The history of programmers trying to use programming languages for data files, configuration, or UI description files, is filled with horror stories. "WHY in the world did we choose Lua for our data files again?" was an interrogation I had not so far in the past, when I was working for a big game company. "HOW did we let this Grunt file grow more than 8000 js lines?" was a more recent one. Programming languages are NOT a good fit for data files. But there may be another way... Today, I want to talk about my favorite part of lisp languages: making your own languages when you need them. By that I don't mean making a big pile of utility functions or macros for the savvy lisper to drool upon, but actually taking the time to think about a file format, may it be completely static, and make it from scratch using s-expressions. Those parentheses may be good for something in the end...