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Dafny

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What Dafny is

Dafny is a programming language for writing specifications alongside code, with a static verifier that checks code against those specifications. Its deductive proof tools cover preconditions, postconditions, lemmas, quantifiers, loop invariants, termination conditions and read/write specifications. The language includes classes, iterators, arrays, tuples, generic and subset types, inductive datatypes, lambdas, and mutable and immutable data structures. Dafny compiles programs to C#, Java, JavaScript, Go and Python, although compiling and running generated code may require tools for the chosen target. Verification requires Dafny and its bundled Z3 tool; Dafny itself is a .NET 8.0 artifact. The project offers a Visual Studio Code extension with verification while typing, compilation and running of .dfy files, syntax highlighting, verification traces, IntelliSense, navigation and hover information. The ecosystem also includes Emacs installation guidance, a formatter, language server, tutorials and reference material. Dafny is free under the MIT License and has builds for Windows, Linux and macOS. The project describes use in academic teaching and research and in industry.

Who it suits

Dafny suits developers who want to specify program behavior and verify code against those specifications. The project also describes use in academic teaching and research and in industry.

What it does well

  • Static verification checks code against written specifications.
  • Proof tools include contracts, invariants and termination conditions.
  • Compiles to five listed target languages.
  • Free under the MIT License.

What to know first

  • Verification requires the Z3 tool.
  • Generated programs may require target-specific tools.
  • The tool is a .NET 8.0 artifact.

Getting started

  1. Visit the Dafny website and consult its installation guide.
  2. Install Dafny for Windows, Linux, or macOS.
  3. Install Z3 for verification.
  4. Use the Visual Studio Code extension or follow the Emacs installation guidance.
  5. Add tools for your selected compilation target when compiling or running generated programs.

Verdict

Dafny combines a programming language with specification and deductive verification tools. Users should account for Z3 and any additional tools needed for their compilation targets.

Questions

Is Dafny free?

Yes. Dafny has a free plan and is licensed under the MIT License.

Which operating systems does Dafny support?

The installation guide lists Windows, Linux, and macOS builds.

Which languages can Dafny compile to?

Dafny can compile programs to C#, Java, JavaScript, Go, and Python. Compiling and running generated programs may require tools for the selected target.

Does verification require other software?

Yes. Dafny verification requires the Z3 tool. The Dafny tool itself is a .NET 8.0 artifact.

Is Dafny open source?

Dafny is licensed under the MIT License.

Where can users get help?

The project directs users to its Zulip channel for questions and GitHub for issue reports.

Facts from Dafny's own pages

Audience
Dafny is used in academia for teaching and research and in industry, including by teams at Amazon. · dafny.org, 5 Oct 2026
Compilation targets
Dafny compiles code to C#, Java, JavaScript, Go, and Python, with more targets in development. · dafny.org, 5 Oct 2026
Dependencies
The Dafny tool is a .NET 8.0 artifact, and the Z3 tool is required to use Dafny for verification. · dafny.org, 5 Oct 2026
Desktop support
The installation guide lists Windows, Linux, and macOS binary builds and tests Dafny on supported operating system versions. · dafny.org, 5 Oct 2026
IDE integrations
The Dafny ecosystem includes a Visual Studio Code extension powered by a Language Server Protocol implementation, plus a code formatter. · dafny.org, 5 Oct 2026
Intended users
The project describes Dafny as used in academia for teaching and research and in industry, including by teams at Amazon. · dafny.org, 5 Oct 2026
Known target limits
The installation guide describes Rust support as partial and growing and C++ support as rudimentary and special-purpose. · dafny.org, 5 Oct 2026
Language features
The language supports classes, iterators, arrays, tuples, generic and subset types, inductive datatypes, lambdas, and mutable and immutable data structures. · dafny.org, 5 Oct 2026
Learning resources
The Dafny documentation links to a getting-started tutorial, reference manual, FAQs, error explanations, verification optimization guide, style guide, and examples. · dafny.org, 5 Oct 2026
License
The Dafny software is licensed under the MIT License. · github.com, 5 Oct 2026
Other editor integration
The installation guide describes using Dafny with Emacs as well as Visual Studio Code. · dafny.org, 5 Oct 2026
Platforms
Installation instructions cover Windows, Linux, and macOS; the project repository also lists binary downloads for FreeBSD. · dafny.org, 5 Oct 2026
Product
Dafny is a verification-aware programming language with native specification support and a static program verifier. · dafny.org, 5 Oct 2026
Proof features
Its proof toolbox includes quantifiers, calculational proofs, lemmas, preconditions, postconditions, termination conditions, loop invariants, and read/write specifications. · dafny.org, 5 Oct 2026
Purpose
It helps developers write code that can be verified against specifications to reduce the risk of late-stage bugs. · dafny.org, 5 Oct 2026
Requirements
The Dafny tool is a .NET 8.0 artifact, and Dafny plus its bundled Z3 tool are sufficient for verification; compiling and running generated programs may require additional tools. · dafny.org, 5 Oct 2026
Security reporting
The project asks people who discover a potential security issue to notify its security contact instead of creating a public GitHub issue. · github.com, 5 Oct 2026
Support
The project directs users to its Zulip channel for questions and GitHub for issue reports. · github.com, 5 Oct 2026
Target requirements
Compiling generated code requires tools for the selected target language, such as .NET for C#, Node.js for JavaScript, and Go tools for Go. · dafny.org, 5 Oct 2026
Tooling
The Dafny ecosystem includes compilers, IDE plugins, a language server, a code formatter, a reference manual, and tutorials. · dafny.org, 5 Oct 2026
VS Code
The VS Code extension supports compiling and running .dfy files, verification while typing, syntax highlighting, verification traces, IntelliSense, go to definition, and hover information. · marketplace.visualstudio.com, 5 Oct 2026
VS Code features
The VS Code extension supports verification while typing, compiling and running .dfy files, syntax highlighting, verification traces, IntelliSense, go to definition, and hover information. · marketplace.visualstudio.com, 5 Oct 2026
What it does
Dafny is a verification-aware programming language with native support for recording specifications and a static program verifier that checks code against them. · dafny.org, 5 Oct 2026

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