X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcSimplify.lhs;h=cf2974856180d35d367c8ee77ed5844524fe2ca3;hp=8a5ad1a9bffac92c77e99bcb43fe75e0746891dc;hb=432b9c9322181a3644083e3c19b7e240d90659e7;hpb=5656eb8f9bc7ee43da889da4847856a0f70d9461 diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs index 8a5ad1a..cf29748 100644 --- a/compiler/typecheck/TcSimplify.lhs +++ b/compiler/typecheck/TcSimplify.lhs @@ -16,6 +16,8 @@ module TcSimplify ( tcSimplifyDeriv, tcSimplifyDefault, bindInstsOfLocalFuns, + + tcSimplifyStagedExpr, misMatchMsg ) where @@ -58,6 +60,7 @@ import Util import SrcLoc import DynFlags import FastString + import Control.Monad import Data.List \end{code} @@ -656,7 +659,7 @@ tcSimplifyInfer doc tau_tvs wanted ; gbl_tvs <- tcGetGlobalTyVars ; let preds1 = fdPredsOfInsts wanted' gbl_tvs1 = oclose preds1 gbl_tvs - qtvs = grow preds1 tau_tvs1 `minusVarSet` gbl_tvs1 + qtvs = growInstsTyVars wanted' tau_tvs1 `minusVarSet` gbl_tvs1 -- See Note [Choosing which variables to quantify] -- To maximise sharing, remove from consideration any @@ -665,7 +668,7 @@ tcSimplifyInfer doc tau_tvs wanted ; extendLIEs free -- To make types simple, reduce as much as possible - ; traceTc (text "infer" <+> (ppr preds1 $$ ppr (grow preds1 tau_tvs1) $$ ppr gbl_tvs $$ + ; traceTc (text "infer" <+> (ppr preds1 $$ ppr (growInstsTyVars wanted' tau_tvs1) $$ ppr gbl_tvs $$ ppr gbl_tvs1 $$ ppr free $$ ppr bound)) ; (irreds1, binds1) <- tryHardCheckLoop doc bound @@ -706,7 +709,13 @@ tcSimplifyInfer doc tau_tvs wanted -- Then b is fixed by gbl_tvs, so (C a b) will be in free, and -- irreds2 will be empty. But we don't want to generalise over b! ; let preds2 = fdPredsOfInsts irreds2 -- irreds2 is zonked - qtvs = grow preds2 tau_tvs2 `minusVarSet` oclose preds2 gbl_tvs2 + qtvs = growInstsTyVars irreds2 tau_tvs2 `minusVarSet` oclose preds2 gbl_tvs2 + --------------------------------------------------- + -- BUG WARNING: there's a nasty bug lurking here + -- fdPredsOfInsts may return preds that mention variables quantified in + -- one of the implication constraints in irreds2; and that is clearly wrong: + -- we might quantify over too many variables through accidental capture + --------------------------------------------------- ; let (free, irreds3) = partition (isFreeWhenInferring qtvs) irreds2 ; extendLIEs free @@ -861,7 +870,7 @@ Note [NO TYVARS] The excitement comes when simplifying the bindings for h. Initially try to simplify {y @ [[t1]] t2, 0 @ t1}, with initial qtvs = {t2}. -From this we get t1:=:t2, but also various bindings. We can't forget +From this we get t1~t2, but also various bindings. We can't forget the bindings (because of [LOOP]), but in fact t1 is what g is polymorphic in. @@ -971,17 +980,17 @@ 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) +-- 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 +998,38 @@ 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 + tci_given = eq_givens ++ dict_givens, + -- 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 +1040,11 @@ 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 +1060,7 @@ 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 +1071,8 @@ 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} @@ -1096,7 +1115,9 @@ checkLoop env wanteds ; env' <- zonkRedEnv env ; wanteds' <- zonkInsts wanteds - ; (improved, binds, irreds) <- reduceContext env' wanteds' + ; (improved, tybinds, binds, irreds) + <- reduceContext env' wanteds' + ; execTcTyVarBinds tybinds ; if null irreds || not improved then return (irreds, binds) @@ -1201,30 +1222,104 @@ 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. + +There is a complication though. Suppose there are equalities + instance (Eq a, a~b) => Num (a,b) +Then we normalise the 'givens' wrt the equalities, so the original +given "this" dictionary is cast to one of a different type. So it's a +bit trickier than before to identify the "special" dictionary whose +superclasses must not be added. See test + indexed-types/should_run/EqInInstance + +We need a persistent property of the dictionary to record this +special-ness. Current I'm using the InstLocOrigin (a bit of a hack, +but cool), which is maintained by dictionary normalisation. +Specifically, the InstLocOrigin is + NoScOrigin +then the no-superclass thing kicks in. WATCH OUT if you fiddle +with InstLocOrigin! + \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") + + -- Note [Recursive instances and superclases] + ; no_sc_loc <- getInstLoc NoScOrigin + ; let no_sc_this = setInstLoc this no_sc_loc + + ; let env = RedEnv { red_doc = pprInstLoc loc, + red_try_me = try_me, + red_givens = no_sc_this : givens, + red_stack = (0,[]), + red_improve = False } -- No unification vars + + ; (irreds,binds1) <- checkLoop env sc_wanteds ; let (tidy_env, tidy_irreds) = tidyInsts irreds - ; reportNoInstances tidy_env (Just (loc, givens)) tidy_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 + try_me _ = ReduceMe -- Try hard, so we completely solve the superclass + -- constraints right here. See Note [SUPERCLASS-LOOP 1] \end{code} @@ -1345,7 +1440,7 @@ tcSimplifyRestricted -- Used for restricted binding groups tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds -- Zonk everything in sight = do { traceTc (text "tcSimplifyRestricted") - ; wanteds' <- zonkInsts wanteds + ; wanteds_z <- zonkInsts wanteds -- 'ReduceMe': Reduce as far as we can. Don't stop at -- dicts; the idea is to get rid of as many type @@ -1356,8 +1451,9 @@ 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) - ; (_imp, _binds, constrained_dicts) <- reduceContext env wanteds' + ; let env = mkNoImproveRedEnv doc (\_ -> ReduceMe) + ; (_imp, _tybinds, _binds, constrained_dicts) + <- reduceContext env wanteds_z -- Next, figure out the tyvars we will quantify over ; tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs) @@ -1404,9 +1500,10 @@ 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' + ; (_imp, tybinds, binds, irreds) <- reduceContext env wanteds_z + ; execTcTyVarBinds tybinds -- See "Notes on implicit parameters, Question 4: top level" ; ASSERT( all (isFreeWrtTyVars qtvs) irreds ) -- None should be captured @@ -1524,9 +1621,9 @@ 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} @@ -1583,14 +1680,23 @@ tcSimplifyIPs given_ips wanteds -- Unusually for checking, we *must* zonk the given_ips ; let env = mkRedEnv doc try_me given_ips' - ; (improved, binds, irreds) <- reduceContext env wanteds' + ; (improved, tybinds, binds, irreds) <- reduceContext env wanteds' + ; execTcTyVarBinds tybinds - ; 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 +1704,7 @@ 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 +1779,9 @@ 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_stack :: (Int, [Inst]) -- Recursion stack (for err msg) -- See Note [RedStack] } @@ -1700,6 +1807,14 @@ mkRedEnv doc try_me givens 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_stack = (0,[]), + red_improve = True } + mkNoImproveRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv -- Do not do improvement; no givens mkNoImproveRedEnv doc try_me @@ -1709,11 +1824,11 @@ mkNoImproveRedEnv doc try_me 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 @@ -1755,6 +1870,7 @@ discharge with the explicit instance. reduceContext :: RedEnv -> [Inst] -- Wanted -> TcM (ImprovementDone, + TcTyVarBinds, -- Type variable bindings TcDictBinds, -- Dictionary bindings [Inst]) -- Irreducible @@ -1781,10 +1897,11 @@ reduceContext env wanteds0 givens = red_givens env ; (givens', wanteds', - normalise_binds, - eq_improved) <- tcReduceEqs givens wanteds + tybinds, + normalise_binds) <- tcReduceEqs givens wanteds ; traceTc $ text "reduceContext: tcReduceEqs result" <+> vcat - [ppr givens', ppr wanteds', ppr normalise_binds] + [ppr givens', ppr wanteds', ppr tybinds, + ppr normalise_binds] -- Build the Avail mapping from "given_dicts" ; (init_state, _) <- getLIE $ do @@ -1793,9 +1910,13 @@ reduceContext env wanteds0 } -- Solve the *wanted* *dictionary* constraints (not implications) - -- This may expose some further equational constraints... + -- This may expose some further equational constraints in the course + -- of improvement due to functional dependencies if any of the + -- involved unifications gets deferred. ; 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 @@ -1806,8 +1927,12 @@ reduceContext env wanteds0 -- 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 @@ -1816,19 +1941,26 @@ reduceContext env wanteds0 -- Collect all irreducible instances, and determine whether we should -- 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 uncovered extra equalities. We will try to solve them - -- in the next iteration. + -- improvement (i.e., bindings for type variables). + -- (2) If we reduced dictionaries (i.e., got dictionary bindings), + -- they may have exposed further opportunities to normalise + -- family applications. See Note [Dictionary Improvement] + -- + -- NB: We do *not* go around for new extra_eqs. Morally, we should, + -- but we can't without risking non-termination (see #2688). By + -- not going around, we miss some legal programs mixing FDs and + -- TFs, but we never claimed to support such programs in the + -- current implementation anyway. ; let all_irreds = dict_irreds ++ implic_irreds ++ extra_eqs avails_improved = availsImproved avails + eq_improved = anyBag (not . isCoVarBind) tybinds improvedFlexible = avails_improved || eq_improved - extraEqs = (not . null) extra_eqs - improved = improvedFlexible || extraEqs + reduced_dicts = not (isEmptyBag dict_binds) + improved = improvedFlexible || reduced_dicts -- improvedHint = (if avails_improved then " [AVAILS]" else "") ++ - (if eq_improved then " [EQ]" else "") ++ - (if extraEqs then " [EXTRA EQS]" else "") + (if eq_improved then " [EQ]" else "") ; traceTc (text "reduceContext end" <+> (vcat [ text "----------------------", @@ -1836,6 +1968,7 @@ reduceContext env wanteds0 text "given" <+> ppr givens, text "wanted" <+> ppr wanteds0, text "----", + text "tybinds" <+> ppr tybinds, text "avails" <+> pprAvails avails, text "improved =" <+> ppr improved <+> text improvedHint, text "(all) irreds = " <+> ppr all_irreds, @@ -1845,10 +1978,13 @@ reduceContext env wanteds0 ])) ; return (improved, + tybinds, normalise_binds `unionBags` dict_binds `unionBags` implic_binds, all_irreds) } + where + isCoVarBind (TcTyVarBind tv _) = isCoVar tv tcImproveOne :: Avails -> Inst -> TcM ImprovementDone tcImproveOne avails inst @@ -1906,6 +2042,44 @@ mkEqnMsg (pred1,from1) (pred2,from2) tidy_env ; 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 +2102,12 @@ 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 +2120,18 @@ 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 +2153,46 @@ 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 +2230,6 @@ 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 +2298,32 @@ 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 +2335,6 @@ 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]) @@ -2163,17 +2348,17 @@ reduceImplication env -- SLPJ Sept 07: what if improvement happened inside the checkLoop? -- Then we must iterate the outer loop too! + ; 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 - all (not . isEqInst) wanteds - -- we may have instantiated a cotv - -- => must make a new implication constraint! + didntSolveWantedEqs -- no instantiated cotv - ; traceTc $ text "reduceImplication condition" <+> ppr backOff - - -- Progress is no longer measered by the number of bindings ; if backOff then -- No progress - -- If there are any irreds, we back off and do nothing return (emptyBag, [orig_implic]) else do { (simpler_implic_insts, bind) @@ -2183,26 +2368,29 @@ 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, @@ -2388,6 +2576,9 @@ 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 @@ -2434,12 +2625,18 @@ addWanted want_scs avails wanted rhs_expr wanteds avail = Rhs rhs_expr wanteds addGiven :: Avails -> Inst -> TcM Avails -addGiven avails given = addAvailAndSCs AddSCs avails given (Given given) - -- Always add superclasses for 'givens' - -- - -- 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', - -- so the assert isn't true +addGiven avails given + = addAvailAndSCs want_scs avails given (Given given) + where + want_scs = case instLocOrigin (instLoc given) of + NoScOrigin -> NoSCs + _other -> AddSCs + -- Conditionally add superclasses for 'given' + -- 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', so the assert isn't true \end{code} \begin{code} @@ -2659,6 +2856,7 @@ disambiguate doc interactive dflags insts where extended_defaulting = interactive || dopt Opt_ExtendedDefaultRules dflags + -- See also Trac #1974 ovl_strings = dopt Opt_OverloadedStrings dflags unaries :: [(Inst, Class, TcTyVar)] -- (C tv) constraints @@ -2705,12 +2903,16 @@ disambigGroup :: [Type] -- The default types -> TcM () -- Just does unification, to fix the default types disambigGroup default_tys dicts - = try_default default_tys + = do { mb_chosen_ty <- try_default default_tys + ; case mb_chosen_ty of + Nothing -> return () + Just chosen_ty -> do { unifyType chosen_ty (mkTyVarTy tyvar) + ; warnDefault dicts chosen_ty } } where (_,_,tyvar) = ASSERT(not (null dicts)) head dicts -- Should be non-empty classes = [c | (_,c,_) <- dicts] - try_default [] = return () + try_default [] = return Nothing try_default (default_ty : default_tys) = tryTcLIE_ (try_default default_tys) $ do { tcSimplifyDefault [mkClassPred clas [default_ty] | clas <- classes] @@ -2720,10 +2922,7 @@ disambigGroup default_tys dicts -- For example, if Real a is reqd, but the only type in the -- default list is Int. - -- After this we can't fail - ; warnDefault dicts default_ty - ; unifyType default_ty (mkTyVarTy tyvar) - ; return () -- TOMDO: do something with the coercion + ; return (Just default_ty) -- TOMDO: do something with the coercion } @@ -2810,7 +3009,8 @@ tcSimplifyDeriv orig tyvars theta ; (irreds, _) <- tryHardCheckLoop doc wanteds ; let (tv_dicts, others) = partition ok irreds - ; addNoInstanceErrs others + (tidy_env, tidy_insts) = tidyInsts others + ; reportNoInstances tidy_env Nothing [alt_fix] tidy_insts -- See Note [Exotic derived instance contexts] in TcMType ; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars) @@ -2824,6 +3024,8 @@ tcSimplifyDeriv orig tyvars theta ok dict | isDict dict = validDerivPred (dictPred dict) | otherwise = False + alt_fix = vcat [ptext (sLit "use a standalone 'deriving instance' declaration instead,"), + ptext (sLit "so you can specify the instance context yourself")] \end{code} @@ -2838,7 +3040,7 @@ tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it tcSimplifyDefault theta = do wanteds <- newDictBndrsO DefaultOrigin theta (irreds, _) <- tryHardCheckLoop doc wanteds - addNoInstanceErrs irreds + addNoInstanceErrs irreds if null irreds then return () else @@ -2847,6 +3049,26 @@ tcSimplifyDefault theta = do doc = ptext (sLit "default declaration") \end{code} +@tcSimplifyStagedExpr@ performs a simplification but does so at a new +stage. This is used when typechecking annotations and splices. + +\begin{code} + +tcSimplifyStagedExpr :: ThStage -> TcM a -> TcM (a, TcDictBinds) +-- Type check an expression that runs at a top level stage as if +-- it were going to be spliced and then simplify it +tcSimplifyStagedExpr stage tc_action + = setStage stage $ do { + -- Typecheck the expression + (thing', lie) <- getLIE tc_action + + -- Solve the constraints + ; const_binds <- tcSimplifyTop lie + + ; return (thing', const_binds) } + +\end{code} + %************************************************************************ %* * @@ -2914,7 +3136,7 @@ addNoInstanceErrs :: [Inst] -- Wanted (can include implications) -> TcM () addNoInstanceErrs insts = do { let (tidy_env, tidy_insts) = tidyInsts insts - ; reportNoInstances tidy_env Nothing tidy_insts } + ; reportNoInstances tidy_env Nothing [] tidy_insts } reportNoInstances :: TidyEnv @@ -2922,14 +3144,15 @@ reportNoInstances -- Nothing => top level -- Just (d,g) => d describes the construct -- with givens g + -> [SDoc] -- Alternative fix for no-such-instance -> [Inst] -- What is wanted (can include implications) -> TcM () -reportNoInstances tidy_env mb_what insts - = groupErrs (report_no_instances tidy_env mb_what) insts +reportNoInstances tidy_env mb_what alt_fix insts + = groupErrs (report_no_instances tidy_env mb_what alt_fix) insts -report_no_instances :: TidyEnv -> Maybe (InstLoc, [Inst]) -> [Inst] -> TcM () -report_no_instances tidy_env mb_what insts +report_no_instances :: TidyEnv -> Maybe (InstLoc, [Inst]) -> [SDoc] -> [Inst] -> TcM () +report_no_instances tidy_env mb_what alt_fixes insts = do { inst_envs <- tcGetInstEnvs ; let (implics, insts1) = partition isImplicInst insts (insts2, overlaps) = partitionWith (check_overlap inst_envs) insts1 @@ -2947,7 +3170,7 @@ report_no_instances tidy_env mb_what insts complain_implic inst -- Recurse! = reportNoInstances tidy_env (Just (tci_loc inst, tci_given inst)) - (tci_wanted inst) + alt_fixes (tci_wanted inst) check_overlap :: (InstEnv,InstEnv) -> Inst -> Either Inst SDoc -- Right msg => overlap message @@ -2995,13 +3218,13 @@ report_no_instances tidy_env mb_what insts = vcat [ addInstLoc insts $ sep [ ptext (sLit "Could not deduce") <+> pprDictsTheta insts , nest 2 $ ptext (sLit "from the context") <+> pprDictsTheta givens] - , show_fixes (fix1 loc : fixes2) ] + , show_fixes (fix1 loc : fixes2 ++ alt_fixes) ] | otherwise -- Top level = vcat [ addInstLoc insts $ ptext (sLit "No instance") <> plural insts <+> ptext (sLit "for") <+> pprDictsTheta insts - , show_fixes fixes2 ] + , show_fixes (fixes2 ++ alt_fixes) ] where fix1 loc = sep [ ptext (sLit "add") <+> pprDictsTheta insts