X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcSimplify.lhs;h=bed09325acde46c7fa3d695f4a4bfb732ca49a07;hp=0a4fe0733624cb20f4e8308f1e1d4492692594e3;hb=HEAD;hpb=972bf5f61cebb29ffd6c86453f3571c2bc138392 diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs index 0a4fe07..bed0932 100644 --- a/compiler/typecheck/TcSimplify.lhs +++ b/compiler/typecheck/TcSimplify.lhs @@ -1,7 +1,7 @@ \begin{code} module TcSimplify( simplifyInfer, - simplifyDefault, simplifyDeriv, + simplifyDefault, simplifyDeriv, simplifyRule, simplifyTop, simplifyInteractive ) where @@ -15,10 +15,12 @@ import TcType import TcSMonad import TcInteract import Inst -import Unify( niFixTvSubst, niSubstTvSet ) +import Id ( evVarPred ) +import Unify ( niFixTvSubst, niSubstTvSet ) import Var import VarSet import VarEnv +import Coercion import TypeRep import Name @@ -49,7 +51,7 @@ simplifyTop :: WantedConstraints -> TcM (Bag EvBind) -- but when there is nothing to quantify we don't wrap -- in a degenerate implication, so we do that here instead simplifyTop wanteds - = simplifyCheck SimplCheck wanteds + = simplifyCheck (SimplCheck (ptext (sLit "top level"))) wanteds ------------------ simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind) @@ -61,7 +63,8 @@ simplifyDefault :: ThetaType -- Wanted; has no type variables in it -> TcM () -- Succeeds iff the constraint is soluble simplifyDefault theta = do { wanted <- newFlatWanteds DefaultOrigin theta - ; _ignored_ev_binds <- simplifyCheck SimplCheck (mkFlatWC wanted) + ; _ignored_ev_binds <- simplifyCheck (SimplCheck (ptext (sLit "defaults"))) + (mkFlatWC wanted) ; return () } \end{code} @@ -75,27 +78,29 @@ simplifyDefault theta \begin{code} simplifyDeriv :: CtOrigin - -> [TyVar] - -> ThetaType -- Wanted - -> TcM ThetaType -- Needed + -> PredType + -> [TyVar] + -> ThetaType -- Wanted + -> TcM ThetaType -- Needed -- Given instance (wanted) => C inst_ty -- Simplify 'wanted' as much as possibles -- Fail if not possible -simplifyDeriv orig tvs theta - = do { tvs_skols <- tcInstSuperSkolTyVars tvs -- Skolemize - -- One reason is that the constraint solving machinery - -- expects *TcTyVars* not TyVars. Another is that - -- when looking up instances we don't want overlap - -- of type variables +simplifyDeriv orig pred tvs theta + = do { tvs_skols <- tcInstSkolTyVars tvs -- Skolemize + -- The constraint solving machinery + -- expects *TcTyVars* not TyVars. + -- We use *non-overlappable* (vanilla) skolems + -- See Note [Overlap and deriving] ; let skol_subst = zipTopTvSubst tvs $ map mkTyVarTy tvs_skols subst_skol = zipTopTvSubst tvs_skols $ map mkTyVarTy tvs + doc = parens $ ptext (sLit "deriving") <+> parens (ppr pred) ; wanted <- newFlatWanteds orig (substTheta skol_subst theta) ; traceTc "simplifyDeriv" (ppr tvs $$ ppr theta $$ ppr wanted) ; (residual_wanted, _binds) - <- runTcS SimplInfer NoUntouchables $ + <- runTcS (SimplInfer doc) NoUntouchables $ solveWanteds emptyInert (mkFlatWC wanted) ; let (good, bad) = partitionBagWith get_good (wc_flat residual_wanted) @@ -111,6 +116,31 @@ simplifyDeriv orig tvs theta ; return (substTheta subst_skol min_theta) } \end{code} +Note [Overlap and deriving] +~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider some overlapping instances: + data Show a => Show [a] where .. + data Show [Char] where ... + +Now a data type with deriving: + data T a = MkT [a] deriving( Show ) + +We want to get the derived instance + instance Show [a] => Show (T a) where... +and NOT + instance Show a => Show (T a) where... +so that the (Show (T Char)) instance does the Right Thing + +It's very like the situation when we're inferring the type +of a function + f x = show [x] +and we want to infer + f :: Show [a] => a -> String + +BOTTOM LINE: use vanilla, non-overlappable skolems when inferring + the context for the derived instance. + Hence tcInstSkolTyVars not tcInstSuperSkolTyVars + Note [Exotic derived instance contexts] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In a 'derived' instance declaration, we *infer* the context. It's a @@ -222,7 +252,7 @@ simplifyInfer top_lvl apply_mr name_taus wanteds -- Step 2 -- Now simplify the possibly-bound constraints ; (simpl_results, tc_binds0) - <- runTcS SimplInfer NoUntouchables $ + <- runTcS (SimplInfer (ppr (map fst name_taus))) NoUntouchables $ simplifyWithApprox (zonked_wanteds { wc_flat = perhaps_bound }) ; when (insolubleWC simpl_results) -- Fail fast if there is an insoluble constraint @@ -522,7 +552,7 @@ simplifyRule name tv_bndrs lhs_wanted rhs_wanted -- variables; hence *no untouchables* ; (lhs_results, lhs_binds) - <- runTcS SimplRuleLhs untch $ + <- runTcS (SimplRuleLhs name) untch $ solveWanteds emptyInert zonked_lhs ; traceTc "simplifyRule" $ @@ -564,7 +594,8 @@ simplifyRule name tv_bndrs lhs_wanted rhs_wanted -- Hence the rather painful ad-hoc treatement here ; rhs_binds_var@(EvBindsVar evb_ref _) <- newTcEvBinds - ; rhs_binds1 <- simplifyCheck SimplCheck $ + ; let doc = ptext (sLit "rhs of rule") <+> doubleQuotes (ftext name) + ; rhs_binds1 <- simplifyCheck (SimplCheck doc) $ WC { wc_flat = emptyBag , wc_insol = emptyBag , wc_impl = unitBag $ @@ -718,22 +749,26 @@ solve_wanteds inert wanted@(WC { wc_flat = flats, wc_impl = implics, wc_insol = unsolved_implics } -givensFromWanteds :: CanonicalCts -> Bag FlavoredEvVar --- Extract the *wanted* ones from CanonicalCts --- and make them into *givens* -givensFromWanteds = foldrBag getWanted emptyBag +givensFromWanteds :: SimplContext -> CanonicalCts -> Bag FlavoredEvVar +-- Extract the Wanted ones from CanonicalCts and conver to +-- Givens; not Given/Solved, see Note [Preparing inert set for implications] +givensFromWanteds _ctxt = foldrBag getWanted emptyBag where getWanted :: CanonicalCt -> Bag FlavoredEvVar -> Bag FlavoredEvVar getWanted cc givens - | not (isCFrozenErr cc) - , Wanted loc <- cc_flavor cc - , let given = mkEvVarX (cc_id cc) (Given (setCtLocOrigin loc UnkSkol)) - = given `consBag` givens - | otherwise - = givens -- We are not helping anyone by pushing a Derived in! - -- Because if we could not solve it to start with - -- we are not going to do either inside the impl constraint - + | pushable_wanted cc + = let given = mkEvVarX (cc_id cc) (mkGivenFlavor (cc_flavor cc) UnkSkol) + in given `consBag` givens -- and not mkSolvedFlavor, + -- see Note [Preparing inert set for implications] + | otherwise = givens + + pushable_wanted :: CanonicalCt -> Bool + pushable_wanted cc + | not (isCFrozenErr cc) + , isWantedCt cc + = isEqPred (evVarPred (cc_id cc)) -- see Note [Preparing inert set for implications] + | otherwise = False + solveNestedImplications :: InertSet -> CanonicalCts -> Bag Implication -> TcS (Bag FlavoredEvVar, Bag Implication) @@ -743,14 +778,18 @@ solveNestedImplications just_given_inert unsolved_cans implics | otherwise = do { -- See Note [Preparing inert set for implications] -- Push the unsolved wanteds inwards, but as givens - traceTcS "solveWanteds: preparing inerts for implications {" empty - - ; let pushed_givens = givensFromWanteds unsolved_cans + + ; simpl_ctx <- getTcSContext + + ; let pushed_givens = givensFromWanteds simpl_ctx unsolved_cans tcs_untouchables = filterVarSet isFlexiTcsTv $ tyVarsOfEvVarXs pushed_givens -- See Note [Extra TcsTv untouchables] - ; (_, inert_for_implics) <- solveInteract just_given_inert pushed_givens + ; traceTcS "solveWanteds: preparing inerts for implications {" + (vcat [ppr tcs_untouchables, ppr pushed_givens]) + + ; (_, inert_for_implics) <- solveInteract just_given_inert pushed_givens ; traceTcS "solveWanteds: } now doing nested implications {" $ vcat [ text "inerts_for_implics =" <+> ppr inert_for_implics @@ -901,6 +940,42 @@ We were not able to solve (a ~w [beta]) but we can't just assume it as given because the resulting set is not inert. Hence we have to do a 'solveInteract' step first. +Finally, note that we convert them to [Given] and NOT [Given/Solved]. +The reason is that Given/Solved are weaker than Givens and may be discarded. +As an example consider the inference case, where we may have, the following +original constraints: + [Wanted] F Int ~ Int + (F Int ~ a => F Int ~ a) +If we convert F Int ~ Int to [Given/Solved] instead of Given, then the next +given (F Int ~ a) is going to cause the Given/Solved to be ignored, casting +the (F Int ~ a) insoluble. Hence we should really convert the residual +wanteds to plain old Given. + +We need only push in unsolved equalities both in checking mode and inference mode: + + (1) In checking mode we should not push given dictionaries in because of +example LongWayOverlapping.hs, where we might get strange overlap +errors between far-away constraints in the program. But even in +checking mode, we must still push type family equations. Consider: + + type instance F True a b = a + type instance F False a b = b + + [w] F c a b ~ gamma + (c ~ True) => a ~ gamma + (c ~ False) => b ~ gamma + +Since solveCTyFunEqs happens at the very end of solving, the only way to solve +the two implications is temporarily consider (F c a b ~ gamma) as Given (NB: not +merely Given/Solved because it has to interact with the top-level instance +environment) and push it inside the implications. Now, when we come out again at +the end, having solved the implications solveCTyFunEqs will solve this equality. + + (2) In inference mode, we recheck the final constraint in checking mode and +hence we will be able to solve inner implications from top-level quantified +constraints nonetheless. + + Note [Extra TcsTv untouchables] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Furthemore, we record the inert set simplifier-generated unification @@ -956,7 +1031,8 @@ solveCTyFunEqs cts ; return (niFixTvSubst ni_subst, unsolved_can_cts) } where - solve_one (cv,tv,ty) = setWantedTyBind tv ty >> setWantedCoBind cv ty + solve_one (cv,tv,ty) = do { setWantedTyBind tv ty + ; setCoBind cv (mkReflCo ty) } ------------ type FunEqBinds = (TvSubstEnv, [(CoVar, TcTyVar, TcType)]) @@ -999,7 +1075,7 @@ getSolvableCTyFunEqs untch cts , not (tv `elemVarSet` niSubstTvSet tv_subst (tyVarsOfTypes xis)) -- Occurs check: see Note [Solving Family Equations], Point 2 - = ASSERT ( not (isGiven fl) ) + = ASSERT ( not (isGivenOrSolved fl) ) (cts_in, extendFunEqBinds feb cv tv (mkTyConApp tc xis)) dflt_funeq (cts_in, fun_eq_binds) ct