the type for cache validation callbacks.
validate (key,old) is called with
old = None if the key is not in the cache yet and has to be computed. It is called with
old=Some data if the key is already in the cache. The validator has to decide whether to return the same value or recompute it (perhas because it is expired / no longer fresh).
val create : int -> 'a t
create n creates a LRU/2Q that can hold at most
with_cache cache f key acts like
f key, except it might make fewer calls to
f when the
key is already in the cache. On successful termination of
f key the result is stored in the cache.
f key must always return the same value when called with the same key. Cached values never expire, they are only removed if the cache capacity is exceeded, see cache replacement policy.
with_validator cache validator key calls
validator each time
key is accessed. If the
key is not in the cache
validator (key, None) is called and it should compute the value associated with
key. If the
key is already in the cache then
validator [key, Some value] is called and
validator should decide whether the
value is still fresh, or if it should be recomputed. The value returned by
validator in either case is stored in the
validate (key,_) is allowed to return different values when called with the same key again, provided it properly validates the lifetime of old entries. Values are removed from the cache when its capacity is exceeded, see cache replacement policy.
get cache key retrieves the
key from the
cache if it exists. The key's position in LRU is updated