[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



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