X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypes%2FFunDeps.lhs;h=a7e9f19b064d9ea6931460dae45a0d7d50e67f43;hb=235edf36cc202bb21c00d0e5e05ebf076fb0542e;hp=30549dd62b1e82f8a6edebf6480ca44d5cf4a2ea;hpb=0f5731ee92009fe43e9cc0bd276b4562e0c37089;p=ghc-hetmet.git

diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs
index 30549dd..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,
@@ -76,21 +83,29 @@ uniform thing is to return {t}.
 
 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
@@ -138,7 +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;