[ruby-core:117560] [Ruby master Feature#20300] Hash: set value and get pre-existing value in one call

["mame (Yusuke Endoh) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20300 has been updated by mame (Yusuke Endoh).


@matz said he was not sure what a name is good for this method because its true motivation is unclear.

It was originally intended as a method to improve the efficiency of `Set#add?`, but the use case was shifted to a method for thread safety. This history makes the use case less persuasive.
If the main purpose is thread safety, we want to respect the terminology of the parallel computing area. If it is just an internal method for efficiency, a long and verbose name may be preferred. (If it is a daily-use method, a short and convenient name may be preferred.)

You may want to explain the concrete example of the use case of thread safety for Hash value exchange, this proposed API is really sufficient for that example use case, and what API and name are given to similar feature in other languages.

----------------------------------------
Feature #20300: Hash: set value and get pre-existing value in one call
https://bugs.ruby-lang.org/issues/20300#change-107959

* Author: AMomchilov (Alexander Momchilov)
* Status: Open
----------------------------------------
When using a Hash, sometimes you want to set a new value, **and** see what was already there. Today, you **have** to do this in two steps:

```ruby
h = { k: "old value" }

# 1. Do a look-up for `:k`.
old_value = h[:k]
# 2. Do another look-up for `:k`, even though we just did that!
h[:k] = "new value"

use(old_value)
```

This requires two separate `Hash` look-ups for `:k`. This is fine for symbols, but is expensive if computing `#hash` or `#eql?` is expensive for the key. It's impossible to work around this today from pure Ruby code.

One example use case is `Set#add?`. See https://bugs.ruby-lang.org/issues/20301 for more details.

I propose adding `Hash#exchange_value`, which has semantics are similar to this Ruby snippet:

```ruby
class Hash
  # Exact method name TBD.
  def exchange_value(key, new_value)
    old_value = self[key]
    self[key] = new_value
    old_value
  end
end
```

... except it'll be implemented in C, with modifications to `tbl_update` that achieves this with a hash-lookup. 

I'm opening to alternative name suggestions. @nobu came up with `exchange_value`, which I think is great.

Here's a PR with a PoC implementation: https://github.com/ruby/ruby/pull/10092

```ruby
h = { k: "old value" }

# Does only a single hash look-up
old_value = h.exchange_value(:k, "new value")

use(old_value)
```



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