Http Client in Prolog

Here’s an idiomatic Prolog example demonstrating the concept of an HTTP client:

:- use_module(library(http/http_client)).
:- use_module(library(http/http_open)).

main :-
    % Make an HTTP GET request
    http_get('https://example.com', Response, []),
    
    % Print the response status
    status_code(Response, Status),
    format('Response status: ~w~n', [Status]),
    
    % Print the first 5 lines of the response body
    setup_call_cleanup(
        http_open('https://example.com', In, []),
        read_lines(In, 5),
        close(In)
    ).

% Helper predicate to read and print lines
read_lines(_, 0) :- !.
read_lines(In, N) :-
    read_line_to_string(In, Line),
    (   Line == end_of_file
    ->  true
    ;   format('~w~n', [Line]),
        N1 is N - 1,
        read_lines(In, N1)
    ).

% Helper predicate to extract status code
status_code(Response, Code) :-
    member(status(Code, _), Response).

This Prolog example demonstrates how to make an HTTP GET request and process the response. Let’s break it down:

1. We use the http_client and http_open libraries, which provide HTTP functionality in Prolog.

2. The main/0 predicate is our entry point:

   • It makes an HTTP GET request using http_get/3.
   • It extracts and prints the response status code.
   • It then makes another request using http_open/3 to read the first 5 lines of the response body.

3. The read_lines/2 predicate is a helper that reads and prints a specified number of lines from an input stream.

4. The status_code/2 predicate extracts the status code from the response.

To run this program:

1. Save the code in a file, e.g., http_client.pl.
2. Make sure you have SWI-Prolog installed with the http pack.
3. Load the file in SWI-Prolog:

$ swipl http_client.pl

4. Query the main/0 predicate:

?- main.

This will execute the HTTP GET request and display the results.

Note that Prolog’s approach to HTTP clients is quite different from imperative languages. It uses predicates and backtracking to handle operations, which is characteristic of Prolog’s logical programming paradigm. The example demonstrates how to use Prolog’s built-in HTTP libraries to perform web requests in a declarative manner.