Errors in Prolog

% In Prolog, error handling is typically done through failure and backtracking.
% However, we can simulate Go-like error handling using compound terms.

% Define a predicate that may produce an error
f(Arg, Result, Error) :-
    (   Arg =:= 42
    ->  Result = -1,
        Error = error("can't work with 42")
    ;   Result is Arg + 3,
        Error = nil
    ).

% Define sentinel errors
err_out_of_tea(error("no more tea available")).
err_power(error("can't boil water")).

% Define a predicate that may produce different errors
make_tea(Arg, Error) :-
    (   Arg =:= 2
    ->  err_out_of_tea(Error)
    ;   Arg =:= 4
    ->  Error = error("making tea", err_power(SubError))
    ;   Error = nil
    ).

% Main predicate
main :-
    % Demonstrate f/3
    forall(
        member(I, [7, 42]),
        (   f(I, R, E),
            (   E == nil
            ->  format('f worked: ~w~n', [R])
            ;   format('f failed: ~w~n', [E])
            )
        )
    ),
    
    % Demonstrate make_tea/2
    forall(
        between(0, 4, I),
        (   make_tea(I, Err),
            (   Err == nil
            ->  writeln('Tea is ready!')
            ;   handle_tea_error(Err)
            )
        )
    ).

% Helper predicate to handle tea-related errors
handle_tea_error(Error) :-
    (   Error = error("no more tea available")
    ->  writeln('We should buy new tea!')
    ;   Error = error("making tea", SubError),
        SubError = err_power(_)
    ->  writeln('Now it is dark.')
    ;   format('unknown error: ~w~n', [Error])
    ).

% To run the program, consult this file and call main/0.

This Prolog code demonstrates error handling in a way that’s conceptually similar to the Go example. Here’s an explanation of the key points:

In Prolog, we use compound terms to represent errors. A successful result is represented by nil, while an error is represented by error(Message) or error(Message, SubError) for wrapped errors.
The f/3 predicate simulates a function that may return an error. It takes an argument and returns both a result and a potential error.
Sentinel errors are defined as facts, such as err_out_of_tea/1 and err_power/1.
The make_tea/2 predicate demonstrates how to return different types of errors, including wrapped errors.
The main/0 predicate serves as the entry point of the program. It demonstrates how to use f/3 and make_tea/2, and how to handle their potential errors.
Error handling is done by pattern matching on the error term. The handle_tea_error/1 predicate shows how to check for specific error types, including wrapped errors.

To run this program, you would typically save it to a file (e.g., errors.pl), consult it in your Prolog interpreter, and then call the main/0 predicate:

?- [errors].
?- main.
f worked: 10
f failed: error(can't work with 42)
Tea is ready!
Tea is ready!
We should buy new tea!
Tea is ready!
Now it is dark.

This example demonstrates how to implement error handling in Prolog in a way that’s somewhat similar to Go’s approach, despite the significant differences between the two languages.

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