s c h e m e w i k i . o r g

(define (fold-lines-in-file filename proc init . mode)
  (with-input-from-file
   filename
   (lambda () (apply fold-lines proc init (current-input-port) mode))))

(define (fold-lines proc init . port+mode)
  (let while ((accum init))
    (let ((line (apply read-line port+mode)))
      (unless (eof-object? line)
        (while (proc line accum))))))