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,
However, consider
class D a b c | b->c
- f x = e -- Generates constraint (D s Int t)
+ 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
+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.
+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
\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;