It seems to me APL and family are mostly useful for numerical work on data that is possibly queried (efficiently at that) from a database. A very naïve way would be to say it's a glorified Excel. Is there something I'm missing ? Are there other domains one could use it for ?
Although the "glorified Excel" statement makes me cringe, I would agree that APL's strong point is numerical work on data. I wouldn't use APL to write, for example, an operating system.
When Ken Iverson wrote APL in the sixties, numerical work was one of the big uses of computers (after the typical finance applications such as billing, inventory, accounts payable/receivable, general ledger), and APL filled that niche quite well. When I was working at IBM in the nineties investment banks were still using APL for some of their modeling.
APL also took a different evolutionary path than many of today's languages, operator precedence for example. APL's rule was simple: evaluate left-to-right. I didn't need to remember that exponentiation has a higher precedence than multiplication which in turn has a higher precedence than addition (those were the easy ones to remember; the hard ones were bit operations, logical operations — some languages had both high-precedence logical operations ("&&" and "||") as well as low-precedence ("and" and "or"), which further muddied the waters). I found Reverse Polish Notation (RPN) the most elegant when determining operator precedence, with APL's a close second.
Another difference was the way that the code was maintained. For example, my father, who was also an APL developer, often said, "APL is write-only code". Rarely would one modify someone else's code; instead, you'd re-write the function entirely. Thus the most important aspect of writing APL code was documenting what the function took as input and what it spat out.
But I'm glossing over one of the most important aspects of APL: it made you think about the problem space differently: instead of nested for-loops, you'd think of vectors and matrices. The manner in which you thought about the problem was incredibly succinct, similar to APL's vocabulary.
There was an extremely impressive video a year or two back of someone who had built a compiler in APL. While I don't code in it, it changed my view of APL and its power.
HN was fairly caught up with it for a month or so, and a flood of APL posts.
Just look for Aaron Hsu Co-Dfns on GitHub, talks on YouTube, and on HN...I think his username here is arcfide. Look for his submissions. He's probably 100x better at coding than me and never leaves notepad.
Historically it was spreadsheets that killed (financier) interest in APL (which was going gangbusters at the time). They were much easier for this crowd. APL does actually work for non numerical stuff. There is the famous example of OS360 being written in a couple pages of APL code. The model was actually used at IBM and at the time was heavily used there. Iverson went on to shepherd J, a much sophisticated language. K, another much modern offshoot, in the mean time has taken over the financial (trader) world.
