X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcInteract.lhs;h=f0edcc97f49106a1ddec382dba3271db02173e88;hb=cd2f5397bc1345fc37706168c268a8bd37af7f2f;hp=c3d7a9e3aabc11070a9bc57e900d78d7108439ef;hpb=2072edcfe180f617d8f9f8990f682589c4e35082;p=ghc-hetmet.git diff --git a/compiler/typecheck/TcInteract.lhs b/compiler/typecheck/TcInteract.lhs index c3d7a9e..f0edcc9 100644 --- a/compiler/typecheck/TcInteract.lhs +++ b/compiler/typecheck/TcInteract.lhs @@ -1,7 +1,7 @@ \begin{code} module TcInteract ( solveInteract, AtomicInert, - InertSet, emptyInert, extendInertSet, extractUnsolved, solveOne, + InertSet, emptyInert, updInertSet, extractUnsolved, solveOne, listToWorkList ) where @@ -15,6 +15,7 @@ import Type import TypeRep import Id +import VarEnv import Var import TcType @@ -35,7 +36,7 @@ import Outputable import TcRnTypes import TcErrors import TcSMonad -import qualified Bag as Bag +import Bag import qualified Data.Map as Map import Maybes @@ -44,7 +45,7 @@ import FastString ( sLit ) import DynFlags \end{code} -Note [InsertSet invariants] +Note [InertSet invariants] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ An InertSet is a bag of canonical constraints, with the following invariants: @@ -81,18 +82,39 @@ now we do not distinguish between given and solved constraints. Note that we must switch wanted inert items to given when going under an implication constraint (when in top-level inference mode). +Note [InertSet FlattenSkolemEqClass] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The inert_fsks field of the inert set contains an "inverse map" of all the +flatten skolem equalities in the inert set. For instance, if inert_cts looks +like this: + + fsk1 ~ fsk2 + fsk3 ~ fsk2 + fsk4 ~ fsk5 + +Then, the inert_fsks fields holds the following map: + fsk2 |-> { fsk1, fsk3 } + fsk5 |-> { fsk4 } +Along with the necessary coercions to convert fsk1 and fsk3 back to fsk2 +and fsk4 back to fsk5. Hence, the invariants of the inert_fsks field are: + + (a) All TcTyVars in the domain and range of inert_fsks are flatten skolems + (b) All TcTyVars in the domain of inert_fsk occur naked as rhs in some + equalities of inert_cts + (c) For every mapping fsk1 |-> { (fsk2,co), ... } it must be: + co : fsk2 ~ fsk1 + +The role of the inert_fsks is to make it easy to maintain the equivalence +class of each flatten skolem, which is much needed to correctly do spontaneous +solving. See Note [Loopy Spontaneous Solving] \begin{code} -- See Note [InertSet invariants] data InertSet = IS { inert_cts :: Bag.Bag CanonicalCt , inert_fsks :: Map.Map TcTyVar [(TcTyVar,Coercion)] } --- inert_fsks contains the *FlattenSkolem* equivalence classes. --- inert_fsks extra invariants: --- (a) all TcTyVars in the domain and range of inert_fsks are flatten skolems --- (b) for every mapping tv1 |-> (tv2,co), co : tv2 ~ tv1 + -- See Note [InertSet FlattenSkolemEqClass] --- newtype InertSet = IS (Bag.Bag CanonicalCt) instance Outputable InertSet where ppr is = vcat [ vcat (map ppr (Bag.bagToList $ inert_cts is)) , vcat (map (\(v,rest) -> ppr v <+> text "|->" <+> hsep (map (ppr.fst) rest)) @@ -100,20 +122,9 @@ instance Outputable InertSet where ) ] - - emptyInert :: InertSet emptyInert = IS { inert_cts = Bag.emptyBag, inert_fsks = Map.empty } - -extendInertSet :: InertSet -> AtomicInert -> InertSet --- Simply extend the bag of constraints rebuilding an inert set -extendInertSet (IS { inert_cts = cts - , inert_fsks = fsks }) item - = IS { inert_cts = cts `Bag.snocBag` item - , inert_fsks = fsks } - - updInertSet :: InertSet -> AtomicInert -> InertSet -- Introduces an element in the inert set for the first time updInertSet (IS { inert_cts = cts, inert_fsks = fsks }) @@ -125,13 +136,13 @@ updInertSet (IS { inert_cts = cts, inert_fsks = fsks }) FlatSkol {} <- tcTyVarDetails tv2 = let cts' = cts `Bag.snocBag` item fsks' = Map.insertWith (++) tv2 [(tv1, mkCoVarCoercion cv)] fsks + -- See Note [InertSet FlattenSkolemEqClass] in IS { inert_cts = cts', inert_fsks = fsks' } updInertSet (IS { inert_cts = cts , inert_fsks = fsks }) item = let cts' = cts `Bag.snocBag` item in IS { inert_cts = cts', inert_fsks = fsks } - foldlInertSetM :: (Monad m) => (a -> AtomicInert -> m a) -> a -> InertSet -> m a foldlInertSetM k z (IS { inert_cts = cts }) = Bag.foldlBagM k z cts @@ -143,7 +154,7 @@ extractUnsolved is@(IS {inert_cts = cts}) getFskEqClass :: InertSet -> TcTyVar -> [(TcTyVar,Coercion)] --- Precondition: tv is a FlatSkol +-- Precondition: tv is a FlatSkol. See Note [InertSet FlattenSkolemEqClass] getFskEqClass (IS { inert_cts = cts, inert_fsks = fsks }) tv = case lkpTyEqCanByLhs of Nothing -> fromMaybe [] (Map.lookup tv fsks) @@ -427,7 +438,15 @@ spontaneousSolveStage workItem inerts , sr_inerts = inerts , sr_stop = Stop } } -{-- + +{-- This is all old code, but does not quite work now. The problem is that due to + Note [Loopy Spontaneous Solving] we may have unflattened a type, to be able to + perform a sneaky unification. This unflattening means that we may have to recanonicalize + a given (solved) equality, this is why the result of trySpontaneousSolve is now a list + of constraints (instead of an atomic solved constraint). We would have to react all of + them once again with the worklist but that is very tiresome. Instead we throw them back + in the worklist. + | isWantedCt workItem -- Original was wanted we have now made him given so -- we have to ineract him with the inerts again because @@ -463,6 +482,8 @@ spontaneousSolveStage workItem inerts -- Note, just passing the inerts through for the skolem equivalence classes trySpontaneousSolve :: WorkItem -> InertSet -> TcS (Maybe SWorkList) trySpontaneousSolve (CTyEqCan { cc_id = cv, cc_flavor = gw, cc_tyvar = tv1, cc_rhs = xi }) inerts + | isGiven gw + = return Nothing | Just tv2 <- tcGetTyVar_maybe xi = do { tch1 <- isTouchableMetaTyVar tv1 ; tch2 <- isTouchableMetaTyVar tv2 @@ -522,8 +543,42 @@ where (fsk := E alpha, on the side). Now, if we spontaneously *solve* it and keep it as wanted. In inference mode we'll end up quantifying over (alpha ~ Maybe (E alpha)) Hence, 'solveWithIdentity' performs a small occurs check before -actually solving. But this occurs check *must look through* flatten -skolems. +actually solving. But this occurs check *must look through* flatten skolems. + +However, it may be the case that the flatten skolem in hand is equal to some other +flatten skolem whith *does not* mention our unification variable. Here's a typical example: + +Original wanteds: + g: F alpha ~ F beta + w: alpha ~ F alpha +After canonicalization: + g: F beta ~ f1 + g: F alpha ~ f1 + w: alpha ~ f2 + g: F alpha ~ f2 +After some reactions: + g: f1 ~ f2 + g: F beta ~ f1 + w: alpha ~ f2 + g: F alpha ~ f2 +At this point, we will try to spontaneously solve (alpha ~ f2) which remains as yet unsolved. +We will look inside f2, which immediately mentions (F alpha), so it's not good to unify! However +by looking at the equivalence class of the flatten skolems, we can see that it is fine to +unify (alpha ~ f1) which solves our goals! + +A similar problem happens because of other spontaneous solving. Suppose we have the +following wanteds, arriving in this exact order: + (first) w: beta ~ alpha + (second) w: alpha ~ fsk + (third) g: F beta ~ fsk +Then, we first spontaneously solve the first constraint, making (beta := alpha), and having +(beta ~ alpha) as given. *Then* we encounter the second wanted (alpha ~ fsk). "fsk" does not +obviously mention alpha, so naively we can also spontaneously solve (alpha := fsk). But +that is wrong since fsk mentions beta, which has already secretly been unified to alpha! + +To avoid this problem, the same occurs check must unveil rewritings that can happen because +of spontaneously having solved other constraints. + Note [Avoid double unifications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -531,23 +586,17 @@ The spontaneous solver has to return a given which mentions the unified unificat variable *on the left* of the equality. Here is what happens if not: Original wanted: (a ~ alpha), (alpha ~ Int) We spontaneously solve the first wanted, without changing the order! - given : a ~ alpha [having unifice alpha := a] + given : a ~ alpha [having unified alpha := a] Now the second wanted comes along, but he cannot rewrite the given, so we simply continue. At the end we spontaneously solve that guy, *reunifying* [alpha := Int] -We avoid this problem by orienting the given so that the unification variable is on the left. -[Note that alternatively we could attempt to enforce this at canonicalization] +We avoid this problem by orienting the given so that the unification +variable is on the left. [Note that alternatively we could attempt to +enforce this at canonicalization] -Avoiding double unifications is yet another reason to disallow touchable unification variables -as RHS of type family equations: F xis ~ alpha. Consider having already spontaneously solved -a wanted (alpha ~ [b]) by setting alpha := [b]. So the inert set looks like: - given : alpha ~ [b] -And now a new wanted (F tau ~ alpha) comes along. Since it does not react with anything -we will be left with a constraint (F tau ~ alpha) that must cause a unification of -(alpha := F tau) at some point (either in spontaneous solving, or at the end). But alpha -is *already* unified so we must not do anything to it. By disallowing naked touchables in -the RHS of constraints (in favor of introduced flatten skolems) we do not have to worry at -all about unifying or spontaneously solving (F xis ~ alpha) by unification. +See also Note [No touchables as FunEq RHS] in TcSMonad; avoiding +double unifications is the main reason we disallow touchable +unification variables as RHS of type family equations: F xis ~ alpha. \begin{code} ---------------- @@ -556,161 +605,122 @@ solveWithIdentity :: InertSet -> TcS (Maybe SWorkList) -- Solve with the identity coercion -- Precondition: kind(xi) is a sub-kind of kind(tv) --- See [New Wanted Superclass Work] to see why we do this for *given* as well +-- Precondition: CtFlavor is Wanted or Derived +-- See [New Wanted Superclass Work] to see why solveWithIdentity +-- must work for Derived as well as Wanted solveWithIdentity inerts cv gw tv xi - | not (isGiven gw) - = do { m <- passOccursCheck inerts tv xi - ; case m of - Nothing -> return Nothing - Just (xi_unflat,coi) -- coi : xi_unflat ~ xi - -> do { traceTcS "Sneaky unification:" $ + = do { tybnds <- getTcSTyBindsMap + ; case occurCheck tybnds inerts tv xi of + Nothing -> return Nothing + Just (xi_unflat,coi) -> solve_with xi_unflat coi } + where + solve_with xi_unflat coi -- coi : xi_unflat ~ xi + = do { traceTcS "Sneaky unification:" $ vcat [text "Coercion variable: " <+> ppr gw, text "Coercion: " <+> pprEq (mkTyVarTy tv) xi, text "Left Kind is : " <+> ppr (typeKind (mkTyVarTy tv)), text "Right Kind is : " <+> ppr (typeKind xi) - ] - ; setWantedTyBind tv xi_unflat -- Set tv := xi_unflat - ; cv_given <- newGivOrDerCoVar (mkTyVarTy tv) xi_unflat xi_unflat - ; let flav = mkGivenFlavor gw UnkSkol - ; (cts, co) <- case coi of - ACo co -> do { can_eqs <- canEq flav cv_given (mkTyVarTy tv) xi_unflat - ; return (can_eqs, co) } - IdCo co -> return $ - (singleCCan (CTyEqCan { cc_id = cv_given - , cc_flavor = mkGivenFlavor gw UnkSkol - , cc_tyvar = tv, cc_rhs = xi } - -- xi, *not* xi_unflat! - ), co) - ; case gw of - Wanted {} -> setWantedCoBind cv co - Derived {} -> setDerivedCoBind cv co - _ -> pprPanic "Can't spontaneously solve *given*" empty - - -- See Note [Avoid double unifications] - - -- The reason that we create a new given variable (cv_given) instead of reusing cv - -- is because we do not want to end up with coercion unification variables in the givens. - ; return (Just cts) } - } - | otherwise - = return Nothing - - -passOccursCheck :: InertSet -> TcTyVar -> TcType -> TcS (Maybe (TcType,CoercionI)) --- passOccursCheck inerts tv ty --- Traverse the type and make sure that 'tv' does not appear under --- some flatten skolem. If it appears under some flatten skolem --- look in that flatten skolem equivalence class to see if you can --- find a different flatten skolem to use, which does not mention the --- variable. --- Postcondition: Just (ty',coi) <- passOccursCheck tv ty + ] + ; setWantedTyBind tv xi_unflat -- Set tv := xi_unflat + ; cv_given <- newGivOrDerCoVar (mkTyVarTy tv) xi_unflat xi_unflat + ; let flav = mkGivenFlavor gw UnkSkol + ; (cts, co) <- case coi of + ACo co -> do { can_eqs <- canEq flav cv_given (mkTyVarTy tv) xi_unflat + ; return (can_eqs, co) } + IdCo co -> return $ + (singleCCan (CTyEqCan { cc_id = cv_given + , cc_flavor = mkGivenFlavor gw UnkSkol + , cc_tyvar = tv, cc_rhs = xi } + -- xi, *not* xi_unflat because + -- xi_unflat may require flattening! + ), co) + ; case gw of + Wanted {} -> setWantedCoBind cv co + Derived {} -> setDerivedCoBind cv co + _ -> pprPanic "Can't spontaneously solve *given*" empty + -- See Note [Avoid double unifications] + ; return (Just cts) } + +occurCheck :: VarEnv (TcTyVar, TcType) -> InertSet + -> TcTyVar -> TcType -> Maybe (TcType,CoercionI) +-- Traverse @ty@ to make sure that @tv@ does not appear under some flatten skolem. +-- If it appears under some flatten skolem look in that flatten skolem equivalence class +-- (see Note [InertSet FlattenSkolemEqClass], [Loopy Spontaneous Solving]) to see if you +-- can find a different flatten skolem to use, that is, one that does not mention @tv@. +-- +-- Postcondition: Just (ty', coi) = occurCheck binds inerts tv ty -- coi :: ty' ~ ty --- NB: I believe there is no need to do the tcView thing here -passOccursCheck is tv (TyConApp tc tys) - = do { tys_mbs <- mapM (passOccursCheck is tv) tys - ; case allMaybes tys_mbs of - Nothing -> return Nothing - Just tys_cois -> - let (tys',cois') = unzip tys_cois - in return $ - Just (TyConApp tc tys', mkTyConAppCoI tc cois') - } -passOccursCheck is tv (PredTy sty) - = do { sty_mb <- passOccursCheckPred tv sty - ; case sty_mb of - Nothing -> return Nothing - Just (sty',coi) -> return (Just (PredTy sty', coi)) - } - where passOccursCheckPred tv (ClassP cn tys) - = do { tys_mbs <- mapM (passOccursCheck is tv) tys - ; case allMaybes tys_mbs of - Nothing -> return Nothing - Just tys_cois -> - let (tys', cois') = unzip tys_cois - in return $ - Just (ClassP cn tys', mkClassPPredCoI cn cois') - } - passOccursCheckPred tv (IParam nm ty) - = do { mty <- passOccursCheck is tv ty - ; case mty of - Nothing -> return Nothing - Just (ty',co') - -> return (Just (IParam nm ty', - mkIParamPredCoI nm co')) - } - passOccursCheckPred tv (EqPred ty1 ty2) - = do { mty1 <- passOccursCheck is tv ty1 - ; mty2 <- passOccursCheck is tv ty2 - ; case (mty1,mty2) of - (Just (ty1',coi1), Just (ty2',coi2)) - -> return $ - Just (EqPred ty1' ty2', mkEqPredCoI coi1 coi2) - _ -> return Nothing - } - -passOccursCheck is tv (FunTy arg res) - = do { arg_mb <- passOccursCheck is tv arg - ; res_mb <- passOccursCheck is tv res - ; case (arg_mb,res_mb) of - (Just (arg',coiarg), Just (res',coires)) - -> return $ - Just (FunTy arg' res', mkFunTyCoI coiarg coires) - _ -> return Nothing - } - -passOccursCheck is tv (AppTy fun arg) - = do { fun_mb <- passOccursCheck is tv fun - ; arg_mb <- passOccursCheck is tv arg - ; case (fun_mb,arg_mb) of - (Just (fun',coifun), Just (arg',coiarg)) - -> return $ - Just (AppTy fun' arg', mkAppTyCoI coifun coiarg) - _ -> return Nothing - } - -passOccursCheck is tv (ForAllTy tv1 ty1) - = do { ty1_mb <- passOccursCheck is tv ty1 - ; case ty1_mb of - Nothing -> return Nothing - Just (ty1',coi) - -> return $ - Just (ForAllTy tv1 ty1', mkForAllTyCoI tv1 coi) - } - -passOccursCheck _is tv (TyVarTy tv') - | tv == tv' - = return Nothing - -passOccursCheck is tv (TyVarTy fsk) - | FlatSkol ty <- tcTyVarDetails fsk - = do { zty <- zonkFlattenedType ty -- Must zonk as it contains unif. vars - ; occ <- passOccursCheck is tv zty - ; case occ of - Nothing -> go_down_eq_class $ getFskEqClass is fsk - Just (zty',ico) -> return $ Just (zty',ico) - } - where go_down_eq_class [] = return Nothing - go_down_eq_class ((fsk1,co1):rest) - = do { occ1 <- passOccursCheck is tv (TyVarTy fsk1) - ; case occ1 of - Nothing -> go_down_eq_class rest - Just (ty1,co1i') - -> return $ Just (ty1, mkTransCoI co1i' (ACo co1)) } -passOccursCheck _is _tv ty - = return (Just (ty,IdCo ty)) - -{-- -Problematic situation: -~~~~~~~~~~~~~~~~~~~~~~ - Suppose we have a flatten skolem f1 := F f6 - Suppose we are chasing for 'alpha', and: - f6 := G alpha with eq.class f7,f8 - - Then we will return F f7 potentially. ---} - - +-- NB: The returned type ty' may not be flat! +occurCheck ty_binds inerts the_tv the_ty + = ok emptyVarSet the_ty + where + -- If (fsk `elem` bad) then tv occurs in any rendering + -- of the type under the expansion of fsk + ok bad this_ty@(TyConApp tc tys) + | Just tys_cois <- allMaybes (map (ok bad) tys) + , (tys',cois') <- unzip tys_cois + = Just (TyConApp tc tys', mkTyConAppCoI tc cois') + | isSynTyCon tc, Just ty_expanded <- tcView this_ty + = ok bad ty_expanded -- See Note [Type synonyms and the occur check] in TcUnify + ok bad (PredTy sty) + | Just (sty',coi) <- ok_pred bad sty + = Just (PredTy sty', coi) + ok bad (FunTy arg res) + | Just (arg', coiarg) <- ok bad arg, Just (res', coires) <- ok bad res + = Just (FunTy arg' res', mkFunTyCoI coiarg coires) + ok bad (AppTy fun arg) + | Just (fun', coifun) <- ok bad fun, Just (arg', coiarg) <- ok bad arg + = Just (AppTy fun' arg', mkAppTyCoI coifun coiarg) + ok bad (ForAllTy tv1 ty1) + -- WARNING: What if it is a (t1 ~ t2) => t3? It's not handled properly at the moment. + | Just (ty1', coi) <- ok bad ty1 + = Just (ForAllTy tv1 ty1', mkForAllTyCoI tv1 coi) + + -- Variable cases + ok bad this_ty@(TyVarTy tv) + | tv == the_tv = Nothing -- Occurs check error + | not (isTcTyVar tv) = Just (this_ty, IdCo this_ty) -- Bound var + | FlatSkol zty <- tcTyVarDetails tv = ok_fsk bad tv zty + | Just (_,ty) <- lookupVarEnv ty_binds tv = ok bad ty + | otherwise = Just (this_ty, IdCo this_ty) + + -- Check if there exists a ty bind already, as a result of sneaky unification. + -- Fall through + ok _bad _ty = Nothing + + ----------- + ok_pred bad (ClassP cn tys) + | Just tys_cois <- allMaybes $ map (ok bad) tys + = let (tys', cois') = unzip tys_cois + in Just (ClassP cn tys', mkClassPPredCoI cn cois') + ok_pred bad (IParam nm ty) + | Just (ty',co') <- ok bad ty + = Just (IParam nm ty', mkIParamPredCoI nm co') + ok_pred bad (EqPred ty1 ty2) + | Just (ty1',coi1) <- ok bad ty1, Just (ty2',coi2) <- ok bad ty2 + = Just (EqPred ty1' ty2', mkEqPredCoI coi1 coi2) + ok_pred _ _ = Nothing + + ----------- + ok_fsk bad fsk zty + | fsk `elemVarSet` bad + -- We are already trying to find a rendering of fsk, + -- and to do that it seems we need a rendering, so fail + = Nothing + | otherwise + = firstJusts (ok new_bad zty : map (go_under_fsk new_bad) fsk_equivs) + where + fsk_equivs = getFskEqClass inerts fsk + new_bad = bad `extendVarSetList` (fsk : map fst fsk_equivs) + + ----------- + go_under_fsk bad_tvs (fsk,co) + | FlatSkol zty <- tcTyVarDetails fsk + = case ok bad_tvs zty of + Nothing -> Nothing + Just (ty,coi') -> Just (ty, mkTransCoI coi' (ACo co)) + | otherwise = pprPanic "go_down_equiv" (ppr fsk) \end{code} @@ -972,10 +982,12 @@ doInteractWithInert | fl2 `canRewrite` fl1 && tv2 `elemVarSet` tyVarsOfType xi1 = do { rewritten_eq <- rewriteEqRHS (cv2,tv2,xi2) (cv1,fl1,tv1,xi1) ; mkIRContinue workItem DropInert rewritten_eq } --- Finally, if workitem is a flatten equivalence class constraint and the --- inert is a wanted constraints, even when the workitem cannot rewrite the --- inert, drop the inert out because you may have to reconsider solving him --- using the equivalence class you created. + +-- Finally, if workitem is a Flatten Equivalence Class constraint and the +-- inert is a wanted constraint, even when the workitem cannot rewrite the +-- inert, drop the inert out because you may have to reconsider solving the +-- inert *using* the equivalence class you created. See note [Loopy Spontaneous Solving] +-- and [InertSet FlattenSkolemEqClass] | not $ isGiven fl1, -- The inert is wanted or derived isMetaTyVar tv1, -- and has a unification variable lhs @@ -984,7 +996,7 @@ doInteractWithInert = mkIRContinue workItem DropInert (singletonWorkList inert) --- Fall-through case for all other cases +-- Fall-through case for all other situations doInteractWithInert _ workItem = noInteraction workItem -------------------------------------------- @@ -1108,15 +1120,6 @@ rewriteEqLHS which (co1,xi1) (cv2,gw,xi2) mkSymCoercion co1 `mkTransCoercion` mkCoVarCoercion cv2 ; mkCanonical gw cv2' } --- -> --- if isWanted gw then --- do { cv2' <- newWantedCoVar xi1 xi2 --- ; setWantedCoBind cv2 $ --- co1 `mkTransCoercion` mkCoVarCoercion cv2' --- ; return cv2' } --- else newGivOrDerCoVar xi1 xi2 $ --- mkSymCoercion co1 `mkTransCoercion` mkCoVarCoercion cv2 --- ; mkCanonical gw cv2' } solveOneFromTheOther :: (EvVar, CtFlavor) -> CanonicalCt -> TcS InteractResult @@ -1782,19 +1785,18 @@ new given work. There are several reasons for this: Now suppose that we have: given: C a b wanted: C a beta - By interacting the given we will get that (F a ~ b) which is not + By interacting the given we will get given (F a ~ b) which is not enough by itself to make us discharge (C a beta). However, we - may create a new given equality from the super-class that we promise - to solve: (F a ~ beta). Now we may interact this with the rest of - constraint to finally get: - given : beta ~ b - + may create a new derived equality from the super-class of the + wanted constraint (C a beta), namely derived (F a ~ beta). + Now we may interact this with given (F a ~ b) to get: + derived : beta ~ b But 'beta' is a touchable unification variable, and hence OK to - unify it with 'b', replacing the given evidence with the identity. + unify it with 'b', replacing the derived evidence with the identity. - This requires trySpontaneousSolve to solve given equalities that - have a touchable in their RHS, *in addition* to solving wanted - equalities. + This requires trySpontaneousSolve to solve *derived* + equalities that have a touchable in their RHS, *in addition* + to solving wanted equalities. Here is another example where this is useful. @@ -1883,5 +1885,3 @@ matchClassInst clas tys loc } } \end{code} - -