Issue #12770 has been reported by Derek Kniffin.

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Feature #12770: Hash#left_merge
https://bugs.ruby-lang.org/issues/12770

* Author: Derek Kniffin
* Status: Open
* Priority: Normal
* Assignee: 
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I would like a Hash method that does the following:

````ruby
a = {a: 1, b: nil, c: nil, d: nil}
b = {a: 1, b: 2, c: nil}
a.left_merge(b) # => {a: 1, b: 2, c: nil, d: nil}
```` 

So, it takes the first hash, and for any values that are nil, if there's a value for that key in the second hash, fill in the value from the second hash.

I've searched around a bit, and I haven't found this anywhere, so I'd like to propose a new one: `Hash#left_merge`. I've also got a first draft of the method definition:

````ruby
class Hash
  def left_merge(new_hash)
    merge(new_hash) { |_, old_v, new_v| old_v || new_v }
  end
end
````



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