And the spinner implementation:
(define p
(let ((index 0)
(spinner-chars "|\\-/"))
(define (spin)
(set! index (+ index 1))
(if (>= index (string-length spinner-chars))
(set! index 0))
(display (array-ref spinner-chars index))
(display "\b"))
(make-soft-port
(vector
(lambda (c) (if (char=? c #\newline) (spin) (write c))) ; putc
(lambda (s) (if (string-contains s "\n") (spin))) ; puts
(lambda () #t) ; flush
(lambda () #f) ; getc
(lambda () #t)) ; close
"w")))(define (f) (display "\n" p) (f)) (f)
