Clojure defmacro 丢失元数据 [英] Clojure defmacro loses metadata

问题描述

我正在尝试创建一个小 Clojure 宏，它 def 是一个带有类型提示的字符串:

I am trying to create a little Clojure macro that defs a String with a type hint:

(defmacro def-string [name value]
  `(def ^String ~name ~value))

(def-string db-host-option "db-host")

当我macroexpand它时，类型提示丢失了:

When I macroexpand it, the type hint is lost:

(macroexpand '(def-string db-host-option "db-host"))
;=> (def db-host-option "db-host")

不要介意暗示这一点的类型的智慧.

Never mind the wisdom of type hinting this.

为什么宏会丢失元数据?如何编写此宏或任何包含元数据的宏?

Why is the macro losing the metadata? How do I write this macro, or any that includes metadata?

推荐答案

^ 是一个阅读器宏.defmacro 永远不会看到它.提示放在列表 (unquote name) 中.例如将 (meta ^String 'x) 与 (meta ' ^String x) 进行比较以查看效果.

^ is a reader macro. defmacro never gets to see it. The hint is put on the list (unquote name). Compare for example (meta ^String 'x) with (meta ' ^String x) to see the effect.

您需要将提示放在符号上.

You need to put the hint on the symbol.

(defmacro def-string
  [name value]
  `(def ~(vary-meta name assoc :tag `String) ~value))

以及用法:

user=> (def-string foo "bar")
#'user/foo
user=> (meta #'foo)
{:ns #<Namespace user>, :name foo, :file "NO_SOURCE_PATH", :line 5, :tag java.lang.String}

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