I am learning about Functional Programming

(define (parse initial callback)
  ;; Construct and return a purely-functional parser of length-ten
  ;; strings.

  ;; `initial' is the initial data to process.
  ;; `callback' is a function to call with length-ten strings.

  ;; The return value is a function which accepts data as an
  ;; argument. The return value of *that* is a function with an
  ;; identical signature. When you want to push more data into the
  ;; parser, always pass it to the most recently returned function.

  (lambda (data)
    (let ([buf (string-append initial data)])
      (if (>= (string-length buf) 10)
          (begin
            (callback (substring buf 0 10))
            (parse (substring buf 10) callback))
          (parse buf callback)))))

(((((parse "hello" write) " world") " radix") " is") " here!!!!!!!!!!!!!!!")