pub trait FromParam<'a>: Sized {
    type Error: Debug;

    fn from_param(param: &'a str) -> Result<Self, Self::Error>;
Expand description

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.


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:

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:

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.

    • Primitive types: f32, f64, isize, i8, i16, i32, i64, i128, usize, u8, u16, u32, u64, u128, bool
    • IpAddr and SocketAddr types: IpAddr, Ipv4Addr, Ipv6Addr, SocketAddrV4, SocketAddrV6, SocketAddr
    • NonZero* types: NonZeroI8, NonZeroI16, NonZeroI32, NonZeroI64, NonZeroI128, NonZeroIsize, NonZeroU8, NonZeroU16, NonZeroU32, NonZeroU64, NonZeroU128, NonZeroUsize

    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.

  • &str, String

    This implementation always returns successfully.

    Returns the percent-decoded path segment with invalid UTF-8 byte sequences replaced by � U+FFFD.

  • 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.


Say you want to parse a segment of the form:


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<Self, Self::Error> {
        // We can convert `param` into a `str` since we'll check every
        // character for safety later.
        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.is_ascii_alphabetic()) {
            return Err(param);

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

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

fn hello(key_val: MyParam) -> String {

Required Associated Types

The associated error to be returned if parsing/validation fails.

Required Methods

Parses and validates an instance of Self from a path parameter string or returns an Error if parsing or validation fails.

Implementations on Foreign Types