Rust Error E0628: Generators with More Than One Parameter

Understanding Rust Error E0628

Rust Error E0628 occurs when a generator is defined with more than one parameter. Currently, Rust generators support at most one explicit parameter, and using more than one parameter in a generator will result in a compilation error.

Example of Erroneous Code

Here is an example that demonstrates the error:

#![feature(generators, generator_trait)]

fn main() {
    let generator = |a: i32, b: i32| {
        // error: too many parameters for a generator
        // Allowed only 0 or 1 parameter
        yield a;
    };
}

In this example, the generator has two parameters, which is not allowed in Rust. Hence, the compiler will produce the E0628 error.

Fixing the Error

To fix the error, ensure that the generator has a maximum of one parameter. If more than one value needs to be passed, consider using a tuple or a struct to pass multiple values as a single parameter.

Example of Fixed Code

Here is an example where the error is fixed by using a tuple to pass multiple values as a single parameter:

#![feature(generators, generator_trait)]

fn main() {
    let generator = |(a, b): (i32, i32)| {
        yield a;
        yield b;
    };
}

With this change, the generator now has a single parameter, which is a tuple containing both values. The E0628 error will no longer occur, and the code will compile successfully.

Possible Alternatives

If using a tuple or a struct is not suitable for your use case, consider using a closure instead of a generator, as closures in Rust can have multiple explicit parameters.

Example of Closure Alternative

Here's an example of a closure with multiple parameters:

fn main() {
    let closure = |a: i32, b: i32| {
        a + b
    };

    let result = closure(1, 2);
    println!("Result: {}", result);
}

This code will compile and run successfully since closures can have multiple parameters in Rust.