That project obviously has been dead for a long time, but its successor Extempore [0] seems to still be in somewhat active development. And btw. it runs on Linux, MacOS and Windows.

[0] https://extemporelang.github.io/

Impromptu was pretty nice as a scripting env for Core Graphics (I sued it as such at some point).

I also coordinated an effort to save it from being deleted from Wikipedia, along with Andrew Sorensen's "Study in Keith" video.

Andrew who's also the author of Impromptu and Extempore, had gifted the recording of a live coding performance to Wikipedia, under a CC license. Some editors wanted to have it removed because it was "personal research". Go figure... Sanity prevailed thankfully.

Edit: also, you should listen to "A study in Keith", it's a damn good piece. You can follow along as Andrew tweaks the grammar/Markov chains that make up the music, but it isn't necessary. The music starts at 1:56 (since the piece is written from scratch, there are a couple minutes of silent video while the author sets up the initial code).

https://upload.wikimedia.org/wikipedia/commons/c/c5/Study_in...

Unfortunately OSX only. Too bad.