问题

我试图使用Clojure定义一个宏,其工作原理与java中的类似 它应该能够有一个绑定表单,例如[变量值],可以绑定到closable的实例。

Java中的示例

try (Socket s = new Socket()) {
s.accept();
} catch(Exception e) {}

在这段代码中,套接字“s”会自动关闭,就像有一个明确的finally子句一样

finally {
if (s != null) s.close();
}

尝试的解决方案

;I am defining my macro. It can either take one argument(expression) or can take two arguments(expression and a vector with two elements(a variable and value)
;expression is a form expression and should be able to be evaluated
(defmacro safe 
    ; if a vector and also an expression is passed into the macro
    [s NewS expression]
    ;I am defining my try block, and unquoting it with a ' so that the try block is not executed within the macro
    `(try   
        ;I am letting the variable(NewS) be equal to the value(s)
        (let [s NewS]
        ;I am trying the expression to see if it is valid and printing the exception if there is one
        (try 
            ~expression (catch Exception e (str "caught exception: " (.getMessage e))))
        )
        ;I am checking if my value is an instance of a java closeable
        (instance? java.util.Closeable s)
        ;I am catching the exception from the let statement if there is one
        (catch Exception e (str "caught exception: " (.getMessage e)))
    )
    ;if only an expression is passed into the macro
    [expression]
    `(try
        ~expression (catch Exception e (str "caught exception: " (.getMessage e)))
    )
)

输入和输出示例

user> (def v (safe (/ 1 0)))
user> v
#<ArithmeticException java.lang.ArithmeticException: Divide by zero>
user> (def v (safe (/ 10 2)))
user> v
5
user> (def v (safe [s (FileReader. (File. "file.txt"))] (.read s)))
user> v
105 ; first byte of file file.txt
user> (def v (safe [s (FileReader. (File. "missing-file"))] (. s read)))
user> v
#<FileNotFoundException java.io.FileNotFoundException:
missing-file (No such file or directory)>

错误消息

当我将这些示例输入输入到我的主函数中时,我得到了一个我不理解的编译器异常。

CompilerException clojure.lang.ArityException: Wrong number of args (1) passed to: core/safe, compiling:(/private/var/folders/6f/q7lhngtn45q_xpzd_24gjp2h0000gn/T/form-init2350735096437822603.clj:1:8) 

我不知道我可以在这个宏中调整什么,但我不能让它不返回错误。

使现代化

这个解决方案几乎可以工作

(defmacro safe
    ([[s NewS] expression]
    `(try  
        (let [~s ~NewS] ~expression) (catch Exception e# (str "caught exception: " (.getMessage e#)))
    ))
    ([expression]
    `(try
        ~expression (catch Exception e# (str "caught exception: " (.getMessage e#)))
    ))
)

但以下测试失败

(defn-主 “我还没有做太多……还没有。” [&参数] (导入java.io.FileReader java.io.File) (def v(safe[s(FileReader.(File.txt”))](.read s))) (println v) )

user$ lein run
caught exception: file.txt (No such file or directory)
user$ cat file.txt 
teast