On 24 April 2013 03:33, Andrei Alexandrescu <SeeWebsiteForEmail@erdani.org> wrote:
On 4/23/13 12:04 PM, Manu wrote:
On 24 April 2013 00:24, Andrei Alexandrescu
<SeeWebsiteForEmail@erdani.org <mailto:SeeWebsiteForEmail@erdani.org>>

wrote:


        The very point of this DIP is to not create
        syntax-driven features, instead better define existing ones that
        make
        sense on their own so they can be used for same purpose.


    It's a new feature, no two ways about it. It overlaps ref and auto
    ref without any palpable benefit and defines yet another way to
    achieve the same thing as auto ref. On this ground alone the
    proposal has a large problem.


How does it overlap ref? It simply justifies the argument with an extra
constraint and isn't tied to 'ref' at all, it's just useful in conjunction.

The best setup would be:

1. To take lvalues by reference, write "ref".

2. To take lvalues and rvalues by reference, write "auto ref".

That's not a good setup at all. It still doesn't make sense.
There's nothing 'automatic' about it, I've specified ref, it is ref, there's no other choice.
And it relies on a major breaking change to ref, which restricts the functionality of ref by default.

Everything else is superfluous and puts the burden of justification on the proposer. With DIP36, the setup would be:

1. To take lvalues by reference, write "ref".

2. To take lvalues and rvalues by reference:

2.1. Is it a template? Then write "auto ref".

2.2. Is it a non-template? Then write "scope ref".

Stop talking about r-values, rather, consider safety of passing temporaries. This problem has nothing to do with r-values, this is equally unsafe:
  void f(ref int x);
  int x;
  f(x);

It's precisely the same problem, and should be fixed with the same solution.

I would rewrite your list as such:

1. 'un-safely' pass a value (may not be a local), write 'ref'
2. safely pass a value (may be a local), write 'scope ref'
3. if you are concerned with templates do you:
3.a. want unsafe auto-ref, type 'auto ref' (only non-locals would generate 'ref')
3.b. want safe auto-ref, type 'scope auto ref'

The automatic selection of ref for templates is fairly unrelated to this issue.

I can't agree that it overlaps auto-ref at all. They're fundamentally
different concepts. auto-ref is a template concept; it selects the
ref-ness based on the received arg. 'auto ref', ie, 'automatic
ref-ness'. It makes no sense on a non-template situation.
I'm still completely amazed that the very reason this DIP makes perfect
sense to me(/us) is the same reason you have a problem with it.

I don't know how to respond to this. To me is it painfully obvious DIP 36 is poor language design and fails to solve a variety of issues, such as clarifying lifetime of temporaries, safety, and returning ref from functions.

Lifetime of temporaries is the most basic of principles. A local lives the life of the function in which it is defined.
Safety is the whole point, and intrinsic to the proposal; safety by explicit specification, thus the programmer retains the option.

If a function receives 'scope ref', and wants to return it, I think it should also return 'scope ref' (I guess this wasn't defined in the DIP), otherwise it would be considered an escape.

            2. The proposal is sketchy and does not give many details,
            such as the
            lifetime of temporaries bound to scope ref objects.


        It can't because lifetime of temporaries is not defined in D at
        all and
        suck stuff needs to be consistent. It is not really different from a
        lifetime of struct literal temporary used for pass-by-value.


    A proposal aimed at binding rvalues to references must address
    lifetime of temporaries as a central concern.


It's not an r-value, it's a standard stack-allocated temporary. It's
lifetime is identical to any other local.
The reason it's not detailed in the proposal is because it adds no such
new feature, and makes no changes. The lifetime of a local is well
understood.

Currently rvalues are destroyed immediately after the call they are passed into. DIP 36 would need to change that, but fails to specify it.

Again, I think it was presumed (I can't conceive any other approach), and certainly Kenji read it that way, because his code appears to do just that.

            3. The relationship with auto ref is insufficiently
            described, e.g.
            there should be clarification on why auto ref cannot be
            improved to
            fulfill the desired role.


        auto ref is a template-world entity. If by "improved" you mean
        "completely reworked" than sure, I can add this rationale. Will
        do today.


    I think we should focus on
    http://d.puremagic.com/issues/__show_bug.cgi?id=9238

    <http://d.puremagic.com/issues/show_bug.cgi?id=9238> and on making
    ref safe.


I don't believe it's possible to make ref safe. Can you suggest any
vision for this?

http://d.puremagic.com/issues/show_bug.cgi?id=9238


It's unsafe by definition... you are passing a pointer of unknown origin
to a function that could do anything with that pointer.
Hence 'scope ref', which appropriately restricts what the callee is able
to do with it.

Our intent is to make "ref" always scoped and reserve non-scoped uses to pointers. We consider this good language design: we have unrestricted pointers for code that doesn't care much about safety, and we have "ref" which is almost as powerful but sacrifices a teeny bit of that power for the sake of guaranteed safety. Safety is guaranteed by making sure "ref" is always scoped (references can be passed down but never escape their bound value).

That's a massive breaking change...

[response in other thread, we need to stop repeating in 2 threads I think]