The current implementation of append_name_if uses an undefined variable filter.
Maybe we could replace the example by something like that:

<typename FilterFunction>
auto append_name_if(FilterFunction filter) {
  return [filter](std::vector<std::string> previously_collected, const person_t &person) {
    if (filter(person))
      previously_collected.push_back(name(person));
    return previously_collected;
  };
}

// ...
template <typename FilterFunction, typename Iterator>
std::vector<std::string> names_for(
        Iterator people_begin,
        Iterator people_end,
        FilterFunction filter)
{
    return std::accumulate(
        people.cbegin(),
        people.cend(),
        std::vector<std::string>{},
        append_name_if(filter));
}

The assumption here is that filter is a free-standing function available in the scope. This snippet is meant to demonstrate the accumulation, I couldn't use lambdas yet, since they are introduced in the next chapter.

I'll see whether I can make this explicit to avoid confusion.

Thanks for the report!

OK I see. Yes, it would be too soon in chronology to use a function that returns a lambda here.
But, yes, maybe, a declaration of a free-standing function filter could make the code clearer (at least for me ).