1 files changed, 6 insertions, 6 deletions
diff --git a/DSL/Util.hs b/DSL/Util.hs
index 75d38af..44d272c 100644
--- a/DSL/Util.hs
+++ b/DSL/Util.hs
@@ -28,20 +28,20 @@ runChecks fs s
 hcf :: Machine -> String -> IO Machine
 hcf m msg = putStrLn msg >> return m{ ok=False }
 
-appendProcTable :: [ProcSpec] -> ProcTable -> Maybe ProcTable
+appendProcTable :: [ProcSpec] -> ProcTable -> Either String ProcTable
 appendProcTable x y = go x
   where
-    go []           = Just y
+    go []           = Right y
     go ((n, bs):ps) = go ps >>= f
       where
         f acc
-          | member n acc = Nothing
-          | otherwise    = Just $ insert n bs acc
+          | member n acc = Left $ "Duplicate PROC definition: " ++ n
+          | otherwise    = Right $ insert n bs acc
 
-mergeProcTables :: ProcTable -> ProcTable -> Maybe ProcTable
+mergeProcTables :: ProcTable -> ProcTable -> Either String ProcTable
 mergeProcTables x = appendProcTable $ toList x
 
-buildProcTable :: [ProcSpec] -> Maybe ProcTable
+buildProcTable :: [ProcSpec] -> Either String ProcTable
 buildProcTable ps = appendProcTable ps empty
 
 unreachable :: a