The SO answer is easy to follow right until the final operator “/“ where it just goes “magic”. That’s less than helpful as the rest is quite straightforward aside from the syntax.
Its very short description of the two-arguments version make it sound like a fold, but that wouldn’t generalise to what looks like a cartesian product, would it?
If I understand it right "*/" results in a function that applies "*" between its arguments. "~" takes the function f(x) on its left and the argument y on its right and turns it into f(y, y).
So the right side of the tilde evaluates to a range 1..10. I think the tilde then turns its left and right into "*/"(1..10, 1..10), resulting in 1..10 * 1..10
My problem with this is not “J has an operator that does exactly what you want” it’s “J has operators which do entirely different things depending on the arity”, you can’t intuit the behaviour of the binary / from the unary, they are for all intents and purposes unrelated.
As a result, the original mathexchange answer introducing / by explaining its unary behaviour (despite that being used nowhere in the answer) is solely a source of confusion.
Its very short description of the two-arguments version make it sound like a fold, but that wouldn’t generalise to what looks like a cartesian product, would it?