Writing to a file or stdout in Rust

Question

I'm learning Rust, and I'm somewhat stumped.

I'm trying to give the user the option of writing output to stdout or to a supplied filename.

I started with the example code that's given for using extra::getopts located here. From there, in the do_work function, I'm trying to do this:

use std::io::stdio::stdout;
use std::io::buffered::BufferedWriter;

fn do_work( input: &str, out: Option<~str> ) {
    println!( "Input:  {}", input );
    println!( "Output: {}", match out {
        Some(x) => x,
        None    => ~"Using stdout"
    } );
    let out_writer = BufferedWriter::new( match out {
        // I know that unwrap is frowned upon, 
        // but for now I don't want to deal with the Option.
        Some(x) => File::create( &Path::new( x ) ).unwrap(),
        None    => stdout()
    } );
    out_writer.write( bytes!( "Test output\n" ) );
}

But it outputs the following error:

test.rs:25:43: 28:6 error: match arms have incompatible types: expected `std::io::fs::File` but found `std::io::stdio::StdWriter` (expected struct std::io::fs::File but found struct std::io::stdio::StdWriter)
test.rs:25     let out_writer = BufferedWriter::new( match out {
test.rs:26         Some(x) => File::create( &Path::new( x ) ).unwrap(),
test.rs:27         None    => stdout()
test.rs:28     } );
test.rs:25:22: 25:41 error: failed to find an implementation of trait std::io::Writer for [type error]
test.rs:25     let out_writer = BufferedWriter::new( match out {
                            ^~~~~~~~~~~~~~~~~~~

But I don't understand what the issue is because both File and StdWriter implement the Writer Trait. Can someone explain what I'm doing wrong?

Thanks!

Answer 1

Yes, both implement Write, but the problem is BufWriter is expecting a type T that implements Writer, and that T can't be File and Stdout at the same time.

You must cast both to the common type (either Box<dyn Write> or &dyn Write, but since you cannot return references you have to use Box):

fn do_work(input: &str, out: Option<String>) {
    let mut out_writer: Box<dyn Write> = BufWriter::new(match out {
        Some(ref x) => Box::new(File::create(&Path::new(x)).unwrap()),
        None => Box::new(stdout()),
    });
    out_writer.write(b"Test output\n").unwrap();
}

You should also handle errors properly, not just using unwrap (used in example for simplicity).

Answer 2

A lot has changed in Rust since 2014, so here is an answer that works for me using Rust 1.15.1:

let out_writer = match out {
    Some(x) => {
        let path = Path::new(x);
        Box::new(File::create(&path).unwrap()) as Box<dyn Write>
    }
    None => Box::new(io::stdout()) as Box<dyn Write>,
};

This is pretty much the same as @Arjan's answer, except that ~ was replaced by Box, and some names have changed. I'm leaving out BufferedWriter, but if you want that, I believe it is now named BufWriter.