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assorted ramblings by Jamis Buck

Playing with Constants, Methods, and Superclasses

24 March 2015 — A few curious Rubyisms of dubious use, which may yet be worth knowing about — 3-minute read

File this one under “Ruby Tricks of Questionable Usefulness.” Still, it ought to be admitted that even questionably-useful tricks can sometimes inspire unexpectedly-creative solutions. To that end, I present the following, which I first saw used years ago in Why the Lucky Stiff’s Camping web microframework. (Kudos to @vinbarnes for helping me remember that!)

Let’s begin by observing the following (possibly unexpected) feature of Ruby. Did you know that you can have a constant and a method with the exact same name, and they won’t collide?

Check this out.

Sum = 0

def Sum(*args)
  args.inject(Sum) { |s,i| s + i }

p Sum      #-> 0
p Sum(1)   #-> 1
p Sum(1,2) #-> 3

See that? The first time we display Sum, we’re actually printing the value of the constant. But as soon as we start adding arguments to the call, that’s when we start hitting the method.

No errors, no warnings, no symbols shadowing other symbols. Just crazy Ruby fun.

The next (seemingly unrelated) tidbit is this: did you know that when you define a class, the bit where you specify the superclass is actually an expression? We usually just put a constant there, but it can be anything (as long as it evaluates to a class).

class Shape

circle_is_shape = true
class Circle < (circle_is_shape ? Shape : Object)

p Circle.superclass #-> Shape

I know what you’re thinking. “Whoa, that’s pretty cool! But why would anyone ever want to do that?”

I’m glad you asked, because that segues neatly into the third little Ruby trick of the day.

You probably already knew that defining a class is the same as creating a new Class object and assigning it to a constant, right?

# This:
class Shape

# is the same as this:
Shape =

This means that our class names are just constants…and we’ve already seen that we can have methods that share those same names. Further, we’ve also seen that the superclass expression in a class definition can be any expression at all… We can stick a method invocation in there!


class Shape

def Shape(which)
  require "shapes/#{which}"

class Square < Shape

class Circle < Shape(:ellipse)

So, we have Shape declared as both a constant (our base Shape class) and a method, where the method just does some work to load a hypothetical class from a file derived from the parameter.

Then, you can see that we can declare subclasses of Shape as normal, with the Square class being a typical example. However, we can get tricky. See that? Circle is a subclass of whatever is returned by our Shape method.

So what? Well, for one thing, it means you can do crazy things like data-driven class hierarchies, where an external configuration file lets you specify (for instance) the type of spline to be used, or the orientation and number of points on a star:

require 'yaml'
config = YAML.load_file('definitions.yml')

class Spline < Shape(config[:spline])

class Star < Shape(config[:star])

As I said at the beginning, though, this may not have any real practical use. There are certainly other (possibly less-obscure) ways to accomplish this same thing. Still, you have to admit, it’s rather fun to think about!