Dr. Racket中调试和REPL模式之间的不同行为：修改常量

Question

I am trying to code my own unit-test library in scheme. So far, I have the following code in the definition window:

#lang scheme
(define all-tests '())

(define-syntax make-tests
  (syntax-rules (->)
    [(_ test-name function (args ... -> result) ...)
     (begin
       (define test-name
         (list function (list (list args ...) ...) (list result ...)))
       (set! all-tests (cons test-name all-tests)))]))

Using Dr. Racket, when I submit the following code into the REPL:

> (make-tests tests + (1 2 -> 3))
set!: assignment disallowed;
 cannot modify a constant
  constant: all-tests

However when I tried to debug it in Dr. Racket:

(debug)> (make-tests tests + (1 2 -> 3))

(debug)> tests
(#<procedure:+> ((1 2)) (3))
(debug)> all-tests
((#<procedure:+> ((1 2)) (3)))

So, for some reason, in Dr. Racket v6.1, my code works in debug mode, but does not work when using the REPL. What is going on, and how can I debug my code?

Answer 1

Based on the comment on this previous answer : If a module doesn't mutate a binding you cannot mutate it from outside the module. Running code in the REPL is not the same as running code in the module. A racket source in Racket language is a module and #lang scheme is not Scheme but synonym for #lang racket .

So to get the behaviour you would like you can replace (define all-tests '()) with:

(define all-tests #f)
(set! all-tests '())   ; quick fix that makes all-test mutable

And it will work. You may dig deeper into racket documentation about it too.

Answer 2

You can Un-check the option Enforce constant definition In the choose language menu in DrRacket In the Dynamic properties box.

And as a note to your macro. Why do you specify a name for the test? The name is not placed in the list, and aside from defining it to cons it to all-tests it is never used. You can use a let instead. Unless of course your are going to do something with tests later.

(define-syntax make-tests
  (syntax-rules (->)
    [(_ function (args ... -> result) ...)
       (let
        ((test (list function (list (list args ...) ...) (list result ...))))
         (set! all-tests (cons test all-tests)))]))