It´s not so difficult as it looks: is is just a function that returns a function that returns a function. If you are used to object orientation, think of matrix as instansiation, m as an object, and the returned setter as a method. With the arc abbreviations, you could also write (m.settersym val 0 0) rather than having to modify =, but personally I would like to have some setter macro to make the syntax more natural (By the way: this is exactly how you would do a matrix implmentation in C++, only with templates in stead of macros)
^^ Again, somebody has to write the nastiness ^^. The main difficulty really is the fact that you have a polymorphic function that dispatches based on number of parameters as well as their contents.
Although I suppose you could actually use --warning-blatant-self-proclamation-- my p-m: macro:
(p-m:def matrix
(,(s (is s settersym)))
writerfunction
(i j)
(readerfunction i j)
x (err "argument error"))
