Lambda the Ultimate

Rubato is a functional language for composing music based on Topos theory. I've never used it, but the documentation, particularly the PHD thesis of G. Milmeister The Rubato Composer Music Software - Component Based Implementation of a Functorial Concept Architecture , describes the use of Topos theory for a specialized programming language for music composition. It's very well written and it's mapping of Topos Theory concepts to programming concepts has helped me to understand some Topos Theory concepts in a more concrete fashion (which is possibly a mistake!). There is also a book The Topos of Music which looks interesting.