Code shared from the Rust Playground

playground.rs

use std::collections::HashMap;
use std::hash::Hash;

pub struct CacheMap<F, T, V>
    where F: Fn(&T) -> V, T: Hash + Eq + Copy, V: Copy
{
    operation: F,
    results: HashMap<T, V>,
}

impl<F, T, V> CacheMap<F, T, V>
    where F: Fn(&T) -> V, T: Hash + Eq + Copy, V: Copy
{
    pub fn new(f: F) -> CacheMap<F, T, V> {
        CacheMap {
            operation: f,
            results: HashMap::new(),
        }
    }

    pub fn value (&mut self, arg: &T) -> V {
        match self.results.get(arg) {
            Some(val) => *val,
            None => {
                let val = (self.operation)(arg);
                self.results.insert(*arg, val);
                val
            }
        }
    }
}

fn main() {}