chicken scheme - how do i temporarily capture data sent to standard output

I have a procedure that calls (display "foo")

I want to write a unit test for it, to confirm that it's sending the correct data there but display sends its input to Standard Output:

(define (display x #!optional (port ##sys#standard-output))
  (##sys#check-output-port port #t 'display)
  (##sys#print x #f port) )

Question: In other languages I might redefine standard output as something that just writes to a variable, and then set it back after the test. Is that the correct thing to do in chicken ? If so, how? If not, then what is the correct thing to do?

Note: passing something else in to display as a second parameter isn't an option because i'd have to alter the method I'm unit testing to do so.

Solution

The port is an optional second argument which defaults to standard output.

You can do one of two things to send it to a string. The first way is to create a string port and pass it to display as the optional argument to use instead of the standard output port:

(use ports)
(call-with-output-string
  (lambda (my-string-port)
    (display "foo" my-string-port)))

The second is to temporarily bind the current output port to a string port:

(use ports)
(with-output-to-string
  (lambda () (display "foo")))

The second way is mostly useful when you're calling procedures that don't accept a port argument, like print, for example.

You can find this in the manual section about string ports.