From 1e7dc0ff98f2f6f25ad773c658eb3317360f5573 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@github.com>
Date: Mon, 14 Dec 2015 17:00:30 -0500
Subject: [PATCH] Define `intersectionWith`.

---
 src/OrderedMap.hs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/OrderedMap.hs b/src/OrderedMap.hs
index 1300f34a1..bf1b51132 100644
--- a/src/OrderedMap.hs
+++ b/src/OrderedMap.hs
@@ -9,6 +9,7 @@ module OrderedMap (
   , empty
   , union
   , unions
+  , intersectionWith
   ) where
 
 data OrderedMap key value = OrderedMap { toList :: [(key, value)] }
@@ -42,3 +43,6 @@ union (OrderedMap a) (OrderedMap b) = OrderedMap $ a ++ filter (not . (`elem` ex
 
 unions :: Eq key => [OrderedMap key value] -> OrderedMap key value
 unions = foldl union empty
+
+intersectionWith :: Eq key => (a -> b -> c) -> OrderedMap key a -> OrderedMap key b -> OrderedMap key c
+intersectionWith combine (OrderedMap a) (OrderedMap b) = OrderedMap $ a >>= (\ (key, value) -> maybe [] (pure . ((,) key) . combine value) $ Prelude.lookup key b)