Binary Lambda Calculus is absolutely tiny [1], with a 29 byte self-interpreter (43 byte for the byte oriented version), and absolutely extensible, since it can

Binary Lambda Calculus is absolutely tiny [1], with a 29 byte self-interpreter (43 byte for the byte oriented version), and absolutely extensible, since it can transform into any other language. [1] https://www.ioccc.org/2012/tromp/hint.html https://www.ioccc.org/2012/tromp/hint.html