From: matthew@...
Date: 2016-09-19T03:00:35+00:00
Subject: [ruby-core:77308] [Ruby trunk Feature#12770] Hash#left_merge

Issue #12770 has been updated by Matthew Kerwin.


Robert A. Heiler wrote:
> I think that the name appears to be a bit strange - if we have a left_merge, do we have a right_merge, an up_merge, a bottom_merge? :)

Given that it comes from SQL's [JOIN](https://en.wikipedia.org/wiki/Join_(SQL)) then theoretically, yes, we could have a #right_merge.  The directionality in the name implies which of the two "operands" is "more important" -- left_merge means that the rule for resolving collisions is: "keep the value from the left hash when it exists", "right_merge" would presumably be "use the value from the right hash when it exists."

i.e.

~~~ruby
class Hash
  def left_merge hsh
    merge(hsh) {|key, left, right| left.nil? ? right : left }
  end

  #def right_merge hsh
  #  merge(hsh) {|key, left, right| right.nil? ? left : right }
  #end
end
~~~

That said, I've never wanted for this functionality to exist in the core. I don't do much of this sort of data mashing in Ruby, though.

----------------------------------------
Feature #12770: Hash#left_merge
https://bugs.ruby-lang.org/issues/12770#change-60547

* 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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