我最近认为我在clojure.spec中遇到了一个bug，因为它在我在没有先在其外部添加一个额外的括号如的情况下将lambda表达式如(fn [x] x)或#(identity %)放入->线程宏时抛出了异常。这是我多年来在Clojure中犯的最常见的“错误”，我听说其他人也有相同的误解。在我这个案例中，“错误”基于我的假设，即lambda表达式是语言中的一个基本的“东西”或值。但实际上，Clojure只是数据，大部分是宏，每个宏都以不同的方式处理其输入数据。
->线程宏将lambda表达式视为一个列表，而不是基础的“函数值”。当执行(-> :hmm (fn [x] x))时，它会被展开为(fn :hmm [x] x)，这显然不是预期的。然而，它对非列表的东西（如符号、关键词等）有一个特殊的便捷情况，它假定这些可以以一个参数的方式行为，并在连接之前将这些值包装在括号中。这个特殊案例增加了价值，因为它解释了用户的意图，并避免了在他们不需要时强制添加括号。
我坚持认为基本lambda形式(fn)和#()应该，并且可以像符号一样相似地处理，通过解释用户的意图。这样做的好处是会使线程宏更简单、更直观，会消除影响众多用户的非必要异常，并将语言推向将lambda形式视为核心“值”的方向。最后一个观点是哲学性的，但在我看来对于使语言“正好工作”至关重要，这是新用户或非专家用户在完成任务时期望的方式。
现有将非列表形式 包装起来的特殊案例已经证明了提供一些用户意图的解释的价值，但现在存在一个不协调，即这个特殊案例和该特殊案例对于语言中最基础且可能是最重要的“值”之一的函数本身的缺失。
您有没有被这个“错误”咬过？
如果您想与现有的 -> 宏并排尝试，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，该补丁处理此使用案例，并且我没有想到它会破坏任何功能。它保留现有的行为，同时增加了新功能。它通过了所有的核心 Clojure 测试，但我还没有在我的外部代码中对其进行测试。在它被认为可能是核心更改之前，这显然是必要的。
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
