I wrote it, but I'm not sure I like it?

example_usage.rb

# Dynamically creating the correct url for downloading elasticsearch package via version number

download_url = "https://download.elasticsearch.org/elasticsearch/elasticsearch/elasticsearch-0.90.10.deb" # can we say elasticsearch one more time?

template = '<%= "https://download.#{@domain}.org/#{@domain}/#{@domain}/#{@domain}-#{@major}.#{@minor}.#{@patch}.deb" %>'
vars = {domain: 'elasticsearch', major: 0, minor: 90, patch: 10}

url_template = UrlTemplate.new(template, vars)

url_template.url == download_url
#=> true

url_template.rb

require 'erb'

class UrlTemplate
  attr_reader :url, :template

  def initialize(template, erb_vars = {})
    @template     = ERB.new(template)
    self.erb_vars = erb_vars

    @url = interpolate_url
  end

  def erb_vars=(value)
    @erb_vars = normalize_erb_vars(value)
  end

private
  def erb_vars
    @erb_vars
  end

  def interpolate_url
    erb_binding_wrapper do |binding|
      @template.result(binding)
    end
  end

  def erb_binding_wrapper(&block)
    set_erb_vars
    value = yield(binding)
    unset_erb_vars
    value
  end

  def set_erb_vars
    erb_vars.each do |erb_var, value|
      instance_variable_set erb_var, value
    end
  end

  def unset_erb_vars
    erb_vars.each do |erb_var, value|
      remove_instance_variable erb_var
    end
  end

  def normalize_erb_vars(erb_vars)
    Hash[
      erb_vars.map { |k, v| ["@#{k}".to_sym, v] }
    ]
  end
end