X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypes%2FFunDeps.lhs;h=6ce932bfe356a0c902d23485add4387bb4ef85fb;hp=b8b97b11e2d3a2835c89cba9840cd995c42d4b5f;hb=50d0293555691012f96259de7f8682b94db58517;hpb=2e9952b703341351298739b5d4f869fbdfc8490e diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs index b8b97b1..6ce932b 100644 --- a/compiler/types/FunDeps.lhs +++ b/compiler/types/FunDeps.lhs @@ -9,8 +9,9 @@ It's better to read it as: "if we know these, then we're going to know these" \begin{code} module FunDeps ( - Equation, pprEquation, - oclose, grow, improve, improveOne, + FDEq (..), + Equation(..), pprEquation, + oclose, improveFromInstEnv, improveFromAnother, checkInstCoverage, checkFunDeps, pprFundeps ) where @@ -20,16 +21,16 @@ module FunDeps ( import Name import Var import Class -import TcGadt import TcType +import Unify import InstEnv import VarSet import VarEnv import Outputable import Util -import ListSetOps +import FastString -import Data.List ( tails ) +import Data.List ( nubBy ) import Data.Maybe ( isJust ) \end{code} @@ -40,6 +41,26 @@ import Data.Maybe ( isJust ) %* * %************************************************************************ + oclose(vs,C) The result of extending the set of tyvars vs + using the functional dependencies from C + + grow(vs,C) The result of extend the set of tyvars vs + using all conceivable links from C. + + E.g. vs = {a}, C = {H [a] b, K (b,Int) c, Eq e} + Then grow(vs,C) = {a,b,c} + + Note that grow(vs,C) `superset` grow(vs,simplify(C)) + That is, simplfication can only shrink the result of grow. + +Notice that + oclose is conservative v `elem` oclose(vs,C) + one way: => v is definitely fixed by vs + + grow is conservative if v might be fixed by vs + the other way: => v `elem` grow(vs,C) + +---------------------------------------------------------- (oclose preds tvs) closes the set of type variables tvs, wrt functional dependencies in preds. The result is a superset of the argument set. For example, if we have @@ -48,60 +69,47 @@ then oclose [C (x,y) z, C (x,p) q] {x,y} = {x,y,z} because if we know x and y then that fixes z. -Using oclose -~~~~~~~~~~~~ -oclose is used - -a) When determining ambiguity. The type - forall a,b. C a b => a -is not ambiguous (given the above class decl for C) because -a determines b. - -b) When generalising a type T. Usually we take FV(T) \ FV(Env), -but in fact we need - FV(T) \ (FV(Env)+) -where the '+' is the oclosure operation. Notice that we do not -take FV(T)+. This puzzled me for a bit. Consider - - f = E - -and suppose e have that E :: C a b => a, and suppose that b is -free in the environment. Then we quantify over 'a' only, giving -the type forall a. C a b => a. Since a->b but we don't have b->a, -we might have instance decls like - instance C Bool Int where ... - instance C Char Int where ... -so knowing that b=Int doesn't fix 'a'; so we quantify over it. - - --------------- - A WORRY: ToDo! - --------------- -If we have class C a b => D a b where .... - class D a b | a -> b where ... -and the preds are [C (x,y) z], then we want to see the fd in D, -even though it is not explicit in C, giving [({x,y},{z})] - -Similarly for instance decls? E.g. Suppose we have - instance C a b => Eq (T a b) where ... -and we infer a type t with constraints Eq (T a b) for a particular -expression, and suppose that 'a' is free in the environment. -We could generalise to - forall b. Eq (T a b) => t -but if we reduced the constraint, to C a b, we'd see that 'a' determines -b, so that a better type might be - t (with free constraint C a b) -Perhaps it doesn't matter, because we'll still force b to be a -particular type at the call sites. Generalising over too many -variables (provided we don't shadow anything by quantifying over a -variable that is actually free in the envt) may postpone errors; it -won't hide them altogether. - +oclose is used (only) when generalising a type T; see extensive +notes in TcSimplify. + +Note [Important subtlety in oclose] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider (oclose (C Int t) {}), where class C a b | a->b +Then, since a->b, 't' is fully determined by Int, and the +uniform thing is to return {t}. + +However, consider + class D a b c | b->c + f x = e -- 'e' generates constraint (D s Int t) + -- \x.e has type s->s +Then, if (oclose (D s Int t) {}) = {t}, we'll make the function +monomorphic in 't', thus + f :: forall s. D s Int t => s -> s + +But if this function is never called, 't' will never be instantiated; +the functional dependencies that fix 't' may well be instance decls in +some importing module. But the top-level defaulting of unconstrained +type variables will fix t=GHC.Prim.Any, and that's simply a bug. + +Conclusion: oclose only returns a type variable as "fixed" if it +depends on at least one type variable in the input fixed_tvs. + +Remember, it's always sound for oclose to return a smaller set. +An interesting example is tcfail093, where we get this inferred type: + class C a b | a->b + dup :: forall h. (Call (IO Int) h) => () -> Int -> h +This is perhaps a bit silly, because 'h' is fixed by the (IO Int); +previously GHC rejected this saying 'no instance for Call (IO Int) h'. +But it's right on the borderline. If there was an extra, otherwise +uninvolved type variable, like 's' in the type of 'f' above, then +we must accept the function. So, for now anyway, we accept 'dup' too. \begin{code} oclose :: [PredType] -> TyVarSet -> TyVarSet oclose preds fixed_tvs - | null tv_fds = fixed_tvs -- Fast escape hatch for common case - | otherwise = loop fixed_tvs + | null tv_fds = fixed_tvs -- Fast escape hatch for common case + | isEmptyVarSet fixed_tvs = emptyVarSet -- Note [Important subtlety in oclose] + | otherwise = loop fixed_tvs where loop fixed_tvs | new_fixed_tvs `subVarSet` fixed_tvs = fixed_tvs @@ -109,8 +117,10 @@ oclose preds fixed_tvs where new_fixed_tvs = foldl extend fixed_tvs tv_fds - extend fixed_tvs (ls,rs) | ls `subVarSet` fixed_tvs = fixed_tvs `unionVarSet` rs - | otherwise = fixed_tvs + extend fixed_tvs (ls,rs) + | not (isEmptyVarSet ls) -- Note [Important subtlety in oclose] + , ls `subVarSet` fixed_tvs = fixed_tvs `unionVarSet` rs + | otherwise = fixed_tvs tv_fds :: [(TyVarSet,TyVarSet)] -- In our example, tv_fds will be [ ({x,y}, {z}), ({x,p},{q}) ] @@ -123,44 +133,6 @@ oclose preds fixed_tvs ] \end{code} -Note [Growing the tau-tvs using constraints] -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -(grow preds tvs) is the result of extend the set of tyvars tvs - using all conceivable links from pred - -E.g. tvs = {a}, preds = {H [a] b, K (b,Int) c, Eq e} -Then grow precs tvs = {a,b,c} - -All the type variables from an implicit parameter are added, whether or -not they are mentioned in tvs; see Note [Implicit parameters and ambiguity] -in TcSimplify. - -See also Note [Ambiguity] in TcSimplify - -\begin{code} -grow :: [PredType] -> TyVarSet -> TyVarSet -grow preds fixed_tvs - | null preds = real_fixed_tvs - | otherwise = loop real_fixed_tvs - where - -- Add the implicit parameters; - -- see Note [Implicit parameters and ambiguity] in TcSimplify - real_fixed_tvs = foldr unionVarSet fixed_tvs ip_tvs - - loop fixed_tvs - | new_fixed_tvs `subVarSet` fixed_tvs = fixed_tvs - | otherwise = loop new_fixed_tvs - where - new_fixed_tvs = foldl extend fixed_tvs non_ip_tvs - - extend fixed_tvs pred_tvs - | fixed_tvs `intersectsVarSet` pred_tvs = fixed_tvs `unionVarSet` pred_tvs - | otherwise = fixed_tvs - - (ip_tvs, non_ip_tvs) = partitionWith get_ip preds - get_ip (IParam _ ty) = Left (tyVarsOfType ty) - get_ip other = Right (tyVarsOfPred other) -\end{code} %************************************************************************ %* * @@ -169,42 +141,67 @@ grow preds fixed_tvs %************************************************************************ -\begin{code} ----------- -type Equation = (TyVarSet, [(Type, Type)]) --- These pairs of types should be equal, for some --- substitution of the tyvars in the tyvar set --- INVARIANT: corresponding types aren't already equal - --- It's important that we have a *list* of pairs of types. Consider --- class C a b c | a -> b c where ... --- instance C Int x x where ... --- Then, given the constraint (C Int Bool v) we should improve v to Bool, --- via the equation ({x}, [(Bool,x), (v,x)]) --- This would not happen if the class had looked like --- class C a b c | a -> b, a -> c - --- To "execute" the equation, make fresh type variable for each tyvar in the set, --- instantiate the two types with these fresh variables, and then unify. --- --- For example, ({a,b}, (a,Int,b), (Int,z,Bool)) --- We unify z with Int, but since a and b are quantified we do nothing to them --- We usually act on an equation by instantiating the quantified type varaibles --- to fresh type variables, and then calling the standard unifier. - -pprEquation (qtvs, pairs) - = vcat [ptext SLIT("forall") <+> braces (pprWithCommas ppr (varSetElems qtvs)), - nest 2 (vcat [ ppr t1 <+> ptext SLIT(":=:") <+> ppr t2 | (t1,t2) <- pairs])] +Each functional dependency with one variable in the RHS is responsible +for generating a single equality. For instance: + class C a b | a -> b +The constraints ([Wanted] C Int Bool) and [Wanted] C Int alpha + FDEq { fd_pos = 1 + , fd_ty_left = Bool + , fd_ty_right = alpha } +However notice that a functional dependency may have more than one variable +in the RHS which will create more than one FDEq. Example: + class C a b c | a -> b c + [Wanted] C Int alpha alpha + [Wanted] C Int Bool beta +Will generate: + fd1 = FDEq { fd_pos = 1, fd_ty_left = alpha, fd_ty_right = Bool } and + fd2 = FDEq { fd_pos = 2, fd_ty_left = alpha, fd_ty_right = beta } + +We record the paremeter position so that can immediately rewrite a constraint +using the produced FDEqs and remove it from our worklist. + + +INVARIANT: Corresponding types aren't already equal +That is, there exists at least one non-identity equality in FDEqs. + +Assume: + class C a b c | a -> b c + instance C Int x x +And: [Wanted] C Int Bool alpha +We will /match/ the LHS of fundep equations, producing a matching substitution +and create equations for the RHS sides. In our last example we'd have generated: + ({x}, [fd1,fd2]) +where + fd1 = FDEq 1 Bool x + fd2 = FDEq 2 alpha x +To ``execute'' the equation, make fresh type variable for each tyvar in the set, +instantiate the two types with these fresh variables, and then unify or generate +a new constraint. In the above example we would generate a new unification +variable 'beta' for x and produce the following constraints: + [Wanted] (Bool ~ beta) + [Wanted] (alpha ~ beta) + +Notice the subtle difference between the above class declaration and: + class C a b c | a -> b, a -> c +where we would generate: + ({x},[fd1]),({x},[fd2]) +This means that the template variable would be instantiated to different +unification variables when producing the FD constraints. + +Finally, the position parameters will help us rewrite the wanted constraint ``on the spot'' ----------- +\begin{code} type Pred_Loc = (PredType, SDoc) -- SDoc says where the Pred comes from -improve :: (Class -> [Instance]) -- Gives instances for given class - -> [Pred_Loc] -- Current constraints; - -> [(Equation,Pred_Loc,Pred_Loc)] -- Derived equalities that must also hold - -- (NB the above INVARIANT for type Equation) - -- The Pred_Locs explain which two predicates were - -- combined (for error messages) +data Equation + = FDEqn { fd_qtvs :: TyVarSet -- Instantiate these to fresh unification vars + , fd_eqs :: [FDEq] -- and then make these equal + , fd_pred1, fd_pred2 :: Pred_Loc } -- The Equation arose from + -- combining these two constraints + +data FDEq = FDEq { fd_pos :: Int -- We use '0' for the first position + , fd_ty_left :: Type + , fd_ty_right :: Type } \end{code} Given a bunch of predicates that must hold, such as @@ -237,142 +234,97 @@ NOTA BENE: \begin{code} -improve inst_env preds - = [ eqn | group <- equivClassesByUniq (predTyUnique . fst) (filterEqPreds preds), - eqn <- checkGroup inst_env group ] - where - filterEqPreds = filter (not . isEqPred . fst) - -- Equality predicates don't have uniques - -- In any case, improvement *generates*, rather than - -- *consumes*, equality constraints - -improveOne :: (Class -> [Instance]) - -> Pred_Loc - -> [Pred_Loc] - -> [(Equation,Pred_Loc,Pred_Loc)] - --- Just do improvement triggered by a single, distinguised predicate - -improveOne inst_env pred@(IParam ip ty, _) preds - = [ ((emptyVarSet, [(ty,ty2)]), pred, p2) - | p2@(IParam ip2 ty2, _) <- preds - , ip==ip2 - , not (ty `tcEqType` ty2)] +instFD_WithPos :: FunDep TyVar -> [TyVar] -> [Type] -> ([Type], [(Int,Type)]) +-- Returns a FunDep between the types accompanied along with their +-- position (<=0) in the types argument list. +instFD_WithPos (ls,rs) tvs tys + = (map (snd . lookup) ls, map lookup rs) + where + ind_tys = zip [0..] tys + env = zipVarEnv tvs ind_tys + lookup tv = lookupVarEnv_NF env tv -improveOne inst_env pred@(ClassP cls tys, _) preds +zipAndComputeFDEqs :: (Type -> Type -> Bool) -- Discard this FDEq if true + -> [Type] + -> [(Int,Type)] + -> [FDEq] +-- Create a list of FDEqs from two lists of types, making sure +-- that the types are not equal. +zipAndComputeFDEqs discard (ty1:tys1) ((i2,ty2):tys2) + | discard ty1 ty2 = zipAndComputeFDEqs discard tys1 tys2 + | otherwise = FDEq { fd_pos = i2 + , fd_ty_left = ty1 + , fd_ty_right = ty2 } : zipAndComputeFDEqs discard tys1 tys2 +zipAndComputeFDEqs _ _ _ = [] + +-- Improve a class constraint from another class constraint +-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +improveFromAnother :: Pred_Loc -- Template item (usually given, or inert) + -> Pred_Loc -- Workitem [that can be improved] + -> [Equation] +-- Post: FDEqs always oriented from the other to the workitem +-- Equations have empty quantified variables +improveFromAnother pred1@(ClassP cls1 tys1, _) pred2@(ClassP cls2 tys2, _) + | tys1 `lengthAtLeast` 2 && cls1 == cls2 + = [ FDEqn { fd_qtvs = emptyVarSet, fd_eqs = eqs, fd_pred1 = pred1, fd_pred2 = pred2 } + | let (cls_tvs, cls_fds) = classTvsFds cls1 + , fd <- cls_fds + , let (ltys1, rs1) = instFD fd cls_tvs tys1 + (ltys2, irs2) = instFD_WithPos fd cls_tvs tys2 + , tcEqTypes ltys1 ltys2 -- The LHSs match + , let eqs = zipAndComputeFDEqs tcEqType rs1 irs2 + , not (null eqs) ] + +improveFromAnother _ _ = [] + + +-- Improve a class constraint from instance declarations +-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +pprEquation :: Equation -> SDoc +pprEquation (FDEqn { fd_qtvs = qtvs, fd_eqs = pairs }) + = vcat [ptext (sLit "forall") <+> braces (pprWithCommas ppr (varSetElems qtvs)), + nest 2 (vcat [ ppr t1 <+> ptext (sLit "~") <+> ppr t2 | (FDEq _ t1 t2) <- pairs])] + +improveFromInstEnv :: (InstEnv,InstEnv) + -> Pred_Loc + -> [Equation] -- Needs to be an Equation because + -- of quantified variables +-- Post: Equations oriented from the template (matching instance) to the workitem! +improveFromInstEnv _inst_env (pred,_loc) + | not (isClassPred pred) + = panic "improveFromInstEnv: not a class predicate" +improveFromInstEnv inst_env pred@(ClassP cls tys, _) | tys `lengthAtLeast` 2 - = instance_eqns ++ pairwise_eqns - -- NB: we put the instance equations first. This biases the - -- order so that we first improve individual constraints against the - -- instances (which are perhaps in a library and less likely to be - -- wrong; and THEN perform the pairwise checks. - -- The other way round, it's possible for the pairwise check to succeed - -- and cause a subsequent, misleading failure of one of the pair with an - -- instance declaration. See tcfail143.hs for an example - where - (cls_tvs, cls_fds) = classTvsFds cls - instances = inst_env cls - rough_tcs = roughMatchTcs tys - - -- NOTE that we iterate over the fds first; they are typically - -- empty, which aborts the rest of the loop. - pairwise_eqns :: [(Equation,Pred_Loc,Pred_Loc)] - pairwise_eqns -- This group comes from pairwise comparison - = [ (eqn, pred, p2) - | fd <- cls_fds - , p2@(ClassP cls2 tys2, _) <- preds - , cls == cls2 - , eqn <- checkClsFD emptyVarSet fd cls_tvs tys tys2 - ] - - instance_eqns :: [(Equation,Pred_Loc,Pred_Loc)] - instance_eqns -- This group comes from comparing with instance decls - = [ (eqn, p_inst, pred) - | fd <- cls_fds -- Iterate through the fundeps first, + = [ FDEqn { fd_qtvs = qtvs, fd_eqs = eqs, fd_pred1 = p_inst, fd_pred2=pred } + | fd <- cls_fds -- Iterate through the fundeps first, -- because there often are none! - , let rough_fd_tcs = trimRoughMatchTcs cls_tvs fd rough_tcs - , ispec@(Instance { is_tvs = qtvs, is_tys = tys_inst, - is_tcs = mb_tcs_inst }) <- instances - , not (instanceCantMatch mb_tcs_inst rough_fd_tcs) - , eqn <- checkClsFD qtvs fd cls_tvs tys_inst tys - , let p_inst = (mkClassPred cls tys_inst, - ptext SLIT("arising from the instance declaration at") - <+> ppr (getSrcLoc ispec)) - ] - -improveOne inst_env eq_pred preds - = [] - ----------- -checkGroup :: (Class -> [Instance]) - -> [Pred_Loc] - -> [(Equation, Pred_Loc, Pred_Loc)] - -- The preds are all for the same class or implicit param - -checkGroup inst_env (p1@(IParam _ ty, _) : ips) - = -- For implicit parameters, all the types must match - [ ((emptyVarSet, [(ty,ty')]), p1, p2) - | p2@(IParam _ ty', _) <- ips, not (ty `tcEqType` ty')] - -checkGroup inst_env clss@((ClassP cls _, _) : _) - = -- For classes life is more complicated - -- Suppose the class is like - -- classs C as | (l1 -> r1), (l2 -> r2), ... where ... - -- Then FOR EACH PAIR (ClassP c tys1, ClassP c tys2) in the list clss - -- we check whether - -- U l1[tys1/as] = U l2[tys2/as] - -- (where U is a unifier) - -- - -- If so, we return the pair - -- U r1[tys1/as] = U l2[tys2/as] - -- - -- We need to do something very similar comparing each predicate - -- with relevant instance decls - - instance_eqns ++ pairwise_eqns - -- NB: we put the instance equations first. This biases the - -- order so that we first improve individual constraints against the - -- instances (which are perhaps in a library and less likely to be - -- wrong; and THEN perform the pairwise checks. - -- The other way round, it's possible for the pairwise check to succeed - -- and cause a subsequent, misleading failure of one of the pair with an - -- instance declaration. See tcfail143.hs for an exmample + , let trimmed_tcs = trimRoughMatchTcs cls_tvs fd rough_tcs + -- Trim the rough_tcs based on the head of the fundep. + -- Remember that instanceCantMatch treats both argumnents + -- symmetrically, so it's ok to trim the rough_tcs, + -- rather than trimming each inst_tcs in turn + , ispec@(Instance { is_tvs = qtvs, is_tys = tys_inst, + is_tcs = inst_tcs }) <- instances + , not (instanceCantMatch inst_tcs trimmed_tcs) + , let p_inst = (mkClassPred cls tys_inst, + sep [ ptext (sLit "arising from the dependency") <+> quotes (pprFunDep fd) + , ptext (sLit "in the instance declaration at") + <+> ppr (getSrcLoc ispec)]) + , (qtvs, eqs) <- checkClsFD qtvs fd cls_tvs tys_inst tys -- NB: orientation + , not (null eqs) + ] + where + (cls_tvs, cls_fds) = classTvsFds cls + instances = classInstances inst_env cls + rough_tcs = roughMatchTcs tys +improveFromInstEnv _ _ = [] + - where - (cls_tvs, cls_fds) = classTvsFds cls - instances = inst_env cls - - -- NOTE that we iterate over the fds first; they are typically - -- empty, which aborts the rest of the loop. - pairwise_eqns :: [(Equation,Pred_Loc,Pred_Loc)] - pairwise_eqns -- This group comes from pairwise comparison - = [ (eqn, p1, p2) - | fd <- cls_fds, - p1@(ClassP _ tys1, _) : rest <- tails clss, - p2@(ClassP _ tys2, _) <- rest, - eqn <- checkClsFD emptyVarSet fd cls_tvs tys1 tys2 - ] - - instance_eqns :: [(Equation,Pred_Loc,Pred_Loc)] - instance_eqns -- This group comes from comparing with instance decls - = [ (eqn, p1, p2) - | fd <- cls_fds, -- Iterate through the fundeps first, - -- because there often are none! - p2@(ClassP _ tys2, _) <- clss, - let rough_tcs2 = trimRoughMatchTcs cls_tvs fd (roughMatchTcs tys2), - ispec@(Instance { is_tvs = qtvs, is_tys = tys1, - is_tcs = mb_tcs1 }) <- instances, - not (instanceCantMatch mb_tcs1 rough_tcs2), - eqn <- checkClsFD qtvs fd cls_tvs tys1 tys2, - let p1 = (mkClassPred cls tys1, - ptext SLIT("arising from the instance declaration at") <+> - ppr (getSrcLoc ispec)) - ] ----------- checkClsFD :: TyVarSet -- Quantified type variables; see note below -> FunDep TyVar -> [TyVar] -- One functional dependency from the class -> [Type] -> [Type] - -> [Equation] + -> [(TyVarSet, [FDEq])] checkClsFD qtvs fd clas_tvs tys1 tys2 -- 'qtvs' are the quantified type variables, the ones which an be instantiated @@ -387,7 +339,7 @@ checkClsFD qtvs fd clas_tvs tys1 tys2 -- tys2 = [Maybe t1, t2] -- -- We can instantiate x to t1, and then we want to force --- (Tree x) [t1/x] :=: t2 +-- (Tree x) [t1/x] ~ t2 -- -- This function is also used when matching two Insts (rather than an Inst -- against an instance decl. In that case, qtvs is empty, and we are doing @@ -401,52 +353,69 @@ checkClsFD qtvs fd clas_tvs tys1 tys2 length tys1 == length clas_tvs , ppr tys1 <+> ppr tys2 ) - case tcUnifyTys bind_fn ls1 ls2 of + case tcUnifyTys bind_fn ltys1 ltys2 of Nothing -> [] - Just subst | isJust (tcUnifyTys bind_fn rs1' rs2') - -- Don't include any equations that already hold. + Just subst | isJust (tcUnifyTys bind_fn rtys1' rtys2') + -- Don't include any equations that already hold. -- Reason: then we know if any actual improvement has happened, -- in which case we need to iterate the solver - -- In making this check we must taking account of the fact that any - -- qtvs that aren't already instantiated can be instantiated to anything + -- In making this check we must taking account of the fact that any + -- qtvs that aren't already instantiated can be instantiated to anything -- at all - -> [] - - | otherwise -- Aha! A useful equation - -> [ (qtvs', zip rs1' rs2')] + -- NB: We can't do this 'is-useful-equation' check element-wise + -- because of: + -- class C a b c | a -> b c + -- instance C Int x x + -- [Wanted] C Int alpha Int + -- We would get that x -> alpha (isJust) and x -> Int (isJust) + -- so we would produce no FDs, which is clearly wrong. + -> [] + + | otherwise + -> [(qtvs', fdeqs)] -- We could avoid this substTy stuff by producing the eqn -- (qtvs, ls1++rs1, ls2++rs2) -- which will re-do the ls1/ls2 unification when the equation is -- executed. What we're doing instead is recording the partial -- work of the ls1/ls2 unification leaving a smaller unification problem - where - rs1' = substTys subst rs1 - rs2' = substTys subst rs2 + where + rtys1' = map (substTy subst) rtys1 + irs2' = map (\(i,x) -> (i,substTy subst x)) irs2 + rtys2' = map snd irs2' + + fdeqs = zipAndComputeFDEqs (\_ _ -> False) rtys1' irs2' + -- Don't discard anything! + -- We could discard equal types but it's an overkill to call + -- tcEqType again, since we know for sure that /at least one/ + -- equation in there is useful) + qtvs' = filterVarSet (`notElemTvSubst` subst) qtvs - -- qtvs' are the quantified type variables - -- that have not been substituted out - -- - -- Eg. class C a b | a -> b - -- instance C Int [y] - -- Given constraint C Int z - -- we generate the equation - -- ({y}, [y], z) + -- qtvs' are the quantified type variables + -- that have not been substituted out + -- + -- Eg. class C a b | a -> b + -- instance C Int [y] + -- Given constraint C Int z + -- we generate the equation + -- ({y}, [y], z) where bind_fn tv | tv `elemVarSet` qtvs = BindMe | otherwise = Skolem - (ls1, rs1) = instFD fd clas_tvs tys1 - (ls2, rs2) = instFD fd clas_tvs tys2 + (ltys1, rtys1) = instFD fd clas_tvs tys1 + (ltys2, irs2) = instFD_WithPos fd clas_tvs tys2 +\end{code} + +\begin{code} instFD :: FunDep TyVar -> [TyVar] -> [Type] -> FunDep Type +-- A simpler version of instFD_WithPos to be used in checking instance coverage etc. instFD (ls,rs) tvs tys = (map lookup ls, map lookup rs) where env = zipVarEnv tvs tys lookup tv = lookupVarEnv_NF env tv -\end{code} -\begin{code} checkInstCoverage :: Class -> [Type] -> Bool -- Check that the Coverage Condition is obeyed in an instance decl -- For example, if we have @@ -537,32 +506,40 @@ badFunDeps :: [Instance] -> Class -> TyVarSet -> [Type] -- Proposed new instance type -> [Instance] badFunDeps cls_insts clas ins_tv_set ins_tys - = [ ispec | fd <- fds, -- fds is often empty + = nubBy eq_inst $ + [ ispec | fd <- fds, -- fds is often empty, so do this first! let trimmed_tcs = trimRoughMatchTcs clas_tvs fd rough_tcs, - ispec@(Instance { is_tcs = mb_tcs, is_tvs = tvs, + ispec@(Instance { is_tcs = inst_tcs, is_tvs = tvs, is_tys = tys }) <- cls_insts, -- Filter out ones that can't possibly match, -- based on the head of the fundep - not (instanceCantMatch trimmed_tcs mb_tcs), + not (instanceCantMatch inst_tcs trimmed_tcs), notNull (checkClsFD (tvs `unionVarSet` ins_tv_set) fd clas_tvs tys ins_tys) ] where (clas_tvs, fds) = classTvsFds clas rough_tcs = roughMatchTcs ins_tys + eq_inst i1 i2 = instanceDFunId i1 == instanceDFunId i2 + -- An single instance may appear twice in the un-nubbed conflict list + -- because it may conflict with more than one fundep. E.g. + -- class C a b c | a -> b, a -> c + -- instance C Int Bool Bool + -- instance C Int Char Char + -- The second instance conflicts with the first by *both* fundeps trimRoughMatchTcs :: [TyVar] -> FunDep TyVar -> [Maybe Name] -> [Maybe Name] -- Computing rough_tcs for a particular fundep --- class C a b c | a c -> b where ... +-- class C a b c | a -> b where ... -- For each instance .... => C ta tb tc --- we want to match only on the types ta, tb; so our +-- we want to match only on the type ta; so our -- rough-match thing must similarly be filtered. --- Hence, we Nothing-ise the tb type right here -trimRoughMatchTcs clas_tvs (ltvs,_) mb_tcs +-- Hence, we Nothing-ise the tb and tc types right here +trimRoughMatchTcs clas_tvs (ltvs, _) mb_tcs = zipWith select clas_tvs mb_tcs where select clas_tv mb_tc | clas_tv `elem` ltvs = mb_tc - | otherwise = Nothing + | otherwise = Nothing \end{code}