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, findGlobals,
12 -- Various unifications
13 unifyTauTy, unifyTauTyList, unifyTauTyLists,
14 unifyFunTy, unifyListTy, unifyPArrTy, unifyTupleTy,
15 unifyKind, unifyKinds, unifyOpenTypeKind, unifyFunKind
19 #include "HsVersions.h"
22 import HsSyn ( HsExpr(..) )
23 import TcHsSyn ( TypecheckedHsExpr, TcPat, mkHsLet,
24 ExprCoFn, idCoercion, isIdCoercion, mkCoercion, (<.>), (<$>) )
25 import TypeRep ( Type(..), SourceType(..), TyNote(..), openKindCon )
27 import TcRnMonad -- TcType, amongst others
28 import TcType ( TcKind, TcType, TcSigmaType, TcRhoType, TcTyVar, TcTauType,
29 TcTyVarSet, TcThetaType, TyVarDetails(SigTv),
31 tcSplitAppTy_maybe, tcSplitTyConApp_maybe,
32 tcGetTyVar_maybe, tcGetTyVar,
33 mkFunTy, tyVarsOfType, mkPhiTy,
34 typeKind, tcSplitFunTy_maybe, mkForAllTys,
35 isHoleTyVar, isSkolemTyVar, isUserTyVar,
36 tidyOpenType, tidyOpenTypes, tidyOpenTyVar, tidyOpenTyVars,
37 eqKind, openTypeKind, liftedTypeKind, isTypeKind, mkArrowKind,
38 hasMoreBoxityInfo, allDistinctTyVars
40 import qualified Type ( getTyVar_maybe )
41 import Inst ( newDicts, instToId, tcInstCall )
42 import TcMType ( getTcTyVar, putTcTyVar, tcInstType, readHoleResult, newKindVar,
43 newTyVarTy, newTyVarTys, newOpenTypeKind, newHoleTyVarTy,
44 zonkTcType, zonkTcTyVars, zonkTcTyVarsAndFV )
45 import TcSimplify ( tcSimplifyCheck )
46 import TysWiredIn ( listTyCon, parrTyCon, mkListTy, mkPArrTy, mkTupleTy )
47 import TcEnv ( tcGetGlobalTyVars, findGlobals )
48 import TyCon ( tyConArity, isTupleTyCon, tupleTyConBoxity )
49 import PprType ( pprType )
50 import Id ( Id, mkSysLocal )
51 import Var ( Var, varName, tyVarKind )
52 import VarSet ( emptyVarSet, unitVarSet, unionVarSet, elemVarSet, varSetElems )
54 import Name ( isSystemName )
55 import ErrUtils ( Message )
56 import BasicTypes ( Boxity, Arity, isBoxed )
57 import Util ( equalLength, notNull )
58 import Maybe ( isNothing )
64 * A hole is always filled in with an ordinary type, not another hole.
66 %************************************************************************
68 \subsection{Subsumption}
70 %************************************************************************
72 All the tcSub calls have the form
74 tcSub expected_ty offered_ty
76 offered_ty <= expected_ty
78 That is, that a value of type offered_ty is acceptable in
79 a place expecting a value of type expected_ty.
81 It returns a coercion function
82 co_fn :: offered_ty -> expected_ty
83 which takes an HsExpr of type offered_ty into one of type
87 type TcHoleType = TcSigmaType -- Either a TcSigmaType,
90 tcSubExp :: TcHoleType -> TcSigmaType -> TcM ExprCoFn
91 tcSubOff :: TcSigmaType -> TcHoleType -> TcM ExprCoFn
92 tcSub :: TcSigmaType -> TcSigmaType -> TcM ExprCoFn
95 These two check for holes
98 tcSubExp expected_ty offered_ty
99 = traceTc (text "tcSubExp" <+> (ppr expected_ty $$ ppr offered_ty)) `thenM_`
100 checkHole expected_ty offered_ty tcSub
102 tcSubOff expected_ty offered_ty
103 = checkHole offered_ty expected_ty (\ off exp -> tcSub exp off)
105 -- checkHole looks for a hole in its first arg;
106 -- If so, and it is uninstantiated, it fills in the hole
107 -- with its second arg
108 -- Otherwise it calls thing_inside, passing the two args, looking
109 -- through any instantiated hole
111 checkHole (TyVarTy tv) other_ty thing_inside
113 = getTcTyVar tv `thenM` \ maybe_ty ->
115 Just ty -> thing_inside ty other_ty
116 Nothing -> traceTc (text "checkHole" <+> ppr tv) `thenM_`
117 putTcTyVar tv other_ty `thenM_`
120 checkHole ty other_ty thing_inside
121 = thing_inside ty other_ty
124 No holes expected now. Add some error-check context info.
127 tcSub expected_ty actual_ty
128 = traceTc (text "tcSub" <+> details) `thenM_`
129 addErrCtxtM (unifyCtxt "type" expected_ty actual_ty)
130 (tc_sub expected_ty expected_ty actual_ty actual_ty)
132 details = vcat [text "Expected:" <+> ppr expected_ty,
133 text "Actual: " <+> ppr actual_ty]
136 tc_sub carries the types before and after expanding type synonyms
139 tc_sub :: TcSigmaType -- expected_ty, before expanding synonyms
140 -> TcSigmaType -- ..and after
141 -> TcSigmaType -- actual_ty, before
142 -> TcSigmaType -- ..and after
145 -----------------------------------
147 tc_sub exp_sty (NoteTy _ exp_ty) act_sty act_ty = tc_sub exp_sty exp_ty act_sty act_ty
148 tc_sub exp_sty exp_ty act_sty (NoteTy _ act_ty) = tc_sub exp_sty exp_ty act_sty act_ty
150 -----------------------------------
151 -- Generalisation case
152 -- actual_ty: d:Eq b => b->b
153 -- expected_ty: forall a. Ord a => a->a
154 -- co_fn e /\a. \d2:Ord a. let d = eqFromOrd d2 in e
156 -- It is essential to do this *before* the specialisation case
157 -- Example: f :: (Eq a => a->a) -> ...
158 -- g :: Ord b => b->b
161 tc_sub exp_sty expected_ty act_sty actual_ty
162 | isSigmaTy expected_ty
163 = tcGen expected_ty (tyVarsOfType actual_ty) (
164 -- It's really important to check for escape wrt the free vars of
165 -- both expected_ty *and* actual_ty
166 \ body_exp_ty -> tc_sub body_exp_ty body_exp_ty act_sty actual_ty
167 ) `thenM` \ (gen_fn, co_fn) ->
168 returnM (gen_fn <.> co_fn)
170 -----------------------------------
171 -- Specialisation case:
172 -- actual_ty: forall a. Ord a => a->a
173 -- expected_ty: Int -> Int
174 -- co_fn e = e Int dOrdInt
176 tc_sub exp_sty expected_ty act_sty actual_ty
177 | isSigmaTy actual_ty
178 = tcInstCall Rank2Origin actual_ty `thenM` \ (inst_fn, body_ty) ->
179 tc_sub exp_sty expected_ty body_ty body_ty `thenM` \ co_fn ->
180 returnM (co_fn <.> inst_fn)
182 -----------------------------------
185 tc_sub _ (FunTy exp_arg exp_res) _ (FunTy act_arg act_res)
186 = tcSub_fun exp_arg exp_res act_arg act_res
188 -----------------------------------
189 -- Type variable meets function: imitate
191 -- NB 1: we can't just unify the type variable with the type
192 -- because the type might not be a tau-type, and we aren't
193 -- allowed to instantiate an ordinary type variable with
196 -- NB 2: can we short-cut to an error case?
197 -- when the arg/res is not a tau-type?
198 -- NO! e.g. f :: ((forall a. a->a) -> Int) -> Int
200 -- is perfectly fine, because we can instantiat f's type to a monotype
202 -- However, we get can get jolly unhelpful error messages.
203 -- e.g. foo = id runST
205 -- Inferred type is less polymorphic than expected
206 -- Quantified type variable `s' escapes
207 -- Expected type: ST s a -> t
208 -- Inferred type: (forall s1. ST s1 a) -> a
209 -- In the first argument of `id', namely `runST'
210 -- In a right-hand side of function `foo': id runST
212 -- I'm not quite sure what to do about this!
214 tc_sub exp_sty exp_ty@(FunTy exp_arg exp_res) _ (TyVarTy tv)
215 = ASSERT( not (isHoleTyVar tv) )
216 getTcTyVar tv `thenM` \ maybe_ty ->
218 Just ty -> tc_sub exp_sty exp_ty ty ty
219 Nothing -> imitateFun tv exp_sty `thenM` \ (act_arg, act_res) ->
220 tcSub_fun exp_arg exp_res act_arg act_res
222 tc_sub _ (TyVarTy tv) act_sty act_ty@(FunTy act_arg act_res)
223 = ASSERT( not (isHoleTyVar tv) )
224 getTcTyVar tv `thenM` \ maybe_ty ->
226 Just ty -> tc_sub ty ty act_sty act_ty
227 Nothing -> imitateFun tv act_sty `thenM` \ (exp_arg, exp_res) ->
228 tcSub_fun exp_arg exp_res act_arg act_res
230 -----------------------------------
232 -- If none of the above match, we revert to the plain unifier
233 tc_sub exp_sty expected_ty act_sty actual_ty
234 = uTys exp_sty expected_ty act_sty actual_ty `thenM_`
238 %************************************************************************
240 \subsection{Functions}
242 %************************************************************************
245 tcSub_fun exp_arg exp_res act_arg act_res
246 = tc_sub act_arg act_arg exp_arg exp_arg `thenM` \ co_fn_arg ->
247 tc_sub exp_res exp_res act_res act_res `thenM` \ co_fn_res ->
248 newUnique `thenM` \ uniq ->
250 -- co_fn_arg :: HsExpr exp_arg -> HsExpr act_arg
251 -- co_fn_res :: HsExpr act_res -> HsExpr exp_res
252 -- co_fn :: HsExpr (act_arg -> act_res) -> HsExpr (exp_arg -> exp_res)
253 arg_id = mkSysLocal FSLIT("sub") uniq exp_arg
254 coercion | isIdCoercion co_fn_arg,
255 isIdCoercion co_fn_res = idCoercion
256 | otherwise = mkCoercion co_fn
258 co_fn e = DictLam [arg_id]
259 (co_fn_res <$> (HsApp e (co_fn_arg <$> (HsVar arg_id))))
260 -- Slight hack; using a "DictLam" to get an ordinary simple lambda
261 -- HsVar arg_id :: HsExpr exp_arg
262 -- co_fn_arg $it :: HsExpr act_arg
263 -- HsApp e $it :: HsExpr act_res
264 -- co_fn_res $it :: HsExpr exp_res
268 imitateFun :: TcTyVar -> TcType -> TcM (TcType, TcType)
270 = ASSERT( not (isHoleTyVar tv) )
271 -- NB: tv is an *ordinary* tyvar and so are the new ones
273 -- Check that tv isn't a type-signature type variable
274 -- (This would be found later in checkSigTyVars, but
275 -- we get a better error message if we do it here.)
276 checkM (not (isSkolemTyVar tv))
277 (failWithTcM (unifyWithSigErr tv ty)) `thenM_`
279 newTyVarTy openTypeKind `thenM` \ arg ->
280 newTyVarTy openTypeKind `thenM` \ res ->
281 putTcTyVar tv (mkFunTy arg res) `thenM_`
286 %************************************************************************
288 \subsection{Generalisation}
290 %************************************************************************
293 tcGen :: TcSigmaType -- expected_ty
294 -> TcTyVarSet -- Extra tyvars that the universally
295 -- quantified tyvars of expected_ty
296 -- must not be unified
297 -> (TcRhoType -> TcM result) -- spec_ty
298 -> TcM (ExprCoFn, result)
299 -- The expression has type: spec_ty -> expected_ty
301 tcGen expected_ty extra_tvs thing_inside -- We expect expected_ty to be a forall-type
302 -- If not, the call is a no-op
303 = tcInstType SigTv expected_ty `thenM` \ (forall_tvs, theta, phi_ty) ->
305 -- Type-check the arg and unify with poly type
306 getLIE (thing_inside phi_ty) `thenM` \ (result, lie) ->
308 -- Check that the "forall_tvs" havn't been constrained
309 -- The interesting bit here is that we must include the free variables
310 -- of the expected_ty. Here's an example:
311 -- runST (newVar True)
312 -- Here, if we don't make a check, we'll get a type (ST s (MutVar s Bool))
313 -- for (newVar True), with s fresh. Then we unify with the runST's arg type
314 -- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
315 -- So now s' isn't unconstrained because it's linked to a.
316 -- Conclusion: include the free vars of the expected_ty in the
317 -- list of "free vars" for the signature check.
319 newDicts SignatureOrigin theta `thenM` \ dicts ->
320 tcSimplifyCheck sig_msg forall_tvs dicts lie `thenM` \ inst_binds ->
323 zonkTcTyVars forall_tvs `thenM` \ forall_tys ->
324 traceTc (text "tcGen" <+> vcat [text "extra_tvs" <+> ppr extra_tvs,
325 text "expected_ty" <+> ppr expected_ty,
326 text "inst ty" <+> ppr forall_tvs <+> ppr theta <+> ppr phi_ty,
327 text "free_tvs" <+> ppr free_tvs,
328 text "forall_tys" <+> ppr forall_tys]) `thenM_`
331 checkSigTyVarsWrt free_tvs forall_tvs `thenM` \ zonked_tvs ->
333 traceTc (text "tcGen:done") `thenM_`
336 -- This HsLet binds any Insts which came out of the simplification.
337 -- It's a bit out of place here, but using AbsBind involves inventing
338 -- a couple of new names which seems worse.
339 dict_ids = map instToId dicts
340 co_fn e = TyLam zonked_tvs (DictLam dict_ids (mkHsLet inst_binds e))
342 returnM (mkCoercion co_fn, result)
344 free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
345 sig_msg = ptext SLIT("expected type of an expression")
350 %************************************************************************
352 \subsection[Unify-exported]{Exported unification functions}
354 %************************************************************************
356 The exported functions are all defined as versions of some
357 non-exported generic functions.
359 Unify two @TauType@s. Dead straightforward.
362 unifyTauTy :: TcTauType -> TcTauType -> TcM ()
363 unifyTauTy ty1 ty2 -- ty1 expected, ty2 inferred
364 = -- The unifier should only ever see tau-types
365 -- (no quantification whatsoever)
366 ASSERT2( isTauTy ty1, ppr ty1 )
367 ASSERT2( isTauTy ty2, ppr ty2 )
368 addErrCtxtM (unifyCtxt "type" ty1 ty2) $
372 @unifyTauTyList@ unifies corresponding elements of two lists of
373 @TauType@s. It uses @uTys@ to do the real work. The lists should be
374 of equal length. We charge down the list explicitly so that we can
375 complain if their lengths differ.
378 unifyTauTyLists :: [TcTauType] -> [TcTauType] -> TcM ()
379 unifyTauTyLists [] [] = returnM ()
380 unifyTauTyLists (ty1:tys1) (ty2:tys2) = uTys ty1 ty1 ty2 ty2 `thenM_`
381 unifyTauTyLists tys1 tys2
382 unifyTauTyLists ty1s ty2s = panic "Unify.unifyTauTyLists: mismatched type lists!"
385 @unifyTauTyList@ takes a single list of @TauType@s and unifies them
386 all together. It is used, for example, when typechecking explicit
387 lists, when all the elts should be of the same type.
390 unifyTauTyList :: [TcTauType] -> TcM ()
391 unifyTauTyList [] = returnM ()
392 unifyTauTyList [ty] = returnM ()
393 unifyTauTyList (ty1:tys@(ty2:_)) = unifyTauTy ty1 ty2 `thenM_`
397 %************************************************************************
399 \subsection[Unify-uTys]{@uTys@: getting down to business}
401 %************************************************************************
403 @uTys@ is the heart of the unifier. Each arg happens twice, because
404 we want to report errors in terms of synomyms if poss. The first of
405 the pair is used in error messages only; it is always the same as the
406 second, except that if the first is a synonym then the second may be a
407 de-synonym'd version. This way we get better error messages.
409 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
412 uTys :: TcTauType -> TcTauType -- Error reporting ty1 and real ty1
413 -- ty1 is the *expected* type
415 -> TcTauType -> TcTauType -- Error reporting ty2 and real ty2
416 -- ty2 is the *actual* type
419 -- Always expand synonyms (see notes at end)
420 -- (this also throws away FTVs)
421 uTys ps_ty1 (NoteTy n1 ty1) ps_ty2 ty2 = uTys ps_ty1 ty1 ps_ty2 ty2
422 uTys ps_ty1 ty1 ps_ty2 (NoteTy n2 ty2) = uTys ps_ty1 ty1 ps_ty2 ty2
424 -- Variables; go for uVar
425 uTys ps_ty1 (TyVarTy tyvar1) ps_ty2 ty2 = uVar False tyvar1 ps_ty2 ty2
426 uTys ps_ty1 ty1 ps_ty2 (TyVarTy tyvar2) = uVar True tyvar2 ps_ty1 ty1
427 -- "True" means args swapped
430 uTys _ (SourceTy (IParam n1 t1)) _ (SourceTy (IParam n2 t2))
431 | n1 == n2 = uTys t1 t1 t2 t2
432 uTys _ (SourceTy (ClassP c1 tys1)) _ (SourceTy (ClassP c2 tys2))
433 | c1 == c2 = unifyTauTyLists tys1 tys2
434 uTys _ (SourceTy (NType tc1 tys1)) _ (SourceTy (NType tc2 tys2))
435 | tc1 == tc2 = unifyTauTyLists tys1 tys2
437 -- Functions; just check the two parts
438 uTys _ (FunTy fun1 arg1) _ (FunTy fun2 arg2)
439 = uTys fun1 fun1 fun2 fun2 `thenM_` uTys arg1 arg1 arg2 arg2
441 -- Type constructors must match
442 uTys ps_ty1 (TyConApp con1 tys1) ps_ty2 (TyConApp con2 tys2)
443 | con1 == con2 && equalLength tys1 tys2
444 = unifyTauTyLists tys1 tys2
446 | con1 == openKindCon
447 -- When we are doing kind checking, we might match a kind '?'
448 -- against a kind '*' or '#'. Notably, CCallable :: ? -> *, and
449 -- (CCallable Int) and (CCallable Int#) are both OK
450 = unifyOpenTypeKind ps_ty2
452 -- Applications need a bit of care!
453 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
454 -- NB: we've already dealt with type variables and Notes,
455 -- so if one type is an App the other one jolly well better be too
456 uTys ps_ty1 (AppTy s1 t1) ps_ty2 ty2
457 = case tcSplitAppTy_maybe ty2 of
458 Just (s2,t2) -> uTys s1 s1 s2 s2 `thenM_` uTys t1 t1 t2 t2
459 Nothing -> unifyMisMatch ps_ty1 ps_ty2
461 -- Now the same, but the other way round
462 -- Don't swap the types, because the error messages get worse
463 uTys ps_ty1 ty1 ps_ty2 (AppTy s2 t2)
464 = case tcSplitAppTy_maybe ty1 of
465 Just (s1,t1) -> uTys s1 s1 s2 s2 `thenM_` uTys t1 t1 t2 t2
466 Nothing -> unifyMisMatch ps_ty1 ps_ty2
468 -- Not expecting for-alls in unification
469 -- ... but the error message from the unifyMisMatch more informative
470 -- than a panic message!
472 -- Anything else fails
473 uTys ps_ty1 ty1 ps_ty2 ty2 = unifyMisMatch ps_ty1 ps_ty2
479 If you are tempted to make a short cut on synonyms, as in this
483 -- NO uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
484 -- NO = if (con1 == con2) then
485 -- NO -- Good news! Same synonym constructors, so we can shortcut
486 -- NO -- by unifying their arguments and ignoring their expansions.
487 -- NO unifyTauTypeLists args1 args2
489 -- NO -- Never mind. Just expand them and try again
493 then THINK AGAIN. Here is the whole story, as detected and reported
494 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
496 Here's a test program that should detect the problem:
500 x = (1 :: Bogus Char) :: Bogus Bool
503 The problem with [the attempted shortcut code] is that
507 is not a sufficient condition to be able to use the shortcut!
508 You also need to know that the type synonym actually USES all
509 its arguments. For example, consider the following type synonym
510 which does not use all its arguments.
515 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
516 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
517 would fail, even though the expanded forms (both \tr{Int}) should
520 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
521 unnecessarily bind \tr{t} to \tr{Char}.
523 ... You could explicitly test for the problem synonyms and mark them
524 somehow as needing expansion, perhaps also issuing a warning to the
529 %************************************************************************
531 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
533 %************************************************************************
535 @uVar@ is called when at least one of the types being unified is a
536 variable. It does {\em not} assume that the variable is a fixed point
537 of the substitution; rather, notice that @uVar@ (defined below) nips
538 back into @uTys@ if it turns out that the variable is already bound.
541 uVar :: Bool -- False => tyvar is the "expected"
542 -- True => ty is the "expected" thing
544 -> TcTauType -> TcTauType -- printing and real versions
547 uVar swapped tv1 ps_ty2 ty2
548 = traceTc (text "uVar" <+> ppr swapped <+> ppr tv1 <+> (ppr ps_ty2 $$ ppr ty2)) `thenM_`
549 getTcTyVar tv1 `thenM` \ maybe_ty1 ->
551 Just ty1 | swapped -> uTys ps_ty2 ty2 ty1 ty1 -- Swap back
552 | otherwise -> uTys ty1 ty1 ps_ty2 ty2 -- Same order
553 other -> uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2
555 -- Expand synonyms; ignore FTVs
556 uUnboundVar swapped tv1 maybe_ty1 ps_ty2 (NoteTy n2 ty2)
557 = uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2
560 -- The both-type-variable case
561 uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2@(TyVarTy tv2)
563 -- Same type variable => no-op
567 -- Distinct type variables
568 -- ASSERT maybe_ty1 /= Just
570 = getTcTyVar tv2 `thenM` \ maybe_ty2 ->
572 Just ty2' -> uUnboundVar swapped tv1 maybe_ty1 ty2' ty2'
576 -> WARN( not (k1 `hasMoreBoxityInfo` k2), (ppr tv1 <+> ppr k1) $$ (ppr tv2 <+> ppr k2) )
577 putTcTyVar tv2 (TyVarTy tv1) `thenM_`
581 -> WARN( not (k2 `hasMoreBoxityInfo` k1), (ppr tv2 <+> ppr k2) $$ (ppr tv1 <+> ppr k1) )
582 putTcTyVar tv1 ps_ty2 `thenM_`
587 update_tv2 = (k2 `eqKind` openTypeKind) || (not (k1 `eqKind` openTypeKind) && nicer_to_update_tv2)
588 -- Try to get rid of open type variables as soon as poss
590 nicer_to_update_tv2 = isUserTyVar tv1
591 -- Don't unify a signature type variable if poss
592 || isSystemName (varName tv2)
593 -- Try to update sys-y type variables in preference to sig-y ones
595 -- Second one isn't a type variable
596 uUnboundVar swapped tv1 maybe_ty1 ps_ty2 non_var_ty2
597 = -- Check that tv1 isn't a type-signature type variable
598 checkM (not (isSkolemTyVar tv1))
599 (failWithTcM (unifyWithSigErr tv1 ps_ty2)) `thenM_`
601 -- Do the occurs check, and check that we are not
602 -- unifying a type variable with a polytype
603 -- Returns a zonked type ready for the update
604 checkValue tv1 ps_ty2 non_var_ty2 `thenM` \ ty2 ->
606 -- Check that the kinds match
607 checkKinds swapped tv1 ty2 `thenM_`
609 -- Perform the update
610 putTcTyVar tv1 ty2 `thenM_`
615 checkKinds swapped tv1 ty2
616 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
617 -- ty2 has been zonked at this stage, which ensures that
618 -- its kind has as much boxity information visible as possible.
619 | tk2 `hasMoreBoxityInfo` tk1 = returnM ()
622 -- Either the kinds aren't compatible
623 -- (can happen if we unify (a b) with (c d))
624 -- or we are unifying a lifted type variable with an
625 -- unlifted type: e.g. (id 3#) is illegal
626 = addErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
630 (k1,k2) | swapped = (tk2,tk1)
631 | otherwise = (tk1,tk2)
637 checkValue tv1 ps_ty2 non_var_ty2
638 -- Do the occurs check, and check that we are not
639 -- unifying a type variable with a polytype
640 -- Return the type to update the type variable with, or fail
642 -- Basically we want to update tv1 := ps_ty2
643 -- because ps_ty2 has type-synonym info, which improves later error messages
648 -- f :: (A a -> a -> ()) -> ()
652 -- x = f (\ x p -> p x)
654 -- In the application (p x), we try to match "t" with "A t". If we go
655 -- ahead and bind t to A t (= ps_ty2), we'll lead the type checker into
656 -- an infinite loop later.
657 -- But we should not reject the program, because A t = ().
658 -- Rather, we should bind t to () (= non_var_ty2).
660 -- That's why we have this two-state occurs-check
661 = zonkTcType ps_ty2 `thenM` \ ps_ty2' ->
662 case okToUnifyWith tv1 ps_ty2' of {
663 Nothing -> returnM ps_ty2' ; -- Success
666 zonkTcType non_var_ty2 `thenM` \ non_var_ty2' ->
667 case okToUnifyWith tv1 non_var_ty2' of
668 Nothing -> -- This branch rarely succeeds, except in strange cases
669 -- like that in the example above
672 Just problem -> failWithTcM (unifyCheck problem tv1 ps_ty2')
675 data Problem = OccurCheck | NotMonoType
677 okToUnifyWith :: TcTyVar -> TcType -> Maybe Problem
678 -- (okToUnifyWith tv ty) checks whether it's ok to unify
681 -- Just p => not ok, problem p
686 ok (TyVarTy tv') | tv == tv' = Just OccurCheck
687 | otherwise = Nothing
688 ok (AppTy t1 t2) = ok t1 `and` ok t2
689 ok (FunTy t1 t2) = ok t1 `and` ok t2
690 ok (TyConApp _ ts) = oks ts
691 ok (ForAllTy _ _) = Just NotMonoType
692 ok (SourceTy st) = ok_st st
693 ok (NoteTy (FTVNote _) t) = ok t
694 ok (NoteTy (SynNote t1) t2) = ok t1 `and` ok t2
695 -- Type variables may be free in t1 but not t2
696 -- A forall may be in t2 but not t1
698 oks ts = foldr (and . ok) Nothing ts
700 ok_st (ClassP _ ts) = oks ts
701 ok_st (IParam _ t) = ok t
702 ok_st (NType _ ts) = oks ts
705 Just p `and` m = Just p
708 %************************************************************************
710 \subsection[Unify-fun]{@unifyFunTy@}
712 %************************************************************************
714 @subFunTy@ and @unifyFunTy@ is used to avoid the fruitless
715 creation of type variables.
717 * subFunTy is used when we might be faced with a "hole" type variable,
718 in which case we should create two new holes.
720 * unifyFunTy is used when we expect to encounter only "ordinary"
721 type variables, so we should create new ordinary type variables
724 subFunTy :: TcHoleType -- Fail if ty isn't a function type
725 -- If it's a hole, make two holes, feed them to...
726 -> (TcHoleType -> TcHoleType -> TcM a) -- the thing inside
727 -> TcM a -- and bind the function type to the hole
729 subFunTy ty@(TyVarTy tyvar) thing_inside
731 = -- This is the interesting case
732 getTcTyVar tyvar `thenM` \ maybe_ty ->
734 Just ty' -> subFunTy ty' thing_inside ;
737 newHoleTyVarTy `thenM` \ arg_ty ->
738 newHoleTyVarTy `thenM` \ res_ty ->
741 thing_inside arg_ty res_ty `thenM` \ answer ->
743 -- Extract the answers
744 readHoleResult arg_ty `thenM` \ arg_ty' ->
745 readHoleResult res_ty `thenM` \ res_ty' ->
747 -- Write the answer into the incoming hole
748 putTcTyVar tyvar (mkFunTy arg_ty' res_ty') `thenM_`
750 -- And return the answer
753 subFunTy ty thing_inside
754 = unifyFunTy ty `thenM` \ (arg,res) ->
758 unifyFunTy :: TcRhoType -- Fail if ty isn't a function type
759 -> TcM (TcType, TcType) -- otherwise return arg and result types
761 unifyFunTy ty@(TyVarTy tyvar)
762 = ASSERT( not (isHoleTyVar tyvar) )
763 getTcTyVar tyvar `thenM` \ maybe_ty ->
765 Just ty' -> unifyFunTy ty'
766 Nothing -> unify_fun_ty_help ty
769 = case tcSplitFunTy_maybe ty of
770 Just arg_and_res -> returnM arg_and_res
771 Nothing -> unify_fun_ty_help ty
773 unify_fun_ty_help ty -- Special cases failed, so revert to ordinary unification
774 = newTyVarTy openTypeKind `thenM` \ arg ->
775 newTyVarTy openTypeKind `thenM` \ res ->
776 unifyTauTy ty (mkFunTy arg res) `thenM_`
781 unifyListTy :: TcType -- expected list type
782 -> TcM TcType -- list element type
784 unifyListTy ty@(TyVarTy tyvar)
785 = getTcTyVar tyvar `thenM` \ maybe_ty ->
787 Just ty' -> unifyListTy ty'
788 other -> unify_list_ty_help ty
791 = case tcSplitTyConApp_maybe ty of
792 Just (tycon, [arg_ty]) | tycon == listTyCon -> returnM arg_ty
793 other -> unify_list_ty_help ty
795 unify_list_ty_help ty -- Revert to ordinary unification
796 = newTyVarTy liftedTypeKind `thenM` \ elt_ty ->
797 unifyTauTy ty (mkListTy elt_ty) `thenM_`
800 -- variant for parallel arrays
802 unifyPArrTy :: TcType -- expected list type
803 -> TcM TcType -- list element type
805 unifyPArrTy ty@(TyVarTy tyvar)
806 = getTcTyVar tyvar `thenM` \ maybe_ty ->
808 Just ty' -> unifyPArrTy ty'
809 _ -> unify_parr_ty_help ty
811 = case tcSplitTyConApp_maybe ty of
812 Just (tycon, [arg_ty]) | tycon == parrTyCon -> returnM arg_ty
813 _ -> unify_parr_ty_help ty
815 unify_parr_ty_help ty -- Revert to ordinary unification
816 = newTyVarTy liftedTypeKind `thenM` \ elt_ty ->
817 unifyTauTy ty (mkPArrTy elt_ty) `thenM_`
822 unifyTupleTy :: Boxity -> Arity -> TcType -> TcM [TcType]
823 unifyTupleTy boxity arity ty@(TyVarTy tyvar)
824 = getTcTyVar tyvar `thenM` \ maybe_ty ->
826 Just ty' -> unifyTupleTy boxity arity ty'
827 other -> unify_tuple_ty_help boxity arity ty
829 unifyTupleTy boxity arity ty
830 = case tcSplitTyConApp_maybe ty of
831 Just (tycon, arg_tys)
833 && tyConArity tycon == arity
834 && tupleTyConBoxity tycon == boxity
836 other -> unify_tuple_ty_help boxity arity ty
838 unify_tuple_ty_help boxity arity ty
839 = newTyVarTys arity kind `thenM` \ arg_tys ->
840 unifyTauTy ty (mkTupleTy boxity arity arg_tys) `thenM_`
843 kind | isBoxed boxity = liftedTypeKind
844 | otherwise = openTypeKind
848 %************************************************************************
850 \subsection{Kind unification}
852 %************************************************************************
855 unifyKind :: TcKind -- Expected
858 unifyKind k1 k2 = uTys k1 k1 k2 k2
860 unifyKinds :: [TcKind] -> [TcKind] -> TcM ()
861 unifyKinds [] [] = returnM ()
862 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenM_`
864 unifyKinds _ _ = panic "unifyKinds: length mis-match"
868 unifyOpenTypeKind :: TcKind -> TcM ()
869 -- Ensures that the argument kind is of the form (Type bx)
870 -- for some boxity bx
872 unifyOpenTypeKind ty@(TyVarTy tyvar)
873 = getTcTyVar tyvar `thenM` \ maybe_ty ->
875 Just ty' -> unifyOpenTypeKind ty'
876 other -> unify_open_kind_help ty
879 | isTypeKind ty = returnM ()
880 | otherwise = unify_open_kind_help ty
882 unify_open_kind_help ty -- Revert to ordinary unification
883 = newOpenTypeKind `thenM` \ open_kind ->
884 unifyKind ty open_kind
888 unifyFunKind :: TcKind -> TcM (Maybe (TcKind, TcKind))
889 -- Like unifyFunTy, but does not fail; instead just returns Nothing
891 unifyFunKind (TyVarTy tyvar)
892 = getTcTyVar tyvar `thenM` \ maybe_ty ->
894 Just fun_kind -> unifyFunKind fun_kind
895 Nothing -> newKindVar `thenM` \ arg_kind ->
896 newKindVar `thenM` \ res_kind ->
897 putTcTyVar tyvar (mkArrowKind arg_kind res_kind) `thenM_`
898 returnM (Just (arg_kind,res_kind))
900 unifyFunKind (FunTy arg_kind res_kind) = returnM (Just (arg_kind,res_kind))
901 unifyFunKind (NoteTy _ ty) = unifyFunKind ty
902 unifyFunKind other = returnM Nothing
905 %************************************************************************
907 \subsection[Unify-context]{Errors and contexts}
909 %************************************************************************
915 unifyCtxt s ty1 ty2 tidy_env -- ty1 expected, ty2 inferred
916 = zonkTcType ty1 `thenM` \ ty1' ->
917 zonkTcType ty2 `thenM` \ ty2' ->
918 returnM (err ty1' ty2')
923 text "Expected" <+> text s <> colon <+> ppr tidy_ty1,
924 text "Inferred" <+> text s <> colon <+> ppr tidy_ty2
927 (env1, [tidy_ty1,tidy_ty2]) = tidyOpenTypes tidy_env [ty1,ty2]
929 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
930 -- tv1 is zonked already
931 = zonkTcType ty2 `thenM` \ ty2' ->
934 err ty2 = (env2, ptext SLIT("When matching types") <+>
935 sep [quotes pp_expected, ptext SLIT("and"), quotes pp_actual])
937 (pp_expected, pp_actual) | swapped = (pp2, pp1)
938 | otherwise = (pp1, pp2)
939 (env1, tv1') = tidyOpenTyVar tidy_env tv1
940 (env2, ty2') = tidyOpenType env1 ty2
944 unifyMisMatch ty1 ty2
945 = zonkTcType ty1 `thenM` \ ty1' ->
946 zonkTcType ty2 `thenM` \ ty2' ->
948 (env, [tidy_ty1, tidy_ty2]) = tidyOpenTypes emptyTidyEnv [ty1',ty2']
949 msg = hang (ptext SLIT("Couldn't match"))
950 4 (sep [quotes (ppr tidy_ty1),
951 ptext SLIT("against"),
952 quotes (ppr tidy_ty2)])
954 failWithTcM (env, msg)
956 unifyWithSigErr tyvar ty
957 = (env2, hang (ptext SLIT("Cannot unify the type-signature variable") <+> quotes (ppr tidy_tyvar))
958 4 (ptext SLIT("with the type") <+> quotes (ppr tidy_ty)))
960 (env1, tidy_tyvar) = tidyOpenTyVar emptyTidyEnv tyvar
961 (env2, tidy_ty) = tidyOpenType env1 ty
963 unifyCheck problem tyvar ty
965 4 (sep [ppr tidy_tyvar, char '=', ppr tidy_ty]))
967 (env1, tidy_tyvar) = tidyOpenTyVar emptyTidyEnv tyvar
968 (env2, tidy_ty) = tidyOpenType env1 ty
970 msg = case problem of
971 OccurCheck -> ptext SLIT("Occurs check: cannot construct the infinite type:")
972 NotMonoType -> ptext SLIT("Cannot unify a type variable with a type scheme:")
977 %************************************************************************
979 \subsection{Checking signature type variables}
981 %************************************************************************
983 @checkSigTyVars@ is used after the type in a type signature has been unified with
984 the actual type found. It then checks that the type variables of the type signature
986 (a) Still all type variables
987 eg matching signature [a] against inferred type [(p,q)]
988 [then a will be unified to a non-type variable]
990 (b) Still all distinct
991 eg matching signature [(a,b)] against inferred type [(p,p)]
992 [then a and b will be unified together]
994 (c) Not mentioned in the environment
995 eg the signature for f in this:
1001 Here, f is forced to be monorphic by the free occurence of x.
1003 (d) Not (unified with another type variable that is) in scope.
1004 eg f x :: (r->r) = (\y->y) :: forall a. a->r
1005 when checking the expression type signature, we find that
1006 even though there is nothing in scope whose type mentions r,
1007 nevertheless the type signature for the expression isn't right.
1009 Another example is in a class or instance declaration:
1011 op :: forall b. a -> b
1013 Here, b gets unified with a
1015 Before doing this, the substitution is applied to the signature type variable.
1017 We used to have the notion of a "DontBind" type variable, which would
1018 only be bound to itself or nothing. Then points (a) and (b) were
1019 self-checking. But it gave rise to bogus consequential error messages.
1022 f = (*) -- Monomorphic
1024 g :: Num a => a -> a
1027 Here, we get a complaint when checking the type signature for g,
1028 that g isn't polymorphic enough; but then we get another one when
1029 dealing with the (Num x) context arising from f's definition;
1030 we try to unify x with Int (to default it), but find that x has already
1031 been unified with the DontBind variable "a" from g's signature.
1032 This is really a problem with side-effecting unification; we'd like to
1033 undo g's effects when its type signature fails, but unification is done
1034 by side effect, so we can't (easily).
1036 So we revert to ordinary type variables for signatures, and try to
1037 give a helpful message in checkSigTyVars.
1040 checkSigTyVars :: [TcTyVar] -> TcM [TcTyVar]
1041 checkSigTyVars sig_tvs = check_sig_tyvars emptyVarSet sig_tvs
1043 checkSigTyVarsWrt :: TcTyVarSet -> [TcTyVar] -> TcM [TcTyVar]
1044 checkSigTyVarsWrt extra_tvs sig_tvs
1045 = zonkTcTyVarsAndFV (varSetElems extra_tvs) `thenM` \ extra_tvs' ->
1046 check_sig_tyvars extra_tvs' sig_tvs
1049 :: TcTyVarSet -- Global type variables. The universally quantified
1050 -- tyvars should not mention any of these
1051 -- Guaranteed already zonked.
1052 -> [TcTyVar] -- Universally-quantified type variables in the signature
1053 -- Not guaranteed zonked.
1054 -> TcM [TcTyVar] -- Zonked signature type variables
1056 check_sig_tyvars extra_tvs []
1058 check_sig_tyvars extra_tvs sig_tvs
1059 = zonkTcTyVars sig_tvs `thenM` \ sig_tys ->
1060 tcGetGlobalTyVars `thenM` \ gbl_tvs ->
1062 env_tvs = gbl_tvs `unionVarSet` extra_tvs
1064 traceTc (text "check_sig_tyvars" <+> (vcat [text "sig_tys" <+> ppr sig_tys,
1065 text "gbl_tvs" <+> ppr gbl_tvs,
1066 text "extra_tvs" <+> ppr extra_tvs])) `thenM_`
1068 checkM (allDistinctTyVars sig_tys env_tvs)
1069 (complain sig_tys env_tvs) `thenM_`
1071 returnM (map (tcGetTyVar "checkSigTyVars") sig_tys)
1074 complain sig_tys globals
1075 = -- "check" checks each sig tyvar in turn
1077 (env2, emptyVarEnv, [])
1078 (tidy_tvs `zip` tidy_tys) `thenM` \ (env3, _, msgs) ->
1080 failWithTcM (env3, main_msg $$ nest 4 (vcat msgs))
1082 (env1, tidy_tvs) = tidyOpenTyVars emptyTidyEnv sig_tvs
1083 (env2, tidy_tys) = tidyOpenTypes env1 sig_tys
1085 main_msg = ptext SLIT("Inferred type is less polymorphic than expected")
1087 check (tidy_env, acc, msgs) (sig_tyvar,ty)
1088 -- sig_tyvar is from the signature;
1089 -- ty is what you get if you zonk sig_tyvar and then tidy it
1091 -- acc maps a zonked type variable back to a signature type variable
1092 = case tcGetTyVar_maybe ty of {
1093 Nothing -> -- Error (a)!
1094 returnM (tidy_env, acc, unify_msg sig_tyvar (quotes (ppr ty)) : msgs) ;
1098 case lookupVarEnv acc tv of {
1099 Just sig_tyvar' -> -- Error (b)!
1100 returnM (tidy_env, acc, unify_msg sig_tyvar thing : msgs)
1102 thing = ptext SLIT("another quantified type variable") <+> quotes (ppr sig_tyvar')
1106 if tv `elemVarSet` globals -- Error (c) or (d)! Type variable escapes
1107 -- The least comprehensible, so put it last
1109 -- get the local TcIds and TyVars from the environment,
1110 -- and pass them to find_globals (they might have tv free)
1111 then findGlobals (unitVarSet tv) tidy_env `thenM` \ (tidy_env1, globs) ->
1112 returnM (tidy_env1, acc, escape_msg sig_tyvar tv globs : msgs)
1115 returnM (tidy_env, extendVarEnv acc tv sig_tyvar, msgs)
1121 -----------------------
1122 escape_msg sig_tv tv globs
1123 = mk_msg sig_tv <+> ptext SLIT("escapes") $$
1124 if notNull globs then
1125 vcat [pp_it <+> ptext SLIT("is mentioned in the environment:"),
1126 nest 2 (vcat globs)]
1128 empty -- Sigh. It's really hard to give a good error message
1129 -- all the time. One bad case is an existential pattern match.
1130 -- We rely on the "When..." context to help.
1132 pp_it | sig_tv /= tv = ptext SLIT("It unifies with") <+> quotes (ppr tv) <> comma <+> ptext SLIT("which")
1133 | otherwise = ptext SLIT("It")
1136 unify_msg tv thing = mk_msg tv <+> ptext SLIT("is unified with") <+> thing
1137 mk_msg tv = ptext SLIT("Quantified type variable") <+> quotes (ppr tv)
1140 These two context are used with checkSigTyVars
1143 sigCtxt :: Id -> [TcTyVar] -> TcThetaType -> TcTauType
1144 -> TidyEnv -> TcM (TidyEnv, Message)
1145 sigCtxt id sig_tvs sig_theta sig_tau tidy_env
1146 = zonkTcType sig_tau `thenM` \ actual_tau ->
1148 (env1, tidy_sig_tvs) = tidyOpenTyVars tidy_env sig_tvs
1149 (env2, tidy_sig_rho) = tidyOpenType env1 (mkPhiTy sig_theta sig_tau)
1150 (env3, tidy_actual_tau) = tidyOpenType env2 actual_tau
1151 sub_msg = vcat [ptext SLIT("Signature type: ") <+> pprType (mkForAllTys tidy_sig_tvs tidy_sig_rho),
1152 ptext SLIT("Type to generalise:") <+> pprType tidy_actual_tau
1154 msg = vcat [ptext SLIT("When trying to generalise the type inferred for") <+> quotes (ppr id),