Fn and Never Primitives

Function types และ never type

Function Pointers

fn main() {
    // Function pointer type
    fn add(a: i32, b: i32) -> i32 { a + b }
    fn multiply(a: i32, b: i32) -> i32 { a * b }

    // fn is a type
    let op: fn(i32, i32) -> i32 = add;
    println!("add(2, 3) = {}", op(2, 3));

    let op: fn(i32, i32) -> i32 = multiply;
    println!("multiply(2, 3) = {}", op(2, 3));

    // Array of function pointers
    let operations: [fn(i32, i32) -> i32; 2] = [add, multiply];

    for (i, op) in operations.iter().enumerate() {
        println!("operations[{}](4, 5) = {}", i, op(4, 5));
    }

    // Function as parameter
    fn apply(f: fn(i32) -> i32, x: i32) -> i32 {
        f(x)
    }

    fn double(x: i32) -> i32 { x * 2 }
    println!("apply(double, 10) = {}", apply(double, 10));

    // Size of function pointer
    println!("Size of fn pointer: {} bytes", std::mem::size_of::<fn()>());
}