I had two days off and one stale itch: I wanted to write a Lisp instead of read another tutorial about one. Not a serious one. Not one that competes with the half-dozen excellent small Lisps already on GitHub. A bonsai Lisp. A Sunday Lisp.
(defn fact [n]
(if (<= n 1) 1
(* n (fact (- n 1)))))By Sunday night I had a tokenizer, a recursive descent parser, an eval loop, and a tiny standard library. About two hundred lines of TypeScript, a handful of ugly tests, and a notebook of notes I will not show anyone.
Why this still feels good
I have been writing software professionally for ten years. Most days the work is shaped by other people’s decisions: a framework’s, a team’s, an org’s. The pleasure of a small Lisp is that for a weekend you decide everything. Whitespace? You decide. Tail calls? You decide. Whether nil is falsy? You decide.
That is not a serious case for production code. It is, however, a serious case for spending an occasional weekend writing something with no users. The shape of one’s taste needs exercise too.