Environment object, needed for all the operations.

Transaction instance. Needed for all operations with data, even reading.

Database instance. Represents a logical table in the database.

Binary blob representing a key or value in the database.

UNIX permissions to set on created files. Zero value means to open existing, but do not create.

Flags for opening and environment.

Flags for a transaction.

Flags for a database.

Flags for all data related operations.

Flags for cursor operations.

Converts an instance to/from the representation needed by libmdbx. This type is used for both keys and values.

Only the Text instance is provided, since it is commonly used as the key when storing/retrieving a value.

For your own types, in general, you will want to use a serialization library such as store, cereal, etc, and apply the newtype deriving via trick.

The Store instance can be defined as:

newtype MdbxItemStore a = MdbxItemStore {
  unwrapStore :: a
}

instance Store a => MdbxItem (MdbxItemStore a) where
  fromMdbxVal item = MdbxItemStore $ fromMdbxStore item
  toMdbxVal item = withMdbxStore (unwrapStore item)

fromMdbxStore :: Store v => MdbxVal -> IO v
fromMdbxStore (MdbxVal size ptr) = do
  bs <- unsafePackCStringLen (castPtr ptr, fromIntegral size)
  decodeIO bs

withMdbxStore :: Store v => v -> (MdbxVal -> IO a) -> IO a
withMdbxStore val fn =
  unsafeUseAsCStringLen bsV $ (ptrV, sizeV) -> do
    let mval = MdbxVal (fromIntegral sizeV) (castPtr ptrV)
    fn mval
  where
    bsV = encode val

This code can be adaptad to other serialization libraries. It is not provided as part of libmdbx-hs itself to avoid forcing dependencies.

Then, to derive the instance for your owwn type:

data User = User {
  _username :: Text,
  _password :: Text
} deriving (Eq, Show, Generic, Store)

deriving via (MdbxItemStore User) instance MdbxItem User

Note: if you plan on using a custom type as the key, be careful if it contains Text or ByteString instances, since these types have a length field which is, in general, before the data. This causes issues when using cursors, since they depend on key ordering and the length field will make shorter instances lower than longer ones, even if the content indicates the opposite. In general, it is simpler to use Text as the key.