[Issue 7177] $ should forward to length by default

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=7177



--- Comment #57 from Andrei Alexandrescu <andrei at erdani.com> 2013-03-31 11:04:45 PDT ---
(In reply to comment #56)
> (In reply to comment #55)
> > @Kenji: I'm a bit unclear on your view on this. You mention you'd prefer a
> > library solution but your pull request seems to go for a in-compiler solution.
> 
> As Steven already mentioned in comment#14, automatically forwarding from $ to
> 'length' would break user-defined containers.

I don't think there's any breakage. My understanding is that those containers
already need to define opDollar. If opDollar is defined there is no change in
semantics.

> The fact clearly represents that we cannot always regard $ as "length".
> Indeed, it is true in built-in arrays and range concept, but isn't true in
> associative arrays and some user-defined containers. So I think that this
> enhancement has a bias toward range concept.

I'd have quite a bit of difficulty agreeing with this. The notion that "$" is a
synonym for "length" predates ranges and has been there for strings and arrays
ever since they were defined. On the contrary, I'd argue that notions such as
length-less ranges and infinite ranges contributed to the notion that opDollar
may be (rarely) something distinct from length.

> And, as far as possible, compiler should be a neutral.

That shouldn't be done to a fault, either. The D language is not neutral on
user-defined expr++ vs. ++expr; it forces both to have similar semantics to
their built-in counterparts. In contrast, the C++ language allows defining
++expr and expr++ with different semantics. But wait, it's worse: (a) C++
forces a net loss of efficiency for expr++ barring heroic optimization efforts
that are not generally applicable, and (b) C++ requires actual boilerplate to
ensure both variants work. I think it is plain that C++ made the wrong decision
and D learned from it and made the right decision.

Similarly, we should not require boilerplate for $ just to convince it to do
what it's always done for arrays and strings. Associative arrays and
user-defined containers are free to disable it or define it, depending on
what's most useful for them. 

> In other words, this is just reasonable for std.range users. Most of D
> programmers would use std.range, but not all.

Again, I find it very difficult to agree with this. $ has always been length
wherever meaningful - long before ranges came up.

> I don't mention that infinite range _should_ have r[n..$], rather mention that
> it is possible.

Yes, and defaulting $ to length does not prevent that. Again, if a range does
define opDollar, that will always be chosen.

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