# `debug_messages`

By default, `debug` and `debug(Group)` messages are only printed when the `debug` flag is turned on. These messages are also suppressed when compiling code with the `optimize` flag turned on. This tool supports selective enabling of `debug` and `debug(Group)` messages in normal and debug modes.

## API documentation

This tool API documentation is available at:

../../apis/library_index.html#debug-messages

For general information on debugging, open in a web browser the following link and consult the debugging section of the User Manual:

../../handbook/userman/debugging.html

## Loading

This tool can be loaded using the query:

    | ?- logtalk_load(debug_messages(loader)).

## Testing

To test this tool, load the `tester.lgt` file:

    | ?- logtalk_load(debug_messages(tester)).

## Usage

The tool provides two sets of predicates. The first set allows enabling and disabling of all `debug` and `debug(Group)` messages for a given component. The second set allows enabling and disabling of `debug(Group)` messages for a given group and component for fine-grained control.

Upon loading the tool, all debug messages are skipped. The user is then expected to use the tool API to selectively enable the messages that will be printed. As an example, consider the following object, part of a `xyz` component:

    :- object(foo).

        :- public([bar/0, baz/0]).
        :- uses(logtalk, [print_message/3]).

        bar :-
            print_message(debug(bar), xyz, @'bar/0 called').

        baz :-
            print_message(debug(baz), xyz, @'baz/0 called').

    :- end_object.

Assuming the object `foo` is compiled and loaded in normal or debug mode, after also loading this tool, `bar/0` and `baz/0` messages will not print any debug messages:

    | ?- {debug_messages(loader), foo}.
    ...
    yes

    | ?- foo::(bar, baz).
    yes

We can then enable all debug messages for the `xyz` component:

    | ?- debug_messages::enable(xyx).
    yes

    | ?- foo::(bar, baz).
    bar/0 called
    baz/0 called
    yes

Or we can selectively enable only debug messages for a specific group:

    | ?- debug_messages::disable(xyx).
    yes

    | ?- debug_messages::enable(xyx, bar).
    yes

    | ?- foo::(bar, baz).
    bar/0 called
    yes