Reading about D: few questions

[Timon Gehr]

On 12/24/2011 08:41 PM, Mr. Anonymous wrote:
> On 24.12.2011 21:22, Andrew Wiley wrote:
>> 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.
>
> Maybe you're right, but a[]++; alone, imo, should compile.

+1.