Because core.cache types are defined in terms of the CacheProtocol protocol and the cache functions adhere to its strictures, any cache types can serve as the basis for another.

Put in simpler terms, to create a cache instance composed of the seed data {:a 1, :b 2} with a FIFO eviction policy and a 5-second entry lifetime, then the following nested cache usage would work:

(def C (-> {:a 1 :b 2}
       (fifo-cache-factory :threshold 2)
       (ttl-cache-factory  :ttl 5000)))

;; used right away

(assoc C :c 42)
;;=> {:b 2, :c 42}

;; used after 5 seconds

(assoc C :d 138)
;;=> {:d 138}

What the code above does is to simply use a FIFOCache instance, seeded with {:a 1, :b 2} as the seed for a TTLCacheQ instance. As shown, within the TTL window, the cache evicts its elements using a FIFO policy. However, once a TTL window has expired the expired elements are also evicted.