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
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)
73 tcInfer tc_infer = withBox openTypeKind tc_infer
77 %************************************************************************
81 %************************************************************************
84 subFunTys :: SDoc -- Somthing like "The function f has 3 arguments"
85 -- or "The abstraction (\x.e) takes 1 argument"
86 -> Arity -- Expected # of args
87 -> BoxyRhoType -- res_ty
88 -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
90 -- Attempt to decompse res_ty to have enough top-level arrows to
91 -- match the number of patterns in the match group
93 -- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
94 -- and the inner call to thing_inside passes args: [a1,...,an], b
95 -- then co_fn :: (a1 -> ... -> an -> b) ~ res_ty
97 -- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
100 {- Error messages from subFunTys
102 The abstraction `\Just 1 -> ...' has two arguments
103 but its type `Maybe a -> a' has only one
105 The equation(s) for `f' have two arguments
106 but its type `Maybe a -> a' has only one
108 The section `(f 3)' requires 'f' to take two arguments
109 but its type `Int -> Int' has only one
111 The function 'f' is applied to two arguments
112 but its type `Int -> Int' has only one
116 subFunTys error_herald n_pats res_ty thing_inside
117 = loop n_pats [] res_ty
119 -- In 'loop', the parameter 'arg_tys' accumulates
120 -- the arg types so far, in *reverse order*
121 -- INVARIANT: res_ty :: *
122 loop n args_so_far res_ty
123 | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
125 loop n args_so_far res_ty
126 | isSigmaTy res_ty -- Do this before checking n==0, because we
127 -- guarantee to return a BoxyRhoType, not a
129 = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet $ \ _ res_ty' ->
130 loop n args_so_far res_ty'
131 ; return (gen_fn <.> co_fn, res) }
133 loop 0 args_so_far res_ty
134 = do { res <- thing_inside (reverse args_so_far) res_ty
135 ; return (idHsWrapper, res) }
137 loop n args_so_far (FunTy arg_ty res_ty)
138 = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
139 ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
140 ; return (co_fn', res) }
142 -- Try to normalise synonym families and defer if that's not possible
143 loop n args_so_far ty@(TyConApp tc tys)
145 = do { (coi1, ty') <- tcNormaliseFamInst ty
147 IdCo -> defer n args_so_far ty
148 -- no progress, but maybe solvable => defer
149 ACo _ -> -- progress: so lets try again
150 do { (co_fn, res) <- loop n args_so_far ty'
151 ; return $ (co_fn <.> coiToHsWrapper (mkSymCoI coi1), res)
155 -- res_ty might have a type variable at the head, such as (a b c),
156 -- in which case we must fill in with (->). Simplest thing to do
157 -- is to use boxyUnify, but we catch failure and generate our own
158 -- error message on failure
159 loop n args_so_far res_ty@(AppTy _ _)
160 = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
161 ; (_, mb_coi) <- tryTcErrs $
162 boxyUnify res_ty (FunTy arg_ty' res_ty')
163 ; if isNothing mb_coi then bale_out args_so_far
164 else do { let coi = expectJust "subFunTys" mb_coi
165 ; (co_fn, res) <- loop n args_so_far (FunTy arg_ty'
167 ; return (co_fn <.> coiToHsWrapper coi, res)
171 loop n args_so_far ty@(TyVarTy tv)
172 | isTyConableTyVar tv
173 = do { cts <- readMetaTyVar tv
175 Indirect ty -> loop n args_so_far ty
177 do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
178 ; res <- thing_inside (reverse args_so_far ++ arg_tys)
180 ; return (idHsWrapper, res) } }
181 | otherwise -- defer as tyvar may be refined by equalities
182 = defer n args_so_far ty
184 mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
185 mk_res_ty [] = panic "TcUnify.mk_res_ty1"
186 kinds = openTypeKind : take n (repeat argTypeKind)
187 -- Note argTypeKind: the args can have an unboxed type,
188 -- but not an unboxed tuple.
190 loop n args_so_far res_ty = bale_out args_so_far
192 -- build a template type a1 -> ... -> an -> b and defer an equality
193 -- between that template and the expected result type res_ty; then,
194 -- use the template to type the thing_inside
195 defer n args_so_far ty
196 = do { arg_tys <- newFlexiTyVarTys n argTypeKind
197 ; res_ty' <- newFlexiTyVarTy openTypeKind
198 ; let fun_ty = mkFunTys arg_tys res_ty'
199 err = error_herald <> comma $$
200 text "which does not match its type"
201 ; coi <- addErrCtxt err $
202 defer_unification False False fun_ty ty
203 ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty'
204 ; return (coiToHsWrapper coi, res)
208 = do { env0 <- tcInitTidyEnv
209 ; res_ty' <- zonkTcType res_ty
210 ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
211 ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
213 mk_msg res_ty n_actual
214 = error_herald <> comma $$
215 sep [ptext SLIT("but its type") <+> quotes (pprType res_ty),
216 if n_actual == 0 then ptext SLIT("has none")
217 else ptext SLIT("has only") <+> speakN n_actual]
221 ----------------------
222 boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
223 -> BoxyRhoType -- Expected type (T a b c)
224 -> TcM ([BoxySigmaType], -- Element types, a b c
225 CoercionI) -- T a b c ~ orig_ty
226 -- It's used for wired-in tycons, so we call checkWiredInTyCon
227 -- Precondition: never called with FunTyCon
228 -- Precondition: input type :: *
230 boxySplitTyConApp tc orig_ty
231 = do { checkWiredInTyCon tc
232 ; loop (tyConArity tc) [] orig_ty }
234 loop n_req args_so_far ty
235 | Just ty' <- tcView ty = loop n_req args_so_far ty'
237 loop n_req args_so_far ty@(TyConApp tycon args)
239 = ASSERT( n_req == length args) -- ty::*
240 return (args ++ args_so_far, IdCo)
242 | isOpenSynTyCon tycon -- try to normalise type family application
243 = do { (coi1, ty') <- tcNormaliseFamInst ty
244 ; traceTc $ text "boxySplitTyConApp:" <+>
245 ppr ty <+> text "==>" <+> ppr ty'
247 IdCo -> defer -- no progress, but maybe solvable => defer
248 ACo _ -> -- progress: so lets try again
249 do { (args, coi2) <- loop n_req args_so_far ty'
250 ; return $ (args, coi2 `mkTransCoI` mkSymCoI coi1)
254 loop n_req args_so_far (AppTy fun arg)
256 = do { (args, coi) <- loop (n_req - 1) (arg:args_so_far) fun
257 ; return (args, mkAppTyCoI fun coi arg IdCo)
260 loop n_req args_so_far (TyVarTy tv)
261 | isTyConableTyVar tv
262 , res_kind `isSubKind` tyVarKind tv
263 = do { cts <- readMetaTyVar tv
265 Indirect ty -> loop n_req args_so_far ty
266 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
267 ; return (arg_tys ++ args_so_far, IdCo) }
269 | otherwise -- defer as tyvar may be refined by equalities
272 (arg_kinds, res_kind) = splitKindFunTysN n_req (tyConKind tc)
274 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc)))
277 -- defer splitting by generating an equality constraint
278 defer = boxySplitDefer arg_kinds mk_res_ty orig_ty
280 (arg_kinds, _) = splitKindFunTys (tyConKind tc)
282 -- apply splitted tycon to arguments
283 mk_res_ty = mkTyConApp tc
285 ----------------------
286 boxySplitListTy :: BoxyRhoType -> TcM (BoxySigmaType, CoercionI)
287 -- Special case for lists
288 boxySplitListTy exp_ty
289 = do { ([elt_ty], coi) <- boxySplitTyConApp listTyCon exp_ty
290 ; return (elt_ty, coi) }
292 ----------------------
293 boxySplitPArrTy :: BoxyRhoType -> TcM (BoxySigmaType, CoercionI)
294 -- Special case for parrs
295 boxySplitPArrTy exp_ty
296 = do { ([elt_ty], coi) <- boxySplitTyConApp parrTyCon exp_ty
297 ; return (elt_ty, coi) }
299 ----------------------
300 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
301 -> TcM ((BoxySigmaType, BoxySigmaType), -- Returns m, a
303 -- If the incoming type is a mutable type variable of kind k, then
304 -- boxySplitAppTy returns a new type variable (m: * -> k); note the *.
305 -- If the incoming type is boxy, then so are the result types; and vice versa
307 boxySplitAppTy orig_ty
311 | Just ty' <- tcView ty = loop ty'
314 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
315 = return ((fun_ty, arg_ty), IdCo)
317 loop ty@(TyConApp tycon _args)
318 | isOpenSynTyCon tycon -- try to normalise type family application
319 = do { (coi1, ty') <- tcNormaliseFamInst ty
321 IdCo -> defer -- no progress, but maybe solvable => defer
322 ACo co -> -- progress: so lets try again
323 do { (args, coi2) <- loop ty'
324 ; return $ (args, coi2 `mkTransCoI` mkSymCoI coi1)
329 | isTyConableTyVar tv
330 = do { cts <- readMetaTyVar tv
332 Indirect ty -> loop ty
333 Flexi -> do { [fun_ty, arg_ty] <- withMetaTvs tv kinds mk_res_ty
334 ; return ((fun_ty, arg_ty), IdCo) } }
335 | otherwise -- defer as tyvar may be refined by equalities
338 tv_kind = tyVarKind tv
339 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
341 liftedTypeKind] -- arg type :: *
342 -- The defaultKind is a bit smelly. If you remove it,
343 -- try compiling f x = do { x }
344 -- and you'll get a kind mis-match. It smells, but
345 -- not enough to lose sleep over.
347 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
349 -- defer splitting by generating an equality constraint
350 defer = do { ([ty1, ty2], coi) <- boxySplitDefer arg_kinds mk_res_ty orig_ty
351 ; return ((ty1, ty2), coi)
354 orig_kind = typeKind orig_ty
355 arg_kinds = [mkArrowKind liftedTypeKind (defaultKind orig_kind),
357 liftedTypeKind] -- arg type :: *
359 -- build type application
360 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
361 mk_res_ty _other = panic "TcUnify.mk_res_ty2"
364 boxySplitFailure actual_ty expected_ty
365 = unifyMisMatch False False actual_ty expected_ty
366 -- "outer" is False, so we don't pop the context
367 -- which is what we want since we have not pushed one!
370 boxySplitDefer :: [Kind] -- kinds of required arguments
371 -> ([TcType] -> TcTauType) -- construct lhs from argument tyvars
372 -> BoxyRhoType -- type to split
373 -> TcM ([TcType], CoercionI)
374 boxySplitDefer kinds mkTy orig_ty
375 = do { tau_tys <- mapM newFlexiTyVarTy kinds
376 ; coi <- defer_unification False False (mkTy tau_tys) orig_ty
377 ; return (tau_tys, coi)
382 --------------------------------
383 -- withBoxes: the key utility function
384 --------------------------------
387 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
388 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
389 -> ([BoxySigmaType] -> BoxySigmaType)
390 -- Constructs the type to assign
391 -- to the original var
392 -> TcM [BoxySigmaType] -- Return the fresh boxes
394 -- It's entirely possible for the [kind] to be empty.
395 -- For example, when pattern-matching on True,
396 -- we call boxySplitTyConApp passing a boolTyCon
398 -- Invariant: tv is still Flexi
400 withMetaTvs tv kinds mk_res_ty
402 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
403 ; let box_tys = mkTyVarTys box_tvs
404 ; writeMetaTyVar tv (mk_res_ty box_tys)
407 | otherwise -- Non-boxy meta type variable
408 = do { tau_tys <- mapM newFlexiTyVarTy kinds
409 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
410 -- Sure to be a tau-type
413 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
414 -- Allocate a *boxy* tyvar
415 withBox kind thing_inside
416 = do { box_tv <- newBoxyTyVar kind
417 ; res <- thing_inside (mkTyVarTy box_tv)
418 ; ty <- {- pprTrace "with_box" (ppr (mkTyVarTy box_tv)) $ -} readFilledBox box_tv
423 %************************************************************************
425 Approximate boxy matching
427 %************************************************************************
430 preSubType :: [TcTyVar] -- Quantified type variables
431 -> TcTyVarSet -- Subset of quantified type variables
432 -- see Note [Pre-sub boxy]
433 -> TcType -- The rho-type part; quantified tyvars scopes over this
434 -> BoxySigmaType -- Matching type from the context
435 -> TcM [TcType] -- Types to instantiate the tyvars
436 -- Perform pre-subsumption, and return suitable types
437 -- to instantiate the quantified type varibles:
438 -- info from the pre-subsumption, if there is any
439 -- a boxy type variable otherwise
441 -- Note [Pre-sub boxy]
442 -- The 'btvs' are a subset of 'qtvs'. They are the ones we can
443 -- instantiate to a boxy type variable, because they'll definitely be
444 -- filled in later. This isn't always the case; sometimes we have type
445 -- variables mentioned in the context of the type, but not the body;
446 -- f :: forall a b. C a b => a -> a
447 -- Then we may land up with an unconstrained 'b', so we want to
448 -- instantiate it to a monotype (non-boxy) type variable
450 -- The 'qtvs' that are *neither* fixed by the pre-subsumption, *nor* are in 'btvs',
451 -- are instantiated to TauTv meta variables.
453 preSubType qtvs btvs qty expected_ty
454 = do { tys <- mapM inst_tv qtvs
455 ; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
458 pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
460 | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
461 | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
462 ; return (mkTyVarTy tv') }
463 | otherwise = do { tv' <- tcInstTyVar tv
464 ; return (mkTyVarTy tv') }
467 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
468 -> BoxyRhoType -- Type to match (note a *Rho* type)
469 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
471 -- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
472 -- "Boxy types: inference for higher rank types and impredicativity"
474 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
475 = go tmpl_tvs tmpl_ty emptyVarSet boxy_ty
477 go t_tvs t_ty b_tvs b_ty
478 | Just t_ty' <- tcView t_ty = go t_tvs t_ty' b_tvs b_ty
479 | Just b_ty' <- tcView b_ty = go t_tvs t_ty b_tvs b_ty'
481 go t_tvs (TyVarTy _) b_tvs b_ty = emptyTvSubst -- Rule S-ANY; no bindings
482 -- Rule S-ANY covers (a) type variables and (b) boxy types
483 -- in the template. Both look like a TyVarTy.
484 -- See Note [Sub-match] below
486 go t_tvs t_ty b_tvs b_ty
487 | isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
488 = go (t_tvs `delVarSetList` tvs) t_tau b_tvs b_ty -- Rule S-SPEC
489 -- Under a forall on the left, if there is shadowing,
490 -- do not bind! Hence the delVarSetList.
491 | isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
492 = go t_tvs t_ty (extendVarSetList b_tvs tvs) b_tau -- Rule S-SKOL
493 -- Add to the variables we must not bind to
494 -- NB: it's *important* to discard the theta part. Otherwise
495 -- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
496 -- and end up with a completely bogus binding (b |-> Bool), by lining
497 -- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
498 -- This pre-subsumption stuff can return too few bindings, but it
499 -- must *never* return bogus info.
501 go t_tvs (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
502 = boxy_match t_tvs arg1 b_tvs arg2 (go t_tvs res1 b_tvs res2)
503 -- Match the args, and sub-match the results
505 go t_tvs t_ty b_tvs b_ty = boxy_match t_tvs t_ty b_tvs b_ty emptyTvSubst
506 -- Otherwise defer to boxy matching
507 -- This covers TyConApp, AppTy, PredTy
514 |- head xs : <rhobox>
515 We will do a boxySubMatchType between a ~ <rhobox>
516 But we *don't* want to match [a |-> <rhobox>] because
517 (a) The box should be filled in with a rho-type, but
518 but the returned substitution maps TyVars to boxy
520 (b) In any case, the right final answer might be *either*
521 instantiate 'a' with a rho-type or a sigma type
522 head xs : Int vs head xs : forall b. b->b
523 So the matcher MUST NOT make a choice here. In general, we only
524 bind a template type variable in boxyMatchType, not in boxySubMatchType.
529 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
530 -> [BoxySigmaType] -- Type to match
531 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
533 -- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
534 -- "Boxy types: inference for higher rank types and impredicativity"
536 -- Find a *boxy* substitution that makes the template look as much
537 -- like the BoxySigmaType as possible.
538 -- It's always ok to return an empty substitution;
539 -- anything more is jam on the pudding
541 -- NB1: This is a pure, non-monadic function.
542 -- It does no unification, and cannot fail
544 -- Precondition: the arg lengths are equal
545 -- Precondition: none of the template type variables appear anywhere in the [BoxySigmaType]
549 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
550 = ASSERT( length tmpl_tys == length boxy_tys )
551 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
552 -- ToDo: add error context?
554 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
556 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
557 = boxy_match tmpl_tvs t_ty boxy_tvs b_ty $
558 boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys subst
559 boxy_match_s tmpl_tvs _ boxy_tvs _ subst
560 = panic "boxy_match_s" -- Lengths do not match
564 boxy_match :: TcTyVarSet -> TcType -- Template
565 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
566 -> BoxySigmaType -- Match against this type
570 -- The boxy_tvs argument prevents this match:
571 -- [a] forall b. a ~ forall b. b
572 -- We don't want to bind the template variable 'a'
573 -- to the quantified type variable 'b'!
575 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
576 = go orig_tmpl_ty orig_boxy_ty
579 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
580 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
582 go ty1 ty2 -- C.f. the isSigmaTy case for boxySubMatchType
584 , (tvs1, _, tau1) <- tcSplitSigmaTy ty1
585 , (tvs2, _, tau2) <- tcSplitSigmaTy ty2
586 , equalLength tvs1 tvs2
587 = boxy_match (tmpl_tvs `delVarSetList` tvs1) tau1
588 (boxy_tvs `extendVarSetList` tvs2) tau2 subst
590 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
592 , not $ isOpenSynTyCon tc1
595 go (FunTy arg1 res1) (FunTy arg2 res2)
596 = go_s [arg1,res1] [arg2,res2]
599 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
600 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
601 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
602 = go_s [s1,t1] [s2,t2]
605 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
606 , boxy_tvs `disjointVarSet` tyVarsOfType orig_boxy_ty
607 , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
608 = extendTvSubst subst tv boxy_ty'
610 = subst -- Ignore others
612 boxy_ty' = case lookupTyVar subst tv of
613 Nothing -> orig_boxy_ty
614 Just ty -> ty `boxyLub` orig_boxy_ty
616 go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
617 -- Example: Tree a ~ Maybe Int
618 -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
619 -- misleading error messages. An even more confusing case is
620 -- a -> b ~ Maybe Int
621 -- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
622 -- from this pre-matching phase.
625 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
628 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
629 -- Combine boxy information from the two types
630 -- If there is a conflict, return the first
631 boxyLub orig_ty1 orig_ty2
632 = go orig_ty1 orig_ty2
634 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
635 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
636 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
637 | tc1 == tc2, length ts1 == length ts2
638 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
640 go (TyVarTy tv1) ty2 -- This is the whole point;
641 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
644 -- Look inside type synonyms, but only if the naive version fails
645 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
646 | Just ty2' <- tcView ty1 = go ty1 ty2'
648 -- For now, we don't look inside ForAlls, PredTys
649 go ty1 ty2 = orig_ty1 -- Default
652 Note [Matching kinds]
653 ~~~~~~~~~~~~~~~~~~~~~
654 The target type might legitimately not be a sub-kind of template.
655 For example, suppose the target is simply a box with an OpenTypeKind,
656 and the template is a type variable with LiftedTypeKind.
657 Then it's ok (because the target type will later be refined).
658 We simply don't bind the template type variable.
660 It might also be that the kind mis-match is an error. For example,
661 suppose we match the template (a -> Int) against (Int# -> Int),
662 where the template type variable 'a' has LiftedTypeKind. This
663 matching function does not fail; it simply doesn't bind the template.
664 Later stuff will fail.
666 %************************************************************************
670 %************************************************************************
672 All the tcSub calls have the form
674 tcSub actual_ty expected_ty
676 actual_ty <= expected_ty
678 That is, that a value of type actual_ty is acceptable in
679 a place expecting a value of type expected_ty.
681 It returns a coercion function
682 co_fn :: actual_ty ~ expected_ty
683 which takes an HsExpr of type actual_ty into one of type
688 tcSubExp :: InstOrigin -> BoxySigmaType -> BoxySigmaType -> TcM HsWrapper
689 -- (tcSub act exp) checks that
691 tcSubExp orig actual_ty expected_ty
692 = -- addErrCtxtM (unifyCtxt actual_ty expected_ty) $
693 -- Adding the error context here leads to some very confusing error
694 -- messages, such as "can't match forall a. a->a with forall a. a->a"
695 -- Example is tcfail165:
696 -- do var <- newEmptyMVar :: IO (MVar (forall a. Show a => a -> String))
697 -- putMVar var (show :: forall a. Show a => a -> String)
698 -- Here the info does not flow from the 'var' arg of putMVar to its 'show' arg
699 -- but after zonking it looks as if it does!
701 -- So instead I'm adding the error context when moving from tc_sub to u_tys
703 traceTc (text "tcSubExp" <+> ppr actual_ty <+> ppr expected_ty) >>
704 tc_sub orig actual_ty actual_ty False expected_ty expected_ty
708 -> BoxySigmaType -- actual_ty, before expanding synonyms
709 -> BoxySigmaType -- ..and after
710 -> InBox -- True <=> expected_ty is inside a box
711 -> BoxySigmaType -- expected_ty, before
712 -> BoxySigmaType -- ..and after
714 -- The acual_ty is never inside a box
715 -- IMPORTANT pre-condition: if the args contain foralls, the bound type
716 -- variables are visible non-monadically
717 -- (i.e. tha args are sufficiently zonked)
718 -- This invariant is needed so that we can "see" the foralls, ad
719 -- e.g. in the SPEC rule where we just use splitSigmaTy
721 tc_sub orig act_sty act_ty exp_ib exp_sty exp_ty
722 = traceTc (text "tc_sub" <+> ppr act_ty $$ ppr exp_ty) >>
723 tc_sub1 orig act_sty act_ty exp_ib exp_sty exp_ty
724 -- This indirection is just here to make
725 -- it easy to insert a debug trace!
727 tc_sub1 orig act_sty act_ty exp_ib exp_sty exp_ty
728 | Just exp_ty' <- tcView exp_ty = tc_sub orig act_sty act_ty exp_ib exp_sty exp_ty'
729 tc_sub1 orig act_sty act_ty exp_ib exp_sty exp_ty
730 | Just act_ty' <- tcView act_ty = tc_sub orig act_sty act_ty' exp_ib exp_sty exp_ty
732 -----------------------------------
733 -- Rule SBOXY, plus other cases when act_ty is a type variable
734 -- Just defer to boxy matching
735 -- This rule takes precedence over SKOL!
736 tc_sub1 orig act_sty (TyVarTy tv) exp_ib exp_sty exp_ty
737 = do { traceTc (text "tc_sub1 - case 1")
738 ; coi <- addSubCtxt orig act_sty exp_sty $
739 uVar True False tv exp_ib exp_sty exp_ty
740 ; traceTc (case coi of
741 IdCo -> text "tc_sub1 (Rule SBOXY) IdCo"
742 ACo co -> text "tc_sub1 (Rule SBOXY) ACo" <+> ppr co)
743 ; return $ coiToHsWrapper coi
746 -----------------------------------
747 -- Skolemisation case (rule SKOL)
748 -- actual_ty: d:Eq b => b->b
749 -- expected_ty: forall a. Ord a => a->a
750 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
752 -- It is essential to do this *before* the specialisation case
753 -- Example: f :: (Eq a => a->a) -> ...
754 -- g :: Ord b => b->b
757 tc_sub1 orig act_sty act_ty exp_ib exp_sty exp_ty
759 = do { traceTc (text "tc_sub1 - case 2") ;
760 if exp_ib then -- SKOL does not apply if exp_ty is inside a box
761 defer_to_boxy_matching orig act_sty act_ty exp_ib exp_sty exp_ty
763 { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ _ body_exp_ty ->
764 tc_sub orig act_sty act_ty False body_exp_ty body_exp_ty
765 ; return (gen_fn <.> co_fn) }
768 act_tvs = tyVarsOfType act_ty
769 -- It's really important to check for escape wrt
770 -- the free vars of both expected_ty *and* actual_ty
772 -----------------------------------
773 -- Specialisation case (rule ASPEC):
774 -- actual_ty: forall a. Ord a => a->a
775 -- expected_ty: Int -> Int
776 -- co_fn e = e Int dOrdInt
778 tc_sub1 orig act_sty actual_ty exp_ib exp_sty expected_ty
779 -- Implements the new SPEC rule in the Appendix of the paper
780 -- "Boxy types: inference for higher rank types and impredicativity"
781 -- (This appendix isn't in the published version.)
782 -- The idea is to *first* do pre-subsumption, and then full subsumption
783 -- Example: forall a. a->a <= Int -> (forall b. Int)
784 -- Pre-subsumpion finds a|->Int, and that works fine, whereas
785 -- just running full subsumption would fail.
786 | isSigmaTy actual_ty
787 = do { traceTc (text "tc_sub1 - case 3")
788 ; -- Perform pre-subsumption, and instantiate
789 -- the type with info from the pre-subsumption;
790 -- boxy tyvars if pre-subsumption gives no info
791 let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
792 tau_tvs = exactTyVarsOfType tau
793 ; inst_tys <- if exp_ib then -- Inside a box, do not do clever stuff
794 do { tyvars' <- mapM tcInstBoxyTyVar tyvars
795 ; return (mkTyVarTys tyvars') }
796 else -- Outside, do clever stuff
797 preSubType tyvars tau_tvs tau expected_ty
798 ; let subst' = zipOpenTvSubst tyvars inst_tys
799 tau' = substTy subst' tau
801 -- Perform a full subsumption check
802 ; traceTc (text "tc_sub_spec" <+> vcat [ppr actual_ty,
803 ppr tyvars <+> ppr theta <+> ppr tau,
805 ; co_fn2 <- tc_sub orig tau' tau' exp_ib exp_sty expected_ty
807 -- Deal with the dictionaries
808 ; co_fn1 <- instCall orig inst_tys (substTheta subst' theta)
809 ; return (co_fn2 <.> co_fn1) }
811 -----------------------------------
812 -- Function case (rule F1)
813 tc_sub1 orig act_sty (FunTy act_arg act_res) exp_ib exp_sty (FunTy exp_arg exp_res)
814 = do { traceTc (text "tc_sub1 - case 4")
815 ; tc_sub_funs orig act_arg act_res exp_ib exp_arg exp_res
818 -- Function case (rule F2)
819 tc_sub1 orig act_sty act_ty@(FunTy act_arg act_res) _ exp_sty (TyVarTy exp_tv)
821 = do { traceTc (text "tc_sub1 - case 5")
822 ; cts <- readMetaTyVar exp_tv
824 Indirect ty -> tc_sub orig act_sty act_ty True exp_sty ty
825 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
826 ; tc_sub_funs orig act_arg act_res True arg_ty res_ty } }
828 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
829 mk_res_ty other = panic "TcUnify.mk_res_ty3"
830 fun_kinds = [argTypeKind, openTypeKind]
832 -- Everything else: defer to boxy matching
833 tc_sub1 orig act_sty actual_ty exp_ib exp_sty expected_ty@(TyVarTy exp_tv)
834 = do { traceTc (text "tc_sub1 - case 6a" <+> ppr [isBoxyTyVar exp_tv, isMetaTyVar exp_tv, isSkolemTyVar exp_tv, isExistentialTyVar exp_tv,isSigTyVar exp_tv] )
835 ; defer_to_boxy_matching orig act_sty actual_ty exp_ib exp_sty expected_ty
838 tc_sub1 orig act_sty actual_ty exp_ib exp_sty expected_ty
839 = do { traceTc (text "tc_sub1 - case 6")
840 ; defer_to_boxy_matching orig act_sty actual_ty exp_ib exp_sty expected_ty
843 -----------------------------------
844 defer_to_boxy_matching orig act_sty actual_ty exp_ib exp_sty expected_ty
845 = do { coi <- addSubCtxt orig act_sty exp_sty $
846 u_tys True False act_sty actual_ty exp_ib exp_sty expected_ty
847 ; return $ coiToHsWrapper coi }
849 -----------------------------------
850 tc_sub_funs orig act_arg act_res exp_ib exp_arg exp_res
851 = do { arg_coi <- addSubCtxt orig act_arg exp_arg $
852 uTysOuter False act_arg exp_ib exp_arg
853 ; co_fn_res <- tc_sub orig act_res act_res exp_ib exp_res exp_res
854 ; wrapper1 <- wrapFunResCoercion [exp_arg] co_fn_res
855 ; let wrapper2 = case arg_coi of
857 ACo co -> WpCo $ FunTy co act_res
858 ; return (wrapper1 <.> wrapper2) }
860 -----------------------------------
862 :: [TcType] -- Type of args
863 -> HsWrapper -- HsExpr a -> HsExpr b
864 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
865 wrapFunResCoercion arg_tys co_fn_res
866 | isIdHsWrapper co_fn_res
871 = do { arg_ids <- newSysLocalIds FSLIT("sub") arg_tys
872 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpApps arg_ids) }
877 %************************************************************************
879 \subsection{Generalisation}
881 %************************************************************************
884 tcGen :: BoxySigmaType -- expected_ty
885 -> TcTyVarSet -- Extra tyvars that the universally
886 -- quantified tyvars of expected_ty
887 -- must not be unified
888 -> ([TcTyVar] -> BoxyRhoType -> TcM result)
889 -> TcM (HsWrapper, result)
890 -- The expression has type: spec_ty -> expected_ty
892 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
893 -- If not, the call is a no-op
894 = do { traceTc (text "tcGen")
895 -- We want the GenSkol info in the skolemised type variables to
896 -- mention the *instantiated* tyvar names, so that we get a
897 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
898 -- Hence the tiresome but innocuous fixM
899 ; ((tvs', theta', rho'), skol_info) <- fixM (\ ~(_, skol_info) ->
900 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
901 -- Get loation from monad, not from expected_ty
902 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty)
903 ; return ((forall_tvs, theta, rho_ty), skol_info) })
906 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
907 text "expected_ty" <+> ppr expected_ty,
908 text "inst ty" <+> ppr tvs' <+> ppr theta' <+> ppr rho',
909 text "free_tvs" <+> ppr free_tvs])
912 -- Type-check the arg and unify with poly type
913 ; (result, lie) <- getLIE (thing_inside tvs' rho')
915 -- Check that the "forall_tvs" havn't been constrained
916 -- The interesting bit here is that we must include the free variables
917 -- of the expected_ty. Here's an example:
918 -- runST (newVar True)
919 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
920 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
921 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
922 -- So now s' isn't unconstrained because it's linked to a.
923 -- Conclusion: include the free vars of the expected_ty in the
924 -- list of "free vars" for the signature check.
926 ; loc <- getInstLoc (SigOrigin skol_info)
927 ; dicts <- newDictBndrs loc theta' -- Includes equalities
928 ; inst_binds <- tcSimplifyCheck loc tvs' dicts lie
930 ; checkSigTyVarsWrt free_tvs tvs'
931 ; traceTc (text "tcGen:done")
934 -- The WpLet binds any Insts which came out of the simplification.
935 dict_vars = map instToVar dicts
936 co_fn = mkWpTyLams tvs' <.> mkWpLams dict_vars <.> WpLet inst_binds
937 ; returnM (co_fn, result) }
939 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
944 %************************************************************************
948 %************************************************************************
950 The exported functions are all defined as versions of some
951 non-exported generic functions.
954 boxyUnify :: BoxyType -> BoxyType -> TcM CoercionI
955 -- Acutal and expected, respectively
957 = addErrCtxtM (unifyCtxt ty1 ty2) $
958 uTysOuter False ty1 False ty2
961 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM [CoercionI]
962 -- Arguments should have equal length
963 -- Acutal and expected types
964 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
967 unifyType :: TcTauType -> TcTauType -> TcM CoercionI
968 -- No boxes expected inside these types
969 -- Acutal and expected types
970 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
971 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
972 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
973 addErrCtxtM (unifyCtxt ty1 ty2) $
974 uTysOuter True ty1 True ty2
977 unifyPred :: PredType -> PredType -> TcM CoercionI
978 -- Acutal and expected types
979 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
980 uPred True True p1 True p2
982 unifyTheta :: TcThetaType -> TcThetaType -> TcM [CoercionI]
983 -- Acutal and expected types
984 unifyTheta theta1 theta2
985 = do { checkTc (equalLength theta1 theta2)
986 (vcat [ptext SLIT("Contexts differ in length"),
987 nest 2 $ parens $ ptext SLIT("Use -fglasgow-exts to allow this")])
988 ; uList unifyPred theta1 theta2
992 uList :: (a -> a -> TcM b)
993 -> [a] -> [a] -> TcM [b]
994 -- Unify corresponding elements of two lists of types, which
995 -- should be of equal length. We charge down the list explicitly so that
996 -- we can complain if their lengths differ.
997 uList unify [] [] = return []
998 uList unify (ty1:tys1) (ty2:tys2) = do { x <- unify ty1 ty2;
999 ; xs <- uList unify tys1 tys2
1002 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
1005 @unifyTypeList@ takes a single list of @TauType@s and unifies them
1006 all together. It is used, for example, when typechecking explicit
1007 lists, when all the elts should be of the same type.
1010 unifyTypeList :: [TcTauType] -> TcM ()
1011 unifyTypeList [] = returnM ()
1012 unifyTypeList [ty] = returnM ()
1013 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
1014 ; unifyTypeList tys }
1017 %************************************************************************
1019 \subsection[Unify-uTys]{@uTys@: getting down to business}
1021 %************************************************************************
1023 @uTys@ is the heart of the unifier. Each arg occurs twice, because
1024 we want to report errors in terms of synomyms if possible. The first of
1025 the pair is used in error messages only; it is always the same as the
1026 second, except that if the first is a synonym then the second may be a
1027 de-synonym'd version. This way we get better error messages.
1029 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
1032 type SwapFlag = Bool
1033 -- False <=> the two args are (actual, expected) respectively
1034 -- True <=> the two args are (expected, actual) respectively
1036 type InBox = Bool -- True <=> we are inside a box
1037 -- False <=> we are outside a box
1038 -- The importance of this is that if we get "filled-box meets
1039 -- filled-box", we'll look into the boxes and unify... but
1040 -- we must not allow polytypes. But if we are in a box on
1041 -- just one side, then we can allow polytypes
1043 type Outer = Bool -- True <=> this is the outer level of a unification
1044 -- so that the types being unified are the
1045 -- very ones we began with, not some sub
1046 -- component or synonym expansion
1047 -- The idea is that if Outer is true then unifyMisMatch should
1048 -- pop the context to remove the "Expected/Acutal" context
1051 :: InBox -> TcType -- ty1 is the *actual* type
1052 -> InBox -> TcType -- ty2 is the *expected* type
1054 uTysOuter nb1 ty1 nb2 ty2
1055 = do { traceTc (text "uTysOuter" <+> ppr ty1 <+> ppr ty2)
1056 ; u_tys True nb1 ty1 ty1 nb2 ty2 ty2 }
1057 uTys nb1 ty1 nb2 ty2
1058 = do { traceTc (text "uTys" <+> ppr ty1 <+> ppr ty2)
1059 ; u_tys False nb1 ty1 ty1 nb2 ty2 ty2 }
1063 uTys_s :: InBox -> [TcType] -- tys1 are the *actual* types
1064 -> InBox -> [TcType] -- tys2 are the *expected* types
1066 uTys_s nb1 [] nb2 [] = returnM []
1067 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { coi <- uTys nb1 ty1 nb2 ty2
1068 ; cois <- uTys_s nb1 tys1 nb2 tys2
1071 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
1075 -> InBox -> TcType -> TcType -- ty1 is the *actual* type
1076 -> InBox -> TcType -> TcType -- ty2 is the *expected* type
1079 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
1080 = do { traceTc (text "u_tys " <+> ppr ty1 <+> text " " <+> ppr ty2)
1081 ; coi <- go outer ty1 ty2
1082 ; traceTc (case coi of
1083 ACo co -> text "u_tys yields coercion: " <+> ppr co
1084 IdCo -> text "u_tys yields no coercion")
1089 go :: Outer -> TcType -> TcType -> TcM CoercionI
1091 do { traceTc (text "go " <+> ppr orig_ty1 <+> text "/" <+> ppr ty1
1092 <+> ppr orig_ty2 <+> text "/" <+> ppr ty2)
1096 go1 :: Outer -> TcType -> TcType -> TcM CoercionI
1097 -- Always expand synonyms: see Note [Unification and synonyms]
1098 -- (this also throws away FTVs)
1100 | Just ty1' <- tcView ty1 = go False ty1' ty2
1101 | Just ty2' <- tcView ty2 = go False ty1 ty2'
1103 -- Variables; go for uVar
1104 go1 outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
1105 go1 outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
1106 -- "True" means args swapped
1108 -- The case for sigma-types must *follow* the variable cases
1109 -- because a boxy variable can be filed with a polytype;
1110 -- but must precede FunTy, because ((?x::Int) => ty) look
1111 -- like a FunTy; there isn't necy a forall at the top
1113 | isSigmaTy ty1 || isSigmaTy ty2
1114 = do { traceTc (text "We have sigma types: equalLength" <+> ppr tvs1 <+> ppr tvs2)
1115 ; checkM (equalLength tvs1 tvs2)
1116 (unifyMisMatch outer False orig_ty1 orig_ty2)
1117 ; traceTc (text "We're past the first length test")
1118 ; tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
1119 -- Get location from monad, not from tvs1
1120 ; let tys = mkTyVarTys tvs
1121 in_scope = mkInScopeSet (mkVarSet tvs)
1122 phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
1123 phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
1124 (theta1,tau1) = tcSplitPhiTy phi1
1125 (theta2,tau2) = tcSplitPhiTy phi2
1127 ; addErrCtxtM (unifyForAllCtxt tvs phi1 phi2) $ do
1128 { checkM (equalLength theta1 theta2)
1129 (unifyMisMatch outer False orig_ty1 orig_ty2)
1131 ; cois <- uPreds False nb1 theta1 nb2 theta2 -- TOMDO: do something with these pred_cois
1132 ; traceTc (text "TOMDO!")
1133 ; coi <- uTys nb1 tau1 nb2 tau2
1135 -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
1136 ; free_tvs <- zonkTcTyVarsAndFV (varSetElems (tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2))
1137 ; ifM (any (`elemVarSet` free_tvs) tvs)
1138 (bleatEscapedTvs free_tvs tvs tvs)
1140 -- If both sides are inside a box, we are in a "box-meets-box"
1141 -- situation, and we should not have a polytype at all.
1142 -- If we get here we have two boxes, already filled with
1143 -- the same polytype... but it should be a monotype.
1144 -- This check comes last, because the error message is
1145 -- extremely unhelpful.
1146 ; ifM (nb1 && nb2) (notMonoType ty1)
1150 (tvs1, body1) = tcSplitForAllTys ty1
1151 (tvs2, body2) = tcSplitForAllTys ty2
1154 go1 outer (PredTy p1) (PredTy p2)
1155 = uPred False nb1 p1 nb2 p2
1157 -- Type constructors must match
1158 go1 _ (TyConApp con1 tys1) (TyConApp con2 tys2)
1159 | con1 == con2 && not (isOpenSynTyCon con1)
1160 = do { cois <- uTys_s nb1 tys1 nb2 tys2
1161 ; return $ mkTyConAppCoI con1 tys1 cois
1163 -- See Note [TyCon app]
1164 | con1 == con2 && identicalOpenSynTyConApp
1165 = do { cois <- uTys_s nb1 tys1' nb2 tys2'
1166 ; return $ mkTyConAppCoI con1 tys1 (replicate n IdCo ++ cois)
1170 (idxTys1, tys1') = splitAt n tys1
1171 (idxTys2, tys2') = splitAt n tys2
1172 identicalOpenSynTyConApp = idxTys1 `tcEqTypes` idxTys2
1173 -- See Note [OpenSynTyCon app]
1175 -- Functions; just check the two parts
1176 go1 _ (FunTy fun1 arg1) (FunTy fun2 arg2)
1177 = do { coi_l <- uTys nb1 fun1 nb2 fun2
1178 ; coi_r <- uTys nb1 arg1 nb2 arg2
1179 ; return $ mkFunTyCoI fun1 coi_l arg1 coi_r
1182 -- Applications need a bit of care!
1183 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
1184 -- NB: we've already dealt with type variables and Notes,
1185 -- so if one type is an App the other one jolly well better be too
1186 go1 outer (AppTy s1 t1) ty2
1187 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
1188 = do { coi_s <- uTys nb1 s1 nb2 s2; coi_t <- uTys nb1 t1 nb2 t2
1189 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1191 -- Now the same, but the other way round
1192 -- Don't swap the types, because the error messages get worse
1193 go1 outer ty1 (AppTy s2 t2)
1194 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
1195 = do { coi_s <- uTys nb1 s1 nb2 s2; coi_t <- uTys nb1 t1 nb2 t2
1196 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1198 -- One or both outermost constructors are type family applications.
1199 -- If we can normalise them away, proceed as usual; otherwise, we
1200 -- need to defer unification by generating a wanted equality constraint.
1202 | ty1_is_fun || ty2_is_fun
1203 = do { (coi1, ty1') <- if ty1_is_fun then tcNormaliseFamInst ty1
1204 else return (IdCo, ty1)
1205 ; (coi2, ty2') <- if ty2_is_fun then tcNormaliseFamInst ty2
1206 else return (IdCo, ty2)
1207 ; coi <- if isOpenSynTyConApp ty1' || isOpenSynTyConApp ty2'
1208 then do { -- One type family app can't be reduced yet
1210 ; ty1'' <- zonkTcType ty1'
1211 ; ty2'' <- zonkTcType ty2'
1212 ; if tcEqType ty1'' ty2''
1214 else -- see [Deferred Unification]
1215 defer_unification outer False orig_ty1 orig_ty2
1217 else -- unification can proceed
1219 ; return $ coi1 `mkTransCoI` coi `mkTransCoI` (mkSymCoI coi2)
1222 ty1_is_fun = isOpenSynTyConApp ty1
1223 ty2_is_fun = isOpenSynTyConApp ty2
1225 -- Anything else fails
1226 go1 outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
1230 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
1232 do { coi <- uTys nb1 t1 nb2 t2
1233 ; return $ mkIParamPredCoI n1 coi
1235 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
1237 do { cois <- uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
1238 ; return $ mkClassPPredCoI c1 tys1 cois
1240 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
1242 uPreds outer nb1 [] nb2 [] = return []
1243 uPreds outer nb1 (p1:ps1) nb2 (p2:ps2) =
1244 do { coi <- uPred outer nb1 p1 nb2 p2
1245 ; cois <- uPreds outer nb1 ps1 nb2 ps2
1248 uPreds outer nb1 ps1 nb2 ps2 = panic "uPreds"
1253 When we find two TyConApps, the argument lists are guaranteed equal
1254 length. Reason: intially the kinds of the two types to be unified is
1255 the same. The only way it can become not the same is when unifying two
1256 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
1257 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
1258 which we do, that ensures that f1,f2 have the same kind; and that
1259 means a1,a2 have the same kind. And now the argument repeats.
1261 Note [OpenSynTyCon app]
1262 ~~~~~~~~~~~~~~~~~~~~~~~
1265 type family T a :: * -> *
1267 the two types (T () a) and (T () Int) must unify, even if there are
1268 no type instances for T at all. Should we just turn them into an
1269 equality (T () a ~ T () Int)? I don't think so. We currently try to
1270 eagerly unify everything we can before generating equalities; otherwise,
1271 we could turn the unification of [Int] with [a] into an equality, too.
1273 Note [Unification and synonyms]
1274 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1275 If you are tempted to make a short cut on synonyms, as in this
1279 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
1280 -- NO = if (con1 == con2) then
1281 -- NO -- Good news! Same synonym constructors, so we can shortcut
1282 -- NO -- by unifying their arguments and ignoring their expansions.
1283 -- NO unifyTypepeLists args1 args2
1285 -- NO -- Never mind. Just expand them and try again
1289 then THINK AGAIN. Here is the whole story, as detected and reported
1290 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1292 Here's a test program that should detect the problem:
1296 x = (1 :: Bogus Char) :: Bogus Bool
1299 The problem with [the attempted shortcut code] is that
1303 is not a sufficient condition to be able to use the shortcut!
1304 You also need to know that the type synonym actually USES all
1305 its arguments. For example, consider the following type synonym
1306 which does not use all its arguments.
1311 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1312 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1313 would fail, even though the expanded forms (both \tr{Int}) should
1316 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1317 unnecessarily bind \tr{t} to \tr{Char}.
1319 ... You could explicitly test for the problem synonyms and mark them
1320 somehow as needing expansion, perhaps also issuing a warning to the
1325 %************************************************************************
1327 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1329 %************************************************************************
1331 @uVar@ is called when at least one of the types being unified is a
1332 variable. It does {\em not} assume that the variable is a fixed point
1333 of the substitution; rather, notice that @uVar@ (defined below) nips
1334 back into @uTys@ if it turns out that the variable is already bound.
1338 -> SwapFlag -- False => tyvar is the "actual" (ty is "expected")
1339 -- True => ty is the "actual" (tyvar is "expected")
1341 -> InBox -- True <=> definitely no boxes in t2
1342 -> TcTauType -> TcTauType -- printing and real versions
1345 uVar outer swapped tv1 nb2 ps_ty2 ty2
1346 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1347 | otherwise = brackets (equals <+> ppr ty2)
1348 ; traceTc (text "uVar" <+> ppr swapped <+>
1349 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1350 nest 2 (ptext SLIT(" <-> ")),
1351 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1352 ; details <- lookupTcTyVar tv1
1355 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1356 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1357 -- The 'True' here says that ty1 is now inside a box
1358 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1362 uUnfilledVar :: Outer
1364 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1365 -> TcTauType -> TcTauType -- Type 2
1367 -- Invariant: tyvar 1 is not unified with anything
1369 uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1370 | Just ty2' <- tcView ty2
1371 = -- Expand synonyms; ignore FTVs
1372 uUnfilledVar False swapped tv1 details1 ps_ty2 ty2'
1374 uUnfilledVar outer swapped tv1 details1 ps_ty2 (TyVarTy tv2)
1375 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1377 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1378 -- this is box-meets-box, so fill in with a tau-type
1379 -> do { tau_tv <- tcInstTyVar tv1
1380 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv)
1383 other -> returnM IdCo -- No-op
1385 | otherwise -- Distinct type variables
1386 = do { lookup2 <- lookupTcTyVar tv2
1388 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
1389 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1392 uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2
1393 = -- ty2 is not a type variable
1395 MetaTv (SigTv _) _ -> rigid_variable
1397 uMetaVar outer swapped tv1 info ref1 ps_ty2 non_var_ty2
1398 SkolemTv _ -> rigid_variable
1401 | isOpenSynTyConApp non_var_ty2
1402 = -- 'non_var_ty2's outermost constructor is a type family,
1403 -- which we may may be able to normalise
1404 do { (coi2, ty2') <- tcNormaliseFamInst non_var_ty2
1406 IdCo -> -- no progress, but maybe after other instantiations
1407 defer_unification outer swapped (TyVarTy tv1) ps_ty2
1408 ACo co -> -- progress: so lets try again
1410 ppr co <+> text "::"<+> ppr non_var_ty2 <+> text "~" <+>
1412 ; coi <- uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2'
1413 ; let coi2' = (if swapped then id else mkSymCoI) coi2
1414 ; return $ coi2' `mkTransCoI` coi
1417 | SkolemTv RuntimeUnkSkol <- details1
1418 -- runtime unknown will never match
1419 = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1420 | otherwise -- defer as a given equality may still resolve this
1421 = defer_unification outer swapped (TyVarTy tv1) ps_ty2
1424 Note [Deferred Unification]
1425 ~~~~~~~~~~~~~~~~~~~~
1426 We may encounter a unification ty1 = ty2 that cannot be performed syntactically,
1427 and yet its consistency is undetermined. Previously, there was no way to still
1428 make it consistent. So a mismatch error was issued.
1430 Now these unfications are deferred until constraint simplification, where type
1431 family instances and given equations may (or may not) establish the consistency.
1432 Deferred unifications are of the form
1435 where F is a type function and x is a type variable.
1437 id :: x ~ y => x -> y
1440 involves the unfication x = y. It is deferred until we bring into account the
1441 context x ~ y to establish that it holds.
1443 If available, we defer original types (rather than those where closed type
1444 synonyms have already been expanded via tcCoreView). This is, as usual, to
1445 improve error messages.
1447 We need to both 'unBox' and zonk deferred types. We need to unBox as
1448 functions, such as TcExpr.tcMonoExpr promise to fill boxes in the expected
1449 type. We need to zonk as the types go into the kind of the coercion variable
1450 `cotv' and those are not zonked in Inst.zonkInst. (Maybe it would be better
1451 to zonk in zonInst instead. Would that be sufficient?)
1454 defer_unification :: Bool -- pop innermost context?
1459 defer_unification outer True ty1 ty2
1460 = defer_unification outer False ty2 ty1
1461 defer_unification outer False ty1 ty2
1462 = do { ty1' <- unBox ty1 >>= zonkTcType -- unbox *and* zonk..
1463 ; ty2' <- unBox ty2 >>= zonkTcType -- ..see preceding note
1464 ; traceTc $ text "deferring:" <+> ppr ty1 <+> text "~" <+> ppr ty2
1465 ; cotv <- newMetaCoVar ty1' ty2'
1466 -- put ty1 ~ ty2 in LIE
1467 -- Left means "wanted"
1468 ; inst <- (if outer then popErrCtxt else id) $
1469 mkEqInst (EqPred ty1' ty2') (Left cotv)
1471 ; return $ ACo $ TyVarTy cotv }
1474 uMetaVar :: Bool -- pop innermost context?
1476 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1479 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1480 -- ty2 is not a type variable
1482 uMetaVar outer swapped tv1 BoxTv ref1 ps_ty2 non_var_ty2
1483 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1484 -- that any boxes in ty2 are filled with monotypes
1486 -- It should not be the case that tv1 occurs in ty2
1487 -- (i.e. no occurs check should be needed), but if perchance
1488 -- it does, the unbox operation will fill it, and the DEBUG
1490 do { final_ty <- unBox ps_ty2
1492 ; meta_details <- readMutVar ref1
1493 ; case meta_details of
1494 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1495 return () -- This really should *not* happen
1498 ; checkUpdateMeta swapped tv1 ref1 final_ty
1502 uMetaVar outer swapped tv1 info1 ref1 ps_ty2 non_var_ty2
1503 = do { -- Occurs check + monotype check
1504 ; mb_final_ty <- checkTauTvUpdate tv1 ps_ty2
1505 ; case mb_final_ty of
1506 Nothing -> -- tv1 occured in type family parameter
1507 defer_unification outer swapped (mkTyVarTy tv1) ps_ty2
1509 do { checkUpdateMeta swapped tv1 ref1 final_ty
1515 uUnfilledVars :: Outer
1517 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1518 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1520 -- Invarant: The type variables are distinct,
1521 -- Neither is filled in yet
1522 -- They might be boxy or not
1524 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1525 = -- see [Deferred Unification]
1526 defer_unification outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1528 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1529 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2) >> return IdCo
1530 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1531 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1) >> return IdCo
1533 -- ToDo: this function seems too long for what it acutally does!
1534 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1535 = case (info1, info2) of
1536 (BoxTv, BoxTv) -> box_meets_box >> return IdCo
1538 -- If a box meets a TauTv, but the fomer has the smaller kind
1539 -- then we must create a fresh TauTv with the smaller kind
1540 (_, BoxTv) | k1_sub_k2 -> update_tv2 >> return IdCo
1541 | otherwise -> box_meets_box >> return IdCo
1542 (BoxTv, _ ) | k2_sub_k1 -> update_tv1 >> return IdCo
1543 | otherwise -> box_meets_box >> return IdCo
1545 -- Avoid SigTvs if poss
1546 (SigTv _, _ ) | k1_sub_k2 -> update_tv2 >> return IdCo
1547 (_, SigTv _) | k2_sub_k1 -> update_tv1 >> return IdCo
1549 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1550 then update_tv1 >> return IdCo -- Same kinds
1551 else update_tv2 >> return IdCo
1552 | k2_sub_k1 -> update_tv1 >> return IdCo
1553 | otherwise -> kind_err >> return IdCo
1555 -- Update the variable with least kind info
1556 -- See notes on type inference in Kind.lhs
1557 -- The "nicer to" part only applies if the two kinds are the same,
1558 -- so we can choose which to do.
1560 -- Kinds should be guaranteed ok at this point
1561 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1562 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1564 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1567 | k2_sub_k1 = fill_from tv2
1568 | otherwise = kind_err
1570 -- Update *both* tyvars with a TauTv whose name and kind
1571 -- are gotten from tv (avoid losing nice names is poss)
1572 fill_from tv = do { tv' <- tcInstTyVar tv
1573 ; let tau_ty = mkTyVarTy tv'
1574 ; updateMeta tv1 ref1 tau_ty
1575 ; updateMeta tv2 ref2 tau_ty }
1577 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1578 unifyKindMisMatch k1 k2
1582 k1_sub_k2 = k1 `isSubKind` k2
1583 k2_sub_k1 = k2 `isSubKind` k1
1585 nicer_to_update_tv1 = isSystemName (Var.varName tv1)
1586 -- Try to update sys-y type variables in preference to ones
1587 -- gotten (say) by instantiating a polymorphic function with
1588 -- a user-written type sig
1592 refineBox :: TcType -> TcM TcType
1593 -- Unbox the outer box of a boxy type (if any)
1594 refineBox ty@(TyVarTy box_tv)
1595 | isMetaTyVar box_tv
1596 = do { cts <- readMetaTyVar box_tv
1599 Indirect ty -> return ty }
1600 refineBox other_ty = return other_ty
1602 refineBoxToTau :: TcType -> TcM TcType
1603 -- Unbox the outer box of a boxy type, filling with a monotype if it is empty
1604 -- Like refineBox except for the "fill with monotype" part.
1605 refineBoxToTau ty@(TyVarTy box_tv)
1606 | isMetaTyVar box_tv
1607 , MetaTv BoxTv ref <- tcTyVarDetails box_tv
1608 = do { cts <- readMutVar ref
1610 Flexi -> fillBoxWithTau box_tv ref
1611 Indirect ty -> return ty }
1612 refineBoxToTau other_ty = return other_ty
1614 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1615 -- Subtle... we must zap the boxy res_ty
1616 -- to kind * before using it to instantiate a LitInst
1617 -- Calling unBox instead doesn't do the job, because the box
1618 -- often has an openTypeKind, and we don't want to instantiate
1620 zapToMonotype res_ty
1621 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1622 ; boxyUnify res_tau res_ty
1625 unBox :: BoxyType -> TcM TcType
1626 -- unBox implements the judgement
1628 -- with input s', and result s
1630 -- It removes all boxes from the input type, returning a non-boxy type.
1631 -- A filled box in the type can only contain a monotype; unBox fails if not
1632 -- The type can have empty boxes, which unBox fills with a monotype
1634 -- Compare this wth checkTauTvUpdate
1636 -- For once, it's safe to treat synonyms as opaque!
1638 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1639 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1640 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1641 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1642 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1643 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1644 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1646 | isTcTyVar tv -- It's a boxy type variable
1647 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1648 = do { cts <- readMutVar ref -- under nested quantifiers
1650 Flexi -> fillBoxWithTau tv ref
1651 Indirect ty -> do { non_boxy_ty <- unBox ty
1652 ; if isTauTy non_boxy_ty
1653 then return non_boxy_ty
1654 else notMonoType non_boxy_ty }
1656 | otherwise -- Skolems, and meta-tau-variables
1657 = return (TyVarTy tv)
1659 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1660 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1661 unBoxPred (EqPred ty1 ty2) = do { ty1' <- unBox ty1; ty2' <- unBox ty2; return (EqPred ty1' ty2') }
1666 %************************************************************************
1668 \subsection[Unify-context]{Errors and contexts}
1670 %************************************************************************
1676 unifyCtxt act_ty exp_ty tidy_env
1677 = do { act_ty' <- zonkTcType act_ty
1678 ; exp_ty' <- zonkTcType exp_ty
1679 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1680 (env2, act_ty'') = tidyOpenType env1 act_ty'
1681 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1684 mkExpectedActualMsg act_ty exp_ty
1685 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1686 text "Inferred type" <> colon <+> ppr act_ty ])
1689 -- If an error happens we try to figure out whether the function
1690 -- function has been given too many or too few arguments, and say so.
1691 addSubCtxt orig actual_res_ty expected_res_ty thing_inside
1692 = addErrCtxtM mk_err thing_inside
1695 = do { exp_ty' <- zonkTcType expected_res_ty
1696 ; act_ty' <- zonkTcType actual_res_ty
1697 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1698 (env2, act_ty'') = tidyOpenType env1 act_ty'
1699 (exp_args, _) = tcSplitFunTys exp_ty''
1700 (act_args, _) = tcSplitFunTys act_ty''
1702 len_act_args = length act_args
1703 len_exp_args = length exp_args
1705 message = case orig of
1707 | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1708 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1709 other -> mkExpectedActualMsg act_ty'' exp_ty''
1710 ; return (env2, message) }
1712 wrongArgsCtxt too_many_or_few fun
1713 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1714 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1715 <+> ptext SLIT("arguments")
1718 unifyForAllCtxt tvs phi1 phi2 env
1719 = returnM (env2, msg)
1721 (env', tvs') = tidyOpenTyVars env tvs -- NB: not tidyTyVarBndrs
1722 (env1, phi1') = tidyOpenType env' phi1
1723 (env2, phi2') = tidyOpenType env1 phi2
1724 msg = vcat [ptext SLIT("When matching") <+> quotes (ppr (mkForAllTys tvs' phi1')),
1725 ptext SLIT(" and") <+> quotes (ppr (mkForAllTys tvs' phi2'))]
1727 -----------------------
1728 unifyMisMatch outer swapped ty1 ty2
1729 | swapped = unifyMisMatch outer False ty2 ty1
1730 | outer = popErrCtxt $ unifyMisMatch False swapped ty1 ty2 -- This is the whole point of the 'outer' stuff
1731 | otherwise = failWithMisMatch ty1 ty2
1735 %************************************************************************
1739 %************************************************************************
1741 Unifying kinds is much, much simpler than unifying types.
1744 unifyKind :: TcKind -- Expected
1747 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1748 | isSubKindCon kc2 kc1 = returnM ()
1750 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1751 = do { unifyKind a2 a1; unifyKind r1 r2 }
1752 -- Notice the flip in the argument,
1753 -- so that the sub-kinding works right
1754 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1755 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1756 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1758 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1759 unifyKinds [] [] = returnM ()
1760 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1762 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1765 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1766 uKVar swapped kv1 k2
1767 = do { mb_k1 <- readKindVar kv1
1769 Flexi -> uUnboundKVar swapped kv1 k2
1770 Indirect k1 | swapped -> unifyKind k2 k1
1771 | otherwise -> unifyKind k1 k2 }
1774 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1775 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1776 | kv1 == kv2 = returnM ()
1777 | otherwise -- Distinct kind variables
1778 = do { mb_k2 <- readKindVar kv2
1780 Indirect k2 -> uUnboundKVar swapped kv1 k2
1781 Flexi -> writeKindVar kv1 k2 }
1783 uUnboundKVar swapped kv1 non_var_k2
1784 = do { k2' <- zonkTcKind non_var_k2
1785 ; kindOccurCheck kv1 k2'
1786 ; k2'' <- kindSimpleKind swapped k2'
1787 -- KindVars must be bound only to simple kinds
1788 -- Polarities: (kindSimpleKind True ?) succeeds
1789 -- returning *, corresponding to unifying
1792 ; writeKindVar kv1 k2'' }
1795 kindOccurCheck kv1 k2 -- k2 is zonked
1796 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1798 not_in (TyVarTy kv2) = kv1 /= kv2
1799 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1802 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1803 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1804 -- If the flag is False, it requires k <: sk
1805 -- E.g. kindSimpleKind False ?? = *
1806 -- What about (kv -> *) :=: ?? -> *
1807 kindSimpleKind orig_swapped orig_kind
1808 = go orig_swapped orig_kind
1810 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1812 ; return (mkArrowKind k1' k2') }
1814 | isOpenTypeKind k = return liftedTypeKind
1815 | isArgTypeKind k = return liftedTypeKind
1817 | isLiftedTypeKind k = return liftedTypeKind
1818 | isUnliftedTypeKind k = return unliftedTypeKind
1819 go sw k@(TyVarTy _) = return k -- KindVars are always simple
1820 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1821 <+> ppr orig_swapped <+> ppr orig_kind)
1822 -- I think this can't actually happen
1824 -- T v = MkT v v must be a type
1825 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1828 kindOccurCheckErr tyvar ty
1829 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1830 2 (sep [ppr tyvar, char '=', ppr ty])
1834 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1835 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1837 unifyFunKind (TyVarTy kvar)
1838 = readKindVar kvar `thenM` \ maybe_kind ->
1840 Indirect fun_kind -> unifyFunKind fun_kind
1842 do { arg_kind <- newKindVar
1843 ; res_kind <- newKindVar
1844 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1845 ; returnM (Just (arg_kind,res_kind)) }
1847 unifyFunKind (FunTy arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1848 unifyFunKind other = returnM Nothing
1851 %************************************************************************
1855 %************************************************************************
1857 ---------------------------
1858 -- We would like to get a decent error message from
1859 -- (a) Under-applied type constructors
1860 -- f :: (Maybe, Maybe)
1861 -- (b) Over-applied type constructors
1862 -- f :: Int x -> Int x
1866 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1867 -- A fancy wrapper for 'unifyKind', which tries
1868 -- to give decent error messages.
1869 -- (checkExpectedKind ty act_kind exp_kind)
1870 -- checks that the actual kind act_kind is compatible
1871 -- with the expected kind exp_kind
1872 -- The first argument, ty, is used only in the error message generation
1873 checkExpectedKind ty act_kind exp_kind
1874 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1877 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1879 Just r -> returnM () ; -- Unification succeeded
1882 -- So there's definitely an error
1883 -- Now to find out what sort
1884 zonkTcKind exp_kind `thenM` \ exp_kind ->
1885 zonkTcKind act_kind `thenM` \ act_kind ->
1887 tcInitTidyEnv `thenM` \ env0 ->
1888 let (exp_as, _) = splitKindFunTys exp_kind
1889 (act_as, _) = splitKindFunTys act_kind
1890 n_exp_as = length exp_as
1891 n_act_as = length act_as
1893 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1894 (env2, tidy_act_kind) = tidyKind env1 act_kind
1896 err | n_exp_as < n_act_as -- E.g. [Maybe]
1897 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1899 -- Now n_exp_as >= n_act_as. In the next two cases,
1900 -- n_exp_as == 0, and hence so is n_act_as
1901 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1902 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1903 <+> ptext SLIT("is unlifted")
1905 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1906 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1907 <+> ptext SLIT("is lifted")
1909 | otherwise -- E.g. Monad [Int]
1910 = ptext SLIT("Kind mis-match")
1912 more_info = sep [ ptext SLIT("Expected kind") <+>
1913 quotes (pprKind tidy_exp_kind) <> comma,
1914 ptext SLIT("but") <+> quotes (ppr ty) <+>
1915 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1917 failWithTcM (env2, err $$ more_info)
1921 %************************************************************************
1923 \subsection{Checking signature type variables}
1925 %************************************************************************
1927 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1928 are not mentioned in the environment. In particular:
1930 (a) Not mentioned in the type of a variable in the envt
1931 eg the signature for f in this:
1937 Here, f is forced to be monorphic by the free occurence of x.
1939 (d) Not (unified with another type variable that is) in scope.
1940 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1941 when checking the expression type signature, we find that
1942 even though there is nothing in scope whose type mentions r,
1943 nevertheless the type signature for the expression isn't right.
1945 Another example is in a class or instance declaration:
1947 op :: forall b. a -> b
1949 Here, b gets unified with a
1951 Before doing this, the substitution is applied to the signature type variable.
1954 checkSigTyVars :: [TcTyVar] -> TcM ()
1955 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1957 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1958 -- The extra_tvs can include boxy type variables;
1959 -- e.g. TcMatches.tcCheckExistentialPat
1960 checkSigTyVarsWrt extra_tvs sig_tvs
1961 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1962 ; check_sig_tyvars extra_tvs' sig_tvs }
1965 :: TcTyVarSet -- Global type variables. The universally quantified
1966 -- tyvars should not mention any of these
1967 -- Guaranteed already zonked.
1968 -> [TcTyVar] -- Universally-quantified type variables in the signature
1969 -- Guaranteed to be skolems
1971 check_sig_tyvars extra_tvs []
1973 check_sig_tyvars extra_tvs sig_tvs
1974 = ASSERT( all isSkolemTyVar sig_tvs )
1975 do { gbl_tvs <- tcGetGlobalTyVars
1976 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1977 text "gbl_tvs" <+> ppr gbl_tvs,
1978 text "extra_tvs" <+> ppr extra_tvs]))
1980 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1981 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1982 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1985 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1986 -> [TcTyVar] -- The possibly-escaping type variables
1987 -> [TcTyVar] -- The zonked versions thereof
1989 -- Complain about escaping type variables
1990 -- We pass a list of type variables, at least one of which
1991 -- escapes. The first list contains the original signature type variable,
1992 -- while the second contains the type variable it is unified to (usually itself)
1993 bleatEscapedTvs globals sig_tvs zonked_tvs
1994 = do { env0 <- tcInitTidyEnv
1995 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1996 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1998 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1999 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
2001 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
2003 check (tidy_env, msgs) (sig_tv, zonked_tv)
2004 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
2006 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
2007 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
2009 -----------------------
2010 escape_msg sig_tv zonked_tv globs
2012 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
2013 nest 2 (vcat globs)]
2015 = msg <+> ptext SLIT("escapes")
2016 -- Sigh. It's really hard to give a good error message
2017 -- all the time. One bad case is an existential pattern match.
2018 -- We rely on the "When..." context to help.
2020 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
2022 | sig_tv == zonked_tv = empty
2023 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
2026 These two context are used with checkSigTyVars
2029 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
2030 -> TidyEnv -> TcM (TidyEnv, Message)
2031 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
2032 = zonkTcType sig_tau `thenM` \ actual_tau ->
2034 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
2035 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
2036 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
2037 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
2038 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
2040 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),