2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[Unify]{Unifier}
6 The unifier is now squarely in the typechecker monad (because of the
7 updatable substitution).
10 module TcUnify ( unifyTauTy, unifyTauTyList, unifyTauTyLists,
11 unifyFunTy, unifyListTy, unifyTupleTy,
12 unifyKind, unifyKinds, unifyOpenTypeKind
15 #include "HsVersions.h"
19 import TypeRep ( Type(..) ) -- friend
20 import Type ( funTyCon, Kind, unboxedTypeKind, boxedTypeKind, openTypeKind,
21 superBoxity, typeCon, openKindCon, hasMoreBoxityInfo,
22 tyVarsOfType, typeKind,
23 mkTyVarTy, mkFunTy, splitFunTy_maybe, splitTyConApp_maybe,
24 isNotUsgTy, splitAppTy_maybe, mkTyConApp,
25 tidyOpenType, tidyOpenTypes, tidyTyVar
27 import TyCon ( TyCon, isTupleTyCon, tupleTyConBoxity, tyConArity )
28 import Name ( hasBetterProv )
29 import Var ( TyVar, tyVarKind, varName, isSigTyVar )
30 import VarSet ( varSetElems )
31 import TcType ( TcType, TcTauType, TcTyVar, TcKind, newBoxityVar,
32 newTyVarTy, newTyVarTys, tcGetTyVar, tcPutTyVar, zonkTcType
36 import BasicTypes ( Arity, Boxity, isBoxed )
37 import TysWiredIn ( listTyCon, mkListTy, mkTupleTy )
42 %************************************************************************
44 \subsection{The Kind variants}
46 %************************************************************************
49 unifyKind :: TcKind -- Expected
53 = tcAddErrCtxtM (unifyCtxt "kind" k1 k2) $
56 unifyKinds :: [TcKind] -> [TcKind] -> TcM s ()
57 unifyKinds [] [] = returnTc ()
58 unifyKinds (k1:ks1) (k2:ks2) = unifyKind k1 k2 `thenTc_`
60 unifyKinds _ _ = panic "unifyKinds: length mis-match"
64 unifyOpenTypeKind :: TcKind -> TcM s ()
65 -- Ensures that the argument kind is of the form (Type bx)
68 unifyOpenTypeKind ty@(TyVarTy tyvar)
69 = tcGetTyVar tyvar `thenNF_Tc` \ maybe_ty ->
71 Just ty' -> unifyOpenTypeKind ty'
72 other -> unify_open_kind_help ty
75 = case splitTyConApp_maybe ty of
76 Just (tycon, [_]) | tycon == typeCon -> returnTc ()
77 other -> unify_open_kind_help ty
79 unify_open_kind_help ty -- Revert to ordinary unification
80 = newBoxityVar `thenNF_Tc` \ boxity ->
81 unifyKind ty (mkTyConApp typeCon [boxity])
85 %************************************************************************
87 \subsection[Unify-exported]{Exported unification functions}
89 %************************************************************************
91 The exported functions are all defined as versions of some
92 non-exported generic functions.
94 Unify two @TauType@s. Dead straightforward.
97 unifyTauTy :: TcTauType -> TcTauType -> TcM s ()
98 unifyTauTy ty1 ty2 -- ty1 expected, ty2 inferred
99 = tcAddErrCtxtM (unifyCtxt "type" ty1 ty2) $
103 @unifyTauTyList@ unifies corresponding elements of two lists of
104 @TauType@s. It uses @uTys@ to do the real work. The lists should be
105 of equal length. We charge down the list explicitly so that we can
106 complain if their lengths differ.
109 unifyTauTyLists :: [TcTauType] -> [TcTauType] -> TcM s ()
110 unifyTauTyLists [] [] = returnTc ()
111 unifyTauTyLists (ty1:tys1) (ty2:tys2) = uTys ty1 ty1 ty2 ty2 `thenTc_`
112 unifyTauTyLists tys1 tys2
113 unifyTauTyLists ty1s ty2s = panic "Unify.unifyTauTyLists: mismatched type lists!"
116 @unifyTauTyList@ takes a single list of @TauType@s and unifies them
117 all together. It is used, for example, when typechecking explicit
118 lists, when all the elts should be of the same type.
121 unifyTauTyList :: [TcTauType] -> TcM s ()
122 unifyTauTyList [] = returnTc ()
123 unifyTauTyList [ty] = returnTc ()
124 unifyTauTyList (ty1:tys@(ty2:_)) = unifyTauTy ty1 ty2 `thenTc_`
128 %************************************************************************
130 \subsection[Unify-uTys]{@uTys@: getting down to business}
132 %************************************************************************
134 @uTys@ is the heart of the unifier. Each arg happens twice, because
135 we want to report errors in terms of synomyms if poss. The first of
136 the pair is used in error messages only; it is always the same as the
137 second, except that if the first is a synonym then the second may be a
138 de-synonym'd version. This way we get better error messages.
140 We call the first one \tr{ps_ty1}, \tr{ps_ty2} for ``possible synomym''.
143 uTys :: TcTauType -> TcTauType -- Error reporting ty1 and real ty1
144 -- ty1 is the *expected* type
146 -> TcTauType -> TcTauType -- Error reporting ty2 and real ty2
147 -- ty2 is the *actual* type
150 -- Always expand synonyms (see notes at end)
151 -- (this also throws away FTVs and usage annots)
152 uTys ps_ty1 (NoteTy _ ty1) ps_ty2 ty2 = uTys ps_ty1 ty1 ps_ty2 ty2
153 uTys ps_ty1 ty1 ps_ty2 (NoteTy _ ty2) = uTys ps_ty1 ty1 ps_ty2 ty2
155 -- Variables; go for uVar
156 uTys ps_ty1 (TyVarTy tyvar1) ps_ty2 ty2 = uVar False tyvar1 ps_ty2 ty2
157 uTys ps_ty1 ty1 ps_ty2 (TyVarTy tyvar2) = uVar True tyvar2 ps_ty1 ty1
158 -- "True" means args swapped
160 -- Functions; just check the two parts
161 uTys _ (FunTy fun1 arg1) _ (FunTy fun2 arg2)
162 = uTys fun1 fun1 fun2 fun2 `thenTc_` uTys arg1 arg1 arg2 arg2
164 -- Type constructors must match
165 uTys ps_ty1 (TyConApp con1 tys1) ps_ty2 (TyConApp con2 tys2)
166 | con1 == con2 && length tys1 == length tys2
167 = unifyTauTyLists tys1 tys2
169 | con1 == openKindCon
170 -- When we are doing kind checking, we might match a kind '?'
171 -- against a kind '*' or '#'. Notably, CCallable :: ? -> *, and
172 -- (CCallable Int) and (CCallable Int#) are both OK
173 = unifyOpenTypeKind ps_ty2
176 = unifyMisMatch ps_ty1 ps_ty2
179 -- Applications need a bit of care!
180 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
181 -- NB: we've already dealt with type variables and Notes,
182 -- so if one type is an App the other one jolly well better be too
183 uTys ps_ty1 (AppTy s1 t1) ps_ty2 ty2
184 = case splitAppTy_maybe ty2 of
185 Just (s2,t2) -> uTys s1 s1 s2 s2 `thenTc_` uTys t1 t1 t2 t2
186 Nothing -> unifyMisMatch ps_ty1 ps_ty2
188 -- Now the same, but the other way round
189 -- Don't swap the types, because the error messages get worse
190 uTys ps_ty1 ty1 ps_ty2 (AppTy s2 t2)
191 = case splitAppTy_maybe ty1 of
192 Just (s1,t1) -> uTys s1 s1 s2 s2 `thenTc_` uTys t1 t1 t2 t2
193 Nothing -> unifyMisMatch ps_ty1 ps_ty2
195 -- Not expecting for-alls in unification
196 -- ... but the error message from the unifyMisMatch more informative
197 -- than a panic message!
199 -- Anything else fails
200 uTys ps_ty1 ty1 ps_ty2 ty2 = unifyMisMatch ps_ty1 ps_ty2
205 If you are tempted to make a short cut on synonyms, as in this
209 uTys (SynTy con1 args1 ty1) (SynTy con2 args2 ty2)
210 = if (con1 == con2) then
211 -- Good news! Same synonym constructors, so we can shortcut
212 -- by unifying their arguments and ignoring their expansions.
213 unifyTauTypeLists args1 args2
215 -- Never mind. Just expand them and try again
219 then THINK AGAIN. Here is the whole story, as detected and reported
220 by Chris Okasaki \tr{<Chris_Okasaki@loch.mess.cs.cmu.edu>}:
222 Here's a test program that should detect the problem:
226 x = (1 :: Bogus Char) :: Bogus Bool
229 The problem with [the attempted shortcut code] is that
233 is not a sufficient condition to be able to use the shortcut!
234 You also need to know that the type synonym actually USES all
235 its arguments. For example, consider the following type synonym
236 which does not use all its arguments.
241 If you ever tried unifying, say, \tr{Bogus Char} with \tr{Bogus Bool},
242 the unifier would blithely try to unify \tr{Char} with \tr{Bool} and
243 would fail, even though the expanded forms (both \tr{Int}) should
246 Similarly, unifying \tr{Bogus Char} with \tr{Bogus t} would
247 unnecessarily bind \tr{t} to \tr{Char}.
249 ... You could explicitly test for the problem synonyms and mark them
250 somehow as needing expansion, perhaps also issuing a warning to the
255 %************************************************************************
257 \subsection[Unify-uVar]{@uVar@: unifying with a type variable}
259 %************************************************************************
261 @uVar@ is called when at least one of the types being unified is a
262 variable. It does {\em not} assume that the variable is a fixed point
263 of the substitution; rather, notice that @uVar@ (defined below) nips
264 back into @uTys@ if it turns out that the variable is already bound.
267 uVar :: Bool -- False => tyvar is the "expected"
268 -- True => ty is the "expected" thing
270 -> TcTauType -> TcTauType -- printing and real versions
273 uVar swapped tv1 ps_ty2 ty2
274 = tcGetTyVar tv1 `thenNF_Tc` \ maybe_ty1 ->
276 Just ty1 | swapped -> uTys ps_ty2 ty2 ty1 ty1 -- Swap back
277 | otherwise -> uTys ty1 ty1 ps_ty2 ty2 -- Same order
278 other -> uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2
280 -- Expand synonyms; ignore FTVs; ignore usage annots
281 uUnboundVar swapped tv1 maybe_ty1 ps_ty2 (NoteTy _ ty2)
282 = uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2
285 -- The both-type-variable case
286 uUnboundVar swapped tv1 maybe_ty1 ps_ty2 ty2@(TyVarTy tv2)
288 -- Same type variable => no-op
292 -- Distinct type variables
293 -- ASSERT maybe_ty1 /= Just
295 = tcGetTyVar tv2 `thenNF_Tc` \ maybe_ty2 ->
297 Just ty2' -> uUnboundVar swapped tv1 maybe_ty1 ty2' ty2'
299 Nothing | tv1_dominates_tv2
301 -> WARN( not (k1 `hasMoreBoxityInfo` k2), (ppr tv1 <+> ppr k1) $$ (ppr tv2 <+> ppr k2) )
302 tcPutTyVar tv2 (TyVarTy tv1) `thenNF_Tc_`
306 -> WARN( not (k2 `hasMoreBoxityInfo` k1), (ppr tv2 <+> ppr k2) $$ (ppr tv1 <+> ppr k1) )
307 (ASSERT( isNotUsgTy ps_ty2 )
308 tcPutTyVar tv1 ps_ty2 `thenNF_Tc_`
313 tv1_dominates_tv2 = isSigTyVar tv1
314 -- Don't unify a signature type variable if poss
315 || k2 == openTypeKind
316 -- Try to get rid of open type variables as soon as poss
317 || varName tv1 `hasBetterProv` varName tv2
318 -- Try to update sys-y type variables in preference to sig-y ones
320 -- Second one isn't a type variable
321 uUnboundVar swapped tv1 maybe_ty1 ps_ty2 non_var_ty2
322 = checkKinds swapped tv1 non_var_ty2 `thenTc_`
323 occur_check non_var_ty2 `thenTc_`
324 ASSERT( isNotUsgTy ps_ty2 )
325 checkTcM (not (isSigTyVar tv1))
326 (failWithTcM (unifyWithSigErr tv1 ps_ty2)) `thenTc_`
328 WARN( not (typeKind non_var_ty2 `hasMoreBoxityInfo` tyVarKind tv1), (ppr tv1 <+> ppr (tyVarKind tv1)) $$
329 (ppr non_var_ty2 <+> ppr (typeKind non_var_ty2)) )
330 tcPutTyVar tv1 non_var_ty2 `thenNF_Tc_`
331 -- This used to say "ps_ty2" instead of "non_var_ty2"
333 -- But that led to an infinite loop in the type checker!
337 -- f :: (A a -> a -> ()) -> ()
341 -- x = f (\ x p -> p x)
343 -- Here, we try to match "t" with "A t", and succeed
344 -- because the unifier looks through synonyms. The occurs
345 -- check doesn't kick in because we are "really" binding "t" to "()",
346 -- but we *actually* bind "t" to "A t" if we store ps_ty2.
347 -- That leads the typechecker into an infinite loop later.
351 occur_check ty = mapTc occur_check_tv (varSetElems (tyVarsOfType ty)) `thenTc_`
355 | tv1 == tv2 -- Same tyvar; fail
356 = zonkTcType ps_ty2 `thenNF_Tc` \ zonked_ty2 ->
357 failWithTcM (unifyOccurCheck tv1 zonked_ty2)
359 | otherwise -- A different tyvar
360 = tcGetTyVar tv2 `thenNF_Tc` \ maybe_ty2 ->
362 Just ty2' -> occur_check ty2'
365 checkKinds swapped tv1 ty2
366 -- We're about to unify a type variable tv1 with a non-tyvar-type ty2.
367 -- We need to check that we don't unify a boxed type variable with an
368 -- unboxed type: e.g. (id 3#) is illegal
369 | tk1 == boxedTypeKind && tk2 == unboxedTypeKind
370 = tcAddErrCtxtM (unifyKindCtxt swapped tv1 ty2) $
375 (k1,k2) | swapped = (tk2,tk1)
376 | otherwise = (tk1,tk2)
382 %************************************************************************
384 \subsection[Unify-fun]{@unifyFunTy@}
386 %************************************************************************
388 @unifyFunTy@ is used to avoid the fruitless creation of type variables.
391 unifyFunTy :: TcType -- Fail if ty isn't a function type
392 -> TcM s (TcType, TcType) -- otherwise return arg and result types
394 unifyFunTy ty@(TyVarTy tyvar)
395 = tcGetTyVar tyvar `thenNF_Tc` \ maybe_ty ->
397 Just ty' -> unifyFunTy ty'
398 other -> unify_fun_ty_help ty
401 = case splitFunTy_maybe ty of
402 Just arg_and_res -> returnTc arg_and_res
403 Nothing -> unify_fun_ty_help ty
405 unify_fun_ty_help ty -- Special cases failed, so revert to ordinary unification
406 = newTyVarTy openTypeKind `thenNF_Tc` \ arg ->
407 newTyVarTy openTypeKind `thenNF_Tc` \ res ->
408 unifyTauTy ty (mkFunTy arg res) `thenTc_`
413 unifyListTy :: TcType -- expected list type
414 -> TcM s TcType -- list element type
416 unifyListTy ty@(TyVarTy tyvar)
417 = tcGetTyVar tyvar `thenNF_Tc` \ maybe_ty ->
419 Just ty' -> unifyListTy ty'
420 other -> unify_list_ty_help ty
423 = case splitTyConApp_maybe ty of
424 Just (tycon, [arg_ty]) | tycon == listTyCon -> returnTc arg_ty
425 other -> unify_list_ty_help ty
427 unify_list_ty_help ty -- Revert to ordinary unification
428 = newTyVarTy boxedTypeKind `thenNF_Tc` \ elt_ty ->
429 unifyTauTy ty (mkListTy elt_ty) `thenTc_`
434 unifyTupleTy :: Boxity -> Arity -> TcType -> TcM s [TcType]
435 unifyTupleTy boxity arity ty@(TyVarTy tyvar)
436 = tcGetTyVar tyvar `thenNF_Tc` \ maybe_ty ->
438 Just ty' -> unifyTupleTy boxity arity ty'
439 other -> unify_tuple_ty_help boxity arity ty
441 unifyTupleTy boxity arity ty
442 = case splitTyConApp_maybe ty of
443 Just (tycon, arg_tys)
445 && tyConArity tycon == arity
446 && tupleTyConBoxity tycon == boxity
448 other -> unify_tuple_ty_help boxity arity ty
450 unify_tuple_ty_help boxity arity ty
451 = newTyVarTys arity kind `thenNF_Tc` \ arg_tys ->
452 unifyTauTy ty (mkTupleTy boxity arity arg_tys) `thenTc_`
455 kind | isBoxed boxity = boxedTypeKind
456 | otherwise = openTypeKind
460 %************************************************************************
462 \subsection[Unify-context]{Errors and contexts}
464 %************************************************************************
470 unifyCtxt s ty1 ty2 tidy_env -- ty1 expected, ty2 inferred
471 = zonkTcType ty1 `thenNF_Tc` \ ty1' ->
472 zonkTcType ty2 `thenNF_Tc` \ ty2' ->
473 returnNF_Tc (err ty1' ty2')
478 text "Expected" <+> text s <> colon <+> ppr tidy_ty1,
479 text "Inferred" <+> text s <> colon <+> ppr tidy_ty2
482 (env1, [tidy_ty1,tidy_ty2]) = tidyOpenTypes tidy_env [ty1,ty2]
484 unifyKindCtxt swapped tv1 ty2 tidy_env -- not swapped => tv1 expected, ty2 inferred
485 -- tv1 is zonked already
486 = zonkTcType ty2 `thenNF_Tc` \ ty2' ->
487 returnNF_Tc (err ty2')
489 err ty2 = (env2, ptext SLIT("When matching types") <+>
490 sep [quotes pp_expected, ptext SLIT("and"), quotes pp_actual])
492 (pp_expected, pp_actual) | swapped = (pp2, pp1)
493 | otherwise = (pp1, pp2)
494 (env1, tv1') = tidyTyVar tidy_env tv1
495 (env2, ty2') = tidyOpenType env1 ty2
499 unifyMisMatch ty1 ty2
500 = zonkTcType ty1 `thenNF_Tc` \ ty1' ->
501 zonkTcType ty2 `thenNF_Tc` \ ty2' ->
503 (env, [tidy_ty1, tidy_ty2]) = tidyOpenTypes emptyTidyEnv [ty1',ty2']
504 msg = hang (ptext SLIT("Couldn't match"))
505 4 (sep [quotes (ppr tidy_ty1),
506 ptext SLIT("against"),
507 quotes (ppr tidy_ty2)])
509 failWithTcM (env, msg)
511 unifyWithSigErr tyvar ty
512 = (env2, hang (ptext SLIT("Cannot unify the type-signature variable") <+> quotes (ppr tidy_tyvar))
513 4 (ptext SLIT("with the type") <+> quotes (ppr tidy_ty)))
515 (env1, tidy_tyvar) = tidyTyVar emptyTidyEnv tyvar
516 (env2, tidy_ty) = tidyOpenType env1 ty
518 unifyOccurCheck tyvar ty
519 = (env2, hang (ptext SLIT("Occurs check: cannot construct the infinite type:"))
520 4 (sep [ppr tidy_tyvar, char '=', ppr tidy_ty]))
522 (env1, tidy_tyvar) = tidyTyVar emptyTidyEnv tyvar
523 (env2, tidy_ty) = tidyOpenType env1 ty