I have been working on Glitch for a while. Not forcefully limited to short tweet-size tunes, it makes generative music from simple chiptunes [1] to something more advanced [2].

I would be happy to answer any questions, if you are interested in!

It's open source and new version is coming soon - https://github.com/naivesound/glitch

[1]: http://bit.ly/2ib97gq (parser seems to trim the last parenthesis, so I had to use URL shortener)

[2]: http://bit.ly/2gQDWKM