Trait FromStr

1.0.0 · Source
pub 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 · Source

type Err

The associated error which can be returned from parsing.

Required Methods§

1.0.0 · 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.

Implementors§

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impl FromStr for IpAddr

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impl FromStr for SocketAddr

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impl FromStr for Month

Parsing a str into a Month uses the format %B.

§Example

use chrono::Month;

assert_eq!("January".parse::<Month>(), Ok(Month::January));
assert!("any day".parse::<Month>().is_err());

The parsing is case-insensitive.

assert_eq!("fEbruARy".parse::<Month>(), Ok(Month::February));

Only the shortest form (e.g. jan) and the longest form (e.g. january) is accepted.

assert!("septem".parse::<Month>().is_err());
assert!("Augustin".parse::<Month>().is_err());
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impl FromStr for Weekday

Parsing a str into a Weekday uses the format %A.

§Example

use chrono::Weekday;

assert_eq!("Sunday".parse::<Weekday>(), Ok(Weekday::Sun));
assert!("any day".parse::<Weekday>().is_err());

The parsing is case-insensitive.

assert_eq!("mON".parse::<Weekday>(), Ok(Weekday::Mon));

Only the shortest form (e.g. sun) and the longest form (e.g. sunday) is accepted.

assert!("thurs".parse::<Weekday>().is_err());
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impl FromStr for IpNet

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impl FromStr for log::Level

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impl FromStr for log::LevelFilter

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impl FromStr for Value

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impl FromStr for bool

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impl FromStr for char

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impl FromStr for f16

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impl FromStr for f32

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impl FromStr for f64

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impl FromStr for i8

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impl FromStr for i16

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impl FromStr for i32

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impl FromStr for i64

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impl FromStr for i128

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impl FromStr for isize

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impl FromStr for u8

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impl FromStr for u16

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impl FromStr for u32

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impl FromStr for u64

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impl FromStr for u128

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impl FromStr for usize

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impl FromStr for ByteString

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impl FromStr for CString

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impl FromStr for String

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impl FromStr for Ipv4Addr

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impl FromStr for Ipv6Addr

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impl FromStr for SocketAddrV4

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impl FromStr for SocketAddrV6

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impl FromStr for NonZero<i8>

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impl FromStr for NonZero<i16>

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impl FromStr for NonZero<i32>

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impl FromStr for NonZero<i64>

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impl FromStr for NonZero<i128>

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impl FromStr for NonZero<isize>

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impl FromStr for NonZero<u8>

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impl FromStr for NonZero<u16>

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impl FromStr for NonZero<u32>

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impl FromStr for NonZero<u64>

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impl FromStr for NonZero<u128>

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impl FromStr for NonZero<usize>

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impl FromStr for OsString

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impl FromStr for PathBuf

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impl FromStr for DateTime<FixedOffset>

Accepts a relaxed form of RFC3339. A space or a ‘T’ are accepted as the separator between the date and time parts. Additional spaces are allowed between each component.

All of these examples are equivalent:

"2012-12-12T12:12:12Z".parse::<DateTime<FixedOffset>>()?;
"2012-12-12 12:12:12Z".parse::<DateTime<FixedOffset>>()?;
"2012-  12-12T12:  12:12Z".parse::<DateTime<FixedOffset>>()?;
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impl FromStr for DateTime<Local>

Accepts a relaxed form of RFC3339. A space or a ‘T’ are accepted as the separator between the date and time parts.

All of these examples are equivalent:

"2012-12-12T12:12:12Z".parse::<DateTime<Local>>()?;
"2012-12-12 12:12:12Z".parse::<DateTime<Local>>()?;
"2012-12-12 12:12:12+0000".parse::<DateTime<Local>>()?;
"2012-12-12 12:12:12+00:00".parse::<DateTime<Local>>()?;
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impl FromStr for DateTime<Utc>

Accepts a relaxed form of RFC3339. A space or a ‘T’ are accepted as the separator between the date and time parts.

All of these examples are equivalent:

"2012-12-12T12:12:12Z".parse::<DateTime<Utc>>()?;
"2012-12-12 12:12:12Z".parse::<DateTime<Utc>>()?;
"2012-12-12 12:12:12+0000".parse::<DateTime<Utc>>()?;
"2012-12-12 12:12:12+00:00".parse::<DateTime<Utc>>()?;
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impl FromStr for NaiveDate

Parsing a str into a NaiveDate uses the same format, %Y-%m-%d, as in Debug and Display.

§Example

use chrono::NaiveDate;

let d = NaiveDate::from_ymd_opt(2015, 9, 18).unwrap();
assert_eq!("2015-09-18".parse::<NaiveDate>(), Ok(d));

let d = NaiveDate::from_ymd_opt(12345, 6, 7).unwrap();
assert_eq!("+12345-6-7".parse::<NaiveDate>(), Ok(d));

assert!("foo".parse::<NaiveDate>().is_err());
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impl FromStr for NaiveDateTime

Parsing a str into a NaiveDateTime uses the same format, %Y-%m-%dT%H:%M:%S%.f, as in Debug.

§Example

use chrono::{NaiveDateTime, NaiveDate};

let dt = NaiveDate::from_ymd_opt(2015, 9, 18).unwrap().and_hms_opt(23, 56, 4).unwrap();
assert_eq!("2015-09-18T23:56:04".parse::<NaiveDateTime>(), Ok(dt));

let dt = NaiveDate::from_ymd_opt(12345, 6, 7).unwrap().and_hms_milli_opt(7, 59, 59, 1_500).unwrap(); // leap second
assert_eq!("+12345-6-7T7:59:60.5".parse::<NaiveDateTime>(), Ok(dt));

assert!("foo".parse::<NaiveDateTime>().is_err());
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impl FromStr for NaiveTime

Parsing a str into a NaiveTime uses the same format, %H:%M:%S%.f, as in Debug and Display.

§Example

use chrono::NaiveTime;

let t = NaiveTime::from_hms_opt(23, 56, 4).unwrap();
assert_eq!("23:56:04".parse::<NaiveTime>(), Ok(t));

let t = NaiveTime::from_hms_nano_opt(23, 56, 4, 12_345_678).unwrap();
assert_eq!("23:56:4.012345678".parse::<NaiveTime>(), Ok(t));

let t = NaiveTime::from_hms_nano_opt(23, 59, 59, 1_234_567_890).unwrap(); // leap second
assert_eq!("23:59:60.23456789".parse::<NaiveTime>(), Ok(t));

// Seconds are optional
let t = NaiveTime::from_hms_opt(23, 56, 0).unwrap();
assert_eq!("23:56".parse::<NaiveTime>(), Ok(t));

assert!("foo".parse::<NaiveTime>().is_err());
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impl FromStr for FixedOffset

Parsing a str into a FixedOffset uses the format %z.

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impl FromStr for HeaderName

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impl FromStr for HeaderValue

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impl FromStr for Method

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impl FromStr for StatusCode

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impl FromStr for Authority

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impl FromStr for PathAndQuery

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impl FromStr for Scheme

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impl FromStr for Uri

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impl FromStr for Name

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impl FromStr for icu_locale_core::extensions::private::other::Subtag

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impl FromStr for icu_locale_core::extensions::transform::key::Key

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impl FromStr for Attribute

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impl FromStr for icu_locale_core::extensions::unicode::key::Key

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impl FromStr for SubdivisionId

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impl FromStr for SubdivisionSuffix

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impl FromStr for Language

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impl FromStr for Region

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impl FromStr for Script

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impl FromStr for icu_locale_core::subtags::Subtag

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impl FromStr for Variant

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impl FromStr for Ipv4Net

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impl FromStr for Ipv6Net

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impl FromStr for JsonPathInst

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impl FromStr for Mime

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impl FromStr for regex::regex::bytes::Regex

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impl FromStr for regex::regex::string::Regex

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impl FromStr for Number

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impl FromStr for tracing_core::metadata::Level

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impl FromStr for tracing_core::metadata::LevelFilter

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impl FromStr for Url

Parse a string as an URL, without a base URL or encoding override.

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impl FromStr for Decimal

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impl<const CAP: usize> FromStr for ArrayString<CAP>

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impl<const N: usize> FromStr for TinyAsciiStr<N>