Non Blocking Channel Operations in Standard ML

In Standard ML, we can implement non-blocking operations using pattern matching and exceptions. While Standard ML doesn’t have built-in channels like Go, we can simulate similar behavior using refs and exceptions.

exception Empty

fun nonBlockingReceive queue = 
    case !queue of
        [] => NONE
      | x::xs => (queue := xs; SOME x)

fun nonBlockingSend queue value =
    queue := value :: !queue

fun main() =
    let
        val messages = ref ([] : string list)
        val signals = ref ([] : bool list)
    in
        (* Non-blocking receive *)
        case nonBlockingReceive messages of
            SOME msg => print("received message " ^ msg ^ "\n")
          | NONE => print("no message received\n");

        (* Non-blocking send *)
        let
            val msg = "hi"
        in
            (nonBlockingSend messages msg;
             print("sent message " ^ msg ^ "\n"))
            handle _ => print("no message sent\n")
        end;

        (* Multi-way non-blocking select *)
        case (nonBlockingReceive messages, nonBlockingReceive signals) of
            (SOME msg, _) => print("received message " ^ msg ^ "\n")
          | (_, SOME sig) => print("received signal " ^ (if sig then "true" else "false") ^ "\n")
          | (NONE, NONE) => print("no activity\n")
    end

val _ = main()

In this Standard ML implementation:

1. We define an Empty exception to simulate a channel that can’t be read from or written to.

2. The nonBlockingReceive function attempts to read from a queue (simulating a channel). If the queue is empty, it returns NONE, otherwise it returns SOME value.

3. The nonBlockingSend function adds a value to a queue (simulating sending to a channel).

4. In the main function, we create two refs to lists to simulate channels: messages for strings and signals for booleans.

5. We implement a non-blocking receive using pattern matching on the result of nonBlockingReceive.

6. For the non-blocking send, we use exception handling. If the send fails, it will raise an exception which we catch and handle.

7. The multi-way non-blocking select is implemented using pattern matching on tuples of nonBlockingReceive results.

To run this program, save it to a file (e.g., non_blocking_operations.sml) and use your Standard ML interpreter:

$ sml non_blocking_operations.sml
no message received
sent message hi
no activity

This example demonstrates how to implement non-blocking operations in Standard ML, simulating behavior similar to Go’s channels and select statements.