# Requests Together, a route's attribute and function signature specify what must be true about a request in order for the route's handler to be called. You've already seen an example of this in action: ```rust #[get("/world")] fn handler() { .. } ``` This route indicates that it only matches against `GET` requests to the `/world` route. Rocket ensures that this is the case before `handler` is called. Of course, you can do much more than specify the method and path of a request. Among other things, you can ask Rocket to automatically validate: * The type of a dynamic path segment. * The type of _many_ dynamic path segments. * The type of incoming data. * The types of query strings, forms, and form values. * The expected incoming or outgoing format of a request. * Any arbitrary, user-defined security or validation policies. The route attribute and function signature work in tandem to describe these validations. Rocket's code generation takes care of actually validating the properties. This section describes how to ask Rocket to validate against all of these properties and more. ## Methods A Rocket route attribute can be any one of `get`, `put`, `post`, `delete`, `head`, `patch`, or `options`, each corresponding to the HTTP method to match against. For example, the following attribute will match against `POST` requests to the root path: ```rust #[post("/")] ``` The grammar for these attributes is defined formally in the [`rocket_codegen`](https://api.rocket.rs/rocket_codegen/) API docs. ### HEAD Requests Rocket handles `HEAD` requests automatically when there exists a `GET` route that would otherwise match. It does this by stripping the body from the response, if there is one. You can also specialize the handling of a `HEAD` request by declaring a route for it; Rocket won't interfere with `HEAD` requests your application handles. ### Reinterpreting Because browsers can only send `GET` and `POST` requests, Rocket _reinterprets_ request methods under certain conditions. If a `POST` request contains a body of `Content-Type: application/x-www-form-urlencoded`, and the form's **first** field has the name `_method` and a valid HTTP method name as its value (such as `"PUT"`), that field's value is used as the method for the incoming request. This allows Rocket applications to submit non-`POST` forms. The [todo example](https://github.com/SergioBenitez/Rocket/tree/v0.4.0-dev/examples/todo/static/index.html.tera#L47) makes use of this feature to submit `PUT` and `DELETE` requests from a web form. ## Dynamic Segments You can declare path segments as dynamic by using angle brackets around variable names in a route's path. For example, if we want to say _Hello!_ to anything, not just the world, we can declare a route like so: ```rust #[get("/hello/")] fn hello(name: &RawStr) -> String { format!("Hello, {}!", name.as_str()) } ``` If we were to mount the path at the root (`.mount("/", routes![hello])`), then any request to a path with two non-empty segments, where the first segment is `hello`, will be dispatched to the `hello` route. For example, if we were to visit `/hello/John`, the application would respond with `Hello, John!`. Any number of dynamic path segments are allowed. A path segment can be of any type, including your own, as long as the type implements the [`FromParam`] trait. Rocket implements `FromParam` for many of the standard library types, as well as a few special Rocket types. For the full list of supplied implementations, see the [`FromParam` API docs]. Here's a more complete route to illustrate varied usage: ```rust #[get("/hello///")] fn hello(name: String, age: u8, cool: bool) -> String { if cool { format!("You're a cool {} year old, {}!", age, name) } else { format!("{}, we need to talk about your coolness.", name) } } ``` [`FromParam`]: https://api.rocket.rs/rocket/request/trait.FromParam.html [`FromParam` API docs]: https://api.rocket.rs/rocket/request/trait.FromParam.html ### Raw Strings You may have noticed an unfamiliar [`RawStr`] type in the code example above. This is a special type, provided by Rocket, that represents an unsanitized, unvalidated, and undecoded raw string from an HTTP message. It exists to separate validated string inputs, represented by types such as `String`, `&str`, and `Cow` types, from unvalidated inputs, represented by `&RawStr`. It provides helpful methods to convert the unvalidated string into a validated one. Because `&RawStr` implements [`FromParam`], it can be used as the type of a dynamic segment, as in the example above. When used as the type of a dynamic segment, a `RawStr` points to a potentially undecoded string. By contrast, a `String` is guaranteed to be decoded. Which you should use depends on whether you want direct but potentially unsafe access to the string (`&RawStr`), or safe access to the string at the cost of an allocation (`String`). [`RawStr`]: https://api.rocket.rs/rocket/http/struct.RawStr.html ## Forwarding Let's take a closer look at the route attribute and signature pair from the last example: ```rust #[get("/hello///")] fn hello(name: String, age: u8, cool: bool) -> String { ... } ``` What if `cool` isn't a `bool`? Or, what if `age` isn't a `u8`? When a parameter type mismatch occurs, Rocket _forwards_ the request to the next matching route, if there is any. This continues until a route doesn't forward the request or there are no remaining routes to try. When there are no remaining routes, a customizable **404 error** is returned. Routes are attempted in increasing _rank_ order. Rocket chooses a default ranking from -4 to -1, detailed in the next section, for all routes, but a route's rank can also be manually set with the `rank` attribute. To illustrate, consider the following routes: ```rust #[get("/user/")] fn user(id: usize) -> T { ... } #[get("/user/", rank = 2)] fn user_int(id: isize) -> T { ... } #[get("/user/", rank = 3)] fn user_str(id: &RawStr) -> T { ... } ``` Notice the `rank` parameters in `user_int` and `user_str`. If we run this application with the routes mounted at the root, requests to `/user/` will be routed as follows: 1. The `user` route matches first. If the string at the `` position is an unsigned integer, then the `user` handler is called. If it is not, then the request is forwarded to the next matching route: `user_int`. 2. The `user_int` route matches next. If `` is a signed integer, `user_int` is called. Otherwise, the request is forwarded. 3. The `user_str` route matches last. Since `` is a always string, the route always matches. The `user_str` handler is called. Forwards can be _caught_ by using a `Result` or `Option` type. For example, if the type of `id` in the `user` function was `Result`, then `user` would never forward. An `Ok` variant would indicate that `` was a valid `usize`, while an `Err` would indicate that `` was not a `usize`. The `Err`'s value would contain the string that failed to parse as a `usize`. By the way, if you were to omit the `rank` parameter in the `user_str` or `user_int` routes, Rocket would emit an error and abort launch, indicating that the routes _collide_, or can match against similar incoming requests. The `rank` parameter resolves this collision. ### Default Ranking If a rank is not explicitly specified, Rocket assigns a default ranking. By default, routes with static paths and query strings have lower ranks (higher precedence) while routes with dynamic paths and without query strings have higher ranks (lower precedence). The table below describes the default ranking of a route given its properties. | static path | query string | rank | example | | ------------- | -------------- | ------ | ------------------- | | yes | yes | -4 | `/hello?world=true` | | yes | no | -3 | `/hello` | | no | yes | -2 | `/?world=true` | | no | no | -1 | `/` | ## Multiple Segments You can also match against multiple segments by using `` in a route path. The type of such parameters, known as _segments_ parameters, must implement [`FromSegments`]. Segments parameters must be the final component of a path: any text after a segments parameter will result in a compile-time error. As an example, the following route matches against all paths that begin with `/page/`: ```rust #[get("/page/")] fn get_page(path: PathBuf) -> T { ... } ``` The path after `/page/` will be available in the `path` parameter. The `FromSegments` implementation for `PathBuf` ensures that `path` cannot lead to [path traversal attacks](https://www.owasp.org/index.php/Path_Traversal). With this, a safe and secure static file server can be implemented in 4 lines: ```rust #[get("/")] fn files(file: PathBuf) -> Option { NamedFile::open(Path::new("static/").join(file)).ok() } ``` [`FromSegments`]: https://api.rocket.rs/rocket/request/trait.FromSegments.html ## Format A route can specify the data format it is willing to accept or respond with using the `format` route parameter. The value of the parameter is a string identifying an HTTP media type. For instance, for JSON data, the string `application/json` can be used. When a route indicates a payload-supporting method (`PUT`, `POST`, `DELETE`, and `PATCH`), the `format` route parameter instructs Rocket to check against the `Content-Type` header of the incoming request. Only requests where the `Content-Type` header matches the `format` parameter will match to the route. As an example, consider the following route: ```rust #[post("/user", format = "application/json", data = "")] fn new_user(user: Json) -> T { ... } ``` The `format` parameter in the `post` attribute declares that only incoming requests with `Content-Type: application/json` will match `new_user`. (The `data` parameter is described in the next section.) Shorthand is also supported for the most common `format` arguments. Instead of using the full Content-Type, `format = "application/json"`, you can also write shorthands like `format = "json"`. For a full list of available shorthands, see the [`ContentType::parse_flexible()`] documentation. When a route indicates a non-payload-supporting method (`HEAD`, `OPTIONS`, and, these purposes, `GET`) the `format` route parameter instructs Rocket to check against the `Accept` header of the incoming request. Only requests where the preferred media type in the `Accept` header matches the `format` parameter will match to the route. As an example, consider the following route: ```rust #[get("/user/", format = "json")] fn user(id: usize) -> Json { ... } ``` The `format` parameter in the `get` attribute declares that only incoming requests with `application/json` as the preferred media type in the `Accept` header will match `user`. If instead the route had been declared as `post`, Rocket would match the `format` against the `Content-Type` header of the incoming response. [`ContentType::parse_flexible()`]: https://api.rocket.rs/rocket/http/struct.ContentType.html#method.parse_flexible ## Request Guards Request guards are one of Rocket's most powerful instruments. As the name might imply, a request guard protects a handler from being called erroneously based on information contained in an incoming request. More specifically, a request guard is a type that represents an arbitrary validation policy. The validation policy is implemented through the [`FromRequest`] trait. Every type that implements `FromRequest` is a request guard. Request guards appear as inputs to handlers. An arbitrary number of request guards can appear as arguments in a route handler. Rocket will automatically invoke the [`FromRequest`] implementation for request guards before calling the handler. Rocket only dispatches requests to a handler when all of its guards pass. As an example, the following dummy handler makes use of three request guards, `A`, `B`, and `C`. An input can be identified as a request guard if it is not named in the route attribute. This is why `param` is not a request guard. ```rust #[get("/")] fn index(param: isize, a: A, b: B, c: C) -> ... { ... } ``` Request guards always fire in left-to-right declaration order. In the example above, the order will be `A` followed by `B` followed by `C`. Failure is short-circuiting; if one guard fails, the remaining are not attempted. To learn more about request guards and implementing them, see the [`FromRequest`] documentation. [`FromRequest`]: https://api.rocket.rs/rocket/request/trait.FromRequest.html [`Cookies`]: https://api.rocket.rs/rocket/http/enum.Cookies.html ### Custom Guards You can implement `FromRequest` for your own types. For instance, to protect a `sensitive` route from running unless an `ApiKey` is present in the request headers, you might create an `ApiKey` type that implements `FromRequest` and then use it as a request guard: ```rust #[get("/sensitive")] fn sensitive(key: ApiKey) -> &'static str { ... } ``` You might also implement `FromRequest` for an `AdminUser` type that authenticates an administrator using incoming cookies. Then, any handler with an `AdminUser` or `ApiKey` type in its argument list is assured to only be invoked if the appropriate conditions are met. Request guards centralize policies, resulting in a simpler, safer, and more secure applications. ### Forwarding Guards Request guards and forwarding are a powerful combination for enforcing policies. To illustrate, we consider how a simple authorization system might be implemented using these mechanisms. We start with two request guards: * `User`: A regular, authenticated user. The `FromRequest` implementation for `User` checks that a cookie identifies a user and returns a `User` value if so. If no user can be authenticated, the guard forwards. * `AdminUser`: A user authenticated as an administrator. The `FromRequest` implementation for `AdminUser` checks that a cookie identifies an _administrative_ user and returns an `AdminUser` value if so. If no user can be authenticated, the guard forwards. We now use these two guards in combination with forwarding to implement the following three routes, each leading to an administrative control panel at `/admin`: ```rust #[get("/admin")] fn admin_panel(admin: AdminUser) -> &'static str { "Hello, administrator. This is the admin panel!" } #[get("/admin", rank = 2)] fn admin_panel_user(user: User) -> &'static str { "Sorry, you must be an administrator to access this page." } #[get("/admin", rank = 3)] fn admin_panel_redirect() -> Redirect { Redirect::to("/login") } ``` The three routes above encode authentication _and_ authorization. The `admin_panel` route only succeeds if an administrator is logged in. Only then is the admin panel displayed. If the user is not an admin, the `AdminUser` route will forward. Since the `admin_panel_user` route is ranked next highest, it is attempted next. This route succeeds if there is _any_ user signed in, and an authorization failure message is displayed. Finally, if a user isn't signed in, the `admin_panel_redirect` route is attempted. Since this route has no guards, it always succeeds. The user is redirected to a log in page. ## Cookies [`Cookies`] is an important, built-in request guard: it allows you to get, set, and remove cookies. Because `Cookies` is a request guard, an argument of its type can simply be added to a handler: ```rust use rocket::http::Cookies; #[get("/")] fn index(cookies: Cookies) -> Option { cookies.get("message") .map(|value| format!("Message: {}", value)) } ``` This results in the incoming request's cookies being accessible from the handler. The example above retrieves a cookie named `message`. Cookies can also be set and removed using the `Cookies` guard. The [cookies example] on GitHub illustrates further use of the `Cookies` type to get and set cookies, while the [`Cookies`] documentation contains complete usage information. [cookies example]: https://github.com/SergioBenitez/Rocket/tree/v0.4.0-dev/examples/cookies ### Private Cookies Cookies added via the [`Cookies::add()`] method are set _in the clear._ In other words, the value set is visible by the client. For sensitive data, Rocket provides _private_ cookies. Private cookies are just like regular cookies except that they are encrypted using authenticated encryption, a form of encryption which simultaneously provides confidentiality, integrity, and authenticity. This means that private cookies cannot be inspected, tampered with, or manufactured by clients. If you prefer, you can think of private cookies as being signed and encrypted. The API for retrieving, adding, and removing private cookies is identical except methods are suffixed with `_private`. These methods are: [`get_private`], [`add_private`], and [`remove_private`]. An example of their usage is below: ```rust /// Retrieve the user's ID, if any. #[get("/user_id")] fn user_id(cookies: Cookies) -> Option { cookies.get_private("user_id") .map(|cookie| format!("User ID: {}", cookie.value())) } /// Remove the `user_id` cookie. #[post("/logout")] fn logout(mut cookies: Cookies) -> Flash { cookies.remove_private(Cookie::named("user_id")); Flash::success(Redirect::to("/"), "Successfully logged out.") } ``` [`Cookies::add()`]: https://api.rocket.rs/rocket/http/enum.Cookies.html#method.add ### Secret Key To encrypt private cookies, Rocket uses the 256-bit key specified in the `secret_key` configuration parameter. If one is not specified, Rocket will automatically generate a fresh key. Note, however, that a private cookie can only be decrypted with the same key with which it was encrypted. As such, it is important to set a `secret_key` configuration parameter when using private cookies so that cookies decrypt properly after an application restart. Rocket emits a warning if an application is run in production without a configured `secret_key`. Generating a string suitable for use as a `secret_key` configuration value is usually done through tools like `openssl`. Using `openssl`, a 256-bit base64 key can be generated with the command `openssl rand -base64 32`. For more information on configuration, see the [Configuration](/guide/configuration) section of the guide. [`get_private`]: https://api.rocket.rs/rocket/http/enum.Cookies.html#method.get_private [`add_private`]: https://api.rocket.rs/rocket/http/enum.Cookies.html#method.add_private [`remove_private`]: https://api.rocket.rs/rocket/http/enum.Cookies.html#method.remove_private ### One-At-A-Time For safety reasons, Rocket currently requires that at most one `Cookies` instance be active at a time. It's uncommon to run into this restriction, but it can be confusing to handle if it does crop up. If this does happen, Rocket will emit messages to the console that look as follows: ``` => Error: Multiple `Cookies` instances are active at once. => An instance of `Cookies` must be dropped before another can be retrieved. => Warning: The retrieved `Cookies` instance will be empty. ``` The messages will be emitted when a violating handler is called. The issue can be resolved by ensuring that two instances of `Cookies` cannot be active at once due to the offending handler. A common error is to have a handler that uses a `Cookies` request guard as well as a `Custom` request guard that retrieves `Cookies`, as so: ```rust #[get("/")] fn bad(cookies: Cookies, custom: Custom) { .. } ``` Because the `cookies` guard will fire before the `custom` guard, the `custom` guard will retrieve an instance of `Cookies` when one already exists for `cookies`. This scenario can be fixed by simply swapping the order of the guards: ```rust #[get("/")] fn good(custom: Custom, cookies: Cookies) { .. } ``` ## Body Data At some point, your web application will need to process body data. Data processing, like much of Rocket, is type directed. To indicate that a handler expects data, annotate it with `data = ""`, where `param` is an argument in the handler. The argument's type must implement the [`FromData`] trait. It looks like this, where `T: FromData`: ```rust #[post("/", data = "")] fn new(input: T) -> String { ... } ``` Any type that implements [`FromData`] is also known as _data guard_. [`FromData`]: https://api.rocket.rs/rocket/data/trait.FromData.html ### Forms Forms are the most common type of data handled in web applications, and Rocket makes handling them easy. Say your application is processing a form submission for a new todo `Task`. The form contains two fields: `complete`, a checkbox, and `description`, a text field. You can easily handle the form request in Rocket as follows: ```rust #[derive(FromForm)] struct Task { complete: bool, description: String, } #[post("/todo", data = "")] fn new(task: Form) -> String { ... } ``` The `Form` type implements the `FromData` trait as long as its generic parameter implements the [`FromForm`] trait. In the example, we've derived the `FromForm` trait automatically for the `Task` structure. `FromForm` can be derived for any structure whose fields implement [`FromFormValue`]. If a `POST /todo` request arrives, the form data will automatically be parsed into the `Task` structure. If the data that arrives isn't of the correct Content-Type, the request is forwarded. If the data doesn't parse or is simply invalid, a customizable `400 - Bad Request` or `422 - Unprocessable Entity` error is returned. As before, a forward or failure can be caught by using the `Option` and `Result` types: ```rust #[post("/todo", data = "")] fn new(task: Option>) -> String { ... } ``` [`FromForm`]: https://api.rocket.rs/rocket/request/trait.FromForm.html [`FromFormValue`]: https://api.rocket.rs/rocket/request/trait.FromFormValue.html #### Lenient Parsing Rocket's `FromForm` parsing is _strict_ by default. In other words, A `Form` will parse successfully from an incoming form only if the form contains the exact set of fields in `T`. Said another way, a `Form` will error on missing and/or extra fields. For instance, if an incoming form contains the fields "a", "b", and "c" while `T` only contains "a" and "c", the form _will not_ parse as `Form`. Rocket allows you to opt-out of this behavior via the [`LenientForm`] data type. A `LenientForm` will parse successfully from an incoming form as long as the form contains a superset of the fields in `T`. Said another way, a `LenientForm` automatically discards extra fields without error. For instance, if an incoming form contains the fields "a", "b", and "c" while `T` only contains "a" and "c", the form _will_ parse as `LenientForm`. You can use a `LenientForm` anywhere you'd use a `Form`. Its generic parameter is also required to implement `FromForm`. For instance, we can simply replace `Form` with `LenientForm` above to get lenient parsing: ```rust #[derive(FromForm)] struct Task { .. } #[post("/todo", data = "")] fn new(task: LenientForm) { .. } ``` [`LenientForm`]: https://api.rocket.rs/rocket/request/struct.LenientForm.html #### Field Renaming By default, Rocket matches the name of an incoming form field to the name of a structure field. While this behavior is typical, it may also be desired to use different names for form fields and struct fields while still parsing as expected. You can ask Rocket to look for a different form field for a given structure field by using the `#[form(field = "name")]` field annotation. As an example, say that you're writing an application that receives data from an external service. The external service `POST`s a form with a field named `type`. Since `type` is a reserved keyword in Rust, it cannot be used as the name of a field. To get around this, you can use field renaming as follows: ```rust #[derive(FromForm)] struct External { #[form(field = "type")] api_type: String } ``` Rocket will then match the form field named `type` to the structure field named `api_type` automatically. #### Field Validation Fields of forms can be easily validated via implementations of the [`FromFormValue`] trait. For example, if you'd like to verify that some user is over some age in a form, then you might define a new `AdultAge` type, use it as a field in a form structure, and implement `FromFormValue` so that it only validates integers over that age: ```rust struct AdultAge(usize); impl<'v> FromFormValue<'v> for AdultAge { type Error = &'v RawStr; fn from_form_value(form_value: &'v RawStr) -> Result { match form_value.parse::() { Ok(age) if age >= 21 => Ok(AdultAge(age)), _ => Err(form_value), } } } #[derive(FromForm)] struct Person { age: AdultAge } ``` If a form is submitted with a bad age, Rocket won't call a handler requiring a valid form for that structure. You can use `Option` or `Result` types for fields to catch parse failures: ```rust #[derive(FromForm)] struct Person { age: Option } ``` The [forms validation](https://github.com/SergioBenitez/Rocket/tree/v0.4.0-dev/examples/form_validation) and [forms kitchen sink](https://github.com/SergioBenitez/Rocket/tree/v0.4.0-dev/examples/form_kitchen_sink) examples on GitHub provide further illustrations. ### JSON Handling JSON data is no harder: simply use the [`Json`](https://api.rocket.rs/rocket_contrib/struct.Json.html) type: ```rust #[derive(Deserialize)] struct Task { description: String, complete: bool } #[post("/todo", data = "")] fn new(task: Json) -> String { ... } ``` The only condition is that the generic type in `Json` implements the `Deserialize` trait from [Serde](https://github.com/serde-rs/json). See the [JSON example] on GitHub for a complete example. [JSON example]: https://github.com/SergioBenitez/Rocket/tree/v0.4.0-dev/examples/json ### Streaming Sometimes you just want to handle incoming data directly. For example, you might want to stream the incoming data out to a file. Rocket makes this as simple as possible via the [`Data`](https://api.rocket.rs/rocket/data/struct.Data.html) type: ```rust #[post("/upload", format = "plain", data = "")] fn upload(data: Data) -> io::Result { data.stream_to_file("/tmp/upload.txt").map(|n| n.to_string()) } ``` The route above accepts any `POST` request to the `/upload` path with `Content-Type: text/plain` The incoming data is streamed out to `tmp/upload.txt`, and the number of bytes written is returned as a plain text response if the upload succeeds. If the upload fails, an error response is returned. The handler above is complete. It really is that simple! See the [GitHub example code](https://github.com/SergioBenitez/Rocket/tree/v0.4.0-dev/examples/raw_upload) for the full crate. ## Query Strings Query strings are handled just like forms. A query string can be parsed into any structure that implements the `FromForm` trait. They are matched against by appending a `?` to the path followed by a static query string or a dynamic parameter ``. For instance, say you change your mind and decide to use query strings instead of `POST` forms for new todo tasks in the previous forms example, reproduced below: ```rust #[derive(FromForm)] struct Task { .. } #[post("/todo", data = "")] fn new(task: Form) -> String { ... } ``` Rocket makes the transition simple: simply declare `` as a query parameter as follows: ```rust #[get("/todo?")] fn new(task: Task) -> String { ... } ``` Rocket will parse the query string into the `Task` structure automatically by matching the structure field names to the query parameters. If the parse fails, the request is forwarded to the next matching route. Parse failures can be captured on a per-field or per-form basis. To catch failures on a per-field basis, use a type of `Option` or `Result` for the given field: ```rust #[derive(FromForm)] struct Task<'r> { description: Result, complete: Option } ``` To catch failures on a per-form basis, change the type of the query string target to either `Option` or `Result`: ```rust #[get("/todo?")] fn new(task: Option) { ... } ``` For a concrete illustration on how to handle query parameters, see [the `query_params` example](https://github.com/SergioBenitez/Rocket/tree/v0.4.0-dev/examples/query_params). ## Error Catchers Routing may fail for a variety of reasons. These include: * A [request guard](#request-guards) returns `Failure`. * A handler returns a [`Responder`](/guide/responses/#responder) that fails. * No matching route was found. If any of these conditions occur, Rocket returns an error to the client. To do so, Rocket invokes the _catcher_ corresponding to the error's status code. A catcher is like a route, except it only handles errors. Rocket provides default catchers for all of the standard HTTP error codes. To override a default catcher, or declare a catcher for a custom status code, use the `catch` attribute, which takes a single integer corresponding to the HTTP status code to catch. For instance, to declare a catcher for `404 Not Found` errors, you'd write: ```rust use rocket::catch; #[catch(404)] fn not_found(req: &Request) -> T { .. } ``` As with routes, the return type (here `T`) must implement `Responder`. A concrete implementation may look like: ```rust #[catch(404)] fn not_found(req: &Request) -> String { format!("Sorry, '{}' is not a valid path.", req.uri()) } ``` Also as with routes, Rocket needs to know about a catcher before it is used to handle errors. The process, known as "registering" a catcher, is similar to mounting a route: call the `register` method with a list of catchers via the `catchers!` macro. The invocation to add the **404** catcher declared above looks like: ```rust rocket::ignite().register(catchers![not_found]) ``` Unlike route request handlers, catchers take exactly zero or one parameters. If the catcher takes a parameter, it must be of type [`&Request`](https://api.rocket.rs/rocket/struct.Request.html) The [error catcher example](https://github.com/SergioBenitez/Rocket/tree/v0.4.0-dev/examples/errors) on GitHub illustrates their use in full.