csv.rs for expressive rust

csv.rs

use std::env;
use std::io::{self, BufReader, BufWriter, BufRead, Write};
use std::fs::File;
use std::process;

enum Error {
    Io(io::Error),
    Program(&'static str),
}

impl From<io::Error> for Error {
    fn from(e: io::Error) -> Error {
        Error::Io(e)
    }
}

impl From<&'static str> for Error {
    fn from(e: &'static str) -> Error {
        Error::Program(e)
    }
}

fn main() {
    let mut args = env::args();
    let program = args.next().expect("cannot get program name");
    let result = match (args.next(), args.next(), args.next(), args.next()) {
        (Some(input), Some(column_name), Some(replacement), Some(output)) => {
            replace_column_in_csv(&input, &output, &column_name, &replacement)
        }
        _ => Err(Error::Program("invalid arguments")),
    };

    match result {
        Ok(()) => process::exit(0),
        Err(Error::Io(io_error)) => eprintln!("{}: I/O Error: {}", program, io_error),
        Err(Error::Program(error)) => eprintln!("{}: {}", program, error),
    }
    process::exit(1);
}

fn replace_column_in_csv(
    input_filename: &str,
    output_filename: &str,
    replaced_column_name: &str,
    replacement_value: &str,
) -> Result<(), Error> {
    let mut input_lines = BufReader::new(File::open(input_filename)?).lines();
    let mut output_file = BufWriter::new(File::create(output_filename)?);

    let column_names = input_lines.next().ok_or("input file is empty")??;
    let replaced_column_index = column_names
        .split(',')
        .position(|name| name == replaced_column_name)
        .ok_or("column name doesn’t exist in the input file")?;
    output_file.write_all(column_names.as_bytes())?;
    output_file.write_all(b"\n")?;

    for line in input_lines {
        let line = line?;
        let mut row = line.split(',').enumerate().map(|(column_index, value)| {
            if column_index == replaced_column_index {
                replacement_value
            } else {
                value
            }
        });
        output_file.write_all(
            row.next().ok_or("empty row")?.as_bytes(),
        )?;
        for value in row {
            output_file.write_all(b",")?;
            output_file.write_all(value.as_bytes())?;
        }
        output_file.write_all(b"\n")?;
    }

    Ok(())
}