You can change the program above and/or load additional Prolog sources into the life server and type this in the toplevel to update the server. Then simply reload the page in your browser to see the changes. Using make/0 should be fairly safe in recent Prolog versions.

?- make.

If a requests ends in failure or an error in running the Prolog query that handles it, the server responds with a 500 internal server error. If it concerns an exception, extra context can be added by loading the library(http/http_error) as below.

:- use_module(library(http/http_error)).

If the error context is insufficient, one can use the graphical tracer by setting a spy-point on the handler or a suspect predicate called by the handler. Spy-points for background threads are set using tspy/1 (_thread_-spy). E.g., see below. After setting the spy-point, reload the page in the browser and the graphical tracer should appear.

?- tspy(say_hi/1).

Progress and debug messages cannot be printed to the current output, because current_output is rebound for catching output for the server. It is strongly encouraged to use library(debug) for this purpose. Statements are added to your code using debug/3:

        ...,
        debug(hello, 'About to say hello', []),
        ...

Such statements are activated using ?- debug(hello). and deactivated using ?- nodebug(hello). and print messages like this:

?- debug(hello).
?- debug(hello, 'About to say hello to ~p', ['the world']).
% About to say hello to the world

If you forgot what predicates are called by your server, you can use Firefox Developer Tools to analyse the traffic. You can also use debug channel http(request). Doing this on our first server and loading '/hello_world' gives this output:

?- debug(http(request)).
% [Thread httpd@5000_1] [2] get /hello_world ...
% [Thread httpd@5000_1] [2] 200 OK (0.002 seconds; 0 bytes)

The development environment is programmed to be able to locate source from the HTTP locations, so the following now helps to edit the source:

?- edit('/hello_world').

See also

- Next: Hello World in HTML