I liked what I saw. I enjoy paring down C code, but J makes my best efforts look as verbose as a legal contract. Introductory J examples are already cryptic enough to induce watering in the untrained eye. In the right hands, a J program can be compressed so densely that it threatens to collapse into a black hole of inscrutability.
I’ve heard about an obfuscated C tattoo, as well as a Lisp tattoo. They should have gotten J tattoos! Instead of Hello World or the Fibonacci sequence, why not sport a terse program that solves a Sudoku, or finds a QR decomposition of a matrix?
More generally, J appears to be ideal when source size matters most. For example, programming via a smartphone with a tiny screen and keyboard.
A day at play with J
J systems are freely available, but I reasoned I’d get a better feel for the language by writing my own interpreter. Thanks to J’s elegant design, I soon got the celebrated "mean=:+/%#" example working. That was enough to gain an appreciation (but also a little contempt) for J’s simple grammar, array handling, and above all, compact notation. A J symbol is worth a thousand machine words. See my J notes.
I kept picking at it for a while, but I think I’ll stop soon. It’d take substantial effort to implement types apart from doubles, not to mention error handling, memory management, arrays with axes of zero length, and fills.
Git repository: a primordial J interpreter.