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, isMetaTyVar,
42 typeKind, tcSplitFunTy_maybe, mkForAllTys, mkAppTy,
43 tidyOpenType, tidyOpenTypes, tidyOpenTyVar, tidyOpenTyVars,
44 pprType, tidyKind, 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, isFunTyCon )
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 -- Precondition: never called with FunTyCon
297 zapToTyConApp tc (Check ty)
298 = ASSERT( not (isFunTyCon tc) ) -- Never called with FunTyCon
299 do { checkWiredInTyCon tc ; unifyTyConApp tc ty } -- NB: fails for a forall-type
301 zapToTyConApp tc (Infer hole)
302 = do { (_, elt_tys, _) <- tcInstTyVars (tyConTyVars tc)
303 ; let tc_app = mkTyConApp tc elt_tys
304 ; writeMutVar hole tc_app
305 ; traceTc (text "zap" <+> ppr tc)
306 ; checkWiredInTyCon tc
309 zapToListTy :: Expected TcType -> TcM TcType -- Special case for lists
310 zapToListTy exp_ty = do { [elt_ty] <- zapToTyConApp listTyCon exp_ty
313 ----------------------
314 unifyTyConApp :: TyCon -> TcType -> TcM [TcType]
316 = ASSERT( not (isFunTyCon tc) ) -- Never called with FunTyCon
317 unify_tc_app (tyConArity tc) True tc ty
318 -- Add a boolean flag to remember whether
319 -- to use the type refinement or not
321 unifyListTy :: TcType -> TcM TcType -- Special case for lists
322 unifyListTy exp_ty = do { [elt_ty] <- unifyTyConApp listTyCon exp_ty
326 unify_tc_app n_args use_refinement tc (NoteTy _ ty)
327 = unify_tc_app n_args use_refinement tc ty
329 unify_tc_app n_args use_refinement tc (TyConApp tycon arg_tys)
331 = ASSERT( n_args == length arg_tys ) -- ty::*
332 mapM (wobblify use_refinement) arg_tys
334 unify_tc_app n_args use_refinement tc (AppTy fun_ty arg_ty)
335 = do { arg_ty' <- wobblify use_refinement arg_ty
336 ; arg_tys <- unify_tc_app (n_args - 1) use_refinement tc fun_ty
337 ; return (arg_tys ++ [arg_ty']) }
339 unify_tc_app n_args use_refinement tc ty@(TyVarTy tyvar)
340 = do { traceTc (text "unify_tc_app: tyvar" <+> pprTcTyVar tyvar)
341 ; details <- condLookupTcTyVar use_refinement tyvar
343 IndirectTv use' ty' -> unify_tc_app n_args use' tc ty'
344 other -> unify_tc_app_help n_args tc ty
347 unify_tc_app n_args use_refinement tc ty = unify_tc_app_help n_args tc ty
349 unify_tc_app_help n_args tc ty -- Revert to ordinary unification
350 = do { (_, elt_tys, _) <- tcInstTyVars (take n_args (tyConTyVars tc))
351 ; let tc_app = mkTyConApp tc elt_tys
352 ; if not (isTauTy ty) then -- Can happen if we call zapToTyConApp tc (forall a. ty)
353 unifyMisMatch ty tc_app
355 { unifyTauTy ty tc_app
356 ; returnM elt_tys } }
359 ----------------------
360 unifyAppTy :: TcType -- Type to split: m a
361 -> TcM (TcType, TcType) -- (m,a)
364 unifyAppTy ty = unify_app_ty True ty
366 unify_app_ty use (NoteTy _ ty) = unify_app_ty use ty
368 unify_app_ty use ty@(TyVarTy tyvar)
369 = do { details <- condLookupTcTyVar use tyvar
371 IndirectTv use' ty' -> unify_app_ty use' ty'
372 other -> unify_app_ty_help ty
376 | Just (fun_ty, arg_ty) <- tcSplitAppTy_maybe ty
377 = do { fun' <- wobblify use fun_ty
378 ; arg' <- wobblify use arg_ty
379 ; return (fun', arg') }
381 | otherwise = unify_app_ty_help ty
383 unify_app_ty_help ty -- Revert to ordinary unification
384 = do { fun_ty <- newTyFlexiVarTy (mkArrowKind liftedTypeKind liftedTypeKind)
385 ; arg_ty <- newTyFlexiVarTy liftedTypeKind
386 ; unifyTauTy (mkAppTy fun_ty arg_ty) ty
387 ; return (fun_ty, arg_ty) }
390 ----------------------
391 wobblify :: Bool -- True <=> don't wobblify
394 -- Return a wobbly type. At the moment we do that by
395 -- allocating a fresh meta type variable.
396 wobblify True ty = return ty -- Don't wobblify
398 wobblify False ty@(TyVarTy tv)
399 | isMetaTyVar tv = return ty -- Already wobbly
401 wobblify False ty = do { uniq <- newUnique
402 ; tv <- newMetaTyVar (mkSysTvName uniq FSLIT("w"))
405 ; return (mkTyVarTy tv) }
409 %************************************************************************
411 \subsection{Subsumption}
413 %************************************************************************
415 All the tcSub calls have the form
417 tcSub expected_ty offered_ty
419 offered_ty <= expected_ty
421 That is, that a value of type offered_ty is acceptable in
422 a place expecting a value of type expected_ty.
424 It returns a coercion function
425 co_fn :: offered_ty -> expected_ty
426 which takes an HsExpr of type offered_ty into one of type
430 -----------------------
431 -- tcSubExp is used for expressions
432 tcSubExp :: Expected TcRhoType -> TcRhoType -> TcM ExprCoFn
434 tcSubExp (Infer hole) offered_ty
435 = do { offered' <- zonkTcType offered_ty
436 -- Note [Zonk return type]
437 -- zonk to take advantage of the current GADT type refinement.
438 -- If we don't we get spurious "existential type variable escapes":
439 -- case (x::Maybe a) of
440 -- Just b (y::b) -> y
441 -- We need the refinement [b->a] to be applied to the result type
442 ; writeMutVar hole offered'
443 ; return idCoercion }
445 tcSubExp (Check expected_ty) offered_ty
446 = tcSub expected_ty offered_ty
448 -----------------------
449 -- tcSubPat is used for patterns
450 tcSubPat :: TcSigmaType -- Pattern type signature
451 -> Expected TcSigmaType -- Type from context
453 -- In patterns we insist on an exact match; hence no CoFn returned
454 -- See Note [Pattern coercions] in TcPat
455 -- However, we can't call unify directly, because both types might be
456 -- polymorphic; hence the call to tcSub, followed by a check for
457 -- equal types. (We can't just check for the identity coercion, because
458 -- in the polymorphic case we might get back something eta-equivalent to
459 -- the identity coercion, but that's not easy to tell.)
461 tcSubPat sig_ty (Infer hole)
462 = do { sig_ty' <- zonkTcType sig_ty
463 ; writeMutVar hole sig_ty' -- See notes with tcSubExp above
466 -- This tcSub followed by tcEqType checks for identical types
467 -- It'd be done more neatly by augmenting the unifier to deal with
468 -- (identically shaped) for-all types.
470 tcSubPat sig_ty (Check exp_ty)
471 = do { co_fn <- tcSub sig_ty exp_ty
472 ; sig_ty' <- zonkTcType sig_ty
473 ; exp_ty' <- zonkTcType exp_ty
474 ; if tcEqType sig_ty' exp_ty' then
477 { (env, msg) <- misMatchMsg sig_ty' exp_ty'
478 ; failWithTcM (env, msg $$ extra) } }
480 extra | isTauTy sig_ty = empty
481 | otherwise = ptext SLIT("Polymorphic types must match exactly in patterns")
486 %************************************************************************
488 tcSub: main subsumption-check code
490 %************************************************************************
492 No holes expected now. Add some error-check context info.
496 tcSub :: TcSigmaType -> TcSigmaType -> TcM ExprCoFn -- Locally used only
497 -- tcSub exp act checks that
499 tcSub expected_ty actual_ty
500 = traceTc (text "tcSub" <+> details) `thenM_`
501 addErrCtxtM (unifyCtxt "type" expected_ty actual_ty)
502 (tc_sub expected_ty expected_ty actual_ty actual_ty)
504 details = vcat [text "Expected:" <+> ppr expected_ty,
505 text "Actual: " <+> ppr actual_ty]
508 tc_sub :: TcSigmaType -- expected_ty, before expanding synonyms
509 -> TcSigmaType -- ..and after
510 -> TcSigmaType -- actual_ty, before
511 -> TcSigmaType -- ..and after
514 -----------------------------------
516 tc_sub exp_sty (NoteTy _ exp_ty) act_sty act_ty = tc_sub exp_sty exp_ty act_sty act_ty
517 tc_sub exp_sty exp_ty act_sty (NoteTy _ act_ty) = tc_sub exp_sty exp_ty act_sty act_ty
519 -----------------------------------
520 -- Generalisation case
521 -- actual_ty: d:Eq b => b->b
522 -- expected_ty: forall a. Ord a => a->a
523 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
525 -- It is essential to do this *before* the specialisation case
526 -- Example: f :: (Eq a => a->a) -> ...
527 -- g :: Ord b => b->b
530 tc_sub exp_sty expected_ty act_sty actual_ty
531 | isSigmaTy expected_ty
532 = tcGen expected_ty (tyVarsOfType actual_ty) (
533 -- It's really important to check for escape wrt the free vars of
534 -- both expected_ty *and* actual_ty
535 \ body_exp_ty -> tc_sub body_exp_ty body_exp_ty act_sty actual_ty
536 ) `thenM` \ (gen_fn, co_fn) ->
537 returnM (gen_fn <.> co_fn)
539 -----------------------------------
540 -- Specialisation case:
541 -- actual_ty: forall a. Ord a => a->a
542 -- expected_ty: Int -> Int
543 -- co_fn e = e Int dOrdInt
545 tc_sub exp_sty expected_ty act_sty actual_ty
546 | isSigmaTy actual_ty
547 = tcInstCall InstSigOrigin actual_ty `thenM` \ (inst_fn, _, body_ty) ->
548 tc_sub exp_sty expected_ty body_ty body_ty `thenM` \ co_fn ->
549 returnM (co_fn <.> inst_fn)
551 -----------------------------------
554 tc_sub _ (FunTy exp_arg exp_res) _ (FunTy act_arg act_res)
555 = tcSub_fun exp_arg exp_res act_arg act_res
557 -----------------------------------
558 -- Type variable meets function: imitate
560 -- NB 1: we can't just unify the type variable with the type
561 -- because the type might not be a tau-type, and we aren't
562 -- allowed to instantiate an ordinary type variable with
565 -- NB 2: can we short-cut to an error case?
566 -- when the arg/res is not a tau-type?
567 -- NO! e.g. f :: ((forall a. a->a) -> Int) -> Int
569 -- is perfectly fine, because we can instantiate f's type to a monotype
571 -- However, we get can get jolly unhelpful error messages.
572 -- e.g. foo = id runST
574 -- Inferred type is less polymorphic than expected
575 -- Quantified type variable `s' escapes
576 -- Expected type: ST s a -> t
577 -- Inferred type: (forall s1. ST s1 a) -> a
578 -- In the first argument of `id', namely `runST'
579 -- In a right-hand side of function `foo': id runST
581 -- I'm not quite sure what to do about this!
583 tc_sub exp_sty exp_ty@(FunTy exp_arg exp_res) _ act_ty
584 = do { (act_arg, act_res) <- unify_fun act_ty
585 ; tcSub_fun exp_arg exp_res act_arg act_res }
587 tc_sub _ exp_ty act_sty act_ty@(FunTy act_arg act_res)
588 = do { (exp_arg, exp_res) <- unify_fun exp_ty
589 ; tcSub_fun exp_arg exp_res act_arg act_res }
591 -----------------------------------
593 -- If none of the above match, we revert to the plain unifier
594 tc_sub exp_sty expected_ty act_sty actual_ty
595 = uTys True exp_sty expected_ty True act_sty actual_ty `thenM_`
598 -----------------------------------
599 -- A helper to make a function type match
600 -- The error message isn't very good, but that's a problem with
601 -- all of this subsumption code
603 = do { (ok, args, res) <- unify_fun_ty True 1 ty
604 ; if ok then return (head args, res)
605 else failWithTc (ptext SLIT("Expecting a function type, but found") <+> quotes (ppr ty))}
609 tcSub_fun exp_arg exp_res act_arg act_res
610 = tc_sub act_arg act_arg exp_arg exp_arg `thenM` \ co_fn_arg ->
611 tc_sub exp_res exp_res act_res act_res `thenM` \ co_fn_res ->
612 newUnique `thenM` \ uniq ->
614 -- co_fn_arg :: HsExpr exp_arg -> HsExpr act_arg
615 -- co_fn_res :: HsExpr act_res -> HsExpr exp_res
616 -- co_fn :: HsExpr (act_arg -> act_res) -> HsExpr (exp_arg -> exp_res)
617 arg_id = mkSysLocal FSLIT("sub") uniq exp_arg
618 coercion | isIdCoercion co_fn_arg,
619 isIdCoercion co_fn_res = idCoercion
620 | otherwise = mkCoercion co_fn
622 co_fn e = DictLam [arg_id]
623 (noLoc (co_fn_res <$> (HsApp (noLoc e) (noLoc (co_fn_arg <$> HsVar arg_id)))))
624 -- Slight hack; using a "DictLam" to get an ordinary simple lambda
625 -- HsVar arg_id :: HsExpr exp_arg
626 -- co_fn_arg $it :: HsExpr act_arg
627 -- HsApp e $it :: HsExpr act_res
628 -- co_fn_res $it :: HsExpr exp_res
634 %************************************************************************
636 \subsection{Generalisation}
638 %************************************************************************
641 tcGen :: TcSigmaType -- expected_ty
642 -> TcTyVarSet -- Extra tyvars that the universally
643 -- quantified tyvars of expected_ty
644 -- must not be unified
645 -> (TcRhoType -> TcM result) -- spec_ty
646 -> TcM (ExprCoFn, result)
647 -- The expression has type: spec_ty -> expected_ty
649 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
650 -- If not, the call is a no-op
651 = do { -- We want the GenSkol info in the skolemised type variables to
652 -- mention the *instantiated* tyvar names, so that we get a
653 -- good error message "Rigid variable 'a' is bound by (forall a. a->a)"
654 -- Hence the tiresome but innocuous fixM
655 ((forall_tvs, theta, rho_ty), skol_info) <- fixM (\ ~(_, skol_info) ->
656 do { (forall_tvs, theta, rho_ty) <- tcSkolType skol_info expected_ty
657 ; span <- getSrcSpanM
658 ; let skol_info = GenSkol forall_tvs (mkPhiTy theta rho_ty) span
659 ; return ((forall_tvs, theta, rho_ty), skol_info) })
662 ; traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
663 text "expected_ty" <+> ppr expected_ty,
664 text "inst ty" <+> ppr forall_tvs <+> ppr theta <+> ppr rho_ty,
665 text "free_tvs" <+> ppr free_tvs,
666 text "forall_tvs" <+> ppr forall_tvs])
669 -- Type-check the arg and unify with poly type
670 ; (result, lie) <- getLIE (thing_inside rho_ty)
672 -- Check that the "forall_tvs" havn't been constrained
673 -- The interesting bit here is that we must include the free variables
674 -- of the expected_ty. Here's an example:
675 -- runST (newVar True)
676 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
677 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
678 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
679 -- So now s' isn't unconstrained because it's linked to a.
680 -- Conclusion: include the free vars of the expected_ty in the
681 -- list of "free vars" for the signature check.
683 ; dicts <- newDicts (SigOrigin skol_info) theta
684 ; inst_binds <- tcSimplifyCheck sig_msg forall_tvs dicts lie
686 ; checkSigTyVarsWrt free_tvs forall_tvs
687 ; traceTc (text "tcGen:done")
690 -- This HsLet binds any Insts which came out of the simplification.
691 -- It's a bit out of place here, but using AbsBind involves inventing
692 -- a couple of new names which seems worse.
693 dict_ids = map instToId dicts
694 co_fn e = TyLam forall_tvs (mkHsDictLam dict_ids (mkHsDictLet inst_binds (noLoc e)))
695 ; returnM (mkCoercion co_fn, result) }
697 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
698 sig_msg = ptext SLIT("expected type of an expression")
703 %************************************************************************
705 \subsection[Unify-exported]{Exported unification functions}
707 %************************************************************************
709 The exported functions are all defined as versions of some
710 non-exported generic functions.
712 Unify two @TauType@s. Dead straightforward.
715 unifyTauTy :: TcTauType -> TcTauType -> TcM ()
716 unifyTauTy ty1 ty2 -- ty1 expected, ty2 inferred
717 = -- The unifier should only ever see tau-types
718 -- (no quantification whatsoever)
719 ASSERT2( isTauTy ty1, ppr ty1 )
720 ASSERT2( isTauTy ty2, ppr ty2 )
721 addErrCtxtM (unifyCtxt "type" ty1 ty2) $
722 uTys True ty1 ty1 True ty2 ty2
724 unifyTheta :: TcThetaType -> TcThetaType -> TcM ()
725 unifyTheta theta1 theta2
726 = do { checkTc (equalLength theta1 theta2)
727 (ptext SLIT("Contexts differ in length"))
728 ; unifyTauTyLists True (map mkPredTy theta1) True (map mkPredTy theta2) }
731 @unifyTauTyList@ unifies corresponding elements of two lists of
732 @TauType@s. It uses @uTys@ to do the real work. The lists should be
733 of equal length. We charge down the list explicitly so that we can
734 complain if their lengths differ.
737 unifyTauTyLists :: Bool -> -- Allow refinements on tys1
739 Bool -> -- Allow refinements on tys2
740 [TcTauType] -> TcM ()
741 -- Precondition: lists must be same length
742 -- Having the caller check gives better error messages
743 -- Actually the caller neve does need to check; see Note [Tycon app]
744 unifyTauTyLists r1 [] r2 [] = returnM ()
745 unifyTauTyLists r1 (ty1:tys1) r2 (ty2:tys2) = uTys r1 ty1 ty1 r2 ty2 ty2 `thenM_`
746 unifyTauTyLists r1 tys1 r2 tys2
747 unifyTauTyLists r1 ty1s r2 ty2s = panic "Unify.unifyTauTyLists: mismatched type lists!"
750 @unifyTauTyList@ takes a single list of @TauType@s and unifies them
751 all together. It is used, for example, when typechecking explicit
752 lists, when all the elts should be of the same type.
755 unifyTauTyList :: [TcTauType] -> TcM ()
756 unifyTauTyList [] = returnM ()
757 unifyTauTyList [ty] = returnM ()
758 unifyTauTyList (ty1:tys@(ty2:_)) = unifyTauTy ty1 ty2 `thenM_`
762 %************************************************************************
764 \subsection[Unify-uTys]{@uTys@: getting down to business}
766 %************************************************************************
768 @uTys@ is the heart of the unifier. Each arg happens twice, because
769 we want to report errors in terms of synomyms if poss. The first of
770 the pair is used in error messages only; it is always the same as the
771 second, except that if the first is a synonym then the second may be a
772 de-synonym'd version. This way we get better error messages.
774 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
777 uTys :: Bool -- Allow refinements to ty1
778 -> TcTauType -> TcTauType -- Error reporting ty1 and real ty1
779 -- ty1 is the *expected* type
780 -> Bool -- Allow refinements to ty2
781 -> TcTauType -> TcTauType -- Error reporting ty2 and real ty2
782 -- ty2 is the *actual* type
785 -- Always expand synonyms (see notes at end)
786 -- (this also throws away FTVs)
787 uTys r1 ps_ty1 (NoteTy n1 ty1) r2 ps_ty2 ty2 = uTys r1 ps_ty1 ty1 r2 ps_ty2 ty2
788 uTys r1 ps_ty1 ty1 r2 ps_ty2 (NoteTy n2 ty2) = uTys r1 ps_ty1 ty1 r2 ps_ty2 ty2
790 -- Variables; go for uVar
791 uTys r1 ps_ty1 (TyVarTy tyvar1) r2 ps_ty2 ty2 = uVar False r1 tyvar1 r2 ps_ty2 ty2
792 uTys r1 ps_ty1 ty1 r2 ps_ty2 (TyVarTy tyvar2) = uVar True r2 tyvar2 r1 ps_ty1 ty1
793 -- "True" means args swapped
796 uTys r1 _ (PredTy (IParam n1 t1)) r2 _ (PredTy (IParam n2 t2))
797 | n1 == n2 = uTys r1 t1 t1 r2 t2 t2
798 uTys r1 _ (PredTy (ClassP c1 tys1)) r2 _ (PredTy (ClassP c2 tys2))
799 | c1 == c2 = unifyTauTyLists r1 tys1 r2 tys2
800 -- Guaranteed equal lengths because the kinds check
802 -- Functions; just check the two parts
803 uTys r1 _ (FunTy fun1 arg1) r2 _ (FunTy fun2 arg2)
804 = uTys r1 fun1 fun1 r2 fun2 fun2 `thenM_` uTys r1 arg1 arg1 r2 arg2 arg2
806 -- Type constructors must match
807 uTys r1 ps_ty1 (TyConApp con1 tys1) r2 ps_ty2 (TyConApp con2 tys2)
808 | con1 == con2 = unifyTauTyLists r1 tys1 r2 tys2
809 -- See Note [TyCon app]
811 -- Applications need a bit of care!
812 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
813 -- NB: we've already dealt with type variables and Notes,
814 -- so if one type is an App the other one jolly well better be too
815 uTys r1 ps_ty1 (AppTy s1 t1) r2 ps_ty2 ty2
816 = case tcSplitAppTy_maybe ty2 of
817 Just (s2,t2) -> uTys r1 s1 s1 r2 s2 s2 `thenM_` uTys r1 t1 t1 r2 t2 t2
818 Nothing -> unifyMisMatch ps_ty1 ps_ty2
820 -- Now the same, but the other way round
821 -- Don't swap the types, because the error messages get worse
822 uTys r1 ps_ty1 ty1 r2 ps_ty2 (AppTy s2 t2)
823 = case tcSplitAppTy_maybe ty1 of
824 Just (s1,t1) -> uTys r1 s1 s1 r2 s2 s2 `thenM_` uTys r1 t1 t1 r2 t2 t2
825 Nothing -> unifyMisMatch ps_ty1 ps_ty2
827 -- Not expecting for-alls in unification
828 -- ... but the error message from the unifyMisMatch more informative
829 -- than a panic message!
831 -- Anything else fails
832 uTys r1 ps_ty1 ty1 r2 ps_ty2 ty2 = unifyMisMatch ps_ty1 ps_ty2
837 When we find two TyConApps, the argument lists are guaranteed equal
838 length. Reason: intially the kinds of the two types to be unified is
839 the same. The only way it can become not the same is when unifying two
840 AppTys (f1 a1):=:(f2 a2). In that case there can't be a TyConApp in
841 the f1,f2 (because it'd absorb the app). If we unify f1:=:f2 first,
842 which we do, that ensures that f1,f2 have the same kind; and that
843 means a1,a2 have the same kind. And now the argument repeats.
848 If you are tempted to make a short cut on synonyms, as in this
852 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
853 -- NO = if (con1 == con2) then
854 -- NO -- Good news! Same synonym constructors, so we can shortcut
855 -- NO -- by unifying their arguments and ignoring their expansions.
856 -- NO unifyTauTypeLists args1 args2
858 -- NO -- Never mind. Just expand them and try again
862 then THINK AGAIN. Here is the whole story, as detected and reported
863 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
865 Here's a test program that should detect the problem:
869 x = (1 :: Bogus Char) :: Bogus Bool
872 The problem with [the attempted shortcut code] is that
876 is not a sufficient condition to be able to use the shortcut!
877 You also need to know that the type synonym actually USES all
878 its arguments. For example, consider the following type synonym
879 which does not use all its arguments.
884 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
885 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
886 would fail, even though the expanded forms (both \tr{Int}) should
889 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
890 unnecessarily bind \tr{t} to \tr{Char}.
892 ... You could explicitly test for the problem synonyms and mark them
893 somehow as needing expansion, perhaps also issuing a warning to the
898 %************************************************************************
900 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
902 %************************************************************************
904 @uVar@ is called when at least one of the types being unified is a
905 variable. It does {\em not} assume that the variable is a fixed point
906 of the substitution; rather, notice that @uVar@ (defined below) nips
907 back into @uTys@ if it turns out that the variable is already bound.
910 uVar :: Bool -- False => tyvar is the "expected"
911 -- True => ty is the "expected" thing
912 -> Bool -- True, allow refinements to tv1, False don't
914 -> Bool -- Allow refinements to ty2?
915 -> TcTauType -> TcTauType -- printing and real versions
918 uVar swapped r1 tv1 r2 ps_ty2 ty2
919 = traceTc (text "uVar" <+> ppr swapped <+> ppr tv1 <+> (ppr ps_ty2 $$ ppr ty2)) `thenM_`
920 condLookupTcTyVar r1 tv1 `thenM` \ details ->
922 IndirectTv r1' ty1 | swapped -> uTys r2 ps_ty2 ty2 r1' ty1 ty1 -- Swap back
923 | otherwise -> uTys r1' ty1 ty1 r2 ps_ty2 ty2 -- Same order
924 DoneTv details1 -> uDoneVar swapped tv1 details1 r2 ps_ty2 ty2
927 uDoneVar :: Bool -- Args are swapped
928 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
929 -> Bool -- Allow refinements to ty2
930 -> TcTauType -> TcTauType -- Type 2
932 -- Invariant: tyvar 1 is not unified with anything
934 uDoneVar swapped tv1 details1 r2 ps_ty2 (NoteTy n2 ty2)
935 = -- Expand synonyms; ignore FTVs
936 uDoneVar swapped tv1 details1 r2 ps_ty2 ty2
938 uDoneVar swapped tv1 details1 r2 ps_ty2 ty2@(TyVarTy tv2)
939 -- Same type variable => no-op
943 -- Distinct type variables
945 = do { lookup2 <- condLookupTcTyVar r2 tv2
947 IndirectTv b ty2' -> uDoneVar swapped tv1 details1 b ty2' ty2'
948 DoneTv details2 -> uDoneVars swapped tv1 details1 tv2 details2
951 uDoneVar swapped tv1 details1 r2 ps_ty2 non_var_ty2 -- ty2 is not a type variable
953 MetaTv ref1 -> do { -- Do the occurs check, and check that we are not
954 -- unifying a type variable with a polytype
955 -- Returns a zonked type ready for the update
956 ty2 <- checkValue tv1 r2 ps_ty2 non_var_ty2
957 ; updateMeta swapped tv1 ref1 ty2 }
959 skolem_details -> unifyMisMatch (TyVarTy tv1) ps_ty2
963 uDoneVars :: Bool -- Args are swapped
964 -> TcTyVar -> TcTyVarDetails -- Tyvar 1
965 -> TcTyVar -> TcTyVarDetails -- Tyvar 2
967 -- Invarant: the type variables are distinct,
968 -- and are not already unified with anything
970 uDoneVars swapped tv1 (MetaTv ref1) tv2 details2
972 MetaTv ref2 | update_tv2 -> updateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
973 other -> updateMeta swapped tv1 ref1 (mkTyVarTy tv2)
974 -- Note that updateMeta does a sub-kind check
975 -- We might unify (a b) with (c d) where b::*->* and d::*; this should fail
979 update_tv2 = k1 `isSubKind` k2 && (k1 /= k2 || nicer_to_update_tv2)
980 -- Update the variable with least kind info
981 -- See notes on type inference in Kind.lhs
982 -- The "nicer to" part only applies if the two kinds are the same,
983 -- so we can choose which to do.
985 nicer_to_update_tv2 = isSystemName (varName tv2)
986 -- Try to update sys-y type variables in preference to ones
987 -- gotten (say) by instantiating a polymorphic function with
988 -- a user-written type sig
990 uDoneVars swapped tv1 (SkolemTv _) tv2 details2
992 MetaTv ref2 -> updateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
993 other -> unifyMisMatch (mkTyVarTy tv1) (mkTyVarTy tv2)
995 uDoneVars swapped tv1 (SigSkolTv _ ref1) tv2 details2
997 MetaTv ref2 -> updateMeta (not swapped) tv2 ref2 (mkTyVarTy tv1)
998 SigSkolTv _ _ -> updateMeta swapped tv1 ref1 (mkTyVarTy tv2)
999 other -> unifyMisMatch (mkTyVarTy tv1) (mkTyVarTy tv2)
1002 updateMeta :: Bool -> TcTyVar -> IORef MetaDetails -> TcType -> TcM ()
1003 -- Update tv1, which is flexi; occurs check is alrady done
1004 updateMeta swapped tv1 ref1 ty2
1005 = do { checkKinds swapped tv1 ty2
1006 ; writeMutVar ref1 (Indirect ty2) }
1010 checkKinds swapped tv1 ty2
1011 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
1012 -- ty2 has been zonked at this stage, which ensures that
1013 -- its kind has as much boxity information visible as possible.
1014 | tk2 `isSubKind` tk1 = returnM ()
1017 -- Either the kinds aren't compatible
1018 -- (can happen if we unify (a b) with (c d))
1019 -- or we are unifying a lifted type variable with an
1020 -- unlifted type: e.g. (id 3#) is illegal
1021 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
1022 unifyKindMisMatch k1 k2
1024 (k1,k2) | swapped = (tk2,tk1)
1025 | otherwise = (tk1,tk2)
1031 checkValue tv1 r2 ps_ty2 non_var_ty2
1032 -- Do the occurs check, and check that we are not
1033 -- unifying a type variable with a polytype
1034 -- Return the type to update the type variable with, or fail
1036 -- Basically we want to update tv1 := ps_ty2
1037 -- because ps_ty2 has type-synonym info, which improves later error messages
1042 -- f :: (A a -> a -> ()) -> ()
1046 -- x = f (\ x p -> p x)
1048 -- In the application (p x), we try to match "t" with "A t". If we go
1049 -- ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
1050 -- an infinite loop later.
1051 -- But we should not reject the program, because A t = ().
1052 -- Rather, we should bind t to () (= non_var_ty2).
1054 -- That's why we have this two-state occurs-check
1055 = zonk_tc_type r2 ps_ty2 `thenM` \ ps_ty2' ->
1056 case okToUnifyWith tv1 ps_ty2' of {
1057 Nothing -> returnM ps_ty2' ; -- Success
1060 zonk_tc_type r2 non_var_ty2 `thenM` \ non_var_ty2' ->
1061 case okToUnifyWith tv1 non_var_ty2' of
1062 Nothing -> -- This branch rarely succeeds, except in strange cases
1063 -- like that in the example above
1064 returnM non_var_ty2'
1066 Just problem -> failWithTcM (unifyCheck problem tv1 ps_ty2')
1069 zonk_tc_type refine ty
1070 = zonkType (\tv -> return (TyVarTy tv)) refine ty
1071 -- We may already be inside a wobbly type t2, and
1072 -- should take that into account here
1074 data Problem = OccurCheck | NotMonoType
1076 okToUnifyWith :: TcTyVar -> TcType -> Maybe Problem
1077 -- (okToUnifyWith tv ty) checks whether it's ok to unify
1080 -- Just p => not ok, problem p
1085 ok (TyVarTy tv') | tv == tv' = Just OccurCheck
1086 | otherwise = Nothing
1087 ok (AppTy t1 t2) = ok t1 `and` ok t2
1088 ok (FunTy t1 t2) = ok t1 `and` ok t2
1089 ok (TyConApp _ ts) = oks ts
1090 ok (ForAllTy _ _) = Just NotMonoType
1091 ok (PredTy st) = ok_st st
1092 ok (NoteTy (FTVNote _) t) = ok t
1093 ok (NoteTy (SynNote t1) t2) = ok t1 `and` ok t2
1094 -- Type variables may be free in t1 but not t2
1095 -- A forall may be in t2 but not t1
1097 oks ts = foldr (and . ok) Nothing ts
1099 ok_st (ClassP _ ts) = oks ts
1100 ok_st (IParam _ t) = ok t
1103 Just p `and` m = Just p
1107 %************************************************************************
1111 %************************************************************************
1113 Unifying kinds is much, much simpler than unifying types.
1116 unifyKind :: TcKind -- Expected
1119 unifyKind LiftedTypeKind LiftedTypeKind = returnM ()
1120 unifyKind UnliftedTypeKind UnliftedTypeKind = returnM ()
1122 unifyKind OpenTypeKind k2 | isOpenTypeKind k2 = returnM ()
1123 unifyKind ArgTypeKind k2 | isArgTypeKind k2 = returnM ()
1124 -- Respect sub-kinding
1126 unifyKind (FunKind a1 r1) (FunKind a2 r2)
1127 = do { unifyKind a2 a1; unifyKind r1 r2 }
1128 -- Notice the flip in the argument,
1129 -- so that the sub-kinding works right
1131 unifyKind (KindVar kv1) k2 = uKVar False kv1 k2
1132 unifyKind k1 (KindVar kv2) = uKVar True kv2 k1
1133 unifyKind k1 k2 = unifyKindMisMatch k1 k2
1135 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
1136 unifyKinds [] [] = returnM ()
1137 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
1139 unifyKinds _ _ = panic "unifyKinds: length mis-match"
1142 uKVar :: Bool -> KindVar -> TcKind -> TcM ()
1143 uKVar swapped kv1 k2
1144 = do { mb_k1 <- readKindVar kv1
1146 Nothing -> uUnboundKVar swapped kv1 k2
1147 Just k1 | swapped -> unifyKind k2 k1
1148 | otherwise -> unifyKind k1 k2 }
1151 uUnboundKVar :: Bool -> KindVar -> TcKind -> TcM ()
1152 uUnboundKVar swapped kv1 k2@(KindVar kv2)
1153 | kv1 == kv2 = returnM ()
1154 | otherwise -- Distinct kind variables
1155 = do { mb_k2 <- readKindVar kv2
1157 Just k2 -> uUnboundKVar swapped kv1 k2
1158 Nothing -> writeKindVar kv1 k2 }
1160 uUnboundKVar swapped kv1 non_var_k2
1161 = do { k2' <- zonkTcKind non_var_k2
1162 ; kindOccurCheck kv1 k2'
1163 ; k2'' <- kindSimpleKind swapped k2'
1164 -- KindVars must be bound only to simple kinds
1165 -- Polarities: (kindSimpleKind True ?) succeeds
1166 -- returning *, corresponding to unifying
1169 ; writeKindVar kv1 k2'' }
1172 kindOccurCheck kv1 k2 -- k2 is zonked
1173 = checkTc (not_in k2) (kindOccurCheckErr kv1 k2)
1175 not_in (KindVar kv2) = kv1 /= kv2
1176 not_in (FunKind a2 r2) = not_in a2 && not_in r2
1179 kindSimpleKind :: Bool -> Kind -> TcM SimpleKind
1180 -- (kindSimpleKind True k) returns a simple kind sk such that sk <: k
1181 -- If the flag is False, it requires k <: sk
1182 -- E.g. kindSimpleKind False ?? = *
1183 -- What about (kv -> *) :=: ?? -> *
1184 kindSimpleKind orig_swapped orig_kind
1185 = go orig_swapped orig_kind
1187 go sw (FunKind k1 k2) = do { k1' <- go (not sw) k1
1189 ; return (FunKind k1' k2') }
1190 go True OpenTypeKind = return liftedTypeKind
1191 go True ArgTypeKind = return liftedTypeKind
1192 go sw LiftedTypeKind = return liftedTypeKind
1193 go sw k@(KindVar _) = return k -- KindVars are always simple
1194 go swapped kind = failWithTc (ptext SLIT("Unexpected kind unification failure:")
1195 <+> ppr orig_swapped <+> ppr orig_kind)
1196 -- I think this can't actually happen
1198 -- T v = MkT v v must be a type
1199 -- T v w = MkT (v -> w) v must not be an umboxed tuple
1202 kindOccurCheckErr tyvar ty
1203 = hang (ptext SLIT("Occurs check: cannot construct the infinite kind:"))
1204 2 (sep [ppr tyvar, char '=', ppr ty])
1206 unifyKindMisMatch ty1 ty2
1207 = zonkTcKind ty1 `thenM` \ ty1' ->
1208 zonkTcKind ty2 `thenM` \ ty2' ->
1210 msg = hang (ptext SLIT("Couldn't match kind"))
1211 2 (sep [quotes (ppr ty1'),
1212 ptext SLIT("against"),
1219 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
1220 -- Like unifyFunTy, but does not fail; instead just returns Nothing
1222 unifyFunKind (KindVar kvar)
1223 = readKindVar kvar `thenM` \ maybe_kind ->
1225 Just fun_kind -> unifyFunKind fun_kind
1226 Nothing -> do { arg_kind <- newKindVar
1227 ; res_kind <- newKindVar
1228 ; writeKindVar kvar (mkArrowKind arg_kind res_kind)
1229 ; returnM (Just (arg_kind,res_kind)) }
1231 unifyFunKind (FunKind arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
1232 unifyFunKind other = returnM Nothing
1235 %************************************************************************
1237 \subsection[Unify-context]{Errors and contexts}
1239 %************************************************************************
1245 unifyCtxt s ty1 ty2 tidy_env -- ty1 expected, ty2 inferred
1246 = zonkTcType ty1 `thenM` \ ty1' ->
1247 zonkTcType ty2 `thenM` \ ty2' ->
1248 returnM (err ty1' ty2')
1250 err ty1 ty2 = (env1,
1253 text "Expected" <+> text s <> colon <+> ppr tidy_ty1,
1254 text "Inferred" <+> text s <> colon <+> ppr tidy_ty2
1257 (env1, [tidy_ty1,tidy_ty2]) = tidyOpenTypes tidy_env [ty1,ty2]
1259 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
1260 -- tv1 and ty2 are zonked already
1263 msg = (env2, ptext SLIT("When matching the kinds of") <+>
1264 sep [quotes pp_expected <+> ptext SLIT("and"), quotes pp_actual])
1266 (pp_expected, pp_actual) | swapped = (pp2, pp1)
1267 | otherwise = (pp1, pp2)
1268 (env1, tv1') = tidyOpenTyVar tidy_env tv1
1269 (env2, ty2') = tidyOpenType env1 ty2
1270 pp1 = ppr tv1' <+> dcolon <+> ppr (tyVarKind tv1)
1271 pp2 = ppr ty2' <+> dcolon <+> ppr (typeKind ty2)
1273 unifyMisMatch ty1 ty2
1274 = do { (env, msg) <- misMatchMsg ty1 ty2
1275 ; failWithTcM (env, msg) }
1278 = do { (env1, pp1, extra1) <- ppr_ty emptyTidyEnv ty1
1279 ; (env2, pp2, extra2) <- ppr_ty env1 ty2
1280 ; return (env2, sep [sep [ptext SLIT("Couldn't match") <+> pp1,
1281 nest 7 (ptext SLIT("against") <+> pp2)],
1282 nest 2 extra1, nest 2 extra2]) }
1284 ppr_ty :: TidyEnv -> TcType -> TcM (TidyEnv, SDoc, SDoc)
1286 = do { ty' <- zonkTcType ty
1287 ; let (env1,tidy_ty) = tidyOpenType env ty'
1288 simple_result = (env1, quotes (ppr tidy_ty), empty)
1291 | isSkolemTyVar tv -> return (env2, pp_rigid tv',
1293 | otherwise -> return simple_result
1295 (env2, tv') = tidySkolemTyVar env1 tv
1296 other -> return simple_result }
1298 pp_rigid tv = ptext SLIT("the rigid variable") <+> quotes (ppr tv)
1300 unifyCheck problem tyvar ty
1302 2 (sep [ppr tidy_tyvar, char '=', ppr tidy_ty]))
1304 (env1, tidy_tyvar) = tidyOpenTyVar emptyTidyEnv tyvar
1305 (env2, tidy_ty) = tidyOpenType env1 ty
1307 msg = case problem of
1308 OccurCheck -> ptext SLIT("Occurs check: cannot construct the infinite type:")
1309 NotMonoType -> ptext SLIT("Cannot unify a type variable with a type scheme:")
1313 %************************************************************************
1317 %************************************************************************
1319 ---------------------------
1320 -- We would like to get a decent error message from
1321 -- (a) Under-applied type constructors
1322 -- f :: (Maybe, Maybe)
1323 -- (b) Over-applied type constructors
1324 -- f :: Int x -> Int x
1328 checkExpectedKind :: Outputable a => a -> TcKind -> TcKind -> TcM ()
1329 -- A fancy wrapper for 'unifyKind', which tries
1330 -- to give decent error messages.
1331 checkExpectedKind ty act_kind exp_kind
1332 | act_kind `isSubKind` exp_kind -- Short cut for a very common case
1335 = tryTc (unifyKind exp_kind act_kind) `thenM` \ (_errs, mb_r) ->
1337 Just r -> returnM () ; -- Unification succeeded
1340 -- So there's definitely an error
1341 -- Now to find out what sort
1342 zonkTcKind exp_kind `thenM` \ exp_kind ->
1343 zonkTcKind act_kind `thenM` \ act_kind ->
1345 let (exp_as, _) = splitKindFunTys exp_kind
1346 (act_as, _) = splitKindFunTys act_kind
1347 n_exp_as = length exp_as
1348 n_act_as = length act_as
1350 (env1, tidy_exp_kind) = tidyKind emptyTidyEnv exp_kind
1351 (env2, tidy_act_kind) = tidyKind env1 act_kind
1353 err | n_exp_as < n_act_as -- E.g. [Maybe]
1354 = quotes (ppr ty) <+> ptext SLIT("is not applied to enough type arguments")
1356 -- Now n_exp_as >= n_act_as. In the next two cases,
1357 -- n_exp_as == 0, and hence so is n_act_as
1358 | isLiftedTypeKind exp_kind && isUnliftedTypeKind act_kind
1359 = ptext SLIT("Expecting a lifted type, but") <+> quotes (ppr ty)
1360 <+> ptext SLIT("is unlifted")
1362 | isUnliftedTypeKind exp_kind && isLiftedTypeKind act_kind
1363 = ptext SLIT("Expecting an unlifted type, but") <+> quotes (ppr ty)
1364 <+> ptext SLIT("is lifted")
1366 | otherwise -- E.g. Monad [Int]
1367 = ptext SLIT("Kind mis-match")
1369 more_info = sep [ ptext SLIT("Expected kind") <+>
1370 quotes (pprKind tidy_exp_kind) <> comma,
1371 ptext SLIT("but") <+> quotes (ppr ty) <+>
1372 ptext SLIT("has kind") <+> quotes (pprKind tidy_act_kind)]
1374 failWithTcM (env2, err $$ more_info)
1378 %************************************************************************
1380 \subsection{Checking signature type variables}
1382 %************************************************************************
1384 @checkSigTyVars@ checks that a set of universally quantified type varaibles
1385 are not mentioned in the environment. In particular:
1387 (a) Not mentioned in the type of a variable in the envt
1388 eg the signature for f in this:
1394 Here, f is forced to be monorphic by the free occurence of x.
1396 (d) Not (unified with another type variable that is) in scope.
1397 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1398 when checking the expression type signature, we find that
1399 even though there is nothing in scope whose type mentions r,
1400 nevertheless the type signature for the expression isn't right.
1402 Another example is in a class or instance declaration:
1404 op :: forall b. a -> b
1406 Here, b gets unified with a
1408 Before doing this, the substitution is applied to the signature type variable.
1411 checkSigTyVars :: [TcTyVar] -> TcM ()
1412 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1414 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM ()
1415 checkSigTyVarsWrt extra_tvs sig_tvs
1416 = zonkTcTyVarsAndFV (varSetElems extra_tvs) `thenM` \ extra_tvs' ->
1417 check_sig_tyvars extra_tvs' sig_tvs
1420 :: TcTyVarSet -- Global type variables. The universally quantified
1421 -- tyvars should not mention any of these
1422 -- Guaranteed already zonked.
1423 -> [TcTyVar] -- Universally-quantified type variables in the signature
1424 -- Guaranteed to be skolems
1426 check_sig_tyvars extra_tvs []
1428 check_sig_tyvars extra_tvs sig_tvs
1429 = ASSERT( all isSkolemTyVar sig_tvs )
1430 do { gbl_tvs <- tcGetGlobalTyVars
1431 ; traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tvs,
1432 text "gbl_tvs" <+> ppr gbl_tvs,
1433 text "extra_tvs" <+> ppr extra_tvs]))
1435 ; let env_tvs = gbl_tvs `unionVarSet` extra_tvs
1436 ; ifM (any (`elemVarSet` env_tvs) sig_tvs)
1437 (bleatEscapedTvs env_tvs sig_tvs sig_tvs)
1440 bleatEscapedTvs :: TcTyVarSet -- The global tvs
1441 -> [TcTyVar] -- The possibly-escaping type variables
1442 -> [TcTyVar] -- The zonked versions thereof
1444 -- Complain about escaping type variables
1445 -- We pass a list of type variables, at least one of which
1446 -- escapes. The first list contains the original signature type variable,
1447 -- while the second contains the type variable it is unified to (usually itself)
1448 bleatEscapedTvs globals sig_tvs zonked_tvs
1449 = do { (env3, msgs) <- foldlM check (env2, []) (tidy_tvs `zip` tidy_zonked_tvs)
1450 ; failWithTcM (env3, main_msg $$ nest 2 (vcat msgs)) }
1452 (env1, tidy_tvs) = tidyOpenTyVars emptyTidyEnv sig_tvs
1453 (env2, tidy_zonked_tvs) = tidyOpenTyVars env1 zonked_tvs
1455 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1457 check (tidy_env, msgs) (sig_tv, zonked_tv)
1458 | not (zonked_tv `elemVarSet` globals) = return (tidy_env, msgs)
1460 = do { (tidy_env1, globs) <- findGlobals (unitVarSet zonked_tv) tidy_env
1461 ; returnM (tidy_env1, escape_msg sig_tv zonked_tv globs : msgs) }
1463 -----------------------
1464 escape_msg sig_tv zonked_tv globs
1466 = vcat [sep [msg, ptext SLIT("is mentioned in the environment:")],
1467 nest 2 (vcat globs)]
1469 = msg <+> ptext SLIT("escapes")
1470 -- Sigh. It's really hard to give a good error message
1471 -- all the time. One bad case is an existential pattern match.
1472 -- We rely on the "When..." context to help.
1474 msg = ptext SLIT("Quantified type variable") <+> quotes (ppr sig_tv) <+> is_bound_to
1476 | sig_tv == zonked_tv = empty
1477 | otherwise = ptext SLIT("is unified with") <+> quotes (ppr zonked_tv) <+> ptext SLIT("which")
1480 These two context are used with checkSigTyVars
1483 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1484 -> TidyEnv -> TcM (TidyEnv, Message)
1485 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1486 = zonkTcType sig_tau `thenM` \ actual_tau ->
1488 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1489 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1490 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1491 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1492 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1494 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),