Invariant is a simple gem that provides Kernel#assert to document your assumptions in code.

Use assertions to prevent the impossible.

Whenever you find yourself thinking "but of course that could never happen," add code to check it. The easiest way to do this is with assertions.

http://pragmatictips.com/33

Add to Gemfile:

gem 'invariant'

Assertions are enabled by default. Better safe than sorry. To disable assertions:

Invariant.disable_assertions

For example, in Rails you may want to disable assertions in Production:

# config/initializers/invariant.rb
Invariant.disable_assertions if Rails.env.production?

Use assert to test a condition:

assert age > 0

Provide an optional message:

assert errors.empty?, "Why do we still have errors?"

You can also test a block of code. This is handy when you're invariant requires several lines of code.

assert do
  one_thing = calculate_something
  other_thing = calculate_something_else
  one_thing > other_thing
end

Blocks also support an optional message:

assert 'That one thing should always be greater' do
  one_thing = calculate_something
  other_thing = calculate_something_else
  one_thing > other_thing
end

A failed assertion raises Invariant::AssertionError which inherits directly from Exception.

An AssertionError should be an exceptional failure. It should not be something a program knows how to recover from. This is why AssertionError is not a StandardError:

  def real_world
    assert 1 == 0, 'We are in the real world'
  rescue
    'We are in the Matrix'
  end

  real_world # => raises Invariant::AssertionError, 'We are in the real world'