FromStr Trait

The FromStr trait lets you define how to parse a string into your custom type. Once implemented, you get .parse() for free.

The Trait

pub trait FromStr {
    type Err;
    fn from_str(s: &str) -> Result<Self, Self::Err>;
}

Example: Parsing a Color

use std::str::FromStr;

#[derive(Debug)]
struct Color {
    r: u8,
    g: u8,
    b: u8,
}

impl FromStr for Color {
    type Err = String;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        // Expect format "r,g,b"
        let parts: Vec<&str> = s.split(',').collect();
        if parts.len() != 3 {
            return Err(format!("expected 3 values, got {}", parts.len()));
        }

        let r = parts[0].trim().parse::<u8>().map_err(|e| e.to_string())?;
        let g = parts[1].trim().parse::<u8>().map_err(|e| e.to_string())?;
        let b = parts[2].trim().parse::<u8>().map_err(|e| e.to_string())?;

        Ok(Color { r, g, b })
    }
}

Usage

Once FromStr is implemented, you can use .parse() with turbofish syntax:

let color: Color = "255, 128, 0".parse().unwrap();
// or
let color = "255, 128, 0".parse::<Color>().unwrap();

This is the same mechanism that makes "42".parse::<i32>() work — the standard library implements FromStr for all the primitive types.