Ad hoc ranges

[Tomek Sowiński]

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.

-- 
Tomek