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 ( mkHsDictLet, 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, tcEqType,
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") <+> speakNOf n_pats (ptext SLIT("argument"))
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") <+> speakNOf 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 if n_actual == 0 then ptext SLIT("has none")
236 else ptext SLIT("has only") <+> speakN n_actual]
238 unify_fun_ty :: Bool -> Arity -> TcRhoType
239 -> TcM (Bool, -- Arity satisfied?
240 [TcSigmaType], -- Arg types found; length <= arity
241 TcRhoType) -- Result type
243 unify_fun_ty use_refinement arity ty
245 = do { res_ty <- wobblify use_refinement ty
246 ; return (True, [], ty) }
248 unify_fun_ty use_refinement arity (NoteTy _ ty)
249 = unify_fun_ty use_refinement arity ty
251 unify_fun_ty use_refinement arity ty@(TyVarTy tv)
252 = do { details <- condLookupTcTyVar use_refinement tv
254 IndirectTv use' ty' -> unify_fun_ty use' arity ty'
255 DoneTv (MetaTv ref) -> ASSERT( liftedTypeKind `isSubKind` tyVarKind tv )
256 -- The argument to unifyFunTys is always a type
257 -- Occurs check can't happen, of course
258 do { args <- mappM newTyFlexiVarTy (replicate arity argTypeKind)
259 ; res <- newTyFlexiVarTy openTypeKind
260 ; writeMutVar ref (Indirect (mkFunTys args res))
261 ; return (True, args, res) }
262 DoneTv skol -> return (False, [], ty)
265 unify_fun_ty use_refinement arity ty
266 | Just (arg,res) <- tcSplitFunTy_maybe ty
267 = do { arg' <- wobblify use_refinement arg
268 ; (ok, args', res') <- unify_fun_ty use_refinement (arity-1) res
269 ; return (ok, arg':args', res') }
271 unify_fun_ty use_refinement arity ty
272 -- Common cases are all done by now
273 -- At this point we usually have an error, but ty could
274 -- be (a Int Bool), or (a Bool), which can match
275 -- So just use the unifier. But catch any error so we just
276 -- return the success/fail boolean
277 = do { arg <- newTyFlexiVarTy argTypeKind
278 ; res <- newTyFlexiVarTy openTypeKind
279 ; let fun_ty = mkFunTy arg res
280 ; (_, mb_unit) <- tryTc (uTys True ty ty True fun_ty fun_ty)
282 Nothing -> return (False, [], ty) ;
284 do { (ok, args', res') <- unify_fun_ty use_refinement (arity-1) res
285 ; return (ok, arg:args', res')
290 ----------------------
291 zapToTyConApp :: TyCon -- T :: k1 -> ... -> kn -> *
292 -> Expected TcSigmaType -- Expected type (T a b c)
293 -> TcM [TcType] -- Element types, a b c
294 -- Insists that the Expected type is not a forall-type
295 -- It's used for wired-in tycons, so we call checkWiredInTyCOn
296 zapToTyConApp tc (Check ty)
297 = do { checkWiredInTyCon tc ; unifyTyConApp tc ty } -- NB: fails for a forall-type
299 zapToTyConApp tc (Infer hole)
300 = do { (tc_app, elt_tys) <- newTyConApp tc
301 ; writeMutVar hole tc_app
302 ; traceTc (text "zap" <+> ppr tc)
303 ; checkWiredInTyCon tc
306 zapToListTy :: Expected TcType -> TcM TcType -- Special case for lists
307 zapToListTy exp_ty = do { [elt_ty] <- zapToTyConApp listTyCon exp_ty
310 ----------------------
311 unifyTyConApp :: TyCon -> TcType -> TcM [TcType]
312 unifyTyConApp tc ty = unify_tc_app True tc ty
313 -- Add a boolean flag to remember whether to use
314 -- the type refinement or not
316 unifyListTy :: TcType -> TcM TcType -- Special case for lists
317 unifyListTy exp_ty = do { [elt_ty] <- unifyTyConApp listTyCon exp_ty
321 unify_tc_app use_refinement tc (NoteTy _ ty)
322 = unify_tc_app use_refinement tc ty
324 unify_tc_app use_refinement tc ty@(TyVarTy tyvar)
325 = do { details <- condLookupTcTyVar use_refinement tyvar
327 IndirectTv use' ty' -> unify_tc_app use' tc ty'
328 other -> unify_tc_app_help tc ty
331 unify_tc_app use_refinement tc ty
332 | Just (tycon, arg_tys) <- tcSplitTyConApp_maybe ty,
334 = ASSERT( tyConArity tycon == length arg_tys ) -- ty::*
335 mapM (wobblify use_refinement) arg_tys
337 unify_tc_app use_refinement tc ty = unify_tc_app_help tc ty
339 unify_tc_app_help tc ty -- Revert to ordinary unification
340 = do { (tc_app, arg_tys) <- newTyConApp tc
341 ; if not (isTauTy ty) then -- Can happen if we call zapToTyConApp tc (forall a. ty)
342 unifyMisMatch ty tc_app
344 { unifyTauTy ty tc_app
345 ; returnM arg_tys } }
348 ----------------------
349 unifyAppTy :: TcType -- Type to split: m a
350 -> TcM (TcType, TcType) -- (m,a)
353 unifyAppTy ty = unify_app_ty True ty
355 unify_app_ty use (NoteTy _ ty) = unify_app_ty use ty
357 unify_app_ty use ty@(TyVarTy tyvar)
358 = do { details <- condLookupTcTyVar use tyvar
360 IndirectTv use' ty' -> unify_app_ty use' ty'
361 other -> unify_app_ty_help ty
365 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
366 = do { fun' <- wobblify use fun_ty
367 ; arg' <- wobblify use arg_ty
368 ; return (fun', arg') }
370 | otherwise = unify_app_ty_help ty
372 unify_app_ty_help ty -- Revert to ordinary unification
373 = do { fun_ty <- newTyFlexiVarTy (mkArrowKind liftedTypeKind liftedTypeKind)
374 ; arg_ty <- newTyFlexiVarTy liftedTypeKind
375 ; unifyTauTy (mkAppTy fun_ty arg_ty) ty
376 ; return (fun_ty, arg_ty) }
379 ----------------------
380 wobblify :: Bool -- True <=> don't wobblify
383 -- Return a wobbly type. At the moment we do that by
384 -- allocating a fresh meta type variable.
385 wobblify True ty = return ty
386 wobblify False ty = do { uniq <- newUnique
387 ; tv <- newMetaTyVar (mkSysTvName uniq FSLIT("w"))
390 ; return (mkTyVarTy tv) }
392 ----------------------
393 newTyConApp :: TyCon -> TcM (TcTauType, [TcTauType])
394 newTyConApp tc = do { (tvs, args, _) <- tcInstTyVars (tyConTyVars tc)
395 ; return (mkTyConApp tc args, args) }
399 %************************************************************************
401 \subsection{Subsumption}
403 %************************************************************************
405 All the tcSub calls have the form
407 tcSub expected_ty offered_ty
409 offered_ty <= expected_ty
411 That is, that a value of type offered_ty is acceptable in
412 a place expecting a value of type expected_ty.
414 It returns a coercion function
415 co_fn :: offered_ty -> expected_ty
416 which takes an HsExpr of type offered_ty into one of type
420 -----------------------
421 -- tcSubExp is used for expressions
422 tcSubExp :: Expected TcRhoType -> TcRhoType -> TcM ExprCoFn
424 tcSubExp (Infer hole) offered_ty
425 = do { offered' <- zonkTcType offered_ty
426 -- Note [Zonk return type]
427 -- zonk to take advantage of the current GADT type refinement.
428 -- If we don't we get spurious "existential type variable escapes":
429 -- case (x::Maybe a) of
430 -- Just b (y::b) -> y
431 -- We need the refinement [b->a] to be applied to the result type
432 ; writeMutVar hole offered'
433 ; return idCoercion }
435 tcSubExp (Check expected_ty) offered_ty
436 = tcSub expected_ty offered_ty
438 -----------------------
439 -- tcSubPat is used for patterns
440 tcSubPat :: TcSigmaType -- Pattern type signature
441 -> Expected TcSigmaType -- Type from context
443 -- In patterns we insist on an exact match; hence no CoFn returned
444 -- See Note [Pattern coercions] in TcPat
445 -- However, we can't call unify directly, because both types might be
446 -- polymorphic; hence the call to tcSub, followed by a check for
447 -- equal types. (We can't just check for the identity coercion, because
448 -- in the polymorphic case we might get back something eta-equivalent to
449 -- the identity coercion, but that's not easy to tell.)
451 tcSubPat sig_ty (Infer hole)
452 = do { sig_ty' <- zonkTcType sig_ty
453 ; writeMutVar hole sig_ty' -- See notes with tcSubExp above
456 -- This tcSub followed by tcEqType checks for identical types
457 -- It'd be done more neatly by augmenting the unifier to deal with
458 -- (identically shaped) for-all types.
460 tcSubPat sig_ty (Check exp_ty)
461 = do { co_fn <- tcSub sig_ty exp_ty
462 ; sig_ty' <- zonkTcType sig_ty
463 ; exp_ty' <- zonkTcType exp_ty
464 ; if tcEqType sig_ty' exp_ty' then
467 { (env, msg) <- misMatchMsg sig_ty' exp_ty'
468 ; failWithTcM (env, msg $$ extra) } }
470 extra | isTauTy sig_ty = empty
471 | otherwise = ptext SLIT("Polymorphic types must match exactly in patterns")
476 %************************************************************************
478 tcSub: main subsumption-check code
480 %************************************************************************
482 No holes expected now. Add some error-check context info.
486 tcSub :: TcSigmaType -> TcSigmaType -> TcM ExprCoFn -- Locally used only
487 -- tcSub exp act checks that
489 tcSub expected_ty actual_ty
490 = traceTc (text "tcSub" <+> details) `thenM_`
491 addErrCtxtM (unifyCtxt "type" expected_ty actual_ty)
492 (tc_sub expected_ty expected_ty actual_ty actual_ty)
494 details = vcat [text "Expected:" <+> ppr expected_ty,
495 text "Actual: " <+> ppr actual_ty]
498 tc_sub :: TcSigmaType -- expected_ty, before expanding synonyms
499 -> TcSigmaType -- ..and after
500 -> TcSigmaType -- actual_ty, before
501 -> TcSigmaType -- ..and after
504 -----------------------------------
506 tc_sub exp_sty (NoteTy _ exp_ty) act_sty act_ty = tc_sub exp_sty exp_ty act_sty act_ty
507 tc_sub exp_sty exp_ty act_sty (NoteTy _ act_ty) = tc_sub exp_sty exp_ty act_sty act_ty
509 -----------------------------------
510 -- Generalisation case
511 -- actual_ty: d:Eq b => b->b
512 -- expected_ty: forall a. Ord a => a->a
513 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
515 -- It is essential to do this *before* the specialisation case
516 -- Example: f :: (Eq a => a->a) -> ...
517 -- g :: Ord b => b->b
520 tc_sub exp_sty expected_ty act_sty actual_ty
521 | isSigmaTy expected_ty
522 = tcGen expected_ty (tyVarsOfType actual_ty) (
523 -- It's really important to check for escape wrt the free vars of
524 -- both expected_ty *and* actual_ty
525 \ body_exp_ty -> tc_sub body_exp_ty body_exp_ty act_sty actual_ty
526 ) `thenM` \ (gen_fn, co_fn) ->
527 returnM (gen_fn <.> co_fn)
529 -----------------------------------
530 -- Specialisation case:
531 -- actual_ty: forall a. Ord a => a->a
532 -- expected_ty: Int -> Int
533 -- co_fn e = e Int dOrdInt
535 tc_sub exp_sty expected_ty act_sty actual_ty
536 | isSigmaTy actual_ty
537 = tcInstCall InstSigOrigin actual_ty `thenM` \ (inst_fn, _, body_ty) ->
538 tc_sub exp_sty expected_ty body_ty body_ty `thenM` \ co_fn ->
539 returnM (co_fn <.> inst_fn)
541 -----------------------------------
544 tc_sub _ (FunTy exp_arg exp_res) _ (FunTy act_arg act_res)
545 = tcSub_fun exp_arg exp_res act_arg act_res
547 -----------------------------------
548 -- Type variable meets function: imitate
550 -- NB 1: we can't just unify the type variable with the type
551 -- because the type might not be a tau-type, and we aren't
552 -- allowed to instantiate an ordinary type variable with
555 -- NB 2: can we short-cut to an error case?
556 -- when the arg/res is not a tau-type?
557 -- NO! e.g. f :: ((forall a. a->a) -> Int) -> Int
559 -- is perfectly fine, because we can instantiate f's type to a monotype
561 -- However, we get can get jolly unhelpful error messages.
562 -- e.g. foo = id runST
564 -- Inferred type is less polymorphic than expected
565 -- Quantified type variable `s' escapes
566 -- Expected type: ST s a -> t
567 -- Inferred type: (forall s1. ST s1 a) -> a
568 -- In the first argument of `id', namely `runST'
569 -- In a right-hand side of function `foo': id runST
571 -- I'm not quite sure what to do about this!
573 tc_sub exp_sty exp_ty@(FunTy exp_arg exp_res) _ act_ty
574 = do { (act_arg, act_res) <- unify_fun act_ty
575 ; tcSub_fun exp_arg exp_res act_arg act_res }
577 tc_sub _ exp_ty act_sty act_ty@(FunTy act_arg act_res)
578 = do { (exp_arg, exp_res) <- unify_fun exp_ty
579 ; tcSub_fun exp_arg exp_res act_arg act_res }
581 -----------------------------------
583 -- If none of the above match, we revert to the plain unifier
584 tc_sub exp_sty expected_ty act_sty actual_ty
585 = uTys True exp_sty expected_ty True act_sty actual_ty `thenM_`
588 -----------------------------------
589 -- A helper to make a function type match
590 -- The error message isn't very good, but that's a problem with
591 -- all of this subsumption code
593 = do { (ok, args, res) <- unify_fun_ty True 1 ty
594 ; if ok then return (head args, res)
595 else failWithTc (ptext SLIT("Expecting a function type, but found") <+> quotes (ppr ty))}
599 tcSub_fun exp_arg exp_res act_arg act_res
600 = tc_sub act_arg act_arg exp_arg exp_arg `thenM` \ co_fn_arg ->
601 tc_sub exp_res exp_res act_res act_res `thenM` \ co_fn_res ->
602 newUnique `thenM` \ uniq ->
604 -- co_fn_arg :: HsExpr exp_arg -> HsExpr act_arg
605 -- co_fn_res :: HsExpr act_res -> HsExpr exp_res
606 -- co_fn :: HsExpr (act_arg -> act_res) -> HsExpr (exp_arg -> exp_res)
607 arg_id = mkSysLocal FSLIT("sub") uniq exp_arg
608 coercion | isIdCoercion co_fn_arg,
609 isIdCoercion co_fn_res = idCoercion
610 | otherwise = mkCoercion co_fn
612 co_fn e = DictLam [arg_id]
613 (noLoc (co_fn_res <$> (HsApp (noLoc e) (noLoc (co_fn_arg <$> HsVar arg_id)))))
614 -- Slight hack; using a "DictLam" to get an ordinary simple lambda
615 -- HsVar arg_id :: HsExpr exp_arg
616 -- co_fn_arg $it :: HsExpr act_arg
617 -- HsApp e $it :: HsExpr act_res
618 -- co_fn_res $it :: HsExpr exp_res
624 %************************************************************************
626 \subsection{Generalisation}
628 %************************************************************************
631 tcGen :: TcSigmaType -- expected_ty
632 -> TcTyVarSet -- Extra tyvars that the universally
633 -- quantified tyvars of expected_ty
634 -- must not be unified
635 -> (TcRhoType -> TcM result) -- spec_ty
636 -> TcM (ExprCoFn, result)
637 -- The expression has type: spec_ty -> expected_ty
639 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
640 -- If not, the call is a no-op
641 = do { -- We want the GenSkol info in the skolemised type variables to
642 -- mention the *instantiated* tyvar names, so that we get a
643 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
644 -- Hence the tiresome but innocuous fixM
645 ((forall_tvs, theta, rho_ty), skol_info) <- fixM (\ ~(_, skol_info) ->
646 do { (forall_tvs, theta, rho_ty) <- tcSkolType skol_info expected_ty
647 ; span <- getSrcSpanM
648 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty) span
649 ; return ((forall_tvs, theta, rho_ty), skol_info) })
652 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
653 text "expected_ty" <+> ppr expected_ty,
654 text "inst ty" <+> ppr forall_tvs <+> ppr theta <+> ppr rho_ty,
655 text "free_tvs" <+> ppr free_tvs,
656 text "forall_tvs" <+> ppr forall_tvs])
659 -- Type-check the arg and unify with poly type
660 ; (result, lie) <- getLIE (thing_inside rho_ty)
662 -- Check that the "forall_tvs" havn't been constrained
663 -- The interesting bit here is that we must include the free variables
664 -- of the expected_ty. Here's an example:
665 -- runST (newVar True)
666 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
667 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
668 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
669 -- So now s' isn't unconstrained because it's linked to a.
670 -- Conclusion: include the free vars of the expected_ty in the
671 -- list of "free vars" for the signature check.
673 ; dicts <- newDicts (SigOrigin skol_info) theta
674 ; inst_binds <- tcSimplifyCheck sig_msg forall_tvs dicts lie
676 ; checkSigTyVarsWrt free_tvs forall_tvs
677 ; traceTc (text "tcGen:done")
680 -- This HsLet binds any Insts which came out of the simplification.
681 -- It's a bit out of place here, but using AbsBind involves inventing
682 -- a couple of new names which seems worse.
683 dict_ids = map instToId dicts
684 co_fn e = TyLam forall_tvs (mkHsDictLam dict_ids (mkHsDictLet inst_binds (noLoc e)))
685 ; returnM (mkCoercion co_fn, result) }
687 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
688 sig_msg = ptext SLIT("expected type of an expression")
693 %************************************************************************
695 \subsection[Unify-exported]{Exported unification functions}
697 %************************************************************************
699 The exported functions are all defined as versions of some
700 non-exported generic functions.
702 Unify two @TauType@s. Dead straightforward.
705 unifyTauTy :: TcTauType -> TcTauType -> TcM ()
706 unifyTauTy ty1 ty2 -- ty1 expected, ty2 inferred
707 = -- The unifier should only ever see tau-types
708 -- (no quantification whatsoever)
709 ASSERT2( isTauTy ty1, ppr ty1 )
710 ASSERT2( isTauTy ty2, ppr ty2 )
711 addErrCtxtM (unifyCtxt "type" ty1 ty2) $
712 uTys True ty1 ty1 True ty2 ty2
714 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
715 unifyTheta theta1 theta2
716 = do { checkTc (equalLength theta1 theta2)
717 (ptext SLIT("Contexts differ in length"))
718 ; unifyTauTyLists True (map mkPredTy theta1) True (map mkPredTy theta2) }
721 @unifyTauTyList@ unifies corresponding elements of two lists of
722 @TauType@s. It uses @uTys@ to do the real work. The lists should be
723 of equal length. We charge down the list explicitly so that we can
724 complain if their lengths differ.
727 unifyTauTyLists :: Bool -> -- Allow refinements on tys1
729 Bool -> -- Allow refinements on tys2
730 [TcTauType] -> TcM ()
731 -- Precondition: lists must be same length
732 -- Having the caller check gives better error messages
733 -- Actually the caller neve does need to check; see Note [Tycon app]
734 unifyTauTyLists r1 [] r2 [] = returnM ()
735 unifyTauTyLists r1 (ty1:tys1) r2 (ty2:tys2) = uTys r1 ty1 ty1 r2 ty2 ty2 `thenM_`
736 unifyTauTyLists r1 tys1 r2 tys2
737 unifyTauTyLists r1 ty1s r2 ty2s = panic "Unify.unifyTauTyLists: mismatched type lists!"
740 @unifyTauTyList@ takes a single list of @TauType@s and unifies them
741 all together. It is used, for example, when typechecking explicit
742 lists, when all the elts should be of the same type.
745 unifyTauTyList :: [TcTauType] -> TcM ()
746 unifyTauTyList [] = returnM ()
747 unifyTauTyList [ty] = returnM ()
748 unifyTauTyList (ty1:tys@(ty2:_)) = unifyTauTy ty1 ty2 `thenM_`
752 %************************************************************************
754 \subsection[Unify-uTys]{@uTys@: getting down to business}
756 %************************************************************************
758 @uTys@ is the heart of the unifier. Each arg happens twice, because
759 we want to report errors in terms of synomyms if poss. The first of
760 the pair is used in error messages only; it is always the same as the
761 second, except that if the first is a synonym then the second may be a
762 de-synonym'd version. This way we get better error messages.
764 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
767 uTys :: Bool -- Allow refinements to ty1
768 -> TcTauType -> TcTauType -- Error reporting ty1 and real ty1
769 -- ty1 is the *expected* type
770 -> Bool -- Allow refinements to ty2
771 -> TcTauType -> TcTauType -- Error reporting ty2 and real ty2
772 -- ty2 is the *actual* type
775 -- Always expand synonyms (see notes at end)
776 -- (this also throws away FTVs)
777 uTys r1 ps_ty1 (NoteTy n1 ty1) r2 ps_ty2 ty2 = uTys r1 ps_ty1 ty1 r2 ps_ty2 ty2
778 uTys r1 ps_ty1 ty1 r2 ps_ty2 (NoteTy n2 ty2) = uTys r1 ps_ty1 ty1 r2 ps_ty2 ty2
780 -- Variables; go for uVar
781 uTys r1 ps_ty1 (TyVarTy tyvar1) r2 ps_ty2 ty2 = uVar False r1 tyvar1 r2 ps_ty2 ty2
782 uTys r1 ps_ty1 ty1 r2 ps_ty2 (TyVarTy tyvar2) = uVar True r2 tyvar2 r1 ps_ty1 ty1
783 -- "True" means args swapped
786 uTys r1 _ (PredTy (IParam n1 t1)) r2 _ (PredTy (IParam n2 t2))
787 | n1 == n2 = uTys r1 t1 t1 r2 t2 t2
788 uTys r1 _ (PredTy (ClassP c1 tys1)) r2 _ (PredTy (ClassP c2 tys2))
789 | c1 == c2 = unifyTauTyLists r1 tys1 r2 tys2
790 -- Guaranteed equal lengths because the kinds check
792 -- Functions; just check the two parts
793 uTys r1 _ (FunTy fun1 arg1) r2 _ (FunTy fun2 arg2)
794 = uTys r1 fun1 fun1 r2 fun2 fun2 `thenM_` uTys r1 arg1 arg1 r2 arg2 arg2
796 -- Type constructors must match
797 uTys r1 ps_ty1 (TyConApp con1 tys1) r2 ps_ty2 (TyConApp con2 tys2)
798 | con1 == con2 = unifyTauTyLists r1 tys1 r2 tys2
799 -- See Note [TyCon app]
801 -- Applications need a bit of care!
802 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
803 -- NB: we've already dealt with type variables and Notes,
804 -- so if one type is an App the other one jolly well better be too
805 uTys r1 ps_ty1 (AppTy s1 t1) r2 ps_ty2 ty2
806 = case tcSplitAppTy_maybe ty2 of
807 Just (s2,t2) -> uTys r1 s1 s1 r2 s2 s2 `thenM_` uTys r1 t1 t1 r2 t2 t2
808 Nothing -> unifyMisMatch ps_ty1 ps_ty2
810 -- Now the same, but the other way round
811 -- Don't swap the types, because the error messages get worse
812 uTys r1 ps_ty1 ty1 r2 ps_ty2 (AppTy s2 t2)
813 = case tcSplitAppTy_maybe ty1 of
814 Just (s1,t1) -> uTys r1 s1 s1 r2 s2 s2 `thenM_` uTys r1 t1 t1 r2 t2 t2
815 Nothing -> unifyMisMatch ps_ty1 ps_ty2
817 -- Not expecting for-alls in unification
818 -- ... but the error message from the unifyMisMatch more informative
819 -- than a panic message!
821 -- Anything else fails
822 uTys r1 ps_ty1 ty1 r2 ps_ty2 ty2 = unifyMisMatch ps_ty1 ps_ty2
827 When we find two TyConApps, the argument lists are guaranteed equal
828 length. Reason: intially the kinds of the two types to be unified is
829 the same. The only way it can become not the same is when unifying two
830 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
831 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
832 which we do, that ensures that f1,f2 have the same kind; and that
833 means a1,a2 have the same kind. And now the argument repeats.
838 If you are tempted to make a short cut on synonyms, as in this
842 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
843 -- NO = if (con1 == con2) then
844 -- NO -- Good news! Same synonym constructors, so we can shortcut
845 -- NO -- by unifying their arguments and ignoring their expansions.
846 -- NO unifyTauTypeLists args1 args2
848 -- NO -- Never mind. Just expand them and try again
852 then THINK AGAIN. Here is the whole story, as detected and reported
853 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
855 Here's a test program that should detect the problem:
859 x = (1 :: Bogus Char) :: Bogus Bool
862 The problem with [the attempted shortcut code] is that
866 is not a sufficient condition to be able to use the shortcut!
867 You also need to know that the type synonym actually USES all
868 its arguments. For example, consider the following type synonym
869 which does not use all its arguments.
874 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
875 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
876 would fail, even though the expanded forms (both \tr{Int}) should
879 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
880 unnecessarily bind \tr{t} to \tr{Char}.
882 ... You could explicitly test for the problem synonyms and mark them
883 somehow as needing expansion, perhaps also issuing a warning to the
888 %************************************************************************
890 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
892 %************************************************************************
894 @uVar@ is called when at least one of the types being unified is a
895 variable. It does {\em not} assume that the variable is a fixed point
896 of the substitution; rather, notice that @uVar@ (defined below) nips
897 back into @uTys@ if it turns out that the variable is already bound.
900 uVar :: Bool -- False => tyvar is the "expected"
901 -- True => ty is the "expected" thing
902 -> Bool -- True, allow refinements to tv1, False don't
904 -> Bool -- Allow refinements to ty2?
905 -> TcTauType -> TcTauType -- printing and real versions
908 uVar swapped r1 tv1 r2 ps_ty2 ty2
909 = traceTc (text "uVar" <+> ppr swapped <+> ppr tv1 <+> (ppr ps_ty2 $$ ppr ty2)) `thenM_`
910 condLookupTcTyVar r1 tv1 `thenM` \ details ->
912 IndirectTv r1' ty1 | swapped -> uTys r2 ps_ty2 ty2 r1' ty1 ty1 -- Swap back
913 | otherwise -> uTys r1' ty1 ty1 r2 ps_ty2 ty2 -- Same order
914 DoneTv details1 -> uDoneVar swapped tv1 details1 r2 ps_ty2 ty2
917 uDoneVar :: Bool -- Args are swapped
918 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
919 -> Bool -- Allow refinements to ty2
920 -> TcTauType -> TcTauType -- Type 2
922 -- Invariant: tyvar 1 is not unified with anything
924 uDoneVar swapped tv1 details1 r2 ps_ty2 (NoteTy n2 ty2)
925 = -- Expand synonyms; ignore FTVs
926 uDoneVar swapped tv1 details1 r2 ps_ty2 ty2
928 uDoneVar swapped tv1 details1 r2 ps_ty2 ty2@(TyVarTy tv2)
929 -- Same type variable => no-op
933 -- Distinct type variables
935 = do { lookup2 <- condLookupTcTyVar r2 tv2
937 IndirectTv b ty2' -> uDoneVar swapped tv1 details1 b ty2' ty2'
938 DoneTv details2 -> uDoneVars swapped tv1 details1 tv2 details2
941 uDoneVar swapped tv1 details1 r2 ps_ty2 non_var_ty2 -- ty2 is not a type variable
943 MetaTv ref1 -> do { -- Do the occurs check, and check that we are not
944 -- unifying a type variable with a polytype
945 -- Returns a zonked type ready for the update
946 ty2 <- checkValue tv1 r2 ps_ty2 non_var_ty2
947 ; updateMeta swapped tv1 ref1 ty2 }
949 skolem_details -> unifyMisMatch (TyVarTy tv1) ps_ty2
953 uDoneVars :: Bool -- Args are swapped
954 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
955 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
957 -- Invarant: the type variables are distinct,
958 -- and are not already unified with anything
960 uDoneVars swapped tv1 (MetaTv ref1) tv2 details2
962 MetaTv ref2 | update_tv2 -> updateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
963 other -> updateMeta swapped tv1 ref1 (mkTyVarTy tv2)
964 -- Note that updateMeta does a sub-kind check
965 -- We might unify (a b) with (c d) where b::*->* and d::*; this should fail
969 update_tv2 = k1 `isSubKind` k2 && (k1 /= k2 || nicer_to_update_tv2)
970 -- Update the variable with least kind info
971 -- See notes on type inference in Kind.lhs
972 -- The "nicer to" part only applies if the two kinds are the same,
973 -- so we can choose which to do.
975 nicer_to_update_tv2 = isSystemName (varName tv2)
976 -- Try to update sys-y type variables in preference to ones
977 -- gotten (say) by instantiating a polymorphic function with
978 -- a user-written type sig
980 uDoneVars swapped tv1 (SkolemTv _) tv2 details2
982 MetaTv ref2 -> updateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
983 other -> unifyMisMatch (mkTyVarTy tv1) (mkTyVarTy tv2)
985 uDoneVars swapped tv1 (SigSkolTv _ ref1) tv2 details2
987 MetaTv ref2 -> updateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
988 SigSkolTv _ _ -> updateMeta swapped tv1 ref1 (mkTyVarTy tv2)
989 other -> unifyMisMatch (mkTyVarTy tv1) (mkTyVarTy tv2)
992 updateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
993 -- Update tv1, which is flexi; occurs check is alrady done
994 updateMeta swapped tv1 ref1 ty2
995 = do { checkKinds swapped tv1 ty2
996 ; writeMutVar ref1 (Indirect ty2) }
1000 checkKinds swapped tv1 ty2
1001 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
1002 -- ty2 has been zonked at this stage, which ensures that
1003 -- its kind has as much boxity information visible as possible.
1004 | tk2 `isSubKind` tk1 = returnM ()
1007 -- Either the kinds aren't compatible
1008 -- (can happen if we unify (a b) with (c d))
1009 -- or we are unifying a lifted type variable with an
1010 -- unlifted type: e.g. (id 3#) is illegal
1011 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1012 unifyKindMisMatch k1 k2
1014 (k1,k2) | swapped = (tk2,tk1)
1015 | otherwise = (tk1,tk2)
1021 checkValue tv1 r2 ps_ty2 non_var_ty2
1022 -- Do the occurs check, and check that we are not
1023 -- unifying a type variable with a polytype
1024 -- Return the type to update the type variable with, or fail
1026 -- Basically we want to update tv1 := ps_ty2
1027 -- because ps_ty2 has type-synonym info, which improves later error messages
1032 -- f :: (A a -> a -> ()) -> ()
1036 -- x = f (\ x p -> p x)
1038 -- In the application (p x), we try to match "t" with "A t". If we go
1039 -- ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
1040 -- an infinite loop later.
1041 -- But we should not reject the program, because A t = ().
1042 -- Rather, we should bind t to () (= non_var_ty2).
1044 -- That's why we have this two-state occurs-check
1045 = zonk_tc_type r2 ps_ty2 `thenM` \ ps_ty2' ->
1046 case okToUnifyWith tv1 ps_ty2' of {
1047 Nothing -> returnM ps_ty2' ; -- Success
1050 zonk_tc_type r2 non_var_ty2 `thenM` \ non_var_ty2' ->
1051 case okToUnifyWith tv1 non_var_ty2' of
1052 Nothing -> -- This branch rarely succeeds, except in strange cases
1053 -- like that in the example above
1054 returnM non_var_ty2'
1056 Just problem -> failWithTcM (unifyCheck problem tv1 ps_ty2')
1059 zonk_tc_type refine ty
1060 = zonkType (\tv -> return (TyVarTy tv)) refine ty
1061 -- We may already be inside a wobbly type t2, and
1062 -- should take that into account here
1064 data Problem = OccurCheck | NotMonoType
1066 okToUnifyWith :: TcTyVar -> TcType -> Maybe Problem
1067 -- (okToUnifyWith tv ty) checks whether it's ok to unify
1070 -- Just p => not ok, problem p
1075 ok (TyVarTy tv') | tv == tv' = Just OccurCheck
1076 | otherwise = Nothing
1077 ok (AppTy t1 t2) = ok t1 `and` ok t2
1078 ok (FunTy t1 t2) = ok t1 `and` ok t2
1079 ok (TyConApp _ ts) = oks ts
1080 ok (ForAllTy _ _) = Just NotMonoType
1081 ok (PredTy st) = ok_st st
1082 ok (NoteTy (FTVNote _) t) = ok t
1083 ok (NoteTy (SynNote t1) t2) = ok t1 `and` ok t2
1084 -- Type variables may be free in t1 but not t2
1085 -- A forall may be in t2 but not t1
1087 oks ts = foldr (and . ok) Nothing ts
1089 ok_st (ClassP _ ts) = oks ts
1090 ok_st (IParam _ t) = ok t
1093 Just p `and` m = Just p
1097 %************************************************************************
1101 %************************************************************************
1103 Unifying kinds is much, much simpler than unifying types.
1106 unifyKind :: TcKind -- Expected
1109 unifyKind LiftedTypeKind LiftedTypeKind = returnM ()
1110 unifyKind UnliftedTypeKind UnliftedTypeKind = returnM ()
1112 unifyKind OpenTypeKind k2 | isOpenTypeKind k2 = returnM ()
1113 unifyKind ArgTypeKind k2 | isArgTypeKind k2 = returnM ()
1114 -- Respect sub-kinding
1116 unifyKind (FunKind a1 r1) (FunKind a2 r2)
1117 = do { unifyKind a2 a1; unifyKind r1 r2 }
1118 -- Notice the flip in the argument,
1119 -- so that the sub-kinding works right
1121 unifyKind (KindVar kv1) k2 = uKVar False kv1 k2
1122 unifyKind k1 (KindVar kv2) = uKVar True kv2 k1
1123 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1125 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1126 unifyKinds [] [] = returnM ()
1127 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1129 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1132 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1133 uKVar swapped kv1 k2
1134 = do { mb_k1 <- readKindVar kv1
1136 Nothing -> uUnboundKVar swapped kv1 k2
1137 Just k1 | swapped -> unifyKind k2 k1
1138 | otherwise -> unifyKind k1 k2 }
1141 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1142 uUnboundKVar swapped kv1 k2@(KindVar kv2)
1143 | kv1 == kv2 = returnM ()
1144 | otherwise -- Distinct kind variables
1145 = do { mb_k2 <- readKindVar kv2
1147 Just k2 -> uUnboundKVar swapped kv1 k2
1148 Nothing -> writeKindVar kv1 k2 }
1150 uUnboundKVar swapped kv1 non_var_k2
1151 = do { k2' <- zonkTcKind non_var_k2
1152 ; kindOccurCheck kv1 k2'
1153 ; k2'' <- kindSimpleKind swapped k2'
1154 -- KindVars must be bound only to simple kinds
1155 -- Polarities: (kindSimpleKind True ?) succeeds
1156 -- returning *, corresponding to unifying
1159 ; writeKindVar kv1 k2'' }
1162 kindOccurCheck kv1 k2 -- k2 is zonked
1163 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1165 not_in (KindVar kv2) = kv1 /= kv2
1166 not_in (FunKind a2 r2) = not_in a2 && not_in r2
1169 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1170 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1171 -- If the flag is False, it requires k <: sk
1172 -- E.g. kindSimpleKind False ?? = *
1173 -- What about (kv -> *) :=: ?? -> *
1174 kindSimpleKind orig_swapped orig_kind
1175 = go orig_swapped orig_kind
1177 go sw (FunKind k1 k2) = do { k1' <- go (not sw) k1
1179 ; return (FunKind k1' k2') }
1180 go True OpenTypeKind = return liftedTypeKind
1181 go True ArgTypeKind = return liftedTypeKind
1182 go sw LiftedTypeKind = return liftedTypeKind
1183 go sw k@(KindVar _) = return k -- KindVars are always simple
1184 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1185 <+> ppr orig_swapped <+> ppr orig_kind)
1186 -- I think this can't actually happen
1188 -- T v = MkT v v must be a type
1189 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1192 kindOccurCheckErr tyvar ty
1193 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1194 2 (sep [ppr tyvar, char '=', ppr ty])
1196 unifyKindMisMatch ty1 ty2
1197 = zonkTcKind ty1 `thenM` \ ty1' ->
1198 zonkTcKind ty2 `thenM` \ ty2' ->
1200 msg = hang (ptext SLIT("Couldn't match kind"))
1201 2 (sep [quotes (ppr ty1'),
1202 ptext SLIT("against"),
1209 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1210 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1212 unifyFunKind (KindVar kvar)
1213 = readKindVar kvar `thenM` \ maybe_kind ->
1215 Just fun_kind -> unifyFunKind fun_kind
1216 Nothing -> do { arg_kind <- newKindVar
1217 ; res_kind <- newKindVar
1218 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1219 ; returnM (Just (arg_kind,res_kind)) }
1221 unifyFunKind (FunKind arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1222 unifyFunKind other = returnM Nothing
1225 %************************************************************************
1227 \subsection[Unify-context]{Errors and contexts}
1229 %************************************************************************
1235 unifyCtxt s ty1 ty2 tidy_env -- ty1 expected, ty2 inferred
1236 = zonkTcType ty1 `thenM` \ ty1' ->
1237 zonkTcType ty2 `thenM` \ ty2' ->
1238 returnM (err ty1' ty2')
1240 err ty1 ty2 = (env1,
1243 text "Expected" <+> text s <> colon <+> ppr tidy_ty1,
1244 text "Inferred" <+> text s <> colon <+> ppr tidy_ty2
1247 (env1, [tidy_ty1,tidy_ty2]) = tidyOpenTypes tidy_env [ty1,ty2]
1249 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1250 -- tv1 and ty2 are zonked already
1253 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1254 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1256 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1257 | otherwise = (pp1, pp2)
1258 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1259 (env2, ty2') = tidyOpenType env1 ty2
1260 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1261 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1263 unifyMisMatch ty1 ty2
1264 = do { (env, msg) <- misMatchMsg ty1 ty2
1265 ; failWithTcM (env, msg) }
1268 = do { (env1, pp1, extra1) <- ppr_ty emptyTidyEnv ty1
1269 ; (env2, pp2, extra2) <- ppr_ty env1 ty2
1270 ; return (env2, sep [sep [ptext SLIT("Couldn't match") <+> pp1,
1271 nest 7 (ptext SLIT("against") <+> pp2)],
1272 nest 2 extra1, nest 2 extra2]) }
1274 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
1276 = do { ty' <- zonkTcType ty
1277 ; let (env1,tidy_ty) = tidyOpenType env ty'
1278 simple_result = (env1, quotes (ppr tidy_ty), empty)
1281 | isSkolemTyVar tv -> return (env2, pp_rigid tv',
1283 | otherwise -> return simple_result
1285 (env2, tv') = tidySkolemTyVar env1 tv
1286 other -> return simple_result }
1288 pp_rigid tv = ptext SLIT("the rigid variable") <+> quotes (ppr tv)
1290 unifyCheck problem tyvar ty
1292 2 (sep [ppr tidy_tyvar, char '=', ppr tidy_ty]))
1294 (env1, tidy_tyvar) = tidyOpenTyVar emptyTidyEnv tyvar
1295 (env2, tidy_ty) = tidyOpenType env1 ty
1297 msg = case problem of
1298 OccurCheck -> ptext SLIT("Occurs check: cannot construct the infinite type:")
1299 NotMonoType -> ptext SLIT("Cannot unify a type variable with a type scheme:")
1303 %************************************************************************
1307 %************************************************************************
1309 ---------------------------
1310 -- We would like to get a decent error message from
1311 -- (a) Under-applied type constructors
1312 -- f :: (Maybe, Maybe)
1313 -- (b) Over-applied type constructors
1314 -- f :: Int x -> Int x
1318 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1319 -- A fancy wrapper for 'unifyKind', which tries
1320 -- to give decent error messages.
1321 checkExpectedKind ty act_kind exp_kind
1322 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1325 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1327 Just r -> returnM () ; -- Unification succeeded
1330 -- So there's definitely an error
1331 -- Now to find out what sort
1332 zonkTcKind exp_kind `thenM` \ exp_kind ->
1333 zonkTcKind act_kind `thenM` \ act_kind ->
1335 let (exp_as, _) = splitKindFunTys exp_kind
1336 (act_as, _) = splitKindFunTys act_kind
1337 n_exp_as = length exp_as
1338 n_act_as = length act_as
1340 err | n_exp_as < n_act_as -- E.g. [Maybe]
1341 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1343 -- Now n_exp_as >= n_act_as. In the next two cases,
1344 -- n_exp_as == 0, and hence so is n_act_as
1345 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1346 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1347 <+> ptext SLIT("is unlifted")
1349 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1350 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1351 <+> ptext SLIT("is lifted")
1353 | otherwise -- E.g. Monad [Int]
1354 = ptext SLIT("Kind mis-match")
1356 more_info = sep [ ptext SLIT("Expected kind") <+>
1357 quotes (pprKind exp_kind) <> comma,
1358 ptext SLIT("but") <+> quotes (ppr ty) <+>
1359 ptext SLIT("has kind") <+> quotes (pprKind act_kind)]
1361 failWithTc (err $$ more_info)
1365 %************************************************************************
1367 \subsection{Checking signature type variables}
1369 %************************************************************************
1371 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1372 are not mentioned in the environment. In particular:
1374 (a) Not mentioned in the type of a variable in the envt
1375 eg the signature for f in this:
1381 Here, f is forced to be monorphic by the free occurence of x.
1383 (d) Not (unified with another type variable that is) in scope.
1384 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1385 when checking the expression type signature, we find that
1386 even though there is nothing in scope whose type mentions r,
1387 nevertheless the type signature for the expression isn't right.
1389 Another example is in a class or instance declaration:
1391 op :: forall b. a -> b
1393 Here, b gets unified with a
1395 Before doing this, the substitution is applied to the signature type variable.
1398 checkSigTyVars :: [TcTyVar] -> TcM ()
1399 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1401 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1402 checkSigTyVarsWrt extra_tvs sig_tvs
1403 = zonkTcTyVarsAndFV (varSetElems extra_tvs) `thenM` \ extra_tvs' ->
1404 check_sig_tyvars extra_tvs' sig_tvs
1407 :: TcTyVarSet -- Global type variables. The universally quantified
1408 -- tyvars should not mention any of these
1409 -- Guaranteed already zonked.
1410 -> [TcTyVar] -- Universally-quantified type variables in the signature
1411 -- Guaranteed to be skolems
1413 check_sig_tyvars extra_tvs []
1415 check_sig_tyvars extra_tvs sig_tvs
1416 = ASSERT( all isSkolemTyVar sig_tvs )
1417 do { gbl_tvs <- tcGetGlobalTyVars
1418 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1419 text "gbl_tvs" <+> ppr gbl_tvs,
1420 text "extra_tvs" <+> ppr extra_tvs]))
1422 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1423 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1424 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1427 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1428 -> [TcTyVar] -- The possibly-escaping type variables
1429 -> [TcTyVar] -- The zonked versions thereof
1431 -- Complain about escaping type variables
1432 -- We pass a list of type variables, at least one of which
1433 -- escapes. The first list contains the original signature type variable,
1434 -- while the second contains the type variable it is unified to (usually itself)
1435 bleatEscapedTvs globals sig_tvs zonked_tvs
1436 = do { (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1437 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1439 (env1, tidy_tvs) = tidyOpenTyVars emptyTidyEnv sig_tvs
1440 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1442 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1444 check (tidy_env, msgs) (sig_tv, zonked_tv)
1445 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1447 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1448 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1450 -----------------------
1451 escape_msg sig_tv zonked_tv globs
1453 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1454 nest 2 (vcat globs)]
1456 = msg <+> ptext SLIT("escapes")
1457 -- Sigh. It's really hard to give a good error message
1458 -- all the time. One bad case is an existential pattern match.
1459 -- We rely on the "When..." context to help.
1461 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1463 | sig_tv == zonked_tv = empty
1464 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1467 These two context are used with checkSigTyVars
1470 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1471 -> TidyEnv -> TcM (TidyEnv, Message)
1472 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1473 = zonkTcType sig_tau `thenM` \ actual_tau ->
1475 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1476 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1477 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1478 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1479 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1481 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),