lisp - Mysterious Racket 错误 : define: unbound identifier; also, 没有#%app 语法转换器绑定(bind)在:define

Question

这个程序产生一个错误:

define: unbound identifier;
 also, no #%app syntax transformer is bound in: define

当粘贴到 REPL 中时(准确地说，最后一行:(displayln (eval-clause clause state)))，它起作用了。在定义窗口中运行时，它会失败。我不知道为什么。

#lang racket
(define *state* '((a false) (b true) (c true) (d false)))
(define *clause* '(a (not b) c))

(define (eval-clause clause state)
  (for ([x state])
    (eval `(define ,(first x) ,(second x))))
  (eval (cons 'or (map eval clause))))

(displayln (eval-clause *clause* *state*))

这也是:

(define (eval-clause clause state)
  (eval `(let ,state ,(cons 'or clause))))

产生

let: unbound identifier;
 also, no #%app syntax transformer is bound in: let

这是我翻译以下 Common Lisp 程序的尝试:Common Lisp wins here?

; (C) 2013 KIM Taegyoon
; 3-SAT problem
; https://groups.google.com/forum/#!topic/lisp-korea/sVajS0LEfoA
(defvar *state* '((a nil) (b t) (c t) (d nil)))
(defvar *clause* '(a (not b) c))

(defun eval-clause (clause state)
  (dolist (x state)
    (set (car x) (nth 1 x)))
  (some #'identity (mapcar #'eval clause)))

(print (eval-clause *clause* *state*))

并且在 Paren :

(set *state* (quote ((a false) (b false) (c true) (d false))))
(set *clause* (quote (a (! b) c)))
(defn eval-clause (clause state)
  (for i 0 (dec (length state)) 1
    (set x (nth i state))
    (eval (list set (nth 0 x) (nth 1 x))))  
  (eval (cons || clause)))
(eval-clause *clause* *state*)

Answer 1

eval 在 Racket 中很棘手。根据 Racket 指南 15.1.2，您需要按如下方式挂接到当前命名空间

(define-namespace-anchor anc)
(define ns (namespace-anchor->namespace anc))

然后在每次调用 eval 时添加 ns:

(define (eval-clause clause state)
  (for ([x state])
    (eval `(define ,(first x) ,(second x)) ns))
  (eval (cons 'or (map (curryr eval ns) clause)) ns))

请注意，正如上面引用的文档中所述，这在 REPL 中不是必需的。

但是，为您的定义创建一个特定的命名空间可能是一个更好的主意，这样它们就不会与您自己的模块定义混淆:

(define my-eval
  (let ((ns (make-base-namespace)))
    (lambda (expr) (eval expr ns))))

(define *state* '((a #f) (b #t) (c #t) (d #f)))
(define *clause* '(a (not b) c))

(define (eval-clause clause state)
  (for ([x state])
    (my-eval `(define ,(first x) ,(second x))))
  (my-eval (cons 'or (map my-eval clause))))

(displayln (eval-clause *clause* *state*))

或者，如果您想继续使用 racket/bool 中的 true 和 false，请定义 my-eval 如下;

(define my-eval
  (let ((ns (make-base-namespace)))
    (parameterize ((current-namespace ns))
      (namespace-require 'racket/bool))
    (lambda (expr) (eval expr ns))))