Question

If Array and String are class definitions, how are these things defined?

def whatever(some_input)
  some_input = Array(some_input)
end

How does that get invoked? Can we add this behaviour to our own classes, or is this only for core classes?

Answer 1

They’re actually methods on Kernel:

>> Kernel.methods.select { |m| m =~ /^[A-Z]/ }
=> [:Array, :Complex, :Float, :Integer, :Rational, :String, :URI]

And Object includes Kernel so these methods are available everywhere. You can make methods whose names begin with an upper case letter, it just isn’t that common.

Nokogiri does similar things with the Nokogiri::HTML and Nokogiri::XML constructors. For example, in Nokogiri you will find this:

module Nokogiri
  class << self
    ###   
    # Parse HTML.  Convenience method for Nokogiri::HTML::Document.parse
    def HTML thing, url = nil, encoding = nil, options = XML::ParseOptions::DEFAULT_HTML, &block
      Nokogiri::HTML::Document.parse(thing, url, encoding, options, &block)
    end
  end  
  #...
end

so HTML is just a standard method with non-standard name.