Lang-music
#lang music is a project that Michael Gunantra and I did for IU's C424 class. It's a music notation language inspired by MML, but instead of manipulating and concatenating strings, it's a structured language with full support for manipulating notes and bars.
It was done in about 500 lines of Racket and outputs MIDI data. The playback routine is still a little buggy but it's a fun prototype to play with nonetheless.