[cmake-developers] CMake alternative language (was: Using CMake as a library from Python)

[Petr Kmoch]

Hi all.

I'd like to voice my opinion as a somewhat advanced CMake user here.

For me, one of the strongest points of CMake is the fact that its project
specification is procedural rather than declarative. In my current job, for
example, we have a significant framework built in CMake which handles a lot
of unique setups we have (which were largely inherited from a previous
inhouse buildsystem). Yes, the end result of our framework is that the
CMakeLists consist mostly of declarative commands from our framework, but
the implementation of these commands is heavily procedural. I am convinced
that if CMake didn't give us the procedural power required to make this
work, we couldn't have adopted it. (I had previously tried emulating bits
of this setup in a more declarative system and failed miserably).

Of course (having written much of this framework I'm talking about above),
I know all too well that a better front-end language would do a world of
good for CMake. I also understand that taking a more declarative approach
could help that, and I'm not opposed to such a change in principle.
However, please take care not to throw the baby out with the bathwater and
nerf the expressiveness of what can be done with CMake (in a procedural
way).

If I understand Brad's suggestion correctly, it would amount to a (possibly
empty) procedural step being used to generate a declarative description of
the buildsystem. This would work well in our scenario, I believe, as long
as that procedural step could be sufficiently modularised on the client
side.

I fully support introducing an alternative input language to CMake and
taking all steps necessary for this to happen, but please do this in a way
which will not restrict what CMake is capable of doing.

Petr

On Fri, Jan 8, 2016 at 5:30 PM, Brad King <brad.king at kitware.com> wrote:

> 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
>
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