在 Clojure 中构建匿名函数映射 [英] Constructing a map on anonymous function in Clojure

问题描述

我只是在学习语言，我有一个简单的问题.为什么这样做(构造 {:key "value"}):

I am just learning the language and I've got a simple question. Why does this work (constructs {:key "value"}):

(#(assoc {} :key %) "value")

但这不会:

(#({:key %}) "value")
ArityException Wrong number of args (0) passed to: PersistentArrayMap  clojure.lang.AFn.throwArity (AFn.java:429)

在 Python 上，后一种语法是完全有效的:

On Python the latter syntax is perfectly valid:

> (lambda v: {'key': v})('value')
{'key': 'value'}

感谢您提供出色的答案，很明显我需要停止将 # 视为 Python 中的 lambda 等价物.

edit: thanks for great answers, it is apparent I need to stop thinking # as equivalent to lambda in Python.

推荐答案

#(f %) 被读者展开为 (fn [%] (f %)>. 同样地，#({:key %}) 被扩展为 (fn [%] ({:key %}).python 等效的将是 lambda v: {'key': v}()，与Clojure版本有完全相同的问题.

#(f %) is expanded by the reader into (fn [%] (f %). Likewise, #({:key %}) is expanded into (fn [%] ({:key %}). The python equivalent of this would be lambda v: {'key': v}(), which has the exact same problem as the Clojure version.

您要查找的内容与 (fn [v] {:key v}) 等价.如果你真的想使用 #(...) 表示法，你可以使用 #(do {:key %}).

What you are looking for is something equivalent to (fn [v] {:key v}). If you really want to use #(...) notation, you could use #(do {:key %}).

顺便说一句，我个人从不使用 #(...).我认为它更难理解(例如这个证据)，并且只比等效的 fn 形式稍微紧凑一些.然后还有 #(...) 表单不能嵌套的限制.

Incidentally, I personally never use #(...). I think it's more difficult to grok (as examples such as this evidence), and is only very slightly more compact than an equivalent fn form. Then there's also the limitation that #(...) forms can not be nested.

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