CODEWITHSUNDEEP
bindingrust
PablodeAcero
PablodeAcero

Reputation: 411

"expected identifier, found (" when trying to bind a tuple variable

I'm playing with Rust a little bit, and got stuck. I've the following function:

fn addOne(h: (i32, i32, i32)){

    let mut (x, y, z) = h;

    (x+1, y+1, z+1)
}

This gives me the following error on compile:

example.rs:2:10: 2:11 error: expected identifier, found `(`
example.rs:2    let mut (x, y, z) = h;

Is there something wrong with the binding? Sorry if it is such a simple question.

Upvotes: 2

Views: 2007

Answers (1)

Shepmaster
Shepmaster

Reputation: 430673

The mut qualifier applies per-variable. When you are destructuring a variable, each variable can be mutable or not:

fn add_one(h: (i32, i32, i32)) -> (i32, i32, i32) {
    let (mut x, mut y, mut z) = h;
    (x+1, y+1, z+1)
}

Of course, you don't need mutability here at all:

fn add_one(h: (i32, i32, i32)) -> (i32, i32, i32) {
    let (x, y, z) = h;
    (x+1, y+1, z+1)
}

You could also choose to destructure in the function signature:

fn add_one((x, y, z): (i32, i32, i32)) -> (i32, i32, i32) {
    (x+1, y+1, z+1)
}

Or use the default tuple accessor methods:

fn add_one(h: (i32, i32, i32)) -> (i32, i32, i32) {
    (h.0 + 1, h.1 + 1, h.2 + 1)
}

If you want to mutate the tuple in place, you are going to need to provide a mutable reference to the tuple. Without knowing more about the problem you are trying to solve, I would actively discourage you from choosing this path, at least until you've profiled and determined you need it:

fn add_one(h: &mut (i32, i32, i32)) {
    h.0 += 1;
    h.1 += 1;
    h.2 += 1;
}

fn main() {
    let mut a = (1, 2, 3);
    add_one(&mut a);
    println!("{:?}", a)
}

There are lots of ways to write code that is meaningful to you!

Upvotes: 6

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