boostorg/hana

Your standard library for metaprogramming

Barrett Adair

@badair

I don't see hash_impl in the documentation -- how do you customize it for your own type?

Jason Rice

@ricejasonf

watch my video :P

you specialize hash_impl for your datatype's tag

the public facing function is just hana::hash

Barrett Adair

@badair

I already did! Don't remember that part. Okay thanks

Jason Rice

@ricejasonf

I'm really curious about the use case you have for it

Barrett Adair

@badair

Let me back up then. Is there a type in Hana that's like hana::type but lets you store a value of that type inside? I didn't find one so I made one myself.

Jason Rice

@ricejasonf

Wouldn't that be just an instance of the value of that type?

Barrett Adair

@badair

Yeah. Does hana::type<T> have a member of type T?

bc that's what I need

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.

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