propensive / probably   0.8.0

Apache License 2.0 Website GitHub

To probe what we can't prove, so the unprovable may become probable; testing for Scala

Scala versions: 2.13 2.12

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To probe what we can't prove, so what isn't provable may be probable

Probably is a testing library designed to unintrusively provide test recording and reporting capabilities to any codebase, regardless of the users’ choices of libraries or programming paradigms. Probably can define and run unit tests and property tests. Its syntax is simple and unexciting, and its execution model has zero magic: it’s the same as for any other program.


  • no framework, reflection or control flow to understand
  • ScalaCheck-style property testing
  • tests may be run multiple times, with results aggregated
  • automatic derivation of arbitrary instances
  • functional API where it matters; impure calls where it's safe and practical

Availability Plan

Probably has not yet been published. The medium-term plan is to build Probably with Fury and to publish it as a source build on Vent. This will enable ordinary users to write and build software which depends on Probably.

Subsequently, Probably will also be made available as a binary in the Maven Central repository. This will enable users of other build tools to use it.

For the overeager, curious and impatient, see building.

Getting Started

Probably defines only two primary types: a mutable Runner for recording test results and reporting back on them, and Test definitions, whose instances are created by the Runner.

Although it is possible to construct and use different Runners, the most typical usage is to use the global singleton Runner called test, because for most purposes only one Runner will be required. Defining a test is simple. For example,

import probably.*

test(t"the sum of two identical integers is divisible by two"):
  val x: Int = 7
  x + x
.assert(_%2 == 0)

Note that the assertion takes a predicate lambda, which operates on the result from evaluating the body of the test. It does not operate on the value directly. This clearly separates the process of running the test from the check which is performed upon it.

When a test definition like this is encountered in running code, its body will be evaluated, and a predicate (defined as a parameter to assert) will be evaluated on the result of the body. The outcome of this will be one of four possibilities:

  • the predicate returns true, and the test passes
  • it returns false, and the test fails
  • an exception is thrown while the body is being evaluated
  • an exception is thrown while the assertion is being evaluated

These different cases will be distinguished in the test report.

It is important to note that a test can be defined anywhere, such as,

  • in the main method of an application
  • in a lazy val
  • inside an asynchronous Task
  • in a parameterized method, testing a property of that parameter
  • in the request handler on a web server
  • in a pattern extractor (unapply method)
  • inside an actor
  • in the Scala REPL
  • as one branch of a case clause
  • nested inside another test

Regardless of where the test is defined, the behavior is always the same: it will be evaluated, checked, and the result will be recorded in the Runner, as a side-effect. Tests may be run more than once (in which case they are recorded more than once, and aggregated) or not at all if, by virtue of some runtime criterion, they are simply not executed. The question of whether the test is executed is the same for

The decision to make the Runner mutable reflects the power of Scala's hybrid nature. The state of the Runner is write-only while the tests are being run, so many of the common concurrency problems which arise with mutable state do not apply. The Runner has one read-only method, report(), which will produce a summary report of the recorded test results. Reports may be produced many times, but normally report() is called just once, at the end. This conscious and careful compromise in functional purity buys convenience: integration of tests does not impose constraints on new code, or require non-local changes to existing code.

Parameterized tests

As tests may appear anywhere, they are easy to parameterize. We could, for example, rewrite the test above like so,

import probably.*

def runTest(x: Int): Unit =
  test(t"the sum of three identical integers is divisible by 3"):
    x + x + x
  .assert(_%3 == 0)


However, if the test were to fail, it would be useful to know what input caused it to fail. Any number of inputs can be logged by including them as additional named parameters after the test name, like this:

import probably.*

def runTest(x: Int): Unit =
  test(t"the sum of three identical integers is divisible by 3", input = x):
    x + x + x
  .assert(_%3 == 0)

The choice of the parameter name input is the user’s choice: any name that is a valid identifier may be chosen. The output from running the above tests will be displayed like this: Screenshot of test results showing input=2147483647

Property-based testing

The ability to run the same test multiple times with different parameters suggests an obvious approach to property-based testing: to run the same test over and over again with a stream of different inputs. Probably also provides the means to generate such streams of increasingly-obscure instances for a variety of primitive types, and will derive generators on-demand for case-class and sealed-trait types for which generators exist for each of the parameters.

import probably.*

case class Person(name: Text, age: Int)[Person](1000).foreach { person =>
  test(t"all persons have realistic ages", v = person):
  .assert { a => a >= 0 && a < 100 }

For a given Seed, the pseudorandom data generated will always be deterministic and hence repeatable.

Command-line Interface

Probably comes with a simple CLI runner for running test suites through the standard shell interface. This works particularly well for objects containing a series of unit tests. To use the command-line interface, create an object which extends Suite, giving the test suite a name. Then implement the run method to execute the tests, in order, like so:

object ProjectTests extends Suite("Project tests"):
  def run(using Runner): Unit =
    test(t"first test"):
      // test body
    .assert(/* predicate */)

The Suite class provides an implementation of a main method, so any object which subclasses Suite may be run from the command line.

Test Expression

Probably provides a second way of defining a test: as an expression. For example,

import probably.*

test(t"check the backup exists"):

This style should look familiar, apart from one superficial difference: the test predicate is applied to a method called check instead of assert. This transforms the test from a statement into an expression, which means that it returns the result of its body, instead of Unit. Note that it returns the value, regardless of whether the test passes or fails, and execution continues.

This confers a few further differences with assertion tests:

  • exceptions thrown inside the body are not caught (but are recorded); exceptions in the check are still caught
  • test expressions cannot be skipped; their return value is necessary for execution to continue

Test Suites

A test suite is a convenient grouping of related tests, and can be launched from a runner (the value test in the following example) like so:

test.suite("integration tests") { test =>
  test(t"end-to-end process"):

Like other tests, a suite has a name, and will be executed at the point it is defined, and like other tests, it will pass or fail (or, produce mixed results). Its body, however, is a lambda which introduces a new Runner instance which will be used to run the tests in the suite. By convention, the new Runner is also named test. This will shadow the outer one, which is usually the desired behavior.

When the test suite completes, its results are aggregated into the report of the runner which spawned it. If you launched it using the CLI, the table of results will show the nested tests indented.

The Runner introduced by the suite method is the same as any other Runner, so further test suites can be defined inside other test suites, making it possible to organise tests into a hierarchy.


Probably is classified as maturescent. For reference, Scala One projects are categorized into one of the following five stability levels:

  • embryonic: for experimental or demonstrative purposes only, without any guarantees of longevity
  • fledgling: of proven utility, seeking contributions, but liable to significant redesigns
  • maturescent: major design decisions broady settled, seeking probatory adoption and refinement
  • dependable: production-ready, subject to controlled ongoing maintenance and enhancement; tagged as version 1.0.0 or later
  • adamantine: proven, reliable and production-ready, with no further breaking changes ever anticipated

Projects at any stability level, even embryonic projects, can still be used, as long as caution is taken to avoid a mismatch between the project's stability level and the required stability and maintainability of your own project.

Probably is designed to be small. Its entire source code currently consists of 795 lines of code.


Probably will ultimately be built by Fury, when it is published. In the meantime, two possibilities are offered, however they are acknowledged to be fragile, inadequately tested, and unsuitable for anything more than experimentation. They are provided only for the necessity of providing some answer to the question, "how can I try Probably?".

  1. Copy the sources into your own project

    Read the fury file in the repository root to understand Probably's build structure, dependencies and source location; the file format should be short and quite intuitive. Copy the sources into a source directory in your own project, then repeat (recursively) for each of the dependencies.

    The sources are compiled against the latest nightly release of Scala 3. There should be no problem to compile the project together with all of its dependencies in a single compilation.

  2. Build with Wrath

    Wrath is a bootstrapping script for building Probably and other projects in the absence of a fully-featured build tool. It is designed to read the fury file in the project directory, and produce a collection of JAR files which can be added to a classpath, by compiling the project and all of its dependencies, including the Scala compiler itself.

    Download the latest version of wrath, make it executable, and add it to your path, for example by copying it to /usr/local/bin/.

    Clone this repository inside an empty directory, so that the build can safely make clones of repositories it depends on as peers of probably. Run wrath -F in the repository root. This will download and compile the latest version of Scala, as well as all of Probably's dependencies.

    If the build was successful, the compiled JAR files can be found in the .wrath/dist directory.


Contributors to Probably are welcome and encouraged. New contributors may like to look for issues marked beginner.

We suggest that all contributors read the Contributing Guide to make the process of contributing to Probably easier.

Please do not contact project maintainers privately with questions unless there is a good reason to keep them private. While it can be tempting to repsond to such questions, private answers cannot be shared with a wider audience, and it can result in duplication of effort.


Probably was designed and developed by Jon Pretty, and commercial support and training on all aspects of Scala 3 is available from Propensive OÜ.


The name Probably acknowledges an appropriate level of confidence in writing tests which probe the functionality of a program, in contrast to using types which (in theory, at least) prove it.

In general, Scala One project names are always chosen with some rationale, however it is usually frivolous. Each name is chosen for more for its uniqueness and intrigue than its concision or catchiness, and there is no bias towards names with positive or "nice" meanings—since many of the libraries perform some quite unpleasant tasks.

Names should be English words, though many are obscure or archaic, and it should be noted how willingly English adopts foreign words. Names are generally of Greek or Latin origin, and have often arrived in English via a romance language.


The logo shows a twenty-sided die, an icosahedron, alluding to probabilistic chance.


Probably is copyright © 2024 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.