the pre-subsumption, *nor* are in 'btvs',
452 -- are instantiated to TauTv meta variables.
454 preSubType qtvs btvs qty expected_ty
455 = do { tys <- mapM inst_tv qtvs
456 ; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
459 pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
461 | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
462 | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
463 ; return (mkTyVarTy tv') }
464 | otherwise = do { tv' <- tcInstTyVar tv
465 ; return (mkTyVarTy tv') }
468 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
469 -> BoxyRhoType -- Type to match (note a *Rho* type)
470 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
472 -- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
473 -- "Boxy types: inference for higher rank types and impredicativity"
475 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
476 = go tmpl_tvs tmpl_ty emptyVarSet boxy_ty
478 go t_tvs t_ty b_tvs b_ty
479 | Just t_ty' <- tcView t_ty = go t_tvs t_ty' b_tvs b_ty
480 | Just b_ty' <- tcView b_ty = go t_tvs t_ty b_tvs b_ty'
482 go t_tvs (TyVarTy _) b_tvs b_ty = emptyTvSubst -- Rule S-ANY; no bindings
483 -- Rule S-ANY covers (a) type variables and (b) boxy types
484 -- in the template. Both look like a TyVarTy.
485 -- See Note [Sub-match] below
487 go t_tvs t_ty b_tvs b_ty
488 | isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
489 = go (t_tvs `delVarSetList` tvs) t_tau b_tvs b_ty -- Rule S-SPEC
490 -- Under a forall on the left, if there is shadowing,
491 -- do not bind! Hence the delVarSetList.
492 | isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
493 = go t_tvs t_ty (extendVarSetList b_tvs tvs) b_tau -- Rule S-SKOL
494 -- Add to the variables we must not bind to
495 -- NB: it's *important* to discard the theta part. Otherwise
496 -- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
497 -- and end up with a completely bogus binding (b |-> Bool), by lining
498 -- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
499 -- This pre-subsumption stuff can return too few bindings, but it
500 -- must *never* return bogus info.
502 go t_tvs (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
503 = boxy_match t_tvs arg1 b_tvs arg2 (go t_tvs res1 b_tvs res2)
504 -- Match the args, and sub-match the results
506 go t_tvs t_ty b_tvs b_ty = boxy_match t_tvs t_ty b_tvs b_ty emptyTvSubst
507 -- Otherwise defer to boxy matching
508 -- This covers TyConApp, AppTy, PredTy
515 |- head xs : <rhobox>
516 We will do a boxySubMatchType between a ~ <rhobox>
517 But we *don't* want to match [a |-> <rhobox>] because
518 (a) The box should be filled in with a rho-type, but
519 but the returned substitution maps TyVars to boxy
521 (b) In any case, the right final answer might be *either*
522 instantiate 'a' with a rho-type or a sigma type
523 head xs : Int vs head xs : forall b. b->b
524 So the matcher MUST NOT make a choice here. In general, we only
525 bind a template type variable in boxyMatchType, not in boxySubMatchType.
530 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
531 -> [BoxySigmaType] -- Type to match
532 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
534 -- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
535 -- "Boxy types: inference for higher rank types and impredicativity"
537 -- Find a *boxy* substitution that makes the template look as much
538 -- like the BoxySigmaType as possible.
539 -- It's always ok to return an empty substitution;
540 -- anything more is jam on the pudding
542 -- NB1: This is a pure, non-monadic function.
543 -- It does no unification, and cannot fail
545 -- Precondition: the arg lengths are equal
546 -- Precondition: none of the template type variables appear anywhere in the [BoxySigmaType]
550 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
551 = ASSERT( length tmpl_tys == length boxy_tys )
552 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
553 -- ToDo: add error context?
555 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
557 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
558 = boxy_match tmpl_tvs t_ty boxy_tvs b_ty $
559 boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys subst
560 boxy_match_s tmpl_tvs _ boxy_tvs _ subst
561 = panic "boxy_match_s" -- Lengths do not match
565 boxy_match :: TcTyVarSet -> TcType -- Template
566 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
567 -> BoxySigmaType -- Match against this type
571 -- The boxy_tvs argument prevents this match:
572 -- [a] forall b. a ~ forall b. b
573 -- We don't want to bind the template variable 'a'
574 -- to the quantified type variable 'b'!
576 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
577 = go orig_tmpl_ty orig_boxy_ty
580 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
581 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
583 go ty1 ty2 -- C.f. the isSigmaTy case for boxySubMatchType
585 , (tvs1, _, tau1) <- tcSplitSigmaTy ty1
586 , (tvs2, _, tau2) <- tcSplitSigmaTy ty2
587 , equalLength tvs1 tvs2
588 = boxy_match (tmpl_tvs `delVarSetList` tvs1) tau1
589 (boxy_tvs `extendVarSetList` tvs2) tau2 subst
591 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
593 , not $ isOpenSynTyCon tc1
596 go (FunTy arg1 res1) (FunTy arg2 res2)
597 = go_s [arg1,res1] [arg2,res2]
600 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
601 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
602 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
603 = go_s [s1,t1] [s2,t2]
606 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
607 , boxy_tvs `disjointVarSet` tyVarsOfType orig_boxy_ty
608 , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
609 = extendTvSubst subst tv boxy_ty'
611 = subst -- Ignore others
613 boxy_ty' = case lookupTyVar subst tv of
614 Nothing -> orig_boxy_ty
615 Just ty -> ty `boxyLub` orig_boxy_ty
617 go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
618 -- Example: Tree a ~ Maybe Int
619 -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
620 -- misleading error messages. An even more confusing case is
621 -- a -> b ~ Maybe Int
622 -- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
623 -- from this pre-matching phase.
626 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
629 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
630 -- Combine boxy information from the two types
631 -- If there is a conflict, return the first
632 boxyLub orig_ty1 orig_ty2
633 = go orig_ty1 orig_ty2
635 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
636 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
637 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
638 | tc1 == tc2, length ts1 == length ts2
639 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
641 go (TyVarTy tv1) ty2 -- This is the whole point;
642 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
645 -- Look inside type synonyms, but only if the naive version fails
646 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
647 | Just ty2' <- tcView ty1 = go ty1 ty2'
649 -- For now, we don't look inside ForAlls, PredTys
650 go ty1 ty2 = orig_ty1 -- Default
653 Note [Matching kinds]
654 ~~~~~~~~~~~~~~~~~~~~~
655 The target type might legitimately not be a sub-kind of template.
656 For example, suppose the target is simply a box with an OpenTypeKind,
657 and the template is a type variable with LiftedTypeKind.
658 Then it's ok (because the target type will later be refined).
659 We simply don't bind the template type variable.
661 It might also be that the kind mis-match is an error. For example,
662 suppose we match the template (a -> Int) against (Int# -> Int),
663 where the template type variable 'a' has LiftedTypeKind. This
664 matching function does not fail; it simply doesn't bind the template.
665 Later stuff will fail.
667 %************************************************************************
671 %************************************************************************
673 All the tcSub calls have the form
675 tcSub expected_ty offered_ty
677 offered_ty <= expected_ty
679 That is, that a value of type offered_ty is acceptable in
680 a place expecting a value of type expected_ty.
682 It returns a coercion function
683 co_fn :: offered_ty ~ expected_ty
684 which takes an HsExpr of type offered_ty into one of type
689 tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
690 -- (tcSub act exp) checks that
692 tcSubExp actual_ty expected_ty
693 = -- addErrCtxtM (unifyCtxt actual_ty expected_ty) $
694 -- Adding the error context here leads to some very confusing error
695 -- messages, such as "can't match forall a. a->a with forall a. a->a"
696 -- Example is tcfail165:
697 -- do var <- newEmptyMVar :: IO (MVar (forall a. Show a => a -> String))
698 -- putMVar var (show :: forall a. Show a => a -> String)
699 -- Here the info does not flow from the 'var' arg of putMVar to its 'show' arg
700 -- but after zonking it looks as if it does!
702 -- So instead I'm adding the error context when moving from tc_sub to u_tys
704 traceTc (text "tcSubExp" <+> ppr actual_ty <+> ppr expected_ty) >>
705 tc_sub SubOther actual_ty actual_ty False expected_ty expected_ty
707 tcFunResTy :: Name -> BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
708 tcFunResTy fun actual_ty expected_ty
709 = traceTc (text "tcFunResTy" <+> ppr actual_ty <+> ppr expected_ty) >>
710 tc_sub (SubFun fun) actual_ty actual_ty False expected_ty expected_ty
713 data SubCtxt = SubDone -- Error-context already pushed
714 | SubFun Name -- Context is tcFunResTy
715 | SubOther -- Context is something else
717 tc_sub :: SubCtxt -- How to add an error-context
718 -> BoxySigmaType -- actual_ty, before expanding synonyms
719 -> BoxySigmaType -- ..and after
720 -> InBox -- True <=> expected_ty is inside a box
721 -> BoxySigmaType -- expected_ty, before
722 -> BoxySigmaType -- ..and after
724 -- The acual_ty is never inside a box
725 -- IMPORTANT pre-condition: if the args contain foralls, the bound type
726 -- variables are visible non-monadically
727 -- (i.e. tha args are sufficiently zonked)
728 -- This invariant is needed so that we can "see" the foralls, ad
729 -- e.g. in the SPEC rule where we just use splitSigmaTy
731 tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
732 = traceTc (text "tc_sub" <+> ppr act_ty $$ ppr exp_ty) >>
733 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
734 -- This indirection is just here to make
735 -- it easy to insert a debug trace!
737 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
738 | Just exp_ty' <- tcView exp_ty = tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty'
739 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
740 | Just act_ty' <- tcView act_ty = tc_sub sub_ctxt act_sty act_ty' exp_ib exp_sty exp_ty
742 -----------------------------------
743 -- Rule SBOXY, plus other cases when act_ty is a type variable
744 -- Just defer to boxy matching
745 -- This rule takes precedence over SKOL!
746 tc_sub1 sub_ctxt act_sty (TyVarTy tv) exp_ib exp_sty exp_ty
747 = do { traceTc (text "tc_sub1 - case 1")
748 ; coi <- addSubCtxt sub_ctxt act_sty exp_sty $
749 uVar True False tv exp_ib exp_sty exp_ty
750 ; traceTc (case coi of
751 IdCo -> text "tc_sub1 (Rule SBOXY) IdCo"
752 ACo co -> text "tc_sub1 (Rule SBOXY) ACo" <+> ppr co)
753 ; return $ coiToHsWrapper coi
756 -----------------------------------
757 -- Skolemisation case (rule SKOL)
758 -- actual_ty: d:Eq b => b->b
759 -- expected_ty: forall a. Ord a => a->a
760 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
762 -- It is essential to do this *before* the specialisation case
763 -- Example: f :: (Eq a => a->a) -> ...
764 -- g :: Ord b => b->b
767 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
769 = do { traceTc (text "tc_sub1 - case 2") ;
770 if exp_ib then -- SKOL does not apply if exp_ty is inside a box
771 defer_to_boxy_matching sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
773 { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ _ body_exp_ty ->
774 tc_sub sub_ctxt act_sty act_ty False body_exp_ty body_exp_ty
775 ; return (gen_fn <.> co_fn) }
778 act_tvs = tyVarsOfType act_ty
779 -- It's really important to check for escape wrt
780 -- the free vars of both expected_ty *and* actual_ty
782 -----------------------------------
783 -- Specialisation case (rule ASPEC):
784 -- actual_ty: forall a. Ord a => a->a
785 -- expected_ty: Int -> Int
786 -- co_fn e = e Int dOrdInt
788 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
789 -- Implements the new SPEC rule in the Appendix of the paper
790 -- "Boxy types: inference for higher rank types and impredicativity"
791 -- (This appendix isn't in the published version.)
792 -- The idea is to *first* do pre-subsumption, and then full subsumption
793 -- Example: forall a. a->a <= Int -> (forall b. Int)
794 -- Pre-subsumpion finds a|->Int, and that works fine, whereas
795 -- just running full subsumption would fail.
796 | isSigmaTy actual_ty
797 = do { traceTc (text "tc_sub1 - case 3")
798 ; -- Perform pre-subsumption, and instantiate
799 -- the type with info from the pre-subsumption;
800 -- boxy tyvars if pre-subsumption gives no info
801 let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
802 tau_tvs = exactTyVarsOfType tau
803 ; inst_tys <- if exp_ib then -- Inside a box, do not do clever stuff
804 do { tyvars' <- mapM tcInstBoxyTyVar tyvars
805 ; return (mkTyVarTys tyvars') }
806 else -- Outside, do clever stuff
807 preSubType tyvars tau_tvs tau expected_ty
808 ; let subst' = zipOpenTvSubst tyvars inst_tys
809 tau' = substTy subst' tau
811 -- Perform a full subsumption check
812 ; traceTc (text "tc_sub_spec" <+> vcat [ppr actual_ty,
813 ppr tyvars <+> ppr theta <+> ppr tau,
815 ; co_fn2 <- tc_sub sub_ctxt tau' tau' exp_ib exp_sty expected_ty
817 -- Deal with the dictionaries
818 -- The origin gives a helpful origin when we have
819 -- a function with type f :: Int -> forall a. Num a => ...
820 -- This way the (Num a) dictionary gets an OccurrenceOf f origin
821 ; let orig = case sub_ctxt of
822 SubFun n -> OccurrenceOf n
823 other -> InstSigOrigin -- Unhelpful
824 ; co_fn1 <- instCall orig inst_tys (substTheta subst' theta)
825 ; return (co_fn2 <.> co_fn1) }
827 -----------------------------------
828 -- Function case (rule F1)
829 tc_sub1 sub_ctxt act_sty (FunTy act_arg act_res) exp_ib exp_sty (FunTy exp_arg exp_res)
830 = do { traceTc (text "tc_sub1 - case 4")
831 ; addSubCtxt sub_ctxt act_sty exp_sty $
832 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
835 -- Function case (rule F2)
836 tc_sub1 sub_ctxt act_sty act_ty@(FunTy act_arg act_res) _ exp_sty (TyVarTy exp_tv)
838 = addSubCtxt sub_ctxt act_sty exp_sty $
839 do { traceTc (text "tc_sub1 - case 5")
840 ; cts <- readMetaTyVar exp_tv
842 Indirect ty -> tc_sub SubDone act_sty act_ty True exp_sty ty
843 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
844 ; tc_sub_funs act_arg act_res True arg_ty res_ty } }
846 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
847 mk_res_ty other = panic "TcUnify.mk_res_ty3"
848 fun_kinds = [argTypeKind, openTypeKind]
850 -- Everything else: defer to boxy matching
851 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty@(TyVarTy exp_tv)
852 = do { traceTc (text "tc_sub1 - case 6a" <+> ppr [isBoxyTyVar exp_tv, isMetaTyVar exp_tv, isSkolemTyVar exp_tv, isExistentialTyVar exp_tv,isSigTyVar exp_tv] )
853 ; defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
856 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
857 = do { traceTc (text "tc_sub1 - case 6")
858 ; defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
861 -----------------------------------
862 defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
863 = do { coi <- addSubCtxt sub_ctxt act_sty exp_sty $
864 u_tys outer False act_sty actual_ty exp_ib exp_sty expected_ty
865 ; return $ coiToHsWrapper coi
868 outer = case sub_ctxt of -- Ugh
872 -----------------------------------
873 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
874 = do { arg_coi <- uTys False act_arg exp_ib exp_arg
875 ; co_fn_res <- tc_sub SubDone act_res act_res exp_ib exp_res exp_res
876 ; wrapper1 <- wrapFunResCoercion [exp_arg] co_fn_res
877 ; let wrapper2 = case arg_coi of
879 ACo co -> WpCo $ FunTy co act_res
880 ; return (wrapper1 <.> wrapper2)
883 -----------------------------------
885 :: [TcType] -- Type of args
886 -> HsWrapper -- HsExpr a -> HsExpr b
887 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
888 wrapFunResCoercion arg_tys co_fn_res
889 | isIdHsWrapper co_fn_res
894 = do { arg_ids <- newSysLocalIds FSLIT("sub") arg_tys
895 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpApps arg_ids) }
900 %************************************************************************
902 \subsection{Generalisation}
904 %************************************************************************
907 tcGen :: BoxySigmaType -- expected_ty
908 -> TcTyVarSet -- Extra tyvars that the universally
909 -- quantified tyvars of expected_ty
910 -- must not be unified
911 -> ([TcTyVar] -> BoxyRhoType -> TcM result)
912 -> TcM (HsWrapper, result)
913 -- The expression has type: spec_ty -> expected_ty
915 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
916 -- If not, the call is a no-op
917 = do { traceTc (text "tcGen")
918 -- We want the GenSkol info in the skolemised type variables to
919 -- mention the *instantiated* tyvar names, so that we get a
920 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
921 -- Hence the tiresome but innocuous fixM
922 ; ((tvs', theta', rho'), skol_info) <- fixM (\ ~(_, skol_info) ->
923 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
924 -- Get loation from monad, not from expected_ty
925 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty)
926 ; return ((forall_tvs, theta, rho_ty), skol_info) })
929 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
930 text "expected_ty" <+> ppr expected_ty,
931 text "inst ty" <+> ppr tvs' <+> ppr theta' <+> ppr rho',
932 text "free_tvs" <+> ppr free_tvs])
935 -- Type-check the arg and unify with poly type
936 ; (result, lie) <- getLIE (thing_inside tvs' rho')
938 -- Check that the "forall_tvs" havn't been constrained
939 -- The interesting bit here is that we must include the free variables
940 -- of the expected_ty. Here's an example:
941 -- runST (newVar True)
942 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
943 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
944 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
945 -- So now s' isn't unconstrained because it's linked to a.
946 -- Conclusion: include the free vars of the expected_ty in the
947 -- list of "free vars" for the signature check.
949 ; loc <- getInstLoc (SigOrigin skol_info)
950 ; dicts <- newDictBndrs loc theta'
951 ; inst_binds <- tcSimplifyCheck loc tvs' dicts lie
953 ; checkSigTyVarsWrt free_tvs tvs'
954 ; traceTc (text "tcGen:done")
957 -- The WpLet binds any Insts which came out of the simplification.
958 dict_vars = map instToVar dicts
959 co_fn = mkWpTyLams tvs' <.> mkWpLams dict_vars <.> WpLet inst_binds
960 ; returnM (co_fn, result) }
962 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
967 %************************************************************************
971 %************************************************************************
973 The exported functions are all defined as versions of some
974 non-exported generic functions.
977 boxyUnify :: BoxyType -> BoxyType -> TcM CoercionI
978 -- Acutal and expected, respectively
980 = addErrCtxtM (unifyCtxt ty1 ty2) $
981 uTysOuter False ty1 False ty2
984 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM [CoercionI]
985 -- Arguments should have equal length
986 -- Acutal and expected types
987 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
990 unifyType :: TcTauType -> TcTauType -> TcM CoercionI
991 -- No boxes expected inside these types
992 -- Acutal and expected types
993 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
994 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
995 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
996 addErrCtxtM (unifyCtxt ty1 ty2) $
997 uTysOuter True ty1 True ty2
1000 unifyPred :: PredType -> PredType -> TcM CoercionI
1001 -- Acutal and expected types
1002 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
1003 uPred True True p1 True p2
1005 unifyTheta :: TcThetaType -> TcThetaType -> TcM [CoercionI]
1006 -- Acutal and expected types
1007 unifyTheta theta1 theta2
1008 = do { checkTc (equalLength theta1 theta2)
1009 (vcat [ptext SLIT("Contexts differ in length"),
1010 nest 2 $ parens $ ptext SLIT("Use -fglasgow-exts to allow this")])
1011 ; uList unifyPred theta1 theta2
1015 uList :: (a -> a -> TcM b)
1016 -> [a] -> [a] -> TcM [b]
1017 -- Unify corresponding elements of two lists of types, which
1018 -- should be of equal length. We charge down the list explicitly so that
1019 -- we can complain if their lengths differ.
1020 uList unify [] [] = return []
1021 uList unify (ty1:tys1) (ty2:tys2) = do { x <- unify ty1 ty2;
1022 ; xs <- uList unify tys1 tys2
1025 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
1028 @unifyTypeList@ takes a single list of @TauType@s and unifies them
1029 all together. It is used, for example, when typechecking explicit
1030 lists, when all the elts should be of the same type.
1033 unifyTypeList :: [TcTauType] -> TcM ()
1034 unifyTypeList [] = returnM ()
1035 unifyTypeList [ty] = returnM ()
1036 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
1037 ; unifyTypeList tys }
1040 %************************************************************************
1042 \subsection[Unify-uTys]{@uTys@: getting down to business}
1044 %************************************************************************
1046 @uTys@ is the heart of the unifier. Each arg occurs twice, because
1047 we want to report errors in terms of synomyms if possible. The first of
1048 the pair is used in error messages only; it is always the same as the
1049 second, except that if the first is a synonym then the second may be a
1050 de-synonym'd version. This way we get better error messages.
1052 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
1055 type SwapFlag = Bool
1056 -- False <=> the two args are (actual, expected) respectively
1057 -- True <=> the two args are (expected, actual) respectively
1059 type InBox = Bool -- True <=> we are inside a box
1060 -- False <=> we are outside a box
1061 -- The importance of this is that if we get "filled-box meets
1062 -- filled-box", we'll look into the boxes and unify... but
1063 -- we must not allow polytypes. But if we are in a box on
1064 -- just one side, then we can allow polytypes
1066 type Outer = Bool -- True <=> this is the outer level of a unification
1067 -- so that the types being unified are the
1068 -- very ones we began with, not some sub
1069 -- component or synonym expansion
1070 -- The idea is that if Outer is true then unifyMisMatch should
1071 -- pop the context to remove the "Expected/Acutal" context
1074 :: InBox -> TcType -- ty1 is the *actual* type
1075 -> InBox -> TcType -- ty2 is the *expected* type
1077 uTysOuter nb1 ty1 nb2 ty2
1078 = do { traceTc (text "uTysOuter" <+> ppr ty1 <+> ppr ty2)
1079 ; u_tys True nb1 ty1 ty1 nb2 ty2 ty2 }
1080 uTys nb1 ty1 nb2 ty2
1081 = do { traceTc (text "uTys" <+> ppr ty1 <+> ppr ty2)
1082 ; u_tys False nb1 ty1 ty1 nb2 ty2 ty2 }
1086 uTys_s :: InBox -> [TcType] -- tys1 are the *actual* types
1087 -> InBox -> [TcType] -- tys2 are the *expected* types
1089 uTys_s nb1 [] nb2 [] = returnM []
1090 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { coi <- uTys nb1 ty1 nb2 ty2
1091 ; cois <- uTys_s nb1 tys1 nb2 tys2
1094 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
1098 -> InBox -> TcType -> TcType -- ty1 is the *actual* type
1099 -> InBox -> TcType -> TcType -- ty2 is the *expected* type
1102 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
1103 = do { traceTc (text "u_tys " <+> ppr ty1 <+> text " " <+> ppr ty2)
1104 ; coi <- go outer ty1 ty2
1105 ; traceTc (case coi of
1106 ACo co -> text "u_tys yields coercion: " <+> ppr co
1107 IdCo -> text "u_tys yields no coercion")
1112 go :: Outer -> TcType -> TcType -> TcM CoercionI
1114 do { traceTc (text "go " <+> ppr orig_ty1 <+> text "/" <+> ppr ty1
1115 <+> ppr orig_ty2 <+> text "/" <+> ppr ty2)
1119 go1 :: Outer -> TcType -> TcType -> TcM CoercionI
1120 -- Always expand synonyms: see Note [Unification and synonyms]
1121 -- (this also throws away FTVs)
1123 | Just ty1' <- tcView ty1 = go False ty1' ty2
1124 | Just ty2' <- tcView ty2 = go False ty1 ty2'
1126 -- Variables; go for uVar
1127 go1 outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
1128 go1 outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
1129 -- "True" means args swapped
1131 -- The case for sigma-types must *follow* the variable cases
1132 -- because a boxy variable can be filed with a polytype;
1133 -- but must precede FunTy, because ((?x::Int) => ty) look
1134 -- like a FunTy; there isn't necy a forall at the top
1136 | isSigmaTy ty1 || isSigmaTy ty2
1137 = do { traceTc (text "We have sigma types: equalLength" <+> ppr tvs1 <+> ppr tvs2)
1138 ; checkM (equalLength tvs1 tvs2)
1139 (unifyMisMatch outer False orig_ty1 orig_ty2)
1140 ; traceTc (text "We're past the first length test")
1141 ; tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
1142 -- Get location from monad, not from tvs1
1143 ; let tys = mkTyVarTys tvs
1144 in_scope = mkInScopeSet (mkVarSet tvs)
1145 phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
1146 phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
1147 (theta1,tau1) = tcSplitPhiTy phi1
1148 (theta2,tau2) = tcSplitPhiTy phi2
1150 ; addErrCtxtM (unifyForAllCtxt tvs phi1 phi2) $ do
1151 { checkM (equalLength theta1 theta2)
1152 (unifyMisMatch outer False orig_ty1 orig_ty2)
1154 ; cois <- uPreds False nb1 theta1 nb2 theta2 -- TOMDO: do something with these pred_cois
1155 ; traceTc (text "TOMDO!")
1156 ; coi <- uTys nb1 tau1 nb2 tau2
1158 -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
1159 ; free_tvs <- zonkTcTyVarsAndFV (varSetElems (tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2))
1160 ; ifM (any (`elemVarSet` free_tvs) tvs)
1161 (bleatEscapedTvs free_tvs tvs tvs)
1163 -- If both sides are inside a box, we are in a "box-meets-box"
1164 -- situation, and we should not have a polytype at all.
1165 -- If we get here we have two boxes, already filled with
1166 -- the same polytype... but it should be a monotype.
1167 -- This check comes last, because the error message is
1168 -- extremely unhelpful.
1169 ; ifM (nb1 && nb2) (notMonoType ty1)
1173 (tvs1, body1) = tcSplitForAllTys ty1
1174 (tvs2, body2) = tcSplitForAllTys ty2
1177 go1 outer (PredTy p1) (PredTy p2)
1178 = uPred False nb1 p1 nb2 p2
1180 -- Type constructors must match
1181 go1 _ (TyConApp con1 tys1) (TyConApp con2 tys2)
1182 | con1 == con2 && not (isOpenSynTyCon con1)
1183 = do { cois <- uTys_s nb1 tys1 nb2 tys2
1184 ; return $ mkTyConAppCoI con1 tys1 cois
1186 -- See Note [TyCon app]
1187 | con1 == con2 && identicalOpenSynTyConApp
1188 = do { cois <- uTys_s nb1 tys1' nb2 tys2'
1189 ; return $ mkTyConAppCoI con1 tys1 (replicate n IdCo ++ cois)
1193 (idxTys1, tys1') = splitAt n tys1
1194 (idxTys2, tys2') = splitAt n tys2
1195 identicalOpenSynTyConApp = idxTys1 `tcEqTypes` idxTys2
1196 -- See Note [OpenSynTyCon app]
1198 -- Functions; just check the two parts
1199 go1 _ (FunTy fun1 arg1) (FunTy fun2 arg2)
1200 = do { coi_l <- uTys nb1 fun1 nb2 fun2
1201 ; coi_r <- uTys nb1 arg1 nb2 arg2
1202 ; return $ mkFunTyCoI fun1 coi_l arg1 coi_r
1205 -- Applications need a bit of care!
1206 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
1207 -- NB: we've already dealt with type variables and Notes,
1208 -- so if one type is an App the other one jolly well better be too
1209 go1 outer (AppTy s1 t1) ty2
1210 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
1211 = do { coi_s <- uTys nb1 s1 nb2 s2; coi_t <- uTys nb1 t1 nb2 t2
1212 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1214 -- Now the same, but the other way round
1215 -- Don't swap the types, because the error messages get worse
1216 go1 outer ty1 (AppTy s2 t2)
1217 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
1218 = do { coi_s <- uTys nb1 s1 nb2 s2; coi_t <- uTys nb1 t1 nb2 t2
1219 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1221 -- One or both outermost constructors are type family applications.
1222 -- If we can normalise them away, proceed as usual; otherwise, we
1223 -- need to defer unification by generating a wanted equality constraint.
1225 | ty1_is_fun || ty2_is_fun
1226 = do { (coi1, ty1') <- if ty1_is_fun then tcNormaliseFamInst ty1
1227 else return (IdCo, ty1)
1228 ; (coi2, ty2') <- if ty2_is_fun then tcNormaliseFamInst ty2
1229 else return (IdCo, ty2)
1230 ; coi <- if isOpenSynTyConApp ty1' || isOpenSynTyConApp ty2'
1231 then do { -- One type family app can't be reduced yet
1233 ; ty1'' <- zonkTcType ty1'
1234 ; ty2'' <- zonkTcType ty2'
1235 ; if tcEqType ty1'' ty2''
1237 else -- see [Deferred Unification]
1238 defer_unification outer False orig_ty1 orig_ty2
1240 else -- unification can proceed
1242 ; return $ coi1 `mkTransCoI` coi `mkTransCoI` (mkSymCoI coi2)
1245 ty1_is_fun = isOpenSynTyConApp ty1
1246 ty2_is_fun = isOpenSynTyConApp ty2
1248 -- Anything else fails
1249 go1 outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
1253 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
1255 do { coi <- uTys nb1 t1 nb2 t2
1256 ; return $ mkIParamPredCoI n1 coi
1258 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
1260 do { cois <- uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
1261 ; return $ mkClassPPredCoI c1 tys1 cois
1263 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
1265 uPreds outer nb1 [] nb2 [] = return []
1266 uPreds outer nb1 (p1:ps1) nb2 (p2:ps2) =
1267 do { coi <- uPred outer nb1 p1 nb2 p2
1268 ; cois <- uPreds outer nb1 ps1 nb2 ps2
1271 uPreds outer nb1 ps1 nb2 ps2 = panic "uPreds"
1276 When we find two TyConApps, the argument lists are guaranteed equal
1277 length. Reason: intially the kinds of the two types to be unified is
1278 the same. The only way it can become not the same is when unifying two
1279 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
1280 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
1281 which we do, that ensures that f1,f2 have the same kind; and that
1282 means a1,a2 have the same kind. And now the argument repeats.
1284 Note [OpenSynTyCon app]
1285 ~~~~~~~~~~~~~~~~~~~~~~~
1288 type family T a :: * -> *
1290 the two types (T () a) and (T () Int) must unify, even if there are
1291 no type instances for T at all. Should we just turn them into an
1292 equality (T () a ~ T () Int)? I don't think so. We currently try to
1293 eagerly unify everything we can before generating equalities; otherwise,
1294 we could turn the unification of [Int] with [a] into an equality, too.
1296 Note [Unification and synonyms]
1297 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1298 If you are tempted to make a short cut on synonyms, as in this
1302 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
1303 -- NO = if (con1 == con2) then
1304 -- NO -- Good news! Same synonym constructors, so we can shortcut
1305 -- NO -- by unifying their arguments and ignoring their expansions.
1306 -- NO unifyTypepeLists args1 args2
1308 -- NO -- Never mind. Just expand them and try again
1312 then THINK AGAIN. Here is the whole story, as detected and reported
1313 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1315 Here's a test program that should detect the problem:
1319 x = (1 :: Bogus Char) :: Bogus Bool
1322 The problem with [the attempted shortcut code] is that
1326 is not a sufficient condition to be able to use the shortcut!
1327 You also need to know that the type synonym actually USES all
1328 its arguments. For example, consider the following type synonym
1329 which does not use all its arguments.
1334 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1335 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1336 would fail, even though the expanded forms (both \tr{Int}) should
1339 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1340 unnecessarily bind \tr{t} to \tr{Char}.
1342 ... You could explicitly test for the problem synonyms and mark them
1343 somehow as needing expansion, perhaps also issuing a warning to the
1348 %************************************************************************
1350 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1352 %************************************************************************
1354 @uVar@ is called when at least one of the types being unified is a
1355 variable. It does {\em not} assume that the variable is a fixed point
1356 of the substitution; rather, notice that @uVar@ (defined below) nips
1357 back into @uTys@ if it turns out that the variable is already bound.
1361 -> SwapFlag -- False => tyvar is the "actual" (ty is "expected")
1362 -- True => ty is the "actual" (tyvar is "expected")
1364 -> InBox -- True <=> definitely no boxes in t2
1365 -> TcTauType -> TcTauType -- printing and real versions
1368 uVar outer swapped tv1 nb2 ps_ty2 ty2
1369 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1370 | otherwise = brackets (equals <+> ppr ty2)
1371 ; traceTc (text "uVar" <+> ppr swapped <+>
1372 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1373 nest 2 (ptext SLIT(" <-> ")),
1374 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1375 ; details <- lookupTcTyVar tv1
1378 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1379 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1380 -- The 'True' here says that ty1 is now inside a box
1381 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1385 uUnfilledVar :: Outer
1387 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1388 -> TcTauType -> TcTauType -- Type 2
1390 -- Invariant: tyvar 1 is not unified with anything
1392 uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1393 | Just ty2' <- tcView ty2
1394 = -- Expand synonyms; ignore FTVs
1395 uUnfilledVar False swapped tv1 details1 ps_ty2 ty2'
1397 uUnfilledVar outer swapped tv1 details1 ps_ty2 (TyVarTy tv2)
1398 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1400 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1401 -- this is box-meets-box, so fill in with a tau-type
1402 -> do { tau_tv <- tcInstTyVar tv1
1403 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv)
1406 other -> returnM IdCo -- No-op
1408 | otherwise -- Distinct type variables
1409 = do { lookup2 <- lookupTcTyVar tv2
1411 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
1412 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1415 uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2
1416 = -- ty2 is not a type variable
1418 MetaTv (SigTv _) _ -> rigid_variable
1420 uMetaVar outer swapped tv1 info ref1 ps_ty2 non_var_ty2
1421 SkolemTv _ -> rigid_variable
1424 | isOpenSynTyConApp non_var_ty2
1425 = -- 'non_var_ty2's outermost constructor is a type family,
1426 -- which we may may be able to normalise
1427 do { (coi2, ty2') <- tcNormaliseFamInst non_var_ty2
1429 IdCo -> -- no progress, but maybe after other instantiations
1430 defer_unification outer swapped (TyVarTy tv1) ps_ty2
1431 ACo co -> -- progress: so lets try again
1433 ppr co <+> text "::"<+> ppr non_var_ty2 <+> text "~" <+>
1435 ; coi <- uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2'
1436 ; let coi2' = (if swapped then id else mkSymCoI) coi2
1437 ; return $ coi2' `mkTransCoI` coi
1440 | SkolemTv RuntimeUnkSkol <- details1
1441 -- runtime unknown will never match
1442 = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1443 | otherwise -- defer as a given equality may still resolve this
1444 = defer_unification outer swapped (TyVarTy tv1) ps_ty2
1447 Note [Deferred Unification]
1448 ~~~~~~~~~~~~~~~~~~~~
1449 We may encounter a unification ty1 = ty2 that cannot be performed syntactically,
1450 and yet its consistency is undetermined. Previously, there was no way to still
1451 make it consistent. So a mismatch error was issued.
1453 Now these unfications are deferred until constraint simplification, where type
1454 family instances and given equations may (or may not) establish the consistency.
1455 Deferred unifications are of the form
1458 where F is a type function and x is a type variable.
1460 id :: x ~ y => x -> y
1463 involves the unfication x = y. It is deferred until we bring into account the
1464 context x ~ y to establish that it holds.
1466 If available, we defer original types (rather than those where closed type
1467 synonyms have already been expanded via tcCoreView). This is, as usual, to
1468 improve error messages.
1470 We need to both 'unBox' and zonk deferred types. We need to unBox as
1471 functions, such as TcExpr.tcMonoExpr promise to fill boxes in the expected
1472 type. We need to zonk as the types go into the kind of the coercion variable
1473 `cotv' and those are not zonked in Inst.zonkInst. (Maybe it would be better
1474 to zonk in zonInst instead. Would that be sufficient?)
1477 defer_unification :: Bool -- pop innermost context?
1482 defer_unification outer True ty1 ty2
1483 = defer_unification outer False ty2 ty1
1484 defer_unification outer False ty1 ty2
1485 = do { ty1' <- unBox ty1 >>= zonkTcType -- unbox *and* zonk..
1486 ; ty2' <- unBox ty2 >>= zonkTcType -- ..see preceding note
1487 ; traceTc $ text "deferring:" <+> ppr ty1 <+> text "~" <+> ppr ty2
1488 ; cotv <- newMetaCoVar ty1' ty2'
1489 -- put ty1 ~ ty2 in LIE
1490 -- Left means "wanted"
1491 ; inst <- (if outer then popErrCtxt else id) $
1492 mkEqInst (EqPred ty1' ty2') (Left cotv)
1494 ; return $ ACo $ TyVarTy cotv }
1497 uMetaVar :: Bool -- pop innermost context?
1499 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1502 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1503 -- ty2 is not a type variable
1505 uMetaVar outer swapped tv1 BoxTv ref1 ps_ty2 non_var_ty2
1506 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1507 -- that any boxes in ty2 are filled with monotypes
1509 -- It should not be the case that tv1 occurs in ty2
1510 -- (i.e. no occurs check should be needed), but if perchance
1511 -- it does, the unbox operation will fill it, and the DEBUG
1513 do { final_ty <- unBox ps_ty2
1515 ; meta_details <- readMutVar ref1
1516 ; case meta_details of
1517 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1518 return () -- This really should *not* happen
1521 ; checkUpdateMeta swapped tv1 ref1 final_ty
1525 uMetaVar outer swapped tv1 info1 ref1 ps_ty2 non_var_ty2
1526 = do { -- Occurs check + monotype check
1527 ; mb_final_ty <- checkTauTvUpdate tv1 ps_ty2
1528 ; case mb_final_ty of
1529 Nothing -> -- tv1 occured in type family parameter
1530 defer_unification outer swapped (mkTyVarTy tv1) ps_ty2
1532 do { checkUpdateMeta swapped tv1 ref1 final_ty
1538 uUnfilledVars :: Outer
1540 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1541 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1543 -- Invarant: The type variables are distinct,
1544 -- Neither is filled in yet
1545 -- They might be boxy or not
1547 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1548 = -- see [Deferred Unification]
1549 defer_unification outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1551 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1552 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2) >> return IdCo
1553 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1554 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1) >> return IdCo
1556 -- ToDo: this function seems too long for what it acutally does!
1557 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1558 = case (info1, info2) of
1559 (BoxTv, BoxTv) -> box_meets_box >> return IdCo
1561 -- If a box meets a TauTv, but the fomer has the smaller kind
1562 -- then we must create a fresh TauTv with the smaller kind
1563 (_, BoxTv) | k1_sub_k2 -> update_tv2 >> return IdCo
1564 | otherwise -> box_meets_box >> return IdCo
1565 (BoxTv, _ ) | k2_sub_k1 -> update_tv1 >> return IdCo
1566 | otherwise -> box_meets_box >> return IdCo
1568 -- Avoid SigTvs if poss
1569 (SigTv _, _ ) | k1_sub_k2 -> update_tv2 >> return IdCo
1570 (_, SigTv _) | k2_sub_k1 -> update_tv1 >> return IdCo
1572 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1573 then update_tv1 >> return IdCo -- Same kinds
1574 else update_tv2 >> return IdCo
1575 | k2_sub_k1 -> update_tv1 >> return IdCo
1576 | otherwise -> kind_err >> return IdCo
1578 -- Update the variable with least kind info
1579 -- See notes on type inference in Kind.lhs
1580 -- The "nicer to" part only applies if the two kinds are the same,
1581 -- so we can choose which to do.
1583 -- Kinds should be guaranteed ok at this point
1584 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1585 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1587 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1590 | k2_sub_k1 = fill_from tv2
1591 | otherwise = kind_err
1593 -- Update *both* tyvars with a TauTv whose name and kind
1594 -- are gotten from tv (avoid losing nice names is poss)
1595 fill_from tv = do { tv' <- tcInstTyVar tv
1596 ; let tau_ty = mkTyVarTy tv'
1597 ; updateMeta tv1 ref1 tau_ty
1598 ; updateMeta tv2 ref2 tau_ty }
1600 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1601 unifyKindMisMatch k1 k2
1605 k1_sub_k2 = k1 `isSubKind` k2
1606 k2_sub_k1 = k2 `isSubKind` k1
1608 nicer_to_update_tv1 = isSystemName (Var.varName tv1)
1609 -- Try to update sys-y type variables in preference to ones
1610 -- gotten (say) by instantiating a polymorphic function with
1611 -- a user-written type sig
1615 refineBox :: TcType -> TcM TcType
1616 -- Unbox the outer box of a boxy type (if any)
1617 refineBox ty@(TyVarTy box_tv)
1618 | isMetaTyVar box_tv
1619 = do { cts <- readMetaTyVar box_tv
1622 Indirect ty -> return ty }
1623 refineBox other_ty = return other_ty
1625 refineBoxToTau :: TcType -> TcM TcType
1626 -- Unbox the outer box of a boxy type, filling with a monotype if it is empty
1627 -- Like refineBox except for the "fill with monotype" part.
1628 refineBoxToTau ty@(TyVarTy box_tv)
1629 | isMetaTyVar box_tv
1630 , MetaTv BoxTv ref <- tcTyVarDetails box_tv
1631 = do { cts <- readMutVar ref
1633 Flexi -> fillBoxWithTau box_tv ref
1634 Indirect ty -> return ty }
1635 refineBoxToTau other_ty = return other_ty
1637 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1638 -- Subtle... we must zap the boxy res_ty
1639 -- to kind * before using it to instantiate a LitInst
1640 -- Calling unBox instead doesn't do the job, because the box
1641 -- often has an openTypeKind, and we don't want to instantiate
1643 zapToMonotype res_ty
1644 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1645 ; boxyUnify res_tau res_ty
1648 unBox :: BoxyType -> TcM TcType
1649 -- unBox implements the judgement
1651 -- with input s', and result s
1653 -- It removes all boxes from the input type, returning a non-boxy type.
1654 -- A filled box in the type can only contain a monotype; unBox fails if not
1655 -- The type can have empty boxes, which unBox fills with a monotype
1657 -- Compare this wth checkTauTvUpdate
1659 -- For once, it's safe to treat synonyms as opaque!
1661 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1662 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1663 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1664 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1665 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1666 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1667 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1669 | isTcTyVar tv -- It's a boxy type variable
1670 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1671 = do { cts <- readMutVar ref -- under nested quantifiers
1673 Flexi -> fillBoxWithTau tv ref
1674 Indirect ty -> do { non_boxy_ty <- unBox ty
1675 ; if isTauTy non_boxy_ty
1676 then return non_boxy_ty
1677 else notMonoType non_boxy_ty }
1679 | otherwise -- Skolems, and meta-tau-variables
1680 = return (TyVarTy tv)
1682 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1683 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1684 unBoxPred (EqPred ty1 ty2) = do { ty1' <- unBox ty1; ty2' <- unBox ty2; return (EqPred ty1' ty2') }
1689 %************************************************************************
1691 \subsection[Unify-context]{Errors and contexts}
1693 %************************************************************************
1699 unifyCtxt act_ty exp_ty tidy_env
1700 = do { act_ty' <- zonkTcType act_ty
1701 ; exp_ty' <- zonkTcType exp_ty
1702 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1703 (env2, act_ty'') = tidyOpenType env1 act_ty'
1704 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1707 mkExpectedActualMsg act_ty exp_ty
1708 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1709 text "Inferred type" <> colon <+> ppr act_ty ])
1712 -- If an error happens we try to figure out whether the function
1713 -- function has been given too many or too few arguments, and say so.
1714 addSubCtxt SubDone actual_res_ty expected_res_ty thing_inside
1716 addSubCtxt sub_ctxt actual_res_ty expected_res_ty thing_inside
1717 = addErrCtxtM mk_err thing_inside
1720 = do { exp_ty' <- zonkTcType expected_res_ty
1721 ; act_ty' <- zonkTcType actual_res_ty
1722 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1723 (env2, act_ty'') = tidyOpenType env1 act_ty'
1724 (exp_args, _) = tcSplitFunTys exp_ty''
1725 (act_args, _) = tcSplitFunTys act_ty''
1727 len_act_args = length act_args
1728 len_exp_args = length exp_args
1730 message = case sub_ctxt of
1731 SubFun fun | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1732 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1733 other -> mkExpectedActualMsg act_ty'' exp_ty''
1734 ; return (env2, message) }
1736 wrongArgsCtxt too_many_or_few fun
1737 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1738 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1739 <+> ptext SLIT("arguments")
1742 unifyForAllCtxt tvs phi1 phi2 env
1743 = returnM (env2, msg)
1745 (env', tvs') = tidyOpenTyVars env tvs -- NB: not tidyTyVarBndrs
1746 (env1, phi1') = tidyOpenType env' phi1
1747 (env2, phi2') = tidyOpenType env1 phi2
1748 msg = vcat [ptext SLIT("When matching") <+> quotes (ppr (mkForAllTys tvs' phi1')),
1749 ptext SLIT(" and") <+> quotes (ppr (mkForAllTys tvs' phi2'))]
1751 -----------------------
1752 unifyMisMatch outer swapped ty1 ty2
1753 = do { (env, msg) <- if swapped then misMatchMsg ty2 ty1
1754 else misMatchMsg ty1 ty2
1756 -- This is the whole point of the 'outer' stuff
1757 ; if outer then popErrCtxt (failWithTcM (env, msg))
1758 else failWithTcM (env, msg)
1763 %************************************************************************
1767 %************************************************************************
1769 Unifying kinds is much, much simpler than unifying types.
1772 unifyKind :: TcKind -- Expected
1775 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1776 | isSubKindCon kc2 kc1 = returnM ()
1778 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1779 = do { unifyKind a2 a1; unifyKind r1 r2 }
1780 -- Notice the flip in the argument,
1781 -- so that the sub-kinding works right
1782 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1783 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1784 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1786 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1787 unifyKinds [] [] = returnM ()
1788 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1790 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1793 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1794 uKVar swapped kv1 k2
1795 = do { mb_k1 <- readKindVar kv1
1797 Flexi -> uUnboundKVar swapped kv1 k2
1798 Indirect k1 | swapped -> unifyKind k2 k1
1799 | otherwise -> unifyKind k1 k2 }
1802 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1803 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1804 | kv1 == kv2 = returnM ()
1805 | otherwise -- Distinct kind variables
1806 = do { mb_k2 <- readKindVar kv2
1808 Indirect k2 -> uUnboundKVar swapped kv1 k2
1809 Flexi -> writeKindVar kv1 k2 }
1811 uUnboundKVar swapped kv1 non_var_k2
1812 = do { k2' <- zonkTcKind non_var_k2
1813 ; kindOccurCheck kv1 k2'
1814 ; k2'' <- kindSimpleKind swapped k2'
1815 -- KindVars must be bound only to simple kinds
1816 -- Polarities: (kindSimpleKind True ?) succeeds
1817 -- returning *, corresponding to unifying
1820 ; writeKindVar kv1 k2'' }
1823 kindOccurCheck kv1 k2 -- k2 is zonked
1824 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1826 not_in (TyVarTy kv2) = kv1 /= kv2
1827 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1830 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1831 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1832 -- If the flag is False, it requires k <: sk
1833 -- E.g. kindSimpleKind False ?? = *
1834 -- What about (kv -> *) :=: ?? -> *
1835 kindSimpleKind orig_swapped orig_kind
1836 = go orig_swapped orig_kind
1838 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1840 ; return (mkArrowKind k1' k2') }
1842 | isOpenTypeKind k = return liftedTypeKind
1843 | isArgTypeKind k = return liftedTypeKind
1845 | isLiftedTypeKind k = return liftedTypeKind
1846 | isUnliftedTypeKind k = return unliftedTypeKind
1847 go sw k@(TyVarTy _) = return k -- KindVars are always simple
1848 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1849 <+> ppr orig_swapped <+> ppr orig_kind)
1850 -- I think this can't actually happen
1852 -- T v = MkT v v must be a type
1853 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1856 kindOccurCheckErr tyvar ty
1857 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1858 2 (sep [ppr tyvar, char '=', ppr ty])
1862 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1863 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1865 unifyFunKind (TyVarTy kvar)
1866 = readKindVar kvar `thenM` \ maybe_kind ->
1868 Indirect fun_kind -> unifyFunKind fun_kind
1870 do { arg_kind <- newKindVar
1871 ; res_kind <- newKindVar
1872 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1873 ; returnM (Just (arg_kind,res_kind)) }
1875 unifyFunKind (FunTy arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1876 unifyFunKind other = returnM Nothing
1879 %************************************************************************
1883 %************************************************************************
1885 ---------------------------
1886 -- We would like to get a decent error message from
1887 -- (a) Under-applied type constructors
1888 -- f :: (Maybe, Maybe)
1889 -- (b) Over-applied type constructors
1890 -- f :: Int x -> Int x
1894 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1895 -- A fancy wrapper for 'unifyKind', which tries
1896 -- to give decent error messages.
1897 -- (checkExpectedKind ty act_kind exp_kind)
1898 -- checks that the actual kind act_kind is compatible
1899 -- with the expected kind exp_kind
1900 -- The first argument, ty, is used only in the error message generation
1901 checkExpectedKind ty act_kind exp_kind
1902 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1905 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1907 Just r -> returnM () ; -- Unification succeeded
1910 -- So there's definitely an error
1911 -- Now to find out what sort
1912 zonkTcKind exp_kind `thenM` \ exp_kind ->
1913 zonkTcKind act_kind `thenM` \ act_kind ->
1915 tcInitTidyEnv `thenM` \ env0 ->
1916 let (exp_as, _) = splitKindFunTys exp_kind
1917 (act_as, _) = splitKindFunTys act_kind
1918 n_exp_as = length exp_as
1919 n_act_as = length act_as
1921 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1922 (env2, tidy_act_kind) = tidyKind env1 act_kind
1924 err | n_exp_as < n_act_as -- E.g. [Maybe]
1925 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1927 -- Now n_exp_as >= n_act_as. In the next two cases,
1928 -- n_exp_as == 0, and hence so is n_act_as
1929 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1930 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1931 <+> ptext SLIT("is unlifted")
1933 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1934 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1935 <+> ptext SLIT("is lifted")
1937 | otherwise -- E.g. Monad [Int]
1938 = ptext SLIT("Kind mis-match")
1940 more_info = sep [ ptext SLIT("Expected kind") <+>
1941 quotes (pprKind tidy_exp_kind) <> comma,
1942 ptext SLIT("but") <+> quotes (ppr ty) <+>
1943 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1945 failWithTcM (env2, err $$ more_info)
1949 %************************************************************************
1951 \subsection{Checking signature type variables}
1953 %************************************************************************
1955 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1956 are not mentioned in the environment. In particular:
1958 (a) Not mentioned in the type of a variable in the envt
1959 eg the signature for f in this:
1965 Here, f is forced to be monorphic by the free occurence of x.
1967 (d) Not (unified with another type variable that is) in scope.
1968 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1969 when checking the expression type signature, we find that
1970 even though there is nothing in scope whose type mentions r,
1971 nevertheless the type signature for the expression isn't right.
1973 Another example is in a class or instance declaration:
1975 op :: forall b. a -> b
1977 Here, b gets unified with a
1979 Before doing this, the substitution is applied to the signature type variable.
1982 checkSigTyVars :: [TcTyVar] -> TcM ()
1983 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1985 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1986 -- The extra_tvs can include boxy type variables;
1987 -- e.g. TcMatches.tcCheckExistentialPat
1988 checkSigTyVarsWrt extra_tvs sig_tvs
1989 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1990 ; check_sig_tyvars extra_tvs' sig_tvs }
1993 :: TcTyVarSet -- Global type variables. The universally quantified
1994 -- tyvars should not mention any of these
1995 -- Guaranteed already zonked.
1996 -> [TcTyVar] -- Universally-quantified type variables in the signature
1997 -- Guaranteed to be skolems
1999 check_sig_tyvars extra_tvs []
2001 check_sig_tyvars extra_tvs sig_tvs
2002 = ASSERT( all isSkolemTyVar sig_tvs )
2003 do { gbl_tvs <- tcGetGlobalTyVars
2004 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
2005 text "gbl_tvs" <+> ppr gbl_tvs,
2006 text "extra_tvs" <+> ppr extra_tvs]))
2008 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
2009 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
2010 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
2013 bleatEscapedTvs :: TcTyVarSet -- The global tvs
2014 -> [TcTyVar] -- The possibly-escaping type variables
2015 -> [TcTyVar] -- The zonked versions thereof
2017 -- Complain about escaping type variables
2018 -- We pass a list of type variables, at least one of which
2019 -- escapes. The first list contains the original signature type variable,
2020 -- while the second contains the type variable it is unified to (usually itself)
2021 bleatEscapedTvs globals sig_tvs zonked_tvs
2022 = do { env0 <- tcInitTidyEnv
2023 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
2024 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
2026 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
2027 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
2029 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
2031 check (tidy_env, msgs) (sig_tv, zonked_tv)
2032 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
2034 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
2035 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
2037 -----------------------
2038 escape_msg sig_tv zonked_tv globs
2040 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
2041 nest 2 (vcat globs)]
2043 = msg <+> ptext SLIT("escapes")
2044 -- Sigh. It's really hard to give a good error message
2045 -- all the time. One bad case is an existential pattern match.
2046 -- We rely on the "When..." context to help.
2048 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
2050 | sig_tv == zonked_tv = empty
2051 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
2054 These two context are used with checkSigTyVars
2057 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
2058 -> TidyEnv -> TcM (TidyEnv, Message)
2059 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
2060 = zonkTcType sig_tau `thenM` \ actual_tau ->
2062 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
2063 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
2064 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
2065 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
2066 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
2068 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),