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fn apply<F: Fn(i32) -> i32>(f: F, x: i32) -> i32 {
    f(x)
}

fn make_multiplier(factor: i32) -> impl Fn(i32) -> i32 {
    move |x| x * factor   // 'factor' is moved into the closure
}

fn main() {
    // Basic closure syntax
    let double = |x: i32| x * 2;
    let add_three = |x| x + 3;      // types inferred from use

println!("double(5) = {}", double(5));
    println!("add_three(5) = {}", add_three(5));

// Closure capturing environment by immutable borrow
    let offset = 10;
    let add_offset = |x| x + offset;
    println!("add_offset(5) = {}  offset still = {offset}", add_offset(5));

// Closure capturing by mutable borrow
    let mut total = 0;
    let mut accumulate = |x: i32| { total += x; };
    accumulate(1); accumulate(2); accumulate(3);
    drop(accumulate);   // release borrow so we can read total again
    println!("total = {total}");

// move closure
    let msg = String::from("hello from closure");
    let greet = move || println!("{msg}");
    greet();
    // println!("{msg}"); // compile error: moved into closure

// Higher-order function
    println!("apply double: {}", apply(double, 7));
    println!("apply |x| x-1: {}", apply(|x| x - 1, 7));

// Returning a closure (factory)
    let times7 = make_multiplier(7);
    println!("times7(6) = {}", times7(6));
}