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PropEr (PROPerty-based testing tool for ERlang) is a QuickCheck-inspired open-source property-based testing tool for Erlang, developed by Manolis Papadakis, Eirini Arvaniti, and Kostis Sagonas. The base PropEr system was written mainly by Manolis Papadakis, and the stateful code testing subsystem by Eirini Arvaniti. Kostis Sagonas has been actively maintaining its code base since 2012.

You can reach PropEr's developers in the following ways:

• on the web: at the project's home page or the project's github page
• by email: take the tool's name (all lowercase), add a @ followed by softlab dot ntua dot gr

We welcome user contributions and feedback (comments, suggestions, feature requests, bug reports, patches, etc.).

Copyright 2010-2025 by Manolis Papadakis, Eirini Arvaniti, and Kostis Sagonas.

This program is distributed under the GPL, version 3 or later. Please see the COPYING file for details.

Traditional testing methodologies essentially involve software testers writing a series of test inputs for their programs, along with their corresponding expected outputs, then running the program with these inputs and observing whether it behaves as expected. This method of testing, while simple and easy to automate, suffers from a few problems, such as:

• Writing test cases by hand is tedious and time consuming.
• It is hard to know whether the test suite covers all aspects of the software under test.

Property-based testing is a novel approach to software testing, where the tester needs only specify the generic structure of valid inputs for the program under test, plus certain properties (regarding the program's behaviour and the input-output relation) which are expected to hold for every valid input. A property-based testing tool, when supplied with this information, should randomly produce progressively more complex valid inputs, then apply those inputs to the program while monitoring its execution, to ensure that it behaves according to its specification, as outlined in the supplied properties.

Here are a few examples of simple properties a user may wish to test, expressed in natural language:

• The program should accept any character string and convert all lowercase letters inside the string to uppercase.
• The program should accept any list of integers. If the input list is at least 4 elements long, the program should return the 4th largest integer in the list, else it should throw an exception.

PropEr is such a property-based testing tool, designed to test programs written in the Erlang programming language. Its focus is on testing the behaviour of pure functions. On top of that, it is equipped with two library modules that can be used for testing stateful code. The input domain of functions is specified through the use of a type system, modeled closely after the type system of the language itself. Properties are written using Erlang expressions, with the help of a few predefined macros.

PropEr is also tightly integrated with Erlang's type language:

• Types expressed in the Erlang type language can be used instead of generators written in PropEr's own type system as input data specifications.
• Generators for ADTs can be constructed automatically using the ADTs' API functions.
• PropEr can test functions automatically, based solely on information provided in their specs.

• Obtain a copy of PropEr's sources. You can either get a tagged version of the tool (look under Tags on github) or you can clone the current code base:

  git clone git@github.com:proper-testing/proper.git

• Compile PropEr: Simply run make if you just want to build PropEr. If you want to do some changes to PropEr or submit some pull request you most likely will want to issue a make test to run its unit tests and a make dialyzer call to also run dialyzer on PropEr's code base. To do the above but also build PropEr's documentation issue a make all call; in that case, you are going to need the syntax_tools application and a recent version of EDoc).

• If you are using Homebrew, you can simply:

  brew install proper

  and continue following the instructions below.

• Add PropEr's base directory to your Erlang library path, using one of the following methods:

  1. ERL_LIBS environment variable: Add the following line to your shell startup file (~/.bashrc in the case of the Bash shell):

     export ERL_LIBS=/full/path/to/proper

  2. Erlang resource file: Add the following line to your ~/.erlang file:

     code:load_abs("/full/path/to/proper").

• Add the following include line to all source files that contain properties:

  -include_lib("proper/include/proper.hrl").

• Compile those source files, preferably with debug_info enabled.

• For each property, run:

  proper:quickcheck(your_module:some_property()).

  See also the section common problems below if you want to run PropEr from EUnit.

To get started on using PropEr, see the tutorials and testing tips provided on PropEr's home page. On the same site you can find a copy of PropEr's API documentation (you can also build this from source if you prefer, by running make doc), as well as links to more resources on property-based testing.

The main issue is that both systems define a ?LET macro. To avoid a potential clash, simply include PropEr's header file before EUnit's. That way, any instance of ?LET will count as a PropEr ?LET.

Another issue is that EUnit captures standard output, so normally PropEr output is not visible when proper:quickcheck() is invoked from EUnit. You can work around this by passing the option {to_file, user} to proper:quickcheck/2. For example:

?assertEqual(true, proper:quickcheck(your_mod:some_prop(), [{to_file, user}])).

This will make PropEr properties visible also when invoked from EUnit.

PropEr's notation and output format has been kept quite similar to that of QuviQ's QuickCheck in order to ease the reuse of existing testing code written for that tool. However, incompatibilities are to be expected, since we never run or owned a copy of QuviQ's QuickCheck and the two programs probably bear little resemblance under the hood. Here we provide a nonexhaustive list of known incompatibilities:

• ?SUCHTHATMAYBE behaves differently in PropEr.
• proper_gen:pick/1 differs from eqc_gen:pick/1 in return value format.
• PropEr handles size differently from QuickCheck.
• proper:module/2 accepts options in the second argument instead of the first; this is for consistency with other module/2 functions in Erlang/OTP.

All the above are from circa 2010. Most likely, there exist many more incompatibilities between the two tools by now.