AbstractMethodError when using def-map-type without defining meta

[cespare]

Potemkin: 0.3.7 Clojure: 1.6.0 Java: 1.7

Having trouble with even the simple example from the README. This is in a fresh repl:

user=> (require '[potemkin.collections :refer [def-map-type]])
nil
user=> 

user=> (def-map-type LazyMap [m]   
  #_=>   (get [_ k default-value]
  #_=>     (if (contains? m k)
  #_=>       (let [v (get m k)]
  #_=>         (if (instance? clojure.lang.Delay v)
  #_=>           @v
  #_=>           v))
  #_=>       default-value))
  #_=>   (assoc [_ k v]
  #_=>     (LazyMap. (assoc m k v)))
  #_=>   (dissoc [_ k]
  #_=>      (LazyMap. (dissoc m k)))
  #_=>   (keys [_]
  #_=>     (keys m)))
user.LazyMap
user=> (LazyMap. {})

AbstractMethodError user.LazyMap.meta_STAR_()Ljava/lang/Object;  user.LazyMap (form-init3281556375762400900.clj:78)
user=> (->LazyMap {})

AbstractMethodError user.LazyMap.meta_STAR_()Ljava/lang/Object;  user.LazyMap (form-init3281556375762400900.clj:78)

Defining a meta method fixes it, but the README implies that it shouldn't be required:

Despite this, there are only four functions which really matter: get, assoc, dissoc, and keys. def-map-type is a variant of deftype which, if those four functions are implemented, will look and act like a Clojure map.

(def-map-type LazyMap [m]   
  (get [_ k default-value]
    (if (contains? m k)
      (let [v (get m k)]
        (if (instance? clojure.lang.Delay v)
          @v
          v))
      default-value))
  (assoc [_ k v]
    (LazyMap. (assoc m k v)))
  (dissoc [_ k]
     (LazyMap. (dissoc m k)))
  (keys [_]
    (keys m)
  (meta [_] nil)) ; this makes the readme example work

[ztellman]

This is because the REPL is calling meta, but you're right that this shouldn't be happening.

[cespare]

Thanks!