Comments

[ghc-hetmet.git] / compiler / typecheck / TcSimplify.lhs

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 3c8160c..8b3724a 100644 (file)
--- 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