bestintegral.rs

fn integral(f: &|f32|->f32, p: u32, a: f32, b: f32) -> f32 {
    if p == 1 {
        (b-a) * ((*f)(a) + 4.0 * (*f)((a+b)/2.0) + (*f)(b))/6.0
    }
    else {
        let mid = (a+b)/2.0;
        integral(f, p-1, a, mid) + integral(f, p-1, mid, b)
    }
}

fn main() {
    println!("{}", integral(&|x| x*x, 10, 1.0, 2.0));
}

hilarintegral.rs

fn integral<'a>(f: 'a |f32|->f32, p: u32, a: f32, b: f32) -> (f32, 'a |f32|->f32) {
    if p == 1 {
        ((b-a) * (f(a) + 4.0 * f((a+b)/2.0) + f(b))/6.0, f)
    } else {
        let mid = (a+b)/2.0;
        let (i1, f) = integral(f, p-1, a, mid);
        let (i2, f) = integral(f, p-1, mid, b);
        (i1+i2, f)
    }
}

fn main() {
    let (i, _) = integral(|x: f32| x*x, 10, 1.0, 2.0);
    println!("{}", i);
}

origintegral.rs

fn integral(f: |f32|->f32, p: u32, a: f32, b: f32) -> f32 {
    match (f, p, a, b) {
        (f, 1, a, b) => (b-a) * (f(a) + 4.0 * f((a+b)/2.0) + f(b))/6.0,
        (f, p, a, b) => {
            let mid = (a+b)/2.0;
            integral(f, p-1, a, mid) +  // note: `f` moved here because it has type `|f32| -> f32`, which is a non-copyable stack closure (capture it in a new closure, e.g. `|x| f(x)`, to override)
            integral(f, p-1, mid, b)  // error: use of moved value: `f`
        }
    }
}

fn main() {
    println!("{}", integral(|x: f32| x*x, 10, 1.0, 2.0));
}