在 Clojurescript 中使用 -?>？

Question

在我的 Clojure 共享源中，我有以下内容（我无耻地偷了）：

(defmacro hey-str [name]
  `(str "hey " ~name))

{:author "Laurent Petit (and others)"
  :doc "Functions/macros variants of the ones that can be found in clojure.core 
            (note to other contrib members: feel free to add to this lib)"}

(defmacro- defnilsafe [docstring non-safe-name nil-safe-name]
  `(defmacro ~nil-safe-name ~docstring
     {:arglists '([~'x ~'form] [~'x ~'form ~'& ~'forms])}
       ([x# form#]
         `(let [~'i# ~x#] (when-not (nil? ~'i#) (~'~non-safe-name ~'i# ~form#))))
     ([x# form# & more#]
         `(~'~nil-safe-name (~'~nil-safe-name ~x# ~form#) ~@more#))))

(defnilsafe 
  "Same as clojure.core/-> but returns nil as soon as the threaded value is nil itself (thus short-circuiting any pending computation).
   Examples :
   (-?> \"foo\" .toUpperCase (.substring 1)) returns \"OO\"
   (-?> nil .toUpperCase (.substring 1)) returns nil
   "
  -> -?>)

(defnilsafe 
  "Same as clojure.core/.. but returns nil as soon as the threaded value is nil itself (thus short-circuiting any pending computation).
   Examples :
   (.?. \"foo\" .toUpperCase (.substring 1)) returns \"OO\"
   (.?. nil .toUpperCase (.substring 1)) returns nil
   "
   .. .?.)

(defnilsafe
  "Same as clojure.core/->> but returns nil as soon as the threaded value is nil itself (thus short-circuiting any pending computation).
   Examples :
   (-?>> (range 5) (map inc)) returns (1 2 3 4 5)
   (-?>> [] seq (map inc)) returns nil
   "
  ->> -?>>)

在我的 Clojurescript 代码中，我有以下内容（I :require-macros as c）

(def a nil)
(def b [])
(def c [{:a 23}])

(js/alert (c/hey-str "Stephen")) ;; should display "hey Stephen"
(js/alert (c/-?> a first :a)) ;; should display nil
(js/alert (c/-?> b first :a)) ;; should display nil
(js/alert (c/-?> c first :a)) ;; should display 23

不幸的是，当我编译时，我得到：

WARNING: Use of undeclared Var webstack.client/-?> at line 56 cljs-src/webstack/client.cljs
WARNING: Use of undeclared Var webstack.client/-?> at line 57 cljs-src/webstack/client.cljs
WARNING: Use of undeclared Var webstack.client/-?> at line 58 cljs-src/webstack/client.cljs

当我在浏览器中打开 javascript 时，我收到“嘿斯蒂芬”警报对话框，但在“嘿斯蒂芬”警报。果然，看看生成的javascript代码，我的js/alert就变成了：

alert([cljs.core.str("hey "), cljs.core.str("Stephen")].join(""));
alert(webstack.client.__QMARK__GT_.call(null, webstack.client.__QMARK__GT_.call(null, webstack.client.a, cljs.core.first), "\ufdd0'a"));
alert(webstack.client.__QMARK__GT_.call(null, webstack.client.__QMARK__GT_.call(null, webstack.client.b, cljs.core.first), "\ufdd0'a"));
alert(webstack.client.__QMARK__GT_.call(null, webstack.client.__QMARK__GT_.call(null, webstack.client.c, cljs.core.first), "\ufdd0'a"))

很明显，我可以使用宏，但是 -?> （和相关）宏的编写方式导致它们无法编译。我需要做什么才能使用这些 -?> .?. 和 `-?>>' 宏？

Answer 1

-?> 对我使用 clojure 1.5.1 和 cljsbuild 0.3.0 有效，但前提是我使用 :use-macros 而不是 :require-macros。当我使用 :require-macros clojurescript 尝试将宏解析为本地命名空间中的 var 时，这是不正确的。我认为您在 clojurescript 中发现了一个错误，为什么不报告它？

(ns test.test
  ;(:use-macros [test.nilsafe :only [hey-str -?>]]) ;uncomment and everything works!
  (:require-macros [test.nilsafe :as tn]))

(def a nil)
(def b [])
(def c [{:a 23}])

(.log js/console (tn/hey-str "Stephen")) ;; should display "hey Stephen"
(.log js/console (tn/-?> a first :a)) ;; should display nil
(.log js/console (tn/-?> b first :a)) ;; should display nil
(.log js/console (tn/-?> c first :a))

让我确定这是一个错误的是，取消注释 :use-macros 将导致文件正确编译，即使我没有从变量中删除 tn/ 范围。

Answer 2

->marco 很简单。

(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
  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)
                       (with-meta `(~(first form) ~x ~@(next form)) (meta form))
                       (list form x))]
        (recur threaded (next forms)))
      x)))

写一个 nil 安全的应该很简单（下面的例子没有经过测试），但是你明白了。

(defmacro -?>
  [x & forms]
  (loop [x x, forms forms]
    (if (and forms (some? x)) ; check nil to exit eariler
      (let [form (first forms)
            threaded (if (seq? form)
                       (with-meta `(~(first form) ~x ~@(next form)) (meta form))
                       (list form x))]
        (recur threaded (next forms)))
      x)))

Answer 3

可能是创建函数而不是从宏中创建另一个宏可以帮助您解决此问题：

(defmacro defnilsafe [docstring non-safe-name nil-safe-name]
  `(defn ~nil-safe-name ~docstring
     ([x# form#]
         (let [i# x#] (when-not (nil? i#) (~non-safe-name i# form#))))
     ([x# form# & more#]
         (apply ~nil-safe-name (~nil-safe-name x# form#) more#))))