From: "nobu (Nobuyoshi Nakada) via ruby-core" <ruby-core@...>
Date: 2024-04-17T04:30:09+00:00
Subject: [ruby-core:117545] [Ruby master Feature#20300] Hash: set value and get pre-existing value in one call

Issue #20300 has been updated by nobu (Nobuyoshi Nakada).


AMomchilov (Alexander Momchilov) wrote in #note-21:
> nobu (Nobuyoshi Nakada) wrote in #note-17:
> > I want `ENV.exchange_value` rather than `Hash#exchange_value`.
> 
> Is there a reason to put it on `ENV`, but *not* on `Hash`? That seems like a needless restriction to me.

`ENV` is a hash-like object, so I'm for `Hash#exchange` too.

----------------------------------------
Feature #20300: Hash: set value and get pre-existing value in one call
https://bugs.ruby-lang.org/issues/20300#change-107938

* 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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