gistfile1.txt

boot.user=> (defn define-piece [{:keys [x-fn y-fn max] :or {x-fn identity, y-fn identity}}]
       #_=>   {:fn (fn [[x y]] [(x-fn x) (y-fn y)]) :max max})
#'boot.user/define-piece
boot.user=> ((:fn (define-piece {:x-fn inc})) [1 2])
[2 2]