From: halostatue@... Date: 2020-12-11T03:53:29+00:00 Subject: [ruby-core:101412] [Ruby master Feature#17342] Hash#fetch_set Issue #17342 has been updated by austin (Austin Ziegler). My emails appear not to be making it through the mailing list gateway into the tickets, so��� #note-15 I���m mostly doing Elixir these days, and one of the caching modules I use has as its main interface `fetch_or_set`, so I think that���s the best choice if this is added. On the other hand, Nobu���s refinement implementation (#note-11) looks pretty good (and I say this not using refinements yet, even though they���ve been around for a while). #note-20 `put_new` doesn���t work like this in Elixir, but it is similar in some ways (put only if the key doesn���t exist). The correct Ruby implementation for this would be something like ```ruby hash.key?(key) || hash[key] = value ``` The advantage of a `fetch_or_set` method is that it can be implemented atomically for multithreading runtimes. ---------------------------------------- Feature #17342: Hash#fetch_set https://bugs.ruby-lang.org/issues/17342#change-89174 * Author: MaxLap (Maxime Lapointe) * Status: Feedback * Priority: Normal ---------------------------------------- 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/ Unsubscribe: