Hi,
I notice the maximum rank of an object in GNU APL is 8.
In APL2, it is 63.
The most common APL FFT algorithm reshapes the input into a hypercube with dimensions of length 2, and makes a pass, operating on successive axes. So if you want people to be able to do FFTs on vectors longer than 256 elements, a somewhat larger limit here would seem to be appropriate.
Just a suggestion...