Implementing User-level Value-weak Hashtables

Value weak hashtables retain only weak references to the values associated with either a strongly or weakly referenced key. Value-weak hashtables automatically remove entries with invalid weak references, which occur when the collector reclaims a weakly referenced value. Value-weak hashtables provide a convenient way to automatically manage a table of information whose entries do not need to exist once the values associated with the keys no longer need to exist. However, most Scheme implementations do not provide value-weak hashtables by default in their base library. Keyweak hashtables are more common. This paper presents an user-level technique for implementing value-weak hashtables that relies on features commonly found or that could be implemented in most implementations. This makes value-weak hashtables a practical reality for most Scheme users without requiring additional work on the implementation code itself. Programmers may, therefore, utilize value-weak hashtables in code that targets a wider range of implementations.