Where is the return keyword ?

There's another case when you must use return keyword in Ruby.

def return_args(a, b)
  a , b  # wrong
end

def return_args(a, b)
  return a , b # correct
end

When a function has multiple return values, the use of return keyword is required here.

I have a very different view on this discussion. I suggest that after learning how return keyword and implicit return work in Ruby, you should get into the habit of omitting return when not required while writing Ruby code. The reason is simple, because this is the coding style the entire Ruby community has agreed upon. You may check out Github's Ruby styling guideline for more reading.

Every programming language has its own styleguide, it often contains a lot of best practices & coding style on how to write better code in that particular language. It might be tempting to bring your old habit from another language over and write the code in whatever style you think it's best, that's a bad idea, it's like moving to a new country but not making an effort to adapt to that country's culture or tradition.

Let me tell you a little story. Some years ago, I was collaborating on a Ruby project with a small team. I made a pull request to refactor a Class by extracting some duplicated code into a helper method. Sth like this.

def helper v1, v2
  # method body
end

This pull request was turned down during the code review because while the change I made was good, the other members of the team didn't like the style there, they insisted that I write it this way instead.

def helper(v1, v2)
  # method body
end

In today's world, programming project is seldom a solo effort, you're writing the code for yourself as much as writing it for others to read. Therefore, all contributors write the code by following one set of coding style rule helps to make the entire codebase cohesive, readable, and maintainable.

Kind of. Ruby has a somewhat unique feature "implicit return"

using return is a good habit to get into if you ever intend to program in other languages since most will require you to explicitly use the return keyword.

I should point out that implicit return isn't unique to Ruby. While many languages (C, Java, Python) only allow explicit return by writing a return statement; in the world of functional programming languages (lisp, Clojure, Scheme ... etc), the return keyword is gone, every function definition uses implicit return. In particular Lisp, a programming language that's almost 60 years old (way older than C), is highly influential in the creation of Ruby, Ruby's creator Matz borrowed many FP concepts from Lisp and implemented 'em into the language, implicit return is one of them.

Hope this helps clear things up.