PropCheck - Property based testing for Elixir

PropCheck is a testing library, that provides a wrapper around PropEr, an Erlang based property testing framework in the spirit of QuickCheck.

Installation

To use PropCheck with your project, add it as a dependency to mix.exs:

defp deps do
  [
    {:propcheck, "~> 1.4", only: [:test, :dev]}
  ]
end

From PropCheck 1.3.0 onwards, we require at least Elixir 1.7 since in Elixir 1.11 function get_stracktrace() is deprecated.

Changes

Relevant changes are documented in the Changelog, on GitHub follow this link.

Basic Usage and Build Configuration

PropCheck allows to define properties, which are automatically executed via ExUnit when running mix test. You find relevant information here:

• Details about the property macro are found in PropCheck.Properties,
• Details about how to specify the property conditions are documented in PropCheck,
• The basic data generators are found in PropCheck.BasicTypes,
• For property testing of state-based systems, take a look at PropCheck.StateM.ModelDSL for the new DSL (since PropCheck 1.1.0-rc1), which is a layer on top of PropCheck.StateM.
• The new targeted property based testing approach (TBPT) employing an automated search strategy towards more interesting test data is described in PropCheck.TargetedPBT.

For PropCheck, there is only one configuration option. All counter examples found are stored in a file, the name of which is configurable in mix.exs as part of the project configuration:

def project() do
  [ # many other options
    propcheck: [counter_examples: "filename"]
  ]
end

Note that the path can also be set as part of the application environment:

config :propcheck, counter_examples: "filename"

If both the project configuration and the application environment are present, the application environment is chosen.

Per default, the counter examples file is stored in the build directory (_build), independent from the build environment, in the file propcheck.ctex. The file can be removed using mix propcheck.clean. Note that this task is only available if PropCheck is also available in :dev. Otherwise, MIX_ENV=test mix propcheck.clean can be used.

Setting PropCheck into Verbose Mode

Properties in PropCheck can be run in quiet or verbose mode. Usually, quiet is the default. To enable verbose mode without requiring to touch the source code, the environment variable PROPCHECK_VERBOSE can be used. If this is set to 1, the forall macro prints verbose output.

Detecting Exceptions and Errors in Tests

PropCheck can attempt to detect and output exceptions in non-verbose mode. This can be done using the detect_exceptions: true option for property or forall, or using the environment variable PROPCHECK_DETECT_EXCEPTIONS. If this environment variable is set to 1, exception detection is enabled globally.

Links to other documentation

The guides for PropEr are an essential set of documents to make full use of PropCheck

• PropEr Home Page
• PropEr Tutorials

Elixir versions of most of PropEr's tutorial material can be found in the test folder on GitHub.

Jesper Andersen and Robert Aloi blog about their thoughts and experiences on using QuickCheck which are (mostly) directly transferable to PropCheck (with the notable exception of concurrency and the new state machine DSL from QuickCheck with the possibility to add requirement tags):

• Jesper Andersen's QuickCheck Advice
• Jesper Andersen's Breaking Erlang Maps (4 part series)
• Roberto Aloi's Notes on Erlang Quickcheck

Rather new introductory resources are

• Fred Hebert's PropEr Testing and
• Fred Hebert's book Property-Based Testing With PropEr, Erlang and Elixir. This book explains the new approach of targeted property based testing (TPBT) very nicely and is way more approachable then the scientific papers regarding TPBT. Some of the Erlang only examples of the book are ported to Elixir and can be found in the test test folder on GitHub.

In contrast to the book, the free website is concerned with Erlang only. The Erlang examples translate easily into Elixir (beside that at least a reading knowledge of Erlang is extremely helpful to survive in the BEAM ecosystem ...). Eventually I will port some of the examples to Elixir and PropCheck and certainly like to accept PRs.

What is not available

PropCheck does not support PropEr's capability to derive automatically type generators from type specifications. This is due to some shortcomings in PropEr, where the specification analysis in certain situation attempt to parse the Erlang source code of the file with the type specification. Apparently, this fails if the types are specified in an Elixir source file.

Effectively this means, that to to support this feature from Elixir, the type management system in PropEr needs to be rewritten completely. Jesper Andersen argues in his aforementioned blog post eloquently that automatically derived type generators are not needed, even more that carefully crafted type generators for specific testing purposes is an essential part of the QuickCheck philosophy. Therefore, this missing feature is not that bad. For the same reason, automatic @spec-checking is of limited value in PropCheck since type generators for functions specification are also generated automatically.

PropCheck has only very limited support for parallel testing since it introduces no new features for concurrency compared to PropEr.

Contributing

Please use the GitHub issue tracker for

• bug reports and for
• submitting pull requests

Test Policy

Before submitting a pull request, please use Credo to ensure code consistency and run mix tests to check PropCheck itself. mix tests is a Mix alias that runs regular tests (via mix test) and some external tests (via another Mix alias mix test_ext) which test PropCheck's side effects, like storing counterexamples or proper output format, that can't be easily tested using regular tests.

Rebase Policy

Ensure that your PR is based on the latest master version by rebasing. If your feature branch is my_feature, then do the following before publishing a pull request:

git checkout master
git pull --rebase
git checkout my_feature
git rebase master

If your PR implementation takes longer such that another PR is merged before your own PR, then you have to repeat the above sequence. It might be that you have fix some conflicts. But now you cannot push your branch again, because you changed the history of your local branch compared to the one published on GitHub. Therefore, you have force-push your branch. Do this with

git push --force-with-lease

A simple git push --force is not allowed, --force-with-lease is more friendly and thus allowed. See details in the Git documentation.

The rationale behind this policy is that we want a simple almost linear history, where each merged PR create a sequence of merge with no parallel work. This history will not show how many active branches are available during development but the sequence of incorporating changes to master. That is the important part and we want to see which commit sequence lead to the specific feature. Merges destroy this linearity. But beware, you can do nasty things with git rebase, therefore stick to the simple procedure explained above to not corrupt your repository.

License

PropCheck is provided under the GPL 3 License due to its intimate use of PropEr (which is licensed under GPL 3). In particular, PropEr's exclusion rule of open source software from copyleft applies here as well as described in this discussion on GitHub.

Personally, I would prefer to use the LPGL to differentiate between extending PropEr and PropCheck as GPL-licensed software and the use of PropEr/PropCheck, which would not be infected by GPL's copyleft. But as long as PropEr does not change its licensing, we have no options. PropCheck is clearly an extension of PropEr, so it is derived work falling under GPL's copyleft. Using LGPL or any other license for PropCheck will not help, since GPL's copyleft overrules other licenses or result in an invalid or at least questionable licensing which does not help anybody.

From my understanding of open source licenses as a legal amateur, the situation is currently as follows: Since PropCheck is a testing framework, the system under test is not infected by the copyleft of GPL, since PropCheck is only a tool used temporarily during development of the system under test. At least, if you don't distribute your system together with the test code and the test libs as a binary. Another friendly approach is to have the tests in a separate project, such that the tests are a real client of the system under test. But again, this is my personal take. In question, please consult a professional legal advisor.