Trait FromStr

1.0.0 (const: unstable) · Source

pub(crate) trait FromStr: Sized {
    type Err;

    // Required method
    fn from_str(s: &str) -> Result<Self, Self::Err>;
}

Expand description

Parse a value from a string

FromStr’s from_str method is often used implicitly, through str’s parse method. See parse’s documentation for examples.

FromStr does not have a lifetime parameter, and so you can only parse types that do not contain a lifetime parameter themselves. In other words, you can parse an i32 with FromStr, but not a &i32. You can parse a struct that contains an i32, but not one that contains an &i32.

§Input format and round-tripping

The input format expected by a type’s FromStr implementation depends on the type. Check the type’s documentation for the input formats it knows how to parse. Note that the input format of a type’s FromStr implementation might not necessarily accept the output format of its Display implementation, and even if it does, the Display implementation may not be lossless so the round-trip may lose information.

However, if a type has a lossless Display implementation whose output is meant to be conveniently machine-parseable and not just meant for human consumption, then the type may wish to accept the same format in FromStr, and document that usage. Having both Display and FromStr implementations where the result of Display cannot be parsed with FromStr may surprise users.

§Examples

Basic implementation of FromStr on an example Point type:

use std::str::FromStr;

#[derive(Debug, PartialEq)]
struct Point {
    x: i32,
    y: i32
}

#[derive(Debug, PartialEq, Eq)]
struct ParsePointError;

impl FromStr for Point {
    type Err = ParsePointError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let (x, y) = s
            .strip_prefix('(')
            .and_then(|s| s.strip_suffix(')'))
            .and_then(|s| s.split_once(','))
            .ok_or(ParsePointError)?;

        let x_fromstr = x.parse::<i32>().map_err(|_| ParsePointError)?;
        let y_fromstr = y.parse::<i32>().map_err(|_| ParsePointError)?;

        Ok(Point { x: x_fromstr, y: y_fromstr })
    }
}

let expected = Ok(Point { x: 1, y: 2 });
// Explicit call
assert_eq!(Point::from_str("(1,2)"), expected);
// Implicit calls, through parse
assert_eq!("(1,2)".parse(), expected);
assert_eq!("(1,2)".parse::<Point>(), expected);
// Invalid input string
assert!(Point::from_str("(1 2)").is_err());

Required Associated Types§

1.0.0 (const: unstable) · Source

type Err

The associated error which can be returned from parsing.

Required Methods§

1.0.0 (const: unstable) · Source

fn from_str(s: &str) -> Result<Self, Self::Err>

Parses a string s to return a value of this type.

If parsing succeeds, return the value inside Ok, otherwise when the string is ill-formatted return an error specific to the inside Err. The error type is specific to the implementation of the trait.

§Examples

Basic usage with i32, a type that implements FromStr:

use std::str::FromStr;

let s = "5";
let x = i32::from_str(s).unwrap();

assert_eq!(5, x);

Dyn Compatibility§

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Trait FromStr Copy item path

§Input format and round-tripping

§Examples

Required Associated Types§

type Err

Required Methods§

fn from_str(s: &str) -> Result<Self, Self::Err>

§Examples

Dyn Compatibility§

Implementors§

impl FromStr for TagMode

type Err = Error

impl FromStr for Num

type Err = Either<ParseIntError, ParseFloatError>

impl FromStr for log::Level

type Err = ParseLevelError

impl FromStr for log::LevelFilter

type Err = ParseLevelError

impl FromStr for serde_json::value::Value

type Err = Error

impl FromStr for Cipher

type Err = LabelError

impl FromStr for Algorithm

type Err = LabelError

impl FromStr for EcdsaCurve

type Err = LabelError

impl FromStr for HashAlg

type Err = LabelError

impl FromStr for KdfAlg

type Err = LabelError

impl FromStr for ssh_key_fork_arti::fingerprint::Fingerprint

type Err = Error

impl FromStr for HostPatterns

type Err = Error

impl FromStr for Marker

type Err = Error

impl FromStr for Month

type Err = InvalidVariant

impl FromStr for Weekday

type Err = InvalidVariant

impl FromStr for toml::value::Value

type Err = Error

impl FromStr for toml::value::Value

type Err = Error

impl FromStr for Item

type Err = TomlError

impl FromStr for toml_edit::value::Value

type Err = TomlError

impl FromStr for Address

type Err = Error

impl FromStr for PaddingLevel

type Err = ParseError

impl FromStr for tor_general_addr::general::SocketAddr

type Err = AddrParseError

impl FromStr for RelayId

type Err = RelayIdError

impl FromStr for RelayIdType

type Err = RelayIdError

impl FromStr for PtTargetAddr

type Err = BridgeAddrError

impl FromStr for AuthCertVersion

type Err = ParseError

impl FromStr for tor_netdoc::doc::netstatus::DirectorySignatureHashAlgo

type Err = ParseError

impl FromStr for SoftwareVersion

type Err = Error

impl FromStr for RelayPlatform

type Err = Error

impl FromStr for tor_netdoc::parse2::poc::netstatus::DirectorySignatureHashAlgo

type Err = ParseError

impl FromStr for NdaNetworkStatusVersion

type Err = ParseError

impl FromStr for RelayFamilyId

type Err = Error

impl FromStr for Ed25519AlgorithmString

type Err = FromStrError

impl FromStr for InternetHost

type Err = InvalidInternetHost

impl FromStr for DigestName

type Err = Void

Trait FromStr