4.5 函数 (Functions)

Functions in Sway are declared with the fn keyword. Let's take a look:

Sway中的函数是用fn关键字声明的。我们一起看一下：

fn equals(first_param: u64, second_param: u64) -> bool {
    first_param == second_param
}

We have just declared a function named equals which takes two parameters: first_param and second_param. The parameters must both be 64-bit unsigned integers.

我们刚刚声明了一个名为 equals的函数，它需要两个参数： first_param和second_param。这些参数必须都是64位无符号整数。

This function also returns a bool value, i.e. either true or false. This function returns true if the two given parameters are equal, and false if they are not. If we want to use this function, we can do so like this:

这个函数还返回一个bool值，即true或false。如果给定的两个参数相等，该函数返回true，如果不相等，则返回false。如果我们想使用这个函数，可以这样做：

fn main() {
    equals(5, 5); // evaluates to `true`
    equals(5, 6); // evaluates to `false`
}

可变参数 (Mutable Parameters)

We can make a function parameter mutable by adding ref mut before the parameter name. This allows mutating the argument passed into the function when the function is called. For example:

我们可以通过在参数名称前添加ref mut来使一个函数参数变得可变。这允许在函数被调用时，对传递到函数中的参数进行变化。例如：

fn increment(ref mut num: u32) {
    let prev = num;
    num = prev + 1u32;
}

This function is allowed to mutate its parameter num because of the mut keyword. In addition, the ref keyword instructs the function to modify the argument passed to it when the function is called, instead of modifying a local copy of it.

这个函数被允许修改其参数num，因为有mut关键字。此外，ref关键字指示该函数在调用时修改传递给它的参数，而不是修改它的本地副本。

    let mut num: u32 = 0;
    increment(num);
    assert(num == 1u32); // The function `increment()` modifies `num`

Note that the variable num itself has to be declared as mutable for the above to compile.

注意，变量num本身必须被声明为可变的，这样才能编译上述内容。

Note It is not currently allowed to use mut without ref or vice versa for a function parameter.

Similarly, ref mut can be used with more complex data types such as:

同样，ref mut可以用于更复杂的数据类型，如：

fn swap_tuple(ref mut pair: (u64, u64)) {
    let temp = pair.0;
    pair.0 = pair.1;
    pair.1 = temp;
}

fn update_color(ref mut color: Color, new_color: Color) {
    color = new_color;
}

We can then call these functions as shown below:

然后，我们可以按照下面的方式调用这些函数：

    let mut tuple = (42, 24);
    swap_tuple(tuple);
    assert(tuple.0 == 24); // The function `swap_tuple()` modifies `tuple.0`
    assert(tuple.1 == 42); // The function `swap_tuple()` modifies `tuple.1`
    let mut color = Color::Red;
    update_color(color, Color::Blue);
    assert(match color {
        Color::Blue => true,
        _ => false,
    }); // The function `update_color()` modifies the color to Blue

Note The only place, in a Sway program, where the ref keyword is valid is before a mutable function parameter.

注意在Sway程序中，ref关键字唯一有效的地方是在可变函数参数之前。