various Rust ways of displaying stuff to the screen

Here is a simple way of doing it:

use std::io::{self, Write};

fn main() {
    io::stdout().write_all(b"some outputn");
}

I do however get this warning when I build it:

$ rustc main.rs
main.rs:4:5: 4:42 warning: unused result which must be used, #[warn(unused_must_use)] on by default
main.rs:4     io::stdout().write_ln(b"some output");
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Let's kill it:

use std::io::{self, Write};

fn main() {
    io::stdout().write_all(b"some output").unwrap();
}

What I did is call unwrap which basically asks our operation to panic! in case of some error. Think of it as a shortcut of:

use std::io::{self, Write};
fn main() {
    match io::stdout().write_all(b"some output") {
        Ok(_) => (),
        Err(_) => panic!(),
    }
}

Some docs:

• std::io::Writer::write_line
• core::result::Result::unwrap
• std::panic!

---

What if we wanted to do some string formatting:

use std::io::{self, Write};

fn main() {
    let second_word = "output"; let text = format!("some {}n",
    second_word); io::stdout().write_all(text.as_bytes()).unwrap();
}

But there is a shortcut for the code above:

use std::io::{self, Write};

fn main() {
    let second_word = "output";
    writeln!(&mut io::stdout(), "some {}", second_word).unwrap();
}

There is an even shorter shortcut:

fn main() {
    let second_word = "output"; println!("some {}", second_word);
}

Question to self: do we get a panic! if println gets an I/O error?

Some docs:

• std::format!
• std::writeln!
• std::println!

---

update: there is now eprintln!, for sending output to stderr

So, why bother with all these many ways when there's a simple println!? Flexibility. One example I can think of is writing to stderr. There is no simple macro for that (println! is for stdout). Short of implementing our own macro, We'd have to do something like the following:

use std::io;

fn main() {
    writeln!(&mut io::stderr(), "some output");
}

You can test that it really goes to stderr by running it like:

$ ./main > /dev/null
some output