For a Haskell programmer, and day where a big thing is implemented without the least scrap of code that touches the IO monad is a good day. And this was a good day for me!

Implemented the p2p protocol for tor hidden services. Its needs are somewhat similar to the external special remote protocol, but the two protocols are not fully overlapping with one-another. Rather than try to unify them, and so complicate both cases, I prefer to reuse as much code as possible between separate protocol implementations. The generating and parsing of messages is largely shared between them. I let the new p2p protocol otherwise develop in its own direction.

But, I do want to make this p2p protocol reusable for other types of p2p networks than tor hidden services. This was an opportunity to use the Free monad, which I'd never used before. It worked out great, letting me write monadic code to handle requests and responses in the protocol, that reads the content of files and resumes transfers and so on, all independent of any concrete implementation.

The whole implementation of the protocol only needed 74 lines of monadic code. It helped that I was able to factor out functions like this one, that is used both for handling a download, and by the remote when an upload is sent to it:

receiveContent :: Key -> Offset -> Len -> Proto Bool
receiveContent key offset len = do
        content <- receiveBytes len
        ok <- writeKeyFile key offset content
        sendMessage $ if ok then SUCCESS else FAILURE
        return ok

To get transcripts of the protocol in action, the Free monad can be evaluated purely, providing the other side of the conversation:

ghci> putStrLn $ protoDump $ runPure (put (fromJust $ file2key "WORM--foo")) [PUT_FROM (Offset 10), SUCCESS]
> PUT WORM--foo
> DATA 90
> bytes
result: True

ghci> putStrLn $ protoDump $ runPure (serve (toUUID "myuuid")) [GET (Offset 0) (fromJust $ file2key "WORM--foo")]
< GET 0 WORM--foo
> PROTO-ERROR must AUTH first
result: ()

Am very happy with all this pure code and that I'm finally using Free monads. Next I need to get down the the dirty business of wiring this up to actual IO actions, and an actual network connection.

Today's work was sponsored by Jake Vosloo on Patreon.