2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section{Type subsumption and unification}
8 -- Full-blown subsumption
9 tcSubOff, tcSubExp, tcGen, subFunTy, TcHoleType,
10 checkSigTyVars, checkSigTyVarsWrt, sigCtxt,
12 -- Various unifications
13 unifyTauTy, unifyTauTyList, unifyTauTyLists,
14 unifyFunTy, unifyListTy, unifyPArrTy, unifyTupleTy,
15 unifyKind, unifyKinds, unifyOpenTypeKind,
18 Coercion, ExprCoFn, PatCoFn,
19 (<$>), (<.>), mkCoercion,
20 idCoercion, isIdCoercion
24 #include "HsVersions.h"
27 import HsSyn ( HsExpr(..) )
28 import TcHsSyn ( TypecheckedHsExpr, TcPat, mkHsLet )
29 import TypeRep ( Type(..), SourceType(..), TyNote(..),
30 openKindCon, typeCon )
32 import TcMonad -- TcType, amongst others
33 import TcType ( TcKind, TcType, TcSigmaType, TcRhoType, TcTyVar, TcTauType,
34 TcTyVarSet, TcThetaType, TyVarDetails(SigTv),
36 tcSplitAppTy_maybe, tcSplitTyConApp_maybe,
37 tcGetTyVar_maybe, tcGetTyVar,
38 mkTyConApp, mkFunTy, tyVarsOfType, mkPhiTy,
39 typeKind, tcSplitFunTy_maybe, mkForAllTys,
40 isHoleTyVar, isSkolemTyVar, isUserTyVar,
41 tidyOpenType, tidyOpenTypes, tidyOpenTyVar, tidyOpenTyVars,
42 eqKind, openTypeKind, liftedTypeKind, isTypeKind,
43 hasMoreBoxityInfo, tyVarBindingInfo, allDistinctTyVars
45 import qualified Type ( getTyVar_maybe )
46 import Inst ( LIE, emptyLIE, plusLIE,
47 newDicts, instToId, tcInstCall
49 import TcMType ( getTcTyVar, putTcTyVar, tcInstType, readHoleResult,
50 newTyVarTy, newTyVarTys, newBoxityVar, newHoleTyVarTy,
51 zonkTcType, zonkTcTyVars, zonkTcTyVarsAndFV, zonkTcTyVar )
52 import TcSimplify ( tcSimplifyCheck )
53 import TysWiredIn ( listTyCon, parrTyCon, mkListTy, mkPArrTy, mkTupleTy )
54 import TcEnv ( TcTyThing(..), tcGetGlobalTyVars, tcLEnvElts )
55 import TyCon ( tyConArity, isTupleTyCon, tupleTyConBoxity )
56 import PprType ( pprType )
57 import Id ( Id, mkSysLocal, idType )
58 import Var ( Var, varName, tyVarKind )
59 import VarSet ( emptyVarSet, unionVarSet, elemVarSet, varSetElems )
61 import Name ( isSystemName, getSrcLoc )
62 import ErrUtils ( Message )
63 import BasicTypes ( Boxity, Arity, isBoxed )
64 import Util ( equalLength )
65 import Maybe ( isNothing )
71 * A hole is always filled in with an ordinary type, not another hole.
73 %************************************************************************
75 \subsection{Subsumption}
77 %************************************************************************
79 All the tcSub calls have the form
81 tcSub expected_ty offered_ty
83 offered_ty <= expected_ty
85 That is, that a value of type offered_ty is acceptable in
86 a place expecting a value of type expected_ty.
88 It returns a coercion function
89 co_fn :: offered_ty -> expected_ty
90 which takes an HsExpr of type offered_ty into one of type
94 type TcHoleType = TcSigmaType -- Either a TcSigmaType,
97 tcSubExp :: TcHoleType -> TcSigmaType -> TcM (ExprCoFn, LIE)
98 tcSubOff :: TcSigmaType -> TcHoleType -> TcM (ExprCoFn, LIE)
99 tcSub :: TcSigmaType -> TcSigmaType -> TcM (ExprCoFn, LIE)
102 These two check for holes
105 tcSubExp expected_ty offered_ty
106 = checkHole expected_ty offered_ty tcSub
108 tcSubOff expected_ty offered_ty
109 = checkHole offered_ty expected_ty (\ off exp -> tcSub exp off)
111 -- checkHole looks for a hole in its first arg;
112 -- If so, and it is uninstantiated, it fills in the hole
113 -- with its second arg
114 -- Otherwise it calls thing_inside, passing the two args, looking
115 -- through any instantiated hole
117 checkHole (TyVarTy tv) other_ty thing_inside
119 = getTcTyVar tv `thenNF_Tc` \ maybe_ty ->
121 Just ty -> thing_inside ty other_ty
122 Nothing -> putTcTyVar tv other_ty `thenNF_Tc_`
123 returnTc (idCoercion, emptyLIE)
125 checkHole ty other_ty thing_inside
126 = thing_inside ty other_ty
129 No holes expected now. Add some error-check context info.
132 tcSub expected_ty actual_ty
133 = traceTc (text "tcSub" <+> details) `thenNF_Tc_`
134 tcAddErrCtxtM (unifyCtxt "type" expected_ty actual_ty)
135 (tc_sub expected_ty expected_ty actual_ty actual_ty)
137 details = vcat [text "Expected:" <+> ppr expected_ty,
138 text "Actual: " <+> ppr actual_ty]
141 tc_sub carries the types before and after expanding type synonyms
144 tc_sub :: TcSigmaType -- expected_ty, before expanding synonyms
145 -> TcSigmaType -- ..and after
146 -> TcSigmaType -- actual_ty, before
147 -> TcSigmaType -- ..and after
148 -> TcM (ExprCoFn, LIE)
150 -----------------------------------
152 tc_sub exp_sty (NoteTy _ exp_ty) act_sty act_ty = tc_sub exp_sty exp_ty act_sty act_ty
153 tc_sub exp_sty exp_ty act_sty (NoteTy _ act_ty) = tc_sub exp_sty exp_ty act_sty act_ty
155 -----------------------------------
156 -- Generalisation case
157 -- actual_ty: d:Eq b => b->b
158 -- expected_ty: forall a. Ord a => a->a
159 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
161 -- It is essential to do this *before* the specialisation case
162 -- Example: f :: (Eq a => a->a) -> ...
163 -- g :: Ord b => b->b
166 tc_sub exp_sty expected_ty act_sty actual_ty
167 | isSigmaTy expected_ty
168 = tcGen expected_ty (tyVarsOfType actual_ty) (
169 -- It's really important to check for escape wrt the free vars of
170 -- both expected_ty *and* actual_ty
171 \ body_exp_ty -> tc_sub body_exp_ty body_exp_ty act_sty actual_ty
172 ) `thenTc` \ (gen_fn, co_fn, lie) ->
173 returnTc (gen_fn <.> co_fn, lie)
175 -----------------------------------
176 -- Specialisation case:
177 -- actual_ty: forall a. Ord a => a->a
178 -- expected_ty: Int -> Int
179 -- co_fn e = e Int dOrdInt
181 tc_sub exp_sty expected_ty act_sty actual_ty
182 | isSigmaTy actual_ty
183 = tcInstCall Rank2Origin actual_ty `thenNF_Tc` \ (inst_fn, lie1, body_ty) ->
184 tc_sub exp_sty expected_ty body_ty body_ty `thenTc` \ (co_fn, lie2) ->
185 returnTc (co_fn <.> mkCoercion inst_fn, lie1 `plusLIE` lie2)
187 -----------------------------------
190 tc_sub _ (FunTy exp_arg exp_res) _ (FunTy act_arg act_res)
191 = tcSub_fun exp_arg exp_res act_arg act_res
193 -----------------------------------
194 -- Type variable meets function: imitate
196 -- NB 1: we can't just unify the type variable with the type
197 -- because the type might not be a tau-type, and we aren't
198 -- allowed to instantiate an ordinary type variable with
201 -- NB 2: can we short-cut to an error case?
202 -- when the arg/res is not a tau-type?
203 -- NO! e.g. f :: ((forall a. a->a) -> Int) -> Int
205 -- is perfectly fine!
207 tc_sub exp_sty exp_ty@(FunTy exp_arg exp_res) _ (TyVarTy tv)
208 = ASSERT( not (isHoleTyVar tv) )
209 getTcTyVar tv `thenNF_Tc` \ maybe_ty ->
211 Just ty -> tc_sub exp_sty exp_ty ty ty
212 Nothing -> imitateFun tv exp_sty `thenNF_Tc` \ (act_arg, act_res) ->
213 tcSub_fun exp_arg exp_res act_arg act_res
215 tc_sub _ (TyVarTy tv) act_sty act_ty@(FunTy act_arg act_res)
216 = ASSERT( not (isHoleTyVar tv) )
217 getTcTyVar tv `thenNF_Tc` \ maybe_ty ->
219 Just ty -> tc_sub ty ty act_sty act_ty
220 Nothing -> imitateFun tv act_sty `thenNF_Tc` \ (exp_arg, exp_res) ->
221 tcSub_fun exp_arg exp_res act_arg act_res
223 -----------------------------------
225 -- If none of the above match, we revert to the plain unifier
226 tc_sub exp_sty expected_ty act_sty actual_ty
227 = uTys exp_sty expected_ty act_sty actual_ty `thenTc_`
228 returnTc (idCoercion, emptyLIE)
231 %************************************************************************
233 \subsection{Functions}
235 %************************************************************************
238 tcSub_fun exp_arg exp_res act_arg act_res
239 = tc_sub act_arg act_arg exp_arg exp_arg `thenTc` \ (co_fn_arg, lie1) ->
240 tc_sub exp_res exp_res act_res act_res `thenTc` \ (co_fn_res, lie2) ->
241 tcGetUnique `thenNF_Tc` \ uniq ->
243 -- co_fn_arg :: HsExpr exp_arg -> HsExpr act_arg
244 -- co_fn_res :: HsExpr act_res -> HsExpr exp_res
245 -- co_fn :: HsExpr (act_arg -> act_res) -> HsExpr (exp_arg -> exp_res)
246 arg_id = mkSysLocal FSLIT("sub") uniq exp_arg
247 coercion | isIdCoercion co_fn_arg,
248 isIdCoercion co_fn_res = idCoercion
249 | otherwise = mkCoercion co_fn
251 co_fn e = DictLam [arg_id]
252 (co_fn_res <$> (HsApp e (co_fn_arg <$> (HsVar arg_id))))
253 -- Slight hack; using a "DictLam" to get an ordinary simple lambda
254 -- HsVar arg_id :: HsExpr exp_arg
255 -- co_fn_arg $it :: HsExpr act_arg
256 -- HsApp e $it :: HsExpr act_res
257 -- co_fn_res $it :: HsExpr exp_res
259 returnTc (coercion, lie1 `plusLIE` lie2)
261 imitateFun :: TcTyVar -> TcType -> NF_TcM (TcType, TcType)
263 = ASSERT( not (isHoleTyVar tv) )
264 -- NB: tv is an *ordinary* tyvar and so are the new ones
266 -- Check that tv isn't a type-signature type variable
267 -- (This would be found later in checkSigTyVars, but
268 -- we get a better error message if we do it here.)
269 checkTcM (not (isSkolemTyVar tv))
270 (failWithTcM (unifyWithSigErr tv ty)) `thenTc_`
272 newTyVarTy openTypeKind `thenNF_Tc` \ arg ->
273 newTyVarTy openTypeKind `thenNF_Tc` \ res ->
274 putTcTyVar tv (mkFunTy arg res) `thenNF_Tc_`
275 returnNF_Tc (arg,res)
279 %************************************************************************
281 \subsection{Generalisation}
283 %************************************************************************
286 tcGen :: TcSigmaType -- expected_ty
287 -> TcTyVarSet -- Extra tyvars that the universally
288 -- quantified tyvars of expected_ty
289 -- must not be unified
290 -> (TcRhoType -> TcM (result, LIE)) -- spec_ty
291 -> TcM (ExprCoFn, result, LIE)
292 -- The expression has type: spec_ty -> expected_ty
294 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
295 -- If not, the call is a no-op
296 = tcInstType SigTv expected_ty `thenNF_Tc` \ (forall_tvs, theta, phi_ty) ->
298 -- Type-check the arg and unify with poly type
299 thing_inside phi_ty `thenTc` \ (result, lie) ->
301 -- Check that the "forall_tvs" havn't been constrained
302 -- The interesting bit here is that we must include the free variables
303 -- of the expected_ty. Here's an example:
304 -- runST (newVar True)
305 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
306 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
307 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
308 -- So now s' isn't unconstrained because it's linked to a.
309 -- Conclusion: include the free vars of the expected_ty in the
310 -- list of "free vars" for the signature check.
312 newDicts SignatureOrigin theta `thenNF_Tc` \ dicts ->
313 tcSimplifyCheck sig_msg forall_tvs dicts lie `thenTc` \ (free_lie, inst_binds) ->
314 checkSigTyVarsWrt free_tvs forall_tvs `thenTc` \ zonked_tvs ->
317 -- This HsLet binds any Insts which came out of the simplification.
318 -- It's a bit out of place here, but using AbsBind involves inventing
319 -- a couple of new names which seems worse.
320 dict_ids = map instToId dicts
321 co_fn e = TyLam zonked_tvs (DictLam dict_ids (mkHsLet inst_binds e))
323 returnTc (mkCoercion co_fn, result, free_lie)
325 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
326 sig_msg = ptext SLIT("When generalising the type of an expression")
331 %************************************************************************
333 \subsection{Coercion functions}
335 %************************************************************************
338 type Coercion a = Maybe (a -> a)
339 -- Nothing => identity fn
341 type ExprCoFn = Coercion TypecheckedHsExpr
342 type PatCoFn = Coercion TcPat
344 (<.>) :: Coercion a -> Coercion a -> Coercion a -- Composition
345 Nothing <.> Nothing = Nothing
346 Nothing <.> Just f = Just f
347 Just f <.> Nothing = Just f
348 Just f1 <.> Just f2 = Just (f1 . f2)
350 (<$>) :: Coercion a -> a -> a
354 mkCoercion :: (a -> a) -> Coercion a
355 mkCoercion f = Just f
357 idCoercion :: Coercion a
360 isIdCoercion :: Coercion a -> Bool
361 isIdCoercion = isNothing
364 %************************************************************************
366 \subsection[Unify-exported]{Exported unification functions}
368 %************************************************************************
370 The exported functions are all defined as versions of some
371 non-exported generic functions.
373 Unify two @TauType@s. Dead straightforward.
376 unifyTauTy :: TcTauType -> TcTauType -> TcM ()
377 unifyTauTy ty1 ty2 -- ty1 expected, ty2 inferred
378 = -- The unifier should only ever see tau-types
379 -- (no quantification whatsoever)
380 ASSERT2( isTauTy ty1, ppr ty1 )
381 ASSERT2( isTauTy ty2, ppr ty2 )
382 tcAddErrCtxtM (unifyCtxt "type" ty1 ty2) $
386 @unifyTauTyList@ unifies corresponding elements of two lists of
387 @TauType@s. It uses @uTys@ to do the real work. The lists should be
388 of equal length. We charge down the list explicitly so that we can
389 complain if their lengths differ.
392 unifyTauTyLists :: [TcTauType] -> [TcTauType] -> TcM ()
393 unifyTauTyLists [] [] = returnTc ()
394 unifyTauTyLists (ty1:tys1) (ty2:tys2) = uTys ty1 ty1 ty2 ty2 `thenTc_`
395 unifyTauTyLists tys1 tys2
396 unifyTauTyLists ty1s ty2s = panic "Unify.unifyTauTyLists: mismatched type lists!"
399 @unifyTauTyList@ takes a single list of @TauType@s and unifies them
400 all together. It is used, for example, when typechecking explicit
401 lists, when all the elts should be of the same type.
404 unifyTauTyList :: [TcTauType] -> TcM ()
405 unifyTauTyList [] = returnTc ()
406 unifyTauTyList [ty] = returnTc ()
407 unifyTauTyList (ty1:tys@(ty2:_)) = unifyTauTy ty1 ty2 `thenTc_`
411 %************************************************************************
413 \subsection[Unify-uTys]{@uTys@: getting down to business}
415 %************************************************************************
417 @uTys@ is the heart of the unifier. Each arg happens twice, because
418 we want to report errors in terms of synomyms if poss. The first of
419 the pair is used in error messages only; it is always the same as the
420 second, except that if the first is a synonym then the second may be a
421 de-synonym'd version. This way we get better error messages.
423 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
426 uTys :: TcTauType -> TcTauType -- Error reporting ty1 and real ty1
427 -- ty1 is the *expected* type
429 -> TcTauType -> TcTauType -- Error reporting ty2 and real ty2
430 -- ty2 is the *actual* type
433 -- Always expand synonyms (see notes at end)
434 -- (this also throws away FTVs)
435 uTys ps_ty1 (NoteTy n1 ty1) ps_ty2 ty2 = uTys ps_ty1 ty1 ps_ty2 ty2
436 uTys ps_ty1 ty1 ps_ty2 (NoteTy n2 ty2) = uTys ps_ty1 ty1 ps_ty2 ty2
438 -- Variables; go for uVar
439 uTys ps_ty1 (TyVarTy tyvar1) ps_ty2 ty2 = uVar False tyvar1 ps_ty2 ty2
440 uTys ps_ty1 ty1 ps_ty2 (TyVarTy tyvar2) = uVar True tyvar2 ps_ty1 ty1
441 -- "True" means args swapped
444 uTys _ (SourceTy (IParam n1 t1)) _ (SourceTy (IParam n2 t2))
445 | n1 == n2 = uTys t1 t1 t2 t2
446 uTys _ (SourceTy (ClassP c1 tys1)) _ (SourceTy (ClassP c2 tys2))
447 | c1 == c2 = unifyTauTyLists tys1 tys2
448 uTys _ (SourceTy (NType tc1 tys1)) _ (SourceTy (NType tc2 tys2))
449 | tc1 == tc2 = unifyTauTyLists tys1 tys2
451 -- Functions; just check the two parts
452 uTys _ (FunTy fun1 arg1) _ (FunTy fun2 arg2)
453 = uTys fun1 fun1 fun2 fun2 `thenTc_` uTys arg1 arg1 arg2 arg2
455 -- Type constructors must match
456 uTys ps_ty1 (TyConApp con1 tys1) ps_ty2 (TyConApp con2 tys2)
457 | con1 == con2 && equalLength tys1 tys2
458 = unifyTauTyLists tys1 tys2
460 | con1 == openKindCon
461 -- When we are doing kind checking, we might match a kind '?'
462 -- against a kind '*' or '#'. Notably, CCallable :: ? -> *, and
463 -- (CCallable Int) and (CCallable Int#) are both OK
464 = unifyOpenTypeKind ps_ty2
466 -- Applications need a bit of care!
467 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
468 -- NB: we've already dealt with type variables and Notes,
469 -- so if one type is an App the other one jolly well better be too
470 uTys ps_ty1 (AppTy s1 t1) ps_ty2 ty2
471 = case tcSplitAppTy_maybe ty2 of
472 Just (s2,t2) -> uTys s1 s1 s2 s2 `thenTc_` uTys t1 t1 t2 t2
473 Nothing -> unifyMisMatch ps_ty1 ps_ty2
475 -- Now the same, but the other way round
476 -- Don't swap the types, because the error messages get worse
477 uTys ps_ty1 ty1 ps_ty2 (AppTy s2 t2)
478 = case tcSplitAppTy_maybe ty1 of
479 Just (s1,t1) -> uTys s1 s1 s2 s2 `thenTc_` uTys t1 t1 t2 t2
480 Nothing -> unifyMisMatch ps_ty1 ps_ty2
482 -- Not expecting for-alls in unification
483 -- ... but the error message from the unifyMisMatch more informative
484 -- than a panic message!
486 -- Anything else fails
487 uTys ps_ty1 ty1 ps_ty2 ty2 = unifyMisMatch ps_ty1 ps_ty2
493 If you are tempted to make a short cut on synonyms, as in this
497 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
498 -- NO = if (con1 == con2) then
499 -- NO -- Good news! Same synonym constructors, so we can shortcut
500 -- NO -- by unifying their arguments and ignoring their expansions.
501 -- NO unifyTauTypeLists args1 args2
503 -- NO -- Never mind. Just expand them and try again
507 then THINK AGAIN. Here is the whole story, as detected and reported
508 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
510 Here's a test program that should detect the problem:
514 x = (1 :: Bogus Char) :: Bogus Bool
517 The problem with [the attempted shortcut code] is that
521 is not a sufficient condition to be able to use the shortcut!
522 You also need to know that the type synonym actually USES all
523 its arguments. For example, consider the following type synonym
524 which does not use all its arguments.
529 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
530 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
531 would fail, even though the expanded forms (both \tr{Int}) should
534 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
535 unnecessarily bind \tr{t} to \tr{Char}.
537 ... You could explicitly test for the problem synonyms and mark them
538 somehow as needing expansion, perhaps also issuing a warning to the
543 %************************************************************************
545 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
547 %************************************************************************
549 @uVar@ is called when at least one of the types being unified is a
550 variable. It does {\em not} assume that the variable is a fixed point
551 of the substitution; rather, notice that @uVar@ (defined below) nips
552 back into @uTys@ if it turns out that the variable is already bound.
555 uVar :: Bool -- False => tyvar is the "expected"
556 -- True => ty is the "expected" thing
558 -> TcTauType -> TcTauType -- printing and real versions
561 uVar swapped tv1 ps_ty2 ty2
562 = traceTc (text "uVar" <+> ppr swapped <+> ppr tv1 <+> (ppr ps_ty2 $$ ppr ty2)) `thenNF_Tc_`
563 getTcTyVar tv1 `thenNF_Tc` \ maybe_ty1 ->
565 Just ty1 | swapped -> uTys ps_ty2 ty2 ty1 ty1 -- Swap back
566 | otherwise -> uTys ty1 ty1 ps_ty2 ty2 -- Same order
567 other -> uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2
569 -- Expand synonyms; ignore FTVs
570 uUnboundVar swapped tv1 maybe_ty1 ps_ty2 (NoteTy n2 ty2)
571 = uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2
574 -- The both-type-variable case
575 uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2@(TyVarTy tv2)
577 -- Same type variable => no-op
581 -- Distinct type variables
582 -- ASSERT maybe_ty1 /= Just
584 = getTcTyVar tv2 `thenNF_Tc` \ maybe_ty2 ->
586 Just ty2' -> uUnboundVar swapped tv1 maybe_ty1 ty2' ty2'
590 -> WARN( not (k1 `hasMoreBoxityInfo` k2), (ppr tv1 <+> ppr k1) $$ (ppr tv2 <+> ppr k2) )
591 putTcTyVar tv2 (TyVarTy tv1) `thenNF_Tc_`
595 -> WARN( not (k2 `hasMoreBoxityInfo` k1), (ppr tv2 <+> ppr k2) $$ (ppr tv1 <+> ppr k1) )
596 putTcTyVar tv1 ps_ty2 `thenNF_Tc_`
601 update_tv2 = (k2 `eqKind` openTypeKind) || (not (k1 `eqKind` openTypeKind) && nicer_to_update_tv2)
602 -- Try to get rid of open type variables as soon as poss
604 nicer_to_update_tv2 = isUserTyVar tv1
605 -- Don't unify a signature type variable if poss
606 || isSystemName (varName tv2)
607 -- Try to update sys-y type variables in preference to sig-y ones
609 -- Second one isn't a type variable
610 uUnboundVar swapped tv1 maybe_ty1 ps_ty2 non_var_ty2
611 = -- Check that tv1 isn't a type-signature type variable
612 checkTcM (not (isSkolemTyVar tv1))
613 (failWithTcM (unifyWithSigErr tv1 ps_ty2)) `thenTc_`
615 -- Do the occurs check, and check that we are not
616 -- unifying a type variable with a polytype
617 -- Returns a zonked type ready for the update
618 checkValue tv1 ps_ty2 non_var_ty2 `thenTc` \ ty2 ->
620 -- Check that the kinds match
621 checkKinds swapped tv1 ty2 `thenTc_`
623 -- Perform the update
624 putTcTyVar tv1 ty2 `thenNF_Tc_`
629 checkKinds swapped tv1 ty2
630 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
631 -- ty2 has been zonked at this stage, which ensures that
632 -- its kind has as much boxity information visible as possible.
633 | tk2 `hasMoreBoxityInfo` tk1 = returnTc ()
636 -- Either the kinds aren't compatible
637 -- (can happen if we unify (a b) with (c d))
638 -- or we are unifying a lifted type variable with an
639 -- unlifted type: e.g. (id 3#) is illegal
640 = tcAddErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
644 (k1,k2) | swapped = (tk2,tk1)
645 | otherwise = (tk1,tk2)
651 checkValue tv1 ps_ty2 non_var_ty2
652 -- Do the occurs check, and check that we are not
653 -- unifying a type variable with a polytype
654 -- Return the type to update the type variable with, or fail
656 -- Basically we want to update tv1 := ps_ty2
657 -- because ps_ty2 has type-synonym info, which improves later error messages
662 -- f :: (A a -> a -> ()) -> ()
666 -- x = f (\ x p -> p x)
668 -- In the application (p x), we try to match "t" with "A t". If we go
669 -- ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
670 -- an infinite loop later.
671 -- But we should not reject the program, because A t = ().
672 -- Rather, we should bind t to () (= non_var_ty2).
674 -- That's why we have this two-state occurs-check
675 = zonkTcType ps_ty2 `thenNF_Tc` \ ps_ty2' ->
676 case okToUnifyWith tv1 ps_ty2' of {
677 Nothing -> returnTc ps_ty2' ; -- Success
680 zonkTcType non_var_ty2 `thenNF_Tc` \ non_var_ty2' ->
681 case okToUnifyWith tv1 non_var_ty2' of
682 Nothing -> -- This branch rarely succeeds, except in strange cases
683 -- like that in the example above
684 returnTc non_var_ty2'
686 Just problem -> failWithTcM (unifyCheck problem tv1 ps_ty2')
689 data Problem = OccurCheck | NotMonoType
691 okToUnifyWith :: TcTyVar -> TcType -> Maybe Problem
692 -- (okToUnifyWith tv ty) checks whether it's ok to unify
695 -- Just p => not ok, problem p
700 ok (TyVarTy tv') | tv == tv' = Just OccurCheck
701 | otherwise = Nothing
702 ok (AppTy t1 t2) = ok t1 `and` ok t2
703 ok (FunTy t1 t2) = ok t1 `and` ok t2
704 ok (TyConApp _ ts) = oks ts
705 ok (ForAllTy _ _) = Just NotMonoType
706 ok (SourceTy st) = ok_st st
707 ok (NoteTy (FTVNote _) t) = ok t
708 ok (NoteTy (SynNote t1) t2) = ok t1 `and` ok t2
709 -- Type variables may be free in t1 but not t2
710 -- A forall may be in t2 but not t1
712 oks ts = foldr (and . ok) Nothing ts
714 ok_st (ClassP _ ts) = oks ts
715 ok_st (IParam _ t) = ok t
716 ok_st (NType _ ts) = oks ts
719 Just p `and` m = Just p
722 %************************************************************************
724 \subsection[Unify-fun]{@unifyFunTy@}
726 %************************************************************************
728 @subFunTy@ and @unifyFunTy@ is used to avoid the fruitless
729 creation of type variables.
731 * subFunTy is used when we might be faced with a "hole" type variable,
732 in which case we should create two new holes.
734 * unifyFunTy is used when we expect to encounter only "ordinary"
735 type variables, so we should create new ordinary type variables
738 subFunTy :: TcHoleType -- Fail if ty isn't a function type
739 -- If it's a hole, make two holes, feed them to...
740 -> (TcHoleType -> TcHoleType -> TcM a) -- the thing inside
741 -> TcM a -- and bind the function type to the hole
743 subFunTy ty@(TyVarTy tyvar) thing_inside
745 = -- This is the interesting case
746 getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
748 Just ty' -> subFunTy ty' thing_inside ;
751 newHoleTyVarTy `thenNF_Tc` \ arg_ty ->
752 newHoleTyVarTy `thenNF_Tc` \ res_ty ->
755 thing_inside arg_ty res_ty `thenTc` \ answer ->
757 -- Extract the answers
758 readHoleResult arg_ty `thenNF_Tc` \ arg_ty' ->
759 readHoleResult res_ty `thenNF_Tc` \ res_ty' ->
761 -- Write the answer into the incoming hole
762 putTcTyVar tyvar (mkFunTy arg_ty' res_ty') `thenNF_Tc_`
764 -- And return the answer
767 subFunTy ty thing_inside
768 = unifyFunTy ty `thenTc` \ (arg,res) ->
772 unifyFunTy :: TcRhoType -- Fail if ty isn't a function type
773 -> TcM (TcType, TcType) -- otherwise return arg and result types
775 unifyFunTy ty@(TyVarTy tyvar)
776 = ASSERT( not (isHoleTyVar tyvar) )
777 getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
779 Just ty' -> unifyFunTy ty'
780 Nothing -> unify_fun_ty_help ty
783 = case tcSplitFunTy_maybe ty of
784 Just arg_and_res -> returnTc arg_and_res
785 Nothing -> unify_fun_ty_help ty
787 unify_fun_ty_help ty -- Special cases failed, so revert to ordinary unification
788 = newTyVarTy openTypeKind `thenNF_Tc` \ arg ->
789 newTyVarTy openTypeKind `thenNF_Tc` \ res ->
790 unifyTauTy ty (mkFunTy arg res) `thenTc_`
795 unifyListTy :: TcType -- expected list type
796 -> TcM TcType -- list element type
798 unifyListTy ty@(TyVarTy tyvar)
799 = getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
801 Just ty' -> unifyListTy ty'
802 other -> unify_list_ty_help ty
805 = case tcSplitTyConApp_maybe ty of
806 Just (tycon, [arg_ty]) | tycon == listTyCon -> returnTc arg_ty
807 other -> unify_list_ty_help ty
809 unify_list_ty_help ty -- Revert to ordinary unification
810 = newTyVarTy liftedTypeKind `thenNF_Tc` \ elt_ty ->
811 unifyTauTy ty (mkListTy elt_ty) `thenTc_`
814 -- variant for parallel arrays
816 unifyPArrTy :: TcType -- expected list type
817 -> TcM TcType -- list element type
819 unifyPArrTy ty@(TyVarTy tyvar)
820 = getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
822 Just ty' -> unifyPArrTy ty'
823 _ -> unify_parr_ty_help ty
825 = case tcSplitTyConApp_maybe ty of
826 Just (tycon, [arg_ty]) | tycon == parrTyCon -> returnTc arg_ty
827 _ -> unify_parr_ty_help ty
829 unify_parr_ty_help ty -- Revert to ordinary unification
830 = newTyVarTy liftedTypeKind `thenNF_Tc` \ elt_ty ->
831 unifyTauTy ty (mkPArrTy elt_ty) `thenTc_`
836 unifyTupleTy :: Boxity -> Arity -> TcType -> TcM [TcType]
837 unifyTupleTy boxity arity ty@(TyVarTy tyvar)
838 = getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
840 Just ty' -> unifyTupleTy boxity arity ty'
841 other -> unify_tuple_ty_help boxity arity ty
843 unifyTupleTy boxity arity ty
844 = case tcSplitTyConApp_maybe ty of
845 Just (tycon, arg_tys)
847 && tyConArity tycon == arity
848 && tupleTyConBoxity tycon == boxity
850 other -> unify_tuple_ty_help boxity arity ty
852 unify_tuple_ty_help boxity arity ty
853 = newTyVarTys arity kind `thenNF_Tc` \ arg_tys ->
854 unifyTauTy ty (mkTupleTy boxity arity arg_tys) `thenTc_`
857 kind | isBoxed boxity = liftedTypeKind
858 | otherwise = openTypeKind
862 %************************************************************************
864 \subsection{Kind unification}
866 %************************************************************************
869 unifyKind :: TcKind -- Expected
873 = tcAddErrCtxtM (unifyCtxt "kind" k1 k2) $
876 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
877 unifyKinds [] [] = returnTc ()
878 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenTc_`
880 unifyKinds _ _ = panic "unifyKinds: length mis-match"
884 unifyOpenTypeKind :: TcKind -> TcM ()
885 -- Ensures that the argument kind is of the form (Type bx)
886 -- for some boxity bx
888 unifyOpenTypeKind ty@(TyVarTy tyvar)
889 = getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
891 Just ty' -> unifyOpenTypeKind ty'
892 other -> unify_open_kind_help ty
895 | isTypeKind ty = returnTc ()
896 | otherwise = unify_open_kind_help ty
898 unify_open_kind_help ty -- Revert to ordinary unification
899 = newBoxityVar `thenNF_Tc` \ boxity ->
900 unifyKind ty (mkTyConApp typeCon [boxity])
904 %************************************************************************
906 \subsection[Unify-context]{Errors and contexts}
908 %************************************************************************
914 unifyCtxt s ty1 ty2 tidy_env -- ty1 expected, ty2 inferred
915 = zonkTcType ty1 `thenNF_Tc` \ ty1' ->
916 zonkTcType ty2 `thenNF_Tc` \ ty2' ->
917 returnNF_Tc (err ty1' ty2')
922 text "Expected" <+> text s <> colon <+> ppr tidy_ty1,
923 text "Inferred" <+> text s <> colon <+> ppr tidy_ty2
926 (env1, [tidy_ty1,tidy_ty2]) = tidyOpenTypes tidy_env [ty1,ty2]
928 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
929 -- tv1 is zonked already
930 = zonkTcType ty2 `thenNF_Tc` \ ty2' ->
931 returnNF_Tc (err ty2')
933 err ty2 = (env2, ptext SLIT("When matching types") <+>
934 sep [quotes pp_expected, ptext SLIT("and"), quotes pp_actual])
936 (pp_expected, pp_actual) | swapped = (pp2, pp1)
937 | otherwise = (pp1, pp2)
938 (env1, tv1') = tidyOpenTyVar tidy_env tv1
939 (env2, ty2') = tidyOpenType env1 ty2
943 unifyMisMatch ty1 ty2
944 = zonkTcType ty1 `thenNF_Tc` \ ty1' ->
945 zonkTcType ty2 `thenNF_Tc` \ ty2' ->
947 (env, [tidy_ty1, tidy_ty2]) = tidyOpenTypes emptyTidyEnv [ty1',ty2']
948 msg = hang (ptext SLIT("Couldn't match"))
949 4 (sep [quotes (ppr tidy_ty1),
950 ptext SLIT("against"),
951 quotes (ppr tidy_ty2)])
953 failWithTcM (env, msg)
955 unifyWithSigErr tyvar ty
956 = (env2, hang (ptext SLIT("Cannot unify the type-signature variable") <+> quotes (ppr tidy_tyvar))
957 4 (ptext SLIT("with the type") <+> quotes (ppr tidy_ty)))
959 (env1, tidy_tyvar) = tidyOpenTyVar emptyTidyEnv tyvar
960 (env2, tidy_ty) = tidyOpenType env1 ty
962 unifyCheck problem tyvar ty
964 4 (sep [ppr tidy_tyvar, char '=', ppr tidy_ty]))
966 (env1, tidy_tyvar) = tidyOpenTyVar emptyTidyEnv tyvar
967 (env2, tidy_ty) = tidyOpenType env1 ty
969 msg = case problem of
970 OccurCheck -> ptext SLIT("Occurs check: cannot construct the infinite type:")
971 NotMonoType -> ptext SLIT("Cannot unify a type variable with a type scheme:")
976 %************************************************************************
978 \subsection{Checking signature type variables}
980 %************************************************************************
982 @checkSigTyVars@ is used after the type in a type signature has been unified with
983 the actual type found. It then checks that the type variables of the type signature
985 (a) Still all type variables
986 eg matching signature [a] against inferred type [(p,q)]
987 [then a will be unified to a non-type variable]
989 (b) Still all distinct
990 eg matching signature [(a,b)] against inferred type [(p,p)]
991 [then a and b will be unified together]
993 (c) Not mentioned in the environment
994 eg the signature for f in this:
1000 Here, f is forced to be monorphic by the free occurence of x.
1002 (d) Not (unified with another type variable that is) in scope.
1003 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1004 when checking the expression type signature, we find that
1005 even though there is nothing in scope whose type mentions r,
1006 nevertheless the type signature for the expression isn't right.
1008 Another example is in a class or instance declaration:
1010 op :: forall b. a -> b
1012 Here, b gets unified with a
1014 Before doing this, the substitution is applied to the signature type variable.
1016 We used to have the notion of a "DontBind" type variable, which would
1017 only be bound to itself or nothing. Then points (a) and (b) were
1018 self-checking. But it gave rise to bogus consequential error messages.
1021 f = (*) -- Monomorphic
1023 g :: Num a => a -> a
1026 Here, we get a complaint when checking the type signature for g,
1027 that g isn't polymorphic enough; but then we get another one when
1028 dealing with the (Num x) context arising from f's definition;
1029 we try to unify x with Int (to default it), but find that x has already
1030 been unified with the DontBind variable "a" from g's signature.
1031 This is really a problem with side-effecting unification; we'd like to
1032 undo g's effects when its type signature fails, but unification is done
1033 by side effect, so we can't (easily).
1035 So we revert to ordinary type variables for signatures, and try to
1036 give a helpful message in checkSigTyVars.
1039 checkSigTyVars :: [TcTyVar] -> TcM [TcTyVar]
1040 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1042 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM [TcTyVar]
1043 checkSigTyVarsWrt extra_tvs sig_tvs
1044 = zonkTcTyVarsAndFV (varSetElems extra_tvs) `thenNF_Tc` \ extra_tvs' ->
1045 check_sig_tyvars extra_tvs' sig_tvs
1048 :: TcTyVarSet -- Global type variables. The universally quantified
1049 -- tyvars should not mention any of these
1050 -- Guaranteed already zonked.
1051 -> [TcTyVar] -- Universally-quantified type variables in the signature
1052 -- Not guaranteed zonked.
1053 -> TcM [TcTyVar] -- Zonked signature type variables
1055 check_sig_tyvars extra_tvs []
1057 check_sig_tyvars extra_tvs sig_tvs
1058 = zonkTcTyVars sig_tvs `thenNF_Tc` \ sig_tys ->
1059 tcGetGlobalTyVars `thenNF_Tc` \ gbl_tvs ->
1061 env_tvs = gbl_tvs `unionVarSet` extra_tvs
1063 checkTcM (allDistinctTyVars sig_tys env_tvs)
1064 (complain sig_tys env_tvs) `thenTc_`
1066 returnTc (map (tcGetTyVar "checkSigTyVars") sig_tys)
1069 complain sig_tys globals
1070 = -- "check" checks each sig tyvar in turn
1072 (env2, emptyVarEnv, [])
1073 (tidy_tvs `zip` tidy_tys) `thenNF_Tc` \ (env3, _, msgs) ->
1075 failWithTcM (env3, main_msg $$ nest 4 (vcat msgs))
1077 (env1, tidy_tvs) = tidyOpenTyVars emptyTidyEnv sig_tvs
1078 (env2, tidy_tys) = tidyOpenTypes env1 sig_tys
1080 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1082 check (tidy_env, acc, msgs) (sig_tyvar,ty)
1083 -- sig_tyvar is from the signature;
1084 -- ty is what you get if you zonk sig_tyvar and then tidy it
1086 -- acc maps a zonked type variable back to a signature type variable
1087 = case tcGetTyVar_maybe ty of {
1088 Nothing -> -- Error (a)!
1089 returnNF_Tc (tidy_env, acc, unify_msg sig_tyvar (quotes (ppr ty)) : msgs) ;
1093 case lookupVarEnv acc tv of {
1094 Just sig_tyvar' -> -- Error (b)!
1095 returnNF_Tc (tidy_env, acc, unify_msg sig_tyvar thing : msgs)
1097 thing = ptext SLIT("another quantified type variable") <+> quotes (ppr sig_tyvar')
1101 if tv `elemVarSet` globals -- Error (c) or (d)! Type variable escapes
1102 -- The least comprehensible, so put it last
1104 -- get the local TcIds and TyVars from the environment,
1105 -- and pass them to find_globals (they might have tv free)
1106 then tcGetEnv `thenNF_Tc` \ ve ->
1107 find_globals tv tidy_env (tcLEnvElts ve) `thenNF_Tc` \ (tidy_env1, globs) ->
1108 returnNF_Tc (tidy_env1, acc, escape_msg sig_tyvar tv globs : msgs)
1111 returnNF_Tc (tidy_env, extendVarEnv acc tv sig_tyvar, msgs)
1117 -----------------------
1118 -- find_globals looks at the value environment and finds values
1119 -- whose types mention the offending type variable. It has to be
1120 -- careful to zonk the Id's type first, so it has to be in the monad.
1121 -- We must be careful to pass it a zonked type variable, too.
1126 -> NF_TcM (TidyEnv, [SDoc])
1128 find_globals tv tidy_env things
1129 = go tidy_env [] things
1131 go tidy_env acc [] = returnNF_Tc (tidy_env, acc)
1132 go tidy_env acc (thing : things)
1133 = find_thing ignore_it tidy_env thing `thenNF_Tc` \ (tidy_env1, maybe_doc) ->
1135 Just d -> go tidy_env1 (d:acc) things
1136 Nothing -> go tidy_env1 acc things
1138 ignore_it ty = not (tv `elemVarSet` tyVarsOfType ty)
1140 -----------------------
1141 find_thing ignore_it tidy_env (ATcId id)
1142 = zonkTcType (idType id) `thenNF_Tc` \ id_ty ->
1143 if ignore_it id_ty then
1144 returnNF_Tc (tidy_env, Nothing)
1146 (tidy_env', tidy_ty) = tidyOpenType tidy_env id_ty
1147 msg = sep [ppr id <+> dcolon <+> ppr tidy_ty,
1148 nest 2 (parens (ptext SLIT("bound at") <+>
1149 ppr (getSrcLoc id)))]
1151 returnNF_Tc (tidy_env', Just msg)
1153 find_thing ignore_it tidy_env (ATyVar tv)
1154 = zonkTcTyVar tv `thenNF_Tc` \ tv_ty ->
1155 if ignore_it tv_ty then
1156 returnNF_Tc (tidy_env, Nothing)
1158 (tidy_env1, tv1) = tidyOpenTyVar tidy_env tv
1159 (tidy_env2, tidy_ty) = tidyOpenType tidy_env1 tv_ty
1160 msg = sep [ppr tv1 <+> eq_stuff, nest 2 bound_at]
1162 eq_stuff | Just tv' <- Type.getTyVar_maybe tv_ty, tv == tv' = empty
1163 | otherwise = equals <+> ppr tv_ty
1164 -- It's ok to use Type.getTyVar_maybe because ty is zonked by now
1166 bound_at = tyVarBindingInfo tv
1168 returnNF_Tc (tidy_env2, Just msg)
1170 -----------------------
1171 escape_msg sig_tv tv globs
1172 = mk_msg sig_tv <+> ptext SLIT("escapes") $$
1173 if not (null globs) then
1174 vcat [pp_it <+> ptext SLIT("is mentioned in the environment:"),
1175 nest 2 (vcat globs)]
1177 empty -- Sigh. It's really hard to give a good error message
1178 -- all the time. One bad case is an existential pattern match.
1179 -- We rely on the "When..." context to help.
1181 pp_it | sig_tv /= tv = ptext SLIT("It unifies with") <+> quotes (ppr tv) <> comma <+> ptext SLIT("which")
1182 | otherwise = ptext SLIT("It")
1185 unify_msg tv thing = mk_msg tv <+> ptext SLIT("is unified with") <+> thing
1186 mk_msg tv = ptext SLIT("Quantified type variable") <+> quotes (ppr tv)
1189 These two context are used with checkSigTyVars
1192 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1193 -> TidyEnv -> NF_TcM (TidyEnv, Message)
1194 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1195 = zonkTcType sig_tau `thenNF_Tc` \ actual_tau ->
1197 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1198 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1199 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1200 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1201 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1203 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),
1206 returnNF_Tc (env3, msg)