# 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
# #[macro_use] extern crate rocket;
# fn main() {}

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
# #[macro_use] extern crate rocket;
# fn main() {}
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 [`serde`]!

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

### Errors

Responders may fail instead of generating a response by returning an `Err` with
a status code. When this happens, Rocket forwards the request to the [error
catcher](../requests/#error-catchers) for that 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
# #[macro_use] extern crate rocket;
# fn main() {}

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/master/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
# #[macro_use] extern crate rocket;
# fn main() {}

use rocket::http::{Header, ContentType};
# type OtherResponder = ();
# type MyType = u8;

#[derive(Responder)]
#[response(status = 500, content_type = "json")]
struct MyResponder {
    inner: OtherResponder,
    header: ContentType,
    more: Header<'static>,
    #[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/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
# #[macro_use] extern crate rocket;
# fn main() {}

use std::io::Cursor;

use rocket::request::Request;
use rocket::response::{self, Response, Responder};
use rocket::http::ContentType;

# struct String(std::string::String);
#[rocket::async_trait]
impl<'r> Responder<'r, 'static> for String {
    fn respond_to(self, _: &'r Request<'_>) -> response::Result<'static> {
        Response::build()
            .header(ContentType::Plain)
            # /*
            .sized_body(self.len(), Cursor::new(self))
            # */
            # .sized_body(self.0.len(), Cursor::new(self.0))
            .ok()
    }
}
```

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

```rust
# #[macro_use] extern crate rocket;
# fn main() {}
#[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
# #[macro_use] extern crate rocket;
# fn main() {}

# use std::path::{Path, PathBuf};
use rocket::response::NamedFile;

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

### `Result`

`Result` is another _wrapping_ responder: a `Result<T, E>` can only be returned
when `T` implements `Responder` and `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
# #[macro_use] extern crate rocket;
# fn main() {}

# use std::path::{Path, PathBuf};
use rocket::response::NamedFile;
use rocket::response::status::NotFound;

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

## 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.

[`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
[`rocket_contrib`]: @api/rocket_contrib/

### Async Streams

The [`stream`] responders allow serving potentially infinite async [`Stream`]s.
A stream can be created from any async `Stream` or `AsyncRead` type, or via
generator syntax using the [`stream!`] macro and its typed equivalents.

The simplest version creates a [`ReaderStream`] from a single `AsyncRead` type.
For example, to stream from a TCP connection, we might write:

```rust
# use rocket::*;
use std::io;
use std::net::SocketAddr;

use rocket::tokio::net::TcpStream;
use rocket::response::stream::ReaderStream;

#[get("/stream")]
async fn stream() -> io::Result<ReaderStream![TcpStream]> {
    let addr = SocketAddr::from(([127, 0, 0, 1], 9999));
    let stream = TcpStream::connect(addr).await?;
    Ok(ReaderStream::one(stream))
}
```

Streams can also be created using generator syntax. The following example
returns an infinite [`TextStream`] that produces one `"hello"` every second:

```rust
# use rocket::get;
use rocket::tokio::time::{self, Duration};
use rocket::response::stream::TextStream;

/// Produce an infinite series of `"hello"`s, one per second.
#[get("/infinite-hellos")]
fn hello() -> TextStream![&'static str] {
    TextStream! {
        let mut interval = time::interval(Duration::from_secs(1));
        loop {
            yield "hello";
            interval.tick().await;
        }
    }
}
```

See the [`stream`] docs for full details on creating streams including notes on
how to detect and handle graceful shutdown requests.

[`stream`]: @api/rocket/response/stream/index.html
[`stream!`]: @api/rocket/response/stream/macro.stream.html
[`Stream`]: https://docs.rs/futures/0.3/futures/stream/trait.Stream.html
[`ReaderStream`]: @api/rocket/response/stream/struct.ReaderStream.html
[`TextStream`]: @api/rocket/response/stream/struct.TextStream.html

### JSON

The [`Json`] responder in 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
# #[macro_use] extern crate rocket;

use rocket::serde::{Serialize, json::Json};

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

#[get("/todo")]
fn todo() -> Json<Task> {
    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 [serialization example] provides further illustration.

[`Json`]: @api/rocket/serde/json/struct.Json.html
[`Serialize`]: https://docs.serde.rs/serde/trait.Serialize.html
[`serde`]: https://serde.rs
[serialization example]: @example/serialization

## 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
# #[macro_use] extern crate rocket;
# #[macro_use] extern crate rocket_contrib;
# fn main() {}

use rocket_contrib::templates::Template;

#[get("/")]
fn index() -> Template {
    # /*
    let context = /* object-like value */;
    # */ let context = ();
    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
# #[macro_use] extern crate rocket;

# use rocket_contrib::templates::Template;

#[launch]
fn rocket() -> _ {
    rocket::build()
        .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 [templating example](@example/templating) uses both Tera and Handlebars
templating to implement the same application.

[`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
# #[macro_use] extern crate rocket;
# fn main() {}

#[get("/<id>/<name>?<age>")]
fn person(id: Option<usize>, name: &str, age: Option<u8>) { /* .. */ }
```

URIs to `person` can be created as follows:

```rust
# #[macro_use] extern crate rocket;

# #[get("/<id>/<name>?<age>")]
# fn person(id: Option<usize>, name: &str, age: Option<u8>) { /* .. */ }

// with unnamed parameters, in route path declaration order
let mike = uri!(person(101, "Mike Smith", Some(28)));
assert_eq!(mike.to_string(), "/101/Mike%20Smith?age=28");

// with named parameters, order irrelevant
let mike = uri!(person(name = "Mike", id = 101, age = Some(28)));
assert_eq!(mike.to_string(), "/101/Mike?age=28");
let mike = uri!(person(id = 101, age = Some(28), name = "Mike"));
assert_eq!(mike.to_string(), "/101/Mike?age=28");

// with a specific mount-point
let mike = uri!("/api", person(id = 101, name = "Mike", age = Some(28)));
assert_eq!(mike.to_string(), "/api/101/Mike?age=28");

// with optional (defaultable) query parameters ignored
let mike = uri!(person(101, "Mike", _));
assert_eq!(mike.to_string(), "/101/Mike");
let mike = uri!(person(id = 101, name = "Mike", age = _));
assert_eq!(mike.to_string(), "/101/Mike");
```

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

```rust,ignore
error: `person` route uri expects 3 parameters but 1 was supplied
 --> examples/uri/main.rs:7:26
  |
7 |     let x = uri!(person("Mike Smith"));
  |                         ^^^^^^^^^^^^
  |
  = note: expected parameters: id: Option <usize>, name: &str, age: Option <u8>
```

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

```rust,ignore
 --> examples/uri/src/main.rs:7:31
  |
7 |     let x = uri!(person(id = "10", name = "Mike Smith", age = Some(10)));
  |                              ^^^^ `FromUriParam<Path, &str>` is not implemented for `usize`
```

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
# #[macro_use] extern crate rocket;
# fn main() {}

use rocket::form::Form;

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

#[post("/user/<id>?<details..>")]
fn add_user(id: usize, details: 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
# #[macro_use] extern crate rocket;

# use rocket::form::Form;

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

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

let link = uri!(add_user(120, UserDetails { age: Some(20), nickname: "Bob".into() }));
assert_eq!(link.to_string(), "/user/120?age=20&nickname=Bob");
```

### Typed URI Parts

The [`Part`] trait categorizes types that mark a part of the URI as either a
[`Path`] or a [`Query`]. Said another way, types that implement `Part` 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 `Part`: `P:
Part`. 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>`.

This division has an effect on how the `uri!` macro can be invoked. In query
parts, for a route type of `Option<T>`, you _must_ supply a type of `Option`,
`Result`, or an ignored `_` to the `uri!` invocation. By contrast, you _cannot_
supply such a type in the path part. This ensures that a valid URI is _always_
generated.

```rust
# #[macro_use] extern crate rocket;

# #[get("/<id>/<name>?<age>")]
# fn person(id: Option<usize>, name: &str, age: Option<u8>) { /* .. */ }

/// Note that `id` is `Option<usize>` in the route, but `id` in `uri!` _cannot_
/// be an `Option`. `age`, on the other hand, _must_ be an `Option` (or `Result`
/// or `_`) as its in the query part and is allowed to be ignored.
let mike = uri!(person(id = 101, name = "Mike", age = Some(28)));
assert_eq!(mike.to_string(), "/101/Mike?age=28");
```

### Conversions

[`FromUriParam`] is used to perform a conversion for each value passed to `uri!`
before it is displayed with `UriDisplay`. If a `T: 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
# use rocket::http::uri::fmt::{FromUriParam, Part};
# struct S;
# type String = S;
impl<'a, P: Part> FromUriParam<P, &'a str> for String {
    type Target = &'a str;
#   fn from_uri_param(s: &'a str) -> Self::Target { "hi" }
}
```

Other conversions to be aware of are:

  * `&T` to `T`
  * `&mut T` to `T`
  * `String` to `&str`
  * `&str` to `&Path`
  * `&str` to `PathBuf`
  * `T` to `Form<T>`

The following conversions only apply to path parts:

  * `T` to `Option<T>`
  * `T` to `Result<T, E>`

The following conversions are implemented only in query parts:

  * `Option<T>` to `Result<T, E>` (for any `E`)
  * `Result<T, E>` to `Option<T>` (for any `E`)

Conversions are transitive. That is, a conversion from `A -> B` and a conversion
`B -> C` implies a conversion from `A -> C`. For instance, a value of type
`&str` can be supplied when a value of type `Option<PathBuf>` is expected:

```rust
# #[macro_use] extern crate rocket;

use std::path::PathBuf;

#[get("/person/<id>/<details..>")]
fn person(id: usize, details: Option<PathBuf>) { /* .. */ }

uri!(person(id = 100, details = "a/b/c"));
```

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

[`Origin`]: @api/rocket/http/uri/struct.Origin.html
[`Part`]: @api/rocket/http/uri/trait.Part.html
[`Uri`]: @api/rocket/http/uri/enum.Uri.html
[`Redirect::to()`]: @api/rocket/response/struct.Redirect.html#method.to
[`uri!`]: @api/rocket/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/derive.UriDisplayPath.html
[`UriDisplayQuery`]: @api/rocket/derive.UriDisplayQuery.html