[plt-scheme] hygienic macros

I am not sure what Eli is getting at.  However, it seems that as written 
_rand_ gains little from being a macro in Scheme.  It would be better as 
a procedure in this instance.

To take advantage of the fact that ideally the macros are expanded at 
load-time.  It would be really nice for the macro to yield the _syntax_ 
of the random choice rather than _code_ that picks the random choice.

Something like this:

(define-syntax rand-pick
  (lambda (stx)
   (syntax-case stx ()
     ((_ exp1 exprs ...)
      (let ((stx-list (syntax-e (syntax (exp1 exprs ...)))))
        (list-ref stx-list (random (length stx-list))))))))

I am sure I missed the issues that Eli was referring to..

-Bruce


michael rice wrote:
>   For list-related administrative tasks:
>   http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> 
> To firm up my understanding of hygienic macros I have
> been trying to translate a few from the old backquote
> syntax to the new define-syntax, case-syntax, etc.
> 
> The latest one I chose is a random-choice macro from
> Paul Graham's ANSI Common Lisp (pg. 170):
> 
> (defmacro random-choice (&rest exprs)
> `(case (random ,(length exprs))
>    , at let ((key -1))
>      (mapcar #'(lambda (expr)
>                  `(,(incf key) ,expr))
>              exprs))))
> 
> 
> My translation(shorter pick replaces random-choice):
> 
> ;randomly pick an expression to evaluate
> (define-syntax pick
>   (lambda (x)
>     (syntax-case x ()
>       ((_ exprs ...)
>        (with-syntax (((index ...)
>                       (let f ((i 0) (ls (syntax-e
> (syntax (exprs ...)))))
>                         (if (null? ls)
>                             '()
>                             (cons i (f (+ i 1) (cdr
> ls)))))))
>          (syntax (pick-help (index ...) (exprs
> ...))))))))
> 
> (define-syntax pick-help
>   (lambda (x)
>     (syntax-case x ()
>       ((_ (index ...) (exprs ...))
>        (with-syntax (((clauses ...)
>                       (map (lambda (i e) (list (list
> i) e))
>                            (syntax-e (syntax (index
> ...)))
>                            (syntax-e (syntax (exprs
> ...))))))
>          (syntax (case (random (length '(clauses
> ...))) clauses ...)))))))
> 
> 
> Eli Barzilay gave me the hint that allowed me to
> complete it and suggested that "There are certain
> issues in your code that might worth a discussion..."
> Pick it apart if you will (pun intended).
> 
> Michael Rice
> 
> 
> 
>  
> 
> 
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