Shadow stream in Racket

shadow-stream.rkt

;; Returns a stream that shadows side-effects of the original stream
;; when accessed after the first time
(define (shadow-stream s)
  (let ([shadow-next #f])
    (thunk (begin
             (when (not shadow-next) 
               (let ([next (s)])
                 (set! shadow-next (cons (car next) (shadow-stream (cdr next))))))
             (cons (car shadow-next) (cdr shadow-next))))))