[Haskell-beginners] ListT + Writer

[Baa]

Hello, everybody!

I can process list in monad style with "do" syntax and to use "guard"
function in the body. Something like:

  fn :: [a] -> [a]
  fn lst = do
    el <- lst
    guard $ condition el
    ...
    return $ change el

How can I do the same but with possibility to call "tell" of "Write"
monad in the fn's body? As I understand it should be:

   ListT (Writer w) Int

for this example?

- but how to write it?
- how to call (run) it?
- and how is it safe ("transformers" package has bug in ListT, so "mtl"
  must be used?)?
- is there other canonical way to do it without to use fold*, recursive
  calls/fix, State/RWS ?


/Cheers