## A.28 library(www_browser): Open a URL in the users browser

This library deals with the highly platform specific task of opening a web page. In addition, is provides a mechanism similar to absolute_file_name/3 that expands compound terms to concrete URLs. For example, the SWI-Prolog home page can be opened using:

    ?- www_open_url(swipl(.)).

www_open_url(+Url)
  Open URL in running version of the users' browser or start a new browser. This predicate tries the following steps:

  1. If a prolog flag (see set_prolog_flag/2) `browser` is set and this is the name of a known executable, use this. The flag may be set to `Command-Mode`, where mode is one of `fg` or `bg`, requesting Command to run in foreground or background mode. Default is `bg`.
  2. On Windows, use `win_shell(open, URL)`
  3. Find a generic‘open' comment. Candidates are `xdg-open`, `open` or `gnome-open`.
  4. If a environment variable `BROWSER` is set and this is the name of a known executable, use this.
  5. Try to find a known browser. @tbd Figure out the right tool in step 3 as it is not uncommon that multiple are installed.
[multifile]known_browser(+FileBaseName, -Compatible)
  True if browser FileBaseName has a remote protocol compatible to Compatible.
expand_url_path(+Spec, -URL)
  Expand URL specifications similar to absolute_file_name/3. The predicate url_path/2 plays the role of file_search_path/2.
  Errors
    `existence_error(url_path, Spec)` if the location is not defined.