Re: perplexing syntax-rules bug

[Panicz Maciej Godek]

It doesn't seem to be a bug in syntax-rules. It seems to appear whenever two consecutive let forms are used with an assoc list argument which starts with identical elements:

(begin

  (let ((l '((a . X)(b . Y)(c . 7))))

    (assoc-set! l 'b 'Z))

  (let ((l '((a . X)(b . Y))))

    l))

===> ((a . X) (b . Z))

 

It is likely that the compiler assumes that both lists can be represented using the same storage, because it doesn't notice that the first object gets mutated.

This qualifies for a serious bug report, but if you wish to get a quick workaround, you can do it like this:

(define-syntax sect

  (syntax-rules ()

    ((sect <name> <expr> ...)

     (let* ((sval* '(skip ((name . <name>) (title . #f))))

               (sval (cdr sval*)))

       (format #t "new sect: ~a\n" (quote <name>))

       (format #t "    sval= ~a\n\n" sval)

       (assq-set! sval 'title "ABC")

       (values)))))

Thanks&regards

2014-12-19 21:14 GMT+01:00 Wette, Matthew R (3441) <address@hidden>:

Sorry to bug,  I can't figure out why "sval" in the second evaluation of "sect" is bound to the "sval" from the first evaluation of "sect".   Anyone understand?   This is guile 2.0.11.    -- Matt

;; bug_syntax.scm

(define-syntax sect

  (syntax-rules ()

    ((sect <name> <expr> ...)

     (let ((sval '((name . <name>) (title . #f))))

       (format #t "new sect: ~a\n" (quote <name>))

       (format #t "    sval= ~a\n\n" sval)

       (assq-set! sval 'title "ABC")

       (values)))))

(sect one (title "Section One"))

(sect two (title "Section Two"))

> (load "bug_syntax.scm")

new sect: one

    sval= ((name . one) (title . #f))

new sect: two

    sval= ((name . two) (title . ABC))