Rocket/site/guide/responses.md

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# Responses
You may have noticed that the return type of a handler appears to be arbitrary,
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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 `Responder`s provided by Rocket.
We'll also briefly discuss how to implement your own `Responder`.
[`Responder`]: https://api.rocket.rs/rocket/response/trait.Responder.html
## Responder
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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.
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[`Response`]: https://api.rocket.rs/rocket/response/struct.Response.html
### Wrapping
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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`:
```rust
struct WrappingResponder<R>(R);
```
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A wrapping responder modifies the response returned by `R` before responding
with that same response. For instance, Rocket provides `Responder`s in the
[`status` module](https://api.rocket.rs/rocket/response/status/index.html) 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:
```rust
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use rocket::response::status;
#[post("/<id>")]
fn new(id: usize) -> status::Accepted<String> {
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status::Accepted(Some(format!("id: '{}'", id)))
}
```
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Similarly, the types in the [`content`
module](https://api.rocket.rs/rocket/response/content/index.html) can be used to
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override the Content-Type of a response. For instance, to set the Content-Type
an `&'static str` to JSON, you can use the [`content::Json`] type as follows:
```rust
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use rocket::response::content;
#[get("/")]
fn json() -> content::Json<&'static str> {
content::Json("{ 'hi': 'world' }")
}
```
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[`Accepted`]: https://api.rocket.rs/rocket/response/status/struct.Accepted.html
[`content::Json`]: https://api.rocket.rs/rocket/response/content/struct.Json.html
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### 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](/guide/requests/#error-catchers) 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
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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
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using the [`Failure`](https://api.rocket.rs/rocket/response/struct.Failure.html)
type. For instance, to forward to the catcher for **406 - Not Acceptable**, you
would write:
```rust
#[get("/")]
fn just_fail() -> Failure {
Failure(Status::NotAcceptable)
}
```
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## Implementations
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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.
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### Strings
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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`:
```rust
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:
```rust
#[get("/string")]
fn handler() -> &'static str {
"Hello there! I'm a string!"
}
```
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### `Option`
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`Option` is _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:
```rust
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#[get("/<file..>")]
fn files(file: PathBuf) -> Option<NamedFile> {
NamedFile::open(Path::new("static/").join(file)).ok()
}
```
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### `Result`
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`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. Revisting 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:
```rust
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 `Read`er type.
* [`status`] - Contains types that override the status code of a response.
* [`Flash`] - Sets a "flash" cookie that is removed when accessed.
[`status`]: https://api.rocket.rs/rocket/response/status/index.html
[`response`]: https://api.rocket.rs/rocket/response/index.html
[`NamedFile`]: https://api.rocket.rs/rocket/response/struct.NamedFile.html
[`Content`]: https://api.rocket.rs/rocket/response/struct.Content.html
[`Redirect`]: https://api.rocket.rs/rocket/response/struct.Redirect.html
[`Stream`]: https://api.rocket.rs/rocket/response/struct.Stream.html
[`Flash`]: https://api.rocket.rs/rocket/response/struct.Flash.html
### 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
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consuming large amounts of memory. Rocket provides the [`Stream`] type, making
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this easy. The `Stream` type can be created from any `Read` type. For example,
to stream from a local Unix stream, we might write:
```rust
#[get("/stream")]
fn stream() -> io::Result<Stream<UnixStream>> {
UnixStream::connect("/path/to/my/socket").map(|s| Stream::from(s))
}
```
[`rocket_contrib`]: https://api.rocket.rs/rocket_contrib/index.html
### 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
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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
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write:
```rust
use rocket_contrib::Json;
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#[derive(Serialize)]
struct Task { ... }
#[get("/todo")]
fn todo() -> Json<Task> { ... }
```
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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.
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The [JSON example on GitHub] provides further illustration.
[`JSON`]: https://api.rocket.rs/rocket_contrib/struct.Json.html
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[`Serialize`]: https://docs.serde.rs/serde/trait.Serialize.html
[`serde`]: https://docs.serde.rs/serde/
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[JSON example on GitHub]: https://github.com/SergioBenitez/Rocket/tree/v0.4.0-dev/examples/json
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### 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:
```rust
#[get("/")]
fn index() -> Template {
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let context = /* object-like value */;
Template::render("index", &context)
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}
```
Templates are rendered with the `render` method. The method takes in the name of
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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.
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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.
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For templates to be properly registered, the template fairing must be attached
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to the instance of Rocket. The [Fairings](/guide/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:
```rust
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fn main() {
rocket::ignite()
.mount("/", routes![...])
.attach(Template::fairing());
}
```
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The [`Template`] API
documentation contains more information about templates, while the [Handlebars
Templates example on
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GitHub](https://github.com/SergioBenitez/Rocket/tree/v0.4.0-dev/examples/handlebars_templates)
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is a fully composed application that makes use of Handlebars templates.
[`Template`]: https://api.rocket.rs/rocket_contrib/struct.Template.html