From: marcandre-ruby-core@... Date: 2020-12-10T07:40:37+00:00 Subject: [ruby-core:101365] [Ruby master Feature#17342] Hash#fetch_set Issue #17342 has been updated by marcandre (Marc-Andre Lafortune). matz (Yukihiro Matsumoto) wrote in #note-16: > Most of the case, `hash[:key] ||= init` works. The exception is that `init` value being false. But it should be rare. We could look into public codebases for better numbers; it is not the most common case, but it is definitely encountered it with some frequency (more with `nil` than `false`). > So could you explain the concrete use-case for `fetch_set()`? One additional use-case would be a *thread-safe equivalent* to `Ractor.current[:my_key] ||= init`. > Besides that I am not fully satisfied with the name `fetch_set()`. Did you consider `fetch_init`? ---------------------------------------- Feature #17342: Hash#fetch_set https://bugs.ruby-lang.org/issues/17342#change-89072 * 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: