An emacs mode for what? I don't use emacs much but would be up for using it it helps me with something. Fairly happily plodding along with just the GNU APL interpreter and akt though (and akt seems to work with vim through invoking `akt vim`).
It has an input method that lets you type the non ASCII characters more easily. The default uses the super key (windows key) like alt uses alt, bit there's also a mode that allows you to prefix things with ., So e.g. to get rho you type .r, and to get an actual . you type .., which I find more ergonomic. And you can launch the repl from inside emacs as well. I'm mostly a vim user myself, and have been using it with spacemacs for a more familiar experience.
Emacs mode for APL, it has things such as keyboard support, compilation support, debugging support etc...