[plt-scheme] more macro

Ivanyi Peter skrev:
> Hi there,
> 
> I have noticed the following with macros. I could use macros
> to remove the quote sign, for example:
> 
> (define-syntax m1
>   (lambda (x)
>     (syntax-case x ()
>       ((_ 'f e1 e2 ...)
>        (syntax (f e1 e2 ...))))))
> 
> Usage:
> (syntax-object->datum (expand-once #'(m1 't a b c d)))
> Result:
> (t a b c d)
> 
> I do not know whether it is a feature or a bug. However if I
> use this
> "feature", then I have a problem when I try to use the quote
> sign 
> inside the macro expansion, for example:
> 
> (define-syntax m2
>   (lambda (x)
>     (syntax-case x ()
>       ((_ 'f e1 e2 ...)
>        (letrec
>          ((vars (map (lambda (x) (list x 'x)) 
>                      (syntax-object->datum (syntax (e1 e2
> ...))))))
>          (with-syntax
>            (((lvars ...) vars))
>            (syntax (lvars ...))))))))
> 
> Usage:
> (syntax-object->datum (expand-once #'(m2 't a b c d)))
> Expected result:
> ((a 'a) (b 'b) (c 'c) (d 'd))
> 
> On the other hand macro m2 would report an error saying:
> quote: illegal use of syntax in: (quote x)


I misunderstood the issue the first time. It might be a bug.
On 301.13 I get:


 > (define-syntax m2
   (lambda (x)
     (syntax-case x ()
       ((_ 'f e1 e2 ...)
        (letrec
            ((vars (map (lambda (x) (list x 'x))
                        (syntax-object->datum (syntax (e1 e2 ...))))))
          (with-syntax
              (((lvars ...) vars))
            (syntax (lvars ...))))))))

 > (syntax-object->datum (expand-once #'(m2 't a b c d)))
(#%app (#%top . m2) 't (#%top . a) (#%top . b) (#%top . c) (#%top . d))

 > (syntax-object->datum (expand #'(m2 't a b c d)))
(#%app (#%top . m2) 't (#%top . a) (#%top . b) (#%top . c) (#%top . d))

> Is this a bug or a feature? I do not really understand what is 
> happening with the quote sign inside these macros.

The problem with

   (syntax-case x ()
     ((_ 'f e1 e2 ...) <template-using-'>))

is that the pattern is equivalent to

   (syntax-case x ()
     ((_ (quote f) e1 e2 ...) <template-using-'>))

which means that in <template-using-'> quote will be a pattern variable
bound to whatever occured before f in the input syntax x. To be
sure ' refers to a quote use

   (syntax-case x (quote)
     ((_ (quote f) e1 e2 ...) <template-using-'>))

In your m2 the quote in 'x is not in a template, so I don't think
it should be a problem in that particular macro. Unless ... Hmm.

-- 
Jens Axel Søgaard