From c5d2d92ce6295d7236c9e8a66a47e741d6f5388b Mon Sep 17 00:00:00 2001
From: "simonpj@microsoft.com" <unknown>
Date: Mon, 18 Sep 2006 00:51:42 +0000
Subject: [PATCH] Comments

---
 compiler/typecheck/TcSimplify.lhs |   24 +++++++++++++++++++++---
 compiler/types/FunDeps.lhs        |    1 +
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 3c8160c..8b3724a 100644
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -86,6 +86,24 @@ import DynFlags		( DynFlags(ctxtStkDepth),
 	Notes on functional dependencies (a bug)
 	--------------------------------------
 
+Consider this:
+
+	class C a b | a -> b
+	class D a b | a -> b
+
+	instance D a b => C a b	-- Undecidable 
+				-- (Not sure if it's crucial to this eg)
+	f :: C a b => a -> Bool
+	f _ = True
+	
+	g :: C a b => a -> Bool
+	g = f
+
+Here f typechecks, but g does not!!  Reason: before doing improvement,
+we reduce the (C a b1) constraint from the call of f to (D a b1).
+
+Here is a more complicated example:
+
 | > class Foo a b | a->b
 | >
 | > class Bar a b | a->b
@@ -257,9 +275,9 @@ any other type variables.
 
 
 
-	--------------------------------------
-		Notes on ambiguity
-	--------------------------------------
+-------------------------------------
+	Note [Ambiguity]
+-------------------------------------
 
 It's very hard to be certain when a type is ambiguous.  Consider
 
diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs
index 9347f5f..7000075 100644
--- a/compiler/types/FunDeps.lhs
+++ b/compiler/types/FunDeps.lhs
@@ -125,6 +125,7 @@ oclose preds fixed_tvs
 
 \begin{code}
 grow :: [PredType] -> TyVarSet -> TyVarSet
+-- See Note [Ambiguity] in TcSimplify
 grow preds fixed_tvs 
   | null preds = fixed_tvs
   | otherwise  = loop fixed_tvs
-- 
1.7.10.4