n-ary transform function

[grisumbras]

The way I see it, it should be something like system::invoke that invokes a callable with N (0+) arguments of which some can be results. In addition, for convenience it should invoke the callable similar to what std::invoke does.

Example

Foo
Processor::compose_abc(A const&, B const&, C const&);

system::result<Foo>
read_config(Processor& p, Config const& config)
{
    auto a = maybe_get_a(config);
    auto b = maybe_get_b(config);
    auto c = maybe_get_c(config);
    return system::invoke(&Processor::compose_abc, p, a, b, c);
}

One interesting question is whether result<void> arguments should be allowed. The semantics would be to ignore them when invoking the callable, but that would complicate implementation.

[pdimov]

I assume the error types would need to match?

[grisumbras]

Yes, definitely.

[pdimov]

https://godbolt.org/z/qencc7cnz

[grisumbras]

Looks pretty good. It doesn't do the invoke thing that allows using pointers to members. Is that planned?

Also, I requested nullary function support, but now I don't remember what for.

[pdimov]

Member function support is something I need to add to the "monadic" operators as well. I'll figure something out.

[pdimov]

It strikes me that in addition to unwrap_and_invoke it would also be nice to have unwrap_and_construct, to avoid the need to obtain the construct<T> helper from somewhere.

This enables things like

    friend result<X> tag_invoke( boost::json::try_value_to_tag<X> const&, boost::json::value const& jv )
    {
        return unwrap_and_construct<X>(
            boost::json::try_value_to< decltype(X::a) >( jv.at("a") ),
            boost::json::try_value_to< decltype(X::b) >( jv.at("b") )
        );
    }