在Clojurescript中使用-?>？

Question

在我的Clojure共享源代码中，我有以下内容(我厚颜无耻地偷走了)：

(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，我得到了“嘿Stephen”警告对话框，但在按下“嘿Stephen”警告上的"ok“之后，立即出现了普遍存在的"Uncaught : Cannot call method 'call‘of undefined”错误。果然，看看生成的javascript代码，我的js/alert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"))

显然，我可以使用宏，但是-?> (和相关的)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

在我使用clojure 1.5.1和cljsbuild 0.3.0时，-?>可以工作，但前提是我使用: use -macros而不是:require macros。当我使用:require- macro clojurescript尝试将宏解析为本地命名空间中的var时，这是不正确的。我想你在clojurescript中发现了一个bug，为什么不上报呢？

(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))

使我确信这是一个bug的事情是，即使我没有从变量中删除tn/ scope，取消对:use-宏的注释也会导致文件正确编译。

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)))

编写一个nil安全的代码应该很简单(下面的例子没有经过测试)，但是你已经理解了它的概念。

(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#))))