Generative

Generative and property-based testing for RSpec

Installation

Add Generative to your Gemfile (or gemspec):

group :test do
  gem 'generative'
  gem 'rspec'
end

...and then run bundle.

Require Generative in your .rspec file:

--color
--format Generative::Formatter
--require generative

(Optional) Require the Generative Rake task in your Rakefile:

require 'rspec/core/rake_task'
require 'generative/rake_task'

task(:default).enhance [:spec, :generative]

RSpec::Core::RakeTask.new
Generative::RakeTask.new

Usage

Specs

In your tests, add a generative block. This is a essentially the same as a context or describe block. Inside the block, define some data as you would a let. Then, write your it/specify blocks as usual (while keeping in mind that the input could be anything).

RSpec.describe String do
  let(:string) { "abc" }

  describe "#reverse" do
    it "reverses" do
      expect(string.reverse).to eq("cba")
    end

    generative do
      data(:string) { rand(12345).to_s }

      it "maintains length" do
        expect(string.reverse.length).to eq(string.length)
      end

      it "is not destructive" do
        expect(string.reverse.reverse).to eq(string)
      end
    end
  end
end

Alternatively, you can modify existing specs in a minimally intrusive way by just adding two tags to an existing context and using the fact that data is just an alias for let:

RSpec.describe String do
  describe "#reverse", generative: true, order: :generative do
    let(:string) { rand(12345).to_s }

    it "maintains length" do
      expect(string.reverse.length).to eq(string.length)
    end

    it "is not destructive" do
      expect(string.reverse.reverse).to eq(string)
    end
  end
end

Running

Generative comes with a generative command, which simply runs rspec with the required arguments:

$ generative
+ GENERATIVE_COUNT=10_000
+ rspec --require generative --format Generative::Formatter --tag generative
Run options: include {:generative=>true}

String
  #length
    generative
      is never negative
  #reverse
    generative
      maintains length
      is not destructive

Finished in 2.28 seconds
30000 examples, 0 failures

If you've added the Generative task to your Rakefile, you can also simply run rake to run all tests, including Generative ones.

Number of Tests

Generative uses the GENERATIVE_COUNT environment variable to control how many tests to run for each example. Both the generative command and the example Rakefile above set it to 10,000.

Formatters

Given the examples above, running rspec will use the default "progress" formatter. Requiring generative will modify this formatter to output blue dots instead of green for generative tests. Generative also includes it's own formatter, which will only display generative test names once, also in blue.

Generators

Generative allows you to register your own custom generators for generative tests. Registering a generator requires two thing, a name and an object that responds to #call.

Whether you just want to use lambdas.

Generative.register_generator(:full_name) { "#{Generative.generate(:string)} #{Generative.generate(:string}" }

Generative.register_generator(:user) { FactoryGirl.build(:user, id: Generative.generate(:integer)) }

A class or module that responds to #call.

class LameGenerator
  def self.call
    random_string
  end
end

Generative.register_generator(:lame, LameGenerator)

Or you want to use a fancy library.

FakeTypeMappingLibrary
  generator_for Bar do
    initializer Float, Fixnum

    accessor :baz, Float
    accessor :quz, NotherClass
  end
end

Generative.register_generator(:bar, Bar }

You can then use your generators using the generate helper.

RSpec.describe String do
  let(:string) { "abc" }

  describe "#reverse" do
    generative do
      data(:string) { generate(:my_fancy_string) }

      it "maintains length" do
        expect(string.reverse.length).to eq(string.length)
      end
    end
  end
end

I heard you like generators so I put some generators in your generators...

If you want to use other generators in your registered generator, do note you have to use Generative's module level method .generate like so...

Generative.register_generator :user do
  User.new( first_name: Generative.generate(:string, max: 100) )
end

Pre-made Generators

Note: At this time Generative provides no pre-made generators. We encourage you to go out and seek good ways of generating data and maybe even package them up as your own gems. If you do, let us know and we'll link to it here.

Degenerate

This library has a few generators for some basic primitives... A great start if you're looking to move quickly.