From 9572b96fa49adfe63f5e0644a6a75bd143c3c5bd Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Thu, 3 Aug 2017 17:04:47 -0400
Subject: [PATCH] Define a MonadError instance for Amb.

---
 src/Data/Amb.hs | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/Data/Amb.hs b/src/Data/Amb.hs
index 3026dfadf..c3e479aa5 100644
--- a/src/Data/Amb.hs
+++ b/src/Data/Amb.hs
@@ -1,5 +1,7 @@
+{-# LANGUAGE MultiParamTypeClasses #-}
 module Data.Amb where
 
+import Control.Monad.Error.Class
 import Data.List.NonEmpty
 import Data.Semigroup
 
@@ -23,3 +25,8 @@ instance Monad (Amb l) where
   return = pure
   None a >>= _ = None a
   Some as >>= f = foldr1 (<>) (f <$> as)
+
+instance MonadError l (Amb l) where
+  throwError = None
+  None a  `catchError` f = f a
+  Some as `catchError` _ = Some as