arximboldi / Lager

.. image:: https://github.com/arximboldi/lager/workflows/test/badge.svg :target: https://github.com/arximboldi/lager/actions?query=workflow%3Atest+branch%3Amaster :alt: Github Actions Badge

.. image:: https://codecov.io/gh/arximboldi/lager/branch/master/graph/badge.svg :target: https://codecov.io/gh/arximboldi/lager :alt: CodeCov Badge

.. image:: https://cdn.jsdelivr.net/gh/arximboldi/lager/doc/_static/sinusoidal-badge.svg :target: https://sinusoid.al :alt: Sinusoidal Engineering badge :align: right

.. raw:: html

.. include:introduction/start

lager is a C++ library to assist value-oriented design_ by implementing the unidirectional data-flow architecture. It is heavily inspired by Elm and Redux_, and enables composable designs by promoting the use of simple value types and testable application logic via pure functions. And you get time-travel for free!

.. _unidirectional data-flow architecture: https://www.exclamationlabs.com/blog/the-case-for-unidirectional-data-flow .. _Elm: https://guide.elm-lang.org/architecture .. _Redux: https://redux.js.org/introduction/getting-started .. _value-oriented design: https://www.youtube.com/watch?v=_oBx_NbLghY

• Documentation (Contents_)
• Code (GitHub_)
• CppRussia-Piter 2019 Talk: Squaring the circle (YouTube <https://www.youtube.com/watch?v=e2-FRFEx8CA>, Slides <https://sinusoid.es/talks/cpprussia19-piter>)
• CppCon 2018 Talk: The most valuable values (YouTube <https://www.youtube.com/watch?v=_oBx_NbLghY>, Slides <https://sinusoid.es/talks/cppcon18>)
• C++ on Sea 2019 Talk: Postmodern immutable data-structures (YouTube <https://www.youtube.com/watch?v=y_m0ce1rzRI>, Slides <https://sinusoid.es/talks/cpponsea19>)

.. _contents: https://sinusoid.es/lager/#contents .. _github: https://github.com/arximboldi/lager

.. raw:: html

 <a href="https://www.patreon.com/sinusoidal">
     <img align="right" src="https://cdn.rawgit.com/arximboldi/immer/master/doc/_static/patreon.svg">
 </a>

This project is part of a long-term vision helping interactive and concurrent C++ programs become easier to write. Help this project's long term sustainability by becoming a patron or buying a sponsorship package: [email protected]

.. include:index/end

Examples

For a guided introductory tour with code samples, please read the architecture overview_ section. Other examples:

.. _architecture overview: https://sinusoid.es/lager/architecture.html

• Counter, a minimalistic example with multiple UIs (link <https://github.com/arximboldi/lager/tree/master/example/counter>_).
• Autopong, a basic game using SDL2 (link <https://github.com/arximboldi/lager/blob/master/example/autopong>_).
• Ewig, a terminal text editor with undo, asynchronous loading, and more (link <https://github.com/arximboldi/ewig>_).

Why?

Most interactive software of the last few decades has been written using an object-oriented interpretation of the Model View Controller_ design. This architecture provides nice separation of concerns, allowing the core application logic to be separate from the UI, and a good sense of modularity. However, its reliance on stateful object graphs makes the software hard to test or parallelize. It's reliance on fine-grained callbacks makes composition hard, resulting in subtle problems that are hard to debug.

Value-based unidirectional data-flow tackles a few of these problems:

• Thanks to immutability_ and value-types, it is very easy to add concurrency as threads can operate on their local copies of the data without mutexes or other flaky synchronization mechanisms. Instead, worker threads communicate their results back by dispatching actions to the main thread.

• The application logic is made of pure functions_ that can be easily tested and are fully reproducible. They interact with the world via special side-effects procedures loosely coupled to the services they need via dependency injection_.

• This also means that data and call-graphs are always trees or DAGs_ (instead of cyclical graphs), with explicit composition that is to trace and debug. You can also always snapshot the state, making undo and time-travel easy peasy!

.. _immutability: https://github.com/arximboldi/immer .. _pure functions: https://en.wikipedia.org/wiki/Pure_function .. _model view controller: https://en.wikipedia.org/wiki/Model%E2%80%93view%E2%80%93controller .. _dependency injection: https://en.wikipedia.org/wiki/Dependency_injection .. _DAGs: https://en.wikipedia.org/wiki/Directed_acyclic_graph

Dependencies

This library is written in C++17 and a compliant compiler and standard library necessary. It is continuously tested_ with GCC 7, but it might work with other compilers and versions.

It also depends on Zug_ and Boost Hana_. Some optional extensions and modules may have other dependencies documented in their respective sections.

.. _Zug: https://github.com/arximboldi/zug/ .. _Boost Hana: https://boostorg.github.io/hana .. _continuously tested: https://travis-ci.org/arximboldi/immer

Usage

This is a header only you can just copy the lager subfolder somewhere in your include path.

Some components, like the time-travelling debugger, also require the installation of extra files.

You can use CMake_ to install the library in your system once you have manually cloned the repository::

mkdir -p build && cd build
cmake .. && sudo make install

.. _nix package manager: https://nixos.org/nix .. _cmake: https://cmake.org/

Development

In order to develop the library, you will need to compile and run the examples, tests and benchmarks. These require some additional tools. The easiest way to install them is by using the Nix package manager_. At the root of the repository just type::

nix-shell

This will download all required dependencies and create an isolated environment in which you can use these dependencies, without polluting your system.

Then you can proceed to generate a development project using CMake_::

mkdir build && cd build
cmake ..

From then on, one may build and run all tests by doing::

make check

License

.. image:: https://raw.githubusercontent.com/arximboldi/lager/docs/doc/_static/mit.png :alt: Boost logo :target: https://opensource.org/licenses/MIT :align: right :width: 140 px

This software is licensed under the MIT license.

The full text of the license is can be accessed via this link <https://opensource.org/licenses/MIT>_ and is also included in the LICENSE file of this software package.