Haskell类型类和多个文件

Question

Suppose I have a simple TCP service which, upon accepting a connection, forks a new thread and calls initializeClient clientSocket .

initializeClient looks like this:

initializeClient clientSocket = do
    clientDataTVar <- atomically $ newTVar (UnregisteredClientData clientSocket)
    _ <- forkIO $ clientSocketReadLoop clientDataTVar

For simplicity, let's suppose there are only 2 possible instances in clientDataTVar:

data UnregisteredClientData = UnregisteredClientData {
  ucdSocket :: Socket
}

data CustomerClientData = CustomerClientData {
  ccdSocket :: Socket,
  ccdName :: Text
}

When an Unregistered client sends a message containing their name, their ClientDataTVar value gets replaced:

writeTVar unregisteredClientDataTVar $ CustomerClientData clientSocket theirName

Edit : The above writeTVar line won't typecheck, because the type of the TVar is TVar UnregisteredClientData . CustomerClientData cannot be written into it, but I'm leaving it here to illustrate what I'm trying to achieve.

at which point, the handler for the command returns and clientSocketReadLoop runs again.

clientSocketReadLoop is only interested in the Socket, so I could do the following:

class ClientData a where
  cdSocket :: a -> Socket

instance ClientData UnregisteredClientData where
  cdSocket = ucdSocket

instance ClientData CustomerClientData where
  cdSocket = ccdSocket

Now, when clientSocketReadLoop reads clientDataTVar, it can call cdSocket clientData and retrieve the Socket, regardless of the underlying type being UnregisteredClientData or CustomerClientData. After it does the recv, it needs to call a handler. Exactly which handler to call depends on the type of user (Unregistered or Customer).

I believe this can be achieved by adding handleMessage to the class and instances:

class ClientData a where
  cdSocket :: a -> Socket
  handleMessage :: a -> String -> IO ()

instance ClientData UnregisteredClientData where
  cdSocket = ucdSocket
  handleMessage unregisteredClientDataTVar msg = (implementation)

instance ClientData CustomerClientData where
  cdSocket = ccdSocket
  handleMessage customerClientDataTVar msg = (implementation)

Thus, the appropriate handler is called.

What I'd like to know is:

Is this a good way to do it? I'd imagine it's a fairly standard scenario, although I use a TVar for ClientData while others might just pass the Socket around. This shouldn't change much.

If it is a good way, is it possible to have the class/instance definitions in one file (Types.hs) and implement the handlers in another file (Client.hs)?

Thanks in advance!

Answer 1

I don't think I would use a type class here. When you have a small and limited set of cases you wish to work on, it's much simpler to just use an algebraic data type.

For example,

data ClientData = ClientData { cdSocket :: Socket, cdName :: Maybe Text }

or if you want something more explicit than a Maybe ,

type Name = Text
data UserInfo = Unregistered | Customer Name
data ClientData = ClientData { cdSocket :: Socket, cdUserInfo :: UserInfo }

You can then just use pattern matching to handle the different cases:

handleMessage clientData msg =
  case cdUserInfo clientData of
    Unregistered  -> -- implementation
    Customer name -> -- implementation