Peter,
The Emacs mode is much more than a simple way of editing functions in an editor. The intention is to transform the standalone GNU APL interpreter into some kind of IDE in a similar way as SLIME does for Common Lisp. The Emacs mode backend provides similar functionality as the Swank backend for Common Lisp.
As you might understand, there is a very tight level of integration there.
I would be very happy if someone would like to write a Vim frontend (for example) utilising the same communications protocol. Currently, however, there is only an implementation for Emacs.
Regards,
Elias