Issue #12770 has been updated by Martin Drst.

Status changed from Open to Feedback
Assignee deleted (Yukihiro Matsumoto)

Derek Kniffin wrote:
> I've also got a first draft of the method definition:
> 
> ````ruby
>   def left_merge(new_hash)
>     merge(new_hash) { |_, old_v, new_v| old_v || new_v }
>   end
> ````

If the definition is as easy as that, it doesn't look really worthwhile to create a new method. There are lots of different ways to merge hashes, and that's what the block can take care of!

However, when I actually use that definition, then for

> ````ruby
> a = {a: 1, b: nil, c: 3, d: nil}
> b = {a: 1, b: 2, c: 4, e:nil}
> a.left_merge(b)
> ````

I get `{ a: 1, b: 2, c: 3, d: nil, e: nil }`
and not `{ a: 1, b: 2, c: 3, d: nil }`.

So what exactly do you want?

----------------------------------------
Feature #12770: Hash#left_merge
https://bugs.ruby-lang.org/issues/12770#change-60536

* Author: Derek Kniffin
* Status: Feedback
* Priority: Normal
* Assignee: 
----------------------------------------
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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