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Subject: Re: [boost] [yap] Review part 1: documentation
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2018-02-12 18:08:24


On 02/12/2018 09:45 AM, Zach Laine wrote:
> On Sun, Feb 11, 2018 at 5:54 PM, Steven Watanabe via Boost <
> boost_at_[hidden]> wrote:
>> <snip>
>>>> tutorial/transform_matching.html:
>>>> - The negate example:
>>>> struct xform
>>>> {
>>>> // This transform negates terminals.
>>>> auto operator() (boost::yap::terminal_tag, double x)
>>>> { return -x; }
>>>> // This transform removes negations.
>>>> auto operator() (boost::yap::negate_tag, double x)
>>>> { return x; }
>>>> }
>>>> seems somewhat confused. The behavior I would expect
>>>> from this transform is that it can match expressions
>>>> of the form `d` or `-d` and will return -d or d respectively.
>>> This gets right at the point of the example. It's maybe a
>> counterintuitive
>>> way to evaluate expressions, but the alternative is worse. The
>> alternative
>>> is that terminal transforms are not auto-applied before a terminal is
>>> unwrapped and passed as a value to a tag-transform call. If that were
>> how
>>> the library worked, there would be no way to write a tag-transform
>> function
>>> that *did* apply the transform, even explicitly, because at that point
>> the
>>> terminal is no longer visible.
>> <snip>
>> - re-wrap the argument in a terminal
>> - call (*this)(terminal_tag(), d)
> In either of those cases, any information you had about the details of the
> terminal are now gone, and you can't get them back. The value that you
> re-wrap or call as above may have come from a terminal that was an lvalue,
> an rvalue, a reference-expression, const, mutable, etc.
> - implement terminal evaluation in a separate
>> function which can be used as needed.
> True. I had to make a decision about which convention violated my sense of
> surprise the least. This is the one I chose. It seems less surprising to
> get stuck with transforms you did not yet get to in the terminals (mainly
> because they happen only since you wrote those into your transform
> explicitly), than it does to get stuck because you lost important
> information about the properties of the unwrapped terminal.
> It may be that the community consensus is that my sense of surprise is
> wrong in this case; I'm open to being persuaded.
> [snip]

  Maybe you should reconsider whether terminals should
be unwrapped in the first place. If you don't unwrap
terminals, then this isn't an issue.

If I'm writing a generic function like:

auto operator()(plus_tag, Expr&& lhs, Expr&& rhs)
  transform(lhs, *this);
  transform(rhs, *this)

then the behavior that I want is for terminals
to be processed by the nested call to transform.
Unwrapping the terminals causes surprising behavior,
which is only sort of fixed by automatically applying
the terminal transform and making transform a no-op
for non-Expressions. In particular, if the terminal
transform returns an Expression, that Expression will
get processed again.

>>> examples/vector.html:
>>>> - return boost::yap::make_terminal(std::move(vec[n]));
>>>> Move is probably wrong as you're working with a reference
>>>> to begin with. (Of course, it doesn't really matter for
>>>> double, but imagine that you have a vector<unique_ptr>
>>>> instead.)
>>> The move is required to force a copy; just because the reference is valid
>>> now doesn't mean it won't dangle later.
>> Sure, but when using the evaluate(transform()) idiom,
>> you're guaranteed that the references returned by transform
>> will not be left dangling.
> Sure. But that's just in this example, and using that (admittedly
> dominant) idiom. I still want to reinforce in the example code how you
> make copies of value types, especially built-in ones.

  Perfect-forwarding through a chain of
transforms is also an important use case that
needs to be explained. I don't mind having
the examples demonstrate copying as long as
they are written such in a way that copying
is really the correct behavior. In this particular
example, I don't think that it is.

In Christ,
Steven Watanabe

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