[cmake-developers] CMake alternative language

[yann suisini]

Hi,

I'm a new user of CMake, but I just want to express my newcomer point of
view.
Honestly , I can feel the power of CMAKE, but it's a real pain to learn ...
I'm using CMAKE for an embedded platform with a non GNU compiler , ant at
the end the CMAKE description is longer than the one I built directly in
Ninja.
I had to write a python script to parse my eclipse project xml to create a
list of sources files usable by CMAKE.
The first thing I thought was: why this is not a part of cmake ? And the
second thing was : why not using the scripting power of an existing
language like Python(or other one)
and add CMAKE as a framework / library ?
Probably a dumb question ! :)

Yann

2016-01-13 10:34 GMT+01:00 Charles Huet <charles.huet at gmail.com>:

> Hi,
>
> > * There is a lot of code out there in the current CMake language so I do
> not
>   think it is realistic to drop it.  I'm not proposing that this change.
>
> I am. (more below)
>
> > * Many projects build elaborate macro/function systems in the CMake
> language
>   in order to end up with a declarative specification listing the actual
>   source files, dependencies, and usage requirements.  I'd like to offer
>   an alternative to this.
>
> In my experience, most of the elaborate macros/functions come either from
> a misunderstanding of some of CMake's internals (scope, link debug/release
> libs, etc) or to circumvent some shortcoming of the CMake language (e.g. no
> return value for functions).
>
> >  I'd like to improve this by *optionally* moving part of the
> specification
>   to a (stateless) declarative format that IDEs can load/edit/save directly
>
> Split the buildsystem in two different languages ? Would the declarative
> part be in a different file ?
> Also, the declarative part in my opinion must take advantage of the
> language.
> For instance, add a source file only for WIN32 systems should be easy in
> said declarative format.
> Using a custom language (based on JSON for instance) would mean to add
> conditionals, which comes back to making a custom language again.
>
>
> To come back to my first point, I understand completely that this would be
> a tremendous change, and the transition would be difficult to say the
> least. But I think it would be more than worth it in the long term.
>
> > The moment you make CMake scriptable in more than one language, you are
> forcing
> > every CMake user to learn that additional language because sooner or
> later he
> > will step into a third-party that is using that additional language.
>
> What I have in mind is to deprecate the current CMake language and replace
> it with another language. So there would be a transition period, but in the
> end there would only be one language again, and a better one.
>
> If CMake transitioned to python (or Lua, or whatever) newcomers to CMake
> would not need learn a new language (or at least learn one that has many
> resources, and can be used for other purposes).
> Old-timers would have to learn a new language (or maybe not, most
> programmers I know have played a bit with python to automate simple tasks),
> but this would be easier than learning CMake was, since there are
> established rules and a more consistent design to it.
>
> Of course I'm not saying this should happen overnight, nor am I saying
> this *must* happen, but I think discussing it can only be beneficial.
>
> I've seen lots of people wonder how to make their CMake scripts more
> efficient, and currently this is a very difficult thing to do, since there
> is no profiling possible.
> And who can say they never spent way too much time trying to understand
> why a variable was not correctly initialized ? When the configure step
> takes about 30 seconds, and all you can do is use MESSAGE() to find what
> happens, this is no walk in the park. A real debugger would do a world of
> good to CMake.
> I have seen some hardly understandable CMake code, and only thanks to the
> git history was I able to understand that the person who wrote the script
> completely misunderstood the CMake language.
>
> >As discussed above if some kind of callback or user-coded function needs
> to
> be included for advanced usage of the declarative spec then we would need
> a language for it.  The current CMake language is not well suited to that
> use case (e.g. no expressions or return values), so an existing alternative
> language should be chosen.
>
> >CMake's current "generator expressions" fill this role somewhat now and
> are
> essentially a sub-language.  As with the main language they grew out of
> something not intended to serve their current full role.  They could be
> superseded by a common alternative generate-time language too.
>
> These points are part of the reason I think a new language should be used,
> if it can cover all of these issues. I'd rather not see a new CMake
> declarative language that might itself grow out and become something
> difficult to grasp.
> Something like generator expressions could be expressed in a language such
> as python or lua, by using objects that get resolved at generate time (or
> functions, or whatever).
>
>
> Cheers
>
> Le lun. 11 janv. 2016 à 21:53, Brad King <brad.king at kitware.com> a écrit :
>
>> Hi Folks,
>>
>> I'm replying directly to my previous post in this thread in order to
>> consolidate
>> responses to related discussion raised in others' responses to it:
>>
>>
>> http://thread.gmane.org/gmane.comp.programming.tools.cmake.devel/15339/focus=15383
>>
>> http://thread.gmane.org/gmane.comp.programming.tools.cmake.devel/15339/focus=15386
>>
>> http://thread.gmane.org/gmane.comp.programming.tools.cmake.devel/15339/focus=15389
>>
>> General comments:
>>
>> * There is a lot of code out there in the current CMake language so I do
>> not
>>   think it is realistic to drop it.  I'm not proposing that this change.
>>
>> * CMake's procedural/imperative design is good as the main entry point to
>>   configuration of a project.  It can do system introspection, file
>> generation,
>>   etc.  I'm not proposing that this change.
>>
>> * Many projects build elaborate macro/function systems in the CMake
>> language
>>   in order to end up with a declarative specification listing the actual
>>   source files, dependencies, and usage requirements.  I'd like to offer
>>   an alternative to this.
>>
>> * Integration with IDEs is currently based on one-way generation (VS IDE
>>   projects, Xcode projects, CodeBlocks, etc.).  Editing the project build
>>   specification requires editing CMake code directly because IDEs cannot
>>   easily pierce CMake's procedural/imperative specification:
>>
>>     https://cmake.org/pipermail/cmake-developers/2016-January/027386.html
>>
>>   I'd like to improve this by *optionally* moving part of the
>> specification
>>   to a (stateless) declarative format that IDEs can load/edit/save
>> directly.
>>
>> Specific responses follow.
>>
>>
>> --------------------------------------------------------------------------------
>>
>> On 01/11/2016 12:24 PM, Charles Huet wrote:
>> > I think these goals aim towards a faster configure, and the ability to
>> > only partly reconfigure, right?
>>
>> Yes.
>>
>> > I know I am largely biased by the project I work on, but I do not see
>> how
>> > parallel evaluation woud be a huge benefit.
>> [snip]
>> > And how would that work with CMakeLists that affect their parent scope ?
>>
>> Evaluation of the imperative language is currently serial for reasons like
>> this, which is why I said it would take semantic changes to enable
>> parallel
>> evaluation. This is not the main point of my proposal so I'd rather not
>> get bogged down in the details of this part of the discussion.
>>
>> >> 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).
>> >
>> > I'm not sure I understand how this could be done without losing a lot of
>> > what CMake offers, such as copying or generating files.
>>
>> I'm not proposing dropping the current imperative capabilities.
>>
>> > I'm leaning towards a declarative approach as it is quite easy to learn
>> > (since you declare objects, and every C++ programmer I know is familiar
>> > with those)
>>
>> Yes.
>>
>> > It seems you are leaning towards pure functional, but I do not see how
>> > this would work with the current way CMake handles variables and scope,
>> > could you elaborate ?
>>
>> While declarative may get us most of the way, advanced users may wish to
>> hook in to generation-time evaluation. A clean way to do that would be
>> to specify a function within the declared values. It would not have to
>> be in a functional language as long as it has no access to anything other
>> than the inputs passed to it during evaluation.
>>
>> I mentioned "functional" mostly as an example of a specification whose
>> evaluation is free of side effects.
>>
>> > To clarify, only the following lines should be considered when looking
>> at the POC.
>> >>     myProject=cmake.Project("MyTestProject")
>> >>     myProject.targets=[cmake.SharedLibrary("testLibrary",["lib.cxx"])]
>>
>> Yes, this is the kind of stuff that can be in a declarative format.
>>
>> > It seems you have in mind to write a new CMake language.
>>
>> No, at most a new specification format that can be used for IDE
>> integration.
>> If some kind of user-coded function were included in the specification it
>> should certainly be in an existing language.
>>
>> > Maybe I should take my POC further
>>
>> I think implementation even of a POC is premature at this point.  We
>> should explore the design space further.
>>
>> > CMake's own buildsystem seems like a good testing ground for this, but
>> > it is a little too big for a first go, do you know of a small
>> CMake-based
>> > project that would be better suited ?
>>
>> Maybe you could find something in our test suite.
>>
>> > I don't have a clear view of what a pure functional CMake would look
>> like,
>> > but if you give me some mock code, I could give a try at bringing some
>> pure
>> > functional language up to the level of my POC and we could use it as a
>> more
>> > concrete discussion support.
>>
>> I have no prototype (nor substantial time to spend on design myself) but
>> I've imagined a declarative format in a well-known syntax (e.g. JSON or
>> one
>> of the lightweight human-friendly choices).  If generate-time
>> functionality
>> is needed then code snippets in e.g. Lua could be included.
>>
>>
>> --------------------------------------------------------------------------------
>>
>> Petr Kmoch wrote:
>> > I'd like to voice my opinion as a somewhat advanced CMake user here.
>>
>> Thanks for joining the discussion with this point of view.
>>
>> > For me, one of the strongest points of CMake is the fact that its
>> project
>> > specification is procedural rather than declarative.
>>
>> Good. We will not be dropping imperative capabilities.
>>
>> > end result of our framework is that the CMakeLists consist mostly of
>> > declarative commands from our framework
>>
>> Yes, many projects have such frameworks. Most of them result in a
>> declarative spec inside calls to their macros/functions. I'd like to
>> formalize such specs in a re-usable 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.
>>
>> Not quite. Yes, the procedural/imperative part would still be the entry
>> point as it is now. However, the declarative part would also be an
>> input, not an output. The procedural part's role would be to compute
>> *parameters* to be used for the evaluation of the declarative spec.
>>
>> For example, imagine the declarative spec somehow encodes that source
>> file "foo.c" is optional based on some condition "FOO". The value of
>> that condition could be computed by the procedural part of the process
>> based on system introspection, command-line options, etc., and then
>> provided to CMake for use in the final evaluation.
>>
>>
>> --------------------------------------------------------------------------------
>>
>> On 01/11/2016 01:21 PM, Pau Garcia i Quiles wrote:
>> > The moment you make CMake scriptable in more than one language, you are
>> forcing
>> > every CMake user to learn that additional language because sooner or
>> later he
>> > will step into a third-party that is using that additional language.
>>
>> I don't think the main "entry point" language should be selectable.
>>
>> As discussed above if some kind of callback or user-coded function needs
>> to
>> be included for advanced usage of the declarative spec then we would need
>> a language for it.  The current CMake language is not well suited to that
>> use case (e.g. no expressions or return values), so an existing
>> alternative
>> language should be chosen.
>>
>> CMake's current "generator expressions" fill this role somewhat now and
>> are
>> essentially a sub-language.  As with the main language they grew out of
>> something not intended to serve their current full role.  They could be
>> superseded by a common alternative generate-time language too.
>>
>> > Also, declarative? Why?
>>
>> See above.
>>
>> > There are already a few declarative build systems
>>
>> Yes, and perhaps we can learn from their formats for the proposed
>> declarative
>> part.
>>
>> > qbs, one of the reasons for its existence was CMake was not declarative
>>
>> IIUC that is exactly because of the fact that an imperative spec cannot
>> be pierced easily for editing in IDEs.
>>
>>
>> --------------------------------------------------------------------------------
>>
>> Thanks all for the discussion so far!
>>
>> -Brad
>>
>> --
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>
> Powered by www.kitware.com
>
> Please keep messages on-topic and check the CMake FAQ at:
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> information on each offering, please visit:
>
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