Operate upon POST request with Guile webserver

[Gentoo Arch]

Hi,

I'd like to ask for your advice.

I'm trying to figure out how Guile webserver works to develop a simple 
BBS  and I'm kinda stuck with POST request.

My Guile script generates a form on any path and the form sends to 
"/post" path, where I can easily render content sent by the form.

(define (show-page request body)
  (if (equal? (get-path request) ; get-path is my other function to retrieve 
the path
              '("post"))
      (values '((content-type . (text/plain)))
               body)
      (respond ; literally a respond template from webserver doc
       `((h1 "Oops!")
     (p (@ (id "test")) "The path: " ,(get-path request))
     (form (@ (method "POST") (action "/post"))
           (label (@ (for "test")) "Content: ")
           (input (@ (id "test") (type "text") (name "content")))
           (input (@ (type "submit") (value "Submit"))))))))

But I would like to operate upon the content the form sends like web 
apps usually do: insert it in the database or simply write to file. So 
my question is how I can proceed with something like that:

(define tf (open-file "test-file.txt" "a"))

(define (show-page request body)
  (if (equal? (get-path request)
              '("post"))
      ((values '((content-type . (text/plain)))
              body)
       (display body tf)); I'd like to write/append body to the file, but it 
does nothing
      (respond
       `((h1 "Oops!")
     (p (@ (id "test")) "The path: " ,(get-path request))
     (form (@ (method "POST") (action "/post"))
           (label (@ (for "test")) "Content: ")
           (input (@ (id "test") (type "text") (name "content")))
           (input (@ (type "submit") (value "Submit"))))))))

If I remove (values), it does write to a file (with a complaint in 
webserver output), but I'd also like to add some kind of redirect from 
"/post" page after the script has written stuff to file and I can't use 
two functions per response.

Could you please advise how I can achieve this functionality?