[Haskell-beginners] Problems with IO actions

[Adam Bergmark]

I have an issue with IO actions.
At the moment I have the following snippet of code, which works as I  
expect it to.

mapM_ (writeChan ch) . lines =<< hGetContents h

That is, it gets each line in the file and writes it to the channel.
Previously I had just:

writeChan ch =<< hGetContents h

but when i readChan this I get the empty string every time.

I have two questions:
  * I'm still a little unsure of why mapM_, sequence and the like are  
necessary, could someone please explain this, or point me somewhere  
where I can read up on it?
  * Why does this problem occur when there is only one action to  
perform?


Cheers
/ Adam Bergmark