As someone suggested me, I can read the logs from Writer and WriterT as computation goes by, 
if the monoid for the Writer  is lazy readable.
This has been true until I tried to put the IO inside WriterT


> {-# LANGUAGE FlexibleContexts #-}
> import Control.Monad.Writer


> k :: (MonadWriter [Int] m) => m [Int]

> k = let f x = tell [x] >> f (x + 1) in f 0


> works :: [Int]
> works = snd $ runWriter k


> hangs :: IO [Int]
> hangs = snd `liftM` runWriterT k  


> main = take 20 `liftM` hangs >>= print



The main hangs both interpreted and compiled on ghc 6.10.1.

The issue is not exposing with IO alone as

main = print "test" >> main 

is a working program.

Thanks for explanations.


paolino