Why is rack response body an array not a string?

Question

a classic hello world example from their doc,

class HelloWorld
  def call(env)
    return [200, {}, [\"Hello world!\"]]
  end
end

my question

Answer 1

Judging from the sentence The Body itself should not be an instance of String, as this will break in Ruby 1.9., I'd assume that the last element of the response has to be an Enumerable. Since Strings stopped including this module in 1.9, you'll need some sort of collection that implements it.

BTW: hello world examples are often not great for figuring out why something is done in a certain way, they are too trivial.