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 `Responder`s provided by Rocket.
We'll also briefly discuss how to implement your own `Responder`.

[`Responder`]: @api/rocket/response/trait.Responder.html

## 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.

[`Response`]: @api/rocket/response/struct.Response.html

### 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`:

```rust
struct WrappingResponder<R>(R);
```

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](@api/rocket/response/status/) 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
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](@api/rocket/response/content/)
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:

```rust
use rocket::response::content;

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

! warning: This is _not_ the same as the [`Json`] in [`rocket_contrib`]!

[`Accepted`]: @api/rocket/response/status/struct.Accepted.html
[`content::Json`]: @api/rocket/response/content/struct.Json.html

### 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](../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
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.

### Status

While not encouraged, you can also forward a request to a catcher manually by
returning a [`Status`] directly. For instance, to forward to the catcher for
**406: Not Acceptable**, you would write:

```rust
use rocket::http::Status;

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

The response generated by `Status` depends on the status code itself. As
indicated above, for error status codes (in range [400, 599]), `Status` forwards
to the corresponding error catcher. The table below summarizes responses
generated by `Status` for these and other codes:

| Status Code Range | Response                              |
|-------------------|---------------------------------------|
| [400, 599]        | Forwards to catcher for given status. |
| 100, [200, 205]   | Empty with given status.              |
| All others.       | Invalid. Errors to `500` catcher.     |

[`Status`]: https://api.rocket.rs/v0.5/rocket/http/struct.Status.html

## Custom Responders

The [`Responder`] trait documentation details how to implement your own custom
responders by explicitly implementing the trait. For most use cases, however,
Rocket makes it possible to automatically derive an implementation of
`Responder`. In particular, if your custom responder wraps an existing
responder, headers, or sets a custom status or content-type, `Responder` can be
automatically derived:

```rust
#[derive(Responder)]
#[response(status = 500, content_type = "json")]
struct MyResponder {
    inner: OtherResponder,
    header: SomeHeader,
    more: YetAnotherHeader,
    #[response(ignore)]
    unrelated: MyType,
}
```

For the example above, Rocket generates a `Responder` implementation that:

  * Set the response's status to `500: Internal Server Error`.
  * Sets the Content-Type to `application/json`.
  * Adds the headers `self.header` and `self.more` to the response.
  * Completes the response using `self.inner`.

Note that the _first_ field is used as the inner responder while all remaining
fields (unless ignored with `#[response(ignore)]`) are added as headers to the
response. The optional `#[response]` attribute can be used to customize the
status and content-type of the response. Because `ContentType` and `Status` are
themselves headers, you can also dynamically set the content-type and status by
simply including fields of these types.

For more on using the `Responder` derive, see the [`Responder` derive]
documentation.

[`Responder` derive]: @api/rocket_codegen/derive.Responder.html

## 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`:

```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!"
}
```

### `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:

```rust
#[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:

```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 and [`rocket_contrib`]
library. 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.
  * [`Json`] - Automatically serializes values into JSON.
  * [`MsgPack`] - Automatically serializes values into MessagePack.
  * [`Template`] - Renders a dynamic template using handlebars or Tera.
  * [`Compress`] - Compresses a response at the HTTP layer.

[`status`]: @api/rocket/response/status/
[`response`]: @api/rocket/response/
[`NamedFile`]: @api/rocket/response/struct.NamedFile.html
[`Content`]: @api/rocket/response/struct.Content.html
[`Redirect`]: @api/rocket/response/struct.Redirect.html
[`Stream`]: @api/rocket/response/struct.Stream.html
[`Flash`]: @api/rocket/response/struct.Flash.html
[`MsgPack`]: @api/rocket_contrib/msgpack/struct.MsgPack.html
[`Compress`]: @api/rocket_contrib/compression/struct.Compress.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
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:

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

[`rocket_contrib`]: @api/rocket_contrib/

### 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:

```rust
use rocket_contrib::json::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.

[`Json`]: @api/rocket_contrib/json/struct.Json.html
[`Serialize`]: https://docs.serde.rs/serde/trait.Serialize.html
[`serde`]: https://docs.serde.rs/serde/
[JSON example on GitHub]: @example/json

## 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 {
    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.

For a template to be renderable, it must first be registered. The `Template`
fairing automatically registers all discoverable templates when attached. The
[Fairings](../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
fn main() {
    rocket::ignite()
        .mount("/", routes![...])
        .attach(Template::fairing());
}
```

Rocket discovers templates 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.

! note: The name of the template _does not_ include its extension.

  For a template file named `index.html.tera`, call `render("index")` and use
  the name `"index"` in templates, i.e, `{% extends "index" %}` or `{% extends
  "base" %}` for `base.html.tera`.

### Live Reloading

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

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](@example/handlebars_templates) is a
fully composed application that makes use of Handlebars templates, while the
[Tera templates example](@example/tera_templates) does the same for Tera.

[`Template`]: @api/rocket_contrib/templates/struct.Template.html
[configurable]: ../configuration/#extras

## Typed URIs

Rocket's [`uri!`] macro allows you to build URIs to routes in your application
in a robust, type-safe, and URI-safe manner. Type or route parameter mismatches
are caught at compile-time, and changes to route URIs are automatically
reflected in the generated URIs.

The `uri!` macro returns an [`Origin`] structure with the URI of the supplied
route interpolated with the given values. Each value passed into `uri!` is
rendered in its appropriate place in the URI using the [`UriDisplay`]
implementation for the value's type. The `UriDisplay` implementation ensures
that the rendered value is URI-safe.

Note that `Origin` implements `Into<Uri>` (and by extension, `TryInto<Uri>`), so
it can be converted into a [`Uri`] using `.into()` as needed and passed into
methods such as [`Redirect::to()`].

For example, given the following route:

```rust
#[get("/person/<name>?<age>")]
fn person(name: String, age: Option<u8>) -> T
```

URIs to `person` can be created as follows:

```rust
// with unnamed parameters, in route path declaration order
let mike = uri!(person: "Mike Smith", 28);
assert_eq!(mike.to_string(), "/person/Mike%20Smith?age=28");

// with named parameters, order irrelevant
let mike = uri!(person: name = "Mike", age = 28);
let mike = uri!(person: age = 28, name = "Mike");
assert_eq!(mike.to_string(), "/person/Mike?age=28");

// with a specific mount-point
let mike = uri!("/api", person: name = "Mike", age = 28);
assert_eq!(mike.to_string(), "/api/person/Mike?age=28");

// with optional (defaultable) query parameters ignored
let mike = uri!(person: "Mike", _);
let mike = uri!(person: name = "Mike", age = _);
assert_eq!(mike.to_string(), "/person/Mike");
```

Rocket informs you of any mismatched parameters at compile-time:

```rust
error: person route uri expects 2 parameters but 1 was supplied
 --> examples/uri/src/main.rs:9:29
  |
9 |     uri!(person: "Mike Smith");
  |                  ^^^^^^^^^^^^
  |
  = note: expected parameters: name: String, age: Option<u8>
```

Rocket also informs you of any type errors at compile-time:

```rust
error: the trait bound u8: FromUriParam<Query, &str> is not satisfied
 --> examples/uri/src/main.rs:9:35
  |
9 |     uri!(person: age = "10", name = "Mike");
  |                        ^^^^ FromUriParam<Query, &str> is not implemented for u8
  |
```

We recommend that you use `uri!` exclusively when constructing URIs to your
routes.

### Ignorables

As illustrated in the previous above, query parameters can be ignored using `_`
in place of an expression in a `uri!` invocation. The corresponding type in the
route URI must implement [`Ignorable`]. Ignored parameters are not interpolated
into the resulting `Origin`. Path parameters are not ignorable.

### Deriving `UriDisplay`

The `UriDisplay` trait can be derived for custom types. For types that appear in
the path part of a URI, derive using [`UriDisplayPath`]; for types that appear
in the query part of a URI, derive using [`UriDisplayQuery`].

As an example, consider the following form structure and route:

```rust
#[derive(FromForm, UriDisplayQuery)]
struct UserDetails<'r> {
    age: Option<usize>,
    nickname: &'r RawStr,
}

#[post("/user/<id>?<details..>")]
fn add_user(id: usize, details: Form<UserDetails>) { .. }
```

By deriving using `UriDisplayQuery`, an implementation of `UriDisplay<Query>` is
automatically generated, allowing for URIs to `add_user` to be generated using
`uri!`:

```rust
uri!(add_user: 120, UserDetails { age: Some(20), nickname: "Bob".into() })
  => "/user/120?age=20&nickname=Bob"
```

### Typed URI Parts

The [`UriPart`] trait categorizes types that mark a part of the URI as either a
[`Path`] or a [`Query`]. Said another way, types that implement `UriPart` are
marker types that represent a part of a URI at the type-level. Traits such as
[`UriDisplay`] and [`FromUriParam`] bound a generic parameter by `UriPart`: `P:
UriPart`. This creates two instances of each trait: `UriDisplay<Query>` and
`UriDisplay<Path>`, and `FromUriParam<Query>` and `FromUriParam<Path>`.

As the names might imply, the `Path` version of the traits is used when
displaying parameters in the path part of the URI while the `Query` version is
used when displaying parameters in the query part of the URI. These distinct
versions of the traits exist exactly to differentiate, at the type-level, where
in the URI a value is to be written to, allowing for type safety in the face of
differences between the two locations. For example, while it is valid to use a
value of `None` in the query part, omitting the parameter entirely, doing so is
_not_ valid in the path part. By differentiating in the type system, both of
these conditions can be enforced appropriately through distinct implementations
of `FromUriParam<Path>` and `FromUriParam<Query>`.

### Conversions

The [`FromUriParam`] is used to perform a conversion for each value passed to
`uri!` before it is displayed with `UriDisplay`. If a `FromUriParam<P, S>`
implementation exists for a type `T` for part URI part `P`, then a value of type
`S` can be used in `uri!` macro for a route URI parameter declared with a type
of `T` in part `P`. For example, the following implementation, provided by
Rocket, allows an `&str` to be used in a `uri!` invocation for route URI
parameters declared as `String`:

```rust
impl<P: UriPart, 'a> FromUriParam<P, &'a str> for String { .. }
```

Other conversions to be aware of are:

  * `&str` to `RawStr`
  * `String` to `&str`
  * `String` to `RawStr`
  * `T` to `Option<T>`
  * `T` to `Result<T, E>`
  * `T` to `Form<T>`
  * `&str` to `&Path`
  * `&str` to `PathBuf`

Conversions _nest_. For instance, a value of type `T` can be supplied when a
value of type `Option<Form<T>>` is expected:

```rust
#[get("/person/<id>?<details>")]
fn person(id: usize, details: Option<Form<UserDetails>>) -> T

uri!(person: id = 100, details = UserDetails { .. })
```

See the [`FromUriParam`] documentation for further details.

[`Origin`]: @api/rocket/http/uri/struct.Origin.html
[`UriPart`]: @api/rocket/http/uri/trait.UriPart.html
[`Uri`]: @api/rocket/http/uri/enum.Uri.html
[`Redirect::to()`]: @api/rocket/response/struct.Redirect.html#method.to
[`uri!`]: @api/rocket_codegen/macro.uri.html
[`UriDisplay`]: @api/rocket/http/uri/trait.UriDisplay.html
[`FromUriParam`]: @api/rocket/http/uri/trait.FromUriParam.html
[`Path`]: @api/rocket/http/uri/enum.Path.html
[`Query`]: @api/rocket/http/uri/enum.Query.html
[`Ignorable`]: @api/rocket/http/uri/trait.Ignorable.html
[`UriDisplayPath`]: @api/rocket_codegen/derive.UriDisplayPath.html
[`UriDisplayQuery`]: @api/rocket_codegen/derive.UriDisplayQuery.html