D array expansion and non-deterministic re-allocation

[Bartosz Milewski]

Andrei Alexandrescu Wrote:

> My concern is the semantics of the language. As it is defined right now, a conforming implementation is free to use a quantum random number generator to decide whether to re-allocate or not. Is it likely? I don't think so; but the non-determinism is part of the semantics of D arrays.
> 
> It would not be difficult to specify in the language definition (and 
> TDPL) that behavior is deterministic for a given platform. I think this 
> has some impact on the freedom of the memory allocator, but probably not 
> major.

Actually this wouldn't fix the problem. Although this would make the program deterministic, it would still exhibit chaotic behavior (and chaos is a pretty good simulator of non-determinism--see random number generators). 

An input string that is one character longer than in the previous run in one part of the program could cause change in allocation in a completely different part of the program (arbitrary long distance coupling). 

Theoretically, the heap is deterministic, but in practice no program should depend on all pointers having exactly the same values from run to run. For all intents and purposes the heap should be treated as non-deterministic. This is why no language bothers to impose determinism on the heap. Neither should D.