# 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); ``` 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("/")] fn new(id: usize) -> status::Accepted { 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 [`rocket_contrib`]! [`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` 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("/")] async fn files(file: PathBuf) -> Option { NamedFile::open(Path::new("static/").join(file)).await.ok() } ``` ### `Result` `Result` is another _wrapping_ responder: a `Result` 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("/")] async fn files(file: PathBuf) -> Result> { 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. * [`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 `AsyncRead` type. For example, to stream from a local Unix stream, we might write: ```rust # #[macro_use] extern crate rocket; # fn main() {} # mod test { use std::net::SocketAddr; use rocket::response::{Stream, Debug}; use rocket::tokio::net::TcpStream; #[get("/stream")] async fn stream() -> Result, Debug> { let addr = SocketAddr::from(([127, 0, 0, 1], 9999)); Ok(TcpStream::connect(addr).await.map(Stream::from)?) } # } ``` [`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` 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; # #[macro_use] extern crate rocket_contrib; # fn main() {} use serde::Serialize; use rocket_contrib::json::Json; #[derive(Serialize)] struct Task { /* .. */ } #[get("/todo")] fn todo() -> Json { 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 # #[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::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` (and by extension, `TryInto`), 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("//?")] fn person(id: Option, name: String, age: Option) { /* .. */ } ``` URIs to `person` can be created as follows: ```rust # #[macro_use] extern crate rocket; # #[get("//?")] # fn person(id: Option, name: String, age: Option) { /* .. */ } // 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 , name: String, age: Option ``` 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` 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::http::RawStr; use rocket::request::Form; #[derive(FromForm, UriDisplayQuery)] struct UserDetails<'r> { age: Option, nickname: &'r RawStr, } #[post("/user/?")] fn add_user(id: usize, details: Form) { /* .. */ } ``` By deriving using `UriDisplayQuery`, an implementation of `UriDisplay` is automatically generated, allowing for URIs to `add_user` to be generated using `uri!`: ```rust # #[macro_use] extern crate rocket; # use rocket::http::RawStr; # use rocket::request::Form; # #[derive(FromForm, UriDisplayQuery)] # struct UserDetails<'r> { # age: Option, # nickname: &'r RawStr, # } # #[post("/user/?")] # fn add_user(id: usize, details: Form) { /* .. */ } 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 [`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` and `UriDisplay`, and `FromUriParam` and `FromUriParam`. 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` and `FromUriParam`. This division has an effect on how the `uri!` macro can be invoked. In query parts, for a route type of `Option`, 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("//?")] # fn person(id: Option, name: String, age: Option) { /* .. */ } /// Note that `id` is `Option` 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 `FromUriParam` 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::{FromUriParam, UriPart}; # struct S; # type String = S; impl<'a, P: UriPart> FromUriParam 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` * `&str` to `RawStr` * `String` to `&str` * `String` to `RawStr` * `&str` to `&Path` * `&str` to `PathBuf` * `T` to `Form` The following conversions only apply to path parts: * `T` to `Option` * `T` to `Result` Conversions _nest_. For instance, a value of type `&T` can be supplied when a value of type `Option>` is expected: ```rust # #[macro_use] extern crate rocket; # use rocket::http::RawStr; # use rocket::request::Form; # #[derive(FromForm, UriDisplayQuery)] # struct UserDetails<'r> { age: Option, nickname: &'r RawStr, } #[get("/person/?")] fn person(id: usize, details: Option>) { /* .. */ } let details = UserDetails { age: Some(20), nickname: "Bob".into() }; uri!(person: id = 100, details = Some(&details) ); ``` 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/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