1.rb

module Truncate
  refine String do
    def truncate(n: 100)
      self[0...n]
    end
  end
end

class Foo
  using Truncate
  
  def foo
    something.truncate
  end

  def bar
    something.truncate
  end

  def baz
    something.truncate
  end
end

2.rb

class Foo
  def foo
    truncate something
  end

  def bar
    truncate something
  end

  def baz
    truncate something
  end

  private

  def truncate(v, n: 100)
    v[0...n]
  end
end