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

Question

How can I flatten a nested list like this:

[1, 2, 3, 4] == flatten [[[1,2],[3]],[[4]]]

Answer 1

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