Common Lisp `case` on `read` with custom package

Question

Paul G 的 OnLisp 中的以下代码如果我运行它在没有自定义包的 REPL 中工作正常。当我定义一个包并将其与 (in-package :mypackage) 一起使用时，它不起作用——它总是采用 case 语句中的 t 情况：

(defun run-node (name)
 (let ((n (gethash name *nodes*)))
  (cond ((node-yes n)
         (format t "~A~%>> " (node-contents n))
         (case (read)
           (yes (run-node (node-yes n))) ; never hits this in package
           (t (run-node (node-no n))))) 
        (t (node-contents n)))))

Answer 1

首先请注意，您正在做的事情非常危险：对 read 的无约束调用可能导致程序执行 任何代码 。如果您必须为用户输入调用read，请以安全的方式调用它：

(with-standard-io-syntax
  (let ((*read-eval* nil)
        (*package* ...))
    (read)))

其次，缩进您的代码以便人们可以阅读它：

(defun run-node (name)
  (let ((n (gethash name *nodes*)))
    (cond ((node-yes n)
           (format t "~A~%>> " (node-contents n))
           (case (read)
             (yes (run-node (node-yes n))) ;never hits this in package
             (t (run-node (node-no n))))) 
          (t (node-contents n)))))

现在我们可以注释您的代码（添加一些最小的 read-defanging）来告诉您错误是什么：

(defun run-node (name)
  (with-standard-io-syntax
    (let ((n (gethash name *nodes*))
          (*read-eval* nil))
      (cond ((node-yes n)
             (format t "~A~%>> " (node-contents n))
             (let ((got (read)))
               (format *debug-io*
                       "~&*package* ~16T~A~%got package~16T~A~%our package~16T~A~%"
                       (package-name *package*)
                       (typecase got
                         (symbol (package-name (symbol-package got)))
                         (t "(not a symbol"))
                       (package-name (symbol-package 'yes)))
               (case got
                 (yes (run-node (node-yes n))) ;never hits this in package
                 (t (run-node (node-no n))))))
            (t (node-contents n))))))

你会发现你读到的符号包和你比较的符号包不一样，所以符号是不同的。

Common Lisp `case` on `read` with custom package

Common Lisp `case` on `read` with custom package

lisp

user-input

sbcl

common-lisp

package