Namespacing module method definition

I accidentally came across this piece of code in Ruby's Postgresql gem:

### Convenience alias for PG::Connection.new.
def self::connect( *args )
  return PG::Connection.new( *args )
end

I played around a bit and turns out that this thing is used just like a normal module class method (it's called like this: PG.connect). In fact we could instead say def self.connect ( #args ) ... end and it would work just the same.

Since self::whatever is a first for me I'm wondering what exactly does self namespaces in such a context and what is its true purpose in this case. Can anyone help shed some light on this?

Solution

:: is the scope resolution operator. So self::connect resolves connect from self. Which means that its equivalent to self.connect. You can see how it works from this very contrived example:

class Foo; end

class Bar
  def Foo::baz
    "Hello World"
  end
end

puts Foo.baz # "Hello World"

Of course we could also just use def Foo.baz to get the exact same result.

Using double colons for method definition is discouraged by the Ruby Style guide:

Do not use :: to define class methods.

# bad
class Foo
  def self::some_method
  end
end

# good
class Foo
  def self.some_method
  end
end

Its also not recommended for anything other than referencing constants and constructors:

Use :: only to reference constants (this includes classes and modules) and constructors (like Array() or Nokogiri::HTML()). Do not use :: for regular method invocation.