Mercury is a logic/functional programming language with a strong, static type system. It is designed for the development of high-performance and reliable software, especially in domains such as real-time systems, safety-critical systems, and program analysis.

Hello, world code example:

:- module hello.
:- interface.
:- import_module io.
:- pred main(io::di, io::uo) is det.

:- implementation.
main(!IO) :-
    io.write_string("Hello world!\n", !IO).

Popularity: Less than 1% of developers are using or have used this language.*
*According to StackOverflow's 2023 survey.

Repositories on GitHub: 746