I slightly refactored my previous version I sent you. Here is the description again of my version as compared to your version. You can disregard my previous version and just read this.
When evaluating your package, I wanted to make the BSON easier to write and read, and I wanted to make use of monads so you don't have to supply the connection and database every time. Furthermore, I wanted to modularize the code a little more, for example, separating admin function from normal query/update functions. Finally, I wanted to add more functionality, like support for replica sets and multiple threads per connection. The end result was a significant rewrite of your package. 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, I created two independent helper modules: Control.Monad.Context and Control.Pipeline.
Control.Monad.Context is just like Control.Monad.Reader.Class except you can access the context of any Reader in the monad stack instead of just the top one as long as the context types are different.
Control.Pipeline gives thread-safe and pipelined access to a socket. When you make a call it sends the request and immediately returns a "promise" of the reply without waiting for the reply. When you read the promise it waits for the reply if it has not already arrived then returns it.
Third, I separated MongoDB into 4 modules: MongoDB.Internal.Protocol, MongoDB.Connection, MongoDB.Query, and MongoDB.Admin.
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 allows you to create a pipelined connection to a specific server or to a master/slave in a replica set.
MongoDB-Query defines the "access" monad that has the current connection and database in context, and all the normal query and update operations you execute within this monad 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