MergeableMap in rust. The idea is to give a store that can "merge" arbitrary associative operations with no GC. That and fool around with rust.

mergeablemap.rs

// compiles with rustc 0.8
use std::hashmap::HashMap;
use std::container::MutableMap;
use std::util::swap;
use std::to_str::ToStr;

trait Semigroup {
   fn plus(&self, that: ~Self) -> ~Self;
}

impl Semigroup for int {
  fn plus(&self, mut that: ~int) -> ~int {
    (*that) += *self;
    return that
  }
}

pub trait MergeableMap<K, V: Semigroup>: MutableMap<K, ~V> {
  fn merge(&mut self, k: K, v: ~V) -> Option<~V> {
    let new_val = match self.find(&k) {
      Some(existing) => existing.plus(v),
      None => v
    };
    self.swap(k, new_val)
  }
}

impl <K: IterBytes + Eq, V: Semigroup> MergeableMap<K, V> for HashMap<K, ~V> {}

// Not sure why this isn't working by default
fn show<T:ToStr>(item: &~T) -> ~str {
  item.to_str()
}

fn main() {
  let mut m : HashMap<~str, ~int> = HashMap::new();
  println(m.merge(~"key1", ~2).map(|it| -> ~str { show(it) }).to_str());
  // prints: None
  println(m.merge(~"key1", ~10).map(|it| -> ~str { show(it) }).to_str());
  // prints: Some(2)
}