macroexpand and nested syntax-rules

[jcubic]

(define-syntax when
  (syntax-rules ()
    ((_ test body ...)
     (let ((tmp test))
        (if tmp
            (begin
              body ...))))))


(let ((alist '((foo . "lorem") (bar . "ipsum") (baz . "dolor")))
      (begin (lambda () 10)))
  (pprint (macroexpand (when (assoc 'bar alist) #void))))

;; ==> (#:let ((#:tmp (assoc (quote bar)
;; ==>                       alist)))
;; ==>       (#:if #:tmp
;; ==>            (#:begin
;; ==>              )
;; ==>            ))

(define-syntax-parameter it (syntax-rules () ((_) (syntax-error "Use outside aif"))))

(define-syntax awhen
  (syntax-rules ()
    ((_ test body ...)
     (let ((tmp test))
       (syntax-parameterize
        ((it (syntax-rules ()
               ((_) tmp))))
        (if tmp
            (begin
              body ...)))))))

(let ((alist '((foo . "lorem") (bar . "ipsum") (baz . "dolor")))
      (begin (lambda () 10)))
  (pprint (macroexpand (awhen (assoc 'bar alist) #void))))
;; ==> syntax-rules: no matching syntax in macro (awhen)

[jcubic]

This is actually a collision of _ binding that is used in nested syntax-rules. It works as code but fail when expanding.

(define-syntax symbol
  (syntax-rules ()
    ((_ name)
     (let-syntax ((name (syntax-rules ()
                       ((_)
                        (list 'name)))))
       (name)))))

(print (symbol hello))
;; ==> (hello)
(pprint (macroexpand (symbol hello)))
;; ==> syntax-rules: no matching syntax in macro (symbol)

[jcubic]

It's not a collision only macroexpand try to evaluate (symbol) inside pattern of syntax-rules.

This is the problem that macroexpand know nothing about nested syntax-rules. That's why they don't expand at all.