我最近认为我在clojure.spec中遇到了一个bug，因为它在我没有先将类似(fn [x] x)或#(identity %)的lambda放入->线程宏（将lambda用额外的圆括号括起来，如((fn [x] x))）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”，我曾从其他人那里听说这是一个关于线程宏的普遍误解。在我这个例子中，“错误”基于我的假设，即lambda是这个语言中的基本“事物”或值。但事实上，Clojure只是一种数据，主要是宏，每个宏都以不同方式处理其输入数据。
当一个lambda作为列表而不是基本“函数值”被->线程宏处理时，它类似于(fn :hmm [x] x)展开，这显然并不是预期结果。但是，它有一个特殊的方便情况，对于非列表的事物，如符号、关键字等，它假设它们可以作为具有一个参数的函数行为，并在线程之前将它们括在括号中。这个特殊情况之所以有价值，就在于它解释了用户的意图，避免了在不需要时强制用户添加括号。
我断言，基本的lambda形式(fn)和#()应该，并且可以类似于符号处理，通过解释用户的意图。这样做的好处是使线程宏更加简单、直观，消除了影响众多用户的错误异常，并使语言朝着将lambda形式视为核心“值”的方向发展。最后一点是哲学性的，但我认为对于使语言“仅仅工作”的方式很重要，即一个新手或非专家用户在完成事情时预期的。
现有将非列表形式括起来的特殊案例已经显示了提供用户意图解释的价值，但现在，在这个特殊案例与对于可能是语言中最基础“值”之一的函数的两种最常见形式的缺乏之间存在不一致。
你是被这个“错误”咬过的吗？
如果您想与现有的->宏进行对比，这里有一个叫做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)))
我已经为->和->>准备了一个补丁，该补丁能够处理这种使用场景，并且我认为它不会对当前的功能造成任何破坏，即在保持现有行为的同时添加新的功能。它通过了所有的核心Clojure测试，但是我还没有测试过它在我的个人代码之外的情况。显然，在将其视为对核心可能的变更之前，这是必须的。
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
