让我们与Clojure中绑定 [英] Let vs. Binding in Clojure

问题描述

据我所知，他们是不同的，因为一期工程设置 *编译路径* 和一个没有。不过，我需要他们为什么不同的帮助。

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.

让创建具有给定的绑定一个新的范围，但结合 ...

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

推荐答案

让创造一些价值词法范围不变的别名。 结合创建一个动态范围的一些瓦尔绑定。

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

动态绑定意味着你的结合表格里面的code和任何code这是code调用（即使不在本地词汇范围内）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.

假设：

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

结合实际上创建了一个动态的瓦尔，但让只屏蔽了该变种与当地的别名：

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

结合可以使用限定名称和让（因为它在瓦尔取值时） 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

让 -introduced绑定是不可变的。 结合 -introduced绑定是线程局部可变：

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

词汇和动态绑定：

Lexical vs. dynamic binding:

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

另请参阅瓦尔，让

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