Somehow I'm picturing this as as four lines of output, the rest being used by a virtual APL keyboard.

Unfortunately this is three of so releases old.

GNU APL is up to 1.5 now and is significantly more stable.

Yep, the “lib” is a .jar, not source code. Isn't that rather strange?

I'm almost tempted to be jealous: Where's the iOS version?

I seem to remember hearing you were not allowed to have interpreters on iOS?

Not quite true. Look at “Codea”, a Lua IDE on iDevices. The sad thing is, Apple wouldn't let you distribute something that produces & runs native code on an iDevice. Which would also be much more tempting :)