(use posix)

(call-with-output-pipe "gnuplot --persist"
                       (lambda (port)
                         (display "plot sin(x)" port)
                         (newline)))

;; => Warning: at toplevel:
;;    (dbg.scm:3) in procedure call to `call-with-input-pipe', expected
;;    argument #2 of type `(procedure (input-port) . *)' but was given
;;    an argument of type `(procedure (output-port) undefined)'