From 0f5731ee92009fe43e9cc0bd276b4562e0c37089 Mon Sep 17 00:00:00 2001 From: "simonpj@microsoft.com" Date: Fri, 29 Jun 2007 21:53:34 +0000 Subject: [PATCH] Many comments about oclose, plus a fix for Trac #1456 There was a rather subtle bug in the way 'oclose' works when generalising top-level function definitions. See Note [Important subtlety in oclose] in FunDeps for an explanatoin. I also tidied up duplication in comments while I was here. --- compiler/typecheck/TcSimplify.lhs | 64 +++++++---------------- compiler/types/FunDeps.lhs | 103 +++++++++++++++++++------------------ 2 files changed, 70 insertions(+), 97 deletions(-) diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs index 773b312..db12011 100644 --- a/compiler/typecheck/TcSimplify.lhs +++ b/compiler/typecheck/TcSimplify.lhs @@ -132,14 +132,9 @@ from. The Right Thing is to improve whenever the constraint set changes at all. Not hard in principle, but it'll take a bit of fiddling to do. - - - -------------------------------------- - Notes on quantification - -------------------------------------- - -Suppose we are about to do a generalisation step. -We have in our hand +Note [Choosing which variables to quantify] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Suppose we are about to do a generalisation step. We have in our hand G the environment T the type of the RHS @@ -162,11 +157,12 @@ Here are the things that *must* be true: (A) Q intersect fv(G) = EMPTY limits how big Q can be (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be -(A) says we can't quantify over a variable that's free in the -environment. (B) says we must quantify over all the truly free -variables in T, else we won't get a sufficiently general type. We do -not *need* to quantify over any variable that is fixed by the free -vars of the environment G. + (A) says we can't quantify over a variable that's free in the environment. + (B) says we must quantify over all the truly free variables in T, else + we won't get a sufficiently general type. + +We do not *need* to quantify over any variable that is fixed by the +free vars of the environment G. BETWEEN THESE TWO BOUNDS, ANY Q WILL DO! @@ -180,38 +176,15 @@ Example: class H x y | x->y where ... So Q can be {c,d}, {b,c,d} +In particular, it's perfectly OK to quantify over more type variables +than strictly necessary; there is no need to quantify over 'b', since +it is determined by 'a' which is free in the envt, but it's perfectly +OK to do so. However we must not quantify over 'a' itself. + Other things being equal, however, we'd like to quantify over as few variables as possible: smaller types, fewer type applications, more -constraints can get into Ct instead of Cq. - - ------------------------------------------ -We will make use of - - fv(T) the free type vars of T - - 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 one way: v `elem` oclose(vs,C) => v is definitely fixed by vs - grow is conservative the other way: if v might be fixed by vs => v `elem` grow(vs,C) - - ------------------------------------------ - -Note [Choosing which variables to quantify] -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -Here's a good way to choose Q: +constraints can get into Ct instead of Cq. Here's a good way to +choose Q: Q = grow( fv(T), C ) \ oclose( fv(G), C ) @@ -245,9 +218,8 @@ all the functional dependencies yet: T = c->c C = (Eq (T c d)) - Now oclose(fv(T),C) = {c}, because the functional dependency isn't - apparent yet, and that's wrong. We must really quantify over d too. - +Now oclose(fv(T),C) = {c}, because the functional dependency isn't +apparent yet, and that's wrong. We must really quantify over d too. There really isn't any point in quantifying over any more than grow( fv(T), C ), because the call sites can't possibly influence diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs index 9af9210..30549dd 100644 --- a/compiler/types/FunDeps.lhs +++ b/compiler/types/FunDeps.lhs @@ -37,6 +37,26 @@ 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 +65,39 @@ 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 -- 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 variales 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. \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 +105,10 @@ 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}) ] -- 1.7.10.4