From ead3abe7fbf33f019549a05ad9dd3cd22ef3adab Mon Sep 17 00:00:00 2001 From: "simonpj@microsoft.com" Date: Wed, 4 Jul 2007 22:22:21 +0000 Subject: [PATCH] Fix Trace #1494 A tricky bug to do with the way the implication constraints are solved in TcSimplify. See Note [Reducing implication constraints]. --- compiler/typecheck/TcSimplify.lhs | 59 +++++++++++++++++++++++++++---------- 1 file changed, 43 insertions(+), 16 deletions(-) diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs index db12011..d481146 100644 --- a/compiler/typecheck/TcSimplify.lhs +++ b/compiler/typecheck/TcSimplify.lhs @@ -47,6 +47,7 @@ import VarEnv import FiniteMap import Bag import Outputable +import Maybes import ListSetOps import Util import SrcLoc @@ -735,7 +736,7 @@ with 'given' implications. So our best approximation is to make (D [a]) part of the inferred context, so we can use that to discharge the implication. Hence -the strange function getImplicWanteds. +the strange function get_dictsin approximateImplications. The common cases are more clear-cut, when we have things like forall a. C a => C b @@ -1902,20 +1903,22 @@ reduceImplication env orig_avails reft tvs extra_givens wanteds inst_loc ppr reft, ppr wanteds, ppr avails ]) ; avails <- reduceList env' wanteds avails - -- Extract the binding + -- Extract the results + -- Note [Reducing implication constraints] ; (binds, irreds) <- extractResults avails wanteds - + ; let (outer, inner) = partition (isJust . findAvail orig_avails) irreds + ; traceTc (text "reduceImplication result" <+> vcat - [ ppr irreds, ppr binds]) + [ ppr outer, ppr inner, ppr binds]) -- We always discard the extra avails we've generated; -- but we remember if we have done any (global) improvement ; let ret_avails = updateImprovement orig_avails avails - ; if isEmptyLHsBinds binds then -- No progress + ; if isEmptyLHsBinds binds && null outer then -- No progress return (ret_avails, NoInstance) else do - { (implic_insts, bind) <- makeImplicationBind inst_loc tvs reft extra_givens irreds + { (implic_insts, bind) <- makeImplicationBind inst_loc tvs reft extra_givens inner ; let dict_ids = map instToId extra_givens co = mkWpTyLams tvs <.> mkWpLams dict_ids <.> WpLet (binds `unionBags` bind) @@ -1924,11 +1927,36 @@ reduceImplication env orig_avails reft tvs extra_givens wanteds inst_loc payload | [wanted] <- wanteds = HsVar (instToId wanted) | otherwise = ExplicitTuple (map (L loc . HsVar . instToId) wanteds) Boxed - -- If there are any irreds, we back off and return NoInstance - ; return (ret_avails, GenInst implic_insts (L loc rhs)) + ; return (ret_avails, GenInst (implic_insts ++ outer) (L loc rhs)) } } \end{code} +Note [Reducing implication constraints] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Suppose we are trying to simplify + (Ord a, forall b. C a b => (W [a] b, D c b)) +where + instance (C a b, Ord a) => W [a] b +When solving the implication constraint, we'll start with + Ord a -> Irred +in the Avails. Then we add (C a b -> Given) and solve. Extracting +the results gives us a binding for the (W [a] b), with an Irred of +(Ord a, D c b). Now, the (Ord a) comes from "outside" the implication, +but the (D d b) is from "inside". So we want to generate a Rhs binding +like this + + ic = /\b \dc:C a b). (df a b dc do, ic' b dc) + depending on + do :: Ord a + ic' :: forall b. C a b => D c b + +The 'depending on' part of the Rhs is important, because it drives +the extractResults code. + +The "inside" and "outside" distinction is what's going on with 'inner' and +'outer' in reduceImplication + + Note [Freeness and implications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ It's hard to say when an implication constraint can be floated out. Consider @@ -2009,7 +2037,7 @@ instance Outputable AvailHow where pprAvail :: AvailHow -> SDoc pprAvail (IsIrred x) = text "Irred" <+> ppr x pprAvail (Given x) = text "Given" <+> ppr x -pprAvail (Rhs rhs bs) = text "Rhs" <+> ppr rhs <+> braces (ppr bs) +pprAvail (Rhs rhs bs) = text "Rhs" <+> sep [ppr rhs, braces (ppr bs)] ------------------------- extendAvailEnv :: AvailEnv -> Inst -> AvailHow -> AvailEnv @@ -2067,8 +2095,8 @@ extractResults (Avails _ avails) wanteds go avails binds irreds (w:ws) = case findAvailEnv avails w of - Nothing -> pprTrace "Urk: extractResults" (ppr w) $ - go avails binds irreds ws + Nothing -> pprTrace "Urk: extractResults" (ppr w) $ + go avails binds irreds ws Just (Given id) | id == w_id -> go avails binds irreds ws @@ -2076,9 +2104,8 @@ extractResults (Avails _ avails) wanteds -- The sought Id can be one of the givens, via a superclass chain -- and then we definitely don't want to generate an x=x binding! - Just (IsIrred id) - | id == w_id -> go (add_given avails w) binds (w:irreds) ws - | otherwise -> go avails (addBind binds w_id (nlHsVar id)) irreds ws + Just (IsIrred _) -> go (add_given avails w) binds (w:irreds) ws + -- | otherwise -> go avails (addBind binds w_id (nlHsVar id)) irreds ws -- The add_given handles the case where we want (Ord a, Eq a), and we -- don't want to emit *two* Irreds for Ord a, one via the superclass chain -- This showed up in a dupliated Ord constraint in the error message for @@ -2330,14 +2357,14 @@ disambiguate doc interactive dflags insts = return (insts, emptyBag) | null defaultable_groups - = do { traceTc (text "disambigutate" <+> vcat [ppr unaries, ppr bad_tvs, ppr defaultable_groups]) + = do { traceTc (text "disambiguate1" <+> vcat [ppr insts, ppr unaries, ppr bad_tvs, ppr defaultable_groups]) ; return (insts, emptyBag) } | otherwise = do { -- Figure out what default types to use default_tys <- getDefaultTys extended_defaulting ovl_strings - ; traceTc (text "disambigutate" <+> vcat [ppr unaries, ppr bad_tvs, ppr defaultable_groups]) + ; traceTc (text "disambiguate1" <+> vcat [ppr insts, ppr unaries, ppr bad_tvs, ppr defaultable_groups]) ; mapM_ (disambigGroup default_tys) defaultable_groups -- disambigGroup does unification, hence try again -- 1.7.10.4