
________________________________________________________________________

This file is part of Logtalk <http://logtalk.org/>  
Copyright 1998-2016 Paulo Moura <pmoura@logtalk.org>

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.
________________________________________________________________________


Overview
--------

The `assertions.lgt` file contains definitions for two meta-predicates,
`assertion/1-2`, which allows using of assertions on your source code
(e.g. when writing unit tests). The `assertions_messages.lgt` file defines
the default message translations generated on assertions succeed, fail, or
throw an exception. For more information on these entities, open the
`docs/tools.html` file in a web browser.


Adding assertions to your source code
-------------------------------------

The `assertion/1` predicate takes a goal as argument. For example,
assuming that you're writing a unit test:

	test(assertions_1) :-
		assertions::assertion(ground(x)),
		2 is 1 + 1.

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

	test(assertions_1) :-
		assertions::assertion(assertions_1, ground(x)),
		2 is 1 + 1.

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 that a `uses/2` directive is being used in the
code that will be expanded to direct `assertion/1-2` 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(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(_File)).
	
		:- multifile(logtalk::message_hook/4).
		:- dynamic(logtalk::message_hook/4).
	
		logtalk::message_hook(Message, error, assertions, _) :-
			parameter(1, File),
			writeq(File, Message), write(File, '.'), nl(File).
	
	:- end_category.


Converting assertion failures into errors
-----------------------------------------

If you want an assertion failure to result in 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 assertion failure message
			% logtalk::message_prefix_stream(error, assertions, Prefix, Stream),
			% logtalk::print_message_tokens(Stream, Prefix, Tokens),
			throw(error(Message, _)).
	
	:- end_category.
