Trait rocket::request::FromParam [−] [src]

pub trait FromParam<'a>: Sized {
    type Error: Debug;
    fn from_param(param: &'a str) -> Result<Self, Self::Error>;
}

Trait to convert a dynamic path segment string to a concrete value.

This trait is used by Rocket's code generation facilities to parse dynamic path segment string values into a given type. That is, when a path contains a dynamic segment <param> where param has some type T that implements FromParam, T::from_param will be called.

Forwarding

If the conversion fails, the incoming request will be forwarded to the next matching route, if any. For instance, consider the following route and handler for the dynamic "/<id>" path:

#[get("/<id>")]
fn hello(id: usize) -> String {
    ...
}

If usize::from_param returns an Ok(usize) variant, the encapsulated value is used as the id function parameter. If not, the request is forwarded to the next matching route. Since there are no additional matching routes, this example will result in a 404 error for requests with invalid id values.

Catching Errors

Sometimes, a forward is not desired, and instead, we simply want to know that the dynamic path segment could not be parsed into some desired type T. In these cases, types of Option<T> or Result<T, T::Error> can be used. These types implement FromParam themselves. Their implementations always return successfully, so they never forward. They can be used to determine if the FromParam call failed and to retrieve the error value from the failed from_param call.

For instance, imagine you've asked for an <id> as a usize. To determine when the <id> was not a valid usize and retrieve the string that failed to parse, you can use a Result<usize, &str> type for the <id> parameter as follows:

#[get("/<id>")]
fn hello(id: Result<usize, &str>) -> String {
    match id {
        Ok(id_num) => format!("usize: {}", id_num),
        Err(string) => format!("Not a usize: {}", string)
    }
}

Provided Implementations

Rocket implements FromParam for several standard library types. Their behavior is documented here.

str vs. String

Paths are URL encoded. As a result, the str FromParam implementation returns the raw, URL encoded version of the path segment string. On the other hand, String decodes the path parameter, but requires an allocation to do so. This tradeoff is similiar to that of form values, and you should use whichever makes sense for your application.

Example

Say you want to parse a segment of the form:

[a-zA-Z]+:[0-9]+

into the following structure, where the string before the : is stored in key and the number after the colon is stored in value:

struct MyParam<'r> {
    key: &'r str,
    value: usize
}

The following implementation accomplishes this:

use rocket::request::FromParam;

impl<'r> FromParam<'r> for MyParam<'r> {
    type Error = &'r str;

    fn from_param(param: &'r str) -> Result<MyParam<'r>, &'r str> {
        let (key, val_str) = match param.find(':') {
            Some(i) if i > 0 => (&param[..i], &param[(i + 1)..]),
            _ => return Err(param)
        };

        if !key.chars().all(|c| (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')) {
            return Err(param);
        }

        val_str.parse().map(|value| {
            MyParam {
                key: key,
                value: value
            }
        }).map_err(|_| param)
    }
}

With the implementation, the MyParam type can be used as the target of a dynamic path segment:

#[get("/<key_val>")]
fn hello(key_val: MyParam) -> String {
    ...
}

Associated Types

The associated error to be returned when parsing fails.

Required Methods

Parses an instance of Self from a dynamic path parameter string or returns an Error if one cannot be parsed.

Implementors