module;            // starts a global module fragment

#include <string>

export module foo; // ends a global module fragment
                   // declares the primary module interface unit for named module 'foo'
                   // starts a module unit purview

export std::string f();

module : private;  // ends the portion of the module interface unit that
                   // can affect the behavior of other translation units
                   // starts a private module fragment

std::string f()
{
    return "foo";
}