boostorg/hana

Your standard library for metaprogramming

Jason Rice

@ricejasonf

using Foo = typename my_type::type;

Do your As and Bs have run-time state?

Barrett Adair

@badair

yes

which is what makes Hana so perfect for my use case

Barrett Adair

@badair

this is what I ended up doing:

// todo this shouldn't be necessary
template<typename T>
struct type_v {
    T value;

    template<typename U>
    constexpr auto equal(U) const {
        return std::is_same<T, U>{};   
    }
};

Jason Rice

@ricejasonf

If it was me I would do hana::pair<hana::type<T>, hana::pair<T, V>>

Barrett Adair

@badair

I did that too

Jason Rice

@ricejasonf

was that no good?

Barrett Adair

@badair

I did both at the same time and made a TODO because I didn't implement Hashable

Jason Rice

@ricejasonf

Hashable is only used to decide what bucket a key/value will go in. It still uses Comparable to decide if the key is a match

Barrett Adair

@badair

Sure, but both are required if buckets are needed

Or, both are required

Jason Rice

@ricejasonf

The default for hana::hash is to just lift it to its hana::type

Barrett Adair

@badair

So by default everything is Hashable?

Jason Rice

@ricejasonf

yup :P

Barrett Adair

@badair

I was getting a weird error about something not being hashable

Jason Rice

@ricejasonf

oh.. maybe I am mistaken

I guess there are only implementations for IntregralConstant, type, and string... maybe others

Jason Rice

@ricejasonf

That means a group_by algorithm would have to return a value that is bothComparable and Hashable

If it was Orderable it could be sorted too

Pramod Srinivasan

@domarps

hey everyone, I just posted my issue about a boost python issue here : https://stackoverflow.com/questions/45576378/boost-python-importerror-undefined-symbol. It would be great if I can get feedback on how to resolve this issue.

Jason Rice

@ricejasonf

This is just a stab in the dark, but it appears that you are not actually linking the library. -L merely adds a directory for the compiler driver to search for libraries (I think).

gcc file1.c -Llibs -lmylib -o outfile

@domarps ^

Pramod Srinivasan

@domarps

I didn't understand that comment @ricejasonf. I believe it is an issue with conflicting linkages between different versions of python

Jason Rice

@ricejasonf

I think you need -lboost_python-py34. The -l option is used for linking but you don't have that which explains the undefined reference.

err.. just -lboost_python

Louis Dionne

@ldionne

@domarps Please note this is a channel for Hana, not for general Boost questions. Please direct such questions to the boost-users mailing list.

Jason Rice

@ricejasonf

Searchable does not refine Foldable

Louis Dionne

@ldionne

Sorry, I know I’m a ghost. I’ll try to reply to the issues within a few days, but I’ve been rushing to prepare a talk for tonight.

Jason Rice

@ricejasonf

I figured you are very busy. Sorry if I am being spammy. I found some other stuff to occupy me while that stuff ferments. :D

Post a link to the video of your talk if there will be one.

I'm sure it would be interesting

Louis Dionne

@ldionne

No you’re not spammy at all, I really appreciate the help and involvement.

Jason Rice

@ricejasonf

:+1:

Louis Dionne

@ldionne

There won’t be a video of the talk since this is a local C++ meetup. It’s my CppCon talk basically.

Jason Rice

@ricejasonf

static_assert(sizeof(hana::tuple<hana::int_<42>>) == 1); Is that expected?

Louis Dionne

@ldionne

Yes. What would you expect? That’s the smallest size it can have, right?

An object can’t have 0 bytes in C++.

Jason Rice

@ricejasonf

Ah, that's right. How about this then:

  static_assert(std::is_empty<hana::tuple<>>{});
  static_assert(!std::is_empty<hana::tuple<hana::int_<42>>>{});

Intuitively, I was expecting the latter to be empty because of ebo. Perhaps I need my own has_runtime_state<T> thing.

Louis Dionne

@ldionne

Ehm, I would expect it to be empty.

I think this would be fixed by boostorg/hana#333

Jason Rice

@ricejasonf

Oh, maybe it's not using ebo. ??

    template <typename ...Xn>
    struct tuple
        : detail::operators::adl<tuple<Xn...>>
        , detail::iterable_operators<tuple<Xn...>>
    {
        basic_tuple<Xn...> storage_;
        using hana_tag = tuple_tag;

Louis Dionne

@ldionne

basic_tuple is using EBO, but it’s missing some opportunities for it. I forgot the exact details since it’s been a while.

Jason Rice

@ricejasonf

Would it make sense to put basic_tuple in a storage wrapper and derive from that in tuple?

Also, would promoting basic_tuple to Sequence be a welcome as a PR?

Jason Rice

@ricejasonf

err not sure if that would be very valuable

Louis Dionne

@ldionne

Also, would promoting basic_tuple to Sequence be a welcome as a PR?

It already is.

Would it make sense to put basic_tuple in a storage wrapper and derive from that in tuple?

Hmm. Interactions with EBO are extremely complex due to the conversions they suddenly enable. I’ll have to dive into it again to fix the issue properly.

If, however, you can make the test cases added in #333 pass with a simpler implementation, I think I’d be happy.

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