From: Ross Paterson <ross@soi.city.ac.uk>
Date: Thu, 17 Jun 2010 22:51:10 +0000 (+0000)
Subject: add Applicative instance for Either (proposal #4095)
X-Git-Url: http://git.megacz.com/?a=commitdiff_plain;h=8abb469bd2210d78da74b334a0f4397be5ac37f6;p=ghc-base.git

add Applicative instance for Either (proposal #4095)

This is not the only possible instance for Either, but this one is
compatible with the usual Monad instance.
---

diff --git a/Control/Applicative.hs b/Control/Applicative.hs
index a2755eb..154b591 100644
--- a/Control/Applicative.hs
+++ b/Control/Applicative.hs
@@ -167,6 +167,11 @@ instance Monoid a => Applicative ((,) a) where
         pure x = (mempty, x)
         (u, f) <*> (v, x) = (u `mappend` v, f x)
 
+instance Applicative (Either e) where
+        pure          = Right
+        Left  e <*> _ = Left e
+        Right f <*> r = fmap f r
+
 -- new instances
 
 newtype Const a b = Const { getConst :: a }