rust/doc/tutorial/task.md

Tasks

Rust supports a system of lightweight tasks, similar to what is found in Erlang or other actor systems. Rust tasks communicate via messages and do not share data. However, it is possible to send data without copying it by making use of unique boxes (still, the data is owned by only one task at a time).

NOTE: As Rust evolves, we expect the Task API to grow and change somewhat. The tutorial documents the API as it exists today.

Spawning a task

Spawning a task is done using the various spawn functions in the module task. Let's begin with the simplest one, task::spawn(), and later move on to the others:

let some_value = 22;
let child_task = task::spawn {||
    std::io::println("This executes in the child task.");
    std::io::println(#fmt("%d", some_value));
};

The argument to task::spawn() is a unique closure of type fn~(), meaning that it takes no arguments and generates no return value. The effect of task::spawn() is to fire up a child task that will execute the closure in parallel with the creator. The result is a task id, here stored into the variable child_task.

Ports and channels

Now that we have spawned a child task, it would be nice if we could communicate with it. This is done by creating a port with an associated channel. A port is simply a location to receive messages of a particular type. A channel is used to send messages to a port. For example, imagine we wish to perform two expensive computations in parallel. We might write something like:

let port = comm::port::<int>();
let chan = comm::chan::<int>(port);
let child_task = task::spawn {||
    let result = some_expensive_computation();
    comm::send(chan, result);
};
some_other_expensive_computation();
let result = comm::recv(port);

Let's walk through this code line-by-line. The first line creates a port for receiving integers:

let port = comm::port::<int>();

This port is where we will receive the message from the child task once it is complete. The second line creates a channel for sending integers to the port port:

let chan = comm::chan::<int>(port);

The channel will be used by the child to send a message to the port. The next statement actually spawns the child:

let child_task = task::spawn {||
    let result = some_expensive_computation();
    comm::send(chan, result);
};

This child will perform the expensive computation send the result over the channel. Finally, the parent continues by performing some other expensive computation and then waiting for the child's result to arrive on the port:

some_other_expensive_computation();
let result = comm::recv(port);