APL versus A versus J versus K?

Question

The array-language landscape, while fascinating, is confusing to no end. Is there a reason to pick one of J or K or APL or A? None of these options seem to be open-sourced --

Answer 1

J is an Executable Mathematical Notation which is input and displayed using ascii characters.

It is also a fully fledged Functional, Object Oriented, Programming Language.

The JSoftware implementation of J (I do not know of any others) is not open source, but it is free to use.

Although the engine is not open source, there is an active user library of J code to which people can contribute.

J has extensive and concise documentation http://www.jsoftware.com/help.htm and forum members happily and rapidly respond to any questions or problems and are keen to help new users become oriented as well as with more advanced problems.

On the Open Source issue by the way, the documentation (specification) for J is extensive and anybody could in theory create an open source project to try to implement J if they wanted to.