Hi Charles, Thanks for your efforts in exploring this topic. CMake's current language grew incrementally out of something that was not originally intended as a programming language. The cmState refactoring Stephen Kelly has started is a huge step toward enabling alternative languages, but there is a long way to go.
A few general thoughts: * One rule we have for CMake is to never expose any public SDK of the C++ implementation structures. We want to be able to rewrite them arbitrarily at any time. Therefore any solution that needs to access the C++ structures must be integrated into CMake upstream and expose functionality only through other languages or file formats. * The cmState infrastructure builds on a "snapshot" design with a goal of being able to "fork" configuration/generation temporarily and then revert back, and to be able to re-start configuration from the middle. These goals may be incompatible with any language whose implementation we do not fully control unless it is allowed to execute only in isolated and independent snippets. These are not hard goals, but it is a trade-off to keep in mind. Stephen may be able to elaborate more on the snapshot approach if needed. * A problem with the current design is that the entire configuration process is logically serial making parallel evaluation hard or impossible. In many cases each add_subdirectory can be processed independently, but this will require semantic changes to allow. On 01/04/2016 02:41 AM, Charles Huet wrote: > I'm trying to be as declarative as possible, because really like how readable > simple QML programs are, and I think it would be perfect for a buildsystem. Ideally most of the specification (sources, libraries, executables, etc.) should be in a pure format that can be evaluated without side effects (e.g. declarative or functional). This rules out both Python and Lua, but the specification format does not have to be the main entry point. There could be some imperative configuration step that does system introspection and then loads the pure specification and evaluates it as needed for the specific environment. If we're going to go through the effort to provide an alternative input format, I think we should strive for this approach because it will be more flexible in the long run. A pure specification format will allow easy loading/saving by other tools, IDEs, etc., without having to process any imperative logic. > Actually, I'm directly using the cmMakefile, because I did not want to wrap > all > the commands, and it seemed backwards to me to wrap them. Yes. Any alternative format should be processed directly into the structures used by the generators. The cmState work has separated the generate-time representation quite a bit from the configuration-time (cmake-language-specific) representation, but I think there is still further work needed to finish that. >> Having said all that, Brad favors Lua I believe, and he favors a different >> approach (which no one is working on as far as I know) to adding a new >> language. So wait to hear from him to know whether it is something that >> would be upstreamable. > > Have any details on the approach in question? See above. Lua has come up several times in the past in particular because its implementation is meant to be small and embeddable. I've thought a few times about how to make Lua scripting available from within the CMake language in a clean way, but that will not be as valuable as the above pure-spec approach. > Here is what my test POC looked like for generating a simple shared library: > > #!/usr/bin/env python > # -*- coding: utf-8 -*- > importcmake > > cmake.init("Ninja","/media/dev/src/cmaketest","/media/dev/build/cmaketest") > myProject=cmake.Project("MyTestProject") > myProject.targets=[cmake.SharedLibrary("testLibrary",["lib.cxx"])] > cmake.generate() I do not think we should have the build specification depend on processing code like this. It is not compatible with cmake-gui where the configuration and generation steps are triggered by the user. However, this does serve as a good POC that we can populate the generator structures with another language. In summary, I think work in this direction should first focus on designing a declarative (or functional) specification format where most of the project information can be specified. Then a cmake-language command can be written to load and evaluate a specification file (as a transition). Finally we could look at replacing the entry-point language with something else. At that point we could have closures passed as parameters to the evaluation of the pure spec in order to get custom generate-time logic. Thanks, -Brad -- Powered by www.kitware.com Please keep messages on-topic and check the CMake FAQ at: http://www.cmake.org/Wiki/CMake_FAQ Kitware offers various services to support the CMake community. For more information on each offering, please visit: CMake Support: http://cmake.org/cmake/help/support.html CMake Consulting: http://cmake.org/cmake/help/consulting.html CMake Training Courses: http://cmake.org/cmake/help/training.html Visit other Kitware open-source projects at http://www.kitware.com/opensource/opensource.html Follow this link to subscribe/unsubscribe: http://public.kitware.com/mailman/listinfo/cmake-developers