badtemplate.rb

class Template
  def initialize(&block)
    @body = block
  end

  def method_missing(v)
    @params[v]
  end

  def render(params)
    @params = params
    instance_eval &@body
  end
end

t = Template.new do
  "My name is #{name} and I am #{age}"
end

puts t.render(name: "alice", age: 27)
puts t.render(name: "bob", age: 6)