动态 var 的 Clojure 绑定未按预期工作 [英] Clojure binding of dynamic var not working as expected

问题描述

据我所知，在动态 var 上设置新绑定会影响该绑定中调用的所有函数，以及从这些函数调用的所有函数.

From what I understand, setting a new binding on a dynamic var affects all functions called within that binding, and all functions called from those functions.

为什么在下面的第一个示例中绑定似乎丢失了?

Why does the binding appear to be lost in the first example below?

(def ^:dynamic *out-dir* "/home/user")

(binding [*out-dir* "/home/dave"] (map #(str *out-dir* %) [1 2 3]))
; gives:    ("/home/user1" "/home/user2" "/home/user3")
; expected: ("/home/dave1" "/home/dave2" "/home/dave3")

(binding [*out-dir* "/home/dave"] (conj (map #(str *out-dir* %) [1 2 3]) *out-dir*))
; gives: ("/home/dave" "/home/dave1" "/home/dave2" "/home/dave3")

推荐答案

这是由惰性引起的 - map 返回一个惰性序列，该序列在绑定内部定义但在外部评估.您需要从内部强制评估:

This is caused by lazyness - map returns a lazy sequence which is defined inside the binding but is evaluated outside. You need to force the evaluation from inside:

(binding [*out-dir* "/home/dave"] 
  (doall (map #(str *out-dir* %) [1 2 3])))

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