add fold method to Option<T>

option_ext.rs

use std::borrow::BorrowMut;

pub trait OptionExt<T> {
    fn fold<S, F>(&self, state: S, folder: F) -> S
    where
        F: FnOnce(S, &T) -> S;
}

impl<T> OptionExt<T> for Option<T> {
    fn fold<S, F>(&self, state: S, folder: F) -> S
    where
        F: (FnOnce(S, &T) -> S),
    {
        match self {
            Some(x) => folder(state, &x),
            None => state,
        }
    }
}

fn main() {
    let opt = Some(10);
    let opt_sum = opt.fold(1, |s, x| s + x);
    println!("opt_sum = {}", opt_sum);

    let opt_list = Some(1..=10);
    let opt_list_sum = opt_list.fold(0, |s, x| x.clone().borrow_mut().fold(s, |acc, y| acc + y));
    println!("opt_list_sum = {}", opt_list_sum);
}