[project @ 2005-07-13 10:16:11 by simonpj]

[ghc-hetmet.git] / ghc / compiler / types / FunDeps.lhs

diff --git a/ghc/compiler/types/FunDeps.lhs b/ghc/compiler/types/FunDeps.lhs
index af42ee9..f0a97c3 100644 (file)
--- a/ghc/compiler/types/FunDeps.lhs
+++ b/ghc/compiler/types/FunDeps.lhs
@@ -245,7 +245,15 @@ checkGroup inst_env clss@((ClassP cls _, _) : _)
        --
        -- We need to do something very similar comparing each predicate
        -- with relevant instance decls
-    pairwise_eqns ++ instance_eqns
+
+    instance_eqns ++ pairwise_eqns
+       -- NB: we put the instance equations first.   This biases the 
+       -- order so that we first improve individual constraints against the
+       -- instances (which are perhaps in a library and less likely to be
+       -- wrong; and THEN perform the pairwise checks.
+       -- The other way round, it's possible for the pairwise check to succeed
+       -- and cause a subsequent, misleading failure of one of the pair with an
+       -- instance declaration.  See tcfail143.hs for an exmample
 
   where
     (cls_tvs, cls_fds) = classTvsFds cls