# ACL2

6 Hacker News commenters recommended ACL2 and 0 criticized it, November 2025 to November 2026. It was recommended in 5 of 13 months.

**Verdict:** Free theorem prover, used for industrial verification and recommended as a better fit than Dafny or Lean for reasoning about software. Critics say the user experience is hard to like.

## Why people recommend it

- Well suited to reasoning about software (2 comments): "I hate the ACL2 user experience, but I think it is fundamentally more fit to reason about software." ([source](https://news.ycombinator.com/item?id=47473781)) "ACL2 might be a better fit in this regime than Dafny or Lean" ([source](https://news.ycombinator.com/item?id=47488555))
- Powerful and capable prover (2 comments): "The counterexample generator is very powerful because it leverages the knowledge base of the theorem prover" ([source](https://news.ycombinator.com/item?id=47489444)) "ACL2 is also VERY powerful and capable." ([source](https://news.ycombinator.com/item?id=46067367))

## What people warn about

- Hard user experience (1 comments): "I hate the ACL2 user experience, but I think it is fundamentally more fit to reason about software." ([source](https://news.ycombinator.com/item?id=47473781))
- Sometimes frustrating to use (1 comments): "the both of these facts can sometimes be frustrating" ([source](https://news.ycombinator.com/item?id=45791772))

## Alternatives hackers recommend

- [Lean 4](https://hackerapproved.com/p/lean.md): 268 recommend
- [Rocq](https://hackerapproved.com/p/rocq.md): 25 recommend
- [Coq](https://hackerapproved.com/p/coq.md): 24 recommend
- [Agda](https://hackerapproved.com/p/agda.md): 19 recommend
- [Isabelle](https://hackerapproved.com/p/isabelle.md): 14 recommend

## Cite as

Hacker Approved, "ACL2", https://hackerapproved.com/p/acl2/ (counts from 1,795,640 Hacker News comments, November 2025 to November 2026).