A simple cacher implemented in Rust. Calculates and stores from a closure for future use. It uses HashMap to the store the data.

main.rs

use std::collections::HashMap;
use std::hash::Hash;
use std::thread;
use std::time::Duration;

struct Cacher<K, V, F> {
    calculator: F,
    value: HashMap<K, V>,
}

impl<'a, F, V, K> Cacher<K, V, F>
where
    F: Fn(&K) -> V,
    K: Eq + Hash,
{
    fn new(calculator: F) -> Cacher<K, V, F> {
        Cacher {
            calculator, 
            value: HashMap::new(),
        }
    }

    fn value(&'a mut self, key: K) -> &'a V {
        self.value.entry(key).or_insert_with_key(|key| (self.calculator)(&key))
    }

}

fn main() {
    let mut i32_str_cacher = Cacher::new(|num: &i32| {
        println!("doing some heavy lifting...");
        thread::sleep(Duration::from_secs(2));
        format!("hello from i32 str cacher: {}", num + 1)
    });

    println!("val: {}", i32_str_cacher.value(1));
    println!("val: {}", i32_str_cacher.value(1));
    println!("val: {}", i32_str_cacher.value(2));
    println!("val: {}", i32_str_cacher.value(2));
    println!("val: {}", i32_str_cacher.value(1));

    let mut str_str_cacher = Cacher::new(|name: &String| {
        println!("doing some heavy lifting...");
        thread::sleep(Duration::from_secs(2));
        format!("hello from str str cacher: {}", name)
    });

    println!("val: {}", str_str_cacher.value("how".to_string()));
    println!("val: {}", str_str_cacher.value("how".to_string()));
    println!("val: {}", str_str_cacher.value("are".to_string()));
    println!("val: {}", str_str_cacher.value("are".to_string()));
    println!("val: {}", str_str_cacher.value("how".to_string()));

}