我最近以为我在clojure.spec中遇到了一个bug，因为它在我将lambda如(fn [x] x)或#(identity %)放入->线程宏时抛出了异常，没有先将其括起来，如((fn [x] x))。这是我多年来在Clojure中最常见的重复“错误”，我听到其他人也表示这是一种关于线程宏的常见误解。在我这个例子中，“错误”是基于我的假设认为lambda是语言中的一个基本“东西”或值。但实际上，Clojure只是数据，主要是宏，每个宏以不同的方式处理输入数据。
->线程宏将lambda视为列表，而不是视为基本“函数值”。当一个执行(-> :hmm (fn [x] x))时，它像这样扩展：(fn :hmm [x] x)，这显然不是预期的结果。但是，它确实有一个特别的便利情况，用于非列表事物，如符号、关键字等，它假设它可以作为具有一个参数的行为，并且在这种情况中将其括起来再进行线程处理。这个特殊情况增加了价值，因为它理解了用户的意图，避免了在不需要的情况下强制用户添加括号。
我断言，基本的lambda形式(fn)和#()应该而且可以像符号一样处理，通过解释用户的意图。这样做的好处是使线程宏更简单、更直观，消除了一个不必要的异常，该异常影响到许多用户，并将语言推进到以处理lambda形式的“基本值”。最后一点虽然是一个哲学问题，但在我看来对于使语言“按预期工作”非常重要，这是新用户或非专家用户在完成任务时预期的。
现有将非列表形式括起来的特殊情况已经证明了提供一些对用户意图的解释的价值，但现在，这个特殊情况与两个最常见形式之一的一个潜在最基本“值”——函数本身——的缺失之间存在不一致。
你是否被这个“错误”咬了？
如果你想与现有的->宏并排尝试，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，它处理了这个用例，并且我没有想到会有破坏性的影响。它保持现有的行为，同时添加了新的功能。它通过了所有的核心Clojure测试，但我还没有在自己的额外代码上测试它。显然，在考虑将其作为核心可能的变化之前，这是必要的。
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)
已记录：https://clojure.atlassian.net/browse/CLJ-2553
