C++17 Result

A Rust inspired Result<V,E> type for C++17.

template<typename V, typename E>
struct Result;

basically a sophisticated wrapper for:

std::variant< ok<V>, error<E> >

Usage synopsis

auto fn() -> Result<std::ifstream, string>;
auto splitLines(std::stringstream&) -> std::vector<string>;

int main() {
    auto f = openFile("hello.txt")
        .orMap([](auto&&) { return openFile("fallback.txt"); }) // invoked only on error
        .andMap([](auto&& in) -> Result<std::stringstream, string> { // invoked only if successful
            std::stringstream out;
            out << in.rdbuf();
            if (out.str().empty()) return error("empty file"s);
            return ok(move(out));
        })
        .andMap(&splitLines) // adds no errors
        .orMap([](auto) -> Result<std::vector<string>, string> {
            return ok(std::vector<string>{});
        });
    for (auto &l : f.unwrap()) std::cout << l << '\n'; // we are sure we have a value
}

see Result.test.cpp for a working example.

Requirements

You will need a C++17 compiler.

• GCC 8.x
• Clang 8.x + libc++
• MSVC2017/2019
• googletest is optional to run the tests

Links

• github/oktal/result a similar result type for C++14
• doc.rust-lang/std/result the rust result type documentation

Status

Even though this seems to work, it is only a proof of concept.

• it works
• std::variant is unusable slow for this kind of use case.
• std::reference_wrapper does not allow comparisons.