Shadowing

Noir allows for inheriting variables' values and re-declaring them with the same name similar to Rust, known as shadowing.

For example, the following function is valid in Noir:

fn main() {
    let x = 5;

    {
        let x = x * 2;
        assert (x == 10);
    }

    assert (x == 5);
}

In this example, a variable x is first defined with the value 5.

The local scope that follows shadows the original x, i.e. creates a local mutable x based on the value of the original x. It is given a value of 2 times the original x.

When we return to the main scope, x once again refers to just the original x, which stays at the value of 5.

Temporal mutability

One way that shadowing is useful, in addition to ergonomics across scopes, is for temporarily mutating variables.

fn main() {
    let age = 30;
    // age = age + 5; // Would error as `age` is immutable by default.

    let mut age = age + 5; // Temporarily mutates `age` with a new value.

    let age = age; // Locks `age`'s mutability again.

    assert (age == 35);
}