Stainless

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What Stainless is

Stainless is a free, open-source framework for verifying Scala software against specifications. It uses deductive verification to check function preconditions and postconditions, array bounds, pattern-match coverage, and whether implementations conform to stated requirements. Stainless can verify termination on all inputs and provide counterexamples when code fails verification. It works with a Pure Scala fragment and translates certain imperative features into Pure Scala concepts. The current frontend supports Scala 3.5.0 and later; Scala 2 is no longer supported. Stainless uses the Inox backend and can work with SMT solvers including Z3 and cvc5, with Princess as a Scala-based fallback. Installation options include standalone releases, Snap, the Arch User Repository, source builds, and an sbt plugin; documentation also covers Docker, Metals, and GitHub Codespaces. The standalone release requires JDK 17 and bundles solver components. Stainless is available on Linux, macOS, and Windows under Apache 2.0. Verification assumes unbounded data types and sufficient stack space, so runtime overflows remain possible.

Who it suits

Stainless suits Scala developers who want deductive checks inside an sbt project or browser-based Codespaces workflow. It is intended for verified Scala software and supports Scala 3.5.0 or later.

What it does well

Free and open source
Checks contracts, bounds, and pattern matches
Can prove termination and provide counterexamples
Supports multiple SMT solvers
Runs on Linux, macOS, and Windows

What to know first

Scala 2 frontend is unsupported
Requires JDK 17 for standalone release
No default shared-heap concurrency encoding
Verified code may still overflow at runtime

Verdict

Stainless provides contract-based verification and counterexamples for modern Scala projects, with several installation and solver choices. Its language and verification assumptions should be understood before relying on guarantees in production.

Facts from Stainless's own pages

Browser workflow: The documentation describes running Stainless in GitHub Codespaces, including through a browser. · epfl-lara.github.io, 8 Oct 2026
Contracts: It checks user-provided function preconditions and postconditions. · epfl-lara.github.io, 8 Oct 2026
Correctness checks: It can statically verify that a program conforms to a specification and cannot crash at runtime. · epfl-lara.github.io, 7 Oct 2026
Counterexamples: When a function does not satisfy its contract, Stainless can return a concrete counterexample. · epfl-lara.github.io, 8 Oct 2026
Editor integration: The project page links installation instructions for standalone use, Docker, sbt, and Metals. · ecocloud.epfl.ch, 7 Oct 2026
Installation: Stainless can be installed from a standalone release, through Snap or the Arch User Repository, built from source, or used as an sbt plugin. · epfl-lara.github.io, 8 Oct 2026
Installation options: The installation documentation lists standalone releases, Snap, source builds, and an sbt plugin. · epfl-lara.github.io, 7 Oct 2026
Intended users: The documentation describes using Stainless within an existing sbt project to check Scala code in enabled modules. · epfl-lara.github.io, 8 Oct 2026
Language limits: The language does not provide a default encoding for concurrency with a shared mutable heap. · epfl-lara.github.io, 7 Oct 2026
License: The repository states that Stainless is released under the Apache 2.0 license. · github.com, 8 Oct 2026
Limitations: The documentation warns that verification assumes unbounded data types and sufficient stack space, so verified code may still encounter stack or heap overflow at runtime. · epfl-lara.github.io, 8 Oct 2026
Purpose: Stainless is a verification framework and tool for a subset of the Scala programming language. · github.com, 8 Oct 2026
Runtime requirement: The installation documentation lists Java 17 JRE as a general requirement. · epfl-lara.github.io, 8 Oct 2026
Safety checks: It checks array bounds and whether pattern matches cover all possible cases. · epfl-lara.github.io, 8 Oct 2026
Scala subset: Stainless verifies a Pure Scala fragment and also supports certain imperative features by translating them into Pure Scala concepts. · epfl-lara.github.io, 7 Oct 2026
Scala support: The current repository README says Stainless supports Scala 3.5.0 and later, and no longer supports the Scala 2 frontend. · github.com, 8 Oct 2026
Solvers: Stainless uses the Inox backend and can use SMT solvers including Z3 and cvc5; Princess is included as a Scala-based fallback solver. · epfl-lara.github.io, 8 Oct 2026
Solver support: Stainless relies on Inox and supports SMT solvers including Z3, cvc5, and Princess. · github.com, 7 Oct 2026
Supported Scala version: The repository says the current frontend supports Scala 3.5.0 and later and no longer supports Scala 2. · github.com, 7 Oct 2026
Termination: Stainless attempts to prove termination and lets users specify a decreasing measure when automatic checking does not suffice. · epfl-lara.github.io, 8 Oct 2026
Use cases: The project page reports use in proving compression and decompression algorithms, aspects of file systems, distributed algorithms, type systems, balanced trees, and student assignments. · ecocloud.epfl.ch, 7 Oct 2026
Who it is for: The documentation describes Stainless as a tool to help developers build verified Scala software. · epfl-lara.github.io, 7 Oct 2026

Where Stainless stands on Freedom251

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