How to flatten IO [[String]]?

Question

I am new to Haskell and have some difficulties wrapping my head around some of it\'s concepts.

While playing around with IO I wanted to flatten an IO [[String]].

Answer 1

You can't get anything "out" of IO, so what you need to do is lift flatten to work inside it. The simplest way to do this is fmap--just like map applies a function over a list, fmap applies a function over any Functor instance, such as IO.

flattenIO xs = fmap flatten xs

In more general cases, you can use do notation to get at things in IO computations. For example:

flattenIO xs = do ys <- xs
                  return (flatten ys)

...which is just a roundabout way of writing fmap in this case.