PropCheck allows you to do Property Testing in Ruby.

Gem Build Status Maintainability RubyDoc

It features:

  • Generators for common datatypes.
  • An easy DSL to define your own generators (by combining existing ones, or completely custom).
  • Shrinking to a minimal counter-example on failure.

TODOs before release

Before releasing this gem on Rubygems, the following things need to be finished:

  • [x] Finalize the testing DSL.
  • [x] Testing the library itself (against known 'true' axiomatically correct Ruby code.)
  • [x] Customization of common settings
    • [x] Filtering generators.
    • [x] Customize the max. of samples to run.
    • [x] Stop after a ludicrous amount of generator runs, to prevent malfunctioning (infinitely looping) generators from blowing up someone's computer.
    • [x] Look into customization of settings from e.g. command line arguments.
  • [x] Good, unicode-compliant, string generators.
  • [ ] Filtering generator outputs.


  • [ ] Basic integration with RSpec. See also!msg/rspec/U-LmL0OnO-Y/iW_Jcd6JBAAJ for progress on this.
    • [ ] aggregate , resize and similar generator-modifying calls (c.f. PropEr's variants of these) which will help with introspection/metrics.
    • [ ] Integration with other Ruby test frameworks.
    • Stateful property testing. If implemented at some point, will probably happen in a separate add-on library.


Add this line to your application's Gemfile:

gem 'prop_check'

And then execute:

$ bundle

Or install it yourself as:

$ gem install prop_check


Using PropCheck for basic testing

Propcheck exposes the forall method. It takes generators as keyword arguments and a block to run. Inside the block, each of the names in the keyword-argument-list is available by its name.

(to be precise: a method on the execution context is defined which returns the current generated value for that name)

Raise an exception from the block if there is a problem. If there is no problem, just return normally.

# testing that Enumerable#sort sorts in ascending order
PropCheck.forall(numbers: array(integer())) do
  sorted_numbers = numbers.sort

  # Check that no number is smaller than the previous number
  sorted_numbers.each_cons(2) do |former, latter| 
    raise "Elements are not sorted! #{latter} is < #{former}" if latter < former


When a failure is found, PropCheck will re-run the block given to forall to test 'smaller' inputs, in an attempt to give you a minimal counter-example, from which the problem can be easily understood.

For instance, when a failure happens with the input x = 100, PropCheck will see if the failure still happens with x = 50. If it does , it will try x = 25. If not, it will try x = 75, and so on.

This means if something only goes wrong for x = 2, the program will try:

  • x = 100(fails),`
  • x = 50`(fails),
  • x = 25(fails),
  • x = 12(fails),
  • x = 6(fails),
  • x = 3(fails),
  • x = 1 (succeeds), x = 2 (fails).

and thus the simplified case of x = 2 is shown in the output.

The documentation of the provided generators explain how they shrink. A short summary:

  • Integers shrink to numbers closer to zero.
  • Negative integers also attempt their positive alternative.
  • Floats shrink similarly to integers.
  • Arrays and hashes shrink to fewer elements, as well as shrinking their elements.
  • Strings shrink to shorter strings, as well as characters earlier in their alphabet.

Writing Custom Generators

PropCheck comes bundled with a bunch of common generators, for:

  • integers
  • floats
  • strings
  • symbols
  • arrays
  • hashes etc.

However, you can easily adapt them to generate your own datatypes:


Always returns the given value. No shrinking.


Allows you to take the result of one generator and transform it into something else.

>> Generators.choose(32..128).map(&:chr).call(10,
=> "S"


Allows you to create one or another generator conditionally on the output of another generator.

>> Generators.integer.bind { |a| Generators.integer.bind { |b| Generator.wrap([a , b]) } }.call(100,
=> [2, 79]


Useful if you want to be able to generate a value to be one of multiple possibilities:

>> Generators.one_of(Generators.constant(true), Generators.constant(false)).sample(5, size: 10, rng:
=> [true, false, true, true, true]

(note that for this example, you can also use Generators.boolean. The example happens to show how it is implemented under the hood.)


If one_of does not give you enough flexibility because you want some results to be more common than others, you can use Generators.frequency which takes a hash of (integer_frequency => generator) keypairs.

>> Generators.frequency(5 => Generators.integer, 1 => Generators.printable_ascii_char).sample(size: 10, rng:
=> [4, -3, 10, 8, 0, -7, 10, 1, "E", 10]


There are even more functions in the Generator class and the Generators module that you might want to use, although above are the most generally useful ones.

PropCheck::Generator documentation PropCheck::Generators documentation


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at . This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.


The gem is available as open source under the terms of the MIT License.

Code of Conduct

Everyone interacting in the PropCheck project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.

Attribution and Thanks

I want to thank the original creators of QuickCheck (Koen Claessen, John Hughes) as well as the authors of many great property testing libraries that I was/am able to use as inspiration. I also want to greatly thank Thomasz Kowal who made me excited about property based testing with his great talk about stateful property testing, as well as Fred Herbert for his great book Property-Based Testing with PropEr, Erlang and Elixir which is really worth the read (regardless of what language you are using).