I’m Wrong Because Ruby and Powershell Are Mainstream

Brad Wilson and Scott Hanselman took me to task for my comment the other day that no “mainstream” language had implemented extension methods:

How mainstream is Ruby on Rails for you? Ruby is a full fledged dynamic language. No hacks for “extension methods” (Brad)

Ya, I kind of blanched at that statement too…method_missing is pretty mainstream… (Scott)

They’re right, Ruby does support the addition (and redefinition I think) of methods on a class at any time. There’s a sample of this in the Classes and Objects chapter of Programming Ruby (aka the pick-axe book) where they add a basic documentation facility “available to any module or class” in Ruby by adding a doc instance method to the Module class.

class Module
  @@docs = Hash.new(nil)
  def doc(str)
    @@docs[self.name] = str
  end
  def Module::doc(aClass)
    # If we’re passed a class or module, convert to string
    # (‘<=’ for classes checks for same class or subtype)
    aClass = aClass.name if aClass.type <= Module
    @@docs[aClass] || “No documentation for #{aClass}”
  end
end

Given how Ruby classes are defined, I think the newly added methods have access to the private data of the class. Extension methods in C#3/VB9 only have access the public interface of the object. But that’s a fairly minor difference.

FYI, Powershell can do this as well, though not as succinctly as Ruby. Scott has an example how you can add a DatePhotoTaken property to System.IO.FileInfo using Omar Shahine’sPhotoLibrary project.

Chalk this up to my continuing ignorance of dynamic languages. I’m working on it, albeit slowly.