Expansion problem

    Hello, Racket's friends!

    I'm trying to define the "do*" special form of Common Lisp by a  
Racket macro. Let us recall that "do*" is analogous to "do", but the  
bindings and updating of variables are done sequentially. A simple  
example where "do" and "do*" do not yield the same result could be:

(do* ((i 0 (+ i 1))
       (j 0 (+ i j 1)))
      ((= i 10) (+ i j))
   (write (cons i j))
   (newline))

    In addition, I allow bindings like (var init step) and (var init),  
processed like (var init var). If I define "do*" as follows:

(define-syntax do*
   (syntax-rules ()
     ((do* ((var init step ...) ...) (test expr-0 ...) body-expr-0 ...)
      (let* ((var init) ...)
        (let loop ()
          (if test
	     (do* "ending" expr-0 ...)
              (begin
                body-expr-0 ... (do* "step" var step ...) ... (loop))))))
     ((do* "ending") (if #f #f))
     ((do* "ending" expr-0) expr-0)
     ((do* "ending" expr-0 ...) (begin expr-0 ...))
     ((do* "step" var) #f)
     ((do* "step" var step) (set! var step))))

that works, but I don't like this solution because of side effects. If I try:

(define-syntax do*
   (syntax-rules ()
     ((do* ((var init step ...) ...) (test expr-0 ...) body-expr-0 ...)
      (let* ((var init) ...)
        (let loop ((var var) ...)
	 (if test
	     (do* "ending" expr-0 ...)
	     (begin
	       body-expr-0 ...
                (let* ((do* "step" var step ...) ...) (loop var ...)))))))
     ((do* "ending") (if #f #f))
     ((do* "ending" expr-0) expr-0)
     ((do* "ending" expr-0 ...) (begin expr-0 ...))
     ((do* "step" var) (var var))
     ((do* "step" var step) (var step))))

that results in the following error:

let*: bad syntax (not an identifier and expression for a binding) in:  
(do* "step" i (+ i 1))

    Please, does it mean that the bindings of a "let*" special form  
cannot be the result of a macro expansion? Many thanks in advance,

J.-M. Hufflen


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