TeXmacs is an impressive project spanning several decades:
It uses Guile as its scripting language and documents created within TeXmacs are represented as Scheme programs.
In addition to providing a LaTeX-quality WYSIWYG editor experience for scientific documents, TeXmacs can also be used as a computer algebra front-end for packages like Mathics, which provides an open-source implementation of the Wolfram programming language used in Mathematica: