Reading about D: few questions

[Andrew Wiley]

On Sat, Dec 24, 2011 at 1:08 PM, Timon Gehr <timon.gehr at gmx.ch> wrote:
> On 12/24/2011 07:00 PM, Andrew Wiley wrote:
>>
>> On Sat, Dec 24, 2011 at 9:50 AM, Timon Gehr<timon.gehr at gmx.ch>  wrote:
>>>
>>> On 12/24/2011 06:18 PM, Andrew Wiley wrote:
>>>
>>>>
>>>> 2011/12/24 Mr. Anonymous<mailnew4ster at gmail.com>:
>>>>
>>>>> On 24.12.2011 19:01, Denis Shelomovskij wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> 23.12.2011 22:51, bearophile пишет:
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> ++a[] works, but a[]++ doesn't.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Already known compiler bug.
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> Is it a joke? Array expression in D are for performance reasons to
>>>>>> generate x2-x100 faster code without any compiler optimisations. Link
>>>>>> to
>>>>>> one of these epic comments (even x100 more epic because of '%' use
>>>>>> instead of 'x###'):
>>>>>>
>>>>>>
>>>>>>
>>>>>> https://github.com/D-Programming-Language/druntime/blob/master/src/rt/arraybyte.d#L1127
>>>>>>
>>>>>>
>>>>>> But `a[]++` should store a copy of `a`, increment elements and return
>>>>>> stored copy. It is hidden GC allocation. We already have a silent
>>>>>> allocation in closures, but here a _really large_ peace of data can be
>>>>>> allocated. Yes, this allocation sometimes can be optimized out but not
>>>>>> always.
>>>>>>
>>>>>> IMHO, D should not have `a[]++` operator.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> Why should it store a copy? o_O
>>>>> I also don't see any allocations in the code on the URL above.
>>>>
>>>>
>>>>
>>>> int a_orig = a++;
>>>> int[] arr_orig = arr[]++;
>>>>
>>>> If ++ is going to be applied to an array, it needs to have the same
>>>> meaning as it does elsewhere. After this operation, arr_orig and arr
>>>> must refer to different arrays for that to be true.
>>>
>>>
>>>
>>> Not necessarily.
>>>
>>> class D{
>>>    int payload;
>>>    D opUnary(string op:"++")(){payload++; return this;}
>>> }
>>>
>>> void main() {
>>>    D d = new D;
>>>    assert(d.payload == 0);
>>>    assert(d++.payload == 1);
>>> }
>>
>>
>> That doesn't match integer semantics:
>> int a = 0;
>> assert(a++ == 0);
>> assert(a == 1);
>
>
> Yes, that was my point.
>

Then I'm not understanding what you're trying to prove.
I'm saying that if we implement a postfix ++ operator for arrays,
keeping the language consistent would require it to make a copy if the
user stores a copy of the original array. I guess it could be argued
that since arrays have hybrid value/reference semantics, no copy
should be made and the original should change.

Actually, looking at it from that angle, a[]++ is fundamentally
ambiguous because it could have value semantics or reference
semantics, so I would argue that we shouldn't have it for that reason.
'++a' and 'a += 1' do not have such ambiguities.