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 (file)
--- 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 }