output

       . |
|  .      
       . |
|  .      
       . |
|  .      
       . |
|  .      
       . |
|  .      
         | .
volley finished
@done

ping-pong.the.hs

-- starts a match of n volleys
volley: (n: Integer) = {
    -- start up the ponger
    proc = { notify |
        pong: notify
    } spawn-with: [self];

    -- start up the pinger
    { ping: n ponger: proc } spawn
} do;

pong: notify = {
    receive match: {
        -- volley is done; finish
        @finished -> {
            "         | ." write;
            "volley finished" write;
            notify ! @done
        } do;

        -- got a ping, send a pong
        @(ping: pid) -> {
            "       . |" write;
            pid ! @pong;
            pong: notify -- loop
        } do
    }
} do;

ping: 0 ponger: pid = pid ! @finished;
ping: n ponger: pid = {
    pid ! @(ping: self);

    -- got a pong; send a ping
    receive match: {
        @pong -> {
            "|  .      " write;
            ping: (n - 1) ponger: pid
        } do
    }
} do;

-- start up the match, doing 5 volleys
volley: 5;

-- wait for the match to complete
receive print;