Question

在clojure中设置默认值的惯用方法是使用merge：

;; `merge` can be used to support the setting of default values
(merge {:foo "foo-default" :bar "bar-default"} 
       {:foo "custom-value"})
;;=> {:foo "custom-value" :bar "bar-default"}

但实际上，默认值通常不是简单常量，而是函数调用。显然，如果它不会被使用，我想避免调用该函数。

到目前为止，我正在做类似的事情：

(defn ensure-uuid [msg]
  (if (:uuid msg)
    msg
    (assoc msg :uuid (random-uuid))))

并应用我的ensure-*函数，例如(-> msg ensure-uuid ensure-xyz)。

这样做的更惯用的方法是什么？我想的是：

(merge-macro {:foo {:bar (expensive-func)} :xyz (other-fn)} my-map)

(associf my-map
  [:foo :bar] (expensive-func)
  :xyz (other-fn))

Answer 1

您可以将delay与force结合使用。

然后您可以合并默认值，例如

(merge {:foo "foo-default" :bar "bar-default" :uuid (delay (random-uuid))}
       {:foo "custom-value" :uuid "abc"})

并使用

访问值

(force (:foo ...))

或

(force (:uuid ...))

只有当你真正需要这个值时（而且只是第一次）才会调用

random-uuid。

您可以将force函数中的get-value电话包裹起来，或类似的内容。

Answer 2

我刚刚调整了condp宏，并写了以下内容：

(defmacro assoc-if-nil
  "Takes a map as the first argument and a succession of key value pairs that
  are used to set the key to value if the key of the map is nil. The value part
  is only evaluated if the key is nil (thus different semantics to (merge)).
  Example:
  (assoc-if-nil {:a {:b :set}}
    [:a :b] :non-def
    [:a :c] :non-def
    :d :non-def)
  ;; =>{:a {:b :set, :c :non-def}, :d :non-def}"
  [m & clauses]
  (assert (even? (count clauses)))
  (let [g (gensym)
        get-fn   (fn[kork] (if (vector? kork) `get-in   `get))
        assoc-fn (fn[kork] (if (vector? kork) `assoc-in `assoc))
        pstep (fn [[kork v]] `(if-not (~(get-fn kork) ~g ~kork)
                                (~(assoc-fn kork) ~g ~kork ~v)
                                ~g))]
    `(let [~g ~m ;; avoid double evaluation
           ~@(interleave (repeat g) (map pstep (partition 2 clauses)))]
       ~g)))

扩展为：

(macroexpand-1 '
(assoc-if-nil m
              [:a :b] :nested
              :d :just-key))

(clojure.core/let
 [G__15391     m
  G__15391
  (clojure.core/if-not
   (clojure.core/get-in G__15391 [:a :b])
   (clojure.core/assoc-in G__15391 [:a :b] :nested)
   G__15391)
  G__15391
  (clojure.core/if-not
   (clojure.core/get G__15391 :d)
   (clojure.core/assoc G__15391 :d :just-key)
   G__15391)]
 G__15391)

合并以设置默认值，但可能是昂贵的函数

2 个答案: