cache

Cache and ScopedCache: effectful lookups shared by key.

A cache runs lookup(key) the first time a key is asked for and shares the outcome, success or failure, with every later caller. Callers asking while a lookup runs wait for that one lookup::

users = yield from fx.Cache.make(fetch_user, capacity=1000, time_to_live=60)
user = yield from users.get(user_id)

• capacity: the most entries kept; the least recently used go first (get, get_option and get_success count as a use; has does not).
• time_to_live: a duration, or a function of the lookup's Exit and the key, read from the Clock when the lookup ends. Zero keeps nothing in a Cache; the default is forever. Expired entries are dropped when next read.
• ScopedCache: each entry's lookup runs in a scope of its own, closed when the entry leaves the cache, and every entry's when the scope the cache was made in closes.

Keys compare with == and hash: use frozen dataclasses or tuples for structured keys.

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