From: simonpj <unknown>
Date: Fri, 19 Mar 2004 13:25:24 +0000 (+0000)
Subject: [project @ 2004-03-19 13:25:24 by simonpj]
X-Git-Tag: Initial_conversion_from_CVS_complete~1952
X-Git-Url: http://git.megacz.com/?a=commitdiff_plain;h=45deb4d603c392ac8f404052e7c68ce12f05538b;p=ghc-hetmet.git

[project @ 2004-03-19 13:25:24 by simonpj]
Add comments
---

diff --git a/ghc/compiler/typecheck/TcSimplify.lhs b/ghc/compiler/typecheck/TcSimplify.lhs
index 91ca3b8..a3e9352 100644
--- a/ghc/compiler/typecheck/TcSimplify.lhs
+++ b/ghc/compiler/typecheck/TcSimplify.lhs
@@ -912,6 +912,22 @@ Plan C (brutal)
   Return the bindings from Step 1.
   
 
+A note about Plan C (arising from "bug" reported by George Russel March 2004)
+Consider this:
+
+      instance (HasBinary ty IO) => HasCodedValue ty
+
+      foo :: HasCodedValue a => String -> IO a
+
+      doDecodeIO :: HasCodedValue a => () -> () -> IO a
+      doDecodeIO codedValue view  
+        = let { act = foo "foo" } in  act
+
+You might think this should work becuase the call to foo gives rise to a constraint
+(HasCodedValue t), which can be satisfied by the type sig for doDecodeIO.  But the
+restricted binding act = ... calls tcSimplifyRestricted, and PlanC simplifies the
+constraint using the (rather bogus) instance declaration, and now we are stuffed.
+I claim this is not really a bug.  
 
 \begin{code}
 tcSimplifyRestricted 	-- Used for restricted binding groups