________________________________________________________________________

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
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`optionals`
===========

This library provides an implementation of *optional terms* with an API
modeled after the Java 8 `Optional` class (originally due to requests by
users working in Logtalk/Java hybrid applications). An optional term is
an opaque compound term that may or may not hold a value. Optional terms
avoid forcing the user to define a representation for the absence of a value
by providing an API with predicates that depend on the presence or absence
of a value. Optional terms also allow separating the code that constructs
optional terms from the code that processes them, which is then free to
deal if necessary and at its convenience with any case where the values
hold by optional terms are not present.


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

Open the [../../docs/library_index.html#optionals](../../docs/library_index.html#optionals)
link in a web browser.


Loading
-------

To load all entities in this library, load the `loader.lgt` file:

	| ?- logtalk_load(optionals(loader)).


Testing
-------

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

	| ?- logtalk_load(optionals(tester)).


Usage
-----

The `optional` object provides constructors for optional terms. For example:

	| ?- optional::of(1, Optional).
	...

The created optional terms can then be passed as parameters to the `optional/1`
parametric object. For example:

	| ?- optional::of(1, Optional), optional(Optional)::or_else(Term, 0).
	Optional = optional(1),
	Term = 1
	yes

	| ?- optional::empty(Optional), optional(Optional)::or_else(Term, 0).
	Optional = empty,
	Term = 0
	yes

The `maybe` object provides types and predicates for type-checking of the
term hold by optional terms. It also provides some  predicates for handling
lists of optional terms.


See also
--------

The `expecteds` library.
