Clojure中with-local-vars和with-bindings之间的区别 [英] Difference between with-local-vars and with-bindings in Clojure

问题描述

Clojure的文档 with-local-vars 和 with-bindings 不足以区分两者。

The documentation for Clojure with-local-vars and with-bindings doesn't suffice for me to distinguish the two. Any hints?

推荐答案

新的 var s由<$临时创建c $ c> with-local-vars 。现有的 var 暂时被 with-bindings 反弹。在两种情况下，绑定都是线程本地的。

New vars are temporarily created by with-local-vars. Existing vars are temporarily rebound by with-bindings. In both cases the bindings are thread-local.

请注意，据我所知， with-bindings 是告诉，主要用作通过使用 get-thread-bindings 返回的映射从另一个上下文传递绑定的助手。当不导入绑定时，类似的函数 binding 会更典型。

Note that with-bindings is, as far as I can tell, primarily useful as a helper to pass bindings from another context by using a map returned by get-thread-bindings. The similar function binding would be more typical when not importing bindings.

说明性示例：

(binding [*out* (new java.io.StringWriter)] 
  (print "world!") (str "hello, " *out*))
;=> "hello, world!"

(with-local-vars [*out* (new java.io.StringWriter)] 
  (print "world!") (str "hello," *out*))
;=> world!"hello,#<Var: --unnamed-->"

(with-local-vars [foo (new java.io.StringWriter)] 
  (.write @foo "world") (str "hello, " @foo))
;=> "hello, world"

(binding [foo (new java.io.StringWriter)] 
  (.write @foo "world") (str "hello, " @foo))
;=> CompilerException java.lang.RuntimeException: 
;     Unable to resolve var: foo in this context...

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