---

jupyter: jupytext: text_representation: extension: .md format_name: markdown format_version: '1.3' jupytext_version: 1.16.7 kernelspec: display_name: Logtalk language: logtalk name: logtalk_kernel ---

<!--

This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 1998-2025 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. -->

persistency

This example illustrates a very simple solution for persisting an object dynamic state across sessions. It uses a plain Prolog file for saving the state and an include/1 directive to automatically restore the saved state when the object is loaded. The saved state file is created if it doesn't exist by the loader.lgt file before loading the example itself. See also the serialization example.

Print Logtalk, Prolog backend, and kernel versions (if running as a notebook):

%versions

Load the example:

logtalk_load(persistency(loader)).

<!-- true. -->

No initial state:

\+ persistency::state(_).

<!-- true. -->

Add some terms:

persistency::(add(a), add(b)).

<!-- true. -->

Enumerate current terms:

persistency::state(S).

<!-- S = a ; S = b ; false. -->

Save the terms to permanent storage:

persistency::save.

<!-- true. -->

Halt the process:

halt.

Restart Logtalk and reload:

logtalk_load(persistency(loader)).

<!-- true. -->

Enumerate current terms:

%%table
persistency::state(S).

<!-- S = a ; S = b ; false. -->

Add another term:

persistency::add(c).

<!-- true. -->

Save the terms to permanent storage:

persistency::save.

<!-- true. -->

Halt the process:

halt.

Restart Logtalk and reload:

logtalk_load(persistency(loader)).

<!-- true. -->

Enumerate current terms:

%%table
persistency::state(S).

<!-- S = a ; S = b ; S = c ; false. -->