Is println! an expression or a statement?

Question

I'm working through the Rust documentation (book) and am confused about it's use of semicolons to separate statements.

In one example it uses the println! macro as a statement terminated by a semicolon:

use std::cmp::Ordering;

fn cmp(a: i32, b: i32) -> Ordering {
    if a < b { Ordering::Less }
    else if a > b { Ordering::Greater }
    else { Ordering::Equal }
}

fn main() {
    let x = 5;
    let y = 10;

    let ordering = cmp(x, y);

    if ordering == Ordering::Less {
        println!("less");
    } else if ordering == Ordering::Greater {
        println!("greater");
    } else if ordering == Ordering::Equal {
        println!("equal");
    }
}

And when using the match expression to simplify it...

use std::cmp::Ordering;

fn cmp(a: i32, b: i32) -> Ordering {
    if a < b { Ordering::Less }
    else if a > b { Ordering::Greater }
    else { Ordering::Equal }
}

fn main() {
    let x = 5;
    let y = 10;

    match cmp(x, y) {
        Ordering::Less => println!("less"),
        Ordering::Greater => println!("greater"),
        Ordering::Equal => println!("equal"),
    }
}

The semicolons are gone, indicating that println! is not a statement, but an expression in this context. I don't understand why... what am I missing?

Answer 1

println!() is a macro expanding to an expression. It has no useful return value, but it is an expression (mostly because almost everything is an expression, including function calls and blocks).

There is a convention — I don't know how common it is, but I for one follow it — to treat ()-producing function calls as pseudo-statements, used only for their side effects, and hence terminate them with a semicolon even when not strictly necessary. This is done in the first snippet.

In the second snippet, we have a match, and match arms expect an expression. One can use a block (Ordering::Less => { println!("less"); }) but this is quite a bit of syntactic noise just to make extra clear that the arms are used for their side effects, so I suppose the author just left it off.