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)
```
--
https://bugs.ruby-lang.org/