[ruby-core:101402] [Ruby master Feature#17342] Hash#fetch_set

Issue #17342 has been updated by MaxLap (Maxime Lapointe).


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.

The problem is not only with `false`, but with `nil` too.

Anytime you want to cache something and nil (or false) is a possible value (often meaning the data is missing), this problem arises, as it means these cases are recalculated each time, to again indicate that the data is missing. 

```ruby
# If there is no config with that internal_id, each time you try to use the 
# cached version, which is meant to be fast, it would actually do a query to the database
def self.cached_by_internal_id(internal_id)
  my_cache[internal_id] ||= Config.where(internal_id: internal_id).first
end
```

It is very easy to forget that nil values means the cache is not going to act as a cache. It probably happens in many places, but is not critical, just makes things a little slower from redoing work many times.

--- 

As for the name, there are other options that were suggested:
* fetch_init
* cache

----------------------------------------
Feature #17342: Hash#fetch_set
https://bugs.ruby-lang.org/issues/17342#change-89163

* 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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