let-alist

gistfile1.txt

(defsyntax (let-alist stx)
  (syntax-case stx ()
    ((macro expr body ...)
     (with-syntax ((alist-ref (stx-identifier #'macro '%%ref)))
       #'(let (alist expr)
           (let-syntax
               ((alist-ref
                 (lambda (stx)
                   (syntax-case stx ()
                     ((_ id)
                      (let (str (symbol->string (stx-e #'id)))
                        (if (eq? (string-ref str 0) #\.)
                          (let (sym (string->symbol (substring str 1 (string-length str))))
                            (with-syntax ((alist (quote-syntax alist))
                                          (elem sym))
                              #'(cond
                                 ((assq 'elem alist) => cdr)
                                 (else
                                  (error "Missing alist element" alist 'elem)))))
                          #'(%#ref id))))))))
             body ...))))))