On Thu, Dec 3, 2020 = at 2:13 PM <eregontp@gmail.com= > wrote:

Issue #17342 has been updated by Ereg= on (Benoit Daloze).

shyouhei (Shyouhei Urabe) wrote in #note-13:
> Yes I=E2=80=99m quite sure he is dead serious.=C2=A0 There=C2=A0 are i= diots who write nonsense codes.=C2=A0 As a language ruby has to be fool-pro= of.

I think such semantics (calling #hash only once) could be part of the metho= d documentation.
OTOH calling `#hash` twice is probably not very expensive, and for some cas= es (e.g. Integer, String) can be optimized by a JIT or by memoizing the has= h on the instance.

I don't like the name `fetch_set` much (does it fetch a Set?).
I think `fetch_or_set` or `cache` are better.

----------------------------------------
Feature #17342: Hash#fetch_set
https://bugs.ruby-lang.org/issues/17342#change-8= 8908

* Author: MaxLap (Maxime Lapointe)
* Status: Open
* 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 b= lock), it also sets the value in the Hash for the given key.

We often use the pattern `cache[key] ||=3D calculation`. This pattern howev= er has a problem when the calculation could return false or nil, as in thos= e case, the calculation is repeated each time.

I believe the best practice in that case is:

```ruby
cache.fetch(key) { cache[key] =3D 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 cl= ean. But as each part gets longer, the need to repeat cache[key] becomes mo= re 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|
=C2=A0 RequestStore.store[key] =3D !MonitorValue.where('date >=3D ?&= #39;, Time.now - 5.minutes).exists?
end

RequestStore.store.fetch_set(:monitor_value_is_delayed?) do
=C2=A0 !MonitorValue.where('date >=3D ?', Time.now - 5.minutes).= exists?
end
```

There is a precedent for such a method: Python has it, but with a quite con= fusing 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 d= oes what is proposed here. https://do= cs.python.org/3/library/stdtypes.html#dict.setdefault

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