class Sass::Value::Function

def ==(other)
  return false unless other.is_a?(Sass::Value::Function)
  if defined?(@id)
    other.compile_context == compile_context && other.id == id
  else
    other.signature == signature && other.callback == callback
  end
end

def assert_function(_name = nil)
  self
end

def hash
  @hash ||= defined?(@id) ? [compile_context, id].hash : [signature, callback].hash
end

def initialize(signature, &callback)
  @signature = signature.freeze
  @callback = callback.freeze
end