Amazing elisp project of the day: an interpreter for Z code! https://github.com/jimm/elisp/blob/master/malyon.el (Z is a common language for text adventure games)
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An ad hoc, informally-specified, bug-ridden, slow implementation of half of a text adventure game -- or as I like to call it, a compiler.
Blogging about my programming language project: choosing the basic syntax!
https://www.garden-lang.org/blog/syntax.html
It's really satisfying to use a profiler for the first time on a project. I always find a big performance win with only a small code change.
It's never the code that I expected to be slow, however!