Saturday, February 20, 2010

NineP is on hackage!

I'm excited about it. There's still work to be done, but now I think there's a good chance to get a lot more feedback about the module as it's in experimental, and hopefully we can get some of these folks interested in distributed systems to use this nice, simple protocol.

Friday, February 19, 2010

9ph package for cabal/hackage

The 9ph code repository now has a fully cabalized module, and I'm waiting on a response to get permission to upload stuff to Hackage. This will be my first shared contribution to the Haskell community, though all I've done is create a simple test program and some of the administrative work around wrapping this code up. If anyone likes this, the credit should go to Tim.

I've got some ideas for enhancements, and ways to make a nice server/client API on top of this lovely encoding/decoding library that Tim created on top of the very excellent binary package for Haskell and the lovely Applicative module.

Monday, February 8, 2010

9ph works!

Finally, since last September, I've gotten around to messing with 9ph again. That's the 9P2000 implementation in Haskell that Tim Newsham basically wrote and sent to me to play around with.

I had started such a project on my own at one point, but Tim totally beat me to it, and rather than just re-implement the wheel, I figured I'd check out what he'd done and checked it in.

Today, I wrote a function that runs an IO program yielding a socket that's connected to a local 9P2000 server (or error'd out actually) running on From there I was able to create messages for Tattach, Topen, and Tread, and could explore the errors and successes of the server I was talking to.

For my experiments I just used the Inferno operating system with

"styxlisten -A 'tcp!*!6872' export /"

This just exports the local / namespace that the process running the styxlisten command can see to the world on TCP protocols listening on port 6872.

This library should be enough to write scripts for what is quickly becoming my favorite X11 window manager (if you must use one) wmii. wmii actually exposes a bit of functionality via a 9P server that you can mount from v9fs, Inferno, or any other 9P client. It'd be nice to be able to write some Haskell to configure wmii I figured, so this might be my first use for this library.

I'm hoping to get back to this again before long. This was good progress. I'm going to check in on Tim and see if he's got any more updates he feels he'd like to push. If not I'll just work from this fork.

Wednesday, February 3, 2010


Iteratee really looks promising on paper, and people using it seem to think it's really great. I've been put off a bit by what looks like a rather complex interface, but decided last night to take a crack at it.

What I've written below is an iteratee wrapper around "cat /etc/passwd" output using runInteractiveCommand.

module Main where

import Control.Monad.Trans
import Data.Iteratee.Base
import Data.Iteratee.Base.StreamChunk (ReadableChunk (..))
import Data.Iteratee.IO.Handle
import System.Process
import System.IO

-- For some reason this signature is wrong, but I'm not sure why...
--handleDriver :: (MonadIO m, ReadableChunk s el) => IterateeG s el m a -> Handle -> m a
handleDriver iter h = do
result <- enumHandle h iter >>= run
liftIO $ hClose h
return result

main :: IO ()
main = do
(_, outp, _, _) <- runInteractiveCommand "/bin/cat /etc/passwd"
handleDriver (stream2list :: IterateeG [] Char IO String) outp >>= putStrLn

handleDriver just runs enumHandle with an iteratee (in my case stream2list) over the Handle, in blocks (not character by character) that are specified by the implementation, returning the result. That result is then printed to stdout by putStrLn.

This is a little bit like interact for Handle in that I could have used something more advanced than stream2list to process the result of "/bin/cat /etc/passwd".

I'm not too excited about the fact that I got the type signature on handleDriver wrong, and I'm also a little bit put off by the type signature on stream2list.

So what's the difference between this approach and an interact-like styled approach? For one iteratees work like the function that's supplied to a fold operation over a collection of data. In this case the collection is being produced dynamically via the enumerator. The iteratees themselves work like little parsers that can be composed in a monadic sense. Errors in IO and termination of a stream get propagated automatically up through the system nicely.

Why is lazy IO then not so great? Look at the signature for interact:

interact :: (String -> String) -> IO ()

Interact is a function that takes a function of String to String and produces IO. This means it, ostensibly consumes all the input on stdin, applies the provided function to convert the whole String to a String, then prints that string to stdout. Now it doesn't have to read the whole of input in one shot, because of lazy evaluation. Strings are [Char], and the list structure in Haskell is non-strict in its construction. It's like pausing the construction of that list to do some processing on it, and going back to it with coroutines, except that the system is doing it behind the scenes.

What happens in interact when an error occurs? How does the pure function of type (String -> String) even know about exceptions in the processing? This is where iteratee is an improvement on traditionally lazy IO.

Let's assume we wanted to write a lazy version of interact for a handle called hInteract.

hInteract :: Handle -> (String -> String) -> IO ()

I believe this function could be used safely as follows:

withFile "/etc/passwd" ReadMode ((flip hInteract) id)

withFile uses bracket internally to ensure that hClose is called on the handle and all seems well. I don't think we necessarily understand how resources get used. Oleg, the father of Iteratee, posts this message a few years back explaining more of the benefits of Iteratee.

However, it seems that there is now a new lazy IO mechanism available that is safe. I've not had any time to check into this, but I plan to in the next coming days.

Having written an Expect-like Monad, I'm interested in the aspects of error handling and precise resource control, because the code I'm writing really needs to be able to run to as close to forever as I can get.