2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Type subsumption and unification
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
17 -- Full-blown subsumption
18 tcSubExp, tcFunResTy, tcGen,
19 checkSigTyVars, checkSigTyVarsWrt, bleatEscapedTvs, sigCtxt,
21 -- Various unifications
22 unifyType, unifyTypeList, unifyTheta,
23 unifyKind, unifyKinds, unifyFunKind,
25 preSubType, boxyMatchTypes,
27 --------------------------------
29 tcInfer, subFunTys, unBox, refineBox, refineBoxToTau, withBox,
30 boxyUnify, boxyUnifyList, zapToMonotype,
31 boxySplitListTy, boxySplitPArrTy, boxySplitTyConApp, boxySplitAppTy,
35 #include "HsVersions.h"
45 import TcRnMonad -- TcType, amongst others
65 %************************************************************************
67 \subsection{'hole' type variables}
69 %************************************************************************
72 tcInfer :: (BoxyType -> TcM a) -> TcM (a, TcType)
74 = do { box <- newBoxyTyVar openTypeKind
75 ; res <- tc_infer (mkTyVarTy box)
76 ; res_ty <- {- pprTrace "tcInfer" (ppr (mkTyVarTy box)) $ -} readFilledBox box -- Guaranteed filled-in by now
77 ; return (res, res_ty) }
81 %************************************************************************
85 %************************************************************************
88 subFunTys :: SDoc -- Somthing like "The function f has 3 arguments"
89 -- or "The abstraction (\x.e) takes 1 argument"
90 -> Arity -- Expected # of args
91 -> BoxyRhoType -- res_ty
92 -> ([BoxySigmaType] -> BoxyRhoType -> TcM a)
94 -- Attempt to decompse res_ty to have enough top-level arrows to
95 -- match the number of patterns in the match group
97 -- If (subFunTys n_args res_ty thing_inside) = (co_fn, res)
98 -- and the inner call to thing_inside passes args: [a1,...,an], b
99 -- then co_fn :: (a1 -> ... -> an -> b) ~ res_ty
101 -- Note that it takes a BoxyRho type, and guarantees to return a BoxyRhoType
104 {- Error messages from subFunTys
106 The abstraction `\Just 1 -> ...' has two arguments
107 but its type `Maybe a -> a' has only one
109 The equation(s) for `f' have two arguments
110 but its type `Maybe a -> a' has only one
112 The section `(f 3)' requires 'f' to take two arguments
113 but its type `Int -> Int' has only one
115 The function 'f' is applied to two arguments
116 but its type `Int -> Int' has only one
120 subFunTys error_herald n_pats res_ty thing_inside
121 = loop n_pats [] res_ty
123 -- In 'loop', the parameter 'arg_tys' accumulates
124 -- the arg types so far, in *reverse order*
125 -- INVARIANT: res_ty :: *
126 loop n args_so_far res_ty
127 | Just res_ty' <- tcView res_ty = loop n args_so_far res_ty'
129 loop n args_so_far res_ty
130 | isSigmaTy res_ty -- Do this before checking n==0, because we
131 -- guarantee to return a BoxyRhoType, not a BoxySigmaType
132 = do { (gen_fn, (co_fn, res)) <- tcGen res_ty emptyVarSet $ \ _ res_ty' ->
133 loop n args_so_far res_ty'
134 ; return (gen_fn <.> co_fn, res) }
136 loop 0 args_so_far res_ty
137 = do { res <- thing_inside (reverse args_so_far) res_ty
138 ; return (idHsWrapper, res) }
140 loop n args_so_far (FunTy arg_ty res_ty)
141 = do { (co_fn, res) <- loop (n-1) (arg_ty:args_so_far) res_ty
142 ; co_fn' <- wrapFunResCoercion [arg_ty] co_fn
143 ; return (co_fn', res) }
145 -- res_ty might have a type variable at the head, such as (a b c),
146 -- in which case we must fill in with (->). Simplest thing to do
147 -- is to use boxyUnify, but we catch failure and generate our own
148 -- error message on failure
149 loop n args_so_far res_ty@(AppTy _ _)
150 = do { [arg_ty',res_ty'] <- newBoxyTyVarTys [argTypeKind, openTypeKind]
151 ; (_, mb_coi) <- tryTcErrs $ boxyUnify res_ty (FunTy arg_ty' res_ty')
152 ; if isNothing mb_coi then bale_out args_so_far
153 else do { case expectJust "subFunTys" mb_coi of
155 ACo co -> traceTc (text "you're dropping a coercion: " <+> ppr co)
156 ; loop n args_so_far (FunTy arg_ty' res_ty')
160 loop n args_so_far (TyVarTy tv)
161 | isTyConableTyVar tv
162 = do { cts <- readMetaTyVar tv
164 Indirect ty -> loop n args_so_far ty
165 Flexi -> do { (res_ty:arg_tys) <- withMetaTvs tv kinds mk_res_ty
166 ; res <- thing_inside (reverse args_so_far ++ arg_tys) res_ty
167 ; return (idHsWrapper, res) } }
169 mk_res_ty (res_ty' : arg_tys') = mkFunTys arg_tys' res_ty'
170 mk_res_ty [] = panic "TcUnify.mk_res_ty1"
171 kinds = openTypeKind : take n (repeat argTypeKind)
172 -- Note argTypeKind: the args can have an unboxed type,
173 -- but not an unboxed tuple.
175 loop n args_so_far res_ty = bale_out args_so_far
178 = do { env0 <- tcInitTidyEnv
179 ; res_ty' <- zonkTcType res_ty
180 ; let (env1, res_ty'') = tidyOpenType env0 res_ty'
181 ; failWithTcM (env1, mk_msg res_ty'' (length args_so_far)) }
183 mk_msg res_ty n_actual
184 = error_herald <> comma $$
185 sep [ptext SLIT("but its type") <+> quotes (pprType res_ty),
186 if n_actual == 0 then ptext SLIT("has none")
187 else ptext SLIT("has only") <+> speakN n_actual]
191 ----------------------
192 boxySplitTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
193 -> BoxyRhoType -- Expected type (T a b c)
194 -> TcM ([BoxySigmaType], -- Element types, a b c
196 -- It's used for wired-in tycons, so we call checkWiredInTyCon
197 -- Precondition: never called with FunTyCon
198 -- Precondition: input type :: *
200 boxySplitTyConApp tc orig_ty
201 = do { checkWiredInTyCon tc
202 ; loop (tyConArity tc) [] orig_ty }
204 loop n_req args_so_far ty
205 | Just ty' <- tcView ty = loop n_req args_so_far ty'
207 loop n_req args_so_far ty@(TyConApp tycon args)
209 = ASSERT( n_req == length args) -- ty::*
210 return (args ++ args_so_far, IdCo)
212 | isOpenSynTyCon tycon -- try to normalise type family application
213 = do { (coi1, ty') <- tcNormaliseFamInst ty
215 IdCo -> defer -- no progress, but maybe solvable => defer
216 ACo _ -> -- progress: so lets try again
217 do { (args, coi2) <- loop n_req args_so_far ty'
218 ; return $ (args, coi2 `mkTransCoI` mkSymCoI coi1)
222 loop n_req args_so_far (AppTy fun arg)
224 = do { (args, coi) <- loop (n_req - 1) (arg:args_so_far) fun
225 ; return (args, mkAppTyCoI fun coi arg IdCo)
228 loop n_req args_so_far (TyVarTy tv)
229 | isTyConableTyVar tv
230 , res_kind `isSubKind` tyVarKind tv
231 = do { cts <- readMetaTyVar tv
233 Indirect ty -> loop n_req args_so_far ty
234 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
235 ; return (arg_tys ++ args_so_far, IdCo) }
237 | otherwise -- defer as tyvar may be refined by equalities
240 (arg_kinds, res_kind) = splitKindFunTysN n_req (tyConKind tc)
242 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc)))
245 -- defer splitting by generating an equality constraint
246 defer = boxySplitDefer arg_kinds mk_res_ty orig_ty
248 (arg_kinds, _) = splitKindFunTys (tyConKind tc)
250 -- apply splitted tycon to arguments
251 mk_res_ty = mkTyConApp tc
253 ----------------------
254 boxySplitListTy :: BoxyRhoType -> TcM (BoxySigmaType, CoercionI)
255 -- Special case for lists
256 boxySplitListTy exp_ty
257 = do { ([elt_ty], coi) <- boxySplitTyConApp listTyCon exp_ty
258 ; return (elt_ty, coi) }
260 ----------------------
261 boxySplitPArrTy :: BoxyRhoType -> TcM (BoxySigmaType, CoercionI)
262 -- Special case for parrs
263 boxySplitPArrTy exp_ty
264 = do { ([elt_ty], coi) <- boxySplitTyConApp parrTyCon exp_ty
265 ; return (elt_ty, coi) }
267 ----------------------
268 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
269 -> TcM ((BoxySigmaType, BoxySigmaType), -- Returns m, a
271 -- If the incoming type is a mutable type variable of kind k, then
272 -- boxySplitAppTy returns a new type variable (m: * -> k); note the *.
273 -- If the incoming type is boxy, then so are the result types; and vice versa
275 boxySplitAppTy orig_ty
279 | Just ty' <- tcView ty = loop ty'
282 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
283 = return ((fun_ty, arg_ty), IdCo)
285 loop ty@(TyConApp tycon args)
286 | isOpenSynTyCon tycon -- try to normalise type family application
287 = do { (coi1, ty') <- tcNormaliseFamInst ty
289 IdCo -> defer -- no progress, but maybe solvable => defer
290 ACo co -> -- progress: so lets try again
291 do { (args, coi2) <- loop ty'
292 ; return $ (args, coi2 `mkTransCoI` mkSymCoI coi1)
297 | isTyConableTyVar tv
298 = do { cts <- readMetaTyVar tv
300 Indirect ty -> loop ty
301 Flexi -> do { [fun_ty, arg_ty] <- withMetaTvs tv kinds mk_res_ty
302 ; return ((fun_ty, arg_ty), IdCo) } }
303 | otherwise -- defer as tyvar may be refined by equalities
306 tv_kind = tyVarKind tv
307 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
309 liftedTypeKind] -- arg type :: *
310 -- The defaultKind is a bit smelly. If you remove it,
311 -- try compiling f x = do { x }
312 -- and you'll get a kind mis-match. It smells, but
313 -- not enough to lose sleep over.
315 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
317 -- defer splitting by generating an equality constraint
318 defer = do { ([ty1, ty2], coi) <- boxySplitDefer arg_kinds mk_res_ty orig_ty
319 ; return ((ty1, ty2), coi)
322 orig_kind = typeKind orig_ty
323 arg_kinds = [mkArrowKind liftedTypeKind (defaultKind orig_kind),
325 liftedTypeKind] -- arg type :: *
327 -- build type application
328 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
329 mk_res_ty _other = panic "TcUnify.mk_res_ty2"
332 boxySplitFailure actual_ty expected_ty
333 = unifyMisMatch False False actual_ty expected_ty
334 -- "outer" is False, so we don't pop the context
335 -- which is what we want since we have not pushed one!
338 boxySplitDefer :: [Kind] -- kinds of required arguments
339 -> ([TcType] -> TcTauType) -- construct lhs from argument tyvars
340 -> BoxyRhoType -- type to split
341 -> TcM ([TcType], CoercionI)
342 boxySplitDefer kinds mkTy orig_ty
343 = do { tau_tys <- mapM newFlexiTyVarTy kinds
344 ; coi <- defer_unification False False (mkTy tau_tys) orig_ty
345 ; return (tau_tys, coi)
350 --------------------------------
351 -- withBoxes: the key utility function
352 --------------------------------
355 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
356 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
357 -> ([BoxySigmaType] -> BoxySigmaType)
358 -- Constructs the type to assign
359 -- to the original var
360 -> TcM [BoxySigmaType] -- Return the fresh boxes
362 -- It's entirely possible for the [kind] to be empty.
363 -- For example, when pattern-matching on True,
364 -- we call boxySplitTyConApp passing a boolTyCon
366 -- Invariant: tv is still Flexi
368 withMetaTvs tv kinds mk_res_ty
370 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
371 ; let box_tys = mkTyVarTys box_tvs
372 ; writeMetaTyVar tv (mk_res_ty box_tys)
375 | otherwise -- Non-boxy meta type variable
376 = do { tau_tys <- mapM newFlexiTyVarTy kinds
377 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
378 -- Sure to be a tau-type
381 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
382 -- Allocate a *boxy* tyvar
383 withBox kind thing_inside
384 = do { box_tv <- newMetaTyVar BoxTv kind
385 ; res <- thing_inside (mkTyVarTy box_tv)
386 ; ty <- {- pprTrace "with_box" (ppr (mkTyVarTy box_tv)) $ -} readFilledBox box_tv
391 %************************************************************************
393 Approximate boxy matching
395 %************************************************************************
398 preSubType :: [TcTyVar] -- Quantified type variables
399 -> TcTyVarSet -- Subset of quantified type variables
400 -- see Note [Pre-sub boxy]
401 -> TcType -- The rho-type part; quantified tyvars scopes over this
402 -> BoxySigmaType -- Matching type from the context
403 -> TcM [TcType] -- Types to instantiate the tyvars
404 -- Perform pre-subsumption, and return suitable types
405 -- to instantiate the quantified type varibles:
406 -- info from the pre-subsumption, if there is any
407 -- a boxy type variable otherwise
409 -- Note [Pre-sub boxy]
410 -- The 'btvs' are a subset of 'qtvs'. They are the ones we can
411 -- instantiate to a boxy type variable, because they'll definitely be
412 -- filled in later. This isn't always the case; sometimes we have type
413 -- variables mentioned in the context of the type, but not the body;
414 -- f :: forall a b. C a b => a -> a
415 -- Then we may land up with an unconstrained 'b', so we want to
416 -- instantiate it to a monotype (non-boxy) type variable
418 -- The 'qtvs' that are *neither* fixed by the pre-subsumption, *nor* are in 'btvs',
419 -- are instantiated to TauTv meta variables.
421 preSubType qtvs btvs qty expected_ty
422 = do { tys <- mapM inst_tv qtvs
423 ; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
426 pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
428 | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
429 | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
430 ; return (mkTyVarTy tv') }
431 | otherwise = do { tv' <- tcInstTyVar tv
432 ; return (mkTyVarTy tv') }
435 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
436 -> BoxyRhoType -- Type to match (note a *Rho* type)
437 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
439 -- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
440 -- "Boxy types: inference for higher rank types and impredicativity"
442 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
443 = go tmpl_tvs tmpl_ty emptyVarSet boxy_ty
445 go t_tvs t_ty b_tvs b_ty
446 | Just t_ty' <- tcView t_ty = go t_tvs t_ty' b_tvs b_ty
447 | Just b_ty' <- tcView b_ty = go t_tvs t_ty b_tvs b_ty'
449 go t_tvs (TyVarTy _) b_tvs b_ty = emptyTvSubst -- Rule S-ANY; no bindings
450 -- Rule S-ANY covers (a) type variables and (b) boxy types
451 -- in the template. Both look like a TyVarTy.
452 -- See Note [Sub-match] below
454 go t_tvs t_ty b_tvs b_ty
455 | isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
456 = go (t_tvs `delVarSetList` tvs) t_tau b_tvs b_ty -- Rule S-SPEC
457 -- Under a forall on the left, if there is shadowing,
458 -- do not bind! Hence the delVarSetList.
459 | isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
460 = go t_tvs t_ty (extendVarSetList b_tvs tvs) b_tau -- Rule S-SKOL
461 -- Add to the variables we must not bind to
462 -- NB: it's *important* to discard the theta part. Otherwise
463 -- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
464 -- and end up with a completely bogus binding (b |-> Bool), by lining
465 -- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
466 -- This pre-subsumption stuff can return too few bindings, but it
467 -- must *never* return bogus info.
469 go t_tvs (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
470 = boxy_match t_tvs arg1 b_tvs arg2 (go t_tvs res1 b_tvs res2)
471 -- Match the args, and sub-match the results
473 go t_tvs t_ty b_tvs b_ty = boxy_match t_tvs t_ty b_tvs b_ty emptyTvSubst
474 -- Otherwise defer to boxy matching
475 -- This covers TyConApp, AppTy, PredTy
482 |- head xs : <rhobox>
483 We will do a boxySubMatchType between a ~ <rhobox>
484 But we *don't* want to match [a |-> <rhobox>] because
485 (a) The box should be filled in with a rho-type, but
486 but the returned substitution maps TyVars to boxy
488 (b) In any case, the right final answer might be *either*
489 instantiate 'a' with a rho-type or a sigma type
490 head xs : Int vs head xs : forall b. b->b
491 So the matcher MUST NOT make a choice here. In general, we only
492 bind a template type variable in boxyMatchType, not in boxySubMatchType.
497 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
498 -> [BoxySigmaType] -- Type to match
499 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
501 -- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
502 -- "Boxy types: inference for higher rank types and impredicativity"
504 -- Find a *boxy* substitution that makes the template look as much
505 -- like the BoxySigmaType as possible.
506 -- It's always ok to return an empty substitution;
507 -- anything more is jam on the pudding
509 -- NB1: This is a pure, non-monadic function.
510 -- It does no unification, and cannot fail
512 -- Precondition: the arg lengths are equal
513 -- Precondition: none of the template type variables appear anywhere in the [BoxySigmaType]
517 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
518 = ASSERT( length tmpl_tys == length boxy_tys )
519 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
520 -- ToDo: add error context?
522 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
524 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
525 = boxy_match tmpl_tvs t_ty boxy_tvs b_ty $
526 boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys subst
527 boxy_match_s tmpl_tvs _ boxy_tvs _ subst
528 = panic "boxy_match_s" -- Lengths do not match
532 boxy_match :: TcTyVarSet -> TcType -- Template
533 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
534 -> BoxySigmaType -- Match against this type
538 -- The boxy_tvs argument prevents this match:
539 -- [a] forall b. a ~ forall b. b
540 -- We don't want to bind the template variable 'a'
541 -- to the quantified type variable 'b'!
543 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
544 = go orig_tmpl_ty orig_boxy_ty
547 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
548 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
550 go ty1 ty2 -- C.f. the isSigmaTy case for boxySubMatchType
552 , (tvs1, _, tau1) <- tcSplitSigmaTy ty1
553 , (tvs2, _, tau2) <- tcSplitSigmaTy ty2
554 , equalLength tvs1 tvs2
555 = boxy_match (tmpl_tvs `delVarSetList` tvs1) tau1
556 (boxy_tvs `extendVarSetList` tvs2) tau2 subst
558 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
560 , not $ isOpenSynTyCon tc1
563 go (FunTy arg1 res1) (FunTy arg2 res2)
564 = go_s [arg1,res1] [arg2,res2]
567 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
568 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
569 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
570 = go_s [s1,t1] [s2,t2]
573 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
574 , boxy_tvs `disjointVarSet` tyVarsOfType orig_boxy_ty
575 , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
576 = extendTvSubst subst tv boxy_ty'
578 = subst -- Ignore others
580 boxy_ty' = case lookupTyVar subst tv of
581 Nothing -> orig_boxy_ty
582 Just ty -> ty `boxyLub` orig_boxy_ty
584 go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
585 -- Example: Tree a ~ Maybe Int
586 -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
587 -- misleading error messages. An even more confusing case is
588 -- a -> b ~ Maybe Int
589 -- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
590 -- from this pre-matching phase.
593 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
596 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
597 -- Combine boxy information from the two types
598 -- If there is a conflict, return the first
599 boxyLub orig_ty1 orig_ty2
600 = go orig_ty1 orig_ty2
602 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
603 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
604 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
605 | tc1 == tc2, length ts1 == length ts2
606 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
608 go (TyVarTy tv1) ty2 -- This is the whole point;
609 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
612 -- Look inside type synonyms, but only if the naive version fails
613 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
614 | Just ty2' <- tcView ty1 = go ty1 ty2'
616 -- For now, we don't look inside ForAlls, PredTys
617 go ty1 ty2 = orig_ty1 -- Default
620 Note [Matching kinds]
621 ~~~~~~~~~~~~~~~~~~~~~
622 The target type might legitimately not be a sub-kind of template.
623 For example, suppose the target is simply a box with an OpenTypeKind,
624 and the template is a type variable with LiftedTypeKind.
625 Then it's ok (because the target type will later be refined).
626 We simply don't bind the template type variable.
628 It might also be that the kind mis-match is an error. For example,
629 suppose we match the template (a -> Int) against (Int# -> Int),
630 where the template type variable 'a' has LiftedTypeKind. This
631 matching function does not fail; it simply doesn't bind the template.
632 Later stuff will fail.
634 %************************************************************************
638 %************************************************************************
640 All the tcSub calls have the form
642 tcSub expected_ty offered_ty
644 offered_ty <= expected_ty
646 That is, that a value of type offered_ty is acceptable in
647 a place expecting a value of type expected_ty.
649 It returns a coercion function
650 co_fn :: offered_ty ~ expected_ty
651 which takes an HsExpr of type offered_ty into one of type
656 tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
657 -- (tcSub act exp) checks that
659 tcSubExp actual_ty expected_ty
660 = -- addErrCtxtM (unifyCtxt actual_ty expected_ty) $
661 -- Adding the error context here leads to some very confusing error
662 -- messages, such as "can't match forall a. a->a with forall a. a->a"
663 -- Example is tcfail165:
664 -- do var <- newEmptyMVar :: IO (MVar (forall a. Show a => a -> String))
665 -- putMVar var (show :: forall a. Show a => a -> String)
666 -- Here the info does not flow from the 'var' arg of putMVar to its 'show' arg
667 -- but after zonking it looks as if it does!
669 -- So instead I'm adding the error context when moving from tc_sub to u_tys
671 traceTc (text "tcSubExp" <+> ppr actual_ty <+> ppr expected_ty) >>
672 tc_sub SubOther actual_ty actual_ty False expected_ty expected_ty
674 tcFunResTy :: Name -> BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
675 tcFunResTy fun actual_ty expected_ty
676 = traceTc (text "tcFunResTy" <+> ppr actual_ty <+> ppr expected_ty) >>
677 tc_sub (SubFun fun) actual_ty actual_ty False expected_ty expected_ty
680 data SubCtxt = SubDone -- Error-context already pushed
681 | SubFun Name -- Context is tcFunResTy
682 | SubOther -- Context is something else
684 tc_sub :: SubCtxt -- How to add an error-context
685 -> BoxySigmaType -- actual_ty, before expanding synonyms
686 -> BoxySigmaType -- ..and after
687 -> InBox -- True <=> expected_ty is inside a box
688 -> BoxySigmaType -- expected_ty, before
689 -> BoxySigmaType -- ..and after
691 -- The acual_ty is never inside a box
692 -- IMPORTANT pre-condition: if the args contain foralls, the bound type
693 -- variables are visible non-monadically
694 -- (i.e. tha args are sufficiently zonked)
695 -- This invariant is needed so that we can "see" the foralls, ad
696 -- e.g. in the SPEC rule where we just use splitSigmaTy
698 tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
699 = traceTc (text "tc_sub" <+> ppr act_ty $$ ppr exp_ty) >>
700 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
701 -- This indirection is just here to make
702 -- it easy to insert a debug trace!
704 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
705 | Just exp_ty' <- tcView exp_ty = tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty'
706 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
707 | Just act_ty' <- tcView act_ty = tc_sub sub_ctxt act_sty act_ty' exp_ib exp_sty exp_ty
709 -----------------------------------
710 -- Rule SBOXY, plus other cases when act_ty is a type variable
711 -- Just defer to boxy matching
712 -- This rule takes precedence over SKOL!
713 tc_sub1 sub_ctxt act_sty (TyVarTy tv) exp_ib exp_sty exp_ty
714 = do { traceTc (text "tc_sub1 - case 1")
715 ; coi <- addSubCtxt sub_ctxt act_sty exp_sty $
716 uVar True False tv exp_ib exp_sty exp_ty
717 ; traceTc (case coi of
718 IdCo -> text "tc_sub1 (Rule SBOXY) IdCo"
719 ACo co -> text "tc_sub1 (Rule SBOXY) ACo" <+> ppr co)
720 ; return $ case coi of
725 -----------------------------------
726 -- Skolemisation case (rule SKOL)
727 -- actual_ty: d:Eq b => b->b
728 -- expected_ty: forall a. Ord a => a->a
729 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
731 -- It is essential to do this *before* the specialisation case
732 -- Example: f :: (Eq a => a->a) -> ...
733 -- g :: Ord b => b->b
736 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
738 = do { traceTc (text "tc_sub1 - case 2") ;
739 if exp_ib then -- SKOL does not apply if exp_ty is inside a box
740 defer_to_boxy_matching sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
742 { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ _ body_exp_ty ->
743 tc_sub sub_ctxt act_sty act_ty False body_exp_ty body_exp_ty
744 ; return (gen_fn <.> co_fn) }
747 act_tvs = tyVarsOfType act_ty
748 -- It's really important to check for escape wrt
749 -- the free vars of both expected_ty *and* actual_ty
751 -----------------------------------
752 -- Specialisation case (rule ASPEC):
753 -- actual_ty: forall a. Ord a => a->a
754 -- expected_ty: Int -> Int
755 -- co_fn e = e Int dOrdInt
757 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
758 -- Implements the new SPEC rule in the Appendix of the paper
759 -- "Boxy types: inference for higher rank types and impredicativity"
760 -- (This appendix isn't in the published version.)
761 -- The idea is to *first* do pre-subsumption, and then full subsumption
762 -- Example: forall a. a->a <= Int -> (forall b. Int)
763 -- Pre-subsumpion finds a|->Int, and that works fine, whereas
764 -- just running full subsumption would fail.
765 | isSigmaTy actual_ty
766 = do { traceTc (text "tc_sub1 - case 3")
767 ; -- Perform pre-subsumption, and instantiate
768 -- the type with info from the pre-subsumption;
769 -- boxy tyvars if pre-subsumption gives no info
770 let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
771 tau_tvs = exactTyVarsOfType tau
772 ; inst_tys <- if exp_ib then -- Inside a box, do not do clever stuff
773 do { tyvars' <- mapM tcInstBoxyTyVar tyvars
774 ; return (mkTyVarTys tyvars') }
775 else -- Outside, do clever stuff
776 preSubType tyvars tau_tvs tau expected_ty
777 ; let subst' = zipOpenTvSubst tyvars inst_tys
778 tau' = substTy subst' tau
780 -- Perform a full subsumption check
781 ; traceTc (text "tc_sub_spec" <+> vcat [ppr actual_ty,
782 ppr tyvars <+> ppr theta <+> ppr tau,
784 ; co_fn2 <- tc_sub sub_ctxt tau' tau' exp_ib exp_sty expected_ty
786 -- Deal with the dictionaries
787 -- The origin gives a helpful origin when we have
788 -- a function with type f :: Int -> forall a. Num a => ...
789 -- This way the (Num a) dictionary gets an OccurrenceOf f origin
790 ; let orig = case sub_ctxt of
791 SubFun n -> OccurrenceOf n
792 other -> InstSigOrigin -- Unhelpful
793 ; co_fn1 <- instCall orig inst_tys (substTheta subst' theta)
794 ; return (co_fn2 <.> co_fn1) }
796 -----------------------------------
797 -- Function case (rule F1)
798 tc_sub1 sub_ctxt act_sty (FunTy act_arg act_res) exp_ib exp_sty (FunTy exp_arg exp_res)
799 = do { traceTc (text "tc_sub1 - case 4")
800 ; addSubCtxt sub_ctxt act_sty exp_sty $
801 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
804 -- Function case (rule F2)
805 tc_sub1 sub_ctxt act_sty act_ty@(FunTy act_arg act_res) _ exp_sty (TyVarTy exp_tv)
807 = addSubCtxt sub_ctxt act_sty exp_sty $
808 do { traceTc (text "tc_sub1 - case 5")
809 ; cts <- readMetaTyVar exp_tv
811 Indirect ty -> tc_sub SubDone act_sty act_ty True exp_sty ty
812 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
813 ; tc_sub_funs act_arg act_res True arg_ty res_ty } }
815 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
816 mk_res_ty other = panic "TcUnify.mk_res_ty3"
817 fun_kinds = [argTypeKind, openTypeKind]
819 -- Everything else: defer to boxy matching
820 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty@(TyVarTy exp_tv)
821 = do { traceTc (text "tc_sub1 - case 6a" <+> ppr [isBoxyTyVar exp_tv, isMetaTyVar exp_tv, isSkolemTyVar exp_tv, isExistentialTyVar exp_tv,isSigTyVar exp_tv] )
822 ; defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
825 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
826 = do { traceTc (text "tc_sub1 - case 6")
827 ; defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
830 -----------------------------------
831 defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
832 = do { coi <- addSubCtxt sub_ctxt act_sty exp_sty $
833 u_tys outer False act_sty actual_ty exp_ib exp_sty expected_ty
834 ; return $ case coi of
839 outer = case sub_ctxt of -- Ugh
843 -----------------------------------
844 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
845 = do { arg_coi <- uTys False act_arg exp_ib exp_arg
846 ; co_fn_res <- tc_sub SubDone act_res act_res exp_ib exp_res exp_res
847 ; wrapper1 <- wrapFunResCoercion [exp_arg] co_fn_res
848 ; let wrapper2 = case arg_coi of
850 ACo co -> WpCo $ FunTy co act_res
851 ; return (wrapper1 <.> wrapper2)
854 -----------------------------------
856 :: [TcType] -- Type of args
857 -> HsWrapper -- HsExpr a -> HsExpr b
858 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
859 wrapFunResCoercion arg_tys co_fn_res
860 | isIdHsWrapper co_fn_res
865 = do { arg_ids <- newSysLocalIds FSLIT("sub") arg_tys
866 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpApps arg_ids) }
871 %************************************************************************
873 \subsection{Generalisation}
875 %************************************************************************
878 tcGen :: BoxySigmaType -- expected_ty
879 -> TcTyVarSet -- Extra tyvars that the universally
880 -- quantified tyvars of expected_ty
881 -- must not be unified
882 -> ([TcTyVar] -> BoxyRhoType -> TcM result)
883 -> TcM (HsWrapper, result)
884 -- The expression has type: spec_ty -> expected_ty
886 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
887 -- If not, the call is a no-op
888 = do { traceTc (text "tcGen")
889 -- We want the GenSkol info in the skolemised type variables to
890 -- mention the *instantiated* tyvar names, so that we get a
891 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
892 -- Hence the tiresome but innocuous fixM
893 ; ((tvs', theta', rho'), skol_info) <- fixM (\ ~(_, skol_info) ->
894 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
895 -- Get loation from monad, not from expected_ty
896 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty)
897 ; return ((forall_tvs, theta, rho_ty), skol_info) })
900 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
901 text "expected_ty" <+> ppr expected_ty,
902 text "inst ty" <+> ppr tvs' <+> ppr theta' <+> ppr rho',
903 text "free_tvs" <+> ppr free_tvs])
906 -- Type-check the arg and unify with poly type
907 ; (result, lie) <- getLIE (thing_inside tvs' rho')
909 -- Check that the "forall_tvs" havn't been constrained
910 -- The interesting bit here is that we must include the free variables
911 -- of the expected_ty. Here's an example:
912 -- runST (newVar True)
913 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
914 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
915 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
916 -- So now s' isn't unconstrained because it's linked to a.
917 -- Conclusion: include the free vars of the expected_ty in the
918 -- list of "free vars" for the signature check.
920 ; loc <- getInstLoc (SigOrigin skol_info)
921 ; dicts <- newDictBndrs loc theta'
922 ; inst_binds <- tcSimplifyCheck loc tvs' dicts lie
924 ; checkSigTyVarsWrt free_tvs tvs'
925 ; traceTc (text "tcGen:done")
928 -- The WpLet binds any Insts which came out of the simplification.
929 dict_vars = map instToVar dicts
930 co_fn = mkWpTyLams tvs' <.> mkWpLams dict_vars <.> WpLet inst_binds
931 ; returnM (co_fn, result) }
933 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
938 %************************************************************************
942 %************************************************************************
944 The exported functions are all defined as versions of some
945 non-exported generic functions.
948 boxyUnify :: BoxyType -> BoxyType -> TcM CoercionI
949 -- Acutal and expected, respectively
951 = addErrCtxtM (unifyCtxt ty1 ty2) $
952 uTysOuter False ty1 False ty2
955 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM [CoercionI]
956 -- Arguments should have equal length
957 -- Acutal and expected types
958 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
961 unifyType :: TcTauType -> TcTauType -> TcM CoercionI
962 -- No boxes expected inside these types
963 -- Acutal and expected types
964 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
965 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
966 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
967 addErrCtxtM (unifyCtxt ty1 ty2) $
968 uTysOuter True ty1 True ty2
971 unifyPred :: PredType -> PredType -> TcM CoercionI
972 -- Acutal and expected types
973 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
974 uPred True True p1 True p2
976 unifyTheta :: TcThetaType -> TcThetaType -> TcM [CoercionI]
977 -- Acutal and expected types
978 unifyTheta theta1 theta2
979 = do { checkTc (equalLength theta1 theta2)
980 (vcat [ptext SLIT("Contexts differ in length"),
981 nest 2 $ parens $ ptext SLIT("Use -fglasgow-exts to allow this")])
982 ; uList unifyPred theta1 theta2
986 uList :: (a -> a -> TcM b)
987 -> [a] -> [a] -> TcM [b]
988 -- Unify corresponding elements of two lists of types, which
989 -- should be of equal length. We charge down the list explicitly so that
990 -- we can complain if their lengths differ.
991 uList unify [] [] = return []
992 uList unify (ty1:tys1) (ty2:tys2) = do { x <- unify ty1 ty2;
993 ; xs <- uList unify tys1 tys2
996 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
999 @unifyTypeList@ takes a single list of @TauType@s and unifies them
1000 all together. It is used, for example, when typechecking explicit
1001 lists, when all the elts should be of the same type.
1004 unifyTypeList :: [TcTauType] -> TcM ()
1005 unifyTypeList [] = returnM ()
1006 unifyTypeList [ty] = returnM ()
1007 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
1008 ; unifyTypeList tys }
1011 %************************************************************************
1013 \subsection[Unify-uTys]{@uTys@: getting down to business}
1015 %************************************************************************
1017 @uTys@ is the heart of the unifier. Each arg occurs twice, because
1018 we want to report errors in terms of synomyms if possible. The first of
1019 the pair is used in error messages only; it is always the same as the
1020 second, except that if the first is a synonym then the second may be a
1021 de-synonym'd version. This way we get better error messages.
1023 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
1026 type SwapFlag = Bool
1027 -- False <=> the two args are (actual, expected) respectively
1028 -- True <=> the two args are (expected, actual) respectively
1030 type InBox = Bool -- True <=> we are inside a box
1031 -- False <=> we are outside a box
1032 -- The importance of this is that if we get "filled-box meets
1033 -- filled-box", we'll look into the boxes and unify... but
1034 -- we must not allow polytypes. But if we are in a box on
1035 -- just one side, then we can allow polytypes
1037 type Outer = Bool -- True <=> this is the outer level of a unification
1038 -- so that the types being unified are the
1039 -- very ones we began with, not some sub
1040 -- component or synonym expansion
1041 -- The idea is that if Outer is true then unifyMisMatch should
1042 -- pop the context to remove the "Expected/Acutal" context
1045 :: InBox -> TcType -- ty1 is the *actual* type
1046 -> InBox -> TcType -- ty2 is the *expected* type
1048 uTysOuter nb1 ty1 nb2 ty2
1049 = do { traceTc (text "uTysOuter" <+> ppr ty1 <+> ppr ty2)
1050 ; u_tys True nb1 ty1 ty1 nb2 ty2 ty2 }
1051 uTys nb1 ty1 nb2 ty2
1052 = do { traceTc (text "uTys" <+> ppr ty1 <+> ppr ty2)
1053 ; u_tys False nb1 ty1 ty1 nb2 ty2 ty2 }
1057 uTys_s :: InBox -> [TcType] -- tys1 are the *actual* types
1058 -> InBox -> [TcType] -- tys2 are the *expected* types
1060 uTys_s nb1 [] nb2 [] = returnM []
1061 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { coi <- uTys nb1 ty1 nb2 ty2
1062 ; cois <- uTys_s nb1 tys1 nb2 tys2
1065 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
1069 -> InBox -> TcType -> TcType -- ty1 is the *actual* type
1070 -> InBox -> TcType -> TcType -- ty2 is the *expected* type
1073 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
1074 = do { traceTc (text "u_tys " <+> ppr ty1 <+> text " " <+> ppr ty2)
1075 ; coi <- go outer ty1 ty2
1076 ; traceTc (case coi of
1077 ACo co -> text "u_tys yields coercion: " <+> ppr co
1078 IdCo -> text "u_tys yields no coercion")
1083 go :: Outer -> TcType -> TcType -> TcM CoercionI
1085 do { traceTc (text "go " <+> ppr orig_ty1 <+> text "/" <+> ppr ty1
1086 <+> ppr orig_ty2 <+> text "/" <+> ppr ty2)
1090 go1 :: Outer -> TcType -> TcType -> TcM CoercionI
1091 -- Always expand synonyms: see Note [Unification and synonyms]
1092 -- (this also throws away FTVs)
1094 | Just ty1' <- tcView ty1 = go False ty1' ty2
1095 | Just ty2' <- tcView ty2 = go False ty1 ty2'
1097 -- Variables; go for uVar
1098 go1 outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
1099 go1 outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
1100 -- "True" means args swapped
1102 -- The case for sigma-types must *follow* the variable cases
1103 -- because a boxy variable can be filed with a polytype;
1104 -- but must precede FunTy, because ((?x::Int) => ty) look
1105 -- like a FunTy; there isn't necy a forall at the top
1107 | isSigmaTy ty1 || isSigmaTy ty2
1108 = do { traceTc (text "We have sigma types: equalLength" <+> ppr tvs1 <+> ppr tvs2)
1109 ; checkM (equalLength tvs1 tvs2)
1110 (unifyMisMatch outer False orig_ty1 orig_ty2)
1111 ; traceTc (text "We're past the first length test")
1112 ; tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
1113 -- Get location from monad, not from tvs1
1114 ; let tys = mkTyVarTys tvs
1115 in_scope = mkInScopeSet (mkVarSet tvs)
1116 phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
1117 phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
1118 (theta1,tau1) = tcSplitPhiTy phi1
1119 (theta2,tau2) = tcSplitPhiTy phi2
1121 ; addErrCtxtM (unifyForAllCtxt tvs phi1 phi2) $ do
1122 { checkM (equalLength theta1 theta2)
1123 (unifyMisMatch outer False orig_ty1 orig_ty2)
1125 ; cois <- uPreds False nb1 theta1 nb2 theta2 -- TOMDO: do something with these pred_cois
1126 ; traceTc (text "TOMDO!")
1127 ; coi <- uTys nb1 tau1 nb2 tau2
1129 -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
1130 ; free_tvs <- zonkTcTyVarsAndFV (varSetElems (tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2))
1131 ; ifM (any (`elemVarSet` free_tvs) tvs)
1132 (bleatEscapedTvs free_tvs tvs tvs)
1134 -- If both sides are inside a box, we are in a "box-meets-box"
1135 -- situation, and we should not have a polytype at all.
1136 -- If we get here we have two boxes, already filled with
1137 -- the same polytype... but it should be a monotype.
1138 -- This check comes last, because the error message is
1139 -- extremely unhelpful.
1140 ; ifM (nb1 && nb2) (notMonoType ty1)
1144 (tvs1, body1) = tcSplitForAllTys ty1
1145 (tvs2, body2) = tcSplitForAllTys ty2
1148 go1 outer (PredTy p1) (PredTy p2)
1149 = uPred False nb1 p1 nb2 p2
1151 -- Type constructors must match
1152 go1 _ (TyConApp con1 tys1) (TyConApp con2 tys2)
1153 | con1 == con2 && not (isOpenSynTyCon con1)
1154 = do { cois <- uTys_s nb1 tys1 nb2 tys2
1155 ; return $ mkTyConAppCoI con1 tys1 cois
1157 -- See Note [TyCon app]
1158 | con1 == con2 && identicalOpenSynTyConApp
1159 = do { cois <- uTys_s nb1 tys1' nb2 tys2'
1160 ; return $ mkTyConAppCoI con1 tys1 (replicate n IdCo ++ cois)
1164 (idxTys1, tys1') = splitAt n tys1
1165 (idxTys2, tys2') = splitAt n tys2
1166 identicalOpenSynTyConApp = idxTys1 `tcEqTypes` idxTys2
1167 -- See Note [OpenSynTyCon app]
1169 -- Functions; just check the two parts
1170 go1 _ (FunTy fun1 arg1) (FunTy fun2 arg2)
1171 = do { coi_l <- uTys nb1 fun1 nb2 fun2
1172 ; coi_r <- uTys nb1 arg1 nb2 arg2
1173 ; return $ mkFunTyCoI fun1 coi_l arg1 coi_r
1176 -- Applications need a bit of care!
1177 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
1178 -- NB: we've already dealt with type variables and Notes,
1179 -- so if one type is an App the other one jolly well better be too
1180 go1 outer (AppTy s1 t1) ty2
1181 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
1182 = do { coi_s <- uTys nb1 s1 nb2 s2; coi_t <- uTys nb1 t1 nb2 t2
1183 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1185 -- Now the same, but the other way round
1186 -- Don't swap the types, because the error messages get worse
1187 go1 outer ty1 (AppTy s2 t2)
1188 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
1189 = do { coi_s <- uTys nb1 s1 nb2 s2; coi_t <- uTys nb1 t1 nb2 t2
1190 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1192 -- One or both outermost constructors are type family applications.
1193 -- If we can normalise them away, proceed as usual; otherwise, we
1194 -- need to defer unification by generating a wanted equality constraint.
1196 | ty1_is_fun || ty2_is_fun
1197 = do { (coi1, ty1') <- if ty1_is_fun then tcNormaliseFamInst ty1
1198 else return (IdCo, ty1)
1199 ; (coi2, ty2') <- if ty2_is_fun then tcNormaliseFamInst ty2
1200 else return (IdCo, ty2)
1201 ; coi <- if isOpenSynTyConApp ty1' || isOpenSynTyConApp ty2'
1202 then do { -- One type family app can't be reduced yet
1204 ; ty1'' <- zonkTcType ty1'
1205 ; ty2'' <- zonkTcType ty2'
1206 ; if tcEqType ty1'' ty2''
1208 else -- see [Deferred Unification]
1209 defer_unification outer False orig_ty1 orig_ty2
1211 else -- unification can proceed
1213 ; return $ coi1 `mkTransCoI` coi `mkTransCoI` (mkSymCoI coi2)
1216 ty1_is_fun = isOpenSynTyConApp ty1
1217 ty2_is_fun = isOpenSynTyConApp ty2
1219 -- Anything else fails
1220 go1 outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
1224 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
1226 do { coi <- uTys nb1 t1 nb2 t2
1227 ; return $ mkIParamPredCoI n1 coi
1229 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
1231 do { cois <- uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
1232 ; return $ mkClassPPredCoI c1 tys1 cois
1234 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
1236 uPreds outer nb1 [] nb2 [] = return []
1237 uPreds outer nb1 (p1:ps1) nb2 (p2:ps2) =
1238 do { coi <- uPred outer nb1 p1 nb2 p2
1239 ; cois <- uPreds outer nb1 ps1 nb2 ps2
1242 uPreds outer nb1 ps1 nb2 ps2 = panic "uPreds"
1247 When we find two TyConApps, the argument lists are guaranteed equal
1248 length. Reason: intially the kinds of the two types to be unified is
1249 the same. The only way it can become not the same is when unifying two
1250 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
1251 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
1252 which we do, that ensures that f1,f2 have the same kind; and that
1253 means a1,a2 have the same kind. And now the argument repeats.
1255 Note [OpenSynTyCon app]
1256 ~~~~~~~~~~~~~~~~~~~~~~~
1259 type family T a :: * -> *
1261 the two types (T () a) and (T () Int) must unify, even if there are
1262 no type instances for T at all. Should we just turn them into an
1263 equality (T () a ~ T () Int)? I don't think so. We currently try to
1264 eagerly unify everything we can before generating equalities; otherwise,
1265 we could turn the unification of [Int] with [a] into an equality, too.
1267 Note [Unification and synonyms]
1268 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1269 If you are tempted to make a short cut on synonyms, as in this
1273 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
1274 -- NO = if (con1 == con2) then
1275 -- NO -- Good news! Same synonym constructors, so we can shortcut
1276 -- NO -- by unifying their arguments and ignoring their expansions.
1277 -- NO unifyTypepeLists args1 args2
1279 -- NO -- Never mind. Just expand them and try again
1283 then THINK AGAIN. Here is the whole story, as detected and reported
1284 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1286 Here's a test program that should detect the problem:
1290 x = (1 :: Bogus Char) :: Bogus Bool
1293 The problem with [the attempted shortcut code] is that
1297 is not a sufficient condition to be able to use the shortcut!
1298 You also need to know that the type synonym actually USES all
1299 its arguments. For example, consider the following type synonym
1300 which does not use all its arguments.
1305 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1306 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1307 would fail, even though the expanded forms (both \tr{Int}) should
1310 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1311 unnecessarily bind \tr{t} to \tr{Char}.
1313 ... You could explicitly test for the problem synonyms and mark them
1314 somehow as needing expansion, perhaps also issuing a warning to the
1319 %************************************************************************
1321 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1323 %************************************************************************
1325 @uVar@ is called when at least one of the types being unified is a
1326 variable. It does {\em not} assume that the variable is a fixed point
1327 of the substitution; rather, notice that @uVar@ (defined below) nips
1328 back into @uTys@ if it turns out that the variable is already bound.
1332 -> SwapFlag -- False => tyvar is the "actual" (ty is "expected")
1333 -- True => ty is the "actual" (tyvar is "expected")
1335 -> InBox -- True <=> definitely no boxes in t2
1336 -> TcTauType -> TcTauType -- printing and real versions
1339 uVar outer swapped tv1 nb2 ps_ty2 ty2
1340 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1341 | otherwise = brackets (equals <+> ppr ty2)
1342 ; traceTc (text "uVar" <+> ppr swapped <+>
1343 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1344 nest 2 (ptext SLIT(" <-> ")),
1345 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1346 ; details <- lookupTcTyVar tv1
1349 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1350 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1351 -- The 'True' here says that ty1 is now inside a box
1352 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1356 uUnfilledVar :: Outer
1358 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1359 -> TcTauType -> TcTauType -- Type 2
1361 -- Invariant: tyvar 1 is not unified with anything
1363 uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1364 | Just ty2' <- tcView ty2
1365 = -- Expand synonyms; ignore FTVs
1366 uUnfilledVar False swapped tv1 details1 ps_ty2 ty2'
1368 uUnfilledVar outer swapped tv1 details1 ps_ty2 (TyVarTy tv2)
1369 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1371 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1372 -- this is box-meets-box, so fill in with a tau-type
1373 -> do { tau_tv <- tcInstTyVar tv1
1374 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv)
1377 other -> returnM IdCo -- No-op
1379 | otherwise -- Distinct type variables
1380 = do { lookup2 <- lookupTcTyVar tv2
1382 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
1383 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1386 uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2
1387 = -- ty2 is not a type variable
1389 MetaTv (SigTv _) _ -> rigid_variable
1391 uMetaVar outer swapped tv1 info ref1 ps_ty2 non_var_ty2
1392 SkolemTv _ -> rigid_variable
1395 | isOpenSynTyConApp non_var_ty2
1396 = -- 'non_var_ty2's outermost constructor is a type family,
1397 -- which we may may be able to normalise
1398 do { (coi2, ty2') <- tcNormaliseFamInst non_var_ty2
1400 IdCo -> -- no progress, but maybe after other instantiations
1401 defer_unification outer swapped (TyVarTy tv1) ps_ty2
1402 ACo co -> -- progress: so lets try again
1404 ppr co <+> text "::"<+> ppr non_var_ty2 <+> text "~" <+>
1406 ; coi <- uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2'
1407 ; let coi2' = (if swapped then id else mkSymCoI) coi2
1408 ; return $ coi2' `mkTransCoI` coi
1411 | SkolemTv RuntimeUnkSkol <- details1
1412 -- runtime unknown will never match
1413 = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1414 | otherwise -- defer as a given equality may still resolve this
1415 = defer_unification outer swapped (TyVarTy tv1) ps_ty2
1418 Note [Deferred Unification]
1419 ~~~~~~~~~~~~~~~~~~~~
1420 We may encounter a unification ty1 = ty2 that cannot be performed syntactically,
1421 and yet its consistency is undetermined. Previously, there was no way to still
1422 make it consistent. So a mismatch error was issued.
1424 Now these unfications are deferred until constraint simplification, where type
1425 family instances and given equations may (or may not) establish the consistency.
1426 Deferred unifications are of the form
1429 where F is a type function and x is a type variable.
1431 id :: x ~ y => x -> y
1434 involves the unfication x = y. It is deferred until we bring into account the
1435 context x ~ y to establish that it holds.
1437 If available, we defer original types (rather than those where closed type
1438 synonyms have already been expanded via tcCoreView). This is, as usual, to
1439 improve error messages.
1441 We need to both 'unBox' and zonk deferred types. We need to unBox as
1442 functions, such as TcExpr.tcMonoExpr promise to fill boxes in the expected
1443 type. We need to zonk as the types go into the kind of the coercion variable
1444 `cotv' and those are not zonked in Inst.zonkInst. (Maybe it would be better
1445 to zonk in zonInst instead. Would that be sufficient?)
1448 defer_unification :: Bool -- pop innermost context?
1453 defer_unification outer True ty1 ty2
1454 = defer_unification outer False ty2 ty1
1455 defer_unification outer False ty1 ty2
1456 = do { ty1' <- unBox ty1 >>= zonkTcType -- unbox *and* zonk..
1457 ; ty2' <- unBox ty2 >>= zonkTcType -- ..see preceding note
1458 ; traceTc $ text "deferring:" <+> ppr ty1 <+> text "~" <+> ppr ty2
1459 ; cotv <- newMetaCoVar ty1' ty2'
1460 -- put ty1 ~ ty2 in LIE
1461 -- Left means "wanted"
1462 ; inst <- (if outer then popErrCtxt else id) $
1463 mkEqInst (EqPred ty1' ty2') (Left cotv)
1465 ; return $ ACo $ TyVarTy cotv }
1468 uMetaVar :: Bool -- pop innermost context?
1470 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1473 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1474 -- ty2 is not a type variable
1476 uMetaVar outer swapped tv1 BoxTv ref1 ps_ty2 non_var_ty2
1477 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1478 -- that any boxes in ty2 are filled with monotypes
1480 -- It should not be the case that tv1 occurs in ty2
1481 -- (i.e. no occurs check should be needed), but if perchance
1482 -- it does, the unbox operation will fill it, and the DEBUG
1484 do { final_ty <- unBox ps_ty2
1486 ; meta_details <- readMutVar ref1
1487 ; case meta_details of
1488 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1489 return () -- This really should *not* happen
1492 ; checkUpdateMeta swapped tv1 ref1 final_ty
1496 uMetaVar outer swapped tv1 info1 ref1 ps_ty2 non_var_ty2
1497 = do { -- Occurs check + monotype check
1498 ; mb_final_ty <- checkTauTvUpdate tv1 ps_ty2
1499 ; case mb_final_ty of
1500 Nothing -> -- tv1 occured in type family parameter
1501 defer_unification outer swapped (mkTyVarTy tv1) ps_ty2
1503 do { checkUpdateMeta swapped tv1 ref1 final_ty
1509 uUnfilledVars :: Outer
1511 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1512 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1514 -- Invarant: The type variables are distinct,
1515 -- Neither is filled in yet
1516 -- They might be boxy or not
1518 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1519 = -- see [Deferred Unification]
1520 defer_unification outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1522 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1523 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2) >> return IdCo
1524 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1525 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1) >> return IdCo
1527 -- ToDo: this function seems too long for what it acutally does!
1528 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1529 = case (info1, info2) of
1530 (BoxTv, BoxTv) -> box_meets_box >> return IdCo
1532 -- If a box meets a TauTv, but the fomer has the smaller kind
1533 -- then we must create a fresh TauTv with the smaller kind
1534 (_, BoxTv) | k1_sub_k2 -> update_tv2 >> return IdCo
1535 | otherwise -> box_meets_box >> return IdCo
1536 (BoxTv, _ ) | k2_sub_k1 -> update_tv1 >> return IdCo
1537 | otherwise -> box_meets_box >> return IdCo
1539 -- Avoid SigTvs if poss
1540 (SigTv _, _ ) | k1_sub_k2 -> update_tv2 >> return IdCo
1541 (_, SigTv _) | k2_sub_k1 -> update_tv1 >> return IdCo
1543 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1544 then update_tv1 >> return IdCo -- Same kinds
1545 else update_tv2 >> return IdCo
1546 | k2_sub_k1 -> update_tv1 >> return IdCo
1547 | otherwise -> kind_err >> return IdCo
1549 -- Update the variable with least kind info
1550 -- See notes on type inference in Kind.lhs
1551 -- The "nicer to" part only applies if the two kinds are the same,
1552 -- so we can choose which to do.
1554 -- Kinds should be guaranteed ok at this point
1555 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1556 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1558 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1561 | k2_sub_k1 = fill_from tv2
1562 | otherwise = kind_err
1564 -- Update *both* tyvars with a TauTv whose name and kind
1565 -- are gotten from tv (avoid losing nice names is poss)
1566 fill_from tv = do { tv' <- tcInstTyVar tv
1567 ; let tau_ty = mkTyVarTy tv'
1568 ; updateMeta tv1 ref1 tau_ty
1569 ; updateMeta tv2 ref2 tau_ty }
1571 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1572 unifyKindMisMatch k1 k2
1576 k1_sub_k2 = k1 `isSubKind` k2
1577 k2_sub_k1 = k2 `isSubKind` k1
1579 nicer_to_update_tv1 = isSystemName (Var.varName tv1)
1580 -- Try to update sys-y type variables in preference to ones
1581 -- gotten (say) by instantiating a polymorphic function with
1582 -- a user-written type sig
1586 refineBox :: TcType -> TcM TcType
1587 -- Unbox the outer box of a boxy type (if any)
1588 refineBox ty@(TyVarTy box_tv)
1589 | isMetaTyVar box_tv
1590 = do { cts <- readMetaTyVar box_tv
1593 Indirect ty -> return ty }
1594 refineBox other_ty = return other_ty
1596 refineBoxToTau :: TcType -> TcM TcType
1597 -- Unbox the outer box of a boxy type, filling with a monotype if it is empty
1598 -- Like refineBox except for the "fill with monotype" part.
1599 refineBoxToTau ty@(TyVarTy box_tv)
1600 | isMetaTyVar box_tv
1601 , MetaTv BoxTv ref <- tcTyVarDetails box_tv
1602 = do { cts <- readMutVar ref
1604 Flexi -> fillBoxWithTau box_tv ref
1605 Indirect ty -> return ty }
1606 refineBoxToTau other_ty = return other_ty
1608 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1609 -- Subtle... we must zap the boxy res_ty
1610 -- to kind * before using it to instantiate a LitInst
1611 -- Calling unBox instead doesn't do the job, because the box
1612 -- often has an openTypeKind, and we don't want to instantiate
1614 zapToMonotype res_ty
1615 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1616 ; boxyUnify res_tau res_ty
1619 unBox :: BoxyType -> TcM TcType
1620 -- unBox implements the judgement
1622 -- with input s', and result s
1624 -- It removes all boxes from the input type, returning a non-boxy type.
1625 -- A filled box in the type can only contain a monotype; unBox fails if not
1626 -- The type can have empty boxes, which unBox fills with a monotype
1628 -- Compare this wth checkTauTvUpdate
1630 -- For once, it's safe to treat synonyms as opaque!
1632 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1633 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1634 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1635 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1636 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1637 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1638 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1640 | isTcTyVar tv -- It's a boxy type variable
1641 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1642 = do { cts <- readMutVar ref -- under nested quantifiers
1644 Flexi -> fillBoxWithTau tv ref
1645 Indirect ty -> do { non_boxy_ty <- unBox ty
1646 ; if isTauTy non_boxy_ty
1647 then return non_boxy_ty
1648 else notMonoType non_boxy_ty }
1650 | otherwise -- Skolems, and meta-tau-variables
1651 = return (TyVarTy tv)
1653 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1654 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1655 unBoxPred (EqPred ty1 ty2) = do { ty1' <- unBox ty1; ty2' <- unBox ty2; return (EqPred ty1' ty2') }
1660 %************************************************************************
1662 \subsection[Unify-context]{Errors and contexts}
1664 %************************************************************************
1670 unifyCtxt act_ty exp_ty tidy_env
1671 = do { act_ty' <- zonkTcType act_ty
1672 ; exp_ty' <- zonkTcType exp_ty
1673 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1674 (env2, act_ty'') = tidyOpenType env1 act_ty'
1675 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1678 mkExpectedActualMsg act_ty exp_ty
1679 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1680 text "Inferred type" <> colon <+> ppr act_ty ])
1683 -- If an error happens we try to figure out whether the function
1684 -- function has been given too many or too few arguments, and say so.
1685 addSubCtxt SubDone actual_res_ty expected_res_ty thing_inside
1687 addSubCtxt sub_ctxt actual_res_ty expected_res_ty thing_inside
1688 = addErrCtxtM mk_err thing_inside
1691 = do { exp_ty' <- zonkTcType expected_res_ty
1692 ; act_ty' <- zonkTcType actual_res_ty
1693 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1694 (env2, act_ty'') = tidyOpenType env1 act_ty'
1695 (exp_args, _) = tcSplitFunTys exp_ty''
1696 (act_args, _) = tcSplitFunTys act_ty''
1698 len_act_args = length act_args
1699 len_exp_args = length exp_args
1701 message = case sub_ctxt of
1702 SubFun fun | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1703 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1704 other -> mkExpectedActualMsg act_ty'' exp_ty''
1705 ; return (env2, message) }
1707 wrongArgsCtxt too_many_or_few fun
1708 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1709 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1710 <+> ptext SLIT("arguments")
1713 unifyForAllCtxt tvs phi1 phi2 env
1714 = returnM (env2, msg)
1716 (env', tvs') = tidyOpenTyVars env tvs -- NB: not tidyTyVarBndrs
1717 (env1, phi1') = tidyOpenType env' phi1
1718 (env2, phi2') = tidyOpenType env1 phi2
1719 msg = vcat [ptext SLIT("When matching") <+> quotes (ppr (mkForAllTys tvs' phi1')),
1720 ptext SLIT(" and") <+> quotes (ppr (mkForAllTys tvs' phi2'))]
1722 -----------------------
1723 unifyMisMatch outer swapped ty1 ty2
1724 = do { (env, msg) <- if swapped then misMatchMsg ty2 ty1
1725 else misMatchMsg ty1 ty2
1727 -- This is the whole point of the 'outer' stuff
1728 ; if outer then popErrCtxt (failWithTcM (env, msg))
1729 else failWithTcM (env, msg)
1734 %************************************************************************
1738 %************************************************************************
1740 Unifying kinds is much, much simpler than unifying types.
1743 unifyKind :: TcKind -- Expected
1746 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1747 | isSubKindCon kc2 kc1 = returnM ()
1749 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1750 = do { unifyKind a2 a1; unifyKind r1 r2 }
1751 -- Notice the flip in the argument,
1752 -- so that the sub-kinding works right
1753 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1754 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1755 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1757 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1758 unifyKinds [] [] = returnM ()
1759 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1761 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1764 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1765 uKVar swapped kv1 k2
1766 = do { mb_k1 <- readKindVar kv1
1768 Flexi -> uUnboundKVar swapped kv1 k2
1769 Indirect k1 | swapped -> unifyKind k2 k1
1770 | otherwise -> unifyKind k1 k2 }
1773 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1774 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1775 | kv1 == kv2 = returnM ()
1776 | otherwise -- Distinct kind variables
1777 = do { mb_k2 <- readKindVar kv2
1779 Indirect k2 -> uUnboundKVar swapped kv1 k2
1780 Flexi -> writeKindVar kv1 k2 }
1782 uUnboundKVar swapped kv1 non_var_k2
1783 = do { k2' <- zonkTcKind non_var_k2
1784 ; kindOccurCheck kv1 k2'
1785 ; k2'' <- kindSimpleKind swapped k2'
1786 -- KindVars must be bound only to simple kinds
1787 -- Polarities: (kindSimpleKind True ?) succeeds
1788 -- returning *, corresponding to unifying
1791 ; writeKindVar kv1 k2'' }
1794 kindOccurCheck kv1 k2 -- k2 is zonked
1795 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1797 not_in (TyVarTy kv2) = kv1 /= kv2
1798 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1801 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1802 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1803 -- If the flag is False, it requires k <: sk
1804 -- E.g. kindSimpleKind False ?? = *
1805 -- What about (kv -> *) :=: ?? -> *
1806 kindSimpleKind orig_swapped orig_kind
1807 = go orig_swapped orig_kind
1809 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1811 ; return (mkArrowKind k1' k2') }
1813 | isOpenTypeKind k = return liftedTypeKind
1814 | isArgTypeKind k = return liftedTypeKind
1816 | isLiftedTypeKind k = return liftedTypeKind
1817 | isUnliftedTypeKind k = return unliftedTypeKind
1818 go sw k@(TyVarTy _) = return k -- KindVars are always simple
1819 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1820 <+> ppr orig_swapped <+> ppr orig_kind)
1821 -- I think this can't actually happen
1823 -- T v = MkT v v must be a type
1824 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1827 kindOccurCheckErr tyvar ty
1828 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1829 2 (sep [ppr tyvar, char '=', ppr ty])
1833 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1834 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1836 unifyFunKind (TyVarTy kvar)
1837 = readKindVar kvar `thenM` \ maybe_kind ->
1839 Indirect fun_kind -> unifyFunKind fun_kind
1841 do { arg_kind <- newKindVar
1842 ; res_kind <- newKindVar
1843 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1844 ; returnM (Just (arg_kind,res_kind)) }
1846 unifyFunKind (FunTy arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1847 unifyFunKind other = returnM Nothing
1850 %************************************************************************
1854 %************************************************************************
1856 ---------------------------
1857 -- We would like to get a decent error message from
1858 -- (a) Under-applied type constructors
1859 -- f :: (Maybe, Maybe)
1860 -- (b) Over-applied type constructors
1861 -- f :: Int x -> Int x
1865 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1866 -- A fancy wrapper for 'unifyKind', which tries
1867 -- to give decent error messages.
1868 -- (checkExpectedKind ty act_kind exp_kind)
1869 -- checks that the actual kind act_kind is compatible
1870 -- with the expected kind exp_kind
1871 -- The first argument, ty, is used only in the error message generation
1872 checkExpectedKind ty act_kind exp_kind
1873 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1876 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1878 Just r -> returnM () ; -- Unification succeeded
1881 -- So there's definitely an error
1882 -- Now to find out what sort
1883 zonkTcKind exp_kind `thenM` \ exp_kind ->
1884 zonkTcKind act_kind `thenM` \ act_kind ->
1886 tcInitTidyEnv `thenM` \ env0 ->
1887 let (exp_as, _) = splitKindFunTys exp_kind
1888 (act_as, _) = splitKindFunTys act_kind
1889 n_exp_as = length exp_as
1890 n_act_as = length act_as
1892 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1893 (env2, tidy_act_kind) = tidyKind env1 act_kind
1895 err | n_exp_as < n_act_as -- E.g. [Maybe]
1896 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1898 -- Now n_exp_as >= n_act_as. In the next two cases,
1899 -- n_exp_as == 0, and hence so is n_act_as
1900 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1901 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1902 <+> ptext SLIT("is unlifted")
1904 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1905 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1906 <+> ptext SLIT("is lifted")
1908 | otherwise -- E.g. Monad [Int]
1909 = ptext SLIT("Kind mis-match")
1911 more_info = sep [ ptext SLIT("Expected kind") <+>
1912 quotes (pprKind tidy_exp_kind) <> comma,
1913 ptext SLIT("but") <+> quotes (ppr ty) <+>
1914 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1916 failWithTcM (env2, err $$ more_info)
1920 %************************************************************************
1922 \subsection{Checking signature type variables}
1924 %************************************************************************
1926 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1927 are not mentioned in the environment. In particular:
1929 (a) Not mentioned in the type of a variable in the envt
1930 eg the signature for f in this:
1936 Here, f is forced to be monorphic by the free occurence of x.
1938 (d) Not (unified with another type variable that is) in scope.
1939 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1940 when checking the expression type signature, we find that
1941 even though there is nothing in scope whose type mentions r,
1942 nevertheless the type signature for the expression isn't right.
1944 Another example is in a class or instance declaration:
1946 op :: forall b. a -> b
1948 Here, b gets unified with a
1950 Before doing this, the substitution is applied to the signature type variable.
1953 checkSigTyVars :: [TcTyVar] -> TcM ()
1954 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1956 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1957 -- The extra_tvs can include boxy type variables;
1958 -- e.g. TcMatches.tcCheckExistentialPat
1959 checkSigTyVarsWrt extra_tvs sig_tvs
1960 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1961 ; check_sig_tyvars extra_tvs' sig_tvs }
1964 :: TcTyVarSet -- Global type variables. The universally quantified
1965 -- tyvars should not mention any of these
1966 -- Guaranteed already zonked.
1967 -> [TcTyVar] -- Universally-quantified type variables in the signature
1968 -- Guaranteed to be skolems
1970 check_sig_tyvars extra_tvs []
1972 check_sig_tyvars extra_tvs sig_tvs
1973 = ASSERT( all isSkolemTyVar sig_tvs )
1974 do { gbl_tvs <- tcGetGlobalTyVars
1975 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1976 text "gbl_tvs" <+> ppr gbl_tvs,
1977 text "extra_tvs" <+> ppr extra_tvs]))
1979 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1980 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1981 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1984 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1985 -> [TcTyVar] -- The possibly-escaping type variables
1986 -> [TcTyVar] -- The zonked versions thereof
1988 -- Complain about escaping type variables
1989 -- We pass a list of type variables, at least one of which
1990 -- escapes. The first list contains the original signature type variable,
1991 -- while the second contains the type variable it is unified to (usually itself)
1992 bleatEscapedTvs globals sig_tvs zonked_tvs
1993 = do { env0 <- tcInitTidyEnv
1994 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1995 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1997 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1998 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
2000 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
2002 check (tidy_env, msgs) (sig_tv, zonked_tv)
2003 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
2005 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
2006 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
2008 -----------------------
2009 escape_msg sig_tv zonked_tv globs
2011 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
2012 nest 2 (vcat globs)]
2014 = msg <+> ptext SLIT("escapes")
2015 -- Sigh. It's really hard to give a good error message
2016 -- all the time. One bad case is an existential pattern match.
2017 -- We rely on the "When..." context to help.
2019 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
2021 | sig_tv == zonked_tv = empty
2022 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
2025 These two context are used with checkSigTyVars
2028 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
2029 -> TidyEnv -> TcM (TidyEnv, Message)
2030 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
2031 = zonkTcType sig_tau `thenM` \ actual_tau ->
2033 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
2034 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
2035 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
2036 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
2037 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
2039 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),