---

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 how to implement aspects (as in Aspect-Oriented Programming) using the event-driven programming support and hot patching using categories. The example reuses the bank money transfer AOP example as found in e.g.

https://en.wikipedia.org/wiki/Aspect-oriented_programming

The following entities are defined:

• bank (object defining a bank with a transfer predicate)
• accounts (defining balance, withdraw, and deposit predicates)
• security (hot patching category limiting maximum amount per transfer)
• logging (event monitor object logging transfers) Note that we could also have defined security as a monitor object with a before/3 event handle that would make a transfer fail when the maximum amount is exceeded. The downside of this alternative is that we should not rely in the calling order of event handlers and thus the check could be applied before or after the logging message that a transfer is being attempted.