On 02/04/12 23:44, Timon Gehr wrote:
> On 02/04/2012 11:23 PM, Artur Skawina wrote:
>> On 02/04/12 22:20, Timon Gehr wrote:
>>> On 02/04/2012 06:55 PM, Artur Skawina wrote:
>>>> Semi-related quiz:
>>>>
>>>>      immutable(char)[] a = "a";
>>>>      const    (char)[] b = "b";
>>>>
>>>>      auto aa = a ~ a;
>>>>      auto bb = b ~ b;
>>>>      auto ab = a ~ b;
>>>>
>>>>      writeln("aa: ", typeid(aa), "  bb: ", typeid(bb), "  ab: ", typeid(ab));
>>>>
>>>> And the question is: How many people, who have not already been bitten by this, will give the correct answer to: "What will this program print?"?
>>>>
>>>
>>> I think this is covered in this issue:
>>>
>>> http://d.puremagic.com/issues/show_bug.cgi?id=7311
>>>
>>> But feel free to open a more specific enhancement/bug report.
>>
>> Apparently, there's already a bug open for everything.
>> I'm not sure if it's a good or bad thing. :)
>>
>> I don't think there's one correct answer here - you're right that unique const
>> does not really make sense. But is mutable (non-const) really better than
>> immutable? It depends, sometimes you will want one, sometimes the other.
>> I first ran into this while doing a custom string class - there it was the cause
>> of the one and only cast - (string ~ const(char)[]) can obviously still be a
>> string, but the compiler won't accept it without a cast.
>> I'm not sure how often you'll want the result of concatenation to be mutable,
>> compared to immutable. Anyway, the result really is "unique", not mutable, const
>> or immutable, at least until it is converted to one of those, hence the solution
>> described below.
>>
> 
> 
> Well, string = string ~ const(char)[] and char[] = string ~ const(char)[] should work, regardless of the type of immutable[] ~ const[]. The compiler can track the uniqueness of the data at the expression level without actually introducing a type modifier. There is precedent: Array literals are covariant, because it is safe. Do you want to open the enhancement or should I do it? (I really thought I already had an issue open for this...)

Please do; i've been avoiding filing dmd bugs, because i'm only using gdc, not dmd. (so i can't even easily check if the issue still exists in the current tree)

artur