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
               ((var-ref
                 (syntax-rules ()
                   ((_ id) (%%ref id)))))
             (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 ((elem sym))
                                #'(cond
                                   ((assq 'elem alist) => cdr)
                                   (else
                                    (error "Missing alist element" alist 'elem)))))
                            #'(var-ref id))))))))
               body ...)))))))