I've personally found GNU APL to be a bit flimsy, and not some of the design choices they made in it to be a bit odd. It also deviates from the ISO standard, if that matters to you.
Dyalog is the only implementation that is robust and production ready that is still actively maintained. I would suggest learning with something other than the GNU implementation.
Dyalog is the only implementation that is robust and production ready that is still actively maintained. I would suggest learning with something other than the GNU implementation.