2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section{Type subsumption and unification}
8 -- Full-blown subsumption
9 tcSubPat, tcSubExp, tcSub, tcGen,
10 checkSigTyVars, checkSigTyVarsWrt, bleatEscapedTvs, sigCtxt,
12 -- Various unifications
13 unifyTauTy, unifyTauTyList, unifyTheta,
14 unifyKind, unifyKinds, unifyFunKind,
17 --------------------------------
19 Expected(..), tcInfer, readExpectedType,
20 zapExpectedType, zapExpectedTo, zapExpectedBranches,
21 subFunTys, unifyFunTys,
22 zapToListTy, unifyListTy,
23 zapToTyConApp, unifyTyConApp,
27 #include "HsVersions.h"
29 import HsSyn ( HsExpr(..) , MatchGroup(..), HsMatchContext(..),
30 hsLMatchPats, pprMatches, pprMatchContext )
31 import TcHsSyn ( mkHsLet, mkHsDictLam,
32 ExprCoFn, idCoercion, isIdCoercion, mkCoercion, (<.>), (<$>) )
33 import TypeRep ( Type(..), PredType(..), TyNote(..) )
35 import TcRnMonad -- TcType, amongst others
36 import TcType ( TcKind, TcType, TcSigmaType, TcRhoType, TcTyVar, TcTauType,
37 TcTyVarSet, TcThetaType, Expected(..), TcTyVarDetails(..),
38 SkolemInfo( GenSkol ), MetaDetails(..),
39 pprTcTyVar, isTauTy, isSigmaTy, mkFunTy, mkFunTys, mkTyConApp,
40 tcSplitAppTy_maybe, tcSplitTyConApp_maybe,
41 tyVarsOfType, mkPhiTy, mkTyVarTy, mkPredTy,
42 typeKind, tcSplitFunTy_maybe, mkForAllTys, mkAppTy,
43 tidyOpenType, tidyOpenTypes, tidyOpenTyVar, tidyOpenTyVars,
44 pprType, tidySkolemTyVar, isSkolemTyVar )
45 import Kind ( Kind(..), SimpleKind, KindVar, isArgTypeKind,
46 openTypeKind, liftedTypeKind, mkArrowKind,
47 isOpenTypeKind, argTypeKind, isLiftedTypeKind, isUnliftedTypeKind,
48 isSubKind, pprKind, splitKindFunTys )
49 import Inst ( newDicts, instToId, tcInstCall )
50 import TcMType ( condLookupTcTyVar, LookupTyVarResult(..),
51 tcSkolType, newKindVar, tcInstTyVars, newMetaTyVar,
52 newTyFlexiVarTy, zonkTcKind, zonkType, zonkTcType, zonkTcTyVarsAndFV,
53 readKindVar, writeKindVar )
54 import TcSimplify ( tcSimplifyCheck )
55 import TcIface ( checkWiredInTyCon )
56 import TcEnv ( tcGetGlobalTyVars, findGlobals )
57 import TyCon ( TyCon, tyConArity, tyConTyVars )
58 import TysWiredIn ( listTyCon )
59 import Id ( Id, mkSysLocal )
60 import Var ( Var, varName, tyVarKind )
61 import VarSet ( emptyVarSet, unitVarSet, unionVarSet, elemVarSet, varSetElems )
63 import Name ( Name, isSystemName, mkSysTvName )
64 import ErrUtils ( Message )
65 import SrcLoc ( noLoc )
66 import BasicTypes ( Arity )
67 import Util ( notNull, equalLength )
73 * A hole is always filled in with an ordinary type, not another hole.
75 %************************************************************************
77 \subsection{'hole' type variables}
79 %************************************************************************
82 newHole = newMutVar (error "Empty hole in typechecker")
84 tcInfer :: (Expected ty -> TcM a) -> TcM (a,ty)
86 = do { hole <- newHole
87 ; res <- tc_infer (Infer hole)
88 ; res_ty <- readMutVar hole
89 ; return (res, res_ty) }
91 readExpectedType :: Expected ty -> TcM ty
92 readExpectedType (Infer hole) = readMutVar hole
93 readExpectedType (Check ty) = returnM ty
95 zapExpectedType :: Expected TcType -> Kind -> TcM TcTauType
96 -- In the inference case, ensure we have a monotype
97 -- (including an unboxed tuple)
98 zapExpectedType (Infer hole) kind
99 = do { ty <- newTyFlexiVarTy kind ;
100 writeMutVar hole ty ;
103 zapExpectedType (Check ty) kind
104 | typeKind ty `isSubKind` kind = return ty
105 | otherwise = do { ty1 <- newTyFlexiVarTy kind
108 -- The unify is to ensure that 'ty' has the desired kind
109 -- For example, in (case e of r -> b) we push an OpenTypeKind
112 zapExpectedBranches :: MatchGroup id -> Expected TcRhoType -> TcM (Expected TcRhoType)
113 -- If there is more than one branch in a case expression,
114 -- and exp_ty is a 'hole', all branches must be types, not type schemes,
115 -- otherwise the order in which we check them would affect the result.
116 zapExpectedBranches (MatchGroup [match] _) exp_ty
117 = return exp_ty -- One branch
118 zapExpectedBranches matches (Check ty)
120 zapExpectedBranches matches (Infer hole)
121 = do { -- Many branches, and inference mode,
122 -- so switch to checking mode with a monotype
123 ty <- newTyFlexiVarTy openTypeKind
124 ; writeMutVar hole ty
125 ; return (Check ty) }
127 zapExpectedTo :: Expected TcType -> TcTauType -> TcM ()
128 zapExpectedTo (Check ty1) ty2 = unifyTauTy ty1 ty2
129 zapExpectedTo (Infer hole) ty2 = do { ty2' <- zonkTcType ty2; writeMutVar hole ty2' }
130 -- See Note [Zonk return type]
132 instance Outputable ty => Outputable (Expected ty) where
133 ppr (Check ty) = ptext SLIT("Expected type") <+> ppr ty
134 ppr (Infer hole) = ptext SLIT("Inferring type")
138 %************************************************************************
140 \subsection[Unify-fun]{@unifyFunTy@}
142 %************************************************************************
144 @subFunTy@ and @unifyFunTy@ is used to avoid the fruitless
145 creation of type variables.
147 * subFunTy is used when we might be faced with a "hole" type variable,
148 in which case we should create two new holes.
150 * unifyFunTy is used when we expect to encounter only "ordinary"
151 type variables, so we should create new ordinary type variables
154 subFunTys :: HsMatchContext Name
156 -> Expected TcRhoType -- Fail if ty isn't a function type
157 -> ([Expected TcRhoType] -> Expected TcRhoType -> TcM a)
160 subFunTys ctxt (MatchGroup (match:null_matches) _) (Infer hole) thing_inside
161 = -- This is the interesting case
162 ASSERT( null null_matches )
163 do { pat_holes <- mapM (\ _ -> newHole) (hsLMatchPats match)
164 ; res_hole <- newHole
167 ; res <- thing_inside (map Infer pat_holes) (Infer res_hole)
169 -- Extract the answers
170 ; arg_tys <- mapM readMutVar pat_holes
171 ; res_ty <- readMutVar res_hole
173 -- Write the answer into the incoming hole
174 ; writeMutVar hole (mkFunTys arg_tys res_ty)
176 -- And return the answer
179 subFunTys ctxt group@(MatchGroup (match:matches) _) (Check ty) thing_inside
180 = ASSERT( all ((== n_pats) . length . hsLMatchPats) matches )
181 -- Assertion just checks that all the matches have the same number of pats
182 do { (pat_tys, res_ty) <- unifyFunTys msg n_pats ty
183 ; thing_inside (map Check pat_tys) (Check res_ty) }
185 n_pats = length (hsLMatchPats match)
187 FunRhs fun -> ptext SLIT("The equation(s) for") <+> quotes (ppr fun)
188 <+> ptext SLIT("have") <+> speakN n_pats <+> ptext SLIT("arguments")
189 LambdaExpr -> sep [ ptext SLIT("The lambda expression")
190 <+> quotes (pprSetDepth 1 $ pprMatches ctxt group),
191 -- The pprSetDepth makes the abstraction print briefly
192 ptext SLIT("has") <+> speakN n_pats <+> ptext SLIT("arguments")]
193 other -> pprPanic "subFunTys" (pprMatchContext ctxt)
196 unifyFunTys :: SDoc -> Arity -> TcRhoType -> TcM ([TcSigmaType], TcRhoType)
197 -- Fail if ty isn't a function type, otherwise return arg and result types
198 -- The result types are guaranteed wobbly if the argument is wobbly
200 -- Does not allocate unnecessary meta variables: if the input already is
201 -- a function, we just take it apart. Not only is this efficient, it's important
202 -- for (a) higher rank: the argument might be of form
203 -- (forall a. ty) -> other
204 -- If allocated (fresh-meta-var1 -> fresh-meta-var2) and unified, we'd
205 -- blow up with the meta var meets the forall
207 -- (b) GADTs: if the argument is not wobbly we do not want the result to be
210 Error messages from unifyFunTys
211 The first line is passed in as error_herald
213 The abstraction `\Just 1 -> ...' has two arguments
214 but its type `Maybe a -> a' has only one
216 The equation(s) for `f' have two arguments
217 but its type `Maybe a -> a' has only one
219 The section `(f 3)' requires 'f' to take two arguments
220 but its type `Int -> Int' has only one
222 The function 'f' is applied to two arguments
223 but its type `Int -> Int' has only one
226 unifyFunTys error_herald arity ty
227 -- error_herald is the whole first line of the error message above
228 = do { (ok, args, res) <- unify_fun_ty True arity ty
229 ; if ok then return (args, res)
230 else failWithTc (mk_msg (length args)) }
233 = error_herald <> comma $$
234 sep [ptext SLIT("but its type") <+> quotes (pprType ty),
235 ptext SLIT("has only") <+> speakN n_actual]
237 unify_fun_ty :: Bool -> Arity -> TcRhoType
238 -> TcM (Bool, -- Arity satisfied?
239 [TcSigmaType], -- Arg types found; length <= arity
240 TcRhoType) -- Result type
242 unify_fun_ty use_refinement arity ty
244 = do { res_ty <- wobblify use_refinement ty
245 ; return (True, [], ty) }
247 unify_fun_ty use_refinement arity (NoteTy _ ty)
248 = unify_fun_ty use_refinement arity ty
250 unify_fun_ty use_refinement arity ty@(TyVarTy tv)
251 = do { details <- condLookupTcTyVar use_refinement tv
253 IndirectTv use' ty' -> unify_fun_ty use' arity ty'
254 DoneTv (MetaTv ref) -> ASSERT( liftedTypeKind `isSubKind` tyVarKind tv )
255 -- The argument to unifyFunTys is always a type
256 -- Occurs check can't happen, of course
257 do { args <- mappM newTyFlexiVarTy (replicate arity argTypeKind)
258 ; res <- newTyFlexiVarTy openTypeKind
259 ; writeMutVar ref (Indirect (mkFunTys args res))
260 ; return (True, args, res) }
261 DoneTv skol -> return (False, [], ty)
264 unify_fun_ty use_refinement arity ty
265 | Just (arg,res) <- tcSplitFunTy_maybe ty
266 = do { arg' <- wobblify use_refinement arg
267 ; (ok, args', res') <- unify_fun_ty use_refinement (arity-1) res
268 ; return (ok, arg':args', res') }
270 unify_fun_ty use_refinement arity ty
271 -- Common cases are all done by now
272 -- At this point we usually have an error, but ty could
273 -- be (a Int Bool), or (a Bool), which can match
274 -- So just use the unifier. But catch any error so we just
275 -- return the success/fail boolean
276 = do { arg <- newTyFlexiVarTy argTypeKind
277 ; res <- newTyFlexiVarTy openTypeKind
278 ; let fun_ty = mkFunTy arg res
279 ; (_, mb_unit) <- tryTc (uTys True ty ty True fun_ty fun_ty)
281 Nothing -> return (False, [], ty) ;
283 do { (ok, args', res') <- unify_fun_ty use_refinement (arity-1) res
284 ; return (ok, arg:args', res')
289 ----------------------
290 zapToTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
291 -> Expected TcSigmaType -- Expected type (T a b c)
292 -> TcM [TcType] -- Element types, a b c
293 -- Insists that the Expected type is not a forall-type
294 -- It's used for wired-in tycons, so we call checkWiredInTyCOn
295 zapToTyConApp tc (Check ty)
296 = do { checkWiredInTyCon tc ; unifyTyConApp tc ty } -- NB: fails for a forall-type
298 zapToTyConApp tc (Infer hole)
299 = do { (tc_app, elt_tys) <- newTyConApp tc
300 ; writeMutVar hole tc_app
301 ; traceTc (text "zap" <+> ppr tc)
302 ; checkWiredInTyCon tc
305 zapToListTy :: Expected TcType -> TcM TcType -- Special case for lists
306 zapToListTy exp_ty = do { [elt_ty] <- zapToTyConApp listTyCon exp_ty
309 ----------------------
310 unifyTyConApp :: TyCon -> TcType -> TcM [TcType]
311 unifyTyConApp tc ty = unify_tc_app True tc ty
312 -- Add a boolean flag to remember whether to use
313 -- the type refinement or not
315 unifyListTy :: TcType -> TcM TcType -- Special case for lists
316 unifyListTy exp_ty = do { [elt_ty] <- unifyTyConApp listTyCon exp_ty
320 unify_tc_app use_refinement tc (NoteTy _ ty)
321 = unify_tc_app use_refinement tc ty
323 unify_tc_app use_refinement tc ty@(TyVarTy tyvar)
324 = do { details <- condLookupTcTyVar use_refinement tyvar
326 IndirectTv use' ty' -> unify_tc_app use' tc ty'
327 other -> unify_tc_app_help tc ty
330 unify_tc_app use_refinement tc ty
331 | Just (tycon, arg_tys) <- tcSplitTyConApp_maybe ty,
333 = ASSERT( tyConArity tycon == length arg_tys ) -- ty::*
334 mapM (wobblify use_refinement) arg_tys
336 unify_tc_app use_refinement tc ty = unify_tc_app_help tc ty
338 unify_tc_app_help tc ty -- Revert to ordinary unification
339 = do { (tc_app, arg_tys) <- newTyConApp tc
340 ; if not (isTauTy ty) then -- Can happen if we call zapToTyConApp tc (forall a. ty)
341 unifyMisMatch ty tc_app
343 { unifyTauTy ty tc_app
344 ; returnM arg_tys } }
347 ----------------------
348 unifyAppTy :: TcType -- Type to split: m a
349 -> TcM (TcType, TcType) -- (m,a)
352 unifyAppTy ty = unify_app_ty True ty
354 unify_app_ty use (NoteTy _ ty) = unify_app_ty use ty
356 unify_app_ty use ty@(TyVarTy tyvar)
357 = do { details <- condLookupTcTyVar use tyvar
359 IndirectTv use' ty' -> unify_app_ty use' ty'
360 other -> unify_app_ty_help ty
364 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
365 = do { fun' <- wobblify use fun_ty
366 ; arg' <- wobblify use arg_ty
367 ; return (fun', arg') }
369 | otherwise = unify_app_ty_help ty
371 unify_app_ty_help ty -- Revert to ordinary unification
372 = do { fun_ty <- newTyFlexiVarTy (mkArrowKind liftedTypeKind liftedTypeKind)
373 ; arg_ty <- newTyFlexiVarTy liftedTypeKind
374 ; unifyTauTy (mkAppTy fun_ty arg_ty) ty
375 ; return (fun_ty, arg_ty) }
378 ----------------------
379 wobblify :: Bool -- True <=> don't wobblify
382 -- Return a wobbly type. At the moment we do that by
383 -- allocating a fresh meta type variable.
384 wobblify True ty = return ty
385 wobblify False ty = do { uniq <- newUnique
386 ; tv <- newMetaTyVar (mkSysTvName uniq FSLIT("w"))
389 ; return (mkTyVarTy tv) }
391 ----------------------
392 newTyConApp :: TyCon -> TcM (TcTauType, [TcTauType])
393 newTyConApp tc = do { (tvs, args, _) <- tcInstTyVars (tyConTyVars tc)
394 ; return (mkTyConApp tc args, args) }
398 %************************************************************************
400 \subsection{Subsumption}
402 %************************************************************************
404 All the tcSub calls have the form
406 tcSub expected_ty offered_ty
408 offered_ty <= expected_ty
410 That is, that a value of type offered_ty is acceptable in
411 a place expecting a value of type expected_ty.
413 It returns a coercion function
414 co_fn :: offered_ty -> expected_ty
415 which takes an HsExpr of type offered_ty into one of type
419 -----------------------
420 -- tcSubExp is used for expressions
421 tcSubExp :: Expected TcRhoType -> TcRhoType -> TcM ExprCoFn
423 tcSubExp (Infer hole) offered_ty
424 = do { offered' <- zonkTcType offered_ty
425 -- Note [Zonk return type]
426 -- zonk to take advantage of the current GADT type refinement.
427 -- If we don't we get spurious "existential type variable escapes":
428 -- case (x::Maybe a) of
429 -- Just b (y::b) -> y
430 -- We need the refinement [b->a] to be applied to the result type
431 ; writeMutVar hole offered'
432 ; return idCoercion }
434 tcSubExp (Check expected_ty) offered_ty
435 = tcSub expected_ty offered_ty
437 -----------------------
438 -- tcSubPat is used for patterns
439 tcSubPat :: TcSigmaType -- Pattern type signature
440 -> Expected TcSigmaType -- Type from context
442 -- In patterns we insist on an exact match; hence no CoFn returned
443 -- See Note [Pattern coercions] in TcPat
444 -- However, we can't call unify directly, because both types might be
445 -- polymorphic; hence the call to tcSub, followed by a check for
446 -- the identity coercion
448 tcSubPat sig_ty (Infer hole)
449 = do { sig_ty' <- zonkTcType sig_ty
450 ; writeMutVar hole sig_ty' -- See notes with tcSubExp above
453 tcSubPat sig_ty (Check exp_ty)
454 = do { co_fn <- tcSub sig_ty exp_ty
456 ; if isIdCoercion co_fn then
459 unifyMisMatch sig_ty exp_ty }
464 %************************************************************************
466 tcSub: main subsumption-check code
468 %************************************************************************
470 No holes expected now. Add some error-check context info.
474 tcSub :: TcSigmaType -> TcSigmaType -> TcM ExprCoFn -- Locally used only
475 -- tcSub exp act checks that
477 tcSub expected_ty actual_ty
478 = traceTc (text "tcSub" <+> details) `thenM_`
479 addErrCtxtM (unifyCtxt "type" expected_ty actual_ty)
480 (tc_sub expected_ty expected_ty actual_ty actual_ty)
482 details = vcat [text "Expected:" <+> ppr expected_ty,
483 text "Actual: " <+> ppr actual_ty]
486 tc_sub :: TcSigmaType -- expected_ty, before expanding synonyms
487 -> TcSigmaType -- ..and after
488 -> TcSigmaType -- actual_ty, before
489 -> TcSigmaType -- ..and after
492 -----------------------------------
494 tc_sub exp_sty (NoteTy _ exp_ty) act_sty act_ty = tc_sub exp_sty exp_ty act_sty act_ty
495 tc_sub exp_sty exp_ty act_sty (NoteTy _ act_ty) = tc_sub exp_sty exp_ty act_sty act_ty
497 -----------------------------------
498 -- Generalisation case
499 -- actual_ty: d:Eq b => b->b
500 -- expected_ty: forall a. Ord a => a->a
501 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
503 -- It is essential to do this *before* the specialisation case
504 -- Example: f :: (Eq a => a->a) -> ...
505 -- g :: Ord b => b->b
508 tc_sub exp_sty expected_ty act_sty actual_ty
509 | isSigmaTy expected_ty
510 = tcGen expected_ty (tyVarsOfType actual_ty) (
511 -- It's really important to check for escape wrt the free vars of
512 -- both expected_ty *and* actual_ty
513 \ body_exp_ty -> tc_sub body_exp_ty body_exp_ty act_sty actual_ty
514 ) `thenM` \ (gen_fn, co_fn) ->
515 returnM (gen_fn <.> co_fn)
517 -----------------------------------
518 -- Specialisation case:
519 -- actual_ty: forall a. Ord a => a->a
520 -- expected_ty: Int -> Int
521 -- co_fn e = e Int dOrdInt
523 tc_sub exp_sty expected_ty act_sty actual_ty
524 | isSigmaTy actual_ty
525 = tcInstCall InstSigOrigin actual_ty `thenM` \ (inst_fn, _, body_ty) ->
526 tc_sub exp_sty expected_ty body_ty body_ty `thenM` \ co_fn ->
527 returnM (co_fn <.> inst_fn)
529 -----------------------------------
532 tc_sub _ (FunTy exp_arg exp_res) _ (FunTy act_arg act_res)
533 = tcSub_fun exp_arg exp_res act_arg act_res
535 -----------------------------------
536 -- Type variable meets function: imitate
538 -- NB 1: we can't just unify the type variable with the type
539 -- because the type might not be a tau-type, and we aren't
540 -- allowed to instantiate an ordinary type variable with
543 -- NB 2: can we short-cut to an error case?
544 -- when the arg/res is not a tau-type?
545 -- NO! e.g. f :: ((forall a. a->a) -> Int) -> Int
547 -- is perfectly fine, because we can instantiate f's type to a monotype
549 -- However, we get can get jolly unhelpful error messages.
550 -- e.g. foo = id runST
552 -- Inferred type is less polymorphic than expected
553 -- Quantified type variable `s' escapes
554 -- Expected type: ST s a -> t
555 -- Inferred type: (forall s1. ST s1 a) -> a
556 -- In the first argument of `id', namely `runST'
557 -- In a right-hand side of function `foo': id runST
559 -- I'm not quite sure what to do about this!
561 tc_sub exp_sty exp_ty@(FunTy exp_arg exp_res) _ act_ty
562 = do { (act_arg, act_res) <- unify_fun act_ty
563 ; tcSub_fun exp_arg exp_res act_arg act_res }
565 tc_sub _ exp_ty act_sty act_ty@(FunTy act_arg act_res)
566 = do { (exp_arg, exp_res) <- unify_fun exp_ty
567 ; tcSub_fun exp_arg exp_res act_arg act_res }
569 -----------------------------------
571 -- If none of the above match, we revert to the plain unifier
572 tc_sub exp_sty expected_ty act_sty actual_ty
573 = uTys True exp_sty expected_ty True act_sty actual_ty `thenM_`
576 -----------------------------------
577 -- A helper to make a function type match
578 -- The error message isn't very good, but that's a problem with
579 -- all of this subsumption code
581 = do { (ok, args, res) <- unify_fun_ty True 1 ty
582 ; if ok then return (head args, res)
583 else failWithTc (ptext SLIT("Expecting a function type, but found") <+> quotes (ppr ty))}
587 tcSub_fun exp_arg exp_res act_arg act_res
588 = tc_sub act_arg act_arg exp_arg exp_arg `thenM` \ co_fn_arg ->
589 tc_sub exp_res exp_res act_res act_res `thenM` \ co_fn_res ->
590 newUnique `thenM` \ uniq ->
592 -- co_fn_arg :: HsExpr exp_arg -> HsExpr act_arg
593 -- co_fn_res :: HsExpr act_res -> HsExpr exp_res
594 -- co_fn :: HsExpr (act_arg -> act_res) -> HsExpr (exp_arg -> exp_res)
595 arg_id = mkSysLocal FSLIT("sub") uniq exp_arg
596 coercion | isIdCoercion co_fn_arg,
597 isIdCoercion co_fn_res = idCoercion
598 | otherwise = mkCoercion co_fn
600 co_fn e = DictLam [arg_id]
601 (noLoc (co_fn_res <$> (HsApp (noLoc e) (noLoc (co_fn_arg <$> HsVar arg_id)))))
602 -- Slight hack; using a "DictLam" to get an ordinary simple lambda
603 -- HsVar arg_id :: HsExpr exp_arg
604 -- co_fn_arg $it :: HsExpr act_arg
605 -- HsApp e $it :: HsExpr act_res
606 -- co_fn_res $it :: HsExpr exp_res
612 %************************************************************************
614 \subsection{Generalisation}
616 %************************************************************************
619 tcGen :: TcSigmaType -- expected_ty
620 -> TcTyVarSet -- Extra tyvars that the universally
621 -- quantified tyvars of expected_ty
622 -- must not be unified
623 -> (TcRhoType -> TcM result) -- spec_ty
624 -> TcM (ExprCoFn, result)
625 -- The expression has type: spec_ty -> expected_ty
627 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
628 -- If not, the call is a no-op
629 = do { -- We want the GenSkol info in the skolemised type variables to
630 -- mention the *instantiated* tyvar names, so that we get a
631 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
632 -- Hence the tiresome but innocuous fixM
633 ((forall_tvs, theta, rho_ty), skol_info) <- fixM (\ ~(_, skol_info) ->
634 do { (forall_tvs, theta, rho_ty) <- tcSkolType skol_info expected_ty
635 ; span <- getSrcSpanM
636 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty) span
637 ; return ((forall_tvs, theta, rho_ty), skol_info) })
640 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
641 text "expected_ty" <+> ppr expected_ty,
642 text "inst ty" <+> ppr forall_tvs <+> ppr theta <+> ppr rho_ty,
643 text "free_tvs" <+> ppr free_tvs,
644 text "forall_tvs" <+> ppr forall_tvs])
647 -- Type-check the arg and unify with poly type
648 ; (result, lie) <- getLIE (thing_inside rho_ty)
650 -- Check that the "forall_tvs" havn't been constrained
651 -- The interesting bit here is that we must include the free variables
652 -- of the expected_ty. Here's an example:
653 -- runST (newVar True)
654 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
655 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
656 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
657 -- So now s' isn't unconstrained because it's linked to a.
658 -- Conclusion: include the free vars of the expected_ty in the
659 -- list of "free vars" for the signature check.
661 ; dicts <- newDicts (SigOrigin skol_info) theta
662 ; inst_binds <- tcSimplifyCheck sig_msg forall_tvs dicts lie
664 ; checkSigTyVarsWrt free_tvs forall_tvs
665 ; traceTc (text "tcGen:done")
668 -- This HsLet binds any Insts which came out of the simplification.
669 -- It's a bit out of place here, but using AbsBind involves inventing
670 -- a couple of new names which seems worse.
671 dict_ids = map instToId dicts
672 co_fn e = TyLam forall_tvs (mkHsDictLam dict_ids (mkHsLet inst_binds (noLoc e)))
673 ; returnM (mkCoercion co_fn, result) }
675 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
676 sig_msg = ptext SLIT("expected type of an expression")
681 %************************************************************************
683 \subsection[Unify-exported]{Exported unification functions}
685 %************************************************************************
687 The exported functions are all defined as versions of some
688 non-exported generic functions.
690 Unify two @TauType@s. Dead straightforward.
693 unifyTauTy :: TcTauType -> TcTauType -> TcM ()
694 unifyTauTy ty1 ty2 -- ty1 expected, ty2 inferred
695 = -- The unifier should only ever see tau-types
696 -- (no quantification whatsoever)
697 ASSERT2( isTauTy ty1, ppr ty1 )
698 ASSERT2( isTauTy ty2, ppr ty2 )
699 addErrCtxtM (unifyCtxt "type" ty1 ty2) $
700 uTys True ty1 ty1 True ty2 ty2
702 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
703 unifyTheta theta1 theta2
704 = do { checkTc (equalLength theta1 theta2)
705 (ptext SLIT("Contexts differ in length"))
706 ; unifyTauTyLists True (map mkPredTy theta1) True (map mkPredTy theta2) }
709 @unifyTauTyList@ unifies corresponding elements of two lists of
710 @TauType@s. It uses @uTys@ to do the real work. The lists should be
711 of equal length. We charge down the list explicitly so that we can
712 complain if their lengths differ.
715 unifyTauTyLists :: Bool -> -- Allow refinements on tys1
717 Bool -> -- Allow refinements on tys2
718 [TcTauType] -> TcM ()
719 -- Precondition: lists must be same length
720 -- Having the caller check gives better error messages
721 -- Actually the caller neve does need to check; see Note [Tycon app]
722 unifyTauTyLists r1 [] r2 [] = returnM ()
723 unifyTauTyLists r1 (ty1:tys1) r2 (ty2:tys2) = uTys r1 ty1 ty1 r2 ty2 ty2 `thenM_`
724 unifyTauTyLists r1 tys1 r2 tys2
725 unifyTauTyLists r1 ty1s r2 ty2s = panic "Unify.unifyTauTyLists: mismatched type lists!"
728 @unifyTauTyList@ takes a single list of @TauType@s and unifies them
729 all together. It is used, for example, when typechecking explicit
730 lists, when all the elts should be of the same type.
733 unifyTauTyList :: [TcTauType] -> TcM ()
734 unifyTauTyList [] = returnM ()
735 unifyTauTyList [ty] = returnM ()
736 unifyTauTyList (ty1:tys@(ty2:_)) = unifyTauTy ty1 ty2 `thenM_`
740 %************************************************************************
742 \subsection[Unify-uTys]{@uTys@: getting down to business}
744 %************************************************************************
746 @uTys@ is the heart of the unifier. Each arg happens twice, because
747 we want to report errors in terms of synomyms if poss. The first of
748 the pair is used in error messages only; it is always the same as the
749 second, except that if the first is a synonym then the second may be a
750 de-synonym'd version. This way we get better error messages.
752 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
755 uTys :: Bool -- Allow refinements to ty1
756 -> TcTauType -> TcTauType -- Error reporting ty1 and real ty1
757 -- ty1 is the *expected* type
758 -> Bool -- Allow refinements to ty2
759 -> TcTauType -> TcTauType -- Error reporting ty2 and real ty2
760 -- ty2 is the *actual* type
763 -- Always expand synonyms (see notes at end)
764 -- (this also throws away FTVs)
765 uTys r1 ps_ty1 (NoteTy n1 ty1) r2 ps_ty2 ty2 = uTys r1 ps_ty1 ty1 r2 ps_ty2 ty2
766 uTys r1 ps_ty1 ty1 r2 ps_ty2 (NoteTy n2 ty2) = uTys r1 ps_ty1 ty1 r2 ps_ty2 ty2
768 -- Variables; go for uVar
769 uTys r1 ps_ty1 (TyVarTy tyvar1) r2 ps_ty2 ty2 = uVar False r1 tyvar1 r2 ps_ty2 ty2
770 uTys r1 ps_ty1 ty1 r2 ps_ty2 (TyVarTy tyvar2) = uVar True r2 tyvar2 r1 ps_ty1 ty1
771 -- "True" means args swapped
774 uTys r1 _ (PredTy (IParam n1 t1)) r2 _ (PredTy (IParam n2 t2))
775 | n1 == n2 = uTys r1 t1 t1 r2 t2 t2
776 uTys r1 _ (PredTy (ClassP c1 tys1)) r2 _ (PredTy (ClassP c2 tys2))
777 | c1 == c2 = unifyTauTyLists r1 tys1 r2 tys2
778 -- Guaranteed equal lengths because the kinds check
780 -- Functions; just check the two parts
781 uTys r1 _ (FunTy fun1 arg1) r2 _ (FunTy fun2 arg2)
782 = uTys r1 fun1 fun1 r2 fun2 fun2 `thenM_` uTys r1 arg1 arg1 r2 arg2 arg2
784 -- Type constructors must match
785 uTys r1 ps_ty1 (TyConApp con1 tys1) r2 ps_ty2 (TyConApp con2 tys2)
786 | con1 == con2 = unifyTauTyLists r1 tys1 r2 tys2
787 -- See Note [TyCon app]
789 -- Applications need a bit of care!
790 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
791 -- NB: we've already dealt with type variables and Notes,
792 -- so if one type is an App the other one jolly well better be too
793 uTys r1 ps_ty1 (AppTy s1 t1) r2 ps_ty2 ty2
794 = case tcSplitAppTy_maybe ty2 of
795 Just (s2,t2) -> uTys r1 s1 s1 r2 s2 s2 `thenM_` uTys r1 t1 t1 r2 t2 t2
796 Nothing -> unifyMisMatch ps_ty1 ps_ty2
798 -- Now the same, but the other way round
799 -- Don't swap the types, because the error messages get worse
800 uTys r1 ps_ty1 ty1 r2 ps_ty2 (AppTy s2 t2)
801 = case tcSplitAppTy_maybe ty1 of
802 Just (s1,t1) -> uTys r1 s1 s1 r2 s2 s2 `thenM_` uTys r1 t1 t1 r2 t2 t2
803 Nothing -> unifyMisMatch ps_ty1 ps_ty2
805 -- Not expecting for-alls in unification
806 -- ... but the error message from the unifyMisMatch more informative
807 -- than a panic message!
809 -- Anything else fails
810 uTys r1 ps_ty1 ty1 r2 ps_ty2 ty2 = unifyMisMatch ps_ty1 ps_ty2
815 When we find two TyConApps, the argument lists are guaranteed equal
816 length. Reason: intially the kinds of the two types to be unified is
817 the same. The only way it can become not the same is when unifying two
818 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
819 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
820 which we do, that ensures that f1,f2 have the same kind; and that
821 means a1,a2 have the same kind. And now the argument repeats.
826 If you are tempted to make a short cut on synonyms, as in this
830 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
831 -- NO = if (con1 == con2) then
832 -- NO -- Good news! Same synonym constructors, so we can shortcut
833 -- NO -- by unifying their arguments and ignoring their expansions.
834 -- NO unifyTauTypeLists args1 args2
836 -- NO -- Never mind. Just expand them and try again
840 then THINK AGAIN. Here is the whole story, as detected and reported
841 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
843 Here's a test program that should detect the problem:
847 x = (1 :: Bogus Char) :: Bogus Bool
850 The problem with [the attempted shortcut code] is that
854 is not a sufficient condition to be able to use the shortcut!
855 You also need to know that the type synonym actually USES all
856 its arguments. For example, consider the following type synonym
857 which does not use all its arguments.
862 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
863 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
864 would fail, even though the expanded forms (both \tr{Int}) should
867 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
868 unnecessarily bind \tr{t} to \tr{Char}.
870 ... You could explicitly test for the problem synonyms and mark them
871 somehow as needing expansion, perhaps also issuing a warning to the
876 %************************************************************************
878 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
880 %************************************************************************
882 @uVar@ is called when at least one of the types being unified is a
883 variable. It does {\em not} assume that the variable is a fixed point
884 of the substitution; rather, notice that @uVar@ (defined below) nips
885 back into @uTys@ if it turns out that the variable is already bound.
888 uVar :: Bool -- False => tyvar is the "expected"
889 -- True => ty is the "expected" thing
890 -> Bool -- True, allow refinements to tv1, False don't
892 -> Bool -- Allow refinements to ty2?
893 -> TcTauType -> TcTauType -- printing and real versions
896 uVar swapped r1 tv1 r2 ps_ty2 ty2
897 = traceTc (text "uVar" <+> ppr swapped <+> ppr tv1 <+> (ppr ps_ty2 $$ ppr ty2)) `thenM_`
898 condLookupTcTyVar r1 tv1 `thenM` \ details ->
900 IndirectTv r1' ty1 | swapped -> uTys r2 ps_ty2 ty2 r1' ty1 ty1 -- Swap back
901 | otherwise -> uTys r1' ty1 ty1 r2 ps_ty2 ty2 -- Same order
902 DoneTv details1 -> uDoneVar swapped tv1 details1 r2 ps_ty2 ty2
905 uDoneVar :: Bool -- Args are swapped
906 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
907 -> Bool -- Allow refinements to ty2
908 -> TcTauType -> TcTauType -- Type 2
910 -- Invariant: tyvar 1 is not unified with anything
912 uDoneVar swapped tv1 details1 r2 ps_ty2 (NoteTy n2 ty2)
913 = -- Expand synonyms; ignore FTVs
914 uDoneVar swapped tv1 details1 r2 ps_ty2 ty2
916 uDoneVar swapped tv1 details1 r2 ps_ty2 ty2@(TyVarTy tv2)
917 -- Same type variable => no-op
921 -- Distinct type variables
923 = do { lookup2 <- condLookupTcTyVar r2 tv2
925 IndirectTv b ty2' -> uDoneVar swapped tv1 details1 b ty2' ty2'
926 DoneTv details2 -> uDoneVars swapped tv1 details1 tv2 details2
929 uDoneVar swapped tv1 details1 r2 ps_ty2 non_var_ty2 -- ty2 is not a type variable
931 MetaTv ref1 -> do { -- Do the occurs check, and check that we are not
932 -- unifying a type variable with a polytype
933 -- Returns a zonked type ready for the update
934 ty2 <- checkValue tv1 r2 ps_ty2 non_var_ty2
935 ; updateMeta swapped tv1 ref1 ty2 }
937 skolem_details -> unifyMisMatch (TyVarTy tv1) ps_ty2
941 uDoneVars :: Bool -- Args are swapped
942 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
943 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
945 -- Invarant: the type variables are distinct,
946 -- and are not already unified with anything
948 uDoneVars swapped tv1 (MetaTv ref1) tv2 details2
950 MetaTv ref2 | update_tv2 -> updateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
951 other -> updateMeta swapped tv1 ref1 (mkTyVarTy tv2)
952 -- Note that updateMeta does a sub-kind check
953 -- We might unify (a b) with (c d) where b::*->* and d::*; this should fail
957 update_tv2 = k1 `isSubKind` k2 && (k1 /= k2 || nicer_to_update_tv2)
958 -- Update the variable with least kind info
959 -- See notes on type inference in Kind.lhs
960 -- The "nicer to" part only applies if the two kinds are the same,
961 -- so we can choose which to do.
963 nicer_to_update_tv2 = isSystemName (varName tv2)
964 -- Try to update sys-y type variables in preference to ones
965 -- gotten (say) by instantiating a polymorphic function with
966 -- a user-written type sig
968 uDoneVars swapped tv1 (SkolemTv _) tv2 details2
970 MetaTv ref2 -> updateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
971 other -> unifyMisMatch (mkTyVarTy tv1) (mkTyVarTy tv2)
973 uDoneVars swapped tv1 (SigSkolTv _ ref1) tv2 details2
975 MetaTv ref2 -> updateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
976 SigSkolTv _ _ -> updateMeta swapped tv1 ref1 (mkTyVarTy tv2)
977 other -> unifyMisMatch (mkTyVarTy tv1) (mkTyVarTy tv2)
980 updateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
981 -- Update tv1, which is flexi; occurs check is alrady done
982 updateMeta swapped tv1 ref1 ty2
983 = do { checkKinds swapped tv1 ty2
984 ; writeMutVar ref1 (Indirect ty2) }
988 checkKinds swapped tv1 ty2
989 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
990 -- ty2 has been zonked at this stage, which ensures that
991 -- its kind has as much boxity information visible as possible.
992 | tk2 `isSubKind` tk1 = returnM ()
995 -- Either the kinds aren't compatible
996 -- (can happen if we unify (a b) with (c d))
997 -- or we are unifying a lifted type variable with an
998 -- unlifted type: e.g. (id 3#) is illegal
999 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1000 unifyKindMisMatch k1 k2
1002 (k1,k2) | swapped = (tk2,tk1)
1003 | otherwise = (tk1,tk2)
1009 checkValue tv1 r2 ps_ty2 non_var_ty2
1010 -- Do the occurs check, and check that we are not
1011 -- unifying a type variable with a polytype
1012 -- Return the type to update the type variable with, or fail
1014 -- Basically we want to update tv1 := ps_ty2
1015 -- because ps_ty2 has type-synonym info, which improves later error messages
1020 -- f :: (A a -> a -> ()) -> ()
1024 -- x = f (\ x p -> p x)
1026 -- In the application (p x), we try to match "t" with "A t". If we go
1027 -- ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
1028 -- an infinite loop later.
1029 -- But we should not reject the program, because A t = ().
1030 -- Rather, we should bind t to () (= non_var_ty2).
1032 -- That's why we have this two-state occurs-check
1033 = zonk_tc_type r2 ps_ty2 `thenM` \ ps_ty2' ->
1034 case okToUnifyWith tv1 ps_ty2' of {
1035 Nothing -> returnM ps_ty2' ; -- Success
1038 zonk_tc_type r2 non_var_ty2 `thenM` \ non_var_ty2' ->
1039 case okToUnifyWith tv1 non_var_ty2' of
1040 Nothing -> -- This branch rarely succeeds, except in strange cases
1041 -- like that in the example above
1042 returnM non_var_ty2'
1044 Just problem -> failWithTcM (unifyCheck problem tv1 ps_ty2')
1047 zonk_tc_type refine ty
1048 = zonkType (\tv -> return (TyVarTy tv)) refine ty
1049 -- We may already be inside a wobbly type t2, and
1050 -- should take that into account here
1052 data Problem = OccurCheck | NotMonoType
1054 okToUnifyWith :: TcTyVar -> TcType -> Maybe Problem
1055 -- (okToUnifyWith tv ty) checks whether it's ok to unify
1058 -- Just p => not ok, problem p
1063 ok (TyVarTy tv') | tv == tv' = Just OccurCheck
1064 | otherwise = Nothing
1065 ok (AppTy t1 t2) = ok t1 `and` ok t2
1066 ok (FunTy t1 t2) = ok t1 `and` ok t2
1067 ok (TyConApp _ ts) = oks ts
1068 ok (ForAllTy _ _) = Just NotMonoType
1069 ok (PredTy st) = ok_st st
1070 ok (NoteTy (FTVNote _) t) = ok t
1071 ok (NoteTy (SynNote t1) t2) = ok t1 `and` ok t2
1072 -- Type variables may be free in t1 but not t2
1073 -- A forall may be in t2 but not t1
1075 oks ts = foldr (and . ok) Nothing ts
1077 ok_st (ClassP _ ts) = oks ts
1078 ok_st (IParam _ t) = ok t
1081 Just p `and` m = Just p
1085 %************************************************************************
1089 %************************************************************************
1091 Unifying kinds is much, much simpler than unifying types.
1094 unifyKind :: TcKind -- Expected
1097 unifyKind LiftedTypeKind LiftedTypeKind = returnM ()
1098 unifyKind UnliftedTypeKind UnliftedTypeKind = returnM ()
1100 unifyKind OpenTypeKind k2 | isOpenTypeKind k2 = returnM ()
1101 unifyKind ArgTypeKind k2 | isArgTypeKind k2 = returnM ()
1102 -- Respect sub-kinding
1104 unifyKind (FunKind a1 r1) (FunKind a2 r2)
1105 = do { unifyKind a2 a1; unifyKind r1 r2 }
1106 -- Notice the flip in the argument,
1107 -- so that the sub-kinding works right
1109 unifyKind (KindVar kv1) k2 = uKVar False kv1 k2
1110 unifyKind k1 (KindVar kv2) = uKVar True kv2 k1
1111 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1113 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1114 unifyKinds [] [] = returnM ()
1115 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1117 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1120 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1121 uKVar swapped kv1 k2
1122 = do { mb_k1 <- readKindVar kv1
1124 Nothing -> uUnboundKVar swapped kv1 k2
1125 Just k1 | swapped -> unifyKind k2 k1
1126 | otherwise -> unifyKind k1 k2 }
1129 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1130 uUnboundKVar swapped kv1 k2@(KindVar kv2)
1131 | kv1 == kv2 = returnM ()
1132 | otherwise -- Distinct kind variables
1133 = do { mb_k2 <- readKindVar kv2
1135 Just k2 -> uUnboundKVar swapped kv1 k2
1136 Nothing -> writeKindVar kv1 k2 }
1138 uUnboundKVar swapped kv1 non_var_k2
1139 = do { k2' <- zonkTcKind non_var_k2
1140 ; kindOccurCheck kv1 k2'
1141 ; k2'' <- kindSimpleKind swapped k2'
1142 -- KindVars must be bound only to simple kinds
1143 -- Polarities: (kindSimpleKind True ?) succeeds
1144 -- returning *, corresponding to unifying
1147 ; writeKindVar kv1 k2'' }
1150 kindOccurCheck kv1 k2 -- k2 is zonked
1151 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1153 not_in (KindVar kv2) = kv1 /= kv2
1154 not_in (FunKind a2 r2) = not_in a2 && not_in r2
1157 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1158 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1159 -- If the flag is False, it requires k <: sk
1160 -- E.g. kindSimpleKind False ?? = *
1161 -- What about (kv -> *) :=: ?? -> *
1162 kindSimpleKind orig_swapped orig_kind
1163 = go orig_swapped orig_kind
1165 go sw (FunKind k1 k2) = do { k1' <- go (not sw) k1
1167 ; return (FunKind k1' k2') }
1168 go True OpenTypeKind = return liftedTypeKind
1169 go True ArgTypeKind = return liftedTypeKind
1170 go sw LiftedTypeKind = return liftedTypeKind
1171 go sw k@(KindVar _) = return k -- KindVars are always simple
1172 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1173 <+> ppr orig_swapped <+> ppr orig_kind)
1174 -- I think this can't actually happen
1176 -- T v = MkT v v must be a type
1177 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1180 kindOccurCheckErr tyvar ty
1181 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1182 2 (sep [ppr tyvar, char '=', ppr ty])
1184 unifyKindMisMatch ty1 ty2
1185 = zonkTcKind ty1 `thenM` \ ty1' ->
1186 zonkTcKind ty2 `thenM` \ ty2' ->
1188 msg = hang (ptext SLIT("Couldn't match kind"))
1189 2 (sep [quotes (ppr ty1'),
1190 ptext SLIT("against"),
1197 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1198 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1200 unifyFunKind (KindVar kvar)
1201 = readKindVar kvar `thenM` \ maybe_kind ->
1203 Just fun_kind -> unifyFunKind fun_kind
1204 Nothing -> do { arg_kind <- newKindVar
1205 ; res_kind <- newKindVar
1206 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1207 ; returnM (Just (arg_kind,res_kind)) }
1209 unifyFunKind (FunKind arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1210 unifyFunKind other = returnM Nothing
1213 %************************************************************************
1215 \subsection[Unify-context]{Errors and contexts}
1217 %************************************************************************
1223 unifyCtxt s ty1 ty2 tidy_env -- ty1 expected, ty2 inferred
1224 = zonkTcType ty1 `thenM` \ ty1' ->
1225 zonkTcType ty2 `thenM` \ ty2' ->
1226 returnM (err ty1' ty2')
1228 err ty1 ty2 = (env1,
1231 text "Expected" <+> text s <> colon <+> ppr tidy_ty1,
1232 text "Inferred" <+> text s <> colon <+> ppr tidy_ty2
1235 (env1, [tidy_ty1,tidy_ty2]) = tidyOpenTypes tidy_env [ty1,ty2]
1237 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1238 -- tv1 and ty2 are zonked already
1241 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1242 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1244 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1245 | otherwise = (pp1, pp2)
1246 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1247 (env2, ty2') = tidyOpenType env1 ty2
1248 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1249 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1251 unifyMisMatch ty1 ty2
1252 = do { (env1, pp1, extra1) <- ppr_ty emptyTidyEnv ty1
1253 ; (env2, pp2, extra2) <- ppr_ty env1 ty2
1254 ; let msg = sep [sep [ptext SLIT("Couldn't match") <+> pp1, nest 7 (ptext SLIT("against") <+> pp2)],
1255 nest 2 extra1, nest 2 extra2]
1257 failWithTcM (env2, msg) }
1259 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
1261 = do { ty' <- zonkTcType ty
1262 ; let (env1,tidy_ty) = tidyOpenType env ty'
1263 simple_result = (env1, quotes (ppr tidy_ty), empty)
1266 | isSkolemTyVar tv -> return (env2, pp_rigid tv',
1268 | otherwise -> return simple_result
1270 (env2, tv') = tidySkolemTyVar env1 tv
1271 other -> return simple_result }
1273 pp_rigid tv = ptext SLIT("the rigid variable") <+> quotes (ppr tv)
1275 unifyCheck problem tyvar ty
1277 2 (sep [ppr tidy_tyvar, char '=', ppr tidy_ty]))
1279 (env1, tidy_tyvar) = tidyOpenTyVar emptyTidyEnv tyvar
1280 (env2, tidy_ty) = tidyOpenType env1 ty
1282 msg = case problem of
1283 OccurCheck -> ptext SLIT("Occurs check: cannot construct the infinite type:")
1284 NotMonoType -> ptext SLIT("Cannot unify a type variable with a type scheme:")
1288 %************************************************************************
1292 %************************************************************************
1294 ---------------------------
1295 -- We would like to get a decent error message from
1296 -- (a) Under-applied type constructors
1297 -- f :: (Maybe, Maybe)
1298 -- (b) Over-applied type constructors
1299 -- f :: Int x -> Int x
1303 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1304 -- A fancy wrapper for 'unifyKind', which tries
1305 -- to give decent error messages.
1306 checkExpectedKind ty act_kind exp_kind
1307 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1310 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1312 Just r -> returnM () ; -- Unification succeeded
1315 -- So there's definitely an error
1316 -- Now to find out what sort
1317 zonkTcKind exp_kind `thenM` \ exp_kind ->
1318 zonkTcKind act_kind `thenM` \ act_kind ->
1320 let (exp_as, _) = splitKindFunTys exp_kind
1321 (act_as, _) = splitKindFunTys act_kind
1322 n_exp_as = length exp_as
1323 n_act_as = length act_as
1325 err | n_exp_as < n_act_as -- E.g. [Maybe]
1326 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1328 -- Now n_exp_as >= n_act_as. In the next two cases,
1329 -- n_exp_as == 0, and hence so is n_act_as
1330 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1331 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1332 <+> ptext SLIT("is unlifted")
1334 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1335 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1336 <+> ptext SLIT("is lifted")
1338 | otherwise -- E.g. Monad [Int]
1339 = ptext SLIT("Kind mis-match")
1341 more_info = sep [ ptext SLIT("Expected kind") <+>
1342 quotes (pprKind exp_kind) <> comma,
1343 ptext SLIT("but") <+> quotes (ppr ty) <+>
1344 ptext SLIT("has kind") <+> quotes (pprKind act_kind)]
1346 failWithTc (err $$ more_info)
1350 %************************************************************************
1352 \subsection{Checking signature type variables}
1354 %************************************************************************
1356 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1357 are not mentioned in the environment. In particular:
1359 (a) Not mentioned in the type of a variable in the envt
1360 eg the signature for f in this:
1366 Here, f is forced to be monorphic by the free occurence of x.
1368 (d) Not (unified with another type variable that is) in scope.
1369 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1370 when checking the expression type signature, we find that
1371 even though there is nothing in scope whose type mentions r,
1372 nevertheless the type signature for the expression isn't right.
1374 Another example is in a class or instance declaration:
1376 op :: forall b. a -> b
1378 Here, b gets unified with a
1380 Before doing this, the substitution is applied to the signature type variable.
1383 checkSigTyVars :: [TcTyVar] -> TcM ()
1384 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1386 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1387 checkSigTyVarsWrt extra_tvs sig_tvs
1388 = zonkTcTyVarsAndFV (varSetElems extra_tvs) `thenM` \ extra_tvs' ->
1389 check_sig_tyvars extra_tvs' sig_tvs
1392 :: TcTyVarSet -- Global type variables. The universally quantified
1393 -- tyvars should not mention any of these
1394 -- Guaranteed already zonked.
1395 -> [TcTyVar] -- Universally-quantified type variables in the signature
1396 -- Guaranteed to be skolems
1398 check_sig_tyvars extra_tvs []
1400 check_sig_tyvars extra_tvs sig_tvs
1401 = ASSERT( all isSkolemTyVar sig_tvs )
1402 do { gbl_tvs <- tcGetGlobalTyVars
1403 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1404 text "gbl_tvs" <+> ppr gbl_tvs,
1405 text "extra_tvs" <+> ppr extra_tvs]))
1407 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1408 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1409 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1412 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1413 -> [TcTyVar] -- The possibly-escaping type variables
1414 -> [TcTyVar] -- The zonked versions thereof
1416 -- Complain about escaping type variables
1417 -- We pass a list of type variables, at least one of which
1418 -- escapes. The first list contains the original signature type variable,
1419 -- while the second contains the type variable it is unified to (usually itself)
1420 bleatEscapedTvs globals sig_tvs zonked_tvs
1421 = do { (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1422 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1424 (env1, tidy_tvs) = tidyOpenTyVars emptyTidyEnv sig_tvs
1425 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1427 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1429 check (tidy_env, msgs) (sig_tv, zonked_tv)
1430 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1432 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1433 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1435 -----------------------
1436 escape_msg sig_tv zonked_tv globs
1438 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1439 nest 2 (vcat globs)]
1441 = msg <+> ptext SLIT("escapes")
1442 -- Sigh. It's really hard to give a good error message
1443 -- all the time. One bad case is an existential pattern match.
1444 -- We rely on the "When..." context to help.
1446 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1448 | sig_tv == zonked_tv = empty
1449 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1452 These two context are used with checkSigTyVars
1455 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1456 -> TidyEnv -> TcM (TidyEnv, Message)
1457 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1458 = zonkTcType sig_tau `thenM` \ actual_tau ->
1460 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1461 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1462 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1463 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1464 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1466 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),