[gsliepen]

Rust's FromStr only deals with parsing a single object. However, ideally std::scan() would be an exact counterpart of std::print() and would be able to parse multiple objects. I totally agree that the C way of passing references to already existing variables is not great. Ideally you return a tuple of objects, but then it becomes very annoying to specify the types. Maybe something like this?

    auto [value, text, goose] = std::scan<int, std::string, Goose>(input, "{} {} {}");

A halfway solution would be to have the hypothetical std::scan() take references to std::optional<>s or std::expected<>s:

    std::optional<int> value;
    std::optional<std::string> text;
    std::optional<Goose> goose;
    /* auto result = */ std::scan(input, "{} {} {}", value, text, goose);

The latter would be type safe, close to how scanf() works, but less satisfying from a functional programming standpoint.

Orthogonal to that, adding support for scanning a Goose would be just like how you add a formatter for it, and would be quite similar to a Rust trait. One could imagine having to define something like this:

    template<>
    struct std::scanner<Goose> {
        constexpr auto parse(std::format_parse_context& ctx) {…}
        auto scan(std::format_context& ctx) const -> std::optional<Goose> {…}
    };