X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypes%2FFunDeps.lhs;h=0bea32fe57b124df03882711e3d6a6e45471b410;hb=ced4c13ea3577e01556a2f76c2cc458c0be6c83c;hp=9af92107eeff13ef6aa69079633f61f441469b5e;hpb=eb2bf7ad9f967861da2e19ff71a80428c7c2df28;p=ghc-hetmet.git diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs index 9af9210..0bea32f 100644 --- a/compiler/types/FunDeps.lhs +++ b/compiler/types/FunDeps.lhs @@ -9,8 +9,8 @@ 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, improveOne, + Equation, pprEquation, + oclose, improveFromInstEnv, improveFromAnother, checkInstCoverage, checkFunDeps, pprFundeps ) where @@ -20,13 +20,16 @@ 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 FastString + +import Data.List ( nubBy ) import Data.Maybe ( isJust ) \end{code} @@ -37,6 +40,26 @@ 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 @@ -45,60 +68,47 @@ 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 @@ -106,8 +116,10 @@ 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}) ] @@ -120,44 +132,6 @@ 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} %************************************************************************ %* * @@ -188,9 +162,10 @@ type Equation = (TyVarSet, [(Type, Type)]) -- We usually act on an equation by instantiating the quantified type varaibles -- to fresh type variables, and then calling the standard unifier. +pprEquation :: Equation -> SDoc pprEquation (qtvs, pairs) - = vcat [ptext SLIT("forall") <+> braces (pprWithCommas ppr (varSetElems qtvs)), - nest 2 (vcat [ ppr t1 <+> ptext SLIT(":=:") <+> ppr t2 | (t1,t2) <- pairs])] + = vcat [ptext (sLit "forall") <+> braces (pprWithCommas ppr (varSetElems qtvs)), + nest 2 (vcat [ ppr t1 <+> ptext (sLit "~") <+> ppr t2 | (t1,t2) <- pairs])] \end{code} Given a bunch of predicates that must hold, such as @@ -225,6 +200,21 @@ NOTA BENE: \begin{code} type Pred_Loc = (PredType, SDoc) -- SDoc says where the Pred comes from +improveFromInstEnv :: (Class -> [Instance]) + -> Pred_Loc + -> [(Equation,Pred_Loc,Pred_Loc)] +-- Improvement from top-level instances +improveFromInstEnv _inst_env pred + = improveOne _inst_env pred [] -- TODO: Refactor to directly use instance_eqnd? + +improveFromAnother :: Pred_Loc + -> Pred_Loc + -> [(Equation,Pred_Loc,Pred_Loc)] +-- Improvement from another local (given or wanted) constraint +improveFromAnother pred1 pred2 + = improveOne (\_ -> []) pred1 [pred2] -- TODO: Refactor to directly use pairwise_eqns? + + improveOne :: (Class -> [Instance]) -- Gives instances for given class -> Pred_Loc -- Do improvement triggered by this -> [Pred_Loc] -- Current constraints @@ -234,7 +224,7 @@ improveOne :: (Class -> [Instance]) -- Gives instances for given class -- combined (for error messages) -- Just do improvement triggered by a single, distinguised predicate -improveOne inst_env pred@(IParam ip ty, _) preds +improveOne _inst_env pred@(IParam ip ty, _) preds = [ ((emptyVarSet, [(ty,ty2)]), pred, p2) | p2@(IParam ip2 ty2, _) <- preds , ip==ip2 @@ -281,11 +271,12 @@ improveOne inst_env pred@(ClassP cls tys, _) preds , not (instanceCantMatch inst_tcs trimmed_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)) + sep [ ptext (sLit "arising from the dependency") <+> quotes (pprFunDep fd) + , ptext (sLit "in the instance declaration at") + <+> ppr (getSrcLoc ispec)]) ] -improveOne inst_env eq_pred preds +improveOne _ _ _ = [] @@ -307,7 +298,7 @@ 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 @@ -457,7 +448,8 @@ 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 = inst_tcs, is_tvs = tvs, is_tys = tys }) <- cls_insts, @@ -470,19 +462,26 @@ badFunDeps cls_insts clas ins_tv_set 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 -> 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, tc; 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 (_,rtvs) 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` rtvs = Nothing - | otherwise = mb_tc + select clas_tv mb_tc | clas_tv `elem` ltvs = mb_tc + | otherwise = Nothing \end{code}