Feature to get or add value to an associative array.

[Nick Treleaven]

On Sunday, 15 April 2018 at 22:52:47 UTC, Giles Bathgate wrote:
> The function provides a means to get a value corresponding to 
> the key, but if the value doesn't exist it will evaluate the 
> lazy argument to create a new value, add this to the 
> associative array and then return it.
>
> auto p = lookup.getOrAdd("giles", new Person);

Thanks for making this pull, I've thought about solving this 
before. I think the function needs to provide a way to tell if 
the value was already present. I also think it's more ergonomic 
not to have to use a lazy argument, and probably more efficient, 
so I had in mind:

Value* slot(AA aa, K key, scope bool* inserted = null);

bool inserted;
auto p = aa.slot("key", &inserted);
if (inserted) {
   ...
   *p = new Value(...);
}
// use *p

This pattern needs a pointer to be returned, instead of using 
`ref`. Note that `&inserted` is valid in @safe code, but only 
with -dip1000. I called the function `slot` because it always 
returns the address of the slot which the value is stored in.