Question

(ns protocols-records-learning.core)

(defprotocol Hit-points
  "Able to be harmed by environment interaction."
  (hit? [creature hit-roll] "Checks to see if hit.")
  (damage [creature damage-roll] "Damages target by damage-roll, negated by per-implementation factors.")
  (heal [creature heal-roll] "Heals creature by specified amount."))

(defrecord Human [ac, health, max-health]
  Hit-points
  (hit? [creature hit-roll] (>= hit-roll ac))
  (damage [creature damage-roll] (if (pos? damage-roll) (Human. ac (- health damage-roll) max-health)))
  (heal [creature heal-roll] (if (pos? heal-roll)
                    (if (>= max-health (+ heal-roll health))
                      (Human. ac max-health max-health)
                      (Human. ac (+ heal-roll health) max-health)))))

(def ryan (atom (Human. 10 4 4)))

(defn hurt-ryan
  "Damage Ryan by two points."
  [ryan]
  (swap! ryan (damage 2)))

Leads to error:

Exception in thread “main” java.lang.IllegalArgumentException: No single method: damage of interface: protocols_records_learning.core.Hit_points found for function: damage of protocol: Hit-points (core.clj:34)

Can somebody explain this error, and what is causing it, and how to properly change the atom?

Answer 1

This should work.

(defn hurt-ryan
  "Damage Ryan by two points."
  [ryan]
  (swap! ryan damage 2))

Note, the removed pair of parens around damage. The error message is a clunky try to tell you that Clojure didn’t find a version of damage with the arity of 1. The parens around damage try to do exactly that: call damage with one argument (the 2).

Improvement of the error messages is an ongoing task, which has arrived at protocols, yet.