Ad hoc ranges

[Andrei Alexandrescu]

On 1/21/11 7:35 PM, Tomek Sowiński wrote:
> Andrei Alexandrescu napisał:
>
>>> Like I said, anything that doesn't bother to expose range-interfaced iterators and is not performance critical is
>>> considered a target for ad hoc ranges. Working with non-D libraries, or libraries ported to D but preserving
>>> mother-language idioms. Tasks like traversing a tree of GUI widgets, or business specific objects where defining
>>> proper ranges rarely happens and is use-case driven in practice. I expect they could be of some use in unittesting
>>> as mock input. Vaguely related: educational -- ad hoc ranges read almost like a for loop so the learning curve for
>>> ranges in general is eased off.
>>>
>>> Adding them to Phobos is an interesting idea. We need to evaluate their worth, though.
>>>
>>> Everybody: if you could write up a one-liner like range(empty, popFront, front), what would you use it for?
>>
>> How about a singleton range - a range with exactly one element. It could
>> be done with repeat(x, 1) but let's try it with your function as a
>> warm-up exercise.
>
> If x is nullable, range(x, x=null, x); it destroys x, though. Otherwise the state must be held separately on the stack.
>
> bool empty;
> auto r = range(empty, empty=true, x);
>
> So repeat(x, 1) wins this one. I think such nuggets can better be expressed as a degenerate case of existing facilities. I envision ad hoc ranges at places where no iteration is defined and a one-off range struct doesn't pay. Like database-backed entities which don't conform to any clear-cut data structure, but if you squint you see it's sort of a tree, and you may just be able to e.g. walk through children recursively fetching only active ones from DB, traverse columns of interest, and dump their content to a grid component which takes an arbitrary range of values. And all this can be wrapped in std.parallelism to overlap DB round trips.

I think the challenge here is to figure out where to store the state. 
The idiom makes it difficult for the delegates to communicate state to 
one another.

Andrei