Comment by ferd

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))))