我最近认为我在 clojure.spec 中遇到了一个bug，因为它在我的lambda表达式（例如(fn [x] x) 或 #(identity %)）未先用额外的括号包裹（如 ((fn [x] x))）而放入-> 线程宏中时抛出了异常。这是我多年来在Clojure中犯的最常见的重复“错误”，我也从其他人那里听到这关于线程宏的常见误解。在我这个例子中，这个“错误”是基于我的假设认为lambda是语言中的一个基本“事物”或值。但实际上，Clojure只是数据，大部分是宏，每个宏以不同的方式处理其输入数据。
-> 线程宏将lambda视为一个列表，而不是一个基本的“函数值”。当一个执行(-> :hmm (fn [x] x))时，它会像这样展开：(fn :hmm [x] x)这显然不是预期的结果。然而，它有一个特殊方便的情况，用于非列表事物，如符号、关键字等，它假设这是可以以一个参数为行为的事物，在编写前将其包裹在括号中。这个特殊案例增加了价值，因为它解释了用户的意图，并避免了强制用户在不必要时添加括号。
我坚持认为基本lambda形式 (fn) 和 #() 应该并且可以像符号一样被类似处理，通过解释用户的意图。这样做带来的价值会使线程宏更简单、更直观，会消除影响大量用户的不必要的异常，并使该语言更接近将lambda形式视为核心“值”。最后一点是一个哲学问题，但我认为这对使语言“顺其自然”很重要，就像新用户或非专家用户期待的那样，当他们完成工作时。
现有的针对非列表形式的包装特殊案例已经证明了提供一些对用户意图的解释的价值，但现在，这个特殊案例与其他两种最常见的表单（可能是语言中最基础的“价值观”之一的功能本身）之间存在不一致。
您是否曾被这种“错误”困扰过？
如果您想尝试将它与现有的 -> 宏并排使用，这里有一个名为 t-> 的版本
(defmacro t->
"Threads the expr through the forms. Inserts x as the
second item in the first form, making a list of it if it is a lambda or not a
list already. If there are more forms, inserts the first form as the
second item in second form, etc."
{:added "1.0"}
[x & forms]
(loop [x x, forms forms]
(if forms
(let [form (first forms)
threaded (if (and (seq? form) (not (#{'fn 'fn*} (first form))))
(with-meta `(~(first form) ~x ~@(next form)) (meta form))
(list form x))]
(recur threaded (next forms)))
x)))
我为 -> 和 ->> 准备了一个补丁，它处理了这个用例，并且我相信它没有破坏性的影响，保持了现有行为，同时增加了新的功能。它通过了所有核心Clojure测试，但我还没有在我的代码之外测试它。显然，在它被认为是核心可能的变化之前，这将是必需的。
From 686831062a574486413022af31e8c7a07b78cd24 Mon Sep 17 00:00:00 2001
From: Thomas Spellman <[email protected]>
Date: Mon, 13 Jan 2020 20:39:45 -0800
Subject: [PATCH] thread macros
---
src/clj/clojure/core.clj | 12 ++++++------
test/clojure/test_clojure/macros.clj | 12 ++++++++++++
2 files changed, 18 insertions(+), 6 deletions(-)
diff --git a/src/clj/clojure/core.clj b/src/clj/clojure/core.clj
index 8e98e072..fe43289b 100644
--- a/src/clj/clojure/core.clj
+++ b/src/clj/clojure/core.clj
@@ -1670,42 +1670,42 @@
(. (. System (getProperties)) (get \"os.name\"))
but is easier to write, read, and understand."
{:added "1.0"}
([x form] `(. ~x ~form))
([x form & more] `(.. (. ~x ~form) ~@more)))
(defmacro ->
- "Threads the expr through the forms. Inserts x as the
- second item in the first form, making a list of it if it is not a
+ "Threads the expr through the forms. Inserts x as the second item
+ in the first form, making a list of it if it is a lambda or not a
list already. If there are more forms, inserts the first form as the
second item in second form, etc."
{:added "1.0"}
[x & forms]
(loop [x x, forms forms]
(if forms
(let [form (first forms)
- threaded (if (seq? form)
+ threaded (if (and (seq? form) (not (#{'fn 'fn*} (first form))))
(with-meta `(~(first form) ~x ~@(next form)) (meta form))
(list form x))]
(recur threaded (next forms)))
x)))
(defmacro ->>
- "Threads the expr through the forms. Inserts x as the
- last item in the first form, making a list of it if it is not a
+ "Threads the expr through the forms. Inserts x as the last item
+ in the first form, making a list of it if it is a lambda or not a
list already. If there are more forms, inserts the first form as the
last item in second form, etc."
{:added "1.1"}
[x & forms]
(loop [x x, forms forms]
(if forms
(let [form (first forms)
- threaded (if (seq? form)
+ threaded (if (and (seq? form) (not (#{'fn 'fn*} (first form))))
(with-meta `(~(first form) ~@(next form) ~x) (meta form))
(list form x))]
(recur threaded (next forms)))
x)))
(def map)
(defn ^:private check-valid-options
diff --git a/test/clojure/test_clojure/macros.clj b/test/clojure/test_clojure/macros.clj
index ce17bb38..9fb1fa9e 100644
--- a/test/clojure/test_clojure/macros.clj
+++ b/test/clojure/test_clojure/macros.clj
@@ -106,8 +106,20 @@
(is (nil? (loop []
(as-> 0 x
(when-not (zero? x)
(recur))))))
(is (nil? (loop [x nil] (some-> x recur))))
(is (nil? (loop [x nil] (some->> x recur))))
(is (= 0 (loop [x 0] (cond-> x false recur))))
(is (= 0 (loop [x 0] (cond->> x false recur)))))
+
+(deftest ->lambda-test
+ (is (= 'a (-> 'a ((fn [x] x)))))
+ (is (= 'a (-> 'a (fn [x] x))))
+ (is (= 'a (-> 'a #(identity %))))
+ (is (= 'a (-> 'a (#(identity %))))))
+
+(deftest ->>lambda-test
+ (is (= 'a (->> 'a ((fn [x] x)))))
+ (is (= 'a (->> 'a (fn [x] x))))
+ (is (= 'a (->> 'a #(identity %))))
+ (is (= 'a (->> 'a (#(identity %))))))
--
2.21.0 (Apple Git-122.2)
记录：https://clojure.atlassian.net/browse/CLJ-2553
