SWI-Prolog -- http_public_url/2

Documentation
- Reference manual
- Packages
  - SWI-Prolog HTTP support
    - The HTTP server libraries
      - library(http/http_host): Obtain public server location
        
        http_public_url/2
        
        http_public_host_url/2
        
        http_public_host/4
        
        http_current_host/4

Availability::- use_module(library(http/http_host)).

Source

[det]http_public_url(+Request, -URL)

True when URL is an absolute URL for the current request. Typically, the login page should redirect to this URL to avoid losing the session.