在不运行函数的情况下检查Clojure前提条件？ [英] Checking Clojure pre-conditions without running the function?

问题描述

我有一个函数可以执行某些（可能是冗长的）工作（defn workwork [x] ...）和其他一些函数来检查调用是否会成功时间（defn workwork-precondition-1 [x] ...）。

I have one function that does some (possibly lengthy) work (defn workwork [x] ...) and some other functions to check if the call will succeed ahead of time (defn workwork-precondition-1 [x] ...).

前提条件函数应为每次调用 work （例如，使用：pre ）进行评估。前提条件函数也应该在一个函数中收集（和编辑），并直接提供给客户端代码（例如，禁用按钮）。

The precondition functions should be evaluated every time workwork is called (e.g. using :pre). The precondition functions should also be collected (and:ed) in a single function and made available to client code directly (e.g. to disable a button).

惯用语避免代码重复的同时在Clojure中解决此问题的方法？

Which is the idiomatic way to solve this in Clojure while avoiding code duplication?

特别是，有什么方法可以在不运行函数主体的情况下评估函数的前提条件吗？

In particular, is there any way to evaluate the pre-conditions of a function without running the function body?

推荐答案

您只需将前提条件收集到一个函数中即可：

You can just collect your preconditions into a function:

(defn foo-pre [x]
  (even? x))

然后在：pre 样式的前提条件：

Then call the function in a :pre-style precondition:

(defn foo [x]
  {:pre [(foo-pre x)]}
  …)

使用 defn 引入的函数，您可以从Var：上的元数据中提取：pre 样式的前提条件>

For functions introduced using defn, you can extract the :pre-style preconditions from the metadata on the Var:

(-> #'foo meta :arglists first meta)
;= {:pre [(foo-pre x)]}

对于：arglists

这里有两个警告：

1. 自动生成的：arglists 条目元数据可能会被覆盖。覆盖：arglists 会导致上述有用的自动生成的元数据被丢弃。

1. The automatically-generated :arglists entry in the Var's metadata maybe be overridden. Overriding :arglists results in the above kind of useful automatically-generated metadata to be thrown out.

上面的（->＃'foo meta…） {：pre [（foo-pre x）]} 值c $ c>表达式包含 foo-pre 作为文字 symbol –您有责任弄清楚它在 foo 的定义点。 （这可能或不可能-例如， foo 可以在顶级<$中包含 defn c $ c> let 或 letfn 形式，其中 foo-pre 是本地函数。 ）

The {:pre [(foo-pre x)]} value returned by the above (-> #'foo meta …) expression contains foo-pre as a literal symbol – it'd be your responsibility to figure out which function it referred to at foo's point of definition. (This may or may not be possible – for example foo could be defn'd inside a top-level let or letfn form, with foo-pre a local function.)

最后，匿名函数可以使用：pre 和：post ，但是目前没有从函数本身中提取它们的机制。

And finally, anonymous functions may use :pre and :post, but there is currently no mechanism for extracting them from the function itself.

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