guile-website-example/app/handlers/example/hello-world.scm

(library (handlers hello-world)
  (export hello-world-handler)
  (import
    (except (rnrs base) let-values error)
    (only (guile)
          lambda* λ)))


(define hello-world-handler
  (lambda (request request-body)
    "A handler for a route."
    ;; A handler must return 2 values: The header and body
    ;; of the response.
    (values
     ;; Return the headers as first value (the bare
     ;; minimum).
     '((content-type . (text/plain)))
     ;; Then the response body. This is an example for
     ;; returning a string as second value, instead of a
     ;; procedure, which takes an output port.
     "Hello World!")))