Re: [Haskell-cafe] WriterT [w] IO is not lazy in reading [w]

On Thu, 2009-01-01 at 02:17 -0800, Ryan Ingram wrote:

2008/12/31 Paolino <paolo.veronelli@gmail.com>:

...

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

Assuming you have Control.Monad.State.Lazy (which I think is the default), here is the difference: [...] There's a much simpler way of understanding why you must wait for an IO computation to finish before you can use the result. In IO you can throw exceptions (IOErrors), so you must wait until an IO computation is finished to know that there is any result at all.