At 1:29 PM +0100 2/4/11, Tim Baumgartner wrote:

Hi community,

In the following code, the main function only wants to print the first 5 numbers of a huge list. But since the computation is not lazy, this doesn't work in a satisfactory way.

content :: Int -> IO [Int] content i = do fs <- files i ds <- directories i fss <- mapM content ds return $ fs ++ concat fss

files i = return [1..i]

directories i = return [1..i-1]

main = content 1000000 >>= print . take 5

Now I'd like to know if it's possible to make this "IO" lazy, using unsafeInterleaveIO. I tried to do it, failed. Perhaps someone can help.

In your simplified example, it suffices to wrap unsafeInterleaveIO around (mapM content ds). In a more realistic case, you'd need to add some `unsafeInterleaveIO`s in `files` and `directories`, too. Dean P.S. You might also want to look into iteratees, which accomplish a similar goal in a more robust way.

I guess another weakness of this code is the bad performance of "dfs ++ concat fss", but currently I don't care about that.

Thanks for any help Tim