[cmake-developers] Adding argument "OPTIONAL" to find_package() and add_subdirectory

[Alexander Neundorf]

On Tuesday, June 07, 2011 02:34:06 PM Eric Noulard wrote:
> 2011/6/7 Alexander Neundorf <neundorf at kde.org>:
> > On Monday, June 06, 2011 03:26:03 PM Brad King wrote:
> >> On 06/04/2011 06:30 AM, Alexander Neundorf wrote:
> [...]
> 
> >> > What do you think about adding the keyword OPTIONAL to
> >> > add_subdirectory ?
> >> > 
> >> > Both have been proven useful, the one for find_package() especially
> >> > for packagers.
> >> 
> >> Ditto previous response.  These commands are primitives.  We should not
> >> extend them with features unrelated to their basic purpose.
> > 
> > While this is correct, it also keeps cmake a bit low-level.
> > For this reason, we created such macros in KDE.
> > Now our developers write stuff outside KDE, so either they can't use it,
> > or they create copies of these files.
> > This may be ok, but having 50 or 100 versions of these files and macros
> > around in the net, some probably differing slightly, is also not a nice
> > situation.
> 
> Then it is possible to create a new CMake module,
> 
> say
> 
>  UseEnhancedConfigure.cmake
> 
> which could be included in CMake as a contributed module maintained by KDE
> dev. This new module would define something like:
> 
> optional_find_package().
> optional_add_subdirectory()
> 
> this would make the feature available upstream, thus available outside KDE
> and does not add extra feature to builtin configure.

I can't think of any reason why somebody would want to use 
find_package(...without REQUIRED) instead of optional_find_package().

Can somebody else see a reason ?


> > These two OPTION features in this email are IMO features which are useful
> > in many projects and which would make using cmake-based projects easier
> > for users (people compiling the software), since they could expect that
> > if packages can be disabled, that this will be done via an option with a
> > name which always follows the same scheme.
> 
> I totally agree with that but I think it does not implies this has to be
> done by builtin find_package/add_subdirectory ?

Well, it doesn't have to.
But if packagers requested this for something in KDE, they will also want this 
for non-KDE projects.
And I see no reason why this shouldn't be the default for _every_ non-REQUIRED 
package.
This would make the options available for cmake-based projects more 
consistent.
This issue comes up regularly from new users.

So, I don't see a reason why such an option should not be the default 
behaviour and why I should rely on an external extension.

Alex