Clojure：解构时预报告失败值

Question

在此 SO post 之后，我想在我的函数中打印前提条件的值。但是在以下情况下它对我来说失败了（可能是解构）：

我有一个 dir? 辅助函数（可以跳过这个）：

(defn dir? [s]
  "returns true if the string passed is is an existing directory"
  (->> (clojure.java.io/file s)
       ((juxt #(.exists %) #(.isDirectory %)))
       (every? true?)))

它工作得很好，使用 is 宏，我得到了一些很好的错误消息，我可以在其中看到测试和传递的参数：

(is (dir? (io/file "resources/static"))) ;; => true

(is (dir? (io/file "resources/statice"))) ;; typo, error below

FAIL in clojure.lang.PersistentList$EmptyList@1 (boot.user4515592986834245937.clj:86) expected: (dir? (io/file "resources/statice")) actual: (not (dir? #object[java.io.File 0x6730a420 "resources/statice"]))

但是，当尝试在先决条件 :pre 中使用它时，我得到一个难看的错误：

(defn make-something
  [&{:keys [dir]
     :or {dir "default-dir"}}]
  {:pre [(is (dir? (clojure.java.io/file dir)))]}
  ;;... do something with these
 )

(make-something :dir "resources/statices") ;; doesn't exist

clojure.lang.Compiler$CompilerException: java.lang.AssertionError: Assert failed: (is (dir? (io/file dir))), compiling:(boot.user4515592986834245937.clj:80:12) java.lang.AssertionError: Assert failed: (is (dir? (io/file dir)))

我怎样才能像上面那样在我的函数中得到一个很好的错误消息？

以防万一，我使用的是 Clojure 1.7。

Answer 1

您需要检查您的代码（dir? 函数）。以下代码段对我有用：

(require '[clojure.java.io :as io])

(defn dir? [s]
  (let [dir (io/file s)]
    (and (.exists dir)
         (.isDirectory dir))))

(defn make-something
  [& {:keys [dir] :or {dir "default-dir"}}]
  {:pre [(is (dir? dir))]}
  (println dir))

(make-something :dir "/tmp")
out => /tmp
ret => nil

(make-something :dir "/xxxx")
FAIL in clojure.lang.PersistentList$EmptyList@1 (form-init3332271312167175068.clj:1)
expected: (dir? dir)
actual: (not (dir? "/xxxx"))

AssertionError Assert failed: (is (dir? dir))  user/make-sth (form-init3332271312167175068.clj:1)

Clojure：解构时预报告失败值

Clojure :pre report failing value when destructuring

error-handling

clojure

preconditions