How to handle errors from the Read::read_to_end method? - error-handling

Question

I am reading a file and parsing the data:
fn main() {
parse_torrentfile("ubuntu-16.04.1-server-amd64.torrent");
}
fn parse_file(filename: &'static str) {
let mut f = File::open(&Path::new(filename)).unwrap();
let mut v: Vec<u8> = Vec::new();
let file_content = f.read_to_end(&mut v);
println!("{:?}", file_content);
}
If ubuntu-16.04.1-server-amd64.torrent is present, this works well, but if it isn't present, this error occurs:
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 2, message: "No such file or directory" } }'
How can I print "Not Exist File" or "process file parsing"?

Answer 1

You should be matching on the Result instead of calling unwrap() or expect(). Once you've got the error, you can either print it out or look at the ErrorKind if you want to handle different errors differently.
use std::fs::File;
use std::path::Path;
use std::io::Read;
fn main() {
parse_file("doesnt_exist.txt");
}
fn parse_file(filename: &'static str) {
let mut f = match File::open(&Path::new(filename)) {
Ok(f) => f,
Err(e) => {
use std::io::ErrorKind::*;
println!("Got error: {}", e);
match e.kind() {
NotFound => {
println!("File not found");
}
k => {
println!("Error: {:?}", k);
}
}
return;
}
};
let mut v: Vec<u8> = Vec::new();
let file_content = f.read_to_end(&mut v);
println!("{:?}", file_content);
}
Playground

Answer 2

Probably the most idiomatic approach would be propagating the result outside parse_file, printing the error in main.
use std::fs::File;
use std::io::{Read, Error as IoError};
use std::path::Path;
fn main() {
match parse_file("ubuntu-16.04.1-server-amd64.torrent") {
Err(e) => println!("{}", e),
_ => {}
}
}
fn parse_file(filename: &'static str) -> Result<(), IoError> {
let mut f = File::open(&Path::new(filename))?;
let mut v: Vec<u8> = Vec::new();
let file_content = f.read_to_end(&mut v);
println!("{:?}", file_content);
Ok(())
}
If you want to print the error in one line and then propagating it, you could do something like this instead:
fn main() {
parse_file("ubuntu-16.04.1-server-amd64.torrent");
}
fn parse_file(filename: &'static str) -> Result<(), IoError> {
let mut f = File::open(&Path::new(filename)).map_err(|e| {
println!("{}", e);
e
})?;
let mut v: Vec<u8> = Vec::new();
let file_content = f.read_to_end(&mut v);
println!("{:?}", file_content);
Ok(())
}