match Expressions

match can be used to check a value against one or more options:

fn main() {
    let val = 1;
    match val {
        1 => println!("one"),
        10 => println!("ten"),
        100 => println!("one hundred"),
        _ => {
            println!("something else");
        }
    }
}

Like if expressions, match can also return a value;

fn main() {
    let flag = true;
    let val = match flag {
        true => 1,
        false => 0,
    };
    println!("The value of {flag} is {val}");
}
This slide should take about 5 minutes.

  • match arms are evaluated from top to bottom, and the first one that matches has its corresponding body executed.

  • There is no fall-through between cases the way that switch works in other languages.

  • The body of a match arm can be a single expression or a block. Technically this is the same thing, since blocks are also expressions, but students may not fully understand that symmetry at this point.

  • match expressions need to be exhaustive, meaning they either need to cover all possible values or they need to have a default case such as _. Exhaustiveness is easiest to demonstrate with enums, but enums haven’t been introduced yet. Instead we demonstrate matching on a bool, which is the simplest primitive type.

  • This slide introduces match without talking about pattern matching, giving students a chance to get familiar with the syntax without front-loading too much information. We’ll be talking about pattern matching in more detail tomorrow, so try not to go into too much detail here.

More to Explore

  • To further motivate the usage of match, you can compare the examples to their equivalents written with if. In the second case matching on a bool an if {} else {} block is pretty similar. But in the first example that checks multiple cases, a match expression can be more concise than if {} else if {} else if {} else.

  • match also supports match guards, which allow you to add an arbitrary logical condition that will get evaluated to determine if the match arm should be taken. However talking about match guards requires explaining about pattern matching, which we’re trying to avoid on this slide.