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, 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
195 -- It's used for wired-in tycons, so we call checkWiredInTyCOn
196 -- Precondition: never called with FunTyCon
197 -- Precondition: input type :: *
199 boxySplitTyConApp tc orig_ty
200 = do { checkWiredInTyCon tc
201 ; loop (tyConArity tc) [] orig_ty }
203 loop n_req args_so_far ty
204 | Just ty' <- tcView ty = loop n_req args_so_far ty'
206 loop n_req args_so_far (TyConApp tycon args)
208 = ASSERT( n_req == length args) -- ty::*
209 return (args ++ args_so_far)
211 loop n_req args_so_far (AppTy fun arg)
213 = loop (n_req - 1) (arg:args_so_far) fun
215 loop n_req args_so_far (TyVarTy tv)
216 | isTyConableTyVar tv
217 , res_kind `isSubKind` tyVarKind tv
218 = do { cts <- readMetaTyVar tv
220 Indirect ty -> loop n_req args_so_far ty
221 Flexi -> do { arg_tys <- withMetaTvs tv arg_kinds mk_res_ty
222 ; return (arg_tys ++ args_so_far) }
225 mk_res_ty arg_tys' = mkTyConApp tc arg_tys'
226 (arg_kinds, res_kind) = splitKindFunTysN n_req (tyConKind tc)
228 loop _ _ _ = boxySplitFailure (mkTyConApp tc (mkTyVarTys (tyConTyVars tc))) orig_ty
230 ----------------------
231 boxySplitListTy :: BoxyRhoType -> TcM BoxySigmaType -- Special case for lists
232 boxySplitListTy exp_ty = do { [elt_ty] <- boxySplitTyConApp listTyCon exp_ty
236 ----------------------
237 boxySplitAppTy :: BoxyRhoType -- Type to split: m a
238 -> TcM (BoxySigmaType, BoxySigmaType) -- Returns m, a
239 -- If the incoming type is a mutable type variable of kind k, then
240 -- boxySplitAppTy returns a new type variable (m: * -> k); note the *.
241 -- If the incoming type is boxy, then so are the result types; and vice versa
243 boxySplitAppTy orig_ty
247 | Just ty' <- tcView ty = loop ty'
250 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
251 = return (fun_ty, arg_ty)
254 | isTyConableTyVar tv
255 = do { cts <- readMetaTyVar tv
257 Indirect ty -> loop ty
258 Flexi -> do { [fun_ty,arg_ty] <- withMetaTvs tv kinds mk_res_ty
259 ; return (fun_ty, arg_ty) } }
261 mk_res_ty [fun_ty', arg_ty'] = mkAppTy fun_ty' arg_ty'
262 mk_res_ty other = panic "TcUnify.mk_res_ty2"
263 tv_kind = tyVarKind tv
264 kinds = [mkArrowKind liftedTypeKind (defaultKind tv_kind),
266 liftedTypeKind] -- arg type :: *
267 -- The defaultKind is a bit smelly. If you remove it,
268 -- try compiling f x = do { x }
269 -- and you'll get a kind mis-match. It smells, but
270 -- not enough to lose sleep over.
272 loop _ = boxySplitFailure (mkAppTy alphaTy betaTy) orig_ty
275 boxySplitFailure actual_ty expected_ty
276 = unifyMisMatch False False actual_ty expected_ty
277 -- "outer" is False, so we don't pop the context
278 -- which is what we want since we have not pushed one!
282 --------------------------------
283 -- withBoxes: the key utility function
284 --------------------------------
287 withMetaTvs :: TcTyVar -- An unfilled-in, non-skolem, meta type variable
288 -> [Kind] -- Make fresh boxes (with the same BoxTv/TauTv setting as tv)
289 -> ([BoxySigmaType] -> BoxySigmaType)
290 -- Constructs the type to assign
291 -- to the original var
292 -> TcM [BoxySigmaType] -- Return the fresh boxes
294 -- It's entirely possible for the [kind] to be empty.
295 -- For example, when pattern-matching on True,
296 -- we call boxySplitTyConApp passing a boolTyCon
298 -- Invariant: tv is still Flexi
300 withMetaTvs tv kinds mk_res_ty
302 = do { box_tvs <- mapM (newMetaTyVar BoxTv) kinds
303 ; let box_tys = mkTyVarTys box_tvs
304 ; writeMetaTyVar tv (mk_res_ty box_tys)
307 | otherwise -- Non-boxy meta type variable
308 = do { tau_tys <- mapM newFlexiTyVarTy kinds
309 ; writeMetaTyVar tv (mk_res_ty tau_tys) -- Write it *first*
310 -- Sure to be a tau-type
313 withBox :: Kind -> (BoxySigmaType -> TcM a) -> TcM (a, TcType)
314 -- Allocate a *boxy* tyvar
315 withBox kind thing_inside
316 = do { box_tv <- newMetaTyVar BoxTv kind
317 ; res <- thing_inside (mkTyVarTy box_tv)
318 ; ty <- {- pprTrace "with_box" (ppr (mkTyVarTy box_tv)) $ -} readFilledBox box_tv
323 %************************************************************************
325 Approximate boxy matching
327 %************************************************************************
330 preSubType :: [TcTyVar] -- Quantified type variables
331 -> TcTyVarSet -- Subset of quantified type variables
332 -- see Note [Pre-sub boxy]
333 -> TcType -- The rho-type part; quantified tyvars scopes over this
334 -> BoxySigmaType -- Matching type from the context
335 -> TcM [TcType] -- Types to instantiate the tyvars
336 -- Perform pre-subsumption, and return suitable types
337 -- to instantiate the quantified type varibles:
338 -- info from the pre-subsumption, if there is any
339 -- a boxy type variable otherwise
341 -- Note [Pre-sub boxy]
342 -- The 'btvs' are a subset of 'qtvs'. They are the ones we can
343 -- instantiate to a boxy type variable, because they'll definitely be
344 -- filled in later. This isn't always the case; sometimes we have type
345 -- variables mentioned in the context of the type, but not the body;
346 -- f :: forall a b. C a b => a -> a
347 -- Then we may land up with an unconstrained 'b', so we want to
348 -- instantiate it to a monotype (non-boxy) type variable
350 -- The 'qtvs' that are *neither* fixed by the pre-subsumption, *nor* are in 'btvs',
351 -- are instantiated to TauTv meta variables.
353 preSubType qtvs btvs qty expected_ty
354 = do { tys <- mapM inst_tv qtvs
355 ; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
358 pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
360 | Just boxy_ty <- lookupTyVar pre_subst tv = return boxy_ty
361 | tv `elemVarSet` btvs = do { tv' <- tcInstBoxyTyVar tv
362 ; return (mkTyVarTy tv') }
363 | otherwise = do { tv' <- tcInstTyVar tv
364 ; return (mkTyVarTy tv') }
367 :: TcTyVarSet -> TcType -- The "template"; the tyvars are skolems
368 -> BoxyRhoType -- Type to match (note a *Rho* type)
369 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
371 -- boxySubMatchType implements the Pre-subsumption judgement, in Fig 5 of the paper
372 -- "Boxy types: inference for higher rank types and impredicativity"
374 boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
375 = go tmpl_tvs tmpl_ty emptyVarSet boxy_ty
377 go t_tvs t_ty b_tvs b_ty
378 | Just t_ty' <- tcView t_ty = go t_tvs t_ty' b_tvs b_ty
379 | Just b_ty' <- tcView b_ty = go t_tvs t_ty b_tvs b_ty'
381 go t_tvs (TyVarTy _) b_tvs b_ty = emptyTvSubst -- Rule S-ANY; no bindings
382 -- Rule S-ANY covers (a) type variables and (b) boxy types
383 -- in the template. Both look like a TyVarTy.
384 -- See Note [Sub-match] below
386 go t_tvs t_ty b_tvs b_ty
387 | isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
388 = go (t_tvs `delVarSetList` tvs) t_tau b_tvs b_ty -- Rule S-SPEC
389 -- Under a forall on the left, if there is shadowing,
390 -- do not bind! Hence the delVarSetList.
391 | isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
392 = go t_tvs t_ty (extendVarSetList b_tvs tvs) b_tau -- Rule S-SKOL
393 -- Add to the variables we must not bind to
394 -- NB: it's *important* to discard the theta part. Otherwise
395 -- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
396 -- and end up with a completely bogus binding (b |-> Bool), by lining
397 -- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
398 -- This pre-subsumption stuff can return too few bindings, but it
399 -- must *never* return bogus info.
401 go t_tvs (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
402 = boxy_match t_tvs arg1 b_tvs arg2 (go t_tvs res1 b_tvs res2)
403 -- Match the args, and sub-match the results
405 go t_tvs t_ty b_tvs b_ty = boxy_match t_tvs t_ty b_tvs b_ty emptyTvSubst
406 -- Otherwise defer to boxy matching
407 -- This covers TyConApp, AppTy, PredTy
414 |- head xs : <rhobox>
415 We will do a boxySubMatchType between a ~ <rhobox>
416 But we *don't* want to match [a |-> <rhobox>] because
417 (a) The box should be filled in with a rho-type, but
418 but the returned substitution maps TyVars to boxy
420 (b) In any case, the right final answer might be *either*
421 instantiate 'a' with a rho-type or a sigma type
422 head xs : Int vs head xs : forall b. b->b
423 So the matcher MUST NOT make a choice here. In general, we only
424 bind a template type variable in boxyMatchType, not in boxySubMatchType.
429 :: TcTyVarSet -> [TcType] -- The "template"; the tyvars are skolems
430 -> [BoxySigmaType] -- Type to match
431 -> TvSubst -- Substitution of the [TcTyVar] to BoxySigmaTypes
433 -- boxyMatchTypes implements the Pre-matching judgement, in Fig 5 of the paper
434 -- "Boxy types: inference for higher rank types and impredicativity"
436 -- Find a *boxy* substitution that makes the template look as much
437 -- like the BoxySigmaType as possible.
438 -- It's always ok to return an empty substitution;
439 -- anything more is jam on the pudding
441 -- NB1: This is a pure, non-monadic function.
442 -- It does no unification, and cannot fail
444 -- Precondition: the arg lengths are equal
445 -- Precondition: none of the template type variables appear anywhere in the [BoxySigmaType]
449 boxyMatchTypes tmpl_tvs tmpl_tys boxy_tys
450 = ASSERT( length tmpl_tys == length boxy_tys )
451 boxy_match_s tmpl_tvs tmpl_tys emptyVarSet boxy_tys emptyTvSubst
452 -- ToDo: add error context?
454 boxy_match_s tmpl_tvs [] boxy_tvs [] subst
456 boxy_match_s tmpl_tvs (t_ty:t_tys) boxy_tvs (b_ty:b_tys) subst
457 = boxy_match tmpl_tvs t_ty boxy_tvs b_ty $
458 boxy_match_s tmpl_tvs t_tys boxy_tvs b_tys subst
459 boxy_match_s tmpl_tvs _ boxy_tvs _ subst
460 = panic "boxy_match_s" -- Lengths do not match
464 boxy_match :: TcTyVarSet -> TcType -- Template
465 -> TcTyVarSet -- boxy_tvs: do not bind template tyvars to any of these
466 -> BoxySigmaType -- Match against this type
470 -- The boxy_tvs argument prevents this match:
471 -- [a] forall b. a ~ forall b. b
472 -- We don't want to bind the template variable 'a'
473 -- to the quantified type variable 'b'!
475 boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
476 = go orig_tmpl_ty orig_boxy_ty
479 | Just t_ty' <- tcView t_ty = go t_ty' b_ty
480 | Just b_ty' <- tcView b_ty = go t_ty b_ty'
482 go ty1 ty2 -- C.f. the isSigmaTy case for boxySubMatchType
484 , (tvs1, _, tau1) <- tcSplitSigmaTy ty1
485 , (tvs2, _, tau2) <- tcSplitSigmaTy ty2
486 , equalLength tvs1 tvs2
487 = boxy_match (tmpl_tvs `delVarSetList` tvs1) tau1
488 (boxy_tvs `extendVarSetList` tvs2) tau2 subst
490 go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
492 , not $ isOpenSynTyCon tc1
495 go (FunTy arg1 res1) (FunTy arg2 res2)
496 = go_s [arg1,res1] [arg2,res2]
499 | Just (s1,t1) <- tcSplitAppTy_maybe t_ty,
500 Just (s2,t2) <- tcSplitAppTy_maybe b_ty,
501 typeKind t2 `isSubKind` typeKind t1 -- Maintain invariant
502 = go_s [s1,t1] [s2,t2]
505 | tv `elemVarSet` tmpl_tvs -- Template type variable in the template
506 , boxy_tvs `disjointVarSet` tyVarsOfType orig_boxy_ty
507 , typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
508 = extendTvSubst subst tv boxy_ty'
510 = subst -- Ignore others
512 boxy_ty' = case lookupTyVar subst tv of
513 Nothing -> orig_boxy_ty
514 Just ty -> ty `boxyLub` orig_boxy_ty
516 go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
517 -- Example: Tree a ~ Maybe Int
518 -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
519 -- misleading error messages. An even more confusing case is
520 -- a -> b ~ Maybe Int
521 -- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
522 -- from this pre-matching phase.
525 go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
528 boxyLub :: BoxySigmaType -> BoxySigmaType -> BoxySigmaType
529 -- Combine boxy information from the two types
530 -- If there is a conflict, return the first
531 boxyLub orig_ty1 orig_ty2
532 = go orig_ty1 orig_ty2
534 go (AppTy f1 a1) (AppTy f2 a2) = AppTy (boxyLub f1 f2) (boxyLub a1 a2)
535 go (FunTy f1 a1) (FunTy f2 a2) = FunTy (boxyLub f1 f2) (boxyLub a1 a2)
536 go (TyConApp tc1 ts1) (TyConApp tc2 ts2)
537 | tc1 == tc2, length ts1 == length ts2
538 = TyConApp tc1 (zipWith boxyLub ts1 ts2)
540 go (TyVarTy tv1) ty2 -- This is the whole point;
541 | isTcTyVar tv1, isBoxyTyVar tv1 -- choose ty2 if ty2 is a box
544 -- Look inside type synonyms, but only if the naive version fails
545 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
546 | Just ty2' <- tcView ty1 = go ty1 ty2'
548 -- For now, we don't look inside ForAlls, PredTys
549 go ty1 ty2 = orig_ty1 -- Default
552 Note [Matching kinds]
553 ~~~~~~~~~~~~~~~~~~~~~
554 The target type might legitimately not be a sub-kind of template.
555 For example, suppose the target is simply a box with an OpenTypeKind,
556 and the template is a type variable with LiftedTypeKind.
557 Then it's ok (because the target type will later be refined).
558 We simply don't bind the template type variable.
560 It might also be that the kind mis-match is an error. For example,
561 suppose we match the template (a -> Int) against (Int# -> Int),
562 where the template type variable 'a' has LiftedTypeKind. This
563 matching function does not fail; it simply doesn't bind the template.
564 Later stuff will fail.
566 %************************************************************************
570 %************************************************************************
572 All the tcSub calls have the form
574 tcSub expected_ty offered_ty
576 offered_ty <= expected_ty
578 That is, that a value of type offered_ty is acceptable in
579 a place expecting a value of type expected_ty.
581 It returns a coercion function
582 co_fn :: offered_ty ~ expected_ty
583 which takes an HsExpr of type offered_ty into one of type
588 tcSubExp :: BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
589 -- (tcSub act exp) checks that
591 tcSubExp actual_ty expected_ty
592 = -- addErrCtxtM (unifyCtxt actual_ty expected_ty) $
593 -- Adding the error context here leads to some very confusing error
594 -- messages, such as "can't match forall a. a->a with forall a. a->a"
595 -- Example is tcfail165:
596 -- do var <- newEmptyMVar :: IO (MVar (forall a. Show a => a -> String))
597 -- putMVar var (show :: forall a. Show a => a -> String)
598 -- Here the info does not flow from the 'var' arg of putMVar to its 'show' arg
599 -- but after zonking it looks as if it does!
601 -- So instead I'm adding the error context when moving from tc_sub to u_tys
603 traceTc (text "tcSubExp" <+> ppr actual_ty <+> ppr expected_ty) >>
604 tc_sub SubOther actual_ty actual_ty False expected_ty expected_ty
606 tcFunResTy :: Name -> BoxySigmaType -> BoxySigmaType -> TcM HsWrapper -- Locally used only
607 tcFunResTy fun actual_ty expected_ty
608 = traceTc (text "tcFunResTy" <+> ppr actual_ty <+> ppr expected_ty) >>
609 tc_sub (SubFun fun) actual_ty actual_ty False expected_ty expected_ty
612 data SubCtxt = SubDone -- Error-context already pushed
613 | SubFun Name -- Context is tcFunResTy
614 | SubOther -- Context is something else
616 tc_sub :: SubCtxt -- How to add an error-context
617 -> BoxySigmaType -- actual_ty, before expanding synonyms
618 -> BoxySigmaType -- ..and after
619 -> InBox -- True <=> expected_ty is inside a box
620 -> BoxySigmaType -- expected_ty, before
621 -> BoxySigmaType -- ..and after
623 -- The acual_ty is never inside a box
624 -- IMPORTANT pre-condition: if the args contain foralls, the bound type
625 -- variables are visible non-monadically
626 -- (i.e. tha args are sufficiently zonked)
627 -- This invariant is needed so that we can "see" the foralls, ad
628 -- e.g. in the SPEC rule where we just use splitSigmaTy
630 tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
631 = traceTc (text "tc_sub" <+> ppr act_ty $$ ppr exp_ty) >>
632 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
633 -- This indirection is just here to make
634 -- it easy to insert a debug trace!
636 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
637 | Just exp_ty' <- tcView exp_ty = tc_sub sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty'
638 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
639 | Just act_ty' <- tcView act_ty = tc_sub sub_ctxt act_sty act_ty' exp_ib exp_sty exp_ty
641 -----------------------------------
642 -- Rule SBOXY, plus other cases when act_ty is a type variable
643 -- Just defer to boxy matching
644 -- This rule takes precedence over SKOL!
645 tc_sub1 sub_ctxt act_sty (TyVarTy tv) exp_ib exp_sty exp_ty
646 = do { traceTc (text "tc_sub1 - case 1")
647 ; coi <- addSubCtxt sub_ctxt act_sty exp_sty $
648 uVar True False tv exp_ib exp_sty exp_ty
649 ; traceTc (case coi of
650 IdCo -> text "tc_sub1 (Rule SBOXY) IdCo"
651 ACo co -> text "tc_sub1 (Rule SBOXY) ACo" <+> ppr co)
652 ; return $ case coi of
657 -----------------------------------
658 -- Skolemisation case (rule SKOL)
659 -- actual_ty: d:Eq b => b->b
660 -- expected_ty: forall a. Ord a => a->a
661 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
663 -- It is essential to do this *before* the specialisation case
664 -- Example: f :: (Eq a => a->a) -> ...
665 -- g :: Ord b => b->b
668 tc_sub1 sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
670 = do { traceTc (text "tc_sub1 - case 2") ;
671 if exp_ib then -- SKOL does not apply if exp_ty is inside a box
672 defer_to_boxy_matching sub_ctxt act_sty act_ty exp_ib exp_sty exp_ty
674 { (gen_fn, co_fn) <- tcGen exp_ty act_tvs $ \ _ body_exp_ty ->
675 tc_sub sub_ctxt act_sty act_ty False body_exp_ty body_exp_ty
676 ; return (gen_fn <.> co_fn) }
679 act_tvs = tyVarsOfType act_ty
680 -- It's really important to check for escape wrt
681 -- the free vars of both expected_ty *and* actual_ty
683 -----------------------------------
684 -- Specialisation case (rule ASPEC):
685 -- actual_ty: forall a. Ord a => a->a
686 -- expected_ty: Int -> Int
687 -- co_fn e = e Int dOrdInt
689 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
690 -- Implements the new SPEC rule in the Appendix of the paper
691 -- "Boxy types: inference for higher rank types and impredicativity"
692 -- (This appendix isn't in the published version.)
693 -- The idea is to *first* do pre-subsumption, and then full subsumption
694 -- Example: forall a. a->a <= Int -> (forall b. Int)
695 -- Pre-subsumpion finds a|->Int, and that works fine, whereas
696 -- just running full subsumption would fail.
697 | isSigmaTy actual_ty
698 = do { traceTc (text "tc_sub1 - case 3")
699 ; -- Perform pre-subsumption, and instantiate
700 -- the type with info from the pre-subsumption;
701 -- boxy tyvars if pre-subsumption gives no info
702 let (tyvars, theta, tau) = tcSplitSigmaTy actual_ty
703 tau_tvs = exactTyVarsOfType tau
704 ; inst_tys <- if exp_ib then -- Inside a box, do not do clever stuff
705 do { tyvars' <- mapM tcInstBoxyTyVar tyvars
706 ; return (mkTyVarTys tyvars') }
707 else -- Outside, do clever stuff
708 preSubType tyvars tau_tvs tau expected_ty
709 ; let subst' = zipOpenTvSubst tyvars inst_tys
710 tau' = substTy subst' tau
712 -- Perform a full subsumption check
713 ; traceTc (text "tc_sub_spec" <+> vcat [ppr actual_ty,
714 ppr tyvars <+> ppr theta <+> ppr tau,
716 ; co_fn2 <- tc_sub sub_ctxt tau' tau' exp_ib exp_sty expected_ty
718 -- Deal with the dictionaries
719 -- The origin gives a helpful origin when we have
720 -- a function with type f :: Int -> forall a. Num a => ...
721 -- This way the (Num a) dictionary gets an OccurrenceOf f origin
722 ; let orig = case sub_ctxt of
723 SubFun n -> OccurrenceOf n
724 other -> InstSigOrigin -- Unhelpful
725 ; co_fn1 <- instCall orig inst_tys (substTheta subst' theta)
726 ; return (co_fn2 <.> co_fn1) }
728 -----------------------------------
729 -- Function case (rule F1)
730 tc_sub1 sub_ctxt act_sty (FunTy act_arg act_res) exp_ib exp_sty (FunTy exp_arg exp_res)
731 = do { traceTc (text "tc_sub1 - case 4")
732 ; addSubCtxt sub_ctxt act_sty exp_sty $
733 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
736 -- Function case (rule F2)
737 tc_sub1 sub_ctxt act_sty act_ty@(FunTy act_arg act_res) _ exp_sty (TyVarTy exp_tv)
739 = addSubCtxt sub_ctxt act_sty exp_sty $
740 do { traceTc (text "tc_sub1 - case 5")
741 ; cts <- readMetaTyVar exp_tv
743 Indirect ty -> tc_sub SubDone act_sty act_ty True exp_sty ty
744 Flexi -> do { [arg_ty,res_ty] <- withMetaTvs exp_tv fun_kinds mk_res_ty
745 ; tc_sub_funs act_arg act_res True arg_ty res_ty } }
747 mk_res_ty [arg_ty', res_ty'] = mkFunTy arg_ty' res_ty'
748 mk_res_ty other = panic "TcUnify.mk_res_ty3"
749 fun_kinds = [argTypeKind, openTypeKind]
751 -- Everything else: defer to boxy matching
752 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty@(TyVarTy exp_tv)
753 = do { traceTc (text "tc_sub1 - case 6a" <+> ppr [isBoxyTyVar exp_tv, isMetaTyVar exp_tv, isSkolemTyVar exp_tv, isExistentialTyVar exp_tv,isSigTyVar exp_tv] )
754 ; defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
757 tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
758 = do { traceTc (text "tc_sub1 - case 6")
759 ; defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
762 -----------------------------------
763 defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
764 = do { coi <- addSubCtxt sub_ctxt act_sty exp_sty $
765 u_tys outer False act_sty actual_ty exp_ib exp_sty expected_ty
766 ; return $ case coi of
771 outer = case sub_ctxt of -- Ugh
775 -----------------------------------
776 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res
777 = do { arg_coi <- uTys False act_arg exp_ib exp_arg
778 ; co_fn_res <- tc_sub SubDone act_res act_res exp_ib exp_res exp_res
779 ; wrapper1 <- wrapFunResCoercion [exp_arg] co_fn_res
780 ; let wrapper2 = case arg_coi of
782 ACo co -> WpCo $ FunTy co act_res
783 ; return (wrapper1 <.> wrapper2)
786 -----------------------------------
788 :: [TcType] -- Type of args
789 -> HsWrapper -- HsExpr a -> HsExpr b
790 -> TcM HsWrapper -- HsExpr (arg_tys -> a) -> HsExpr (arg_tys -> b)
791 wrapFunResCoercion arg_tys co_fn_res
792 | isIdHsWrapper co_fn_res
797 = do { arg_ids <- newSysLocalIds FSLIT("sub") arg_tys
798 ; return (mkWpLams arg_ids <.> co_fn_res <.> mkWpApps arg_ids) }
803 %************************************************************************
805 \subsection{Generalisation}
807 %************************************************************************
810 tcGen :: BoxySigmaType -- expected_ty
811 -> TcTyVarSet -- Extra tyvars that the universally
812 -- quantified tyvars of expected_ty
813 -- must not be unified
814 -> ([TcTyVar] -> BoxyRhoType -> TcM result)
815 -> TcM (HsWrapper, result)
816 -- The expression has type: spec_ty -> expected_ty
818 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
819 -- If not, the call is a no-op
820 = do { traceTc (text "tcGen")
821 -- We want the GenSkol info in the skolemised type variables to
822 -- mention the *instantiated* tyvar names, so that we get a
823 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
824 -- Hence the tiresome but innocuous fixM
825 ; ((tvs', theta', rho'), skol_info) <- fixM (\ ~(_, skol_info) ->
826 do { (forall_tvs, theta, rho_ty) <- tcInstSkolType skol_info expected_ty
827 -- Get loation from monad, not from expected_ty
828 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty)
829 ; return ((forall_tvs, theta, rho_ty), skol_info) })
832 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
833 text "expected_ty" <+> ppr expected_ty,
834 text "inst ty" <+> ppr tvs' <+> ppr theta' <+> ppr rho',
835 text "free_tvs" <+> ppr free_tvs])
838 -- Type-check the arg and unify with poly type
839 ; (result, lie) <- getLIE (thing_inside tvs' rho')
841 -- Check that the "forall_tvs" havn't been constrained
842 -- The interesting bit here is that we must include the free variables
843 -- of the expected_ty. Here's an example:
844 -- runST (newVar True)
845 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
846 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
847 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
848 -- So now s' isn't unconstrained because it's linked to a.
849 -- Conclusion: include the free vars of the expected_ty in the
850 -- list of "free vars" for the signature check.
852 ; loc <- getInstLoc (SigOrigin skol_info)
853 ; dicts <- newDictBndrs loc theta'
854 ; inst_binds <- tcSimplifyCheck loc tvs' dicts lie
856 ; checkSigTyVarsWrt free_tvs tvs'
857 ; traceTc (text "tcGen:done")
860 -- The WpLet binds any Insts which came out of the simplification.
861 dict_vars = map instToVar dicts
862 co_fn = mkWpTyLams tvs' <.> mkWpLams dict_vars <.> WpLet inst_binds
863 ; returnM (co_fn, result) }
865 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
870 %************************************************************************
874 %************************************************************************
876 The exported functions are all defined as versions of some
877 non-exported generic functions.
880 boxyUnify :: BoxyType -> BoxyType -> TcM CoercionI
881 -- Acutal and expected, respectively
883 = addErrCtxtM (unifyCtxt ty1 ty2) $
884 uTysOuter False ty1 False ty2
887 boxyUnifyList :: [BoxyType] -> [BoxyType] -> TcM [CoercionI]
888 -- Arguments should have equal length
889 -- Acutal and expected types
890 boxyUnifyList tys1 tys2 = uList boxyUnify tys1 tys2
893 unifyType :: TcTauType -> TcTauType -> TcM CoercionI
894 -- No boxes expected inside these types
895 -- Acutal and expected types
896 unifyType ty1 ty2 -- ty1 expected, ty2 inferred
897 = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
898 ASSERT2( not (isBoxyTy ty2), ppr ty2 )
899 addErrCtxtM (unifyCtxt ty1 ty2) $
900 uTysOuter True ty1 True ty2
903 unifyPred :: PredType -> PredType -> TcM CoercionI
904 -- Acutal and expected types
905 unifyPred p1 p2 = addErrCtxtM (unifyCtxt (mkPredTy p1) (mkPredTy p2)) $
906 uPred True True p1 True p2
908 unifyTheta :: TcThetaType -> TcThetaType -> TcM [CoercionI]
909 -- Acutal and expected types
910 unifyTheta theta1 theta2
911 = do { checkTc (equalLength theta1 theta2)
912 (vcat [ptext SLIT("Contexts differ in length"),
913 nest 2 $ parens $ ptext SLIT("Use -fglasgow-exts to allow this")])
914 ; uList unifyPred theta1 theta2
918 uList :: (a -> a -> TcM b)
919 -> [a] -> [a] -> TcM [b]
920 -- Unify corresponding elements of two lists of types, which
921 -- should be of equal length. We charge down the list explicitly so that
922 -- we can complain if their lengths differ.
923 uList unify [] [] = return []
924 uList unify (ty1:tys1) (ty2:tys2) = do { x <- unify ty1 ty2;
925 ; xs <- uList unify tys1 tys2
928 uList unify ty1s ty2s = panic "Unify.uList: mismatched type lists!"
931 @unifyTypeList@ takes a single list of @TauType@s and unifies them
932 all together. It is used, for example, when typechecking explicit
933 lists, when all the elts should be of the same type.
936 unifyTypeList :: [TcTauType] -> TcM ()
937 unifyTypeList [] = returnM ()
938 unifyTypeList [ty] = returnM ()
939 unifyTypeList (ty1:tys@(ty2:_)) = do { unifyType ty1 ty2
940 ; unifyTypeList tys }
943 %************************************************************************
945 \subsection[Unify-uTys]{@uTys@: getting down to business}
947 %************************************************************************
949 @uTys@ is the heart of the unifier. Each arg occurs twice, because
950 we want to report errors in terms of synomyms if possible. The first of
951 the pair is used in error messages only; it is always the same as the
952 second, except that if the first is a synonym then the second may be a
953 de-synonym'd version. This way we get better error messages.
955 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
959 -- False <=> the two args are (actual, expected) respectively
960 -- True <=> the two args are (expected, actual) respectively
962 type InBox = Bool -- True <=> we are inside a box
963 -- False <=> we are outside a box
964 -- The importance of this is that if we get "filled-box meets
965 -- filled-box", we'll look into the boxes and unify... but
966 -- we must not allow polytypes. But if we are in a box on
967 -- just one side, then we can allow polytypes
969 type Outer = Bool -- True <=> this is the outer level of a unification
970 -- so that the types being unified are the
971 -- very ones we began with, not some sub
972 -- component or synonym expansion
973 -- The idea is that if Outer is true then unifyMisMatch should
974 -- pop the context to remove the "Expected/Acutal" context
977 :: InBox -> TcType -- ty1 is the *actual* type
978 -> InBox -> TcType -- ty2 is the *expected* type
980 uTysOuter nb1 ty1 nb2 ty2
981 = do { traceTc (text "uTysOuter" <+> ppr ty1 <+> ppr ty2)
982 ; u_tys True nb1 ty1 ty1 nb2 ty2 ty2 }
984 = do { traceTc (text "uTys" <+> ppr ty1 <+> ppr ty2)
985 ; u_tys False nb1 ty1 ty1 nb2 ty2 ty2 }
989 uTys_s :: InBox -> [TcType] -- tys1 are the *actual* types
990 -> InBox -> [TcType] -- tys2 are the *expected* types
992 uTys_s nb1 [] nb2 [] = returnM []
993 uTys_s nb1 (ty1:tys1) nb2 (ty2:tys2) = do { coi <- uTys nb1 ty1 nb2 ty2
994 ; cois <- uTys_s nb1 tys1 nb2 tys2
997 uTys_s nb1 ty1s nb2 ty2s = panic "Unify.uTys_s: mismatched type lists!"
1001 -> InBox -> TcType -> TcType -- ty1 is the *actual* type
1002 -> InBox -> TcType -> TcType -- ty2 is the *expected* type
1005 u_tys outer nb1 orig_ty1 ty1 nb2 orig_ty2 ty2
1006 = do { traceTc (text "u_tys " <+> ppr ty1 <+> text " " <+> ppr ty2)
1007 ; coi <- go outer ty1 ty2
1008 ; traceTc (case coi of
1009 ACo co -> text "u_tys yields coercion: " <+> ppr co
1010 IdCo -> text "u_tys yields no coercion")
1015 go :: Outer -> TcType -> TcType -> TcM CoercionI
1017 do { traceTc (text "go " <+> ppr orig_ty1 <+> text "/" <+> ppr ty1
1018 <+> ppr orig_ty2 <+> text "/" <+> ppr ty2)
1022 go1 :: Outer -> TcType -> TcType -> TcM CoercionI
1023 -- Always expand synonyms: see Note [Unification and synonyms]
1024 -- (this also throws away FTVs)
1026 | Just ty1' <- tcView ty1 = go False ty1' ty2
1027 | Just ty2' <- tcView ty2 = go False ty1 ty2'
1029 -- Variables; go for uVar
1030 go1 outer (TyVarTy tyvar1) ty2 = uVar outer False tyvar1 nb2 orig_ty2 ty2
1031 go1 outer ty1 (TyVarTy tyvar2) = uVar outer True tyvar2 nb1 orig_ty1 ty1
1032 -- "True" means args swapped
1034 -- The case for sigma-types must *follow* the variable cases
1035 -- because a boxy variable can be filed with a polytype;
1036 -- but must precede FunTy, because ((?x::Int) => ty) look
1037 -- like a FunTy; there isn't necy a forall at the top
1039 | isSigmaTy ty1 || isSigmaTy ty2
1040 = do { traceTc (text "We have sigma types: equalLength" <+> ppr tvs1 <+> ppr tvs2)
1041 ; checkM (equalLength tvs1 tvs2)
1042 (unifyMisMatch outer False orig_ty1 orig_ty2)
1043 ; traceTc (text "We're past the first length test")
1044 ; tvs <- tcInstSkolTyVars UnkSkol tvs1 -- Not a helpful SkolemInfo
1045 -- Get location from monad, not from tvs1
1046 ; let tys = mkTyVarTys tvs
1047 in_scope = mkInScopeSet (mkVarSet tvs)
1048 phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
1049 phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
1050 (theta1,tau1) = tcSplitPhiTy phi1
1051 (theta2,tau2) = tcSplitPhiTy phi2
1053 ; addErrCtxtM (unifyForAllCtxt tvs phi1 phi2) $ do
1054 { checkM (equalLength theta1 theta2)
1055 (unifyMisMatch outer False orig_ty1 orig_ty2)
1057 ; cois <- uPreds False nb1 theta1 nb2 theta2 -- TOMDO: do something with these pred_cois
1058 ; traceTc (text "TOMDO!")
1059 ; coi <- uTys nb1 tau1 nb2 tau2
1061 -- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
1062 ; free_tvs <- zonkTcTyVarsAndFV (varSetElems (tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2))
1063 ; ifM (any (`elemVarSet` free_tvs) tvs)
1064 (bleatEscapedTvs free_tvs tvs tvs)
1066 -- If both sides are inside a box, we are in a "box-meets-box"
1067 -- situation, and we should not have a polytype at all.
1068 -- If we get here we have two boxes, already filled with
1069 -- the same polytype... but it should be a monotype.
1070 -- This check comes last, because the error message is
1071 -- extremely unhelpful.
1072 ; ifM (nb1 && nb2) (notMonoType ty1)
1076 (tvs1, body1) = tcSplitForAllTys ty1
1077 (tvs2, body2) = tcSplitForAllTys ty2
1080 go1 outer (PredTy p1) (PredTy p2)
1081 = uPred False nb1 p1 nb2 p2
1083 -- Type constructors must match
1084 go1 _ (TyConApp con1 tys1) (TyConApp con2 tys2)
1085 | con1 == con2 && not (isOpenSynTyCon con1)
1086 = do { cois <- uTys_s nb1 tys1 nb2 tys2
1087 ; return $ mkTyConAppCoI con1 tys1 cois
1089 -- See Note [TyCon app]
1090 | con1 == con2 && identicalOpenSynTyConApp
1091 = do { cois <- uTys_s nb1 tys1' nb2 tys2'
1092 ; return $ mkTyConAppCoI con1 tys1 (replicate n IdCo ++ cois)
1096 (idxTys1, tys1') = splitAt n tys1
1097 (idxTys2, tys2') = splitAt n tys2
1098 identicalOpenSynTyConApp = idxTys1 `tcEqTypes` idxTys2
1099 -- See Note [OpenSynTyCon app]
1101 -- Functions; just check the two parts
1102 go1 _ (FunTy fun1 arg1) (FunTy fun2 arg2)
1103 = do { coi_l <- uTys nb1 fun1 nb2 fun2
1104 ; coi_r <- uTys nb1 arg1 nb2 arg2
1105 ; return $ mkFunTyCoI fun1 coi_l arg1 coi_r
1108 -- Applications need a bit of care!
1109 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
1110 -- NB: we've already dealt with type variables and Notes,
1111 -- so if one type is an App the other one jolly well better be too
1112 go1 outer (AppTy s1 t1) ty2
1113 | Just (s2,t2) <- tcSplitAppTy_maybe ty2
1114 = do { coi_s <- uTys nb1 s1 nb2 s2; coi_t <- uTys nb1 t1 nb2 t2
1115 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1117 -- Now the same, but the other way round
1118 -- Don't swap the types, because the error messages get worse
1119 go1 outer ty1 (AppTy s2 t2)
1120 | Just (s1,t1) <- tcSplitAppTy_maybe ty1
1121 = do { coi_s <- uTys nb1 s1 nb2 s2; coi_t <- uTys nb1 t1 nb2 t2
1122 ; return $ mkAppTyCoI s1 coi_s t1 coi_t }
1124 -- One or both outermost constructors are type family applications.
1125 -- If we can normalise them away, proceed as usual; otherwise, we
1126 -- need to defer unification by generating a wanted equality constraint.
1128 | ty1_is_fun || ty2_is_fun
1129 = do { (coi1, ty1') <- if ty1_is_fun then tcNormaliseFamInst ty1
1130 else return (IdCo, ty1)
1131 ; (coi2, ty2') <- if ty2_is_fun then tcNormaliseFamInst ty2
1132 else return (IdCo, ty2)
1133 ; coi <- if isOpenSynTyConApp ty1' || isOpenSynTyConApp ty2'
1134 then do { -- One type family app can't be reduced yet
1136 ; ty1'' <- zonkTcType ty1'
1137 ; ty2'' <- zonkTcType ty2'
1138 ; if tcEqType ty1'' ty2''
1140 else -- see [Deferred Unification]
1141 defer_unification outer False orig_ty1 orig_ty2
1143 else -- unification can proceed
1145 ; return $ coi1 `mkTransCoI` coi `mkTransCoI` (mkSymCoI coi2)
1148 ty1_is_fun = isOpenSynTyConApp ty1
1149 ty2_is_fun = isOpenSynTyConApp ty2
1151 -- Anything else fails
1152 go1 outer _ _ = unifyMisMatch outer False orig_ty1 orig_ty2
1156 uPred outer nb1 (IParam n1 t1) nb2 (IParam n2 t2)
1158 do { coi <- uTys nb1 t1 nb2 t2
1159 ; return $ mkIParamPredCoI n1 coi
1161 uPred outer nb1 (ClassP c1 tys1) nb2 (ClassP c2 tys2)
1163 do { cois <- uTys_s nb1 tys1 nb2 tys2 -- Guaranteed equal lengths because the kinds check
1164 ; return $ mkClassPPredCoI c1 tys1 cois
1166 uPred outer _ p1 _ p2 = unifyMisMatch outer False (mkPredTy p1) (mkPredTy p2)
1168 uPreds outer nb1 [] nb2 [] = return []
1169 uPreds outer nb1 (p1:ps1) nb2 (p2:ps2) =
1170 do { coi <- uPred outer nb1 p1 nb2 p2
1171 ; cois <- uPreds outer nb1 ps1 nb2 ps2
1174 uPreds outer nb1 ps1 nb2 ps2 = panic "uPreds"
1179 When we find two TyConApps, the argument lists are guaranteed equal
1180 length. Reason: intially the kinds of the two types to be unified is
1181 the same. The only way it can become not the same is when unifying two
1182 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
1183 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
1184 which we do, that ensures that f1,f2 have the same kind; and that
1185 means a1,a2 have the same kind. And now the argument repeats.
1187 Note [OpenSynTyCon app]
1188 ~~~~~~~~~~~~~~~~~~~~~~~
1191 type family T a :: * -> *
1193 the two types (T () a) and (T () Int) must unify, even if there are
1194 no type instances for T at all. Should we just turn them into an
1195 equality (T () a ~ T () Int)? I don't think so. We currently try to
1196 eagerly unify everything we can before generating equalities; otherwise,
1197 we could turn the unification of [Int] with [a] into an equality, too.
1199 Note [Unification and synonyms]
1200 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1201 If you are tempted to make a short cut on synonyms, as in this
1205 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
1206 -- NO = if (con1 == con2) then
1207 -- NO -- Good news! Same synonym constructors, so we can shortcut
1208 -- NO -- by unifying their arguments and ignoring their expansions.
1209 -- NO unifyTypepeLists args1 args2
1211 -- NO -- Never mind. Just expand them and try again
1215 then THINK AGAIN. Here is the whole story, as detected and reported
1216 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
1218 Here's a test program that should detect the problem:
1222 x = (1 :: Bogus Char) :: Bogus Bool
1225 The problem with [the attempted shortcut code] is that
1229 is not a sufficient condition to be able to use the shortcut!
1230 You also need to know that the type synonym actually USES all
1231 its arguments. For example, consider the following type synonym
1232 which does not use all its arguments.
1237 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
1238 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
1239 would fail, even though the expanded forms (both \tr{Int}) should
1242 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
1243 unnecessarily bind \tr{t} to \tr{Char}.
1245 ... You could explicitly test for the problem synonyms and mark them
1246 somehow as needing expansion, perhaps also issuing a warning to the
1251 %************************************************************************
1253 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
1255 %************************************************************************
1257 @uVar@ is called when at least one of the types being unified is a
1258 variable. It does {\em not} assume that the variable is a fixed point
1259 of the substitution; rather, notice that @uVar@ (defined below) nips
1260 back into @uTys@ if it turns out that the variable is already bound.
1264 -> SwapFlag -- False => tyvar is the "actual" (ty is "expected")
1265 -- True => ty is the "actual" (tyvar is "expected")
1267 -> InBox -- True <=> definitely no boxes in t2
1268 -> TcTauType -> TcTauType -- printing and real versions
1271 uVar outer swapped tv1 nb2 ps_ty2 ty2
1272 = do { let expansion | showSDoc (ppr ty2) == showSDoc (ppr ps_ty2) = empty
1273 | otherwise = brackets (equals <+> ppr ty2)
1274 ; traceTc (text "uVar" <+> ppr swapped <+>
1275 sep [ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1 ),
1276 nest 2 (ptext SLIT(" <-> ")),
1277 ppr ps_ty2 <+> dcolon <+> ppr (typeKind ty2) <+> expansion])
1278 ; details <- lookupTcTyVar tv1
1281 | swapped -> u_tys outer nb2 ps_ty2 ty2 True ty1 ty1 -- Swap back
1282 | otherwise -> u_tys outer True ty1 ty1 nb2 ps_ty2 ty2 -- Same order
1283 -- The 'True' here says that ty1 is now inside a box
1284 DoneTv details1 -> uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1288 uUnfilledVar :: Outer
1290 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1291 -> TcTauType -> TcTauType -- Type 2
1293 -- Invariant: tyvar 1 is not unified with anything
1295 uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2
1296 | Just ty2' <- tcView ty2
1297 = -- Expand synonyms; ignore FTVs
1298 uUnfilledVar False swapped tv1 details1 ps_ty2 ty2'
1300 uUnfilledVar outer swapped tv1 details1 ps_ty2 (TyVarTy tv2)
1301 | tv1 == tv2 -- Same type variable => no-op (but watch out for the boxy case)
1303 MetaTv BoxTv ref1 -- A boxy type variable meets itself;
1304 -- this is box-meets-box, so fill in with a tau-type
1305 -> do { tau_tv <- tcInstTyVar tv1
1306 ; updateMeta tv1 ref1 (mkTyVarTy tau_tv)
1309 other -> returnM IdCo -- No-op
1311 | otherwise -- Distinct type variables
1312 = do { lookup2 <- lookupTcTyVar tv2
1314 IndirectTv ty2' -> uUnfilledVar outer swapped tv1 details1 ty2' ty2'
1315 DoneTv details2 -> uUnfilledVars outer swapped tv1 details1 tv2 details2
1318 uUnfilledVar outer swapped tv1 details1 ps_ty2 non_var_ty2
1319 = -- ty2 is not a type variable
1321 MetaTv (SigTv _) _ -> rigid_variable
1323 uMetaVar outer swapped tv1 info ref1 ps_ty2 non_var_ty2
1324 SkolemTv _ -> rigid_variable
1327 | isOpenSynTyConApp non_var_ty2
1328 = -- 'non_var_ty2's outermost constructor is a type family,
1329 -- which we may may be able to normalise
1330 do { (coi2, ty2') <- tcNormaliseFamInst non_var_ty2
1332 IdCo -> -- no progress, but maybe after other instantiations
1333 defer_unification outer swapped (TyVarTy tv1) ps_ty2
1334 ACo co -> -- progress: so lets try again
1336 ppr co <+> text "::"<+> ppr non_var_ty2 <+> text "~" <+>
1338 ; coi <- uUnfilledVar outer swapped tv1 details1 ps_ty2 ty2'
1339 ; let coi2' = (if swapped then id else mkSymCoI) coi2
1340 ; return $ coi2' `mkTransCoI` coi
1343 | SkolemTv RuntimeUnkSkol <- details1
1344 -- runtime unknown will never match
1345 = unifyMisMatch outer swapped (TyVarTy tv1) ps_ty2
1346 | otherwise -- defer as a given equality may still resolve this
1347 = defer_unification outer swapped (TyVarTy tv1) ps_ty2
1350 Note [Deferred Unification]
1351 ~~~~~~~~~~~~~~~~~~~~
1352 We may encounter a unification ty1 = ty2 that cannot be performed syntactically,
1353 and yet its consistency is undetermined. Previously, there was no way to still
1354 make it consistent. So a mismatch error was issued.
1356 Now these unfications are deferred until constraint simplification, where type
1357 family instances and given equations may (or may not) establish the consistency.
1358 Deferred unifications are of the form
1361 where F is a type function and x is a type variable.
1363 id :: x ~ y => x -> y
1366 involves the unfication x = y. It is deferred until we bring into account the
1367 context x ~ y to establish that it holds.
1369 If available, we defer original types (rather than those where closed type
1370 synonyms have already been expanded via tcCoreView). This is, as usual, to
1371 improve error messages.
1373 We need to both 'unBox' and zonk deferred types. We need to unBox as
1374 functions, such as TcExpr.tcMonoExpr promise to fill boxes in the expected
1375 type. We need to zonk as the types go into the kind of the coercion variable
1376 `cotv' and those are not zonked in Inst.zonkInst. (Maybe it would be better
1377 to zonk in zonInst instead. Would that be sufficient?)
1380 defer_unification :: Bool -- pop innermost context?
1385 defer_unification outer True ty1 ty2
1386 = defer_unification outer False ty2 ty1
1387 defer_unification outer False ty1 ty2
1388 = do { ty1' <- unBox ty1 >>= zonkTcType -- unbox *and* zonk..
1389 ; ty2' <- unBox ty2 >>= zonkTcType -- ..see preceding note
1390 ; traceTc $ text "deferring:" <+> ppr ty1 <+> text "~" <+> ppr ty2
1391 ; cotv <- newMetaCoVar ty1' ty2'
1392 -- put ty1 ~ ty2 in LIE
1393 -- Left means "wanted"
1394 ; inst <- (if outer then popErrCtxt else id) $
1395 mkEqInst (EqPred ty1' ty2') (Left cotv)
1397 ; return $ ACo $ TyVarTy cotv }
1400 uMetaVar :: Bool -- pop innermost context?
1402 -> TcTyVar -> BoxInfo -> IORef MetaDetails
1405 -- tv1 is an un-filled-in meta type variable (maybe boxy, maybe tau)
1406 -- ty2 is not a type variable
1408 uMetaVar outer swapped tv1 BoxTv ref1 ps_ty2 non_var_ty2
1409 = -- tv1 is a BoxTv. So we must unbox ty2, to ensure
1410 -- that any boxes in ty2 are filled with monotypes
1412 -- It should not be the case that tv1 occurs in ty2
1413 -- (i.e. no occurs check should be needed), but if perchance
1414 -- it does, the unbox operation will fill it, and the DEBUG
1416 do { final_ty <- unBox ps_ty2
1418 ; meta_details <- readMutVar ref1
1419 ; case meta_details of
1420 Indirect ty -> WARN( True, ppr tv1 <+> ppr ty )
1421 return () -- This really should *not* happen
1424 ; checkUpdateMeta swapped tv1 ref1 final_ty
1428 uMetaVar outer swapped tv1 info1 ref1 ps_ty2 non_var_ty2
1429 = do { -- Occurs check + monotype check
1430 ; mb_final_ty <- checkTauTvUpdate tv1 ps_ty2
1431 ; case mb_final_ty of
1432 Nothing -> -- tv1 occured in type family parameter
1433 defer_unification outer swapped (mkTyVarTy tv1) ps_ty2
1435 do { checkUpdateMeta swapped tv1 ref1 final_ty
1441 uUnfilledVars :: Outer
1443 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
1444 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
1446 -- Invarant: The type variables are distinct,
1447 -- Neither is filled in yet
1448 -- They might be boxy or not
1450 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (SkolemTv _)
1451 = -- see [Deferred Unification]
1452 defer_unification outer swapped (mkTyVarTy tv1) (mkTyVarTy tv2)
1454 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (SkolemTv _)
1455 = checkUpdateMeta swapped tv1 ref1 (mkTyVarTy tv2) >> return IdCo
1456 uUnfilledVars outer swapped tv1 (SkolemTv _) tv2 (MetaTv info2 ref2)
1457 = checkUpdateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1) >> return IdCo
1459 -- ToDo: this function seems too long for what it acutally does!
1460 uUnfilledVars outer swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2)
1461 = case (info1, info2) of
1462 (BoxTv, BoxTv) -> box_meets_box >> return IdCo
1464 -- If a box meets a TauTv, but the fomer has the smaller kind
1465 -- then we must create a fresh TauTv with the smaller kind
1466 (_, BoxTv) | k1_sub_k2 -> update_tv2 >> return IdCo
1467 | otherwise -> box_meets_box >> return IdCo
1468 (BoxTv, _ ) | k2_sub_k1 -> update_tv1 >> return IdCo
1469 | otherwise -> box_meets_box >> return IdCo
1471 -- Avoid SigTvs if poss
1472 (SigTv _, _ ) | k1_sub_k2 -> update_tv2 >> return IdCo
1473 (_, SigTv _) | k2_sub_k1 -> update_tv1 >> return IdCo
1475 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1476 then update_tv1 >> return IdCo -- Same kinds
1477 else update_tv2 >> return IdCo
1478 | k2_sub_k1 -> update_tv1 >> return IdCo
1479 | otherwise -> kind_err >> return IdCo
1481 -- Update the variable with least kind info
1482 -- See notes on type inference in Kind.lhs
1483 -- The "nicer to" part only applies if the two kinds are the same,
1484 -- so we can choose which to do.
1486 -- Kinds should be guaranteed ok at this point
1487 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1488 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1490 box_meets_box | k1_sub_k2 = if k2_sub_k1 && nicer_to_update_tv1
1493 | k2_sub_k1 = fill_from tv2
1494 | otherwise = kind_err
1496 -- Update *both* tyvars with a TauTv whose name and kind
1497 -- are gotten from tv (avoid losing nice names is poss)
1498 fill_from tv = do { tv' <- tcInstTyVar tv
1499 ; let tau_ty = mkTyVarTy tv'
1500 ; updateMeta tv1 ref1 tau_ty
1501 ; updateMeta tv2 ref2 tau_ty }
1503 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1504 unifyKindMisMatch k1 k2
1508 k1_sub_k2 = k1 `isSubKind` k2
1509 k2_sub_k1 = k2 `isSubKind` k1
1511 nicer_to_update_tv1 = isSystemName (Var.varName tv1)
1512 -- Try to update sys-y type variables in preference to ones
1513 -- gotten (say) by instantiating a polymorphic function with
1514 -- a user-written type sig
1518 refineBox :: TcType -> TcM TcType
1519 -- Unbox the outer box of a boxy type (if any)
1520 refineBox ty@(TyVarTy box_tv)
1521 | isMetaTyVar box_tv
1522 = do { cts <- readMetaTyVar box_tv
1525 Indirect ty -> return ty }
1526 refineBox other_ty = return other_ty
1528 refineBoxToTau :: TcType -> TcM TcType
1529 -- Unbox the outer box of a boxy type, filling with a monotype if it is empty
1530 -- Like refineBox except for the "fill with monotype" part.
1531 refineBoxToTau ty@(TyVarTy box_tv)
1532 | isMetaTyVar box_tv
1533 , MetaTv BoxTv ref <- tcTyVarDetails box_tv
1534 = do { cts <- readMutVar ref
1536 Flexi -> fillBoxWithTau box_tv ref
1537 Indirect ty -> return ty }
1538 refineBoxToTau other_ty = return other_ty
1540 zapToMonotype :: BoxySigmaType -> TcM TcTauType
1541 -- Subtle... we must zap the boxy res_ty
1542 -- to kind * before using it to instantiate a LitInst
1543 -- Calling unBox instead doesn't do the job, because the box
1544 -- often has an openTypeKind, and we don't want to instantiate
1546 zapToMonotype res_ty
1547 = do { res_tau <- newFlexiTyVarTy liftedTypeKind
1548 ; boxyUnify res_tau res_ty
1551 unBox :: BoxyType -> TcM TcType
1552 -- unBox implements the judgement
1554 -- with input s', and result s
1556 -- It removes all boxes from the input type, returning a non-boxy type.
1557 -- A filled box in the type can only contain a monotype; unBox fails if not
1558 -- The type can have empty boxes, which unBox fills with a monotype
1560 -- Compare this wth checkTauTvUpdate
1562 -- For once, it's safe to treat synonyms as opaque!
1564 unBox (NoteTy n ty) = do { ty' <- unBox ty; return (NoteTy n ty') }
1565 unBox (TyConApp tc tys) = do { tys' <- mapM unBox tys; return (TyConApp tc tys') }
1566 unBox (AppTy f a) = do { f' <- unBox f; a' <- unBox a; return (mkAppTy f' a') }
1567 unBox (FunTy f a) = do { f' <- unBox f; a' <- unBox a; return (FunTy f' a') }
1568 unBox (PredTy p) = do { p' <- unBoxPred p; return (PredTy p') }
1569 unBox (ForAllTy tv ty) = ASSERT( isImmutableTyVar tv )
1570 do { ty' <- unBox ty; return (ForAllTy tv ty') }
1572 | isTcTyVar tv -- It's a boxy type variable
1573 , MetaTv BoxTv ref <- tcTyVarDetails tv -- NB: non-TcTyVars are possible
1574 = do { cts <- readMutVar ref -- under nested quantifiers
1576 Flexi -> fillBoxWithTau tv ref
1577 Indirect ty -> do { non_boxy_ty <- unBox ty
1578 ; if isTauTy non_boxy_ty
1579 then return non_boxy_ty
1580 else notMonoType non_boxy_ty }
1582 | otherwise -- Skolems, and meta-tau-variables
1583 = return (TyVarTy tv)
1585 unBoxPred (ClassP cls tys) = do { tys' <- mapM unBox tys; return (ClassP cls tys') }
1586 unBoxPred (IParam ip ty) = do { ty' <- unBox ty; return (IParam ip ty') }
1587 unBoxPred (EqPred ty1 ty2) = do { ty1' <- unBox ty1; ty2' <- unBox ty2; return (EqPred ty1' ty2') }
1592 %************************************************************************
1594 \subsection[Unify-context]{Errors and contexts}
1596 %************************************************************************
1602 unifyCtxt act_ty exp_ty tidy_env
1603 = do { act_ty' <- zonkTcType act_ty
1604 ; exp_ty' <- zonkTcType exp_ty
1605 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1606 (env2, act_ty'') = tidyOpenType env1 act_ty'
1607 ; return (env2, mkExpectedActualMsg act_ty'' exp_ty'') }
1610 mkExpectedActualMsg act_ty exp_ty
1611 = nest 2 (vcat [ text "Expected type" <> colon <+> ppr exp_ty,
1612 text "Inferred type" <> colon <+> ppr act_ty ])
1615 -- If an error happens we try to figure out whether the function
1616 -- function has been given too many or too few arguments, and say so.
1617 addSubCtxt SubDone actual_res_ty expected_res_ty thing_inside
1619 addSubCtxt sub_ctxt actual_res_ty expected_res_ty thing_inside
1620 = addErrCtxtM mk_err thing_inside
1623 = do { exp_ty' <- zonkTcType expected_res_ty
1624 ; act_ty' <- zonkTcType actual_res_ty
1625 ; let (env1, exp_ty'') = tidyOpenType tidy_env exp_ty'
1626 (env2, act_ty'') = tidyOpenType env1 act_ty'
1627 (exp_args, _) = tcSplitFunTys exp_ty''
1628 (act_args, _) = tcSplitFunTys act_ty''
1630 len_act_args = length act_args
1631 len_exp_args = length exp_args
1633 message = case sub_ctxt of
1634 SubFun fun | len_exp_args < len_act_args -> wrongArgsCtxt "too few" fun
1635 | len_exp_args > len_act_args -> wrongArgsCtxt "too many" fun
1636 other -> mkExpectedActualMsg act_ty'' exp_ty''
1637 ; return (env2, message) }
1639 wrongArgsCtxt too_many_or_few fun
1640 = ptext SLIT("Probable cause:") <+> quotes (ppr fun)
1641 <+> ptext SLIT("is applied to") <+> text too_many_or_few
1642 <+> ptext SLIT("arguments")
1645 unifyForAllCtxt tvs phi1 phi2 env
1646 = returnM (env2, msg)
1648 (env', tvs') = tidyOpenTyVars env tvs -- NB: not tidyTyVarBndrs
1649 (env1, phi1') = tidyOpenType env' phi1
1650 (env2, phi2') = tidyOpenType env1 phi2
1651 msg = vcat [ptext SLIT("When matching") <+> quotes (ppr (mkForAllTys tvs' phi1')),
1652 ptext SLIT(" and") <+> quotes (ppr (mkForAllTys tvs' phi2'))]
1654 -----------------------
1655 unifyMisMatch outer swapped ty1 ty2
1656 = do { (env, msg) <- if swapped then misMatchMsg ty2 ty1
1657 else misMatchMsg ty1 ty2
1659 -- This is the whole point of the 'outer' stuff
1660 ; if outer then popErrCtxt (failWithTcM (env, msg))
1661 else failWithTcM (env, msg)
1666 %************************************************************************
1670 %************************************************************************
1672 Unifying kinds is much, much simpler than unifying types.
1675 unifyKind :: TcKind -- Expected
1678 unifyKind (TyConApp kc1 []) (TyConApp kc2 [])
1679 | isSubKindCon kc2 kc1 = returnM ()
1681 unifyKind (FunTy a1 r1) (FunTy a2 r2)
1682 = do { unifyKind a2 a1; unifyKind r1 r2 }
1683 -- Notice the flip in the argument,
1684 -- so that the sub-kinding works right
1685 unifyKind (TyVarTy kv1) k2 = uKVar False kv1 k2
1686 unifyKind k1 (TyVarTy kv2) = uKVar True kv2 k1
1687 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1689 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1690 unifyKinds [] [] = returnM ()
1691 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1693 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1696 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1697 uKVar swapped kv1 k2
1698 = do { mb_k1 <- readKindVar kv1
1700 Flexi -> uUnboundKVar swapped kv1 k2
1701 Indirect k1 | swapped -> unifyKind k2 k1
1702 | otherwise -> unifyKind k1 k2 }
1705 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1706 uUnboundKVar swapped kv1 k2@(TyVarTy kv2)
1707 | kv1 == kv2 = returnM ()
1708 | otherwise -- Distinct kind variables
1709 = do { mb_k2 <- readKindVar kv2
1711 Indirect k2 -> uUnboundKVar swapped kv1 k2
1712 Flexi -> writeKindVar kv1 k2 }
1714 uUnboundKVar swapped kv1 non_var_k2
1715 = do { k2' <- zonkTcKind non_var_k2
1716 ; kindOccurCheck kv1 k2'
1717 ; k2'' <- kindSimpleKind swapped k2'
1718 -- KindVars must be bound only to simple kinds
1719 -- Polarities: (kindSimpleKind True ?) succeeds
1720 -- returning *, corresponding to unifying
1723 ; writeKindVar kv1 k2'' }
1726 kindOccurCheck kv1 k2 -- k2 is zonked
1727 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1729 not_in (TyVarTy kv2) = kv1 /= kv2
1730 not_in (FunTy a2 r2) = not_in a2 && not_in r2
1733 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1734 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1735 -- If the flag is False, it requires k <: sk
1736 -- E.g. kindSimpleKind False ?? = *
1737 -- What about (kv -> *) :=: ?? -> *
1738 kindSimpleKind orig_swapped orig_kind
1739 = go orig_swapped orig_kind
1741 go sw (FunTy k1 k2) = do { k1' <- go (not sw) k1
1743 ; return (mkArrowKind k1' k2') }
1745 | isOpenTypeKind k = return liftedTypeKind
1746 | isArgTypeKind k = return liftedTypeKind
1748 | isLiftedTypeKind k = return liftedTypeKind
1749 | isUnliftedTypeKind k = return unliftedTypeKind
1750 go sw k@(TyVarTy _) = return k -- KindVars are always simple
1751 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1752 <+> ppr orig_swapped <+> ppr orig_kind)
1753 -- I think this can't actually happen
1755 -- T v = MkT v v must be a type
1756 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1759 kindOccurCheckErr tyvar ty
1760 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1761 2 (sep [ppr tyvar, char '=', ppr ty])
1765 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1766 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1768 unifyFunKind (TyVarTy kvar)
1769 = readKindVar kvar `thenM` \ maybe_kind ->
1771 Indirect fun_kind -> unifyFunKind fun_kind
1773 do { arg_kind <- newKindVar
1774 ; res_kind <- newKindVar
1775 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1776 ; returnM (Just (arg_kind,res_kind)) }
1778 unifyFunKind (FunTy arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1779 unifyFunKind other = returnM Nothing
1782 %************************************************************************
1786 %************************************************************************
1788 ---------------------------
1789 -- We would like to get a decent error message from
1790 -- (a) Under-applied type constructors
1791 -- f :: (Maybe, Maybe)
1792 -- (b) Over-applied type constructors
1793 -- f :: Int x -> Int x
1797 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1798 -- A fancy wrapper for 'unifyKind', which tries
1799 -- to give decent error messages.
1800 -- (checkExpectedKind ty act_kind exp_kind)
1801 -- checks that the actual kind act_kind is compatible
1802 -- with the expected kind exp_kind
1803 -- The first argument, ty, is used only in the error message generation
1804 checkExpectedKind ty act_kind exp_kind
1805 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1808 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1810 Just r -> returnM () ; -- Unification succeeded
1813 -- So there's definitely an error
1814 -- Now to find out what sort
1815 zonkTcKind exp_kind `thenM` \ exp_kind ->
1816 zonkTcKind act_kind `thenM` \ act_kind ->
1818 tcInitTidyEnv `thenM` \ env0 ->
1819 let (exp_as, _) = splitKindFunTys exp_kind
1820 (act_as, _) = splitKindFunTys act_kind
1821 n_exp_as = length exp_as
1822 n_act_as = length act_as
1824 (env1, tidy_exp_kind) = tidyKind env0 exp_kind
1825 (env2, tidy_act_kind) = tidyKind env1 act_kind
1827 err | n_exp_as < n_act_as -- E.g. [Maybe]
1828 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1830 -- Now n_exp_as >= n_act_as. In the next two cases,
1831 -- n_exp_as == 0, and hence so is n_act_as
1832 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1833 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1834 <+> ptext SLIT("is unlifted")
1836 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1837 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1838 <+> ptext SLIT("is lifted")
1840 | otherwise -- E.g. Monad [Int]
1841 = ptext SLIT("Kind mis-match")
1843 more_info = sep [ ptext SLIT("Expected kind") <+>
1844 quotes (pprKind tidy_exp_kind) <> comma,
1845 ptext SLIT("but") <+> quotes (ppr ty) <+>
1846 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1848 failWithTcM (env2, err $$ more_info)
1852 %************************************************************************
1854 \subsection{Checking signature type variables}
1856 %************************************************************************
1858 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1859 are not mentioned in the environment. In particular:
1861 (a) Not mentioned in the type of a variable in the envt
1862 eg the signature for f in this:
1868 Here, f is forced to be monorphic by the free occurence of x.
1870 (d) Not (unified with another type variable that is) in scope.
1871 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1872 when checking the expression type signature, we find that
1873 even though there is nothing in scope whose type mentions r,
1874 nevertheless the type signature for the expression isn't right.
1876 Another example is in a class or instance declaration:
1878 op :: forall b. a -> b
1880 Here, b gets unified with a
1882 Before doing this, the substitution is applied to the signature type variable.
1885 checkSigTyVars :: [TcTyVar] -> TcM ()
1886 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1888 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1889 -- The extra_tvs can include boxy type variables;
1890 -- e.g. TcMatches.tcCheckExistentialPat
1891 checkSigTyVarsWrt extra_tvs sig_tvs
1892 = do { extra_tvs' <- zonkTcTyVarsAndFV (varSetElems extra_tvs)
1893 ; check_sig_tyvars extra_tvs' sig_tvs }
1896 :: TcTyVarSet -- Global type variables. The universally quantified
1897 -- tyvars should not mention any of these
1898 -- Guaranteed already zonked.
1899 -> [TcTyVar] -- Universally-quantified type variables in the signature
1900 -- Guaranteed to be skolems
1902 check_sig_tyvars extra_tvs []
1904 check_sig_tyvars extra_tvs sig_tvs
1905 = ASSERT( all isSkolemTyVar sig_tvs )
1906 do { gbl_tvs <- tcGetGlobalTyVars
1907 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1908 text "gbl_tvs" <+> ppr gbl_tvs,
1909 text "extra_tvs" <+> ppr extra_tvs]))
1911 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1912 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1913 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1916 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1917 -> [TcTyVar] -- The possibly-escaping type variables
1918 -> [TcTyVar] -- The zonked versions thereof
1920 -- Complain about escaping type variables
1921 -- We pass a list of type variables, at least one of which
1922 -- escapes. The first list contains the original signature type variable,
1923 -- while the second contains the type variable it is unified to (usually itself)
1924 bleatEscapedTvs globals sig_tvs zonked_tvs
1925 = do { env0 <- tcInitTidyEnv
1926 ; let (env1, tidy_tvs) = tidyOpenTyVars env0 sig_tvs
1927 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1929 ; (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1930 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1932 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1934 check (tidy_env, msgs) (sig_tv, zonked_tv)
1935 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1937 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1938 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1940 -----------------------
1941 escape_msg sig_tv zonked_tv globs
1943 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1944 nest 2 (vcat globs)]
1946 = msg <+> ptext SLIT("escapes")
1947 -- Sigh. It's really hard to give a good error message
1948 -- all the time. One bad case is an existential pattern match.
1949 -- We rely on the "When..." context to help.
1951 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1953 | sig_tv == zonked_tv = empty
1954 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1957 These two context are used with checkSigTyVars
1960 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1961 -> TidyEnv -> TcM (TidyEnv, Message)
1962 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1963 = zonkTcType sig_tau `thenM` \ actual_tau ->
1965 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1966 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1967 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1968 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1969 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1971 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),