F*

What F* is

F* is a free, open-source programming language for functional and effectful code, designed around proving program properties. Its type system combines dependent types with automated proof support based on SMT solving, as well as interactive theorem proving with tactics. F* programs compile to OCaml by default. With KaRaMeL or Vale, selected fragments can also be extracted to F#, C, WebAssembly, or assembly. The language is implemented in F* and bootstrapped with OCaml, and it is distributed under the Apache 2.0 license.

F* runs on Windows, Linux, and macOS, and can be installed from binaries, through OPAM, Docker, or Nix, or by building from source. The project provides an online book, browser examples and exercises, and a Low* tutorial for a low-level subset that can compile to C through KaRaMeL. For community help, it points users to GitHub Discussions and a public Zulip forum, and also lists a mailing list and maintainer contact email.

F* is used in security-focused development: Project Everest develops secure communication software with it, and HACL* provides cryptographic primitives written in F* and extracted to C. The project reports that code from HACL*, ValeCrypt, and EverCrypt appears in production projects including Firefox, the Linux kernel, Python, mbedTLS, and WireGuard.

Who it suits

F* suits programmers who need a proof-oriented language for functional or effectful programming and can work with theorem proving and SMT-based automation. Its installation options and learning materials support both local development and self-hosted use; its security projects also make it relevant to people working on high-assurance software.

Limits

The free F* plan provides the language and listed installation options. Programs compile to OCaml by default; extraction to F#, C, WebAssembly, or assembly uses KaRaMeL or Vale.

Getting started

1. Choose Windows, Linux, or macOS as your platform.
2. Install a binary, use OPAM, Docker, or Nix, or build from source.
3. Start with the online book or browser examples and exercises.
4. Use the Low* tutorial if you want to explore its low-level subset and C compilation through KaRaMeL.
5. Ask questions through GitHub Discussions or the public Zulip forum.

Verdict

Pick F* if your work calls for a proof-oriented language with both automated and interactive theorem proving, or for code that can be extracted to several targets. Consider whether its proof-focused approach fits your project and whether you want to install binaries, use a package-based route, or build from source.