# `assertions`

The `assertions.lgt` file contains definitions for two meta-predicates, `assertion/1-2`, which allows the use of assertions on your source code to print warning and error messages (using the message printing mechanism). The `assertions_messages.lgt` file defines the default message translations generated when assertions succeed, fail, or throw an exception.

## API documentation

This tool API documentation is available at:

../../docs/library_index.html#assertions

## Loading

This tool can be loaded using the query:

    | ?- logtalk_load(assertions(loader)).

## Testing

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

    | ?- logtalk_load(assertions(tester)).

## Adding assertions to your source code

The `assertion/1` predicate takes a goal as argument. For example:

    foo(L) :-
        assertions::assertion(non_empty_list(L)),
        ...

The `assertion/2` predicate takes as arguments a term for passing context information and a goal. Using again a unit test as an example:

    foo(L) :-
        assertions::assertion(foo_list_alerts, non_empty_list(L)),
        ...

When using a large number of assertions, you can use a lighter syntax by adding a `uses/2` directive. For example:

    :- uses(assertions, [assertion/1, assertion/2]).

## Automatically adding file and line context information to assertions

The `assertions/1` parametric object can be used as a hook object to automatically add file and line context information, represented by the term `file_lines(File, BeginLine-EndLine)`, to calls to the `assertion/1` predicate by goal-expanding it to calls to the `assertion/2` predicate (the expansion assumes that a `uses/2` directive is being used in the code that will be expanded to direct `assertion/1` calls to the `assertions` object). For example, assuming the file using assertions is named `source`, it would be compiled and loaded using the call:

    logtalk_load(source, [hook(assertions(debug))])

## Suppressing assertion calls from source code

The `assertions/1` parametric object can be used as a hook object to suppress calls to the `assertion/1-2` predicates using goal-expansion (the expansion assumes `assertions::assertion/1-2` messages). For example, assuming the file using assertions is named `source`, it would be compiled and loaded using the call:

    logtalk_load(source, [hook(assertions(production))])

## Redirecting assertion failure messages

By default, assertion failures and errors are printed to the standard output stream. These messages, however, can be intercepted by defining the `logtalk::message_hook/4` multifile predicate. For example:

    :- category(redirect_assertions_messages).

        :- multifile(logtalk::message_hook/4).
        :- dynamic(logtalk::message_hook/4).

        logtalk::message_hook(Message, error, assertions, _) :-
            writeq(my_log_file, Message), write(my_log_file, '.\n').

    :- end_category.

## Converting assertion failures into errors

If you want an assertion failure to result in a failure or a runtime error, you can intercept the assertion failure messages, optionally still printing them, and throw an error. For example:

    :- category(assertions_failures_to_errors).

        :- multifile(logtalk::message_hook/4).
        :- dynamic(logtalk::message_hook/4).

        logtalk::message_hook(Message, error, assertions, Tokens) :-
            % uncomment the next two lines to also print the default message
            % logtalk::message_prefix_stream(error, assertions, Prefix, Stream),
            % logtalk::print_message_tokens(Stream, Prefix, Tokens),
            throw(error(Message, _)).

    :- end_category.

In alternative, if you want assertions to always trigger an exception, use instead the `lgtunit` tool `assertions/1-2` public predicates.