Lisp Macros - Pond’ring Aloud

Mar 18, 2025

In Lisp, there are only 3 kinds[+] of things

Macros look like functions. A table in the compiler differentiates between macros and functions.

With these imaginary definitions...

defmacro m2 (x) = m1 (x)

defmacro m1 (x) = f(x)

deffunction f (x) = ...

evaluate the following code:

f (41)

m2 (42)

f (43)

Macro expand ---->

f (41)

m1 (42)

f (43)

Re-expand, from where we left off ---- >

f (41)

f (42)

f (43)

The Lisp evaluator repeatedly expands macros until only functions (and atoms) remain. Then, it evaluates the program.

[+] This is not quite true, but, is a good enough approximation for this article.

Programming Simplicity