CSV rust

use std::fs;
use std::io::BufRead;
use std::io::Write;
use std::io;
use std::env;


fn replace_col(inpath: &str, outpath: &str, column: &str,
                   replacement: &str) -> io::Result<()>
{
    let infile = fs::File::open(inpath)?;
    let reader = io::BufReader::new(infile);
    let outfile = fs::File::create(outpath)?;
    let mut writer = io::BufWriter::new(outfile);
    let mut replacenum: usize = 0;
    let mut lines = reader.lines();
    // handle header
    if let Some(line) = lines.next() { // could be empty file
        let header = line?; // could be IO error
        let mut splitted = header.split(',');
        // search for column label
        if let Some(colnum) = splitted.position(|colname| colname == column) {
            replacenum = colnum;
        }
        else {
            return Err(io::Error::new(
                io::ErrorKind::InvalidData, "Column not found"));
        }
        // pass header through unchanged
        writer.write_fmt(format_args!("{}\n", header))?;
    }
    for line in lines {
        let content = line?; // could be IO error
        let splitted = content.splitn(replacenum + 2, ',');
        // figure out where to insert the replacement string
        let mut replaced = splitted.enumerate()
            .map(|(colnum, orig)|
                 if colnum == replacenum { replacement }
                 else { orig });
        // first column
        if let Some(value) = replaced.next() { // could be empty string
            writer.write(value.as_bytes())?;
        }
        // all following columns. Include comma separator
        for value in replaced {
            writer.write_fmt(format_args!(",{}", value))?;
        }
        writer.write(b"\n")?;
    }
    writer.flush()?;
    Ok({})
}

fn main()
{
    let mut args = env::args().skip(1); // skip program name
    let inpath = args.next().expect("Need input file path");
    let column = args.next().expect("Need column label");
    let replacement = args.next().expect("Need replacement value");
    let outpath = args.next().expect("Need output file path");
    replace_col(&inpath, &outpath, &column, &replacement).expect("IO Error");
}