[racket] macro and set! problem

From: Skeptic . (skeptic2000 at hotmail.com)
Date: Sat Jul 10 08:36:52 EDT 2010

I tried to do :
(define tests #f)(set! tests '())
but I still got the problem.

> From: cce at ccs.neu.edu
> Date: Sat, 10 Jul 2010 09:07:07 +0100
> Subject: Re: [racket] macro and set! problem
> To: skeptic2000 at hotmail.com
> CC: users at racket-lang.org
> I think the issue is that any variable, V, in a module compiled
> without a (set! V E) expression somewhere in it is compiled as
> immutable. If the only set! in the module is in a macro that has not
> been called yet, your variable is being compiled as immutable. If you
> put a set! of the variable somewhere inside the module -- even just
> (set! V V) as the last form in the module -- the variable should be
> compiled as mutable and the macro should work in its original form.
> I have not tested this, so it may not be right, but it is what I
> recall about how set! works with modules.
> Carl Eastlund
> On Sat, Jul 10, 2010 at 5:52 AM, Skeptic .  wrote:
>> Hi,
>> I was trying to come up with a naive implementation of check-expect/test and I found out that if a macro expands to a set! on a module-level variable directly, an error will be given when using the macro from another module. A simple fix seems to instead have the macro to expands to a function call that performs the set!.
>> I'm not sure what to think about it and I would like to hear why it works that way or what I have missed.
>> Here's what I came up with in a few minutes :
>> (provide check-expect test)
>> (define tests '())
>> (define-syntax check-expect  (λ (stx)    (syntax-case stx ()      [(_ exp res)        #'(add-test (list (delay exp) res #'stx))])))
>> (define add-test  (λ (test) (set! tests (cons test tests))))   ; add-test is there because having check-expect expands to set! doesn't work when called from another module.
>> (define test  (λ ()    (printf "Ran ~a checks.\n" (length tests))    (let* ([failed (filter (λ (test) (not (equal? (force (car test)) (cadr test)))) (reverse tests))]           [nbfailed (length failed)])      (if (> nbfailed 0)          (begin            (printf "~a tests failed:\n" nbfailed)            (for-each (λ (ft)                         (let ([ft-stx (caddr ft)])                        (printf "Actual value ~a differs from ~a, the expected value.\n ~a\n" (force (car ft)) (cadr ft)                                 (format "In ~a at line ~a column ~a" (syntax-source ft-stx) (syntax-line ft-stx) (syntax-column ft-stx))))) failed))          (printf "All test passed!\n"))      )))
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