Is there a function to flatten a nested list of elements?

As others have pointed out, concat :: [[a]] -> [a] is the function you are looking for, and it can't flatten nested lists of arbitrary depth. You need to call it multiple times to flatten it down to the desired level.

The operation does generalize to other monads, though. It is then known as join, and has the type Monad m => m (m a) -> m a.

Prelude Control.Monad> join [[1, 2], [3, 4]]
[1,2,3,4]    
Prelude Control.Monad> join (Just (Just 3))
Just 3
Prelude Control.Monad.Reader> join (+) 21
42

As hammar pointed out, join is the "monadic" way to flatten a list. You can use the do-Notation as well to write easily flatten functions of several levels:

flatten xsss = do xss <- xsss
                  xs <- xss
                  x <- xs
                  return x