D array expansion and non-deterministic re-allocation

[Bartosz Milewski]

Andrei Alexandrescu Wrote:

> Bartosz Milewski wrote:
> > Andrei Alexandrescu Wrote:
> > 
> >> How about creating a struct Value!T that transforms T (be it an
> >> array or a class) into a value type? Then if you use Value!(int[]),
> >> you're effectively dealing with values throughout (even though
> >> internally they might be COWed). Sometimes I also see a need for
> >> DeepValue!T which would e.g. duplicate transitively arrays of
> >> arrays and full object trees. For the latter we need some more
> >> introspection though. But we have everything in the laguage to make
> >> Value and DeepValue work with arrays.
> >> 
> >> What do you think?
> > 
> > I'm afraid this would further muddle the message: "If you want safe
> > arrays, use the Value device; if you want to live dangerously, use
> > the built in type."
> 
> I think the message is "If you want values, use Value. If you want 
> slices, use slices." To me that's rather a clear message.
> 
> > I'd rather see the reverse: D arrays are safe to
> > use.
> 
> But that's backwards. You can do Value with slices. You can't do slices 
> with values. The logical primitive to pick is the slice.
> 

I know you don't like analogies, but for me it sounds like an advice to a  C++ programmer: if you want to control destruction, use unique_ptr (or shared_ptr). This, BTW, is how I program in C++ because I can't live with memory leaks and double-deletions. 

What would your advice be to somebody who is learning D as her first language. Don't use arrays directly, use Value!(T[]) ? 

Should libraries be written in terms of Value!(T[])? 

> > They have the usual reference semantics of static arrays. But if
> > you expand them, the sharing goes away and you get a unique reference
> > to a copy. This is the "no gotcha" semantics, totally predictable,
> > easy to reason about.
> > 
> > How the compiler supports that semantics while performing clever
> > optimizations is another story. It's fine if this part is hard. The
> > language can even impose complexity requirements, if you are sure
> > that they are possible to implement (it's enough to have one
> > compliant implementation to prove this point).
> 
> Well the problem is we don't have that. It's more difficult to "be fine" 
> if the onus of implementation is on you.
> 
> > By the way, what are the library algorithms that rely on O(1)
> > behavior of array append?
> 
> I don't know, but there are plenty of algorithms out there that rely on 
> that.

I thought you had concrete examples.