I too like K better but it is nice to see another APL implementation and a free one at that. Also note that dyalog has made their commercial APL available on the RaspberryPi.

The open source "kona" is close to K version3 (no longer available from kx.com as they have moved on to Q, another very powerful array / database language). So far kona is only 7.2K lines of C code though admittedly very dense code (it is sort of like writing K in C!).