Bleat a bit more informatively in unionLists

1 files changed, 6 insertions, 2 deletions

diff --git a/compiler/utils/ListSetOps.lhs b/compiler/utils/ListSetOps.lhs
index f5e0eae9a0..83334fbb28 100644
--- a/compiler/utils/ListSetOps.lhs
+++ b/compiler/utils/ListSetOps.lhs
@@ -19,6 +19,8 @@ module ListSetOps (
 
    ) where
 
+#include "HsVersions.h"
+
 import Outputable
 import Unique
 import UniqFM
@@ -41,9 +43,11 @@ insertList :: Eq a => a -> [a] -> [a]
 insertList x xs | isIn "insert" x xs = xs
                 | otherwise          = x : xs
 
-unionLists :: (Eq a) => [a] -> [a] -> [a]
+unionLists :: (Outputable a, Eq a) => [a] -> [a] -> [a]
 -- Assumes that the arguments contain no duplicates
-unionLists xs ys = [x | x <- xs, isn'tIn "unionLists" x ys] ++ ys
+unionLists xs ys 
+  = WARN(length xs > 100 || length ys > 100, ppr xs $$ ppr ys)
+    [x | x <- xs, isn'tIn "unionLists" x ys] ++ ys
 
 minusList :: (Eq a) => [a] -> [a] -> [a]
 -- Everything in the first list that is not in the second list: