[cmake-developers] ExternalProject: Use native paths as substitute for directory tokens

[James Johnston]

> -----Original Message-----
> From: cmake-developers [mailto:cmake-developers-bounces at cmake.org]
> On Behalf Of Kislinskiy, Stefan
> Sent: Thursday, August 27, 2015 13:44
> To: Brad King; CHEVRIER, Marc; David Cole
> Cc: cmake-developers at cmake.org
> Subject: Re: [cmake-developers] ExternalProject: Use native paths as
> substitute for directory tokens
> 
> This is pretty nice from the perspective of an engineer. But I wonder if
it
> wouldn't be kind of over-engineered in the sense that it would be a rather
> hidden feature for the purpose? -- I probably wouldn't think of searching
for
> a generator expression in the documentation when I have trouble with the
> replacement of directory tokens. Then again, there could be a hint in the
> ExternalProject documentation which would be fine I guess. One way or
> another, I would be happy if we could determine what the patch should
> provide exactly so that we come to an end. :-) Thank you for all the
feedback
> so far.

I think Brad raises an excellent point here though I hadn't thought of...
implementing the underlying functionality as a generator expression allows
the problem to be solved outside of the scope of ExternalProject when it
arises...  For example if I add_custom_command as a custom build step and
need to pass a native path argument to it - a path known only at generate
time.  I guess it is not possible right now?

> Another approach is to introduce a generator expression to transform the
> path slash style.  Then it could be used both for ExternalProject and in
other
> custom commands.  E.g.
> 
>  $<PATH_FOR_SHELL:<SOURCE_DIR>/bootstrap${shell_ext}>
> 
> In this case <SOURCE_DIR> would be replaced with forward slashes by
> ExternalProject but then at generate time CMake would evaluate the genex
> to convert slashes as needed.

I would vote naming it TO_NATIVE_PATH instead of PATH_FOR_SHELL, for
consistency with the existing parameter in the file command.  There could
then also be a corresponding TO_CMAKE_PATH gen-exp, although I wonder if it
would be used much.  (Then again, do people even use file(TO_CMAKE_PATH)
very much?) 

James