[kindohm]

If you're interested in rhythms and are open to using samples, you could take a look at Tidal (http://tidal.lurk.org). Tidal is a DSL embedded in Haskell, and can be used to very quickly create patterns of sound: everything from house music to chaotic breakcore to abstract textures. It's great for rhythm-based performance (and composition). It isn't as good at melody, but it does have some support for MIDI output and melodic expression, and with some effort your library of samples can support melody easily. I found Tidal's learning curve to be very shallow, but I think it depends on how you perceive music and rhythm. The Tidal community is working on a better install experience right now (in the meantime you'll need to compile a few things from source, etc). It primarily supports Emacs and Atom (Emacs appears to be the most stable).

Edit: Tidal is great for both live-coding and static-composition scenarios. In my opinion, it's ideal for live coding performance because minimal code is needed to get sound going quickly.