A library for reading and writing CDB (Constant Database) files.

CDB files are immutable key-value stores, designed for extremely fast and memory-efficient construction and lookup. They can be as large as 4GB, and at no point in their construction or use must all data be loaded into memory. CDB files can contain multiple values for a given key.

For more information on the CDB file format, please see: http://cr.yp.to/cdb.html

Using hs-cdb should be fairly straightforward. Here's a simple example:

   printStuff :: IO ()
   printStuff = do
     cdb <- cdbInit "my.cdb"
     let foo = cdbGet cdb "foo"
     let bars = cdbGetAll cdb "bar"
     maybe (putStrLn "Not found") putStrLn foo
     mapM_ putStrLn bars

The CDB will be automatically cleaned up by the garbage collector after use.

The only sticking point may be the use of the Packable and Unpackable classes. This allows the hs-cdb interface to be both generic (so your CDB can store effectively any kind of data) but also convenient in the common case of plaintext data. Internally, hs-cdb uses ByteStrings, but it will automatically pack and unpack keys and values to suit the types you're using in your program. In particular, in an instance is provided for String, so hs-cdb can use Strings as keys and values transparently.

Writing a CDB is just as straightforward:

   makeCDB :: IO ()
   makeCDB = cdbMake "my.cdb" $ do
     cdbAdd "foo" "this is the data associated with foo"
     cdbAddMany [("bar1", "bar1data"), ("bar2", "bar2data")]

Again, hs-cdb automatically closes the files after use. Moreover, in CDB tradition, hs-cdb actually creates a CDB named file.cdb by first writing it to file.cdb.tmp, and then atomically renaming it over file.cdb. This means that readers never need to pause when you're regenerating a CDB.

Note that the CDBMake monad is nothing more than a State wrapper around the IO monad, so you can use IO commands with liftIO from Control.Monad.State.