[FEATURE] support kumi::map without a return type

[ldalessa]

Is your feature request related to a problem? Please describe.
When I have tuples of references it makes sense to map them without returning anything. Currently providing an invocable that returns void results in a compilation error.

Describe the solution you'd like
Specialize on the return type of the function object, if void then don't try and produce a tuple.

Describe alternatives you've considered
The current workaround is to return something trivial, like 0.

Additional context
https://godbolt.org/z/b56T3ebjd

[jfalcou]

map is basically transform. I think you want to call for_each : https://godbolt.org/z/vz9Gqdavr

[ldalessa]

In this case I have a zip-like thing happening and really do want map-like functionality.

std::vector<int> a, b, c, d;
kumi::tuple l = { std::ref(a), std::ref(b) };
kumi::tuple r = {std::ref(c), std::ref(d) };
kumi::map([](auto&& l, auto&& r) {
    return l[0] = r[0]; // don't want to return
}, l, r);

[ldalessa]

It's possible that this should just be a separate function named zip_with.

[DenisYaroshevskiy]

I fail to see how

kumi::map([](auto&& l, auto&& r) {
    return l[0] = r[0]; // don't want to return
    }, l, r);

is not:

kumi::for_each([](auto&& l, auto&& r) {
   l[0] = r[0];
 }, l, r);

Is that you want it to be called map? I mean, it's not rly map.

[ldalessa]

If for_each supports zipping then that’s what I want. I don’t want map, that was my original complaint.

I’m unfamiliar with the use of algorithms called for_each that provide this zip functionality. I think it’s more traditionally called zip_with. Now I know what kumi calls it is though, I’m happy.