I just read this article on APL http://vector.org.uk/art10011550 Anyone used APL? What's it like writing a file parser or a UI in APL?

I was an early user of APL. It is an extremely powerful and compact language. Promoted by IBM in the early 70's, it originally ran as a terminal language under a time sharing mainframe. Its lamentable decision to use an invented alphabet was finally corrected by Iverson late in his life, and he rebooted it as J to compete against Mathematica. J continues onwards, and is a more modern version of APL. Keep in mind that APL was effectively a character graphics type of interface, and in its day was leaps and bounds ahead of any other system. APL is so powerful and dense, it is what many of us call a "write-only" language, where anyone but the original author has little chance of understanding a program of any size.