gistfile1.scm

(define-syntax symbol-append
  (syntax-rules ()
    [(_ sym ...)
     (string->symbol (string-append (symbol->string sym) ...))]))

(define-syntax (define-ustruct stx)
  (define (make-updater-name struct-name field-name)
    ;; How do I get symbol-append to be defined here?
    (symbol-append struct-name '-update- field-name))
  (define (make-updater-proc name-stx field-stx)
    (let* ((field-name (syntax->datum field-stx))
           (struct-name (syntax->datum name-stx))
           (update-proc (datum->syntax name-stx
                                       (make-updater-name struct-name
                                                          field-name))))
      #`(define (#,update-proc val s)
          (struct-copy #,name-stx s [#,field-stx val]))))

  (syntax-case stx ()
      ((_ name fields kws ...)
       #`(begin (define-struct name fields kws ...)
                #,@(syntax-map (lambda (f)
                                 (make-updater-proc #'name f))
                               #'fields)))))