Use a stable ordering for the export list in the interface

The export list was ordered according to the whim of FastStrings,
which meant that interface fingerprints could change for no good
reason, causing spurious recompilation.

diff --git a/compiler/iface/MkIface.lhs b/compiler/iface/MkIface.lhs
index c929d96..17a379b 100644 (file)
--- a/compiler/iface/MkIface.lhs
+++ b/compiler/iface/MkIface.lhs
@@ -923,11 +923,14 @@ mkIfaceAnnotation (Annotation { ann_target = target, ann_value = serialized }) =
 \begin{code}
 mkIfaceExports :: [AvailInfo]
                -> [(Module, [GenAvailInfo OccName])]
-  -- Group by module and sort by occurrence
-  -- This keeps the list in canonical order
+                  -- Group by module and sort by occurrence
 mkIfaceExports exports
   = [ (mod, eltsFM avails)
-    | (mod, avails) <- fmToList groupFM
+    | (mod, avails) <- sortBy (stableModuleCmp `on` fst) (fmToList groupFM)
+                       -- NB. the fmToList is in a random order,
+                       -- because Ord Module is not a predictable
+                       -- ordering.  Hence we perform a final sort
+                       -- using the stable Module ordering.
     ]
   where
        -- Group by the module where the exported entities are defined