I must ask why runWriterT k :: State s (a,[Int]) is working.
Looks like I could runIO the same way I evalState there.
In that case I wouldn't wait for the State s action to finish.
Thanks
2008/12/31 Derek Elkins
On Wed, 2008-12-31 at 21:48 +0100, Paolino wrote:
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
runWriterT :: MonadWriter w m a => WriterT w m a -> m (a, w)
which is to say runWriterT k :: IO (a, [Int])
It's not going to return anything until the IO action terminates, which is to say never.