1 Normalisation of type terms relative to type instances as well as
2 normalisation and entailment checking of equality constraints.
6 -- type normalisation wrt to toplevel equalities only
9 -- instance normalisation wrt to equalities
13 misMatchMsg, failWithMisMatch,
18 #include "HsVersions.h"
30 import TypeRep ( Type(..) )
40 import SrcLoc ( Located(..) )
50 %************************************************************************
52 Normalisation of types wrt toplevel equality schemata
54 %************************************************************************
56 Unfold a single synonym family instance and yield the witnessing coercion.
57 Return 'Nothing' if the given type is either not synonym family instance
58 or is a synonym family instance that has no matching instance declaration.
59 (Applies only if the type family application is outermost.)
61 For example, if we have
63 :Co:R42T a :: T [a] ~ :R42T a
65 then 'T [Int]' unfolds to (:R42T Int, :Co:R42T Int).
68 tcUnfoldSynFamInst :: Type -> TcM (Maybe (Type, Coercion))
69 tcUnfoldSynFamInst (TyConApp tycon tys)
70 | not (isOpenSynTyCon tycon) -- unfold *only* _synonym_ family instances
73 = do { -- we only use the indexing arguments for matching,
74 -- not the additional ones
75 ; maybeFamInst <- tcLookupFamInst tycon idxTys
76 ; case maybeFamInst of
77 Nothing -> return Nothing
78 Just (rep_tc, rep_tys) -> return $ Just (mkTyConApp rep_tc tys',
79 mkTyConApp coe_tc tys')
81 tys' = rep_tys ++ restTys
82 coe_tc = expectJust "TcTyFuns.tcUnfoldSynFamInst"
83 (tyConFamilyCoercion_maybe rep_tc)
87 (idxTys, restTys) = splitAt n tys
88 tcUnfoldSynFamInst _other = return Nothing
91 Normalise 'Type's and 'PredType's by unfolding type family applications where
92 possible (ie, we treat family instances as a TRS). Also zonk meta variables.
94 tcNormaliseFamInst ty = (co, ty')
98 -- |Normalise the given type as far as possible with toplevel equalities.
99 -- This results in a coercion witnessing the type equality, in addition to the
102 tcNormaliseFamInst :: TcType -> TcM (CoercionI, TcType)
103 tcNormaliseFamInst = tcGenericNormaliseFamInst tcUnfoldSynFamInst
106 Generic normalisation of 'Type's and 'PredType's; ie, walk the type term and
107 apply the normalisation function gives as the first argument to every TyConApp
108 and every TyVarTy subterm.
110 tcGenericNormaliseFamInst fun ty = (co, ty')
113 This function is (by way of using smart constructors) careful to ensure that
114 the returned coercion is exactly IdCo (and not some semantically equivalent,
115 but syntactically different coercion) whenever (ty' `tcEqType` ty). This
116 makes it easy for the caller to determine whether the type changed. BUT
117 even if we return IdCo, ty' may be *syntactically* different from ty due to
118 unfolded closed type synonyms (by way of tcCoreView). In the interest of
119 good error messages, callers should discard ty' in favour of ty in this case.
122 tcGenericNormaliseFamInst :: (TcType -> TcM (Maybe (TcType, Coercion)))
123 -- what to do with type functions and tyvars
124 -> TcType -- old type
125 -> TcM (CoercionI, TcType) -- (coercion, new type)
126 tcGenericNormaliseFamInst fun ty
127 | Just ty' <- tcView ty = tcGenericNormaliseFamInst fun ty'
128 tcGenericNormaliseFamInst fun (TyConApp tyCon tys)
129 = do { (cois, ntys) <- mapAndUnzipM (tcGenericNormaliseFamInst fun) tys
130 ; let tycon_coi = mkTyConAppCoI tyCon ntys cois
131 ; maybe_ty_co <- fun (mkTyConApp tyCon ntys) -- use normalised args!
132 ; case maybe_ty_co of
133 -- a matching family instance exists
135 do { let first_coi = mkTransCoI tycon_coi (ACo co)
136 ; (rest_coi, nty) <- tcGenericNormaliseFamInst fun ty'
137 ; let fix_coi = mkTransCoI first_coi rest_coi
138 ; return (fix_coi, nty)
140 -- no matching family instance exists
141 -- we do not do anything
142 Nothing -> return (tycon_coi, mkTyConApp tyCon ntys)
144 tcGenericNormaliseFamInst fun (AppTy ty1 ty2)
145 = do { (coi1,nty1) <- tcGenericNormaliseFamInst fun ty1
146 ; (coi2,nty2) <- tcGenericNormaliseFamInst fun ty2
147 ; return (mkAppTyCoI nty1 coi1 nty2 coi2, mkAppTy nty1 nty2)
149 tcGenericNormaliseFamInst fun (FunTy ty1 ty2)
150 = do { (coi1,nty1) <- tcGenericNormaliseFamInst fun ty1
151 ; (coi2,nty2) <- tcGenericNormaliseFamInst fun ty2
152 ; return (mkFunTyCoI nty1 coi1 nty2 coi2, mkFunTy nty1 nty2)
154 tcGenericNormaliseFamInst fun (ForAllTy tyvar ty1)
155 = do { (coi,nty1) <- tcGenericNormaliseFamInst fun ty1
156 ; return (mkForAllTyCoI tyvar coi, mkForAllTy tyvar nty1)
158 tcGenericNormaliseFamInst fun ty@(TyVarTy tv)
160 = do { traceTc (text "tcGenericNormaliseFamInst" <+> ppr ty)
161 ; res <- lookupTcTyVar tv
164 do { maybe_ty' <- fun ty
166 Nothing -> return (IdCo, ty)
168 do { (coi2, ty'') <- tcGenericNormaliseFamInst fun ty'
169 ; return (ACo co1 `mkTransCoI` coi2, ty'')
172 IndirectTv ty' -> tcGenericNormaliseFamInst fun ty'
176 tcGenericNormaliseFamInst fun (PredTy predty)
177 = do { (coi, pred') <- tcGenericNormaliseFamInstPred fun predty
178 ; return (coi, PredTy pred') }
180 ---------------------------------
181 tcGenericNormaliseFamInstPred :: (TcType -> TcM (Maybe (TcType,Coercion)))
183 -> TcM (CoercionI, TcPredType)
185 tcGenericNormaliseFamInstPred fun (ClassP cls tys)
186 = do { (cois, tys')<- mapAndUnzipM (tcGenericNormaliseFamInst fun) tys
187 ; return (mkClassPPredCoI cls tys' cois, ClassP cls tys')
189 tcGenericNormaliseFamInstPred fun (IParam ipn ty)
190 = do { (coi, ty') <- tcGenericNormaliseFamInst fun ty
191 ; return $ (mkIParamPredCoI ipn coi, IParam ipn ty')
193 tcGenericNormaliseFamInstPred fun (EqPred ty1 ty2)
194 = do { (coi1, ty1') <- tcGenericNormaliseFamInst fun ty1
195 ; (coi2, ty2') <- tcGenericNormaliseFamInst fun ty2
196 ; return (mkEqPredCoI ty1' coi1 ty2' coi2, EqPred ty1' ty2') }
200 %************************************************************************
202 Normalisation of instances wrt to equalities
204 %************************************************************************
207 tcReduceEqs :: [Inst] -- locals
209 -> TcM ([Inst], -- normalised locals (w/o equalities)
210 [Inst], -- normalised wanteds (including equalities)
211 TcDictBinds, -- bindings for all simplified dictionaries
212 Bool) -- whether any flexibles where instantiated
213 tcReduceEqs locals wanteds
214 = do { let (local_eqs , local_dicts) = partition isEqInst locals
215 (wanteds_eqs, wanteds_dicts) = partition isEqInst wanteds
216 ; eqCfg1 <- normaliseEqs (local_eqs ++ wanteds_eqs)
217 ; eqCfg2 <- normaliseDicts False local_dicts
218 ; eqCfg3 <- normaliseDicts True wanteds_dicts
219 ; eqCfg <- propagateEqs (eqCfg1 `unionEqConfig` eqCfg2
220 `unionEqConfig` eqCfg3)
221 ; finaliseEqsAndDicts eqCfg
226 %************************************************************************
228 Equality Configurations
230 %************************************************************************
232 We maintain normalised equalities together with the skolems introduced as
233 intermediates during flattening of equalities as well as
235 !!!TODO: We probably now can do without the skolem set. It's not used during
236 finalisation in the current code.
239 -- |Configuration of normalised equalities used during solving.
241 data EqConfig = EqConfig { eqs :: [RewriteInst] -- all equalities
242 , locals :: [Inst] -- given dicts
243 , wanteds :: [Inst] -- wanted dicts
244 , binds :: TcDictBinds -- bindings
245 , skolems :: TyVarSet -- flattening skolems
248 addSkolems :: EqConfig -> TyVarSet -> EqConfig
249 addSkolems eqCfg newSkolems
250 = eqCfg {skolems = skolems eqCfg `unionVarSet` newSkolems}
252 addEq :: EqConfig -> RewriteInst -> EqConfig
253 addEq eqCfg eq = eqCfg {eqs = eq : eqs eqCfg}
255 unionEqConfig :: EqConfig -> EqConfig -> EqConfig
256 unionEqConfig eqc1 eqc2 = EqConfig
257 { eqs = eqs eqc1 ++ eqs eqc2
258 , locals = locals eqc1 ++ locals eqc2
259 , wanteds = wanteds eqc1 ++ wanteds eqc2
260 , binds = binds eqc1 `unionBags` binds eqc2
261 , skolems = skolems eqc1 `unionVarSet` skolems eqc2
264 emptyEqConfig :: EqConfig
265 emptyEqConfig = EqConfig
270 , skolems = emptyVarSet
273 instance Outputable EqConfig where
274 ppr (EqConfig {eqs = eqs, locals = locals, wanteds = wanteds, binds = binds})
275 = vcat [ppr eqs, ppr locals, ppr wanteds, ppr binds]
278 The set of operations on an equality configuration. We obtain the initialise
279 configuration by normalisation ('normaliseEqs'), solve the equalities by
280 propagation ('propagateEqs'), and eventually finalise the configuration when
281 no further propoagation is possible.
284 -- |Turn a set of equalities into an equality configuration for solving.
286 -- Precondition: The Insts are zonked.
288 normaliseEqs :: [Inst] -> TcM EqConfig
290 = do { ASSERTM2( allM isValidWantedEqInst eqs, ppr eqs )
291 ; traceTc $ ptext (sLit "Entering normaliseEqs")
293 ; (eqss, skolemss) <- mapAndUnzipM normEqInst eqs
294 ; return $ emptyEqConfig { eqs = concat eqss
295 , skolems = unionVarSets skolemss
299 -- |Flatten the type arguments of all dictionaries, returning the result as a
300 -- equality configuration. The dictionaries go into the 'wanted' component if
301 -- the second argument is 'True'.
303 -- Precondition: The Insts are zonked.
305 normaliseDicts :: Bool -> [Inst] -> TcM EqConfig
306 normaliseDicts isWanted insts
307 = do { traceTc $ ptext (sLit "Entering normaliseDicts") <+>
308 ptext (if isWanted then sLit "[Wanted]" else sLit "[Local]")
309 ; (insts', eqss, bindss, skolemss) <- mapAndUnzip4M (normDict isWanted)
311 ; return $ emptyEqConfig { eqs = concat eqss
312 , locals = if isWanted then [] else insts'
313 , wanteds = if isWanted then insts' else []
314 , binds = unionManyBags bindss
315 , skolems = unionVarSets skolemss
319 -- |Solves the equalities as far as possible by applying propagation rules.
321 propagateEqs :: EqConfig -> TcM EqConfig
322 propagateEqs eqCfg@(EqConfig {eqs = todoEqs})
323 = do { traceTc $ hang (ptext (sLit "Entering propagateEqs:"))
326 ; propagate todoEqs (eqCfg {eqs = []})
329 -- |Finalise a set of equalities and associated dictionaries after
330 -- propagation. The returned Boolean value is `True' iff any flexible
331 -- variables, except those introduced by flattening (i.e., those in the
332 -- `skolems' component of the argument) where instantiated. The first returned
333 -- set of instances are the locals (without equalities) and the second set are
334 -- all residual wanteds, including equalities.
336 -- Remove all identity dictinary bindings (i.e., those whose source and target
337 -- dictionary are the same). This is important for termination, as
338 -- TcSimplify.reduceContext takes the presence of dictionary bindings as an
339 -- indicator that there was some improvement.
341 finaliseEqsAndDicts :: EqConfig
342 -> TcM ([Inst], [Inst], TcDictBinds, Bool)
343 finaliseEqsAndDicts (EqConfig { eqs = eqs
348 = do { traceTc $ ptext (sLit "finaliseEqsAndDicts")
349 ; (eqs', subst_binds, locals', wanteds') <- substitute eqs locals wanteds
350 ; (eqs'', improved) <- instantiateAndExtract eqs'
351 ; final_binds <- filterM nonTrivialDictBind $
352 bagToList (subst_binds `unionBags` binds)
354 ; ASSERTM2( allM isValidWantedEqInst eqs'', ppr eqs'' )
355 ; return (locals', eqs'' ++ wanteds', listToBag final_binds, improved)
358 nonTrivialDictBind (L _ (VarBind { var_id = ide1
359 , var_rhs = L _ (HsWrap _ (HsVar ide2))}))
360 = do { ty1 <- zonkTcType (idType ide1)
361 ; ty2 <- zonkTcType (idType ide2)
362 ; return $ not (ty1 `tcEqType` ty2)
364 nonTrivialDictBind _ = return True
368 %************************************************************************
370 Normalisation of equalities
372 %************************************************************************
374 A normal equality is a properly oriented equality with associated coercion
375 that contains at most one family equality (in its left-hand side) is oriented
376 such that it may be used as a reqrite rule. It has one of the following two
379 (1) co :: F t1..tn ~ t (family equalities)
380 (2) co :: x ~ t (variable equalities)
382 Variable equalities fall again in two classes:
384 (2a) co :: x ~ t, where t is *not* a variable, or
385 (2b) co :: x ~ y, where x > y.
387 The types t, t1, ..., tn may not contain any occurrences of synonym
388 families. Moreover, in Forms (2) & (3), the left-hand side may not occur in
389 the right-hand side, and the relation x > y is an arbitrary, but total order
392 !!!TODO: We may need to keep track of swapping for error messages (and to
393 re-orient on finilisation).
397 = RewriteVar -- Form (2) above
398 { rwi_var :: TyVar -- may be rigid or flexible
399 , rwi_right :: TcType -- contains no synonym family applications
400 , rwi_co :: EqInstCo -- the wanted or given coercion
402 , rwi_name :: Name -- no semantic significance (cf. TcRnTypes.EqInst)
403 , rwi_swapped :: Bool -- swapped orientation of original EqInst
405 | RewriteFam -- Forms (1) above
406 { rwi_fam :: TyCon -- synonym family tycon
407 , rwi_args :: [Type] -- contain no synonym family applications
408 , rwi_right :: TcType -- contains no synonym family applications
409 , rwi_co :: EqInstCo -- the wanted or given coercion
411 , rwi_name :: Name -- no semantic significance (cf. TcRnTypes.EqInst)
412 , rwi_swapped :: Bool -- swapped orientation of original EqInst
415 isWantedRewriteInst :: RewriteInst -> Bool
416 isWantedRewriteInst = isWantedCo . rwi_co
418 rewriteInstToInst :: RewriteInst -> TcM Inst
419 rewriteInstToInst eq@(RewriteVar {rwi_var = tv})
420 = deriveEqInst eq (mkTyVarTy tv) (rwi_right eq) (rwi_co eq)
421 rewriteInstToInst eq@(RewriteFam {rwi_fam = fam, rwi_args = args})
422 = deriveEqInst eq (mkTyConApp fam args) (rwi_right eq) (rwi_co eq)
424 -- Derive an EqInst based from a RewriteInst, possibly swapping the types
427 deriveEqInst :: RewriteInst -> TcType -> TcType -> EqInstCo -> TcM Inst
428 deriveEqInst rewrite ty1 ty2 co
429 = do { co_adjusted <- if not swapped then return co
430 else mkSymEqInstCo co (ty2, ty1)
434 , tci_co = co_adjusted
435 , tci_loc = rwi_loc rewrite
436 , tci_name = rwi_name rewrite
440 swapped = rwi_swapped rewrite
441 (left, right) = if not swapped then (ty1, ty2) else (ty2, ty1)
443 instance Outputable RewriteInst where
444 ppr (RewriteFam {rwi_fam = fam, rwi_args = args, rwi_right = rhs, rwi_co =co})
445 = hsep [ ppr co <+> text "::"
446 , ppr (mkTyConApp fam args)
450 ppr (RewriteVar {rwi_var = tv, rwi_right = rhs, rwi_co =co})
451 = hsep [ ppr co <+> text "::"
458 The following functions turn an arbitrary equality into a set of normal
459 equalities. This implements the WFlat and LFlat rules of the paper in one
460 sweep. However, we use flexible variables for both locals and wanteds, and
461 avoid to carry around the unflattening substitution \Sigma (for locals) by
462 already updating the skolems for locals with the family application that they
463 represent - i.e., they will turn into that family application on the next
464 zonking (which only happens after finalisation).
466 In a corresponding manner, normDict normalises class dictionaries by
467 extracting any synonym family applications and generation appropriate normal
470 Whenever we encounter a loopy equality (of the form a ~ T .. (F ...a...) ...),
471 we drop that equality and raise an error if it is a wanted or a warning if it
475 normEqInst :: Inst -> TcM ([RewriteInst], TyVarSet)
476 -- Normalise one equality.
478 = ASSERT( isEqInst inst )
479 go ty1 ty2 (eqInstCoercion inst)
481 (ty1, ty2) = eqInstTys inst
483 -- look through synonyms
484 go ty1 ty2 co | Just ty1' <- tcView ty1 = go ty1' ty2 co
485 go ty1 ty2 co | Just ty2' <- tcView ty2 = go ty1 ty2' co
487 -- left-to-right rule with type family head
488 go (TyConApp con args) ty2 co
490 = mkRewriteFam False con args ty2 co
492 -- right-to-left rule with type family head
493 go ty1 ty2@(TyConApp con args) co
495 = do { co' <- mkSymEqInstCo co (ty2, ty1)
496 ; mkRewriteFam True con args ty1 co'
499 -- no outermost family
501 = do { (ty1', co1, ty1_eqs, ty1_skolems) <- flattenType inst ty1
502 ; (ty2', co2, ty2_eqs, ty2_skolems) <- flattenType inst ty2
503 ; let ty12_eqs = ty1_eqs ++ ty2_eqs
504 sym_co2 = mkSymCoercion co2
506 ; (co', ty12_eqs') <- adjustCoercions co co1 sym_co2 eqTys ty12_eqs
507 ; eqs <- checkOrientation ty1' ty2' co' inst
508 ; if isLoopyEquality eqs ty12_eqs'
509 then do { if isWantedCo (tci_co inst)
511 addErrCtxt (ptext (sLit "Rejecting loopy equality")) $
514 warnDroppingLoopyEquality ty1 ty2
515 ; return ([], emptyVarSet) -- drop the equality
518 return (eqs ++ ty12_eqs',
519 ty1_skolems `unionVarSet` ty2_skolems)
522 mkRewriteFam swapped con args ty2 co
523 = do { (args', cargs, args_eqss, args_skolemss)
524 <- mapAndUnzip4M (flattenType inst) args
525 ; (ty2', co2, ty2_eqs, ty2_skolems) <- flattenType inst ty2
526 ; let co1 = mkTyConApp con cargs
527 sym_co2 = mkSymCoercion co2
528 all_eqs = concat args_eqss ++ ty2_eqs
529 eqTys = (mkTyConApp con args', ty2')
530 ; (co', all_eqs') <- adjustCoercions co co1 sym_co2 eqTys all_eqs
531 ; let thisRewriteFam = RewriteFam
536 , rwi_loc = tci_loc inst
537 , rwi_name = tci_name inst
538 , rwi_swapped = swapped
540 ; return $ (thisRewriteFam : all_eqs',
541 unionVarSets (ty2_skolems:args_skolemss))
544 -- If the original equality has the form a ~ T .. (F ...a...) ..., we will
545 -- have a variable equality with 'a' on the lhs as the first equality.
546 -- Then, check whether 'a' occurs in the lhs of any family equality
547 -- generated by flattening.
548 isLoopyEquality (RewriteVar {rwi_var = tv}:_) eqs
549 = any inRewriteFam eqs
551 inRewriteFam (RewriteFam {rwi_args = args})
552 = tv `elemVarSet` tyVarsOfTypes args
553 inRewriteFam _ = False
554 isLoopyEquality _ _ = False
556 normDict :: Bool -> Inst -> TcM (Inst, [RewriteInst], TcDictBinds, TyVarSet)
557 -- Normalise one dictionary or IP constraint.
558 normDict isWanted inst@(Dict {tci_pred = ClassP clas args})
559 = do { (args', cargs, args_eqss, args_skolemss)
560 <- mapAndUnzip4M (flattenType inst) args
561 ; let rewriteCo = PredTy $ ClassP clas cargs
562 eqs = concat args_eqss
563 pred' = ClassP clas args'
565 then -- don't generate a binding if there is nothing to flatten
566 return (inst, [], emptyBag, emptyVarSet)
568 ; (inst', bind) <- mkDictBind inst isWanted rewriteCo pred'
569 ; eqs' <- if isWanted then return eqs else mapM wantedToLocal eqs
570 ; return (inst', eqs', bind, unionVarSets args_skolemss)
572 normDict _isWanted inst
573 = return (inst, [], emptyBag, emptyVarSet)
574 -- !!!TODO: Still need to normalise IP constraints.
576 checkOrientation :: Type -> Type -> EqInstCo -> Inst -> TcM [RewriteInst]
577 -- Performs the occurs check, decomposition, and proper orientation
578 -- (returns a singleton, or an empty list in case of a trivial equality)
579 -- NB: We cannot assume that the two types already have outermost type
580 -- synonyms expanded due to the recursion in the case of type applications.
581 checkOrientation ty1 ty2 co inst
584 -- look through synonyms
585 go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
586 go ty1 ty2 | Just ty2' <- tcView ty2 = go ty1 ty2'
588 -- identical types => trivial
591 = do { mkIdEqInstCo co ty1
595 -- two tvs, left greater => unchanged
596 go ty1@(TyVarTy tv1) ty2@(TyVarTy tv2)
598 = mkRewriteVar False tv1 ty2 co
600 -- two tvs, right greater => swap
602 = do { co' <- mkSymEqInstCo co (ty2, ty1)
603 ; mkRewriteVar True tv2 ty1 co'
606 -- only lhs is a tv => unchanged
607 go ty1@(TyVarTy tv1) ty2
608 | ty1 `tcPartOfType` ty2 -- occurs check!
609 = occurCheckErr ty1 ty2
611 = mkRewriteVar False tv1 ty2 co
613 -- only rhs is a tv => swap
614 go ty1 ty2@(TyVarTy tv2)
615 | ty2 `tcPartOfType` ty1 -- occurs check!
616 = occurCheckErr ty2 ty1
618 = do { co' <- mkSymEqInstCo co (ty2, ty1)
619 ; mkRewriteVar True tv2 ty1 co'
622 -- type applications => decompose
624 | Just (ty1_l, ty1_r) <- repSplitAppTy_maybe ty1 -- won't split fam apps
625 , Just (ty2_l, ty2_r) <- repSplitAppTy_maybe ty2
626 = do { (co_l, co_r) <- mkAppEqInstCo co (ty1_l, ty2_l) (ty1_r, ty2_r)
627 ; eqs_l <- checkOrientation ty1_l ty2_l co_l inst
628 ; eqs_r <- checkOrientation ty1_r ty2_r co_r inst
629 ; return $ eqs_l ++ eqs_r
631 -- !!!TODO: would be more efficient to handle the FunApp and the data
632 -- constructor application explicitly.
634 -- inconsistency => type error
636 = ASSERT( (not . isForAllTy $ ty1) && (not . isForAllTy $ ty2) )
639 mkRewriteVar swapped tv ty co = return [RewriteVar
643 , rwi_loc = tci_loc inst
644 , rwi_name = tci_name inst
645 , rwi_swapped = swapped
648 flattenType :: Inst -- context to get location & name
649 -> Type -- the type to flatten
650 -> TcM (Type, -- the flattened type
651 Coercion, -- coercion witness of flattening wanteds
652 [RewriteInst], -- extra equalities
653 TyVarSet) -- new intermediate skolems
654 -- Removes all family synonyms from a type by moving them into extra equalities
658 -- look through synonyms
659 go ty | Just ty' <- tcView ty = go ty'
661 -- type variable => nothing to do
663 = return (ty, ty, [] , emptyVarSet)
665 -- type family application
666 -- => flatten to "gamma :: F t1'..tn' ~ alpha" (alpha & gamma fresh)
667 go ty@(TyConApp con args)
669 = do { (args', cargs, args_eqss, args_skolemss) <- mapAndUnzip4M go args
670 ; alpha <- newFlexiTyVar (typeKind ty)
671 ; let alphaTy = mkTyVarTy alpha
672 ; cotv <- newMetaCoVar (mkTyConApp con args') alphaTy
673 ; let thisRewriteFam = RewriteFam
676 , rwi_right = alphaTy
677 , rwi_co = mkWantedCo cotv
678 , rwi_loc = tci_loc inst
679 , rwi_name = tci_name inst
683 mkTyConApp con cargs `mkTransCoercion` mkTyVarTy cotv,
684 thisRewriteFam : concat args_eqss,
685 unionVarSets args_skolemss `extendVarSet` alpha)
686 } -- adding new unflatten var inst
688 -- data constructor application => flatten subtypes
689 -- NB: Special cased for efficiency - could be handled as type application
690 go (TyConApp con args)
691 = do { (args', cargs, args_eqss, args_skolemss) <- mapAndUnzip4M go args
692 ; return (mkTyConApp con args',
693 mkTyConApp con cargs,
695 unionVarSets args_skolemss)
698 -- function type => flatten subtypes
699 -- NB: Special cased for efficiency - could be handled as type application
701 = do { (ty_l', co_l, eqs_l, skolems_l) <- go ty_l
702 ; (ty_r', co_r, eqs_r, skolems_r) <- go ty_r
703 ; return (mkFunTy ty_l' ty_r',
706 skolems_l `unionVarSet` skolems_r)
709 -- type application => flatten subtypes
711 -- | Just (ty_l, ty_r) <- repSplitAppTy_maybe ty
712 = do { (ty_l', co_l, eqs_l, skolems_l) <- go ty_l
713 ; (ty_r', co_r, eqs_r, skolems_r) <- go ty_r
714 ; return (mkAppTy ty_l' ty_r',
717 skolems_l `unionVarSet` skolems_r)
720 -- forall type => panic if the body contains a type family
721 -- !!!TODO: As long as the family does not contain a quantified variable
722 -- we might pull it out, but what if it does contain a quantified
724 go ty@(ForAllTy _ body)
725 | null (tyFamInsts body)
726 = return (ty, ty, [] , emptyVarSet)
728 = panic "TcTyFuns.flattenType: synonym family in a rank-n type"
730 -- we should never see a predicate type
732 = panic "TcTyFuns.flattenType: unexpected PredType"
734 adjustCoercions :: EqInstCo -- coercion of original equality
735 -> Coercion -- coercion witnessing the left rewrite
736 -> Coercion -- coercion witnessing the right rewrite
737 -> (Type, Type) -- types of flattened equality
738 -> [RewriteInst] -- equalities from flattening
739 -> TcM (EqInstCo, -- coercion for flattened equality
740 [RewriteInst]) -- final equalities from flattening
741 -- Depending on whether we flattened a local or wanted equality, that equality's
742 -- coercion and that of the new equalities produced during flattening are
744 adjustCoercions (Left cotv) co1 co2 (ty_l, ty_r) all_eqs
745 -- wanted => generate a fresh coercion variable for the flattened equality
746 = do { cotv' <- newMetaCoVar ty_l ty_r
747 ; writeMetaTyVar cotv $
748 (co1 `mkTransCoercion` TyVarTy cotv' `mkTransCoercion` co2)
749 ; return (Left cotv', all_eqs)
752 adjustCoercions co@(Right _) _co1 _co2 _eqTys all_eqs
753 -- local => turn all new equalities into locals and update (but not zonk)
755 = do { all_eqs' <- mapM wantedToLocal all_eqs
756 ; return (co, all_eqs')
759 mkDictBind :: Inst -- original instance
760 -> Bool -- is this a wanted contraint?
761 -> Coercion -- coercion witnessing the rewrite
762 -> PredType -- coerced predicate
763 -> TcM (Inst, -- new inst
764 TcDictBinds) -- binding for coerced dictionary
765 mkDictBind dict isWanted rewriteCo pred
766 = do { dict' <- newDictBndr loc pred
767 -- relate the old inst to the new one
768 -- target_dict = source_dict `cast` st_co
769 ; let (target_dict, source_dict, st_co)
770 | isWanted = (dict, dict', mkSymCoercion rewriteCo)
771 | otherwise = (dict', dict, rewriteCo)
773 -- co :: dict ~ dict'
774 -- hence, if isWanted
775 -- dict = dict' `cast` sym co
777 -- dict' = dict `cast` co
778 expr = HsVar $ instToId source_dict
779 cast_expr = HsWrap (WpCast st_co) expr
780 rhs = L (instLocSpan loc) cast_expr
781 binds = instToDictBind target_dict rhs
782 ; return (dict', binds)
787 -- gamma :: Fam args ~ alpha
788 -- => alpha :: Fam args ~ alpha, with alpha := Fam args
789 -- (the update of alpha will not be apparent during propagation, as we
790 -- never follow the indirections of meta variables; it will be revealed
791 -- when the equality is zonked)
792 wantedToLocal :: RewriteInst -> TcM RewriteInst
793 wantedToLocal eq@(RewriteFam {rwi_fam = fam,
795 rwi_right = alphaTy@(TyVarTy alpha)})
796 = do { writeMetaTyVar alpha (mkTyConApp fam args)
797 ; return $ eq {rwi_co = mkGivenCo alphaTy}
799 wantedToLocal _ = panic "TcTyFuns.wantedToLocal"
803 %************************************************************************
805 Propagation of equalities
807 %************************************************************************
809 Apply the propagation rules exhaustively.
812 propagate :: [RewriteInst] -> EqConfig -> TcM EqConfig
813 propagate [] eqCfg = return eqCfg
814 propagate (eq:eqs) eqCfg
815 = do { optEqs <- applyTop eq
818 -- Top applied to 'eq' => retry with new equalities
819 Just (eqs2, skolems2)
820 -> propagate (eqs2 ++ eqs) (eqCfg `addSkolems` skolems2)
822 -- Top doesn't apply => try subst rules with all other
823 -- equalities, after that 'eq' can go into the residual list
825 -> do { (eqs', eqCfg') <- applySubstRules eq eqs eqCfg
826 ; propagate eqs' (eqCfg' `addEq` eq)
830 applySubstRules :: RewriteInst -- currently considered eq
831 -> [RewriteInst] -- todo eqs list
832 -> EqConfig -- residual
833 -> TcM ([RewriteInst], EqConfig) -- new todo & residual
834 applySubstRules eq todoEqs (eqConfig@EqConfig {eqs = resEqs})
835 = do { (newEqs_t, unchangedEqs_t, skolems_t) <- mapSubstRules eq todoEqs
836 ; (newEqs_r, unchangedEqs_r, skolems_r) <- mapSubstRules eq resEqs
837 ; return (newEqs_t ++ newEqs_r ++ unchangedEqs_t,
838 eqConfig {eqs = unchangedEqs_r}
839 `addSkolems` (skolems_t `unionVarSet` skolems_r))
842 mapSubstRules :: RewriteInst -- try substituting this equality
843 -> [RewriteInst] -- into these equalities
844 -> TcM ([RewriteInst], [RewriteInst], TyVarSet)
846 = do { (newEqss, unchangedEqss, skolemss) <- mapAndUnzip3M (substRules eq) eqs
847 ; return (concat newEqss, concat unchangedEqss, unionVarSets skolemss)
851 = do { -- try the SubstFam rule
852 optEqs <- applySubstFam eq1 eq2
854 Just (eqs, skolems) -> return (eqs, [], skolems)
856 { -- try the SubstVarVar rule
857 optEqs <- applySubstVarVar eq1 eq2
859 Just (eqs, skolems) -> return (eqs, [], skolems)
861 { -- try the SubstVarFam rule
862 optEqs <- applySubstVarFam eq1 eq2
864 Just eq -> return ([eq], [], emptyVarSet)
865 Nothing -> return ([], [eq2], emptyVarSet)
866 -- if no rule matches, we return the equlity we tried to
867 -- substitute into unchanged
871 Attempt to apply the Top rule. The rule is
875 co' :: [s1/x1, .., sm/xm]s ~ t with co = g s1..sm |> co'
877 where g :: forall x1..xm. F u1..um ~ s and [s1/x1, .., sm/xm]u1 == t1.
879 Returns Nothing if the rule could not be applied. Otherwise, the resulting
880 equality is normalised and a list of the normal equalities is returned.
883 applyTop :: RewriteInst -> TcM (Maybe ([RewriteInst], TyVarSet))
885 applyTop eq@(RewriteFam {rwi_fam = fam, rwi_args = args})
886 = do { optTyCo <- tcUnfoldSynFamInst (TyConApp fam args)
888 Nothing -> return Nothing
889 Just (lhs, rewrite_co)
890 -> do { co' <- mkRightTransEqInstCo co rewrite_co (lhs, rhs)
891 ; eq' <- deriveEqInst eq lhs rhs co'
892 ; liftM Just $ normEqInst eq'
899 applyTop _ = return Nothing
902 Attempt to apply the SubstFam rule. The rule is
904 co1 :: F t1..tn ~ t & co2 :: F t1..tn ~ s
906 co1 :: F t1..tn ~ t & co2' :: t ~ s with co2 = co1 |> co2'
908 where co1 may be a wanted only if co2 is a wanted, too.
910 Returns Nothing if the rule could not be applied. Otherwise, the equality
911 co2' is normalised and a list of the normal equalities is returned. (The
912 equality co1 is not returned as it remain unaltered.)
915 applySubstFam :: RewriteInst
917 -> TcM (Maybe ([RewriteInst], TyVarSet))
918 applySubstFam eq1@(RewriteFam {rwi_fam = fam1, rwi_args = args1})
919 eq2@(RewriteFam {rwi_fam = fam2, rwi_args = args2})
920 | fam1 == fam2 && tcEqTypes args1 args2 &&
921 (isWantedRewriteInst eq2 || not (isWantedRewriteInst eq1))
922 -- !!!TODO: tcEqTypes is insufficient as it does not look through type synonyms
923 -- !!!Check whether anything breaks by making tcEqTypes look through synonyms.
924 -- !!!Should be ok and we don't want three type equalities.
925 = do { co2' <- mkRightTransEqInstCo co2 co1 (lhs, rhs)
926 ; eq2' <- deriveEqInst eq2 lhs rhs co2'
927 ; liftM Just $ normEqInst eq2'
932 co1 = eqInstCoType (rwi_co eq1)
934 applySubstFam _ _ = return Nothing
937 Attempt to apply the SubstVarVar rule. The rule is
939 co1 :: x ~ t & co2 :: x ~ s
941 co1 :: x ~ t & co2' :: t ~ s with co2 = co1 |> co2'
943 where co1 may be a wanted only if co2 is a wanted, too.
945 Returns Nothing if the rule could not be applied. Otherwise, the equality
946 co2' is normalised and a list of the normal equalities is returned. (The
947 equality co1 is not returned as it remain unaltered.)
950 applySubstVarVar :: RewriteInst
952 -> TcM (Maybe ([RewriteInst], TyVarSet))
953 applySubstVarVar eq1@(RewriteVar {rwi_var = tv1})
954 eq2@(RewriteVar {rwi_var = tv2})
956 (isWantedRewriteInst eq2 || not (isWantedRewriteInst eq1))
957 = do { co2' <- mkRightTransEqInstCo co2 co1 (lhs, rhs)
958 ; eq2' <- deriveEqInst eq2 lhs rhs co2'
959 ; liftM Just $ normEqInst eq2'
964 co1 = eqInstCoType (rwi_co eq1)
966 applySubstVarVar _ _ = return Nothing
969 Attempt to apply the SubstVarFam rule. The rule is
971 co1 :: x ~ t & co2 :: F s1..sn ~ s
973 co1 :: x ~ t & co2' :: [t/x](F s1..sn) ~ s
974 with co2 = [co1/x](F s1..sn) |> co2'
976 where x occurs in F s1..sn. (co1 may be local or wanted.)
978 Returns Nothing if the rule could not be applied. Otherwise, the equality
979 co2' is returned. (The equality co1 is not returned as it remain unaltered.)
982 applySubstVarFam :: RewriteInst -> RewriteInst -> TcM (Maybe RewriteInst)
983 applySubstVarFam eq1@(RewriteVar {rwi_var = tv1})
984 eq2@(RewriteFam {rwi_fam = fam2, rwi_args = args2})
985 | tv1 `elemVarSet` tyVarsOfTypes args2
986 = do { let co1Subst = substTyWith [tv1] [co1] (mkTyConApp fam2 args2)
987 args2' = substTysWith [tv1] [rhs1] args2
988 lhs2 = mkTyConApp fam2 args2'
989 ; co2' <- mkRightTransEqInstCo co2 co1Subst (lhs2, rhs2)
990 ; return $ Just (eq2 {rwi_args = args2', rwi_co = co2'})
995 co1 = eqInstCoType (rwi_co eq1)
997 applySubstVarFam _ _ = return Nothing
1001 %************************************************************************
1003 Finalisation of equalities
1005 %************************************************************************
1007 Exhaustive substitution of all variable equalities of the form co :: x ~ t
1008 (both local and wanted) into the left-hand sides of all other equalities. This
1009 may lead to recursive equalities; i.e., (1) we need to apply the substitution
1010 implied by one variable equality exhaustively before turning to the next and
1011 (2) we need an occurs check.
1013 We also apply the same substitutions to the local and wanted class and IP
1016 NB: Given that we apply the substitution corresponding to a single equality
1017 exhaustively, before turning to the next, and because we eliminate recursive
1018 equalities, all opportunities for subtitution will have been exhausted after
1019 we have considered each equality once.
1022 substitute :: [RewriteInst] -- equalities
1023 -> [Inst] -- local class dictionaries
1024 -> [Inst] -- wanted class dictionaries
1025 -> TcM ([RewriteInst], -- equalities after substitution
1026 TcDictBinds, -- all newly generated dictionary bindings
1027 [Inst], -- local dictionaries after substitution
1028 [Inst]) -- wanted dictionaries after substitution
1029 substitute eqs locals wanteds = subst eqs [] emptyBag locals wanteds
1031 subst [] res binds locals wanteds
1032 = return (res, binds, locals, wanteds)
1033 subst (eq@(RewriteVar {rwi_var = tv, rwi_right = ty, rwi_co = co}):eqs)
1034 res binds locals wanteds
1035 = do { traceTc $ ptext (sLit "TcTyFuns.substitute:") <+> ppr tv <+>
1036 ptext (sLit "->") <+> ppr ty
1037 ; let coSubst = zipOpenTvSubst [tv] [eqInstCoType co]
1038 tySubst = zipOpenTvSubst [tv] [ty]
1039 ; eqs' <- mapM (substEq eq coSubst tySubst) eqs
1040 ; res' <- mapM (substEq eq coSubst tySubst) res
1041 ; (lbinds, locals') <- mapAndUnzipM
1042 (substDict eq coSubst tySubst False)
1044 ; (wbinds, wanteds') <- mapAndUnzipM
1045 (substDict eq coSubst tySubst True)
1047 ; let binds' = unionManyBags $ binds : lbinds ++ wbinds
1048 ; subst eqs' (eq:res') binds' locals' wanteds'
1050 subst (eq:eqs) res binds locals wanteds
1051 = subst eqs (eq:res) binds locals wanteds
1053 -- We have, co :: tv ~ ty
1054 -- => apply [ty/tv] to right-hand side of eq2
1055 -- (but only if tv actually occurs in the right-hand side of eq2)
1056 substEq (RewriteVar {rwi_var = tv, rwi_right = ty})
1058 | tv `elemVarSet` tyVarsOfType (rwi_right eq2)
1059 = do { let co1Subst = mkSymCoercion $ substTy coSubst (rwi_right eq2)
1060 right2' = substTy tySubst (rwi_right eq2)
1062 RewriteVar {rwi_var = tv2} -> mkTyVarTy tv2
1063 RewriteFam {rwi_fam = fam,
1064 rwi_args = args} ->mkTyConApp fam args
1065 ; co2' <- mkLeftTransEqInstCo (rwi_co eq2) co1Subst (left2, right2')
1067 RewriteVar {rwi_var = tv2} | tv2 `elemVarSet` tyVarsOfType ty
1068 -> occurCheckErr left2 right2'
1069 _ -> return $ eq2 {rwi_right = right2', rwi_co = co2'}
1076 -- We have, co :: tv ~ ty
1077 -- => apply [ty/tv] to dictionary predicate
1078 -- (but only if tv actually occurs in the predicate)
1079 substDict (RewriteVar {rwi_var = tv})
1080 coSubst tySubst isWanted dict
1082 , tv `elemVarSet` tyVarsOfPred (tci_pred dict)
1083 = do { let co1Subst = PredTy (substPred coSubst (tci_pred dict))
1084 pred' = substPred tySubst (tci_pred dict)
1085 ; (dict', binds) <- mkDictBind dict isWanted co1Subst pred'
1086 ; return (binds, dict')
1090 substDict _ _ _ _ dict
1091 = return (emptyBag, dict)
1092 -- !!!TODO: Still need to substitute into IP constraints.
1095 For any *wanted* variable equality of the form co :: alpha ~ t or co :: a ~
1096 alpha, we instantiate alpha with t or a, respectively, and set co := id.
1097 Return all remaining wanted equalities. The Boolean result component is True
1098 if at least one instantiation of a flexible was performed.
1101 instantiateAndExtract :: [RewriteInst] -> TcM ([Inst], Bool)
1102 instantiateAndExtract eqs
1103 = do { let wanteds = filter (isWantedCo . rwi_co) eqs
1104 ; wanteds' <- mapM inst wanteds
1105 ; let residuals = catMaybes wanteds'
1106 improved = length wanteds /= length residuals
1107 ; residuals' <- mapM rewriteInstToInst residuals
1108 ; return (residuals', improved)
1111 inst eq@(RewriteVar {rwi_var = tv1, rwi_right = ty2, rwi_co = co})
1115 = doInst (rwi_swapped eq) tv1 ty2 co eq
1118 | Just tv2 <- tcGetTyVar_maybe ty2
1120 = doInst (not $ rwi_swapped eq) tv2 (mkTyVarTy tv1) co eq
1122 inst eq = return $ Just eq
1124 doInst _swapped _tv _ty (Right ty) _eq
1125 = pprPanic "TcTyFuns.doInst: local eq: " (ppr ty)
1126 doInst swapped tv ty (Left cotv) eq
1127 = do { lookupTV <- lookupTcTyVar tv
1128 ; uMeta swapped tv lookupTV ty cotv
1131 -- meta variable has been filled already
1132 -- => ignore (must be a skolem that was introduced by flattening locals)
1133 uMeta _swapped _tv (IndirectTv _) _ty _cotv
1136 -- type variable meets type variable
1137 -- => check that tv2 hasn't been updated yet and choose which to update
1138 uMeta swapped tv1 (DoneTv details1) (TyVarTy tv2) cotv
1140 = panic "TcTyFuns.uMeta: normalisation shouldn't allow x ~ x"
1143 = do { lookupTV2 <- lookupTcTyVar tv2
1146 uMeta swapped tv1 (DoneTv details1) ty cotv
1148 uMetaVar swapped tv1 details1 tv2 details2 cotv
1151 ------ Beyond this point we know that ty2 is not a type variable
1153 -- signature skolem meets non-variable type
1154 -- => cannot update (retain the equality)!
1155 uMeta _swapped _tv (DoneTv (MetaTv (SigTv _) _)) _non_tv_ty _cotv
1158 -- updatable meta variable meets non-variable type
1159 -- => occurs check, monotype check, and kinds match check, then update
1160 uMeta swapped tv (DoneTv (MetaTv _ ref)) non_tv_ty cotv
1161 = do { -- occurs + monotype check
1162 ; mb_ty' <- checkTauTvUpdate tv non_tv_ty
1166 -- normalisation shouldn't leave families in non_tv_ty
1167 panic "TcTyFuns.uMeta: unexpected synonym family"
1169 do { checkUpdateMeta swapped tv ref ty' -- update meta var
1170 ; writeMetaTyVar cotv ty' -- update co var
1175 uMeta _ _ _ _ _ = panic "TcTyFuns.uMeta"
1177 -- uMetaVar: unify two type variables
1178 -- meta variable meets skolem
1180 uMetaVar swapped tv1 (MetaTv _ ref) tv2 (SkolemTv _) cotv
1181 = do { checkUpdateMeta swapped tv1 ref (mkTyVarTy tv2)
1182 ; writeMetaTyVar cotv (mkTyVarTy tv2)
1186 -- meta variable meets meta variable
1187 -- => be clever about which of the two to update
1188 -- (from TcUnify.uUnfilledVars minus boxy stuff)
1189 uMetaVar swapped tv1 (MetaTv info1 ref1) tv2 (MetaTv info2 ref2) cotv
1190 = do { case (info1, info2) of
1191 -- Avoid SigTvs if poss
1192 (SigTv _, _ ) | k1_sub_k2 -> update_tv2
1193 (_, SigTv _) | k2_sub_k1 -> update_tv1
1195 (_, _) | k1_sub_k2 -> if k2_sub_k1 && nicer_to_update_tv1
1196 then update_tv1 -- Same kinds
1198 | k2_sub_k1 -> update_tv1
1199 | otherwise -> kind_err
1200 -- Update the variable with least kind info
1201 -- See notes on type inference in Kind.lhs
1202 -- The "nicer to" part only applies if the two kinds are the same,
1203 -- so we can choose which to do.
1205 ; writeMetaTyVar cotv (mkTyVarTy tv2)
1209 -- Kinds should be guaranteed ok at this point
1210 update_tv1 = updateMeta tv1 ref1 (mkTyVarTy tv2)
1211 update_tv2 = updateMeta tv2 ref2 (mkTyVarTy tv1)
1213 kind_err = addErrCtxtM (unifyKindCtxt swapped tv1 (mkTyVarTy tv2)) $
1214 unifyKindMisMatch k1 k2
1218 k1_sub_k2 = k1 `isSubKind` k2
1219 k2_sub_k1 = k2 `isSubKind` k1
1221 nicer_to_update_tv1 = isSystemName (Var.varName tv1)
1222 -- Try to update sys-y type variables in preference to ones
1223 -- gotten (say) by instantiating a polymorphic function with
1224 -- a user-written type sig
1226 uMetaVar _ _ _ _ _ _ = panic "uMetaVar"
1230 %************************************************************************
1234 %************************************************************************
1236 The infamous couldn't match expected type soandso against inferred type
1237 somethingdifferent message.
1240 eqInstMisMatch :: Inst -> TcM a
1242 = ASSERT( isEqInst inst )
1243 setErrCtxt ctxt $ failWithMisMatch ty_act ty_exp
1245 (ty_act, ty_exp) = eqInstTys inst
1246 InstLoc _ _ ctxt = instLoc inst
1248 -----------------------
1249 failWithMisMatch :: TcType -> TcType -> TcM a
1250 -- Generate the message when two types fail to match,
1251 -- going to some trouble to make it helpful.
1252 -- The argument order is: actual type, expected type
1253 failWithMisMatch ty_act ty_exp
1254 = do { env0 <- tcInitTidyEnv
1255 ; ty_exp <- zonkTcType ty_exp
1256 ; ty_act <- zonkTcType ty_act
1257 ; failWithTcM (misMatchMsg env0 (ty_act, ty_exp))
1260 misMatchMsg :: TidyEnv -> (TcType, TcType) -> (TidyEnv, SDoc)
1261 misMatchMsg env0 (ty_act, ty_exp)
1262 = let (env1, pp_exp, extra_exp) = ppr_ty env0 ty_exp
1263 (env2, pp_act, extra_act) = ppr_ty env1 ty_act
1264 msg = sep [sep [ptext (sLit "Couldn't match expected type") <+> pp_exp,
1266 ptext (sLit "against inferred type") <+> pp_act],
1267 nest 2 (extra_exp $$ extra_act)]
1272 ppr_ty :: TidyEnv -> TcType -> (TidyEnv, SDoc, SDoc)
1274 = let (env1, tidy_ty) = tidyOpenType env ty
1275 (env2, extra) = ppr_extra env1 tidy_ty
1277 (env2, quotes (ppr tidy_ty), extra)
1279 -- (ppr_extra env ty) shows extra info about 'ty'
1280 ppr_extra :: TidyEnv -> Type -> (TidyEnv, SDoc)
1281 ppr_extra env (TyVarTy tv)
1282 | isTcTyVar tv && (isSkolemTyVar tv || isSigTyVar tv) && not (isUnk tv)
1283 = (env1, pprSkolTvBinding tv1)
1285 (env1, tv1) = tidySkolemTyVar env tv
1287 ppr_extra env _ty = (env, empty) -- Normal case
1290 Warn of loopy local equalities that were dropped.
1293 warnDroppingLoopyEquality :: TcType -> TcType -> TcM ()
1294 warnDroppingLoopyEquality ty1 ty2
1295 = do { env0 <- tcInitTidyEnv
1296 ; ty1 <- zonkTcType ty1
1297 ; ty2 <- zonkTcType ty2
1298 ; let (env1 , tidy_ty1) = tidyOpenType env0 ty1
1299 (_env2, tidy_ty2) = tidyOpenType env1 ty2
1300 ; addWarnTc $ hang (ptext (sLit "Dropping loopy given equality"))
1301 2 (quotes (ppr tidy_ty1 <+> text "~" <+> ppr tidy_ty2))