guile-examples/shell/example-06-concurrent-output.scm

(import
 ;; ports
 (ice-9 popen)
 (ice-9 textual-ports)
 (ice-9 binary-ports)
 (ice-9 exceptions)
 ;; receive form
 ;; (ice-9 receive)
 ;; pattern matching
 (ice-9 match)
 ;; concurrency
 (ice-9 threads)
 (ice-9 futures)
 ;; let-values form
 (srfi srfi-11))


(define-values (in out)
  (match-let ([(in . out) (pipe)])
    ;; make out line buffered
    (setvbuf out 'none)
    (values in out)))


(define seconds
  (λ (s)
    (* s (expt 10 6))))


(define endless-writer
  (λ (out)
    (let loop ()
      ;; (simple-format #t "writer: writing message to output port\n")
      (put-string out "Hello!\n")
      ;; forcing the output should be unnecessary
      ;; (force-output out)
      (usleep (seconds 1))
      (loop))
    'never))


(define reader
  (lambda* (in-port #:key (bytes-count 1024) (out-port (current-output-port)))
    "Read from an IN-PORT and write to OUT-PORT, BYTES-COUNT
bytes at a time."
    (let loop ([bv (get-bytevector-n in-port bytes-count)])
      ;; (simple-format #t "reader: reading from in-port\n")
      (unless (eof-object? bv)
        (put-bytevector out-port bv)
        (loop (get-bytevector-n in-port bytes-count))))))


(define line-reader
  (lambda* (in-port #:key (out-port (current-output-port)))
    "Read from an IN-PORT and write to OUT-PORT."
    (let loop ([line (get-line in-port)])
      ;; (simple-format #t "reader: reading from in-port\n")
      (unless (eof-object? line)
        (simple-format out-port "~a\n" line)
        ;;(put-string out-port (string-append line "\n"))
        (loop (get-line in-port))))))


;; Start writing and reading endlessly.
(define writer-future (future (endless-writer out)))
(define reader-future (future (line-reader in #:out-port (current-output-port))))

;; Read from in-port, which should be the corresponding one to the
;; out-port, to which the endless-writer writes its output. Output to
;; the current output port, so that the output is visible. Read some
;; number of bytes at once. Limits the amount of memory needed for the
;; string.