Stainless

Hosted at https://github.com/epfl-lara/stainless ; mirrored at https://gitlab.epfl.ch/lara/stainless . Check out also Inox and Bolts.

Verification framework for a subset of the Scala programming language. See the tutorial.

Please note that this repository uses git submodules, so you need to either:

• clone it with the --recursive option, or
• run $ git submodule update --init --recursive after cloning.

Please note that Stainless does not support Scala 2 frontend anymore, only Scala 3.5.0 and later. The latest release that supports Scala 2.13 frontend is the v0.9.8.7.

Quick start

We test mostly on Ubuntu; on Windows, you can get sufficient text-based Ubuntu environment by installing Windows Subsystem for Linux (e.g. wsl --install, then wsl --install -d ubuntu). Ensure you have a Java version ready (it can be headless); on Ubuntu sudo apt install openjdk-17-jdk-headless suffices.

Once ready, Download the latest stainless-dotty-standalone release for your platform. Unzip the archive, and run Stainless through the stainless script. Stainless expects a list of space-separated Scala files to verify but also has other Command-line Options.

To check if everything works, you may create a file named HelloStainless.scala next to the stainless script with the following content:

import stainless.collection._
object HelloStainless {
  def myTail(xs: List[BigInt]): BigInt = {
    require(xs.nonEmpty)
    xs match {
      case Cons(h, _) => h
      // Match provably exhaustive
    }
  }
}

and run stainless HelloStainless.scala. If all goes well, Stainless should report something along the lines:

[  Info  ]   ┌───────────────────┐
[  Info  ] ╔═╡ stainless summary ╞════════════════════════════════════════════════════════════════════╗
[  Info  ] ║ └───────────────────┘                                                                    ║
[  Info  ] ║ HelloStainless.scala:6:5:   myTail  body assertion: match exhaustiveness  nativez3   0,0 ║
[  Info  ] ╟┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄╢
[  Info  ] ║ total: 1    valid: 1    (0 from cache) invalid: 0    unknown: 0    time:     0,0         ║
[  Info  ] ╚══════════════════════════════════════════════════════════════════════════════════════════╝
[  Info  ] Shutting down executor service.

If you see funny symbols instead of beautiful ASCII art, run Stainless with the --no-colors option for clean ASCII output with a standardized error message format.

The release archive of Stainless only requires JDK17. In particular, it needs neither a Scala compiler nor SBT. It is shipped with Z3 4.12.2+, cvc5 1.0.8+ and Princess (branch compiled for Scala 3). If Z3 API is not found, use option --solvers=smt-z3 to rely on the executable instead of API call to z3.

SBT Stainless plugin

Alternatively, one may integrate Stainless with SBT. To do so, download sbt-stainless, and move it to the directory of the project. Assuming the project's structure is:

MyProject
├── build.sbt
├── project
│   └── build.properties
├── sbt-stainless.zip
└── src/

Unzipping sbt-stainless.zip should yield the following:

MyProject
├── build.sbt
├── project
│   ├── build.properties
│   └── lib                     <--------
│       └── sbt-stainless.jar   <--------
├── sbt-stainless.zip
├── src/
└── stainless/                  <--------

That is, it should create a stainless/ directory and a lib/ directory with project/. If, instead, the unzipping process creates a sbt-stainless/ directory containing the lib/project/ and stainless/ directories, these should be moved according to the above structure.

Finally, the plugin must be explicitly enabled for projects in build.sbt desiring Stainless with .enablePlugins(StainlessPlugin). For instance:

ThisBuild / version := "0.1.0"

ThisBuild / scalaVersion := "3.5.0"

lazy val myTestProject = (project in file("."))
  .enablePlugins(StainlessPlugin) // <--------
  .settings(
    name := "Test"
  )

Verification occurs with the usual compile command.

Note that this method only ships the Princess SMT solver. Z3 and cvc5 can still be used if their executable can be found in the $PATH.

Build and Use

To start quickly, install a JVM and use a recent release. To build the project, run sbt universal:stage. If all goes well, scripts are generated for the front end:

• frontends/dotty/target/universal/stage/bin/stainless-dotty

Use one of these scripts as you would use scalac to compile Scala files. The default behavior of Stainless is to formally verify files, instead of generating JVM class files. See frontends/benchmarks/verification/valid/ and related directories for some benchmarks and bolts repository for a larger collection. More information is available in the documentation links.

SSH and VSCode

Visual Studio Code offers a feature allowing to connect to a host over SSH and edit code located on this host. This is useful to edit code on a remote machine using the local Visual Studio Code editor and running Stainless on this remote machine. See this official documentation to learn more about this feature.

If you have access to a remote machine over SSH, this is the recommended way to use Stainless. Please note you have to install Stainless on the remote machine following the instructions above.

Github Codespaces

Github Codespaces To allow running Stainless with only a browser, we have provided a sample repository to use Stainless with Github Codespaces. Github Codespaces are cloud machines that can be access via Visual Studio Code locally or in the browser. In our experience (as of October 2023), this flow works well, given the provided Ubuntu Linux virtual machines with 16GB of RAM and substantial processing power. Please see this repository for further details.

Further Documentation and Learning Materials

To get started, see videos:

• ASPLOS'22 tutorial
• FMCAD'21 tutorial
• Formal Verification Course: Getting Started, Tutorial 1 Tutorial 2 Tutorial 3 Tutorial 4, Assertions, Unfolding, Dispenser Example
• Keynote at Lambda Days'20
• Keynote at ScalaDays'17 Copenhagen

or see local documentation chapters, such as:

• Introduction to Stainless
• Installation
• Getting Started
• Command-line Options
• Mini Tutorial

There is also a Stainless EPFL Page.

License

Stainless is released under the Apache 2.0 license. See the LICENSE file for more information.

Relation to Inox

Stainless relies on Inox to solve the various queries stemming from program verification. Inox supports model-complete queries in a feature-rich fragment that lets Stainless focus on program transformations and soundness of both contract and termination checking and uses its own reasoning steps, as well as invocations to solvers (theorem provers) z3, cvc5, and Princess.