Url Parsing in Prolog

Our URL parsing program demonstrates how to parse and extract components from a URL string in Prolog.

:- use_module(library(uri)).

main :-
    % We'll parse this example URL, which includes a
    % scheme, authentication info, host, port, path,
    % query params, and query fragment.
    S = "postgres://user:pass@host.com:5432/path?k=v#f",
    
    % Parse the URL and ensure there are no errors.
    uri_parse(S, URI),
    
    % Accessing the scheme is straightforward.
    uri_component(URI, scheme, Scheme),
    writeln(Scheme),
    
    % User contains all authentication info; we'll extract
    % username and password separately.
    uri_component(URI, user, User),
    writeln(User),
    uri_component(URI, username, Username),
    writeln(Username),
    uri_component(URI, password, Password),
    writeln(Password),
    
    % The Host contains both the hostname and the port,
    % if present. We'll extract them separately.
    uri_component(URI, host, Host),
    writeln(Host),
    uri_component(URI, hostname, Hostname),
    writeln(Hostname),
    uri_component(URI, port, Port),
    writeln(Port),
    
    % Here we extract the path and the fragment after
    % the #.
    uri_component(URI, path, Path),
    writeln(Path),
    uri_component(URI, fragment, Fragment),
    writeln(Fragment),
    
    % To get query params, we can use the search component.
    % It returns a list of Key-Value pairs.
    uri_component(URI, search, Search),
    writeln(Search),
    
    % We can also get the raw query string.
    uri_component(URI, query, Query),
    writeln(Query).

Running our URL parsing program shows all the different pieces that we extracted:

$ swipl -q -t main -s url_parsing.pl
postgres
user:pass
user
pass
host.com:5432
host.com
5432
/path
f
[k-v]
k=v

In this Prolog version:

We use the uri library, which provides URL parsing functionality.
The uri_parse/2 predicate is used to parse the URL string into a URI structure.
We use uri_component/3 to extract various components from the parsed URI.
Prolog doesn’t have a direct equivalent to Go’s url.ParseQuery, but the search component returns a list of Key-Value pairs, which is similar.
Error handling in Prolog is typically done through predicate success or failure, rather than explicit error returns as in Go.
Prolog uses write predicates (writeln/1) for output instead of a fmt.Println function.

This example demonstrates how to work with URLs in Prolog, showing that while the syntax and approach differ from Go, similar functionality can be achieved.

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Url Parsing in Prolog