> Chess is a lot trickier than it looks. It has so many rules: castling, en passant, pawn promotion, pinning, the discovered check, and the deadlock case of stalemate.

Nit: Pinning and the discovered check are not really rules, but rather names of tactics.

I can't wait to show this to my manager next time he asks why it's taking three weeks to build a simple CRUD app.

"Look, if this guys TLA+ logic struggles to model a 1,500-year-old game without crying over a French pawn-capture rule, you can't expect me to integrate Stripe billing without a few state invariant violations."

Shameless plug: a code walkthru modeling the rules of chess, ment as an exercise/teaching functional programming (in Clojure):

https://neuroning.com/boardgames-exercise/notebooks/walkthro...

The implementation makes it really easy to add new piece types or rules. For example, here's the full logic for rooks (sans castling):

  (defn expand-pmove-for-rook [pmove]
    (->> pmove
      (expand-pmove-dirs [↑ ↓ ← →])
      (pmoves-discard #(or (pmove-on-same-player-piece? %)
                           (pmove-changed-direction? %)))
      (map pmoves-finish-capturing-opponent-piece)
      (pmoves-finish-and-continue))))

While I think everything written in this post is correct, what really is starting bothering me is this over-focus/attention on data even when what you want to express is behavior, let me explain:

The post talks about "transition invariants" that should be somehow different from "state invariants" yet it describe them as:

> These are predicates over a <<state, next-state>> pair ...

i.e. it still is about state, but I find it much more useful to focus on behavior so instead of thinking about how state transition you focus on what the program is allowed to perform, regardless of the underlying data structure.

What I mean is that I'd like the code to tell me why a certain piece can't do such move instead of why it cannot transition it's position to another position and basically dumping its state in my head and there I have to execute the program myself.

Anyone know what language is being used in the blogpost?

If you like this, you're probably gonna like this: https://en.wikipedia.org/wiki/Chessboard_complex

That king promotion rule sounds like it made the game more fun.

Screenshots of code? In 2026?...

One king per color?

side question, which CS class(es) teach about invariants?