This is very important: you cannot.

But you can still get your hands on one inside a do block.  As in, if you have tree :: IO [XmlTree], then you can say, eg.

printTree = do

    t <- tree

    print t

Inside this block, t is an [XmlTree], and it is passed to print. Intutively, when you see

 x <- y

inside a do block, if y :: IO a, for some type a, then x :: a.

But there is no function that will get you from IO [XmlTree] -> [XmlTree], you have to use binding like this.

Luke