Re: [boost] [Hana] Formal review for Hana

Subject: Re: [boost] [Hana] Formal review for Hana
From: Louis Dionne (ldionne.2_at_[hidden])
Date: 2015-06-14 15:19:13

Paul Fultz II <pfultz2 <at> yahoo.com> writes:

> Here some points I see as I have been reviewing Hana. I see you have made
> some changes on github, so maybe some of this doesn't apply anymore.

The changes I made are only to develop, so whatever you say is still
applicable to master.

> - Dot shouldn't be used in names, underscores should be used instead.

This will be modified, since it was asked for by 3 different persons so far.
For future reference, you can refer to [1] for the status of this issue.

> - `fold` and `reverse_fold` should be preferred over `fold_right` and
>   ``fold_left`. This is more familiar to C++ programmers.

First, to my knowledge, the only libraries that even define fold and/or
reverse_fold are Fusion and MPL, so it's not like there was an undeniable
precedent for using those names instead of something else in C++. But
even then, `fold` and `reverse_fold` functions are provided for consistency
with those libraries, so I really don't see what's the problem. If you
prefer those names, you can use them and they have exactly the same
semantics as their Fusion counterpart.

> - Concepts are capitalized, however, models of a concept should not be
>   capitalized(such as `IntregralConstant`, `Either`, `Lazy`, `Optional`,
>   `Tuple`, etc)

`IntegralConstant`, `Tuple`, etc... are tags used for tag dispatching,
like Fusion's `vector_tag` & friends. I can't use a non-capitalized
version as-is, because it's going to clash with `integral_constant`.
Also, I find that using something like `tuple_tag` is uglier than using
`Tuple`. Consider for example

    make<tuple_tag>(xs...)
    to<tuple_tag>(xs)

    make<Tuple>(xs...)
    to<Tuple>(xs)

I find that using `Tuple` just leads to prettier code. Also, since Hana
does not specify the type of all of its containers, we refer to them by their
tag instead of their (unspecified) type. So for example, I talk of a Hana
`Range`, not a Hana `range`, because the latter is not a type. If I was to
use the name `range_tag` instead of `Range`, I couldn't do that as easily.
Fusion does not have this problem because all of its containers have well
specified types, so you can always talk about a `fusion::vector` and it is
always understood that this includes `fusion::vector3`, but Hana does not
have that.

> - IntregralConstant is very strange. In Hana, its not a concept(even though
>   its capitalized), but rather a so called "data type". Furthermore, because
>   of this strangeness it doesn't interoperate with other
>   IntregralConstants(such as from Tick) even though all the operators are
>   defined.

IntegralConstant is not a concept, that's true. The fact that it does not
interoperate with Tick's IntegralConstants out-of-the-box has nothing to
do with that, however. You must make your Tick integral constants a model
of Hana's Constant concept for it to work. See the gist at [2] for how to
do that.

> - The `.times` seems strange and should be a free function so it works with
>   any IntregralConstant.

The idea of having a `.times` member function comes from Ruby [3].
I personally think it is expressive and a simple tool for a simple job.
The syntax is also much cleaner than using a non-member function. Consider

    int_<10>.times([]{ std::cout << "foo" << std::endl; });
    times(int_<10>, []{ std::cout << "foo" << std::endl; });

However, as we've been discussing in this issue [4], I might add an
equivalent non-member function.

> - In the section 'Taking control of SFINAE', seems like it could be solved a
>   lot simpler and easier using `overload_linear`.

First, `overload_linearly` is an implementation detail, since I've moved
the Functional module out of the way to give me the possibility of using
Fit in the future. However, your point is valid; I could also write the
following instead:

    auto optionalToString = hana::overload_linearly(
        [](auto&& x) -> decltype(x.toString()) { return x.toString(); },
        [](auto&&) -> std::string { return "toString not defined"; }
    );

This approach is fine for the optionalToString function, which is rather
simple. I wanted to show how to use Optional to control compile-time
empty-ness in complex cases, so I'll just expand this section or change
the example to something that is really better solved using Optional.

Thanks for the heads up; you can refer to this issue [5] in the future.

> - Concepts refer to 'superclasses' these should be listed either as
>   refinements or listed under the requirements section(which seem to be
>   missing). It would be nicer if the concepts were documented like how they
>   are at cppreference: http://en.cppreference.com/w/cpp/concept

This was fixed on develop. I now use the term "Refined concept" instead of
"Superclass". Regarding concept requirements, they are listed in the
"minimal complete definition" section of each concept. Then, semantic
properties that must be satisfied are explained in the "laws" section.

> - Concepts make no mention of minimum type requirement such as
>   MoveConstructible.

I believe the right place to put this would be in the documentation of
concrete models like `Tuple`, but not in the concepts (like `Sequence`).
Hana's concepts operate at a slightly higher level and they do not really
have a notion of storage. But I agree that it is necessary to document
these requirements. Please refer to this issue [6] for status.

> - Organization of documentation could be better. Its nice showing what
>   algorithms when the user views a concept, but it would be better if all
>   the algorithms could be viewed together.

I assume you are talking about the reference documentation and not the
tutorial. I agree that it could be easier to look for algorithms. There
are also other quirks I'd like to see gone. The problem is that Doxygen
is pretty inflexible, and I'm already tweaking it quite heavily. I'm
considering either using a different tool completely or making some
changes in the organization of the reference.

Your more precise comment about viewing algorithms on their own page is
already tracked by this issue [7].

> - For compile-time `Iterable` sequence(which is all you support right now),
>   `is_empty` can be inferred, and should be optional.

How can it be inferred?

> - Overall, I think the Concepts could be simplified. They seem to be too
>   complicated, and it leads to many surprises which seem to not make
>   sense(such as using `Range` or `String` with `concat` or using
>   `tick::integral_constant`).

1. Concatenating ranges does not make sense. A Hana Range is a contiguous
   sequence of compile-time integers. What happens when you concatenate
   `make_range(0_c, 3_c)` with `make_range(6_c, 10_c)`? It's not contiguous
   anymore, so it's not a Range anymore.

2. Concatenating strings makes complete sense, indeed. This could be handled
   very naturally by defining a `Monoid` model, but it was not done because
   I did not like using `+` for concatenating strings :-). I opened this
   issue [8] to try and find a proper resolution.

3. Tick's integral_constants can be handled as I explained in the
   gist at [2].

As a fundamental library, Hana was designed to be very general and extensible
in ways I couldn't possibly foresee. Hence, I could have stuck with Fusion's
concept hierarchy (minus iterators), but that would have been less general
than what I was aiming for. Also, Hana is slightly biased towards functional
programming, and it reflects in the concepts. If that is what you mean by
"complicated", then I'd say this generality and power is a feature rather
than a bug.

I would really like to know specifically which concepts you find too
complicated or superfluous. There are definitely things that could be
improved, but in general I am very content with the current hierarchy
and I think this is one of Hana's strengths, to be frank.

> - Currently, none of the algorithms are constrained, instead it uses
>   `static_assert`, which I think is bad for a library that is targeting
>   modern compilers.

People have mixed opinions about this. I personally think the last thing you
want is for an overload to SFINAE-out because of some failure deep down the
call chain, considering we're working with heterogeneous objects. I think
the best way to go is to fail very fast and very explicitly with a nice
static_assert message, which is what Hana tries very hard to do.

I also think the minority of people that would benefit from having SFINAE
friendly algorithms is largely outweighted by the majority of non template
metaprogramming gurus (likely not reading this list) who would rather have
a nice and helpful `static_assert` message explaining what they messed up.

Also, there's the problem that being SFINAE-friendly could hurt compile-time
performance, because everytime you call an algorithm we'd have to check
whether it's going to compile. However, because we're working with dependent
types, checking whether the algorithm compiles requires doing the algorithm
itself, which is in general costly. We could however emulate this by using
the Tag system. For example, `fold_left` could be defined as:

    template <typename Xs, typename State, typename F,
        typename = std::enable_if_t<models<Foldable, Xs>()>
    >
    constexpr decltype(auto) fold_left(Xs&& xs, State&& state, F&& f) {
        // ...
    }

This would give us an approximative SFINAE-friendliness, but like I said
above I think the best approach is to fail loud and fast.

> - It would be nice if the use of variable templates would be optional(and
>   not used internally), since without inline variables, it can lead to ODR
>   violations and executable bloat.

Without variable templates, we would have to write `type<T>{}`, `int_<1>{}`,
etc.. all of the time instead of `type<T>` and `int_<1>`. Sure, that's just
two characters, but considering you couldn't even rely on what is `type<T>`
(if it were a type, since `decltype(type<T>)` is currently unspecified),
we're really not gaining much. In short; no variable templates means a less
usable library, without much benefits (see next paragraph).

Regarding variable templates and ODR, I thought variable templates couldn't
lead to ODR violations? I know global function objects (even constexpr) can
lead to ODR violations, but I wasn't aware about the problem for variable
templates. I would appreciate if you could show me where the problem lies
more specifically. Also, for reference, there's a defect report [9] related
to global constexpr objects, and an issue tracking this problem here [10].

Finally, regarding executable bloat, we're talking about stateless constexpr
objects here. At worst, we're talking 1 byte per object. At best (and most
likely), we're talking about 0 bytes because of link time optimizations.
Otherwise, I could also give internal linkage to the global objects and they
would probably be optimized away by the compiler itself, without even
requiring LTO. Am I dreaming?

> Overall, I would like to see Hana compile on more compilers before it gets
> accepted into boost(currently it doesn't even compile on my macbook).

What compiler did you try to compile it with? Also, an important GCC bug
that was preventing Hana from working properly on GCC was fixed a couple
of days ago, so I'm going to try to finish the port ASAP and I'm fairly
confident that it should work on GCC 5.2.

Thanks for all your comments.

Regards,
Louis

[1]: https://github.com/ldionne/hana/issues/114
[2]: https://gist.github.com/ldionne/32f61a7661d219ca834d#file-main-cpp
[3]: http://ruby-doc.org/core-1.9.3/Integer.html#method-i-times
[4]: https://github.com/ldionne/hana/issues/100
[5]: https://github.com/ldionne/hana/issues/115
[6]: https://github.com/ldionne/hana/issues/116
[7]: https://github.com/ldionne/hana/issues/82
[8]: https://github.com/ldionne/hana/issues/117
[9]: http://www.open-std.org/JTC1/SC22/WG21/docs/cwg_active.html#2104
[10]: https://github.com/ldionne/hana/issues/76