The great slice debate -- should slices be separated from arrays?

[Steven Schveighoffer]

On Tue, 24 Nov 2009 11:07:43 -0500, Andrei Alexandrescu  
<SeeWebsiteForEmail at erdani.org> wrote:

> Steven Schveighoffer wrote:
>> In many other posts, people have been festering over dropping T[new]  
>> and not having a reference array type.  The argument is (and forgive me  
>> if this is a strawman, someone from the other side can post if I'm  
>> incorrect): If we make arrays a separate type from slices, and only  
>> allow appending on arrays, then we solve the stomping problem and the  
>> hard-to-determine reallocating problem.  For those who are unfamiliar  
>> with these problems, I'll try to outline them at the bottom of the post.
>>  I contend that even if we make arrays a separate type, even if arrays  
>> are made a true reference type, slices will still suffer from the same  
>> hard-to-determine reallocation problem *unless* we make slices fatter.
>>  My proof is as simple as an example.  Assume 'Array' is the new type  
>> for an array:
>>  auto a = new Array!(int)(15); // allocate an array of 15 integers, all  
>> 0
>> auto s = a[0..5];
>> a ~= [1,2,3,4,5];
>> a[0] = 1.
>>  Now, what does s[0] equal?
>
> Array may include a field
>
>     bool sliceExtracted;
>
> that is set to true whenever you take a slice from the array and set to  
> false whenever the array's data is reallocated. The array's  
> documentation could mention that ~= is amortized constant if there are  
> no intervening slicing operations.

We weren't talking about performance, but I think I know what you are  
saying: To solve the hard-to-determine reallocation problem, you propose  
that any array which has had a slice taken from it, will reallocate on  
append even if it could grow into the block?  That seems like a rather  
blunt solution...

if(arr[0.."hello".length] == "hello")
    arr ~= "xyz"; // oops, reallocate because we took a slice earlier.

Wouldn't this make slicing combined with appending unusable for  
performance reasons?  Then the only time you want to use slices is if you  
are done appending to the array, and then even today's implementation  
wouldn't cause the hard-to-determine problem for code written that way.

Maybe I misunderstood you, let me know.

-Steve