Why is Enumerable#each_with_object deprecated?

This is rather an answer to a denial of the presupposition of your question, and is also to make sure what it is.

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The each_with_object method saves you extra key-strokes. Suppose you are to create a hash out of an array. With inject, you need an extra h in:

array.inject({}){|h, a| do_something_to_h_using_a; h} # <= extra `h` here

but with each_with_object, you can save that typing:

array.each_with_object({}){|a, h| do_something_to_h_using_a} # <= no `h` here

So it is good to use it whenever possible, but there is a restriction. As I also answered in "How to group by count in array without using loop",

• When the initial element is a mutable object such as an Array, Hash, String, you can use each_with_object.

• When the initial element is an immutable object such as Numeric, you have to use inject:

  sum = (1..10).inject(0) {|sum, n| sum + n} # => 55
  

There's no note in the Ruby trunk source code, the method is still there (contrary to that page's claims), and there's been no talk of it on the mailing list that I can find.

APIdock is simply confused. The point where APIdock says it was deprecated is actually the earliest version with the method in the standard library (rather than just being an ActiveSupport backport extension), and Rails disables its version if you're using a Ruby that has the method, so APIdock appears to be confused by the method migrating between projects.

Well, that seems a bit weird.

Even Agile Rails writes somewhere:

"The Ruby 1.9 each_with_object method was found to be so handy that the Rails crew backported it to Ruby 1.8 for you".

This seems like an error in APIdock ? I don't see any reason why it would be.