It's "very very minorly" (that's the only commit message used) difficult to follow, but I'm pretty sure ATS2 (Postiats) is in maintenance mode (activity is build changes), with ATS3 (Xanadu) https://github.com/githwxi/ATS-Xanadu being the focus of active development.
Abstract Tree Syntax
Looks interesting. Actively developed : https://github.com/githwxi/ATS-Postiats
It's "very very minorly" (that's the only commit message used) difficult to follow, but I'm pretty sure ATS2 (Postiats) is in maintenance mode (activity is build changes), with ATS3 (Xanadu) https://github.com/githwxi/ATS-Xanadu being the focus of active development.