boostorg/hana

Your standard library for metaprogramming

Jason Rice

@ricejasonf

nor should you need to "remove reference"

err the "remove reference" comment I was looking at the old code in the chat

hana::at_key(result, MemberName{})

Hoss Ajallooiean

@katetsu

i tried that

but still geetting errors

auto& member = boost::hana::at_key(result, mp_at_c<decltype(member_name_var), I>);

I being the index

https://godbolt.org/z/Ezd6ad

Jason Rice

@ricejasonf

break that line up

using MemberName = the error will be more obvious

you are passing a type where an expression is expected

Hoss Ajallooiean

@katetsu

exactly

Jason Rice

@ricejasonf

hence MemberName{} <- note the default construction

Hoss Ajallooiean

@katetsu

i see

ok so the default ctor creates the compile time string value?

vs the type is compile type hana string?

Jason Rice

@ricejasonf

same as decltype("timer"_s){}

or hana::string<'t', 'i', 'm', 'e', 'r'>{}

Hoss Ajallooiean

@katetsu

ok got it thanks a lot:)

Hoss Ajallooiean

@katetsu

hi @ricejasonf, do you have any idea what this compiler error means? https://godbolt.org/z/f1MPT3

Jason Rice

@ricejasonf

the key is not in the map

when calling at_key

internally it uses find which returns an optional hence the weird error

the key you are passing it my::not_found

Jason Rice

@ricejasonf

since you are not using I for anything other than getting the value inside the variant...

why not just use std::visit

you could use hana::overload_linearly and give a lambda that takes a my::not_found as its parameter and then a generic lambda that handles the compile-time string

Hoss Ajallooiean

@katetsu

Thanks a lot.

Adam Boseley

@aboseley

Is it possible to extract the types from a std::variant?
https://godbolt.org/z/oddP5T94G

Adam Boseley

@aboseley

This looks to work. Is there a better way

template <typename... Types> 
constexpr auto getVariantTypes(std::variant<Types...>&& v){
  return hana::tuple_t<Types...>;
}

Jason Rice

@ricejasonf

There's also Boost.Mp11 mp_rename<Var, hana::tuple>. It depends on what you want to do with the types.

Adam Boseley

@aboseley

I'd like to extract the types out of a std::variant, and put then the equivalent of a hana type list. The mp_rename shuffles the types from the variant to a hana::tuple, but I think I still need to wrap them in a hana::type_c. Which I struggling to do. This is easy to do for boost::variant but there isn't matching support for std::variant that I can see

Jason Rice

@ricejasonf

I suggest not using tuple_t. hana::experimental::types does the same thing without wrapping individuals types in type as that tends to create a lot of unnecessary type instantiations. If all you are doing is type computations then you don't want to use hana::tuple either as that has several member templates that are expensive to instantiate (relatively).

If you really wanted to you could do mp_transform<hana::type, mp_rename<Var, hana::tuple>>>

Adam Boseley

@aboseley

Thanks for the advise!

Adam Boseley

@aboseley

When introspecting on user defined types, is there a way to get back the string "name" of the struct, we pass in as the first to parameter to BOOST_HANA_ADAPT_STRUCT, like we do for each of its members?

Jason Rice

@ricejasonf

no, I think you would have to roll your own macro for that

There is this: https://github.com/Manu343726/ctti

...or just adding a static-constexpr char const* name manually

Adam Boseley

@aboseley

Thanks again. I've actually been adding a static-constexpr string_view to the parent structure to hold the name. It just duplicates what I'm already passing to the macro, and wanted to make sure I wasn't missing so other hana magic for getting it back.

Enrico Maria De Angelis

@Aster89

Hi, here's a question that's been puzzling me for a while: https://stackoverflow.com/questions/67858207/is-there-a-way-to-express-the-function-application-operator-function-with-hana

Gleb Popov

@arrowd

Just wanted to share your question here, but you already did that.

Enrico Maria De Angelis

@Aster89

@arrowd , my question? :D

Gleb Popov

@arrowd

Yep.

Enrico Maria De Angelis

@Aster89

Good, if I'm lucky someone will offer a bounty on it :D

Enrico Maria De Angelis

@Aster89

@arrowd , I've added a few more bits to the story too

It'd be nice to have Louis Dionne's opinion on it :D

Pavel Krasnopevtsev

@pkrasnop_twitter

I wonder whether it is expected that boost::hana::map equality operator will actually use copy constructor to construct all objects inside it (which is pretty expensive in my case). Apparently this happens because of boost::hana::just which effectively triggers a copy or am I missing something?

Jason Rice

@ricejasonf

Could you post a minimal use case that reproduces this?

Pavel Krasnopevtsev

@pkrasnop_twitter

@ricejasonf , https://godbolt.org/z/Kf5ex7PKE

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