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  • Jan 31 15:06

    ldionne on gh-pages

    Update benchmarks to 490dbf6 fo… (compare)

  • Jan 24 07:47

    ldionne on gh-pages

    Update benchmarks to 490dbf6 fo… (compare)

  • Jan 23 02:04
    pthom commented #432
  • Jan 18 12:46

    ldionne on gh-pages

    Update benchmarks to 490dbf6 fo… (compare)

  • Jan 16 22:27
    ricejasonf commented #434
  • Jan 05 06:22
    ricejasonf commented #330
  • Jan 03 11:40

    ldionne on gh-pages

    Update benchmarks to 490dbf6 fo… (compare)

  • Jan 02 00:33
    ricejasonf closed #434
  • Jan 02 00:33
    ricejasonf commented #434
  • Jan 02 00:04
    ricejasonf opened #434
  • Dec 27 2018 13:11

    ldionne on gh-pages

    Update benchmarks to 490dbf6 fo… (compare)

  • Dec 22 2018 11:56
    pthom commented #432
  • Dec 22 2018 11:55
    pthom commented #432
  • Dec 21 2018 15:48
    pthom synchronize #432
  • Dec 21 2018 09:19
    sdebionne opened #433
  • Dec 21 2018 00:08
    ricejasonf commented #432
  • Dec 21 2018 00:03
    ricejasonf commented #432
  • Dec 20 2018 23:36
    pthom commented #432
  • Dec 20 2018 23:36
    pthom commented #432
  • Dec 20 2018 23:13
    ricejasonf commented #432
Jason Rice
@ricejasonf
It would be handy to have a version of partial or capture that captures by reference.
Barrett Adair
@badair
Indeed!
Maik Klein
@MaikKlein
okay it seems clang3.7 on arch is broken
Louis Dionne
@ldionne
@ricejasonf Technically, you can achieve this with std::ref, but you’ll end up with reference wrappers instead of actual references.
Jason Rice
@ricejasonf
@ldionne Yes, I'm going to play around with that.
Louis Dionne
@ldionne
References are damn pests. They are very useful, but they just don’t behave like objects… Same problem in the STL.
Barrett Adair
@badair
I agree 100%. reference_wrapper is a decent bandaid, but the lack of ability to :: them makes them unappealing for metaprogramming.
The fact that you dont get implicit conversions on the SR operator isn't reference wrapper's fault, though
Louis Dionne
@ldionne
SR operator?
Barrett Adair
@badair
Scope resolution? Thats what I've always called it. ::
Louis Dionne
@ldionne
Oh, I couldn’t understand the accronym
Barrett Adair
@badair
Sorry, on mobile
Louis Dionne
@ldionne
No problem
Jason Rice
@ricejasonf
I played around with hana::partial and std::ref. It yielded nothing other than a slightly slower compile time. I also forgot that my stuff is, for the most part, empty. (Although the whole tree probably adds up to like 50-100 bytes)
Jason Rice
@ricejasonf
It also couldn't be used in only some of the constexprs which was weird. https://gist.github.com/ricejasonf/78f62e74d9dec0d72bbf
Barrett Adair
@badair
Are you familiar with the implementation of std::ref/cref and std::reference_wrapper?
If not, it might bring some clarity to issues you encounter. It's been some time since I've read the specs on them, but they are actually really simple
Barrett Adair
@badair
Is the code you posted
(Sorry, didn't mean to send that)
Barrett Adair
@badair
I honestly don't see a way to implement a perfect forwarding version of hana::partial without resorting to heavy type manipulation and lots of additional code I think
(Man this chat thing is touchy)
...I think it's probably in Hana's best interest to document hana::partial as a value-only tool, and to use a reference_wrapper for references
Barrett Adair
@badair
And to forget about a perfect forwarding partial. I know how I would implement it, but it would be a lot of code for an edge use case. The idea of reference forwarding in a partial application is kind of silly, since the reference needs to be in scope anyway.
TLDR; +1 for reference_wrapper
Barrett Adair
@badair
I will try to implement a perfect forwarding partial application in my project when i finish the changes I'm in the middle of making. I will let you know if I succeed
Louis Dionne
@ldionne

I think a perfect forwading partial would be easy to achieve. Go look at the rest of partial's implementation, but we would most likely need the following only:

template <std::size_t ...n, typename F, typename ...X>
struct partial_t<std::index_sequence<n...>, F, X...> {
    // ... same as before ...

    // Here, instead of storing actual values, store references
    basic_tuple<F, X&&...> storage_;

    // ... same as before ...
};

So I don't think the problem is the implementation complexity, but rather the use case we'd have for this, and how to make this variant of partial available without making the interface more complex (so without adding partial_ref).

Barrett Adair
@badair
how would you put lvalue references in the tuple above? X&& is not a universal reference - right?
Thats the heart of the problem, afaik
Louis Dionne
@ldionne
I think it would work if make_partial did not decay the arguments
Since X would be a reference type (some T&), and then X&& would be T& &&, which collapses to T& by reference collapsing rules.
Barrett Adair
@badair
That makes sense
Same logic as std::forward
Louis Dionne
@ldionne
Yyes.
Barrett Adair
@badair
Wouldn't you still need to move-construct from rvalue references? For safety? And then re-move on the re-forward?
Or probably const ref on the re-forward
That's a difficult decision, since you can't bind crefs to rrefs, but you can do the opposite. So it depends on the signature of the hosted callable
Right?
Did that make sense?
Louis Dionne
@ldionne
partial would only hold references, and it would assume that whatever the references point to are in scope for the whole lifetime of the partial object. So there would be no need to move-construct any actual objects. One way to think of it is like if partial was holding pointers to external objects. No need to move construct anything there.
Now, if you had const refs and you did not use decay when creating partial_t<…>, you would end up with T const& &&, which collapses to T const&, and it would still work. At least, AFAICT.
Note that we wouldn’t be using std::cref or std::ref at all to implement this.
Barrett Adair
@badair
When are destructors called on prvalues? E.g. the second argument to hana::partial(f, foo{})
That was my concern
You know more about storing references to rvalues than I do.
Barrett Adair
@badair
And yes, we are on the same page - i meant for "cref" to be shorthand for const lvalue reference, not the library function. That was confusing
Louis Dionne
@ldionne
Ok, in hana::partial(f, foo{}), hana::partial would store a rvalue-reference to the foo{} temporary. Of course, this reference is valid exactly until the temporary is destructed. However, the lifetime of a temporary is until the end of the enclosing expression. So, for example, consider some_algorithm(sequence, hana::partial(f, foo{})). hana::partial holds a rvalue reference to the foo{} temporary. Then, this partial is used by some_algorithm, which is fine because the temporary is still in scope. Then, the algorithm returns and the temporary goes out of scope, which is fine.
Basically, what you couldn’t do if we stored rvalue references in partial is the following: auto p = hana::partial(f, foo{}); use(p);. This is because by the time p is used (and hence the rvalue reference to foo{} accessed), foo{} has gone out of scope.
Barrett Adair
@badair
Ok, that's what I thought. So you just wouldn't keep the partial object around. Got it
Louis Dionne
@ldionne
Right.
That is, at least, my understanding of temporary lifetime.