[ruby-core:116978] [Ruby master Feature#20300] Hash: set value and get pre-existing value in one call

["matheusrich (Matheus Richard) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20300 has been updated by matheusrich (Matheus Richard).


Some other examples from other languages:

- [Java](https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html#put-K-V-) and [Kotlin](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-hash-map/put.html) call this `put`.
- [Groovy](https://docs.groovy-lang.org/latest/html/groovy-jdk/java/util/Map.html) calls it `putAt`
- [Elixir](https://hexdocs.pm/elixir/1.12/Map.html#get_and_update/3) has an interesting `get_and_update`, which is very descriptive, but also accepts a function argument, which can be used to operate on the old value before returning it. I could see us supporting both cases:

```rb
hash.get_and_update(:key, :new_value)
hash.get_and_update(:key) { |old| old.upcase }
```

- [Clojure](https://clojuredocs.org/clojure.core/update-in) does something similar with `update-in`.
- [Racket](https://docs.racket-lang.org/reference/hashtables.html#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._hash-update%29%29) calls it `hash-update`.
- [Haskell](https://hackage.haskell.org/package/containers-0.4.0.0/docs/Data-Map.html) calls this `insertLookupWithKey` 😅


---

I like `exchange_value` too. While a bit more verbose, having the `_value` suffix opens the opportunity to add a similar method for keys in the future

Another suggestion is `replace_value`, since we already have `replace` on hash.

----------------------------------------
Feature #20300: Hash: set value and get pre-existing value in one call
https://bugs.ruby-lang.org/issues/20300#change-107026

* 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#update_value`, which has semantics are similar to this Ruby snippet:

```ruby
class Hash
  # Exact method name TBD.
  def update_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.update_value(:k, "new value")

use(old_value)
```



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