gistfile1.rkt

(define-syntax (val stx)
  (syntax-case stx ()
    [(_ id v)
     #`(define-syntax id
         (make-set!-transformer
          (lambda (stx)
            (syntax-case stx (set!)
              [(set! id new-v) #'(error 'id "~a is not mutable" (symbol->string 'id))]
              [id (identifier? #'id) #'v]))))]))



(val bar (lambda (x) x))
(bar 10)