X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypes%2FFunDeps.lhs;h=a2bb3dfaf0f936d0dd17025699d0ca4f23e844ba;hp=5ece0471884ec94b704736a60133a1328c7ce17f;hb=ad94d40948668032189ad22a0ad741ac1f645f50;hpb=1c64864faf17b32ecaefa37fe8aa8fac5304f830 diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs index 5ece047..a2bb3df 100644 --- a/compiler/types/FunDeps.lhs +++ b/compiler/types/FunDeps.lhs @@ -1,37 +1,40 @@ - +% +% (c) The University of Glasgow 2006 % (c) The GRASP/AQUA Project, Glasgow University, 2000 % -\section[FunDeps]{FunDeps - functional dependencies} + +FunDeps - functional dependencies It's better to read it as: "if we know these, then we're going to know these" \begin{code} +{-# OPTIONS -w #-} +-- The above warning supression flag is a temporary kludge. +-- While working on this module you are encouraged to remove it and fix +-- any warnings in the module. See +-- http://hackage.haskell.org/trac/ghc/wiki/CodingStyle#Warnings +-- for details + module FunDeps ( Equation, pprEquation, - oclose, grow, improve, + oclose, grow, improveOne, checkInstCoverage, checkFunDeps, pprFundeps ) where #include "HsVersions.h" -import Name ( Name, getSrcLoc ) -import Var ( TyVar ) -import Class ( Class, FunDep, pprFundeps, classTvsFds ) -import TcGadt ( tcUnifyTys, BindFlag(..) ) -import Type ( substTys, notElemTvSubst ) -import Coercion ( isEqPred ) -import TcType ( Type, PredType(..), tcEqType, - predTyUnique, mkClassPred, tyVarsOfTypes, tyVarsOfPred ) -import InstEnv ( Instance(..), InstEnv, instanceHead, classInstances, - instanceCantMatch, roughMatchTcs ) +import Name +import Var +import Class +import TcGadt +import TcType +import InstEnv import VarSet import VarEnv import Outputable -import Util ( notNull ) -import List ( tails ) -import Maybe ( isJust ) -import ListSetOps ( equivClassesByUniq ) +import Util +import Data.Maybe ( isJust ) \end{code} @@ -41,6 +44,26 @@ import ListSetOps ( equivClassesByUniq ) %* * %************************************************************************ + 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 @@ -49,60 +72,47 @@ 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 @@ -110,8 +120,10 @@ 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}) ] @@ -124,24 +136,43 @@ 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 --- See Note [Ambiguity] in TcSimplify grow preds fixed_tvs - | null preds = fixed_tvs - | otherwise = loop 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 pred_sets + 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 - pred_sets = [tyVarsOfPred pred | pred <- preds] + (ip_tvs, non_ip_tvs) = partitionWith get_ip preds + get_ip (IParam _ ty) = Left (tyVarsOfType ty) + get_ip other = Right (tyVarsOfPred other) \end{code} %************************************************************************ @@ -152,7 +183,6 @@ 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 @@ -177,16 +207,6 @@ type Equation = (TyVarSet, [(Type, Type)]) pprEquation (qtvs, pairs) = vcat [ptext SLIT("forall") <+> braces (pprWithCommas ppr (varSetElems qtvs)), nest 2 (vcat [ ppr t1 <+> ptext SLIT(":=:") <+> ppr t2 | (t1,t2) <- pairs])] - ----------- -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) \end{code} Given a bunch of predicates that must hold, such as @@ -219,81 +239,72 @@ 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 - ----------- -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 +type Pred_Loc = (PredType, SDoc) -- SDoc says where the Pred comes from + +improveOne :: (Class -> [Instance]) -- Gives instances for given class + -> Pred_Loc -- Do improvement triggered by this + -> [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) +-- 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)] + +improveOne inst_env pred@(ClassP cls tys, _) preds + | 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 exmample - + -- 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, p1, p2) - | fd <- cls_fds, - p1@(ClassP _ tys1, _) : rest <- tails clss, - p2@(ClassP _ tys2, _) <- rest, - eqn <- checkClsFD emptyVarSet fd cls_tvs tys1 tys2 + = [ (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, p1, p2) - | fd <- cls_fds, -- Iterate through the fundeps first, + = [ (eqn, p_inst, pred) + | 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)) + , 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) + , 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 + = [] + + checkClsFD :: TyVarSet -- Quantified type variables; see note below -> FunDep TyVar -> [TyVar] -- One functional dependency from the class -> [Type] -> [Type] @@ -464,11 +475,11 @@ badFunDeps :: [Instance] -> Class badFunDeps cls_insts clas ins_tv_set ins_tys = [ ispec | fd <- fds, -- fds is often empty 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) ] @@ -478,16 +489,16 @@ badFunDeps cls_insts clas ins_tv_set ins_tys 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 types ta, tc; so our -- rough-match thing must similarly be filtered. -- Hence, we Nothing-ise the tb type right here -trimRoughMatchTcs clas_tvs (ltvs,_) mb_tcs +trimRoughMatchTcs clas_tvs (_,rtvs) mb_tcs = zipWith select clas_tvs mb_tcs where - select clas_tv mb_tc | clas_tv `elem` ltvs = mb_tc - | otherwise = Nothing + select clas_tv mb_tc | clas_tv `elem` rtvs = Nothing + | otherwise = mb_tc \end{code}