Ruby module given arguments calls a method?

Question

I'm confused about what's going on in the Nokogiri docs.

As far as I can tell, if

require 'nokogiri'
some_html = "<html><body><h1>Mr. Belvedere Fan Club</h1></body></html>"

then these three lines do the same thing:

html_doc = Nokogiri::HTML::Document.parse(some_html)
html_doc = Nokogiri::HTML.parse(some_html)
html_doc = Nokogiri::HTML(some_html)

The second is just a convenience method for the first. But to my non-Ruby eyes, the third looks like it's passing an argument to a module, not a method. I realize that Ruby has constructors, but I thought they took the form Class.new, not Module(args). What's going on here?

interesting question BTW

Pablo Fernandez
– Pablo Fernandez

2011-07-30 02:58:20 +00:00
Commented Jul 30, 2011 at 2:58 — Pablo Fernandez
– Pablo Fernandez, Commented Jul 30, 2011 at 2:58

Maurício Linhares · Accepted Answer · 2011-07-30 03:29:49Z

8

It's just syntax sugar, look at the Nokogiri::HTML module definition:

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

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

      ####
      # Parse a fragment from +string+ in to a NodeSet.
      def fragment string, encoding = nil
        HTML::DocumentFragment.parse string, encoding
      end
    end

    # Instance of Nokogiri::HTML::EntityLookup
    NamedCharacters = EntityLookup.new
  end
end

First, they define a class method at the Nokogiri module called HTML (yes, Ruby allows you to do that), then they define the module Nokogiri::HTML and in there they define the class method parse.

Most people don't know but the :: operator can also be used to perform method calls:

"my_string"::size #will print 9

answered Jul 30, 2011 at 3:29

Maurício Linhares

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2 Comments

Maurício Linhares Over a year ago

For method calls, no, but you have to use :: when trying to access constants, like Nokogiri::HTML::Document.

bsamek Over a year ago

Sorry, deleted my question in the comments. I asked whether there is a difference between . and ::. Thanks Mauricio!

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