Derive a valid Ix instance for data Foo = Foo Int Int

The old one didn't satisfy the axioms. See trac #2158 for details.

diff --git a/compiler/typecheck/TcGenDeriv.lhs b/compiler/typecheck/TcGenDeriv.lhs
index 6d9fc55..f248674 100644 (file)
--- a/compiler/typecheck/TcGenDeriv.lhs
+++ b/compiler/typecheck/TcGenDeriv.lhs
@@ -730,7 +730,11 @@ gen_Ix_binds tycon
       = mk_easy_FunBind tycon_loc unsafeIndex_RDR 
                [nlTuplePat [con_pat as_needed, con_pat bs_needed] Boxed, 
                 con_pat cs_needed] 
-               (mk_index (zip3 as_needed bs_needed cs_needed))
+        -- We need to reverse the order we consider the components in
+        -- so that
+        --     range (l,u) !! index (l,u) i == i   -- when i is in range
+        -- (from http://haskell.org/onlinereport/ix.html) holds.
+               (mk_index (reverse $ zip3 as_needed bs_needed cs_needed))
       where
        -- index (l1,u1) i1 + rangeSize (l1,u1) * (index (l2,u2) i2 + ...)
        mk_index []        = nlHsIntLit 0