Issue #17342 has been updated by Dan0042 (Daniel DeLorme). It's confusing that this ticket, #17342, is striked out even though the status is "Feedback". Another name for this could be `Hash#add`; at least that's the terminology used in memcached and I believe some other caching servers. ---------------------------------------- Feature #17342: Hash#fetch_set https://bugs.ruby-lang.org/issues/17342#change-107000 * Author: MaxLap (Maxime Lapointe) * Status: Feedback ---------------------------------------- I would like to propose adding the `fetch_set` method to `Hash`. It behaves just like `fetch`, but when using the default value (2nd argument or the block), it also sets the value in the Hash for the given key. We often use the pattern `cache[key] ||= calculation`. This pattern however has a problem when the calculation could return false or nil, as in those case, the calculation is repeated each time. I believe the best practice in that case is: ```ruby cache.fetch(key) { cache[key] = calculation } ``` With my suggestion, it would be: ```ruby cache.fetch_set(key) { calculation } ``` In these examples, each part is very short, so the `fetch` case is still clean. But as each part gets longer, the need to repeat cache[key] becomes more friction. Here is a more realistic example: ```ruby # Also using the key argument to the block to avoid repeating the # long symbol, adding some indirection RequestStore.store.fetch(:monitor_value_is_delayed?) do |key| RequestStore.store[key] = !MonitorValue.where('date >= ?', Time.now - 5.minutes).exists? end RequestStore.store.fetch_set(:monitor_value_is_delayed?) do !MonitorValue.where('date >= ?', Time.now - 5.minutes).exists? end ``` There is a precedent for such a method: Python has it, but with a quite confusing name: `setdefault(key, default_value)`. This does not set a default for the whole dictionary as the name would make you think, it really just does what is proposed here. https://docs.python.org/3/library/stdtypes.html#dict.setdefault -- https://bugs.ruby-lang.org/