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Hi Scott,
Thanks for writing the Haskell driver for MongoDB! It functions well but I basically rewrote it in an attempt to factor it nicely and support additional functionality like multiple threads using the same connection. I hope you like it! You can find it on my fork of your repository at http://github.com/TonyGen/mongoDB.
First, I separated out BSON into its own package, since it can be used as an interchange format independent of MongoDB. You can find this new package on Github at http://github.com/TonyGen/bson-haskell and on Hackage at http://hackage.haskell.org/package/bson. I also made the BSON easier to write and view. For example, you can write: ["a" =: 1, "b" =: "hello", "c" =: [1,2,3]], and it shows as: [a: 1, b: "hello", c: [1,2,3]].
Second, for modularity, I separated MongoDB into 5 modules: MongoDB-Internal-Connection, MongoDB-Internal-Protocol, MongoDB-Connection, MongoDB-Query, and MongoDB-Admin.
MongoDB-Internal-Connection defines a connection with multi-threaded support via two monads, one with shared access to a connection (Task), and one with exclusive access to a connection (Op). This module also exposes low-level writing and reading bytes inside the Op monad for MongoDB-Internal-Protocol to use. This module is not intended for the application-programmer use, and maybe should be a hidden module inside cabal, but for now it is not.
MongoDB-Internal-Protocol defines the MongoDB Wire Protocol (http://www.mongodb.org/display/DOCS/Mongo+Wire+Protocol). It defines the messages the the client and server send back and forth to each other. Again, this module is not intended for the application-programmer use, and maybe should be a hidden module inside cabal, but for now it is not.
MongoDB-Connection re-exports Connection, and Task and Op monads from MongoDB-Internal-Connection but without the low-level read and write bytes functions. It also adds support for replica-sets, which will replace replica-pairs in the next release of MongoDB coming out soon. I had to make two connection modules (MongoDB-Internal-Connection and MongoDB-Connection) because connecting to a replica set requires quering its config info, which requires us to import MongoDB-Query, which recursively imports MongoDB-Internal-Protocol then MongoDB-Internal-Connection. I could have used mutual dependent modules (via .hs-boot) but felt that violated the layered approach I was going for.
MongoDB-Query defines all the normal query and update operations like find, findOne, count, insert, modify, delete, group, mapReduce, allDatabases, allCollections, runCommand, etc.
MongoDB-Admin defines all the administration operations like validateCollection, ensureIndex, dropIndex, addUser, copyDatabase, dbStats, setProfilingLevel, etc.
Finally, the top-level MongoDB module simply re-exports MongoDB-Connection, MongoDB-Query, and MongoDB-Admin, along with Data.Bson from the bson package.
I updated your TODO list, removing items I completed, added items that were missing, and added back items I removed from the code like lazy list from a cursor (I am skeptical of lazy I/O, but we could add it back).
I also update your two tutorials to reflect this new API.
I hope you like these changes! Let me know your feedback, and I hope we can both maintain it in the future.
Cheers, Tony Hannan 10gen Inc. Creators of MongoDB