Why we need opApply (Was: Can we drop static struct initializers?)

[dsimcha]

== Quote from Max Samukha (spambox at d-coding.com)'s article
> On Sat, 21 Nov 2009 18:51:40 +0000 (UTC), dsimcha <dsimcha at yahoo.com>
> wrote:
> >== Quote from Max Samukha (spambox at d-coding.com)'s article
> >> On Fri, 20 Nov 2009 15:30:48 -0800, Walter Bright
> >> <newshound1 at digitalmars.com> wrote:
> >> >Yigal Chripun wrote:
> >> >> what about foreach_reverse ?
> >> >
> >> >No love for foreach_reverse? <tear>
> >> And no mercy for opApply
> >
> >opApply **must** be kept!!!!  It's how my parallel foreach loop works.  This would
> >be **impossible** to implement with ranges.  If opApply is removed now, I will
> >fork the language over it.
> I guess it is possible:
> uint[] numbers = new uint[1_000];
> pool.parallel_each!((size_t i){
>         numbers[i] = i;
>     })(iota(0, numbers.length));
> Though I agree it's not as cute but it is faster since the delegate is
> called directly. Or did I miss something?

I'm sorry, but I put a lot of work into getting parallel foreach working, and I
also have a few other pieces of code that depend on opApply and could not (easily)
be rewritten in terms of ranges.  I feel very strongly that opApply and ranges
accomplish different enough goals that they should both be kept.

opApply is good when you **just** want to define foreach syntax and nothing else,
with maximum flexibility as to how the foreach syntax is implemented.  Ranges are
good when you want to solve a superset of this problem and define iteration over
your object more generally, giving up some flexibility as to how this iteration
will be implemented.

Furthermore, ranges don't allow for overloading based on the iteration type.  For
example, you can't do this with ranges:

foreach(char[] line; file) {}  // Recycles buffer.
foreach(string line; file) {}  // Doesn't recycle buffer.

They also don't allow iterating over more than one variable, like:
foreach(var1, var2, var3; myObject) {}

Contrary to popular belief, opApply doesn't even have to be slow.  Ranges can be
as slow as or slower than opApply if at least one of the three functions (front,
popFront, empty) is not inlined.   This actually happens in practice.  For
example, based on reading disassemblies and the code to inline.c, neither front()
nor popFront() in std.range.Take is ever inlined.  If the range functions are
virtual, none of them will be inlined.

Just as importantly, I've confirmed by reading the disassembly that LDC is capable
of inlining the loop body of opApply at optimization levels >= O3.  If D becomes
mainstream, D2 will eventually also be implemented on a compiler that's smart
enough to do stuff like this.  To remove opApply for performance reasons would be
to let the capabilities of DMD's current optimizer influence long-term design
decisions.

If anyone sees any harm in keeping opApply other than a slightly larger language
spec, please let me know.  Despite its having been superseded by ranges for a
subset of use cases (and this subset, I will acknowledge, is better handled by
ranges), I actually think the flexibility it gives in terms of how foreach can be
implemented makes it one of D's best features.