Rocket/site/guide/5-responses.md

Responses

You may have noticed that the return type of a handler appears to be arbitrary, and that's because it is! A value of any type that implements the Responder trait can be returned, including your own. In this section, we describe the Responder trait as well as several useful Responders provided by Rocket. We'll also briefly discuss how to implement your own Responder.

Responder

Types that implement Responder know how to generate a Response from their values. A Response includes an HTTP status, headers, and body. The body may either be fixed-sized or streaming. The given Responder implementation decides which to use. For instance, String uses a fixed-sized body, while File uses a streamed response. Responders may dynamically adjust their responses according to the incoming Request they are responding to.

Wrapping

Before we describe a few responders, we note that it is typical for responders to wrap other responders. That is, responders can be of the following form, where R is some type that implements Responder:

struct WrappingResponder<R>(R);

A wrapping responder modifies the response returned by R before responding with that same response. For instance, Rocket provides Responders in the status module that override the status code of the wrapped Responder. As an example, the Accepted type sets the status to 202 - Accepted. It can be used as follows:

use rocket::response::status;

#[post("/<id>")]
fn new(id: usize) -> status::Accepted<String> {
    status::Accepted(Some(format!("id: '{}'", id)))
}

Similarly, the types in the content module can be used to override the Content-Type of a response. For instance, to set the Content-Type of &'static str to JSON, you can use the content::Json type as follows:

use rocket::response::content;

#[get("/")]
fn json() -> content::Json<&'static str> {
    content::Json("{ 'hi': 'world' }")
}

Errors

Responders may fail; they need not always generate a response. Instead, they can return an Err with a given status code. When this happens, Rocket forwards the request to the error catcher for the given status code.

If an error catcher has been registered for the given status code, Rocket will invoke it. The catcher creates and returns a response to the client. If no error catcher has been registered and the error status code is one of the standard HTTP status code, a default error catcher will be used. Default error catchers return an HTML page with the status code and description. If there is no catcher for a custom status code, Rocket uses the 500 error catcher to return a response.

While not encouraged, you can also forward a request to a catcher manually by using the Failure type. For instance, to forward to the catcher for 406 - Not Acceptable, you would write:

#[get("/")]
fn just_fail() -> Failure {
    Failure(Status::NotAcceptable)
}

Implementations

Rocket implements Responder for many types in Rust's standard library including String, &str, File, Option, and Result. The Responder documentation describes these in detail, but we briefly cover a few here.

Strings

The Responder implementations for &str and String are straight-forward: the string is used as a sized body, and the Content-Type of the response is set to text/plain. To get a taste for what such a Responder implementation looks like, here's the implementation for String:

impl Responder<'static> for String {
    fn respond_to(self, _: &Request) -> Result<Response<'static>, Status> {
        Response::build()
            .header(ContentType::Plain)
            .sized_body(Cursor::new(self))
            .ok()
    }
}

Because of these implementations, you can directly return an &str or String type from a handler:

#[get("/string")]
fn handler() -> &'static str {
    "Hello there! I'm a string!"
}

Option

Option is a wrapping responder: an Option<T> can only be returned when T implements Responder. If the Option is Some, the wrapped responder is used to respond to the client. Otherwise, a error of 404 - Not Found is returned to the client.

This implementation makes Option a convenient type to return when it is not known until process-time whether content exists. For example, because of Option, we can implement a file server that returns a 200 when a file is found and a 404 when a file is not found in just 4, idiomatic lines:

#[get("/<file..>")]
fn files(file: PathBuf) -> Option<NamedFile> {
    NamedFile::open(Path::new("static/").join(file)).ok()
}

Result

Result is a special kind of wrapping responder: its functionality depends on whether the error type E implements Responder.

When the error type E implements Responder, the wrapped Responder in Ok or Err, whichever it might be, is used to respond to the client. This means that the responder can be chosen dynamically at run-time, and two different kinds of responses can be used depending on the circumstances. Revisiting our file server, for instance, we might wish to provide more feedback to the user when a file isn't found. We might do this as follows:

use rocket::response::status::NotFound;

#[get("/<file..>")]
fn files(file: PathBuf) -> Result<NamedFile, NotFound<String>> {
    let path = Path::new("static/").join(file);
    NamedFile::open(&path).map_err(|_| NotFound(format!("Bad path: {}", path)))
}

If the error type E does not implement Responder, then the error is simply logged to the console, using its Debug implementation, and a 500 error is returned to the client.

Rocket Responders

Some of Rocket's best features are implemented through responders. You can find many of these responders in the response module. Among these are:

• Content - Used to override the Content-Type of a response.
• NamedFile - Streams a file to the client; automatically sets the Content-Type based on the file's extension.
• Redirect - Redirects the client to a different URI.
• Stream - Streams a response to a client from an arbitrary Reader type.
• status - Contains types that override the status code of a response.
• Flash - Sets a "flash" cookie that is removed when accessed.

Streaming

The Stream type deserves special attention. When a large amount of data needs to be sent to the client, it is better to stream the data to the client to avoid consuming large amounts of memory. Rocket provides the Stream type, making this easy. The Stream type can be created from any Read type. For example, to stream from a local Unix stream, we might write:

#[get("/stream")]
fn stream() -> io::Result<Stream<UnixStream>> {
    UnixStream::connect("/path/to/my/socket").map(|s| Stream::from(s))
}

JSON

The JSON responder in rocket_contrib allows you to easily respond with well-formed JSON data: simply return a value of type Json<T> where T is the type of a structure to serialize into JSON. The type T must implement the Serialize trait from serde, which can be automatically derived.

As an example, to respond with the JSON value of a Task structure, we might write:

use rocket_contrib::Json;

#[derive(Serialize)]
struct Task { ... }

#[get("/todo")]
fn todo() -> Json<Task> { ... }

The JSON type serializes the structure into JSON, sets the Content-Type to JSON, and emits the serialized data in a fixed-sized body. If serialization fails, a 500 - Internal Server Error is returned.

The JSON example on GitHub provides further illustration.

Templates

Rocket includes built-in templating support that works largely through a Template responder in rocket_contrib. To render a template named "index", for instance, you might return a value of type Template as follows:

#[get("/")]
fn index() -> Template {
    let context = /* object-like value */;
    Template::render("index", &context)
}

Templates are rendered with the render method. The method takes in the name of a template and a context to render the template with. The context can be any type that implements Serialize and serializes into an Object value, such as structs, HashMaps, and others.

Rocket searches for a template with the given name in the configurable template_dir directory. Templating support in Rocket is engine agnostic. The engine used to render a template depends on the template file's extension. For example, if a file ends with .hbs, Handlebars is used, while if a file ends with .tera, Tera is used.

When your application is compiled in debug mode (without the --release flag passed to cargo), templates are automatically reloaded when they are modified. This means that you don't need to rebuild your application to observe template changes: simply refresh! In release builds, reloading is disabled.

For templates to be properly registered, the template fairing must be attached to the instance of Rocket. The Fairings sections of the guide provides more information on fairings. To attach the template fairing, simply call .attach(Template::fairing()) on an instance of Rocket as follows:

fn main() {
    rocket::ignite()
      .mount("/", routes![...])
      .attach(Template::fairing());
}

The Template API documentation contains more information about templates, including how to customize a template engine to add custom helpers and filters. The Handlebars Templates example on GitHub is a fully composed application that makes use of Handlebars templates.