________________________________________________________________________

This file is part of Logtalk <https://logtalk.org/>  
Copyright 1998-2021 Paulo Moura <pmoura@logtalk.org>  
SPDX-License-Identifier: Apache-2.0

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
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`assertions`
============

The `assertions.lgt` file contains definitions for two meta-predicates,
`assertion/1-2`, which allows using 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 on assertions succeed, fail, or throw an exception.


API documentation
-----------------

This tool API documentation is available at:

[../../docs/library_index.html#assertions](../../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.
