[r6rs-discuss] [Formal] Implementing Common Lisp's defmacro is not straigtforward.

[Pascal Costanza]

On 21 Sep 2006, at 17:54, Eli Barzilay wrote:

> On Sep 17, Pascal Costanza wrote:
>
>> Full description of the issue:
>>
>> Section 17.6, page 113, last paragraph, states that "Using lisp-
>> transformer, defining a basic version of Common Lisp's defmacro is a
>> straightforward exercise."
>>
>> This is misleading.  [...]
>
> I don't think so.  I read "basic version" as the basic version that is
> commonly found in some Schemes -- no destructuring and no
> environments.

The text says "basic version of Common Lisp's defmacro", not "basic  
version of some Schemes' defmacro."

The current text can be interpreted as if what lisp-transformer does  
were all that Common Lisp's defmacro basically does. And that's  
wrong. Implementing a full-fledged version of Common Lisp's defmacro  
is certainly not a straightforward exercise.

The text can also be interpreted differently, but that's why I said  
the text is "misleading", not "wrong."

>   Basically this (from an old post):
>
>   (define macros '())
>
>   (define nothing (list "nothing"))
>
>   (define (preprocess-expr expr)
>     (cond
>      ((not (pair? expr)) expr)
>      ((eq? 'defmacro (car expr))
>       (set! macros
>             (cons (cons (cadr expr)
>                         (eval `(lambda ,@(cddr expr))))
>                   macros))
>       nothing)
>      ((assq (car expr) macros) =>
>       (lambda (m) (preprocess-expr (apply (cdr m) (cdr expr)))))
>      (else (cons (preprocess-expr (car expr))
>                  (preprocess-expr (cdr expr))))))
>
>   (define (preprocess)
>     (let loop ((expr (read)))
>       (if (not (eof-object? expr))
>         (let ((expr (preprocess-expr expr)))
>           (if (not (eq? nothing expr))
>             (begin (write expr) (newline)))
>           (loop (read))))))

Huh? The text in R6RS is about implementing a subset of defmacro in  
terms of syntax-case.


Pascal

-- 
Pascal Costanza, mailto:pc at p-cos.net, http://p-cos.net
Vrije Universiteit Brussel, Programming Technology Lab
Pleinlaan 2, B-1050 Brussel, Belgium