[cmake-developers] find_package module-only policy

[Brad King]

On 2/20/2012 5:43 PM, Brad King wrote:
> The new find_package option will be called "CONFIG", please.

Another possibility is to require the mode keywords to come as the first
argument, before the package name, just like many other commands:

   find_package(CONFIG Foo)

That will clearly distinguish it from other options that affect the
settings within Config mode.

> I'm still not happy about warning without a CONFIG or NO_MODULE
> keyword. I think the message wording can be made good enough to
> solve this problem, especially when the new CONFIG and MODULE
> keywords can be used *optionally* to get even better messages.

Let's focus first on the wording of the current message and see
what we can get since it's needed either way and we can always
take step (3) later.

Here is another idea for the message wording for the default mode
when no Find module is present.  It explains exactly why CMake is
looking for a package configuration file and who is expected to
provide what:

  CMake Error at CMakeLists.txt:7 (find_package):
    By not providing "Findecm.cmake" in CMAKE_MODULE_PATH the caller
    has asked CMake to find a package configuration file provided by
    "ecm", but CMake did not find one.

    Could not find a package configuration file provided by "ecm"
    with any of the names:

      ecmConfig.cmake
      ecm-config.cmake

    Add the installation prefix of "ecm" to CMAKE_PREFIX_PATH or
    set "ecm_DIR" to a directory containing one of the above files.
    If "ecm" provides separate development package or SDK be sure
    it has been installed.

The first paragraph explains how CMake is interpreting the intention
of the caller.  This puts the blame or credit at the call site in
the project code where it belongs both when it is a bug and when it
is intentional.  It can be dropped in explicit CONFIG mode:

  CMake Error at CMakeLists.txt:7 (find_package):
    Could not find a package configuration file provided by "ecm"
    with any of the names:

      ecmConfig.cmake
      ecm-config.cmake

    Add the installation prefix of "ecm" to CMAKE_PREFIX_PATH or
    set "ecm_DIR" to a directory containing one of the above files.
    If "ecm" provides separate development package or SDK be sure
    it has been installed.

In explicit MODULE mode the message can leave out the package config
file text altogether (we can add more detail too):

  CMake Error at CMakeLists.txt:7 (find_package):
    No "Findecm.cmake" module is present in CMAKE_MODULE_PATH.

-Brad