Using singleton objects as default arguments in Ruby

Sometimes you would like to define a method which takes an optional argument, but the programmer might pass nil. And your code needs to distinguish between the value not being provided (default value) and nil. How can it be achieved?

The usual solution for default value is to define them as nil or other empty/zero values which makes sense such as 0 or a string, an empty array, etc.

class Foo
  def bar(one, two: nil)
    # ...
  end
end

But what if you need to distinguish between nil and no value being provided? What if you want to distinguish between:

foo.bar(:something, two: nil)

and

foo.bar(:something)

Here is the solution. Define a single, unique object and use it as a default. And instead of checking if the passed argument is nil check if that’s the singleton object or not.

class Foo
  NOT_PROVIDED = Object.new

  def bar(one, two: NOT_PROVIDED)
    puts one.inspect
    if two == NOT_PROVIDED
      puts "not provided"
    else
      puts two.inspect
    end
  end

  private_constant :NOT_PROVIDED
end

using private_constant is not necessary but I like to remind Ruby devs that we can use it for ages and that we can have private classes that way as well.

Foo.new.bar(1)
1
not provided

Foo.new.bar(1, two: 2)
1
2

You could use a symbol (:not_provided) or number or anything else that’s unique in ruby, but in general methods (such as assert_changes described below) they could be valid objects to be provided as an argument. So the best way to solve it, is to use a unique object that nobody can pass as an argument.

Here is how Rails is using it to implement assert_changes:

assert_changes :@object, from: nil, to: :foo do
  @object = :foo
end

assert_changes -> { object.counter }, from: 0, to: 1 do
  object.increment
end
UNTRACKED = Object.new
def assert_changes(expression, message = nil, from: UNTRACKED, to: UNTRACKED, &block)
  exp = if expression.respond_to?(:call)
    expression
  else
   -> { eval(expression.to_s, block.binding) }
  end

  before = exp.call
  retval = yield

  unless from == UNTRACKED
    error = "#{expression.inspect} isn't #{from.inspect}"
    error = "#{message}.\n#{error}" if message
    assert from === before, error
  end

  after = exp.call

  if to == UNTRACKED
    error = "#{expression.inspect} didn't changed"
    error = "#{message}.\n#{error}" if message
    assert_not_equal before, after, error
  else
    error = "#{expression.inspect} didn't change to #{to}"
    error = "#{message}.\n#{error}" if message
    assert to === after, error
  end

  retval
end

I guess I prefer the rspec approach

expect do
  object.increment
end.to change{ object.counter }.from(0).to(1)

but I admire the assert_changes implementation which uses UNTRACKED object.

Although, it’s kind of similar to using boolean arguments, which often is an indicator that 2 separate methods should be defined. So instead of foo(1, true) and foo(1, false), it is often argued it’s better to just have foo(1) and bar(1) and I usually agree with this guideline. However, in case of assert_changes the usage of named arguments and singleton object seems OK to me.

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