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tinybcast

This is a small haskell program implementing a udp broadcast chat client.

bcast

usage

  • You can run with..
    • bcast username to broadcast on a LAN, or..
    • bcast username listenHost listenPort [destHost destPort ...] to specify alternate destination addresses.
  • Type words, use the left, right, home, end keys to move the cursor.
  • Press enter to send.
  • Press escape to quit. (/quit isn't a command, just for show)

architecture

threads

There are five pieces of state (boxes), and five threads (colored arrows).

state (boxes)

  1. A vty is used to get events from the user and display the UI.
  2. A TVar appState is the content which is displayed in the UI.
  3. A TChan netOutbox is the outgoing event buffer.
  4. A TChan netInbox is the incoming event buffer.
  5. A socket is used to send and receive on the network.

threads (colored arrows)

  1. frontendInput: Block until a vty event, apply the event to appState, optionally emit to netOutbox.
    • Not all vty events require a network event. When typing into the buffer, events are handled by applying to appState directly so the UI can update. When you press the "Enter" key, the buffer is cleared and a network event is emitted.
  2. fontendDisplay: Render appState to vty and then block until appState changes.
  3. backendSend: Block until a netOutbox event, broadcast it to the network (and also copy it over to netInbox).
    • We copy events directly from the outbox to the inbox because the user shouldn't have to wait for the network to get feedback from the UI. If you're implementing a message-delivery protocol, then you might be required to order outgoing messages, and so this behavior might be incorrect.
  4. backendReceive: Block until the network receives a packet, deserialize it and emit to netInbox.
  5. protocolReordering: Block until a netInbox event and apply it to appState.
    • This is kind of a silly thread that doesn't do very much but shuffle things between references. It's somewhat of a placeholder for if you were implementing a message-delivery protocol. Much of the time you'd need to reorder and delay received messages before delivering them to the application. This thread might be where that code would go?

dependencies

  • vty provides the UI.
  • network provides the sockets.
  • async provides threads.
  • stm provides mutable references which can be accessed atomically.