Add-on Resources

The presentation

My presentation in a zip-file. It’s made in Keynote for Mac and contains animations that are not likely to work elsewhere.

A proof-of-concept implementation of Web Prolog written for clarity

Proof-of-concept implementations of Web Prolog have been written before, but they have been complex in a way that has tended to hide its most essential features. This is the first PoC that has been written for optimal understandability in mind, which should make it easier to implement Web Prolog in Prolog systems other than SWI-Prolog. It does mean, however, that it is also less complete, and does not, for example, implement the node option or the load_* options.

Here’s how to install the PoC, run the tests, play with the stateless HTTP API, and do some Erlang-style programming in Web Prolog:

  1. Install the latest SWI-Prolog for your platform.
  2. Source code for the PoC is here, in a zip-file.
  3. Unzip and do 
    $ cd trinity-poc

    and then

    $ swipl trinity.pl

  4. Tests are available in the same directory. Just do

    ?- [trinity_tests].

    and then run:

    ?- test.

    The tests should all pass.

  5. To run a node with its stateless HTTP API, you need to do:

    ?- node(3010).
% Started server at http://localhost:3010/
true.
?-

    Warning: Do not expose the node to the whole world at this point, as it is not secure!

    You may want to take the node’s stateless HTTP API for a trial run by selecting (or just entering) the following URI in a web browser:

    http://localhost:3010/call?goal=member(X,[a,b])&format=prolog

    In the browser’s window, you should then see the following:

    success([member(a,[a,b]),member(b,[a,b])],false)

    Then try this:

    http://localhost:3010/call?goal=member(X,[a,b])&template=X&offset=0&limit=1&format=prolog

    and you should see

    success([a],true)

    Then this, where offset is set to 1:

    http://localhost:3010/call?goal=member(X,[a,b])&template=X&offset=1&limit=1&format=prolog

    and you should see

    success([b],false)

    From you Prolog prompt, you should also be able to test rpc/2-3, like so:

    ?- rpc('http://localhost:3010', member(X,[a,b])).
X = a ;
X = b.
?-

    and so:

    ?- rpc('http://localhost:3010', member(X,[a,b]),[limit(1)]).
X = a ;
% New HTTP request from offset 1
X = b.
?-

    Here is a test that shows that the spurious recomputation is eliminated:

    ?- time(rpc('http://localhost:3010', (sleep(1), X=foo ; X=bar), [limit(1)])).
% 2,040 inferences, 0.001 CPU in 1.006 seconds (0% CPU, 1485798 Lips)
X = foo ;
% New HTTP request from offset 1
% 1,861 inferences, 0.001 CPU in 0.004 seconds (23% CPU, 1975584 Lips)
X = bar.
?-

    If you want to test the naive implementation of the HTTP API (the one you may be able to implement in Scryer Prolog at this point in time) you need to replace compute_answer/5 with two lines (see ≈ line 590). If you do, note that computing the solution X=bar will take just as long as computing the solution X=foo since spurious recomputations are no longer eliminated.

  6. Your Prolog top-level can be used as a shell. Try this for example:

    ?- toplevel_spawn(Pid, [
       monitor(true)
   ]).
Pid = '42103437'.
   
?- toplevel_call($Pid, between(1,5,I), [
       template(I),
       limit(2)
   ]).
Pid = '42103437'.
   
?- flush.
Shell got success('42103437',[1,2],true)
true.
   
?- toplevel_next($Pid,[
       limit(10)
   ]).
Pid = '42103437'.
   
?- flush.
Shell got success('42103437',[3,4,5],false)
Shell got down('42103437',true)
true.
   
?-

    You’ll find a lot more examples in the draft chapters linked above, and most of the examples that work in the PoC is included in the file trinity_programs.pl.

    Examples that do not work:

    1. Examples that use the node option,
    2. examples that use the load_* options,
    3. and probably a few others…

A book extract