foreach, an analogy

[Gregor Richards]

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