Steve Losh's Blog

Using the CLOS Dependent Maintenance Protocol

Part 3 in a series of short posts about fun Common Lisp Macros.

Somewhere between tiny shell scripts and full projects.

I’ve gotten a bunch of emails asking for advice on how to learn Common Lisp in the present day. I decided to write down all the advice I’ve been giving through email and social media posts in the hopes that someone might find it useful.
One disclaimer up front: this is a road to Common Lisp, not the road to Common Lisp. It’s what I followed (without some of the dead ends) and has a lot of my personal opinions baked in, but it is by no means the only way to learn the language.

I haven’t been writing much lately because I’ve been in the process of switching my life over to Linux from OS X. I finally managed to get my blog infrastructure running again, so let’s take a look at some deceptively-tricky Common Lisp macros: when-let and if-let (and their let* counterparts).
Introduction I first heard of the when-let and if-let macros when I learned Clojure a few years ago. They’re used like let to bind values to variables, but if a value is nil then when-let will return nil without running the body, and if-let will execute its alternative branch.

I haven’t written anything in a while. But after seeing the metaprogramming video on Computerphile the other day I felt the urge to write about Lisp again, so I decided to do a small series of posts on fun/useful little Common Lisp macros I’ve made over the past couple of years.
The Macro The first macro we’ll look at is gathering:
(defmacro gathering (&body body) "Run `body` to gather some things and return a fresh list of them.

In 2014 I spent two weeks in Scotland for my birthday. I rented a car and traveled around, photographing as much as I could.

This project started as a joke.
I posted a few photographs of plants on Instagram alongside my other photos. Purely by accident I posted one plant every three images, and because Instagram shows thumbnails in three columns it ended up forming a nice line of sepia-toned thumbnails on my profile.
I decided to keep up the rhythm for a while because it looked nice, and two years later I’m still making plant photos.

Plants Iceland Scotland

In 2015 I quit my job and moved to Iceland for graduate school. Two years, one Master’s degree, and ten thousand photographs later I moved back to the states.