Trait rocket::request::FromParam

pub trait FromParam<'a>: Sized {
    type Error: Debug;
    fn from_param(param: &'a RawStr) -> 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, &RawStr> type for the <id> parameter as follows:

#[get("/<id>")]
fn hello(id: Result<usize, &RawStr>) -> 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.

• f32, f64, isize, i8, i16, i32, i64, usize, u8, u16, u32, u64, bool, IpAddr, Ipv4Addr, Ipv6Addr, SocketAddrV4, SocketAddrV6, SocketAddr

  A value is parsed successfully if the from_str method from the given type returns successfully. Otherwise, the raw path segment is returned in the Err value.

• &RawStr

  This implementation always returns successfully.

  The path segment is passed directly with no modification.

• String

  Percent decodes the path segment. If the decode is successful, the decoded string is returned. Otherwise, an Err with the original path segment is returned.

• Cow

  Percent decodes the path segment, allocating only when necessary. If the decode is successful, the decoded string is returned. Otherwise, an Err with the original path segment is returned.

• Option<T> where T: FromParam

  This implementation always returns successfully.

  The path segment is parsed by T's FromParam implementation. If the parse succeeds, a Some(parsed_value) is returned. Otherwise, a None is returned.

• Result<T, T::Error> where T: FromParam

  This implementation always returns successfully.

  The path segment is parsed by T's FromParam implementation. The returned Result value is returned.

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;
use rocket::http::RawStr;

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

    fn from_param(param: &'r RawStr) -> Result<Self, Self::Error> {
        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

type Error: Debug

The associated error to be returned when parsing fails.

Required Methods

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

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

Implementations on Foreign Types

impl<'a> FromParam<'a> for String

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<String, Self::Error>

impl<'a> FromParam<'a> for Cow<'a, str>

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Cow<'a, str>, Self::Error>

impl<'a> FromParam<'a> for i8

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for i16

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for i32

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for i64

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for isize

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for u8

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for u16

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for u32

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for u64

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for usize

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for f32

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for f64

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for bool

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for IpAddr

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for Ipv4Addr

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for Ipv6Addr

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for SocketAddrV4

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for SocketAddrV6

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a> FromParam<'a> for SocketAddr

type Error = &'a RawStr

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a, T: FromParam<'a>> FromParam<'a> for Result<T, T::Error>

type Error = !

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

impl<'a, T: FromParam<'a>> FromParam<'a> for Option<T>

type Error = !

fn from_param(param: &'a RawStr) -> Result<Self, Self::Error>

Implementors

• impl<'a> FromParam<'a> for &'a RawStr type Error = ();