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
68 %************************************************************************
70 \subsection{'hole' type variables}
72 %************************************************************************
75 tcInfer :: (BoxyType -> TcM a) -> TcM (a, TcType)
76 tcInfer tc_infer = withBox openTypeKind tc_infer
80 %************************************************************************
84 %************************************************************************
87 subFunTys :: SDoc -- Somthing like "The function f has 3 arguments"
88 -- or "The abstraction (\x.e) takes 1 argument"
89 -> Arity -- Expected # of args
90 -> BoxyRhoType -- res_ty
91 -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
93 -- Attempt to decompse res_ty to have enough top-level arrows to
94 -- match the number of patterns in the match group
96 -- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
97 -- and the inner call to thing_inside passes args: [a1,...,an], b
98 -- then co_fn :: (a1 -> ... -> an -> b) ~ res_ty
100 -- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
103 {- Error messages from subFunTys
105 The abstraction `\Just 1 -> ...' has two arguments
106 but its type `Maybe a -> a' has only one
108 The equation(s) for `f' have two arguments
109 but its type `Maybe a -> a' has only one
111 The section `(f 3)' requires 'f' to take two arguments
112 but its type `Int -> Int' has only one
114 The function 'f' is applied to two arguments
115 but its type `Int -> Int' has only one
119 subFunTys error_herald n_pats res_ty thing_inside
120 = loop n_pats [] res_ty
122 -- In 'loop', the parameter 'arg_tys' accumulates
123 -- the arg types so far, in *reverse order*
124 -- INVARIANT: res_ty :: *
125 loop n args_so_far res_ty
126 | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
128 loop n args_so_far res_ty
129 | isSigmaTy res_ty -- Do this before checking n==0, because we
130 -- guarantee to return a BoxyRhoType, not a
132 = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet $ \ _ res_ty' ->
133 loop n args_so_far res_ty'
134 ; return (gen_fn <.> co_fn, res) }
136 loop 0 args_so_far res_ty
137 = do { res <- thing_inside (reverse args_so_far) res_ty
138 ; return (idHsWrapper, res) }
140 loop n args_so_far (FunTy arg_ty res_ty)
141 = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
142 ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
143 ; return (co_fn', res) }
145 -- Try to normalise synonym families and defer if that's not possible
146 loop n args_so_far ty@(TyConApp tc tys)
148 = do { (coi1, ty') <- tcNormaliseFamInst ty
150 IdCo -> defer n args_so_far ty
151 -- no progress, but maybe solvable => defer
152 ACo _ -> -- progress: so lets try again
153 do { (co_fn, res) <- loop n args_so_far ty'
154 ; return $ (co_fn <.> coiToHsWrapper (mkSymCoI coi1), res)
158 -- res_ty might have a type variable at the head, such as (a b c),
159 -- in which case we must fill in with (->). Simplest thing to do
160 -- is to use boxyUnify, but we catch failure and generate our own
161 -- error message on failure
162 loop n args_so_far res_ty@(AppTy _ _)
163 = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
164 ; (_, mb_coi) <- tryTcErrs $
165 boxyUnify res_ty (FunTy arg_ty' res_ty')
166 ; if isNothing mb_coi then bale_out args_so_far
167 else do { let coi = expectJust "subFunTys" mb_coi
168 ; (co_fn, res) <- loop n args_so_far (FunTy arg_ty'
170 ; return (co_fn <.> coiToHsWrapper coi, res)
174 loop n args_so_far ty@(TyVarTy tv)
175 | isTyConableTyVar tv
176 = do { cts <- readMetaTyVar tv
178 Indirect ty -> loop n args_so_far ty
180 do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
181 ; res <- thing_inside (reverse args_so_far ++ arg_tys)
183 ; return (idHsWrapper, res) } }
184 | otherwise -- defer as tyvar may be refined by equalities
185 = defer n args_so_far ty
187 mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
188 mk_res_ty [] = panic "TcUnify.mk_res_ty1"
189 kinds = openTypeKind : take n (repeat argTypeKind)
190 -- Note argTypeKind: the args can have an unboxed type,
191 -- but not an unboxed tuple.
193 loop n args_so_far res_ty = bale_out args_so_far
195 -- build a template type a1 -> ... -> an -> b and defer an equality
196 -- between that template and the expected result type res_ty; then,
197 -- use the template to type the thing_inside
198 defer n args_so_far ty
199 = do { arg_tys <- newFlexiTyVarTys n argTypeKind
200 ; res_ty' <- newFlexiTyVarTy openTypeKind
201 ; let fun_ty = mkFunTys arg_tys res_ty'
202 err = error_herald <> comma $$
203 text "which does not match its type"
204 ; coi <- addErrCtxt err $
205 defer_unification False False fun_ty ty
206 ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty'
207 ; return (coiToHsWrapper coi, res)
211 = do { env0 <- tcInitTidyEnv
212 ; res_ty' <- zonkTcType res_ty
213 ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
214 ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
216 mk_msg res_ty n_actual
217 = error_herald <> comma $$
218 sep [ptext SLIT("but its type") <+> quotes (pprType res_ty),
219 if n_actual == 0 then ptext SLIT("has none")
220 else ptext SLIT("has only") <+> speakN n_actual]
224 ----------------------
225 boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
226 -> BoxyRhoType -- Expected type (T a b c)
227 -> TcM ([BoxySigmaType], -- Element types, a b c
228 CoercionI) -- T a b c ~ orig_ty
229 -- It's used for wired-in tycons, so we call checkWiredInTyCon
230 -- Precondition: never called with FunTyCon
231 -- Precondition: input type :: *
233 boxySplitTyConApp tc orig_ty
234 = do { checkWiredInTyCon tc
235 ; loop (tyConArity tc) [] orig_ty }
237 loop n_req args_so_far ty
238 | Just ty' <- tcView ty = loop n_req args_so_far ty'
240 loop n_req args_so_far ty@(TyConApp tycon args)
242 = ASSERT( n_req == length args) -- ty::*
243 return (args ++ args_so_far, IdCo)
245 | isOpenSynTyCon tycon -- try to normalise type family application
246 = do { (coi1, ty') <- tcNormaliseFamInst ty
247 ; traceTc $ text "boxySplitTyConApp:" <+>
248 ppr ty <+> text "==>" <+> ppr ty'
250 IdCo -> defer -- no progress, but maybe solvable => defer
251 ACo _ -> -- progress: so lets try again
252 do { (args, coi2) <- loop n_req args_so_far ty'
253 ; return $ (args, coi2 `mkTransCoI` mkSymCoI coi1)
257 loop n_req args_so_far (AppTy fun arg)
259 = do { (args, coi) <- loop (n_req - 1) (arg:args_so_far) fun
260 ; return (args, mkAppTyCoI fun coi arg IdCo)
263 loop n_req args_so_far (TyVarTy tv)
264 | isTyConableTyVar tv
265 , res_kind `isSubKind` tyVarKind tv
266 = do { cts <- readMetaTyVar tv
268 Indirect ty -> loop n_req args_so_far ty
269 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
270 ; return (arg_tys ++ args_so_far, IdCo) }
272 | otherwise -- defer as tyvar may be refined by equalities
275 (arg_kinds, res_kind) = splitKindFunTysN n_req (tyConKind tc)
277 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc)))
280 -- defer splitting by generating an equality constraint
281 defer = boxySplitDefer arg_kinds mk_res_ty orig_ty
283 (arg_kinds, _) = splitKindFunTys (tyConKind tc)
285 -- apply splitted tycon to arguments
286 mk_res_ty = mkTyConApp tc
288 ----------------------
289 boxySplitListTy :: BoxyRhoType -> TcM (BoxySigmaType, CoercionI)
290 -- Special case for lists
291 boxySplitListTy exp_ty
292 = do { ([elt_ty], coi) <- boxySplitTyConApp listTyCon exp_ty
293 ; return (elt_ty, coi) }
295 ----------------------
296 boxySplitPArrTy :: BoxyRhoType -> TcM (BoxySigmaType, CoercionI)
297 -- Special case for parrs
298 boxySplitPArrTy exp_ty
299 = do { ([elt_ty], coi) <- boxySplitTyConApp parrTyCon exp_ty
300 ; return (elt_ty, coi) }
302 ----------------------
303 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
304 -> TcM ((BoxySigmaType, BoxySigmaType), -- Returns m, a
306 -- If the incoming type is a mutable type variable of kind k, then
307 -- boxySplitAppTy returns a new type variable (m: * -> k); note the *.
308 -- If the incoming type is boxy, then so are the result types; and vice versa
310 boxySplitAppTy orig_ty
314 | Just ty' <- tcView ty = loop ty'
317 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
318 = return ((fun_ty, arg_ty), IdCo)
320 loop ty@(TyConApp tycon _args)
321 | isOpenSynTyCon tycon -- try to normalise type family application
322 = do { (coi1, ty') <- tcNormaliseFamInst ty
324 IdCo -> defer -- no progress, but maybe solvable => defer
325 ACo co -> -- progress: so lets try again
326 do { (args, coi2) <- loop ty'
327 ; return $ (args, coi2 `mkTransCoI` mkSymCoI coi1)
332 | isTyConableTyVar tv
333 = do { cts <- readMetaTyVar tv
335 Indirect ty -> loop ty
336 Flexi -> do { [fun_ty, arg_ty] <- withMetaTvs tv kinds mk_res_ty
337 ; return ((fun_ty, arg_ty), IdCo) } }
338 | otherwise -- defer as tyvar may be refined by equalities
341 tv_kind = tyVarKind tv
342 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
344 liftedTypeKind] -- arg type :: *
345 -- The defaultKind is a bit smelly. If you remove it,
346 -- try compiling f x = do { x }
347 -- and you'll get a kind mis-match. It smells, but
348 -- not enough to lose sleep over.
350 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
352 -- defer splitting by generating an equality constraint
353 defer = do { ([ty1, ty2], coi) <- boxySplitDefer arg_kinds mk_res_ty orig_ty
354 ; return ((ty1, ty2), coi)
357 orig_kind = typeKind orig_ty
358 arg_kinds = [mkArrowKind liftedTypeKind (defaultKind orig_kind),
360 liftedTypeKind] -- arg type :: *
362 -- build type application
363 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
364 mk_res_ty _other = panic "TcUnify.mk_res_ty2"
367 boxySplitFailure actual_ty expected_ty
368 = unifyMisMatch False False actual_ty expected_ty
369 -- "outer" is False, so we don't pop the context
370 -- which is what we want since we have not pushed one!
373 boxySplitDefer :: [Kind] -- kinds of required arguments
374 -> ([TcType] -> TcTauType) -- construct lhs from argument tyvars
375 -> BoxyRhoType -- type to split
376 -> TcM ([TcType], CoercionI)
377 boxySplitDefer kinds mkTy orig_ty
378 = do { tau_tys <- mapM newFlexiTyVarTy kinds
379 ; coi <- defer_unification False False (mkTy tau_tys) orig_ty
380 ; return (tau_tys, coi)
385 --------------------------------
386 -- withBoxes: the key utility function
387 --------------------------------
390 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
391 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
392 -> ([BoxySigmaType] -> BoxySigmaType)
393 -- Constructs the type to assign
394 -- to the original var
395 -> TcM [BoxySigmaType] -- Return the fresh boxes
397 -- It's entirely possible for the [kind] to be empty.
398 -- For example, when pattern-matching on True,
399 -- we call boxySplitTyConApp passing a boolTyCon
401 -- Invariant: tv is still Flexi
403 withMetaTvs tv kinds mk_res_ty
405 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
406 ; let box_tys = mkTyVarTys box_tvs
407 ; writeMetaTyVar tv (mk_res_ty box_tys)
410 | otherwise -- Non-boxy meta type variable
411 = do { tau_tys <- mapM newFlexiTyVarTy kinds
412 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
413 -- Sure to be a tau-type
416 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
417 -- Allocate a *boxy* tyvar
418 withBox kind thing_inside
419 = do { box_tv <- newBoxyTyVar kind
420 ; res <- thing_inside (mkTyVarTy box_tv)
421 ; ty <- {- pprTrace "with_box" (ppr (mkTyVarTy box_tv)) $ -} readFilledBox box_tv
426 %************************************************************************
428 Approximate boxy matching
430 %************************************************************************
433 preSubType :: [TcTyVar] -- Quantified type variables
434 -> TcTyVarSet -- Subset of quantified type variables
435 -- see Note [Pre-sub boxy]
436 -> TcType -- The rho-type part; quantified tyvars scopes over this
437 -> BoxySigmaType -- Matching type from the context
438 -> TcM [TcType] -- Types to instantiate the tyvars
439 -- Perform pre-subsumption, and return suitable types
440 -- to instantiate the quantified type varibles:
441 -- info from the pre-subsumption, if there is any
442 -- a boxy type variable otherwise
444 -- Note [Pre-sub boxy]
445 -- The 'btvs' are a subset of 'qtvs'. They are the ones we can
446 -- instantiate to a boxy type variable, because they'll definitely be
447 -- filled in later. This isn't always the case; sometimes we have type
448 -- variables mentioned in the context of the type, but not the body;
449 -- f :: forall a b. C a b => a -> a
450 -- Then we may land up with an unconstrained 'b', so we want to
451 -- instantiate it to a monotype (non-boxy) type variable
453 -- The 'qtvs' that are *neither* fixed by the pre-subsumption, *nor* are in 'btvs',
454 -- are instantiated to TauTv meta variables.
456 preSubType qtvs btvs qty expected_ty
457 = do { tys <- mapM inst_tv qtvs
458 ; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
461 pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
463 | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
464 | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
465 ; return (mkTyVarTy tv') }
466 | otherwise = do { tv' <- tcInstTyVar tv
467 ; return (mkTyVarTy tv') }
470 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
471 -> BoxyRhoType -- Type to match (note a *Rho* type)
472 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
474 -- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
475 -- "Boxy types: inference for higher rank types and impredicativity"
477 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
478 = go tmpl_tvs tmpl_ty emptyVarSet boxy_ty
480 go t_tvs t_ty b_tvs b_ty
481 | Just t_ty' <- tcView t_ty = go t_tvs t_ty' b_tvs b_ty
482 | Just b_ty' <- tcView b_ty = go t_tvs t_ty b_tvs b_ty'
484 go t_tvs (TyVarTy _) b_tvs b_ty = emptyTvSubst -- Rule S-ANY; no bindings
485 -- Rule S-ANY covers (a) type variables and (b) boxy types
486 -- in the template. Both look like a TyVarTy.
487 -- See Note [Sub-match] below
489 go t_tvs t_ty b_tvs b_ty
490 | isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
491 = go (t_tvs `delVarSetList` tvs) t_tau b_tvs b_ty -- Rule S-SPEC
492 -- Under a forall on the left, if there is shadowing,
493 -- do not bind! Hence the delVarSetList.
494 | isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
495 = go t_tvs t_ty (extendVarSetList b_tvs tvs) b_tau -- Rule S-SKOL
496 -- Add to the variables we must not bind to
497 -- NB: it's *important* to discard the theta part. Otherwise
498 -- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
499 -- and end up with a completely bogus binding (b |-> Bool), by lining
500 -- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
501 -- This pre-subsumption stuff can return too few bindings, but it
502 -- must *never* return bogus info.
504 go t_tvs (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
505 = boxy_match t_tvs arg1 b_tvs arg2 (go t_tvs res1 b_tvs res2)
506 -- Match the args, and sub-match the results
508 go t_tvs t_ty b_tvs b_ty = boxy_match t_tvs t_ty b_tvs b_ty emptyTvSubst
509 -- Otherwise defer to boxy matching
510 -- This covers TyConApp, AppTy, PredTy
517 |- head xs : <rhobox>
518 We will do a boxySubMatchType between a ~ <rhobox>
519 But we *don't* want to match [a |-> <rhobox>] because
520 (a) The box should be filled in with a rho-type, but
521 but the returned substitution maps TyVars to boxy
523 (b) In any case, the right final answer might be *either*
524 instantiate 'a' with a rho-type or a sigma type
525 head xs : Int vs head xs : forall b. b->b
526 So the matcher MUST NOT make a choice here. In general, we only
527 bind a template type variable in boxyMatchType, not in boxySubMatchType.
532 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
533 -> [BoxySigmaType] -- Type to match
534 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
536 -- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
537 -- "Boxy types: inference for higher rank types and impredicativity"
539 -- Find a *boxy* substitution that makes the template look as much
540 -- like the BoxySigmaType as possible.
541 -- It's always ok to return an empty substitution;
542 -- anything more is jam on the pudding
544 -- NB1: This is a pure, non-monadic function.
545 -- It does no unification, and cannot fail
547 -- Precondition: the arg lengths are equal
548 -- Precondition: none of the template type variables appear anywhere in the [BoxySigmaType]
552 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
553 = ASSERT( length tmpl_tys == length boxy_tys )
554 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
555 -- ToDo: add error context?
557 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
559 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
560 = boxy_match tmpl_tvs t_ty boxy_tvs b_ty $
561 boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys subst
562 boxy_match_s tmpl_tvs _ boxy_tvs _ subst
563 = panic "boxy_match_s" -- Lengths do not match
567 boxy_match :: TcTyVarSet -> TcType -- Template
568 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
569 -> BoxySigmaType -- Match against this type
573 -- The boxy_tvs argument prevents this match:
574 -- [a] forall b. a ~ forall b. b
575 -- We don't want to bind the template variable 'a'
576 -- to the quantified type variable 'b'!
578 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
579 = go orig_tmpl_ty orig_boxy_ty
582 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
583 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
585 go ty1 ty2 -- C.f. the isSigmaTy case for boxySubMatchType
587 , (tvs1, _, tau1) <- tcSplitSigmaTy ty1
588 , (tvs2, _, tau2) <- tcSplitSigmaTy ty2
589 , equalLength tvs1 tvs2
590 = boxy_match (tmpl_tvs `delVarSetList` tvs1) tau1
591 (boxy_tvs `extendVarSetList` tvs2) tau2 subst
593 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
595 , not $ isOpenSynTyCon tc1
598 go (FunTy arg1 res1) (FunTy arg2 res2)
599 = go_s [arg1,res1] [arg2,res2]
602 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
603 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
604 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
605 = go_s [s1,t1] [s2,t2]
608 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
609 , boxy_tvs `disjointVarSet` tyVarsOfType orig_boxy_ty
610 , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
611 = extendTvSubst subst tv boxy_ty'
613 = subst -- Ignore others
615 boxy_ty' = case lookupTyVar subst tv of
616 Nothing -> orig_boxy_ty
617 Just ty -> ty `boxyLub` orig_boxy_ty
619 go _ (TyVarTy tv) | isMetaTyVar tv
620 = subst -- Don't fail if the template has more info than the target!
621 -- Otherwise, with tmpl_tvs = [a], matching (a -> Int) ~ (Bool -> beta)
622 -- would fail to instantiate 'a', because the meta-type-variable
623 -- beta is as yet un-filled-in
625 go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
626 -- Example: Tree a ~ Maybe Int
627 -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
628 -- misleading error messages. An even more confusing case is
629 -- a -> b ~ Maybe Int
630 -- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
631 -- from this pre-matching phase.
634 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
637 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
638 -- Combine boxy information from the two types
639 -- If there is a conflict, return the first
640 boxyLub orig_ty1 orig_ty2
641 = go orig_ty1 orig_ty2
643 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
644 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
645 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
646 | tc1 == tc2, length ts1 == length ts2
647 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
649 go (TyVarTy tv1) ty2 -- This is the whole point;
650 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
653 go ty1 (TyVarTy tv2) -- Symmetrical case
654 | isTcTyVar tv2, isBoxyTyVar tv2
657 -- Look inside type synonyms, but only if the naive version fails
658 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
659 | Just ty2' <- tcView ty1 = go ty1 ty2'
661 -- For now, we don't look inside ForAlls, PredTys
662 go ty1 ty2 = orig_ty1 -- Default
665 Note [Matching kinds]
666 ~~~~~~~~~~~~~~~~~~~~~
667 The target type might legitimately not be a sub-kind of template.
668 For example, suppose the target is simply a box with an OpenTypeKind,
669 and the template is a type variable with LiftedTypeKind.
670 Then it's ok (because the target type will later be refined).
671 We simply don't bind the template type variable.
673 It might also be that the kind mis-match is an error. For example,
674 suppose we match the template (a -> Int) against (Int# -> Int),
675 where the template type variable 'a' has LiftedTypeKind. This
676 matching function does not fail; it simply doesn't bind the template.
677 Later stuff will fail.
679 %************************************************************************
683 %************************************************************************
685 All the tcSub calls have the form
687 tcSub actual_ty expected_ty
689 actual_ty <= expected_ty
691 That is, that a value of type actual_ty is acceptable in
692 a place expecting a value of type expected_ty.
694 It returns a coercion function
695 co_fn :: actual_ty ~ expected_ty
696 which takes an HsExpr of type actual_ty into one of type
701 tcSubExp :: InstOrigin -> BoxySigmaType -> BoxySigmaType -> TcM HsWrapper
702 -- (tcSub act exp) checks that
704 tcSubExp orig actual_ty expected_ty
705 = -- addErrCtxtM (unifyCtxt actual_ty expected_ty) $
706 -- Adding the error context here leads to some very confusing error
707 -- messages, such as "can't match forall a. a->a with forall a. a->a"
708 -- Example is tcfail165:
709 -- do var <- newEmptyMVar :: IO (MVar (forall a. Show a => a -> String))
710 -- putMVar var (show :: forall a. Show a => a -> String)
711 -- Here the info does not flow from the 'var' arg of putMVar to its 'show' arg
712 -- but after zonking it looks as if it does!
714 -- So instead I'm adding the error context when moving from tc_sub to u_tys
716 traceTc (text "tcSubExp" <+> ppr actual_ty <+> ppr expected_ty) >>
717 tc_sub orig actual_ty actual_ty False expected_ty expected_ty
721 -> BoxySigmaType -- actual_ty, before expanding synonyms
722 -> BoxySigmaType -- ..and after
723 -> InBox -- True <=> expected_ty is inside a box
724 -> BoxySigmaType -- expected_ty, before
725 -> BoxySigmaType -- ..and after
727 -- The acual_ty is never inside a box
728 -- IMPORTANT pre-condition: if the args contain foralls, the bound type
729 -- variables are visible non-monadically
730 -- (i.e. tha args are sufficiently zonked)
731 -- This invariant is needed so that we can "see" the foralls, ad
732 -- e.g. in the SPEC rule where we just use splitSigmaTy
734 tc_sub orig act_sty act_ty exp_ib exp_sty exp_ty
735 = traceTc (text "tc_sub" <+> ppr act_ty $$ ppr exp_ty) >>
736 tc_sub1 orig act_sty act_ty exp_ib exp_sty exp_ty
737 -- This indirection is just here to make
738 -- it easy to insert a debug trace!
740 tc_sub1 orig act_sty act_ty exp_ib exp_sty exp_ty
741 | Just exp_ty' <- tcView exp_ty = tc_sub orig act_sty act_ty exp_ib exp_sty exp_ty'
742 tc_sub1 orig act_sty act_ty exp_ib exp_sty exp_ty
743 | Just act_ty' <- tcView act_ty = tc_sub orig act_sty act_ty' exp_ib exp_sty exp_ty
745 -----------------------------------
746 -- Rule SBOXY, plus other cases when act_ty is a type variable
747 -- Just defer to boxy matching
748 -- This rule takes precedence over SKOL!
749 tc_sub1 orig act_sty (TyVarTy tv) exp_ib exp_sty exp_ty
750 = do { traceTc (text "tc_sub1 - case 1")
751 ; coi <- addSubCtxt orig act_sty exp_sty $
752 uVar True False tv exp_ib exp_sty exp_ty
753 ; traceTc (case coi of
754 IdCo -> text "tc_sub1 (Rule SBOXY) IdCo"
755 ACo co -> text "tc_sub1 (Rule SBOXY) ACo" <+> ppr co)
756 ; return $ coiToHsWrapper coi
759 -----------------------------------
760 -- Skolemisation case (rule SKOL)
761 -- actual_ty: d:Eq b => b->b
762 -- expected_ty: forall a. Ord a => a->a
763 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
765 -- It is essential to do this *before* the specialisation case
766 -- Example: f :: (Eq a => a->a) -> ...
767 -- g :: Ord b => b->b
770 tc_sub1 orig act_sty act_ty exp_ib exp_sty exp_ty
771 | isSigmaTy exp_ty = do
772 { traceTc (text "tc_sub1 - case 2") ;
773 if exp_ib then -- SKOL does not apply if exp_ty is inside a box
774 defer_to_boxy_matching orig act_sty act_ty exp_ib exp_sty exp_ty
776 { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ _ body_exp_ty ->
777 tc_sub orig act_sty act_ty False body_exp_ty body_exp_ty
778 ; return (gen_fn <.> co_fn) }
781 act_tvs = tyVarsOfType act_ty
782 -- It's really important to check for escape wrt
783 -- the free vars of both expected_ty *and* actual_ty
785 -----------------------------------
786 -- Specialisation case (rule ASPEC):
787 -- actual_ty: forall a. Ord a => a->a
788 -- expected_ty: Int -> Int
789 -- co_fn e = e Int dOrdInt
791 tc_sub1 orig act_sty actual_ty exp_ib exp_sty expected_ty
792 -- Implements the new SPEC rule in the Appendix of the paper
793 -- "Boxy types: inference for higher rank types and impredicativity"
794 -- (This appendix isn't in the published version.)
795 -- The idea is to *first* do pre-subsumption, and then full subsumption
796 -- Example: forall a. a->a <= Int -> (forall b. Int)
797 -- Pre-subsumpion finds a|->Int, and that works fine, whereas
798 -- just running full subsumption would fail.
799 | isSigmaTy actual_ty
800 = do { traceTc (text "tc_sub1 - case 3")
801 ; -- Perform pre-subsumption, and instantiate
802 -- the type with info from the pre-subsumption;
803 -- boxy tyvars if pre-subsumption gives no info
804 let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
805 tau_tvs = exactTyVarsOfType tau
806 ; inst_tys <- if exp_ib then -- Inside a box, do not do clever stuff
807 do { tyvars' <- mapM tcInstBoxyTyVar tyvars
808 ; return (mkTyVarTys tyvars') }
809 else -- Outside, do clever stuff
810 preSubType tyvars tau_tvs tau expected_ty
811 ; let subst' = zipOpenTvSubst tyvars inst_tys
812 tau' = substTy subst' tau
814 -- Perform a full subsumption check
815 ; traceTc (text "tc_sub_spec" <+> vcat [ppr actual_ty,
816 ppr tyvars <+> ppr theta <+> ppr tau,
818 ; co_fn2 <- tc_sub orig tau' tau' exp_ib exp_sty expected_ty
820 -- Deal with the dictionaries
821 ; co_fn1 <- instCall orig inst_tys (substTheta subst' theta)
822 ; return (co_fn2 <.> co_fn1) }
824 -----------------------------------
825 -- Function case (rule F1)
826 tc_sub1 orig act_sty (FunTy act_arg act_res) exp_ib exp_sty (FunTy exp_arg exp_res)
827 = do { traceTc (text "tc_sub1 - case 4")
828 ; tc_sub_funs orig act_arg act_res exp_ib exp_arg exp_res
831 -- Function case (rule F2)
832 tc_sub1 orig act_sty act_ty@(FunTy act_arg act_res) _ exp_sty (TyVarTy exp_tv)
834 = do { traceTc (text "tc_sub1 - case 5")
835 ; cts <- readMetaTyVar exp_tv
837 Indirect ty -> tc_sub orig act_sty act_ty True exp_sty ty
838 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
839 ; tc_sub_funs orig act_arg act_res True arg_ty res_ty } }
841 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
842 mk_res_ty other = panic "TcUnify.mk_res_ty3"
843 fun_kinds = [argTypeKind, openTypeKind]
845 -- Everything else: defer to boxy matching
846 tc_sub1 orig act_sty actual_ty exp_ib exp_sty expected_ty@(TyVarTy exp_tv)
847 = do { traceTc (text "tc_sub1 - case 6a" <+> ppr [isBoxyTyVar exp_tv, isMetaTyVar exp_tv, isSkolemTyVar exp_tv, isExistentialTyVar exp_tv,isSigTyVar exp_tv] )
848 ; defer_to_boxy_matching orig act_sty actual_ty exp_ib exp_sty expected_ty
851 tc_sub1 orig act_sty actual_ty exp_ib exp_sty expected_ty
852 = do { traceTc (text "tc_sub1 - case 6")
853 ; defer_to_boxy_matching orig act_sty actual_ty exp_ib exp_sty expected_ty
856 -----------------------------------
857 defer_to_boxy_matching orig act_sty actual_ty exp_ib exp_sty expected_ty
858 = do { coi <- addSubCtxt orig act_sty exp_sty $
859 u_tys True False act_sty actual_ty exp_ib exp_sty expected_ty
860 ; return $ coiToHsWrapper coi }
862 -----------------------------------
863 tc_sub_funs orig act_arg act_res exp_ib exp_arg exp_res
864 = do { arg_coi <- addSubCtxt orig act_arg exp_arg $
865 uTysOuter False act_arg exp_ib exp_arg
866 ; co_fn_res <- tc_sub orig act_res act_res exp_ib exp_res exp_res
867 ; wrapper1 <- wrapFunResCoercion [exp_arg] co_fn_res
868 ; let wrapper2 = case arg_coi of
870 ACo co -> WpCo $ FunTy co act_res
871 ; return (wrapper1 <.> wrapper2) }
873 -----------------------------------
875 :: [TcType] -- Type of args
876 -> HsWrapper -- HsExpr a -> HsExpr b
877 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
878 wrapFunResCoercion arg_tys co_fn_res
879 | isIdHsWrapper co_fn_res
884 = do { arg_ids <- newSysLocalIds FSLIT("sub") arg_tys
885 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpApps arg_ids) }
890 %************************************************************************
892 \subsection{Generalisation}
894 %************************************************************************
897 tcGen :: BoxySigmaType -- expected_ty
898 -> TcTyVarSet -- Extra tyvars that the universally
899 -- quantified tyvars of expected_ty
900 -- must not be unified
901 -> ([TcTyVar] -> BoxyRhoType -> TcM result)
902 -> TcM (HsWrapper, result)
903 -- The expression has type: spec_ty -> expected_ty
905 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
906 -- If not, the call is a no-op
907 = do { traceTc (text "tcGen")
908 -- We want the GenSkol info in the skolemised type variables to
909 -- mention the *instantiated* tyvar names, so that we get a
910 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
911 -- Hence the tiresome but innocuous fixM
912 ; ((tvs', theta', rho'), skol_info) <- fixM (\ ~(_, skol_info) ->
913 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
914 -- Get loation from monad, not from expected_ty
915 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty)
916 ; return ((forall_tvs, theta, rho_ty), skol_info) })
919 traceTc (text "tcGen" <+> vcat [
920 text "extra_tvs" <+> ppr extra_tvs,
921 text "expected_ty" <+> ppr expected_ty,
922 text "inst ty" <+> ppr tvs' <+> ppr theta'
924 text "free_tvs" <+> ppr free_tvs])
926 -- Type-check the arg and unify with poly type
927 ; (result, lie) <- getLIE (thing_inside tvs' rho')
929 -- Check that the "forall_tvs" havn't been constrained
930 -- The interesting bit here is that we must include the free variables
931 -- of the expected_ty. Here's an example:
932 -- runST (newVar True)
933 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
934 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
935 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
936 -- So now s' isn't unconstrained because it's linked to a.
937 -- Conclusion: include the free vars of the expected_ty in the
938 -- list of "free vars" for the signature check.
940 ; loc <- getInstLoc (SigOrigin skol_info)
941 ; dicts <- newDictBndrs loc theta' -- Includes equalities
942 ; inst_binds <- tcSimplifyCheck loc tvs' dicts lie
944 ; checkSigTyVarsWrt free_tvs tvs'
945 ; traceTc (text "tcGen:done")
948 -- The WpLet binds any Insts which came out of the simplification.
949 dict_vars = map instToVar dicts
950 co_fn = mkWpTyLams tvs' <.> mkWpLams dict_vars <.> WpLet inst_binds
951 ; return (co_fn, result) }
953 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
958 %************************************************************************
962 %************************************************************************
964 The exported functions are all defined as versions of some
965 non-exported generic functions.
968 boxyUnify :: BoxyType -> BoxyType -> TcM CoercionI
969 -- Acutal and expected, respectively
971 = addErrCtxtM (unifyCtxt ty1 ty2) $
972 uTysOuter False ty1 False ty2
975 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM [CoercionI]
976 -- Arguments should have equal length
977 -- Acutal and expected types
978 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
981 unifyType :: TcTauType -> TcTauType -> TcM CoercionI
982 -- No boxes expected inside these types
983 -- Acutal and expected types
984 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
985 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
986 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
987 addErrCtxtM (unifyCtxt ty1 ty2) $
988 uTysOuter True ty1 True ty2
991 unifyPred :: PredType -> PredType -> TcM CoercionI
992 -- Acutal and expected types
993 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
994 uPred True True p1 True p2
996 unifyTheta :: TcThetaType -> TcThetaType -> TcM [CoercionI]
997 -- Acutal and expected types
998 unifyTheta theta1 theta2
999 = do { checkTc (equalLength theta1 theta2)
1000 (vcat [ptext SLIT("Contexts differ in length"),
1001 nest 2 $ parens $ ptext SLIT("Use -fglasgow-exts to allow this")])
1002 ; uList unifyPred theta1 theta2
1006 uList :: (a -> a -> TcM b)
1007 -> [a] -> [a] -> TcM [b]
1008 -- Unify corresponding elements of two lists of types, which
1009 -- should be of equal length. We charge down the list explicitly so that
1010 -- we can complain if their lengths differ.
1011 uList unify [] [] = return []
1012 uList unify (ty1:tys1) (ty2:tys2) = do { x <- unify ty1 ty2;
1013 ; xs <- uList unify tys1 tys2
1016 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
1019 @unifyTypeList@ takes a single list of @TauType@s and unifies them
1020 all together. It is used, for example, when typechecking explicit
1021 lists, when all the elts should be of the same type.
1024 unifyTypeList :: [TcTauType] -> TcM ()
1025 unifyTypeList [] = return ()
1026 unifyTypeList [ty] = return ()
1027 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
1028 ; unifyTypeList tys }
1031 %************************************************************************
1033 \subsection[Unify-uTys]{@uTys@: getting down to business}
1035 %************************************************************************
1037 @uTys@ is the heart of the unifier. Each arg occurs twice, because
1038 we want to report errors in terms of synomyms if possible. The first of
1039 the pair is used in error messages only; it is always the same as the
1040 second, except that if the first is a synonym then the second may be a
1041 de-synonym'd version. This way we get better error messages.
1043 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
1046 type SwapFlag = Bool
1047 -- False <=> the two args are (actual, expected) respectively
1048 -- True <=> the two args are (expected, actual) respectively
1050 type InBox = Bool -- True <=> we are inside a box
1051 -- False <=> we are outside a box
1052 -- The importance of this is that if we get "filled-box meets
1053 -- filled-box", we'll look into the boxes and unify... but
1054 -- we must not allow polytypes. But if we are in a box on
1055 -- just one side, then we can allow polytypes
1057 type Outer = Bool -- True <=> this is the outer level of a unification
1058 -- so that the types being unified are the
1059 -- very ones we began with, not some sub
1060 -- component or synonym expansion
1061 -- The idea is that if Outer is true then unifyMisMatch should
1062 -- pop the context to remove the "Expected/Acutal" context
1065 :: InBox -> TcType -- ty1 is the *actual* type
1066 -> InBox -> TcType -- ty2 is the *expected* type
1068 uTysOuter nb1 ty1 nb2 ty2
1069 = do { traceTc (text "uTysOuter" <+> ppr ty1 <+> ppr ty2)
1070 ; u_tys True nb1 ty1 ty1 nb2 ty2 ty2 }
1071 uTys nb1 ty1 nb2 ty2
1072 = do { traceTc (text "uTys" <+> ppr ty1 <+> ppr ty2)
1073 ; u_tys False nb1 ty1 ty1 nb2 ty2 ty2 }
1077 uTys_s :: InBox -> [TcType] -- tys1 are the *actual* types
1078 -> InBox -> [TcType] -- tys2 are the *expected* types
1080 uTys_s nb1 [] nb2 [] = return []
1081 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { coi <- uTys nb1 ty1 nb2 ty2
1082 ; cois <- uTys_s nb1 tys1 nb2 tys2
1085 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
1089 -> InBox -> TcType -> TcType -- ty1 is the *actual* type
1090 -> InBox -> TcType -> TcType -- ty2 is the *expected* type
1093 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
1094 = do { traceTc (text "u_tys " <+> ppr ty1 <+> text " " <+> ppr ty2)
1095 ; coi <- go outer ty1 ty2
1096 ; traceTc (case coi of
1097 ACo co -> text "u_tys yields coercion: " <+> ppr co
1098 IdCo -> text "u_tys yields no coercion")
1103 go :: Outer -> TcType -> TcType -> TcM CoercionI
1105 do { traceTc (text "go " <+> ppr orig_ty1 <+> text "/" <+> ppr ty1
1106 <+> ppr orig_ty2 <+> text "/" <+> ppr ty2)
1110 go1 :: Outer -> TcType -> TcType -> TcM CoercionI
1111 -- Always expand synonyms: see Note [Unification and synonyms]
1112 -- (this also throws away FTVs)
1114 | Just ty1' <- tcView ty1 = go False ty1' ty2
1115 | Just ty2' <- tcView ty2 = go False ty1 ty2'
1117 -- Variables; go for uVar
1118 go1 outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
1119 go1 outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
1120 -- "True" means args swapped
1122 -- The case for sigma-types must *follow* the variable cases
1123 -- because a boxy variable can be filed with a polytype;
1124 -- but must precede FunTy, because ((?x::Int) => ty) look
1125 -- like a FunTy; there isn't necy a forall at the top
1127 | isSigmaTy ty1 || isSigmaTy ty2
1128 = do { traceTc (text "We have sigma types: equalLength" <+> ppr tvs1 <+> ppr tvs2)
1129 ; unless (equalLength tvs1 tvs2)
1130 (unifyMisMatch outer False orig_ty1 orig_ty2)
1131 ; traceTc (text "We're past the first length test")
1132 ; tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
1133 -- Get location from monad, not from tvs1
1134 ; let tys = mkTyVarTys tvs
1135 in_scope = mkInScopeSet (mkVarSet tvs)
1136 phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
1137 phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
1138 (theta1,tau1) = tcSplitPhiTy phi1
1139 (theta2,tau2) = tcSplitPhiTy phi2
1141 ; addErrCtxtM (unifyForAllCtxt tvs phi1 phi2) $ do
1142 { unless (equalLength theta1 theta2)
1143 (unifyMisMatch outer False orig_ty1 orig_ty2)
1145 ; cois <- uPreds False nb1 theta1 nb2 theta2 -- TOMDO: do something with these pred_cois
1146 ; traceTc (text "TOMDO!")
1147 ; coi <- uTys nb1 tau1 nb2 tau2
1149 -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
1150 ; free_tvs <- zonkTcTyVarsAndFV (varSetElems (tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2))
1151 ; when (any (`elemVarSet` free_tvs) tvs)
1152 (bleatEscapedTvs free_tvs tvs tvs)
1154 -- If both sides are inside a box, we are in a "box-meets-box"
1155 -- situation, and we should not have a polytype at all.
1156 -- If we get here we have two boxes, already filled with
1157 -- the same polytype... but it should be a monotype.
1158 -- This check comes last, because the error message is
1159 -- extremely unhelpful.
1160 ; when (nb1 && nb2) (notMonoType ty1)
1164 (tvs1, body1) = tcSplitForAllTys ty1
1165 (tvs2, body2) = tcSplitForAllTys ty2
1168 go1 outer (PredTy p1) (PredTy p2)
1169 = uPred False nb1 p1 nb2 p2
1171 -- Type constructors must match
1172 go1 _ (TyConApp con1 tys1) (TyConApp con2 tys2)
1173 | con1 == con2 && not (isOpenSynTyCon con1)
1174 = do { cois <- uTys_s nb1 tys1 nb2 tys2
1175 ; return $ mkTyConAppCoI con1 tys1 cois
1177 -- See Note [TyCon app]
1178 | con1 == con2 && identicalOpenSynTyConApp
1179 = do { cois <- uTys_s nb1 tys1' nb2 tys2'
1180 ; return $ mkTyConAppCoI con1 tys1 (replicate n IdCo ++ cois)
1184 (idxTys1, tys1') = splitAt n tys1
1185 (idxTys2, tys2') = splitAt n tys2
1186 identicalOpenSynTyConApp = idxTys1 `tcEqTypes` idxTys2
1187 -- See Note [OpenSynTyCon app]
1189 -- Functions; just check the two parts
1190 go1 _ (FunTy fun1 arg1) (FunTy fun2 arg2)
1191 = do { coi_l <- uTys nb1 fun1 nb2 fun2
1192 ; coi_r <- uTys nb1 arg1 nb2 arg2
1193 ; return $ mkFunTyCoI fun1 coi_l arg1 coi_r
1196 -- Applications need a bit of care!
1197 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
1198 -- NB: we've already dealt with type variables and Notes,
1199 -- so if one type is an App the other one jolly well better be too
1200 go1 outer (AppTy s1 t1) ty2
1201 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
1202 = do { coi_s <- uTys nb1 s1 nb2 s2; coi_t <- uTys nb1 t1 nb2 t2
1203 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1205 -- Now the same, but the other way round
1206 -- Don't swap the types, because the error messages get worse
1207 go1 outer ty1 (AppTy s2 t2)
1208 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
1209 = do { coi_s <- uTys nb1 s1 nb2 s2; coi_t <- uTys nb1 t1 nb2 t2
1210 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1212 -- One or both outermost constructors are type family applications.
1213 -- If we can normalise them away, proceed as usual; otherwise, we
1214 -- need to defer unification by generating a wanted equality constraint.
1216 | ty1_is_fun || ty2_is_fun
1217 = do { (coi1, ty1') <- if ty1_is_fun then tcNormaliseFamInst ty1
1218 else return (IdCo, ty1)
1219 ; (coi2, ty2') <- if ty2_is_fun then tcNormaliseFamInst ty2
1220 else return (IdCo, ty2)
1221 ; coi <- if isOpenSynTyConApp ty1' || isOpenSynTyConApp ty2'
1222 then do { -- One type family app can't be reduced yet
1224 ; ty1'' <- zonkTcType ty1'
1225 ; ty2'' <- zonkTcType ty2'
1226 ; if tcEqType ty1'' ty2''
1228 else -- see [Deferred Unification]
1229 defer_unification outer False orig_ty1 orig_ty2
1231 else -- unification can proceed
1233 ; return $ coi1 `mkTransCoI` coi `mkTransCoI` (mkSymCoI coi2)
1236 ty1_is_fun = isOpenSynTyConApp ty1
1237 ty2_is_fun = isOpenSynTyConApp ty2
1239 -- Anything else fails
1240 go1 outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
1244 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
1246 do { coi <- uTys nb1 t1 nb2 t2
1247 ; return $ mkIParamPredCoI n1 coi
1249 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
1251 do { cois <- uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
1252 ; return $ mkClassPPredCoI c1 tys1 cois
1254 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
1256 uPreds outer nb1 [] nb2 [] = return []
1257 uPreds outer nb1 (p1:ps1) nb2 (p2:ps2) =
1258 do { coi <- uPred outer nb1 p1 nb2 p2
1259 ; cois <- uPreds outer nb1 ps1 nb2 ps2
1262 uPreds outer nb1 ps1 nb2 ps2 = panic "uPreds"
1267 When we find two TyConApps, the argument lists are guaranteed equal
1268 length. Reason: intially the kinds of the two types to be unified is
1269 the same. The only way it can become not the same is when unifying two
1270 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
1271 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
1272 which we do, that ensures that f1,f2 have the same kind; and that
1273 means a1,a2 have the same kind. And now the argument repeats.
1275 Note [OpenSynTyCon app]
1276 ~~~~~~~~~~~~~~~~~~~~~~~
1279 type family T a :: * -> *
1281 the two types (T () a) and (T () Int) must unify, even if there are
1282 no type instances for T at all. Should we just turn them into an
1283 equality (T () a ~ T () Int)? I don't think so. We currently try to
1284 eagerly unify everything we can before generating equalities; otherwise,
1285 we could turn the unification of [Int] with [a] into an equality, too.
1287 Note [Unification and synonyms]
1288 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1289 If you are tempted to make a short cut on synonyms, as in this
1293 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
1294 -- NO = if (con1 == con2) then
1295 -- NO -- Good news! Same synonym constructors, so we can shortcut
1296 -- NO -- by unifying their arguments and ignoring their expansions.
1297 -- NO unifyTypepeLists args1 args2
1299 -- NO -- Never mind. Just expand them and try again
1303 then THINK AGAIN. Here is the whole story, as detected and reported
1304 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1306 Here's a test program that should detect the problem:
1310 x = (1 :: Bogus Char) :: Bogus Bool
1313 The problem with [the attempted shortcut code] is that
1317 is not a sufficient condition to be able to use the shortcut!
1318 You also need to know that the type synonym actually USES all
1319 its arguments. For example, consider the following type synonym
1320 which does not use all its arguments.
1325 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1326 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1327 would fail, even though the expanded forms (both \tr{Int}) should
1330 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1331 unnecessarily bind \tr{t} to \tr{Char}.
1333 ... You could explicitly test for the problem synonyms and mark them
1334 somehow as needing expansion, perhaps also issuing a warning to the
1339 %************************************************************************
1341 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1343 %************************************************************************
1345 @uVar@ is called when at least one of the types being unified is a
1346 variable. It does {\em not} assume that the variable is a fixed point
1347 of the substitution; rather, notice that @uVar@ (defined below) nips
1348 back into @uTys@ if it turns out that the variable is already bound.
1352 -> SwapFlag -- False => tyvar is the "actual" (ty is "expected")
1353 -- True => ty is the "actual" (tyvar is "expected")
1355 -> InBox -- True <=> definitely no boxes in t2
1356 -> TcTauType -> TcTauType -- printing and real versions
1359 uVar outer swapped tv1 nb2 ps_ty2 ty2
1360 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1361 | otherwise = brackets (equals <+> ppr ty2)
1362 ; traceTc (text "uVar" <+> ppr swapped <+>
1363 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1364 nest 2 (ptext SLIT(" <-> ")),
1365 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1366 ; details <- lookupTcTyVar tv1
1369 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1370 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1371 -- The 'True' here says that ty1 is now inside a box
1372 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1376 uUnfilledVar :: Outer
1378 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1379 -> TcTauType -> TcTauType -- Type 2
1381 -- Invariant: tyvar 1 is not unified with anything
1383 uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1384 | Just ty2' <- tcView ty2
1385 = -- Expand synonyms; ignore FTVs
1386 uUnfilledVar False swapped tv1 details1 ps_ty2 ty2'
1388 uUnfilledVar outer swapped tv1 details1 ps_ty2 (TyVarTy tv2)
1389 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1391 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1392 -- this is box-meets-box, so fill in with a tau-type
1393 -> do { tau_tv <- tcInstTyVar tv1
1394 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv)
1397 other -> return IdCo -- No-op
1399 | otherwise -- Distinct type variables
1400 = do { lookup2 <- lookupTcTyVar tv2
1402 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
1403 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1406 uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2
1407 = -- ty2 is not a type variable
1409 MetaTv (SigTv _) _ -> rigid_variable
1411 uMetaVar outer swapped tv1 info ref1 ps_ty2 non_var_ty2
1412 SkolemTv _ -> rigid_variable
1415 | isOpenSynTyConApp non_var_ty2
1416 = -- 'non_var_ty2's outermost constructor is a type family,
1417 -- which we may may be able to normalise
1418 do { (coi2, ty2') <- tcNormaliseFamInst non_var_ty2
1420 IdCo -> -- no progress, but maybe after other instantiations
1421 defer_unification outer swapped (TyVarTy tv1) ps_ty2
1422 ACo co -> -- progress: so lets try again
1424 ppr co <+> text "::"<+> ppr non_var_ty2 <+> text "~" <+>
1426 ; coi <- uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2'
1427 ; let coi2' = (if swapped then id else mkSymCoI) coi2
1428 ; return $ coi2' `mkTransCoI` coi
1431 | SkolemTv RuntimeUnkSkol <- details1
1432 -- runtime unknown will never match
1433 = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1434 | otherwise -- defer as a given equality may still resolve this
1435 = defer_unification outer swapped (TyVarTy tv1) ps_ty2
1438 Note [Deferred Unification]
1439 ~~~~~~~~~~~~~~~~~~~~
1440 We may encounter a unification ty1 = ty2 that cannot be performed syntactically,
1441 and yet its consistency is undetermined. Previously, there was no way to still
1442 make it consistent. So a mismatch error was issued.
1444 Now these unfications are deferred until constraint simplification, where type
1445 family instances and given equations may (or may not) establish the consistency.
1446 Deferred unifications are of the form
1449 where F is a type function and x is a type variable.
1451 id :: x ~ y => x -> y
1454 involves the unfication x = y. It is deferred until we bring into account the
1455 context x ~ y to establish that it holds.
1457 If available, we defer original types (rather than those where closed type
1458 synonyms have already been expanded via tcCoreView). This is, as usual, to
1459 improve error messages.
1461 We need to both 'unBox' and zonk deferred types. We need to unBox as
1462 functions, such as TcExpr.tcMonoExpr promise to fill boxes in the expected
1463 type. We need to zonk as the types go into the kind of the coercion variable
1464 `cotv' and those are not zonked in Inst.zonkInst. (Maybe it would be better
1465 to zonk in zonInst instead. Would that be sufficient?)
1468 defer_unification :: Bool -- pop innermost context?
1473 defer_unification outer True ty1 ty2
1474 = defer_unification outer False ty2 ty1
1475 defer_unification outer False ty1 ty2
1476 = do { ty1' <- unBox ty1 >>= zonkTcType -- unbox *and* zonk..
1477 ; ty2' <- unBox ty2 >>= zonkTcType -- ..see preceding note
1478 ; traceTc $ text "deferring:" <+> ppr ty1 <+> text "~" <+> ppr ty2
1479 ; cotv <- newMetaCoVar ty1' ty2'
1480 -- put ty1 ~ ty2 in LIE
1481 -- Left means "wanted"
1482 ; inst <- (if outer then popErrCtxt else id) $
1483 mkEqInst (EqPred ty1' ty2') (Left cotv)
1485 ; return $ ACo $ TyVarTy cotv }
1488 uMetaVar :: Bool -- pop innermost context?
1490 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1493 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1494 -- ty2 is not a type variable
1496 uMetaVar outer swapped tv1 BoxTv ref1 ps_ty2 non_var_ty2
1497 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1498 -- that any boxes in ty2 are filled with monotypes
1500 -- It should not be the case that tv1 occurs in ty2
1501 -- (i.e. no occurs check should be needed), but if perchance
1502 -- it does, the unbox operation will fill it, and the debug code
1504 do { final_ty <- unBox ps_ty2
1505 ; when debugIsOn $ do
1506 { meta_details <- readMutVar ref1
1507 ; case meta_details of
1508 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1509 return () -- This really should *not* happen
1512 ; checkUpdateMeta swapped tv1 ref1 final_ty
1516 uMetaVar outer swapped tv1 info1 ref1 ps_ty2 non_var_ty2
1517 = do { -- Occurs check + monotype check
1518 ; mb_final_ty <- checkTauTvUpdate tv1 ps_ty2
1519 ; case mb_final_ty of
1520 Nothing -> -- tv1 occured in type family parameter
1521 defer_unification outer swapped (mkTyVarTy tv1) ps_ty2
1523 do { checkUpdateMeta swapped tv1 ref1 final_ty
1529 uUnfilledVars :: Outer
1531 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1532 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1534 -- Invarant: The type variables are distinct,
1535 -- Neither is filled in yet
1536 -- They might be boxy or not
1538 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1539 = -- see [Deferred Unification]
1540 defer_unification outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1542 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1543 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2) >> return IdCo
1544 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1545 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1) >> return IdCo
1547 -- ToDo: this function seems too long for what it acutally does!
1548 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1549 = case (info1, info2) of
1550 (BoxTv, BoxTv) -> box_meets_box >> return IdCo
1552 -- If a box meets a TauTv, but the fomer has the smaller kind
1553 -- then we must create a fresh TauTv with the smaller kind
1554 (_, BoxTv) | k1_sub_k2 -> update_tv2 >> return IdCo
1555 | otherwise -> box_meets_box >> return IdCo
1556 (BoxTv, _ ) | k2_sub_k1 -> update_tv1 >> return IdCo
1557 | otherwise -> box_meets_box >> return IdCo
1559 -- Avoid SigTvs if poss
1560 (SigTv _, _ ) | k1_sub_k2 -> update_tv2 >> return IdCo
1561 (_, SigTv _) | k2_sub_k1 -> update_tv1 >> return IdCo
1563 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1564 then update_tv1 >> return IdCo -- Same kinds
1565 else update_tv2 >> return IdCo
1566 | k2_sub_k1 -> update_tv1 >> return IdCo
1567 | otherwise -> kind_err >> return IdCo
1569 -- Update the variable with least kind info
1570 -- See notes on type inference in Kind.lhs
1571 -- The "nicer to" part only applies if the two kinds are the same,
1572 -- so we can choose which to do.
1574 -- Kinds should be guaranteed ok at this point
1575 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1576 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1578 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1581 | k2_sub_k1 = fill_from tv2
1582 | otherwise = kind_err
1584 -- Update *both* tyvars with a TauTv whose name and kind
1585 -- are gotten from tv (avoid losing nice names is poss)
1586 fill_from tv = do { tv' <- tcInstTyVar tv
1587 ; let tau_ty = mkTyVarTy tv'
1588 ; updateMeta tv1 ref1 tau_ty
1589 ; updateMeta tv2 ref2 tau_ty }
1591 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1592 unifyKindMisMatch k1 k2
1596 k1_sub_k2 = k1 `isSubKind` k2
1597 k2_sub_k1 = k2 `isSubKind` k1
1599 nicer_to_update_tv1 = isSystemName (Var.varName tv1)
1600 -- Try to update sys-y type variables in preference to ones
1601 -- gotten (say) by instantiating a polymorphic function with
1602 -- a user-written type sig
1606 refineBox :: TcType -> TcM TcType
1607 -- Unbox the outer box of a boxy type (if any)
1608 refineBox ty@(TyVarTy box_tv)
1609 | isMetaTyVar box_tv
1610 = do { cts <- readMetaTyVar box_tv
1613 Indirect ty -> return ty }
1614 refineBox other_ty = return other_ty
1616 refineBoxToTau :: TcType -> TcM TcType
1617 -- Unbox the outer box of a boxy type, filling with a monotype if it is empty
1618 -- Like refineBox except for the "fill with monotype" part.
1619 refineBoxToTau ty@(TyVarTy box_tv)
1620 | isMetaTyVar box_tv
1621 , MetaTv BoxTv ref <- tcTyVarDetails box_tv
1622 = do { cts <- readMutVar ref
1624 Flexi -> fillBoxWithTau box_tv ref
1625 Indirect ty -> return ty }
1626 refineBoxToTau other_ty = return other_ty
1628 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1629 -- Subtle... we must zap the boxy res_ty
1630 -- to kind * before using it to instantiate a LitInst
1631 -- Calling unBox instead doesn't do the job, because the box
1632 -- often has an openTypeKind, and we don't want to instantiate
1634 zapToMonotype res_ty
1635 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1636 ; boxyUnify res_tau res_ty
1639 unBox :: BoxyType -> TcM TcType
1640 -- unBox implements the judgement
1642 -- with input s', and result s
1644 -- It removes all boxes from the input type, returning a non-boxy type.
1645 -- A filled box in the type can only contain a monotype; unBox fails if not
1646 -- The type can have empty boxes, which unBox fills with a monotype
1648 -- Compare this wth checkTauTvUpdate
1650 -- For once, it's safe to treat synonyms as opaque!
1652 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1653 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1654 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1655 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1656 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1657 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1659 | isTcTyVar tv -- It's a boxy type variable
1660 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1661 = do { cts <- readMutVar ref -- under nested quantifiers
1663 Flexi -> fillBoxWithTau tv ref
1664 Indirect ty -> do { non_boxy_ty <- unBox ty
1665 ; if isTauTy non_boxy_ty
1666 then return non_boxy_ty
1667 else notMonoType non_boxy_ty }
1669 | otherwise -- Skolems, and meta-tau-variables
1670 = return (TyVarTy tv)
1672 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1673 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1674 unBoxPred (EqPred ty1 ty2) = do { ty1' <- unBox ty1; ty2' <- unBox ty2; return (EqPred ty1' ty2') }
1679 %************************************************************************
1681 \subsection[Unify-context]{Errors and contexts}
1683 %************************************************************************
1689 unifyCtxt act_ty exp_ty tidy_env
1690 = do { act_ty' <- zonkTcType act_ty
1691 ; exp_ty' <- zonkTcType exp_ty
1692 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1693 (env2, act_ty'') = tidyOpenType env1 act_ty'
1694 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1697 mkExpectedActualMsg act_ty exp_ty
1698 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1699 text "Inferred type" <> colon <+> ppr act_ty ])
1702 -- If an error happens we try to figure out whether the function
1703 -- function has been given too many or too few arguments, and say so.
1704 addSubCtxt orig actual_res_ty expected_res_ty thing_inside
1705 = addErrCtxtM mk_err thing_inside
1708 = do { exp_ty' <- zonkTcType expected_res_ty
1709 ; act_ty' <- zonkTcType actual_res_ty
1710 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1711 (env2, act_ty'') = tidyOpenType env1 act_ty'
1712 (exp_args, _) = tcSplitFunTys exp_ty''
1713 (act_args, _) = tcSplitFunTys act_ty''
1715 len_act_args = length act_args
1716 len_exp_args = length exp_args
1718 message = case orig of
1720 | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1721 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1722 other -> mkExpectedActualMsg act_ty'' exp_ty''
1723 ; return (env2, message) }
1725 wrongArgsCtxt too_many_or_few fun
1726 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1727 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1728 <+> ptext SLIT("arguments")
1731 unifyForAllCtxt tvs phi1 phi2 env
1732 = return (env2, msg)
1734 (env', tvs') = tidyOpenTyVars env tvs -- NB: not tidyTyVarBndrs
1735 (env1, phi1') = tidyOpenType env' phi1
1736 (env2, phi2') = tidyOpenType env1 phi2
1737 msg = vcat [ptext SLIT("When matching") <+> quotes (ppr (mkForAllTys tvs' phi1')),
1738 ptext SLIT(" and") <+> quotes (ppr (mkForAllTys tvs' phi2'))]
1740 -----------------------
1741 unifyMisMatch outer swapped ty1 ty2
1742 | swapped = unifyMisMatch outer False ty2 ty1
1743 | outer = popErrCtxt $ unifyMisMatch False swapped ty1 ty2 -- This is the whole point of the 'outer' stuff
1744 | otherwise = failWithMisMatch ty1 ty2
1748 %************************************************************************
1752 %************************************************************************
1754 Unifying kinds is much, much simpler than unifying types.
1757 unifyKind :: TcKind -- Expected
1760 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1761 | isSubKindCon kc2 kc1 = return ()
1763 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1764 = do { unifyKind a2 a1; unifyKind r1 r2 }
1765 -- Notice the flip in the argument,
1766 -- so that the sub-kinding works right
1767 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1768 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1769 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1771 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1772 unifyKinds [] [] = return ()
1773 unifyKinds (k1:ks1) (k2:ks2) = do unifyKind k1 k2
1775 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1778 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1779 uKVar swapped kv1 k2
1780 = do { mb_k1 <- readKindVar kv1
1782 Flexi -> uUnboundKVar swapped kv1 k2
1783 Indirect k1 | swapped -> unifyKind k2 k1
1784 | otherwise -> unifyKind k1 k2 }
1787 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1788 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1789 | kv1 == kv2 = return ()
1790 | otherwise -- Distinct kind variables
1791 = do { mb_k2 <- readKindVar kv2
1793 Indirect k2 -> uUnboundKVar swapped kv1 k2
1794 Flexi -> writeKindVar kv1 k2 }
1796 uUnboundKVar swapped kv1 non_var_k2
1797 = do { k2' <- zonkTcKind non_var_k2
1798 ; kindOccurCheck kv1 k2'
1799 ; k2'' <- kindSimpleKind swapped k2'
1800 -- KindVars must be bound only to simple kinds
1801 -- Polarities: (kindSimpleKind True ?) succeeds
1802 -- returning *, corresponding to unifying
1805 ; writeKindVar kv1 k2'' }
1808 kindOccurCheck kv1 k2 -- k2 is zonked
1809 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1811 not_in (TyVarTy kv2) = kv1 /= kv2
1812 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1815 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1816 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1817 -- If the flag is False, it requires k <: sk
1818 -- E.g. kindSimpleKind False ?? = *
1819 -- What about (kv -> *) :=: ?? -> *
1820 kindSimpleKind orig_swapped orig_kind
1821 = go orig_swapped orig_kind
1823 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1825 ; return (mkArrowKind k1' k2') }
1827 | isOpenTypeKind k = return liftedTypeKind
1828 | isArgTypeKind k = return liftedTypeKind
1830 | isLiftedTypeKind k = return liftedTypeKind
1831 | isUnliftedTypeKind k = return unliftedTypeKind
1832 go sw k@(TyVarTy _) = return k -- KindVars are always simple
1833 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1834 <+> ppr orig_swapped <+> ppr orig_kind)
1835 -- I think this can't actually happen
1837 -- T v = MkT v v must be a type
1838 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1841 kindOccurCheckErr tyvar ty
1842 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1843 2 (sep [ppr tyvar, char '=', ppr ty])
1847 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1848 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1850 unifyFunKind (TyVarTy kvar) = do
1851 maybe_kind <- readKindVar kvar
1853 Indirect fun_kind -> unifyFunKind fun_kind
1855 do { arg_kind <- newKindVar
1856 ; res_kind <- newKindVar
1857 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1858 ; return (Just (arg_kind,res_kind)) }
1860 unifyFunKind (FunTy arg_kind res_kind) = return (Just (arg_kind,res_kind))
1861 unifyFunKind other = return Nothing
1864 %************************************************************************
1868 %************************************************************************
1870 ---------------------------
1871 -- We would like to get a decent error message from
1872 -- (a) Under-applied type constructors
1873 -- f :: (Maybe, Maybe)
1874 -- (b) Over-applied type constructors
1875 -- f :: Int x -> Int x
1879 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1880 -- A fancy wrapper for 'unifyKind', which tries
1881 -- to give decent error messages.
1882 -- (checkExpectedKind ty act_kind exp_kind)
1883 -- checks that the actual kind act_kind is compatible
1884 -- with the expected kind exp_kind
1885 -- The first argument, ty, is used only in the error message generation
1886 checkExpectedKind ty act_kind exp_kind
1887 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1890 (_errs, mb_r) <- tryTc (unifyKind exp_kind act_kind)
1892 Just r -> return () ; -- Unification succeeded
1895 -- So there's definitely an error
1896 -- Now to find out what sort
1897 exp_kind <- zonkTcKind exp_kind
1898 act_kind <- zonkTcKind act_kind
1900 env0 <- tcInitTidyEnv
1901 let (exp_as, _) = splitKindFunTys exp_kind
1902 (act_as, _) = splitKindFunTys act_kind
1903 n_exp_as = length exp_as
1904 n_act_as = length act_as
1906 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1907 (env2, tidy_act_kind) = tidyKind env1 act_kind
1909 err | n_exp_as < n_act_as -- E.g. [Maybe]
1910 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1912 -- Now n_exp_as >= n_act_as. In the next two cases,
1913 -- n_exp_as == 0, and hence so is n_act_as
1914 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1915 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1916 <+> ptext SLIT("is unlifted")
1918 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1919 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1920 <+> ptext SLIT("is lifted")
1922 | otherwise -- E.g. Monad [Int]
1923 = ptext SLIT("Kind mis-match")
1925 more_info = sep [ ptext SLIT("Expected kind") <+>
1926 quotes (pprKind tidy_exp_kind) <> comma,
1927 ptext SLIT("but") <+> quotes (ppr ty) <+>
1928 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1930 failWithTcM (env2, err $$ more_info)
1933 %************************************************************************
1935 \subsection{Checking signature type variables}
1937 %************************************************************************
1939 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1940 are not mentioned in the environment. In particular:
1942 (a) Not mentioned in the type of a variable in the envt
1943 eg the signature for f in this:
1949 Here, f is forced to be monorphic by the free occurence of x.
1951 (d) Not (unified with another type variable that is) in scope.
1952 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1953 when checking the expression type signature, we find that
1954 even though there is nothing in scope whose type mentions r,
1955 nevertheless the type signature for the expression isn't right.
1957 Another example is in a class or instance declaration:
1959 op :: forall b. a -> b
1961 Here, b gets unified with a
1963 Before doing this, the substitution is applied to the signature type variable.
1966 checkSigTyVars :: [TcTyVar] -> TcM ()
1967 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1969 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1970 -- The extra_tvs can include boxy type variables;
1971 -- e.g. TcMatches.tcCheckExistentialPat
1972 checkSigTyVarsWrt extra_tvs sig_tvs
1973 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1974 ; check_sig_tyvars extra_tvs' sig_tvs }
1977 :: TcTyVarSet -- Global type variables. The universally quantified
1978 -- tyvars should not mention any of these
1979 -- Guaranteed already zonked.
1980 -> [TcTyVar] -- Universally-quantified type variables in the signature
1981 -- Guaranteed to be skolems
1983 check_sig_tyvars extra_tvs []
1985 check_sig_tyvars extra_tvs sig_tvs
1986 = ASSERT( all isSkolemTyVar sig_tvs )
1987 do { gbl_tvs <- tcGetGlobalTyVars
1988 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1989 text "gbl_tvs" <+> ppr gbl_tvs,
1990 text "extra_tvs" <+> ppr extra_tvs]))
1992 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1993 ; when (any (`elemVarSet` env_tvs) sig_tvs)
1994 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1997 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1998 -> [TcTyVar] -- The possibly-escaping type variables
1999 -> [TcTyVar] -- The zonked versions thereof
2001 -- Complain about escaping type variables
2002 -- We pass a list of type variables, at least one of which
2003 -- escapes. The first list contains the original signature type variable,
2004 -- while the second contains the type variable it is unified to (usually itself)
2005 bleatEscapedTvs globals sig_tvs zonked_tvs
2006 = do { env0 <- tcInitTidyEnv
2007 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
2008 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
2010 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
2011 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
2013 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
2015 check (tidy_env, msgs) (sig_tv, zonked_tv)
2016 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
2018 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
2019 ; return (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
2021 -----------------------
2022 escape_msg sig_tv zonked_tv globs
2024 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
2025 nest 2 (vcat globs)]
2027 = msg <+> ptext SLIT("escapes")
2028 -- Sigh. It's really hard to give a good error message
2029 -- all the time. One bad case is an existential pattern match.
2030 -- We rely on the "When..." context to help.
2032 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
2034 | sig_tv == zonked_tv = empty
2035 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
2038 These two context are used with checkSigTyVars
2041 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
2042 -> TidyEnv -> TcM (TidyEnv, Message)
2043 sigCtxt id sig_tvs sig_theta sig_tau tidy_env = do
2044 actual_tau <- zonkTcType sig_tau
2046 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
2047 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
2048 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
2049 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
2050 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
2052 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),