For instance, you can't put the "break" of a "for" loop in a helper function called out of the for loop. It has to be enclosed in the for loop.
We can think of the loop as a macro; Lisp would implement it as such.
When macros transform certain expressions enclosed in the macro call, all the syntax has to be right there, enclosed in the call.
In other words macros are "local syntactic transformations". Why I'm putting that in quotes is that this is the exact phrase used in the paper "Macros that Reach Out and Touch Somewhere" by George Kiczales et al.
Kiczales describes a macro system which peforms global program transformations, allowing macros to act on code that is not enclosed in them; i.e. bring about nonlocal transformations.
"In this paper, we present a new kind of macro, called a data path macro, in which transformations can take place at any point along the dataflow path that includes the macro invocation."
This is pretty exotic. I think nbody has done anything like it, and they never released their code. They left unsolved problems documented in section 5, Future Work.
