Coq
Coq is a proof assistant and formal verification tool. Its main uses include the development and verification of formal mathematical proofs, program correctness, and specification of software systems.
Hello, world code example:
Require Import Coq.Strings.String.
Eval compute in ("Hello world!"%string).
Popularity: Less than 1% of developers are using or have used this language.*
*According to StackOverflow's 2023 survey.
Repositories on GitHub: 5,850