On Sun, 26 Jun 2011 15:02:07 -0400
David Place
On Jun 26, 2011, at 1:29 PM, Manfred Lotz wrote:
When I stumbled upon lazy IO newbie-wise I was pointed to withFile resp. bracket by Daniel Fischer and now that I know how to do it it seems fine to me. It also alerted me to pay more attention to lazyness as this is a Haskell immanent thingie.
Of course, it is possible to use hGetContents with withFile. You can still get into trouble because hGetContents is unsafe. Beginners get tripped up trying to do something like the following getting unexpected results. (i remember I did.)
print10 = do contents <- withFile "/usr/share/dict/words" ReadMode (\h -> hGetContents h) print $ take 10 contents
So, you have to do this keeping in mind a rather procedural model of the evaluation of the lazy data structures. I feel this is not very declarative or intuitive.
print10' = do h <- openFile "/usr/share/dict/words" ReadMode contents <- hGetContents h print $ take 10 contents hClose h
Iteratee IO provides a declarative way to do this that is safe, compositional and efficient. But not yet very pretty. In haskell cafe, John Lato says that he is working on that.
Thanks for showing this. I have to admit that I'm as a beginner not in a position to judge about the merits of Iteratee IO versus the standard way. Just wanted to point out that the way I work thru the XML files in my particular task seems to work fine. I will watch what happens to Iteratee IO. Is there any problem in the code snippet I pasted? If so I would like to get a hint of course. -- Thanks, Manfred