The builder pattern

Some data structures are complicated to construct, due to their construction needing:

which can easily lead to a large number of distinct constructors with many arguments each.

If T is such a data structure, consider introducing a T builder:

  1. Introduce a separate data type TBuilder for incrementally configuring a T value. When possible, choose a better name: e.g. Command is the builder for Process.
  2. The builder constructor should take as parameters only the data required to to make a T.
  3. The builder should offer a suite of convenient methods for configuration, including setting up compound inputs (like slices) incrementally. These methods should return self to allow chaining.
  4. The builder should provide one or more "_terminal_" methods for actually building a T.

The builder pattern is especially appropriate when building a T involves side effects, such as spawning a task or launching a process.

In Rust, there are two variants of the builder pattern, differing in the treatment of ownership, as described below.

Non-consuming builders (preferred):

In some cases, constructing the final T does not require the builder itself to be consumed. The follow variant on std::io::process::Command is one example:

// NOTE: the actual Command API does not use owned Strings;
// this is a simplified version.

pub struct Command {
    program: String,
    args: Vec<String>,
    cwd: Option<String>,
    // etc

impl Command {
    pub fn new(program: String) -> Command {
        Command {
            program: program,
            args: Vec::new(),
            cwd: None,

    /// Add an argument to pass to the program.
    pub fn arg<'a>(&'a mut self, arg: String) -> &'a mut Command {

    /// Add multiple arguments to pass to the program.
    pub fn args<'a>(&'a mut self, args: &[String])
                    -> &'a mut Command {

    /// Set the working directory for the child process.
    pub fn cwd<'a>(&'a mut self, dir: String) -> &'a mut Command {
        self.cwd = Some(dir);

    /// Executes the command as a child process, which is returned.
    pub fn spawn(&self) -> IoResult<Process> {

Note that the spawn method, which actually uses the builder configuration to spawn a process, takes the builder by immutable reference. This is possible because spawning the process does not require ownership of the configuration data.

Because the terminal spawn method only needs a reference, the configuration methods take and return a mutable borrow of self.

The benefit

By using borrows throughout, Command can be used conveniently for both one-liner and more complex constructions:

// One-liners

// Complex configuration
let mut cmd = Command::new("/bin/ls");

if size_sorted {


Consuming builders:

Sometimes builders must transfer ownership when constructing the final type T, meaning that the terminal methods must take self rather than &self:

// A simplified excerpt from std::task::TaskBuilder

impl TaskBuilder {
    /// Name the task-to-be. Currently the name is used for identification
    /// only in failure messages.
    pub fn named(mut self, name: String) -> TaskBuilder { = Some(name);

    /// Redirect task-local stdout.
    pub fn stdout(mut self, stdout: Box<Writer + Send>) -> TaskBuilder {
        self.stdout = Some(stdout);
        //   ^~~~~~ this is owned and cannot be cloned/re-used

    /// Creates and executes a new child task.
    pub fn spawn(self, f: proc():Send) {
        // consume self

Here, the stdout configuration involves passing ownership of a Writer, which must be transferred to the task upon construction (in spawn).

When the terminal methods of the builder require ownership, there is a basic tradeoff:

Under the rubric of making easy things easy and hard things possible, all builder methods for a consuming builder should take and returned an owned self. Then client code works as follows:

// One-liners
TaskBuilder::new().named("my_task").spawn(proc() { ... });

// Complex configuration
let mut task = TaskBuilder::new();
task = task.named("my_task_2"); // must re-assign to retain ownership

if reroute {
    task = task.stdout(mywriter);

task.spawn(proc() { ... });

One-liners work as before, because ownership is threaded through each of the builder methods until being consumed by spawn. Complex configuration, however, is more verbose: it requires re-assigning the builder at each step.