From: "rubyFeedback (robert heiler) via ruby-core" <ruby-core@...>
Date: 2024-02-26T10:38:53+00:00
Subject: [ruby-core:116948] [Ruby master Feature#20300] Hash: set value and get pre-existing value in one call

Issue #20300 has been updated by rubyFeedback (robert heiler).


Not so sure about these names. Note that get_and_set() may have people wonder why it is not set_and_get(). :D

In regards to .update_value() people could ask why the old value is returned rather than the
new one.

I guess exchange_value is a bit more specific than the other variants, as people can say "I put in value x, as the new value, and get, in exchange, value y which was the old one".

I don't have a better name suggestion though, but just from the alternatives so far I think nobu's suggestion is at the least better than the other ones, in my opinion.

I don't have any particular opinion on the suggested functionality; in my own code I kind of keep setters and getters usually separate, or, e. g. first set and then get (or get first, assign to a variable, then set to a new value). It would be nice if we could have some kind of "universal idiom" for the desired functionality though - could make it easier if different programming languages, at the least OOP-centric ones, would use the same name/terminology for that. That would make naming things easier too, or at the least more consistent across programming languages, just like setters and getters are fairly well-established idioms in OOP-centric languages (ruby is kind of multi-paradigm; I don't know how functional programming languages solve this issue of setting and obtaining a new or old value in one go, so I can't comment on that).

----------------------------------------
Feature #20300: Hash: set value and get pre-existing value in one call
https://bugs.ruby-lang.org/issues/20300#change-106988

* 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)
```



-- 
https://bugs.ruby-lang.org/
 ______________________________________________
 ruby-core mailing list -- ruby-core@ml.ruby-lang.org
 To unsubscribe send an email to ruby-core-leave@ml.ruby-lang.org
 ruby-core info -- https://ml.ruby-lang.org/mailman3/postorius/lists/ruby-core.ml.ruby-lang.org/