rust main fn read dir result error cannot compile

Question

I am newbie on rust and trying an example to read dir on Win10 from:
https://rust-lang-nursery.github.io/rust-cookbook/file/dir.html

However the code cannot compile due to return result is expecting 2 type arguments:

std::io::Error  
std::time::SystemTimeError

use std::fs;

fn main() -> Result<()> {
    let current_dir = "C:/temp/";

    println!(
        "Entries modified in the last 24 hours in {:?}:",
        current_dir
    );

    for entry in fs::read_dir(current_dir)? {
        let entry = entry?;
        let path = entry.path();

        let metadata = fs::metadata(&path)?;
        let last_modified = metadata.modified()?.elapsed()?.as_secs();

        if last_modified < 24 * 3600 && metadata.is_file() {
            println!(
                "Last modified: {:?} seconds, is read only: {:?}, size: {:?} bytes, filename: {:?}",
                last_modified,
                metadata.permissions().readonly(),
                metadata.len(),
                path.file_name().ok_or("No filename")?
            );
        }
    }

    Ok(())
}

Here is the playground link:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=3af53ffa4d1cc6ae4c774f36514dd657

Masklinn Masklinn · Accepted Answer · 2020-06-11T08:22:48

If you click the "expand" button in the top-right corner of the snippet, you'll see that there are bits which are hidden by default:

use error_chain::error_chain;

use std::{env, fs};

error_chain! {
    foreign_links {
        Io(std::io::Error);
        SystemTimeError(std::time::SystemTimeError);
    }
}

fn main() -> Result<()> {
   // ...

those bits turn out to be quite relevant because by default error_chain! automatically generates a Result alias such that you only need to specify the "Ok" parameter and the other is the generated error type (similar to std::io::Result, which is a typedef for std::result::Result<T, std::io::Error>).

So you need to either expand the snippet before copying it (error_chain is available on the playground) or hand-roll the Result alias... and possibly error type though here you could just define type Result<T> = Result<T, Box<dyn std::error::Error>> and it should work.

This behaviour (of the cookbook) is explained in the about page's a note about error handling section though I can't say I'm fond of it, given cookbooks are generally not things you read start-to-end.

rust main fn read dir result error cannot compile

1 Answers