RFC on range design for D2

[Benji Smith]

Sergey Gromov wrote:
> Benji Smith <dlanguage at benjismith.net> wrote:
>> Bill Baxter wrote:
>>> Hmm, for FSMs you can't really define a good end state.  There may not
>>> be any particular end state. ... ah, but wait I forgot.  That's the
>>> beauty of a range -- the end "state" doesn't have to be a "state" per
>>> se.  It can be any predicate you want it to be.  "Range" is misleading
>>> in this case.  This is one of those cases where you just have to
>>> remember "range" means "current value plus stopping criterion".
>> That's what I was saying earlier.
>>
>> I think the mechanics are good. And for contiguous, sequential 
>> containers, the word "range" is great. For other types of containers, or 
>> other selection/iteration scenarios, you can shoehorn your mental model 
>> into the "range" metaphor. But it's weird.
> 
> It seems to me like a misuse of ranges.  Do you really want to iterate 
> over a state machine?  FSM is a mailbox with a 'message' hole.  You put 
> messages into it and it does things.  How do you iterate over a mailbox?

Well, no. Not when you put it like that.

The example I posted earlier went something like this:

    MarkovModel<ApplicationState> model = ...;
    for (ApplicationState state : model) {
       state.doStuff();
    }

It's not a bad abstraction. The model handles all of the semantics of 
calculating the transition probabilities and selecting the next state, 
so that the foreach loop doesn't have to muss with those details.

Yeah, it's a total misuse of the "range" metaphor, and that's exactly 
what I'm saying. In Java, where I implemented this project, an 
"iterator" is a tiny bit of logic for returning objects in a 
(potentially endless, potentially reversible) sequence, primarily to 
support looping constructs. Just because there's no underlying range of 
objects doesn't mean they're not iterable.

Of course, Java iterators are *much* more limited constructs than these 
new D ranges. But I still think the concept has merit. And, like you 
said, calling them ranges makes them seem stupid. Because they're not 
ranges.

--benji