Logging in Prolog

Here’s the translation of the Go logging example to Prolog, formatted in Markdown suitable for Hugo:

Our first example demonstrates how to perform logging in Prolog. We’ll use the print/1 and write/1 predicates for basic output, and create custom predicates for more advanced logging functionality.

:- use_module(library(format)).

% Simple logging
log_message(Message) :-
    get_time(Timestamp),
    format_time(atom(FormattedTime), '%Y-%m-%d %H:%M:%S', Timestamp),
    format('~w: ~w~n', [FormattedTime, Message]).

% Logging with custom prefix
log_with_prefix(Prefix, Message) :-
    get_time(Timestamp),
    format_time(atom(FormattedTime), '%Y-%m-%d %H:%M:%S', Timestamp),
    format('~w ~w: ~w~n', [Prefix, FormattedTime, Message]).

% Logging to a file
log_to_file(Filename, Message) :-
    get_time(Timestamp),
    format_time(atom(FormattedTime), '%Y-%m-%d %H:%M:%S', Timestamp),
    open(Filename, append, Stream),
    format(Stream, '~w: ~w~n', [FormattedTime, Message]),
    close(Stream).

% Main predicate to demonstrate logging
main :-
    % Basic logging
    log_message('standard logger'),

    % Logging with microsecond precision
    get_time(Timestamp),
    format_time(atom(FormattedTime), '%Y-%m-%d %H:%M:%S.%f', Timestamp),
    format('~w: with micro~n', [FormattedTime]),

    % Logging with custom prefix
    log_with_prefix('my:', 'from mylog'),
    log_with_prefix('ohmy:', 'from mylog'),

    % Logging to a file
    log_to_file('log.txt', 'hello'),

    % Reading from the file and printing
    read_file('log.txt', Content),
    format('from file: ~w', [Content]).

% Helper predicate to read file contents
read_file(Filename, Content) :-
    open(Filename, read, Stream),
    read_string(Stream, _, Content),
    close(Stream).

To run this Prolog program, you would typically save it in a file (e.g., logging.pl) and then consult it in your Prolog interpreter. Here’s how you might run it:

?- [logging].
?- main.

Sample output (the date and time emitted will depend on when the example ran):

2023-08-22 10:45:16: standard logger
2023-08-22 10:45:16.904141: with micro
my: 2023-08-22 10:45:16: from mylog
ohmy: 2023-08-22 10:45:16: from mylog
from file: 2023-08-22 10:45:16: hello

This Prolog example demonstrates basic logging functionality:

1. We define a simple log_message/1 predicate that logs a message with a timestamp.
2. The log_with_prefix/2 predicate allows logging with a custom prefix.
3. log_to_file/2 demonstrates how to log messages to a file.
4. We show how to get microsecond precision in timestamps.
5. The main/0 predicate demonstrates the usage of these logging functions.

Note that Prolog doesn’t have built-in modules for advanced logging like Go’s slog. For more complex logging needs in Prolog, you might need to implement additional predicates or use external libraries if available.

The concept of structured logging (as seen with slog in the Go example) doesn’t have a direct equivalent in standard Prolog. However, you could potentially implement a similar concept by defining predicates that format log messages as structured data (e.g., in a list or compound term format) if needed for your application.