Fix from [benh].

deriv.rs

use std::f64::consts::PI;
 
fn deriv(f: |f64| -> f64, g: |(|f64| -> f64)|) {
    g(|x: f64| -> f64 {
        let epsilon = 0.0000000001;
        (f(x + epsilon) - f(x)) / epsilon
    });
}
 
fn main() {
    println!("cos(pi) = {}", PI.cos());
    deriv(|x|x.sin(), |myCos| {
        println!("cos(pi) = {}", myCos(PI))
    });
}