Custom Errors in Prolog

Our first example demonstrates how to create custom errors in Prolog. While Prolog doesn’t have a built-in error type like Go, we can create our own error representation using compound terms.

:- use_module(library(format)).

% Define a custom error structure
arg_error(Arg, Message) :- 
    format(atom(ErrorMsg), '~w - ~w', [Arg, Message]).

% Main function equivalent
f(42, _, Error) :- 
    !, 
    arg_error(42, 'can\'t work with it', Error).
f(Arg, Result, _) :- 
    Result is Arg + 3.

% Equivalent to main function
main :-
    f(42, _, Error),
    (   nonvar(Error)
    ->  arg_error(Arg, Message, Error),
        format('~w~n', [Arg]),
        format('~w~n', [Message])
    ;   format('No error occurred~n')
    ).

In this Prolog version:

1. We define a custom error structure arg_error/2 that takes an argument and a message.

2. The f/3 predicate is our main function. It returns an error when the input is 42, otherwise it adds 3 to the input.

3. In the main/0 predicate, we call f/3 with 42 and check if an error occurred.

4. If an error occurred (i.e., Error is not a variable), we extract the argument and message from the error and print them.

To run this program, you would typically save it in a file (e.g., custom_errors.pl) and then consult it in a Prolog interpreter:

?- [custom_errors].
?- main.
42
can't work with it

This example demonstrates how to create and handle custom errors in Prolog. While the approach is different from Go, it achieves a similar result of creating and using custom error types.