But aren't these just domain-specific languages? It doesn't make sense to compare them to Ruby/Python or C/Java, because they don't solve the same problem. It's like saying that sed is more expressive than C.
Can you write a web server or a source control system in APL/J/K? I don't think so. You can compare Python and C because you can write a source control system in both (e.g. Mercurial and git).
> But aren't these just domain-specific languages?
No. Why would you think that?
> Can you write a web server or a source control system in APL/J/K?
Yes.
I'm not aware of a source control system written in any of them, but K's standard web server is implemented in K; there are several databases written in each of them - flipdb in APL, kdb+ in K, JDB in J. There are compilers written in each. There are raytracers, spreadsheets and sudoku solvers.
http://nsl.com/ contains a lot of K examples, some of them annotated. jsoftware.com is your source for J examples.
Can you write a web server or a source control system in APL/J/K? I don't think so. You can compare Python and C because you can write a source control system in both (e.g. Mercurial and git).