Idiom for setting a variable in top-level scope

Question

I have a CMakeLists.txt which sometimes serves as a project top-level list file and is sometimes used with add_subdirectory().

Now, for reasons, this file needs to set a variable in the top-level scope. The way I achieve this right now is:

if (PROJECT_IS_TOP_LEVEL)
    set(FOO "bar")
else()
    set(FOO "bar" PARENT_SCOPE)
endif()

but this is verbose and redundant. What would be a better idiom for doing this?