Returns true if the cache contains a value for the supplied key, false otherwise.
Returns an empty version of this specific cache implementation.
Returns an option containing the value stored for the supplied key or None if the key is not present in the cache.
Returns an iterator of all key-value pairs inside of this cache.
Touches the cache with the supplied key.
Touches the cache with the supplied key. If the key is present in the cache, the cache calls "hit" on the key. If the key is missing, the cache adds fn(k) to itself.