ortac — OCaml RunTime Assertion Checking

ortac is a tool to convert an OCaml module interface with Gospel specifications into code to check those specifications.

There are various ways to check specifications, all provided by plugins.

The QCheck-STM plugin generates a program using QCheck-STM that will test the module by running random functions calls and checking they give consistent results with the model provided in the Gospel specification; see the dedicated README for details.

This repository also contains two other experimental plugins, that are not yet stable. Expect rough edges if you venture to try them out:

• the wrapper plugin generates wrapper modules, i.e. modules exposing the original interface but instrumenting all function calls with assertions on arguments and on results, either normal or exceptional; see the dedicated README for details,
• the monolith plugin generates programs using Monolith that will test a module by comparing it against an instrumented version of that module; see the dedicated README for details.

At its core, Ortac provides a way to convert the executable fragment of Gospel into OCaml code. This core functionality is used by all plugins.

Installation

The easiest way to try ortac out is to opam install its main packages directly from the opam repository:

opam install ortac-qcheck-stm ortac-runtime

This will install the following OPAM packages:

• ortac-core.opam which provides the ortac command-line tool and the core functionality used by all plugins,
• ortac-runtime.opam which provides the support library for the code generated by Ortac plugins,
• ortac-qcheck-stm.opam which provides the QCheck-STM plugin for the ortac command-line tool.

Alternatively, you can install the packages directly from this repository using opam pin:

opam pin add -y https://github.com/ocaml-gospel/ortac.git

Besides the first three packages, this will also install the following experimental packages:

• ortac-wrapper.opam which provides the wrapper plugin for the ortac command-line tool,
• ortac-monolith.opam which provides the Monolith plugin for the ortac command-line tool,
• ortac-runtime-monolith.opam which provides the support library for the code generated with the Monolith plugin.
• ortac-dune.opam which provides a dune rule generator.

Even using opam pin, you can install only some of those packages by explicitly mentioning which package you want to install, for instance:

$ opam pin add ortac-core https://github.com/ocaml-gospel/ortac.git

To check that it is correctly installed, simply run ortac. If you installed all packages, you should get:

$ ortac
ortac: required COMMAND name is missing, must be one of 'monolith', 'qcheck-stm' or 'wrapper'.
Usage: ortac COMMAND …
Try 'ortac --help' for more information.

Quick start

or: How to use Ortac to test whether the specifications of a module hold.

QCheck-STM plugin

The QCheck-STM plugin can generate a standalone executable that will test the consistency between the behaviour of the functions of the module and their model, as provided by the Gospel specifications. Look at the dedicated README for the QCheck-STM plugin to see how it can be used.

Monolith plugin

The Monolith plugin can generate a standalone executable that will try to falsify the Gospel specifications of a module by stress-testing the code. Look in the dedicated README for the Monolith plugin to see how it can be used.

Wrapper plugin

The wrapper plugin can be used to generate a wrapper module that will expose the same interface as the original module but instrumenting all function calls with assertions corresponding to the Gospel specifications. Look in the dedicated README for the wrapper plugin to see how it can be used.

Supported Gospel

The main general limitation on what Gospel specifications are supported comes from the translation from Gospel into OCaml: it is only possible to translate the executable fragment of the Gospel language. In particular, it does not support quantifications except in the very specific patterns in which they quantify over an integer within explicit bounds such as:

forall i. a < i < b -> ...
exists i. a < i < b /\ ...

(the pattern matching tolerates some small variations in the way bounds are stated, such as b >= i > a, etc. but not much more).

Additionally, each plugin has its own set of constraints on what it can handle. See the dedicated READMEs for details.