From b52d0975be192c19eb353728d4fc8f1d02e542ff Mon Sep 17 00:00:00 2001
From: "gwern0@gmail.com" <unknown>
Date: Fri, 8 Jan 2010 21:44:55 +0000
Subject: [PATCH] Control.Monad: +void :: f a -> f () See
 http://hackage.haskell.org/trac/ghc/ticket/3292 Turns m a
 -> m (). Lets one call functions for their side-effects
 without having to get rid of their return values with '>>
 return ()'. Very useful in many contexts (parsing, IO
 etc.); particularly good for 'forkIO' and 'forM_', as they
 demand return types of 'IO ()' though most interesting IO
 functions return non-().

---
 Control/Monad.hs |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Control/Monad.hs b/Control/Monad.hs
index a1d7d26..407bfc4 100644
--- a/Control/Monad.hs
+++ b/Control/Monad.hs
@@ -190,6 +190,10 @@ f >=> g     = \x -> f x >>= g
 forever     :: (Monad m) => m a -> m b
 forever a   = a >> forever a
 
+-- | @'void' value@ discards or ignores the result of evaluation, such as the return value of an 'IO' action.
+void :: Functor f => f a -> f ()
+void = fmap (const ())
+
 -- -----------------------------------------------------------------------------
 -- Other monad functions
 
-- 
1.7.10.4