X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcSimplify.lhs;h=534c5d0601c8e74bd841b1c86da8a845f78ba43d;hp=b074437106bdb41714166034f798222d02502fb4;hb=ebec49fed627b7dd17e297ddc79a9c677a2ce538;hpb=e891720545a2f088cc48ad62bad7c5b2ad7d183f diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs index b074437..534c5d0 100644 --- a/compiler/typecheck/TcSimplify.lhs +++ b/compiler/typecheck/TcSimplify.lhs @@ -971,17 +971,17 @@ makeImplicationBind :: InstLoc -> [TcTyVar] -> [Inst] -> [Inst] -> TcM ([Inst], TcDictBinds) -- Make a binding that binds 'irreds', by generating an implication --- constraint for them, *and* throwing the constraint into the LIE +-- constraint for them. +-- -- The binding looks like -- (ir1, .., irn) = f qtvs givens -- where f is (evidence for) the new implication constraint --- f :: forall qtvs. {reft} givens => (ir1, .., irn) --- qtvs includes coercion variables +-- f :: forall qtvs. givens => (ir1, .., irn) +-- qtvs includes coercion variables. -- -- This binding must line up the 'rhs' in reduceImplication makeImplicationBind loc all_tvs - givens -- Guaranteed all Dicts - -- or EqInsts + givens -- Guaranteed all Dicts or EqInsts irreds | null irreds -- If there are no irreds, we are done = return ([], emptyBag) @@ -989,28 +989,38 @@ makeImplicationBind loc all_tvs = do { uniq <- newUnique ; span <- getSrcSpanM ; let (eq_givens, dict_givens) = partition isEqInst givens - eq_tyvar_cos = mkTyVarTys (varSetElems $ tyVarsOfTypes $ map eqInstType eq_givens) - -- Urgh! See line 2187 or thereabouts. I believe that all these - -- 'givens' must be a simple CoVar. This MUST be cleaned up. - ; let name = mkInternalName uniq (mkVarOcc "ic") span + -- extract equality binders + eq_cotvs = map eqInstType eq_givens + + -- make the implication constraint instance + name = mkInternalName uniq (mkVarOcc "ic") span implic_inst = ImplicInst { tci_name = name, tci_tyvars = all_tvs, tci_given = (eq_givens ++ dict_givens), - tci_wanted = irreds, tci_loc = loc } - ; let -- only create binder for dict_irreds - (_, dict_irreds) = partition isEqInst irreds + -- same order as binders + tci_wanted = irreds, + tci_loc = loc } + + -- create binders for the irreducible dictionaries + dict_irreds = filter (not . isEqInst) irreds dict_irred_ids = map instToId dict_irreds lpat = mkBigLHsPatTup (map (L span . VarPat) dict_irred_ids) - rhs = L span (mkHsWrap co (HsVar (instToId implic_inst))) - co = mkWpApps (map instToId dict_givens) - <.> mkWpTyApps eq_tyvar_cos - <.> mkWpTyApps (mkTyVarTys all_tvs) - bind | [dict_irred_id] <- dict_irred_ids = VarBind dict_irred_id rhs - | otherwise = PatBind { pat_lhs = lpat, - pat_rhs = unguardedGRHSs rhs, - pat_rhs_ty = hsLPatType lpat, - bind_fvs = placeHolderNames } + + -- create the binding + rhs = L span (mkHsWrap co (HsVar (instToId implic_inst))) + co = mkWpApps (map instToId dict_givens) + <.> mkWpTyApps eq_cotvs + <.> mkWpTyApps (mkTyVarTys all_tvs) + bind | [dict_irred_id] <- dict_irred_ids + = VarBind dict_irred_id rhs + | otherwise + = PatBind { pat_lhs = lpat + , pat_rhs = unguardedGRHSs rhs + , pat_rhs_ty = hsLPatType lpat + , bind_fvs = placeHolderNames + } + ; traceTc $ text "makeImplicationBind" <+> ppr implic_inst ; return ([implic_inst], unitBag (L span bind)) } @@ -1021,11 +1031,11 @@ tryHardCheckLoop :: SDoc -> TcM ([Inst], TcDictBinds) tryHardCheckLoop doc wanteds - = do { (irreds,binds) <- checkLoop (mkRedEnv doc try_me []) wanteds + = do { (irreds,binds) <- checkLoop (mkInferRedEnv doc try_me) wanteds ; return (irreds,binds) } where - try_me _ = ReduceMe AddSCs + try_me _ = ReduceMe -- Here's the try-hard bit ----------------------------------------------------------- @@ -1041,7 +1051,7 @@ gentleCheckLoop inst_loc givens wanteds where env = mkRedEnv (pprInstLoc inst_loc) try_me givens - try_me inst | isMethodOrLit inst = ReduceMe AddSCs + try_me inst | isMethodOrLit inst = ReduceMe | otherwise = Stop -- When checking against a given signature -- we MUST be very gentle: Note [Check gently] @@ -1052,8 +1062,8 @@ gentleInferLoop doc wanteds = do { (irreds, binds) <- checkLoop env wanteds ; return (irreds, binds) } where - env = mkRedEnv doc try_me [] - try_me inst | isMethodOrLit inst = ReduceMe AddSCs + env = mkInferRedEnv doc try_me + try_me inst | isMethodOrLit inst = ReduceMe | otherwise = Stop \end{code} @@ -1098,7 +1108,7 @@ checkLoop env wanteds ; (improved, binds, irreds) <- reduceContext env' wanteds' - ; if not improved then + ; if null irreds || not improved then return (irreds, binds) else do @@ -1201,30 +1211,87 @@ Alas! Alack! We can do the same for (instance D Int): ds2 = $p1 dc And now we've defined the superclass in terms of itself. - -Solution: never generate a superclass selectors at all when -satisfying the superclass context of an instance declaration. - Two more nasty cases are in tcrun021 tcrun033 +Solution: + - Satisfy the superclass context *all by itself* + (tcSimplifySuperClasses) + - And do so completely; i.e. no left-over constraints + to mix with the constraints arising from method declarations + + +Note [Recursive instances and superclases] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider this code, which arises in the context of "Scrap Your +Boilerplate with Class". + + class Sat a + class Data ctx a + instance Sat (ctx Char) => Data ctx Char + instance (Sat (ctx [a]), Data ctx a) => Data ctx [a] + + class Data Maybe a => Foo a + + instance Foo t => Sat (Maybe t) + + instance Data Maybe a => Foo a + instance Foo a => Foo [a] + instance Foo [Char] + +In the instance for Foo [a], when generating evidence for the superclasses +(ie in tcSimplifySuperClasses) we need a superclass (Data Maybe [a]). +Using the instance for Data, we therefore need + (Sat (Maybe [a], Data Maybe a) +But we are given (Foo a), and hence its superclass (Data Maybe a). +So that leaves (Sat (Maybe [a])). Using the instance for Sat means +we need (Foo [a]). And that is the very dictionary we are bulding +an instance for! So we must put that in the "givens". So in this +case we have + Given: Foo a, Foo [a] + Watend: Data Maybe [a] + +BUT we must *not not not* put the *superclasses* of (Foo [a]) in +the givens, which is what 'addGiven' would normally do. Why? Because +(Data Maybe [a]) is the superclass, so we'd "satisfy" the wanted +by selecting a superclass from Foo [a], which simply makes a loop. + +On the other hand we *must* put the superclasses of (Foo a) in +the givens, as you can see from the derivation described above. + +Conclusion: in the very special case of tcSimplifySuperClasses +we have one 'given' (namely the "this" dictionary) whose superclasses +must not be added to 'givens' by addGiven. That is the *whole* reason +for the red_given_scs field in RedEnv, and the function argument to +addGiven. + \begin{code} -tcSimplifySuperClasses +tcSimplifySuperClasses :: InstLoc + -> Inst -- The dict whose superclasses + -- are being figured out -> [Inst] -- Given -> [Inst] -- Wanted -> TcM TcDictBinds -tcSimplifySuperClasses loc givens sc_wanteds +tcSimplifySuperClasses loc this givens sc_wanteds = do { traceTc (text "tcSimplifySuperClasses") ; (irreds,binds1) <- checkLoop env sc_wanteds ; let (tidy_env, tidy_irreds) = tidyInsts irreds ; reportNoInstances tidy_env (Just (loc, givens)) tidy_irreds ; return binds1 } where - env = mkRedEnv (pprInstLoc loc) try_me givens - try_me _ = ReduceMe NoSCs - -- Like tryHardCheckLoop, but with NoSCs + env = RedEnv { red_doc = pprInstLoc loc, + red_try_me = try_me, + red_givens = this:givens, + red_given_scs = add_scs, + red_stack = (0,[]), + red_improve = False } -- No unification vars + add_scs g | g==this = NoSCs + | otherwise = AddSCs + + try_me _ = ReduceMe -- Try hard, so we completely solve the superclass + -- constraints right here. See Note [SUPERCLASS-LOOP 1] \end{code} @@ -1356,7 +1423,7 @@ tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds -- BUT do no improvement! See Plan D above -- HOWEVER, some unification may take place, if we instantiate -- a method Inst with an equality constraint - ; let env = mkNoImproveRedEnv doc (\_ -> ReduceMe AddSCs) + ; let env = mkNoImproveRedEnv doc (\_ -> ReduceMe) ; (_imp, _binds, constrained_dicts) <- reduceContext env wanteds' -- Next, figure out the tyvars we will quantify over @@ -1404,7 +1471,7 @@ tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds ; let is_nested_group = isNotTopLevel top_lvl try_me inst | isFreeWrtTyVars qtvs inst, (is_nested_group || isDict inst) = Stop - | otherwise = ReduceMe AddSCs + | otherwise = ReduceMe env = mkNoImproveRedEnv doc try_me ; (_imp, binds, irreds) <- reduceContext env wanteds' @@ -1524,9 +1591,9 @@ tcSimplifyRuleLhs wanteds } -- Sigh: we need to reduce inside implications - red_env = mkRedEnv doc try_me [] + red_env = mkInferRedEnv doc try_me doc = ptext (sLit "Implication constraint in RULE lhs") - try_me inst | isMethodOrLit inst = ReduceMe AddSCs + try_me inst | isMethodOrLit inst = ReduceMe | otherwise = Stop -- Be gentle \end{code} @@ -1585,12 +1652,20 @@ tcSimplifyIPs given_ips wanteds ; let env = mkRedEnv doc try_me given_ips' ; (improved, binds, irreds) <- reduceContext env wanteds' - ; if not improved then + ; if null irreds || not improved then ASSERT( all is_free irreds ) do { extendLIEs irreds ; return binds } - else - tcSimplifyIPs given_ips wanteds } + else do + -- If improvement did some unification, we go round again. + -- We start again with irreds, not wanteds + -- Using an instance decl might have introduced a fresh type + -- variable which might have been unified, so we'd get an + -- infinite loop if we started again with wanteds! + -- See Note [LOOP] + { binds1 <- tcSimplifyIPs given_ips' irreds + ; return $ binds `unionBags` binds1 + } } where doc = text "tcSimplifyIPs" <+> ppr given_ips ip_set = mkNameSet (ipNamesOfInsts given_ips) @@ -1598,7 +1673,7 @@ tcSimplifyIPs given_ips wanteds -- Simplify any methods that mention the implicit parameter try_me inst | is_free inst = Stop - | otherwise = ReduceMe NoSCs + | otherwise = ReduceMe \end{code} @@ -1673,8 +1748,11 @@ data RedEnv , red_try_me :: Inst -> WhatToDo , red_improve :: Bool -- True <=> do improvement , red_givens :: [Inst] -- All guaranteed rigid - -- Always dicts + -- Always dicts & equalities -- but see Note [Rigidity] + + , red_given_scs :: Inst -> WantSCs -- See Note [Recursive instances and superclases] + , red_stack :: (Int, [Inst]) -- Recursion stack (for err msg) -- See Note [RedStack] } @@ -1697,6 +1775,16 @@ mkRedEnv :: SDoc -> (Inst -> WhatToDo) -> [Inst] -> RedEnv mkRedEnv doc try_me givens = RedEnv { red_doc = doc, red_try_me = try_me, red_givens = givens, + red_given_scs = const AddSCs, + red_stack = (0,[]), + red_improve = True } + +mkInferRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv +-- No givens at all +mkInferRedEnv doc try_me + = RedEnv { red_doc = doc, red_try_me = try_me, + red_givens = [], + red_given_scs = const AddSCs, red_stack = (0,[]), red_improve = True } @@ -1705,15 +1793,16 @@ mkNoImproveRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv mkNoImproveRedEnv doc try_me = RedEnv { red_doc = doc, red_try_me = try_me, red_givens = [], + red_given_scs = const AddSCs, red_stack = (0,[]), red_improve = True } data WhatToDo - = ReduceMe WantSCs -- Try to reduce this - -- If there's no instance, add the inst to the - -- irreductible ones, but don't produce an error - -- message of any kind. - -- It might be quite legitimate such as (Eq a)! + = ReduceMe -- Try to reduce this + -- If there's no instance, add the inst to the + -- irreductible ones, but don't produce an error + -- message of any kind. + -- It might be quite legitimate such as (Eq a)! | Stop -- Return as irreducible unless it can -- be reduced to a constant in one step @@ -1783,10 +1872,13 @@ reduceContext env wanteds0 wanteds', normalise_binds, eq_improved) <- tcReduceEqs givens wanteds + ; traceTc $ text "reduceContext: tcReduceEqs result" <+> vcat + [ppr givens', ppr wanteds', ppr normalise_binds] -- Build the Avail mapping from "given_dicts" ; (init_state, _) <- getLIE $ do - { init_state <- foldlM addGiven emptyAvails givens' + { init_state <- foldlM (addGiven (red_given_scs env)) + emptyAvails givens' ; return init_state } @@ -1794,52 +1886,49 @@ reduceContext env wanteds0 -- This may expose some further equational constraints... ; let (wanted_implics, wanted_dicts) = partition isImplicInst wanteds' ; (avails, extra_eqs) <- getLIE (reduceList env wanted_dicts init_state) + -- The getLIE is reqd because reduceList does improvement + -- (via extendAvails) which may in turn do unification ; (dict_binds, bound_dicts, dict_irreds) <- extractResults avails wanted_dicts - ; traceTc $ text "reduceContext extractresults" <+> vcat + ; traceTc $ text "reduceContext: extractResults" <+> vcat [ppr avails, ppr wanted_dicts, ppr dict_binds] -- Solve the wanted *implications*. In doing so, we can provide -- as "given" all the dicts that were originally given, -- *or* for which we now have bindings, -- *or* which are now irreds - ; let implic_env = env { red_givens = givens ++ bound_dicts ++ - dict_irreds } + -- NB: Equality irreds need to be converted, as the recursive + -- invocation of the solver will still treat them as wanteds + -- otherwise. + ; let implic_env = env { red_givens + = givens ++ bound_dicts ++ + map wantedToLocalEqInst dict_irreds } ; (implic_binds_s, implic_irreds_s) <- mapAndUnzipM (reduceImplication implic_env) wanted_implics ; let implic_binds = unionManyBags implic_binds_s implic_irreds = concat implic_irreds_s -- Collect all irreducible instances, and determine whether we should - -- go round again. We do so in either of three cases: + -- go round again. We do so in either of two cases: -- (1) If dictionary reduction or equality solving led to -- improvement (i.e., instantiated type variables). - -- (2) If we managed to normalise any dicts, there is merit in going - -- around gain, because reduceList may be able to get further. - -- (3) If we uncovered extra equalities. We will try to solve them + -- (2) If we uncovered extra equalities. We will try to solve them -- in the next iteration. + -- (3) If we reduced dictionaries (i.e., got dictionary bindings), + -- they may have exposed further opportunities to normalise + -- family applications. See Note [Dictionary Improvement] + ; let all_irreds = dict_irreds ++ implic_irreds ++ extra_eqs - improvedFlexible = availsImproved avails || - eq_improved - improvedDicts = not $ isEmptyBag normalise_binds + avails_improved = availsImproved avails + improvedFlexible = avails_improved || eq_improved extraEqs = (not . null) extra_eqs - improved = improvedFlexible || improvedDicts || extraEqs - -{- Old story - -- Figure out whether we should go round again. We do so in either - -- two cases: - -- (1) If any of the mutable tyvars in givens or irreds has been - -- filled in by improvement, there is merit in going around - -- again, because we may make further progress. - -- (2) If we managed to normalise any dicts, there is merit in going - -- around gain, because reduceList may be able to get further. - - ; improvedMetaTy <- anyM isFilledMetaTyVar $ varSetElems $ - tyVarsOfInsts (givens ++ all_irreds) - ; let improvedDicts = not $ isEmptyBag normalise_binds - improved = improvedMetaTy || improvedDicts - -} + reduced_dicts = not (isEmptyBag dict_binds) + improved = improvedFlexible || extraEqs || reduced_dicts + -- + improvedHint = (if avails_improved then " [AVAILS]" else "") ++ + (if eq_improved then " [EQ]" else "") ++ + (if extraEqs then " [EXTRA EQS]" else "") ; traceTc (text "reduceContext end" <+> (vcat [ text "----------------------", @@ -1848,7 +1937,7 @@ reduceContext env wanteds0 text "wanted" <+> ppr wanteds0, text "----", text "avails" <+> pprAvails avails, - text "improved =" <+> ppr improved, + text "improved =" <+> ppr improved <+> text improvedHint, text "(all) irreds = " <+> ppr all_irreds, text "dict-binds = " <+> ppr dict_binds, text "implic-binds = " <+> ppr implic_binds, @@ -1873,39 +1962,88 @@ tcImproveOne avails inst -- Avails has all the superclasses etc (good) -- It also has all the intermediates of the deduction (good) -- It does not have duplicates (good) - -- NB that (?x::t1) and (?x::t2) will be held separately in avails - -- so that improve will see them separate + -- NB that (?x::t1) and (?x::t2) will be held separately in + -- avails so that improve will see them separate ; traceTc (text "improveOne" <+> ppr inst) ; unifyEqns eqns } -unifyEqns :: [(Equation,(PredType,SDoc),(PredType,SDoc))] +unifyEqns :: [(Equation, (PredType, SDoc), (PredType, SDoc))] -> TcM ImprovementDone unifyEqns [] = return False unifyEqns eqns = do { traceTc (ptext (sLit "Improve:") <+> vcat (map pprEquationDoc eqns)) - ; mapM_ unify eqns - ; return True } + ; improved <- mapM unify eqns + ; return $ or improved + } where unify ((qtvs, pairs), what1, what2) - = addErrCtxtM (mkEqnMsg what1 what2) $ do - (_, _, tenv) <- tcInstTyVars (varSetElems qtvs) - mapM_ (unif_pr tenv) pairs - unif_pr tenv (ty1,ty2) = unifyType (substTy tenv ty1) (substTy tenv ty2) + = addErrCtxtM (mkEqnMsg what1 what2) $ + do { let freeTyVars = unionVarSets (map tvs_pr pairs) + `minusVarSet` qtvs + ; (_, _, tenv) <- tcInstTyVars (varSetElems qtvs) + ; mapM_ (unif_pr tenv) pairs + ; anyM isFilledMetaTyVar $ varSetElems freeTyVars + } + + unif_pr tenv (ty1, ty2) = unifyType (substTy tenv ty1) (substTy tenv ty2) + + tvs_pr (ty1, ty2) = tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2 pprEquationDoc :: (Equation, (PredType, SDoc), (PredType, SDoc)) -> SDoc -pprEquationDoc (eqn, (p1, _), (p2, _)) = vcat [pprEquation eqn, nest 2 (ppr p1), nest 2 (ppr p2)] +pprEquationDoc (eqn, (p1, _), (p2, _)) + = vcat [pprEquation eqn, nest 2 (ppr p1), nest 2 (ppr p2)] mkEqnMsg :: (TcPredType, SDoc) -> (TcPredType, SDoc) -> TidyEnv -> TcM (TidyEnv, SDoc) mkEqnMsg (pred1,from1) (pred2,from2) tidy_env - = do { pred1' <- zonkTcPredType pred1; pred2' <- zonkTcPredType pred2 - ; let { pred1'' = tidyPred tidy_env pred1'; pred2'' = tidyPred tidy_env pred2' } + = do { pred1' <- zonkTcPredType pred1 + ; pred2' <- zonkTcPredType pred2 + ; let { pred1'' = tidyPred tidy_env pred1' + ; pred2'' = tidyPred tidy_env pred2' } ; let msg = vcat [ptext (sLit "When using functional dependencies to combine"), nest 2 (sep [ppr pred1'' <> comma, nest 2 from1]), nest 2 (sep [ppr pred2'' <> comma, nest 2 from2])] ; return (tidy_env, msg) } \end{code} +Note [Dictionary Improvement] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +In reduceContext, we first reduce equalities and then class constraints. +However, the letter may expose further opportunities for the former. Hence, +we need to go around again if dictionary reduction produced any dictionary +bindings. The following example demonstrated the point: + + data EX _x _y (p :: * -> *) + data ANY + + class Base p + + class Base (Def p) => Prop p where + type Def p + + instance Base () + instance Prop () where + type Def () = () + + instance (Base (Def (p ANY))) => Base (EX _x _y p) + instance (Prop (p ANY)) => Prop (EX _x _y p) where + type Def (EX _x _y p) = EX _x _y p + + data FOO x + instance Prop (FOO x) where + type Def (FOO x) = () + + data BAR + instance Prop BAR where + type Def BAR = EX () () FOO + +During checking the last instance declaration, we need to check the superclass +cosntraint Base (Def BAR), which family normalisation reduced to +Base (EX () () FOO). Chasing the instance for Base (EX _x _y p), gives us +Base (Def (FOO ANY)), which again requires family normalisation of Def to +Base () before we can finish. + + The main context-reduction function is @reduce@. Here's its game plan. \begin{code} @@ -1928,6 +2066,12 @@ reduceList env@(RedEnv {red_stack = (n,stk)}) wanteds state -- Base case: we're done! reduce :: RedEnv -> Inst -> Avails -> TcM Avails reduce env wanted avails + + -- We don't reduce equalities here (and they must not end up as irreds + -- in the Avails!) + | isEqInst wanted + = return avails + -- It's the same as an existing inst, or a superclass thereof | Just _ <- findAvail avails wanted = do { traceTc (text "reduce: found " <+> ppr wanted) @@ -1940,18 +2084,18 @@ reduce env wanted avails Stop -> try_simple (addIrred NoSCs); -- See Note [No superclasses for Stop] - ReduceMe want_scs -> do -- It should be reduced + ReduceMe -> do -- It should be reduced { (avails, lookup_result) <- reduceInst env avails wanted ; case lookup_result of - NoInstance -> addIrred want_scs avails wanted + NoInstance -> addIrred AddSCs avails wanted -- Add it and its superclasses - GenInst [] rhs -> addWanted want_scs avails wanted rhs [] + GenInst [] rhs -> addWanted AddSCs avails wanted rhs [] GenInst wanteds' rhs -> do { avails1 <- addIrred NoSCs avails wanted ; avails2 <- reduceList env wanteds' avails1 - ; addWanted want_scs avails2 wanted rhs wanteds' } } + ; addWanted AddSCs avails2 wanted rhs wanteds' } } -- Temporarily do addIrred *before* the reduceList, -- which has the effect of adding the thing we are trying -- to prove to the database before trying to prove the things it @@ -1973,10 +2117,46 @@ reduce env wanted avails \end{code} +Note [RECURSIVE DICTIONARIES] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider + data D r = ZeroD | SuccD (r (D r)); + + instance (Eq (r (D r))) => Eq (D r) where + ZeroD == ZeroD = True + (SuccD a) == (SuccD b) = a == b + _ == _ = False; + + equalDC :: D [] -> D [] -> Bool; + equalDC = (==); + +We need to prove (Eq (D [])). Here's how we go: + + d1 : Eq (D []) + +by instance decl, holds if + d2 : Eq [D []] + where d1 = dfEqD d2 + +by instance decl of Eq, holds if + d3 : D [] + where d2 = dfEqList d3 + d1 = dfEqD d2 + +But now we can "tie the knot" to give + + d3 = d1 + d2 = dfEqList d3 + d1 = dfEqD d2 + +and it'll even run! The trick is to put the thing we are trying to prove +(in this case Eq (D []) into the database before trying to prove its +contributing clauses. + Note [SUPERCLASS-LOOP 2] ~~~~~~~~~~~~~~~~~~~~~~~~ -But the above isn't enough. Suppose we are *given* d1:Ord a, -and want to deduce (d2:C [a]) where +We need to be careful when adding "the constaint we are trying to prove". +Suppose we are *given* d1:Ord a, and want to deduce (d2:C [a]) where class Ord a => C a where instance Ord [a] => C [a] where ... @@ -2014,42 +2194,6 @@ Now we implement the Right Solution, which is to check for loops directly when adding superclasses. It's a bit like the occurs check in unification. -Note [RECURSIVE DICTIONARIES] -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -Consider - data D r = ZeroD | SuccD (r (D r)); - - instance (Eq (r (D r))) => Eq (D r) where - ZeroD == ZeroD = True - (SuccD a) == (SuccD b) = a == b - _ == _ = False; - - equalDC :: D [] -> D [] -> Bool; - equalDC = (==); - -We need to prove (Eq (D [])). Here's how we go: - - d1 : Eq (D []) - -by instance decl, holds if - d2 : Eq [D []] - where d1 = dfEqD d2 - -by instance decl of Eq, holds if - d3 : D [] - where d2 = dfEqList d3 - d1 = dfEqD d2 - -But now we can "tie the knot" to give - - d3 = d1 - d2 = dfEqList d3 - d1 = dfEqD d2 - -and it'll even run! The trick is to put the thing we are trying to prove -(in this case Eq (D []) into the database before trying to prove its -contributing clauses. - %************************************************************************ %* * @@ -2118,20 +2262,32 @@ Note that -- -- Note on coercion variables: -- - -- The extra given coercion variables are bound at two different sites: + -- The extra given coercion variables are bound at two different + -- sites: + -- -- -) in the creation context of the implication constraint -- the solved equational constraints use these binders -- -- -) at the solving site of the implication constraint - -- the solved dictionaries use these binders + -- the solved dictionaries use these binders; -- these binders are generated by reduceImplication -- + -- Note [Binders for equalities] + -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + -- To reuse the binders of local/given equalities in the binders of + -- implication constraints, it is crucial that these given equalities + -- always have the form + -- cotv :: t1 ~ t2 + -- where cotv is a simple coercion type variable (and not a more + -- complex coercion term). We require that the extra_givens always + -- have this form and exploit the special form when generating binders. reduceImplication env orig_implic@(ImplicInst { tci_name = name, tci_loc = inst_loc, tci_tyvars = tvs, - tci_given = extra_givens, tci_wanted = wanteds }) + tci_given = extra_givens, tci_wanted = wanteds + }) = do { -- Solve the sub-problem - ; let try_me _ = ReduceMe AddSCs -- Note [Freeness and implications] + ; let try_me _ = ReduceMe -- Note [Freeness and implications] env' = env { red_givens = extra_givens ++ red_givens env , red_doc = sep [ptext (sLit "reduceImplication for") <+> ppr name, @@ -2143,13 +2299,6 @@ reduceImplication env [ ppr (red_givens env), ppr extra_givens, ppr wanteds]) ; (irreds, binds) <- checkLoop env' wanteds - ; let (extra_eq_givens, extra_dict_givens) = partition isEqInst extra_givens - -- SLPJ Sept 07: I think this is bogus; currently - -- there are no Eqinsts in extra_givens - dict_ids = map instToId extra_dict_givens - - -- Note [Reducing implication constraints] - -- Tom -- update note, put somewhere! ; traceTc (text "reduceImplication result" <+> vcat [ppr irreds, ppr binds]) @@ -2163,11 +2312,17 @@ reduceImplication env -- SLPJ Sept 07: what if improvement happened inside the checkLoop? -- Then we must iterate the outer loop too! - ; traceTc (text "reduceImplication condition" <+> ppr ((isEmptyLHsBinds binds) || (null irreds))) + ; didntSolveWantedEqs <- allM wantedEqInstIsUnsolved wanteds + -- we solve wanted eqs by side effect! + + -- Progress is no longer measered by the number of bindings + -- If there are any irreds, but no bindings and no solved + -- equalities, we back off and do nothing + ; let backOff = isEmptyLHsBinds binds && -- no new bindings + (not $ null irreds) && -- but still some irreds + didntSolveWantedEqs -- no instantiated cotv --- Progress is no longer measered by the number of bindings - ; if (isEmptyLHsBinds binds) && (not $ null irreds) then -- No progress - -- If there are any irreds, we back off and do nothing + ; if backOff then -- No progress return (emptyBag, [orig_implic]) else do { (simpler_implic_insts, bind) @@ -2177,26 +2332,29 @@ reduceImplication env -- case. After all, we only try hard to reduce at top level, or -- when inferring types. - ; let dict_wanteds = filter (not . isEqInst) wanteds - -- TOMDO: given equational constraints bug! - -- we need a different evidence for given - -- equations depending on whether we solve - -- dictionary constraints or equational constraints - - eq_tyvars = varSetElems $ tyVarsOfTypes $ map eqInstType extra_eq_givens - -- SLPJ Sept07: this looks Utterly Wrong to me, but I think - -- that current extra_givens has no EqInsts, so - -- it makes no difference - co = wrap_inline -- Note [Always inline implication constraints] - <.> mkWpTyLams tvs - <.> mkWpLams eq_tyvars - <.> mkWpLams dict_ids - <.> WpLet (binds `unionBags` bind) - wrap_inline | null dict_ids = idHsWrapper - | otherwise = WpInline - rhs = mkLHsWrap co payload - loc = instLocSpan inst_loc - payload = mkBigLHsTup (map (L loc . HsVar . instToId) dict_wanteds) + ; let -- extract Id binders for dicts and CoTyVar binders for eqs; + -- see Note [Binders for equalities] + (extra_eq_givens, extra_dict_givens) = partition isEqInst + extra_givens + eq_cotvs = map instToVar extra_eq_givens + dict_ids = map instToId extra_dict_givens + + -- Note [Always inline implication constraints] + wrap_inline | null dict_ids = idHsWrapper + | otherwise = WpInline + co = wrap_inline + <.> mkWpTyLams tvs + <.> mkWpTyLams eq_cotvs + <.> mkWpLams dict_ids + <.> WpLet (binds `unionBags` bind) + rhs = mkLHsWrap co payload + loc = instLocSpan inst_loc + -- wanted equalities are solved by updating their + -- cotv; we don't generate bindings for them + dict_bndrs = map (L loc . HsVar . instToId) + . filter (not . isEqInst) + $ wanteds + payload = mkBigLHsTup dict_bndrs ; traceTc (vcat [text "reduceImplication" <+> ppr name, @@ -2382,6 +2540,9 @@ extractResults (Avails _ avails) wanteds = return (binds, bound_dicts, irreds) go binds bound_dicts irreds done (w:ws) + | isEqInst w + = go binds bound_dicts (w:irreds) done' ws + | Just done_ids@(done_id : rest_done_ids) <- lookupFM done w = if w_id `elem` done_ids then go binds bound_dicts irreds done ws @@ -2427,9 +2588,10 @@ addWanted want_scs avails wanted rhs_expr wanteds where avail = Rhs rhs_expr wanteds -addGiven :: Avails -> Inst -> TcM Avails -addGiven avails given = addAvailAndSCs AddSCs avails given (Given given) - -- Always add superclasses for 'givens' +addGiven :: (Inst -> WantSCs) -> Avails -> Inst -> TcM Avails +addGiven want_scs avails given = addAvailAndSCs (want_scs given) avails given (Given given) + -- Conditionally add superclasses for 'givens' + -- See Note [Recursive instances and superclases] -- -- No ASSERT( not (given `elemAvails` avails) ) because in an instance -- decl for Ord t we can add both Ord t and Eq t as 'givens',