units/src/conv/mod.rs

use std::error;
use std::fmt;
use std::convert::Infallible;

pub mod bytes;

pub trait Conversion
{
    const UNIT: &'static str;

    type Output: fmt::Display;
    type Unit: fmt::Display = Self::Output;
    type Error: Into<ConversionError> = ConversionError;

    fn convert(&self, input: &str) -> Result<(Self::Output, Option<Self::Unit>), Self::Error>;
}

#[derive(Debug)]
pub struct ConversionError(String);

impl error::Error for ConversionError{}
impl fmt::Display for ConversionError
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result
    {
	write!(f, "conversion failed: {}", self.0)
    }
}

impl From<Infallible> for ConversionError
{
    fn from(from: Infallible) -> Self
    {
	match from {}
    }
}

fn print_output<T,U>((value, units): (T, Option<U>))
where T: fmt::Display,
      U: fmt::Display
{
    println!("{}", value);
    if let Some(units) = units
    {
	eprintln!("{}", units);
    }
}

/// Dispatch a conversion of `value` for type `unit` and print the result to stdout.
pub fn dispatch_out(unit: impl AsRef<str>, value: impl AsRef<str>) -> Result<(), ConversionError>
{
    match unit.as_ref() {
	bytes::Bytes::UNIT => print_output(bytes::Bytes.convert(value.as_ref())?),
	name => return Err(ConversionError(format!("Unknown unit {}", name))),
    }

    Ok(())
}