Is there a reverse-syntax version of "include?" in core/std-lib?

Question

I commonly use the [1, 2, 3].include? foo idiom. There is nothing inherently wrong with this, but for readability's sake, it would be nice to be able to write foo.is_in? [1, 2, 3]. Does the core or standard library have a method that will do this?

Answer 1

No there is no such method in the standard. Implement it on your own:

class Object
  def is_in? a
    return a.include?(self)
  end
end

Note in the above code I never check the type of a so you will get an error if include? is not defined for a.

Answer 2

Not exactly in the same way as you want, but you can rewrite this

if [1, 2, 3].include? foo
  do_this
else
  do_that
end

to this:

case foo; when 1, 2, 3
  do_this
else
  do_that
end