calling from other languages

[mvolkmann]

Is it possible to evaluate a Prolog goal from another programming language using Scryer for the evaluation?
I know some Prolog implementations have libraries for doing this including calling from JavaScript or running in an HTTP server.

[triska]

There is ongoing work for making Scryer available as a library, see especially #225 and #1880.

Regarding HTTP: You can easily create an HTTP server with Scryer Prolog, using library(http/http_server):

https://www.scryer.pl/http/http_server.html

Such a server can receive queries and respond with answers, I think such a setup and separation is easiest to debug, and I would recommend it if possible.

[mvolkmann]

Here is a basic example of using the http/http_server library. When I run this I get "Warning: overwriting http_listen/2" and the server doesn't work. Am I using the library incorrectly?

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

text_handler(_, Response) :-
  http_status_code(Response, 200),
  http_body(Response, text("Welcome to Scryer Prolog!")).

http_listen(8081, [
  get(echo, text_handler)
]).

[triska]

The library provides the predicate http_listen/2 which you can invoke, whereas your current code provides an alternative definition of this predicate.

[mvolkmann]

Ah, that makes sense. I copied code from main documentation for the library at https://www.scryer.pl/http/http_server.html.
It seems like the documentation should either precede that line with :- or wrap it in an initialization like the following. But this also does not work. I get the warning "initialization/1 failed for: user:http_listen(8081,[get(echo,text_handler)])". I verified that I don't have anything else listening on port 8081. Not sure what the issue is.

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

text_handler(_, Response) :-
  http_status_code(Response, 200),
  http_body(Response, text("Welcome to Scryer Prolog!")).

:- initialization(
  % GET /echo
  http_listen(8081, [
    get(echo, text_handler)
  ])
).

[david-sitsky]

I think what you want is this:

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

text_handler(_, Response) :-
  http_status_code(Response, 200),
  http_body(Response, text("Welcome to Scryer Prolog!")).

initialization :-
  % GET /echo
  http_listen(8081, [
    get(echo, text_handler)
  ]).

Then from the prompt, do the following:

?- initialization.
Listening at 0.0.0.0:8081

Sure enough, visiting http://localhost:8081/echo works and I see the "Welcome to Scryer Prolog!" text. However the prompt shows an error soon after. The browser (chrome) seem to request favicon.ico which presumably also needs a handler?

?- initialization.
Listening at 0.0.0.0:8081
2023-07-31 (16:49:09) get /echo
2023-07-31 (16:49:09) get /favicon.ico
   error(type_error(list,'$dropped_value'),must_be/2).

However using eww from emacs doesn't have that issue.

[mvolkmann]

It's not clear me why using :- initialization(( ... )). doesn't work, but your approach does work for me. Thanks!

[triska]

Yes, well spotted, that should work, but currently does not! I think this is due to a rather subtle issue involving meta_predicate/1 and initialization/1, I have filed #1936 for this issue!

[mvolkmann]

@david-sitsky When I start the HTTP server using the approach you suggested, pressing ctrl-c or ctrl-d in the top level does not stop it. The only way I've found to stop it is to use the killall command. Does that match your experience?

[triska]

@mvolkmann: There is an existing issue about this: #485.