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Feature #20300

Updated by AMomchilov (Alexander Momchilov) 9 months ago

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 
   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.  

 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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