Formatting

We've seen that formatting is specified via a format string:

• format!("{}", foo) -> "3735928559"
• format!("{:x}", foo) -> "0xdeadbeef"

The same variable (foo) can be formatted differently depending on which argument type is used: x vs unspecified.

This formatting functionality is implemented via traits, and there is one trait for each argument type. The most common formatting trait is Display, which handles cases where the argument type is left unspecified: {} for instance.

use core::fmt::{Formatter, Display};
use core::fmt;

#[derive(Drop)]
struct City {
    name: ByteArray,
    // Latitude
    lat: i32,
    // Longitude
    lon: i32,
}

impl CityDisplay of Display<City> {
    // `f` is a buffer, and this method must write the formatted string into it.
    fn fmt(self: @City, ref f: Formatter) -> Result<(), fmt::Error> {
        let lat_c = if *self.lat >= 0 {
            'N'
        } else {
            'S'
        };
        let lon_c = if *self.lon >= 0 {
            'E'
        } else {
            'W'
        };

        // `write!` is like `format!`, but it will write the formatted string
        // into a buffer (the first argument).
        write!(f, "{}: {}'{} {}'{}", self.name, *self.lat, lat_c, *self.lon, lon_c)
    }
}

#[derive(Debug, Copy, Drop)]
struct Color {
    red: u8,
    green: u8,
    blue: u8,
}

fn main() {
    let dublin = City { name: "Dublin", lat: 53, lon: -6 };
    let oslo = City { name: "Oslo", lat: 59, lon: 10 };
    let vancouver = City { name: "Vancouver", lat: 49, lon: -123 };

    println!("{}", dublin);
    println!("{}", oslo);
    println!("{}", vancouver);

    let colors = array![
        Color { red: 128, green: 255, blue: 90 },
        Color { red: 0, green: 3, blue: 254 },
        Color { red: 0, green: 0, blue: 0 },
    ];

    let mut i = 0;
    loop {
        if i >= colors.len() {
            break;
        }
        // Switch this to use {} once you've added an implementation
        // for fmt::Display.
        println!("{:?}", *colors.at(i));
        i += 1;
    }
}

You can view a full list of formatting traits and their argument types in the core::fmt documentation.

See also:

core::fmt