Re: [boost] Hana Views and References

Subject: Re: [boost] Hana Views and References
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2015-06-27 12:08:55

Le 27/06/15 16:48, Louis Dionne a écrit :
>> Joel de Guzman <djowel <at> gmail.com> writes:
>> [...]
>>
>> Your tests focused on a mere subset of Fusion which is the least
>> optimized: fusion::vector. Your tests do not cover other areas
>> such as views, and other containers.
> Frankly, I thought fusion::vector would be the most efficient container.
> Isn't vector documented as such? Quoting from the documentation:
>
>      vector is the simplest of the Fusion sequence container (a vector with N
>      elements is just a struct with N members), and in many cases the most
>      efficient.
>
>
>> I can imagine a case, when say
>> you push_back or insert where views should shine. It's the difference
>> between copy by value and pass by reference. Try doing that with large
>> tuple elements (those without resources and are not efficiently
>> moved). Actually, have you tried reverse with large tuples like
>> bitmaps and stuff?
>>
>> Hmmmm, how did you test reverse in Fusion BTW? Fusion reverse should
>> return a view with ZERO(!) copy. Did you create another vector from the
>> reverse_view? I guess so, since your test subject is Fusion vector.
>> In that case, you are not doing it right! You don't copy to another
>> vector. You should use the results as-is. Lazily.
>>
>> For that matter, the same is true with transform! Fusion returns
>> transform_view. You should get zero overhead with those because
>> they are lazy. If you are copying back to a fusion vector, then
>> you are not doing it right. Have you tried accessing, say only
>> the Nth element instead of (presumably) copying back to a vector
>> before accessing the Nth element? or how about transforming only
>> a range within the container?
>>
>> It seems you are doing your tests unfairly using eager evaluation
>> (Hana) instead of lazy evaluation (Fusion).
> There seems to be a larger issue here. That issue is: How to benchmark
> lazy computations and eager computations? To illustrate the problem,
> consider the following "benchmark":
>
> transforming a vector in C++:
>
>      #include <algorithm>
>      #include <vector>
>
>      int main() {
>          std::vector<int> xs(1000);
>          std::vector<int> ys; ys.reserve(xs.size());
>          std::transform(xs.begin(), xs.end(), std::back_inserter(ys), [](int
> x) {
>              return x + 1;
>          });
>      }
>
> transforming a list in Haskell:
>
>      import Data.List
>
>      main = do
>          let xs = take 1000 (repeat 0)
>          let ys = fmap (+1) xs
>          return ()
>
> Written this way, the Haskell code will always be faster because it does not
> do anything, since it is lazy. However, that does not tell us much about
> what we're interested in. What we'd like to know is how the above code
> behaves when I _actually_ access the elements of the lazy list. So, of
> course I copy the result into vectors when benchmarking Fusion algorithms,
> since otherwise there's nothing to benchmark!
>
> I think this ought to be explained in the documentation, but I don't think
> it
> invalidates Hana's claims. For equivalent functionality (i.e. when accessing
> all the members of the resulting container, which is assumed to usually be
> the case), Hana will perform as good as Fusion.
>
>
Instead of using a fusion::vector as a return of transform or reverse, 
you could just iterate on the result to e.g. calculate something simple.

Vicente