// x is borrowed here
fn fn1 ( x: &i32 ) -> i32 {
    return 3*x*x + 4*x + 3i32;
}

fn fn2 ( _x: &i32 ) -> i32 {
    return 1;
}

// takes an x and a function, which takes an integer and returns an integer
fn call_fn ( x: &i32, fct: &dyn Fn(&i32)->i32 ) -> i32 {
    return fct(x);
}

fn main() {
    let vals = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20];
    // {} inserts a value, :>width$ inside specifies how this value is presented, 
    //    -> its aligned to the right (>), the parameter "width" specifies how many spaces are emitted.
    println!("{:>width$} | {:>width$} | {:>width$}", "x", "fn1(x)", "fn2(x)", width=10);
    println!("-----------+------------+-----------");
    
    // using a borrowed vals here also makes x borrowed.
    for x in &vals {
        println!("{:>width$} | {:>width$} | {:>width$}", x, call_fn( x, &fn1 ), call_fn( x, &fn2 ), width=10)
    }
}