---

This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 1998-2023 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.

To load this example and for sample queries, please see the SCRIPT.txt file.

This example illustrates using expected terms to call a conjunction of goals where any of them may cause an error condition without using the traditional catch/throw mechanism. This solution enables us to:

1. Abstract the error handling code that would be required to test if the previous goal generated an error before calling the next goal in the conjunction.
2. Allow the next goal to decide what to do with the result passed by the previous goal. For example, recovering from an unexpected error or just passing the unexpected error to the next goal.
3. Allow postponing handling an error in one of the goals, facilitating composition (e.g., adding more goals to a conjunction of goals or using some of the goals in a different context).
4. Handle a goal failure by passing the reason for the failure to the next goal, thus providing a common solution to handle both failures and errors.

   The original example is taken from standardization proposals and online discussions on expected values in the context of other OOP languages such as C++. For example:

   https://blog.tartanllama.xyz/optional-expected/

   For details please see the comments in the cascade.lgt source file. The cascade_dcgs.lgt file contains an alternative implementation of the cascade object using DCGs.

   See also the missing_data and books examples.