Parsing & Conversion

Parse strings to types with str::parse / FromStr. Convert to string via to_string, Display, or format!.

Recipe

fn parse_port(s: &str) -> Result<u16, std::num::ParseIntError> {
    s.trim().parse()
}

When to reach for this: Config, CLI args, protocol fields, user input validation.

Working Example

use std::str::FromStr;
 
#[derive(Debug)]
struct Color { r: u8, g: u8, b: u8 }
 
impl FromStr for Color {
    type Err = &'static str;
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let mut parts = s.split(',');
        let r: u8 = parts.next().ok_or("r")?.parse().map_err(|_| "r")?;
        let g: u8 = parts.next().ok_or("g")?.parse().map_err(|_| "g")?;
        let b: u8 = parts.next().ok_or("b")?.parse().map_err(|_| "b")?;
        Ok(Color { r, g, b })
    }
}
 
fn main() {
    println!("{:?}", "255,128,0".parse::<Color>());
}

What this demonstrates:

• Custom FromStr for domain type
• Turbofish parse::<Color>()
• Error type choice &'static str or custom enum

Deep Dive

From/Into for infallible conversions. TryFrom for fallible. parse requires FromStr.

Gotchas

• unwrap parse - Panic on bad input. Fix: ? or match.
• Locale number formats - parse not locale-aware. Fix: Dedicated crate.
• Trim forgotten - Leading spaces fail int parse. Fix: trim().
• Partial parse consumption - from_str should consume all or document remainder.
• Lossy utf8 - from_utf8_lossy not parsing numbers.

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Alternatives

FAQs

Related

• Strings Basics
• Formatting
• Error Handling Basics
• clap

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.