foreach, an analogy

[Bruno Medeiros]

Gregor Richards wrote:
> Bill Baxter wrote:
>> I like analogies, so here goes one.  If you don't just skip it.  :-)
>>
>> Say you've built a house and you're almost done, but you find one 
>> doorway with a door that's about an inch too narrow for the frame. 
>> You'd really like to cover that gap somehow.  You could just leave it 
>> as-is.  Sure.  You've got *most* of the doorway covered after all.  
>> It's not like a criminal could sneak in through a one-inch gap.  
>> Still, it could get pretty cold in the winter, so you'd really like to 
>> fix it.
>>
>> Clearly the best solution long term is to get a bigger door.
>>
>> But you don't have a bigger door.  You do, however, have a clever 
>> solution: take another door, identical to the first, and hang it on 
>> the *other side* of the frame to cover the gap.  Perfect!  The gap is 
>> covered!
>>
>> But hmm.  Now there are two doors.  Either of them alone *almost* 
>> enough to cover the doorway.  You don't really _need_ two doors doing 
>> almost exactly the same thing.
>>
>> It's not so bad, though.  At least in the summer, when you don't need 
>> it, you can just leave that second door open and out of the way. Even 
>> take it off its hinges and put it in the basement.
>>
>> That second door, as I'm sure you've figured out, is foreach_reverse.
>>
>> And really I do believe it's not so bad.  It's an ugly hack, like 
>> hanging a second door to fill a small gap, but if you don't want to 
>> iterate backwards over arrays as fast as possible, then you're free to 
>> ignore it, put that door in the basement, and use whatever technique 
>> you prefer.
>>
>> However, I still hope the plan is to one day get a bigger door.
>>
>> --bb
> 
> The bigger door is 'for'.  'foreach' is nothing but a convenient wrapper 
> around 'for'.  And don't you OOphiles go telling me that your fancy 
> class foo that has iteration /needs/ 'foreach':
>   for (auto bar = foo.begin(); !(bar is null); bar = foo.iterate(bar))
> Is it less pretty than foreach?  Yeah.  That's why foreach exists.  But 
> don't go saying that the reverse foreach is a band-aid patch, because 
> both forms are just convenience wrappers around the far more powerful 
> and useful 'for'.  Your small door is actually the screen door.
> 
>  - Gregor Richards

Not so. "'foreach' is nothing but a convenient wrapper around 'for'", 
yes, true, but 'foreach_reverse' is just a wrapper around this:
   foreach(Foo f; &aggregate.opApplyReverse) { ...
which is not more complicated that the 'foreach_reverse' form. As such 
'foreach_reverse' is a wrapper, but not a particularly convenient one, 
unlike 'foreach' which is.
As of DMD now, the only advantage in 'foreach_reverse' is ephemerous: it 
allows efficient reverse iteration of arrays. But couldn't the compiler 
easily detect this:
   foreach(Foo f; &fooarray.opApplyReverse) { ...
and and compile it as if it were a:
   foreach_reverse(Foo f; fooarray) { ...

-- 
Bruno Medeiros - MSc in CS/E student
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D