boostorg/hana

Your standard library for metaprogramming

Jason Rice

@ricejasonf

:heart_eyes: nice! ty

that's an interesting way to initialize a map

Louis Dionne

@ldionne

Yes, quite. I used something similar for a library I was writing for a client recently.

Works well, but it is annoying to have to copy the arguments into the map.

The restriction of not having references in containers is a f*cking pain, but I don’t see an easy way around it. Just like std::vector won’t allow for references.

Jason Rice

@ricejasonf

ah.. so it is not zero cost

Louis Dionne

@ldionne

Well, you can move-in the arguments and it should be pretty-much zero cost. But it’s not zero cost strictly-speaking.

But if you pass arrays as named parameters, they’ll have to be moved (which may be inefficient).

Or you can store the arguments with std::reference_wrappers, but then you’ll have to use args[“a”_param].get() inside the function.

Jason Rice

@ricejasonf

Would it be possible to add that auto operator= to type and integral_constant?

This message was deleted

Jason Rice

@ricejasonf

You mentioned some things about improving map before. I would like to discuss it, when you are not too busy.

Louis Dionne

@ldionne

Would it be possible to add that auto operator= to type and integral_constant?

You mean to create pairs? I think that's clearly out of Hana's scope, since Hana is a general purpose library, not a named parameter library. The right approach would be for you to create wrappers over type and integral_constant. Or more generally, I would probably do something like

template <typename Param>
struct param_t {
    // define operator=
};

template <typename Param>
constexpr param_t<Param> param{};

Then, you can use the thing as

f(param<hana::int_<3>> = "three", param<hana::type<int>> = "int", ...);

The syntactic overhead is very reasonable, and it clearly says what it's doing. Just my .02.

You mentioned some things about improving map before. I would like to discuss it, when you are not too busy.

I'd be happy to discuss it. I'll try to put my thoughts on paper and I'll create an issue documenting them. Then, we can talk about it either here or on GitHub. Sounds reasonable?

Jason Rice

@ricejasonf

:thumbsup:

Jason Rice

@ricejasonf

About the param thing, yeah, creating a wrapper is simple enough and I will do that, but I was speaking to constructing maps in general. (nbd of course)

constexpr auto foo = hana::type_c<int>;
constexpr auto bar = hana::type_c<double>;

hana::make_map(
    foo       = "foo",
    bar       = "bar",
    "foo"_s   = "foo",
    "bar"_s   = "bar",
    5_c       = "five",
    42_c      = "forty-two"
);

that's probably not that useful though

Louis Dionne

@ldionne

But then the following fails, because assignment is not what we expect it to be:

hana::tuple<int, int, hana::type<char>> t{1, 2, {}};
hana::tuple<int, int, hana::type<char>> u{3, 4, {}};
t = u;

Actually, it will even compile but fail to do the right thing. Since type, integral_constant & al are stateless this is OK strictly speaking (assignment has no effect anyway), but I really think this is conceptually wrong.

Jason Rice

@ricejasonf

yeah I guess that would be an abuse :sweat_smile:

Louis Dionne

@ldionne

@ricejasonf See #223 regarding hana::map optimizations.

@ricejasonf It’s not an easy one to tackle, but if you’re willing to give it a shot I’ll provide guidance and help as needed.

Btw, this is likely the single most important improvement that can be done to Hana right now. But it’s a chunk of work.

Jason Rice

@ricejasonf

@ldionne Yes, I definitely want to take a crack at this. :D I'll have to do some digging as to how you are automating benchmarks and their results.

Louis Dionne

@ldionne

Just ask if you need guidance.

Jason Rice

@ricejasonf

Do you know how to get cmake to look for ruby in /usr/local/bin ?

Louis Dionne

@ldionne

Hmm, it should probably look in that place on its own.

1 minute

Jason Rice

@ricejasonf

oh sorry.. I had to remove CMakeCache.txt I think

I was thinking there was a RUBY_ROOT option or something

Louis Dionne

@ldionne

I don’t see any RUBY_ROOT option in the documentation of the FindRuby.cmakemodule. If it does not work ,you can try adding /usr/local/bin to CMAKE_FIND_ROOT_PATH.

Jason Rice

@ricejasonf

heh.. I apt-get remove --purge ruby'd and then realized it was probably just stuck in CMakeCache.txt.

Louis Dionne

@ldionne

Does it work now?

Jason Rice

@ricejasonf

yep

sorry to bother you

Louis Dionne

@ldionne

No problem, really. The benchmarking system isn’t the prettiest thing, so don’t hesitate to let me know if you don’t understand or run into problems.

And if you have ideas to improve it, let me know.

Jason Rice

@ricejasonf

is there a limit to size on your static doc site?

var hana = github.getRepo('boostorg', 'hana');
    hana.getRef('heads/datasets', function(err, sha) {
      var repo = "https://cdn.rawgit.com/boostorg/hana/" + sha + "/release/clang-3.6.2/";
      $(".benchmark-chart").each(function(index, div) {
        var dataset = div.getAttribute("damplement efficiently, because it requires stopping at the first element which satisfies the given predicate. For the same reason, modern techniques don't really help us here, so this algorithm constitutes a good test of the implementation quality of Hana, without taking into account the free lunch given to use by C++14.
          Number of elementsTime (s)Compile-time behavior of findta-dataset");
        $.getJSON(repo + dataset, function(options) {
          Hana.initChart($(div), options);
        });
      });

that div.getAttribute argument looks awfully suspicious

Jason Rice

@ricejasonf

It looks like repo points to an absolute url which means the charts aren't coming from the same build

Louis Dionne

@ldionne

The charts in the documentation are always pointing to the latest charts. I know, this is wrong.

Couldn’t figure out how to do it differently.

But I can’t find that div.getAttribute, where is it?

What this "damplement efficiently, because it require…”?

Jason Rice

@ricejasonf

lol.. I was guessing that it is some kind of code generation bug

an attribute name is like class or style or something :laughing:

Louis Dionne

@ldionne

Where did you find this?

Jason Rice

@ricejasonf

the top of index.html

yeah the prod site actually says var dataset = div.getAttribute("data-dataset");

Louis Dionne

@ldionne

My local build/doc/index.html is also fine, showing ”data-dataset”.

Jason Rice

@ricejasonf

it's also statically set in header.html..

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