Clojure 中的让与绑定 [英] Let vs. Binding in Clojure

问题描述

我知道它们是不同的，因为一种用于设置 *compile-path* 而另一种则没有.但是，我需要帮助解释为什么它们不同.

I understand that they're different since one works for setting *compile-path* and one doesn't. However, I need help with why they're different.

let 使用给定的绑定创建一个新的作用域，但是 binding...?

let creates a new scope with the given bindings, but binding...?

推荐答案

let 为某个值创建一个词法作用域的不可变别名.binding 为某些 Var 创建一个动态范围的绑定.

let creates a lexically scoped immutable alias for some value. binding creates a dynamically scoped binding for some Var.

动态绑定意味着 binding 表单中的代码以及该代码调用的任何代码(即使不在本地词法范围内)都将看到新的绑定.

Dynamic binding means that the code inside your binding form and any code which that code calls (even if not in the local lexical scope) will see the new binding.

给定:

user> (def ^:dynamic x 0)
#'user/x

binding 实际上为 Var 创建了一个动态绑定，但 let 只用本地别名隐藏 var:

binding actually creates a dynamic binding for a Var but let only shadows the var with a local alias:

user> (binding [x 1] (var-get #'x))
1
user> (let [x 1] (var-get #'x))
0

binding 可以使用限定名称(因为它对 Var 进行操作)而 let 不能:

binding can use qualified names (since it operates on Vars) and let can't:

user> (binding [user/x 1] (var-get #'x))
1
user> (let [user/x 1] (var-get #'x))
; Evaluation aborted.
;; Can't let qualified name: user/x

let 引入的绑定是不可变的.binding 引入的绑定是线程本地可变的:

let-introduced bindings are not mutable. binding-introduced bindings are thread-locally mutable:

user> (binding [x 1] (set! x 2) x)
2
user> (let [x 1] (set! x 2) x)
; Evaluation aborted.
;; Invalid assignment target

词法绑定与动态绑定:

user> (defn foo [] (println x))
#'user/foo
user> (binding [x 1] (foo))
1
nil
user> (let [x 1] (foo))
0
nil

另见 Vars、让.

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