# "until" loop - My first elisp macro

Published on Dec 19, 2021, by Junji Zhi

Recently I was reading On Lisp by Paul Graham and got interested in using macros in Emacs Lisp. I decided to play with it.

Here’s my first macro: a until loop:

(defmacro until (test &rest body)
(while (not ,test)
,@body))


Think of until as a negative while. The loop runs until the test becomes true.

To use it:

(setq jz/counter 1)
(setq jz/res '())
(until (>= jz/counter 5)
(setq jz/counter (+ 1 jz/counter))
(setq jz/res (cons jz/counter jz/res)))

;; jz/res => (5 4 3 2)


One note about defining macros: If we want to splice the rest of the lisp statement, we need two things:

• Add a &rest in the macro argument List. It means that there are arbitrary number of statements there, and we wrap them all into body.
• use comma-at, or ,@ to splice the body. It is similar to the spread operator ...` in Javascript ES6 that can unwrap an object into another.

I’m not sure whether this is the most efficient way to define macros. But it definitely kicks off an interesting adventure!

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