1 files changed, 12 insertions, 0 deletions
diff --git a/src/option_key_fn.sml b/src/option_key_fn.sml
new file mode 100644
index 00000000..27ba9138
--- /dev/null
+++ b/src/option_key_fn.sml
@@ -0,0 +1,12 @@
+functor OptionKeyFn(K : ORD_KEY)
+        : ORD_KEY where type ord_key = K.ord_key option = struct
+
+type ord_key = K.ord_key option
+
+val compare =
+ fn (NONE, NONE) => EQUAL
+  | (NONE, _) => LESS
+  | (_, NONE) => GREATER
+  | (SOME x, SOME y) => K.compare (x, y)
+
+end