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 }
1102 uTys_s :: InBox -> [TcType] -- tys1 are the *actual* types
1103 -> InBox -> [TcType] -- tys2 are the *expected* types
1105 uTys_s _ [] _ [] = return []
1106 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { coi <- uTys nb1 ty1 nb2 ty2
1107 ; cois <- uTys_s nb1 tys1 nb2 tys2
1108 ; return (coi:cois) }
1109 uTys_s _ _ _ _ = panic "Unify.uTys_s: mismatched type lists!"
1113 -> InBox -> TcType -> TcType -- ty1 is the *actual* type
1114 -> InBox -> TcType -> TcType -- ty2 is the *expected* type
1117 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
1118 = do { traceTc (text "u_tys " <+> vcat [sep [ braces (ppr orig_ty1 <+> text "/" <+> ppr ty1),
1120 braces (ppr orig_ty2 <+> text "/" <+> ppr ty2)],
1122 ; coi <- go outer orig_ty1 ty1 orig_ty2 ty2
1123 ; traceTc (case coi of
1124 ACo co -> text "u_tys yields coercion:" <+> ppr co
1125 IdCo -> text "u_tys yields no coercion")
1129 bale_out :: Outer -> TcM a
1130 bale_out outer = unifyMisMatch outer
1131 -- We report a mis-match in terms of the original arugments to
1132 -- u_tys, even though 'go' has recursed inwards somewhat
1134 -- Note [Unifying AppTy]
1135 -- A case in point is unifying (m Int) ~ (IO Int)
1136 -- where m is a unification variable that is now bound to (say) (Bool ->)
1137 -- Then we want to report "Can't unify (Bool -> Int) with (IO Int)
1138 -- and not "Can't unify ((->) Bool) with IO"
1140 go :: Outer -> TcType -> TcType -> TcType -> TcType -> TcM CoercionI
1141 -- Always expand synonyms: see Note [Unification and synonyms]
1142 -- (this also throws away FTVs)
1143 go _ sty1 ty1 sty2 ty2
1144 | Just ty1' <- tcView ty1 = go (Unify False ty1' ty2 ) sty1 ty1' sty2 ty2
1145 | Just ty2' <- tcView ty2 = go (Unify False ty1 ty2') sty1 ty1 sty2 ty2'
1147 -- Variables; go for uVar
1148 go outer _ (TyVarTy tyvar1) sty2 ty2 = uVar outer False tyvar1 nb2 sty2 ty2
1149 go outer sty1 ty1 _ (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 sty1 ty1
1150 -- "True" means args swapped
1152 -- The case for sigma-types must *follow* the variable cases
1153 -- because a boxy variable can be filed with a polytype;
1154 -- but must precede FunTy, because ((?x::Int) => ty) look
1155 -- like a FunTy; there isn't necy a forall at the top
1157 | isSigmaTy ty1 || isSigmaTy ty2
1158 = do { traceTc (text "We have sigma types: equalLength" <+> ppr tvs1 <+> ppr tvs2)
1159 ; unless (equalLength tvs1 tvs2) (bale_out outer)
1160 ; traceTc (text "We're past the first length test")
1161 ; tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
1162 -- Get location from monad, not from tvs1
1163 ; let tys = mkTyVarTys tvs
1164 in_scope = mkInScopeSet (mkVarSet tvs)
1165 phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
1166 phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
1167 (theta1,tau1) = tcSplitPhiTy phi1
1168 (theta2,tau2) = tcSplitPhiTy phi2
1170 ; addErrCtxtM (unifyForAllCtxt tvs phi1 phi2) $ do
1171 { unless (equalLength theta1 theta2) (bale_out outer)
1172 ; _cois <- uPreds outer nb1 theta1 nb2 theta2 -- TOMDO: do something with these pred_cois
1173 ; traceTc (text "TOMDO!")
1174 ; coi <- uTys nb1 tau1 nb2 tau2
1176 -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
1177 ; free_tvs <- zonkTcTyVarsAndFV (varSetElems (tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2))
1178 ; when (any (`elemVarSet` free_tvs) tvs)
1179 (bleatEscapedTvs free_tvs tvs tvs)
1181 -- If both sides are inside a box, we are in a "box-meets-box"
1182 -- situation, and we should not have a polytype at all.
1183 -- If we get here we have two boxes, already filled with
1184 -- the same polytype... but it should be a monotype.
1185 -- This check comes last, because the error message is
1186 -- extremely unhelpful.
1187 ; when (nb1 && nb2) (notMonoType ty1)
1191 (tvs1, body1) = tcSplitForAllTys ty1
1192 (tvs2, body2) = tcSplitForAllTys ty2
1195 go outer _ (PredTy p1) _ (PredTy p2)
1196 = uPred outer nb1 p1 nb2 p2
1198 -- Non-synonym type constructors must match
1199 go _ _ (TyConApp con1 tys1) _ (TyConApp con2 tys2)
1200 | con1 == con2 && not (isOpenSynTyCon con1)
1201 = do { cois <- uTys_s nb1 tys1 nb2 tys2
1202 ; return $ mkTyConAppCoI con1 tys1 cois
1204 -- Family synonyms See Note [TyCon app]
1205 | con1 == con2 && identicalOpenSynTyConApp
1206 = do { cois <- uTys_s nb1 tys1' nb2 tys2'
1207 ; return $ mkTyConAppCoI con1 tys1 (replicate n IdCo ++ cois)
1211 (idxTys1, tys1') = splitAt n tys1
1212 (idxTys2, tys2') = splitAt n tys2
1213 identicalOpenSynTyConApp = idxTys1 `tcEqTypes` idxTys2
1214 -- See Note [OpenSynTyCon app]
1216 -- Functions; just check the two parts
1217 go _ _ (FunTy fun1 arg1) _ (FunTy fun2 arg2)
1218 = do { coi_l <- uTys nb1 fun1 nb2 fun2
1219 ; coi_r <- uTys nb1 arg1 nb2 arg2
1220 ; return $ mkFunTyCoI fun1 coi_l arg1 coi_r
1223 -- Applications need a bit of care!
1224 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
1225 -- NB: we've already dealt with type variables and Notes,
1226 -- so if one type is an App the other one jolly well better be too
1227 go outer _ (AppTy s1 t1) _ ty2
1228 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
1229 = do { coi_s <- go outer s1 s1 s2 s2 -- NB recurse into go
1230 ; coi_t <- uTys nb1 t1 nb2 t2 -- See Note [Unifying AppTy]
1231 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1233 -- Now the same, but the other way round
1234 -- Don't swap the types, because the error messages get worse
1235 go outer _ ty1 _ (AppTy s2 t2)
1236 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
1237 = do { coi_s <- go outer s1 s1 s2 s2
1238 ; coi_t <- uTys nb1 t1 nb2 t2
1239 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1241 -- If we can reduce a family app => proceed with reduct
1242 -- NB1: We use isOpenSynTyCon, not isOpenSynTyConApp as we also must
1243 -- defer oversaturated applications!
1245 -- NB2: Do this *after* trying decomposing applications, so that decompose
1246 -- (m a) ~ (F Int b)
1247 -- where F has arity 1
1248 go _ _ ty1@(TyConApp con1 _) _ ty2
1249 | isOpenSynTyCon con1
1250 = do { (coi1, ty1') <- tcNormaliseFamInst ty1
1252 IdCo -> defer -- no reduction, see [Deferred Unification]
1253 _ -> liftM (coi1 `mkTransCoI`) $ uTys nb1 ty1' nb2 ty2
1256 go _ _ ty1 _ ty2@(TyConApp con2 _)
1257 | isOpenSynTyCon con2
1258 = do { (coi2, ty2') <- tcNormaliseFamInst ty2
1260 IdCo -> defer -- no reduction, see [Deferred Unification]
1261 _ -> liftM (`mkTransCoI` mkSymCoI coi2) $
1262 uTys nb1 ty1 nb2 ty2'
1265 -- Anything else fails
1266 go outer _ _ _ _ = bale_out outer
1268 defer = defer_unification outer False orig_ty1 orig_ty2
1272 uPred :: Outer -> InBox -> PredType -> InBox -> PredType -> TcM CoercionI
1273 uPred _ nb1 (IParam n1 t1) nb2 (IParam n2 t2)
1275 do { coi <- uTys nb1 t1 nb2 t2
1276 ; return $ mkIParamPredCoI n1 coi
1278 uPred _ nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
1280 do { cois <- uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
1281 ; return $ mkClassPPredCoI c1 tys1 cois
1283 uPred outer _ _ _ _ = unifyMisMatch outer
1285 uPreds :: Outer -> InBox -> [PredType] -> InBox -> [PredType]
1287 uPreds _ _ [] _ [] = return []
1288 uPreds outer nb1 (p1:ps1) nb2 (p2:ps2) =
1289 do { coi <- uPred outer nb1 p1 nb2 p2
1290 ; cois <- uPreds outer nb1 ps1 nb2 ps2
1293 uPreds _ _ _ _ _ = panic "uPreds"
1298 When we find two TyConApps, the argument lists are guaranteed equal
1299 length. Reason: intially the kinds of the two types to be unified is
1300 the same. The only way it can become not the same is when unifying two
1301 AppTys (f1 a1)~(f2 a2). In that case there can't be a TyConApp in
1302 the f1,f2 (because it'd absorb the app). If we unify f1~f2 first,
1303 which we do, that ensures that f1,f2 have the same kind; and that
1304 means a1,a2 have the same kind. And now the argument repeats.
1306 Note [OpenSynTyCon app]
1307 ~~~~~~~~~~~~~~~~~~~~~~~
1310 type family T a :: * -> *
1312 the two types (T () a) and (T () Int) must unify, even if there are
1313 no type instances for T at all. Should we just turn them into an
1314 equality (T () a ~ T () Int)? I don't think so. We currently try to
1315 eagerly unify everything we can before generating equalities; otherwise,
1316 we could turn the unification of [Int] with [a] into an equality, too.
1318 Note [Unification and synonyms]
1319 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1320 If you are tempted to make a short cut on synonyms, as in this
1324 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
1325 -- NO = if (con1 == con2) then
1326 -- NO -- Good news! Same synonym constructors, so we can shortcut
1327 -- NO -- by unifying their arguments and ignoring their expansions.
1328 -- NO unifyTypepeLists args1 args2
1330 -- NO -- Never mind. Just expand them and try again
1334 then THINK AGAIN. Here is the whole story, as detected and reported
1335 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1337 Here's a test program that should detect the problem:
1341 x = (1 :: Bogus Char) :: Bogus Bool
1344 The problem with [the attempted shortcut code] is that
1348 is not a sufficient condition to be able to use the shortcut!
1349 You also need to know that the type synonym actually USES all
1350 its arguments. For example, consider the following type synonym
1351 which does not use all its arguments.
1356 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1357 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1358 would fail, even though the expanded forms (both \tr{Int}) should
1361 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1362 unnecessarily bind \tr{t} to \tr{Char}.
1364 ... You could explicitly test for the problem synonyms and mark them
1365 somehow as needing expansion, perhaps also issuing a warning to the
1370 %************************************************************************
1372 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1374 %************************************************************************
1376 @uVar@ is called when at least one of the types being unified is a
1377 variable. It does {\em not} assume that the variable is a fixed point
1378 of the substitution; rather, notice that @uVar@ (defined below) nips
1379 back into @uTys@ if it turns out that the variable is already bound.
1383 -> SwapFlag -- False => tyvar is the "actual" (ty is "expected")
1384 -- True => ty is the "actual" (tyvar is "expected")
1386 -> InBox -- True <=> definitely no boxes in t2
1387 -> TcTauType -> TcTauType -- printing and real versions
1390 uVar outer swapped tv1 nb2 ps_ty2 ty2
1391 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1392 | otherwise = brackets (equals <+> ppr ty2)
1393 ; traceTc (text "uVar" <+> ppr outer <+> ppr swapped <+>
1394 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1395 nest 2 (ptext (sLit " <-> ")),
1396 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1397 ; details <- lookupTcTyVar tv1
1400 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1401 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1402 -- The 'True' here says that ty1 is now inside a box
1403 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1407 uUnfilledVar :: Outer
1409 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1410 -> TcTauType -> TcTauType -- Type 2
1412 -- Invariant: tyvar 1 is not unified with anything
1414 uUnfilledVar _ swapped tv1 details1 ps_ty2 ty2
1415 | Just ty2' <- tcView ty2
1416 = -- Expand synonyms; ignore FTVs
1417 let outer' | swapped = Unify False ty2' (mkTyVarTy tv1)
1418 | otherwise = Unify False (mkTyVarTy tv1) ty2'
1419 in uUnfilledVar outer' swapped tv1 details1 ps_ty2 ty2'
1421 uUnfilledVar outer swapped tv1 details1 _ (TyVarTy tv2)
1422 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1424 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1425 -- this is box-meets-box, so fill in with a tau-type
1426 -> do { tau_tv <- tcInstTyVar tv1
1427 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv)
1430 _ -> return IdCo -- No-op
1432 | otherwise -- Distinct type variables
1433 = do { lookup2 <- lookupTcTyVar tv2
1435 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
1436 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1439 uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2
1440 = -- ty2 is not a type variable
1442 MetaTv (SigTv _) _ -> rigid_variable
1443 MetaTv info ref1 -> uMetaVar outer swapped tv1 info ref1 ps_ty2 non_var_ty2
1444 SkolemTv _ -> rigid_variable
1447 | isOpenSynTyConApp non_var_ty2
1448 = -- 'non_var_ty2's outermost constructor is a type family,
1449 -- which we may may be able to normalise
1450 do { (coi2, ty2') <- tcNormaliseFamInst non_var_ty2
1452 IdCo -> -- no progress, but maybe after other instantiations
1453 defer_unification outer swapped (TyVarTy tv1) ps_ty2
1454 ACo co -> -- progress: so lets try again
1456 ppr co <+> text "::"<+> ppr non_var_ty2 <+> text "~" <+>
1458 ; coi <- uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2'
1459 ; let coi2' = (if swapped then id else mkSymCoI) coi2
1460 ; return $ coi2' `mkTransCoI` coi
1463 | SkolemTv RuntimeUnkSkol <- details1
1464 -- runtime unknown will never match
1465 = unifyMisMatch outer
1466 | otherwise -- defer as a given equality may still resolve this
1467 = defer_unification outer swapped (TyVarTy tv1) ps_ty2
1470 Note [Deferred Unification]
1471 ~~~~~~~~~~~~~~~~~~~~
1472 We may encounter a unification ty1 = ty2 that cannot be performed syntactically,
1473 and yet its consistency is undetermined. Previously, there was no way to still
1474 make it consistent. So a mismatch error was issued.
1476 Now these unfications are deferred until constraint simplification, where type
1477 family instances and given equations may (or may not) establish the consistency.
1478 Deferred unifications are of the form
1481 where F is a type function and x is a type variable.
1483 id :: x ~ y => x -> y
1486 involves the unfication x = y. It is deferred until we bring into account the
1487 context x ~ y to establish that it holds.
1489 If available, we defer original types (rather than those where closed type
1490 synonyms have already been expanded via tcCoreView). This is, as usual, to
1491 improve error messages.
1493 We need to both 'unBox' and zonk deferred types. We need to unBox as
1494 functions, such as TcExpr.tcMonoExpr promise to fill boxes in the expected
1495 type. We need to zonk as the types go into the kind of the coercion variable
1496 `cotv' and those are not zonked in Inst.zonkInst. (Maybe it would be better
1497 to zonk in zonInst instead. Would that be sufficient?)
1500 defer_unification :: Outer
1505 defer_unification outer True ty1 ty2
1506 = defer_unification outer False ty2 ty1
1507 defer_unification outer False ty1 ty2
1508 = do { ty1' <- unBox ty1 >>= zonkTcType -- unbox *and* zonk..
1509 ; ty2' <- unBox ty2 >>= zonkTcType -- ..see preceding note
1510 ; traceTc $ text "deferring:" <+> ppr ty1 <+> text "~" <+> ppr ty2
1511 ; cotv <- newMetaCoVar ty1' ty2'
1512 -- put ty1 ~ ty2 in LIE
1513 -- Left means "wanted"
1514 ; inst <- popUnifyCtxt outer $
1515 mkEqInst (EqPred ty1' ty2') (Left cotv)
1517 ; return $ ACo $ TyVarTy cotv }
1522 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1525 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1526 -- ty2 is not a type variable
1528 uMetaVar outer swapped tv1 BoxTv ref1 ps_ty2 ty2
1529 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1530 -- that any boxes in ty2 are filled with monotypes
1532 -- It should not be the case that tv1 occurs in ty2
1533 -- (i.e. no occurs check should be needed), but if perchance
1534 -- it does, the unbox operation will fill it, and the debug code
1536 do { final_ty <- unBox ps_ty2
1537 ; meta_details <- readMutVar ref1
1538 ; case meta_details of
1539 Indirect _ -> -- This *can* happen due to an occurs check,
1540 -- just as it can in checkTauTvUpdate in the next
1541 -- equation of uMetaVar; see Trac #2414
1542 -- Note [Occurs check]
1543 -- Go round again. Probably there's an immediate
1544 -- error, but maybe not (a type function might discard
1545 -- its argument). Next time round we'll end up in the
1546 -- TauTv case of uMetaVar.
1547 uVar outer swapped tv1 False ps_ty2 ty2
1548 -- Setting for nb2::InBox is irrelevant
1550 Flexi -> do { checkUpdateMeta swapped tv1 ref1 final_ty
1554 uMetaVar outer swapped tv1 _ ref1 ps_ty2 _
1555 = do { -- Occurs check + monotype check
1556 ; mb_final_ty <- checkTauTvUpdate tv1 ps_ty2
1557 ; case mb_final_ty of
1558 Nothing -> -- tv1 occured in type family parameter
1559 defer_unification outer swapped (mkTyVarTy tv1) ps_ty2
1561 do { checkUpdateMeta swapped tv1 ref1 final_ty
1566 {- Note [Occurs check]
1568 An eager occurs check is made in checkTauTvUpdate, deferring tricky
1569 cases by calling defer_unification (see notes with
1570 checkTauTvUpdate). An occurs check can also (and does) happen in the
1571 BoxTv case, but unBox doesn't check for occurrences, and in any case
1572 doesn't have the type-function-related complexity that
1573 checkTauTvUpdate has. So we content ourselves with spotting the potential
1574 occur check (by the fact that tv1 is now filled), and going round again.
1575 Next time round we'll get the TauTv case of uMetaVar.
1579 uUnfilledVars :: Outer
1581 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1582 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1584 -- Invarant: The type variables are distinct,
1585 -- Neither is filled in yet
1586 -- They might be boxy or not
1588 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1589 = -- see [Deferred Unification]
1590 defer_unification outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1592 uUnfilledVars _ swapped tv1 (MetaTv _ ref1) tv2 (SkolemTv _)
1593 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2) >> return IdCo
1594 uUnfilledVars _ swapped tv1 (SkolemTv _) tv2 (MetaTv _ ref2)
1595 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1) >> return IdCo
1597 -- ToDo: this function seems too long for what it acutally does!
1598 uUnfilledVars _ swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1599 = case (info1, info2) of
1600 (BoxTv, BoxTv) -> box_meets_box >> return IdCo
1602 -- If a box meets a TauTv, but the fomer has the smaller kind
1603 -- then we must create a fresh TauTv with the smaller kind
1604 (_, BoxTv) | k1_sub_k2 -> update_tv2 >> return IdCo
1605 | otherwise -> box_meets_box >> return IdCo
1606 (BoxTv, _ ) | k2_sub_k1 -> update_tv1 >> return IdCo
1607 | otherwise -> box_meets_box >> return IdCo
1609 -- Avoid SigTvs if poss
1610 (SigTv _, _ ) | k1_sub_k2 -> update_tv2 >> return IdCo
1611 (_, SigTv _) | k2_sub_k1 -> update_tv1 >> return IdCo
1613 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1614 then update_tv1 >> return IdCo -- Same kinds
1615 else update_tv2 >> return IdCo
1616 | k2_sub_k1 -> update_tv1 >> return IdCo
1617 | otherwise -> kind_err >> return IdCo
1619 -- Update the variable with least kind info
1620 -- See notes on type inference in Kind.lhs
1621 -- The "nicer to" part only applies if the two kinds are the same,
1622 -- so we can choose which to do.
1624 -- Kinds should be guaranteed ok at this point
1625 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1626 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1628 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1631 | k2_sub_k1 = fill_from tv2
1632 | otherwise = kind_err
1634 -- Update *both* tyvars with a TauTv whose name and kind
1635 -- are gotten from tv (avoid losing nice names is poss)
1636 fill_from tv = do { tv' <- tcInstTyVar tv
1637 ; let tau_ty = mkTyVarTy tv'
1638 ; updateMeta tv1 ref1 tau_ty
1639 ; updateMeta tv2 ref2 tau_ty }
1641 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1642 unifyKindMisMatch k1 k2
1646 k1_sub_k2 = k1 `isSubKind` k2
1647 k2_sub_k1 = k2 `isSubKind` k1
1649 nicer_to_update_tv1 = isSystemName (Var.varName tv1)
1650 -- Try to update sys-y type variables in preference to ones
1651 -- gotten (say) by instantiating a polymorphic function with
1652 -- a user-written type sig
1656 refineBox :: TcType -> TcM TcType
1657 -- Unbox the outer box of a boxy type (if any)
1658 refineBox ty@(TyVarTy box_tv)
1659 | isMetaTyVar box_tv
1660 = do { cts <- readMetaTyVar box_tv
1663 Indirect ty -> return ty }
1664 refineBox other_ty = return other_ty
1666 refineBoxToTau :: TcType -> TcM TcType
1667 -- Unbox the outer box of a boxy type, filling with a monotype if it is empty
1668 -- Like refineBox except for the "fill with monotype" part.
1669 refineBoxToTau (TyVarTy box_tv)
1670 | isMetaTyVar box_tv
1671 , MetaTv BoxTv ref <- tcTyVarDetails box_tv
1672 = do { cts <- readMutVar ref
1674 Flexi -> fillBoxWithTau box_tv ref
1675 Indirect ty -> return ty }
1676 refineBoxToTau other_ty = return other_ty
1678 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1679 -- Subtle... we must zap the boxy res_ty
1680 -- to kind * before using it to instantiate a LitInst
1681 -- Calling unBox instead doesn't do the job, because the box
1682 -- often has an openTypeKind, and we don't want to instantiate
1684 zapToMonotype res_ty
1685 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1686 ; _ <- boxyUnify res_tau res_ty
1689 unBox :: BoxyType -> TcM TcType
1690 -- unBox implements the judgement
1692 -- with input s', and result s
1694 -- It removes all boxes from the input type, returning a non-boxy type.
1695 -- A filled box in the type can only contain a monotype; unBox fails if not
1696 -- The type can have empty boxes, which unBox fills with a monotype
1698 -- Compare this wth checkTauTvUpdate
1700 -- For once, it's safe to treat synonyms as opaque!
1702 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1703 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1704 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1705 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1706 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1707 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1709 | isTcTyVar tv -- It's a boxy type variable
1710 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1711 = do { cts <- readMutVar ref -- under nested quantifiers
1713 Flexi -> fillBoxWithTau tv ref
1714 Indirect ty -> do { non_boxy_ty <- unBox ty
1715 ; if isTauTy non_boxy_ty
1716 then return non_boxy_ty
1717 else notMonoType non_boxy_ty }
1719 | otherwise -- Skolems, and meta-tau-variables
1720 = return (TyVarTy tv)
1722 unBoxPred :: PredType -> TcM PredType
1723 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1724 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1725 unBoxPred (EqPred ty1 ty2) = do { ty1' <- unBox ty1; ty2' <- unBox ty2; return (EqPred ty1' ty2') }
1730 %************************************************************************
1734 %************************************************************************
1737 unifyMisMatch :: Outer -> TcM a
1738 unifyMisMatch (Unify is_outer ty1 ty2)
1739 | is_outer = popErrCtxt $ failWithMisMatch ty1 ty2 -- This is the whole point of the 'outer' stuff
1740 | otherwise = failWithMisMatch ty1 ty2
1742 popUnifyCtxt :: Outer -> TcM a -> TcM a
1743 popUnifyCtxt (Unify True _ _) thing = popErrCtxt thing
1744 popUnifyCtxt (Unify False _ _) thing = thing
1746 -----------------------
1747 unifyCtxt :: TcType -> TcType -> TidyEnv -> TcM (TidyEnv, SDoc)
1748 unifyCtxt act_ty exp_ty tidy_env
1749 = do { act_ty' <- zonkTcType act_ty
1750 ; exp_ty' <- zonkTcType exp_ty
1751 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1752 (env2, act_ty'') = tidyOpenType env1 act_ty'
1753 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1756 mkExpectedActualMsg :: Type -> Type -> SDoc
1757 mkExpectedActualMsg act_ty exp_ty
1758 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1759 text "Inferred type" <> colon <+> ppr act_ty ])
1762 -- If an error happens we try to figure out whether the function
1763 -- function has been given too many or too few arguments, and say so.
1764 addSubCtxt :: InstOrigin -> TcType -> TcType -> TcM a -> TcM a
1765 addSubCtxt orig actual_res_ty expected_res_ty thing_inside
1766 = addErrCtxtM mk_err thing_inside
1769 = do { exp_ty' <- zonkTcType expected_res_ty
1770 ; act_ty' <- zonkTcType actual_res_ty
1771 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1772 (env2, act_ty'') = tidyOpenType env1 act_ty'
1773 (exp_args, _) = tcSplitFunTys exp_ty''
1774 (act_args, _) = tcSplitFunTys act_ty''
1776 len_act_args = length act_args
1777 len_exp_args = length exp_args
1779 message = case orig of
1781 | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1782 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1783 _ -> mkExpectedActualMsg act_ty'' exp_ty''
1784 ; return (env2, message) }
1786 wrongArgsCtxt too_many_or_few fun
1787 = ptext (sLit "Probable cause:") <+> quotes (ppr fun)
1788 <+> ptext (sLit "is applied to") <+> text too_many_or_few
1789 <+> ptext (sLit "arguments")
1792 unifyForAllCtxt :: [TyVar] -> Type -> Type -> TidyEnv -> TcM (TidyEnv, SDoc)
1793 unifyForAllCtxt tvs phi1 phi2 env
1794 = return (env2, msg)
1796 (env', tvs') = tidyOpenTyVars env tvs -- NB: not tidyTyVarBndrs
1797 (env1, phi1') = tidyOpenType env' phi1
1798 (env2, phi2') = tidyOpenType env1 phi2
1799 msg = vcat [ptext (sLit "When matching") <+> quotes (ppr (mkForAllTys tvs' phi1')),
1800 ptext (sLit " and") <+> quotes (ppr (mkForAllTys tvs' phi2'))]
1805 %************************************************************************
1809 %************************************************************************
1811 Unifying kinds is much, much simpler than unifying types.
1814 unifyKind :: TcKind -- Expected
1817 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1818 | isSubKindCon kc2 kc1 = return ()
1820 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1821 = do { unifyKind a2 a1; unifyKind r1 r2 }
1822 -- Notice the flip in the argument,
1823 -- so that the sub-kinding works right
1824 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1825 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1826 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1828 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1829 unifyKinds [] [] = return ()
1830 unifyKinds (k1:ks1) (k2:ks2) = do unifyKind k1 k2
1832 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1835 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1836 uKVar swapped kv1 k2
1837 = do { mb_k1 <- readKindVar kv1
1839 Flexi -> uUnboundKVar swapped kv1 k2
1840 Indirect k1 | swapped -> unifyKind k2 k1
1841 | otherwise -> unifyKind k1 k2 }
1844 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1845 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1846 | kv1 == kv2 = return ()
1847 | otherwise -- Distinct kind variables
1848 = do { mb_k2 <- readKindVar kv2
1850 Indirect k2 -> uUnboundKVar swapped kv1 k2
1851 Flexi -> writeKindVar kv1 k2 }
1853 uUnboundKVar swapped kv1 non_var_k2
1854 = do { k2' <- zonkTcKind non_var_k2
1855 ; kindOccurCheck kv1 k2'
1856 ; k2'' <- kindSimpleKind swapped k2'
1857 -- KindVars must be bound only to simple kinds
1858 -- Polarities: (kindSimpleKind True ?) succeeds
1859 -- returning *, corresponding to unifying
1862 ; writeKindVar kv1 k2'' }
1865 kindOccurCheck :: TyVar -> Type -> TcM ()
1866 kindOccurCheck kv1 k2 -- k2 is zonked
1867 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1869 not_in (TyVarTy kv2) = kv1 /= kv2
1870 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1873 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1874 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1875 -- If the flag is False, it requires k <: sk
1876 -- E.g. kindSimpleKind False ?? = *
1877 -- What about (kv -> *) ~ ?? -> *
1878 kindSimpleKind orig_swapped orig_kind
1879 = go orig_swapped orig_kind
1881 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1883 ; return (mkArrowKind k1' k2') }
1885 | isOpenTypeKind k = return liftedTypeKind
1886 | isArgTypeKind k = return liftedTypeKind
1888 | isLiftedTypeKind k = return liftedTypeKind
1889 | isUnliftedTypeKind k = return unliftedTypeKind
1890 go _ k@(TyVarTy _) = return k -- KindVars are always simple
1891 go _ _ = failWithTc (ptext (sLit "Unexpected kind unification failure:")
1892 <+> ppr orig_swapped <+> ppr orig_kind)
1893 -- I think this can't actually happen
1895 -- T v = MkT v v must be a type
1896 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1899 kindOccurCheckErr :: Var -> Type -> SDoc
1900 kindOccurCheckErr tyvar ty
1901 = hang (ptext (sLit "Occurs check: cannot construct the infinite kind:"))
1902 2 (sep [ppr tyvar, char '=', ppr ty])
1906 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1907 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1909 unifyFunKind (TyVarTy kvar) = do
1910 maybe_kind <- readKindVar kvar
1912 Indirect fun_kind -> unifyFunKind fun_kind
1914 do { arg_kind <- newKindVar
1915 ; res_kind <- newKindVar
1916 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1917 ; return (Just (arg_kind,res_kind)) }
1919 unifyFunKind (FunTy arg_kind res_kind) = return (Just (arg_kind,res_kind))
1920 unifyFunKind _ = return Nothing
1923 %************************************************************************
1925 \subsection{Checking signature type variables}
1927 %************************************************************************
1929 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1930 are not mentioned in the environment. In particular:
1932 (a) Not mentioned in the type of a variable in the envt
1933 eg the signature for f in this:
1939 Here, f is forced to be monorphic by the free occurence of x.
1941 (d) Not (unified with another type variable that is) in scope.
1942 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1943 when checking the expression type signature, we find that
1944 even though there is nothing in scope whose type mentions r,
1945 nevertheless the type signature for the expression isn't right.
1947 Another example is in a class or instance declaration:
1949 op :: forall b. a -> b
1951 Here, b gets unified with a
1953 Before doing this, the substitution is applied to the signature type variable.
1956 checkSigTyVars :: [TcTyVar] -> TcM ()
1957 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1959 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1960 -- The extra_tvs can include boxy type variables;
1961 -- e.g. TcMatches.tcCheckExistentialPat
1962 checkSigTyVarsWrt extra_tvs sig_tvs
1963 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1964 ; check_sig_tyvars extra_tvs' sig_tvs }
1967 :: TcTyVarSet -- Global type variables. The universally quantified
1968 -- tyvars should not mention any of these
1969 -- Guaranteed already zonked.
1970 -> [TcTyVar] -- Universally-quantified type variables in the signature
1971 -- Guaranteed to be skolems
1973 check_sig_tyvars _ []
1975 check_sig_tyvars extra_tvs sig_tvs
1976 = ASSERT( all isTcTyVar sig_tvs && all isSkolemTyVar sig_tvs )
1977 do { gbl_tvs <- tcGetGlobalTyVars
1978 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1979 text "gbl_tvs" <+> ppr gbl_tvs,
1980 text "extra_tvs" <+> ppr extra_tvs]))
1982 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1983 ; when (any (`elemVarSet` env_tvs) sig_tvs)
1984 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1987 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1988 -> [TcTyVar] -- The possibly-escaping type variables
1989 -> [TcTyVar] -- The zonked versions thereof
1991 -- Complain about escaping type variables
1992 -- We pass a list of type variables, at least one of which
1993 -- escapes. The first list contains the original signature type variable,
1994 -- while the second contains the type variable it is unified to (usually itself)
1995 bleatEscapedTvs globals sig_tvs zonked_tvs
1996 = do { env0 <- tcInitTidyEnv
1997 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1998 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
2000 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
2001 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
2003 main_msg = ptext (sLit "Inferred type is less polymorphic than expected")
2005 check (tidy_env, msgs) (sig_tv, zonked_tv)
2006 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
2008 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
2009 ; return (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
2011 -----------------------
2012 escape_msg :: Var -> Var -> [SDoc] -> SDoc
2013 escape_msg sig_tv zonked_tv globs
2015 = vcat [sep [msg, ptext (sLit "is mentioned in the environment:")],
2016 nest 2 (vcat globs)]
2018 = msg <+> ptext (sLit "escapes")
2019 -- Sigh. It's really hard to give a good error message
2020 -- all the time. One bad case is an existential pattern match.
2021 -- We rely on the "When..." context to help.
2023 msg = ptext (sLit "Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
2025 | sig_tv == zonked_tv = empty
2026 | otherwise = ptext (sLit "is unified with") <+> quotes (ppr zonked_tv) <+> ptext (sLit "which")
2029 These two context are used with checkSigTyVars
2032 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
2033 -> TidyEnv -> TcM (TidyEnv, Message)
2034 sigCtxt id sig_tvs sig_theta sig_tau tidy_env = do
2035 actual_tau <- zonkTcType sig_tau
2037 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
2038 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
2039 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
2040 sub_msg = vcat [ptext (sLit "Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
2041 ptext (sLit "Type to generalise:") <+> pprType tidy_actual_tau
2043 msg = vcat [ptext (sLit "When trying to generalise the type inferred for") <+> quotes (ppr id),