Over the last week or so, I’ve been playing around with some of the minor details of jlouis’ Haskell-Torrent code. The main pieces I’ve been playing with have been the more binary centric pieces: the torrent file en/decoding and since Wednesday, the wire protocol.

After trying to compile the code using GHC 6.12.1, I noticed that a rather strange dependency: HCodecs. Investigating further, I realised that all that was being used was its fairly nice binary builder and parser, and was being used in the parsing and construction of bencoded files. This seemed to me to be the domain of one of my favourite packages, binary. I also noticed that when trying to parse the sample torrent file jlouis had provided, there was binary data that could not be represented in a String type, because there were byte sequences that could not be parses into Char’s.

So I went off to try and implement the BCode module using Data.Binary. The encoding was really simple and straight forward, but parsing was another matter. Binary has no real error handeling, which meant that decoding things like lists where you keep adding elements to the list until you reach an ‘e’, was made quite verbose due to this fact. After implementing a (semi) working module, I went in search of a more parsec like interface to binary parsing. This is when someone on #Haskell informed me of cereal, a package based on binary, which added an Alternative instance, and an Either String a return type for better error reporting.

I fell in love. This was exactly what I was after, I could create my own combinators like many and many1, with the same ease I could with parsec. Using cereal, I was able to make a really clear, concise parser, which should also be quite efficient.

I will post some of this code at a later time, when i’m not writing on my iPhone, and show you just what I mean. I’m also considering releasing the BCode module as a package of it’s own, if anyone else feels it might me useful to them. Either that, or I might write up a module of useful combinators for use with cereal.

Until next time, happy new year and I hope this year treats you all better than the last one did.

– Al