zug is a C++ library providing transducers. Transducers are composable sequential transformations independent of the source. They are extremely lightweight, and can be used to express algorithms over pull-based sequences (iterators, files) but also push based sequences (signals, events, asynchronous streams) in a generic way.
- Documentation (Contents)
- Code (GitHub)
- CppCon 2015 Talk: Transducers: from Clojure to C++ (YouTube, Slides)
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Example
auto xf = zug::filter([](int x) { return x > 0; })
| zug::map([](int x) { return std::to_string(x); });Here xf is a transducer, a transformation over a sequence of integers,
resulting in a sequence of strings. Note, however, that this transformation
makes no reference to whatever it is transforming. In fact, we can apply it in
many ways.
Transforming a range
auto data1 = std::vector<int>{3, -2, 42, -10};
auto data2 = zug::into(std::vector<std::string>{}, xf, data1);
assert(data2 == {"3", "42"});As a lazy iterator
auto data1 = std::vector<int>{ ... };
auto data2 = zug::sequence(xf, data1);
std::copy(data2.begin(), data2.end(), ...);Generators and sinks
zug::run(zug::read<int>(std::cin) | xf |
zug::write(std::cout));Reads integers from the terminal and outputs back the positive ones.
Transforming cursors
The library is used in Lager, a library implementing the unidirectional data-flow architecture for C++ interactive applications. It is used to treat reactive values as a temporal sequence that can be transformed in arbitrary ways. For example:
auto x = lager::state<int>{42};
auto y = lager::reader<std::string>{x.xform(xf)}
y.watch([] (auto&& v) { std::cout << v << std::endl; });
x.set(10); // outputs: 10
x.set(-2); // no outputWhy?
You have learn Sean Parent's lesson: No Raw Loops. Instead of iterating over sequences directly, you use STL algoriths like transform, filter, etc, or even better, the new ranges library.
However, what if you have a sequence that can not be easily or efficiently expressed as an iterator? Then, you may have to reimplement all these algorithms again, on top of whatever sequence abstraction you have invented, for example, see RxCpp... Or you use transducers.
Transducers are generic algorithmic transformations, in a way that is completely agnostic of the actual sequence that is being transformed. As a library author, you can add transducer support for your library, and automatically get access to our wide collection of transducers and allow your users to simply write their own.
Dependencies
This library is written in C++14 and a compliant compiler is necessary. It is continuously tested with Clang 3.8 and GCC 6, but it might work with other compilers and versions.
If compiling with C++14 and using skip, boost variant is required.
For C++17 and above, no external library is necessary and there are no other requirements.
Usage
This is a header only library but to be configured correctly you need to run CMake first:
mkdir -p build && cd build cmake ..
Or you can just copy the zug subfolder somewhere in your include path.
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
This software is licensed under the Boost Software License 1.0.
The full text of the license is can be accessed via this link and is also included
in the LICENSE file of this software package.