module Temple::Mixins::EngineDSL

def element(args, block)
  name = args.shift
  if Class === name
    filter = name
    name = filter.name.to_sym
  end
  raise(ArgumentError, 'First argument must be Class or Symbol') unless Symbol === name
  if block
    raise(ArgumentError, 'Class and block argument are not allowed at the same time') if filter
    filter = block
  end
  filter ||= args.shift
  case filter
  when Proc
    # Proc or block argument
    # The proc is converted to a method of the engine class.
    # The proc can then access the option hash of the engine.
    raise(ArgumentError, 'Too many arguments') unless args.empty?
    raise(ArgumentError, 'Proc or blocks must have arity 1') unless filter.arity == 1
    method_name = "FILTER #{name}"
    if Class === self
      define_method(method_name, &filter)
      [name, instance_method(method_name)]
    else
      (class << self; self; end).class_eval { define_method(method_name, &filter) }
      [name, method(method_name)]
    end
  when Class
    # Class argument (e.g Filter class)
    # The options are passed to the classes constructor.
    local_options = Hash === args.last ? args.pop : nil
    raise(ArgumentError, 'Only symbols allowed in option filter') unless args.all? {|o| Symbol === o }
    [name, filter, args, local_options]
  else
    # Other callable argument (e.g. Object of class which implements #call or Method)
    # The callable has no access to the option hash of the engine.
    raise(ArgumentError, 'Class or callable argument is required') unless filter.respond_to?(:call)
    [name, filter]
  end
end