Given a class like:

class Thing
  CONSTANT = 10
  # other code
end

And another like:

class Other
  def initialize
    @thing = Thing.new
  end

  def method
    puts CONSTANT
  end
end

Is it possible to extend Forwardable to have Other's CONSTANT delegate to Thing's constant?

I have tried extendingSingleForwardable (see docs here ) like so:

class Other
  extend SingleForwardable

  def initialize
    @thing = Thing.new
  end

  def method
    puts CONSTANT
  end

  def_single_delegator :@thing, :CONSTANT
end

and:

def_single_delegator :@thing, :constant

but neither worked, am I getting the syntax wrong? If not, is there another way?

This one is answered by the documentation (bold emphasis mine):

def_single_delegator(accessor, method, new_name=method)

Defines a method method which delegates to accessor (i.e. it calls the method of the same name in accessor). If new_name is provided, it is used as the name for the delegate method.

So, no, you can only delegate message sends, not constant lookup.