2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Type subsumption and unification
10 -- Full-blown subsumption
12 checkSigTyVars, checkSigTyVarsWrt, bleatEscapedTvs, sigCtxt,
14 -- Various unifications
15 unifyType, unifyTypeList, unifyTheta,
16 unifyKind, unifyKinds, unifyFunKind,
17 preSubType, boxyMatchTypes,
19 --------------------------------
21 tcInfer, subFunTys, unBox, refineBox, refineBoxToTau, withBox,
22 boxyUnify, boxyUnifyList, zapToMonotype,
23 boxySplitListTy, boxySplitPArrTy, boxySplitTyConApp, boxySplitAppTy,
27 #include "HsVersions.h"
37 import TcRnMonad -- TcType, amongst others
59 %************************************************************************
61 \subsection{'hole' type variables}
63 %************************************************************************
66 tcInfer :: (BoxyType -> TcM a) -> TcM (a, TcType)
67 tcInfer tc_infer = withBox openTypeKind tc_infer
71 %************************************************************************
75 %************************************************************************
78 subFunTys :: SDoc -- Something like "The function f has 3 arguments"
79 -- or "The abstraction (\x.e) takes 1 argument"
80 -> Arity -- Expected # of args
81 -> BoxySigmaType -- res_ty
82 -> Maybe UserTypeCtxt -- Whether res_ty arises from a user signature
83 -- Only relevant if we encounter a sigma-type
84 -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
86 -- Attempt to decompse res_ty to have enough top-level arrows to
87 -- match the number of patterns in the match group
89 -- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
90 -- and the inner call to thing_inside passes args: [a1,...,an], b
91 -- then co_fn :: (a1 -> ... -> an -> b) ~ res_ty
93 -- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
96 {- Error messages from subFunTys
98 The abstraction `\Just 1 -> ...' has two arguments
99 but its type `Maybe a -> a' has only one
101 The equation(s) for `f' have two arguments
102 but its type `Maybe a -> a' has only one
104 The section `(f 3)' requires 'f' to take two arguments
105 but its type `Int -> Int' has only one
107 The function 'f' is applied to two arguments
108 but its type `Int -> Int' has only one
112 subFunTys error_herald n_pats res_ty mb_ctxt thing_inside
113 = loop n_pats [] res_ty
115 -- In 'loop', the parameter 'arg_tys' accumulates
116 -- the arg types so far, in *reverse order*
117 -- INVARIANT: res_ty :: *
118 loop n args_so_far res_ty
119 | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
121 loop n args_so_far res_ty
122 | isSigmaTy res_ty -- Do this before checking n==0, because we
123 -- guarantee to return a BoxyRhoType, not a
125 = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet mb_ctxt $ \ _ res_ty ->
126 loop n args_so_far res_ty
127 ; return (gen_fn <.> co_fn, res) }
129 loop 0 args_so_far res_ty
130 = do { res <- thing_inside (reverse args_so_far) res_ty
131 ; return (idHsWrapper, res) }
133 loop n args_so_far (FunTy arg_ty res_ty)
134 = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
135 ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
136 ; return (co_fn', res) }
138 -- Try to normalise synonym families and defer if that's not possible
139 loop n args_so_far ty@(TyConApp tc _)
141 = do { (coi1, ty') <- tcNormaliseFamInst ty
143 IdCo -> defer n args_so_far ty
144 -- no progress, but maybe solvable => defer
145 ACo _ -> -- progress: so lets try again
146 do { (co_fn, res) <- loop n args_so_far ty'
147 ; return $ (co_fn <.> coiToHsWrapper (mkSymCoI coi1), res)
151 -- res_ty might have a type variable at the head, such as (a b c),
152 -- in which case we must fill in with (->). Simplest thing to do
153 -- is to use boxyUnify, but we catch failure and generate our own
154 -- error message on failure
155 loop n args_so_far res_ty@(AppTy _ _)
156 = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
157 ; (_, mb_coi) <- tryTcErrs $
158 boxyUnify res_ty (FunTy arg_ty' res_ty')
159 ; if isNothing mb_coi then bale_out args_so_far
160 else do { let coi = expectJust "subFunTys" mb_coi
161 ; (co_fn, res) <- loop n args_so_far (FunTy arg_ty'
163 ; return (co_fn <.> coiToHsWrapper coi, res)
167 loop n args_so_far ty@(TyVarTy tv)
168 | isTyConableTyVar tv
169 = do { cts <- readMetaTyVar tv
171 Indirect ty -> loop n args_so_far ty
173 do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
174 ; res <- thing_inside (reverse args_so_far ++ arg_tys)
176 ; return (idHsWrapper, res) } }
177 | otherwise -- defer as tyvar may be refined by equalities
178 = defer n args_so_far ty
180 mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
181 mk_res_ty [] = panic "TcUnify.mk_res_ty1"
182 kinds = openTypeKind : take n (repeat argTypeKind)
183 -- Note argTypeKind: the args can have an unboxed type,
184 -- but not an unboxed tuple.
186 loop _ args_so_far _ = bale_out args_so_far
188 -- Build a template type a1 -> ... -> an -> b and defer an equality
189 -- between that template and the expected result type res_ty; then,
190 -- use the template to type the thing_inside
191 defer n args_so_far ty
192 = do { arg_tys <- newFlexiTyVarTys n argTypeKind
193 ; res_ty' <- newFlexiTyVarTy openTypeKind
194 ; let fun_ty = mkFunTys arg_tys res_ty'
195 err = error_herald <> comma $$
196 text "which does not match its type"
197 ; coi <- addErrCtxt err $
198 defer_unification (Unify False fun_ty ty) False fun_ty ty
199 ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty'
200 ; return (coiToHsWrapper coi, res)
204 = do { env0 <- tcInitTidyEnv
205 ; res_ty' <- zonkTcType res_ty
206 ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
207 ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
209 mk_msg res_ty n_actual
210 = error_herald <> comma $$
211 sep [ptext (sLit "but its type") <+> quotes (pprType res_ty),
212 if n_actual == 0 then ptext (sLit "has none")
213 else ptext (sLit "has only") <+> speakN n_actual]
217 ----------------------
218 boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
219 -> BoxyRhoType -- Expected type (T a b c)
220 -> TcM ([BoxySigmaType], -- Element types, a b c
221 CoercionI) -- T a b c ~ orig_ty
222 -- It's used for wired-in tycons, so we call checkWiredInTyCon
223 -- Precondition: never called with FunTyCon
224 -- Precondition: input type :: *
226 boxySplitTyConApp tc orig_ty
227 = do { checkWiredInTyCon tc
228 ; loop (tyConArity tc) [] orig_ty }
230 loop n_req args_so_far ty
231 | Just ty' <- tcView ty = loop n_req args_so_far ty'
233 loop n_req args_so_far ty@(TyConApp tycon args)
235 = ASSERT( n_req == length args) -- ty::*
236 return (args ++ args_so_far, IdCo)
238 | isOpenSynTyCon tycon -- try to normalise type family application
239 = do { (coi1, ty') <- tcNormaliseFamInst ty
240 ; traceTc $ text "boxySplitTyConApp:" <+>
241 ppr ty <+> text "==>" <+> ppr ty'
243 IdCo -> defer -- no progress, but maybe solvable => defer
244 ACo _ -> -- progress: so lets try again
245 do { (args, coi2) <- loop n_req args_so_far ty'
246 ; return $ (args, coi2 `mkTransCoI` mkSymCoI coi1)
250 loop n_req args_so_far (AppTy fun arg)
252 = do { (args, coi) <- loop (n_req - 1) (arg:args_so_far) fun
253 ; return (args, mkAppTyCoI fun coi arg IdCo)
256 loop n_req args_so_far (TyVarTy tv)
257 | isTyConableTyVar tv
258 , res_kind `isSubKind` tyVarKind tv
259 = do { cts <- readMetaTyVar tv
261 Indirect ty -> loop n_req args_so_far ty
262 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
263 ; return (arg_tys ++ args_so_far, IdCo) }
265 | otherwise -- defer as tyvar may be refined by equalities
268 (arg_kinds, res_kind) = splitKindFunTysN n_req (tyConKind tc)
270 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc)))
273 -- defer splitting by generating an equality constraint
274 defer = boxySplitDefer arg_kinds mk_res_ty orig_ty
276 (arg_kinds, _) = splitKindFunTys (tyConKind tc)
278 -- apply splitted tycon to arguments
279 mk_res_ty = mkTyConApp tc
281 ----------------------
282 boxySplitListTy :: BoxyRhoType -> TcM (BoxySigmaType, CoercionI)
283 -- Special case for lists
284 boxySplitListTy exp_ty
285 = do { ([elt_ty], coi) <- boxySplitTyConApp listTyCon exp_ty
286 ; return (elt_ty, coi) }
288 ----------------------
289 boxySplitPArrTy :: BoxyRhoType -> TcM (BoxySigmaType, CoercionI)
290 -- Special case for parrs
291 boxySplitPArrTy exp_ty
292 = do { ([elt_ty], coi) <- boxySplitTyConApp parrTyCon exp_ty
293 ; return (elt_ty, coi) }
295 ----------------------
296 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
297 -> TcM ((BoxySigmaType, BoxySigmaType), -- Returns m, a
299 -- If the incoming type is a mutable type variable of kind k, then
300 -- boxySplitAppTy returns a new type variable (m: * -> k); note the *.
301 -- If the incoming type is boxy, then so are the result types; and vice versa
303 boxySplitAppTy orig_ty
307 | Just ty' <- tcView ty = loop ty'
310 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
311 = return ((fun_ty, arg_ty), IdCo)
313 loop ty@(TyConApp tycon _args)
314 | isOpenSynTyCon tycon -- try to normalise type family application
315 = do { (coi1, ty') <- tcNormaliseFamInst ty
317 IdCo -> defer -- no progress, but maybe solvable => defer
318 ACo _ -> -- progress: so lets try again
319 do { (args, coi2) <- loop ty'
320 ; return $ (args, coi2 `mkTransCoI` mkSymCoI coi1)
325 | isTyConableTyVar tv
326 = do { cts <- readMetaTyVar tv
328 Indirect ty -> loop ty
329 Flexi -> do { [fun_ty, arg_ty] <- withMetaTvs tv kinds mk_res_ty
330 ; return ((fun_ty, arg_ty), IdCo) } }
331 | otherwise -- defer as tyvar may be refined by equalities
334 tv_kind = tyVarKind tv
335 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
337 liftedTypeKind] -- arg type :: *
338 -- The defaultKind is a bit smelly. If you remove it,
339 -- try compiling f x = do { x }
340 -- and you'll get a kind mis-match. It smells, but
341 -- not enough to lose sleep over.
343 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
345 -- defer splitting by generating an equality constraint
346 defer = do { ([ty1, ty2], coi) <- boxySplitDefer arg_kinds mk_res_ty orig_ty
347 ; return ((ty1, ty2), coi)
350 orig_kind = typeKind orig_ty
351 arg_kinds = [mkArrowKind liftedTypeKind (defaultKind orig_kind),
353 liftedTypeKind] -- arg type :: *
355 -- build type application
356 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
357 mk_res_ty _other = panic "TcUnify.mk_res_ty2"
360 boxySplitFailure :: TcType -> TcType -> TcM (a, CoercionI)
361 boxySplitFailure actual_ty expected_ty = failWithMisMatch actual_ty expected_ty
364 boxySplitDefer :: [Kind] -- kinds of required arguments
365 -> ([TcType] -> TcTauType) -- construct lhs from argument tyvars
366 -> BoxyRhoType -- type to split
367 -> TcM ([TcType], CoercionI)
368 boxySplitDefer kinds mk_ty orig_ty
369 = do { tau_tys <- mapM newFlexiTyVarTy kinds
370 ; let ty1 = mk_ty tau_tys
371 ; coi <- defer_unification (Unify False ty1 orig_ty) False ty1 orig_ty
372 ; return (tau_tys, coi)
377 --------------------------------
378 -- withBoxes: the key utility function
379 --------------------------------
382 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
383 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
384 -> ([BoxySigmaType] -> BoxySigmaType)
385 -- Constructs the type to assign
386 -- to the original var
387 -> TcM [BoxySigmaType] -- Return the fresh boxes
389 -- It's entirely possible for the [kind] to be empty.
390 -- For example, when pattern-matching on True,
391 -- we call boxySplitTyConApp passing a boolTyCon
393 -- Invariant: tv is still Flexi
395 withMetaTvs tv kinds mk_res_ty
397 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
398 ; let box_tys = mkTyVarTys box_tvs
399 ; writeMetaTyVar tv (mk_res_ty box_tys)
402 | otherwise -- Non-boxy meta type variable
403 = do { tau_tys <- mapM newFlexiTyVarTy kinds
404 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
405 -- Sure to be a tau-type
408 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
409 -- Allocate a *boxy* tyvar
410 withBox kind thing_inside
411 = do { box_tv <- newBoxyTyVar kind
412 ; res <- thing_inside (mkTyVarTy box_tv)
413 ; ty <- {- pprTrace "with_box" (ppr (mkTyVarTy box_tv)) $ -} readFilledBox box_tv
418 %************************************************************************
420 Approximate boxy matching
422 %************************************************************************
425 preSubType :: [TcTyVar] -- Quantified type variables
426 -> TcTyVarSet -- Subset of quantified type variables
427 -- see Note [Pre-sub boxy]
428 -> TcType -- The rho-type part; quantified tyvars scopes over this
429 -> BoxySigmaType -- Matching type from the context
430 -> TcM [TcType] -- Types to instantiate the tyvars
431 -- Perform pre-subsumption, and return suitable types
432 -- to instantiate the quantified type varibles:
433 -- info from the pre-subsumption, if there is any
434 -- a boxy type variable otherwise
436 -- Note [Pre-sub boxy]
437 -- The 'btvs' are a subset of 'qtvs'. They are the ones we can
438 -- instantiate to a boxy type variable, because they'll definitely be
439 -- filled in later. This isn't always the case; sometimes we have type
440 -- variables mentioned in the context of the type, but not the body;
441 -- f :: forall a b. C a b => a -> a
442 -- Then we may land up with an unconstrained 'b', so we want to
443 -- instantiate it to a monotype (non-boxy) type variable
445 -- The 'qtvs' that are *neither* fixed by the pre-subsumption, *nor* are in 'btvs',
446 -- are instantiated to TauTv meta variables.
448 preSubType qtvs btvs qty expected_ty
449 = do { tys <- mapM inst_tv qtvs
450 ; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
453 pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
455 | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
456 | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
457 ; return (mkTyVarTy tv') }
458 | otherwise = do { tv' <- tcInstTyVar tv
459 ; return (mkTyVarTy tv') }
462 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
463 -> BoxyRhoType -- Type to match (note a *Rho* type)
464 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
466 -- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
467 -- "Boxy types: inference for higher rank types and impredicativity"
469 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
470 = go tmpl_tvs tmpl_ty emptyVarSet boxy_ty
472 go t_tvs t_ty b_tvs b_ty
473 | Just t_ty' <- tcView t_ty = go t_tvs t_ty' b_tvs b_ty
474 | Just b_ty' <- tcView b_ty = go t_tvs t_ty b_tvs b_ty'
476 go _ (TyVarTy _) _ _ = emptyTvSubst -- Rule S-ANY; no bindings
477 -- Rule S-ANY covers (a) type variables and (b) boxy types
478 -- in the template. Both look like a TyVarTy.
479 -- See Note [Sub-match] below
481 go t_tvs t_ty b_tvs b_ty
482 | isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
483 = go (t_tvs `delVarSetList` tvs) t_tau b_tvs b_ty -- Rule S-SPEC
484 -- Under a forall on the left, if there is shadowing,
485 -- do not bind! Hence the delVarSetList.
486 | isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
487 = go t_tvs t_ty (extendVarSetList b_tvs tvs) b_tau -- Rule S-SKOL
488 -- Add to the variables we must not bind to
489 -- NB: it's *important* to discard the theta part. Otherwise
490 -- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
491 -- and end up with a completely bogus binding (b |-> Bool), by lining
492 -- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
493 -- This pre-subsumption stuff can return too few bindings, but it
494 -- must *never* return bogus info.
496 go t_tvs (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
497 = boxy_match t_tvs arg1 b_tvs arg2 (go t_tvs res1 b_tvs res2)
498 -- Match the args, and sub-match the results
500 go t_tvs t_ty b_tvs b_ty = boxy_match t_tvs t_ty b_tvs b_ty emptyTvSubst
501 -- Otherwise defer to boxy matching
502 -- This covers TyConApp, AppTy, PredTy
509 |- head xs : <rhobox>
510 We will do a boxySubMatchType between a ~ <rhobox>
511 But we *don't* want to match [a |-> <rhobox>] because
512 (a) The box should be filled in with a rho-type, but
513 but the returned substitution maps TyVars to boxy
515 (b) In any case, the right final answer might be *either*
516 instantiate 'a' with a rho-type or a sigma type
517 head xs : Int vs head xs : forall b. b->b
518 So the matcher MUST NOT make a choice here. In general, we only
519 bind a template type variable in boxyMatchType, not in boxySubMatchType.
524 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
525 -> [BoxySigmaType] -- Type to match
526 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
528 -- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
529 -- "Boxy types: inference for higher rank types and impredicativity"
531 -- Find a *boxy* substitution that makes the template look as much
532 -- like the BoxySigmaType as possible.
533 -- It's always ok to return an empty substitution;
534 -- anything more is jam on the pudding
536 -- NB1: This is a pure, non-monadic function.
537 -- It does no unification, and cannot fail
539 -- Precondition: the arg lengths are equal
540 -- Precondition: none of the template type variables appear anywhere in the [BoxySigmaType]
544 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
545 = ASSERT( length tmpl_tys == length boxy_tys )
546 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
547 -- ToDo: add error context?
549 boxy_match_s :: TcTyVarSet -> [TcType] -> TcTyVarSet -> [BoxySigmaType]
550 -> TvSubst -> TvSubst
551 boxy_match_s _ [] _ [] subst
553 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
554 = boxy_match tmpl_tvs t_ty boxy_tvs b_ty $
555 boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys subst
556 boxy_match_s _ _ _ _ _
557 = panic "boxy_match_s" -- Lengths do not match
561 boxy_match :: TcTyVarSet -> TcType -- Template
562 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
563 -> BoxySigmaType -- Match against this type
567 -- The boxy_tvs argument prevents this match:
568 -- [a] forall b. a ~ forall b. b
569 -- We don't want to bind the template variable 'a'
570 -- to the quantified type variable 'b'!
572 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
573 = go orig_tmpl_ty orig_boxy_ty
576 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
577 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
579 go ty1 ty2 -- C.f. the isSigmaTy case for boxySubMatchType
581 , (tvs1, ps1, tau1) <- tcSplitSigmaTy ty1
582 , (tvs2, ps2, tau2) <- tcSplitSigmaTy ty2
583 , equalLength tvs1 tvs2
584 , equalLength ps1 ps2
585 = boxy_match (tmpl_tvs `delVarSetList` tvs1) tau1
586 (boxy_tvs `extendVarSetList` tvs2) tau2 subst
588 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
590 , not $ isOpenSynTyCon tc1
593 go (FunTy arg1 res1) (FunTy arg2 res2)
594 = go_s [arg1,res1] [arg2,res2]
597 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
598 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
599 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
600 = go_s [s1,t1] [s2,t2]
603 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
604 , boxy_tvs `disjointVarSet` tyVarsOfType orig_boxy_ty
605 , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
606 = extendTvSubst subst tv boxy_ty'
608 = subst -- Ignore others
610 boxy_ty' = case lookupTyVar subst tv of
611 Nothing -> orig_boxy_ty
612 Just ty -> ty `boxyLub` orig_boxy_ty
614 go _ (TyVarTy tv) | isTcTyVar tv && isMetaTyVar tv
615 -- NB: A TyVar (not TcTyVar) is possible here, representing
616 -- a skolem, because in this pure boxy_match function
617 -- we don't instantiate foralls to TcTyVars; cf Trac #2714
618 = subst -- Don't fail if the template has more info than the target!
619 -- Otherwise, with tmpl_tvs = [a], matching (a -> Int) ~ (Bool -> beta)
620 -- would fail to instantiate 'a', because the meta-type-variable
621 -- beta is as yet un-filled-in
623 go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
624 -- Example: Tree a ~ Maybe Int
625 -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
626 -- misleading error messages. An even more confusing case is
627 -- a -> b ~ Maybe Int
628 -- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
629 -- from this pre-matching phase.
632 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
635 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
636 -- Combine boxy information from the two types
637 -- If there is a conflict, return the first
638 boxyLub orig_ty1 orig_ty2
639 = go orig_ty1 orig_ty2
641 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
642 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
643 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
644 | tc1 == tc2, length ts1 == length ts2
645 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
647 go (TyVarTy tv1) _ -- This is the whole point;
648 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
651 go _ (TyVarTy tv2) -- Symmetrical case
652 | isTcTyVar tv2, isBoxyTyVar tv2
655 -- Look inside type synonyms, but only if the naive version fails
656 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
657 | Just ty2' <- tcView ty1 = go ty1 ty2'
659 -- For now, we don't look inside ForAlls, PredTys
660 go _ _ = orig_ty1 -- Default
663 Note [Matching kinds]
664 ~~~~~~~~~~~~~~~~~~~~~
665 The target type might legitimately not be a sub-kind of template.
666 For example, suppose the target is simply a box with an OpenTypeKind,
667 and the template is a type variable with LiftedTypeKind.
668 Then it's ok (because the target type will later be refined).
669 We simply don't bind the template type variable.
671 It might also be that the kind mis-match is an error. For example,
672 suppose we match the template (a -> Int) against (Int# -> Int),
673 where the template type variable 'a' has LiftedTypeKind. This
674 matching function does not fail; it simply doesn't bind the template.
675 Later stuff will fail.
677 %************************************************************************
681 %************************************************************************
683 All the tcSub calls have the form
685 tcSub actual_ty expected_ty
687 actual_ty <= expected_ty
689 That is, that a value of type actual_ty is acceptable in
690 a place expecting a value of type expected_ty.
692 It returns a coercion function
693 co_fn :: actual_ty ~ expected_ty
694 which takes an HsExpr of type actual_ty into one of type
699 tcSubExp :: InstOrigin -> BoxySigmaType -> BoxySigmaType -> TcM HsWrapper
700 -- (tcSub act exp) checks that
702 tcSubExp orig actual_ty expected_ty
703 = -- addErrCtxtM (unifyCtxt actual_ty expected_ty) $
704 -- Adding the error context here leads to some very confusing error
705 -- messages, such as "can't match forall a. a->a with forall a. a->a"
706 -- Example is tcfail165:
707 -- do var <- newEmptyMVar :: IO (MVar (forall a. Show a => a -> String))
708 -- putMVar var (show :: forall a. Show a => a -> String)
709 -- Here the info does not flow from the 'var' arg of putMVar to its 'show' arg
710 -- but after zonking it looks as if it does!
712 -- So instead I'm adding the error context when moving from tc_sub to u_tys
714 traceTc (text "tcSubExp" <+> ppr actual_ty <+> ppr expected_ty) >>
715 tc_sub orig actual_ty actual_ty False expected_ty expected_ty
719 -> BoxySigmaType -- actual_ty, before expanding synonyms
720 -> BoxySigmaType -- ..and after
721 -> InBox -- True <=> expected_ty is inside a box
722 -> BoxySigmaType -- expected_ty, before
723 -> BoxySigmaType -- ..and after
725 -- The acual_ty is never inside a box
726 -- IMPORTANT pre-condition: if the args contain foralls, the bound type
727 -- variables are visible non-monadically
728 -- (i.e. tha args are sufficiently zonked)
729 -- This invariant is needed so that we can "see" the foralls, ad
730 -- e.g. in the SPEC rule where we just use splitSigmaTy
732 tc_sub orig act_sty act_ty exp_ib exp_sty exp_ty
733 = traceTc (text "tc_sub" <+> ppr act_ty $$ ppr exp_ty) >>
734 tc_sub1 orig act_sty act_ty exp_ib exp_sty exp_ty
735 -- This indirection is just here to make
736 -- it easy to insert a debug trace!
738 tc_sub1 :: InstOrigin -> BoxySigmaType -> BoxySigmaType -> InBox
739 -> BoxySigmaType -> Type -> TcM HsWrapper
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 (Unify True act_sty exp_sty) 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 Nothing $ \ _ 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 _ 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 _ (FunTy act_arg act_res) exp_ib _ (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 _ = 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 :: InstOrigin -> TcType -> TcType -> InBox
858 -> TcType -> TcType -> TcM HsWrapper
859 defer_to_boxy_matching orig act_sty actual_ty exp_ib exp_sty expected_ty
860 = do { coi <- addSubCtxt orig act_sty exp_sty $
861 u_tys (Unify True act_sty exp_sty)
862 False act_sty actual_ty exp_ib exp_sty expected_ty
863 ; return $ coiToHsWrapper coi }
865 -----------------------------------
866 tc_sub_funs :: InstOrigin -> TcType -> BoxySigmaType -> InBox
867 -> TcType -> BoxySigmaType -> TcM HsWrapper
868 tc_sub_funs orig act_arg act_res exp_ib exp_arg exp_res
869 = do { arg_coi <- addSubCtxt orig act_arg exp_arg $
870 uTysOuter False act_arg exp_ib exp_arg
871 ; co_fn_res <- tc_sub orig act_res act_res exp_ib exp_res exp_res
872 ; wrapper1 <- wrapFunResCoercion [exp_arg] co_fn_res
873 ; let wrapper2 = case arg_coi of
875 ACo co -> WpCast $ FunTy co act_res
876 ; return (wrapper1 <.> wrapper2) }
878 -----------------------------------
880 :: [TcType] -- Type of args
881 -> HsWrapper -- HsExpr a -> HsExpr b
882 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
883 wrapFunResCoercion arg_tys co_fn_res
884 | isIdHsWrapper co_fn_res
889 = do { arg_ids <- newSysLocalIds (fsLit "sub") arg_tys
890 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpApps arg_ids) }
895 %************************************************************************
897 \subsection{Generalisation}
899 %************************************************************************
902 tcGen :: BoxySigmaType -- expected_ty
903 -> TcTyVarSet -- Extra tyvars that the universally
904 -- quantified tyvars of expected_ty
905 -- must not be unified
906 -> Maybe UserTypeCtxt -- Just ctxt => this polytype arose directly
907 -- from a user type sig
908 -- Nothing => a higher order situation
909 -> ([TcTyVar] -> BoxyRhoType -> TcM result)
910 -> TcM (HsWrapper, result)
911 -- The expression has type: spec_ty -> expected_ty
913 tcGen expected_ty extra_tvs mb_ctxt thing_inside -- We expect expected_ty to be a forall-type
914 -- If not, the call is a no-op
915 = do { traceTc (text "tcGen")
916 ; ((tvs', theta', rho'), skol_info) <- instantiate expected_ty
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 $
928 thing_inside tvs' rho'
930 -- Check that the "forall_tvs" havn't been constrained
931 -- The interesting bit here is that we must include the free variables
932 -- of the expected_ty. Here's an example:
933 -- runST (newVar True)
934 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
935 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
936 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
937 -- So now s' isn't unconstrained because it's linked to a.
938 -- Conclusion: include the free vars of the expected_ty in the
939 -- list of "free vars" for the signature check.
941 ; loc <- getInstLoc (SigOrigin skol_info)
942 ; dicts <- newDictBndrs loc theta' -- Includes equalities
943 ; inst_binds <- tcSimplifyCheck loc tvs' dicts lie
945 ; checkSigTyVarsWrt free_tvs tvs'
946 ; traceTc (text "tcGen:done")
949 -- The WpLet binds any Insts which came out of the simplification.
950 dict_vars = map instToVar dicts
951 co_fn = mkWpTyLams tvs' <.> mkWpLams dict_vars <.> WpLet inst_binds
952 ; return (co_fn, result) }
954 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
956 instantiate :: TcType -> TcM (([TcTyVar],ThetaType,TcRhoType), SkolemInfo)
957 instantiate expected_ty
958 | Just ctxt <- mb_ctxt -- This case split is the wohle reason for mb_ctxt
959 = do { let skol_info = SigSkol ctxt
960 ; stuff <- tcInstSigType True skol_info expected_ty
961 ; return (stuff, skol_info) }
963 | otherwise -- We want the GenSkol info in the skolemised type variables to
964 -- mention the *instantiated* tyvar names, so that we get a
965 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
966 -- Hence the tiresome but innocuous fixM
967 = fixM $ \ ~(_, skol_info) ->
968 do { stuff@(forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
969 -- Get loation from *monad*, not from expected_ty
970 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty)
971 ; return (stuff, skol_info) }
976 %************************************************************************
980 %************************************************************************
982 The exported functions are all defined as versions of some
983 non-exported generic functions.
986 boxyUnify :: BoxyType -> BoxyType -> TcM CoercionI
987 -- Acutal and expected, respectively
988 boxyUnify ty1 ty2 = addErrCtxtM (unifyCtxt ty1 ty2) $
989 uTysOuter False ty1 False ty2
992 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM [CoercionI]
993 -- Arguments should have equal length
994 -- Acutal and expected types
995 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
998 unifyType :: TcTauType -> TcTauType -> TcM CoercionI
999 -- No boxes expected inside these types
1000 -- Acutal and expected types
1001 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
1002 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
1003 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
1004 addErrCtxtM (unifyCtxt ty1 ty2) $
1005 uTysOuter True ty1 True ty2
1008 unifyPred :: PredType -> PredType -> TcM CoercionI
1009 -- Acutal and expected types
1010 unifyPred p1 p2 = uPred (Unify False (mkPredTy p1) (mkPredTy p2)) True p1 True p2
1012 unifyTheta :: TcThetaType -> TcThetaType -> TcM [CoercionI]
1013 -- Acutal and expected types
1014 unifyTheta theta1 theta2
1015 = do { checkTc (equalLength theta1 theta2)
1016 (vcat [ptext (sLit "Contexts differ in length"),
1017 nest 2 $ parens $ ptext (sLit "Use -XRelaxedPolyRec to allow this")])
1018 ; uList unifyPred theta1 theta2
1022 uList :: (a -> a -> TcM b)
1023 -> [a] -> [a] -> TcM [b]
1024 -- Unify corresponding elements of two lists of types, which
1025 -- should be of equal length. We charge down the list explicitly so that
1026 -- we can complain if their lengths differ.
1027 uList _ [] [] = return []
1028 uList unify (ty1:tys1) (ty2:tys2) = do { x <- unify ty1 ty2;
1029 ; xs <- uList unify tys1 tys2
1032 uList _ _ _ = panic "Unify.uList: mismatched type lists!"
1035 @unifyTypeList@ takes a single list of @TauType@s and unifies them
1036 all together. It is used, for example, when typechecking explicit
1037 lists, when all the elts should be of the same type.
1040 unifyTypeList :: [TcTauType] -> TcM ()
1041 unifyTypeList [] = return ()
1042 unifyTypeList [_] = return ()
1043 unifyTypeList (ty1:tys@(ty2:_)) = do { _ <- unifyType ty1 ty2
1044 ; unifyTypeList tys }
1047 %************************************************************************
1049 \subsection[Unify-uTys]{@uTys@: getting down to business}
1051 %************************************************************************
1053 @uTys@ is the heart of the unifier. Each arg occurs twice, because
1054 we want to report errors in terms of synomyms if possible. The first of
1055 the pair is used in error messages only; it is always the same as the
1056 second, except that if the first is a synonym then the second may be a
1057 de-synonym'd version. This way we get better error messages.
1059 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
1062 type SwapFlag = Bool
1063 -- False <=> the two args are (actual, expected) respectively
1064 -- True <=> the two args are (expected, actual) respectively
1066 type InBox = Bool -- True <=> we are inside a box
1067 -- False <=> we are outside a box
1068 -- The importance of this is that if we get "filled-box meets
1069 -- filled-box", we'll look into the boxes and unify... but
1070 -- we must not allow polytypes. But if we are in a box on
1071 -- just one side, then we can allow polytypes
1073 data Outer = Unify Bool TcType TcType
1074 -- If there is a unification error, report these types as mis-matching
1075 -- Bool = True <=> the context says "Expected = ty1, Acutal = ty2"
1076 -- for this particular ty1,ty2
1078 instance Outputable Outer where
1079 ppr (Unify c ty1 ty2) = pp_c <+> pprParendType ty1 <+> ptext (sLit "~")
1080 <+> pprParendType ty2
1082 pp_c = if c then ptext (sLit "Top") else ptext (sLit "NonTop")
1085 -------------------------
1086 uTysOuter :: InBox -> TcType -- ty1 is the *actual* type
1087 -> InBox -> TcType -- ty2 is the *expected* type
1089 -- We've just pushed a context describing ty1,ty2
1090 uTysOuter nb1 ty1 nb2 ty2
1091 = do { traceTc (text "uTysOuter" <+> ppr ty1 <+> ppr ty2)
1092 ; u_tys (Unify True ty1 ty2) nb1 ty1 ty1 nb2 ty2 ty2 }
1094 uTys :: InBox -> TcType -> InBox -> TcType -> TcM CoercionI
1095 -- The context does not describe ty1,ty2
1096 uTys nb1 ty1 nb2 ty2
1097 = do { traceTc (text "uTys" <+> ppr ty1 <+> ppr ty2)
1098 ; u_tys (Unify False ty1 ty2) nb1 ty1 ty1 nb2 ty2 ty2 }
1103 -> InBox -> [TcType] -- tys1 are the *actual* types
1104 -> InBox -> [TcType] -- tys2 are the *expected* types
1106 uTys_s outer nb1 tys1 nb2 tys2
1109 go [] [] = return []
1110 go (ty1:tys1) (ty2:tys2) = do { coi <- uTys nb1 ty1 nb2 ty2
1111 ; cois <- go tys1 tys2
1112 ; return (coi:cois) }
1113 go _ _ = unifyMisMatch outer
1114 -- See Note [Mismatched type lists and application decomposition]
1118 -> InBox -> TcType -> TcType -- ty1 is the *actual* type
1119 -> InBox -> TcType -> TcType -- ty2 is the *expected* type
1122 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
1123 = do { traceTc (text "u_tys " <+> vcat [sep [ braces (ppr orig_ty1 <+> text "/" <+> ppr ty1),
1125 braces (ppr orig_ty2 <+> text "/" <+> ppr ty2)],
1127 ; coi <- go outer orig_ty1 ty1 orig_ty2 ty2
1128 ; traceTc (case coi of
1129 ACo co -> text "u_tys yields coercion:" <+> ppr co
1130 IdCo -> text "u_tys yields no coercion")
1134 bale_out :: Outer -> TcM a
1135 bale_out outer = unifyMisMatch outer
1136 -- We report a mis-match in terms of the original arugments to
1137 -- u_tys, even though 'go' has recursed inwards somewhat
1139 -- Note [Unifying AppTy]
1140 -- A case in point is unifying (m Int) ~ (IO Int)
1141 -- where m is a unification variable that is now bound to (say) (Bool ->)
1142 -- Then we want to report "Can't unify (Bool -> Int) with (IO Int)
1143 -- and not "Can't unify ((->) Bool) with IO"
1145 go :: Outer -> TcType -> TcType -> TcType -> TcType -> TcM CoercionI
1146 -- Always expand synonyms: see Note [Unification and synonyms]
1147 -- (this also throws away FTVs)
1148 go _ sty1 ty1 sty2 ty2
1149 | Just ty1' <- tcView ty1 = go (Unify False ty1' ty2 ) sty1 ty1' sty2 ty2
1150 | Just ty2' <- tcView ty2 = go (Unify False ty1 ty2') sty1 ty1 sty2 ty2'
1152 -- Variables; go for uVar
1153 go outer _ (TyVarTy tyvar1) sty2 ty2 = uVar outer False tyvar1 nb2 sty2 ty2
1154 go outer sty1 ty1 _ (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 sty1 ty1
1155 -- "True" means args swapped
1157 -- The case for sigma-types must *follow* the variable cases
1158 -- because a boxy variable can be filed with a polytype;
1159 -- but must precede FunTy, because ((?x::Int) => ty) look
1160 -- like a FunTy; there isn't necy a forall at the top
1162 | isSigmaTy ty1 || isSigmaTy ty2
1163 = do { traceTc (text "We have sigma types: equalLength" <+> ppr tvs1 <+> ppr tvs2)
1164 ; unless (equalLength tvs1 tvs2) (bale_out outer)
1165 ; traceTc (text "We're past the first length test")
1166 ; tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
1167 -- Get location from monad, not from tvs1
1168 ; let tys = mkTyVarTys tvs
1169 in_scope = mkInScopeSet (mkVarSet tvs)
1170 phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
1171 phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
1172 (theta1,tau1) = tcSplitPhiTy phi1
1173 (theta2,tau2) = tcSplitPhiTy phi2
1175 ; addErrCtxtM (unifyForAllCtxt tvs phi1 phi2) $ do
1176 { unless (equalLength theta1 theta2) (bale_out outer)
1177 ; _cois <- uPreds outer nb1 theta1 nb2 theta2 -- TOMDO: do something with these pred_cois
1178 ; traceTc (text "TOMDO!")
1179 ; coi <- uTys nb1 tau1 nb2 tau2
1181 -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
1182 ; free_tvs <- zonkTcTyVarsAndFV (varSetElems (tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2))
1183 ; when (any (`elemVarSet` free_tvs) tvs)
1184 (bleatEscapedTvs free_tvs tvs tvs)
1186 -- If both sides are inside a box, we are in a "box-meets-box"
1187 -- situation, and we should not have a polytype at all.
1188 -- If we get here we have two boxes, already filled with
1189 -- the same polytype... but it should be a monotype.
1190 -- This check comes last, because the error message is
1191 -- extremely unhelpful.
1192 ; when (nb1 && nb2) (notMonoType ty1)
1196 (tvs1, body1) = tcSplitForAllTys ty1
1197 (tvs2, body2) = tcSplitForAllTys ty2
1200 go outer _ (PredTy p1) _ (PredTy p2)
1201 = uPred outer nb1 p1 nb2 p2
1203 -- Non-synonym type constructors must match
1204 go outer _ (TyConApp con1 tys1) _ (TyConApp con2 tys2)
1205 | con1 == con2 && not (isOpenSynTyCon con1)
1206 = do { traceTc (text "utys1" <+> ppr con1 <+> (ppr tys1 $$ ppr tys2))
1207 ; cois <- uTys_s outer nb1 tys1 nb2 tys2
1208 ; return $ mkTyConAppCoI con1 tys1 cois
1210 -- Family synonyms See Note [TyCon app]
1211 | con1 == con2 && identicalOpenSynTyConApp
1212 = do { traceTc (text "utys2" <+> ppr con1 <+> (ppr tys1' $$ ppr tys2'))
1213 ; cois <- uTys_s outer nb1 tys1' nb2 tys2'
1214 ; return $ mkTyConAppCoI con1 tys1 (replicate n IdCo ++ cois)
1218 (idxTys1, tys1') = splitAt n tys1
1219 (idxTys2, tys2') = splitAt n tys2
1220 identicalOpenSynTyConApp = idxTys1 `tcEqTypes` idxTys2
1221 -- See Note [OpenSynTyCon app]
1223 -- Functions; just check the two parts
1224 go _ _ (FunTy fun1 arg1) _ (FunTy fun2 arg2)
1225 = do { coi_l <- uTys nb1 fun1 nb2 fun2
1226 ; coi_r <- uTys nb1 arg1 nb2 arg2
1227 ; return $ mkFunTyCoI fun1 coi_l arg1 coi_r
1230 -- Applications need a bit of care!
1231 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
1232 -- NB: we've already dealt with type variables and Notes,
1233 -- so if one type is an App the other one jolly well better be too
1234 -- See Note [Mismatched type lists and application decomposition]
1235 go outer _ (AppTy s1 t1) _ ty2
1236 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
1237 = do { coi_s <- go outer s1 s1 s2 s2 -- NB recurse into go...
1238 ; coi_t <- uTys nb1 t1 nb2 t2 -- See Note [Unifying AppTy]
1239 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1241 -- Now the same, but the other way round
1242 -- Don't swap the types, because the error messages get worse
1243 go outer _ ty1 _ (AppTy s2 t2)
1244 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
1245 = do { coi_s <- go outer s1 s1 s2 s2
1246 ; coi_t <- uTys nb1 t1 nb2 t2
1247 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1249 -- If we can reduce a family app => proceed with reduct
1250 -- NB1: We use isOpenSynTyCon, not isOpenSynTyConApp as we also must
1251 -- defer oversaturated applications!
1253 -- NB2: Do this *after* trying decomposing applications, so that decompose
1254 -- (m a) ~ (F Int b)
1255 -- where F has arity 1
1256 go _ _ ty1@(TyConApp con1 _) _ ty2
1257 | isOpenSynTyCon con1
1258 = do { (coi1, ty1') <- tcNormaliseFamInst ty1
1260 IdCo -> defer -- no reduction, see [Deferred Unification]
1261 _ -> liftM (coi1 `mkTransCoI`) $ uTys nb1 ty1' nb2 ty2
1264 go _ _ ty1 _ ty2@(TyConApp con2 _)
1265 | isOpenSynTyCon con2
1266 = do { (coi2, ty2') <- tcNormaliseFamInst ty2
1268 IdCo -> defer -- no reduction, see [Deferred Unification]
1269 _ -> liftM (`mkTransCoI` mkSymCoI coi2) $
1270 uTys nb1 ty1 nb2 ty2'
1273 -- Anything else fails
1274 go outer _ _ _ _ = bale_out outer
1276 defer = defer_unification outer False orig_ty1 orig_ty2
1280 uPred :: Outer -> InBox -> PredType -> InBox -> PredType -> TcM CoercionI
1281 uPred _ nb1 (IParam n1 t1) nb2 (IParam n2 t2)
1283 = do { coi <- uTys nb1 t1 nb2 t2
1284 ; return $ mkIParamPredCoI n1 coi }
1285 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
1287 = do { traceTc (text "utys3" <+> ppr c1 <+> (ppr tys2 $$ ppr tys2))
1288 ; cois <- uTys_s outer nb1 tys1 nb2 tys2
1289 ; return $ mkClassPPredCoI c1 tys1 cois }
1290 uPred outer _ _ _ _ = unifyMisMatch outer
1292 uPreds :: Outer -> InBox -> [PredType] -> InBox -> [PredType]
1294 uPreds _ _ [] _ [] = return []
1295 uPreds outer nb1 (p1:ps1) nb2 (p2:ps2) =
1296 do { coi <- uPred outer nb1 p1 nb2 p2
1297 ; cois <- uPreds outer nb1 ps1 nb2 ps2
1300 uPreds _ _ _ _ _ = panic "uPreds"
1303 Note [Mismatched type lists and application decomposition]
1304 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1305 When we find two TyConApps, you might think that the argument lists
1306 are guaranteed equal length. But they aren't. Consider matching
1307 w (T x) ~ Foo (T x y)
1308 We do match (w ~ Foo) first, but in some circumstances we simply create
1309 a deferred constraint; and then go ahead and match (T x ~ T x y).
1310 This came up in Trac #3950.
1313 (a) either we must check for identical argument kinds
1314 when decomposing applications,
1316 (b) or we must be prepared for ill-kinded unification sub-problems
1318 Currently we adopt (b) since it seems more robust -- no need to maintain
1321 Note [OpenSynTyCon app]
1322 ~~~~~~~~~~~~~~~~~~~~~~~
1325 type family T a :: * -> *
1327 the two types (T () a) and (T () Int) must unify, even if there are
1328 no type instances for T at all. Should we just turn them into an
1329 equality (T () a ~ T () Int)? I don't think so. We currently try to
1330 eagerly unify everything we can before generating equalities; otherwise,
1331 we could turn the unification of [Int] with [a] into an equality, too.
1333 Note [Unification and synonyms]
1334 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1335 If you are tempted to make a short cut on synonyms, as in this
1339 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
1340 -- NO = if (con1 == con2) then
1341 -- NO -- Good news! Same synonym constructors, so we can shortcut
1342 -- NO -- by unifying their arguments and ignoring their expansions.
1343 -- NO unifyTypepeLists args1 args2
1345 -- NO -- Never mind. Just expand them and try again
1349 then THINK AGAIN. Here is the whole story, as detected and reported
1350 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1352 Here's a test program that should detect the problem:
1356 x = (1 :: Bogus Char) :: Bogus Bool
1359 The problem with [the attempted shortcut code] is that
1363 is not a sufficient condition to be able to use the shortcut!
1364 You also need to know that the type synonym actually USES all
1365 its arguments. For example, consider the following type synonym
1366 which does not use all its arguments.
1371 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1372 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1373 would fail, even though the expanded forms (both \tr{Int}) should
1376 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1377 unnecessarily bind \tr{t} to \tr{Char}.
1379 ... You could explicitly test for the problem synonyms and mark them
1380 somehow as needing expansion, perhaps also issuing a warning to the
1385 %************************************************************************
1387 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1389 %************************************************************************
1391 @uVar@ is called when at least one of the types being unified is a
1392 variable. It does {\em not} assume that the variable is a fixed point
1393 of the substitution; rather, notice that @uVar@ (defined below) nips
1394 back into @uTys@ if it turns out that the variable is already bound.
1398 -> SwapFlag -- False => tyvar is the "actual" (ty is "expected")
1399 -- True => ty is the "actual" (tyvar is "expected")
1401 -> InBox -- True <=> definitely no boxes in t2
1402 -> TcTauType -> TcTauType -- printing and real versions
1405 uVar outer swapped tv1 nb2 ps_ty2 ty2
1406 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1407 | otherwise = brackets (equals <+> ppr ty2)
1408 ; traceTc (text "uVar" <+> ppr outer <+> ppr swapped <+>
1409 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1410 nest 2 (ptext (sLit " <-> ")),
1411 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1412 ; details <- lookupTcTyVar tv1
1415 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1416 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1417 -- The 'True' here says that ty1 is now inside a box
1418 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1422 uUnfilledVar :: Outer
1424 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1425 -> TcTauType -> TcTauType -- Type 2
1427 -- Invariant: tyvar 1 is not unified with anything
1429 uUnfilledVar _ swapped tv1 details1 ps_ty2 ty2
1430 | Just ty2' <- tcView ty2
1431 = -- Expand synonyms; ignore FTVs
1432 let outer' | swapped = Unify False ty2' (mkTyVarTy tv1)
1433 | otherwise = Unify False (mkTyVarTy tv1) ty2'
1434 in uUnfilledVar outer' swapped tv1 details1 ps_ty2 ty2'
1436 uUnfilledVar outer swapped tv1 details1 _ (TyVarTy tv2)
1437 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1439 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1440 -- this is box-meets-box, so fill in with a tau-type
1441 -> do { tau_tv <- tcInstTyVar tv1
1442 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv)
1445 _ -> return IdCo -- No-op
1447 | otherwise -- Distinct type variables
1448 = do { lookup2 <- lookupTcTyVar tv2
1450 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
1451 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1454 uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2
1455 = -- ty2 is not a type variable
1457 MetaTv (SigTv _) _ -> rigid_variable
1458 MetaTv info ref1 -> uMetaVar outer swapped tv1 info ref1 ps_ty2 non_var_ty2
1459 SkolemTv _ -> rigid_variable
1462 | isOpenSynTyConApp non_var_ty2
1463 = -- 'non_var_ty2's outermost constructor is a type family,
1464 -- which we may may be able to normalise
1465 do { (coi2, ty2') <- tcNormaliseFamInst non_var_ty2
1467 IdCo -> -- no progress, but maybe after other instantiations
1468 defer_unification outer swapped (TyVarTy tv1) ps_ty2
1469 ACo co -> -- progress: so lets try again
1471 ppr co <+> text "::"<+> ppr non_var_ty2 <+> text "~" <+>
1473 ; coi <- uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2'
1474 ; let coi2' = (if swapped then id else mkSymCoI) coi2
1475 ; return $ coi2' `mkTransCoI` coi
1478 | SkolemTv RuntimeUnkSkol <- details1
1479 -- runtime unknown will never match
1480 = unifyMisMatch outer
1481 | otherwise -- defer as a given equality may still resolve this
1482 = defer_unification outer swapped (TyVarTy tv1) ps_ty2
1485 Note [Deferred Unification]
1486 ~~~~~~~~~~~~~~~~~~~~
1487 We may encounter a unification ty1 = ty2 that cannot be performed syntactically,
1488 and yet its consistency is undetermined. Previously, there was no way to still
1489 make it consistent. So a mismatch error was issued.
1491 Now these unfications are deferred until constraint simplification, where type
1492 family instances and given equations may (or may not) establish the consistency.
1493 Deferred unifications are of the form
1496 where F is a type function and x is a type variable.
1498 id :: x ~ y => x -> y
1501 involves the unfication x = y. It is deferred until we bring into account the
1502 context x ~ y to establish that it holds.
1504 If available, we defer original types (rather than those where closed type
1505 synonyms have already been expanded via tcCoreView). This is, as usual, to
1506 improve error messages.
1508 We need to both 'unBox' and zonk deferred types. We need to unBox as
1509 functions, such as TcExpr.tcMonoExpr promise to fill boxes in the expected
1510 type. We need to zonk as the types go into the kind of the coercion variable
1511 `cotv' and those are not zonked in Inst.zonkInst. (Maybe it would be better
1512 to zonk in zonInst instead. Would that be sufficient?)
1515 defer_unification :: Outer
1520 defer_unification outer True ty1 ty2
1521 = defer_unification outer False ty2 ty1
1522 defer_unification outer False ty1 ty2
1523 = do { ty1' <- unBox ty1 >>= zonkTcType -- unbox *and* zonk..
1524 ; ty2' <- unBox ty2 >>= zonkTcType -- ..see preceding note
1525 ; traceTc $ text "deferring:" <+> ppr ty1 <+> text "~" <+> ppr ty2
1526 ; cotv <- newMetaCoVar ty1' ty2'
1527 -- put ty1 ~ ty2 in LIE
1528 -- Left means "wanted"
1529 ; inst <- popUnifyCtxt outer $
1530 mkEqInst (EqPred ty1' ty2') (Left cotv)
1532 ; return $ ACo $ TyVarTy cotv }
1537 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1540 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1541 -- ty2 is not a type variable
1543 uMetaVar outer swapped tv1 BoxTv ref1 ps_ty2 ty2
1544 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1545 -- that any boxes in ty2 are filled with monotypes
1547 -- It should not be the case that tv1 occurs in ty2
1548 -- (i.e. no occurs check should be needed), but if perchance
1549 -- it does, the unbox operation will fill it, and the debug code
1551 do { final_ty <- unBox ps_ty2
1552 ; meta_details <- readMutVar ref1
1553 ; case meta_details of
1554 Indirect _ -> -- This *can* happen due to an occurs check,
1555 -- just as it can in checkTauTvUpdate in the next
1556 -- equation of uMetaVar; see Trac #2414
1557 -- Note [Occurs check]
1558 -- Go round again. Probably there's an immediate
1559 -- error, but maybe not (a type function might discard
1560 -- its argument). Next time round we'll end up in the
1561 -- TauTv case of uMetaVar.
1562 uVar outer swapped tv1 False ps_ty2 ty2
1563 -- Setting for nb2::InBox is irrelevant
1565 Flexi -> do { checkUpdateMeta swapped tv1 ref1 final_ty
1569 uMetaVar outer swapped tv1 _ ref1 ps_ty2 _
1570 = do { -- Occurs check + monotype check
1571 ; mb_final_ty <- checkTauTvUpdate tv1 ps_ty2
1572 ; case mb_final_ty of
1573 Nothing -> -- tv1 occured in type family parameter
1574 defer_unification outer swapped (mkTyVarTy tv1) ps_ty2
1576 do { checkUpdateMeta swapped tv1 ref1 final_ty
1581 {- Note [Occurs check]
1583 An eager occurs check is made in checkTauTvUpdate, deferring tricky
1584 cases by calling defer_unification (see notes with
1585 checkTauTvUpdate). An occurs check can also (and does) happen in the
1586 BoxTv case, but unBox doesn't check for occurrences, and in any case
1587 doesn't have the type-function-related complexity that
1588 checkTauTvUpdate has. So we content ourselves with spotting the potential
1589 occur check (by the fact that tv1 is now filled), and going round again.
1590 Next time round we'll get the TauTv case of uMetaVar.
1594 uUnfilledVars :: Outer
1596 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1597 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1599 -- Invarant: The type variables are distinct,
1600 -- Neither is filled in yet
1601 -- They might be boxy or not
1603 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1604 = -- see [Deferred Unification]
1605 defer_unification outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1607 uUnfilledVars _ swapped tv1 (MetaTv _ ref1) tv2 (SkolemTv _)
1608 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2) >> return IdCo
1609 uUnfilledVars _ swapped tv1 (SkolemTv _) tv2 (MetaTv _ ref2)
1610 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1) >> return IdCo
1612 -- ToDo: this function seems too long for what it acutally does!
1613 uUnfilledVars _ swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1614 = case (info1, info2) of
1615 (BoxTv, BoxTv) -> box_meets_box >> return IdCo
1617 -- If a box meets a TauTv, but the fomer has the smaller kind
1618 -- then we must create a fresh TauTv with the smaller kind
1619 (_, BoxTv) | k1_sub_k2 -> update_tv2 >> return IdCo
1620 | otherwise -> box_meets_box >> return IdCo
1621 (BoxTv, _ ) | k2_sub_k1 -> update_tv1 >> return IdCo
1622 | otherwise -> box_meets_box >> return IdCo
1624 -- Avoid SigTvs if poss
1625 (SigTv _, _ ) | k1_sub_k2 -> update_tv2 >> return IdCo
1626 (_, SigTv _) | k2_sub_k1 -> update_tv1 >> return IdCo
1628 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1629 then update_tv1 >> return IdCo -- Same kinds
1630 else update_tv2 >> return IdCo
1631 | k2_sub_k1 -> update_tv1 >> return IdCo
1632 | otherwise -> kind_err >> return IdCo
1634 -- Update the variable with least kind info
1635 -- See notes on type inference in Kind.lhs
1636 -- The "nicer to" part only applies if the two kinds are the same,
1637 -- so we can choose which to do.
1639 -- Kinds should be guaranteed ok at this point
1640 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1641 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1643 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1646 | k2_sub_k1 = fill_from tv2
1647 | otherwise = kind_err
1649 -- Update *both* tyvars with a TauTv whose name and kind
1650 -- are gotten from tv (avoid losing nice names is poss)
1651 fill_from tv = do { tv' <- tcInstTyVar tv
1652 ; let tau_ty = mkTyVarTy tv'
1653 ; updateMeta tv1 ref1 tau_ty
1654 ; updateMeta tv2 ref2 tau_ty }
1656 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1657 unifyKindMisMatch k1 k2
1661 k1_sub_k2 = k1 `isSubKind` k2
1662 k2_sub_k1 = k2 `isSubKind` k1
1664 nicer_to_update_tv1 = isSystemName (Var.varName tv1)
1665 -- Try to update sys-y type variables in preference to ones
1666 -- gotten (say) by instantiating a polymorphic function with
1667 -- a user-written type sig
1671 refineBox :: TcType -> TcM TcType
1672 -- Unbox the outer box of a boxy type (if any)
1673 refineBox ty@(TyVarTy box_tv)
1674 | isMetaTyVar box_tv
1675 = do { cts <- readMetaTyVar box_tv
1678 Indirect ty -> return ty }
1679 refineBox other_ty = return other_ty
1681 refineBoxToTau :: TcType -> TcM TcType
1682 -- Unbox the outer box of a boxy type, filling with a monotype if it is empty
1683 -- Like refineBox except for the "fill with monotype" part.
1684 refineBoxToTau (TyVarTy box_tv)
1685 | isMetaTyVar box_tv
1686 , MetaTv BoxTv ref <- tcTyVarDetails box_tv
1687 = do { cts <- readMutVar ref
1689 Flexi -> fillBoxWithTau box_tv ref
1690 Indirect ty -> return ty }
1691 refineBoxToTau other_ty = return other_ty
1693 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1694 -- Subtle... we must zap the boxy res_ty
1695 -- to kind * before using it to instantiate a LitInst
1696 -- Calling unBox instead doesn't do the job, because the box
1697 -- often has an openTypeKind, and we don't want to instantiate
1699 zapToMonotype res_ty
1700 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1701 ; _ <- boxyUnify res_tau res_ty
1704 unBox :: BoxyType -> TcM TcType
1705 -- unBox implements the judgement
1707 -- with input s', and result s
1709 -- It removes all boxes from the input type, returning a non-boxy type.
1710 -- A filled box in the type can only contain a monotype; unBox fails if not
1711 -- The type can have empty boxes, which unBox fills with a monotype
1713 -- Compare this wth checkTauTvUpdate
1715 -- For once, it's safe to treat synonyms as opaque!
1717 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1718 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1719 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1720 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1721 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1722 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1724 | isTcTyVar tv -- It's a boxy type variable
1725 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1726 = do { cts <- readMutVar ref -- under nested quantifiers
1728 Flexi -> fillBoxWithTau tv ref
1729 Indirect ty -> do { non_boxy_ty <- unBox ty
1730 ; if isTauTy non_boxy_ty
1731 then return non_boxy_ty
1732 else notMonoType non_boxy_ty }
1734 | otherwise -- Skolems, and meta-tau-variables
1735 = return (TyVarTy tv)
1737 unBoxPred :: PredType -> TcM PredType
1738 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1739 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1740 unBoxPred (EqPred ty1 ty2) = do { ty1' <- unBox ty1; ty2' <- unBox ty2; return (EqPred ty1' ty2') }
1745 %************************************************************************
1749 %************************************************************************
1752 unifyMisMatch :: Outer -> TcM a
1753 unifyMisMatch (Unify is_outer ty1 ty2)
1754 | is_outer = popErrCtxt $ failWithMisMatch ty1 ty2 -- This is the whole point of the 'outer' stuff
1755 | otherwise = failWithMisMatch ty1 ty2
1757 popUnifyCtxt :: Outer -> TcM a -> TcM a
1758 popUnifyCtxt (Unify True _ _) thing = popErrCtxt thing
1759 popUnifyCtxt (Unify False _ _) thing = thing
1761 -----------------------
1762 unifyCtxt :: TcType -> TcType -> TidyEnv -> TcM (TidyEnv, SDoc)
1763 unifyCtxt act_ty exp_ty tidy_env
1764 = do { act_ty' <- zonkTcType act_ty
1765 ; exp_ty' <- zonkTcType exp_ty
1766 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1767 (env2, act_ty'') = tidyOpenType env1 act_ty'
1768 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1771 mkExpectedActualMsg :: Type -> Type -> SDoc
1772 mkExpectedActualMsg act_ty exp_ty
1773 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1774 text "Inferred type" <> colon <+> ppr act_ty ])
1777 -- If an error happens we try to figure out whether the function
1778 -- function has been given too many or too few arguments, and say so.
1779 addSubCtxt :: InstOrigin -> TcType -> TcType -> TcM a -> TcM a
1780 addSubCtxt orig actual_res_ty expected_res_ty thing_inside
1781 = addErrCtxtM mk_err thing_inside
1784 = do { exp_ty' <- zonkTcType expected_res_ty
1785 ; act_ty' <- zonkTcType actual_res_ty
1786 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1787 (env2, act_ty'') = tidyOpenType env1 act_ty'
1788 (exp_args, _) = tcSplitFunTys exp_ty''
1789 (act_args, _) = tcSplitFunTys act_ty''
1791 len_act_args = length act_args
1792 len_exp_args = length exp_args
1794 message = case orig of
1796 | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1797 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1798 _ -> mkExpectedActualMsg act_ty'' exp_ty''
1799 ; return (env2, message) }
1801 wrongArgsCtxt too_many_or_few fun
1802 = ptext (sLit "Probable cause:") <+> quotes (ppr fun)
1803 <+> ptext (sLit "is applied to") <+> text too_many_or_few
1804 <+> ptext (sLit "arguments")
1807 unifyForAllCtxt :: [TyVar] -> Type -> Type -> TidyEnv -> TcM (TidyEnv, SDoc)
1808 unifyForAllCtxt tvs phi1 phi2 env
1809 = return (env2, msg)
1811 (env', tvs') = tidyOpenTyVars env tvs -- NB: not tidyTyVarBndrs
1812 (env1, phi1') = tidyOpenType env' phi1
1813 (env2, phi2') = tidyOpenType env1 phi2
1814 msg = vcat [ptext (sLit "When matching") <+> quotes (ppr (mkForAllTys tvs' phi1')),
1815 ptext (sLit " and") <+> quotes (ppr (mkForAllTys tvs' phi2'))]
1820 %************************************************************************
1824 %************************************************************************
1826 Unifying kinds is much, much simpler than unifying types.
1829 unifyKind :: TcKind -- Expected
1832 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1833 | isSubKindCon kc2 kc1 = return ()
1835 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1836 = do { unifyKind a2 a1; unifyKind r1 r2 }
1837 -- Notice the flip in the argument,
1838 -- so that the sub-kinding works right
1839 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1840 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1841 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1843 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1844 unifyKinds [] [] = return ()
1845 unifyKinds (k1:ks1) (k2:ks2) = do unifyKind k1 k2
1847 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1850 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1851 uKVar swapped kv1 k2
1852 = do { mb_k1 <- readKindVar kv1
1854 Flexi -> uUnboundKVar swapped kv1 k2
1855 Indirect k1 | swapped -> unifyKind k2 k1
1856 | otherwise -> unifyKind k1 k2 }
1859 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1860 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1861 | kv1 == kv2 = return ()
1862 | otherwise -- Distinct kind variables
1863 = do { mb_k2 <- readKindVar kv2
1865 Indirect k2 -> uUnboundKVar swapped kv1 k2
1866 Flexi -> writeKindVar kv1 k2 }
1868 uUnboundKVar swapped kv1 non_var_k2
1869 = do { k2' <- zonkTcKind non_var_k2
1870 ; kindOccurCheck kv1 k2'
1871 ; k2'' <- kindSimpleKind swapped k2'
1872 -- KindVars must be bound only to simple kinds
1873 -- Polarities: (kindSimpleKind True ?) succeeds
1874 -- returning *, corresponding to unifying
1877 ; writeKindVar kv1 k2'' }
1880 kindOccurCheck :: TyVar -> Type -> TcM ()
1881 kindOccurCheck kv1 k2 -- k2 is zonked
1882 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1884 not_in (TyVarTy kv2) = kv1 /= kv2
1885 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1888 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1889 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1890 -- If the flag is False, it requires k <: sk
1891 -- E.g. kindSimpleKind False ?? = *
1892 -- What about (kv -> *) ~ ?? -> *
1893 kindSimpleKind orig_swapped orig_kind
1894 = go orig_swapped orig_kind
1896 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1898 ; return (mkArrowKind k1' k2') }
1900 | isOpenTypeKind k = return liftedTypeKind
1901 | isArgTypeKind k = return liftedTypeKind
1903 | isLiftedTypeKind k = return liftedTypeKind
1904 | isUnliftedTypeKind k = return unliftedTypeKind
1905 go _ k@(TyVarTy _) = return k -- KindVars are always simple
1906 go _ _ = failWithTc (ptext (sLit "Unexpected kind unification failure:")
1907 <+> ppr orig_swapped <+> ppr orig_kind)
1908 -- I think this can't actually happen
1910 -- T v = MkT v v must be a type
1911 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1914 kindOccurCheckErr :: Var -> Type -> SDoc
1915 kindOccurCheckErr tyvar ty
1916 = hang (ptext (sLit "Occurs check: cannot construct the infinite kind:"))
1917 2 (sep [ppr tyvar, char '=', ppr ty])
1921 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1922 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1924 unifyFunKind (TyVarTy kvar) = do
1925 maybe_kind <- readKindVar kvar
1927 Indirect fun_kind -> unifyFunKind fun_kind
1929 do { arg_kind <- newKindVar
1930 ; res_kind <- newKindVar
1931 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1932 ; return (Just (arg_kind,res_kind)) }
1934 unifyFunKind (FunTy arg_kind res_kind) = return (Just (arg_kind,res_kind))
1935 unifyFunKind _ = return Nothing
1938 %************************************************************************
1940 \subsection{Checking signature type variables}
1942 %************************************************************************
1944 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1945 are not mentioned in the environment. In particular:
1947 (a) Not mentioned in the type of a variable in the envt
1948 eg the signature for f in this:
1954 Here, f is forced to be monorphic by the free occurence of x.
1956 (d) Not (unified with another type variable that is) in scope.
1957 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1958 when checking the expression type signature, we find that
1959 even though there is nothing in scope whose type mentions r,
1960 nevertheless the type signature for the expression isn't right.
1962 Another example is in a class or instance declaration:
1964 op :: forall b. a -> b
1966 Here, b gets unified with a
1968 Before doing this, the substitution is applied to the signature type variable.
1971 checkSigTyVars :: [TcTyVar] -> TcM ()
1972 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1974 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1975 -- The extra_tvs can include boxy type variables;
1976 -- e.g. TcMatches.tcCheckExistentialPat
1977 checkSigTyVarsWrt extra_tvs sig_tvs
1978 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1979 ; check_sig_tyvars extra_tvs' sig_tvs }
1982 :: TcTyVarSet -- Global type variables. The universally quantified
1983 -- tyvars should not mention any of these
1984 -- Guaranteed already zonked.
1985 -> [TcTyVar] -- Universally-quantified type variables in the signature
1986 -- Guaranteed to be skolems
1988 check_sig_tyvars _ []
1990 check_sig_tyvars extra_tvs sig_tvs
1991 = ASSERT( all isTcTyVar sig_tvs && all isSkolemTyVar sig_tvs )
1992 do { gbl_tvs <- tcGetGlobalTyVars
1993 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1994 text "gbl_tvs" <+> ppr gbl_tvs,
1995 text "extra_tvs" <+> ppr extra_tvs]))
1997 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1998 ; when (any (`elemVarSet` env_tvs) sig_tvs)
1999 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
2002 bleatEscapedTvs :: TcTyVarSet -- The global tvs
2003 -> [TcTyVar] -- The possibly-escaping type variables
2004 -> [TcTyVar] -- The zonked versions thereof
2006 -- Complain about escaping type variables
2007 -- We pass a list of type variables, at least one of which
2008 -- escapes. The first list contains the original signature type variable,
2009 -- while the second contains the type variable it is unified to (usually itself)
2010 bleatEscapedTvs globals sig_tvs zonked_tvs
2011 = do { env0 <- tcInitTidyEnv
2012 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
2013 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
2015 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
2016 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
2018 main_msg = ptext (sLit "Inferred type is less polymorphic than expected")
2020 check (tidy_env, msgs) (sig_tv, zonked_tv)
2021 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
2023 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
2024 ; return (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
2026 -----------------------
2027 escape_msg :: Var -> Var -> [SDoc] -> SDoc
2028 escape_msg sig_tv zonked_tv globs
2030 = vcat [sep [msg, ptext (sLit "is mentioned in the environment:")],
2031 nest 2 (vcat globs)]
2033 = msg <+> ptext (sLit "escapes")
2034 -- Sigh. It's really hard to give a good error message
2035 -- all the time. One bad case is an existential pattern match.
2036 -- We rely on the "When..." context to help.
2038 msg = ptext (sLit "Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
2040 | sig_tv == zonked_tv = empty
2041 | otherwise = ptext (sLit "is unified with") <+> quotes (ppr zonked_tv) <+> ptext (sLit "which")
2044 These two context are used with checkSigTyVars
2047 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
2048 -> TidyEnv -> TcM (TidyEnv, Message)
2049 sigCtxt id sig_tvs sig_theta sig_tau tidy_env = do
2050 actual_tau <- zonkTcType sig_tau
2052 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
2053 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
2054 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
2055 sub_msg = vcat [ptext (sLit "Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
2056 ptext (sLit "Type to generalise:") <+> pprType tidy_actual_tau
2058 msg = vcat [ptext (sLit "When trying to generalise the type inferred for") <+> quotes (ppr id),