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copied!<p>pelotom's answer is the straightforward one. But not the fun one! <code>sequence</code> is the Haskell function that one can think of as flipping the order of type constructors between a list and a monad. </p> <p><code>sequence :: (Monad m) => [m a] -> m [a]</code></p> <p>Now what you want is, so to speak, to flip the order of type constructors between a <code>Maybe</code> and a monad. Data.Traversable exports a <code>sequence</code> function with just that capacity!</p> <p><code>Data.Traversable.sequence :: (Traversable t, Monad m) => t (m a) -> m (t a)</code></p> <p>This can specialize to <code>Maybe (IO ()) -> IO (Maybe ())</code> like in your example.</p> <p>Hence: </p> <pre><code>Prelude Data.Traversable> Data.Traversable.sequence (fmap print $ Nothing) Nothing Prelude Data.Traversable> Data.Traversable.sequence (fmap print $ Just 123) 123 Just () </code></pre> <p>Note that there's also a <code>sequenceA</code> function which is slightly more general, working not just on Monads but all Applicatives.</p> <p>So why use this approach? For <code>Maybe</code> the approach that takes it apart explicitly is fine. But what about a bigger data structure -- a <code>Map</code> for example? In that case, <code>traverse</code>, <code>sequenceA</code> and friends from <code>Data.Traversable</code> can be real handy.</p> <p>Edit: as Ed'ka notes, <code>traverse :: Applicative f => (a -> f b) -> t a -> f (t b)</code> and so one can just write <code>traverse print $ Just 123</code>.</p>

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