CODEWITHSUNDEEP
rust
Flavius
Flavius

Reputation: 13816

Conditions in a match arm without destructuring

use std::cmp::Ordering;

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

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

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

How to use match with conditions, without destructuring (because there's nothing to destructure), in the function cmp above?

The code has been adapted from the well-known example in the book which uses only if/else, however, it does not work:

src/main.rs:5:9: 5:10 error: unexpected token: `_`
src/main.rs:5         _ if a < b => Ordering::Less,
                      ^
Could not compile `match_ordering`.

I am using rustc 1.0.0-nightly (3ef8ff1f8 2015-02-12 00:38:24 +0000).

This would work:

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

but it would use destructuring. Is there any other way, or is this just the most idiomatic, clean way of writing it?

Upvotes: 1

Views: 1445

Answers (1)

huon
huon

Reputation: 102216

You need to match on something, i.e. match { ... } isn't valid because there needs to be something between the match and the {. Since you don't care about the value itself, matching on unit, (), should be fine: match () { ... }, e.g.:

match () {
    _ if a < b => Ordering::Less,
    _ if a > b => Ordering::Greater,
    _ => Ordering::Equal
}

(To be strict: match { _ => ... } is actually trying to parse the { ... } as the match head (i.e. the expression being matched on), and _ isn't valid at the start of an expression, hence the error.)

On idiomacity: I personally think expressing a series of conditions with short results (like this) is fine with match and a series of _ if ...s as you have done.

Upvotes: 6

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