[commit: ghc] : Comments only (6c4c33f)

[Simon Marlow]

Repository : ssh://darcs.haskell.org//srv/darcs/ghc

On branch  : 

http://hackage.haskell.org/trac/ghc/changeset/6c4c33fc9cc4d9a4727a63f3a9681f7200354750

>---------------------------------------------------------------

commit 6c4c33fc9cc4d9a4727a63f3a9681f7200354750
Author: simonpj at microsoft.com <unknown>
Date:   Thu Feb 17 17:58:40 2011 +0000

    Comments only

>---------------------------------------------------------------

 compiler/typecheck/TcCanonical.lhs |   16 ++++++++++++++++
 1 files changed, 16 insertions(+), 0 deletions(-)

diff --git a/compiler/typecheck/TcCanonical.lhs b/compiler/typecheck/TcCanonical.lhs
index 8668d90..1974143 100644
--- a/compiler/typecheck/TcCanonical.lhs
+++ b/compiler/typecheck/TcCanonical.lhs
@@ -1001,6 +1001,22 @@ itself, and so on.
 %*                                                                      *
 %************************************************************************
 
+When we spot an equality arising from a functional dependency,
+we now use that equality (a "wanted") to rewrite the work-item
+constraint right away.  This avoids two dangers
+
+ Danger 1: If we send the original constraint on down the pipeline
+           it may react with an instance declaration, and in delicate
+	   situations (when a Given overlaps with an instance) that
+	   may produce new insoluble goals: see Trac #4952
+
+ Danger 2: If we don't rewrite the constraint, it may re-react
+           with the same thing later, and produce the same equality
+           again --> termination worries.
+
+To achieve this required some refactoring of FunDeps.lhs (nicer
+now!).  
+
 \begin{code}
 rewriteWithFunDeps :: [Equation]
                    -> [Xi] -> CtFlavor