custom-format

This crate extends the standard formatting syntax with custom format specifiers, by providing custom formatting macros.

It uses : (a space and a colon) as a separator before the format specifier, which is not a syntax currently accepted and allows supporting standard specifiers in addition to custom specifiers.

Example

use custom_format::{CustomFormat, CustomFormatter};

use core::fmt;

pub struct DateTime {
    year: i32,
    month: u8,
    month_day: u8,
    hour: u8,
    minute: u8,
    second: u8,
    nanoseconds: u32,
}

impl CustomFormat for DateTime {
    fn fmt(&self, f: &mut fmt::Formatter, spec: &str) -> fmt::Result {
        match spec {
            // Year with pad for at least 4 digits
            "%Y" => write!(f, "{:04}", self.year),
            // Year % 100 (00..99)
            "%y" => write!(f, "{:02}", (self.year % 100).abs()),
            // Month of the year, zero-padded (01..12)
            "%m" => write!(f, "{:02}", self.month),
            // Day of the month, zero-padded (01..31)
            "%d" => write!(f, "{:02}", self.month_day),
            // Hour of the day, 24-hour clock, zero-padded (00..23)
            "%H" => write!(f, "{:02}", self.hour),
            // Minute of the hour (00..59)
            "%M" => write!(f, "{:02}", self.minute),
            // Second of the minute (00..60)
            "%S" => write!(f, "{:02}", self.second),
            // Nanosecond (9 digits)
            "%9N" => write!(f, "{:09}", self.nanoseconds),
            // Date (%m/%d/%y)
            "%D" => write!(
                f,
                "{}/{}/{}",
                CustomFormatter::new(self, "%m"),
                CustomFormatter::new(self, "%d"),
                CustomFormatter::new(self, "%y")
            ),
            // The ISO 8601 date format (%Y-%m-%d)
            "%F" => write!(
                f,
                "{}-{}-{}",
                CustomFormatter::new(self, "%Y"),
                CustomFormatter::new(self, "%m"),
                CustomFormatter::new(self, "%d")
            ),
            // 24-hour time (%H:%M:%S)
            "%T" => write!(
                f,
                "{}:{}:{}",
                CustomFormatter::new(self, "%H"),
                CustomFormatter::new(self, "%M"),
                CustomFormatter::new(self, "%S")
            ),
            // Incorrect format
            _ => Err(fmt::Error),
        }
    }
}

fn main() {
    let date_time = DateTime {
        year: 1836,
        month: 5,
        month_day: 18,
        hour: 23,
        minute: 45,
        second: 54,
        nanoseconds: 123456789,
    };

    // Expands to:
    //
    // match (&(date_time), &("The date time is")) {
    //     (arg0, arg1) => {
    //         ::std::println!(
    //             "{0}: {1}-{2}-{3} {4}:{5}:{6}.{7}",
    //             arg1,
    //             ::custom_format::CustomFormatter::new(arg0, "%Y"),
    //             ::custom_format::CustomFormatter::new(arg0, "%m"),
    //             ::custom_format::CustomFormatter::new(arg0, "%d"),
    //             ::custom_format::CustomFormatter::new(arg0, "%H"),
    //             ::custom_format::CustomFormatter::new(arg0, "%M"),
    //             ::custom_format::CustomFormatter::new(arg0, "%S"),
    //             ::custom_format::CustomFormatter::new(arg0, "%9N")
    //         )
    //     }
    // }
    //
    // Output: "The date time is: 1836-05-18 23:45:54.123456789"
    //
    custom_format::println!(
        "{1}: {0 :%Y}-{0 :%m}-{0 :%d} {0 :%H}:{0 :%M}:{0 :%S}.{0 :%9N}",
        date_time,
        "The date time is"
    );
}