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copied!<p>You will be disappointed, but the <code>>>=</code> in <code>IO</code> monad isn't that interesting. To quote the GHC source:</p> <pre><code>{- | A value of type @'IO' a@ is a computation which, when performed, does some I\/O before returning a value of type @a@. There is really only one way to \"perform\" an I\/O action: bind it to @Main.main@ in your program. When your program is run, the I\/O will be performed. It isn't possible to perform I\/O from an arbitrary function, unless that function is itself in the 'IO' monad and called at some point, directly or indirectly, from @Main.main@. 'IO' is a monad, so 'IO' actions can be combined using either the do-notation or the '>>' and '>>=' operations from the 'Monad' class. -} newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #)) </code></pre> <p>That means <code>IO</code> monad is declared as the instance of <strike><code>State</code></strike> <code>State#</code> monad, <strike>and its <code>>>=</code> is defined there (and its implementation is fairly easy to guess)</strike>.</p> <p>See <a href="http://www.haskell.org/haskellwiki/IO_inside#Welcome_to_the_RealWorld.2C_baby_:.29" rel="nofollow noreferrer">IO inside</a> article on Haskell wiki for more details about the <code>IO</code> monad. It is also helpful to look at the <a href="http://www.haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#t:IO" rel="nofollow noreferrer">Haskell docs</a>, where every position has small "Source" link on the right.</p> <p><strong>Update:</strong> And there goes another disappointment, which is my answer, because I didn't notice the '#' in <code>State#</code>. However <code>IO</code> behaves like <code>State</code> monad carrying abstract <code>RealWorld</code> state</p> <p>As @ehird wrote <code>State#</code> is compiler's internal and <code>>>=</code> for the <code>IO</code> monad is defined in <a href="http://hackage.haskell.org/package/base-4.7.0.2/docs/src/GHC-Base.html#bindIO" rel="nofollow noreferrer">GHC.Base</a> module:</p> <pre><code>instance Monad IO where {-# INLINE return #-} {-# INLINE (>>) #-} {-# INLINE (>>=) #-} m >> k = m >>= \ _ -> k return = returnIO (>>=) = bindIO fail s = failIO s returnIO :: a -> IO a returnIO x = IO $ \ s -> (# s, x #) bindIO :: IO a -> (a -> IO b) -> IO b bindIO (IO m) k = IO $ \ s -> case m s of (# new_s, a #) -> unIO (k a) new_s </code></pre>

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