Ruby syntax question: Rational(a, b) and Rational.new!(a, b)

Question

Today I came across the strange ruby syntax in the Rational class:

Rational(a,b)

(Notice the absence of the .new()portion comp

Answer 1

The method Rational() is actually an instance method defined outside of the class Rational. It therefore becomes an instance method of whatever object loads the library 'rational' (normally main:Object) in the same way that 'puts' does, for example.

By convention this method is normally a constructor for the class of the same name.

Answer 2

All you have to do is declare a global function with the same name as your class. And that is what rational.rb does:

def Rational(a, b = 1)
  if a.kind_of?(Rational) && b == 1
    a
  else
    Rational.reduce(a, b)
  end
end

to make the constructor private:

private :initialize

and similarly for the new method:

private_class_method :new

I suppose Rational.new could be kept public and made to do what Rational() does, but having a method that turns its arguments into instances is consistent with Array(), String(), etc. It's a familiar pattern that's easy to implement and understand.