Question

I have a struct like this:

class Item < Struct.new(:url, :list)
  def list
    @list ||= Array.new
  end
end

I found out today that the .list() and [:list] returns different things:

i = Item.new
#=> #<struct Item url=nil, list=nil>
i.list
#=> []
i[:list]
#=> nil
i.list << 1
#=> [1]
i.list += [2]
#=> [1, 2]
i.list
#=> [1]
i[:list]
#=> [1, 2]

Why is this and how can I write my struct to have default empty array properly?

Answer 1

Assuming you want the other advantages of Struct and need to stick with it, you could write your own initialize method:

class Item < Struct.new(:url, :list)
  def initialize(url, list = nil)
    self.url  = url
    self.list = Array(list)
  end
end

Array() will ensure anything passed in will be put in an array if it’s not one already, and will return an empty array ([]) if nil is the argument.