X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypes%2FFunDeps.lhs;h=a7e9f19b064d9ea6931460dae45a0d7d50e67f43;hb=3b1438a9757639d7f37f10e1237e2369ca0ebe4a;hp=9af92107eeff13ef6aa69079633f61f441469b5e;hpb=eb2bf7ad9f967861da2e19ff71a80428c7c2df28;p=ghc-hetmet.git diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs index 9af9210..a7e9f19 100644 --- a/compiler/types/FunDeps.lhs +++ b/compiler/types/FunDeps.lhs @@ -8,6 +8,13 @@ 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/Commentary/CodingStyle#Warnings +-- for details + module FunDeps ( Equation, pprEquation, oclose, grow, improveOne, @@ -37,6 +44,26 @@ 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 +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 @@ -106,8 +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}) ] @@ -137,7 +153,7 @@ See also Note [Ambiguity] in TcSimplify \begin{code} grow :: [PredType] -> TyVarSet -> TyVarSet grow preds fixed_tvs - | null preds = real_fixed_tvs + | null preds = fixed_tvs | otherwise = loop real_fixed_tvs where -- Add the implicit parameters;