On 07/27/12 20:47, Stuart wrote:
> On Friday, 27 July 2012 at 15:09:38 UTC, Graham Fawcett wrote:
>> On Friday, 27 July 2012 at 13:10:46 UTC, Stuart wrote:
>>> On Friday, 27 July 2012 at 03:00:25 UTC, Brad Anderson wrote:
>>>>
>>>> D equivalent: iota(0, int.max, 2).map!(a => /* do something with even
>>>> numbers */)();
>>>
>>> I think you're missing the point. The purpose isn't to generate a sequence
>>> of numbers, but to illustrate how the Yield keyword is used in VB.NET.
>>> Sure, getting a sequence of numbers may be straightforward, but what about
>>> a lazy-populated list of all files on a computer? That can be done using
>>> Yield - and more importantly, WRITTEN like a normal synchronous function.
>>> Let's see you do that with map.
>>
>> That's easy:
>>
>> [...elided code...]
>> auto entries = dirEntries(BASE_DIR, SpanMode.breadth);
>
> Ah, but that depends upon the pre-existence of the dirEntries() function. I
> think perhaps you're missing the point - which is that "Yield" allows you to
> WRITE a function synchronously which will then be executed lazily, as an
> iterator. What you have demonstrated there is USING a lazy function. How
> would I write, in D, a function that would lazily assemble some data and
> return it as a lazy collection? I mean, without calling existing lazy
> functions.
A more or less direct translation of your original example would be:
auto infiniteSequence1(T, S)(T startValue, S step) {
struct IS {
auto opApply(scope int delegate(ref T) yield) {
while (1)
if (auto r = yield(startValue))
return r;
else
startValue += step;
}
}
return IS();
}
void main() {
import std.stdio;
foreach(n; infiniteSequence(2, 2)) {
writeln(n);
if (n>13)
break;
}
}
and a D-style range based version could look like this:
auto infiniteSequence(T, S)(T startValue, S step) {
static struct IS {
T startValue;
S step;
enum empty = false;
@property ref front() { return startValue; }
void popFront() { front += step; }
}
return IS(startValue, step);
}
which both are as lazy as it gets. Returning direntries would be
done similarly. Is this just about the syntax (which isn't much
different)?
artur
