From f8afe6208a5d497a8c90b1a901d251431209f5b6 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@github.com>
Date: Fri, 11 Mar 2016 16:49:24 -0500
Subject: [PATCH] MaybeBoth is a Crosswalk.

---
 src/Data/Functor/Both.hs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/Data/Functor/Both.hs b/src/Data/Functor/Both.hs
index 6e2c29a7e..ee1ebbb0b 100644
--- a/src/Data/Functor/Both.hs
+++ b/src/Data/Functor/Both.hs
@@ -1,6 +1,7 @@
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 module Data.Functor.Both where
 
+import Data.Align
 import Data.Bifunctor
 import Data.Bifunctor.These
 import Prelude hiding (zipWith, fst, snd)
@@ -53,3 +54,6 @@ newtype MaybeBoth a = MaybeBoth { runMaybeBoth :: Both (Maybe a) }
 instance Applicative MaybeBoth where
   pure = MaybeBoth . pure . Just
   MaybeBoth (Both (f, g)) <*> MaybeBoth (Both (a, b)) = MaybeBoth (both (f <*> a) (g <*> b))
+
+instance Crosswalk MaybeBoth where
+  crosswalk f (MaybeBoth ab) = runBothWith (alignWith (MaybeBoth . maybeBothOfThese)) (maybe nil f <$> ab)