From 3dd4d12af82e4427068ded23d8d96c201e835d89 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Mon, 24 Apr 2017 13:28:56 -0400
Subject: [PATCH] Define the strengthening of a single-member union.

---
 src/Data/Functor/Union.hs | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/Data/Functor/Union.hs b/src/Data/Functor/Union.hs
index ed2cfdc55..6fee9f877 100644
--- a/src/Data/Functor/Union.hs
+++ b/src/Data/Functor/Union.hs
@@ -25,6 +25,11 @@ unwrapU :: (ComonadCofree (Union fs) w, InUnion fs f) => w a -> Maybe (f (w a))
 unwrapU = prj . unwrap
 
 
+strengthen :: Union '[f] a -> f a
+strengthen (Here f) = f
+strengthen _ = undefined
+
+
 -- Classes
 
 class InUnion (fs :: [* -> *]) (f :: * -> *) where