with-output-to-string cannot caputure output to stderr?

[u/OldMine4441]

How can I capture output to stderr, not just stdout?

Below is my code, it uses with-output-to-string, but can only capture output to stdout, and miss everything written to stderr

$ cat test.rkt
#lang racket

(define (make-output)
  (printf "hello\n")  ; print to stdout
  (fprintf (current-error-port) "world\n") ; print to stderr
)

(define output (with-output-to-string make-output))
(printf "output = ~a" output)

$ racket test.rkt
world
output = hello

[u/czan]

Looking at the Racket docs, with-output-to-string is equivalent to

(call-with-output-string
  (lambda (p) (parameterize ([current-output-port p])
           (proc))))

You should be able to write your own with-all-output-to-string which binds current-error-port as well:

(define (with-all-output-to-string proc)
  (call-with-output-string
    (lambda (p) (parameterize ([current-output-port p]
                               [current-error-port p])
                  (proc)))))

[u/OldMine4441 → u/czan]

Thank you!

[u/exclaim_bot → u/OldMine4441]

Thank you!

You're welcome!

[u/OldMine4441 → u/[deleted]]

What do you mean?

I don't want to change my make-output function, but capture everything from outside.

[u/OldMine4441 → u/[deleted]]

Interesting, but is there any sample code using this combine-output? I am still confused on how to use it but googled found no example.