2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section{Type subsumption and unification}
8 -- Full-blown subsumption
9 tcSub, tcGen, subFunTy,
10 checkSigTyVars, sigCtxt, sigPatCtxt,
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,
29 mkHsDictApp, mkHsTyApp, mkHsLet )
30 import TypeRep ( Type(..), SourceType(..), TyNote(..),
31 openKindCon, typeCon )
33 import TcMonad -- TcType, amongst others
34 import TcType ( TcKind, TcType, TcSigmaType, TcPhiType, TcTyVar, TcTauType,
35 TcTyVarSet, TcThetaType,
37 tcSplitAppTy_maybe, tcSplitTyConApp_maybe,
38 tcGetTyVar_maybe, tcGetTyVar,
39 mkTyConApp, mkTyVarTys, mkFunTy, tyVarsOfType, mkRhoTy,
40 typeKind, tcSplitFunTy_maybe, mkForAllTys,
41 isHoleTyVar, isSkolemTyVar, isUserTyVar, allDistinctTyVars,
42 tidyOpenType, tidyOpenTypes, tidyOpenTyVar, tidyOpenTyVars,
43 eqKind, openTypeKind, liftedTypeKind, isTypeKind,
44 hasMoreBoxityInfo, tyVarBindingInfo
46 import qualified Type ( getTyVar_maybe )
47 import Inst ( LIE, emptyLIE, plusLIE, mkLIE,
50 import TcMType ( getTcTyVar, putTcTyVar, tcInstType,
51 newTyVarTy, newTyVarTys, newBoxityVar, newHoleTyVarTy,
52 zonkTcType, zonkTcTyVars, zonkTcTyVar )
53 import TcSimplify ( tcSimplifyCheck )
54 import TysWiredIn ( listTyCon, parrTyCon, mkListTy, mkPArrTy, mkTupleTy )
55 import TcEnv ( TcTyThing(..), tcExtendGlobalTyVars, tcGetGlobalTyVars, tcLEnvElts )
56 import TyCon ( tyConArity, isTupleTyCon, tupleTyConBoxity )
57 import PprType ( pprType )
58 import CoreFVs ( idFreeTyVars )
59 import Id ( mkSysLocal, idType )
60 import Var ( Var, varName, tyVarKind )
61 import VarSet ( elemVarSet, varSetElems )
63 import Name ( isSystemName, getSrcLoc )
64 import ErrUtils ( Message )
65 import BasicTypes ( Boxity, Arity, isBoxed )
66 import Util ( isSingleton, equalLength )
67 import Maybe ( isNothing )
72 %************************************************************************
74 \subsection{Subsumption}
76 %************************************************************************
79 tcSub :: TcSigmaType -- expected_ty; can be a type scheme;
80 -- can be a "hole" type variable
81 -> TcSigmaType -- actual_ty; can be a type scheme
82 -> TcM (ExprCoFn, LIE)
85 (tcSub expected_ty actual_ty) checks that
86 actual_ty <= expected_ty
87 That is, that a value of type actual_ty is acceptable in
88 a place expecting a value of type expected_ty.
90 It returns a coercion function
91 co_fn :: actual_ty -> expected_ty
92 which takes an HsExpr of type actual_ty into one of type
96 tcSub expected_ty actual_ty
97 = traceTc (text "tcSub" <+> details) `thenNF_Tc_`
98 tcAddErrCtxtM (unifyCtxt "type" expected_ty actual_ty)
99 (tc_sub expected_ty expected_ty actual_ty actual_ty)
101 details = vcat [text "Expected:" <+> ppr expected_ty,
102 text "Actual: " <+> ppr actual_ty]
105 tc_sub carries the types before and after expanding type synonyms
108 tc_sub :: TcSigmaType -- expected_ty, before expanding synonyms
109 -> TcSigmaType -- ..and after
110 -> TcSigmaType -- actual_ty, before
111 -> TcSigmaType -- ..and after
112 -> TcM (ExprCoFn, LIE)
114 -----------------------------------
116 tc_sub exp_sty (NoteTy _ exp_ty) act_sty act_ty = tc_sub exp_sty exp_ty act_sty act_ty
117 tc_sub exp_sty exp_ty act_sty (NoteTy _ act_ty) = tc_sub exp_sty exp_ty act_sty act_ty
119 -----------------------------------
120 -- "Hole type variable" case
121 -- Do this case before unwrapping for-alls in the actual_ty
123 tc_sub _ (TyVarTy tv) act_sty act_ty
125 = -- It's a "hole" type variable
126 getTcTyVar tv `thenNF_Tc` \ maybe_ty ->
129 Just ty -> -- Already been assigned
130 tc_sub ty ty act_sty act_ty ;
132 Nothing -> -- Assign it
133 putTcTyVar tv act_sty `thenNF_Tc_`
134 returnTc (idCoercion, emptyLIE)
137 -----------------------------------
138 -- Generalisation case
139 -- actual_ty: d:Eq b => b->b
140 -- expected_ty: forall a. Ord a => a->a
141 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
143 -- It is essential to do this *before* the specialisation case
144 -- Example: f :: (Eq a => a->a) -> ...
145 -- g :: Ord b => b->b
148 tc_sub exp_sty expected_ty act_sty actual_ty
149 | isSigmaTy expected_ty
150 = tcGen expected_ty (
151 \ body_exp_ty -> tc_sub body_exp_ty body_exp_ty act_sty actual_ty
152 ) `thenTc` \ (gen_fn, co_fn, lie) ->
153 returnTc (gen_fn <.> co_fn, lie)
155 -----------------------------------
156 -- Specialisation case:
157 -- actual_ty: forall a. Ord a => a->a
158 -- expected_ty: Int -> Int
159 -- co_fn e = e Int dOrdInt
161 tc_sub exp_sty expected_ty act_sty actual_ty
162 | isSigmaTy actual_ty
163 = tcInstType actual_ty `thenNF_Tc` \ (tvs, theta, body_ty) ->
164 newDicts orig theta `thenNF_Tc` \ dicts ->
166 inst_fn e = mkHsDictApp (mkHsTyApp e (mkTyVarTys tvs))
169 tc_sub exp_sty expected_ty body_ty body_ty `thenTc` \ (co_fn, lie) ->
170 returnTc (co_fn <.> mkCoercion inst_fn, lie `plusLIE` mkLIE dicts)
174 -----------------------------------
177 tc_sub _ (FunTy exp_arg exp_res) _ (FunTy act_arg act_res)
178 = tcSub_fun exp_arg exp_res act_arg act_res
180 -----------------------------------
181 -- Type variable meets function: imitate
183 -- NB 1: we can't just unify the type variable with the type
184 -- because the type might not be a tau-type, and we aren't
185 -- allowed to instantiate an ordinary type variable with
188 -- NB 2: can we short-cut to an error case?
189 -- when the arg/res is not a tau-type?
190 -- NO! e.g. f :: ((forall a. a->a) -> Int) -> Int
192 -- is perfectly fine!
194 tc_sub exp_sty exp_ty@(FunTy exp_arg exp_res) _ (TyVarTy tv)
195 = getTcTyVar tv `thenNF_Tc` \ maybe_ty ->
197 Just ty -> tc_sub exp_sty exp_ty ty ty
198 Nothing -> imitateFun tv exp_sty `thenNF_Tc` \ (act_arg, act_res) ->
199 tcSub_fun exp_arg exp_res act_arg act_res
201 tc_sub _ (TyVarTy tv) act_sty act_ty@(FunTy act_arg act_res)
202 = getTcTyVar tv `thenNF_Tc` \ maybe_ty ->
204 Just ty -> tc_sub ty ty act_sty act_ty
205 Nothing -> imitateFun tv act_sty `thenNF_Tc` \ (exp_arg, exp_res) ->
206 tcSub_fun exp_arg exp_res act_arg act_res
208 -----------------------------------
210 -- If none of the above match, we revert to the plain unifier
211 tc_sub exp_sty expected_ty act_sty actual_ty
212 = uTys exp_sty expected_ty act_sty actual_ty `thenTc_`
213 returnTc (idCoercion, emptyLIE)
216 %************************************************************************
218 \subsection{Functions}
220 %************************************************************************
223 tcSub_fun exp_arg exp_res act_arg act_res
224 = tcSub act_arg exp_arg `thenTc` \ (co_fn_arg, lie1) ->
225 tcSub exp_res act_res `thenTc` \ (co_fn_res, lie2) ->
226 tcGetUnique `thenNF_Tc` \ uniq ->
228 -- co_fn_arg :: HsExpr exp_arg -> HsExpr act_arg
229 -- co_fn_res :: HsExpr act_res -> HsExpr exp_res
230 -- co_fn :: HsExpr (act_arg -> act_res) -> HsExpr (exp_arg -> exp_res)
231 arg_id = mkSysLocal SLIT("sub") uniq exp_arg
232 coercion | isIdCoercion co_fn_arg,
233 isIdCoercion co_fn_res = idCoercion
234 | otherwise = mkCoercion co_fn
236 co_fn e = DictLam [arg_id]
237 (co_fn_res <$> (HsApp e (co_fn_arg <$> (HsVar arg_id))))
238 -- Slight hack; using a "DictLam" to get an ordinary simple lambda
239 -- HsVar arg_id :: HsExpr exp_arg
240 -- co_fn_arg $it :: HsExpr act_arg
241 -- HsApp e $it :: HsExpr act_res
242 -- co_fn_res $it :: HsExpr exp_res
244 returnTc (coercion, lie1 `plusLIE` lie2)
246 imitateFun :: TcTyVar -> TcType -> NF_TcM (TcType, TcType)
248 = ASSERT( not (isHoleTyVar tv) )
249 -- NB: tv is an *ordinary* tyvar and so are the new ones
251 -- Check that tv isn't a type-signature type variable
252 -- (This would be found later in checkSigTyVars, but
253 -- we get a better error message if we do it here.)
254 checkTcM (not (isSkolemTyVar tv))
255 (failWithTcM (unifyWithSigErr tv ty)) `thenTc_`
257 newTyVarTy openTypeKind `thenNF_Tc` \ arg ->
258 newTyVarTy openTypeKind `thenNF_Tc` \ res ->
259 putTcTyVar tv (mkFunTy arg res) `thenNF_Tc_`
260 returnNF_Tc (arg,res)
264 %************************************************************************
266 \subsection{Generalisation}
268 %************************************************************************
271 tcGen :: TcSigmaType -- expected_ty
272 -> (TcPhiType -> TcM (result, LIE)) -- spec_ty
273 -> TcM (ExprCoFn, result, LIE)
274 -- The expression has type: spec_ty -> expected_ty
276 tcGen expected_ty thing_inside -- We expect expected_ty to be a forall-type
277 -- If not, the call is a no-op
278 = tcInstType expected_ty `thenNF_Tc` \ (forall_tvs, theta, phi_ty) ->
280 -- Type-check the arg and unify with poly type
281 thing_inside phi_ty `thenTc` \ (result, lie) ->
283 -- Check that the "forall_tvs" havn't been constrained
284 -- The interesting bit here is that we must include the free variables
285 -- of the expected_ty. Here's an example:
286 -- runST (newVar True)
287 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
288 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
289 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
290 -- So now s' isn't unconstrained because it's linked to a.
291 -- Conclusion: include the free vars of the expected_ty in the
292 -- list of "free vars" for the signature check.
294 tcExtendGlobalTyVars free_tvs $
295 tcAddErrCtxtM (sigCtxt forall_tvs theta phi_ty) $
297 newDicts SignatureOrigin theta `thenNF_Tc` \ dicts ->
298 tcSimplifyCheck sig_msg forall_tvs dicts lie `thenTc` \ (free_lie, inst_binds) ->
299 checkSigTyVars forall_tvs free_tvs `thenTc` \ zonked_tvs ->
302 -- This HsLet binds any Insts which came out of the simplification.
303 -- It's a bit out of place here, but using AbsBind involves inventing
304 -- a couple of new names which seems worse.
305 dict_ids = map instToId dicts
306 co_fn e = TyLam zonked_tvs (DictLam dict_ids (mkHsLet inst_binds e))
308 returnTc (mkCoercion co_fn, result, free_lie)
310 free_tvs = tyVarsOfType expected_ty
311 sig_msg = ptext SLIT("When generalising the type of an expression")
316 %************************************************************************
318 \subsection{Coercion functions}
320 %************************************************************************
323 type Coercion a = Maybe (a -> a)
324 -- Nothing => identity fn
326 type ExprCoFn = Coercion TypecheckedHsExpr
327 type PatCoFn = Coercion TcPat
329 (<.>) :: Coercion a -> Coercion a -> Coercion a -- Composition
330 Nothing <.> Nothing = Nothing
331 Nothing <.> Just f = Just f
332 Just f <.> Nothing = Just f
333 Just f1 <.> Just f2 = Just (f1 . f2)
335 (<$>) :: Coercion a -> a -> a
339 mkCoercion :: (a -> a) -> Coercion a
340 mkCoercion f = Just f
342 idCoercion :: Coercion a
345 isIdCoercion :: Coercion a -> Bool
346 isIdCoercion = isNothing
349 %************************************************************************
351 \subsection[Unify-exported]{Exported unification functions}
353 %************************************************************************
355 The exported functions are all defined as versions of some
356 non-exported generic functions.
358 Unify two @TauType@s. Dead straightforward.
361 unifyTauTy :: TcTauType -> TcTauType -> TcM ()
362 unifyTauTy ty1 ty2 -- ty1 expected, ty2 inferred
363 = -- The unifier should only ever see tau-types
364 -- (no quantification whatsoever)
365 ASSERT2( isTauTy ty1, ppr ty1 )
366 ASSERT2( isTauTy ty2, ppr ty2 )
367 tcAddErrCtxtM (unifyCtxt "type" ty1 ty2) $
371 @unifyTauTyList@ unifies corresponding elements of two lists of
372 @TauType@s. It uses @uTys@ to do the real work. The lists should be
373 of equal length. We charge down the list explicitly so that we can
374 complain if their lengths differ.
377 unifyTauTyLists :: [TcTauType] -> [TcTauType] -> TcM ()
378 unifyTauTyLists [] [] = returnTc ()
379 unifyTauTyLists (ty1:tys1) (ty2:tys2) = uTys ty1 ty1 ty2 ty2 `thenTc_`
380 unifyTauTyLists tys1 tys2
381 unifyTauTyLists ty1s ty2s = panic "Unify.unifyTauTyLists: mismatched type lists!"
384 @unifyTauTyList@ takes a single list of @TauType@s and unifies them
385 all together. It is used, for example, when typechecking explicit
386 lists, when all the elts should be of the same type.
389 unifyTauTyList :: [TcTauType] -> TcM ()
390 unifyTauTyList [] = returnTc ()
391 unifyTauTyList [ty] = returnTc ()
392 unifyTauTyList (ty1:tys@(ty2:_)) = unifyTauTy ty1 ty2 `thenTc_`
396 %************************************************************************
398 \subsection[Unify-uTys]{@uTys@: getting down to business}
400 %************************************************************************
402 @uTys@ is the heart of the unifier. Each arg happens twice, because
403 we want to report errors in terms of synomyms if poss. The first of
404 the pair is used in error messages only; it is always the same as the
405 second, except that if the first is a synonym then the second may be a
406 de-synonym'd version. This way we get better error messages.
408 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
411 uTys :: TcTauType -> TcTauType -- Error reporting ty1 and real ty1
412 -- ty1 is the *expected* type
414 -> TcTauType -> TcTauType -- Error reporting ty2 and real ty2
415 -- ty2 is the *actual* type
418 -- Always expand synonyms (see notes at end)
419 -- (this also throws away FTVs)
420 uTys ps_ty1 (NoteTy n1 ty1) ps_ty2 ty2 = uTys ps_ty1 ty1 ps_ty2 ty2
421 uTys ps_ty1 ty1 ps_ty2 (NoteTy n2 ty2) = uTys ps_ty1 ty1 ps_ty2 ty2
423 -- Variables; go for uVar
424 uTys ps_ty1 (TyVarTy tyvar1) ps_ty2 ty2 = uVar False tyvar1 ps_ty2 ty2
425 uTys ps_ty1 ty1 ps_ty2 (TyVarTy tyvar2) = uVar True tyvar2 ps_ty1 ty1
426 -- "True" means args swapped
429 uTys _ (SourceTy (IParam n1 t1)) _ (SourceTy (IParam n2 t2))
430 | n1 == n2 = uTys t1 t1 t2 t2
431 uTys _ (SourceTy (ClassP c1 tys1)) _ (SourceTy (ClassP c2 tys2))
432 | c1 == c2 = unifyTauTyLists tys1 tys2
433 uTys _ (SourceTy (NType tc1 tys1)) _ (SourceTy (NType tc2 tys2))
434 | tc1 == tc2 = unifyTauTyLists tys1 tys2
436 -- Functions; just check the two parts
437 uTys _ (FunTy fun1 arg1) _ (FunTy fun2 arg2)
438 = uTys fun1 fun1 fun2 fun2 `thenTc_` uTys arg1 arg1 arg2 arg2
440 -- Type constructors must match
441 uTys ps_ty1 (TyConApp con1 tys1) ps_ty2 (TyConApp con2 tys2)
442 | con1 == con2 && equalLength tys1 tys2
443 = unifyTauTyLists tys1 tys2
445 | con1 == openKindCon
446 -- When we are doing kind checking, we might match a kind '?'
447 -- against a kind '*' or '#'. Notably, CCallable :: ? -> *, and
448 -- (CCallable Int) and (CCallable Int#) are both OK
449 = unifyOpenTypeKind ps_ty2
451 -- Applications need a bit of care!
452 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
453 -- NB: we've already dealt with type variables and Notes,
454 -- so if one type is an App the other one jolly well better be too
455 uTys ps_ty1 (AppTy s1 t1) ps_ty2 ty2
456 = case tcSplitAppTy_maybe ty2 of
457 Just (s2,t2) -> uTys s1 s1 s2 s2 `thenTc_` uTys t1 t1 t2 t2
458 Nothing -> unifyMisMatch ps_ty1 ps_ty2
460 -- Now the same, but the other way round
461 -- Don't swap the types, because the error messages get worse
462 uTys ps_ty1 ty1 ps_ty2 (AppTy s2 t2)
463 = case tcSplitAppTy_maybe ty1 of
464 Just (s1,t1) -> uTys s1 s1 s2 s2 `thenTc_` uTys t1 t1 t2 t2
465 Nothing -> unifyMisMatch ps_ty1 ps_ty2
467 -- Not expecting for-alls in unification
468 -- ... but the error message from the unifyMisMatch more informative
469 -- than a panic message!
471 -- Anything else fails
472 uTys ps_ty1 ty1 ps_ty2 ty2 = unifyMisMatch ps_ty1 ps_ty2
478 If you are tempted to make a short cut on synonyms, as in this
482 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
483 -- NO = if (con1 == con2) then
484 -- NO -- Good news! Same synonym constructors, so we can shortcut
485 -- NO -- by unifying their arguments and ignoring their expansions.
486 -- NO unifyTauTypeLists args1 args2
488 -- NO -- Never mind. Just expand them and try again
492 then THINK AGAIN. Here is the whole story, as detected and reported
493 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
495 Here's a test program that should detect the problem:
499 x = (1 :: Bogus Char) :: Bogus Bool
502 The problem with [the attempted shortcut code] is that
506 is not a sufficient condition to be able to use the shortcut!
507 You also need to know that the type synonym actually USES all
508 its arguments. For example, consider the following type synonym
509 which does not use all its arguments.
514 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
515 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
516 would fail, even though the expanded forms (both \tr{Int}) should
519 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
520 unnecessarily bind \tr{t} to \tr{Char}.
522 ... You could explicitly test for the problem synonyms and mark them
523 somehow as needing expansion, perhaps also issuing a warning to the
528 %************************************************************************
530 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
532 %************************************************************************
534 @uVar@ is called when at least one of the types being unified is a
535 variable. It does {\em not} assume that the variable is a fixed point
536 of the substitution; rather, notice that @uVar@ (defined below) nips
537 back into @uTys@ if it turns out that the variable is already bound.
540 uVar :: Bool -- False => tyvar is the "expected"
541 -- True => ty is the "expected" thing
543 -> TcTauType -> TcTauType -- printing and real versions
546 uVar swapped tv1 ps_ty2 ty2
547 = traceTc (text "uVar" <+> ppr swapped <+> ppr tv1 <+> (ppr ps_ty2 $$ ppr ty2)) `thenNF_Tc_`
548 getTcTyVar tv1 `thenNF_Tc` \ maybe_ty1 ->
550 Just ty1 | swapped -> uTys ps_ty2 ty2 ty1 ty1 -- Swap back
551 | otherwise -> uTys ty1 ty1 ps_ty2 ty2 -- Same order
552 other -> uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2
554 -- Expand synonyms; ignore FTVs
555 uUnboundVar swapped tv1 maybe_ty1 ps_ty2 (NoteTy n2 ty2)
556 = uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2
559 -- The both-type-variable case
560 uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2@(TyVarTy tv2)
562 -- Same type variable => no-op
566 -- Distinct type variables
567 -- ASSERT maybe_ty1 /= Just
569 = getTcTyVar tv2 `thenNF_Tc` \ maybe_ty2 ->
571 Just ty2' -> uUnboundVar swapped tv1 maybe_ty1 ty2' ty2'
575 -> WARN( not (k1 `hasMoreBoxityInfo` k2), (ppr tv1 <+> ppr k1) $$ (ppr tv2 <+> ppr k2) )
576 putTcTyVar tv2 (TyVarTy tv1) `thenNF_Tc_`
580 -> WARN( not (k2 `hasMoreBoxityInfo` k1), (ppr tv2 <+> ppr k2) $$ (ppr tv1 <+> ppr k1) )
581 putTcTyVar tv1 ps_ty2 `thenNF_Tc_`
586 update_tv2 = (k2 `eqKind` openTypeKind) || (not (k1 `eqKind` openTypeKind) && nicer_to_update_tv2)
587 -- Try to get rid of open type variables as soon as poss
589 nicer_to_update_tv2 = isUserTyVar tv1
590 -- Don't unify a signature type variable if poss
591 || isSystemName (varName tv2)
592 -- Try to update sys-y type variables in preference to sig-y ones
594 -- Second one isn't a type variable
595 uUnboundVar swapped tv1 maybe_ty1 ps_ty2 non_var_ty2
596 = -- Check that tv1 isn't a type-signature type variable
597 checkTcM (not (isSkolemTyVar tv1))
598 (failWithTcM (unifyWithSigErr tv1 ps_ty2)) `thenTc_`
600 -- Do the occurs check, and check that we are not
601 -- unifying a type variable with a polytype
602 -- Returns a zonked type ready for the update
603 checkValue tv1 ps_ty2 non_var_ty2 `thenTc` \ ty2 ->
605 -- Check that the kinds match
606 checkKinds swapped tv1 ty2 `thenTc_`
608 -- Perform the update
609 putTcTyVar tv1 ty2 `thenNF_Tc_`
614 checkKinds swapped tv1 ty2
615 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
616 -- ty2 has been zonked at this stage, which ensures that
617 -- its kind has as much boxity information visible as possible.
618 | tk2 `hasMoreBoxityInfo` tk1 = returnTc ()
621 -- Either the kinds aren't compatible
622 -- (can happen if we unify (a b) with (c d))
623 -- or we are unifying a lifted type variable with an
624 -- unlifted type: e.g. (id 3#) is illegal
625 = tcAddErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
629 (k1,k2) | swapped = (tk2,tk1)
630 | otherwise = (tk1,tk2)
636 checkValue tv1 ps_ty2 non_var_ty2
637 -- Do the occurs check, and check that we are not
638 -- unifying a type variable with a polytype
639 -- Return the type to update the type variable with, or fail
641 -- Basically we want to update tv1 := ps_ty2
642 -- because ps_ty2 has type-synonym info, which improves later error messages
647 -- f :: (A a -> a -> ()) -> ()
651 -- x = f (\ x p -> p x)
653 -- In the application (p x), we try to match "t" with "A t". If we go
654 -- ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
655 -- an infinite loop later.
656 -- But we should not reject the program, because A t = ().
657 -- Rather, we should bind t to () (= non_var_ty2).
659 -- That's why we have this two-state occurs-check
660 = zonkTcType ps_ty2 `thenNF_Tc` \ ps_ty2' ->
661 case okToUnifyWith tv1 ps_ty2' of {
662 Nothing -> returnTc ps_ty2' ; -- Success
665 zonkTcType non_var_ty2 `thenNF_Tc` \ non_var_ty2' ->
666 case okToUnifyWith tv1 non_var_ty2' of
667 Nothing -> -- This branch rarely succeeds, except in strange cases
668 -- like that in the example above
669 returnTc non_var_ty2'
671 Just problem -> failWithTcM (unifyCheck problem tv1 ps_ty2')
674 data Problem = OccurCheck | NotMonoType
676 okToUnifyWith :: TcTyVar -> TcType -> Maybe Problem
677 -- (okToUnifyWith tv ty) checks whether it's ok to unify
680 -- Just p => not ok, problem p
685 ok (TyVarTy tv') | tv == tv' = Just OccurCheck
686 | otherwise = Nothing
687 ok (AppTy t1 t2) = ok t1 `and` ok t2
688 ok (FunTy t1 t2) = ok t1 `and` ok t2
689 ok (TyConApp _ ts) = oks ts
690 ok (ForAllTy _ _) = Just NotMonoType
691 ok (SourceTy st) = ok_st st
692 ok (NoteTy (FTVNote _) t) = ok t
693 ok (NoteTy (SynNote t1) t2) = ok t1 `and` ok t2
694 -- Type variables may be free in t1 but not t2
695 -- A forall may be in t2 but not t1
697 oks ts = foldr (and . ok) Nothing ts
699 ok_st (ClassP _ ts) = oks ts
700 ok_st (IParam _ t) = ok t
701 ok_st (NType _ ts) = oks ts
704 Just p `and` m = Just p
707 %************************************************************************
709 \subsection[Unify-fun]{@unifyFunTy@}
711 %************************************************************************
713 @subFunTy@ and @unifyFunTy@ is used to avoid the fruitless
714 creation of type variables.
716 * subFunTy is used when we might be faced with a "hole" type variable,
717 in which case we should create two new holes.
719 * unifyFunTy is used when we expect to encounter only "ordinary"
720 type variables, so we should create new ordinary type variables
723 subFunTy :: TcSigmaType -- Fail if ty isn't a function type
724 -> TcM (TcType, TcType) -- otherwise return arg and result types
725 subFunTy ty@(TyVarTy tyvar)
727 = getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
729 Just ty -> subFunTy ty
730 Nothing | isHoleTyVar tyvar
731 -> newHoleTyVarTy `thenNF_Tc` \ arg ->
732 newHoleTyVarTy `thenNF_Tc` \ res ->
733 putTcTyVar tyvar (mkFunTy arg res) `thenNF_Tc_`
736 -> unify_fun_ty_help ty
739 = case tcSplitFunTy_maybe ty of
740 Just arg_and_res -> returnTc arg_and_res
741 Nothing -> unify_fun_ty_help ty
744 unifyFunTy :: TcPhiType -- Fail if ty isn't a function type
745 -> TcM (TcType, TcType) -- otherwise return arg and result types
747 unifyFunTy ty@(TyVarTy tyvar)
748 = getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
750 Just ty' -> unifyFunTy ty'
751 Nothing -> unify_fun_ty_help ty
754 = case tcSplitFunTy_maybe ty of
755 Just arg_and_res -> returnTc arg_and_res
756 Nothing -> unify_fun_ty_help ty
758 unify_fun_ty_help ty -- Special cases failed, so revert to ordinary unification
759 = newTyVarTy openTypeKind `thenNF_Tc` \ arg ->
760 newTyVarTy openTypeKind `thenNF_Tc` \ res ->
761 unifyTauTy ty (mkFunTy arg res) `thenTc_`
766 unifyListTy :: TcType -- expected list type
767 -> TcM TcType -- list element type
769 unifyListTy ty@(TyVarTy tyvar)
770 = getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
772 Just ty' -> unifyListTy ty'
773 other -> unify_list_ty_help ty
776 = case tcSplitTyConApp_maybe ty of
777 Just (tycon, [arg_ty]) | tycon == listTyCon -> returnTc arg_ty
778 other -> unify_list_ty_help ty
780 unify_list_ty_help ty -- Revert to ordinary unification
781 = newTyVarTy liftedTypeKind `thenNF_Tc` \ elt_ty ->
782 unifyTauTy ty (mkListTy elt_ty) `thenTc_`
785 -- variant for parallel arrays
787 unifyPArrTy :: TcType -- expected list type
788 -> TcM TcType -- list element type
790 unifyPArrTy ty@(TyVarTy tyvar)
791 = getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
793 Just ty' -> unifyPArrTy ty'
794 _ -> unify_parr_ty_help ty
796 = case tcSplitTyConApp_maybe ty of
797 Just (tycon, [arg_ty]) | tycon == parrTyCon -> returnTc arg_ty
798 _ -> unify_parr_ty_help ty
800 unify_parr_ty_help ty -- Revert to ordinary unification
801 = newTyVarTy liftedTypeKind `thenNF_Tc` \ elt_ty ->
802 unifyTauTy ty (mkPArrTy elt_ty) `thenTc_`
807 unifyTupleTy :: Boxity -> Arity -> TcType -> TcM [TcType]
808 unifyTupleTy boxity arity ty@(TyVarTy tyvar)
809 = getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
811 Just ty' -> unifyTupleTy boxity arity ty'
812 other -> unify_tuple_ty_help boxity arity ty
814 unifyTupleTy boxity arity ty
815 = case tcSplitTyConApp_maybe ty of
816 Just (tycon, arg_tys)
818 && tyConArity tycon == arity
819 && tupleTyConBoxity tycon == boxity
821 other -> unify_tuple_ty_help boxity arity ty
823 unify_tuple_ty_help boxity arity ty
824 = newTyVarTys arity kind `thenNF_Tc` \ arg_tys ->
825 unifyTauTy ty (mkTupleTy boxity arity arg_tys) `thenTc_`
828 kind | isBoxed boxity = liftedTypeKind
829 | otherwise = openTypeKind
833 %************************************************************************
835 \subsection{Kind unification}
837 %************************************************************************
840 unifyKind :: TcKind -- Expected
844 = tcAddErrCtxtM (unifyCtxt "kind" k1 k2) $
847 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
848 unifyKinds [] [] = returnTc ()
849 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenTc_`
851 unifyKinds _ _ = panic "unifyKinds: length mis-match"
855 unifyOpenTypeKind :: TcKind -> TcM ()
856 -- Ensures that the argument kind is of the form (Type bx)
857 -- for some boxity bx
859 unifyOpenTypeKind ty@(TyVarTy tyvar)
860 = getTcTyVar tyvar `thenNF_Tc` \ maybe_ty ->
862 Just ty' -> unifyOpenTypeKind ty'
863 other -> unify_open_kind_help ty
866 | isTypeKind ty = returnTc ()
867 | otherwise = unify_open_kind_help ty
869 unify_open_kind_help ty -- Revert to ordinary unification
870 = newBoxityVar `thenNF_Tc` \ boxity ->
871 unifyKind ty (mkTyConApp typeCon [boxity])
875 %************************************************************************
877 \subsection[Unify-context]{Errors and contexts}
879 %************************************************************************
885 unifyCtxt s ty1 ty2 tidy_env -- ty1 expected, ty2 inferred
886 = zonkTcType ty1 `thenNF_Tc` \ ty1' ->
887 zonkTcType ty2 `thenNF_Tc` \ ty2' ->
888 returnNF_Tc (err ty1' ty2')
893 text "Expected" <+> text s <> colon <+> ppr tidy_ty1,
894 text "Inferred" <+> text s <> colon <+> ppr tidy_ty2
897 (env1, [tidy_ty1,tidy_ty2]) = tidyOpenTypes tidy_env [ty1,ty2]
899 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
900 -- tv1 is zonked already
901 = zonkTcType ty2 `thenNF_Tc` \ ty2' ->
902 returnNF_Tc (err ty2')
904 err ty2 = (env2, ptext SLIT("When matching types") <+>
905 sep [quotes pp_expected, ptext SLIT("and"), quotes pp_actual])
907 (pp_expected, pp_actual) | swapped = (pp2, pp1)
908 | otherwise = (pp1, pp2)
909 (env1, tv1') = tidyOpenTyVar tidy_env tv1
910 (env2, ty2') = tidyOpenType env1 ty2
914 unifyMisMatch ty1 ty2
915 = zonkTcType ty1 `thenNF_Tc` \ ty1' ->
916 zonkTcType ty2 `thenNF_Tc` \ ty2' ->
918 (env, [tidy_ty1, tidy_ty2]) = tidyOpenTypes emptyTidyEnv [ty1',ty2']
919 msg = hang (ptext SLIT("Couldn't match"))
920 4 (sep [quotes (ppr tidy_ty1),
921 ptext SLIT("against"),
922 quotes (ppr tidy_ty2)])
924 failWithTcM (env, msg)
926 unifyWithSigErr tyvar ty
927 = (env2, hang (ptext SLIT("Cannot unify the type-signature variable") <+> quotes (ppr tidy_tyvar))
928 4 (ptext SLIT("with the type") <+> quotes (ppr tidy_ty)))
930 (env1, tidy_tyvar) = tidyOpenTyVar emptyTidyEnv tyvar
931 (env2, tidy_ty) = tidyOpenType env1 ty
933 unifyCheck problem tyvar ty
935 4 (sep [ppr tidy_tyvar, char '=', ppr tidy_ty]))
937 (env1, tidy_tyvar) = tidyOpenTyVar emptyTidyEnv tyvar
938 (env2, tidy_ty) = tidyOpenType env1 ty
940 msg = case problem of
941 OccurCheck -> ptext SLIT("Occurs check: cannot construct the infinite type:")
942 NotMonoType -> ptext SLIT("Cannot unify a type variable with a type scheme:")
947 %************************************************************************
949 \subsection{Checking signature type variables}
951 %************************************************************************
953 @checkSigTyVars@ is used after the type in a type signature has been unified with
954 the actual type found. It then checks that the type variables of the type signature
956 (a) Still all type variables
957 eg matching signature [a] against inferred type [(p,q)]
958 [then a will be unified to a non-type variable]
960 (b) Still all distinct
961 eg matching signature [(a,b)] against inferred type [(p,p)]
962 [then a and b will be unified together]
964 (c) Not mentioned in the environment
965 eg the signature for f in this:
971 Here, f is forced to be monorphic by the free occurence of x.
973 (d) Not (unified with another type variable that is) in scope.
974 eg f x :: (r->r) = (\y->y) :: forall a. a->r
975 when checking the expression type signature, we find that
976 even though there is nothing in scope whose type mentions r,
977 nevertheless the type signature for the expression isn't right.
979 Another example is in a class or instance declaration:
981 op :: forall b. a -> b
983 Here, b gets unified with a
985 Before doing this, the substitution is applied to the signature type variable.
987 We used to have the notion of a "DontBind" type variable, which would
988 only be bound to itself or nothing. Then points (a) and (b) were
989 self-checking. But it gave rise to bogus consequential error messages.
992 f = (*) -- Monomorphic
997 Here, we get a complaint when checking the type signature for g,
998 that g isn't polymorphic enough; but then we get another one when
999 dealing with the (Num x) context arising from f's definition;
1000 we try to unify x with Int (to default it), but find that x has already
1001 been unified with the DontBind variable "a" from g's signature.
1002 This is really a problem with side-effecting unification; we'd like to
1003 undo g's effects when its type signature fails, but unification is done
1004 by side effect, so we can't (easily).
1006 So we revert to ordinary type variables for signatures, and try to
1007 give a helpful message in checkSigTyVars.
1010 checkSigTyVars :: [TcTyVar] -- Universally-quantified type variables in the signature
1011 -> TcTyVarSet -- Tyvars that are free in the type signature
1012 -- Not necessarily zonked
1013 -- These should *already* be in the free-in-env set,
1014 -- and are used here only to improve the error message
1015 -> TcM [TcTyVar] -- Zonked signature type variables
1017 checkSigTyVars [] free = returnTc []
1018 checkSigTyVars sig_tyvars free_tyvars
1019 = zonkTcTyVars sig_tyvars `thenNF_Tc` \ sig_tys ->
1020 tcGetGlobalTyVars `thenNF_Tc` \ globals ->
1022 checkTcM (allDistinctTyVars sig_tys globals)
1023 (complain sig_tys globals) `thenTc_`
1025 returnTc (map (tcGetTyVar "checkSigTyVars") sig_tys)
1028 complain sig_tys globals
1029 = -- "check" checks each sig tyvar in turn
1031 (env2, emptyVarEnv, [])
1032 (tidy_tvs `zip` tidy_tys) `thenNF_Tc` \ (env3, _, msgs) ->
1034 failWithTcM (env3, main_msg $$ vcat msgs)
1036 (env1, tidy_tvs) = tidyOpenTyVars emptyTidyEnv sig_tyvars
1037 (env2, tidy_tys) = tidyOpenTypes env1 sig_tys
1039 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1041 check (tidy_env, acc, msgs) (sig_tyvar,ty)
1042 -- sig_tyvar is from the signature;
1043 -- ty is what you get if you zonk sig_tyvar and then tidy it
1045 -- acc maps a zonked type variable back to a signature type variable
1046 = case tcGetTyVar_maybe ty of {
1047 Nothing -> -- Error (a)!
1048 returnNF_Tc (tidy_env, acc, unify_msg sig_tyvar (quotes (ppr ty)) : msgs) ;
1052 case lookupVarEnv acc tv of {
1053 Just sig_tyvar' -> -- Error (b)!
1054 returnNF_Tc (tidy_env, acc, unify_msg sig_tyvar thing : msgs)
1056 thing = ptext SLIT("another quantified type variable") <+> quotes (ppr sig_tyvar')
1060 if tv `elemVarSet` globals -- Error (c) or (d)! Type variable escapes
1061 -- The least comprehensible, so put it last
1063 -- a) get the local TcIds and TyVars from the environment,
1064 -- and pass them to find_globals (they might have tv free)
1065 -- b) similarly, find any free_tyvars that mention tv
1066 then tcGetEnv `thenNF_Tc` \ ve ->
1067 find_globals tv tidy_env (tcLEnvElts ve) `thenNF_Tc` \ (tidy_env1, globs) ->
1068 find_frees tv tidy_env1 [] (varSetElems free_tyvars) `thenNF_Tc` \ (tidy_env2, frees) ->
1069 returnNF_Tc (tidy_env2, acc, escape_msg sig_tyvar tv globs frees : msgs)
1072 returnNF_Tc (tidy_env, extendVarEnv acc tv sig_tyvar, msgs)
1075 -----------------------
1076 -- find_globals looks at the value environment and finds values
1077 -- whose types mention the offending type variable. It has to be
1078 -- careful to zonk the Id's type first, so it has to be in the monad.
1079 -- We must be careful to pass it a zonked type variable, too.
1084 -> NF_TcM (TidyEnv, [SDoc])
1086 find_globals tv tidy_env things
1087 = go tidy_env [] things
1089 go tidy_env acc [] = returnNF_Tc (tidy_env, acc)
1090 go tidy_env acc (thing : things)
1091 = find_thing ignore_it tidy_env thing `thenNF_Tc` \ (tidy_env1, maybe_doc) ->
1093 Just d -> go tidy_env1 (d:acc) things
1094 Nothing -> go tidy_env1 acc things
1096 ignore_it ty = not (tv `elemVarSet` tyVarsOfType ty)
1098 -----------------------
1099 find_thing ignore_it tidy_env (ATcId id)
1100 = zonkTcType (idType id) `thenNF_Tc` \ id_ty ->
1101 if ignore_it id_ty then
1102 returnNF_Tc (tidy_env, Nothing)
1104 (tidy_env', tidy_ty) = tidyOpenType tidy_env id_ty
1105 msg = sep [ppr id <+> dcolon <+> ppr tidy_ty,
1106 nest 2 (parens (ptext SLIT("bound at") <+>
1107 ppr (getSrcLoc id)))]
1109 returnNF_Tc (tidy_env', Just msg)
1111 find_thing ignore_it tidy_env (ATyVar tv)
1112 = zonkTcTyVar tv `thenNF_Tc` \ tv_ty ->
1113 if ignore_it tv_ty then
1114 returnNF_Tc (tidy_env, Nothing)
1116 (tidy_env1, tv1) = tidyOpenTyVar tidy_env tv
1117 (tidy_env2, tidy_ty) = tidyOpenType tidy_env1 tv_ty
1118 msg = sep [ptext SLIT("Type variable") <+> quotes (ppr tv1) <+> eq_stuff, nest 2 bound_at]
1120 eq_stuff | Just tv' <- Type.getTyVar_maybe tv_ty, tv == tv' = empty
1121 | otherwise = equals <+> ppr tv_ty
1122 -- It's ok to use Type.getTyVar_maybe because ty is zonked by now
1124 bound_at = tyVarBindingInfo tv
1126 returnNF_Tc (tidy_env2, Just msg)
1128 -----------------------
1129 find_frees tv tidy_env acc []
1130 = returnNF_Tc (tidy_env, acc)
1131 find_frees tv tidy_env acc (ftv:ftvs)
1132 = zonkTcTyVar ftv `thenNF_Tc` \ ty ->
1133 if tv `elemVarSet` tyVarsOfType ty then
1135 (tidy_env', ftv') = tidyOpenTyVar tidy_env ftv
1137 find_frees tv tidy_env' (ftv':acc) ftvs
1139 find_frees tv tidy_env acc ftvs
1142 escape_msg sig_tv tv globs frees
1143 = mk_msg sig_tv <+> ptext SLIT("escapes") $$
1144 if not (null globs) then
1145 vcat [pp_it <+> ptext SLIT("is mentioned in the environment:"),
1146 nest 2 (vcat globs)]
1147 else if not (null frees) then
1148 vcat [ptext SLIT("It is reachable from the type variable(s)") <+> pprQuotedList frees,
1149 nest 2 (ptext SLIT("which") <+> is_are <+> ptext SLIT("free in the signature"))
1152 empty -- Sigh. It's really hard to give a good error message
1153 -- all the time. One bad case is an existential pattern match
1155 is_are | isSingleton frees = ptext SLIT("is")
1156 | otherwise = ptext SLIT("are")
1157 pp_it | sig_tv /= tv = ptext SLIT("It unifies with") <+> quotes (ppr tv) <> comma <+> ptext SLIT("which")
1158 | otherwise = ptext SLIT("It")
1160 vcat_first :: Int -> [SDoc] -> SDoc
1161 vcat_first n [] = empty
1162 vcat_first 0 (x:xs) = text "...others omitted..."
1163 vcat_first n (x:xs) = x $$ vcat_first (n-1) xs
1166 unify_msg tv thing = mk_msg tv <+> ptext SLIT("is unified with") <+> thing
1167 mk_msg tv = ptext SLIT("Quantified type variable") <+> quotes (ppr tv)
1170 These two context are used with checkSigTyVars
1173 sigCtxt :: [TcTyVar] -> TcThetaType -> TcTauType
1174 -> TidyEnv -> NF_TcM (TidyEnv, Message)
1175 sigCtxt sig_tyvars sig_theta sig_tau tidy_env
1176 = zonkTcType sig_tau `thenNF_Tc` \ actual_tau ->
1178 (env1, tidy_sig_tyvars) = tidyOpenTyVars tidy_env sig_tyvars
1179 (env2, tidy_sig_rho) = tidyOpenType env1 (mkRhoTy sig_theta sig_tau)
1180 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1181 msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tyvars tidy_sig_rho),
1182 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1185 returnNF_Tc (env3, msg)
1187 sigPatCtxt bound_tvs bound_ids tidy_env
1188 = returnNF_Tc (env1,
1189 sep [ptext SLIT("When checking a pattern that binds"),
1190 nest 4 (vcat (zipWith ppr_id show_ids tidy_tys))])
1192 show_ids = filter is_interesting bound_ids
1193 is_interesting id = any (`elemVarSet` idFreeTyVars id) bound_tvs
1195 (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
1196 ppr_id id ty = ppr id <+> dcolon <+> ppr ty
1197 -- Don't zonk the types so we get the separate, un-unified versions