Ruby use case for nil, equivalent to Python None or JavaScript undefined

Question

How does Ruby's nil manifest in code? For example, in Python you might use None for a default argument when it refers to another argument, but in Ruby you can refer to other arguments in the arg list (see this question). In JS, undefined pops up even more because you can't specify default arguments at all. Can you give an example of how RubyNone pops up and how it's dealt with?

I'm not looking for just an example using nil. Preferably it would be a real code snippet which had to use nil for some reason or other.

Answer 1

Ruby's nil and Python's None are equivalent in the sense that they represent the absence of a value. However, people coming from Python may find some behavior surprising. First, Ruby returns nil in situations Python raises an exception:

Accessing arrays and hashes:

[1, 2, 3][999] # nil. But [].fetch(0) raises an IndexError
{"a" => 1, "b" => 2}["nonexistent"] # nil. But {}.fetch("nonexistent") raises an IndexError

Instance instance variables:

class MyClass
  def hello
    @thisdoesnotexist
  end
end

MyClass.new.hello #=> nil

The second remarkable fact is that nil is an object with lots of methods. You can even convert it to an integer, float or string:

nil.to_i # 0
nil.to_f # 0.0 # But Integer(nil) or Float(nil) raise a TypeError.
nil.to_s # ""